The use of current transformers

In summary, the conversation discusses using an existing CT for a new capacitor bank and the potential issues with using it for both the bank's controller and the main switch board ammeter. The importance of checking the controller manual and measuring the burden of the CT is emphasized, as well as the potential risks of using the same CT for both purposes. It is suggested to consult the manufacturer for confirmation and to consider using separate CTs for indication and protective relaying.
  • #1
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.
 
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  • #2
Hi Shahram. :welcome:

This is a power-factor correction capacitor bank?
 
  • #3
NascentOxygen said:
Hi Shahram. :welcome:

This is a power-factor correction capacitor bank?
Hi
yes, this is a power factor correction capacitor bank.
 
  • #4
Shahram said:
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?
 
  • #5
Asymptotic said:
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).
 
  • #6
Shahram said:
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.
 
  • #7
Asymptotic said:
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
 
  • #8
Shahram said:
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
Asymptotic said:
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.
 
  • #9
Shahram said:
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.
 
  • #10
Asymptotic said:
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.
 
  • #11
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.
 
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  • #12
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
 
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  • #13
russ_watters said:
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.
 
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  • #14
jim hardy said:
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?
 
  • #15
Shahram said:
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
 
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  • #16
jim hardy said:
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
 

1. What is the purpose of a current transformer?

A current transformer is used to measure and monitor the flow of electric current in a circuit. It is also used to step down high currents to a level that can be easily measured by instruments such as ammeters and energy meters.

2. How does a current transformer work?

A current transformer works by using a primary winding, which is connected in series with the circuit carrying the current to be measured. This current induces a magnetic field in the core of the transformer, which in turn, induces a current in the secondary winding. The secondary current is proportional to the primary current and can be measured by instruments.

3. What are the advantages of using current transformers?

Current transformers offer a number of advantages, including accurate and reliable current measurement, electrical isolation between the primary and secondary circuits, and the ability to step down high currents to a level that can be safely measured.

4. What are the different types of current transformers?

There are two main types of current transformers: measuring current transformers and protective current transformers. Measuring current transformers are used for accurate measurement and monitoring of currents, while protective current transformers are used for detecting and protecting against overcurrents and faults in the circuit.

5. How do you select the right current transformer for a specific application?

The selection of a current transformer depends on factors such as the type and magnitude of the current to be measured, the accuracy required, and the frequency of the current. It is important to carefully consider these factors and consult manufacturer specifications before selecting a current transformer for a specific application.

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