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Investigation of Energy Storage Batteries in Stability Enforcement of Low Inertia Active Distribution Network

Investigation of Energy Storage Batteries in Stability Enforcement of Low Inertia Active... The inherent intermittency of renewable power generation poses one of the great challenges to the future smart grid. With incentives and subsidies, the penetration level of small-scale renewable energy into power grids is sharply increasing worldwide. Battery energy storage systems (BESS) are used to curtail the extra power during low demand times. These energy storage systems are capable of absorbing and delivering real power to the grid. The increased penetration level of inverter-based distributed generation (DG) reduces the inertia of the grid and thus affects the transient stability of the network. This paper discusses and investigates the impact of BESS on distribution networks’ stability with high penetration levels of inverter based DG. The obtained results show that proper charging and discharging schemes of the BESS can enhance the transient stability of the network. Fast switching between charging and discharging mode would be helpful during transient fault disturbance to keep the system in a balanced condition. . . . Keywords Renewable distributed generation Energy storage battery Low inertia distribution network Transient stability Introduction common, but power system operator would always keep this deviation as low as possible. Synchronous generators and tur- Conventional power systems are dominated by large rotating bines provide the rotating mass and supply/consume kinetic turbines and synchronous generators, which provide the inertia energy to/from the electric grid during a frequency deviation, and damping effects for stability requirements. A power system Δf. The supplied/consumed kinetic energy is proportional to is known to be transiently stable if it is able to regain its stability the rate of change of the frequency [3]. During transient stabil- after a disturbance [1]. The frequency of a large electric power ity events, the inertia constant H of synchronous generator system is maintained within an acceptable range by the rota- curtails the frequency deviation. This phenomenon slows down tional mass of many synchronous generators connected by tie- the frequency dynamics which increases the response time for lines in the network [2]. The power system frequency is in a transient events such as system faults, power plant outages, and stable state when active power produced by generators is equal- sudden disconnection of loads. Penetration of inverter-based ly consumed by the total system load along with transmission Renewable Energy Sources (RES) such as Photovoltaic (PV) and distribution network losses. This balance should be main- units and wind turbines in power systems have been increasing tained as much as possible. During normal steady system op- rapidly over time. The increase of RES penetration results in an eration, minor frequency deviations from its nominal value are equivalent decrease in conventional generators and thus the rotational inertia in the system becomes very low. This can lead to serious effects on the system’s frequency deviation [4]. * K. N. Bangash The stored kinetic energy (E ) in the rotating mass of the kin kbangash@aus.edu conventional synchronous generator help regulates the fre- * M. E. A. Farrag quency deviation by slowing down the frequency dynamics. Mohamed.Farrag@gcu.ac.uk The rotational energy is given as: Electrical Department, American University of Sharjah, 2 E ¼ JðÞ 2π f ð1Þ kin m Sharjah, United Arab Emirates Department of Engineering, School of Engineering and Built where J is the moment of inertia of synchronous generator and Environment, Glasgow Caledonian University, Glasgow, UK f is the frequency of the machine. The inertia constant H for a Faculty of Industrial Education, Helwan University, Helwan, Egypt 1 Page 2 of 12 Technol Econ Smart Grids Sustain Energy (2019) 4:1 synchronous machine is the ratio of the kinetic energy of ro- the unusable electric energy that generated by those sources tating masses and generator rated power, given by: and stream its power back to the grid during truncated gener- ation. This will help the system to accommodate the average E JðÞ 2π f kin m load scheme as a favor compared to peak load scheme. Peak H ¼ ¼ ð2Þ S 2S B B demand on the UK grid is expected to severely increase by 2050 due to abrupt demand from electric vehicles market, where H is the time typically in the range of 2~10 s, during especially after the adoption of no new petrol-based vehicles which the machine can supply its rated power exclusively to be bought after 2030 and also due to household heating through its stored kinetic energy. The classical swing equation ascents. Energy storage technologies could produce savings given in (3) describes the inertial response of the synchronous of £10 bn a year by 2050 in the UK [9]. ABB and UK Power generator following a power imbalance. Networks developed a dynamic energy storage solution that 2HS supports power quality during disturbances and support inter- ˙ ˙ ˙ E ¼ JðÞ 2π f  f ¼  f ¼ðÞ P −P ð3Þ kin mec e m m m mittence of wind power generation [10]. Integration of renewable generation in the main grid can where P is the mechanical power supplied by the generator mec change the system reliability and security measures. System and P as the electric power demand. The rotational inertia of operator would always be busy to shut down or synchronize the power system increases with the increase of the synchro- the conventional generator depending upon intermittency of nous generators number. The inertia constant H is inversely the solar and wind power generation. This could alter the real proportional to the frequency dynamics. Therefore, the higher and reactive power reserve that is required for system opera- the inertia constant H the lower is the frequency deviation. tion and stability. Wind farms are usually built at locations The high penetration level of inverter-based distributed with strong winds. Thus, the selection of the wind farm loca- generation (DG) with no rotating mass decreases the rotational tion is limited and the grid has to be designed accordingly. inertia of power systems. Thus, low inertia inverter-based DG High penetration of wind power production to existing grid in small power networks would lead to high deviations in system can overload transmission and distribution network voltage and frequency during large disturbances [5]. The time [11–13]. Moreover, the operational switching combinations frame for different frequency stability issues varies from a few of embedded DG would make the fault current calculations seconds to several minutes or sometimes hours. Table 1 de- more complex as compared when the power flow is one way scribes the time frame for potential frequency stability issues. and passive demand [14]. For system security, sufficient operational generation re- Power network is a composite system, which is susceptible to serves are required to compensate for the unexpected loss of disturbances. The power system is transiently stable, if it re- generation or sudden load rejection. The inadequate opera- mains intact in synchronism and survives after grid faults such tional reserve would consequently lead to frequency instabil- as the short circuit in transmission lines. Transient stability de- ity that can trigger automatic load shedding. In European pends on the type of disturbance like fault duration, fault type, countries, whose networks are interconnected, standard EN operating conditions and system characteristics [15]. The rotor 50160 specifies 50 Hz ±1% (49.5~50.5 Hz) for 95% of the of synchronous generators must remain synchronized with the week and [+4%, −6%] (52~47 Hz) in the event of major dis- rotating magnetic field of the stator after a fault. The generators turbances [7]. During low load consumption and renewable must come back into synchronism with the rest of the system power generation is reasonably high, the load dispatcher may after oscillations once the fault is cleared. If they cannot come restrict or curtail energy generated from the renewable sources back into synchronism, the generators will experience cascade to maintain the system standard frequency within limits, tripping with an increased chance of blackout [16–18]. which may be considered as a waste of energy [8]. Battery Nagaraju Pogaku et al. [19] developed the modeling of au- energy storage systems (BESS) can be used as a tool to absorb tonomous operation of inverter-based microgrids to analyze the Table 1 Time Frame for frequency stability issues [6] Time Potential issues Contribution Few Seconds Low system inertia, low primary control response time Synchronous and asynchronous generators (and motors) 10–30 Seconds Inadequate primary reserve All primarily controlled generators in the system Slow activation time of primary reserve 5–15 min. Inadequate secondary reserve Load shared by Generators in the area of disturbance Slow activation time of secondary reserve is regulated by load dispatch center More than 15 min. Insufficient regulatory Reserve All generators contracted for this service Activation is manual, no frequency response Technol Econ Smart Grids Sustain Energy (2019) 4:1 Page 3 of 12 1 oscillatory modes for its poor damping. The developed model 0.8 pu DG should remain connected to the grid for 0.5 s. includes inverter low and high-frequency dynamics, network The duration is increased to 2.5 