Power conversion device

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

Power Conversion Device

[0001] FIELD An embodiment of the present invention relates to a power conversion device.

[0002] BACKGROUND ART There is a power conversion device that is connected to a solar panel and also to an AC power system, converts DC power supplied from the solar panel into AC power compatible with the power system, and supplies the converted AC power to a load or the power system.

[0003] The power conversion device stops power conversion operation during the nighttime hours when the power generated by the solar panels is low, and then starts power conversion operation in response to an increase in the power generated by the solar panels in the morning hours.

[0004] The DC voltage of a solar panel drops at night and gradually rises after sunrise as the amount of solar radiation increases. The power conversion device activates power conversion when the DC voltage of the solar panel remains within a specified range for a predetermined period of time. However, there are cases where the power conversion device cannot activate power conversion, for example, early in the morning on a winter day when the outside temperature is low.

[0005] The open-circuit voltage of a 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. For this reason, the open-circuit voltage of a solar panel tends to be high in the early morning of winter when the outside temperature is low.

[0006] In this way, when the open circuit voltage of the solar panel is high, the DC voltage of the solar panel gradually rises from sunrise, goes from below the lower limit of the specified range to above the lower limit of the specified range, and then may exceed the upper limit of the specified range within a specified 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 as the temperature rises, and the power conversion device will not be able to start power conversion until the open-circuit voltage of the solar panel falls below the upper limit of the specified range. This will result in wasted power generated by the solar panel. For example, this could lead to a decrease in the amount of power sold to the power grid or an increase in the amount of power purchased from the power grid.

[0008] For this reason, it is desirable for the power conversion device to be able to more appropriately start the power conversion operation even when the outside temperature is low.

[0009] JP 2011-91931 A

[0010] The embodiments of the present invention provide a power conversion device that can more appropriately start the power conversion operation even when the outside temperature is low.

[0011] According to an embodiment of the present invention, there is provided a power conversion system including: a converter connected to a solar panel and also connected to an AC power grid, converting DC power supplied from the solar panel into AC power compatible with the power grid and supplying the converted AC power to the power grid; a DC switch provided between the solar panel and the converter, switching 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 grid and the converter, switching between a state in which the converter is connected to the power grid and a state in which the converter is disconnected from the power grid; a charge storage element that smooths the DC voltage supplied from the solar panel to the converter; and a control circuit that controls the power conversion operation by the converter and controls the opening and closing of each of the DC switch and the AC switch, 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 conversion device having a cold weather response mode that controls the power conversion operation by the converter, and when the DC voltage of the solar panel falls below the specified range, stops the power conversion operation by the converter and opens the DC switch and the AC switch; the control circuit closes the DC switch when the DC voltage of the solar panel becomes equal to or higher than a predetermined voltage that is lower than a lower limit of the specified range, and after closing the DC switch, closes the AC switch when the DC voltage of the solar panel becomes equal to or higher than a lower limit of the specified range, and after closing the AC switch, starts voltage control that controls the operation of the converter so that the DC voltage of the charge storage element is constant at the maximum value of the AC voltage of the power grid; and after starting the voltage control, when the DC power of the solar panel becomes equal to or higher than a predetermined power, switches the operation of the converter from the voltage control operation to an operation of supplying AC power to the power grid side based on the DC power of the solar panel, thereby starting the converter.

[0012] According to an embodiment of the present invention, a power conversion device is provided that can more appropriately start the power conversion operation even when the outside air temperature is low.

[0013] Fig. 4(a) and Fig. 4(b) are graphs that schematically show a specific example of a solar panel. Fig. 4(b) is a block diagram that schematically shows a modified example of a power conversion device according to an embodiment. Fig. 4(a) is a flow chart that schematically shows an example of the operation of a control circuit. Fig. 4(b) is a graph that schematically shows a specific example of a solar panel. Fig. 4(b) is a block diagram that schematically shows a modified example of a power conversion device according to an embodiment.

[0014] Each embodiment will be described below with reference to the drawings. The drawings are schematic or conceptual, and the relationship between the thickness and width of each part, the size ratio between parts, etc., are not necessarily the same as those in reality. Even when the same part is shown, the dimensions and ratios may be different depending on the drawing. In this specification and each drawing, elements similar to those previously described with reference to the previous drawings are designated by the same reference numerals, and detailed descriptions thereof will be omitted as appropriate.

