The present invention generally relates to the con¬ trol of three-phase electrode furnaces, the positions of the electrodes being controlled on the basis of impedance or resistance values which are determined on the basis of measured current and voltage values. The invention is particularly applicable to three-phase electric arc fur¬ naces.

When melting down scrap in the frequently occurring type of three-phase electric arc furnace which is charged with cold scrap by batches, unstable electric arcs bounc¬ ing between different pieces of scrap and the respective electrodes arise during the initial melt-down of the material from the respective scrap cage. This phenomenon is caused by the electric arcs, which are extinguished each time the current of the respective phase passes a zero crossing (twice a second at 50 Hz AC), requiring higher ignition voltage to be reignited against the cold pieces of scrap, as compared with the condition when the charge has been heated. Reignition of the electric arcs occurs in the position where at present the resistance is lowest. This implies irregular operation and higher inductive resistance. The final result will be a delayed increase of the active furnace power to the desired maxi¬ mum level that the furnace transformer and the connected mains permit.

Similar conditions may also occur when scrap falls down, which in itself is a normal occurrence when scrap high up in the furnace falls down into the empty space which is formed when charged scrap is being melted down from below. Scrap falling down constitutes a disturbance, whereby the impedance in the secondary circuits can be sharply reduced by direct contact between electrodes and pieces of scrap. This leads to a reduction of the active power and the operation of the furnace may again become unstable, since some time after the scrap falling down

the electric arcs operate against the colder scrap that has fallen down.

The disadvantages described above could be counter¬ acted by selecting, in case of unstable operation, a fur- nace working point which gives a higher current/voltage ratio, whereby the ratio of reactance to impedance in the secondary circuits increases. In practice, this could be carried out by selecting a lower impedance or resistance desired value for the electrode control, which means that the furnace is operated with greater displacement between current and voltage, which in turn results in higher available voltage for reignition of the electric arc after the zero crossing of the current, whereby the effective burning time of the electric arcs increases and the operation of the furnace becomes more stable.

The problem of such a mode of action is that reli¬ able information is required as to when and how sharply such a decrease should be made in order to obtain the desired effect. For instance, decreasing the impedance desired value, if this is not necessary in order to obtain stable operation, or decreasing the impedance desired value to an unnecessarily high degree, implies unnecessarily high consumption of electrodes, while the power take-off will not be optimum. As soon as the electric arcs are burning steadily, the maximally available transformer power should instead be drawn without unnecessarily high current, which usual¬ ly implies a power factor just above Z . In disturbances of the above-mentioned kind, it would be advantageous to reduce the impedances of the phases as much as is requir¬ ed to obtain a more stable reignition and increased active power owing to longer burning time of the elec¬ tric arcs. The problem is that there is no satisfactory control means. The object of the present invention is to eliminate the above-mentioned problems and to provide a method

which enables both more stable operation and higher power take-off.

This object is achieved by a method having the fea¬ tures set forth in the accompanying claims. According to a first aspect of the invention, use is made of the knowledge that the content of harmonic compo¬ nents of the phase currents can be used to provide data for selecting suitable control desired values.

According to a second aspect of the invention, use is made of the knowledge that the current and voltage values that are used for determining the control desired values should be based on the fundamental frequency, i.e. as far as possible free from harmonic interference. This is particularly important when actual values are deter- mined by the so-called MC method which is disclosed in, for instance, PCT Publication WO 85/03834, to which refe¬ rence is now made and whose content should be considered incorporated herewith by this reference.

A preferred embodiment of the invention is based on a combination of the MC method, in an application for controlling the electrodes in three-phase furnaces, in which instantaneous values for current and voltage are obtained by sampling, and known mathematical methods for dividing up the variation over time of these current and voltage values in a number of sinusoidal functions based on the fundamental tone of the system, and with a number of harmonic functions in addition, so-called Fourier ana¬ lysis, the results subsequently being used for actual value determination based on the fundamental frequency and for determination of the relative harmonic content (current k-factor) which is known from the alternating current theory and which has appeared to be a suitable control parameter, whose value serves as a basis for decreasing the impedance or resistance desired value of the control system to obtain a more stable operation, higher power take-off and a more rapid increase of the power towards the maximum value when starting and after

a disturbance has occurred. According to the invention, the decrease of the impedance/resistance desired value could be carried out in one or more steps to a number of lower levels, the decrease suitably occurring with a certain delay such that short disturbances should not affect the stability of controlling. This also applies when returning to the normal level of the desired value.

