Mitigation of Power Quality Problems in a Wind Driven Isolated Generator by using Static Series Compensator

Mitigation of Power Quality Problems in a Wind Driven Isolated Generator by using Static Series Compensator

Pragya Guru

PG Scholar

Department of Electrical Engineering Samrat Ashok Technological Institute Vidisha, (M.P.) India

C. S. Sharma

Associate Professor Department of Electrical Engineering Samrat Ashok Technological Institute

Vidisha, (M.P.) India

Abstract This paper deals with the performance of a system consisting of a three-phase self excited induction generator (also known as Isolated generator) with static compensator (STATCOM) for feeding the static resistive-capacitive load and investigated in a wind energy conversion system to mitigate the power quality problems such as poor voltage, voltage and current harmonics due to sudden change in load. The cost effective STATCOM providing stable operation, was designed by connecting additional shunt capacitance with the load. A three- phase, insulated gate bipolar transistor (IGBT) based current- control voltage source converter (CC-VSC) known as STATCOM, is used for harmonic elimination caused by sudden changes in load or due to faults. The STATCOM control algorithm was realized by controlling the source current using two control loops with proportional integral (PI) controller, one for SEIG controlling (the SEIG terminal voltage) and the other for maintaining the DC bus voltage to generate the reference current. Here the proposed electrical system is modeled and simulated in MATLAB using Simulink tool box and studied. On the basis of this model, different characteristics of SEIG with STATCOM are analyzed which shows its suitability in SEIG in generating stations like wind energy system.

Keywords Wind energy system, SEIG (Self Excited Induction Generator), Static Compensator (STATCOM), Voltage Regulation, Voltage source Converter (VSC)

1. INTRODUCTION

   The renewable energy resources such as micro, mini hydro and wind are being harnessed to generate electrical energy; however the usage of induction generator for this purpose is getting considerable attention [1-3]. The presence of wind power generation is likely to influence the operation of the existing power system networks, especially the power system stability. The SEIG as reported by Bassett and Potter in 1935

   [1] is a squirrel cage induction machine with suitable capacitor bank at stator terminal. In the other manner, an externally driven squirrel cage induction machine with its stator terminals connected to a reactive power source (capacitor bank) is popularly known as SEIG [4].

   When an SEIG is driven by prime mover such as biomass, biogas, and biodiesel engine or wind turbine, the frequency of the generated voltage is almost constant from no load to full load. The fast response, improved switching features and the low cost of the power converters have attracted the researchers to explore their applications for SEIG. But poor voltage regulation has been the major drawback of an SEIG in its

   applications. Hence the terminal voltage of an SEIG needs to be regulated during load perturbations. Several methods for voltage regulation have been reported for SEIG-based autonomous power generation systems. The methods reported in [5-8] have employed the passive element for the voltage regulation. L. Shridhar and B. Singh used the short shunt compensation method for a three-phase SEIG. With the development of fast acting self-commutating switches and PWM techniques, voltage source converter based static reactive compensators STATCOM [9-14] have been evolved. The performance of SEIG-STATCOM system has been discussed for linear loads. There is also available comparative study on operating performance of static series compensated three-phase self excited induction generator with SVC and STATCOM [15].

   Contrary to static compensators (STATCOM) has been used effectively in power system for mitigation of resonance, reactive power control, voltage regulation etc [16-22]. The schematic of three-phase STATCOM comprise six pulse Insulated gate bipolar transistor (IGBT) based pulse width modulated with a suitable sized capacitor at DC link. It modulates the effective impedance of the line by injecting a controllable AC voltages in quadrature with the line current and emulates an inductor (or capacitor) when the injected voltage is quadrature leading (or lagging) to the line current.

   In this paper, the studies have been carried out on SEIG- SATCOM system for R-C load. For controlling the voltage a static compensator (STATCOM) is used as a reactive power compensator along with harmonic elimination. With the employed control technique the STATCOM is found to capable of generating/absorbing controllable reactive power and maintaining constant voltage during change in load and to damp out the oscillations and to make system stable.

