Oil type Transformer had come to existence since 1892 or more than 100 years ago. In the beginning, the Oil type Transformers were "Open Tank" type. It had air inlet and outlet for the expansion of the oil volume. The Oil volume goes up and down according to the Temperature of the Oil. We can say that the Transformer is "Breathing".
The Open Tank type Transformers were improved in time. A small oil reserved tank is connected above the Transformer Main Tank. This is called Conservator type Transformer. In order to prevent the danger of moisture and oxygen come in contact with the oil, "Silica Gel", desiccant agents are connected to the conservator tank.
Oil will be filled in the Main Tank and outflow into the conservator tank. The oil level will not be more than half of the conservator tank.
When Transformers are energized, the temperature of the windings will increase. The surrounding oil will be hot and oil volume will increase according to the increasing temperature. The oil will expand at the maximum of 7% of the total oil volume in the tank. The air in the conservator tank will be pushed out to the atmosphere by the increasing volume of oil.
When the transformer cooled down, the volume of oil will decrease. The outside air will be sucked into the conservator to balance the pressure. During the cool down (by the decreasing of the load,
or the cooler ambient temperature, or by rain water), the moisture in the air will enter into the conservator. We need the Silica Gel to help prevent the moisture enter into the transformer, but cannot prevent totally.
For Conservator Type Transformer, we recommend to test the oil at least once a year. The best time to get the oil sample for testing is after the Rainy Season.
In order to prevent moisture entering into the transformer totally, a Rubber Bag (Rubber Air Cell or Rubber Diaphragm) will be placed in the conservator tank. This Rubber Bag will act as a partition to prevent the oil come in contact with the air, but it is still flexible for the oil expansion. The Rubber Bag Conservator system usually will be used in large Power Transformer.