3kV insulated CTs on 11kV cable! 1

[hilleard]

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

 

[tulum]

Are they window type CT's?

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)

 

[hilleard]

Thanks for the reply Tulum

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

 

[CastleHydro]

Tulum is correct. I want to add that in this type of application, you must be careful with armour and shields on the cable. Grounds should not normally pass through the window with the conductor, but if they must, as in the case of stress-cone terminations then the ground lead must be routed back through the window to neutralize the effect of any ground currents. If the shield is not grounded at the terminating end then it is okay for it to pass through the CT as there won't be any current in it.

 

[busbar]

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

http://www.powellelectric.com/pdf/ptb/booklet 1-91.pdf

[~3MB]

 

[jbartos]

Suggestion: If in ANSI/IEEE Territories, Reference:
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

 

[tulum]

I just came across another hit for the same article as above... so FYI here it is...

http://www.netaworld.org/files/ItemFileA345.pdf

 

[jbartos]

Comment: Supposing that the insulated conductor has BIL=120kV and the 600V CT has BIL=10kV, the total BIL for the insulation system is approximately BILtot=120kV+10kV=130kV,
which means that there is nothing to be worry about

 

[stevenal]

It seems that heavy trimming of this thread has left the word that adding the two BILs is sufficient to check the safety. I still disagree.

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.

 

[jraef]

Yes, but if the CTs described by the OP are used inside of factory assembled equipment, unshielded cables are usually used between components.

"Venditori de oleum-vipera non vigere excordis populi"

 

[stevenal]

Agreed. If construction is part of a tested and listed assembly, no problem.

 

[wilkin]

The cable screen is non magnetic and can therfore be left in place but the earth for the screen must be returned through the ct for earthing.
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.