Grid-connected power conversion system

Abstract

Provided is a grid-connected power conversion system that comprises a plurality of power conversion devices that convert power supplied from a plurality of distributed power sources to alternating-current power that is compatible with a power grid. When stopping the operation of a principal circuit unit of the power conversion devices, a controller of the power conversion devices performs an operation that switches a plurality of switching elements of the principal circuit unit for a prescribed period of time and then stops the operation of the principal circuit unit to suppress output of current from the principal circuit unit and inflow of current to the principal circuit unit from the power grid. The present invention thereby provides a grid-connected power conversion system that can suppress the occurrence of circulating current between a plurality of power conversion devices by means of a simpler configuration.

Description

Grid-connected power conversion system

[0001] FIELD An embodiment of the present invention relates to a grid-tied power conversion system.

[0002] A grid-connected power conversion system including a plurality of power conversion devices is known. The plurality of power conversion devices are provided corresponding to a plurality of distributed power sources, respectively. Each power conversion device is connected to each distributed power source and is also connected in parallel to the power grid. Each power conversion device is also connected in parallel to, for example, a load. Each power conversion device converts power supplied from each distributed power source into AC power compatible with the power grid and supplies the converted AC power to the load or the power grid. In this way, the grid-connected power conversion system is connected to the power grid and can supply AC power to the load and supply surplus power from the distributed power sources to the power grid.

[0003] Each power conversion device has a charge storage element that stores DC power based on power supplied from multiple distributed power sources, multiple switching elements that convert the DC power stored in the charge storage element into AC power, and multiple rectifier elements connected in anti-parallel to each of the multiple switching elements, and by switching the multiple switching elements, the power supplied from the multiple distributed power sources is converted into AC power that is compatible with the power grid.

[0004] In such a grid-connected power conversion system, when the operation of each power conversion device is stopped, there may be variations in the timing at which each power conversion device stops. If variations in the timing at which each power conversion device stops, a high-frequency circulating current (cross current) caused by the operation of the power conversion device that is still operating may flow into the power conversion device that has stopped operating, and an overvoltage may occur in the charge storage element of the power conversion device that has stopped operating.

[0005] High-frequency components, such as those caused by the switching of each switching element, are superimposed on the AC power supplied from each power conversion device to the power grid. Furthermore, relatively large stray capacitances exist between the distributed power sources and the ground, and between the ground and the conductors connecting the distributed power sources and the power conversion devices. Therefore, if there is variation in the timing at which each power conversion device shuts down, the high-frequency current superimposed on the AC power supplied to the power grid flows in a circular fashion between the operating power conversion device and the stopped power conversion device via the ground. The high-frequency current returns to the operating power conversion device via, for example, the rectifying element, charge storage element, stray capacitance between the ground and the stopped power conversion device, and the ground. This may charge the charge storage element of the stopped power conversion device, resulting in an overvoltage.

[0006] There is a concern that an overvoltage occurring in a charge storage element of a power conversion device may cause a failure of the power conversion device.

[0007] For this reason, it has been proposed to connect each power conversion device to the power grid via an isolation transformer or reactor, thereby suppressing the generation of circulating current between each power conversion device even when there is variation in the timing at which each power conversion device stops.

[0008] However, in a configuration in which the power conversion devices are connected via an isolation transformer or a reactor, the number of components increases due to the isolation transformer or the reactor, which leads to an increase in the size and cost of the entire system.

[0009] For this reason, it is desirable that the generation of circulating current between the power conversion devices in a grid-connected power conversion system be suppressed with a simpler configuration.

[0010] Patent No. 6752401

[0011] An embodiment of the present invention provides a grid-connected power conversion system that can suppress the generation of circulating current between a plurality of power conversion devices with a simpler configuration.

[0012] According to an embodiment of the present invention, a power conversion system is provided which is connected to a plurality of distributed power sources and is connected in parallel to an AC power grid, and the plurality of power conversion devices each include a main circuit unit connected to at least one of the plurality of distributed power sources and converting power supplied from the at least one connected distributed power source into AC power compatible with the power grid, and a control unit which controls the power conversion operation by the main circuit unit, and the main circuit unit includes a charge storage element which stores DC power based on the power supplied from the at least one distributed power source, a plurality of switching elements which convert the DC power stored in the charge storage element into AC power, and a plurality of switching elements which are connected in anti-parallel to each of the plurality of switching elements. and a rectifying element, and by switching the plurality of switching elements, converts power supplied from the at least one distributed power source into AC power corresponding to the power grid, wherein the control unit controls the power conversion operation by the main circuit unit by controlling the switching of the plurality of switching elements, and when stopping the operation of the main circuit unit, causes the main circuit unit to perform a predetermined operation for a predetermined period from the timing at which it was decided to stop the operation of the main circuit unit, and then stops the operation of the main circuit unit, wherein the predetermined operation is an operation of switching the plurality of switching elements so as to suppress the output of current from the main circuit unit and the inflow of current from the power grid to the main circuit unit.

[0013] According to an embodiment of the present invention, a grid-connected power conversion system that can suppress the generation of circulating current between a plurality of power conversion devices with a simpler configuration is provided.

[0014] 1 is a block diagram schematically illustrating a grid-connected power conversion system according to an embodiment; 2 is a block diagram schematically illustrating an example of a power conversion device according to an embodiment;

[0015] 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.

