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Use of solar PV inverters during night-time for voltage regulation and stability of the utility grid

Use of solar PV inverters during night-time for voltage regulation and stability of the utility grid Abstract Photovoltaic (PV) inverters are vital components for future smart grids. Although the popularity of PV-generator installations is high, their effective performance remains low. Certain inverters are designed to operate in volt-ampere reactive (VAR) mode during the night. Yet, this approach is ineffective due to the consumption of active power from the grid (as internal losses) and the regulation necessity of the direct-current (DC) bus. This paper will demonstrate the operation of a PV inverter in reactive power-injection mode when solar energy is unavailable. The primary focus is on the design of the inverter controller with respect to the synchronous rotating frame control method. The proposed novel method enables an inverter to inject the required level of reactive power to regulate the voltage levels of the utility grid within specified limits. In the process, the inverter does not absorb active power from the grid for its internal operation. The presented model has the ability to inject ≤2 kVAR of reactive power at zero power factor without absorbing active power from the grid. Simulation and hardware models of the inverter were developed and tested for different reactive loads in which the hardware model represented the real-world application. The reactive power injection of the two models ran at zero power factor and produced the expected outcomes for their corresponding independent reactive loads. Henceforth, it was evident that the proposed method can enhance the efficiency of an inverter and ensure the stability of the utility grid to which it is connected. Open in new tabDownload slide solar, energy system and policy, energy storage, distributed energy and smart grid, photovoltaic, reactive power, night-time usage Introduction Grid-tie inverters can be regarded as the main component in both renewable-energy conversion systems and smart grid systems. They can convert renewable energy into power that then can be fed to the utility grid as long as the renewable source exists. For photovoltaic (PV) inverters, solar energy must be there to generate active power. Otherwise, the inverter will remain idle during the night. The idle behaviour reduces the efficiency of the PV inverter. However, if there is a mechanism to use such inverters in a different way at night, its efficiency can be increased. The main motivation of the current research is to address the above issue and increase the efficiency of the inverter by using it during the night as well. Generally, when the reactive power is insufficient in the utility grid, it is not possible to push the power demand by loads through the lines as the voltage sags [1]. Using the inverter as a reactive power generator by operating it as a volt-ampere reactive (VAR) compensator is a potential way of solving the above issue of voltage sag [2]. The rapid increase in using PV inverters can be used to regulate the grid voltage and it will reduce the extra cost of installing capacitor banks. Currently, there are multiple ongoing research applications and experiments focusing on this general concept of using a PV inverter as a VAR compensator [3–5]. The PV generators consume a certain amount of active power for their internal operation and losses. Once the PV generators are used as reactive power generators, they will absorb active power from the grid that subsequently reduces the effectiveness of the concept. Although there are multiple research studies that address the use of PV inverters as reactive compensators during the night, none of them has addressed the specific issue of consuming active power from the grid [6, 7]. Most of the previous designs are planned to absorb the active power requirement from the grid to their internal operation. The novel method in the current research focuses on addressing these issues and developing an efficient method that can enhance the grid stability and usage of PV inverters. The novel control method introduced in this paper allows PV inverters to operate in pure reactive power-injection mode. The inverter is enhanced with the ability to work in this mode without absorbing any active power from the grid to compensate for its internal operation with losses and to regulate the direct-current (DC) bus voltage, which is the main novelty of this design. The novel concept is associated with a battery bank to compensate for the internal operation of the inverter. Specifically, this system is designed to inject only the required amount of reactive power by operating it in zero power factor. Further, the efficient use of the inverter can be increased by using it during the night. Overall, the concept introduced here expands the use of PV inverters and helps to maintain and regulate the voltage within the declared limits. Furthermore it can maintain the stability of the utility grid to which it is connected. Mainly, the synchronous rotating frame control method is utilized to develop the novel controlling method as it successfully defines the system performance under both steady-state and transient operations [8]. Also, the synchronous rotating frame control method is valid for any instantaneous difference between the voltage and the current. The details of the novel method are demonstrated in a simulation model and in a hardware development model with experimental results to validate the workability. The paper is organized as follows. Section 1 describes the active and reactive power injection using PV inverters. This is followed by Section 2, a discussion on the reactive power requirement of the grid and night-time reactive power injection. After that, Section 3 presents the novel control mechanism in detail. Further on, Section 4 discusses the simulation model and the hardware implementation under several grid conditions. The final section of this paper presents conclusions. 1 Active and reactive power control In the modern day, the PV inverters are being developed under the interconnection standards such as IEEE 1547, which do not allow for voltage regulations [9]. However, a majority of manufacturers of PV inverters tend to enhance their products with reactive power absorbing or injecting capabilities without exceeding their voltage ratings. The PV inverters theoretically can be developed as reactive power supporters, the same as the static compensators (STATCOMs) that the industrial standards do not address [10]. Typical PV inverters are designed to be disconnected at night. Alternatively, it is possible to use its reactive power capability when there is no active power generation. Typically, renewable generators like wind and solar individually follow a reactive power or a power factor set point, which can be tuned at the plant level for Volt/VAR regulating. Doing so will allow them to be involved in voltage controlling of the system. Reactive power control and the power factor control that are connected to the grid are not usually used in large power plants as they can result in improper response to system voltage variations, which could reduce the voltage stability of the system. However, reactive power and power factor control can be considered suitable options for distributed generators. 1.1 Reactive power requirement on the system The load demand can be increased due to their dynamic behaviour once they are connected to the transmission system [11]. Then the voltage level will drop at the load end together with the feeders. Hence the system fails to push the power through the transmission lines according to the demand of local loads. The voltage sag can be considered one of the most irritating events that can occur in the power system. However, it can be mitigated by injecting the desired amount of reactive power into the system. In other words, the voltage level of the power systems and transmission lines can be controlled by injecting or absorbing the reactive power from the distributed sources. To describe the combination between the voltage and the reactive power—a relationship that can be developed using the droop control equations for grid voltage and combining them to embed a general impedance. The connection between the voltage and the reactive power can be demonstrated as shown in the following equation [12]: Q−Q0=− 1kqα2+1(V−V0)(1) where α denotes the ratio R/X, which gives the grid impedance ratio (α = R/X). Generally, the grid impedance R + jX must be known to control the reactive power as well as the active power. As the variables R and X appear separately when deriving Equation 1, the ratio R/X can be used. Hence this value can be used to evaluate the required amount of reactive power. The frequency component of the above relationship is omitted since the distributed generator (DG) unit is grid-following and it cannot be controlled by local DGs. The parameter V gives the instantaneous grid voltage, while Q denotes the reactive power of the DG unit. V0 equals the nominal grid voltage. Q0 can be explained as a non-zero set point imposed by the transmission system operator. The parameter kq determines the amount of reactive power control for a certain grid voltage deviation from its nominal value (this parameter has the unit of Volt/VAR). The setting of this parameter has a vital impact on the grid voltage control and allows great flexibility [13]. Furthermore, the droop parameter is set to the preference or could also be altered by the network operator. In brief, this parameter is set to the level at which the DG units participate in the primary control of the grid voltage. 1.2 Reactive power control Accordingly, a variance of grid voltage from the declared value will create a reactive power demand. Generally, a grid-connected PV inverter can be programmed to inject and absorb the reactive power. Hence, both the overvoltage and undervoltage conditions can be regulated using the reactive power control ability. The dq components theory, which will be described in Section 2, can be used to perform the controlling mechanism efficiently [14]. The q component of the current can be regulated to control the reactive power injection instantaneously as it is used to represent the reactive power component. 1.3 Night-time reactive power-injection mode In general, the grid-tie inverters are equipped with DC-to-DC converters that regulate the DC bus voltage [15]. Once the active power is unavailable during the night, the DC-to-DC conversion becomes inactive. However, a developed control scheme with an energy-storage system can allow the inverter to operate in the reactive power mode even without the PV panels harvesting solar energy. Subsequently, the inverter can be programmed to operate as a VAR compensator to inject only the required reactive power, which will regulate the voltage at the load end. 2 Controlling mechanism The controlling mechanism of the novel concept with a background study is described under this topic. Further, the methods used for the design are described in detail. 2.1 Overview of different modelling techniques PV inverters in current power systems are utilizing several controlling techniques with the purpose of controlling the power. Table 1 shows a few controlling methods with their characteristics. Table 1: Special characteristics of inverter controlling mechanisms Controlling method . Special characteristics . 1. Current loop under single-axis 𝑑q rotating coordinate system Does not behave the positive and negative sequence decomposition of currents 2. Current loop under dual 𝑑q rotating coordinate system Has a dynamic process of transient underabig fault of asymmetrical transient grid system voltage 3. Active damping compensation feedback control strategy Cannot understand the damping control for unbalanced situations and also affects the system stability 4. Proportional-resonant current controller When the system is under grid voltage, the asymmetrical fault improved upon dynamic regulation 5. Hysteresis control method The switching frequency of pulse width modulation (PWM) can vary 6. The peak current control method Inductor currents track the basic sinusoidal waveforms 7. Stationary reference frame control Capable of removing the steady-state error between the controlled signal and reference signal 8. Synchronous rotating frame control Valid for any instantaneous difference of voltage or current and effectively describes the system performance under both steady-state and transient operations Controlling method . Special characteristics . 1. Current loop under single-axis 𝑑q rotating coordinate system Does not behave the positive and negative sequence decomposition of currents 2. Current loop under dual 𝑑q rotating coordinate system Has a dynamic process of transient underabig fault of asymmetrical transient grid system voltage 3. Active damping compensation feedback control strategy Cannot understand the damping control for unbalanced situations and also affects the system stability 4. Proportional-resonant current controller When the system is under grid voltage, the asymmetrical fault improved upon dynamic regulation 5. Hysteresis control method The switching frequency of pulse width modulation (PWM) can vary 6. The peak current control method Inductor currents track the basic sinusoidal waveforms 7. Stationary reference frame control Capable of removing the steady-state error between the controlled signal and reference signal 8. Synchronous rotating frame control Valid for any instantaneous difference of voltage or current and effectively describes the system performance under both steady-state and transient operations Open in new tab Table 1: Special characteristics of inverter controlling mechanisms Controlling method . Special characteristics . 1. Current loop under single-axis 𝑑q rotating coordinate system Does not behave the positive and negative sequence decomposition of currents 2. Current loop under dual 𝑑q rotating coordinate system Has a dynamic process of transient underabig fault of asymmetrical transient grid system voltage 3. Active damping compensation feedback control strategy Cannot understand the damping control for unbalanced situations and also affects the system stability 4. Proportional-resonant current controller When the system is under grid voltage, the asymmetrical fault improved upon dynamic regulation 5. Hysteresis control method The switching frequency of pulse width modulation (PWM) can vary 6. The peak current control method Inductor currents track the basic sinusoidal waveforms 7. Stationary reference frame control Capable of removing the steady-state error between the controlled signal and reference signal 8. Synchronous rotating frame control Valid for any instantaneous difference of voltage or current and effectively describes the system performance under both steady-state and transient operations Controlling method . Special characteristics . 1. Current loop under single-axis 𝑑q rotating coordinate system Does not behave the positive and negative sequence decomposition of currents 2. Current loop under dual 𝑑q rotating coordinate system Has a dynamic process of transient underabig fault of asymmetrical transient grid system voltage 3. Active damping compensation feedback control strategy Cannot understand the damping control for unbalanced situations and also affects the system stability 4. Proportional-resonant current controller When the system is under grid voltage, the asymmetrical fault improved upon dynamic regulation 5. Hysteresis control method The switching frequency of pulse width modulation (PWM) can vary 6. The peak current control method Inductor currents track the basic sinusoidal waveforms 7. Stationary reference frame control Capable of removing the steady-state error between the controlled signal and reference signal 8. Synchronous rotating frame control Valid for any instantaneous difference of voltage or current and effectively describes the system performance under both steady-state and transient operations Open in new tab From the controlling methods presented, the synchronous rotating frame controlling method with proportional-integral (PI) controller is used to develop the novel control method of pure reactive injecting [16]. The proposed approach is to model the power systems on the basis of dq0 quantities, which is not as general as abc-based models and is advantageous mainly when the network and units are symmetrically configured. The three-phase rotating reference frame control strategy is given in Fig. 1 as the general structure. Here, initially, the measured three-phase stationary current waveforms are converted into α and β components using the Clarke transformation in which the output will be a two-coordinated time-variant system [17]. After that, the Park transformation is used to convert α and β components from the stationary frame to the synchronous rotating frame in the form of d and q [17]. The name ‘synchronous’ rotating reference frame is given to this form since the dq components are arranged to rotate synchronously with the power line frequency. Then the d and q components are moderated according to the reactive power requirement. The modification technique is described in detail under the system modelling topic. After doing the required modifications in the d and q components, the rotating synchronous frame is reconverted to the stationary frame using inverse transformation. Finally, the controller will generate gate signals for the switching device using moderated waveforms. Fig. 1: Open in new tabDownload slide General structure of the three-phase rotating reference frame control strategy. Generally, the grid-tied inverters are designed to deliver regulated power from sources in renewable-energy applications, which may have oscillated input voltage levels. Hence, they must generate a fixed output voltage in a fixed frequency to the load or to the power system. Therefore, the output frequency is locked with the grid frequency at the installation and the consumer does not have the capability to change it. 2.2 System modelling A novel pure reactive power-injection mode has been invented in this research with the aim of balancing the power system by regulating the voltage level within the declared limits. The design was done mainly by considering the power output adjustment and VAR