MV design guide - Schneider Electric

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