CT magnetization
CT magnetization
(OP)
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?
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?






RE: CT magnetization
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
RE: CT magnetization
RE: CT magnetization
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
RE: CT magnetization
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.
RE: CT magnetization
RE: CT magnetization
Hope this is not too confusing - I do not make many comments on this forum.