3-level inverter, refrigeration system and power converter

Abstract

Reduces current pulsation in the DC bus. [Solution] The 3-level inverter 10 includes a capacitor 12, a switch section having a plurality of switches Sa1-Sc4 provided between the P side of the DC bus, the N side of the DC bus, one terminal of the capacitor 12, and an AC line, and a controller that selects the ON and OFF states of the plurality of switches Sa1-Sc4. At least one of the plurality of switches Sa1-Sc4 is provided in all paths between the other terminal of the capacitor 12 and the other side of the P side or N side of the DC bus, and the controller 15 selects the ON and OFF states of the plurality of switches Sa1-Sc4 such that the second AC component, which is the first frequency AC component included in the second power between the capacitor 12 and the switch section 11, is greater than the first AC component, which is the first frequency AC component included in the first power between the DC bus and the 3-level inverter 10.

Description

[Technical Field]

[0001] This disclosure relates to a three-level inverter, a refrigeration system, and a power converter. [Background technology]

[0002] Patent Document 1 describes a control device for a multilevel inverter that is applied to a multilevel inverter comprising a plurality of capacitors connected in series and a switch electrically connected to the capacitors and the armature winding of a rotating electric machine, and controls the switch to output one of a plurality of voltages that can be output from the series connection of the plurality of capacitors, wherein the series connection of the plurality of capacitors can be connected in parallel to a power supply, and an electrical device can be connected in parallel to a target capacitor which is some of the plurality of capacitors, and the control device comprises a determination unit that determines whether the supplyable current value to the electrical device is less than the required current value of the electrical device, and a control unit that controls the switch to increase the current flowing through the armature winding to a greater value than when it is determined that the supplyable current value is less than the required current value if it is determined that the supplyable current value is less than the required current value. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2024-078294 [Overview of the project] [Problems that the invention aims to solve]

[0004] In an inverter that converts DC power to AC power, fluctuations in the inverter's load cause fluctuations in the DC power. These fluctuations in DC power cause the current in the DC bus to pulsate, undesirably affecting the DC power supply. Here, the fluctuation in DC power is the AC component superimposed on the DC component.

[0005] This disclosure aims to reduce the pulsation of the DC bus current in a 3-level inverter by varying the intermediate potential or the current at the intermediate potential according to the AC component. [Means for solving the problem]

[0006] The three-level inverter in the first aspect is a three-level inverter configured to convert power from a DC bus to power from the k-phase (k is a natural number of 2 or more) of an AC line, the three-level inverter comprising: a capacitor with one terminal connected to either the P-side or N-side of the DC bus; a switch unit having a plurality of switches provided between the P-side of the DC bus, the N-side of the DC bus, the other terminal of the capacitor, and the AC line; and a controller for selecting the on and off states of the plurality of switches, wherein at least one of the plurality of switches is provided in all paths between the other terminal of the capacitor and the other P-side or N-side of the DC bus, and the controller selects the on and off states of the plurality of switches such that the second AC component, which is the AC component of the first frequency included in the second power between the capacitor and the switch unit, is greater than the first AC component, which is the AC component of the first frequency included in the first power between the DC bus and the three-level inverter. This reduces the pulsation of the DC bus current. A three-level inverter in the second aspect is a three-level inverter in the first aspect, wherein the controller selects the on and off states of the plurality of switches to supply power from the DC bus to the capacitor. This allows the capacitor to be charged and an intermediate potential to be generated. A three-level inverter in the third aspect is a three-level inverter in the first or second aspect, wherein the controller selects the on and off states of the plurality of switches such that the second DC voltage of the capacitor is less than the first DC voltage of the DC bus. This prevents unintended power outflow from the capacitor to the DC bus. The third-level inverter of the fourth aspect is a third-level inverter of any one of the first to third aspects, wherein the first frequency is the fundamental wave component of the AC voltage of the AC line, an integer multiple of the fundamental wave component, or an integer fraction of the fundamental wave component. This reduces the pulsation of the DC bus current, which fluctuates at a frequency of the fundamental wave component of the AC voltage of the AC line, an integer multiple of the fundamental wave component, or an integer fraction of the fundamental wave component. The third-level inverter of the fifth aspect is a third-level inverter of any one of the first to fourth aspects, configured to supply power to a rotating electric machine that drives the compression mechanism of a compressor, wherein the first frequency is the frequency of the load torque of the compression mechanism. This reduces the pulsation of the DC bus current caused by fluctuations in the load torque of the compression mechanism of the compressor. The 3-level inverter of the sixth aspect is a 3-level inverter of any one of the first to fourth aspects, configured to supply power to an AC power source, wherein the first frequency is the fundamental wave component of the AC voltage of the AC power source, an integer multiple of the fundamental wave component, or an integer fraction of the fundamental wave component. This reduces the pulsation of the DC bus current caused by fluctuations in the power of the AC line. The refrigeration system according to the seventh aspect comprises a three-level inverter according to any one of the first to sixth aspects, and a compressor equipped with a rotating electric motor that is powered by the three-level inverter. This makes it possible to reduce the pulsation of the current in the DC bus of the refrigeration system. The power converter according to the eighth aspect comprises a three-level inverter according to any one of the first to sixth aspects, and a DC power supply that supplies power to the three-level inverter. This makes it possible to reduce the current pulsation of the DC power supply of the power converter. [Brief explanation of the drawing]

