When the number of sub-modules (SMs) in modular multilevel converter (MMC) is large, the simulation speed usually slows down significantly. In this paper, we propose a fast electromagnetic transient (EMT) modeling to solve this problem. In this paper, the equivalent MMC model is derived based on Thevein theorem, on the basis of which, control strategy is designed. Nearest Level Modulation (NLM) is selected as modulation strategy of MMC. Output current and circulating current are selected as the controlled variables of MMC closed-loop control. In this paper, after constructing the EMT model, simulation tests are conducted under three operation conditions in MATLAB/Simulink software to verify the feasibility of proposed model. It can be drawn that compared with original circuit, the proposed model accelerates the simulation speed of MMC and performs excellently in output current tracking and SM capacitor voltage balance.

MMC is widely used in high-voltage and high-power applications due to its advantages of high modularization, long maintenance interval and low harmonic content of output waveform. Aiming at the internal power imbalance control problem of three-phase symmetrical DC / AC MMC, an internal power imbalance control strategy Firstly, the bridge arm energy and circulation are decomposed into components. According to the mathematical expression of the bridge arm energy, the positive and negative sequence circulation instructions for controlling the power imbalance in the internal phase of the converter are derived. Then, energy conservation of DC side and AC side, the circulating current command for controlling the power imbalance between phases inside the converter is derived. Under the requirement of output current control.By injecting the above circulating current, The optimal input sub-module is determined based on voltage sorting, enabling precise management of the internal power imbalance within DC/AC-MMC systems. Subsequently, simulations are conducted to internal imbalance control strategy, demonstrating its practical applicability.

Conventional cascaded H-bridge power amplifier (CHB-PA) with N H-bridge power modules (HBPMs) could create 2N+1 level for output voltage at most and the output voltage level directly affects the sinusoidal characteristic and fidelity performance of power amplifier. With the same number of cascaded HBPMs, this paper proposes an asymmetrical cascaded multilevel power amplifier (ACM-PA) in which the voltage of one HBPM is one-third of the voltage of the other HBPMs, reducing the withstand voltage level of the HBPM. A virtual carrier phase shift pulse width modulation (VCPS-PWM) strategy is also proposed for ACM-PA to increase the output voltage level up to 6N-3 with the HBPM number as the same as conventional CHB-PA. Comparative simulation and experimental results are included to validate that the proposed ACM-PA and VCPS-PWM could obviously increase the output voltage level and decrease the total harmonic distortion (THD) of load current, improving the fidelity performance of power amplifier.

Due to its unique topology, modular multilevel converter (MMC) is very suitable for AC/AC conversion applications in distribution network and has broad development prospects. Aiming at the problems of multivariable coupling and multi-objective accurate control of single-phase direct AC/AC-MMC, a model predictive control (MPC) strategy based on current decomposition model is proposed in this paper. Firstly, according to the relationship between bridge arm electrical quantity and input and output, the equivalent current decomposition model of decoupling input circuit, output circuit and circulating circuit is derived. Then, the evaluation function with the output current, input current, circulating current and submodule (SM) capacitor voltage as the rolling optimization objective is established, the rolling optimization process of MPC is carried out according to the equivalent current model and simplified by selecting the optimal bridge arm levels, so as to realize the multi-objective accurate and fast control of AC/AC-MMC. Finally, the proposed MPC control strategy is simulated and compared with the traditional MPC control strategy and PI control strategy to verify the feasibility and effectiveness of the proposed control method.