04-07-2012, 11:40 AM
MITIGATION OF POWER QUALITY DISTURBANCES
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ABSTRACT
Power Quality is a major concern of our modern industries and other consumers. Poor quality of supply
will affect the performance of customer equipment such as computers, microprocessors adjustable speed
drives, power electronic devices, life saving equipment in hospitals, etc. and result in heavy financial losses
to customers due to loss of production or breakdown in industries or loss of life in a hospital. The two
major power quality disturbances are voltage sag and harmonic distortion. In the event of voltage sag, due
to insufficient energy, equipments may malfunction or trip.
INTRODUCTION
Electrical fault in a distribution network is almost
impossible to avoid. Main causes for power
quality disturbances may be due to insulation
failure, tree falling, birds contact, lightning or a
fault on an adjacent feeder. These disturbances
may be in the form of voltage sag, swells, voltage
imbalances, transients, interruptions and
harmonics which can cause problems to the
industries ranging from malfunctioning of
equipments to complete plant shutdowns.
METHODOLOGY
The methodology undertaken is to investigate
how power quality disturbances affect certain
sensitive equipment in industries, commercial
centres, major hospitals, etc.
The objective is to design a device which will
perform dual functions of mitigation of voltage
sag and suppression of harmonics dynamically
and simultaneously. The device proposed is
named as Quality Power Provider (QPP) which
possesses certain features of the Dynamic Voltage
Restorer (DVR). QPP is designed to perform the
above dual functions with low power and energy
ratings compared to an Uninterrupted Power
Supply (UPS) or shunt connected Static
Compensator (STATCOM).
Modeling and simulation of hospital network
The simulated model of the detailed 132/11KV
JAWA distribution network modeled using
PSS/ADEPT. The complete model of JAWA
network is given in Figure 1 Appendix A. The
segment of above network that supplies Ipoh
hospital 11KV switching station is shown in
Figure 2.
The PSS/ADEPT software was used to study the
load flow and voltage sag magnitudes at various
nodes under fault conditions. Four types of faults,
namely, Single Line to Ground Fault (SLGF),
Line to Line Fault (LLF), Double Line to Ground
Fault (DLGF) and 3 Line to Ground Fault (3LGF)
were created at Node JawaT2 (Appendix A) and
voltages were measured.
Harmonic detection and compensation
The sinusoidal waveform generated by the sine
wave generator is compared with harmonic
distorted load voltage and synchronized with the
PLL. As the series filter works as a fundamental
sinusoidal current source, it automatically
generates a harmonic voltage equal to the
harmonic voltage drop. The voltage drop will be
modulated with the triangular wave of high
frequency that will generate the firing pulse to be
used for the VSI switching operation which will
produce the pulse output ac voltage.
SIMULATIONS AND RESULTS
In the simulation, a 11KV three-phase supply was
used, and the load was connected at the 400Volts
side. The QPP is design to maintain the voltage at
the sensitive load at its rated value and sinusoidal
waveform, during sag or harmonics or both at the
source side or at the load side.
The following are the various simulation
experiments conducted to check the performance
of the QPP. Test results for LGF, DLGF and
without any disturbances are shown in Appendix
D. Experiment was conducted with QPP both in
off and on mode, with sag and harmonics load
connected. In the entire above test, the QPP is
able to maintain the voltage at the load at its rated
value and sinusoidal.