Static Voltage and Frequency Regulation of Standalone Wind Energy Conversion System

Transcription

1 Indan Journal of Scence and Technology, Vol 9(29), DOI: /jst/2016/v929/79403, August 2016 ISSN (Prnt) : ISSN (Onlne) : Statc Voltage and Frequency Regulaton of Standalone Wnd Energy Converson System Prakash Kumar and D. K. Palwala Rajasthan Techncal Unversty, Kota , Rajasthan, Inda; prakash.ucertu@gmal.com, dheerajpalwala@gmal.com Abstract Background/Objectve: Ths paper presents regulaton of Self Excted Inducton Generator (SEIG) as asynchronous generator n standalone mode for wnd energy converson system (WECS). Methods/Statstcal Analyss: The proposed controller conssts of Voltage Source Converter (VSC) havng bdrectonal actve & reactve power flow control, ntegrated wth Battery Storage Unt (BSU), connected n shunt. It compensates for varable reactve power wth varable actve power. Ratng of nducton machne has been obtaned by short crcut test, open crcut test and synchronous speed test. Fndngs: SEIG have relatve advantage over conventonal synchronous generator. However, vo1ltage and frequency regulaton are among prme challenges n ts practcal applcaton as standalone generator. Voltage and frequency of SEIG depends upon speed of rotor, shunt capactor and load. A sutable control scheme needs to be developed to ensure mnmum varaton n voltage and frequency for varable nput and electrcal load. Voltage regulaton has been acheved by adjustng reactve power provded by statc compensator consstng of nductor a VSC and dc bus capactor. Concluson/Improvement: The smulaton results show that voltage and frequency of SEIG-WECS have neglgble varaton for resstve, reactve, balanced, unbalanced and nonlnear load under varyng wnd speed and consumer load. It elmnates harmonc contents and balances the connected electrcal load. Keywords: Battery Storage Unt, Standalone Generator, Self Excted Asynchronous Generator, Voltage Source Converter, Voltage and Frequency Regulaton 1. Introducton Squrrel cage asynchronous motor can be used to make SEIG by connectng approprate value of capactor at ts stator termnals 1 and drvng the rotor by sutable speed prme mover. It has relatve advantages over tradtonal synchronous generators n terms of rugged constructon, lower mantenance, smple operaton and brushless constructon 2. However, voltage and frequency regulaton are among nherent obstacle n ts practcal applcaton as asynchronous generator n varable speed drve applcaton. Voltage and frequency of SEIG, count on rotor speed, connected load and power factor (p.f.) of load 3. Synchronous generators have better effcency n fxed speed drve applcatons 4, but nducton generators are preferred for varable drve applcatons. Lterature ncludes applcaton of nducton machne as doubly fed nducton generator 5,6, SEIG 7 and assocated controllers 8,9 to work under stochastc nature of wnd energy 10. Voltage and frequency of generator can be regulated by connectng fxed shunt capactors and electroncally swtched nductances3. Seres connected capactors provde addtonal VARs wth ncreasng load whle shunt capactors supply reactve power demand of SEIG 11. Seres connected capactors can generate sub-synchronous resonance whereas the parallel connected capactors can cause poor voltage regulaton for constant 12,13 and varable 14,15 load applcatons. The output frequency of SEIG-Wnd Energy Converson System (WECS) n standalone mode can be regulated by asynchronous ac-dc-ac lnk power converter ncorporatng matrx converter 16. Controlled Pulse Wdth Modulated (PWM) gate sgnal s connected tovsc wth BSU can be used to regulate voltage and frequency 17. Ths Author for correspondence

