Dry-type transformer

Dry-type transformers are all transformers that do not contain any liquid insulating materials. The heat loss is dissipated by self-cooling or by additional fans and cooling units.

A transformer consists of a magnetic circuit, known as a core, and has at least two current-carrying windings with a fixed number of turns. The windings facing the electrical voltage (mains voltage) are referred to as the primary side (primary coil), the side with the consumer and the electrical load is referred to as the secondary side (secondary coil). The manufacturing technology for the core and the quality of the transformer core used have an effect on the magnetic circuit. Ideally, the magnetic circuit (magnetic field) should generate low eddy current losses and have low remagnetization losses (hysteresis losses).

Another aspect is the resistance in the winding. Winding losses can only be reduced with layered and ordered windings on the primary and secondary coils and the best winding metal. The voltage is regulated by the number of turns on the coil. The current strength determines the diameter of the winding metal.

With the exception of silver, copper has the best conductivity value with γ = 56, while aluminum only has γ = 36. Aluminum follows with a gap of around 35 percent. Copper is therefore the best metal and aluminum “only” the second best of the technically and economically usable conductor materials for electrical energy. All other metals cannot be considered as conductors, and alloys generally have a considerably lower conductivity than pure metals. Silver or gold are ruled out completely due to their high price.

The core consists of electrical sheets insulated on both sides and stacked on top of each other. The insulation of the coil to the core is ensured by a sufficient air gap. The insulation between the upper and lower voltage is ensured by insulating material and/or a large air gap. A vertical air flow along the coil surface and in the space between the cooling channels in the coils ensures the necessary dissipation of heat loss. Due to convection, the required air flow is generated automatically (this is referred to as cooling type AN – Air Natural) or it is additionally amplified with fans (cooling type AF – Air Forced).

Dry-type transformers are always used when oil-filled transformers cannot be used due to increased fire load or increased risk to nature and water or may only be put into operation with extensive safety measures. The use and operation of dry transformers is therefore environmentally friendly, but in terms of thermal properties this technology is at a disadvantage compared to oil-filled transformers. The windings are insulated with dry insulating materials, usually using a cast resin process in the case of larger construction capacities. Due to the difficulty in dissipating heat loss, cast resin transformers may only be manufactured for self-cooling up to an output of 40 MVA, and above that only with additional fans.

Dry-type transformers are mainly used as distribution transformers and power transformers (energy distribution and power supply) in the low-voltage and medium-voltage grid up to a power range of 40 MVA and are limited to a maximum operating voltage of 36 kV. The limitation of the power range is necessary because the insulating media used have a lower dielectric strength than transformer oil. For outdoor installation, a housing must always protect the transformer against moisture and dirt. As the insulation of these transformers is provided by the surrounding air, the distances between the winding and the core and between the high voltage and the low voltage must be increased at high installation heights (according to the standard > 1,000 m) in order to compensate for the dielectric strength of the air, which decreases with the pressure.