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POWER QUALITY IMPROVEMENT IN DISTRIBUTION
SYSTEM USING SAPF
Niranjana B1, Selvanayakam A2, Geethamani R2 –
Assistant Professor, Sri Krishna College of Engineering and Technology, Coimbatore, India.
Abstract The major issues in power systems are created due to the
non-linear characteristics and fast switching of power
electronic equipments. Power quality issues are becoming
stronger because of sensitive equipments. The proposed PQ
theory is used for calculating the reference compensating
currents which is required to inject into the network at the
connected point of non-linear load. Switching scheme of
compensator is provided by comparing the reference
compensating currents obtained from PQ theory and
compensator currents. To meet a non linear loads it is
necessary to inject compensating current to maintain the
reactive power and to bring the source current waveform as
sinusoidal. Shunt active power filter have been carried out
for current harmonic reduction and reactive power
compensation and its done by simulating three phase four
wire system and three phase three wire system. So the
power factor has been improved by attaining source voltage
and source current in phase.
Keywords: Power quality, PQ theory,
compensator currents, Shunt Active Power
Filter, harmonic reduction, etc.
I INTRODUCTION
In recent years, the advancement in the technology,
specifically the evolution of power electronics
applications based on semiconductor switches (diode
and thyristor rectifiers, electronic starters, UPS and
HVDC systems, arc furnaces etc) has fetched many
technical eases and economical profits, but it has
concurrently introduced new challenges for power
system operation studies. To appreciate the maximum
asset utilization, secure and reliable operation needs
to be maintained regarding various aspects of power
system operation. The electrical transmission system
identifies devices such as power electronic circuitry
used for power conversion as non-linear load.
A nonlinear element in a power system is described
by the introduction of a distortion due to their non-
ideal characteristics. Nonlinear loads, including;
Switched Mode Power Supplies, Variable Frequency
Drives (VFD), Adjustable Speed Drives (ASD), and
Uninterruptable Power Supply (UPS), present a
special challenge to successful delivery of high
quality power under all operating conditions. With
the increased number of power electronic system
connected to the mains, the systems have become
more sensitive to supply voltage and current
distortions. Distorted voltages and currents have
many harmful effects such as resonance problem
arises between the supply inductances and
capacitances leading to over-currents and over-
voltages.
Thermal and mechanical insulation stresses occurs in
transformers because of heat losses I2 Z increases
due to distorted current. System powering phase to
neutral connected loads can also occur detrimental
effects. Sequencing of operation depends on a zero
crossing for timing, mis-operation due to voltage
distortion. Rapidly changing or varying industrial
loads such as electric arc furnaces, welding machines,
alternators, rolling mills and motors may also give
rise to supply voltage fluctuations which might cause
tripping of equipment. Ideally, Pure sinusoidal waves
are in AC systems, both voltage and current, because
of non-linear loads, the characteristics of voltage and
current may vary from the ideal sinusoidal wave.
II LITERATURE REVIEW
The degradation in power quality causes adverse
economical impact on the utilities and customers.
Power quality issues due to harmonics presented in
current and voltage which is solved by the use of
Hybrid Series Active Power Filter (HSAPF). HSAPF
is more robust and stable because of the Sliding
mode controller-2. An accurate averaged model of
three-phase HSAPF is also derived in this paper.
The literature study for the thesis work starts with the
location of the harmonic emerging because of the
utilization of non-linear loads. The primary
wellsprings of harmonic currents and voltages are
because of control and energy transformation systems
included in the power electronic devices, for
example, chopper, cyclo-converter, rectifier 3 and so
forth. Sources of harmonics in energy transformation
devices such as power factor improvement and
voltage controller devices of motor, traction and
power converters, high-voltage direct-current power
International Journal of Pure and Applied MathematicsVolume 119 No. 17 2018, 1259-1269ISSN: 1314-3395 (on-line version)url: http://www.acadpubl.eu/hub/Special Issue http://www.acadpubl.eu/hub/
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converters, battery-charging systems, wind and solar-
powered dc/ac converters, static-var compensators,
direct energy devices fuel cells, control of heating
elements storage, batteries which require dc/ac power
converters. The harmonic currents and voltages were
measured utilizing an element indicator analyzer by
M. Etezadi-Amoli, and plotted at for divers
substations. Because of utilization of non-linear loads
like chopper, rectifier and so forth the load current
gets contorted, which is clarified pleasantly by
Robert considering harmonic study.
