ABB RELION 670 Series Technical Manual page 140

Section 6
Differential protection
e xte rna l
fa ult
re gion
IEC05000190 V1 EN-US
Figure 39: Operation of the internal/external fault discriminator for internal fault with CT
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
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
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
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.
134
Dire ctiona l Compa ris on Crite rion: Inte rna l fa ult a s s e e n from the HV s ide
120
150
180
210
240
HV s ide contribution to the tota l ne ga tive s e que nce diffe re ntia l curre nt in kA
Dire ctiona l limit (within the re gion de limite d by ± 60 de gre e s is inte rna l fa ult)
saturation
90
60
35 ms
30
0.5 kA
330
1.0 kA
300
1.5 kA
270
1MRK 502 066-UUS B
e xcurs ion
from 0 de gre e s
due to CT
s a tura tion
de finite ly
a n inte rna l
fa ult
0
trip c o mmand
in 12 ms
Inte rna l fa ult
de cla re d 7 ms
a fte r inte rna l
fa ult occure d
en05000190.vsd
IminNegSeq , and the
M13039-320 v5
M13039-323 v6
Technical manual