Phase-Unsynchronized Power Decoupling Control of MMC Based on Feedback Linearization

The traditional power decoupling control of modular multilevel converter (MMC) is based on the phase synchronization by the phase-locked loop (PLL) without considering MMC intricate inner dynamics, which jeopardizes the power decoupling performance and system stability. To improve the performance, a nonlinear phase-unsynchronized power decoupling control for MMC is proposed without PLL. At first, the developed model illustrates that the MMC power coupling is affected by the ac current as well as the MMC capacitor voltages. Then, through the input-output linearization method, the MMC output powers are decoupled without the phase synchronization. Involving integral control elements, the dynamics of MMC output powers are designed as second-order inertial systems so that the control parameters can be easily determined. To verify the developed model, a comparison of MMC dynamics with the proposed control is conducted between the numerical solution in MATLAB and the EMT simulation in PSCAD/EMTDC. Moreover, four cases in simulations and four cases in the experimental prototype demonstrate the desired phase-unsynchronized performance and power decoupling of the proposed method compared with the conventional $dq$ control. At last, the system zero dynamic stability is investigated and the influences of MMC control parameters, ac-side voltage, dc-side voltage, and operation points are analyzed to evaluate the stability performance of the proposed method.