The rating of an EHV transmission line

When an EHV transmission line (let's say 345 kV or 400 kV, for example) is rated at 1,000 MW, is this 1,000 MW:- a. the rating which should not be exceeded in normal service, with the aim of eliminating the possibility that a failure on this line might initiate a cascade failure affecting the entire transmission system, causing a widespread blackout. Or is the 1,000 MW b. the rating that applies under emergency conditions only, for example when neighbouring parallel lines have already tripped out of service. If this is the case, the line might be restricted to (for example) 700 MW under all normal circumstances, and 1,000 MW would be allowed only in an emergency.
My problem is that I am looking at a technical specification sheet, but I do not know which interpretation of "MW rating" is the correct one.
Richard Chambers, Leeds UK. ===============================

???? 18/8/2017 8:47 ??, ? Richard Chambers ??????:
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It's been since 2003 I graduated so I'll try my best to answer. A search for my old textbooks was fruitless. Short transmission lines (~60 km) have thermal rating. medium length (~60 km to 220 km) have voltage drop. Long lines (like yours) have stability. So the rating is for the line to be stable. A heavy-duty 400 kV line can go well in 1200 A ballpark.

Hi Richard,
Before I retired I was an Infrastructure Planning Engineer for a UK DNO. Ou r aim was to 'make the asset sweat'. At 132kV, the rating of a circuit was the minimum continuous rating. Why minimum, you ask? The circuit rating ch anges with ambient temperature, so the Summer Rating is less than the Winte r Rating. The rating can also be pushed by the shape of the load curve. You can supply a higher maximum demand if the load factor is low.
You can also push the rating, by as much as two, for short term switching t ransients.
The circuit rating is likely to be set by one, or a few pinch points. For e xample, I had one 132kV circuit which crossed a railway line. The maximum l ine sag at that crossing, set the rating for whole 25km circuit. I was able to raise the circuit rating by 10MW, by changing the insulator strings on the bottom phase on each of the two circuits, on the two adjacent towers, f
aintenance budget!
Regards
Peter

I should have also mentioned that circuit rating and switchgear rating are two very different things. The circuit rating is more of a guide that can b e passed for short durations. A switchgear rating should never be exceeded, unless you can persuade the manufacturer to change the rating.

I should have also mentioned that circuit rating and switchgear rating are two very different things. The circuit rating is more of a guide that can be passed for short durations. A switchgear rating should never be exceeded, unless you can persuade the manufacturer to change the rating. ======================================Peter
Thank you for your interesting reply. I ought to tell you that the reason for my question is that for most of my working life I have been initially an engineer concerned with the operation of generators, then a tariff metering engineer concerned with operating the meters that set the bills that the generating company sends to its customers. The tariff metering is also continuously streamed to Grid Control to enable them to keep the grid stable. For this latter purpose I need to improve my understanding of the transmission system, but so far lack the necessary experience.
I remember reading a few years ago about cascade failures of entire transmission systems. If I remember correctly -- and it is quite possible I havegot it wrong -- the grid is interconnected like the mesh of a fishing net. This arrangement provides alternative routs for the electricity in the event of a failure of one of the individual lines. If one of the transmission lines suffers a fault and trips out, the current of that line will divert itself onto the neighbouring lines in this mesh. If you are operating the grid system with all the lines operating at their full rating, the diverted current may increase the current in the relief line to such an extent that the relief line also trips out because of overcurrent. This second tripping then diverts even more current onto another neighbouring line, tripping it too. Now you have started an avalanche of tripped lines, and the whole grid system might collapse by successive trippings. I think it was in 2002 that this type of fault caused a complete blackout in NE USA and spread to Ontario, Canada. Shortly after that fault there was another smaller cascade failure in the SE of England, but this was confined to the Distribution System and did not spread to the National Grid. There was also one in Italy/Switzerland in the same year.
To prevent this type of cascade failure from occurring (I think I remember reading), a proper system must never allow all the lines to operate simultaneously at their full rating. Depending upon calculations of how much power might be diverted, the actual maximum operational level of every line must be less than the thermal rating of that line. If a line has only one other line in parallel with it, a transmission line with a thermal rating of 1,000 MW would normally be operated at a maximum allowable load of 500 MW (or something like that -- have I got it right?). If the mesh provides three alternative routes for the power, the individual lines might be operated at up to 750 MW (or something).
I have done Google searches on this, and I have looked in various books on transmission systems, but I can find nothing sufficiently detailed to help me.
My confusion comes from looking at the Transmission System Data Catalogue for Yorkshire and the North East, published by National Grid, which lists a particular transmission line (of interest to me) as rating 1800 MW. I need to know whether this is the thermal rating, or whether it is the maximum power they would dare to send through that particular line. If 1800 MW is the thermal rating, they might dare only to use it up to a maximum of 1200 MW. So my question boils down to "What do they mean by the rating of a transmission line, in this sense?".
Thank you again for your help.
Richard Chambers Leeds UK. ==============================

The power rating in MVA needs to be defined according to the regional stand ard at a defined atmospheric condition. e.g. 35'C and with solar heat. The re will be a rating for over current defined perhaps at 102 to 120% for nor mal to emergency conditions depending on seasonal ambient.
Thus the utilization or margin of derating of this capacity must consider t he heuristic fluctuations and mesh capacity margin of the grid for expected variations and with provisions for isolation protection for faults and lo ad sustainability for nuclear generators.

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. ===========================================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. =============================================

???? 1/10/2017 7:49 ??, ? Richard Chambers ??????:
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You don't say what is the voltage of the transmission line.Usually the operation conditions of redundant transmission lines depends on the current demand. If demand at receiving end is eg 2000 MW then each of the 4 redundant lines, will be, of course operated at 500 MW each. If demand requires all 4 lines to be at their full capacity, then a trip will most probably lead to a blackout. During our visit (field trip) with the local college at the local substation which is also the Center of Load Distribution they told us that if a major transmission line under heavy load trips it will most probably lead to a major blackout. For instance the 3 North-South transmission lines from the browncoal revier in Kozan to Athens are 400 kV, 500 km each, double circuit and heavy duty. Each one has cost billions of drachmas. And when the man responsible for our "tour" in the substation showed us an 150 kV circuit breaker and its control panel, that had only 2 switches, Local/Remote and Trip/Close I asked what will happen if I turn this to trip, will I shut down half of iraklion, he told us all of Crete most probably.HTH.