Power supply

Abstract

To provide a power supply capable of supplying stable power to a load.SOLUTION: A power supply 100 is connected to a three-phase AC power supply 200 and supplies power to a load 210. The power supply 100 includes a transformer 10 that transforms the voltage of the power input from the three-phase AC power supply 200, a series transformer 20 that is provided on the secondary side of the transformer 10 and controls the voltage of the power supplied from the transformer 10 to the load 210, and a semiconductor power conversion unit 30 that controls the voltage on the primary side of the series transformer 20.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a power supply device, and more particularly to a power supply device that is connected to a power system and supplies power to a load. [Background technology]

[0002] 2. Description of the Related Art Power supply devices that are connected to a power grid and supply power to a load are known (see, for example, Patent Document 1).

[0003] The above-mentioned Patent Document 1 discloses an electric furnace transformer including a main transformer to which power is supplied from a power system, and a series transformer. The main transformer includes a primary winding, a secondary winding, and a tap winding. The series transformer includes an excitation winding connected to the tap winding. When power is supplied from the power system to the main transformer, a voltage obtained by adding the voltage of the secondary winding of the main transformer and the voltage on the secondary side of the series transformer is applied to the load. The electric furnace transformer of the above-mentioned Patent Document 1 also includes a tap changer that switches a tap terminal provided on the tap winding. When the tap terminal is switched, the voltage applied to the excitation winding of the series transformer changes. As a result, the voltage applied to the load changes. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Special Publication No. 6-105659 Summary of the Invention [Problem to be solved by the invention]

[0005] However, in the above-mentioned Patent Document 1, a tap winding is provided to change the voltage applied to the load. Here, a relatively long time is required to switch the tap terminal provided on the tap winding. Therefore, the voltage output from the electric furnace transformer cannot be changed quickly, and the voltage output from the electric furnace transformer cannot follow a sudden change in the load. This causes a problem that a stable power supply cannot be provided to the load.

[0006] The present invention has been made to solve the above-mentioned problems, and one object of the present invention is to provide a power supply device capable of supplying stable power to a load. [Means for solving the problem]

[0007] In order to achieve the above-mentioned object, a power supply device according to one aspect of the present invention is a power supply device that is connected to a power system and supplies power to a load, and includes a transformer that transforms the voltage of power input from the power system, a series transformer that is provided on the secondary side of the transformer and controls the voltage of the power supplied from the transformer to the load, and a semiconductor power conversion unit that controls the voltage on the primary side of the series transformer.

[0008] As described above, the power supply device according to one aspect of the present invention includes a semiconductor power conversion unit that controls the voltage on the primary side of the series transformer. As a result, the semiconductor power conversion unit can change the voltage outputted at a relatively high speed compared to the tap changing method, so that even if the load changes suddenly, the voltage applied from the power supply device to the load can be adjusted to a desired voltage. Also, unlike the tap changing method that can only change the voltage stepwise, the semiconductor power conversion unit can change the output voltage linearly (continuously). As a result, even if the load changes suddenly, the voltage outputted from the power supply device can be adjusted to a desired voltage. Therefore, the power supply device can supply stable power to the load. Also, while the tap changing method requires inspection and replacement of the tap changer due to mechanical switching of the tap terminal, the semiconductor power conversion unit does not require inspection and replacement, unlike the tap changing method. Therefore, the user's trouble of inspection and replacement can be saved. Also, the reactive power of the power supplied from the power supply device fluctuates due to a sudden change in the load. In the power supply device according to one aspect, since the semiconductor power conversion unit is provided, the semiconductor power conversion unit generates reactive power, so that at least a part of the fluctuation of the reactive power contained in the power supplied from the power supply device can be compensated for. As a result, the voltage fluctuation occurring in the power grid can be suppressed, and the deterioration of the quality of the power can be suppressed.

[0009] In the power supply device according to the above aspect, the semiconductor power conversion unit preferably includes an inverter unit, and the voltage on the primary side of the series transformer is controlled by power output from the inverter unit to the series transformer. Thus, by controlling on / off of a switching element included in the inverter unit, the voltage applied to the load can be adjusted to a desired voltage even if fluctuations occur in the load.

[0010] In this case, preferably, a voltage detector for detecting a voltage on the secondary side of the transformer is further provided, and the semiconductor power conversion unit controls the voltage output from the inverter unit based on the voltage detected by the voltage detector, thereby making it possible to adjust the voltage output from the power supply device to a desired voltage.

