Power conditioner

Abstract

To provide a power conditioner that prevents damage to a power supply connected to a faulty circuit while suppressing capacitor overvoltage.SOLUTION: The power conditioner includes a control device, a first DC / DC converter connected to a first power supply and having a first switching element, a second DC / DC converter connected to a power supply different from the first power supply and having a second switching element, an inverter, a capacitor connected in parallel between the DC / DC converter and the inverter, and a diode preventing a current flow from the inverter to the DC / DC converter. The control device detects the current flowing toward the first power source in the first DC / DC converter and, further, when the voltage of the capacitor is at or above a first voltage threshold, supplies a second control signal to the second switching element such that the period during which the second switching element is turned on becomes longer.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a power conditioner. [Background technology]

[0002] Conventionally, a power conditioner that converts DC power from solar cells into AC power of a utility frequency connected to a grid has been used in a solar power generation system. Also, a solar cell string, which is made up of a group of directly connected solar cell panels, is connected in parallel to the power conditioner in a multi-input type solar power generation system (see Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 2019-161892 Summary of the Invention [Problem to be solved by the invention]

[0004] In a power conditioner in such a multi-input solar power generation system, if a fault occurs in a circuit connected to a certain string and current is allowed in the reverse direction from normal, power is input from other strings to the solar cells connected to that circuit via the faulted circuit. Furthermore, in the power conditioner, a capacitor is placed between the DC / DC converter on the solar cell string side and the inverter on the load side. If current is allowed in the reverse direction, current also flows into this capacitor, which could cause the capacitor voltage to rise above the allowable range.

[0005] One aspect of the disclosed technology aims to provide a power conditioner that can prevent damage to a power supply connected to a faulty circuit and suppress capacitor overvoltage. [Means for solving the problem]

[0006] One aspect of the disclosed technology is exemplified by the following power conditioner: The power conditioner includes a control device, a converter set including at least one first DC / DC converter connected to a first power supply and having a first switching element that is on / off controlled in response to a control signal from the control device, and a second DC / DC converter connected to a power supply other than the first power supply and connected in parallel to the first DC / DC converter and having a second switching element that is on / off controlled in response to the control signal from the control device, an inverter connected to the first DC / DC converter and the second DC / DC converter, a capacitor connected in parallel between the first DC / DC converter and the inverter, and a diode arranged in series between the first DC / DC converter and the second DC / DC converter and the capacitor, for preventing a current from flowing from the inverter to the first DC / DC converter and the second DC / DC converter. When the control device detects a current flowing toward the first power supply in the first DC / DC converter, it sets the control signal to be supplied to the second switching element to a first control signal having a first duty ratio at which the current flowing toward the first power supply is equal to or less than a predetermined current value, and when the control device detects the current flowing toward the first power supply in the first DC / DC converter and the voltage of the capacitor is equal to or greater than a first voltage threshold, it supplies to the second switching element a second control signal that lengthens the period during which the second switching element is turned on compared to when the first control signal is supplied.

[0007] When the power conditioner detects a current flowing toward the first power source, the power conditioner supplies the control signal to the second switching element as the first control signal, which reduces the current flowing toward the first power source to a predetermined current value or less. Because the current flowing toward the first power source is reduced to the predetermined current value or less, damage to the first power source and a voltage increase in the capacitor due to the inflow of the current flowing toward the first power source are suppressed. Furthermore, when the voltage of the capacitor is equal to or greater than a first voltage threshold, the power conditioner supplies the second switching element with a second control signal, which lengthens the period during which the second switching element is turned on compared to when the first control signal is supplied. Supplying the second control signal to the second switching element further reduces the current flowing toward the first power source. Therefore, the power conditioner suppresses damage to the power source and a voltage increase in the capacitor. Here, the first power source and the another power source may be solar cells.

[0008] The power conditioner may further include the following feature: The control device continues supplying the second control signal to the second switching element until the voltage of the capacitor becomes equal to or lower than a second threshold value that is lower than the first voltage threshold value. The power conditioner having such a feature can prevent a process from being repeated, in which the control signal supplied to the second switching element is returned from the second control signal to the first control signal duty ratio, and then immediately returned to the second control signal.