s if the measured line-line dynamics and load dynamics. These inverter models allow voltage is 0.87 pu. The DG should not be tripped as long as achieving the required stability margin for reliable grid opera- the frequency is above 47 Hz and below 50.5 Hz as recom- tion.Sonietal. [20] developed a controller for the inverters to mended by the Engineering Recommendation G83/2, G59/1 regulate the frequency after large frequency deviations in given in Table 2,[30, 31]. microgrids. Anurag K Srivastava et al. [23] investigated the In this work, with the integration of inverter based DG, use of energy storage systems, such as batteries and ultra- synchronous generator is disconnected for economic load dis- capacitors to improve the transient stability of a system pene- patch. Transient stability is executed on Matlab and the ability trated with DG. Small synchronous and induction generators of the remaining synchronous generator to recover back after a are used to represent DG. Since these DG are not inverter-based fault is analyzed. During high DG output power, Energy DG, the inertia of the system has increased and thus the tran- Storage battery (ESB) are required to be in a charged mode sient stability is improved. Sebastián and Alzola [24]presented to allow for a higher share of real power production from SM, the modeling and testing of an islanded Wind-Diesel Hybrid which leads to an increased kinetic energy and better stable System (WDHS) with Ni–MH BESS. The constant speed stall operation. Energy storage battery is used to balance the gen- controlled wind turbine generator comprises an induction gen- eration and demand, and improve transient stability. erator is directly connected to the autonomous grid. The simu- This research analyses the impact of extra penetration of lations show that the system dynamics are significantly im- inverter-based PVs and wind power at UK 11 kV distribution proved by using BESS. Monshizadeh et al. [25]usedaDC- feeder, and its impacts on system stability. This paper is orga- side capacitor of the inverter as energy storage to mimic the nized as follows; section I, describes the background informa- kinetic energy of a synchronous generator. Though the DC-side tion about the work and the impact of inertia-less DGs on capacitor is an essential element in most inverters, capacitors system transient stability. Section II, illustrates the modeling cannot supply power for a long time. Unlike capacitors, batte- of a UK distribution system with rooftop PVs and wind power ries can supply power for longer times. Jaber Alipoor et al. [26] generation. Section III, presents the transient analysis of DG used the alternating inertia technique to find the right value of and BESS. Finally, conclusions are presented in Section IV. the inertia using Virtual Synchronous Generator (VSG) to pro- duce virtual acceleration or deceleration during oscillations. Soni et al. [20] proposed a control technique for inverter- General Problem and System Description based DG to improve the power management and frequency response of an isolated microgrid. The droop gain of the invert- A typical home load profile of UK on a bright sunny day er is determined as a function of the frequency deviation. The during the summer season is used for simulation. The rooftop inverter supplies higher power to add virtual inertia to the sys- PVoutput power rating is 3 kW. The patterns of the daily load tem and lower the frequency deviation. cycle, PV and wind power production are illustrated in Fig. 1 One of the major concerns for using batteries is that they [32, 33]. The wind profile is obtained from a weather station will degrade faster if also participate in grid operation, such as that is attached to one of Caledonian Buildings in Glasgow. demand response, peak shaving and frequency regulation. Shi In the UK, power demand is less than DG (PVs and Wind) et al. [21] proposed a joint optimization framework for using power production, particularly at the middle of the day during BESS in both frequency regulation and peak shaving for com- the summer season. Moreover, during peak load timing, PV mercial customers. The results show that cost savings are larg- power production is null, while wind power generation drops er if batteries are used for multiple purposes rather than devot- down as shown in Fig. 1. High penetration of renewable DG ing them to a single application. Bian et al. [22] developed a can cause reverse power flow, which results in unpredictable method to estimate the demand side contributions to system short-circuit current. Therefore, changes in the settings of the inertia of Great Britain (GB) power system. Grid-connected installed protective relays are required. Moreover, single synchronous motors in industries provide inertia. The demand phase PV power production can cause unbalance voltage [34]. side can contribute an average of 1.75 s inertia constant. National grid centre would like to decommit expensive However, the Variable Frequency Drives (VFD) used to con- generator units for economic load dispatch during high trol the motors decrease the demand side inertia though they help to enhance the efficiency. In the case of PV-systems and Table 2 Frequency protection settings [31] inverter based wind generators, the inverter acts like an active/ Parameter Trip setting Trip time reactive current source with no “rotor angle”, which could introduce transient rotor angle instability [27]. Over frequency 50.5 Hz (50 Hz +1%) 0.5 s According to G83/2 and G59/1 Engineering Under frequency 47 Hz (50 Hz −6%) 0.5 s Recommendations [28, 29], if the line-line voltage reaches 1 Page 4 of 12 Technol Econ Smart Grids Sustain Energy (2019) 4:1 Fig. 1 Domestic load profile, PV and wind generation (average summer day) renewable power generation and less load demand. The inertia the high penetration of inverter-based renewable DG. During of the system will be reduced by decommitting some synchro- the charging mode of the batteries, the transient stability is nous machines from the power system. Sudden connection/ increased, due to the increased share of power production disconnection of loads or temporary short circuits may lead to from SM which increases the kinetic energy. oscillations, and sometimes the remaining synchronous ma- Synchronous generators of 2 MVA, 400 V, 50 Hz, chines may lose synchronization from the system. The exces- 112 kg.m , 1500 rpm driven by a diesel engine are selected sive electricity generated by the DG during low load con- for simulation. Figure 2 describes the block diagram of the sumption and high renewable power generation can be stored diesel engine generator set. The diesel engine is coupled with in the battery storage installed at the LV distribution network. the synchronous machine. The engine’s speed is maintained at This power can be supplied back during high power consump- 1500 rpm with speed regulator/governor. Increased real power tion times. In this paper, the use of BESS to enforce stability in load on the synchronous generator, results in an imbalance low inertia systems is investigated. This can be the bi-product torque on the coupled synchronous generator rotor and the of batteries other than storage. crankshaft. This results in deceleration of the engine and In the future smart grid, more and more renewable DG causes an equivalent reduction in generated frequency. would replace conventional synchronous machines (SM). The engine speed regulator acts as a feedback controller Power systems would also shift from centralized to and is used to regulate the engine’s speed. The regulator re- decentralized topologies in the form of the microgrid. In a sponds by increasing the fuel rate, with this the engine’sgross microgrid, the generation should be balanced with the load output torque is increased, resulting in the engine speed and and the losses in the network. The generation in microgrids frequency to run at rated value. The difference between the is composed of conventional SMs driven by diesel engines in actual and desired speed is used as the input to the proportion- addition to low-inertia renewable DG. In case of disturbance, al, integral plus derivative (PID) controller as shown in Fig. 3. the SMs in the microgrid may lose their synchronization due The regulator uses a discrete variable-gain PID controller to to the low inertia in the system. This challenge is investigated determine the desired quantity of fuel to be delivered to the in this paper using a typical 11 kV UK distribution feeder with next available cylinder. local conventional SMs. The feeder is disconnected from the The voltage regulator controls the field current to the excit- main grid forming a microgrid. It is expected that renewable er and keeps the generator voltage constant. Figure 4 depicts DG would be installed in the feeder and accordingly, the need the voltage control system. The voltage regulator control sys- for expensively operated SMs would be reduced. In this paper, tem is implemented with a Proportional, Integral (PI) control- the idea of home connected energy storage battery is used to ler to stabilize the voltage by controlling reactive power cope with the stability issue due to the low inertia created by (VAR). A sudden increase in generator real power results Fig. 2 Block diagram of diesel Diesel Synchronous Power engine generator Engine Generator Network Speed Excitation Voltage Regulator System Regulator Fuel Speed Voltage Technol Econ Smart Grids Sustain Energy (2019) 4:1 Page 5 of 12 1 1 +T1s 1 +T1s Discrete – Delay 2 k 1 + 1/(1+T*s) 2e6 Time 1 + T2s Integrator 1 + T2s Pmec Gain K wref (pu) pu2Was Discrete Torque Discrete Gas flow (pu) Lead–Lag1 Lead–Lag ……..Regulator ……… …Engine... ……………………Throle actuator…………….