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2.2.3.3 Zero-sequence current elimination
In addition to compensating for the phase angle between the primary and secondary currents of the
transforemer, also phase angle matching prevents unnecessary operation due to zero-sequence
current during an external earth fault, such as in the following cases.
Case 1:
When an external fault occurs at the star-connected side of the transformer shown in Figure
2.2.3.2, a zero-sequence current flows in star-connected side, but the zero-sequence current at the
delta-side circulates in the delta winding. The zero-sequence current is only fed into the star
winding side of the DIFT which is star-connected at the CT secondary, thus causing the DIFT to
operate incorrectly. In α-method phase matching, the zero-sequence current is eliminated from a
relay input current as described above. In β-method phase matching, the zero-sequence current is
eliminated from the relay input current by Delta connection on the Star-winding side.
Since the DIFT provides a function to eliminate the zero-sequence current by software, the DIFT
is insensitive the fault described.
Figure 2.2.3.2 External Earth Fault at the Star-connected side of a Transformer
Case 2:
When the delta winding of a power transformer is earthed through an earthing transformer as
shown in Figure 2.2.3.3 and the earthing transformer is located within the differential protection
zone, in case of an external earth fault the zero-sequence current flows only on the delta side of the
power transformer and appears as a differential current.
Figure 2.2.3.3 External Earth Fault at the Delta-winding side of a Transformer with
in-zone Earthing Transformer
Transforme
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