Inverter device

Abstract

To provide an inverter device capable of suppressing deviation of the neutral point potential.SOLUTION: An inverter device A1 converts DC power output from a DC power supply into AC power and outputs the AC power. The inverter device A1 includes an inverter circuit 2 (multilevel inverter circuit) including a voltage dividing part that divides a power supply voltage of a DC power supply 1 to generate a neutral point potential, and a control circuit 5 that controls the inverter circuit 2 so that an output voltage of the inverter device A1 becomes a target value of the output voltage. The control circuit 5 includes a deviation detection part that detects whether deviation of the neutral point potential has occurred, and decreases the target value of the output voltage when the deviation detection part detects occurrence of the deviation of the neutral point potential.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to an inverter device that converts DC power output from a DC power supply into AC power and outputs the AC power. [Background technology]

[0002] Conventionally, three-level inverters capable of outputting voltages at three different levels have been known. For example, Patent Document 1 discloses an example of a grid-connected inverter system equipped with a multilevel inverter circuit. The grid-connected inverter system is connected to a power grid by a switch, converts DC power output by a DC power supply into AC power, and supplies the AC power to the grid. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2012-70498 Summary of the Invention [Problem to be solved by the invention]

[0004] Some grid-connected inverter systems operate independently by disconnecting from the power grid during power outages or abnormalities. During this independent operation, the inverter acts as a voltage source to supply power to the load, controlling the output of AC voltage but not AC current. Therefore, when a magnetizing inrush current occurs, a bias in the neutral point potential can occur. This bias in the neutral point potential can distort the output AC voltage, causing overvoltage detection and resulting in operation shutdown. In the worst case, this can even cause the inverter to fail. Furthermore, magnetizing inrush current is not limited to inverters used in grid-connected inverter systems, but can also occur in inverters used for other purposes.

[0005] The present disclosure has been made in view of the above circumstances, and has an object to provide an inverter device that can suppress bias in neutral point potential. [Means for solving the problem]

[0006] The inverter device provided by the present disclosure is an inverter device that converts DC power output by a DC power supply into AC power and outputs it, and is equipped with a multilevel inverter circuit including a voltage divider unit that divides the power supply voltage of the DC power supply to generate a neutral point potential, and a control circuit that controls the multilevel inverter circuit so that the output voltage of the inverter device becomes a target value for the output voltage, and the control circuit includes a bias detection unit that detects whether or not a bias has occurred in the neutral point potential, and when the bias detection unit detects the occurrence of a bias in the neutral point potential, it lowers the target value.

[0007] In a preferred embodiment of the inverter device, the inverter device further includes a first voltage detection unit that detects a first voltage that is a potential difference between the potential of the positive electrode of the DC power supply and the neutral point potential, and a second voltage detection unit that detects a second voltage that is a potential difference between the potential of the negative electrode of the DC power supply and the neutral point potential, and the bias detection unit determines that a bias has occurred in the neutral point potential when the absolute value of the voltage difference between the first voltage and the second voltage is equal to or greater than a threshold value.

[0008] In a preferred embodiment of the inverter device, the bias detection unit further detects the degree of bias of the neutral point potential depending on the magnitude of the voltage difference, and the control circuit reduces the target value more significantly the greater the degree of bias of the neutral point potential.

[0009] In a preferred embodiment of the inverter device, the multilevel inverter circuit is a three-level inverter circuit in which the voltage of each phase has three levels of potential, and the voltage divider unit includes two capacitors connected in series between the positive and negative electrodes of the DC power supply, and the two capacitors have the same capacitance.

[0010] In a preferred embodiment of the inverter device, when the bias detection unit detects the occurrence of bias in the neutral point potential and no longer detects the occurrence of bias in the neutral point potential, the control circuit returns the target value from the reduced value to the original value. [Effects of the Invention]

[0011] According to the inverter device of the present disclosure, when the control circuit detects the occurrence of a neutral point potential bias, it reduces the target value of the output voltage. With this configuration, if a neutral point potential bias occurs due to the occurrence of a magnetizing inrush current, the output voltage of the inverter device is reduced. This allows the inverter device of the present disclosure to suppress neutral point potential bias caused by the magnetizing inrush current. Furthermore, because the inverter device can suppress neutral point potential bias, distortion of the output voltage (AC voltage) caused by neutral point potential bias is suppressed, and operation shutdowns and inverter failures due to overvoltage detection can be suppressed. [Brief explanation of the drawings]

[0012] [Figure 1] 1 is a block diagram showing an inverter device according to a first embodiment. [Figure 2] 1 is a circuit diagram showing an internal configuration of an inverter circuit of an inverter device according to a first embodiment. [Figure 3] 2 is a block diagram showing the internal configuration of a control circuit of the inverter device according to the first embodiment. FIG. [Figure 4] FIG. 4 is a block diagram showing an inverter device according to a modified example of the first embodiment. [Figure 5] FIG. 10 is a block diagram showing the internal configuration of a control circuit of an inverter device according to a second embodiment. [Figure 6] 6A and 6B are diagrams showing a correspondence table between the degree of imbalance and the voltage difference (FIG. 6A) and a correspondence table between the degree of imbalance and the correction amount (amount of reduction) (FIG. 6B), which are stored in a control circuit in an inverter device according to a second embodiment. [Figure 7] 10 is a graph showing the relationship between the degree of bias (voltage difference) and the target voltage of an inverter device according to a modified example of the second embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0013] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the inverter device of the present disclosure will be described below with reference to the accompanying drawings. In the following, identical or similar components will be designated by the same reference numerals, and redundant description will be omitted.

[0014] 1 shows an inverter device A1 according to a first embodiment. The inverter device A1 is used, for example, in a power conditioner, and converts DC power input from a DC power source 1 into AC power and outputs the AC power. Note that application examples of the inverter device A1 of the present disclosure are not limited to power conditioners.

