chapter 5 power quality improvement by using power...

Chapter 5 Power Quality Improvement by using Power Active Filters

Ph.D Thesis submitted to Jawaharlal Nehru Technological University Anantapur

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CHAPTER 5

POWER QUALITY IMPROVEMENT BY USING POWER ACTIVE

FILTERS

5.1 POWER QUALITY IMPROVEMENT

This chapter deals with the harmonic elimination in Power System by adopting various methods.

Due to the development of Power Electronics technology, more Power Electronics appliances

are used, which leads the serious harmonics pollutions. Using Shunt Active Filters we can

eliminate these kinds of harmonics. The development of new Shunt Active Filter is presented.

The concept of proposed Shunt Active Filter and its operating Principle, Control Theory is also

discussed. The filtering scheme provides harmonics suppression at the source so that the source

will supply high quality power to linear load. The „Power Quality‟ has become the „buzzword‟

in the last one decade due to increase in quality-sensitive load, like computers, non-linear

switched devices, which are the sources of disturbance to create poor power quality &

awareness of implications of power quality.

5.2 INTRODUCTION

Quality means customer satisfaction, which cannot be defined absolutely. It is defined with

reference to consumer expectations. The quality of a product is thus measured by using yardstick

of consumer satisfaction. Electric power quality is satisfaction of its customer; a consumer is

satisfied if he is able to use power through his equipment and devices to serve his purpose. This

is possible only if his equipment and devices have Electromagnetic Compatibility (EMC) with

the supply quality; Thus, EMC is the measure of power quality. The SIMULINK/MATLAB is a

highly developed graphical user interface simulation tool. It has proved instrumental in



87

implementing the graphical based controller. The Simulation tool has been used to perform the

modeling and simulation of the customer power controller for a wide range of operating

conditions. The Simulation results of the proposed filter are discussed.

The objective of this chapter develops the proposed new shunt active power filter for the

current harmonics suppression using SIMULINK / MATALAB for power quality

improvement.[42]

5.3 FILTERS

Filters are used to restrict the flow of harmonics current in the Power Systems. It is a LC

circuit, which passes all frequencies in its pass bands and stops all frequencies in its stop

bands. There are two basic types of filters. The simplest method of harmonics filtering is with

passive filters. It uses the reactive storage components, namely capacitors and inductors. It has

two types. Shunt passive filter is the Combination of L and C elements, which are connected in

parallel with the line. It will restrict the flow of harmonics through the line. Fig-5.1 shows the

configuration of Shunt passive Filter.

Fig 5.1 Shunt passive filters Fig 5.2 Series passive filters

Series passive filter is the combination of L & C in parallel, which are connected in series with

the supply as in Fig 5.2, which has the ability to eliminate harmonics amplification of shunt

supply

Non-linear load

L c

Non-linear

load

c

L

supply



88

passive filter. The series active filter needs a much smaller kVA rating than a conventional

shunt active filter, and as a result, the combined system has good filtering characteristics and

high efficiency. This chapter presents a design of the shunt passive filter that makes possible a

great reduction in the required kVA rating of the series active filter. It can minimize the peak

voltage across the series active filter and reduce the required kVA rating of the filter to 60

percent. A computer simulation geared to practical applications of large three-phase. Thyristor

rectifiers are used to compare the compensation characteristics of the optimized system with

those of a combined system that uses a conventional shunt passive filter. Active Filters are

newly emerging devices for harmonics filtering, which will use Controllable Sources to cancel

the harmonics in the Power Systems. The basic principle of operation of an Active Filter is to

inject a suitable non-sinusoidal voltage and currents in to the system in order to achieve a clean

voltage and current waveforms at the point of filtering.

