CHAPTER 1

INTRODUCTION

1.1 GENERAL

Power system is designed to operate at frequency of 50 Hz. However, certain types

of loads such as nonlinear loads produce currents and voltages with frequencies that are

integer multiples of the 50 Hz fundamental frequency. These higher frequencies are form

of electrical distortion known as power system harmonics. Power system harmonics are

produced by nonlinear loads or devices that draw no sinusoidal currents. Examples of

common sources of harmonics are transformers, adjustable speed drives, power electronics

loads and so forth. The harmonic producing loads can be divided into three main

categories that are, large number of distributed non-linear components of small ratings;

large and continuously and randomly varying nonlinear loads; and large static power

converters and transmission system-level power electronic devices. Harmonics can be

much more deteriorate if other power quality problems such as resonance and voltage

unbalance also occur at the same time. These non-fundamental frequencies or harmonic

distortions can be classified into two types, notably voltage harmonic and current

harmonic. The effects of voltage harmonic distortions are voltage sag, swell and

fluctuation, which are mainly caused by sudden loading of the system at point of common

coupling, large neutral currents due to unbalanced loading, improper grounding etc. The

primary effects of current harmonic distortions on the other hand are power factor

reduction, poor utilization of distribution wiring plant, high current flow in the neutral line

of four-wire three phase system, excessive over heating of line cable etc(1).

Harmonic pollutions in the power system can be mitigated by adding passive

and/or active power filters to the system; utilizes auto-transformer to cancel low order

harmonics; and using phase shifted on the secondary transformer and/or those that had

high level reactance between primary and secondary windings (2). Traditionally, passive

shunt LC filters is used in suppressing harmonics in power system. Nevertheless, it has

1

three obvious limitations which are; the source impedance strongly influences the filtering

characteristics, parallel resonance may occur with the source impedance, and bulky in size

with fixed compensation characteristics. To solve the limitations of passive filter, active

power filter is introduced. Basically, active power filter is a power electronic converter

incorporating energy-storage element. There are two types of active power filter which are

shunt active power filter and series active power filter. These two typical active filters are

distinguished by connection with the power lines. By means of shunt connected active

power filters, which can be regarded as a kind of source current compensating for the

current harmonics drawn by non-linear loads. However, the cost of shunt active filters is

relatively high and they are not preferable for a large-scale system since the power

capacity of the filter is directly proportional to the load current to be compensated. In

addition, their compensating performance is better in the current-type harmonic source

than in the voltage-type harmonic source (3). Thus, to overcome harmonics distortion

generated by voltage fed type harmonics-producing loads, the series active filter was

introduced at the end of the 1980s. Such filter acts as a kind of harmonic isolator, since it

provides high impedance for the harmonics while providing zero impedance for the

fundamental. Besides, it reduces the need for protection of the loads because it injects

series harmonic voltage source into the supply line through injecting transformer.

In addition, the series active power filter can regulate the point of common

coupling voltage at a desired value by controlling the inverter output in order to

compensate distorted utility voltage. Combination system of series active filter and passive

filter is introduced to complement each others. Since series active filter behaves like an

active impedance, which not causing any voltage drop for the fundamental component,

instead it forces the load current harmonics into the passive filter. Therefore, series active

filter improves the filtering characteristics of passive filter and load power factor in such

way of compensating the the reactive power required by the load. On the other hand, such

combination system makes possible to significantly reduce the rating of the series active

filter. Hence, this configuration is also well known as hybrid active power filter since it

inherits the efficiency of passive filters and the improved performance of active filter.(4)

proposed adaptive fuzzy dividing frequency-control method for hybrid active power filter

is an example for a new control technique in compensating current harmonics. Considering

all advantages and limitations of the combined system, the objective of this simulation

studies is to mitigate source current harmonics drawn by typical harmonics-producing

2

loads specifically three phase diode rectifier with smoothing dc capacitor to comply with

international standards such as IEEE519-1992 and IEC61000, and evaluates the

performance of the combined filter system. In this thesis, a control scheme based on SRF

is introduced. It works well by compensating source current harmonics and source voltage

unbalance using a series hybrid active power filter. For unbalance voltage compensation,

the desired fundamental component is derived from positive sequence component of the

unbalance voltage. Then, an all-pass filter is implemented, giving a desired phase shift

without magnitude reduction.

The derived reference fundamental component then is used as fundamental current

and voltage reference for current harmonics compensation. Next, these calculated

compensated control signals is fed to the SVPWM-VSI in order to generate the

compensating voltages for mitigation of source current harmonics. The organization of this

thesis begins with the system configuration, followed by control scheme, before coming to

the results and discussion, and ended with conclusions and acknowledgement.

1.2 LITERATURE SURVEY

PAKDEL (2007) reveals the basis of harmonic suppression by using a control

scheme with three topologies for harmonic suppression and power factor correction in a

single-phase system with a diode rectifier load. It is assumed that inductive or capacitive

loads are connected to the dc side of the diode rectifier.

RAGHAVENDHIRAN (2003) compares the performance of the active filter based

on the novel current compensation technique with that of the filter based on sliding mode

control law method.

ALI SARDAR (2009) presents minimization of harmonics using power active

filters, which satisfy the current harmonic suppression for the mprovement in power

quality and the techniques considered include here is the shunt & series active power

filters.

MAHALEKSHMI (2010) proposes that the current harmonics can be compensated

by means of using the Shunt Active Power Filter, Passive Power Filter and the

combination of both. The system has the function of voltage stability, and harmonic

3

suppression. The reference current can be calculated by dq transformation. An improved

generalized integrator control was proposed to improve the performance of APF. The

above all the concepts have been developed in his thesis.

MORAN (2000) describes the different power quality problems in distribution

systems and their solutions with power electronics based equipment.

1.3 OBJECTIVE OF THE THESIS

The main objective of the thesis is to suppress the harmonics by using series hybrid

power filter in electrical power system with non linear loads in order to develop the power

quality of the system.

1.4 ORGANIZATION OF THESIS

The thesis is organized into five chapters. The description about each chapter is as

follows:

Chapter 1 presents the introductory part and the objective of the thesis. It also

explains about the various suggestions given by various authors.

Chapter 2 presents the description about harmonics and filters.

Chapter 3 deals with the modeling of the system and the control scheme used to

suppress the harmonics.

Chapter 4 describes the system performance without and with filter and the

simulation result for both the cases.

Chapter 5 describes the summary of the work done.

1.5 SUMMARY

In this chapter, how the power system is affected by harmonics and what are the

precautions and preventive measures taken by usage of filters and it has been explained

by different literature survey.

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CHAPTER 2

HARMONICS AND FILTERS

2.1 INTRODUCTION FOR HARMONICS

A harmonic of a wave is a component frequency of the signal that is an integer

multiple of the fundamental frequency, i.e. if the fundamental frequency is f, the

harmonics have frequencies 2f, 3f, 4f, . . . etc. The harmonics have the property that they

are all periodic at the fundamental frequency; therefore the sum of harmonics is also

periodic at that frequency. Harmonic frequencies are equally spaced by the width of the

fundamental frequency and can be found by repeatedly adding that frequency. For

example, if the fundamental frequency is 25 Hz, the frequencies of the harmonics are: 50

Hz, 75 Hz, 100 Hz, etc. The presence of harmonics does not mean that the factory or office

cannot run properly. Like other power quality phenomena, it depends on the “stiffness” of

the power distribution system and the susceptibility of the equipment. As shown below,

there are a number of different types of equipment that can have disoperation or failures

due to high harmonic voltage and/or current levels. In addition, one factory may be the

source of high harmonics but able to run properly. This harmonic pollution is often carried

back onto the electric utility distribution system, and may effect facilities on the same

system which are more susceptible

Some typical types of equipment susceptible to harmonic pollution include:

- Excessive neutral current, resulting in overheated neutrals. The odd triplen harmonics in

three phase wye circuits are actually additive in the neutral. This is because the harmonic

number multiplied by the 120 degree phase shift between phases is a integer multiple of

360 degrees.

5

Figure 2.1. Additive Third Harmonics.

Incorrect reading meters, including induction discW-hr meters and averaging type

current meters.

Reduced true PF, where PF= Watts/VA.

