JP7664638B2 - DC Bus Control System - Google Patents

Description

The present disclosure relates to a DC bus control system.

In recent years, power supply systems that utilize renewable energy sources such as solar, wind, and wave power have been attracting attention as alternative sources of power to fossil and nuclear energy, and some of these are already in practical use.

The power generated by this type of power supply system varies greatly depending on the weather, season, location, etc. For this reason, in order to maintain the voltage of the DC bus to which the power supply system is connected within a specified tolerance range, it is desirable to connect power sources such as solar cells and wind power generators to the DC bus via a large-capacity power converter with a wide input range. However, in that case, increasing the capacity of the power converter leads to an increase in the size, complexity, and cost of the entire system.

The present applicant has proposed a control system for efficiently controlling power fluctuations in a DC bus caused by fluctuations in the input power source and load (Patent Document 1). In the control system of Patent Document 1, a main stabilization device controls the DC bus voltage based on a storage capacity index, and controls storage devices such as fuel cells and water electrolysis cells.

The storage capacity index used here is the state of charge (SOC) of the storage device in the main stabilization device, and is calculated as a value obtained by integrating the charge and discharge current. The storage device used in the main stabilization device is generally unable to extract all of the energy used for charging by discharging. Furthermore, it is not possible to accurately calculate the proportion of energy extracted by discharging. This results in a discrepancy between the storage capacity (calculated value) used for control and the actual storage capacity. As a result, each time the main stabilization device repeatedly charges and discharges the storage device, the discrepancy between the storage capacity of the storage device and the calculated value increases, and in the case of excessive discharging, the storage capacity is depleted, and in the case of excessive charging, the storage capacity is saturated. As a result, it becomes impossible to control the DC bus, and the system cannot operate.

To avoid this problem, it is possible to measure the efficiency of charging and discharging and correct the charging rate, but accurate correction is difficult because charging and discharging efficiency generally depends on temperature, current, etc. Therefore, a discrepancy will eventually arise between the calculated amount of stored energy and the actual value, making it difficult to operate the system stably over the long term.

In addition, an accurate value for the amount of stored energy can be obtained by stopping charging and discharging and measuring it, but this hinders continuous operation.

International Publication No. 2019/103059

The present invention aims to provide a control system that can efficiently control power fluctuations in a DC bus caused by fluctuations in the input power source or load, and can operate stably and continuously for a long period of time by avoiding the depletion of the storage capacity of the main stabilization device or the stop of control due to saturation.

One aspect of the present invention is a DC bus control system for controlling power fluctuations of a DC bus connecting an input power source and a load, comprising a main stabilization device having a first charge/discharge element and a first power converter, and at least one semi-stabilization device having a second charge/discharge element, a charging element, or a discharging element and a second power converter, the first power converter being configured to determine a bus voltage target value and to bidirectionally transmit and receive 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 being configured to determine a bus voltage target value and to transmit and receive 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, or a current target value is calculated according to a difference between a threshold value for charging or discharging of a discharging element and the voltage of the DC bus, and DC power is exchanged between the second charging/discharging element, charging element, or discharging element and the DC bus so that a current equal to the current target value flows through the second charging/discharging element, charging element, or discharging element, and the first power converter calculates the bus voltage target value based on a target value corresponding to a first storage amount index of the first charging/discharging element and a second storage amount index calculated by a method different from the first storage amount index of the first charging/discharging element.

According to the present invention, it is possible to efficiently control power fluctuations in the DC bus caused by fluctuations in the input power source and load, and to realize control that keeps the storage capacity of the main stabilization device constant over the long term, thereby providing a control system that can operate continuously and stably over the long term.

1 is an overall configuration diagram of a DC bus control system according to an embodiment; FIG. 13 is a configuration diagram showing another example of a semi-stabilizing device in the embodiment. 1 is a block diagram showing a configuration example of a power converter in a solar power generation system; FIG. 2 is a block diagram showing an example of a configuration of a power converter in a main stabilizing device. 2 is a block diagram showing an example of the configuration of a power converter in a semi-stabilizing device; 2 is a block diagram showing an example of the configuration of a power converter in a semi-stabilizing device; 1 is a conceptual diagram showing a schematic relationship between the charging/discharging power of a power storage device, the input power of a water electrolysis cell, the output power of a fuel cell, and the bus voltage. FIG. 4 is an explanatory diagram of the operation of the main stabilizing device. FIG. 4 is an explanatory diagram of the operation of the main stabilizing device. FIG. 4 is an explanatory diagram of the operation of the main stabilizing device. FIG. 4 is an explanatory diagram of the operation of the semi-stabilizing device. FIG. 4 is an explanatory diagram of the operation of the semi-stabilizing device. FIG. 4 is an explanatory diagram of the operation of the semi-stabilizing device. FIG. 4 is an explanatory diagram of the operation of the semi-stabilizing device. FIG. 4 is a diagram showing a simulation result of the system according to the first embodiment. FIG. 13 is a diagram showing a simulation result of a system according to a conventional example. FIG. 1 is a diagram for explaining a system problem according to a conventional example. FIG. 1 is a diagram for explaining a system problem according to a conventional example. FIG. 11 is a diagram showing a simulation result of the system according to the second embodiment.

