Power conversion system, drive system, and startup method

Abstract

A power conversion system according to an embodiment is provided with a reactor between the DC side of a converter and the DC side of an inverter, and is provided with a transformer between the inverter and a synchronous motor of a load. This power conversion system comprises an inverter and a control unit. The inverter includes a thyristor configured as a full-bridge thyristor, and generates a three-phase AC by means of a control. When starting the synchronous motor using the inverter, the control unit interrupts the AC current of the inverter by performing a control for switching the thyristor at a specific phase multiple times during a half cycle of the three-phase AC.

Description

Power conversion system, drive system and starting method

[0001] The present invention relates to a power conversion system, a drive system, and a starting method.

[0002] A power conversion system includes a converter that converts three-phase AC power into DC power, a DC reactor that smooths the DC power, an inverter that converts the DC power supplied from the converter via the DC reactor into three-phase AC power of a desired frequency, and a transformer that transforms the AC output by the inverter. When the inverter starts a synchronous motor using the AC power transformed by the aforementioned transformer, there have been cases where the starting torque is insufficient and the synchronous motor cannot be started.

[0003] Japanese Unexamined Patent Publication No. 2003-61380

[0004] An object of the present invention is to provide a power conversion system, a drive system, and a starting method for starting a synchronous motor driven via a transformer.

[0005] In the power conversion system of an embodiment, a reactor is provided between the DC side of the converter and the DC side of the inverter, and a transformer is provided between the inverter and the synchronous motor serving as a load. The power conversion system includes the inverter and a control unit. The inverter includes thyristors configured in a full-bridge type, and generates three-phase AC through control. When starting the synchronous motor using the inverter, the control unit intermittently switches the alternating current of the inverter by controlling switching of the thyristors of a specific phase multiple times during a half cycle of the three-phase alternating current.

[0006] A schematic diagram of the power conversion device of the embodiment. A schematic diagram of the inverter of the embodiment. A time chart schematically showing the ideal operation of the inverter of the embodiment. A time chart showing the startup state observed when the inverter was actually operated. A diagram for explaining the current waveform flowing through the transformer 5 based on the drive method of the embodiment. A diagram for explaining the current waveform flowing through the transformer 5 based on the drive method of the comparative example. A diagram for explaining the difference in current waveforms flowing through the transformer 5 based on the respective drive methods of the embodiment and the comparative example. A schematic diagram of the power conversion system 100A of the embodiment.

[0007] The following describes the power conversion system, drive system, and startup method of the embodiment. In the following description, being electrically connected may simply be referred to as "being connected." The minute fixed values ​​in the embodiment may include 0. In this specification, "based on XX" means "based on at least XX," and also includes cases where it is based on another element in addition to XX. Furthermore, "based on XX" is not limited to cases where XX is used directly, but also includes cases where it is based on calculations or processing performed on XX. "XX" is any element (for example, any information).

[0008] The power converter shown below is an example of a Load Commutated Inverter (LCI). Below, we will describe an example of the procedure for starting a stopped synchronous motor using this power converter.

[0009] Figure 1 is a schematic diagram of the power conversion system 100 according to the embodiment. Figure 2 is a schematic diagram of the inverter 4 according to the embodiment. As shown in Figure 1, the power conversion system 100 comprises transformers 1 and 5, a converter 2, a DC reactor 3, an inverter 4, and a control unit 20. The transformer 1 converts the three-phase AC voltage supplied from the AC power source PS into a three-phase AC voltage of a predetermined voltage value and supplies it to the converter 2.

[0010] Converter 2 is a three-phase full-wave rectifier circuit including at least six thyristors, which converts three-phase AC power from transformer 1 into variable-voltage DC power. DC reactor 3 is connected between the positive output terminal 2a of converter 2 and the positive input terminal 4a of inverter 4 to smooth the DC current. The negative output terminal 2b of converter 2 and the negative input terminal 4b of inverter 4 are connected to each other. Another DC reactor 3 may be connected between the negative output terminal 2b of converter 2 and the negative input terminal 4b of inverter 4.

