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 switches, based on a power supply state of the AC system, from a voltage constant control mode that maintains a voltage of the DC power transmission line constant to a supply power decrease mode that decreases electric power supplied to the AC system, and the second conversion unit performs AC voltage decrease control that decreases a voltage of the AC power converted by the first conversion unit when determining that the third conversion unit has switched to the supply power decrease mode.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 switches from a constant voltage control mode that keeps the voltage of the DC transmission line constant to a reduced supply power mode that reduces the power supplied to the AC system based on the power supply status of the AC system, and the second conversion unit performs AC voltage reduction control that reduces the voltage of the AC power converted by the first conversion unit when it determines that the third conversion unit has switched to the reduced supply power mode.

[0006] Furthermore, in a DC power transmission system according to one embodiment of the present invention, when the second conversion unit determines that the third conversion unit has switched to the reduced supply power mode, the second conversion unit performs the AC voltage reduction control and outputs a mode switching instruction to the third conversion unit to switch from the reduced supply power mode to the constant voltage control mode, and the third conversion unit switches from the reduced supply power mode to the constant voltage control mode based on the mode switching instruction output by the second conversion unit.

[0007] In the DC power transmission system according to one aspect of the present invention, the second conversion unit determines that the third conversion unit has been switched to the supplied power reduction mode when detecting an increase in voltage of the DC power transmission line.

[0008] Furthermore, in a DC power transmission system according to one embodiment of the present invention, the third conversion unit transmits a mode switching signal to the second conversion unit indicating that the mode has been switched to the reduced supply power mode, and when the second conversion unit receives the mode switching signal from the third conversion unit, the second conversion unit determines that the third conversion unit has been switched to the reduced supply power mode.

[0009] In addition, in the DC power transmission system according to one embodiment of the present invention, in the reduced supply power mode, the third conversion unit dissipates excess power in the DC power transmission line by generating heat using a braking chopper circuit that connects the DC power transmission line lines via a resistor and a switch.

[0010] In the DC power transmission system according to one aspect of the present invention, at least one of the second conversion unit and the third conversion unit causes a power storage unit that stores power to store surplus power in the reduced power supply mode.

[0011] One aspect of the present invention is a control method for 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, the control method including: switching, by the third conversion unit, from a constant voltage control mode that keeps the voltage of the DC transmission line constant to a reduced supply power mode that reduces the power supplied to the AC system based on the power supply status of the AC system; and performing AC voltage reduction control by the second conversion unit that reduces the voltage of the AC power converted by the first conversion unit when it is determined that the third conversion unit has switched to the reduced supply power mode. [Effects of the Invention]

[0012] 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]

[0013] [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] FIG. 10 is a diagram illustrating an example of transient stability control. [Figure 3] FIG. 4 is a diagram showing an example of AC voltage drop tolerance of the present embodiment. [Figure 4]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 5] FIG. 2 is a diagram illustrating an example of a chopper cell circuit according to the present embodiment. [Figure 6] FIG. 1 is a diagram illustrating an example of a full-bridge cell circuit according to an embodiment of the present invention. [Figure 7] 1 is a diagram illustrating an example of an AC / DC conversion circuit according to an embodiment of the present invention; [Figure 8] FIG. 2 is a diagram illustrating an example of the flow of operations of the DC power transmission system according to the present embodiment. [Figure 9] FIG. 10 is a diagram illustrating a modified example of the power supply system. [Figure 10] FIG. 1 is a diagram illustrating an example of the configuration of a conventional DC power transmission system. DETAILED DESCRIPTION OF THE INVENTION

[0014] 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. The power supply system 1 is a general term for the power supply system 1a and a power supply system 1b, which is a modified example described later. In the following description, the power supply system 1a will also be simply referred to as the power supply system 1.

[0015] In order to operate the AC system 30 stably, the following requirements must be met. (1) The voltage, current, frequency, etc. are at a level that will not damage the devices that make up the AC system 30. (2) Even if voltage, current, frequency, etc. fluctuate due to an accident or other reason, the system must be able to return to a state without fluctuations.

