On-Line Compensation For On-Load Tap-Changer Position; Differential Current Alarm - ABB RELION 670 Series Applications Manual

1MRK 504 152-UEN B
heavy internal faults with severely saturated current transformers this differential protection
operates well below one cycle, since the harmonic distortions in the differential currents do
not slow down the differential protection operation. Practically, an unrestrained operation is
achieved for all internal faults.
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 indicate 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

On-line compensation for on-load tap-changer position

The Transformer differential function in the IED has a built-in facility to on-line compensate for
on-load tap-changer operation. The following parameters which are set under general settings
are related to this compensation feature:
• Parameter
located. The following options are available:
When value
exist and it will disregard all other parameters related to first OLTC
• Parameter
position 1)
• Parameter
example, 11 for OLTC with 21 positions) This tap position shall correspond to the values
for rated current and voltage set for that winding
• Parameter
example, 21 for OLTC with 21 positions)
• Parameter
voltage for that winding is obtained (for example, position with maximum number of
turns)
• Parameter
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.
6.1.3.8

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
Application manual
LocationOLTC1 defines the winding where first OLTC (OLTC1) is physically
Not Used is selected the differential function will assume that OLTC1 does not
LowTapPosOLTC1 defines the minimum end tap position for OLTC1 (typically
RatedTapOLTC1 defines the rated (for example, mid) position for OLTC1 (for
HighTapPsOLTC1 defines the maximum end tap position for OLTC1 (for
TapHighVoltTC1 defines the end position for OLTC1 where highest no-load
StepSizeOLTC1 defines the voltage change per OLTC1 step (for example, 1.5%)
Not Used / Winding 1 / Winding 2 / Winding 3 .
Section 6
Differential protection
M15266-326 v5
M15266-337 v6
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