Power conversion device

Abstract

Provided is a power conversion device comprising: a converter that is provided between an electrode of an AC arc furnace and an input transformer; and a control device that controls the operation of the converter. The converter has: a converter that converts AC power supplied from the input transformer into DC power; and an inverter that converts the DC power converted by the converter into AC power and outputs the converted AC power to the electrode side. Thus, provided is a power conversion device for an AC arc furnace, said power conversion device being capable of enhancing, with a simpler configuration, the power supply quality of a power system to which the AC arc furnace is connected.

Description

Power Conversion Device

[0001] FIELD An embodiment of the present invention relates to a power conversion device.

[0002] There is an alternating current arc furnace (AC arc furnace) that melts a metal material (e.g., scrap) by supplying AC power to electrodes and generating an arc discharge between the electrodes and the metal material. The AC arc furnace includes transformers such as an input transformer and an output transformer, converts AC power supplied from a power grid into AC power of a desired magnitude using the transformer, and supplies the converted AC power to the electrodes.

[0003] In the AC arc furnaces described above, there is a risk that the power quality of the connected power system may be degraded due to reactive power, voltage flicker, harmonics, etc. generated by the AC arc furnace. For this reason, in AC arc furnaces, a reactive power compensator (such as a self-excited SVCS or a separately excited SVC), a harmonic filter, a power factor correction capacitor, etc. are provided on the secondary side of the input transformer to improve the power quality.

[0004] In recent years, due to the carbon neutral background, steel companies have been converting their steel manufacturing process into CO ２ The shift from blast furnace steelmaking, which has a high level of emissions, to electric arc furnace steelmaking is underway. ２ This allows for emissions to be reduced to, for example, about one-fourth. Against this backdrop, steel companies are being asked to increase the size of their AC arc furnaces in order to maintain production volumes.

[0005] However, when an AC arc furnace is enlarged, the reactive power generated also increases, which raises concerns that equipment such as a reactive power compensator and a harmonic filter will also become larger, resulting in increased equipment costs and installation space. For this reason, there is a demand for AC arc furnaces that can improve power supply quality with a simpler configuration.

[0006] JP 2008-210145 A JP 2014-204522 A JP 2017-118600 A

[0007] An embodiment of the present invention provides a power conversion device for an AC arc furnace that can improve the power quality of a power system to which an AC arc furnace is connected with a simpler configuration.

[0008] According to an embodiment of the present invention, there is provided a power conversion device for use in an AC arc furnace including an electrode that generates an arc discharge between itself and a metal material, and an input transformer that steps down AC power supplied from a power grid, the power conversion device being provided between the electrode and the input transformer, the power conversion device comprising: a converter that converts AC power supplied from the input transformer into DC power; and an inverter that converts the DC power converted by the converter into AC power and outputs the converted AC power to the electrode side, the inverter having a plurality of switching elements that convert the DC power converted by the converter into AC power by switching of the plurality of switching elements, and the control device controlling the switching of the plurality of switching elements of the inverter, thereby controlling the conversion of DC power to AC power by the inverter.

[0009] According to an embodiment of the present invention, a power conversion device for an AC arc furnace is provided that can reduce the need to provide ancillary equipment such as a reactive power compensator (flicker suppression equipment), a harmonic filter, a power factor improvement capacitor (phase modifying equipment), etc. in the AC arc furnace, and can improve the power supply quality of the power system to which the AC arc furnace is connected with a simpler configuration.

[0010] FIG. 1 is a block diagram schematically showing an AC arc furnace and a power conversion device according to a first embodiment. FIG. 2 is a block diagram schematically showing an example of a converter according to the first embodiment. FIG. 3 is a block diagram schematically showing an AC arc furnace and a power conversion device according to a second embodiment. FIG. 4 is a block diagram schematically showing an AC arc furnace and a power conversion device according to a third embodiment. FIG. 5 is a block diagram schematically showing an AC arc furnace and a power conversion device according to a fourth embodiment. FIG. 6 is a block diagram schematically showing an AC arc furnace and a power conversion device according to a fifth embodiment. FIG. 7 is a block diagram schematically showing a modified example of a converter.

[0011] Each embodiment will be described below 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 those in reality. Even when the same part is shown, the dimensions and ratios may be different depending on the drawing. In this specification and each drawing, elements similar to those previously described with reference to the previous drawings are designated by the same reference numerals, and detailed descriptions thereof will be omitted as appropriate.