[0015] 1 is a block diagram illustrating a power conversion device according to an embodiment. As illustrated in FIG. 1, the power conversion device 10 includes 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 conversion device 10) is connected to the power grid 4 via a transformer 6 or the like, for example.

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

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

[0019] The converter 12 does not necessarily have to be connected to a load. For example, the converter 12 may be connected only to the power grid 4 and supply converted AC power only to the power grid 4. The power conversion device 10 may be applied to, for example, a self-consumption system in which power generated by the solar panel 2 is supplied only to a 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 depending on the power grid 4 and 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 open and close the electrical path between the solar panel 2 and the converter 12, thereby switching between a state in which the converter 12 is connected to the solar panel 2 and a state in which the converter 12 is disconnected from the solar panel 2.

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

[0023] The AC switches 22a to 22c are provided between the power grid 4 and the converter 12, and switch between a state in which the converter 12 is connected to the power grid 4 and a state in which the converter 12 is disconnected from the power grid 4 by opening and closing the electrical path between the power grid 4 and the converter 12.

[0024] The DC switches 21a and 21b and the AC switches 22a to 22c are connected to the control circuit 14. The opening and closing of each of the DC switches 21a and 21b and the AC switches 22a to 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 and 21b and the AC switches 22a to 22c.

[0025] When the AC power output by the converter 12 is three-phase AC power, the power conversion device 10 includes three AC switches 22a to 22c corresponding to the respective phases of the three-phase AC power. However, the number of AC switches provided in the power conversion device 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 conversion device 10 further includes a charge storage element 23, a rectifying element 24, and a filter circuit 25. The charge storage element 23 smoothes 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 rectifying element 24 is provided between the solar panel 2 and the charge storage element 23. The rectifying element 24 suppresses reverse flow of power from the converter 12 and the charge storage element 23 to the solar panel 2. The rectifying element 24 is, for example, a backflow prevention diode.

[0028] The filter circuit 25 is provided between the power grid 4 and the converter 12 and suppresses 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 grid 4 and the converter 12. The capacitor 25b is connected, for example, between the connection point between the power grid 4 and the inductor 25a and a portion at a common potential (for example, a ground terminal). For example, if the AC power output from the converter 12 is three-phase AC power, the inductor 25a and the capacitor 25b are provided for each of the three phases.

[0029] The power conversion device 10 further includes 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 provided, for example, between the solar panel 2 and the DC switches 21a and 21b. This enables the DC voltage detector 30 to detect the magnitude of the DC voltage of 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. The AC voltage detectors 32a to 32c detect, for example, 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. The AC current detectors 33a to 33c detect, for example, 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 conversion device 10 further includes a temperature sensor 40. The temperature sensor 40 measures the outside air temperature. The temperature sensor 40 is provided, for example, near the solar panel 2 and measures the outside air temperature around the solar panel 2. The temperature sensor 40 is connected to the control circuit 14. The temperature sensor 40 measures the outside air temperature and inputs the measurement result of the outside air temperature 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 power conversion operation 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 around noon, and then gradually drops towards sunset. The control circuit 14 stops the power conversion operation by the converter 12 when the DC voltage of the solar panel 2 drops in the evening or at night and falls below the specified range. Then, the control circuit 14 starts the power conversion operation by the converter 12 when the DC voltage of the solar panel 2 gradually rises after sunrise and falls within the specified range.

[0036] The control circuit 14 has two operation modes, a normal mode and a cold weather mode, as start-up operation modes for starting the power conversion operation by the converter 12 .

[0037] The normal mode is a normal operating mode used when the outside air temperature is equal to or higher than a predetermined temperature. The cold weather mode is a cold weather operating mode used when the outside air temperature is lower than 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 changeable based on a signal input to the control circuit 14 from, for example, an external device or an operation unit.

[0038] Fig. 2 is a flow chart showing an example of the operation of the control circuit. Fig. 3 is a graph showing an example of the operation of the cold weather mode. Fig. 3 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 outside air temperature is equal to or higher than a predetermined temperature based on the outside air temperature measured by the temperature sensor 40 (step S101 in FIG. 2). When the power conversion operation by the converter 12 is stopped, the control circuit 14 opens the DC switches 21a and 21b and the AC switches 22a to 22c to disconnect the converter 12 from the solar panel 2 and the power grid 4.

