Page 112 - Power Quality in Electrical Systems

Power Harmonic Filters 95

Practical Considerations in the Use of
Passive Filters
Passive power harmonic filter design requires a number of practical
considerations. There is no unique solution to the design problem, so in
each case a careful trade-off analysis must be performed. Practical con-
siderations include the following:
■ Tuning: The harmonic filter sections are tuned below the harmonic
frequency to prevent the filter frequency from shifting upward if one
or more capacitors fail and their fuses blow. Typical orders are 4.85
for the 5th harmonic; 6.7 for the 7th; and 10.6 for the 11th harmonic.
■ Protection: Capacitors are protected by fuses in small groups to
minimize the effect of fuse blowing. The whole filter can be divided into
assemblies, each protected by a circuit breaker.
■ Switching: Filters provide fundamental frequency reactive power
(vars). Portions of the filter can be switched off at times of light load
to limit overvoltage.
■ Tolerances: Capacitors and inductors must be specified so that the
combination of ratings (L and C) does not result in resonance at an
undesired frequency. In other words, we do not want positive peaks in
the filter impedance curves to correspond with harmonic frequencies.
■ Rating: The current rating of the inductors and the voltage rating
of the capacitors must include the fundamental and harmonic com-
ponents.
■ Location: Filters should be located electrically close to the nonlin-
ear load that produces the harmonic currents.
■ Detuning: A change in system impedance or component variations
due to aging or temperature can result in some detuning of the har-
monic filter.

Active harmonic filters
Active harmonic filters have been a growing area of research in recent
years, due to improvements in switch technology, and also because of cost
issues associated with filter components [6.6–6.8]. Especially at high power
levels, the cost of magnetic and capacitive components can be high. High-
frequency switching devices, including the metal-oxide semiconductor
field-effect transistor (MOSFET) and insulated gate bipolar transistor
(IGBT) have emerged in recent years with high current and voltage
ratings. These devices switch on and off with fast switching speeds.
Thus, high-frequency converters can be designed with good power deliv-
ery efficiency using them.