power quality improvement in distribution system using …

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).

International Journal of Pure and Applied Mathematics Special Issue

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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


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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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