ABB RELION REX640 Technical Manual page 737

1MRS759142 C
REX640
Technical Manual
GUID-96BACB22-3697-4039-B961-BCA4C418F954 V1 EN
Figure 405: 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.
Ω
R 0 00865 . 200 m
= ⋅
m
m
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First the stabilizing voltage is calculated based on
A
6 770
⋅
U
15 3 .
(
= ⋅ Ω +
s
1000
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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-048EDA28-1CDD-4936-BD35-710CC1811206 V1 EN
V
78 7 .
I mA
35 8 5 .
= ⋅ ≈
m
323 V
GUID-EA42DD81-FCC5-44D8-A301-F7A44E0EBD95 V1 EN
1 73 .
≈ Ω
V
1 73 . 78 7 .
)
Ω ≈
mA
Section 4
Protection functions
(Equation 176)
Equation 166.
(Equation 177)
(Equation 178)
(Equation 179)
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