4.3.8.8 Example calculations for high-impedance differential protection
The example shows the calculations for the
Operate value
setting, stabilizing
resistor value (R
s
) and required knee point voltage (U
kn
) of the CTs.
Table 725: Protected generator values
Quantity Value
S
n
8 MVA
U
n
6 kV
I
n
770 A
I
kmax
4620 A (6 × I
n
) out-of-zone fault
I
kmaxin
9.24 kA (12 × I
n
) in-zone fault
Table 726: Assumed CT data
Quantity Value
CT 1000/1 A
R
in
15.3 Ω
U
kn
323 V
I
e
35 mA (at U
kn
)
Figure 464: 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 2.5 mm
2
. Resistance at 75°C is 0.00865 Ω/m.
1MRS759142 F
Protection functions
REX640
Technical Manual
815
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