Improving the Stability of Standalone MMCs by Shaping the AC Side Impedance Using Insertion Index Compensation

Due to the significant coupling of the internal dynamics on the ac-side in a modular multilevel converter (MMC), its terminal characteristics are quite different from a conventional two-level voltage source converter (TL-VSC). Thus, the stability analysis and controller parameters design of the MMC without ignoring internal dynamics must adopt complicated modeling approaches, like the harmonic state-space (HSS) method, which doesn’t have an intuitive insight. Therefore, to simplify the modeling process and give a clear physical meaning, this paper proposes a decoupling method with ac-side impedance shaping by feedforwarding the capacitor voltages to compensate the arm insertion indices, which can also improve the stability of the MMC. Consequently, the ac-side impedance characteristics of the MMC are similar to that of the TL-VSCs, and the stability and controller parameters can be design-oriented by using a simple averaged state-space model. Finally, the effectiveness of the theoretical analysis and correctness of the proposed approach are verified by time-domain simulations in PSCAD/EMTDC.