Tolerance Band Adaptation Method for Dynamic Operation of Grid-Connected Modular Multilevel Converters

The use of modular multilevel converters (MMC) in high-voltage direct current (HVdc) transmission systems has grown significantly in the past decade. The efficiency, cell capacitor voltage ripple, and dynamic performance are three contradictory aspects of the MMC which are related to the converter switching scheme. Previously introduced tolerance band (TB)-based schemes enable efficient and simple control for grid-connected MMCs. This paper addresses the dynamic operation of TB switching schemes by proposing a dynamic boundary setting technique for steady-state operation and a switching scheme scheduling controller for transient fault handling. The performance of proposed methods are validated in a realistic point-to-point HVdc link, modeled in real-time digital simulator where two converters with 512 cells per arm are implemented. Utilizing the proposed methods will enable efficient implementation of TB-based schemes for different operating points, and also a robust transient fault handling.