… Pmec_Diesel Delay w_pu rad/s to pu Fig. 3 Speed control system loads torque higher than the engine torque. The engine speed into the network. This can represent inverter based DGs in decreases because the engine governor cannot respond quick- which the inverter controls the amount of P and Q generated ly. Speed regulator increases the fuel supplied to the engine by the variable speed/frequency wind turbines or the dc PV once deceleration is detected. The generated voltage is also panels. The distribution network only receives P and Q proportional to engine speed so the generator terminal voltage from the inverter, and therefore, a detailed model of wind decreases due to armature reaction and internal voltage drops. turbine and PV panels are not simulated in this work. BESS The voltage regulator compensates this drop by increasing the is considered as a source of active power consumption, generator excitation field current. three-phase dynamic load block is used to model ESB in Two synchronous generators are connected at one terminal Grid to Battery (G2B) mode. The negative sign is assigned of the UK feeder as shown in Fig. 5. The voltage regulator and to the output of the Simulink three-phase dynamic load the exciter control the voltage at the SMs’ terminals. All these block to model the BESS in Battery to Grid (B2G) mode. parameters are available in standard built-in models for SM in In the B2G mode, the battery is utilized to balance the fre- the power systems library of Simulink/Matlab. Lumped load quency or avert stability margins by injecting active power representing domestic power consumption is connected at into the system. The emulator with Li-ion battery charac- seven 11/0.4 kV, 500 kVA transformers. The consumer teristics was tested on IEEE-24 bus system using OPAL-RT lumped load (cluster of 384 homes) consists of a 140 kW at real-time simulator [36]. The ESS showed 80 ms frequency each 11/0.4 kV distribution transformer. A 500 kW extra load response time for major imbalance while keeping the state is connected adjacent to the SMs bus. Single synchronous gener- of charge (SoC) to 50%. Almost, 100 ms time delay is used ator (2 MVA) is feeding a total of 1480 kW load while the home in this research, whenever the battery is switched from G2B load is considered at unity power factor. The input from the in- into B2G mode. Small size Lithium-ion batteries ranging verter based DG is kept constant during a fault. Transient stability from 1to10kWh areavailable to maximize thePVcon- of the SM is investigated and hunting oscillation is observed. sumption by storing electricity during off-peak times [37]. DG power production is injected using a three-phase ABB has worked with UK Power Networks to develop dynamic load block of Simscape power systems library dynamic energy solution that ensures, power supply due [35]. The active reactive powers of the load are defined by to intermittent nature of renewable power, storing excessive an external Simulink® vector of two signals. By assigning a renewable energy generation, improve power quality dur- negative sign to external Simulink vector, three-phase dy- ing fault and also afford dynamic voltage control for eight namic load block can inject a prescribed amount of P and Q million homes and businesses in the UK [38]. PI Vf_Exiter (pu) Vt_ref (pu) Vt (pu) Vf pu to volts Discrete PI Mag 1/z Controller abc Vabc_SM Vt_SM From BSM Voltage measurement measurement Fig. 4 Voltage regulator control system 1 Page 6 of 12 Technol Econ Smart Grids Sustain Energy (2019) 4:1 Fig. 5 Single 11 kV UK feeder with diesel generators and home load Simulation Study SMs. P does not recover back instantly, once the fault is mec removed due to the inertia. The power reached the maximum Base Case – Two SMs at t = 5.7 s and an oscillation is observed. During the fault, the frequency continued to increase and reached 51 Hz when the A UK 11 kV feeder, shown in Fig. 5, is connected to two SMs fault is cleared at 5.3 s. The two generators have enough syn- driven by a diesel engine and simulated without DG. The chronizing torques to remain in synchronism with the system performed simulations indicate that the critical clearing time when the fault is removed after 0.3 s. The voltage at the fault for one SM without DG is 0.3 s while with two SMs connect- point is zero during the fault. Moreover, each SM contributed ed, the critical clearing time is 2.0 s as the inertia of the system a fault current of 740 amps upon the occurrence of the fault has increased. A three-phase temporary fault is applied be- and decreased to 600 amps when the fault is cleared. tween bus B3 and B4. The fault is applied at t =5.0 s and cleared at t =5.3 s. Distribution Feeder with Two SM and DG The traces of P representing the mechanical power of mec the SMs, the speed of the SM, frequency and fault location Two SMs are connected at one terminal of the 11 kV feeder voltage are shown in Fig. 6a. During the short circuit, the and one DG of a capacity equal to the total domestic load mechanical power (P ) of the SMs drops down because (7 × 140 kW) is connected at the other end of the feeder, mec the part of the feeder beyond the fault is no longer fed by representing a wind farm as shown in the Fig. 7. The traces (a) (b) Pmec DG Output Power 1,250 Pmec 3 4 5 5.3 5.6 5.9 6.2 6.5 7 8 9 10 3 4 5 5.3 5.6 5.9 6.2 6.5 7 8 9 10 Speed 1.1 1.1 Speed 0.9 0.9 3 4 5 5.3 5.6 5.9 6.2 6.5 7 8 9 10 3 4 5 5.3 5.6 5.9 6.2 6.5 7 8 9 10 51 Frequency Frequency 3 4 5 5.3 5.6 5.9 6.2 6.5 7 8 9 10 3 4 5 5.3 5.6 5.9 6.2 6.5 7 8 9 10 1.5 1.5 1 1 Voltage 0.5 0.5 Voltage 0 0 3 4 5 5.3 5.6 5.9 6.2 6.5 7 8 9 10 3 4 5 5.3 5.6 5.9 6.2 6.5 7 8 9 10 Time in Seconds Time in Seconds Fig. 6 Response of the generator power, speed, frequency and voltage before, during and after the fault Per Unit Per Unit kW Hz Per Unit Per Unit kW Hz Technol Econ Smart Grids Sustain Energy (2019) 4:1 Page 7 of 12 1 Fig. 7 Single 11 kV UK feeder with diesel generators and wind farm of P representing the mechanical power of the SM, DG system experienced a power swing because the kinetic energy mec output power, the speed of the SM, frequency and fault stored in the rotor increases the rotor speed. The frequency location voltage are shown in Fig. 6b. SMs and DG are deliv- almost reached the maximum limit i.e. 52 Hz at t = 5.6 s. The ering 340 kW and 980 kW respectively before the fault. traces of the DG output power also illustrates that the output During the short circuit, the SMs started to feed the bus up power of inverter based DG is constant during the fault be- to the fault point that was previously fed by DG. P in- cause the controller of the inverter does not allow power shar- mec creased from 340 to 1250 kW, while the speed and frequency ing as in SMs. dropped down. When the fault is cleared, the SMss power descended and due to inertia it did not settle at 340 kW but Fault Analysis with Low Inertia decreased to 0 kW. At t = 6.1 s, the SM power started to rise and stabilized at t = 6.5 s. The speed regulator maintains a One SM is replaced by wind turbine for economic load dis- speed of 1.0 p.u and nominal frequency to 50 Hz after a tran- patch. A short circuit fault is applied at t = 5 s to investigate the sient period of 1.5 s. The system is stabilized after 2 s. system performance under the low inertia of the inverter based The two generators have enough synchronizing torques to renewable DG. During the short circuit, a single generator remain in synchronism with the system when the fault is re- with reduced inertia is hunting instead of supplying power to moved at t = 5.3 s. Moreover, once the fault is cleared, the the feeder up to the fault point as shown in Fig. 8. Finally, P mec Fig. 8 Transient response of low Pmec DG Output Power inertia network without ESB 3 4 5 5.3 5.6 6.15 8 9 10 Speed 0.75 0.5 3 4 5 5.3 5.6 6.15 8 9 10 Frequency 3 4 5 5.3 5.6 6.15 8 9 10 1.5 Voltage 3 4 5 5.3 5.6 6.15 8 9 10 Time in Seconds Per Unit Hz Per Unit kW 1 Page 8 of 12 Technol Econ Smart Grids Sustain Energy (2019) 4:1 Fig. 9 Single 11 kV UK feeder with diesel generator, wind farm and energy storage battery dropped down to 150 kW when the short circuit is cleared at half of the power produced by the wind turbines is consumed t = 5.3 s. After the clearance of the short circuit, P is sup- by the batteries. mec posed to return back to 550 kW, but with low inertia, it deliv- During the fault period, P dropped down from 1120 kW mec ered maximum capable power. Speed and frequency oscillated since the SM is only feeding power to the fault point. Due to till t = 6.15 s and then finally lost synchronism. With a low the low inertia, P does not stop instantly and reached 0 at mec inertia system, the remaining SM was unable to regain syn- t = 5.3 s, as shown in Fig. 10. The frequency and the speed chronization back after the fault was cleared. The simulations have increased because of the amount of electrical active pow- performed on this configuration showed that the critical fault er that the generator exports is not the same as the amount of clearing time is 0.05 s. Moreover, the single SM contributed a mechanical power it imports. P started to increase after the mec fault current of 485 amps at the onset of the fault and de- clearance of the fault and delivered its maximum power be- creased to 370 amps when the fault is cleared. tween t =5.7 s to t = 6.3 s. At t = 6.3 s, P started to decrease mec and settled