Boost connection circuit, power conversion system

Abstract

To provide a boost connection circuit capable of precisely determining the operating state of a power converter connected to a rear stage.SOLUTION: A boost circuit (11) is capable of boosting a voltage of DC power output from a second DC power supply (PV2) having an open circuit voltage lower than a first DC power supply (PV1). A confluent output wire (Wm) is a wire in which an output wire (Wb) of the boost circuit (11) and an output wire (W1) of the first DC power supply (PV1) are merged, and is connected to a power converter (20) that converts DC power supplied from a boost connection circuit (10) to AC power. A control unit (12) is configured to determine whether the power converter (20) is in operation or in stop according to a current output from the boost connection circuit (10) detected by a current sensor (A2).SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present disclosure relates to a boost connection circuit and a power conversion system for integrating outputs of a plurality of DC power supplies with different output voltages. [Background technology]

[0002] As renewable energy has been gaining attention, the use of photovoltaic power generation systems is expanding. When using an indoor centralized power conditioner in a photovoltaic power generation system in which multiple solar cell strings are installed, a junction box is installed in front of the power conditioner to integrate the direct current generated by the multiple solar cell strings.

[0003] Depending on the shape of the roof, etc., it may not be possible to standardize the number of solar cell modules that make up each of the multiple solar cell strings. In such cases, a junction box with a boost function is installed in front of the power conditioner. The junction box with a boost function has the function of boosting the voltage of a solar cell string with a small number of solar cell modules connected in series to the voltage of a standard solar cell string and integrating the direct current of each solar cell string (for example, see Patent Documents 1 and 2). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2016-10232 A [Patent Document 2] JP 2013-218503 A Summary of the Invention [Problem to be solved by the invention]

[0005] A junction box with a boost function needs to perform boost operation when the downstream power conditioner is operating, but needs to stop boost operation when the downstream power conditioner is stopped. If the junction box with a boost function performs boost operation while the power conditioner is stopped, the output voltage of the junction box with a boost function will rise to the limiter value, leading to deterioration of the components inside the junction box with a boost function and an input overvoltage error of the power conditioner.

[0006] The present disclosure has been made in consideration of these circumstances, and has an object to provide a boost connection circuit and a power conversion system that can determine with high accuracy the operating state of a power conversion device connected in a downstream stage. [Means for solving the problem]

[0007] In order to solve the above problems, a boost connection circuit according to an embodiment of the present disclosure includes a boost circuit capable of boosting a voltage of DC power output from a second DC power source having an open circuit voltage lower than that of a first DC power source, an output wiring of the boost circuit, a junction output wiring in which the output wiring of the first DC power source is junctioned, a control unit for controlling the boost circuit, and a current sensor for detecting a current output from the boost connection circuit. The junction output wiring is connected to a power conversion device that converts the DC power supplied from the boost connection circuit into AC power, and the control unit determines whether the power conversion device is operating or stopped according to the current detected by the current sensor. Effect of the Invention

[0008] According to the present disclosure, it is possible to realize a boost connection circuit that can determine with high accuracy the operating state of a power conversion device connected downstream. [Brief description of the drawings]

[0009] [Figure 1] 1 is a diagram illustrating a configuration example of a solar power generation system according to an embodiment. [Diagram 2] FIG. 2 is a diagram illustrating an example of a circuit configuration of a boost circuit. [Diagram 3]FIG. 1 is a diagram showing the power-voltage characteristics (PV curve) of a solar cell module. [Figure 4] FIG. 11 is a diagram for explaining a method for determining whether a booster is in a power conditioning operation state according to a first comparative example. [Diagram 5] FIG. 11 is a diagram for explaining a method for determining whether a booster is in a power conditioning operation state according to a second comparative example. [Figure 6] 10A to 10C are diagrams for explaining a method for determining whether or not a booster is in operation, according to an embodiment. [Figure 7] 5 is a flowchart showing a flow of a booster power conditioning operation determination process according to an embodiment. [Figure 8] FIG. 1 is a diagram showing a configuration example of a solar power generation system according to a first modified example. [Figure 9] FIG. 11 is a diagram illustrating a configuration example of a solar power generation system according to a second modification. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] Fig. 1 is a diagram showing a configuration example of a photovoltaic power generation system according to an embodiment. The photovoltaic power generation system shown in Fig. 1 is a centralized photovoltaic power generation system, and includes a plurality of solar cell strings PV1 and PV2 and a power conversion system 1. The power conversion system 1 includes a junction box with a boost function (hereinafter, appropriately referred to as a boost junction box or simply a booster) 10 and a power conversion device 20.

