IAn IAt
IAn
IAt
Idiff
ANSI0700018_3_en.vsd
ANSI07000018 V3 EN-US
Figure 52: Internal fault
IAn IAt
External fault: IAn = - IAt
IAnIAt
Idiff = 0
en07000019-2_ansi.vsd
ANSI07000019 V2 EN-US
Figure 53: External fault
Generator differential protection GENPDIF (87G) function uses two mutually independent
characteristics to which magnitudes of the three fundamental frequency RMS differential currents
are compared at each execution of the differential protection function. These two characteristics
divide, each of them independently, the operate current – restrain current plane into two regions:
the operate (trip) region and the restrain (block) region, as shown in figure
55. Two kinds of
protection are obtained:
• the non-stabilized (instantaneous unrestrained) differential protection
• the stabilized differential protection
The non-stabilized (instantaneous) differential protection is used for very high differential
currents, where it must be beyond any doubt, that the fault is internal. This limit, (defined by the
1MRK 502 066-UUS B Section 6
Differential protection
155
Technical manual
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