A Cacade Multilevel STATCOM for Wind Generation Sytem S. D. Gamini Jayainghe School of Electrical and Electronic Engineering Nanyang Technogical Univerity 5 Nanyang Avenue, Singapore 639798 han34@ntu.edu.g D. M. Vilathgamuwa Senior Member, IEEE School of Electrical and Electronic Engineering Nanyang Technogical Univerity 5 Nanyang Avenue, Singapore 639798 emahinda@ntu.edu.g U. K. Madawala Senior Member, IEEE Department of Electrical & Computer Engineering, Univerity of Auckland, Auckland, New Zealand u.madawala@auckland.ac.nz Abtract--Thi paper preent a novel STATCOM configuration for voltage quality improvement in wind power generation ytem. The propoed STATCOM i formed by cacading two 3-level inverter, bulk inverter and conditioning inverter, through a coupling tranformer. Both inverter are powered by dc-link capacitor and they are charged by a mall amount of active power drawn from the grid. To minimize witching loe, the high power bulk inverter operate at low frequency while low power high frequency conditioning inverter i ued to uppre harmonic content produced by the bulk inverter output. With only 24 witche thi topology can yntheize a nine level inverter, if the dc-link voltage ratio i maintained at 3:1. Modulation and control technique have been developed to meet thi requirement. Reactive power of the STATCOM i controlled to mitigate voltage ag or well caued by udden wind change. Simulation and experimental reult are preented to verify the efficacy of the propoed modulation and control technique ued in the STATCOM. Index Term--Cacade Multilevel converter, STATCOM, Neutral point potential balancing. I. INTRODUCTION The Static Synchronou Compenator (STATCOM) i a Flexible AC Tranmiion Sytem (FACTS) device for reactive power compenation and voltage regulation that ha played an important role in power indutry ince 198. Fat dynamic repone of STATCOM make them uitable for mitigating potential hort term voltage fluctuation that lat for few hundred of milliecond particularly in wind generation ytem. Furthermore, it ha been recognized that STATCOM have advantage over the conventional Static Var Compenator (SVC) a they generate le harmonic current and require a much maller reactor. Conventional STATCOM ue two-level Voltage Source Converter (VSC) for power proceing. A compared to the two-level VSC configuration, multilevel configuration i more uitable for STATCOM realization ince it provide higher ac-ide voltage level and improved waveform with reduced harmonic ditortion. Multilevel converter topologie uch a diode-clamped converter, flying-capacitor converter and cacading converter have been made a STATCOM power proceor o far. The converter ued in thi paper i a combination of diode clamped and cacading converter and i configured by cacading two unit of diode-clamped three level inverter. Thi configuration offer uperior performance and ha been ued in high power motor drive application [1] [2]. V dc V dcx o C 2 C 1 C X2 C X1 a b Bulk inverter c i a i b i c + + + v a v b v c - - - Grid Wind Farm Coupling Tranformer Conditioning inverter Fig. 1. Schematic of the propoed STATCOM (V dc : V dcx = 3:1) PCC Fig. 1 how the chematic of the propoed STATCOM in which the two inverter, Bulk Inverter and Conditioning Inverter are connected at the end of a coupling tranformer econdary winding. The dc-link of each inverter conit of two capacitor. They are charged by active power drawn from the grid. The bulk inverter operate at a low frequency producing quare wave output with required amplitude and phae while high frequency low power conditioning inverter i ued to make the output waveform mooth and cloer to inuoidal in hape. To obtain the maximal ditention for thi cacade-3/3 inverter, conditioning inverter dc-link voltage hould be maintained at one third of the bulk inverter dc-link voltage [2]. Under uch a condition, thee two inverter effectively produce a ninelevel inverter with only 24 witche. The mot important feature of thi converter i that, a the high power bulk inverter operate at low frequency it can be contructed uing 1323
