1

CHAPTER 1

INTRODUCTION

1.1POWER QUALITY

Power quality is a set of electrical boundaries that allows equipment to

function in its intended manner without significant loss of performance or life

expectancy.

Use of Non-Linear loads and devices in power systems is expected to grow

rapidly. These loads inject harmonic currents into the power system. Active

filtering of electric power has now become a mature technology for harmonic and

reactive power compensation in two-wire (single phase), three-wire (three phase

without neutral), and four-wire (three phase with neutral) ac power networks with

non-linear loads. Current harmonics are one of the most common power quality

problems and are usually resolved by the use of shunt passive of active filters.

1.2 POWER QUALITY PROBLEMS

1.2.1 Voltage sags (or dips)

Causes: Faults on the transmission or distribution network (most of the times on

parallel feeders).Connection of heavy loads and start-up of large motors.

Consequences: Malfunction of information technology equipment, namely

microprocessor-based control systems (PCs, PLCs, etc) that may lead to a process

stoppage. Disconnection and loss of efficiency in electric rotating machines.

2

1.2.2 Very short interruptions

Causes: Mainly due to the opening and automatic re-closure of protection devices

to decommission a faulty section of the network. The main fault causes are

insulation failure, lightning and insulator flashover.

Consequences: Tripping of protection devices, loss of information and

malfunction of data processing equipment. Stoppage of sensitive equipment, such

as PCs, PLCs, if theyre not prepared to deal with this situation.

1.2.3. Long interruptions

Causes: Equipment failure in the power system network, storms and objects (trees,

cars, etc) striking liner poles, fire, human error, bad coordination or failure of

protection devices.

Consequences: Stoppage of all equipment.

1.2.4. Voltage spike

Causes: Lightning, switching of lines or power factor correction capacitors,

disconnection of heavy loads.

Consequences: Destruction of components (particularly electronic components)

and of insulation materials, data processing errors or data loss, electromagnetic

interference.

1.2.5. Voltage swell

Causes: Start/stop of heavy loads, badly dimensioned power sources, badly

regulated transformers (mainly during off-peak hours).

3

Consequences: Data loss, flickering of lighting and screens, stoppage or damage

of sensitive equipment, if the voltage values are too high.

1.2.6. Harmonic distortion

Causes: Electric machines working above the knee of the magnetization curve

(magnetic saturation), arc furnaces, welding machines, rectifiers, and DC brush

motors. All non-linear loads, such as power electronics equipment including ASDs,

switched mode power supplies, data processing equipment, high efficiency

lighting.

Consequences: Increased probability in occurrence of resonance, neutral overload

in 3-phase systems, overheating of all cables and equipment, loss of efficiency in

electric machines, electromagnetic interference with communication systems, and

errors in measuring harmonics when using average reading meters, nuisance

tripping of thermal protections.

1.2.7. Voltage fluctuation

Causes: Arc furnaces, frequent start/stop of electric motors (for instance

elevators), oscillating loads.

Consequences: Most consequences are common to under voltages. The most

perceptible consequence is the flickering of lighting and screens, giving the

impression of unsteadiness of visual perception.

1.2.8. Noise

Causes: Electromagnetic interferences provoked by Hertzian waves such as

microwaves, television diffusion, and radiation due to welding machines, arc

furnaces, and electronic equipment. Improper grounding may also be a cause.

4

Consequences: Disturbances on sensitive electronic equipment, usually not

destructive. May cause data loss and data processing errors.

Fig1.1 Solution to power quality problems

1.3 HARMONICS

"Harmonics are sinusoidal voltages or currents having frequencies that are

integer multiple of the supply frequency". It is becoming a major concern for

electric utility company and consumers. It is produced by power electronics and

other equipments which are called non-linear loads. Examples of nonlinear loads

are computers, fluorescent lamp and television in residential while variable speed

drives, inverters and arc furnaces which are mostly common in industrial areas.

Increasing numbers of these loads in electrical system for the purpose of, such as

improving energy efficiency, has caused an increase in harmonics pollution. These

loads draw non-sinusoidal current from the system. The waveform is normally

periodic according to supply frequency which is either 50Hz or 60Hz depending on

the country.

5

Effect of high level of voltage or current harmonics can cause transformer

heating, nuisance tripping of fuse, circuit breaker and protective devices, high

current in neutral conductor and distorted voltage waveform. Capacitors are

sensitive to harmonic voltage while transformers are sensitive to current

harmonics. There are many researches which study the effect of harmonics which

affects both utility and consumers. Greater concerns have been expressed by

industries which have equipment or processes that are sensitive to distortion on the

supply voltage which affect their plant operation and productivity. Resonance is

another problem related to harmonics. It occurs when harmonic current produced

by non-linear load interacts with system impedance to produce high harmonic

voltage.

All triplen harmonics (odd multiples of three i.e. 3, 9, 15 ) is zero

sequence and cannot flow in a balanced three-wire systems or loads. Therefore, the

delta-wye-grounded transformer at the entrance of industrial plant can block the

triplen harmonic from entering utility distribution system. However, triplen

harmonic current flows in neutral conductor and are three times in magnitude.

Fig. 1.2Harmonically related sine wave

6

1.4 HARMONIC STANDARDS

Institute of Electrical and Electronics Engineers (IEEE) has come out with

standards and guidelines regarding harmonics. One of the standards, IEEE

Standard 519-1992, provides comprehensive recommended guidelines on

investigation, assessment and measurement of harmonics in power system. The

standard includes steady state limits on current harmonic and harmonic voltages at

all system voltage levels. The limit was set for a steady state operation and for

worst case scenario.

Another international standards and conformity assessment body,

International Electro technical Commission (IEC), produced a standard, IEC

61000-3-6, which also provides guidelines to address harmonics issue with sets of

steady state limits. Both standards are in common where the limits were derived

based on a basic principle of insuring voltage quality and shared responsibility

between utility and customer (Halpin, 2005).

