When two or more generators are running in parallel and if one of the units starts drawing power from the main bus bar, it can lead to reverse power flow. It can cause overloading of the other power supply unit and hence leads to the preferential trip or may lead to total power failure(Blackout). The faulty unit may continue to draw power from the main bus bar and go for a motoring effect and RPM will soot up which leads to over-speed trips or in the worst case some mechanical failure to the prime mover.
To avoid the motoring of the generator, a reverse power trip is installed. It is basically a relay made up of a lightweight non-magnetic Aluminium disc between two soft laminated electromagnetic iron cores.
The upper magnet is wound with a Voltage Coil (PT) which is supplied from one phase & artificial neutral of generator output. The other output magnet is wound with a Current Coil (CT) connected to the same phase as the voltage in the upper electromagnet. Voltage Coil has more number of turns, hence more inductive value and induced current that lag in the coil by an angle of 90°. On the other hand, the current coil has less number of turns so less inductive value & less induced current.
Both the upper and lower section produces magnetic fields because they are current-carrying conductors. But the Induced current in PT lags more than in CT so the magnetic field produced in the upper section will be weaker than the lower section & both magnetic fields will have a difference of 90°.
When both fields pass through the Aluminium disc, it produces an eddy current, and torque is generated that tries to rotate the disc. Under normal power flow, the trip contact on the disc is open and rotation is restricted by stoppers but if a reverse power starts to flow the disc is rotated in the opposite direction, and moves away from the stoppers in the direction of the trip contact activates the trip.