SU1737768A1 - Device for power supply of electric arc furnaces - Google Patents

Description

to contactor 28, windings 23 to contactor 29, windings 24 to clamps of contactor 30.

Transformers 1-3 are equipped with their own additional windings 31-33, which are phase-connected in a triangle circuit.

Features of the connection of transformer windings, determining the electromagnetic processes in them and in the power supply network, follow from the circuit diagram of FIG. 1. In particular, the serial connection of transformer windings causes a multiple decrease in current shocks during operational short circuits of the arc furnace both in the secondary winding of the corresponding furnace transformer and in the supply network due to the almost instantaneous redistribution of voltages between the phases of the primary windings of the transformers of the corresponding group. If, for example, due to the collapse of the charge or another reason, one of the phases of the secondary winding 8 of the transformer 2 and the furnace 14 connected to it was practically shorted, then it will almost instantly occur:

a decrease in the voltage of the corresponding phase of the primary winding 20 of the furnace transformer 2 due to the redistribution of the voltage between this primary winding 20 and the windings 19, 21 of the transformers 1.3;

eliminating the conditions for the occurrence of a shock in the supply network due to such an operational short circuit of the furnace 14;

smoothing the possible fluctuations of the total reactive power consumed from the network by these electric arc furnaces. Next, the electromagnetic control system for lifting the rods of the electric furnace will work, and after the elimination of this short-circuit of the furnace, the normal relation between the voltages on the series-connected primary windings of the transformers 1-3 will be restored. Similar electromagnetic processes occur in another group of transformers 4-6 and furnaces 16-18 attached to them.

During operation of the device, various modes of simultaneously operating furnaces can take place: one furnace operates in the melting mode, and the other in the refining mode, etc. In addition, there may be a temporary introduction from the operation of any furnace without disturbing the working conditions of other furnaces; for this, the circuit has three-phase contactors 25-30, with which the primary can be shorted

winding of the corresponding furnace transformer.

The features of this device are associated with uneven load

in the three-phase secondary circuit of a furnace transformer. When analyzing the electromagnetic processes in the supply network and the three-phase furnace transformers connected to it, we will

0 suppose the system of linear voltages of the source - supply network - symmetrical, and its internal resistance to zero. In these conditions will take place:

asymmetry of the stress system

5 secondary winding of the transformer with uneven load;

asymmetry of the voltage system on the primary windings of the same transformer;

0symmetry of voltage systems

the primary windings of other furnace transformers of a group common to them with a phase-phase connection of the primary windings, caused both by the symmetry of the system of their total voltage and by the uneven load of the secondary winding of one of these three-phase transformers;

caused by this asymmetry of systems

0 voltages on the secondary windings of transformers of the same group. Consequently, the phase unevenness of the load of at least one furnace transformer causes the asymmetry of the voltages of the secondary winding of even a transformer with a uniform load due to the asymmetry of the voltages on its primary winding noted above.

The violation of the symmetry of the voltage system on the delta-connected secondary winding of the furnace transformer, caused by the unbalanced melting mode of the fired arc furnace, will result in:

5k violation of the symmetry of the voltage system on the secondary windings of other furnace transformers of one of their groups; to non-simultaneous passage through zero or maximum values

0 third current harmonics in separate phases of the secondary winding of the furnace transformer;

to the possibility of closing the magnetic flux from the current of the third harmonic in the phase of the secondary winding located on (one of the rods of the transformer magnetic conductor, through the adjacent rod of this transformer magnetic conductor;

induction of the EMF components in the primary winding of the transformer by the flow components from the third and multiple harmonics of the current in the secondary winding of the transformer;

to the impossibility of considering in these conditions a connection by a triangle of the secondary winding as an adequate means of protecting the primary windings of furnace transformers from various kinds of unpleasantnesses associated with these higher harmonics;

transmission to the primary circuit of the third and multiple voltage and current harmonics.

It is known that a system of voltages or currents of a threefold (and multiple) frequency forms a zero phase sequence if the corresponding base frequency system forms a direct phase sequence.

Phase-to-serial connection of the primary windings 22-24 of the furnace-transformer group 4-6 in a triangle corresponds to the symmetry of the system of total voltages of the fundamental frequency on the sides of the triangle. Such a connection for the third voltage harmonics, excited in the primary windings of transformers of one group with the same current harmonics in the secondary windings of transformers, should be considered as a short circuit, which:

creates the conditions for the demagnetizing effect of the flux linkages of the individual phases of the primary winding of each transformer with respect to the third components (and their multiplications) of the current harmonics in the secondary windings:

therefore, it is possible to consider each of the furnace transformers to be in short-circuit mode for the third and multiple harmonics of current in the windings of such a transformer;

eliminates the conditions for the occurrence of voltages in the windings 22-24 of this group of transformers 4-6.

Also, the system of voltages applied to each beam of a star by a phase-series connection of primary windings 19-21 of a group of transformers 1-3 is also symmetrical. If in the beginning it is conditional not to take into account the additional windings 31-33 of this group of transformers, then we can note:

the same direction of the third harmonic of the total voltage on each beam of the star of a phase-to-phase connection of the primary windings 19-21 relative to the output terminals of the same name of such a connection;

the mutual compensation of the component stresses on the series connection of any pair of rays of such a star from the third and multiple

harmonics;

the resulting idle mode of each of the transformers 1-3 1-3 for the third and multiple of the highest harmonics of current in the secondary windings 7-9 furnace

transformers 1-3;

unacceptably large overvoltages on the primary windings of 19-21 transformers of this group, expressed by the sum of a large number of terms, each of which is expressed by the product of the final high harmonic of the current by the very large inductive idle resistance for the corresponding increased frequency component. The value of

each higher (third or multiple) harmonic of the current is almost unchanged, since the higher harmonic of the current is a source of current, not voltage.

The danger of such an emergency situation for isolating the primary windings 19-21 disappears due to the use of additional windings 31-33 in this group of transformers, which are connected in triangle in phase-series. In this case, the third and multiple harmonics of the current will be closed inside such a circuit as components of zero phase following.

For the components of zero phase following from the third and multiple higher harmonics of current, such a connection of additional windings should be considered as a short-circuited circuit, which creates

the conditions for the demagnetizing effect of the flux linkages of the individual phases of the additional winding of each of these transformers with respect to the higher harmonic components of the fluxes,

excited by the third and multiple of the highest harmonic currents in the secondary winding of the transformer;

allows you to consider each of the furnace transformers 1-3 being

in the short circuit mode for the third and multiple harmonics of the current;

eliminates overvoltages on the windings of 19-21 transformers of this group.

Claims (1)

The invention The device for powering electric arc furnaces, containing two groups of three-phase furnace transformers with the same type connected by secondary windings, in order to improve the quality of consumed energy, the primary windings of one group of furnace transformers are phase-wise sequentially connected to a common triangle circuit, the primary windings of the second group of transformers are phase-wise connected to a star and each second transformer the group is equipped with an additional three-phase winding, the conclusions of all the additional windings are connected in phase to a common triangle. 1ShShGG 4 5, t w t t M