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CT Heating Problem

03/25/2018 10:34 PM

We have an 6.3 KV switchboard installed at our plant site. One of the MV panel, which generator incoming panel with breaker rating of 2500amperes has heating problem in it. There are 4 single core SWA cable terminating at each phase. the armor of cable is grounded at switchboard end only. the CT for middle phase has always burning marks on its bottom surface. The thermography of switch-gear rear side revealed that plate on which CT are mounted is at 80 degree C. While on top of all CT the temp is about 56 degree C on surface with temp gradient decreasing away from middle mounting plate.
We appreciate if you can help out in determining root cause of heating.

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#1

Re: CT Heating Problem

03/25/2018 10:59 PM

The first thing I would check is the connections for integrity....there might be a loose connection or degradation of the connection surfaces...How old is the installation? What protection systems are in place? What testing has been done?

You could have excess current in the neutral wire, usually caused by high zero sequence harmonic components.

I also might check the power factor....

https://www.electrotechnik.net/2010/04/what-are-reasons-for-transformer.html

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#2
In reply to #1

Re: CT Heating Problem

03/25/2018 11:18 PM

Dear SolarEagle

Thank you for your prompt support.

We have checked CT connections they are tight and healthy. The CTs are connected on line side of generator and are used for differential protection of 18MW turbo generator. We have replaced middle phase CT back in 2017 but last month there was again tripping on differential on starting 01 motor. The fault record from G60 protection relay revealed anomaly in signature of fault current OF "S" phase line side CT. We removed CT and saw heating same marks again on bottom surface of CT. The removed CT was tested and its excitation curve was not normal (attached)

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#3

Re: CT Heating Problem

03/26/2018 3:00 AM

Sure. Where does the quotation go to, please?

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#4
In reply to #3

Re: CT Heating Problem

03/26/2018 5:12 AM

The CT in which are facing heating problem is “middle” phase Line Side CT used for differential protection of 18MW turbo Generator TG-702. In normal condition there are two turbo generators TG-701/702 operating in parallel with about 60%loading on each generator. When only 01 turbo generator operates (during maintenance of other one) we face problem of incorrect tripping of generator on differential protection 87G-2. During analysis of fault record on G60 relay we have always observed that the signature of middle phase on line side CT is different from other phases and different from neutral side CTs. In March 2017 we replace all line side CT with new GE make equivalent CT which are installed on neutral side of generator. During replacement in march 2017 there was observation that middle phase CT had burning marks on its bottom surface and was cracked due to heat, this caused change in core properties of CT hence different saturation curves and secondary current signature/envelop. Last month again same observation repeated and again middle phase CT was found heated and caused tripping (fault report attached). The faulty CT report is also attached along with healthy CTs

  1. The currents observed on G60 relay during normal operation are as follow:

Line Side CT:

R Phase: 1,249Amp

S Phase: 1246 Amp

T Phase: 1256 Amp

Neutral Side CT:

R Phase: 1,274Amp

S Phase: 1271Amp

T Phase: 1275Amp

CT Ratio : 3000/5

  1. The neutral side CT are installed in 1980, Line Side “R” & “T” PHASE CT installed in March, 2017 and “S” phase replaced last month.
  2. The CT are in operation therefore secondary current directly cannot be measures however the current read on relay are mentioned against Sr. 1
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#5

Re: CT Heating Problem

03/26/2018 8:05 AM

Looking at the IR photograph, I am suspicious of where the heat is coming from. Unfortunately, the photo is zoomed in too much, but it looks like the higher heat is below the CTs, not the CTs themselves. Is that true? If so, the CTs are probably not the problem.

What are they mounted on? I can see the bolts that hold them to some sort of brace, which appears to be what is hot. Is it metal? If it is metallic, it is possible that there is some sort of eddy current being induced in it which is heating it up. I have seen similar heating effects in 13.8kV switchgear. I would recommend investigating that. If that is so, I would recommend that the metal support brace be replaced with one made of fiberglass or other insulating composite (Glastic® or similar).

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#6
In reply to #5

Re: CT Heating Problem

03/26/2018 8:41 AM

Yes we also suspect that CT is not directly a problem . . it is heat that changes core properties of CT . . The heat is suspected to be generated from metallic mounting support/brace. The single core cable generates fulx that induce eddy current in support.

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#7
In reply to #6

Re: CT Heating Problem

03/26/2018 11:33 AM

Maybe it's just the mounting position configuration that's the problem...Might try mounting in a row instead of trefoil...the brackets look like aluminum which normally should be ok....

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#8
In reply to #7

Re: CT Heating Problem

03/26/2018 1:26 PM

There probably isn't space available to mount the CTs in a row. But the metal doesn't heat by itself - there is something causing it. That needs to be determined. If it isn't heat that is conducted from something else, then it is "generated" - most likely by induction in some way. The simplest way to eliminate that is replace the metal with something non-metallic. We have numerous times used Glastic rod/sheet or fiberglass channel to support CTs and other components in switchgear, and have had excellent results. That's the direction I would go if I were tasked with this situation.

