DC power transmission system and control method

Abstract

To provide a DC power transmission system that can be controlled to satisfy system constraints.SOLUTION: A DC power transmission system comprises: a first conversion unit that converts electric power supplied from a power source into AC power; a second conversion unit that converts the AC power converted by the first conversion unit into DC power and outputs the converted DC power to a DC power transmission line; and a third conversion unit that converts the DC power supplied from the DC power transmission line into AC power and outputs the converted AC power to an AC system, wherein the third conversion unit measures a frequency of the AC system, the second conversion unit adjusts a frequency of an AC power transmission line that connects the first conversion unit and the second conversion unit to match the frequency or a frequency previously associated with the frequency, and the first conversion unit performs frequency adjusting rate control for the power source based on the frequency of the AC power transmission line.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a DC power transmission system and a control method. [Background technology]

[0002] BACKGROUND ART Conventionally, techniques have been disclosed for stably transmitting DC power generated by renewable energy (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2023-060571 Summary of the Invention [Problem to be solved by the invention]

[0004] However, the conventional technology disclosed in Patent Document 1 has a problem in that it is not possible to perform control that satisfies grid constraints such as transient stability and frequency. The present invention has been made in view of the above, and aims to provide a DC power transmission system and a control method that can perform control that satisfies grid constraints such as transient stability and frequency. [Means for solving the problem]

[0005] One aspect of the present invention is a DC power transmission system including a first conversion unit that converts power supplied from a power source into AC power, a second conversion unit that converts the AC power converted by the first conversion unit into DC power and outputs the converted DC power to a DC transmission line, and a third conversion unit that converts the DC power supplied from the DC transmission line into AC power and outputs the converted AC power to an AC system, wherein the third conversion unit measures the frequency of the AC system, the second conversion unit matches the frequency of the AC transmission line connecting the first conversion unit and the second conversion unit to the frequency or a frequency that is pre-assigned to the frequency, and the first conversion unit performs frequency adjustment factor control on the power source based on the frequency of the AC transmission line.

[0006] In the DC power transmission system according to one aspect of the present invention, the second conversion unit obtains information indicating the frequency of the AC system from the third conversion unit through communication.

[0007] In addition, in a DC power transmission system according to one embodiment of the present invention, the pre-associated frequency is a frequency indicated as a frequency corresponding to the frequency of the AC system by relationship information indicating a predetermined correspondence between the reference frequency of the AC system and the reference frequencies of the second conversion unit, the first conversion unit, and the power source.

[0008] One aspect of the present invention is a DC power transmission system including a first conversion unit that converts power supplied from a power source into AC power, a second conversion unit that converts the AC power converted by the first conversion unit into DC power and outputs the converted DC power to a DC transmission line, and a third conversion unit that converts the DC power supplied from the DC transmission line into AC power and outputs the converted AC power to an AC system, wherein the third conversion unit measures the frequency of the AC system, and the second conversion unit converts the frequency of an AC transmission line connecting the first conversion unit and the second conversion unit to the frequency or a frequency previously corresponding to the frequency. a control method executed by a DC power transmission system in which the third conversion unit measures the frequency of the AC system; the second conversion unit matches the frequency of the AC transmission line connecting the first conversion unit and the second conversion unit to the frequency or a frequency previously associated with the frequency; and the first conversion unit performs frequency adjustment rate control on the power source based on the frequency of the AC transmission line. [Effects of the Invention]

[0009] According to the present invention, it is possible to provide a DC power transmission system that can be controlled to satisfy grid constraints. [Brief explanation of the drawings]

[0010] [Figure 1] 1 is a diagram illustrating an example of the configuration of a power supply system according to an embodiment of the present invention. [Figure 2] 1 is a diagram illustrating an example of a functional configuration of a DC power transmission system according to an embodiment of the present invention. [Figure 3] FIG. 2 is a diagram illustrating an example of a chopper cell circuit according to the present embodiment. [Figure 4] FIG. 1 is a diagram illustrating an example of a full-bridge cell circuit according to an embodiment of the present invention. [Figure 5] 1 is a diagram illustrating an example of an AC / DC conversion circuit according to an embodiment of the present invention; [Figure 6] FIG. 4 is a diagram illustrating an example of a flow of frequency adjustment rate control according to the present embodiment. [Figure 7] FIG. 10 is a diagram illustrating a modified example of the power supply system. [Figure 8] FIG. 1 is a diagram illustrating an example of the configuration of a conventional DC power transmission system. DETAILED DESCRIPTION OF THE INVENTION

