VAMP 265 - 7.3. Calculating the Stabilizing Resistance RS, VDR Value and Actual Sensitivity

7.3 Calculating the stabilizing
resistance RS, VDR value and actual
sensitivity
7 Applications
Technical description
156
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VM265.EN011
7.3. Calculating the stabilizing resistance
R
S
, VDR value and actual sensitivity
7.3.1. Value of stabilizing resistor R
S
The voltage V
S
(Figure 7.1-1) is:
Equation 7.3.1-1
WCT
PRIM
SEC
MAXTS
RR
CT
CT
IV 
I
MAXT
=
Maximum through fault current not to cause an REF trip
1
CT
SEC
=
Nominal secondary current of the CT
CT
PRI
=
Nominal primary current of the CT
R
CT
=
Resistance of CT secondary.
R
W
=
Total resistance of wiring, connections etc.
The CT should be of class X (see chapter 7.2.1) and the knee
point voltage should be twice the calculated V
S
.
The stabilizing resistor R
S
is calculated as:
Equation 7.3.1-2
R
V
I
S
S
Set

I
Set
= Setting value of the relay as secondary value.
1
Selecting a low value helps to achieve more sensitivity and helps to avoid
the usage of a voltage limiting VDR. An unselective earth fault pick-up/trip
is not always a problem if a fast overcurrent stage will clear the fault
anyway.