Inverter-side current feedback control (ICFC) has been extensively adopted in distributed generation systems because of its simple implementation and better consistency with actual operating conditions. At the same time, when there are a large number of background harmonics in the grid, capacitor voltage feedforward (CVF) is usually added to the system for its suppression. This method could make the system keep the first-order characteristics under the analogue control. However, under digital control, due to the digital control delay, a reverse resonance peak will be generated in the loop gain, which makes the system unstable in the weak grid. In order to solve the aforementioned problems, this study proposes a feedforward phase compensation method of LCL grid-connected inverter based on the all-pass filter (AF). By introducing AF into the CVF channel, the phase lag in the range of reverse resonance peak frequency is compensated, so as to enhance the robustness of the system in the weak grid. At the same time, this study gives the detailed design process of the proposed method. Experimental results on a 3-kW prototype are provided, and the effectiveness of the proposed control method is verified..

A new perspective of impedance matching is proposed to study the power flow operation of the three-phase four-wire (3P4W) unified power quality conditioner (UPQC) system. Specifically, the UPQC system is equivalent to an adjustable impedance network, when external conditions change, such as grid voltage sag/swell and unbalanced loads, the node impedances in the network will be dynamically matched. On the one hand, the three nodes of the series converter (SC) will be matched as positive or negative impedances with the change of grid voltages, which means that the SC operates in a rectifier or inverter state to absorb or transmit the energy. On the other hand, to compensate the unbalanced load currents, the three nodes of the parallel converter (PC) will be matched as positive and negative impedances at the same time, which means that the PC operates in both rectifier and inverter state to absorb and transmit the energy. After matching the node impedances, the system's output node impedances are equal to the load impedances, thus achieving the power balance of the whole system. Experimental results of the power flow and impedance matching from a laboratory-scale prototype hardware are presented to evaluate the correctness of the impedance analysis.

This paper presents a cascaded full-bridge resonant inverter configuration for different material vessel induction cooking. The proposed inverter configuration features simultaneous heating of three different material vessels, and independent power control. In this proposed work, three different induction heating loads are simultaneously operated at their respective resonant frequencies. Vessels of iron, steel, and aluminium materials are used. The output powers are independently controlled by an asymmetric duty cycle control. The proposed inverter is designed and hardware prototype has been implemented. Experimental results are validated with simulation results.

In this study, a non-isolated three-port high step-up/step-down DC–DC converter is proposed. The proposed converter has the capability of cancelling the input current ripple in the high current port, moreover, the voltage conversion ratios between the high voltage and low voltage are increased by coupled inductors. All the ports of the proposed converter have a common ground. In addition, any of the three ports can be used as an input voltage source, which can supply the other two ports. The number of DC voltage ports of the proposed converter can be more increased by using more inductor-switch cells. In this study, the theoretical results of the proposed converter are obtained and the voltage conversion ratios of the output ports, the voltage and current stress on switches, average currents of inductors, and the required condition of cancelling input current ripple at the low voltage side are calculated for the proposed three-port converter. The voltage gains of the developed converter with four ports are also calculated. Finally, the theoretical results are reconfirmed by the experimental results for 28 V input/150 and 460 V output ports.

DC transformer plays a significant role in DC distributed network. This study proposes a modular multilevel dynamic DC transformer (MMDT), which can realise smooth control of transformer ratio, multi-level voltage conversion, and reliable fault isolation without high-frequency transformers and extra loss. The inner circuit, formed by the capacitors, not only realises the energy exchange between different sides but also provides the capability of fault isolation. In the proposed MMDT, the voltage transforming is realised by switching to different numbers of control units (CUs) on each side. To realise multi-level voltage transforming, the multi-port MMDT is developed. Furthermore, this study offers analyses of the current and voltage characteristics in steady-state and investigates the parameter design method in detail. Finally, the simulation and experiment tests are conducted to verify the feasibility and superiority of the proposed MMDT.