2 Statc Voltage and Frequency Regulaton of Standalone Wnd Energy Converson System scheme presents low frequency harmoncs at low rotor speeds. VSC based controller can be used wth varable speed prme mover prme mover to meet dynamcally varyng real and reactve 18 load power demand connected by the sde of SEIG stator to regulate fxed supply frequency. A seres capactor has been used to reduce usage of statc compensator (STATCOM) for regulatng voltage n3-phase SEIG 19. SEIG conned to varable speed prme mover and a VSC based controller can mantan actve power at generator end for reducng burden of suppled reactve power to load 20. Generaton from SEIG s substantally depends on p.f. of connected load due to ts dependence on reactve current 21. Load control unt doesn t balance large reactve power requrement varaton. STATCOM balances the phase current and thus works as a load balancer 22. The controller regulates actve and reactve power to regulate frequency and voltage respectvely. STATCOM can be used to work as reactve power compensator. It can also regulate voltage and frequency wth ncrease n load current and sgnfcantly elmnateharmoncs 23. Standalone WECS usng a squrrel cage SEIG could be one of the attractve optons for small power generaton system. A bdrectonal VS-PWM wth a capactor and swtched resstor at dc bus provdes a reference voltage and frequency for the nducton generator 24,25. The converter can compensate for reactve power to regulate the termnal voltage 26,27. However, t can only absorb the actve power. Ths paper presents bdrectonal real and reactve power flow management scheme to govern system voltage and frequency for varable generator speed drve and connected load. Proposed bdrectonal converter compensates varable reactve power to regulate the termnal voltage. It absorbs actve power durng lght load or hgher wnd speed and delvers the actve power durng hgh load or lower wnd speed wth BSU for SEIG-WECS n order to obtan frequency and voltage regulaton for varable mechancal power as well as varyng load condtons. 2. System Descrpton System confguraton and control scheme of SEIG- WECS has been shown n Fgure 1 and Fgure 2 respectvely. The wnd turbne has been connected to SEIG rotor through gearng and couplng arrangement. Stator sde of SEIG conssts of delta connected capactor bank, to meet reactve power demand, n parallel wth VSC and the consumer load. Capactor bank provdes necessary exctaton to generate electrcal power at rated voltage under no load condton. VSC controller meets addtonal exctaton requred for regulatng voltage. VSC absorbs or njects the actve power dependng upon frequency error (e f ). VSC absorbs actve power conserved n BSU, when frequency of the system s more than reference value. Conversely, VSC njects actve power when system frequency decreases due to ncrease n load or decrease n wnd speed. Reactve power s also compensated from VSC as a functon of termnal voltage error. As the termnal voltage drops, the termnal voltage error makes the VSC converter to nject reactve power. Thus VSC provdes real and reactve power flow n both rectfer and nverter mode. DC bus capactor s connected n parallel wth BSU, whch suppresses the rpples appearng at the DC sde of VSC. 3. Mathematcal Modelng The proposed control scheme has been shown n Fgure 2. Three phase reference current ( a, b and c ) conssts of two components vz. drect axs or actve component ( ad, V wnd Fgure 1. Frequency PI controller f ref f sys e f 3-phase PLL frequency measurement V abc SEIG } v a } v b C C C a b c Capactor Bank lb ly lr Consumer Load sc sb sa S 1 L f, R f Schematc dagram of 3-Φ SEIG-WECS. vd 2 nd order LPF Actve Power Component Ampltude Calculaton (nstantaneous value) p vd Fgure 2. MATLAB based Controller subsystem of SEIGwnd energy converson system S 2 VSC S 3 S 4 Calculaton of reference current (nphase components) V a V b V c Peak valve & unt templet generator Vm,ref V m e v S 5 C dc S 6 BSU V dc Hysteress current controller Quadrature reactve components of reference source current AC Voltage controller S 1 S 2 S 3 S 4 S 5 S 6 a b + + aq c bq cq W a W b W c Quasrature unt vector calculaton mq Battery Bank 2 Vol 9 (29) August Indan Journal of Scence and Technology