Overheated transformers, especially delta windings where triplen harmonics

generated on the load side of a delta-wye transformer will circulate in the primary

side. Some type of losses go up as the square of harmonic value (such as skin effect

and eddy current losses). This is also true for solenoid coils and lighting ballasts.

6

Zero, negative sequence voltages on motors and generators. In a balanced system,

voltage harmonics can either be positive (fundamental, 4th, 7th,...), negative (2nd,

5th, 8th...) or zero (3rd, 6th, 9th,...) sequencing values. This means that the voltage

at that particular frequency tries to rotate the motor forward, backward, or neither

(just heats up the motor), respectively. There is also heating from increased losses

as in a transformer.

Nuisance operation of protective devices, including false tripping of relays and

failure of a UPS to transfer properly, especially if controls incorporate zero-

crossing sensing circuits.

Bearing failure from shaft currents through uninsulated bearings of electric motors.

Blown-fuses on PF correction caps, due to high voltage and currents from

resonance with line impedance.

Mis-operation or failure of electronic equipment

If there are voltage subharmonics in the range of 1-30Hz, the effect on lighting is

called flicker. This is especially true at 8.8Hz, where the human eye is most

sensitive, and just 0.5% variation in the voltage is noticeable with some types of

lighting.

2.2 POWER QUALITY

Power quality is defined as the concept of powering and grounding sensitive

equipment in a matter that is suitable to the operation of that equipment.

There are many different reasons for the enormous increase in the interest in power

quality. Some of the main reasons are:

Electronic and power electronic equipment has especially become much more

sensitive. Equipment has become less tolerant of voltage quality disturbances,

production processes have become less tolerant of incorrect of incorrect operation

of equipment, and companies have become less tolerant of production stoppages.

The main perpetrators are interruptions and voltage dips, with the emphasis in

discussions and in the literature being on voltage dips and short interruptions. High

frequency transients do occasionally receive attention as causes of equipment

malfunction.

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Equipment produces more current disturbances than it used to do. Both low and

high power equipment is more and more powered by simple power electronic

converters which produce a broad spectrum of distortion. There are indications that

the harmonic distortion in the power system is rising, but no conclusive results are

obtained due to the lack of large scale surveys.

The deregulation of the electricity industry has led to an increased need for quality

indicators. Customers are demanding, and getting, more information on the voltage

quality they can expect.

Also energy efficient equipment is an important source of power quality

disturbance. Adjustable speed drives and energy saving lamps are both important

sources of waveform distortion and are also sensitive to certain type of power

quality disturbances. When these power quality problems become a barrier for the

large scale introduction of environmentally friendly sources and users’ equipment,

power quality becomes an environmental issue with much wider consequences than

the currently merely economic issues.

2.3 NEED FOR POWER QUALITY IMPROVEMENT

1. Equipment has become less tolerant of voltage quality disturbances, production

processes have become less tolerant of incorrect of incorrect operation of equipment, and

companies have become less tolerant of production stoppages. Note that in many

discussions only the first problem is mentioned, whereas the latter two may be at least

equally important .All this leads to much higher costs than before being associated with

even a very short duration disturbance. The main perpetrators are interruptions and voltage

dips, with the emphasis in discussions and in the literature being on voltage dips and short

interruptions. High frequency transients do occasionally receive attention as causes of

equipment malfunction but are generally not well exposed in the literature.

2. Equipment produces more current disturbances than it used to do. Both low and

high power equipment is more and more powered by simple power electronic converters

which produce a broad spectrum of distortion. There are indications that the harmonic

distortion in the power system is rising, but no conclusive results are obtained due to the

lack of large scale surveys.

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3. The deregulation of the electricity industry has led to an increased need for

quality indicators. Customers are demanding, and getting, more information on the voltage

quality they can expect. Some issues of the interaction between deregulation and power

quality are discussed.

4. Also energy efficient equipment is an important source of power quality

disturbance. Adjustable speed drives and energy saving lamps are both important sources

of waveform distortion and are also sensitive to certain type of power quality disturbances.

When these power quality problems become a barrier for the large scale introduction of

environmentally friendly sources and users’ equipment, power quality becomes an

environmental issue with much wider consequences than the currently merely economic

issues.

2.4 POWER QUALITY STANDARDS

2.4.1 Purpose of Standardization

Standards that define the quality of the supply have been present for decades

already. Almost any country has standards defining the margins in which frequency and

voltage are allowed to vary. Other standards limit harmonic current and voltage distortion,

voltage fluctuations, and duration of an interruption. There are three reasons for

developing power quality standards.

2.4.2 The European Voltage Characteristics Standard

European standard 50160 [80] describes electricity as a product, including its

shortcomings. It gives the main characteristics of the voltage at the customer's supply

terminals in public low-voltage and medium-voltage networks under normal operating

conditions. Some disturbances are just mentioned, for others a wide range of typical values

are given, and for some disturbances actual voltage characteristics are given.

Voltage variation: Standard EN 50160 gives limits for some variations. For each of these

variations the value is given which shall not be exceeded for 95% of the time. The

measurement should be performed with a certain averaging window. The length of this

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window is 10 minutes for most variations; thus very short time scales are not considered

in.

Voltage magnitude: 95% of the 10-minute averages during one week shall be within

±10% of the nominal voltage of 230 V.

TABLE 2.1 Voltage Characteristics as Published by Goteborg Energi

Phenomenon Basic Level

Magnitude variations Voltage shall be between 207 and 244 V

Voltage unbalance Up to 2%

Voltage fluctuations Not exceeding the flicker curve

Frequency In between 49.5 and 50.5 Hz

2.5 POWER QUALITY TERMINOLOGY

DSTATCOM means Distribution Static Compensator. STATCOM is a static VAR

generator, whose output is varied so as to maintain or control specific parameters of the

electric power system.

Sag is a decrease in rms voltage or currents to between 0.1 to 0.9 p.u at the power

frequency for duration of from 0.5 cycles to 1 minute.

Balanced Sag is an equal drop in the rms value of voltage in the three-phases of a

three-phase system or at the terminals of three-phase equipment for duration up to a few

minutes.

Voltage dip is sudden reduction in the supply voltage by a value of more than 10%

of the reference value, fallowed by a voltage recovery after a short period of time.

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Unbalanced Fault is a short circuit or open circuit fault in which not all three

phases are equally involved.

Voltage Tolerance it is the immunity of a piece of equipment against voltage

magnitude variations (Sags, Swells and Interruptions) and short duration over voltages.

Duration (of Voltage Sag) it is the time during which the voltage deviates

significantly from the ideal voltage.

Critical Distance is the distance at which a short-circuit fault will lead to a voltage

sag of a given magnitude for a given load position.

Current Disturbance it is a variation of event during which the current in the

system or at the equipment terminals deviates from the ideal sine wave.

Voltage Disturbance it is a variation of event during which the voltage in the

system or at the equipment terminals deviates from the ideal sine wave.

Power Quality it is the study or description of both voltage and current

disturbances. Power quality can be seen as the combination of voltage quality and current

quality.

Interruption is the voltage event in which the voltage is zero during a certain time.

The time during which the voltage is zero is referred to as the “duration” of the

interruption. (OR) A voltage magnitude event with a magnitude less than 10% of the

nominal voltage.

Over Voltage is an abnormal voltage higher than the normal service voltage, such

as might be caused from switching and lightning surges. (OR) Abnormal voltage between

two points of a system that is greater than the highest value appearing between the same

two points under normal service conditions.

Under Voltage is a voltage event in which the rms voltage is outside its normal

operating margin for a certain period of time. (OR) A voltage magnitude event with a

magnitude less than the nominal rms voltage, and a duration exceeding 1 minute.

Swell it is a momentary increase in the rms voltage or current to between 1.1 and

1.8pu delivered by the mains, outside of the normal tolerance, with a duration of more than

one cycle and less than few seconds.

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Recovery Time is the time interval needed for the voltage or current to return to its

normal operating value, after a voltage or current event.

Fault is an event occurs on the power system and it effects the normal operation of

the power system.

Voltage Fluctuation is a special type of voltage variation in which the voltage

shows changes in the magnitude and/or phase angle on a time scale of seconds or less.

Severe voltage fluctuations lead to light flicker.

