Bushing for a Liquid Nitrogen Cryostat - A Novel Field Grading Material

Abstract

The number of applications of high-temperature superconductors (HTS) in the power grid increases gradually. In order to achieve an efficient cooling system, HTS are put inside of a cryostat. In many cases the cooling medium can be used as insulation medium as well. With HTS, liquid nitrogen is an applicable cooling medium.

In order to achieve safe and uninterrupted operation, liquid nitrogen has to be tested for its insulation properties in a variety of conditions, such as different pressures, temperatures and even mixtures of different states of matter. One of these properties is the breakdown voltage which is tested with a certain electrode arrangement inside a cryostat. Usually a bushing is required to apply the high voltage to the electrode setup, as cryostats are often made of metal and thus on ground potential. The bushing generally consists of a field grading, a solid insulation and a feedthrough. The testing procedure for a certain setup is often time intensive as it requires a thorough preparation prior to testing, a cooldown phase, the testing time itself and subsequently a reheating phase. Especially the cooldown phase is time intensive and done very slowly and carefully, even with rates as low as 2 K/h. This is due to the fact that the classic electric field grading and feedthrough in the bushing are usually made of metals which exhibit a different thermal contraction constant than the insulation material of the bushing. Thus, a too fast cooldown can lead to delamination between solid insulation and metal parts.

In this work a bushing designed for lightning impulse voltages of up to 200 kV is presented. It features a field grading and feedthrough design which reduces the problem of different thermal contraction constants by using a specially prepared composite material. It allows for cooldown from room temperature to liquid nitrogen temperatures (ΔT ≈ 200 K) in approximately 90 min, which represents a rate of 133 K/h. It has been tested extensively in the high voltage lab with AC peak voltages up to 170 kV and has been subjected to vibrations. Furthermore, partial discharge tests at commissioning have been done to deduce the partial discharge behaviour.