Power system switching device and power system switching method for ac power source

Abstract

To provide a power system switching device and a power system switching method for an AC power source which can switch power systems of the AC power source without stopping an AC motor by a simple device configuration even when power supply to the AC motor is blocked.SOLUTION: A power system switching device for an AC power source comprises: a first power system which supplies power from the AC power source to an AC motor; a second power system which is connected in parallel with the first power system to supply power in the same phase as power of the first power system from the AC power source to the AC motor; an uninterruptible power supply device which is connected between the AC power source and the AC motor in the second power system, stores power from the AC power source and supplies power to the AC motor; an overlap type power system switcher which switches a supply path of power to be supplied to the AC motor between the first power system and the uninterruptible power supply device; and a control unit which controls a switching operation of the overlap type power system switcher.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a power supply system switching device and a power supply system switching method for an AC power supply. [Background technology]

[0002] Electric motors installed in power generation plants or production lines involving the processing of hazardous materials, etc., include those that cannot tolerate even a brief interruption in power supply during a power outage, and those that must be started up immediately as emergency power sources in the event of a power outage.

[0003] For example, consider a case where an integrated steelworks has to temporarily shut down its steelmaking plant due to equipment repairs or other problems. In this case, in a granulator that cools and solidifies molten iron from a blast furnace, if the motor that drives the belt conveyor that discharges granulated iron from the cooling tank stops due to a power outage, the discharge of the granulated iron from the cooling tank will stagnate, creating a risk of a steam explosion. Therefore, in such a production line, it is not acceptable to cut off the power supply even for a short time during a power outage.

[0004] In computers, controllers, etc., uninterruptible power supplies (UPS or CVCF) that output a constant voltage and constant frequency are used as a countermeasure when the power supply is cut off due to a power outage. However, for AC motors, the starting current is large, so the capacity of the uninterruptible power supply becomes large, which causes a problem of increasing equipment costs.

[0005] In some cases, a diesel generator is used as a backup power source, and the power supply system is switched from commercial power to the diesel generator in the event of a power outage. However, it can take several tens of seconds for the backup power source to start up, which creates an unavoidable risk of trouble in the above-mentioned production line.

[0006] In response to this, Patent Document 1 discloses a power outage countermeasure technology for an AC motor drive device, in which power from an AC power source is rectified using a rectifier to obtain DC power, which is then converted to AC power using an inverter that performs variable voltage / variable frequency control and supplied to the AC motor.

[0007] Specifically, Patent Document 1 discloses an AC motor drive device that includes an energy storage means for storing power supplied from an AC power source, and a step-up chopper for boosting the power stored in the energy storage means as a DC voltage. This AC motor drive device stores power in the energy storage means when the AC power source is normal, and supplies power from the energy storage means to the AC motor via the step-up chopper, a diode, and an inverter when the AC power source is out of order. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] JP 2010-154650 A Summary of the Invention [Problem to be solved by the invention]

[0009] However, the above-mentioned conventional technology has the following problems. The technology described in Patent Document 1 is directed to a device that converts power from an AC power source into DC power by a rectifier, converts it to AC power by an inverter, and supplies it to an AC motor. In this case, a boost chopper that boosts the power supplied from the energy storage means to a DC voltage is connected between the rectifier and the inverter via a diode.

[0010] Therefore, in order to switch the power supply system during a power outage, auxiliary devices such as a rectifier, an inverter, a boost chopper, and a diode are required, which causes the AC motor drive device to become larger as a whole. In addition, there is also a problem that the increase in the number of devices constituting the AC motor drive device increases the labor burden for maintenance and the risk of failure.

[0011] The present invention has been made to solve the above-mentioned problems, and an object of the present invention is to provide a power supply system switching device and a power supply system switching method for AC power supplies that are capable of switching the power supply system of an AC power supply with a simple device configuration without stopping the AC motor, even when the power supply to the AC motor is cut off. [Means for solving the problem]

[0012] In order to solve the above-mentioned problems and achieve the object, a power supply system switching device for an AC power supply according to the present invention is a power supply system switching device for an AC power supply that switches a power supply system for supplying power from an AC power supply to an AC motor, and includes: a first power supply system that supplies power from the AC power supply to the AC motor; a second power supply system that is connected in parallel with the first power supply system and supplies power in phase with the power of the first power supply system from the AC power supply to the AC motor; an uninterruptible power supply device that is connected between the AC power supply and the AC motor in the second power supply system and stores power from the AC power supply and supplies power to the AC motor; an overlap-type power supply system switch that switches a supply path of power to be supplied to the AC motor between the first power supply system and the uninterruptible power supply; and a control unit that controls a switching operation of the overlap-type power supply system switch.

