Most power system circuit elements are primarily inductive and, therefore, the presence of shunt capacitors used for power-factor correction or harmonic filtering can cause cyclic energy transfer between the inductive and capacitive elements at the natural frequency of resonance.

At this frequency the inductive and capacitive reactances are equal.The combination of inductive (L) and capacitive (C) elements as viewed from a bus of interest, generally the bus at which harmonic currents are injected by a nonlinear load (source bus), can result in either a series resonance (L and C in series) or a parallel resonance (L and C in parallel).

At either series or parallel resonance, the net impedance is resistive. In harmonic studies, it is essential that the driving-point impedance, as seen from the harmonic source bus (or other bus of interest), be examined to identify the series and parallel resonance frequencies and resulting impedances.

In practical electrical systems, PF correction capacitors are utilized to offset the power factor penalty imposed by the utility. This can create an abnormal situation, because the combination of capacitors and inductive elements in the system can result in either series or parallel resonance or a combination of both depending upon the system configuration.

Usually parallel resonance occurs more often because capacitor banks act in parallel with system impedance (inductive); this can be a matter of concern if the resonant frequency happens to be close
to one of the frequencies generated by the harmonic sources in the system.

The result of a series resonance may be the flow of unexpected amounts of harmonic currents through certain elements. A common manifestation of excessive harmonic current flow is inadvertent relay operation, burned fuses, and overheating of cables, etc.

The result of a parallel resonance may be the presence of excessive harmonic voltages across network elements. A common manifestation of excessive harmonic voltages is capacitor or insulation failure.

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