Before the 1960s, dry-type transformers mainly used B-class insulation in open-ventilated designs, with the product model designated as SG. At that time, foil coils were not yet available, so the low-voltage coils were mostly multi-strand wound in layers or spiral configurations, and the high-voltage coils were of the disc type. The conductors used were double glass-fiber-wrapped wires or single glass-fiber-wrapped wires with alkyd enamel coating. Most other insulation materials were phenolic glass fiber materials. The impregnation process involved using B-class insulation varnish to impregnate the high and low-voltage coils under ambient temperature and pressure, followed by medium-temperature drying (with drying temperatures not exceeding 130°C). While this type of dry-type transformer represented a significant improvement in fire resistance over oil-immersed transformers, its moisture and pollution resistance performance was concerning. Consequently, production of this type of transformer has ceased. Nevertheless, the successful design of electrical, magnetic, and thermal calculations and structural layout provided a solid foundation for the subsequent development of new H-class insulated open-ventilated dry transformers.
In the United States, certain transformer manufacturers, such as FPT Corporation in Virginia, developed a dry-type transformer that uses DuPont's NOMEX® aramid as the primary insulation material. FPT's product comes in two models: FB type with an insulation system rated at 180°C (H-class) and FH type with an insulation system rated at 220°C (C-class), with coil temperature rises of 115K (125K in China) and 150K, respectively. The low-voltage coils use foil or multi-strand layer windings, with turn-to-turn and layer-to-layer insulation made of NOMEX®. The high-voltage coils are disc-type, with conductors wrapped in NOMEX® paper. Instead of using conventional spacer blocks between coil discs, comb-shaped spacers are employed, effectively halving the peak voltage between discs and significantly enhancing the axial short-circuit resistance of the high-voltage coils, though this increases winding difficulty and manufacturing time. The high and low-voltage coils are concentrically wound to enhance mechanical strength. Some designs also use NOMEX® insulation boards as spacers and blocks. The insulation cylinders between high and low-voltage coils are made from 0.76mm thick NOMEX® paperboard. The impregnation process involves multiple cycles of vacuum pressure impregnation (VPI) and high-temperature drying (with drying temperatures reaching 180-190°C). At FPT, this type of transformer is manufactured up to a maximum voltage of 34.5kV and a maximum capacity of 10,000kVA. This technology has been UL-certified in the U.S.
In China, some transformer manufacturers have adopted NOMEX® insulation materials and relevant manufacturing specifications from DuPont (such as HV-1 or HV-2) and Reliatran® transformer technical standards to produce H-class insulated SG-type dry-type transformers, similar to FPT’s FB type. However, unlike FPT, domestic products do not impregnate the entire transformer body, only the coils. Although impregnating the transformer as the whole body provides better overall sealing, it is less aesthetically pleasing and requires that all relevant product testing be completed before treatment. Additionally, the impregnating varnish can easily become contaminated, making the coil-only impregnation process more reasonable in China.
In Europe, the development of dry-type transformers has been more diverse. In addition to epoxy resin vacuum casting and winding processes, other types of transformers similar to China's SCR-type non-cast solid insulation encapsulated transformers and SG-type open ventilated dry transformers have been developed. In the 1970s, a Swedish factory used NOMEX® insulation material to develop open-ventilated dry transformers. Later, another factory used glass fiber and DMD to replace NOMEX® insulation material, reducing material costs. The coil structure was similar to the early B-class insulated products, with multi-strand or foil-wound low-voltage and disc-type high-voltage coils. Turn insulation was made of glass fiber, and spacers were made of ceramic. Other insulation parts used modified diphenyl ether resin glass cloth laminates (cylinders) or modified polyamide-imide laminated glass cloth boards (cylinders), DMD, SMC, and other materials. The coil processing method was VI vacuum impregnation, without pressure application during impregnation. The technical keys to this process were the reasonable selection of impregnation varnish (resin) and impregnation process and the production of ceramic parts. Ordinary ceramics are brittle, unglazed ceramics that are prone to moisture and can crack under uneven stress or heat. Therefore, they must have very high density and hardness, which currently can only be achieved through imports.