[0015] As shown in FIG. 1, inverter device A1 is connected to power system B via circuit breaker CB. When circuit breaker CB is closed, inverter device A1 is connected to power system B and performs grid-connected operation. On the other hand, when circuit breaker CB is open, inverter device A1 is disconnected from power system B and performs isolated operation (non-grid-connected operation). Circuit breaker CB is opened in the event of a power outage or abnormality in power system B. FIG. 1 shows the state in which inverter device A1 is disconnected from power system B. During isolated operation, inverter device A1 supplies power to load L using DC power source 1.

[0016] As shown in FIG. 1 , the inverter device A1 includes an inverter circuit 2, a filter circuit 3, a transformer circuit 4, a control circuit 5, a voltage sensor 6, a first voltage detection unit 71, and a second voltage detection unit 72. A DC power supply 1 is connected to the input side of the inverter circuit 2. The inverter circuit 2 is a three-phase inverter, and the inverter circuit 2, filter circuit 3, and transformer circuit 4 are connected in series by output lines for output voltages of U, V, and W phases, in this order. The output lines are connected to a three-phase power grid B. The output lines are also connected to a load L. A control circuit 5 is connected to the inverter circuit 2. During stand-alone operation, the inverter device A1 converts DC power output by the DC power supply 1 into AC power using the inverter circuit 2 and supplies the AC power to the load L via the filter circuit 3 and the transformer circuit 4. Note that the configuration of the inverter device A1 is not limited to this. For example, a so-called transformerless system may be used in which a DC / DC converter circuit is provided between the DC power supply 1 and the inverter circuit 2 instead of the transformer circuit 4.

[0017] The DC power supply 1 outputs DC power and includes, for example, a power storage device. The power storage device is, for example, a lithium ion battery, but may also be a fuel cell, a lead storage battery, or an electric double layer capacitor. Instead of a power storage device, the DC power supply 1 may also be a solar cell (which converts solar energy into electrical energy to generate DC power), or a device that converts AC power generated by a diesel engine generator, a micro gas turbine generator, a wind turbine generator, or the like into DC power and outputs it. The power supply voltage of the DC power supply 1 is not particularly limited, but will be described as 600 V in this embodiment.

[0018] The inverter circuit 2 converts the power supply voltage (DC voltage) input from the DC power supply 1 into an AC voltage and outputs it to the filter circuit 3. In this embodiment, the inverter circuit 2 is a three-phase PWM-controlled inverter equipped with switching elements, and is a three-level inverter circuit in which the output phase voltage of each phase has three levels of potential. The inverter circuit 2 converts the DC voltage input from the DC power supply 1 into an AC voltage by switching each switching element on and off based on a PWM signal P input from the control circuit 5.

[0019] The filter circuit 3 removes high-frequency components caused by switching from the AC voltage input from the inverter circuit 2. The filter circuit 3 includes a low-pass filter (not shown) made up of a reactor and a capacitor. The AC voltage from which the high-frequency components have been removed by the filter circuit 3 is output to the transformer circuit 4. Note that the configuration of the filter circuit 3 is not limited to this, and any well-known filter circuit for removing high-frequency components may be used.

[0020] The transformer circuit 4 steps up or steps down the AC voltage output from the filter circuit 3 to a level substantially equal to the system voltage of the power system B.

[0021] The voltage sensor 6 detects the output voltage Vout of the inverter device A1. The voltage sensor 6 is disposed between the transformer circuit 4 and the circuit breaker CB at the output end of the inverter device A1. The detected output voltage Vout is output to the control circuit 5 as an output detection signal.

[0022] The control circuit 5 generates a PWM signal P that controls the switching of the switching elements of the inverter circuit 2 and is realized by, for example, a microcomputer. The control circuit 5 receives detection signals from a voltage sensor 6 and various sensors (not shown) and outputs the PWM signal P to the inverter circuit 2. The control circuit 5 controls the output of the inverter device A1 using a current control method during grid-connected operation and a voltage control method during stand-alone operation. For example, during stand-alone operation, the control circuit 5 generates the PWM signal P based on a set target voltage (e.g., effective value) of the output voltage and an output detection signal input from the voltage sensor 6. The target voltage (e.g., effective value) is not limited, but will be described as 300 V in this embodiment. The inverter circuit 2 switches each switching element on and off based on the input PWM signal P, thereby outputting a phase voltage corresponding to the set target voltage. A detailed description of the control circuit 5 will be given later.

[0023] Next, the internal configuration and detailed description of the inverter circuit 2 will be provided with reference to Fig. 2. Fig. 2 is a circuit diagram showing the internal configuration of the inverter circuit 2. Fig. 2 also shows a first voltage detection unit 71 and a second voltage detection unit 72, which will also be described. As described above, the inverter circuit 2 is a three-phase PWM-controlled three-level inverter circuit.

[0024] 2, the inverter circuit 2 includes 12 switching elements S1 to S12, 12 free-wheeling diodes D1 to D12, and a voltage divider 21. In this embodiment, IGBTs (Insulated Gate Bipolar Transistors) are used as the switching elements S1 to S12. Note that the switching elements S1 to S12 are not limited to IGBTs and may be bipolar transistors, MOSFETs, reverse-blocking thyristors, etc.

[0025] The voltage dividing unit 21 divides the power supply voltage of the DC power supply 1 to generate a neutral point potential. The voltage dividing unit 21 includes two voltage dividing capacitors C1 and C2. The voltage dividing capacitors C1 and C2 have the same capacitance and equally divide the power supply voltage input from the DC power supply 1. The voltage dividing capacitors C1 and C2 are connected in series at point O and in parallel between point P, which is connected to the positive electrode of the DC power supply 1, and point N, which is connected to the negative electrode. When the voltage dividing capacitors C1 and C2 have the same capacitance, the potential at point O is intermediate between the potentials at points N and P. Therefore, when the negative electrode of the DC power supply 1 is grounded, the potential at point N is "0." If the potential of the positive electrode of the DC power supply 1, i.e., the potential at point P, is "E," the potential at point O becomes "(1 / 2)E," which is intermediate between the potential at point N (0) and the potential at point P (E). The types of voltage-dividing capacitors C1 and C2 are not limited. Point O is the neutral point, and the potential at point O is the neutral point potential. In an example where the negative electrode of DC power supply 1 is grounded and the potential at point N is "0", if the power supply voltage of DC power supply 1 is 600 V, the neutral point potential is 300 V. If the negative electrode of DC power supply 1 is not grounded and the potential at point N is not "0", the neutral point potential is offset by the potential at point N.

