A High and Low Frequency Hybrid Modular Multilevel Converter for Output Performance Optimization and Its Control Strategy

The traditional modular multilevel converter (MMC) generally adopts a single type of device. The typical Si-based MMC (Si-MMC) has low cost, while SiC-based MMC (SiC-MMC) has high efficiency. In addition, under the same specifications, the price of SiC MOSFET devices is usually 5~8 times that of Si IGBT devices. It is difficult to effectively balance the output performance, cost, and efficiency of the medium voltage MMC. This paper proposes a new MMC (NMMC) topology. Each arm of the proposed NMMC contains a high-frequency sub-module featuring a heterogeneous cross-connect module (HCCM) and a low-frequency sub-module composed of N−1 half-bridge converters. The high-frequency bridge arm of HCCM adopts SiC MOSFET devices, while the commutation bridge arm and low-frequency sub-module adopt Si IGBT devices. Firstly, the NMMC topology employs a high-low frequency hybrid modulation strategy, providing low switching loss of SiC MOSFET devices and low on-state loss of Si IGBT devices. Secondly, the energy fluctuation mechanism and working state of the high-frequency sub-module in different modes are analyzed. A specific capacitor voltage balancing strategy is proposed for the high-frequency sub-module, and a detailed voltage balancing control flow chart is given. Simulation and experiments are carried out to verify the proposed topology, modulation strategy, and voltage balancing strategy. The proposed structure achieves 2N+1 levels. Its high-frequency switching action is mainly concentrated in SiC MOSFET devices, while Si IGBT devices work in the low-frequency state. The total loss at different equivalent switching frequencies and output power is analyzed on the Matlab/Simulink and PLECS joint simulation platform. Taking the rated load power of 1.5 MW and the equivalent switching frequency of feq=20 kHz as an example, the total loss of the proposed NMMC is 37.5%, lower than that of Si-MMC and close to that of SiC-MMC. The proposed topology is 80% lower cost than the Si-MMC and 51.8% lower than the SiC-MMC. The following conclusions can be drawn: (1) The NMMC topology outputs 2N+1 levels with only one high-frequency sub-module per arm, reducing the number of SiC MOSFET devices and overall device cost. (2) The high-frequency action primarily involves SiC MOSFET devices, while the low-frequency action of Si IGBT devices is adopted. Under high equivalent switching frequency and output power, the operation efficiency of the proposed scheme is significantly enhanced. (3) The energy fluctuation mechanism of the high-frequency sub-module DC capacitor is analyzed, and a specific voltage balance strategy is proposed, which can effectively maintain the stability of the high and low-frequency sub-module capacitor voltage. © 2024 China Machine Press. All rights reserved.