current from the W2 side compensated for eventual power transformer phase shift andtransferred to the power transformer W1 side. These negative sequence currentcontributions are phasors, which are further used in directional comparisons, tocharacterize a fault as internal or external. See section "Internal/external faultdiscriminator" for more information.The magnitude of the negative sequence differential current (IDNSMAG) can be readas a service value from the function. At it is also available as an output from thedifferential protection function block. Also, it can be connected to the disturbancerecorder and automatically recorded during any external or internal fault condition.6.1.3.8 Internal/external fault discriminatorThe internal/external fault discriminator is a very powerful and reliablesupplementary criterion to the traditional differential protection. It is recommendedthat this feature shall be always used (that is, On) when protecting three-phase powertransformers. The internal/external fault discriminator detects even minor faults, witha high sensitivity and at high speed, and at the same time discriminates with a highdegree of dependability between internal and external faults.The algorithm of the internal/external fault discriminator is based on the theory ofsymmetrical components. Already in 1933, Wagner and Evans in their famous book"Symmetrical Components" have stated that:1.Source of the negative-sequence currents is at the point of fault,NS NS NSE I Z= - ×EQUATION1254 V1 EN (Equation 25)2. Negative-sequence currents distribute through the negative-sequencenetwork3. Negative-sequence currents obey the first Kirchhoff"s lawThe internal/external fault discriminator responds to the magnitudes and the relativephase angles of the negative-sequence fault currents at the different windings of theprotected power transformer. The negative sequence fault currents must first bereferred to the same phase reference side, and put to the same magnitude reference.This is done by the matrix expression (see equation 23).Operation of the internal/external fault discriminator is based on the relative positionof the two phasors representing the winding one (W1) and winding two (W2) negativesequence current contributions, respectively, defined by expression shown inequation 23. It performs a directional comparison between these two phasors. Takinginto account the phase rotation transformation the relative phase displacementbetween the two negative sequence current phasors is calculated. In case of three-winding power transformers, a little more complex algorithm is applied, with twodirectional tests. The overall directional characteristic of the internal/external faultdiscriminator is shown in figure 33, where the directional characteristic is defined bytwo setting parameters:1MRK 502 048-UEN A Section 6Differential protection97Technical manual
