CT mag curve testing

[Chiefgrunt]

Can someone explain to me the importance of CT mag curve testing.
I'm currently working on a job where the commisioning engineer is carrying out these tests on a new installation and I'm a little confused as to why!!

 

[Milliamp]

The engineer is trying to verify that the correct CT is installed in the equipment. See the post below - Current transformer saturation.

 

[scottf]

It serves a number of purposes:

1) For protection CTs, to verify the performance is close to what would be expected for the protection class rating. For metering CTs, not much can be told about performance from an excitation test.

2) The results can be compared against the theoretical curve (from the manufacturer). If the knee-point voltage is significantly lower than the theoretical value, it could indicate a shorted turn on the secondary winding. A shorted turn would effect the performance of the CT in service and, depending on the CT design, could cause internal over-heating leading to failure.

3) You may be able to pick it up with other tests, but the excitation test would confirm that there is continuity in the secondary winding.