Is 27/59 Relay Used for "Tripping" the Feeder?

It depends on application and location of the transformer in the network.

ROCOf this is " rate of change of frequency "function-this function of the relay is used when two different buses and synchornised.

yes

many protection are equiped with over/under frequency protection, I can suggest SEPAM 80 from Square-D schneider group

The 27/59 Under/Over Voltage relay has different functions for a transmission and distribution systems.

In Power system we have to use these relays on variuos applications for proper function of the system, these relays have to trip the breakers it is protecting if it senses such a condition. both these conditions are very serious and a important also for power system protection.

A low voltage below 90% & a high voltage above 120% are dangerous damaging various electrical equipments. All Equipments are designed to work only in this range of voltage.

In low voltage distribution systems these relays are directly co-ordinated with the LV breakers for tripping. in transmisssion system they are used as secondary relays for tripping.

Rate of change of frequency is the threshold range of frequency's available for frequency compensation as per the manufacturer's. it may vary upon the applications in the transmission lines.

If you talk about frequency relay 81 it is having a different kind of application.