to 1120 kW at t =10 s. Impacts of Energy Storage Battery on Low Inertia It is obvious that when the fault is cleared, the impact of Network power flow from DG is less compared to the case where ESB are not used. Batteries are consuming the power produced by Energy storage battery (4x140kW) is connected besides the the DG. At t = 6 s the frequency reached 46.4 Hz with an wind farm in charging mode as shown in Fig. 9.More than initial dip of output power reaching zero. With the slow Fig. 10 Transient response of low Pmec DG Output Power Battery Power Consumption inertia network with ESB -560 4 5 5.3 5.7 6.3 6.85 8 9 10 1.1 Speed 1.05 0.94 4 5 5.3 5.7 6.3 6.85 8 9 10 Frequency 46.3 4 5 5.3 5.7 6.3 6.85 8 9 10 1.5 Voltage 4 5 5.3 5.7 6.3 6.85 8 9 10 Time in Seconds Per Unit Hz Per Unit kW Technol Econ Smart Grids Sustain Energy (2019) 4:1 Page 9 of 12 1 (a) (b) 3,000 Pmec DG Output Power Battery Power 2,500 2,500 Pmec DG Output Power Battery Power Consumption 2,000 2,000 1,000 1,000 -500 -500 3 4 5 5.3 5.6 5.9 7 8 9 10 3 4 5 5.3 5.6 5.9 6.7 8 9 10 Speed 0.75 Speed 0.8 0.5 0.3 3 4 5 5.3 5.6 5.9 6.7 8 9 10 3 4 5 5.3 5.6 5.9 7 8 9 10 Frequency Frequency 3 4 5 5.3 5.6 5.9 7 8 9 10 3 4 5 5.3 5.6 5.9 6.7 8 9 10 1.5 Voltage 1.5 Voltage 0 0 3 4 5 5.3 5.6 5.9 7 8 9 10 3 4 5 5.3 5.6 5.9 6.7 8 9 10 Time in Seconds Time in Seconds Fig. 11 Transient response with DG disconnected hunting frequency, the SM is able to recover back. Once the Transient Response by Switching the Battery fault is cleared, the machine speed begins to stabilize and the from G2B into B2G machine begins to export more active power (kW) compared to the pre-fault condition. This is because the machine has In this case, the wind DG is disconnected intentionally at the stored up kinetic energy in the rotor during the fault. end of the fault duration (t = 5.3 s) and the battery is switched from charging to discharging mode as shown in Fig. 11b. At t = 5.4 s, the battery delivered 500 kW to the system instead of Transient Analysis with Disconnection of Wind consumption. Switching of the battery from −500 to 500 kW Turbine has injected 1000 kW to the system. The batteries have re- duced the power demand from the system by 1000 kW that In this case, the wind DG is disconnected intentionally at the would increase the kinetic energy as per Eq. (3). The frequen- end of short circuit time i.e. t = 5.3 s, to investigate the stability cy dropped down to 43.5 Hz at t = 6.3 s but recovered back to of the system. Figure 11ashows theobtained results. When 50 Hz at t = 7.2 s. With minor oscillation, the system is able to the DG is disconnected at t = 5.3 s, the SM takes the share of recover and become stable at t =10 s. DG and P increased to 1700 kW at t = 5.5 s. The rotor Simulation is also performed to find the optimal power of mec speed decreased and with low kinetic energy as well as active ESB required in the G2B mode to keep the system synchro- power reserve, it did not recover back and the SM is nize after disturbance. ESB requires 40 kWof power in charg- desynchronized. In the previous case, the system remained ing mode to keep the system stable and synchronized as intact after the transient time because the battery load was depicted in Table 3. System stability and reliability is the bi- incorporated. In the next case, the battery will be switched to product of ESB, otherwise, the main function of ESB is to the discharge mode i.e. battery to grid mode to analyze the store excessive power and give back to the grid especially impact of the battery on the stability. during peak hour load demand. Table 3 Optimal ESB power to ESB power (kW) Synchronous machine Remarks keep the system stable status after the fault 10 De-synchronized ESB is connected adjacent to 20 De-synchronized Wind farm/Solar park in the G2B mode 30 De-synchronized 40 Synchronised Per Unit Hz Per Unit kW Per Unit Hz Per Unit kW 1 Page 10 of 12 Technol Econ Smart Grids Sustain Energy (2019) 4:1 (a) (b) 2500 Pmec Total DG Output Power 0 Pmec Total DG Output Power Battery Power Consumption 3 4 5 5.3 6 7 8 9 10 11 12 -1000 3 4 5 5.3 5.6 5.9 7 8 9 10 Speed 0.75 1.05 0.5 Speed 3 4 5 5.3 6 7 8 9 10 11 12 0.92 3 4 5 5.3 5.6 5.9 7 8 9 10 Frequency Frequency 3 4 5 5.3 6 7 8 9 10 11 12 46.2 3 4 5 5.3 5.6 5.9 7 8 9 10 1.5 1.5 Voltage Voltage 0 0 3 4 5 5.3 5.6 5.9 7 8 9 10 3 4 5 5.3 6 7 8 9 10 11 12 Time in Seconds Time in Seconds Fig. 12 Transient response with Roof top DGs Roof-Top DG in Low Inertia System consequently, P of the synchronous generator has also in- mec creased to keep the generation and demand in balance. Thus, In this case, a 140 kW capacity of DG is connected at the LV the kinetic energy has apparently increased. Less oscillation is side of every 500kVA distribution transformer. This resembles observed after the fault clearance as shown in Fig. 12b. The domestic rooftop DG. The total power consumption is sup- system is recovered back to steady state condition at t=10 s. plied by domestic renewable DG, which makes the power The frequency is below 47 Hz for 0.5 s between t=5.84 to t = needed from the 500 kVA transformer negligible. Only one 6.34 s. This result shows that with DG and batteries, the fre- SM is connected in the system. With low inertia, the system is quency fulfilled the frequency engineering recommendation unable to stabilize after the fault clearance and desynchronized given in Table 2. The batteries were able to recover back the at t = 7 s as shown in Fig. 12a. system and eliminated the need to disconnect the DG which will lower the system frequency and deteriorates the conditions further. Impact of Roof-Top DG with Energy Storage Battery Batteries in charging mode have increased the kinetic en- ergy of SM as it has to increase its power to satisfy the de- A simulation is performed for the case when rooftop DG and mand. This property is useful for SM to resynchronize partic- batteries are connected to the system. At every LV side of the ularly in transient state conditions as illustrated in Table 4. 500 kVA transformers, 140 kW batteries are connected in charg- Simulation is performed for stability to find the total opti- ing mode. Batteries consume 1000 kW extra power from the mal power required by ESB in G2B mode. Rooftop DGs system. This has increased the electrical demand and Table 4 Comparison of scenarios Scenario 1 Base Case 2 SM without DG 2 SM’s have enough inertia to recover back after the fault is removed. Scenario 2 Two SM and Inverter based DG Oscillation is increased after the fault, as two ways of power flow, the share of power production from SM is low resulting in less kinetic energy, but the system has recovered back. Scenario 3 One SM with Inverter based DG The share of power production from SM is less and less kinetic energy. With low inertia, SM was unable to re-synchronize after the fault is removed. Scenario 4 ESB is connected besides DG The share of power flow from DG on the feeder is less as ESB is consuming the power produced by DG. SM is able to recover back because more power production that results in more kinetic energy. Scenario 5 DG is disconnected With Low active power reserve, SM does not recover back and ESB is still in charging mode. Scenario 6 ESB switched from G2B into B2G mode Switching ESB into B2G mode has injected real power into the system thus virtually injected inertia to the system. SM is able to resynchronize. Scenario 7 Roof top DGs with one SM Share of power production from SM is less so less kinetic energy. With low inertia, SM was unable to resynchronize after the fault is removed. Scenario 8 ESB connected to Roof top DGs ESB in charging mode has reduced the share of power from DGs and SM has gained kinetic energy by increasing power production. SM is recovered back. Per Unit Hz Per Unit kW Per Unit Hz Per Unit kW Technol Econ Smart Grids Sustain Energy (2019) 4:1 Page 11 of 12 1 Table 5 Optimal ESB power to keep the system stable with rooftop DGs ESB power (kW) Total ESB power (kW) Synchronous machine Remarks in individual LV feeder of seven LV feeder status after the fault 14 98 Synchronized ESB is connected at every 11/0.4 7 49 Synchronized kV transformer LV side in the G2B mode 2.8 19.6 Synchronised 1.6 11.2 Synchronised 0.8 5.6 De-synchronized Open Access This article is distributed under the terms of the Creative required 11.2 kW power of ESB to keep the system stable as Commons Attribution 4.0 International License (http:// shown in Table 5. This power is much less than the power creativecommons.org/licenses/by/4.0/), which permits unrestricted use, required when wind farm with ESB is connected to one end of distribution, and reproduction in any medium, provided you give appro- the feeder as described in Table 3. priate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. References Conclusion 1. Kundur P, Paserba J, Ajjarapu V et al (2004) Definition and classi- In this research, the impact of the energy storage battery on the fication of power system stability. IEEE/CIGRE joint task force on transient stability of the synchronous generator is analyzed. stability terms and definitions. IEEE Trans Power Syst 19(3):1387– Firstly, the transient stability of UK 11 kV system with two 1401 2. Skopik F, Smith P (2015) Smart grid security innovation solutions SM is analyzed. With enough inertia and kinetic energy, SM is st for a modernized grid. 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Investigation of Energy Storage Batteries in Stability Enforcement of Low Inertia Active Distribution Network

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Springer Journals
