Grid Impedance Shaping Methods To Improve Robustness Of Grid-Connected Inverter Against Grid Impedance Variation

Due to energy crisis and environmental pollution, there is a growing interest in renewable energy represented by wind energy and solar energy. As an interface between the renewable energy power generation unit and grid, LCL-type grid-connected inverter is implemented to transform the electric energy exported by renewable energy power generation unit into AC electric energy which can be fed into by grid. To ensure the stability of the system, the inverter often takes capacitor current feedback to damp the resonnance introduced by LCL filter. However, when the inverter adopts digital control, the damping effect would be weakened by the control delays introduced by digital control. Consequently, the robustness of the inverter against grid impedance variation becomes worse, which could even cause instability of the inverter. In this thesis, methods to improve the robustness of grid-connected inverter against grid impedance variation are analyzed.In this paper, the Norton equivalent model of digitally controlled LCL-type grid-connected inverter is proposed by the equivalent transformation of control block diagram, the impedance-based stability criterion is derived from the model.To improve robustness of grid-connected inverter against grid impedance variation, the impedance shaping methods based on passive circuit are proposed in this paper, including switching capacitor and adding a resistor and capacitor at the point of common coupling, to ensure that the shaped grid impedance and output impedance of inverter always satisfy the impedance-based stability criterion. With the proposed methods, the grid-connected inverter can be adapted to the wide variation of grid impedance. These methods have the advantages of simple realization, high reliability and low cost.On the basis of switching capacitor method, the impedance shaping methods based on active capacitor converter is proposed in this paper. By appropriately controlling the current of the active capacitor converter, its port characteristic will behave as capacitor near the resonance frequency. This method equivalently adjusts grid impedance, to ensure that the shaped grid impedance and output impedance of inverter always satisfy the impedance-based stability criterion. Comparing to the methods based on passive circuit, this method nearly does not introduce loss and can avoid fundamental reactive power.Finally, the theoretical analysis is validated by the experimental results from a 6k W LCL-type single-phase grid-connected inverter.