Resumo:
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.