3 Prakash Kumar and D. K. Palwala bd and cd ) to control frequency and quadrature axs or reactve component ( aq, bq and cq ) to control termnal voltage. Control scheme has been desgned to regulate termnal voltage, frequency, reactve power retenton, load reconclaton, harmonc eradcaton and battery current. Voltage error e v has been calculated by comparng peak voltage ampltude (v m ) and ampltude of reference AC voltage (v m,ref ). Error e v s processed through voltage PI controller to obtan quadrature current reference ( q ) as reactve component of reference current. Unt templates (u a, u b and u c ) and quadrature unt template (w a, w b and w c ) are computed by three phase AC lne voltages (v a ) and ther peak ampltude (v m ). Instantaneous magntude of frequency (f) s calculated by phase locked loop control. Frequency error (e f ) s estmated by subtractng estmated frequency (f) from reference frequency (f ref ). Frequency error s processed through frequency PI controller. Maxmum generator current (I G,max. ) s calculated by dvson of rated power output of SEIG to rated termnal voltage. In order to generate real part of reference current component ( d ), obtaned outcome of frequency PI controller s subtracted from maxmum generator current. The product of output references ( d and q ) and unt templates s algebracally summed to attan reference source currents ( a, b and c ). Obtaned reference source current s compared wth actual lne currents ( a, b and c ) and passed n hysteress current controller to get PWM swtchng pulses to feed VSC. 3.1 Unt Template Generator The peak ampltude of voltages (V m ) s calculated as V = {(2 / 3)( v + v + v )} / (1) m a b c Where, v a are termnal lne voltages of SEIG. The unt vectors n phase wth v a are computed as ua = va Vm ub = vb V (2) m u = v V c c m Unt vector n quadrature wth v a s: w = ( u + u ) 3 a b c w = 3u 2 + ( u u ) 2 3 (3) b a b c w = 3u 2 + ( u u ) 2 3 c a b c AC voltage error at n th samplng tme s calculated as: e ( n) = V ( n) V ( n) (4) v m, ref m Where, V m,ref (n) s reference peak ampltude of AC voltage. 3.2 Reference Source Current The outcome of voltage PI controller to regulate AC voltage at stator termnals of SEIG at n th samplng tme s: ( n) = ( n 1) + K [ e ( n) e ( n 1)] + K e ( n) (5) q q P v v I v Where, K P and K I are gan constants of voltage PI controller; e v (n) and e v (n-1) are AC peak voltage errors at n th and (n-1) th samplng tme; q (n-1) s magntude of reference source current at (n-1) th samplng tme. Quadrature reactve component of the reference source current s estmated by: = w aq q a = w bq q b = w cq q c Frequency error e f (n) s gven by: (6) e ( n) = f( n) f ( n) (7) f Where, f(n) and f ref (n) are system frequency and the reference frequency respectvely at n th samplng tme. Frequency error s passed by frequency PI controller and output at n th samplng tme t s gven by: ( n) = ( n 1) + K [ e ( n) e ( n 1)] + K e ( n) (8) d d P f f I f Where, K P and K I are gan constants of the frequency PI controller; e f (n) and e f (n-1) are frequency errors at the n th and (n-1) th samplng tme; d (n-1) s output of frequency PI controller at (n-1) th samplng tme. Actve component of reference s obtaned by subtractng output of frequency PI controller from maxmum generator current.e. ref I ( n) = I ( n) I ( n) (9) d G, Max. d The nstantaneous n phase actve reference source current component s calculated by: ad = u d a bd = u d b (10) = u cd d c Vol 9 (29) August Indan Journal of Scence and Technology 3

4 Statc Voltage and Frequency Regulaton of Standalone Wnd Energy Converson System Thus from equaton (6) and equaton (10), reference source current can be obtaned as: = + a ad aq = + b bd bq = + c cd cq (11) Requred PWM pulses for VSC s calculated by comparng reference source current and obtaned load current; and passng error n hysteress current controller. = a, error a a = b, error b b = c, error c c (12) Hysteress current controller generates PWM pulses for VSC usng these error values of current. 4. Modelng of Wnd Turbne Generator WTGS conssts of asynchronous SEIG as generator unt. The synchronous speed test data for ar gap voltage and magnetzng current s expermentally estmated for smulaton of saturaton n SEIG. MATLAB based model of SEIG-wnd energy converson system has been consdered. 7.5 kw, 400 V, 50 Hz, delta connected nducton machne has been used as SEIG. Capactor bank of 36 µf per phase connected n delta has been used at stator end of machne for obtanng a 6 kw power output at rated voltage and rated speed. The power output of SEIG-WECS can be gven as: P = (13) 3 0.5ρ AKPvwnd Where, ρ s specfc densty of ar, A s swept area of the blades, K P s power coeffcent and v wnd s wnd speed n m/s. Power coeffcent K p s the functon of tp speed rato (TSR, λ). For a constant ptch angle (β ), t may be gven as: K = K {( K / λ ) K β K } e + K λ (14) P Where, ( K5 / λ ) / λ [1/ ( λ K β)} { K / ( β 1)} and β = = (15) For the constants K 1 = , K 2 = 116, K 3 = 0.4, K 4 = 5, K 5 = 21, K 6 = , K 7 = 0.08, K 8 = 0.035; polynomal relatonshp curve for K P and λ at fxed degree ptch angle (β = 0), provdes utmost value of K P as 0.48 for a maxmum TSR. Ths gves hghest mechancal power avalable for wnd turbne. 5. Results and Dscusson The SEIG-WECS wth energy storage system for varable electrcal load and nput wnd speed has been smulated and results have been analyzed. Fgure 3 shows the SEIG- WECS supplyng balanced 3-phase resstance load. Transent waveform from n fgures 3 to 6 ncludes SEIG lne voltage (V abc ), lne current (I abc ), load current (I L,abc ), VSC current (I s,abc ), peak termnal voltage (V m ), frequency (f), wnd velocty (v wnd ), battery current (I BSU ), DC bus voltage (V dc ) and generator, load & battery power (P). The SEIG has been connected wth exctaton capactor to generate 6 kw at rated speed. At tme t=1.55 sec., a resstve load of 4 kw has been connected. On ncorporatng resstance load, voltage and frequency reman constant and current supplyng to BSU reduces VSC current subsequently. At tme t=1.7 sec., an addtonal resstve load of 4 kw has been connected.e. total load connected to SEIG- WECS system becomes 8 kw. Then after an addtonal 2 kw load s suppled from BSU and battery bank current becomes negatve. The power curve relaton shows that the generated power remans constant at 6 kw whle power to battery bank becomes postve when load s less than 6 kw and become negatve when load power s more than 6 kw. Fgure 4 shows transent waveform of SEIG-WECS for unbalanced reactve load. A 3 kw, 0.8 laggng p.f. balanced reactve load has been appled at t=1.55 sec. At t=1.65 sec., a sngle phase resstance load s added and total load becomes unbalanced. Applcaton of reactve unbalanced load, the voltage and frequency remans constant. The SEIG Fgure 3. Transent waveform of SEIG-WECS for applcaton of 8kW resstve load applcaton. 4 Vol 9 (29) August Indan Journal of Scence and Technology

