phasing


Postby rustyriv » Sun Nov 02, 2008 3:37 pm

can you guys help me with a question i have been struggling with for sometime, it concerns our incoming power supply from the grid, if you can imagine two or many more power stations all supplying power to the same network of transformers, cables etc. how are they all kept in phase at the same time? would i not be correct in saying that should one power station become out of phase i.e. the two sinewaves become out of sync. with one another would this not turn this power station into a giant electric motor? being driven by the others? or at least leading to a massive short circuit? bearing in mind the many different types of methods used to generate power how would you for example be able to so accurately control the speed the speed of the alternator in a hydro-electric power station to the degree which would be required to maintain that the sinewaves line up with one another? there must be a simple explanation to this problem but i would be very interested to know what it is, regards.
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Postby ericmark » Sun Nov 02, 2008 8:23 pm

Looking at a smaller system there are two methods of synchronising. A large central unit can control each generator and this controls the power delivered by each engine. This system keeps the frequency spot on. The more common system relies on the droop on each engine this is the difference between no load speed and full load speed so long as each engine has the same droop characteristics then once they are synchronised they will share load. Neither of these systems will work with a national grid but it is more like the former with a central control asking each power station for power. The direct short is a problem as although it will be cleared it will take a few seconds to disconnect the offending part of the system and it is important that power stations do not drop out of the system when this happens but continue to generate this has caused big problems for wind generators where they used the power generated to also control the system so zero volts meant no control and they dropped out. Also each power station has to agree to produce so much extra power when demand increases and the wind turbines wanted to just give the maximum they could to the grid. The national grid had to insist all suppliers worked under the same rules which made it impossible to have one small wind generator but it had to be done as wind farms so they can comply with the rules. Of course from time to time a generator will fail and there is normally a reverse power system incorporated which will auto disconnect the generator.
This caused a lot of problems with the 750KVA generators I worked on as we had step up and step down transformers and the inrush of one transformer needed 6 of the 12 generators to be running but with so little load until the first of the 9 transformers were put on line as one added generators it was very easy to put too much power on the system from one which would knock out another on reverse power. Once first transformer was on line then adding more generators was a lot easier. Normally step up transformers would not be required and the generators would produce power at 3.3kV but this was on the building of an airport on the Falklands and we wanted to be able to mix and match generators as the site was built and bit by bit the main 1500kVA generators took over the work.
Some power stations do allow tours and they will explain. I have worked on a few the first with 2 x 750MW generators (Sizewell) and the second with 4 x 600MW generators (Connah’s Quay) and also at the other Connah’s Quay on Long Gannet but although I renewed and fitted AVR’s etc. one did not really get involved with how they would be run once completed as an electrician I did not have my degree then we only knew how to read plans and do as we were told. It was the smaller power stations where one learnt more and although I have set up and run many systems using droop the only system which used a central control was in Hassi R’mel in Algeria back in 1981-2 and my memory of this is now rather vague. 4 x MTU powered Siemens generators at 750kW each and a 250kW Mercedes powered one to keep hospital powered if there was system failure. All containerised quite a nice set-up.
Eric
ericmark

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