Power Transformer Neutral CT: CT Insulation Level 3

[Distribution73]

Hi!
I need to specify a current transformer that is to be connected to the neutral of an existing 34.5kV : 4.16kV (delta - wye) power transformer. The CT is intended for installation at the wye neutral connection (4.16kV side) which is directly connected to ground.
In principle I was thinking of using a simple 600V rated ring type CT, however I was wondering if I could be missing something with regards to the CT insulation level, even though it is installed around a wire directly connected to ground.
Your feedback is much appreciated!
Thank you!

 

[cuky2000]

Nameplate Examples:
1) CT per IEC Std: Link

2) CT per ANSI/IEEE Std: Link

 

[scottf]

It is common in the US to use a 5 or 15kV CT, but I have never understand why a 600V CT cannot be used on a grounded-wye neutral.

 

[Coco_HueHueHue]

Same as scottf in Canada.
But i also don't understand why you can't use a 600V CT if you are solidly grounded.

I have also seen isolated bus with cold or heat shrink and a 600V CT.

 

[argotier]

When you say ring type CT you mean a toroidal or window type CT? Like this:

If so, the insulation is provided by the transformer bushing (or cable, if you use an insulated cable to go through the CT window) and you could use a 600 V CT.

If there is a live wire to go through the CT window... Im not so sure.

Edit: Standards and regulations requirements aside, the minimum insulation you need for a ground connection is the maximum voltage that arise in case of a ground fault. All grounds, even the solidly grounded, have an impedance (very low usually), wich together with the ground conductors impedance (also low), will cause a voltage drop when the ground fault current (usually high) circulates.

Hope it helps.

 

[RRaghunath]

If the CT is window type CT surrounding the insulated cable connecting the neutral to earth, 600V CT should do. This is because the main insulation is provided by the HV cable.
If the CT is in the neutral terminal box surrounding the bare neutral conductor, the CT has to have same insulation level as the neutral bushing / neutral terminal. This shall be same as that for line terminal/bushing as per IEC 60076-3 for voltages not exceeding 72.5kV (with uniform insulation).

 

[stevenal]

Think carefully about where you put weak links in any system. Sure the neutral is at ground potential normally, but is there any condition of maintenance or failure where that might change? When I apply a neutral CT (not a slipover), I match or exceed the neutral bushing BIL.

 

[Distribution73]

Thank you very much for the very helpful feedback. It is much appreciated!