CT off by 26% 4

[stevenal]

Tripped on bus differential for a through fault last week. When troubleshooting, we found one of the 1200:5 CTs was actually 1200:3.7. That explains the trip, but what would cause such inaccuracy? Demagnetizing did not help. All taps on the multi-ratio CT had the same error. Has anyone seen this?

 

[scottf]

The material really depends on who the manufacturer is. Tri-Flex is the normal material used that is subject to absorbing moisture. In our designs we use a mix of tri-flex and mylar.

The best way is to specify that the wrapping be a heat-cured fiberglass tape, as opposed to the "electrical tape" type vinyl tape. The cured fiberglass tape is much better against moisture intrusion.

 

[stevenal]

Update:

Manufacturer suggested baking the CTs. Also said they normally test by placing on a metal plate and meggering from winding to plate. We did so, and the X CT had a high reading while the Y CT read zero. The external wrapping is a fiberglass tape. Will be contacting the manufacturer for a replacement.

 

[RRaghunath]

If the secondary were 2.88A instead of 3.7A, I would have thought that the CTs are supposed to be connected in delta (2.88multiplied by Sqrt3 is 5A). But, looking at the thread of mails, it seems to be CT accuracy/saturation issue.

 

[stevenal]

Update:

Manufacturer sent replacement CT at no charge, even though breaker is out of warranty. The two CTs on the bushing still interact. Waiting for better weather to troubleshoot.

I've attached a picture of the one that failed. We believe the problem may be right here where the external wires are connected. Uninsulated butt splices are covered with a cloth tube. The dark spots are mildew stains. We're thinking we can replace the splices with insulated butt splices, and have a good spare. Maybe we'll get some heat shrink butt splices from the local marine supply. Whatcha think scottf?

 

[scottf]

Was the sleeving shown not covering the crimp?

In my experience, it's not normal to use insulated crimps (most manufacturers don't like them as they're more likely to be crimped poorly), but rather the crimps shown and then cover with sleeving. That kind of sleeving is pretty good stuff.

Are you at liberty to say whom the manufacturer is?

My gut feel is that the crimps probably aren't your problem, but it's hard to say.

 

[stevenal]

scottf,

The sleeve was pulled back for the picture, but the loose fitting sleeve does nothing to keep water out. The mildew stains indicate the area has been continuously wet. I think if the connectors were water tight, we'd pass the meggar test with the whole CT submerged.

Meramec CT on an Alstom breaker. We have many similar units with no problems. Our latest delivery came from the manufacturer with plastic covering the poly part of the bushing and no other protection. Breakers were wet and filthy from road sand and salt after going over mountain passes in our recent challenging weather. CTs tested okay straight off the truck.

 

[ELEP]

-scottf
Is it possible to identify interturn shorting by excitation test ?. This is is new information for me. I think excitation test is perform to idetify CT saturation ?

We generally use precision widing resistance meter to identify interturn shorting.

Let's have a look at the stevenal question one more time: difference in CT ratio; 1200:5 CTs was but actually 1200:3.7;

Do you think 1 or 2 no. of interturn shortings ?.

 

[scottf]

ELEP-

I doubt a single turn short on a 1200:5A CT would show up on a secondary resistance reading. 1200:5 = 240:1, therefore 1 shorted turn would be approx. a 0.4% difference, which is well inside the difference you would see between like units, i.e. manufacturing tolerance.

A shorted turn will show up on secondary excitation tests. The knee-point voltage will drop noticeably and the excitation current will increase.

Note that a shorted turn won't simply result in secondary current changing by a factor of 1 turn difference. That's because the shorted turn causes local saturation of the core and throws off the magnetic properties.

 

[waross]

Re: the mildew near the tap connections. I found out the hard way that mildew is conductive.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[oldfieldguy]

The shorted turn will quite often show up in the excitation test, especially compared to similar CT's. The one with shorted turns will have a lower saturation volfor a given current. Connect a variable voltage source to the secondary terminals and raise the voltage while monitoring the current from the source. Stop at one amp and record the voltage. Compare with the others.

It is not unusual for the "shorted turn" to be a conductive path through the window of the CT brought about by the shifting of conductive supporting members. I've found those inside OCB's and transformers.

old field guy

 

[stevenal]

Update:
Since February, we've been waiting for weather and schedules to come together. Finally got out this morning. The problem is that CTs that test fine in the shop do not test okay once they are installed on the breaker. This morning we found ratios were unacceptable on both CTs. As we prepared to begin disassembly again, I noticed the arrester bracket touching the shroud creating a ground loop. Although the factory (and cranky108 and scottf) told us way back to look for ground loops, I failed to notice this contact previously. The breaker had been retrofitted with factory supplied arresters and brackets following a damaging lightning incident. The other two phases had brackets that were still very close, but not touching. A small adjustment, and the ratio was correct on both CTs. I wonder why the manufacturer built these brackets to sit so close. In any case, I think we finally have this thing solved. Not sure at this point if the original problem had more to do with the loop or the moisture intrusion, or if the loop even existed at the time of misoperation. We've torn the thing apart and put it back together a few times since then. Thanks for all the help.

 

[scottf]

Stevenal-

Glad you found your problem.

I've seen such problems in the past and once I read your last post I thought "I should have told him to check the arrester bracket a lot earlier". Sorry I didn't pass that along sooner...it's a somewhat common cause of creating a loop with the shroud around bushing CTs.

 

[dpc]

I'm not picturing this at all. How does the arrester bracket interact with the bushing CT?

 

[scottf]

The shroud that covers the CTs in this design is aluminum. The shroud is designed to be isolated from the base that supports the CTs, such that it does not form a loop around the CT.

The arrester bracket touched the shroud and base, thus connecting them together and forming the loop.

In theory, the loop forms an additional 1 turn secondary winding and would carry (or attempt to) the primary current flowing in the bushing. This has a countering effect to the CT secondary winding and produces errors as mentioned above.

 

[oldfieldguy]

I have found this problem before in brand new S**mens GCB's. there was a bit of casting flash completing the circuit. We discovered the problem in normal commissioning tests and corrected it by knocking the offending bit of metal down with the blade of a screwdriver.

Another incidence of similar nature was a found in an OCB that tripped on an out of zone fault. In that case the CT retainer plate had shifted during normal operation and contacted the metal portion of the bushing, completing the offending loop. It saturated at less than half the expected voltage as compared with similar units on the other phases. Again, easy fix, but big problem before it was found.


old field guy

 

[stevenal]

dpc,

Here is a visual aid. There is now an air gap, but small.

 

[oldfieldguy]

Yep! That'll do it. I'd hate to think of what an inadvertent contact from vibration would do during normal operations...

old field guy

 

[dpc]

Scott and Steven,

Thanks for the explanation and the photo. I was thinking about an internal transformer CT under the oil. I should have re-read the thread.

Cheers,

Dave