The power rating in MVA needs to be defined according to the regional
standard at a defined atmospheric condition. e.g. 35'C and with solar heat.
There will be a rating for over current defined perhaps at 102 to 120% for
normal to emergency conditions depending on seasonal ambient.

Thus the utilization or margin of derating of this capacity must consider
the heuristic fluctuations and mesh capacity margin of the grid for expected
variations and with provisions for isolation protection for faults and load
sustainability for nuclear generators.
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Thank you for your answer, which comes closest to the information I want,
but which I do not quite understand to the level of specificness that I
require. Could you please provide the following clarification.

Suppose that, in a particular part of the system, we have two double
circuits (A and B) in parallel running from north to south for a distance of
100km, but separated from each other by 50 km (say) in the east-west
direction. (It might help you understand what I am asking if you draw this).
At both the northern and the southern end there is another double circuit,
each of length 50 km, connecting A and B together.

The double circuit A consists of circuit A1 and circuit A2 using the same
transmission towers, and each rated at 1,000MW, making a total rating for A
of 2,000MW. Similarly, B1 and B2 are each rated at 1,000MW, so that the
total rating of double circuit B is 2,000MW. The total capacity of the
entire system for north-south flow is therefore 4,000MW.

Suppose that the system is operating at full capacity of 4,000MW north-south
flow. Now suppose that an aeroplane crashes into double circuit B, taking
both B1 and B2 completely out of service. The 2000MW that had been flowing
in B now re-routes to A, which will now carry atotal of 4000MW. There will
be 2000 MW on A1 and 2000 MW on A2, each of which is rated at only 1000MW.

If A1 and A2 now trip by overcurrent, the accident could start a cascade
failure and a nationwide blackout. This is undoubtedly the worst thing that
could ever happen to a transmission system.

What I specifically want to know is:
a. Does a rating of 1000MW mean that the line A1 (for example) can actually
be allowed to operate under all normal (non-emergency) circumstances at its
full rating of 1000MW? In which case, the Grid Operation Department must be
100% confident that it can re-route the load flows, or shed up to 2000MW of
load sufficiently quickly to prevent a serious cascade failure.
or
b. Does a rating of 1000MW per single circuit mean that the Grid Control
Centre would never allow A1, A2, B1 and B2 to operate at above 500MW each
under normal non-emergency conditions? If this is the case, the aeroplane
accident would cause the load flow in A1 and A2 to rise from 500MW each to
an emergency value of 1000MW each, but this is still within the rating of
each line and there would be no question of any line tripping, and no
possibility of a cascade failure.

So my question boils down to: What is the maximum load flow in circuit A1
that Grid Control would allow under normal conditions, when A1,A2, B1 and B2
are all fully operational under non-emergency conditions?
Does a rating of 1000MW imply that the line can carry up to 1000MW under
normal non-emergancy conditions, or does it mean that it would never
normally carry more than 500 MW?

A complicated question, but I have tried to specified it as precisely as
possible.

Richard Chambers Leeds UK.
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