Power Conversion Equipment

Abstract

To provide a power conversion apparatus capable of appropriately setting a limit value for an output current of an inverter.SOLUTION: A power conversion apparatus comprises an inverter and a control device. The inverter includes a plurality of switching elements to convert a DC voltage into an AC voltage. The control device controls a switching operation of the plurality of switching elements so that an AC current flowing in the inverter is accorded with a current command value. The control device includes a setting unit and a limiter circuit. The setting unit sets a limit value for the current command value depending on the DC voltage. The limiter circuit restricts the current command value within a range according to the limit value. The setting unit increases the limit value as the DC voltage becomes lower.SELECTED DRAWING: Figure 5

Description

[Technical field]

[0001] The present disclosure relates to a power conversion device. [Background technology]

[0002] Japanese Patent Laid-Open Publication No. 1-295680 (Patent Document 1) discloses an overcurrent protection method for an inverter. In Patent Document 1, the output current of the inverter is detected based on a two-stage detection standard, and when the output current exceeds a lower standard, the output current is suppressed. Also, when the output current exceeds the lower standard for a predetermined time or when the output current exceeds the upper standard, the inverter is stopped. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 1-295680 Summary of the Invention [Problem to be solved by the invention]

[0004] According to Patent Document 1, the overcurrent protection function of the inverter is improved by providing two levels of overcurrent detection levels. However, depending on the lower reference current level, while the inverter can be protected, there is a concern that it may be difficult to fully utilize the current drive capability of the semiconductor switching elements that constitute the inverter.

[0005] Therefore, a main object of the present disclosure is to provide a power conversion device capable of appropriately setting a limit value for the output current of an inverter. [Means for solving the problem]

[0006] A power conversion device according to an aspect of the present disclosure includes an inverter and a control device. The inverter includes a plurality of switching elements and converts a DC voltage into an AC voltage. The control device controls the switching operation of the plurality of switching elements so that an AC current flowing through the inverter matches a current command value. The control device includes a setting unit and a limiter circuit. The setting unit sets a limit value for the current command value according to the magnitude of the DC voltage. The limiter circuit limits the current command value within a range according to the limit value. The setting unit increases the magnitude of the limit value as the DC voltage decreases. Effect of the Invention

[0007] According to the present disclosure, it is possible to provide a power conversion device capable of appropriately setting a limit value for an inverter output current. [Brief description of the drawings]

[0008] [Figure 1] 1 is a circuit block diagram showing a configuration example of an uninterruptible power supply to which a power conversion device according to a first embodiment is applied. [Diagram 2] 2 is a circuit block diagram showing a configuration of the inverter shown in FIG. 1 and its peripheral parts. [Diagram 3] 3 is a diagram for explaining the operation of the inverter shown in FIG. 2. [Figure 4] 3 is a circuit block diagram showing a configuration of an inverter control circuit shown in FIG. 2. [Diagram 5] 5 is a circuit block diagram showing a configuration of a voltage command unit shown in FIG. 4. [Figure 6] 6 is a diagram for explaining a method of setting a current command limit value in a limit value setting unit shown in FIG. 5. FIG. [Figure 7] 6 is a diagram illustrating an example of the configuration of a limit value setting unit illustrated in FIG. 5. [Figure 8] 10 is a circuit block diagram showing a configuration of an inverter and its peripheral parts included in an uninterruptible power supply according to a second embodiment. FIG. [Figure 9] 9 is a circuit block diagram showing a configuration of an inverter control circuit shown in FIG. 8. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. In the drawings, the same or corresponding parts are designated by the same reference characters, and the description thereof will not be repeated in principle.

[0010] [Embodiment 1] <Applications of power conversion devices> Fig. 1 is a circuit block diagram showing a configuration example of an uninterruptible power supply 1 to which a power conversion device according to embodiment 1 is applied. The uninterruptible power supply 1 is configured to once convert three-phase AC power from a commercial AC power supply 21 into DC power, convert the DC power into three-phase AC power, and supply it to a load 24. In Fig. 1 and Fig. 2, for the sake of simplicity of the drawings and explanation, only a portion of the circuit corresponding to one phase (e.g., U phase) of the three phases (U phase, V phase, W phase) is shown.

[0011] 1, an uninterruptible power supply 1 includes an AC input terminal T1, a bypass input terminal T2, a battery terminal T3, and an AC output terminal T4. The AC input terminal T1 receives AC power of a commercial frequency from a commercial AC power supply 21. The bypass input terminal T2 receives AC power of a commercial frequency from a bypass AC power supply 22. The bypass AC power supply 22 may be a commercial AC power supply or a generator.

[0012] The battery terminal T3 is connected to a battery 23. The battery 23 stores DC power. The battery 23 corresponds to one embodiment of a "power storage device." A capacitor may be connected in place of the battery 23. The AC output terminal T4 is connected to a load 24. The load 24 is driven by AC power.

