Electrical tree characteristics of epoxy resin under AC voltage at 77 K

Abstract

The insulation structure which is mainly composed of epoxy resin is a key part of the superconducting magnet system. Due to the special operating environments, superconducting coils must address the challenges of cryogenic liquids and various kinds of high voltages. In this paper, the electrical tree characteristics of epoxy resin at 77 K were investigated. An experimental setup with a partial discharge (PD) device and a cryostat filled with liquid nitrogen (LN2) was set up. Optical microscopy and confocal Raman spectrometer were applied to investigate the accumulation of degradation products within the tree channels. Each group of samples was tested at a range of AC voltages (50 Hz) from 8 kV rms (root mean square) to 20 kV rms and the PD experiments were carried out at both room temperature and 77 K. The results indicated that the color of electrical trees was darker and the tree structures were more complicated at 77 K compared with those at room temperature. The weak reaction between oxygen and the degradation products within the tree channels was responsible for the darker tree channels at 77 K. The density of electrical trees increased and the damage to materials caused by electrical trees was more serious at 77 K. Meanwhile, the tree inception probability decreased and the tree growth was inhibited obviously at 77 K. It’s also revealed that the degradation products within the tree channels were confirmed to be graphitic carbon based on the corresponding Raman spectra. The size of graphitic domains within the tree channels at 77 K was more than 2 times of that at room temperature, which contributed to the tree growth acceleration in the later growth stage at 77 K.