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Copyright © 2018 by The Author(s)
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Energy; Energy Systems; Power Electronics, Electrical Machines and Networks; Energy Policy, Economics and Management
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2199-4706
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10.1007/s40866-018-0059-4
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Abstract

The inherent intermittency of renewable power generation poses one of the great challenges to the future smart grid. With incentives and subsidies, the penetration level of small-scale renewable energy into power grids is sharply increasing worldwide. Battery energy storage systems (BESS) are used to curtail the extra power during low demand times. These energy storage systems are capable of absorbing and delivering real power to the grid. The increased penetration level of inverter-based distributed generation (DG) reduces the inertia of the grid and thus affects the transient stability of the network. This paper discusses and investigates the impact of BESS on distribution networks’ stability with high penetration levels of inverter based DG. The obtained results show that proper charging and discharging schemes of the BESS can enhance the transient stability of the network. Fast switching between charging and discharging mode would be helpful during transient fault disturbance to keep the system in a balanced condition. . . . Keywords Renewable distributed generation Energy storage battery Low inertia distribution network Transient stability Introduction common, but power system operator would always keep this deviation as low as possible. Synchronous generators and tur- Conventional power systems are dominated by large rotating bines provide the rotating mass and supply/consume kinetic turbines and synchronous generators, which provide the inertia energy to/from the electric grid during a frequency deviation, and damping effects for stability requirements. A power system Δf. The supplied/consumed kinetic energy is proportional to is known to be transiently stable if it is able to regain its stability the rate of change of the frequency [3]. During transient stabil- after a disturbance [1]. The frequency of a large electric power ity events, the inertia constant H of synchronous generator system is maintained within an acceptable range by the rota- curtails the frequency deviation. This phenomenon slows down tional mass of many synchronous generators connected by tie- the frequency dynamics which increases the response time for lines in the network [2]. The power system frequency is in a transient events such as system faults, power plant outages, and stable state when active power produced by generators is equal- sudden disconnection of loads. Penetration of inverter-based ly consumed by the total system load along with transmission Renewable Energy Sources (RES) such as Photovoltaic (PV) and distribution network losses. This balance should be main- units and wind turbines in power systems have been increasing tained as much as possible. During normal steady system op- rapidly over time. The increase of RES penetration results in an eration, minor frequency deviations from its nominal value are equivalent decrease in conventional generators and thus the rotational inertia in the system becomes very low. This can lead to serious effects on the system’s frequency deviation [4]. * K. N. Bangash The stored kinetic energy (E ) in the rotating mass of the kin kbangash@aus.edu conventional synchronous generator help regulates the fre- * M. E. A. Farrag quency deviation by slowing down the frequency dynamics. Mohamed.Farrag@gcu.ac.uk The rotational energy is given as: Electrical Department, American University of Sharjah, 2 E ¼ JðÞ 2π f ð1Þ kin m Sharjah, United Arab Emirates Department of Engineering, School of Engineering and Built where J is the moment of inertia of synchronous generator and Environment, Glasgow Caledonian University, Glasgow, UK f is the frequency of the machine. The inertia constant H for a Faculty of Industrial Education, Helwan University, Helwan, Egypt 1 Page 2 of 12 Technol Econ Smart Grids Sustain Energy (2019) 4:1 synchronous machine is the ratio of the kinetic energy of ro- the unusable electric energy that generated by those sources tating masses and generator rated power, given by: and stream its power back to the grid during truncated gener- ation. This will help the system to accommodate the average E JðÞ 2π f kin m load scheme as a favor compared to peak load scheme. Peak H ¼ ¼ ð2Þ S 2S B B demand on the UK grid is expected to severely increase by 2050 due to abrupt demand from electric vehicles market, where H is the time typically in the range of 2~10 s, during especially after the adoption of no new petrol-based vehicles which the machine can supply its rated power exclusively to be bought after 2030 and also due to household heating through its stored kinetic energy. The classical swing equation ascents. Energy storage technologies could produce savings given in (3) describes the inertial response of the synchronous of £10 bn a year by 2050 in the UK [9]. ABB and UK Power generator following a power imbalance. Networks developed a dynamic energy storage solution that 2HS supports power quality during disturbances and support inter- ˙ ˙ ˙ E ¼ JðÞ 2π f  f ¼  f ¼ðÞ P −P ð3Þ kin mec e m m m mittence of wind power generation [10]. Integration of renewable generation in the main grid can where P is the mechanical power supplied by the generator mec change the system reliability and security measures. System and P as the electric power demand. The rotational inertia of operator would always be busy to shut down or synchronize the power system increases with the increase of the synchro- the conventional generator depending upon intermittency of nous generators number. The inertia constant H is inversely the solar and wind power generation. This could alter the real proportional to the frequency dynamics. Therefore, the higher and reactive power reserve that is required for system opera- the inertia constant H the lower is the frequency deviation. tion and stability. Wind farms are usually built at locations The high penetration level of inverter-based distributed with strong winds. Thus, the selection of the wind farm loca- generation (DG) with no rotating mass decreases the rotational tion is limited and the grid has to be designed accordingly. inertia of power systems. Thus, low inertia inverter-based DG High penetration of wind power production to existing grid in small power networks would lead to high deviations in system can overload transmission and distribution network voltage and frequency during large disturbances [5]. The time [11–13]. Moreover, the operational switching combinations frame for different frequency stability issues varies from a few of embedded DG would make the fault current calculations seconds to several minutes or sometimes hours. Table 1 de- more complex as compared when the power flow is one way scribes the time frame for potential frequency stability issues. and passive demand [14]. For system security, sufficient operational generation re- Power network is a composite system, which is susceptible to serves are required to compensate for the unexpected loss of disturbances. The power system is transiently stable, if it re- generation or sudden load rejection. The inadequate opera- mains intact in synchronism and survives after grid faults such tional reserve would consequently lead to frequency instabil- as the short circuit in transmission lines. Transient stability de- ity that can trigger automatic load shedding. In European pends on the type of disturbance like fault duration, fault type, countries, whose networks are interconnected, standard EN operating conditions and system characteristics [15]. The rotor 50160 specifies 50 Hz ±1% (49.5~50.5 Hz) for 95% of the of synchronous generators must remain synchronized with the week and [+4%, −6%] (52~47 Hz) in the event of major dis- rotating magnetic field of the stator after a fault. The generators turbances [7]. During low load consumption and renewable must come back into synchronism with the rest of the system power generation is reasonably high, the load dispatcher may after oscillations once the fault is cleared. If they cannot come restrict or curtail energy generated from the renewable sources back into synchronism, the generators will experience cascade to maintain the system standard frequency within limits, tripping with an increased chance of blackout [16–18]. which may be considered as a waste of energy [8]. Battery Nagaraju Pogaku et al. [19] developed the modeling of au- energy storage systems (BESS) can be used as a tool to absorb tonomous operation of inverter-based microgrids to analyze the Table 1 Time Frame for frequency stability issues [6] Time Potential issues Contribution Few Seconds Low system inertia, low primary control response time Synchronous and asynchronous generators (and motors) 10–30 Seconds Inadequate primary reserve All primarily controlled generators in the system Slow activation time of primary reserve 5–15 min. Inadequate secondary reserve Load shared by Generators in the area of disturbance Slow activation time of secondary reserve is regulated by load dispatch center More than 15 min. Insufficient regulatory Reserve All generators contracted for this service Activation is manual, no frequency response Technol Econ Smart Grids Sustain Energy (2019) 4:1 Page 3 of 12 1 oscillatory modes