[0026] The first voltage detection unit 71 detects a first voltage V1, which is the potential difference between the positive electrode potential (potential at point P) and the neutral point potential (potential at point O) of the DC power supply 1. The first voltage V1 corresponds to the voltage across the voltage-dividing capacitor C1. The first voltage detection unit 71 outputs the detected first voltage V1 to the control circuit 5 as a first voltage detection signal.

[0027] The second voltage detection unit 72 detects a second voltage V2, which is the potential difference between the neutral point potential (potential at point O) and the negative electrode potential of the DC power supply 1 (potential at point N). The second voltage V2 corresponds to the voltage across the voltage-dividing capacitor C2. The second voltage detection unit 72 outputs the detected second voltage V2 to the control circuit 5 as a second voltage detection signal.

[0028] Switching elements S1 and S4 are connected in series, with the emitter terminal of switching element S1 connected to the collector terminal of switching element S4. The collector terminal of switching element S1 is connected to point P, and the emitter terminal of switching element S4 is connected to point N, forming a bridge structure. Similarly, switching elements S2 and S5 are connected in series to form a bridge structure, and switching elements S3 and S6 are connected in series to form a bridge structure. Because switching elements S1, S2, and S3 are connected to the positive side of DC power supply 1, they may be referred to as "positive-side switches Sp" when not distinguishing between them. On the other hand, switching elements S4, S5, and S6 are connected to the negative side of DC power supply 1, so they may be referred to as "negative-side switches Sn" when not distinguishing between them. The PWM signals P (Pup, Pvp, Pwp, Pun, Pvn, and Pwn) output from the control circuit 5 are input to the base terminals of the switching elements S1 to S6, respectively.

[0029] The bridge structure formed by switching elements S1 and S4 is the U-phase arm, the bridge structure formed by switching elements S2 and S5 is the V-phase arm, and the bridge structure formed by switching elements S3 and S6 is the W-phase arm. A U-phase output line is connected to connection point U between switching elements S1 and S4 of the U-phase arm, a V-phase output line is connected to connection point V between switching elements S2 and S5 of the V-phase arm, and a W-phase output line is connected to connection point W between switching elements S3 and S6 of the W-phase arm.

[0030] The connection point U is connected to point O via an intermediate switch consisting of switching elements S7 and S8. The switching elements S7 and S8 are connected in series with their collector terminals connected. The emitter terminal of the switching element S7 is connected to point O, and the emitter terminal of the switching element S8 is connected to point U. Similarly, the connection point V is connected to point O via an intermediate switch consisting of switching elements S9 and S10. The switching elements S9 and S10 are connected with their collector terminals connected, with the emitter terminal of the switching element S9 connected to point O, and the emitter terminal of the switching element S10 connected to point V. The connection point W is also connected to point O via an intermediate switch consisting of switching elements S11 and S12. The switching elements S11 and S12 are connected with their collector terminals connected, with the emitter terminal of the switching element S11 connected to point O, and the emitter terminal of the switching element S12 connected to point W. The switching elements S7 and S8 perform on / off operations at the same timing, connecting points O and U in the on state and not connecting them in the off state. Similarly, the switching elements S9 and S10 perform on / off operations at the same timing, connecting points O and V in the on state and not connecting them in the off state. The switching elements S11 and S12 also perform on / off operations at the same timing, connecting points O and W in the on state and not connecting them in the off state. When it is not necessary to distinguish between the intermediate switches, they may be referred to as the "intermediate switch So." The PWM signal P (Puo, Pvo, Pwo) output from the control circuit 5 is input to the base terminals of the switching elements S7 and S8, the base terminals of the switching elements S9 and S10, and the base terminals of the switching elements S11 and S12, respectively.

[0031] Each of the switching elements S1 to S12 is switched between an ON state and an OFF state based on the PWM signal P. When the positive switch Sp is in an ON state and the negative switch Sn and the intermediate switch So are in an OFF state, the potential of the output line of the corresponding phase becomes the potential of point P (i.e., the potential "E" on the positive side of the DC power supply 1). When the negative switch Sn is in an ON state and the positive switch Sp and the intermediate switch So are in an OFF state, the potential of the output line of the corresponding phase becomes the potential of point N (i.e., the potential "0" on the negative side of the DC power supply 1). When the intermediate switch So is in an ON state and the positive switch Sp and the negative switch Sn are in an OFF state, the potential of the output line of the corresponding phase becomes the potential of point O (i.e., the potential "(1 / 2)E" midway between the positive and negative sides of the DC power supply 1). As a result, the output phase voltage output from each output line has three levels of potential: "E", the potential on the positive side of the DC power supply 1, "0", the potential on the negative side, and an intermediate potential of "(1 / 2)E". In addition, the output line voltage, which is the voltage between the output lines, has five levels of potential.

[0032] The free wheel diodes D1 to D12 are connected in antiparallel between the collector terminals and emitter terminals of the switching elements S1 to S12, respectively. That is, the anode terminals of the free wheel diodes D1 to D12 are connected to the emitter terminals of the switching elements S1 to S12, respectively, and the cathode terminals of the free wheel diodes D1 to D12 are connected to the collector terminals of the switching elements S1 to S12, respectively. The free wheel diodes D1 to D12 are used to prevent a high reverse voltage caused by the back electromotive force generated by switching the switching elements S1 to S12 from being applied to the switching elements S1 to S12. There are no particular restrictions on the type of free wheel diodes D1 to D12.

