CT strikes again 2

[230842]

I've opened a low voltage CT rated 300/5A to see internal construction details and have observed the secondary is composed by two parallel windings: One has 60 turns (which seems right) but the other has only 59 turns and is constructed with a thinner wire.
Can anybody explain this? Julian

 

[tgott]

Page 8 of ABB's Instrument Transformers Technical Information and Application Guide states, "The parallel auxiliary winding effectively keeps the leakage flux out of the core so that the leakage reactance in the equivalent circuit is effectively ahead of the exciting branches. This simplifies the calculation of the current by-passed through Z0 and Zi. (They can be combined...)"

http://www.abb.com/global/abbzh/abb...EC96AC155B585256A47005E8FE7/$File/IT Book.pdf

 

[230842]

Thanks togtt. I think your post do no answer my question: it refers to a ring type core CT as displayed in Figs 4B and 4D of ABB's document; the additional winding is directly made close and around the core and its turns are mixed with secondary ones; it is made with a significatively thinner wire (approx .3 to.5 mm against 1.6 to 2.0 mm diameter of secondary) and it has one turn less (59) than secondary (60) Julian

 

[scottf]

23084-

The parallel winding is probably for "turns correction". If you can tell me the accuracy rating, I can probably tell you for sure why they did it. It is a common practice.

 

[230842]

scottf

The accuracy is rated class .2 Julian

 

[scottf]

It's turns correction for sure then. By using a parallel winding, the load line can be shifted up or down within the accuracy rectangle for IEC rated cores (parallelogram for IEEE). The size wire is picked to get cose to a resistance value that is needed to assist in shifting the load-line.

The load line is defined as a line drawn from the low-burden to the fuill burden point at a given current inside a reactangle defined by the ratio error and phase-angle error of the accuracy class.

The idea behind turns correction, is that the core can be smaller than if core correction wasn't used. So it's used to either meet space constraints, or to reduce cost.

 

[230842]

Scottf:
Your answer looks quite reasonable. Do you know any book, paper or site to get technical information on the matter?
I thank you very much. Julian

 

[scottf]

Julian-

Off hand, I do not know where you can find any information about this practice in detail. If I didn't work for an instrument transformer manufacturer, I don't think I would know about it either.

Sorry...

 

[busbar]

scottf-- join up so we can give you a star! This group appreciates sage wisdom.

 

[scottf]

Thanks Busbar....I'm joined up now.

 

[230842]

Wellcome Scottf. I wish my star is the first.
Since no documentatios is apparently availabe, I'll try to investigate myself on the matter to find out the technical reasons that could justify that correction procedure. Does it perhaps provide a correction equivalent to a fraction of one turn?. Julian

 

[scottf]

Julian-

Yes, it does have the effect of altering the number of turns and hence the term "turns correction". The technique is not just limited to factions of turns. There are actually many ways to use turns correction and parallel windings, depending on the design/space requirements.

 

[dpc]

I have an old Westinghouse book on transformers that describes an "equalizer" winding that is used in CTs to keep leakage flux out of the core. But it is wound separately on the core and then connected in parallel to the main winding.

As I understand it (which is questionable), the goal is to force the leakage flux out of the core so that it does not appreciable increase the exciting current. Since the two windings are connected in parallel, the voltage on the two coils is the same, hence the flux in each coil must be nearly equal. If leakage flux attempts to flow in the core through the equalizer winding, the induced current in the equalizer winding will oppose this flux and tend to force it out of the core into the air.

Not sure if this is related at all to the parallel winding described above, but it was new to me, so I thought others might be interested.

 

[scottf]

dpc-

I don't quite follow what your saying, but to answer it is not the same as what I was mentioning before.

You mention "the goal is to force the leakage flux out of the core so that it does not appreciable increase the exciting current". Leakeage flux by definition is the flux that "leaks" from the core/coil and does not contribute to the inducing of current in a winding. In general, the reduction of leakage flux (stray reactance) is always a "good" thing. At least a good thing in terms of accuracy of CTs. I don't believe the assertion that an increase in leakage flux will decrease the excitation current is correct.

Looking over what I have written in previous posts, I think I may have used some terms interchangably (due to internal company sayings), that may be confusing.

Turns correction is simply altering the number of turns on a winding to make up for core defincieny (i.e. not ideal core sizing). This is not commonly used on CTs. More common on VT primary winding design.

Parallel correction is what I should have called this. This is where 1 winding is wound in parallel with another winding. One winding is called the correction winding and is used to shift the accuracy load line up (i.e. make the RCF [ratio correction factor] more positive without changing the phase angle error). The size wire used in the correction winding and the main winding are determined to control the impact of each winding on the total winding (i.e. total after they are in parallel). Typically, the correction winding (one with fewer turns) will be the smaller wire.

 

[230842]

Hi. After some investigation I've got two first approach formulas giving the turn number correction produced by the use of the auxiliary parallel winding technique:
- N, R: Number of turns and resistance of main winding
- Na; Ra: Number of turns and resistance of auxiliary parallel winding
- Nc; Rc: Number of turns and resistance of equivalent winding after correction
Nc = (Ra*N^2 + R*Na^2)/(Ra*N + R*Na)
Rc = R*(Ra*N^2 + R*Na^2)/((Ra^N + R*Na)^2)
These formulas are aproximate since they use resistance values instead of impedance ones. Julian

 

[busbar]

scottf--

As time permits, could you look over:

http://www.themeterguy.com/cgi-bin/yabb/YaBB.pl

 

[scottf]

busbar...I've been looking over that site recently...is pretty good, but not too much activity.