[0009] The power conditioner may further include the following feature: The control device continues supplying the second control signal to the second switching element for a predetermined period after starting supply of the second control signal to the second switching element. The power conditioner having such a feature can further reduce the voltage of the capacitor by supplying the second control signal to the second switching element for the predetermined period.

[0010] The power conditioner may further include the following feature: The duty ratio of the second control signal is 100%. By setting the duty ratio of the second control signal to 100%, current flow from the second DC / DC converter to the first power supply and the capacitor is prohibited. Therefore, the power conditioner can more easily reduce the voltage of the capacitor. [Effects of the Invention]

[0011] According to the disclosed technology, it is possible to provide a power conditioner that can prevent damage to a power supply connected to a faulty circuit and suppress capacitor overvoltage. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 1 is a schematic configuration diagram of a solar cell system including a power conditioner, which is an example of a power conversion device according to this embodiment. [Figure 2] FIG. 2 shows a boost chopper, which is an example of a DC / DC converter. [Figure 3] FIG. 3 is a first diagram schematically illustrating a current flow when a short-circuit failure occurs in the diodes of some of the boost choppers. [Figure 4] FIG. 4 is a second diagram schematically illustrating a current flow when a short-circuit failure occurs in the diodes of some of the boost choppers. [Figure 5] FIG. 5 is a diagram illustrating an example of a processing flow of the power conditioner according to the embodiment. [Figure 6] FIG. 6 is a diagram illustrating a change in the DC link voltage in the embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0013] <Application example> Hereinafter, application examples of the present invention will be described with reference to the drawings. For example, the present invention is applied to a device shown in FIG. This invention is applied to a power conditioner equipped with boost choppers 2-1 to 2-4, which are an example of DC / DC converters connected in parallel as shown in the figure. In boost chopper 2-1, current flows in the direction of solid arrow A2 due to freewheel diode 25-1 under normal conditions, but if freewheel diode 25-1 shorts out, there is a possibility that current (hereinafter also referred to as reverse current) will flow in the opposite direction to normal conditions (towards solar cell string PV-1 in Figure 1) as shown by dashed arrow A3.

[0014] If an excessive current flows into the solar cell connected to the boost chopper 2-1 in the direction of the dashed arrow A3, it may be damaged by heat generation, etc. However, depending on the characteristics of the solar cell, if the current or power flows into the solar cell, it will not necessarily be damaged, as long as it is below a certain amount.

[0015] In this application example, the current or power flowing into the solar cell string PV-1 is suppressed to a first threshold value that is as small as possible, thereby preventing damage to the solar cells and enabling the use of power from other solar cells. One control method for achieving this condition is to set the duty ratio of the PWM signals that drive and control the IGBTs 24-2 to 24-4 of the boost choppers 2-2 to 2-4 to a duty ratio (first duty ratio) that makes the reverse current equal to or smaller than the first threshold value.

[0016] 1, in this application example, a capacitor 6 that smoothes the outputs of the boost choppers 2-1 to 2-4 is arranged in parallel between the output sides of the boost choppers 2-1 to 2-4 and the inverter 3. Furthermore, a diode 7 that suppresses current flow from the load side to the photovoltaic strings PV-1 to PV-4 when switching of the IGBTs 24-1 to 24-4 is not being performed is arranged between the output sides of the boost choppers 2-1 to 2-4 and the capacitor 6.

[0017] When a current flows in the direction of dashed arrow A3 due to a short-circuit fault in freewheeling diode 25-1, current also flows into capacitor 6, as shown by arrow A1 in FIG. 1 . Then, because the discharge of capacitor 6 is prevented by diode 7, the voltage of capacitor 6 rises. If the current continues to flow into capacitor 6, there is a risk that an overvoltage will occur across capacitor 6. Hereinafter, in this specification, the voltage of capacitor 6 will also be referred to as the DC link voltage.

[0018] Therefore, in this application example, when the DC link voltage becomes equal to or higher than a predetermined voltage threshold, the duty ratio of the PWM signal that drives and controls the IGBTs 24-2 to 24-4 of the boost choppers 2-2 to 2-4 is set to a second duty ratio that is higher than the first duty ratio, thereby suppressing the flow of current into the capacitor 6 and, ultimately, suppressing the rise in the DC link voltage.

