1.4 Electronic governing - part 5
For turbines where there is a narrow range detector, a separate wide range speed-sensing circuit must be provided. Typically, this comprises a diode pump circuit, giving a linear conversion of frequency to voltage over a required range from barring speed up to the maximum synchronous speed. Wide range governing systems may operate to control the run-up by means of either the governing valves or the HP stop valves.
Run-up on the governor valves has the advantage that the HP steam chest can be 'warmed through' prior to steam admission to the turbine, but this arrangement requires very sensitive control at small valve openings in order to provide a smooth increase in speed. Control of the HP stop valves permits the use of a built-in pilot valve for steam control of the run-up phase. Since this has a lower-gain valve position/steam flow characteristic, the required sensitivity of the valve position controller is less critical.
The typical facilities required by the operator are:
- The ability to preset the terminal speed.
- A 'hold' button so that the speed can be held at the current value.
- A 'release' button to allow the run-up to continue.
- The ability to preselect speed ramp rates to any one of several values (usually five).
- An automatic run-up mode; this may be selected as an alternative to the fixed ramping rate which requires close operator supervision of machine parameters.