[0013] In order to solve the above-mentioned problems and achieve the object, a power supply system switching method for AC power sources according to the present invention uses the above-mentioned power supply system switching device for AC power sources and includes: a start-up step in which, at the start-up of an AC motor, the control unit supplies power to the AC motor only from a first power supply system; an overlap step in which, after the start-up step, the control unit supplies power to the AC motor from both the first power supply system and the second power supply system; and an operation step in which, after the overlap step, the control unit supplies power to the AC motor only from the second power supply system.

[0014] Further, in the power supply system switching method of the present invention, in the above-described invention, the control unit performs a transition from the startup step to the overlap step and a transition from the overlap step to the operation step in accordance with a preset time from startup of the AC motor. Effect of the Invention

[0015] According to the power supply system switching device and power supply system switching method for an AC power supply of the present invention, even if the power supply to an AC motor is cut off, it is possible to switch the power supply system of the AC power supply with a simple device configuration without stopping the AC motor. [Brief description of the drawings]

[0016] [Figure 1] FIG. 1 is a diagram showing a schematic configuration of a power supply system transfer device for AC power supplies according to an embodiment of the present invention. [Diagram 2] FIG. 2 is a diagram showing a schematic configuration of a continuous inverter type uninterruptible power supply. [Diagram 3] FIG. 3 is a diagram showing a schematic configuration of an uninterruptible power supply using a momentary power sag compensation (MPC) system. [Figure 4] FIG. 4 is a flowchart showing the flow of a method for switching AC power systems using a power system switching device for AC power according to an embodiment of the present invention. [Diagram 5] FIG. 5 is a diagram showing a schematic diagram of a change in starting current. [Figure 6] FIG. 6 is a diagram showing changes over time in the control signal output from the control unit to the overlap type power system switch. [Figure 7] FIG. 7 is a diagram showing a schematic configuration example of a power supply system transfer device for AC power supplies in the case where two AC motors are driven by one AC power supply. [Figure 8] FIG. 8 is a diagram showing a schematic configuration of a granulator according to an embodiment of the present invention. [Figure 9]FIG. 9 is a diagram showing an embodiment of the present invention, in which an uninterruptible power supply is disposed between an AC power supply and an AC motor by a normal connection method. [Figure 10] FIG. 10 is a diagram showing an example of an input voltage to an AC motor when power supply from an AC power source is interrupted according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] A power supply system switching device and a power supply system switching method for an AC power supply according to an embodiment of the present invention will be described with reference to the drawings. Note that the present invention is not limited to the following embodiment, and the components in the following embodiment include those that are replaceable and easy for a person skilled in the art, or those that are substantially the same.

[0018] <AC power supply switching device> A power supply system transfer device for AC power supply according to an embodiment of the present invention will be described with reference to Fig. 1 to Fig. 3. The power supply system transfer device for AC power supply according to the embodiment switches a power supply system that supplies power from an AC power supply to an AC motor. The power supply system transfer device 10 according to the embodiment is disposed between an AC power supply 1 and an AC motor 2.

[0019] The AC power supply 1 supplies the power required to drive the AC motors installed in the production line. The AC power supply 1 outputs, for example, three-phase AC and supplies an AC voltage of 200 to 440 V and a frequency of 50 Hz or 60 Hz. The AC power supply 1 also supplies power for driving one or more AC motors 2. Therefore, the power supplied by the AC power supply 1 is set according to the total output of the AC motors 2 to be driven. The AC power supply 1 supplies, for example, 100 to 2000 kVA of power.

[0020] The power supply transfer device 10 for AC power comprises a first power supply system 3, a second power supply system 4, an uninterruptible power supply (UPS) 5, an overlap type power supply transfer switch 6, and a control unit .

[0021] The first power supply system 3 supplies an AC voltage (power) from the AC power supply 1 to the AC motor 2. The first power supply system 3 is, for example, a cable for wiring, and is selected according to the power from the AC power supply 1.

[0022] The second power supply system 4 is connected in parallel with the first power supply system 3, and supplies an AC voltage (power) having the same phase as the power of the first power supply system 3 from the AC power source 1 to the AC motor 2. If an element (e.g., a capacitor, etc.) that changes the phase of the AC power source 1 is not interposed between the AC power source 1 and the AC motor 2 in the second power supply system 4, the power supplied from the second power supply system 4 to the AC motor 2 and the power supplied from the first power supply system 3 to the AC motor 2 will be in phase. Furthermore, if AC power is not supplied to the second power supply system 4 from a power source other than the AC power source 1, a transformer, etc., the power supplied from the second power supply system 4 to the AC motor 2 and the power supplied from the first power supply system 3 to the AC motor 2 will be in phase.