[0012] 1 is a block diagram schematically illustrating an AC arc furnace and a power conversion device according to a first embodiment. As shown in Fig. 1, an AC arc furnace 2 includes a furnace body 3, electrodes 4, an output transformer 5, a furnace circuit breaker 6, an input transformer 7, and a power conversion device 10.

[0013] The furnace body 3 has a space capable of accommodating a metal material (e.g., scrap). The electrodes 4 generate an arc discharge between the metal material contained in the furnace body 3 and the electrodes 4 based on the supplied AC power, thereby melting the metal material contained in the furnace body 3. The electrodes 4 are configured to be able to be raised and lowered by a lifting mechanism (not shown) or the like so that an appropriate distance required for generating an arc discharge can be maintained between the electrodes 4 and the metal material, even when the height of the metal material changes due to melting, for example.

[0014] The AC power supplied to the electrodes 4 is, for example, three-phase AC power. The AC arc furnace 2 has, for example, three electrodes 4 corresponding to each phase of the three-phase AC power. However, the AC power supplied to the electrodes 4 is not limited to three-phase AC power and may be any AC power. The electrodes 4 may have any configuration that is capable of generating an arc discharge between them and the metal material based on the supplied AC power.

[0015] The primary side of the input transformer 7 is connected to the power grid PS. The input transformer 7 steps down the AC power supplied from the power grid PS. The input transformer 7, for example, steps down the magnitude of the AC voltage supplied from the power grid PS to a voltage corresponding to the equipment on the premises of the AC arc furnace 2. The secondary side of the input transformer 7 is connected to, for example, the power conversion device 10. In other words, the input transformer 7 steps down the magnitude of the AC voltage supplied from the power grid PS to an AC voltage corresponding to the power conversion device 10, and supplies the stepped-down AC power to the power conversion device 10.

[0016] The power conversion device 10 is provided between the input transformer 7 and the electrode 4. The power conversion device 10 is provided, for example, between the input transformer 7 and the output transformer 5. The power conversion device 10 converts AC power supplied from the input transformer 7 into DC power, then converts the DC power back into AC power, and outputs the converted AC power to the electrode 4 side. The power conversion device 10, for example, supplies the converted AC power to the output transformer 5. The power conversion device 10 also changes the magnitude of the AC voltage supplied to the output transformer 5 depending on the process of the AC arc furnace 2, etc. The capacity range of the AC power supplied from the power conversion device 10 to the output transformer 5 is, for example, several tens to several hundreds of MVA.

[0017] The furnace circuit breaker 6 is provided between the power conversion device 10 and the output transformer 5. The furnace circuit breaker 6 switches between a state in which AC power can be supplied to the output transformer 5 and a state in which AC power supply to the output transformer 5 is cut off.

[0018] The output transformer 5 is provided between the power conversion device 10 and the electrode 4. More specifically, the output transformer 5 is provided between the furnace circuit breaker 6 and the electrode 4. The output transformer 5 steps down the AC power supplied from the power conversion device 10 and supplies the stepped-down AC power to the electrode 4. The AC power (load conditions) supplied from the output transformer 5 to the electrode 4 is, for example, several hundred volts and several tens of kA.

[0019] The electrode 4 generates an arc discharge between the electrode 4 and the metal material contained in the furnace body 3 based on the AC power supplied from the output transformer 5, as described above, and melts the metal material contained in the furnace body 3.

[0020] As described above, the power conversion device 10 is used in an AC arc furnace 2 that includes an electrode 4 that generates an arc discharge between itself and a metal material, an output transformer 5 that supplies AC power to the electrode 4, and an input transformer 7 that steps down the AC power supplied from the power grid PS. The power conversion device 10 is provided between the output transformer 5 and the input transformer 7, and converts the AC power supplied from the input transformer 7 into DC power, then converts the DC power back into AC power, and supplies the converted AC power to the output transformer 5.

[0021] The power conversion device 10 includes a converter 12 and a control device 14. The converter 12 is provided between the electrode 4 and the input transformer 7. The converter 12 is provided, for example, between the output transformer 5 and the input transformer 7. The power conversion device 10 includes, for example, a plurality of converters 12 provided between the output transformer 5 and the input transformer 7. In other words, the power conversion device 10 includes a plurality of converters 12 connected in parallel between the output transformer 5 and the input transformer 7.

