Iaetsd minimization of voltage sags and swells using dvr

MINIMIZATION OF VOLTAGE SAGS AND SWELLS USING DVR
N.VISWANATH Dr. K. RAMA SUDHA
PG Scholar Professor
Department Of Electrical Engineering,
Andhra University,
Visakhapatnam,
Andhra Pradesh
ABSTRACT: - Power quality problem is an
occurrence of non-standard voltage, current or
frequency that results in a failure or a
disoperation of end user equipments. Utility
distribution networks, sensitive industrial loads
and critical commercial operations suffer from
various types of outages and service
interruptions which can cost significant
financial losses. 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 present work is to identify the
prominent concerns in this area and hence the
measures that can enhance the quality of the
power are recommended. This work describes
the techniques of correcting the supply voltage
sag, swell and interruption in a distributed
system. 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 most effective devices, both
of them based on the VSC principle.
KEY WORDS: Dynamic voltage restorer,
Voltage Sag and swell, PWM Generator.
I INTRODUCTION
The quality of output power delivered
from the utilities has become a major concern. The
most concerning disturbance affecting power
quality is voltage sag. Voltage sag is a sudden
drop in the Root Mean Square (RMS) [1] voltage
and is usually characterized by the retained voltage.
The major source of voltage sag is short- circuits
on the utility lines. Faults from the disturbed
process will generate a momentary voltage sag
[2][3] in the electrical environment to the end user.
The Dynamic Voltage Restorer (DVR) is an
effective Custom Power device which is used to
mitigate the impacts of voltage sags on sensitive
loads in distribution systems. DVR is used for
balancing the load voltage due to harmonics and
unbalancing at the source end, in order to eliminate
switching transients. DVR has to inject voltages
with large magnitude, which is completely
undesirable. By varying load voltage angle, if the
required nominal voltage is injected at the system
frequency, the control operation will be efficient.
To realize this, a method for estimating the
frequency from the sampled injected voltage signal
has been presented.
DVR consists of energy storage device,
pulse width modulation inverter, LC filter and
series transformer. Pulse Width Modulated (PWM)
control technique is applied for inverter switching
to produce a three phase 50 Hz sinusoidal voltages
at the load terminals. The PWM scheme which is
used to synthesize the injected voltage generates
switching frequency harmonics must be prevented
from entering into the utility and customer system.
A low-pass filter is introduced to accomplish this
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function. Literature shows that a number of
techniques are available for improving power
quality problems and frequency estimation to
measure the signals which are available in distorted
form. Least mean square, Kalman filtering,
Discrete Fourier transform, Smart discrete Fourier
Transform and Newton method are some of the
techniques shown in literature. Faults in the
distribution system may cause voltage sag or swell
in the large parts of the system. Voltage sag and
swell can cause sensitive equipment to fail and
create a large current unbalance that trips the
circuit breakers. These effects can be very
expensive for the customer, to avoid equipment
damage. There are many different methods to
mitigate voltage sags and swells, but the use of a
DVR is considered to be the most cost efficient
method. DVR with PI controller has a simple
structure and offers a satisfactory performance
over a wide range of operating conditions. The
main problem of Conventional Controllers [3][4] is
the correct tuning of the controller gains. When
there are variations in the system parameters and
operating conditions, the controller may not
provide the required control performance with fixed
gains.
Power Quality problem is the main
concern in electricity industry. Power Quality
includes a wide range of disturbances such as
voltage sags/swells, flicker, harmonics distortion,
impulse transient, and interruptions. And the
majority of power quality problems are due to
different fault conditions. These conditions cause
voltage sag. Voltage sag can occur at any instant of
time, with amplitude ranging from 10-90% and a
duration lasting for half a cycle to one minute. It is
generally caused by faults in the power system
and characterized by its magnitude and duration.
The duration of voltage sag depends on clearance
of fault by using protective devices. Power
Electronics based devices installed at medium
voltage level for mitigation of power quality
phenomenon, known as “Custom Power Devices”,
able to deliver customized solution to power
quality problems. Voltage sag and interruption
mitigating devices are normally connected between
the supply and the load.
Dynamic voltage restorer [5] is a series
connected device designed to maintain a constant
RMS voltage across a sensitive load. The structure
of DVR is shown in Fig. I. The DVR consists of:
Voltage Source Inverters: Voltage Source Inverters
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 transformers: 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 devices: 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.
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s
t
Fig 1: Structure of Dynamic Voltage Restorer
II COMPENSATION OF VOLTAGE SAG
USING DVR
The single line diagram of test system is
shown in Fig.2. The voltage source is connected
to a feeder with an impedance of
Rs +jXS (1)
The load is balanced and the impedance of the
load is given by
RL + jXL (2)
Fig 3 shows the test system with 3phase fault.
VL is the source voltage in volts
vt is voltage at point of common coupling in volts.
Rs+jXs is impedance of the feeder in ohms
VL is the load voltage in voltage
RL
+jXL is the load impedance in ohms.