In order to lessen the harmonics and to enhance the
power factor of the ac loads inactive L–C channels
were utilized and additionally capacitors were
utilized. At the same time the aloof channels have a
few drawbacks like settled recompense, vast size and
reverberation issue. Numerous exploration work
advancement are created to avoid harmonic issues on
the APF channels or active power line conditioners
APLC's are essentially arranged into two sorts,
specifically, single stage (2-wireassociation), three-
stage (3-wire and 4-wire association) arrangements to
meet the necessities of the nonlinear loads in the
dissemination systems. Single-stage loads, for
example, local lights, TVs, ventilation systems, and
laser printers carry on as nonlinear loads and reason
harmonics in the power frame work. Numerous
setups, for example, the active arrangement channel,
active shunt channel, and blending of shunt and
arrangement channel has been produced.
III Research Methodology - Power Quality Issues:
The supply interruption is the most severe power
quality issue which affects all the equipments
connected to the electrical grid. However other
problems, described below and illustrated in Figure 1
to 7, beyond of leading to some equipments
malfunction, can also damage them:
Harmonic distortion: when non-linear loads are
connected to the electrical grid, the current that flows
through the lines contains harmonics, and the
resulting voltage drops caused by the harmonics on
the lines impedances causes distortion on the feeding
voltages.
Fig. 1 Harmonic Distortion Noise (electromagnetic interference): corresponds
to high frequency electromagnetic noise, which can,
for instance, be produced by the fast switching of
electronic power converters.
Fig.2 Noise
Inter-harmonics: appear with the presence of
current components that are not related to the
fundamental frequency. These components can be
produced by arc furnaces or by cyclo-converters
(equipments that, being fed at 50 HZ, allow to
synthesize output voltages and currents with inferior
frequency).
Fig. 3 Inter Harmonics
Momentary Interruption: occurs, for instance,
when the electrical system has automatic reset circuit
breakers, that opens when a fault occurs, closing
automatically after some milliseconds (and is kept
closed if the short-circuit is extinguished).
Flicker: It happens due to intermittent variations of
certain loads, causing voltage fluctuations (which
results, for instance, in oscillations on electric light
intensity).
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Fig. 4 Momentary Interruption
Voltage Sag: can be caused, for instance, by a
momentary short-circuit at another branch of the
same electrical system, which is eliminated after
some milliseconds by the opening of the branch
circuit breaker.
Fig. 5 Voltage Sag
Voltage swell: can be caused, by fault situations or
by commutation operations of equipments connected
to the electrical grid.
Fig. 6 Voltage Swell
Fig. 7 Flicker Notches: These occur due to loads which consume
currents with abrupt periodical variations (like
rectifiers with capacitive or inductive filter).
HARMONIC POLLUTION EFFECTS
Besides wave shape distortion, presence of harmonics
on energy distribution lines causes problems on
equipments & components of electrical system,
namely:
Heating, pulsed torque, audible noise and
life span reduction of rotating electrical
machines
Undue firing of power semiconductors in
controlled rectifiers and voltage regulators
Operation problems on protection relays,
circuit breakers and fuses;
Increased losses on the electrical conductors
Considerable increase of the capacitor’s
thermal dissipation, leading to dielectric
deterioration
Life span reduction of lamps and luminous
intensity fluctuation (flicker - when sub-
harmonics occur)
Errors on the energy meters and other
measurement devices
Electromagnetic interference in
communication equipments
Malfunction or operation flaws in electronic
equipment connected to the electrical grid,
such as computers, programmable logic
controllers (PLCs), control systems
commanded by microcontrollers, etc. (these
devices often control fabrication processes).
REAL CASES OF PROBLEMS CAUSED BY
HARMONICS
A new computation system was installed in an
insurance company building.
Once the system was turned on, the main circuit
breaker opened, putting all system off-line. After
several verifications, the engineers discovered that
the interruption had been cause by an excessive value
of current in the neutral wire of the three phase
system. Despite the system being balanced, the
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neutral wire current had a value equal to 65% of the
value of phase current, which led to the triggering of
the circuit-breaker, since the neutral wire current
relay was set to 50% of the value of phase current. It
should be highlighted that in a balanced three-phase
system, the neutral current must be equal to zero.
However, when the current is distorted, contrarily to
what normally occurs, the current harmonics multiple
of three are summed in the neutral wire, instead of
canceling each other. Studies demonstrate that neutral
currents have increased in commercial buildings.
This is due to the growing use of electronic
equipment, such as computers, printers, copiers,
faxes, etc. Those equipments use single-phase
rectifier at their entrance, which consume 3rd order
current harmonics, such as the 3rd, 9th and 15th
harmonics. In order to avoid neutral wire heating
problems, these must be oversized, or, even better,
the 3rd order harmonics must be compensated. In
another documented case, an electrical power
distribution company reported a 300kVA transformer
break down, whose load did not exceed its rated
apparent power. The transformer was replaced by an
identical one, but it started to show the same
problems shortly after. These transformer’s loads
mainly consisted of electronic variable speed drives
for electric motors, which current consumption has a
large harmonic content.
