1.3.3 Governor speed reference
The governor speed reference is the prime input by which the turbine can be controlled, either by the operator or by one or more external automatic controlling functions. The range of speeds over which the reference can be set will depend on whether the turbine is synchronised or unsynchronised.
Prior to synchronisation, the speed reference range will extend from the lowest controllable speed (typically 3% of nominal) up to the highest speed at which the turbine-generator must be capable of synchronisation (104%). Once synchronised, the speed reference will be limited to the range 94-106%.
Different maximum rates of increase or decrease of the speed reference also apply. Prior to synchronisation, the rate of increase may be selected by the operator or it may be a quite rapid fixed rate which is compatible with the maximum conceivable rate at which the turbine speed could be increased if warmed through and all conditions were at an optimum. In this latter case, the setpoint would either be manually raised by the operator in small steps, subject to the observed conditions, or raised by a separate automatic run-up system. The rate of decrease is usually a single fixed rate.
When the turbine-generator comes under control of the auto-synchroniser, or if manual synchronisation is to be performed, another rate-of-change of speed reference will apply. This must be compatible with the autosynchronising unit and is usually designed to give fast and certain synchronisation.
When synchronised and under operator control, the rate-of-change is conventionally set to give full travel from -6% to +6% of nominal speed in 2 minutes. The governor speed reference now acts as a load reference and, if a 4% droop is applicable, this means that the equivalent rate of load change is 2.5% per second. Whether or not this rate-of-change can be sustained is dependent on the steam supply system, so the operator must take care to control the appropriate parameters if a prolonged increase or decrease is required.
An automatic unit controller is generally used to raise or lower the governor speed setpoint in preference to the operator. In this case a specific rate, usually slower than 2.5% load per second, will apply.