[0016] 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:

[0017] (1) Heat capacity (2) Voltage (3) Transient stability (4) Steady-state stability (5) Frequency (6) Fault current

[0018] 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.

[0019] 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.

[0020] In the following description, the term "power supply status" refers to, for example, the status of the voltage, current, frequency, etc. of the AC system 30. Furthermore, the term "power supply status has changed" refers to, for example, a case where the voltage, current, frequency, etc. of the AC system 30 has changed from a steady state (for example, a state where no fault has occurred).

[0021] In a DC transmission system such as the DC transmission system 10, unlike a conventional AC transmission system that connects a power plant 40 to an AC system 30, DC-AC conversion is essential between the DC transmission system 10 and the AC system 30. 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 (for example, voltage, current, frequency, etc.) of the AC system 30 changes, the power is not transferred to the electric wires on the DC side (i.e., the DC transmission system 10 side) across the AC-DC converter. Changes in supply conditions may not be apparent. 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 a synchronous generator in an existing power plant 40 by reducing transient stability and reducing frequency fluctuations through pseudo-inertia.

[0022] [Conventional DC transmission system] Here, a conventional DC power transmission system will be described with reference to FIG. 10 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 power to an AC power transmission line 14.

[0023] The AC transmission line 14 is a bus 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 a bus, when the plurality of power sources 20 are operated in parallel.

[0024] 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 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.

[0025] 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.

[0026] [Transient stability control] As described above, fluctuations occur in the synchronous generator of the power plant 40 due to an accident or the like in the AC system 30. If the energy generated by the fluctuations in the synchronous generator of the power plant 40 becomes (acceleration energy > deceleration energy), stable continuous operation becomes impossible. In order to reduce acceleration energy and increase deceleration energy, it is effective to quickly reduce the amount of electricity (i.e., power generation output) flowing from the power plant 40 into the AC grid 30 after an accident occurs in the AC grid 30. Here, "quickly" means within about several hundred milliseconds after the accident occurs.

[0027] Furthermore, it is preferable that the power plant 40, which has reduced its power output due to an accident, return its power output to normal after a certain period of time has elapsed. Here, the certain period of time refers to about one second after the power output reduction control is initiated.

[0028] That is, transient stability refers to the stability of the power supply state of the AC system 30 for a period of several hundred milliseconds to several seconds after the occurrence of a fault. The DC power transmission system 10 of this embodiment controls the transient stability as described above.

[0029] Figure 2 is a diagram showing an example of transient stability control. Steady-state control is performed from time t0 to time t1 in the figure. At time t1, control to limit power generation output following the occurrence of an accident (i.e., power generation output reduction control) is initiated. At time t2, power generation output reduction control is restored to steady-state control. The time from time t1 to time t2 is the fixed time mentioned above, for example, about 1 second.

[0030] On the other hand, in the control of the power source 20 (for example, the wind power generation device 22), the AC voltage drop tolerance is given as a constraint.

[0031] 3 is a diagram showing an example of AC voltage drop tolerance in this embodiment. The solid line in the figure is a requirement line for AC voltage drop tolerance. If the residual voltage of the output of the power source 20 falls within the continuous operation region above this requirement line, the power source 20 can operate stably. Therefore, when performing power generation output reduction control of the power source 20, if the operating conditions are set above the requirement line (for example, the operation setting line indicated by the dashed line), the power source 20 can operate stably.

[0032] The DC power transmission system 10 of this embodiment performs control to maintain transient stability even when the power supply situation of the AC system 30 changes, for example, when an accident occurs in the AC system 30. The configuration of this DC power transmission system 10 will be described with reference to Fig. 4.

[0033] [Functional configuration of DC power transmission system 10] 4 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.

[0034] [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 voltage fluctuations and frequency fluctuations in the AC system 30. In this case, the third conversion unit 13 detects the occurrence of voltage fluctuations and frequency fluctuations in the AC system 30.