[0016] 1 is a block diagram illustrating a grid-connected power conversion system according to an embodiment. As illustrated in FIG. 1, the grid-connected power conversion system 10 includes a plurality of power conversion devices 12 and a control device 14.

[0017] The multiple power conversion devices 12 are provided corresponding to the multiple solar cell panels 2 (distributed power sources), respectively. However, the number of the multiple power conversion devices 12 does not necessarily have to be the same as the number of the multiple solar cell panels 2 (the number of multiple distributed power sources). For example, two power conversion devices 12 may be connected to one solar cell panel 2, or conversely, one power conversion device 12 may be connected to two solar cell panels 2.

[0018] Each power conversion device 12 is connected to a respective solar cell panel 2 and is also connected in parallel to the AC power grid 4. The input side of each power conversion device 12 is connected to the respective solar cell panel 2. The output side of each power conversion device 12 is connected to a connection point CP and is connected to the power grid 4 via the connection point CP. In other words, the output sides of each power conversion device 12 are connected to each other via the connection point CP.

[0019] Each power conversion device 12 is connected in parallel to the power system 4 via, for example, a transformer 6. The transformer 6 is provided between the connection point CP and the power system 4. The output side of each power conversion device 12 is connected to, for example, the secondary side of the transformer 6. The primary side of the transformer 6 is connected to the power system 4. In this way, each power conversion device 12 is connected to the power system 4 via the transformer 6.

[0020] The grid-connected power conversion system 10 further includes, for example, a plurality of switches 16. The plurality of switches 16 are provided corresponding to the plurality of power conversion devices 12, respectively. The plurality of switches 16 are provided between the plurality of power conversion devices 12 and the power grid 4. More specifically, the plurality of switches 16 are provided between the output side of each of the plurality of power conversion devices 12 and the connection point CP. Each of the plurality of switches 16 has an on state in which the plurality of power conversion devices 12 are connected to the power grid 4, and an off state in which the plurality of power conversion devices 12 are disconnected from the power grid 4. Thus, by selectively opening the plurality of switches 16, only a desired power conversion device 12 among the plurality of power conversion devices 12 can be paralleled off from the power grid 4.

[0021] On the other hand, in the grid-connected power conversion system 10, no isolation transformer or reactor is provided between the output side of each power conversion device 12 and the connection point CP. The output sides of each power conversion device 12 are electrically connected to each other without insulation such as magnetic coupling. For example, the output sides of each power conversion device 12 are electrically connected to each other when each switch 16 is closed.

[0022] For example, each power conversion device 12 is connected in parallel to the power grid 4 and also connected in parallel to the load 8. For example, each power conversion device 12 is connected to the load 8 via a connection point CP. The load 8 may be any load that requires a supply of AC power.

[0023] Each power conversion device 12 converts the power supplied from each solar cell panel 2 into AC power compatible with the power grid 4, and supplies the converted AC power to the load 8 or the power grid 4. In this example, each power conversion device 12 converts the DC power supplied from each solar cell panel 2 into AC power compatible with the power grid 4. In other words, the DC power supplied from each solar cell panel 2 is DC power generated by each solar cell panel 2. The AC power of the power grid 4 is, for example, three-phase AC power. Each power conversion device 12 converts the power supplied from each solar cell panel 2 into three-phase AC power compatible with the power grid 4. However, the AC power of the power grid 4 is not limited to three-phase AC power and may be any AC power.

[0024] As a result, the grid-connected power conversion system 10 can be connected to the power grid 4 to supply AC power to the load 8 , and can also supply surplus power from each solar cell panel 2 to the power grid 4 .

[0025] It should be noted that each power conversion device 12 does not necessarily have to be connected to the load 8. Each power conversion device 12 may be connected only to the power grid 4, for example, and supply converted AC power only to the power grid 4.

[0026] The control device 14 controls the power conversion operation of the multiple power conversion devices 12. The control device 14 also controls, for example, the switching between an open state and an on state of the multiple switches 16. The control device 14 communicates with, for example, a higher-level controller, and controls the operation of each power conversion device 12 and each switch 16 based on commands input from the higher-level controller.

[0027] 2 is a block diagram illustrating an example of a power conversion device according to an embodiment. As illustrated in FIG. 2, the power conversion device 12 includes a main circuit unit 30 and a control unit 32.

[0028] The main circuit unit 30 is connected to at least one of the plurality of solar cell panels 2. The main circuit unit 30 converts the power supplied from the connected solar cell panel 2 into AC power compatible with the power grid 4, and supplies the converted AC power to the load 8 or the power grid 4.

[0029] The main circuit unit 30 has, for example, a plurality of first terminals 41a, 41b, a plurality of second terminals 42a to 42c, a plurality of switching elements 44a to 44d, a plurality of rectifying elements 46a to 46d, and a plurality of charge storage elements 48a, 48b.