Fig 5.3 Shunt active filters Fig 5.4 Series active filters

Shunt Active Filter is connected in parallel to the load. The system configuration is shown in

Fig 5.3. It consists of a Voltage Source Inverter and a filter inductor connected in series. It

performs a harmonics current suppression to the line. Where as series active filter shown in Fig

source

Non-linear loads

Active

filter

source

Active

filter

Non-

linear loads



89

5.4, is connected in series with the load. The major advantages of the Series Active Filter are, it

maintains the output voltage waveform as sinusoidal and balances the three-phase voltage.[43]

5.4 HARMONICS MEASUREMENT

5.4.1 Importance of monitoring PQ

In a case study where the end-user equipment knocked off-line 30 times in 9 months but there

were only five operations on the utility substation breaker. There were so many events, which

will result in end-user problems that never show up in the utility statistics. One example is

capacitor switching, which is quite common and normal on the utility system, but can cause

transient over-voltage that disrupt manufacturing of machinery. Another is a momentary fault

any where in the system that causes voltage to sag briefly at the location of the customer,

which might cause an adjustable-speed drive or a distributed generator to trip off, but the utility

will have no indication that anything will miss on the feeder unless it has power quality

monitor installed.

5.4.2 Harmonics study

If a case study is conducted with two transformers, in which first transformer is supplying non-

linear load and there are 4 feeders on this transformer, whose rating is 2MVA which were

supplying the non-linear loads. The harmonics distortion can be observed from Fig 5.5.

Fig 5.5 Current Harmonics



90

Fig 5.6 Current waveform

Table 5.1 Current Harmonic

Amp

Harmonics

A B C N

THD%f 15.2 16.0 13.2 20.6

H3%f 0.8 1.4 0.7 5.0

H5%f 14.5 15.1 12.6 4.6

H7%f 14.1 4.8 3.5 4.3

H9%f 0.8 0.4 0.5 17.8

H11%f 0.6 0.9 0.8 2.9

H13%f 0.9 0.9 0.9 2.7

H15%f 0.1 0.1 0.1 1.9

Fig 5.7 Voltage waveform

Time

Current

Voltage ↑

Time →



91

Fig 5.8 Voltage Harmonics

Table 5.2 Voltage Harmonic

Table 5.3 Power & Energy

P/V/I/pf A B C Total

kW 0.53 0.43 0.56 1.52

kVA 0.55 0.45 0.58 1.58

kVAr 0.15 0.13 0.15 0.43

pf 0.96 0.96 0.97 0.96

(least)

Irms 2.2 1.9 2.4

Vrms 244.1 243.9 243.9

Volt

Harmonics

A B C N

THD%f 4.1 4.4 4.2 67.6

H3%f 0.2 0.2 0.2 6.0

H5%f 4.0 4.2 4.0 59.5

H7%f 0.8 0.9 0.8 20.0

H9%f 0.3 0.2 0.3 21.6

H11%f 0.3 0.3 0.2 8.2

H13%f 0.2 0.3 0.3 4.4

H15%f 0.1 0.0 0.1 1.3

Time →

Voltage ↑

Harmonics



92

It can be observed from the above waveforms and tables that the transformer contains current

harmonics of around 16% and voltage harmonics of 4%. Major harmonics in Current

waveform are 5th

and 7th

harmonics. Hence it is strongly recommended to mitigate the 5th

and

7th

harmonics in current waveform.

5.4.3 Power Quality Evaluation

Transform rating: 2 MVA

Electrical System = 415V,50 Hz, 3 Ph 3 Wires (With capacitor bank switched ON)

Apparent Power, S = 860.23 kVA

Real Power, P = 700 kW

Reactive Power, Q = 500 kVAR

Power Factor, pf = 0.97(lagging) (5.1)

Source Voltage, V

(Phase-A), VA = 243 Vrms THDVfund, A = 4.1%

(Phase-B), VB = 243 Vrms THDVfund, B = 4.4%

(Phase-C), VC = 243 Vrms THDVfund, C = 4.2% (5.2)

Load Current, IL

(Phase-A), IL, A = 1500 A THDIfund, A = 15.2%

(Phase-B), IL, B = 1260 A THDIfund, B = 16.0%

(Phase-C), IL, C = 1560 A THDIfund, C = 13.2%

(Neutral), IL, N = 120 A THDIfund, N = 20.6% (5.3)

Power & Current are calculated by using CT Ratio 400:1

The total harmonic current for each phase is calculated as follows

IH = THDIfund × (iL÷ (1+THDIfund2))



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IH, A = 0.152 × (1500÷ (1+0.1522)) = 225.41A

IH, B = 0.160 × (1260÷ (1+0.162)) = 199.06A

IH, C = 0.132 × (1560÷ (1+0.1322)) = 204.14A

IH, N = 0.206 × (120÷ (1+0.2062)) = 242.10 A (5.4)

Hence from the above calculation it is observed that the transformer contains a harmonics

current of 200A per phase. Hence it is strongly recommended to reduce harmonics currents.