[0011] 1 is a diagram showing an example of the configuration of a power supply system 1 according to this embodiment. The power supply system 1 is made up of a DC transmission system 10, a power source 20, an AC system 30, and a power plant 40. The power supply system 1 supplies power generated by power generation facilities such as the power plant 40 and the power source 20 to demand facilities (not shown) such as factories and homes via the AC system 30, which is made up of substations, transmission lines, distribution lines (none of which are shown), and the like. The power source 20 also includes a device that consumes power and reduces its power consumption depending on the power supply status of the AC grid 30, thereby making the reduced amount of power appear to be produced relatively. For example, the power source 20 includes a water electrolysis device that produces hydrogen, a data center where many servers operate, a server cooling device, and the like.

[0012] In order to operate the AC system 30 stably, the following requirements must be met. (1) The AC voltage, current, frequency, etc. are at a level that will not damage the devices that make up the AC system 30. (2) Even if AC voltage, current, frequency, etc. fluctuate due to an accident or other reason, the system must be able to return to a state without fluctuations.

[0013] When power generation facilities and demand facilities are connected to the AC system 30, it is necessary to maintain the AC system 30 in a state where it can operate stably. The constraints for stably operating the AC system 30 are called system constraints. System constraints can be classified into the following categories according to electrical phenomena:

[0014] (1) Heat capacity (2) AC voltage (3) Transient stability (4) Steady-state stability (5) Frequency (6) Fault current

[0015] In recent years, technological development of renewable energy sources such as solar and wind power has progressed. Aiming to make these renewable energies the main power source of the AC grid 30, the development of offshore wind power plants, for example, is underway. When an offshore wind power plant is installed offshore, the distance between the renewable energy power source and the AC grid 30 increases. When the distance between the renewable energy power source and the AC grid 30 increases, DC transmission, which can transmit power using two wires (positive and negative), can reduce the amount of wire material and is advantageous in terms of construction costs and maintainability of the transmission line compared to AC transmission using three-phase wires. For DC transmission, a DC transmission system such as the DC transmission system 10 described above is used.

[0016] An existing power plant 40 (for example, a thermal power plant, a hydroelectric power plant, or a nuclear power plant) supplies AC power generated by a synchronous generator to an AC system 30. If an accident occurs in the AC system 30 and the power supply status of the AC system 30 changes, the synchronous generator acts to absorb the change and stabilizes the power supply status of the AC system 30.

[0017] In the following description, the term "power supply status" refers to, for example, the status of the AC voltage, AC current, frequency, etc. of the AC system 30. Furthermore, the term "power supply status has changed" refers to, for example, a case where the AC voltage, AC current, frequency, etc. of the AC system 30 has changed from a steady state (for example, a state where no fault has occurred).

[0018] Unlike conventional AC transmission systems that connect a power plant 40 to an AC system 30, a DC transmission system such as the DC transmission system 10 requires DC-AC conversion within the DC transmission system 10. In DC-AC conversion, an AC-DC converter is placed between the DC side and the AC side. In this case, even if an accident occurs in the AC system 30 and the power supply status of the AC system 30 (e.g., AC voltage, AC current, frequency, etc.) changes, the change in the power supply status may not appear in the power source of the AC system 14 across the DC-AC converter. Therefore, when connecting the DC power transmission system 10 to the AC grid 30 as a main power source, it is desirable to make the DC power transmission system 10 behave similarly to the synchronous motor of an existing power plant 40 by reducing transient stability and reducing frequency fluctuations through pseudo-inertia.

[0019] [Conventional DC transmission system] Here, a conventional DC power transmission system will be described with reference to FIG. 8 is a diagram showing an example of the configuration of a conventional DC power transmission system. In the conventional DC power transmission system, a first conversion unit 91 converts power generated by a power source 20 (for example, a wind power generation device 22) into AC power and outputs the converted power to an AC power transmission line 14.