Both lay the responsibility on consumer to limit the penetration of current

harmonic into power system while utility company is responsible to limit harmonic

voltage at point of common coupling (PCC). According to IEEE definition, point

of common coupling is a point anywhere in the entire system where utility and

consumer can have access for direct measurement and the indices is meaningful to

both.

Example of steady state harmonic voltage limit from IEEE Std. 519-1992 at

PCC for medium voltage level (< 69 kV) is 5% THD and 3% individual voltage

distortion. In reality, harmonic is time-variant and it changes over time due to

several factors. Both standards recognize this condition and allow the limits to be

exceeded for short duration. IEC has provided a set of time-varying limits based on

7

percentile over a period of time i.e. 95th and 99

th for very short time (3 second) and

short time (10 minute) aggregate measurements.

1.5 ESTIMATION OF HARMONICS

IEEE PES Winter Meeting 1998 provides basic harmonic theory which

according to Fourier theorem, periodic non-sinusoidal or complex voltage (Figure

1.2) or current waveforms can be represented by the sum of a series of multiple

frequency terms of varying magnitudes and phases as shown in equation (1.1).

)]cos([)( 0 nn qtnaatf (1.1)

Where,

na is the magnitude of the nth

harmonic frequency

oa is the d.c. component

nq is the phase angle of the nth

harmonic frequency

is the fundamental frequency

n=1, 2, 3.

Fig 1.3 Harmonic Current and Voltage Distortion

a.

b) Resulting voltage distortion due to non-sinusoidal current

Non-linear current

Supply

voltage

(a) (b)

V

time

Distorted

Voltage

waveform time

8

Harmonic is measured using total harmonic distortion (THD) which is also

known as distortion factor and can be applied to current and voltage. It is a square-

root of sum of all harmonic magnitudes over the fundamental. Equation (1.2)

shows the calculation for voltage total harmonic distortion (THDv).

1

2

2

V

V

THDn

n

V

(1.2)

where:

1V is the magnitude of fundamental frequency voltage

nV is the magnitude of nth harmonic frequency voltage

For a balanced three-phase network with three-phase non-linear loads,

harmonic current or voltage has phase sequences. Equations (1.3) until (1.7)

describe the equation for each phase for the first three harmonics.

)3sin()2sin()sin()( 332211 tItItIti oooa (1.3) )3

63sin()

3

42sin()

3

2sin()( 332211

tItItIti ooob (1.4) )3

63sin()

3

42sin()

3

2sin()( 332211

tItItIti oooc (1.5) where:

nI is the nth current harmonic magnitude

o is the fundamental frequency

9

nis the nth harmonic phase angle

n= 1,2,3

Equation (1.4) and (1.5) can also be described as follows:

)03sin()3

22sin()

3

2sin()( 332211 tItItIti ooob (1.6)

)03sin()3

22sin()

3

2sin()( 332211 tItItIti oooc (1.7)

magnitude of all phases for all harmonic frequencies is equal for a balanced system.

Looking at equations (1.3), (1.6) and (1.7), the first harmonic or the fundamental is

positive sequence since ib(t) lags ia(t) by 120o and ic(t) leads ia(t) by 120

o. The second

harmonic is negative sequence since and ib(t) leads ia(t) by 120o and ic(t) lags ia(t) by

120o. The third harmonic is zero sequence since ib(t) and ic(t) are in phase with ia(t). The

sequence pattern for each harmonic order is shown in table 1.1

Table 1.1Harmonic Phase Sequence

Harmonic order Phase Sequence

1 +

2 -

3 0

4 +

10

5 -

6 0

7 +

8 -

9 0

10 +

11 -

12 0

13 +

15 0

1.6 INSTRUMENT USED FOR MEASURING HARMONICS

1.6.1 HARMONICANALYZERS

Harmonic analyzers or harmonic meters are relatively simple instruments for

measuring and recording harmonic distortion data. Typically, harmonic analyzers

11

contain meter with a waveform display screen, voltage leads, and current probes.

Some of the analyzers are handheld devices and others are intended for tabletop

use. Some instruments provide a snapshot of the waveform and harmonic

distortion pertaining to the instant during which the measurement is made. Other

instruments are capable of recording snapshots as well as a continuous record of

harmonic distortion over time. This particular instrument is a single-phase

measurement device capable of being used in circuits of up to 600 V.

1.6.2 TRANSIENT DISTURBANCE ANALYZERS

Transient-disturbance analyzers are advanced data acquisition devices for

capturing, storing, and presenting short-duration, sub cycle power system

disturbances. As one might expect, the sampling rates for these instruments are

high. It is not un typical for transient-disturbance recorders to have sampling rates

in the range of 2 to4 million samples per second. Higher sampling rates provide

greater accuracy in describing transient events in terms of their amplitude and

frequency content.

1.6.3 OSCILLOSCOPES

Oscilloscopes are useful for measuring repetitive high-frequency waveforms

or waveforms containing superimposed high-frequency noise on power and control

circuits. Oscilloscopes have sampling rates far higher than transient-disturbance

analyzers. Oscilloscopes with sampling rates of several hundred million samples

per-second are common. This allows the instrument to accurately record recurring

noise and high-frequency waveforms. Such data are not within the capabilities of

harmonic analyzers and transient-disturbance recorders. Digital storage

oscilloscopes have the ability to capture and store waveform data for later use.

Using multiple-channel, digital storage oscilloscopes, more than one electrical

12

parameter may be viewed and stored. The noise in the ground circuit was

responsible for production shut down at this facility.

1.6.4 DATA LOGGERS AND CHART RECORDERS

Data loggers and chart recorders are sometimes used to record voltage,

current, demand, and temperature data in electrical power systems. Data loggers

and chart recorders are slow-response devices that are useful for measuring steady-

state data over a long period of time. These devices record one sample of the event

at predetermined duration, such as 1 sec, 2 sec, 5 sec, etc. The data are normally

stored within the loggers until they are retrieved for analysis. The data from data

loggers and chart recorders are sufficient for determining variation of the voltage

or current at a particular location over an extended period and if there is no need to

determine instantaneous changes in the values.