[0023] The uninterruptible power supply 5 is connected between the AC power supply 1 and the AC motor 2 in the second power supply system 4, stores power from the AC power supply 1, and supplies power to the AC motor 2. In this case, the second power supply system 4 is composed of a second power supply system 4a that connects the AC power supply 1 and the uninterruptible power supply 5, and a second power supply system 4b that connects the uninterruptible power supply 5 and the AC motor 2. If the uninterruptible power supply 5 does not include an element or the like that changes the phase of power between the second power supply system 4a and the second power supply system 4b, the power supplied from the second power supply system 4b to the AC motor 2 and the power supplied from the first power supply system 3 to the AC motor 2 will be in phase.

[0024] The uninterruptible power supply 5 is a device that converts AC voltage to DC voltage, stores the voltage in a storage battery, and, in the event of a power outage in the AC power source 1, converts the power stored in the storage battery to AC voltage and supplies the power. As the uninterruptible power supply 5, a multiple power compensator (MPC) type and a continuous inverter type are known, but it is preferable to use an uninterruptible power supply using the multiple power compensator (MPC) type in the embodiment of the present invention.

[0025] Fig. 2 shows a schematic configuration of a continuous inverter type uninterruptible power supply 5. The uninterruptible power supply 5 in the figure includes a rectifier (converter) 56 that converts AC voltage supplied via the second power supply system 4a into DC voltage, a storage battery 53 that stores the power converted into DC, and an inverter 57 that converts the DC power stored in the storage battery 53 into AC.

[0026] As an embodiment of the present invention, it is also possible to apply a continuous inverter type uninterruptible power supply 5. However, in a continuous inverter type uninterruptible power supply 5, power efficiency may decrease due to conversion from DC to AC by the inverter 57. In addition, the inverter 57 may cause a power phase shift between the second power supply system 4a and the second power supply system 4b. For this reason, it is preferable to apply a momentary sag compensation (MPC) system to the uninterruptible power supply 5.

[0027] 3 shows a schematic configuration of an uninterruptible power supply 5 using a momentary power sag compensation (MPC) system. An AC voltage is supplied to the uninterruptible power supply 5 via a second power supply system 4a. An AC voltage is output from the uninterruptible power supply 5 via a second power supply system 4b. The uninterruptible power supply 5 includes a high-speed cutoff switch 51, a bidirectional converter 52, and a storage battery 53.

[0028] The high speed shutoff switch (HSS) 51 is an element that quickly shuts off a circuit when an abnormality occurs in the AC input voltage. When the power supply from the AC power source 1 is cut off, the high speed shutoff switch 51 quickly shuts off the second power supply system 4a and the circuit on the secondary side of the high speed shutoff switch 51.

[0029] If the high-speed cut-off switch 51 is not cut off when the power supply from the AC power source 1 is cut off, power will also be supplied from the storage battery 53 to the second power supply system 4a via the bidirectional converter 52. This may cause an accident due to a short circuit when the AC power source 1 recovers from a power outage. The high-speed cut-off switch 51 is provided in order to prevent such an accident. However, the high-speed cut-off switch 51 is an element that functions as a switch, and does not cause a phase shift in the AC power between the second power supply system 4a and the second power supply system 4b.

[0030] The bidirectional converter 52 is a device that functions both as a converter that converts the AC voltage supplied via the second power supply system 4a into DC, and as an inverter that converts the DC power stored in the storage battery 53 into AC.

[0031] The storage battery 53 stores the power converted to DC by the bidirectional converter 52. As the storage battery 53, for example, a lead storage battery may be used, and a battery that has about 252 cells that output a voltage of 2 V and can output a nominal voltage of 504 V and an AC voltage of 420 V may be used.

[0032] It is preferable to use a storage battery 53 that can continue discharging for 5 to 10 minutes. The reason why the discharge time of the storage battery 53 is set to 5 minutes or more is that it is only necessary to supply power to the AC motor 2 until the power supply from the AC power source 1 or another alternative power source is resumed. In addition, a storage battery 53 with a discharge time of more than 10 minutes has a large capacity and is therefore not economical.

[0033] The uninterruptible power supply 5 should be provided with a dedicated ground as necessary. This is to prevent the uninterruptible power supply 5 from malfunctioning due to noise generated from other devices or other power supply systems. It is also to prevent noise generated from the high-speed cutoff switch 51 and the bidirectional converter 52 from affecting other devices.