Below, an embodiment of the present invention is explained with reference to the figures.

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 solar power generation system 10 and a wind power generation system 20, which are renewable energy power supply systems. These power generation systems 10 and 20 are connected in parallel, and their output sides are connected to a DC bus 70. The solar power generation system 10 includes a solar cell 11 and a power converter 12, and the wind power generation system 20 includes a wind power generator 21 and a power converter 22.

The input power source may be any power source. If the input power source is a renewable energy power source system, it may be one that utilizes energy such as wave power or geothermal power in addition to the above, or it may be a power source system such as hydroelectric (small hydroelectric) power generation, tidal power generation, tidal current power generation, or temperature difference power generation. It may also be a combination of these, including the above.

Furthermore, the number of power supply systems connected in parallel with each other is not particularly limited.

The DC bus 70 is connected to the main stabilizing device 30 and the sub-stabilizing devices 40, 50, and 60, as well as to a load 90.

The main stabilization device 30 sets a variable bus voltage target value within a predetermined tolerance range centered on the reference bus voltage (the reference voltage of the DC bus 70), and controls the charging and discharging of the storage device 31 by operating the power converter 32 so that the output voltage on the DC bus 70 side matches the bus voltage target value.

In addition, the semi-stabilizing device 40 calculates input/output current target values 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 input/output current target value, thereby controlling the charging and discharging of the storage device 41.

Here, the power storage devices 31 and 41 are, for example, batteries (secondary batteries), electric double layer capacitors, capacitors, flywheels, redox flow batteries, etc. Furthermore, the power converters 32 and 42 are, for example, isolated DC/DC converters or choppers, etc., and are capable of transmitting and receiving DC power in both directions as shown by the arrows.

In the semi-stabilizing device 50, the power converter 52 performs DC/DC conversion so that the input/output current matches the input/output current target value calculated based on the difference between the charging threshold and the voltage of the DC bus, and supplies DC power to the water electrolysis cell 51 (a type of charging operation), electrolyzing water to generate hydrogen gas and oxygen gas. In addition, the semi-stabilizing 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 type of discharging operation), and at that time, the power converter 62 performs DC/DC conversion so that the input/output current matches the input/output current target value calculated based on the difference between the discharge threshold and the voltage of the DC bus.

The configurations of the semi-stabilizing device 50 and semi-stabilizing device 60 described above are merely illustrative, and an alternative to the water electrolysis cell 51 may be a means for electrochemically producing C-H bonds (CH4, C2H4, etc.) or alcohol by reducing carbon dioxide, or a means for producing ammonia by reducing nitrogen, and an alternative to the fuel cell 61 may be a fuel cell that uses alcohol or the like, or a power generation means that burns chemical substances (hydrogen, C-H bonds, alcohol, ammonia, etc.) to rotate a turbine or the like.

2 shows another example of the configuration of the semi-stabilizing device. As shown in the figure, the semi-stabilizing device 50A may be an integrated structure in which the semi-stabilizing devices 50 and 60 share the hydrogen storage device 53.

In Figure 1, the power storage devices 31 and 41 are capable of absorbing (charging) and releasing (discharging) DC power. In addition, the water electrolysis cell 51 (and the hydrogen storage device 53 in Figure 2) converts DC power into gas and stores it, and the fuel cell 61 (and the hydrogen storage device 53) is capable of generating electricity by converting gas into DC power. The power storage devices 31 and 41 constitute the charge/discharge elements, the water electrolysis cell 51 (and the hydrogen storage device 53) constitutes the charge element, and the fuel cell 61 (and the hydrogen storage device 53) constitutes the discharge element.

As described above, each of the stabilizing devices 30, 40, 50, and 60 can be regarded as a power buffer that transmits and receives DC power to and from the DC bus 70 through the operation of the power converters 32, 42, 52, and 62. The main stabilizing device 30 and the semi-stabilizing device 40 are power buffers with a charging and discharging function, the semi-stabilizing device 50 is a power buffer with a charging function, and the semi-stabilizing device 60 is a power buffer with a discharging function.

