VAMP 265 - Page 163

Technical description
7 Applications
7.4 Current Transformer Selection
VM265.EN011
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163
Formula to solve needed CT power SN
By replacing the complex power terms with corresponding
resistances in Equation 7.4.1-3 we get,
Equation 7.4.2-2
LWCT
NCT
ALFA
RRR
RR
kk



where the nominal burden resistance is
2
secNCT
N
N
I
S
R 
R
CT
=
Winding resistance (See Figure 7.4.1-1)
R
W
=
Wiring resistance (from CT to the relay and back)
R
L
=
Resistance of the protection relay input
S
N
=
Nominal burden of the CT
I
NCTsec
=
Nominal secondary current of the CT
By solving S
N
and substituting k
A
according Equation 7.4.2-2 ,
we get
Equation 7.4.2-3
 
2
secNCTCTLWCT
NCTALF
NT raSET
N
IRRRR
Ik
IcI
S






