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. |