DC bus control system

Abstract

This DC bus control system controls power fluctuation in a DC bus which is connected between an input power supply and a load. The DC bus control system includes: a main stabilization device that has a first charging / discharging element and a first power converter; and a plurality of sub-stabilization devices that each have a second charging / discharging element, a charging element, or a discharging element, and a second power converter. The plurality of sub-stabilization devices each include: a first sub-stabilization device that has a charging / discharging element and the second power converter; and at least one second sub-stabilization device that has a charging element or a discharging element and the second power converter. The response speed of the first sub-stabilization device is determined such that the charging amount of the charging element or the discharging amount of the discharging element in the second sub-stabilization device is changed at a predetermined time constant.

Description

Technical Field

[0001] The present disclosure relates to a DC bus control system.

Background Art

[0002] In recent years, as alternative power sources to fossil energy and nuclear energy, power systems using renewable energy such as sunlight, wind power, and wave power have attracted attention, and some of them have already been put into practical use.

[0003] In this type of power system, the generated power varies greatly depending on the weather, season, location, etc. Therefore, in order to maintain the voltage of the DC bus to which the power system is connected within a predetermined allowable range, it is desirable to connect power sources such as solar cells and wind turbines to the DC bus via a power converter with a wide input range and large capacity. However, in that case, increasing the capacity of the power converter will lead to an increase in the size, complexity, and cost of the entire system.

[0004] The applicant of the present application has proposed a control system for efficiently controlling the power fluctuations of the DC bus caused by fluctuations in the input power supply and load (Patent Document 1). In the control system of Patent Document 1, the main stabilization device controls the DC bus voltage based on the power storage amount index and controls the fuel cell, water electrolysis cell, and power storage device. According to the control system of Patent Document 1, it is possible to efficiently control the power fluctuations of the DC bus caused by fluctuations in the input power supply and load.

[0005] However, the control system of Patent Document 1 aims at early stabilization of the DC voltage and does not take into account the deterioration of the water electrolysis cell (charging element) and fuel cell (discharging element). Water electrolysis cells and fuel cells will deteriorate if they are operated with excessive or insufficient current or voltage, or if the fluctuations in current or voltage are too rapid. And when the deterioration of the water electrolysis cell or fuel cell occurs, long-term continuous operation will be hindered.

Prior Art Documents

Patent Documents

[0006] [Patent Document 1] International Publication No. 2019 / 103059 [Summary of the Invention] [Problems to be Solved by the Invention]

[0007] An object of the present invention is to provide a control system that can efficiently control power fluctuations in a DC bus caused by fluctuations in an input power source or a load and can operate stably over a long period of time. [Means for Solving the Problems]

[0008] A DC bus control system for controlling power fluctuations in a DC bus connecting an input power source and a load includes a main stabilization device having a first charge / discharge element and a first power converter, and a plurality of quasi-stabilization devices each having a second charge / discharge element, a charging element, or a discharging element and a second power converter. The first power converter obtains a bus voltage target value and is configured to bidirectionally transfer DC power between the first charge / discharge element and the DC bus so that the voltage of the DC bus matches the bus voltage target value. The second power converter obtains a current target value according to the difference between a threshold value related to charging or discharging of the second charge / discharge element, the charging element, or the discharging element and the voltage of the DC bus, and is configured to transfer DC power between the second charge / discharge element, the charging element, or the discharging element and the DC bus so that a current equal to the current target value flows through the second charge / discharge element, the charging element, or the discharging element. The plurality of quasi-stabilization devices include a first quasi-stabilization device having a charge / discharge element and a second power converter, and at least one second quasi-stabilization device having a charging element or a discharging element and a second power converter. The response speed of the first quasi-stabilization device is determined such that the charge amount of the charging element or the discharge amount of the discharging element of the second quasi-stabilization device changes with a predetermined time constant. [Effects of the Invention]

[0009] According to the present invention, it is possible to efficiently control power fluctuations in a DC bus caused by fluctuations in an input power supply or a load, and to prevent deterioration by slowing down the response speed of a charging element or a discharging element of a quasi-stabilizing device, thereby providing a control system that can be operated stably over a long period of time.

Brief Description of the Drawings

[0010]

Figure 1

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Figure 8B

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Figure 10A

Figure 10B

Embodiments for Carrying Out the Invention

[0011] Hereinafter, embodiments of the present invention will be described with reference to the drawings.

[0012] <First Embodiment> FIG. 1 is an overall configuration diagram of a DC bus control system according to this embodiment. The DC bus control system shown in FIG. 1 includes, as input power sources, a photovoltaic power generation system 10 and a wind power generation system 20, which are renewable energy power source systems. These power generation systems 10 and 20 are connected in parallel and their output sides are connected to a DC bus 70. The photovoltaic power generation system 10 includes a solar cell 11 and a power converter 12, and the wind power generation system 20 includes a wind turbine 21 and a power converter 22.

