 Open Access
 Total Downloads : 184
 Authors : Pragya Guru, C. S. Sharma
 Paper ID : IJERTV4IS100452
 Volume & Issue : Volume 04, Issue 10 (October 2015)
 DOI : http://dx.doi.org/10.17577/IJERTV4IS100452
 Published (First Online): 26102015
 ISSN (Online) : 22780181
 Publisher Name : IJERT
 License: This work is licensed under a Creative Commons Attribution 4.0 International License
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 threephase self excited induction generator (also known as Isolated generator) with static compensator (STATCOM) for feeding the static resistivecapacitive 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 (CCVSC) 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)

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 [13]. 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 SEIGbased autonomous power generation systems. The methods reported in [58] have employed the passive element for the voltage regulation. L. Shridhar and B. Singh used the short shunt compensation method for a threephase SEIG. With the development of fast acting selfcommutating switches and PWM techniques, voltage source converter based static reactive compensators STATCOM [914] have been evolved. The performance of SEIGSTATCOM system has been discussed for linear loads. There is also available comparative study on operating performance of static series compensated threephase 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 [1622]. The schematic of threephase 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 RC 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.

SYSTEM CONFIGURATION & CONTROL STRATEGY

SEIGSTATCOM System
The block diagram of SEIGSTATCOM 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 SEIGSTATCOM system with control technique
reference supply currents through carrierless hysteresis current controller. The delta connected threephase 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.

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 inphase unit
vectors ua ub and ucare threephase 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 inphase 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 qcomponent 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.

SEIG modeling
This model is developed in qd 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

Gate signal generation

iga isaigb isbigc iscT
(4)
output of the PI controller (I*smd) determines the amplitude of the active current. Multiplication of inphase 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 Inphase 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 nonlinear load draws
nonsinusoidal currents which causes harmonics to be injected into the generating system. Under unbalanced load conditions,
be derived using a quadrature transformation of the inphase 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.


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 threephase 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 (n1)th instant and
I * is the amplitude of the quadrature component of the
i* i* i
smq n1
reference source current at the (n1)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
Inphase 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 inphase components of the reference source currents are computed as:


MATLAB BASED MODELLING
i* I *
u ; i*
I *
u ; i*
I * u
(14)
The MATLAB model of the SEIGSTATCOM 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 3leg voltage source converter and a DC link capacitor. The SEIG is coupled with STATCOM through an Lfilter. 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:

RESULT AND DISCUSSION
The performance of SEIGSTATCOM system with static RC 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 SEIGSTATCOM 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.

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 SEIGSTATCOM combination promises a potential application for isolated power generation using renewable energy sources in remote areas with improved power quality.
Table1 THD of SEIGSTATCOM system with RC load
STATCOM in SEIG System 

Vg 
Ig 

THD% 
0.20 
1.99 

Simulaton result of SEIG with increased load
Fig. 4 SEIG output rotor speed, Vs & Is without STATCOM

Simulation Result of SEIG with increased load and STATCOM.
Fig. 5 SEIG output rotor speed, Vs, Is & FFT window with STATCOM
APPENDIX

Generator parameters:
18 kW, 415V, Yconnected, 50 Hz, 4pole, 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

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