Investigation of low temperature and electric stress behaviour of different natural ester liquids

Summary

Natural ester liquid is increasingly used as alternative to conventional mineral oil in distribution transformer insulation. It shows advantageous behaviour in terms of fire safety and environmental sustainability. Natural ester liquid and mineral oil show comparable dielectric strength in homogeneous and slightly inhomogeneous fields. Over the last years, many studies compared natural ester liquid to mineral oil in the commonly used test procedures for dielectric strength to give a large data base for transformer manufacturers. Data is necessary to decide whether adaption of transformer design for natural ester filled transformers is needed or if safe operation under high levels of insulation stress is given with natural ester filled distribution transformers. In order to create necessary data by a field test, Netze BW, a German distribution system operator, installed hermetically sealed distribution transformers with different power classes filled with different natural ester insulation liquids at selected places in the distribution grid where special conditions like low temperatures or heavy load apply. Laboratory investigations presented in this contribution are the first project milestone and will be compared to field test data after one year of operation. Different chemical structure of natural ester liquid leading to higher viscosity is a reason that makes one think of how a natural ester filled distribution transformer behaves at cold start where insulation liquid is not liquid anymore or whether multiple cold starts effect material condition and thus in long terms breakdown voltage. Another question is whether it is valid to talk about “natural ester liquid” compared to mineral oil or if differences between different natural ester liquids affect important liquid properties differently. Results show most significant differences between the investigated insulation liquids for low temperature investigations. High electric stress tests show comparable results for investigated liquids. To transfer results of this study to other natural ester liquids, each liquid needs to be subjected to a selected series of insulation tests.

Additional informations

Authors

S. TENBOHLEN, E. JUNGE, M. KONERMANN

Keywords

distribution transformer, dielectric breakdown, arc discharge, oil insulation, vegetable oil