INTRODUCTION:
The
various power quality
problems are due
to the increasing use of non
linear and power
electronic loads.Harmonics and
voltage distortion occur
due to these
loads. The power quality
problems can cause
malfunctioning of sensitive equipments,
protection and relay
system.Distribution system is
mainly affected by
voltage sag and swell
power quality issue.
Short circuits, lightning
strokes, faults and inrush
currents are the
causes of voltage
sags. Start/stop of heavy
loads, badly dimensioned
power sources, badly regulated transformers, single line to
ground fault on the system lead to voltage
swells. Voltage sag is a decrease of
the normal voltage level between 10 and
90% of the nominal rms voltage at the power frequency, for durations of 0,5
cycle to 1 minute. Voltage swells are momentary increase of the voltage, at the power
frequency, outside the
normal tolerances, with duration of more than one cycle and
typically less than a few seconds .
The use of
custom power devices
is one of the most
efficient method to
mitigate voltage sag
and swells.
There are many custom power devices. Each of which
has its own benefits and
limitations. Custom power
device(CPD) is a powerful tool based on semiconductor switches concept
to protect sensitive
loads if there
is a disturbance
from power line.
Among the several
novel CPD, the
Dynamic Voltage Restorer
(DVR) are now
becoming more established
in industry to mitigate
the impact of
voltage disturbances on sensitive loads.
DYNAMIC
VOLTAGE RESTORER:
Dynamic
voltage restorer is
a static var
device that has
applications in a
variety of transmission
and distribution systems. It
is a series
compensation device, which
protects sensitive electric
load from power
quality problems such
as voltage sags, swells, unbalance and distortion
through power electronic controllers
that use voltage
source converters (VSC). The
first DVR was installed in North America in 1996 - a
12.47 kV system
located in Anderson,
South Carolina.
Since then,
DVRs have been applied to protect critical loads in utilities,
semiconductor and food
processing. Today, the
dynamic voltage restorer
is one of
the most effective
PQ devices in solving voltage sag
problems.
PRINCIPLE
OF DVR:
The
basic principle of
the dynamic voltage
restorer is to
inject a voltage of required magnitude and frequency, so that it can restore the load side voltage to the
desired amplitude and waveform even
when the source
voltage is unbalanced
or distorted. Generally, it
employs a gate turn off thyristor (GTO)
solid state power electronic switches
in a pulse
width modulated (PWM)
inverter structure. The DVR can generate
or absorb independently
controllable real and reactive
power at the load side. In other
words, the DVR is made of a solid state
DC to AC switching power converter that injects a set of three phase AC output voltages in series and
synchronism with the distribution line
voltages. Dynamic voltage restorer
is a series
connected device designed
to maintain a
constant RMS voltage
across a sensitive load.
SYSTEMATIC
DIAGRAM OF DVR:
COMPONENTS
OF DYNAMIC VOLTAGE RESTRORER:
The DVR consists of:
A. Voltage
Source Converter (VSC)
B. Injection
Transformer
C. Passive
Filters
D. Energy
storage device/ Control system
The description of each
component of DVR is given below
Voltage
Source Converter (VSC):
Voltage Source
Converter converts the dc voltage
from the energy storage
unit to a
controllable three phase
ac voltage. The inverter
switches are normally
fired using a
sinusoidal Pulse Width Modulation scheme.
Injection
Transformer:
Injection transformers
used in the DVR plays a crucial role in ensuring the
maximum reliability and
effectiveness of the restoration scheme.
It is connected
in series with
the distribution feeder.
Passive
Filters:
Passive Filters are
placed at the high voltage side of the DVR to
filter the harmonics.
These filters are
placed at the
high voltage side as
placing the filters
at the inverter
side introduces phase angle
shift which can
disrupt the control
algorithm.
Energy storage device/ Control system:
Examples of
energy storage devices
are dc capacitors, batteries, super-capacitors, superconducting magnetic
energy Storage and flywheels. The capacity of energy storage device has
a big impact on the compensation capability of the system. Compensation of
real power is
essential when large
voltage sag occurs.
VOLTAGE
INJECTION METHODS
Voltage injection
or compensation methods
by means of a
DVR depend upon
the limiting factors
such as; DVR
power ratings, various conditions
of load, and
different types of voltage
sags. Some loads
are sensitive towards
phase angel jump and some are
sensitive towards change in magnitude and others are
tolerant to these.