management. There are several research outcomes that have been presented about the concept, although they have several drawbacks such as consuming active power from the grid and outsized transient time as discussed in the introduction. Those drawbacks interrupt the achievement of the maximum efficiency. Therefore, the following points are considered the key requirements of the proposed design: (i) Injection of pure reactive power during the night or day according to the requirement of the grid. (ii) The inverter must not consume power for its internal usage from the utility grid. (iii) The amount of reactive power injection should depend on the requirement of the grid. 2.3 Design framework The inverter is modified with the aim of providing solutions for the drawbacks mentioned above. The power factor is decreased to zero to inject only the reactive power during the times when there is no solar energy. Especially, even during the day, if the power system needs reactive power, the novel function can be enabled to inject the required reactive power support into the grid. Fig. 2 presents the flow chart of the control system of the novel mechanism. Fig. 2: Open in new tabDownload slide The flow chart on the reactive power-injection concept. Mainly the dq-frame transformation theory is used for the novel method. According to the flow chart in Fig. 2, initially, the system detects the grid current and voltage using relevant sensors. Then the sensed three-phase voltage and current waveforms are transformed into the dq format as described previously. Afterwards, the q component of the current (Iq) and the d component of the current (Id), which represent the reactive power and the active power respectively, can be analysed with the reference value and the required changes to its parameters can be made using PI controllers. The reference value for Id is fixed as zero. It terminated the d component as the output must be purely reactive. The reference value of Iq is decided by the reference generator. As shown in the flow chart, the grid voltage is compared with the declared value. Then the reference current generator will decide the reference value for the q component, which will then decide the amount of reactive power. After the modified dq component of the waveforms is reconverted as a three-phase signal, it can be used to control the gate of the power metal-oxide semiconductor field-effect transistors (MOSFET) inside the inverter [18]. A majority of the existing research studies have introduced their models to consume a minor amount of active power from the utility grid for internal components of the inverter and their losses in typical operation. Hence this novel system in the current study is designed to consume the internal power requirements and losses from the battery storage of the inverter. The design is capable of injecting reactive power during the day as opposed to the night during conditions in which the system voltage level decreases. The key fact used in the current design is the maintenance of the output reactive power according to the requirement of the system. The closed-loop control system gives regular feedback on the output to regulate it with the current status. 2.4 Reference currents generation One way of injecting and controlling the reactive power is by changing the angle and the magnitude of the inverter voltage. Further, the controlling can be done using the q component current (Iq) in the voltage-oriented control method [19]. The feed-forward controller is used to determine the two reference current components Id and Iq using a PI controller acting on the grid voltage error. In other words, the difference between the current grid voltage and the declared reference value of the voltage is considered to generate the Iq reference. Also, as the design is set to inject the reactive power without injecting the active power, the active current component must be at the zero level in order to stop the active power from being injected. The Iq component must be non-zero in order to generate the reactive power at the same time. Therefore, a mechanism to generate an active current reference signal (Id*) is not necessary. A three-phase system voltage of 400 Vrms is selected as the declared voltage value for the system. The reactive power requirement can be detected by checking the system voltage as the voltage sags in the absence of reactive power. The comparison between the current value and the reference value of the voltage functioned by the current reference generator is graphically shown in Fig. 3. Fig. 3: Open in new tabDownload slide Generating of the Iq reference. Subsequently, the generated dq current reference components are passed to the current control loop as shown in Fig. 3. Finally, the moderated dq control signals are fed to the dq to the three-phase converter to generate the sinusoidal pulse width modulation (SPWM) gate signal. Hence the output waveforms will provide pure reactive power output that can be used to maintain the stability in the utility grid. 3 Simulation results In this section, the MATLAB®/Simulink® simulation model of the novel design is presented by considering three different scenarios of the power system. The design will be validated with the results at the end. 3.1 Simulation model The novel function has been developed using the MATLAB®/Simulink® platform. This three-phase grid-connected inverter is designed based on the techniques and functional developments that were described in Sections 1 and 2. The schematic block diagram of the simulation design is given in Fig. 4. The power source for this design is the battery storage as the design is based on the night-time during which the solar panels are not operational. Fig. 4: Open in new tabDownload slide Block diagram of the simulation design. Initially the three-phase voltages and currents are sensed from the grid as shown in the block diagram. Then they are converted into dq format and sent to the controller at which point the signals were moderated according to the reference values as described previously. Afterwards, they are reconverted to three signals and sent through the SPWM generator to generate the six distinct gate drive signals. Specific functioning abilities of the model are as follows: (i) The inverter has the ability to detect the grid voltage levels and deviations of the system. (ii) The inverter is designed to start the reactive power injection once the grid voltage level goes lower than the declared RMS voltage of 400 V. (iii) Reactive power injection will rely on the grid requirement up to the maximum capability of the inverter. 3.2 Simulation specifications The design specifications of the simulation model are shown in Table 2. Table 2: Simulation specifications Parameter . Value . DC battery voltage 48 V Maximum output current 5 A Rated three-phase AC voltage 400 V Power rating 2 kVA Reactive power capability 1.9 kVAR Supply duration 7 hours System frequency 50 Hz Switching frequency 10 kHz Power factor 0 Filter inductance 16 mH Filter capacitance 2.2 μF Parameter . Value . DC battery voltage 48 V Maximum output current 5 A Rated three-phase AC voltage 400 V Power rating 2 kVA Reactive power capability 1.9 kVAR Supply duration 7 hours System frequency 50 Hz Switching frequency 10 kHz Power factor 0 Filter inductance 16 mH Filter capacitance 2.2 μF Open in new tab Table 2: Simulation specifications Parameter . Value . DC battery voltage 48 V Maximum output current 5 A Rated three-phase AC voltage 400 V Power rating 2 kVA Reactive power capability 1.9 kVAR Supply duration 7 hours System frequency 50 Hz Switching frequency 10 kHz Power factor 0 Filter inductance 16 mH Filter capacitance 2.2 μF Parameter . Value . DC battery voltage 48 V Maximum output current 5 A Rated three-phase AC voltage 400 V Power rating 2 kVA Reactive power capability 1.9 kVAR Supply duration 7 hours System frequency 50 Hz Switching frequency 10 kHz Power factor 0 Filter inductance 16 mH Filter capacitance 2.2 μF Open in new tab 3.3 Simulation results The design simulation is carried out based on three different situations with different load conditions. The tested system three-phase voltage is 400 Vrms at 50 Hz of frequency. Fig. 5 shows the three-phase voltage and current waveforms from the inverter at the point of common coupling (PCC). Fig. 5: Open in new tabDownload slide Voltage and current waveforms from the inverter. The required amount of reactive power for a certain grid voltage deviation from its nominal value can be determined using Equation 1. According to Equation 1, the required reactive power depends on the grid impedance ratio, kq and the voltage deviation. In general, the grid impedance ratio for medium transmission lines is ~0.8, which can be considered a constant value [20]. The low kq value results in strong control of the reactive power. Therefore, kq could also be considered a constant. Hence it can be concluded that the reactive power deviation is directly proportional to the voltage level changes. Therefore, Equation 1 can be modified as the following equation (kq = 0.5): Q−Q0=− 10.50.82+1(V−V0) Q−Q0=−1.561(V−V0)(2) The results of the simulation are shown below for three different cases. Case 1: Constant reactive load (smaller than the reactive capability) The active and reactive power of the load is set to 10 kW and 1000 VAR, respectively. The inverter maintains its active power as zero to feed pure reactive power to the grid efficiently. Output waveforms of the active and reactive powers of the system are shown in Fig. 6. Fig. 6: Open in new tabDownload slide The (a) reactive and (b) active powers at the PCC—Case 1. Fig. 6a shows the behaviour of the amount of reactive power in the system. The inverter injects the entire reactive power requirement while the reactive supply of the main generator remains at the zero level as the requirement is within the reactive power-injection capability of the inverter. Active power injection of the inverter remains at zero while the entire active power requirement is fulfilled by the main generator, as shown in Fig. 6b. Case 2: Constant reactive load (greater than the reactive capability) Similarly to the previous case, the active power requirement of the load end remains at 10 kW, since the design is not focusing on the active powers. But the reactive power requirement at the load end increases up to 5000 VAR in this case. The output waveform of the system is depicted in Fig. 7. Fig. 7: Open in new tabDownload slide Output waveform of the (a) reactive and (b) active powers at the PCC—Case 2. As Fig. 7a shows, the reactive power requirement of the system has increased to 5000 VAR, which is greater than the maximum capability of the inverter. Therefore, the inverter supplies 2000 VAR in maximum capability while the main generator supplies the rest. The active power usage remains the same as in Case 1. Case 3: Variable reactive load Here, the voltage sag condition is considered as described in Fig. 8. In the beginning, the voltage level is within the declared value of 230 V. The inverter feeds a low amount of reactive power (<90 VAR). After 0.3 s, the voltage level drops by 14 V without the inverter. Fig. 8a shows both waveforms of the voltage level changing with both the inverter and without the inverter. With the injection of the required reactive power from the inverter, the voltage level does not drop more than 8 V. Fig. 8b shows the reactive power injection from the inverter according to the voltage level. Nearly 18 VAR of reactive power is injected into the system to repair the voltage sag. The active power injection or absorption does not happen during this time period. Fig. 8: Open in new tabDownload slide (a) Voltage and (b) output waveform of the active and reactive powers at the PCC—Case 3. The designed inverter has the ability to inject reactive power for nearly 6 hours in its maximum capability if the inverter is powered by a battery with a 442-Ah capacity. Retaining the active power at zero in Fig. 8b indicates that the inverter has the ability to inject pure reactive power without consuming active power from the grid. Finally, the results validated that this inverter model can be used during the night as a pure reactive power generator without consuming any active power from the grid. Two assumptions were considered for the design. The first assumption is that the voltage level of the battery bank is always at the maximum level and the second assumption is that the active power of the load remains constant during the simulation time. 4 Hardware development and results The hardware implementation with output results of the novel three-phase inverter model is discussed in this section. Fig. 9 shows the block diagram with the main components of the hardware modelled inverter. A photograph of the novel inverter circuit design is shown in Fig. 10. Fig. 9: Open in new tabDownload slide The block diagram of the hardware modelled inverter. Fig. 10: Open in new tabDownload slide General view of the experimental set-up. A scaled-down model without a transformer is developed here with the purpose of testing the working ability of the novel function on a hardware platform, with the specifications shown in Table 3. Table 3: Specifications of the scaled-down model Parameter . Scaled-down value . Input DC voltage 10 V Output AC voltage 6.9 Vrms Maximum output current 1 A Maximum output power 11.9 W Operating frequency 50 Hz Maximum operating temperature 70 ˚C Parameter . Scaled-down value . Input DC voltage 10 V Output AC voltage 6.9 Vrms Maximum output current 1 A Maximum output power 11.9 W Operating frequency 50 Hz Maximum operating temperature 70 ˚C Open in new tab Table 3: Specifications of the scaled-down model Parameter . Scaled-down value . Input DC voltage 10 V Output AC voltage 6.9 Vrms Maximum output current 1 A Maximum output power 11.9 W Operating frequency 50 Hz Maximum operating temperature 70 ˚C Parameter . Scaled-down value . Input DC voltage 10 V Output AC voltage 6.9 Vrms Maximum output current 1 A Maximum output power 11.9 W Operating frequency 50 Hz Maximum operating temperature 70 ˚C Open in new tab 4.1 Controlling the microcontroller The key aspect of this research is developing the inverter controller. Furthermore, it can be considered the most important section of a PV inverter. Generally, the controller consists of a microcontroller. Various types of microcontrollers with different specifications are available in the market that can be used in the hardware implementation. Specifications such as processing speed and input and output resolutions are the main parameters that must be considered when selecting the suitable controller for the hardware development. After doing performance analysis, the C2000 Launchpad-TMS320F28027F development kit was selected as the microcontroller [21]. It has a resolution of 12 bits as the analogue input resolution of the analogue to digital converter and 16 bits of resolution for the output pulse width modulation (PWM). It has a clock speed of 200 MHz, which is a very large value compared to the other microcontrollers. C or C++ programming languages can be used with the Code Composer Studio software for programming of the C2000 microcontroller. Alternatively, the programming could be easily done using the MATLAB®/Simulink® software, which comprises an embedded code support package within it. The program that was simulated previously in Section 3 can be directly converted into the form of machine language via Code Composer Studio and the controller can be programmed using this plug-in on MATLAB®. In order to program the controller, this converted method is used by matching the inputs and outputs according to the appropriate resolutions of the microcontroller. 4.2 Inputs sensing The controller must be fed with the three-phase current and voltage waveforms for the closed-loop operation of the inverter. The considered three-phase system has a voltage of 230 Vrms in one phase to neutral and the sensor must be compatible with this AC voltage. The ZMPT101B sensor module has been selected as the most suitable sensor for this case, since it is a very accurate and efficient one. AC voltage from 0 to 250 Vrms can be sensed with this sensor and the output analogue signal can vary between 0 and 5 V. Similarly to the voltage consideration, there are various kinds of sensors available in the market for sensing the current with different techniques such as the Hall-effect technique or using current transformers. The ACS712 sensor was selected as the most suitable current sensor for the operation of sensing the three-phase current. It is an advanced fully integrated Hall-effect-based linear current sensor. The sensor comes with different current sensing ranges: 0–5, 0–20 and 0–30 A. The sensor with a range of 0–5 A was selected to sense the three-phase currents in the current design as the testing current is <5 A. 4.3 Main circuit The main power electronic components of the circuit are listed in Table 4. Table 4: Main components of the circuit design Main component . Model . Power MOSFET IRF540, n-channel MOSFET driver-integrated circuit (IC) IR2110 Schottky diodes BYV26C—ultra-fast soft-recovery avalanche rectifier diode Main component . Model . Power MOSFET IRF540, n-channel MOSFET driver-integrated circuit (IC) IR2110 Schottky diodes BYV26C—ultra-fast soft-recovery avalanche rectifier diode Open in new tab Table 4: Main components of the circuit design Main component . Model . Power MOSFET IRF540, n-channel MOSFET driver-integrated circuit (IC) IR2110 Schottky diodes BYV26C—ultra-fast soft-recovery avalanche rectifier diode Main component . Model . Power MOSFET IRF540, n-channel MOSFET driver-integrated circuit (IC) IR2110 Schottky diodes BYV26C—ultra-fast soft-recovery avalanche rectifier diode Open in new tab The main circuit contains three separate driving arms. The three phases are assembled using three IR2110 half-bridge driver integrated circuits (ICs). The IC consists of an integrated high-side drive, which is specialized for driving the MOSFET pairs of high and low sides. Each IR2110 IC has its own signal input pins (at the high side in (HIN) and low side in (LIN)) which will connect with the output pins of the microcontroller to get the switching signal. The arrangement of the main components of the hardware circuit diagram of one inverter leg (Phase A) is shown in Fig. 11. Fig. 11: Open in new tabDownload slide Main components of the circuit diagram together with one inverter leg. The final implemented three-phase main inverter circuit board is shown in Fig. 12. Fig. 12: Open in new tabDownload slide Hardware implemented circuit. 