[0040] When the control circuit 14 determines that the temperature is equal to or higher than the predetermined temperature, it operates in the normal mode and starts up the converter 12 in the normal mode (step S102 in FIG. 2).

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

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

[0043] After closing the DC switches 21a and 21b, the control circuit 14 performs synchronization control to control the operation 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 grid 4 (step S105 in FIG. 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 grid 4. At this time, the magnitude of the AC voltage of the power grid 4 may be detected by a voltage detector or may be obtained through communication with a higher-level controller or the like.

[0044] 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 grid 4 becomes equal to or less than a predetermined value, the control circuit 14 closes the AC switches 22a to 22c (step S106 in FIG. 2). As a result, the AC power output from the converter 12 is supplied to the power grid 4. In other words, the power conversion operation by the converter 12 is initiated.

[0045] After starting up the converter 12, the control circuit 14 controls the operation of the converter 12, for example, by a maximum power point tracking (MPPT) control method that causes the DC power supplied from the solar panel 2 to the converter 12 to track the maximum power point (step S107 in FIG. 2 ). After starting up the converter 12, the control circuit 14 causes the converter 12 to supply AC power to the power grid 4 based on the DC power of the solar panel 2. The operation of supplying AC power to the power grid 4 based on the DC power of the solar panel 2 is not limited to the maximum power point tracking operation, and may be any operation that can appropriately supply AC power to the power grid 4 based on the DC power of the solar panel 2.

[0046] On the other hand, when the control circuit 14 determines that the temperature is lower than the predetermined temperature, it operates in the cold weather response mode and starts up the converter 12 in the cold weather response mode (step S108 in FIG. 2).

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

[0048] When the control circuit 14 determines that the voltage is equal to or higher than the predetermined voltage Vth, it closes the DC switches 21a and 21b (step S110 in FIG. 2, time t1 in FIG. 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 higher than the lower limit Vmin of the specified range (step S111 in FIG. 2).

[0050] When the control circuit 14 determines that the voltage is equal to or greater than the lower limit value Vmin of the specified range, it closes the AC switches 22a to 22c (step S112 in FIG. 2, time t2 in FIG. 3).

[0051] After closing the AC switches 22a to 22c, 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 grid 4 (step S113 in FIG. 2). More specifically, when the DC voltage of the charge storage element 23 is Vdc and the effective value of the AC voltage of the power grid 4 is Vac, the control circuit 14 controls the operation of the converter 12 so that Vdc remains constant at Vdc = √2 × Vac. This makes it possible to suppress power flow 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 the solar panel 2 becomes equal to or greater than the lower limit value Vmin of the specified range, the DC current supplied from the solar panel 2 is low, and it is possible that sufficient DC power is not being obtained from the 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 less than the predetermined power, 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, in response to determining that the power is equal to or greater than the predetermined power level, the control circuit 14 switches the operation of the converter 12 from voltage control to maximum power point tracking control (step S107 in FIG. 2 , time t3 in FIG. 3 ). In other words, in response to determining that the power is equal to or greater than the predetermined power level, the control circuit 14 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 start up even in the cold weather mode.

[0056] 4(a) and 4(b) are graphs schematically showing a specific example of a solar panel. FIG. 4(a) shows a schematic example of the relationship between the DC voltage and DC power of the solar panel 2. FIG. 4(b) shows a schematic example of the relationship between the DC voltage and DC current of the solar panel 2. FIG. 4(a) also shows four characteristics, CT1 to CT4, obtained by varying the temperature of the solar panel 2. Among the characteristics CT1 to CT4, the temperature of the solar panel 2 for characteristic CT1 is the highest, followed by characteristic CT2, characteristic CT3, and characteristic CT4 in that order. In other words, the temperature of the solar panel 2 for characteristic CT4 is the lowest.

[0057] 4A, the DC voltage (open circuit voltage) of the solar panel 2 tends to increase as the temperature of the solar panel 2 decreases. For this reason, the open circuit voltage of the solar panel 2 tends to increase in the early morning hours in winter when the outside temperature is low.

[0058] In this way, if an attempt is made to start the converter 12 in normal mode when the open circuit voltage of the solar panel 2 is high, the DC voltage of the solar panel 2 will gradually rise from sunrise, and may go from below the lower limit value Vmin of the specified range to above the lower limit value Vmin of the specified range, and then exceed the upper limit value Vmax of the specified range within a specified time.