[0035] When there is no fault in the AC system 30 (i.e., under normal circumstances), the third conversion unit 13 operates in an automatic voltage regulator (AVR) mode that keeps the voltage of the DC transmission line 15 constant depending on the power supply status of the AC system 30. On the other hand, if an accident occurs in the AC system 30 and it is possible that (acceleration energy > deceleration energy) occurs, the third conversion unit 13 operates in a supply power reduction mode that reduces the active power output to the AC system connection line 16.

[0036] In the supply power reduction mode, the third conversion unit 13 may operate in an automatic power regulator (APR) mode to reduce the active power output to the AC grid connection line 16, thereby reducing the output to the AC grid connection line 16. In addition, in the supply power reduction mode, the third conversion unit 13 may operate in an AVR mode to increase the voltage of the DC transmission line 15, thereby reducing the output to the AC grid connection line 16.

[0037] That is, after a fault occurs in the AC system 30, the third conversion unit 13 quickly reduces the amount of electricity (more specifically, active power) flowing from the power source 20 to the AC system 30. In this way, the DC power transmission system 10 reduces acceleration energy and increases deceleration energy, thereby ensuring continued stable operation.

[0038] In this embodiment, a case will be described as an example in which the third conversion unit 13 operates in the APR mode to reduce the active power output to the AC system connection line 16.

[0039] When the third conversion unit 13 operates in a supply power reduction mode (for example, APR mode) and reduces the output of the AC grid connection line 16, excess energy flows from the power source 20 to the third conversion unit 13, causing a rise in the voltage value of the DC transmission line 15. 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 to suppress a rise in the voltage of the DC transmission line 15. The configurations of the first conversion unit 11 and the second conversion unit 12 will be described.

[0040] [Functional configuration of each conversion unit] The first conversion unit 11 is installed in close proximity to the power source 20, converts the DC 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.

[0041] 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).

[0042] The first control unit 110 includes, as its functional unit, a first voltage control unit 111. The first voltage control unit 111 monitors the voltage of the AC transmission line 14. When the voltage of the AC transmission line 14 drops, the power generation output of the power source 20 drops.

[0043] 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.

[0044] 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). The second control unit 210 includes, as its functional units, a second voltage control unit 211, a mode determination unit 212, and a mode return instruction unit 213.

[0045] The mode determination unit 212 determines whether the operation mode of the third conversion unit 13 is a constant voltage control mode (e.g., AVR mode; the same applies in the following description) or a supply power reduction mode (e.g., APR mode; the same applies in the following description). The method by which the mode determination unit 212 determines the operating mode of the third conversion unit 13 may be either a method of directly obtaining the operating mode of the third conversion unit 13 by communicating with the third conversion unit 13, or a method of estimating (i.e., indirectly obtaining) the operating mode of the third conversion unit 13 from changes in the voltage of the DC transmission line 15. When the mode determination unit 212 determines that the operation mode of the third conversion unit 13 is the supply power reduction mode, it instructs the second voltage control unit 211 to absorb surplus energy and reduce the voltage of the AC transmission line 14.

[0046] After the mode determination unit 212 determines that the mode is the reduced power supply mode, the mode return instruction unit 213 outputs an instruction to return to the constant voltage control mode to the third conversion unit 13 after a certain time has elapsed. Here, the certain time is about one second after the transition to the reduced power supply mode.

[0047] The second voltage control unit 211 controls the second conversion circuit 220 based on the determination result of the mode determination unit 212. That is, in the case of the constant voltage control mode (i.e., the normal case), the second voltage control unit 211 controls the voltage of the AC transmission line 14 to follow the voltage of the DC transmission line 15. In the supply power reduction mode (i.e., when the third conversion unit 13 reduces the power supplied to the AC grid connection line 16), the second voltage control unit 211 controls the voltage of the AC transmission line 14 to reduce.

[0048] 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.

[0049] 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.

[0050] 5 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.

[0051] 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.

[0052] 6 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.

[0053] 4, 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.

