Frequency-Adaptive Energy Control for Grid-Forming MMCs Under Unbalanced Conditions

This paper presents an energy control strategy for a Modular Multilevel Converter (MMC) which is suitable for grid-forming operation under unbalanced conditions. Frequency-adaptive notch filters based on Second Order Generalized Integrators (SOGIs) are applied for extracting the average components of the energy sum and energy difference feedback signals of each phase. The frequency-adaptivity is provided by utilizing the internal frequency defined by the synchronization mechanism of the grid-forming control. This approach ensures applicability for islanded operation and other weak grid conditions where large frequency variations can occur. A comparative analysis between the frequency-adaptive filtering and an implementation based on conventional fixed width moving average filters (MAF) is presented. The SOGI-based implementation allows for reducing the delay in the energy feedback signals compared to the MAF-based reference case, thus ensuring a more damped response. The performance of the proposed energy control strategy is comprehensively demonstrated by time-domain simulations of an HVDC terminal and by experimental testing on a 50 kVA MMC prototype. Both grid connected and islanded conditions are considered together with 5 different strategies for controlling the negative sequence currents during unbalanced conditions.