4.4 LCL filter The L and C filter parameters can be calculated using the transfer function of the LCL filter. The following equation shows the transfer function of the LCL filter in which Ig is the grid current, Vi is filter of the input voltage, L1 + L2 = L and Lp=L1L2L1+L2[22]: IgVi=1SL (1+ S2CLp )(3) As a rule of thumb, the reactive power (Q) absorbed by the filter capacitor is limited to 5% of the rated power [23]. Hence, the following equation can be derived considering the reactance to find the capacitor value of C in which S represents the apparent power, V represents the phase voltage and f represents the system frequency: C=0.05 SV22 π f(4) Then the inductance value at the switching frequency can be calculated using the transfer function given by Equation 3. Finally, the value of the inductance can be calculated using the following equation in which ω = 2πf: L=|1ωswIg (sw)Vi (sw)(1−ωsw2ωres2)|(5) The LCL filter is implemented in order to smooth the output waveform by considering the values calculated above as well as the available component values in the market. Three Mylar capacitors are used here with a capacitance of 2.2 μF and a rated voltage of 400 V. Two sets of three copper coil ferrite core inductors to the value of 16 mH are used with the maximum current rating of 1 A. 4.5 Full inverter The connection diagram of the full inverter circuit is shown in Fig. 13. Initially, it senses the six waveforms of currents and voltages from the grid and passes them to the analogue input pins of the controller board by reducing them to signals with low amplitudes (0–5 V). Once the controller processes the signals, the six output PWM pins release the SPWM switching signals to the MOSFET driver IC as high and low side signal pairs. Then the driver ICs catch the signals and operate the two assigned power MOSFETs by switching their gate pins to receive the final AC output waveform from the circuit. Finally, the smoothed three-phase output signal can be obtained by sending it through the LCL filter. Fig. 13: Open in new tabDownload slide Full inverter diagram. 4.6 Output results The three-phase output waveforms with other parameters from the designed inverter are observed using the oscilloscope, power meter and multimeter. Active and reactive power outputs were observed under several different resistive and inductive load conditions. The obtained two waveforms of the three-phase voltage output using a two-channel oscilloscope are shown in Fig. 14. The frequency of the output waveforms remains the same as the grid frequency, which is 50 Hz. The output peak value of the voltage is 7.8 V and the phase shift is 120˚ between the two waveforms. The reactive power-injection mode of the design must have the ability to inject the reactive power depending on the load requirement. Therefore, the novel model is tested with different inductive loads and the values of the power outputs were observed using power meters. Fig. 14: Open in new tabDownload slide The three-phase outputs on the two-channel oscilloscope. The observed results were analysed and presented graphically in Fig. 15. Initially, the model was tested with pure resistive load. Therefore, the initial point of the inductive load was shown as zero and the active power was ~0.18 W with a unity power factor. Thereafter, the load fed by the inverter was gradually increased and the active and reactive powers with the power factor from the inverter output were observed. The reactive power increased with the load and the power factor decreased proportionally. The mechanism for the moderation of the sensed voltage and current waveforms using the novel controller can be validated using the results obtained here as the inverter has the ability to change the power factor over a wide range. If the design was tested further with large inductive loads, it would be able to run in zero power factor as shown in the simulation. The design was tested only with minor inductive loads with the available components, which does not affect adversely the final outcome. However, the results show that this system can achieve a broad range of power factors. In other words, the model can inject reactive power according to the system requirement. The hardware experimental results show good system performance and acceptable agreement with the simulation results well. However, there are minor variances between the simulation and experimental results due to the real-world environment in the experimental prototype test, such as electromagnetic interference, and particularly in the presence of the line resistances. Furthermore, the obtained results prove the feasibility of the proposed novel control technique. Fig. 15: Open in new tabDownload slide Graphical representation of the output results of the hardware model. 4.7 Design specifications The design specifications for the scaled-down novel experimented model were presented at the beginning of Section 4. The specifications of the full-scale actual model that were acquired using the tested model are presented in Table 5. Table 5: Specifications of the full-scale model Parameter . Value . Active power capability 2 kVA Reactive power capability 1.9 kVAR Maximum output current 5 A Operating voltage 400 V DC inverter input voltage 96 V DC battery voltage 48 V Output waveform frequency 50 Hz Three-phase transformer 96/400 V PWM switching frequency 10 kHz Parameter . Value . Active power capability 2 kVA Reactive power capability 1.9 kVAR Maximum output current 5 A Operating voltage 400 V DC inverter input voltage 96 V DC battery voltage 48 V Output waveform frequency 50 Hz Three-phase transformer 96/400 V PWM switching frequency 10 kHz Open in new tab Table 5: Specifications of the full-scale model Parameter . Value . Active power capability 2 kVA Reactive power capability 1.9 kVAR Maximum output current 5 A Operating voltage 400 V DC inverter input voltage 96 V DC battery voltage 48 V Output waveform frequency 50 Hz Three-phase transformer 96/400 V PWM switching frequency 10 kHz Parameter . Value . Active power capability 2 kVA Reactive power capability 1.9 kVAR Maximum output current 5 A Operating voltage 400 V DC inverter input voltage 96 V DC battery voltage 48 V Output waveform frequency 50 Hz Three-phase transformer 96/400 V PWM switching frequency 10 kHz Open in new tab 5 Conclusion An implemented simulation model as well as an experimental hardware set-up using power electronic components based on the novel concept of designing and controlling renewable-energy generation units were presented in this paper. The obtained results of the simulation and hardware model have been presented with a comparison in order to validate the effectiveness of the proposed novel control method. The smartness of the proposed controlling method is accomplished by regulating the required reactive power injection during the night to the utility grid without using the communication capability with other inverters in the system or without using the centralized administration service. This function can help not only at night, but also once the utility grid needs reactive power for the grid stability. The controlling mechanism performs based on a dq rotating frame controller with moderated techniques. Furthermore, the power electronic components of the inverter have the ability to work without absorbing the active power from the grid. The proposed mechanism helps to reduce the voltage sags by injecting the required reactive power in order to increase the voltage level and maintain it within the declared limits. Overall, the experimented results validated that the novel design can enhance the efficiency of the smart inverter by using it during the night and improve the stability of the power system. Additionally, a new considerable payment method can be implemented with new regulations associated with this concept for distributed inverter owners who inject the required reactive power into the system with the purpose of improving the grid stability. With respect to future developments, an actual scaled model that can be connected directly to the utility grid can be constructed using the obtained results and the proposed scaled-down hardware test model. Conflict of interest statement None declared. References [1] Shen Y , Cui M, Wang Q, et al. Comprehensive reactive power support of DFIG adapted to different depth of voltage sags . Energies , 2017 , 10 : 808 . Google Scholar Crossref Search ADS WorldCat [2] Wang L , Lao K, Lam C, et al. Delta-connected static var compensator (SVC) based hybrid active power filter (SVC-HAPF) and its control method . In: IECON 2017—43rd Annual Conference of the IEEE Industrial Electronics Society, Beijing, China, 29 October 2017–1 November 2017 , 6384 – 6388 . [3] Gandhi O , Rodriguez-Gallegos C, Reindl T, et al. Competitiveness of PV inverter as a reactive power compensator considering inverter lifetime reduction. 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Reliability analysis of single-phase PV inverters with reactive power injection. In: IEEE Energy Conversion Congress and Exposition, Montreal , Canada , 20–24 September 2015 , 2132 – 2139 . [8] Costa IDL , Brandao DI, Junior LM, et al. Analysis of stationary- and synchronous-reference frames for three-phase three-wire grid-connected converter AC current regulators . Energies , 2021 , 14 : 8348 . Google Scholar Crossref Search ADS WorldCat [9] Enayati B. The Revised IEEE1547 Standard. West Roxbury, MA, USA : National Grid , 2019 . Google Scholar Google Preview OpenURL Placeholder Text WorldCat COPAC [10] Varma RK , Siavashi EM. PV-STATCOM: a new smart inverter for voltage control in distribution systems . IEEE Trans Sustainable Energy , 2018 , 9 : 1681 – 1691 . Google Scholar Crossref Search ADS WorldCat [11] Xu Y , Milanović JV. Day-ahead prediction and shaping of dynamic response of demand at bulk supply points . IEEE Trans Power Syst , 2016 , 31 : 3100 – 3108 . 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Modified PI controller with improved steady-state performance and comparison with PR controller on direct matrix converters . Chinese Journal of Electrical Engineering , 2019 , 5 : 53 – 66 . Google Scholar Crossref Search ADS WorldCat [17] O’Rourke CJ , Qasim MM, Overlin MR, et al. A geometric interpretation of reference frames and transformations: Dq0, Clarke, and Park . IEEE Trans Energy Convers , 2019 , 34 : 2070 – 2083 . Google Scholar Crossref Search ADS WorldCat [18] Kaur R , Gupta P, Singh J. A review on power MOSFET device structures . International Journal of Applied Science and Engineering , 2017 , 5 : 208 – 218 . Google Scholar OpenURL Placeholder Text WorldCat [19] Adhikari J , Prasanna IV and Panda SK. Voltage oriented control of the three-level Vienna rectifier using vector control method . In: 2016 IEEE Applied Power Electronics Conference and Exposition (APEC), Long Beach, CA, USA , 20 March 2016 , 9 – 16 . [20] Thompson MJ , Somani A. A tutorial on calculating source impedance ratios for determining line length . In: 2015 68th Annual Conference for Protective Relay Engineers, College Station, TX, USA , 30 March 2015–2 April 2015 , 833 – 884 . [21] LAUNCHXL-F28027 . Texas Instruments Incorporated , 2019 . https://www.ti.com/tool/LAUNCHXL-F28027 (12 July 2022, date last accessed). Google Scholar Google Preview OpenURL Placeholder Text WorldCat COPAC [22] Dursun M , Dosoglu MK. LCL filter design for grid connected three-phase inverter . In: 2nd International Symposium on Multidisciplinary Studies and Innovative Technologies (ISMSIT), Ankara, Turkey , 19–21 October 2018 , 1 – 4 . [23] Zhou D , Wang H, Blaabjerg F. Reactive power impacts on LCL filter capacitor lifetime in grid-connected inverter . IEEE Open Journal of Power Electronics , 2020 , 1 : 139 – 148 . Google Scholar Crossref Search ADS WorldCat © The Author(s) 2022. Published by Oxford University Press on behalf of National Institute of Clean-and-Low-Carbon Energy This is an Open Access article distributed under the terms of the Creative Commons Attribution-NonCommercial License (https://creativecommons.org/licenses/by-nc/4.0/), which permits non-commercial re-use, distribution, and reproduction in any medium, provided the original work is properly cited. For commercial re-use, please contact journals.permissions@oup.com © The Author(s) 2022. Published by Oxford University Press on behalf of National Institute of Clean-and-Low-Carbon Energy http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Clean Energy Oxford University Press

Use of solar PV inverters during night-time for voltage regulation and stability of the utility grid

Clean Energy , Volume 6 (4): 13 – Aug 1, 2022

Use of solar PV inverters during night-time for voltage regulation and stability of the utility grid

Clean Energy , Volume 6 (4): 13 – Aug 1, 2022

Abstract

Abstract Photovoltaic (PV) inverters are vital components for future smart grids. Although the popularity of PV-generator installations is high, their effective performance remains low. Certain inverters are designed to operate in volt-ampere reactive (VAR) mode during the night. Yet, this approach is ineffective due to the consumption of active power from the grid (as internal losses) and the regulation necessity of the direct-current (DC) bus. This paper will demonstrate the operation of a PV inverter in reactive power-injection mode when solar energy is unavailable. The primary focus is on the design of the inverter controller with respect to the synchronous rotating frame control method. The proposed novel method enables an inverter to inject the required level of reactive power to regulate the voltage levels of the utility grid within specified limits. In the process, the inverter does not absorb active power from the grid for its internal operation. The presented model has the ability to inject ≤2 kVAR of reactive power at zero power factor without absorbing active power from the grid. Simulation and hardware models of the inverter were developed and tested for different reactive loads in which the hardware model represented the real-world application. The reactive power injection of the two models ran at zero power factor and produced the expected outcomes for their corresponding independent reactive loads. Henceforth, it was evident that the proposed method can enhance the efficiency of an inverter and ensure the stability of the utility grid to which it is connected. Open in new tabDownload slide solar, energy system and policy, energy storage, distributed energy and smart grid, photovoltaic, reactive power, night-time usage Introduction Grid-tie inverters can be regarded as the main component in both renewable-energy conversion systems and smart grid systems. They can convert renewable energy into power that then can be fed to the utility grid as long as the renewable source exists. For photovoltaic (PV) inverters, solar energy must be there to generate active power. Otherwise, the inverter will remain idle during the night. The idle behaviour reduces the efficiency of the PV inverter. However, if there is a mechanism to use such inverters in a different way at night, its efficiency can be increased. The main motivation of the current research is to address the above issue and increase the efficiency of the inverter by using it during the night as well. Generally, when the reactive power is insufficient in the utility grid, it is not possible to push the power demand by loads through the lines as the voltage sags [1]. Using the inverter as a reactive power generator by operating it as a volt-ampere reactive (VAR) compensator is a potential way of solving the above issue of voltage sag [2]. The rapid increase in using PV inverters can be used to regulate the grid voltage and it will reduce the extra cost of installing capacitor banks. Currently, there are multiple ongoing research applications and experiments focusing on this general concept of using a PV inverter as a VAR compensator [3–5]. The PV generators consume a certain amount of active power for their internal operation and losses. Once the PV generators are used as reactive power generators, they will absorb active power from the grid that subsequently reduces the effectiveness of the concept. Although there are multiple research studies that address the use of PV inverters as reactive compensators during the night, none of them has addressed the specific issue of consuming active power from the grid [6, 7]. Most of the previous designs are planned to absorb the active power requirement from the grid to their internal operation. The novel method in the current research focuses on addressing these issues and developing an efficient method that can enhance the grid stability and usage of PV inverters. The novel control method introduced in this paper allows PV inverters to operate in pure reactive power-injection mode. The inverter is enhanced with the ability to work in this mode without absorbing any active power from the grid to compensate for its internal operation with losses and to regulate the direct-current (DC) bus voltage, which is the main novelty of this design. The novel concept is associated with a battery bank to compensate for the internal operation of the inverter. Specifically, this system is designed to inject only the required amount of reactive power by operating it in zero power factor. Further, the efficient use of the inverter can be increased by using it during the night. Overall, the concept introduced here expands the use of PV inverters and helps to maintain and regulate the voltage within the declared limits. Furthermore it can maintain the stability of the utility grid to which it is connected. Mainly, the synchronous rotating frame control method is utilized to develop the novel controlling method as it successfully defines the system performance under both steady-state and transient operations [8]. Also, the synchronous rotating frame control method is valid for any instantaneous difference between the voltage and the current. The details of the novel method are demonstrated in a simulation model and in a hardware development model with experimental results to validate the workability. The paper is organized as follows. Section 1 describes the active and reactive power injection using PV inverters. This is followed by Section 2, a discussion on the reactive power requirement of the grid and night-time reactive power injection. After that, Section 3 presents the novel control mechanism in detail. Further on, Section 4 discusses the simulation model and the hardware implementation under several grid conditions. The final section of this paper presents conclusions. 