Dual-threshold graded desaturation protection for IGBTs in VSC-HVDC converter valve submodules

A dual-threshold graded desaturation protection strategy is proposed to address differences in the short-circuit current evolution and protection requirements of insulated-gate bipolar transistors (IGBTs) under different short-circuit conditions in converter valve submodules for voltage-source-converter-based high-voltage direct-current (VSC-HVDC) transmission. The high-threshold channel uses independently configured detection-circuit parameters and a shortened blanking time to provide fast protection under low-inductance short-circuit conditions. The low-threshold channel uses a trip level determined from the device output characteristics and requires the detection voltage to remain above this level for a specified duration to identify slowly rising overcurrents under high-inductance short-circuit conditions. A first-order dynamic model of the detection voltage is established to derive coordination constraints among the detection threshold, response time constant, and confirmation delay. Experiments are conducted for three protection configurations: conventional high-threshold protection, optimized high-threshold protection, and low-threshold protection. The results show that the optimized high-threshold protection reduces the protection operating time by approximately 47% compared with the conventional scheme, while the low-threshold protection effectively clears faults during slowly rising overcurrent conditions.