Power converter

Abstract

Provided is a power conversion device comprising: a converter that is connected to a solar panel and also connected to an alternating current power system and that converts direct current power supplied from the solar panel to alternating current power corresponding to the power system and supplies the converted alternating current power to the power system side; and a control circuit for controlling the operation of power conversion by the converter. The control circuit has a cold-weather operation mode for: engaging a direct current switch when a direct current voltage of the solar panel reaches or exceeds a predetermined voltage that is lower than a lower limit value of a stipulated range; when the direct current voltage of the solar panel reaches or exceeds the lower limit value of the stipulated range, engaging an alternating current switch and controlling operation of the converter so that a direct current voltage of a charge accumulation element will be constant at a maximum value of an alternating current voltage of the power system; and starting up the converter when the direct current power of the solar panel reaches or exceeds a predetermined power. Provided thereby is a power conversion device making it possible to more appropriately start up operation of power conversion even when the outside air temperature is low.

Description

Technical Field

[0001] Embodiments of the present invention relate to a power conversion device.

Background Art

[0002] There is a power conversion device that is connected to a solar panel and an AC power grid, converts DC power supplied from the solar panel into AC power corresponding to the power grid, and supplies the converted AC power to a load or the power grid.

[0003] The power conversion device stops the power conversion operation during a time period when the power generation power of the solar panel is low at night. Then, the power conversion device starts the power conversion operation in response to an increase in the power generation power of the solar panel in the morning time period.

[0004] The DC voltage of the solar panel decreases at night and gradually increases in response to an increase in the solar radiation amount after sunrise. The power conversion device starts the power conversion operation when the DC voltage of the solar panel is within a specified range for a predetermined time or more. At this time, for example, in the early morning of winter when the outside air temperature is low, the power conversion device may not be able to start the power conversion operation.

[0005] The open-circuit voltage of the solar panel is inversely proportional to the temperature, and the lower the temperature of the solar panel, the higher the open-circuit voltage of the solar panel. Therefore, in the early morning of winter when the outside air temperature is low, the open-circuit voltage of the solar panel tends to be high.

[0006] Thus, when the open-circuit voltage of the solar panel is high, the DC voltage of the solar panel gradually rises from sunrise, and after becoming not less than the lower limit value of the specified range from a state less than the lower limit value of the specified range, it may exceed the upper limit value of the specified range within a predetermined time.

[0007] If the DC voltage of a solar panel exceeds the upper limit of the specified range, the temperature of the solar panel will rise, and the power converter will be unable to start power conversion until the open-circuit voltage of the solar panel drops below the upper limit of the specified range. This results in wasted power generated by the solar panel. For example, it can lead to a decrease in the amount of electricity sold to the power grid or an increase in the amount of electricity purchased from the power grid.

[0008] Therefore, it is desirable that power conversion devices be designed to enable more appropriate power conversion operation even when the outside temperature is low. [Prior art documents] [Patent Documents]

[0009] [Patent Document 1] Japanese Patent Publication No. 2011-91931 [Overview of the project] [Problems that the invention aims to solve]

[0010] Embodiments of the present invention provide a power conversion device that can more appropriately initiate power conversion operations even when the outside temperature is low. [Means for solving the problem]

