“Comparison Analysis of AC Voltage Controllers Based
on Experimental and Simulated Application Studies”
Hamdy. A. Ashour and Rania. A. Ibrahim
Arab Academy for Science & Technology
Department of Electrical & Computer Control Engineering, 1029 Miami, Alexandria, Egypt
hashour@aast.edu, rania_assem@alexseeds.com
heating, melting, arc furnace, transformer tap changing,
cycloconverters, wind turbines, power factor improvement,
flexible transmission systems (FACTS), static switches, AC
motors control and operation [1-13]. The operation of the AC
voltage controllers have been explained in literatures [5-8].
This paper introduces a comparison between eleven possible
configurations of the AC voltage controllers based on
experimental and simulation analysis. Principle of operation
is reviewed, experimental setup and software simulation are
introduced, a detailed comparison has been carried out and
two different application case studies have been discussed for
practical validation.
Abstract- This paper introduces a detailed comparison between
possible connections of AC voltage controllers. For each
configuration, the experimental setup is implemented and the
corresponding simulation program is presented using Simulink
under Matlab. The simulated and experimental instantaneous
voltage and current waveforms in case of resistive and inductive
loads are matched well, validating the simulation comparison
for analysis. The comparison analysis includes the required
number of devices and isolated gate signals, which determines
the complexity and the size, hence the overall cost. Also
harmonic spectrum, total harmonic distortion, effective rms
value, dc offset and the control range are compared to specify
the performance. The implementation of a fixed- capacitor
thyristor controlled reactor (FC-TCR) and three phase
induction motor starters (SOFT STARTER) as two application
case studies of AC voltage regulators has been discussed.
Experimental and simulation results have been obtained and
well correlated, showing the effectiveness of such configurations
in the fields of control of reactive power flow and in the field of
controlling the starting performance of three phase induction
motor.
II. AC VOLTAGE CONTROLLERS:
If a thyristor switch is connected between an AC supply
and load, the power flow can be controlled by varying the
rms value of the AC voltage applied to the load. This type of
power circuit is known as an AC voltage controller
(regulator). Since the input voltage is AC, the thyristor is line
commutated, so there is no need of extra commutation
circuitry and the circuits for AC voltage controllers are
simple and relatively inexpensive. AC switch can be
implemented either using a single triac with a single isolated
gate circuit but for lower power applications, using two back
to back thyristors with two isolated gate circuits, using two
diodes and two thyristors with a single isolated gate circuit,
or with four diodes and a single thyristor with a single
isolated gate circuit. The power flow to the load can be
controlled by the ON-OFF or phase angle control techniques.
For the ON-OFF control, the rms output voltage for resistive
load can be expressed as [6]:
LIST OF SYMBOLS:
Vo
rms
Vs
n, m
k
α
β
Θ
δ
: rms output load voltage
: rms supply voltage
: Number of ON & OFF cycles
: Duty cycle
: Delay angle
: Extinction angle
: Load angle
: Conduction angle ( δ = β − α )
I FC −TCR , V FC −TCR : FC-TCR current and voltage
BFC −TCR
: Compensator susceptance
Bc
: Capacitor susceptance
BTCR
: Inductor susceptance as a function of
delay angle
: Maximum inductor susceptance
: Angular frequency
: Capacitance & inductance value
Bmax
ω
C,L
1
 n 2π
2
n
2
Vorms = 
=Vs k
(1)
∫ 2Vs sin ωtd(ωt) =Vs
(
)
2
n
+
m
m
+n
π
0


While for the phase control Vo can be expressed as:
rms
1
2
1
 2 2π
1 
sin2α  2
(2)
Vorms =  ∫ 2Vs2 sin2 ωtd(ωt) =Vs  π −α +

π
π
2
2 
 
 0

The ON-OFF type of control can be applied in applications
having mechanical inertia and high thermal time constant
such as industrial heating and speed control of small motors,
while the phase control can be utilized in many industrial
applications such as motor starters, transformer tap changing
and static VAR compensators. In case of inductive load, the
current will not be in phase with the controlled voltage. In
this case, in order to ensure full control of the AC voltage, a
single gate pulse should be replaced with continuous train of
pulses and the range of delay angle α is limited within the
range of:
I. INTRODUCTION:
The AC voltage controller can be considered as a voltage
regulator device by which the root mean square load voltage
(rms), hence the power flow, can be set and maintained
constant at a certain desired value. The recent developments
achieved in the field of power electronics, control techniques
and microprocessors have introduced such AC voltage
controllers for the applications of power ranges from few
watts up to fractions of megawatts, such as light dimmers,
79
θ ≤α ≤π
While (2) maybe then re- written as:
(3)
1
1
2 β
2
 1  sin2α sin2β  2
Vorms =  ∫ 2Vs2 sin2 ωtd(ωt) =Vs  δ +
+
 (4)
2
2 
2π α

π 
The AC voltage controllers can be configured to be used
either in the single phase low power domestic applications, or
in the three phase high power industrial applications.
