Regulated AC/DC/AC power supply using scott transformer
M. Moussa a, H. Hesham, and Yasser G. Dessouky*
a
Arab Academy for Science and Technology and Maritime Transport, Miami, P.O. Box: 1029, Alexandria, Egypt.
Abstract
In today's industry, it is necessary to convert power for equipment used in environments where dissimilar voltages and frequencies are
the norm. Static frequency converters or industrial power supplies are used for converting either 50Hz or 60Hz utility line power to 400Hz
power. They are more efficient than motor-generator sets. In addition, they offer harmonic cancellation, power factor correction, phase
conversion, voltage conversion with balanced, smooth, and controlled power output. Many varied applications in power electronics require
sinusoidal outputs at frequency 400Hz. This paper describes the design, simulation and implementation of a power converter topology and
control techniques for realizing sinusoidal output systems. A 150 KVA 3-phase power supply, whose line voltage and frequency are 440V
ase are used to
convert the dc voltage to get two phase AC power supply which is converted via a Scott transformer to a three phase, whose line voltage
and frequency are 440V and 400 Hz. A resonant filter is used to eliminate harmonics. Feedback signals from load voltage and dc link
current are used to control the rectifier so as to maintain constant voltage at variable load conditions. The system is theoretically analyzed
and experimentally verified.
Keywords: Static converters, Power supplies, Scott transformer, Resonant filter, Center tapped inverter.
*
Corresponding author. Tel: +2001001234411
E-mail: mona.moussa@yahoo.com
ygdessouky@yahoo.com
1. Introduction
2.2. DC Link Filter
Power supplies are among the most important
components of any industrial application. Standard power
supply is designed to optimize the power required, resulting
in maximized efficiency, power factor and load regulation.
Industrial power supplies are used for applications such as:
aircraft power supplies, paper mill, laser power supplies,
radar/sonar power supplies, battery charger, and marine
propulsion systems [1-3]. In this paper, an industrial
application is considered where the (6) MVA from the
synchronous generator of a ship is used to supply different
loads on board. A power converter is designed to supply
150 KVA of this total power to special loads such as Gyro
system and other navigation equipments. The converter,
shown in Figure 1, employs two stages of power
conversion. In the first stage, the fixed frequency ac supply
voltage is rectified to create the required dc bus by using
thyristor phase controlled rectifier. In the second stage, the
dc bus voltage is inverted at the required output frequency
by using two half-bridge inverters 90º phases shifted. The
Scott-transformer connection allowed 2-φ to 3-φ
components to be interconnected, which adds an advantage
to this power supply of having a relatively low cost because
of using only two center tap inverters switched at power
frequency with no PMW on the switches, meaning lower
losses and voltage stresses where the DC link voltage is
controlled using bridge rectifier.
The function of the dc link filter is to attenuate the
rectifier output voltage harmonics across the link inductor
Lo and to sink the inverter input current harmonics into the
link capacitor Co. However, attenuation of the rectifier
output voltage harmonics across Lo creates additional
ripple current into Co, while the sinking of the inverter
input current harmonics into Co gives rise to additional
ripple voltage across Lo. Therefore, both filter components
(Lo and Co) are affected by both harmonic sources. The
size and cost of this dc filter is determined by the rated
system power, rated dc bus voltage, and the specified levels
of THD in the link input current, and link output voltage.
To smooth the dc voltage, a dc link filter is used whose
parameters are designed to be (Lo = 5mH and Co=
22000F) [5].
2. System description
This static converter contains controlled rectifier, DC
link filter, Scott transformer, single phase-inverter and
series-parallel resonant filter. A description of these
components is as follows:
2.1. Three phase fully controlled bridge converter
The phase controlled rectifier is obtained by six
thyristors. Continuous control over the output dc voltage is
obtained by controlling the conduction interval of each
thyristor. The load harmonic voltage increases considerably
as the average value goes down. The input current contains
only odds harmonics of the input frequency other than the
triplex harmonics. In this system, the three-phase supply,
whose line voltage and frequency are 440V and 60Hz, is
converted to dc voltage via controlled rectifier where the
conduction interval to control the dc voltage from (425 V)
to (510 V) from 10% to 120% of the full load respectively
[4].