[0011] According to an embodiment of the present invention, the present invention comprises: a converter connected to a solar panel and an AC power system, which converts DC power supplied from the solar panel into AC power corresponding to the power system and supplies the converted AC power to the power system; a DC switch provided between the solar panel and the converter, which switches between a state in which the converter is connected to the solar panel and a state in which the converter is disconnected from the solar panel; an AC switch provided between the power system and the converter, which switches between a state in which the converter is connected to the power system and a state in which the converter is disconnected from the power system; a charge storage element that smooths the DC voltage supplied from the solar panel to the converter; and a control circuit that controls the operation of power conversion by the converter and controls the opening and closing of the DC switch and the AC switch, respectively, wherein the control circuit sets a specified range for the DC voltage of the solar panel, and when the DC voltage of the solar panel is within the specified range, A power converter is provided having a cold-weather mode that controls the operation of the converter to convert power, stops the operation of the converter to convert power when the DC voltage of the solar panel falls below the specified range, opens the DC switch and the AC switch, the control circuit closes the DC switch when the DC voltage of the solar panel becomes a predetermined voltage lower than the lower limit of the specified range, closes the AC switch after closing the DC switch and when the DC voltage of the solar panel becomes equal to or greater than the lower limit of the specified range, starts voltage control to control the operation of the converter so that the DC voltage of the charge storage element becomes constant at the maximum value of the AC voltage of the power system, and after starting the voltage control, switches the operation of the converter from the voltage control operation to an operation of supplying AC power to the power system based on the DC power of the solar panel when the DC power of the solar panel becomes equal to or greater than the predetermined power, thereby activating the converter. [Effects of the Invention]

[0012] According to embodiments of the present invention, a power conversion device is provided that can more appropriately initiate power conversion operations even when the outside temperature is low. [Brief explanation of the drawing]

[0013] [Figure 1] This is a schematic block diagram showing a power conversion device according to an embodiment. [Figure 2] This is a flowchart that schematically represents an example of how a control circuit works. [Figure 3] This graph schematically illustrates an example of how the cold-weather mode operates. [Figure 4] Figures 4(a) and 4(b) are schematic graphs illustrating an example of solar panel identification. [Figure 5] This is a schematic block diagram showing a modified example of the power conversion device according to the embodiment. [Modes for carrying out the invention]

[0014] Each embodiment will be described below with reference to the drawings. Please note that the drawings are schematic or conceptual, and the relationships between the thickness and width of each part, as well as the ratios of the sizes of the parts, are not necessarily identical to those of reality. Furthermore, even when representing the same part, the dimensions and ratios may differ between drawings. In this specification and in each figure, elements similar to those described above are denoted by the same reference numerals, and detailed explanations are omitted as appropriate.

[0015] Figure 1 is a schematic block diagram showing a power conversion device according to an embodiment. As shown in Figure 1, the power conversion device 10 comprises a converter 12 and a control circuit 14.

[0016] The converter 12 is connected to the solar panel 2 and also to the AC power grid 4. The converter 12 (power converter 10) is connected to the power grid 4 via, for example, a transformer 6.

[0017] The converter 12 converts the DC power supplied from the solar panel 2 into AC power corresponding to the power grid 4, and supplies the converted AC power to the power grid 4 side. The control circuit 14 controls the operation of power conversion by the converter 12.

[0018] The converter 12 is connected to the power grid 4, for example, and is connected to a load (not shown) via the connection point with the power grid 4. The converter 12 supplies AC power to the load in association with the power grid 4, and supplies the surplus power of the solar panel 2 to the power grid 4. The power conversion device 10 is used, for example, in a system-connected power conversion system.

[0019] Note that the converter 12 does not necessarily have to be connected to the load. The converter 12 may be connected only to the power grid 4, for example, and supply the converted AC power only to the power grid 4. The power conversion device 10 may be applied to, for example, a self-consumption type system that supplies the generated power of the solar panel 2 only to the load.

[0020] The AC power of the power grid 4 is three-phase AC power. The converter 12 converts the DC power supplied from the solar panel 2 into three-phase AC power. However, the AC power converted by the converter 12 may be single-phase AC power or the like. The AC power converted by the converter 12 may be any AC power according to the power grid 4 or the load.

[0021] The power conversion device 10 further includes DC switches 21a and 21b, and AC switches 22a to 22c. The DC switches 21a and 21b are provided between the solar panel 2 and the converter 12, and switch between a state where the converter 12 is connected to the solar panel 2 and a state where the converter 12 is disconnected from the solar panel 2 by opening and closing the circuit between the solar panel 2 and the converter 12.

[0022] The power conversion device 10 includes, for example, a pair of DC switches 21a and 21b provided in the high-potential circuit between the solar panel 2 and the converter 12, and in the low-potential circuit between the solar panel 2 and the converter 12. The DC switches may be provided in, for example, only one of the high-potential circuit and the low-potential circuit.

