1.5 Governor valve relays - part 1
The output of the electronic speed governor requires conversion from a current signal to a mechanical force of sufficient power to actuate the steam valves. It is the function of the governor relays to perform this duty.
The hydraulic relay has no competitor as a force amplifier for steam valve actuation. It has the capability to develop the high power needed for rapid valve opening or closing, without a significant deadband or hysteresis.
The usual arrangement for electronic governing is to employ a single separate relay for each steam valve, the conversion being carried out by a proprietary electrohydraulic servo-valve. There are many different forms of valve relay, of which three are used on CEGB turbines with electronic governing. These will be referred to as Types 1, 2 and 3 in the descriptions which follow. Types 1 and 2 use the same proprietary servo-valve which will be described first. The servo-valve used with the Type 3 relay is included in the description of the relay.
Figure 2.18 shows a servo-valve in cross-section, together with a diagrammatic representation of how it is connected in a simple position-loop controlling a primary ram. The feedback to the amplifier in the governing system is provided by a linear variable differential transformer (LVDT).