Inverter admittance phase-reshaping strategy for enhancing its robustness to grid impedance

Abstract

Harmonic resonances around the intersection frequency is triggered with the stability margin decreasing to zero and the system will be unstable with the stability margin reducing to be negative. For suppressing harmonics, the phase difference between the inverter admittance and grid admittance should be lower than 120° at their intersection. Based on this principle, a phase-reshaping strategy aiming to revise the phase of inverter admittance without using any real-time information of grid impedance or resonance frequency is proposed. The strategy is realized based on the feedforward of the voltage at the point of common coupling (PCC). In the feedforward, a lag network with an adjustable proportional element is employed to modify the admittance phase and the second-order notch filter is utilized to compensate for the admittance magnitude of the fundamental frequency. After adopting the proposed strategy, the system phase margin is always higher than 60°, which will yield effective suppression to harmonics around the intersection frequency. The validity of the proposed strategy is verified by experimental results.