Extended-term dynamic simulation using variable time step integration

The application of the Extended Time Scale Stability Analysis program (EXSTAB) is illustrated. The program was developed for the numerical simulation of power system phenomena spanning time periods from several seconds to several hours and involving slow events, such as load cycles, as well as fast transients, such as line trips. The integration algorithm in this problem automatically reduces the time step of integration to capture fast transients and gradually increases it as the transients decay. The algorithm is based on the simultaneous solution of the differential and algebraic equations describing a power system, using a numerically stable, implicit, low-order integrator similar to the trapezoidal rule. Recognizing that the simulation of severe transients over periods of time of a few seconds is handled more efficiently using explicit methods with fixed time steps, the program automatically switches to an explicit integrator following the occurrence of a severe disturbance. The program automatically returns to an implicit mode once the severe transient is over.<>