The determination of the stability regions via the controlling unstable equilibrium points in AC-DC system transient stability analysis

In this paper, a sequential integrated AC-DC system transient stability analysis is accomplished by taking into account the variation of the amplitude of the bus voltages during transient. This variation of the bus voltages is provided by power-flow analysis of the AC-DC system with respect to the modified active power, since the active power transferred through the DC line changes at each integration step. Thus, the AC-DC transient stability analysis is accomplished considering the true value of the ratio of the reactive power absorbed from the converters to the active power transferred through DC line by the sequential approach. In this study, the DC line is taken into consideration as a monopolar one and is also represented as the current injections to the buses where it is connected and its effect is reflected at the internal buses in the form of additional bus power injections. The system loads are also represented as current injections to the buses where they are connected. The effect of these is again reflected at the internal buses in the form of additional bus power injections. Thus, the problem in not constructing a valid Lyapunov function due to the transfer conductance is eliminated.