Interaction of SF6 and Vaccuum Circuit-Breakers with Power Transformers: Characterization of Reignition Behavior and Transient Stress Analysis

Abstract

In this thesis, switching tests on a 125.6 kV power transformer were carried out with a live-tank SF6 and vacuum circuit-breaker to investigate transient voltages occurring in the form of circuit-breaker reignitions from the transformer’s point of view. Theory on basic switching and reignition behavior was elaborated and the problem of fast non-conventional voltage stress was discussed in relation to the power transformer. A special sensor, using a capacitive bushing as a high frequency voltage divider, was conceived, designed, practically implemented and its frequency response determined, in order to record the transient voltages near the power transformer terminals. The reignition behavior of the two circuit-breakers was investigated for no-load transformer and shunt-reactor switching scenarios. Furthermore, tests were carried out to prove the effectiveness of an RC-snubber circuit for mitigating reignitions. Meaningful parameters were defined for the reignitions that occurred during the tests, which were then used for a detailed evaluation. In addition to a statistical evaluation of the reignition behavior of a selected test scenario, the voltage stress caused by the reignitions was finally discussed from the transformer's point of view. Results have shown that reignitions occurred throughout all scenarios, whereas the configuration with the vacuum circuit-breaker for shunt-reactor switching caused by far the highest number of reignitions. All events fall into the category of fast-front overvoltages, with risetimes between 404 ns and 2409 ns. The maximum identified steepness of a reignition is 532.8 kV/µs, with reignition processes generating overvoltages of up to 2.48 p.u. with respect to the applied test voltage. All multiple reignitions have shown a wide range of repetition frequencies with values between 1.2 kHz and 415.5 kHz.

Datum der Bewilligung	2024
Originalsprache	Englisch
Betreuer/-in	Peter Zeller (Betreuer*in)