Transformers are essential components of any power distribution network. They are needed to convert current into a suitable form before it can be transported or used.

The core of the transformer needed for this conversion consists of efficient grain-oriented (GO) electrical steel strip, which is also known under a variety of names such as silicon steel and transformer steel. Transformers can be generally divided into two types, large power transformers and the smaller distribution transformers.

The use of standard grades of CGO (conventional grain-oriented) steel is generally used for distribution transformers, while the highly-efficient HGO (highly permeable grain-oriented) grades are preferred for power transformers.

Grain-oriented electrical steel strip is particularly wellsuited for use in transformers, as in static applications the magnetic fluxes run in defined directions. The grain oriented electrical steel strip has outstanding magnetic properties in the direction of rolling, due to a preferential orientation of the grains. The strict orientation of the easily-magnetizable grains, known as “Goss” orientation (see figure Perfect Goss orientation of the grains in the direction of rolling), makes the magnetization processes in the direction of rolling very favorable in terms of energy. 

To reduce eddy current losses and improve energy efficiency, extremely thin material (0.18 – 0.35 mm) is used, grain sizes up to several cm are selected and the material is alloyed with around 3 % Si.

Compared to NGO electrical steel strip, the production of GO electrical steel requires a highly complex, multistage heat treatment process. The complete production workflow can be seen in the figure shown below. It is very important that the individual heat treatment steps are perfectly matched with the alloying system in order to achieve the best magnetic properties. EBNER offers the facilities described below for this application:

HICON/H2® decarburizing and coating line (DCL)

After cold rolling, grain-oriented electrical steel strip is heat treated in a continuous furnace to decarburize and recrystallize it. In EBNER HICON/H2® decarburizing lines, this process takes place in a finely-tuned and precisely-maintained furnace atmosphere. The highly efficient and consistent decarburization process inhibits magnetic aging, and ensures the long-term efficiency of a transformer. Optionally, ammonia can also be used to nitride special HGO (highly-permeable grain-oriented) grades in a nitriding zone following decarburization, which improves magnetic characteristics even further during the high-temperature anneal. The strip is then cooled slowly and evenly, and coated with a magnesium oxide layer in either an in-line or off-line process. EBNER offers these lines in cooperation with selected strip handling partners.

THE STRENGTHS OF AN EBNER FACILITY, COMPARED TO STANDARD DESIGNS, PROVIDE SIGNIFICANT ADVANTAGES TO OUR CUSTOMERS AND INCLUDE THE FOLLOWING:

Precise atmosphere flow, separation of atmospheres and exact control of the atmosphere humidification and supply system.

• Perfect dewpoint control using the humidifier
• Controlled convection to ensure even decarburization
• Low CO levels to increase the decarburization rate
• No contamination of the atmospheres from the different technological zones

Extremely low carbon contents (< 0.0030 % C, 30 ppm) ensure the best possible magnetic properties without aging.

The very latest burner/radiant tube technology and rapid heating-up systems for high heating gradients to achieve efficient decarburization and improved texture (crystalline orientation).

HITT high temperature bell annealer facility (HBAF)
For high-temperature anneals (HTA), EBNER has developed the HITT (High Temperature & Tight) line of 2-stack bell annealers High-temperature anneals use secondary recrystallization to form grains with the magnetically advantageous Goss texture (3). The high processing
temperatures (above 1150 °C) and straight hydrogen atmospheres also remove sulfur and nitrogen from the material. First, a drying phase (1) is used to dry the MgO coating applied during an upstream process, which inhibits the formation of stickers in the wraps at high workload space temperatures by forming a Forsterite layer (2). The processing steps (1 – 5) of the high-temperature anneal are depicted schematically below.