F
n
the rated accuracy limit factor corresponding to the rated burden S
n
I
2n
the rated secondary current of the CT
R
in
the secondary internal resistance of the CT
S
n
the volt-amp rating of the CT
The formulas are based on choosing the CTs according to
Equation 182
,
which results an absolutely stable scheme. In some cases, it is possible
to achieve stability with knee point voltages lower than stated in the
formulas. The conditions in the network, however, have to be known well
enough to ensure the stability.
1. If U
k
≥ 2 x U
s
, fast protection relay operation is secure.
2. If U
k
≥ 1.5 x U
s
and < 2 x U
s
, protection relay operation can be
slightly prolonged and should be studied case by case.
If U
k
< 1.5 x U
s
, the protection relay operation is jeopardized.
Another CT has to be chosen.
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
= × + +
( )
≈ ×
(Equation 187)
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)
(Equation 188)
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 protection relay is tested.
If R
s
was smaller, the VDR could be avoided. However, the value of R
s
depends on
the 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.
1MRS759142 F
Protection functions
REX640
Technical Manual
767
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