Cartridges for transformer dehumidification

Wednesday March 22nd, 2017

Moisture greatly effects the life expectancy of an electric transformer: it not only deteriorates the dielectric properties of insulating materials (paper and oil) but also reduces the resistance to oxidation (aging) and the mechanical strength of the paper. That is why it is recommended to maintain moisture levels of the insulating system to a minimum.

Description and functioning principle

The on-line dehydration cartridges remove water from the oil and the cellulose material whilst the transformer is in operation.

The use of the device slows down the cellulose insulation’s aging and degradation, improves the dielectric properties of the oil and therefore increases the transformer’s reliability.

The device is connected hydraulically to the transformer case; A hydraulic pump makes the oil circulate through a bed of molecular sieves that attract and retain the water dissolved in the oil. The reduction of the water content in the oil causes the desorption of the water contained in the cellulose insulation, as it “migrates” to the oil in order to replenish the hydrostatic equilibrium. In this way the water is gradually extracted from the cellulose and adsorbed by the molecular sieves.

The cartridges are designed to create a slow, gradual, and non-invasive dehydration process, therefore it is also suitable for very old equipment and featuring particularly fragile solid insulators; aiming to extend the equipment’s residual thermal life and reduce the risk of untimely failure.


The dehydration system can remove about 10 liters of water. Absorption time, and so the efficiency of the device, varies depending on the hydration level (water content) and the transformer’s operating temperature.

  • Applied to a device with a hydration level of 4% in the cellulose, and operating at high temperatures in a particularly humid environment, the system reaches saturation in about 6-9 months.
  • Applied on a device with a hydration level of 1% in the cellulose, and operating at 50 ° C, the TRASEC CL3A system can last up to 24 months before saturation.
  • On a new, dry appliance (hydration < 0. 5%), the system can last up to 5 years before saturation.

Installation and commissioning

The oil suction pipe is connected to the lower part of the case or to the oil’s required cooling circuit; the delivery pipe is connected to the upper part of the case or the required cooling circuit.


The dehydrating device absorbs the water dissolved in the oil until it is saturated. The cartridges can be inserted with two sensors to measure the concentration of capacitive equivalent water dissolved (CL3AM).

The level of dissolved water contained in the oil can be viewed on site or on a remote display by transmitting the sensors‘ output signal. Alternatively, the performance of the system can be measured by indirect periodical monitoring (by means of on-line senors or by laboratory testing) the concentration of the water dissolved in the oil within the transformer casing.

The picture on the left shows the dehydration cartridges with IP65 steel box joined to the frame, which houses the monitoring system. The local video display is housed inside the metal box. The device is equipped with a visual indicator stating the flow of oil in circulation.

Replacing and regenerating the molecular sieves

After reaching the saturation point, the cylinders containing the molecular sieves must be replaced and sent to be regenerated. Sea Marconi staff or the user himself can replace the cylinders. During regeneration, the weight of the molecular sieves is compared to their original weight (dry material); the weight difference corresponds to the amount of absorbed water.

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