Hierarchical Direct Power Control of Modular Multilevel Converter for Tundish Heating

In this paper, the full bridge-based modular multilevel converter (MMC) is discussed for the application of tundish induction heating power supply (TIHPS). TIHPS features three-phase to single-phase ac/ac conversion and variable output frequency, and has intense demand for the power transmission stability and output current control precision. MMC is suitable for this application and especially the bulky and costly transformer and ac filter in the conventional topology can be dispensable. Therefore, it is interesting to investigate the applicable control method for MMC-TIHPS. According to the instantaneous power theory, a hierarchical direct power control based on the improved deadbeat current control is presented to assure the energy balance inside and outside MMC-TIHPS and reduce the sensitiveness of the current controller to parameter uncertainties. The proposed control strategy dispenses with the phase-locked loop, and does not exist the coupling among control variables of internal and external currents. Then, parameters design of energy controller is studied for stability analysis. Finally, MMC-TIHPS and proposed control method are verified by experimental results of a down-scaled physical prototype.