[0013] The uninterruptible power supply 1 further includes electromagnetic contactors 2, 8, 14, and 16, current detectors 3 and 11, capacitors 4, 9, and 13, reactors 5 and 12, a converter 6, a bidirectional chopper 7, an inverter 10, a semiconductor switch 15, an operation unit 17, and a control device 18.

[0014] The electromagnetic contactor 2 and the reactor 5 are connected in series between the AC input terminal T1 and the input node of the converter 6. The capacitor 4 is connected to a node N1 between the electromagnetic contactor 2 and the reactor 5. The electromagnetic contactor 2 is turned on when the uninterruptible power supply 1 is in use, and is turned off, for example, during maintenance of the uninterruptible power supply 1.

[0015] The instantaneous value of the AC input voltage Vi appearing at the node N1 is detected by the control device 18. The presence or absence of a power outage is determined based on the instantaneous value of the AC input voltage Vi. The current detector 3 detects the AC input current Ii flowing through the node N1, and provides the control device 18 with a signal Iif indicating the detected value.

[0016] Capacitor 4 and reactor 5 form a low-pass filter that passes commercial frequency AC power from commercial AC power supply 21 to converter 6 and prevents a switching frequency signal generated in converter 6 from passing to commercial AC power supply 21.

[0017] Converter 6 is controlled by control device 18, and during normal operation when AC power is supplied from commercial AC power supply 21, converts the AC power into DC power and outputs it to DC line L1. During a power outage when the supply of AC power from commercial AC power supply 21 is stopped, operation of converter 6 is stopped. The output voltage of converter 6 can be controlled to a desired value. Converter 6 corresponds to one embodiment of "converter".

[0018] A capacitor 9 is connected to the DC line L1 and smoothes the voltage of the DC line L1. An instantaneous value of a DC voltage VDC appearing on the DC line L1 is detected by a control device 18. The DC line L1 is connected to a high-voltage side node of a bidirectional chopper 7, and a low-voltage side node of the bidirectional chopper 7 is connected to a battery terminal T3 via an electromagnetic contactor 8.

[0019] The electromagnetic contactor 8 is turned on when the uninterruptible power supply 1 is in use, and is turned off, for example, during maintenance of the uninterruptible power supply 1 and the battery 23. The instantaneous value of the inter-terminal voltage VB of the battery 23 appearing at the battery terminal T3 is detected by the control device 18.

[0020] The bidirectional chopper 7 is controlled by the control device 18, and during normal operation, it stores the DC power generated by the converter 6 in the battery 23, and during a power outage, it supplies the DC power of the battery 23 to the inverter 10 via the DC line L1.

[0021] When storing DC power in the battery 23, the bidirectional chopper 7 steps down the DC voltage VDC of the DC line L1 and supplies it to the battery 23. When supplying DC power from the battery 23 to the inverter 10, the bidirectional chopper 7 steps up the terminal voltage VB of the battery 23 and outputs it to the DC line L1. The DC line L1 is connected to the input node of the inverter 10.

[0022] The inverter 10 is controlled by the control device 18, and converts DC power supplied from the converter 6 or the bidirectional chopper 7 via the DC line L1 into AC power of a commercial frequency and outputs the converted power. That is, the inverter 10 normally converts DC power supplied from the converter 6 via the DC line L1 into AC power, and during a power outage, converts DC power supplied from the battery 23 via the bidirectional chopper 7 into AC power. The output of the inverter 10 can be controlled to a desired value. The inverter 10 corresponds to one embodiment of "inverter".

[0023] An output node 10a of the inverter 10 is connected to a first terminal of a reactor 12, and a second terminal (node ​​N2) of the reactor 12 is connected to an AC output terminal T4 via an electromagnetic contactor 14. A capacitor 13 is connected to the node N2.

[0024] The current detector 3 detects the instantaneous value of the output current Io of the inverter 10, and provides a signal Iof indicating the detected value to the control device 18. The instantaneous value of the AC output voltage Vo appearing at the node N2 is detected by the control device 18.

[0025] Reactor 12 and capacitor 13 form a low-pass filter that allows commercial frequency AC power generated by inverter 10 to pass to AC output terminal T4 and prevents a switching frequency signal generated by inverter 10 from passing to AC output terminal T4.

[0026] The electromagnetic contactor 14 is controlled by a control device 18, and is turned on in an inverter power supply mode in which the AC power generated by the inverter 10 is supplied to a load 24, and is turned off in a bypass power supply mode in which the AC power from the bypass AC power supply 22 is supplied to the load 24.

[0027] The semiconductor switch 15 includes a thyristor and is connected between the bypass input terminal T2 and the AC output terminal T4. The electromagnetic contactor 16 is connected in parallel with the semiconductor switch 15. The semiconductor switch 15 is controlled by a control device 18, and is normally turned off, and is instantly turned on when the inverter 10 fails to supply AC power from the bypass AC power supply 22 to the load 24. The semiconductor switch 15 is turned off after a predetermined time has elapsed since it was turned on.

