6.1.2.6 External/Internal fault discriminator M15266-269 v10The external/internal fault discriminator operation is based on the relative position of the twophasors (in case of a two-winding transformer) representing the W1 and W2 negativesequence current contributions, defined by matrix expression see the technical referencemanual. It practically performs a directional comparison between these two phasors.In order to perform a directional comparison of the two phasors their magnitudes must behigh enough so that one can be sure that they are due to a fault. On the other hand, in order toguarantee a good sensitivity of the internal/external fault discriminator, the value of thisminimum limit must not be too high. Therefore this limit value (IMinNegSeq) is settable in therange from 1% to 20% of the differential protections IBasecurrent, which is in our case thepower transformer HV side rated current. The default value is 4%. Only if the magnitude ofboth negative sequence current contributions are above the set limit, the relative positionbetween these two phasors is checked. If either of the negative sequence currentcontributions, which should be compared, is too small (less than the set value forIMinNegSeq), no directional comparison is made in order to avoid the possibility to produce awrong decision.This magnitude check, guarantees stability of the algorithm when the power transformer isenergized. In cases where the protected transformer can be energized with a load connectedon the LV side (e.g. a step-up transformer in a power station with directly connected auxiliarytransformer on its LV side) the value for this setting shall be increased to at least 12%. This isnecessary in order to prevent unwanted operation due to LV side currents during thetransformer inrush.The settingNegSeqROA represents the so-called Relay Operate Angle, which determines theboundary between the internal and external fault regions. It can be selected in the range from30 degrees to 90 degrees, with a step of 1 degree. The default value is 60 degrees. The defaultsetting 60 degrees somewhat favors security in comparison to dependability. If the user hasno well-justified reason for another value, 60 degrees shall be applied.If the above conditions concerning magnitudes are fulfilled, the internal/external faultdiscriminator compares the relative phase angle between the negative sequence currentcontributions from the HV side and LV side of the power transformer using the following tworules :• If the negative sequence currents contributions from HV and LV sides are in phase or atleast in the internal fault region, the fault is internal.• If the negative sequence currents contributions from HV and LV sides are 180 degrees outof phase or at least in the external fault region, the fault is external.Under external fault condition and with no current transformer saturation, the relative angle istheoretically equal to 180 degrees. During internal fault and with no current transformersaturation, the angle shall ideally be 0 degrees, but due to possible different negativesequence source impedance angles on HV and LV side of power transformer, it may differsomewhat from the ideal zero value.The internal/external fault discriminator has proved to be very reliable. If a fault is detected,that is, START signals set by ordinary differential protection, and at the same time theinternal/external fault discriminator characterizes this fault as an internal, any eventualblocking signals produced by either the harmonic or the waveform restraints are ignored.If the bias current is more than 110% of IBase, the negative sequence threshold (IMinNegSeq)is increased internally.. This assures response times of the differential protection below onepower system cycle (below 20 ms for 50 Hz system) for all more severe internal faults. Even forheavy internal faults with severely saturated current transformers this differential protectionoperates well below one cycle, since the harmonic distortions in the differential currents donot slow down the differential protection operation. Practically, an unrestrained operation isachieved for all internal faults.Section 6 1MRK 504 158-UEN ADifferential protection68Application manual
