The Research On Photovoltaic Gird-Connected Inverter Based On Z-source Topology

The problem of energy has become more serious as the social economy develops rapidly in recent years. As the most promising energy method, the use of solar energy is taking a more important role in generating electricity than before. The research and development of solar energy would have an active impact on the social economy. From the overview of photovoltaic on grid system, this paper made a detailed analysis of the principle of single-level voltage source inverter, voltage source inverter with Boost circuit and Z source inverter, summarized and compared their advantages and disadvantages, ultimately chose the Z source inverter circuit with more excellent performance. After analyzing the topology, operation principle and waveform of Z source inverter, this paper build model for each part of the system, based on state space average method, then combine them together. Against the shortcomings of the model, a unified model was proposed. This paper also introduced the modulation principle of SPWM and SVPWM as well as injection method of shoot-through zero vector. This article introduced principle and realization of single-phase SVPWM modulation combined with shoot-through segment injection based on DSP+FPGA hardware platform. Based on modeling results and SVPWM, a multi-loop control system was designed. And the current inner loop was improved by adding a repeated control link, effectively reducing the grid current harmonic distortion, achieved good control effect. Finally, experiments were conducted on voltage source Z Source on grid Inverter platform, which is controlled by DSP+ FPGA, ideal results were gained. The main contents include:1. Make an analysis of the composition and function of parts in the photovoltaic system. Introduce the topologies of inverter, which is the core of a photovoltaic on grid system, and their applications, a detailed analysis of topology, working principle and characteristics of three common-used inverters was also conducted. Focus on the causes, hazards, and suppression method of second harmonic interference of Z source inverter bus voltage. Against the three inverter, from the aspects of the switching devices power (SDP), the size of DC side components, photovoltaic module or fuel battery and inverter capacity and system efficiency, a full comparison and analysis were conducted. Finally, according to the comparison results and the actual design requirements of this article, Z source inverter was selected. And the advantages and disadvantages of Z source inverter were summarized.2. Modeling methods commonly used in power electronics were briefly introduced and concluded. The modeling results and state space averaging method used in Z source inverter were introduced in emphases. Z source inverter was divided into Z sources boost sub-circuit and the inverter bridge to model the two parts respectively. After analyzing the shortage of this method, a unified modeling approach was proposed and specific modeling models and modeling results were given.3. Introduce principle and generation of two main modulation methods-SPWM and SVPWM applied to inverter. Several injection methods of SPWM and SVPWM shoot-through zero vector in the Z source inverter were analyzed and the advantages and disadvantages of several control methods were summarized. Finally, against the switching frequency doubling problem caused by bringing in shoot-through zero vectors in the traditional SPWM modulation, principle of single-phase SVPWM as well as the shoot-through segment control methods used in this paper were introduced. And the method of generating the single-phase SVPWM by FPGA was given.4. Analyze the control strategy of Z source inverter in photovoltaic power generation system. In terms of circuit structure and control objectives, the entire control system consists of three closed-loop structure:the inner grid AC current loop, the outer bus voltage loop, MPPT shoot-through zero vector loop. Based on the analysis of control principle of inner current loop, for the problem of bus voltage second harmonic interference in Z source inverter, the repetitive control theory was proposed to improve the performance of inner loop. The method of testing Z source capacitor voltage and calculating shoot-through zero vectors on line was used to control the bus voltage directly to improve the control accuracy of the system.5. Analyze the design of Z Source Inverter hardware system, DSP+FPGA hardware platform and simulation system. Hardware system design includes control system based on DSP+FPGA and design of main parameters of Z source inverter. DSP+ FPGA hardware platform design includes coordinated control strategy between DSP and FPGA and the function structure of DSP and FPGA, Simulation system design includes the modeling of each part in Z source inverter.6. Based on experimental platform of Z source inverter, in which DSP+FPGA is controlling core and demonstration power station of NanKai University as a basis, the simulation and experimental results were analyzed and compared, the simulation results and experimental results agree with each other, the rationality and effectiveness of the system was verified.