[0023] The AC switches 22a to 22c are installed between the power system 4 and the converter 12, and by opening and closing the circuit between the power system 4 and the converter 12, they switch between a state in which the converter 12 is connected to the power system 4 and a state in which the converter 12 is disconnected from the power system 4.

[0024] The DC switches 21a, 21b and the AC switches 22a-22c are connected to the control circuit 14. The opening and closing of each of the DC switches 21a, 21b and the AC switches 22a-22c is switched based on the control of the control circuit 14. In other words, the control circuit 14 controls the opening and closing of each of the DC switches 21a, 21b and the AC switches 22a-22c.

[0025] When the AC power output by the converter 12 is three-phase AC power, the power converter 10 is equipped with three AC switches 22a to 22c, each corresponding to one of the phases of the three-phase AC power. However, the number of AC switches provided in the power converter 10 is not limited to this, and may be any number corresponding to the configuration of the AC power output by the converter 12.

[0026] The power converter 10 further comprises a charge storage element 23, a rectifier element 24, and a filter circuit 25. The charge storage element 23 smooths the DC voltage supplied from the solar panel 2 to the converter 12. The charge storage element 23 is, for example, a smoothing capacitor.

[0027] The rectifier element 24 is provided between the solar panel 2 and the charge storage element 23. The rectifier element 24 suppresses the reverse flow of power from the converter 12 and the charge storage element 23 to the solar panel 2. The rectifier element 24 is, for example, a reverse current prevention diode.

[0028] The filter circuit 25 is provided between the power system 4 and the converter 12 to suppress harmonic components of the AC power output from the converter 12, making the output waveform closer to a sine wave. The filter circuit 25 includes, for example, an inductor 25a and a capacitor 25b. The inductor 25a is connected, for example, between the power system 4 and the converter 12. The capacitor 25b is connected, for example, between the connection point between the power system 4 and the inductor 25a and a point at a common potential (for example, the ground terminal). The inductor 25a and the capacitor 25b are provided for each of the three phases, for example, when the AC power output by the converter 12 is three-phase AC power.

[0029] The power converter 10 further comprises a DC voltage detector 30, a DC current detector 31, AC voltage detectors 32a to 32c, and AC current detectors 33a to 33c.

[0030] The DC voltage detector 30 detects the magnitude of the DC voltage supplied from the solar panel 2 to the converter 12. The DC voltage detector 30 is installed, for example, between the solar panel 2 and the DC switches 21a and 21b. This allows the DC voltage detector 30 to detect the magnitude of the DC voltage from the solar panel 2 even when the DC switches 21a and 21b are open. The DC voltage detector 30 is connected to the control circuit 14. The DC voltage detector 30 inputs the detection result of the magnitude of the DC voltage supplied from the solar panel 2 to the converter 12 to the control circuit 14.

[0031] The DC current detector 31 detects the magnitude of the DC current supplied from the solar panel 2 to the converter 12. The DC current detector 31 is connected to the control circuit 14. The DC current detector 31 inputs the detection result of the magnitude of the DC current supplied from the solar panel 2 to the converter 12 to the control circuit 14.

[0032] The AC voltage detectors 32a to 32c detect the magnitude of the AC voltage output from the converter 12. For example, the AC voltage detectors 32a to 32c detect the magnitude of each phase of the three-phase AC voltage output from the converter 12. The AC voltage detectors 32a to 32c are connected to the control circuit 14. The AC voltage detectors 32a to 32c input the detection results of the magnitude of the AC voltage output from the converter 12 to the control circuit 14.

[0033] The AC current detectors 33a to 33c detect the magnitude of the AC current output from the converter 12. For example, the AC current detectors 33a to 33c detect the magnitude of each phase of the three-phase AC current output from the converter 12. The AC current detectors 33a to 33c are connected to the control circuit 14. The AC current detectors 33a to 33c input the detection results of the magnitude of the AC current output from the converter 12 to the control circuit 14.