[0011] In addition to the positive input terminal 4a and negative input terminal 4b described above, the inverter 4 is equipped with three output terminals 4c to 4e. The three output terminals 4c to 4e of the inverter 4 are connected to the R-phase terminal 30a, S-phase terminal 30b, and T-phase terminal 30c of the synchronous motor 30, respectively. As shown in Figure 2, the inverter 4 constitutes a three-phase power conversion circuit including at least six thyristors. Thyristors U, V, W, X, Y, and Z are examples of the six thyristors described above.

[0012] For example, the anodes of thyristors U, V, and W are all connected to the positive input terminal 4a, and their cathodes are connected to output terminals 4c to 4e, respectively. The anodes of thyristors X, Y, and Z are each connected to output terminals 4c to 4e, respectively, and their cathodes are all connected to the negative input terminal 4b.

[0013] The inverter 4 generates three-phase AC power for driving the synchronous motor 30 by conducting the above-mentioned thyristor at a predetermined phase, thereby converting the DC power supplied from the converter 2 via the DC reactor 3 into variable frequency, variable voltage three-phase AC power.

[0014] The transformer 5 converts the three-phase AC voltage supplied from the inverter 4 into a three-phase AC voltage of a predetermined voltage value and supplies it to the synchronous motor 30.

[0015] The control unit 20 controls the converter 2 using signals from the position detector 7. General methods may be applied to the control of the converter 2 by the control unit 20. After startup, the control unit 20 controls the inverter 4 using signals from the position detector 7. General methods may be applied to the control of the inverter 4 by the control unit 20 after startup.

[0016] Referring to Figures 3 and 4, the ideal operation of the synchronous motor 30 during constant-speed operation will be explained.

[0017] Figure 3 is a time chart schematically showing the ideal operation of the inverter 4. Figure 3(a) shows the three-phase AC voltages Eu, Ev, and Ew, which are the AC output voltages of the inverter 4. These waveforms are approximately equal to the synchronous motor drive voltage waveforms when converted using the transformer's transformation ratio. Figure 3(b) shows the inverter gate signals, i.e., the conducting thyristors among the six thyristors U, V, W, X, Y, and Z of the inverter 4. Figure 3(c) shows the DC currents flowing through each input terminal (4a, 4b) of the inverter 4. Figures 3(d) to (f) show the armature currents flowing through the U, V, and W phases of the synchronous motor 30, respectively.

[0018] In Figures 3(a) to 3(f), the point where the line voltages Eu-Ev, Ev-Ew, and Ew-Eu are 0V is the reference point for the phase control angle γ, where γ = 0 degrees. A firing command signal is given to a predetermined thyristor at a time when the phase is advanced by a desired angle γ1 from the reference point. For example, a firing command signal is given to thyristor V while thyristor U is conducting, and then to thyristor W while thyristor V is conducting. Similarly, a firing command signal is given to thyristor X while thyristor Z is conducting, and then to thyristor Y while thyristor X is conducting.

[0019] The control unit 20 generates a line voltage synchronized with the rotation of the synchronous motor 30 by controlling the current path flowing through the synchronous motor 30 by igniting six thyristors U, V, W, X, Y, and Z in a predetermined order of two at a time in accordance with the rotation of the synchronous motor 30.

[0020] By the way, in the main circuit configuration of this embodiment, a transformer 5 is placed between the inverter 4 and the synchronous motor 30. Therefore, the inverter 4 drives an inductive load. The inductance of the above-mentioned inductive load is larger than the inductance of the synchronous motor 30 alone, and phenomena such as voltage fluctuations may occur depending on the operating conditions.

[0021] Figure 4 is a time chart showing the startup state observed when the inverter 4 was actually operated. Figures 4(c) and 4(d) correspond to the DC current in Figure 3(c) and the U-phase current in Figure 3(d) mentioned above. In particular, a transient response due to driving an inductive load is observed in the peak value of the current during the starting stage.

