1.4 Electronic governing - part 2
Failures caused by spurious valve opening can be designed to be less likely than failures resulting in valve closure. For instance, loss of hydraulic or electrical supplies can be made to close the valve by a closing spring or by a preset electrical null bias.
In practice, the most likely effect on operation will be a fault causing a load reduction or a complete loss of load. It will be seen from the block schematic diagram (Fig 2.11) that single faults in a valve channel can cause one valve to close, giving a partial loss of load. A complete loss of load would require a failure in the common processing part of the governor. There are thus strong grounds for enhancing the reliability of this part of the governor by providing redundancy or monitoring techniques. These are described later in this section. The probability of not achieving a complete loss of load is therefore another important governor characteristic.
Because redundancy or monitoring is normally employed, when faults occur in the common processing part of the governor, they will be annunciated to the operator so that maintenance can be carried out. Faults can also occur in individual valve channels causing only a partial loss of load. These can usually be repaired on-line without interfering with operation or registering in any of the previous definitions of reliability. They will, however, be embraced in the overall 'mean time between failures' statistics for the governor and will obviously be of importance to the user of governing systems.