Page 104 - Power Quality in Electrical Systems

Power Harmonic Filters 87

Figure 6.13b, we see the characteristic peak and valley of this type of
harmonic filter. Harmonic filters have also been used to reduce har-
monic interference with telephone systems.

Multisection filters
We can use multiple harmonic filter sections (Figure 6.14) to reduce the
effects of higher-order harmonics generated by nonlinear loads con-
nected to the PCC. The filter is designed to attenuate higher-order
harmonics such as the 5th, 7th, and 11th that are generated by the
nonlinear load. Generally, the filter components are tuned a few percent
below the harmonic frequency [6.2] to account for component varia-
tions, temperature variations, component aging, and system changes.
Figure 6.15a shows a filter designed to attenuate the 5th, 7th, and
11th harmonics. In this design example, each filter section is tuned
4 percent below the filtered harmonic. The series resonant frequencies
of the three series resonant circuits are

1 1
f 5 5 5 288 Hz
1
26
26
2p2L C 1 2p 2s500 3 10 ds611 3 10 d
1
1 1
f 5 5 26 26 5 403 Hz
2
2p2L C 2 2p 2s500 3 10 ds312 3 10 d
2
1 1
f 3 5 5 26 26 5 634 Hz
2p2L 3 C 3 2p 2s500 3 10 ds126 3 10 d
These are the frequencies at which we expect significant attenua-
tion, as evidenced in the PSPICE plot of Figure 6.15b. We also see peak-
ing at frequencies below the three series-resonant frequencies. This is
characteristic of harmonic single-tuned filters and we need to be

I hc
PCC
+

Z Z
I h Z 1 Z 2 Z 3 L S V h
+ Utility source
− −
Converter Filter Linear loud
Figure 6.14 A multiple section filter. The harmonic generating load is
modeled as a current source of value I h .