[0011] The first solar cell string PV1 includes a plurality of solar cell modules (solar panels) connected in series. The second solar cell string PV2 includes solar cell modules having a smaller number of series connections than the first solar cell string PV1. For example, the first solar cell string PV1 may include five solar cell modules, and the second solar cell string PV2 may include three solar cell modules.

[0012] Each solar cell module includes a number of solar cells connected in series. The solar cells can convert light energy directly into DC power by utilizing the photovoltaic effect. The solar cells can be heterojunction solar cells, polycrystalline silicon solar cells, single crystal silicon solar cells, thin film silicon solar cells, compound solar cells, etc.

[0013] The boost connection box 10 includes a boost circuit 11, a control unit 12, a first input terminal IN1, a second input terminal IN2, a first switch RY1, a second switch RY2, a backflow prevention diode D1, an output terminal OUT, an input voltage sensor V1, an input current sensor A1, an output voltage sensor V2, an output current sensor A2 and a temperature sensor T1.

[0014] The second solar cell string PV2 has a smaller number of series connections than the first solar cell string PV1, and therefore has a lower open circuit voltage than the first solar cell string PV1. The first solar cell string PV1 is connected to a first input terminal IN1, and power input to the first input terminal IN1 is supplied to an output wiring W1. A first switch RY1 and a backflow prevention diode D1 are connected to the output wiring W1. The second solar cell string PV2 is connected to a second input terminal IN2, and power input to the second input terminal IN2 is supplied to the output wiring W2 via the second switch RY2, and is further connected to a boost circuit 11.

[0015] The output wiring W1 of the first solar cell string PV1 and the output wiring Wb of the boost circuit 11 are joined at a terminal block (not shown), and a joined output wiring Wm connects the terminal block to an input terminal of the power conversion device 20. Hereinafter, in this specification, the circuit between the first solar cell string PV1 and the terminal block is referred to as a standard circuit system, and the circuit between the second solar cell string PV2 and the terminal block is referred to as a boost circuit system.

[0016] The boost circuit 11 is a DC / DC converter capable of boosting the voltage of the DC power output from the second solar cell string PV2. The boost circuit 11 boosts the output voltage of the second solar cell string PV2 to the voltage of a standard circuit system, thereby integrating the output power of the first solar cell string PV1 and the output power of the second solar cell string PV2 and supplying the integrated power to the power conversion device 20 via the output terminal OUT.

[0017] The input voltage sensor V1 detects the voltage of the output wiring W2 of the second solar cell string PV2 and outputs it to the control unit 12. The input current sensor A1 detects the current flowing in the output wiring W2 of the second solar cell string PV2 and outputs it to the control unit 12. The output voltage sensor V2 detects the voltage of the junction output wiring Wm and outputs it to the control unit 12. The output current sensor A2 detects the current flowing in the junction output wiring Wm and outputs it to the control unit 12. The temperature sensor T1 detects the temperature inside the boost junction box 10 and outputs it to the control unit 12.

[0018] The input voltage sensor V1 and the output voltage sensor V2 include, for example, a voltage dividing resistor and a differential amplifier. The input current sensor A1 and the output current sensor A2 include, for example, a CT sensor or a Hall sensor. The temperature sensor T1 includes, for example, a thermocouple or a thermistor.

