Delta-Polygon Autotransformer Based 24-Pulse Rectifier for Switching Mode Power Supply
DOI: 10.12928/TELKOMNIKA.v14i1.2652 431
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_lzjtu@163.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.
(2)
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 V/200 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 (Lf, Cf) 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
(3)
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 (Va, Vb, Vc). Then, the four sets of
three-phase voltages generated by the proposed autotransformer, namely (Va1,Vb1, Vc1)，(Va2,
Vb2, Vc2)，(Va3, Vb3, Vc3), and (Va4, Vb4, Vc4), 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:
V 0 ,V V -120,V V 120
Va b c (2)
3V 30 ,V 3V 90 ,V 3V 150
Vab bc ca (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:
7.5, 1 -112.5, 1 127.5
1 V V V V V
Va b c (4)
-7.5 , 2 -127.5 , 2 112.5
2 V V V V V
Va b c (5)
22.5, 3 -97.5, 3 142.5
3 V V V V V
Va b c (6)
-22.5 , 4 -142.5 , 4 97.5
4 V V V V V
(4)
Then, voltages of phase ‘a’ can be written as:
bc ca a
a V kV kV
V
2 1
1 (8)
bc ab
a
a V kV k V
V
2 1
2 (9)
bc ca a
a V kV k V
V3 1 3 4 (10)
bc ab
a
a V kV k V
V
4 3
2
4 (11)
1 2
2k1k2 k3k4k5 (12)
Substituting Equation (2) to (7) into Equation (8) to (12), the values of k1, k2, k3, k4, and k5 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
(5)
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 MATLAB/Simulink 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.
(6)
4. Results and Discussion
To demonstrate the superiority of the proposed 24-pulse delta-polygon connected autotransformer, various 24-pulse autotransformers are constructed and simulated. Comparison of the kVA rating and power quality indices obtained from various 24-pulse AC-DC converters based on auto-wound transformer is tabulated in Table 1. To exhibit its strong harmonic mitigation ability, performance of the 24-pulse AC-DC converter has been compared with 6-, 12-, and 18- pulse AC-DC converters12-, as shown in Table 2. Moreover12-, the SMPS load of the proposed 24-pulse rectifier has been varied and its effect on various power quality indices is also studied and the results are given in Table 3. Simulated waveforms of supply current/voltage and output current/voltage of the existing 6-pulse rectifier and the proposed 24-pulse rectifier supplying SMPS at full load are shown in Figure 7 and Figure 8, respectively. Figure 9 and Figure 10 depict waveforms of supply current and its harmonic spectrum of the proposed 24-pulse rectifier at light load and under full load, respectively. Additionally, variation of THD of input line current and power factor with load perturbation on the 6-, 12-, 18-, and 24-pulse rectifiers is shown in Figure 11.
It can be observed from Table 1 that the proposed delta-polygon autotransformer based 24-pulse rectifier system lead to the smallest kVA rating and improved PF and THD of utility current as compared to other autotransformer structures. The proposed approach needs an autotransformer of 2.929 kVA (only 19.5% of the output load), resulting in reduced cost and improved efficiency of the whole system. It can also be obtained from Table 1 that the THD of input line current is smaller for the star auto-connected transformer than the proposed delta-polygon connected autotransformer, but the magnetic rating is about 33% higher. Table 2 indicates that, at full load, the THD of supply current of these topologies range from 31.88% to 5.63%, whereas the PF of these converters vary in the range of 0.993 to 0.998.
