JP2020048314A - Reactive power compensator - Google Patents

Abstract

To provide a reactive power compensator that controls an increase in bus voltage to enable an operation to be continued safely even in the case where an AC arc furnace is brought into single-phase energization.SOLUTION: A reactive power compensator according to an embodiment is connected to a bus to which a three-phase alternating current is supplied, together with an AC arc furnace. The reactive power compensator includes: a power converter that injects reactive power into the bus; and a control device that generates a first control signal for supplying certain reactive power to the bus, generates a second control signal for supplying reactive power that controls flicker of the bus, and selects either one of the first control signal and the second control signal for supply to the power converter. The control device compares data on the current with a predetermined threshold value, and in the case where the current in any of the phases falls below the threshold value, supplies the first control signal to the power converter, and in the case where data on the current in all the phases is more than or equal to the threshold value, supplies the second control signal to the power converter.SELECTED DRAWING: Figure 1

Description

  An embodiment of the present invention relates to a reactive power compensator connected to a power supply system of an AC arc furnace.

  In a system to which an AC arc furnace for steelmaking is connected, voltage flicker becomes a problem due to a steep change in reactive power. Therefore, a flicker suppressing reactive power compensator is connected to the bus connected to the AC arc furnace for the purpose of keeping the voltage flicker within the regulation value.

  In such a system, in order to improve the power factor by compensating for the delayed reactive power of the AC arc furnace, a capacitor facility such as a phase advance capacitor is often provided. If the single-phase power is supplied during the operation of the AC arc furnace, a ferrification effect in which the voltage of the system increases due to a decrease in delayed reactive power may occur, and an excessive voltage may be applied to capacitor equipment and the like.

  Further, it is known that a power system generally has a resonance point and an anti-resonance point determined by a reactance component due to a transformer or an electric wire connected to the system and a capacitance component such as a capacitor facility. When the AC arc furnace becomes single-phase energized, the system voltage rises due to the reduction of delayed reactive power, and harmonic current flows out to the bus.At the anti-resonance point, the impedance of the system rises significantly, so the bus If the flowing harmonic current contains a component close to the frequency of the anti-resonance point, the voltage of the system is greatly distorted, and the voltage value further increases. For this reason, there is a possibility that a further overvoltage may be applied to devices, devices, and the like connected to the same bus.

JP-A-2017-11860 JP 2014-87207 A

  An embodiment of the present invention provides a reactive power compensator that can suppress a rise in bus voltage and continue operation safely even when an AC arc furnace that normally operates by three-phase current supply becomes single-phase current supply.

  The reactive power compensator according to the embodiment is connected to a bus to which three-phase alternating current is supplied, together with an AC arc furnace. The reactive power compensator generates a power converter that injects reactive power into the bus, and a first control signal for supplying a constant reactive power to the bus, and generates a reactive power that suppresses flicker of the bus. A control device for generating a second control signal to be supplied, selecting one of the first control signal and the second control signal, and supplying the selected signal to the power converter. The control device detects the current of each phase of the AC arc furnace, compares the data of the detected current of each phase with a preset threshold, and determines whether the current of any phase is When the voltage falls below a threshold value, the first control signal is supplied to the power converter, and when data of currents of all phases is equal to or larger than the threshold value, the second control signal is supplied to the power converter. To supply.

  In the present embodiment, a reactive power compensator capable of suppressing an increase in the bus voltage and continuing the operation safely even when the AC arc furnace that normally operates by three-phase energization becomes single-phase energized is realized.

FIG. 1 is a schematic block diagram illustrating a reactive power compensator according to an embodiment. FIGS. 2A and 2B are block diagrams illustrating a part of the reactive power compensator according to the embodiment. It is a block diagram which illustrates a part of reactive power compensation apparatus of a reference example.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
The drawings are schematic or conceptual, and the relationship between the thickness and the width of each part, the size ratio between the parts, and the like are not necessarily the same as actual ones. In addition, even when the same part is represented, the dimensions and ratios may be represented differently depending on the drawings.
In the specification and the drawings of the present application, the same reference numerals are given to the same elements as those described above with respect to the already described drawings, and the detailed description will be appropriately omitted.

