Abstract: Electromechanical transient simulation is an important tool for large-scale power system analysis. Existing electromechanical transient models for power electronic equipment generally have ideal characteristics of current source, which cannot reflect the impact of equipment stability and capacity constraints on system transient characteristics. In the background of power electronics dominated power systems, the above deficiencies reduce the simulation accuracy, which restricts the determination of power grid operation limit and renewable energy transmission capacity. In this paper, the unitary mathematical model and control system of static var generator（SVG） is derived based on the rotating coordinate frame and the corresponding electromechanical transient model is established. On the premise that the modeling accuracy is consistent with the electromagnetic transient model, the main differences between electromechanical transient simulation and electromagnetic transient simulation are compared based on the mode analysis method, which reveals the capability boundary and applicable scope of the electromagnetic transient simulation. The results show that the electromechanical transient simulation can reveal the power angle, frequency, and voltage stability characteristics related to fundamental-frequency power energy flow, but cannot reflect the resonance or broadband oscillation characteristics strongly related to power grid dynamics.