The impact of switching capacitor banks with very high inrush current on switchgear

Capacitor banks are installed in an increasing number in order to control power quality issues in the transmission and distribution networks. Due to load fluctuation, switching of capacitor banks is normally a daily operation. Although the current to be switched (e.g. the normal load current) is far below the maximum capability of circuit breakers, the transient current upon making (the so-called inrush current) has proven to be a major challenge for circuit breakers. The often very high value of (inrush) current flowing during the closing (pre-) arc between breaker contacts is potentially harmful for the contact system. The IEC circuit breaker 62271-100 standard specifies 20 kA peak while energizing (an) additional bank(s) to those already energized, the so-called back-to-back configuration. It will be demonstrated that three-phase energization with full inrush current cannot be reliably performed in test-circuits. Statistics will be presented on the number of (transmission, distribution) circuit breakers that were tested for this duty. The probability of a late breakdown in vacuum, after energization with inrush current, is rising with rated voltage. Absence of late breakdown of vacuum interrupters after capacitive current switching is especially challenging at higher voltage levels, and is a main barrier to develop vacuum interrupters for transmission voltages having very low probability of re-strike. It was observed that in SF6 circuit breakers, the very intense pre-arc can damage the nozzle, whereas in vacuum circuit breakers, the inrush current arc may deteriorate the dielectric withstand of the switching gap, sometime leading to (late) breakdown after load current interruption. A new measurement method is described to monitor the field electron emission (FEE) current that flows in a pulsating manner in vacuum gaps after current interruption. This measurement system is able to deal (and measure) currents varying as wide as nine decades, from full breakdown currents of several tens of kA to FEE currents of tens of µA). Research tests in full-power test-circuits (following the IEC standard) with a number of prototype vacuum interrupters of different geometry and contact material show a very large range (from micro-amperes to milli-amperes) of current during recovery voltage after load current interruption. It was observed that the load current at longer arcing times reduces the electrical emission activity of the contact surfaces. Large inrush current increases the FEE current. No relationship between steady state FEE current intensity and breakdown probability could be established.