Research On Snubber Circuits For High Power NPC Converter

With the continuous development of high voltage and large current semiconductor power devices,AC-DC-AC voltage type PWM variable frequency speed regulation technology has been greatly developed in the field of steel rolling.Based on the most commonly used 4.5kV/4kA asymmetric IGCT semiconductor devices,rated capacity of single NPC three-level inverter has now reached 10MVA.If a higher capacity converter is needed,the total capacity of 10MVA converters for multiple parallel can achieve 20MVA,30MVA and even higher.In this way,the implementation of the project is simple,but the cost is high,it can not meet the market demand of the intermediate capacity and waste the system resources.In order to meet the need of intermediate inverters,snubber circuit for IGCT and IEGT were studied to design snubber circuits which can improve the capacity of NPC three-level converter.Based on the PSIM simulation software,using the curve fitting method to build a functional model of IGCT and IEGT semiconductor devices.Using these models,commutation process and loss distribution of NPC three-level IGCT converter and IEGT converter snubber circuits were studied.The loss of converters using two different power devices is compared and analyzed.The effect of different positions of the absorption capacitor is analyzed by single device simulation test.According to the simulation results,the reason why the snubber circuit can reduce the loss of the device is analyzed theoretically.Finally,this paper proposes the design idea of classical clamping RLCD with additional RC snubber circuit to improve the capacity of converter.For engineering practice,RLCD+2RC snubber circuit which can effectively improve the rated capacity of NPC three-level IGCT converter is designed.