It is recommended to add oil to transformers through transformer oil treatment systems. 110 kV transformers are filled with oil at the temperature above 10oC with no vacuum. The oil purification system is connected to a tap in the bottom of the transformer tank, while the air outlets in the lid are opened. Transformer oil tank is filled at the rate of approximately 1.5 – 3 tons per hour until oil is visible in the openings of the plugs, at which time the plugs are closed and tightened. If the transformer is equipped with an expansion tank, filling continues until oil level reached the markings in the oil indicator of the expansion tank. The oil is allowed to settle for at least 12 hours, then the plugs are again opened to let the accumulated air escape. 150 – 500 kV transformers with no air tight oil protection are filled with oil at the temperature of 40 – 50oC with residual pressure in the tank of no more than 665 Pa. After vacuum is reached in the transformer, the vacuum pump is not stopped; the oil is injected into the transformer tank through an oil purification system via the vent in the transformer oil tank lid at the rate of no more than 3 tons an hour.

Entering the core, the oil is sprayed, which help to remove moisture and gas. The corresponding residual pressure must be maintained throughout the filling process.

Pressure filters and centrifuges are not recommended for the process of adding oil to the transformer under vacuum, since these systems are connected to the atmosphere.

The oil hose must be oil tight and be able to withstand complete vacuum. Oil level in the transformer is usually controlled by temporary oil level indicators made of glass tubes connected to the top and the bottom parts of the transformer. It is, however, recommended to control the level with two vacuum meters. One vacuum meter is connected to the empty space above the oil, while the other is installed in the bottom valve of the transformer. The height of oil column in meters above the point of installation of the lower vacuum meter is easily calculated with this formula: H = (p1 – p2)/0.9, where p1 and p2 are the indications of the vacuum meters in Pa.

For practical calculations, oil density is assumed to be 0.9 x 103 kg per m3. Oil is added under vacuum to the level of 150 – 200 mm from the top of the lid, until all core insulation components are covered with oil. Then oil flow is stopped, and the space above the oil surface is vacuumed for 10 hours. Then the vacuum pump is stopped, and air is allowed into the space above the oil through an air drier. The core is left with atmospheric pressure for 5 hours. Oil filling is followed by adding more oil through a connector for oil adding installed in the expansion tank. Oil is added to the level indicated by the marks in the extension tank without vacuum after instllation of all oil filled components (coolers, exhaust etc) and installation of the expansion tank. After 12 hours, air is again allowed to escape from all plugs in the lid or in the components. The process of adding oil to transformers delivered with a partial oil load is similar, if the oil has not been drained entirely during installation. Before filling oil into transformers equipped with membrane oil protection, a gas relay with a stop valve and other components must be installed on the lid; this will require venting of the tank. The transformer is filled with degassed oil under residual tank pressure, through a slide valve in the lower part of the tank to 100 – 200 mm below the lid of the tank. Flow rate of the oil is unlimited. The space above the oil is vacuumed under the corresponding pressure for 2 hours. When expansion tank is installed with the flexible gas relay shell and connecting pipelines with stop valves, oil is added to the expansion tank to the highest possible level. The cutoff valve and the stop vent between the gas relay and the expansion tank are manually opened, the connecting pipelines are filled with oil, and the air is released through the relay’s air release vent. The stop valve separating the gas relay from the tank is then closed, and oil from the expansion tank is added to the main transformer tank. If necessary, more oil is added to the expansion tank. The required oil level is set in the expansion tank and then air discharge plugs on the tank and components are open and the remaining air is released.