1.4 Electronic governing - part 8
A characteristic feature of most electronic governors is that they exercise individual control over each steam control valve in the turbine in response to an overall steam demand signal. This permits on-load testing of individual valves and also fault monitoring with the capability to close a valve in the event of a failure.
Particular mention is made here of the characteristics of the valve position loop and its significance in achieving the desired control of turbine load.
Given constant steam inlet conditions, the power output of a turbine is a linear function of steam flow passing through the no-load point at a small percentage of maximum steam flow. This characteristic is known as the Willans line and is shown in Fig 2.16 (a). In the conventional condensing turbine used for driving a power generator, the pressure drop across the turbine is also directly proportional to the steam flow through it.
The desired characteristic is therefore that the steam demand input to the valve position controller should have a linear relationship with steam flow. However, inherent features of valve design are non-linear so that a valve linearising function has to be introduced into the governor to restore the required linearity. In particular, the valve area/steam flow relationship is non-linear for values of pressure ratio (across the valve) less than the critical value. Refer to Kearton, page 628  and the typical characteristic is shown in Fig 2.16 (b). Depending on the valve shape, there will also be a non-linear relationship between valve lift and valve area, giving the characteristic shown in Fig 2.16 (c). To linearise these two effects, the steam demand is shaped by linearising circuits to give a characteristic of the form shown in Fig 2.16 (d). Additional versine effects arising from the lever arrangement driving the valve position pick-off may also be included in the compensation. Each individual valve controller has three-channel valve steam demand signals as input. Its function is to take a majority vote of these signals to form a signal demand, to provide linearisation and to provide position control of the turbine steam valve, using the signal derived from a valve position transducer as feedback. Additionally, monitoring of majority voter operation and of servo-valve or position transducer faults is carried out, with the capability of automatic shutdown in the event of loss of accurate valve position control.