Current Transformer Sizing


by Josh111
Tags: current, sizing, transformer
Josh111
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#1
Aug22-09, 08:19 PM
P: 44
I am using a digital genset controller that to run and protect a generator. This controller has, in addition to other functions, timed overcurrent protection (1-30 seconds). The controller uses 5 amp secondaries. The generator is 83 amp. The 100 amp breaker I plan to use has a instantaneous trip rating of 300 amp.

My question is:

Since it is timed overcurrent, it may be possible that the amperage will be higher than full load amp for a while before there is a shutdown. Is it necessary, therefore to size the CTs at a rating higher than 100/5 to prevent them from burning up during a overcurrent situation? What percent, if any should they be oversized.
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vk6kro
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#2
Aug23-09, 03:45 AM
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This controller has, in addition to other functions, timed overcurrent protection (1-30 seconds).

This sounds like you get to choose the overcurrent time. Maybe it is to let a motor start up knowing it will only draw excess current for a short time. If you know the generator can handle a short term overload, you might want to do this. You wouldn't want all the lights to go out every time the fridge turned on.

You have to look at this from a heat and also from a voltage point of view and examine the actual components involved. I don't think this is possible with the information you have given.

A current transformer might generate twice the output voltage for double the current and this may do damage that a little bit overheating will not do. So you might like to get someone to check this out for you.
Josh111
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#3
Aug23-09, 05:25 AM
P: 44
Yes the generator can handle temporary overcurrent because it has a high insulation rating.
My question remains in regards to the sizing of the current transformer.

vk6kro
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#4
Aug23-09, 09:53 PM
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Current Transformer Sizing


You have to look at this from a heat and also from a voltage point of view and examine the actual components involved. I don't think this is possible with the information you have given.

Ok, so,
What is the current transformer doing now? What actual load is on it?

If the voltage out of it doubled, what would that do to the components connected to it?

How big is it physically?

How hot does it get normally?

How hot is it allowed to get?

Would an extra 10 degrees C rise in temperature for about 2 minutes make any difference to its long term survival?

How long would you need to make the time for the timed overcurrent protection?
Would 5 seconds be enough?
negitron
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#5
Aug23-09, 10:32 PM
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You need to find out the rating factor of the CT to be used. The rating factor is how many times the nominal current (in this case, 100 amps) the CT can handle and still provide a valid measurement (i.e., not saturate). Typically, they're available with rating factors between 1.0 and 3.0.

EDIT: I wanted to add there is also a VA rating on the CT. It is possible, depending on burden resistor selection, to drive a CT to destruction, even if it's well within its rating factor. Burden resistor selection is key.
Josh111
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#6
Aug24-09, 09:28 PM
P: 44
I am using relay type CTs not metering CTs. My understanding is that when CTs are overloaded their accuracy is compromised even if the time is 5 seconds. Thus I am wondering if it is common practice to over rate CTs so yhat they will not generate exessive voltage and therefore current as someone said earlier.


I understand that synchronous generators are usually capable of generating 300% of nameplate current except that the stator will overheat over short time. Thus I am wondering if it would be good to size CTs at 300% of full load amps.
negitron
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#7
Aug24-09, 09:44 PM
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The voltage they generate depends solely upon the burden resistor. So, if you want to keep the voltage down, you use a smaller burden. The current will remain fixed by the ratio over a wide range of resistances. If you use a 100:5 CT, you will have 5 amps at full rated primary current (100 amps) whether you use a .02 ohm resistor or .5 ohms. In the former case, the voltage will be V = IR = 5 *.02 = .1 V; in the latter, it will be 2.5 V.
Josh111
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#8
Aug24-09, 10:01 PM
P: 44
I am still wondering how to size the CTs as far as the amp/turns go. Is it at full load amp or above full load amp?
negitron
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#9
Aug24-09, 10:11 PM
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Again, that's going to depend on the CT ratings. Check the spec sheet or contact the manufacturer for their recommendations.


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