I. Introduction
With the increasing of renewable generations, power electronic interfaced power devices are replacing electro-magnetic power devices (e.g. synchronous generators) [1]–[2]. Due to the existence of energy storage elements with various energy storage levels and relevant multi-timescale controls, fast dynamics that close to the time scale of AC network dynamics have been occurring in the internal-voltage amplitude/frequency (A/F) at the port of power devices, which causes the increasingly significant impact of dynamic characteristics of AC network on system dynamic behaviors. Growing occurrences of related oscillations in power systems are seriously endangering the system safe and normal operation [3]–[5]. Therefore, it is urgent to realize a universal system dynamic analysis considering dynamic characteristics of AC network, in which small disturbance relationship and its linearization of AC network as fundaments should be concerned at first.