With its Geafol cast-resin transformers, Siemens Power Transmission and Distribution (PTD) is increasingly developing into a specialist for high power ratings in the medium voltage range. By the end of September 2007, the world’s largest cast-resin transformer with a rated power capacity of 40 MVA left the factory in Kirchheim/Teck, Germany. The developers and designers were able to successfully design a cast-resin transformer to this power specification. Two of the new 40-MVA transformers will be used on the experimental and test line for high voltage direct current transmission systems at Siemens PTD in Erlangen.

Comprehensive and elaborate routine testing as well as type and special tests performed on the largest cast-resin transformer to date have shown that in many cases, this transformer variant can provide an alternative to liquid-filled transformers in this power category. The design required a number of special developments. For example, whereas the higher voltage windings of cast-resin transformers of lower power rating are made of one cast, the high voltage winding of the 50-metric ton transformer, which has a rated power capacity of 40 MVA, each consists of six coil sections which are connected together to form one winding. This winding has special cooling ducts designed to dissipate heat.

Compared with oil-insulated transformers of the same power capacity, the advantages are that Geafol cast-resin insulated transformers are practically maintenance-free as well as being flame retardant and self-extinguishing, they can be recycled at relatively low cost, and can be designed for very low temperatures. And because they require little fire and water protection, they can be installed almost anywhere. In many cases, they require less area for installation than comparable liquid-filled transformers. Versions with reduced no-load and short-circuit losses also increase efficiency and thus lead to lower operating costs. The insulation consists of an environmentally friendly mixture of epoxy resin and quartz powder which does not produce toxic gases even when exposed to an electric arc. The use of cast-resin insulated transformers in many cases avoids the restrictions that apply to oil-filled transformers, while retaining properties such as operational safety and long service life.