[0020] The AC transmission line 14 is an electric wire on the power source 20 side to which a plurality of power sources 20 (for example, wind power generation devices 22) are connected. That is, the first conversion unit 91 plays a role in controlling the generated power to be fed into the AC transmission line 14, which is an electric wire, when the plurality of power sources 20 are operated in parallel.

[0021] The second conversion unit 92 converts AC power supplied from the AC transmission line 14 into DC power and outputs the DC power to the DC transmission line 15. The DC transmission line 15 is an electric line that transmits DC power over long distances. The third conversion unit 93 converts the DC power supplied from the DC transmission line 15 into AC power that matches the AC voltage and frequency of the AC system 30 and outputs it to the AC system 30 via the AC system connection line 16. For example, in the case of offshore wind power generation, the second conversion unit 92 is an AC / DC converter installed offshore, and the third conversion unit 93 is an AC / DC converter installed on land.

[0022] In this conventional power supply system, the wind turbine generator 22 generates power according to wind conditions. That is, the output of the wind turbine generator 22 is determined by the wind conditions. The second converter 92 generates the AC voltage and frequency of the AC transmission line 14 based on the output of the wind turbine generator 22 determined by the wind conditions. Next, the third converter 93 performs control to keep the DC voltage of the DC transmission line 15 constant. That is, in the case of a conventional power supply system, the output of the first conversion unit 91, the second conversion unit 92, and the third conversion unit 93 are controlled in this order to follow the output fluctuation of the wind power generation device 22. Figure 2

[0023] [Functional configuration of DC power transmission system 10] 2 is a diagram illustrating an example of a functional configuration of the DC power transmission system 10 according to the present embodiment. As described above, the DC power transmission system 10 includes the first conversion unit 11, the second conversion unit 12, and the third conversion unit 13. In this embodiment, the DC power transmission system 10 also includes the first conversion unit 11 and the AC power transmission line 14, which are arranged closer to the power source 20 than the DC power transmission line 15 and the second conversion unit 12.

[0024] [Regarding the third conversion unit 13] The third conversion unit 13 of this embodiment has a function of monitoring the power supply status of the AC system 30. For example, the power supply system 1 includes devices that detect the occurrence of an accident in the AC system 30 and devices that cut off the power supply when an accident occurs (for example, a protective relay system such as a stabilization relay, a high-speed circuit breaker in a substation, etc.). The third conversion unit 13 detects the occurrence of an accident in the AC system 30 by monitoring the operation of these devices. The third conversion unit 13 may also have a function of monitoring AC voltage fluctuations and frequency fluctuations in the AC system 30. In this case, the third conversion unit 13 detects the occurrence of AC voltage fluctuations and frequency fluctuations in the AC system 30.

[0025] In the DC power transmission system 10 of this embodiment, the first conversion unit 11 and the second conversion unit 12 operate in cooperation with the third conversion unit 13. The configurations of the first conversion unit 11 and the second conversion unit 12 will be described.

[0026] [Functional configuration of each conversion unit] The first conversion unit 11 is installed in close proximity to the power source 20, converts the power generated by the power source 20 into AC power, and outputs the AC power to the AC transmission line . Here, the power source 20 is, for example, a solar power generation device 21, a wind power generation device 22, or a power storage device 23. In the DC power transmission system 10, the power source 20 may be of any type as long as it is connected to the first conversion unit 11 so as to supply a power generation output.

[0027] The first conversion unit 11 includes a first control unit 110 and a first conversion circuit 120. The first control unit 110 is, for example, a computer device, and includes a CPU (Central Processing Unit) that operates based on a program stored in a storage unit (not shown).

[0028] The first control unit 110 includes a first voltage control unit 111 as a functional unit. The first voltage control unit 111 may be referred to as an active power output control unit. As an active power output control unit, the first voltage control unit 111 controls active power output in accordance with the frequency of the AC transmission line 14.

[0029] The first conversion circuit 120 is a known DC-AC conversion circuit or AC-AC conversion circuit, and includes, for example, a semiconductor switch. The input side of the first conversion circuit 120 is connected to the power source 20, and the output side is connected to the AC transmission line 14. The first conversion circuit 120 converts the power output from the power source 20 into AC power and outputs it to the AC transmission line 14 by controlling the semiconductor switch on and off based on the control of the first control unit 110.