FIG. 1 is a schematic block diagram illustrating a reactive power compensator according to the present embodiment.
As shown in FIG. 1, the reactive power compensator 10 of the embodiment is connected to a bus 101. The AC arc furnace is connected to the bus 101 and operates by receiving power supply from the bus 101. The reactive power compensator 10 injects power to be compensated into the bus 101 to reduce voltage flicker of the bus 101 generated by the arc furnace. The reactive power compensator 10 injects a reactive power corresponding to the reduced delayed reactive power into the bus 101 when the delayed reactive power decreases due to the single-phase operation of the arc furnace, and suppresses the increase in the bus voltage due to the advanced reactive power. Suppress.

  The bus 101 is connected to an AC arc furnace and the reactive power compensator 10, as well as a phase advance capacitor 40. The phase-advancing capacitor 40, which is a phase adjustment facility, improves the power factor lowered by the AC arc furnace. In addition, although not shown, an existing thyristor controlled reactor (TCR) may be interconnected to the bus 101 as another phase adjustment facility. In that case, a higher-order harmonic filter for suppressing harmonics by the TCR is also connected to the bus 101.

  The impedance of the bus 101, the devices and devices connected to the bus 101, and the system including the power system 1 has frequency characteristics, and the frequency at the resonance point or the anti-resonance point depends on the characteristics. Respectively. At the resonance point, series resonance occurs, and the impedance of the system is significantly reduced. At the anti-resonance point, parallel resonance occurs, and the impedance of the system rises significantly.

  In the AC arc furnace, the electrode 4 is connected to the bus 101 via the furnace transformer 3. The electrode 4 is arranged to be separated upward from the scrap 6 charged in the furnace body 5. The electrode 4 generates an arc discharge between the electrode 4 and a scrap 6 inserted in a fixed furnace body 5. The electrode 4 is raised and lowered with respect to the scrap 6 by an electrode lifting mechanism (not shown) so as to maintain the arc discharge.

  The bus 101 is supplied with three-phase AC power from the power system 1. The circuit from the electrode lifting mechanism and the electrode 4 to the bus 101 is provided for each of three phases. The electrode lifting / lowering mechanism is configured to connect the electrode 4 of each phase and the scrap 6 so that the impedance determined by the phase voltage applied to the electrode 4 corresponding to each phase of the three-phase alternating current and the line current flowing through the electrode 4 becomes constant. Control the distance between.

  As described above, the impedance of each phase is controlled to a substantially constant value by the electrode lifting / lowering mechanism, but the state of the arc discharge constantly fluctuates, and fluctuates more rapidly than the response speed of the electrode lifting / lowering mechanism. There is. In some cases, the impedance of one of the phases becomes larger than the impedance of the other phase, and the three-phase alternating current cannot be maintained in a balanced state, resulting in imbalance. When the unbalance increases, the power system 1 supplies power to the arc furnace substantially in a single phase. Therefore, the power supplied to the arc furnace decreases, and the delayed reactive power supplied to the arc furnace also decreases.

  The reactive power compensator 10 of the embodiment monitors the line current of each phase and determines whether the AC arc furnace is operating with three-phase current or single-phase current. When determining that the AC arc furnace is operating with single-phase energization, the reactive power compensator 10 switches from the flicker suppression control operation to a constant reactive power compensation operation. In the constant reactive power compensating operation, the reactive power compensating device 10 injects the delayed reactive power reduced by the operation of the AC arc furnace by single-phase conduction into the bus 101 to improve the power factor.

A specific configuration example of the reactive power compensator 10 will be described.
The reactive power compensating device 10 includes a control device 20 and a reactive power compensating device (power converter) 30 for flicker suppression. Control device 20 supplies a control signal to flicker suppressing reactive power compensating device 30 based on the data of the voltage of bus 101 and the data of the current flowing through electrode 4. As the control signal, either a flicker compensation control signal (second control signal) for performing flicker compensation control or a constant reactive power control signal (first control signal) for performing constant reactive power control is selected. Is output. The flicker suppression reactive power compensator 30 generates reactive power to be compensated based on either the flicker compensation control signal or the constant reactive power control signal, and injects the generated appropriate reactive power into the bus 101.

  The control device 20 includes a constant reactive power control circuit 21, a flicker suppression control circuit 22, a selection circuit 23, and a switching circuit 24.

