New modified staircase modulation and capacitor balancing strategy of 21-level modular multilevel converter for HVDC transmission systems

In this paper, a modified staircase modulation is proposed which enables more efficient use of the DC link voltage. This is very preferable feature for HVDC transmission systems as the 15% lower DC link voltage can be used (lower number of cells). In the second part of the paper the capacitor balancing of modular multilevel converter (MMC) converter is investigated. Balancing becomes a significant issue when the number of the converter levels is high. For instance, with a 401-level MMC converter, the converter comprises 400 cells per arm per phase. This means that 2400 capacitors have to be measured and fed back to the controller for balancing. The system will not be only impractical but also unreliable and complex. A new capacitor balancing strategy is proposed to enhance the system reliability and eliminate the capacitor voltage sensors. A simple example of 7-level converter is presented to explain basic principles of the proposed technique. The proposed method performance is validated by studying a 21-level MMC based VSC-HVDC system.