[0033] The inverter circuit 2 configured as described above is of a T-type NPC (Neutral Point Clamped) type. Note that the configuration of the inverter circuit 2 described above is merely an example, and is not limited to this as long as it is a three-level inverter circuit. For example, the inverter circuit 2 may be a three-level inverter circuit of a diode-clamped NPC type. However, a three-level inverter circuit of a T-type NPC type for the inverter circuit 2 has fewer elements through which current passes, enabling lower loss to be achieved, and also reducing the number of power supplies required for the gate drive circuit (control circuit 5).

[0034] Next, the internal configuration and detailed description of the control circuit 5 will be given with reference to Fig. 3. Fig. 3 is a block diagram showing the internal configuration of the control circuit 5, and is a functional block diagram for performing voltage control during stand-alone operation. For current control during grid-connected operation, a well-known method (for example, the method described in Patent Document 1) is used.

[0035] The control circuit 5 includes an imbalance detection unit 51, a target setting unit 52, a voltage control unit 53, and a PWM signal generation unit 54. The control circuit 5 may also have a configuration for detecting an overcurrent, a ground fault, a short circuit, an isolated operation, etc., and stopping the operation of the inverter circuit 2, a configuration for maximum power point tracking, etc.

[0036] The bias detection unit 51 detects whether or not a bias in the neutral point potential occurs. In this embodiment, the bias detection unit 51 includes a difference calculation unit 511 and a detection determination unit 512, as shown in FIG.

[0037] The difference calculation unit 511 calculates the voltage difference (e.g., V1-V2) between the first voltage V1 and the second voltage V2 based on the first voltage signal input from the first voltage detection unit 71 and the second voltage signal input from the second voltage detection unit 72, and defines the absolute value of the voltage difference as the voltage difference.

[0038] The detection / determination unit 512 determines that a bias in the neutral point potential has occurred when the voltage difference (the absolute value of the voltage difference between the first voltage V1 and the second voltage V2) calculated by the difference calculation unit 511 is equal to or greater than a predetermined threshold (hereinafter referred to as the "determination threshold"). The determination threshold is a threshold for determining whether a bias in the neutral point potential has occurred, and may be a value according to the specifications of the inverter device A1, a value obtained experimentally, or a value according to the power supply voltage or a set target voltage of the DC power supply 1. However, the determination threshold is set to a value equal to or less than the maximum difference ΔVmax. The maximum difference ΔVmax is the maximum value of the voltage difference allowed by the inverter device A1 depending on the conditions of the inverter circuit 2 and the load L, etc. For example, in an example where the power supply voltage of the DC power supply 1 is 600 V, the maximum difference ΔVmax is 60 V (10% of the power supply voltage of the DC power supply 1, 600 V), and the determination threshold is 30 V. The maximum difference ΔVmax and the determination threshold are not limited to these values. When the bias detection unit 51 detects a bias in the neutral point potential, it outputs a bias detection signal to the voltage control unit 53. The bias detection signal may be output only when a bias in the neutral point potential occurs, or may include a signal indicating that a bias in the neutral point potential is occurring and a signal indicating that a bias is not occurring.

[0039] The target value (target voltage) of the output voltage Vout of the inverter device A1 is stored in advance in the target setting unit 52. In this embodiment, as described above, the target voltage is, for example, 300 V. The target setting unit 52 outputs the stored target voltage to the voltage control unit 53. Note that the target voltage may be configured so that it can be changed to any value by the user of the inverter device A1.

[0040] The voltage control unit 53 generates a control signal for controlling the inverter circuit 2, using the target voltage input from the target setting unit 52 and the output voltage signal (output voltage Vout) input from the voltage sensor 6. In this embodiment, since the inverter circuit 2 is PWM controlled, the control signal is a PWM signal P. The voltage control unit 53 includes a target correction unit 531 and a feedback control unit 532.

[0041] The target correction unit 531 corrects the target voltage input from the target setting unit 52 based on the bias detection signal input from the bias detection unit 51. The target correction unit 531 corrects the target voltage when the bias detection unit 51 determines, based on the bias detection signal, that a bias has occurred in the neutral point potential. In the following description, the target voltage set by the target setting unit 52 is referred to as the "set target voltage," and the target voltage corrected by the target correction unit 531 is referred to as the "corrected target voltage." The corrected target voltage may be a value according to the specifications of the inverter device A1, a value obtained experimentally, or a value according to the power supply voltage of the DC power supply 1 or the set target voltage. The corrected target voltage is not limited in any way and may be, for example, 150 V, which is half the set target voltage (300 V). If a bias in the neutral point potential occurs, the target correction unit 531 outputs a corrected target voltage to the feedback control unit 532, and if a bias in the neutral point potential does not occur, the target correction unit 531 outputs a set target voltage to the feedback control unit 532.

[0042] The feedback control unit 532 performs feedback control based on the deviation between the output voltage signal (output voltage Vout) input from the voltage sensor 6 and the target voltage (set target voltage or corrected target voltage) input from the target correction unit 531. That is, when a bias in the neutral point potential occurs, the feedback control unit 532 performs feedback control using the corrected target voltage, and when a bias in the neutral point potential does not occur, the feedback control unit 532 performs feedback control using the set target voltage. The feedback control unit 532 generates a command value signal through the feedback control. The command value signal is a signal for controlling the switching (switching between on and off) of each of the switching elements S1 to S12 of the inverter circuit 2. The feedback control unit 532 outputs the generated command value signal to the PWM signal generation unit 54. If necessary, the feedback control unit 532 may amplify the command value signal before outputting it, or may convert it into a signal for commanding the waveform of the phase voltage output by the inverter device A1 before outputting it.