[0013] The input power source may be any one. When the input power source is a renewable energy power source system, in addition to those described above, it may be one that utilizes energy such as wave power or geothermal energy, or may be a power source system such as hydro (small hydro) power generation, tidal power generation, tidal current power generation, or temperature difference power generation. Also, including those described above, combinations of these may be used.

[0014] Furthermore, the number of power source systems connected in parallel to each other is not particularly limited.

[0015] A main stabilization device 30 and quasi-stabilization devices 40, 50, and 60 are connected to the DC bus 70, and a load 90 is also connected.

[0016] The main stabilization device 30 sets a variable bus voltage target value within a predetermined allowable range centered on a reference bus voltage (the reference voltage of the DC bus 70), and operates the power converter 32 so that the output voltage on the DC bus 70 side matches the bus voltage target value, and controls the charge and discharge of the energy storage device 31.

[0017] Further, the quasi-stabilization device 40 calculates a target value of the input / output current based on the difference between the charge / discharge threshold value and the voltage of the DC bus, and operates the power converter 42 so that the input / output current matches the target value of the input / output current, thereby controlling the charge / discharge of the power storage device 41.

[0018] Here, the power storage devices 31 and 41 are, for example, batteries (secondary batteries), electric double layer capacitors, capacitors, flywheels, or redox flow batteries, etc. Further, the power converters 32 and 42 are, for example, isolated DC / DC converters or choppers, etc., and can transfer DC power bidirectionally as indicated by the arrows.

[0019] The quasi-stabilization device 50 performs DC / DC conversion by the power converter 52 so that the input / output current matches the target value of the input / output current calculated based on the difference between the charge threshold value and the voltage of the DC bus, and supplies DC power to the water electrolysis cell 51 (a kind of charging operation), electrolyzing water to generate hydrogen gas and oxygen gas. Further, the quasi-stabilization device 60 supplies the DC power generated by the electrochemical reaction of the fuel cell 61 to the DC bus 70 via the power converter 62 (a kind of discharging operation), and at this time, the power converter 62 performs DC / DC conversion so that the input / output current matches the target value of the input / output current calculated based on the difference between the discharge threshold value and the voltage of the DC bus.

[0020] The configurations of the quasi-stabilization device 50 and the quasi-stabilization device 60 described above are merely exemplary. As a replacement for the water electrolysis cell 51, electrochemically, means for reducing carbon dioxide to produce C-H bonds (such as CH4, C2H4, etc.) or alcohol, or means for reducing nitrogen to produce ammonia may be used. As a replacement for the fuel cell 61, a fuel cell using alcohol or the like, or power generation means for burning chemical substances (such as hydrogen, C-H systems, alcohol, ammonia, etc.) to rotate a turbine or the like may also be acceptable.

[0021] Figure 2 shows another configuration example of the quasi-stabilization device. As shown in the figure, a quasi-stabilization device 50A having an integrated structure in which the aforementioned quasi-stabilization devices 50 and 60 share a hydrogen storage device 53 may be used.

[0022] In FIG. 1, the power storage devices 31 and 41 can absorb (charge) and discharge DC power. Further, the water electrolysis cell 51 (and the hydrogen storage device 53 in FIG. 2) can convert DC power into gas and store it, and the fuel cell 61 (and also the hydrogen storage device 53) can perform a power generation operation of converting gas into DC power. The power storage devices 31 and 41 constitute charge / discharge elements, the water electrolysis cell 51 (and the hydrogen storage device 53) constitutes a charging element, and the fuel cell 61 (and the hydrogen storage device 53) constitutes a discharging element.

[0023] As described above, each of the stabilization devices 30, 40, 50, and 60 can be regarded as a power buffer that exchanges DC power with the DC bus 70 by the operations of the power converters 32, 42, 52, and 62. The main stabilization device 30 and the sub-stabilization device 40 are power buffers having a charge / discharge function, the sub-stabilization device 50 is a power buffer having a charging function, and the sub-stabilization device 60 is a power buffer having a discharging function.

[0024] Note that only one main stabilization device 30 having a function of setting the bus voltage target value is sufficient, but the number of sub-stabilization devices may be provided as many as necessary according to the number of parallel power systems and the required power of the load 90.

[0025] The monitoring and indicating device 80 collects state information (voltage, current, temperature, etc.) of each power generation system 10 and 20, the main stabilization device 30, and the sub-stabilization devices 40, 50, and 60 to perform state monitoring and operation monitoring, and generates operation commands (start / stop commands, etc.) and charge / discharge threshold value commands, etc. for each part based on these monitoring results. Between the monitoring and indicating device 80 and each of the above-described parts, various monitoring signals and commands can be transmitted and received by wire or wirelessly.

[0026] The load 90 may be a DC load such as a DC motor, or a DC / AC converter that converts DC power into AC power and its AC load. Further, an AC power system may be connected to the DC bus 70 via a DC / AC converter.

[0027] Next, the configuration of each part in FIG. 1 will be described. In the configuration of FIG. 1, a photovoltaic power generation system 10 and a wind power generation system 20 are provided as input power sources.

