Why we need directional relay:In my power plant while doing operations duty we used to face several faults, reverse power was one of it. Reverse power that is power flows in wrong direction. When such thing occurs, the alternator becomes motor taking power from the grid, this situation is called “Motoring action of alternator”
This problem is so frequent, as it is due to the failure of prime mover where there are lots of simple reason behind, such as stopping of fuel, stopping of air(oxygen supply) etc. I am giving example of diesel engine, because reverse power is only lethal to diesel engine.
Think about it, a normal engine is running 750RPM, its dangerous speed is 880RPM. When it is driven by alternator due to motoring action there is surely going to be mechanical seizure or breakdown of engine. [alvaa]The only way to stop this is to forced trip the alternator breaker, if it is stuck then shut down the whole power plant. You must be thinking, as there why not stop the engine. The answer to this is that the only way to stop a diesel/ fuel engine to stop the supply of fuel to the engine. Then do the math.
AS Directional relay is the combination of a basic relay and a directional element. So it can sense power plus its flow of direction. That’s it. Its prime function should be clear as of it is.
So What is directional relay:
A relay capable of sensing both power flow or current flow and its direction is called directional relay. Directional relays are used most extensively to recognize the difference between current being supplied in one direction. That is it has the capability to sense the direction of power or current flow.