Calculation of the transient stability boundary of AC/DC inter-connected system based on the stability margin of dynamic energy Calculation of transient stability boundary of AC/DC inter-connected system

Concerning the transient power angle stability problem in AC/DC inter-connected system caused by DC line grounding fault, a transient stability analysis method based on the stability margin of dynamic energy is proposed. First, the analytical expressions of the dynamic energy function before alpha retard, during alpha retard and after alpha retard are derived. According to the topologies of system and the integration intervals of energy function in different stages, the expressions of the acceleration energy, position energy and margin energy are obtained, which, respectively, reflect the rotating speed of system, the swing range of power angle and the system's ability of resisting in stability. Then, the surplus energy in the system is calculated by subtracting the sum of the acceleration energy and position energy from the margin energy, and the stability margin of the system is obtained. And then, the stability boundary of the system is derived. It is revealed that, if the stability margin is exactly right 0 at the end moment of alpha retard, the system is critical stable. Finally, hardware-in-the-loop tests are conducted in RT-LAB platform. The simulation results verify that, the stability margin reaches the minimal value at the end moment of alpha retard, and this value can clearly reflect the stability of system. Besides, the earlier the alpha retard starts or the longer the distance between the fault point and the rectifier outlet is, the better it is for the stability of system.