[0028] The electromagnetic contactor 16 is turned off in the inverter power supply mode and turned on in the bypass power supply mode. Moreover, the electromagnetic contactor 16 is turned on when the inverter 10 fails and supplies AC power from the bypass AC power supply 22 to the load 24. In other words, when the inverter 10 fails, the semiconductor switch 15 is instantly turned on for a predetermined time and the electromagnetic contactor 16 is turned on. This makes it possible to prevent the semiconductor switch 15 from being damaged due to overheating.

[0029] The operation unit 17 includes a plurality of buttons operated by a user of the uninterruptible power supply 1, a display that displays various information, etc. By operating the operation unit 17, the user can turn the power supply of the uninterruptible power supply 1 on and off, and select either the bypass power supply mode or the inverter power supply mode.

[0030] The control device 18 incorporates a processor 180 and a memory 182, and controls each device of the uninterruptible power supply 1 based on information stored in the memory 182 and information from each sensor. That is, the control device 18 detects whether or not a power outage has occurred based on the detected value of the AC input voltage Vi, and controls the converter 6 and the inverter 10 in synchronization with the phase of the AC input voltage Vi.

[0031] Furthermore, the control device 18 normally controls the converter 6 so that the DC voltage VDC becomes the desired reference voltage VDCr, and stops the operation of the converter 6 in the event of a power outage.

[0032] Furthermore, the control device 18 normally controls the bidirectional chopper 7 so that the inter-terminal voltage VB of the battery 23 becomes the desired reference voltage VBr, and during a power outage, controls the bidirectional chopper 7 so that the DC voltage VDC becomes the desired reference voltage VDCr.

[0033] Next, a description will be given of the operation of the uninterruptible power supply 1. When the inverter power supply mode is selected during normal operation, the semiconductor switch 15 and the electromagnetic contactor 16 are turned off, and the electromagnetic contactors 2, 8, and 14 are turned on.

[0034] AC power supplied from a commercial AC power source 21 is converted into DC power by the converter 6. The DC power generated by the converter 6 is stored in a battery 23 by a bidirectional chopper 7 and is also supplied to the inverter 10. The inverter 10 converts the DC power supplied from the converter 6 into AC power and supplies it to a load 24. The load 24 is driven by the AC power supplied from the inverter 10.

[0035] When the supply of AC power from the commercial AC power source 21 is stopped, that is, when a power outage occurs, the operation of the converter 6 is stopped and DC power from the battery 23 is supplied to the inverter 10 by the bidirectional chopper 7. The inverter 10 converts the DC power from the bidirectional chopper 7 into AC power and supplies it to the load 24. Therefore, while the DC power is stored in the battery 23, the operation of the load 24 can be continued.

[0036] Furthermore, if the inverter 10 fails during the inverter power supply mode, the semiconductor switch 15 is instantly turned on, the electromagnetic contactor 14 is turned off, and the electromagnetic contactor 16 is turned on. This allows AC power from the bypass AC power supply 22 to be supplied to the load 24 via the semiconductor switch 15 and the electromagnetic contactor 16, and the operation of the load 24 continues. After a certain period of time, the semiconductor switch 15 is turned off, preventing the semiconductor switch 15 from being damaged due to overheating.

[0037] Fig. 2 is a circuit block diagram showing the configuration of the inverter 10 and its peripheral parts shown in Fig. 1. In Fig. 2, a positive-side DC line L1 and a negative-side DC line L2 are connected between the converter 6 and the inverter 10. A capacitor 9 is connected between the DC lines L1 and L2.

[0038] Under normal circumstances, the converter 6 converts the AC input voltage Vi from the commercial AC power supply 21 into a DC voltage VDC and outputs it between the DC lines L1 and L2. During a power outage, the operation of the converter 6 is stopped, and the bidirectional chopper 7 boosts the terminal voltage VB of the battery 23 and outputs the DC voltage VDC between the DC lines L1 and L2.

[0039] The inverter 10 includes IGBTs (Insulated Gate Bipolar Transistors) Q1-Q4 and diodes D1-D4. The IGBTs Q1-Q4 correspond to an example of a "switching element." As the switching elements, any self-extinguishing switching element such as a MOSFET (Metal Oxide Semiconductor Field Effect Transistor) can be used other than the IGBT.

[0040] The collectors of IGBTs Q1 and Q3 are both connected to a DC line L1, and their emitters are connected to output nodes 10a and 10b, respectively. The collectors of IGBTs Q2 and Q4 are both connected to output nodes 10a and 10b, and their emitters are both connected to a DC line L2. Diodes D1 to D4 are connected in anti-parallel to IGBTs Q1 to Q4, respectively. Each of diodes D1 to D4 is provided to pass a freewheel current when the corresponding IGBT is off. When the switching element is a MOSFET, the freewheel diode is formed of a parasitic diode (body diode).