[0028] The photovoltaic power generation system 10 and the wind power generation system 20 have a common function in that the generated power using renewable energy is converted into DC power by the power converters 12 and 22 and supplied to the DC bus 70. For this reason, hereinafter, the photovoltaic power generation system 10 will be described as an example.

[0029] FIG. 3 is a block diagram showing a configuration example of the power converter 12 in the photovoltaic power generation system 10. This power converter 12 includes a DC / DC conversion unit 12A and a control circuit 12B.

[0030] The DC / DC conversion unit 12A converts the DC output voltage of the solar cell 11 into a DC voltage of a predetermined magnitude by the operation of semiconductor switching elements and outputs it to the DC bus 70, and is configured by, for example, a boost chopper.

[0031] In the control circuit 12B that controls the DC / DC conversion unit 12A, the output voltage and current of the solar cell 11 are detected by a voltage detector 12a and a current detector 12b, and these detected values are input to the MPPT control unit 12c. The MPPT control unit 12c searches for the maximum output point of the solar cell 11 by a hill climbing method or the like and outputs it to the voltage / current control unit 12d.

[0032] The voltage / current control unit 12d sends drive pulses generated by PWM (pulse width modulation) control or the like to the drive circuit 12e, and the drive circuit 12e turns on and off the semiconductor switching elements of the DC / DC conversion unit 12A based on the above drive pulses.

[0033] Also, the voltage of the DC bus 70 is detected by a voltage detector 12f, and this bus voltage detection value is input to a comparison unit 12g together with a bus voltage target value sent from a main stabilization device 30 described later. The comparison unit 12g generates a control signal according to the deviation between the bus voltage detection value and the bus voltage target value and outputs it to the voltage / current control unit 12d.

[0034] Based on the above control signal, the voltage and current control unit 12d calculates a drive pulse to make the detected bus voltage value match the target bus voltage value. For example, when the detected bus voltage value exceeds the target bus voltage value, it performs a control operation to lower the output voltage of the DC / DC conversion unit 12A (including operation stop).

[0035] FIG. 4 is a block diagram showing a configuration example of a power converter 32 in the main stabilizer 30. This power converter 32 includes a DC / DC conversion unit 32A and a control circuit 32B.

[0036] The DC / DC conversion unit 32A has a function of bidirectionally transferring DC power between the DC bus 70 and the power storage device 31 to control the charge and discharge of the power storage device 31, and is composed of an isolated DC / DC converter, a chopper, etc. equipped with semiconductor switching elements. A sensor 31a for detecting voltage, current, and temperature is installed in the power storage device 31.

[0037] The configuration of the control circuit 32B is as follows.

[0038] The voltage detector 32a detects the voltage of the DC bus 70, and the bus voltage target value calculation unit 32b calculates the bus voltage target value according to the first power storage amount index of the power storage device 31. The calculation method of the bus voltage target value will be described later.

[0039] As the above power storage amount index, for example, the charge rate (SOC: State of Charge) obtained by integrating the charge and discharge current of the power storage device 31 detected by the sensor 31a can be used.

[0040] The offset calculation unit 32c calculates an offset of the bus voltage target value based on the difference between the second power storage amount index of the power storage device 31 and the target value of the second power storage amount index. The second power storage amount index is an index of the power storage amount of the power storage device 31 obtained by a method different from the first power storage amount index. For example, the terminal voltage (battery voltage) of the power storage device 31 detected by the sensor 31a can be used.

[0041] The offset calculation unit 32c obtains the above-mentioned offset, for example, as a value corresponding to the integrated value of the difference between the second power storage amount index and its target value. More specifically, the offset is obtained as a value obtained by multiplying a value obtained by integrating the difference between the second power storage amount index and its target value by a predetermined gain.

[0042] Here, when the magnitude (absolute value) of the charge and discharge current of the power storage device 31 is greater than a predetermined value, the integration operation may not be performed, and the integration operation may be performed only when the magnitude of the charge and discharge current is smaller than the predetermined value. That is, the offset may be obtained as a value corresponding to a value obtained by integrating the difference between the second power storage amount index and the target value of the power storage amount index only when the magnitude of the charge and discharge current of the power storage device 31 is smaller than the predetermined value. The predetermined value here is determined, for example, as a value such that if the magnitude of the charge and discharge current of the power storage device 31 is smaller than that value, it can be expected that the second power storage amount index (for example, the terminal voltage) accurately represents the power storage amount of the power storage device 31.

[0043] The deviation between the bus voltage target value and the bus voltage detection value is calculated by the subtractor 32d, and further, the offset is subtracted from the voltage deviation by the subtractor 32e, and the voltage deviation after offset correction is input to the charge and discharge control unit 32f.

