Small-signal Stability Analysis and Improvement with Phase-shift Phase-locked Loop Based on Back Electromotive Force Observer for VSC-HVDC in Weak Grids

Voltage source converter based high-voltage direct current (VSC-HVDC) transmission technology has been extensively employed in power systems with a high penetration of renewable energy resources. However, connecting voltage (VSC) to an AC weak grid may cause the system become unstable. In this paper, phase-shift phase-locked loop (PS-PLL) is proposed wherein back electromotive force (BEMF) observer added conventional (PLL). The BEMF used observe infinite stationary $\alpha\beta$ frame, which avoids problem inaccurate observations xmlns:xlink="http://www.w3.org/1999/xlink">$dq$ frame that are caused by output phase angle errors PLL. VSC using PS-PLL can operate as if it facing strong grid, thus enhancing stability VSC-HVDC system. only needs be properly modified on basis traditional PLL, makes easy implement. addition, because difficult obtain exact impedance influence short-circuit ratio (SCR) estimation performance also studied. effectiveness verified small-signal analysis and time-domain simulation.