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 a voltage source converter (VSC) to an AC weak grid may cause the converter system to become unstable. In this paper, a phase-shift phase-locked loop (PS-PLL) is proposed wherein a back electromotive force (BEMF) observer is added to the conventional phase-locked loop (PLL). The BEMF observer is used to observe the voltage of the infinite grid in the stationary
$\alpha\beta$
frame, which avoids the problem of inaccurate observations of the grid voltage in the
$dq$
frame that are caused by the output phase angle errors of the PLL. The VSC using the PS-PLL can operate as if it is facing a strong grid, thus enhancing the stability of the VSC-HVDC system. The proposed PS-PLL only needs to be properly modified on the basis of a traditional PLL, which makes it easy to implement. In addition, because it is difficult to obtain the exact impedance of the grid, the influence of short-circuit ratio (SCR) estimation errors on the performance of the PS-PLL is also studied. The effectiveness of the proposed PS-PLL is verified by the small-signal stability analysis and time-domain simulation.