Microgrid: Islanding and Grid Connected Mode of Operation Based on Unified Control Strategy

Transcription

1 International Journal of Eletrial Eletronis omputers & Mehanial Engineering (IJEEM) ISSN: Volume 2 Issue 6 ǁ De IJEEM journal of Eletrial Engineering (ijeem-jee) Mirogrid: Islanding and Grid onneted Mode of Operation ased on Unified ontrol Strategy Radha haitanya 1, Shaik Mahu Jani 2 1 M.Teh Student, Priyadarshini Institute of Tehnology & Management, Guntur 2 ssistant Professor, Priyadarshini Institute of Tehnology & Management, Guntur radhahaitanya229@gmail.om strat This projet presents a unified ontrol strategy that enales oth islanded and grid-tied operations of three-phase inverter indistriuted generation, with no need for swithing etween two orresponding ontrollers or ritial islanding detetion. The proposed ontrol strategy omposes of an inner indutor urrent loop, and a novel voltage loop in the synhronous referene frame. The inverter is regulated as a urrent soure just y the inner indutor urrent loop in grid-tied operation, and the voltage ontroller is automatially ativated to regulate the load voltage upon the ourrene of islanding. Furthermore, the waveforms of the grid urrent in the grid-tied mode and the load voltage in the islanding mode are distorted under nonlinear loal load with the onventional strategy. Finally, the effetiveness of the proposed ontrol strategy is validated y the simulation and experimental results. Index Terms Distriuted generation (DG), islanding mode, load urrent, seamless transfer, three-phase inverter, unified ontrol strategy. I. INTRODUTION Distriuted generation (DG) is emerging as a viale alternative when renewale or nononventional energy resoures are availale, suh as wind turines, photovoltai arrays, fuel ells, miro turines. Most of these resoures are onneted to the utility through power eletroni interfaing onverters, i.e., three-phase inverter. Moreover, DG is a suitale form to offer high reliale eletrial power supply, as it is ale to operate either in the grid-tied mode or in the islanded mode. In the grid-tied operation, DG deliveries power to the utility and the loal ritial load. Upon the ourrene of utility outage, the islanding is formed. Under this irumstane, the DG must e tripped and ease to energize the portion of utility as soon as possile. However, in order to improve the power reliaility of some loal ritial ontinue to feed the loal ritial load. The load voltage is key issue of these two operation modes, eause it is fixed y the utility in the grid-tied operation, and formed y the DG in the islanded mode, respetively. Therefore, upon the happening of islanding, DG must take over the load voltage as soon as possile, in order to redue the transient in the load voltage. nd this issue rings a hallenge for the operation of DG. Droop-ased ontrol is used widely for the power sharing of parallel inverters, whih is alled as voltage mode ontrol in this paper, and it an also e applied to DG to realize the power sharing etween DG and utility in the grid-tied mode. In this situation, the inverter is always regulated as a voltage soure y the voltage loop, and the quality of the load voltage an e guaranteed during the transition of operation modes. However, the limitation of this approah is that the dynami performane is poor, eause the andwidth of the external power loop, realizing droop ontrol, is muh lower than the voltage loop. Moreover, the grid urrent is not ontrolled diretly, and the issue of the inrush grid urrent during the transition from the islanded mode to the grid-tied mode always exists, even though phase-loked loop (PLL) and the virtual indutane are adopted. The inverter is ontrolled as a urrent soure y one sets of a ontroller in the grid-tied mode, while as a voltage soure y the other sets of ontroller in the islanded mode. s the voltage loop or urrent loop is just utilized in this approah, a nie dynami performane an e ahieved. esides, the output urrent is diretly ontrolled in the grid-tied mode, and the inrush grid urrent is almost eliminated. In the hyrid voltage and urrent mode ontrol, there is a need to swith the ontroller when the operation mode