THD results are obtained by FFT test.

5.5 HARMONIC ANALYSIS

The case study conducted on 3 transformers in which one of them is of 1.5MVA, which drives

a load of 50 kW with variable frequency. It is observed from Fig 5.1 that, the transformer is

supplying a highly non-linear load current of 25% THD. In which major harmonics are 5th

and

7th

.

5.5.1 Power Quality Evaluation

Transformer rating :(1.5MVA)

Electrical System = 415 V, 50 Hz, 3 Phase 3 Wires

Apparent Power, S = 834 kVA

Real Power, P = 815 kW

Reactive Power, Q =180 kVAr

Power Factor, p f = 0.98(lagging) (5.5)



94

Source Voltage, V

(Phase-A), V = 399.2 Vrms THDVfund, A = 4..5%

(Phase-B), VB = 397.5 Vrms THDVfund, B = 4.2%

(Phase-C), VC = 397.1 Vrms THDVfund, C = 4.3% (5.6)

Load Current, IL

(Phase-A), IL, A = 1260 A THDIfund, A = 25.2%

(Phase-B), IL, B = 1260 A THDIfund, B = 24.5%

(Phase-C), IL, C = 1140 A THDIfund, C = 25.0% (5.7)

Power & Current are calculated by using CT Ratio 600:1

The total harmonic current for each phase is calculated as follows

IH = THDIfund × (iL÷ (1+THDIfund2)

IH, A= 0.252 × (1260÷ (1+0.2522)) = 307.89A

IH, B = 0.245 × (1260÷ (1+0.2452)) = 299.83A

IH, C = 0.25 × (1140÷ (1+0.252)) = 276.49A (5.8)

Hence from the above calculation it is observed that the transformer contains a harmonics

current of 300A per phase. Hence it is strongly recommended to reduce harmonic currents

[44].THD results are obtained by FFT test.

5.6 MECHANISM OF ACTIVE HARMONIC FILTER

A 100 Amp Active Harmonics filter may be installed in PDB2 of Plant III supplied by Tx1.

The Plant III has got 30 Ring frames, each ring frame having VFD (Variable Frequency Drive)

of 50 kW rating. 30 Ring frames have been grouped into four groups. A Power Distribution



95

Board (PDB) supports each group. PDB2 supports 8 numbers of ring frames. Single line

diagram of the same installation is given below in Fig 5.9. They were facing a Problem of high

current harmonics more than 25% in PDB2. Tripping of the Circuit breaker and higher

temperature of cable and transformer (Tx1). The 100 Amp APF Units may be installed across

the load of PDB2.

Fig 5.9 Single Line Diagram of Active Filter

Transformer: 4 MVA, 33 kV /440 V, /,

PFC: power factor correction capacitors (50 kVAr cap Bank)

Solid-state harmonics filter rating: 415 V, 50 Hz, 3-Ph-3 wire, 100 Amp AHF.

VFD Rating: Each VFD is of 50 kW, 440 V, 50 Hz (5.9)

Incomer Source

point

VFD 8

VFD 2

VFD 1

PDB 2

Other

loads

PFC

Solid state

Harmoni-

cs filter

Transformer

Tx1

Vp

Ip Is

IH IL

Vs

Is1



96

Slight Distortion in current wave is, because of higher harmonics current presents in the

system than the rating of the AHF installed at the time of measurement. Due to variation in

load sometimes-Harmonics current exceeds rating of AHF & work in full correction mode

voltage waveform.

5.7 RELATIVE STUDY

Easy Installation.

Installation without affecting the Production.

Time required to install the AHF is less than 45 Min.

User Friendly control Panel.

Maintenance can be done easily without disturbing load efforts.

Current transformers are to be connected at load side for Current Sensing.

R, Y, & B terminals of AHF to be connected In shunt with the Load point.

Current THD: Before AHF Installed Current THD (I THD) was: 30-31% After AHF

Installed Current THD (I THD) brought down to: 5-6%.

