CT magnetization

[sivajith]

Hi,


When we perform continuity check (ductor test) on a busbar we inject 100Amps current DC on to the circuit. Now if this circuit has a CT will this magnetize the CT?. When we perform CT testing we normally demagnetise the CT but if subsequently we perform this continuity check will it magnetize the CT?

 

[submonkey]

Hi,

I would expect that all CTs will be magnetised to the maximum
extent supported by their core remanence - about 80% for solid
iron (standard) CTs and about 10% for air gapped CTs.

It's probably prudent to demagnetise the CTs after the test.

Note that there are conditions which occur in service which
would fully magnetise CTs, including:

- Some through fault conditions for feeder or transformer CTs.
- Nearly any in-zone trip of a high impedance scheme.

I doubt that it is common practise for those CTs to be de-magnetised
prior to restoration, and life seems to continue.

Thanks,
Alan

 

[godel123]

if you inject DC==> no induction

 

[waross]

A CT responds to the magnetic field surrounding a current carrying conductor. Induction is the result of changing magnetic field strength.
With slowly applied DC there is almost no induction, but the magnetic field is still there.
The magnetic effect of 100 Amps DC may be similar to the worst case of about 71 Amps AC. There may be some residual magnetism but saturation is unlikely.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[scottf]

Applying DC to the CTs will most certainly magnetize the CT to some degree. If it's not overly burdensome, it would be recommended to de-magnetize the CTs after such a test.

Assuming these CTs are for protection, having them magnetized lowers the current at which the CT will saturate. Overtime the magnetism will work itself out due to varying current levels, but if you have a transient event right after re-energizing the CTs after magnetizing, the CTs very likely may not perform as rated.

 

[godel123]

indeed , passing a direct current will magnetize the CT.

 

[CanadianKid]

As most utilities perform DC resistance tests on Busbar and switchgear which has CTs within the test loop, and because you are applying DC current to the primary, the level of magnetization occurring is actually a Volt Seconds affect. You have 1 turn and the voltage across that turn has a smaller affect than we see on power transformers. Although safe practice is to demagnetize, it should be stated that most times, demagnetizing is not conducted when applying relatively small DC currents (versus rated CT AC rated current). The magnetizing of a CT under load when faults occur is typically a much more "magnetizing" event, and may times, we still do not demagnetize. Once in service, the 100A DC from testing is typically washed out (cancelled), but some high current fault events will leave CTs highly magnetized, which will typically causes measurement errors or nuisance faults. This form of magnetization is more difficult to demagnetize.
Hope this is not too confusing - I do not make many comments on this forum.