Current Transformer Sizing

In summary: Generally, for short term overcurrent situations, it's a good idea to have a higher rating factor (i.e. oversized) CT to prevent any damage or accuracy issues. In summary, the conversation discusses the use of a digital genset controller with timed overcurrent protection and the use of a 100 amp breaker with a 300 amp instantaneous trip rating. The question is raised about the necessary size of the current transformers (CTs) to prevent damage during overcurrent situations. The conversation also delves into the importance of considering heat and voltage in choosing the appropriate CTs, as well as the impact of burden resistor selection. Ultimately, the recommendation is to check the rating factor of the CTs and consider using a higher rating factor for
  • #1
Josh111
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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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  • #2
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.
 
  • #3
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.
 
  • #4
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?
 
  • #5
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.
 
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  • #6
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.
 
  • #7
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.
 
  • #8
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?
 
  • #9
Again, that's going to depend on the CT ratings. Check the spec sheet or contact the manufacturer for their recommendations.
 

What is a current transformer and how does it work?

A current transformer is a type of instrument transformer that is used to measure or monitor the flow of electric current in a circuit. It works by stepping down the primary current (usually high) to a lower, measurable value that can then be used to indicate the amount of current flowing through the circuit.

Why is it important to properly size a current transformer?

Properly sizing a current transformer is crucial in order to obtain accurate measurements and prevent damage to the transformer and the equipment it is connected to. Oversized or undersized current transformers can lead to errors in measurements and can cause overheating or malfunctioning of the equipment.

What factors should be considered when sizing a current transformer?

When sizing a current transformer, factors such as the primary current, secondary current, burden rating, accuracy class, and frequency should be taken into account. The primary current is the maximum amount of current that the transformer will be exposed to, while the secondary current is the desired output. The burden rating and accuracy class determine the accuracy of the transformer, and the frequency is important for ensuring compatibility with the circuit.

How is the burden rating of a current transformer determined?

The burden rating of a current transformer is determined by the total load of the connected instruments or devices. This includes the resistance, inductance, and capacitance of the circuit, as well as any additional instruments that may be connected to the transformer. It is important to select a transformer with a burden rating that is equal to or greater than the total load of the circuit to ensure accurate measurements.

Are there any standards for sizing current transformers?

Yes, there are several standards that provide guidelines for sizing current transformers, such as IEEE Std C57.13 and IEC 60044-1. These standards specify the accuracy classes and burden ratings for different applications, and also provide formulas for calculating the appropriate transformer size based on the primary and secondary currents, and the frequency of the circuit.

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