Different possible configurations of the AC voltage
controllers are depicted in fig. 1 and will be compared
through the next sections.
(a) General block diagram
Figure 3: Simulation software
III. EXPERIMENTAL SETUP AND SIMULATION SOFTWARE:
The simulation analysis has been carried out using Simulink
under Matlab version 6.5 which provides strong power
electronics and analysis toolboxes. The system is simulated in
a modular form, as shown in fig. 3, typical to the
experimental block diagram for clear comparison and has an
additional block for harmonic analysis. This simulation block
calculates the Fourier coefficients, the total harmonic
distortion (THD), the effective rms value, the DC offset
component and then plots the harmonic spectrum as will be
demonstrated in the next sections.
A general block diagram for the experimental setup is shown
in fig. 2-a while an example of practical connections is shown
in fig. 2-b. The setup is built in a modular form and consists
of a variable power supply, a variable RLC load bank, a
synchronizing and isolating firing gate signals, a controlled
set point (delay angle α) and a group of individual diodes and
thyristors power electronic devices. The setup is reliable and
flexible to be reconnected to get different configurations
shown in fig. 1.
T1
T1
T1
A
Io
+
Load
Vs
Io
+
VCA
Load
Vs
D1
−
(a)
Load
C
Load
T4 T5
(g)
A
T1
T2
T4 T
2
Load
T3
T6
B
T3
(i)
C
T1
T5
B
B
T6
T2
T6
Load
A
Load
D4
Load
T5
(f)
Load
C
(h)
T3
B
Load
C
Load
Load
(e)
D2
T4
Load
D2
T1
D6
Load
C
A
Load
(c)
D6
T5
C
T5
Load
Load
Load
A
B
T6
T5
B
(d)
T3
Load
T2
T3
T2
Load
IV. COMPARISON ANALYSIS:
A
T5
D2
Load
T4
T3
T1
Bn
T6
Load
T1
IC
VBC
D4
Load
T3
D6
C
C
A
Load
T4
Load
T5
IB
T1
A
D4
T3
B
V AB
(b)
T1
A
Load
IA
B
T2
−
T1
(b) Example of simulation program (Conf. c)
Load
C
(j)
T6
Load
Load
For each configuration in fig. 1, the experimental setup is
reconnected and the experimental waveforms are obtained
using storage scope, then the simulation is reconfigured to
obtain the corresponding simulated waveforms for
comparison and analysis. An example of the experimental
connections and the corresponding Simulink simulated
program for one of the possible three phase configurations
are illustrated in fig. 2-b and (3-b) respectively. For the single
phase configuration, the gate signals (1, 2) are shifted by 180º
while for the three phase configurations the gate (1, 2, 3, 4, 5,
6) are shifted by 60º. For all configurations, the comparison is
carried out for load voltage waveforms at delay angle α=108º
and in two cases: unity power factor (resistive load) and 0.6
lagging power factor (resistive and inductive load).
The comparison depicted includes the followings:
A. Experimental and Simulation Waveforms:
These waveforms shown in fig. 4 are for clear comparison
and simulation validation. The scales of time, voltage and
current of the experimental waveforms are typical for these
shown in the corresponding simulation graphs. A good
agreement between the simulated and experimental
waveforms can be seen in fig. 4 for different configurations.
