If there are two feeders included in the zone of protection of REFPDIF, then therespective bias current is found as the relatively highest of the following currents, thatis, those which are connected in an application:1current[1] = max (I3PW1CT1)CTFactorPri1×EQUATION1526 V1 EN (Equation 27)1current[3] = max (I3PW2CT1)CTFactorSec1×EQUATION1528 V1 EN (Equation 28)current[5] = INEQUATION1530 V1 EN (Equation 29)The bias current is thus generally equal to none of the input currents. If all primaryratings of the CTs were equal to IBase, then the bias current would be equal to thehighest current in Amperes. IBase shall be set equal to the rated current of theprotected winding where REFPDIF function is applied.6.2.7.4 Detection of external earth faultsExternal faults are more common than internal earth faults for which the restrictedearth-fault protection should operate. It is important that the restricted earth-faultprotection remains stable during heavy external earth and phase-to-phase faults, andalso when such a heavy external fault is cleared by some other protection such asovercurrent, or earth-fault protection. The conditions during a heavy external fault,and particularly immediately after the clearing of such a fault may be complex. Thecircuit breaker’s poles may not open exactly at the same moment, some of the CTsmay still be highly saturated, and so on.The detection of external earth faults is based on the fact that for such a fault a highneutral current appears first, while a false differential current only appears if one ormore current transformers saturate.For an internal earth fault, a true differential current develops immediately, while foran external fault it only develops if a CT saturates. If a trip request comes first, beforean external fault could be positively established, then it must be an internal fault.If an external earth fault has been detected, then the REFPDIF is temporarilydesensitized.Directional criterionThe directional criterion is applied in order to positively distinguish between internaland external earth faults. This check is an additional criterion, which should preventmalfunctions at heavy external earth faults, and during the disconnection of suchfaults by other protections. Earth faults on lines connecting the power transformeroccur much more often than earth faults on a power transformer winding. It isimportant therefore that the Restricted earth-fault protection, low impedanceSection 6 1MRK 502 048-UEN ADifferential protection116Technical manual
