Axial magnetic field strength needed for a 126kV single-break vacuum circuit breaker during symmetrical current switching

The objective of this paper is to investigate how strong axial magnetic field (AMF) strength is needed in test duty T100s for a 126 kV single-break vacuum circuit breaker (VCB) as interrupting a rated short circuit current of 40 kA rms in light of arcing contact gap and arc duration. The AMF was generated by a pair of 2/3 coil AMF contacts. First, test duty T100s of the 126 kV single-break VCB were carried out 20 times with arcing times from 1.1 ms to 20.1 ms at 40 kA rms. With a pre-set opening displacement curve of the movable electrode, the contact gaps at arc extinguishing increased from 5 mm to 51 mm according to the arcing times from 1.1 ms to 20.1 ms. Then the AMF distributions of the 126 kV single-break VCB at different contact gaps were simulated to analyze the interrupting experimental results. The results showed that the shortest arcing contact gap window exists between 22 mm and 31 mm and the longest arcing contact gap is 49 mm for a successful current interruption. The shortest arcing contact gap window corresponded to the maximum AMF per kilo-ampere of the breaking peak current from 7.5 mT/kA to 8.6 mT/kA. And the longest arcing contact gap corresponded to the maximum AMF per kilo-ampere of the breaking peak current of 5.7 mT/kA.