Electrical-thermal coupling characteristics of pre-insertion resistors in AC filter circuit breaker for UHV grid.

The ultra-high voltage (UHV) AC/DC grid can provide a platform for sustainable power worldwide. To improve the bus voltage quality of the UHV AC system, AC filters are frequently switched into the UHV grid through circuit breakers with pre-insertion resistors. The pre-insertion resistors suppress inrush currents and operate over-voltage during switching. In this paper, we establish a macro and micro model of the pre-insertion resistor based on its temperature coefficient and micro-morphology. We simulate and analyze its electric-thermal coupling characteristics under standard closing and short-circuit faults. After the simulation model and physical comparison analysis, we find that under a usual closing surge, the electric field distribution of the pre-insertion resistor is uniform and undergoes a slight rise in temperature. However, under a short circuit fault, the temperature rise is drastic and exceeds the maximum allowable temperature, causing glassy melt in some parts of the resistor. Considering the volume ratio of each component of the resistor, a two-dimensional cross-sectional simulation model of the resistor is established to simulate the electric-thermal characteristics of the microstructure of the resistor, and insinuates that the current is concentrated in the carbon channel. That is mainly due to the uneven distribution of carbon material and may lead the local temperature to exceed the maximum allowable temperature and damage the resistor.