[0019] The control unit 12 controls the boost circuit 11 based on the voltage, current, and temperature input from each sensor. The control unit 12 can determine whether the output voltage of the second solar cell string PV2 is equal to or higher than a voltage at which boosting can be started, based on the voltage detected by the input voltage sensor V1. The control unit 12 can optimize the power generation of the second solar cell string PV2 by controlling the boost operation of the boost circuit 11 and controlling the voltage detected by the input voltage sensor V1 and the current detected by the input current sensor A1 (details will be described later).

[0020] Using the temperature detected by temperature sensor T1, control unit 12 monitors whether the temperature of each component in boost junction box 10 exceeds an allowable range due to the boost operation of boost circuit 11. If the detected temperature exceeds the allowable range during the boost operation of boost circuit 11, control unit 12 stops the boost operation of boost circuit 11 to protect the components in boost junction box 10. Even when the boost operation of boost circuit 11 is stopped, if the detected temperature exceeds the allowable range, control unit 12 does not start the boost operation of boost circuit 11 until the detected temperature falls within the allowable range.

[0021] The control unit 12 can be realized by a combination of hardware and software resources, or by hardware resources alone. Examples of hardware resources that can be used include analog elements, microcontrollers, DSPs, ROMs, RAMs, ASICs, FPGAs, and other LSIs. Examples of software resources that can be used include programs such as firmware.

[0022] Fig. 2 is a diagram showing an example of a circuit configuration of the boost circuit 11. The boost circuit 11 shown in Fig. 2 is a boost chopper including an input capacitor C1, a reactor L1, a diode D2, a switching element S1, and an output capacitor C2.

[0023] An input voltage sensor V1 and a smoothing input capacitor C1 are connected between the positive and negative wiring of the output wiring W2 of the second solar cell string PV2. A reactor L1 is inserted in the positive wiring of the output wiring W2 of the second solar cell string PV2. An input current sensor A1 is installed on the negative wiring of the output wiring W2 of the second solar cell string PV2. It may also be installed on the positive wiring.

[0024] A switching element S1 and a smoothing output capacitor C2 are connected between the positive and negative wiring of the output wiring Wb of the boost circuit 11. A diode D2 is connected in series to the positive wiring between the switching element S1 and the output capacitor C2 of the boost circuit 11. A reactor L1 is connected to the node between the switching element S1 and the diode D2. The diode D2 prevents a reverse current from flowing from the output side of the boost circuit 11.

[0025] The switching element S1 may be an insulated gate bipolar transistor (IGBT) or a metal-oxide-semiconductor field-effect transistor (MOSFET). The reactor L1 stores and releases energy based on the output current from the second solar cell string PV2 according to the on / off state of the switching element S1.

[0026] The control unit 12 can control the boost ratio by controlling the on / off ratio (duty ratio) of the switching element S1. The control unit 12 can perform MPPT (Maximum Power Point Tracking) control of the boost circuit 11 so that the output power (power generation) of the second solar cell string PV2 is maximized.

[0027] 3 is a diagram showing the power-voltage characteristics (PV curve) of a solar cell module. In the voltage range between the open circuit voltage Voc of the solar cell module and the maximum output operating voltage Vpm at which the module operates at the maximum output power Pmax, the output power P increases as the operating voltage V is lowered. In the voltage range below the maximum output operating voltage Vpm, the output power P decreases as the operating voltage V is lowered. In MPPT control, the operating voltage V is controlled so as to maintain the maximum output power Pmax.

[0028] The control unit 12 detects the output power of the second photovoltaic string PV2 based on the output voltage of the second photovoltaic string PV2 detected by the input voltage sensor V1 and the output current of the second photovoltaic string PV2 detected by the input current sensor A1. The control unit 12 generates a voltage command value for maximizing the output power of the second photovoltaic string PV2 based on the relationship between the output voltage and output power of the second photovoltaic string PV2.

