harmonics study in ac to dc drives

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International Journal of Emerging Technology and Advanced Engineering

Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 3, Issue 2, February 2013)

350

Analysis of AC-DC Converter Based on Power Factor and THD

Shiney.S.Varghese1, Sincy George 2

Department of Electrical Engineering, Fr. C.R. Institute of Technology, Vashi.

Abstract — Harmonic distortion and low power factor in

power systems caused by power converters have been a great

concern. To overcome these problems several converters and

control schemes have been proposed in recent years. This

work is proposed to identify the power converters with low

cost and high efficiency for three phase systems. In this paper

a comparative analysis of improved quality three phase AC to

DC converters has been investigated using MATLAB

SIMULATION. Analysis has been done to verify that the one

cycle controlled AC/DC converter has improved quality with

respect to power factor and THD as compared to diode

rectifiers and thyristor controlled rectifiers. Simulation resultsof three types of AC o DC converter are presented in this

paper.

I. I NTRODUCTION

Three phase rectifiers have a wide range of application

like electrochemical processes, arc furnaces, adjustable

speed drives etc.[1]. Diode rectifiers and thyristor bridge

converters were traditionally employed to obtain dc voltage

from ac utility. The problems with these converters were

that they used to pollute the utility with low order

harmonics, which are difficult to filter [2]. The current

spectrum of three phase converters consists of odd

multiples, in pairs of 6n 1, with decaying amplitude for increasing order, where n is the harmonic order [3]. With

thyristors, instead of the diodes, the firing angle delays the

start of the conducting of the current. This will affect the

active and reactive power taken from the supply, i.e. the

power factor. It is observed that the power factor is low,

resulting in more current being drawn from the utility. As

the distortion increases, the THD (Total Harmonic

Distortion) is indefinitely large. In an electric power

system, a load with a low power factor draws more current

for the same amount of useful power transferred. The

higher currents increase the energy lost in the distributionsystem, and require larger wires and other equipment. Non-

linear loads create harmonic currents in addition to theoriginal (fundamental frequency) AC current. The simplest

way to control the harmonic current is to use a passive

filter. This filter reduces the harmonic current, so that the

non-linear device looks like a linear load. The power factor

can be brought to near unity. The filters require large-value

high-current inductors, which are bulky and expensive.

However using PWM ac to DC converter by controlling

the switching of switching device, it is possible to achieve

nearly unity power and sinusoidal input current. This paper

compares the various circuit of AC to DC converters and

analyses the PWM bidirectional converter working on One

Cycle Control technique [4]. The PWM control structure

does not require a PLL, thus making its operation simple.

Detailed simulation studies are carried out on the

mentioned converters to verify the effectiveness of the

proposed one cycle control scheme.

II. POWER FACTOR A ND THD FOR AC-DC CONVERTER

The distortion of the normal sine wave by non-linear

loads is created by harmonics. Harmonics are related to the

fundamental frequency and are defined as whole number

multiples of the fundamental frequency. THD of a signal is

a measurement of the harmonic distortion present and is

defined as the ratio of the sum of all harmonic components

of the voltage or current waveform compared against the

fundamental component of the voltage or current wave.

1

2

2

1

22

3

2

2.....

I

I

I

I I I

THD

N

N

N

N

(1)

Where I N is the magnitude of Nth order harmonic

component of current.

Power factor is a measurement of how efficiently a facility

uses the electrical energy and is given as:

pf pf nt Displaceme Distortion PF

(2)

Where,

pf nt Displaceme = Cos (3)

Non-linear loads have large values of THD, and cause

considerable distortion to the normal sine wave. The more

the sine wave gets distorted, the lower the total power

factor becomes. Usually, total power factor is associated

only with the phase displacement of the voltage waveform

to the current waveform, but harmonics also affect the total

power factor.





