Bushing CT Grounds

[111R]

I've never seen a cutaway of a bushing CT or had the opportunity to take one apart.

Is the iron core of the bushing CT grounded to the breaker frame? Or, is it insulated from the case? If grounded, it seems like this would affect leakage flux due to another low reluctance path being present.

Does it make sense to check insulation resistance on bushing CTs between the secondary winding and ground or the primary bus and secondary? I can see how resistance between a primary and secondary is useful for a wound PT or CT where the windings are in close proximity, but I'm not sure if it's necessary on a bushing CT.

 

[randyman]

Bushing CTs made in USA, in general, do not have a ground connection from the core. The core is fully insulated from the secondary winding. Dry-type BCTs which are found on HV power circuit breakers are usually installed under the bushing flange and protected by an aluminum shell isolated from the BCTs. And since the BCTs are along the ground shank of the bushing they are fully isolated from the primary conductor as well.

Resistance checking between pri and sec is probably a waste of time. Between sec and ground will be like chekcing the lead resistance to ground.

If the BCTs are potted in an aluminum shell or case, then resistance check between sec and case may be worth the effort.

About the only other resistance check that could find potential problems would be between BCTs. In many cases there are more than one BCT stacked on the bushing.

 

[prc]

The secondary core of EHV CTs and core of BCTs are not grounded and kept floating.But core of a power transformer is always grounded.In case of power transformer, core will be subjected to electric field and floating potential can be developed.But in case of CT, the core will be seeing earth electrode inside and earthed tank outside and may not be seeing the electric field.By earthing it will not form any low reluctance path for flux.

Usually CT secondray winding is checked for insulation resistance with 1 kV megger, with acceptance value of 500 Meghohms.

 

[prc]

Clarification- What I mentioned for EHV CTs is only for dead tank CT designs. I dont know the arrangement for live tank CTs.Probably core is kept at Live line potential

 

[inpran]

We carry out insulation resistance check with voltage levels mentioned by prc for secondary to ground. But have not carried out between primary and secondary.

 

[LeifEricson]

As stated by Randyman and prc, BCT cores are not typically connected to ground at least in the US. Secondary winding resistance/Megger/hipot testing is a pretty std test for the breaker mfrs and during commissioning. It provides assurance that there is adequate insulation between the secondary circuit and ground. Many problems will not show up in other (lower voltage) routine tests.