Harmonic distortion on the power system is
caused by nonlinear devices that produce distorted or nonsinusoidal waveforms.
Examples include electronically controlled devices (such as rectifiers and power
controllers), arcing loads (such as arc furnaces and arc welders), and magnetic
devices to a lesser degree (such as rotating ac machinery and transformers).
Excessive harmonic voltage and/or current
can cause damage to equipment and the electrical system. IEEE Std 519-199210
gives application guidelines. One of the common ways of controlling harmonic
distortion is to place a passive shunt harmonic filter close to the harmonic
producing load(s).
The harmonic-producing device can generally
be viewed as a source of harmonic current. The objective of the harmonic filter
is to shunt some of the harmonic current from the load into the filter thereby reducing the amount of harmonic
current that flows into the power system.
The simplest type of shunt harmonic filter
is a series inductance/capacitance (LC) circuit. More complex harmonic filters
may involve multiple LC circuits, some of which may also include a resistor.
Key filter design considerations include
the following:
a) Reactive power (kilovar) requirements
b) Harmonic limitations
c) Normal system conditions, including
ambient harmonics
d) Normal harmonic filter conditions
e) Contingency system conditions, including
ambient harmonics
f) Contingency harmonic filter conditions
These considerations can be grouped into
performance and rating criteria. The performance criteria relate to normal
expected operating conditions and include capacitive reactive power
requirements, harmonic limitations, normal system conditions, and normal
harmonic filter conditions.
The rating criteria relate to unusual
conditions that may place a more severe duty on the equipment. These unusual
conditions include contingency system conditions and contingency harmonic
filter conditions.
Under the contingency conditions, it may be
acceptable to have a more relaxed harmonic limitation.
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