  A constant reactive power control circuit (shown as a constant Q control circuit) 21 is connected to the output of the instrument transformer 7 and the output of the current transformer 2, and outputs data of each phase voltage of the bus 101 and each current flowing through the electrode 4. Input phase line current data. The constant reactive power control circuit 21 outputs a constant reactive power control signal based on data of each phase voltage of the bus 101 and data of each line current flowing through the electrode 4. The constant reactive power control signal includes a reactive power command value corresponding to the reduced delayed reactive power. According to the output reactive power command value, the flicker suppressing reactive power compensator 30 injects delayed reactive power corresponding to the delayed reactive power reduced by the operation of the AC arc furnace by single-phase conduction into the bus 101, and Improve rates.

  The flicker suppression control circuit 22 is connected to the output of the instrument transformer 7 and the output of the current transformer 2, and inputs data of each phase voltage of the bus 101 and data of line current of each phase flowing through the electrode 4. . The flicker suppression control circuit 22 outputs a flicker compensation control signal based on the data of each phase voltage of the bus 101 and the data of each phase line current flowing through the electrode 4. The flicker compensation control signal includes a reactive power command value for suppressing flicker on the bus 101.

  Outputs of the constant reactive power control circuit 21 and the flicker suppression control circuit 22 are connected to a first input and a second input of the switching circuit 24, respectively. The switching circuit 24 supplies either the output of the constant reactive power control circuit 21 or the output of the flicker suppression control circuit 22 to the flicker suppression reactive power compensator 30 based on the output of the selection circuit 23.

  The selection circuit 23 receives line current data of each phase and compares the input line current data with a preset threshold value. When the data of the line currents of all phases is equal to or larger than the threshold value, the selection circuit 23 connects the switching circuit 24 to the second input. An output of the flicker suppression control circuit 22 is connected to a flicker suppression reactive power compensator 30 via a second input of the switching circuit 24. When the line current data of each phase is lower than any one threshold value, the selection circuit 23 connects the switching circuit 24 to the first input. An output of the constant reactive power control circuit 21 is connected to a flicker suppressing reactive power compensator 30 via a first input of the switching circuit 24.

  In the AC arc furnace, the elevation of the electrode 4 is controlled for each phase of the three-phase AC, and the impedance is controlled to be constant. As described above, in the reactive power compensator 10 of the embodiment, when the impedance constant control is normally performed on all three phases, the control signal generated by the flicker suppression control circuit 22 is used to suppress flicker. Based on this, a reactive power compensation operation is performed.

  On the other hand, when the single-phase energization is performed, the amount of power input to the arc furnace decreases, so that the delayed reactive power decreases and the power factor of the system decreases. The reactive power compensator 10 according to the embodiment generates a reactive power command value corresponding to the amount of decrease in delayed reactive power, and injects the delayed reactive power into the bus 101.

  The flicker suppressing reactive power compensator (power converter) 30 generates a voltage and a current so that the reactive power corresponds to the supplied reactive power command value, and supplies the generated voltage and current to the bus 101. The flicker suppressing reactive power compensating device 30 is preferably a self-excited power converter, and an appropriate circuit system is adopted according to the power capacity and voltage supplied to the bus 101.

The operation of the reactive power compensator 10 according to the embodiment will be described.
FIGS. 2A and 2B are block diagrams illustrating a part of the reactive power compensator according to the embodiment.
FIGS. 2A and 2B are configuration examples of the selection circuit. As shown in FIG. 2A, the selection circuit 23 includes comparators 231R, 231S, 231T, on-delay circuits 232R, 232S, 232T, and an AND circuit 233.

  The comparators 231R, 231S, 231T are connected to the current transformer 2 of each phase. Each of the comparators 231R, 231S, and 231T includes a preset threshold value Is. Each of the comparators 231R, 231S, and 231T outputs an H-level signal when the line current of each phase is equal to or greater than the threshold Is. Each of the comparators 231R, 231S, and 231T outputs an L-level signal when the line current of each phase is smaller than the threshold Is.

  As the threshold value Is, an appropriate value is selected and set for each arc furnace. The threshold value Is is set by, for example, actually measuring the current value when the arc discharge is stably continued in the arc furnace, and is set to, for example, 50% of the discharge time.

