High DGA 11

[powerzizo]

Hi All
we have two identical generators transformers 420MVA(18/220kV). both transformers has almost the same service time 14 years and got high DGA and buchholz alarms one year in between.
both transformers are currently under inspection by OEM and initial findings were burnt cable in phase -W- HV leads to OLTC at 10CM distance from the HV winding.
another two similar transformers are still in service with almost the same operating conditions and service time.we are concerned if the in service transformers will suffer the same issue shortly. The routine DGA we do every six months and results comes clear for the in service transformers . the two faulty transformers most recent DGA before the failure were also normal one or two months before the buchholz activation.
are there other recommended tests should we do to predict this type of failures,noting that we do tan delta, DC resistance, turns ratio every year and results come normal?

due to long down time to repair any of the two faulty transformers, and considering long lead time to get new transformer, may an expert advise me the fastest way to find similar capacity transformer to minimize the shutdown time?

 

[prc]

Poor crimping, I can understand.But how uneven strands can result in burning? What happened to DBDS test result?
Coming to drying-the only way to dry cellulose insulation is by vacuum and heat. In LFH drying, you are able to apply both simultaneously to get very fast drying. No-load current cannot heat anything-it is negligible.Under vacuum you cannot apply any voltage-it is disaster-remember Paschen's law.
To speed up drying,if the following type of heating is applied along with vacuum, it can speed up drying.
(a) put few turns of cable all around outside the tank(touching it) and apply suitable voltage. Eddy currents will appear on tank, heating the insulation from outside.
(b)spray hot oil through sprinklers in to tank with vacuum maintained over it.
Hot oil circulate for 10-12 hours- drain oil quickly, feeding dry air or nitrogen of low dew point say-45C -keep for 12 hours - measure dew point -it will give the moisture level in paper- apply vacuum for 24 hours- oil fill -hot oil circulate
Repeat above cycle for 2-3 times till satisfactory dew point is got.
(a) can be provided through out (b) only during vacuum period
By this way, satisfactory dry=ness will be got within two weeks.

 

[powerzizo]

Thank you.
Just to clarify,in which condition step a can be provided?and what level of voltage?

 

[prc]

The tank eddy heating can be applied through out the process, only taking care that surface temperature not exceeding 90C, lest the painting may get damaged. During hot oil circulation and air filled condition, it will transfer part of the heat to active part. During vacuum stage, it will transfer heat inside by radiation. The cable loop is done at same level as winding height and heating is controlled by the current in the cable. Applicable for all voltages.

 

[powerzizo]

Thank you prc, some photos for the repair to explain the poor crimping and uneven number of strands issues.
DBDS in all transformers between 100 to 180 ppm-Oil corrosive test was positive, passivator content 35ppm and we will top up to 100ppm.

IS there a standard procedure somewhere for the drying method that you have described?

 

[prc]

This external heating can be done by electric heaters also. It seems eddy heating of tank is popular in Russia.

 

[powerzizo]

Thank you