5 The interval when
IdMinHigh
is active is set to 60 s because a power transformer is included in
the protected zone. As both IEDs process the same currents, both must have the same value
set for
IdMinHigh
.
6
The unrestrained operate (differential) current limit should greater than the highest through fault
current. This current appears at a three-phase short circuit on the 33 kV side of the transformer
and can be calculated as:
220
2.75
3 (7.0 15.0 24.2)
Through
I kA= =
× + +
EQUATION1423 V1 EN-US (Equation 5)
With a safety margin of 20%:
1.2
1.2 2.75 3.30
5.50
0.6 0.6
Through
I
kA kA
Idunre
Ibase kA kA
×
×
= = = =
EQUATION1424 V1 EN-US (Equation 6)
7 The cross-block logic should always be active when there is a power transformer in the
protected zone.
Setting example with a small tap transformer in the protected zone
A typical example is shown in Figure 36
IEC12000193-2-en.vsd
3Id> 3Id>
SA
1700
MVA
SA
1280
MVA
10MVA
ek=10%
138/10kV
3Id>
3Id>
A B
IEC12000193 V2 EN-US
Figure 36: Setting example
Input data to calculation:
• Apparent source power at A side: Ss
A
= 1700 MVA
• Line impedance from A to tap: Zl
A
= 2.8 Ω
• Line impedance from tap to B side: Zl
B
= 1.2 Ω
Section 4 1MRK 505 382-UEN B
Analog and binary signal transfer for line differential protection
44 Communication set-up 670/650 series 2.2
Application Guide
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