5 Prakash Kumar and D. K. Palwala currents also reman balanced. VSC compensates reactve power demand of load. It also works as load balancer. Fgure 5 represents transent waveform for nonlnear dode rectfer load wth 100 ohms and 100 mh at DC sde. Another 3-phase balanced resstance load of 4 kw has been added at tme t=1.7 sec. Wth applcaton of nonlnear load, voltage and frequency of SEIG-WECS remans constant, as obtaned n case of lnear and unbalanced load applcaton. SEIG voltage and current remans snusodal and VSC elmnates the harmonc components generated by the nonlnear load. The generated output power remans constant and the addtonal actve power demand s suppled form BSU. The harmonc analyss shows that the total harmonc dstorton (THD) of SEIG voltage and current s obtaned as 1.6% and 2.2% respectvely whle the load current THD s 26.2%. Fgure 6 revels performance of system under varable wnd speed and constant resstve load. At tme t=1.55 sec., balanced 3-phase resstve load of 4 kw s appled. At tme t=1.65 sec., wnd velocty s changed from 10 m/s Fgure 6. Transent waveform of SEIG-WECS for varable wnd speed and resstve load. to 12 m/s. Increase n wnd speed ncreases the power output of the generator whle voltage and frequency remans constant. Generator current also ncreases. At tme t=1.85 sec. wnd speed reduces from 12 m/s to 8 m/s, whch s below the base speed. Reducton n wnd speed reduces power output of SEIG. The generator current also reduces accordngly. At tme t=2.1 sec., load s fully removed. Voltage and frequency remans constant wth change n wnd speed and wth change n consumer load. BSU absorbs addtonal actve power and delvers when load demand s ncreased from rated generaton. 6. Concluson Fgure 4. Transent waveform of SEIG-WECS for applcaton of unbalanced reactve load. The applcaton of AWECS as standalone system has been desgned and analyzed under varyng consumer load and wnd speed. A VSC based voltage and frequency regulaton has been proposed and valdated wth smulaton for WECS wth SEIG n standalone mode for frequency control ncorporatng BSU. Proposed controller has bdrectonal power flow capablty of real and reactve power compensaton. It regulates voltage and frequency of SEIG-WECS unt under varable electrcal load and wnd speed. VSC works as voltage regulator, harmoncs remover and load equalzer for varyng consumer load. Thus for the proposed system the voltage and frequency remans constant for resstve, reactve, balanced, unbalanced, nonlnear loads under varyng wnd speed and consumer load. Hence the system wll be benefcal for feedng solated loads. 7. Reference Fgure 5. Transent waveform of SEIG-WECS for applcaton of nonlnear load. 1. Chan TF. Capactance requrements of self-excted nducton generators. IEEE Transacton on Energy Converson. 1993; 8(2): Vol 9 (29) August Indan Journal of Scence and Technology 5

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