The slight differences noticed between waveforms,
particularly for inductive loads, could be due to the difference
between switching performance of the actual and simulated
devices.
T3
(k)
Figure 1: Different circuit configurations for the implementation of AC voltage
controllers
B.
Number of Devices:
Lower number reduces the cost; hence the cheapest
configuration is the single phase configuration (a) while the
cheaper one in the three phase is configuration (k). However,
(a) General block diagram
the SCR current rating of configuration (k) should be 2
higher than the other three phase configurations. For three
phase, since the devices are connected at the phases for
(b) Example of actual connections (conf. c)
Figure 2: Experimental set-up
80
configurations (i, j, k), and not at the lines like others, they
could have lower current and higher voltage ratings.
components and the THD, since the output waveform tends
to be sine wave.
C.
Number of Isolated Gates:
Increasing this number complicates the circuits, increases
the size and the overall cost. Configuration (a) is the best
from this point of view as it needs only one gate signal, while
the three phase configurations (d, f, h, k) require only three
isolated signals rather than six required by the rest.
D.
Number of Load Terminals:
For the three phase configurations (g, h, i, j, k), the six
terminals of the load should be available and not connected
as star or delta. This could limit the applications of these
configurations according to the nature of the available loads.
It is not the case for other configurations which require loads
with only three terminals and are suitable for most three
phase loads (connected or not connected as star or delta).
(a) Conf. a
E.
Effective rms Value and the DC Offset:
These values are calculated in per unit for the supply
voltage taken as a base voltage and for a certain delay angle
α=108º and the load impedance is the base for current
calculation. Due to the presence of diodes in configurations
(a, d, f, h) or the possible forced path through the ON SCR in
configuration (k), the output voltage and input current are
asymmetrical containing a DC component. This is very clear
in the single phase configuration (a) which also has a limited
range of control as the Vorms can be only varied from 0.7 p.u.
to 1 p.u. If there is a magnetic element, such as a transformer,
such DC component may cause saturation problems. For
these reasons, these configurations, named as unidirectional,
are more suitable for resistive loads, such as heating and
lighting applications. Configurations (b, c, e, g, i, j) are
bidirectional control and the waveforms are symmetrical
along the x-axis, containing no DC component. These
configurations are most suitable for AC motor controls and
power system applications.
(b) Conf. b
(c) Conf. c &g
(d) Conf. d &h
F.
Harmonic Spectrum and Total Harmonic Distortion
(THD):
Harmonics may cause problems particularly for motors
(negative torque) and power systems (resonance and noise
interference). Harmonics may be useful in some applications,
such as heating, since the effective rms may be increased.
Configurations (c, g) and (d, h) produce similar waveforms
respectively as seen from fig. 4-c & 4-d. From fig. 4-a & 4-b
it can be seen that the unidirectional configurations introduce
even and odd harmonics while the bidirectional
configurations introduce only odd harmonics due to the
symmetrical positive and negative parts of the waveforms.
Even harmonics may cause problems in motor applications. It
can be also seen that the inductive load increases the value of
harmonic components and THD values due to the distortion
in the waveforms. The triplen harmonics will be disappeared
in the line values for the delta connected loads
(configurations e, f, i). It should be noted that the control
range of the delay angle may change the voltage wave shapes
and fig. 6 depicts the different voltage wave shapes at
different delay angles using resistive load for clarity.
