| Compared to the current mainstream like fossil fuels, nuclear, wind, hydro and biomass power generation technology, application of solar photovoltaic (PV power generation technology) is green and clean, safe and reliable, with no geographical restrictions, no harm to the environment and no occupation of land and grain, etc., which enables it to become the hope of all mankind to achieve sustainable development.The PV grid-connected inverter converts PV cells’output direct current to alternating current and feeds it into the grid. As core device in this energy conversion and control process, it affects and decides the system’s stability, security, reliability, efficiency, service life and cost.In recent years, domestic PV power generation is developing rapidly and has gained the government’s strong promotion. This dissertation chooses to start from the single-phase voltage source inverter bridge PV grid to conduct in-depth study on three key technologies, namely, the output waveform quality control under nonlinear load conditions, high and low voltage ride-through capability under sudden grid voltage change, and parallel control with no interconnection. On such basis, the rapidly developing three-phase four-leg inverter control technology is studied.Firstly, starting from the output side of the PV grid-connected inverter, this dissertation studies its output waveform quality control. On the basis of analyzing causes of DC injection problems and the existing treatment methods, an instantaneous algorithm of DC component is proposed, which includes preset link, to achieve instantaneous calculation of DC component by every carrier cycle. And, based on this algorithm, this dissertation proposes a PV grid-connected inverter DC injection instantaneous suppression strategy through integrating repetitive control and PI control, which can effectively suppress DC component of various causes, including that caused by non-linear loads. Unified compensation of PV grid-connected inverter output DC component and harmonic injection is achieved through establishing dual repetitive control loop based on this strategy. The feasibility and effectiveness of the strategy is verified through constructing two types of nonlinear loads-half-wave load and uncontrolled rectifier load-in the experiments.This dissertation then extends the research to the grid side of PV grid-connected inverters and studies high and low voltage ride-through technology. Based on the analysis of the ride-through requirements in China, we propose a control strategy suitable for domestic single-phase PV grid-connected inverters, which enables them to have high and low voltage ride-through capability and the ability to dynamically adjust active output and reactive compensation according to the need. By introducing the instantaneous prediction algorithm, the strategy achieves instantaneous grid voltage amplitude tracking; by introducing voltage feed-forward control, the strategy achieves suppression of sudden change of grid-connected current at the moment of sudden grid voltage change. Through analyzing inverter active output and reactive compensation control, this dissertation sets the control principles of Crowbar circuit in the strategy. The feasibility and effectiveness of the strategy is verified through experiments in which sudden grid voltage change is simulated.This dissertation then takes into account the trend for PV power to develop towards high-power distributed power supply mode and studies parallel technique with no interconnection of PV grid-connected inverters. Analyzing the traditional PQ droop method and parallel control conditions of inverters and taking into account the special characteristics of PV grid-connected inverters, we propose a parallel control strategy with no interconnection based on improved voltage control, establish and improve voltage control equation and achieve such parallel control through the " voltage-current" dual-loop structure. We also analyze equivalent output impedance difference of each inverter and the negative effects of the grid connection impedance difference on parallel control without interconnection. We improve this through introducing virtual impedance. The feasibility and effectiveness of the strategy is verified through experiments in which sudden change of system load and inverter connection impedance difference are simulated. Moreover, experiments to suppress DC circulating current are conducted, combining the research on PV grid-connected inverter output waveform quality control.Finally, this dissertation studies the rapidly developing control technology of the three-phase four-leg inverter specific to the current growing number of unbalanced load and non-linear load. It analyzes the unbalanced load suppression mechanism of three-phase four-leg inverters, compares the control principles of3D SVM in abc coordinate system with that in αβγ coordinate system, and gets rid of the invalid tetrahedron in3D SVM in abc coordinate system. By integrating repetitive control and PI control in dqO coordinate system, it achieves effective control of three-phase four-leg inverters under unbalanced load and nonlinear load conditions. In this dissertation, with combination of PV grid-connected inverter output waveform quality control research, the feasibility and effectiveness of the strategy is verified by constructing unbalanced load and nonlinear load in the experiments. |