2.3. Scott transformer
A Scott-transformer, shown in Figure 2 is used to drive
three phase current from a two phase source. It consists of
a center tapped transformer T 1 and an 86.6% tapped
transformer T2 on the 3-φ side of the circuit. The primaries
of both transformers are connected to the 2-φ voltages. One
end of the T2 86.6% secondary winding is a 3-φ output, the
other end is connected to the T 1 secondary center tap. Both
ends of the T1 secondary are the other two 3-φ connections
[6].
To compensate for the voltage drop in the internal
impedance of the different parts of the system, the Scott
transformer is a step up whose turns ratio is 1: 1.2 [7].
2.4. Single-phase centre-tapped transformer inverter
An alternating load voltage can be generated from a dc
source by the use of a centre-tapped transformer as shown
in Figure 3 [8]. Basically, by switching the two switches,
the dc source is connected in alternative senses to the two
halves of the transformer primary, so inducing a square
wave voltage across the load in the transformer secondary
[9].
For loads whose current is out of phase with the voltage,
anti-parallel diodes feedback the stored load energy during
those periods when the current reverses relative to the
voltage. Two square wave center-tapped-transformer
inverters are used whose output voltages are perpendicular
(90 separation) which are the two phase voltage sources of
the Scott transformer to get three-phase output voltages
[10].
Controlled
Bridge
Rectifier
Center Tap
Inverter
DC Link
Filter
Scott
Transformer
Resonant
Filter
3-ph power
supply
Load
Gate Drive and
Control Circuits
DC Current
Feed Back
Load Voltage
Feed Back
Fig. 1 AC/DC/AC Power supply.
Fig. 2 Scott-transformer converts 2-φ to 3-φ.
capacitor bank overloading, additional heating and losses in
AC machines, increased probability of relay malfunctions,
disturbances in solid-state and microprocessor based
systems, interference with telecommunication systems [14].
The resonant arm filter, shown in Figure 4, is more
appropriate to attenuate low order harmonics. Both the
series arm L1C1 and the parallel arm L2C2 are tuned to the
inverter output frequency. The series arm presents zero
impedance to the fundamental frequency, but finite
increasing impedance to higher frequencies [15]. The
parallel arm presents infinite impedance at the fundamental
frequency, but reducing impedance to higher frequencies.
Taking the fundamental frequency
(1)
AC
Voltage
Making C1 = AC2 and L2 = AL1, and setting ω = nω , where
n is the order of the harmonic. The filter transfer function is
then given by [16]:
(2)
Fig. 3 Centre-tapped transformer inverter.
2.5. Harmonic and Filters
Harmonic distortion of voltages and currents in power
systems are caused by the presence of non-linear loads in
the system that produce distorted current. Using Fourier
analysis, these distorted voltages and currents can be
described in terms of harmonics. The harmonics in the
lower frequency band are the most significant [11,13]. A
few of the major effects of the harmonics are as follows:
The output voltage of the inverter, and then through the
Scott transformer, is 400Hz, 180 conduction square wave
whose major harmonic is the third (n=3) which equals to
33.3% of the fundamental [17]. If this value is to be
attenuated to 4%, then the value of the gain (A) of (2) is
determined to be 0.76.
The value of the reactance (nω L1) is taken to be less
than the load impedance (150KVA, 440 Vline, 0.8 PF) to
avoid excessive load voltage changes when the load varies.
Take nω L1 = 1.29, then the filter parameters are given in
Table 1
3. Simulation
A prototype system is used to simulate the proposed
system using MATLAB software as shown in Figure 5.
The SIMULINK model starts at 10% full load till 0.8 sec
when the full load is connected. Then at 1.2 sec, the supply
is over-loaded by another 20% of full load. The results are
shown in Fig. 6 to Fig. 10.
Fig. 6 and Fig. 7 show that the load is supplied by
almost a sinusoidal current at almost constant and
sinusoidal voltage at different load conditions. The
resonant filter reduced the third harmonic in the voltage
and in the current at almost 4% and 1% of the fundamental
respectively, and reduced the THD in the voltage and
current to (4.4  0.7)% and (2.2  0.4)% respectively. The
third harmonic appeared, despite three-phase load nature,
because the impact of two-phase connection in the Scott
transformer. Fig. 8 and Fig. 9 show that the system has a
good time response to regulate load voltage at sudden load
change. However, the disadvantage of power converter is
the harmonic input to the incoming source, this is shown in
Fig. 10, where the fifth harmonic is more than 20% of the
fundamental and the THD is greater than 25%. However, if
the supply is critical, a method to improve supply power
quality could be implemented.
