Example Calculations For Busbar High-Impedance Differential Protection - ABB RELION 620 Series Technical Manual

1MAC504801-IB E
4.6.4.7
620 series ANSI
Technical Manual
( )
û = 2 2 U U − U
kn max kn
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U the knee point voltage of the CT
kn
The VDR is recommended when the peak voltage û ≥ 2kV, which is the insulation level
for which the protection relay is tested.
The maximum fault current in case of a fault inside the zone is considered to be 12.6 kA
in primary, CT is of 1250/5 A (ratio n = 240), knee point voltage is 81 V and the stabilizing
resistor is 330 Ohms.
12600
A
330
U
= × Ω =
max
240
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∧
u 2 2 81 17325 81
( )
= × × −
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As the peak voltage û is 3.2 kV, VDR must be used. If the R
avoided. However, the value of R
voltage of the protection relay. Thus, either a higher setting should be used or the
stabilizing voltage should be lowered.
As the peak voltage û = 3.2 kV, VDR must be used. If the R
avoided.
The value of R depends on the protection relay operating current and the stabilizing
s
voltage. Therefore, either a higher setting in the protection relay or a lower stabilizing
voltage must be used.

Example calculations for busbar high-impedance differential protection

The protected object in the example for busbar differential protection is a single-bus
system with two zones of protection.
17325
V
3 34 . kV
≈
depends on the operation current and stabilizing
s
Section 4
Protection functions
(Equation 62)
(Equation 63)
(Equation 64)
is smaller, VDR could be
s
is smaller, VDR can be
s
483