[0022] In the power conversion device 10, the output capacity can be adjusted as desired by adjusting the number of parallel connections of the multiple converters 12. In the power conversion device 10, for example, by increasing the number of parallel connections of the multiple converters 12, the output capacity of AC power supplied to the output transformer 5 can be increased while suppressing an increase in the withstand voltage of elements used in each converter 12. However, the number of converters 12 provided in the power conversion device 10 may be any number. The number of converters 12 provided in the power conversion device 10 may be one.

[0023] Each converter 12 includes a converter 20 and an inverter 22. Each converter 12 is, for example, a BTB (Back To Back) converter. The converter 20 converts AC power supplied from the input transformer 7 into DC power. The inverter 22 converts the DC power converted by the converter 20 into AC power and supplies the converted AC power to the output transformer 5. As described above, the inverter 22 also changes the magnitude of the AC voltage supplied to the output transformer 5 depending on the process of the AC arc furnace 2, etc.

[0024] The control device 14 controls the operation of each converter 12. More specifically, the control device 14 controls the operation of each converter 20 and inverter 22 of each converter 12. The control device 14 controls the conversion of AC power to DC power by the converter 20, and also controls the conversion of DC power to AC power by the inverter 22. The control device 14 also controls the operation of the inverter 22, thereby controlling the magnitude of the AC voltage output from the inverter 22.

[0025] Furthermore, the control device 14 can change the number of converters 12 in operation, for example, by individually controlling the operation of the multiple converters 12. The control device 14 determines the number of converters 12 to operate so as to achieve the highest efficiency depending on the load, for example. In other words, the control device 14 enables degenerate operation of the multiple converters 12. This can further reduce the power consumption of the power conversion device 10, for example. However, the control device 14 does not necessarily have to be able to change the number of converters 12 to operate.

[0026] Fig. 2 is a block diagram schematically illustrating an example of a converter according to the first embodiment. Fig. 2 schematically illustrates an example of one of the multiple converters 12. Since the configurations of the converters 12 are substantially the same, only the configuration of one converter 12 will be described.

[0027] As shown in FIG. 2, the converter 20 has a plurality of AC input terminals 30a to 30c, a pair of DC output terminals 31a and 31b, a plurality of switching elements 32a to 32f, and a plurality of rectifying elements 33a to 33f.

[0028] The converter 20 has, for example, three AC input terminals 30a to 30c corresponding to the respective phases of the three-phase AC power. The converter 20 also has, for example, six switching elements 32a to 32f connected in a three-phase full bridge configuration, and six rectifying elements 33a to 33f connected in anti-parallel to the six switching elements 32a to 32f, respectively.

[0029] The connection point between switching element 32a and switching element 32b is connected to AC input terminal 30a. The connection point between switching element 32c and switching element 32d is connected to AC input terminal 30b. The connection point between switching element 32e and switching element 32f is connected to AC input terminal 30c.

[0030] In addition, both ends of the series-connected switching elements 32a and 32b, both ends of the series-connected switching elements 32c and 32d, and both ends of the series-connected switching elements 32e and 32f are connected to a pair of DC output terminals 31a and 31b, respectively.

[0031] Converter 20 is connected to input transformer 7 via AC input terminals 30a to 30c. Converter 20 converts AC power supplied from input transformer 7 to AC input terminals 30a to 30c into DC power by switching switching elements 32a to 32f, and outputs the converted DC power from a pair of DC output terminals 31a, 31b.

[0032] The converter 20 is, for example, a PWM rectifier circuit. Each of the switching elements 32a to 32f may be, for example, a self-excited switching element such as an IGBT. However, the configuration of the converter 20 is not limited to the above. The converter 20 may be, for example, a single-phase full-bridge circuit, a three-level converter such as an NPC (Neutral-Point-Clamped) type, or a multilevel converter such as an MMC (Modular Multilevel Converter) type. The converter 20 may be configured in any way that can convert AC power supplied from the input transformer 7 into DC power by switching the multiple switching elements 32a to 32f. Each of the switching elements 32a to 32f may be, for example, a separately-excited switching element such as a thyristor.

[0033] The inverter 22 has a pair of DC input terminals 40a, 40b, a plurality of AC output terminals 41a to 41c, a plurality of switching elements 42a to 42f, and a plurality of rectifying elements 43a to 43f.

[0034] The inverter 22 has, for example, three AC output terminals 41 a to 41 c corresponding to the respective phases of the three-phase AC power. The inverter 22 also has, for example, six switching elements 42 a to 42 f connected in a three-phase full bridge configuration, and six rectifying elements 43 a to 43 f connected in anti-parallel to the six switching elements 42 a to 42 f, respectively.