2 of DG is hanged. During the interval from the ourrene of utility outage and swithing the ontroller to voltage mode, the load voltage is neither fixed y the utility, nor regulated y the DG, and the length of the time interval is determined y the islanding detetion proess. Therefore, the main issue in this approah is that it makes the quality of the load voltage heavily reliant on the speed and auray of the islanding detetion method. When the nonlinear loal load is fed, the harmoni omponent of the load urrent will fully flow into the utility. single-phase DG, whih injets harmoni urrent into the utility for mitigating the harmoni omponent of the grid urrent, the voltage mode ontrol is enhaned y ontrolling the DG to emulate a resistane at the harmoni frequeny, and then the harmoni urrent flowing into utility an e mitigated. In the islanded mode, the nonlinear load may distort the load voltage, many ontrol shemes have een proposed to improve the quality of the load voltage, inluding a multi loop ontrol method, resonant ontrollers, sliding mode ontrol. However, existing ontrol strategies, dealing with the nonlinear loal load in DG, mainly fous on either the quality of the grid urrent in the gridtied mode or the one of the load voltage in the islanded mode, and improving oth of them y a unified ontrol strategy is seldom. Third, the proposed ontrol strategy is enhaned y introduing a unified load urrent feed forward, in order to deal with the issue aused y the nonlinear loal load; this sheme is implemented y adding the load urrent into the referene of the inner urrent loop. In the grid-tied mode, the DG injets harmoni urrent into the grid for ompensating the harmoni omponent of the grid urrent, and thus, the harmoni omponent of the grid urrent will e mitigated. Moreover, the enefit of the proposed load urrent feed forward an e extended into the islanded operation mode, due to the improved quality of the load voltage. II. PROPOSED ONTROL STRTEGY. Power Stage This paper presents a unified ontrol strategy for a three phase inverter in DG to operate in oth islanded and grid-tied modes. The shemati diagram of the DG ased on the proposed ontrol strategy is shown y Fig. 1. The DG is equipped with a three-phase interfae inverter terminated with a L filter. The primary energy is onverted to the eletrial energy, whih is then onverted to d y the front-end power onverter, and the output d voltage is regulated y it. In the a side of inverter, the loal ritial load is onneted diretly. The inverter transfer swith Si is ontrolled y the DG, the utility protetion swith Su is governed y the utility. When the utility is normal, oth swithes Si and Su are ON, and the DG in the grid-tied mode injets power to the utility. When the utility is in fault, the swith Su is tripped y the utility instantly, and then the islanding is formed. fter the islanding has een onfirmed y the DG with the islanding detetion sheme, the swith Si is disonneted, and the DG is transferred from the grid-tiedmode to the islanded mode. When the utility is restored, the DG should e resynhronized with the utility first, and then the swith Si is turned ON to onnet the DG with the grid.. asi Idea With the hyrid voltage and urrent mode ontrol, the inverter is ontrolled as a urrent soure to generate the referene power PDG + jqdg in the grid-tied mode. nd its output power PDG + jqdg should e the sum of the power injeted to the grid Pg + jqg and the load demand Pload +jqload, whih an e expressed as follows y assuming that the load is represented as a parallel RL iruit: Pload =(3/2) ( V 2mR) (1) Qload =(3/2) V 2m(1/ωL ω) (2) In (1) and (2), Vm and ω represent the amplitude and frequeny of the load voltage, respetively. When the nonlinear loal load is fed, it an still e equivalent to the parallel RL iruit y just taking aount of the fundamental omponent. During the time interval from the instant of islanding happening to the moment of swithing the ontrol system to voltage mode ontrol, the load voltage is neither fixed y the utility nor regulated y the inverter, so the load voltage may drift from the normal range. nd this phenomenon an e explained as elow y the power relationship. During this time interval, the inverter is still ontrolled as a urrent soure, and