[0054] 7 is a diagram showing an example of an AC / DC conversion circuit 500 according to this embodiment. The AC / DC conversion circuit 500 includes an upper arm 501 and a lower arm 502 in which chopper cell circuits 400 or full-bridge cell circuits 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.

[0055] 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.

[0056] 4, 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.

[0057] That is, the second conversion circuit 220 can control the voltage of the AC transmission line 14 by 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 second power storage unit 221, and the like.

[0058] 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). The third control unit 310 includes, as its functional units, a third voltage control unit 311, a power supply status determination unit 312, and a mode switching unit 313.

[0059] The power supply status determination unit 312 determines the power supply status of the AC system 30. The power supply status determination unit 312 monitors the voltage fluctuations and frequency fluctuations of the AC system 30, the operation of the stabilization relay, the operation of the high-speed circuit breaker, and the like, and determines whether or not an accident has occurred in the AC system 30. Based on the determination result of the power supply status determination unit 312, the mode switching unit 313 switches the power control mode of the third conversion unit 13 between a constant voltage control mode and a supply power reduction mode. The third voltage control unit 311 controls the third conversion circuit 320 in a constant voltage control mode or a reduced power supply mode based on the operation of the mode switching unit 313 .

[0060] 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.

[0061] 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.

[0062] 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.

[0063] 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 described above, the resistor provided in the braking chopper 511, the storage battery provided in the third power storage unit 321, and the like.

[0064] [Operation of DC power transmission system 10] FIG. 8 is a diagram showing an example of the flow of operations of the DC power transmission system 10 of this embodiment. (Step S310) The power supply status determination unit 312 of the third conversion unit 13 monitors the power supply status of the AC system 30. The power supply status determination unit 312 outputs the monitoring result of the power supply status to the mode switching unit 313. (Step S320) Mode switching unit 313 determines whether the power supply status output from power supply status determination unit 312 in step S310 is a status in which a transition to a supply power reduction mode is necessary. If mode switching unit 313 determines that a transition to a supply power reduction mode is necessary, it switches the power control mode to the supply power reduction mode. (Step S330) The third voltage control unit 311 reduces the output to the AC system connection line 16 in the supply power reduction mode.

[0065] That is, the third conversion unit 13 switches from the constant voltage control mode to the supply power reduction mode based on the power supply status of the AC system 30. As described above, the AVR mode is an example of a constant voltage control mode in which the voltage of the DC transmission line 15 is kept constant. The APR mode is an example of a power supply reduction mode in which the output to the AC grid connection line 16 is quickly reduced, resulting in an increase in the voltage of the DC transmission line 15 (DC transmission line).

[0066] That is, based on the power supply status of the AC system 30, the third conversion unit 13 switches from a constant voltage control mode (e.g., AVR mode) that keeps the voltage of the DC transmission line 15 (DC transmission line) constant to a supply power reduction mode (e.g., APR mode) that increases the voltage of the DC transmission line 15 (DC transmission line). As a result, surplus power is generated in the DC transmission line 15, and the voltage of the DC transmission line 15 rises.

[0067] (Step S340) The third conversion unit 13 operates the braking chopper 511 of the third conversion circuit 320 to cause the resistor to consume excess power on the DC transmission line 15 as heat, thereby stabilizing the transient state of the DC transmission line 15.

[0068] That is, in the supply power reduction mode (APR mode), the third conversion unit 13 causes excess power of the DC transmission line 15 (DC transmission line) to be consumed as heat by a braking chopper 511 (braking chopper circuit) that connects the lines of the DC transmission line 15 (DC transmission line) via a resistor and a switch.

[0069] (Step S350) The third conversion unit 13 causes the capacitor included in the chopper cell circuit 400 of the third conversion circuit 320 to absorb surplus power from the DC transmission line 15. The capacitor included in the chopper cell circuit 400 of the third conversion circuit 320 is also referred to as an MMC cell capacitor. The MMC cell capacitor corresponds to the power storage unit 403 of the chopper cell circuit 400 or the power storage unit 453 of the full-bridge cell circuit 450. The third conversion unit 13 may cause the third power storage unit 321 to absorb surplus power from the DC transmission line 15.

