ABB REG650 User 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 35 for an example of a heavy internal fault with transient CT
saturation.
en05000190.vsd
0.5 kA
30
210
60
240
90
270
120
300
150
330
180
0
HV side contribution to the total negative sequence differential current in kA
Directional limit (within the region delimited by ± 60 degrees is internal fault)
1.0 kA
1.5 kA
definitely
an internal
fault
Internal fault
declared 7 ms
after internal
fault occured
trip co mmand
in 12 ms
excursion
from 0 degrees
due to CT
saturation
external
fault
region
35 ms
Directional Comparis on Criterion: Internal fault as seen from the HV side
IEC05000190 V1 EN
Figure 35: 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.
6.1.3.9 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
Section 6 1MRK 502 048-UEN A
Differential protection
100
Technical manual

ABB REG650 Specifications

General

Brand	ABB
Model	REG650
Category	Protection Device
Language	English