A new approach to mitigate nuisance tripping of PWM ASDs due to utility capacitor switching transients (CSTs)

Utility capacitor-switching transients (CSTs) are responsible for numerous nuisance trippings of PWM adjustable speed drives (ASDs) and result in costly down times. During a CST event, the DC link voltage of the ASD can momentarily rise to greater than 1.3 p.u. resulting in nuisance tripping. In this paper, a new approach to mitigate nuisance tripping of PWM ASDs is discussed. In this approach, the soft-charge resistor available in most ASDs, is momentarily introduced in the series path of the power flow to effectively damping the CST. Further, damping effect is electronically adjusted. The proposed approach demonstrates a method that can be incorporated within an ASD to electronically damp the oscillatory transient generated during a CST event. The advantages of the proposed approach are: (i) electronic damping for CST event is achieved by low cost modifications to ASD hardware; (ii) it adapts to several utility resonance conditions. The required additional hardware for ASDs to be immune to CSTs, can be viewed as an add-on option. This paper discusses the analysis, simulation and experimental performance on a 480 V, 16 kVA commercial ASD equipment.