The Impact of Heat on Current Transformers

 The Impact of Heat on CTs
Heat can have a serious impact on the rating factor of a current transformer. The IEEE C57.13 standard addresses the “de-rating” of a current transformer’s rating factor in Section 4.5 (see image to the left). A standard CT has a published rating factor that is guaranteed at 30° C and a second rating factor for 55° C. The rating factor at 55° C will always be lower than it will be at 30°.

If we look at the top line of the accompanying chart, you will see an example of a CT with a RF of 4 at 30° that derates to RF 3 at 55° C.  This is very typical of most CTs. What this means for a utility is that in high ambient temperature applications, they will not be able to use the full RF of a CT.

Is there anything that can be done to eliminate this impact?  Yes!  The most typical high ambient temperature application that utilities face is in a padmount transformer.  In this application an oval shaped CT is placed over the 4″ bus bar inside the transformer.  Between the heat generated by the bus bar and the heat generated by being trapped in a closed environment, the ambient temperature can soar well above 55° C.  Peak Demand’s COV-6 CT has a special insert that allows it to keep the full rating factor up to 85° C allowing utilities to take advantage of the full RF.  Of course the Alta Series High Accuracy, Extended Range CT also has the ability to operate at 85° C and get the benefits of high accuracy.

COV-6

COV-6 Current Transformer

 

Article by Steve Lindsay

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