VAMP 265, Operation And Configuration Instructions

Page: 156 / 220

7.3 Calculating the stabilizing
resistance RS, VDR value and actual
sensitivity 7 Applications Technical description
156
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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
W CT
PRIM
SEC
MAXT S
R R
CT
CT
I V
 
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.