In addition, only one main stabilization device 30 having the function of setting the bus voltage target value is required, but only the required number of semi-stabilization devices need to be installed depending on the number of parallel connections in the power supply system and the power requirements of the load 90.

The monitoring and instruction device 80 collects status information (voltage, current, temperature, etc.) of each power generation system 10 and 20, the main stabilization device 30, and the semi-stabilization devices 40, 50, and 60 to monitor their status and operation, and generates operation commands (start/stop commands, etc.) for each part and charge/discharge threshold commands, etc., based on the results of these monitoring. Various monitoring signals and commands can be transmitted and received between the monitoring and instruction device 80 and each of the above-mentioned parts via wired or wireless communication.

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. An AC power system may also be connected to the DC bus 70 via a DC/AC converter.

Next, the configuration of each part in Fig. 1 will be described. The configuration in Fig. 1 has a solar power generation system 10 and a wind power generation system 20 as input power sources.

The photovoltaic power generation system 10 and the wind power generation system 20 have a common function in that they convert the generated power using renewable energy into DC power by the power converters 12 and 22 and supply it to the DC bus 70. For this reason, the following explanation will be given using the photovoltaic power generation system 10 as an example.

Figure 3 is a block diagram showing an example configuration of a power converter 12 in a solar power generation system 10. This power converter 12 includes a DC/DC conversion unit 12A and a control circuit 12B.

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 a semiconductor switching element and outputs it to the DC bus 70, and is composed of, for example, a boost chopper.

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.

The voltage/current control unit 12d sends a drive pulse 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 drive pulse.

In addition, 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 the main stabilization device 30 described below. 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 the control signal to a voltage/current control unit 12d.

Based on the control signal, the voltage/current control unit 12d calculates a drive pulse that matches the bus voltage detection value with the bus voltage target value. For example, when the bus voltage detection value exceeds the bus voltage target value, the voltage/current control unit 12d performs a control operation to reduce the output voltage of the DC/DC conversion unit 12A (including stopping operation).

Figure 4 is a block diagram showing an example of the configuration of the power converter 32 in the main stabilization device 30. This power converter 32 includes a DC/DC conversion section 32A and a control circuit 32B.

The DC/DC conversion unit 32A has a function of controlling the charging and discharging of the power storage device 31 by bidirectionally transmitting and receiving DC power between the DC bus 70 and the power storage device 31, and is composed of an insulated DC/DC converter equipped with a semiconductor switching element, a chopper, etc. The power storage device 31 is equipped with a sensor 31a that detects voltage, current, and temperature.

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

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 storage amount index of the storage device 31. The method of calculating the bus voltage target value will be described later.

As the above-mentioned stored energy indicator, for example, the state of charge (SOC) obtained by integrating the charging and discharging current of the storage device 31 detected by the sensor 31a can be used.

The offset integration unit 32c calculates the cumulative deviation of the bus voltage target value based on the difference (offset) between the second storage amount index of the storage device 31 and the target value of the second storage amount index. The second storage amount index is an index of the storage amount of the storage device 31 that is calculated by a method different from that of the first storage amount index, and may be, for example, the terminal voltage (battery voltage) of the storage device 31 detected by the sensor 31a.

The target value of the second storage amount index can be a value of the second storage amount index corresponding to a predetermined storage amount or storage rate. The predetermined storage amount or storage rate can be appropriately determined according to the requirements of the system, and for example, a storage amount of 50% of full charge can be adopted, but is not limited to this. For example, the predetermined storage amount or storage rate can be 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, or 80% of full charge, or can be appropriately determined according to the purpose and use within the range of 47% to 54%, 45% to 55%, 40% to 60%, or 30% to 70% of full charge.

The offset integration unit 32c calculates the cumulative deviation as a value corresponding to the integral of the difference between the second storage amount index and its target value, for example. More specifically, the cumulative deviation is calculated by multiplying the integral of the difference between the second storage amount index and its target value by a predetermined gain.

Here, when the magnitude (absolute value) of the charge/discharge current of the storage device 31 is greater than a predetermined value, the integral calculation is not performed, and the integral calculation is performed only when the magnitude of the charge/discharge current is smaller than the predetermined value. In other words, the cumulative deviation is obtained by integrating the difference between the second storage amount index and the target value of the storage amount index only when the magnitude of the charge/discharge current of the storage device 31 is smaller than a predetermined value. The predetermined value here is determined, for example, as a value that can be expected to accurately represent the storage amount of the storage device 31 when the magnitude of the charge/discharge current of the storage device 31 is smaller than that value.

The deviation between the bus voltage target value and the bus voltage detection value is calculated by subtractor 32d, and the cumulative deviation is added to the voltage deviation by adder 32e. The output of subtractor 32d is a new bus voltage target value corrected by the cumulative deviation, and the new bus voltage target value is input to charge/discharge control unit 32f.

