Operating Area of Modular Multilevel Converter Station Considering the Constraint of Internal Dynamics

The study of operating area of modular multilevel converter (MMC) plays an important role in the parameter design of modular multilevel converter based high voltage direct current (MMC-HVDC) system. This paper studies the influence of AC system parameters and internal dynamics of MMC on the operating area of MMC. Firstly, a steady-state model connecting with AC system is developed based on a single-terminal system, and a new method to calculate the maximum value of electrical quantities with non-sinusoidal waveforms is proposed. Then, based on the point-scanning method, the operating area of MMC is obtained when the constraints of internal dynamic and external variables are considered. The influences of circulating current suppression control (CCSC) strategy and short-circuit ratio (SCR) of AC system are studied respectively, and the main constraints to confine the operating area are determined. Finally, the boundary control is designed for power over-limit situation. Simulation results in PSCAD/EMTDC verify the validity and accuracy of the proposed calculation method and the boundary control.