[0070] That is, in the reduced power supply mode, the third conversion unit 13 stores the surplus power in the power storage unit (the MMC cell capacitor or the third power storage unit 321) that stores power.

[0071] (Step S210) Meanwhile, the mode determination unit 212 of the second conversion unit 12 detects that the third conversion unit 13 has transitioned to the supply power reduction mode.

[0072] Here, the third conversion unit 13 and the second conversion unit 12 may be connected via a communication line, allowing them to exchange information about the power control mode. In this case, the mode determination unit 212 may detect that the third conversion unit 13 has transitioned to the reduced power supply mode by acquiring information about the power control mode from the mode switching unit 313 of the third conversion unit 13 via the communication line. In this case, in step S320, the third conversion unit 13 notifies the second conversion unit 12 via the communication line that the third conversion unit 13 has transitioned to the reduced power supply mode. The mode determination unit 212 of the second conversion unit 12 determines that the third conversion unit 13 has transitioned to the reduced power supply mode based on the transition information to the reduced power supply mode received via the communication line.

[0073] That is, the third conversion unit 13 transmits a mode switching signal indicating that the mode has been switched to the reduced power supply mode to the second conversion unit 12. When the second conversion unit 12 receives the mode switching signal from the third conversion unit 13, it determines that the third conversion unit 13 has been switched to the reduced power supply mode.

[0074] Furthermore, the mode determination unit 212 may detect that the third conversion unit 13 has transitioned to the reduced power supply mode by monitoring voltage fluctuations on the DC transmission line 15. In this case, when the mode determination unit 212 detects a voltage increase on the DC transmission line 15, it determines that the third conversion unit 13 has transitioned to the reduced power supply mode.

[0075] That is, when the second conversion unit 12 detects an increase in the voltage of the DC transmission line 15 (DC transmission line), it determines that the third conversion unit 13 has been switched to the supply power reduction mode.

[0076] (Step S220) When the mode determination unit 212 determines that the third conversion unit 13 has transitioned to the supply power reduction mode, the second voltage control unit 211 causes the MMC cell capacitor of the second conversion circuit 220 or the second power storage unit 221 to absorb the surplus power of the DC transmission line 15. As a result, the voltage of the DC transmission line 15 decreases.

[0077] That is, in the reduced power supply mode, the second conversion unit 12 causes the power storage unit that stores power to store surplus power.

[0078] When the second conversion unit 12 determines that the third conversion unit 13 has been switched to the supplied power reduction mode, the second conversion unit 12 performs AC voltage reduction control to reduce the voltage of the AC power converted by the first conversion unit 11.

[0079] (Step S110) When the voltage of the AC transmission line 14 drops, the power generation output of the power source 20 drops. As a result, when the third conversion unit 13 reduces the output of the AC system connection line 16 due to a change in the power supply situation of the AC system 30, the second conversion unit 12 and the first conversion unit 11 sequentially reduce their outputs to ensure continued stable operation in a transient state.

[0080] (Step S230) The mode return instruction unit 213 of the second conversion unit 12 outputs an instruction to return to the power control mode to the third conversion unit 13 after a predetermined time (for example, one second) has elapsed since step S210.

[0081] That is, when the second conversion unit 12 determines that the third conversion unit 13 has switched to the supply power reduction mode, it performs AC voltage reduction control and outputs a mode switching instruction to the third conversion unit 13 to switch from the supply power reduction mode to the constant voltage control mode.

[0082] (Step S360) Returning to FIG. 8, when the mode switching unit 313 of the third conversion unit 13 receives an instruction to switch to the constant voltage control mode from the mode return instruction unit 213, it returns the power control mode from the reduced power supply mode to the constant voltage control mode.

[0083] That is, the third conversion unit 13 switches from the supply power reduction mode to the constant voltage control mode based on the mode switching instruction output by the second conversion unit 12.