1 Active and reactive power control In the modern day, the PV inverters are being developed under the interconnection standards such as IEEE 1547, which do not allow for voltage regulations [9]. However, a majority of manufacturers of PV inverters tend to enhance their products with reactive power absorbing or injecting capabilities without exceeding their voltage ratings. The PV inverters theoretically can be developed as reactive power supporters, the same as the static compensators (STATCOMs) that the industrial standards do not address [10]. Typical PV inverters are designed to be disconnected at night. Alternatively, it is possible to use its reactive power capability when there is no active power generation. Typically, renewable generators like wind and solar individually follow a reactive power or a power factor set point, which can be tuned at the plant level for Volt/VAR regulating. Doing so will allow them to be involved in voltage controlling of the system. Reactive power control and the power factor control that are connected to the grid are not usually used in large power plants as they can result in improper response to system voltage variations, which could reduce the voltage stability of the system. However, reactive power and power factor control can be considered suitable options for distributed generators. 1.1 Reactive power requirement on the system The load demand can be increased due to their dynamic behaviour once they are connected to the transmission system [11]. Then the voltage level will drop at the load end together with the feeders. Hence the system fails to push the power through the transmission lines according to the demand of local loads. The voltage sag can be considered one of the most irritating events that can occur in the power system. However, it can be mitigated by injecting the desired amount of reactive power into the system. In other words, the voltage level of the power systems and transmission lines can be controlled by injecting or absorbing the reactive power from the distributed sources. To describe the combination between the voltage and the reactive power—a relationship that can be developed using the droop control equations for grid voltage and combining them to embed a general impedance. The connection between the voltage and the reactive power can be demonstrated as shown in the following equation [12]: Q−Q0=− 1kqα2+1(V−V0)(1) where α denotes the ratio R/X, which gives the grid impedance ratio (α = R/X). Generally, the grid impedance R + jX must be known to control the reactive power as well as the active power. As the variables R and X appear separately when deriving Equation 1, the ratio R/X can be used. Hence this value can be used to evaluate the required amount of reactive power. The frequency component of the above relationship is omitted since the distributed generator (DG) unit is grid-following and it cannot be controlled by local DGs. The parameter V gives the instantaneous grid voltage, while Q denotes the reactive power of the DG unit. V0 equals the nominal grid voltage. Q0 can be explained as a non-zero set point imposed by the transmission system operator. The parameter kq determines the amount of reactive power control for a certain grid voltage deviation from its nominal value (this parameter has the unit of Volt/VAR). The setting of this parameter has a vital impact on the grid voltage control and allows great flexibility [13]. Furthermore, the droop parameter is set to the preference or could also be altered by the network operator. In brief, this parameter is set to the level at which the DG units participate in the primary control of the grid voltage. 1.2 Reactive power control Accordingly, a variance of grid voltage from the declared value will create a reactive power demand. Generally, a grid-connected PV inverter can be programmed to inject and absorb the reactive power. Hence, both the overvoltage and undervoltage conditions can be regulated using the reactive power control ability. The dq components theory, which will be described in Section 2, can be used to perform the controlling mechanism efficiently [14]. The q component of the current can be regulated to control the reactive power injection instantaneously as it is used to represent the reactive power component. 1.3 Night-time reactive power-injection mode In general, the grid-tie inverters are equipped with DC-to-DC converters that regulate the DC bus voltage [15]. Once the active power is unavailable during the night, the DC-to-DC conversion becomes inactive. However, a developed control scheme with an energy-storage system can allow the inverter to operate in the reactive power mode even without the PV panels harvesting solar energy. Subsequently, the inverter can be programmed to operate as a VAR compensator to inject only the required reactive power, which will regulate the voltage at the load end. 2 Controlling mechanism The controlling mechanism of the novel concept with a background study is described under this topic. Further, the methods used for the design are described in detail. 2.1 Overview of different modelling techniques PV inverters in current power systems are utilizing several controlling techniques with the purpose of controlling the power. Table 1 shows a few controlling methods with their characteristics. Table 1: Special characteristics of inverter controlling mechanisms Controlling method . Special characteristics . 1. Current loop under single-axis 𝑑q rotating coordinate system Does not behave the positive and negative sequence decomposition of currents 2. Current loop under dual 𝑑q rotating coordinate system Has a dynamic process of transient underabig fault of asymmetrical transient grid system voltage 3. Active damping compensation feedback control strategy Cannot understand the damping control for unbalanced situations and also affects the system stability 4. Proportional-resonant current controller When the system is under grid voltage, the asymmetrical fault improved upon dynamic regulation 5. Hysteresis control method The switching frequency of pulse width modulation (PWM) can vary 6. The peak current control method Inductor currents track the basic sinusoidal waveforms 7. Stationary reference frame control Capable of removing the steady-state error between the controlled signal and reference signal 8. Synchronous rotating frame control Valid for any instantaneous difference of voltage or current and effectively describes the system performance under both steady-state and transient operations Controlling method . Special characteristics . 1. Current loop under single-axis 𝑑q rotating coordinate system Does not behave the positive and negative sequence decomposition of currents 2. Current loop under dual 𝑑q rotating coordinate system Has a dynamic process of transient underabig fault of asymmetrical transient grid system voltage 3. Active damping compensation feedback control strategy Cannot understand the damping control for unbalanced situations and also affects the system stability 4. Proportional-resonant current controller When the system is under grid voltage, the asymmetrical fault improved upon dynamic regulation 5. Hysteresis control method The switching frequency of pulse width modulation (PWM) can vary 6. The peak current control method Inductor currents track the basic sinusoidal waveforms 7. Stationary reference frame control Capable of removing the steady-state error between the controlled signal and reference signal 8. Synchronous rotating frame control Valid for any instantaneous difference of voltage or current and effectively describes the system performance under both steady-state and transient operations Open in new tab Table 1: Special characteristics of inverter controlling mechanisms Controlling method . Special characteristics . 1. Current loop under single-axis 𝑑q rotating coordinate system Does not behave the positive and negative sequence decomposition of currents 2. Current loop under dual 𝑑q rotating coordinate system Has a dynamic process of transient underabig fault of asymmetrical transient grid system voltage 3. Active damping compensation feedback control strategy Cannot understand the damping control for unbalanced situations and also affects the system stability 4. Proportional-resonant current controller When the system is under grid voltage, the asymmetrical fault improved upon dynamic regulation 5. Hysteresis control method The switching frequency of pulse width modulation (PWM) can vary 6. The peak current control method Inductor currents track the basic sinusoidal waveforms 7. Stationary reference frame control Capable of removing the steady-state error between the controlled signal and reference signal 8. Synchronous rotating frame control Valid for any instantaneous difference of voltage or current and effectively describes the system performance under both steady-state and transient operations Controlling method . Special characteristics . 1. Current loop under single-axis 𝑑q rotating coordinate system Does not behave the positive and negative sequence decomposition of currents 2. Current loop under dual 𝑑q rotating coordinate system Has a dynamic process of transient underabig fault of asymmetrical transient grid system voltage 3. Active damping compensation feedback control strategy Cannot understand the damping control for unbalanced situations and also affects the system stability 4. Proportional-resonant current controller When the system is under grid voltage, the asymmetrical fault improved upon dynamic regulation 5. Hysteresis control method The switching frequency of pulse width modulation (PWM) can vary 6. The peak current control method Inductor currents track the basic sinusoidal waveforms 7. Stationary reference frame control Capable of removing the steady-state error between the controlled signal and reference signal 8. Synchronous rotating frame control Valid for any instantaneous difference of voltage or current and effectively describes the system performance under both steady-state and transient operations Open in new tab From the controlling methods presented, the synchronous rotating frame controlling method with proportional-integral (PI) controller is used to develop the novel control method of pure reactive injecting [16]. The proposed approach is to model the power systems on the basis of dq0 quantities, which is not as general as abc-based models and is advantageous mainly when the network and units are symmetrically configured. The three-phase rotating reference frame control strategy is given in Fig. 1 as the general structure. Here, initially, the measured three-phase stationary current waveforms are converted into α and β components using the Clarke transformation in which the output will be a two-coordinated time-variant system [17]. After that, the Park transformation is used to convert α and β components from the stationary frame to the synchronous rotating frame in the form of d and q [17]. The name ‘synchronous’ rotating reference frame is given to this form since the dq components are arranged to rotate synchronously with the power line frequency. Then the d and q components are moderated according to the reactive power requirement. The modification technique is described in detail under the system modelling topic. After doing the required modifications in the d and q components, the rotating synchronous frame is reconverted to the stationary frame using inverse transformation. Finally, the controller will generate gate signals for the switching device using moderated waveforms. Fig. 1: Open in new tabDownload slide General structure of the three-phase rotating reference frame control strategy. Generally, the grid-tied inverters are designed to deliver regulated power from sources in renewable-energy applications, which may have oscillated input voltage levels. Hence, they must generate a fixed output voltage in a fixed frequency to the load or to the power system. Therefore, the output frequency is locked with the grid frequency at the installation and the consumer does not have the capability to change it. 2.2 System modelling A novel pure reactive power-injection mode has been invented in this research with the aim of balancing the power system by regulating the voltage level within the declared limits. The design was done mainly by considering the power output adjustment and VAR management. There are several research outcomes that have been presented about the concept, although they have several drawbacks such as consuming active power from the grid and outsized transient time as discussed in the introduction. Those drawbacks interrupt the achievement of the maximum efficiency. Therefore, the following points are considered the key requirements of the proposed design: (i) Injection of pure reactive power during the night or day according to the requirement of the grid. (ii) The inverter must not consume power for its internal usage from the utility grid. (iii) The amount of reactive power injection should depend on the requirement of the grid. 2.3 Design framework The inverter is modified with the aim of providing solutions for the drawbacks mentioned above. The power factor is decreased to zero to inject only the reactive power during the times when there is no solar energy. Especially, even during the day, if the power system needs reactive power, the novel function can be enabled to inject the required reactive power support into the grid. Fig. 2 presents the flow chart of the control system of the novel mechanism. Fig. 2: Open in new tabDownload slide The flow chart on the reactive power-injection concept. Mainly the dq-frame transformation theory is used for the novel method. According to the flow chart in Fig. 2, initially, the system detects the grid current and voltage using relevant sensors. Then the sensed three-phase voltage and current waveforms are transformed into the dq format as described previously. Afterwards, the q component of the current (Iq) and the d component of the current (Id), which represent the reactive power and the active power respectively, can be analysed with the reference value and the required changes to its parameters can be made using PI controllers. The reference value for Id is fixed as zero. It terminated the d component as the output must be purely reactive. The reference value of Iq is decided by the reference generator. As shown in the flow chart, the grid voltage is compared with the declared value. Then the reference current generator will decide the reference value for the q component, which will then decide the amount of reactive power. After the modified dq component of the waveforms is reconverted as a three-phase signal, it can be used to control the gate of the power metal-oxide semiconductor field-effect transistors (MOSFET) inside the inverter [18]. A majority of the existing research studies have introduced their models to consume a minor amount of active power from the utility grid for internal components of the inverter and their losses in typical operation. Hence this novel system in the current study is designed to consume the internal power requirements and losses from the battery storage of the inverter. The design is capable of injecting reactive power during the day as opposed to the night during conditions in which the system voltage level decreases. The key fact used in the current design is the maintenance of the output reactive power according to the requirement of the system. The closed-loop control system gives regular feedback on the output to regulate it with the current status. 2.4 Reference currents generation One way of injecting and controlling the reactive power is by changing the angle and the magnitude of the inverter voltage. Further, the controlling can be done using the q component current (Iq) in the voltage-oriented control method [19]. The feed-forward controller is used to determine the two reference current components Id and Iq using a PI controller acting on the grid voltage error. In other words, the difference between the current grid voltage and the declared reference value of the voltage is considered to generate the Iq reference. Also, as the design is set to inject the reactive power without injecting the active power, the active current component must be at the zero level in order to stop the active power from being injected. The Iq component must be non-zero in order to generate the reactive power at the same time. Therefore, a mechanism to generate an active current reference signal (Id*) is not necessary. A three-phase system voltage of 400 Vrms is selected as the declared voltage value for the system. The reactive power requirement can be detected by checking the system voltage as the voltage sags in the absence of reactive power. The comparison between the current value and the reference value of the voltage functioned by the current reference generator is graphically shown in Fig. 3. Fig. 3: Open in new tabDownload slide Generating of the Iq reference. Subsequently, the generated dq current reference components are passed to the current control loop as shown in Fig. 3. Finally, the moderated dq control signals are fed to the dq to the three-phase converter to generate the sinusoidal pulse width modulation (SPWM) gate signal. Hence the output waveforms will provide pure reactive power output that can be used to maintain the stability in the utility grid. 3 Simulation results In this section, the MATLAB®/Simulink® simulation model of the novel design is presented by considering three different scenarios of the power system. The design will be validated with the results at the end. 3.1 Simulation model The novel function has been developed using the MATLAB®/Simulink® platform. This three-phase grid-connected inverter is designed based on the techniques and functional developments that were described in Sections 1 and 2. The schematic block diagram of the simulation design is given in Fig. 4. The power source for this design is the battery storage as the design is based on the night-time during which the solar panels are not operational. Fig. 4: Open in new tabDownload slide Block diagram of the simulation design. Initially the three-phase voltages and currents are sensed from the grid as shown in the block diagram. Then they are converted into dq format and sent to the controller at which point the signals were moderated according to the reference values as described previously. Afterwards, they are reconverted to three signals and sent through the SPWM generator to generate the six distinct gate drive signals. Specific functioning abilities of the model are as follows: (i) The inverter has the ability to detect the grid voltage levels and deviations of the system. (ii) The inverter is designed to start the reactive power injection once the grid voltage level goes lower than the declared RMS voltage of 400 V. (iii) Reactive power injection will rely on the grid requirement up to the maximum capability of the inverter. 