Design of three-phase grid-connected inverter with LCL filter

The LCL filter has better high frequency attenuation than L filter in the same size, and the form through L filter connecting the grid can reduce the current surge caused by the capaciter of LC filter. But the LCL filter causes voltage or current resonce spikes easily to bring about the system unstability. The split capacitor grid-connected control strategy with LCL filter simplifies the grid-connected inverter from third-order system to first-order system in this paper, so the resonce disappears. An active islanding detection method by injecting a disturbance current into the system through the quadrature axis(q-axis) is presented. The disturbance singal injection caused frequency deviation of the voltage at the point of common coupling(PCC) car detect islanding. The model by Matlab/Simlink is built. The simulation shows the control method introduced is reasonable and the active islanding detection method can detect the islanding phenomenon in 150ms, always meets the two-second UL detection requirement. At last, a 10kW experimental equipment is set up, and the result indicates that the split capacitor control strategy simplifies the control system and the inverter has high power factor and low harmonic.