Reducing the delay angle α reduces the value of harmonic
(e) Conf. e
(f) Conf. f
81
(g) Conf. i
THD = 96.43%
RMS= 1.25 p.u
DC= -2e-3 p.u
THD = 82.95%
RMS= 82.95 p.u
DC= -7e-4 p.u
(f) Conf. f
THD = 147.6%
RMS= 0.47 p.u
DC= 0 p.u
(h) Conf. j
THD = 206.4%
RMS= 0.97p.u
DC= 0 p.u
(g) Conf. i
THD = 105.1%
RMS= 0.33 p.u
DC= 0 p.u
(i) Conf. k
Figure 4: Simulation and experimental load phase voltage and current waveforms
for different configurations. Graphs in sequence are:
Left graphs: SIMULATION
Right graphs: EXPERIMENTAL
1st = Voltage, R -load
1st = Voltage, R -load
nd
2 = Current, R- load
2nd = Current, R- load
3rd= Voltage, RL- load
3rd= Voltage, RL- load
4th= Current, RL- load
4th= Current, RL- load
Scales: Voltage: 50 V / div, Current: 1 A / div, Time: 0.01 sec/div
THD = 67.68%
RMS= 0.8 p.u
DC= -0.3 p.u
THD = 186.8%
RMS= 0.48 p.u
DC= 0 p.u
(h) Conf. j
THD = 82.85%
RMS= 0.68 p.u
DC= -1e-3 p.u
THD = 99.8%
RMS= 0.76 p.u
DC= -2e-3 p.u
(i) Conf. k
Figure 5: Harmonic analysis for output voltage for different configurations
Left: R- load
Right: RL- load
THD = 69.89%
RMS= 0.86 p.u
DC= -0.21 p.u
α =40º
α=80º
(a) Conf. a
THD = 87.2%
RMS=0.54 p.u
DC=0 p.u
THD = 129.1%
RMS= 0.67 p.u
DC= 0 p.u
α=120º
(Conf. a)
(Conf. b)
(Conf. c & g)
(Conf. j)
(Conf. k)
α=40º
(b) Conf. b
α=80º
THD = 104.9%
RMS= 0.33 p.u
DC= 0 p.u
THD = 266.9%
RMS= 0.69 p.u
DC= 0 p.u
α=120º
(Conf. i)
α=40
(c) Conf. c & g
THD = 82.82%
RMS= 0.68 p.u
DC= -1e-3 p.u
THD = 105%
RMS= 0.74 p.u
DC= -1e-3 p.u
α=80
α=120
(Conf. d & h)
(Conf. e)
(Conf. f)
(d) Conf. d & h
Figure 6: Effect of varying delay angle on output voltage for different AC
voltage controllers on R-load.
THD = 100.2%
RMS= 0.57 p.u
DC= 0 p.u
THD = 254%
RMS= 0.96 p.u
DC= 0 p.u
V. APPLICATION CASE STUDIES:
Two different application case studies using AC voltage
controllers will be discussed through the following sections
(e) Conf. e
82
practical implemented setup for both the 3 SCRs and 6 SCRs
corresponding to those simulated waveforms in fig. 9 & 10 at
no-load. The differences between the experimental and
simulation waveforms are due to the increase of α was done
manually unlike the simulation.
A.
Static Power Factor Improvement (FC-TCR):
Recently, the AC voltage controllers have been utilized in
the field of power system quality and flexible AC
transmission FACTS [9-11]. Unlike traditional shunt reactive
elements, a fixed capacitor– thyristor- controlled reactor (FCTCR) is able to rapidly and smoothly supply or absorb
reactive power by controlling the firing delay angles of
thyristors. As shown in fig. 7, each branch has a fixed
capacitor and two anti- parallel thyristors controlled in series
with an inductor. For such configuration, (5) can be written
from [10].
I FC −TCR = V jB FC −TCR
B FC −TCR = BC − BTCR , BC = ωC
(5)
2α 1
1


BTCR (α ) = B max  2 −
− sin 2α , B max =
π
π
ω
L


Fig. 7-c shows the operating characteristics and the
susceptance ( BFC −TCR ) of this type of compensator based on
(5) and it can be seen that VAR (reactive power) production
as well as VAR absorption is possible by varying the delay
angle of thyristors; hence the power factor changes from
leading to lagging. The firing gates of the thyristors are
synchronized with the capacitance voltage and can be varied
from 90º to 180º.
From (5) and for L= 448 mH, C= 18 µF, ω =314.15 rad/sec,
then:
if
α=90º BFC-TCR = Bc - Bmax= (1.45 e-3) mho.
or if α=180º BFC-TCR= Bc = (5.65e-3 ) mho.