[0034] The power converter 10 further includes a temperature sensor 40. The temperature sensor 40 measures the ambient temperature. The temperature sensor 40 is installed, for example, near the solar panel 2 and measures the ambient temperature around the solar panel 2. The temperature sensor 40 is connected to the control circuit 14. The temperature sensor 40 measures the ambient temperature and inputs the measurement result to the control circuit 14.

[0035] The control circuit 14 sets a specified range for the DC voltage of the solar panel 2. The control circuit 14 controls the operation of the power conversion by the converter 12 when the DC voltage of the solar panel 2 is within the specified range. The DC voltage of the solar panel 2 gradually rises from sunrise, reaches its highest point around noon, and then gradually decreases towards sunset. When the DC voltage of the solar panel 2 drops in the evening or at night and falls below the specified range, the control circuit 14 stops the power conversion operation by the converter 12. Then, when the DC voltage of the solar panel 2 gradually rises after sunrise and comes within the specified range, the control circuit 14 starts the power conversion operation by the converter 12.

[0036] The control circuit 14 has two operating modes, a normal mode and a cold-weather mode, which are startup operating modes for initiating the power conversion operation by the converter 12.

[0037] The normal mode is the standard operating mode used when the ambient temperature is above a predetermined temperature. The cold-weather mode is a cold-weather operating mode used when the ambient temperature is below a predetermined temperature. The predetermined temperature is, for example, 0°C. However, the predetermined temperature is not limited to this and may be any temperature. The predetermined temperature may be changed based on signals input to the control circuit 14 from, for example, an external device or an operating unit.

[0038] Figure 2 is a flowchart that schematically represents an example of the operation of a control circuit. Figure 3 is a schematic graph illustrating an example of how the cold-weather mode operates. Figure 3 schematically shows an example of the DC voltage and DC current supplied from the solar panel 2 to the converter 12.

[0039] When the power conversion operation by the converter 12 is stopped, the control circuit 14 determines whether the ambient temperature is above a predetermined temperature based on the ambient temperature measured by the temperature sensor 40 (step S101 in Figure 2). When the power conversion operation by the converter 12 is stopped, the control circuit 14 opens the DC switches 21a, 21b and the AC switches 22a to 22c, disconnecting the converter 12 from the solar panel 2 and the power grid 4.

[0040] If the control circuit 14 determines that the temperature is above a predetermined level, it operates in normal mode and starts the converter 12 in normal mode (step S102 in Figure 2).

[0041] In normal mode, the control circuit 14 determines whether the DC voltage of the solar panel 2 is within a specified range for a predetermined period of time or longer (step S103 in Figure 2). In other words, the control circuit 14 determines whether the state in which the DC voltage of the solar panel 2 is between the lower limit Vmin and the upper limit Vmax (see Figure 3) of the specified range has continued for a predetermined period of time or longer.

[0042] The control circuit 14 closes the DC switches 21a and 21b when it determines that the DC voltage of the solar panel 2 has been within a specified range for a predetermined time or longer (step S104 in Figure 2). As a result, the DC power from the solar panel 2 is supplied to the converter 12.

[0043] After closing the DC switches 21a and 21b, the control circuit 14 performs synchronous control of the converter 12 so that the magnitude of the AC voltage output from the converter 12 is the same as the magnitude of the AC voltage of the power system 4 (step S105 in Figure 2). In other words, the control circuit 14 controls the operation of the converter 12 so that the amplitude and phase of the AC voltage output from the converter 12 are synchronized with the amplitude and phase of the AC voltage of the power system 4. At this time, the magnitude of the AC voltage of the power system 4 may be detected by a voltage detector or obtained by communication with a higher-level controller or the like.

[0044] The control circuit 14 closes the AC switches 22a to 22c when the difference between the magnitude of the AC voltage output from the converter 12 and the magnitude of the AC voltage of the power system 4 falls below a predetermined value (step S106 in Figure 2). As a result, the AC power output from the converter 12 is supplied to the power system 4. In other words, the power conversion operation by the converter 12 is activated.

