Analysis of Loss and Efficiency for Cascaded H-Bridge Converters with Different Modulation Strategies and Control Methods

Cascaded H-bridge (CHB) multilevel inverter uses multiple H-bridge modules to form a combined power unit to achieve multilevel output voltage. Because the DC power of each sub-module is independently controlled by the rectifier unit connected by the low-voltage winding of the phase-shifting transformer, its capacitance ripple is basically not affected by the output power and frequency. Therefore, it has been widely used in high-voltage high-power motor drives and other high-voltage power conversion systems. In this paper, the junction temperature and loss of the device in CHB converters are modeled by using the curve fitting function of PLECS, combined with the characteristics of the device switching process. The effects of modulation strategies and control methods on the device loss, junction temperature and the overall system efficiency are studied for CHB converter under different operating conditions. The advantages and disadvantages of different control methods are obtained for the rectifier-side CHB with the CHB inverter under the 1600kW power level. According to the junction temperature and loss of the device, different modulation strategies can be employed at different operation conditions. The simulation has been conducted for analysis and validation of the modulation strategies and control methods.