device like GTO, ETO or IGCT to reduce witching loe. On the other hand, the conditioning inverter, which act to compenate low order harmonic produced by the bulk inverter, can be contructed uing more common device like IGBT. Thi paper decribe control and modulation method for the cacaded multilevel converter, dc-link capacitor voltage regulation and in particular STATCOM controller. The STATCOM controller, propoed to mitigate potential voltage ag and well in a Permanent Magnet Synchronou Generator (PMSG) baed wind energy converion ytem, i decribed in Part II. A detailed analyi of the inverter controller i given in Part III. Permanent Magnet Synchronou Generator model and the related controller i decribed in Part IV. Simulation and experiment reult are given in Part V and VI to how the efficacy of the propoed STATCOM. II. STATCOM CONTROLLER Fig. 2 how the controller block diagram of the STATCOM that control the active and reactive power tranfer between the grid and the STATCOM. The meaured grid voltage V d, in ynchronou reference frame, i compared with the reference V d and the error ignal produced i then fed into a PI controller that generate a current reference i q for the inner current controller loop. The STATCOM ac ide current are meaured and tranferred to the ynchronou reference frame in the form of direct and quadrature component. The direct component i related to the real power exchange while quadrature component i related to the reactive power exchange. Therefore, the bulk inverter dc-link voltage, V dc can be maintained by controlling the direct component of the STATCOM current. To regulate the grid voltage, the quadrature component of the STATCOM current i controlled [3]-[4]. The current controller output, v d and v q are the reference voltage for the inverter. With thee reference, amplitude A m and the angle α m of the STATCOM output voltage can be calculated uing (1) and (2). Conequently, the STATCOM inverter controller generate output voltage with required amplitude and phae. The intantaneou angle θ, of the phae voltage vector, i obtained through a Phae Locked Loop (PLL). 1 2 2 A m = vd + vq Vdc (1) 1 v d α = m tan +θ vd (2) Vdc v π co β f = (3) 1 π A m β = co Vdc (4) Fig. 2. Schematic control diagram of the STATCOM connected in a wind generator ytem III. PQ COMPENSATION BASED CONDITIONING INVERTER CONTROLLER Fig. 3 how the bulk inverter voltage variation at point a with repect to point o i.e. V ao. It ha three level called Poitive, Zero and Negative. Angle β i called the firing angle and it determine the amplitude of the fundamental component, v f, according to (3). A the conditioning inverter only mitigate the harmonic produced by the bulk inverter, it doe not change the amplitude of the fundamental output. Therefore, the fundamental amplitude of the bulk inverter output voltage govern the amplitude of the STATCOM output voltage. In other word reference amplitude A m, generated by the STATCOM controller, hould be equal to v f. To meet that condition firing angle β i calculated according to (4). Therefore, by controlling the firing angle β, of the bulk inverter output, the STATCOM output voltage can be controlled [2]. After calculating the firing angle, next tep i to find uitable bulk vector. For that, pace vector diagram of the cacade multilevel inverter ha to be tudied. Fig. 3. Bulk inverter line to neutral point voltage The cacade inverter pace vector witching diagram, for the dc- link voltage ratio 3:1 (V dc = 3V dcx ), i hown in Fig. 4. Darker dot in the diagram how the witching tate of the bulk inverter and are known a bulk vector. They can be categorized into five group. 1) Large vector 2, 22, 2, 22, 2 and 22, 2) Medium vector 21, 12, 21, 12, 12 and 21, 3) Negative mall vector 1, 11, 1 11, 1324
1, and 11, 4) Poitive mall vector 211, 221, 121, 122, 112 and 212 and 5) Zero vector, 111 and 222. Therefore, altogether there are 27 bulk vector marked by darker dot. Each darker dot i the origin of another maller hexagonal pattern which repreent the witching tate of the conditioning inverter. They are the vector of the conditioning inverter and are imply known a conditioning vector. Thee mall hexagon alo have the ame vector pattern a the bulk inverter. A a reult of thi, 27X27 different vector can be identified for thi cacade-3/3 inverter. But ome of them overlap a hown in Fig. 4. The darker dot are marked with correponding witching tate. The witching tate 2 mean both upper witche are turned on. Similarly, the witching tate 1 mean middle two witche are turned on and the witching tate mean the lower two witche are turned on. (a) (b) (c) Fig. 4. Cacade inverter pace vector diagram for V dc : V dcx = 3:1 A given reference voltage vector can be yntheized by combining a bulk vector and three conditioning vector. A mentioned in the introduction, the bulk inverter produce quare wave output by witching from one bulk vector to another lowly. For < β <3, only large and medium bulk vector are ued to yntheize the bulk inverter voltage a hown in Fig. 5(a). Sub hexagonal vector pattern are omitted in thee diagram a the focu at thi point i only on bulk vector. For β =3, only medium bulk vector are ued a in Fig. 5(b). For 3 < β <6, both medium and mall bulk vector are ued and Fig. 5(c) how the alternative ue of mall and medium bulk vector under thi condition [2]. [ vqiq vdid] [ v i v i ] 3 P = 2 + Q = 2 3 q d d q (5) (6) + 2 Px