[0035] The connection point between switching element 42a and switching element 42b is connected to AC output terminal 41a. The connection point between switching element 42c and switching element 42d is connected to AC output terminal 41b. The connection point between switching element 42e and switching element 42f is connected to AC output terminal 41c.

[0036] In addition, both ends of the serially connected switching elements 42a and 42b, both ends of the serially connected switching elements 42c and 42d, and both ends of the serially connected switching elements 42e and 42f are connected to a pair of DC input terminals 40a and 40b, respectively.

[0037] The inverter 22 is connected to a pair of DC output terminals 31a, 31b of the converter 20 via a pair of DC input terminals 40a, 40b. The inverter 22 is also connected to the output transformer 5 (furnace circuit breaker 6) via each of the AC output terminals 41a to 41c. The inverter 22 converts DC power supplied from the converter 20 to the pair of DC input terminals 40a, 40b into AC power by switching each of the switching elements 42a to 42f, and outputs the converted AC power from each of the AC output terminals 41a to 41c.

[0038] As a result, the inverter 22 can convert AC power supplied from the input transformer 7 into DC power, and then convert the DC power back into AC power, and supply the converted AC power to the output transformer 5. However, the configuration of the inverter 22 is not limited to the above, and any configuration may be used that can convert DC power supplied from the converter 20 to a pair of DC input terminals 40a, 40b into AC power by switching a plurality of switching elements 42a to 42f, and supply the converted AC power to the output transformer 5. The inverter 22 may be, for example, a single-phase full-bridge circuit, a three-level converter such as an NPC (Neutral-Point-Clamped) type, or a multilevel converter such as an MMC (Modular Multilevel Converter) type.

[0039] Each of the switching elements 42a to 42f may be a self-excited switching element such as an IGBT, etc. Each of the switching elements 42a to 42f may be any element that can appropriately convert DC power to AC power by switching.

[0040] 2 , each converter 12 further includes, for example, a charge storage element 24. The charge storage element 24 is provided between the converter 20 and the inverter 22 and suppresses fluctuations in the voltage of the DC power converted by the converter 20. For example, a capacitor is used as the charge storage element 24. In other words, the charge storage element 24 is a smoothing capacitor. However, the charge storage element 24 is not limited to a capacitor and may be any element capable of suppressing fluctuations in the voltage of the DC power converted by the converter 20.

[0041] The charge storage element 24 is provided, for example, between a pair of DC output terminals 31 a, 31 b of the converter 20 and between a pair of DC input terminals 40 a, 40 b of the inverter 22. As a result, the charge storage element 24 is charged based on the DC power output from the converter 20, and fluctuations in the voltage of the DC power converted by the converter 20 are suppressed by the charge storage element 24. However, the charge storage element 24 is provided as needed and can be omitted. The charge storage element 24 may be provided as appropriate depending on, for example, the circuit configuration of the converter 12.

[0042] The control device 14 transmits a control signal to the converter 20 to control the switching of each of the switching elements 32 a to 32 f of the converter 20 , thereby controlling the conversion of AC power to DC power by the converter 20 .

[0043] At this time, the control device 14 controls the switching of the switching elements 32a to 32f of the converter 20 so that the power factor on the input side (the power system PS side) of the converter 20 becomes 1, for example.

[0044] However, the control of each of the switching elements 32a to 32f of the converter 20 by the control device 14 is not limited to the control that makes the power factor 1. The control of each of the switching elements 32a to 32f of the converter 20 by the control device 14 may be any control that can appropriately control the conversion of AC power to DC power by the converter 20.

[0045] Furthermore, the control device 14 transmits a control signal to the inverter 22 and controls the switching of each of the switching elements 42a to 42f of the inverter 22, thereby controlling the conversion of DC power to AC power by the inverter 22. At this time, the control device 14 controls the magnitude of the AC voltage output from the inverter 22 by controlling the switching intervals of each of the switching elements 42a to 42f, etc.

