Discussion

Posted by LYES BENZERROUG in Linkedin. Click title to go to linkedin discussion

Transformer Failure 60KV, 40 MVA

I invite all experts to give their opinion on the case, the DGA is normal except low oxygen econsommation. What are complementary tests to be done to see if the active part was hit

The icident is caused by initiation of the two stork 60kV terminals levels TRS 60/30

Response

Hi Lyes

This is a very interesting case.

From the analysis used in the tool LEDT, there is clear indication of a fault condition experienced (See figure - movement from point A to B). However, it is felt that the source of the fault is external. The internal increase in H2 and CH4 was probably due to the top tank lid being hot. There is also some ethane to support this. See this link for more details on the LEDT method. http://powertransformerhealth.blogspot.co.za/p/ledt.html

It seems like this was a bushing failure or something caused the bushing to fail. I would presume that the Buchholz did not operate. I would hope that if there was differential protection which operated. In light of this the internals may be ok.

I would suggest the following on site tests:

Inspect the transformer externally to establish all areas affected. Check the tap changer for any obvious damage.

Insulation resistance – use a 500/1000 V megger on the phase to phase and phase to ground, the neutral also to ground. It must be clear.

Also test the Core to ensure that it is earthed at a single point. Remove the single earth strap from the core of the transformer and apply a DC voltage with a normal insulation resistance tester. The voltage used should not exceed 500V DC.

If the phases are still intact then these connections (HV, LV, and Neutral) can be used for these further tests.

Winding resistance - to check for any abnormalities due to loose connections, broken strands and high contact resistance in tap changers.

Impedance test - to determine if any winding movement has occurred, which usually occurs during heavy through fault current or mechanical damage during transport or installation.

Magnetization current test - to detect defects in the magnetic core structure, shifting of windings, failure in the turn-to-turn insulation or problems in the tap change device.

SFRA - test should be finally done to establish if there was any winding movement/deformation/shorts, Core problems related to movement/grounding/screens, Mechanical faults relating to clamping structures/connections.

Depending if the transformer passes the above tests you can do an onsite internal inspection but you must create a workshop/clean environment for internal inspection.

• ·         Internal Inspection - Be careful not to contaminate the internal insulation.

• ·        Check the bushing connections on the internal, tap changer, all connections, and lead windings for any problems.

Depending on how old this transformer is (I only see 2 oil samples so it seems new) the paper DP can be tested. If it is on average above 600 then I would suggest reusing. A transformer repair company in your area can then advise on the way forward. You can then source new bushings or if you have spares they can be used. Make sure the damage of the transformer tank allows for this.

I must say the above is based on the limited information that you have provided. I would suggest that a more detailed assessment be made on-site to establish if the above recommendations are still feasible.

Please provide details on what you managed to establish.

Good Luck.

Nad