iq Qxi d v = qx 2 2 3 iq + id (7) 2 Px id Qxi q v = dx 2 2 3 iq + id (8) Fig. 5. Bulk vector election for (a) < β <3, (b) β =3, (c) 3 < β <6 Fig. 6 how the block diagram of the conditioning inverter controller where v q and v d are calculated baed on the bulk inverter witching tate S a, S b and S c. The d-q ynchronou reference frame current i q and i d are calculated uing meaured STATCOM phae current i b and i c. The intantaneou active power (P) and reactive power (Q) delivered by the bulk inverter are calculated uing (5) and (6). The DC value of P and Q are obtained by paing intantaneou vale through low pa filter a hown in Fig. 6. The difference between the filtered and intantaneou value of P and Q repreent the compenation power which hould be upplied by the conditioning inverter [2]. They are denoted by P x and Q x repectively. Thoe active and reactive power compenation value are then ued to calculate required voltage of the conditioning inverter uing invere power olution (7) and (8). Negative ign appear in (7) and (8) a current in the conditioning inverter are oppoite to the load current direction. The reference voltage in (7) and (8) are then paed through a PWM block to obtain witching tate of the conditioning inverter [5]. A redundant tate election algorithm i ued to balance the conditioning inverter capacitor voltage. A PI controller i ued to regulate the active power drawn from the bulk inverter in a way uch that conditioning inverter dc-link voltage i maintained at 1/3 of the bulk inverter dc-link voltage for the nine level operation. Bulk inverter dc-link voltage regulation i carried out by the STATCOM controller hown in Fig.2. There the PI controller, which generate the current reference i d, i ued to maintain the bulk inverter dc-link voltage. 1325
V. SIMULATION RESULTS 1 Wind Speed (m/) 8 6 4 4.5 x 14 4 3.5 (a) Fig.6. Conditioning inverter controller block diagram IV. PMSG CONTROLLER Permanent Magnet Synchronou Generator (PMSG) have been gaining acceptance in direct coupled lowpeed wind generation application a they are highly efficient and relatively mall in diameter [6]. Therefore, to tet the ability of the propoed STATCOM to mitigate potential voltage variation, caued by udden wind change, a PMSG baed wind energy converion ytem i ued. Fig. 7 how the controller block diagram of the PMSG. For maximum power point tracking, the wind turbine model in the controller generate a reference peed for the PMSG uing the meaured wind peed and the optimum Tip Speed Ratio of the turbine. The difference of the actual peed and it reference i then fed into a peed regulator. A the peed i controlled by regulating the electrical torque, the current of the grid interfacing inverter are controlled in a manner that the deired torque, which indirectly track the optimal peed of the PMSG. Current (A) Current (A) (b) 8 6 iq 4 iq 2 id 5-5 5 (c) (d) -5 5-5 3 x 1 4 (e) (f) 2-2.3.31.32.33.34.35.36.37.38.39 22 (g) 2 18 75 (h) 7 65 Fig. 7. PMSG controller block diagram 6 (i) 1326
Fig. 8. Dynamic behavior of the ytem for tep change in wind peed, (a) Wind peed, (b) PCC voltage retoration in ynchronou reference frame, (c) STATCOM active and reactive current component, (d) STATCOM current - phae a, (e) STATCOM output voltage - phae a, (f) voltage at PCC, (g) an exaggerated view of the STATCOM output voltage, (h) Bulk inverter capacitor voltage, (i) Conditioning inverter capacitor voltage VI. EXPERIMENTAL RESULTS Fig. 8 how the dynamic behavior of the STATCOM for a tep change in wind peed. A hown in Fig. 8(a), at t = 3 m, wind peed i changed from 9.3 m -1 to 6.3 m -1, and again changed to 8.1 m -1 at t = 7m. Thee udden variation make the voltage at PCC to deviate from it et value. The STATCOM detect thoe deviation and inject appropriate amount of reactive power to bring the voltage to the et value. Fig. 8(b) how the reult of voltage retoration attempt of the STATCOM. Without the STATCOM being connected, magnitude and duration of voltage variation would well exceed acceptable level. Therefore, it can be een that fat acting STATCOM can effectively mitigate voltage fluctuation caued by udden wind change in wind generation ytem. Fig. 8(c) how the variation of the reactive current reference i q and the actual reactive current i q of the STATCOM. The reactive current i q follow the reference with a mall delay. A mall active current, i d, i drawn from the grid to compenate witching loe in the STATCOM and to maintain the bulk inverter dc-link voltage. Fig. 8(d) how variation of the STATCOM current of phae a. STATCOM output voltage and the voltage at PCC are hown in Fig. 8(e) and Fig. 8(f) repectively. A magnified view of the STATCOM output voltage variation from t = 3m to t = 39m i given in Fig. 