Logic Diagram - ABB REG670 Technical Manual

1MRK502052-UEN B
6.1.7.2
Technical manual
If an open CT is detected and the output OPENCT set to 1, then all the differential
functions are blocked, except the unrestrained (instantaneous) differential. An alarm
signal is also produced after a settable delay (tOCTAlarmDelay) to report to
operational personnel for quick remedy actions once the open CT is detected. When
the open CT condition is removed (that is, the previously open CT is reconnected), the
functions remain blocked for a specified interval of time, which is also defined by a
setting (tOCTResetDelay). This is to prevent an eventual mal-operation 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 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 tOCTAlarmDelay.
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 L1; 2= Phase L2; 3=
Phase L3).
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 120%
• 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.

Logic diagram

The simplified internal logics, for transformer differential protection are shown in the
following figures.
Section 6
Differential protection
135