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Overhauling of Electrical Power transformer

Overhauling of transformer is required either due to ageing of transformer or due to fault of transformer & after rectification of fault following steps should be followed for better overhauling.


Draining of complete oil in tank or barrels before draining the oil, the oil in the transformer tank should be heated up by a separate hot oil circulation at around 70’C. So that when the oil is totally drained out, the core & winding assembly will have temperature higher than ambient. This will note be exposed without oil for more than 12 hrs. Before this period either the assembly should be filled with new oil or nitrogen or should be subjected to vacuum. Before draining the oil, the external surface of tank & fittings should be cleaned to locate the point of leakage


Overhauling of OLTC which includes:


Hot oil washing of diverter & selector chamber after untanking diverter switches.


Untanking diverter switches


Cleaning of fixed & moving contacts


Tightening of all the clamping hardware


Replacing all the oil sealing gaskets


Greasing of gears in driving mechanism


Replacement lubricating oil in gear bo


Washing of core coil assembly by hot oil jet to remove sludge carbon Deposition and any other foreign particles deposited over it


Tightening of core studs & checking its IR value


Tightening pressure screws for axial tightness of coil


Attending to any minor abnormalities on core coil assembly


Paper covering to main leads wherever required


Flushing of radiators


Cleaning the tank inside by hot oil jet without untanking core & winding assembly


Preparation & replacement of all the oil sealing gaskets made from rubberised cork sheet confirming to grade RC80C of IS 4253(Part-2) 1960 for whole transformer.


Drying out of core & winding as per procedure laid down as under:


Equalizing diverter chamber of OLTC with main tank, so that the diverter chamber is not subjected to pressure during vaccum.Vaccum pulling by a separate vacuumed  pump up to 700mm or more, provided the tank & cover does not have permanent deflection due to vaccum.For 12 hrs after attending 700mm vaccum.After every vacuum cycle, the quantity of water collected in the condensate of the vacuum system is to be measured & noted for the purpose of knowing the rate of water extracted from the core & winding insulation. The new oil in storage tank is filtered by two stage high vacuum filter machine of minimum 4500LPH capacity & impregnated in transformer tank under vacuum. After oil filling the oil in the transformer tank is filtered to raise the temperature, to 65-70 “C. After raising the temperature, the oil is drained out & dry nitrogen having 99.5% dryness is injected. The nitrogen is kept in the transformer for about 12 to 24 hrs. at least 2/3 cycles to be given.IR value to be measured after each cycle.


Filtration of Oil


After the drying out cycles are over the oil in the transformer is filtered till the BDV & water content values are achieved as required by IS 1866-1983.During this final filtration, a separate degassing column with separate vacuum pump should be used to get better dehydration of oil. During filtration the radiator bank should also be filtered separately or along with filtration should two stage, high vacuums machine of minimum 4500LPH capacity. After filtration the oil will be tested for BDV in presence of GETCO engineer site. The BDV value should meet requirement as per IS 1866-1983.After filtration the oil sample will be tested for water content (PPM) at ERDA. The results should meet requirement as per IS 1866-1983.


Dismantling of all the fittings of the transformer such as HT & LT bushing, conservator, explosion vent, Buchholz relay, radiators, top cover, inspection cover etc. including hiring of crane.

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