  The outputs of the comparators 231R, 231S, 231T are connected to an AND circuit 233 via on-delay circuits 232R, 232S, 232T, respectively. The on-delay circuits 232R, 232S, and 232T output an H-level signal when the H-level signal is longer than a preset on-delay time. The on-delay circuits 232R, 232S, and 232T output an L-level signal when the H-level signal is shorter than the on-delay time and when an L-level signal is input.

  That is, the selection circuit 23 outputs an H-level signal for each phase current as long as the state in which the data of the line current of each phase is equal to or more than the threshold Is continues beyond the on-delay time. Outputs an L-level signal.

  By setting the on-delay time, it is possible to prevent the logical value output from oscillatingly fluctuating in response to a sharp current fluctuation due to arc discharge.

  AND circuit 233 outputs an H-level signal when all inputs (outputs of all on-delay circuits) are at H level. The AND circuit 233 outputs an L level signal when any of the inputs is at the L level.

  When the output of the AND circuit 233, that is, the output of the selection circuit 23 is at the H level, the switching circuit 24 is connected to the flicker suppression control circuit 22. The output of the selection circuit 23 connects the switching circuit 24 to the constant reactive power control circuit 21 when the output is at the L level.

  As shown in FIG. 2B, one on-delay circuit 233 may be provided at the output of the AND circuit 233. That is, the selection circuit 23a includes comparators 231R, 231S, 231T provided for each phase, an AND circuit 233, and an on-delay circuit 234. The outputs of the comparators 231R, 231S, 231T are connected to the AND circuit 233 Have been. An on-delay circuit 234 is connected to the output of the AND circuit 233.

  In this case, in the selection circuit 23a, when the output of the AND circuit 233 is at the H level and the H level state continues for the ON delay time or longer, the ON delay circuit 234, that is, the selection circuit 23a outputs the H level signal. I do. When AND circuit 233 outputs an H-level signal for a shorter time than the on-delay time, and when AND circuit 233 outputs an L-level signal, selection circuit 23a outputs an L-level signal.

  In this way, the reactive power compensator 10 of the embodiment monitors the data of the current of each phase of the three-phase alternating current, and when it detects that the current value is small in one phase, it is a single-phase operation. It operates to inject the insufficient delayed reactive power into the bus 101.

The effect of the reactive power compensator 10 of the embodiment will be described.
In the reactive power compensator 10 of the embodiment, the selection circuit 23 determines whether or not the data of the line current of all phases is equal to or larger than the threshold Is. The selection circuit 23 determines that the three-phase alternating current is supplied and the arc discharge control by the constant impedance control is performed in all the phases when the line current data of all the phases is equal to or larger than the threshold value Is. In this case, since the reactive power command value for suppressing flicker is generated, the reactive power compensator 10 can appropriately suppress the voltage flicker of the bus 101.

  On the other hand, if the line current data of any of the three phases falls below the threshold value Is, it is determined that the arc furnace is operating in a single phase, and the delay invalidated by the single-phase operation is reduced. A reactive power command value corresponding to the power is generated. Therefore, the reactive power compensator 10 improves the power factor by compensating for the delayed reactive power reduced by the single-phase operation of the arc furnace.

  As described above, the phase-advancing capacitor 40 is connected to the bus 101 to which the AC arc furnace is connected in order to compensate for the delayed reactive power caused by the AC arc furnace. The system has a reactance component such as a transformer or a wire of the system and a capacitance component such as a capacitor facility including the phase advance capacitor 40.

  Further, the existing busbar 101 is provided with an existing TCR, and in many cases, a harmonic filter or the like for removing harmonics by the TCR is provided, so that an impedance network including an AC arc furnace and these facilities can be constructed. In an AC arc furnace, the frequency at the anti-resonance point due to the parallel resonance of the impedance network of the system including the AC arc furnace may be very close to twice the frequency of the AC power supply.

  When the AC arc furnace operates with single-phase energization, delayed reactive power decreases, the power factor decreases, and a relatively large harmonic current flows into the bus 101. This harmonic current has a waveform like a sine wave on which a direct current is superimposed, and contains a second harmonic component.

  That is, when the arc furnace is operated by single-phase conduction, a harmonic current containing a second harmonic component flows into the bus 101, and the impedance of the system having an anti-resonance point close to the frequency of the second harmonic causes the bus 101 Voltage is greatly distorted. For this reason, the voltage value of the bus 101 may excessively increase, and an excessively large voltage may be applied to devices, devices, and the like connected to the bus 101, which may have a bad influence such as breakage.

