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A.4 Current Transformer Requirements
299
7ST6 Manual
E50417-G1176-C251-A3
A.4
Current Transformer Requirements
The requirements to the feeder current transformers are normally determined by the
overcurrent protection, especially by the setting of the high-set stage. Furthermore,
there is a minimum requirement, which is an experimental value.
The recommendations for dimensioning are in accordance with IEC 60044-1.
For the conversion of the requirements to the knee-point voltage and other transformer
categories, reference is made to IEC 60044-6, BS 3938 and ANSI/IEEE C 57.13.
A.4.1
Overcurrent Factors
Operational and
Nominal Overcur-
rent Factor
Calculation
Example according
to IEC 60044-1
Required minimum operational
overcurrent factor
minimum 20
where
n'
minimum operational overcurrent factor
I
>>
PU
primary pickup value of the high-set
stage
I
pN
primary CT rated current
Resulting rated overcurrent factor
where
n
Rated overcurrent factor
R
BC
connected burden (device and connec-
tion lead)
R
BN
Rated load
R
Ct
Internal transformer load
I
sN
= 1 A
n' = 20
R
BC
= 0.6
Ω
(device and connection leads)
R
Ct
= 3
Ω
R
BN
= 5
Ω
(5 VA)
n with 10 selected,
therefore: 5P10. 5 VA
with
I
sN
= secondary nominal transformer current
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