[0019] <Embodiment> Hereinafter, an embodiment will be described with reference to the drawings. Fig. 1 is a schematic configuration diagram of a solar cell system including a power conditioner 1 which is an example of a power conversion device according to this embodiment. A plurality of solar cell strings PV-1 to PV-4 are connected to the power conditioner 1. The power conditioner 1 includes DC / DC converters 2-1 to 2-4 which convert DC voltages output from the solar cell strings PV-1 to PV-4, and an inverter 3 which converts the converted DC voltages into AC voltage. When the solar cell strings PV-1 to PV-4 are not distinguished from one another, they are also referred to as solar cell strings PV. When the DC / DC converters 2-1 to 2-4 are not distinguished from one another, they are also referred to as DC / DC converters 2.

[0020] The power conditioner 1 further includes a relay 5, which is an example of a switching means for switching a circuit that connects the output from the inverter 3 to a commercial power supply or a load (not shown). The power conditioner 1 includes the DC / DC converters 2-1 to 2-4, the inverter 3, and the relay 5. In FIG. 1, four photovoltaic strings PV-1 to PV-4 are connected to the power conditioner 1, but the number of photovoltaic strings connected in parallel is not limited to this, and it is sufficient that there is at least a plurality.

[0021] Furthermore, in this embodiment, a capacitor 6 that smoothes the output of the DC / DC converters 2-1 to 2-4 is arranged in parallel between the output sides of the DC / DC converters 2-1 to 2-4 and the inverter 3. A voltage sensor 8 that measures the voltage of the capacitor 6 (i.e., the DC link voltage) is provided for the capacitor 6. The output of the voltage sensor 8 is input to the control device 4. Furthermore, between the output sides of the DC / DC converters 2-1 to 2-4 and the capacitor 6, a diode 7 that suppresses current flow from the load side to the photovoltaic strings PV-1 to PV-4 when switching of the IGBTs 24-1 to 24-4 is not being performed is arranged on the N side of the photovoltaic strings PV-1 to PV-4.

[0022] FIG. 2 shows a boost chopper, which is an example of the DC / DC converters 2-1 to 2-4. In FIG. 2, connections to the photovoltaic string PV-1 and the inverter 3 are omitted. The DC / DC converter 2 is a mechanism that converts the DC voltage of the photovoltaic string PV to a predetermined DC voltage and adjusts the operating point of the photovoltaic string PV, and is not limited to a boost chopper. The boost chopper 2-1 is connected to the P side and N side of the photovoltaic string PV-1, and a current sensor 21-1 that detects the direction and magnitude of the current is provided on the P side. The output of the current sensor 21-1 is input to the control device 4. A capacitor 22-1 is connected in parallel to the output side of the current sensor 21-1, and an inductor 23-1 is connected in series to the P side.

[0023] An IGBT 24-1, which is an example of a switching element, is connected in parallel to the output side of the inductor 23-1. An anti-parallel diode is connected to the IGBT 24-1. A freewheeling diode 25-1, with the output side of the solar cell string PV-1 as the forward direction, is connected in series to the P side of the output side of the IGBT 24-1, and a capacitor 26-1 is connected in parallel to the output side. The gate of the IGBT 24-1 is connected to the control device 4 via a drive circuit 27-1, and a PWM control signal is supplied. The other boost choppers 2-2 to 2-4 have the same configuration, so they are denoted by the same reference numerals and their description will be omitted. Note that, although the capacitor 22-1 is connected to the output side of the current sensor 21-1 in FIG. 2, the current sensor 21-1 may be connected in series to the inductor 23-1 after the capacitor 22-1.

[0024] When the boost chopper 2-1 is in a normal state, the freewheel diode 25-1 allows current to flow from the photovoltaic string PV-1 to the output side as indicated by the solid arrow A2, and prevents reverse current from flowing from the output side to the photovoltaic string PV-1. However, if a short circuit occurs in the freewheel diode 25-1 of the boost chopper 2-1 for some reason, a current in the opposite direction to the normal state as indicated by the dashed arrow A3 is allowed to flow from the output side to the photovoltaic string PV-1. In such a case, if the IGBTs 24-2 to 24-4 in the other boost choppers 2-2 to 2-4 are driven as in a normal state or switching is stopped, there is a possibility that the power output from the other boost choppers 2-2 to 2-4 will be input to the photovoltaic string PV-1 side through the boost chopper 2-1 as indicated by the dashed arrow A3.

