CT equivalent model

[dvdinesh]

Does anybody have an equivalent model for a current transformer, please send it to me at dvdinesh@rediffmail.com.
Thanks in advance,
Regards
Dinesh.

 

[electricpete]

Primary ===>=I1=======>=I2'=>=
| |
| |
jXm Zb
| |
|Im' |
| |
===================

Where
I1 = primary current
I2' = secondary current referred to primary
Im' = magnetizing current - flowing down the page
I2' = I1'-Im'
Note that Im' represents an error current in the following sense: As Im' approaches zero, I2' approaches I1.
Xm = magnetizing reactance
Zb = burden (load) impedance

 

[dvdinesh]

Thanks Electric pete for your reply. Please let me know, How to find the jXm (core loss factor)?
Thanks once again.
Regards,
Dinesh.

 

[scottf]

The core loss and excitation current are functions of the core material used and the core size. You would most certainly have to get that data from the manufacturer or measure it from excitation data and some other tests.

One question...why do you need to know this? Are you trying to model CT performance, or are you looking for something to use from a system modeling perspective?

 

[dvdinesh]

Scottf, Thanks for showing interest. I am working on a project on RCCB(Residual current circuit breaker). The RCCB uses a CT to detect the fault current(earth leakage current). there is an existing product which is using a strip wound toroidal core made of Nickel-iron alloy(Ni-86%)[the commertial name is Mu Metal). That CT is not working fine in giving enough voltage for primary current of 150mA. So I am trying to develop new CT which can meet my requirement. Meanwhile I thought of finding out the effect of increasing the no of secondary turns(windings) on the behaviour of CT, so I asked for equivalent model, so that I can predict some of the things. I have an equivalent model of CT, which suggests that number of secondary windings can be increased only up to an optimal value, beyond which, the power delivered by the CT to the load may come down. The size of the CT is: Outer Dia = 24.6mm, Inner Dia = 13.13mm and Height = 20.16mm. The dimensions of the New CT has to be in that order only.The number of secondary windings used in the existing CT is 600 and primary windings two.
Any info on, How the dimensions of the CT affect its behaviour? and other details regarding CT design will help me a lot. Any websites giving information on this?
Thanks in advance..

 

[scottf]

The ratio does not seem to be adequate (i.e. too high) for this application. You mentioned 600 T on the secondary and 2T on the primary...which is 300:1A (or 1500:5A). Why would you not use a 1:1A or 5:5 A ratio and increase the number of primary turns substantially. This would increase the voltage output at 150 mA primary current. Depending on the size wire used, it may not increase the overall size dramatically.

As for dimensions, the larger the cross-sectional area of the core, the lower the nominal operating flux.

If it were me, I would focus on the ratio.

 

[peebee]

I agree scottf, and that is exactly the standard solution for this application. You can't use a CT sized for phase currents on a residual ground current relay. I'd suggest talking to a switchboard manufacturer for additional tips on sizing.

 

[dvdinesh]

The wire used for the primary current is 3.5mm2 teflon isolated flexible wire. It is supposed to carry a rated current of 40A. Because of the diameter of the wire and the limited innner diameter of the core, it is difficult to use more than two turns. Also, heating effect will increase with the increase in the length of the wire.
I am also looking at other core material like "Nanocrystaline" which is more permeable that "Mu metal".
I will be thankful for any sugession or information on CTs suitable for this application.

 

[scottf]

Why does the CT have to carry a rated current of 40A in the primary and you're trying to trip a relay at 150 mA? Also, why is the heat generated by the primary winding a concern for placement in the neutral. If you're stuck on remaining with the ratio you mentioned previously, you're not going to find a core material that will make much of a difference. Your A/turn at 150 mA will simply not be enough. if you want to adequately relay at primary currents of 150 mA, you're going to have to use a lower ratio CT. I don't think that size core is going to work very well.

Also, are you saying that 3.5mm2 wire is good to carry 40A continuously?

Good luck.