3 its output power is kept almost unhanged. However, the power injeted to utility dereases to zero rapidly, and then the power onsumed y the load will e imposed to the output power of DG. If oth ative power Pg and reative power Qg injeted into the grid are positive in the gridtiedmode, then Pload and Qload will inrease after the islanding happens, and the amplitude and frequeny of the load voltage will rise and drop, respetively, aording to (1) and (2). III. OPERTION PRINIPLE OF DG The operation priniple of DG with the proposed unified ontrol strategy will e illustrated in detail in this setion, and there are in total four states for the DG, inluding the grid-tied mode, transition from the grid-tied mode to the islanded mode, the islanded mode, and transition from the islanded mode to the grid-tied mode.. Grid-Tied Mode When the utility is normal, the DG is ontrolled as a urrent soure to supply given ative and reative power y the indutor urrent loop, the ative and reative power an e given y the urrent referene of D- and Q-axis independently. First, the phase angle of the utility voltage is otained y the PLL, whih onsists of a Park transformation expressed y, a PI ompensator, a limiter, and an integrator. Seond, the filter indutor urrent, whih has een transformed into SRF y the Park transformation, is fed ak and ompared with the indutor urrent referene ilref dq, and the indutor urrent is regulated to trak the referene ilref dq y the PI ompensator GI. The referene of the indutor urrent loop ilref dq seems omplex and it is explained as elow. It is assumed that the utility is stiff, and the three-phase utility voltage. y the Park transformation, the utility voltage is transformed into the SRF, whih is shown as vgd = Vg os(θ θ) vgq=vgsin(θ θ) (5) vgq is regulated to zero y the PLL, so vgd equals the magnitude of the utility voltage Vg. s the filter apaitor voltage equals the utility voltage in the gird-tied mode, vd equals the magnitude of the utility voltage Vg, and vq equals zero, too.. Transition from the Grid-Tied Mode to the Islanded Mode When the utility swith Su opens, the islanding happens, and the amplitude and frequeny of the load voltage will drift due to the ative and reative power mismath etween the DG and the load demand. The transition an e divided into two time interval. The first time interval is from the instant of turning off Su to the instant of turning off Si when islanding is onfirmed. The seond time interval egins from the instant of turning off inverter swith Si. When the islanding happens, the loal load must asor the extra power injeted to the grid, as the output power of inverter is not hanged instantaneously. ording to (1), the magnitude of the load voltage Vm will rise with the inrease of Pload. t the same time, the angle frequeny ω should drop, in order to onsume more reative power with (2). Therefore, the result through the power relationship oinides with the previous analysis. The seond time interval of the transition egins from the instant when the swith Si is open after the islanding has een onfirmed y the islanding detetion method. If the swith Si opens, the load voltage va is independent with the grid voltage vga. t the same time, vga will redue to zero theoretially as the swith Su has opened. Then, the input of the ompensator GPLL eomes zero and the angle frequeny is invariale and fixed to the value at the end of the first interval. Under this irumstane, vdq is regulated y the voltage loop, and the inverter is ontrolled to e a voltage soure. With the previous analysis, it an e onluded that the drift of the amplitude and frequeny in the load voltage is restrited in the given range when islanding happens. nd the inverter is transferred from the urrent soure operation mode to the voltage soure operation mode autonomously. In the hyrid voltage and urrent mode ontrol, the time delay of islanding detetion is ritial to the drift of the frequeny and magnitude in the load voltage, eause the drift is worse with the inrease of the delay time. However, this phenomenon is avoided in the proposed ontrol strategy.. Islanded Mode In the islanded mode, swithing Si and Su are oth in OFF state. The PLL annot trak the

4 utility voltage normally, and the angle frequeny is fixed. In this situation, the DG is ontrolled as a voltage soure, eause voltage ompensator GV D and GV Q an regulate the load voltage vdq. The voltage referenes indand Q-axis are Vmax and zero, respetively. nd the magnitude of the load voltage equals to Vmax approximately, whih will e analyzed in Setion IV. onsequently, the ontrol diagram of the threephase inverter in the islanded mode an e simplified. the load urrent illdq is partial referene of the indutor urrent loop. So, if there is disturane in the load urrent, it will e suppressed quikly y the indutor urrent loop, and a stiff load voltage an e ahieved. D. Transition From the Islanded Mode to the Grid-Tied Mode If the utility is restored and the utility swith Su is ON, the DG should e onneted with utility y turning on swith Si. However, several preparation steps should e performed efore turning on swith Si. First, as soon as utility