winding movement detected 1

[collies99]

I just tested a 16 MVA unit that was subjected by 2 - 100 MVA 3-phase bolted fault on a down stream bus. One occured last week and the other occured last May.

Ratio and resitance data is normal.

Capacitance bridge data showa a 16% increase for SEC - GND capacitance an 8% increase in SEC - (PRI & GND) values.

Latest DGA shown no change. This can be expected in the movement occured on the last fault and not the faaul in May and even then. it may not be dectable by DGA analysis.

I am suggesting a SFRA test on the faulted unit and do a comparitive analysis for a SFRA test on its sistem unit.

Another DGA is expecte in one month to determine in general heating of the core may result of the winding shift.

The unit has been back in service for 1 week now.

Thoughts anyone ? and thanks.

 

[DTR2011]

I am suggesting a SFRA test on the faulted unit and do a comparitive analysis for a SFRA test on its sistem unit.

That is a very good next step. Try to have the best test connections you can have.

 

[prc]

It looks like winding had movement.Apart from DGA you can mesure leakage impedance between primary and secondary by applying low voltage on primary and shorting secondary. Compare this value with previous bench mark value or test report value.

 

[collies99]

Great idea on the leakage reactance. I will add that to the tests and compare also to its sister unit.

Just to add that winding resistance did not change so no deformation of the winding cross-sectiona area were noticeable and may be suggestive that this was a minor movement which may not impact winding / insulation life.

I'll get a better feel after these test are completed.

Thanks all.

 

[prc]

No you cannot take such a lenient view.Most probably the fault current wound have caused a buckling of secondary winding to core.Considering the large change in C, the buckling wound be quite bad.No change in resistance will occur in this failure mode. Once the movement is confirmed by leakage reactance measurement, better to shift the unit for repair.Otherwise it may lead to a catastrophic failure causing extensive internal damage.