[0034] The uninterruptible power supply 5 may be provided with a bypass circuit that is directly connected from the second power supply system 4a to the second power supply system 4b, as necessary. This bypass circuit is disconnected by a switch when the uninterruptible power supply 5 is in operation. When performing maintenance and inspection of the uninterruptible power supply 5, the second power supply system 4a and the second power supply system 4b may be short-circuited by the switch so that the power supplied from the AC power source 1 is not supplied to the bidirectional converter 52 and the storage battery 53 of the uninterruptible power supply 5.

[0035] The overlap type power system switch 6 switches the supply path of power supplied to the AC motor 2 between the first power system 3 and the second power system 4 connected to the uninterruptible power supply 5. The overlap type power system switch 6 is a power system switch that is capable of overlapping power supply, temporarily overlapping powers supplied from different power systems in the same phase and supplying the power to a load.

[0036] The overlap type power supply system changeover switch 6 is a switch with an electrode structure configured to allow the power supplied from the first power supply system 3 and the second power supply system 4b to overlap when switching between the power supplied from the first power supply system 3 to the AC motor 2 and the power supplied from the second power supply system 4b to the AC motor 2. By using such an overlap type power supply system changeover switch 6, the power supplied to the AC motor 2 is not interrupted and AC power is supplied continuously.

[0037] The overlap type power system switch 6 may have, for example, a rated voltage of AC 660V, a rated current of 400A, and three poles. The overlap type power system switch 6 includes a first switch 61 that opens and closes the power supplied to the AC motor 2 via the first power system 3, and a second switch 62 that opens and closes the power supplied to the AC motor 2 via the second power system 4b. The overlap type power system switch 6 is configured such that the first switch 61 and the second switch 62 are opened and closed based on a control signal from the outside.

[0038] The control unit 7 is a control device that controls the switching operation of the overlap type power system switch 6. The control unit 7 controls the opening and closing of the first switch 61 and the opening and closing of the second switch 62, thereby controlling the time during which the supply from the first power system 3 to the AC motor 2 and the supply from the second power system 4b overlap (overlap time). The control unit 7 also includes, for example, a timer, and outputs a control signal for opening and closing the first switch 61 and the second switch 62 according to a preset elapsed time.

[0039] The control unit 7 controls the first switch 61 to be closed (ON) and the second switch 62 to be open (OFF) when starting the AC motor 2. This makes it possible to set the startup power required when starting the AC motor 2 to be supplied from the AC power source 1 (direct input).

[0040] Then, the control unit 7 controls both the first switch 61 and the second switch 62 to be closed (ON) after the AC motor 2 is started, thereby enabling power to be supplied to the AC motor 2 from both the first power supply system 3 and the second power supply system 4b in an overlapping manner. As a result, power is supplied to the operation of the AC motor 2 from both the first power supply system 3 and the second power supply system 4b, and an AC voltage is supplied to the uninterruptible power supply 5 from the second power supply system 4a, so that power is stored in the storage battery 53.

[0041] Furthermore, the control unit 7 controls the first switch 61 to be open (OFF) and controls the second switch 62 to be closed (ON). As a result, power is supplied from the second power supply system 4b to the operation of the AC motor 2, and AC voltage is supplied from the second power supply system 4a to the uninterruptible power supply 5, so that the storage battery 53 continues to store electricity.

[0042] When the AC motor 2 is operated in this manner, if the power supply from the AC power source 1 is interrupted due to a power outage, the power stored in the storage battery 53 of the uninterruptible power supply 5 is converted to an AC voltage by the bidirectional converter 52. Then, since the power supply from the second power supply system 4b to the AC motor 2 continues, the power supply to the AC motor 2 is not interrupted even if a power outage occurs.

[0043] Furthermore, such a device configuration eliminates the need to supply startup power to the AC motor 2 from the uninterruptible power supply 5 via the second power supply system 4b during a power outage. Therefore, compared to a method in which the power supply system is switched after a power outage occurs, the capacity of the uninterruptible power supply 5 can be kept low, and the power supply system transfer device 10 can be made smaller.

[0044] <How to switch AC power supply systems> A method for switching between power supply systems of AC power supplies using the above-mentioned power supply system switching device 10 will be described with reference to Figs.

[0045] In the power supply system switching method for AC power supplies according to the embodiment of the present invention, first, when starting the AC motor 2, the control unit 7 supplies power to the AC motor 2 only from the first power supply system 3 (start-up step S1). The start-up step S1 is a process that is started in a state in which the AC motor 2 is stopped and is performed until the AC motor 2 is started.

