Recently, the mica/epoxy insulation, which has been used in rotating machines for over a hundred years, is now being replaced by a new concept using HV cross-linked polyethylene (XLPE) cables. Dryformer is a new oil-free HV transformer based on cable technology first used in revolutionary new generator, Powerformer. Forced-air cooled, it has innovative windings made from XLPE cables with circular conductors.
Dryformer is designed at present for primary voltages of 36 kV–145 kV and power ratings up to 150 MVA. The absence of oil means that there is no risk of ground or water pollution in the event of damage and a less risk of fire or explosion. Therefore, Dryformer can be sited closer to the consumer, for example below ground and in urban or eco-logically sensitive locations. As the electric field is fully contained within the XLPE cable and the cable surface is at ground potential, Dryformer offers unique opportunities for optimizing power transformer design.
By using the state-of-the-art of cable technology, XLPE cable can have electric field strengths upto 15 KV/mm. From a manufacturing perspective, the Dryformer has the considerable advantage of having the insulation system built up at the cable factory (unlike in oil/paper insulation, where a thorough drying out process using a combination of high temperature and vacuum and quick assembly is required).
Following the discovery of high temperature superconducting (HTS) materials in 1986, several studies looked into the feasibility of HTS transformers. Previously developed low-temperature superconductors (LTS) required cooling by liquid helium to about 4.2 K, with advanced cryogenic technology that is expensive both in terms of cost and of refrigeration power expended per unit of heat power removed from the cryostat. The technology for the new materials, which is based on liquid nitrogen (LN2) at temperatures up to about 78 K, is simpler and cheaper, and the ratio refrigeration power used to heat removed is reduced from over 1,000 to about 25.
The feasibility of HTS transformers is also considered from the viewpoints of high performance of the windings and lower cost of refrigeration. HTS transformers have; higher efficiency, emergency overload capability up to twice the normal rating, has lower leakage reactance and improved voltage regulation and reliability. These recent innovations have decreased the need or in some cases completely removed the need for transformer oil, which in the next five years will crucially hamper the demand of these oils as well.
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