The use of current transformers

[Shahram]

Hello everyone,

Recently, i have designed a capacitor bank for a company which they already have their main switch board with metering stuff. The ammeter of the main switch board has its own CTs but for the capacitor bank I need to install a CT separately. The spec of existing CTs for main switch board is type AL4- 15VA- cl 0.5 FS 5 and 2000/5 A. There is a step down transformer with apparent power of 1250 KVA and capacitor bank sized around 460 Kvar. My question is could i use one of the existing CTs (say phase R) for the capacitor bank's multi function and the ammeter of the main switch board simultaneously? Thanks very much in advance for any help.

 

[NascentOxygen]

Hi Shahram.

This is a power-factor correction capacitor bank?

 

[Shahram]

Hi Shahram.

This is a power-factor correction capacitor bank?

Hi
yes, this is a power factor correction capacitor bank.

 

[Asymptotic]

The ammeter of the main switch board has its own CTs but for the capacitor bank I need to install a CT separately.

Is this a contactor switched capacitor bank with a power factor controller using current transformer feedback?

 

[Shahram]

Is this a contactor switched capacitor bank with a power factor controller using current transformer feedback?

yes, this is a capacitor bank which is being switched by contactors with a controller (regulator).

 

[Asymptotic]

yes, this is a capacitor bank which is being switched by contactors with a controller (regulator).

Carefully and thoroughly study the power factor controller manual. The controllers I'm familiar with have CTs measuring current through each phase, potential transformers measuring line-to-line voltage across each phase, and use this information to determine how many cap banks must be turned on to obtain the power factor set point.

Transformer and CT phasing and 'polarity' must be correct. If they aren't, chances are none of the cap banks will turn on, or all of them will turn on and remain on. If the controller was designed to use 5 amp CTs then it may be possible to use the existing CTs depending upon the burden.

 

[Shahram]

Carefully and thoroughly study the power factor controller manual. The controllers I'm familiar with have CTs measuring current through each phase, potential transformers measuring line-to-line voltage across each phase, and use this information to determine how many cap banks must be turned on to obtain the power factor set point.

Transformer and CT phasing and 'polarity' must be correct. If they aren't, chances are none of the cap banks will turn on, or all of them will turn on and remain on. If the controller was designed to use 5 amp CTs then it may be possible to use the existing CTs depending upon the burden.

Thanks very much for your reply i will find and read that manual for sure. We assumed that the load is dispatched symmetrically between 3 phases that's why we are going to use only one CT for the capacitor bank's controller specifically (not for the main switch board right after Transformer which certainly needs 3 CTs and so on). You mentioned burden, do you mean the burden of secondary winding? how can i measure that? and do you think saturation of the CT will not happen if we use one CT for 2 purposes (capacitor bank controller and switch board ammeter? Thanks again

 

[Shahram]

Thanks very much for your reply i will find and read that manual for sure. We assumed that the load is dispatched symmetrically between 3 phases that's why we are going to use only one CT for the capacitor bank's controller specifically (not for the main switch board right after Transformer which certainly needs 3 CTs and so on). You mentioned burden, do you mean the burden of secondary winding? how can i measure that? and do you think saturation of the CT will not happen if we use one CT for 2 purposes (capacitor bank controller and switch board ammeter? Thanks again

Bt

Carefully and thoroughly study the power factor controller manual. The controllers I'm familiar with have CTs measuring current through each phase, potential transformers measuring line-to-line voltage across each phase, and use this information to determine how many cap banks must be turned on to obtain the power factor set point.

Transformer and CT phasing and 'polarity' must be correct. If they aren't, chances are none of the cap banks will turn on, or all of them will turn on and remain on. If the controller was designed to use 5 amp CTs then it may be possible to use the existing CTs depending upon the burden.

Btw, i just remembered that the manufacturer of the capacitor bank used a controller that uses only one CT.

 

[Asymptotic]

Thanks very much for your reply i will find and read that manual for sure. We assumed that the load is dispatched symmetrically between 3 phases that's why we are going to use only one CT for the capacitor bank's controller specifically (not for the main switch board right after Transformer which certainly needs 3 CTs and so on). You mentioned burden, do you mean the burden of secondary winding? how can i measure that? and do you think saturation of the CT will not happen if we use one CT for 2 purposes (capacitor bank controller and switch board ammeter? Thanks again

Do a search on "current transformer burden". Adding the controller may add too much burden to the existing CT secondary. If so, install another CT to be used exclusively by the controller.

 

[Shahram]

Do a search on "current transformer burden". Adding the controller may add too much burden to the existing CT secondary. If so, install another CT to be used exclusively by the controller.

Thanks a lot, I will check CT burden.

 

[russ_watters]

Ali, you should check with the manufacturer of the capacitor bank/controller. They should know if what you are doing will work on their equipment.

 

[jim hardy]

Be aware that some places use one set of CT's for indication and control
and another set for protective relaying.
They're kept separate because of the importance of protective relays to equipment and personnel safety.

That you plan to use a CT that already drives an indicator 'feels' reassuring. Be doggone sure you don't disturb your protective relay schemes.

old jim

 

[Shahram]

Ali, you should check with the manufacturer of the capacitor bank/controller. They should know if what you are doing will work on their equipment.

Thanks for your reply, i have already talked to the manufacturer of the capacitor bank and they didn't give any useful information to solve this issue. I should ask for its manual from them.

 

[Shahram]

Be aware that some places use one set of CT's for indication and control
and another set for protective relaying.
They're kept separate because of the importance of protective relays to equipment and personnel safety.

That you plan to use a CT that already drives an indicator 'feels' reassuring. Be doggone sure you don't disturb your protective relay schemes.

old jim

Thanks very much jim, if I'm not mistaken cl 0.5 FS 5 indicates that the CT's I'm trying to use are not for protective purposes. Am I correct?

 

[jim hardy]

if I'm not mistaken cl 0.5 FS 5 indicates that the CT's I'm trying to use are not for protective purposes. Am I correct?

That's a 68 page document that I'm not going to study at length. You should have posted a link to it - here's the one i found...
https://w3.siemens.com/powerdistribution/global/sitecollectiondocuments/en/mv/indoor-devices/protective-measuring-transformers-m4/catalogue-protective-and-measuring-transformers-m4_en.pdf
Pages 9 and 10 indeed suggest that your transformer is designed for non-protective application.
It's encumbent on you though to make sure a predecessor hasn't mistakenly mixed control with protection so that you won't compound an existing error.
If you find that situation you must apprise your client.

So you've got to learn at least the minimal details of whatever machine it is you're about to modify.
That's just everyday being a 'thorough and responsible engineer'.old jim

 

[Shahram]

That's a 68 page document that I'm not going to study at length. You should have posted a link to it - here's the one i found...
https://w3.siemens.com/powerdistribution/global/sitecollectiondocuments/en/mv/indoor-devices/protective-measuring-transformers-m4/catalogue-protective-and-measuring-transformers-m4_en.pdf
Pages 9 and 10 indeed suggest that your transformer is designed for non-protective application.
It's encumbent on you though to make sure a predecessor hasn't mistakenly mixed control with protection so that you won't compound an existing error.
If you find that situation you must apprise your client.

So you've got to learn at least the minimal details of whatever machine it is you're about to modify.
That's just everyday being a 'thorough and responsible engineer'.old jim

Thanks very much indeed, you gave me a great information. Yes, you are right i should make sure if they didn't mix control with protection mistakenly. Thanks again Jim