Such configuration of the FC-TCR has been experimentally
connected and simulated using Matlab. Fig. 8 shows the
experimental and the corresponding simulation waveforms of
the FC-TCR voltage and current for different delay angles. It
can be seen that experimental and simulation waveforms are
matched and show the validity of controlling the flow of
reactive power. This configuration could be utilized to
replace the traditional bank of capacitors in many
applications such as power factor improvement, power
system voltage and reactive power control, voltage control of
induction generator and performance optimization of three
phase induction motor operated from a single phase supply.
(a) Three phase
connection
(b) Per phase
connection
(c) Characteristic curves
Figure 7: Configuration and characteristics of the FC-TCR
Voltage
Current
α= 125º (lagging)
Voltage
Current
(a) Experimental results
α= 133º ≈ in phase
α= 125º (lagging)
α= 133º ≈ in phase
Voltage
Current
α= 160º (leading)
α= 160º (leading)
(b) Simulation Results
Figure 8: Effect of changing delay angle on the FC-TCR voltage and
current waveforms
CH1 voltage: 20 V / div, CH2 current: 0.2 A / div, Time: 0.01 sec/div
B.
Three Phase Induction Motor Starters (Soft Starters):
Controlling the starting performance of three phase
induction motor has become one of the major concerns in
industrial fields [12 -13]. The purpose is to control the
starting voltage, current and torque as desired. Configurations
(e, f) in fig. 1 were utilized to examine the starting
performance on a three phase delta connected induction
motor of 0.75 kW, 75 V, 1.5 A, 50Hz, both experimentally
and using simulation. As seen from the results depicted in fig.
9 & 10, configuration (e) produces symmetrical wave forms
for both voltage and currents, unlike configuration (f) which
produces unsymmetrical voltage and current waveforms. The
unsymmetrical waveforms caused by the usage of only 3
SCRs is the reason for the appearance of odd and even
harmonics, DC voltage and current, torque pulsations and a
longer time to reach the desired speed as can be seen from
fig. 9. Heat loss in configuration (f) is higher than
configuration (e) due to the higher rms value of current. Fig.
11 & 12 depict the experimental waveforms obtained for the
(a) Line voltage
(b) Line current
(c) Motor speed
Losses=
5.4 watt
(d) Average heat losses
83
Losses=
5 watt
VI. CONCLUSION:
A comparison study for different configurations of AC
voltage controllers has been introduced through this work.
Experimental and simulation waveforms are matched and
validated for the configurations. The analysis showed that
unidirectional configurations, having a combination of diodes
and thyristors, produced even and odd harmonics and also
contained a DC offset hence they are most suitable for
heating, melting and welding applications, while the
bidirectional configurations are suitable for AC motors,
power systems and electrical drives applications due to the
waveforms symmetry of the controlled voltages. This paper
also validates the analysis of each configuration for any value
of delay angle and control range. Using the AC voltage
controllers in reactive power control and controlling the
starting performance of three phase induction motor through
FC-TCR and SOFT STARTERS respectively as two
application case studies of AC regulators are demonstrated by
the aid of experimental and simulation results.
(e) Developed torque
Figure 9: Simulation of three phase induction motor using 6 and 3 SCRS with
alpha ramp from 0 to 220 V at 1 sec (0.5 N.m. loading)
Left: 6 SCRs
Right: 3 SCRs
(a) Phase voltage
(b) Line current
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(c) Phase current
THD= 241.5%
DC=0
THD= 226.5%
DC=0.05
(d) Harmonic Analysis
Figure 10: Simulation waveforms of three phase induction motor at α=130º
Left: 6 SCRs
Right: 3 SCRs
(a) Phase voltage and phase current
(b) Line voltage and line current
Figure 11: Experimental Waveforms of Three Phase Induction motor
CH1 voltage: 50 V / div, CH2 current: 0.75 A / div, Time: 0.01 sec/div
Left: 6 SCRs
Right: 3 SCRs
(a) Phase voltage and line current
(b) Phase voltage and phase current
Figure 12: Experimental waveforms of three phase induction motor
α=130º
CH1 voltage: 50 V / div, CH2 current: 1 A / div, Time: 0.01 sec/div
Left: 6 SCRs
Right: 3 SCRs
84