1.7 HARMONIC FILTERS

Harmonic filters are used to eliminate the harmonic distortion caused by

nonlinear loads. Specifically, harmonic filters are designed to attenuate or in some

filters eliminate the potentially dangerous effects of harmonic currents active

within the power distribution system. Filters can be designed to trap these currents

and, through the use of a series of capacitors, coils, and resistors, shunt them to

ground. A filter may contain several of these elements, each designed to

compensate a particular frequency or an array of frequencies.

1.8 TYPES OF HARMONIC FILTERS

Filters are often the most common solution that is used to mitigate

harmonics from a power system. Unlike other solutions, filters offer a simpler

inexpensive alternative with high benefits. There are three different types of filters

13

each offering their own unique solution to reduce and eliminate harmonics. These

harmonic filters are broadly classified into passive, active and hybrid structures.

The choice of filter used is dependent upon the nature of the problem and the

economic cost associated with implementation.

1.8.1 PASSIVE FILTERS

A passive filter is composed of only passive elements such as inductors,

capacitors and resistors thus not requiring any operational amplifiers. Passive

filters are inexpensive compared with most other mitigating devices. Its structure

may be either of the series or parallel type. The structure chosen for

implementation depends on the type of harmonic source present. Internally, they

cause the harmonic current to resonate at its frequency. Through this approach, the

harmonic currents are attenuated in the LC circuits tuned to the harmonic orders

requiring filtering. This prevents the severe harmonic currents travelling upstream

to the power source causing increased widespread problems.

Shunt passive filters have been widely used because of their low cost and

low loss. This is because the components are common but also because no active

elements are required. The performances of the filters are very sensitive to the

power system impedance and series or parallel resonance with the power system

impedance may occur. Also, the effective compensation with the variation of the

voltage cannot be carried out with passive filters.

1.8.1.1 PARALLEL-PASSIVE FILTER:

The configuration shown in figure 1.4 is that of a parallel passive filter (PPF). The

PPF contains resonant LC tuned components corresponding to a particular

14

harmonic frequency. The filter is designed to provide a high impedance block at

the load or harmonic current source. This high impedance path effectively blocks

currents of the tuned harmonic order, thus acting as a harmonic current sink.

Fig.1.4 Basic parallel-passive filter for current-source nonlinear loads

1.8.1.2 SERIES-PASSIVE FILTER

The configuration shown in figure is that of a series passive filter (SPF).

Unlike the PPF, the SPF acts like a current harmonic dam providing high

impedance blocks to the harmonic voltages of a specific order which is tuned by

the resonant LC components. In figure 1.5, three resonant passive filters are

connected in series of which each LC component is tuned for the respective 5th,

7th and 11th harmonic orders. The three resonant circuits provide a high

impedance path specifically designed to block the 5th, 7th and 11th harmonic

orders respectively. The ultimate circuit of the SPF is an inductor.

15

Fig. 1.5 Basic series-passive filter for voltage-source nonlinear loads

1.8.2 ACTIVE FILTERS

The technology of active power filter has been developed during the past

two decades reaching maturity for harmonics compensation, reactive power, and

voltage balance in ac power networks. All active power filters are developed with

pulse width modulated (PWM) converters (current-source or voltage-source

inverters). The current-fed PWM inverter bridge structure behaves as a non-

sinusoidal current source to meet the harmonic current requirement of the non-

linear load. It has a self-supported dc reactor that ensures the continuous

circulation of the dc current. They present good reliability, but have important

losses and require higher values of parallel capacitor filters at the ac terminals to

remove unwanted current harmonics. Moreover, they cannot be used in multilevel

or multistep modes configurations to allow compensation in higher power ratings.

The other converter used in active power filter topologies is the PWM

voltage-source inverter (PWM-VSI). This converter is more convenient for active

power filtering applications since it is lighter, cheaper, and expandable to

multilevel and multi step versions, to improve its performance for high power

rating compensation with lower switching frequencies. The PWMVSI has to be

16

connected to the ac mains through coupling reactors. An electrolytic capacitor

keeps a dc voltage constant and ripple free.

.

Fig 1.6 Active power filter topology

The different active power filter topologies are shown Shunt active power

filters are widely used to compensate current harmonics, reactive power, and load

current unbalanced. It can also be used as a static VAR generator in power system

networks for stabilizing and improving voltage profile. Series active power filters

is connected before the load in series with the ac mains, through a coupling

transformer to eliminate voltage harmonics and to balance and regulate the

terminal voltage of the load or line.

The hybrid configuration is a combination of series active filter and passive

shunt filter .This topology is very convenient for the compensation of high power

17

systems, because the rated power of the active filter is significantly reduced (about

10% of the load size), since the major part of the hybrid filter consists of the

passive shunt LC filter used to compensate lower order current harmonics and

reactive power at fundamental frequency. Due to the operation constraint, shunt or

series active power filters can compensate only specific power quality problems.

Therefore, the selection of the type of active power filter to improve power quality

depends on the source of the problem as given in the table

Table 1.2 Active filter solution to power quality problem

18

CHAPTER-2

SHUNT ACTIVE FILTERS

2.1 INTRODUCTION

Active power filters provide a cost effective and flexible solution for system

harmonic control. Although the predominance of electronic equipment in our

professional environment makes work more convenient, these devices complicate

demands on facility wiring and power utilities. Most facilities employ employ a

variety of devices such as multiple switch mode power supplies, motors, fans, and

other non linear loads. Among the adverse effects of multiple non linear loads are

voltage distortion, excessive neutral return currents, reduced utilization of available

power, and power factor penalties. Harmonic currents in particular are receiving

more attention as a critical power quality concern, with an estimated 60percent of

electricity now passing through non linear loads. Ironically, the equipment used to

boost productivity and efficiency is also generating increases in non productive

power consumption, power pollution, and low power factor.

An active filter compensates for harmonics and corrects the power factor by

supplying the harmonic currents drawn by non linear loads and it is controlled to

generate the required compensation currents ,so the mains only needs to supply

the fundamental current and of good quality.

