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7UT613/63x Manual
C53000-G1176-C160-2
Figure 2-37 Definition of current direction with transverse differential protection
The currents flow into the protected object even in case of healthy operation, in con-
trast to all other applications. For this reason, the polarity of one current transformer
set must be reversed, i.e. you must set a „wrong“ polarity, as described in Subsection
2.1.4 under „Current Transformer Data for 3-Phase Measuring Locations“.
Starpoint Condi-
tioning
If the differential protection is used as generator or motor protection, the starpoint con-
dition need not be considered even if the starpoint of the machine is earthed (high- or
low-resistant). The phase currents are always equal at both measuring points in case
of an external fault. With internal faults, the fault current results always in a differential
current.
Nevertheless, increased earth fault sensitivity can be achieved by the„Restricted Earth
Fault Protection“ (see section 2.3) or the „High-impedance Differential Protection“ (see
section 2.7) .
2.2.4 Differential Protection for Shunt Reactors
If current transformers are available for each phase at both sides of a shunt reactor,
the same considerations apply for series reactors.
In most cases, current transformers are installed in the lead phases and in the star-
point connection (see figure 2-38). In this case, comparison of the zero sequence cur-
rents is reasonable. The „Restricted Earth Fault Protection“ is most suitable for this ap-
plication (see 2.3).
If current transformers are installed in the line at both sides of the connection point of
the reactor (see figure 2-38), the same conditions as for auto-transformers apply. Such
an arrangement is therefore treated like an auto-transformer.
A neutral earthing reactor (starpoint former) outside the protected zone of a power
transformer can be treated as a separate protected object provided it is equipped with
current transformers like a shunt reactor. The difference is that the starpoint former
has a low impedance for zero sequence currents.
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