VSR relay is used in generating station, but its angle is is phase +11 or -11TO +13 TO -13 why?

Vector surge Relay is for fast de-coupling of Generatot in case of Mains failure / Grid disturbances.

It compares the vector angle between each phases when TG is Synchronised with Grid.When Del-Thita changes Generator Rotor angle increases, means TG is Synchronised but Loosing Synchronism but Still Synchronised & need to be isolated if Delta-Thita Crosses Tripping Limit due to grid Disturbances/ Grid Failure. It sence Del-Thita( Vector angle of Phases) within 60 mili seconds & to Trip Islanding Breaker it Takes 150-160 Miliseconds to Safe Guard of TG.

Similar thing may happen in case of Auto closing of TG with Grid .

For low impedence Mains maximum Phase angle differance( Del Thita) setting should be 4-6 Degree.

And for High Impedence Mains maximum Phase angle differance( Del Thita) setting should be 10-12 Degree.