How mains decoupling relay (mains monitoring relay) detects mains failure?

Mains decoupling Relay is extensively used in parallel operation of Generators & Grid.

Say a Generator(Tg) is synchronised with Grid through a Breaker.now we take the3 phase PT input from the breaker PT, into Mains Decoupling Relay for safe Islanding of Tg in case of Fault in Grid , by tripping the Breaker.

Now this relay act for :-

(A) Under/ Over Voltage.

 ( B) Under/Over frequency

  (C) df/dt  ( Delta increase/ Decrease of Voltage with respect of Time)

  (D) Vector Surge :

When both Tg & Grid connected parallelly, their Voltage Vector are in same phase&3 phase  are120 Degree apart. Now sudden fault in Grid, will produce Vector displacement Del-Thita to increase from Zero-Degree to2,3,4,5,6,7......more, between Tg & Grid( Del Thita is an angle between TG & Grid Voltage).

In that condition if Tg is  still synchronised( TG looses synchronism but still Synchronised), it will put enormous electromechanical Stresses on TG, results Alternator Rotor Damage,Or in the Turbine blade damage, Vibration etc etc.

To restrict this we use Mains decoupling Relay. Like MRN-2, MRG-2,MRN-3 are extensively used for safe parallel Operation, in case of Mains failure protection.