ABB ret650 - Differential Current Alarm; Open CT Detection

• If any comparison indicates an internal fault, then it is an internal fault.
• If any comparison indicates an external fault, then it is an external fault
If one of the windings is not connected, the algorithm automatically reduces to the
two-winding version. Nevertheless, the whole power transformer is protected,
including the non-connected winding.
6.1.3.7 Differential current alarm
M15266-337 v6
Differential protection continuously monitors the level of the fundamental
frequency differential currents and gives an alarm if the pre-set value is
simultaneously exceeded in all three phases. This feature can be used to monitor
the integrity of on-load tap-changer compensation within the differential function.
The threshold for the alarm pickup level is defined by setting parameter
IDiffAlarm. This threshold should be typically set in such way to obtain operation
when on-load tap-changer measured value within differential function differs for
more than two steps from the actual on-load tap-changer position. To obtain such
operation set parameter IDiffAlarm equal to two times the on-load tap-changer step
size (For example, typical setting value is 5% to 10% of base current). Set the time
delay defined by parameter tAlarmDelay two times longer than the on-load tap-
changer mechanical operating time (For example, typical setting value 10s).
6.1.3.8 Open CT detection
M15266-340 v5
The Transformer differential function has a built-in, advanced open CT detection
feature. This feature can block the unexpected operation created by the
Transformer differential function in case of open CT secondary circuit under
normal load condition. An alarm signal can also be issued to station operational
personnel to make remedy action once the open CT condition is detected.
The following setting parameters are related to this feature:
• Setting parameter OpenCTEnable enables/disables this feature
• Setting parameter tOCTAlarmDelay defines the time delay after which the
alarm signal will be given
• Setting parameter tOCTReset defines the time delay after which the open CT
condition will reset once the defective CT circuits have been rectified
• Once the open CT condition has been detected, then all the differential
protection functions are blocked except the unrestraint (instantaneous)
differential protection
The outputs of open CT condition related parameters are listed below:
• OpenCT: Open CT detected
• OpenCTAlarm: Alarm issued after the setting delay
• OpenCTIN: Open CT in CT group inputs (1 for input 1 and 2 for input 2)
• OpenCTPH: Open CT with phase information (1 for phase L1, 2 for phase L2,
3 for phase L3)
1MRK 504 169-UEN A Section 6
Differential protection
Transformer protection RET650 2.2 IEC 83
Application manual