Differential Current Alarm; Open Ct Detection - ABB RELION 650 Series Applications Manual

1MRK 504 158-UEN A
External faults happen ten to hundred times more often than internal ones as far as the power
transformers are concerned. If a disturbance is detected and the internal/external fault
discriminator characterizes this fault as an external fault, the conventional additional criteria
are posed on the differential algorithm before its trip is allowed. This assures high algorithm
stability during external faults. However, at the same time the differential function is still
capable of tripping quickly for evolving faults.
The principle of the internal/external fault discriminator can be extended to autotransformers
and transformers with three windings. If all three windings are connected to their respective
networks then three directional comparisons are made, but only two comparisons are
necessary in order to positively determine the position of the fault with respect to the
protected zone. The directional comparisons which are possible, are: W1 - (W2+W3) and W2 -
(W1+W3). The rule applied by the internal/external fault discriminator in case of three-winding
power transformers is:
• 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.
6.1.2.7

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
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
example, typical setting value is 5% to 10% of base current). Set the time delay defined by
parameter
time (For example, typical setting value 10s).
6.1.2.8

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
• Setting parameter
be given
• Setting parameter
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:
Application manual
IDiffAlarm . This threshold should be typically set in such way to
IDiffAlarm equal to two times the on-load tap-changer step size (For
tAlarmDelay two times longer than the on-load tap-changer mechanical operating
OpenCTEnable enables/disables this feature
tOCTAlarmDelay defines the time delay after which the alarm signal will
tOCTReset defines the time delay after which the open CT condition
Section 6
Differential protection
M15266-337 v6
M15266-340 v5
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