3.2 Simulation specifications The design specifications of the simulation model are shown in Table 2. Table 2: Simulation specifications Parameter . Value . DC battery voltage 48 V Maximum output current 5 A Rated three-phase AC voltage 400 V Power rating 2 kVA Reactive power capability 1.9 kVAR Supply duration 7 hours System frequency 50 Hz Switching frequency 10 kHz Power factor 0 Filter inductance 16 mH Filter capacitance 2.2 μF Parameter . Value . DC battery voltage 48 V Maximum output current 5 A Rated three-phase AC voltage 400 V Power rating 2 kVA Reactive power capability 1.9 kVAR Supply duration 7 hours System frequency 50 Hz Switching frequency 10 kHz Power factor 0 Filter inductance 16 mH Filter capacitance 2.2 μF Open in new tab Table 2: Simulation specifications Parameter . Value . DC battery voltage 48 V Maximum output current 5 A Rated three-phase AC voltage 400 V Power rating 2 kVA Reactive power capability 1.9 kVAR Supply duration 7 hours System frequency 50 Hz Switching frequency 10 kHz Power factor 0 Filter inductance 16 mH Filter capacitance 2.2 μF Parameter . Value . DC battery voltage 48 V Maximum output current 5 A Rated three-phase AC voltage 400 V Power rating 2 kVA Reactive power capability 1.9 kVAR Supply duration 7 hours System frequency 50 Hz Switching frequency 10 kHz Power factor 0 Filter inductance 16 mH Filter capacitance 2.2 μF Open in new tab 3.3 Simulation results The design simulation is carried out based on three different situations with different load conditions. The tested system three-phase voltage is 400 Vrms at 50 Hz of frequency. Fig. 5 shows the three-phase voltage and current waveforms from the inverter at the point of common coupling (PCC). Fig. 5: Open in new tabDownload slide Voltage and current waveforms from the inverter. The required amount of reactive power for a certain grid voltage deviation from its nominal value can be determined using Equation 1. According to Equation 1, the required reactive power depends on the grid impedance ratio, kq and the voltage deviation. In general, the grid impedance ratio for medium transmission lines is ~0.8, which can be considered a constant value [20]. The low kq value results in strong control of the reactive power. Therefore, kq could also be considered a constant. Hence it can be concluded that the reactive power deviation is directly proportional to the voltage level changes. Therefore, Equation 1 can be modified as the following equation (kq = 0.5): Q−Q0=− 10.50.82+1(V−V0) Q−Q0=−1.561(V−V0)(2) The results of the simulation are shown below for three different cases. Case 1: Constant reactive load (smaller than the reactive capability) The active and reactive power of the load is set to 10 kW and 1000 VAR, respectively. The inverter maintains its active power as zero to feed pure reactive power to the grid efficiently. Output waveforms of the active and reactive powers of the system are shown in Fig. 6. Fig. 6: Open in new tabDownload slide The (a) reactive and (b) active powers at the PCC—Case 1. Fig. 6a shows the behaviour of the amount of reactive power in the system. The inverter injects the entire reactive power requirement while the reactive supply of the main generator remains at the zero level as the requirement is within the reactive power-injection capability of the inverter. Active power injection of the inverter remains at zero while the entire active power requirement is fulfilled by the main generator, as shown in Fig. 6b. Case 2: Constant reactive load (greater than the reactive capability) Similarly to the previous case, the active power requirement of the load end remains at 10 kW, since the design is not focusing on the active powers. But the reactive power requirement at the load end increases up to 5000 VAR in this case. The output waveform of the system is depicted in Fig. 7. Fig. 7: Open in new tabDownload slide Output waveform of the (a) reactive and (b) active powers at the PCC—Case 2. As Fig. 7a shows, the reactive power requirement of the system has increased to 5000 VAR, which is greater than the maximum capability of the inverter. Therefore, the inverter supplies 2000 VAR in maximum capability while the main generator supplies the rest. The active power usage remains the same as in Case 1. Case 3: Variable reactive load Here, the voltage sag condition is considered as described in Fig. 8. In the beginning, the voltage level is within the declared value of 230 V. The inverter feeds a low amount of reactive power (<90 VAR). After 0.3 s, the voltage level drops by 14 V without the inverter. Fig. 8a shows both waveforms of the voltage level changing with both the inverter and without the inverter. With the injection of the required reactive power from the inverter, the voltage level does not drop more than 8 V. Fig. 8b shows the reactive power injection from the inverter according to the voltage level. Nearly 18 VAR of reactive power is injected into the system to repair the voltage sag. The active power injection or absorption does not happen during this time period. Fig. 8: Open in new tabDownload slide (a) Voltage and (b) output waveform of the active and reactive powers at the PCC—Case 3. The designed inverter has the ability to inject reactive power for nearly 6 hours in its maximum capability if the inverter is powered by a battery with a 442-Ah capacity. Retaining the active power at zero in Fig. 8b indicates that the inverter has the ability to inject pure reactive power without consuming active power from the grid. Finally, the results validated that this inverter model can be used during the night as a pure reactive power generator without consuming any active power from the grid. Two assumptions were considered for the design. The first assumption is that the voltage level of the battery bank is always at the maximum level and the second assumption is that the active power of the load remains constant during the simulation time. 4 Hardware development and results The hardware implementation with output results of the novel three-phase inverter model is discussed in this section. Fig. 9 shows the block diagram with the main components of the hardware modelled inverter. A photograph of the novel inverter circuit design is shown in Fig. 10. Fig. 9: Open in new tabDownload slide The block diagram of the hardware modelled inverter. Fig. 10: Open in new tabDownload slide General view of the experimental set-up. A scaled-down model without a transformer is developed here with the purpose of testing the working ability of the novel function on a hardware platform, with the specifications shown in Table 3. Table 3: Specifications of the scaled-down model Parameter . Scaled-down value . Input DC voltage 10 V Output AC voltage 6.9 Vrms Maximum output current 1 A Maximum output power 11.9 W Operating frequency 50 Hz Maximum operating temperature 70 ˚C Parameter . Scaled-down value . Input DC voltage 10 V Output AC voltage 6.9 Vrms Maximum output current 1 A Maximum output power 11.9 W Operating frequency 50 Hz Maximum operating temperature 70 ˚C Open in new tab Table 3: Specifications of the scaled-down model Parameter . Scaled-down value . Input DC voltage 10 V Output AC voltage 6.9 Vrms Maximum output current 1 A Maximum output power 11.9 W Operating frequency 50 Hz Maximum operating temperature 70 ˚C Parameter . Scaled-down value . Input DC voltage 10 V Output AC voltage 6.9 Vrms Maximum output current 1 A Maximum output power 11.9 W Operating frequency 50 Hz Maximum operating temperature 70 ˚C Open in new tab 4.1 Controlling the microcontroller The key aspect of this research is developing the inverter controller. Furthermore, it can be considered the most important section of a PV inverter. Generally, the controller consists of a microcontroller. Various types of microcontrollers with different specifications are available in the market that can be used in the hardware implementation. Specifications such as processing speed and input and output resolutions are the main parameters that must be considered when selecting the suitable controller for the hardware development. After doing performance analysis, the C2000 Launchpad-TMS320F28027F development kit was selected as the microcontroller [21]. It has a resolution of 12 bits as the analogue input resolution of the analogue to digital converter and 16 bits of resolution for the output pulse width modulation (PWM). It has a clock speed of 200 MHz, which is a very large value compared to the other microcontrollers. C or C++ programming languages can be used with the Code Composer Studio software for programming of the C2000 microcontroller. Alternatively, the programming could be easily done using the MATLAB®/Simulink® software, which comprises an embedded code support package within it. The program that was simulated previously in Section 3 can be directly converted into the form of machine language via Code Composer Studio and the controller can be programmed using this plug-in on MATLAB®. In order to program the controller, this converted method is used by matching the inputs and outputs according to the appropriate resolutions of the microcontroller. 4.2 Inputs sensing The controller must be fed with the three-phase current and voltage waveforms for the closed-loop operation of the inverter. The considered three-phase system has a voltage of 230 Vrms in one phase to neutral and the sensor must be compatible with this AC voltage. The ZMPT101B sensor module has been selected as the most suitable sensor for this case, since it is a very accurate and efficient one. AC voltage from 0 to 250 Vrms can be sensed with this sensor and the output analogue signal can vary between 0 and 5 V. Similarly to the voltage consideration, there are various kinds of sensors available in the market for sensing the current with different techniques such as the Hall-effect technique or using current transformers. The ACS712 sensor was selected as the most suitable current sensor for the operation of sensing the three-phase current. It is an advanced fully integrated Hall-effect-based linear current sensor. The sensor comes with different current sensing ranges: 0–5, 0–20 and 0–30 A. The sensor with a range of 0–5 A was selected to sense the three-phase currents in the current design as the testing current is <5 A. 4.3 Main circuit The main power electronic components of the circuit are listed in Table 4. Table 4: Main components of the circuit design Main component . Model . Power MOSFET IRF540, n-channel MOSFET driver-integrated circuit (IC) IR2110 Schottky diodes BYV26C—ultra-fast soft-recovery avalanche rectifier diode Main component . Model . Power MOSFET IRF540, n-channel MOSFET driver-integrated circuit (IC) IR2110 Schottky diodes BYV26C—ultra-fast soft-recovery avalanche rectifier diode Open in new tab Table 4: Main components of the circuit design Main component . Model . Power MOSFET IRF540, n-channel MOSFET driver-integrated circuit (IC) IR2110 Schottky diodes BYV26C—ultra-fast soft-recovery avalanche rectifier diode Main component . Model . Power MOSFET IRF540, n-channel MOSFET driver-integrated circuit (IC) IR2110 Schottky diodes BYV26C—ultra-fast soft-recovery avalanche rectifier diode Open in new tab The main circuit contains three separate driving arms. The three phases are assembled using three IR2110 half-bridge driver integrated circuits (ICs). The IC consists of an integrated high-side drive, which is specialized for driving the MOSFET pairs of high and low sides. Each IR2110 IC has its own signal input pins (at the high side in (HIN) and low side in (LIN)) which will connect with the output pins of the microcontroller to get the switching signal. The arrangement of the main components of the hardware circuit diagram of one inverter leg (Phase A) is shown in Fig. 11. Fig. 11: Open in new tabDownload slide Main components of the circuit diagram together with one inverter leg. The final implemented three-phase main inverter circuit board is shown in Fig. 12. Fig. 12: Open in new tabDownload slide Hardware implemented circuit. 4.4 LCL filter The L and C filter parameters can be calculated using the transfer function of the LCL filter. The following equation shows the transfer function of the LCL filter in which Ig is the grid current, Vi is filter of the input voltage, L1 + L2 = L and Lp=L1L2L1+L2[22]: IgVi=1SL (1+ S2CLp )(3) As a rule of thumb, the reactive power (Q) absorbed by the filter capacitor is limited to 5% of the rated power [23]. Hence, the following equation can be derived considering the reactance to find the capacitor value of C in which S represents the apparent power, V represents the phase voltage and f represents the system frequency: C=0.05 SV22 π f(4) Then the inductance value at the switching frequency can be calculated using the transfer function given by Equation 3. Finally, the value of the inductance can be calculated using the following equation in which ω = 2πf: L=|1ωswIg (sw)Vi (sw)(1−ωsw2ωres2)|(5) The LCL filter is implemented in order to smooth the output waveform by considering the values calculated above as well as the available component values in the market. Three Mylar capacitors are used here with a capacitance of 2.2 μF and a rated voltage of 400 V. Two sets of three copper coil ferrite core inductors to the value of 16 mH are used with the maximum current rating of 1 A. 4.5 Full inverter The connection diagram of the full inverter circuit is shown in Fig. 13. Initially, it senses the six waveforms of currents and voltages from the grid and passes them to the analogue input pins of the controller board by reducing them to signals with low amplitudes (0–5 V). Once the controller processes the signals, the six output PWM pins release the SPWM switching signals to the MOSFET driver IC as high and low side signal pairs. Then the driver ICs catch the signals and operate the two assigned power MOSFETs by switching their gate pins to receive the final AC output waveform from the circuit. Finally, the smoothed three-phase output signal can be obtained by sending it through the LCL filter. Fig. 13: Open in new tabDownload slide Full inverter diagram. 4.6 Output results The three-phase output waveforms with other parameters from the designed inverter are observed using the oscilloscope, power meter and multimeter. Active and reactive power outputs were observed under several different resistive and inductive load conditions. The obtained two waveforms of the three-phase voltage output using a two-channel oscilloscope are shown in Fig. 14. The frequency of the output waveforms remains the same as the grid frequency, which is 50 Hz. The output peak value of the voltage is 7.8 V and the phase shift is 120˚ between the two waveforms. The reactive power-injection mode of the design must have the ability to inject the reactive power depending on the load requirement. Therefore, the novel model is tested with different inductive loads and the values of the power outputs were observed using power meters. Fig. 14: Open in new tabDownload slide The three-phase outputs on the two-channel oscilloscope. The observed results were analysed and presented graphically in Fig. 15. Initially, the model was tested with pure resistive load. Therefore, the initial point of the inductive load was shown as zero and the active power was ~0.18 W with a unity power factor. Thereafter, the load fed by the inverter was gradually increased and the active and reactive powers with the power factor from the inverter output were observed. The reactive power increased with the load and the power factor decreased proportionally. The mechanism for the moderation of the sensed voltage and current waveforms using the novel controller can be validated using the results obtained here as the inverter has the ability to change the power factor over a wide range. If the design was tested further with large inductive loads, it would be able to run in zero power factor as shown in the simulation. The design was tested only with minor inductive loads with the available components, which does not affect adversely the final outcome. However, the results show that this system can achieve a broad range of power factors. In other words, the model can inject reactive power according to the system requirement. The hardware experimental results show good system performance and acceptable agreement with the simulation results well. However, there are minor variances between the simulation and experimental results due to the real-world environment in the experimental prototype test, such as electromagnetic interference, and particularly in the presence of the line resistances. Furthermore, the obtained results prove the feasibility of the proposed novel control technique. Fig. 15: Open in new tabDownload slide Graphical representation of the output results of the hardware model. 