Research On Key Technology For Grid-Connected Photovoltaic System Based On Improved Quasi-Z-Source Inverter

Along with the rapid development of national economy and traditional energy crisis is increasingly more serious, the green renewable energy has get rapid development. Solar energy gets more and more attention because of its advantages such as low cost, non-pollution, inexhaustible and so on. The grid-connected photovoltaic (PV) power generation system is the main trend of the development of solar power generation technology today, which has important theoretical and practical significance on the remission of world energy crisis and the improvement of environment pollution problem. In order to further improve the efficiency and the safe stability of grid-connected PV power generation system, this paper selected the three-phase grid-connected PV power generation system as the research object, the key technology for grid-connected PV system was studied deeply and systematically, such as inverter topology, maximum power point tracking (MPPT) algorithm under partially shaded condition, fast islanding detection algorithm and low voltage ride through (LVRT) control technology.Quasi-Z-source inverter (qZSI) is an improved circuit topology basis on Z-source inverter (ZSI), and it has been widely applied in the field of new energy power generation because of it overcome the inherent defects of ZSI. But qZSI has the same boost ability as ZSI, so it is slightly insufficient to the grid-connected PV system which has a wide range of output voltage. Therefore, in order to enhance the boost ability of qZSI, this paper proposed an improved qZSI (TqZSI) through applying tapped-inductor into traditional qZSI. The boost ability of inverter can be enhanced significantly and the capacitor voltage stress can be reduced through changing the turn ratio of tapped-inductor. The topology structure and working principle of TqZSI was analyzed in detail, and compared with qZSI and switched-inductor qZSI (SL-qZSI) from aspects of boost ability and device voltage stress. Small signal model of the improved quasi-Z-source network was derived and established, and the influences of different parameters on system performance were analyzed. The simulation results indicated that TqZSI has higher boost ability and lower device voltage stress compared with qZSI and SL-qZSI.MPPT is one of the essential functions for grid-connected PV power generation system. Considering the output characteristic of PV array has multi local maximum power points and traditional MPPT methods fail to identify global maximum power point under partial shading condition. The simulation model of shadowed PV array was established by Matlab in this paper, and the distribution rule of local maximum power points was summarized through simulating and analyzing the output characteristic of PV array under different shading conditions. Then a modified MPPT algorithm based on the distribution rule was proposed. The proposed MPPT algorithm can accurately track the global maximum power point under uniform illumination and partial shading conditions, increasing the efficiency of grid-connected PV system.With the continuous development of grid-connected PV system, islanding problem becomes more and more serious. According to the technology regulations of photovoltaic power station connected with power grid, the small sized grid-connected PV power station should have islanding detection function. Considering conventional passive detection methods have large non-detection zone and active detection methods affect the power quality. The output characteristic of grid-connected PV inverter under islanding condition, the dynamic changes of voltage unbalance factor at the point of common coupling (PCC) and its relationship with the active current was analyzed in this paper. Then a novel islanding detection method based on positive feedback between active current and voltage unbalance factor was proposed, which by detecting the voltage unbalance factor at PCC to determine islanding occured. Under normal condition, the introduction of positive feedback loop between voltage unbalance factor and active current hardly affects the output power quality of grid-connected PV inverter, and under islanding condition, the positive feedback loop makes the voltage unbalance factor at PCC deviate from normal range quickly, realizing no non-detection zone and non-devastating islanding detection. And the determined condition of false islanding was set to avoid incorrect operation under false islanding situation, which could improve the operation stability of grid-connected PV system. Because the voltage unbalance factor is related with amplitude of negative sequence voltage and positive sequence voltage, so the positive and negative sequence separation method based on decoupling was analyzed and its engineering realization was provided.State Grid Corporation of China (SGCC) stipulates that medium and large scale grid-connected PV power station should have low voltage ride through (LVRT) capacity in order to raise power transmission quality and maintain stability of system. The mathematical model of grid-connected PV inverter under grid voltage symmetrical and unsymmetrical dip condition was established in this paper to analyze the output characteristics under those two conditions. The LVRT control strategy that suitable for TqZSI with restraining negative sequence current under grid voltage unsymmetrical dip condition was studied. The positive sequence component and negative sequence component of grid-connected PV inverter output current is controlled respectively through adopting parallel structure of positive sequence converter and negative sequence converter in order to restrain the influence of negative current and waveform distortion. Meanwhile, a setting method of active current and reactive current of grid-connected PV inverter during low voltage time was proposed in order to provide reactive power support for grid.Finally, the experiment platform for grid-connected PV inverter based on digital signal processor TMS320F28335 was set up. Experimental researchs for TqZSI, rapid islanding detection algorithm and LVRT control strategy were completed on the experimental platform. And the experimental results indicate the correctness and validity of the theoretical analysis and control strategy.