Voltage THD: Before AHF Installed Current THD (I THD) was: 6-7% After AHF Installed

Current THD (I THD) brought down to: 4-5%.

Improved power factor (up to Unity) without power factor correction capacitors.[45]



97

Table 5.4 Variation of Parameters

Parameter Before AHF

installation

(Cap Bank ON)

After AHF

installation

(Cap Bank OFF)

Avg Amp 307.88 294.68

Avg kVA 221.57 212.28

Max kVA 278 258

kW 211.6 210.8

pf 0 .955 0 .994

kVAr 67.11 19.81

I THD 24.5% 6.5%

V THD 6.4% 5.3%

Voltage 415.5 415.5

Table 5.5 Transformer Primary Side

Parameter

Before AHF

Installation

After AHF

Installation

ITHD 19.6 16.3%

VTHD 2.7% 2.4%

Voltage 33KV 33KV

PF at TX6 0.976 0.986

Max kVA 1590 1540

Table 5.6 Transformer Secondary Side

Parameter Before AHF

Installation

After AHF

Installation

ITHD 20.5% 16%

VTHD 4.7% 4.2%



98

5.8 ACTIVE FILTER CONTROL ALGORITHM

Fig 5.10 Block diagram of the system

Fig 5.11 Shunt Active Filter

5.8.1 Description of System configuration

The system configuration of the proposed shunt active filter is shown in the Fig 5.10. It

consists of a three-phase source, which is connected to a three-phase non-linear diode bridge

rectifier circuit. The Shunt Active Power Filter is connected in shunt with the load. It consists

Three phase

voltage source

Non-linear

loads

Active filter

ia

ib ic

Source

3-ph

isource iload

Load

3-ph

icomp

Coupling

coil

Inverter

module

DC bus Vdc

Shunt Active Filter

iload

icomp ref

Vsource

P

controller



99

of the voltage source inverter in series with the inductor and capacitor. The triggering for the

inverter circuit is given through the control circuit. The inverter can be implemented by IGBTs

operating in hard-switched Pulse-Width Modulation (PWM) mode to provide sufficient

bandwidth for the filtering function.

5.8.2 Operating Principle

Three-phase bridge rectifier with RL loads (non-linear loads) is connected to the three phase

three wire distribution system as shown in Fig 5.11. Due to the nature of the non-linear loads,

harmonics are injected in to the system. Shunt Active Power Filter is connected in shunt with

the load to suppress the harmonics. The Voltage Source Inverter (VSI) generates a

compensating harmonics current in to the phase conductors through the inductor and capacitor

sets connected in series with it. The generated harmonics currents cancel each other with out

affecting the fundamental part of the source current.

5.8.3 Control Strategy

There are three stages in the active filtering technology. In the first stage the essentials voltage

and current signals are sensed using power transformers and current transformers to gather

accurate system information. In the second stage, compensating commands in terms of current

or voltage levels are derived based on control methods and AF configuration. In the third stage

of control, the gating signals for the solid-state devices of the AF are generated using PWM

techniques. In this we have used the instantaneous p-q theory for deriving the compensating

signal. [46]

5.8.4 Control Algorithm

The generalized theory of the instantaneous reactive power in three phase circuits is also

known as instantaneous power theory, or P-Q theory. It is based on instantaneous values in



100

three-phase power systems with or without neutral wire, and is valid for steady state or

transitory operations, as well as for generator voltage and current waveforms. The p-q theory

consists of an algebraic transformation of the three-phase voltages and currents in the a-b-c co-

ordinates to the --0 coordinates, followed by the calculation of the

(5.10)

p-q theory instantaneous power components. As explained above the first step in p-q theory is

to transfer the a-b-c frame of voltage and currents into --0 coordinates.

(5.11)

The instantaneous powers “p” and “q” are calculated using the equations given below

p0 = v0•i0 instantaneous zero sequence power.

p = v•i+v•i instantaneous real power.

q = v• i-v• i instantaneous imaginary power

v0 1/ 2 1/ 2 1/ 2 va

v = 2/3 ● 1 -1/2 -1/2 ● vb

v 0 3/2 -3/2 vc

i0 1/ 2 1/ 2 1/ 2 ia

i = 2/3 ● 1 -1/2 -1/2 ● ib

i 0 3/2 -3/2 ic



101

(5.12)

Where p =p + p ~

q =q + q ~

p0 =p0 + p ~0

p0 = Mean value of the instantaneous zero sequence power –corresponds to the energy per time

unity which is transferred from the power supply to the load through the zero sequence

components of voltage and current.

p ~0 = Alternated value of the instantaneous zero sequence power. It means the energy per time

unity is exchanged between the power supply and the load through the zero sequence

components. The zero-sequence power only exists in three-phase system with neutral wire.

