DC power transmission systems, methods, and programs

Abstract

To reduce power loss in a bipolar DC power transmission system. [Solution] The higher-level control device 10 in the DC power transmission system 100 measures the power of the return line 40G, calculates a total return line power value 110C which is the sum of the power of each return line 40G, calculates a correction value to correct the control amount so that the total return line power value 110C approaches zero, and corrects the command value of the control amount with the correction value, thereby obtaining the control amount for the positive-side forward converter 111CP and the negative-side forward converter 111CN.

Description

Technical Field

[0001] The present invention relates to a DC power transmission system, method, and program, and for example, relates to a bipolar DC power transmission system, method, and program.

Background Art

[0002] A bipolar DC power transmission system is used to connect solar power generation facilities, wind power generation facilities, etc. to a commercial power system (see, for example, Patent Document 1).

[0003] In addition, a bipolar DC power transmission system is used as one element that constitutes frequency conversion equipment. Frequency conversion equipment is equipment that connects commercial power systems with different frequencies.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] A bipolar DC power transmission system has a converter for the positive electrode and a converter for the negative electrode at the converter on the power transmission side and the converter on the power reception side, respectively. Also, between the converter on the power transmission side and the converter on the power reception side, they are connected by three DC power transmission lines: the main line for the positive electrode, the main line for the negative electrode, and the return line.

[0006] Generally, the converter at the converter on the power transmission side is controlled to transmit power at a fixed ratio such as 1:1 between the positive electrode and the negative electrode due to the characteristics of control. The converter at the converter on the power reception side is also controlled to receive power at a fixed ratio such as 1:1 between the positive electrode and the negative electrode, similar to the converter at the converter on the power transmission side. Therefore, normally, no current flows through the return line.

[0007] In a bipolar DC power transmission system, multiple receiving-side converters may be connected to a single transmitting-side converter station. In this case, due to accidents or inspections, some converters at the receiving-side converter stations may be shut down while others continue to operate. At this time, the ratio of the number of operating positive-electrode converters to the number of operating negative-electrode converters at the receiving-side converter station (hereinafter also referred to as the "number ratio") changes. This change in the number ratio causes a power difference between the positive-electrode converters and the negative-electrode converters, even in receiving-side converter stations where converters are not shut down. When a power difference occurs between the positive-electrode converters and the negative-electrode converters, current flows in the return line, resulting in significant power loss.

[0008] This invention has been made in view of the above problems, and aims to reduce power loss in a bipolar DC power transmission system. [Means for solving the problem]

[0009] A DC power transmission system according to a typical embodiment of the present invention includes a transmitting-side converter station comprising a positive-side forward converter that converts AC power to positive-electrode DC power according to a given control amount, and a negative-side forward converter that converts AC power to negative-electrode DC power according to a given control amount; a plurality of receiving-side converter stations comprising a positive-side reverse converter that converts DC power transmitted from the positive-side forward converter to AC power, and a negative-side reverse converter that converts DC power transmitted from the negative-side forward converter to AC power; a DC power transmission line that includes a positive-electrode main line, a negative-electrode main line, and a return line, is provided corresponding to each of the receiving-side converter stations, and connects the transmitting-side converter station and the receiving-side converter station; and a control system that controls at least one of the transmitting-side converter station and the receiving-side converter station. The system comprises a higher-level control device, the higher-level control device includes a storage unit, a control amount calculation unit that calculates the control amount of the positive-side forward converter and the negative-side forward converter and provides the control amount to the positive-side forward converter and the negative-side forward converter, a return line power measurement unit that measures the power of the return line of each of the DC transmission lines and stores the measurement result in the storage unit, a return line total power calculation unit that calculates a return line total power value which is the sum of the power of each of the return lines and stores the calculation result in the storage unit, and a correction value calculation unit that calculates a correction value for correcting the control amount so that the return line total power value approaches zero, the control amount calculation unit calculates the control amount by correcting the command value of the control amount with the allocated value. [Effects of the Invention]

[0010] According to the present invention, it is possible to reduce power loss in a bipolar DC power transmission system. [Brief explanation of the drawing]

