Drying transformer windings

During prolonged operation, transportation, or installation, the solid insulation of transformers becomes moistened due to environmental exposure. As a result, the insulation resistance in the windings drops, and power losses and the tangent of the loss angle increase. For this reason, drying transformers is considered a significant and important procedure.

The materials used for insulation are, for the most part, hygroscopic. Thus, paper, by its very nature, possesses a large number of capillaries, which allows it to readily absorb moisture from the air. The oil, which is meant to protect the entire active part of the transformer from potential gas and moisture ingress, also does not provide absolute impermeability. In this way, even the oil-impregnated part of the transformer becomes moistened when exposed to the air.

To prevent malfunctions and accidents, mandatory technical maintenance is carried out on power lines once every six months. Drying transformers is one of its most important stages.
Most often, the choice of method is determined by the production capacities available to the organisation, as well as the rated power and voltage class of the power equipment.

A number of key methods are used for drying transformers:

• the cold vacuum method. This is performed using special equipment equipped with a chamber for freezing moisture out of the oil. During vacuuming, the transformer is heated, but the insulation temperature must not fall below 20 degrees;
• the thermo-vacuum diffusion method. This method is most commonly used to dry the insulation of power equipment with an operating voltage of 110–750 kV. Oil heated to a temperature of 80–85 degrees is directed into the windings. In doing so, it transfers the acquired heat to the insulation. Subsequently, the oil is drained from the transformer and vacuumed for two days;
• oil spraying. For this method, the oil is heated to 80 degrees and sprayed onto the functional area of the power equipment, thereby heating it. This method is quite time-consuming and can take several days depending on the degree of wetting;
• the repeated drying method. The principle of the operation is based on spraying (as in the previous method) heated oil onto the insulation under alternating vacuum conditions. At the same time, dry heated air is periodically introduced into the tank. The procedure is carried out until the insulation temperature rises to 80–90 degrees;
• the hot air drying method for transformers. Air heated to 100 degrees
  is directed at the active part of the transformer, thereby heating the windings and the shunt. This method is used very rarely, however, a hot air blower is still used as auxiliary equipment when blowing through with other methods.

GlobeCore has developed several types of units that effectively combat the elimination of moisture in the active component of a transformer. These are the mobile units “Iney” and “Sukhovey”.
The “Iney” type of equipment is designed for drying and vacuuming the solid insulation of transformers under de-pressurised conditions during their installation or commissioning.
Drying transformers with the “Iney” type unit is carried out in the following manner. A vacuum hose with a diameter of 150 mm is connected to the tank. First, vacuuming is performed using a forevacuum pump, which reduces the air pressure in the tank to 400 Pa. Then, to create a deeper vacuum, a booster injector is used. Before the start of vacuuming, the unit is cooled to –70 degrees. It takes approximately half an hour for the low-temperature trap to reach this level when the ambient temperature is +20 degrees.

The “Sukhovey” assembly is used during the installation, repair, and maintenance of power equipment. As a rule, transformers are delivered to the installation site in a disassembled state, and their assembly and connection to the power grid are carried out there. To prevent moisture from entering the shunt windings in the event of a breach of the transformer’s hermetic seal during its integration into the power line, the “Sukhovey” unit is connected to it. It supplies hot air to the tank, thereby preventing moisture from reaching the windings of the magnetic core. In addition, this assembly is used when oil is drained from the transformer during scheduled or repair de-pressurisation of the equipment.

These two units from GlobeCore – “Iney” and “Sukhovey” – are designed for drying transformer windings. The main difference in their application can be considered the fact that the former is specialised for drying moisture that has previously penetrated the windings of the magnetic core, while the latter is for preventing its entry into the transformer tank during de-pressurisation.