TELKOMNIKA, Vol.14, No.2, June 2016, pp. 431~439 ISSN: 1693-6930, accredited A by DIKTI, Decree No: 58DIKTIKep2013
DOI: 10.12928TELKOMNIKA.v14i1.2652
431
Received September 6, 2015; Revised December 28, 2015; Accepted January 7, 2016
Delta-Polygon Autotransformer Based 24-Pulse Rectifier for Switching Mode Power Supply
Chun-ling Hao, Xiao-qiang Chen, Hao Qiu
School of Automation and Electrical Engineering, Lanzhou Jiaotong University, Anning West Road No. 88, Anning District, Lanzhou, 730070, China
Corresponding author, e-mail: hcl_lzjtu163.com
Abstract
In medium and high power capacity switching mode power supply SMPS, power quality at the AC side is often severely distorted. In this paper, a small magnetic rating delta-polygon autotransformer
based 24-pulse rectifier feeding SMPS is designed, constructed, and simulated for harmonic mitigation. Various auto-wound transformers for the 24-pulse AC-DC converter are discussed and compared in terms
of magnetic rating and power quality indices, in order that the optimal autotransformer structure can be chosen. The effect of load variation on the proposed 24-pulse rectifier is also analyzed. Moreover,
performance of the 6-, 12-, and 18-pulse rectifiers based on delta-polygon autotransformer are studied through comparison. Results demonstrate that the total harmonic distortion of utility current is lower than
6.10 and unity power factor is achieved under varying load.
Keywords: autotransformer; multipulse rectifier; harmonic mitigation; power quality Copyright © 2016 Universitas Ahmad Dahlan. All rights reserved.
1. Introduction
In recent years, medium capacity high frequency SMPS has been widely used in computers, telecommunications, aerospace, and welding, etc [1]. However, severe power
quality problems exist in the utility interface where a three-phase diode rectifier with nonlinear characteristics is commonly used as the front end of SMPS. To reduce the adverse effect of
harmonics in the AC mains, international organizations have issued strict power quality standards, such as IEEE519-1992
[2], and IEC 61000-3-2 [3].
In order to improve the power quality, various methods have been studied. Passive, active, or hybrid filters [4-6] are used to compensate harmonics in existing equipment. However,
the ratings of these filters are usually close to the output load, and the control strategy is complex. For new high-power equipment, multipulse rectifier method
[7-9] is preferred at design stage. Because it not only can reduce harmonic contents in the power system, but also has the
advantages of low cost, simple structure, durability, and reliability [8]. Furthermore, multipulse rectifiers based on autotransformer can further reduce cost, volume, and loss of the entire
system [9], since the windings are interconnected and only a small portion of the total kilo-volt-
ampere kVA of the load is transferred through magnetic coupling. The asymmetric autotransformer is applied to an 18-pulse AC-DC converter in [10], and interphase reactor IPR
and zero sequence blocking transformer ZSBT is not needed in this scheme. In [11], a universal formula to design delta- and wye-wound autotransformer in 12- and 18-pulse rectifier
system is introduced, and turns ratio calculation and polarity judgment process of all windings is simplified by using that formula. In [12], structure of the 12-pulse delta type autotransformer is
optimized by analyzing the influence of voltage transformation ratio, and the optimal structure of the transformer with the smallest capacity and the least windings is presented. However, the
total harmonic distortion THD of utility current in the aforementioned 12- and 18 - pulse AC-DC converters exceeds acceptable limits at light load. Therefore, the pulse number should be
further increased to improve various power quality indices, at the expense of increased cost and complexity of the system. Pulse doubling was employed in zigzag autotransformer based 12-
pulse rectifier for harmonic mitigation in [13]. In [14], a modified zigzag autotransformer based 24-pulse AC-DC converter is designed. Compared with rectifier systems with higher pulse
number [15-17], 24-pulse AC-DC converter is more economical and efficient.
ISSN: 1693-6930
TELKOMNIKA Vol. 14, No. 2, June 2016 : 431 – 439
432 Power supply equipments applied to computers and telecommunications require
rectifier system with electrical isolation and adjustable DC link voltage. Thus, to obtain the desired topology, a simple, efficient, and robust system structure is given in [18]. Multipulse
rectifier systems based on phase-shifting autotransformer to feed SMPS are studied in [19-21]. However, these topologies have a common drawback: under light load, the THD of input line
current exceeds limiting values. The effect of different 18-pulse autotransformer configurations on power quality parameters is discussed in [20], and delta-polygon connected autotransformer
is proved to have the smallest magnetic rating and the optimal power quality indices.
This paper designs and constructs a 24-pulse rectifier system based on delta-polygon autotransformer supplying telecommunication power supply. The structures of various 24-pulse
auto-connected transformers are modeled, analyzed, and compared, and delta-polygon autotransformer is found to be the optimum in terms of magnetic rating and power quality
indices. Simulation results of four different AC-DC converters, such as THD of supply current, THD of supply voltage at the point of common coupling PCC, distortion factor DF,
displacement factor DPF, and power factor PF, are presented and compared. 2. Research Method
2.1. Configuration of the 24-pulse Approach
A schematic diagram of a medium capacity SMPS 60 V200 A using a full-bridge DC chopper with a 6-pulse AC–DC converter as the front end is shown in Figure 1. The THD of AC
mains current and the PF of this 6-pulse rectifier does not conform to IEEE Std. 519-1992. Therefore, to reduce the THD of supply current and to improve the PF, a 24-pulse rectifier
scheme is proposed, as shown in Figure 2.
