The Research Of LCL Filter Resonance Problem In Three-Phase Grid-Connected Inverter

With the increase of photovoltaic power, L filter has been unable to satisfy the design requirements of high-power grid-connected inverters. In high-power grid-connected inverters, LCL filter has gradually replaced L filter and been widely used, with its advantages of good filtering performance and small volume. However, the application of LCL filter has two difficulties. One is its parameter design problem and the other one is its resonance problem. Based on this, the LCL filter resonance problem in three-phase grid-connected inverters is researched in this paper.First, the switch mathematical model of three-phase grid-connected inverter with LCL as its filter is established. And then, the working characteristics of LCL filter and the influence of parameters changing on the resonant frequency and the high frequency switching ripple attenuation is analyzed in detail. And at last, LCL filter parameters are designed.Contraposing the problem of resonance for LCL filter, two control strategies are studied, which are state feedback active damping control and split capacitance control. State feedback control can configure the poles of the controllability system at arbitrary position. A simple and intuitionistic pole placement algorithm is used in this paper to place the poles of the controlled object in a reasonable range, so as to achieve an controlled object that is expected. And then the parameters of the PI regulator is designed with comprehensively considering the phase margin, amplitude margin and overshoot of the closed-loop system. Finally, the influence of inductance changing on the system stability and the anti-interference characteristics of the closed-loop system are analyzed. Split capacitor control can eliminate the resonance of closed-loop control system. Comparing with other active damping methods, it simplifies the structure of the control system and increases the bandwidth of the system. However, the capacitor C2 and the inductor L2 on grid side that are out of the control loop will in resonance. Therefore, a method that adding a damping resistor in C2 branch is proposed in this paper to suppress the resonance. The theoretical analysis and simulation about this two control strategies are finished with MATLAB software, and the simulation results verify the theoretical analysis.Finally, in accordance with the requirements of the research, the structure of the main circuit and control circuit are designed, and a three-phase grid-connected inverter system experimental platform with LCL filter as its interface is built. TMS320F28335 is chosen as its main controller chip of the digital control system. The experiments of state feedback active damping control and split capacitor control verify that this two control strategies are all effective.