Research On The Control Strategy Of The Three-phase Four-leg Photovoltaic Inverter For Household

Asymmetric load and nonlinear load are commonly in the household photovoltaic power supply system. Therefore inverter is required to have better ability to resist load imbalance and to adapt to nonlinear load. Three-phase four-leg inverter has strong ability to handle unbalanced load by providing a pathway for zero sequence current. The optimal control of three-phase four-leg inverter under asymmetric load and nonlinear load is mainly studied in this paper aimed at household photovoltaic system applications.Firstly, the typical structure of household photovoltaic power supply system and commonly used three-phase four-wire inverters are introduced. The application characteristics, modulation and control methods of three-phase four-leg inverter are expounded. The requirement for inverter under unbalanced load and nonlinear load is introduced.To facilitate the analysis of control system, the models of three-phase four-leg inverter in stationary and rotating coordinate frame are established. The effect of static coordinate model decoupling control for inverter is analyzed. The effect for bus capacitor voltage ripple of three-phase four-leg inverter under unbalanced load condition is analyzed. In this paper a voltage modulation method of carrier-based PWM is described. This modulation method is the same as the space vector modulation of dc voltage utilization, but more simple.A double closed-loop control with proportional resonant controller under static a-b-c coordinate system for three-phase four-leg inverter is analyzed and verified by simulation. The parameters of voltage and current regulator are designed and the digital realization methods are researched. A nonlinear control strategy with current feed-forward is proposed by analyzing the reason of the inverter output waveform distortion caused by nonlinear load. The validity of the proposed control strategy for unbalanced load and nonlinear load is verified by simulation.An experimental platform with 3k VA three-phase four-leg photovoltaic inverter based on TMS320F28335 controlling is designed. The design principle of hardware and software is introduced. The control performance experiments for three-phase four-leg photovoltaic inverter are completed under unbalanced load and nonlinear load. The good steady and dynamic performance of the proposed nonlinear control based on load current feed-forward is verified by experimental results.