The voltage, current, temperature, and charge/discharge thresholds of the storage device 31 are input to the charge/discharge control unit 32f, which generates a drive pulse by performing PWM control or the like while taking into account this input information so that the bus voltage detection value matches the bus voltage target value. The drive circuit 32g turns on and off the semiconductor switching elements of the DC/DC conversion unit 32A in accordance with the drive pulse. The DC/DC conversion unit 32A controls the charge/discharge of the storage device 31 as described above so as to match the bus voltage detection value to the bus voltage target value.

In general, the amount of stored electricity in the storage device 31 can be estimated approximately from its terminal voltage. The terminal voltage also varies depending on the magnitude of the charge/discharge current, but when the charge/discharge current is sufficiently small (below the above-mentioned predetermined value), the terminal voltage can be considered to represent the amount of stored electricity in the storage device 31. When a steady state in which the change in power generation or power consumption is small is achieved as a result of control by the main stabilization device, the charge/discharge current of the main stabilization device becomes 0. Therefore, the terminal voltage when the steady state is achieved and the charge/discharge current is small can be used as an index representing the actual (accurate) amount of stored electricity in the storage device 31. In this embodiment, the difference between the terminal voltage and its target value is integrated, and the obtained integral value is added to the target value to control the bus voltage as a new target value. This control based on the integral value of the error can be considered as integral control of PID control when the control based on the amount of stored electricity is viewed as proportional control. As a result, the offset of the steady-state electricity storage index can be removed. In other words, the amount of stored electricity in the storage device 31 can be maintained at the target value of the second storage amount index, and the actual amount of stored electricity in the storage device 31 does not run out or overflow, allowing stable continuous operation for a long period of time.

The control circuit 32B (charge/discharge control unit 32f) may also stop charging/discharging the storage device 31 for a very short period of time that does not affect the control of the system, forcibly setting a period during which the charge/discharge current is zero. The length of this period is set to a length that allows the terminal voltage of the storage battery to stabilize and represent the actual amount of stored energy. The control circuit 32B can be expected to achieve the same effect as above by setting a period during which the charge/discharge current is zero on a regular or irregular basis, measuring the terminal voltage of the storage device 31 during this period as a second indicator of stored energy, integrating the difference with the target amount of stored energy, and using it for control.

The charge/discharge threshold of the storage device 31 may be set by the control circuit 32B itself, or may be received as a command from the monitoring/indicating device 80 of Figure 1.

Figure 5 is a block diagram showing an example of the configuration of the power converter 42 in the semi-stabilizing device 40 of Figure 1. This power converter 42 includes a DC/DC conversion unit 42A and a control circuit 42B. The power converter 42 has a function similar to that of the power converter 32 of Figure 4A in that it transmits and receives DC power in both directions between the DC bus 70 and the storage device 41. The storage device 41 is provided with a sensor 41a that detects voltage, current, and temperature, similar to the storage device 31. The control circuit 42B includes a voltage detector 42a, a comparator 42b, a subtractor 42c, a charge/discharge control unit 42d, and a drive circuit 42e.

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

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

Figure 6 is a block diagram showing an example of the configuration of the power converter 52 in the semi-stabilizing device 50. This power converter 52 includes a DC/DC conversion unit 52A and a control circuit 52B.

The DC/DC conversion unit 52A has the function of converting the DC power of the DC bus 70 to a predetermined magnitude and supplying it to the water electrolysis cell 51, and is composed of an insulated DC/DC converter equipped with a semiconductor switching element, a chopper, etc. 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.

The control circuit 52B that controls the DC/DC conversion unit 52A is generally configured in the same manner as the control circuit 42B in Figure 5.

6, the voltage of the DC bus 70 is detected by the voltage detector 52a, and the deviation between the charging threshold and the bus voltage detection value is calculated by the subtractor 52c, and this voltage deviation is input to the charging control unit 52d. The bus voltage detection value is input to the comparison unit 52b together with the charging threshold, and the comparison unit 52b outputs a charging command to the charging control unit 52d when the bus voltage detection value exceeds the charging threshold. Here, the charging threshold corresponds to the starting voltage of electrolysis by the water electrolysis cell 51. In other words, the charging threshold is a threshold related to the charging of the water electrolysis cell 51.

The charging control unit 52d calculates the input/output current target value based on the voltage deviation input from the subtractor 52c, 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. The drive circuit 52e turns on and off the semiconductor switching element of the DC/DC conversion unit 52A in accordance with the drive pulse, thereby supplying DC power to the water electrolysis cell 51 to electrolyze water.

