Neutral CT voltage rating on wye connected 138kV reactors?

[NAZ55]

What should be the voltage rating for the netural CT on 138kV wye connected reactors?

 

[NAZ55]

more specifically I am looking for an insulation level.

 

[scottf]

Typically 15 kV class CTs are used for such applications.

Reactors wouldn't typically fail shorted, so it's reasonable to assume the neutral won't float to the line voltage.

 

[NAZ55]

scottf how about 600V class?

 

[scottf]

I typically wouldn't use 600V class on this type of application. In theory they would work fine, however any kind of neutral voltage rise starts to push the insulation limits pretty quickly.

15 kV CTs run about $500. Since they're being used to protect 138 kV reactors and other equipment, it seems like a no-brain-er to me.

Also, 600V CTs are typically window-type, although wound-type are available in certain designs, however, wound-type are generally not rated for outdoor installation. Depending on the ratio you need, window-type CTs probably won't work. Generally I see ratios of 5:5A, 10:5A, etc... used on neutral applications. You can't get those in a window CT.

 

[NAZ55]

I apologize that I wasn't able to respond back to your comment earlier, but my question is how did you come up with 15kV rating? why not 35kV etc?
Since the neutral would be at a ground potential just like a X0 bushing shouldn't 600V class work?
Also, what kind of neutral voltage rise are you referring to?
Thanks

 

[dpc]

Agree with scottf - we would use 15 kV class - assuming a grounded systems, obviously.

Ground potential can vary from one location to another especially if there is a grounded 138 kV conductor nearby.

Google "earth potential rise" or "ground potential rise".

"The more the universe seems comprehensible, the more it also seems pointless." -- Steven Weinberg

 

[NAZ55]

dpc,

Let me see if I can word my question differently.
Are you all aware of any IEEE standard addressing the issue in my initial post?

I am not really after what you would use or what you think is a no brainer. What I am after is a standard which would back up /justify the use of either 600V or 15kV voltage class.

I will appreciate your advice.

Thanks
Zeeshan

 

[scottf]

zazmat-

There is no standard that I'm aware of the specifically spells out the insulation class to be used for neutral applications of reactor banks (I work for an instr transformer manufacturer and I used to work for a company that made reactors).

You need to be aware of what voltages can be present on the neutral during transients. Because of the possible voltages present, 15 kV class ITs are normally used on grounded neutral applications. I believe the idea is that neutral voltages will not float up nearly that high and if they do, the CT is the least of your worries.

 

[Kiribanda]

zazmat,

In case of a power transformer, if the neutral is solidly grounded, the MINIMUM BIL level (per CSA-C88)of the neutral bushing should be 95 kV.Based on that we specify 110 kV BIL for the neutral bushing.

Assuming a set of WYE connected 138 kV reactors is equivalent to a wye connected power transformer, the BIL of the neutral bushing can be justified equal to 95 kV minimum.

Based on above, I will specify a BIL level of 110 kV for the netral CT. Now you will find that for a 13.8 kV (15 kV)nominal system, the BIL level is 110 kV per ANSI C57.12.

Does it make sense?