STATCOM Based Voltage Regulator for SelfExcited Induction Generator Feeding Non-Linear Loads Bhim Singh, S. S. Murthy and Sushma Gupta discrete type and inject harmonics in the generating system. By the invention of solid-state self-commutating devices, it is possible to make a static, noiseless voltage regulator, which can provide continuously variable reactive power to the SEIG with varying load to keep terminal voltage constant. This system called STATCOM has specific benefits compared to SVC [9], Schauder and Mehta [10] have derived governing equations of STATCOM to determine the response of the STATCOM. Singh and Shilpakar [11] have proposed an analysis of solid-state voltage regulator for SEIG with static load. Wekhande and Agrawal [12] have proposed a controller to regulate three-phase AC output voltage of the SEIG with varying rotor speed, transient load conditions and reactive loads. Miranda et al. [13] have proposed static VAR compensator for electrical pumping system driven by induction generator. Kuo and Wang [14-15] have described a method of voltage control of SEIG under unbalanced/nonKey Words: Self-excited induction generator, Non-linear loads, linear load using current controlled voltage source inverter. STATCOM, load balancing, harmonic elimination. However, due to extensive use of solid-state devices, energy can be saved by adjustable speed drives used in pump, I. INTRODUCTION compressor, air-conditioner etc. and other domestic appliances In remote areas, plenty of non-conventional energy sources such as TV, computer, SMPS, UPS etc. Three-phase and are available. These non-conventional energy sources are single-phase rectifiers are the front-end converter of aboveidentified as potential prime movers for the generating mentioned system, which is non-linear in nature. These nonsystems. Externally driven induction machine operates as a linear loads draw non-sinusoidal currents (fundamental along self-excited induction generator (SEIG) with its excitation with harmonics) from the generating system therefore they requirements being met by a capacitor bank connected across inject the harmonics in the system. The SEIG is an isolated its terminals. The SEIG has advantages like simplicity, system, which is small in size and the injected harmonics may maintenance free, absence of DC, brushless etc. as compared pollute generated voltage. The STATCOM eliminates the to the conventional synchronous generator. harmonics, provides load balancing and supplies the reactive A considerable reported literature exists on steady state and power to the load- and generator. In this paper, authors are transient analysis of SEIG under balanced/unbalanced presenting a simple mathematical modelling for the transient resistive, reactive and motor loads. In [1-3] d-q axes modeling analysis of the SEIG-STATCOM system under balanced/ is reported for the transient analysis of SEIG. Wang and Deng unbalanced three-phase and single-phase non-linear loads [4] have presented the transient performance of the SEIG (rectifier with R and R-C load) and simulated results show that under unbalanced excitation system. Jain et al. [5] have given the SEIG-STATCOM system behaves an ideal supply under a generalized model for the transient analysis of SEIG under these unbalanced non-linear loads. symmetrical and unsymmetrical conditions. A major disadvantage of SEIG is its poor voltage II. SYSTEM CONFIGURATION AND CONTROL regulation requires a variable capacitance bank to maintain SCHEME constant terminal voltage under varying loads. Attempts have The schematic diagram of SEIG with excitation capacitor, been made to maintain constant terminal voltage by fixed STATCOM, load and control scheme is shown in Fig. 1, capacitor and thyristor controlled inductor (SVC) [6], Excitation capacitors are selected to generate the rated voltage saturable-core reactor [7] and short-shunt connection [8], of SEIG at no load. The additional demand of reactive power However, voltage regulation provided by these schemes is of is fulfilled using the STATCOM under varying loads. The
Abstract—This paper deals with the performance analysis of static compensator (STATCOM) based voltage regulator for self-excited induction generators (SEIGs) supplying non-linear loads. In practice, a number of loads are non-linear in nature and therefore they inject harmonics in the generating systems. The SEIG being a weak isolated system, its performance is very much affected by these harmonics. The additional drawbacks of SEIG are poor voltage regulation and it requires adjustable reactive power source with varying load to maintain constant terminal voltage. A three-phase insulated gate bipolar transistor (IGBT) based current controlled voltage source inverter known as STATCOM is used for harmonic elimination and it provides required reactive power for the SEIG with varying loads to maintain constant terminal voltage. A dynamic model of the SEIGSTATCOM feeding non-linear loads using stationary d-q axes reference frame is developed for predicting the behavior of the system under transient conditions The simulated results show that SEIG terminal voltage is maintained constant even with non-linear balanced and unbalanced loads and free from harmonics using STATCOM based voltage regulator
Bhim Singh (e-mail; bsinghfBee.iitd,acini. S,S. Murthy (e-mail:
[email protected]) and Sushma Gupta (e-mail: sush
[email protected] are with the Department of Electrical Engineering, Indian Institute of Technology, Delhi, Huaz Khas New Delhi-16, INDIA.
