The large-scale integration of renewable energy sources into the power grid has significantly changed traditional grid characteristics, posing great challenges to the stability of inverters. To ensure high stability and superior dynamic performance, grid-connected inverters shall dynamically adapt their control strategies from grid-following (GFL) to grid-forming (GFM) control. While existing studies primarily focus on mode transitions between GFL control and Virtual Synchronous Generator (VSG) control with voltage and current dual-loop structures, seamless switching between GFL and alternative GFM control, such as direct voltage control, remains unexplored. To address this gap, this paper proposes a smooth switching method for transitioning between GFL control and direct voltage control in the grid-connected operation. The proposed approach integrates a novel modulation voltage based pre-synchronization strategy with an adaptive adjustment method to simultaneously minimize the impact current and switching time during mode transition. Time-domain simulations are carried out in MATLAB/Simulink to validate the effectiveness and robustness of the proposed switching method under various operational scenarios. Results demonstrate that the method ensures an impact current ≤ 20% of the rated current, and a switching time ≤ 200 ms across varying operational conditions.

This article introduces a modeling method for the reverse biased electric field of junction barrier Schottky (JBS) diodes, utilizing the Schwarz-Christoffel transformation. Building upon prior research on JBS diodes electric field modeling, this approach is rooted in a purely theoretical derivation, avoiding the dependence on conclusions from simulation software—a limitation of earlier modeling methods. In this study, complex boundary conditions are transformed mathematically into simpler ones to make it possible to solve for the electric field. Then, the analytic solution of the electric field distribution is obtained by using the superposition theorem. To validate this modeling method, the electric field distribution from this model is compared with results from simulation software, and a way of applying the analytic solution of the electric field distribution in commercial simulation software is given.

With the rapid deployment of Distributed Energy Resource (DER), advanced inverter functions such as voltage support have been developed. However, with conflict control targets, the voltage support control is concerned to degrade the effectiveness of the islanding detection, which is yet fully investigated. Given the wide application of Slip-Mode Frequency Shift (SMS), this paper takes it as an example of islanding detection method. Firstly, the mechanism of islanded operation in a RLC island system is analyzed, and the relationship between basic electrical quantities is explored. Then the small signal model of such islanding system with the voltage support and SMS control is constructed, and its eigenvalues and sensitivity to parameter change are analyzed to investigate the influence of voltage support on the effectiveness of SMS. Finally, HIL experiments are conducted to verify the above analysis. The results show that during islanded operation, the voltage support control enhances the islanding detection and helps reduce the islanding detection time. The key affecting parameters are identified as the droop gain of voltage support control, open-loop response time and SMS coefficient.

The three-phase four-leg four-wire three-level (3P4L4W 3L) inverters are widely used in mediate-to-high power applications for supplying power to unbalanced and/or nonlinear ac loads. To save filter inductance, a neutral inductor is often inserted in the neutral wire for reducing the ripple of the common-mode (CM) current at the high switching frequency. The filter inductance and the neutral inductance are yet not specifically designed for the 3P4L4W 3L inverter. For obtaining the minimum required inductance, a complete design methodology is proposed for calculating the ac filter inductance and neutral inductance. Finally, simulation results are presented to verify the feasibility the proposed optimized design for the filter inductors and the neutral inductor.