I. Introduction
The growing penetration of the power electronics-interfaced renewable energy sources has led to the emergence of power-electronic-based power systems into electrical grids, such as wind farms [1], photovoltaic power plants [2], and micro-grids [3]. Those systems are featured with the high efficiency and flexibility in electric power generation and distribution, but are also more susceptible to the network disturbances in a wide frequency range [4]. In inverter-fed power systems, a number of oscillation phenomena have been reported, which includes the low-frequency oscillations caused by the droop-enabled power control [5], and grid synchronization control [6]. In addition, the harmonic-frequency oscillations from hundreds of hertz to several kilohertz may happen, which is caused by the interaction of the inner control loops as well as the inductive or capacitive behavior of inverters [7]. It is undoubtable that these oscillation phenomena will weaken the power quality and cause the instability of power system.