1.4 Electronic governing - part 3
Figure 2.12 shows a detailed block diagram of a typical governing system. The main feature of an analogue system is that each function is performed by a discrete set of circuit components, whose sole task is to perform that function by converting input signals into output signals whose amplitude (in voltage or current terms) is directly proportional to the required transfer function.
This feature is particularly suited to the adoption of a modular approach to the hardware, where a function (or a group of functions) are performed by a module and the output (or outputs) passed on to following modules. This permits a simple approach to fault finding by monitoring signal levels.
A three-channel system can be made to satisfy the reliability requirements established for governing systems. If a failure occurs in one channel, it causes the output of that channel to deviate from the other two channels. Detection circuits are incorporated, in addition to the three processing channels, to identify failed channels and to disconnect them before they can contribute an erroneous output. Because the detection circuits determine the output on the basis of the correctly operating channels, they are often called 'majority voting circuits'. Figure 2.13 shows the block diagram of Fig 2.12 divided into three independent parallel channels. The central processing part of the governor terminates where the three outputs of each linearising network are monitored by the final majority rating circuits associated with the steam valve servos. These circuits are extremely simple and therefore reliable. They have the characteristic that for small discrepancies in the input signals, the output signal is the average of the inputs whilst, for large discrepancies, the mode of action becomes that of a median-selector.
It will be noted from the diagram that the exhaust pressure unloader is isolated from the main signal path by a triplex majority-voting system. Since transducer failures are a major factor in system reliability, this gives a further measure of protection for the basic governor which will continue to operate as a full three-channel system.
Each valve servo amplifier is ostensibly single-channel and the precise configuration is closely associated with the hydraulic system used to amplify the electronic signals into forces large enough to position the steam valves. The variety of configurations employed are fully described in Section 1.5 of this chapter. For the moment, it should be noted that a degree of self-monitoring is provided so that a faulty valve, hydraulic relay or servo amplifier will identify itself to the operator and apply a closing signal to the valve. Inter-valve monitoring is performed in some governors, since it is usual for all valves of one type (e.g., HP governor valves) to have the same percentage opening, within limits. However, this form of monitoring must be vetoed during on-load testing of valves where only one valve of a set is closed at any time.