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5
The interval when
IdMinHigh is active is set to 60 s because a power transformer is
included in the protected circuit. As both IEDs process the same currents, both must have
the same value set for
IdMinHigh.
6
The unrestrained operate differential current value shall be 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 32)
With a safety margin of 20% we get:
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 33)
7
The cross block logic shall always be active when there is a power transformer in the
protected zone.
Setting example for a small Tap-off transformer
A typical example can be as per single line diagram below.
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 64: Setting example
Input data to the 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
Apparent source power at B side: Ss
B
= 1280 MVA
Base current of differential current protection: I
Base
= 42 A
1MRK 505 343-UEN B
Section 6
Differential protection
137
Application manual
Summary of Contents for Relion 670 series
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