Investigation on the 7.5MVA IGCT-based three-level NPC convertor

In this paper an investigation and development on a set of 7.5MVA large power three-level neutral point clamped convertor system is introduced. The convertor include five parts: rectifier, discharge circuit, chopper, dc-link, inverter. The rectifier of the convertor consist of the three phases diode rectifier bridge in series with thyristor rectifier bridge. The discharge circuit consists of one thyristor bridge in series with a resistor. The chopper consists of one IGCT bridge in series of resistors. The dc-link consists of multiple capacitors. The inverter of the convertor used a kind of topology of each power bridge having a separate snubber circuit. The convertor used modularization design to reduce the stray inductance and its bulk. The control system of the convertor adopted the VMIC core system based on VME bus and DSP microcomputer control board to generate the three-level voltage space vector modulation pulse. The protection system of the convertor could take fast and effective protection action aim at overvoltage and overcurrent and other faults occurred. The experiment of the convertor performance showed that the maximum of the direct voltage was 5000 V and the maximum of the output current was 2500 A.