[0007] [Figure 1] This figure shows an example of a power control system equipped with a 3-level inverter according to this embodiment. [Figure 2] This figure shows the results for Example 1 and Comparative Example 1. [Figure 3] This figure illustrates switch control example 1 in Embodiment 1. (a) shows the first switch state, and (b) shows the second switch state. [Figure 4] This figure illustrates switch control example 2 in Example 1. (a) shows the third switch state, and (b) shows the fourth switch state. [Figure 5] This figure shows a 3-level inverter of a modified example 1 of Example 1. [Figure 6] This figure shows a 3-level inverter of a modified example 3 of Example 1. [Figure 7] This is a diagram showing the power control system of Example 2. [Figure 8] This diagram shows a conventional 3-level inverter. [Modes for carrying out the invention]

[0008] The embodiments will be described in detail below with reference to the attached drawings. Figure 1 is a diagram showing an example of a power control system equipped with a 3-level inverter according to this embodiment. The power control system 1 comprises a 3-level inverter 10, a DC bus 20, a DC power supply 21, and a motor 30. The DC bus 20 is a bus connected to the DC power supply 21. The positive side (positive electrode side) of the DC power supply 21 is denoted as the P side, and the negative side (negative electrode side) is denoted as the N side. The 3-level inverter 10 is an inverter that takes three potential levels as input, using the potential of the P side, the potential of the N side, and the potential between the P side potential and the N side potential (hereinafter referred to as the intermediate potential) of the DC bus 20. Generally, 3-level inverters are known to reduce power loss, i.e., achieve higher efficiency, compared to 2-level inverters that take two potential levels, the P side potential and the N side potential, as input. In the circuit, the point that supplies the intermediate potential is denoted as the neutral point O.

[0009] The three-level inverter 10 according to the present embodiment includes a switch unit 11, a capacitor 12, and a controller 15. The three-level inverter 10 converts DC power supplied from a DC bus 20 into, for example, three-phase AC power, and supplies the converted three-phase AC power to a motor 30. Hereinafter, it is assumed that the three-level inverter 10 converts power into three-phase AC power, and the three phases are referred to as a-phase, b-phase, and c-phase. A current path through which the three-phase AC power of the a-phase, b-phase, and c-phase flows is referred to as an AC line, and currents flowing through the a-phase, b-phase, and c-phase are referred to as current i a , i b , i c . A power conversion device can be configured by the three-level inverter 10 and a DC power supply 21 that supplies power to the three-level inverter.

[0010] The switch unit 11 is configured by a bidirectional switch type circuit, and includes switch groups corresponding to the three phases of a-phase, b-phase, and c-phase. The a-phase includes switches S a1 , S a2 , S a3 and S a4 , that is, a switch group including four switches. Each of the switches S a1 , S a2 , S a3 , S a4 has a corresponding diode D a1 , D a2 , D a3 , D a4 . Similarly, the b-phase includes a switch group consisting of switches S b1 , S b2 , S b3 , S b4 and diodes D b1 , D b2 , D b3 , D b4 , and the c-phase includes a switch group consisting of switches S c1 , S c2 , S c3 , S c4 and diodes D c1 , D c2 , D c3 , D c4 .

[0011] Each switch S a1 ~S a4 S b1 ~S b4 S c1 ~S c4 N-channel MOSFETs are used. Therefore, the high-potential terminal of each switch is the drain, and the low-potential terminal is the source. Each diode D a1 ~D a4 , D b1 ~D b4 , D c1 ~D c4 As such, an N-channel MOSFET body diode is used. Each diode D a1 ~D a4 , D b1 ~D b4 , D c1 ~D c4 Each switch S a1 ~S a4 S b1 ~S b4 S c1 ~S c4 It is connected so that current flows from the source to the drain.

[0012] Switch S a1 S b1 and S c1 The drain is connected to the P side of DC bus 20. Switch S a1 S b1 and S c1 The sources are, respectively, Switch S a4 S b4 and S c4 It is connected to the drain of switch S. a4 S b4 and S c4 The source is connected to the N side of the DC bus 20. The motor 30 includes a-phase, b-phase, and c-phase connection parts (not shown). Switch S a1 and S a4 The connection point is connected to the a-phase connection of motor 30. Switch S b1 and S b4 The connection point is connected to the b-phase connection of motor 30. Switch Sc1 and S c4 is connected to the c-phase connection portion of the motor 30. An a-phase coil is connected to the a-phase connection portion of the motor 30, a b-phase coil is connected to the b-phase connection portion, and a c-phase coil is connected to the c-phase connection portion. In FIG. 1, a Y-connection is illustrated as an example of the wiring of the motor 30, but the wiring is not limited thereto, and a Δ-connection may also be adopted. For the connection method in the case of the Δ-connection, the above description may be read appropriately mutatis mutandis.

[0013] Switch S a2 and S a3 and diode D a2 and D a3 , a bidirectional switch that conducts and blocks current in two directions is formed. The drains of switch S a2 and S a3 are connected to each other. The source of switch S a2 is connected to the connection point between switch S a1 and S a4 . The source of switch S a3 is connected to the neutral point O. When switch S a2 and S a3 are on, bidirectional current flow is permitted, and when switch S a2 and S a3 are off, bidirectional current flow is blocked.

[0014] Switch S b2 and S b3 and diode D b2 and D b3 , a bidirectional switch that conducts and blocks current in two directions is formed. The drains of switch S b2 and S b3 are connected to each other. The source of switch S b2 is connected to the connection point between switch S b1 and S b4 . The source of switch S b3 is connected to the neutral point O. When switch S b2 and S b3 are on, bidirectional current flow is permitted, and when switch S b2and S b3 When this is turned off, the flow of current in both directions is blocked.

