22-10-2012, 03:18 PM
Control Techniques for Power Quality Improvement in Delta Connected Load using DSTATCOM
Control Techniques.pdf (Size: 552.81 KB / Downloads: 36)
Abstract
Distribution Static Compensator (DSTATCOM)
is a custom power device used for power quality
improvement in distribution system. The control techniques
applied to the DSTATCOM play a major role in its
performance. This paper presents comparative study of the
reference current generation techniques using voltage source
converter based DSTATCOM for reactive power
compensation, source current balancing and harmonic
mitigation in delta connected load. Different control
techniques such as Instantaneous reactive power theory,
Synchronous reference Frame theory and Symmetrical
component theory have been used in this paper. Reference
currents generated by control techniques have been tracked
by the compensator in a hysteresis band control scheme.
Dynamic simulation of the DSTATCOM for different
control strategies has been carried out in
MATLAB/Simpower system environment. Simulation
results demonstrate the performance and feasibility of
various control techniques for DSTACOM.
INTRODUCTION
Reactive power compensation of non-linear and/or
poor power factor loads is an important issue in the
modern distribution system. Moreover situation worsens
in the presence of unbalanced loads. Excessive reactive
power demand increases feeder losses and reduces active
power flow capability of the distribution system where as
unbalancing affects the operation of transformers and
generators [1]. DSTATCOM has been used extensively
for reactive power compensation, load balancing and
harmonic mitigation in the distribution system [2].
The objective of the compensating scheme is to supply
the oscillating component of power such that the dc
component can be supplied by the source. The
performance of DSTATCOM depends on the control
algorithm used for extraction of reference current
components. For this purpose, many control algorithms
have been reported in literature, and some of these are
Instantaneous Reactive Power theory (IRP) [3-5],
interpretations and modifications on IRP [6-9],
Synchronous Reference Frame theory (SRF) [10],
Symmetrical Component theory (SC) [11], current
compensation using dc bus voltage regulation
,computation based on per phase basis and scheme based
on neural network techniques. This paper presents
comparative study of IRP [3] ,SRF [10] and SC[11] used
for harmonic mitigation, reactive power compensation
and load balancing in delta connected load using voltage
source converter based DSTATCOM.
CONTROL ALGORITHEMS
The basic compensator scheme for delta connected
load is shown in Fig 1. In this scheme, the compensator is
represented by ideal current sources. Practical
implementation of compensator is given in section III.
The aim of control scheme is to generate the reference
current waveforms which are to be injected to serve the
required objective. The compensator would produce
desired results as long as its bandwidth is sufficient to
follow the fluctuations in the load.
SYSTEM CONFIGURATION
The current source shown in Fig.1 can be implemented
by a three-phase VSC that has been operated in hysteresis
band control mode. The controller tracks the reference
current generated by control algorithms. The standard
configuration of a three-phase Voltage source converter,
consisting of three poles of two switches each, is
unsuitable for this purpose. In case of standard
configuration of a three-phase VSC, the sum of converter
output currents is zero. It is not desirable for unbalanced
load. It would result in the flow of neutral current of the
load in source neutral. It would cause distortion of source
currents. In this paper, VSC configuration [13] has been
used for delta connected load. Configuration used in this
paper as shown in Fig. 5 is bank of three single-phase
VSC. Each VSC is connected to the network through an
isolating transformer. The purpose of including the
transformer is to provide isolation between the converters.
This prevents the dc storage capacitor from being shorted
through switches in different converters. It is to be noted
that the capacitor must be precharged to sufficiently high
value to obtain satisfactory tracking performance.
CONCLUSION
This paper presents a comparative analysis of three
control strategies used for DSTATCOM installed in threephase
three-wire distribution system for unbalanced and
distorted load. SRF and SC methods results in excellent
reactive power compensation resulting in unity power
factor for both linear and non-linear unbalanced load.
Balancing of source currents has been achieved most
effectively by SC and IRP methods .SRF method results
in slight unbalance for both linear and non-linear
unbalanced load. THD in all three- phase source currents
has been found to be under IEEE prescribed limits for all
three methods. SC method has shown superior
performance for both linear and non-linear unbalanced
load from point of view of reactive power compensation,
load balancing and harmonic mitigation.