[0046] As described above, in the power conversion device 10 according to the present embodiment, the converter 12 includes a converter 20 and an inverter 22. As a result, the power conversion device 10 uses the converter 20 and the inverter 22 to suppress reactive power fluctuations and other factors generated by the AC arc furnace 2, thereby preventing reactive power fluctuations, voltage flicker, harmonics, and other factors from leaking into the power grid PS. Therefore, an AC arc furnace 2 using the power conversion device 10 can reduce the need for a reactive power compensator, harmonic filter, power factor correction capacitor, and other components provided on the secondary side of the input transformer 7. For example, these pieces of equipment can be eliminated, or the output capacity of these pieces of equipment can be reduced. Furthermore, when the converter 12 includes a charge storage element 24, as in the example shown in FIG. 2 , the charge storage element 24 can absorb reactive power fluctuations and other factors generated by the AC arc furnace 2, thereby further preventing reactive power fluctuations, voltage flicker, harmonics, and other factors from leaking into the power grid PS.

[0047] For example, even if the AC arc furnace 2 is enlarged, it is possible to prevent the equipment such as the reactive power compensator and harmonic filter from becoming larger, which would lead to an increase in equipment costs and installation space.

[0048] The power conversion device 10 according to this embodiment can improve the power quality of the power system PS connected to the AC arc furnace 2 with a simpler configuration than a configuration that requires equipment such as a reactive power compensator and a harmonic filter. The power conversion device 10 according to this embodiment can reduce the need for equipment such as a reactive power compensator and a harmonic filter, and can improve the power quality of the power system PS while suppressing increases in equipment costs and installation space.

[0049] Furthermore, the power conversion device 10 according to this embodiment can be applied to an existing AC arc furnace system by replacing equipment such as a reactive power compensator and a harmonic filter. In this case, the output transformer 5, the input transformer 7, and the like can be used as they are. In this way, the power conversion device 10 according to this embodiment can be easily incorporated into an existing system when updating an existing AC arc furnace system, for example.

[0050] Furthermore, in the power conversion device 10 according to this embodiment, the control device 14 controls the magnitude of the AC voltage output from the inverter 22 by controlling the switching of each of the switching elements 42a to 42f of the inverter 22. As a result, in the power conversion device 10 according to this embodiment, for example, a turns ratio adjustment tap of the output transformer 5 can be eliminated. This can simplify the configuration of the output transformer 5, for example, and prevent increases in cost and size of the output transformer 5. For example, the configuration of the AC arc furnace 2 can be simplified. Furthermore, maintenance of the turns ratio adjustment tap of the output transformer 5 is eliminated, and the effort and costs associated with tap maintenance can be reduced.

[0051] Furthermore, in the power conversion device 10 according to this embodiment, the control device 14 controls the switching of each of the switching elements 32a to 32f of the converter 20 so that the power factor on the input side (the power grid PS side) of the converter 20 becomes 1. As a result, the power conversion device 10 according to this embodiment can further reduce the need for equipment for adjusting the power factor, such as a power factor correction capacitor provided on the secondary side of the input transformer 7. For example, the equipment for adjusting the power factor can be eliminated, or the output capacity of the equipment for adjusting the power factor can be reduced. Therefore, it is possible to further suppress increases in equipment costs and installation space while improving the power quality of the power grid PS.

[0052] Second Embodiment Fig. 3 is a block diagram schematically illustrating an AC arc furnace and a power conversion device according to a second embodiment. As shown in Fig. 3, in a power conversion device 10a of an AC arc furnace 2a, a converter 12a further includes an inverter transformer 26. Note that components that are substantially the same in function and configuration as those in the first embodiment are designated by the same reference numerals, and detailed description thereof will be omitted.

[0053] The inverter transformer 26 is provided between the inverter 22 and the electrode 4. The inverter transformer 26 is provided, for example, between the inverter 22 and the output transformer 5 (furnace circuit breaker 6). The inverter transformer 26 boosts the AC power supplied from the inverter 22 and outputs the boosted AC power to the electrode 4. The inverter transformer 26 supplies the boosted AC power to the output transformer 5, for example.

[0054] In this example, the output transformer 5 steps down the AC power supplied from the inverter transformer 26 and supplies it to the electrodes 4 .

[0055] As described above, in the power conversion device 10a according to this embodiment, the converter 12a further includes an inverter transformer 26. By boosting the voltage using the inverter transformer 26, the output of the converter 12a can be made higher voltage, thereby reducing impedance constraints up to the output transformer 5. For example, compared to when AC power is supplied to the output transformer 5 at low voltage and high current, even when the cable between the converter 12a (inverter transformer 26) and the output transformer 5 is long, the effects of loss due to cable impedance can be reduced. Therefore, compared to when AC power is supplied to the output transformer 5 at low voltage and high current, the distance between the converter 12a (inverter transformer 26) and the output transformer 5 can be increased, thereby reducing constraints on the placement of the converter 12a. For example, this eliminates the need to place the converter 12a in close proximity to the furnace body 3.