[0084] As described above, in the DC power transmission system 10 of this embodiment, the third conversion unit 13 detects a change in the power supply status of the AC system 30, and performs output restriction in the order of the third conversion unit 13, the second conversion unit 12, and the first conversion unit 11. The DC power transmission system 10 configured in this manner can quickly eliminate fluctuations in the AC system 30 in a transient state. Furthermore, to eliminate fluctuations in the transient state, the DC power transmission system 10 of this embodiment utilizes power absorption functions such as MMC cell capacitors and braking choppers that have conventionally been provided in the third conversion unit 13 and the second conversion unit 12. Therefore, the DC power transmission system 10 of this embodiment can be realized without large capital investment. As described above, according to the DC power transmission system 10 of this embodiment, if a fault occurs in the AC system 30 and the power supply status of the AC system 30 changes, the DC power transmission system 10 can stabilize the power supply status of the AC system 30 in a transient state by behaving in the same way as a conventional synchronous generator. Therefore, the DC power transmission system 10 can be connected to the AC system 30 as a main power source together with or instead of a conventional synchronous generator.

[0085] The topology of the power supply system 1 shown in FIG. 1 is an example, and is not limited to this. Fig. 9 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. 9. 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.

[0086] As a modification of the topology of the power supply system 1, the power supply system 1 may have a configuration in which the above-described first conversion unit 11 is replaced with an AC system operated by a general electricity transmission and distribution company. As another variant, the power supply system 1 may be configured such that an AC system operated by a general electricity transmission and distribution company is interposed between the second conversion unit 12 and the first conversion unit 11 described above.

[0087] 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.

[0088] 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.

[0089] 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]

[0090] 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...energy 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, 211...second voltage control unit, 212...mode determination unit, 213...mode return instruction unit, 220...second conversion circuit, 221...second energy storage unit, 310...third control unit, 311... Third voltage control unit, 312...power supply status determination unit, 313...mode switching unit, 320...third conversion 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, 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 switches from a constant voltage control mode in which a voltage of the DC transmission line is kept constant to a supply power reduction mode in which power supplied to the AC system is reduced, based on a power supply status of the AC system; The second conversion unit performs AC voltage reduction control to reduce the voltage of the AC power converted by the first conversion unit when it is determined that the third conversion unit has been switched to the supply power reduction mode. DC transmission system.

2. when it is determined that the third conversion unit has been switched to the supply power reduction mode, the second conversion unit performs the AC voltage reduction control and outputs a mode switching instruction to the third conversion unit to switch from the supply power reduction mode to the constant voltage control mode; The third conversion unit switches from the supply power reduction mode to the constant voltage control mode based on the mode switching instruction output by the second conversion unit. The DC power transmission system according to claim 1 .

3. The second conversion unit determines that the third conversion unit has been switched to the supply power reduction mode when detecting an increase in the voltage of the DC transmission line. The DC power transmission system according to claim 1 .

4. the third conversion unit transmits a mode switching signal indicating that the second conversion unit has switched to the power supply reduction mode, When the second conversion unit receives the mode switching signal from the third conversion unit, the second conversion unit determines that the third conversion unit has been switched to the reduced power supply mode. The DC power transmission system according to claim 1 .

5. In the reduced power supply mode, the third conversion unit dissipates excess power of the DC transmission line by generating heat using a braking chopper circuit that connects the DC transmission line via a resistor and a switch. The DC power transmission system according to claim 1 .

6. At least one of the second conversion unit and the third conversion unit stores surplus power in a power storage unit that stores power in the reduced power supply mode. The DC power transmission system according to claim 1 .

7. 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 control method for a DC power transmission system comprising: switching, by the third conversion unit, from a constant voltage control mode in which the voltage of the DC transmission line is kept constant to a supply power reduction mode in which the power supplied to the AC system is reduced, based on a power supply status of the AC system; When the second conversion unit determines that the third conversion unit has switched to the supply power reduction mode, the second conversion unit performs AC voltage reduction control to reduce the voltage of the AC power converted by the first conversion unit. A control method comprising:

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