[0029] For example, the control unit 12 searches for an operating point of the maximum output power Pmax by changing the operating voltage V by a predetermined step width according to a hill-climbing method. For example, on the left side of the operating point of the maximum output power Pmax in Fig. 3, a voltage command value is generated for shifting the current operating voltage V to the right, and on the right side of the operating point of the maximum output power Pmax, a voltage command value is generated for shifting the current operating voltage V to the left. When the operating point of the maximum output power Pmax is captured, the control unit 12 generates a voltage command value so as to maintain the operating point of the maximum output power Pmax. The switching element S1 of the boost circuit 11 performs switching operation in response to a drive signal based on the generated voltage command value.

[0030] Returning to Fig. 1, the power conversion device 20 is a power conditioner that converts DC power into AC power in a solar power generation system. The power conversion device 20 includes a DC / DC converter 21, an inverter 22, and a control unit 23.

[0031] The DC / DC converter 21 is a converter capable of adjusting the voltage of the DC power integrated by the boost junction box 10. For example, a boost chopper as shown in Fig. 2 can be used as the DC / DC converter 21. The DC / DC converter 21 is connected to the inverter 22 via a DC bus.

[0032] The inverter 22 can convert the DC power supplied from the DC / DC converter 21 into AC power and output the converted AC power to a commercial power system (hereinafter referred to as system) 2 via a distribution board (not shown). Note that a load (not shown) in the house is connected to the distribution board, and the inverter 22 can also supply the converted AC power to the load via the distribution board.

[0033] The control unit 23 performs overall control of the entire power conversion device 20. The control unit 23 can be realized by a combination of hardware resources and software resources, or by hardware resources alone. Analog elements, microcontrollers, DSPs, ROMs, RAMs, ASICs, FPGAs, and other LSIs can be used as hardware resources. Programs such as firmware can be used as software resources.

[0034] The control unit 23 performs MPPT control of the DC / DC converter 21 so that the integrated output power of the first solar cell string PV1 and the second solar cell string PV2 is maximized. The control unit 23 controls the inverter 22 so that the voltage of the DC bus between the DC / DC converter 21 and the inverter 22 maintains a target value. Specifically, the control unit 23 generates a current command value for matching the voltage of the DC bus with the target value. When the voltage of the DC bus is higher than the target value, the control unit 23 generates a current command value for increasing the output power of the inverter 22, and when the voltage of the DC bus is lower than the target value, the control unit 23 generates a current command value for decreasing the output power of the inverter 22. The inverter 22 performs switching operation in response to a drive signal based on the generated current command value.

[0035] When the power conversion device 20 (hereinafter referred to as the power conditioner) is operating, the booster needs to perform boost operation to supply the power generated by the second solar cell string PV2 to the power conditioner, but when the power conditioner is stopped, the boost operation needs to be stopped.

[0036] If the booster operates without considering the state of the inverter, the voltage will repeatedly rise and fall inside the booster, leading to deterioration of the components inside the booster. Also, if the input voltage of the inverter rises due to an unnecessary increase in the output voltage of the booster, an input overvoltage error may be issued on the inverter side.

[0037] Therefore, the booster needs to check whether the boost start conditions inside the booster are satisfied, as well as detect the operating state of the inverter to check whether the inverter is operating. The boost start conditions inside the booster include, for example, whether the input voltage of the second solar cell string PV2 is equal to or higher than the voltage that starts boosting, and whether the component temperatures inside the booster are within the normal temperature range.

[0038] Normally, the inverter and the booster are not connected by communication or signal lines to notify the inverter's operating status, but only by a DC power line. A method can be considered to determine the inverter's operating status based on the voltage state of this DC power line.