Table 1. Comparison of magnetic rating and power quality indices for various auto-wound transformer based 24-pulse rectifiers
Transformer type Sr (kVA) K (%) THDi (%) PF
Star 7.860 52.4 4.16 0.991
Fork 7.380 49.2 5.07 0.992
Scott 4.605 30.7 4.80 0.994
Hexagon 4.005 26.7 5.10 0.992
Polygon 3.705 24.7 5.09 0.992
Zigzag 3.495 23.3 4.62 0.996
Delta 4.695 31.3 10.3 0.997
Delta-Polygon 2.925 19.5 4.64 0.998
Modified Scott 4.695 31.3 4.88 0.994
Modified Hexagon 3.180 21.2 5.17 0.991
Modified Polygon 3.255 21.7 5.24 0.995
Modified Zigzag 3.270 21.8 4.68 0.992
Notes: Sr: the magnetic rating of an autotransformer; K (%): percentage of the total output
power; THDi (%): THD of utility current; PF: power factor
For 6-pulse AC-DC converter, the THD of AC mains current is 31.88% under full load, which degenerates to 78.75% at light load condition (20% of the load rating), and the PF is 0.993 and 0.983, respectively, under these conditions. The THD of supply voltage is 9.89% at full load. Apparently, 6-pulse AC-DC converter injects large harmonic components into the power grid. For 12-pulse AC-DC converter, the THD of supply current and the PF at full load are 12.13% and 0.996, respectively. Meanwhile, the THD of input line current is 14.91% and the PF is 0.987 under light load. For applications where the requirement for harmonic components is more stringent, an 18-pulse AC-DC converter is generally favored. As tabulated in Table 2, the THD of utility current is 7.33%, the THD of utility voltage is 3.91%, and the PF is 0.997 at full load, while the THD of the supply current is 8.51% and the PF is 0.991 under light load. Therefore, for 12- and 18-pulse rectifiers, the THD of AC mains current at light load cannot conform to the IEEE Std. 519-1992. To further improve the power quality indices, a 24-pulse AC-DC converter based on delta-polygon autotransformer has been designed and simulated. The THD of AC mains current under full load and light load as given in Table 2 is 5.63% and 6.03% and the PF under these conditions is 0.998 and 0.997, respectively. As shown in Table 3, the THD of supply current are found to vary in the range of 6.03% to 5.63% with the variation of
(7)
the SMPS load and the PF is close to unity at varying load conditions. Therefore, the proposed 24-pulse harmonic mitigator can adhere to the requirements of IEEE Std. 519-1992.
Table 2. Comparison of power quality indices for 6-, 12-, 18-, and 24-pulse rectifiers
TOPO LOGY
THDv (%) THDi (%) Is (A) PF DPF DF
FL FL LL FL LL FL LL FL LL FL LL
6-pulse 9.89 31.88 78.75 21.4 5.5 0.993 0.983 0.999 0.997 0.994 0.986
12-pulse 3.85 12.13 14.91 20.3 5.3 0.996 0.987 0.999 0.998 0.997 0.989
18-pulse 3.91 7.33 8.51 20.0 5.0 0.997 0.991 0.999 0.999 0.997 0.992
24-pulse 3.37 5.63 6.03 19.9 4.9 0.998 0.997 0.999 0.998 0.998 0.998
Notes: THDv (%): THD of utility voltage; Is (A): RMS value of input line current; DF: distortion factor; DPF: displacement power factor; FL: full load; LL: light load.
Table 3. Effect of load variation on power quality parameters of the proposed 24-pulse rectifier
Load (%) THDv (%) THDi (%) Is (A) PF DPF DF
20% 1.31 6.03 4.9 0.997 0.998 0.998
40% 2.06 6.06 8.7 0.997 0.998 0.998
60% 2.53 5.88 12.5 0.998 0.999 0.998
80% 3.12 5.78 16.3 0.998 0.999 0.998
100% 3.37 5.63 19.9 0.998 0.999 0.998
Figure 7. Waveforms of supply current/voltage and output current/voltage of the 6-pulse rectifier supplying SMPS at full load
Figure 8. Waveforms of supply current/voltage and output current/voltage of the proposed 24-pulse rectifier supplying SMPS at full load
It can be observed from Figure 7 and Figure 8 that supply current/voltage of the 6-pulse AC-DC converter are severely distorted, while those of the proposed 24-pulse AC-DC converter are quasi-sine wave. Besides, ripples in the output current/voltage of the 6-pulse rectifier are relatively large, whereas the 24-pulse rectifier has negligible output ripples. The 24-stpped current waveforms shown in Figure 9 and Figure 10 indicate that the delta-polygon auto-connected transformer based 24-pulse harmonic mitigator is capable of eliminating less than 23rd harmonics in the utility current, leading to simple DC link filter design. It can also be observed from Figure 11 that the greatly improved performance of the proposed 24-pulse
(8)
rectifier system makes power quality indices such as THDi and PF satisfactory for various load
conditions.
