Arc, Submerged Arc, Ladle, Induction, Upto 10 MVA,Upto 33kV,ONAN/OFWF
Cooling A high current furnace transformer is provided having the
low-voltage windings arranged in a closed delta configuration within the
transformer enclosure. Bus bar terminals are mounted exterior to the
enclosure for providing electrical connection with the low-voltage
ONAN /ONAF cooling with conventional pressed steel radiators. The range
includes up to 33 KV systems and as per the site requirements.
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
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
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.