Analysis of power quality improvement in grid

IJRET: International Journal of Research in Engineering and Technology eISSN: 2319-1163 | pISSN: 2321-7308
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Volume: 02 Issue: 10 | Oct-2013, Available @ http://www.ijret.org 106
ANALYSIS OF POWER QUALITY IMPROVEMENT IN GRID
CONNECTED WIND DRIVEN INDUCTION GENERATOR AT VARIOUS
LOAD CONDITIONS
Sk. Baji babu1
, M. Balasubba Reddy2
, T.Madhu3
1
M. Tech Scholar (P.E), 2
Professor & H.O.D, 3
Assistant Professor, Department of EEE, Prakasam Engineering College,
Kandukur, Andhra Pradesh, India,
bajibabu207@gmail.com, balasubbareddy@yahoo.com, tallapanenimadhu@gmail.com
Abstract
As a promising renewable alternative, the wind power is one of the significant sources of generation. Reactive power compensation
and harmonic reduction in a low voltage distribution networks for integration of wind power to the grid are the main issues addressed
in this paper. This paper proposes a control scheme based on instaneous Pq theory for compensating the reactive power requirement
of a three phase grid connected wind driven induction generator as well as the harmonics produced by the non linear load connected
to the PCC using STATCOM. The proposed control scheme is simulated using MATLAB/SIMULINK. The Simulation results are
presented in this paper.
Keywords- wind power, distribution network, induction generator, STATCOM, reactive power.
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1. INTRODUCTION
The integration of wind power to grid introduces power
quality issues, which mainly consist of voltage regulation and
reactive power compensation Induction machines are mostly
used as generators in wind power based generations. Induction
generators draw reactive power from the grid to which they
are connected. Therefore, the integration of wind power to
power system networks is one of the main concerns of the
power system engineers. The addition of wind power into the
electric grid affect's the power quality [I].During the last few
years, power electronic technology plays an important role in
distributed generation and integration of in d energy
generation into the electric grid[2]. A non-linear load on a
power system is typically a rectifier (such as used in a power
supply), some kind of arc discharge device such as fluorescent
lamp, electric welding machine, or arc furnace because current
in these systems is interrupted by a switching action, the
current contains frequency components that are multiples of
the power system frequency. It changes the shape of the
current wave form from a sine wave to some other form and
also create harmonic currents in addition to the
original(fundamental frequency) AC current. The most used
unit to reduce the harmonics produced by the non linear load.
Compensate for reactive power in the power systems are either
synchronous condensers or shunt capacitors, the latter either
with mechanical switches or with thyristor switch, as in Static
VAR Compensator (SVC). The disadvantage of using shunt
Capacitor is that the reactive power supplied is proportional to
the square of the voltage. Consequently, the reactive power
supplied from the capacitors decreases rapidly when the
voltage decreases [3] .To overcomes the above disadvantages;
STATCOM is best suited for reactive power compensation
and harmonic reduction. It is based on a controllable voltage
source converter (VSC). By control of the voltage source
converter output voltage in relation to the grid voltage, the
voltage source converter will appear as a generator or absorber
of reactive power [6].Fig I show the block diagram of grid
connected system. In this 3-phase separately excited induction
generator feeding non linear load has been presented. A
STATCOM is connected at the point of common coupling
with this system in order to compensate the reactive power
requirements of induction generator as well as load and also to
reduce the harmonics produced by the non linear load.

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The proposed STATCOM control scheme for grid connected
wind energy generation for power quality improvement has
following objective.
This paper is organized as follows. Section II discusses
reactive power requirements of induction generator. Section
III, IV and V discuss wind driven induction generator with
STATCOM, reference current generation and hysteresis
current controller for STATCOM respectively.
2. REACTIVE POWER REQUIREMENTS OF
INDUCTION GENERATOR
The induction generators are widely used, due to the
advantage of cost effectiveness, robustness, ruggedness,
simplicity and requirement of no brush and commutators.
However; induction generators require reactive power for
magnetization. When the generated active power of an
induction generator is varied due to wind, absorbed reactive
power and terminal voltage of an induction generator can be
significantly affected.
A Three phase induction generator feeding a non linear load as
well as connected to the grid is simulated using
MATLAB/Simulink to study the reactive power requirement
of IG and also harmonics produced by the non linear load.
Simulation diagram of this system is shown in Fig2.
3. WIND DRIVEN INDUCTION GENARATOR
WITH STATCOM
During the operation induction generator draws reactive power
from the grid for its magnetization. Non linear load distorts the
grid current waveform and also increase the harmonic
component. Due to this, grid current is not in phase with the
grid voltage and its wave shape is also different from sine
wave which is shown in fig 3.
Hence the power factor is not unity. Reactive power
requirement of induction generator and load is supplied by the
grid. Grid reactive power is the sum of both induction
generator and load reactive power is shown in Table I. Grid
reactive power, load current and induction generator current is
shown in fig 4, 5 and 6 respectively.

