Open Ct Detection - ABB Relion 670 Series Technical Reference Manual

1MRK 505 208-UEN B
5.1.4.1
Technical reference manual
Differential protection zones include a sensitive operational level. This sensitive
operational level is designed to be able to detect internal busbar earth faults in low
impedance earthed power systems (that is, power systems where the earth-fault
current is limited to a certain level, typically between 300A and 2000A primary by
a neutral point reactor or resistor). Alternatively, this sensitive level can be used
when high sensitivity is required from busbar differential protection (that is,
energizing of the bus via long line).
Overall operating characteristic of Busbar differential protection is shown in
figure 49.
Diff Oper Level
Sensitive Oper Level
IEC06000142 V1 EN
Figure 49: Operating characteristic
Where:
I Iin represents the RMS value of the incoming current to the differential protection zone
in
I Id represents the RMS value of the differential current of the differential protection zone
d
s = 0.53 the operating slope for the differential function is fixed to 0.53 in the algorithm and can not
be changed by the user

Open CT detection

The innovative measuring algorithm provides stability for open or short-circuited
main CT secondary circuits, which means that no separate check zone is actually
necessary. Start current level for open CT detection can usually be set to detect the
open circuit condition for the smallest CT. This built-in feature allows the
protection terminal to be set very sensitive, even to a lower value than the
maximum CT primary rating in the station. At detection of problems in CT
Sensitive
differential
protection
Operate
region
Sens Iin Block
s=0.53
Section 5
Differential protection
Differential protection
operation characteristic
I [Primary Amps]
in
en06000142.vsd
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