ABB RELION 620 Series Technical Manual page 604

Section 4
Protection functions
598
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Figure 308: Example calculation for high-impedance differential protection (only
one phase is presented in detail)
The length of the secondary circuit loop is 200 m and the area of the cross-section is
2
2.5 mm . Resistance at 75°C is 0.00865 Ω/m.
R . m
0 00865 200
= Ω ÷ ⋅
m
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First the stabilizing voltage is calculated based on
A
6 770
⋅
U . .
( 15 3 1 73
= Ω +
s
1000
GUID-07DF3FB1-366D-41FB-BDE7-A6A143C31DCC V1 EN
In this case the requirement for the current transformer knee point voltage is fulfilled
because U > 2U .
kn s
The magnetising curve of the CT is assumed to be linear. The magnetizing current at
the stabilizing voltage can be estimated.
U
s
I I
= ⋅
m e
U
kn
GUID-551E9FD0-3F1A-47C1-9ED5-10336EEC2104 V1 EN
. V
78 7
I mA . mA
35 8 5
= ⋅ ≈
m
V
323
GUID-43ED47B8-4C9B-4CF9-BA13-62806B454A29 V1 EN
m . Ω
1 73
≈
. V
) 78 7
Ω ≈
1MRS757644 F
(Equation 115)
Equation 105.
(Equation 116)
(Equation 117)
(Equation 118)
Technical Manual
620 series