25-08-2017, 09:32 PM
Power Quality Improvement Using DVR
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Abstract
Voltage sags and swells in the medium and low voltage distribution grid are considered to
be the most frequent type of power quality problems based on recent power quality studies. Their
impact on sensitive loads is severe. The impact ranges from load disruptions to substantial economic
losses up to millions of dollars. Different solutions have been developed to protect sensitive loads
against such disturbances but the DVR is considered to be the most efficient and effective solution. Its
appeal includes lower cost, smaller size and its dynamic response to the disturbance. This research
described DVR principles and voltage restoration methods for balanced and/or unbalanced voltage
sags and swells in a distribution system. Simulation results were presented to illustrate and understand
the performances of DVR under voltage sags/swells conditions.
INTRODUCTION
Dynamic voltage restores (DVRs) are now
becoming more established in industry to reduce the
impact of voltage dips on sensitive loads
[1-3]
. A voltage
dip is commonly defined as any low voltage drop event
between 10% and 90% of the nominal RMS voltage,
lasting between 0.5 cycles and 1 min
[4]
. In comparison
with interruptions, voltage dips affect a large number of
customers and for some cases may cause extremely
serious problems. Voltage dips are one of the most
occurring power quality problems. They occur more
often and cause severe problems and economical losses.
There are different ways to mitigate voltage dips, swells
and interruptions in transmission and distribution
systems. At present, a wide range of very flexible
controllers which capitalize on newly available power
electronics components are emerging for custom power
applications. Among these, the distribution static
compensator and the dynamic voltage restorer are the
most effective devices; both of them based on the
voltage source converter (SVC) principle
[5]
.
Figure 1 shows a typical DVR series connected
topology. The DVR essentially consists of a series
inverter (VSI), inverter output filter and an energy
storage device connected to the DC link.
DYNAMIC VOLTAGE RESTORER
A Dynamic Voltage Restorer (DVR) is a recently
proposed series connected solid state device that injects
voltage into the system in order to regulate the load side
voltage. The DVR was first installed in 1996
[6]
. It is
normally installed in a distribution system between the
supply and the critical load feeder
[7]
. Its primary
function is to rapidly boost up the load-side voltage in
the event of a disturbance in order to avoid any power
disruption to that load
[8,9]
. There are various circuit
topologies and control schemes that can be used to
implement a DVR. In addition to voltage sags and
swells compensation, DVR can also perform other tasks
such as: line voltage harmonics compensation,
reduction of transients in voltage and fault current
limitations.
The general configuration of the DVR consists of
an Injection/Booster transformer, a Harmonic filter, a
Voltage Source Converter (VSC), DC charging circuit
and a Control and Protection system as shown in Fig
CONVENTIONAL DVR VOLTAGE INJECTION METHODS
The possibility of compensating voltage sag can be
limited by a number of factors including finite DVR
power rating, different load conditions and different
types of voltage sag. Some loads are very sensitive to
phase angle jump and others are tolerant to it.
Therefore, the control strategy depends on the type of
load characteristics. There are three distinguishing
methods to inject DVR compensating voltage:
SIMULATION
In order to show the performance of the DVR in
voltage sags and swells mitigation, a simple distribution
network was simulated using MATLAB (Fig. 1). A
DVR was connected to the system through a series
transformer with a capability to insert a maximum
voltage of 50% of the phase to ground system nominal
voltage. In this simulation the In-Phase Compensation
(IPC) method was used. The load considered in the
study is a 5.5 MVA capacity with 0.92 p.f, lagging.
CONCLUSION
The simulation results showed clearly the
performance of the DVR in mitigating voltage sags and
swells. The DVR handled both balanced and
unbalanced situations without any difficulties and
injected the appropriate voltage component to correct
rapidly any anomaly in the supply voltage to keep the
load voltage balanced and constant at the nominal
value. The efficiency and the effectiveness in voltage
sags/swells compensation showed by the DVR makes
him an interesting power quality device compared to
other custom power devices