[0046] The start-up step S1 is started in a state where the control unit 7 controls the first switch 61 to be closed (ON) and the second switch 62 to be open (OFF). This allows the setting so that the start-up power required at the start of the AC motor 2 is supplied (directly) from the AC power source 1. In general, at the start-up of the AC motor 2, a start-up current larger than the rated current of the AC motor 2 flows for a short period of time (for example, 5 to 8 seconds).

[0047] Fig. 5 is a schematic diagram showing the change in the starting current. As can be seen from the figure, in the process in which the AC motor 2 starts and the rotation speed increases, a starting current of about 3 to 7 times the rated current flows for a short period of time. Therefore, if an uninterruptible power supply is used to restore the AC motor 2 to a steady operating state when the AC motor 2 stops or slows down due to a power outage, an uninterruptible power supply with a capacity excessive to the rated output of the AC motor 2 is required.

[0048] For example, when the starting current is three times the rated current of the AC motor 2, if the power required for steady operation is 200 kVA, the operation of the AC motor 2 cannot be immediately restored to the steady state unless the capacity of the uninterruptible power supply is about 600 kVA. Therefore, in the start-up step S1, the start-up power required for starting the AC motor 2 is supplied from the AC power source 1.

[0049] Next, after starting the AC motor 2, the control unit 7 supplies power to the AC motor 2 from both the first power supply system 3 and the second power supply system 4b (overlap step S2). In the overlap step S2, the control unit 7 controls both the first switch 61 and the second switch 62 to be closed (ON), and overlaps the power supply to the AC motor 2 from the first power supply system 3 and the second power supply system 4b. In other words, power of the same phase is supplied from both the first power supply system 3 and the second power supply system 4b to the operation of the AC motor 2. As a result, an AC voltage is supplied from the second power supply system 4a to the uninterruptible power supply 5, and the power converted from AC to DC by the bidirectional converter 52 is stored in the storage battery 53.

[0050] The control unit 7 has a preset time (overlap time) during which the power supply to the AC motor 2 from the first power supply system 3 and the second power supply system 4b overlap, and the overlap step S2 ends when the set overlap time has elapsed.

[0051] Next, after the overlap step S2, the control unit 7 supplies power to the AC motor 2 only from the second power supply system 4b (operation step S3). In the operation step S3, the first switch 61 is controlled to be open (OFF) and the second switch 62 is controlled to be closed (ON) based on a switching signal from the control unit 7, so that power is supplied to the AC motor 2 only from the second power supply system 4b. As a result, AC power is supplied from the second power supply system 4a to the uninterruptible power supply 5, and the storage of power in the storage battery 53 is continued. Also, the AC motor 2 is driven by the power supplied from the AC power source 1 via the second power supply system 4b.

[0052] Then, in operation step S3, when the power supply from the AC power source 1 is cut off due to a power outage, the secondary circuit is cut off by the high-speed cut-off switch 51 of the uninterruptible power supply 5, and the power stored in the storage battery 53 is converted to an AC voltage by the bidirectional converter 52. This allows the power supply from the second power supply system 4b to the AC motor 2 to continue.

[0053] Since the high-speed cutoff switch 51 operates in, for example, 2 ms or less, the power supply to the AC motor 2 is not substantially cut off. If the operation of the AC motor 2 does not stop, there is no need for the uninterruptible power supply 5 to supply the startup power required to start the AC motor 2. Therefore, the capacity of the uninterruptible power supply 5 can be kept low, and the power supply transfer device 10 can be made smaller in size.

[0054] In the above-described method for switching power supply systems for AC power sources, it is preferable that the control unit 7 transitions from the startup step S1 to the overlap step S2 and from the overlap step S2 to the operation step S3 in accordance with a preset time from the startup of the AC motor 2.

[0055] 6 is a diagram showing the change over time of the control signal output by the control unit 7 to the overlap type power system switch 6. The control unit 7 sends a switching signal for transition from the startup step S1 to the overlap step S2 to the overlap type power system switch 6 at a preset time T1 from the startup of the AC motor 2. Then, the control unit 7 sends a switching signal for transition from the overlap step S2 to the operation step S3 to the overlap type power system switch 6 at a preset time T2 after the predetermined overlap time has elapsed. In this case, the control unit 7 may use a timer to set the elapsed times T1 and T2 from the startup of the AC motor 2, and send the switching signal to the overlap type power system switch 6.

