1.4 Electronic governing - part 1
Traditionally, the early mechanical and mechanical-hydraulic governors, performed their main function of maintaining the safety of the unit reliably. The provision of series emergency stop and governor valves has provided protection against the occasional failure of a valve to respond to its control signal, while the failure mode of the mechanical speed error sensing arrangement has generally been to deteriorate slowly in performance rather than to fail suddenly.
Even so, the provision of two sets of steam valves in series, with one set operated directly by a separate protection system has also negated the effect of a sudden failure. The reliability of mechanical governing systems has therefore not been closely monitored in the past.
Before discussing the configuration of the electro-hydraulic governing systems current on modern turbine plant, it is necessary to understand the meaning of reliability in this context. The general definition of reliability is the probability of successfully achieving a desired task, usually over some specified time period. Taking the total task of the governing and protection systems to prevent a dangerous overspeed, the probability of success in this must be of such a high order that failure is never encountered. This is achieved by providing completely independent governing and protection systems, using different hardware and different techniques for speed sensing, operating by different routes to the two series-connected sets of valve gear. Obviously, the reliability of the speed governor should be as high as possible to achieve this and may be defined as the probability of not exceeding the overspeed trip setpoint. However, in any general governing system, the valve position output has three failure modes. A valve can close down, open up or stick in its current position. It is only when either of the last two occurs and is followed by an event leading to a sudden unloading of the turbine, for example, the opening of the circuit-breaker on a turbine driving a generator, that the overspeed trip is brought into operation. There is also the possibility, in an uncoupled turbine, that a failure will occur through an excessive speed demand.