voltage is restored, the PLL will trak the phase of the utility voltage. s a result, the phase angle of the load voltage va will follow the grid voltage vga. If the load voltage va is in phase with the utility voltage, vgd will equal the magnitude of the utility voltage the magnitude of the load voltage Vmax is larger than the utility voltage magnitude Vg, the voltage referene Vref will e hanged to Vg y toggling the seletor S from terminals 1 to 2. s a result, the load voltage will equal to the utility voltage in oth phase and magnitude. IV. NLYSIS ND DESIGN In this setion, the three-phase inverter with the proposed ontrol strategy is analyzed and designed in oth steady state and transient state. In the steady state, the operation points of DG in oth grid-tied and islanded modes are analyzed, and the limiters and referenes are seleted. In the transient state, ompensators in oth indutor urrent loop and the external voltage loop are designed ased on the small-signal model, and the impat of the load urrent feed forward is analyzed as well. 2) Seletion of Referenes and Limiters: In the grid-tied mode, the ative power injeted into the grid Pg is given y the urrent referene Igref d, and it is the upper value of the limiter in D-axis. Therefore, the seletion of Igref d depends on the power rating of the inverter. s a result, the reative power Qg annot e very large, in order to make the magnitude of the load voltage within the normal range in the islanded mode. In the grid-tied mode, Vmax should e larger than the magnitude of the utility voltage Vg. t the same time, the load In order to guarantee that the PLL operates normally in the grid-tied mode, the utility angle frequeny ω should not touh the upper value ωmax or lower value ωmin of the limier in the PLL. esides, the angle frequeny ω is restrited etween ωmax and ωmin in the islanded mode, and it should not drift from the normal value too far. So, ωmax and ωmin are seleted as the maximum and minimum angle frequenies allowed y the utility standard.. Transient State 1) Small-Signal Model of the Power Stage: efore the ompensators in the voltage and urrent loops are designed and the transient performane is analyzed, the three-phase inverter in the DG needs to e modeled. ording to the power stage 1, the d-link voltage Vd is regulated y the front-end onverter in DG. Then, it is assumed that the d voltage Vd is very stiff, and its dynami is not onerned in this paper. Then, it an e found that there are ouplings introdued y the indutor Lf and apaitors f etween D and Q-axes. 2) Design and nalysis of the urrent Loop: The indutor urrent loop should operate normally to regulate the indutor urrent loop in oth islanded and grid-tied modes. In the islanded mode, the small-signal model of the ontrol-to-urrent an e otained aording to (28), whih is shown as Gid1(s)=ˆIL(s)ˆd(s)=Vd2 ss2lff+srlf+1.(29) in the grid-tied mode, the dynami of the apaitor f is ignored due to the stiff utility, and the small-signal model of the ontrol-to-urrent. 3) Design and nalysis of the Voltage Loop: The voltage loop just operates in the islanded mode to regulate the load voltage, and the simplified lok diagram, wheregi (s) andgvi(s) denote the losed-loop transfer funtion of an indutor loop and the impedane of the filter apaitor f, respetively. 4) Impat of Load urrent Feed forward: In, the load urrentˆill is a part of the indutor urrent referene, and the disturane from the load

5 In1 In2 Out1 Va Ia a Va Ia a urrent an e suppressed y the indutor urrent loop diretly. To evaluate the effet of the load urrent feed forward in the islanded mode, the transfer funtion of the output impedane is derived. it an e found that an extra fator [1 Gi (s)] appears in the output impedane with load urrent feed forward, and the magnitude of the output impedane will e redued in the low frequeny range eause the gain of the losedloop transfer funtion Gi (s) loses to unity in the andwidth of the urrent loop. The ode plot of the output impedane of these two onditions, and it an e seen that the magnitude of the output impedane is redued from d to 600 Hz with the load urrent feed forward. onsequently, the quality of the load voltage va will e improved with the load urrent feed forward. In the grid-tied mode, the indutor urrent is regulated y the indutor urrent loop diretly, and the indutor urrent referene is mainly omposed y the urrent referene Igref dq, and the load urrent illdq. If the load urrent is not fed forward, the output urrent iodq of the inverter will e fixed y