[0011] [Figure 1] This figure shows a schematic configuration of a power conversion system including a DC power transmission system according to the first embodiment of the present invention. [Figure 2] This figure shows a detailed configuration of the area around the converter shown in Figure 1. [Figure 3] This figure shows the functional block configuration of a higher-level control device according to the first embodiment of the present invention. [Figure 4]This diagram shows the hardware configuration of the higher-level control unit. [Figure 5] This diagram shows the controlled objects of a DC power transmission system. [Figure 6] This diagram shows the control block of a DC power transmission system. [Figure 7] This flowchart shows a method for controlling the distribution of power in a DC power transmission system using a higher-level control device. [Figure 8] This graph shows an example of how the power that a converter that has stopped operating was receiving is distributed to the positive and negative electrode converters that are still in operation. [Figure 9] This graph shows the change in the operating point of an inverter when the distribution of the received power to the inverter is changed by the DC power transmission system. [Figure 10] This figure shows the functional block configuration of a higher-level control device according to a second embodiment of the present invention. [Figure 11] This graph shows the change in droop control characteristics before and after changing the gain of the inverse converter. [Figure 12] This flowchart shows a method for controlling the distribution of transmitted and received power in a DC power transmission system by a higher-level control device according to a second embodiment of the present invention. [Modes for carrying out the invention]

[0012] 1. Overview of the Embodiment First, a general overview of a typical embodiment of the invention disclosed in this application will be provided. In the following description, as an example, the reference numerals in the drawings corresponding to the components in each embodiment are indicated in parentheses.

[0013] [1] A DC power transmission system (100) according to one aspect of the present invention includes a power transmission-side converter station (20) comprising a positive-side forward converter (111CP) that converts AC power (PPS) to positive DC power according to a given control amount, and a negative-side forward converter (111CN) that converts AC power (PNS) to negative DC power according to a given control amount, and a positive-side reverse converter (111IP) that converts the DC power transmitted from the positive-side forward converter (111CP) to AC power (PPR), and the negative-side forward converter A plurality of receiving-side converter stations (30) each comprising a negative-side reverse converter (111IN) that converts DC power transmitted from a converter (111CN) to AC power (PNR), a DC transmission line including a positive main line (40P), a negative main line (40N), and a return line (40G), provided corresponding to each receiving-side converter station (30), and connecting the transmitting-side converter station (20) and the receiving-side converter station (30), and at least one of the transmitting-side converter station (20) and the receiving-side converter station (30) The system includes a higher-level control device (10) that controls the system, the higher-level control device (10) includes a storage unit (11), a control amount calculation unit (12) that calculates the control amount for the positive-side forward converter (111CP) and the negative-side forward converter (111CN) and provides the control amount to the positive-side forward converter (111CP) and the negative-side forward converter (111CN), and measures the power of the return line (40G) among the DC transmission lines (40P, 40N, 40G) and stores the measured result in the storage unit (11). The system includes a return line power measurement unit (13), a return line total power calculation unit (14) that calculates a return line total power value (110C), which is the sum of the power of each return line (40G), and stores the calculated result in the storage unit (11), and a correction value calculation unit (16) that calculates a correction value to correct the control amount so that the return line total power value (110C) approaches zero, and the control amount calculation unit (12) calculates the control amount by correcting the command value of the control amount with the correction value.

[0014] 〔2〕In the DC power transmission system (100) described in 〔1〕 above, the upper control device (10) has a gain change unit (17) that changes the gain. The gain change unit (17) is configured such that for the power receiving side converter station (30) in which either the positive side inverter (111IP) or the negative side inverter (111IN) is in a stopped state and the other is in a droop control state, the gain of the positive side inverter (111IP) or the negative side inverter (111IN) in the droop control state is changed so that it receives power at a constant power, and it is preferable to update the value of the gain stored in the storage unit (11).