[0041] An output node 10a of the inverter 10 is connected to a node N2 via a reactor 12, and an output node 10b is connected to a neutral point NP. A capacitor 13 is connected between the node N2 and the neutral point NP.

[0042] IGBTQ1, Q4 and IGBTQ2, Q3 are alternately turned on. When IGBTQ1, Q3 are turned on and IGBTQ2, Q3 are turned off, the positive terminal (DC line L1) of capacitor 9 is connected to output node 10a via IGBTQ1, and output node 10b is connected to the negative terminal (DC line L2) of capacitor 13 via IGBTQ4, and the voltage between the terminals of capacitor 9 is output between output nodes 10a and 10b. That is, a positive DC voltage is output between output nodes 10a and 10b.

[0043] When IGBTs Q2 and Q3 are turned on and IGBTs Q1 and Q4 are turned off, the positive terminal (DC line L1) of capacitor 9 is connected to output node 10b via IGBT Q3, and output node 10a is connected to the negative terminal (DC line L2) of capacitor 9 via IGBT Q2, and the voltage across the terminals of capacitor 9 is output between output nodes 10b and 10a. That is, a negative DC voltage is output between output nodes 10a and 10b.

[0044] The on / off of the above-mentioned IGBTs Q1 to Q4 is controlled by an inverter control circuit 30. The inverter control circuit 30 is included in the control device .

[0045] Here, we will explain problems in the operation of the inverter 10. Figure 3 is a diagram for explaining the operation of the inverter 10 shown in Figure 2.

[0046] 3, the dashed line indicates the waveform of a current command value Io* which is a target value of the output current Io of the inverter 10. As will be described later, the current command value Io* is generated based on a reference AC voltage Vor. The current command value Io* is a sinusoidal signal of commercial frequency, and its phase is synchronized with the phase of the reference AC voltage Vor of the corresponding phase (here, U phase) of the three phases (U phase, V phase, W phase).

[0047] 3, the solid line shows the waveform of the output current Io (here, U-phase current) of the inverter 10 detected by the current detector 11. The lower part of Fig. 3 shows the waveform of the output voltage Vo between the output nodes 10a and 10b of the inverter 10.

[0048] In the inverter control circuit 30, a voltage command value is generated so that the deviation ΔIo between the current command value Ior and the output current Io detected by the current detector 11 becomes 0, and the on / off of the IGBTs Q1 to Q4 of the inverter 10 is controlled based on the generated voltage command value.

[0049] However, since the reactor 12 is connected to the output nodes 10a and 10b of the inverter 10, the output current Io of the inverter 10 has a waveform in which a high-frequency (corresponding to a switching frequency) ripple current Iripple is superimposed on a sine wave signal according to the current command value Ior. Therefore, as shown in Fig. 3, the positive peak value of the output current Io is larger than the positive peak value of the current command value Io* by the amount of the ripple current Iripple. Also, the negative peak value of the output current Io is smaller than the negative peak value of the current command value Io*.

[0050] In the inverter 10, a limit value of the output current Io (hereinafter also referred to as the "output current limit value") is set to a current value lower than an overcurrent detection level determined according to the rated current of the IGBT as a measure to prevent an overcurrent from flowing through the IGBTs Q1 to Q4 and damaging the IGBTs Q1 to Q4. The output current limit value is defined by an upper limit value (positive value) of the output current Io and a lower limit value (negative value) of the output current Io.

[0051] 3, Iovr indicates the positive side overcurrent detection level, and Io_lim indicates the positive side output current limit value (i.e., the upper limit value of the output current Io). Although not shown in the figure, the negative side output current limit value Io_lim (i.e., the lower limit value of the output current Io) is greater than the negative side overcurrent detection level Iovr.

[0052] In controlling the inverter 10, the output current Io of the inverter 10 is limited according to the output current limit value Io_lim, thereby suppressing the occurrence of an overcurrent in the IGBT. Specifically, in the inverter control circuit 30, a limit value of the current command value Io* (hereinafter also referred to as a "current command limit value") is set in advance according to the output current limit value Io_lim. The inverter control circuit 30 is configured to limit the current command value Io* within a range according to this current command limit value. As a result, the output current Io is controlled according to the limited current command value Io*, and as a result, it is possible to keep the output current Io within a range according to the output current limit value Io_lim.

[0053] How to set the current command limit value is important in order to realize such an overcurrent protection function while maximizing the current drive capability of the switching elements of the inverter 10. This embodiment provides a method for setting the current command limit value.

[0054] In the inverter 10, as the DC voltage VDC applied to the input node increases, the peak value of the sine wave signal in the output current Io increases. Therefore, even when the inverter 10 is operated under the maximum DC voltage VDC assumed for the operation of the uninterruptible power supply 1, it is necessary to set the current command limit value so that the output current Io according to the current command value Io* does not exceed the range according to the output current limit value Io_lim.

