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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 most concerning disturbance
quality is voltage sag. Voltage sag is a sudden
ABSTRACT: - Power quality problem is an
drop in the Root Mean Square (RMS) [1] voltage
occurrence of non-standard voltage, current or
and is usually characterized by the retained voltage.
frequency that results in a failure or a
The major source of voltage sag is short- circuits
disoperation of end user equipments. Utility
on the utility lines. Faults from the disturbed
distribution networks, sensitive industrial loads
process will generate a momentary voltage sag
and critical commercial operations suffer from various
types
interruptions
of which
outages can
and
cost
[2][3] in the electrical environment to the end user.
service
The Dynamic Voltage Restorer (DVR) is an
significant
effective Custom Power device which is used to
financial losses. With the restructuring of power systems and
affecting power
mitigate the impacts of voltage sags on sensitive
with shifting trend towards
loads in distribution systems. DVR is used for
distributed and dispersed generation, the issue
balancing the load voltage due to harmonics and
of power quality is going to take newer
unbalancing at the source end, in order to eliminate
dimensions. The present work is to identify the
switching transients. DVR has to inject voltages
prominent concerns in this area and hence the
with large
measures that can enhance the quality of the
magnitude,
which is completely
undesirable. By varying load voltage angle, if the
power are recommended. This work describes
required nominal voltage is injected at the system
the techniques of correcting the supply voltage
frequency, the control operation will be efficient.
sag, swell and interruption in a distributed
To realize this, a method for estimating the
system. At present, a wide range of very flexible
frequency from the sampled injected voltage signal
controllers, which capitalize on newly available
has been presented.
power electronics components, are emerging for custom power applications. Among these, the
DVR consists of energy storage device,
distribution static compensator and the dynamic
pulse width modulation inverter, LC filter and
voltage restorer are most effective devices, both
series transformer. Pulse Width Modulated (PWM)
of them based on the VSC principle.
control technique is applied for inverter switching to produce a three phase 50 Hz sinusoidal voltages
KEY WORDS: Dynamic voltage restorer,
at the load terminals. The PWM scheme which is
Voltage Sag and swell, PWM Generator.
used to synthesize the injected voltage generates switching frequency harmonics must be prevented
I INTRODUCTION
from entering into the utility and customer system. The quality of output power delivered
A low-pass filter is introduced to accomplish this
from the utilities has become a major concern. The
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INTERNATIONAL CONFERENCE ON CURRENT INNOVATIONS IN ENGINEERING AND TECHNOLOGY
ISBN: 378 - 26 - 138420 - 5
function. Literature shows that a number of
voltage level for mitigation of power quality
techniques are available for improving power
phenomenon, known as “Custom Power Devices�,
quality problems and frequency estimation to
able to deliver customized solution to power
measure the signals which are available in distorted
quality problems.
form. Least mean square, Kalman filtering,
mitigating devices are normally connected between
Discrete Fourier transform, Smart discrete Fourier
the supply and the load.
Voltage sag and interruption
Transform and Newton method are some of the Dynamic voltage restorer [5] is a series
techniques shown in literature. Faults in the
connected device designed to maintain a constant
distribution system may cause voltage sag or swell
RMS voltage across a sensitive load. The structure
in the large parts of the system. Voltage sag and
of DVR is shown in Fig. I. The DVR consists of:
swell can cause sensitive equipment to fail and create a large current unbalance that trips the
Voltage Source Inverters: Voltage Source Inverters
circuit breakers. These effects can be very
converts the dc voltage from the energy storage
expensive for the customer, to avoid equipment
unit to a controllable three phase ac voltage. The
damage. There are many different methods to
inverter switches are normally fired using a
mitigate voltage sags and swells, but the use of a
sinusoidal Pulse Width Modulation scheme.
