Relevancy of IEC Requirements Related to Switching Cable and Line Charging Currents for Medium Voltage Vacuum Circuit Breakers (VCB)

An analysis of switching processes at interruption of lightly loaded urban and rural networks with the aid of VCB has been provided. It has been found that typical loading of the distribution feeders result in virtual absence of recovery voltage shift being the main reason for non-sustained disruptive discharges (NSDD). Truly unloaded situation is interruption of section located between neighbouring points of isolation in cable line, but 95% of lengths of these sections are less than 3km that is substantially shorter than ~50km lengths considered by standard. This difference results in different duration of current pulses relevant for VCB closing in real and test cases. Estimation of the probability of contact micro welding has shown that in real case probability of contact welding is much lowers which results in lower probability of NSDD. On the basis of the above irrelevancy of the current IEC requirement with regard to general purpose VCB has been stated. Alternative test procedure for special purpose VCB has also been offered