[0055] 3, the output current Io of the inverter 10 includes a ripple current Iripple. The ripple current Iripple can be expressed by the following equation (1). Iripple=VDC / (2L)×1 / fsw / 2 (1) In the formula (1), L is the inductance of the reactor 12, and fsw is the switching frequency of the IGBTs Q1 to Q4.

[0056] As can be seen from equation (1), the magnitude of the ripple current Iripple is determined by the DC voltage VDC and the inductance L. However, since the inductance L is a fixed value, the ripple current Iripple changes solely in response to the DC voltage VDC. As the DC voltage VDC increases, the ripple current Iripple also increases.

[0057] In this way, as the DC voltage VDC increases, the peak value of the sine wave signal in the output current Io increases, and the ripple current Iripple also increases. Therefore, it is required to set the current command limit value so that the output current Io does not exceed the output current limit value Io_lim even under the maximum DC voltage VDC in operation.

[0058] On the other hand, in the operation of the uninterruptible power supply 1, the DC voltage VDC is usually controlled to a voltage value lower than the maximum voltage assumed in the above-mentioned operation. Therefore, the peak value of the sine wave signal and the ripple current Iripple are both smaller in current value than when the DC voltage VDC is at the maximum voltage. Therefore, when the current command limit value is set in consideration of the maximum voltage of the DC voltage VDC as described above, the output current Io of the inverter 10 is designed to have a sufficient margin with respect to the output current limit value Io_lim in actual operation. In other words, the problem is that the current driving capability of the switching element of the inverter 10 cannot be maximized.

[0059] Therefore, in this embodiment, the current command limit value is variably set according to the magnitude of the DC voltage VDC, thereby making it possible to maximize the current driving capability of the switching elements of the inverter 10 while realizing the overcurrent protection function.

[0060] FIG. 4 is a circuit block diagram showing a configuration of inverter control circuit 30 shown in FIG.

[0061] 4, inverter control circuit 30 includes a voltage command unit 31, a triangular wave generator 32, a comparator 33, a buffer 34, an inverter 35, and gate drive circuits 36 and 37. These functional configurations are realized by processor 181 executing a predetermined program in control device 18 shown in FIG.

[0062] The voltage command unit 31 generates a sinusoidal voltage command value Vo# based on the instantaneous value of the AC output voltage Vo appearing at the node N2 (FIG. 1) and the output signal Iof of the current detector 11 (FIG. 1). The phase of the voltage command value Vo# is synchronized with the phase of the AC input voltage Vi of the corresponding phase (here, the U phase) among the three phases (U phase, V phase, and W phase).

[0063] The triangular wave generator 32 outputs a triangular wave signal Cu1 having a frequency sufficiently higher than the commercial frequency. The comparator 33 compares the voltage command value Vo# from the voltage command unit 31 with the triangular wave signal Cu1 from the triangular wave generator 32, and outputs a PWM (Pulse Width Modulation) signal Au1 indicating the comparison result. The buffer 34 provides the PWM signal Au1 to the gate drive circuits 36 and 37. The inverter 35 inverts the PWM signal Au1 to generate a PWM signal Bu1 and provides it to the gate drive circuits 36 and 37.

[0064] The gate drive circuit 36 ​​generates gate drive signals VG1 and VG2 for turning on and off the IGBTs Q1 and Q2 based on the PWM signals Au1 and Bu1. The gate drive circuit 37 generates gate drive signals VG3 and VG4 for turning on and off the IGBTs Q3 and Q4 based on the PWM signals Au1 and Bu1.

[0065] FIG. 5 is a circuit block diagram showing a configuration of the voltage command unit 31 shown in FIG.

[0066] 5, voltage command unit 31 includes a reference voltage generating unit 40, a voltage control unit 42, a limiter circuit 44, a subtractor 46, a current control unit 48, an adder 50, a voltage detector 56, and a limit value setting unit 60.

[0067] The reference voltage generating unit 40 generates a three-phase reference AC voltage Vor. The reference AC voltage Vor of each phase is a sine wave signal of a commercial frequency. The voltage control unit 42 generates a three-phase current command value Io* based on the reference AC voltage Vor from the reference voltage generating unit 40.

[0068] The limiter circuit 44 generates a current command value Io# by limiting the current command value Io* generated by the voltage control unit 42 to a range according to the current command limit value Io*_lim. The range according to the current command limit value Io*_lim is specified by an upper limit value (+Io*_lim) and a lower limit value (-Io*_lim).

[0069] In the limiter circuit 44, when the peak value (absolute value) of the current command value Io* is equal to or smaller than the current command limit value Io*_lim, Io* becomes Io# as is. When the peak value (absolute value) of the current command value Io* is greater than the current command limit value Io*_lim, Io*_lim becomes the peak value (absolute value) of Io#.