[0022] The current flowing through the transformer 5 will be explained with reference to Figures 5A, 5B, and 6. Figure 5A is a diagram illustrating the current waveform flowing through the transformer 5 based on the driving method of the embodiment. Figure 5B is a diagram illustrating the current waveform flowing through the transformer 5 based on the driving method of the comparative example. Figure 6 is a diagram illustrating the difference in current waveforms flowing through the transformer 5 based on the respective driving methods of the embodiment and the comparative example. Note that the waveforms shown in Figures 5A, 5B, and 6 were generated by simulation.

[0023] Figure 5A shows the rectangular pulse current waveform flowing on the primary side of the transformer 5 as a dashed line, and the current waveform flowing on the secondary side of the transformer 5 as a solid line. The current waveform flowing on the secondary side is a primary response waveform that responds to the rectangular pulse current. The waveforms shown in Figures 5A and 5B model conditions such as when the synchronous motor 30 is starting up at a relatively slow speed (relatively low frequency).

[0024] Figure 5B shows the driving method of the comparative example. According to the driving method of the comparative example, due to the relationship between the frequency characteristics of the transformer 5 and the switching period of the current waveform, even if the above-mentioned rectangular pulse current is flowing on the primary side, the current flowing on the secondary side is limited to a portion of the time width allocated to that pulse current. The portion of the time width allocated to the pulse current refers to the range from the timing when the pulse waveform rises in the positive direction until the current value decreases by a predetermined time constant.

[0025] In contrast, as shown in Figure 5A, according to the driving method of this embodiment, the inverter 4 intermittently supplies current to the primary side of the transformer 5 by increasing the number of pulses (referred to as divided pulses P1, P2, P3, P4, P5, P6, P7, P8) with narrower pulse widths compared to the comparative example, per AC cycle. For example, the control unit 20 shows the change in current value when a specific thyristor corresponding to a specific phase is switched four times during half a cycle of the three-phase AC. There is no limit to this number of times (number of pulses), and it may be changed as appropriate. For example, it is possible to switch a specific thyristor four or more times during half a cycle of the three-phase AC.

[0026] Figure 6 shows the current waveforms flowing on the secondary side of the transformer 5, superimposed for the embodiment and the comparative example. As can be seen from Figure 6, it was confirmed that the current flowing on the secondary side of the transformer 5 increases with the driving method of this embodiment compared to the comparative example.

[0027] According to the above embodiment, the power conversion system includes a reactor between the DC side of the converter and the DC side of the inverter, and a transformer between the inverter and the synchronous motor of the load. The power conversion system comprises an inverter and a control unit. The inverter includes a thyristor configured in a full-bridge type and generates three-phase alternating current by control. When the control unit starts the synchronous motor using the inverter, it intermittently switches the alternating current of the inverter by controlling the thyristor of a specific phase multiple times during half a cycle of the three-phase alternating current. This enables the synchronous motor driven via the transformer to be started.

[0028] (Second Embodiment) An embodiment of the power conversion system 100A will be described. Figure 7 is a schematic diagram of the power conversion system 100A of the embodiment. The power conversion system 100 described in Figure 1 above can be configured as the power conversion system 100A. For example, in the power conversion system 100A, a DC reactor 3 is provided between the DC side of the converter 2 and the DC side of the inverter 4, and a transformer 5A is provided between the inverter 4 and the synchronous motor 30 of the load. In the power conversion system 100A, a DC reactor 13 is provided between the DC side of the converter 12 and the DC side of the inverter 14, and a transformer 5A is provided between the inverter 14 and the synchronous motor 30 of the load. This transformer 5A has two primary windings. When the control unit 20A starts the synchronous motor 30 using the inverter 4 or the inverter 14, it may start the synchronous motor 30 by intermittently switching the AC current of the inverter 4, for example, by controlling the switching of a specific phase thyristor multiple times during half a cycle of the three-phase AC. Note that the inverter 14 may be used instead of the inverter 4 for starting. For configurations other than those described above, refer to the first embodiment. As a result, the power conversion system 100A can start the synchronous motor 30 driven via the transformer 5A.

