Note: Descriptions are shown in the official language in which they were submitted.
1
1 "DIRECT-ARC EhECTRIC FURNACE FED WITH CONTROLhED
2 CURRENT AND METHOD TO FEED A DIRECT-ARC FURNACE WITS
3 CONTROhLED CURRENT"
This invention concerns a three-phase direct-arc electric
6 furnace fed with a controlled current and also a method to
7 feed with controlled current a three-phase direct-arc
8 furnace.
g This invention is applied to three-phase electric arc
furnaces for the smelting of metals and iron and alloys
11 thereof in particular.
12 Direct-arc electric furnaces are mainly used at the
13 present time for the smelting and re-smelting of steel and
14 are almost all three-phase furnaces.
During the last twenty years the power of each furnace has
16 increased considerably, passing from unit powers of 16 MW
17 and 20 MVA to powers greater than 85 MW and 120 MVA.
18 These high powers entail for the supply network great
19 problems of disturbances in the voltage (flicker) as well as
considerable phase shifts due to the inductive loads.
21 To correct the phase differences due to these inductive
22 loads and reduce the voltage fluctuations the modern
23 compensation technology makes use of variable compensators
24 of reactive power, which are operated with controlled
diodes.
26 The principle of regulation is shown in Fig.1 and is as
27 follows.
28 Three inductors are placed in parallel connection with the
29 three-phase medium-voltage line, which is the supply point
of the strongly inductive loads of the furnace: these
31 inductors are supplied by means of thyristors T, the firing
32 angle of which is controlled on the basis of the current
33 detected by the device SI.
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1 This regulation system keeps constant and balanced at zero
2 the total reactive power employed by the furnace, the
3 inductors L1 and L2 and the batteries of power-factor
4 corrective capacitors CR, Which are all connected to the
medium voltage supply line.
6 The batteries of power-factor corrective capacitors CR
7 with the addition of suitable inductors are made to perform
S also the function of filtration of the harmonics generated
9 by the furnace and by the compensation system.
Instead, the active power of the furnace arcs is regulated
11 by changing the height of the electrodes by means of
12 suitable hydraulic assemblies GI, by trying to keep the
13 resistances of the arcs constant.
14 To overcome the difficulties and some of the shortcomings
Which this indirect type of regulation of the absorbed
16 current entails, direct-current furnaces have recently been
17 produced, and with this type there is one single electrode
18 and the return of the current is entrusted to the shell of
19 the furnace.
The supply current of the arc is provided by a
21 rectification assembly made of controlled diodes or '
22 thyristors. This system involves two substantial drawbacks.
23 On the one hand there is the difficulty of obtaining the
29 return route of the current, while on the other hand there
is a strong' generation of harmonics of an odd number by the
26 rectification system.
27 To obviate these serious drawbacks of both types of arc
28 furnace, the present applicant has designed, tested and
29 embodied the present invention, which has as its objective
the preset purpose.
31 The direct-arc electric furnace fed with controlled
32 current and the method to feed with controlled current a
33 direct-arc furnace are set forth and characterized in the
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1 respective main claims, whereas the dependent claims
2 describe variants of the idea of the solution.
3 According to the invention the control mechanism acts
9 directly on the arc current of the furnace so as to
determine the operating point and reduce disturbances. This
6 is in contrast with the state of the art, which lets the
7 current in the furnace evolve freely and be controlled only
8 by the hydraulic system regulating the length of the arc,
9 while /the anti-flicker control system thereafter seeks to
regularise the situation towards the mains supply side.
11 Whereas the present three-phase arc furnaces are usually
12 connected to a compensation system which works independently
13 and in parallel connection with the furnace, the three arcs
19 of the furnace according to this invention are fed by
imparting to each arc a first basic current restricted by a
16 first inductor L1 according to one idea of the solution.
17 A second current is superimposed on this first current by
18 a second inductor L2; the second current is operated and
19 regulated by means of a thyristor T by a transfer function,
which takes into account the operating state of the arc by
21 analysing the value and/or the initial slope or trend of
22 that first basic current.