[0015] Switch S c2 and S c3 and diode D c2 and D c3 This forms a bidirectional switch that allows for the flow and interruption of current in both directions. Switch S c2 and S c3 The drains of these two switches are connected. Switch S c2 The source is Switch S c1 and S c4 It is connected to the connection point of switch S. c3 The source is connected to the neutral point O. Switch S c2 and S c3 When it is ON, bidirectional current flow is permitted, switch S c2 and S c3 When this is turned off, the flow of current in both directions is blocked. In this embodiment, an N-channel MOSFET was used as an example of a switch in the 3-level inverter, but the switch is not limited to this. For example, a P-channel MOSFET, a heterojunction FET (HEMT), a bipolar transistor, or an insulated-gate bipolar transistor (IGBT) may be used as a switch. In this embodiment, the body diode of an N-channel MOSFET was used as an example of a diode in the 3-level inverter, but the diode is not limited to this. For example, a diode connected in parallel with the switch may be used.

[0016] Capacitor 12 has one terminal connected to the N side of the DC bus 20 and the other terminal connected to the neutral point O. The neutral point O is connected to switch S a1 ~S a3 S b1 ~S b3 S c1 ~S c3It is connected to the P side of the DC bus via this. In other words, one terminal of capacitor 12 is connected to the N side of the DC bus 20, and at least one switch is connected to the entire path between the other terminal of capacitor 12 and the P side of the DC bus. Here, connection means an electrical connection, and includes, for example, a configuration in which a resistor or inductor is interposed between capacitor 12 and the N side of the DC bus 20. It also includes a configuration in which a resistor or inductor is interposed between capacitor 12 and the neutral point O.

[0017] The voltage across capacitor 12 is v n This is how it is expressed. The voltage across capacitor 12 is also the potential of neutral point O. The potential of neutral point O relative to the potential of the N side of DC bus 20 is the intermediate potential v n This is how it is written. The current flowing through capacitor 12 is i n This is how it is written. The current flowing through capacitor 12 is called the intermediate current i. n It is sometimes called the intermediate current i n Here, the direction of current flow from the N side of the DC bus 20 to the neutral point O is defined as positive (+). In other words, the intermediate current i n At the neutral point O, the direction in which current flows from the capacitor 12 to the switch section 11 is defined as positive (+).

[0018] The controller 15 controls each switch S of the switch unit 11. a1 ~S a4 S b1 ~S b4 S c1 ~S c4 The controller selects the ON and OFF states and converts the DC power supplied from the DC power supply 21 into three-phase AC power. The controller 15 uses sensors (not shown) to determine the voltage of the capacitor 12, the voltage of the DC bus 20, and the currents i of the a-phase, b-phase, and c-phase of the AC line. a i b i c The controller 15 is composed of, for example, a CPU, RAM, ROM, etc.

[0019] The DC bus 20 is connected to the DC power supply 21. As the DC power supply 21, any known configuration can be appropriately selected, such as a storage battery, a converter connected to a storage battery, a converter connected to a solar (PV) panel, a converter connected to a generator, or a converter connected to an AC power supply (such as a diode bridge). The voltage output by the DC power supply 21 is the DC voltage v dc This is how it is written. DC voltage v dc This is the potential of the P side of the DC bus 20 relative to the potential of the N side of the DC bus 20. The current output by the DC power supply 21 is the DC current i. dc This is how it is written. DC current i dc Here, the direction from the N side of the DC bus 20 to the P side of the DC bus 20 is defined as positive (+). In other words, the DC current i dc In the P side of the DC bus 20, the direction in which current flows from the DC power supply 21 to the switch unit 11 is defined as positive (+).

[0020] (Example 1) The circuit configuration of Example 1 is the power control system 1 shown in Figure 1. The motor 30 is driven by three-phase AC power supplied from the 3-level inverter 10. The motor 30 can be exemplified as a permanent magnet synchronous motor (PMSM) built into the compressor. The motor 30 can also be exemplified as a synchronous motor other than a permanent magnet synchronous motor, or an induction motor. The motor 30 is an example of a rotating electric machine. The compressor is provided, for example, in a refrigeration system. Examples of refrigeration systems include air conditioners such as cooling-only units, heating-only units, and air conditioners that switch between cooling and heating. Examples of refrigeration systems include water heaters and chiller units. Furthermore, examples of refrigeration systems include cooling devices that cool the air inside refrigerators, freezers, display cases, containers, etc.

[0021] If motor 30 is a permanent magnet synchronous motor (PMSM) built into the compressor, the compressor repeats the cycle of suction, compression, and discharge during one rotation of motor 30. The load is small during the suction process, but large during the compression process. Therefore, the load torque fluctuates during one rotation of motor 30. In other words, the load torque pulsates according to the rotation angle of motor 30. When the load torque of motor 30 pulsates, a DC current i supplied from the DC power supply 21 flows through the DC bus 20. dc It pulsates. DC current i dc The DC current i may pulsate at a frequency of the fundamental wave component of the AC voltage in the AC line, or an integer multiple of the fundamental wave component, or an integer fraction of the fundamental wave component. Here, "AC voltage in the AC line" corresponds to the line voltage of the motor, and "fundamental wave component of the AC voltage in the AC line" corresponds to the electrical angular frequency of the motor. Furthermore, assuming that the motor 30 exemplified in Example 1 has 3 pole pairs, and that the pulsation of the load torque during one rotation of the motor is considered as one period, then the DC current i dc It pulsates at a frequency one-third of the fundamental wave component. This DC current i dc The pulsation causes the DC power to pulsate, which has an undesirable effect on the DC power supply 21 that supplies the DC power. Therefore, in this embodiment, the intermediate potential v n or intermediate current i n By causing it to pulsate, a direct current i dc Reduces pulsation. Intermediate potential v n or intermediate current i n To cause pulsation, the neutral point O is connected only to the N side of the DC bus 20 via capacitor 12.

