This article presents a modified modular multilevel converter (MMC) equipped with the middle-cells and control algorithm for medium voltage applications. It is intended to reduce the submodule (SM) capacitor voltage ripples by the third-order harmonic voltage injection (THVI) and second-order harmonic current injection (SHCI) without the common-mode voltage injected on the ac-side. On the one hand, using the proposed algorithm, the independent power degrees of freedom can be produced to eliminate the dominant fundamental part in arm powers, which can dramatically reduce the SM capacitor voltage ripples. On the other hand, the smaller amplitude of the SHCI can be obtained, which decreases the power losses. Meanwhile, by properly controlling the proposed middle-cells of the modified MMC, the compensated parts can be produced to eliminate the common-mode voltages caused by the THVI. Additionally, the arm power analysis of the modified MMC under different power factors is conducted, and the obtained results are discussed. The mathematical models of the SM capacitor voltages for the half-bridge based SMs and the proposed middle-cells in the modified MMC are also analyzed. Finally, the effectiveness of the theoretical analysis of the modified MMC is verified by simulation and experimental results.

DC microgrids have attracted more and more attention with its universality, high efficiency and potential application market. However, DC fault protection is still a challenge to the development of DC microgrids. In this paper, a novel bidirectional DC solid-state circuit breaker based on coupled inductor is introduced. It can realize bidirectional energy flow and bidirectional isolation and interruption of short-circuit faults with fewer components. The working principle of the topology and the working process under different conditions are described, and some properties of the novel circuit breaker are explored, including minimum fault current ramp rate and load step analysis, which provides a reference for the selection of component parameters. Finally, the feasibility and performance of the novel bidirectional circuit breaker is evaluated by simulations and experiments.

Interleaved LLC resonant converters are widely used in various fields. However, interleaved LLC converters under Pulse Frequency Modulation (PFM) will lose the regulation of individual phases, causing a load sharing problem. Existing load sharing solutions have limitations; for example, phase shedding and current sharing cannot be realized at the same time. This paper proposed a novel current sharing method for interleaved full-bridge LLC resonant converters. Based on Zero-Vector-Injection, the voltage applied to the resonant tank is controlled to compensate for the difference in gain caused by component tolerance. The modulation strategy is proposed to maintain soft switching after Zero-Vector-Injection, and the phase shedding technique is also used to improve the efficiency at a light load. The detailed theoretical analysis and implementation method are proposed and validated using simulations. Experiments are also carried out to verify the feasibility of the proposed strategy based on a 2-phase 1.8 kW prototype.

The electromagnetic wave propagation through plasma medium is one of the most important research fields in computational electromagnetics. A numerical formulation based on both the auxiliary differential equation (ADE) and the precise-integration time-domain (PITD) method for solving the plasma problems is proposed to break through the Courant-Friedrich-Levy (CFL) limit on the time-step size in a finite-difference time-domain (FDTD) simulation. In this new method, the current density J is introduced as the auxiliary variable to deal with the complex permittivity of the plasma which is dependent on the frequency, and the precise integration (PI) technique makes the selectable maximum time-step size become much larger and removes the impact of the time-step size to the numerical dispersion error. Numerical experimentations of the typical plasma problems verify and validate the reliability of the proposed formulation. Through the numerical results, it can be found that the maximum allowable time-step size of the new method is much larger than that of the CFL limit of the FDTD method, and the calculation error of the new method is nearly independent of the time-step size. As a consequence, the execution time is significantly reduced by using a larger time-step size.

A modified precise-integration time-domain (PITD) formulation is presented to model the wave propagation in magnetized plasma based on the auxiliary differential equation (ADE). The most prominent advantage of this algorithm is using a time-step size which is larger than the maximum value of the Courant–Friedrich–Levy (CFL) condition to achieve the simulation with a satisfying accuracy. In this formulation, Maxwell’s equations in magnetized plasma are obtained by using the auxiliary variables and equations. Then, the spatial derivative is approximated by the second-order finite-difference method only, and the precise integration (PI) scheme is used to solve the resulting ordinary differential equations (ODEs). The numerical stability and dispersion error of this modified method are discussed in detail in magnetized plasma. The stability analysis validates that the simulated time-step size of this method can be chosen much larger than that of the CFL condition in the finite-difference time-domain (FDTD) simulations. According to the numerical dispersion analysis, the range of the relative error in this method is ${10}^{-6}$ to $5\times {10}^{-4}$ when the electromagnetic wave frequency is from 1 GHz to 100 GHz. More particularly, it should be emphasized that the numerical dispersion error is almost invariant under different time-step sizes which is similar to the conventional PITD method in the free space. This means that with the increase of the time-step size, the presented method still has a lower computational error in the simulations. Numerical experiments verify that the presented method is reliable and efficient for the magnetized plasma problems. Compared with the formulations based on the FDTD method, e.g., the ADE-FDTD method and the JE convolution FDTD (JEC-FDTD) method, the modified algorithm in this paper can employ a larger time step and has simpler iterative formulas so as to reduce the execution time. Moreover, it is found that the presented method is more accurate than the methods based on the FDTD scheme, especially in the high frequency range, according to the results of the magnetized plasma slab. In conclusion, the presented method is efficient and accurate for simulating the wave propagation in magnetized plasma.