2.2 OPERATION

A shunt Active Filters(APF)is a device connected in parallel to group of

loads.APF cancels the reactive and harmonics currents drawn by the load so as to

make supply current sinusoidal. Active Power Filter plays a vital role in present

day liberalized energy market. Active Power Filter is explored for executing

19

different power conditioning function simultaneously along with harmonic

elimination due to increase in non-linear and unbalanced load at the point of

coupling. Shunt active filters are by far the most widely accept and dominant filter

of choice in most industrial processes. The active filter is connected in parallel at

the PCC and is fed from the main power circuit. The objective of the shunt active

filter is to supply opposing harmonic current to the nonlinear load effectively to

reduce net harmonic current. This means that the supply signals remain purely

fundamental. Shunt filters also have the additional benefit of contributing to

reactive power compensation and balancing of three-phase currents. Since the

active filter is connected in parallel to the PCC, only the compensation current plus

a small amount of active fundamental current is carried in the unit. For an

increased range of power ratings, several shunt active filters can be combined

together to withstand higher currents. This configuration consists of four distinct

categories of circuit, namely inverter configurations, switched-capacitor circuits,

lattice-structured filters and voltage-regulator-type filters.

Fig 2.1 Shunt active filter

20

In the SAPF compensator system, the transformer inductance Lts with filter

capacitor Cf forms a filter shown in Fig. 2.2(a). Here inductance Lts is the sum of

leakage inductance of primary and secondary of the injection transformer, iinv is the

inverter output current, iLoad is load current and ic is the capacitor branch (filter)

current. vinv is the inverter output voltage whose polarity will depend upon the

switching signal obtained by hysteresis controller. The LC combination leads to a

second order characteristic equation and hence the SAPF voltage trajectory is a

second order response between the two boundaries. The parabolic trajectory can be

attributed in this nature. Due to these oscillations over the capacitor voltage, when

it reaches upper or lower boundary, a linear return is not ensured even though a

reverse dc voltage is applied using control algorithm. This may lead to frequent

violation, results in poor quality of load voltage with increased THD.

Fig 2.2 (a) Conventional Filter Circuit (b) its tracking response to band

controller

21

Table 2.1 Summary of shunt Active filter

A Shunt Active Filters (SAF) has following advantages:

Controlled as a current source with a simple control algorithm,

Its operation is not affected by supply voltage harmonics,

Can be installed as a black box,

Can be installed as parallel units to obtain higher kVA rating,

Has the same power circuit and equal control algorithm to PWM

Rectifier. Therefore, has possibility of system integration with

active front-ends,

Do not create displacement factor problems.

22

CHAPTER 3

PULSE WIDTH MODULATION

3.1 INTRODUCTION:

In Pulse Width Modulation, variable speed drives are increasingly applied in

many new industrial applications that require superior performance. Recently,

developments in power electronics and semiconductor technology have lead

improvements in power electronic systems. Hence, different circuit configurations

namely multilevel inverters have become popular and considerable interest by

researcher are given on them. Variable voltage and frequency supply to a.c drives

is invariably obtained from a three-phase voltage source inverter. A number of

Pulse width modulation (PWM) schemes are used to obtain variable voltage and

frequency supply. The most widely used PWM schemes for three-phase voltage

source inverters are carrier-based sinusoidal PWM and space vector PWM

(SVPWM).

Output voltage from an inverter can also be adjusted by exercising a control

within the inverter itself. The most efficient method of doing this is by pulse-width

modulation control used within an inverter. In this method, a fixed dc input voltage

is given to the inverter and a controlled ac output voltage is obtained by adjusting

the on and off periods of the inverter components. This is the most popular method

of controlling the output voltage and this method is termed as Pulse-Width

Modulation (PWM) Control.

The advantages possessed by PWM techniques are as under:

(i) The output voltage control with this method can be obtained without any

additional components.

23

(ii) In this method, lower order harmonics can be eliminated or minimized along

with its output voltage control. As higher order harmonics can be filtered easily,

the filtering requirements are minimized.

The main disadvantage of this method is that SCRs are expensive as they must

possess low turn-on and turn-off times. PWM inverters are quite popular in

industrial applications. PWM techniques are characterized by constant amplitude

pulses. The width of these pulses is however modulated to obtain inverter output

voltage control and to reduce its harmonic content.

3.2 SINUSOIDAL PULSE WIDTH MODULATION:

In SPWM methods in order to produce 120 out-of-phase load voltages,

three modulating signals that are 120 out of phase are used. Fig. 8 shows the ideal

waveforms of three-phase VSI SPWM. In order to use a single carrier signal and

preserve the features of the PWM technique, the normalized carrier frequency mf

should be an odd multiple of 3. Thus, all phase voltages (vaN, vbN, and vcN) are

identical but 120 out of phase without even harmonics; moreover, harmonics at

frequencies a multiple of 3 are identical in amplitude and phase in all phases. For

instance, if the ninth harmonic in phase aN is

Thus, the ac output line voltage vab= vaN- vbN will not contain the ninth

harmonic. Therefore, for odd multiple of 3 values of the normalized carrier

frequency mf, the harmonics in the ac output voltage appear at normalized

frequencies fhcentred around mfand its multiples, specifically, at

h = lmf k l=1, 2

24

Where l =1, 3, 5...for k = 2, 4, 6 ; and l =2, 4, 6.for k =1, 5, 7 .;

such that h is not a multiple of 3. Therefore, the harmonics will be at mf 2, mf

4 . . . 2mf 1, 2mf 5 . . ., 3mf 2, 3mf 4. . ., 4mf 1, 4mf 5 . . .

For nearly sinusoidal ac load current, the harmonics in the dc link current are

at frequencies given by

h = lmf k 1 l=1, 2.

Where l = 0, 2, 4. for k=1, 5, 7.and l =1, 3, 5. for k = 2, 4, 6 . such

that h = l * mf k is positive and not a multiple of 3. For instance, Fig. 7h shows

the sixth harmonic (h = 6), which is due to h = (l * 9) - 2 1 = 6.

