the protection from operating during an overexcitation condition of a power
transformer. If the ratio of the 5
th
harmonic to the fundamental in the dif
ferential
current is above a settable limit, the operation is restrained. It is recommended to
use I5/I1Ratio = 25% as default value if there are no special reasons to choose
another setting.
6.2.3.5 Protections based on the directional criterion
GUID-0C213808-629F-4A58-BB70-C37DBAFEEE1E v1
Two kinds of protections based on the directional criterion are integrated into the
railway differential protection function:
• The directional unrestrained differential protection
• The directional sensitive differential protection
These two subfunctions are based on the internal-external fault discriminator,
which is a supplementary criterion to the traditional differential protection. The
internal/external fault discriminator detects even minor faults, with a high
sensitivity and at high speed and, at the same time, discriminates between internal
and external faults with a high degree of dependability. The goal is to be able to
quickly switch off the faulty power transformer before the fault develops into a
severe one (for example, a fault involving the iron core). This feature should
always be used when protecting a railway power transformer (by setting DirDiffEn
= On).
6.2.4 Setting examples
GUID-812455C4-D231-4E71-814F-E83FBBB0461B v1
For the transformer in Figure
31, there are two solutions for balancing the
transformer differential protection.
IEC16000067-1-en.vsd
15kV132kV
I
U
v
I
U
I
s
S
V
R
IEC16000067 V1 EN-US
Figure 31: A railway transformer with rating 16MVA, 132/15kV
Solution 1 (see Figure
37 for CT connections and ACT configuration):
Connection to the preprocessing blocks:
1MRK 506 375-UEN A Section 6
Differential protection
Railway application RER670 2.2 IEC 83
Application manual
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