Research On Low-cost And Low-power Grid-connected Photovoltaic Inverters

The AC-module (A photovoltaic module with an integral grid-connected dc to ac inverter) structure is one of the development trends of photovoltaic systems, but the relatively high losses and system cost are in the way of its widespread application. This paper studied the flyback current-source inverter for grid-connected photovoltaic system and also the MPPT (Maximum Power Point Tracking) control technology.Based on the current source high frequency link technology, this paper described the operation principle of flyback current-source grid-connected inverter in detail, meanwhile a complete steady state analysis is presented, including the relation of voltage and current, expressions for peak device stresses. Then the formula for losses calculating is derived. It shows that the losses of flyback-type inverter are closely related with transformer's turns-ratio. The turns-ratio impacts the choice of power semiconductors and the design of transformer, and affects the inverter efficiency ultimately. Based on the analysis model of losses, this paper proposed the optimal design considerations. The design method of key parameters is also given. The experimental results validate the optimal design method is effective.Based on establishing the mathematical model of solar cell and analysising the known MPPT technologies, by simulating the solar cell with diode, the MPPT technique for small power applications is studied. Because the similar temperature characteristics between diodes and solar cells, the forward voltage drop of diode can be sampled to provide the MPPT reference. Adopting this method, good MPPT performance can be obtained even when the environmental temperature is varied.The simulation and experimental results validate that the low cost flyback-type current source inverter for grid-connected photovoltaic system is with good MPPT result, high efficiency, as well as high in-grid current power factor and low current THD.