[0025] If the power input to the solar cell string PV-1 is excessive, it may be damaged by heat generation. Furthermore, if a short circuit occurs in the freewheeling diode 25-1 of the boost chopper 2-1, the currents output from the other boost choppers 2-2 to 2-4 will also flow into the capacitor 6, as shown by the arrow A1 in Fig. 1. Because the discharge of the capacitor 6 is prevented by the diode 7, the voltage of the capacitor 6 will increase due to the current flowing into the capacitor 6, and there is a risk that the capacitor 6 will become overvoltage.

[0026] The following describes a case where a short-circuit fault occurs in the freewheel diode 25-1 of the boost chopper 2-1, but the same applies when a short-circuit fault occurs in the diode of any of the boost choppers, and also when a system in which three or more solar cell strings are connected has multiple diodes that have short-circuit faults while at least one or more other chopper units are operating normally. Information about the occurrence of the short-circuit fault and the location of the fault may be displayed on a display unit (not shown) provided in the power conditioner 1 or on a display unit of a terminal such as a PC connected via a network. Here, the converter set 9 includes DC / DC converters 2-1 to 2-4, i.e., the boost choppers 2-1 to 2-4.

[0027] When a reverse current is detected due to a short-circuit fault in the freewheeling diode 25-1 of the boost chopper 2-1, the control device 4 controls the duty ratio of the PWM control signal supplied to the IGBTs 24-2 to 24-4 of the boost choppers 2-2 to 2-4 to suppress the reverse current. FIG. 3 is a first diagram that schematically shows the current flow when a short-circuit fault occurs in the diodes of some of the boost choppers. FIG. 3 also shows the current flow when the duty ratio of the PWM signal supplied to the IGBTs of the normally operating boost choppers is set to 100% when a short-circuit fault occurs in the diodes of some of the boost choppers. In FIG. 3, the boost choppers 2-3 to 2-4 are omitted from the drawing to avoid complication.

[0028] 3, the duty ratio of the PWM signal supplied to the IGBT 24-2 of the boost chopper 2-2 is set to 100%. As a result, a current flows as indicated by an arrow A4, suppressing the current flow from the boost chopper 2-2 to the boost chopper 2-1, and ultimately suppressing the current flow to the capacitor 6. Therefore, the DC link voltage gradually decreases as the power stored in the capacitor 6 flows to the inverter 3 and the like.

[0029] Fig. 4 is a second diagram that schematically illustrates the current flow when a short-circuit fault occurs in the diode of one of the boost choppers. Fig. 4 schematically illustrates the current flow when a short-circuit fault occurs in the diode of one of the boost choppers and the duty ratio of the PWM signal supplied to the IGBT of the boost chopper that is operating normally is set to a first duty ratio. In Fig. 4, as in Fig. 3, the boost choppers 2-3 to 2-4 are omitted from the illustration to avoid complication of the drawing.

[0030] The first duty ratio may be set to a high value within the range in which the IGBT can be switched. The high duty ratio within the range in which the IGBT can be switched varies depending on the IGBT product. The high duty ratio within the range in which the IGBT can be switched is, for example, 98% to 99%.

[0031] In the example of Fig. 4, as shown by arrow A6, current flows from boost chopper 2-2 to boost chopper 2-1. Because a high duty ratio is set within the switching range, the current flowing from boost chopper 2-2 to boost chopper 2-1 is lower than during normal operation of the power conditioner 1. Part of the current flowing into boost chopper 2-1 flows to photovoltaic string PV-1, and the other part flows to capacitor 6. As a result, the DC link voltage gradually rises.

[0032] When the DC link voltage increases, the power conditioner 1 suppresses current flow to the boost chopper 2-1 in which a short-circuit fault has occurred, as shown in Fig. 3, and when the DC link voltage decreases, the power conditioner 1 allows current flow to the boost chopper 2-1 in which a short-circuit fault has occurred, as shown in Fig. 4. By switching between these control modes, the power conditioner 1 supplies power from the photovoltaic strings PV-1 to PV-4 to the load while suppressing increases in the DC link voltage.

