[diff-trafo-schaltgranpass-yd5-020926-rei, 1, en_GB]
Figure 2-28 Matching the transformer vector group, example Yd5 (magnitudes not considered)
Since there is no point earthed within the protected zone, no considerable zero sequence current can be
produced within the protected zone in case of an earth fault outside the protected zone, regardless whether
or not the system starpoint is earthed anywhere else in the system. In case of an earth fault within the
protected zone, a zero sequence current may occur at a measuring location if the system starpoint is earthed
anywhere else or another earth fault is present in the system (double earth fault in a non-earthed system).
Thus, zero sequence currents are of no concern for the stability of the differential protection as they cannot
occur in case of external faults.
However, in case of internal earth faults, the zero sequence currents are practically fully included in the differ-
ential quantity because they pass the measuring points from outside. Even higher earth fault sensitivity during
internal earth fault is possible by means of the time overcurrent protection for zero sequence currents (Section
2.4.1 General) and/or the single-phase time overcurrent protection (Section 2.7 Single-Phase Time Overcur-
rent Protection).
Earthed Starpoint
Differential protection makes use of the fact that the total of all currents flowing into the protected object is
zero in healthy operation. If the starpoint of a power transformer winding is connected to earth, a current can
flow into the protected zone across this earth connection in case of earth faults. Consequently, this current
should be included in the current processing in order to obtain a complete image of the in-flowing quantities.
Figure 2-29 shows an external earth fault which produces an out-flowing zero sequence current (–Ι
L3
= –3 ·
Ι
0
), which corresponds with the in-flowing starpoint current (Ι
SP
= 3 · Ι
0
). As a result, these currents cancel
each other out.
Functions
2.2 Differential Protection
SIPROTEC 4, 7UT6x, Manual 103
C53000-G1176-C230-5, Edition 09.2016
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