what advantage does knowing the knee of a CT curve provide in maintenance?

[anorris2010]

Hello,

Before our unit got Megger's MRCT, we tested Current transformers with Insulation-resistance and turns ratio. I'm not against Megger's battery of tests as they are easy enough to run all at once. I can understand polarity will verify the last technician connected the CT correctly and resistance will verify no shorts or opens in the windings. But what advantage does knowing the Knee volts and current values during maintenance provide?

[Kalbi_Rob]

Hello,

Before our unit got Megger's MRCT, we tested Current transformers with Insulation-resistance and turns ratio. I'm not against Megger's battery of tests as they are easy enough to run all at once. I can understand polarity will verify the last technician connected the CT correctly and resistance will verify no shorts or opens in the windings. But what advantage does knowing the Knee volts and current values during maintenance provide?

In accordance with NETA 7.10.1.b: only insulation resistance testing is required for maintenance testing of CTs.

That being said, it does not hurt to perform the optional tests.

I highly recommend reading up on CT saturation as the Knee provides the point at which the CT goes into saturation. We determine this value during acceptance/maintenance testing to verify damage has not occurred to the core (such as physical or overheating). If damage occurs in the core, then the magnetic flux created can change the saturation point of the CT, causing erroneous readings at higher loads, or be affected by a different burden value than originally designed. That varied burden could cause the CT to go into saturation not as engineered, causing erroneous readings, improper protection, and potential damage to the CT and surrounding equipment or connected equipment.

[anorris2010]

Thank you, I see the resistance measurements and insulation resistance are recommended electrical tests. I can also see how saturation could result in erroneous readings and improper protection at higher loads. I just never thought unseen physical damage or overheating would change the saturation point. Still, I agree if there is a signification shift in CT saturation from acceptance to a maintenance test, there is reason to believe damage has occurred and the system could no longer function as engineered. I will take your recommendation to read more on CT saturation.

In accordance with NETA 7.10.1.b: only insulation resistance testing is required for maintenance testing of CTs.

That being said, it does not hurt to perform the optional tests.

I highly recommend reading up on CT saturation as the Knee provides the point at which the CT goes into saturation. We determine this value during acceptance/maintenance testing to verify damage has not occurred to the core (such as physical or overheating). If damage occurs in the core, then the magnetic flux created can change the saturation point of the CT, causing erroneous readings at higher loads, or be affected by a different burden value than originally designed. That varied burden could cause the CT to go into saturation not as engineered, causing erroneous readings, improper protection, and potential damage to the CT and surrounding equipment or connected equipment.