Igref dq. s a result, the disturane of the load urrent will e fully injeted into the utility, with the load urrent feed forward, the disturane of the load urrent an e ompensated y the inverter, and the transfer funtion from the load urrent to the grid urrent. Vd Va1 Va v + - Ia1 Ia a + + g 1 2 a a reaker1 a a a a Utility GRid LL filter a vi a Grid V-I ritial Load Iaload Ia Vd Iaload12 Disrete, Ts = 5e-005 s sopes Vaload powergui Fig.1 Simulation for proposed sheme Vaload12 V. SIMULTION ND EXPERIMENTL RESULTS. Simulation Results To investigate the feasile of the proposed ontrol strategy, the simulation has een done in PSIM. The power rating of a three-phase inverter is 3kWin the simulation. TheRMS of the rated phase voltage are 115 V, and the voltage referene Vmax is set as 10% higher than the rated value. The rated utility frequeny is 50 Hz, and the upper and the lower values of the limiter in the PLL are given as 0.2 Hz higher and lower than the rated frequeny, respetively. In the grid-tied mode, the dynami performane of the onventional voltage mode ontrol and the proposed unified ontrol strategy is ompared y stepping down the grid urrent referene from 9 to 5. The simulation result of the voltage mode ontrol is shown in Fig.2, and the urrent referene is hanged at the moment of 14 s. It is found that dynami proess lasts until around and the time interval of the dynami proess is less than 5 ms. omparing the simulation results aove, it an e seen that the dynami performane of the proposed unified ontrol strategy is etter than the onventional voltage mode ontrol. During the transition from the gridtied mode to the islanded mode, the proposed unified ontrol strategy is ompared with the hyrid voltage and urrent mode ontrol, and the simulation senario is shown as follows: 1) Initially, the utility is normal, and the DG is onneted with the utility; 2) at 0.5 s, islanding happens; and 3) at 0.52 s, the islanding is onfirmed. Fig. 3, presents the simulate results with the proposed unified ontrol strategy. Initially, the magnitude of grid urrent is 9 and follows the urrent referene Igref dq. The

6 magnitude and frequeny of the load voltage are held y the utility. fter the islanding happens, the amplitude of the load voltage inreases a little to follow the voltage referene Vmax, and the output urrent of DG dereases autonomously to math the load power demand. omparing the simulation results aove, it an e found that the voltage quality is improved deeply y the proposed ontrol strategy in the transition from the grid-tied mode to the islanded mode, and the speed of the islanding detetion is no more ritial. Then, the magnitude of the load voltage is regulated to equal the utility voltage. t the moment of 350 ms, the swith Si is turned on, and the urrent injeted into the grid iga inreases smoothly without huge inrush urrent, and the load voltage is stale during the transition. When DG feeds nonlinear load in the grid-tied mode, It an e seen that with the load urrent feed forward, there is harmoni ompo ent in the indutor urrent ila, and the harmonis omponent in the grid urrent is redued. The THD of the grid urrent under different power of the nonlinear load and different amplitude of the grid fundamental urrent is investigated. It an e found that with the inrease of the load power, the THD of the grid urrent rises, and the THD of the grid urrent is redued with the load urrent feed forward. In the power of the nonlinear load is set at around 600 W and the amplitude of the grid fundamental urrent is hanged. It an e seen that the THD of the grid urrent an e mitigated at different magnitude of the grid fundamental urrent. (a) () Fig.2 onventional model of operation (a) () Fig.3 Proposed mode of operation VI. ONLUSION unified ontrol strategy was proposed for threephase inverter in DG to operate in oth islanded and grid-tied modes, with no need for swithing etween two different ontrol arhitetures or ritial islanding detetion. novel voltage ontroller was presented. It is inativated in the grid-tied mode, and the DG operates as a urrent soure with fast dynami performane. Upon the utility outage, the voltage ontroller an automatially e ativated to regulate the load voltage. Moreover, a novel load urrent feed forward was proposed, and it an improve the waveform quality of oth the grid urrent in the grid-tied mode and the load voltage in the islanded mode. The proposed unified ontrol strategy was verified y the simulation results.

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