Stability Improvement for Three-Phase Grid-Connected Converters Through Impedance Reshaping in Quadrature-Axis

• Published in: IEEE transactions on power electronics Vol. 33; no. 10; pp. 8365 - 8375
• Main Authors: Fang, Jingyang, Li, Xiaoqiang, Li, Hongchang, Tang, Yi
• Format: Journal Article
• Language: English
• Published: New York IEEE 01.10.2018; The Institute of Electrical and Electronics Engineers, Inc. (IEEE)
• Subjects: Control stability; Distributed generation; Electric converters; Energy conversion; Energy storage; Frequency conversion; Impedance; Phase locked loops; phase-locked-loop (PLL); power converter; Power converters; Power system stability; Reactive power; Stability; Stability analysis; stability improvement; Storage systems; Synchronism; synchronous frame; Time-frequency analysis; Voltage control; Waveforms
• ISSN: 0885-8993; 1941-0107; 1941-0107
• DOI: 10.1109/TPEL.2017.2777972

Three-phase AC-DC and DC-AC power converters have been extensively employed as grid-interfaces in various applications, e.g., distributed generation and energy storage systems. In these applications, power converters should always synchronize with the mains grid so that active and/or reactive power can properly be regulated while maintaining desired waveforms of grid currents. Grid synchronization necessitates accurate information of grid voltages, which is normally obtained through phase-locked-loops (PLLs). However, the employment of PLLs may bring in stability concerns. Previous research revealed that the inclusion of PLLs shapes the impedance of power converters into a negative resistance in the quadrature-axis (q-axis), and this should be responsible for instability. To resolve the instability issue caused by PLLs, 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 by directly relating the q -axis voltage to the q-axis current reference. As a result, the presented three-phase power conversion system can operate stably even under a severely weak grid condition, which are verified by simulation and experimental results.