1.4 Electronic governing - part 6
Therefore, as in the case of the narrow range governor, these requirements are conveniently carried out by a motorised potentiometer suitably controlled by the operator. More recent practice employs the electronic equivalent of the motorised potentiometer (shown in Fig 2.14).
The speed control on the operator's terminal provides an input to two signal comparators C1 and C2, whose other input is equivalent to the demanded speed. If there is no difference between these two speed demands, neither oscillator is activated and the counter retains its existing count, giving the defined demand signal at the output of the digital-to-analogue converter (DAC). If the operator demands a higher terminal speed, the appropriate comparator activates its associated oscillator to increase the count until the DAC output reaches the terminal speed selected by the operator. The effect is that the DAC output is ramped-up until it reaches the terminal speed, the rate of increase being proportional to the oscillator frequency, set by the run-up rate control which is also on the operator's desk. Although typically there is a choice of five runup rates, the 'lower' oscillator is normally fixed to give a single fast rate-of-speed decrease.
The demanded speed is compared with the measured speed in the speed error amplifier, the gain of which is set such that a 10% change in valve position is achieved for about 1% change in turbine speed error. This implies a lower loop gain than the normal 4% droop of the narrow range governor, giving a large stability margin. The speed error amplifier acts through either the stop or governing valve controllers to give precise control of the actual turbine speed to that selected by the operator. Since the turbine-generator remains unsynchronised during run-up, the loading is light and with near normal steam pressures only a small proportion of the full valve opening is necessary to achieve run-up to speed. Often a steam demand limit is applied during run-up and if this limit is exceeded the operator is alerted to a possible fault.