2.6 VOLTAGE SOURCE CONVERTERS (VSC)

A voltage-source converter is a power electronic device, which can generate a

sinusoidal voltage with any required magnitude, frequency and phase angle. Voltage

source converters are widely used in adjustable-speed drives, but can also be used to

mitigate voltage dips. The VSC is used to either completely replace the voltage or to inject

the ‘missing voltage’. The ‘missing voltage’ is the difference between the nominal voltage

and the actual.

2.7 POWER QUALITY DISTURBANCES

2.7.1 Voltage sags

Major Causes : Faults, Starting of large loads, and brown-out recovery. Major Consequences: Shorts, accelerated aging, loss of data or stability, Process interrupts, etc.

2.7.2 Capacitor Switching Transients

Major Causes : A power factor correction method

Major Consequences: Insulation breakdown or spark over, semi conductor device

damage, shorts, accelerated aging, loss of data or stability.

12

2.7.3 Harmonics

Major Causes : Power electronic equipment, arcing, transformer saturation.

Major Consequences: Equipment Over heating, high voltage/current,

Protective device operations.

2.7.4 Lightning Transients

Major Causes : Lightning strikes

Major Consequences: Insulation breakdown or spark over, semi conductor

device damage, shorts, accelerated aging, loss of data

or stability.

2.7.5 High Impedance faults

Major Causes : Fallen conductors, trees (fail to establish a permanent return

path)

Major Consequences: Fire, threats to personal safety.

2.8 PRINCIPLES FOR IMPROVING POWER QUALITY

From the discussion already presented, it is evident that for improving power

quality, the steps given in fig (4) have to be taken. As also pointed out, the appropriate

decomposition of power for purposes of both identification and control of the distortion

elimination by filters has to be achieved. Since it is essential to use clear and consistent

terminology, the term non-active power filter will be used for equipment that eliminates

non-active power. The actual types of these filters are to be discussed in a further chapter

of this paper.

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Figure 2.2 Improving power quality by distortion elimination.

The non-active power filters to be used can be divided into the classes of input

converters, dynamic filters and tuned impedance filters. Theses principles and the control

requirements will now be discussed shortly.

Figure 2.3 Principle of input converter to eliminate distortion loads on the power network.

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Identify distortion by usingAppropriate power theory

Decide on method ofDistortion elimination

Equipment with appropriate power electronic input converters.(Dynamic input filters)

Dynamic filters for distortion elimination

Tuned impedance filters

Type of filter:SVC, PWM(Series or parallel) hybrid, undefined

2.9 IMPROVEMENT TECHNIQUES

To improve the power quality, some devices need to be installed at a suitable

location. These devices are called custom power devices, which make sure that customers

get pre specified quality and reliability of supply. The compensating devices compensate a

load, i.e its power factor, unbalance conditions or improve the power quality of supplied

voltage, etc. some of the power quality improving techniques are given as below.

2.9.1 Harmonics

Harmonic Filters may be used to mitigate, and in some cases, eliminate problems

created by power system harmonics. Non-linear loads such as rectifiers, converters, home

electronic appliances, and electric arc furnaces cause harmonics giving rise to extra losses

in power equipment such as transformers, motors and capacitors. They can also cause

other, probably more serious problems, when interfering with control systems and

electronic devices. Installing filters near the harmonic sources can effectively reduce

harmonics. For large, easily identifiable sources of harmonics, conventional filters

designed to meet the demands of the actual application are the most cost efficient means of

eliminating harmonics. These filters consist of capacitor banks with suitable tuning

reactors and damping resistors. For small and medium size loads, active filters, based on

power electronic converters with high switching frequency, may be a more attractive

solution.

Benefits

Eliminates harmonics

Improved Power Factor

Reduced Transmission Losses

Increased Transmission Capability

Improved Voltage Control

Improved Power Quality

Other applications

Shunt Capacitors

2.9.2 Voltage Flickers

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Voltage flicker can become a significant problem for power distributors when large

motor loads are introduced in remote locations. Installation of a series capacitor in the

feeder strengthens the network and allows such load to be connected to existing lines,

avoiding more significant investment in new substations or new distribution lines.

The use of the MiniCap on long distribution feeders provides self-regulated

reactive power compensation that efficiently reduces voltage variations during large motor

starting.

Benefits

Reduced voltage fluctuations (flicker)

Improved voltage profile along the line

Easier starting of large motors

Self-regulation

2.9.3 Bottlenecks

Bottlenecks may be relieved by the use of Series Compensation. Longer lines tend

to have stability-constrained capacity limitations as opposed to the higher thermal

constraints of shorter lines. Series Compensation has the net effect of reducing

transmission line series reactance, thus effectively reducing the line length. Series

Compensation also offers additional power transfer capability for some thermal-

constrained bottlenecks by balancing the loads among the parallel lines. The power

transfer between two-area interconnected systems is limited to 1500MW due to stability

constraints. Additional electricity can be delivered between them if Series Compensation is

applied to increase the maximum stability limits.

Benefits

Increased Power Transfer Capability

Additional flexibility in Grid Operation

Improved Grid Voltage Control

Lower Transmission Losses

Improved Transient Stability

Other applications

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Power Flow Control

Transient Stability Improvement

2.9.4 Shunt Capacitors

Regulation of the power factor to increase the transmission capability and reduce

transmission losses. Shunt capacitors are primarily used to improve the power factor in

transmission and distribution networks, resulting in improved voltage regulation, reduced

network losses, and efficient capacity utilization. Figure shows a plot of terminal voltage

versus line loading for a system that has a shunt capacitor installed at the load bus.

Improved transmission voltage regulation can be obtained during heave power transfer

conditions when the system consumes a large amount of reactive power that must be

replaced by compensation. At the line surge impedance loading level, the shunt capacitor

would decrease the line losses by more than 35%. In distribution and industrial systems, it

is common to use shunt capacitors to compensate for the highly inductive loads, thus

achieving reduced delivery system losses and network voltage drop.

Benefits

Improved power factor

Reduced transmission losses

Increased transmission capability

Improved voltage control

Improved power quality

Other applications

Harmonic Filters

2.9.5 Shunt Reactor

The primary purpose of the shunt reactor is to compensate for capacitive charging

voltage, a phenomenon getting more prominent for increasing line voltage. Long high-

voltage transmission lines and relatively short cable lines (since a power cable has high

capacitance to earth) generate a large amount of reactive power during light power transfer

conditions which must be absorbed by compensation. Otherwise, the receiving terminals of

the transmission lines will exhibit a “voltage rise” characteristic and many types of power

17

equipment cannot withstand such over voltages. A better fine tuning of the reactive power

can be made by the use of a tap changer in the shunt reactor. It can be possible to vary the

reactive power between 50 to 100 % of the needed power.

Benefits

Simple and robust customer solution with low installation costs and minimum

maintenance

No losses from an intermediate transformer when feeding reactive

compensation from a lower voltage level.

No harmonics created which may require filter banks.

2.9.6 SVC

Static VAR Compensators are used in transmission and distribution networks

mainly providing dynamic voltage support in response to system disturbances and

balancing the reactive power demand of large and fluctuating industrial loads. A Static

VAR Compensator is capable of both generating and absorbing variable reactive power

continuously as opposed to discrete values of fixed and switched shunt capacitors or

reactors. Further improved system steady state performance can be obtained from SVC

applications. With continuously variable reactive power supply, the voltage at the SVC bus

may be maintained smoothly over a wide range of active power transfers or system loading

conditions. This entails the reduction of network losses and provision of adequate power

quality to the electric energy end-users.

The traction system is a major source of unbalanced loads. Electrification of

railways, as an economically attractive and environmentally friendly investment in

infrastructure, has introduced an unbalanced and heavy distorted load on the three-phase

transmission grid. Without compensation, this would result in significant unbalanced

voltage affecting most neighboring utility customers. The SVC can elegantly be used to

counteract the unbalances and mitigate the harmonics such that the power quality within

the transmission grid is not impaired.

Static Var Compensators are mainly used to perform voltage and reactive power

regulation. However, when properly placed and controlled, SVCs can also effectively

counteract system oscillations. A SVC, in effect, has the ability to increase the damping

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factor (typically by 1-2 MW per Mvar installed) on a bulk power system which is

experiencing power oscillations. It does so by effectively modulating its reactive power

output such that the regulated SVC bus voltage would increase the system damping

capability.