C1
Vo
Fig. 4 Parallel-series resonant arm filter.
Table 1
Filter parameters
C2 = 500F
L1 = 0.41mH
L2 = 0.31mH
2.6. Feedback control system
The system has two PID controllers fed from the two
feedback cascaded loop, namely, the outer loop from the
load voltage and the inner loop from the current of the dc
link filter. These controllers regulate the load voltage at
constant level of 440V from almost no load to 120% full
load.
g
g
+
+
A
i
-
E
C
A
Discrete,
Ts = 1e-005 s.
1+
+2
+
B
+
v
-
1.1
E
2
1
alpha_deg
CA
Vb
C
C
c
C
c
E
Vc
180
20
Voltage
FFT window: 3 of 600 cycles of selectedController
signal
Current
PID
Controller
1
3
FFT window: 3 of 600 cycles of selected signal
440
node 10
10
200
node 10
-100
-20
-200
0.401
0.402
0.403
0.404
0.405
0.406
0.407
10
0
-10
-20
0.403
0.404
Time (s)
0.405
0.406
0.407
80
100
60
0
40
-100
-200
20
0
0.9
0
500
1000
1500
2000
2500
Frequency (Hz)
3000
3500
4000
0.901
0.902
0.903
0.904
Time (s)
0.905
0.906
0.907
0.904
Time (s)
0.905
0.906
0.907
Max. current = 263.5A , THD= 2.48%
100
80
80
dament al)
100
60
80
60
40
20
0
(b)
Max. current = 27.65 A, THD= 1.79%
ament al)
0.903
Max. current = 263.5 , THD= 2.48%
(a)
60
0.902
100
200
Mag (% of Fundamental)
Mag (% of Fundamental)
20
0.402
0.901
Max. current = 27.65 , THD= 1.79%
100
0.401
0.9
Fig. Ti5me (s) Simulink Block Diagram of the overall system.
Output current ia(t) at 100%F.L
Output current ia(t) at 10%F.L
0.4
node 10
0
-10
0.4
node 10
100
PID
0
A
B
2
C
1+
+3
C
Synchronized
6-Pulse Generator
Va
b
C
1.1
g
+
v
-
a
B
+2
E
C
pulses
A
b
A
BC
a
B
B
+
v
-
g
A
B
C
AB
A
B
C
+
v
-
A
A
C
B
g
C
-K-
-
C
A
B
B
C1 = 380F
C
C2
A
L2
B
Vi
C
L1
0
500
1000
1500
2000
Frequency (Hz)
2500
3000
3500
4000
200
0
-200
1.4
1.401
1.402
1.403
1.404
Time (s)
1.405
1.406
1.407
Output current ia(t) at 120% F.L
Max. current = 313.3 , THD= 2.66%
DC Current (A)
100
DC Voltage (volt)
600
Mag (% of Fundamental)
200
1.4
1.401
1.402
1.403
1.404
Time (s)
1.405
1.406
600
40
400
FFT window: 3 of 600 cycles of selected signal
0
0
200 0
(c)
200
200
20
1.407
400
500
1000
1500
2000
Frequency (Hz)
2500
3000
3500
4000
0
0
0.5
80
60
0.403
0.404
Time (s)
0.405
0.406
FFT window: 3 of 600 cycles of selected signal
0
of
500
0.901
1000
0.902
15000.903
20000.904
Frequency
Time (s)(Hz)
25000.905
-40
0.5 4 0.405 0.41 0.415 0.42 0.425 0.43 0.435 0.44 0.445
Time (s)
30000.906 3500 0.907 4000
(a)
20
0
Output load voltage at 100% F.L
0
Max. voltage = 355.2 , THD= 4.93%
0.905
0.906
0.907
0
20
-200
01.4
5001.401
10001.402
15001.403
Fundamental (400Hz) = 355.2 , THD= 4.93%
60
1.4
1000
2000
1.401
1.402
3000
80
60
40
4000 5000 6000 7000 8000
20
Frequency (Hz)
0
1.403 1.404 1.405 1.406 1.407
0
Time (s)
(c)
40
500
1000
1500
2500
3000
20080
0
1000
2000
3000
4000 5000
Frequency (Hz)
6000
7000
400
100
60
200
50
0
400
0.5
0
1.5 0
1
Time (s)
0.5
1
Time (s)
(a)
20
(b)
3000
4000 5000
Frequency (Hz)
6000
7000
8000
20
Fundamental)
10
2000
of
15
1000
g (%
of
Fundamental)
20 0
15
10
800
700
1.435
800
1.44
900
1.445
1000
700
800
900
1000