[0056] The elapsed time T1 from the start-up of the AC motor 2 is set to be longer than the time required for the AC motor 2 to be driven to reach a steady state after starting. The elapsed time T1 from the start-up of the AC motor 2 may be set to, for example, about 10 to 20 seconds. The overlap time may be set to a time sufficient for the power supplied via the second power supply system 4b to be supplied to the storage battery 53, and may be set to, for example, about 0.5 to 2.0 seconds.

[0057] <Another embodiment of the power supply transfer device> Another embodiment of the power supply system transfer device for AC power supplies according to the present invention can be applied to a case where a plurality of AC motors are driven by one AC power supply 1. Fig. 7 is a diagram showing a schematic configuration example of a power supply system transfer device for AC power supplies when two AC motors are driven by one AC power supply.

[0058] 7 is an AC power supply system switching device that switches between power supply systems that supply power to two AC motors 2A and 2B from one AC power supply 1. The power supply system switching device 11 includes first power supply systems 3A and 3B that supply power to the AC motors 2A and 2B from one AC power supply 1. The power supply system switching device 11 also includes a second power supply system 4 (4a, 4b) that is connected in parallel with the first power supply systems 3A and 3B and supplies power in phase with the power of the first power supply systems 3A and 3B from the AC power supply 1 to the AC motors 2A and 2B.

[0059] The power supply system transfer device 11 is connected between the AC power source 1 of the second power supply system 4 (4a, 4b) and the AC motors 2A, 2B, and includes an uninterruptible power supply 5 that stores power from the AC power source 1 and supplies power to the AC motors 2A, 2B. The power supply system transfer device 11 also includes overlap type power supply system switchers 6A, 6B that switch the supply path of power to be supplied to the AC motors 2A, 2B between the first power supply systems 3A, 3B and the uninterruptible power supply 5. The power supply system transfer device 11 also includes control units 7A, 7B that control the switching operation of the overlap type power supply system switchers 6A, 6B.

[0060] The method for switching power supply systems of AC power supplies using the power supply system switching device 11 shown in Fig. 7 is executed according to the flow of Fig. 4. That is, in a start-up step S1, when the AC motors 2A, 2B are started, power is supplied to the AC motors 2A, 2B from the first power supply systems 3A, 3B based on commands from the control units 7A, 7B.

[0061] Next, in an overlap step S2, after the AC motors 2A, 2B are started, power is supplied to the AC motors 2A, 2B from the first power supply system 3A, 3B and the second power supply system 4 (4a, 4b) based on a command from the control units 7A, 7B. Then, in an operation step S3, after the overlap step S2, power is supplied to the AC motors 2A, 2B only from the second power supply system 4 (4a, 4b) based on a command from the control units 7A, 7B. Note that the control units 7A, 7B may be configured as a single control device to control the overlap type power supply system changeover switches 6A, 6B.

[0062] As described above, if the power supply from the AC power source 1 is interrupted due to a power outage while the operation step S3 is continuing, the power supply from the uninterruptible power supply 5 to the AC motors 2A, 2B via the second power supply system 4b continues. This means that the power supply to the AC motors 2A, 2B is not substantially interrupted. Furthermore, if the operation of the AC motors 2A, 2B does not stop, there is no need for the uninterruptible power supply 5 to supply the startup power required to start the AC motors 2A, 2B, so the capacity of the uninterruptible power supply 5 can be kept low.

[0063] <Example> As an embodiment of the present invention, an example will be described in which the power supply system switching device of the AC power source according to the present invention is applied to an AC motor that serves as a drive source for a belt conveyor constituting the granular pig iron mill, which produces granular pig iron by cooling molten pig iron from a blast furnace and solidifying it into granules.

[0064] 8 is a diagram showing a schematic configuration of the granulator 20. The granulator 20 mainly includes a tundish 21, a spray head 22, a granulator chute 23, a granulator pit 24, a mesh conveyor 25, and a stacking conveyor 26.

[0065] The tundish 21 temporarily stores molten iron from the blast furnace and supplies it to the spray head 22. The spray head 22 sprays the molten iron toward the granular pig iron chute 23 while dispersing it into particles of about 3 to 25 mm in size.

[0066] The granulated pig iron chute 23 is a funnel-shaped device that receives the molten pig iron sprayed from the spray head 22 and turned into droplets. The granulated pig iron chute 23 is positioned so as to be submerged in cooling water 28 stored in the granulated pig iron pit 24. The molten pig iron sprayed into the granulated pig iron chute 23 is cooled by the cooling water 28. As a result, the molten molten pig iron becomes molten pig iron granules (granulated pig iron), which are discharged from the bottom of the granulated pig iron chute 23 onto the mesh conveyor 25.

