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Current Transformer Sizing
- Thread starter EEWVU2002
- Start date
Get a manual for the sort of relay you are looking to use and find the CT requirements for the relay from there.
Most relay manufacturers (Schweitzer Engineering Laboratories and Alstom as two examples) have the manuals for all of their relays on their websites so check them out.
If it is an older electromechanical relay, the relay manufacturer will still be able to provide a hard copy of the information.
Some engineering texts cover the subject OK, such as:
"Protection of Industrial Power Systems" by T Davies
Most relay manufacturers (Schweitzer Engineering Laboratories and Alstom as two examples) have the manuals for all of their relays on their websites so check them out.
If it is an older electromechanical relay, the relay manufacturer will still be able to provide a hard copy of the information.
Some engineering texts cover the subject OK, such as:
"Protection of Industrial Power Systems" by T Davies
- Thread starter
- #6
To size CTs for meeting depends a little on the standard in use, but the following should apply pretty good for IEC and IEEE.
First, you need to know your maximum current. Then you need an idea of the expected nominal and minimum current.
The trick is to pick the ratio so that ideally, the minimum expected current is above the ratio. I.e. if the minimum expected current is 125A, then pick a 100:5A or 100:1A (depending on where you are) ratio.
Then, pick a rating factor (full name is continuous thermal rating factor) that will take the continuous rating past your maximum current.
Example, if your minimum current is 125A and the maximum current is 250A, pick a 100:5A raio with a rating factor of 3.0, which means the CT can take up to 300A.
Rating factors are based on an average ambient temperature of 30 deg C. They are derated for average ambients above 30 deg C. Depending on the voltage class of the CT rating factors up to 4.0 are normally available.
The reason for trying to keep the ratio below the minimum expected current is because the base accuracy rating is normally valid from the rated current up to the rating factor.
The next is to pick the accuracy and burden required. For the IEEE markets, accuracy classes of 0.3% and 0.15% are common for revenue metering. For the IEC world, 0.2% and 0.5% classes are normal. Then figure your burden requirements, which is the burden of the meter, plus the round trip leads. In IEEE they are expessed in ohms, like B0.9 means up to 0.9 ohms connected to the CT. In the IEC world burden is expressed in VA, like 10 VA, or 20 VA. Since most IEC CTs have 1A rated secondaries, the VA is normally the same as the ohms at rated current.
Hope this helps.
First, you need to know your maximum current. Then you need an idea of the expected nominal and minimum current.
The trick is to pick the ratio so that ideally, the minimum expected current is above the ratio. I.e. if the minimum expected current is 125A, then pick a 100:5A or 100:1A (depending on where you are) ratio.
Then, pick a rating factor (full name is continuous thermal rating factor) that will take the continuous rating past your maximum current.
Example, if your minimum current is 125A and the maximum current is 250A, pick a 100:5A raio with a rating factor of 3.0, which means the CT can take up to 300A.
Rating factors are based on an average ambient temperature of 30 deg C. They are derated for average ambients above 30 deg C. Depending on the voltage class of the CT rating factors up to 4.0 are normally available.
The reason for trying to keep the ratio below the minimum expected current is because the base accuracy rating is normally valid from the rated current up to the rating factor.
The next is to pick the accuracy and burden required. For the IEEE markets, accuracy classes of 0.3% and 0.15% are common for revenue metering. For the IEC world, 0.2% and 0.5% classes are normal. Then figure your burden requirements, which is the burden of the meter, plus the round trip leads. In IEEE they are expessed in ohms, like B0.9 means up to 0.9 ohms connected to the CT. In the IEC world burden is expressed in VA, like 10 VA, or 20 VA. Since most IEC CTs have 1A rated secondaries, the VA is normally the same as the ohms at rated current.
Hope this helps.
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- #8
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