Optimisation of LCL filter based on closed-loop total harmonic distortion calculation model of the grid-connected inverter

A novel optimisation method of LCL filter based on closed-loop total harmonic distortion (THD) calculation model of the grid-connected inverter is proposed in this study. If the parameters of the LCL filter are large enough, the current harmonics can be significantly inhibited. However, the size, weight and price of the filter may increase. If the parameters of the LCL filter are too small, the attenuation of the current harmonics may be not enough. In the proposed method, the target of optimisation is to obtain the minimum value of the inductors on the premise of the allowed current harmonics. The harmonics should be analysed and the control-loop should be taken into consideration. According to this, a closed-loop THD calculation model is established. Generalised reduced gradient (GRG) method is applied to do the optimisation based on the model. With the optimised parameters of LCL filter, the size, weight and price of the filter can be reduced. The relationship between THD and the parameters of the LCL filter is analysed. The principle of GRG is discussed. Finally an 18 kVA three-phase grid-connected inverter with the optimised LCL filter has been established and tested. Simulation and experimental results verify the theoretical analysis.