Reactive power compensation device

Abstract

To provide a reactive power compensation device for an arc furnace which prevents a voltage of a bus from rising.SOLUTION: A reactive power compensation device comprises a control device and a power converter. The control device detects a voltage and a current of a power system to which an arc furnace is connected, calculates reactive power and outputs a calculated command value to the power converter. The command value includes a first command value corresponding to a fluctuated portion of a phase and a second command value corresponding to a non-fluctuated portion of the phase. In a case where a magnitude of power that the power converter outputs is smaller than a predetermined value, the first command value and the second command value are added and outputted by the control device. In the case where the magnitude of power is equal to or larger than the predetermined value, a predetermined first setting value is set to the second command value, and the first command value and the second command value are added and outputted by the control device.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The embodiments of the present invention relate to a reactive power compensation device primarily used in electric arc furnaces for steel making. [Background technology]

[0002] In a power system to which a steelmaking arc furnace is connected, a sudden load change in the arc furnace causes a reactive power fluctuation, which generates a voltage flicker. Customers who have an arc furnace as a load are required to keep the voltage flicker within a prescribed regulation value. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 8-265974 Summary of the Invention [Problem to be solved by the invention]

[0004] The reactive power compensator generates reactive power to offset the fluctuations in reactive power that cause voltage flicker, and is connected to the busbar of the power system to which the steelmaking arc furnace is connected. Leading-phase equipment such as leading-phase capacitors and harmonic filters are connected to the busbar to improve the power factor.

[0005] In such an arc furnace, when the load decreases, the lagging reactive power of the bus becomes smaller, and the leading reactive power of the leading-phase equipment may become excessive. In this case, the excessive leading reactive power increases the bus voltage, and the increased voltage may exceed the rated voltage value of the equipment and devices connected to the bus.

[0006] Conventionally, reactive power compensators for suppressing flicker only compensate for fluctuations in reactive power, and therefore may not be able to fully compensate for leading reactive power even if leading reactive power occurs due to variations in leading-phase equipment.

[0007] The embodiments of the present invention have been made to solve the above-mentioned problems, and an object of the present invention is to provide a reactive power compensation device for an arc furnace that prevents a voltage rise on a bus bar. [Means for solving the problem]

[0008] An embodiment of the present invention is a reactive power compensator that compensates for reactive power of a busbar of a power system to which an arc furnace is connected. The reactive power compensator includes a control device that receives detection values ​​of a first voltage measured by a potential transformer connected to the busbar, a first current flowing through the electrode, and a second current flowing through a leading-phase equipment connected to the busbar, calculates a first reactive power supplied in the entire system based on the detection values, and calculates a command value corresponding to the first reactive power, and a power converter that generates and outputs a second reactive power to be output from a converter based on the command value. The control device separates the first reactive power into a fluctuating component and a non-fluctuating component of reactive power over time and calculates the first reactive power, and when the magnitude of the first reactive power is smaller than a first predetermined value, adds a first command value corresponding to the fluctuating component and a second command value corresponding to the non-fluctuating component to calculate a new command value and outputs it to the power converter, and when the first reactive power is equal to or greater than the first predetermined value, adds a first setting value that is set in advance to the second command value and outputs it to the power converter as a new command value. Effect of the Invention

[0009] According to an embodiment, a reactive power compensation device for an arc furnace is provided that prevents a voltage rise on a busbar. [Brief description of the drawings]

[0010] [Figure 1] 1 is a schematic block diagram illustrating a reactive power compensation device according to an embodiment; [Diagram 2] 2(a) to 2(c) are graphs for explaining the operation of the reactive power compensation device of FIG. [Diagram 3]3(a) to 3(c) are graphs for explaining the operation of the reactive power compensation device of FIG. [Figure 4] 4 is an example of a flowchart for explaining an operation of a reactive power compensation device according to an embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] Hereinafter, an embodiment will be described with reference to the drawings. The drawings are schematic or conceptual, and the relationship between the thickness and width of each part, the size ratio between parts, etc. are not necessarily the same as in reality. Even when the same part is shown, the dimensions and ratios of each part may be different depending on the drawing. In this specification and each drawing, elements similar to those described above with reference to the previous drawings are given the same reference numerals and detailed descriptions thereof will be omitted as appropriate.

[0012] FIG. 1 is a schematic block diagram illustrating a reactive power compensation device according to an embodiment. FIG. 1 shows a reactive power compensator 100, as well as components constituting an arc furnace and power supply facility elements that supply power to the arc furnace. 1, the reactive power compensator 100 is connected to a bus 11 via a transformer 8. The bus 11 is connected to a power system 10. The reactive power compensator 100 generates a desired reactive power and injects the generated reactive power into the bus 11.

