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Determining Desiccant Breather Sizing for Electrical Transformers

During the last 100 years, technical knowledge made huge improvements, nevertheless most of desiccant breathers, use on electrical transformers, have the same design than in beginning of the past century.
While many in the field may think it is a simple matter of determining the dielectric oil volume and whether the breather in question needs a high airflow capacity.


In electrical transformer applications, the size of the desiccant breather depends principally on the volume of the conservator air reservoir - not the volume of the oil reservoir. Desiccant breathers are installed on the transformer's conservator air reservoir, which is frequently situated above the oil reservoir.

The conservator air reservoir is usually large enough that expanding dielectric oil will not reach the total volume.

For example: if the volume of the conservator air reservoir is 4,000 litres, the expansion of the dielectric oil will never reach the total volume of the conservator air reservoir.

Temperature Fluctuations

Temperature fluctuations affect the air volume in the conservator air reservoir, which affects one's choice of breathers.

For example: the working temperature of an electrical transformer may fluctuate between an average ambient temperature of 20ºC to a maximum average temperature in a load condition of 80°C. It takes a minimum of 48 hours to reach the maximum temperature from the ambient temperature. Also take into consideration how many times a day the transformer is off and for how long; in this case, once a day for a few hours. It is realistic to consider that the average maximum daily temperature variation will be about or less than 30°C, which will not greatly expand the oil in the oil reservoir but will have a direct impact on the volume of air in the conservator air reservoir.


The maximum volume of air, both inhaled and exhaled from the transformer, is the total volume of the conservator air reservoir.

To determine airflow, we divide the total volume of the conservator air reservoir over the operating time.

Using our previous examples, we have to divide the volume of the conservator air reservoir by 6 hours and you will see that, in all cases, the airflow is lower than 10 l/mn.

Note: On a Des-Case standard desiccant breather used on electrical transformer, it is necessary to open two vent holes (at 180° from each other). Those two holes allow a maximum airflow of 150 l/mn, or 15 times the airflow needed by most of existing electrical transformers.

To calculate which Des-Case breather would work best to protect your electrical transformer, please open those files:
Oil volume variation on electrical trasformers
.pdf Pour ouvrir ce fichier vous aurez besoin deInstaller Foxit pdf ReaderInstaller pdf Acrobat Reader Taille : 180.83 Ko
Calculation Table for electrical transformer applications
Format .xls Pour ouvrir ce fichier vous aurez besoin deMicrosoft Ecxel® Taille : 22.53 Ko