Figure 9. Waveforms of supply current and its harmonic spectrum of the proposed 24-pulse rectifier supplying SMPS at light load
Figure 10. Waveforms of supply current and its harmonic spectrum of the proposed 24-pulse rectifier supplying SMPS at full load
Figure 11. Variation of THD of input line current and power quality with load of the 6-, 12-, 18-, and 24-pulse rectifiers
5. Conclusion
The delta-polygon autotransformer based 24-pulse AC-DC converter has been designed, constructed and simulated to demonstrate its superiority for feeding a 12 kW SMPS in this paper. There is a drastic improvement in the THD of AC mains current and PF under wide operating range of the load. Up to 21st harmonics in the supply current are eliminated completely. The proposed converter needs an autotransformer of only 19.5% of the load rating, resulting in a circuit of lower cost, loss, weight, and space compared with other types of autotransformer based 24-pulse AC-DC converter. Thus, the proposed 24-pulse rectifier system can be properly applied to medium and high power capacity SMPS.
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Acknowledgements
This project is supported by the Science and Technology Plan Project of Gansu Province (145RJZA098).
Reference
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[3] IEC. 61000-3-2. Limits for Harmonic Current Emissions. Geneva: IEC; 2014.
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(1)
bc ca a
a
V
k
V
k
V
V
3
1
3
4(10)
bc ab
a
a
V
k
V
k
V
V
4 3
2
4
(11)
1
2
2
k
1
k
2
k
3
k
4
k
5
(12)
Substituting Equation (2) to (7) into Equation (8) to (12), the values of
k
1,
k
2,
k
3,
k
4, and
k
5are 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
(2)
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 MATLAB/Simulink 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.
(3)
proposed 24-pulse rectifier has been varied and its effect on various power quality indices is
also studied and the results are given in Table 3. Simulated waveforms of supply current/voltage
and output current/voltage of the existing 6-pulse rectifier and the proposed 24-pulse rectifier
supplying SMPS at full load are shown in Figure 7 and Figure 8, respectively. Figure 9 and
Figure 10 depict waveforms of supply current and its harmonic spectrum of the proposed
24-pulse rectifier at light load and under full load, respectively. Additionally, variation of THD of
input line current and power factor with load perturbation on the 6-, 12-, 18-, and 24-pulse
rectifiers is shown in Figure 11.
It can be observed from Table 1 that the proposed delta-polygon autotransformer based
24-pulse rectifier system lead to the smallest kVA rating and improved PF and THD of utility
current as compared to other autotransformer structures. The proposed approach needs an
autotransformer of 2.929 kVA (only 19.5% of the output load), resulting in reduced cost and
improved efficiency of the whole system. It can also be obtained from Table 1 that the THD of
input line current is smaller for the star auto-connected transformer than the proposed
delta-polygon connected autotransformer, but the magnetic rating is about 33% higher. Table 2
indicates that, at full load, the THD of supply current of these topologies range from 31.88% to
5.63%, whereas the PF of these converters vary in the range of 0.993 to 0.998.
Table 1. Comparison of magnetic rating and power quality indices for various auto-wound
transformer based 24-pulse rectifiers
Transformer type Sr (kVA) K (%) THDi (%) PF
Star 7.860 52.4 4.16 0.991
Fork 7.380 49.2 5.07 0.992
Scott 4.605 30.7 4.80 0.994
Hexagon 4.005 26.7 5.10 0.992
Polygon 3.705 24.7 5.09 0.992
Zigzag 3.495 23.3 4.62 0.996
Delta 4.695 31.3 10.3 0.997
Delta-Polygon 2.925 19.5 4.64 0.998
Modified Scott 4.695 31.3 4.88 0.994
Modified Hexagon 3.180 21.2 5.17 0.991
Modified Polygon 3.255 21.7 5.24 0.995
Modified Zigzag 3.270 21.8 4.68 0.992
Notes: Sr: the magnetic rating of an autotransformer; K (%): percentage of the total output power; THDi (%): THD of utility current; PF: power factor
For 6-pulse AC-DC converter, the THD of AC mains current is 31.88% under full load,
which degenerates to 78.75% at light load condition (20% of the load rating), and the PF is
0.993 and 0.983, respectively, under these conditions. The THD of supply voltage is 9.89% at
full load. Apparently,
6-pulse AC-DC converter injects large harmonic components into the
power grid. For 12-pulse AC-DC converter, the THD of supply current and the PF at full load are
12.13% and 0.996, respectively. Meanwhile, the THD of input line current is 14.91% and the PF
is 0.987 under light load. For applications where the requirement for harmonic components is
more stringent, an 18-pulse AC-DC converter is generally favored. As tabulated in Table 2, the
THD of utility current is 7.33%, the THD of utility voltage is 3.91%, and the PF is 0.997 at full
load, while the THD of the supply current is 8.51% and the PF is 0.991 under light load.