[0015] [3] A method for reducing power loss according to one aspect of the present invention is a method for controlling a DC power transmission system (100), the DC power transmission system (100) comprising a transmission-side converter station (20) which includes a positive-side forward converter (111CP) that converts AC power (PPS) to positive DC power according to a given control amount, and a negative-side forward converter (111CN) that converts AC power (PNS) to negative DC power according to a given control amount, and the positive-side forward converter A plurality of receiving-side converter stations (30) each include a positive-side inverse converter (111IP) that converts DC power transmitted from (111CP) to AC power (PPR), and a negative-side inverse converter (111IN) that converts DC power transmitted from the negative-side forward converter (111CN) to AC power (PNR), and a positive terminal main line (40P), a negative terminal main line (40N), and a return line (40G), each of which is provided in correspondence with the receiving-side converter station (30), and the transmitting-side converter station ( 20) and a DC transmission line connecting the power receiving side converter station (30) comprises a control amount calculation step (S12, S13) which calculates the control amount of the positive-side forward converter (111CP) and the negative-side forward converter (111CN) and provides the control amount to the positive-side forward converter (111CP) and the negative-side forward converter (111CN), and a return line power measurement step (S 1) The method includes a return line total power calculation step (S2) for calculating a return line total power value (110C) which is the sum of the power of each return line (40G), and a correction value calculation step (S10) for calculating a correction value to correct the control amount so that the return line total power value (110C) approaches zero, wherein the control amount calculation steps (S12, S13) include a step of calculating the control amount by correcting the command value of the control amount with the correction value.

[0016] 〔4〕A program (1021) according to an aspect of the present invention includes a positive-side forward converter (111CP) that converts alternating-current power (PPS) into direct-current power of the positive electrode according to a given control amount, and a negative-side forward converter (111CN) that converts alternating-current power (PNS) into direct-current power of the negative electrode according to a given control amount, a power transmission-side converter station (20) including the above, a positive-side reverse converter (111IP) that converts the direct-current power transmitted from the positive-side forward converter (111CP) into alternating-current power (PPR), and a negative-side reverse converter (111IN) that converts the direct-current power transmitted from the negative-side forward converter (111CN) into alternating-current power (PNR), a plurality of power reception-side converter stations (30) including the above, a main line (40P) of the positive electrode, a main line (40N) of the negative electrode, and a return line (40G), provided corresponding to each of the power reception-side converter stations (30), a direct-current transmission line connecting the power transmission-side converter station (20) and the power reception-side converter station (30), and a higher-level control device (10) that controls at least one of the power transmission-side converter station (20) and the power reception-side converter station (30). In a direct-current power transmission system (100), it is a program (1021) for causing the higher-level control device (10) to execute, including a control amount calculation step (S12, S13) for calculating the control amounts of the positive-side forward converter (111CP), the negative-side forward converter (111CN), the positive-side reverse converter (111IP), and the negative-side reverse converter (111IN), and giving the control amounts to the positive-side forward converter (111CP) and the negative-side forward converter (111CN), a return line power measurement step (S1) for measuring the power of the return line (40G) among each of the direct-current transmission lines (40P, 40N, 40G), a return line power total value calculation step (S2) for calculating a return line power total value (110C) that is the total of the powers of each of the return lines (40G), and a correction value calculation step (S10) for calculating a correction value for correcting the control amount so that the return line power total value (110C) approaches zero. The control amount calculation step (S12, S13) includes a step of calculating the control amount by correcting the command value of the control amount with the correction value.

[0017] 2. Specific examples of embodiments Specific examples of embodiments of the present invention will be described below with reference to the drawings. In the following description, common components in each embodiment will be denoted by the same reference numerals, and repeated explanations will be omitted. It should also be noted that the drawings are schematic, and the dimensional relationships and ratios of each element may differ from reality. There may also be parts where the dimensional relationships and ratios differ between drawings.

[0018] <<First Embodiment>> <<Outline Configuration of the Power Conversion System>> Figure 1 is a diagram showing a schematic configuration of a power conversion system including a DC power transmission system according to the first embodiment of the present invention.

[0019] The power conversion system 500 shown in Figure 1 comprises a transmitting AC system 200, a receiving AC system 300, and a DC transmission system 100. The transmitting AC system 200 is connected to the DC transmission system 100. The receiving AC system 300 is connected to the DC transmission system 100. The power conversion system 500 is a system in which the AC power transmitted by the transmitting AC system 200 is converted into DC power by the DC transmission system 100, the DC power converted by the DC transmission system 100 is converted back into AC power, and then the AC power is transmitted to the receiving AC system 300.

