Eli5: How do you synchronize a whole national grid?

91 viewsEngineeringOther

Back in 2015 Turkey has been synchronized to the European grid.
Now my question is, how do you synchronize a whole country to another country? Adjusting the voltage is pretty simple by using transformers, but what about the frequency and phase angle? While synchronizing a generator to the grid is just a matter of changing the rpm of the generator, but obviously you can’t do that, in order to change the frequency of the grid, because if your generator is to slow it will be powered by the grid. So how do you synchronize a whole country?

In: Engineering

6 Answers

Anonymous 0 Comments

that is how you do it. you very slowly change the rpm of all generators on the grid until they match the grid you want to connect with.

Anonymous 0 Comments

I’m not sure but I think what you say “*if your generator is to slow it will be powered by the grid.*” is the answer. If your generator slows down the grid will pull on it and speed it up. If your generator speeds up and tries to push on the grid, the grid will resist and slow it down a little. And together all the generators keep each other in sync. You can see a lab bench example of syncing two generators here: https://www.youtube.com/watch?v=RGPCIypib5Q

The European grid is 50 Hz, you can see it flickering on lights or hear it and get close to it by manual faster/slower adjustments, I don’t know what equipment can speed or slow a generator but [Synchroscopes (Wikipedia)](https://en.wikipedia.org/wiki/Synchroscope) can measure how in/out of sync two power feeds are, so there must be a way to feed that into the regulation of a generator. The Wikipedia page says they are used for syncrhonising power generators onto a grid and describes basically a human adjusting the settings until the Synchroscope says the new generator is in sync, before connecting them. I wouldn’t be surprised if that’s automated nowadays.

One of the problems people consider in the event of a disaster which shuts down a national grid completely is how do you get it started again? Some power stations take power to start them up (e.g. water pumps, coal conveyor belts, control computers, water pumps, etc.) and they all need to come online in sync. In that situation it’s known as a [Black start](https://en.wikipedia.org/wiki/Black_start) and an example is given there – battery to start a diesel generator at a hydro plant, diesel generator runs the control systems and powers some of the valves to start water flowing, water generates power to bring the whole hydro plant online and energise the local grid, a bigger gas, coal or nuclear plant starts up from that and merges into the grid.

> if your generator is to slow it will be powered by the grid

Something like this was happening across Europe for a couple of months in 2018; the European grid is 50Hz between many countries, and industrial devices and microwaves and cookers use that for clock ticks. When Kosovo declared independence from Serbia and Serbia refused to recognise it, that left some problems over who was paying for what power and what infrastructure which ended up dragging the entire European grid down below 50Hz for a sustained time, and putting clocks up to 6 minutes out. – https://www.wired.com/story/clock-slow-europe-electricity-power-serbia-kosovo/

You can see a graph and details of the European power frequency tracked here: https://gridradar.net/en/mains-frequency and here: https://www.mainsfrequency.com/

And you can see really cool electricity maps here! https://app.electricitymaps.com/map

This doesn’t totally answer your question how they are sync’ed, but hopefully it’s relevant and interesting enought to be allowed.

Anonymous 0 Comments

I can’t find any info because I don’t know the right search terms, but I’m aware of a type of synchronous generator that has electronically switchable stator windings. This allows the control system to effectively rotate the stator field, thus adjusting the relative rotational speed between the rotor and the stator. This means the output can be in phase over a wide range of rotor RPM which is useful in many renewable energy fields. Very cool tech, and I wish I had a link to post.

Anonymous 0 Comments

You essentially do it the same way as if you connect a new set of generators to the grid: carefully adjust the frequency of most of them, very slowly, until it matches the target. The phase angle is particularly simple as it changes when the frequency is even the tiniest bit off.

To adjust the frequency one can add slightly to much or little power, generators then adjust by changing the frequency. We can even see that within a single grid when the burden gets too high, it causes the frequency to decline.

At the moment both frequency and angle match, you connect the two grids. The classical device for this are [Synchroscopes](https://en.wikipedia.org/wiki/Synchroscope), you can find a few nice videos of those on the web. [Here’s a modern variant](https://www.youtube.com/watch?v=FI_07YVS3tY) and this can also be automated.

After connection each grid then drives the other(‘s generators), so their frequency stabilizes further and wipes out remaining discrepancies.

Anonymous 0 Comments

When you connect two generators together, they become locked together. They both turn at the same speed and they remain phase locked together.

If one is slower than the other, it will act like a motor, pulling power from the other one to speed up. The other one will slow down under load. In practice, you want this to be a controlled process. If you just randomly connect a generator to the grid (which is much bigger and much stronger than one generator), it will yank the newly connected generator so hard that something will probably break.

So, if you want to connect a generator to the grid you have to synchronise it. You measure the grid voltage and generator voltage. You then adjust the generator voltage, by adjusting the generator exciter, until it matches. You then measure the grid frequency and phase, and the generator frequency and phase. You adjust the generator speed up or down by adjusting the throttle, until the frequency and phase match very closely. Then once they are matched, you close the circuit breaker to make the connection. Once connected, they become locked together.

Once synchronised, the generator throttle control controls the amount of power exported to the grid, and doesn’t do much to frequency. The generator exciter control controls the amount of reactive power imported or exported to the grid.

It’s very similar when you synchronise 2 grids. You have a circuit breaker connecting them together. You keep it open.

You adjust the grid voltages until they match by adjusting “reactive power” – which can be done by having individual generators on each grid adjust their excitation controls, or by using separate “compensation” equipment on each grid.

The you adjust the frequency and phase of each grid. This is done exactly the same way as adjusting frequency and phase of a generator. You adjust the throttle controls on the individual generators, and this will very gradually shift the grid frequency and phase. The once the frequency and phase are matched, the circuit breaker is closed joining the two grids and the two grids will now stay synced by transferring power across the link.

Anonymous 0 Comments

Add more fuel to your generator (or turn on some generators), and the grid runs at a higher voltage and faster frequency. Take fuel away from your generator (or turn off some generators), and the grid runs at a lower voltage and slower frequency.

Make fine adjustments until you’re at almost the right frequency. Then the two grids will very slowly shift between “in phase” and “out of phase”.

When you’re in phase, quickly hit the big button that closes a massive circuit breaker and hooks the grids together.

Once the grids are together, everything will run at the same frequency. If you were very slightly slow, then a tiny bit of electricity will flow toward you, speeding you up and slowing everyone else down. If you were very slightly fast, then a tiny bit of electricity will flow from you, slowing you down and speeding everyone else up. Once everything’s connected, the grid synchronizes itself.

If you mess it up, and you connect the grids when you’re too far out of phase or frequency, a huge amount of electricity will flow, and instead of a gentle correction, your generator will experience a massive force and a hard jolt that will probably break it. Then you’ll get to enjoy days or weeks of downtime, a six-to-eight-figure repair bill, and some very difficult questions from the government. Probably the electric utility regulator, but if you mess up really big, the government will also invite you to a hearing, and you’ll have a chance to explain to a bunch of angry politicians why your actions resulted in millions of people without power.

[Here’s a video of a guy syncing a 400 kW hydro plant to the grid](https://www.youtube.com/watch?v=xGQxSJmadm0).