Research On Key Problem Of Photovoltaic Grid Connected Inverter

Grid-connected inverter is the main energy conversion device as well as the most complex part of the photovoltaic(PV) generation system. When compared to the other part of PV generation system, grid-connected inverter suffers from the highest failure rate, which is caused both by software and hardware design. The performance of grid-connected inverter will directly affect the economic efficiency of PV system. The key techniques such as maximum power point tracking(MPPT) algorithm, neutral point potential balance of three-phase three-level inverter, power control strategies under normal state and unbalanced grid state, common-mode leakage current are discussed in this paper, which will improve the performance and reliability of the system.The main contents and contributions are as follows:1. The characteristics of photovoltaic cells are studied, after which the principle of MPPT algorithm in two-stage PV generation system is analyzed. A MPPT method based on current predictive controller is presented in this part, which enhances the rapid response of MPPT algorithm.2. On the basis of the structure of NPC three-level inverter, this part introduces the main causes that leads to the imbalance of neutral point potential in DC side. A neutral point voltage controlling method is proposed to meet the target of restricting the fluctuations of neutral point potential within reasonable limits.3. The voltage oriented control(VOC) and direct power control strategy are discussed under normal grid state. Two control strategies can improve the static and dynamic performances of the system while the latter strategy works better in the aspect of dynamic response. The suppression of negative sequence current/active power fluctuation/reactive power fluctuation/both active and reactive power fluctuation are studied under unbalanced grid state, which will strengthen the ability during grid-fault period for three-phase inverters.4. The mechanism and suppression measures of common-mode leakage current in transformerless grid-connected PV inverter are studied in this part. Leakage current can be effectively reduced either by increasing the levels to reduce the rate of change in common mode voltage or by increasing the exchange of bypass capacitors or increasing the high-frequency impedance of leakage current loop.5. This paper designs a 20 KW NPC three-level three-phase inverter. The hardware includes the main circuit, control unit, the signal sampling unit and driving protection unit of the system. The software includes the main loop program, vector control program, direct power control program and the SVPWM program.