Nowadays, in order to avoid transformers break
down, or reduced life span, it’s important to know the
harmonic distortion of the currents delivered to the
load by them. In function of that value, it will be
applied to the transformer a power derating factor
(factor K). This means, in function of the harmonic
distortion value, the rated power value of the
transformer is reduced.
POWER QUALITY PROBLEMS-SOLUTIONS
The solution for some of the power quality problems
can be overcome by using the below devices or
equipments:
The UPSs (Uninterruptable Power Supplies)
or emergency generators are the only
solution for long interruptions in the
electrical power supply
Transient Voltage Surge Suppressors
guarantee protection against transient
phenomena which cause voltage spikes in
the lines
The electromagnetic interference filters
guarantee that polluting equipment does not
propagate the high frequency noise to the
electrical grid
Isolation transformers with electrostatic
shield offer not only galvanic isolation, but
also avoid the propagation of voltage spikes
to the secondary winding.
IV BLOCK DIAGRAM
Fig.8 Block Diagram
CONTROLLED SINGLE-PHASE HALF WAVE
RECTIFIER
By replacing the silicon controlled rectifier
(SCR) instead of diode from the uncontrolled rectifier
this results in controlled half-wave rectifier of
Figure.9. The SCR behaves similarly to a diode in
that it is a one-way device and will block current
flow in the negative direction. However, it will not
conduct in the forward direction until an appropriate
trigger signal has been applied to its gate. In reality,
there will be a short delay before the SCR turns “on”
even after it has been adequately triggered; but for
the purpose of explanation, the SCR in Figure.9 can
be considered ideal and will turn “on” immediately
when triggered. The angle at which the SCR is
triggered is commonly called the firing angle.
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Fig.9 Controlled half-wave rectifier circuit with an
ideal SCR and a resistive load
CONTROLLED SINGLE-PHASE FULL WAVE
RECTIFIER
By replacing the diodes in uncontrolled rectifier with
SCRs, a controlled full-wave rectifier is created as
shown in Figure.10. Thyristors are gated in pairs with
the firing angle of time t1. All SCRs are initially
“off”. In the positive half cycle of the source voltage
waveform, SCR1 and SCR4 are gated “on” while
SCR2 and SCR3 remain “off” due to being reverse
biased. In the negative half cycle of the source
voltage waveform, SCR2 and SCR3 are gated “on”
while SCR1 and SCR4 remain “off”.
Fig.10 Controlled full-wave rectifier circuit with
ideal SCRs and resistive load
TWELVE PULSE CONVERTER
The 12-pulse method has been also used for reduced
facility harmonics distortion. In these case two set of
non linear load are fed by two phase shifted
transformer winding with the using of twelve pulse
converter 5th and 7th harmonics can be cancellation
on primary side of transformer. From H= np±1 so
that 11th and 13th harmonics are present. 12-pulse
rectifier 5th, 7th 90% cancelled still has 11th, 13th,
17th, 19th, etc,
Fig.11 Twelve pulse converter
A three phase bridge rectifier gives a six pulse output
voltage. When two three phase bridge rectifier are
connected either in series or parallel combination, it
gives 12 pulses output voltage which is better than
that of three-phase bridge rectifier in terms of
rectifier efficiency and has also lower harmonics
content present in it (i.e low ripple content). And the
most important point to be cared about when two
three phase bridge rectifier are connected in series is
that the input supply voltage supplied to first and
second three phase bridge rectifier must have phase
displacement of 30 degrees and this can be
introduced by the use of the star-star transformer and
Star-Delta transformer.
When two three phase bridge rectifier are connected
in a parallel combination, it is necessary to insert an
interphase reactance between the bridges in order to
adjust the instantaneous voltage difference between
two bridges.
SHUNT ACTIVE POWER FILTER
Fig.12 Basic structure of shunt active power filter
The higher and lower order harmonics in the power
system especially harmonics below switching
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frequency are filtered by using active power filters. A
non-linear load draws non-sinusoidal current from
the network, it is considered to be harmonic current
sources.
Shunt Active power Filter works as a current source
which produces harmonic currents opposite to
harmonic currents produced by the non-linear load.
Parallel connection of Shunt Active Power Filter and
non-linear loads compensate the harmonic current, so
that the network is loaded only with fundamental
current.
IMPLEMENTATION OF SAPF
The implementation of effective hysteresis control
technique for shunt active power filter has been done
in three steps. In the first step, the required load
current and source voltage signals are measured to
know the exact information about the system studied.
