Transient stability analysis of AC/DC power system based on transient energy function

In this paper, a transient energy function for AC/DC power systems is used to evaluate critical clearing times with consideration of the DC dynamics. The generator angular speeds, rotor angles and voltages at the HVDC link's ac buses are explicit variables in the transient energy function. A method for computing the transient energy function numerically is proposed. In the method, the angular speeds and rotor angles deemed as slow dynamics are integrated along sustained fault trajectories, and the integral path of the DC dynamics treated as fast dynamics are acquired by some modifications. The problems in applying potential energy boundary surface (PEBS) method to the computation of critical clearing time are also discussed. The critical clearing times for 4-generator and 10-generator AC/DC systems obtained using the transient energy function approximate well to those with the step-by-step time simulation.