Is it OK to add power factor capacitors to a power system that supplies motor drives? PQM-103

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).

How does source impedance affect harmonic distortion? PQM_102

If we consider an individual non-linear load such as a 6-pulse rectifier, the input harmonic current distortion is largely a function of effective source impedance. Generally speaking, the higher the total input impedance, the lower the harmonic current distortion.  However, there is a point of diminishing returns. Typical effective impedance values range from about 0.5% (corresponding to about 100% THD-i) to about 5% (corresponding to about 35% impedance). If a system had a total effective impedance of 10%, current distortion would be about 26% THD-I. But, if the impedance was due to AC reactance, then the corresponding ac voltage drop (about 10%) would be too high for this to be a practical solution. While nameplate impedance (%) is based on full rated current, effective impedance is based on actual circuit (fundamental) current.  If a 500KVA, 5% impedance transformer supplies power to a 300KVA motor drive, then although nameplate impedance states 5%, the effective impedance will be about 3% (5% times 300/500) and harmonic current distortion at full load will be about 45% THD-i.  Current distortion, as a percentage of fundamental current, will increase as load is reduced (lower effective impedance), although the rms value of harmonic current will decrease.

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