Transformer tank pressure test during the manufacturing 1

[krisys]

Is there any thumb rule to quickly estimate what should be the rated static pressure of the tank of a 132/34.5kV, 140MVA transformer, with the conservator at a height of 2.5 meter.

IEC 60076-1 states that the tank shall be tested for the pressure of at least 30 kPa over the normal liquid pressure.

By applying the above standard, the static pressure in this case is expected to be about 2.5 mtr x 0.8 = 2 bar, assuming the specific gravity of oil is 0.8. Hence the test pressure shall be
= 2 bar + 30 kPa
= 2 bar + 0.3 bar
= 2.3 bar

I would appreciate if someone from the fraternity could validate or comment on the above.

 

[prc]

krisys, 2.5 metre oil head gives approx o.2 bar and not 2 bar pressure.

The leak test in a transformer is done at various stages. As per IEC 60076-1 ed3.0-2011, it shall be with 30kPa over the normal oil head in transformer for 24 hours for trfs >20 MVA or >72.5 kV. For lower ratings pressure test is for 8 hours (clause 11.8)

When tank is fabricated, it is done with water filling and equivalent pressure generated at tank bottom.

At factory and at site (if demanded by user) it is done by applying dry air pressure inside the air bag or over the oil level in conservator. Another way is to pressurize oil inside the tank after closing valve to conservator using oil filter machine so as to get equivalent pressure at tank bottom. This will avoid air dissolving in to oil.

 

[ScottyUK]

Don't forget that on a transformer of this size it is common to require that the tank be capable of withstanding a fairly high vacuum to allow use of a mobile oil processor. This may be more onerous than the pressure withstand requirement.

 

[prc]

Scotty this test is done not for checking pressure with stand strength, but as a leak test and IEC calls it as Leak test or Tightness test (clause 11.8 of IEC 60076-1)

IEC has other tests also for the points raised by scotty.

Clause 11.9- Vacuum deflection test
Clause 11.10 -Pressure deflection test
Clause 11.11- Vacuum tightness test at site

 

[Jebb]

If any tests are done on site, be sure to use caution if the transformer contains a separate tap changer compartment in the main tank. It is possible to rupture the tap changer with pressure (or vacuum) if it is not applied to both at the same time.