8(g). With the 9-level operation of the VSC, harmonic ditortion of the STATCOM output voltage i greatly reduced. The bulk inverter and conditioning inverter dc- link voltage regulation i hown in Fig. 8(h) and Fig. 8(i) repectively. Note that the conditioning inverter voltage i maintained at one third of the bulk inverter dc link voltage for 9-level operation. The reult how a good performance of the propoed PMSG, STATCOM and inverter control ytem. Table I. Sytem parameter of the imulated ytem Fundamental frequency f = 5Hz STATCOM Interfacing Reitance R f =.5Ω STATCOM Interfacing Inductance L f =.5mH PCC voltage v l = 69kV Coupling tranformer turn ratio 4.16 : 69 DC-Link voltage V dc = 4V DC-Link capacitor C 1,C 2,C X1,C X2 C = 22μF STATCOM power rating 5MVA Conditioning inverter witching 5kHz frequency Am 2 unit/div Current 1A/div Voltage 7V/div Voltage 7V/div (a) (b) (c) (d) (e) 1327
Angle.5 rad/div Voltage 1V/div Current.5A/div (f) (g) (h) ource i ued to yntheize the PMSG baed wind energy converion ytem. Three tep change are introduced on it output voltage v n to oberve the dynamic repone of the STACOM a in Fig. 9(b). The ytem of Fig. 9(a) i initially at teady tate and when a change of the PCC voltage v l i ened an appropriate amount of reactive current i injected to retore it to the nominal value a hown in Fig. 9(d). The reult of thi retoration attempt i hown in Fig. 9(c). The variation of modulation index A m and power angle α m i hown in Fig. 9(e) and Fig. 9(f) repectively. At the beginning of thi proce, voltage V d i at it nominal value. Therefore, reactive power injection i not required. Once a change in voltage V d i ened the controller tart to inject reactive power to bring V d back to the nominal value. DClink capacitor voltage variation are hown in Fig. 9(g). A mall active current, i d, i drawn from the grid a hown in Fig.9(h) to maintain capacitor voltage and replenih power lo due to witching and reitive component of the coupling tranformer. An elaborated view of the inverter output voltage and current i hown in Fig. 9(i) and Fig. 9(j) repectively. It can be een from thee reult that the cacade multilevel STATCOM controller perform exceedingly well in tranient and teady tate condition Voltage 2V/div Current.5A/div (i) (j) Fig. 9. Experimental reult, (a) experiment etup, (b) PCC voltage without STATCOM, (c) PCC voltage with STATCOM, (d) injected reactive current, (e) amplitude A m (f) power angle, (g) bulk and conditioning inverter dc-link voltage, (h) active current, (i) STATCOM output voltage v a, (j) STATCOM output current Table II. Sytem parameter of the experimental etup Fundamental frequency f = 5Hz STATCOM Interfacing Reitance R = 2Ω STATCOM Interfacing Inductance L = 3mH Line reitance R n = 4Ω Line Inductance L n = 34mH Load reitance R l = 55Ω Load Inductance L n = 11mH DC-Link voltage V dc = 6V DC-Link capacitor C 1,C 2,C X1,C X2 C = 22μF Nominal value of V d 35V The laboratory prototype of the cacade multilevel STATCOM i et up a in Fig. 9(a). A programmable AC VII. CONCLUSIONS A novel STATCOM, a a olution for potential voltage quality problem related to wind power generation ytem, i preented. The related modulation and control technique are explained in detail. The propoed STATCOM how good dynamic repone for voltage variation by retoring the PCC voltage to it deired value. The imulation and experimental reult how good performance of the dc-link voltage regulation and capacitor voltage balancing technique. Thee reult indicate that the propoed cacade multilevel STATCOM can improve the quality of the output of wind generation ytem with reduced harmonic and witching loe. VIII. REFERENCES [1] K.A. Corzine, et.al. Control of cacaded multilevel inverter, IEEE Tran. on Power Electron., Vol.19, No. 3, pp. 732-738, 24. [2] S. Lu, K.A. Corzine, Advanced Control and Analyi of Cacaded Multilevel Converter Baed on P-Q Compenation, IEEE Tran. on Power Electron., vol.22, no. 4, pp. 1242-1252, 27. [3] Wanki Min; Joonki Min; Jaeho Choi, Control of STATCOM uing cacade multilevel inverter for high power application Proc. of the International Conference on Power Electronic and Drive Sytem, 1999 PEDS99, Volume 2, Iue, 1999, pp. 871 876, Vol. 2. [4] C. Schauder and H. Mehta, Vector analyi and control of advanced tatic VAR compenator Proc. Int. Elect. Eng. C, Vol.14, No. 4, pp. 299-36, Jul. 1993. [5] K. A. Corzine and J. R. Baker, Multilevel voltage-ource duty-cycle modulation: Analyi and implementation, IEEE Tran. Ind. Electron., Vol. 49, No. 5, pp. 19 116, Oct. 22. [6] D. M. Vilathgamuwa, Wang Xiaoyu, C. J. Gajanayake, Z-ource converter baed grid-interface for variable-peed permanent magnet wind turbine generator, Proc. Power Electronic Specialit Conference, 28, PESC 28. 1328