The DC/DC conversion unit 52A operates to control the DC power supplied to the water electrolysis cell 51 through the above operation, while matching the input/output current to the input/output current target values.

Regarding the semi-stabilizing device 60 in Fig. 1, the power generation operation by the fuel cell 61 can be considered as a discharge operation, and the water electrolysis cell 51, charge threshold, and charge control unit 52d of the semi-stabilizing device 50 shown in Fig. 6 can be replaced with the fuel cell 61, discharge threshold, and discharge control unit, respectively. In this case, the discharge threshold corresponds to the starting voltage for power generation by the fuel cell 61.

In the semi-stabilizing device 60, when the bus voltage detection value falls below the discharge threshold, 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.

The DC/DC conversion unit operates to control the power generated by the fuel cell 61 through the above operation while matching the input and output currents to the input and output current target values.

The water electrolysis cell 51 and fuel cell 61 are also provided with sensors for detecting voltage, current, temperature, etc., and these detection values are input to the charge control unit 52d and discharge control unit, but for convenience, the above sensors are omitted from the illustration.

In addition, the charge threshold and discharge threshold may be set by each control circuit itself, or may be received as instructions from the monitoring and indicating device 80.

The configurations and operations of the power converters 12, 32, 42, and 52, particularly the control circuits 12B, 32B, 42B, and 52B, shown in Figures 3 to 6 are merely illustrative and do not in any way limit the technical scope of the present invention, and it goes without saying that configurations different from these may be adopted.

Next, Figure 7 is a conceptual diagram that shows the charging and discharging power of the storage device 41 of the semi-stabilizer 40 according to the voltage of the DC bus 70, the input power of the water electrolysis cell 51 of the semi-stabilizer 50, and the output power of the fuel cell 61 of the semi-stabilizer 60. The horizontal width of the triangular symbol in Figure 7 indicates the magnitude of each power, and the wider the width, the larger the power value.

Figure 7 illustrates an example in which the input power source is a renewable energy power source system, such as the solar power generation system 10 and/or the wind power generation system 20 in Figure 1. The charging and discharging operations of each part are controlled according to the voltage of the DC bus 70 to which the generated power is supplied and the charging and discharging thresholds of the storage device 41, the water electrolysis cell 51, and the fuel cell 61, etc.

For example, as shown in (a) for the storage device 41, the higher the bus voltage is above the charge threshold of the storage device 41, the greater the charging power supplied to the storage device 41, and the lower the bus voltage is below the discharge threshold of the storage device 41, the greater the discharging power released from the storage device 41. Similarly, the higher the bus voltage is above the charge 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 discharge threshold of the fuel cell 61, the greater the discharging power generated by the fuel cell 61.

(b) for the storage device 41 is a case where the charge threshold and discharge threshold are set lower than (a) according to the reference bus voltage, and (c) is a case where the charge threshold and discharge threshold are set higher than (a). Similar threshold setting change operations are also possible for the charge threshold of the water electrolysis cell 51 and the discharge threshold of the fuel cell 61.

In this way, by controlling the charge and discharge operations by changing the charge and discharge thresholds of the storage device 41, the water electrolysis cell 51, and the fuel cell 61, it is possible to individually adjust the DC power exchanged between the DC bus 70 and the semi-stabilizing devices 40, 50, and 60. In other words, it is possible to finely control the operation of each of them as a power buffer.

As mentioned above, the charge threshold and discharge threshold can be changed based on instructions from the monitoring and indicating device 80 or by the power converters 42, 52, and 62 themselves.

Figures 8A and 8B are diagrams illustrating the operation of the main stabilizing device 30.

As shown by the dashed line (thick line) in Fig. 8A, the main stabilization device 30 transmits and receives DC power between the DC bus 70 and the storage device 31, and controls the charging and discharging of the storage device 31. The control circuit 32B in the power converter 32 sets a bus voltage target value based on a first storage amount index (e.g., charging rate) of the storage device 31, for example, according to the characteristics shown in Fig. 8B.

This bus voltage target value is set within the allowable range of the voltage of the DC bus 70 so that it is higher the larger the first energy storage index is and lower the smaller the first energy storage index is, and the control circuit 32B controls the DC/DC conversion unit 32A so that the bus voltage detection value matches this bus voltage target value.

Figures 9A and 9B are diagrams illustrating the operation of semi-stabilizing devices 40 and 50.

As shown by the dashed line (thick line) in Figure 9A, the power converter 42 of the semi-stabilized device 40 charges the storage device 41 using the DC power from the DC bus 70, and the power converter 52 of the semi-stabilized device 50 supplies the DC power from the DC bus 70 to the water electrolysis cell 51 to electrolyze water.