351

Harmonic problems are generally caused by non linear

loads such as adjustable speed drives, arcing devices,

electronic ballast and switching power supplies. They cancause the nearby equipments to malfunction, voltage

distortion and trigger a resonance with the utility. The

relation between distortion power factor and THD is given

by:

pf Distortion =21

1

THD

(4)

Distortion power factor takes into account the harmonic

currents that do not contribute to the real work produced by

the load where as displacement power factor relates to the

displacement between the system current and voltage

waveform. Based on these two parameters the following

AC/DC converters are analyzed.

a. Three phase diode rectifiers

A six- pulse uncontrolled diode rectifier with a dc load,

Rl is shown in Fig 1.Three phase diode rectifiers are often

used in Industry to provide the dc input voltage for motor

drives and dc-to-dc converters [5]. These rectifiers are

extremely robust and present low cost, but draw non-

sinusoidal currents or reactive power from the source,

deteriorating the electrical power system quality [6].

Fig. 1 Six-pulse uncontrolled diode rectifier

It is seen that the fundamental component of current is in

phase with the supply voltage and hence the displacement

power factor is unity. Harmonic currents present in supply

current consist of 3,5,7,9 …..order of harmonics with

magnitude ......

7

,

,5

,

3

753 I I I Hence the distortion power

factor will be low which results in poor power factor. The

harmonic current injections affect the power system by

distorting the bus voltage at the point of common coupling.

These aspects have a negative influence on both power

factor and power quality. The current THD for diode

rectifiers is usually high at 30% and the power factor is

0.954[6]. It is seen that although the displacement power

factor is unity, the distortion power factor is high due to

large harmonic content resulting in low power factor.

Hence if we use diode rectifiers for high power

application, filters are to be used to improve power factor

so as to make input current a sinusoidal one. B. Three -phase controlled rectifier

The thyristor valves are used for conversion of AC into a

controlled DC and thus are the central component of any

HVDC converter station. They are also used in various

classes of railway rolling systems so that fine control of the

traction motors can be achieved. A phase controlled

rectifier is accomplished by replacing the diodes in a 6-

pulse rectifier with thyristors. Since a thyristor needs a

triggering pulse for transition from nonconducting to

conducting state, the phase angle at which the thyristor

starts to conduct can be delayed. A six pulse controlled

rectifier using thyristor is shown in Fig. 2

Fig. 2 Six-pulse controlled bridge rectifier

With firing angle =0, the input current waveform for

controlled and uncontrolled rectifier will be the same. As

is increased, distortion in the current waveform also

increases. As it is known that displacement power factor

for thyristor rectifier will be coshence total power factor

will be low. As seen in three phase diode rectifier,

harmonic currents present in supply current consist of

3,5,7,9 …..order of harmonics. Hence the distortion power

factor will be high which results in poor power factor. The

poor power factor causes high apparent current and the

absolute harmonic currents are higher than those with a

diode rectifier. Hence, in the case of controlled rectifiers,

both capacitor banks and passive filters are required to

make the input current nearly sinusoidal.

C. PWM (pulse width modulation) AC-DC converter PWM converters shift the frequency of the dominant

harmonics to a higher value where they can be easily

filtered. The control structure of three-phase six switch

PWM converter consists of an outer voltage control loop

and an inner current control loop. The current controller

senses the input current and compares it with a sinusoidal

current reference. To obtain this current reference the phase

information of the utility voltages or current is required.





352

This information is obtained by employing either a phase

lock loop (PLL) or a current phase observer digital

technique [8]. To simplify the control structure of thesegrids connected system, one – cycle-control (OCC) based

ac-to dc converters have been opted. A three phase PWM

AC-DC converter with a resistive load R L is shown in Fig

3.

Fig. 3 Three phase PWM AC-DC converter [3]

D. One cycle control scheme for PWM AC to DC

converter

Working of a three phase PWM converter using one

cycle control is explained in this section.PWM AC to DC

converter can be used where we need a regulated or

controlled DC output. The advantage of PWM rectifier

compared to thyristor controlled rectifier is that while

regulating the output voltage, it also maintains the input

current nearly sinusoidal thus improving power factor near

to unity. The control block diagram of one cycle controller for phase a is shown in Fig. 4. The dc link voltage Vo is

sensed and compared with the desired value Vo*.The error

(Vo*- Vo) is processed by a proportional – integral (PI)

controller to generate Vm.A bipolar saw tooth waveform of

amplitude Vm and having a time period of Ts is

synthesized, using an integrator.