2. SYSTEM CONFIGURATION & CONTROL STRATEGY

   1. SEIG-STATCOM System

      The block diagram of SEIG-STATCOM system along with the control scheme for generating reference current signals and subsequent gating signals for generating IGBTs are depicted.

      The figure 1 shows the system configuration of self excited induction generator, consisting three leg IGBT based voltage source converter and consumer load. The six gating signals for IGBTs of STATCOM are obtained by comparing sensed and

      Fig. 1 Representation of SEIG-STATCOM system with control technique

      reference supply currents through carrier-less hysteresis current controller. The delta connected three-phase capacitor bank is used for the generator excitation and value of an excitation capacitor is selected to generate the rated voltage at no load. The isolated asynchronous generator or SEIG generates constant power and when consumer load changes; the STATCOM is used to regulate the voltage due to load changes.

      STATCOM consists of IGBT based current controlled 3- leg VSC, DC bus capacitor and AC inductors. The output of the VSC is connected through the AC filtering inductors to SEIG terminals. The DC bus Capacitor is used to filter voltage ripples and provides self supporting Dc bus. Fig. 2 shows the control scheme for STATCOM. The proposed control scheme is based on indirect current control and deploys two control loops for generating reference supply currents.

   2. Control strategy

      The fig. 2 shows the control strategy of the proposed voltage controller for SEIG. The control scheme of STATCOM to regulate the terminal voltage of SEIG which is based on the generation of source currents has two main components, in- phase and quadrature, with AC voltage. The in-phase unit

      vectors ua ub and ucare three-phase sinusoidal functions, computed by dividing the AC voltages va vb and vcby their

      amplitude Vt. Another set of Quadrature unit vectors ( wa wb and wc ) are sinusoidal functions obtained from in-phase vectors (ua, ub and uc). To regulate the AC terminals voltage (Vt), it is sensed and compared with the reference voltage. The voltage error is processed in the Proportional Integral (PI) controller. The output of the PI controller (I*smq) for the AC voltage drop control loop determines the amplitude of the reactive current to be generated by the STATCOM. Multiplication of Quadrature unit vectors ( wa wb and wc ) with the output of the PI based Ac voltage controller (I*smq) yields the q-component of the ref. source currents (i*saq, i*sbq and i*scq). To provide a self supporting DC bus for STATCOM, its DC bus voltage is sensed and then compared with the DC reference voltage.

      1. SEIG modeling

         This model is developed in q-d reference frame considering the effect of oth main and cross flux saturation and is expressed as

         pi L1vriGi

         (2)

         p r

         P Tp Tem

         2J

         (3)

         Where v, i, r, L, G

         Appendix.

         and

         Tem

         are defined in

         The SEIG voltages (vga , vgb and vgc) from shunt capacitance bank are expressed as

         Fig. 2 Control Scheme for STATCOM

         The error voltage is processed in another PI controller. The

         p vga vgb vgcT 1

         Chs

      2. Gate signal generation

   iga isaigb isbigc iscT

   (4)

   output of the PI controller (I*smd) determines the amplitude of the active current. Multiplication of in-phase unit vectors (ua, ub and uc) with the output of the PI controller (I*smd) yields in phase quadrature with (i*sad, i*sbd and i*scd). The instantaneous summation of quadrature and In-phase component gives the reference source currents (i*sa, i*sb and i*sc), both are

   The IGBT gate signals are derived by relating equation of the PI controller, reference supply current an hysteresis current controller.

   Three phase voltages at SEIG terminals (vga , vgb and vgc) are considered sinusoidal and hence there amplitude is

   computed as

   compared with the sensed line current (isa, isb and isc). These current error signals are amplified and then compared with the

   triangular carrier wave.

