A technique for improving transient stability in lightly loaded systems by real and reactive power dispatching

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Modern EHV transmission lines are characterized during light load periods by an enormous amount of reactive power production. In order to absorb this surplus of reactive power, system generators must be run underexcited, thus reducing their own and the overall system stability margins. To cope with this problem, a technique is discussed which provides for an optimal real and reactive power allocation, so as to minimize in so far as possible the risk of system instability. This technique is carried on in the following two phases: a) redispatching the real load among the generators according to their ratings and their transient-important characteristics such as inertia constant and transient reactance. b) minimizing reactive power generation and real losses by means of TCUL tap settings. To illustrate the proposed method and the associated computer programs, a three-bus and a nine-bus multimachine systems have first been subjected to this optimal dispatching technique, and then tested for a series of severe three-phase faults. The results show a considerable improvement in terms of critical switching times, thus proving the technique as being actually effective.