Development of Full Frequency Bandwidth Measurement of VFTO in UHV GIS

In order to study the characteristics of very fast transient overvoltage (VFTO) in ultra-high voltage (UHV) gas-insulated switchgear (GIS), the authors have developed a full frequency bandwidth measurement system based on a porthole-type capacitive voltage divider. Three kinds of measurement circuits for the measurement system are studied and the influence of cable length on measurement results is analyzed in this paper. The calibration of the measurement system using step response was conducted. The results show that the measurement system of the capacitive voltage divider integrated with an impedance converter has a bandwidth ranging from 0.003 Hz to 300 MHz at least. It can be used to measure the full VFTO during DS operation perfectly. A full-scale UHV DS (disconnect switchgear) is used to establish the test circuit. The measurement system with good anti-electromagnetic interference ability was constructed. Four different measurement locations are chosen on which to install the measurement system. An approximate total of 3000 closing and opening operation tests were conducted. Due to the wide frequency band of the measurement system, it is possible to obtain the full VFTO waveform accurately and conveniently. According to the waveforms, the key parameters of VFTO, such as trapped charge voltage, peak value of VFTO, breakdown characteristic of DS, frequency spectrum of transient waveform, and the overvoltage coefficient could be obtained.