STATCOM acts as a source of lagging or leading current to maintain the constant terminal voltage with variation in load. The STATCOM consists of a three-phase IGBT based current
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controlled voltage source inverter, DC bus capacitor and AC inductors. The output of the inverter is connected through the AC filtering inductor to the SE1G terminals. The DC bus capacitor is used as an energy storage device and provides self-supporting DC bus. The control technique to regulate the terminal voltage of the SEIG is based on the control of source currents (which have two components in-phase and quadrature with AC voltage). The in-phase unit vectors (u,, m, and u^) are threephase sinusoidal functions, computed by dividing the AC voltages v,, vb and vd by their amplitude V,. Another set of quadrature unit vectors (w,, wb and wc) is sinusoidal function obtained from iri-phase vectors (u,, ub and u,). To regulate AC terminal voltage (V,) it is sensed and compared with the reference voltage. The voltage error is processed in the PI controller. The output of the PI controller (I™,) for AC voltage control loop decides the amplitude of reactive current to be generated by the STATCOM. Multiplication of quadrature unit vectors (w,, vit and wc) with the output of PI based AC voltage controller (Ismq) yields the quadrature component of the source reference currents (ia,*, i*,* and i*,,*). To provide selfsupporting DC bus of STATCOM, its DC bus voltage is sensed and compared with DC reference voltage. The error voltage is processed in another PI controller. The output of the PI controller (I,md) decides the amplitude of active current. Multiplication of in-phase unit vectors (u,, n, and uj with output of PI controller (Ismd) yields the in-phase component of the source reference currents (isad*, i5bd*and iscd*). The sum of quadrature and in-phase components is the source reference currents (isa*, iSb* and L*}, which are compared with the source line current (i^, u and i«) in PWM current controller to generate switching signal for the devices of STATCOM.
(4) (5)
1) Quadrature Component of Source Reference Currents: The AC voltage error V^at the n"1 sampling instant is: V^V^-V*,, (6) Where Vm, is the amplitude of reference AC terminal voltage and V,w is the amplitude of the sensed three-phase AC voltage
Non-linear load draws non- sinusoidal currents (fundamental as well as harmonics) due to which harmonics produced are in the generating system. Under unbalanced loading conditions, SEIG currents may be unbalanced (produce positive and negative sequence) due to which machine is to be derated. STATCOM is able to filter out the harmonics and balance the unbalanced load resulting in balanced currents and voltages of the SEIG. III. MODELLING OF SEIG-STATCOM SYSTEM Mathematical model of SEIG-STATCOM system contains the modeling of SEIG and STATCOM and is as follows.
A. Modelling of control scheme ofSTA TCOM Different components of SEIG-STATCOM system shown in Fig. 1 are modeled as follows. Three-phase voltages at the SEIG terminals (v,, vb and vc) are considered sinusoidal and hence their amplitude is computed as: V,= \(2/3)(vi1+W+^)}"2 (1) The unit vector in phase with vM vb and vc are derived as: u,= v,/V,; u, = v,/Vr; uc = vc/V, (2) The unit vectors in quadrature with v,, vb and vt may be derived using a quadrature transformation of the in-phase unit vectors ua, Ut, and u, [ 11 ] as: ' (3)
Fig. 1 Schematic diagram of (he proposed scheme of SEIG-STATCOM
at the SEIG terminals at n'h instant. The output of the PI controller (I*™q(n)) for maintaining AC terminal voltage constant at the n"1 sampling instant is expressed as: I*™*« = I*! r»-I) + K,, { Vaf.-, - Vcn |)} + Kia V^n, (7) Where K^, and Kia are the proportional and integral gain constants of the proportional integral (PI) controller, Vj, w and V Wri) are the voltage errors in nth and (n-l)' h instant and I'micn-i) is the amplitude of quadrature component of the source reference current at (n-1)"1 instant. The quadrature components of the source reference currents are estimated as: i'sa, = I*Smq wa; 1% = !*„, wb; i*«, = I*™, wc . (8)
2) In-Phase Component of Source Reference Currents:
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The DC bus voltage error V t e at n'h sampling instant is; (9)
Rearranging the eqn. (21) and eqn. (24) it results in: Lf pira - Lf picb = v, - eab - Rf ic. + Rf i* (25) Lf pica + 2 Lf picb = vb - ek - Rt ica - 2 Rf itb (26) Hence, the STATCOM current derivatives are obtained by solving the eqn (25) and (26) as: l*smd{T0 = I*stntl(ii-1> + Kpd { Vdcttfn) ~ Vdccrm-l)} + Kid V i t a ^ ) ( 1 0) (27) I*md(n) is considered as the amplitude of active source current. pi a = {(vb - e*) + 2 (v, - eab) - 3 R, iB}/(3Lr) Kpj and K,d are the proportional and integral gain constants of. pi* = {(vb - ej - (v. - esb) - 3 Rf i_}/