[0070] The subtractor 46 obtains a deviation ΔIo=Io#-Io between the current command value Io# and the output current Io detected by the current detector 11. The current control unit 48 generates a voltage command value Vo* so that the deviation ΔIo becomes 0. The current control unit 48 generates the voltage command value Vo* by amplifying the deviation ΔIo according to proportional control or proportional-integral control, for example.

[0071] An adder 50 adds the voltage command value Vo* and the detection value of the AC output voltage Vo to generate a voltage command value Vo#.

[0072] The voltage detector 56 detects an instantaneous value of the DC voltage VDC appearing on the DC line L1. The limit value setting unit 60 sets a current command limit value Io*_lim and provides it to the limiter circuit 44. The limit value setting unit 60 is configured to set the current command limit value Io*_lim based on the detection value of the DC voltage VDC by the voltage detector 56, as described below.

[0073] FIG. 6 is a diagram for explaining a method of setting the current command limit value Io*_lim in the limit value setting unit 60 shown in FIG.

[0074] An example of the relationship between the current command limit value Io*_lim and the DC voltage VDC is shown in Fig. 6. In addition to the above relationship, the overcurrent detection level Iovr and the output current limit value Io_lim are also shown in Fig. 5. In Fig. 6, the current command limit value Io*_lim, the overcurrent detection level Iovr, and the output current limit value Io_lim are all shown as absolute values.

[0075] The overcurrent detection level Iovr is determined according to the rated current of a switching element (for example, an IGBT) constituting the inverter 10. When the magnitude of the output current Io detected by the current detector 11 exceeds the overcurrent detection level Iovr, the control device 18 (inverter control circuit 30) stops the operation of the inverter 10 by turning off all of the IGBTs Q1 to Q4. Furthermore, the control device 18 stops the operation of the converter 6, instantly turns on the semiconductor switch 15, turns off the electromagnetic contactor 14, and turns on the electromagnetic contactor 16. This switches the uninterruptible power supply 1 from the inverter power supply mode to the bypass power supply mode. The control device 18 turns off the semiconductor switch 15 after a certain time.

[0076] The output current limit value Io_lim is set to a value that prevents the output current Io of the inverter 10 from exceeding the overcurrent detection level Iovr, taking into consideration the ripple current Iripple at the maximum DC voltage VDC (maximum voltage VDC_max in the figure) during operation of the uninterruptible power supply 1.

[0077] 6, the current command limit value Io*_lim is variably set according to the magnitude of the DC voltage VDC. Specifically, a minimum value Ili_min of the current command limit value Io*_lim is set corresponding to the maximum DC voltage VDC (corresponding to the maximum voltage VDC_max in the figure) during operation of the uninterruptible power supply 1. This minimum value Ilim_min is set so that the magnitude (absolute value) of the peak value of the output current Io when the DC voltage VDC is the maximum voltage VDC_max is equal to or less than the magnitude (absolute value) of the output current limit value Io_lim.

[0078] As described above, the ripple current Iripple contained in the output current Io is determined by the inductance L of the reactor 12 and the switching frequency fsw of the switching elements in the inverter 10. Therefore, the minimum value Ilim_min can be set appropriately according to the maximum operational voltage VDC_max of the uninterruptible power supply 1, the inductance L of the reactor 12, and the switching frequency fsw.

[0079] The current command limit value Io*_lim is set so that the current value increases as the DC voltage VDC decreases from the maximum voltage VDC_max. Thus, as the DC voltage VDC increases toward the maximum voltage VDC, the current command limit value Io*_lim decreases toward the minimum value Ilim_min.

[0080] In the relationship shown in Figure 6, the magnitude of the current command limit value Io*_lim for each DC voltage VDC is set so that the magnitude (absolute value) of the peak value of the output current Io of the inverter 10 receiving the DC voltage VDC at its input node is less than or equal to the magnitude (absolute value) of the output current limit value Io_lim.

[0081] 6, when the DC voltage VDC is lower than the maximum voltage VDC_max, the current command limit value Io*_lim is set to a value greater than the minimum value Ilim_min, and the limiter circuit 44 relaxes the restriction on the current command value Io*. As a result, as the DC voltage VDC decreases, the amplitude of the sine wave contained in the output current Io of the inverter 10 can be increased within the range defined by the output current limit value Io_lim. This makes it possible to utilize the current drive capability of the switching elements constituting the inverter 10 while realizing an overcurrent protection function.

[0082] FIG. 7 is a diagram showing an example of the configuration of the limit value setting unit 60 shown in FIG.

[0083] 7, limit value setting unit 60 has a subtractor 62, a multiplier 64, and an adder 66. Subtractor 62 receives DC voltage VDC detected by voltage detector 56 and maximum voltage VDC_max. Maximum voltage VDC_max is the maximum voltage during operation of uninterruptible power supply 1. Subtractor 62 obtains deviation ΔVDC=VDC-VDC_max between DC voltage VDC and maximum voltage VDC_max.