for its poor damping. The developed model 0.8 pu DG should remain connected to the grid for 0.5 s. includes inverter low and high-frequency dynamics, network The duration is increased to 2.5 s if the measured line-line dynamics and load dynamics. These inverter models allow voltage is 0.87 pu. The DG should not be tripped as long as achieving the required stability margin for reliable grid opera- the frequency is above 47 Hz and below 50.5 Hz as recom- tion.Sonietal. [20] developed a controller for the inverters to mended by the Engineering Recommendation G83/2, G59/1 regulate the frequency after large frequency deviations in given in Table 2,[30, 31]. microgrids. Anurag K Srivastava et al. [23] investigated the In this work, with the integration of inverter based DG, use of energy storage systems, such as batteries and ultra- synchronous generator is disconnected for economic load dis- capacitors to improve the transient stability of a system pene- patch. Transient stability is executed on Matlab and the ability trated with DG. Small synchronous and induction generators of the remaining synchronous generator to recover back after a are used to represent DG. Since these DG are not inverter-based fault is analyzed. During high DG output power, Energy DG, the inertia of the system has increased and thus the tran- Storage battery (ESB) are required to be in a charged mode sient stability is improved. Sebastián and Alzola [24]presented to allow for a higher share of real power production from SM, the modeling and testing of an islanded Wind-Diesel Hybrid which leads to an increased kinetic energy and better stable System (WDHS) with Ni–MH BESS. The constant speed stall operation. Energy storage battery is used to balance the gen- controlled wind turbine generator comprises an induction gen- eration and demand, and improve transient stability. erator is directly connected to the autonomous grid. The simu- This research analyses the impact of extra penetration of lations show that the system dynamics are significantly im- inverter-based PVs and wind power at UK 11 kV distribution proved by using BESS. Monshizadeh et al. [25]usedaDC- feeder, and its impacts on system stability. This paper is orga- side capacitor of the inverter as energy storage to mimic the nized as follows; section I, describes the background informa- kinetic energy of a synchronous generator. Though the DC-side tion about the work and the impact of inertia-less DGs on capacitor is an essential element in most inverters, capacitors system transient stability. Section II, illustrates the modeling cannot supply power for a long time. Unlike capacitors, batte- of a UK distribution system with rooftop PVs and wind power ries can supply power for longer times. Jaber Alipoor et al. [26] generation. Section III, presents the transient analysis of DG used the alternating inertia technique to find the right value of and BESS. Finally, conclusions are presented in Section IV. the inertia using Virtual Synchronous Generator (VSG) to pro- duce virtual acceleration or deceleration during oscillations. Soni et al. [20] proposed a control technique for inverter- General Problem and System Description based DG to improve the power management and frequency response of an isolated microgrid. The droop gain of the invert- A typical home load profile of UK on a bright sunny day er is determined as a function of the frequency deviation. The during the summer season is used for simulation. The rooftop inverter supplies higher power to add virtual inertia to the sys- PVoutput power rating is 3 kW. The patterns of the daily load tem and lower the frequency deviation. cycle, PV and wind power production are illustrated in Fig. 1 One of the major concerns for using batteries is that they [32, 33]. The wind profile is obtained from a weather station will degrade faster if also participate in grid operation, such as that is attached to one of Caledonian Buildings in Glasgow. demand response, peak shaving and frequency regulation. Shi In the UK, power demand is less than DG (PVs and Wind) et al. [21] proposed a joint optimization framework for using power production, particularly at the middle of the day during BESS in both frequency regulation and peak shaving for com- the summer season. Moreover, during peak load timing, PV mercial customers. The results show that cost savings are larg- power production is null, while wind power generation drops er if batteries are used for multiple purposes rather than devot- down as shown in Fig. 1. High penetration of renewable DG ing them to a single application. Bian et al. [22] developed a can cause reverse power flow, which results in unpredictable method to estimate the demand side contributions to system short-circuit current. Therefore, changes in the settings of the inertia of Great Britain (GB) power system. Grid-connected installed protective relays are required. Moreover, single synchronous motors in industries provide inertia. The demand phase PV power production can cause unbalance voltage [34]. side can contribute an average of 1.75 s inertia constant. National grid centre would like to decommit expensive However, the Variable Frequency Drives (VFD) used to con- generator units for economic load dispatch during high trol the motors decrease the demand side inertia though they help to enhance the efficiency. In the case of PV-systems and Table 2 Frequency protection settings [31] inverter based wind generators, the inverter acts like an active/ Parameter Trip setting Trip time reactive current source with no “rotor angle”, which could introduce transient rotor angle instability [27]. Over frequency 50.5 Hz (50 Hz +1%) 0.5 s According to G83/2 and G59/1 Engineering Under frequency 47 Hz (50 Hz −6%) 0.5 s Recommendations [28, 29], if the line-line voltage reaches 1 Page 4 of 12 Technol Econ Smart Grids Sustain Energy (2019) 4:1 Fig. 1 Domestic load profile, PV and wind generation (average summer day) renewable power generation and less load demand. The inertia the high penetration of inverter-based renewable DG. During of the system will be reduced by decommitting some synchro- the charging mode of the batteries, the transient stability is nous machines from the power system. Sudden connection/ increased, due to the increased share of power production disconnection of loads or temporary short circuits may lead to from SM which increases the kinetic energy. oscillations, and sometimes the remaining synchronous ma- Synchronous generators of 2 MVA, 400 V, 50 Hz, chines may lose synchronization from the system. The exces- 112 kg.m , 1500 rpm driven by a diesel engine are selected sive electricity generated by the DG during low load con- for simulation. Figure 2 describes the block diagram of the sumption and high renewable power generation can be stored diesel engine generator set. The diesel engine is coupled with in the battery storage installed at the LV distribution network. the synchronous machine. The engine’s speed is maintained at This power can be supplied back during high power consump- 1500 rpm with speed regulator/governor. Increased real power tion times. In this paper, the use of BESS to enforce stability in load on the synchronous generator, results in an imbalance low inertia systems is investigated. This can be the bi-product torque on the coupled synchronous generator rotor and the of batteries other than storage. crankshaft. This results in deceleration of the engine and In the future smart grid, more and more renewable DG causes an equivalent reduction in generated frequency. would replace conventional synchronous machines (SM). The engine speed regulator acts as a feedback controller Power systems would also shift from centralized to and is used to regulate the engine’s speed. The regulator re- decentralized topologies in the form of the microgrid. In a sponds by increasing the fuel rate, with this the engine’sgross microgrid, the generation should be balanced with the load output torque is increased, resulting in the engine speed and and the losses in the network. The generation in microgrids frequency to run at rated value. The difference between the is composed of conventional SMs driven by diesel engines in actual and desired speed is used as the input to the proportion- addition to low-inertia renewable DG. In case of disturbance, al, integral plus derivative (PID) controller as shown in Fig. 3. the SMs in the microgrid may lose their synchronization due The regulator uses a discrete variable-gain PID controller to to the low inertia in the system. This challenge is investigated determine the desired quantity of fuel to be delivered to the in this paper using a typical 11 kV UK distribution feeder with next available cylinder. local conventional SMs. The feeder is disconnected from the The voltage regulator controls the field current to the excit- main grid forming a microgrid. It is expected that renewable er and keeps the generator voltage constant. Figure 4 depicts DG would be installed in the feeder and accordingly, the need the voltage control system. The voltage regulator control sys- for expensively operated SMs would be reduced. In this paper, tem is implemented with a Proportional, Integral (PI) control- the idea of home connected energy storage battery is used to ler to stabilize the voltage by controlling reactive power cope with the stability issue due to the low inertia created by (VAR). A sudden increase in generator real power results Fig. 2 Block diagram of diesel Diesel Synchronous Power engine generator Engine Generator Network Speed Excitation Voltage Regulator System Regulator Fuel Speed Voltage Technol Econ Smart Grids Sustain Energy (2019) 4:1 Page 5 of 12 1 1 +T1s 1 +T1s Discrete – Delay 2 k 1 + 1/(1+T*s) 2e6 Time 1 + T2s Integrator 1 + T2s Pmec Gain K wref (pu) pu2Was Discrete Torque Discrete Gas flow (pu) Lead–Lag1 Lead–Lag ……..Regulator ……… …Engine... ……………………Throle actuator…………….