[0022] DC current i dc To reduce the pulsation, the switch section 11 of the 3-level inverter 10 is controlled in the following manner. (1) The torque of motor 30 is made to pulsate in response to the pulsation of the load torque of motor 30. More specifically, during one cycle of the pulsation of the load torque of motor 30, the power supplied to motor 30 is increased when the load torque of motor 30 is large, and the power supplied to motor 30 is decreased when the load torque of motor 30 is small. This suppresses fluctuations in the rotational speed of motor 30 and reduces vibration of the compressor. (2) DC current i dc The intermediate current i is greater than the pulsation (AC component) n This increases the pulsation (AC component). This results in a potential difference (DC voltage v) between the P potential and N potential of the DC bus 20. dc It suppresses fluctuations in ). (3) Intermediate potential v n When the DC voltage v is greater than 0, dc To make it smaller than that. In other words, the intermediate potential v n DC voltage v dc The system is controlled to stay within a certain range. This prevents unintended power outflow from the capacitor 12 to the DC bus 20 and the DC power supply 21. At this time, the switch section 11 of the 3-level inverter 10 is also controlled as follows. (a) Power is supplied to the motor 30 from the DC bus 20 and the capacitor 12. (b) AC power is supplied between the capacitor 12 and the motor 30. The AC power is controlled to be greater than the AC power contained in the power in (a) above (i.e., the AC power contained in the power in (a) above is reduced). (c) In order to secure an intermediate potential for capacitor 12, DC power is supplied to capacitor 12 from the DC bus 20 or motor 30.

[0023] (Comparative example) Figure 8 shows a 3-level inverter 310 according to the prior art (see Patent Document 1) as a comparative example (controller not shown). The DC bus 20 is also shown in Figure 8. Each switch S a1 ~S a4 S b1 ~S b4 S c1 ~S c4The connection is the same as in the 3-level inverter of this embodiment. However, capacitor 13 is provided between the P side of the DC bus 20 and the neutral point O. In the conventional 3-level inverter, capacitors 12 and 13 connected in series provide the DC voltage v of the DC power supply 21. dc The voltage is divided, resulting in an intermediate potential v n This is generated.

[0024] The results of Example 1 are shown in Figures 2(a) to (f). Furthermore, the results using the conventional 3-level inverter 310 shown in Figure 8 are presented as Comparative Example 1 in Figures 2(g) to (l). The horizontal axis in Figures 2(a) to (l) represents time (Time in ms). Figure 2(a) shows the load torque Tm of motor 30 (permanent magnet synchronous motor: PMSM) in Example 1. The load torque Tm changes periodically, showing a maximum value of 1.5 Nm around times 12 ms and 36 ms, and showing approximately 0 Nm around times 0 ms, 24 ms, and 48 ms. Figure 2(g) shows the load torque Tm of motor 30 (permanent magnet synchronous motor: PMSM) in Comparative Example 1. Similar fluctuations in load torque Tm were observed as in Figure 2(a) of Example 1.

[0025] Figure 2(b) shows the current i of phase a in Example 1. a , the current i of phase b b and the current i of phase c c This figure shows the current i during the time period when the load torque Tm of motor 30 is large. a i b and i c It had grown larger. Figure 2(h) shows the current i of phase a in Comparative Example 1. a , the current i of phase b b and the current i of phase c c This figure shows the current i during the time period when the load torque Tm of the motor 30 is large. Similar to Figure 2(b) of Example 1, a i b and i c It had grown larger.

[0026] Figure 2(c) shows the d-axis current i of the motor 30 (permanent magnet synchronous motor: PMSM) in the dq rotation coordinate system. d and q-axis current i q This shows the d-axis current i due to fluctuations in the load torque Tm of motor 30. d and q-axis current i q Pulsation was observed. Figure 2(i) shows the d-axis current i of the dq rotation coordinate system of motor 30 (permanent magnet synchronous motor: PMSM) in Comparative Example 1. d and q-axis current i q This shows the d-axis current i due to fluctuations in the load torque Tm of motor 30, similar to Figure 2(c) of Example 1. d and q-axis current i q Pulsation was observed. Comparing Figure 2(c) and Figure 2(i), i q They are changing in the same way. However, i d A difference can be seen. In Figure 2(i) of Comparative Example 1, at around 0ms, 24ms, and 48ms, when the load torque Tm of motor 30 is small (almost 0 Nm), i d This is 0A. In contrast, in Figure 2(c) of Example 1, the load torque Tm of motor 30 becomes smaller (almost 0Nm), i q When the value of becomes small, i d The motor is controlled to flow in this manner. By controlling it in this way, a motor current of a predetermined level or higher can be supplied even when the load torque is near 0 Nm, and the intermediate current i n This ensures that capacitor 12 can be charged. In contrast, in Figure 2(i) of Comparative Example 1, the motor current becomes small around the load torque of 0 Nm, and the intermediate current i n The value becomes too small, leading to a problem where capacitor 12 cannot be charged.