[0084] A multiplier 64 calculates a correction amount ΔIlim of the current command limit value Io*_lim by multiplying the deviation ΔVDC by a coefficient k. The coefficient k corresponds to the gradient of the straight line representing the relationship between the DC voltage VDC and the current command limit value Io*_lim shown in FIG.

[0085] An adder 66 calculates the current command limit value Io*_lim by adding the correction amount ΔIlim to the minimum value Ilim_min of the current command limit value Io*_lim.

[0086] Note that, although Figures 6 and 7 show an example in which the current command limit value Io*_lim is changed (decreased) linearly in response to an increase in the DC voltage VDC, the current command limit value Io*_lim may be changed non-linearly (for example, in a curve) or in a stepwise manner in response to a change in the DC voltage VDC.

[0087] Furthermore, the control device (inverter control circuit 30) can store data indicating the relationship between the current command limit value Io*_lim and the DC voltage VDC in an internal memory. The limit value setting unit 60 can set the current command limit value Io*_lim based on the detection value of the DC voltage VDC by referring to the data. The data may be in the form of a table or a function.

[0088] As described above, according to the power conversion device of the first embodiment, the limit value Io*_lim of the current command value of the output current of the inverter 10 is set to increase as the DC voltage VDC input to the inverter 10 decreases. This makes it possible to maximize the current driving capability of the inverter 10 while realizing the overcurrent protection function.

[0089] [Embodiment 2] 8 is a circuit block diagram showing an inverter 10A and its peripheral parts included in an uninterruptible power supply according to embodiment 2. The uninterruptible power supply according to embodiment 2 differs from the uninterruptible power supply 1 according to embodiment 1 in that the converter 6, the bidirectional chopper 7, the inverter 10, and the inverter control circuit 30 are replaced with a converter 6A, a bidirectional chopper 7A, an inverter 10A, and an inverter control circuit 30A, respectively.

[0090] 8, three DC lines L1 to L3 are connected between the converter 6A and the inverter 10A. The DC line L2 is connected to a neutral point NP and set to a neutral point voltage (for example, 0 V). The capacitor 9 includes two capacitors 9a and 9b. The capacitor 9a is connected between the DC lines L1 and L2. The capacitor 9b is connected between the DC lines L2 and L3.

[0091] During normal operation when AC power is supplied from the commercial AC power supply 21, the converter 6A converts the AC power from the commercial AC power supply 21 into DC power and supplies it to the DC lines L1 to L3. At this time, the converter 6A charges each of the capacitors 9a and 9b so that the DC voltage VDCa between the DC lines L1 and L2 becomes the reference voltage VDCr, and the DC voltage VDCb between the DC lines L2 and L3 becomes the reference voltage VDCr.

[0092] The voltages of the DC lines L1, L2, and L3 are set to a positive DC voltage (+VDCr), a neutral point voltage (0V), and a negative DC voltage (-VDCr), respectively. In the event of a power outage in which the supply of AC power from the commercial AC power supply 21 is stopped, the operation of the converter 6A is stopped.

[0093] In normal operation, the bidirectional chopper 7A stores the DC power generated by the converter 6A in the battery 23. At this time, the bidirectional chopper 7A charges the battery 23 so that the terminal voltage VB of the battery 23 becomes equal to the reference voltage VBr.

[0094] During a power outage, the bidirectional chopper 7A supplies DC power from the battery 23 to the inverter 10A. At this time, the bidirectional chopper 7A charges each of the capacitors 9a and 9b so that each of the inter-terminal voltages VDCa and VDCb of the capacitors 9a and 9b becomes a reference voltage VDCr.

[0095] Under normal conditions, inverter 10A converts the DC power generated by converter 6A into AC power of commercial frequency and supplies it to load 24. At this time, inverter 10A generates AC output voltage Vo of commercial frequency based on the positive DC voltage, neutral point voltage, and negative DC voltage supplied from DC lines L1 to L3.

[0096] Inverter 10A includes IGBTs Q21-Q24 and diodes D21-D24. The collector of IGBT Q21 is connected to DC line L1, and its emitter is connected to output node 10a. The collectors of IGBTs Q22 and Q24 are connected to each other, and their emitters are connected to DC line L2 and output node 10a, respectively. The collector of IGBT Q23 is connected to output node 10a, and its emitter is connected to DC line L3. Diodes D21-D24 are connected in anti-parallel to IGBTs Q21-Q24, respectively. Output node 10a is connected to node N2 via reactor 12.

[0097] In inverter 10A, during a first period, IGBTs Q23 and Q24 are turned off and on, respectively, and IGBTs Q21 and Q22 are alternately turned on, and during a second period, IGBTs Q21 and Q22 are turned off and on, respectively, and IGBTs Q23 and Q24 are alternately turned on.

[0098] Fig. 9 is a circuit block diagram showing a configuration of inverter control circuit 30A shown in Fig. 8. Inverter control circuit 30A is included in control device 18.