[0033] <Control method> 5 is a diagram showing an example of a processing flow of the power conditioner 1 according to the embodiment. Hereinafter, an example of a processing flow of the power conditioner 1 will be described with reference to FIG.

[0034] In step S1, it is determined whether a reverse current has occurred. The control device 4 detects the current (including the direction) using, for example, the current sensors 21-1 to 21-4 of the step-up choppers 2-1 to 2-4. The control device 4 determines whether a reverse current has occurred based on the direction of the detected current. If a reverse current has occurred (YES in step S1), the process proceeds to step S2. If a reverse current has not occurred (NO in step S1), the process of step S1 is repeated.

[0035] In step S2, the control device 4 sets the duty ratio of the PWM signals supplied to the IGBTs 24-2 to 24-4 of the boost choppers 2-2 to 2-4 to a first duty ratio. The first duty ratio is set, for example, as high as possible within a range in which the IGBTs 24-2 to 24-4 can be switched. The first duty ratio is, for example, 98% to 99%.

[0036] In step S3, the control device 4 detects the DC link voltage by means of the voltage sensor 8. The control device 4 determines whether the detected DC link voltage is equal to or higher than a first voltage threshold. The first voltage threshold is appropriately determined according to, for example, the voltage that the capacitor 6 can withstand. If it is equal to or higher than the first voltage threshold (YES in step S3), the process proceeds to step S4. If it is lower than the first voltage threshold (NO in step S3), the process of step S3 is repeated.

[0037] In step S4, the control device 4 sets the duty ratio of the PWM signal supplied to the IGBTs 24-2 to 24-4 of the boost choppers 2-2 to 2-4 to a second duty ratio that is larger than the first duty ratio. The second threshold is, for example, 100%.

[0038] In step S5, the control device 4 determines whether the DC link voltage is equal to or lower than a second voltage threshold that is lower than the first voltage threshold, or whether a predetermined period has elapsed since the duty ratio of the PWM signal supplied to the IGBTs 24-2 to 24-4 of the boost choppers 2-2 to 2-4 was set to be equal to or higher than the second threshold. The second voltage threshold is determined based on, for example, the voltage at which the capacitor 6 can be stably operated. If the DC link voltage is equal to or lower than the second voltage threshold, or if the predetermined period has elapsed (YES in step S5), the process proceeds to step S2. If neither the condition that the DC link voltage is equal to or lower than the second voltage threshold nor the condition that the predetermined period has elapsed is satisfied (NO in step S5), the process of step S5 is repeated.

[0039] <Change in DC link voltage> FIG. 6 is a diagram illustrating the change in the DC link voltage in the embodiment. The vertical axis in FIG. 6 illustrates voltage, and the horizontal axis illustrates time. As can be understood by referring to FIG. 6, the first voltage threshold is set lower than the overvoltage of the capacitor 6, and the second voltage threshold is set lower than the first voltage threshold.

[0040] During the period T1, the control device 4 detects the reverse current and sets the duty ratio of the PWM signals supplied to the IGBTs 24-2 to 24-4 of the boost choppers 2-2 to 2-4 to the first duty ratio. Due to the occurrence of the reverse current, the DC link voltage rises.

[0041] During period T2, the DC link voltage reaches the first voltage threshold, and the control device 4 sets the duty ratio of the PWM signals supplied to the IGBTs 24-2 to 24-4 of the boost choppers 2-2 to 2-4 to a second duty ratio. By setting the duty ratio to the second duty ratio, current flow from the boost choppers 2-2 to 2-4 to the boost chopper 2-1 is prohibited, and the DC link voltage drops.

[0042] During period T3, since a predetermined period has elapsed since the duty ratio was set to the second duty ratio, the control device 4 sets the duty ratio of the PWM signals supplied to the IGBTs 24-2 to 24-4 of the boost choppers 2-2 to 2-4 to the first duty ratio. By setting the duty ratio to the first duty ratio, current flows from the boost choppers 2-2 to 2-4 to the boost chopper 2-1, and the DC link voltage increases.

