DQ Impedance Reshaping of Three-Phase Power-Controlled Grid-Connected Inverter for Low-Frequency Stability Improvement Under Weak Grid Condition

Phase-locked loop (PLL) is commonly used to synchronize the phase angle of the injected current of voltage source grid-connected inverters (GCIs) with that of the voltage at point of common coupling. However, the quadrature-axis component of the dq impedance model of the GCIs presents negative resistance characteristics in low-frequency range due to the usage of the PLL, which may lead to low-frequency instability phenomena if the GCIs work under weak grid condition. This paper presents a dq impedance reshaping method of powercontrolled GCIs to eliminate the negative effect of PLL on lowfrequency stability. The dq impedance models of the GCIs under current and power control modes are first established using complex vector and complex transfer function theory. On its basis, the negative effects of PLL on current control loop and power control loop are theoretically derived. A grid voltage feedforward loop is then designed in the control system of the powercontrolled GCIs, where the parameters of the feed-forward loop are calculated. The effectiveness of the proposed dq impedance reshaping method is validated by frequency scanning results and time-domain simulation results. Index Terms—Grid-connected inverter, impedance reshaping, low-frequency stability, negative resistance, phase-locked loop, power control loop.