[0044] The charge / discharge control unit 32f receives the voltage, current, temperature, and charge / discharge threshold values of the power storage device 31. The charge / discharge control unit 32f generates drive pulses by performing PWM control or the like while considering these input information so that the detected bus voltage value matches the bus voltage target value. The drive circuit 32g turns on and off the semiconductor switching element of the DC / DC conversion unit 32A according to the above drive pulses. The DC / DC conversion unit 32A controls the charging and discharging of the power storage device 31 as described above to make the detected bus voltage value match the bus voltage target value.

[0045] Generally, the power storage amount of the power storage device 31 can be roughly estimated from its terminal voltage. Although the terminal voltage also varies depending on the magnitude of the charge / discharge current, when the charge / discharge current is sufficiently small (when it is below the above predetermined value), the terminal voltage can be regarded as representing the power storage amount of the power storage device 31. Therefore, by integrating the difference between the terminal voltage and its target value only when the charge / discharge current is small, similar to the integral control of PID control, the offset of the steady-state power storage index can be removed. That is, the power storage amount of the power storage device 31 can be maintained at the target value of the second power storage amount index, and long-term continuous operation can be stably performed without the actual power storage amount of the power storage device 31 running out or overflowing.

[0046] Note that the charge / discharge threshold value of the power storage device 31 may be set by the control circuit 32B itself or received as a command from the monitoring / indicating device 80 in FIG. 1.

[0047] FIG. 5 is a block diagram showing a configuration example of the power converter 42 in the quasi-stabilization device 40 of FIG. 1. This power converter 42 includes a DC / DC conversion unit 42A and a control circuit 42B. The power converter 42 has the same function as the power converter 32 in FIG. 4A in that it bidirectionally transfers DC power between the DC bus 70 and the power storage device 41. The power storage device 41 is provided with a sensor 41a for detecting voltage, current, and temperature, similar to the power storage device 31. The control circuit 42B includes a voltage detector 42a, a comparison unit 42b, a subtractor 42c, a charge / discharge control unit 42d, and a drive circuit 42e.

[0048] The power converter 42 shown in Fig. 5 differs from the power converter 32 in Fig. 4A in the following respects. The control circuit 42B calculates the target input / output current value of the charge / discharge control unit 42d based on the deviation between the charge / discharge threshold value and the detected bus voltage value. The charge / discharge control unit 42d further performs charge / discharge control on the power storage device 41 so that the input / output current of the DC / DC conversion unit 42A matches the target input / output current value. Here, the charge / discharge threshold value may be a threshold value related to the charge / discharge of the power storage device 41 (charge threshold value and discharge threshold value), and the target input / output current value may be determined according to the difference between the threshold value and the voltage of the DC bus 70.

[0049] Furthermore, the comparison unit 42b provided in the control circuit 42B compares the charge / discharge threshold value of the power storage device 41 with the detected bus voltage value, and outputs a charge command or a discharge command according to the magnitude relationship between the charge threshold value or the discharge threshold value and the detected bus voltage value to control the operation of the charge / discharge control unit 42d. Note that the charge / discharge threshold value may be set by the control circuit 42B itself or received as a command from the monitoring / indicating device 80.

[0050] Fig. 6 is a block diagram showing a configuration example of the power converter 52 in the quasi-stabilization device 50. This power converter 52 includes a DC / DC conversion unit 52A and a control circuit 52B.

[0051] The DC / DC conversion unit 52A has a function of converting the DC power of the DC bus 70 into a predetermined magnitude and supplying it to the water electrolysis cell 51, and is composed of an isolated DC / DC converter or a chopper equipped with semiconductor switching elements. The water electrolysis cell 51 electrolyzes water using the DC power supplied from the DC / DC conversion unit 52A and stores the generated hydrogen gas in an external storage device (not shown), in other words, performs a kind of charging operation.

[0052] The control circuit 52B that controls the DC / DC conversion unit 52A is generally configured in the same way as the control circuit 42B in Fig. 5.

[0053] That is, in the control circuit 52B of FIG. 6, the voltage detector 52a detects the voltage of the DC bus 70, and the subtractor 52c calculates the deviation between the charging threshold value and the bus voltage detection value, and this voltage deviation is input to the charge control unit 52d. Further, the bus voltage detection value is input to the comparison unit 52b together with the charging threshold value, and the comparison unit 52b outputs a charging command to the charge control unit 52d when the bus voltage detection value exceeds the charging threshold value. Here, the charging threshold value corresponds to the starting voltage of electrolysis by the water electrolysis cell 51. That is, the above charging threshold value is a threshold value related to the charging of the water electrolysis cell 51.

[0054] The charge control unit 52d calculates the input / output current target value based on the voltage deviation input from the subtractor 52c, and generates a drive pulse as a charging command so that the input / output current of the DC / DC conversion unit 52A matches the input / output current target value, and outputs it to the drive circuit 52e. In the drive circuit 52e, the semiconductor switching element of the DC / DC conversion unit 52A is turned on and off according to the above drive pulse, thereby supplying DC power to the water electrolysis cell 51 to electrolyze water.

[0055] The DC / DC conversion unit 52A operates so as to control the DC power supplied to the water electrolysis cell 51 by the above operation and to match the input / output current with the input / output current target value.