[0011] In the power supply device according to the above aspect, preferably, a main winding connected to the load and an auxiliary winding are provided on the secondary side of the transformer, and the semiconductor power conversion unit controls the voltage on the primary side of the series transformer using the power input from the auxiliary winding as a power source. Thus, the semiconductor power conversion unit transforms the power input from the auxiliary winding, making it possible to easily adjust the voltage output from the power supply device.

[0012] In the power supply device according to the above aspect, the semiconductor power conversion unit preferably controls the voltage on the primary side of the series transformer using power input from an external power supply as a power source. This makes it possible to easily adjust the voltage output from the power supply device by the semiconductor power conversion unit transforming the power input from the external power supply.

[0013] In the power supply device according to the above aspect, the load preferably includes an arc furnace that melts the material to be melted by electric power. The arc furnace includes an electrode to which electric power is supplied from the power supply device, and a furnace body in which the material to be melted is placed. In the arc furnace, if the distance between the electrode of the arc furnace and the material to be melted is appropriate, a stable arc discharge occurs between the electrode and the material to be melted. However, if the distance between the electrode and the material to be melted is too close, an excessive current flows between the electrode and the material to be melted, and if the distance between the electrode and the material to be melted is too far, an arc discharge does not occur between the electrode and the material to be melted. In addition, the distance between the electrode and the material to be melted changes as the material to be melted changes. Due to these factors, the current flowing through the electrode of the arc furnace changes relatively rapidly. Therefore, adjusting the voltage output from the power supply device to a desired voltage by providing a semiconductor power conversion unit is particularly effective for an arc furnace in which the current flowing through the electrode changes relatively rapidly.

[0014] In this case, it is preferable to further include a current detector for detecting a current flowing through the secondary side of the transformer in order to adjust the position of the electrode of the arc furnace relative to the material to be melted. By adjusting the position of the electrode so that the current detected by the current detector has a predetermined current value, a stable arc discharge can be generated between the electrode and the material to be melted.

[0015] In the power supply device according to the above aspect, the semiconductor power conversion unit preferably includes a converter unit, an inverter unit, and a power storage unit provided between the converter unit and the inverter unit. With this, even if harmonics are generated in the voltage output from the power supply device to the load due to the occurrence of an arc discharge, the power stored in the power storage unit is used as a power source to output the harmonic components from the inverter unit, thereby suppressing an increase in harmonics contained in the voltage of the power system. Furthermore, even if the voltage of the power supply system drops instantaneously, the power stored in the power storage unit is used as a power source to output a voltage from the inverter unit, thereby stabilizing the voltage output to the load. Effect of the Invention

[0016] The power supply device of the present invention can supply stable power to a load. [Brief description of the drawings]

[0017] [Figure 1] FIG. 1 is a diagram showing a power supply device according to a first embodiment. [Diagram 2] FIG. 11 is a diagram showing a power supply device according to a second embodiment. [Diagram 3] FIG. 11 is a diagram showing a power supply device according to a third embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, preferred embodiments of the present invention will be described with reference to the accompanying drawings.

[0019] [First embodiment] The configuration of a power supply device 100 according to a first embodiment of the present invention will be described with reference to Fig. 1. The power supply device 100 is connected to a three-phase AC power supply 200 and configured to supply power to a load 210. The three-phase AC power supply 200 is an example of the "power system" in the claims.

[0020] As shown in FIG. 1, the power supply device 100 includes a transformer 10, a series transformer 20, a semiconductor power conversion unit 30, a voltage detector 40, and a current detector 50. The transformer 10 transforms the voltage of the power input from a three-phase AC power supply 200. The transformer 10 is configured by a transformer. A primary winding 11 is provided on the primary side of the transformer 10. A main winding 12 and an auxiliary winding 13 are provided on the secondary side of the transformer 10. The primary winding 11 and the main winding 12 on the secondary side form a main transformer. The main winding 12 is connected to a load 210. Specifically, one end of the main winding 12 is connected to the load 210, and the other end is connected to the series transformer 20. The auxiliary winding 13 is connected to the semiconductor power conversion unit 30.

