Voltage Ripple Suppression Methods for the Capacitor in Modular Multilevel Converter Submodules Employing a Reversed Pulse Width Modulation-Switching Channel

Abstract

Modular multilevel converters (MMCs) will be widely applied in onboard integrated power systems due to their high levels of electric power output and good-quality sine waveform outputs. However, the capacitor voltage of MMCs fluctuates greatly because the charge–discharge process of the capacitor is continuous when the system is working. In order to reduce voltage ripples efficiently, a capacitor voltage ripple-suppression strategy employing a reversed PWM switching channel is proposed in this paper. For one pair of the upper and lower arm submodules, a switching channel is built. Then, the highest ripple voltage possible can be offset since the voltage fluctuation direction of the upper and the lower arm capacitors would be reversed. In addition, a clamping capacitor is added to the switching channel to further suppress the fluctuation voltage by 78%. Compared to traditional large capacitance suppression methods, only 12% capacitance is used in the proposed method. The reliability and power density of the proposed MMC both increased, and there are no additional losses compared with previous voltage ripple suppression methods. The effectiveness of the proposed voltage ripple suppression strategy is verified by simulation results.

Model Predictive PI Circulating Current Control for Modular Multilevel Converter

“…Controlling circulating current can be obtained through suppressing the second-order harmonic currents [9][10][11][12][13][14][15][16][17][18][19] and minimizing voltage ripple in sub-module capacitor [20][21][22][23][24]. The most commonly used active suppression method is to use the Park transform to design traditional proportional integral (PI) control based on negative sequence second harmonic coordinate transformation [9,10].…”

“…Injecting methods are feasible to suppress circulating currents, in which the second harmonics are injected in three phases to reduce capacitor voltage ripples, since the circulating currents decrease with the capacitor voltage ripples [20][21][22][23][24]. However, lookup tables are required to produce the amplitude and phase for injection currents references.…”