Modelagem e análise da influência da fast valving na estabilidade transitória de sistemas elétricos de potência

Abstract

The transient stability of power systems can be enhanced with the use of Fast Valving (FV) in steam turbines. Fast Valving control has been calling attention around the world due the fact that power systems are operating near their stability limits and due the difficulties in expanding the transmission systems. In Brazil it happens either. The number of thermoelectric power plants in Brazilian power system is increasing, so it is necessary to use accurate mathematical models to represent them. Mathematical models used in stability studies are reaching a sophisticated level due to power systems size and topology, so it demands the development of new techniques to assure stability, predicting with simulations the behavior of the systems. Fast Valving is one of the least expensive controls for stability, but one of the biggest difficulties is that there are not mathematical models available for it at the moment, just articles and books that mention how FV actuates, that is, a quick mechanical power reduction of the steam turbine when a large power oscillation happens. So the main purpose of this master dissertation is to develop mathematical models of FV and analyze its behavior in electric power systems. The fast reduction of mechanical power happens due to the fast closure of the intercept valves (IV) or the fast closure of control and intercept valves together. The actuation must start in the minimum time after the detection, for example, of a fault and then valves must reopen totally or partially in a short period of time. FV principle is strictly related to the equal area criterion: when a fault or load shedding happens, the electrical power suddenly reduces, to avoid a possible loss of synchronism mechanical power must be reduced with the fast closure of the steam turbine valves. The fast reduction of mechanical power increases the deccelerating area, and reduces the accelerating area. According to the equal area criterion, if the deccelerating area is bigger than the accelerating area, the system will not lose synchronism. There are several philosophies to actuate FV and the combined utilization of some methods is considered an optimization. Usually FV actuation is made by special solenoids and there are keys in the dispatch office that permits to choose whether the valves will be available to actuate or not. The advantage of FV is that with its actuation the thermal unit does not need to be disconnected of the system in many cases, avoiding resynchronisation and besides FV raises the critical clearing time mainly in weak transmission networks.