Parallel resonance in electrical power systems occurs when the system inductance is in parallel with power factor correction capacitors at a given location. If the resonance frequency matches or is close to a current frequency created by non-linear loads such as an adjustable frequency drive, significant distortion can result leading to a wide range of problems.

The size of the capacitor and the equivalent impedance at the capacitor location is used to determine the resonance.  If capacitors are switched in steps, meaning the amount of kVAR online can vary, then the resonance can vary.  Similarly, if the source impedance varies, the resonance also can vary. 

Each combination of capacitor steps should be considered when determining the parallel resonance. Changes to the utility short circuit current can affect the equivalent impedance and consequently the resonance. 

Watch this video as I explain how the combination of capacitors and system impedance can affect parallel resonance frequency. 

This video is from my live 32 hour Electrical Power System Engineering Class and also the on-demand class: Análisis de factores de potencia y armónicos