MV design guide - Schneider Electric
MV design guide - Schneider Electric
MV design guide - Schneider Electric
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Switchgear<br />
definition<br />
Current transformer<br />
In the case of an ambient temperature greater than 40°C for the CT,<br />
the CT's nominal current (Ipn) must be greater than Ips multiplied by the<br />
derating factor corresponding to the cubicle.<br />
As a general rule, the derating is of 1% Ipn per degree above 40°C.<br />
(See "Derating" chapter in this <strong>guide</strong>).<br />
Rated thermal short-circuit current (Ith)<br />
The rated thermal short-circuit current is generally the rms. value of<br />
the installation's maximum short-circuit current and the duration of<br />
this is generally taken to be equal to 1 s.<br />
Example:<br />
c Ssc = 250 <strong>MV</strong>A<br />
c U = 15 kV<br />
3<br />
3<br />
I th 1 s<br />
=<br />
Ssc • 10<br />
=<br />
250 • 10<br />
= 9 600 A<br />
U • e 15 • e<br />
c Each CT must be able to withstand the short-circuit current which can<br />
flow through its primary circuit both thermally and dynamically until the<br />
fault is effectively broken.<br />
c If Ssc is the network short-circuit power expressed in <strong>MV</strong>A, then:<br />
Ith =<br />
Ssc<br />
U • e<br />
c When the CT is installed in a fuse protected cubicle, the Ith to use<br />
is equal to 80 Ir.<br />
c If 80 Ir > Ith 1 s for the disconnecting device,<br />
then Ith 1 s for the CT = Ith 1 s for the device.<br />
Overcurrent coefficient (Ksi)<br />
Knowing this allows us to know whether a CT will be easy to<br />
manufacture or otherwise.<br />
c It is equal to:<br />
Ksi =<br />
Ith 1 s<br />
Ipr<br />
c The lower Ksi is, the easier the CT will be to manufacture.<br />
A high Ksi leads to over-dimensioning of the primary winding's section.<br />
The number of primary turns will therefore be limited together with the<br />
induced electromotive force; the CT will be even more difficult to produce.<br />
Order of magnitude<br />
ksi<br />
Manufacture<br />
Ksi < 100<br />
standard<br />
100 < Ksi < 300 sometimes difficult for certain<br />
secondary characteristics<br />
100 < Ksi < 400 difficult<br />
400 < Ksi < 500 limited to certain secondary characteristics<br />
Ksi > 500<br />
very often impossible<br />
A CT's secondary circuit must be adapted to constraints related to its use,<br />
either in metering or in protection applications.<br />
56 Merlin Gerin <strong>MV</strong> <strong>design</strong> <strong>guide</strong> <strong>Schneider</strong> <strong>Electric</strong>