Reshaping Quadrature-Axis Impedance of Three-Phase Grid-Connected Converters for Low-Frequency Stability Improvement

When employed as grid-interfaces, three-phase AC-DC and DC-AC power converters should always synchronize with the power grid so that active and/or reactive power can properly be regulated while maintaining desired grid-injected currents. Grid synchronization necessitates information of grid voltages, which is normally obtained through phase-locked-loops (PLLs). However, the employment of PLLs may bring in stability concerns, because it shapes the impedance of power converters into a negative resistance in the quadrature-axis (q-axis). To resolve the instability issues, this paper proposes an impedance controller for reshaping the q-axis impedance into a positive resistance in the low-frequency band. Without any extra burden on system hardware, the proposed controller can easily be implemented through directly relating the q-axis voltage to the q-axis current reference. As a result, the presented three-phase power conversion system operates stably even under a severely weak grid condition, as already been verified by simulation and experimental results.