By utilising the relative harmonic content according to the invention, it is thus achieved that in unstable electric arcs, the desired value in controlling of elec¬ trodes is adjusted in the manner described above so as to obtain higher average power, i.e. the power curve is closer to the ideal curve, which facilitates higher pro¬ duction owing to shorter charge time and lower operat- ing expenses owing to, inter alia, the lower power and electrode consumption caused by reduced time-dependent losses.

The invention will now be described in more detail by means of an embodiment, reference being made to the accompanying drawings, in which

Fig. 1 is a schematic diagram of a three-phase electric arc furnace, which has a conventional electrode control system supplemented in a simple manner so as to allow the furnace to be controlled according to the invention;

Fig. 2 schematically shows input signals to and out¬ put signals from an adaptation unit for a measuring and controlling computer used according to the invention; and Figs 3a and 3b are schematic diagrams of typical power curves in conventional controlling and controlling according to the invention and the associated adjusting of desired values, respectively, when controlling accord¬ ing to the invention.

There is shown in Fig. 1 a bottom 1 of a conven- tional three-phase electric arc furnace (not shown), in which three electrodes 2, 3, 4 are arranged. The elec¬ trodes are via a conventional furnace transformer 5 con-

nected to the respective corresponding phases R, S and T of a 50 Hz three-phase network. A measuring transformer 7 is connected to current transformers 8 on the primary side of the furnace transformer 5 and feeds, via trans- formers 9 and via a respective AC/DC circuit 10 current measured values ir(k), is(k), it(k) (k stands for con¬ ventional) to a conventional regulator 11. Only the cir¬ cuit arrangement for ir(k) is shown for the purpose of clarification. Besides, the same applies to entire Fig. 1, when there are signals relating to three diffe¬ rent phases or impedances.

The furnace transformer 5 has different tap changer positions (LK-positions) and the associated LK signal is supplied to the measuring transformer 7 and the regulator 11 to make it possible to take the LK position involved into consideration.

Voltage measuring transformers 15 are connected to the secondary side of the furnace transformer 5 and to the furnace bottom 1 via an LP filter 16. The trans- formers 15 feed, via a respective AC/DC circuit 17, a phase voltage value (relative to the furnace bottom 1) urO, usO and utO to the regulator 11. This is also sup¬ plied with an impedance or resistance desired value (El value). The regulator 11 is arranged to conventionally supply a respective electrode control output signal on its output 19 for electrode control, as indicated at 21.

A modification according to the invention implies that the control signals of the regulator 11 are supplied in a controlled manner via a measuring and controlling computer MRD (reginR-regoutR, reginS-regoutS and reginT- regoutT) . The computer MRD is supplied with signals and emits signals via an adaptation unit APE (consisting of APE I and APE II), as illustrated in more detail in Fig. 2. Phase current signals ir, is, it to the computer MRD are taken via additional current transformers 25 on the primary side of the furnace transformer 5 (before the measuring transformer 7 which follows the LK position)

and are converted to the secondary side in the computer MRD while taking the LK position into consideration, about which the computer MRD obtains information via the LK signal. Voltage signals urs, ust, uts, urO, usO and utO to the computer MRD are taken from the measuring transfor¬ mers 15 parallel to the phase voltage signals to the regulator 11.

All current and voltage signals supplied to the com- puter MRD pass through the respective anti-alias filters in the adaptation unit APE, such that faults in the sub¬ sequent sampling are avoided.

In a preferred embodiment, the input signals to the computer MRD are sampled evenly distributed and such that each input signal is sampled typically 48 times per 50 Hz period. To achieve optimum accuracy in the subsequent de¬ terminations, the measured values of one sampling should refer to the same point of time. This can be achieved by suitable interpolation between measured values for one and the same input, such that all measured values belong¬ ing to one sampling are "moved" to a point of time which is mutual for the sampling.