23 According to a variant, besides the analysis of this value
29 and/or initial slope the state of the electrical magnitudes
in question at various points in the plant and in particular
26 the position of the tap changer under load of the
27 transformer are also analysed.
28 According to a variant of the idea of the solution a
29 saturable reactor RS may be suitably employed instead of the
inductors L1 and L2 and thyri,stor T.
31 According to the invention the power-factor corrective
32 capacitors, which also operate as filters for the
33 absorption of the harmonics generated by the furnace in
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1 relation to the mains supply network, are placed in parallel
2 connection on the medium voltage bus bar in a manner fully
3 analogous to the one that takes place in the state of the
4 art, but have much less high values of capacity.
With reference to the attached figures, which are given as
6 a non-restrictive example, Fig.l shows the state of the art.
7 Fig.2 shows the invention and enables the difference of
8 the invention from the state of the art to be understood,
9 while ~Figs.3 and 4 show variants of the idea of the
solution.
' 11 Let us now look in detail at the state of the art and the
12 content of the invention.
13 The invention differs from the state of the art in the
14 zone between the medium voltage line and the furnace
transformer.
16 In Fig.l the inductor L1 has the purpose of optimising and
17 making more flexible by an exact choice of its value the
18 operating point of the furnace with reference to the usable
19 transferred power, to the length of the arc, to the current
intensity and to the radiation index.
21 The dimensioning of the inductor L1 is carried out by
22 determining an operating point that balances the contrasting
23 requirements of ensuring an adequate transfer of power and
29 an arc current high enough for the technological
requirements of the smelting process and, at the same time,
26 of limiting the peaks of current in the event of a short ,
27 circuit of the electrodes.
28 The choice of the inductor L1 affects indirectly the arc
29 heat radiation, which must vary between a minimum value
imposed by considerations of efficiency of production and a
31 maximum value imposed by restrictions of the wear on the
32 refractory lining and by observing the relative safety
33 limits.
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1 It is then important to compensate the reactive power of
2 an inductive type absorbed by the furnace.
3 The necessary reactive capacitive compensation power is
4 obtained by connecting in parallel to the medium voltage
line a fixed bank of power-factor corrective capacitors CR
6 (usually star-connected with neutral insulated or earthed)
7 and a variable inductance obtained with a fixed inductor L2
8 and a thyristor-operated valve T (inductor controlled by
9 thyristors). The connection for the inductors L2 is the
triangle connection.
11 Inductors LF are also provided to act as a filter and are
12 placed in series to the thyristors T and capacitors CR.
13 The need for a variable compensation is justified not so
14 much by the requirement of obtaining at the supply point an
average power factor greater than that imposed by the power
16 supply authority but rather by the possibility of
17 compensating very quickly the peaks of absorbed reactive
18 power which are responsible for the disturbance of a flicker
19 type in the supply mains network.
The dimensioning of the bank of capacitors CR and the
21 inductor L2 is carried out according to the maximum reactive
22 power required by the furnace (equal to the furnace short
23 circuit power) corrected by a coefficient greater than 1 due
24 to the incomplete compensation of the compensation assembly
described (CR+L2+T).
26 Inductors LF are also provided which have the task of a
27 filter and are positioned in series to the thyristors T and
28 capacitors CR.
29 Measurement of the reactive currents absorbed by each
supply phase of the furnace is carried out for each phase by
31 a device S1, which generates the signal of feed-back of the
32 control system of the arc current.
33 At all times the capacitive current absorbed by the banks
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1 of capacitors CR must be balanced with the inductive
2 currents absorbed by the furnace and by the inductor L2
3 controlled by the thyristors T.
4 A second regulation device referenced with GI in the
figure concerns the geometry of the electric circuit that
6 controls the arc resistance.