The identical conclusions can be drawn for the operation at small and large

values of mf as for the single-phase configurations. However, because the

maximum amplitude of the fundamental phase voltage in the linear region (ma

25

DISADVANTAGES OF SPWM:

Sinusoidal PWM has been a very popular technique used in AC motor

control. This relatively unsophisticated method employs a triangular carrier wave

modulated by a sine wave and the points of intersection determine the switching

Fig 3.1 The three-phase VSI. Ideal waveforms for the SPWM (ma = 0.8, mf = 0.9):

(a) carrier and modulating signals; (b) switch S1 state; (c)switch S3 state; (d) ac output voltage; (e) ac output voltage spectrum; (f) ac output current; (g) dc current; (h) dc current spectrum; (i) switch S1

current; (j) diode D1 current.

26

points of the power devices in the inverter. However, this method is unable to

make full use of the inverters supply voltage and the asymmetrical nature of the

PWM switching characteristics produces relatively high harmonic distortion in the

supply

3.3 SPACE VECTOR MODULATION:

Space Vector PWM (SVPWM) is a more sophisticated technique for

generating a fundamental sine wave that provides a higher voltage to the motor and

lower total harmonic distortion, it is also compatible for use in vector control

(Field orientation) of AC motors.

Three phase voltage-fed PWM inverters are recently showing growing

popularity for multi-megawatt industrial drive applications. The main reasons for

this popularity are easy sharing of large voltage between the series devices and the

improvement of the harmonic quality at the output as compared to a two level

inverter. In the lower end of power, GTO devices are being replaced by IGBTs

because of their rapid evolution in voltage and current ratings and higher switching

frequency.

The Space Vector Pulse Width Modulation of a three level inverter provides

the additional advantage of superior harmonic quality and larger under-modulation

range that extends the modulation factor to 90.7% from the traditional value of

78.5% in Sinusoidal Pulse Width Modulation.

The desired three phase voltages at the output of the inverter could be

represented by an equivalent vector V rotating in the counter clock wise direction

as shown in Fig. 3.2. The magnitude of this vector is related to the magnitude of

27

the output voltage and the time this vector takes to complete one revolution is the

same as the fundamental time period of the output voltage.

Let us consider the situation when the desired line-to-line output voltage

vector V is in sector 1 as shown in Fig. 3.4. This vector could be synthesized by

the pulse-width modulation (PWM) of the two adjacent SSVs V1 (pnn) and V2

(ppn), the duty cycle of each being d1 and d2, respectively, and the zero vector (

V7(nnn) / V8(ppp) ) of duty cycle d0:

Fig 3.2 Output voltage vector in the

plane

Fig 3.3 Output line voltages in time domain

28

Where, 0 m 0.866, is the modulation index. This would correspond to a maximum line-to-

line voltage of 1.0Vg, which is 15% more than conventional sinusoidal PWM as shown.

Fig 3.4 Synthesis of the required output voltage vector in sector1

All SVM schemes and most of the other PWM algorithms use Eqns. (1) and

(2) for the output voltage synthesis. The modulation algorithms that use non-

adjacent SSVs have been shown to produce higher THD and/or switching losses

and are not analyzed here, although some of them, e.g. hysteresis, can be very

simple to implement and can provide faster transient response. The duty cycles d1,

d2, and d0, are uniquely determined by Eqns. (1) and (2) , the only difference

between PWM schemes that use adjacent vectors is the choice of the zero vector(s)

and the sequence in which the vectors are applied within the switching cycle.

d1V1+ d2V2=V=m Vg ................(1)

d1+d2+d0=1 ................(2)

29

3.4 ADVANTAGES OF SPACE VECTOR MODULATUON:

Space Vector Modulation for a three phase UPS inverter makes it possible to

adapt the switching behaviour to different situations such as: half load, full load,

linear load, non-linear load, static load, pulsating load, etc. In combination with a

zigzag three phase transformer in the output this provides the following

advantages:

Very low values can be reached for the output voltage THD (

30

inverter does exactly the opposite, it converts the DC voltage back into AC voltage

to supply the critical load. In case the utility fails or is out of tolerance, the rectifier

will be switched off and the batteries will supply the inverter with DC voltage.

A UPS is usually supplying a critical load, mostly equipment like computers,

mainframes, medical equipment, etc.. Such equipment is sensitive to disturbances

on the utility. For example, a dip in the utility voltage can cause hard disks to

crash.

Fig 3.5 Block diagram of UPS

Disturbances on the utility can not only damage equipment, but also cause

productivity losses and discontinuity in processes. Medical equipment like digital

X/ray and CT scanners are pulsating loads, which means that there are significant

variations in the current. These loads are switched on and off all the time, within

milliseconds. If such equipment is protected with a UPS, the inverter of that UPS

should be able to maintain the output voltage wave shape (sine wave) within

tolerance, in order to not jeopardize the functionality of the equipment

31

Similar performance is required for non-linear loads where the output

current is different in shape than the output voltage. Typically computers are

equipped with power supplies that act as non-linear loads.

With such requirements, the expectations of the performance of the inverter

of the UPS are high. Conventional ways to control the inverter provide good

results, but many times the UPS needs to be oversized to maintain the

performance. More advanced ways of control are required to prevent this and to

make a UPS cost effective.

The main task of the control system in a UPS unit is to minimize the output

voltage total harmonic distortion in different loading profiles. In addition, it should

provide the proper mechanism to recharge the battery set and maintain high input

power factor and low total input current harmonic distortion. Other factors

considered for a good control technique are nearly zero steady-state inverter output

voltage error, good voltage regulation, robustness, fast transient response, and

protection of the inverter against overload under linear/non-linear loads. The most

common switching technique is Sinusoidal PWM. This method can be utilized for

both single-phase and three phase systems. The advantage of this method is low

output voltage harmonic and robustness. This strategy uses a single feedback loop

to provide well-regulated output voltage with low THD. The feedback control can

be continuous or discontinuous. Analog techniques are used in continuous

approach.

