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)
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.
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.
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 results
of three types of AC o DC converter are presented in this
paper.
II. POWER FACTOR AND 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.
I. INTRODUCTION
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 distribution
system, and require larger wires and other equipment. Nonlinear loads create harmonic currents in addition to the
original (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.
N
THD 
Where
I
2
2
I
2
3
 .....  I
I1
2
N

I
N 2
I1
2
N
(1)
IN 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  Distortion pf  Displacement pf
(2)
Where,
Displacement pf = 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.
350
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)
Harmonic problems are generally caused by non linear
loads such as adjustable speed drives, arcing devices,
electronic ballast and switching power supplies. They can
cause 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:
Distortion pf =
1
1  THD2
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 6pulse 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
(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 nonsinusoidal currents or reactive power from the source,
deteriorating the electrical power system quality [6].
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 coshence 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.
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
I3 I5 I7
, , ...... Hence the distortion power
3 5, 7
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.
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.
351
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)
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 tooth
magnitude) switch S2 is on. The expression for rising slope
(K1) of the source current signal is as follows [2]:
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 these
grids 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 RL is shown in Fig
3.
K1 
Rs (Vs  Vo
L

(5)
Where,
Rs-Shunt current sensing gain,
Vs-Single phase source voltage,
Vo- Averaged dc link output voltage
L-Boost Inductors
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. 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]
K2 
Rs (Vs  Vo
L

(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 (iA, iB, iC) which is
scaled using Rs with the saw tooth to determine the
switching pulses for the six switches.
Fig. 4 Control scheme for one cycle controlled converter
352
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)
TABLE II
MEASURED VALUES OF VARIOUS PARAMETERS OF DIODE
RECTIFIER
III. SIMULATION RESULTS
To analyze the performance of the various AC to DC
converters, detailed simulations are carried out on
MATLAB platform. The converters are compared in terms
of input current, power factor and THD.
Active
Power(P)
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:
4598
230 V
Source voltage frequency
50 Hz
DC link capacitor
500 F
Load Resistance

4672
P.F
Current THD
0.98
106.7%
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
TABLE I
PARAMETERS USED FOR SIMULATING THREE PHASE
DIODE RECTIFIER
Supply voltage(phase-Neutral)
Apparent
Power (S)
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:
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.
TABLE III
PARAMETERS USED FOR SIMULATING THREE PHASE
CONTROLLED RECTIFIER
Supply voltage(phase-Neutral)
230 V
Source voltage frequency
50 Hz
Load Resistance
30
Firing angle 
30 degrees
Fig.6 Waveform of source current (phase a)
Fig.8 Waveform of source current (phase a)
Fig.7 Harmonic spectrum of input current (phase a)
353
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)
These harmonic currents are injected back into the
supply system where they interact adversely with power
system equipments like capacitors, transformers, motors
causing additional losses, overheating and overloading.
TABLE IV
MEASURED VALUES OF VARIOUS PARAMETERS OF THREE
PHASE CONTROLLED RECTIFIERS
Active
Power(P)
2293
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.
Apparent Power
(S)
3130
P.F
Current THD%
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.
354
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TABLE V
PARAMETERS USED FOR SIMULATING THE ONE CYLE
CONTROLLER BASED AC-DC CONVERTER
Supply voltage(phase-Neutral)
230 V
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)
0.08
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
Active
Power(P)
Apparent
Power (S)
8338
8368
P.F
Current THD
0.99
8.61%
Input voltage and current waveform(phase a)
400
300
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.
200
100
0
-100
-200
-300
-400
0.4
0.41
0.42
0.43
0.44
0.45
0.46
0.47
0.48
0.49
0.5
IV. CONCLUSION
Fig.11 Waveform of Input voltage and current (phase a)
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.
Current waveforms in phases a , b , and c
50
40
30
20
10
0
-10
-20
-30
-40
-50
0.46
0.465
0.47
0.475
0.48
0.485
0.49
0.495
0.5
Fig. 12 Current waveforms in phases a, b, and c
The input voltage and current waveform (Fig. 11) shows
that one cycle controlled converter draws a near –
sinusoidal input current while providing a regulated output
dc voltage.
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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)
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