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DQ Impedance Reshaping of Three-Phase Power-Controlled Grid-Connected Inverter for Low-Frequency Stability Improvement Under Weak Grid Condition Weihua Zhou∗ , Yanbo Wang∗ , Raymundo E. Torres-Olguin† and Zhe Chen∗ ∗Department of Energy Technology, Aalborg University, Aalborg, Denmark
Abstract—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.
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