An Internal Thermal Distribution Balance Control Between the Top and Bottom Devices in Modular Multilevel Converter

The reliability of semiconductor devices directly determines the safe operation of the system. However, temperature is one of the key factors that affect the reliability of semiconductors. Since the dc component of arm current in modular multilevel converter results in the unbalanced thermal stress between the top and bottom devices in the submodule (SM), which causes the discrepancy in the lifetime of four devices inside the SM, and seriously threatens the reliability of the system. To solve this problem, an internal thermal distribution balance control between top and bottom devices is proposed in this article. The proposed control method has the advantage of reducing the junction temperature deviation between the top and bottom devices without affecting the electrical characteristics of the ac and dc ports. Moreover, the voltage stress of all devices is reduced especially for active power transfer, which alleviates the risk of overvoltage failure. The SM lifetime under the proposed control is increased by 14.2% based on full-scale simulation. Finally, a downscale experimental platform is built to verify the effectiveness of the proposed control method.