… Pmec_Diesel Delay w_pu rad/s to pu Fig. 3 Speed control system loads torque higher than the engine torque. The engine speed into the network. This can represent inverter based DGs in decreases because the engine governor cannot respond quick- which the inverter controls the amount of P and Q generated ly. Speed regulator increases the fuel supplied to the engine by the variable speed/frequency wind turbines or the dc PV once deceleration is detected. The generated voltage is also panels. The distribution network only receives P and Q proportional to engine speed so the generator terminal voltage from the inverter, and therefore, a detailed model of wind decreases due to armature reaction and internal voltage drops. turbine and PV panels are not simulated in this work. BESS The voltage regulator compensates this drop by increasing the is considered as a source of active power consumption, generator excitation field current. three-phase dynamic load block is used to model ESB in Two synchronous generators are connected at one terminal Grid to Battery (G2B) mode. The negative sign is assigned of the UK feeder as shown in Fig. 5. The voltage regulator and to the output of the Simulink three-phase dynamic load the exciter control the voltage at the SMs’ terminals. All these block to model the BESS in Battery to Grid (B2G) mode. parameters are available in standard built-in models for SM in In the B2G mode, the battery is utilized to balance the fre- the power systems library of Simulink/Matlab. Lumped load quency or avert stability margins by injecting active power representing domestic power consumption is connected at into the system. The emulator with Li-ion battery charac- seven 11/0.4 kV, 500 kVA transformers. The consumer teristics was tested on IEEE-24 bus system using OPAL-RT lumped load (cluster of 384 homes) consists of a 140 kW at real-time simulator [36]. The ESS showed 80 ms frequency each 11/0.4 kV distribution transformer. A 500 kW extra load response time for major imbalance while keeping the state is connected adjacent to the SMs bus. Single synchronous gener- of charge (SoC) to 50%. Almost, 100 ms time delay is used ator (2 MVA) is feeding a total of 1480 kW load while the home in this research, whenever the battery is switched from G2B load is considered at unity power factor. The input from the in- into B2G mode. Small size Lithium-ion batteries ranging verter based DG is kept constant during a fault. Transient stability from 1to10kWh areavailable to maximize thePVcon- of the SM is investigated and hunting oscillation is observed. sumption by storing electricity during off-peak times [37]. DG power production is injected using a three-phase ABB has worked with UK Power Networks to develop dynamic load block of Simscape power systems library dynamic energy solution that ensures, power supply due [35]. The active reactive powers of the load are defined by to intermittent nature of renewable power, storing excessive an external Simulink® vector of two signals. By assigning a renewable energy generation, improve power quality dur- negative sign to external Simulink vector, three-phase dy- ing fault and also afford dynamic voltage control for eight namic load block can inject a prescribed amount of P and Q million homes and businesses in the UK [38]. PI Vf_Exiter (pu) Vt_ref (pu) Vt (pu) Vf pu to volts Discrete PI Mag 1/z Controller abc Vabc_SM Vt_SM From BSM Voltage measurement measurement Fig. 4 Voltage regulator control system 1 Page 6 of 12 Technol Econ Smart Grids Sustain Energy (2019) 4:1 Fig. 5 Single 11 kV UK feeder with diesel generators and home load Simulation Study SMs. P does not recover back instantly, once the fault is mec removed due to the inertia. The power reached the maximum Base Case – Two SMs at t = 5.7 s and an oscillation is observed. During the fault, the frequency continued to increase and reached 51 Hz when the A UK 11 kV feeder, shown in Fig. 5, is connected to two SMs fault is cleared at 5.3 s. The two generators have enough syn- driven by a diesel engine and simulated without DG. The chronizing torques to remain in synchronism with the system performed simulations indicate that the critical clearing time when the fault is removed after 0.3 s. The voltage at the fault for one SM without DG is 0.3 s while with two SMs connect- point is zero during the fault. Moreover, each SM contributed ed, the critical clearing time is 2.0 s as the inertia of the system a fault current of 740 amps upon the occurrence of the fault has increased. A three-phase temporary fault is applied be- and decreased to 600 amps when the fault is cleared. tween bus B3 and B4. The fault is applied at t =5.0 s and cleared at t =5.3 s. Distribution Feeder with Two SM and DG The traces of P representing the mechanical power of mec the SMs, the speed of the SM, frequency and fault location Two SMs are connected at one terminal of the 11 kV feeder voltage are shown in Fig. 6a. During the short circuit, the and one DG of a capacity equal to the total domestic load mechanical power (P ) of the SMs drops down because (7 × 140 kW) is connected at the other end of the feeder, mec the part of the feeder beyond the fault is no longer fed by representing a wind farm as shown in the Fig. 7. The traces (a) (b) Pmec DG Output Power 1,250 Pmec 3 4 5 5.3 5.6 5.9 6.2 6.5 7 8 9 10 3 4 5 5.3 5.6 5.9 6.2 6.5 7 8 9 10 Speed 1.1 1.1 Speed 0.9 0.9 3 4 5 5.3 5.6 5.9 6.2 6.5 7 8 9 10 3 4 5 5.3 5.6 5.9 6.2 6.5 7 8 9 10 51 Frequency Frequency 3 4 5 5.3 5.6 5.9 6.2 6.5 7 8 9 10 3 4 5 5.3 5.6 5.9 6.2 6.5 7 8 9 10 1.5 1.5 1 1 Voltage 0.5 0.5 Voltage 0 0 3 4 5 5.3 5.6 5.9 6.2 6.5 7 8 9 10 3 4 5 5.3 5.6 5.9 6.2 6.5 7 8 9 10 Time in Seconds Time in Seconds Fig. 6 Response of the generator power, speed, frequency and voltage before, during and after the fault Per Unit Per Unit kW Hz Per Unit Per Unit kW Hz Technol Econ Smart Grids Sustain Energy (2019) 4:1 Page 7 of 12 1 Fig. 7 Single 11 kV UK feeder with diesel generators and wind farm of P representing the mechanical power of the SM, DG system experienced a power swing because the kinetic energy mec output power, the speed of the SM, frequency and fault stored in the rotor increases the rotor speed. The frequency location voltage are shown in Fig. 6b. SMs and DG are deliv- almost reached the maximum limit i.e. 52 Hz at t = 5.6 s. The ering 340 kW and 980 kW respectively before the fault. traces of the DG output power also illustrates that the output During the short circuit, the SMs started to feed the bus up power of inverter based DG is constant during the fault be- to the fault point that was previously fed by DG. P in- cause the controller of the inverter does not allow power shar- mec creased from 340 to 1250 kW, while the speed and frequency ing as in SMs. dropped down. When the fault is cleared, the SMss power descended and due to inertia it did not settle at 340 kW but Fault Analysis with Low Inertia decreased to 0 kW. At t = 6.1 s, the SM power started to rise and stabilized at t = 6.5 s. The speed regulator maintains a One SM is replaced by wind turbine for economic load dis- speed of 1.0 p.u and nominal frequency to 50 Hz after a tran- patch. A short circuit fault is applied at t = 5 s to investigate the sient period of 1.5 s. The system is stabilized after 2 s. system performance under the low inertia of the inverter based The two generators have enough synchronizing torques to renewable DG. During the short circuit, a single generator remain in synchronism with the system when the fault is re- with reduced inertia is hunting instead of supplying power to moved at t = 5.3 s. Moreover, once the fault is cleared, the the feeder up to the fault point as shown in Fig. 8. Finally, P mec Fig. 8 Transient response of low Pmec DG Output Power inertia network without ESB 3 4 5 5.3 5.6 6.15 8 9 10 Speed 0.75 0.5 3 4 5 5.3 5.6 6.15 8 9 10 Frequency 3 4 5 5.3 5.6 6.15 8 9 10 1.5 Voltage 3 4 5 5.3 5.6 6.15 8 9 10 Time in Seconds Per Unit Hz Per Unit kW 1 Page 8 of 12 Technol Econ Smart Grids Sustain Energy (2019) 4:1 Fig. 9 Single 11 kV UK feeder with diesel generator, wind farm and energy storage battery dropped down to 150 kW when the short circuit is cleared at half of the power produced by the wind turbines is consumed t = 5.3 s. After the clearance of the short circuit, P is sup- by the batteries. mec posed to return back to 550 kW, but with low inertia, it deliv- During the fault period, P dropped down from 1120 kW mec ered maximum capable power. Speed and frequency oscillated since the SM is only feeding power to the fault point. Due to till t = 6.15 s and then finally lost synchronism. With a low the low inertia, P does not stop instantly and reached 0 at mec inertia system, the remaining SM was unable to regain syn- t = 5.3 s, as shown in Fig. 10. The frequency and the speed chronization back after the fault was cleared. The simulations have increased because of the amount of electrical active pow- performed on this configuration showed that the critical fault er that the generator exports is not the same as the amount of clearing time is 0.05 s. Moreover, the single SM contributed a mechanical power it imports. P started to increase after the mec fault current of 485 amps at the onset of the fault