Studies On The Topology And Control Strategies Of A Single-Phase Grid-Tie Coupled-Magnetic Full-bridge Soft-switching Inverter

Because of the world energy crisis and environmental deterioration, it is becoming the primary task for human being to exploit the renewable and green energy sources for the sake of sustainable development. With the technological progress of solar cell and power electronics, the photovoltaic technology and industry is not only an important supplement of current energy consumption, but also the potential main energy source in the future. Recently, the photovoltaic system constantly develops with the tendency of lower cost, higher efficiency and higher power density. The main application form of solar energy will be the grid-connected photovoltaic system.This paper is mainly to conduct a research on a single-phase grid-tie coupled-magnetic full-bridge soft-switching inverter. On that basis, control strategy research of a single-phase grid-tie hard-switching inverter under grid-connected and stand-alone mode is conducted. At the same time, software design is accordingly conducted. The main work is as follows:1. Basic cell topology and operating principle of coupled magnetic soft-switching circuit are analysed in detail. A simulation circuit of the single-phase coupled magnetic full-bridge soft-switching inverter is presented. Simulations have been conducted with Pspice, and the results prove the superiority of the proposed soft-switching inverter design.2. The double polarity SPWM control method is selected to generate switching control signals of the inverter in this paper with the analysis of various kinds of current control methods. The inverter employs different control strategies under different modes: under the grid-connected mode, the load voltage is determined by the grid. The inverter works as a current source, and the output current is directly controlled by a current closed-loop control strategy; under the stand-alone mode, the inverter controls output voltage employing a double closed-loop control strategy which consists of a voltage outer loop and a current inner loop. Simulations for a single-phase hard-switching inverter working under the two modes respectively have been conducted with MATLAB Simulink. At the same time, a single-phase software phase-locked loop circuit is proposed making grid-tie output current to track grid voltage, and the simulation results verify its validity.3. Software design of PID, ADC, PLL, MPPT, SPWM and main program is introduced and the software flow charts are given. At last, analyse experimental results of the grid-tie inverter prototype.Based on the design of the coupled-magnetic soft-switching inverter and PSPICE simulations, the main switching devices achieve soft-switching characteristics and lower switching losses, and show the superiority; through the grid-tie and stand-alone computer simulations and preliminary experiments of the grid-tie inverter, the control strategies are verified to achieve reasonableness and effectiveness.