An Exact Method to Compute Time Delay Margin for Stability of Time- Delayed Generator Excitation Control System (original) (raw)

This paper investigates the effect of time delays on the stability of a generator excitation control system compensated with a stabilizing transformer known as rate feedback stabilizer to damp out oscillations. The time delays are due to the use of measurement devices and communication links for data transfer. A direct and exact method based on Rekasius substitution is proposed to determine the maximum amount of time delay known as delay margin that the system can tolerate without loosing its stability. The proposed method starts with the determination of all possible purely imaginary characteristic roots for any positive time delay. To achieve this, Rekasius substitution is first used to convert the transcendental characteristic equation into a polynomial. Then, Routh stability criterion is applied to determine the critical root, the corresponding oscillation frequency and the delay margin for stability. It is found that the excitation control system becomes unstable when the time delay crosses certain critical values. Theoretical delay margins are computed for a wide range of controller gains and their accuracy are verified by using Matlab/Simulink. Results also indicate that the addition of a stabilizing transformer to the excitation system increases the delay margin and improves the system damping significantly.