The need for the VDR depends on certain conditions.
First, voltage U
max
, ignoring the CT saturation during the fault, is calculated with
the equation
U
I
n
R R R
I
n
R
k in
in m s
k in
smax
max max
= × + +
( )
≈ ×
GUID-CB54C30A-C69D-4C59-B9B3-44530319D1CE V1 EN (Equation 66)
I
kmaxin
the maximum fault current inside the zone, in primary amps
n the turns ration of the CT
R
in
the internal resistance of the CT in ohms
R
m
the resistance of the longest loop of the CT secondary circuit, in ohms
R
s
the resistance of the stabilized resistor, in ohms
Next, the peak voltage û, which includes the CT saturation, is estimated with the
formula (given by P.Mathews, 1955)
GUID-0FBE4CDF-8A7C-4574-8325-C61E61E0C55C V1 EN (Equation 67)
U
kn
the knee point voltage of the CT
The VDR is recommended when the peak voltage û ≥ 2kV, which is the insulation
level for which the IED is tested.
If R
s
was smaller, the VDR could be avoided. However, the value of R
s
depends on
the IED operation current and stabilizing voltage. Thus, either a higher setting must
be used in the IED or the stabilizing voltage must be lowered.
Section 4 1YHT530004D05 D
Protection functions
440 615 series
Technical Manual
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