Example Calculations For High-Impedance Differential Protection - ABB RELION REX640 Technical Manual

Section 4
Protection functions
4.3.8.8
730
As the peak voltage ȗ = 3.2 kV, VDR must be used. In some cases, VDR can be
avoided if R is smaller. The value of R
s
current and stabilizing voltage. Thus, a higher setting in the protection relay must be
used or the stabilizing voltage lowered.
Flux-balancing principle
When the function block is used with the flux-balancing principle, there are no extra
requirements for the measuring devices. The core-balance transformers used in an
ordinary overcurrent protection are adequate here as well.

Example calculations for high-impedance differential protection

The example shows the calculations for the Operate value setting, stabilizing resistor
value (R ) and required knee point voltage (U
s
Table 651: Protected generator values
Quantity
S
n
U
n
I
n
I
kmax
I
kmaxin
Table 652: Assumed CT data
Quantity
CT
R
in
U
kn
I
e
depends on the protection relay operation
s
) of the CTs.
kn
Value
8 MVA
6 kV
770 A
4620 A (6 × I ) out-of-zone fault
n
9.24 kA (12 × I ) in-zone fault
n
Value
1000/1 A
15.3 Ω
323 V
35 mA (at U )
kn
1MRS759142 C
REX640
Technical Manual