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

Figures 10A and 10B are diagrams illustrating the operation of semi-stabilizing devices 40 and 60.

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

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

According to the DC bus control system of this embodiment, in response to changes in the power input/output balance of the entire system, the system first responds by inputting/outputting current to the main stabilization device 30 (storage device 31), which has a fast response speed, thereby mitigating the response to other devices. In this case, when current flows into the main stabilization device 30, the first storage amount index increases as the storage device 31 charges, and the bus voltage increases. Conversely, when current flows out of the main stabilization device 30, the first storage amount index decreases as the storage device 31 discharges, 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.

On the other hand, the semi-stabilizer increases or decreases the amount of current flowing into the charging or discharging element in response to fluctuations in the bus voltage, thereby charging or discharging. This operation is performed by the power converter of the semi-stabilizer.

This series of operations is performed so that the current input and output to the primary stabilizer 30 is zero, and as a result, the 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 current inflow and outflow of the semi-stabilizer reaches a constant value.

According to this embodiment, the operation of each device is performed in an analog manner using the voltage of the DC bus line as a signal, so that system control can be performed by distributed processing based on the operation of each device, and centralized overall control is not necessary. As a result, the problem of errors that arise when centralized management is performed can also be eliminated. The reaction speed of the main stabilizer needs to follow the power fluctuations of the system, but since the main stabilizer absorbs fast fluctuations, the operation of the semi-stabilizer can be relatively slow without any problems, and since the direction of the current can also be controlled, it can be used with 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 operating voltage threshold of these devices and the ratio of current inflow and outflow to the DC bus voltage, it is possible to use multiple devices as semi-stabilizers. In addition, since the operating signal of each device is the DC bus voltage, it is relatively easy to increase or decrease the number of semi-stabilizers.

Furthermore, according to this embodiment, the main stabilization device 30 determines the target value of the DC bus voltage based on the second storage capacity index of the storage device 31, so that the storage capacity of the storage device 31 can be kept constant, enabling stable long-term continuous operation.

Below, we explain the simulations conducted to verify this effect.

Figure 11 is a diagram showing the results of a simulation of the control system of this embodiment. In this simulation, the state of charge (SOC) obtained by integrating the charge/discharge current of the storage device 31 was used as the first storage amount index, and the terminal voltage (battery voltage) of the storage device 31 was used as the second storage amount index. In addition, 50V was set as the target value of the second storage amount index.

Figure 11(a) shows the time series changes in the power generated by the solar cell 1101, the power source (input) power 1102, and the load (output) power 1103. Figure 11(b) shows the power consumed by the water electrolysis cell 1104 and the power generated by the fuel cell 1105. When there is a net input power to the system, electrolysis (charging) is performed in the water electrolysis cell, and when there is a net output power, a direct current is supplied (discharged) by the electrochemical reaction of hydrogen in the fuel cell.

11(c) shows the time series changes in the terminal voltage (battery voltage) 1106 and the charge/discharge current 1107 of the storage device 31. It can be seen that the terminal voltage 1106 of the storage device 31 is controlled to approach its target value of 50 V, and is 50 V when the storage device 31 is not being charged or discharged, i.e., when the charge/discharge current is zero.

Figure 12 shows a similar simulation result as a comparative example in which the power converter 32 of the main stabilization device 30 does not have the offset integration unit 32c (i.e., the configuration of Patent Document 1). Figure 12(a) shows the time series changes in the generated power 1201 of the solar cell, the power source (input) power 1202, and the load (output) power 1203, which are the same changes as those in Figure 11(a). Figure 12(b) shows the power consumption 1204 in the water electrolysis cell and the power generation 1205 in the fuel cell, which show almost the same changes as those in Figure 11(b). Figure 12(c) shows the time series changes in the terminal voltage (battery voltage) 1206 and the charge/discharge current 1207 of the storage device 31. In this comparative example, the generated power and power consumption fluctuate, and the voltage 1206 (an index of the actual amount of stored power) of the storage device 31 decreases every time charging or discharging occurs.

As described above, in the DC bus control system according to this embodiment, the power converter 32 in the main stabilization device 30 has an offset integration section 32c, which removes the steady-state offset from the target value of the storage capacity index (battery voltage), so that the battery voltage of the storage device 31, i.e., the storage capacity, can be kept constant. Also, by not performing an integral calculation of the battery voltage for offset calculation when the charge/discharge current of the storage device 31 is large, control based on an inaccurate storage capacity can be avoided, and stable operation can be performed. As described above, in the DC bus control system according to this embodiment, the storage device 31 of the main stabilization device 30 can maintain a constant storage capacity for a long period of time, so that stable continuous operation for a long period of time is possible.