[0043] The PWM signal generating unit 54 generates a PWM signal P based on a carrier signal (e.g., a triangular wave signal) of a predetermined frequency (e.g., 4 kHz) generated internally and a command value signal input from the feedback control unit 532, and outputs the PWM signal P to the inverter circuit 2. The PWM signal P includes a PWM signal Pup input to the base terminal of the switching element S1, a PWM signal Pvp input to the base terminal of the switching element S2, a PWM signal Pwp input to the base terminal of the switching element S3, a PWM signal Pun input to the base terminal of the switching element S4, a PWM signal Pvn input to the base terminal of the switching element S5, and a PWM signal Pwn input to the base terminal of the switching element S6. The PWM signal P also includes a PWM signal Puo input to the base terminals of the switching elements S7 and S8, a PWM signal Pvo input to the base terminals of the switching elements S9 and S10, and a PWM signal Pwo input to the base terminals of the switching elements S11 and S12.

[0044] An example of operation of the inverter device A1 configured as above during stand-alone operation will now be described. In this example of operation, using the above-mentioned numerical example, it is assumed that the power supply voltage of the DC power supply 1 is 600V and the target value (set target voltage) of the output voltage Vout is 300V. In this case, if there is no bias in the neutral point potential, the neutral point potential will be 300V (i.e., the first voltage V1 is 300V and the second voltage V2 is 300V). The above-mentioned determination threshold is 30V. Furthermore, if there is bias in the neutral point potential, the corrected target voltage (the target voltage corrected by the target corrector 531) is 150V.

[0045] In this example, if no magnetizing inrush current or the like occurs in the inverter device A1 (if the inverter device A1 is in a normal state), the voltage difference (absolute value of the voltage difference) between the first voltage V1 and the second voltage V2 does not exceed the above-mentioned determination threshold value (30 V). At this time, the bias detection unit 51 determines that no bias in the neutral point potential has occurred (no bias in the neutral point potential is detected), and the target correction unit 531 outputs a set target voltage (300 V) to the feedback control unit 532. As a result, the control circuit 5 controls the inverter circuit 2 so that the output voltage Vout becomes the set target voltage, and the inverter device A1 outputs an output voltage Vout of 300 V.

[0046] On the other hand, when an excitation inrush current or the like occurs in the inverter device A1 and the voltage difference (absolute value of the voltage difference) between the first voltage V1 and the second voltage V2 becomes equal to or greater than the judgment threshold value (30 V), the bias detection unit 51 determines that a bias in the neutral point potential has occurred (detects a bias in the neutral point potential). Therefore, the target correction unit 531 corrects the target voltage input from the target setting unit 52, and the target correction unit 531 outputs a corrected target voltage (150 V) to the feedback control unit 532. As a result, the control circuit 5 controls the inverter circuit 2 so that the output voltage Vout becomes the corrected target voltage, and the inverter device A1 outputs an output voltage Vout of 150 V.

[0047] When a neutral point potential deviation occurs due to the magnetizing inrush current, the control circuit 5 detects the neutral point potential deviation and reduces the output voltage Vout. The reduction in the output voltage Vout suppresses the neutral point potential deviation due to the magnetizing inrush current, thereby reducing distortion that may occur in the output voltage Vout. Furthermore, the magnetizing inrush current converges over time, gradually eliminating the neutral point potential deviation. This reduces the voltage difference between the first voltage V1 and the second voltage V2, making the voltage difference between the first voltage V1 and the second voltage V2 less than the determination threshold. In other words, in the control circuit 5, the deviation detection unit 51 does not detect the neutral point potential deviation, and the voltage control unit 53 returns the target voltage to its original value (the set target voltage). This restores the output voltage Vout of the inverter device A1, and the inverter device A1 returns to its normal operating state. In this way, the neutral point potential deviation is suppressed in the inverter device A1. In addition, the bias detection unit 51 (detection / determination unit 512) may provide hysteresis between the determination threshold for determining that a bias in the neutral point potential has occurred and the determination threshold for determining that the bias in the neutral point potential has been resolved (changed from an occurring state to no longer occurring). In this case, frequent switching between detection and non-detection of the bias in the neutral point potential is suppressed, thereby suppressing frequent changes in the output voltage.

[0048] The inverter device A1 has the following functions and effects.

[0049] In the inverter device A1, the inverter circuit 2 includes a voltage divider 21 that divides the power supply voltage of the DC power supply 1 to generate a neutral point potential (potential at point O), and the control circuit 5 includes a bias detector 51 that detects whether or not a bias in the neutral point potential (potential at point O) has occurred. When the bias detector 51 detects the occurrence of a bias in the neutral point potential, the control circuit 5 reduces the target value (target voltage) of the output voltage Vout. With this configuration, for example, if a bias in the neutral point potential occurs due to the occurrence of a magnetizing inrush current, the output voltage Vout of the inverter device A1 decreases. As a result, as described above, the inverter device A1 can suppress the bias in the neutral point potential caused by the magnetizing inrush current. Furthermore, since the inverter device A1 can suppress the bias in the neutral point potential, distortion of the output voltage (AC voltage) due to the bias in the neutral point potential is suppressed, and operation shutdowns and inverter failures due to the detection of an overvoltage can be suppressed. Therefore, even if an excitation inrush current occurs during stand-alone operation, causing an imbalance in the neutral point potential, the inverter device A1 will be able to continue stand-alone operation, enabling a more robust supply of power to the load L.

[0050] In the inverter device A1, the bias detection unit 51 detects a bias in the neutral point potential when a voltage difference (absolute value of the voltage difference) between a first voltage V1, which is the potential difference between the potential of the positive electrode of the DC power supply 1 (the potential at point P) and the neutral point potential (the potential at point O), and a second voltage V2, which is the potential difference between the neutral point potential and the potential of the negative electrode of the DC power supply 1 (point N), is equal to or greater than a threshold value. The first voltage V1 is detected by a first voltage detection unit 71, and the second voltage V2 is detected by a second voltage detection unit 72. In this embodiment, the voltage dividing unit 21 generates the neutral point potential using two voltage-dividing capacitors C1 and C2, which have the same capacitance. In this case, when a bias in the neutral point potential does not occur, the neutral point potential is exactly half the potential difference between the potential of the positive electrode of the DC power supply 1 and the potential of the negative electrode of the DC power supply 1 (the power supply voltage of the DC power supply 1), and the first voltage V1 and the second voltage V2 are equal to each other. On the other hand, if a bias in the neutral point potential occurs, a difference occurs between the first voltage V1 and the second voltage V2. Therefore, the inverter device A1 can detect the bias in the neutral point potential by comparing the voltage difference (absolute value of the voltage difference) between the first voltage V1 and the second voltage V2 with the determination threshold.

