Adjustable speed motor drives (ASDs. VFDs, VSDs) are
non-linear loads that produce harmonic current distortion during their normal
operation. Harmonics are currents flowing at frequencies which are multiples of
the fundamental frequency (ie: 50Hz, 60Hz).
Since capacitive reactance (ohms) decreases as frequency increases,
capacitors provide a low impedance path for harmonic currents. This can result
in the power factor capacitors absorbing significant harmonic currents and
becoming overloaded, leading to premature failure. Capacitance also combines with system
impedance (ie: power transformer) to form a tuned circuit. If this circuit is tuned near one of the
harmonic frequencies present, then a resonance condition occurs and either
harmonic current or voltage can be amplified.
When adding power factor capacitors to a power system shared
by both across the line started motors and adjustable speed drives, there are
two choices. 1) apply harmonic filters
to the drives, sufficient to suppress the system harmonic voltage
distortion to negligible levels, 2) use a detuned capacitor system (consists
of a detuning reactor in series with capacitor to force the tuning point below
the lowest order harmonic).