[0030] The multiple first terminals 41a, 41b are connected to the solar cell panel 2. The multiple first terminals 41a, 41b are terminals for receiving power from the solar cell panel 2. In other words, the multiple first terminals 41a, 41b are input terminals. In this example, the main circuit unit 30 (power conversion device 12) has a pair of first terminals 41a, 41b corresponding to the DC power of the solar cell panel 2. One first terminal 41a is connected to the high-potential output terminal of the solar cell panel 2. The other first terminal 41b is connected to the low-potential output terminal of the solar cell panel 2. As a result, the DC power output from the solar cell panel 2 is supplied to the main circuit unit 30 via the pair of first terminals 41a, 41b.

[0031] The second terminals 42a to 42c are connected to the power grid 4 and the load 8. The second terminals 42a to 42c are terminals for supplying converted AC power to the power grid 4 or the load 8. In other words, the second terminals 42a to 42c are output terminals. In this example, the main circuit unit 30 (power conversion device 12) has three second terminals 42a to 42c corresponding to the three-phase AC power of the power grid 4.

[0032] In this example, the main circuit section 30 has a pair of charge storage elements 48a, 48b connected in series between a pair of first terminals 41a, 41b.

[0033] Each of the switching elements 44a to 44d is a semiconductor switching element such as an IGBT or a MOSFET. Each of the switching elements 44a to 44d has a pair of main terminals and a control terminal, and has an ON state in which current flows between the pair of main terminals, and an OFF state in which current flow between the pair of main terminals is blocked. The OFF state does not necessarily have to be a state in which current flow between the pair of main terminals is completely blocked, but may also be a state in which a weak current flows between the pair of main terminals that does not affect the operation of the main circuit unit 30. In other words, the OFF state is a state in which the magnitude of the current flowing between the pair of main terminals is smaller than in the ON state. Each of the switching elements 44a to 44d is not limited to the above, and may be any element that can be switched between an ON state and an OFF state.

[0034] The switching elements 44a, 44b are connected in series between the pair of first terminals 41a, 41b. In other words, the switching elements 44a, 44b are provided in parallel with the pair of charge storage elements 48a, 48b. When the switching elements 44a, 44b are turned on, the direction of current flow is from the first terminal 41a on the high potential side to the first terminal 41b on the low potential side.

[0035] The switching elements 44c and 44d are connected in series between the connection point of the pair of charge storage elements 48a and 48b and the connection point of the switching elements 44a and 44b. When the switching element 44c is turned on, the direction of current flow is from the connection point of the switching elements 44a and 44b to the connection point of the charge storage elements 48a and 48b. When the switching element 44d is turned on, the direction of current flow is from the connection point of the charge storage elements 48a and 48b to the connection point of the switching elements 44a and 44b. Thus, the direction of current flow when the switching element 44c is turned on is opposite to the direction of current flow when the switching element 44d is turned on.

[0036] Each of the rectifying elements 46a to 46d is connected in anti-parallel to each of the switching elements 44a to 44d. The direction of current flowing through each of the rectifying elements 46a to 46d is opposite to the direction of current flowing when each of the switching elements 44a to 44d is turned on. Each of the rectifying elements 46a to 46d is, for example, a free wheel diode.

[0037] The connection point of the switching elements 44a, 44b is connected to the connection point of the charge storage elements 48a, 48b via the switching elements 44c, 44d, and is also connected to the second terminal 42a, so that the second terminal 42a (the connection point of the switching elements 44a, 44b) serves as an AC output point for one phase of the three-phase AC power output from the main circuit unit 30.

[0038] Fig. 2 shows only the configuration of one phase of the three-phase AC power output from the main circuit unit 30 (power conversion device 12). The configurations of the other phases of the main circuit unit 30 are substantially the same as the configuration shown in Fig. 2, so detailed description thereof will be omitted. Note that the charge storage elements 48a, 48b may be used in common for the circuits of each phase, for example.

[0039] In this example, the main circuit unit 30 is a three-level inverter of a bidirectional switch type, which controls the switching of the switching elements 44a to 44d, thereby converting the DC power supplied from the solar panel 2 into three-phase AC power.

[0040] However, the configuration of the main circuit unit 30 is not limited to the above, and the main circuit unit 30 may be, for example, a two-level inverter.

[0041] In this example, a solar cell panel 2 is shown as an example of a distributed power source. The distributed power source is not limited to the solar cell panel 2, and may be, for example, another power generation device such as a wind power generation device or a geothermal power generation device, or may be a power storage device. The power supplied from the distributed power source to the main circuit unit 30 is not limited to DC power, and may be AC ​​power or the like. The main circuit unit 30 may be configured, for example, to convert AC power supplied from the distributed power source to DC power, and further convert the DC power into AC power compatible with the power grid 4.

[0042] The power supplied from the distributed power sources to the main circuit unit 30 is not limited to power generated by the distributed power sources, but may be power stored in a distributed power source (power storage device). Furthermore, if the distributed power source is a power storage device, the main circuit unit 30 may further have a function of converting AC power supplied from the power grid 4 into DC power compatible with the power storage device and storing the converted DC power in the power storage device.

[0043] In this example, the main circuit unit 30 has a pair of charge storage elements 48a, 48b. The number of charge storage elements provided in the main circuit unit 30 is not limited to two, and may be one, three, or more. The DC power stored in the charge storage element may be DC power supplied from a distributed power source, or may be DC power obtained by converting AC power supplied from the distributed power source using a rectifier circuit or the like. The charge storage element may be configured in any way that can store DC power based on power supplied from multiple distributed power sources. The number of multiple switching elements may be any number that can convert power supplied from the distributed power sources into AC power compatible with the power grid 4.

