Phase-reshaping strategy for enhancing grid-connected inverter robustness to grid impedance

As the grid inductor increases the phase difference between the inverter admittance and the grid admittance increases at their intersection frequency showing that the harmonic amplification coefficient is enhanced. When the phase difference exceeds 180 degrees the admittance ratio cannot satisfy Nyquist criterion and the system becomes unstable. To address this problem a phase-reshaping strategy aiming to revise the phase of the inverter admittance in a wide frequency region without using any real-time information referring to the grid impedance or the resonant frequency is proposed. The strategy is realised by the combination of the feedforward of the voltage at the point of common coupling (PCC) and the harmonic compensators. In the feedforward, a phase lag network is employed to modify the admittance phase in a wide frequency region. The harmonic compensator in parallel to the current controller is used to offset the magnitude rise brought in by the PCC voltage feedforward. After adopting the proposed strategy, the upper limit of the phase difference is constrained at 150 degrees and the magnitude of the inverter admittance is lower than that of the original one at low-order odd-harmonic frequency. The validity of the proposed strategy is verified by the experimental results.