I have been an EE for over forty years and never did find out the right answer to this one....
How do power-stations and transformer switching stations ensure that the power they are feeding into the grid is in-phase with the existing power on the lines.
I know they are VERY serious about setting the line frequency to a ridiculously good accuracy. However, obviously, you can not connect a power line to another line that is 180degress out of phase. Even a small deviation would presumably cause a huge drain on the system and generate a rather strange, and out-of-spec AC waveform.
OK I can imagine a solution at the power station that uses the target line frequency to synchronize the alternators before flipping the switch perhaps. However, that switching station 100km away maybe switching onto a line from a different alternator that is much closer or farther away and consequently at a different point in the phase cycle...
How do they do that...
Note his is NOT the same as "How to synchronize a generator on the electrical grid?" That article only pertains to a local generator and is not, in my mind, the same as the main power grid and transformer switching.
Answer
Before connecting a generator to the grid, they spin it up to more or less the right speed. Then they hook what is basically a voltmeter between a generator phase, and the corresponding line phase. They adjust the generator drive until the observed voltage is
a) very slowly changing (frequency difference below some threshold) and
b) drops below some low voltage threshold (phase difference close enough so the power flow that results when they throw the big switch is manageable).
Once the generator is connected to the grid, it always stays in phase. If not driven mechanically, it will act as a motor. The amount of power it draws from or exports to the grid is controlled by how hard it is driven mechanically.
Each generator is connected to its local part of the grid, synced to its local frequency. There will be a slight phase difference between the generator and the local grid. If the generator is supplying power to the grid, its phase will be slightly in advance. The larger the power input to the generator, the larger the phase difference, and the larger will be the power exported to the grid.
This 'power flow follows phase difference' extends to whole areas of the grid. If there is a large load in the south, the generators in the south will slow down initially, retarding their phase with respect to the north. This phase difference will create a power flow from north to south.
Where you have a nationwide grid, the management strive very hard never to let any significant part become 'islanded' from the other part. Once they drift apart in phase, it may take a long time before they can be brought together again, as the phase matching will need to be exquisitely accurate to avoid a huge power flow at the time of connection.
Where two separately controlled grids are to be connected, say by the Anglo-French undersea cable, it is done with DC. It is easy at the receiving end to synchronise the inverters to the grid.
Keeping the grid in phase with an average of 50 cycles per second over the course of a day, is simply done by feeding in more or less power, to speed or slow the grid frequency respectively, usually at night when there's a bit more slack in the demand.
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