The three-phase four-leg inverter (TPFLI) provides unbalanced voltage to a load or injects unbalanced current to a grid to compensate unbalanced current. However, it has a high switching loss because of the fourth leg. This paper presents a discontinuous pulse-width modulation (DPWM) method that minimizes the per-phase switching loss of the TPFLI. The discontinuous three-dimensional space-vector pulse-width modulation (3D SVPWM) can be implemented in the TPFLI by injecting the same offset voltage as the conventional DPWM of three-phase three-leg inverter (TPTLI). However, the conventional DPWM is unsuitable for TPFLI because of its unbalanced current. We demonstrate that the discontinuous phase can be chosen according to the offset direction when a voltage reference vector is specified. Based on this result, a new per-phase minimum-loss discontinuous PWM strategy is developed and compared with the conventional DPWM. This method can also be applied to the TPTLI and implemented by injecting an offset voltage generated using the inverter phase voltage reference and phase current. Furthermore, the lifespan of a TPFLI can be extended by preventing deterioration of a specific leg. The validity of the proposed method is verified through simulation and experiments.

An input‐series quasi‐resonant (QR) flyback high‐voltage switching mode power supply (SMPS) based on an integrated transformer has been designed and analysed. It is capable of converting a high DC input voltage to lower DC voltage to power the control components of high‐power converters in a DC microgrid. An integrated transformer, combined with identical duty cycle control, is adopted to achieve automatic equalization of the series modules effectively even without employing any voltage or current equalization control strategy, of which effect is analysed theoretically. The design of the control system, based on QR flyback SMPS is given, which is implemented by peak current control, and a DCM small‐signal model for the designed SMPS is established, which indicates that the system has high stability and accuracy to ensure normal operation across a broad range of input voltages. Finally, the validity of the theoretical analysis and the proposed method is demonstrated by the development of a 48 W SMPS experimental prototype, which is composed of three input‐series modules. The overall efficiency of the prototype has been improved due to the adoption of QR. The proposed design and analysis methods can be extended to other input‐series isolated DC/DC topologies.

For the efficient power allocation of the distribution line, several studies on intelligent distribution management systems that enable connection or separation between distribution networks are being conducted. However, in case the systems are linked together and if the voltage and phase of each distribution network are different, uncontrolled circulating current and inrush current flow into the line. To resolve this, we introduced the active phase controller (APC) as a solution and performed an analysis using the capacity calculation method for the APC connected to each distribution network. When connecting lines between distribution networks, the phase of each contact was ensured to be the same. Specifically, the phase value compensated by each APC is different because the amounts of active power and reactive power to be controlled are also different. In this study, we analyzed the APC capacity according to the impedance state of the distribution line and calculated the APC output voltage for the phase matching of each distribution network at the connection point. As a result, the minimum design capacity of APC was derived; moreover, simulation results were used to validate the analysis.