Transformer conservator tanks

Author: Lawrence Kirchner


What are they for?

The transformer conservator tank acts similarly to the overflow tank on the radiator of a car. As the transformer load cycles, so does the oil temperature. All fluids, including transformer oil, change volume when the temperature changes. Common transformer oil changes volume by 0.07% per degree Celsius. For example, 12,000 gallons of transformer oil at 25°C would expand to approximately 12,420 gallons at 75°C. Equation:((75-25) x 12000 x 0.0007pu))+12000. This extra 420 gallons must be temporarily stored somewhere until the time when the transformer load drops and the oil contracts.

This explains why the Liquid Level Gauge mounted on the conservator tank has markings in Celsius. So, would the following liquid level indication be correct?


This is a trick question. The answer would be “Yes” if the transformer were off line or very lightly loaded in a moderate ambient. The answer would be “The transformer is low on oil” if the transformer was loaded to 100% of nameplate rating during summer in Arizona. The average oil temperature must be considered when evaluating the liquid level. 

Typically, when the conservator is filled to the 25°C mark, the tank is approximately 25-35% full. This leaves enough room for oil expansion when the transformer is operating at full load and the average oil temperature is in the range of 70-90°C. Inversely, there would be enough oil in reserve to satisfy the conditions when the transformer is taken off line in an ambient well below freezing.

The bladder and liquid level gauge

Modern conservator type oil preservation systems incorporate an oil resistant bladder (bag) to separate the insulating oil from the outside air. Transformer oil is hygroscopic and therefore tends to absorb humidity. It is imperative that the oil remain dry to maintain necessary electrical properties. As the oil level in the conservator tank rises and falls, outside air is expelled or taken in. The bladder conforms to the space in the tank not occupied by the oil. The bladder floats on the oil. The liquid level float assembly rides up under the bladder as the oil rises and falls.


The bladder life is typically 15-20 years due to dry rot. Keeping the air dry inside the bladder will extend the bladder life. This is commonly done by connecting the bladder breather tube to a desiccant breather.

Confirming correct liquid level

There are two field methods commonly used to confirm that the actual oil level in the conservator agrees with the gauge reading and average oil temperature. 

  1. A measuring device or stick can be gently inserted into the bag to measure the distance from the top of the tank to the oil level. The manufacturer of the transformer can provide data to confirm the proper level given the average oil temperature. This method is also helpful in determining if the bladder is leaking. If the bladder is not leaking, there should be no oil on the measuring stick/device.
  2. A clear hose can be connected to the bottom of the conservator (or main tank). The open end shall be raised above the top of the conservator tank. Consult manufacturer’s data.

Tank pressure(s)

The conservator is physically elevated above the main tank to provide necessary head pressure to assure a positive main tank pressure regardless of temperature. A slight positive pressure on the main tank assures that water or air will not be drawn into the transformer in the event of a leak.



  1. Not all conservator tanks are physically braced for vacuum. Consult manufacturer before pulling vacuum.
  2. It is often necessary to slightly pressurize the bladder to bleed air from between the bladder and the tank. Do this with caution and do not exceed +2 psi. Over pressurizing the bladder without first closing the valve between the conservator and the main tank could result in the main tank Pressure Relief Device activating and creating an oil spill.
  3. Draining a transformer without first opening the equalizing valve between the conservator tank and the bladder will result in sucking the bladder into the piping to the main tank. This often results in severe damage to the bladder. This could also happen as a result of not enough oil in the transformer, or in the event of a major main tank leak.