[0099] 9, inverter control circuit 30A includes a voltage command unit 31, triangular wave generators 82 and 83, comparators 84 and 85, buffers 86 and 87, inverters 88 and 89, and gate drive circuits 90 and 91. These functional configurations are realized by processor 181 executing a predetermined program in control device 18 shown in FIG.

[0100] The voltage command unit 31 generates a sinusoidal voltage command value Vo# based on the instantaneous value of the AC output voltage Vo appearing at the node N2 (FIG. 1) and the output signal Iof of the current detector 11 (FIG. 1). The voltage command unit 31 has a configuration similar to that of the voltage command unit 31 (see FIG. 5) in the first embodiment. In the second embodiment, the voltage detector 56 detects a DC voltage VDC which is the sum of the inter-terminal voltages VDCa and VDCb of the capacitors 9a and 9b.

[0101] The triangular wave generator 82 outputs a triangular wave signal Cu1a having a frequency sufficiently higher than the commercial frequency. The triangular wave generator 83 outputs a triangular wave signal Cu1b having the same phase and frequency as the triangular wave signal Cu1a.

[0102] The comparator 84 compares the voltage command value Vo# from the voltage command unit 31 with the triangular wave signal Cu1a from the triangular wave generator 82, and outputs a PWM signal φ1 indicating the comparison result. The buffer 86 provides the PWM signal φ1 to a gate drive circuit 90. The inverter 88 inverts the PWM signal φ1 to generate a PWM signal φ2 and provides it to the gate drive circuit 90.

[0103] The comparator 85 compares the voltage command value Vo# from the voltage command unit 31 with the triangular wave signal Cu1b from the triangular wave generator 83, and outputs a PWM signal φ3 indicating the comparison result. The buffer 87 provides the PWM signal φ3 to a gate drive circuit 91. The inverter 89 inverts the PWM signal φ3 to generate a PWM signal φ4 and provides it to the gate drive circuit 91.

[0104] Gate drive circuit 90 generates gate drive signals VG21, VG22 for turning on and off IGBTs Q21, Q22 based on PWM signals φ1, φ2. Gate drive circuit 91 generates gate drive signals VG23, VG24 for turning on and off IGBTs Q23, Q24 based on PWM signals φ3, φ4.

[0105] As described above, also in the second embodiment, the limit value Io*_lim of the current command value of the output current of the inverter 10A is set to increase as the DC voltage VDC input to the inverter 10A decreases. Therefore, the same effects as those in the first embodiment can be obtained.

[0106] The embodiments disclosed herein should be considered to be illustrative and not restrictive in all respects. The scope of the present invention is defined by the claims, not the above description, and is intended to include all modifications within the meaning and scope of the claims. [Explanation of symbols]

[0107] 1 Uninterruptible power supply, 2,8,14,16 Electromagnetic contactor, 3,11 Current detector, 4,9,13 Capacitor, 5,12 Reactor, 6,6A Converter, 7,7A Bidirectional chopper, 10,10A Inverter, 15 Semiconductor switch, 17 Operation unit, 18 Control device, 23 Battery, 30,30A Inverter control circuit, 31 Voltage command unit, 32,82,83 Triangular wave generator, 33,84,85 Comparator, 34,86,87 Buffer, 35,88,89 Inverter, 36,37,90,91 Gate drive circuit, 40 Reference voltage generation unit, 42 Voltage control unit, 44 Limiter circuit, 46,62 Subtractor, 48 Current control unit, 50,66 Adder, 60 Limit value setting unit, 64 Multiplier, 181 Processor, 182 Memory, T1 AC input terminal, T2 bypass input terminal, T3 battery terminal, T4 AC output terminal, Q1 to Q4, Q21 to Q24 IGBTs, D1 to D4, D21 to D24 diodes, L1 to L3 DC lines, Io output current, Iripple ripple current, Iovr overcurrent detection level, Io_lim output current limit value, Io*_lim current command limit value.

Claims

1. A power conversion device, an inverter including a plurality of switching elements for converting a DC voltage into an AC voltage; a reactor connected between an output node of the inverter and an AC output terminal; a control device that controls switching operations of the plurality of switching elements so that an output current of the inverter coincides with a current command value; The control device includes: a setting unit that sets a limit value of the current command value in accordance with the magnitude of the DC voltage; a limiter circuit that limits the current command value within a range that conforms to the limit value, a setting unit that sets a minimum value of the limit value based on a maximum value of the DC voltage during operation of the power conversion device, an inductance of the reactor, and a switching frequency of the multiple switching elements, and increases the magnitude of the limit value from the minimum value of the limit value as the DC voltage decreases from the maximum value.

2. a voltage detector for detecting the DC voltage, The power conversion device according to claim 1 , wherein the setting unit sets the limit value by adding a correction amount corresponding to a deviation between the detection value of the voltage detector and the maximum value to the minimum value of the limit value.

3. The power conversion device according to claim 1 , further comprising a converter that converts an AC voltage into a DC voltage and outputs the DC voltage to the inverter.

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