Tree initiation characteristics of epoxy resin in Ln2 for superconducting magnet insulation

The superconducting magnet system is the key part of international thermonuclear experimental reactor (ITER) device which ensures the feasibility of fusion energy application. Epoxy resin has been used as the adhesives and electrical insulation in the magnet system. Since ITER device operates in the liquid nitrogen environment and is driven by a pulse power, the insulation made of epoxy resin faces the challenges of extremely low temperature and pulse voltage. This paper investigated the influence of low temperature on the electrical tree characteristics in epoxy resin. Positive pulse voltage with the frequency of 400 Hz was applied on the needle-plate geometry electrodes. The amplitudes of the pulse voltage were 10, 12 and 14 kV. In order to simulate the practical defects, a metal needle was inserted into the samples and the distance between the tip and plane was 2 mm. The experimental temperature ranged from 30 to -196 °C. The experimental results indicate that the low temperature have a significant influence on the electrical tree characteristics. Different tree structures in epoxy resin are observed with the changing temperatures. The density of the tree branches increases as the temperature decreases, while the tree growth rate presents an opposite trend. It is also revealed that the time to breakdown become long under the low temperature. However, once the breakdown occurs, a wide breakdown channels will be left in epoxy resin, which means great destruction to the electrical insulation. In addition, when the samples are under the same ambient temperature, the probability of tree initiation shows a significant increase and electrical trees grow faster in the following period with application of higher pulse voltage. The higher-energy charges may be responsible for the high growth rate.