In a nutshell, differential protection works like this; first, a 'protected zone' must be identified. This can be a transformer, motor, generator, busbars, basically anything that either carries or utilizes electricity.
Two sets of CT's are then placed at the ends of this zone, back to back. Polarity is important here. These are connected to a relay of some sort. The older style looked alot like a disc-type watthour meter, except the disc only spun about 1/4 turn, then made a contact which tripped a breaker. In this type, CT ratios had to match voltage ratios across a transformer, or taps on the relays were used. The newer electronic type can be programmed for any ratio, as well as the phase shift that exists across a wye to delta transformer. The relay is looking for a balance of current between the two sets of CT's.
If this protection scheme is properly connected, the CT secondary currents will cancel each other our, resulting in zero current between them.
In the event of a fault somewhere in the 'protected zone', one set of CT's will see more current than the other, and similar difference of current will appear between the secondaries. The relay will see this and trip the breaker that feeds the setup.
We say 'differential protection of a transformer', when in reality, the transformer isn't protected very much at all. The equipment that feeds the transformer is what's really protected. In order for this scheme to trip, a fault has already occurred in the 'protected zone'. The only protection for the transformer is a smaller explosion. lol.
You've likely installed hundreds of differential protection devices and not even known it. A GFI breaker or receptacle is just such a device. It looks at current going out the hot, and compares it to the neutral. Any difference indicates a problem, and it trips its 'breaker'. That's why you can't share neutrals on GFI circuits.
P.S. I'm just a journeyman who works with this stuff alot, the above is very basic. The engineers around here can explain it much better.
Rob.