[0067] The mesh conveyor 25 is a device that transports molten pig iron granules using a metal mesh belt. The mesh belt is used to remove cooling water 28 from the belt when the molten pig iron granules are discharged from the granular pig iron pit 24. The temperature of the molten pig iron granules is approximately 500°C when they are discharged onto the mesh conveyor 25. The molten pig iron granules discharged onto the mesh conveyor 25 are discharged outside the granular pig iron pit 24.

[0068] Outside the granular pig iron pit 24, there is provided a stacking conveyor 26 for transporting the molten pig iron granules transported by the mesh conveyor 25. The stacking conveyor 26 transports the molten pig iron granules to a predetermined yard 27.

[0069] The granulator 20 is used to continuously produce molten iron supplied from a blast furnace as molten iron granules when the operation of the iron-making process is temporarily stopped. Therefore, the granulator 20 needs to be constantly operating while molten iron is being supplied from the blast furnace.

[0070] In particular, when the mesh conveyor 25 and stacking conveyor 26 constituting the granular iron machine 20 stop, the molten iron granules are retained in the granular iron pit 24. When the molten iron granules accumulate at the bottom of the granular iron chute 23, the molten iron granules are difficult to cool, and the temperature of the molten iron granules may exceed 650°C. When the temperature of the molten iron granules exceeds 650°C, there is a risk of a steam explosion occurring due to the sudden vaporization of the cooling water 28 sealed between the molten iron granules. Therefore, even if a power outage occurs, it is not permitted for the mesh conveyor 25 and stacking conveyor 26 to stop.

[0071] In addition, considering the risk of a steam explosion, even if the mesh conveyor 25 or stacking conveyor 26 stops temporarily, the stop time must be 15 seconds or less. Therefore, we studied a method of switching the power supply system so that the conveyors (mesh conveyor 25 and stacking conveyor 26) do not stop even if the power supply from the AC power source is stopped.

[0072] First, we considered using a diesel generator to generate power as a backup power source to supply drive power to the conveyor. However, it took about 20 seconds to start up the diesel generator, and we found that this did not reduce the risk of a steam explosion. We also considered an example in which an uninterruptible power supply was installed in the usual way between the AC power source 1 and the AC motors 2A and 2B.

[0073] 9 shows an example (comparative example) in which an uninterruptible power supply 72 is arranged between an AC power supply 1 and AC motors 2A and 2B by a normal connection method. That is, the power supply system transfer device 70 shown in the figure is an example in which it is connected in series between the AC power supply 1 and AC motors 2A and 2B.

[0074] 9 includes a power supply system 71 (71a, 71b) that supplies power to AC motors 2A, 2B from an AC power source 1. In the power supply system transfer device 70, an uninterruptible power supply 72 is connected to the power supply system 71a that supplies power from the AC power source 1, and is configured so that power is supplied from the uninterruptible power supply 72 to the AC motors 2A, 2B via the power supply system 71b.

[0075] In the normal power supply system shown in Figure 9, when the power supply from AC power source 1 is cut off due to a power outage, a change-over switch inside uninterruptible power supply 72 switches power supply from the storage battery inside uninterruptible power supply 72 to AC motors 2A, 2B.

[0076] Table 1 shows the specifications of the AC motors that drive the mesh conveyor 25 and the stacking conveyor 26.

[0077] [Table 1]

[0078] The AC motor that drives the mesh conveyor 25 has a motor capacity of 55 kW and a rated current of 120 A. The AC motor that drives the stacking conveyor 26 has a motor capacity of 37 kW and a rated current of 81 A. Meanwhile, the starting currents required to start these conveyors from a stopped state to an operating state are 549 A and 348 A, respectively.

[0079] 9, when starting the mesh conveyor 25 and the stacking conveyor 26, it is necessary to supply a starting current to the AC motors 2A and 2B via the uninterruptible power supply 72, and it is necessary to install a device capable of supplying a total current of 897 A as the uninterruptible power supply 72. This increases the equipment cost of the power supply transfer device 70.

[0080] It is also possible to start the AC motors 2A and 2B individually by disposing an electromagnetic contactor between the uninterruptible power supply 72 of the power supply system transfer device 70 shown in Fig. 9 and the AC motors 2A and 2B. However, even in this case, it is necessary to install an uninterruptible power supply 72 capable of supplying a current of 549 A in order to start the mesh conveyor 25, which requires a large starting current.

