A Critical Look At The Equal-Area Criterion For Analyzing Transient Stability: Is What We Have All Learned Correct?

The equal-area criterion is a graphical method for analyzing transient stability that has been known since the 1920s. It is intuitive, easy to understand, and used primarily in academic education. However, the integral of power P over transient angle   used to calculate the acceleration area Facc and deceleration area Fdec in the P- -diagram has no physical significance, since it describes power but not energy. This leads to the further question of what these areas actually represent. In this paper the energy changes during rotor acceleration and deceleration and their relationships to the areas Facc and Fdec are analyzed and obvious contradictions between assumptions and used swing equation, such as the usual assumption of constant turbine power, are explained. Key findings are that the areas Facc and Fdec only represent a small portion of the energy exchanged during the acceleration or deceleration phase, and that a M- -representation (M≙torque) should be used instead of the usual P- -diagrams in order to remain physically correct.