Staying in Phase On The Grid

Question

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 1

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.

Answer 2

You're confusing an accurate number of cycles over a 24 hour period with very rigid instantaneous frequency control. That's not how it's done in most places.

The frequency is maintained at around its nominal frequency by matching generation to load - all the time that the load is greater than the generation, the frequency will be (very) gradually falling, and all the time the load is less than the generation the frequency will be increasing.

The inertia is enormous and, in general, both load and generation change fairly gradually, so there's lots of time to make adjustments to generators (or loads, where people have contracted to control their loads in this way) to keep the system balanced. The frequency is allowed to drift between various limits (operational and regulatory).

In the UK at least, the correct number of cycles per day is maintained by keeping track of 'real time' and 'grid time', and the grid is run a bit fast or a bit slow to make sure they don't get too far apart.

There are accurate frequency references in use within the grid control system - that's what they're comparing with/measuring against, but the grid itself isn't phase/frequency-locked to them in any direct way.

At the bottom left of the big display in this image is a graph with a vertical wiggly yellow trace - that's the frequency of the UK National grid for a while before the photo was taken - as you can see it's not locked to anything very tightly, though the graph is probably only about ±0.3 Hz.

Answer 3

They use a Synchroscope. I have seen this done in power plant control rooms.

https://en.wikipedia.org/wiki/Synchroscope

Answer 4

Having parts of an individual power system run at different phase angles from other parts is routine and unavoidable. This is not a problem until it is necessary to re-connect parts. In the Utility where I worked, the service people at the site would connect a phase-meter to each of the parts. Due to the difference in phase, the phase-meter would run like a clock, indicating the instantaneous phase difference. The person doing the connection (by means of an electrically-actuated circuit breaker, usually) would simply time the breaker closure for the instant at which the phase-meter showed zero phase difference. Since this zero-point occurs every few seconds, it is not difficult to catch it. We even used this with our HVDC Back-to-Back converter station; it works very, very well.

Answer 5

20 years ago, just after uni, I worked on a company doing exactly this.

It used to be that there were all sorts of complex phase-adjustment circuits with complex analogue electronics. These days that's generally not the case.

What my company back then specialised in was high-voltage AC/DC conversion technology. They built the first cross-channel link, and various HVDC links round the world since then. (Over long distances the losses in cables due to reactance are significant, so DC gives more efficient transmission.) When the DC gets turned back into AC (with what's essentially a very high power, very smooth inverter) you can synchronise the timing so that the resulting AC is exactly in phase with the local grid.

As this got more efficient with better high-power electronics, what people realised was that it had become more efficient to convert from DC to AC and back to DC again than it was to use any alternative methods. The result is called a "back-to-back converter". Where a cross-channel link would have miles of cable between the AC-to-DC and DC-to-AC converters, a back-to-back scheme just has a few feet of extremely thick busbar.

Of course the conversion is not 100% efficient, so the electronics are mounted on water-cooled heatsinks and the whole thing is pretty carefully monitored. But it's efficient enough that the losses are perfectly acceptable in exchange for the power going into the grid perfectly in phase.

Answer 6

In the USA, the grids are managed by Independent System Operators (ISOs). The ISOs are somewhat like a stock market. They negotiate how much power each generating station supplies to the grid. In addition to the buy/sell transactions, they monitor and manage the performance of the grid. When a generator is connected, it matches the voltage, frequency and phase at the local connection point. Then it connects, but does not immediately supply power. It negotiate price, power level and rate of power increase with the ISO. That is my understanding of the basic system operation.

Answer 7

Back in the day (1979) just after university I worked at a UK generator manuafacturer, and in the test lab (this was for smaller equipment) they used the crossed lights method to simplify the 'voltage measurement' that others have mentioned.

Basically they connected L1-L1 via a lamp, which needed to go out (zero volts / in phase) before closure, and a crossed lamp L2 (gen) - L3 (grid) which had to go to maximim first. Once the phase difference lamp was 'out' the connection relay / contactor / switch could be thrown.

There were various apocryphal stories about things that had gone wrong in various places which were educational!