On Partial Discharge/Corona Considerations for Low Voltage Switchgear and Controlgear

Electrical field and partial discharge (PD, corona) considerations usually are not in the foreground with low voltage switch- and control gear as per the product standards of IEC, UL and CSA. The design is mainly based on creepage and clearance dimensions, material categories described through CTI and RTI values. IEC product standards refer to the subject of insulation coordination as per the horizontal IEC 60664-family in their normative references. These standards, in principle, require PD measurements for rated voltages exceeding 500 VAC at room temperature. IEC 60664-2 gives some rules for consideration and electrical dimensioning of solid insulation. Miniaturization accompanied by increased power density and raised operational voltage as well as (non-sinusoidal) power supply generated by pulse width modulated (PWM) converters require to reconsider the relevance of electrical field stress in low voltage equipment. With variable frequency drive (VFD) fed motors reflected voltage waves were observed, causing partial discharge effects and insulation damages. Meanwhile motors and cables are available as VFD-proof versions. PD measurements on commercial manual motor controllers with sinusoidal voltages at both room temperature and a more severe elevated temperature related to the maximal operational temperature of the internal components (bimetal trip units) are presented. Conclusions are drawn and recommendations for future designs and testing are proposed.