SVC is used most frequently for compensation of disturbances generated by the

Electrical Arc Furnaces (EAF). With a well-designed SVC, disturbances such as flicker

from the EAF are mitigated Flicker, the random variation in light intensity from

incandescent lamps caused by the operating of nearby fluctuating loads on the common

electric supply grid, is highly irritating for those affected. The random voltage variations

can also be disturbing to other process equipment fed from the same grid. The proper

mitigation of flicker is therefore a matter of power quality improvement as well as an

improvement to human environment.

Benefits

Increased Power Transfer Capability

Additional flexibility in Grid Operation

Improved Grid Voltage Stability

Improved Grid Voltage Control

Improved Power Factor

Other applications

Power Oscillation Damping

Power Quality (Flicker Mitigation, Voltage Balancing)

Grid voltage support

2.9.7 STATCOM

STATCOM, when connected to the grid, can provide dynamic voltage support in

response to system disturbances and balance the reactive power demand of large and

fluctuating industrial loads. A STATCOM is capable of both generating and absorbing

variable reactive power continuously as opposed to discrete values of fixed and switched

shunt capacitors or reactors. With continuously variable reactive power supply, the voltage

at the STATCOM bus may be maintained smoothly over a wide range of system operation

19

conditions. This entails the reduction of network losses and provision of sufficient power

quality to the electric energy end-users.

STATCOM® is an effective method used to attack the problem of flicker. The

unbalanced, erratic nature of an electric arc furnace (EAF) causes significant fluctuating

reactive power demand, which ultimately results in irritating electric lamp flicker to

neighboring utility customers. In order to stabilize voltage and reduce disturbing flicker

successfully, it is necessary to continuously measure and compensate rapid changes by

means of extremely fast reactive power compensation.

STATCOM® uses voltage source converters to improve furnace productivity

similar to a traditional SVC while offering superior voltage flicker mitigation due to fast

response time. Similar to SVC, the STATCOM can elegantly be used to restore voltage

and current balance in the grid, and to mitigate voltage fluctuations generated by the

traction loads.

Benefits

Increased Power Transfer Capability

Additional flexibility in Grid Operation

Improved Grid Voltage Stability

Improved Grid Voltage Control

Improved Power Factor

Eliminated Flicker

Harmonic Filtering

Voltage Balancing

Power Factor Correction

Furnace/mill Process Productivity Improvement

Other applications

Power Quality (Flicker Mitigation, Voltage Balancing)

Grid Voltage Support

2.9.8 Shunt Active Filters

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The various nonlinear loads like Adjustable Speed Drives (ASD’s), bulk rectifiers,

furnaces, computer supplies, etc. draw non sinusoidal currents containing harmonics from

the supply which in turn causes voltage harmonics. Harmonic currents result in increased

power system losses, excessive heating in rotating machinery, interference with nearby

communication circuits and control circuits, etc.

It has become imperative to maintain the sinusoidal nature of voltage and currents

in the power system. Various international agencies like IEEE and IEC have issued

standards, which put limits on various current and voltage harmonics. The limits for

various current and voltage harmonics specified by IEEE-519 for various frequencies are

given in Table 3.1 and Table 3.2.

Table 2.2

IEEE 519 Voltage Limits

Bus VoltageMinimum Individual

Harmonic Components (%)

Maximum

THD (%)

69 kV and below 3 5

115 kV to 161 kV 1.5 2.5

Above 161 Kv 1 1.5

The objectives and functions of active power filters have expanded from reactive

power compensation, voltage regulation, etc. to harmonic isolation between utilities and

consumers, and harmonic damping throughout the distribution as harmonics propagate

through the system. Active power filters are either installed at the individual consumer

premises or at substation and/or on distribution feeders. Depending on the compensation

objectives, various types of active power filter topologies have evolved, a proper briefing

provided in further.

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Table 2.3

IEEE 519 Current Limits

SCR=Isc/Il h<11 11 to 17 17 to 23 23 to 35 35<h THD

<20 4.0 2.0 1.5 0.6 0.3 5.0

20 – 50 7.0 3.5 2.5 1.0 0.5 8.0

50 -100 10.0 4.5 4.0 1.5 0.7 12.0

100 – 1000 12.0 5.5 5.0 2.0 1.0 15.0

>1000 15.0 7.0 6.0 2.5 1.4 20.0

2.10 CLASSIFICATIONS OF ACTIVE POWER FILTERS

2.10.1 Converter based classification

Current Source Inverter (CSI) Active Power Filter (Fig 3.1) and Voltage Source

Inverter Active Power Filter (VSI) (Fig 3.2) are two classifications in this category.

Current Source Inverter behaves as a nonsinusoidal current source to meet the harmonic

current requirement of the nonlinear loads. A diode is used in series with the self-

commutating device (IGBT) for reverse voltage blocking. However, GTO-based

configurations do not need the series diode, but they have restricted frequency of

switching. They are considered sufficiently reliable, but have higher losses and require

higher values of parallel ac power capacitors. Moreover, they cannot be used in multilevel

or multistep modes to improve performance in higher ratings.

The other converter used as an AF is a voltage-fed PWM inverter structure, as

shown in Fig 3.2. It has a self-supporting dc voltage bus with a large dc capacitor. It has

become more dominant, since it is lighter, cheaper, and expandable to multilevel and

multistep versions, to enhance the performance with lower switching frequencies. It is

more popular in UPS-based applications, because in the presence of mains, the same

Inverter bridge can be used as an AF to eliminate harmonics of critical nonlinear loads.

22

2.10.2 Topology based Classification

AF’s can be classified based on the topology used as series or shunt filters, and

unified power quality conditioners use a combination of both. Combinations of active

series and passive shunt filtering are known as hybrid filters. Fig 3.3 is an example of an

active shunt filter, which is most widely used to eliminate current harmonics, reactive

power compensation (also known as STATCOM, and balancing unbalanced currents. It is

mainly used at the load end, because current harmonics are injected by nonlinear loads. It

injects equal compensating currents, opposite in phase, to cancel harmonics and/or reactive

components of the nonlinear load current at the point of connection. It can also be used as

a static VAR generator (STATCOM) in the power system network for stabilizing and

improving the voltage profile.

Fig 2.4 Current fed type AF Fig 2.5 Voltage fed type AF

Fig 2.4 shows the basic block of a stand-alone active series filter. It is connected

before the load in series with the mains, using a matching transformer, to eliminate voltage

harmonics, and to balance and regulate the terminal voltage of the load or line. It has been

used to reduce negative-sequence voltage and regulate the voltage on three-phase systems.

It can be installed by electric utilities to compensate voltage harmonics and to damp out

harmonic propagation caused by resonance with line impedances and passive shunt

compensators.

Fig 2.4 shows the basic block of a stand-alone active series filter. It is connected

before the load in series with the mains, using a matching transformer, to eliminate voltage

harmonics, and to balance and regulate the terminal voltage of the load or line. It has been

23

used to reduce negative-sequence voltage and regulate the voltage on three-phase systems.

It can be installed by electric utilities to compensate voltage harmonics and to damp out

harmonic propagation caused by resonance with line impedances and passive shunt

compensators.

Fig 2.5 shows the hybrid filter, which is a combination of an active series filter and

passive shunt filter. It is quite popular because the solid-state devices used in the active

series part can be of reduced size and cost (about 5% of the load size) and a major part of

the hybrid filter is made of the passive shunt L–C filter used to eliminate lower order

harmonics. It has the capability of reducing voltage and current harmonics at a reasonable

cost.

Fig 2.6 Shunt-type AF Fig 2.7 Series-type AF

Fig 2.8 Hybrid filter Fig 2.9 Unified Power Quality Conditioner

24

Fig 2.9 shows a unified power quality conditioner (also known as a universal AF),

which is a combination of active shunt and active series filters. The dc-link storage

element (either inductor or dc-bus capacitor) is shared between two current-source or

voltage-source bridges operating as active series and active shunt compensators. It is used

in single-phase as well as three-phase configurations. It is considered an ideal AF, which

eliminates voltage and current harmonics and is capable of giving clean power to critical

and harmonic-prone loads, such as computers, medical equipment, etc. It can balance and

regulate terminal voltage and eliminate negative-sequence currents. Its main drawbacks are

its large cost and control complexity because of the large number of solid-state devices

involved.