[0056] Regarding the metastable state stabilizer 60 in FIG. 1, considering the power generation operation by the fuel cell 61 as a discharge operation, the water electrolysis cell 51, the charging threshold value, and the charge control unit 52d of the metastable state stabilizer 50 shown in FIG. 6 may be replaced with the fuel cell 61, the discharge threshold value, and the discharge control unit, respectively. The discharge threshold value in this case corresponds to the starting voltage of power generation by the fuel cell 61.

[0057] In the metastable state stabilizer 60, when the bus voltage detection value falls below the discharge threshold value, a drive pulse corresponding to a discharge command is output to the discharge control unit to operate the DC / DC conversion unit, and the power generated by the fuel cell 61 is supplied to the DC bus 70 via the DC / DC conversion unit.

[0058] The DC / DC converter operates to control the generated power of the fuel cell 61 by the above operation while matching the input / output current to the input / output current target value.

[0059] Sensors for detecting voltage, current, temperature, etc. are also provided in the water electrolysis cell 51 and the fuel cell 61, and these detected values are input to the charge control unit 52d and the discharge control unit. For the sake of convenience, the illustration of these sensors is omitted.

[0060] Also, the charge threshold and the discharge threshold may be set by each control circuit itself or may be received as a command from the monitoring / indicating device 80.

[0061] The configurations and operations of the power converters 12, 32, 42, and 52 shown in FIGS. 3 to 6, particularly the control circuits 12B, 32B, 42B, and 52B, are merely exemplary and in no way limit the technical scope of the present invention. Needless to say, configurations different from these may be adopted.

[0062] Next, FIG. 7 is a conceptual diagram schematically showing the charge / discharge power of the power storage device 41 of the quasi-stabilizing device 40, the input power of the water electrolysis cell 51 of the quasi-stabilizing device 50, and the output power of the fuel cell 61 of the quasi-stabilizing device 60 according to the voltage of the DC bus 70. The horizontal width of the triangular symbol in FIG. 7 indicates the magnitude of each power, and the wider the width, the larger the power value.

[0063] FIG. 7 exemplifies the case where the input power source is a renewable energy power system. The renewable energy power system is, for example, the solar power generation system 10 and / or the wind power generation system 20 in FIG. 1. The charge / discharge operations of each part are controlled according to the voltage of the DC bus 70 to which these generated powers are supplied, the charge / discharge thresholds of the power storage device 41, the water electrolysis cell 51, and the fuel cell 61, etc.

[0064] For example, as shown in (a) regarding the power storage device 41, the higher the bus voltage is above the charging threshold of the power storage device 41, the greater the charging power supplied to the power storage device 41. Also, the lower the bus voltage is below the discharging threshold of the power storage device 41, the greater the discharging power discharged from the power storage device 41. Similarly, the higher the bus voltage is above the charging threshold of the water electrolysis cell 51, the greater the charging power supplied to the water electrolysis cell 51. And the lower the bus voltage is below the discharging threshold of the fuel cell 61, the greater the discharging power generated from the fuel cell 61.

[0065] Regarding (b) of the power storage device 41, when the charging threshold and the discharging threshold are set lower than those in (a) according to the reference bus voltage, (c) is the case where the charging threshold and the discharging threshold are set higher than those in (a). A similar operation of changing the threshold settings is also possible for the charging threshold of the water electrolysis cell 51 and the discharging threshold of the fuel cell 61.

[0066] In this way, by changing the charging thresholds and discharging thresholds of the power storage device 41, the water electrolysis cell 51, and the fuel cell 61 to control the charge and discharge operations, the DC power transferred between the DC bus 70 and the quasi-stabilization devices 40, 50, and 60 can be individually adjusted. In other words, it is possible to finely control the operation as each power buffer.

[0067] As described above, the change of the charging threshold and the discharging threshold can be based on a command from the monitoring and indicating device 80, or can be performed by the power converters 42, 52, and 62 themselves.

[0068] Figures 8A and 8B are operation explanatory diagrams of the main stabilization device 30.

[0069] As shown by the dashed line (thick line) in Figure 8A, the main stabilization device 30 transfers DC power between the DC bus 70 and the power storage device 31 and controls the charge and discharge of the power storage device 31. The control circuit 32B in the power converter 32 sets the bus voltage target value based on the first power storage amount index (e.g., charge rate) of the power storage device 31 according to the characteristics shown in Figure 8B, for example.

[0070] This bus voltage target value may be set such that within the allowable range of the voltage of the DC bus 70, the higher the first power storage amount index, the higher the value, and the lower the first power storage amount index, the lower the value. The control circuit 32B controls the DC / DC conversion unit 32A so that the bus voltage detection value matches this bus voltage target value.

[0071] Alternatively, an upper limit and a lower limit may be set for the bus voltage target value. The upper limit is set to a value corresponding to the maximum operation of charging the water electrolysis cell or the battery (a value indicating the maximum operation) or a value slightly higher than that. The lower limit is set to a value corresponding to the maximum operation of discharging the fuel cell or the battery (a value indicating the maximum operation) or a value slightly lower than that.

