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.
U
A
V
max
=
×
=
12600
240
330
17325
Ω
GUID-83BA854A-8BF2-4E60-A6F7-90E79458B6E5 V1 EN
(Equation 9)
u
kV
∧
=
×
×
−
(
)
≈
2 2 81 17325 81
3 34
.
GUID-9BA35770-2458-4CE0-9E06-5D56CAA21B57 V1 EN
(Equation 10)
As the peak voltage û is 3.2 kV, VDR must be used. If the R
s
is smaller, VDR could be
avoided. However, the value of R
s
depends on the operation current and stabilizing
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
s
is smaller, VDR can be
avoided.
The value of R
s
depends on the protection relay operating current and the stabilizing
voltage. Therefore, either a higher setting in the protection relay or a lower stabilizing
voltage must be used.
Section 4
1MRS757455 E
Requirements for measurement transformers
62
REB611
Application Manual