2.10.3 Supply-System-Based Classification

This classification of AF’s is based on the supply and/or the load system having

single-phase (two wire) and three-phase (three wire or four wire) systems. There are many

nonlinear loads, such as domestic appliances, connected to single-phase supply systems.

Some three-phase nonlinear loads are without neutral, such as ASD’s, fed from three-wire

supply systems. There are many nonlinear single-phase loads distributed on four-wire

three-phase supply systems, such as computers, commercial lighting, etc. Hence, AF’s

may also be classified accordingly as two-wire, three-wire, and four-wire types.

1) Two-Wire AF’s

Two-wire (single phase) AF’s are used in all three modes as active series, active

shunt, and a combination of both as unified line conditioners. Both converter

configurations, current-source PWM bridge with inductive energy storage element and

voltage-source PWM bridge with capacitive dc-bus energy storage elements, are used to

form two-wire AF circuits. In some cases, active filtering is included in the power

conversion stage to improve input characteristics at the supply end.

2) Three-Wire AF’s

Three-phase three-wire nonlinear loads, such as ASD’s, are major applications of

solid-state power converters and, lately, many ASD’s, etc., incorporate AF’s intheir front-

end design. A large number of publications have appeared on three-wire AF’s with

different configurations. All the configurations shown in Figs 3.1–3.6 are developed, in

three-wire AF’s, with three wires on the ac side and two wires on the dc side. Active shunt

25

AF’s are developed in the current-fed type (Fig 3.1) or voltage-fed type with single-stage

(Fig 3.2) or multi-step/multilevel and multi-series configurations. Active shunt AF’s are

also designed with three single-phase AF’s with isolation transformers [18] for proper

voltage matching, independent phase control, and reliable compensation with unbalanced

systems. Active series filters are developed for stand-alone mode (Fig 3.4) or hybrid mode

with passive shunt filters (Fig 3.5). The latter (hybrid) has become quite popular to reduce

the size of power devices and cost of the overall system. A combination of active series

and active shunt is used for unified power quality conditioners (Fig 3.6) and universal

filters.

3) Four-Wire AF’s

A large number of single-phase loads may be supplied from three-phase mains with

neutral conductor. They cause excessive neutral current, harmonic and reactive power

burden, and unbalance. To reduce these problems, four-wire AF’s have been attempted.

They have been developed as: 1) active shunt mode with current feed and voltage feed; 2)

active series mode; and 3) hybrid form with active series and passive shunt mode.

2.10.4 Compensated Variable Based Classification

(1) Harmonic Compensation

(2) Multiple Compensation

This is the most important system parameter requiring compensation in power

systems and it is subdivided into voltage- and current-harmonic compensation. The

compensation of voltage and current harmonics is interrelated.

Different combinations of the above systems can be used to improve the

effectiveness of filters. The following are the most frequently used combinations.

Harmonic currents with Reactive power compensation.

Harmonic voltages with Reactive power compensation.

Harmonic currents and voltages.

Harmonic currents and voltages with reactive-power compensation.

26

2.10.5 Voltage Type Vs Current Type APF’s

A clear trend for preferred type of APF’s does not exist. A choice depends on

source of distortion at the specified bus, equipment cost, and amount of correction desired.

Voltage-type has an advantage in that they can be readily expanded in parallel to

increase their combined rating. Their combined switching rate can be increased if they are

carefully controlled so that their individual switching times do not coincide. Therefore,

using parallel voltage-type converters with out increasing individual converter switching

rates can eliminate higher order harmonics. Voltage type converters are lighter and less

expansive than current-type converters.

The main drawback of voltage-type converters lies in the increased complexity of

their control system. For systems with several connected in parallel, this complexity is

greatly increased.

Current-type converters have advantages of excellent current controllability, easy

protection and high reliability over Voltage source APF. More over CSI topology has

superior characteristics compared to VSI topology in terms of direct injected current,

which result in a faster response in time varying load environment and lower dc energy

storage requirement. The drawback of the current source APF is larger power losses of the

dc-link inductor. However, the current-type active power filter will become more attractive

when the super conducting coils are available in the future. Losses are less important in

low- power applications but very important in high power applications.

Since they are easily expandable, voltage type APF’s are likely to be used for

network wide compensation. Current type APF’s will continue to popular for single-node

distortion problems. In other words, electric utility interest will likely to be focused on

voltage type converters, while industrial users likely to use both type of converters.

2.11 Operation of Three Phase Active Power Filters

In recent years, the power quality of the AC main system has become a great

concern due to the rapidly increased number of electronic equipment. In order to reduce

the harmonic contamination in power lines and improve the transmission efficiency Active

power filters become essential. A current source is connected in parallel with nonlinear

load and controlled to generate the harmonic currents needed for the load.

27

The basic configuration of a three-phase three-wire active power filter is shown in

Fig 2.9 The diode bridge rectifier is used as an ideal harmonic generator to study the

performance of the Active filter. The current-controlled voltage-source inverter (VSI) is

shown connected at the load end. This PWM inverter consists of six switches with

antiparallels diode across each switch. The capacitor is designed in order to provide DC

voltage with acceptable ripples. In order to assure the filter current at any instant, the DC

voltage Vdc must be at least equal to 3/2 of the peak value of the line AC mains voltage.

2.11.1 Sources of Harmonic Current

The main sources of harmonic current are at present the phase angle controlled

rectifiers and inverters. These are often called static power converters. These devices take

AC power and convert it to another form, sometimes back to AC power at the same or

different frequency, based on the firing scheme. The firing scheme refers to the controlling

mechanism that determines how and when current is conducted. One major variation is the

phase angle at which conduction begins and ends. A typical such converter is the

switching-type power supplies found in most personal computers and peripheral

equipment, such as printers. While they offer many benefits in size, weight and cost, the

large increase of this type of equipment over the past fifteen years is largely responsible

for the increased attention to harmonics. Figure shows how a switching-type power supply

works. The AC voltage is converted into a DC voltage, which is further converted into

other voltages that the equipment needs to run. The rectifier consists of semi-conductor

devices (such as diodes) that only conduct current in one direction. In order to do so, the

voltage on the one end must be greater than the other end. These devices feed current into

a capacitor, where the voltage value on the cap at any time depends on how much energy

is being taken out by the rest of the power supply. When the input voltage value is

higher than voltage on the capacitor, the diode will conduct current through it. This results

in a current waveform and harmonic spectrum . Obviously, this is not a pure sinoidal

waveform with only a 60 Hz frequency component.

28

Fig.2.10 Output current waveform

Fig.2.11 Harmonic spectrum

Fluorescent lights can be the source of harmonics, as the ballasts are non-linear

inductors. The third harmonic is the predominate harmonic in this case. As previously

mentioned, the third harmonic current from each phase in a four-wire wye or star system

will be additive in the neutral, instead of cancelling out Some of the newer electronic

29

ballasts have very significant harmonic problems, as they operate somewhat like a

switching power supply, but can result in current harmonic distortion levels over 30%.

Low power, AC voltage regulators for light dimmers and small induction motors

adjust the phase angle or point on the wave where conduction occurs. Medium power

converters are used for motor control in manufacturing and railroad applications, and

include such equipment as ASDs (adjustable speed drives) and VFDs (variable frequency

drives). Metal reduction operations, like electric arc furnaces, and high voltage DC

transmission employ large power converters, in the 2-20MVA rating. This type of 3-phase

equipment may also cause other types of power quality problems. When the semiconductor

device is supposed to turn-off, it does not do so abruptly. This happens under “naturally”

commutated conditions, where the voltage that was larger on the anode side compared to

the cathode is now the opposite. This occurs each cycle as the voltage waveform goes

through the sine waveform. It also happens under “forced” commutation conditions, where

the semi-conductor device has a “gate”-type control mechanism built in to it. This

commutation period is a time when two semiconductor devices are both conducting

current at the same time, effectively shorting one phase to the other and resulting in large

current transients. When transformers are first energized, the current drawn is different

from the steady state condition. This is caused by the inrush of the magnetizing current.

The harmonics during this period varies over time. Some harmonics have zero value for

part of the time, and then increase for a while before returning to zero. An unbalanced

transformer (where either the output current, winding impedance, or input voltage on each

leg are not equal) will cause harmonics, as will overvoltage saturation of a transformer.