[0013] An electrode 1a is connected to the busbar 11 via a furnace transformer 2. The electrode 1a is positioned away from the scrap 1b charged in the furnace 1. An arc discharge occurs between the electrode 1a and the scrap 1b. The scrap 1b is heated by the heat generated by the arc discharge. The power input to the scrap 1b is controlled by keeping the distance between the electrode 1a and the scrap 1b appropriate using an electrode lifting device (not shown) and by switching the tap of the furnace transformer 2, etc.

[0014] A potential transformer 5 is connected to the bus bar 11. The potential transformer 5 detects and outputs a voltage (first voltage) v0 of the bus bar 11.

[0015] A leading-phase equipment 3 is connected to the busbar 11. The leading-phase equipment 3 includes a leading-phase capacitor and is installed so as to lead the reactive power of the busbar 11 in order to suppress a drop in the busbar voltage due to lagging reactive power generated by the power input to the AC arc furnace.

[0016] Current transformers 4 and 9 are provided on the primary side of the furnace transformer 2 and the phase-leading equipment 3, respectively. The current transformer 4 detects a current i1 (first current) flowing through the primary side of the furnace transformer 2. The current transformer 9 detects a current (second current) i2 flowing through the phase-leading equipment 3.

[0017] The output of the potential transformer 5, the output of the current transformer 4, and the output of the current transformer 9 are connected to the reactive power compensator 100. Data on the voltage v0 of the bus 11, the current i1 flowing through the furnace, and the current i2 flowing through the leading-phase equipment 3 are input to the reactive power compensator 100. The reactive power compensator 100 generates a command value for the required reactive power based on each data of the voltage v0, the currents i1, and i2, and injects the required reactive power QSVC into the bus 11 based on the generated command value.

[0018] The configuration and operation of the reactive power compensator 100 will be described. The reactive power compensation device 100 includes a power converter 6 and a control device 7. The control device 7 is connected to the power converter 6. The control device 7 calculates a reactive power command value based on each piece of data of a voltage v0 and currents i1 and i2 that are sequentially input. The control device 7 outputs the sequentially calculated reactive power command value to the power converter 6. The control device 7 is, for example, a programmable controller.

[0019] The power converter 6 is, for example, a converter formed of a self-excited power conversion circuit. The power converter 6 injects reactive power generated in accordance with a reactive power command value output from the control device 7 into the bus 11.

[0020] In the reactive power compensation device 100 according to the embodiment, the control device 7 calculates the reactive power QF of the arc furnace using data on the voltage v0 and the current i1. The control device 7 also calculates the reactive power QC of the leading-phase equipment using data on the voltage v0 and the current i2. The control device 7 calculates a reactive power command value based on the reactive power QF of the arc furnace and the reactive power QC of the leading-phase equipment. Specifically, the control device 7 calculates the sum of the reactive powers QF and QC to calculate the reactive power Q to be compensated. The control device 7 calculates and outputs a command value corresponding to the reactive power Q to be compensated.

[0021] The control device 7 outputs the calculated command value for reactive power to the power converter 6.

[0022] The operation of the reactive power compensator 100 will now be described in more detail. 2(a) to 2(c) are graphs for explaining the operation of the reactive power compensation device of FIG. In FIG. 2(a) to FIG. 2(c), the horizontal axis represents time, and on the vertical axis, the positive direction represents the magnitude of leading reactive power, and the negative direction represents the magnitude of lagging reactive power.

[0023] As shown in Fig. 2(a), in the leading-phase equipment 3, the reactive power has a fixed delay. The reactive power QF among the power input to the arc furnace lags as a whole, and the delay varies with time. Flicker occurs due to the fluctuation in the delay of the reactive power QF.

[0024] The reactive power Q of the bus 11 is the sum of QF and QC, and the reactive power Q is leading overall. If the reactive power Q in the case of Fig. 2(a) is left unattended, the voltage of the bus 11 will rise, and if the voltage value exceeds the rated value of the devices and equipment connected to the bus 11, damage to the devices and equipment may occur.

[0025] As shown in Fig. 2(b), the control device 7 calculates a command value corresponding to reactive power QSVC that cancels out Q = QF + QC, and outputs it to the power converter 6. Since the power converter 6 injects the reactive power QSVC into the bus 11, the reactive power Q' of the bus 11 becomes the sum of the reactive power Q before compensation and the reactive power QSVC output by the reactive power compensator 100, and in this example, the delay of the reactive power is cancelled out and becomes 0. In this case, the voltage of the bus 11 is set to an appropriate voltage, and a voltage suitable for the rated voltage is applied to the devices and equipment connected to the bus 11.