[0039] 4 is a diagram for explaining a method for determining whether or not a booster is in operation according to Comparative Example 1. In the following explanation, it is assumed that the boost start condition inside the booster is satisfied. When the power conditioner is stopped, the voltage detected by the output voltage sensor V2 is usually the open circuit voltage Voc of the first solar cell string PV1.

[0040] As shown in Figures 3 and 4, when the inverter operates and starts outputting power to grid 2, the input voltage of the inverter drops due to MPPT control. Since the input voltage of the inverter is almost equal to the output voltage of the booster, the input voltage of the inverter, which is controlled by the inverter, is detected by the output voltage sensor V2 of the booster.

[0041] When the voltage detected by the output voltage sensor V2 drops from the open circuit voltage Voc of the first solar cell string PV1 and falls below the judgment voltage for a certain period of time or longer, the booster determines that the power conditioner has operated and starts boost operation.

[0042] FIG. 5 is a diagram for explaining a method of determining whether or not the inverter is operating in a booster according to Comparative Example 2. When the inverter is stopped as described above, the output voltage sensor V2 detects the open circuit voltage Voc of the first solar cell string PV1. In Comparative Example 2, the control unit 12 of the booster causes the boost circuit 11 to temporarily perform a boost operation. When the boost operation starts, the output voltage of the booster detected by the output voltage sensor V2 rises to the limiter value Vlim of the boost circuit 11, and the boost operation stops. The control unit 12 temporarily stops the boost operation of the boost circuit 11, and restarts the boost operation after a few minutes. When the boost operation is repeatedly stopped and restarted, the booster determines that the inverter is stopped.

[0043] When the inverter starts to operate, it starts to output power to grid 2, and the input voltage of the inverter drops due to MPPT control. When the booster starts boost operation in this state, the output voltage of the booster does not rise to the limiter value Vlim of the boost circuit 11, and the boost operation can continue. When the boost operation continues, the booster determines that the inverter is operating.

[0044] The inverter detects the open circuit voltage Voc of the first solar cell string PV1 and reflects the detected open circuit voltage Voc in control such as MPPT control. If the booster performs boost operation while the inverter is stopped, the input voltage of the inverter will rise up to the limiter value Vlim of the output voltage of the boost circuit 11, which may result in erroneous recognition of the actual open circuit voltage Voc of the first solar cell string PV1.

[0045] In the inverter operation determination methods according to Comparative Examples 1 and 2, the inverter may malfunction due to an erroneous recognition of the input voltage. Examples of inverter malfunctions include a malfunction of the input overvoltage detection function, a malfunction of MPPT control due to an erroneous recognition of the open circuit voltage Voc, and a malfunction of stopping or restoring output control (also called output suppression) due to an erroneous recognition of the input voltage.

[0046] Furthermore, in the inverter operation determination method of Comparative Example 1, even if the inverter starts operating and starts outputting power to grid 2, depending on the rate at which solar radiation increases, the drop in the inverter input voltage due to MPPT control may not reach the determination voltage of the booster, and the booster may not be able to recognize that the inverter has started operating.

[0047] Conversely, if the solar radiation rises once and then suddenly falls, the voltage detected by the output voltage sensor V2 may fall to the judgment voltage even though the inverter is not operating, and the booster may mistakenly recognize that the inverter has started operating.

[0048] 6 is a diagram for explaining a method for determining whether a booster inverter is operating according to an embodiment. In this embodiment, the inverter operation is determined based on the current detected by the output current sensor A2, not on the voltage detected by the output voltage sensor V2. The output voltage of the booster may change due to solar radiation, but the output current of the booster does not flow unless the inverter is operating, so the effects of solar radiation fluctuations can be eliminated.

[0049] In a low solar radiation state, the inverter may repeatedly start and stop, and accordingly the booster may repeatedly start and stop boosting. As a countermeasure to this, a time element may be added to determine whether the inverter is operating stably.