2.11.2 Occurrence of Harmonics

Wherever the aforementioned equipment is used, one can suspect that harmonics are

present. The amount of voltage harmonics will often depend on the amount of harmonic

currents being drawn by the load, and the source impedance, which includes all of the

wiring and transformers back to the source of the electricity. Ohm’s Law says that Voltage

equals Current multipled by Impedance. This is true for harmonic values as well. If the

source harmonic impedance is very low (often referred to as a “stiff” system) then the

harmonic currents will result in lower harmonic voltages than if the source impedance

were high (such as found with some types of isolation transformers). Like any power

30

quality investigation, the search can begin at the equipment effected by the problem or at

the point-of-common-coupling (PCC), where the utility service meets the building

distribution system. If only one piece of equipment is effected (or suspected), it is often

easier to start the monitoring process there. If the source is suspected to be from the utility

service side (such is the case when there is a neighboring factory that is known to generate

high harmonics), then monitoring usually begins at the PCC.

The phase voltages and currents, as well as the neutral-to-ground voltage and

neutral current should be monitored, where possible. This will aid in pinpointing problems,

or detecting marginal systems. Monitoring the neutral will often show a high 3rd harmonic

value, indicating the presence of non-linear loads in the facility.

2.11.3 Detection of harmonics

Hand-held harmonic meters can be useful tools for making spot checks for known

harmonic problems. However, harmonic values will often change during the day, as

different loads are turned on and off within the facility or in other facilities on the same

electric utility distribution system. This requires the use of a harmonic monitor or power

quality monitor with harmonic capabilities which can record the harmonic values over a

period of time. Power Quality Monitor with Harmonic Analysis Typically, monitoring will

last for one business cycle. A business cycle is how long it takes for the normal operation

of the plant to repeat itself. For example, if a plant runs three identical shifts, seven days a

week, then a business cycle would be eight hours.

More typically, a business cycle is one week, as different operations take place on a

Monday, when the plant equipment is restarted after being off over the weekend, then on a

Wednesday, or a Saturday, when only a skelton crew may be working. Certain types of

loads also generate typical harmonic spectrum signatures, that can point the investigator

towards the source. This is related to the number of pulses, or paths of conduction. The

general equation is h = ( n * p ) +/- 1, where h is the harmonic number, n is any integer

(1,2,3,..) and p is the number of pulses in the circuit, and the magnitude decreases as the

ration of 1/h (1/3, 1/5, 1/7, 1/9,...). Table 4 shows examples of such.

2.11.4 Problems Faced in Harmonics

31

Most electrical loads (except half-wave rectifiers) produce symmetrical current

waveforms, which means that the positive half of the waveform looks like a mirror image

of the negative half. This results in only odd harmonic values being present. Even

harmonics will disrupt this half-wave symmetry. The presence of these even harmonics

should cause the investigator to suspect there is a half-wave rectifier on the ciruit. This

also result from a full wave rectifier when one side of the rectifier has blown or damaged

components. Early detection of this condition in a UPS system can prevent a complete

failure when the load is switched onto back-up power. To determine what is normal or

acceptable levels, a number of standards have been developed by various organizations.

ANSI/IEEE C57.110 Recommended Practice for Establishing Transformer Compatibility

when supplying non sinusoidal load currents is a useful document for determining how

much a transformer should be derated from its nameplate rating when operating in the

presence of harmonics. There are two parameters typically used, called K-factor and TDF .

Some power quality harmonic monitors will automatically calculate these values. IEEE

519-1992 Recommended Practices and Requirements for Harmonic Control in Electrical

Power Systems provides guidelines from determining what are acceptable limits.

For voltage harmonics, the voltage level of the system is used to determine the

limits. At the higher voltages, more customers will be effective, hence, the lower limits.

The European Community has also developed susceptibility and emission limits for

harmonics. Formerly known as the 555-2 standard for appliances of less than 16 A, a more

encompassing set of standards under IEC 1000-4-7 are now in effect.

2.11.5 Precaution to Prevent Harmonics

Care should be undertaken to make sure that the corrective action taken to

minimize the harmonic problems don’t actually make the system worse. This can be the

result of resonance between harmonic filters, PF correcting capacitors and the system

impedance. Isolating harmonic pollution devices on separate circuits with or without the

use of harmonic filters are typical ways of mitigating the effects of such. Loads can be

relocated to try to balance the system better. Neutral conductors should be properly sized.

Phase conductors are particularly important with some modular office partition-type walls,

which can exhibit high impedance values. The operating limits of transformers and motors

should be derated, in accordance with industry standards from IEEE, ANSI and NEMA on

32

such. Use of higher pulse converters, such as 24-pulse rectifiers, can eliminate lower

harmonic values, but at the expense of creating higher harmonic values.

2.12 INTRODUCTION FOR FILTERS

Electronic filters are electronic circuits which perform signal processing functions,

specifically to remove unwanted frequency components from the signal, to enhance

wanted ones, or both. Electronic filters can be:

passive or active

analog or digital

high-pass , low-pass, band pass, band-reject (band reject; notch), or all-pass.

discrete-time (sampled) or continuous-time

linear or non-linear

infinite impulse response (IIR type) or finite impulse response (FIR type)

2.12.1 Passive Filter

The passive filters have been used as a conventional solution to solve harmonic

currents problems, but they present some disadvantages: they only filter the frequencies

they were previously tuned for; its operation cannot be limited to a certain load or group of

loads; resonance can occur due to the interaction between the passive filters and others

loads, with unexpected results.

Passive implementations of linear filters are based on combinations of resistors (R),

inductors (L) and capacitors (C). These types are collectively known as passive filters,

because they do not depend upon an external power supply and/or they do not contain

active components such as transistors.

Inductors block high-frequency signals and conduct low-frequency signals, while

capacitors do the reverse. A filter in which the signal passes through an inductor, or in

which a capacitor provides a path to ground, presents less attenuation to low-frequency

signals than high-frequency signals and is a low-pass filter. If the signal passes through a

capacitor, or has a path to ground through an inductor, then the filter presents less

33

attenuation to high-frequency signals than low-frequency signals and is a high-pass filter.

Resistors on their own have no frequency-selective properties, but are added to inductors

and capacitors to determine the time-constants of the circuit, and therefore the frequencies

to which it responds.

The inductors and capacitors are the reactive elements of the filter. The number of

elements determines the order of the filter. In this context, an LC tuned circuit being used

in a band-pass or band-stop filter is considered a single element even though it consists of

two components.

At high frequencies (above about 100 megahertz), sometimes the inductors consist

of single loops or strips of sheet metal, and the capacitors consist of adjacent strips of

metal. These inductive or capacitive pieces of metal are called stubs.

Fig.2.12 A low-pass electronic filter

A low-pass electronic filter realized by an RC circuit

The simplest passive filters, RC and RL filters, include only one reactive element, except

hybrid LC filter which is characterized by inductance and capacitance integrated in one

element.

An L filter consists of two reactive elements, one in series and one in parallel.

Fig.2.13 Low-pass π filter

34

Fig.2.14 High-pass T filter

Three-element filters can have a 'T' or 'π' topology and in either geometries, a low-

pass, high-pass, band-pass, or band-stop characteristic is possible. The components can be

chosen symmetric or not, depending on the required frequency characteristics. The high-

pass T filter in the illustration has a very low impedance at high frequencies, and a very

high impedance at low frequencies. That means that it can be inserted in a transmission

line, resulting in the high frequencies being passed and low frequencies being reflected.

Likewise, for the illustrated low-pass π filter, the circuit can be connected to a transmission

line, transmitting low frequencies and reflecting high frequencies. Using m-derived filter

sections with correct termination impedances, the input impedance can be reasonably

constant in the pass band.

Multiple element filters are usually constructed as a ladder network. These can be

seen as a continuation of the L,T and π designs of filters. More elements are needed when

it is desired to improve some parameter of the filter such as stop-band rejection or slope of

transition from pass-band to stop-band.

2.12.2 Active Filter

With this filter it is possible to effectively compensate the harmonic currents and

the reactive power (correcting power factor to the unity), and also to balance the power

supply currents (distributing the loads for the three-phases in equal form, and

compensating zero sequence current).

An active filter is a type of analog electronic filter, distinguished by the use of one

or more active components i.e. voltage amplifiers or buffer amplifiers. Typically this will

be a vacuum tube, or solid-state (transistor or operational amplifier).