[0021] The series transformer 20 is provided on the secondary side of the transformer 10, and controls the voltage of the power supplied from the transformer 10 to the load 210. Specifically, the series transformer 20 includes a primary winding 21 and a secondary winding 22. The primary winding 21 is connected to the semiconductor power conversion unit 30. The secondary winding 22 is connected to the other end side of the main winding 12.

[0022] Here, in the first embodiment, the semiconductor power conversion unit 30 controls the voltage on the primary side of the series transformer 20. The semiconductor power conversion unit 30 also controls the voltage on the primary side of the series transformer 20 using the power input from the auxiliary winding 13 as a power source. Specifically, the semiconductor power conversion unit 30 includes a converter unit 31 and an inverter unit 32. The converter unit 31 is connected to the auxiliary winding 13. The converter unit 31 is configured, for example, by a diode. The converter unit 31 converts the AC power input from the auxiliary winding 13 into DC power. The inverter unit 32 is connected to the primary winding 21 of the series transformer 20. The inverter unit 32 is configured, for example, by a plurality of semiconductor switching elements. The inverter unit 32 converts the DC power input from the converter unit 31 into AC power and outputs it to the primary winding 21 of the series transformer 20. That is, in the first embodiment, the voltage on the primary side of the series transformer 20 is controlled by the power output from the inverter unit 32 to the series transformer 20. The converter unit 31 may be a PWM converter.

[0023] The transformer 10 and the series transformer 20 are disposed, for example, inside a housing 60. A terminal 61 is provided in the housing 60, and the three-phase AC power supply 200 and the transformer 10 are connected via the terminal 61. The main winding 12 and the load 210 are connected via the terminal 61. The auxiliary winding 13 and the converter unit 31 are connected via the terminal 61. The inverter unit 32 and the series transformer 20 are connected via the terminal 61.

[0024] In the first embodiment, the voltage detector 40 detects the voltage on the secondary side of the transformer 10. The voltage detector 40 is connected to a wiring that connects the main winding 12 on the secondary side of the transformer 10 and the load 210, and inputs the detected voltage value to the voltage regulator 42. The power supply device 100 is also provided with a voltage setter 41 and a voltage regulator 42. A command is output from the voltage regulator 42 to the semiconductor power conversion unit 30 so that the voltage detected by the voltage detector 40 becomes the voltage value (VCC) set in the voltage setter 41. In the semiconductor power conversion unit 30, the semiconductor switching element of the inverter unit 32 is turned on and off based on the command input from the voltage regulator 42, and the inverter unit 32 outputs a voltage according to the command. As a result, the voltage detected by the voltage detector 40 becomes equal to the voltage value set in the voltage setter 41.

[0025] Moreover, in the first embodiment, the load 210 includes an arc furnace 211 that melts the object to be melted 220 by electric power. The arc furnace 211 is, for example, a three-phase AC arc furnace. The arc furnace 211 includes an electrode 212 to which electric power is supplied from the power supply device 100, and a furnace body 213 in which the object to be melted 220 is placed. The object to be melted 220 is, for example, iron. When the arc furnace 211 is a three-phase AC arc furnace, three electrodes 212 are provided. In the arc furnace 211, a stable arc discharge is generated between the electrode 212 and the object to be melted 220, and the object to be melted 220 is melted by heat generated by the arc discharge. However, in the arc furnace 211, if the distance between the electrode 212 and the object to be melted 220 is too short, an excessive current flows between the electrode 212 and the object to be melted 220, and if the distance between the electrode 212 and the object to be melted 220 is too far, an arc discharge does not occur between the electrode 212 and the object to be melted 220. In addition, the distance between the electrode 212 and the object to be melted 220 changes as the object to be melted 220 melts. The change in the distance between the electrode 212 and the object to be melted 220 hinders stable arc discharge.

[0026] Also provided are a semiconductor power conversion unit 70 and an electric motor 71 for adjusting the position of the electrode 212 relative to the object to be melted 220. The semiconductor power conversion unit 70 outputs power input from a three-phase AC power source 230 to the electric motor 71. The electrode 212 is raised and lowered by the driving force of the electric motor 71, and the distance between the electrode 212 and the object to be melted 220 is adjusted. The electric motor 71 includes a pulse generator 72, and the raising and lowering speed of the electrode 212 is controlled based on the rotation speed of the electric motor 71.