[0050] As shown in Fig. 6, if the output current Aout of the booster exceeds the start judgment current Ast and continues for the start judgment time Tst or longer, the booster determines that the inverter is operating and starts boost operation. If the output current Aout of the booster remains below the stop judgment current Asp and continues for the stop judgment time Tsp or longer, the booster determines that the inverter is stopped and stops boost operation.

[0051] FIG. 7 is a flowchart showing the flow of the power conditioning operation determination process of the booster according to the embodiment. When the power supply of the booster is on (Y of S10), first, it is checked whether the current booster is in a boosting operation or is stopped (S11), and if the boosting operation is stopped (Y of S11), the control unit 12 of the booster determines whether the operation start condition of the booster (excluding the current condition) is satisfied (S12). Examples of the operation start condition of the booster include the input voltage of the boost circuit 11 detected by the input voltage sensor V1 being equal to or higher than the boost start voltage (e.g., 40V), the temperature inside the booster detected by the temperature sensor T1 being lower than the set temperature for component protection (e.g., 80°C), and no abnormality occurring in the booster. If all of these three conditions are satisfied, the control unit 12 determines that the operation start condition (excluding the current condition) is satisfied (Y of S12). If any one of these three conditions is not satisfied, the control unit 12 determines that the operation start conditions (excluding the current condition) are not satisfied (N in S12).

[0052] When the operation start conditions (excluding the current condition) are satisfied (Y in S12), the control unit 12 judges whether the output current Aout detected by the output current sensor A2 has exceeded the start judgment current Ast for the start judgment time Tst or more (S13). When the output current Aout has exceeded the start judgment current Ast for the start judgment time Tst or more (Y in S13), the control unit 12 starts the boost operation of the boost circuit 11 (S14).

[0053] If the booster is in boost operation in step S11 (N in S11), the control unit 12 determines whether or not the operation stop conditions (excluding the current condition) of the booster are satisfied (S15). Examples of the operation stop conditions of the booster include the input voltage of the boost circuit 11 detected by the input voltage sensor V1 being equal to or lower than the boost stop voltage (e.g., 30 V), the temperature inside the booster detected by the temperature sensor T1 being equal to or higher than a set temperature for component protection (e.g., 90° C.), and an abnormality occurring in the booster. If any one of these three conditions is satisfied, the control unit 12 determines that the operation stop conditions (excluding the current condition) are satisfied (Y in S15). If all of these three conditions are not satisfied, the control unit 12 determines that the operation stop conditions (excluding the current condition) are not satisfied (N in S15).

[0054] When the operation stop conditions (excluding the current condition) are satisfied (Y in S15), the control unit 12 stops the boost operation of the boost circuit 11 (S18). When the operation stop conditions (excluding the current condition) are not satisfied (N in S15), the control unit 12 judges whether the output current Aout detected by the output current sensor A2 is a current less than the stop judgment current Asp that continues for the stop judgment time Tsp or more (S16). When the output current Aout is a current less than the stop judgment current Asp that continues for the stop judgment time Tsp or more (Y in S16), the control unit 12 stops the boost operation of the boost circuit 11 (S18). When the current does not continue for the stop judgment time Tsp or more (N in S16), the control unit 12 continues the boost operation of the boost circuit 11 (S17).

[0055] When the power supply to the booster is turned off (N in S10), the power conditioner operation determination process ends.

[0056] As described above, according to this embodiment, the operating state of the power conversion device 20 is determined based on the output current of the boost junction box 10 detected by the output current sensor A2, rather than the output voltage of the boost junction box 10 detected by the output voltage sensor V2, thereby making it possible to determine with high accuracy the operating state of the power conversion device 20. This prevents the boost junction box 10 from starting or continuing a boost operation while the power conversion device 20 is in a stopped state, suppresses deterioration of the components in the boost junction box 10 due to an increase in the output voltage of the boost junction box 10, and reduces malfunctions of the power conversion device 20.

