THE BREAKDOWN PROCESS OF SYNTACTIC FOAM UNDER AC STRESS AT CRYOGENIC TEMPERATURE

Summary

Liquid nitrogen (LN2) based insulation systems for superconducting power components are state of the art, but electrically feature some disadvantages. One disadvantage is the significant reduction of the breakdown field strength, if thermal bubble formation within the LN2 occurs during operation. This paper deals with syntactic foam as a solid insulation composite, which can be an alternative to LN2 based insulation systems for superconducting power components. Syntactic foam consists of a polymeric matrix and embedded hollow microspheres with mean diameters of several 10 µm. The embedded hollow microspheres feature a reduced thermal contraction, a lower density and a lower relative permittivity compared to the pure matrix material. Several syntactic foams are investigated regarding their breakdown field strength under AC voltage stress and within a temperature range of 65 – 77 K, in which superconducting power components commonly operate. Within this temperature range LN2 is in its liquid state of aggregation. To form syntactic foam epoxy resin (ER) and unsaturated polyester resin (UPR) serve as matrix materials. The hollow microspheres used in these investigations are made of glass and silanized glass. By comparing the results of measurements at liquid nitrogen temperature (LNT) with those at ambient temperature (293 K), the influence of temperature on the breakdown field strength of syntactic foam can be estimated. Additional optical investigations of brittle fractures performed with a laserscanning microscope give information about the mechanical state of syntactic foam at LNT. By means of the investigation results a model of the AC breakdown process of syntactic foam at LNT is developed.

Additional informations

Authors

Pang Lei, Yang Xiaolei, Guo Can, van de Ploeg