DVR is considered to be the most cost efficient method. DVR with PI controller has a simple
Injection
structure and offers a satisfactory performance
used in the DVR plays a crucial role in
over a wide range of operating conditions. The
ensuring
main problem of Conventional Controllers [3][4] is
effectiveness of the restoration scheme. It is
the correct tuning of the controller gains. When
connected in series with the distribution feeder.
there are variations in the system parameters and
transformers: Injection
the
maximum
transformers
reliability
and
Passive Filters: Passive Filters are placed at the
operating conditions, the controller may not
high voltage side of the DVR to filter the
provide the required control performance with fixed
harmonics. These filters are placed at the high
gains.
voltage side as placing the filters at the inverter side introduces phase angle shift which can disrupt
Power Quality problem is the main
the control algorithm.
concern in electricity industry. Power Quality includes a wide range of disturbances such as
Energy storage devices: Examples of energy
voltage sags/swells, flicker, harmonics distortion,
storage devices are dc capacitors, batteries, super-
impulse transient, and interruptions. And the
capacitors,
majority of power quality problems are due to
superconducting
magnetic
energy
Storage and flywheels. The capacity of energy
different fault conditions. These conditions cause
storage
voltage sag. Voltage sag can occur at any instant of
device
compensation
time, with amplitude ranging from 10-90% and a
has
a
capability
big of
impact the
on
the
system.
Compensation of real power is essential when large
duration lasting for half a cycle to one minute. It is
voltage sag occurs.
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
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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
Fig 1: Structure of Dynamic Voltage Restorer
and
reactive
power
compensation
are
generally classified as pre-sag compensation, inII COMPENSATION OF VOLTAGE SAG
phase compensation and energy optimization
USING DVR
technique. The single line diagram of DVR connected in the distribution system
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. Fig.3. single line diagram of dynamic voltage restorer
VL is the source voltage in volts s
connected to distribution system
vt is voltage at point of common coupling in volts. t
When the source
Rs+ jX s is impedance of the feeder in ohms
V
L
voltage drops
or
increases, the dynamic voltage restorer injects a series
is the load voltage in voltage
voltage through the injection transformer
so that the desired load [11] voltage magnitude can be maintained. The series injected voltage of
RL+ jX L is the load impedance in ohms.
the DVR, Vk can be written as:
Is is the source current and IL is the load current
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)
Fig.2. Single line diagram of test system
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INTERNATIONAL CONFERENCE ON CURRENT INNOVATIONS IN ENGINEERING AND TECHNOLOGY
III Function of DVR:
ISBN: 378 - 26 - 138420 - 5
contribute to the losses. The DVR will be most of the time in this mode. In boost mode (VDVR>0),
The main function of a DVR is the protection
of
sensitive
loads
from
the DVR is injecting a compensation voltage
voltage
through the booster transformer due to a detection
sags/swells coming from the network. Therefore as
of a supply voltage disturbance.
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 Fig.4.Equivalent Circuit of DVR
level through the booster transformer. Figure 4 shows the equivalent circuit of The DVR works independently of the type
the DVR, when the source voltage is drop or
of fault or any event that happens in the system,
increase, the DVR injects a series voltage Vinj
provided that the whole system remains connected
through the injection transformer [9][10] so that the
to the supply grid, i.e. the line breaker does not trip.
desired load voltage magnitude VL can be
For most practical cases, a more economical design
maintained.
can be achieved by only compensating the positive The series injected voltage of the DVR
and negative sequence [7] components of the
can be written as
voltage disturbance seen at the input of the DVR. This option is Reasonable because for a typical
Vinj = VL + VS
distribution bus configuration, the zero sequence part of a disturbance will not pass through the step
(5)
Where;
down transformer because of infinite impedance for VL is the desired load voltage magnitude
this component.
VS is the source voltage during sags/swells
The DVR has two modes of operation
condition.
which are: standby mode and boost mode. In standby mode (VDVR=0), the booster transformer’s
The load current ILoad is given by,
low voltage winding is shorted through the converter. No switching of semiconductors occurs
=
in this mode of operation, because the individual
(
± ∗
)
(6)
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
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Pulse-width modulation (PWM): PWM also works well with digital
Pulse-width modulation (PWM), or pulse-duration
controls, which, because of their on/off nature, can
modulation (PDM), is a modulation technique that
easily set the needed duty cycle. PWM has also
controls the width of the pulse, formally the pulse
been used in certain communication systems where
duration, based on modulator signal information.
its duty cycle has been used to convey information
Although this modulation technique can be used to
over a communications channel.
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
Fig 5: An Example of PWM in an Ac Motor Driver
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.
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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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[11] S. S. Choi, B. H. Li, and D. M. Vilathgamuwa,“Dynamic
VI CONCLUSION
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