[0020] The transmission-side AC system 200 is an AC system that inputs AC power to the DC transmission system 100. The transmission-side AC system 200 is, for example, a power generation facility that does not have a synchronizing force, such as a solar power generation facility or a wind power generation facility, or a commercial power system with a constant frequency. In this embodiment, the transmission-side AC system 200 includes a wind turbine generator 201 and a transformer 202. The receiving AC power system 300 is an AC power system that receives AC power output from the DC power transmission system 100. The receiving AC system 300 is, for example, a commercial power system, and in particular a commercial power system having a different frequency from the commercial power system connected to the transmitting AC system 200.

[0021] The DC power transmission system 100 is a system that converts AC power transmitted from the transmitting AC system 200 into DC power, converts the converted DC power back into AC power, and transmits the converted AC power to the receiving AC system 300.

[0022] For example, if the transmitting AC system 200 is a power generation facility that does not have synchronizing power, such as a solar power generation facility or a wind power generation facility, and the receiving AC system 300 is a commercial power system, the DC transmission system 100 functions as a power converter.

[0023] Furthermore, for example, if the transmitting AC system 200 is a commercial power system with a constant frequency, and the receiving AC system 300 is a commercial power system with a different frequency from the commercial power system connected to the transmitting AC system 200, the DC transmission system 100 functions as a frequency conversion device. Note that the transmitting AC system 200 and the receiving AC system 300 are not limited to the examples described above.

[0024] Specifically, the DC power transmission system 100 comprises a higher-level control device 10, a power transmission converter station 20, a power receiving converter station 30, and a DC power transmission line 40.

[0025] The higher-level control device 10 is an information processing device that monitors and controls the operating status and transmitted power of the power transmission station 20, as well as the operating status and received power of the power receiving station 30. Further details will be described later.

[0026] The transmission-side converter station 20 is equipment that converts AC power input from the transmission-side AC system 200 into DC power. The transmission-side converter station 20 is connected to the receiving-side converter station 30 via the DC transmission line 40. In Figure 1, the DC transmission system 100 has one transmission-side converter station 20, but is not limited to this. That is, the DC transmission system 100 may have one or more transmission-side converter stations 20.

[0027] The power transmission side converter station 20 includes a forward converter 111C, a converter station protection control panel 112C, and a converter control panel 113C. Note that the forward converter 111C and the inverse converter 111I, which will be described later, are sometimes collectively referred to as converter 111. Figure 2 shows a detailed configuration of the area around the converter in Figure 1. As shown in Figure 2, the forward converter 111C is a semiconductor power converter that converts AC power input from the transmission AC system 200 to DC power according to a control amount given by the higher-level control device 10. The forward converter 111C comprises a positive forward converter 111CP and a negative forward converter 111CN. The positive-side forward converter 111CP is a semiconductor power converter that converts AC power input from the transmission AC system 200 to positive DC power according to a control amount provided by the higher-level control device 10. The negative-side forward converter 111CN is a semiconductor power converter that converts AC power input from the transmission AC system 200 to negative DC power according to a control amount provided by the higher-level control device 10.

[0028] The converter station protection control panel 112C is a device that constitutes the converter control device 120C, and controls the operating status of the forward converter 111C and the power transmission in accordance with commands input from the higher-level control device 10. The converter station protection control panel 112C comprises a converter station control panel and a converter station protection panel. The converter station protection control panel 112C outputs signals (hereinafter also referred to as "control signals") to the converter control panel 113C for controlling the operating status of the forward converter 111C and the power transmission in response to commands input from the higher-level control device 10 via the converter station control panel. In addition, the converter station protection control panel 112C outputs signals (hereinafter also referred to as "monitoring signals") to the higher-level control device 10 via the converter station control panel 113C that transmit the operating status of the forward converter 111C and the status of the power transmission.

[0029] The converter station protection panel outputs protection detection device operation information, which is input from a protection detection device (not shown), to the converter station control panel and the converter control panel 113C. The converter control panel 113C is a component of the converter control device 120C and controls the operating state and power transmission of the forward converter 111C in accordance with control signals input from the converter station control panel. The converter control panel 113C also outputs monitoring signals to the converter station control panel. Furthermore, the converter control panel 113C controls the operating state and power transmission of the forward converter 111C in accordance with protection detection device operation information input from the converter station protection panel.