[0056] In the power conversion device 10a according to this embodiment, the degree of freedom in the placement of the converter 12a can be increased. Furthermore, by increasing the degree of freedom in the placement of the converter 12a, it becomes easier to reuse an existing output transformer 5, for example, when updating an AC arc furnace. The power conversion device 10a according to this embodiment can also make it easier to update an existing AC arc furnace.

[0057] Third Embodiment Fig. 4 is a block diagram schematically illustrating an AC arc furnace and a power conversion device according to a third embodiment. As shown in Fig. 4, in a power conversion device 10b of an AC arc furnace 2b, a converter 12b further includes a converter transformer 28.

[0058] Converter transformer 28 is provided between input transformer 7 and converter 20. Converter transformer 28 steps down the AC power supplied from input transformer 7 and supplies the stepped-down AC power to converter 20.

[0059] As described above, in the power conversion device 10b according to this embodiment, the converter 12b further includes a converter transformer 28. By stepping down the voltage using the converter transformer 28, the output of the input transformer 7 can be made higher voltage, thereby reducing impedance constraints up to the converter 12b (converter transformer 28). For example, compared to when AC power is supplied to the converter 12b at low voltage and high current, even when the cable between the input transformer 7 and the converter 12b (converter transformer 28) is long, the effects of loss due to cable impedance can be reduced. Therefore, compared to when AC power is supplied to the converter 12b at low voltage and high current, the distance between the input transformer 7 and the converter 12b can be increased, thereby reducing constraints on the placement of the converter 12b. For example, the need to place the converter 12b immediately adjacent to the input transformer 7 can be eliminated.

[0060] Furthermore, in the power conversion device 10b according to this embodiment, for example, by adjusting the transformation ratio with the converter transformer 28, it is possible to more easily reuse the existing input transformer 7 when updating an AC arc furnace. The power conversion device 10b according to this embodiment can also make it easier to update an existing AC arc furnace.

[0061] 5 is a block diagram showing a schematic diagram of an AC arc furnace and a power conversion device according to a fourth embodiment. As shown in FIG. 5, in a power conversion device 10c of an AC arc furnace 2c, a converter 12c further includes an inverter transformer 26 and a converter transformer 28.

[0062] This allows the power conversion device 10c to have a higher degree of freedom in the placement of the converter 12c. For example, it is no longer necessary to place the converter 12c in the immediate vicinity of the furnace body 3 or the input transformer 7.

[0063] Furthermore, the power conversion device 10c can more easily utilize the existing output transformer 5 and the existing input transformer 7. The power conversion device 10c according to this embodiment can also more easily update an existing AC arc furnace.

[0064] On the other hand, in the power conversion device 10 described in relation to the first embodiment, the number of parts of the converter 12 can be reduced, and increases in facility costs and installation space can be further suppressed. The power conversion device 10 described in relation to the first embodiment can improve the power quality of the power system PS to which the AC arc furnace 2 is connected with a simpler configuration.

[0065] Fifth Embodiment Fig. 6 is a block diagram schematically illustrating an AC arc furnace and a power conversion device according to a fifth embodiment. As shown in Fig. 6, in an AC arc furnace 2d, a converter 12d of a power conversion device 10d has an inverter transformer 26, and the output transformer 5 is omitted. In this way, when the inverter transformer 26 is provided in the converter 12d, the output transformer 5 may be omitted. It is sufficient that at least one of the output transformer 5 and the inverter transformer 26 is provided.

[0066] 7 is a block diagram showing a modified example of the converter 20a. As shown in FIG. 7, the converter 20a has a transformer 35 and four rectifier circuits 36a to 36d instead of the multiple switching elements 32a to 32f and the multiple rectifier elements 33a to 33f.

[0067] The transformer 35 has a primary winding w1 and four secondary windings w2a to w2d. The transformer 35 is, for example, a three-phase transformer. The primary winding w1 is connected to each of the AC input terminals 30a to 30c. In other words, the primary winding w1 is connected to the input transformer 7 via each of the AC input terminals 30a to 30c. The primary winding w1 receives AC power input from the input transformer 7. The primary winding w1 is, for example, delta-connected.