[0072] When the upper limit and the lower limit are set for the bus voltage target value, the relationship between the first power storage amount index and the target value determined by the bus voltage target value calculation unit 32b is as shown in FIG. 8C. Since the first power storage amount index only takes values between the lower limit value and the upper limit value, the bus voltage target value also takes a value corresponding to this range. By providing an upper limit for the first power storage amount index, even when the generated power is excessive, the system can be returned to a normal state in a relatively short time after the situation is resolved. When the load power is excessive, the same effect as above can be obtained because the first power storage amount index is provided with a lower limit.

[0073] Note that when the generated power or the load power is excessive, a difference occurs between the first power storage amount index (a value obtained by calculation and used for control) and the actual power storage amount of the actual power storage device 31. However, in the DC bus control system according to the present embodiment, the main stabilization device 30 determines the bus voltage target value in consideration of the integral value of the difference between the second power storage amount index (battery voltage) and its target value by the offset calculation unit 32c. Therefore, the difference between the calculated power storage amount index (the first power storage amount index) and the actual power storage amount as described above can be automatically eliminated in the long term.

[0074] FIGS. 9A and 9B are operation explanatory diagrams of the sub-stabilization devices 40 and 50.

[0075] As shown by the dashed line (thick line) in FIG. 9A, the power converter 42 of the quasi-stabilization device 40 charges the power storage device 41 using the DC power of the DC bus 70, and the power converter 52 of the quasi-stabilization device 50 supplies the DC power of the DC bus 70 to the water electrolysis cell 51 to electrolyze water.

[0076] The charging characteristics in this case are as shown in FIG. 9B, and the power converters 42 and 52 are respectively controlled so that the charging current increases as the voltage of the DC bus 70 becomes higher than the charging threshold of the power storage device 41 or the water electrolysis cell 51.

[0077] FIGS. 10A and 10B are operation explanatory diagrams of the quasi-stabilization devices 40 and 60.

[0078] As shown by the dashed line (thick line) in FIG. 10A, the power converter 42 of the quasi-stabilization device 40 discharges the power storage device 41 to supply DC power to the DC bus 70, and the power converter 62 of the quasi-stabilization device 60 operates the fuel cell 61 to generate power to supply DC power to the DC bus 70.

[0079] The discharge characteristics in this case are as shown in FIG. 10B, and the power converters 42 and 62 are respectively controlled so that the discharge current increases as the voltage of the DC bus 70 becomes lower than the discharge threshold of the power storage device 41 or the fuel cell 61.

[0080] According to the DC bus control system according to this embodiment, in response to a change in the input / output power balance of the entire system, first, the input / output of current to the main stabilization device 30 (power storage device 31) with a fast response speed is used to cope with the change, thereby relaxing the response to other devices. At this time, when current flows into the main stabilization device 30, the first power storage amount index increases due to the charging of the power storage device 31, and the bus voltage rises. Conversely, when current flows out from the main stabilization device 30, the first power storage amount index decreases due to the discharge of the power storage device 31, and the bus voltage drops. These operations are performed by the power converter 32 (DC / DC conversion unit 32A and control circuit 32B) of the main stabilization device 30.

[0081] On the one hand, the quasi-stabilization device increases or decreases the amount of current flowing into the charging element or the discharging element in response to fluctuations in the bus voltage, and charging or discharging is performed. This operation is carried out by the power converter of the quasi-stabilization device.

[0082] These series of operations are performed so that the input and output of current to and from the main stabilization device 30 become zero, and as a result, the total sum of the current input and output to the entire system is controlled to be zero. Therefore, at a certain DC bus voltage, a steady state is reached when the inflow and outflow of the current of the quasi-stabilization device reach a certain value.

[0083] According to this embodiment, since the operations of each device are performed analogously using the voltage of the DC bus line as a signal, system control can be performed by distributed processing based on the operations of each device, and centralized overall control is not necessary. As a result, the problem of errors, which is a problem in the case of performing centralized management, can also be solved. The reaction speed of the main stabilization device needs to follow the power fluctuations of the system. However, since the main stabilization device absorbs rapid fluctuations, the operation of the quasi-stabilization device can be relatively slow without problems. Also, since the direction of its current can be controlled, it can be used even for devices that can only absorb or release power in one direction, such as water electrolysis cells and fuel cells, and are not suitable for high-speed output fluctuations. Furthermore, by changing the setting of the operation voltage threshold of these devices and the ratio of current inflow and outflow to the DC bus voltage, it becomes possible to use a plurality of devices as quasi-stabilization devices. Also, since the operation signal of each device is the DC bus voltage, the increase or decrease of the quasi-stabilization device can be performed relatively easily.

[0084] Furthermore, according to this embodiment, since the main stabilization device 30 determines the target value of the DC bus voltage based on the second power storage amount index of the power storage device 31, the power storage amount of the power storage device 31 can be kept constant, and long-term continuous operation can be stably performed.

