Unrestrained, And Sensitive Negative Sequence Protections - ABB REG650 Technical Manual

Section 5
Differential protection
5.1.3.9
86
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
However, it shall be noted that additional security measures are implemented in the
internal/external fault discriminator algorithm in order to guaranty 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. At 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 to the traditional power
transformer differential protection.
The unrestrained negative sequence differential protection
If one or more start signals have been set by the traditional differential protection
algorithm, because one or more of the fundamental frequency differential currents
entered the operate region on the operate - restrain characteristic then the
unrestrained negative sequence protection is activated. So, this protection is not
independent of the traditional restrained differential protection - it is activated after
the first start signal has been placed.
90
60
35 ms
30
0.5 kA
330
1.0 kA
300
1.5 kA
270
1MRK 502 034-UEN -
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
Technical Manual