[0044] For example, if the power supplied from the distributed power sources is three-phase AC power, the main circuit unit 30 has three first terminals. For example, if the AC power of the power grid 4 is single-phase AC power, the main circuit unit 30 has two second terminals. In this way, the number of the multiple first terminals may be set appropriately depending on the power supplied from the distributed power sources, etc. The number of the multiple second terminals may be set appropriately depending on the AC power of the power grid 4, etc.

[0045] In this way, the configuration of main circuit unit 30 may be set appropriately depending on the configuration of the distributed power sources, the power supplied from the distributed power sources, etc. Main circuit unit 30 may have any configuration that includes at least a charge storage element that stores DC power based on the power supplied from the connected distributed power sources, a plurality of switching elements that convert the DC power stored in the charge storage element into AC power, and a plurality of rectifier elements connected in anti-parallel to each of the plurality of switching elements, and that can convert the power supplied from the connected distributed power sources into AC power compatible with power grid 4 by switching the plurality of switching elements.

[0046] The control unit 32 controls the power conversion operation by the main circuit unit 30. The control unit 32 is connected, for example, to the control terminals of each of the switching elements 44a to 44d, and controls the power conversion operation by the main circuit unit 30 by controlling the switching of each of the switching elements 44a to 44d. The control unit 32 is also connected, for example, to the control device 14, and controls the operation of the main circuit unit 30 based on commands input from the control device 14. The control unit 32 receives, for example, a command for an output target value from the control device 14, and controls the operation of the main circuit unit 30 so as to supply AC power of a magnitude corresponding to the output target value to the load 8 or the power system 4.

[0047] In the example shown in Figure 2, when the voltage of the DC power supplied from the solar cell panel 2 between the pair of first terminals 41a, 41b is E, the voltage of each charge storage element 48a, 48b is E / 2.

[0048] At this time, by turning on the switching elements 44a and 44d and turning off the switching elements 44b and 44c, a voltage of E / 2 from the charge storage element 48a is output to the second terminal 42a. By turning on the switching elements 44c and 44d and turning off the switching elements 44a and 44b, the voltage (0 V) at the neutral point of the charge storage elements 48a and 48b is output to the second terminal 42a. Then, by turning off the switching elements 44a and 44d and turning on the switching elements 44b and 44c, a voltage of −E / 2 from the charge storage element 48b is output to the second terminal 42a.

[0049] In this way, in this example, by controlling the switching of each of the switching elements 44a to 44d, it is possible to output voltages of three levels, E / 2, 0, and -E / 2, from the second terminals 42a to 42c.

[0050] The control unit 32 controls the switching of the switching elements 44a to 44d as described above, and stops the operation of the main circuit unit 30 when a command to stop operation is input from the control device 14 while the main circuit unit 30 is performing power conversion. The control unit 32 stops the operation of the main circuit unit 30, for example, by turning off each of the switching elements 44a to 44d. In other words, the state in which the operation of the main circuit unit 30 is stopped is a gate block state in which each of the switching elements 44a to 44d is set to the off state. In other words, the state in which the operation of the main circuit unit 30 is stopped is a grid-connection standby state in which the supply of AC power from the main circuit unit 30 to the power grid 4 and the load 8 is stopped.

[0051] Furthermore, when the distributed power source is a solar cell panel 2, the control unit 32 monitors, for example, the amount of power generated by the solar cell panel 2. The control unit 32 is connected, for example, to a voltage detector (not shown) and monitors the amount of power generated by the solar cell panel 2 based on the DC voltage of the solar cell panel 2 detected by the voltage detector. When the amount of power generated by the solar cell panel 2 is equal to or greater than a predetermined threshold, the control unit 32 controls the operation of the main circuit unit 30 and supplies AC power from the main circuit unit 30 to the load 8 or the power grid 4. When the amount of power generated by the solar cell panel 2 is less than the predetermined threshold, the control unit 32 stops the operation of the main circuit unit 30. When the amount of power generated by the solar cell panel 2 changes from a state equal to or greater than the predetermined threshold to a state less than the threshold due to factors such as sunset or clouds while controlling the operation of the main circuit unit 30, the control unit 32 stops the operation of the main circuit unit 30. Note that the method for monitoring the amount of power generated by the solar cell panel 2 is not limited to the above, and any method that can appropriately monitor the amount of power generated by the solar cell panel 2 may be used.

[0052] When stopping the operation of the main circuit unit 30, the control unit 32 does not stop the operation of the main circuit unit 30 immediately in response to a command to stop operation or a decrease in the amount of power generated by the solar panel 2, but rather causes the main circuit unit 30 to perform a predetermined operation for a predetermined period from the timing at which it is decided to stop the operation of the main circuit unit 30, and then stops the operation of the main circuit unit 30. The timing at which it is decided to stop the operation of the main circuit unit 30 is, for example, the timing at which it receives an input of a command to stop operation or the timing at which it detects a decrease in the amount of power generated.

