Oil pillow is a type of oil storage device used in transformers. Its function is to allow excess oil to flow into the oil pillow when the transformer's load increases and the oil temperature rises, causing the oil in the tank to expand. On the contrary, when the temperature decreases, the oil in the oil pillow will flow into the oil tank again, playing an automatic role in adjusting the oil level. That is, the oil pillow plays a role in storing and replenishing oil, ensuring that the oil tank is filled with oil. At the same time, due to the installation of oil pillows, the contact surface between the transformer and the air is reduced, and the moisture, dust, and oxidized oil dirt absorbed from the air are deposited in the sedimentation device at the bottom of the oil pillow, greatly slowing down the deterioration rate of the transformer oil.

Structure of oil pillow: The main body of the oil pillow is a cylindrical container welded from steel plates, with a volume of approximately 10% of the fuel tank capacity. The oil pillow is horizontally installed at the top of the oil tank, and the oil inside is connected to the transformer oil tank through the gas relay's connecting pipeline, allowing the oil level to rise and fall freely with temperature changes. Under normal circumstances, the lowest oil level in the oil pillow should be higher than the raised seat of the high-pressure casing. For casing with a connected structure, the lowest oil level in the oil pillow should be higher than the top of the casing. A glass oil level gauge (or oil level gauge) is installed on the side of the oil pillow, which can observe the changes in the oil level inside the oil pillow at any time.

There are three forms of transformer oil pillows: corrugated, capsule, and diaphragm.

1. Capsule type oil pillow, with rubber capsules inside to separate transformer oil from the external atmosphere and provide space for thermal expansion and contraction of transformer oil.

Install an oil resistant nylon rubber diaphragm bag inside the oil pillow, with the inside of the diaphragm bag connected to the atmosphere through a moisture absorber (respirator) and the outside in contact with insulating oil. When the oil temperature in the transformer oil tank rises and expands, the oil level in the oil pillow also rises, and the diaphragm bag exhausts outward. On the contrary, when the oil level inside the oil pillow drops, the diaphragm bag sucks in air from the outside, automatically balancing the pressure on the inside and outside of the bag. During the suction and exhaust process of the oil pillow, the insulating oil does not come into direct contact with the atmosphere. In addition, the connecting pipe between the oil pillow and the explosion-proof cylinder has been removed, reducing the oil level of the explosion-proof cylinder to the lowest oil level of the oil pillow (or removing the explosion-proof cylinder and replacing it with a pressure release device). In this way, a pressure oil bag made of oil resistant rubber film installed at the bottom of the oil pillow separates the oil pillow from the oil displayed on the oil label. The pressure oil bag often bears outward pressure to prevent atmospheric infiltration into the fuel tank, and the fuel tank becomes completely sealed. The structure of diaphragm type sealed oil pillow is basically the same as that of ordinary oil pillow. If the diaphragm bag cracks, the sealed oil pillow will become a regular oil pillow in operation.

2. Diaphragm type oil pillows use rubber diaphragms to separate transformer oil from the external atmosphere and provide space for thermal expansion and contraction of transformer oil.

The diaphragm type oil pillow consists of two semi cylindrical bodies, with a semi-circular thin film inside the cabinet. The film is fixed around the edge of the cabinet and floats on the oil surface, separating it from the atmosphere as the oil surface changes. For diaphragm type oil pillows, a magnetic oil gauge can be installed, and the roller of the oil gauge linkage mechanism is not subject to any resistance on the film, allowing for free and flexible extension and contraction. The upper part of the magnetometer has a junction box and a switch inside. When the oil level of the oil pillow reaches the highest or lowest position, the switch automatically closes and sends an alarm signal.

3. Ripple type oil pillow, a metal expander composed of metal corrugated sheets, separates transformer oil from the external atmosphere and provides space for thermal expansion and contraction of transformer oil.

The corrugated oil pillow is divided into two types: internal oil type and external oil type. The internal oil type has better performance but larger volume. Adopting advanced stainless steel corrugated tube compensation technology, replacing the capsules in old-fashioned oil pillows, to achieve volume compensation of transformer insulation oil and isolation from the outside world, preventing moisture absorption and oxidation of insulation oil. When the volume expands due to heating, the corrugated tube is compressed and moves towards the fixed end; When the oil level is too high, the bellows will compress to a certain extent and trigger an alarm; When the volume of insulation oil shrinks due to cooling, the corrugated tube elongates on its own under atmospheric pressure. The corrugated oil pillow can be divided into two types: external oil type and internal oil type.