[0030] The receiving-side converter station 30 is equipment that converts the DC power transmitted from the transmitting-side converter station 20 into AC power, and then transmits the AC power to the receiving-side AC system 300. The receiving-side converter station 30 is connected to the transmitting-side converter station 20 via a DC transmission line 40. Furthermore, the receiving-side converter station 30 is connected to other receiving-side converter stations 30 via a connecting line 50. In Figure 1, the DC power transmission system 100 has two receiving-side converter stations 30, but it is not limited to this. That is, the DC power transmission system 100 may have one or more receiving-side converter stations 30. The power transmission converter station 20 and the power receiving converter station 30 may be installed together at one location, or they may be installed at different locations.

[0031] The power receiving side converter station 30 includes an inverse converter 111I, a converter station protection control panel 112I, and a converter control panel 113I.

[0032] The inverse converter 111I is a semiconductor power converter that converts the DC power transmitted from the forward converter 111C into AC power and then transmits the AC power to the receiving AC system 300. The inverse converter 111I comprises a positive inverse converter 111IP and a negative inverse converter 111IN. The positive-side inverse converter 111IP is a semiconductor power converter that converts the DC power transmitted from the positive-side forward converter 111CP into positive-side AC power and then transmits the AC power to the receiving-side AC system 300. The negative-side inverse converter 111IN is a semiconductor power converter that converts the DC power transmitted from the negative-side forward converter 111CN into negative-side AC power and then transmits the AC power to the receiving-side AC system 300.

[0033] The converter station protection control panel 112I is a device that constitutes the converter control device 120I, and controls the operating status of the inverse converter 111I and the power transmission in accordance with commands input from the higher-level control device 10. The converter station protection control panel 112I comprises a converter station control panel and a converter station protection panel. The converter station protection control panel 112I outputs signals (hereinafter also referred to as "control signals") to the converter control panel 113I for controlling the operating status of the inverse converter 111I and the power transmission in response to commands input from the higher-level control device 10 via the converter station control panel. In addition, the converter station protection control panel 112I outputs signals (hereinafter also referred to as "monitoring signals") to the higher-level control device 10 via the converter station control panel 113I that transmit the operating status of the inverse converter 111I and the status of the power transmission.

[0034] The converter station protection panel outputs protection detection device operation information, which is input from a protection detection device (not shown), to the converter station control panel and the converter control panel 113I. The converter control panel 113I is a component of the converter control device 120I and controls the operating status and power transmission of the inverse converter 111I in accordance with control signals input from the converter station control panel. The converter control panel 113I also outputs monitoring signals to the converter station control panel. Furthermore, the converter control panel 113I controls the operating status and power transmission of the inverse converter 111I in accordance with protection detection device operation information input from the converter station protection panel.

[0035] The DC transmission line 40 is a transmission line that transmits DC power generated by the forward converter 111C to the reverse converter 111I. The DC transmission line 40 connects the power transmission station 20 on the transmitting side and the power reception station 30 on the receiving side. The DC transmission line 40 is, for example, an OF cable (Oil-Filled Cable) or a CV cable (Cross-Linked Polyethylene Insulated Vinyl Sheath Cable). The DC transmission line 40 can be installed in any location, such as underground or underwater. The DC transmission line 40 connects the transmitting converter station 20 and the receiving converter station 30. The DC transmission line 40 is provided in correspondence to each of the multiple receiving converter stations 30 connected to the transmitting converter station 20. The DC transmission line 40 comprises a positive main line 40P, a negative main line 40N, and a return line 40G.

[0036] The positive terminal main line 40P is a transmission line that transmits the DC power generated by the positive forward converter 111CP to the positive reverse converter 111IP. The negative terminal main line 40N is a transmission line that transmits the DC power generated by the negative forward converter 111CN to the negative reverse converter 111IN. The return line 40G is a transmission line that transmits the power that flows when there is a difference between the power transmitted between the positive forward converter 111CP and the positive reverse converter 111IP and the power transmitted between the negative forward converter 111CN and the negative reverse converter 111IN (hereinafter also referred to as "return line power") between the forward converter 111C and the reverse converter 111I.

