20-07-2012, 11:35 AM
Power Quality Improvement In Distribution System Using D-Statcom
In Transmission Lines
Power Quality Improvement.pdf (Size: 453.34 KB / Downloads: 262)
Abstract –
A Power quality problem is an occurrence manifested as a nonstandard voltage, current or frequency that results in a
failure or a mis-operation of end user equipments. Utility distribution networks, sensitive industrial loads and critical
commercial operations suffer from various types of outages and service interruptions which can cost significant financial
losses. With the restructuring of power systems and with shifting trend towards distributed and dispersed generation, the issue
of power quality is going to take newer dimensions. In developing countries like India, where the variation of power
frequency and many such other determinants of power quality are themselves a serious question, it is very vital to take
positive steps in this direction. The present work is to identify the prominent concerns in this area and hence the measures that
can enhance the quality of the power are recommended.
This paper presents the enhancement of voltage sags/swell; harmonic distortion and low power factor using
Distribution Static Compensator (D-STATCOM). The model is based on the Voltage Source Converter (VSC) principle. The
D-STATCOM injects a current into the system to mitigate the voltage sags/swell. to improve harmonic distortion and low
power factor. The simulations were performed using MATLAB SIMULINK version R2009b.
INTRODUCTION
An electric distribution system is part of an
electric system between the bulk power source or
sources and the consumer’s service switches. The
bulk power sources are located in or near the load
area to be served by the distribution system and may
be either generating stations or power substations
supplied over transmission lines.stributionsystems
can, in general, be divided into six parts, namely, sub
transmission circuits, distribution substations,
distribution or primary feeders, distribution
transformers, secondary circuits or secondary’s, and
consumer’s service connections and meters or
consumer’s services. One of the most common power
quality problems today is voltage sag/swell. It is
often set only by two parameters, depth/magnitude
and duration. The voltage sag/swell magnitude is
ranged from 10% to 90% of nominal voltage and
with duration from half a cycle to 1 min. In a threephase
system voltage sag is by nature a three-phase
phenomenon.
VOLTAGE SOURCE CONVERTER
(VSC)
A voltage-source converter is a power
electronic device, which can generate a sinusoidal
voltage with any required magnitude, frequency and
phase angle. Voltage source converters are widely
used in adjustable-speed drives, but can also be used
to mitigate voltage dips. The VSC is used to either
completely replace the voltage or to inject the
‘missing voltage’. The ‘missing voltage’ is the
difference between the nominal voltage and the
actual. The converter is normally based on some kind
of energy storage, which will supply the converter
with a DC voltage. The solid-state electronics in the
converter is then switched to get the desired output
voltage. Normally the VSC is not only used for
voltage sag/swell mitigation, but also for other power
quality issues, e.g. flicker and harmonics.
CONTROLLER
The aim of the control scheme is to maintain
constant voltage magnitude at the point where a
sensitive load is connected, under system
disturbances. The control system only measures the
r.m.s voltage at the load point, i.e., no reactive power
measurements are required. The VSC switching
strategy is based on a sinusoidal PWM technique
which offers simplicity and good response. Since
custom power is a relatively low-power application,
PWM methods offer a more flexible option than the
Fundamental Frequency Switching (FFS) methods
favored in FACTS applications. Besides, high
switching frequencies can be used to improve on the
efficiency of the converter, without incurring
significant switching losses.
The controller input is an error signal obtained
from the reference voltage and the value rms of the
terminal voltage measured. Such error is processed
by a PI controller the output is the angle δ, which is
provided to the PWM signal generator. It is important
to note that in this case, indirectly controlled
converter, there is active and reactive power
exchange with the network simultaneously: an error
signal is obtained by comparing the reference voltage
with the rms voltage measured at the load point. The
PI controller process the error signal generates the
required angle to drive the error to zero, i.e., the load
rms voltage is brought back to the reference voltage.
Distribution Static Compensator
(DSTATCOM)
A D-STATCOM (Distribution Static
Compensator), which is schematically depicted in
Figure-5.1, consists of a two-level Voltage Source
Converter (VSC), a dc energy storage device, a
coupling transformer connected in shunt to the
distribution network through a coupling transformer.
The VSC converts the dc voltage across the storage
device into a set of three-phase ac output voltages.
These voltages are in phase and coupled with the ac
system through the reactance of the coupling
transformer. Suitable adjustment of the phase and
magnitude of the D-STATCOM output voltages
allows effective control of active and reactive power
exchanges between the D-STATCOM and the ac
system. Such configuration allows the device to
absorb or generate controllable active and reactive
power.
METHODOLOGY
To enhance the performance of distribution
system, D-STATCOM was connected to the
distribution system. D-STATCOM was designed
using MATLAB simulink version R2009b.
The test system shown in figure 6.1
comprises a 230kV, 50Hz transmission system,
represented by a Thevenin equivalent, feeding into
the primary side of a 3-winding transformer
connected in Y/Y/Y, 230/11/11 kV. A varying load is
connected to the 11 kV, secondary side of the
transformer. A two-level D-STATCOM is connected
to the 11 kV tertiary winding to provide
instantaneous voltage support at the load point. A 750
μF capacitor on the dc side provides the DSTATCOM
energy storage capabilities. Circuit
Breaker is used to control the period of operation of
the D-STATCOM.
CONCLUSION
A single-phase to ground fault, three-phase
fault and voltage swell are occurred in a time period
of 500ms-900ms at different inductive and capacitive
loads. D-STATCOM is designed by the combination
of two-level VSC and PWM-based control. Here the
voltage measurement is controlled by PWMcontroller.
So by using D-STATCOM sag of 13%,
interruption of 25% and swell of 11% conditions are
mitigated.