Design of Natural Damping Control for Three-phase Grid-Connected Voltage Source Inverters with LCL-Filters

Nowadays, researches have been carried out on reducing the total harmonics distortion when a voltage source inverter (VSI) is connected to the grid through a filter. LCL-filter reduces the harmonics contents when the switching frequencies are low and also the values of the inductance are low. Another advantage of LCL-filter is it has two possible current feedbacks that can be use for control. In this thesis, the current at the inverter side will be use for feedback control and no additional sensor is required. There was an observation that a natural damping term is present in the control loop, when the inverter side current is used for feedback control, instead of using active or passive damping technique. Steps to determine the values of the inductors at grid and inverter side are presented, so that ideal damping can be naturally obtained by using just the inverter side current control. When the steps to determine values of the inductors are not fulfilled, a notch filter with second order is used to extract the damping information from the converter current, then tuned the system damping with a compensation gain and due to its free of fundamental component, the compensation strategy will not cause an over modulation problem. Simulation results for both strategy are shown using Matlab Simulink to prove the effectiveness of the proposed control strategy. Keywords: Voltage source inverters (VSI), LCL filter, current control and resonance damping.