Second Embodiment
In the DC bus control system according to the first embodiment, when the generated power is within the power absorption capacity of the electrolysis cell and the power consumption of the load is within the power generation capacity of the fuel cell, the charge/discharge current of the power storage device 31 of the main stabilization device 30 is controlled to zero, thereby enabling stable operation. When the generated power exceeds the power absorption capacity of the electrolysis cell or the power consumption of the load exceeds the power generation capacity of the fuel cell, the system can continue to operate by having the power storage device 31 absorb or supply power without abnormal shutdown.

However, even if the generated power or load power returns to within the capacity of the water electrolysis cell or fuel cell, discharging or charging from the storage device 31 of the main stabilization device 30 continues until the excess stored power is eliminated. Therefore, the control capability of the system is temporarily reduced until the excess stored power is eliminated. This point is explained below.

Note that the following explanation using Figures 13 to 15 shows the operation of a configuration in which the main stabilization device 30 power converter 32 does not have an offset integration section 32c (i.e., the configuration of Patent Document 1).

Figure 13 is a diagram explaining the operation of the system when the generated power and load power are within the capacity of the water electrolysis cell and fuel cell. Figure 13(a) shows the time variations in generated power 1301 and load power 1302. Figure 13(b) shows the time variations in power consumption 1303 of the water electrolysis cell and power generated 1304 in the fuel cell. Figure 13(c) shows the time variations in the control target value 1305 of the bus voltage. Figure 13(d) shows the time variations in voltage 1306 and current 1307 of the storage device 31.

As shown in Figure 13, when the generated power and load power are within the appropriate range, the charging and discharging current of the storage device 31 is kept at zero, and the water electrolysis cell and fuel cell operate according to the generated power and load power.

Figure 14 is a diagram explaining the operation of the system when the generated power exceeds the power absorption capacity of the water electrolysis cell. Figure 14(a) shows the time variations in generated power 1401 and load power 1402. Figure 14(b) shows the time variations in power consumption 1403 of the water electrolysis cell and power generated in the fuel cell 1404. Figure 14(c) shows the time variations in voltage 1405 and current 1406 of the storage device 31. Figure 14(d) shows the time variations in the control target value 1407 of the bus voltage.

Because the generated power exceeds the power absorption capacity of the water electrolysis cell, the storage device 31 absorbs the surplus power, and the amount of power stored in the storage device 31 increases. Even if the generated power returns to within the capacity of the system, the storage device 31 of the main stabilization device 30 continues to discharge until the excess amount of power stored in the storage device 31 is eliminated (period T1 in the figure). During this time, power is supplied only from the storage device 31, and power is not supplied from the fuel cell. There is no power supply from the fuel cell because the bus voltage remains high. During this time, if the system supplies load power, it cannot supply power that exceeds the power supply capacity of the storage device 31 of the main stabilization device 30, and the system will deviate from the control of the main controller.

Here we have described the case where the generated power is excessive, but the same problem occurs when the load power is excessive.

In this embodiment, we address this issue as follows:

The bus voltage target value calculation unit 32b determines the bus voltage target value based on the first storage amount index of the storage device 31. Here, the first storage amount index is a value obtained by integrating the charge/discharge current of the storage device 31. In this embodiment, an upper limit and a lower limit are set for the first storage amount index. The upper limit is a value corresponding to the maximum operation of charging the water electrolysis cell or the battery (a value indicating maximum operation) or a value slightly higher than that. The lower limit is a value corresponding to the maximum operation of discharging the fuel cell or the battery (a value indicating maximum operation) or a value slightly lower than that.

As a result, the relationship between the first storage amount index and the target value determined by the bus voltage target value calculation unit 32b is as shown in Figure 8C. Since the first storage amount index can only take values between the lower limit value and the upper limit value, the bus voltage target value also takes values corresponding to this range.

Figure 15 is a diagram explaining the operation of the system when power exceeding the power absorption capacity of the water electrolysis cell is supplied when the upper and lower limits are set for the first power storage index as described above. Note that in this case, the offset integration unit 32c does not integrate the offset and add it.

Figure 15(a) shows the time variations in generated power 1501 and load power 1502, which are the same as Figure 14(a). Figure 15(b) shows the time variations in power consumption 1503 of the water electrolysis cell and power generation 1504 in the fuel cell. Figure 15(c) shows the time variations in voltage 1505 and current 1506 of the storage device 31. Figure 15(e) shows the time variations in the control target value 1507 of the bus voltage. Note that the vertical scale in Figure 15(d) is significantly different from that of Figure 14(e).

