A coherency based rescheduling method for dynamic security

For power system online dynamic security analysis (DSA), preventive control or remedial action should be an integral part of the function if instability for a contingency is detected. Research done so far in online remedial action has been in rescheduling generation and most of the suggested methods for determining such preventive control use the sensitivities of the stability energy margin to the generator power injections. For any approach using the energy margin, computation time is a concern and a somewhat different approach was suggested that determines control action by maximizing coherency of the generators without the use of sensitivities. In this paper, a new coherency based sensitivity method is proposed for generation rescheduling. Different coherency indices have been defined and then compared by ranking the contingencies according to these indices as well as the energy margin index. Since the coherency indices are always functions of the rotor angles, the sensitivity trajectories of a coherency index, such as the most critical rotor angle, with respect to changes of generation can be calculated at every time step of the integration process. This paper suggests that these sensitivities calculated shortly after fault clearing be used for rescheduling the generation. The calculation of these sensitivities are obviously faster than the calculation of the energy margin sensitivities. This paper also shows, with test results using several different systems, that the rescheduling achieved by this method provides the necessary remedial action. It is also noted that this method is intuitively more direct as it uses the sensitivities of the worst affected generator angles for rescheduling.