   Vt 2 3va2 vb2 vc2

   (5)

   If the amplified current error signal is equal to or greater than the triangular carrier wave, the lower circuit of the inverter phase is turned ON and the upper device turned OFF. If the amplified current less than or equal to the triangular carrier wave the lower device of the inverter phase is turned

   as:

   The unit vectors in phase with va vb and vc are derived

   ua vaVt ; ub vbVt ; uc vcVt (6) The unit vectors in quadrature with va vb and vc may

   OFF and the upper device turned ON. A non-linear load draws

   non-sinusoidal currents which causes harmonics to be injected into the generating system. Under unbalanced load conditions,

   be derived using a quadrature transformation of the in-phase unit vectors ua ub and uc as

   SEIG currents may be unbalanced which may cause the machine to be derated. STATCOM is able to filter out the harmonics and balance the unbalanced load resulting in balanced and sinusoidal currents and voltages in the generator.

3. SYSTEM MODELLING

   wa – ua 3 uc 3

   wb 3 2 ub 2 3

   wb – 3 ua 2 ub uc 2 3

   (7)

   (8)

   (9)

   The system consist SEIG, STATCOM with associated control technique and load. The dynamic model of system

   Quadrature component of reference source currents:

   The AC voltage error at the nth sampling instant is

   components are briefed herewith.

   Ver n Vtref n Vt n

   (10)

   A. Filter

   Te inductive filter is used to remove the high frequency components from the output voltage of VSC. The size of the inductive filter is governed by the allowable ripples in the compensation currents. The inductance value depends on the switching frequency fs and peak ripple current irpp with

   associate ripple band Krp and is expressed as,

   Where, Vtref n is the amplitude of the reference AC terminal voltage.

   Vt n is amplitude of the sensed three-phase AC voltage t

   the SEIG terminals at the nth instant.

   smq n

   The output of the PI controller I * for maintaining constant AC terminal voltage at the nth sampling instant is expressed as:

   I* I*

   1 K paV V

   KiaV

   (11)

   smq n

   smq n

   er n

   er n1

   er n

   3

   Where K and K are the proportional and integral gain

   vbat

   pa ia

   2

   L

   6afsKrpirpp

   (1)

   constants of the proportional integral (PI) controller. Ver n

   and Ver n1are the voltage error at nth and (n-1)th instant and

   I * is the amplitude of the quadrature component of the

   i* i* i

   smq n1

   reference source current at the (n-1)th instant. The quadrature components of the reference source currents are computed as:

   saerr

   i

   *

   sberr

   i*

   sa

   i

   *

   sb

   i*

   sa

   • isb

   • i

     i* I *

     w ; i*

     I *

     w ; i*

     I * w

     (12)

     scerr

     sc sc

     saq

     smq a

     sbq

     smq b

     scq

     smq c

     The error signals are amplified and then compared with the triangular carrier wave. If the amplified phase a current error

     In-phase component of Reference source currents:

     The error in DC voltage of the STATCOM

     signal is greater than triangular wave signal switch S4 (lower device) is ON and switch S1 (upper device) is OFF. If the

     Vdcer n Vdcerf n Vdc n

     amplitude current error signal corresponding to

     *

     i

     saerr

     is less

     Where Vdcer n the reference DC voltage and

     Vdc nis the

     than the triangular wave the signal switch S1 is ON and switch

     sensed DC link voltage of the STATCOM. The output of the PI controller for maintaining the DC bus voltage of the STATCOM at the nth sampling instant is expressed as:

     S4 is OFF. Similar logic applies to other two phases of VSC of STATCOM.

     D. Load Model

     I * I *

     K V V

     K V

     (13)

     The study is carried out for series connected resistive

     smd n

     smd n1

     pd dcer n

     dcer n1

     id dcer n

     capacitive load. The modeling equation for load is:

     I * is considered to be amplitude of the active

     smd n

     source current. K pd and Kid are the proportional and

     pvcla

     vclb

     vclc

     T 1

     R C

     vga

   • vca

   • vclavgb

   • vcb

   • vcbavgc

   • vcc

   • vclcT

   integral gain constants of the DC bus proportional integral f f

   (PI) controller.

