ISSN: 1693-6930
TELKOMNIKA Vol. 14, No. 2, June 2016 : 431 – 439
436
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 converters, as shown in Table 2. Moreover, 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 currentvoltage
and output currentvoltage 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 S
r
kVA K
THD
i
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: S
r
: the magnetic rating of an autotransformer; K : percentage of the total output power; THD
i
: 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
TELKOMNIKA ISSN: 1693-6930
Delta-Polygon Autotransformer Based 24-Pulse Rectifier for Switching Mode… Chun-ling Hao 437
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
THD
v
THD
i
I
s
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 THD
v
THD
i
I
s
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 currentvoltage and output currentvoltage of the 6-pulse rectifier supplying SMPS at full load
Figure 8. Waveforms of supply currentvoltage and output currentvoltage of the proposed 24- pulse rectifier supplying SMPS at full load
It can be observed from Figure 7 and Figure 8 that supply currentvoltage 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 currentvoltage 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
rd
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
ISSN: 1693-6930
TELKOMNIKA Vol. 14, No. 2, June 2016 : 431 – 439
438 rectifier system makes power quality indices such as THD
i
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
