1current[4] = max (I3PW2CT2)CTFactorSec2×EQUATION1529 V1 EN (Equation 33)current[5] = INEQUATION1530 V1 EN (Equation 34)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.6.4.7.4 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.An external earth fault is thus assumed to have occurred when a high neutral currentsuddenly appears, while at the same time the differential current Idiff remains low, atleast for a while. This condition must be detected before a trip request is placed withinREFPDIF. Any search for external fault is aborted if a trip request has been placed. Acondition for a successful detection is that it takes not less than 4ms for the first CT tosaturate.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 detected, 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 impedance1MRK502052-UEN B Section 6Differential protection173Technical manual