[0053] The predetermined operation is an operation of switching each of the switching elements 44a to 44d of the main circuit unit 30 so as to suppress the output of current from the main circuit unit 30 to the power grid 4 (load 8) and the inflow of current from the power grid 4 to the main circuit unit 30. The predetermined operation is, for example, an operation of switching each of the switching elements 44a to 44d in a state where the output current command value of the main circuit unit 30 is set to 0. The predetermined operation is, for example, an operation of operating the main circuit unit 30 at 0 output.

[0054] For example, the control device 14 switches the switching elements 44a to 44d of the main circuit unit 30 in a predetermined operation so that the voltage waveform of the AC power output from the main circuit unit 30 matches the voltage waveform of the power grid 4. This allows the control device 14 to switch the switching elements 44a to 44d of the main circuit unit 30 so as to suppress the output of current from the main circuit unit 30 and the inflow of current from the power grid 4 to the main circuit unit 30.

[0055] However, the predetermined operation is not limited to the above, and may be any operation that can suppress the output of current from the main circuit unit 30 to the power grid 4 and the inflow of current from the power grid 4 to the main circuit unit 30.

[0056] For example, the control device 14 disconnects the power conversion device 12 whose main circuit unit 30 has stopped operating from the power grid 4 by switching the switch 16 connected to the power conversion device 12 whose main circuit unit 30 has stopped operating from an on state to an open state after the main circuit unit 30 of each power conversion device 12 has performed a predetermined operation for a predetermined period of time and then stopped operating.

[0057] The control unit 32 also has a Fault Ride Through (FRT) function that allows the main circuit unit 30 to continue operation for a certain period of time even when an abnormality (system fault) such as an instantaneous drop occurs in the AC power of the power system 4. Requirements for the FRT function are, for example, "continuing operation without gate blocking in response to an instantaneous voltage drop of 20% or more that lasts for one second or less, and restoring output to 80% or more before the voltage drop within 0.1 seconds after voltage recovery," and "responding with gate blocking in response to a voltage drop of less than 20% that lasts for one second or less, and restoring output to 80% or more before the voltage drop within one second after voltage recovery."

[0058] The control unit 32 detects an abnormality in the AC power of the power grid 4, and causes the main circuit unit 30 to operate the FRT function in response to the detection of the abnormality in the AC power of the power grid 4. If the abnormality in the AC power of the power grid 4 is resolved within a certain time period after the detection of the abnormality in the AC power of the power grid 4, the control unit 32 returns from the FRT function to normal operation in response to the detection of the elimination of the abnormality in the AC power of the power grid 4. The normal operation is an operation of converting the power supplied from each solar cell panel 2 into AC power compatible with the power grid 4 and supplying the converted AC power to the load 8 or the power grid 4. As described above, the certain time period is, for example, one second. The certain time period is a time period specified in the FRT function.

[0059] On the other hand, if the abnormality in the AC power of the power system 4 is not resolved within a certain period of time after the detection of the abnormality in the AC power of the power system 4, the control unit 32 causes the main circuit unit 30 to perform the above-mentioned specified operation for a certain period of time, and then stops the operation of the main circuit unit 30, in accordance with the passage of the certain period of time.

[0060] Note that detection of an abnormality in the AC power of the power system 4 can be performed, for example, by providing a voltmeter or ammeter (not shown) to detect the voltage and current of the AC power of the power system 4. The determination of whether or not the AC power of the power system 4 is abnormal may be performed by the control unit 32 based on the voltage and current detection results, or by a higher-level controller such as the control device 14. The control unit 32 may detect an abnormality in the AC power of the power system 4 by making a determination based on the voltage and current detection results, or may receive an abnormality detection signal from a higher-level controller and detect an abnormality in the AC power of the power system 4 based on the input of the abnormality detection signal. The method of detecting an abnormality in the AC power of the power system 4 by the control unit 32 is not limited to the above, and any method that can appropriately detect an abnormality in the AC power of the power system 4 may be used. However, the control unit 32 does not necessarily have to have the FRT function. The control unit 32 does not necessarily have to detect an abnormality in the AC power of the power system 4.

[0061] As described above, in the grid-connected power conversion system 10 according to this embodiment, when the control unit 32 stops the operation of the main circuit unit 30, it causes the main circuit unit 30 to perform a predetermined operation for a predetermined period of time from the timing at which it has decided to stop the operation of the main circuit unit 30, and then stops the operation of the main circuit unit 30.

[0062] If the operation of the main circuit unit 30 is stopped immediately without performing the specified operation as described above, and there is variation in the timing at which the main circuit unit 30 of each power conversion device 12 stops, the high-frequency circulating current (cross current) associated with the operation of the main circuit unit 30 of the power conversion device 12 that is still operating may flow into the main circuit unit 30 of the power conversion device 12 that has stopped operating, and an overvoltage may be generated in the charge storage elements 48a, 48b of the main circuit unit 30 of the power conversion device 12 that has stopped operating.

[0063] High-frequency components associated with the switching of the switching elements 44a to 44d are superimposed on the AC power supplied from each power conversion device 12 to the power grid 4. The high-frequency circulating current is, for example, an AC current with a higher frequency than the frequency of the AC current supplied from each power conversion device 12 to the power grid 4. Furthermore, as shown in Fig. 1 , a relatively large stray capacitance Cs exists between the solar cell panel 2 (distributed power source) and the ground, and between the conductor connecting the solar cell panel 2 and the power conversion device 12 and the ground.

