I. Introduction
The past few years have witnessed an exponential growth of power converters in electric grids[l]. Unlike synchronous generators, power converters requires PLLs to track grid phase [2]. However, in weak grids, the small-signal instability (or harmonic instability) may happen due to the existence of PLL. It is reported that a higher PLL bandwidth yields a wider frequency range of negative resistance behavior [3], [4], which could further introduce sideband oscillations [5]. Many research works have thus been made to mitigate the influence of PLL on system stability.