Page 58 - Power Electronic Control in Electrical Systems

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48 Power systems engineering ± fundamental concepts

It is evident that P in and P out are determined quite independently. Yet in the steady
state they must be essentially equal, otherwise energy would be accumulating some-
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where in the transmission system. The power system operator can control P in but s/he
has no control over P out , since customers can connect and disconnect loads at will.
The power system operator does not even have any practical means of measuring
P out for the entire system, and in any case, even if this parameter was available, there
may be several generating stations in the system, so it appears to be somewhat
arbitrary as to what contributions should be supplied by the individual generating
stations at any instant.
In the short term (i.e. over a period of a fraction of a second), it is the frequency
control that ensures that P in ˆ P out , and this control is effected by maintaining the
speed of the generators extremely close to the nominal value. Suppose the power
system is in a steady state and P in ˆ P out . Suppose that the load increases so that
more power is taken from the system, tending to make P out > P in . The prime mover
and the generator will tend to slow down. Therefore the prime mover has a governor
(i.e. a valve controller) that increases P in when the frequency is below the rated value,
and decreases P in when the frequency is above the rated value.
In an isolated power system with only one generator, the governor has a relatively
simple job to do, to maintain the speed of the generator at the correct synchronous
speed to hold the frequency constant. But what happens in a power system with
multiple generators? In this case usually there is a mixture of power stations. The
large ones which produce the most economical power are usually best operated at
constant power for long periods, without varying their contribution to P in . Apart
from the economics, one reason for this is that if the power is varied, the temperature
distribution in the turbine, boiler, and generator will be affected, and `thermal
cycling' is considered undesirable in these very large machines. So these generators
have a relatively steep or insensitive governor characteristic, such that the frequency
would have to change by quite a large amount to change the contribution to P in
(`Quite a large amount' might mean only a fraction of 1 Hz). Elsewhere in the power
system, or sometimes in the same power station, there are special generators assigned
to the taskof frequency control. These generators have very flat governor character-
istics such that a tiny change in frequency will cause a large swing in power. They are
usually gas turbine powered, up to 20 MW or so, but very large rapid-response
generators are sometimes built into hydro-electric pumped-storage schemes. For
example, the Dinorwic power station in North Wales has a rating of 1800 MW and
can change from zero to maximum power in a few tens of seconds.
The rapid-response generators in a large interconnected power system (such as the
United Kingdom system) are used for frequency control in the short term (over a few
minutes or hours). They provide a time buffer to allow the larger power stations to
vary their contribution. As the total system load changes during the day, the fre-
quency is maintained almost constant, within 0.1 Hz. Averaged over 24 hours, the
frequency is kept virtually dead accurate .

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Losses in the transmission system are assumed to be negligible for the purposes of this discussion.
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In fossil-fuel power stations two-pole generators predominate, and the speed is 3000 rev/min in a 50-Hz
system or 3600 rev/min in a 60-Hz system. In nuclear power stations, four-pole generators are more
common, running at 1500 rev/min (1800 rev/min at 60 Hz). In hydro plants, the generators have larger
numbers of poles with speeds in the range 100±1000 rev/min.