Study on the mechanism of AC tie-line power fluctuation for a two-area interconnected power system

The expression of the tie-line power fluctuation of a two-area interconnected power system at the moment of power impact is derived in this paper. The power fluctuation process caused by a disturbance is divided into three stages, namely before, during and after the disturbance, and the dynamic equation for each stage is established and the variation of the state point is derived in this study. Based on the theory of the distribution of impact power in the power systems, the change of the tie-line power of a two-area interconnected power system after a disturbance is analyzed. After a disturbance, the tie-line power takes on damped oscillation of inter-area oscillation mode, and the steady-state power value is determined by the total inertia time constant of the generators in both areas. Meanwhile, the expression of power fluctuation, which is similar to the step response of a second-order system and the formula for the peak power during the first power swing are derived. The peak power is mainly determined by the ratio of total inertia time constants of the two-area power system's units and the damping ratio of the inter-area fluctuation mode. The result effectiveness of the proposed equations is verified by the simple system simulation method and the analysis on the generator-tripping contingency of the North China Power Grid and Central China Power Grid interconnected system. The study's conclusions are helpful for operators and engineers to master dynamic characteristics of power fluctuation on AC tie-lines of two-area interconnected power systems. The study provides the scientific basis for arranging operating schemes and control measures of interconnected power systems.