Overvoltage Estimation by Stray Inductances During Turn-off of a 500 kV/25 kA DC Circuit Breaker

Metal oxide varistors (MOVs) are usually applied to clamp the voltage across a dc circuit breaker (DCCB) after fault current elimination. This letter focuses on the transient process of a DCCB turn-off and has three contributions. First, it reveals that snubber circuit inductance, the internal and external stray inductances of an MOV could cause a significant overvoltage across a DCCB, which exceeds 47.5% of its nominal clamping voltage. The overvoltage mechanism is clearly analyzed based on the working principle of a DCCB, providing theoretical equations to estimate the overvoltage magnitude. Second, a nonlinear model of an MOV is proposed to simplify the calculation and provide an accurate estimation result. Third, a new arrangement of series submodules with discrete MOVs is proposed to suppress the overvoltage. A hybrid 500 kV/25 kA DCCB prototype with series insulated gate bipolar transistors (IGBTs) is implemented to validate the proposed analysis. The series-IGBT rating voltage is 1440 kV, and the MOV with a nominal clamping voltage of 800 kV is used to protect IGBTs. However, a 25 kA turn-off experiment clearly shows that the overvoltage increased to 1203 kV by an external stray inductance of 39.5 μH and di/dt of 1480 A/μs. The relative error between measurements and simulations is within 3%. Besides, simulations on the new arrangement show the overvoltage is decreased by 23.4% than before.