Transformer Drying

Power transformers are an essential part of the electrical grids which are used for the transfer of electrical energy produced by generators of power stations to industrial and private consumers.

The electrical efficiency and reliability of the transformers are critical for the stability of the electrical grids. For the insulation of high voltage power transformers and medium size distribution transformers, mineral oil and cellulose materials are commonly used. Due to their hygroscopic properties, transformer oil and cellulose based materials (thermally upgraded kraft paper or transformer board) absorb moisture from the environment during the manufacturing process. Moisture in the oil and paper reduces the dielectric properties of the transformer dramatically and must be removed before commissioning of the transformer. This drying process is performed under vacuum, during several steps of the transformer manufacturing cycle.

Application requirements

  • High peak pumping speed
  • High water vapor pumping capacity
  • High resistance to corrosion
  • Customized solutions with accessories for increased lifetime

Process flow of a transformer production

Process flow of a transformer production

Overview about vacuum applications during the transformer production process

Production stepProcess
Drying of single winding/winding block    Hot Air drying, Vapour Phase drying
Active part dryingVapour Phase drying
Evacuation of transformer tankAfter tanking of active part and before oil filling
Oil purificationDegassing and de-moistening of transformer oil

Hot Air drying
The Hot Air drying process is mainly used for drying of high voltage transformers windings or for drying od the active part of smaller distribution transformer. Process consists of hot air circulation followed by pumping down of the heating chamber to a pressure below < 1hPa. The design criteria of the pumping speed depends on the weight of the insulation material. As a common rule, average pumping speed of 1,000 m³/h is required for every ton of insulation material to be dried.

Vapour Phase drying (VPD)
Forty years ago, Pfeiffer Vacuum was one of the leading companies in the development of this technology so we have acquired a wide experience. VPD process operates under vacuum in the absence of oxygen, in the range of 0,1 to 0,2 hPa. Heat transfer to the active part is achieved by solvent vapour. The solvent is evaporated and injected in the heated chamber, and condenses on the surface of the active part. The heat transfer of the condensing vapour is dramatically higher than the heat transfer by forced convection. As a consequence the total drying time of the active part is more than 50 % faster in comparison to the Hot Air drying process.

Evacuation of Transformer Tanks
Typical transformer final assembly can take up to 18 hours. Even if the active parts have already been dried, during this time, a re-moistering of the insulation material occurs at the surface. Putting the transformer tank under vacuum (average pressure of 1 hPa) is sufficient for removing the surface moisture. Furthermore, transformer tank pump down is used to evacuate any remaining air pockets that could be present in dead volumes of the transformer.

Oil purification
Transformer oil is essential for the proper functioning of a transformer and is used to improve the dielectric characteristics of the insulation material by means of impregnation in order to dissipate the heat generated by the electrical transformation process.

For degassing and drying purposes, oil is heated up to about 60°C and sprayed into a vacuum tight vessel (degasser) with packing material. At a pressure level in the range of 1 to 2 hPa, water vapor and dissolved gases are evacuated by the pumping unit.

Product portfolio
A typical vacuum pump unit for transformer drying consists of one or several stages of Roots-type blowers and a primary pump.

For hot air drying plants, a two-stage Roots pumping station is usually used, either based on a rotary vane pump (CombiLine WU) or a dry screw pump (CombiLine WH). The required pumping capacity usually varies from 1,000 to 4,000 m3/h, depending on the size of transformer. VPD plants typically operate at low pressure, so three-stage Roots pumping stations with typical pumping speeds of 8,000 to 12,000 m3/h are generally used. For the evacuation of the transformer tank, a mobile two-stage vacuum unit is used. We strongly recommend the use of a screw pump as a backing pump in combination with the HiLobe, for lower operating costs and minimal space requirements (CombiLine RH).

For the oil purification process, a single-stage rotary vane pump from our HenaLine series may suffice. If a higher pumping speed is required, our CombiLine WU series is the perfect choice.

For all transformer drying applications, we can also provide customized Roots pumping stations adapted to your specific needs. Please contact our experts!

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