Field Emission Currents in Vacuum Breakers after Capacitive Switching

Field electron emission current measurements during the recovery phase of vacuum interrupters are carried out to understand the origin of restrikes in case of interrupting capacitive loads. A special shunt is designed for this measurement. To compensate the capacitive current component due to the stray capacitance of the interrupter, hardware (electronic, on-line) as well as software (off-line) processing are developed for different applications. Measurement and analysis of field emission currents (0.01-5 mA) from current zero until the moment of restrike are performed both in an experimental circuit as well as in full-power test-circuits with commercially available vacuum circuit breakers (up to 36 kV rated voltage). In this context, the influence of capacitive breaking currents on the dielectric recovery of the vacuum gap and the contacts surface condition is studied. Furthermore, the influence of pre-arcing at contact closing under inrush currents in the range of several kilo-amperes and kilo-hertz on the field emission characteristics after capacitive current switching is investigated. The number of making operations as well as the amplitude of the inrush current is varied. A clear relation between inrush current during closing and field emission current after interruption was established.