International Journal of Engineering Trends and Technology (IJETT) - Volume4 Issue6- June 2013
Mitigation of Sag Using DVR with Neural Network
Controller
B.Indhuja1, Ms. M.Bhavani 2
1
PG scholar, Assistant professor2
Dept. of Electrical and electronics Engineering
Anna University, Chennai: Regional center, Madurai.
Madurai, Tamilnadu, India.
Abstract—
The Dynamic Voltage Restorer is the most common device
used in the distribution systems to protect the consumers against
sudden changes in voltage magnitude. In this paper the design of
DVR by using Neural Network controller is proposed in order to
mitigate the voltage sags to improve the transient response and
eliminate the study state error. The proposed method is applied
with some disturbances in the load side. The capability of the
proposed DVR has been tested to limit the fault current. Here
the DVR access virtual impedance with the aim of protecting
PCC voltage during fault without any problem in real power
injection to the DVR. Simulation results show the capability of
the DVR to control the emergency conditions of the distribution
system.
Index Terms—DVR, voltage sag, Neural Network controller.
I. INTRODUCTION
Power Quality is the relative frequency and severity of
deviations in the incoming power supplied to electrical
equipment from the steady 50 Hz sinusoidal waveform. These
deviations may affect the safe or reliable operation of
equipment such as computers. Thus terms like “poor power
quality” mean that there is ample deviation from norms in the
power supply that may cause equipment malfunction or
failure. In certain commercial and industrial electrical
applications, it is critical that high quality and uninterrupted
power be supplied; for fear that significant economic losses
can be incurred..
The reason for demanding high quality un-interruptible
power during production process is mainly because of the
modern manufacturing and process equipment that operate at
high efficiency requires stable and defect free power supply
for the successful operation of their machines. Machines,
sensitive to power supply variations are to be designed more
precisely. For instance, some instruments like adjustable speed
drives and automation devices are come under here and also
power electronic equipments are fall in to the above
category.[1][5]
Manufacturing cost and the reliability of those solid state
devices have been improved as new technologies emerged.
So, the protection devices which include such solid state
devices can be purchased at a reasonable price with superior
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performance than the conventional electrical or pneumatic
devices available in the market. The Uninterruptible Power
Supplies, Dynamic Voltage Restorers and Active Power Filter
are examples for commonly used custom power devices.
Among those Active Power Filter is used to mitigate harmonic
problems occurring due to non-linear loading conditions,
whereas Uninterruptible Power Supply and Dynamic Voltage
Restorer are used to compensate for voltage sag and surge
conditions.[2][4]
Voltage sag may occur from single phase to three phases.
But it has been found that single phase voltage sags are
routine and most frequent in the power industry. Thus, the
industries that use single and three phase supply will undergo
several interruptions during their production process and they
are forced to use some form of voltage compensation
equipment. As soon as the fault occurs the action of DVR
starts. On event of fault which results in voltage sag, the
magnitude reduction is accompanied by phase angle shift and
the remaining voltage magnitude with respective phase angle
shift is provided by the DVR.[6][10]
Employing minimum active voltage injection mode in the
DVR with some phase angle shift in the post fault voltage can
result in miraculous use of DVR. If active voltage is less
prominent in DVR then it can be delivered to the load for
maintaining stability.[7] Considering this, a transition process
is proposed such that voltage restoration is achieved by
injecting the voltage difference between the pre sag and the in
sag (source side) voltages during the initial first cycle or so the
sag. When the sag voltage pharos is available, the injection
voltage is controlled to move progressively from the in phase
injection point to the corresponding minimum active voltage
injection point.
The initial voltage injection magnitude and phase angle of
DVR can be categorized into different cases considering the
injection limit that will be discussed further. The simulation of
various 1 phase and 3 phase faults are done using MATLAB.
The present project deals with only voltage sag, voltage swell
can be simulated in same way. The simulation results show
the very good performance of the controller theoretically. The
performance of DVR theoretically is tested. Therefore this
project has contributed a strong knowledge to the research and
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International Journal of Engineering Trends and Technology (IJETT) - Volume4 Issue6- June 2013
development targeting industrial application to compensate
the single-phase voltage sags and 3 phase balanced voltage
sags.
A) VOLTAGE SAG
Over the last fifteen years, based on how the power quality
instruments measure voltage sags and swells the definitions
have been developed. [1] Power system communities’ state
sags or dips as a reduction in voltage.
Surges are now called as swells, except that the voltage
exceeds a particular user-defined high limit.
B) SINGLE PHASE SAGS
The frequently occurring voltage sags are single
phase events which are basically due to a phase to ground
fault occurring somewhere on the system. On other feeders
from the same substation this phase to ground fault appears
as single phase voltage sag. Typical cases are due to
lightning strikes. It is common to see single phase voltage
sags to 30% of nominal voltage or less in industrial plants.
