19-09-2013, 03:34 PM
Modeling and solution of Power Quality Problems
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Abstract
A Power quality problem is an occurrence
manifested as a nonstandard voltage, current or
frequency that results in a failure or a miss-operation
offend use equipments .Utility distribution networks,
sensitive industrial loads, and critical commercial
operations all suffer from various types ofoutages and
service interruptions which can costsignificant
financial 1oss per incident based on process dow n-
time, lost production, idle work forces, and other
factors. 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. The aim therefore, in this work, is
to identify the prominent concerns in the area and
thereby to recommend measures that can enhance the
quality of the power, keeping in mind their economic
viability and technical repercussions. In this paper
electromagnetic transient studies are presented for the
following two custom power controllers: the
distribution static compensator (D-STATCOM), and
the dynamic voltage restorer (DVR). Comprehensive
results are presented to assess the performance of each
device as a potential custom power solution.
INTRODUCTION
ower quality is certainly a major concern in the present
era; it becomes especially important with the
introduction of sophisticated devices, whose
performance is very sensitive to the quality of power
supply. Modern industrial processes are based a large
amount of electronic devices such as programmable
logic controllers and adjustable speed drives. The
electronic devices are very sensitive to disturbances
[1] and thus industrial loads become less tolerant to
power quality problems such as voltage dips, voltage
swells, and harmonics. Voltage dips are considered
one of the most severe disturbances to the industrial
equipmen t. A paper machine can be affected by
disturbances of 10% voltage drop lasting for 100ms.
SOLUTIONS TO POWER QUALITY
PROBLEMS
There are two approaches to the mitigation of power
quality problems. The solution to the power quality
can be done from customer side or from utility side
[4]. First approach is called load conditioning, which
ensures that the equipment is less sensitive to powe r
disturbances, allowing the operation even under
significant voltage distortion. The other solution is to
install line conditioning systems that suppress or
counteracts the power system disturbances. A flexible
and versatile solution to voltage quality problems is
offered by active power filters. Currently they are
based on PWM converters and connect to low and
medium voltage distribution system in shunt or in
series. Series active power filters must operate in
conjunction with shunt passive filters in ord er to
compensate load current harmonics. Shunt active
power filters operate as a controllable current source
and series active power filters operates as a
controllable voltage source. Both schemes are
implemented preferable with voltage source PWM
inverter s [5], with a dc bus having a reactive element
such as a capacitor. Active power filters can perform
one or more of the functions required to compensate
power systems and improving power quality.
MODELING OF CUSTOM POWER
DEVICES AND SIMULATION RESULTS
As mentioned in the previous section that custom
power devices could be the effective means to
overcome some of the major power quality problems
by the way of injecting active and/or reactive power(s)
into the system [9]-[11]. This section of the paper
deals with the modeling of DSTATCOM and DVR.
Consequently some case will be taken up for analysis
and performance comparison of these devices. The
modeling approach adopted in the paper is graphical in
nature, as opposed to mathematical models embedded
in code using a high-level computer language. The
well-developed graphic facilities available in an
industry standard power system package, namely,
MATLAB (/Simulink) [12], is used to conduct all
aspects of model implementation and to carry out
extensive simulation studies. The control scheme for
these devices is shown in Fig.1. The controller input is
an error signal obtained from the reference voltage and
the value rms of the terminal voltage measured.
CONCLUSIONS
Power quality measures can be applied both at the user
end and also at the utility level. The work identifies
some important measures that can be applied at the
utility level without much system upset (or design
changes).This pape r has presented models of custom
power equipment, namely D - STATCOM, DVR, and
applied them to mitigate voltage dip which is very
prominent as per utilities are concerned. The highly
developed graphic facilities available in MATLAB
SIMULINK were used to cond uct all aspects of
model implementation and to carry out extensive
simulation studies on test systems. A new PWM-based
control scheme has been implemented to control the
electronic valves in the two –level VSC used in the D-
STATCOM and DVR. As opposed to fundamental
frequency switching schemes already available in the
MATLAB SIMULINK. This characteristic makes it
ideally suitable for low-voltage custom power
applications. It was observed that in case of
DSTATCOM capacity for power compensation and
voltage regulation depends mainly on the rating of the
dc storage device. The simulation results presented
shows good accuracy with results reported in index
journals.