Step-change in generator mechanical power: comprehensive analysis

This paper presents comprehensive analysis of the effects of the generator mechanical power (Pm) change on its stability using the famous Equal Area Criterion (EAC) method providing solved examples and plots. An interactive software (PSS_EAC) for the comprehension of transient stability using the equal area criterion method and time domain simulation (TD) method, developed by the authors, was used for analysis. Generator input mechanical power (P _m) step-change disturbance is very important for analysis of generator stability assessment. A sudden loss of pressure, failure of intake valves and other controls can cause a sudden step-change in Pm. This can further cause transient stability problems. Although this is usually not a practical concern, since the Pm typically changes very slowly, this scenario cannot be ruled out. Various text-books have illustrated the concept but not much detailed examples exist on the effects of P _m-change. The examples presented demonstrate the effects of P _m-change on system stability. Effects of line conductances, machine damping and inertia have also been considered during the analysis. The various equations involved in coding the problem have been given. The cases involve both a sudden increase in P _m (Figure 1) and a sudden decrease in P _m at time t=O ⁺. The (Figure Presented) Figure 1. Equal area criterion (Pm-Change: Sudden Increase) (Figure Presented) Figure 2. Accel./Dec. areas vs. P _m' greater the step-change in P _m, the farther the maximum swing on machine angle (δ). This fact has been elaborated with swing plots. The transient stability margin and acceleration/deceleration plots (Figure 2) are also given. It can be concluded from the results that since the generator first swing is the most important, inclusion of damping term in the swing equation has very little effect on the maximum swing angle. Similarly the effect of high R/X ratio lines shows limitations of the EAC method and there must be an upper bound on the ratio in order to get convincing results using the EAC method. A higher machine inertia constant will help in the overall stability of the machine undergoing disturbance. Although EAC method gives conservative results, this paper has provided the various details of analysis for comprehension of the generator stability assessment, and the various aspects of dynamic performance of power system at the level of SMIB system.