Page 113 - Power Quality in Electrical Systems

96 Chapter Six

Passive filters can also introduce harmonic peaking (as we have
demonstrated earlier), and the resonant networks can have high cur-
rents and voltages, resulting in high VAr ratings for the components. An
alternative to passive filters are active filters, where power electronics
components are used to actively inject harmonics to cancel harmonics
in the line current. This method has been used in the past in lower
power electronics applications [6.9].
The diagram of one type of active harmonic filter is shown in Figure 6.20.
Note that the nonlinear load draws harmonic current, I , from the power
h
source. The active compensator senses the harmonic current and injects
a compensation current, I , which cancels the harmonic current. The net
c
supply current, I , contains only the fundamental. The compensator
s
switches at a very high frequency compared to the fundamental fre-
quency—hence, the VA rating of the energy storage devices in the com-
pensator can be minimized.
Purported advantages of active filters are [6.6]:

■ Superior filtering performance: The active filters are generally
under microprocessor control and hence can be tuned to a particular

I I
Supply s h
LOAD
I c

i s I h

Active
compensator

I c

Figure 6.20 A typical active harmonic filter installation. The goal is to have
the supply current be the only fundamental harmonic. The load draws har-
monic current, I h , and the active compensator injects a current, I c , to
cancel the harmonic content in the line current.