and de- clearance of the fault and delivered its maximum power be- creased to 370 amps when the fault is cleared. tween t =5.7 s to t = 6.3 s. At t = 6.3 s, P started to decrease mec and settled to 1120 kW at t =10 s. Impacts of Energy Storage Battery on Low Inertia It is obvious that when the fault is cleared, the impact of Network power flow from DG is less compared to the case where ESB are not used. Batteries are consuming the power produced by Energy storage battery (4x140kW) is connected besides the the DG. At t = 6 s the frequency reached 46.4 Hz with an wind farm in charging mode as shown in Fig. 9.More than initial dip of output power reaching zero. With the slow Fig. 10 Transient response of low Pmec DG Output Power Battery Power Consumption inertia network with ESB -560 4 5 5.3 5.7 6.3 6.85 8 9 10 1.1 Speed 1.05 0.94 4 5 5.3 5.7 6.3 6.85 8 9 10 Frequency 46.3 4 5 5.3 5.7 6.3 6.85 8 9 10 1.5 Voltage 4 5 5.3 5.7 6.3 6.85 8 9 10 Time in Seconds Per Unit Hz Per Unit kW Technol Econ Smart Grids Sustain Energy (2019) 4:1 Page 9 of 12 1 (a) (b) 3,000 Pmec DG Output Power Battery Power 2,500 2,500 Pmec DG Output Power Battery Power Consumption 2,000 2,000 1,000 1,000 -500 -500 3 4 5 5.3 5.6 5.9 7 8 9 10 3 4 5 5.3 5.6 5.9 6.7 8 9 10 Speed 0.75 Speed 0.8 0.5 0.3 3 4 5 5.3 5.6 5.9 6.7 8 9 10 3 4 5 5.3 5.6 5.9 7 8 9 10 Frequency Frequency 3 4 5 5.3 5.6 5.9 7 8 9 10 3 4 5 5.3 5.6 5.9 6.7 8 9 10 1.5 Voltage 1.5 Voltage 0 0 3 4 5 5.3 5.6 5.9 7 8 9 10 3 4 5 5.3 5.6 5.9 6.7 8 9 10 Time in Seconds Time in Seconds Fig. 11 Transient response with DG disconnected hunting frequency, the SM is able to recover back. Once the Transient Response by Switching the Battery fault is cleared, the machine speed begins to stabilize and the from G2B into B2G machine begins to export more active power (kW) compared to the pre-fault condition. This is because the machine has In this case, the wind DG is disconnected intentionally at the stored up kinetic energy in the rotor during the fault. end of the fault duration (t = 5.3 s) and the battery is switched from charging to discharging mode as shown in Fig. 11b. At t = 5.4 s, the battery delivered 500 kW to the system instead of Transient Analysis with Disconnection of Wind consumption. Switching of the battery from −500 to 500 kW Turbine has injected 1000 kW to the system. The batteries have re- duced the power demand from the system by 1000 kW that In this case, the wind DG is disconnected intentionally at the would increase the kinetic energy as per Eq. (3). The frequen- end of short circuit time i.e. t = 5.3 s, to investigate the stability cy dropped down to 43.5 Hz at t = 6.3 s but recovered back to of the system. Figure 11ashows theobtained results. When 50 Hz at t = 7.2 s. With minor oscillation, the system is able to the DG is disconnected at t = 5.3 s, the SM takes the share of recover and become stable at t =10 s. DG and P increased to 1700 kW at t = 5.5 s. The rotor Simulation is also performed to find the optimal power of mec speed decreased and with low kinetic energy as well as active ESB required in the G2B mode to keep the system synchro- power reserve, it did not recover back and the SM is nize after disturbance. ESB requires 40 kWof power in charg- desynchronized. In the previous case, the system remained ing mode to keep the system stable and synchronized as intact after the transient time because the battery load was depicted in Table 3. System stability and reliability is the bi- incorporated. In the next case, the battery will be switched to product of ESB, otherwise, the main function of ESB is to the discharge mode i.e. battery to grid mode to analyze the store excessive power and give back to the grid especially impact of the battery on the stability. during peak hour load demand. Table 3 Optimal ESB power to ESB power (kW) Synchronous machine Remarks keep the system stable status after the fault 10 De-synchronized ESB is connected adjacent to 20 De-synchronized Wind farm/Solar park in the G2B mode 30 De-synchronized 40 Synchronised Per Unit Hz Per Unit kW Per Unit Hz Per Unit kW 1 Page 10 of 12 Technol Econ Smart Grids Sustain Energy (2019) 4:1 (a) (b) 2500 Pmec Total DG Output Power 0 Pmec Total DG Output Power Battery Power Consumption 3 4 5 5.3 6 7 8 9 10 11 12 -1000 3 4 5 5.3 5.6 5.9 7 8 9 10 Speed 0.75 1.05 0.5 Speed 3 4 5 5.3 6 7 8 9 10 11 12 0.92 3 4 5 5.3 5.6 5.9 7 8 9 10 Frequency Frequency 3 4 5 5.3 6 7 8 9 10 11 12 46.2 3 4 5 5.3 5.6 5.9 7 8 9 10 1.5 1.5 Voltage Voltage 0 0 3 4 5 5.3 5.6 5.9 7 8 9 10 3 4 5 5.3 6 7 8 9 10 11 12 Time in Seconds Time in Seconds Fig. 12 Transient response with Roof top DGs Roof-Top DG in Low Inertia System consequently, P of the synchronous generator has also in- mec creased to keep the generation and demand in balance. Thus, In this case, a 140 kW capacity of DG is connected at the LV the kinetic energy has apparently increased. Less oscillation is side of every 500kVA distribution transformer. This resembles observed after the fault clearance as shown in Fig. 12b. The domestic rooftop DG. The total power consumption is sup- system is recovered back to steady state condition at t=10 s. plied by domestic renewable DG, which makes the power The frequency is below 47 Hz for 0.5 s between t=5.84 to t = needed from the 500 kVA transformer negligible. Only one 6.34 s. This result shows that with DG and batteries, the fre- SM is connected in the system. With low inertia, the system is quency fulfilled the frequency engineering recommendation unable to stabilize after the fault clearance and desynchronized given in Table 2. The batteries were able to recover back the at t = 7 s as shown in Fig. 12a. system and eliminated the need to disconnect the DG which will lower the system frequency and deteriorates the conditions further. Impact of Roof-Top DG with Energy Storage Battery Batteries in charging mode have increased the kinetic en- ergy of SM as it has to increase its power to satisfy the de- A simulation is performed for the case when rooftop DG and mand. This property is useful for SM to resynchronize partic- batteries are connected to the system. At every LV side of the ularly in transient state conditions as illustrated in Table 4. 500 kVA transformers, 140 kW batteries are connected in charg- Simulation is performed for stability to find the total opti- ing mode. Batteries consume 1000 kW extra power from the mal power required by ESB in G2B mode. Rooftop DGs system. This has increased the electrical demand and Table 4 Comparison of scenarios Scenario 1 Base Case 2 SM without DG 2 SM’s have enough inertia to recover back after the fault is removed. Scenario 2 Two SM and Inverter based DG Oscillation is increased after the fault, as two ways of power flow, the share of power production from SM is low resulting in less kinetic energy, but the system has recovered back. Scenario 3 One SM with Inverter based DG The share of power production from SM is less and less kinetic energy. With low inertia, SM was unable to re-synchronize after the fault is removed. Scenario 4 ESB is connected besides DG The share of power flow from DG on the feeder is less as ESB is consuming the power produced by DG. SM is able to recover back because more power production that results in more kinetic energy. Scenario 5 DG is disconnected With Low active power reserve, SM does not recover back and ESB is still in charging mode. Scenario 6 ESB switched from G2B into B2G mode Switching ESB into B2G mode has injected real power into the system thus virtually injected inertia to the system. SM is able to resynchronize. Scenario 7 Roof top DGs with one SM Share of power production from SM is less so less kinetic energy. With low inertia, SM was unable to resynchronize after the fault is removed. Scenario 8 ESB connected to Roof top DGs ESB in charging mode has reduced the share of power from DGs and SM has gained kinetic energy by increasing power production. SM is recovered back. Per Unit Hz Per Unit kW Per Unit Hz Per Unit kW Technol Econ Smart Grids Sustain Energy (2019) 4:1 Page 11 of 12 1 Table 5 Optimal ESB power to keep the system stable with rooftop DGs ESB power (kW) Total ESB power (kW) Synchronous machine Remarks in individual LV feeder of seven LV feeder status after the fault 14 98 Synchronized ESB is connected at every 11/0.4 7 49 Synchronized kV transformer LV side in the G2B mode 2.8 19.6 Synchronised 1.6 11.2 Synchronised 0.8 5.6 De-synchronized Open Access This article is distributed under the terms of the Creative required 11.2 kW power of ESB to keep the system stable as Commons Attribution 4.0 International License (http:// shown in Table 5. This power is much less than the power creativecommons.org/licenses/by/4.0/), which permits unrestricted use, required when wind farm with ESB is connected to one end of distribution, and reproduction in any medium, provided you give appro- the feeder as described in Table 3. priate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. References Conclusion 1. Kundur P, Paserba J, Ajjarapu V et al (2004) Definition and classi- In this research, the impact of the energy storage battery on the fication of power system stability. IEEE/CIGRE joint task force on transient stability of the synchronous generator is analyzed. stability terms and definitions. IEEE Trans Power Syst 19(3):1387– Firstly, the transient stability of UK 11 kV system with two 1401 2. Skopik F, Smith P (2015) Smart grid security innovation solutions SM is analyzed. With enough inertia and kinetic energy, SM is st for a modernized grid. 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