Study on Transient Overvoltage and Multi-Point Secondary Discharge in Ultra-Long GIL During Power Frequency Withstand Test

The Gas Insulated Transmission Line (GIL), known for its excellent environmental adaptability and minimal ecological impact, has emerged as the inevitable choice for power transmission in extreme environments such as crossing rivers, snowy mountains, uninhabited areas, and natural reserves. However, as the application scope of long-distance GIL continues to expand, the issues of multiple partial discharge induced by breakdown traveling waves and difficulties in pinpointing locations are becoming increasingly prominent, significantly impeding the efficiency of on-site operation and maintenance. This study focuses on the 3.6 km 550 kV long-distance GIL planned for the Anhui section of the Ganze transmission project. We established an ATP-EMTP simulation model for the power frequency withstand test to investigate the transient wave propagation characteristics of overvoltage at different locations of the GIL breakdown. We evaluated the overvoltage waveforms and insulation threat levels of key components such as the end sleeves and voltage transformers of the long-distance GIL, and proposed targeted risk prevention and control techniques based on frequency matching. The results indicate that during a GIL breakdown, transient overvoltages with peak values reaching up to 2.24 p.u. (equivalent to $\mathbf{1 0 0 7. 2 ~ k V}$) occur at the tail-end bushing due to the reflection and refraction of transient waves along the longdistance GIL, leading to a higher probability of multi-point secondary discharges. After installing a resistive-capacitive filter structure at both ends of the long-distance GIL equipment, the voltage peak at the GIL tail sleeve can be reduced to 1.57 p.u. (equivalent to 702.79 kV), with an average decrease of 30.22 %. Different resistive-capacitive matching configurations result in varying degrees of attenuation in transient overvoltage peaks, playing a certain inhibitory role in reducing breakdown-induced secondary discharge. The findings of this study have significant implications for on-site protection and efficiency enhancement in long-distance GIL AC withstand tests.