Furthermore, the system must have unbalanced voltages and currents and/or 3rd

harmonics in

both voltage and current of at least one phase.

p = Mean value of the instantaneous real power, corresponds to the energy per time unity,

which is transferred from power supply to the load, through a-b-c coordinates, in a balanced

way (it is the desired power component).

p ~ = Alternated value of the instantaneous real power, it is the energy per time unity, that is

exchanged between the power supply and the load, through a-b-c coordinates.

q = Instantaneous imaginary power, corresponds to the power that is exchanged between the

phases of the load. This component does not imply any transference or exchange of energy

between the power supply and the load, but is responsible for the existence of undesirable

p v v i

= •

q -v v i



102

currents, which circulates between the system phases. In the case of a balanced sinusoidal

voltage supply and a balanced with or without harmonics, q(the mean value of the

instantaneous imaginary power) is equal to the conventional reactive power (q=3.V.I1.sin)

Fig 5.12 Shunt Active Filters with Linear Loads

As in Fig 5.12 , is usually desirable p-q theory power component. The other quantities can

be compensated using a shunt active filter. Can be compensated without the need of any

power supply in the shunt active filter. This quantity is delivered from the power supply to the

load through the active filter. This means that the energy previously transferred from the

source to the load through the zero sequence components of voltage and current, is now

delivered in a balanced way from the source phases. It is also concluded from Fig 5.12 that the

active filter DC Bus is necessary to compensate input power Pi and out put power po, since

these quantities must be stored in this component at one moment to be later delivered to the

load. The instantaneous imaginary power (q), which includes the conventional reactive power,

is compensated without the contribution of the DC Bus. This means that, the size of the DC

Vc

Vb

Va

120

120

Power supply

Shunt Active

power filter

N

C

Vdc

Linear loads Balanced

unbalanced

Non-linear loads Balanced

unbalance



103

Bus does not dependent on the amount of reactive power to be compensated. The

compensation reference currents in --0 components can be calculated by using the equations

(4.13) & (5.14)

(5.13)

Since zero sequence current must be compensated, the reference compensation current in the 0

coordinate is i0 itself. ic0 = i0, In order to get the compensating currents in a-b-c reference

frame the inverse transformation of --0 to a-b-c is applied.

(5.14)

Fig 5.13 Active & Reactive power control

ic

1

v - v

p

-p0

= ________________

ic

v2+v

2 v v q

ica

1/ 2 1 0 ic0

icb

= 2/3 ● 1/ 2 -1/2 3/2 ● ic

icc

1/ 2 -1/2 -3/2 ic



104

5.8.5 Features of p-q theory

The above Fig 5.13 synthesizes the reference current calculations of instantaneous p-q theory

It is inherently a 3-phase system theory

It is based on instantaneous values, allowing excellent dynamic response

Its calculations are relatively simple (it only includes algebraic expressions that can be

Implemented using standard processors).

It can be applied to any three-phase system (balanced or unbalanced, with or without

Harmonics in both voltages and currents) [47]

5.9 SIMULATION RESULTS

The SIMUINK/MATLAB (version 7) is a highly developed graphical user interface simulation

tool. It has proved instrumental in implementing the graphical based controller. The Simulation

tool has been used to perform the modeling and simulation of the custom power controller for a

wide range of operating conditions. The Simulation results of the proposed filter are discussed

in this chapter. The Proposed scheme of Shunt Active Filter is shown in Fig 5.14. For both

with and without filter. The active filter is connected in parallel with the 3 phase supply. The

load taken for the simulation is Diode and Thyristor bridge rectifier with RL load. The Tables

5.7 & 5.8 show the values of R and L for both diode and thyristor bridge rectifiers respectively.



