Transients in medium voltage switchgear: measurements, models and coupling

Circuit breaker operations represent a basic and important source for severe transients in the case of medium voltage substations, in contrast to the high voltage situation where corresponding severe transients are caused by disconnecter operations. Thus the power system parameters such as the load conditions or the switchgear components mainly determine amplitude and time behaviour of medium voltage switchgear transients. An extensive set of transient measurements were performed taking into account various types of switching operations, various types of loads and both types of switchgear, i.e., air- and gas-insulated. Results of measurements were compared with theoretically derived transient parameters such as the time behaviour or the relevant frequency contributions. Since the essential effects of such transients have to be considered in connection with secondary electronic equipment the coupling to secondary lines was also investigated in detail. This was done for switchgear configurations with conventional c.t. and v.t. Instrument transformers as well as for configurations provided with a new technology of instrument transformers, the so-called sensors.