[0085] <Configuration for extending lifespan> Here, a configuration for preventing deterioration of the water electrolysis cell 51 and the fuel cell 61 and realizing long-term continuous operation will be described.

[0086] In order to prevent deterioration of the water electrolysis cell 51 and the fuel cell 61, it is important to avoid rapid power fluctuations and to avoid operation at excessive or insufficient power. More specifically, for each individual cell of the water electrolysis cell 51, voltage application control is performed such that the maximum is 3 V (more preferably, the maximum is 2.5 V, and the rated voltage is 2.0 V), and the time from zero to the rated voltage is 30 seconds or more (more preferably, 60 seconds or more, and even more preferably, 5 minutes or more), and it is desirable not to apply a voltage change faster than this. Further, for each individual cell of the fuel cell 61, the voltage is set to 0.8 V or less and 0.6 V or more, the load is always 15% or more (if a state where the load is 15% or less continues for 5 minutes or more, the fuel cell is stopped), and load control is performed such that the time from zero to the rated voltage is 30 seconds or more (more preferably, 60 seconds or more, and even more preferably, 5 minutes or more), and it is desirable that the voltage fluctuation occurs within the range of 0.7 V to 0.8 V over 1 second or more.

[0087] Therefore, in the present embodiment, the time constant and the maximum power (current) or the minimum power (current) of the water electrolysis cell 51 and the fuel cell 61 are controlled as follows.

[0088] FIG. 7B is a diagram showing the relationship between the bus voltage and the power (electric power) in the water electrolysis cell 51. In the water electrolysis cell 51, the power is constant at a predetermined power W1 min (current I1 min ) while the bus voltage is between 0 and the first threshold value V min . The power (current) gradually increases while the bus voltage is between the first threshold value V min and the second threshold value V max . When the bus voltage is the second threshold value V max or higher, the power is constant at W1 max (current I1 max ). Here, the threshold values Vmin, Vmax and the power W1 min , W1 max (current I1 min , I1 max ) can be set based on a command from the monitoring and indicating device 80 or by the power converter 52.

[0089] The time constant of the response of such a water electrolysis cell 51 is given as follows. [Number] Here, I1 max is the current corresponding to the upper limit value provided for the first power storage amount index. The proportional gain K p is the ratio when converting the bus voltage to the current of the metastable state device, and can be set by the power converter 52.

[0090] Similarly, FIG. 7C is a diagram showing the relationship between the bus voltage and the power (electric power) in the fuel cell 61. The fuel cell 61 has a predetermined power W2 min between the bus voltage from 0 to the first threshold value V max (current I2 max ), is constant, and the power (current) gradually decreases between the bus voltage from the first threshold value V min to the second threshold value V max , and the power W2 max is constant at the bus voltage of the second threshold value V min (current I2 min ). Here, the threshold values Vmin, Vmax and the power W2 min , W2 max (current I2 min , I2 max ) can be set based on a command from the monitoring and indicating device 80 or by the power converter 62.

[0091] The time constant in the fuel cell is also given as follows and can be set in the same manner as the water electrolysis cell 51. [Number]

[0092] Here, the power storage device 41 of the quasi-stabilization device 40 is a power storage device capable of bidirectionally transmitting and receiving DC power like a supercapacitor, and its time constant is approximately equal to the time constant of the power storage device 31 of the main stabilization device 30. In the present disclosure, "approximately equal" means that one is within plus or minus 30% of the other, but it may also be plus or minus 10% or plus or minus 3%.

[0093] By having the power storage device 41 with approximately the same time constant as the power storage device 31 of the main stabilization device 30, the water electrolysis cell 51 and the fuel cell 61 respond with the time constant described above. Here, the maximum current, minimum current, and gain are determined so that the time taken for the water electrolysis cell 51 and the fuel cell to reach the rated voltage from the minimum voltage is 30 seconds or more, more preferably 60 seconds or more, and even more preferably 5 minutes or more, thereby suppressing the deterioration of the water electrolysis cell 51 and the fuel cell 61 and achieving long-term stable operation.

[0094] In the above description, an example where there is only one quasi-stabilization device having a charge / discharge element has been described, but there may be a plurality of quasi-stabilization devices having charge / discharge elements. For example, there may be a quasi-stabilization device 40A having a power storage device (charge / discharge element) with approximately the same time constant as the power storage device 31 of the main stabilization device 30, and a quasi-stabilization device 40B having a time constant smaller than that of the power storage device 31 of the main stabilization device 30. For example, it is conceivable to use a supercapacitor for the quasi-stabilization device 40A and a secondary battery for the quasi-stabilization device 40B. By using a plurality of quasi-stabilization devices with different time constants, the control of the time constants of the water electrolysis cell 51 and the fuel cell 61 becomes simpler.

[0095] In the above example, the quasi-stabilization device 40A corresponds to the first quasi-stabilization device, the quasi-stabilization devices 50 and 60 each correspond to the second quasi-stabilization device, and the quasi-stabilization device 40B corresponds to the third quasi-stabilization device.