[0064] Therefore, if there is variation in the timing at which the main circuit units 30 of the power conversion devices 12 are stopped, a current with high-frequency components superimposed on the AC power supplied to the power grid 12 flows in a circulating manner via the ground between the operating power conversion device 12 and the stopped power conversion device 12. The current with high-frequency components returns to the operating power conversion device 12 via, for example, the rectifier element 46a, the charge storage elements 48a, 48b of the main circuit unit 30 of the stopped power conversion device 12, the stray capacitance Cs between the ground and the ground, and the like. This may charge the charge storage elements 48a, 48b of the stopped power conversion device 12, causing an overvoltage.

[0065] In Fig. 1, the dashed lines show an example of a circulating current that flows in a circulating manner via the ground between the operating power conversion device 12 and the stopped power conversion device 12 when the main circuit unit 30 of the upper power conversion device 12 has stopped operating and the main circuit unit 30 of the lower power conversion device 12 is operating. Fig. 1 shows an example of a current that circulates counterclockwise via each power conversion device 12 and the ground.

[0066] 2 , a high-frequency circulating current generated by the main circuit unit 30 of the operating power conversion device 12 may flow into the charge storage elements 48a, 48b via the second terminal 42a and the rectifying element 46a of the main circuit unit 30 of the stopped power conversion device 12, potentially causing overvoltage in the charge storage elements 48a, 48b. Furthermore, if the distributed power source is a solar panel 2, the voltage of the DC power supplied from the solar panel 2 may transition from an operating voltage to an open-circuit voltage when the power conversion device 12 stops operating, potentially causing the voltages of the charge storage elements 48a, 48b to become higher than they were during operation. If a circulating current flows through the rectifying element 46a when the voltages of the charge storage elements 48a, 48b are high, the rectifying element 46a may suffer overvoltage breakdown.

[0067] Furthermore, when the distributed power source is a solar cell panel 2, for example, the amount of power generated by the solar cell panel 2 may drop to near a threshold value during dusk, causing the power conversion device 12 to alternate between operating the main circuit unit 30 and stopping the operation of the main circuit unit 30. When multiple solar cell panels 2 are installed, there is a possibility that the amount of power generated by each solar cell panel 2 will vary, and the timing at which the main circuit unit 30 of each power conversion device 12 stops operating will also vary. In this case, if the main circuit unit 30 is operated when the charge storage elements 48a, 48b are overvoltage, there is a possibility that each of the switching elements 44a to 44d will malfunction.

[0068] In the grid-connected power conversion system 10 according to the present embodiment, when the operation of the main circuit unit 30 is stopped, the main circuit unit 30 is caused to perform an operation to suppress the output of current from the main circuit unit 30 to the power grid 4 (load 8) and the inflow of current from the power grid 4 to the main circuit unit 30. By causing the main circuit unit 30 to perform an operation to suppress the output and inflow of current (hereinafter referred to as a zero-output operation operation) before stopping the main circuit unit 30, even if there is variation in the timing at which the main circuit units 30 of the power conversion devices 12 are stopped, it is possible to prevent a circulating current, which is caused by the operation of the main circuit unit 30 of the power conversion device 12 that is still operating, from flowing to the main circuit unit 30 of the power conversion device 12 that has stopped operating. For example, by causing the main circuit unit 30 to perform a zero-output operation operation, it is possible to prevent a current from flowing through the rectifying element 46a connected in anti-parallel to the switching element 44a, and thereby prevent a circulating current from flowing between the power conversion devices 12.

[0069] In this way, the grid-connected power conversion system 10 according to this embodiment can suppress the generation of circulating current without providing an isolation transformer, reactor, or the like between the output side of each power conversion device 12 and the connection point CP. Therefore, the grid-connected power conversion system 10 according to this embodiment can suppress the generation of circulating current between the multiple power conversion devices 12 with a simpler configuration. For example, it is possible to suppress an increase in the size and cost of the entire system due to an increase in the number of components such as isolation transformers and reactors.

[0070] Furthermore, by having the main circuit unit 30 perform zero-output operation before shutting down, even when the amount of power generated by the solar panel 2 repeatedly fluctuates between being below the threshold and being above the threshold, such as during dusk, the generation of circulating current can be suppressed, and the switching elements 44a to 44d can be prevented from suffering overvoltage breakdown or the like.

[0071] The length of the predetermined period during which the zero-output operation is performed is set, for example, longer than the length of the period during which the amount of power generated by the solar cell panel 2 repeatedly changes between a state below the threshold and a state above the threshold. The length of the period during which the amount of power generated by the solar cell panel 2 repeatedly changes between a state below the threshold and a state above the threshold, such as during dusk, is, for example, about 5 to 10 seconds. Therefore, the length of the predetermined period during which the zero-output operation is performed is set, for example, to 10 seconds or more. This makes it possible to appropriately suppress the generation of circulating current even when, for example, the timing at which the amount of power generated by the solar cell panel 2 repeatedly changes between a state below the threshold and a state above the threshold differs for each of the multiple power conversion devices 12.