It is also caused by tree branches and birds contact.
C) PHASE TO PHASE SAGS:
The two Phase or Phase to phase sags may be caused
by the height tree branches and adverse weather. Like in
single phase it is also caused by animals or vehicle collision
with utility poles. These types of sags typically appear on
other feeders from the same substations.
D) THREE PHASE SAGS:
These are caused by switching or tripping of a three
phase circuit breaker and by switch or by a recloser which will
create three phase voltage sag on other lines fed from the same
substation. Symmetrical 3 phase sags arise from starting large
motors and they account for less than 20% of all sag events
and are usually confined to an industrial plant or its immediate
neighbors.
E) REQUIREMENTS OF DVR
ENERGY STORAGE UNIT
Various devices such as Flywheels, Lead acid
batteries, Superconducting Magnetic energy storage (SMES)
and Super-Capacitors can be used as energy storage devices.
The main function of these energy storage units is to provide
the desired real power during voltage sag. The amount of
active power generated by the energy storage device is a key
factor, as it decides the compensation ability of DVR. Among
all others, lead batteries are popular because of their high
response during charging and discharging. But the discharge
rate is based on the chemical reaction rate of the battery so
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that the available energy inside the battery is determined by
its discharge rate.
VOLTAGE SOURCE INVERTER
Generally Pulse-Width Modulated Voltage Source
Inverter (PWMVSI) is used. In the previous section we saw
that an energy storage device generates a DC voltage. To
convert this DC voltage into an AC voltage a Voltage Source
Inverter is used. In order to boost the magnitude of voltage
during sag in DVR power circuit, a step up voltage injection
transformer is used with low rating VSI.
PASSIVE FILTERS
To convert the PWM inverted pulse waveform into a
sinusoidal waveform, low pass passive filters are used. In
order to achieve this it is necessary to eliminate the higher
order harmonic components during DC to AC conversion in
Voltage Source Inverter which will also distort the
compensated output voltage. These filters which play a vital
role can be placed either on high voltage side i.e. load side or
on low voltage side i.e. inverter side of the injection
transformers. We can avoid higher order harmonics from
passing through the voltage transformer by placing the filters
in the inverter side. Thus it also reduces the stress on the
injection transformer. One of the problems which arise when
placing the filter in the inverter side is that there might be a
phase shift and voltage drop in the inverted output. So this
could be resolved by placing the filter in the load side. But
this would allow higher order harmonic currents to penetrate
to the secondary side of the transformer, so transformer with
higher rating is essential.
VOLTAGE INJECTION TRANSFORMERS
The primary side of the injection transformer is
connected in series to the distribution line, while the
secondary side is connected to the DVR power circuit[2].
Now 3 single phase transformers or 1 three phase transformer
can be used for 3 phase DVR whereas 1 single phase
transformer can be used for 1 phase DVR. The type of
connection used for 3 phase DVR if 3 single phase
transformers are used is called “Delta-Delta” type connection.
If a winding is missing on primary and secondary side then
such a connection is called “Open-Delta” connection which is
as widely used in DVR systems.
F) DVR OPERATING MODES
The difference between the pre sag voltage and the
sag voltage is injected by the DVR. This is done by supplying
the real power from the energy storage element and the
reactive power. Due to the ratings of DC energy storage and
the voltage injection transformer ratio the maximum
capability of DVR is limited. The magnitude of the injected
voltage can be controlled individually in the case of three
single-phase DVRs. With the network voltages the injected
voltages are made synchronized (i.e. same frequency and the
phase angle).
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International Journal of Engineering Trends and Technology (IJETT) - Volume4 Issue6- June 2013
During the normal operation as there is no sag, DVR
will not supply any voltage to the load. It will be in a standby
mode or it operates in the self-charging mode if the energy
storage device is fully charged. The energy storage device
can be charged either from the power supply itself or from a
different source.
II PROPOSED WORK
The design of DVR consists of series connected
injection transformer, a voltage source inverter, harmonic
filter, and an energy storage device. Here, the design of DVR
consists of Neural Network controller and its is used to tune
the DVR. It maintains the system without any steady state
error. Single line to ground fault has been introduced on the
source side. Due to this fault the sag occurs. In order to
mitigate the sag DVR is introduced into the system having
Neural Network controller.
III SIMULATION RESULTS
Voltage(v)
The performance of the DVR with Neural Network
controller is shown in the graph.
Time(sec)
FIG 1 OUTPUT OF DVR
IV CONCLUSION
In order to show the performance of DVR in
mitigation of voltage sags, the given test system is simulated
using MATLAB. A DVR is connected to a system through a
series transformer with capability to insert maximum voltage
of phase to ground voltage. DVR handles both balanced and
unbalanced situations without any difficulties. The proposed
DVR is simple and having fast response. The proposed
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Neural Network controller technique is used to tune the DVR
easily and used to maintain the system in steady state without
any error.
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