Therefore, for 12- and 18-pulse rectifiers, the THD of AC mains current at light load cannot
conform to the IEEE Std. 519-1992. To further improve the power quality indices, a 24-pulse
AC-DC converter based on delta-polygon autotransformer has been designed and simulated.
The THD of AC mains current under full load and light load as given in Table 2 is 5.63% and
6.03% and the PF under these conditions is 0.998 and 0.997, respectively. As shown in Table 3,
the THD of supply current are found to vary in the range of 6.03% to 5.63% with the variation of
(4)
the SMPS load and the PF is close to unity at varying load conditions. Therefore, the proposed
24-pulse harmonic mitigator can adhere to the requirements of IEEE Std. 519-1992.
Table 2. Comparison of power quality indices for 6-, 12-, 18-, and 24-pulse rectifiers
TOPOLOGY
THDv (%) THDi (%) Is (A) PF DPF DF
FL FL LL FL LL FL LL FL LL FL LL
6-pulse 9.89 31.88 78.75 21.4 5.5 0.993 0.983 0.999 0.997 0.994 0.986
12-pulse 3.85 12.13 14.91 20.3 5.3 0.996 0.987 0.999 0.998 0.997 0.989
18-pulse 3.91 7.33 8.51 20.0 5.0 0.997 0.991 0.999 0.999 0.997 0.992
24-pulse 3.37 5.63 6.03 19.9 4.9 0.998 0.997 0.999 0.998 0.998 0.998
Notes: THDv (%): THD of utility voltage; Is (A): RMS value of input line current; DF: distortion factor; DPF: displacement power factor; FL: full load; LL: light load.
Table 3. Effect of load variation on power quality parameters of the proposed 24-pulse rectifier
Load (%) THDv (%) THDi (%) Is (A) PF DPF DF
20% 1.31 6.03 4.9 0.997 0.998 0.998
40% 2.06 6.06 8.7 0.997 0.998 0.998
60% 2.53 5.88 12.5 0.998 0.999 0.998
80% 3.12 5.78 16.3 0.998 0.999 0.998
100% 3.37 5.63 19.9 0.998 0.999 0.998
Figure 7. Waveforms of supply current/voltage and output current/voltage of the 6-pulse rectifier
supplying SMPS at full load
Figure 8. Waveforms of supply current/voltage and output current/voltage of the proposed
24-pulse rectifier supplying SMPS at full load
It can be observed from Figure 7 and Figure 8 that supply current/voltage of the 6-pulse
AC-DC converter are severely distorted, while those of the proposed 24-pulse AC-DC converter
are quasi-sine wave. Besides, ripples in the output current/voltage of the 6-pulse rectifier are
relatively large, whereas the 24-pulse rectifier has negligible output ripples. The 24-stpped
current waveforms shown in Figure 9 and Figure 10 indicate that the delta-polygon
auto-connected transformer based 24-pulse harmonic mitigator is capable of eliminating less than
23
rdharmonics in the utility current, leading to simple DC link filter design. It can also be
observed from Figure 11 that the greatly improved performance of the proposed 24-pulse
(5)
Figure 9. Waveforms of supply current and its harmonic spectrum of the proposed 24-pulse
rectifier supplying SMPS at light load
Figure 10. Waveforms of supply current and its harmonic spectrum of the proposed 24-pulse
rectifier supplying SMPS at full load
Figure 11. Variation of THD of input line current and power quality with load of the 6-, 12-, 18-,
and 24-pulse rectifiers
5. Conclusion
The delta-polygon autotransformer based 24-pulse AC-DC converter has been
designed, constructed and simulated to demonstrate its superiority for feeding a 12 kW SMPS in
this paper. There is a drastic improvement in the THD of AC mains current and PF under wide
operating range of the load. Up to 21
stharmonics in the supply current are eliminated
completely. The proposed converter needs an autotransformer of only 19.5% of the load rating,
resulting in a circuit of lower cost, loss, weight, and space compared with other types of
autotransformer based 24-pulse AC-DC converter. Thus, the proposed 24-pulse rectifier system
can be properly applied to medium and high power capacity SMPS.
(6)
Acknowledgements
This project is supported by the Science and Technology Plan Project of Gansu
Province (145RJZA098).
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