ABB REG670 Technical Manual page 136

Section 6
Differential protection
130
different negative sequence source impedance angles on the W1 and W2 sides of the
protected power transformer, it may differ somewhat from the ideal 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 36 for an example of a heavy internal fault with transient CT 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, 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 start signals
have been set by the traditional differential protection algorithm. This happens
90
60
35 ms
30
0.5 kA
330
1.0 kA
300
1.5 kA
270
1MRK502052-UEN 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
Technical manual