| Furnace transformers are used to step down from voltages between 11 and 33 kV to levels of
several hundred volts only. This results in massive secondary currents. As an example a 30
MVA unit at 150 V would result in a secondary current of 115 kilo Ampere. For these high
secondary currents special bushings are required to connect to the bus-bars. These bushings
are specified with very specific arrangements to suit the bus-bar arrangement and cooling
system. Furnace bus-bars are mostly water cooled.
Due to the high secondary currents and resistive losses the furnace layout is such as to limit the
bus-bar length. The furnace transformers are then located close to the furnace itself and if they
are single phase units, arranged in a triangle around the furnace. This means that there is a high
risk of fire, a high ambient temperature and this whole set up is located at a level associated with
the third story of the building. The location above ground level encourages single phase units
due to the structural limitations of the buildings. To reduce the fire risk the transformers are
contained in rooms, which adds to the high ambient temperature.
Furnace transformers are very much in a production environment. Loading of these transformers
is then very close to rated values and even beyond. This demands very reliable transformers.
Shutdowns due to transformer problems are frowned upon. When shutdowns occur, the problem
needs to be solved quickly. This in turn calls for good accessibility of the tap changer and other
parts of the transformer. Large inspection covers in close proximity to the tap changer are often
specified.
Due to the nature of the process furnace transformers are specified with large tapping ranges.
Thirty tap positions is not uncommon. Adding to the wide tapping range is the utilisation of the
tap changer. Some users require up to 800 operations of the tap changer per day. This
demands high maintainability and efforts to increase the maintenance intervals. On line tap
changer oil filters are thus essential. To reduce downtime further, plug-in type diverters are
specified. This allows a quick changeover of the diverter switch and an overhaul in a workshop
environment with more time at hand.
Another aspect of the process is the large number of short circuits that these transformers are
subjected to every day. Transformers associated with open arc furnaces can be subjected to a
number of short circuits per melt as the material being melted collapses across the electrodes.
Bus-bar flashovers are also a fact of life on most furnace installations. To add to this peril,
furnace transformers are required to have a lower than normal impedance. This gives rise to
higher over-current factors. A very robust design in terms of the transformers’ ability to withstand
the dynamic effects of repeated short circuits is required. Minimum impedance values for
furnace transformers of the core type are in the order of 4 – 5 %. To achieve lower values, one
would need a shell type transformer. Upper levels for impedance could be any value from 10 to
24 % depending on the configuration and tapping range. |
