MV design guide - Schneider Electric

Switchgear
definition
Current transformer
Please note!
Never leave a CT in
an open circuit.
This is intended to provide a secondary circuit with a current
proportional to the primary current.
Transformation ratio (Kn)
Ipr
Kn = = N2
Isr N1
N.B.: current transformers must be in conformity with standard IEC 185 but can also be defined
by standards BS 3938 and ANSI.
c It comprises one or several primary windings around one or several
secondary windings each having their own magnetic circuit, and all being
encapsulated in an insulating resin.
c It is dangerous to leave a CT in an open circuit because dangerous
voltages for both people and equipment may appear across its terminals.
Primary circuit characteristics
according to IEC standards
Rated frequency (fr)
A CT defined at 50 Hz can be installed on a 60 Hz network.
Its precision is retained. The opposite is not true.
Rated primary circuit voltage (Upr)
c General case:
Rated CT voltage ≥ rated installation voltage
The rated voltage sets the equipment insulation level (see "Introduction"
chapter of this guide). Generally, we would choose the rated CT voltage
based on the installation operating voltage U, according to the chart:
U 3.3 5 5.5 6 6.6 10 11 13.8 15 20 22 30 33
Upr
7.2 kV
cable or busduct
Core balance CT
(sheathed or not sheathed busduct)
insulator
air
insulator
12 kV
17.5 kV
24 kV
36 kV
c Special case:
If the CT is a core balance CT installed on a busduct or on a cable.
The dielectric insulation is provided by the cable or busducting insulation
and the air located between them. The core balance CT is itself insulated.
54 Merlin Gerin MV design guide Schneider Electric