German CT Symbol 1

[Kyle2022]

Good afternoon,

I am attempting to figure out what is meant by the CT symbol in the attached picture. I work in the US and I have never seen a CT shown like this, especially with two polarity marks.

Thank you!
Kyle

 

[scottf]

That shows 2 CTs connected in series (means 2 CTs per phase).

Not anything "German" about it per se. That's how it would normally be shown in the US too.

Now....why it's done like that, I have no idea.

 

[Kyle2022]

Yeah, I guess that's the real question. I figured it showed the 2 CTs in series. Why is it done at all?

Thanks for the response scottf.

 

[scottf]

Based on the 1-line, would be to effectively double the burden rating over that of the individual CT, e.g. the set would be rated C800 instead of the C400 individually.

Not sure what voltage class these CTs are, but with a ratio of 8000:5A, C800 should be pretty easy to achieve in one unit. Hard to say without seeing the rest of it, but it looks odd to me.

 

[Skogsgurra]

I would say that it isn't even typical German. The polarity dots are usually not used in German diagrams. And if they are used, they are not placed on the lines (like connections dots), but one or two mm off.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[ScottyUK]

Agree with Skogs - it's unusual to see the polarity dot on the line instead of along side it. What is the company who produced the drawing? The numbering system has the look of ABB.


----------------------------------

Sometimes I only open my mouth to swap feet...

 

[controlsdude]

I believe the polarity dot represents the start of the winding and the other end is the finish end of the winding. I have seen this used in radar equipment alot but that was over 20 years ago.

I do remember it in school but can not recall the exact meaning.

 

[Kyle2022]

The drawing is from Siemens Germany, which is why I figured this was some sort of German and/or IEC symbol. Apparently this is not the case. That aside, do you agree with scottf as to why it is used OR do you believe this is a mistake altogether?

Thank you. I appreciate your responses.

 

[controlsdude]

Scottf seems to be saying that since both CT's are in series and that the alignment is start to finish (CT1) and then the start to finish (CT2) (if this is a true statement for the dot alignment). Then they would add both coil in series to give you the next series of CT winding. But why not just buy the next size CT and only use one? Then their is the question of insulation rating for the CT brought up by Scottf

I thought the reason why the dots mattered in radar was that Xl for the winding was used for tuned circuits. If you did not follow the dot, you would be 180 degrees out of phase. But in this case your just trying to get a fractional current representing a large current on that phase, but not checking phase. Is this correct?

 

[DanDel]

To me, it looks like it might be a multi-tap CT.

 

[dpc]

Agree with Scottf - the two CTs are placed in series to increase their rating. The two C400s become effectively a C800 CT. I have done this where there were unused bushing CTs and we wanted to improve short circuit fidelity of the CTs.

It is covered in J.L. Blackburn's Protective Relaying book.

 

[davidbeach]

Looks to me like two CTs in series, to gain the higher voltage rating as suggested by scottf. I'm puzzled though by why the polarity marks would be causing any confusion, they look perfectly normal.

 

[Kyle2022]

Thanks davidbeach. The polarity marks do look perfectly normal. I was just confused why there were two sets. I thought maybe someone was trying to depict a different type of CT (bushing or otherwise). Now that I understand these are placed in series to increase their rating, all is OK.

Thank you.

 

[oldfieldguy]

Could also be a dual winding arrangement wherein you connect in parallel for one ration, series for another.

old field guy

 

[BJC]

"The two C400s become effectively a C800 CT."
I don't see how that would work?
You might get a greater burden but if you connect two 800:5 cts in series and run 800 amps through the primary you won't get 10 amps out of it.
I think it's just a connection to mitigate the effects of a spare transformer. If you have a spare on the bushing of a breaker you can't leave it open and if you short is it just gets hot.

 

[davidbeach]

With the two CTs in series, each sees only half the voltage developed across the burden. Half the voltage for a given burden is equivalent twice the voltage class on the CT. Its an old trick from the electromechanical relay days. Probably applied where a C800 wouldn't physically fit but the X/R and available fault current were both high enough that a single C400 would saturate undesirably.

 

[scottf]

BJC-

A few points.

As davidbeach correctly points out, protection accuracy is basically a statement of how much voltage the CT can develop on the secondary without saturating. A CT rated C400 can develop up to 400V on the secondary without saturating (not exactly the whole story, but good enough for this discussion).

Look back at the diagram, if each CT develops 400V across its secondary and the CTs are connected in series, then 800V is developed across the pair. Hence the C800 rating as a pair.

"You might get a greater burden but if you connect two 800:5 cts in series and run 800 amps through the primary you won't get 10 amps out of it."

Remember a CT is effectively a constant current source. The burden rating is not related to the current being put out of the secondary at even given time, i.e. a unit rated with twice the burden rating, doesn't put out twice the secondary current....it puts out twice the secondary voltage, or rather can be pushed up to twice the secondary voltage.

"I think it's just a connection to mitigate the effects of a spare transformer. If you have a spare on the bushing of a breaker you can't leave it open and if you short is it just gets hot."

I very much doubt this is the case and if it is, it is a very poor practice. The heat developed in the second CT is essentially the same whether it's shorted or has a burden connected to it. The heat generated in a CT is dominated by the I^2R losses in the secondary. Technically, a CT with a burden across it will generate slightly more heat, as the cores losses will be higher, i.e. operating at a higher flux density.

 

[RRaghunath]

Well summed up Scottf. I fully agree with you.

A star for you.