[0043] During a period T4, the DC link voltage reaches the first voltage threshold, and the control device 4 sets the duty ratio of the PWM signals supplied to the IGBTs 24-2 to 24-4 of the boost choppers 2-2 to 2-4 to a second duty ratio. By setting the duty ratio to the second duty ratio, current flow from the boost choppers 2-2 to 2-4 to the boost chopper 2-1 is prohibited, and the DC link voltage drops.

[0044] <Effects of the embodiment> In this embodiment, when a reverse current occurs, the duty ratio of the PWM signals supplied to the IGBTs 24-2 to 24-4 of the boost choppers 2-2 to 2-4 is set to a first duty ratio, thereby reducing the current flowing into the photovoltaic string PV-1 and the capacitor 6 to a first threshold or less. Therefore, according to this embodiment, it is possible to prevent a breakdown of the photovoltaic string PV-1 and to make the voltage rise of the capacitor 6 gentler.

[0045] In this embodiment, when the DC link voltage exceeds the first voltage threshold due to a reverse current, the duty ratio of the PWM signals supplied to the IGBTs 24-2 to 24-4 of the boost choppers 2-2 to 2-4 is set to a second duty ratio, thereby suppressing current flow from the boost choppers 2-2 to 2-4 to the boost chopper 2-1. As a result, no current flows into the capacitor 6 either, and the voltage of the capacitor 6 can be reduced.

[0046] Here, the second duty ratio is preferably 100%. By setting the second duty ratio to 100%, current flow from the boost choppers 2-2 to 2-4 to the boost chopper 2-1 is prohibited, and the voltage of the capacitor 6 can be effectively reduced.

[0047] <Modification> In the embodiment, when a short-circuit fault occurs in the freewheel diode 25-1 of the boost chopper 2-1, the duty ratio of the PWM signal supplied to the IGBTs 24-2 to 24-4 of the boost choppers 2-2 to 2-4 is set to the second duty ratio. However, the duty ratio of the PWM signal supplied to all the IGBTs 24-2 to 24-4 of the normal boost choppers 2-2 to 2-4 is not limited to the second duty ratio, and the duty ratio of the PWM signal supplied to some of the IGBTs 24-2 to 24-4 of all the normal boost choppers 2-2 to 2-4 may be set to the second duty ratio. Even when such a configuration is adopted, the current flowing into the capacitor 6 can be suppressed.

[0048] In the embodiment, the photovoltaic strings PV-1 to PV-4 are given as examples of power sources connected to the power conditioner 1, but the power sources connected to the power conditioner 1 are not limited to the photovoltaic strings PV-1 to PV-4.

[0049] In the embodiment, when the DC link voltage reaches the first voltage threshold, the duty ratio of the PWM signals supplied to the IGBTs 24-2 to 24-4 of the boost choppers 2-2 to 2-4 is set to a second duty ratio higher than the first duty ratio. When the DC link voltage reaches the first voltage threshold, the duty ratio is not limited to being higher than the first duty ratio, but may be set such that the period during which the IGBTs 24-2 to 24-4 are turned on within a predetermined period is longer than when a PWM signal with the first duty ratio is supplied.

