The bias current is the highest current of all separate input currents to REFPDIF (87N),that is, of current in phase A, phase B, phase C, and the current in the neutral point(designated as IN in figure 44 and in figure 45).If there are two feeders included in the zone of protection of REFPDIF (87N), then therespective bias current is found as the relatively highest of the following currents, that is,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 primary ratingsof the CTs were equal to IBase, then the bias current would be equal to the highest currentin Amperes. IBase shall be set equal to the rated current of the protected winding whereREFPDIF (87N) function is applied.6.2.7.4 Detection of external ground faultsExternal faults are more common than internal ground faults for which the restrictedground fault protection should operate. It is important that the restricted ground faultprotection remains stable during heavy external ground and phase-to-phase faults, andalso when such a heavy external fault is cleared by some other protection such asovercurrent, or ground fault protection. The conditions during a heavy external fault, andparticularly immediately after the clearing of such a fault may be complex. The circuitbreaker’s poles may not open exactly at the same moment, some of the CTs may still behighly saturated, and so on.The detection of external ground 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 or morecurrent transformers saturate.For an internal ground fault, a true differential current develops immediately, while for anexternal fault it only develops if a CT saturates. If a trip request comes first, before anexternal fault could be positively established, then it must be an internal fault.If an external ground fault has been detected, then the REFPDIF (87N) is temporarilydesensitized.Section 6 1MRK 502 048-UUS ADifferential protection126Technical manual
