ABB RELION 620 Series Technical Manual page 573

1MRS757644 F
620 series
Technical Manual
The need for the VDR depends on certain conditions.
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
n
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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)
(
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 IED is tested.
If R was smaller, the VDR could be avoided. However, the value of R
s
IED operation current and stabilizing voltage. Thus, either a higher setting must be
used in the IED or the stabilizing voltage must be lowered.
I
k max in
R R
)
+ ≈ ×
m s s
n
)
Section 4
Protection functions
(Equation 91)
(Equation 92)
depends on the
s
567