[0045] After activating the converter 12, the control circuit 14 controls the operation of the converter 12 by, for example, a control method called Maximum Power Point Tracking (MPPT) that makes the DC power supplied from the solar panel 2 to the converter 12 track the maximum power point (step S107 in Figure 2). After activating the converter 12, the control circuit 14 causes the converter 12 to supply AC power to the power system 4 based on the DC power of the solar panel 2. The operation of supplying AC power to the power system 4 based on the DC power of the solar panel 2 is not limited to the operation of Maximum Power Point Tracking, but can be any operation that can appropriately supply AC power to the power system 4 based on the DC power of the solar panel 2.

[0046] On the other hand, if the control circuit 14 determines that the temperature is below a predetermined level, it operates in cold-weather mode and starts the converter 12 in cold-weather mode (step S108 in Figure 2).

[0047] In cold-weather mode, the control circuit 14 determines whether the DC voltage of the solar panel 2 is greater than or equal to a predetermined voltage Vth, which is lower than the lower limit Vmin of the specified range (step S109 in Figure 2).

[0048] The control circuit 14 closes the DC switches 21a and 21b when it determines that the voltage is above a predetermined voltage Vth (step S110 in Figure 2, time t1 in Figure 3).

[0049] After closing the DC switches 21a and 21b, the control circuit 14 determines whether the DC voltage of the solar panel 2 is equal to or greater than the lower limit Vmin of the specified range (step S111 in Figure 2).

[0050] The control circuit 14 closes the AC switches 22a to 22c in accordance with its determination that the value is greater than or equal to the lower limit Vmin of the specified range (step S112 in Figure 2, time t2 in Figure 3).

[0051] After the AC switches 22a to 22c are closed, the control circuit 14 performs voltage control to control the operation of the converter 12 so that the DC voltage of the charge storage element 23 remains constant at the maximum value of the AC voltage of the power system 4 (step S113 in Figure 2). More specifically, the control circuit 14 controls the operation of the converter 12 so that the DC voltage of the charge storage element 23 is Vdc and the effective value of the AC voltage of the power system 4 is Vac, and Vdc = √2 × Vac. This makes it possible to suppress the inflow of power from the AC side to the DC side of the converter 12 even when the AC switches 22a to 22c are closed without synchronizing the AC side of the converter 12.

[0052] As shown in Figure 3, immediately after the DC voltage of solar panel 2 exceeds the lower limit Vmin of the specified range, the DC current supplied from solar panel 2 is low, and it is possible that sufficient DC power is not being obtained from solar panel 2.

[0053] Therefore, after starting voltage control, the control circuit 14 calculates the DC power of the solar panel 2 based on the detection results of the DC voltage detector 30 and the DC current detector 31, and determines whether the DC power of the solar panel 2 is equal to or greater than a predetermined power (step S114 in Figure 2).

[0054] If the control circuit 14 determines that the power is below a predetermined level, it continues voltage control to keep the DC voltage of the charge storage element 23 constant at the maximum value of the AC voltage of the power system 4.

[0055] Then, when the control circuit 14 determines that the power is above a predetermined level, it switches the operation of the converter 12 from voltage control to maximum power point tracking control (step S107 in Figure 2, time t3 in Figure 3). In other words, when the control circuit 14 determines that the power is above a predetermined level, it causes the converter 12 to supply AC power to the power grid 4 based on the DC power of the solar panel 2. This allows the converter 12 to be activated even in cold-weather mode.

[0056] Figures 4(a) and 4(b) are schematic graphs illustrating an example of solar panel identification. Figure 4(a) schematically shows an example of the relationship between the DC voltage and DC power of solar panel 2. Figure 4(b) schematically shows an example of the relationship between the DC voltage and DC current of solar panel 2. Figure 4(a) also shows four characteristics CT1 to CT4 of solar panel 2 with varying temperatures. Of the characteristics CT1 to CT4, the temperature of solar panel 2 is highest for characteristic CT1, and decreases in the order of characteristic CT2, characteristic CT3, and characteristic CT4. In other words, the temperature of solar panel 2 is lowest for characteristic CT4.

