Effective Transient-Free Capacitor Switching (TFCS) for Large Motor Starting on MV Systems

Capacitor bank switching on medium voltage power systems has been in practice for nearly half a century. However, capacitor bank switching causes high inrush currents and voltage transients [as high as 3.0 per unit (p.u.)] in the power distribution system potentially damaging electrical equipment, impacting the system reliability and power quality. Additionally, capacitor switching transients affect the life of the capacitor itself and can cause damage or sudden failure. Pipeline substations are often located in remote areas with insufficient short circuit capacity available on the distribution system to start a large medium voltage induction motor. In many such instances, the end user often ends up using a medium voltage soft-starter, a higher rated MVA transformer in conjunction with a reactor start, or a low inrush motor with less than ideal motor efficiency. This paper introduces a transient-free synchronous capacitor switching system for large motor starting on MV power distribution systems that can be used with a traditional capacitor bank system while restricting the voltage transient to less than 1.05 p.u. during capacitor switching. This solution is economically efficient and more reliable compared with other motor starting methods. Alternatively, in lieu of a low inrush, low-efficiency induction motor in some applications, a motor start solution of transient-free capacitor switching, and capacitor bank with regular induction machine can be applied more economically.