ABB RELION REX640 Technical Manual page 767

1MRS759142 F
F
n
I
2n
R
in
S
n
The need for the VDR depends on certain conditions.
First, voltage U
the equation
U
max
I
kmaxin
n
R
in
R
m
R
s
Next, the peak voltage û, which includes the CT saturation, is estimated with the
formula (given by P.Mathews, 1955)
û =
U
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
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.
REX640
Technical Manual
the rated accuracy limit factor corresponding to the rated burden S
the rated secondary current of the CT
the secondary internal resistance of the CT
the volt-amp rating of the CT
The formulas are based on choosing the CTs according to
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 ≥ 2 x U
k
2. If U ≥ 1.5 x U
k
slightly prolonged and should be studied case by case.
If U < 1.5 x U
k
Another CT has to be chosen.
, ignoring the CT saturation during the fault, is calculated with
max
I
k max in
(
= × R + R +
in m
n
the maximum fault current inside the zone, in primary amps
the turns ration of the CT
the internal resistance of the CT in ohms
the resistance of the longest loop of the CT secondary circuit, in ohms
the resistance of the stabilized resistor, in ohms
( )
2 2 U U − U
kn max kn
the knee point voltage of the CT
was smaller, the VDR could be avoided. However, the value of R
s
, fast protection relay operation is secure.
s
and < 2 x U , protection relay operation can be
s s
, the protection relay operation is jeopardized.
s
I
k max in
)
R ≈ × R
s s
n
Protection functions
n
Equation 182
,
(Equation 187)
(Equation 188)
depends on
s
767