Practical solving of the EMC-problems caused by switching high currents of vacuum breakers in a synthetic test circuit

This paper describes the EMC-problems associated with a synthetic test circuit used for testing of current interrupters and shows solutions to handle the electromagnetic interference. Commonly, synthetic test circuits use a rotating machine serving as an energy source. However, for this study a series resonant circuit was applied composed of a capacitor bank (max. energy content 2.4 MJ) connected to adapted air coils to generate a test circuit with a frequency of 50 Hz. Due to the demand of space of the test arrangement the components of the set-up are placed in three rooms which are connected with bushings and high voltage single-conductor cables. The high test current (up to 25 kA) causes a strong magnetic field which influences the test equipment and other facilities. As a result induced voltages may lead to measuring errors or even destroy electronic equipment. To minimise this influence a lot of different measures were taken to protect the equipment against induced overvoltages and to reduce inductive coupling. For example the measuring signals are transmitted to a shielded chamber by optical fibres and the earth connection is realised without any loop. After redesigning the arrangement interference voltages were reduced to small and acceptable values.