> I didn't realise it would be the same across the UK. It has to be, that's the definition of a power grid. The relative phase can have minute variations and

> I didn't realise it would be the same across the UK. It has to be, that's the definition of a power grid. The relative phase can have minute variations and you get massive power flows as a result, which result in the frequency across the whole grid being the same. I believe the whole UK is a single grid. The US is divided into three: The eastern interconnect, the western interconnect, and Texas. (Insert meme here.) There are a few DC-links that allow power to flow between them, but due to the expense of rectifying and inverting all that power (necessary for the frequency and phase to vary), they are rare. It's much cheaper to connect directly at AC, even though this requires that everyone on the system run at the same frequency. Amusingly, this also means that if you could (theoretically arrange enough long shafts and bearings and neglect the curvature of the earth and) orient all the generators on the grid in a line, a single shaft could run through all of them, and the shaft would not twist -- they all spin in precise synchrony since they are, effectively, a single machine. When a new generator is brought on-line, it has to be phase-matched to the grid. These days there are probably fancy PLCs to handle it, but the simplest instrument is called a synchroscope, which is a motor driven by the phase relationship between two AC waves. You can find these on YouTube, they're pretty neat. Basically if the isolated generator is running slower than the grid, the synchroscope needle spins one way, and if it's running faster, the needle spins the other way. You adjust the throttle on the generator until it's running _ever so slightly_ faster, like 60.00Hz on the grid and 60.10Hz on the isolated generator, (50.xx in the UK) which will make the 'scope revolve once per ten seconds. Then you wait until the needle is pointing straight up which indicates zero phase shift, and slam the switches to connect it to the grid, and that's that! If you did it just right, there's a minimal shudder and the generator now begins sending power into the grid, its frequency constrained to 60.00Hz and the extra throttle showing up as positive power flow. If you did it wrong, there's a tremendous amount of torque exerted on the rotating machinery by the grid power, which may simply force it into compliance, or in more extreme cases, may shred millions of dollars of equipment in an instant. Bigger synch setups have lockouts that won't even allow the button to be pressed unless the phase is _pretty close_. The most colossal kind of cockup is when someone wired the synchroscope wrong, so the rotation still works (it has to, that's impossible to screw up) but in-phase would be represented by some needle angle other than straight up. Joining the grid 120 degrees out would be a career-limiting event for someone.