The sinusoidal PWM (SPWM) can be of natural sampling type, average

type, or instantaneous type .In natural sampling type, the peak value of the output

voltage is detected and compared with a reference voltage in order to obtain the

error, which is used to control the reference to the modulator. The average

32

approach is basically the same; but, the sensed voltage is converted to an average

value and after that, is compared with a reference signal. These approaches control

only the amplitude of the output voltage and are good only at high frequencies. In

an instantaneous voltage feedback SPWM control, the output voltage is

continuously compared with the reference signal improving the dynamic

performance of the UPS inverter.

A typical block diagram of a three-phase DC/AC inverter for UPS systems

and SPWM switching control technique is shown in the above fig 3.5. The

disadvantage of this method is lack of flexibility for non-linear loads. Other

programmed PWM techniques such as selective harmonic elimination, minimum

THD, minimum loss, minimum current ripple, and reduced acoustic noise may be

used for the inverter. Better performance even with non-linear and step-changing

loads can be achieved by multiple control loop strategies.

As shown in Fig 3.6, there are two control loops: an outer and an inner. The

outer control loop uses the output voltage as a feedback signal, which is compared

with a reference signal. The error is compensated by a PI-integrator to achieve

stable output voltage under steady-state operation. This error is also used as a

reference signal for the inner current regulator loop, which uses the inductor or the

capacitor output filter current as the feedback signal. The minor current loop

ensures fast dynamic responses enabling good performance with non-linear or

step-changing loads.

33

Fig 3.6 (a) Configuration of a three-phase DC/AC inverter for UPS systems and (b) simple

voltage controller using PWM technique.

34

CHAPTER 4

MATLAB SIMULINK

4.1 MATLAB INTRODUCTION:

MATLAB is a high-performance language for technical computing. It

integrates computation, visualization, and programming in an easy-to-use

environment where problems and solutions are expressed in familiar mathematical

notation. Typical uses include

Math and computation

Algorithm development

Data acquisition

Modeling, simulation, and prototyping

Data analysis, exploration, and visualization

Scientific and engineering graphics

Application development, including graphical user interface building

MATLAB is an interactive system whose basic data element is an array that

does not require dimensioning. This allows you to solve many technical computing

problems, especially those with matrix and vector formulations, in a fraction of the

time it would take to write a program in a scalar non interactive language such as C

or FORTRAN.

The name MATLAB stands for matrix laboratory. MATLAB was originally

written to provide easy access to matrix software developed by the LINPACK and

35

EISPACK projects. Today, MATLAB engines incorporate the LAPACK and

BLAS libraries, embedding the state of the art in software for matrix computation

MATLAB has evolved over a period of years with input from many users. In

university environments, it is the standard instructional tool for introductory and

advanced courses in mathematics, engineering, and science. In industry, MATLAB

is the tool of choice for high-productivity research, development, and analysis.

MATLAB features a family of add-on application-specific solutions called

toolboxes Very important to most users of MATLAB, toolboxes allow you to learn

and apply specialized technology. Toolboxes are comprehensive collections of

MATLAB functions (M-files) that extend the MATLAB environment to solve

particular classes of problems. Areas in which toolboxes are available include

signal processing, control, systems, neural networks, fuzzy logic, wavelets,

simulation, and many others.

4.2 FEATURES OF MATLAB AND SIMULINK

Matlab (*.m):

Schematic (Easy to model complicated systems)

Not easy to change parameters

Simulink (*.mdl):

Only text code (Not easy to model complicated systems)

Easy to edit figures

Cannot edit figures

36

Matlab (*.m) + Simulink (*.mdl): Best choice

Schematic: Simulink

Easy to change parameters: Matlab (m file for parameter initialization)

Edit figures: Simulink (To Workspace)

Matlab (m file for plot)

Simulink

Control System Toolbox

Fuzzy Logic Toolbox

Embedded Target for Motorola MCP555

Embedded Target for TI C6000 DSP

Fixed-Point Blockset

MPC Blocks

NCD Blockset

Neural Network Blockset

Real-Time Windows Target

Real-Time Workshop

Report Generator

S-function demos

37

SimMechanics

SimPowerSystems

Simulink Extras

4.3 SIMPOWER SYSTEMS

SimPower Systems and other products of the Physical Modeling product

family work together with Simulink to model electrical, mechanical, and control

systems. SimPower Systems operates in the Simulink environment. Therefore,

before starting this user's guide, you should be familiar with Simulink. For help

with Simulink, see the Simulink documentation. Or, if you apply Simulink to

signal processing and communications tasks (as opposed to control system design

tasks), see the Signal Processing Block set documentation.

4.4 ROLE OF SIMULATION IN DESIGN

Electrical power systems are combinations of electrical circuits and

electromechanical devices like motors and generators. Engineers working in this

discipline are constantly improving the performance of the systems. Requirements

for drastically increased efficiency have forced power system designers to use

power electronic devices and sophisticated control system concepts that tax

traditional analysis tools and techniques. Further complicating the analyst's role is

the fact that the system is often so nonlinear that the only way to understand it is

through simulation .Land-based power generation from hydroelectric, steam, or

other devices is not the only use of power systems. A common attribute of these

systems is their use of power electronics and control systems to achieve their

performance objectives. SimPower Systems is a modern design tool that allows

scientists and engineers to rapidly and easily build models that simulate power

38

systems. SimPower Systems uses the Simulink environment, allowing you to build

a model using simple click and drag procedures. Not only can you draw the circuit

topology rapidly, but your analysis of the circuit can include its interactions with

mechanical, thermal, control, and other disciplines. This is possible because all the

electrical parts of the simulation interact with the extensive Simulink modeling

library. Since Simulink uses MATLAB as its computational engine, designers

can also use MATLAB toolboxes and Simulink block sets. SimPower Systems and

Sim Mechanics share a special Physical Modelling block and connection line

interface.

4.5 SIMPOWER SYSTEMS LIBRARIES:

The libraries contain models of typical power equipment such as

transformers, lines, machines, and power electronics. These models are proven

ones coming from textbooks, and their validity is based on the experience of the

Power Systems Testing and Simulation Laboratory of Hydro-Qubec, a large

North American utility located in Canada, and also on the experience of Evolve de

Technologies superior and Universities Laval.

