DEFINITE DISTANCE PROTECTION
This relay operates at a constant time for all faults independent of the distance of occurrence of fault. For its application consider Fig 10 which represents a power station feeding transforming sub-station. The power transformer is protected independently by inverse-time over current relay. Let, it be required to provide a relay at the input side so that it wilt trip the circuit instantaneously for any fault on the transmission line and at the same time permit the use of time delayed inverse-time over current relay for the transformer This cannot be achieved with the use of instantaneous over current relay set to trip the circuit for a certain minimum short circuit current because the fault current depends on the number of sections in service and the number of sources feeding the fault. So, the setting of the relay remains uncertain because of variation of the fault current
In order to surpass this difficulty the fact that the ratio of voltage to current or impedance at the input remains constant irrespective of fault current and can well be used for protection. Thus, a definite distance relay, adjusted to operate when the ratio of voltage to current is equal to slightly less than impedance of the transmission line, can be used at the input terminals.
Written by John on March 22nd, 2009 with
2 comments.
Read more articles on Feeder protection and Power System and Switchgear&Protection.
sunil
#1. February 9th, 2010, at 5:09 AM.
gud job
thanx