Investigation of Critical Fault Clearing Time by Applying Different Excitation System Models

Evaluation of power system stability involves rotor angle stability which is done by determining the Critical Fault Clearing Time (CFCT). That is the maximum time a severe disturbance can be applied without the system losing its stability. Important factors to consider are power angle, moment of inertia of the whole power plant, X/R ratio in the net and which excitation system is being used for the synchronous generator. Prolonging the CFCT by changing reactance in the net or increasing the inertia constant by upgrading the power plant have been proven to be effective but expensive methods. The purpose of this paper is to understand CFCT and how tuning the excitation system can be a cost-effective method to extend it. Tuning in this paper done by monitoring the step responses on three different excitation models. Modelling and calculation are executed in a simulation software where a fault is simulated on one power line with one single 54.5 MW generator. CFCT is then first calculated with standard settings and later with adjusted settings. The rotor angle stability shows minor improvement after the tuning with different results for all three exciters.