[0029] (Third Embodiment) The drive system 100B of the embodiment will now be described. The power conversion system 100 shown in Figure 1 above can be configured as the drive system 100B. For example, in the drive system 100B for the synchronous motor 30, a DC reactor 3 is provided between the DC side of the converter 2 and the DC side of the inverter 4, and a transformer 5 is provided between the inverter and the synchronous motor 30 of the load. When the control unit 20 starts the synchronous motor 30 using the inverter 4, it is preferable to start the synchronous motor 30 by intermittently switching the AC current of the inverter 4 by controlling the switching of a thyristor of a specific phase multiple times during half a cycle of the three-phase AC. For configurations other than those described above, refer to the first embodiment. As a result, the drive system 100B can start the synchronous motor 30 driven via the transformer 5. The drive system 100B may also be configured using a transformer 5A as in the power conversion system 100A shown in Figure 7.

[0030] According to at least one embodiment described above, the power conversion system includes a reactor between the DC side of the converter and the DC side of the inverter, and a transformer between the inverter and the synchronous motor of the load. The power conversion system comprises an inverter and a control unit. The inverter includes a thyristor configured in a full-bridge type and generates three-phase alternating current by control. When the control unit starts the synchronous motor using the inverter, it intermittently switches the alternating current of the inverter by controlling the thyristor of a specific phase multiple times during half a cycle of the three-phase alternating current. This enables the synchronous motor driven via the transformer to be started.

[0031] Although several embodiments of the present invention have been described above, these embodiments are presented as examples and are not intended to limit the scope of the invention. These novel embodiments can be implemented in various other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. These embodiments and their variations are included in the scope and spirit of the invention, as well as in the claims of the invention and its equivalents. Furthermore, the embodiments described above can be implemented in combination with each other.

[0032] For example, when the timing for the thyristor of inverter 4 to commutate arrives, the control unit 20 adjusts the phase at which the thyristor of converter 2 (rectifier side) is fired to match the above timing, thereby reducing the DC current flowing through converter 2 to zero. In this state, the control unit 20 fires the next thyristor of inverter 4 at a predetermined timing to commutate it. In the above control, the control unit 20 may control the pulse width of the DC current by switching control parameters.

[0033] 1. 5. 5A Transformer 2. 12. Converter 3. 13. DC Reactor 4. 14. Inverter 20. 20A Control Unit 30. Synchronous Motor 100. 100A Power Conversion System 100B Drive System PS. AC Power Supply

Claims

1. A power conversion system comprising a reactor provided between the DC side of a converter and the DC side of an inverter, and a transformer provided between the inverter and a synchronous motor of a load, the power conversion system comprising an inverter that includes a thyristor configured in a full-bridge type and generates three-phase AC by control, and a control unit that, when starting the synchronous motor using the inverter, intermittently interrupts the AC current of the inverter by controlling the switching of a thyristor of a specific phase multiple times during half a cycle of the three-phase AC.

2. The power conversion system according to claim 1, wherein the control unit switches a specific thyristor corresponding to a specific phase four or more times during half a cycle of the three-phase alternating current.

3. A drive system for a synchronous motor, wherein a reactor is provided between the DC side of a converter and the DC side of an inverter, and a transformer is provided between the inverter and the synchronous motor of the load, the drive system comprising: an inverter that includes a thyristor configured in a full-bridge type and generates a three-phase alternating current by control; and a control unit that, when starting the synchronous motor using the inverter, starts the synchronous motor by intermittently interrupting the alternating current of the inverter by switching a thyristor of a specific phase multiple times during half a cycle of the three-phase alternating current.

4. A method for starting a synchronous motor using a power conversion system in which a reactor is provided between the DC side of a converter and the DC side of an inverter, and a transformer is provided between the inverter and the synchronous motor of a load, wherein the method for starting the synchronous motor uses an inverter that includes a thyristor configured in a full-bridge type and generates a three-phase AC by control, and includes a control that switches a thyristor of a specific phase multiple times during half a cycle of the three-phase AC to interrupt the AC current of the inverter.

5. The startup method according to claim 4, comprising switching a specific thyristor corresponding to a specific phase four or more times during half a cycle of the three-phase alternating current.

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