[0059] For example, in the characteristic CT4 of FIG. 4A, the open-circuit voltage exceeds the upper limit Vmax of the specified range. At such a temperature, if an attempt is made to start the converter 12 in normal mode, 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, in normal mode operation, the temperature of the solar panel 2 increases as the temperature rises, and the converter 12 cannot be started until the open-circuit voltage of the solar panel 2 drops below the upper limit Vmax of the specified range. This raises concerns about wasted power generated by the solar panel 2.

[0060] In contrast, in the power conversion device 10 according to this embodiment, the control circuit 14 has a cold weather operation mode as a startup operation mode for starting the power conversion operation by the converter 12 .

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

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

[0063] 4B, the DC voltage of the solar panel 2 decreases as the DC current of the solar panel 2 increases. After the converter 12 is started, the output of the solar panel 2 flows to the power grid 4 through the converter 12. Therefore, after the converter 12 is started, the DC voltage of the solar panel 2 can be prevented from exceeding the upper limit value Vmax of the specified range. As described above, even if the converter 12 is started in the cold weather response mode, by operating the converter 12 and supplying AC power to the power grid 4 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 value Vmax of the specified range and causing the converter 12 to stop operating after the converter 12 is started.

[0064] Therefore, the power conversion device 10 according to this embodiment can prevent the generation of wasted power by the solar panel 2. For example, the operating time of the power conversion device 10 can be increased in the early morning in winter when the outside temperature is low. This makes it possible to further increase the amount of power sold to the power grid 4 and further reduce the amount of power purchased from the power grid 4, for example.

[0065] 5 is a block diagram showing a modified example of the power conversion device according to the embodiment. As shown in FIG. 5, in the power conversion device 10a, the temperature sensor 40 is omitted, and the control circuit 14 receives an external signal as an input. Note that components that are substantially the same in function and configuration as those in the above embodiment are designated by the same reference numerals, and detailed description thereof will be omitted.

[0066] The control circuit 14 receives an external signal from an external device, such as a host controller, by communicating with the external device. The control circuit 14 then switches between the normal mode and the cold weather mode based on the input external signal.

[0067] In this way, the control circuit 14 is not limited to a configuration in which the normal mode and the cold weather response mode are switched based on the outside air temperature measured by the temperature sensor 40, but may be configured to switch between the normal mode and the cold weather response mode based on an external signal. For example, the outside air temperature may be measured by an external device.

[0068] The external signal may be input, for example, from an operation unit connected to the control circuit 14. The normal mode and the cold weather mode may be switched manually, for example, by inputting an external signal based on the operation of the operation unit.

[0069] In this way, the switching between the normal mode and the cold weather mode does not necessarily have to be based on the outside temperature. For example, if it is estimated that the outside temperature will be low the previous day or at night, the cold weather mode may be executed in advance by inputting an external signal, so that the cold weather mode is operated regardless of the actual outside temperature that early morning.

[0070] Furthermore, the control circuit 14 does not necessarily have to have a normal mode. The control circuit 14 may start the converter 12 only by operating in the cold weather response mode. On the other hand, when operating in the cold weather response mode, turning on the AC switches 22a to 22c without synchronizing the AC side of the converter 12 may cause a large inrush current to flow through the capacitor 25b of the filter circuit 25, which may accelerate deterioration of the capacitor 25b.

[0071] By providing the control circuit 14 with a normal mode and a cold weather response mode and operating in the cold weather response mode only when the outside temperature is low, it is possible to suppress, for example, deterioration of the capacitor 25b, etc. For example, even when the outside temperature is low, it is possible to more appropriately start up the converter 12 and further increase the reliability of the power conversion device.

[0072] This embodiment includes the following aspects: (Supplementary Note 1) A converter connected to a solar panel and an AC power grid, converting DC power supplied from the solar panel into AC power compatible with the power grid and supplying the converted AC power to the power grid, a DC switch provided between the solar panel and the converter, switching 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 grid and the converter, switching between a state in which the converter is connected to the power grid and a state in which the converter is disconnected from the power grid, a charge storage element that smooths the DC voltage supplied from the solar panel to the converter, and a control circuit that controls the power conversion operation by the converter and controls the opening and closing of each of the DC switch and the AC switch. the control circuit sets a specified range for the DC voltage of the solar panel, and controls the power conversion operation by the converter when the DC voltage of the solar panel is within the specified range, and stops the power conversion operation by the converter and opens the DC switch and the AC switch when the DC voltage of the solar panel falls below the specified range; the control circuit: closes the DC switch when the DC voltage of the solar panel becomes equal to or higher than a predetermined voltage lower than the lower limit of the specified range; after closing the DC switch, closes the AC switch when the DC voltage of the solar panel becomes equal to or higher than the lower limit of the specified range; and after closing the AC switch, starts voltage control that controls the operation of the converter so that the DC voltage of the charge storage element is constant at the maximum value of the AC voltage of the power grid; A power conversion device having a cold weather response mode that starts the converter by switching the operation of the converter from the voltage control operation to an operation of supplying AC power to the power grid side based on the DC power of the solar panel when the DC power of the solar panel becomes equal to or greater than a predetermined power after the voltage control has started.