[0030] The second conversion unit 12 includes a second control unit 210, a second conversion circuit 220, and a second power storage unit 221. The second control unit 210 is, for example, a computer device, and includes a CPU (Central Processing Unit) that operates based on a program stored in a storage unit (not shown).

[0031] The second control unit 210 controls, for example, the second conversion circuit 220. That is, in the case of a constant voltage control mode in which the second control unit 210 operates under constant voltage control, which is a process for keeping the voltage of the DC transmission line 15 constant (that is, in the normal case), the second control unit 210 controls the frequency of the AC transmission line 14.

[0032] The second conversion circuit 220 is a known AC-DC conversion circuit and includes, for example, a semiconductor switch. The input side of the second conversion circuit 220 is connected to the AC transmission line 14, and the output side is connected to the DC transmission line 15. The second conversion circuit 220 converts AC power supplied from the AC transmission line 14 into DC power and outputs it to the DC transmission line 15 by controlling the semiconductor switch to turn on and off based on the control of the second control unit 210.

[0033] As an example, the second conversion circuit 220 includes a semiconductor switch circuit such as a chopper cell circuit 400 or a full-bridge cell circuit 450.

[0034] 3 is a diagram showing an example of a chopper cell circuit 400 of this embodiment. The chopper cell circuit 400 includes an upper semiconductor switch 401, a lower semiconductor switch 402, and a power storage unit 403 (e.g., a capacitor), and constitutes a single-phase inverter circuit. Depending on the combination of switching patterns of the upper semiconductor switch 401 and the lower semiconductor switch 402, the chopper cell circuit 400 can output either the voltage across the power storage unit 403 (also referred to as capacitor voltage Vcc) or 0 (zero) [V] as the output voltage Vo.

[0035] Specifically, the chopper cell circuit 400 can set the output voltage Vo to the capacitor voltage Vcc [V] by turning on the upper semiconductor switch 401 and turning off the lower semiconductor switch 402. The chopper cell circuit 400 can set the output voltage Vo to 0 (zero) [V] by turning off the upper semiconductor switch 401 and turning on the lower semiconductor switch 402. The chopper cell circuit 400 is also called a half-bridge cell circuit.

[0036] 4 is a diagram showing an example of a full-bridge cell circuit 450 of this embodiment. The full-bridge cell circuit 450 includes an upper semiconductor switch 451, a lower semiconductor switch 452, and a power storage unit 453, and constitutes a single-phase inverter circuit. By combining the switching patterns of the upper semiconductor switch 451 and the lower semiconductor switch 452, the full-bridge cell circuit 450 can output either ±Vcb [V] or 0 (zero) [V] as the output voltage Vo, where Vcb is the voltage across the power storage unit 453.

[0037] 2, the second conversion circuit 220 includes an AC / DC conversion circuit 500 in which the above-described chopper cell circuit 400 or full-bridge cell circuit 450 are connected in series. The AC / DC conversion circuit 500 is a so-called MMC (Modular Multilevel Converter) and is capable of converting an AC waveform, which is a multilevel waveform, into DC.

[0038] 5 is a diagram illustrating an example of an AC / DC conversion circuit 500 according to this embodiment. The AC / DC conversion circuit 500 includes an AC / DC conversion circuit having an upper arm 501 and a lower arm 502 in which chopper cells 400 or full-bridge cells 450 are connected in series, and a braking chopper 511 (or braking chopper 512). The braking chopper 511 includes a resistor and a semiconductor switch (for example, an IGBT; Insulated Gate Bipolar Transistor), and is provided on the DC side of the AC / DC conversion circuit 500. The braking chopper 511 can consume the DC power transmitted from the power source 20 (for example, the offshore wind power generation device 22) in a resistor for a short period of time such as several tens to several hundreds of milliseconds.

[0039] The AC / DC conversion circuit 500 may also include a capacitor 513 electrically connected in parallel to the braking chopper 511. The capacitor 513 stores surplus power in the second conversion circuit 220 and discharges power that is insufficient in the second conversion circuit 220, thereby supplementing the operation of the second conversion circuit 220.

[0040] 2, the second power storage unit 221 is a device having a power storage function, such as a storage battery or a capacitor. The second power storage unit 221 stores surplus power in the second conversion circuit 220 and discharges power that is insufficient in the second conversion circuit 220, thereby supplementing the operation of the second conversion circuit 220. In addition, when the second conversion circuit 220 includes the above-mentioned capacitor 513, the capacitor 513 may function as the second power storage unit 221.

