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The capacitive-coupling grid-connected inverter (CGCI) is coupled to the point of common coupling via a second-order LC branch. Its operational voltage is much lower than that of a conventional inductive-coupling grid-connected inverter (IGCI) when it serves as a multifunctional inverter to compensate reactive power and transfer active power simultaneously. It is a promising solution for micro-grid and building-integrated distributed generator systems. A quasiproportional-resonant (quasi-PR) controller is applied to reduce steady-state current tracking errors of the CGCI in this paper. The quasi-PR controller generates the voltage reference for use of carrier-based pulse width modulation, which can effectively reduce output current ripples. The second-order coupling impedance of the CGCI causes its modeling and controller design to differ from that of the conventional IGCI. A comprehensive design method for the quasi-PR controller in a CGCI is developed. The quasi-PR controller is also compared with a proportional-integration current controller. Simulation results are provided to verify the effectiveness of the quasi-PR controller and its design method in a CGCI. The current tracking errors are greatly reduced when the quasi-PR controller rather than the proportional-integration controller is applied. Experimental results are also provided to validate the CGCI as a multifunctional grid-connected inverter.

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Co-phase traction power supply system provides continuous power to traction loads without neutral sections. In order to reduce system unbalance, compensate reactive power and harmonics, a railway power conditioner (RPC) operates together with traction transformer in each substation. In the past study, the RPC is designed to achieve three-phase balance and unity power factor (PF) at the grid side. As a result, its rating is high. According to the power quality tariff plan in China, the penalty for reactive power can be avoided if the PF is higher than 0.9. In this study, a grid-side PF of 0.9 is achieved via different control approaches after analysis. Among these approaches, the rating of the RPC in the worst case is more than twice that in the best case. Hence, selection of a suitable control parameter is necessary. The minimum rating of the RPC is achieved by setting the power angle of phases A and B lagging and the power angle of phase C leading under partial compensation. The rating of the RPC is reduced to 70% by setting PF to 0.95 instead of 1. Simulation and experimental results are provided to show the validity of the modelling, design and control method.

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This study proposes a capacitive-coupling grid-connected inverter (CGCI), which consists of a full-bridge singlephase inverter coupled to a power grid via one capacitor in series with an inductor. The fundamental-frequency impedance of the coupling branch is capacitive. In contrast with the conventional inductive-coupling grid-connected inverter (IGCI), this structure provides an alternative interface for use between a low-voltage DC microgrid and an AC grid. A comparison between the CGCI and the IGCI is performed. It is concluded that the CGCI is able to transfer active power and provide lagging reactive power at an operational voltage much lower than that of the IGCI. This reduces the system's initial cost and operational losses, as well as the energy stored in the DC-link capacitor. The CGCI has been analysed and a DC voltage selection method is proposed. Using this method, the DC-link voltage of the CGCI remains at approximately of 50% of the peak grid voltage. In addition, a P-unit current controller is proposed for use with the CGCI, as a proportional-integral controller is not suitable. Finally, simulation and experiments show the effectiveness of the proposed approach.