[0057] As shown in Figure 4(a), the DC voltage (open-circuit voltage) of solar panel 2 tends to increase as the temperature of solar panel 2 decreases. Therefore, the open-circuit voltage of solar panel 2 tends to be higher in the early morning during winter when the outside temperature is low.

[0058] Thus, when attempting to start the converter 12 in normal mode while the open-circuit voltage of the solar panel 2 is high, the DC voltage of the solar panel 2 may gradually rise from sunrise, go from being below the lower limit Vmin of the specified range to being above the lower limit Vmin of the specified range, and then exceed the upper limit Vmax of the specified range within a predetermined time.

[0059] For example, in the characteristic CT4 shown in Figure 4(a), the open-circuit voltage exceeds the upper limit Vmax of the specified range. When attempting to start the converter 12 in normal mode at such a temperature, the upper limit Vmax of the specified range may be exceeded within a predetermined time, making it impossible to start the converter 12. In this case, under normal operation, the converter 12 cannot be started until the temperature of the solar panel 2 rises with the temperature increase and the open-circuit voltage of the solar panel 2 drops below the upper limit Vmax of the specified range. This raises concerns that the power generated by the solar panel 2 may be wasted.

[0060] In contrast, the power converter 10 according to this embodiment has a cold-weather compatible mode as an operating mode at startup for initiating the power conversion operation by the converter 12.

[0061] In cold-weather mode, when the DC voltage of the solar panel 2 falls to a predetermined voltage Vth or higher, which is lower than the lower limit Vmin of the specified range, the DC switches 21a and 21b are turned on. After the DC switches 21a and 21b are turned on, when the DC voltage of the solar panel 2 falls to a predetermined voltage Vmin or higher, the AC switches 22a to 22c are turned on. After the AC switches 22a to 22c are turned on, voltage control is started to control the operation of the converter 12 so that the DC voltage of the charge storage element 23 remains constant at the maximum value of the AC voltage of the power system 4. After the voltage control is started, when the DC power of the solar panel 2 exceeds a predetermined power, the operation of the converter 12 is switched from voltage control to supplying AC power to the power system 4 based on the DC power of the solar panel 2, thereby starting the converter 12.

[0062] In this way, in cold-weather mode, the converter 12 is connected to the solar panel 2 and the power grid 4 before the DC voltage of the solar panel 2 exceeds the upper limit Vmax of the specified range, and the converter 12 is operated by voltage control. This allows the converter 12 to be started more appropriately even in the early mornings of winter when the outside temperature is low.

[0063] Furthermore, as shown in Figure 4(b), the DC voltage of the solar panel 2 decreases in accordance with the increase in the DC current of the solar panel 2. Also, after the converter 12 is started, the output of the solar panel 2 flows to the power grid 4 side through the converter 12. Therefore, after the converter 12 is started, it is possible to prevent the DC voltage of the solar panel 2 from exceeding the upper limit Vmax of the specified range. As described above, even if the converter 12 is started in cold-weather mode, by operating the converter 12 and supplying AC power to the power grid 4 side based on the DC power of the solar panel 2, it is possible to prevent the DC voltage of the solar panel 2 from exceeding the upper limit Vmax of the specified range after the converter 12 is started, which would cause the converter 12 to stop working.

[0064] Therefore, the power converter 10 according to this embodiment can suppress wasted power generated by the solar panel 2. For example, the operating time of the power converter 10 can be increased, such as in the early morning during winter when the outside temperature is low. This makes it possible to, for example, increase the amount of electricity sold to the power grid 4 or further reduce the amount of electricity purchased from the power grid 4.

[0065] Figure 5 is a schematic block diagram showing a modified example of the power conversion device according to the embodiment. As shown in Figure 5, in the power converter 10a, the temperature sensor 40 is omitted, and the control circuit 14 receives the input of an external signal. Components that are substantially the same in function and configuration as those in the above embodiment are denoted by the same reference numerals, and detailed explanations are omitted.

