Active Cancelation of Equivalent Grid Impedance for Improving Stability and Injected Power Quality of Grid-Connected Inverter Under Variable Grid Condition

An active grid impedance cancelator using the concept of series active filter to suppress the effect of the grid disturbance and stabilize the single-phase grid-connected inverters with an inductive-capacitive-inductive filter operating under variable grid condition is presented. Harmonic interaction between the inverter and the grid is thereby avoided owing to the cancelation of equivalent grid impedance. More importantly, the impedance cancelator offers an active damping function to ease the heavy burden of the inverter control, such as power control, phase locked loop, current regulation, impedance shaping, etc. The impedance cancelator is a full-bridge dc-ac converter having no passive inductive-capacitive filter. It is connected in series with the inverter output and is operated as a negative virtual grid impedance. As the volt-ampere rating of the impedance cancelator is low, the efficiency of the entire system is not sacrificed. Starting with the impedance-based analytical modeling method, the basic principle of equivalent grid impedance cancelator is derived and studied. Then, the digital control strategy and the modeling of equivalent grid impedance cancelator are examined. The experimental results of a prototype cancellator for a single-phase inverter are favorably compared with theoretical predictions.