7.1.3.5 Cross-blocking between phasesBasic definition of the cross-blocking is that one of the three phases can blockoperation (that is, tripping) of the other two phases due to the harmonic pollution of thedifferential current in that phase (waveform, 2nd or 5th harmonic content). In thealgorithm the user can control the cross-blocking between the phases via the settingparameter CrossBlockEn. When parameter CrossBlockEn is set to On, cross blockingbetween phases will be introduced. There are no time related settings involved, but thephase with the operating point above the set bias characteristic will be able to cross-block the other two phases if it is self-blocked by any of the previously explainedrestrained criteria. As soon as the operating point for this phase is below the set biascharacteristic cross blocking from that phase will be inhibited. In this way cross-blocking of the temporary nature is achieved. It should be noted that this is the default(recommended) setting value for this parameter. When parameter CrossBlockEn is setto Off, any cross blocking between phases will be disabled.7.1.3.6 External/Internal fault discriminatorThe external/internal fault discriminator operation is based on the relative position ofthe two phasors (in case of a two-winding transformer) representing the W1 and W2negative sequence current contributions, defined by matrix expression see thetechnical reference manual. It practically performs a directional comparison betweenthese two phasors.In order to perform a directional comparison of the two phasors their magnitudes mustbe high enough so that one can be sure that they are due to a fault. On the other hand,in order to guarantee a good sensitivity of the internal/external fault discriminator, thevalue of this minimum limit must not be too high. Therefore this limit value(IMinNegSeq) is settable in the range from 1% to 20% of the differential protectionsIBasecurrent, which is in our case the power transformer HV side rated current. Thedefault value is 4%. Only if the magnitude of both negative sequence currentcontributions are above the set limit, the relative position between these two phasorsis checked. If either of the negative sequence current contributions, which should becompared, is too small (less than the set value for IMinNegSeq), no directionalcomparison is made in order to avoid the possibility to produce a wrong decision.This magnitude check, guarantees stability of the algorithm when the powertransformer is energized. In cases where the protected transformer can be energizedwith a load connected on the LV side (e.g. a step-up transformer in a power stationwith directly connected auxiliary transformer on its LV side) the value for this settingshall be increased to at least 12%. This is necessary in order to prevent unwantedoperation due to LV side currents during the transformer inrush.The setting NegSeqROA represents the so-called Relay Operate Angle, whichdetermines the boundary between the internal and external fault regions. It can beselected in the range from 30 degrees to 90 degrees, with a step of 1 degree. The defaultvalue is 60 degrees. The default setting 60 degrees somewhat favors security incomparison to dependability. If the user has no well-justified reason for another value,60 degrees shall be applied.1MRK 502 071-UEN - Section 7Differential protectionGenerator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062 133Application manual