The control method which is preferably used in connection with the invention is, as mentioned above, the so-called MC method (disclosed in, for example,

WO 85/03834). This requires, however, that current must flow in all three phases to enable application of delta- star transformation. When starting, it is therefore suit¬ able to control in conventional manner by using the regu- lator 11, whose control signals then pass through the computer MRD and thus can be monitored. After starting, a change is made to the MC method, in which sampled measur¬ ed values are used for determining actual values and con¬ trol output values regout. The sampled measured values are processed as instantaneous values, on the basis of which determinations are carried out for each 50 Hz period.

In the computer MRD occurs a fundamental frequency determination for the currents and voltages that are used for producing impedance or resistance actual values. This implies that controlling by the MC method, using delta- star transformation for obtaining phase impedances or resistances, can be carried out, although the respective current and voltage curves during the first part of the melt-down phase have appeared to deviate significantly from the ideal sinusoidal shape. It has proved advantageous to use, for the electrode control, as actual and desired value the real part, i.e. the resistance, of each star impedance. This technique of controlling has proved to result in excellent adaptabi¬ lity, i.e. disturbances in, for instance, scrap falling down and collapsing are rapidly compensated for, and the control becomes stable.

To make the furnace operate at the desired "cosø", for the purpose of achieving, for instance, maximum power during the melt-down phase, the resistance desired value is selected in conformity therewith, taking into conside¬ ration that with unchanged reactance (X) each resistance value (R) corresponds to a given cosø value:

Moreover, the computer MEd effects a determination of the content of harmonic components of the phase currents by determination of the relative harmonic content, i.e. the ratio of the effective value of the harmonics to the effective value of the entire current. In case of high relative harmonic content, the utilised desired value is decreased. As a result, it is compensated for the extra reactance caused by the content of harmonic components. The desired value is returned to normal, when the rela- tive harmonic content has declined to the normal level. The increase as well as the return of the desired value according to the size of the relative harmonic content preferably occurs stepwise. This relative-harmonic-con-

tent-dependent control of the desired value has appeared to be important for the possibility of rapidly reaching full power when starting, in scrap falling down etc.

For the determinations of the fundamental frequency and the relative harmonic content, use is advantageously made of the per se well-known Fourier analysis.

Figs 3a and 3b, which are related to each other in terms of time, illustrate the effect of controlling according to the invention compared with conventional controlling. At the point of time t, power is connected

(however, it could also happen that at this point of time a change was made to the highest LK position) . The curve 31 illustrates very schematically how the power may vary in connection with conventional control, when, owing to lack of stability, it has been chosen to operate with a desired value Z0, which does not fully correspond to the maximally available power, which is obtained at a slight¬ ly higher desired value Zl. As is apparent, the power increases relatively slowly and reaches a maximum value which is lower than the maximally available power. At the point of time t3, scrap falls down, followed by a sub¬ stantial decrease in power. It takes a relatively long time before the highest power has again been reached. The melt-down operation is finished at the point of time t5. In controlling according to the invention, there occurs a rapid and sharp decrease, controlled by the relative harmonic content, of the desired value, which together with the impedance determination based on the fundamental frequency causes the power curve to rise more rapidly towards the maximally available power level. As this level approaches, a stepwise return of the impedance desired value to Zl occurs. At the point of time t2, scrap falls down, which leads to an increase of the rela¬ tive harmonic content and a new reduction of the impe- dance desired value. The cut in the power curve will thus be considerably smaller owing to a quicker return to the maximum power level. The melt-down will be finished at a

significantly earlier point of time t4 and at a clearly smaller amount of consumed energy.

In case of other disturbances than large amounts of scrap falling down, it may be convenient also to carry out the decrease of the desired value stepwise.

The more stable operation obtained when using the invention may thus allow the power curve to lie closer to the maximally available power level, while the increase to the maximum level can occur considerably more rapidly than before.