[0051] In the first embodiment, the control circuit 5 (detection / determination unit 512) determines whether or not a bias in the neutral point potential has occurred based on the voltage difference (absolute value of the voltage difference) between the first voltage V1 and the second voltage V2. However, the method for determining whether or not a bias in the neutral point potential has occurred is not limited to this. For example, whether or not a bias in the neutral point potential has occurred may be determined based on either the first voltage V1 or the second voltage V2. FIG. 4 shows an inverter device A11 according to such a modification. The inverter device A11 shown in FIG. 4 will be described taking as an example a case where whether or not a bias in the neutral point potential has occurred based on the second voltage V2. As shown in FIG. 4, the inverter device A11 differs from the inverter device A1 in that it does not include a first voltage detection unit 71.

[0052] In the inverter device A11, the bias detection unit 51 receives a second voltage detection signal from the second voltage detection unit 72 and detects a bias in the neutral point potential based on the second voltage V2. In the inverter device A11, for example, a reference potential of the neutral point potential is stored in the difference calculation unit 511. The reference potential is the value of the neutral point potential under normal conditions, and in this embodiment, it is half the value of the potential difference (power supply voltage of the DC power supply 1) between the positive pole (point P) of the DC power supply 1 and the negative pole (point N) of the DC power supply 1. The difference calculation unit 511 calculates the absolute value of the difference between the second voltage V2 and the reference potential as a voltage difference. The detection determination unit 512 compares the voltage difference calculated by the difference calculation unit 511 with a determination threshold, and determines that a bias in the neutral point potential has occurred if the voltage difference is equal to or greater than the determination threshold. Note that if the aforementioned reference potential is 300 V, the determination threshold in this modification is 15 V. The voltage difference calculated by the difference calculation unit 511 is the voltage difference (absolute value) between the first voltage V1 and the second voltage V2 in the inverter device A1, whereas it is the voltage difference (absolute value) between the second voltage V2 and the reference potential in the inverter device A11. As a result, the determination threshold of the inverter device A11 is half of the determination threshold of the inverter device A1. In this way, the bias detection unit 51 of the inverter device A11 determines that a bias in the neutral point potential has occurred.

[0053] Alternatively, the bias detection unit 51 of the inverter device A11 may detect a bias in the neutral point potential as follows. For example, if the second voltage V2 is outside a set determination range, it may be determined that a bias in the neutral point potential has occurred. The determination range is set as a reference potential ± a determination threshold. For example, if the aforementioned reference potential is 300 V and the determination threshold is 15 V, the determination range is 285 V or more and 315 V or less. In other words, the detection / determination unit 512 may determine that a bias in the neutral point potential has occurred when the second voltage V2 is less than 285 V or greater than 315 V. In this example, the bias detection unit 51 does not need to include the difference calculation unit 511.

[0054] In the inverter device A11 according to this modification, the bias detection unit 51 detects the bias of the neutral point potential based on the second voltage V2. This configuration eliminates the need for the first voltage detection unit 71 that detects the first voltage V1. Therefore, the inverter device A11 can have a simpler configuration than the inverter device A1.

[0055] In the above variant, an example was given of determining whether or not a bias in the neutral point potential has occurred in accordance with the second voltage V2, but it is also possible to determine whether or not a bias in the neutral point potential has occurred in accordance with the first voltage V1 instead of the second voltage V2.

[0056] 5 shows an inverter device A2 according to a second embodiment. The inverter device A2 differs from the inverter device A1 in the configurations of an imbalance detection unit 51 and a voltage control unit 53.

[0057] In the inverter device A2, the bias detection unit 51 detects the bias of the neutral point potential and also detects the degree of bias of the neutral point potential. As shown in Fig. 5, the bias detection unit 51 of the inverter device A2 further includes a degree determination unit 513, as compared with the bias detection unit 51 of the inverter device A1.

[0058] The degree determination unit 513 determines the degree of bias of the neutral point potential from the voltage difference. For example, the degree determination unit 513 stores the correspondence table shown in FIG. 6(a) and determines the level of the bias degree depending on the magnitude of the voltage difference calculated by the difference calculation unit 511. For example, the degree determination unit 513 determines the voltage difference as level 0 when it is equal to or greater than 0 V and less than 30 V, as level 1 when it is equal to or greater than 30 V and less than 40 V, as level 2 when it is equal to or greater than 40 V and less than 50 V, and as level 3 when it is equal to or greater than 50 V. Level 0 corresponds to the voltage difference at which the detection determination unit 512 determines that no bias of the neutral point potential has occurred. Note that the number of levels of the bias degree and the range of the voltage difference corresponding to each level are not limited to these. The degree determination unit 513 outputs the determined bias degree to the voltage control unit 53 (target correction unit 531).

