Energy Harmonic Stability Analysis of Energy-Balancing Control for Modular Multilevel Converter

The energy-balancing control (EBC) plays a crucial role in the proper functioning of the modular multilevel converter (MMC). However, regarding the stability analysis of MMC energy balance, the discussion on internal energy balance is usually ignored, whereas it has been demonstrated in the existing literature that the capacitor energy harmonics may affect the system stability. This article considers the impact of capacitor energy harmonics on EBC and obtains its steady-state time-invariant form. Based on this, small-signal modeling of both dc-side EBC and ac-side EBC of the MMC is conducted, and the stability of EBC is systematically discussed. Through eigenvalue and participation factor analysis, it is revealed that the coupling caused by energy harmonics in sum and differential energy control contributes to the poor stability of EBC. To address this stability issue, parameter selection recommendations for EBC are proposed. By adopting these recommendations, the dynamic performance of EBC can be maximized while ensuring system stability. The accuracy of the obtained conclusions and the effectiveness of the proposed parameter selection recommendations are verified through simulations and experiments.