[0041] The third conversion unit 13 includes a third control unit 310, a third conversion circuit 320, and a third power storage unit 321. The third control unit 310 is, for example, a computer device, and includes a CPU (Central Processing Unit) that operates based on a program stored in a storage unit (not shown).

[0042] The third control unit 310 determines, for example, the power supply status of the AC system 30. The third control unit 310 monitors frequency fluctuations and the like of the AC system 30, and determines the power supply status of the AC system 30 (for example, whether or not the frequency is increasing or decreasing).

[0043] The third conversion circuit 320 is a known DC-AC conversion circuit and includes, for example, a semiconductor switch. The input side of the third conversion circuit 320 is connected to the DC transmission line 15, and the output side is connected to the AC grid connection line 16. The third conversion circuit 320 converts DC power supplied from the DC transmission line 15 into AC power and outputs it to the AC grid connection line 16 by controlling the semiconductor switch to be on or off based on the control of the third control unit 310.

[0044] The third conversion circuit 320, like the second conversion circuit 220, includes an AC / DC conversion circuit 500 in which the above-mentioned chopper cell circuit 400 or full-bridge cell circuit 450 is connected in series. In other words, the third conversion circuit 320 is a so-called MMC, and is capable of outputting a multilevel waveform.

[0045] The third power storage unit 321 is a device having a power storage function, such as a storage battery or a capacitor, similar to the second power storage unit 221. The third power storage unit 321 stores surplus power in the third conversion circuit 320 and discharges power that is insufficient in the third conversion circuit 320, thereby supplementing the operation of the third conversion circuit 320. In addition, when the third conversion circuit 320 includes the above-mentioned capacitor 513, the capacitor 513 may function as the third power storage unit 321.

[0046] That is, the third conversion circuit 320 can control the voltage of the DC transmission line 15 using the capacitor provided in the AC / DC conversion circuit 500, the resistor provided in the braking chopper 511, the storage battery provided in the third power storage unit 321, and the like.

[0047] [Frequency regulation rate control] So far, we have explained how to resolve fluctuations in a transient state of the AC system 30. Next, we will explain how the DC power transmission system 10 controls the frequency adjustment rate of the AC system 30. Here, the power source 20 and the first conversion unit 11 are equipped with a frequency regulation ratio control function. The frequency regulation ratio control function is a function that causes the power source 20 and the first conversion unit 11 to stabilize the frequency of the AC grid 30 by behaving in a manner similar to the frequency stabilization action achieved by the governor-free function of a synchronous generator in the power plant 40. For example, the wind power generation device 22 and the first conversion unit 11 have the function of increasing the output of the wind power generator to stabilize the frequency when the frequency of the AC transmission line 14 drops, and decreasing the output of the wind power generator to stabilize the frequency when the frequency of the AC transmission line 14 rises. While the output of the wind turbine generator is being increased, more of the energy produced by the rotation of the wind turbine is taken out by the output, and so the rotation speed of the wind turbine decreases. Generally, the design upper limit time for which the output of the wind turbine generator can be increased is determined in advance by the specifications of the wind turbine generator 22 and the first conversion unit 11. While the output of the wind turbine generator is being decreased, less of the energy produced by the rotation of the wind turbine is taken out by the output, and so the rotation speed of the wind turbine increases. The frequency adjustment rate control function of the power source 20 and the first conversion unit 11 is realized by monitoring the frequency of the AC system (or AC transmission line) connected to the first conversion unit 11.

[0048] In the DC power transmission system 10 of this embodiment, the power source 20 (first conversion unit 11) and the AC system 30 are connected via the DC transmission line 15, and are not directly connected. Therefore, in order to operate the frequency adjustment ratio control functions of the power source 20 and the first conversion unit 11, it is necessary to transmit the frequency fluctuation of the AC system 30 to the first conversion unit 11.

[0049] The DC power transmission system 10 of this embodiment transmits frequency fluctuations of the AC system 30 to the first conversion unit 11, thereby operating the frequency adjustment ratio control functions of the power source 20 and the first conversion unit 11. The frequency adjustment ratio control by the DC power transmission system 10 will be described with reference to Fig. 6 .

