VAMP 265 - Example

7.3 Calculating the stabilizing
resistance RS, VDR value and actual
sensitivity
7 Applications
Technical description
158
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VM265.EN011
7.3.4. Example
CT
=
2000/1
V
KP
= 100 V
I
MAXT
=
16 kA = 8 x I
n
I
REF
=
5 % = 50 mA
Setting value scaled to secondary
level
R
CT
=
6 
R
W
=
0.4 

I
MAXF
=
25 kA
Maximum secondary voltage during a through fault (Equation
7.3.1-1):
V 2.514.06
2000
1
16000 
S
V
Conclusion: The knee point voltage of 100 V is acceptable being
about twice the V
S
.
Serial resistance for the relay input (Equation 7.3.1-2):
R
S
  
512
005
1024 1000
.
.
 
Maximum peak voltage during inside fault using a linear model
for CT (Equation 7.3.2-1):
kV 6.12)10004.06(
2000
1
25000 
p
V
Approximation of peak voltage during inside fault using a non-
linear model for a saturating CT (Equation 7.3.2-2):
kV 2.3)10012600(10022 
sp
V
This is a too high value and a VDR must be used to reduce the
voltage below 3 kV.
A zinc oxide varistor (i.e. VDR, METROSIL) of 1 kV will limit
the voltage. Using a 400 J model allows two 20 VA CTs feeding
ten times their nominal power during one second before the
energy capacity of the varistor is exceeded.