Output Voltage Modulation for Cascaded H-Bridge Inverters including Thermal and Battery SoC Balancing in Vehicle Traction Applications

Abstract

In traction applications, two-level inverters are commonly used. Multilevel inverters can provide benefits in efficiency and output voltage quality, as known for power system applications. This work analyzes different aspects of cascaded H-bridge multilevel inverters especially for traction applications. The inverter is simulated, considering efficiency and harmonic distortions, for the whole operating region of a permanent magnet synchronous machine. The findings are verified using a representative laboratory setup. Compared to a two-level inverter, the analyzed cascaded H-bridge multilevel inverter allows for significant increases in efficiency as inverter losses are approximately halved. Different switching strategies are analyzed, such as pulse width modulated switching and fundamental frequency switching with selective harmonic elimination. A strategy is developed, combining the advantages of both switching techniques. Additionally, a thermal balancing strategy is proposed, resulting in a reduction of the temperature differences between switches with up to 67%. Furthermore, cascaded H-bridge multilevel inverters allow for novel possibilities of balancing the individual battery packs. Here, different state of charge balancing strategies are proposed and experimentally validated.