E+C August 2018

TRANSFORMERS + SUBSTATIONS

T he primary factor differentiating these units from the more common liquid-filled transformers is that their electrical core and coils are cooled by normal air ventila- tion, rather than by oil, silicone or other liquid; ducts in the windings allow heat to be dissipated into the air. The construction of dry-type transformers therefore differs considerably from liq- uid-filled units, and separate industry standards apply to the ways they are designed, built and maintained. Containing no oil means higher levels of safety, making dry-type transformers more suitable for indoor, underground and marine applications, but they can operate in a range of hostile outdoor environments as well. They do not require fire-proof vaults, catch basins or facilities to vent toxic gases, and can be easily installed within buildings in close proximity to the load itself; this improves the overall regulation of the system and reduces costly losses in the secondary lines. As a key component of the electrical distribution system, a dry-type transformer must be carefully specified and chosen according to its capacity (its kVA rating), its voltage rating (primary and secondary voltage of the transformer), and its insulation system, which must accommodate the maximum ambient temperature, plus the average wind- ing temperature rise, plus the differential between the average winding temperature rise and the highest temperature of the winding. Voltage ratings There are numerous specifications in which dry-type transformers can be sourced. Large units – commonly available in sizes from 500 kVA to 10 MVA – are usually fed by medium voltage power systems between 3,3 kV and 33 kV, with secondary voltage ratings of 400 V, 550 V, 690 V or 720 V three phase. Insulation methods As the windings of dry-type transformers use air for cooling, there are several treatment techniques to insulate and protect the windings from dirt, moisture, corrosive fumes and conductive dust. Each construction method is suited for particular applications or operating environ- ments; the main types of insulation are open-wound, vacuum pressure impregnated (VPI) and cast coil. Open-wound transformers – open to help ensure penetration of the varnish – are constructed using a dip-and-bake method, in which the conductor coils are pre-heated, The use of dry-type transformers has, in recent decades, extended be- yond niche applications into more universal use, with benefits ranging from safety and reliability to low installation costs and no oil-spillage risk. But what makes a dry-type transformer so different, and what types are available? Understanding dry-type transformers David Claassen, Trafo Power Solutions

Electricity + Control

AUGUST 2018

17