Case Study – How Proactive Electrical Maintenance team saved a Power Transformer

Hello Connections,

Your valuable 10 minutes may avoid breakdown of Electrical Equipment in your plant

Today, we are going to present a case study of ‘POWER TRANSFORMER’, which we never heard off or thought that such even case may happen in plants but happened in one. However, we wish to share the experience, so that the problem Maintenance Engineers faced in that plant, may not be faced by others or can be avoided.

This case study shows, how Pro-active Engineers, preventive Maintenance & Routine testing activities can save your Electrical Equipments and Revenue worth crores.

Power transformer of rating 31.5 MVA, 66/11 kV is used for power distribution to utilities through an 11 kV switch-gear, they were having three Transformers of identical rating, in which all three transformers are in charged condition & the transformer which we are talking about is used as a standby transformer.

The transformers were in charged condition, predominantly the 1st and 3rd transformers were in a loaded condition comparative with the 2nd one. Almost one year passed, all three transformers were functioning normally, with a load of 10 to 20% of its rated capacity.

Incident of Fault :

As a yearly Preventive maintenance activity, the all 3 transformers maintenance has been carried out, everything was found in order, the mineral oil samples of all three transformers were collected and has been sent to the nearest Lab for analysis of DGA, BDV & water content.

The DGA test results of Transformer#2 was found abnormal compared with the past test results (first result before charging), the C2H4(Ethylene) content was found immense, as per IEEE C57.104 standards it should be less than 50 ppm. But where has in the test result, the content of C2H4 was found around 402 ppm, almost 8 times the specified limit as per IEEE.

The Maintenance team didn’t take any final verdict on shutting down the transformer, for more assurance, they have sent the oil samples to another four labs just for checking the actual increase in C2H4 by authenticating with their own lab results, the samples were sent to two private labs, one to OEM and one sample to ERDA, all the results were not the same, moreover there is a significant change in C2H4 as compared with others.

Below is the results of Oil test : -

COMPARISON OF DGA RESULTS

The Transformer is de-energized & OEM representative has been called out to the plant location to further scrutinize the case, the site team performed some electrical tests to ensure the healthiness, the test results were found in order.

The oil has been drained out of the transformer, the manhole covers were removed, the core yoke and tank shorting point disconnected/separated, IR test conducted between the separated points (Core & yoke frame) with respect to ground, a voltage of 2500 Volts is applied to check the healthiness of insulation.

As there is a complication to enter inside the transformer due to the space concern between the body and the core-coil assembly, the inspection window in three sides was opened for conducting the above test.

While conducting Insulation resistance test the voltage is notwithstanding/ building up normally, all of a sudden, the team noticed some chattering sound from inside the transformer, all the team members were inspecting the transformer from all three sides of inspection window, but the chattering sound/ spark is not accurately visible.

OEM recommended dispatching the transformer to their works for further investigation, the transformer is dispatched from the site and the site team also reached to OEM works for inspection.

CCA OF THE SAID TRANSFORMER

The core has been lifted from the body and visual inspection performed, no abnormalities found, Insulation resistance between core & yoke frame conducted @ 2500 volts, chattering sound observed.

Finally, case was concluded, one washer found resting on the bottom of a center limb of core and touching the yoke frame resulting in sparking/chattering.

Pic of the washer which was resting in the bottom of the center limb.

OMG! From where this washer came inside the transformer?

We can't judge this now, however, this washer may have gone inside the transformer in OEM workplace itself or else in site during installation of the transformer, we may say that it was sheer negligence or poor workmanship. But this can happen

After removal of washer Insulation resistance test conducted the values were found more than 500 Mega-ohms.

The transformer has been packed and testing has been done, everything found normal, the same has been commissioned and taken in service.

Conclusion that can be drawn from this Incident :

Conclusion which maintenance team of that plant came up with, which may not be feasible, but if they have done it, they could have saved lot of time and money.

Below is the conclusion drawn by the maintenance team of that plant are as below

Points to be considered before & after commissioning of the Transformer (related to this case study) :-

1.    After receipt of the transformer at your premises, don’t forget to check the insulation resistance of Core & Yoke frames earthing point individually with respect to the ground/ body.

2.    If these points are not available outside the transformer body, then the manhole cover to be removed and all these three points to be isolated and insulation resistance to be conducted as stated in point no 1.

3.    DGA to be carried out before charging, after charging (no load 24hrs), in a loaded condition (after 24 hrs) & after a month, where online DGA is not available.

4.    DGA / Oil test report should be cross verified (if the report is not within prescribed limits) with a minimum of One NABL accredited labs, before concluding.

5.    While erecting transformer and its accessories, one should be very cautious of handling materials and one must double check that nothing is fell inside the transformer tank, before boxing up & oil filtration/ charging.

Hope it will help you, also share this with other Electrical Engineers you know.

Any other suggestions are welcomed.

Regards

Avinash Singh