A50-Transformer Oil in Service-Case Histories-Part 5

Case History No.12

Hydrogen generation without internal fault – stray gassing-hydrogen gas evolution

Year 2006-2007

1.?80 MVA 765 kV Shunt reactors were undergoing loss measurement (by calorimetric method, using oil to water heat exchangers) followed by temperature rise test at factory (under ONAN, pressed steel radiators??in place). During these tests, dissolved gas analysis was done continuously to monitor any incipient fault. When calorimetric loss measurement was done for 19 hours, hydrogen level in oil remained steady at 12.5 ppm. But during the subsequent temperature rise test with??radiators, there was a linear increase of hydrogen generation of the order of 11 ppm/day. Even though this rate was less than the IEEE standard rate of 0.8 ppm/hour (DGA during factory tests- IEC 61181 ed2.0-2007 / IEEE C57.130 -2015) customer had some apprehensions of internal fault. It was then established from separate tests on radiator bank assembly that hydrogen generation was from the partially cured internal painting in radiators. (Celerol 914 Reaction primer) Gas generation disappeared when the internal painting was hot air dried or with passage of time (eg: used radiators).

?2.??Some 420 kV class new oil filled current transformers dispatched from factory to site were showing continuous hydrogen generation during storage, even before energization.

?The lower tank (dead tank design) of CT was of steel fabrication, with hot dip galvanizing both inside and outside. This hydrogen generation disappeared when epoxy painted tanks were used instead of hot dip galvanized tanks. Problem was due to the poor-quality control of hot dip galvanizing process. Hydrogen generation disappeared when defectively galvanized tanks were heated up overnight in an oven before assembling.

?Lessons:

1)???Free water present in oil can corrode the cut edges of transformer core laminations, generating hydrogen, even when transformer is not energized. (Doble Conference 65th- 1998 Sec8-3 P Christenen, G Ohlsson, The behavior of moisture and free water in Power Transformers)

2)???In case of core hot spot temperature reaching in the range of 120-200 degrees C, (say during over excitation) decomposition of thin oil film between core steel laminations can generate hydrogen by the catalytic action of CRGO steel. Certain grades of stainless steel also can act as a catalyst for hydrogen generation. These reactions require liberal presence of oxygen in oil (CIGRE 1998-Paper 12-206).

3)???Hot dip galvanized metal surface in contact with oil can generate hydrogen in oil. This can be severe with imperfect galvanizing process or poor process quality control.

4)???Mixing of oil leaked out from diverter switch of OLTC (through failed oil seals of diverter switch) can show presence of hydrogen in transformer tank oil along with other hydro-carbon gases.

5)???Internal paint applied inside of transformer tank/radiators, such as alkyd resins and modified polyurethanes containing fatty acids in their formulation may form hydrogen gas in contact with transformer oil. This is especially so if the paint is not dried fully before coming in to contact with oil

6)???Gases may also be produced by the exposure of oil to sunlight or may be generated during repair welding of transformer tank under oil filled condition.

7)???Stray gassing of oil is defined by CIGRE (TB No.296-2006 Recent Developments on the interpretation of DGA in transformers) as the formation of gases in oil heated to moderate temperatures (less than 200 °C). H2, CH4 and C2H6 (hydrogen, methane and ethane) may be formed in transformers at such temperatures or as a result of oil oxidation, depending on chemical structure of oil used. Stray gassing is a non-damage fault and can be evaluated using test methods described in Clause 3.1.1 &3.1.2 of TB 296 and ASTM D 7150-2013 Standard Test method for determination of gases characteristic of insulating liquids under thermal stress. Stray gassing of oil has been observed in some cases to be enhanced by the presence in oil of a metal passivator or other additives.

(Clause 4.3 & 4.4 of IEC 60599-2015 Interpretation of DGA)

Case History No.13

Transformer operation without oil

Year 1928

After seven years of operation, a US mid-western utility reported to the OEM that fumes are coming out of a transformer in service. When service engineer from transformer factory visited site and inspected the transformer, he found the unit was working without oil! The instruction manual was inside the tank, tied to a lead wire. ?The concerned forgot to fill oil in transformer during the installation work. Since the load on transformer was very low, transformer worked without problems (thanks, to the over-designing followed in those days). When the load gradually increased with years, fumes came out through the breather. Original oil drums sent with the transformer were located in stores and used for filling the transformer.

(Reference: Thomas J Blalock, Transformers at Pittsfield: A history of the General Electric large power transformer plant at Pittsfield, Massachusetts, Gateway Press, Baltimore, MD, USA,1998, Page 30)

?Case History No.14

Miscibility of water with transformer oil

It is well known that transformer oil will not easily get mixed up with water. In the early days of transformers, this characteristic was used by some clever engineers to overcome oil shortage noticed at site. It is claimed (may be folklore!) the chief engineer compensated oil shortage by filling water through the lower valve on tank, but well below the windings. Since water is heavier than oil, it remained at tank bottom and transformer was energized. After a few days of working, on receipt of oil water was replaced and transformer was put back in to service.

https://www.dhirubhai.net/pulse/a46-transformer-oil-service-case-histories-part-1-p-ramachandran

https://www.dhirubhai.net/pulse/47-transformer-oil-service-case-histories-part-2-p-ramachandran

https://www.dhirubhai.net/pulse/48-transformer-oil-service-case-histories-part-3-p-ramachandran

https://www.dhirubhai.net/pulse/a49-transformer-oil-service-case-histories-part-4-p-ramachandran

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