Unrestrained, And Sensitive Negative Sequence Protections - ABB RET650 Technical Manual

1MRK 504 135-UEN A
6.1.4.9
Technical manual
zero value. However, during heavy faults, CT saturation might cause the measured
phase angle to differ from 180 degrees for an external, and from 0 degrees for an
internal fault. See figure
saturation.
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
e xte rna l
fa ult
re gion
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)
IEC05000190 V1 EN
Figure 36: 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

Two sub functions are based on the internal/external fault discriminator and have the
ability to trip a faulty power transformer, are parts to the traditional power transformer
differential protection.
The unrestrained negative sequence differential protection
The unrestrained negative sequence protection is activated if one or more start signals
have been set by the traditional differential protection algorithm. This happens
36 for an example of a heavy internal fault with transient CT
90
60
35 ms
0.5 kA
1.0 kA
300
1.5 kA
270
Section 6
Differential protection
e xcurs ion
from 0 de gre e s
due to CT
s a tura tion
30
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
330
a fte r inte rna l
fa ult occure d
en05000190.vsd
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