4.7 Design specifications The design specifications for the scaled-down novel experimented model were presented at the beginning of Section 4. The specifications of the full-scale actual model that were acquired using the tested model are presented in Table 5. Table 5: Specifications of the full-scale model Parameter . Value . Active power capability 2 kVA Reactive power capability 1.9 kVAR Maximum output current 5 A Operating voltage 400 V DC inverter input voltage 96 V DC battery voltage 48 V Output waveform frequency 50 Hz Three-phase transformer 96/400 V PWM switching frequency 10 kHz Parameter . Value . Active power capability 2 kVA Reactive power capability 1.9 kVAR Maximum output current 5 A Operating voltage 400 V DC inverter input voltage 96 V DC battery voltage 48 V Output waveform frequency 50 Hz Three-phase transformer 96/400 V PWM switching frequency 10 kHz Open in new tab Table 5: Specifications of the full-scale model Parameter . Value . Active power capability 2 kVA Reactive power capability 1.9 kVAR Maximum output current 5 A Operating voltage 400 V DC inverter input voltage 96 V DC battery voltage 48 V Output waveform frequency 50 Hz Three-phase transformer 96/400 V PWM switching frequency 10 kHz Parameter . Value . Active power capability 2 kVA Reactive power capability 1.9 kVAR Maximum output current 5 A Operating voltage 400 V DC inverter input voltage 96 V DC battery voltage 48 V Output waveform frequency 50 Hz Three-phase transformer 96/400 V PWM switching frequency 10 kHz Open in new tab 5 Conclusion An implemented simulation model as well as an experimental hardware set-up using power electronic components based on the novel concept of designing and controlling renewable-energy generation units were presented in this paper. The obtained results of the simulation and hardware model have been presented with a comparison in order to validate the effectiveness of the proposed novel control method. The smartness of the proposed controlling method is accomplished by regulating the required reactive power injection during the night to the utility grid without using the communication capability with other inverters in the system or without using the centralized administration service. This function can help not only at night, but also once the utility grid needs reactive power for the grid stability. The controlling mechanism performs based on a dq rotating frame controller with moderated techniques. Furthermore, the power electronic components of the inverter have the ability to work without absorbing the active power from the grid. The proposed mechanism helps to reduce the voltage sags by injecting the required reactive power in order to increase the voltage level and maintain it within the declared limits. Overall, the experimented results validated that the novel design can enhance the efficiency of the smart inverter by using it during the night and improve the stability of the power system. Additionally, a new considerable payment method can be implemented with new regulations associated with this concept for distributed inverter owners who inject the required reactive power into the system with the purpose of improving the grid stability. With respect to future developments, an actual scaled model that can be connected directly to the utility grid can be constructed using the obtained results and the proposed scaled-down hardware test model. Conflict of interest statement None declared. References [1] Shen Y , Cui M, Wang Q, et al. Comprehensive reactive power support of DFIG adapted to different depth of voltage sags . Energies , 2017 , 10 : 808 . Google Scholar Crossref Search ADS WorldCat [2] Wang L , Lao K, Lam C, et al. Delta-connected static var compensator (SVC) based hybrid active power filter (SVC-HAPF) and its control method . In: IECON 2017—43rd Annual Conference of the IEEE Industrial Electronics Society, Beijing, China, 29 October 2017–1 November 2017 , 6384 – 6388 . [3] Gandhi O , Rodriguez-Gallegos C, Reindl T, et al. Competitiveness of PV inverter as a reactive power compensator considering inverter lifetime reduction. In: 12th International Photovoltaic Power Generation and Smart Energy Conference & Exhibition (SNEC 2018) , Shanghai, China , 28 May 2018 , 150 : 74 – 82 . [4] Howlader AM , Sadoyama S, Roose SLR, Sepasi S. Distributed voltage regulation using Volt-VAR controls of a smart PV inverter in a smart grid: an experimental study . Journal of Renewable Energy , 2018 , 127 : 145 – 157 . Google Scholar Crossref Search ADS WorldCat [5] Liu Y , Qin W, Han X, et al. Distribution network voltage control by active power/reactive power injection from PV inverters . In: 13th IEEE Conference on Industrial Electronics and Applications (ICIEA), Wuhan, China , 31 June 2018 , 543 – 547 . [6] Varma RK , Rahman SA, Mahendra AC, et al. Novel nighttime application of PV solar farms as STATCOM (PV-STATCOM). In: IEEE Power and Energy Society General Meeting , San Diego, CA, USA , 22 July 2012 , 1 – 8 . [7] Anurag A , Yang Y, Blaabjerg F. Reliability analysis of single-phase PV inverters with reactive power injection. In: IEEE Energy Conversion Congress and Exposition, Montreal , Canada , 20–24 September 2015 , 2132 – 2139 . [8] Costa IDL , Brandao DI, Junior LM, et al. Analysis of stationary- and synchronous-reference frames for three-phase three-wire grid-connected converter AC current regulators . Energies , 2021 , 14 : 8348 . Google Scholar Crossref Search ADS WorldCat [9] Enayati B. The Revised IEEE1547 Standard. West Roxbury, MA, USA : National Grid , 2019 . Google Scholar Google Preview OpenURL Placeholder Text WorldCat COPAC [10] Varma RK , Siavashi EM. PV-STATCOM: a new smart inverter for voltage control in distribution systems . IEEE Trans Sustainable Energy , 2018 , 9 : 1681 – 1691 . Google Scholar Crossref Search ADS WorldCat [11] Xu Y , Milanović JV. Day-ahead prediction and shaping of dynamic response of demand at bulk supply points . IEEE Trans Power Syst , 2016 , 31 : 3100 – 3108 . Google Scholar Crossref Search ADS WorldCat [12] De Kooning J , de Vyver JV, De Kooning JDM, et al. Grid voltage control with distributed generation using online grid impedance estimation . Sustainable Energy Grids Networks , 2016 , 5 : 70 – 77 . Google Scholar Crossref Search ADS WorldCat [13] Calderaro V , Galdi V, Lamberti F, et al. A smart strategy for voltage control ancillary service in distribution networks . IEEE Trans Power Syst , 2015 , 30 : 494 – 502 . Google Scholar Crossref Search ADS WorldCat [14] Macana CA , Mojica-Nava E, Pota HR, et al. A novel compact dq-reference frame model for inverter-based microgrids . Electronics , 2019 , 8 : 1326 . Google Scholar Crossref Search ADS WorldCat [15] Ragab AS , Saad NH and El-Sattar AA. LLC resonant DC-DC converter for grid-connected PV system . In: 12th International Conference on Computer Engineering and Systems (ICCES) , Cairo, Egypt , 19–20 December 2017 , 279 – 285 . [16] Zhang J , Li L, Dorrell DG, et al. Modified PI controller with improved steady-state performance and comparison with PR controller on direct matrix converters . Chinese Journal of Electrical Engineering , 2019 , 5 : 53 – 66 . Google Scholar Crossref Search ADS WorldCat [17] O’Rourke CJ , Qasim MM, Overlin MR, et al. A geometric interpretation of reference frames and transformations: Dq0, Clarke, and Park . IEEE Trans Energy Convers , 2019 , 34 : 2070 – 2083 . Google Scholar Crossref Search ADS WorldCat [18] Kaur R , Gupta P, Singh J. A review on power MOSFET device structures . International Journal of Applied Science and Engineering , 2017 , 5 : 208 – 218 . Google Scholar OpenURL Placeholder Text WorldCat [19] Adhikari J , Prasanna IV and Panda SK. Voltage oriented control of the three-level Vienna rectifier using vector control method . In: 2016 IEEE Applied Power Electronics Conference and Exposition (APEC), Long Beach, CA, USA , 20 March 2016 , 9 – 16 . [20] Thompson MJ , Somani A. A tutorial on calculating source impedance ratios for determining line length . In: 2015 68th Annual Conference for Protective Relay Engineers, College Station, TX, USA , 30 March 2015–2 April 2015 , 833 – 884 . [21] LAUNCHXL-F28027 . Texas Instruments Incorporated , 2019 . https://www.ti.com/tool/LAUNCHXL-F28027 (12 July 2022, date last accessed). Google Scholar Google Preview OpenURL Placeholder Text WorldCat COPAC [22] Dursun M , Dosoglu MK. LCL filter design for grid connected three-phase inverter . In: 2nd International Symposium on Multidisciplinary Studies and Innovative Technologies (ISMSIT), Ankara, Turkey , 19–21 October 2018 , 1 – 4 . [23] Zhou D , Wang H, Blaabjerg F. Reactive power impacts on LCL filter capacitor lifetime in grid-connected inverter . IEEE Open Journal of Power Electronics , 2020 , 1 : 139 – 148 . Google Scholar Crossref Search ADS WorldCat © The Author(s) 2022. Published by Oxford University Press on behalf of National Institute of Clean-and-Low-Carbon Energy This is an Open Access article distributed under the terms of the Creative Commons Attribution-NonCommercial License (https://creativecommons.org/licenses/by-nc/4.0/), which permits non-commercial re-use, distribution, and reproduction in any medium, provided the original work is properly cited. For commercial re-use, please contact journals.permissions@oup.com © The Author(s) 2022. Published by Oxford University Press on behalf of National Institute of Clean-and-Low-Carbon Energy

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Copyright © 2022 National Institute of Clean-and-Low-Carbon Energy
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Abstract

Abstract Photovoltaic (PV) inverters are vital components for future smart grids. Although the popularity of PV-generator installations is high, their effective performance remains low. Certain inverters are designed to operate in volt-ampere reactive (VAR) mode during the night. Yet, this approach is ineffective due to the consumption of active power from the grid (as internal losses) and the regulation necessity of the direct-current (DC) bus. This paper will demonstrate the operation of a PV inverter in reactive power-injection mode when solar energy is unavailable. The primary focus is on the design of the inverter controller with respect to the synchronous rotating frame control method. The proposed novel method enables an inverter to inject the required level of reactive power to regulate the voltage levels of the utility grid within specified limits. In the process, the inverter does not absorb active power from the grid for its internal operation. The presented model has the ability to inject ≤2 kVAR of reactive power at zero power factor without absorbing active power from the grid. Simulation and hardware models of the inverter were developed and tested for different reactive loads in which the hardware model represented the real-world application. The reactive power injection of the two models ran at zero power factor and produced the expected outcomes for their corresponding independent reactive loads. Henceforth, it was evident that the proposed method can enhance the efficiency of an inverter and ensure the stability of the utility grid to which it is connected. Open in new tabDownload slide solar, energy system and policy, energy storage, distributed energy and smart grid, photovoltaic, reactive power, night-time usage Introduction Grid-tie inverters can be regarded as the main component in both renewable-energy conversion systems and smart grid systems. They can convert renewable energy into power that then can be fed to the utility grid as long as the renewable source exists. For photovoltaic (PV) inverters, solar energy must be there to generate active power. Otherwise, the inverter will remain idle during the night. The idle behaviour reduces the efficiency of the PV inverter. However, if there is a mechanism to use such inverters in a different way at night, its efficiency can be increased. The main motivation of the current research is to address the above issue and increase the efficiency of the inverter by using it during the night as well. Generally, when the reactive power is insufficient in the utility grid, it is not possible to push the power demand by loads through the lines as the voltage sags [1]. Using the inverter as a reactive power generator by operating it as a volt-ampere reactive (VAR) compensator is a potential way of solving the above issue of voltage sag [2]. The rapid increase in using PV inverters can be used to regulate the grid voltage and it will reduce the extra cost of installing capacitor banks. Currently, there are multiple ongoing research applications and experiments focusing on this general concept of using a PV inverter as a VAR compensator [3–5]. The PV generators consume a certain amount of active power for their internal operation and losses. Once the PV generators are used as reactive power generators, they will absorb active power from the grid that subsequently reduces the effectiveness of the concept. Although there are multiple research studies that address the use of PV inverters as reactive compensators during the night, none of them has addressed the specific issue of consuming active power from the grid [6, 7]. Most of the previous designs are planned to absorb the active power requirement from the grid to their internal operation. The novel method in the current research focuses on addressing these issues and developing an efficient method that can enhance the grid stability and usage of PV inverters. The novel control method introduced in this paper allows PV inverters to operate in pure reactive power-injection mode. The inverter is enhanced with the ability to work in this mode without absorbing any active power from the grid to compensate for its internal operation with losses and to regulate the direct-current (DC) bus voltage, which is the main novelty of this design. The novel concept is associated with a battery bank to compensate for the internal operation of the inverter. Specifically, this system is designed to inject only the required amount of reactive power by operating it in zero power factor. Further, the efficient use of the inverter can be increased by using it during the night. Overall, the concept introduced here expands the use of PV inverters and helps to maintain and regulate the voltage within the declared limits. Furthermore it can maintain the stability of the utility grid to which it is connected. Mainly, the synchronous rotating frame control method is utilized to develop the novel controlling method as it successfully defines the system performance under both steady-state and transient operations [8]. Also, the synchronous rotating frame control method is valid for any instantaneous difference between the voltage and the current. The details of the novel method are demonstrated in a simulation model and in a hardware development model with experimental results to validate the workability. The paper is organized as follows. Section 1 describes the active and reactive power injection using PV inverters. This is followed by Section 2, a discussion on the reactive power requirement of the grid and night-time reactive power injection. After that, Section 3 presents the novel control mechanism in detail. Further on, Section 4 discusses the simulation model and the hardware implementation under several grid conditions. The final section of this paper presents conclusions. 1 Active and reactive power control In the modern day, the PV inverters are being developed under the interconnection standards such as IEEE 1547, which do not allow for voltage regulations [9]. However, a majority of manufacturers of PV inverters tend to enhance their products with reactive power absorbing or injecting capabilities without exceeding their voltage ratings. The PV inverters theoretically can be developed as reactive power supporters, the same as the static compensators (STATCOMs) that the industrial standards do not address [10]. Typical PV inverters are designed to be disconnected at night. Alternatively, it is possible to use its reactive power capability when there is no active power generation. Typically, renewable generators like wind and solar individually follow a reactive power or a power factor set point, which can be tuned at the plant level for Volt/VAR regulating. Doing so will allow them to be involved in voltage controlling of the system. Reactive power control and the power factor control that are connected to the grid are not usually used in large power plants as they can result in improper response to system voltage variations, which could reduce the voltage stability of the system. However, reactive power and power factor control can be considered suitable options for distributed generators. 1.1 Reactive power requirement on the system The load demand can be increased due to their dynamic behaviour once they are connected to the transmission system [11]. Then the voltage level will drop at the load end together with the feeders. Hence the system fails to push the power through the transmission lines according to the demand of local loads. The voltage sag can be considered one of the most irritating events that can occur in the power system. However, it can be mitigated by injecting the desired amount of reactive power into the system. In other words, the voltage level of the power systems and transmission lines can be controlled by injecting or absorbing the reactive power from the distributed sources. To describe the combination between the voltage and the reactive power—a relationship that can be developed using the droop control equations for grid voltage and combining them to embed a general impedance. The connection between the voltage and the reactive power can be demonstrated as shown in the following equation [12]: Q−Q0=− 1kqα2+1(V−V0)(1) where α denotes the ratio R/X, which gives the grid impedance ratio (α = R/X). Generally, the grid impedance R + jX must be known to control the reactive power as well as the active power. As the variables R and X appear separately when deriving Equation 1, the ratio R/X can be used. Hence this value can be used to evaluate the required amount of reactive power. The frequency component of the above relationship is omitted since the distributed generator (DG) unit is grid-following and it cannot be controlled by local DGs. The parameter V gives the instantaneous grid voltage, while Q denotes the reactive power of the DG unit. V0 equals the nominal grid voltage. Q0 can be explained as a non-zero set point imposed by the transmission system operator. The parameter kq determines the amount of reactive power control for a certain grid voltage deviation from its nominal value (this parameter has the unit of Volt/VAR). The setting of this parameter has a vital impact on the grid voltage control and allows great flexibility [13]. Furthermore, the droop parameter is set to the preference or could also be altered by the network operator. In brief, this parameter is set to the level at which the DG units participate in the primary control of the grid voltage. 