ABB RELION REX640 Technical Manual page 687

1MRS759142 C
REX640
Technical Manual
First, voltage U , ignoring the CT saturation during the fault, is calculated with the
max
equation
I
k max in
U R R
(
= ⋅ +
max in m
n
GUID-C3B7E72F-852E-4707-8E45-0C0BA3F89754 V1 EN
I the maximum fault current inside the zone, in primary amps
kmaxin
n the turns ration of the CT
R the internal resistance of the CT in ohms
in
R the resistance of the longest loop of the CT secondary circuit, in ohms
m
R the resistance of the stabilized resistor, in ohms
s
Next, the peak voltage û, which includes the CT saturation, is estimated with the
formula (given by P.Mathews, 1955)
ß 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.
If R was smaller, the VDR could be avoided. However, the value of R
s
protection relay operation current and stabilizing voltage. Thus, either a higher setting
must be used in the protection relay or the stabilizing voltage must be lowered.
I
k max in
R R
)
+ ≈ ⋅
s s
n
)
Section 4
Protection functions
(Equation 134)
(Equation 135)
depends on the
s
681