[0037] Furthermore, the power transmission converter station 20 may also be equipped with a protection detection device and a DC circuit breaker in addition to the above-described configuration. For example, the protection detection device has a protective relay that detects abnormal conditions such as overvoltage and overcurrent, and instructs the DC circuit breaker to open as needed. In addition, the converter station protection control panel 112C outputs information regarding the operation of the protection detection device, which is input from the converter station protection panel, to the higher-level control device 10 via the converter station control panel. For example, the DC circuit breaker performs closing and opening operations based on instructions from the converter control panel 113C. The DC circuit breaker also performs opening operations based on instructions from the protection detection device. The protective detection device and DC circuit breaker may be installed upstream or downstream of the point where the positive main line 40P, the negative main line 40N, and the return line 40G branch off toward the converter station 30 on each receiving side.

[0038] Similarly, the power receiving converter station 30 may also include a protection detection device and a DC circuit breaker in addition to the above-described configuration. The protection detection device has a protective relay that detects abnormal conditions such as overvoltage and overcurrent, and instructs the DC circuit breaker to open as needed. The converter station protection control panel 112I outputs information regarding the operation of the protection detection device, which is input from the converter station protection panel, to the higher-level control device 10 via the converter station control panel. The DC circuit breaker performs closing and opening operations based on instructions from the converter control panel 113I. The DC circuit breaker also performs opening operations based on instructions from the protection detection device.

[0039] <<Configuration of the higher-level control unit>> Figure 3 is a diagram showing the functional block configuration of a higher-level control device according to the first embodiment of the present invention. Figure 4 shows the hardware configuration of the higher-level control unit.

[0040] The higher-level control unit 10 is a program processing unit that performs data processing according to programs stored in a storage device such as a PC (Personal Computer), server, tablet terminal, and smartphone. The higher-level control unit 10 includes hardware resources such as an arithmetic unit 101, a storage device 102, an input device 103, an I / F (Interface) device 104, an output device 105, and a bus 106.

[0041] The arithmetic unit 101 is composed of processors such as a CPU (Central Processing Unit) and a DSP (Digital Signal Processor). The storage device 102 has a storage area for storing programs that cause the arithmetic unit 101 to perform various data processing operations, and data such as parameters and calculation results used in the data processing by the arithmetic unit 101, and is composed of, for example, ROM (Read Only Memory), RAM (Random Access Memory), HDD, and flash memory.

[0042] Here, program 1021 includes a program for causing the computer to function as a higher-level control unit 10. For example, program 1021 is a program for realizing the control (native application) of the DC power transmission system 100 according to this embodiment, and may be downloaded in advance from an external device (e.g., an external storage medium) and stored in the storage device 102 within the higher-level control unit 10, or it may be stored on an external server (including on the internet).

[0043] Furthermore, data 1022 includes data such as various parameters necessary for controlling the DC power transmission system 100 by program 1021, and data such as the control results of the DC power transmission system 100 by the higher-level control device 10. For example, data 1022 includes transmission-side converter information 110A, receiving-side converter information 110B, return line total power value 110C, etc.

[0044] Furthermore, program 1021 and data 1022 may be distributed via a network, or they may be written to a computer-readable storage medium such as a CD-ROM and distributed therein.

[0045] The input device 103 is a functional unit that detects information input from the outside and consists of, for example, a keyboard, mouse, pointing device, buttons, or touch panel. The I / F device 104 is a functional unit that sends and receives information to and from the outside and consists of a communication control circuit, input / output ports, antenna, etc., for wired or wireless communication.

[0046] The output device 105 is a functional unit that outputs information obtained through data processing by the arithmetic unit 101. Examples of output devices 105 include external storage devices such as SSDs (Solid State Drives) and HDDs (Hard Disk Drives), and display devices such as console units. The bus 106 is a functional unit that interconnects the arithmetic unit 101, storage device 102, input device 103, I / F device 104, and output device 105, enabling data exchange between these devices.

