A CASE STUDY – ABNORMAL DGA & TRANSFORMER TRIPPING

This is a 50 MVA 132/33 KV POWER TRFR for a SEB. It was made in the year 2012 -13 & commissioned successfully in Sep 2013.

The transformer was working satisfactorily till Nov 2013. In Dec 2013; it tripped on Buchholz Relay. DGA was done & Abnormality observed in Results. Low voltage tests were performed & found OK. Oil was drained, total inspection inside the Tank was done & No Fault was found .

Oil filled, all precommissioning tests were performed & the Trfr was Re commissioned.

Same problem was observed after a week - Trfr Alarm / Tripping with Buchholz Relay. Low voltage tests were performed and found in order .

 The Trfr was brought back to works and "Core + Coils" Assy was Detanked after oil drainage. Further Individual Coils of all the three phases were dismantled. Burning marks were observed on Press Board Wraps – on core limb sides – in all the three phases, having almost same Intensity .

On detailed study , the Problematic Phenomenon is concluded as below -

 The diameter of the core is 700mm. As per standard practice; a cooling duct has been provided in the center pocket of the core. Also the two parts of this duct have been shorted by a U shaped Copper link. These copper links have been provided at regular intervals in Top & Bottom Yokes of Core as well in Limbs.

Top & Bottom Yoke Copper Links are OK but the U clamps - provided in the Trfr limbs - are in very High Flux ZONE. This caused heating of these clamps, burning of adjoining Press Board wraps, Gas formation , Buchholz operation & Trfr tripping .

 Core Limbs U clamps have been removed ,necessary rectification done, all Routine Tests have been done at works & Trfr shipped to site. Trfr was commissioned successfully in March 2014 & giving Satisfactory Service till date.

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