[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)
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R1<br />
Es<br />
4<br />
1<br />
R2<br />
R3<br />
FIGURE 2.4.14 Three-phase bridge rectifier.<br />
N<br />
DC Winding<br />
6<br />
R2<br />
3<br />
R1<br />
2 Es<br />
2<br />
R3<br />
5<br />
R2<br />
LQ<br />
Figure 2.4.14 shows a three-phase bridge rectifier. A periodic switching converts electrical power from<br />
one form to another. A dc voltage with superimposed high-frequency ripple is produced. The ripple<br />
voltage consists of the supply voltage with a frequency of multiples of six times the fundamental frequency.<br />
This can be done using either diode rectifiers or thyristors.<br />
Energy balance and Fourier analysis of the square waves confirms that each 6n harmonic in the dc<br />
voltage requires harmonic currents of frequencies of 6n + 1 and 6n – 1 in the ac line. The magnitude of<br />
the harmonic current is essentially inversely proportional to the harmonic number, or a value of 1/h.<br />
This is true of all converters. Once the number of pulses is determined from a converter, the harmonics<br />
generated will begin to be generated on either side of the pulse number. So a six-pulse converter begins<br />
to generate harmonics on the 5th and 7th harmonic, then again on the 11th and 13th harmonic, and so<br />
on. A 12-pulse converter will begin to generate harmonics on the 11th and 13th harmonic, and again<br />
on the 23rd and 25th harmonic, and so on. In order to reduce harmonics, topologies are developed to<br />
increase the number of pulses. This can be done using more-sophisticated converters or by using multiple<br />
converters with phase shifts. From this, we can express the above in a general formula for all pulse<br />
converters as:<br />
h = kq 1 (2.4.6)<br />
where<br />
k = any integer<br />
q = converter pulse number<br />
Table 2.4.1 shows a variety of theoretical harmonics that are produced based on the pulse number of<br />
the rectifier system. It should be emphasized that these are theoretical maximum magnitudes. In reality,<br />
harmonics are generally reduced due to the impedances in the transformer and system, which block the<br />
flow of harmonics. In fact, harmonics may need to be expressed at different values, depending on the<br />
load. On the other hand, due to system interaction and circuit topology, sometimes the harmonic current<br />
values are greater than the ideal values for certain harmonic values. The interaction of devices such as<br />
filters and power-factor correction capacitors can have a great affect on the harmonic values. These<br />
interactions are not always predictable. This is also one of the problems faced by filter designers. This<br />
has increasingly led toward multisecondary transformers that cancel the harmonics at their primary<br />
terminals, although the transformer generally still has to deal with the harmonics.<br />
Id<br />
Es<br />
−<br />
+<br />
Ed<br />
R3<br />
H2<br />
I2<br />
H1 H3<br />
EL<br />
AC Winding<br />
Instantaneous<br />
DC Volts + to −<br />
TABLE 2.4.1 Theoretical Harmonic Currents Present in Input Current to a Typical Static <strong>Power</strong><br />
Rectifier, Per Unit of the Fundamental Current<br />
Harmonic Order<br />
Rectifier Pulse Number<br />
6 12 18 24 30 36 48<br />
5 0.2000 — — — — — —<br />
7 0.1429 — — — — — —<br />
11 0.0909 0.0909 — — — — —<br />
13 0.0769 0.0769 — — — — —<br />
17 0.0588 — 0.0588 — — — —<br />
19 0.0526 — 0.0526 — — — —<br />
23 0.0435 0.0435 — 0.0435 — — —<br />
25 0.0400 0.0400 — 0.0400 — — —<br />
29 0.0345 — — — 0.0345 — —<br />
31 0.0323 — — — 0.0323 — —<br />
35 0.0286 0.0286 0.0286 — — 0.0286 —<br />
37 0.0270 0.0270 0.0270 — — 0.0270 —<br />
41 0.0244 — — — — — —<br />
43 0.0233 — — — — — —<br />
47 0.0213 0.0213 — 0.0213 — — 0.0213<br />
49 0.0204 0.0204 — 0.0204 — — 0.0204<br />
A quick study of Table 2.4.1 shows that the harmonic currents are greatly reduced as the pulse number<br />
increases. Of course, the cost of the rectifiers and transformers increases as the complexity of the design<br />
increases. One must determine what level of power quality one can afford or what level is necessary to<br />
meet the limits given by IEEE 519.<br />
2.4.7 Harmonic Spectrum<br />
One requirement of ANSI/IEEE C57.18.10 is that the specifying engineer supply the harmonic load<br />
spectrum to the transformer manufacturer. There are too many details of circuit operation, system<br />
parameters, and other equipment, such as power-factor correction capacitors, for the transformer manufacturer<br />
to be able to safely assume a harmonic spectrum with complete confidence. The requirement<br />
to specify the harmonic spectrum is of utmost importance. Indeed, the problem of harmonic heating<br />
was the primary reason for the creation of ANSI/IEEE C57.18.10, which shows the theoretical harmonic<br />
spectrum similar to that presented in Table 2.4.1, also in theoretical values. The 25th harmonic was used<br />
as the cutoff point, since the theoretical values were given in the table. Table 1 of ANSI/IEEE C57.18.10<br />
provides very conservative values. Consider what would happen if we used the theoretical values of<br />
harmonic current and allowed the spectrum to go on and on. The harmonic-loss factor from such an<br />
exercise would be infinity for any pulse system. A reasonable cutoff point and accurate harmonic spectrum<br />
are necessary to properly design transformers for harmonic loads. While it may be prudent to be<br />
somewhat conservative, the specifying engineer must recognize the cost of being overly conservative. If<br />
the transformer spectrum is underestimated, the transformer may overheat. If the harmonic spectrum<br />
is estimated at too high a value, the transformer will be overdesigned, and the user will invest more<br />
capital in the transformer than is warranted.<br />
It is also not possible to simply specify the total harmonic distortion (THD) of the current in order<br />
to specify the harmonics. Many different harmonic-current spectra can give the same THD, but the<br />
harmonic-loss factor may be different for each of them.<br />
Total percent harmonic distortion, as it relates to current, can be expressed as:<br />
H<br />
<br />
h2<br />
I THD = I h2 /I 12 100% (2.4.7)<br />
© 2004 by CRC Press LLC<br />
© 2004 by CRC Press LLC