REQUIREMENTS FOR VACUUM CIRCUIT-BREAKERS UNDER CAPACITIVE SWITCHING DUTY

Capacitive current switching means a specific operating condition for vacuum circuit-breakers. It combines high inrush-currents at connection of a capacitive load with considerable low breaking currents at its disconnection. Though the event of a restrike under recovery voltage stress can cause voltage escalation a reliable dielectric performance is essential. This leads to a reconsideration of the vacuum circuit-breaker design for capacitive switching duties. The behavior of various contact materials and designs are studied during series of tests, representing complete switching cycles at 24 kV system voltage comprising making operation and current interruption followed by the subsequent capacitive recovery voltage. The pre-arcing behavior and the occurrence of restrikes are observed as indication for the alteration of the dielectric condition of the contact system during the test series. In comparison of both kinds of breakdowns a significant difference in the breakdown field strength appeared. Considering the occurrence of restrikes these events can be observed as late as several hundreds of milliseconds after current interruption. In view of the reconsideration of an appropriate contact design the choice of contact material takes centre stage since the tested contact types of various materials show a different behavior in their pre-arcing and restrike performance. The relevant material properties affecting the deterioration of the contact surface during a number of capacitive switching operations and the probability of a breakdown must be taken into account accurately