Research of thermal protection characteristics for circuit breakers considering nonlinear electro-thermal-structural coupling

Abstract

Thermal protection is designed in the circuit breaker to interrupt current flow when fault current is detected. To investigate the thermal protection characteristics, a nonlinear electro-thermal-structural model considering the nonlinearities of bimetal deformations have been developed. The coefficients of thermal expansion (CTE) have been measured at different temperatures. Measurement results show that the CTE of thermal sensitive layer of bimetal demonstrates temperature-dependent variation when subjected to different temperatures. The CTE increases from 13.2 ppm/°C at 20 °C to 25.3 ppm/°C at 380 °C, which is equivalent to 91.7% variation in the tested temperature range. Based on the CTE measurements, three different overload protection tests have been conducted according to the requirements of IEC 60898 standard. Simulation results show that the protection unit can be effectively triggered within specified time range under different testing conditions. Comparisons between simulations and experiments show that model predictions with linear CTE assumption only describe expansions at specified temperature. When temperature changes with time, the proposed nonlinear simulation framework is capable to prediction both deformations and temperature changes with higher accuracy.