[0047] Next, we will describe in detail each functional block of the higher-level control unit 10.

[0048] As shown in Figure 3, the higher-level control device 10 includes a control amount calculation unit 12, a return line power measurement unit 13, a return line total power calculation unit 14, a converter station information monitoring unit 15, a correction value calculation unit 16, a power transmission distribution setting unit 16A, and a storage unit 11 as functional blocks for realizing control of the DC power transmission system 100.

[0049] These functional blocks are realized through the cooperation of the aforementioned hardware resources and software that constitute the higher-level control unit 10. Specifically, in the higher-level control unit 10, the arithmetic unit 101 performs various calculations according to the program 1021 and data 1022 stored in the memory device 102, and controls the memory device 102, input device 103, I / F device 104, output device 105, and bus 106 in the higher-level control unit 10, thereby realizing the above functional blocks in the higher-level control unit 10 (control amount calculation unit 12, return line power measurement unit 13, return line total power calculation unit 14, converter station information monitoring unit 15, correction value calculation unit 16, transmission power distribution setting unit 16A, and memory unit 11). At least one of the above functional blocks may be realized by a dedicated circuit.

[0050] The control variable calculation unit 12 is a functional unit that calculates the control variables for the positive-side forward converter 111CP and the negative-side forward converter 111CN, and provides these control variables to the positive-side forward converter 111CP and the negative-side forward converter 111CN. The control variable calculation unit 12 can also calculate the control variables for the positive inverse converter 111IP and the negative inverse converter 111IN, and provide these control variables to the positive inverse converter 111IP and the negative inverse converter 111IN. Furthermore, the control variable calculation unit 12 calculates the control variable by correcting the command value of the control variable with a correction value calculated by the correction value calculation unit 16, which will be described later.

[0051] As a specific example, when the AC power transmitted from the AC power transmission system 200 is converted to DC power, the control amount calculation unit 12 calculates for each positive-side forward converter 111CP a control amount that is less than or equal to the rated capacity of the positive-side forward converter 111CP and should be transmitted by the positive-side forward converter 111CP (hereinafter also referred to as "positive electrode transmitted power"), and assigns a control amount to each positive-side forward converter 111CP. Similarly, when the AC power transmitted from the AC power system 200 on the transmission side is converted to DC power, the control amount calculation unit 12 calculates for each transmission-side converter station 20 a control amount that is less than or equal to the rated capacity of the negative-side forward converter 111CN and should be transmitted by the negative-side forward converter 111CN (hereinafter also referred to as "negative electrode transmitted power"), and assigns a control amount to each negative-side forward converter 111CN.

[0052] Furthermore, the control amount calculation unit 12 corrects the command value of the control amount (hereinafter also referred to as "pre-correction positive electrode transmission power") using a correction value calculated by the correction value calculation unit 16, which will be described later, and then calculates the control amount of the positive-side forward converter 111CP (hereinafter also referred to as "corrected positive electrode transmission power") and assigns the control amount to each positive-side forward converter 111CP.

[0053] Similarly, the control amount calculation unit 12 corrects the command value of the control amount (hereinafter also referred to as "negative electrode transmission power before correction") using the correction value calculated by the correction value calculation unit 16, which will be described later, and then calculates the control amount of the negative-side forward converter 111CN (hereinafter also referred to as "negative electrode transmission power after correction") and assigns the control amount to each negative-side forward converter 111CN.

[0054] The return line power measurement unit 13 is a functional unit that measures the power of the return line 40G among the DC transmission lines 40P, 40N, and 40G. For example, the return line power measurement unit 13 measures the power passing through the return line 40G for each return line 40G connected to each receiving-side converter station 30. Alternatively, the return line power measurement unit 13 may measure the current flowing through the return line 40G and the voltage to ground of the return line 40G for each return line 40G connected to each receiving-side converter station 30, and then calculate the power for each return line 40G connected to each receiving-side converter station 30. Furthermore, the return line power measurement unit 13 stores the measured power of the return line 40G in the storage unit 11.

