Abstract: Low-frequency oscillations in traditional power systems dominated by synchronous generators (SG) are usually analyzed by using the electromechanical model, the quasi-steady state model and algebraic-equation power flow descriptions. Whereas sub-synchronous oscillations or wide-frequency-band oscillations in power-electronic-based power systems are studied with the electromagnetic model, the dynamic network and differential-equation power flow descriptions. Aiming at general oscillations of power systems under different frequencies after small-disturbance instability, this paper develops a unified form of external characteristics of power devices, including not only SG, but also voltage source converter (VSC), which is dominant in power-electronic-based power systems. The power characteristic equations for both dynamic and static networks are derived, and a clear physical picture for power transmission on the network is established. Based on these studies, it is found that the dependence of power on oscillation frequency can be divided into three different regions, including low-frequency region (below 10Hz), the resonance region (from 10Hz to 200Hz), and the high-frequency region (above 200Hz). It is also found that the static network description for low-frequency oscillations is applicable. Whereas for sub-synchronous and high-frequency oscillations, the dynamic network description has to be considered. All these theoretic analytical results have been well verified by extensive simulations in two-machine systems and multi-machine systems.