Fig. 4 Control scheme for one cycle controlled converter

The saw tooth waveform is compared with the „a‟ phase

supply current (isa). At every rising edge of the clock pulse

the source current increases (isa is greater than saw toothmagnitude) switch S2 is on. The expression for rising slope

(K 1) of the source current signal is as follows [2]:

L

VoVs R K s

(1

(5)

Where,

Rs-Shunt current sensing gain,

Vs-Single phase source voltage,

Vo- Averaged dc link output voltage

L-Boost Inductors

Fig. 5 Phase currents along with saw tooth waveform for one cycle

control method [3]

At every rising edge of the clock pulse switches S2, S4,

S6 are turned on, and the source current increases. The

comparator compares the inductor current with the saw

tooth waveform and it determines the turning on of S1, S3,

and S5.

Here the value of drooping source current slope is given

by [2]

L

VoVs R K s

(

2

(6)

The logic of generating the switching pulses using one

cycle control method is shown in Fig 5. As seen the

comparator compares the source current (i A , i B , iC ) which is

scaled using R s with the saw tooth to determine the

switching pulses for the six switches.





353

III. SIMULATION R ESULTS

To analyze the performance of the various AC to DC

converters, detailed simulations are carried out onMATLAB platform. The converters are compared in terms

of input current, power factor and THD.

A. Three phase diode rectifiers

The six-pulse diode rectifier shown in Fig 1 is simulated

in simulink. In this circuit, six diode switches are used to

obtain a constant dc output voltage. The parameters used

for simulation are as shown in Table I:

TABLE I

PARAMETERS USED FOR SIMULATING THREE PHASE

DIODE RECTIFIER

Supply voltage(phase-Neutral) 230 V

Source voltage frequency 50 Hz

DC link capacitor 500 F

Load Resistance

It is seen that the input source current has two peaks

which is due to the high value of the capacitor. As the value

of the capacitor is increased further, more distortion is

observed. From the harmonic spectrum, it can be analyzed

that a high amount of harmonic currents is present in the

supply current consisting of 3,5,7,9 …..order of harmonics.

Fig.6 Waveform of source current (phase a)

Fig.7 Harmonic spectrum of input current (phase a)

TABLE II

MEASURED VALUES OF VARIOUS PARAMETERS OF DIODE

RECTIFIER

The simulation results of diode rectifier show that the

input current is peaky in nature. In this case the

displacement power factor is unity but due to harmonics the

distortion power factor is low. The total power factor is

seen to 0.98 with the THD of 106.7%. as shown in TABLE

II

B. Three –

phase controlled rectifier Consider a three phase controlled rectifier shown in Fig

2.The circuit is same as the three phase diode rectifier

where the diode are replaced by thyristor valves. For

simplicity capacitors are not connected and the firing angle

is assumed to be 30 degrees. The parameters used for

simulation are as given in Table III:

TABLE III

PARAMETERS USED FOR SIMULATING THREE PHASE

CONTROLLED RECTIFIER


Source voltage frequency 50 Hz

Load Resistance 30

Firing angle 30 degrees

Fig.8 Waveform of source current (phase a)

Active

Power(P)

Apparent

Power (S)

P.F Current THD

4598 4672 0.98 106.7%





354

Fig. 9 Harmonic spectrum of input current (phase a)

The simulated result of the three phase controlled

rectifier shows that the nature of input source is non

sinusoidal and the fundamental component is lagging the

voltage. It is seen from the harmonic spectrum, that a high

amount of odd multiples of harmonic current is present in

the supply.

These harmonic currents are injected back into the

supply system where they interact adversely with power

system equipments like capacitors, transformers, motorscausing additional losses, overheating and overloading.

TABLE IV

MEASURED VALUES OF VARIOUS PARAMETERS OF THREE

PHASE CONTROLLED RECTIFIERS

Active

Power(P)

Apparent Power

(S)

P.F Current THD%

2293 3130 0.73 53.05 %

Due to the presence of high amount of harmonics in

supply current the distortion power factor is quite low. The

total power factor is seen to 0.73 with the THD of 53.7%.

Fig.10. Simulation Circuit Diagram of OCC based ac-dc Converter

C. One cycle controlled ac-dc converter

PWM converters are used to overcome the problem of

low order harmonic as observed in three phase controlled

rectifier. Consider the three phase six pulse boost converter

shown in Fig 3 for simulation. The control scheme consists

of PI controller, clock, integrator and a flip flop.