[0066] The control circuit 14 receives external signals from external devices, such as a higher-level controller, by communicating with them. Based on the received external signals, the control circuit 14 switches between normal mode and cold-weather mode.

[0067] Thus, the configuration of the control circuit 14 is not limited to a configuration that switches between normal mode and cold-weather mode based on the ambient temperature measured by the temperature sensor 40, but may also be configured to switch between normal mode and cold-weather mode based on an external signal. For example, the ambient temperature may be measured by an external device.

[0068] Furthermore, external signals may be input, for example, from an operating unit connected to the control circuit 14. Switching between normal mode and cold-weather mode may be done manually, for example, by inputting an external signal based on the operation of the operating unit.

[0069] Thus, the normal mode and cold-weather mode do not necessarily have to be switched based on the outside temperature. For example, if it is estimated the outside temperature will be low in the early morning the day before or during the night, the cold-weather mode can be pre-instructed by inputting an external signal, so that the cold-weather mode operates regardless of the actual outside temperature in the early morning.

[0070] Furthermore, the control circuit 14 does not necessarily have a normal mode. The control circuit 14 may start the converter 12 only in cold-weather mode. On the other hand, in cold-weather mode, by closing the AC switches 22a to 22c without synchronizing the AC side of the converter 12, there are concerns that, for example, a large inrush current may flow through the capacitor 25b of the filter circuit 25, accelerating the deterioration of the capacitor 25b.

[0071] By providing the control circuit 14 with a normal mode and a cold-weather mode, and only activating the cold-weather mode when the outside temperature is low, it is possible to suppress, for example, the degradation of the capacitor 25b. For example, it is possible to enable the converter 12 to start up more appropriately even when the outside temperature is low, while also improving the reliability of the power converter.

[0072] This embodiment includes the following aspects. (Note 1) A converter that is connected to a solar panel and also to an AC power grid, converts the DC power supplied from the solar panel into AC power corresponding to the power grid, and supplies the converted AC power to the power grid. A DC switch is provided between the solar panel and the converter, which switches between a state in which the converter is connected to the solar panel and a state in which the converter is disconnected from the solar panel. An AC switch is provided between the power system and the converter, and switches between a state in which the converter is connected to the power system and a state in which the converter is disconnected from the power system. A charge storage element that smooths the DC voltage supplied from the solar panel to the converter, A control circuit controls the operation of power conversion by the converter and controls the opening and closing of the DC switch and the AC switch, respectively. Equipped with, The control circuit sets a specified range for the DC voltage of the solar panel, controls the power conversion operation of the converter when the DC voltage of the solar panel is within the specified range, and stops the power conversion operation of the converter when the DC voltage of the solar panel falls below the specified range, and opens the DC switch and the AC switch. The aforementioned control circuit is When the DC voltage of the solar panel becomes above a predetermined voltage that is lower than the lower limit of the specified range, the DC switch is closed. After the DC switch is turned on, when the DC voltage of the solar panel becomes equal to or greater than the lower limit of the specified range, the AC switch is turned on. After the AC switch is turned on, voltage control is initiated to control the operation of the converter so that the DC voltage of the charge storage element remains constant at the maximum value of the AC voltage of the power system. A power converter having a cold-weather mode that, after the voltage control has been started, switches the operation of the converter from the voltage control operation to the operation of supplying AC power to the power grid based on the DC power of the solar panel when the DC power of the solar panel exceeds a predetermined power, thereby activating the converter.

[0073] (Note 2) The aforementioned control circuit is Depending on whether the DC voltage of the solar panel remains within the specified range for a predetermined time or longer, the DC switch is closed. After the DC switch is closed, synchronous control is performed to control the operation of the converter so that the magnitude of the AC voltage output from the converter is the same as the magnitude of the AC voltage of the power system. The power conversion device according to Appendix 1, further having a normal mode in which the converter is activated by turning on the AC switch when the difference between the magnitude of the AC voltage output from the converter and the magnitude of the AC voltage of the power system falls below a predetermined value.