[0059] In the voltage control unit 53, the target correction unit 531 changes the correction amount (decrease amount) of the target voltage depending on the degree of bias input from the degree determination unit 513. For example, the target correction unit 531 stores a correspondence table shown in FIG. 6(b). The target correction unit 531 determines the correction amount based on the correspondence table shown in FIG. 6(b). In the example shown in FIG. 6(b), when the degree of bias is level 0, the target correction unit 531 outputs a correction amount of 0 V, i.e., a set target voltage without correction, to the feedback control unit 532. Furthermore, when the degree of bias is level 1, the target correction unit 531 sets a correction amount of −150 V and outputs a corrected target voltage of 150 V. When the degree of bias is level 2, the target correction unit 531 sets a correction amount of −200 V and outputs a corrected target voltage of 100 V. When the degree of bias is level 3, the target correction unit 531 sets a correction amount of −250 V and outputs a corrected target voltage of 50 V to the feedback control unit 532. In this way, the larger the degree of bias, the more the target voltage of the output voltage Vout is reduced by the target corrector 531. As a result, the output voltage Vout from the inverter device A2 is reduced in accordance with the degree of bias of the neutral point potential.

[0060] In the inverter device A2, similar to the inverter device A1, the control circuit 5 reduces the target voltage of the output voltage Vout when the bias detection unit 51 detects the occurrence of a bias in the neutral point potential. Therefore, similar to the inverter device A1, the inverter device A2 can suppress the bias in the neutral point potential. Furthermore, since the inverter device A2 can suppress the bias in the neutral point potential, distortion of the output voltage (AC voltage) caused by the bias in the neutral point potential is suppressed, and operation stoppage or inverter failure due to the detection of an overvoltage can be suppressed. Therefore, even if, for example, a magnetizing inrush current occurs during the stand-alone operation and the neutral point potential is biased, the inverter device A2 can continue the stand-alone operation and can more robustly supply power to the load L. Furthermore, the inverter device A2 has the same configuration as the inverter device A1, and thus achieves the same effects as the inverter device A1.

[0061] In the inverter device A2, the control circuit 5 (target correction unit 531) reduces the target voltage more significantly the greater the degree of bias in the neutral point potential. With this configuration, if the bias in the neutral point potential is large, the output voltage will be reduced more significantly, thereby enhancing the effect of suppressing the bias in the neutral point potential. On the other hand, if the bias in the neutral point potential is small, the output voltage will be reduced less, so the output voltage will not be reduced unnecessarily. In other words, in the inverter device A2, it is possible to suppress unnecessary reductions in the output voltage while adequately ensuring the effect of suppressing the bias in the neutral point potential.

[0062] In the second embodiment, the control circuit 5 (target correction unit 531) changes the correction amount (decrease amount) in a stepwise manner, but the method for changing the correction amount (decrease amount) is not limited to this. For example, the control circuit 5 may change the correction amount (decrease amount) linearly. FIG. 7(a) is a diagram for explaining an inverter device according to such a modified example, and is a graph showing the relationship between the bias degree (voltage difference) and the target voltage (corrected target voltage). In FIG. 7(a), the horizontal axis represents the bias degree (voltage difference), and the vertical axis represents the target voltage (corrected target voltage).

[0063] In the example shown in FIG. 7(a), the target voltage is determined relative to the degree of bias (voltage difference) as follows: First, when the degree of bias is 0 (i.e., when the voltage difference is 0V), the correction amount (decrease amount) of the target voltage is 0V, and the target voltage is the set target voltage Vset. Second, as the degree of bias (voltage difference) increases, the correction amount (decrease amount) of the target voltage increases linearly, and the corrected target voltage decreases linearly. Third, when the degree of bias (voltage difference) is the maximum difference ΔVmax, the corrected target voltage is 0V.

[0064] In the inverter device according to this modification, the degree determination unit 513 outputs the voltage difference calculated by the difference calculation unit 511 as the degree of bias to the target correction unit 531. The target correction unit 531 stores an arithmetic expression shown in FIG. 7A that expresses the relationship between the degree of bias (voltage difference) and the target voltage. The target correction unit 531 calculates the target voltage (corrected target voltage) using the input degree of bias and the arithmetic expression, and outputs the target voltage to the feedback control unit 532. Specifically, if the set target voltage Vset is 300 V and the maximum difference ΔVmax is 60 V, in the example shown in FIG. 7A, the corrected target voltage will decrease by 50 V every time the degree of bias (voltage difference) increases by 10 V. Therefore, the arithmetic expression shown in the following equation (1) is set in the target correction unit 531. Then, the target correction unit 531 calculates a target voltage by substituting the voltage difference input from the difference calculation unit 511 into the following equation (1), and outputs the target voltage to the feedback control unit 532. Target voltage (corrected target voltage) = -(50 / 10) × deviation + 300 (1)

[0065] In the inverter device according to this modification, the correction amount (reduction amount) of the target voltage can be changed linearly according to the bias degree (voltage difference), which allows for more precise adjustment of the balance between the effect of suppressing the bias of the neutral point potential and the suppression of unnecessary reductions in the output voltage.

[0066] In the above modification, the relationship between the degree of bias (voltage difference) and the target voltage (corrected target voltage) shown in FIG. 7(a) is merely an example and is not limiting. For example, the relationship shown in FIG. 7(b) may be used. In the example shown in FIG. 7(b), when the degree of bias (voltage difference) is between 0 and a predetermined value ΔVx, the correction amount (decrease amount) of the target voltage is 0 V, i.e., the set target voltage Vset is used as the target voltage. After the predetermined value ΔVx, as the degree of bias (voltage difference) increases, the correction amount (decrease amount) of the target voltage increases linearly, and the corrected target voltage decreases linearly. In this way, until the degree of bias (voltage difference) reaches a certain predetermined value (ΔVx in the example shown in FIG. 7(b)), the target voltage may be set to the set target voltage Vset without being corrected.

[0067] In the second embodiment (including the modified example), degree determination unit 513 determines the degree of bias from the voltage difference calculated by difference calculation unit 511. However, it may be configured as follows: difference calculation unit 511 outputs the calculated voltage difference as is to target correction unit 531. Then, target correction unit 531 may determine the correction amount (amount of decrease) of the target voltage from the input voltage difference instead of the degree of bias, and output the target voltage (set target voltage or corrected target voltage) to feedback control unit 532.

