Instrumentation for high-speed digital recording of switching surges

Three sophisticated monitoring systems were developed and used to make unattended measurements of over 600 switching surges on 33 motors at 11 different utilities. The authors describe in detail one of the recording systems which relies on ECL (emitted coupler logic) digital electronics and high-speed commercial digitizers to measure the multiple, fast-risetime, high-voltage surges typically created by vacuum circuit breaker switching operations. This three-channel monitoring system is capable of digitizing a voltage waveform at 5 ns intervals and has 64 K bytes of memory per channel. The digitizer sampling rate is varied in real time to allow high-speed recording of up to 256 transients. Wideband capacitive voltage dividers with a flat response from below power frequencies to 20 MHz were designed to bolt directly onto the motor terminals and were used to couple the surges to the monitoring system. Results from typical field measurements made with this system on motors controlled by vacuum switchgear are also reported