Voltage Rating of Neutral CT

[appunni]

Sir,
I have seen that 11 kV CTs are used as the neutral CT on the 66 kV side of 66/11kv transformers (solidly earthed, star connected transformers) for Restricted Earth Fault protection. I know that the voltage in the neutral point of the transformer during fault conditions is very less. Is the procedure of installing of low voltage rated CTs in transformer neutral correct? What is the minimum voltage required for Neutral CT in 220 kV side of a 220 /11kv star (solidly earthed) delta transformer? Is there any CT, which is called ‘Neutral CT’? If there is, what is meant by its voltage rating?

 

[electricpete]

I do know that the neutral bushing is often a lower voltage rating and BIL rating than the line terminal bushings.

I'm not sure all the factors considered in establishing those ratings or how they apply to CT's.

 

[jbartos]

Suggestion: CTs have their BILs. If the neutral CT is over neutral conductor with insulation, then the CT BIL and conductor BIL add up. E.g. the neutral CT can be only 600V with 10000V BIL if the neutral conductor is insulated with its high BIL. If not, there are CTs available with very high BILs as indicated in ANSI/IEEE Std. C57.13 Instrument Transformers.

 

[DanDel]

The neutral CT, along with associated switchgear and cabling, is typically rated for the highest voltage it would see during a L-G fault; which is the L-N voltage rating. This CT is usually used for a ground overcurrent relay, typically with a high or low resistance grounded system.
A 220kV wye-connected winding would have a 220kV/1.73 = 127kV L-N voltage. If there was a L-G fault, there would be 127kV across the neutral grounding resistor, therefore, the CT and associated equipment should be appropriately rated.

 

[peterb]

Apunni -
Can you advise whether the CT in question is a bushing CT installed in the transformer neutral bushing or a separately mounted post type CT?
For a bushing type CT, the CT is installed in a well in the transformer bushing and the insulated bushing lead passes through it. At the point of installation, the primary insulation is provided by the bushing itself and the current transformer has low voltage (600V) insulation. Note that the CT is installed in close proximity to the grounded bushing well.
For a separate post type CT, the voltage rating would be derived from the expected maximum value under fault conditions. I suspect that the 11kV rating is the minimum available for a CT physically suitable for separate mounting external to the transformer.

 

[johnspark]

-Appunni
If the 66kV winding is solidly earthed to the earth grid of the substation, then up to now I assume you have been using 1000v rated wire (70mm2?) to earth the star point.
Therefore, if you now place this cable inside a CT you will be able to use a 1000 volt insulated REF CT.

If you are earthing the star point via a resistor, certainly you need the REF CT voltage rating to be at least equal to one phase to ground voltage (As described by DanDel).

cheers John