Dual Harmonic Injection for Reducing the Submodule Capacitor Voltage Ripples of Hybrid MMC

Abstract

Reducing the capacitor voltage ripples of the half-bridge sub-modules (HBSM) and full-bridge sub-modules (FBSM) in a hybrid modular multilevel converter (MMC) is expected to reduce the capacitance, volume and costs. To address this issue, this paper proposes a dual harmonic injection method which injects the second harmonic circulating current and third order harmonic voltage into the conventional MMC control. Firstly, the mathematical model of the proposed control is established and analyzed. Then, the general strategy of determining the amplitude and phase angle of each injection component is proposed to suppress the fluctuations of the fundamental and double frequency instantaneous power. The proposed strategy can achieve the optimal power fluctuation suppression under various operating conditions, which also has the advantage of reducing the voltage fluctuation difference between HB and FB SMs. The correctness and effectiveness of the proposed strategy are verified in simulations in PSCAD/EMTDC. Index Terms-Hybrid modular multilevel converter (MMC); second harmonic circulating current; third harmonic voltage; dual harmonic injection; fluctuation suppression. NOMENCLATURE Symbols: Descriptions Udc Pole-to-pole dc voltage. Uac Amplitude of the AC-side phase voltage. Im Amplitude of the AC-side phase current. ω Fundamental angular frequency.

Optimal Second-Harmonic Current Injection for Capacitor Voltage Fluctuation Reduction in Hybrid MMCs Under Grid-Side SLG Faults

“…A dual harmonic current injection strategy for reducing the SM voltage fluctuation in hybrid MMCs has been proposed in [26]. However, it aims to mitigate the SM voltage fluctuation under the steady-state operation.…”

“…However, it aims to mitigate the SM voltage fluctuation under the steady-state operation. In order to compare strategy in [26] with the proposed strategy. Simulation results of the strategy in [26] and the proposed strategy after an SLG fault occurs in phase A are given in Fig.…”

“…In order to compare strategy in [26] with the proposed strategy. Simulation results of the strategy in [26] and the proposed strategy after an SLG fault occurs in phase A are given in Fig. 10.…”

“…6. It can be calculated that the voltage fluctuation mitigation rates of phases A, B, and C with the strategy in [26] are 4.09%, 6.8% and 32.34%, while the proposed strategy achieves 33.14%, 29.9% and 66%. Simulation results show that the strategy in [26] performs less effective in suppressing the capacitor voltage fluctuation under grid-side SLG faults compared to the proposed strategy in this paper.…”

“…Moreover, these methods do not apply to the hybrid MMC because of its special converter configuration of mixed HB-and FB-SMs. A dual harmonic current injection strategy for reducing the SM voltage fluctuation in hybrid MMCs has been proposed in [26]. However, the work mainly focuses on SM voltage fluctuation mitigation under steady-state operation.…”