VAMP 265 - Page 164

7.4 Current Transformer Selection
7 Applications
Technical description
164
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VM265.EN011
Example 1
Transformer:
16 MVA YNd11 Z
k
= 10%
110 kV / 21 kV (84 A / 440 A)
CT’s on HW side:
100/5 5P10
Winding resistance R
CT
= 0.07 
(RCT depends on the CT type, INCT and power rating. Let's say that the
selected CT type, 100 A and an initial guess of 15 VA yields to 0.07 .)
Safety factor c = 4.
(Transformer differential, earthed Y.)
CTs on LV side:
500/5 5P10
(Max. Short circuit curent is 4400 A = 8.8 x 500 A)
Winding resistance R
CT
= 0.28 
(RCT depends on the CT type, INCT and power rating. Let's say that the
selected CT type, 500 A and an initial guess of 15 VA yields to 0.28 )
Safety factor c = 3.
(Transformer differential, .)
Differential current setting of the non-stabilized stage I>>:
I
SET
= 9 x I
N
R
L
=
0.008 Typical burden of a VAMP relay current input.
R
WHV
=
0.138 Wiring impedance of high voltage side.
(2x16 m, 4 mm
2
)
R
WLV
=
0.086 Wiring impedance of low voltage side.
(2x10 m, 4 mm
2
)
The needed CT power on HV side will be
 
6.14507.0008.0138.007.0
10010
8494
2











N
S
VA
 15 VA is a good choice for HV side.
And on the LV side
 
2.15528.0008.0086.028.0
50010
44093
2











N
S
VA
 15 VA is a good choice for LV side.