Using the dc link information and the control scheme,

switching pulses are obtained for the six switches.

Simulation of PWM converter with its control circuit is

shown in Fig .10. The parameters used for simulation are

given in TABLE V.





355

TABLE V

PARAMETERS USED FOR SIMULATING THE ONE CYLE

CONTROLLER BASED AC-DC CONVERTER

0.4 0.41 0.42 0.43 0.44 0.45 0.46 0.47 0.48 0.49 0.5-400

-300

-200

-100

0

100

200

300

400Input voltageand current waveform(phasea)

Fig.11 Waveform of Input voltage and current (phase a)

0.46 0.465 0.47 0.475 0.48 0.485 0.49 0.495 0.5-50

-40

-30

-20

-10

0

10

20

30

40

50

Current waveforms inphases a, b, andc

Fig. 12 Current waveforms in phases a, b, and c

The input voltage and current waveform (Fig. 11) showsthat one cycle controlled converter draws a near –

sinusoidal input current while providing a regulated output

dc voltage.

Fig. 13 Harmonic spectrum of input current (phase a)

As seen from the harmonic spectrum of input current the

current THD is very low as compared to diode and thyristor

rectifiers.

TABLE VI

MEASURED VALUES OF VARIOUS PARAMETERS OF ONE

CYCLE CONTROLLED AC TO DC CONVERTER

In PWM controlled AC-DC converter (by one cycle

control), it is seen that the supply voltage and input current

are in phase. The voltage and current being in phase the

displacement power factor is unity. Due to presence of low

harmonics (8.61%), the distortion power factor is high. As

seen the resultant power factor is high at 0.99.

IV. CONCLUSION PWM converters are employed to overcome the problem

of low order harmonics as observed in thyristor bridge

converter, which pollute the utility. By using one-cycle

controlled method for ac-to-dc converter, the control

structure can be simplified as this control technique does

not require the service of the PLL. In this scheme the

switching frequency of the power semiconductor devices is

held constant, which is an added advantage for medium and

high power applications. The simulation results prove the

effectiveness of control technique where the usage of filters

can be eliminated.

REFERENCES[1] Rohit Gupta,Ruchika,“ A Study of AC/DC Converter with Improved

Power Factor and Low Harmonic Distortion” Inter national Journal

on Computer Science and Engineering (IJCSE)

[2] Dharmraj.V.Ghodke,Kishore Chatterji,B.G Fernandez “Modified

One cycle controlled Bidirectional High Power Factor AC-to DC”,

IEEE Transaction on Industrial Electronics,Vol 55,No-6,June 2008


Source voltage frequency 50Hz

Inductance of boost reactor 2mH

DC link capacitor 500 F

Switching frequency 10kHz

One cycle controller proportional

gain(Kp)0.08

One cycle controller proportional

gain(Ki) 0.08Active

Power(P)

Apparent

Power (S)

P.F Current THD

8338 8368 0.99 8.61%





356

[3] Keyue MSmedley,Slbodan Cuk“One cycle controlled of switching

Converters”, IEEE Transaction on Power Electronics,Vol 10,No-6,November 1995

[4] K. L. Lian Member, B. K. Perkins, and P. W. Lehn,“Harmonic

Analysis of a Three-Phase Diode Bridge”NSERC,University of

Toronto

[5] ABB drives“Technical guide No. 6 ,Guide to harmonics with

AC drives”

[6] Shweta Srivastava, Sanjiv Kumar,“Comparative analysis of

improved quality three phase ac/dc converter” International Journalof Emerging Technology and Advanced Engineering (ISSN 2250-

2459, Volume 2, Issue9, September 2012

[7] Sreeraj.E.S, Kishore Chatterjee “ Power Factor Improvement in One

Cycle Controlled Converter”IEEE ISIE 2006, July 9-12, 2006,

Montreal, Quebec, Canada

[8] Rajesh Ghosh, G. Narayanan,“Control of Three-Phase, Four-Wire

PWM Rectifier ”, IEEE Transaction on Power Electronics,Vol

23,No-1,January 2008.

harmonics study in ac to dc drives

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