[0072] For example, when the amount of power generated by the solar cell panel 2 falls below a threshold and the control unit 30 is causing the main circuit unit 30 to perform zero-output operation, if the amount of power generated by the solar cell panel 2 returns to or above the threshold, the control unit 30 stops the zero-output operation and causes the main circuit unit 30 to perform normal operation. In this way, the length of the predetermined period during which the zero-output operation is performed is set longer than the length of the period during which the amount of power generated by the solar cell panel 2 repeatedly changes between a state below the threshold and a state above the threshold. If the amount of power generated by the solar cell panel 2 returns to or above the threshold during the zero-output operation, the control unit 30 stops the zero-output operation and causes the main circuit unit 30 to return to normal operation. In this case, when the amount of power generated by the solar cell panel 2 repeatedly changes between a state below the threshold and a state above the threshold, the main circuit unit 30 repeats normal operation and zero-output operation. This makes it possible to more appropriately suppress the generation of circulating current even when the timing at which the amount of power generated by the solar cell panel 2 repeatedly changes between a state below the threshold and a state above the threshold differs for each of the multiple power conversion devices 12.

[0073] For example, when one of the multiple power conversion devices 12 stops operating due to a decrease in the amount of power generated by the solar cell panel 2, the control device 14 may input a command to stop operation to each power conversion device 12, thereby simultaneously stopping the operation of the main circuit unit 30 of each power conversion device 12. However, in this case, the power conversion device 12 that has excess power generation capacity of the solar cell panel 2 will also be stopped, which may result in an inability to effectively utilize the power generated by the multiple solar cell panels 2. For example, this may result in an increase in the amount of power purchased from the power grid 4 or a decrease in the amount of power sold to the power grid 4.

[0074] In the grid-connected power conversion system 10 according to this embodiment, by causing the main circuit unit 30 to perform zero-power operation before shutting down, the main circuit unit 30 of the power conversion device 12 in which the amount of power generated by the solar cell panel 2 has decreased can be caused to perform zero-power operation, while the main circuit unit 30 of the power conversion device 12 in which the amount of power generated by the solar cell panel 2 has surplus power can continue normal operation. Therefore, compared to a case in which the control device 14 inputs a command to shut down operation to each power conversion device 12 and simultaneously shuts down the operation of the main circuit unit 30 of each power conversion device 12, the power generated by the multiple solar cell panels 2 can be used more effectively. For example, an increase in power purchased from the power grid 4 and a decrease in power sold to the power grid 4 can be suppressed.

[0075] Furthermore, when a command to stop operation is input from the control device 14 to each power conversion device 12, there is a possibility that variations in the timing at which the operation of each power conversion device 12 is stopped may occur due to control delays, etc. By having the main circuit unit 30 perform zero output operation before stopping, even when variations occur in the input timing of the command from the control device 14, it is possible to suppress the occurrence of circulating current and prevent failure of the main circuit unit 30.

[0076] Furthermore, the inventors of the present application have found through careful study that when the FRT function is operated, variations tend to occur in the timing at which the main circuit unit 30 of each power conversion device 12 is stopped. In the grid-connected power conversion system 10 according to this embodiment, the control unit 32 causes the main circuit unit 30 to operate the FRT function in response to detection of an abnormality in the AC power of the power system 4, and if the abnormality in the AC power of the power system 4 is not resolved within a predetermined time from the detection of the abnormality, causes the main circuit unit 30 to perform zero output operation for a predetermined period of time and then stops the operation of the main circuit unit 30 in response to the elapse of the predetermined time.

[0077] This makes it possible to appropriately suppress the occurrence of circulating currents between multiple power conversion devices 12 and the resulting failures due to overvoltage in each power conversion device 12 even when the FRT function is operating.

[0078] Furthermore, after the main circuit unit 30 of each power conversion device 12 performs a predetermined operation for a predetermined period of time and then stops the operation, the control device 14 switches the switch 16 connected to the power conversion device 12 whose main circuit unit 30 has stopped operating from the closed state to the open state, thereby paralleling off the power conversion device 12 whose main circuit unit 30 has stopped operating from the power grid 4. By switching the switch 16 connected to the power conversion device 12 whose main circuit unit 30 has stopped operating from the closed state to the open state in this manner, it is possible to more appropriately suppress, for example, the inflow of circulating current into the power conversion device 12 whose main circuit unit 30 has stopped operating. Furthermore, by switching each switch 16 to the open state, it is possible to suppress, for example, the unintended inflow of AC power from the power grid 4 into each power conversion device 12.

[0079] The predetermined period during which the predetermined operation to suppress the output of current from each power electronics device 12 and the inflow of current from the power grid 4 to each power electronics device 12 is performed may be set to any period that allows appropriate suppression of the generation of circulating current between each power electronics device 12. It is more preferable to set the predetermined period to, for example, the minimum time that allows appropriate suppression of the generation of circulating current. This makes it possible to appropriately suppress the generation of circulating current while preventing the time until the operation of each power electronics device 12 is stopped from becoming unnecessarily long.

