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Zero Sequence Elimination

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To explain this concept, consider a transformer with a vector group of YNd5.

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In the transformer with the specified vector group, the secondary vectors relative to the primary are derived using the following equations.

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Using these equations, the schematic of the winding connections and the direction of the currents

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on the secondary side can be drawn. For example, in the first step, the connection of the windings and the direction of currents for phase A' is shown as illustrated

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Finally, the connections will look as shown in the following diagram.

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Given the figure above, the matrix of currents on the star side is as shown below.

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The overall transfer matrix from the secondary side to the primary side is also as shown:

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For YNd5, it is represented as shown below.

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In the differential protection of a transformer with the given vector group and a 1:1 turn ratio

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if an earth fault occurs outside the protective zone, the relay will operate

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In the event of an earth fault outside the protection zone, the currents shown will be present on both sides

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According to the equation above, if a fault with current If occurs in phase C

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the corresponding winding on the secondary side will have If/sqrt(3) flowing through it.

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According to the differential protection convention, the currents entering the transformer are considered positive

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and those leaving the transformer are negative

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Therefore, the matrices shown will apply for the primary side and the transfer from secondary to primary

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The differential current of the above will be as shown below:

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With these differential currents, the relay will trip

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However, if the zero-sequence current from the YN side is eliminated, the primary side current will be as shown

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With the changes made, the differential currents will be as shown and will remain stable:

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However, if the fault is inside the protection zone, the current seen by the CTs on the YN side will be zero:

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Therefore, for an earth fault within the protective zone, the differential will be as shown below, and the relay will trip.