1.2 Reactive power control Accordingly, a variance of grid voltage from the declared value will create a reactive power demand. Generally, a grid-connected PV inverter can be programmed to inject and absorb the reactive power. Hence, both the overvoltage and undervoltage conditions can be regulated using the reactive power control ability. The dq components theory, which will be described in Section 2, can be used to perform the controlling mechanism efficiently [14]. The q component of the current can be regulated to control the reactive power injection instantaneously as it is used to represent the reactive power component. 1.3 Night-time reactive power-injection mode In general, the grid-tie inverters are equipped with DC-to-DC converters that regulate the DC bus voltage [15]. Once the active power is unavailable during the night, the DC-to-DC conversion becomes inactive. However, a developed control scheme with an energy-storage system can allow the inverter to operate in the reactive power mode even without the PV panels harvesting solar energy. Subsequently, the inverter can be programmed to operate as a VAR compensator to inject only the required reactive power, which will regulate the voltage at the load end. 2 Controlling mechanism The controlling mechanism of the novel concept with a background study is described under this topic. Further, the methods used for the design are described in detail. 2.1 Overview of different modelling techniques PV inverters in current power systems are utilizing several controlling techniques with the purpose of controlling the power. Table 1 shows a few controlling methods with their characteristics. Table 1: Special characteristics of inverter controlling mechanisms Controlling method . Special characteristics . 1. Current loop under single-axis 𝑑q rotating coordinate system Does not behave the positive and negative sequence decomposition of currents 2. Current loop under dual 𝑑q rotating coordinate system Has a dynamic process of transient underabig fault of asymmetrical transient grid system voltage 3. Active damping compensation feedback control strategy Cannot understand the damping control for unbalanced situations and also affects the system stability 4. Proportional-resonant current controller When the system is under grid voltage, the asymmetrical fault improved upon dynamic regulation 5. Hysteresis control method The switching frequency of pulse width modulation (PWM) can vary 6. The peak current control method Inductor currents track the basic sinusoidal waveforms 7. Stationary reference frame control Capable of removing the steady-state error between the controlled signal and reference signal 8. Synchronous rotating frame control Valid for any instantaneous difference of voltage or current and effectively describes the system performance under both steady-state and transient operations Controlling method . Special characteristics . 1. Current loop under single-axis 𝑑q rotating coordinate system Does not behave the positive and negative sequence decomposition of currents 2. Current loop under dual 𝑑q rotating coordinate system Has a dynamic process of transient underabig fault of asymmetrical transient grid system voltage 3. Active damping compensation feedback control strategy Cannot understand the damping control for unbalanced situations and also affects the system stability 4. Proportional-resonant current controller When the system is under grid voltage, the asymmetrical fault improved upon dynamic regulation 5. Hysteresis control method The switching frequency of pulse width modulation (PWM) can vary 6. The peak current control method Inductor currents track the basic sinusoidal waveforms 7. Stationary reference frame control Capable of removing the steady-state error between the controlled signal and reference signal 8. Synchronous rotating frame control Valid for any instantaneous difference of voltage or current and effectively describes the system performance under both steady-state and transient operations Open in new tab Table 1: Special characteristics of inverter controlling mechanisms Controlling method . Special characteristics . 1. Current loop under single-axis 𝑑q rotating coordinate system Does not behave the positive and negative sequence decomposition of currents 2. Current loop under dual 𝑑q rotating coordinate system Has a dynamic process of transient underabig fault of asymmetrical transient grid system voltage 3. Active damping compensation feedback control strategy Cannot understand the damping control for unbalanced situations and also affects the system stability 4. Proportional-resonant current controller When the system is under grid voltage, the asymmetrical fault improved upon dynamic regulation 5. Hysteresis control method The switching frequency of pulse width modulation (PWM) can vary 6. The peak current control method Inductor currents track the basic sinusoidal waveforms 7. Stationary reference frame control Capable of removing the steady-state error between the controlled signal and reference signal 8. Synchronous rotating frame control Valid for any instantaneous difference of voltage or current and effectively describes the system performance under both steady-state and transient operations Controlling method . Special characteristics . 1. Current loop under single-axis 𝑑q rotating coordinate system Does not behave the positive and negative sequence decomposition of currents 2. Current loop under dual 𝑑q rotating coordinate system Has a dynamic process of transient underabig fault of asymmetrical transient grid system voltage 3. Active damping compensation feedback control strategy Cannot understand the damping control for unbalanced situations and also affects the system stability 4. Proportional-resonant current controller When the system is under grid voltage, the asymmetrical fault improved upon dynamic regulation 5. Hysteresis control method The switching frequency of pulse width modulation (PWM) can vary 6. The peak current control method Inductor currents track the basic sinusoidal waveforms 7. Stationary reference frame control Capable of removing the steady-state error between the controlled signal and reference signal 8. Synchronous rotating frame control Valid for any instantaneous difference of voltage or current and effectively describes the system performance under both steady-state and transient operations Open in new tab From the controlling methods presented, the synchronous rotating frame controlling method with proportional-integral (PI) controller is used to develop the novel control method of pure reactive injecting [16]. The proposed approach is to model the power systems on the basis of dq0 quantities, which is not as general as abc-based models and is advantageous mainly when the network and units are symmetrically configured. The three-phase rotating reference frame control strategy is given in Fig. 1 as the general structure. Here, initially, the measured three-phase stationary current waveforms are converted into α and β components using the Clarke transformation in which the output will be a two-coordinated time-variant system [17]. After that, the Park transformation is used to convert α and β components from the stationary frame to the synchronous rotating frame in the form of d and q [17]. The name ‘synchronous’ rotating reference frame is given to this form since the dq components are arranged to rotate synchronously with the power line frequency. Then the d and q components are moderated according to the reactive power requirement. The modification technique is described in detail under the system modelling topic. After doing the required modifications in the d and q components, the rotating synchronous frame is reconverted to the stationary frame using inverse transformation. Finally, the controller will generate gate signals for the switching device using moderated waveforms. Fig. 1: Open in new tabDownload slide General structure of the three-phase rotating reference frame control strategy. Generally, the grid-tied inverters are designed to deliver regulated power from sources in renewable-energy applications, which may have oscillated input voltage levels. Hence, they must generate a fixed output voltage in a fixed frequency to the load or to the power system. Therefore, the output frequency is locked with the grid frequency at the installation and the consumer does not have the capability to change it. 2.2 System modelling A novel pure reactive power-injection mode has been invented in this research with the aim of balancing the power system by regulating the voltage level within the declared limits. The design was done mainly by considering the power output adjustment and VAR management. There are several research outcomes that have been presented about the concept, although they have several drawbacks such as consuming active power from the grid and outsized transient time as discussed in the introduction. Those drawbacks interrupt the achievement of the maximum efficiency. Therefore, the following points are considered the key requirements of the proposed design: (i) Injection of pure reactive power during the night or day according to the requirement of the grid. (ii) The inverter must not consume power for its internal usage from the utility grid. (iii) The amount of reactive power injection should depend on the requirement of the grid. 2.3 Design framework The inverter is modified with the aim of providing solutions for the drawbacks mentioned above. The power factor is decreased to zero to inject only the reactive power during the times when there is no solar energy. Especially, even during the day, if the power system needs reactive power, the novel function can be enabled to inject the required reactive power support into the grid. Fig. 2 presents the flow chart of the control system of the novel mechanism. Fig. 2: Open in new tabDownload slide The flow chart on the reactive power-injection concept. Mainly the dq-frame transformation theory is used for the novel method. According to the flow chart in Fig. 2, initially, the system detects the grid current and voltage using relevant sensors. Then the sensed three-phase voltage and current waveforms are transformed into the dq format as described previously. Afterwards, the q component of the current (Iq) and the d component of the current (Id), which represent the reactive power and the active power respectively, can be analysed with the reference value and the required changes to its parameters can be made using PI controllers. The reference value for Id is fixed as zero. It terminated the d component as the output must be purely reactive. The reference value of Iq is decided by the reference generator. As shown in the flow chart, the grid voltage is compared with the declared value. Then the reference current generator will decide the reference value for the q component, which will then decide the amount of reactive power. After the modified dq component of the waveforms is reconverted as a three-phase signal, it can be used to control the gate of the power metal-oxide semiconductor field-effect transistors (MOSFET) inside the inverter [18]. A majority of the existing research studies have introduced their models to consume a minor amount of active power from the utility grid for internal components of the inverter and their losses in typical operation. Hence this novel system in the current study is designed to consume the internal power requirements and losses from the battery storage of the inverter. The design is capable of injecting reactive power during the day as opposed to the night during conditions in which the system voltage level decreases. The key fact used in the current design is the maintenance of the output reactive power according to the requirement of the system. The closed-loop control system gives regular feedback on the output to regulate it with the current status. 2.4 Reference currents generation One way of injecting and controlling the reactive power is by changing the angle and the magnitude of the inverter voltage. Further, the controlling can be done using the q component current (Iq) in the voltage-oriented control method [19]. The feed-forward controller is used to determine the two reference current components Id and Iq using a PI controller acting on the grid voltage error. In other words, the difference between the current grid voltage and the declared reference value of the voltage is considered to generate the Iq reference. Also, as the design is set to inject the reactive power without injecting the active power, the active current component must be at the zero level in order to stop the active power from being injected. The Iq component must be non-zero in order to generate the reactive power at the same time. Therefore, a mechanism to generate an active current reference signal (Id*) is not necessary. A three-phase system voltage of 400 Vrms is selected as the declared voltage value for the system. The reactive power requirement can be detected by checking the system voltage as the voltage sags in the absence of reactive power. The comparison between the current value and the reference value of the voltage functioned by the current reference generator is graphically shown in Fig. 3. Fig. 3: Open in new tabDownload slide Generating of the Iq reference. Subsequently, the generated dq current reference components are passed to the current control loop as shown in Fig. 3. Finally, the moderated dq control signals are fed to the dq to the three-phase converter to generate the sinusoidal pulse width modulation (SPWM) gate signal. Hence the output waveforms will provide pure reactive power output that can be used to maintain the stability in the utility grid. 3 Simulation results In this section, the MATLAB®/Simulink® simulation model of the novel design is presented by considering three different scenarios of the power system. The design will be validated with the results at the end. 3.1 Simulation model The novel function has been developed using the MATLAB®/Simulink® platform. This three-phase grid-connected inverter is designed based on the techniques and functional developments that were described in Sections 1 and 2. The schematic block diagram of the simulation design is given in Fig. 4. The power source for this design is the battery storage as the design is based on the night-time during which the solar panels are not operational. Fig. 4: Open in new tabDownload slide Block diagram of the simulation design. Initially the three-phase voltages and currents are sensed from the grid as shown in the block diagram. Then they are converted into dq format and sent to the controller at which point the signals were moderated according to the reference values as described previously. Afterwards, they are reconverted to three signals and sent through the SPWM generator to generate the six distinct gate drive signals. Specific functioning abilities of the model are as follows: (i) The inverter has the ability to detect the grid voltage levels and deviations of the system. (ii) The inverter is designed to start the reactive power injection once the grid voltage level goes lower than the declared RMS voltage of 400 V. (iii) Reactive power injection will rely on the grid requirement up to the maximum capability of the inverter. 3.2 Simulation specifications The design specifications of the simulation model are shown in Table 2. Table 2: Simulation specifications Parameter . Value . DC battery voltage 48 V Maximum output current 5 A Rated three-phase AC voltage 400 V Power rating 2 kVA Reactive power capability 1.9 kVAR Supply duration 7 hours System frequency 50 Hz Switching frequency 10 kHz Power factor 0 Filter inductance 16 mH Filter capacitance 2.2 μF Parameter . Value . DC battery voltage 48 V Maximum output current 5 A Rated three-phase AC voltage 400 V Power rating 2 kVA Reactive power capability 1.9 kVAR Supply duration 7 hours System frequency 50 Hz Switching frequency 10 kHz Power factor 0 Filter inductance 16 mH Filter capacitance 2.2 μF Open in new tab Table 2: Simulation specifications Parameter . Value . DC battery voltage 48 V Maximum output current 5 A Rated three-phase AC voltage 400 V Power rating 2 kVA Reactive power capability 1.9 kVAR Supply duration 7 hours System frequency 50 Hz Switching frequency 10 kHz Power factor 0 Filter inductance 16 mH Filter capacitance 2.2 μF Parameter . Value . DC battery voltage 48 V Maximum output current 5 A Rated three-phase AC voltage 400 V Power rating 2 kVA Reactive power capability 1.9 kVAR Supply duration 7 hours System frequency 50 Hz Switching frequency 10 kHz Power factor 0 Filter inductance 16 mH Filter capacitance 2.2 μF Open in new tab 3.3 Simulation results The design simulation is carried out based on three different situations with different load conditions. The tested system three-phase voltage is 400 Vrms at 50 Hz of frequency. Fig. 5 shows the three-phase voltage and current waveforms from the inverter at the point of common coupling (PCC). Fig. 5: Open in new tabDownload slide Voltage and current waveforms from the inverter. The required amount of reactive power for a certain grid voltage deviation from its nominal value can be determined using Equation 1. According to Equation 1, the required reactive power depends on the grid impedance ratio, kq and the voltage deviation. In general, the grid impedance ratio for medium transmission lines is ~0.8, which can be considered a constant value [20]. The low kq value results in strong control of the reactive power. Therefore, kq could also be considered a constant. Hence it can be concluded that the reactive power deviation is directly proportional to the voltage level changes. Therefore, Equation 1 can be modified as the following equation (kq = 0.5): Q−Q0=− 10.50.82+1(V−V0) Q−Q0=−1.561(V−V0)(2) The results of the simulation are shown below for three different cases. Case 1: Constant reactive load (smaller than the reactive capability) The active and reactive power of the load is set to 10 kW and 1000 VAR, respectively. The inverter maintains its active power as zero to feed pure reactive power to the grid efficiently. Output waveforms of the active and reactive powers of the system are shown in Fig. 6. Fig. 6: Open in new tabDownload slide The (a) reactive and (b) active powers at the PCC—Case 1. Fig. 6a shows the behaviour of the amount of reactive power in the system. The inverter injects the entire reactive power requirement while the reactive supply of the main generator remains at the zero