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The shunt connected STATCOM is connected to the PCC
through interfacing inductors. The induction generator and
load is also connected to the PCC [13]. The STATCOM
compensator output is controlled, so as to maintain the power
quality norms in the grid system
In earlier case reactive power requirement of induction
generator and load is supplied by the grid, excess of power
feeding the load is fed back to the grid which is shown in
Table I. But in the proposed system with STATCOM, reactive
power requirement of induction generator and load is supplied
by the STATCOM instead of grid. The STATCOM injects a
compensating current of variable magnitude and frequency
component at the PCC [8]-[10]. Simulation diagram of
separately excited induction generator feeding non linear load
with STATCOM is shown in fig 8.
4. REFERENCE CURRENTGENERATION FOR
STATCOM
Reference current for the STATCOM is generated based on
instantaneous reactive power theory [7]-[lO]. A STATCOM
injects the compensation current which is a sum of reactive
component current of IG, non-linear load and harmonic
component current of non-linear load.
Pq theory gives a generalized definition of instantaneous
reactive power, which is valid for sinusoidal or non sinusoidal,
balanced or unbalanced, three-phase power systems with or
without zero sequence currents and/or voltages.

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Fig.9 shows the three phase power system, instantaneous
Voltages, va, Vb, vc in volts and instantaneous currents, ia, ib,
icin amps of a three phase system are expressed as
instantaneous space vectors ' v' and 'i' given by(l)
'p' is the instantaneous active power of a three-phase circuit
in Watts, given by (2)
P= v.I ----- (2).
Instantaneous active power of a three-phase circuit 'p' is the
scalar product of instantaneous voltage and current. It is the
product of the sum of three phase voltages and current, given
by(3) .
P= va ia + vb ib + vc ic----------- (3)
Instantaneous active power consists of average component and
oscillatory component as given by (4).
P= Pdc + Pac----------------- (4)
'Pdc' is the average component of instantaneous active power
in watts and 'Pac' is the oscillatory component of instantaneous
active power in watts. 'q' is the instantaneous reactive power of
a three-phase circuit in VAR, given by(5)
--------------(5)
Instantaneous reactive power of a three-phase circuit 'q' is the
vector product of instantaneous voltage and current, given by
(6).
Total current is the sum of instantaneous active, reactive and
harmonic component of current is given by (7)
Here ip, iq, ih are instantaneous active, reactive and harmonic
component of currents respectively.
'ip 'is the instantaneous active component current in amps
given by (8)
Since it is a non linear load reactive component and
harmonic component current are used as a reference
current for STATCOM. The reference current for the three
phases as given by (9), (10), (11).
5. HYSTERESIS CONTROLLER FOR STATCOM
The current control scheme for STATCOM is using a
"hysteresis current controller." Using this technique, the
controller keeps the STATCOM current between boundaries
of hysteresis area and gives correct switching signals for
STATCOM operation [11]-[12].
It is a feedback current control method where the actual
current tracks the reference current within a hysteresis band.
The current controller generates the firing pulses to the VSI by
comparing the reference and actual current.
The hysteresis current control scheme for generating the
switching signals to the STATCOM is shown in Fig.10. If the
current exceeds the upper limit of the hysteresis band, upper
switch of the inverter arm is turned off and the lower switch is
turned on. As a result, the current start to decay. If the current
crosses the lower limit of the hysteresis band, the lower switch
of the inverter arm is turned off and the upper switch is turned
on.

