From: noone@nospam.com   
      
   Στις 1/10/2017 7:49 μμ, ο Richard Chambers έγραψε:   
   > "Anthony Stewart"  wrote in message   
   > news:97a403f7-1552-4e0e-840d-0a8fd072f08b@googlegroups.com...   
   > 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.   
   > ===============================================   
   >   
   >   
   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.   
      
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