Experience on natural ester filled transformer at cold climate condition

The selection of insulation liquids to be used in power transformers in cold climate region has focused on comparing the pour points and the minimum ambient temperature registered in the location of transformer installation. On that base if the customer specification indicates that the minimum temperature during transformer outage period would be below the pour point of a specified insulation liquid, this liquid is usually excluded from transformer commission. On the other hand, labs experience shows that the liquid could be frozen at a temperature, which is higher than the pour point, usually known as freezing point, and which increase the restrictions for liquid selection. On a real scale of a transformer, the challenge considers the dielectric and mechanical limits of the design. There are three phases the transformer goes through after installation: storage, energizing and loading. The experience shows that the most critical moment of energizing is during solid-liquid transition phase, in which release of bubbles is expected causing partial discharge leads to dielectric failure. Mechanically if the pressure release during de-freezing the liquid exceed the design limitation, mechanical damage is to be expected specially in the cooling system. This identifies that the time to start freezing is a function of cooling temperature and location presented in the ratio of liquid volume to the surface area exposed to ambient. If the storage time could be limited at a certain subzero temperature so that the liquid is not starting to freeze, the energization is safe to start, and no issue are expected. The current research presents testing of natural ester filled transformer for approval of the liquid at -15°C, while it has a pour point at -30°C. Analyzing of the thermal time constant of the liquid during storage and after energizing phase gives more space for the liquid application even under lower temperature and more time for outage periods. Moreover, the liquid temperature distribution profile shows a minimum temperature above zero degrees, which ensure the possibility of operating the transformer at no-load safely, even if the temperature goes down closer to pour point. This state-of-the-art experience proven that challenge of operating power transformer at extreme cold temperature can be handled if all influencing factor are considered and controlled as long as the liquid does not start solidification.