[0027] Figure 2(d) shows the intermediate current i of Example 1. n This is shown. Also, Figure 2(e) shows the intermediate potential v n This shows that in Example 1, the intermediate current i n and the intermediate potential v n The on and off states of each switch are controlled so that the DC current i pulsates significantly. As a result, as shown in Figure 2(f),dc The pulsation was reduced. In Example 1, the intermediate current i n and the intermediate potential v n Both were pulsated, but depending on the situation, the intermediate current i n and the intermediate potential v n It is acceptable to make only one of them pulsate.

[0028] Figure 2(j) shows the intermediate current i of Comparative Example 1. n This is shown. Also, Figure 2(k) shows the intermediate potential v n This shows that in Comparative Example 1, the intermediate current i n and the intermediate potential v n The DC current i remained almost constant and did not change. As a result, as shown in Figure 2(l), dc It began to pulsate.

[0029] In a conventional 3-level inverter 310, capacitors 12 and 13 control the DC voltage v of the DC power supply 21. dc The voltage is divided, resulting in an intermediate potential v n Therefore, an intermediate potential v is generated. n This is maintained at a nearly constant level. In addition, in conventional 3-level inverters, capacitors 12 and 13 connected in series are connected to the DC bus 20. Therefore, intentionally setting the intermediate potential v n or intermediate current i n When the pulsation is caused, current flows to the DC bus 20 through capacitors 12 and 13, resulting in a DC current i dc It starts to pulsate.

[0030] [Control of the switch in Example 1] In a conventional 3-level inverter 310, when a DC voltage is applied to the DC bus 20, capacitors 12 and 13 are charged to an intermediate potential v n This can generate an intermediate potential v. In other words, capacitors 12 and 13 are charged independently of the switch operation, and the intermediate potential v is generated. nThis can generate the intermediate potential v. However, in the 3-level inverter of Example 1, even if a DC voltage is applied to the DC bus 20, the capacitor 12 will not be charged unless the switch performs a predetermined operation. Therefore, the capacitor 12 is charged to an intermediate potential v. n To generate the signal, it is necessary to control the switch. The following is an example of controlling a switch to supply power from the DC bus 20 to the capacitor 12.

[0031] [Example of switch control 1] Figure 3 is a diagram illustrating switch control example 1. Switch control example 1 is a method of charging the capacitor 12 to a predetermined voltage before starting the motor 30. Figure 3(a) shows the first switch state. The first switch state is switch S a1 S b2 and S b3 The switch is in the ON state. The current flowing in this switched state is shown by a dashed line in Figure 3(a). The DC current supplied from the DC power supply 21 flows through the P side of the DC bus 20, switch S a1 , a-phase coil of motor 30, b-phase coil of motor 30, switch S b2 Switch S b3 The current flows from the neutral point O, through capacitor 12, to the N side of the DC bus 20, and to the DC power supply 21. This current charges capacitor 12. At this time, switch S b2 It's fine if it's in the OFF state. Switch S b2 If it is off, diode D b2 An electric current flows through it.

[0032] Figure 3(b) shows the second switch state. The second switch state is switch S b2 S b3 and S a4 The switch is in the ON state. The second switch state is, relative to the first switch state, switch S b2 and S b3 With the switch left on, a1 Turn off, switch S a4The switch is turned ON. The current flowing in this switched state is shown by a dashed line in Figure 3(b). When switching from the first switch state to the second switch state, the current tries to continue flowing in the same direction due to the inductance component of the motor 30's coil. As a result, the DC current flows from the a-phase coil and the b-phase coil of the motor 30 to the switch S b2 Switch S b3 Neutral point O, capacitor 12, switch S a4 The current flows to the a-phase coil of the motor 30. This current charges the capacitor 12. At this time, switch S b2 and S a4 It's fine if it's in the OFF state. Switch S b2 and S a4 If it is off, diode D b2 and D a4 An electric current flows through it. In the first example of switch control, the circuits shown in Figures 3(a) and (b) operate as a step-down chopper. Since the motor 30 is used as the coil for the step-down chopper, there is no need to prepare a separate coil, which has the advantage of allowing for a smaller circuit.

[0033] Before starting the motor 30, the first switch state and the second switch state are alternately repeated until the capacitor 12 reaches a predetermined voltage. Then, after the capacitor 12 reaches the predetermined voltage, the motor 30 is started and rotated.

[0034] [Switch control example 2] Figure 4 is a diagram illustrating switch control example 2. Switch control example 1 was a method of charging the capacitor 12 to a predetermined voltage before starting the motor 30, but switch control example 2 is a method of charging the capacitor 12 to a predetermined voltage while the motor 30 is running. Figure 4(a) shows the third switch state, and Figure 4(b) shows the fourth switch state. When starting the motor 30, the motor 30 is operated in the third switch state until it reaches a predetermined rotational speed. Then, after the rotational speed of the motor 30 reaches a predetermined value, it is operated in the fourth switch state to charge the capacitor 12.

[0035] In the third switch state shown in Figure 4(a), switch S a1 S a4 S b1 S b4 S c1 and S c4 The switch S repeatedly switches between on and off states, and the 3-level inverter 10 operates in 2 levels (indicated as ON / OFF (switching) in Figure 4(a)). In 2-level operation, switch S a1 and Switch S a4 If one of them is ON, the other will be OFF. Also, switch S b1 and Switch S b4 If one of them is in the ON state, the other will be in the OFF state, and switch S c1 and Switch S c4 When one of them is ON, the other is OFF. Also, in two-level operation, switch S forms a bidirectional switch. a2 and S a3 Switch S b2 and S b3 and switch S c2 and S c3 Both will be turned off (indicated as OFF in Figure 4(a)).

