Choosing a proper CT

[Zientek92]

Hello everyone,

I am new on this forum, new in UK as well.
There is an opprtunity for me to work as commissioning engineer but first I must go through some courses. There is one topic in my protection of transmisson and distribution course which I don't understand in 100%.
Thare is a question from the course:

It is required to specify a CT to be used in conjunction with a time graded overcurrent relay. The CT secondary and the relay are both rated at 1 A. The relay is of electronic type and the relay maximum burden is 0.25 VA at rated current. The total loop resistance between the CT and the relay including the lead wires, CT internal resistance, and relay resistance is 0.5 Ω. The maximum fault current, referred to
the CT secondary is 25 times the relay rating. Determine the CT complete specification assuming the CT burden is 5 VA; i.e. the CT will deliver 5 V at 1 A.

Is here anyone who could answer for this and explain me why the answer is as it is?

Thank you

 

[Lucien Nunes]

Hi and welcome to the forum. We don't like answering course questions outright, as this undermines the purpose of the course. We'd also get every student in the world asking us to answer stuff for them personally, when there are perfectly good study materials out there already. We might however be able to assist by checking your working if you go wrong.

I have to admit to being rather rusty at this too but I'll start you off. It's a protection CT rather than an instrumentation one, so what does that mean in terms of ALF and SF? What accuracy power is needed for that relay? Can you determine the ratio?

Although it asks for the complete specification - some information needed is missing from the question, so you can't give a totally comprehensive answer.

Here's a practical guide to speccing a CT that I've referred to before: http://www.studiecd.dk/cahiers_techniques/Current_transformer_specifications.pdf

 

[timbobelfast]

Looks like a homework question! Need to be careful when it comes to CTs. Tell us your thoughts first before we correct you?!

 

[Zientek92]

Thank you for your answers.

Of course it will be protection CT. Accuracy class 5P ALF 25 if it exists. 5VA burden. Secondary winding nominal current 1A but I don't know how to find out primery nominal current.

I am totally new in protection and the materials I have been given are not fully understandable for me or have no enough information to answer this question. That's why I am looking for help in here.

 

[static zap]

I spotted more reading the question , backwards before I turned off inside !
(How my father spoils a good book - reading the ending 1st)

 

[Pete999]

Thank you for your answers.

Of course it will be protection CT. Accuracy class 5P ALF 25 if it exists. 5VA burden. Secondary winding nominal current 1A but I don't know how to find out primery nominal current.

I am totally new in protection and the materials I have been given are not fully understandable for me or have no enough information to answer this question. That's why I am looking for help in here.

If my memory serves me well, then you will need to specify what ratio you require depending on the meter
1:5 1:10 etc whereby a 1 Amp deflection will show as 5 Amps and 1 Amp deflection will show as 10 Amps and so on, 1:100 etc.