105

Table 5.7 Parameters for Diode Bridge Load

Load parameters Active filter parameters

Resistance =2 ohms IGBT Inverter DC Bus = 900V

Inductance =1.5mH Inverter Coupling Inductance/Phase =1.5mH

Ripple Filter Parameters Rect./phase = 4 ohms

Cap/phase =36 µf

Cap/phase =72 µf

Table 5.8 Parameters For Thyristor Bridge Rectifier Load

As explained in the control algorithm, there are basically three main steps in instantaneous p-q

theory.

First step is converting the voltage and currents in ABC reference (stationary) frame into alpha

beta zero sequence (which is also stationary) frame as shown in Fig 5.15.

Load parameters Active filter parameters

Resistance =2 ohms IGBT Inverter DC Bus = 900V

Inductance =1.5mH Inverter Coupling Inductance/Phase =1.5mH

Ripple Filter Parameters Rect./phase = 4 ohms

Cap/phase =36 µf

Cap/phase =72 µf



106

Second step is to calculate the instantaneous real and reactive powers and extracting reference

compensating currents for compensating the harmonic currents and reactive power at the

source side of the system as shown in Fig 5.16

Third step is to generate the reference compensating current by using an IGBT based voltage

source inverter as shown in Fig 5.14

The results waveform will depict the effectiveness of the active filtering algorithm. It can be

observed from Fig 5.17 that the source current has become a pure sinusoidal wave and it has

been observed form POWER GUI tool that source current THD has been reduced from 24% to

2%. This can be observed from Fig 5.18 that, load power oscillations are reduced with active

filtering.



107

Fig 5.14 Simulated Diagram of Active Harmonic filter



108

Fig 5.15 Simulated Diagram for ABC to Alpha Beta ZeroTransforamtion

Fig 5.16 Simulated Diagram for Reference current calculation



109

Fig 5.17 Source and load current waveforms (Time on x-axis, Vs & IL on y-axis)

Fig 5.18 Source and load power waveforms (Time on x-axis, Ps & PL on y-axis)

The same active filter is connected to thyristor based bridge rectifier which is containing more

harmonics in current compared to the diode bridge rectifier as mentioned in Fig 5.15. The

current THD is reduced from around 44% to 3% hence the designed active filter is effective

with high percentage of THD. The designed active filter is capable of reducing around 50 amps

of harmonics current in load. The simulation diagram for Active harmonics filter with thyristor

based bridge rectifier load is shown in Fig 5.19



110

Fig 5.19 Simulated diagram of Active harmonic filter with thyristor bridge load

Fig 5.20 Shows that the load current waveform has been improved and the source becomes a

pure sinusoidal current waveform. It can also be observed from Fig 2.21 that the power

variation at the load side is reduced compared to source side, in addition to this the active

harmonics filter is capable of supplying the reactive power to the load and power factor at the

source side is also increased.



111

Fig 5.20 Source and load current waveforms (Time on x-axis, Is & IL on y-axis)

Fig 5.21 Source voltage and power waveforms (Time on x-axis, Vs & P on y-axis)



112

5.10 CONCLUSION

The purpose of this chapter is to review the results obtained during the present work before

proceeding with the conclusions of the work done, the objective of the thesis stated in the first

chapter are recalled. The primary objective of this chapter is to model and develop the

proposed new Shunt Active Power Filter for the current harmonics suppression using

SIMULINK/MATLAB (Version 7) simulation tool.

The model of the proposed new Shunt Active Power Filter is realized. The developed new

Shunt Active Power Filtering technology is implemented for a system feeding a non-linear

load. It is simulated using the highly developed graphic tool SIMULINK available in

MATLAB. The results reveal that the proposed new Shunt Active Filtering technology is

simple and effective and is suitable for practical applications.

Table 5.9 Harmonic Distortion of Load Voltage

Dead

Time(µs)

Vab_50Hz

V(rms)

THD

(%)

HS

(%)

H7

(%)

8kHz

(%)

0 380 1.27 0.21 0.13 0.74

5 354.6 2.13 1.43 0.79 0.82

From Table 5.10, it is observed that by the suppression of voltage and current harmonics using

new active filtering technology, the power quality can be improved as discussed in the various

chapters.

chapter 5 power quality improvement by using power...

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