23-05-2012, 10:39 AM
Compensation of harmonic currents generated by computers utilizing an innovative active harmonic conditioner
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introduction
Today, the situation on low-voltage AC systems has become a serious concern.
The quality of electrical power in commercial and industrial installations is
undeniably decreasing.
In addition to external disturbances, such as outages, sags and spikes due to
switching and atmospheric phenomena, there are inherent, internal causes specific
to each site and resulting from the combined use of linear and non-linear loads.
Untimely tripping of protection devices, harmonic overloads, high levels of voltage
and current distortion, temperature rise in conductors and generators all contribute
to reducing the quality and the reliability of a low-voltage AC system.
The above disturbances are well understood and directly related to the proliferation
of loads consuming non-sinusoidal current, referred to as "non-linear loads".
This type of load is used for the conversion, variation and regulation of electrical
power in commercial, industrial and residential installations.
The prospect of a rapid return to linear-load conditions is illusory. Recent studies
show that the consumption of non-linear current will sharply increase in the years
to come.
However, the remarkable progress made by power electronic devices in the recent
years, fast IGBT's, makes it possible to design self adaptable harmonic suppressors
called active harmonic conditioner, known also as active filters. Active harmonic
conditioners are proving to be viable option for controlling harmonic distortion levels
in many applications.
traditional solutions in eliminating harmonic currents
Today, a various panel of harmonic mitigation equipment or solutions is proposed,
but all present some disadvantages. These solutions are listed here after.
This solution does not attempt to eliminate the harmonic currents flowing
in the electrical installation, but rather to "make do" by avoiding the consequences.
When designing a new installation, the idea is to oversize all installation elements
likely to transmit harmonic currents, namely the transformers, cables, circuitbreakers,
engine generator sets and the distribution switchboards. The most widely
implemented solution is oversizing of the neutral conductor.
The result is a major increase in cost.
In existing installations, the most common solution is to derate the electrical
distribution equipment subjected to the harmonic currents. The consequence is
an installation that cannot be used to its full potential.
topologies of active harmonic conditioners
The idea of active harmonic conditioners, also named active filters, is relatively
old, however the lack of an effective technique at a competitive price slowed
its development for a number of years.
Today, the wide-spread use of IGBT components, mastery of their implementation
and the availability of new digital signal processing (DSP) techniques are paving
the way to a much brighter future for the active harmonic conditioner.
The active harmonic conditioner concept uses power electronics to produce
harmonic components which cancel the harmonic components of the non-linear
loads. A number of different topologies are being proposed, whom some of them are
described here after. Within each topologies there are issues of required
components ratings and method of rating the overall conditioner for the loads to be
compensated.
topologies of active harmonic conditioners
This solution, combining an active conditioner and a passive filter, may be either
of the series or parallel type. In certain cases, it may be a cost-effective solution. The
passive filter carries out basic filtering (5th order, for example) and the active
conditioner, through its precise and dynamic technique, covers the other orders.