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#9
In reply to #8

Re: CT Heating Problem

03/26/2018 2:26 PM

Well if the heat is from another source I would be concerned about the properties of any fiberglass supports used, particularly any flammability issues and/or toxic smoke or fumes that may be generated from high heat situation....so anything used for replacement support should be carefully scrutinized and preferably designed for purpose....

http://beetleplastics.com/frp-fire-resistance-fiberglass-reinforced-plastic-and-fires/

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#10
In reply to #9

Re: CT Heating Problem

03/26/2018 3:25 PM

Agreed. That is the first question that needs to be answered. But unless there is something even hotter under the metal brace, it is the source, and needs to be removed. I'm curious to see if the OP gives us any additional or follow-up info...

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#11
In reply to #7

Re: CT Heating Problem

03/26/2018 11:07 PM

The brackets are magnetic therefore not aluminum . . may be they are galvanized iron etc . . there is space limitation so mounting in row is difficult . .

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#12

Re: CT Heating Problem

03/27/2018 4:10 AM

Great information..

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#13
In reply to #11

Re: CT Heating Problem

03/27/2018 5:05 AM

The question are

1) if ct mounting bracket heats up due to induction then why right n left ct remains ok and have no heating marks on them...why only middle ct gets heated and have burning marks at its bottom surface

2) the armor is grounded at switchboard end then can there be any flux to couple with mounting bracket

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#14
In reply to #5

Re: CT Heating Problem

03/27/2018 3:44 PM

there needs to be an air gap or insulation for any conductive material, aluminum, copper or steel, so that there is no magnetic circuit between the 3 individual phase conductors

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#15

Re: CT Heating Problem

03/27/2018 4:46 PM

Apparently the heat is due to eddy current circulation. Check gland plate for non magnetic material and also all brackets .

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#16
In reply to #11

Re: CT Heating Problem

03/27/2018 7:17 PM

There is your answer! One should NEVER use magnetic materials near high current cables. The eddy currents will heat up the magnetic material like you will never believe!!

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#17

Re: CT Heating Problem

03/29/2018 12:53 PM

We can have two possible solutions

1. Replace the mounting brackets with insulated material (we are going for textolite) to eliminate any magnetic coupling / eddy currents. This would solve problem in case heat is due to eddy currents.

2. But in case problem is else where, a concern of high temperature on insulator causing smoke/fumes is there. So we can replace brackets with non magnetic material as a second choice (after confirming existing material to be non magnetic). But this would not completely eliminate problem as even non magnetic material (with less permeability) will have eddy currents induced in it due to changing magnetic fields though magnitude maybe less. So heat will still be there.

Any comments as to which option should v go for?

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#18
In reply to #17

Re: CT Heating Problem

03/30/2018 9:37 AM

The metal in the mounting cannot form a circuit between the individual phases, so just an insulating gap is needed, maybe with a washer or a nylon bolt. See the picture I posted on #14 for the concept, 3 cables passing through a metal plate, which I see I failed to explain about the plate.

This assumes that you have actually found the magnetic circuit causing the heat, which is likely to have existed from the time of the installation, which is how long, and any idea when this started in relation to last modification?

It suspiciously appears to be magnetic circuit heating, but you may have a fault in the current transformer that might not affect the ratio much, or a small crack in the CT insulation tracking high voltage current to ground, maybe look at it in the dark...

It doesn't take much current to produce a large amount of heat, so parasitic circuits with some factor concentrating the path could be a source.

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#19
In reply to #18

Re: CT Heating Problem

03/31/2018 6:09 AM

These mounting brackets are forming a magnetic circuit (CT effect) between three phase conductors (through panel body). See below pic for clarity. We can insulate the joints of mounting brackets with panel body to reduce / eliminate these currents but still induction due to eddy currents would be there as such which will create heat in brackets (We already did this modification on one of other panels with similar CT arrangement, but heat was still there.

This arrangement is as such from installation time (around 35 years ago) along with heat issue. We only got concerned regarding this heat when it starting damaging CTs. We had replaced all CTs in 2016 yet one again got damaged in 2018. So CT is not the problem.

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#20
In reply to #19

Re: CT Heating Problem

04/01/2018 9:27 PM

can you insulate the cross channel at the front part of the picture from the rest of the structural support? This should open the circuit around the 3 conductors.

Shoulder insulators & fiber washers, just in this one place should do it.

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#21
In reply to #20

Re: CT Heating Problem

04/02/2018 2:32 AM

We are going for following modifications for the time being. Will post the results in next week.

1. Insulation of cross channels from panel body to stop circulating currents.

2. Replacement of existing mounting brackets with non magnetic material (after verifying existing material).

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#22
In reply to #21

Re: CT Heating Problem

04/02/2018 10:24 PM

non-magnetic is no help, non-metallic is what you want (low magnetic permeability?), but really just need to open the circuit, like the design with the air slots cut between the 3 phase holes through the aluminum sheet, a design as young engineers we almost put in the field at St. Regis Paper in Bucksport, ME in 1975, someone caught it and had us fix it... There were 3 single conductor 750MCM armored cables coming inside from outdoor switchyard, not 3 conductor groups as typically standard.