35

Active filters have three main advantages over passive filters:

Inductors can be avoided. Passive filters without inductors cannot obtain a high

Q (low damping), but with them are often large and expensive (at low

frequencies), may have significant internal resistance, and may pick up

surrounding electromagnetic signals.

The shape of the response, the Q (Quality factor), and the tuned frequency can

often be set easily by varying resistors, in some filters one parameter can be

adjusted without affecting the others. Variable inductances for low frequency

filters are not practical.

The amplifier powering the filter can be used to buffer the filter from the

electronic components it drives or is fed from, variations in which could

otherwise significantly affect the shape of the frequency response

Fig 2.15 Active Filter

2.13 SUMMARY

In this chapter, the functions of active filter and passive filter are explained. It

also deals with the harmonic present in the system.

36

CHAPTER 3

MODELLING

3.1 INTRODUCTION

Filters are used to restrict the flow of harmonic currents 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. They are active filter and passive filter.

The simplest method of harmonic filtering is with passive filters. It uses the reactive

storage components, namely capacitors and inductors.

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-3.1

shows the configuration of Shunt passive Filter.

The increasing use of power electronics-based loads (adjustable speed drives,

switch mode power supplies, etc.) to improve system efficiency and controllability is

increasing the concern for harmonic distortion levels in end use facilities and on the

overall power system. The application of passive tuned filters creates new system

resonances which are dependent on specific system conditions. In addition, passive filters

often need to be significantly overrated to account for possible harmonic absorption from

the power system. Passive filter ratings must be co-ordinate with reactive power

requirements of the loads and it is often difficult to design the filters to avoid leading

power factor operation for some load conditions. Active filters have the advantage of being

able to compensate for harmonics without fundamental frequency reactive power concerns.

This means that the rating of the active power can be less than a comparable passive filter

for the same non-linear load and the active filter will not introduce system resonances that

can move a harmonic problem from one frequency to another.

37

Fig.3.1The configuration of Shunt passive filter

The active filter concept uses power electronics to produce harmonic current

components that cancel the harmonic current components from the non-linear loads. The

active filter uses power electronic switching to generate harmonic currents that cancel the

harmonic currents from a non-linear load. The active filter configuration investigated in

this lecture is based on a Pulse-Width Modulated (PWM) voltage source inverter that

interfaces to the system through a system interface filter as shown in Figure 3.1 . In this

configuration, the filter is connected in parallel with the load being compensated.

Therefore, the configuration is often referred to as an active parallel or shunt filter. Figure

1 illustrates the concept of the harmonic current cancellation so that the current being

supplied from the source is sinusoidal. The voltage source inverter used in the active filter

makes the harmonic control possible. This inverter uses dc capacitors as the supply and

can switch at a high frequency to generate a signal that will cancel the harmonics from the

non-linear load.

The active filter does not need to provide any real power to cancel harmonic

currents from the load. The harmonic currents to be cancelled show up as reactive power.

Reduction in the harmonic voltage distortion occurs because the harmonic currents flowing

through the source impedance are reduced. Therefore, the dc capacitors and the filter

components must be rated based on the reactive power associated with the harmonics to be

38

cancelled and on the actual current waveform (rms and peak current magnitude) that must

be generated to achieve the cancellation.

The current waveform for cancelling harmonics is achieved with the voltage source

inverter in the current controlled mode and an interfacing filter. The filter provides

smoothing and isolation for high frequency components. The desired current waveform is

obtained by accurately controlling the switching of the Insulated Gate Bipolar Transistors

(IGBTs) in the inverter. Control of the current wave shape is limited by the switching

frequency of the inverter and by the available driving voltage across the interfacing

inductance.

The driving voltage across the interfacing inductance determines the maximum

di/dt that can be achieved by the filter. This is important because relatively high values of

di/dt may be needed to cancel higher order harmonic components. Therefore, there is a

trade-off involved in sizing the interface inductor. A larger inductor is better for isolation

from the power system and protection from transient disturbances. However, the larger

inductor limits the ability of the active filter to cancel higher order harmonics.

3.2 SYSTEM CONFIGURATION

The power circuit topology of a combination series active filter with shunt passive

filters on three phase system, feeding non-linear loads which is a three phase bridge diode

rectifier with smoothing dc capacitor. The designed LC passive filters is single-tuned at

5th and 7th harmonic frequency, both are parallel connected to the power line before the

load. Installation of such passive filter in the vicinity of the non-linear load due to its

responsibility as a harmonic sink path for the tuned harmonic frequency, and also helps in

lowering the power rating of active power filter. A three-phase Voltage Source Inverter

(VSI) is connected in series with the power line via three single phase series coupling

transformer in order to inject the compensation voltage. This VSI is three phase bridge

configuration consist of six Insulated Gate Bipolar Transistor (IGBT) switch connected

with anti-parallel diodes and snubber circuit, is fed by SVPWM control signal from the

controller. Harmonic mitigation by active filters is entirely depends on modulation method

where SVPWM technique has distinct advantages and displays very good characteristics

over this matters. SVPWM has the following advantages over other control schemes in

terms of the use factor of the DC side voltage is high; switching losses are low; in

39

applications such as motor drives it can be conveniently used as flux tracking control or

current control; and also it is easy to digitally implement the modulation scheme. Fig 3.3

indicates the basic switching and sectors applied in SVPWM technique. In order to

implement SVPWM following steps needs to be determined. Firstly, determination of

stationary d-q reference frame vectors Vd, Vq, and reference voltage vector vref and angle;

next step is to determine the time duration T1, T2 and T0; and lastly,

To determine the switching time of each switch applied in VSI. For the

simulation study using Matlab/Simulink, only the configuration of Shunt passive Filter Vd,

Vq, Vref and is required for SVPWM generator block. The turn ratio of the series coupling

transformer connecting the active power filter to the power line is chosen to be 1:1 ratio.

This type of transformer is not only isolate SVPWM-VSI from the power line, but also

matching the voltage and current rating of SVPWM inverter with that of the power line.

The harmonic-producing load is a three-phase diode rectifier with capacitive dc load

having characteristics of voltage-type harmonics. Smoothing dc capacitor capacitance

value is 1000 (μF), while load resistance is 50Ω. System parameters used in the simulation

is depicted on Table 1. Hence, with implementation of Mat lab/Simulink, the simulation

and performance of the system is executed well.

40

Fig 3.2 Series Hybrid Power Filter Circuit Configuration

41

Fig 3.3 Basic switching vectors and sectors

42

Table 3.1: System Parameters

3.3 CONTROL SCHEME

For accurate estimation of the fundamental component, two methods are dominant

and widely used in reference generation control; SRF control theory is utilized for the

proposed system. It has been confirmed that SRF controller achieves significant

performance improvement for active filter implementation without any assumptions

regarding supply power quality. Practically the series active filter is controlled in a manner

where there is no impedance at fundamental frequency and insertion of gain K (Ω)

between source and load at harmonic frequency.

3.3.1 Positive Sequence Component and Source Phase Angle Calculation

Based on series active filter control used in, derivation of balanced positive-

sequence components from unbalanced voltages but without predicted reference voltages,

control of the system begins to be designed. The balanced positive components is derived

from unbalanced sets can be expressed in equation below,

..……. (3.1)

43

The j meaning phase shift angle of 90, is obtained using all-pass filter which is given using

the following equation,

….….. (3.2)

Here, b=377 (rad/s) and c=π/2. The variables ea(+), eb(+), and ec(+), the balanced positive

sequence components is used to derive source phase angle, θ by employing d-q

transformation, as depicted in (3.3),

…….(3.3)

……… (3.4)

3.3.2 Compensation of Source Voltage Unbalance

The equations (3.3) and (3.4) employs that the d-q quantities which are

transformed into SRF based on equation proposed by

………....(3.5)

Where, the superscript “e” means a quantity in SRF.

Thus, required fundamental component of balanced source voltage is obtained from,

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……..(3.6)

Ku is a gain to recover the required fundamental magnitude of source voltage that is

derived based on the following equation,

……..(3.7)

Using (3.6), reference voltage for unbalance compensation is determined by,

………(3.8)

3.3.3 Current Harmonics Compensation Scheme

Two compensation schemes according to harmonic source type will be performed.