Is is the source current and IL is the load current
Fig.2. Single line diagram of test system
DVR is connected between a terminal bus
and load bus. The control technique to be adopted
depends on the type of load as some loads are
sensitive to only magnitude change whereas some
other loads are sensitive to both magnitude and
phase angle shift. Control techniques that utilize
real and reactive power compensation are
generally classified as pre-sag compensation, in-
phase compensation and energy optimization
technique. The single line diagram of DVR
connected in the distribution system
Fig.3. single line diagram of dynamic voltage restorer
connected to distribution system
When the source voltage drops or
increases, the dynamic voltage restorer injects a
series voltage through the injection transformer
so that the desired load [11] voltage magnitude
can be maintained. The series injected voltage of
the DVR, Vk can be written as:
Vk = Vt + Vl (3)
Vk is the series injected voltage in the distribution
system such that it mitigates the voltage sag and
regulates the load bus voltage, Vl to a reference
value Vl
*
. It is pre specified value. The reference
voltage of the DVR can be written as
Vk
*
= Vt + Vl
*
(4)
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III Function of DVR:
The main function of a DVR is the
protection of sensitive loads from voltage
sags/swells coming from the network. Therefore as
shown in Figure, the DVR is located on approach
of sensitive loads. If a fault occurs on other lines,
DVR inserts series voltage [6] VDVR and
compensates load voltage to pre fault value. The
momentary amplitudes of the three injected phase
voltages are controlled such as to eliminate any
detrimental effects of a bus fault to the load voltage
VL. This means that any differential voltages caused
by transient disturbances in the ac feeder will be
compensated by an equivalent voltage generated by
the converter and injected on the medium voltage
level through the booster transformer.
The DVR works independently of the type
of fault or any event that happens in the system,
provided that the whole system remains connected
to the supply grid, i.e. the line breaker does not trip.
For most practical cases, a more economical design
can be achieved by only compensating the positive
and negative sequence [7] components of the
voltage disturbance seen at the input of the DVR.
This option is Reasonable because for a typical
distribution bus configuration, the zero sequence
part of a disturbance will not pass through the step
down transformer because of infinite impedance for
this component.
The DVR has two modes of operation
which are: standby mode and boost mode. In
standby mode (VDVR=0), the booster transformer’s
low voltage winding is shorted through the
converter. No switching of semiconductors occurs
in this mode of operation, because the individual
converter legs [8] are triggered such as to establish
a short-circuit path for the transformer connection.
Therefore, only the comparatively low conduction
losses of the semiconductors in this current loop
contribute to the losses. The DVR will be most of
the time in this mode. In boost mode (VDVR>0),
the DVR is injecting a compensation voltage
through the booster transformer due to a detection
of a supply voltage disturbance.
Fig.4.Equivalent Circuit of DVR
Figure 4 shows the equivalent circuit of
the DVR, when the source voltage is drop or
increase, the DVR injects a series voltage Vinj
through the injection transformer [9][10] so that the
desired load voltage magnitude VL can be
maintained.
The series injected voltage of the DVR
can be written as
Vinj = VL + VS (5)
Where;
VL is the desired load voltage magnitude
VS is the source voltage during sags/swells
condition.
The load current ILoad is given by,
=
( ± ∗ )
(6)
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Pulse-width modulation (PWM):
Pulse-width modulation (PWM), or pulse-duration
modulation (PDM), is a modulation technique that
controls the width of the pulse, formally the pulse
duration, based on modulator signal information.
Although this modulation technique can be used to
encode information for transmission, its main use is
to allow the control of the power supplied to
electrical devices, especially to inertial loads such
as motors. In addition, PWM is one of the two
principal algorithms used in photovoltaic solar
battery chargers, the other being MPPT.
The average value of voltage (and current)
fed to the load is controlled by turning the switch
between supply and load on and off at a fast pace.
The longer the switch is on compared to the off
periods, the higher the power supplied to the load.
The PWM switching frequency has to be
much higher than what would affect the load (the
device that uses the power), which is to say that the
resultant waveform perceived by the load must be
as smooth as possible. Typically switching has to
be done several times a minute in an electric stove,
120 Hz in a lamp dimmer, from few kilohertz
(kHz) to tens of kHz for a motor drive and well into
the tens or hundreds of kHz in audio amplifiers and
computer power supplies.
The term duty cycle describes the
proportion of 'on' time to the regular interval or
'period' of time; a low duty cycle corresponds to
low power, because the power is off for most of the
time. Duty cycle is expressed in percent, 100%
being fully on.
The main advantage of PWM is that
power loss in the switching devices is very low.
When a switch is off there is practically no current,
and when it is on and power is being transferred to
the load, there is almost no voltage drop across the
switch. Power loss, being the product of voltage
and current, is thus in both cases close to zero.
PWM also works well with digital
controls, which, because of their on/off nature, can
easily set the needed duty cycle. PWM has also
been used in certain communication systems where
its duty cycle has been used to convey information
over a communications channel.
Fig 5: An Example of PWM in an Ac Motor Driver
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IV SIMULATION FIGURES
Fig.6. Main Block Diagram of DVR
Fig.7. Control System of the DVR
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V SIMULATION RESULTS
The faults in the three phase source can be eliminated by calculating phase angle θ. But the calculation of θ
becomes complex some times. So by using PWM generator the calculation of the phase angle can be found
easily from the magnitude part only.
The figure8 shows the three phase waveform where fault occur at phase A. By using DVR with PWM
generator the fault is eliminated and the output waveform is shown in figure9.
+
Fig.8.The simulation of the input fault
Fig.9. Simulation result of the output
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VI CONCLUSION
In the simulation study, MATLAB Simulink is used to
simulate the model of dual dynamic voltage restorer is an
effective custom power device for voltage sags and swells
mitigation. The Dual DVR controls under different faults
without any difficulties and injects the appropriate voltage
component to correct rapidly any abnormally in the supply
voltage to keep the load voltage balanced and constant at the
nominal value.
VII REFERENCES
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[3] Choi, S.S.; Li, B.H.; Vilathgamuwa, D.M.(2000) : A Comparative
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