  According to the reactive power compensator 10 of the embodiment, when the delayed reactive power is reduced by the operation of the arc furnace by the single-phase conduction, the reactive power command value corresponding to the reduced delayed reactive power is generated. , Delay reactive power is injected into the bus 101. Therefore, an increase in the bus voltage can be suppressed, and an excessive voltage can be prevented from being applied to devices, devices, and the like connected to the bus 101.

FIG. 3 is a block diagram illustrating a part of the reactive power compensator of the reference example.
The reference example of FIG. 3 is different from the embodiment in that the AND circuit 233 of the selection circuit 23 in the above-described embodiment is an OR circuit 235, and the other points are the same as those in the embodiment. is there.
As shown in FIG. 3, in the selection circuit 123, the signals output from the comparators 231R, 231S, 231T provided for each phase are input to the OR circuit 235 via the ON delay circuits 232R, 232S, 232T. . The selection circuit 123 outputs an L level signal when the current data of all phases is smaller than the threshold Is, and outputs an H level signal in other cases. That is, even if the data of the current of any phase is missing, if the data of the current of the other phase is equal to or more than the threshold Is, the reactive power compensator of the reference example is generated by the flicker suppression control circuit. In addition, a reactive power command value for flicker suppression is continuously generated.

  On the other hand, in an arc furnace, one phase is missing, and the operation by single-phase conduction reduces the delay reactive power of the arc furnace and may cause a harmonic current having a frequency close to the anti-resonance point to flow. . Therefore, the impedance of the power supply system including the bus 101 becomes extremely large, and the voltage of the bus 101 increases.

  As described above, the only change of the selection circuit 23 is that the OR circuit 235 is modified to an AND circuit. Therefore, when the operation of the reactive power compensating device 10 is realized by a program or the like, the reactive power compensating device 10 that can continue the operation safely by suppressing the harmonic current caused by the single-phase energization of the arc furnace with few changes. Can be realized.

  According to the embodiment described above, an AC arc furnace that normally operates by three-phase current supply realizes a reactive power compensating device that can safely continue operation by suppressing a rise in bus voltage even when single-phase current supply is performed. can do.

  Although some embodiments of the present invention have been described above, these embodiments are presented as examples and are not intended to limit the scope of the invention. These new embodiments can be implemented in other various forms, and various omissions, replacements, and changes can be made without departing from the spirit of the invention. These embodiments and modifications thereof are included in the scope and spirit of the invention, and are also included in the scope of the invention described in the claims and the equivalents thereof. The above-described embodiments can be implemented in combination with each other.

  Reference Signs List 1 power system, 2 current transformer, 3 furnace transformer, 4 electrodes, 5 furnace body, 6 scrap, 7 instrument transformer, 10 reactive power compensator, 20 controller, 21 reactive power constant control circuit, 22 flicker Suppression control circuit, 23 selection circuit, 24 switching circuit, 30 Reactive power compensator for flicker suppression, 40 advance capacitor, 101 bus

Claims (3)

A reactive power compensator connected to an AC arc furnace to a bus to which three-phase AC is supplied,
A power converter that injects reactive power into the bus,
A first control signal for supplying a constant reactive power to the bus is generated, a second control signal for supplying a reactive power for suppressing flicker of the bus is generated, and the first control signal and the second control signal are generated. A control device for selecting one of the two control signals and supplying the selected signal to the power converter;
With
The control device detects the current of each phase of the AC arc furnace, compares the data of the detected current of each phase with a preset threshold, and determines whether the current of any phase is When the voltage falls below a threshold value, the first control signal is supplied to the power converter, and when data of currents of all phases is equal to or larger than the threshold value, the second control signal is supplied to the power converter. Reactive power compensator to supply to   The reactive power compensator for an AC arc furnace according to claim 1, wherein a phase-advancing capacitor for improving a lag power factor of the AC arc furnace and a harmonic filter for suppressing harmonics of the phase adjustment equipment are connected to the bus. . In the selection circuit,
The first control signal is supplied to the power converter when current data of any of the phases does not exceed the threshold value for more than a predetermined time,
3. The reactive power compensator according to claim 1, wherein the second control signal is supplied to the power converter when data of currents of all phases is equal to or greater than the threshold for the predetermined time. .

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