[0026] FIG. 2(c) shows a method of calculating the reactive power QSVC output by the reactive power compensator 100. As shown in FIG. 2(c), the reactive power QSVC is the sum of a fluctuation component ΔQ and a non-fluctuation component Qb. The control device 7 separates the fluctuation component and the non-fluctuation component from the calculated reactive power Q of the bus 11, and calculates a command value corresponding to ΔQ for offsetting the fluctuation component and a command value corresponding to Qb for offsetting the non-fluctuation component. The control device 7 adds the command value corresponding to ΔQ and the command value corresponding to Qb to calculate a command value corresponding to QSVC, and outputs it to the power converter 6. The power converter 6 generates reactive power QSVC according to the command value and injects it into the bus 11.

[0027] When the magnitude of the reactive power QSVC injected into the bus 11 is smaller than a predetermined value, the reactive power compensator 100 applies the reactive power QSVC by adding ΔQ and Qb, which is greater than 0. Since the power value that can be output by the power converter 6 is finite, when the magnitude of the reactive power QSVC injected into the bus 11 is equal to or greater than a predetermined value, the non-fluctuation component of the reactive power Qb is reduced or updated to 0, and the fluctuation component of the reactive power ΔQ is output as the reactive power QSVC.

[0028] 3(a) to 3(c) are graphs for explaining the operation of the reactive power compensation device of FIG. 3(a) to 3(c) show the time variation of reactive power when the power value output by the reactive power compensator 100 is smaller than a predetermined value and when it is equal to or greater than the predetermined value. In Fig. 3(a) to 3(c), in period I, the power value output by the reactive power compensator 100 is smaller than the predetermined value. In period II, the power value output by the reactive power compensator 100 is equal to or greater than the predetermined value.

[0029] FIG. 3(a) shows the change in reactive power Q of the bus over time when no reactive power compensation is performed.

[0030] FIG. 3(b) shows the time change of the reactive power QSVC output by the reactive power compensator 100 so as to cancel the reactive power Q in FIG. 3(a).

[0031] FIG. 3(c) shows the time variations of the fluctuation component ΔQ and the non-fluctuation component Qb for outputting the reactive power QSVC in FIG. 3(b).

[0032] As shown in Figures 3(a) to 3(c), in period I, as described in relation to Figures 2(a) to 2(c), the reactive power compensation device 100 adds together the fluctuation component ΔQ of the reactive power and the non-fluctuation component Qb and outputs the sum so as to offset the reactive power Q of the bus bar 11.

[0033] In period II, the magnitude of the reactive power of the bus 11 is equal to or greater than a predetermined value, so the reactive power compensator 100 reduces the non-fluctuation component Qb of the reactive power and outputs Q=ΔQ. As shown in FIG. 3(a), period II represents a period in which the fluctuation of the reactive power Q is large, and in an arc furnace, in a period in which the lagging reactive power QF is large, the input power is large and the lagging reactive power is dominant compared to the leading reactive power. Therefore, although the reactive power after the reactive power compensation is not shown, it is lagging reactive power overall.

[0034] FIG. 4 is an example of a flowchart for explaining the operation of the reactive power compensation device according to the embodiment. In the following description, the symbol of reactive power that can be output by power converter 6 (for example, reactive power QSVC) is assumed to be a command value of reactive power. 4, in step S1, the potential transformer 5 and the current transformers 4 and 9 measure the voltage v0 and the currents i1 and i2, respectively. The control device 7 receives as input data the measured voltage v0 and currents i1 and i2.

[0035] In step S2, the control device 7 calculates the reactive power QF of the arc furnace and the reactive power QC of the leading-phase equipment 3 based on the data of the voltage v0 and the currents i1 and i2, and adds them together to calculate QSVC.

[0036] In step S3, the control device 7 compares the calculated reactive power QSVC with a preset threshold value Qm. If the control device 7 determines that the reactive power QSVC is smaller than the threshold value Qm, the control device 7 transitions the process to step S4. If the control device 7 determines that the reactive power QSVC is equal to or greater than the threshold value Qm, the control device 7 transitions the process to step S5.

[0037] If it is determined in step S3 that the reactive power QSVC is smaller than the threshold value Qm, then in step S4, the control device 7 sets the non-fluctuation component Qb of the reactive power to the calculated non-fluctuation component Qb0.

[0038] When it is determined in step S3 that the reactive power QSVC is equal to or greater than the threshold value Qm, the control device 7 sets the non-fluctuation component Qb of the reactive power to 0 in step S5.