[0027] In the first embodiment, the current detector 50 detects a current flowing through the secondary side of the transformer 10. This current is detected in order to adjust the position of the electrode 212 of the arc furnace 211 relative to the material to be melted 220. The current detector 50 also detects a current flowing between the transformer 10 and the load 210. The power supply device 100 is also provided with a current setter 51 and a current regulator 52. A command is output from the current regulator 52 to the semiconductor power conversion unit 70 for controlling the motor 71 so that the current detected by the current detector 50 becomes the current value (ACC) set by the current setter 51. As a result, the electrode 212 is raised and lowered, and the impedance of the load 210 is adjusted, so that the current detected by the current detector 50 becomes equal to the current value set by the current setter 51.

[0028] Moreover, the voltage regulator 42 adjusts the voltage of the series transformer 20 so that the voltage output from the power supply device 100 becomes the voltage value set in the voltage setting device 41, regardless of the fluctuation of the current flowing through the load 210. Moreover, the current regulator 52 raises and lowers the electrode 212 under the voltage kept constant by the voltage regulator 42, and performs control so that the command of the current regulator 52 and the current flowing through the load 210 match. Moreover, when the command value of the current regulator 52 is 0 A, the electrode 212 is raised and stopped at a position where the arc discharge stops. Moreover, even if the electrode 212 is lowered, if the current detected by the current detector 50 does not increase to the command value of the current regulator 52, the electrode 212 continues to lower. Meanwhile, a limit switch is provided to limit the lowering of the electrode 212, and the electrode 212 is stopped at a predetermined lower limit position. Incidentally, the control speed for raising and lowering the electrode 212 is slower than the control speed for the voltage by the inverter unit 32. However, since the output voltage of the inverter unit 32 is kept constant by the voltage regulator 42, the arc discharge can be generated stably.

[0029] Moreover, the commands set by the voltage setter 41 and the current setter 51 are, for example, effective values. Note that the commands set by the voltage setter 41 and the current setter 51 may be average values ​​or amplitude values. Furthermore, the commands set by the voltage setter 41 and the voltage setter 41 may be fixed values, or may be changed manually or by commands from a higher-level control unit depending on the status of the arc furnace 211.

[0030] Additionally, the voltage regulator 42 and the current regulator 52 may set the same commands for the three-phase voltage and the three-phase current, respectively, or may generate separate commands. By setting separate commands for the three-phase voltage and the three-phase current, the voltage and current are controlled more precisely. Additionally, the voltage regulator 42 and the current regulator 52 each set their commands independently of each other.

[0031] Furthermore, the frequency and phase of the voltage output from semiconductor power conversion unit 30 are the same as the frequency and phase of the voltage applied to load 210. On the other hand, when harmonics are contained in the voltage applied to load 210 due to the occurrence of arc discharge, harmonic components corresponding to the harmonics contained in the voltage applied to load 210 may be output from semiconductor power conversion unit 30. This makes it possible to reduce the influence of harmonics on the voltage of three-phase AC power supply 200.

[0032] [Effects of the first embodiment] In the first embodiment, the following effects can be obtained.

[0033] In the first embodiment, as described above, the semiconductor power conversion unit 30 is provided to control the voltage on the primary side of the series transformer 20. As a result, the semiconductor power conversion unit 30 can change the voltage outputted at a relatively high speed compared to the tap changing method, so that even if the load 210 fluctuates suddenly, the voltage applied from the power supply device 100 to the load 210 can be adjusted to a desired voltage. Moreover, unlike the tap changing method that can only change the voltage stepwise, the semiconductor power conversion unit 30 can change the output voltage linearly (continuously). As a result, even if the load 210 fluctuates suddenly, the voltage outputted from the power supply device 100 can be adjusted to a desired voltage. Therefore, the power supply device 100 can supply stable power to the load 210. Moreover, while the tap changing method requires inspection and replacement of the tap changer due to mechanical switching of the tap terminal, the semiconductor power conversion unit 30 does not require inspection and replacement, unlike the tap changing method. Therefore, the user's trouble of inspection and replacement can be saved. Furthermore, the reactive power of the power supplied from the power supply device 100 fluctuates due to a sudden change in the load 210. In the power supply device 100 according to the first embodiment, the semiconductor power conversion unit 30 is provided, and therefore it is possible to compensate for at least a part of the fluctuation of the reactive power contained in the power supplied from the power supply device 100 by generating reactive power from the semiconductor power conversion unit 30. As a result, the fluctuation of the system voltage of the three-phase AC power supply 200 is suppressed, and deterioration of the quality of the power can be suppressed.

