Modulation-Compensated Power Synchronization Control for Modular Multilevel Converters with Enhanced Performance under Varying SCR

Power synchronization control (PSC), one of the common grid-forming (GFM) controls, is an emerging grid support technique for converters in the converter-dominated power system. However, its application to modular multilevel converters (MMCs) with dynamic grid conditions and varying grid strength, exposed some instability issues. This article studies the essence of the MMC internal voltage coupling and the degree of influence on the reference voltage under different grid short-circuit ratios (SCRs). It is found that the key reason for instability is that the submodule (SM) capacitor voltage modifies the reference voltage through the nonlinear switching process and eventually generates the voltage error, up to 61.3% with high SCR = 25. Based on the above findings, an easy-to-implement modulation compensation is proposed to enhance the PSC-MMC stability in changing SCR scenarios. A simplified selection scheme of the compensation coefficient, which is affected by the practical main circuit parameters is presented according to the established small-signal model (SSM). Both the simulation and the experimental results verify the effectiveness and accuracy of the proposed method.