Differential Current Alarm - ABB RELION RET670 Applications Manual

Section 7
Differential protection
7.1.3.8
7.1.3.9
144
The above parameters are defined for OLTC1. Similar parameters shall be set for
second on-load tap-changer designated with OLTC2 in the parameter names, for three–
winding differential protection.

Differential current alarm

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).
Open CT detection
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 A, 2 for phase B, 3 for
phase C)
1MRK 504 163-UUS A
Transformer protection RET670 2.2 ANSI
Application manual
M15266-337 v6
M15266-340 v5