AC Impedance Modeling of Modular Multilevel Converters Considering Different Control Modes: Similarities and Differences

This paper presents an AC impedance modeling method of modular multilevel converter (MMC) considering different control modes, and its accuracy is verified by simulation. Then the similarities and differences of the MMC impedance model under different control modes are compared. It indicates that low frequency characteristics of MMC impedance are significantly different and the high frequency characteristics are basically the same under different control modes. When studying the low frequency oscillation (or stability) of the MMC-HVDC, outer control loop cannot be ignored; and the impedance model can be simplified to current inner-loop control impedance model when studying the high frequency stability, which is much simpler but still accurate enough.