Low Frequency Oscillation Suppression of Three-Phase Four-Wire Inverter Based on CFM-OSG Phase-Locked Loop

To handle the low frequency oscillations of the three-phase four-wire inverter grid-connected system caused by small phase-locked range, slow locking speed and large steady-state error of the traditional phase-locked loop, an improved phase-locked loop is proposed to suppress low frequency oscillation. Firstly, the paper established a sequence impedance model of three-phase four-wire inverter, and analyzed the impact of the phase-locked loop on low frequency stability in a weak power grid and the mechanism of low frequency oscillations. Secondly, based on the cross-coupling method of the Complex Filter Matrix (CFM)-Orthogonal Signal Generator (OSG), the paper combined a first-order complex filter with a positive-negative sequence voltage component relationship matrix to form an improved phase-locked loop. The improved phase-locked loop is used to reshape the positive sequence, negative sequence and zero sequence impedance of the inverter, which not only increased the impedance amplitude of the inverter system, but also realized low frequency oscillations suppression. Finally, applying the simulation and experimentation, the paper verified the validity of the established sequence impedance and the effectiveness of the proposed improved phase-locked loop to suppress low frequency oscillation in the three-phase four-wire weak power grid.