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#23
In reply to #19

Re: CT Heating Problem

04/04/2018 7:35 AM

Pardon my opinion late in the comments.

The operating principles of avoiding heating where AC cables pass through gland plates are...

  1. Put all cables of the circuit through one hole.
  2. Put all cables through effectively one hole from the magnetic circuit view-point, as well illustrated in post #14.
  3. Make the gland plate and glands of non-magnetic material e.g. aluminium or brass.

Looking at the picture of your CTs, it is evident that if the middle [horizontal] support, to which all 3 CTs are attached, were taken away; then option 1 above would apply.

A little thought shows that effectively you have a single phase 2500 amp rms current loop around that middle support. You do not give dimensions, but I guess the magnetic length round top [or bottom] support through centre support is 1.5 metres. The ampere-turns per metre is 2500/1500 = 1666 At/m.

That is at the right hand side of the chart above, and is well into saturation for mild steel and all other materials - particularly since 1666 At/m was rms- peak will be 2350. Transformers & other electrical machines avoid peak flux densities much above 1.2 - so high iron losses will occur.

So it seems to me that if just the centre horizontal support were aluminium, you would avoid the iron losses and overheating.

I would add that I am assuming that this is a "3 wire" AC circuit, not 4 wire, which might have a "single phase" load at times [not necessarily when you measured!].

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#24
In reply to #23

Re: CT Heating Problem

04/06/2018 12:21 AM

For the time being, both modifications as suggested by me in my earlier post have been done. We haven't been able to find aluminum in our stocks so SS-316L has been used for fabrication of all mounting brackets (03 in total). Existing material of brackets (replaced now by SS-316L) was magnetic as confirmed once we removed them. So fingers crossed (Will post results next week).

PS: We can't put all cables through one hole / mount CTs in a row as suggested in post#14 due to space limitation in the panel.

Really appreciate all feedback / comments on the issue.

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#25
In reply to #24

Re: CT Heating Problem

04/06/2018 2:45 PM

P.S.S. All you needed to do was eliminate the metallic components completing the conductive circuit between the 3 single phase conductors, such as the slots do on the aluminum plate. Insulating one end of the support cross-bars appeared to fulfill that circuit interruption function...

Pretty sure all heating is not eliminated by substituting non-magnetic for magnetic material, non-metallic material is needed in the gap where circuit coupling occurs.

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#26
In reply to #25

Re: CT Heating Problem

04/06/2018 2:57 PM

Yes, most heating will be eliminated by removing any and all magnetic materials from the vicinity of the cables. NO, the current does not circulate in the support structure around the conductor as you have shown.

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#27
In reply to #24

Re: CT Heating Problem

04/09/2018 6:00 PM

HR,

I think you have done the best thing by replacing the supports with stainless steel [not all stainless steels are non-magnetic], although I think just the middle bar would be enough.

I do not think fitting non-magnetic spacers to open up an "air gap" in the magnetic circuit would automatically be effective. An air gap of 1mm takes 800At per Wb/m2 , so 2 mm would be 1600 At - with my guess of your dimension & current giving 1600 At/m, that would halve the flux density in the iron. One could saw off the support 25mm from its end support and replace the end section with aluminium spacer & straps for equivalent mechanical strength, to get a really significant gap & reduction in loss [proportional to B2].
I am doubtfull that just putting a thin Al spacer under each end of the support would be effective - one has to consider that e.g. 50 mm overlap would be 20 times the area of cross section of 2.5 mm thick steel - through this area the flux could pass with much less reluctance, reducing the effective gap. Even a thin spacer would tilt the CT (you would have to put spacers under all the mountings to "even it up").

It has been posted that using aluminium still gets eddy currents (which are bigger because its resistivity is about 0.4 x mild steel, resistivity of stainless is about 7 x mild steel).

Any eddy current loop with emf v volts and resistance r ohms will have loss v2/r watts.

The most important effect is that v is proportional to B, the flux density in the iron.

Since B = μμ0H, taking values from the chart in post #23, mild steel for 1600 At has B = 1.4 Wb/m2

Hence μ = B/μ0H, since μ0 = 4*pi*10-7 , μ = 1.4*107/(1600*4* 3.142)= 696

Thus replacing iron with non-magnetic material [μ = 1] reduces v2 to v2/(696*696).

So losses are reduced by nearly 500000!

Core losses are about 22 watts/kg at 1.6 Wb/m2 peak 50 Hz for t = 0.5 mm thick laminations and proportional to t2, so will increase to 22*52 = 550W/kg for 2.5 mm thick. Noting that 500mm by 100mm by 2.5 mm steel weighs about 1 kg, the loss in the middle support would be approximately 550*2.5/500000 = 0.0025 watt or so for aluminium and much less for stainless steel.

Finally, I thought how one might measure the flux in the middle support, "before" and "after" modification. I suggest that a turns around that support with lead-out to instrument through a "twisted pair" would have an emf proportional to the flux enclosed by the turns. An oscilloscope is suggested, to observe waveform, as well as a meter. The emf I calculate as about 75 millivolt/turn. You will of course, compare the IR temperature plots, before & after.

67model

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