Sizing Current Transformers

[lsuengineer]

I am in the process of sizing current transformers for a 20MVA 69kv-4.16kv power transformer. I am calculating the 69kV full load current to be 167amps and the 4.16kV full load current to be 2776amps. The low side current transformer will be used for differential protection. Can anyone recommend how I would size the current transformers based on the above information?

 

[wbd]

The maximum continuous current rating should be equal to or greater than the rating of the circuit. Therefore, the ratings would be based on standard ratings available:

Voltage Rating: 69kV Current: 200 A
VR: 5kV Current: 3000A

I'm not sure what the standard voltage rating is that encompasses 69kV, but 4160 would be 5kV. 200A and 3000A are standard current ratings.

 

[scottf]

For protection applications, wbd's advise is suitable. However, for metering applications, size the ratio as low as possible and use the rating factor to cover the maximum current expected.

For example, for metering on the 69 kV circuit, a 100:5A with an RF of 2.0 (or better yet, a 75:5A with an RF of 3.0) would be better than a 200:5A ratio.

 

[davidbeach]

If you are doing transformer differential protection, wouldn't the CTs on both sides be used for the differential?

 

[rcw retired EE]

Need to check performance of the low side CT for differential protection on through fault conditions. Sizing the CT for 125% of full load amps may give a CT that saturates for a fault on the 4.16 kV side. The differential relay should not trip for a downstream fault.

Ratio also depends on the type of differential relay. An older electro-mechanical type requires a closer matching of the ratios, than modern digital relays.

Check the relay instruction book for suggested ratios.

 

[SMB1]

Hi,

Also you have to check the CT knee voltage requirement for the used relay. Both CTs must has the same

 

[hogdawg]

For differential current protection, size the c.t.'s as close as possible to the fla.

 

[bacon4life]

The following links have some detailed links on how to avoid saturation of the CTs.

http://www.selinc.com/techpprs/6027.pdf

http://www.selinc.com/techpprs/6107.pdf

http://www.geindustrial.com/products/support/489/get8402.pdf

 

[jghrist]

If your differential relay is a numeric type, then you have a lot more flexibility on CT sizing. Numeric relays allow for a large difference in ratios. Ideally, the maximum CT secondary current will be less than 20 times nominal (100A for 5A CTs) and the nominal current will be about equal to max transformer rating.

You should check that neither CT will saturate for the maximum through fault.