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.

I. Introduction

VFTO phenomena generated during the operation of disconnect switches (DS) in gas-insulated switchgear (GIS) and the threat of VFTO to the insulations of power equipment have been widely investigated since the 1980s, both by tests and by computer simulations; and significant progress has been achieved [1]–[4]. However, in the development of the 1100 kV GIS/hybrid IS used for the ultra-high voltage (UHV) alternating current (ac) transmission project organized by the State Grid Corporation of China, there is still difficulty in the assessment of the VFTO levels in the UHV equipment. The main concern on the insulation safety under VFTO caused inhibition of the DS operation on the hot line.