[0081] Therefore, hereinafter, an example will be described in which the power supply system transfer device of the AC power supply according to the present invention is applied to two AC motors that drive the mesh conveyor 25 and the stacking conveyor 26. In this embodiment, a power supply system transfer device 11 shown in FIG. 7 is configured.

[0082] Specifically, AC motor 2A is an AC motor that drives mesh conveyor 25, and AC motor 2B is an AC motor that drives stacking conveyor 26. An uninterruptible power supply 5 is disposed between AC power source 1 of second power supply system 4 and AC motors 2A and 2B, and stores power from AC power source 1 and supplies power to AC motors 2A and 2B.

[0083] For the overlap type power system changeover switches 6A and 6B, power system changeover switches SSK series LH type (overlap type) manufactured by Kyoritsu Keiki Co., Ltd. are used. The uninterruptible power supply 5 used in the power system changeover device 11 configured in this way does not need to start the mesh conveyor 25 and the stacking conveyor 26 even if a power outage occurs while the conveyors are in operation, so that the capacity can be kept small. Specifically, the uninterruptible power supply 5 may be any device capable of supplying a current of 201A, the total value of the rated current, to the AC motors 2A and 2B.

[0084] 10 shows an example of the input voltage to the AC motor 2A when the power supply from the AC power source 1 is cut off while the AC motors 2A, 2B are operating under a 100% load condition when the power supply system transfer device 11 of the embodiment is used. The input current is a measured value of the current flowing through the first power supply system 3A.

[0085] The DC circuit voltage is the voltage supplied from the storage battery 53 of the uninterruptible power supply 5 to the bidirectional converter 52. The output voltage is the voltage output from the second power supply system 4b to the AC motor 2A, and is the input voltage to the AC motor 2A. It can be seen from FIG. 10 that the power supply from the AC power source 1 is cut off and the input current becomes zero. When the power supply from the AC power source 1 is cut off, the power stored in the storage battery 53 of the uninterruptible power supply 5 is output, so that the DC circuit voltage gradually decreases. On the other hand, the output voltage was not cut off even when the power supply from the AC power source 1 was cut off.

[0086] According to the power supply system switching device and power supply system switching method for AC power supply according to the embodiments described above, even if the power supply to the AC motor is cut off, it is possible to switch the power supply system of the AC power supply with a simple device configuration without stopping the AC motor.

[0087] Although the embodiment of the invention made by the present inventor has been described above, the present invention is not limited by the description and drawings that form a part of the disclosure of the present invention according to the present embodiment. In other words, other embodiments, examples, operation techniques, etc. made by those skilled in the art based on the present embodiment are all included in the scope of the present invention. [Explanation of symbols]

[0088] 1 AC power supply 2,2A,2B AC motor 3,3A,3B 1st power supply system 4,4a,4b 2nd power supply system 5 Uninterruptible power supply (UPS) 51 High-speed cutoff switch 52 Bidirectional Converter 53 Storage battery 56 Rectifier (converter) 57 Inverter 6,6A,6B Overlap type power system switch 61 First Switch 62 Second Switch 7,7A,7B control section 10,11,70 Power system switching device 20 Granular pig iron machine 21 Tundish 22 Spray Head 23 Granular iron chute 24 Granular iron pit 25 Mesh Conveyor 26 Stacking Conveyor 27 yards 71,71a,71b Power system

Claims

1. A power supply system switching device for AC power supplies that switches a power supply system for supplying power from an AC power supply to an AC motor, comprising: a first power supply system that supplies power from the AC power supply to the AC motor; a second power supply system connected in parallel with the first power supply system and configured to supply electric power having the same phase as electric power of the first power supply system from the AC power source to the AC motor; an uninterruptible power supply connected between the AC power supply in the second power supply system and the AC motor, storing power from the AC power supply and supplying power to the AC motor; an overlap type power supply system switching device that switches a supply path of power to be supplied to the AC motor between the first power supply system and the uninterruptible power supply; A control unit that controls a switching operation of the overlap type power supply system switch; A power supply system switching device for AC power supply comprising:

2. Using the power supply system transfer device for AC power supply according to claim 1, a starting step of supplying power to the AC motor only from the first power supply system by a control unit when starting the AC motor; an overlapping step of supplying power to the AC motor from the first power supply system and the second power supply system by the control unit after the starting step; an operating step of supplying power to the AC motor only from the second power supply system by the control unit after the overlapping step; A power supply system switching method for AC power supplies comprising:

3. The control unit performs a transition from the startup step to the overlap step and a transition from the overlap step to the operation step in accordance with a preset time from the startup of the AC motor. The method for switching between power supply systems of AC power sources according to claim 2.

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