  Power flow oscillations can occur in power systems after major disturbances
such as system faults, which may result in significant power swings and even
lead to system collapse. This paper provides a detailed analysis of the
efficacy of the legacy power swing blocking and out-of-step tripping functions
in grid-following voltage-source converter (GFL-VSC) systems. First, the power
swing dynamics of GFL-VSC are characterized, considering the outer power
control loops and phase-locked loop (PLL). It is found that the loss of
synchronism (LOS) of the GFL-VSC system is caused by the divergence of the
output angle of the PLL, which is fundamentally different from that of
synchronous generator (SG)-based systems, whose LOS originates from the
divergence of power angle differences between power sources. Therefore, the
legacy setting principles for power swing blocking and out-of-step tripping
function designed for SG-based systems can not be directly applied to GFL-VSC
systems, and may even malfunction. Time-domain simulations are performed in the
PSCAD/EMTDC platform to validate the correctness of the theoretical analysis.
