Power system stability can be enhanced with use of Fast Valving io steam turbines. Its actuation guarantees a fast reduction of mechanical power in faults and load rejections, reducing the accelerating area (PS) and raising the decelerating area. This article proposes a simple mathematical model for Fast Valving, which is simulated in a Brazilian thirteen bus system.OWER system stability studies evaluate steady state and P dynamical behavior of electrical systems, mathematical models used in these kinds of studies have their complexity raised due to size and topology of nowadays systems, requiring the development of new methods to assure stability, predicting dynamical behavior rhough simulation. Stability studies in power systems are several and in this article onIy electromechanical is considered.The enhancement of stability may be achieved by correct system design and operation, in practice financial considerations determine which features can be implemented to assure system operation in a feasible condition, that is, not so close to its stability limit and not with an excessive generation reserve. So the risk of losing stability can be reduced using additional elements inserted into the system to help smooth the system dynamic response.The stability of an electric power plant, which has as prime movers steam turbines, can be enhanced with the use of Fast Valving (FV). The FV control is calling attention because systems are operating near their stability limits and due to the difficulties in expanding the transmission system. Fast Valving is one of the least expensive controls for stability and its actuation consists on a fast reduction of the mechanical power when a large disturbance happens near to the generator equipped with this device. The fast reduction of mechanical power is originated by fast closure of Control Valves (CV) andor Intercept Valves (IV) of steam turbines, the fast closure must be initiated in the least time possible after the detection of a meaningfbl disturbance, followed by its reopening. The name Early Valve Actuation (EVA) is given when only the IV This work was supported by Capes. is actuated. Simulations and field testes have shown that FV actuation does not have much effect on turbine generator shaft and blades and enhances the stability limit, mainly in weak transmission networks [ 11.
If. STEAM T U R B~E MODELSThere are several kinds of steam turbines, that range from the most common nonreheat straight condensing, tandem compound single and double reheat, to cross compound single and double reheat. A generic mathematical model is shown in Fig. I, the coefficients K, to Ks determine the contributions from various turbine sections and for the cross compound units the individuat shaft mechanical power PmechHp and Pmech, must be explicitly modeled. Time constants associated to each stage are defined by q , the most significant of them represents the element of biggest physical dimension 7' ' , which is the reheater, T,, T6 and T7 represent the high pressure turbine bowl, the crossove...
