Ground Current Analysis And Suppression For Grid-connected Photovoltaic Inverters

As the world economy on growth in energy demand, solar energy as a renewable energy, is obtaining the growing concern of countries in the world. Photovoltaic power generation is an important form to utilize the solar energy. Photovoltaic inverter is the key component of the grid-connected photovoltaic (PV) systems. It is significant to reduce the cost, improve the efficiency, the security and the reliability of the PV inverter. This paper studies the ground current analysis and suppression technique, the efficiency of the three-phase grid-connected photovoltaic inverters.This paper describes the structure, principle, operating characteristics of photovoltaic cells, and the classification of photovoltaic inverters, the causes and hazards of the ground current. An overview of the ground current suppression technique is given.To analyze the ground current suppression of the non-isolated PV inverters, it needs discussion of the parasitic capacitance and its equivalent model. First, this paper analyzes the structure of the solar cell and photovoltaic array. It proposes the mathematical transmission-line model of the photovoltaic array considering the parasitic capacitance. The frequency characteristics of the equivalent model are analyzed. Finally, the measurement results are given.The DC-link capacitors are used to reduce the ground current for non-isolated single-phase full-bridge inverter. The influence on the ground current with different modulation methods is discussed. The ground current of several three-phase grid connected transformerless PV inverters is analyzed. For the single-stage structure, the traditional three-phase inverter, the three-phase half-bridge inverter with split capacitor (3HB-SC), the three-phase neutral-point-clamped inverter (3xNPC), two-level inverter with neutral leg, three-phase four-leg inverter are discussed. The calculation method to estimate the ground current is established. The common-mode models and the Fourier equations of the ground current are given. The change of the ground current is proposed as the parameters vary. For the two-stage structure, the influence of the different front-end boost converters and modulation on the ground current is analyzed. The simulation and the experimental results are given to verify it.Active Common-noise Canceler (ACC) is introduced to suppress the ground current for grid connected transformerless photovoltaic system. Loss analysis is done for ACC. The effect on loss is studied as the parameters and the condition change. The method of optimizing the design is given to reduce the loss and the impact on the system efficiency. Experiment results are presented to verify the effectiveness.The efficiency optimization of the three-phase grid-connected PV inverter is discussed. It analyzes the efficiency of the single-stage structure non-isolated PV inverters, such as the two-level and the three-level inverter. And several two-stage structure topologies with high-frequency isolation and non-isolated topologies are also discussed. The efficiency of the three-phase PV inverter with two-level boost converter is discussed in detail. The impact on the efficiency is analyzed as the DC-link voltage and the interleaved multiplicity change. The simulation results are given to verify it.