900 1000
80
60
FFT window: 3 of 90 cycles of selected signal
40
200
100
1.405
200
1.41
300
1.415
400
1.42
500
600
1.425
1.43
Frequency
Time (s)(Hz)
100
Max. current = 332.7A , THD= 29.00%
0
-20080
100 200 300 400 500 600 700 800 900 1000
Frequency (Hz)
601.4 1.405 1.41 1.415 1.42 1.425 1.43 1.435 1.44 1.445
Time (s)
80
60
40
20
0
0
100
200
300
400
500
600
Frequency (Hz)
(c)
4. Experimental work
8000
150
1000
40
Fig. 10 Instantaneous input supply current waveform and FFT at: (a) 10%
F.L, (b) 100% F.L, and (c) 120% F.L.
Max. current = 399.2 A, THD= 28.28%
20
100
RMS output voltage (volt)
600
Fig. 8 (a) RMS /load current, (b) RMS line voltage.
(%
4000
Fig. 7 Instantaneous output phase voltage waveform and FFT at: (a) 10%
F.L, (b) 100% F.L, and (c) 120% F.L.
20
Fundamental (400Hz) = 358.2 , THD= 5.07%
100
0
ag
3500
Fundament al)
Fundament al)
of
2000
Frequency (Hz)
Fundament al)
0
800
-200 0
700
400 500 600
Frequency (Hz)
900
0.945
Max. current = 399.2 , THD= 28.28%
20
200
100
00
of
200
10020
800
0.94
100
-200
0
20
60
-200
0.9 0.905 0.91 0.915 0.92 0.925 0.93 0.935 0.94 0.945
0
1.40
Time (s)
(b)
40
Input supply current ia(t) at 120% F.L
Mag
Max. voltage = 358.2 , THD= 5.07%
100
700
0.935
Max. current = 332.7 , THD= 29.00%
80
(%
Output load voltage at 120% F.L
40
RMS Current (A)
(%
2500 1.405 3000 1.406 3500 1.407 4000
(b)
2500
Mag
20001.404
Time (s)(Hz)
Frequency
300
0
40200
0
200
500
600
0.925
0.93
Frequency
Time (s)(Hz)
(%
0.904
Time (s)
100
900 1000
400
0.92
Mag
0.903
800
300
0.915
1.5
0
80 0
100 200 300 400 500 600 700 800 900 1000
The systemFrequencyhas
(Hz) been built in the lab as shown in Figure
11, with a scaled down rate of 1.5 kVA to verify the
60
operation,
where the 3-phase 44V, 50 Hz input supply is
rectified using the CD43-40B Dual SCR Isolated POW-RBLOK
Module controlled rectifier. A 2nd order LC filter (L
40
= 5mH, C = 1500µF/470V) smoothes the output DC which
is the input to two single-phase perpendicular centre tap
20
inverters
(switches IRFP150N) to produce two-phase AC
voltages which are converted to 3-phase voltages via the
120%
step up Scott transformer, to make up for the voltage
0
of
0.902
300 Input suppl400y current500ia(t) at 100%600F.L 700
Frequency (Hz)
200
0.91
(%
0.901
200
100
0.905
(a)0 0
Mag
60 0.9
100
0
-200
0
0
0.9
Mag (% of Fundamental)
-80200
FFT window: 3 of 600 cycles of selected signal
60
FFT window: 3 of 90 cycles of selected signal
60
200
40
5 20
Max. current = 36.08 A, THD= 39.59%
200
100
80
Fundament al)
8000
of
7000
Mag (% of Fundamental)
Fundament al)
of
(%
Mag
Fundament al)
of
(%
Mag
1000
2000 Fundament3000al (400Hz) =4000356.8 , THD=50003.73% 6000
Frequency (Hz)
Mag (% of Fundamental)
100
2000
0
100
0
Max. current = 36.08 , THD= 39.59%
100
10
(%
0
20
0
0.90
0.445
15 80
2015
-2100
20040
0.407
(b)
20
40
Mag
0.402
Input supply current ia(t) at 10% F.L
20
Fundamental)
Mag (% of Fundamental)
Fundament al)
of
(%
Mag
0.401
-20
1.5
Time (s)
-200
0.4
(a)
1
Time (s)
0.41 0.415 (b)
0.42 DC
0.425 voltage.
