Study Of Hybrid Clamped Five Level Converter In High Voltage High Power Application

Multilevel converter is one of the main solutions for the high voltage and high power AC driving system nowadays. This paper focuses on the hybrid-clamped five level back to back converter. In the view of the high voltage application requirements, parameter calculation for the main circuit, analysis and structure design, control and protection analysis are completed. Moreover, a inverter phase arm has carried out and tested in this project.Firstly, based on the1.2MVA/3.3kV industrial needs, calculation for the three phase hybrid-clamped five level back to back converter, such as frequency, DC voltage, capacitance and so on, is finished. As for the capacitance calculation, the design method by using large bus capacitors and small clamping capacitors is adopted to minimize the volume of converter’s bridge arm. And in order to meet the requirement of the output voltage, separated heat sinks are used to push the DC bus voltage up to6kV.Secondly, this paper analyzes the characteristics of the converter, and finds the "high frequency current decoupling character", that one bridge can be separated into switched capacitor circuit and NPC four level circuit. Ansoft Q3D simulation software is used for the lamination design of the connecting line in each part, and finally feasible stray inductances are achieved.Next, this paper also presents a suitable three-level-control scheme for the back to back system. And synchronization, communication, and realization for different controller are completed.Finally, one inverter phase is built and tested under2400V DC bus/100A load current. The double pulse test is applied to the NPC four level circuit, and several difference between realistic commutation circuit and the ideal one is discovered."high frequency current decoupling character" is confirmed in the inverter test with inductance load.