[0074] (Note 3) It is further equipped with a temperature sensor to measure the outside temperature. The power converter described in Appendix 2, wherein the control circuit activates the converter in normal mode when the ambient temperature measured by the temperature sensor is above a predetermined temperature, and activates the converter in cold-weather mode when the ambient temperature is below the predetermined temperature.

[0075] (Note 4) The control circuit receives an external signal input and switches between the normal mode and the cold-weather mode based on the input external signal, as described in Appendix 2.

[0076] While several embodiments of the present invention have been described, these embodiments are presented as examples only and are not intended to limit the scope of the invention. These novel embodiments can be carried out in a variety of other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. These embodiments and their variations are included in the scope and spirit of the invention, as well as in the claims and their equivalents. [Explanation of symbols]

[0077] 2…Solar panel, 4…Power system, 6…Transformer, 10, 10a…Power converter, 12…Converter, 14…Control circuit, 21a, 21b…DC switch, 22a~22c…AC switch, 23…Charge storage element, 24…Rectifier element, 25…Filter circuit, 30…DC voltage detector, 31…DC current detector, 32a~32c…AC voltage detector, 33a~33c…AC current detector, 40…Temperature sensor

Claims

1. A converter that is connected to a solar panel and also to an AC power grid, converts the DC power supplied from the solar panel into AC power corresponding to the power grid, and supplies the converted AC power to the power grid. A DC switch is provided between the solar panel and the converter, which switches between a state in which the converter is connected to the solar panel and a state in which the converter is disconnected from the solar panel. An AC switch is provided between the power system and the converter, and switches between a state in which the converter is connected to the power system and a state in which the converter is disconnected from the power system. A charge storage element that smooths the DC voltage supplied from the solar panel to the converter, A control circuit controls the operation of power conversion by the converter and controls the opening and closing of the DC switch and the AC switch, respectively. Equipped with, The control circuit sets a specified range for the DC voltage of the solar panel, controls the power conversion operation of the converter when the DC voltage of the solar panel is within the specified range, and stops the power conversion operation of the converter when the DC voltage of the solar panel falls below the specified range, and opens the DC switch and the AC switch. The aforementioned control circuit is When the DC voltage of the solar panel becomes above a predetermined voltage that is lower than the lower limit of the specified range, the DC switch is closed. After the DC switch is turned on, when the DC voltage of the solar panel becomes equal to or greater than the lower limit of the specified range, the AC switch is turned on. After the AC switch is turned on, voltage control is initiated to control the operation of the converter so that the DC voltage of the charge storage element remains constant at the maximum value of the AC voltage of the power system. A power converter having a cold-weather mode that, after the voltage control has been started, switches the operation of the converter from the voltage control operation to the operation of supplying AC power to the power grid based on the DC power of the solar panel when the DC power of the solar panel exceeds a predetermined power, thereby activating the converter.

2. The aforementioned control circuit is Depending on whether the DC voltage of the solar panel remains within the specified range for a predetermined time or longer, the DC switch is closed. After the DC switch is closed, synchronous control is performed to control the operation of the converter so that the magnitude of the AC voltage output from the converter is the same as the magnitude of the AC voltage of the power system. The power conversion device according to claim 1, further comprising a normal mode in which the converter is activated by turning on the AC switch when the difference between the magnitude of the AC voltage output from the converter and the magnitude of the AC voltage of the power system falls below a predetermined value.

3. It is further equipped with a temperature sensor to measure the outside temperature. The power converter according to claim 2, wherein the control circuit activates the converter in normal mode when the ambient temperature measured by the temperature sensor is above a predetermined temperature, and activates the converter in cold-weather mode when the ambient temperature is below the predetermined temperature.

4. The power conversion device according to claim 2, wherein the control circuit receives an external signal input and switches between the normal mode and the cold-weather mode based on the input external signal.

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