Figure 1. Schematic diagram of a 6-pulse rectifier supplying SMPS
Figure 2. Schematic diagram of the proposed 24-pulse rectifier supplying SMPS The proposed system configuration employs four full-bridge DC choppers to supply the
load. For filtering out high frequency components, a filter L
f
, C
f
is placed between the 6-pulse diode rectifier and the full-bridge DC chopper. Moreover, the high frequency isolated
transformer at output of the full-bridge DC chopper provides electric isolation. As shown in Figure 2, the secondary windings of the four transformers are connected in series, leading to
TELKOMNIKA ISSN: 1693-6930
Delta-Polygon Autotransformer Based 24-Pulse Rectifier for Switching Mode… Chun-ling Hao 433
balanced output current in secondary windings. Output rectifier is chosen to be the full-wave rectifier, so that conduction losses of the output diodes can be reduced. The sum of the
secondary windings’ voltages of the four high frequency transformers forms output voltage of the entire system, and the output voltage is regulated by a proportional and integral PI
controller. 2.2. Design of the Proposed 24-pulse Delta-polygon Autotransformer
According to harmonic elimination principle of multipulse technique, the minimum phase-shifting angle required is depicted in [7]:
converters pulse
- 6
of Number
60 =
angle shifting
- Phase
1 Hence, the phase-shifting angle is 15° among the four sets of three-phase voltages of
the 24-pulse rectifier. Two sets of them are displaced at an angle of ±7.5°, with respect to the utility voltage, while the remaining two sets are displaced at an angle of ±22.5°. Consider
reference voltage is the three-phase balanced supply voltage V
a
, V
b
, V
c
. Then, the four sets of three-phase voltages generated by the proposed autotransformer, namely V
a1
,V
b1
, V
c1
，
V
a2
, V
b2
, V
c2
，
V
a3
, V
b3
, V
c3
, and V
a4
, V
b4
, V
c4
, are phase shifted +7.5°, -7.5°, +22.5°, and -22.5°, respectively. To produce symmetrical pulses and to reduce ripples in output voltage, the
amplitude of these voltages should be equal. The winding arrangement and phasor diagram of the 24-pulse delta-polygon autotransformer is shown in Figure 3.
Figure 3. Winding arrangement and phasor diagram of the proposed autotransformer Assume the three-phase supply voltages applied to the autotransformer as:
120
, 120
- ,
V V
V V
V V
c b
a
2
150
3 ,
90 3
, 30
3 V
V V
V V
V
ca bc
ab
3 Where V is the rms value of input phase voltage.
The four sets of required voltages for the four three-phase diode bridges are:
5 .
127 ,
5 .
12 1
- ,
5 .
7
1 1
1
V V
V V
V V
c b
a
4
5 .
12 1
, 5
. 27
1 -
, 5
. 7
-
2 2
2
V V
V V
V V
c b
a
5
5
. 42
1 ,
5 .
97 -
, 5
. 22
3 3
3
V V
V V
V V
c b
a
6
5 .
97 ,
5 .
42 1
- ,
5 .
22 -
4 4
4
V V
V V
V V
c b
a
7
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434 Then, voltages of phase ‘a’ can be written as:
bc ca
a a
V k
V k
V V
2 1
1
8
bc ab
a a
V k
V k
V V
2 1
2
9
bc ca
a a
V k
V k
V V
4 3
1 3
10
bc ab
a a
V k
V k
V V
4 3
2 4
11
1 2
2
5 4
3 2
1
k
k k
k k
12 Substituting Equation 2 to 7 into Equation 8 to 12, the values of k
1
, k
2
, k
3
, k
4
, and k
5
are calculated to be 0.006, 0.073, 0.045, 0.123, and 0.703, respectively. Furthermore, to obtain the optimal autotransformer structure for the 24-pulse rectifier
supplying SMPS load, various autotransformer configurations are compared, and their winding arrangements and phasor diagrams are shown in Figure 4 and Figure 5, respectively. The turns
ratio of each winding of these autotransformers can also be obtained in a similar way.
Star autotransformer Fork autotransformer
Hexagon autotransformer
Polygon autotransformer
Delta autotransformer Zigzag
autotransformer
Scott autotransformer Delta-polygon autotransformer
Figure 4. Wingding arrangement of various 24-pulse autotransformers
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Delta-Polygon Autotransformer Based 24-Pulse Rectifier for Switching Mode… Chun-ling Hao 435
Star autotransformer Fork autotransformer
Hexagon autotransformer
Polygon autotransformer
Delta autotransformer Zigzag
autotransformer
Scott autotransformer Delta-polygon autotransformer
Figure 5. Phasor diagram of various 24-pulse autotransformers 3. Simulations Based on MATLAB
The MATLABSimulink is used to model and simulate 6-, 12-, 18-, and 24-pulse rectifiers, under the same source and load condition. A three-phase 380V, 50Hz AC voltage
source is adopted as the power supply. The simulation model of the proposed 24-pulse AC-DC converter based on delta-polygon autotransformer is presented in Figure 6. The source
impedance is kept at a practical value of 0.03 pu and the leakage reactance of the autotransformer is set to be 0.05 pu. The simulation results are shown in Figure 7-11 and
Table 1-3.
Figure 6. Simulation model of the delta-polygon autotransformer based 24-pulse rectifier
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TELKOMNIKA Vol. 14, No. 2, June 2016 : 431 – 439
436
4. Results and Discussion
