furnace transformers and reactors design and features - Tamini

egulation on the furnace, Tamini has designed and
supplied a 100 MVA, 225/35-12 kV regulating
transformer, with an independent regulation on each
secondary phase, with the aim to balance the
reactance of the furnace.
In this case, the high voltage primary windings and
the regulating secondary windings are star-and-delta
connected respectively. The transformer also has a
I. Diagram of the 100 MVA EAF regulating transformer for France
1U
1N
1W 1V
V
Delta tertiary winding, which can feed a power
factor compensation and auxiliary circuits.
When the tap-changer on one phase of the LV side
is in a different position compared with those of the
other two phases, then a circulating current is
created in the delta-connected secondary and tertiary
windings and of course it is under control, this situation
occurs either during on-load or no-load operation.
50
T 14 58
59
TI 1 51
TI 9
37
71 72 TI 2
TI 11
55
38
5453
52
68
TI 8 69
2V
U
2U
k
– +
2W
13. A 100 MVA AC-EAF regulating transformer, 220 kV with three
independent tap changers.
k
– +
TI 10
56
TI 13
57
72 73
– +
k
W
3U
3W
67
66
T 17
65
64
T 16
T 15
63 62 6160
3V
FURNACE TRANSFORMERS
The content of this section refers specially to electric
arc furnace transformers for iron and steel works
which are subject to exceptional mechanical and
electrical stresses during melting process.
The design and features described below are
basically the same used for furnace transformers for
electrometallurgical works.
Mechanical Stresses on Winding
During furnace operation, the transformers undergo
thermal and mechanical stresses due either to
frequent short circuits on arc or to continuous
energizing and deenergizing operations during the
daily steel melting process. Continuous stresses and
vibrations may loose the windings if they are not
properly treated and robustly clamped. To avoid any
inconvenience due to such heavy and frequent
stresses, Tamini has adopted improved procedures
during manufacture for winding pressure and
thermal treatment operation; furthermore Tamini has
since long time developed a special windings
clamping system which guarantees an exceptional
resistance against any electrodynamical stresses
even under the most arduous operating conditions.
The system has been used successfully for many
years but it is subject to continuous review for any
possible further improvements.
Electrical Stresses
These are mainly due to the variation in electric arc
overvoltages, which involves either LV bushing, bar
insulation, or the LV winding itself. Electrical stresses
are also caused by a transient state resulting from
FURNACE TRANSFORMERS
AND REACTORS DESIGN AND FEATURES
a sudden disconnectin of HV circuit breakers
expecially if vacuum-type circuit breakers are
installed when the low currents must be cut off (as
for the transformer’s no load current). To overcome
these dangerous overvoltages, RC devices and
surge arresters are frequently used and installed by
the electrical contractor. Nevertheless it is
important that the transformer design itself is
improved. Tamini pays very particular attention to
this problem adopting a very specific manufacturing
procedure suitable to guarantee a stronger
transformer insulating structure and safer operation.
14. One of two 85 MVA 34.5/1.2-0.78 kV AC-EAF transformers for USA
17