[0036] In the fourth switch state shown in Figure 4(b), each switch S of the switch unit 11 a1 ~S a4 S b1 ~S b4 S c1 ~S c4 The voltage repeatedly switches between ON and OFF states, causing the 3-level inverter 10 to operate in 3 levels (indicated as ON / OFF (switching) in Figure 4(b)). While the 3-level inverter 10 is operating in 3 levels, the capacitor 12 is charged, and the intermediate potential v n This generates [something]. When charging the capacitor 12, the 3-level inverter 10 is controlled so that the reactive power of the motor 30 is greater than during normal operation when the capacitor 12 is not being charged.

[0037] (Modification 1 of Example 1) Figure 5 shows a 3-level inverter 110 of Modification 1 (controller 15 is not shown). While the 3-level inverter 10 of Example 1 showed an example using a bidirectional switch circuit, Modification 1 uses a neutral point clamp circuit. Also, in Example 1, each switch was an N-channel MOSFET, but in Modification 1, insulated-gate bipolar transistors (IGBTs) are used.

[0038] Phase a is S 11 S 12 S 13 and S 14 It is equipped with a switch group consisting of four switches S 11 S 12 S 13 S 14 This is a diode D connected in antiparallel. 11 , D 12 , D 13 , D 14 It has. Similarly, phase b has four switches S 21 S 22 S 23 S 24 A group of switches consisting of four diodes D 21 , D 22 , D 23 , D 24 It is equipped with four switches S 31 S 32 S 33 S 34 A group of switches consisting of four diodes D 31 , D 32 , D 33 , D 34 It is equipped with.

[0039] Switch S of phase a 11 The collector is connected to the P side of DC bus 20. Switch S 11 The emitter is switch S 12 It is connected to the collector of switch S. 12 The emitter is switch S 13 It is connected to the collector of switch S. 13The emitter is switch S 14 It is connected to the collector of switch S. 14 The emitter of is connected to the N side of DC bus 20. Furthermore, clamp diode D 15 However, Switch S 11 and S 12 It is connected between the connection point and the neutral point O. Also, clamp diode D 16 However, Switch S 13 and S 14 It is connected between the connection point and the neutral point O. And switch S 12 and S 13 The connection point is connected to the load (for example, the a-phase coil of motor 30) (see Figure 1).

[0040] b-phase switch S 21 The collector is connected to the P side of DC bus 20. Switch S 21 The emitter is switch S 22 It is connected to the collector of switch S. 22 The emitter is switch S 23 It is connected to the collector of switch S. 23 The emitter is switch S 24 It is connected to the collector of switch S. 24 The emitter of is connected to the N side of DC bus 20. Furthermore, clamp diode D 25 However, Switch S 21 and S 22 It is connected between the connection point and the neutral point O. Also, clamp diode D 26 However, Switch S 23 and S 24 It is connected between the connection point and the neutral point O. And switch S 22 and S 23 The connection point is connected to the load.

[0041] c-phase switch S 31 The collector is connected to the P side of DC bus 20. Switch S 31 The emitter is switch S 32It is connected to the collector of switch S. 32 The emitter is switch S 33 It is connected to the collector of switch S. 33 The emitter is switch S 34 It is connected to the collector of switch S. 34 The emitter of is connected to the N side of DC bus 20. Furthermore, clamp diode D 35 However, Switch S 31 and S 32 It is connected between the connection point and the neutral point O. Also, clamp diode D 36 However, Switch S 33 and S 34 It is connected between the connection point and the neutral point O. And switch S 32 and S 33 The connection point is connected to the load.

[0042] Capacitor 12 has one terminal connected to the N side of the DC bus 20 and the other terminal connected to the neutral point O. The neutral point O is connected to clamp diode D. 15 and Switch S 11 It is connected to the P side of the DC bus via [a specific component]. Also, the neutral point O is connected to the clamp diode D. 25 and Switch S 21 It is connected to the P side of the DC bus via [a specific component]. Furthermore, the neutral point O is connected to the clamp diode D. 35 and Switch S 31 It is connected to the P side of the DC bus via this. In other words, one terminal of capacitor 12 is connected to the N side of the DC bus 20, and at least one switch is connected to the entire path between the other terminal of capacitor 12 and the P side of the DC bus.

[0043] As described above, the 3-level inverter 110 of Modified Example 1 has a different configuration from the 3-level inverter 10 of Example 1, but it can achieve the same effects as the 3-level inverter 10 of Example 1. In addition, although insulated-gate bipolar transistors (IGBTs) were used as examples for each switch in Modified Example 1, N-channel MOSFETs may also be used.

[0044] (Modification 2 of Example 1) In Example 1, an example of driving a motor 30 was shown, but other power-consuming loads besides the motor 30 may also be used. In this case, the switch section 11 of the 3-level inverter 10 is controlled in the following respects. (1) The system supplies power to the load that includes both DC and AC components. (2) DC current i dc The intermediate current i is greater than the pulsation (AC component) n This increases the pulsation (AC component). This results in a potential difference (DC voltage v) between the P potential and N potential of the DC bus 20. dc It suppresses fluctuations in ). (3) Intermediate potential v n When the DC voltage v is greater than 0, dc This is made smaller than the specified size. This prevents unintended power leakage from the capacitor 12 to the DC bus 20 and the DC power supply 21. The above points (2) and (3) are the same as in Example 1. At this time, the switch section 11 of the 3-level inverter 10 is also controlled as follows. (a) Power is supplied to the load from the DC bus 20 and the capacitor 12. (b) Power in the form of AC is supplied between the capacitor 12 and the load. The AC component power is controlled to be greater than the AC component included in the power in (a) above (i.e., the AC component included in the power in (a) above is reduced). (c) In order to secure an intermediate potential for capacitor 12, DC power is supplied to capacitor 12 from the DC bus 20 or the load.