[0034] In the first embodiment, as described above, the semiconductor power conversion unit 30 includes the inverter unit 32, and the voltage on the primary side of the series transformer 20 is controlled by the power output from the inverter unit 32 to the series transformer 20. As a result, by controlling the on / off of the switching element included in the inverter unit 32, the voltage applied to the load 210 can be adjusted to a desired voltage even if fluctuations occur in the load 210.

[0035] In the first embodiment, as described above, the power supply device 100 further includes a voltage detector 40 that detects the voltage on the secondary side of the transformer 10. The semiconductor power conversion unit 30 controls the voltage output from the inverter unit 32 based on the voltage detected by the voltage detector 40. This makes it possible to adjust the voltage output from the power supply device 100 to a desired voltage.

[0036] In the first embodiment, as described above, the secondary side of the transformer 10 is provided with the main winding 12 connected to the load 210 and the auxiliary winding 13, and the semiconductor power conversion unit 30 controls the voltage on the primary side of the series transformer 20 using the power input from the auxiliary winding 13 as a power source. In this way, the semiconductor power conversion unit 30 transforms the power input from the auxiliary winding 13, and thus the voltage output from the power supply device 100 can be easily adjusted.

[0037] In the first embodiment, as described above, the load 210 includes an arc furnace 211 that melts the object to be melted 220 by electric power. The arc furnace 211 includes an electrode 212 to which electric power is supplied from the power supply device 100, and a furnace body 213 in which the object to be melted 220 is placed. In the arc furnace 211, if the distance between the electrode 212 of the arc furnace 211 and the object to be melted 220 is appropriate, a stable arc discharge occurs between the electrode 212 and the object to be melted 220. However, if the distance between the electrode 212 and the object to be melted 220 is too close, an excessive current flows between the electrode 212 and the object to be melted 220, and if the distance between the electrode 212 and the object to be melted 220 is too far, an arc discharge does not occur between the electrode 212 and the object to be melted 220. In addition, the distance between the electrode 212 and the object to be melted 220 changes as the object to be melted 220 melts. Due to these factors, the current flowing through the electrode 212 of the arc furnace 211 changes relatively abruptly. Therefore, adjusting the voltage output from the power supply device 100 to a desired voltage by providing the semiconductor power conversion unit 30 is particularly effective for the arc furnace 211 in which the current flowing through the electrode 212 changes relatively abruptly.

[0038] In the first embodiment, as described above, the power supply device 100 further includes a current detector 50 that detects a current flowing through the secondary side of the transformer 10 in order to adjust the position of the electrode 212 of the arc furnace 211 with respect to the material to be melted 220. This makes it possible to stably generate an arc discharge between the electrode 212 and the material to be melted 220 by adjusting the position of the electrode 212 so that the current detected by the current detector 50 has a predetermined current value.

[0039] [Second embodiment] The configuration of a power supply device 110 according to a second embodiment of the present invention will be described with reference to FIG.

[0040] As shown in Fig. 2, in the power supply device 110, power is input to the semiconductor power conversion unit 30 from an external power supply 240. The semiconductor power conversion unit 30 uses the power input from the external power supply 240 as a power source to control the voltage on the primary side of the series transformer 20. Specifically, the converter unit 31 of the semiconductor power conversion unit 30 converts the AC power input from the external power supply 240 into DC power, and the inverter unit 32 converts the DC power output from the converter unit 31 into AC power and outputs it to the primary side of the series transformer 20. The voltage of the three-phase AC power supply 200 and the voltage of the external power supply 240 are different. Other configurations of the second embodiment are similar to those of the first embodiment.

[0041] [Effects of the second embodiment] In the second embodiment, the following effects can be obtained.

[0042] In the second embodiment, as described above, the semiconductor power conversion unit 30 uses the power input from the external power supply 240 as a power source to control the voltage on the primary side of the series transformer 20. As a result, the semiconductor power conversion unit 30 transforms the power input from the external power supply 240, and can easily adjust the voltage output from the power supply device 110. Other effects of the second embodiment are similar to those of the first embodiment.

[0043] [Third embodiment] The configuration of a power supply device 120 according to a third embodiment of the present invention will be described with reference to FIG.

