DerekC
Established Member
That depends on the output of the station and the type of the station.
A nuclear station requires house loads on order of ten percent of it's net output. But ti should be able to provide many of those from it's emergency generators if the regulator approves the use of them in the black start scenario.
What would normally happen is all the circuit breakers connecting the transmission system to the distribution system would be opened, and then staitons with very low house loads would be started up to provide power to start other plants up.
Once a large number of stations are ready the distribution systems can be reconnected stepwise.
An obvious choice for black start is a hydro scheme, because a station service generator can potentially be started by having someone open a sluice gate with a hand crank.
This process is obviously much less difficult and much faster if some partso f the grid remain intact, which is why some modern nuclear stations are designed to suddenly reject full power to the condenser directly, maintaining only enough flow through the steam generator to keep the plant's house loads operational.
This means the station can stand ready to restore the grid as required.
As the fuel costs of nuclear are very small and sometimes negligible, wasting power output for a few hours is a small price to pay for being able to rapidly restore service. Especially as many freshly shut down nuclear reactors are required by xenon poison out to remain shut down for up to three days if they cannot restart immediately.
This had to be done after the Great Storm of 1987 caused the grid to collapse (electrically speaking) in Southern England. I recall reading an article in an IEE journal about it - the only power station left connected in the area was Isle of Grain, feeding a small amount of power locally, so they started from there.