On Thu, 02 May 2013 23:14:28 +0100, Andy Champ wrote:
On 02/05/2013 11:54, The Other Mike wrote:
The voltage at the generator terminals is for all practical purposes (at least
in the context of this discussion) held at a constant level regardless of load,
as is everything across the super grid until the first transformer that has tap
changers.
Voltage to consumers can be reduced without the generators changing their own
terminal voltage. It happens many times a day as load changes (mainly industrial
load)
During the day a transformer may be on tap x of 20, at night on tap y.
During ALL this period the generator is at a constant voltage (around 15kV, the
400kV grid system is at 400kV, the 275kV grid system at 275kV and the 132kV grid
system at 132kV Below that distribution voltage level of 132kV the voltage on
all the lower voltage grids can vary and can be reduced in a number of stages to
lower demand whilst keeping voltage to the consumer within the statutory voltage
limits of 216V - 253V. This is an existing control regime that has existed for
half a century in the UK.
It doesn't matter if the load on the grid is 30GW 50GW or 60GW, wind turbines
are swinging output up and down like a whores drawers, the frequency is low or
high, the voltage at the generator terminals will*always* be controlled towards
a predefined setpoint defined by the manufacturer of the generator maybe 50
years ago.
http://books.google.co.uk/books?id=L7o4TitajRIC&pg=PA11&dq=automatic+voltage +regulator&hl=en&sa=X&ei=TjGCUZ_FDoXy7Abtp4DoDQ&ve d=0CEEQ6AEwAQ#v=onepage&q=automatic%20voltage%20re gulator&f=false
scroll to page 13
Frequency is determined by the speed of rotation. If speed of rotation starts
dropping more steam is admitted It's very similar in response to cruise control
on a relatively flat road.
If more steam is admitted then more fuel has to be burnt to maintain the steam
pressure and temperature.
I have no trouble understanding how intermittency will cause stability
problems. However...
Google won't let me read enough of that book 
So the fiddle with the exciter current to maintain a fixed output
voltage, regardless of load? Surely at some point they must run out of
steam...
What actually controls the grid frequency? Is it the tap changing at the
132kV step down?
Yes, the excitation is essentially just for the volts at the generator terminals
(it does more than that but just accept it just sets the volts for now!)
If the generator is not connected to the grid then the governor controlling the
fuelling on a gas turbine or the steam on a coal /oil / nuke will hold the speed
of rotation at exactly 3000rpm (or 1500rpm for a 4 pole generator)
Connected the generator to the grid and the governor will do all it can to
control the speed of rotation to 3000rpm.
If the load is such that the speed of rotation and thus the frequency starts to
fall then more steam is admitted until a steady state is achieved again.
Then grid frequency is determined by the governor set point on all the
generators connected to the grid at that particular moment. In the UK they all
control to 50Hz.
The extra quantity of steam as a percentage of the nominal value at 50Hz for a
grid frequency of say 49.7Hz is say 1%, at 49.5Hz it's 3% at 49Hz it's 6%.
Beyond this point only a certain number of generators and boilers with enough
overfuelling capacity will be able to support the additional steam will be
available. At 48.5Hz the requirements could increase to say 15%. Also as the
frequency drops the speed of the fans and pumps located at the power station
have a lower speed of rotation and output and efficiency drops off. At a certain
point you can't get water back into the boiler quick enough to maintain water
levels and avoid overheating (like running a kettle dry) At that point the
generator trips to protect the boiler, increasing load on the other generators
on the system at that point. So frequency control and generator response are
very important for system stability To protect the integrity of the grid blocks
of load is shed or voltage reduced 'in advance' at set frequency levels to avoid
generators being grossly overloaded and the frequency dropping out of the
statutory limits. Such events are supposed to be something like once in 10 year
events (last one was in 2008 and previously 2003)
The tap changing purely controls the final voltage the consumer sees.
If a tap changer has say 15 taps then tsomething around the middle one should
ultimately result in 240v to the consumer. In an area with more local load it
will be lower heading towards the lower limit. In areas of lighter load it will
be higher, heading towards the upper limit. Industrial load or very lightly
loaded long overhead lines also have an effect on final terminal voltage.
Changes can be made to the tappings on 'local' transformers to correct for new
housing etc, but this is a manual operation, performed offload. The tap changes
on the bulk supply transformers are performed on load, usually automatically and
many times per day.
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