At the end of September 2007, the world''s largest cast-resin transformer with a rated power capacity of 40 MVA left the Siemens Power factory in Kirchheim / Teck, Germany. 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.

The transformer measuring 4.8 meters long, 2.8 meters wide and - in its transportable state - 4.7 meters high was transported from the Kirchheim factory to Plochingen in Germany by special road transport on September 26, 2007, and from there by ship on to Erlangen. The two 40 MVA cast-resin transformers (20/12.2 kV) will be used on a system test setup for the new Siemens HVDC transmission system HVDC Plus. The test set up consists of a back-to-back arrangement of the power converters with the two converters connected to the two cast-resin transformers on the three-phase side. One of the major advantages of the topology of the new HVDC transmission system is that standard power transformers can be used for it. The experimental and test set up will enable engineers to test the system characteristics of the new converter topology of an HVDC Plus system extensively under realistic conditions. This will also provide the basis for further advances.