[0080] This embodiment includes the following aspects: (Supplementary Note 1) A power conversion system includes a plurality of power conversion devices connected to a plurality of distributed power sources and connected in parallel to an AC power grid, wherein the plurality of power conversion devices include: a main circuit unit connected to at least one of the plurality of distributed power sources and converting power supplied from the at least one connected distributed power source into AC power compatible with the power grid; and a control unit controlling the power conversion operation of the main circuit unit, wherein the main circuit unit includes a charge storage element that stores DC power based on the power supplied from the at least one distributed power source, a plurality of switching elements that convert the DC power stored in the charge storage element into AC power, and a plurality of rectifier elements connected in anti-parallel to each of the plurality of switching elements, and wherein the power supplied from the at least one distributed power source is converted into AC power compatible with the power grid by switching the plurality of switching elements. The control unit controls the power conversion operation by the main circuit unit by controlling the switching of the plurality of switching elements, and when stopping the operation of the main circuit unit, causes the main circuit unit to perform a predetermined operation for a predetermined period from the timing at which it was decided to stop the operation of the main circuit unit, and then stops the operation of the main circuit unit, the predetermined operation being an operation of switching the plurality of switching elements so as to suppress the output of current from the main circuit unit and the inflow of current from the power grid to the main circuit unit.

[0081] (Supplementary Note 2) The grid-connected power conversion system according to Supplementary Note 1, wherein the control unit has a function of continuing operation of the main circuit unit for a certain period of time even when an abnormality occurs in the AC power of the power system, detects an abnormality in the AC power of the power system, and causes the main circuit unit to perform the operation of the function in response to the detection of the abnormality in the AC power of the power system, and if the abnormality in the AC power of the power system is resolved within the certain period of time from the detection of the abnormality in the AC power of the power system, returns from the operation of the function to normal operation in response to the detection of the elimination of the abnormality in the AC power of the power system, and if the abnormality in the AC power of the power system is not resolved by the lapse of the certain period of time from the detection of the abnormality in the AC power of the power system, causes the main circuit unit to perform the predetermined operation for the certain period of time and then stops the operation of the main circuit unit in response to the lapse of the certain period of time.

[0082] (Supplementary Note 3) The grid-connected power conversion system according to Supplementary Note 1 or 2, further comprising: a plurality of switches provided between each of the plurality of power conversion devices and the power grid, the switches having an on state that connects the plurality of power conversion devices to the power grid and an open state that disconnects the plurality of power conversion devices from the power grid; and a control device that controls the power conversion operation by the plurality of power conversion devices and the switching of the plurality of switches between the open state and the on state, wherein the control device switches the switches connected to the power conversion device whose main circuit unit has stopped operating from the on state to the open state.

[0083] 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.

[0084] DESCRIPTION OF SYMBOLS 2...Solar panel (distributed power source), 4...Power system, 6...Transformer, 8...Load, 10...Grid-connected power conversion system, 12...Power conversion device, 14...Control device, 16...Switch, 30...Main circuit section, 32...Control section, 41a, 41b...First terminal, 42a to 42c...Second terminal, 44a to 44d...Switching elements, 46a to 46d...Rectifying elements, 48a, 48b...Charge storage elements, CP...Connection point

Claims

1. A system-connected power conversion system includes a plurality of power conversion devices connected to a plurality of distributed power sources and connected in parallel to an AC power grid. The plurality of power conversion devices include: a main circuit unit connected to at least one of the plurality of distributed power sources and configured to convert power supplied from the connected at least one distributed power source into AC power corresponding to the power grid; and a control unit configured to control an operation of power conversion by the main circuit unit. The main circuit unit includes a charge storage element configured to store DC power based on the power supplied from the at least one distributed power source, a plurality of switching elements configured to convert the DC power stored in the charge storage element into AC power, and a plurality of rectifying elements connected in anti-parallel to each of the plurality of switching elements. The plurality of switching elements are switched to convert the power supplied from the at least one distributed power source into AC power corresponding to the power grid. The control unit controls the switching of the plurality of switching elements to control the operation of power conversion by the main circuit unit, and when stopping the operation of the main circuit unit, after causing the main circuit unit to perform a predetermined operation for a predetermined period from the timing when the stop of the operation of the main circuit unit is determined, the control unit stops the operation of the main circuit unit. The predetermined operation is an operation of switching the plurality of switching elements so as to suppress output of current from the main circuit unit and inflow of current from the power grid into the main circuit unit.

2. The control unit has a function of continuing the operation of the main circuit unit for a certain period even when an abnormality occurs in the AC power of the power system, detects an abnormality in the AC power of the power system, causes the main circuit unit to perform the operation of this function in response to the detection of the abnormality in the AC power of the power system, and when the abnormality in the AC power of the power system is resolved within the certain period from the detection of the abnormality in the AC power of the power system, returns from the operation of this function to the normal operation in response to the detection of the resolution of the abnormality in the AC power of the power system, and when the abnormality in the AC power of the power system is not resolved until the elapse of the certain period from the detection of the abnormality in the AC power of the power system, after causing the main circuit unit to perform the predetermined operation for the predetermined period in response to the elapse of the certain period, stops the operation of the main circuit unit. The system-connected power conversion system according to claim 1.

3. A plurality of switches provided between each of the plurality of power conversion devices and the power system, having an input state for connecting the plurality of power conversion devices to the power system and an open state for disconnecting the plurality of power conversion devices from the power system, and a control device for controlling the operation of power conversion by the plurality of power conversion devices and the switching between the open state and the input state of the plurality of switches. The system-connected power conversion system according to claim 1, further comprising, wherein the control device switches the switch connected to the power conversion device whose main circuit unit has stopped operating from the input state to the open state.

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