Surge Capacitor – The Forgotten Hero

Abstract

In power plants, nearly 90% of installed rotating equipment consists of AC induction motors. Although these motors are generally provided with redundancy, their failure leads to increased maintenance costs, higher spare inventory requirements, and potential operational risks. In most installations, overvoltage protection is limited to surge arresters. While surge arresters effectively clamp peak overvoltages, they do not provide adequate protection for motors connected through long cable runs and operated by Vacuum Circuit Breakers (VCBs). Switching operations of VCBs generate steep-fronted voltage transients that can cause severe electrical stress on motor windings, leading to inter-turn insulation failure. The mitigation of such voltage surges can be significantly improved by the coordinated application of surge capacitors along with surge arresters. This combined protection scheme limits both the peak overvoltage and the rate of rise of voltage (dv/dt), thereby reducing insulation stress, suppressing partial discharge activity, and enhancing the reliability and service life of motors.

Introduction

The text discusses the vulnerability of VPI-insulated medium- and high-voltage motors to steep-fronted voltage transients caused by lightning, long feeder cables, and vacuum circuit breaker (VCB) switching. VCB phenomena such as current chopping, re-strikes, and pre-strikes generate high dv/dt surges that are amplified by cable reflections and lead to non-uniform voltage distribution across stator windings. These stresses cause partial discharge, localized electric field intensification, insulation degradation, inter-turn short circuits, overheating, and ultimately catastrophic motor failure, significantly reducing motor service life.

The text explains that surge arresters, commonly based on metal oxide varistors (MOVs), are effective at clamping high-energy overvoltages and diverting surge energy to ground. However, they have critical limitations: they do not control dv/dt, operate only after a voltage threshold is exceeded, and allow steep-fronted, high-frequency transients to reach motor terminals. As a result, surge arresters alone cannot adequately protect turn-to-turn insulation in motors switched by VCBs and connected through long cables.

To address this gap, the text introduces surge capacitors as a complementary protection device. Connected in shunt at motor terminals, surge capacitors provide instantaneous, continuous protection by limiting dv/dt and absorbing high-frequency surge energy. They reduce wavefront steepness, lower peak voltage, and minimize insulation stress by temporarily storing surge energy, as governed by I=C dv/dtI = C \, dv/dtI=Cdv/dt. Surge capacitors are maintenance-free, long-lasting, environmentally friendly, and typically rated between 0.1 μF and 0.5 μF for 50/60 Hz systems.

Optimal motor protection is achieved through coordinated application of surge capacitors and surge arresters: surge arresters limit surge magnitude, while surge capacitors suppress fast transients and control dv/dt. The text concludes by outlining key characteristics of a high-quality surge capacitor, including PCB-free dielectric fluid, hermetically sealed construction, high partial discharge inception voltage, and low-loss polypropylene elements. Together, these devices provide comprehensive protection for modern high-voltage motor insulation systems.

Conclusion

Conventional protection using surge arresters alone is effective in limiting peak overvoltages but does not adequately control dv/dt or suppress high-frequency oscillations at motor terminals. Consequently, turn-to-turn insulation remains exposed to damaging electrical stress. The coordinated application of surge capacitors in conjunction with surge arresters provides a comprehensive protection scheme. Surge capacitors continuously reduce dv/dt and wavefront steepness by absorbing high-frequency transient energy, while surge arresters safely divert high-magnitude surge energy to ground. This combined approach significantly minimizes insulation stress, suppresses partial discharge activity, and enhances the reliability and service life of motors operated through long cables and VCBs. Therefore, the integrated use of surge capacitors and surge arresters is strongly recommended for medium- and high-voltage motor installations in power plants and industrial facilities to prevent inter-turn insulation failure, reduce maintenance costs, and improve overall system reliability.

References

[1] Marx Electric Inc., Technical Resources on Surge Protection Devices. [2] Eaton Corporation, Surge Protection and Motor Protection Application Guides.

Copyright

Copyright © 2026 Kumar Gautam. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.