[0068] In the first and second embodiments described above, the control circuit 5 mainly detects the bias in the neutral point potential based on the voltage difference (difference in voltage differences) between the first voltage V1 and the second voltage V2. Alternatively, the control circuit 5 may detect the occurrence of a magnetizing inrush current that causes a bias in the neutral point potential, and detect the occurrence of the neutral point potential bias when it detects the occurrence of the magnetizing inrush current.

[0069] In the first and second embodiments, the inverter circuit 2 is a three-level inverter circuit of a T-type NPC type, but the type is not limited as long as it is a three-level inverter circuit. For example, the inverter circuit 2 may be a three-level inverter circuit of a diode-clamped NPC type. However, a three-level inverter circuit of a T-type NPC type for the inverter circuit 2 has fewer elements through which current passes, which can reduce loss, and also reduces the number of power supplies required for the gate drive circuit (control circuit 5).

[0070] In the first and second embodiments, the voltage divider 21 of the inverter circuit 2 divides the power supply voltage of the DC power supply 1 using two voltage-dividing capacitors C1 and C2 with the same capacitance. However, unlike this configuration, the capacitances of the two voltage-dividing capacitors C1 and C2 may be different from each other. However, in this modification, the neutral point potential is not half the power supply voltage of the DC power supply 1, so there is a certain deviation between the first voltage V1 and the second voltage V2 under normal conditions. Therefore, in this modification, it is necessary to calculate the voltage difference taking into account the deviation between the first voltage V1 and the second voltage V2. More specifically, assuming that there is a deviation ΔV (=V1-V2) between the first voltage V1 and the second voltage V2 under normal conditions, the control circuit 5 (difference calculation unit 511) subtracts the deviation ΔV from the voltage difference (V1-V2) between the first voltage V1 and the second voltage V2 and calculates the absolute value of the subtracted value as the voltage difference. For example, if the first voltage V1 is 400V and the second voltage V2 is 200V under normal conditions, the neutral point potential is 200V and the deviation ΔV is 200V. At this time, assume that due to a bias in the neutral point potential, the first voltage V1 becomes 390V and the second voltage V2 becomes 210V. In this case, the voltage difference V1-V2 between the first voltage V1 and the second voltage V2 is 180V. Subtracting the deviation ΔV from this voltage difference results in a value of -20V (180V-200V). Since the absolute value of this value is calculated as the voltage difference, the control circuit 5 (difference calculation unit 511) calculates the voltage difference to be 20V. Thus, in the inverter device of the present disclosure, the two voltage-dividing capacitors C1 and C2 are not limited to those having the same capacitance.

[0071] In the first and second embodiments, the inverter circuit 2 is a three-level inverter circuit. However, the present invention is not limited to this. The inverter circuit 2 may be a multilevel inverter circuit capable of outputting voltages of three or more levels. In this case, voltage-dividing capacitors are added to the voltage-divider unit 21 as needed, depending on the number of levels. In this modification, the bias detector 51 detects the occurrence of a bias in the neutral point potential when the potential of any of the voltage-divider points (neutral points) deviates from the reference potential of the voltage-divider point. When the potential of any of the multiple voltage-divider points is half the power supply voltage of the DC power supply 1, the potential of the voltage-divider point may be used as a reference. The voltage difference may be calculated in the same manner as in the inverter devices A1 and A2, with the potential of the voltage-divider point being used as a reference, and the potential difference with the positive electrode of the DC power supply 1 defined as a first voltage V1 and the potential difference with the negative electrode of the DC power supply 1 defined as a second voltage V2.

[0072] In the above first and second embodiments, the inverter devices A1, A2, etc. have been described as outputting three-phase AC power, but this is not limitative and they may also output single-phase AC power.

[0073] The inverter device according to the present disclosure is not limited to the above-described embodiment, and the specific configuration of each part of the inverter device according to the present disclosure can be freely modified in various ways. [Explanation of symbols]

[0074] A1, A11, A2: inverter device, 1: DC power supply, 2: inverter circuit, 5: control circuit, 21: voltage dividing unit, 51: bias detection unit, 511: difference calculation unit, 512: detection determination unit, 513: degree determination unit, 53: voltage control unit, 531: target correction unit, 71: first voltage detection unit, 72: second voltage detection unit

Claims

1. An inverter device that converts DC power output from a DC power source into AC power and outputs the AC power, a multilevel inverter circuit including a voltage dividing unit that divides a power supply voltage of the DC power supply to generate a neutral point potential; a control circuit that controls the multilevel inverter circuit so that the output voltage of the inverter device becomes a target value of the output voltage; It is equipped with The control circuit includes a bias detection unit that detects whether or not a bias in the neutral point potential has occurred, and when the bias detection unit detects the occurrence of a bias in the neutral point potential, the inverter device reduces the target value.

2. a first voltage detection unit that detects a first voltage that is a potential difference between a potential of a positive electrode of the DC power supply and the neutral point potential; a second voltage detection unit that detects a second voltage that is a potential difference between a potential of the negative electrode of the DC power supply and the neutral point potential; Furthermore, 2. The inverter device according to claim 1, wherein the bias detection unit determines that a bias in the neutral point potential has occurred when an absolute value of a voltage difference between the first voltage and the second voltage is equal to or greater than a threshold value.

3. the bias detection unit further detects a bias degree of the neutral point potential according to the magnitude of the voltage difference, The inverter device according to claim 2 , wherein the control circuit reduces the target value to a greater extent as the degree of bias of the neutral point potential increases.

4. the multilevel inverter circuit is a three-level inverter circuit in which the voltage of each phase has three levels of potential; the voltage dividing unit includes two capacitors connected in series between the positive and negative electrodes of the DC power supply, 4. The inverter device according to claim 1, wherein the capacitances of the two capacitors are the same.

5. 4. The inverter device according to claim 1, wherein the control circuit returns the target value from a reduced value to an original value when the bias detection unit detects the occurrence of bias in the neutral point potential and no longer detects the occurrence of bias in the neutral point potential.

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