The steam flow from the boiler to the turbine of a large conventional fossil fuelled generator is regulated by a valve of considerable size and weight. For the turbogenerator to respond quickly to a requirement to increase or decrease its output, this valve has to be opened or closed as quickly as possible. This is achieved through a hydraulic actuator. The control signal for the hydraulic actuator is provided by the governor. In the past governors were of mechanical nature but modern generators are fitted with electronic governors. The governor's function is to measure the rotational speed of the generator, to compare it to the reference value (50 or 60 Hz) and, based on the error signal, to instruct the hydraulic actuator to open or close the steam valve.
Figure 3.4 shows the action of the governor of a single generator supplying a load with 50 Hz AC. Governors are designed to operate as proportional control systems; i.e. an error must be present between the set point frequency and the actual frequency for the governor to alter the fuel or steam supply. This proportional control is characterized by the line in Figure 3.4 which has a fall or droop of 4% across the operational range. This 4% value has been found through extensive experience to be appropriate for stable governing and is widespread. Governors have the inbuilt facility that allows an operator to adjust the frequency at which the characteristic intercepts the frequency axis, known as the set point. For the line aa the set point is 52 Hz. With this set point, the system frequency is 52 Hz when the generator is supplying no load and is reduced incrementally to 50 Hz when the load increases to rated generator power. Changing the set point to 51Hz moves the line to bb. Set point adjustment is of considerable importance because it allows power system operators to decide how the demand is shared by the generators on the grid, but more will be given on this later.
Figure 3.4 Frequency/power governor characteristic
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