Measurement of field emission current during switching of capacitive current in vacuum

To check the dielectric behavior of vacuum interrupters after capacitive current interruption and find the reasons for possible restrikes, measurements of field emission current during the recovery voltage is performed. An experimental set-up is built up for current interruption test and dielectric measurement. This set-up allows the measurement of very small field emission current in the range of some 10 μA to some mA. A special measurement current shunt is developed for this measurement. To measure the field emission current compensation of the capacitive component of the current which flows due to the stray capacitance of the vacuum interrupter is necessary. In some cases the capacitive component of the current is even much higher than the field emission current. To compensate the capacitive component hardware (electronic, on-line) compensation as well as software compensation (off-line) is developed. Furthermore, with the help of recovery voltage and field emission current measurements for every half cycle of the recovery voltage the Fowler-Nordheim-Equation (FNE) is used to evaluate the field enhancement factor and the emitting areas, which are indicators for the surface condition of the contacts and possibly for the dielectric behavior of the interrupter.