Description of Reference Numerals

[0096] 10: Photovoltaic power generation system 11: Solar cell 12: Power Converter 12A: DC / DC Conversion Unit 12B: Control Circuit 12a, 12f: Voltage Detector 12b: Current Detector 12c: MPPT Control Unit 12d: Voltage - Current Control Unit 12e: Drive Circuit 12g: Comparison Unit 20: Wind Power Generation System 21: Wind Turbine Generator 22: Power Converter 30: Main Stabilization Device 31: Energy Storage Device 31a: Sensor 32: Power Converter 32A: DC / DC Conversion Unit 32B: Control Circuit 32a: Voltage Detector 32b: Bus Voltage Target Value Calculation Unit 32c: Offset Calculation Unit 32d: Subtractor 32e: Subtractor 32f: Charge - Discharge Control Unit 32g: Drive Circuit 40: Quasi - Stabilization Device 41: Energy Storage Device 41a: Sensor 42: Power Converter 42A: DC / DC Conversion Unit 42B: Control Circuit 42a: Voltage Detector 42b: Comparison Unit 42c: Subtractor 42d: Charge - Discharge Control Unit 42e: Drive Circuit 50, 50A: Quasi - Stabilization Device 51: Water Electrolysis Cell 52: Power Converter 52A: DC / DC Conversion Unit 52B: Control Circuit 52a: Voltage Detector 52b: Comparison Unit 52c: Subtractor 52d: Charging Control Unit 52e: Drive Circuit 53: Hydrogen storage device 60: Quasi-stabilization device 61: Fuel cell 62: Power converter 70: DC bus 80: Monitoring and indicating device 90: Load

Claims

1. A DC bus control system for controlling power fluctuations of a DC bus connecting an input power supply and a load, comprising: a main stabilization device having a first charge / discharge element and a first power converter; a plurality of sub-stabilization devices each having a second charge / discharge element, a charging element, or a discharging element and a second power converter; wherein the first power converter is configured to obtain a bus voltage target value and bidirectionally transfer DC power between the first charge / discharge element and the DC bus so that the voltage of the DC bus matches the bus voltage target value; the second power converter is configured to obtain a current target value according to a difference between a threshold related to charging or discharging of the second charge / discharge element, the charging element, or the discharging element and the voltage of the DC bus, and transfer DC power between the second charge / discharge element, the charging element, or the discharging element and the DC bus so that a current equal to the current target value flows through the second charge / discharge element, the charging element, or the discharging element; the plurality of sub-stabilization devices include a first sub-stabilization device having a charge / discharge element and a second power converter, and at least one second sub-stabilization device having a charging element or a discharging element and a second power converter; a response speed of the first sub-stabilization device is determined such that a charge amount of the charging element or a discharge amount of the discharging element of the second sub-stabilization device changes with a predetermined time constant; time constants of the first charge / discharge element of the main stabilization device and the charge / discharge element of the first sub-stabilization device are substantially equal; the plurality of sub-stabilization devices further include a third sub-stabilization device having a charge / discharge element and a second power converter; a time constant of the charge / discharge element of the third sub-stabilization device is set to be smaller than the time constant of the charge / discharge element of the first sub-stabilization device; characterized in that it is a DC bus control system.

2. A DC bus control system for controlling power fluctuations of a DC bus connecting an input power supply and a load, comprising: a main stabilization device having a first charge / discharge element and a first power converter; a plurality of sub-stabilization devices each having a second charge / discharge element, a charging element, or a discharging element and a second power converter; wherein the first power converter is configured to obtain a bus voltage target value and bidirectionally transfer DC power between the first charge / discharge element and the DC bus so that the voltage of the DC bus matches the bus voltage target value; The second power converter obtains a current target value according to the difference between the threshold related to charging or discharging of the second charge / discharge element, charging element, or discharging element and the voltage of the DC bus, and DC power is configured to be exchanged between the second charge / discharge element, charging element, or discharging element and the DC bus so that a current equal to the current target value flows through the second charge / discharge element, charging element, or discharging element. The plurality of quasi-stabilization devices includes a first quasi-stabilization device having a charge / discharge element and a second power converter, and at least one second quasi-stabilization device having a charging element or a discharging element and a second power converter. The response speed of the first quasi-stabilization device is determined such that the charge amount of the charging element or the discharge amount of the discharging element of the second quasi-stabilization device changes with a predetermined time constant. The first power converter obtains the bus voltage target value by subtracting an offset corresponding to a second stored power amount index obtained in a manner different from the first stored power amount index of the first charge / discharge element from a target value corresponding to the first stored power amount index of the first charge / discharge element. A DC bus control system characterized by the above.

3. The second stored power amount index is the terminal voltage of the first charge / discharge element. The offset is a value corresponding to the integrated value of the difference between the second stored power amount index and the target value of the second stored power amount index. The DC bus control system according to claim 2.

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