Three-phase Voltage Source Converters Based on Series-Connected Power devices for Medium Voltage Variable Speed Drives

Abstract

A three-phase 3.3kV/400kVA voltage source converter (VSC) based on series-connected power devices for medium voltage variable speed drives is designed in this paper. The series-connected power devices are balanced using an active clamping circuit, which consists of an auxiliary switch and a capacitor. During the switching period, the power device voltage will be clamped at the voltage of ACM capacitor. By balancing the voltages of ACM capacitors, we can balance the voltages of series-connected power devices as well. The balancing circuit does not affect the switching of main power devices, and the power devices in the prototype switch at 10kHz. The power loss of the balancing circuit is minor. In each arm of the prototype, double-sided cooling modules as well as heatsinks are stacked vertically to series connect 6 power devices. Experimental tests on the prototype have been tested under 1kV with 4 power devices connected in each arm, and the voltage imbalance degree of the 4 series-connected power devices is less than 5%. Thermal simulation results show the junction temperature is 79°C under the rated power 400kVA. The power density of the prototype is 4.3MVA/m3.