Analysis of Internal Energy in GFL-MMCs and a Decoupled Energy Control Scheme

Modular multilevel converters (MMC) with energy-based control schemes have attracted much attention recently. The majority of the proposed control schemes involve making use of the MMC's internal energy to improve the performance of grid-forming/grid-following (GF-/GFL-) functions in steady state or in preventing disturbance propagation between ac and dc side. Yet, the close coupling between the internal energy-related dynamics of an MMC and its ac side stability characteristics has not been addressed. This paper presents a novel energy-based control scheme that decouples the internal energy-related dynamics of a GFL-MMC from its external energy-related control functions (i.e. dc voltage control, active power control). The proposed control scheme can be applied in MMC-based converters, such as STATCOM, HVDC, etc. Time-domain simulation results are presented and demonstrate functionality of the proposed decoupled energy control scheme. Finally, a frequency-domain model of an MMC w.r.t. the closed-loop output impedance of GFL-MMC is presented, which shows that its external characteristic is not influenced by internal energy-related dynamics.