Functions
2.14 Differential Protection and Its Protected Objects
SIPROTEC, 7UM62, Manual
C53000-G1176-C149-7, Release date 03.2010
115
2.14.2 Protected Object Generator or Motor
The following section describes the special features of the generator and motor as the protection objects.
2.14.2.1 Functional Description
Definition and Matching of Measured Quantities
The differential protection function of the 7UM62 can be used as longitudinal or as transverse differential pro-
tection. The operation modes differ from each other only by the definition of the measured currents and the
limits of the protected zone.
Since the current direction is normally defined as positive in the direction of the protected object, the definitions
as illustrated in the following figure result. The protected zone is limited by the CTs in the neutral point of gen-
erator and the CTs at the terminal side. The differential protection feature of the 7UM62 refers all currents to
the rated current of the protected object. The characteristic values of the protected object (apparent power,
rated voltage) and the primary rated currents of the CTs are entered in the protective device. Measured value
matching is therefore limited to current quantity factors.
Due to their predominantly inductive component, faults in the proximity of the generator have relatively high DC
current time constants that cause magnetisation of the current transformers. The CTs should be designed ac-
cordingly (see section 2.14.4).
Figure 2-40 Definition of current direction with longitudinal differential protection
Use as transverse differential protection involves a peculiarity. The definition of the measured currents for this
application is shown in the figure below.
For transverse differential protection, the phases connected in parallel constitute the border between the pro-
tected zone and the network. A differential current appears in this case only, but always, if there is a current
difference within the particular parallel phases, so that a fault current in one phase can be assumed.
Since in this case, for normal operation, all currents flow into the protected object, i.e. the opposite of all other
applications, the polarity must be reversed for one current transformer set, as described in Section 2.5.1 under
"Connection of the Current Transformer Sets".
Figure 2-41 Definition of current direction with transverse differential protection
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