In the second step, by using the instantaneous PQ
theory it determines the reference compensating
currents. In the third step, by using PWM with
10KHZ current control technique the required gating
signals for the solid-state devices are generated. The
PQ Theory is implemented using the MATLAB
SIMULINK block and it is given to the Active Power
Filter (APF).
CONTROL BLOCK
Figure.13 shows the basic algorithm commonly used
for the calculation of the compensating currents. In
this figure, pc and qc are the compensation reference
powers. In general, when the load is nonlinear the
real and imaginary powers can be divided in average
and oscillating components, as shown below.
Fig.13 Algorithm for control block
Due to harmonic components in the load current the
oscillating powers ~p and ~q are the undesirable in
balanced voltage sources. In some situation q is an
undesirable power as well. With the help of
oscillating powers, the compensating currents can be
calculated in the reference frame. Using Clarke
inverse transformation, the harmonic components in
the load can be compensated by injecting the current
through an active filter. This technique has proven to
be very efficient and practical. However, the
compensated currents are not sinusoidal if the voltage
used in the control algorithm is not balanced and
purely sinusoidal. This problem may happen if the
voltage at the point of common connection (PCC) is
distorted or unbalanced and used in the control
algorithm.
THE P-Q THEORY
The p-q Theory is based on α - β transformation, also
known as the Clarke Transformation [Clarke (1943)],
which consists in a real matrix to transform three-
phase voltages and currents into the stationary
reference frame, given by:The inverse transformation
is given by:
Similarly, generic instantaneous three-phase line
currents (ia, ib, ic) can be transformed into α,β and 0
axis.
From zero sequence axis the zero-sequence
components are partitioned based on this
transformation. There will not be any contribution of
zero sequence components in α and β axis. No zero
sequence current components are present when it is
connected to the three phase three wires system and
can be eliminated in the above equations, simplifying
them. The present analysis will be focused on three-
wire systems. Therefore, zero-sequence voltage or
current is not present. In this situation the real and
imaginary powers are given by:
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where, p is the real power and represents the total
energy per time unity in the three-wire three-phase
system ,in terms of components; q is the imaginary
power and has a non-traditional physical meaning
and gives the measure of the quantity of current or
power that owes in each phase without transporting
energy at any instant.
Fig.14 Simulation Circuit Diagram
With this change of signal, for a balanced positive
sequence voltage source and balanced capacitive or
inductive load, the new reactive (imaginary) power
will have the same magnitude and signal of that
calculated using conventional power theory (Q =
3VIsinÁ).
Fig.15 Delta /Star total load current at 0 deg firing
angle and 25 deg
Fig.16Source voltage/Source current /load current
at 0 deg firing angle and 25 deg firing angle
Fig.17 Source voltage/ source current of Phase A
in per unit at 0 deg firing angle and 25deg firing
angle
Fig.18 DC bus voltage at 0 deg firing angle and 25
deg firing angle
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Fig.19 Load current THD at 0 deg firing angle
Fig.20. Source current THD at 0 deg firing angle
Fig.21. Load current THD at 25 deg firing angle
Fig.22. Source current THD at 25 deg firing angle
V Simulation Result and Analysis
Implementation of harmonic current compensation in
a three-phase power system is done using the shunt
active power filter. The Total Harmonic Distortion
(THD) spectrum in the system load current with 0
deg firing angle, which indicate a THD of 16.36%
and The THD of source current is observed to be
1.72% which is within the allowable harmonic limit.
The Total Harmonic Distortion (THD) spectrum in
the system load current with 25 deg firing angle,
which indicate a THD of 35.57% . The THD of
source current is observed to be 2.51% which is
within the allowable harmonic limit.
VI Conclusion
A SAPF with an P-Q control strategy has been
proposed to compensate the line current distortion
generated by a 12-pulse current source thyristor
controlled converter. The SAPF is connected to the
front-end transformer secondary taps to reduce the
filter side voltage. This SAPF control technique is
capable of individually mitigating specific harmonics
orders (5th, 7th, 11th, and 13th), but can be extended
to mitigate higher order components. This improves
the individual harmonic factor of the compensated
harmonic order, THD, and accounts for the delay
effects introduced in the reference and actual signals.
The proposed technique involves two stages;
preparation and operation. The preparation phase
includes the SAPF elements, by determining the
modulating signal information necessary to produce
the desired compensating. In the operation phase, for
a given loading condition, the controller chooses
suitable modulating signal information to compensate
the current with a current controller and avoids its
International Journal of Pure and Applied Mathematics Special Issue
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time response delay, particularly when the switching
frequency is low. Thus the SAPF implicitly
compensates all sources of delays and provides
accurate mitigation of selected harmonic orders in the
supply current, resulting in better harmonic factors in
terms of the IEEE standard.
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