Study On Parameters Design And Resonance Damping Strategy Of Grid-Connected Inverter With LCL-Filter

As a necessary energy transformation interface of distributed generation system, grid-connected inverter can convert the DC electric energy in the energy storage unit and Renewable power generating unit to high quality AC power. Because the inverter’s output currents usually contain the harmonics around switching frequency using pulsewidth modulation(PWM), an appropriate filter should be adopted. The LCL-filter has the smaller total inductance and faster harmonic attenuation rate comparing to the traditional L-filter, so that the size and cost of the system can be reduced. However, resonance peak of the frequency response is not conducive to the stability of system. Therefore, parameter designs and resonance peak suppression are the hotspots in LCLfilter studying.This thesis establish a discrete state-space model of grid-connected inverter based on LCL-filter. Firstly, continuous state-space model of the system is established based on KCL, KVL and related coordinate conversions. Then, the continuous model is discretized. After that, an improved model is established in consideration of the time delay of digital control.The parameters design of LCL-filter is optimized. In the view of the deficiencies in traditional step by step design and relevant improvement methods, optimization design is adopted in this thesis. Optimization variables, constraints and objective function of optimized object is confirmed firstly. Next, the parameter selection is based on constraint random direction method.The system is controlled by state feedback and state observer. The parameters of state feedback matrix and PI controller is determined by pole-placement method. Moreover, through the state observer, the state variables is estimated without increasing any additional sensors, so that the reliability of system is ensured. Simulink simulation model of grid-connected inverter with LCL-filter is built during design process. The simulations reveal that the harmonics around switching frequency can be filtered effectively with state feedback, the resonance peak is inhibited as well. Besides, the system is not influenced by state observer.Robustness of the system is analyzed finally. Because the system performance may change with parameter perturbation, robustness of the system is discussed in this thesis. The results show that the system has a good robustness.