en05000190.vsd
0.5 kA
30
210
60
240
90
270
120
300
150
330
180
0
HV s ide contribution to the tota l negative s equence differentia l curre nt in kA
Directional limit (within the re gion delimite d by ± 60 degree s is inte rna l fa ult)
1.0 kA
1.5 kA
de finitely
an inte rnal
fa ult
Inte rna l fault
de cla red 7 ms
after inte rna l
fa ult occured
trip c ommand
in 12 ms
excurs ion
from 0 degree s
due to CT
s aturation
external
fa ult
region
35 ms
Directiona l Comparis on Crite rion: Interna l fa ult a s s e en from the HV side
IEC05000190 V1 EN-US
Figure 39: Operation of the internal/external fault discriminator for internal fault with CT
saturation
It shall be noted that additional security measures are implemented in the internal/external fault
discriminator algorithm in order to guarantee proper operation with heavily saturated current
transformers. The trustworthy information on whether a fault is internal or external is typically
obtained in about 10ms after the fault inception, depending on the setting
IminNegSeq
, and the
magnitudes of the fault currents. During heavy faults, approximately 5ms time to full saturation of
the main CT is sufficient in order to produce a correct discrimination between internal and
external faults.
Unrestrained, and sensitive negative sequence protections
M13039-320 v5
Two sub functions, which are based on the internal/external fault discriminator with the ability to
trip a faulty power transformer, are parts of the traditional power transformer differential
protection.
The unrestrained negative sequence differential protection
M13039-323 v6
The unrestrained negative sequence protection is activated if one or more pickup signals have
been set by the traditional differential protection algorithm. This happens because one or more of
the fundamental frequency differential currents entered the operate region on the operate -
restrain characteristic. So, this protection is not independent of the traditional restrained
differential protection - it is activated after the first start signal has been placed.
If the same fault has been positively recognized as internal, then the unrestrained negative
sequence differential protection places its own trip request.
Any block signals by the harmonic and/or waveform criteria, which can block the traditional
differential protection are overridden, and the differential protection operates quickly without any
further delay.
Section 6 1MRK 502 066-UUS B
Differential protection
134
Technical manual
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