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Transformer Vacuuming and Transformer Oil Filling

Transformer Vacuuming and Transformer Oil Filling

1.        Advantages of transformer vacuum oiling

a)        It can greatly reduce the content of oil and water in the transformer, thus greatly enhancing the breakdown strength of the transformer oil, that is, reducing the oil gap distance of the body insulation, leading to a significant reduction in the design cost of the transformer;

b)        Due to vacuum oil injection, the gas content in the oil tank is very small, so the probability of pressure relief valve and gas relay action is reduced, ensuring the normal operation of the transformer in the power grid.

2.        Operating procedures for vacuum oil filling

A.        Vacuum pumping/Transformer Evacuation

1)        Premise:

a)        Vacuum treatment and vacuum oil injection are special processes, which shall be strictly controlled, and process confirmation shall be carried out before process operation, including:

i.              The confirmation criteria that the specified process can meet the requirements;

ii.              Recognition of equipment and personnel qualification;

iii.              The specified procedures and process parameters shall be implemented;

iv.              Records of process parameters shall be maintained.

2)        Vacuum treatment can be carried out only after confirming that the sealing performance of transformer and relevant pipeline system is good. The vacuum treatment pipeline is connected to the valve at the end of the main air header of the transformer, or to the vacuum butterfly valve at the top of the tank cover.

3)        Before vacuum treatment, all butterfly valves at the upper and lower pipes of the cooler (including the plate radiator) shall be opened, and the vacuum pump shall be started to operate for vacuum treatment (see the instruction manual of the cooler).

4)        During vacuum treatment, the ambient temperature is lower than 20 ° C or above (when heating with hot oil circulation, the oil temperature should be 20 ° C, and the body should be heated to 50 ° C-70 ° C), and the vacuum degree should be increased evenly.

5)        In case of rain or fog during pumping or vacuum oiling, it is better to stop pumping and store it with oil (the oil sampling test shall conform to the standard).

6)        When the transformer body is exposed for a long time or for other reasons, the vacuum degree shall be maintained for a long time after vacuumizing. It is considered that there is a trace of moisture, and the time to be maintained again after a sunny day depends on the degree of moisture exposure. Generally, it can last for 8-24 hours to facilitate the evaporation of moisture.

After all accessories of transformer are installed. Open all valves of all accessories and components to connect with the body for vacuum pumping, so that all accessories (including cooler and radiator), except oil conservator and gas relay, are connected with the body. If the oil conservator is designed according to full vacuum, the oil conservator and gas relay will also be vacuumized.

After adding vacuum valve and vacuum gauge at the oil inlet valve on the top of the oil tank, connect the vacuum pipeline, so that before vacuuming the oil tank, close the vacuum valve and vacuum the pipeline, so as to find out the vacuum degree that the vacuum system itself can actually reach. The vacuum degree required for the whole vacuum system (including vacuum pump, pipeline, valve, vacuum measuring instrument, etc.) shall be less than 10Pa. If it is greater than 10Pa, check or repair the vacuum treatment system.

Record the start time of vacuum pumping, the time to check the vacuum degree, the corresponding pressure and the time to reach 133Pa during vacuum pumping. Stop vacuuming when the pressure reaches 133pa, check the vacuum measuring instrument every 10 minutes and record the value P1, and check the reading of the vacuum measuring instrument every 30 minutes and record the data P2. Leakage rate through formula η= ﹤ (P2-P1)xV﹥/T

η Is the leakage rate

V is the volume of transformer oil, in litres

T is the direct time interval between P1 and P2, in seconds

The leakage rate shall be less than 2000pa. L/S. If it is greater than this value, the leakage point of the transformer shall be checked and the leakage shall be remedied. If the leakage rate is less than this, open the vacuum pump to continue vacuumizing. Keep this vacuum degree (133pa) under the condition that the vacuum pump continues to operate. The vacuum shall be maintained for not less than 24 hours. It should be pointed out that if the vacuum pump is stopped to maintain vacuum (static maintenance) after the leakage test, it will not only affect the thorough removal of moisture, but also cause local insulation deep layer to be affected by moisture in case of leakage.

B.        Vacuum oil filling/Transformer oiling

The 220kV transformer must be filled with vacuum oil. Other transformers shall also be filled with vacuum oil when conditions permit. The vacuum oil shall comply with the manufacturer's regulations.

Vacuum Oil Purifier shall be used to inject qualified transformer oil. The temperature of oil injection shall be 40~60 ℃ (outlet temperature of Oil Purifier), but shall not be lower than 10 ℃ and higher than 70 ℃. The oil filling speed depends on the vacuum degree, which is generally (3-6) t/h (it is better not to exceed 6000 liters/hour to prevent the oil flow from being electrified). If the vacuum degree decreases, the oil filling speed shall be appropriately reduced. Stop oil injection when the oil level is 100-200mm from the tank top, and continue to maintain vacuum. 220KV transformer shall not be less than 4 hours.


i.              Before oil injection, the transformer oil shall pass the test;

ii.              The residual oil in the transformer shall be drained as far as possible to avoid the decline of new oil quality;

iii.              When filling the transformer with oil, it is forbidden to mix dust, water, moisture and other dirt into the transformer through oil;

iv.              The main body of the transformer shall be in vacuum state during oil injection;

v.              Vacuum oil purifier shall be selected for oil injection equipment. It is prohibited to directly inject oil into transformer with gear oil pump;

vi.              The oil injected into the transformer should preferably be the new oil or operating oil of the same brand as the oil in the original equipment, and its indicators should meet the standards; (new transformer oil also should be purified before filling into transformer)

vii.              Try to control the oil flow speed during oil injection;

viii.              There must be a specially assigned person to monitor the oil level and prepare for the corresponding exhaust.

3.        Operation Guide for Transformer Vacuum Filling

During oil filling, the vacuum pump will continue to operate, and the valve to the oil tank will also be the same as that during vacuum pumping, so that all parts can be vacuumized together with the body. The transformer oil shall be pumped with a Vacuum Oil Purifier. During vacuum oil filling, oil shall be fed from the oil filling valve at the lower part of the oil tank, so that the oil overflows from the outside of the coil to the inside of the coil, which can reduce the opening force of the oil on the coil screen. To prevent pumping oil into the vacuum pump, close the vacuum valve and stop pumping when the oil is injected to the oil level about 200~300 ㎜ from the inlet of the vacuum pump. However, the vacuum oil purifier still does not stop pumping oil until the oil level approaches the place where the gas relay is installed. Then fill the oil higher than the position required by the oil level curve, and then deflate each accessory (bushing, switch, gas relay, conservator, etc.) to the standard position. For transformers with conservator for vacuum pumping, oil shall be injected directly to the standard oil level.

Hot oil circulation: open the butterfly valve between the oil tank and the oil conservator, extract the oil from the lower part of the oil tank, heat it to 65 ± 5 ℃ by the vacuum oil purifier, and then return to the oil tank from the upper part of the oil tank. The oil inside the transformer oil tank shall be circulated at least twice.

PureTech manufactures Vacuum Oil Purifier and Vacuum Pumping Units for transformer maintenance and installation work. We have Double-stage high vacuum transformer oil purifier and Single-stage vacuum Transformer Oil Purifier, flow rates ranges from 600 liters/hour to 18000 liters/hour which can be selected as per user’s needs. Double-stage high vacuum Transformer Oil Purifier also has the function of transformer evacuation.

More information about transformer oil filtration, transformer vacuuming/transformer evacuation, transformer oil testing, etc. please don't hesitate to contact PureTech Oil Filtration.


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