Powering the steel industry

April 2nd, 2019, Published in Articles: Energize

Both metallurgical and other industrial plant processes, as well as large drive procedures depend on reliable, highly specialised transformers. In steel plants, for example, it is crucial to supply high currents for AC and DC electric as well as ladle furnaces. Electrolysis processes operate with heavy-duty rectifiers which are fed by rectifier transformers. Large drive mining applications and variable speed drives depend on converter transformers to supply power to blast furnaces, pump stations, or rolling stock.

Global demand and production capacities increase simultaneously. This calls for higher voltages and currents from powerful transformers to accommodate the most challenging demands and severe working conditions. The transformers are exposed to cyclical loading and high thermal stress. They must withstand frequent overcurrent and overvoltage caused by short circuits in the furnace or tripped by high-voltage circuit breakers. As high currents cause enormous electromagnetic fields, special attention must be paid to prevent transformer overheating and system malfunctions.

An outage can cause a total loss of industrial production, such as when pots or furnaces “freeze.” The outage costs can quickly put even large facilities in financial difficulties. Every single transformer must be precisely tailored to meet individual demands. Low-cost standard solutions can later become very costly for the industrial customer.

Producing 80 tons of steel requires more than 35 000 kWh of power and 44 000 A. It takes 50 minutes to charge, melt, refine, de-slag, and tap each batch of steel. Following each cycle, it should take 60 minutes or less to repeat the process, tap-to-tap. Melting processes require enormous currents and have extraordinarily severe working conditions. They work under high operation currents that are often close to their short-circuit values. At the same time, melting processes face frequent on- and off-switching and tap changes during operation. But, despite being stressed to their limits, an unplanned outage of a furnace transformer has an enormous financial impact and should be prevented at all costs. This calls for extremely robust and reliable transformers.

When designing a power transformer, it is important to consider the options:

  • Direct or indirect regulation or booster
  • On-load or off-load tap changer (ONTC or OLTC)
  • Oil or vacuum type OLTC (also reactor type OLTC)
  • Series reactor (built-in or separate) for long arc stability
  • Air or water-cooled secondary bushing arrangements and designs
  • Internal secondary phase closure (internal closed delta)
  • Special magnetic shield design for each project
  • Oil-forced (OF)- or oil-directed (OD)-cooling

Combining with a series reactor is recommended for improved efficiency and clear and stable reactance, either as a stand-alone unit or incorporated into the tank of the electric arc furnace (EAF) transformer.

Fig. 1: Oil-forced water-cooled (OWF) transformer cooling process.

Measures against impermissible heating

Heating due to winding currents

The currents in the windings cause losses (load losses), which need to be dissipated by the cooling device. A standard of many manufacturers is OFWF (oil forced, water forced) cooling. The pumps of this system circulate the oil between transformer tank and cooling device. The oil flow through the winding stays comparable with ON (oil natural) -cooling. It’s naturally driven by thermal convection. According to IEC the average winding temperature rise at OD-cooling may be 5 K higher than it can be with OF-cooling.

Heating due to high magnetic fields of current carrying parts

The LV connection and the LV bushings are the transformer components with the highest currents. Each current causes a magnetic field, and high currents cause correspondingly large magnetic fields. Magnetic fields cause eddy currents in ferromagnetic materials like transformer tank or cover and also in plant structures. Eddy currents cause additional losses and heating.

Siemens designs have partly unique features to avoid impermissible heating due to high magnetic fields. One of these is the use of non-magnetic steel at structures which are close to current carrying parts. The LV side and partly the side walls of the transformer tank should be made of non-magnetic steel. Also, distances to the cover should be checked and if necessary, the cover should be made of non-magnetic steel at least partly on the LV side. The press beams of the active part supports of the LV connection and bolts of insulation screw connections are also made of non-magnetic steel. Screw connections with magnetic material can cause gassing.

Significant for the usage of non-magnetic steel is also the LV connection in the plant outside the transformer. This will be considered during the design process and, if desired, Siemens can check the structure in the transformer cell to give a recommendation for shielding in the plant.

Many manufacturers use fibreglass reinforced plastic as a material for the LV bushing plate. The bushing plate of Siemens furnace transformers is made of aluminium. The large aluminium plate shields the environment against the magnetic fields of the transformer and the transformer against the magnetic fields of the LV connection in the plant. None of the Siemens tank part designs need additional cooling.

Transformer insulation concept for defined grounding

The insulation concept of Siemens furnace transformers is defined from the core to the bushing plate. The concept decreases and avoids loop currents, which can be the reason for hot spots at the transformer and the structure in the plant. The core and the press beams are insulated separately from each other. Core and press beam earth are brought out separately to a terminal box on the cover or at a side wall of the tank.

The insulation resistances of all components have to pass a quality check during the manufacturing and are recorded. All components have defined earth points and can be grounded in a defined way.

Fig. 2: Typical furnace transformer for the steel industry.

Electrical contacts of furnace transformers are under special stress due to the high currents and short circuit forces. High currents can cause the accelerated aging of electrical contacts. Deficient electrical contacts can cause failures of transformers due to gassing or total failure of a contact. The standard of many manufacturers is the usage of screw or press contacts in furnace transformers. According to Siemens design rules the usage of screw or press contacts is not allowed for high current contacts. Siemens uses only brazed or welded contacts. It is also possible to braze all contacts of the transformer. Only two exceptions are allowed: The screw contacts of the MR OLTCs (On-load tap-changers by Maschinenfabrik Reinhausen) and the screw contacts of the U-tube bushings.

Conclusion

Melting processes are dependent on reliable power supply, yet they represent severe working conditions and require enormous currents. For the steel industry, it is of crucial importance to prevent an unplanned outage. This is why no compromises on the design and quality of furnace transformers should be made.

Contact Jennifer Naidoo, Siemens, Tel 011 652-2795, jennifer.naidoo@siemens.com

 

 

 

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