[0068] Each of the four secondary windings w2a to w2d is magnetically coupled to the primary winding w1. The four secondary windings w2a to w2d transform and output AC power supplied to the primary winding w1 from the input transformer 7. The four secondary windings w2a to w2d are set to output AC power that is shifted in phase by 15 degrees.

[0069] The phases of the AC power output from each of the four secondary windings w2a to w2d are 0°, +30°, +15°, and -15° relative to the phase of the input side. The secondary winding w2a is, for example, delta-connected. This allows the phase of the AC power output from the secondary winding w2a to be set to 0° relative to the phase of the input side. The secondary winding w2b is, for example, Y-connected. This allows the phase of the AC power output from the secondary winding w2b to be set to -30° relative to the phase of the input side. The secondary windings w2c and w2d are, for example, staggered-connected. This allows the phase of the AC power output from the secondary windings w2c and w2d to be set to ±15° relative to the phase of the input side.

[0070] The four rectifier circuits 36a to 36d rectify the AC power output from each of the four secondary windings w2a to w2d. Each of the rectifier circuits 36a to 36d is, for example, a bridge circuit in which six rectifier elements (e.g., diodes) are connected in a three-phase full bridge configuration. In other words, each of the rectifier circuits 36a to 36d is a diode bridge. The output sides of the four rectifier circuits 36a to 36d are connected in series. Both ends of the output sides of the four rectifier circuits 36a to 36d connected in series are connected to a pair of DC output terminals 31a, 31b. As a result, a voltage obtained by adding up the output voltages of the rectifier circuits 36a to 36d is output to the pair of DC output terminals 31a, 31b.

[0071] The converter 20a is a 24-phase rectifier circuit. The converter 20a, which is a 24-phase rectifier circuit, can bring the power factor on the input side close to 1 and also suppress harmonics. This makes it possible to eliminate the need for additional equipment such as a phase modifying device or a harmonic filter, and to make the additional equipment even smaller.

[0072] In this way, converter 20a is not limited to a configuration that converts AC power supplied from input transformer 7 into DC power by switching a plurality of switching elements, but may be a rectifier circuit such as a 24-phase rectifier circuit. The configuration of converter 20a is not limited to the above, and any configuration that can appropriately convert AC power supplied from input transformer 7 into DC power may be used.

[0073] This embodiment includes the following aspects: (Supplementary Note 1) A power conversion device used in an AC arc furnace including an electrode that generates an arc discharge between itself and a metal material, and an input transformer that steps down AC power supplied from a power grid, and is provided between the electrode and the input transformer, the power conversion device comprising: a converter that converts AC power supplied from the input transformer into DC power, and an inverter that converts the DC power converted by the converter into AC power and outputs the converted AC power to the electrode side, the inverter having a plurality of switching elements that convert the DC power converted by the converter into AC power by switching the plurality of switching elements, and the control device controls the conversion of DC power to AC power by the inverter.

[0074] (Supplementary Note 2) The power conversion device according to Supplementary Note 1, wherein the control device controls switching of the plurality of switching elements of the inverter to control a magnitude of the AC voltage output from the inverter.

[0075] (Supplementary Note 3) The power conversion device according to Supplementary Note 1 or 2, wherein the converter has a plurality of switching elements, and converts AC power supplied from the input transformer into DC power by switching the plurality of switching elements, and the control device controls the conversion of AC power to DC power by the converter by controlling the switching of the plurality of switching elements of the converter.

[0076] (Supplementary Note 4) The power conversion device according to Supplementary Note 3, wherein the control device controls switching of the plurality of switching elements of the converter so that a power factor on an input side of the converter becomes 1.

[0077] (Supplementary Note 5) The power conversion device according to Supplementary Note 1 or 2, wherein the converter is a 24-phase rectifier circuit having: a transformer having a primary winding that receives AC power input from the input transformer, and four secondary windings that are magnetically coupled to the primary winding and set to output AC powers that are shifted in phase by 15 degrees each; and four rectifier circuits that rectify the AC power output from each of the four secondary windings and have output sides connected in series.

[0078] (Supplementary Note 6) The power conversion device according to any one of Supplementary Notes 1 to 5, wherein the converter further includes an inverter transformer that is provided between the inverter and the electrode, boosts AC power supplied from the inverter, and outputs the boosted AC power to the electrode side.

[0079] (Supplementary Note 7) The power conversion device according to any one of Supplementary Notes 1 to 5, wherein the converter further includes a converter transformer that is provided between the input transformer and the converter, steps down AC power supplied from the input transformer, and supplies the stepped-down AC power to the converter.