Therefore the control
strategies depend upon the
type of load
characteristics. There are
four different methods of DVR voltage injection which are
i. Pre-sag compensation
method
ii. In-phase
compensation method
iii. In-phase advanced
compensation method
iv. Voltage tolerance
method with minimum energy injection
A. Pre-Sag/Dip Compensation :
The pre-sag
method tracks the
supply voltage continuously
and if it
detects any disturbances
in supply voltage
it will inject the
difference voltage between
the sag or
voltage at PCC and
pre-fault condition, so
that the load
voltage can be restored
back to the
pre-fault condition. Compensation
of voltage sags in the both phase
angle and amplitude sensitive loads
would be achieved by pre-sag compensation method. In this method the
injected active power cannot be controlled and it is
determined by external
conditions such as
the type of faults and load conditions.
VDVR = Vprefault – Vsag
B. In phase Compensation method :
This is the most straight
forward method. In this method the injected voltage
is in phase with
the supply side
voltage irrespective of the
load current and
pre-fault voltage. The
phase angles of the pre-sag and load voltage are different but the
most important criteria
for power quality
that is the
constant magnitude of
load voltage are
satisfied. One of
the advantages of
this method is
that the amplitude
of DVR injection
voltage is minimum
for a certain
voltage sag in comparison with other strategies.
C. In Phase advanced compensation :
In this method the real
power spent by the DVR is decreased
by minimizing the
power angle between
the sag voltage
and load current.
In case of
pre-sag and in-phase compensation method the
active power is
injected into the system during disturbances. The active
power supply is limited stored energy in
the DC links and this part is one of the most
expensive parts of DVR.The minimization of injected energy is
achieved by making
the active power
component zero by having the
injection voltage phasor
perpendicular to the load current phasor.In this method the
values of load current and voltage are
fixed in the system so we can change only the phase of
the sag voltage.
IPAC method uses
only reactive power and unfortunately, not
al1 the sags
can be mitigated without real
power, as a
consequence, this method
is only suitable for a limited
range of sags
D. Voltage
tolerance method with
minimum energy
injection:
A small drop in voltage
and small jump in phase angle can be tolerated
by the load
itself. If the
voltage magnitude lies
between 90%-110% of
nominal voltage and
5%-10% of nominal
state that will not disturb the operation characteristics of loads. Both
magnitude and phase are the control
parameter for this method
which can be achieved by
small energy injection
CONTROL
TECHNIQUES
A. Linear controllers :
The three main voltage
controllers, which have been proposed
in literature, are
Feed forward (open loop),
Feedback (closed loop) and
Multi-loop controller .The feed-forward voltage controller is
the primary choice
for the DVR,
because of its
simplicity and fastness.
The supply voltage
is continuously monitored
and compared with
a reference voltage;
if the difference
exceeds a certain
tolerance, the DVR
injects the required voltage. The drawback of the open
loop controller is the high
steady state error.
In the feedback
control, the load
voltage is measured and compared
with the reference voltage, the missing
voltage is supplied by the DVR at the supply bus in
a feedback loop.
This controller has
the advantage of
accurate response, but it is complex and time-delayed. Multiloop control
is used with an outer voltage loop to control the DVR
voltage and an
inner loop to
control the load
current. This method
has the strengths
of feed-forward and
feedback control strategies,
on the expense
of complexity and
time delay.
B.
Non-linear Controllers:
It appears
that the nonlinear
controller is more
suitable than the linear type since the DVR is truly a
non-linear system due to the
presence of power
semiconductor switches in
the inverter bridge.
The most non-linear
controllers are the
Artificial Neural Networks
(ANN), Fuzzy Logic
(FL) and Space Vector Pulse Width Modulation (SVPWM). ANN control
method has adaptive
and self-organization capacity.
The ANN
has inherent learning
capability that can
give improved precision
by interpolation. FL controllers
are an attractive choice when precise mathematical formulations
are not possible. When a FL controller
is used, the tracking error and transient
overshoots of PWM
can be considerably
reduced. SVPWM control strategy is to adopt a space vector of
the inverter voltage
to get better
performance of the
exchange is gained in low switching frequency conditions.
V. CONCLUSION
By the use
of different control
techniques it is
viewed that DVR
is suitable for
voltage sag and
swell mitigation. The
basic structure of DVR, its operation, compensation methods and
control techniques are
discussed in detail.
DVR has the
advantage of low cost, require
less computational efforts and its control
is simple as
compared to other
methods. DVR provides
simpler implementation for
voltage profile improvement.
Linear controllers provide
simpler operation and
less computational efforts
when compared to
other methods.