   The in-phase components of the reference source currents are computed as:

4. MATLAB BASED MODELLING

   i* I *

   u ; i*

   I *

   u ; i*

   I * u

   (14)

   The MATLAB model of the SEIG-STATCOM system

   sad(n)

   smd a

   sbd(n)

   smd b

   scd(n)

   smd c

   consists of the asynchronous machine i.e. induction generator with capacitor bank and this circuit is realize in MATLAB

   Reference source currents:

   The Reference currents are given as

   version 13. The modeling of SEIG is carried out using squirrel cage 25hp, 415V, 50Hz asynchronous machine and 75kVar delta connected excitation capacitor bank. The STATCOM is

   i

   i

   * *

   sa saq

   i

   i

   * *

   sb sbq

   i

   i

   * *

   sc scq

   *

   sad

   *

   sbd

   *

   scd

   realized with a 3-leg voltage source converter and a DC link capacitor. The SEIG is coupled with STATCOM through an L-filter. The complete simulation model of the SEIG- STATCOM system with load circuit is shown in fig 3. The output wave forms are shown in fig 4.

   PWM current controller:

   The total reference currents are compute with the sensed source currents the ON/OFF switching patterns of the gate signals to the IGBTs are generated from the PWM current controller. The current errors are computed as:

5. RESULT AND DISCUSSION

   The performance of SEIG-STATCOM system with static R-C load is shown in fig. 4 and fig. 5. The SEIG is loaded from 6 to 7 seconds, which results into increase in load currents and decrease in speed r. Due to increase in load, additional reactive power loading on STATCOM and the Vdc

   Fig. 3 Simulink model of SEIG-STATCOM system

   decreases. This increased loading results into corresponding increase in generato load. But Vdc decreases and the later return to its reference value under the PI controller action and maintained voltage constant.

6. CONCLUSION

The design, modeling and simulation of SEIG with STATCOM have been carried out for static load. For STATCOM the operation in capacitive load is already explained. The proposed STATCOM, which is employing simple and easy to implement PI controller assisted technique to calculate the reference supply current, is found to be elegant. The STATCOM improves the voltage regulation by the injection of compensation currents and is able to regulate the terminal voltage of the generator and suppresses the harmonic currents injected by load.

The developed SEIG-STATCOM combination promises a potential application for isolated power generation using renewable energy sources in remote areas with improved power quality.

Table-1 THD of SEIG-STATCOM system with R-C load

1. Simulaton result of SEIG with increased load

   Fig. 4 SEIG output rotor speed, Vs & Is without STATCOM

2. Simulation Result of SEIG with increased load and STATCOM.

Fig. 5 SEIG output rotor speed, Vs, Is & FFT window with STATCOM

APPENDIX

1. Generator parameters:

   18 kW, 415V, Y-connected, 50 Hz, 4-pole, J = 0.0854 kg- m2 cage induction machine.

   Rs = 0.02 pu, Rr = 0.025 pu, Xls=Xlr= 0.048 pu, Lm= 0.1589 pu

2. Load parameters: Active power = 4500W Reactive power = 4000W

REFERENCES

1. Bassett ED, Potter FM. Capacitive excitation for induction generators. AIEE Trans (Elect Eng) 1935;54:540-5.

2. Rahim AHMA, Alam MA, Kandlawala MF, Dynamic performance improvement of an isolated wind turbine induction generator. Concept Electrical Engg. 2009;35(4):594-607

3. Basic M, Vukadinovic D, Petrovic G. Dynamic and pole zero analysis of self excited induction genrator using a noval model with iron losses. Electrical Power Energy Syst. 2012;42:105-18.

