Dry-type transformers are all transformers that do not contain liquid insulating materials. The dissipation of the heat loss is done by self-cooling or by additional fans and cooling devices..
A transformer consists of a magnetic circuit, this is called the core, and has at least two windings with a fixed number of turns through which current flows. The windings facing the electrical voltage (line voltage) are called the primary side (primary coil), and the side with the load and electrical load is called the secondary side (secondary coil). The manufacturing technique for the core and the quality of transformer core used affects the magnetic circuit. The magnetic circuit (magnetic field) should ideally produce low eddy current losses and have low remagnetization losses (hysteresis losses).
Another aspect is the resistances in the winding. Only with layered and ordered windings on the primary coil and the secondary coil and the best winding metal can the winding losses be reduced. The voltage is controlled with the number of turns on the coil. The current determines the diameter of the winding metal.
Copper has the best conductance except for silver with γ = 56. Aluminum, on the other hand, has only γ = 36. So aluminum follows with about 35 percent gap. Thus, copper is 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 of electricity, and alloys generally have considerably lower conductivity than pure metals. Silver or gold are ruled out altogether because of their high price.
The core consists of electrical sheets insulated on both sides and stacked one on top of the other. Insulation of the coil from the core is provided by sufficient air gap. Insulation between the top and bottom voltage is provided by insulating material or/and by large air gap. A vertical air flow along the coil surface and in the space between the inserted cooling channels in the coils ensures the necessary dissipation of the heat loss. Due to convection, the required air flow is generated by itself (this is referred to as AN – Air Natural cooling) or it is additionally boosted with fans (AF – Air Forced cooling).
Dry-type transformers are always used when oil-filled transformers cannot be used due to increased fire load or increased danger to nature and water, or may only be put into operation with elaborate safety measures. Therefore, the use and operation of dry-type transformers is environmentally friendly, but in terms of thermal properties, this technology is at a disadvantage to oil-filled transformers. The windings are insulated with dry insulating materials, and in the case of larger sizes, usually by the cast-resin method. Due to the poor dissipation of heat, cast-resin transformers may only be manufactured for self-cooling up to a power of 40 MVA, and above that only with additional fans.
Dry-type transformers are used up to the power range of 40 MVA in the low-voltage and medium-voltage grid primarily as distribution transformers and power transformers (power distribution and power supply) 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. Since insulation is provided by the surrounding air in these transformers, the distances between winding and core and between high voltage and low voltage must be increased for large installation heights (> 1,000 m according to the standard) to compensate for the dielectric strength of the air, which decreases with pressure.