furnace transformers and reactors design and features - Tamini

G. DC EAF Basic Diagram
H.V.
Smoothing Reactor
H.V.
High Current Connection
Step Down Transformer
Rectifier Transformer
RECTIFIER
High Current Connection
D.C. Furnace
H. 2x3 Phase Bridge Connection With Wye-Delta Secondary Windings
L.V.
RECTIFIER TRANSFORMERS FOR DC
FURNACES
The typical DC furnace electric diagram, with its
main components, including step-down transformer,
furnace transformer, rectifier system and arc furnace,
is shown in the simplified diagram here aside.
The electric system feeding a DC furnace is
substantially different from that foreseen for feeding
an AC furnace as in this case, the furnace is not
directly fed by the furnace transformer but through a
rectifier high current DC connections and a
smoothing reactor.
The DC technology offers satisfactory performances
in terms of electrode consumption and reduction of
network disturbances but with higher investment and
operative costs and with the disadvantage of relying
on delicate DC and associated electronic equipment
which may be a drawback, particularly in a heavy
industrial process. This has not to be
underestimated.
The diagram G. is the most commonly used diagram
for DC furnace transformers: a double six-phasebridge
for a 12-pulse system with two 30° shifted
secondary windings.
For high power, some additional phase shift windings
have to be provided to obtain systems of 18 pulses
or more. This means transformers with 2, 3 or 4
primary windings and 2,3 or 4 secondary windings.
Moreover in a DC furnace transformer the
secondaries have usually to be magnetically
uncoupled in order to reduce the electromagnetic
interference between the different rectifier units and
to reduce problems in thyristors control.
9. A 100 MVA 33/0.77-0.45 kV DC-EAF transformer for China.
The DC furnace transformers can be quite simple for
what concerns the regulating windings as the
voltage change can be made by an off-circuit motordriven
tap-changer.
Sometimes a fixed ratio transformer is used without
any tap-changer: in this case the voltage is regulated
by the thyristor control only. The two systems can
also be used together.
It has to be underlined that the control by thyristors
causes substantial increase of the eddy losses in the
windings and stray losses in the external structures,
due to high harmonic content of the currents.
This is a basic aspect to be considered in DC EAF
transformer design.
An additional important aspect to be considered,
when applicable, is the possible unbalanced
operation of secondary windings, should one or
more bottom electrodes not conducting.
10. Two 70 MVA 30/0.82-0.67 kV DC-EAF transformers for Germany
DC EAF TECHNOLOGY
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