A Modified Clamp-Double Submodule (CD-SM) Based Modular Multilevel Converter (MMC) With Reduced Component Count for HVDC Application

Recent grid codes require a high voltage direct current (HVDC) converter station remains connected and provide reliable operation under various faults. An improved clamp-double submodule (CD-SM) is introduced in this article, which belongs to the modular multilevel converter (MMC) topologies proposed for high voltage direct current (HVDC) systems. The proposed submodule (SM) topology features a reduced number of control switches, lower converter level faults, and DC fault-blocking capability compared to the conventional submodule topologies. A CD-SM consists of five $IGBT$s, two diodes, and two floating capacitors, where capacitor voltages are maintained according to the binary geometric propagation (GP) ratio which enables it to generate a maximum four-level output voltage. The hybrid pulse width modulation (PWM) technique is used to generate desired switching pulses for a converter and the associated voltage balancing control technique maintains the power exchange between the converters. In this article, the dimensioning of the proposed MMC converter, and its performance under different fault conditions is discussed in detail. Further, a quantitative comparison with other submodule topologies in terms of dc fault-blocking capability, output voltage level, and device count is discussed. Simulation in MATLAB/Simulink and their results validate the effectiveness of the proposed topology for MMC based HVDC system.