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As a result, the current gets back into the hysteresis band.
Hence, the actual current is forced to track the reference
current within the hysteresis band. The choice of the current
band depends on the value of compensation current and the
interfacing inductance.
6. SIMULATION RESULTS
The performance of the system is analyzed with and without
STATCOM by switching ON the STATCOM at time t=0.3s.
The simulation parameters considered are given in Table
1.Initially the STACOM current is zero after 0.3seconds the
STATCOM starts tracking the reference current within the
hysteresis band which is shown in Fig 11.Gridcurrent is not in
phase with voltage during
STATCOM OFF condition and in ON condition grid current is
1800 out of phase with voltage, which signifies that the excess
power after feeding the non linear load is fed back to the
source which is shown in fig 12 and 13 respectively.
When STATCOM controller is made ON, at t=O.3s, it starts to
mitigate the grid reactive power without any change in any
other parameters. Instantaneous reactive power value of grid,
load and induction generator is shown in fig 14, 15, and 16
respectively

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CONCLUSIONS
This paper presents the STATCOM-based control Scheme for
reactive power compensation and harmonic reduction in grid
connected wind generating system feeding non linear load.
The control system for the STATCOM is simulated using
MATLAB/SIMULINK. The Simulation results shows the grid
voltage and current are in-phase, making the power factor
unity, which implies that the reactive power demand of
Induction generator and load is no longer, fed by the grid
rather it is supplied by the STATCOM. Also the shape of the
grid current is almost sinusoidal and the %THD has been
improved from 1l.08 % to 5.78 % after compensation. The
proposed control scheme has improved the power quality
requirement of a low voltage grid connected wind driven IG
system feeding a non-linear load.
REFERENCES
[1] .Q. Tande 'Applying Power Quality Characteristics of wind
turbine for Assessing impact on Voltage Quality', Wind
Energy, pp 52, 2002
[2] Z. Chen, E. Spooner, 'Grid Power Quality with Variable
Speed Wind Turbines', IEEE Trans on Energy Conversion,
Vol.16, No .2, pp 148- 154, June 200I.
[3] L. H. Hansen, L. Helle, F. Blaabjerg, E. Ritchie, S. Munk-
Nielsen, H.Binder, P. S0rensen and B. Bak - Jensen
"Conceptual Survey of Generators and Power Electronics for
Wind Turbines ", Ris0 National Laboratory, Roskilde,
Denmark, December 200l.
[4] A. Arulampalam, M. Bames & NJenkins, Power quality
and stability improvement of a wind farm using STATCOM,
Proc. TEE Generation, Transmission & Distribution, Vol. 153,
No.6, 2006, 701-710.
[5] Z.Saad-Saoud, M.1.Lisboa, 1.B.Ekanayake, N. Jenkins &
G.Strbac, Application of STATCOMs to wind farms, Proc.
TEE Generation, Transmission & Distribution, Vo1.I45, No.
5, 1998, 511-516.

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[6] A. Arulampalam, 1.B.Ekanayake & NJenkins, Application
study of a STATCOM with energy storage, Proc. lEE
Generation, Transmission &Distribution, Vol. 150, No. 3,
2003, 373-384.
[7] Fang Zheng Peng, Jih-Sheng Lai, 'Generalized
Instantaneous Reactive Power Theory for Three-phase Power
Systems', IEEE on instrumentation and measurement, vol. 45,
no. I, Feb, 1996
BIOGRAPHIES:
Sk. Baji babu student of
PrakasamEngineering College is currently
pursing M.Tech in Power Electronics
specialization. He completed B.Tech
(EEE) from JNTUK. His area of interests
includes Facts, AC & DC Drives.
M.Bala Subba Reddy, HOD(EEE Dept.,)
in Prakasam Engineering College Pursuing
P.hd in JNTUK.He got M.Tech,POWER
SYSTEMS from NIT, Trichy. He
completed B.Tech (EEE) from Madras
Uiversity. His areas of research interest
includePower Electronics, Electrical
machines and AC & DC drives.
T. Madhu is Asst. Professor in EEE Dept.,
of Prakasam Engineering College,
kandukur. He completed M.Tech,
POWER ELECTRONICS from JNTUK.
He completed B.Tech (EEE) from
JNTUK.His area of interests includes in
the subjects of Machine Modelling, Facts
etc.,

Analysis of power quality improvement in grid

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