Logic Diagram - ABB RELION 670 Series Technical Manual

Section 6
Differential protection
which is also defined by a setting (
after the reconnection of the previously open CT secondary circuit.
The open CT algorithm provides detailed information about the location of the defective CT
secondary circuit. The algorithm clearly indicates the IED side, CT input and phase in which an
open CT condition has been detected. These indications are provided via the following outputs
from the Transformer differential protection function:
1. Output OPENCT provides instant information to indicate that an open CT circuit has been
detected.
2. Output OPENCTAL provides a time-delayed alarm that the open CT circuit has been detected.
Time delay is defined by the parameter
3. Integer output OPENCTIN provides information on the local HMI regarding which open CT
circuit has been detected (1=CT input No 1; 2=CT input No 2).
4. Integer output OPENCTPH provides information on the local HMI regarding in which phase an
open CT circuit has been detected (1=Phase A; 2= Phase B; 3= Phase C).
Once the open CT condition is declared, the algorithm stops to search for further open CT circuits.
It waits until the first open CT circuit has been corrected. Note that once the open CT condition
has been detected, it can be reset automatically within the differential function. It is not possible
to externally reset an open CT condition. To reset the open CT circuit alarm automatically, the
following conditions must be fulfilled:
• Bias current is for at least one minute smaller than 110%
• The open CT condition in the defective CT circuit has been corrected (for example, current
asymmetry disappears)
• The above two conditions are fulfilled for a longer time than defined by the setting parameter
tOCTResetDelay
If an open CT has been detected in a separate group of three CTs, the algorithm is reset either
when the missing current returns to the normal value, or when all three currents become zero.
After the reset, the open CT detection algorithm starts again to search for open CT circuits within
the protected zone.
6.1.7.2

Logic diagram

The simplified internal logics, for transformer differential protection are shown in the following
figures.
138
tOCTResetDelay ). This is to prevent an eventual mal-operation
tOCTAlarmDelay .
1MRK 502 066-UUS B
M13043-3 v8
Technical manual