[0073] (Supplementary Note 2) The power conversion device according to Supplementary Note 1, further having a normal mode in which the control circuit: closes the DC switch in response to the DC voltage of the solar panel being within the specified range for a predetermined time or more; after closing the DC switch, performs synchronous control to control the operation of the converter so that the magnitude of the AC voltage output from the converter becomes the same as the magnitude of the AC voltage of the power grid; and starts the converter by closing 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 grid becomes equal to or less than a predetermined value.

[0074] (Supplementary Note 3) The power conversion device according to Supplementary Note 2, further comprising a temperature sensor for measuring an outside air temperature, wherein the control circuit starts the converter in the normal mode when the outside air temperature measured by the temperature sensor is equal to or higher than a predetermined temperature, and starts the converter in the cold weather response mode when the outside air temperature is below the predetermined temperature.

[0075] (Supplementary Note 4) The power conversion device according to Supplementary Note 2, wherein the control circuit receives an input of an external signal and switches between the normal mode and the cold weather response mode based on the input external signal.

[0076] Although several embodiments of the present invention have been described, these embodiments are presented as examples and are not intended to limit the scope of the invention. These novel embodiments can be embodied in various other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. These embodiments and their modifications are included within the scope and spirit of the invention, and are also included in the scope of the invention and its equivalents as defined in the claims.

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

Claims

1. A converter connected to a solar panel and an AC power grid, converting DC power supplied from the solar panel into AC power compatible with the power grid and supplying the converted AC power to the power grid; a DC switch provided between the solar panel and the converter, switching 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 grid and the converter, switching between a state in which the converter is connected to the power grid and a state in which the converter is disconnected from the power grid; a charge storage element that smooths the DC voltage supplied from the solar panel to the converter; and a control circuit that controls the power conversion operation by the converter and controls the opening and closing of the DC switch and the AC switch, the control circuit sets a specified range for the DC voltage of the solar panel, and controls the power conversion operation by the converter when the DC voltage of the solar panel is within the specified range, and stops the power conversion operation by the converter and opens the DC switch and the AC switch when the DC voltage of the solar panel falls below the specified range; the control circuit: closes the DC switch when the DC voltage of the solar panel becomes equal to or higher than a predetermined voltage lower than the lower limit of the specified range; after closing the DC switch, closes the AC switch when the DC voltage of the solar panel becomes equal to or higher than the lower limit of the specified range; and after closing the AC switch, starts voltage control that controls the operation of the converter so that the DC voltage of the charge storage element is constant at the maximum value of the AC voltage of the power grid; A power conversion device having a cold weather response mode that starts the converter by switching the operation of the converter from the voltage control operation to an operation of supplying AC power to the power grid side based on the DC power of the solar panel when the DC power of the solar panel becomes equal to or greater than a predetermined power after the voltage control has started.

2. The power conversion device according to claim 1, further comprising a normal mode in which the control circuit: closes the DC switch in response to the DC voltage of the solar panel being within the specified range for a predetermined time or more; after closing the DC switch, performs synchronous control to control the operation of the converter so that the magnitude of the AC voltage output from the converter becomes the same as the magnitude of the AC voltage of the power grid; and when the difference between the magnitude of the AC voltage output from the converter and the magnitude of the AC voltage of the power grid becomes equal to or less than a predetermined value, closes the AC switch to start the converter.

3. A power conversion device as described in claim 2, further comprising a temperature sensor for measuring the outside air temperature, wherein the control circuit starts the converter in the normal mode when the outside air temperature measured by the temperature sensor is equal to or higher than a predetermined temperature, and starts the converter in the cold weather response mode when the outside air temperature is below the predetermined temperature.

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

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