7 Known, adequate servo-mechanisms GI of a hydraulic type,
8 for instance, arrange to move the electrodes vertically with
9 the purpose of keeping the impedance of the furnace
constant.
11 Mechanical regulation obviously has time constants
12 distinctly slower than the electrical type regulation
13 described above and is therefore less effective with regard
14 to the effects of electrical disturbances.
Turning next to the invention shown summarily in the
16 diagram of Fig.2, we find that the inductor L1 performs the
17 same function as the analogous component included in the
18 state of the art shown in Fig. 1.
19 The variable inductor is obtained with a fixed inductor L2
and a thyristor-operated valve T and is positioned in
21 parallel to the inductor Ll,thus providing a variable
22 inductance located in series with the furnace mains supply.
23 The device S1 measures the intensity of the current
24 absorbed by the arc and sends a signal that drives the
control system of the thyristors T.
26 It is possible in this way to keep the current absorbed by
27 the furnace constant within broad limits and thus obtain a
28 controlled current supply.
29 Variations in the arc impedance are compensated by
opposing variations of impedance of the equivalent inductor
31 placed in series and consisting of the parallel between L1
32 and L2.
33 If, for instance, the arc tends to die out, the inductance
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1 is reduced to increase the flow of current.
2 If, instead, the electrodes are short-circuited by the
3 scrap being smelted, the inductance of the inductor is
4 brought up to the maximum value so as to limit the resulting
voltage drop in the supply network, that is to say, there is
6 a tendency to correct the cause of the disturbances in the
7 supply network rather than correct their consequences with a
8 static variable compensator, as instead takes place in the
9 state of the art.
The automatic control of the equivalent series inductance,
11 depending on the arc current, therefore forms the innovatory
12 aspect of the configuration shown.
13 The control now illustrated can cooperate also with
14 hydraulic regulation GI of the length of the arc.
Although the two types of control have as their purpose
16 the maintenance of a constant impedance in the furnace side,
17 yet they entail some differences. The geometric regulation
18 GI which affects the position of the electrodes may change,
19 by acting on the length of the arc, only the resistive part
of the impedance, whereas the electrical regulation of the
21 equivalent series inductance (parallel between L1 and L2)
w) 22 changes the reactive part directly and, by taking action on
23 the arc current, acts also on the equivalent resistance.
29 Moreover, the time constants are very different since in
one case actions of a mechanical type are implicated,
26 whereas in the ether case merely electrical actions are
27 involved.
28 Regulation of the equivalent series reactance, being
29 carried out phase by phase, enables also the imbalance of
impedance inherent in the geometry of the secondary circuit
31 of the furnace (from the outputs of the furnace transformer
32 to the arcs) to be corrected and the currents in the three
33 phases to be kept constant, thus overcoming the so-called
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1 "cold phase" and "wild phase" problems.
2 The inductors controlled by means of thyristors T in the
3 state of the art and in the art here proposed are actuated
4 according to the signals coming from the monitor S1, these
signals being processed by a control assembly GC.
6 This control assembly GC in the art here proposed can also
7 receive signals which reflect variations in the other
8 electrical magnitudes in various parts of the circuit. Thus
9 it can receive signals, for instance by means of a
transformer TV, of measurement from the medium voltage line:
11 it can receive signals, for instance, of the position of the
12 electrodes through the control GC; it can also receive
13 signals from other sources, for instance the position of the
14 commutator under load of the transformer and other setting
signals.
16 Moreover, the control assembly GC prevents saturation of
17 the inductors by eliminating the continuous component of
18 currents passing through the same.
19 Power-factor corrective capacitors CR are connected to the
medium voltage line and have the task of correcting the
21 power factor of the reactive component of the power absorbed
22 by the furnace within the limits set by the authority which
23 supplies electrical power.
24 As can be seen in the figures 1 and 2, the configuration
according to the invention provides for the same components
26 as those already comprised in the normal configuration, but
27 these components are employed functionally in a different
28 way.
