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A transformer oil pillow, also known as an expansion tank or breather, is a small auxiliary reservoir connected to the main transformer tank through a pipe or a breathing apparatus.

1. Its primary function is to accommodate the expansion and contraction of transformer oil due to temperature changes. As the oil volume increases with rising temperatures, it flows into the pillow; when temperatures drop and the oil contracts, it is drawn back into the main tank.

2. The oil pillow also helps maintain a positive pressure inside the transformer, preventing the ingress of moisture and contaminants from the atmosphere. It may contain a desiccant breather to filter incoming air and keep the oil dry and clean.
   

3. In addition, oil pillows are transparent or have level indicators, allowing operators to visually monitor the oil level and condition. This can provide early warning signs of leaks, oil contamination, or other issues that might require maintenance.
   

In summary, a transformer oil pillow is a critical component that ensures the proper functioning and longevity of power transformers by managing oil volume changes, preserving oil quality, regulating pressure, and facilitating visual inspection.