Furnace Transformers (EAF)
Electric arc furnaces transformers (EAF) are used to power arc furnaces for the production of steel and special ferroalloys. This applications which require high levels of power that also operate under very severe conditions to frequent over currents and over voltage generated by short circuits in the furnace and the operating of HV circuit-breaker. These applications often require the deployment of three single-phase furnace transformers instead of a single three-phase transformer.
Transformers in such applications must withstand high levels of thermal, mechanical and electrical stress. EAF transformers build as shell-type transformer is an recognized design and technology, and has proven its strength through many decades (our history).
An EAF transformer requires high secondary current (in the order 10 kA+) with the possibility of regulation of the current and voltage. An shell-type or a core-type transformer equipped with an on load tap changer or an off load tap changer connected to the primary windings gives opportunity to optimize the operation of the furnace.
Series reactor can be built with in the same tank as the EAF transformer to save space, infrastructure and costs. A series reactor could also be build with an tap changer to be able to a just the values.
Shell-type or core-type?
There are two basically different ways to build transformers. The difference lies in the design of the core and winding. The two types take their names from the design of the core and are called the shell-type and the core-type. The two different types are shown as single phase units in figure 2. Shell-type is represented by figure 2a has a rectangular core cross-section thus giving the windings a rectangular shape. The windings consist of interleaved high voltage and low voltage discs with insulating shields inserted between. Some of the high voltage discs have tapings for voltage regulation. The low voltage coils are made from solid copper plates. All the windings are divided into parallel groups corresponding to the number of groups in the bus bar system to the furnace.
The core-type transformer in figure 2b as a single phase unit has two legs with an approximately circular cross section. The high voltage and low voltage windings are mounted concentrically on the core legs. The low voltage windings is normally built up of a number of disc coils connected in parallel to bars which again are connected to the low voltage bushings. This arrangement makes it a practical solution placing the low voltage winding outside the high voltage winding.
Advantages and disadvantages
The two construction concepts are rather different and, even if both types may well be adapted to furnace application, there are certain differences with regard to the electrical characteristics. By dividing the high and low voltage discs into groups, as its done in the shell-type transformer, the reactance of the transformer is kept very low (2-3%), whereas the reactance of a corresponding core-type transformer is 2-3 times as high. In furnace installations a low reactance is normally preferred. Also the freedom in choice of voltage steps and the uniform voltage distribution is favorable with shell-type as opposed to core-type. The mechanical strength of the active part of a shell-type transformer is also a great advantage.