0.43 0.435 0.44
Fig. 9 Instantaneous wave: (a) DC 0.405
current,
Max. voltage = 356.8 , THD= 3.73%
40
20
0.5
-40
0.4
100
-200
FFT window: 3 of 90 cycles of selected signal
040
Time (s)
0
Fig. 6 Instantaneous output current waveform and FFT at: (a) 10% F.L, (b)
0.4
0.401
0.402
0.403
0.404
0.405
0.406
0.407
100% F.L, and (c) 120%F.L. Time (s)
Fundamental (400Hz) = 313.3 , THD= 2.66%
Output load voltage at 10% F.L
100
0
60
1.5
0
-200
20080
1
Mag (% of Fundamental)
Mag (% of Fundamental)
-200
800
60
Mag (% of Fundamental)
0
80
0
100 200 300 400 500 600 700 800 900 1000
Frequency (Hz)
drop through the circuit. The load voltage harmonics are
eliminated using the resonant filter (series branch: L = 11
mH, C = 15 µF/220V, and parallel branch: L = 3 mH, C =
45 µF/220V). The supply is loaded with a
(44V/1.5KVA/400Hz) load. To regulate the load output
voltage during loading, a three phase uncontrolled bridge
with a small smoothing capacitor are used to measure the
output load voltage which is fed back to the control circuit
of the controlled rectifier to increase the DC average
voltage through a PI controller. Also, a current limiter is
used in this control circuit to protect the supply from access
loading. To protect the MOSFET switches and the
thyristor, a soft staring technique is used in the firing and
control signals of both circuits. Fig. 12 to Fig. 16 show the
experimental results, where Fig. 12 and Fig. 13, show the
full load steady state line output voltage and current,
respectively, which are sinusoidal. Fig. 14 and Fig. 15
show the voltage across primary of teaser winding of the
Scott transformer and the supply current at steady state,
respectively. Figure 16 shows the transient response of the
DC link voltage when the supply is loaded suddenly from
no load to full load, where the DC voltage is increased
from 40 V to 51 V to regulate the output voltage at its
nominal rated value. The experimental results show the
validity of the supply to produce sinusoidal output voltage.
Fig. 12 Steady State Load Line Voltage.
Fig. 13 Steady State Load Line Current.
Rectifier
and
firing
Scott
Inverter
and
control
trans
Fig. 14 Voltage across Primary of Teaser Transformer.
Filter
Load
Fig. 11 Experimental rig.
Fig. 15 Steady state Supply Current.
Fig. 16 DC Bus voltage.
5. Conclusion
This paper introduced the design, simulation and
implementation of static power converter techniques for
realizing sinusoidal output system. The converter is used to
feed 150KVA, 440V, 400Hz critical loads on a ship from
440V, 60Hz three-phase supply. The controlled rectifier
and dc link filter provide a dc voltage, controlled by
feedback signals from load voltage and dc link current,
which is then converted to three-phase via two centre tap
inverters and a step up Scott transformer. A resonant filter
is designed to eliminate 3rd harmonics and higher. The
system is experimentally verified at 15KVA, 44V. The
simulated and experimental results have been presented to
prove the validity of the system.
Acknowledgements
The authors would like to thank Eng Ahmed EL-Shazly
of the Fox Power Electronics Company for his help and
support during the practical implementation of the rig.
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