[0039] In step S6, the control device 7 adds together the fluctuation component ΔQ of the reactive power and the non-fluctuation component Qb, and outputs the sum.

[0040] In the above specific example, Qb is set to 0 when the magnitude of the reactive power QSVC is equal to or greater than a predetermined value, but the value of Qb is not limited to 0 and may be a constant value according to the power value that can be output by the power converter 6. In addition, multiple threshold values ​​for the magnitude of the reactive power QSVC may be provided according to the magnitude of the reactive power QSVC, and the value of Qb may be set according to the threshold values.

[0041] The effects of the reactive power compensator 100 according to the embodiment will be described. The reactive power compensator 100 according to the embodiment calculates the reactive power to be output by dividing it into a fluctuation component ΔQ and a non-fluctuation component Qb, and injects reactive power having a desired phase into the bus 11 by adding up these command values. Therefore, even if leading reactive power occurs in the bus 11 before compensation, the non-fluctuation component Qb can be set to offset the leading component, thereby preventing the reactive power of the bus 11 from becoming a leading phase. By preventing the reactive power of the compensated bus 11 from becoming leading reactive power, a voltage rise in the bus 11 can be prevented.

[0042] In the reactive power compensator 100, the non-fluctuation component Qb can be switched between valid and invalid depending on the magnitude of the power to be output. The amount of power input to the arc furnace tends to be large in the early stages of melting or when scrap is added. On the other hand, the input power may be relatively small in the boring period or the final stages of melting. When the input power is large, the reactive power fluctuates greatly and flicker is likely to occur. In addition, in a state where the reactive power fluctuates greatly, such as in the early stages of melting or when scrap is added, the overall power becomes lagging reactive power.

[0043] In the reactive power compensation device 100 according to the embodiment, when the magnitude of the output reactive power is equal to or greater than a predetermined value, the non-fluctuation component Qb is set to 0, making it possible to output reactive power sufficient to suppress fluctuations in the reactive power, thereby preventing the occurrence of flicker.

[0044] When the input power is relatively small, such as during the boring period or the final stage of melting, the lagging reactive power on the arc furnace side tends to be small. According to the reactive power compensator 100 of the embodiment, the lagging phase of the non-fluctuation component Qb of the reactive power is appropriately set, thereby making it possible to prevent the reactive power of the bus 11 from becoming a leading reactive power. This makes it possible to prevent the voltage of the bus 11 from increasing under any circumstances, and to prevent failures due to overvoltage of the equipment and devices in the arc furnace facility.

[0045] In this way, a reactive power compensator for an arc furnace can be realized which prevents voltage rise on the busbar.

[0046] Although some embodiments of the present invention have been described, these embodiments are presented as examples and are not intended to limit the scope of the invention. These novel embodiments can be implemented in various other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. These embodiments and their modifications are included in the scope and spirit of the invention, and are included in the scope of the invention and its equivalents described in the claims. [Explanation of symbols]

[0047] 1...furnace, 1a...electrode, 1b...scrap, 2...furnace transformer, 3...phase-leading equipment, 4, 9...current transformer, 5...instrument transformer, 6...power converter, 7...control device, 8...transformer, 10...power system, 11...busbar, 100...var compensator

Claims

1. A reactive power compensator that compensates for reactive power of a busbar of a power system to which an arc furnace is connected, a control device that receives detection values ​​of a first voltage of the bus, a first current flowing through the electrode, and a second current flowing through a leading-phase equipment connected to the bus, calculates a first reactive power of the bus based on the detection values, and calculates a command value corresponding to the first reactive power; a power converter that generates and outputs the first reactive power based on the command value; Equipped with The control device includes: The first reactive power is separated into a reactive power fluctuation component and a non-fluctuation component in time and calculated; When the magnitude of the first reactive power is smaller than a first predetermined value, a first command value corresponding to the fluctuation component and a second command value corresponding to the non-fluctuation component are added to calculate a new command value, and the new command value is output to the power converter; A reactive power compensation device that adds a preset first setting value to the second command value and outputs the result as a new command value to the power converter when the first reactive power is equal to or greater than the first predetermined value.

2. The control device includes: a second predetermined value greater than the first predetermined value; When the first reactive power is equal to or greater than the second predetermined value, adding a second setting value that is set in advance to the second command value and outputting the result as a new command value to the power converter; 2. The reactive power compensation device according to claim 1, wherein the magnitude of the second set value is smaller than the magnitude of the first set value.

3. 3. The reactive power compensation device according to claim 2, wherein the second set value is zero.

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