[0045] (Modification 3 of Example 1) Figure 6 shows a 3-level inverter 210 of Modification 3 (controller 15 is not shown). In the 3-level inverter 10 of Example 1, the capacitor 12 was connected to the neutral point O and the N side of the DC bus 20. In contrast, the 3-level inverter 210 of Modification 3 differs in that the capacitor 13 is connected to the neutral point O and the P side of the DC bus 20. Even with the 3-level inverter 210 of Modification 3, the same effects as the 3-level inverter 10 of Example 1 can be obtained.

[0046] (Example 2) Figure 7 shows the power control system 2 of Example 2. The power control system 2 comprises a 3-level inverter 10, a DC bus 20, and an AC power supply 40. The 3-level inverter 10 of Example 2 has the same configuration as the 3-level inverter 10 of Example 1. Embodiment 1 was configured to supply power to the motor 30. In contrast, Embodiment 2 is configured to be connected to an AC power supply 40, which is a grid power supply, and to transmit power to the AC power supply 40 or receive power from the AC power supply 40.

[0047] The three-level inverter 10 in Example 2 is configured to transmit and receive three-phase unbalanced power. In such a case, the DC power flowing through the DC bus 20 may pulsate at a frequency of the fundamental wave component of the AC voltage of the AC power supply 40, or an integer multiple of the fundamental wave component, or an integer fraction of the fundamental wave component. The AC voltage of the AC power supply 40 is an example of the AC voltage of an AC line. The switch section 11 of the three-level inverter 10 is controlled in the following respects. (1) Transmit and receive three-phase unbalanced power. The three-phase unbalance condition is controlled, for example, according to the unbalance of the load connected to the AC power supply 40 (system power supply) side. (2) DC current i dc The intermediate current i is greater than the pulsation (AC component) n This increases the pulsation (AC component). This results in a potential difference (DC voltage v) between the P potential and N potential of the DC bus 20. dc It suppresses fluctuations in ). (3) Intermediate potential v n When the DC voltage v is greater than 0, dcThis is made smaller than the specified size. This prevents unintended power leakage from the capacitor 12 to the DC bus 20 and the DC power supply 21. The above points (2) and (3) are the same as in Example 1. At this time, the switch section 11 of the 3-level inverter 10 is also controlled as follows. (a) Power is supplied from the DC bus 20 and capacitor 12 to the load connected to the AC power supply 40. (b) Power is supplied and received between the capacitor 12 and the load connected to the AC power supply 40. The AC component power is controlled to be greater than the AC component included in the power in (a) above (i.e., the AC component included in the power in (a) above is reduced). (c) In order to secure an intermediate potential for capacitor 12, DC power is supplied to capacitor 12 from a load connected to the DC bus 20 or AC power supply 40.

[0048] (Modified version of Example 2) The modified 3-level inverter 10 of Example 2 is configured as an active filter that outputs AC component power to cancel out AC component power (high-frequency component) power generated by other devices. In this case, the switch section 11 of the 3-level inverter 10 is controlled in the following respects. (1) Power is transmitted and received, including AC components (high-frequency components). The AC components (high-frequency components) are controlled to cancel out, for example, the AC components (high-frequency components) of other equipment. (2) DC current i dc The intermediate current i is greater than the pulsation (AC component) n This increases the pulsation (AC component). This results in a potential difference (DC voltage v) between the P potential and N potential of the DC bus 20. dc It suppresses fluctuations in ). (3) Intermediate potential v n When the DC voltage v is greater than 0, dc This is made smaller than the specified size. This prevents unintended power leakage from the capacitor 12 to the DC bus 20 and the DC power supply 21. The above points (2) and (3) are the same as in Example 1. At this time, the switch section 11 of the 3-level inverter 10 is also controlled as follows. (a) Power is supplied to other devices from the DC bus 20 and capacitor 12. (b) Power in the form of AC components is supplied between the capacitor 12 and other equipment. The AC component power is controlled to be greater than the AC component included in the power in (a) above (i.e., the AC component included in the power in (a) above is reduced). (c) In order to secure an intermediate potential for capacitor 12, DC power is supplied to capacitor 12 from the DC bus 20 or other equipment.

[0049] The above embodiments show a configuration in which a 3-level inverter is connected to a three-phase AC load or a three-phase AC power supply, but the invention is not limited thereto. The 3-level inverter of this embodiment may include a group of k-phase switches (where k is a natural number of 2 or more) and be connected to a k-phase AC load or a k-phase AC power supply.