[0044] 3, in the power supply device 120, the semiconductor power conversion unit 130 includes a converter unit 31, an inverter unit 32, and a power storage unit 131 provided between the converter unit 31 and the inverter unit 32. The power storage unit 131 is connected to a wiring that connects the converter unit 31 and the inverter unit 32. Also, AC power input from the auxiliary winding 13 is converted to DC power by the converter unit 31, and the converted DC power is stored in the power storage unit 131. Also, the other configurations of the third embodiment are similar to those of the first embodiment.

[0045] [Effects of the third embodiment] In the third embodiment, the following effects can be obtained.

[0046] In the third embodiment, as described above, the semiconductor power conversion unit 130 includes the converter unit 31, the inverter unit 32, and the power storage unit 131 provided between the converter unit 31 and the inverter unit 32. As a result, even if harmonics occur in the voltage output from the power supply device 100 to the load 210 due to the occurrence of arc discharge, the power stored in the power storage unit 131 is used as a power source to output the harmonic components from the inverter unit 32, thereby suppressing an increase in harmonics contained in the system voltage of the three-phase AC power supply 200. Furthermore, even if the voltage of the three-phase AC power supply 200 drops instantaneously, the power stored in the power storage unit 131 is used as a power source to output a voltage from the inverter unit 32, thereby stabilizing the voltage output to the load 210. Note that other effects of the third embodiment are similar to those of the first embodiment.

[0047] [Variations] The embodiments disclosed herein should be considered to be illustrative and not restrictive in all respects. The scope of the present invention is indicated by the claims, not by the description of the embodiments above, and further includes all modifications (variations) within the meaning and scope of the claims.

[0048] In the above first to third embodiments, an example has been shown in which the load 210 includes the arc furnace 211 that melts the object to be melted 220 by electric power, but the present invention is not limited to this. For example, the load may be a load other than the arc furnace 211.

[0049] In the above first to third embodiments, the arc furnace 211 is a three-phase AC arc furnace, but the present invention is not limited to this. For example, the arc furnace may be a DC arc furnace.

[0050] In the above first to third embodiments, an example has been shown in which the semiconductor power conversion unit 30 includes the converter unit 31 and the inverter unit 32, but the present invention is not limited to this. For example, the semiconductor power conversion unit may include only the inverter unit 32 to which DC power is input from a DC power supply. [Explanation of symbols]

[0051] 10. Transformers 12 Main Winding 13 Auxiliary Winding 20 Series Transformer 30 Semiconductor power conversion section 31 Converter section 32 Inverter section 40 Voltage detector 50 Current detector 131 Power storage unit 200 Three-phase AC power supply (power system) 210 Load 211 Arc Furnace 212 Electrode 220 Melted material 240 External power supply 100, 110, 120 power supplies

Claims

1. A power supply device that is connected to a power grid and supplies power to a load, a transformer that transforms a voltage of the power input from the power system; a series transformer provided on a secondary side of the transformer and configured to control a voltage of power supplied from the transformer to the load; a semiconductor power conversion unit that controls a voltage on a primary side of the series transformer.

2. The semiconductor power conversion unit includes an inverter unit, The power supply device according to claim 1 , wherein a voltage on a primary side of the series transformer is controlled by the power output from the inverter unit to the series transformer.

3. A voltage detector for detecting a voltage on a secondary side of the transformer is further provided. The power supply device according to claim 2 , wherein the semiconductor power conversion unit controls the voltage output from the inverter unit based on the voltage detected by the voltage detector.

4. a main winding connected to the load and an auxiliary winding are provided on a secondary side of the transformer; The power supply device according to claim 1 , wherein the semiconductor power conversion unit controls a voltage on a primary side of the series transformer using power input from the auxiliary winding as a power source.

5. The power supply device according to claim 1 , wherein the semiconductor power conversion unit controls a voltage on a primary side of the series transformer using power input from an external power supply as a power source.

6. 2. The power supply device according to claim 1, wherein the load includes an arc furnace that melts a material to be melted by electric power.

7. 7. The power supply device of claim 6, further comprising a current detector for detecting a current flowing in a secondary side of the transformer to adjust a position of an electrode of the arc furnace relative to the material to be melted.

8. The power supply device according to claim 1 , wherein the semiconductor power conversion unit includes a converter unit, an inverter unit, and a power storage unit provided between the converter unit and the inverter unit.

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