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Posted by Don Kelly on April 9, 2009, 3:44 am
Tim Jackson wrote:

Instability, in the sense of the common usage of the term power system
instability"  would not be a problem. It is the sudden large changes in
transmission capability that are problematic- and these are due to
faults on the system- not load changes but splitting the system so that
some areas have excess load and others have a deficiency of load and
inadequate transfer capability to correct this.    With a healthy
system, even if everybody shuts off their water heaters at 5PM, there
should be no problem-as there is sufficient transfer between units to
keep them in synchronism-i.e. stable.  Frequency will  drop or rise
until governors and other regulatory mechanisms take over and restore
frequency and a desirable load distribution. The "two elephant case"
that Daestrom mentioned was a particularly weak tie (insufficient
transfer to keep the two systems together but weak enough to not cause
instability in either system- the string broke without being noticed by
either elephant- and was known to be inadequate  -at that it held for
about 30 minutes if I recall correctly.

The tie in generation is a problem for two reasons- one is a safety
factor but that is not an issue with proper equipment and the other may
be financial- the utility sees that it has to subsidize the private
producer in many situations- they don't want to pay for the transmission
of someone else's energy nor do they want to pay peak or even average
rates for generation that occurs off- peak (on peak the person with the
generator may have little or nothing to spare).  Most utilities have
come to some fair compromise with regard to this.

As for wind power, you will always have to have hot or spinning capacity
on line, ready to take over. This can take over quite quickly- again due
to the coordination of the droops of the governors  on machines on line.
A system that can cope with the sudden loss of a 500MVA machine can cope
with the slower loss of several 10MW wind units without blinking.

Note also  that on the recent- turn off the lights for an hour day, some
regions had load drops of 5 to 10% and coped very nicely.

Don Kelly
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Posted by daestrom on April 10, 2009, 12:31 am


With somewhat larger co-gen installations around here, another issue is
re-calculating the available fault currents.  Putting a small unit of just a
MW or so on the far end of a line *could* require that customer protection
equipment be upgraded.

The extreme case would be if you put in a co-gen and now your neighbor's
service entrance fault current increases to the point where the typical
service entrance equipment can't interrupt it.  He needs to upgrade his
service for his own protection, but it's because of the co-gen you
installed.  Who's to pay?


Posted by Don Kelly on April 10, 2009, 1:27 am
 daestrom wrote:

Unless the neighbor's load is large , I would assume that the capability
of his protection  would be based on an "infinite bus" behind the
transformer supplying him. In that case there would be no requirement.
In cases of industrial co-generation (10MW is in that category) in an
industrial area, that *may* not apply but in that case it would be the
utility side protection that has to be enhanced.
In the "pa and ma" low level co-generation- where the fuss arises- this
would not be a concern unless the co-generation source is on the same
side of the transformer as the  neighbor.

Don Kelly
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Posted by Alistair Gunn on April 5, 2009, 6:52 am
 In alt.energy.renewable Eeyore twisted the electrons to say:

Clearly the National Grid thinks that the UK is a third world country

"System frequency will therefore vary around the 50 Hz target and
National Grid has statutory obligations to maintain the frequency within
+/- 0.5Hz around this level. However, National Grid normally operates
within more stringent 'operational limits' which are set at +/- 0.2Hz."
These opinions might not even be mine ...
Let alone connected with my employer ...

Posted by Ken on April 5, 2009, 7:07 am

Very true.  When the load is heavy, the frequency goes down.
Here in Sweden the variation normally is 49.85 Hz to 50.15 Hz.
Now in the morning it's around 49.87 Hz.

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