Tian Hanlei1, Han Peisong1, Tang Songfeng2, Chen Maolin2, Liang Guozhuang1
1. School of Electrical Engineering Hebei University of Science and Technology Shijiazhuang 050018 China; 2. School of Electrical Engineering Sichuan University Chengdu 610065 China
Abstract：Non-isolated grid-connected inverters in PV systems cause common mode leakage current in the stray capacitance of the PV array to ground due to the direct removal of the isolation transformer, which causes electromagnetic interference and safety problems to the overall system. Among them, the common ground type structure is widely concerned by domestic and foreign scholars because it directly short-circuits the common mode capacitor and theoretically eliminates the common mode leakage current. Meanwhile, the multilevel design can reduce the output voltage harmonic components and device voltage stress, and improve the output power quality and service life. Therefore, a common ground isolated multilevel PV inverter is proposed. Firstly, by introducing a new switched-capacitor network, two capacitors have voltage self-balancing capability, which can effectively avoid output current distortion. Secondly, the negative pole of the DC side is connected to the neutral point of the grid side, which can theoretically eliminate the common mode leakage current from the stray capacitors to the ground. Thirdly, the structure has the capability of boost and reactive power support, which can meet the wide range of input voltage and provide reactive power support for the grid. Compared with the bridge circuit, the proposed circuit has high voltage utilization on the input side and overcomes the dependence on its voltage level and the limitation of weather conditions. In addition, a feedforward space vector modulation (FSVM) strategy is proposed to decouple the output variable from the fluctuating capacitor voltage and to provide a five-level step wave at the output. Meanwhile, the leakage current suppression capability is compared with the conventional H5 and HERIC topologies under the same parameters. Moreover, the topology transmission efficiency is higher than 96.1% at different output voltage levels based on the simulation of the thermal model of PSIM. A 1 kW experimental platform is built for experimental testing to verify the correctness and feasibility of the proposed scheme. The results show that the topology clamps the common mode voltage to zero potential and keeps it constant, and the peak leakage inductance is less than 5 mA. The output current THD is less than 2.56% throughout the dynamic performance tests from 60 V to 100 V and from 100 V to 60 V. The topology operates smoothly in the reactive power compensation test at PF=±0.9. The off-grid output current and voltage THD at fs=20 kHz are 1.89% and 2.52%, respectively. The structure shows that the PV array can be well integrated with the inverter proposed in this paper. The following conclusions are drawn from the modal analysis, gain derivation, comparative analysis, and design considerations: (1) The structure has a strong boost function and capacitor voltage self-balancing capability. (2) The common ground structure achieves effective ground leakage current suppression of the PV cell without an isolation transformer. (3) The topology has reactive power support capability. (4) The structure decouples the output variables from the fluctuating capacitor voltage through a feed-forward space vector modulation strategy to effectively suppress the output current harmonics, which effectively suppresses output current harmonics.
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2023, Vol. 38 
