Author Topic: Current Transformer Rated primary current vs Max. Primary current  (Read 203 times)

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Offline Sultanpepper123

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Current Transformer Rated primary current vs Max. Primary current
« on: September 19, 2019, 02:03:54 pm »
I've been reading up alot on how to work with a CT. Basicly you have a primary current (eg: 100A) and a turns ratio (eg:1:2500) and a load resistance (eg:5 ohms) so now things are straight forward we can calculate the secondary current (40mA) and let's put a burden resistor 3 ohms to get 120mV while not exceeding the maximum power the CT can deliver. Now I'm currently working on an energy meter design, and while looking for the suppliers i'm getting these specs.



As you can tell the supplier is giving out " Rated primary current " and " Max primary current". I've opened a 80A rated energy meter and found out the CT's have a writing " 5 (80)A / 2.5mA/0.1 " , is it 5A or 80A ?

My questions :

1. On an absolute maximum load 100A will this CT not saturate ?

2. How can i work with this thing ? is it supposed to be saturated somehow and still work on both spectrums ?

UPDATE:

Finally got a reply from a supplier. The response is in the attachments below.

« Last Edit: October 08, 2019, 08:01:02 pm by Sultanpepper123 »
 

Offline edpalmer42

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Re: Current Transformer Rated primary current vs Max. Primary current
« Reply #1 on: September 19, 2019, 03:31:36 pm »
You should go to the manufacturer for clarification.  But I would interpret these specs as meaning that your circuit should remain below <Rated Primary Current> during normal operation.  However, under fault conditions, current levels up to <Maximum Primary Current> won't damage the transformer.

Notice that Is * N = Ip .  So that confirms that these values are the normal operating mode.

Ed
 
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Offline Sultanpepper123

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Re: Current Transformer Rated primary current vs Max. Primary current
« Reply #2 on: September 21, 2019, 07:36:13 pm »
You should go to the manufacturer for clarification.  But I would interpret these specs as meaning that your circuit should remain below <Rated Primary Current> during normal operation.  However, under fault conditions, current levels up to <Maximum Primary Current> won't damage the transformer.

Notice that Is * N = Ip .  So that confirms that these values are the normal operating mode.

Ed


I've sent an email to several manufacturers, if i find anything i'll post an update for anybody who may fall in my situation. Thanks for your help BTW.
 

Online soldar

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Re: Current Transformer Rated primary current vs Max. Primary current
« Reply #3 on: September 21, 2019, 09:35:32 pm »
1. On an absolute maximum load 100A will this CT not saturate ?

2. How can i work with this thing ? is it supposed to be saturated somehow and still work on both spectrums ?

You have a serious misconception. Flux from the primary is instantly counteracted by flux from the secondary. Current is not limited by core saturation. It is limited by copper resistance. That is the way forward power transformers work as well. Only flyback transformers store energy. Forward transformers do not saturate because you add load.
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Offline MagicSmoker

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Re: Current Transformer Rated primary current vs Max. Primary current
« Reply #4 on: September 21, 2019, 10:29:37 pm »
1. On an absolute maximum load 100A will this CT not saturate ?

2. How can i work with this thing ? is it supposed to be saturated somehow and still work on both spectrums ?

You have a serious misconception. Flux from the primary is instantly counteracted by flux from the secondary. Current is not limited by core saturation. It is limited by copper resistance. That is the way forward power transformers work as well. Only flyback transformers store energy. Forward transformers do not saturate because you add load.

Forward transformers will saturate from too many Volt*Seconds (Et), however, and this is what limits the maximum output voltage from a conventional CT. This basically means there is an upper limit to the value of the burden resistor for a given primary current. The CT datasheet will almost certainly give the Et rating; if it doesn't then find another CT vendor.
 

Online coppice

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Re: Current Transformer Rated primary current vs Max. Primary current
« Reply #5 on: September 21, 2019, 11:01:23 pm »
I've opened a 80A rated energy meter and found out the CT's have a writing " 5 (80)A / 2.5mA/0.1 " , is it 5A or 80A ?
Look at the IEC620xx specs for energy meters and these CT specs will make sense. The IEC approach is to define something they call a basis current. Its not the maximum current the meter is designed for. Its frequently 1/6 of the maximum current, although it could be any fraction. You will find meters where the basis current is 1/10th, 1/12th, 1/20th and other fractions of the maximum operating current. Its just a reference point, really. No meter is going to be as accurate at small currents as it is at large currents, or as accurate at poor power factors as it is at unity power factor. The IEC specs define an error mask for each class (2%, 1% 0.5%, or 0.1% accurate) of meter that allows for this reality, and defines the maximum spread of error allowed at various fractions of the basis current, for various power factors. Above the basis current the meter needs to maintain the accuracy specified at the basis current.

The CT you are looking at has been designed to meet the requirements for a 0.1% accurate meter with a basis current of 5A and a maximum operating current of 80A. So, for this CT the maximum current can be up to 16 times the basis current, while complying with the IEC specification. At 5A the secondary current will be 2.5mA. There should also be a specification for the maximum burden resistance before the CT fails to give the specified accuracy.
 

Online soldar

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Re: Current Transformer Rated primary current vs Max. Primary current
« Reply #6 on: September 22, 2019, 02:37:03 am »
Forward transformers will saturate from too many Volt*Seconds (Et), however, and this is what limits the maximum output voltage from a conventional CT.

Well, yes, of course, I am referring to using the transformer within its designed parameters. If I make a mistake and connect 230 Vac to the 12 V winding of a transformer it will saturate the core and bad things will happen.  But if I connect 230V to the 230 V primary this magnetizes the core and uses a small (magnetizing) current. I can start increasing current in the secondary and the core will not saturate at any point. The heat in the copper will be what limits transformer capacity. If I can find a way to lower winding resistance then the same core could handle more current and power.
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