Initially, reference generation for voltage compensating for current harmonics due to the

voltage-type harmonic source is derived. The source side

harmonic current, is h is expressed as,

………(3.9)

If esh, vc and vLh are harmonic of source voltage, injected voltage, and load voltage

harmonic, respectively. Whereby, zsh represents source side harmonic impedance. Given

the reference voltage for current harmonic compensation, vc expressed as,

………..(3.10)

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The harmonic source current is suppressed to be zero provided that the source

voltage is assumed to be sinusoidal. Since zsh is so small that can be neglected, and vc has

no fundamental component. Hence, vLh is can be assumed using following equation,

………..(3.11)

Avoiding additional voltage sensor, the estimated load voltage is obtained from,

……...….(3.12)

Here, es and is are source voltage and current respectively, while vL is load

voltage. Source impedance zs is assumed to be negligible because it is usually 2%-5% p.u.

Secondly, reference voltage compensating for the current-type harmonic source is

calculated from the harmonic current comes from the source side, which is equal to

….......….(3.13)

Whereby, 1 is fundamental component of source current. This value is derived

from the following equation,

…….(3.14)

The respective mean value of source current transformed in SRF,

, are given by,

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…......…..(3.15)

Thus by combining eqns. (3.4) & (3.6), new reference voltage for the current

harmonics compensation is formed, which is expressed by,

………..(3.16)

Here ,Kvh is a controller gain which is far bigger than Zf, an equivalent impedance

of the shunt passive filter . To achieve optimized and good performance in controlling the

reference voltage and balance voltage magnitude, Kvh and Kh, both is set to 1.

3.3.4 Control of SVPWM-VSI

Therefore, from (8) and (16) the reference voltages are current harmonics

compensation and unbalance compensation are add together to become,

…….......(3.17)

The resultant reference voltage or control voltage signal then is transformed into d-

q axis, after being transformed into 2 phase quantities from three phase quantities using

Clarke transformation to obtain vd,c and vq,c. These two values is then used to calculate for

the magnitude and angle for reference voltage vector , vref,c. Hence,

………..(3.18)

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………….(3.19)

As a result, these values are used in generating the gating signal fired by

SVPWM.

3.3.5 Passive LC Filter and SVPWM Harmonics Filtering

In order to tuned LC shunt passive filter, resonant frequency for specific harmonic

frequency need to be calculated first, hence, using well-known fact that the passive filter

presents good filtering characteristics around vicinity of the resonant frequency. Given Lf

and Cf are respective filter inductance and capacitance , therefore the equation for resonant

frequency ,fr is as follows,

………..(3.20)

In order to determine the sharpness of tuning frequency or quality factor, Q,

this can be formulated as equation below,

………..(3.21)

Let XL, XC and R represent inductive reactance , capacitive reactance and

resistance respectively. Usually , a value of Q is chosen to be of range between 20 and

100. On the other hand to tune harmonic order, n is calculated using the following

equation,

.……. (3.22)

Even though SVPWM-VSI generates harmonics, usually the magnitude is

very low and PWM harmonics order is higher because it totally depends on switching

frequency. Since fn is tuned frequency, whereas, f1 is the fundamental frequency. Seeing

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from the primary of the series transformer, Zpwm is the amplitude of the sum of the source

impedance, Zs and load impedance, ZL. If “a” is the transformer ratio, thus, corresponding

equation as given above.

At switching frequency, relationship between the ripple filter parameters has to be

satisfied according to the condition below, hence,

……….(3.23)

Where XCr and XLr are the capacitive and inductive reactance of the ripple filter

respectively. The harmonics current caused by switching voltage ripple, Vr are completely

shunted by Cr, the ripple filter capacitor.

Fig.3.4 (Upper trace) Before filtering

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Fig.3.5 (Lower trace) after filtering

Fig 3.6 (Upper trace) THD spectrum before filtering

A simulation study used the following unbalanced phase voltages of the

source.

…………(3.24)

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Fig 3.7 (Lower trace) THD spectrum after filtering

Upper trace shows severely distorted source current harmonics before filtering, but

after filtered by hybrid power filter, the source current looks smooth and clean as shown at

lower trace. This is due to harmonics produced by diode rectifier with RC load flowing

back to the source that in turn affected the source current. Obviously, the current effective

value has been dropped to more than half, while it is measured a total up to 68% content of

total harmonic distortion (THD), where, 2/3 of it is 3rd harmonic alone. The 3rd harmonic

is more dominant than expected 5th or 7th harmonic because of source voltage unbalance

influence. However, after combination filter system is installed on the power circuit,

source current harmonics is significantly decreased due to the harmonics has been

mitigated. Measured THD for filtered source current shows only 0.5% harmonics left in

the current. Hence, source current looks almost fundamental waveform in shape.Two

different condition of bar plots for in total 25 orders of spectrum harmonics which is

calculated using Fast Fourier Transform (FFT) algorithm. Upper trace plot which is the

uncompensated source current owns substantial distorted harmonics spectrum that can be

observed. From the upper plot, it can be seen that dominant harmonics such as 3rd, 5th and

7th are high in magnitude, whereas the much higher order harmonics can be considered

much lower in magnitude. The plot also shows typical harmonic producing load

characteristics that as the order of harmonic increased, the magnitudes are decreasing.

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Provided that for lower trace plot, harmonic order other than fundamental is

hardly to be seen. There is significant reduction of harmonics content in the source current

after filtering. Clearly, the outcome has surpassed well below the standard level of 5% for

IEEE519-1992. Although the passive filters only tuned the 5th and 7th harmonics, the 3 rd

harmonic is mainly compensated through series active filter that provides high resistance

for the harmonics but allowing only fundamental component flows into the source current.

3.4 SUMMARY

The system configuration of shunt active filter and the control scheme for

suppressing harmonics is verified in this chapter.

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CHAPTER-4

DESIGN OF FILTERS

4.1 INTRODUCTION

MATLAB is a high-level technical computing language and interactive

environment for algorithm development, data visualization, data analysis, and numeric

computation. Using the MATLAB product, you can solve technical computing problems

faster than with traditional programming languages, such as C, C++, and FORTRAN.

4.2 SCHEMATIC DIAGRAM OF A SYSTEM WITHOUT SHUNT

ACTIVE FILTER

A simple power system with non linear load is considered and its output waveform

is found out and given below. The simple power system has the source input voltage of

230V and frequency of 50Hz. The non-linear load has the following parameters of

capacitance 1µF, inductance 0.5mH and resistance of 0.5Ω.From the simulation result the

output waveform has total harmonic distortion of 5%.

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Fig 4.1 A system with nonlinear load

4.3 SIMULATION RESULT

4.4 SUMMARY

From the system performance and simulation results, the output waveform with

harmonics is found out without using filters.

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4.5 INTRODUCTION FOR A SYSTEM WITH FILTERS

Various filters are designed in various systems in order to reduce harmonics

present in the system and the value of total harmonic distortions for that system.

4.6 SCHEMATIC DIAGRAM OF THE SYSTEM WITH FILTERS

Fig 4.2 – Three - Phase Thyristor Converter

55

4.7 SIMULATION RESULTS

Fig 4.3 – Three – Phase Thyristor Converter’s current and voltage waveforms

56

Fig 4.4 – Three – Phase Harmonic Filter with total harmonic distortion value

57

Fig 4.5 – Three Phase Harmonic Filters with sampling time period

58

4.8 SUMMARY

The output of the system with filters describes the system without harmonics or

any other voltage unbalance or current unbalance in the output.

59

CHAPTER-5

CONCLUSIONS

Finally, it has been concluded that the simulation of the system presented in this

thesis shows harmonic supression of source current to the level that comply the standard of

IEEE519-1992 and IEC61000 by using filters.

5.1 SUMMARY OF THE THESIS

Initially a simple system is considered with non linear load and the harmonics

present in the system is analyzed without using filters and the output harmonics as well as

the order of the harmonics present in the system are found out from the output results.

Finally, filters are added to the system and the harmonics are deleted and the THD value

is calculated using

5.2 SCOPE FOR FUTURE WORK

1. To implement harmonic blocking compensator in the power system to suppress

harmonics.

2. Output waveform and the order of the harmonics of the system with tunable

band pass filter are evaluated.

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4. An Luo, Zhikang Shuai, Wenji Zhu, Ruixiang Fan, and Chunming Tu,

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