The capabilities of SimPower Systems for modeling a typical electrical

system are illustrated in demonstration files. And for users who want to refresh

their knowledge of power system theory, there are also self-learning case studies.

The SimPower Systems main library, power lib, organizes its blocks into libraries

according to their behaviour.

39

CHAPTER-5

SIMULATION & RESULTS

The simulation circuit of a non linear load system without shunt active filter is

shown in fig 5.1. In this circuit, UPS and Asynchronous motor is taken as non

linear load, its corresponding line current is measured and FFT analysis have been

performed and the results have been shown in fig 5.2 and 5.3.The THD value is

found to be 38.05%.

Fig 5.1 NON LINEAR LOAD SYSTEM WITHOUT SHUNT ACTIVE

FILTER

40

Fig 5.2 LINE CURRENT

Fig 5.3 FFT ANALYSIS WITHOUT SHUNT ACTIVE FILTER

41

The simulation circuit of a non-linear load system with shunt active filter is

shown in fig 5.4. In this method, space vector modulation is used. When the shunt active power filter block is operating it injects currents equal in magnitude but in

phase opposition to harmonic current. This compensates the harmonic distortion

and makes the source current balanced sinusoidal while the load current remains

nonlinear.

In this circuit, UPS and Asynchronous motor is taken as non linear load, its

corresponding line current is measured and FFT analysis have been performed and

the results have been shown in fig 5.5 and 5.6.The THD value is found to be

2.31%.

Fig 5.4 NON LINEAR LOAD SYSTEM WITH SHUNT ACTIVE FILTER

42

Fig 5.5 LINE CURRENT

Fig 5.6 FFT ANALYSIS WITH SHUNT ACTIVE FILTER

43

CHAPTER-6

HARDWARE

6.1. HARDWARE DESCRIPTION

The available AC voltage of 220V is given to the primary side of the

transformer from a source. The transformer is used to step-down the voltage from

an AC voltage of 220V to 24V and 24V is given to the bridge rectifiers. The

rectifier converts the AC voltage into pulsating DC voltage .To filter the pulsations

present in the DC output voltage of the rectifier the capacitor is connected across

them The three-phase inverter is driven by the driver circuit with an angle of 120

mode. The gate signals are controlled by the micro controller PIC 16F84A.

The constant DC voltage as the input supply of micro controller (5V dc) is

obtained by a circuit which consists of rectifier (W04, 300V, 1A), capacitors

(1000F, 25PF).

The drive circuits, which are controlled by micro controller, consist of opto-

coupler. Circuits, which are controlled by microcontroller, consist of opto-coupler

to isolate the circuits of controller and drive circuit operated at different voltage

levels. The input supply for drive circuits is obtained by using the step-down

transformer (230/12)V. The drive- circuit gives 12V to the corresponding gates of

MOSFETs.

The hardware involves the following sections,

Main source circuit

Micro controller circuit

44

Driver circuit

Inverter circuit

6.1.1. MAIN SOURCE CIRCUIT

The circuit consists of the following parts

Power supply circuit

Transformer

Bridge rectifier

6.1.1.1. POWER SUPPLYCIRCUIT

The main source section of hardware unit is shown in Figure 5.2.Here input

voltage is 24v ac which is getting from step down transformer. This 24v is

converted to DC and regulated by means of regulator 7805.After passing through

regulator the voltage maintain constant value.

Fig 6.1 power supply unit

45

6.1.1.2 TRANSFORMER

It is used to step up/step-down the ac supply voltage to suit the requirement

of the electronics devices and the circuit fed by the dc power supply. It also

provides the isolation from the supply line. In this project supply input voltage is

230v ac and output is step-down voltage of 24v ac is shown in Figure 5.3.

Fig 6.2 Rectifier circuit.

6.1.1.3 BRIDGE RECTIFIER

Fig. 6.2 is rectifier circuit, which converts ac voltage in to pulsating dc

voltage. In this diagonally opposite pair of diodes are made to conduct by giving ac

supply. The bridge rectifier converts the given ac voltage to dc voltage.

6.1.1.4 FILTER

The function of the circuit element is to remove the fluctuation /

pulsation (called as ripple) present in the output voltage supplied by the rectifier. It

cannot give a ripple free voltage as that of dc battery, but it approaches so closely

that the power supply performs so well.

46

6.1.1.5 VOLTAGE REGULATOR

Its main function is to keep the terminal voltage of the dc supply constant

even when

Ac input voltage to the transformer varies

The load varies

It is impossible to get 100% constant voltage but minor variation is acceptable.

6.2 INTRODUCTION OF PIC16F84A

The PIC16F84A belongs to the mid-range family of the PIC micro

microcontroller devices. The program memory contains 1K words, which

translates to 1024 instructions, since each 14-bit program memory word is the

same width as each device instruction. The data memory (RAM) contains 68 bytes.

Data EEPROM is 64 bytes. There are also 13 I/O pins that are user-configured on

a pin-to-pin basis. Some pins are multiplexed with other device functions. These

functions include:

External interrupt

Change on PORTB interrupt

Timer0 clock input

6.2.1 High Performance RISC CPU Features:

Only 35 single word instructions to learn

47

All instructions single-cycle except for program branches which are two-

cycle

Operating speed: DC - 20 MHz clock input DC - 200 ns instruction cycle

1024 words of program memory

68 bytes of Data RAM

64 bytes of Data EEPROM

14-bit wide instruction words

8-bit wide data bytes

15 Special Function Hardware registers

Eight-level deep hardware stack

Direct, indirect and relative addressing modes

Four interrupt sources:

- External RB0/INT pin

- TMR0 timer overflow

- PORTB interrupt-on-change

- Data EEPROM write complete

6.2.2PERIPHERAL FEATURES:

13 I/O pins with individual direction control

48

High current sink/source for direct LED drive

- 25 mA sink max. per pin

- 25 mA source max. per pin

TMR0: 8-bit timer/counter with 8-bit programmable prescalar.

