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.
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