[0050] FIG. 6 is a diagram showing an example of the flow of frequency adjustment rate control in this embodiment. (Step S311) The third control unit 310 of the third conversion unit 13 monitors the power supply status of the AC system 30. Here, the power supply status of the AC system 30 is, for example, the frequency of the AC system 30. The third control unit 310 monitors a decrease or increase (i.e., fluctuation) in the frequency of the AC system 30.

[0051] (Step S321) When the third control unit 310 of the third conversion unit 13 detects a fluctuation in the frequency of the AC system 30, the third control unit 310 of the third conversion unit 13 transmits information indicating the frequency of the AC system 30 (hereinafter referred to as "AC frequency information") to the second conversion unit 12. The second conversion unit 12 and the third conversion unit 13 are connected so as to be able to communicate with each other. (Step S211) The second conversion unit 12 acquires the AC frequency information transmitted by the third conversion unit 13. In this way, the second conversion unit 12 acquires the AC system frequency information from the third conversion unit 13 through communication. (Step S221) The second conversion unit 12 causes the first conversion unit 11 to perform frequency adjustment rate control by matching the frequency of the AC transmission line 14 (i.e., the AC power converted by the first conversion unit 11) to the frequency indicated by the AC frequency information.

[0052] (Step S111) When the first voltage control unit 111 of the first conversion unit 11 detects that the frequency of the AC transmission line 14 has increased or decreased, the first voltage control unit 111 controls the frequency adjustment rate for the power source 20.

[0053] As described above, in the DC power transmission system 10, the third conversion unit 13 measures the frequency of the AC system 30, and the second conversion unit 12 matches the frequency of the AC transmission line 14 connecting the first conversion unit 11 and the second conversion unit 12 to the measured frequency. The first conversion unit 11 performs frequency adjustment rate control based on the frequency of the AC transmission line 14.

[0054] As described above, in the DC power transmission system 10 of this embodiment, the second conversion unit 12 reproduces the frequency fluctuation of the AC system 30 in the AC transmission line 14. Therefore, the frequency fluctuation of the AC system 30 is transmitted to the first conversion unit 11, and frequency adjustment ratio control is performed. Therefore, in the DC power transmission system 10, the frequency adjustment ratio control function inherent to the power source 20 and the first conversion unit 11 can be used to stabilize the frequency of the AC system 30. Therefore, the DC power transmission system 10 can provide a DC power transmission system that can be controlled to satisfy system constraints.

[0055] The topology of the power supply system 1 shown in FIG. 1 is an example, and is not limited to this. Fig. 7 is a diagram showing a modified example of the power supply system 1. The power supply system 1 may have a plurality of power supply systems or AC systems 30 connected to each other. For example, the power supply system 1 shown in Fig. 1 corresponds to the power supply system 1a in Fig. 7. In the power supply system 1, in addition to the power supply system 1a, a power supply system 1b and a power supply system 1c may also be connected to each other. The DC power transmission system 10 of this embodiment can also be applied to the power supply system 1 configured in this manner. In this case, the frequency stabilization effect achieved by operating the frequency adjustment ratio control of the power source 20 can be arbitrarily distributed to the AC systems 1a, 1b, and 1c.

[0056] The reference frequency of the AC system 30 does not necessarily have to match the reference frequencies of the second conversion unit 12, the first conversion unit 11, and the power source 20. The reference frequency is a preset frequency that satisfies system constraints. If the reference frequencies do not match, the second conversion unit 12 may match the frequency of the AC transmission line 14 to a frequency that is pre-associated with the frequency indicated by the AC system frequency information. In such a case, even if there is a difference in the reference frequencies, frequency fluctuations are suppressed by frequency adjustment ratio control. The frequency that is pre-associated with the frequency indicated by the AC system frequency information is, specifically, a frequency indicated by the relationship information as a frequency corresponding to the frequency indicated by the AC system frequency information. The relationship information is information indicating a pre-defined correspondence between the reference frequency of the AC system 30 and the reference frequencies of the second conversion unit 12, the first conversion unit 11, and the power source 20.

[0057] Therefore, more specifically, based on the AC system frequency information and the relationship information, the second conversion unit 12 may match the frequency of the AC transmission line 14 to the frequency indicated by the relationship information as a frequency corresponding to the frequency indicated by the AC system frequency information.

