Does that mean all power stations have to be in sync with each other? If so how is this achieved?
Yes, they do. Each generating station receives a timing signal from their Energy Management System that essentially tells the generator if it is synchronized to the grid frequency or not.
Grid frequency is primarily driven from a standard and consistent time source, such as from an atomic clock.
Is it true about every third house, I've never seen this?
Kinda true if you really stretch things, but not really. See below.
Does this mean we effectively have 3 grids overlaid on top of each other?
Definitely not. It's one grid using three phases. 1
The basics of what you were told are essentially correct. For a host of reasons, it is more efficient and practical to transfer AC power in three phases.
And yes, if the phases cross and connect with each other,
Bad Things really do happen. At a minimum, it will trip the protective relays and shut the area down for a while. The momentary arc that's generated when they cross can be quite spectacular though and can easily destroy large portions of trees that were silly enough to have their branches provide the phase crossing mechanism.
The extent of
Bad Things also depends upon the voltage level of the line and amount of current being carried.
You don't see every third house going dark because of the way distribution laterals are setup. There are two main approaches to feeding distribution loads:
- All three phases transferred along the distribution lateral (aka balanced)
- A single phase transferred along the distribution lateral.
The single phase per lateral approach is easier to put in place but is less efficient because the loads tend not to balance well across the various laterals coming out of the distribution substation.
It's easier to balance out distribution laterals by transferring all three phases along the length of the line. However, power is delivered to blocks of houses and each block is fed off a single phase from the distribution line. You can verify this if you have overhead lines by looking for the step down transformer (it's a big metal can hanging off of the power pole) and verifying that only one phase feeds into the can. 2, 3
My understanding is that residential service in North America is almost always single phase. The primary driver behind this is that most residential grade motors are single phase, and the demand simply isn't there to justify the additional equipment expense from delivering two phases to a residential drop.
1 I'm explicitly ignoring things like regional interconnections such as what is seen in North America. Then you could argue for there being multiple grids, but that's not really germane to your question.
2 You can identify if you have a 3 phase distribution feed in your neighborhood by looking at the wires at the very top of the distribution power pole. Three wires spread across arms at the top of the pole generally mean a 3 phase service in the area.
3If you see a cluster of metal cans and all three phases feeding into those cans then you've likely spotted a capacitor bank which is used to condition the power on the line. It's not used for stepping down the voltage for residential service.