SU1681367A1 - Method of controlling three-phase frequency converter - Google Patents

Description

one

(21) 4358450/07

(22) 04 01 88

(46) 09.30.91. №36

(71) All-Union Scientific Research, Design and Technological Institute of Power Semiconductor Devices

(72) C. G. Mashyn and S. A. Morgulev (53) 621.316.727 (088.8)

(56) US Patent No. 4388108, cl. H 02 M 5/27, 1983.

USSR Author's Certificate No. 1272958, cl. H 05 B 7/144, 1986.

(54) METHOD OF MANAGING THREE-PHASE DIRECT FREQUENCY CONVERTER

(57) The invention relates to electrical engineering and can be used to power

low-frequency three-electrode electric furnaces The invention virtually eliminates electrodynamic loads on current-carrying elements of the device when the polarity of the current changes while maintaining a high power factor of the converter due to the fact that the current changes smoothly when the valve groups are turned on and off, for example, according to a linear law 0.01-0.05 period of low frequency, for which in each control unit 11, 12, 13 an intensity setting block 14, a comparator 20, an OR 21c element are entered by connecting. At the same time, to maintain a high cos setting, the switching frequency of the groups and, accordingly, the currents in the electrodes is set within 0.04-0.004 Hz using the frequency setting unit 22. 2 Il.

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The invention relates to electrical engineering, in particular, to converter equipment, mainly to methods and power sources of electric arc furnaces and can be used in electrothermal installations containing a three-electrode electric furnace, which is powered from a three-phase direct current converter of the industrial frequency to low frequency current.

The aim of the invention is to reduce the vibrations of the electrodes, elements of the furnace and conductors while maintaining high energy performance of the electric furnace.

FIG. 1 shows an electrical diagram of a device for powering a three-electrode electric furnace according to the proposed method; in fig. 2 - time diagrams of currents of valve sets and furnace electrodes.

The device (Fig. 1) contains a three-phase power supply transformer 1, to the secondary windings of which the valve sets 2-4 are connected. Each valve set contains two groups of anti-parallel-connected valves connected via a three-phase bridge circuit. The low frequency terminals of the valve sets A, X, B, U, C through the conductors 5-7 are connected to the electrodes A, C, C of the furnace according to the triangle on the electrodes.

The bath of the furnace is shown in the replacement circuit with series-connected resistive and inductive resistances of 8NO.

The control unit of each valve set 11-13 contains successively connected intensity setting unit 14, comparison unit 15, current regulator 16, phase-shifting unit 17, shunting unit 18 of control units 11-13 connected to control inputs of sets valves, and their inputs are connected to the voltage of the current set of the valve set, the output of the sensor 19 current sensor 19.

Each control unit 11-13 contains a comparator 20 and an OR element 21. The output of the intensity setting node is also connected via a comparator 20 to the first input of the OR element 21, the second input of which is connected to the input of the intensity setting section 14 associated with the corresponding output block 22 sets the frequency of the current, and the output of the OR element is connected to the second input of the driver 18 pulses.

The device works as follows.

The network voltage through the supply transformer 1 is fed to the inputs of valve sets 2-4, which, by the command of the control blocks 11-13, transform

current frequency of the frequency in the current with a frequency of 0.04-0.004 Hz. The converted current through the bifilar conductors 5–7 is supplied to electrodes A, B, C of the electric furnace. The current frequency is set by block 22, which, with the help of control blocks 11-13, turns on in turn the valve groups in each set 2-4 after turning off the same group of leading phase set 4, 2, 3. The duration of current flow in groups is set by block 22 and can vary from 0 to 1/2 of the low frequency period, which is set within 0.04-0.004 Hz. The symmetrical mode of operation of the furnace corresponds to the duration of the flow of current in groups of valves equal to 1/3 of the low frequency period (Fig. 2).

The current in each electrode (d, IB, ic in Fig. 2) is obtained by adding the currents of two valve sets (ia, s, or s, m. Or k te in fig. 2).

5 At the input of the units 14 for setting the current intensity of the blocks 11-13 of the control, set the voltage for setting the current Uz corresponding to the full unlocking of the valves. The input of the intensity setting node 14 simultaneously with the voltage of the current setting receives a signal for setting the frequency of the current, which is square-wave pulses with a duration of 1/3 of the low frequency period, the frequency of which is twice

5 above the frequency of change of polarity of the current in the electrodes. Along the leading edge of the rectangular pulse, the node 14 of a given intensity generates a rising voltage at the output, and on a predetermined front, falling values. The current intensity setting unit 14 provides a smooth rise and fall of the current regardless of the state of the furnace bath during Tn and tc (Fig. 2). The rest of the valves

5 is fully open, and with a significant decrease in the resistance of the bath for technological reasons, the current regulator 16 limits the current, which is signaled by sound and light.

0 Depending on the voltage at the output of the current regulator 16, the phase-shifting device 17 changes the phase of the pulses unlocking the valves.

The logical pulse shaping units form pulses for unlocking the valve sets, the phase of which is determined by the phase shifter 17, and the pulse repetition rate is set by the current frequency setting unit 22. Impulses at the output of block 18 formation

The pulses appear under the condition of the coincidence of the pulses at the output of the phase shifter 17 and the pulses of the current frequency setting unit 22, the presence of an enable signal and the signal from the intensity setting unit 14. Pulses are removed from each valve group under the condition that the current in this group decreases to zero or to a predetermined threshold.

Technical and economic advantages of the proposed device are visible in comparison with the prototype.

The proposed technical solution, when operating in the range of 0.04–0.004 Hz, ensures, without degrading the power factor (cos p), a smooth change in the polarity of the current in the electrodes, which makes it possible to reduce vibrations of the structural elements of the electrothermal installation, i.e., electrodynamic overloads.

In this mode of operation, the reactance of the conductors and the furnace bath is practically reduced to zero both in the fundamental harmonic and in the higher order harmonics, which can be neglected in the selected frequency range, which ensures the preservation of high energy indices.

Claims (1)

Invention Formula A method of controlling a three-phase direct frequency converter for supplying industrial electric furnaces, including three valve sets, each consisting of the first and second valve groups, consists in forming and serving to the control inputs of the valve sets of unlocking sequences with a predetermined 5 frequency, lower than the frequency of the supply network, the gates of each set during the first half-cycle of the low frequency are fed and removed by unlocking pulses, and during the second half-period the bottom The frequency band serves and removes the pulses of unlocking on the valves of the second group, measures the amplitude of the current of the valve groups, changes the phases of the pulses of unlocking of the valves, as a result of which they limit the measured 5 current amplitude, characterized in that, in order to reduce the electrodynamic loads on the conductors and electrodes of the electric furnace when the polarity of the current in the electrodes is changed, the polarity of the output current 0 converter is changed with a frequency of 0.04-0.004 Hz by feeding with the specified frequency of sequences of pulses unlocked alternately for each valve set, while at polarity reversal 5 current in the valve set reduces the current of the switched off group of valves and increases the current of the switched on group of valves during the 0.01-0.05 period of low frequency linearly by changing the phase 0 pulses are unlocked by valves, and the pulses from each group of valves are removed after the current amplitude of the corresponding valve group is reduced to a predetermined value. tn tc ABOUT N  la / GI Tdrf l Fig 2