[0058] It should be noted that equipment manufacturers and system suppliers do not need to change the reference frequency of AC transmission line 14 to match the reference frequency adopted in the country or area to which they are connected.

[0059] Although the embodiments of the present invention have been described above in detail with reference to the drawings, the specific configuration is not limited to this embodiment and can be appropriately modified without departing from the spirit of the present invention. Furthermore, the above-described embodiments can be appropriately combined without departing from the spirit of the present invention.

[0060] Each of the above-mentioned devices has a computer built in. The processes of each of the above-mentioned devices are stored in the form of a program on a computer-readable recording medium, and the computer reads and executes this program to perform the above-mentioned processes. Here, computer-readable recording medium refers to a magnetic disk, magneto-optical disk, CD-ROM, DVD-ROM, semiconductor memory, etc. Alternatively, the computer program may be distributed to a computer via a communication line, and the computer that receives the program may execute the program.

[0061] The program may also be for realizing part of the above-mentioned functions. Furthermore, the above-mentioned functions may be realized in combination with a program already recorded in the computer system, that is, a so-called differential file (differential program). [Explanation of symbols]

[0062] 1...power supply system, 10...DC transmission system, 11...first conversion unit, 12...second conversion unit, 13...third conversion unit, 14...AC transmission line, 15...DC transmission line, 16...AC system connection line, 20...power source, 21...photovoltaic power generation device, 22...wind power generation device, 23...electricity storage device, 30...AC system, 40...power plant, 110...first control unit, 111...first voltage control unit, 120...first conversion circuit, 210...second control unit, 220...second conversion circuit, 221...second electricity storage unit, 310...third control unit, 320...third conversion circuit circuit, 321...third power storage unit, 400...chopper cell circuit, 401...upper semiconductor switch, 402...lower semiconductor switch, 403...power storage unit, 450...full bridge cell circuit, 451...upper semiconductor switch, 452...lower semiconductor switch, 453...power storage unit, 500...AC / DC conversion circuit, 501...upper arm, 502...lower arm, 511...braking chopper, 512...braking chopper, 513...capacitor, 91...conventional first conversion unit, 92...conventional second conversion unit, 93...conventional third conversion unit

Claims

1. a first conversion unit that converts power supplied from a power source into AC power; a second conversion unit that converts the AC power converted by the first conversion unit into DC power and outputs the converted DC power to a DC transmission line; a third conversion unit that converts DC power supplied from the DC transmission line into AC power and outputs the converted AC power to an AC system; A DC power transmission system comprising: The third conversion unit measures the frequency of the AC system, the second conversion unit matches the frequency of an AC transmission line connecting the first conversion unit and the second conversion unit to the frequency or a frequency previously associated with the frequency; The first conversion unit performs frequency adjustment rate control on the power source based on the frequency of the AC transmission line. DC transmission system.

2. The second conversion unit obtains information indicating the frequency of the AC system from the third conversion unit through communication. The DC power transmission system according to claim 1 .

3. The pre-associated frequency is a frequency indicated as a frequency corresponding to the frequency of the AC system by relationship information indicating a predetermined correspondence relationship between a reference frequency of the AC system and reference frequencies of the second conversion unit, the first conversion unit, and the power source.

3. The DC power transmission system according to claim 1 or 2.

4. a first conversion unit that converts power supplied from a power source into AC power; a second conversion unit that converts the AC power converted by the first conversion unit into DC power and outputs the converted DC power to a DC transmission line; a third conversion unit that converts DC power supplied from the DC transmission line into AC power and outputs the converted AC power to an AC system; A DC power transmission system comprising: The third conversion unit measures the frequency of the AC system, the second conversion unit matches the frequency of an AC transmission line connecting the first conversion unit and the second conversion unit to the frequency or a frequency previously associated with the frequency; The first conversion unit performs frequency adjustment rate control on the power source based on the frequency of the AC transmission line. A control method executed by a DC power transmission system, comprising: The third conversion unit measures the frequency of an AC system; The second conversion unit causes a frequency of an AC transmission line connecting the first conversion unit and the second conversion unit to match the frequency or a frequency previously associated with the frequency; The first conversion unit performs frequency adjustment rate control on the power source based on the frequency of the AC transmission line; A control method comprising:

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