29 The different use of the components with a resulting
different dimensioning entails some substantial
31 constructional savings.
32 The comparison of the two solutions can be carried out
33 with an equal active power supplied to the furnace and with
1 equal disturbances of a flicker type generated in the supply
2 mains.
3 The inductor L1, although having a greater reactance, is
4 normally charged in the present invention only by part of
the operating current of the furnace. Its dimensioning as
6 regards power, and therefore as regards cost, is about 30
7 to 40% of that required in the configuration of Fig.1 when
8 a safety factor due to short-circuit overlaads is taken into
9 account.
The power-factor corrective capacitors CR too show a
~,' 11 considerable reduction. In fact, in the case of Fig.l the
12 reactive capacitive power is dimensioned, as we have already
13 noted, at a greater value than the short-circuit power at
14 the electrodes. In the case of Fig.2 it is only necessary
to correct the power factor of a part of the reactive power
16 absorbed by the furnace at the operating point. The
17 resulting reduction is about 70 to 800.
18 The value of the inductor L2 controlled by the valve
19 operated by thyristors T could be taken as being nil at a
theoretical level so as to permit the maximum range of
21 regulation of the furnace series reactance.
22 Technological reasons linked to the smelting process and
23 to the embodiment of the valve T entail, if a precautionary
29 factor is applied,a reduction of about 80 to 90% as compared
to the embodiment of Fig. 1.
26 The valve T itself, being dimensioned according to the
27 invention at lower voltages and less high currents, shows a
2$ reduction in dimensioning by a factor of about 40 to 50%.
29 This value is obtained by calculating the product of the
maximum voltage applied to the valve T multiplied by the
31 maximum current which passes through it.
32 Besides the constructional savings achieved on the
33 components it is necessary to take into account an
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1 improvement in the working costs, this improvement being due
2 substantially to the reduced electrical variability of the
3 arc.
4 In fact, the increased stability of the current and its
uniformity in the three phases achieve a greater efficiency
6 of the production process, less wear of the electrodes and
7 refractory lining and smaller electrodynamic stresses in the
8 event of short circuits.
9 ~ Since the idea at the base of the invention is the
automatic control of the reactance in series to the circuit
11 of the furnace side, it is possible to pick out some
12 variants which achieve this control in a different way.
13 A first variant shown in Fig.3a provides for elimination
14 of the inductor L2 but retention of the valve operated by
thyristors T: as we saw earlier, the inclusion of the
16 inductor L2 is not substantial and, in fact, is due to
17 technological reasons.
18 Fig.3b shows another variant which provides for regulation
19 of a non-continuous type but in steps. From the inductor L1
are branched some intermediate tappings the activation of
21 which is entrusted to thyristor-controlled switches. This
22 variant makes a simpler control possible but does not permit
23 fine regulation of the furnace-side impedance and therefore
24 of the phase current.
Fig.4 shows a variant that provides for use of the
26 saturable reactor RS instead of L1, L2 and T.
27 The saturable reactor, excited by a suitable, constant
28 direct current supplied by the control circuit GC, has the
29 characteristic of providing a low reactance value for small
values of current, lower than the nominal current IN of the
31 furnace, and a high reactance value at higher currents.
32 In this way is obtained the effect of restricting
33 considerably the extent of overcurrents and therefore of
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1 voltage flicker.
2 This solution has the merit of not requiring a complicated
3 control system GC; in fact, when the direct current
4 corresponding to the IN of the operating point of the
furnace has been set, the saturable reactor limits the
6 overcurrents automatically.
7 The control circuit GC fixes the current of excitation of
8 the saturable reactor according to the operating point of
9 the furnace.
To obtain this function, the regulator GC is interfaced
11 with the regulator of the height of the electrodes GI and
12 with the tap changer of voltage under load of the furnace
13 transformer.
14 According to a variant the regulator GC not only makes
reference to signals coming from the regulator GI but also
16 analyses the state of the electrical magnitudes involved at
17 various points in the plant.