6.2.3 Special Microcontroller Features:

10,000 erase/write cycles Enhanced FLASH Program memory typical

10,000,000 typical erase/write cycles EEPROM Data memory typical

EEPROM Data Retention > 40 years

In-Circuit Serial Programming (ICSP) via two pins

Power-on Reset (POR), Power-up Timer (PWRT), Oscillator Start-up

Timer (OST)

Watchdog Timer (WDT) with its own On-Chip RC Oscillator for reliable

operation

Code protection

Power saving SLEEP mode

Selectable oscillator options

6.2.4 CMOS Enhanced FLASH/EEPROM Technology:

Low power, high speed technology

Fully static design

49

Wide operating voltage range:

- Commercial: 2.0V to 5.5V

- Industrial: 2.0V to 5.5V

Low power consumption:

- < 2 mA typical @ 5V, 4 MHz

- 15 mA typical @ 2V, 32 kHz

- < 0.5 mA typical standby current @ 2V

Fig 6.3 Pin Diagram of PIC 16F84A

6.3 DRIVER CIRCUIT IR2110

Floating channel designed for bootstrap operation

Fully operational to +500V or +600V

Tolerant to negative transient voltage dv/dt immune

50

Gate drive supply range from 10 to 20V

Under-voltage lockout for both channels

3.3V logic compatible

Separate logic supply range from 3.3V to 20V

Logic and power ground 5V offset

CMOS Schmitt-triggered inputs with pull-down

Cycle by cycle edge-triggered shutdown logic

Matched propagation delay for both channels

Outputs in phase with inputs

Fig 6.4 Driver Circuit

6.3.1DESCRIPTION

The IR2110/IR2113 are high voltage, high speed power MOSFET and IGBT

drivers with independent high and low side referenced output channels.

51

Proprietary HVIC and latch immune CMOS technologies enable ruggedized

monolithic construction. Logic inputs are compatible with standard CMOS or

LSTTL output, down to 3.3V logic. The output drivers feature a high pulse current

buffer stage designed for minimum driver cross-conduction. Propagation delays

are matched to simplify use in high frequency applications. The floating channel

can be used to drive an N-channel power MOSFET or IGBT in the high side

configuration which operates up to 500 or 600 volts.

6.4 MOSFET:

The N-channel MOSFET enhancement mode silicon gate power field effect

transistor is an advanced power MOSFET designed, tested, and guaranteed to

withstand a specified level of energy in the breakdown avalanche mode of

operation. All of these power MOSFETs are designed for applications such as

switching regulators, switching converter, motor -drives, relay drivers and drivers

for high power bipolar switching transistors requiring high speed and low gate

drive power.

6.4.1 FEATURES:

8A,500V

Rds=0.850

Single pulse avalanche energy rated

SOA is power dissipation limited

52

Fig 6.5 Hardware Kit Diagram

Fig 6.6 Source Current waveform

53

CHAPTER 7

CONCLUSION

This project presents a shunt active power filter as a reliable and cost-

effective solution to power quality problems.

The filter presents good dynamic and steady-state response and it can be a

much better solution for current harmonics compensation than the conventional

approach (capacitors to correct the power factor and passive filters to compensate

for current harmonics). Besides, the shunt active filter can also compensate for

load current unbalances. Therefore this active filter allows the power source to see

an unbalanced reactive non-linear load, as a symmetrical resistive load.

The proposed method allows the use of a large number of low-power active

filters in the same facility, close to each problematic load (or group of loads),

avoiding the circulation of current harmonics. This solution reduces the power

lines losses and voltage drops, and avoids voltage distortions at the loads terminals.

The simulation results show that the active power filter can filter harmonic

currents. The THD of the non-linear system with shunt active filter is evaluated

and is found to be within limits.

54

CHAPTER 8

APPENDIX

CODING FOR PIC MICROCONTROLLER

org 000h

bsf STATUS, 5

movlw 00h

start:movlw 51h

movwf PORTB

call delay

movlw 01h

movwf PORTB

call delay

movlw 03h

movwf PORTB

call delay

movlw 0ah

movwf PORTB

call delay

movlw 18h

movwf PORTB

55

call delay

movlw 20h

movwf PORTB

call delay

Goto start

delay:

movlw 03h

movwf del2

loop1: movlw 245

movwf del

loop: nop

nop

nop

nop

nop

decfsz del, 1

Goto loop

decfsz del2, 1

Goto loop1

Return

56

HARDWARE IMPLEMENTATION

57

CHAPTER 9

REFERENCES

1. .Md. AshfanoorKabir and Upal Mahbub Synchronous Detection and Digital

control of Shunt Active Power Filter in Power Quality Improvement Department

of EEE Bangladesh University Dhaka, Bangladesh,2011 IEEE

2. M. George and K. P. Basu, Three-phase shunt active power filter, American

Journal of Applied Sciences, vol. 5 (8), pp. 909916, 2008.

3. Joao Afonso, Mauricio Aredes, Edson Watanabe, Julio Martins Shunt active

filter for power quality improvement International Conference UIE 2000

Electricity for a Sustainable Urban Development Lisboa, Portugal, 1-4

November 2001, pp. 683-691.

4. Dorin O. Neacsu SPACE VECTOR MODULATION An Introduction 27th

Annual Conference of the IEEE Industrial Electronics Society,2001.

5. Zhuo Fang, Yang Jun, Wang Zhaoan and Hu Junfei, Active power filter for

three-phase four-wire system, Journal of XiAnJiaotong University, vol.34,

pp.8790, March 2000.

6. R.C. Dugan, M.F. Mc Granaghan, S. Santoso and H.W. Beaty, Electrical Power

Systems Quality, McGraw-Hill, 2004.

7. Power Electronics, Circuits, Devices, and Applications, 2nd ed., MH Rashid,

Prentice Hall 1993.

8. C. Sankaran, Power Quality. CRC press, 2001.

58

CHAPTER 10

CONFERENCE DETAILS

Published paper in National Conference on Electrical Engineering Trends

(NCEET 2K12).

Organised by Department of EEE Bhajarang Engineering College.

Date: 7th April 2012