level as the requirement is within the reactive power-injection capability of the inverter. Active power injection of the inverter remains at zero while the entire active power requirement is fulfilled by the main generator, as shown in Fig. 6b. Case 2: Constant reactive load (greater than the reactive capability) Similarly to the previous case, the active power requirement of the load end remains at 10 kW, since the design is not focusing on the active powers. But the reactive power requirement at the load end increases up to 5000 VAR in this case. The output waveform of the system is depicted in Fig. 7. Fig. 7: Open in new tabDownload slide Output waveform of the (a) reactive and (b) active powers at the PCC—Case 2. As Fig. 7a shows, the reactive power requirement of the system has increased to 5000 VAR, which is greater than the maximum capability of the inverter. Therefore, the inverter supplies 2000 VAR in maximum capability while the main generator supplies the rest. The active power usage remains the same as in Case 1. Case 3: Variable reactive load Here, the voltage sag condition is considered as described in Fig. 8. In the beginning, the voltage level is within the declared value of 230 V. The inverter feeds a low amount of reactive power (<90 VAR). After 0.3 s, the voltage level drops by 14 V without the inverter. Fig. 8a shows both waveforms of the voltage level changing with both the inverter and without the inverter. With the injection of the required reactive power from the inverter, the voltage level does not drop more than 8 V. Fig. 8b shows the reactive power injection from the inverter according to the voltage level. Nearly 18 VAR of reactive power is injected into the system to repair the voltage sag. The active power injection or absorption does not happen during this time period. Fig. 8: Open in new tabDownload slide (a) Voltage and (b) output waveform of the active and reactive powers at the PCC—Case 3. The designed inverter has the ability to inject reactive power for nearly 6 hours in its maximum capability if the inverter is powered by a battery with a 442-Ah capacity. Retaining the active power at zero in Fig. 8b indicates that the inverter has the ability to inject pure reactive power without consuming active power from the grid. Finally, the results validated that this inverter model can be used during the night as a pure reactive power generator without consuming any active power from the grid. Two assumptions were considered for the design. The first assumption is that the voltage level of the battery bank is always at the maximum level and the second assumption is that the active power of the load remains constant during the simulation time. 4 Hardware development and results The hardware implementation with output results of the novel three-phase inverter model is discussed in this section. Fig. 9 shows the block diagram with the main components of the hardware modelled inverter. A photograph of the novel inverter circuit design is shown in Fig. 10. Fig. 9: Open in new tabDownload slide The block diagram of the hardware modelled inverter. Fig. 10: Open in new tabDownload slide General view of the experimental set-up. A scaled-down model without a transformer is developed here with the purpose of testing the working ability of the novel function on a hardware platform, with the specifications shown in Table 3. Table 3: Specifications of the scaled-down model Parameter . Scaled-down value . Input DC voltage 10 V Output AC voltage 6.9 Vrms Maximum output current 1 A Maximum output power 11.9 W Operating frequency 50 Hz Maximum operating temperature 70 ˚C Parameter . Scaled-down value . Input DC voltage 10 V Output AC voltage 6.9 Vrms Maximum output current 1 A Maximum output power 11.9 W Operating frequency 50 Hz Maximum operating temperature 70 ˚C Open in new tab Table 3: Specifications of the scaled-down model Parameter . Scaled-down value . Input DC voltage 10 V Output AC voltage 6.9 Vrms Maximum output current 1 A Maximum output power 11.9 W Operating frequency 50 Hz Maximum operating temperature 70 ˚C Parameter . Scaled-down value . Input DC voltage 10 V Output AC voltage 6.9 Vrms Maximum output current 1 A Maximum output power 11.9 W Operating frequency 50 Hz Maximum operating temperature 70 ˚C Open in new tab 4.1 Controlling the microcontroller The key aspect of this research is developing the inverter controller. Furthermore, it can be considered the most important section of a PV inverter. Generally, the controller consists of a microcontroller. Various types of microcontrollers with different specifications are available in the market that can be used in the hardware implementation. Specifications such as processing speed and input and output resolutions are the main parameters that must be considered when selecting the suitable controller for the hardware development. After doing performance analysis, the C2000 Launchpad-TMS320F28027F development kit was selected as the microcontroller [21]. It has a resolution of 12 bits as the analogue input resolution of the analogue to digital converter and 16 bits of resolution for the output pulse width modulation (PWM). It has a clock speed of 200 MHz, which is a very large value compared to the other microcontrollers. C or C++ programming languages can be used with the Code Composer Studio software for programming of the C2000 microcontroller. Alternatively, the programming could be easily done using the MATLAB®/Simulink® software, which comprises an embedded code support package within it. The program that was simulated previously in Section 3 can be directly converted into the form of machine language via Code Composer Studio and the controller can be programmed using this plug-in on MATLAB®. In order to program the controller, this converted method is used by matching the inputs and outputs according to the appropriate resolutions of the microcontroller. 4.2 Inputs sensing The controller must be fed with the three-phase current and voltage waveforms for the closed-loop operation of the inverter. The considered three-phase system has a voltage of 230 Vrms in one phase to neutral and the sensor must be compatible with this AC voltage. The ZMPT101B sensor module has been selected as the most suitable sensor for this case, since it is a very accurate and efficient one. AC voltage from 0 to 250 Vrms can be sensed with this sensor and the output analogue signal can vary between 0 and 5 V. Similarly to the voltage consideration, there are various kinds of sensors available in the market for sensing the current with different techniques such as the Hall-effect technique or using current transformers. The ACS712 sensor was selected as the most suitable current sensor for the operation of sensing the three-phase current. It is an advanced fully integrated Hall-effect-based linear current sensor. The sensor comes with different current sensing ranges: 0–5, 0–20 and 0–30 A. The sensor with a range of 0–5 A was selected to sense the three-phase currents in the current design as the testing current is <5 A. 4.3 Main circuit The main power electronic components of the circuit are listed in Table 4. Table 4: Main components of the circuit design Main component . Model . Power MOSFET IRF540, n-channel MOSFET driver-integrated circuit (IC) IR2110 Schottky diodes BYV26C—ultra-fast soft-recovery avalanche rectifier diode Main component . Model . Power MOSFET IRF540, n-channel MOSFET driver-integrated circuit (IC) IR2110 Schottky diodes BYV26C—ultra-fast soft-recovery avalanche rectifier diode Open in new tab Table 4: Main components of the circuit design Main component . Model . Power MOSFET IRF540, n-channel MOSFET driver-integrated circuit (IC) IR2110 Schottky diodes BYV26C—ultra-fast soft-recovery avalanche rectifier diode Main component . Model . Power MOSFET IRF540, n-channel MOSFET driver-integrated circuit (IC) IR2110 Schottky diodes BYV26C—ultra-fast soft-recovery avalanche rectifier diode Open in new tab The main circuit contains three separate driving arms. The three phases are assembled using three IR2110 half-bridge driver integrated circuits (ICs). The IC consists of an integrated high-side drive, which is specialized for driving the MOSFET pairs of high and low sides. Each IR2110 IC has its own signal input pins (at the high side in (HIN) and low side in (LIN)) which will connect with the output pins of the microcontroller to get the switching signal. The arrangement of the main components of the hardware circuit diagram of one inverter leg (Phase A) is shown in Fig. 11. Fig. 11: Open in new tabDownload slide Main components of the circuit diagram together with one inverter leg. The final implemented three-phase main inverter circuit board is shown in Fig. 12. Fig. 12: Open in new tabDownload slide Hardware implemented circuit. 4.4 LCL filter The L and C filter parameters can be calculated using the transfer function of the LCL filter. The following equation shows the transfer function of the LCL filter in which Ig is the grid current, Vi is filter of the input voltage, L1 + L2 = L and Lp=L1L2L1+L2[22]: IgVi=1SL (1+ S2CLp )(3) As a rule of thumb, the reactive power (Q) absorbed by the filter capacitor is limited to 5% of the rated power [23]. Hence, the following equation can be derived considering the reactance to find the capacitor value of C in which S represents the apparent power, V represents the phase voltage and f represents the system frequency: C=0.05 SV22 π f(4) Then the inductance value at the switching frequency can be calculated using the transfer function given by Equation 3. Finally, the value of the inductance can be calculated using the following equation in which ω = 2πf: L=|1ωswIg (sw)Vi (sw)(1−ωsw2ωres2)|(5) The LCL filter is implemented in order to smooth the output waveform by considering the values calculated above as well as the available component values in the market. Three Mylar capacitors are used here with a capacitance of 2.2 μF and a rated voltage of 400 V. Two sets of three copper coil ferrite core inductors to the value of 16 mH are used with the maximum current rating of 1 A. 4.5 Full inverter The connection diagram of the full inverter circuit is shown in Fig. 13. Initially, it senses the six waveforms of currents and voltages from the grid and passes them to the analogue input pins of the controller board by reducing them to signals with low amplitudes (0–5 V). Once the controller processes the signals, the six output PWM pins release the SPWM switching signals to the MOSFET driver IC as high and low side signal pairs. Then the driver ICs catch the signals and operate the two assigned power MOSFETs by switching their gate pins to receive the final AC output waveform from the circuit. Finally, the smoothed three-phase output signal can be obtained by sending it through the LCL filter. Fig. 13: Open in new tabDownload slide Full inverter diagram. 4.6 Output results The three-phase output waveforms with other parameters from the designed inverter are observed using the oscilloscope, power meter and multimeter. Active and reactive power outputs were observed under several different resistive and inductive load conditions. The obtained two waveforms of the three-phase voltage output using a two-channel oscilloscope are shown in Fig. 14. The frequency of the output waveforms remains the same as the grid frequency, which is 50 Hz. The output peak value of the voltage is 7.8 V and the phase shift is 120˚ between the two waveforms. The reactive power-injection mode of the design must have the ability to inject the reactive power depending on the load requirement. Therefore, the novel model is tested with different inductive loads and the values of the power outputs were observed using power meters. Fig. 14: Open in new tabDownload slide The three-phase outputs on the two-channel oscilloscope. The observed results were analysed and presented graphically in Fig. 15. Initially, the model was tested with pure resistive load. Therefore, the initial point of the inductive load was shown as zero and the active power was ~0.18 W with a unity power factor. Thereafter, the load fed by the inverter was gradually increased and the active and reactive powers with the power factor from the inverter output were observed. The reactive power increased with the load and the power factor decreased proportionally. The mechanism for the moderation of the sensed voltage and current waveforms using the novel controller can be validated using the results obtained here as the inverter has the ability to change the power factor over a wide range. If the design was tested further with large inductive loads, it would be able to run in zero power factor as shown in the simulation. The design was tested only with minor inductive loads with the available components, which does not affect adversely the final outcome. However, the results show that this system can achieve a broad range of power factors. In other words, the model can inject reactive power according to the system requirement. The hardware experimental results show good system performance and acceptable agreement with the simulation results well. However, there are minor variances between the simulation and experimental results due to the real-world environment in the experimental prototype test, such as electromagnetic interference, and particularly in the presence of the line resistances. Furthermore, the obtained results prove the feasibility of the proposed novel control technique. Fig. 15: Open in new tabDownload slide Graphical representation of the output results of the hardware model. 4.7 Design specifications The design specifications for the scaled-down novel experimented model were presented at the beginning of Section 4. The specifications of the full-scale actual model that were acquired using the tested model are presented in Table 5. Table 5: Specifications of the full-scale model Parameter . Value . Active power capability 2 kVA Reactive power capability 1.9 kVAR Maximum output current 5 A Operating voltage 400 V DC inverter input voltage 96 V DC battery voltage 48 V Output waveform frequency 50 Hz Three-phase transformer 96/400 V PWM switching frequency 10 kHz Parameter . Value . Active power capability 2 kVA Reactive power capability 1.9 kVAR Maximum output current 5 A Operating voltage 400 V DC inverter input voltage 96 V DC battery voltage 48 V Output waveform frequency 50 Hz Three-phase transformer 96/400 V PWM switching frequency 10 kHz Open in new tab Table 5: Specifications of the full-scale model Parameter . Value . Active power capability 2 kVA Reactive power capability 1.9 kVAR Maximum output current 5 A Operating voltage 400 V DC inverter input voltage 96 V DC battery voltage 48 V Output waveform frequency 50 Hz Three-phase transformer 96/400 V PWM switching frequency 10 kHz Parameter . Value . Active power capability 2 kVA Reactive power capability 1.9 kVAR Maximum output current 5 A Operating voltage 400 V DC inverter input voltage 96 V DC battery voltage 48 V Output waveform frequency 50 Hz Three-phase transformer 96/400 V PWM switching frequency 10 kHz Open in new tab 5 Conclusion An implemented simulation model as well as an experimental hardware set-up using power electronic components based on the novel concept of designing and controlling renewable-energy generation units were presented in this paper. The obtained results of the simulation and hardware model have been presented with a comparison in order to validate the effectiveness of the proposed novel control method. The smartness of the proposed controlling method is accomplished by regulating the required reactive power injection during the night to the utility grid without using the communication capability with other inverters in the system or without using the centralized administration service. This function can help not only at night, but also once the utility grid needs reactive power for the grid stability. The controlling mechanism performs based on a dq rotating frame controller with moderated techniques. Furthermore, the power electronic components of the inverter have the ability to work without absorbing the active power from the grid. The proposed mechanism helps to reduce the voltage sags by injecting the required reactive power in order to increase the voltage level and maintain it within the declared limits. Overall, the experimented results validated that the novel design can enhance the efficiency of the smart inverter by using it during the night and improve the stability of the power system. Additionally, a new considerable payment method can be implemented with new regulations associated with this concept for distributed inverter owners who inject the required reactive power into the system with the purpose of improving the grid stability. With respect to future developments, an actual scaled model that can be connected directly to the utility grid can be constructed using the obtained results and the proposed scaled-down hardware test model. Conflict of interest statement None declared. References [1] Shen Y , Cui M, Wang Q, et al. Comprehensive reactive power support of DFIG adapted to different depth of voltage sags . Energies , 2017 , 10 : 808 . Google Scholar Crossref Search ADS WorldCat [2] Wang L , Lao K, Lam C, et al. Delta-connected static var compensator (SVC) based hybrid active power filter (SVC-HAPF) and its control method . In: IECON 2017—43rd Annual Conference of the IEEE Industrial Electronics Society, Beijing, China, 29 October 2017–1 November 2017 , 6384 – 6388 . [3] Gandhi O , Rodriguez-Gallegos C, Reindl T, et al. Competitiveness of PV inverter as a reactive power compensator considering inverter lifetime reduction. 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Published by Oxford University Press on behalf of National Institute of Clean-and-Low-Carbon Energy This is an Open Access article distributed under the terms of the Creative Commons Attribution-NonCommercial License (https://creativecommons.org/licenses/by-nc/4.0/), which permits non-commercial re-use, distribution, and reproduction in any medium, provided the original work is properly cited. For commercial re-use, please contact journals.permissions@oup.com © The Author(s) 2022. Published by Oxford University Press on behalf of National Institute of Clean-and-Low-Carbon Energy

Journal

Clean EnergyOxford University Press

Published: Aug 1, 2022

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