[0080] (Supplementary Note 8) The power conversion device according to any one of Supplementary Notes 1 to 5, wherein the converter further includes: a converter transformer provided between the input transformer and the converter, which steps down the AC power supplied from the input transformer and supplies the stepped-down AC power to the converter; and an inverter transformer provided between the inverter and the electrode, which steps up the AC power supplied from the inverter and outputs the stepped-up AC power to the electrode side.

[0081] (Supplementary Note 9) The power conversion device according to any one of Supplementary Notes 1 to 8, wherein a plurality of the converters are provided between the electrode and the input transformer.

[0082] (Supplementary Note 10) The power conversion device according to Supplementary Note 9, wherein the control device is capable of changing the number of the converters in operation by individually controlling the operations of the plurality of converters.

[0083] Although several 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 embodied 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 within the scope and spirit of the invention, and are also included in the scope of the invention and its equivalents as defined in the claims.

[0084] DESCRIPTION OF SYMBOLS 2, 2a to 2d... AC arc furnace, 3... furnace body, 4... electrode, 5... output transformer, 6... furnace circuit breaker, 7... input transformer, 10, 10a to 10d... power conversion device, 12, 12a to 12d... converter, 14... control device, 20... converter, 22... inverter, 24... charge storage element, 26... inverter transformer, 28... converter transformer, 30a to 30c... AC input terminal, 31a, 31b... DC output terminal, 32a to 32f... switching element, 33a to 33f... rectifier element, 35... transformer, 36a to 36d... rectifier circuit, 40a, 40b... DC input terminal, 41a to 41c... AC output terminal, 42a to 42f... switching element, 43a to 43f... rectifying elements, PS... power system

Claims

1. A power conversion device used in an AC arc furnace equipped with an electrode that generates an arc discharge between a metal material and an input transformer that steps down AC power supplied from a power system, and is provided between the electrode and the input transformer, the power conversion device comprising: a converter provided between the electrode and the input transformer; and a control device that controls the operation of the converter, the converter having: a converter that converts the AC power supplied from the input transformer into DC power; and an inverter that converts the DC power converted by the converter into AC power and outputs the converted AC power to the electrode side, the inverter having a plurality of switching elements that convert the DC power converted by the converter into AC power by switching the plurality of switching elements, and the control device that controls the conversion of DC power to AC power by the inverter by controlling the switching of the plurality of switching elements of the inverter.

2. The power conversion device according to claim 1, wherein said control device controls the magnitude of the AC voltage output from said inverter by controlling the switching of said plurality of switching elements of said inverter.

3. A power conversion device as claimed in claim 1, wherein the converter has a plurality of switching elements, and converts AC power supplied from the input transformer into DC power by switching the plurality of switching elements, and the control device controls the conversion of AC power to DC power by the converter by controlling the switching of the plurality of switching elements of the converter.

4. The power conversion device according to claim 3, wherein the control device controls the switching of the plurality of switching elements of the converter so that the power factor on the input side of the converter becomes unity.

5. The power conversion device according to claim 1, wherein the converter is a 24-phase rectifier circuit having: a transformer having a primary winding that receives AC power input from the input transformer, and four secondary windings that are magnetically coupled to the primary winding and set to output AC powers shifted in phase by 15 degrees each; and four rectifier circuits that rectify the AC powers output from each of the four secondary windings and have their output sides connected in series.

6. A power conversion device according to claim 1, wherein the converter further comprises an inverter transformer provided between the inverter and the electrode, for boosting the AC power supplied from the inverter and outputting the boosted AC power to the electrode side.

7. A power conversion device according to claim 1, wherein the converter further comprises a converter transformer provided between the input transformer and the converter, for stepping down the AC power supplied from the input transformer and supplying the stepped-down AC power to the converter.

8. A power conversion device as claimed in claim 1, wherein the converter further comprises: a converter transformer provided between the input transformer and the converter, for stepping down the AC power supplied from the input transformer and supplying the stepped-down AC power to the converter; and an inverter transformer provided between the inverter and the electrode, for stepping up the AC power supplied from the inverter and outputting the stepped-up AC power to the electrode side.

9. The power conversion device according to claim 1, wherein a plurality of said converters are provided between said electrode and said input transformer.

10. The power conversion device according to claim 9, wherein the control device is capable of changing the number of converters in operation by individually controlling the operation of the plurality of converters.

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