[0050] Below, aspects of the disclosed technology are further described with reference to symbols. <Appendix 1> a control device (4); a first DC / DC converter (2-1) connected to a first power source (PV-1) and having a first switching element (24-1) that is on / off controlled in response to a control signal from the control device (4); a converter set (9) including at least one second DC / DC converter (2-2 to 2-4) each connected to a power supply (PV-2 to PV-4) different from the first power supply (PV-1) and connected in parallel with the first DC / DC converter (2-1), the second DC / DC converter having a second switching element (24-2 to 24-4) that is on / off controlled in response to the control signal from the control device (4); an inverter (3) connected to the first DC / DC converter (2-1) and the second DC / DC converters (2-2 to 2-4); a capacitor (6) connected in parallel between the first DC / DC converter (2-1) and the second DC / DC converters (2-2 to 2-4) and the inverter (3); a diode (7) arranged in series between the first DC / DC converter (2-1) and the second DC / DC converters (2-2 to 2-4) and the capacitor (6), for preventing a current from flowing from the inverter (3) to the first DC / DC converter (2-1) and the second DC / DC converters (2-2 to 2-4), The control device (4) when the first DC / DC converter (2-1) detects a current flowing toward the first power supply (PV-1), the control signal to be supplied to the second switching elements (24-2 to 24-4) is set to a first control signal having a first duty ratio such that the current flowing toward the first power supply (PV-1) is equal to or less than a predetermined current value; a current flowing toward the first power source (PV-1) is detected in the first DC / DC converter (2-1), and when the voltage of the capacitor (6) is equal to or higher than a first voltage threshold, a second control signal is supplied to the second switching elements (24-2 to 24-4) such that a period during which the second switching elements are turned on is longer than when the first control signal is supplied; Power conditioner (1). <Appendix 2> The control device (4) The supply of the second control signal to the second switching elements (24-2 to 24-4) is continued until the voltage of the capacitor (6) becomes equal to or lower than a second threshold value that is lower than the first voltage threshold value. A power conditioner (1) as described in Appendix 1. <Appendix 3> The control device (1) continuing to supply the second control signal to the second switching elements (24-2 to 24-4) for a predetermined period after starting to supply the second control signal to the second switching elements (24-2 to 24-4); A power conditioner (1) as described in Appendix 1. <Appendix 4> The duty ratio of the second control signal is 100%. 1. A power conditioner (1) as defined in appendix 1 or 2. <Appendix 5> The first power source and the other power source include solar cells (PV-1 to PV-4). 1. A power conditioner (1) according to any one of appendices 1 to 4. [Explanation of symbols]

[0051] 1. Power conditioner 2-1 DC / DC converter 2-2 DC / DC Converter 2-3 DC / DC converter 2-4 DC / DC Converter 4. Control device 5. Relay 6. Capacitor 7. Diode 8. Voltage sensor 9 Converter Set 21-1 Current sensor 21-2 Current Sensor 21-3 Current Sensor 21-4 Current Sensor 22-1 Capacitor 22-2 Capacitor 22-3 Capacitor 22-4 Capacitor 23-1 Inductor 23-2 Inductor 23-3 Inductor 23-4 Inductor 24-1 IGBT 24-2 IGBT 24-3 IGBT 24-4 IGBT 25-1 Freewheeling diode 25-2 Freewheeling diode 25-3 Freewheeling Diode 25-4 Freewheeling Diode 26-1 Capacitor 26-2 Capacitor 26-3 Capacitor 26-4 Capacitor 27-1 Drive circuit 27-2 Drive circuit 27-3 Drive circuit 27-4 Drive circuit 3. Inverter PV-1 Solar Cell String PV-2 Solar Cell String PV-3 Solar Cell String PV-4 Solar Cell String

Claims

1. a control device; a first DC / DC converter connected to a first power supply and having a first switching element that is on / off controlled in response to a control signal from the control device; a converter set including at least one second DC / DC converter connected to a power supply different from the first power supply and connected in parallel with the first DC / DC converter, the second DC / DC converter having a second switching element that is on / off controlled in response to the control signal from the control device; an inverter connected to the first DC / DC converter and the second DC / DC converter; a capacitor connected in parallel between the first DC / DC converter and the inverter, and between the second DC / DC converter and the inverter; a diode disposed in series between the first DC / DC converter and the capacitor, and between the second DC / DC converter and the capacitor, for preventing a current from flowing from the inverter to the first DC / DC converter and the second DC / DC converter; The control device when the first DC / DC converter detects a current flowing toward the first power supply, the control signal to be supplied to the second switching element is set to a first control signal having a first duty ratio at which the current flowing toward the first power supply is equal to or less than a predetermined current value; a second control signal is supplied to the second switching element when the voltage of the capacitor is equal to or higher than a first voltage threshold, the second control signal being supplied to the second switching element such that the second switching element is kept on for a longer period than when the first control signal is supplied; Power conditioner.

2. The control device continuing to supply the second control signal to the second switching element until the voltage of the capacitor becomes equal to or lower than a second threshold value that is lower than the first voltage threshold value; The power conditioner according to claim 1 .

3. The control device continuing to supply the second control signal to the second switching element for a predetermined period after starting to supply the second control signal to the second switching element; The power conditioner according to claim 1 .

4. The duty ratio of the second control signal is 100%. The power conditioner according to claim 1 .

5. the first power source and the other power source include solar cells; The power conditioner according to any one of claims 1 to 4.

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