4. E. D. Bassettt and F. M. Potter, Capacitive excitation for induction generators, Trans. Amer. Inst. Elect. Eng., vol. 54, no. 5 pp. 540-550, may 1935.

5. H.Rai,A.Tandan,S.Murthy,B.Singh,andB.Singh,Voltage regulation

   of self excited induction generator using passive elements, in Proc . IEEE Int. Conf. Elect. Mach. Drives, Sep.1993, pp.240245.

6. L.Shridhar,B.Singh,andC.Jha,Transient performance of the self regulated short shunt self excited induction generator, IEEE Trans. Energy Convers, vol. 10, no. 2, pp. 261267, Jun.1995.

7. E Bim E, Szajiner J. Burian Y. Voltage compensation of an induction generator with long shunt connection, IEEE Trans Energy convers 1989;4(3):526-30

8. L.Shridhar, B.Singh, C.Jha,B.Singh,andS.Murthy,Selection of ca- pacitors for the self regulated short shunt self excited induction generator, IEEE Trans. Energy Convers. vol.10,no.1,pp.10 17,Mar.1995.

9. B.Singh and L.Shilpakar,Analysis of a novel solid state voltage regulator for a self-excitedinduction generator, IEE Proc.Generat., Transmiss.Distrib.,vol.145,no.6,pp.647655,Nov.1998.

10. S.-C.Kuo and L.Wang,Analysis of voltage control for a self-excited induction generator using a current-controlled voltage source inverter(CC-VSI), IEE Proc.Generat., Transmiss. Distrib.,vol. 148, no. 5, pp.431438,Sep.2001.

11. B. Singh, S. Murthy, and S. Gupta, STATCOM-based voltage reg- ulator for self-excited induction generator feeding nonlinear loads,IEEE Trans. Ind. Electron.,vol. 53, no. 5, pp. 14371452, Oct.2006.

12. G.DastagirandL.A.C.Lopes,Voltageandfrequencyregulationofastand- alone self excited induction generator,in Proc. IEEE Electr.Power Conf.,2007, pp.502506.

13. W.-L.Chenand Y. Y.Hsu,Controller design for an induction generator driven by a variable-speed wind turbine,IEEE Trans. Energy Convers.,vol.21,no.3,pp.625635,Sep.2006.

14. W.-L.Chen,Y.-H.Lin,H.-S.Gau,andC.-H.Yu,STATCOM controls for a self excited induction generator feeding random loads,IEEETrans.PowerDel.,vol.23,no.4,pp.22072215,Oct.2008.

15. Yogesh K. Chauhan, Sanjay K. Jain, Bhim Singh. Operating performance of static series compensated three-phase self-excited induction generator, electrical power and energy systems 49 (2013) 137-148

16. Reddy IP, Sanker Ram BV VC with PLC voltage regulation for enhancement of transient stability of multi machine power system. Int J Eng Intell system 2011;19(1):21-30.

17. Li Shuhui, Xu Ling, Timothy AH. Control of VSC-based STATCOM using conventional and direct-current vector control strategies. Electr Power Energy Syst 2013;43:17586.

18. Zhan CJ, Wu XG, Kromlidis S, Ramachandramurthy VK, Barnes M, Jenkins N, et al. Two electrical models of the lead-acid battery used in a dynamic voltage restorer. IEE Proc Gener Transm Distrib 2003;150(2):17582.

19. Singh B, Verma V, Chandra A, Al-Haddad K. Hybrid lters for power quality improvement. IEE Proc Gener Transm Distrib 2005;152(3):36578.

20. Bongiorno M, Angquest L, Svensson J. A novel control strategy for subsynchronous resonance mitigation using SSSC. IEEE Trans Power Delivery 2008;23(2):103341.

21. Ghorbani A, Mozaffari B, Ranjbar AM. Application of subsynchronous damping Controller (SSDC) to STATCOM. Elect. Power energy syst 2012;43;418-2