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plants and loads brings advantages:
􀂃 There is less dependence on any one generating plant
􀂃 Consumer demand is less variable
􀂃 The proportion of spinning reserve can be reduced
􀂃 Efficient plant can be more fully utilised.
Fig. 4.2 Major transmission routes at 400kV and 275kV in England and Wales
The England and Wales system is connected to Scotland (and on to Ireland) and France
(and on to the rest of Europe).
S4: System Frequency & Voltage
41 EEE8044: Fundamentals
4.3 Control of System Frequency
For a synchronous generator, the frequency of the terminal voltage is proportional to thegenerator speed. In an interconnected system there is a common working frequency, whichdefines the speed of every generator. As the consumer demand increases, more electricalpower is produced by each generator.Viewed on an hour by hour basis, the 24-hour load demand curve shown in Fig. 4.1 has afairly smooth characteristic representing the average behaviour of customers. However, acloser look over say a 10 minute period, or a minute by minute or a second by second
basis, will show that the load demand curve is characterised by a random variationsuperimposed on the hourly average. At any instant, it is practically impossible to exactlymatch the power output of the generators to the actual load demand. If the power output is
greater than the demand the speed of the generators will increase, thus increasing thefrequency of the supply. Similarly, if the demand exceeds the power output at any instantthe speed will drop with a corresponding reduction in frequency. For example, an increasein load without a corresponding increase in generation would result in a power imbalance.
The load power must be taken from somewhere, and the only possible source is the kineticenergy of the rotating masses connected to the machine rotors. The speed will thus drop asthe kinetic energy is being consumed.The frequency is thus not constant but varies continuously as a result of the instantaneouspower mismatch or imbalance between supply and demand. In a large interconnectedpower system the frequency is continuously monitored to within very narrow limits ofaccuracy and action is taken to correct any long term rise or fall by automatic regulation ofgenerator input power in a manner that ensures the most economic system operation.
Fig. 4.3 shows a block diagram for governor control in a large steam-turbine power station.The main steam valve is controlled by the governor and has a direct, fast effect on the highpressure turbine output.
steam valve
P=40%
BOILER
governor control
high pressure
turbine
reheat
intermediate
turbine
pressure
P=30%
low pressure GENERATOR
turbines
P=30%
Fig. 4.3 Governor control
A typical governor characteristic for a large steam turbogenerator is shown in Fig. 4.4showing a 4% drop in speed between no load and full load. Another feature of thegovernor system is the ability to control the main valve positions independently of speed
S4: System Frequency & Voltage
42 EEE8044: Fundamentals
producing the family of characteristics shown in Fig. 4.5 and allowing the power output of
the turbine at a given speed to be adjusted at will.
power output
speed
0
1 pu
1.04 pu
1 pu
Fig. 2.4 Typical governor characteristics
1 pu
speed
1.04 pu
0.96 pu
1 pu
power output
nominal spped
P1 P