Study On The Control Strategies For Photovoltaic Inverter And Islanding Detection

Energy shortage and environmental degradation is growing bad in twenty-first centuryof global problems. With the development of human civilization, global industrial’sdemand for energy is also growing. Traditional energy sources, such as coal, oil and naturalgas, will eventually diappered. Humanity is facing a seriously energy crisis. We have tofind new alternative energy as soon as possible. The energy issue has become the socialproblems related to the national economy and national security.Firstly, we introduced the research background and significance of photovoltaictechnology. Through analyzing the advantages and disadvantages of existed main circuittopologys, we choose a single-stage and three-phase photovoltaic inverters with afrequency transforme. There is transformer isolation, on the one hand, can prevent theleakage current because of the parasitic capacitance of PV injection into the grid. Otherhand, when the bus voltage of photovoltaic cells is low, it can play the role of boost.According the circuit topology that has been chosen to analysis the control strategy,including the maximum power point tracking (MPPT), islanding detection andanti-islanding protection, and tracking control of net current, etc. All functions areachieved in the three-phase inverter. MPPT using the new control method which isproposed by laboratory to achieve no oscillation at the maximum power point (MPP),When the external environment changes rapidly, a miscarriage of justice can avoid. Andthe tracking control of photovoltaic inverter requirs the output current and the grid voltagehave the same frequency and the same phase.Analysis and comparison of the advantages and disadvantages of various detectionmethods and application of islanding. A new active islanding detection method is proposedin this paper. This approach achieved by imposing a sinusoidal current changes in thecommand current. When the frequency of current exceeds specified thresholds with thelinear load, we can detect whether there has been an island effect after grid is broken byjudging the frequency of load voltage. The new method belongs open-loop control, and no blind spot detection.Finally, we complete digital implementation of three-phase photovoltaic invertercontrol system by ST’s ARM with Cortex-M3core. It Includs PWM generation module,MPPT, the dynamic controller and command current’s digital realization.We have a preliminary experimental verification for the main circuit of this topologyand control algorithm. The result of experimen confirmed the feasibility of this program, aswell as the correctness of theoretical analysis.