[0057] In this embodiment, the boost junction box 10, which connects multiple solar cell strings (PV1, PV2) in parallel, and the power converter 20 are separate devices. This allows the combination of the boost junction box 10 and the power converter 20 to accommodate various installation configurations of solar cell strings, and allows various installation forms to be accommodated with a small number of types of devices. Therefore, the cost of the device including the boost junction box 10 and the power converter 20 can be reduced.

[0058] The present disclosure has been described above based on the embodiments. The embodiments are merely examples, and it will be understood by those skilled in the art that various modifications are possible in the combination of each component and each processing step, and that such modifications are also within the scope of the present disclosure.

[0059] FIG. 8 is a diagram showing a configuration example of a solar power generation system according to Modification 1. FIG. 9 is a diagram showing a configuration example of a solar power generation system according to Modification 2. FIG. 1 shows an example in which the output current sensor A2 is installed on the junction output wiring Wm. In this regard, the standard circuit output current sensor A3 may be installed on the output wiring W1 of the standard circuit system instead of the output current sensor A2. Modification 1 shown in FIG. 8 is an example in which the sensor is installed on the positive wiring side of the output wiring W1 of the standard circuit system. Modification 2 shown in FIG. 9 is an example in which the sensor is installed on the negative wiring side of the output wiring W1 of the standard circuit system.

[0060] The control unit 12 can detect the output current Aout of the booster by adding up the current flowing through the boost circuit system detected by the input current sensor A1 and the current flowing through the standard circuit system detected by the standard circuit output current sensor A3.

[0061] In the above embodiment, in order to simplify the explanation, an example has been described in which one first solar cell string PV1 is connected to the standard circuit system, and one second solar cell string PV2 is connected to the boost circuit system. In this regard, a plurality of first solar cell strings PV1 may be connected in parallel to the standard circuit system. In this case, a first switch RY1 and a backflow prevention diode D1 are provided in each string. The current (power) output from the plurality of first solar cell strings PV1 to the junction output wiring Wm is the sum of the output currents (output power) of the plurality of first solar cell strings PV1.

[0062] Similarly, a plurality of second solar cell strings PV2 may be connected in parallel to the boost circuit system. In this case, a second switch RY2 and a boost circuit 11 are provided for each string. The current (power) output from the plurality of boost circuits 11 to the junction output wiring Wm is the sum of the output currents (output power) of the plurality of second solar cell strings PV2.

[0063] In the above embodiment, an example has been described in which the first solar cell string PV1 and the second solar cell string PV2, which has an open circuit voltage lower than that of the first solar cell string PV1, are connected to the boost junction box 10. In this regard, a DC power source other than a plurality of solar cells having different open circuit voltages can also be connected to the boost junction box 10. For example, a first storage battery pack and a second storage battery pack, which has an open circuit voltage lower than that of the first storage battery pack, may be connected to the boost junction box 10. The second storage battery pack is a storage battery pack in which the number of cells or modules connected in series is smaller than that of the first storage battery pack.

[0064] The embodiment may be specified by the following items.

