3kV insulated CTs on 11kV cable!
3kV insulated CTs on 11kV cable!
(OP)
Good afternoon all,
Could anyone advise me as to whether there are any circumstances where it is accpetable to use instrument (power monitoring) current transformers with an insulation level rating of 3kV on an 11kV cable? The insulation level of this cable is 13kV
Thanks for your help people
Could anyone advise me as to whether there are any circumstances where it is accpetable to use instrument (power monitoring) current transformers with an insulation level rating of 3kV on an 11kV cable? The insulation level of this cable is 13kV
Thanks for your help people






RE: 3kV insulated CTs on 11kV cable!
It is a common practice to apply window CTs rated 600v to systems with higher voltages (4160,12470, 13800,14400, etc).
This practice is done by passing FULLY INSULATED conductors through the window (which seems to be your case).
The conductor insulation functions as the CT primary insulation providing a fully rated installation.
Hope this helps...
IEEE Std 242-2001 page 51, paragraph e)
RE: 3kV insulated CTs on 11kV cable!
These CTs are best known to me as "ring" type CTs. The conductor is fully insulated and does indeed pass through the CT.
Anymore information from other forum member would be greatly appreciated
RE: 3kV insulated CTs on 11kV cable!
RE: 3kV insulated CTs on 11kV cable!
For ANSI C37-type installations, one discussion is Technical Brief #85 from Powell Electrical Manufacturing Company – …600 Volt Class Current Transformers in Medium Voltage Switchgear www.powellelectric.com/pdf/ptb/booklet%201-91.pdf [~3MB]
RE: 3kV insulated CTs on 11kV cable!
ANSI/IEEE C57.13-1978
Table 7 "Basic Impulse Insulation Levels for Current Transformers"
states:
Nominal BILL,
System and Full Wave
Voltage (kV) Crest kV
======== ===========
13.8 110 or 95
RE: 3kV insulated CTs on 11kV cable!
http://www.netaworld.org/files/ItemFileA345.pdf
RE: 3kV insulated CTs on 11kV cable!
which means that there is nothing to be worry about
RE: 3kV insulated CTs on 11kV cable!
Cables and ground level equipment of this voltage level are not just insulated to sufficient BIL, but are also shielded. This is because insulation, like anything else in a circuit, must be at some potential or other. The grounded shield, though, is at ground potential. The potential in between is a gradient from the phase voltage to ground. Proper terminations this gradient even. If shield conductors are stripped back to avoid a CT window prior to terminating, the gradient is altered. By how much is the question. There are probably ways of simulating and testing this, but I see little reason to bother. The double back approach that CastleHydro described makes sure that only the phase current is measured, and keeps the shield where it should be. Each BIL stands by itself, nothing to add.
RE: 3kV insulated CTs on 11kV cable!
"Venditori de oleum-vipera non vigere excordis populi"
RE: 3kV insulated CTs on 11kV cable!
RE: 3kV insulated CTs on 11kV cable!
With the cable earth screen intact the voltage level for the ct is .72/3kv.
If the cable is not screened the air between the cable and the ct will discharge leading to electrical failure of the ct.