[0050] (Effects of the embodiment) The 3-level inverter 10 of this embodiment is configured to convert the power of a DC bus 20 and the power of the k-phase (k is a natural number of 2 or more) of an AC line, and the 3-level inverter 10 includes a capacitor 12 with one terminal connected to either the P-side or N-side of the DC bus 20, a switch unit 11 having a plurality of switches provided between the P-side of the DC bus 20, the N-side of the DC bus 20, the other terminal of the capacitor 12, and the AC line, and the ON state and OFF state of the plurality of switches The system includes a selector controller 15, and all paths between the other terminal of the capacitor 12 and the other P-side or N-side of the DC bus 20 are provided with at least one of the plurality of switches. The controller 15 selects the on and off states of the plurality of switches such that the second AC component, which is the first frequency AC component included in the second power between the capacitor 12 and the switch unit 11, is greater than the first AC component, which is the first frequency AC component included in the first power between the DC bus 20 and the 3-level inverter 10. This reduces the pulsation of the current in the DC bus 20.

[0051] In this embodiment, the controller 15 selects the on and off states of the plurality of switches in order to supply power to the capacitor 12 from the DC bus 20. This allows the capacitor 12 to be charged and an intermediate potential to be generated.

[0052] In this embodiment, the controller 15 selects the on and off states of the plurality of switches so that the second DC voltage of the capacitor 12 is lower than the first DC voltage of the DC bus 20. This prevents unintended power outflow from the capacitor 12 to the DC bus 20.

[0053] In the three-level inverter 10 of this embodiment, the first frequency is the fundamental wave component of the AC voltage of the AC line, an integer multiple of the fundamental wave component, or an integer fraction of the fundamental wave component. This makes it possible to reduce the pulsation of the current of the DC bus 20, which fluctuates at a frequency of the fundamental wave component of the AC voltage of the AC line, an integer multiple of the fundamental wave component, or an integer fraction of the fundamental wave component.

[0054] The three-level inverter 10 of this embodiment is configured to supply power to a rotating electric machine that drives the compression mechanism of a compressor, and the first frequency is the frequency of the load torque of the compression mechanism. This makes it possible to reduce the pulsation of the current in the DC bus 20 caused by fluctuations in the load torque of the compression mechanism of the compressor. Examples of compression mechanisms include reciprocating type, rotary type, and scroll type.

[0055] The three-level inverter 10 of this embodiment is configured to supply power to an AC power supply 40, and the first frequency is the fundamental wave component of the AC voltage of the AC power supply 40, or an integer multiple of the fundamental wave component, or an integer fraction of the fundamental wave component. This makes it possible to reduce the pulsation of the current in the DC bus 20 caused by fluctuations in the power of the AC line.

[0056] The refrigeration system of this embodiment includes a 3-level inverter 10 and a compressor equipped with a rotating electric motor that is powered by the 3-level inverter 10. This makes it possible to reduce current pulsation in the DC bus 20 of the refrigeration system.

[0057] The power conversion device of this embodiment includes a 3-level inverter 10 and a DC power supply 21 that supplies power to the 3-level inverter 10. This makes it possible to reduce the current pulsation of the DC power supply 21 of the power conversion device.

[0058] Although embodiments have been described above, the technical scope of this disclosure is not limited to the embodiments described above. It is clear from the claims that combinations of two or more of the above embodiments, as well as various modifications or improvements to the above embodiments, are also included in the technical scope of this disclosure. [Explanation of Symbols]

[0059] 1, 2…Power control system, 10, 110, 210, 310…3-level inverter, 11…Switch section, 12, 13…Capacitor, 15…Controller, 20…DC bus, 21…DC power supply, 30…Motor, 40…AC power supply, O…Neutral point, i n ...intermediate current, v dc ...DC voltage, v n ...intermediate potential

Claims

1. A three-level inverter configured to convert power from a DC bus and power from the k-phase (where k is a natural number greater than or equal to 2) of an AC line, The aforementioned three-level inverter is A capacitor with one terminal connected to either the P side or the N side of the DC bus, A switch section having a plurality of switches is provided between the P side of the DC bus, the N side of the DC bus, the other terminal of the capacitor, and the AC line. The system includes a controller that selects the on and off states of the plurality of switches, All paths between the other terminal of the capacitor and the other P-side or N-side of the DC bus are provided with at least one of the plurality of switches. The controller selects the on and off states of the plurality of switches such that the second AC component, which is the first frequency AC component included in the second power between the capacitor and the switch unit, is greater than the first AC component, which is the first frequency AC component included in the first power between the DC bus and the three-level inverter. 3-level inverter.

2. The controller selects the on and off states of the plurality of switches in order to supply power to the capacitor from the DC bus. The three-level inverter according to claim 1.

3. The controller selects the on and off states of the plurality of switches such that the second DC voltage of the capacitor is less than the first DC voltage of the DC bus. The three-level inverter according to claim 1.

4. The first frequency is the fundamental wave component of the AC voltage of the AC line, or an integer multiple of the fundamental wave component, or an integer fraction of the fundamental wave component. The three-level inverter according to claim 1.

5. The three-level inverter is configured to supply power to a rotating electric machine that drives the compression mechanism of the compressor. The first frequency is the frequency of the load torque of the compression mechanism. The three-level inverter according to claim 1.

6. The aforementioned three-level inverter is configured to supply power to an AC power source, The first frequency is the fundamental wave component of the AC voltage of the AC power supply, or an integer multiple of the fundamental wave component, or an integer fraction of the fundamental wave component. The three-level inverter according to claim 1.

7. A three-level inverter according to any one of claims 1 to 6, A compressor equipped with a rotating electric machine that receives power from the aforementioned three-level inverter, A refrigeration device equipped with a refrigerator.

8. A three-level inverter according to any one of claims 1 to 6, A DC power supply that provides power to the aforementioned three-level inverter, A power conversion device equipped with this device.

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