[0065] [Item 1] a boost circuit (11) capable of boosting a voltage of DC power output from a second DC power source (PV2) having an open circuit voltage lower than that of a first DC power source (PV1); a joined output wiring (Wb) in which an output wiring (Wb) of the boost circuit (11) and an output wiring (W1) of the first DC power source (PV1) are joined together; A control unit (12) for controlling the boost circuit (11); a current sensor (A2) or (A1, A3) for detecting a current output from the boost connection circuit (10); The junction output wiring (Wb) is connected to a power converter (20) that converts DC power supplied from the boost connection circuit (10) into AC power, The boost connection circuit (10) is characterized in that the control unit (12) determines whether the power conversion device (20) is operating or stopped depending on the current detected by the current sensor (A2) or (A1, A3). This makes it possible to determine the operating state of the power conversion device (20) with high accuracy. [Item 2] The boost connection circuit (10) according to item 1, characterized in that the control unit (12) starts operation of the boost circuit (11) when a current flowing from the boost connection circuit (10) to the power conversion device (20) becomes equal to or greater than a first threshold value. This makes it possible to determine with high accuracy when the power conversion device (20) starts operating. [Item 3] The boost connection circuit (10) according to item 2, characterized in that the control unit (12) stops operation of the boost circuit (11) when a current flowing from the boost connection circuit (10) to the power conversion device (20) becomes equal to or less than a second threshold value that is lower than the first threshold value. This makes it possible to determine with high accuracy whether the power conversion device (20) has stopped operating. [Item 4] 4. The boost connection circuit (10) according to any one of items 1 to 3, wherein the current sensor (A2) is installed on the junction output wiring (Wb). This makes it possible to directly detect the current flowing from the boost connection circuit (10) to the power converter (20). [Item 5] The boost connection circuit (10) according to any one of items 1 to 3, characterized in that the current sensors (A1, A3) are installed on an output wiring (W1) of the first DC power supply (PV1) and an output wiring of the second DC power supply (PV2). This makes it possible to detect the current flowing from the boost connection circuit (10) to the power converter (20) without providing a current sensor (A2) in the merged output wiring (Wb). [Item 6] 6. The boost connection circuit (10) according to any one of items 1 to 5, wherein the first DC power supply (PV1) and the second DC power supply (PV2) are solar cells (PV1, PV2). According to this, by determining the operating state of the power conversion device (20) based on the current rather than the voltage output from the boost connection circuit (10), it is possible to avoid erroneous detection due to the influence of MPPT control of the power conversion device (20). [Item 7] A boost connection circuit (10) according to any one of items 1 to 6; a power converter (20) that converts DC power supplied from the boost connection circuit (10) into AC power; A power conversion system (1) comprising: This makes it possible to determine the operating state of the power conversion device (20) with high accuracy. [Explanation of symbols]

[0066] 1 power conversion system, 2 systems, 10 boost junction box, 11 boost circuit, 12 control unit, 20 power conversion device, 21 DC / DC converter, 22 inverter, 23 control unit, PV1 first solar cell string, PV2 second solar cell string, RY1 first switch, RY2 second switch, D1 reverse current prevention diode, V1 input voltage sensor, V2 output voltage sensor, A1 input current sensor, A2 output current sensor, A3 standard circuit output current sensor, T1 temperature sensor, W1, W2, Wb output wiring, Wm combined output wiring, L1 reactor, C1 input capacitor, C2 output capacitor, D2 diode, S1 switching element.

Claims

1. a boost circuit capable of boosting a voltage of DC power output from a second DC power source having an open circuit voltage lower than that of the first DC power source; a joined output wiring in which an output wiring of the boost circuit and an output wiring of the first DC power source are joined together; A control unit for controlling the boost circuit; a current sensor for detecting a current output from the boost connection circuit; the junction output wiring is connected to a power conversion device that converts the DC power supplied from the boost connection circuit into AC power; The control unit starts operation of the boost circuit that has been stopped when a current flowing from the boost connection circuit to the power conversion device becomes equal to or greater than a first threshold value.

2. 2. The boost connection circuit according to claim 1, wherein the control unit stops operation of the boost circuit when a current flowing from the boost connection circuit to the power conversion device becomes equal to or lower than a second threshold value that is lower than the first threshold value.

3. 3. The boost connection circuit according to claim 1, wherein the current sensor is disposed on the junction output wiring.

4. 3. The boost connection circuit according to claim 1, wherein the current sensor is provided on an output wiring of the first DC power supply and an output wiring of the second DC power supply.

5. 5. The boost connection circuit according to claim 1, wherein the first DC power source and the second DC power source are solar cells.

6. A boost connection circuit according to any one of claims 1 to 5; a power conversion device that converts DC power supplied from the boost connection circuit into AC power; A power conversion system comprising:

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