Experimental Research on Very-Fast Transient Overvoltage in 1100-kV Gas-Insulated Switchgear

Very-fast transient overvoltage (VFTO) generated during switching of the disconnector in gas-insulated switchgear (GIS) may threaten the safety operation of electrical equipment. As the rated voltage of GIS increases, the difference between the level of VFTO and its lightning impulse withstand voltage gradually decreases. Hence, VFTO becomes the main dielectric stress to switchgear optimization and insulation coordination for ultra-high voltage (UHV) systems. To obtain the characteristics of VFTO in UHV GIS, two full-scale 1100-kV GIS test circuits were established in China. Two types of capacitive sensors were developed to measure VFTO generated in two test circuits. The measurement systems based on both types of the sensors were calibrated by a special calibration device in laboratories and onsite. The measurable frequency range of the systems is 0.1 Hz ~ 100 MHz and their division ratios are stable. More than 2000 tests by switching the disconnector in GIS had been carried out in the test circuits, and VFTO waveforms were captured by the measurement systems. From the test results, the waveform properties, the striking number, the predominant frequency, and distribution of the trapped charge voltage for full-voltage waveforms, the typical characteristics and oscillation coefficient of single-strike waveform, the distribution of VFTO magnitude was analyzed. The mitigation of VFTO by a damping resistor in the disconnector was illustrated. The maximum magnitude of VFTO was 2.27 p.u., and it was mitigated to 1.33 p.u. by a damping resistor of 500 Ω. The achieved characteristics of VFTO in the UHV GIS have become the basis of VFTO simulation and insulation coordination for engineering design.