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.
7.1.3.7 On-line compensation for on-load tap-changer position
M15266-326 v5
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 LocationOLTC1 defines the winding where first OLTC (OLTC1) is
physically located. The following options are available: Not Used / Winding 1 /
Winding 2 / Winding 3. When value Not Used is selected the differential function
will assume that OLTC1 does not exist and it will disregard all other parameters
related to first OLTC
• Parameter LowTapPosOLTC1 defines the minimum end tap position for OLTC1
(typically position 1)
• Parameter RatedTapOLTC1 defines the rated (for example, mid) position for
OLTC1 (for 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 HighTapPsOLTC1 defines the maximum end tap position for OLTC1
(for example, 21 for OLTC with 21 positions)
• Parameter TapHighVoltTC1 defines the end position for OLTC1 where highest no-
load voltage for that winding is obtained (for example, position with maximum
number of turns)
• Parameter StepSizeOLTC1 defines the voltage change per OLTC1 step (for
example, 1.5%)
1MRK 502 071-UUS A Section 7
Differential protection
Generator protection REG670 2.2 ANSI and Injection equipment REX060, REX061, REX062 141
Application manual
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