As these results show, the bus voltage drops in a short time after the state in which the generated power exceeds the power absorption capacity of the water electrolysis cell is resolved, charging in the water electrolysis cell ends, and power supply from the fuel cell becomes possible. In other words, by setting an upper limit on the first power storage index, even if the generated power is excessive, the system can be returned to a normal state in a relatively short time after the state is resolved.

When the load power is excessive, a lower limit is set for the first storage capacity indicator, thereby achieving the same effect as described above.

As shown in FIG. 15(c), after the system control is restored, a difference occurs between the first storage amount index (a value obtained by calculation and used for control) and the actual storage amount of the storage device 31. If such errors in the storage amount accumulate, there is a high possibility that long-term operation of the system will be hindered.

In the DC bus control system according to this embodiment, the main stabilization device 30 determines the bus voltage target value by using the offset integration unit 32c to take into account the integral value (accumulated deviation) of the difference between the second battery storage index (battery voltage) and the target value. Therefore, the difference between the calculated battery storage index (first battery storage index) and the actual battery storage amount as described above can be automatically eliminated in the long term.

In other words, according to this embodiment, if excessive power generation or consumption occurs, normal control can be restored in a short time after the problem is resolved, and stable continuous operation for a long period of time is possible.

10: Solar 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 power generator 22: Power converter 30: Main stabilization device 31: Power 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 integration unit 32d: Subtractor 32e: Adder 32f: Charge/discharge control unit 32g: Drive circuit 40: Semi-stabilizing device 41: Power storage device 41a: Sensor 42: Power converter 42A: DC/DC conversion unit 42B: Control circuit 42a: Voltage detector 42b: Comparator 42c: Subtractor 42d: Charge/discharge control unit 42e: Drive circuits 50, 50A: Semi-stabilizing device 51: Water electrolysis cell 52: Power converter 52A: DC/DC conversion unit 52B: Control circuit 52a: Voltage detector 52b: Comparator 52c: Subtractor 52d: Charge control unit 52e: Drive circuit 53: Hydrogen storage device 60: Semi-stabilizing device 61: Fuel cell 62: Power converter 70: DC bus 80: Monitoring/indication device 90: Load

Claims (9)

A DC bus control system for controlling power fluctuations of a DC bus connecting an input power source and a load, comprising:
a primary stabilization device having a first charging/discharging element and a first power converter;
at least one quasi-stabilizing device having a second charge/discharge element, a charging element, or a discharging element and a second power converter;
the first power converter is configured to obtain a bus voltage target value, and to bidirectionally exchange DC power between the first charging/discharging element and the DC bus such that a voltage of the DC bus coincides with the bus voltage target value;
the second power converter is configured to calculate a current target value according to a difference between a threshold value related to charging or discharging of the second charging/ discharging element, the charging element, or the discharging element and the voltage of the DC bus, and to exchange DC power between the second charging/discharging element, the charging element, or the discharging element and the DC bus such that a current equal to the current target value flows through the second charging/discharging element, the charging element, or the discharging element;
the first power converter determines the bus voltage target value based on a target value corresponding to a first power storage amount index of the first charging/discharging element and a second power storage amount index determined by a method different from the first power storage amount index of the first charging/discharging element;
1. A DC bus control system comprising: the second charge amount indicator is a terminal voltage of the first charge/discharge element,
the bus voltage target value is calculated by adding a value corresponding to an accumulated deviation obtained by integrating a difference between the second power storage amount index and a target value of the second power storage amount index to a target value corresponding to a first power storage amount index of the first charging/discharging element;
2. The DC bus control system of claim 1. The cumulative deviation is a value obtained by integrating a difference between the second storage amount index and a target value of the second storage amount index only when the magnitude of the charging/discharging current of the first charging/discharging element is smaller than a predetermined value.
3. The DC bus control system of claim 2. the first power converter provides a period during which charging/discharging of the first charging/discharging element is not performed;
The cumulative deviation is a value obtained by integrating a difference between the second power storage amount index and a target value of the second power storage amount index over the period.
4. A DC bus control system according to claim 2 or 3. The first storage amount index is a value obtained by integrating the charge/discharge current of the first charge/discharge element.
5. A DC bus control system according to any one of claims 1 to 4. A predetermined upper limit and a lower limit are set for the first storage amount indicator.
6. The DC bus control system of claim 5. The upper limit is a value corresponding to a maximum operation of charging the second charging/discharging element or the charging element,
The lower limit is a value corresponding to the maximum discharge operation of the second charge/discharge element or the discharge element;
7. The DC bus control system of claim 6. 8. The DC bus control system of claim 1, further comprising a renewable energy power supply system as the input power source. The DC bus control system of claim 8, further comprising the load.

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