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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