Research On Operation Control Of Grid Connected Photovoltaic Power System

In renewable energy system, photovoltaic power generation has wide application and research significance. Based on the analysis and summary of domestic and foreign research status of photovoltaic grid-connected power generation system, this paper constructed a PV grid-connected system experimental platform, and main techniques on operation control were studied.First of all, a two-stage structure is used. The first stage is an interleaved boost converter and the second stage is single-phase H bridge inverter. For two case-that the output connected to either L filter or LCL filter, main circuit parameters are analyzed and designed.Secondly, the overall control scheme is confirmed that the first stage of the boost converter achieves maximum power point tracking function and the second stage inverter achieves the function of regulate DC bus voltage and current control. The system startup sequence, the maximum power point tracking algorithm (MPPT), island detection technology, DC bus voltage regulation loop design, overload operation and mode switch, digital phase-locked loop and current control techniques are analyzed and designed in detail, to optimize the system performance. Advantages and disadvantages of common MPPT algorithm are analyzed and a variable-step incremental conductance method is used to meet the tracking speed and precision. Islanding detection principles are analyzed and a two-direction positive feedback active frequent drift detection algorithm is proposed to detect island effectively. Modeling and compensation parameter design of the DC bus voltage loop are done. Mechanism of produce of third harmonic is analyzed and gives the voltage loop piecewise control strategy. Overload operation and mode switch are analyzed and designed, also, start-up sequence, implementation of PLL, system protection are designed. The system is stable and has excellent performance.To the grid-connected current control part, for the case that the output is connected to the L filter, modeling and controller design are done. Grid voltage interfering is analyzed and propose two methods to inhibit grid voltage impaction. Advantages and disadvantages of the two methods are analyzed and a combined control scheme is given.For the case that the output is connected to the LCL filter, the existence of resonant peak in LCL filter and commonly used passive and active damping method are analyzed. Based on the analysis of splitting capacitor feedback control, two novel control strategies are proposed. One is repetitive control scheme based on feedback of splitting capacitor. This strategy retains the advantages of splitting feedback control and repetitive control, meanwhile, the design of repetitive control compensator is simplified. The other is active damping control scheme based on feedback of splitting capacitor. This strategy can effectively restrain the potential grid-connected harmonic current which may exist, enhancing the robustness of the system.Finally, a2kW prototype and experimental platform were built. Experimental results verify the validity of the grid-connected PV system and control strategy.