Cutting the effects of harmonics can be a headache, particularly in
environments where the supply network is weak. Geoff Brown offers a
solution: go on the attack with an active filter!
Harmonics are the villains of electric power systems and the catalogue of
woe they can cause is a long one - from overheating of transformers,
cables, motors and generators, to causing electronic displays and
lighting to flicker, circuit breakers to trip, computers to fail and
metering to give false readings. A number of methods can be used to
combat the problem, filtering being one of them. The simplest type is the
passive filter, which comes in two main types. If harmonic levels are not
too great, sufficient filtering can be achieved by using de-tuned power
factor correction banks, equipment principally intended to provide
reactive power. For the right application, this solution can be both
cost-effective and reliable.
When harmonic levels are too high for a de-tuned solution, however,
fine-tuned filters can be used, sometimes in combination with high-pass
filters. These shunt harmonic currents through a filter-link designed to
have a low impedance at a given frequency compared with the rest of the
network, provided by series-connected capacitive and reactive components.
Although a simple solution, passive filters do have several drawbacks:
* A separate filter link is required for each harmonic frequency that
needs to be filtered.
* The degree of filtering is given by its impedance in relation to all
other impedances in the network. The degree of filtering will therefore
vary according to variations in the network impedance.
* The amount of harmonic current absorbed by the filter is not
controlled, but is the result of the impedances, the generated harmonic
current and any harmonic distortion originating from the feeding
network.
Passive filters also generate reactive power. This means that the network
characteristics need to be well known to allow the passive filter to be
correctly dimensioned. However, even more important is that all later
changes in the network will alter the behaviour of the filter. A new
converter, a new neighbouring factory or changes in the utility network
may all be enough to overload the filter. A filter can be modified for
the new situation, but this may be difficult in practice.
Until recently, the reactive power generation of a passive filter was not
really a disadvantage, since the need for filtering was always
accompanied by a need for reactive power compensation. However, most of
today's ac drives are built with diode rectifiers, with inherently high
power factor, and so provide only small amounts of reactive power;
attempting to filter the harmonics with reactive power generating passive
filters rapidly leads to overcompensation. But the link between reactive
power generation and filtering is already a control problem, since their
relationships vary. If filters are switched based on reactive power
demand, then the harmonics may overload the filter. If the switching
strategy is based on harmonic loading, then the reactive power
compensation may become incorrect.
Active filters
In contrast to the passive filter, the active filter measures the
harmonic currents and generates a harmonic compensating current of
opposing phase, thus causing cancellation. And as the active filter can
be controlled, combined with the active generation of the compensating
harmonic currents, the system cannot be overloaded. Harmonic currents
exceeding the capacity of the filter will remain on the network, but the
filter will operate and eliminate all harmonic current up to its capacity.
The current generator and the control system are the two most important
parts of an active filter. For today's current generators, the most
practical solution is based on the use of insulated gate bipolar
transistors (IGBTs). The