Dual-stage High-efficiency Soft-switching Grid-connected Inverter

With the consumption of the fossil energy, lots of problems concerning about the development of the society show up. Especially the energy crisis and environment pollution turn serious day by day, so almost all the countries pay attention to the new energy resources. The new energy resources will be the main energy of the future, because it is clean, endless and extensive. But new energy has not been popular by now due to its high cost, especially the photovoltaic power generation. All the researchers focus on the increase of the PV battery conversion efficiency, meanwhile the efficiency of the grid-connected inverter causes their attention too. The increase of the inverter efficiency will acquire more electricity power, so the generation cost turns down relatively, which benefit the development of the solar photovoltaic power generation.Photovoltaic grid-connected inverter is the core point of grid-connected solar power system energy conversion and control. This paper focuses on the control strategies and the novel circuit topology of dual-stage PV grid-connected soft-switching inverter.First, as to dual-stage PV grid-connected inverter, MPPT function can be applied to the first-stage DC/DC converter or the second-stage DC/AC inverter. As a result, two different MPPT control strategies emerge. This paper designs the main circuit parameters of PV gird-connected inverter and the regulator of two different control strategies. The MPPT function uses the track method of perturb and observe with variable step to achieve fast track under fast-changing environment and tiny perturb track under slow-changing environment. At last, PSIM software is used to simulate the two different control strategies. And the advantages and disadvantages of the two strategies is compared from the stability of the DC voltage and the control performance of the MPPT.Second, soft-switching technology is the necessary option considering the miniaturization of the power electronic converter and the high power density. The first stage DC/DC converter applies the phase-shift PWM three-phase six-leg interleaving DC/DC converter which suit to high power applications. This topology is easy to achieve soft-switching function and high transformation ratio. As to the second-stage DC/AC inverter, a novel zero-voltage switching inverter topology using two coupled magnetic in one resonant pole is chosen after comparing the different soft-switching inverter topology. Its advantages contains:low auxiliary switch current stress, no limitation on capacitor balance, reliability reset of magnetizing current. This paper analyses the principle of the two soft-switching converters, and simulations are presented to verify the validity of the two topologies.Finally, based on the theory analysis and simulations, the prototype of the dual-stage PV grid-connected soft-switching inverter is designed. The parameter design of the two conversion stages and the selection of the switches are presented. And the drive circuit is designed respectively for the two stages. The first stage use UCC3895as the control IC combined with FIFO control logic to provide the phase-shifting interleaving PWM signals. The second stage use TMS320F28335as the controller to finish the MPPT function and provide the control signals for the main switches and the auxiliary switches. At last, design the crucial element of the second stage---coupled magnetic, especially. The experiment results verify the validity of the theoretical and simulation analysis.