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16
Requirements
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All kinds of conventional magnetic core CTs are possible to be used with REx 5xx ter-
minals if they fulfil the requirements that correspond to the above specified according
to the IEC 60044-6 standard. From the different standards and available data for relay-
ing applications it is possible to approximately calculate a secondary e.m.f. of the CT.
It is then possible to compare this to the required secondary e.m.f. E
the CT fulfils the requirements. The requirements according to some other standards are
specified below.
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A CT according to IEC 60044-1 is specified by the secondary limiting e.m.f. E
value of the E
is approximately equal to E
2max
≈
E
E
al
2max
The current transformers must have a secondary limiting e.m.f. E
following:
>
E
maximum of E
2max
alreq
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A CT according to BS is often specified by the rated knee-point e.m.f. E
ue of the E
is lower than E
kneeBS
a general relation between the E
of the E
value. Therefore, the rated equivalent limiting secondary e.m.f. E
al
specified according to BS can be estimated to:
≈
⋅
E
1.2 E
alBS
kneeBS
The current transformers must have a rated knee-point e.m.f. E
following:
⋅
>
1.2 E
maximum of E
kneeBS
according to IEC 60044-6.
al
according to IEC 60044-6. It is not possible to give
al
and the E
but normally the E
kneeBS
al
alreq
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and judge if
alreq
. The
2max
(Equation 3)
that fulfills the
2max
(Equation 4)
. The val-
kneeBS
is 80 to 85%
kneeBS
for a CT
alBS
(Equation 5)
that fulfills the
kneeBS
(Equation 6)
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