05-10-2012, 03:26 PM
A STATCOM-Control Scheme for Grid Connected Wind Energy System for Power Quality Improvement
A STATCOM-Control.pdf (Size: 742.23 KB / Downloads: 172)
Abstract
Injection of the wind power into an electric grid
affects the power quality. The performance of the wind turbine
and thereby power quality are determined on the basis of measurements
and the norms followed according to the guideline
specified in International Electro-technical Commission standard,
IEC-61400. The influence of the wind turbine in the grid
system concerning the power quality measurements are-the active
power, reactive power, variation of voltage, flicker, harmonics,
and electrical behavior of switching operation and these are
measured according to national/international guidelines. The
paper study demonstrates the power quality problem due to
installation of wind turbine with the grid. In this proposed scheme
STATic COMpensator (STATCOM) is connected at a point of
common coupling with a battery energy storage system (BESS) to
mitigate the power quality issues. The battery energy storage is
integrated to sustain the real power source under fluctuating wind
power. The STATCOM control scheme for the grid connected
wind energy generation system for power quality improvement
is simulated using MATLAB/SIMULINK in power system block
set. The effectiveness of the proposed scheme relives the main
supply source from the reactive power demand of the load and the
induction generator. The development of the grid co-ordination
rule and the scheme for improvement in power quality norms as
per IEC-standard on the grid has been presented.
INTRODUCTION
TO have sustainable growth and social progress, it is necessary
to meet the energy need by utilizing the renewable
energy resources like wind, biomass, hydro, co-generation,
etc In sustainable energy system, energy conservation and
the use of renewable source are the key paradigm. The need
to integrate the renewable energy like wind energy into power
system is to make it possible to minimize the environmental
impact on conventional plant [1]. The integration of wind energy
into existing power system presents a technical challenges
and that requires consideration of voltage regulation, stability,
power quality problems. The power quality is an essential customer-
focused measure and is greatly affected by the operation
of a distribution and transmission network. The issue of
power quality is of great importance to the wind turbine [2].
Voltage Variation
The voltage variation issue results from the wind velocity
and generator torque. The voltage variation is directly related
to real and reactive power variations. The voltage variation is
commonly classified as under:
• Voltage Sag/Voltage Dips.
• Voltage Swells.
• Short Interruptions.
• Long duration voltage variation.
The voltage flicker issue describes dynamic variations in the
network caused by wind turbine or by varying loads. Thus the
power fluctuation from wind turbine occurs during continuous
operation. The amplitude of voltage fluctuation depends on grid
strength, network impedance, and phase-angle and power factor
of the wind turbines. It is defined as a fluctuation of voltage in
a frequency 10–35 Hz. The IEC 61400-4-15 specifies a flicker
meter that can be used to measure flicker directly.
GRID COORDINATION RULE
The AmericanWind Energy Association (AWEA) led the effort
in the united state for adoption of the grid code for the interconnection
of the wind plants to the utility system. The first grid
code was focused on the distribution level, after the blackout in
the United State in August 2003. The United State wind energy
industry took a stand in developing its own grid code for contributing
to a stable grid operation. The rules for realization of
grid operation of wind generating system at the distribution network
are defined as-per IEC-61400-21. The grid quality characteristics
and limits are given for references that the customer
and the utility grid may expect. According to Energy-Economic
Law, the operator of transmission grid is responsible for the organization
and operation of interconnected system [6].
TOPOLOGY FOR POWER QUALITY IMPROVEMENT
The STATCOM based current control voltage source inverter
injects the current into the grid in such a way that the source
current are harmonic free and their phase-angle with respect to
source voltage has a desired value. The injected current will
cancel out the reactive part and harmonic part of the load and
induction generator current, thus it improves the power factor
and the power quality. To accomplish these goals, the grid voltages
are sensed and are synchronized in generating the current
command for the inverter. The proposed grid connected
system is implemented for power quality improvement at point
of common coupling (PCC), as shown in Fig. 1.
CONTROL SCHEME
The control scheme approach is based on injecting the currents
into the grid using “bang-bang controller.” The controller
uses a hysteresis current controlled technique. Using such technique,
the controller keeps the control system variable between
boundaries of hysteresis area and gives correct switching signals
for STATCOM operation.
The control system scheme for generating the switching signals
to the STATCOM is shown in Fig. 3.
The control algorithm needs the measurements of several
variables such as three-phase source current , DC voltage
, inverter current with the help of sensor. The current
control block, receives an input of reference current and
actual current are subtracted so as to activate the operation
of STATCOM in current control mode [16]–[18].
Power Quality Improvement
It is observed that the source current on the grid is affected
due to the effects of nonlinear load and wind generator, thus
purity of waveform may be lost on both sides in the system.
The inverter output voltage under STATCOM operation with
load variation is shown in Fig. 8. The dynamic load does affect
the inverter output voltage. The source current with and without
STATCOM operation is shown in Fig. 9. This shows that the
unity power factor is maintained for the source power when the
STATCOM is in operation. The current waveform before and
after the STATCOM operation is analyzed. The Fourier analysis
of thiswaveform is expressed and theTHD of this source current
at PCC without STATCOM is 4.71%, as shown in Fig. 10.
The power quality improvement is observed at point of
common coupling, when the controller is in ON condition. The
STATCOM is placed in the operation at 0.7 s and source current
waveform is shown in Fig. 11 with its FFT. It is shown that the
THD has been improved considerably and within the norms of
the standard.
CONCLUSION
The paper presents the STATCOM-based control scheme for
power quality improvement in grid connected wind generating
system and with non linear load. The power quality issues and
its consequences on the consumer and electric utility are presented.
The operation of the control system developed for the
STATCOM-BESS in MATLAB/SIMULINK for maintaining
the power quality is simulated. It has a capability to cancel out
the harmonic parts of the load current. It maintains the source
voltage and current in-phase and support the reactive power
demand for the wind generator and load at PCC in the grid
system, thus it gives an opportunity to enhance the utilization
factor of transmission line. The integrated wind generation
and STATCOM with BESS have shown the outstanding performance.
Thus the proposed scheme in the grid connected
system fulfills the power quality norms as per the IEC standard
61400-21.