[0055] The return line power total calculation unit 14 is a functional unit that calculates the return line power total value 110C, which is the sum of the power of each return line 40G measured by the return line power measurement unit 13. Specifically, the return line power total calculation unit 14 calculates the sum of the power of each return line 40G connected to each receiving-side converter station 30, as measured by the return line power measurement unit 13. For example, in this embodiment, the return line power total value calculation unit 14 calculates the return line power total value 110C, which is the sum of the power of each return line 40G, by subtracting the negative electrode power total value (the sum of the power flowing through the negative electrode main line 40N, located between the negative electrode main line 40N, located between the negative electrode main line 40N, located between the negative electrode main line 40IN, located between the negative electrode main line 40N, located between the negative electrode main line 40IN, from the positive electrode power total value, which is the sum of the power flowing through the positive electrode main line 40P, located between the positive electrode main line 40P, located between the positive electrode main line 40P, located between the positive electrode main line 111CP, located between the positive electrode main line 111CP, located between the positive electrode main line 40P, located between the negative electrode main line 111CN, located between the negative electrode main line 40IN, located between the negative electrode main line 40IN, located between the negative electrode main line 40IN, located between the negative electrode main line 111CN, located between the negative electrode main line 111IN, located between the negative electrode main line 111CN, located between the negative electrode main line 111IN, located between the negative electrode main line 40 Furthermore, the return line power total value calculation unit 14 stores the calculated return line power total value 110C in the storage unit 11.

[0056] The return line power total calculation unit 14 calculates the return line power total value 110C using only the power passing through the return line 40G connected to the receiving side converter station 30 where both the positive-side inverter 111IP and the negative-side inverter 111IN are in a droop control state, based on the operating status monitoring results monitored by the converter station information monitoring unit 15 (described later). In other words, when calculating the total return power value 110C, the power passing through the return line 40G connected to the receiving-side converter station 30 where either the positive-side inverter 111IP or the negative-side inverter 111IN is stopped, as measured by the operating status monitoring results monitored by the converter station information monitoring unit 15 described later, and the power passing through the return line 40G connected to the receiving-side converter station 30 where both the positive-side inverter 111IP and the negative-side inverter 111IN are in a constant power control state, are excluded from the total return power value 110C.

[0057] The converter station information monitoring unit 15 is a functional unit that monitors whether the operating status of the positive-side inverter 111IP and the negative-side inverter 111IN is stopped or powered. Furthermore, the converter information monitoring unit 15 stores the monitoring results of the operating status of the positive-side inverse converter 111IP and the negative-side inverse converter 111IN in the storage unit 11.

[0058] A stopped state refers to a condition in which the inverse converter 111I is unable to convert the positive or negative power transmitted from the forward converter 111C into AC power due to an accident, inspection, or other reasons. The power receiving state refers to the state in which the inverter 111I can convert the positive and negative power transmitted from the forward converter 111C into AC power. The power receiving state includes the constant power control state and the droop control state.

[0059] The constant power control state refers to a state in which the inverse converter 111I always converts the transmitted power transmitted from the power transmission station 20 on the transmission side into a constant AC power (active power), even if a disturbance occurs in the power conversion system 500. The droop control state refers to the state in which the inverter 111I converts the transmitted power transmitted from the power transmission station 20 to AC power while adjusting the positive and negative power received to follow the fluctuations in the power conversion system 500 when fluctuations occur in the power conversion system 500 (hereinafter also referred to as the "droop characteristic"). The positive and negative power received are changed as needed based on the gain value set in the inverter 111I. The gain value set for the inverse converter 111I is stored in the memory unit 11, which will be described later.

[0060] The gain, in particular, is the reciprocal of the slope of the droop control, which is the value that determines the change in DC voltage corresponding to the change in transmitted power. When the positive-side inverter 111IP and the negative-side inverter 111IN are in a constant power control state or a droop control state, they control the positive-electrode power and negative-electrode power themselves according to these states, so there is no need for the control amount calculation unit 12 to calculate the control amount.

[0061] The correction value calculation unit 16 is a functional unit that calculates a correction value to correct the control amount so that the total return line power value 110C approaches zero. The correction value calculation unit 16 includes a power transmission power distribution setting unit 16A.

[0062] The power transmission power distribution setting unit 16A is a functional unit that sets the distribution of power transmitted by the positive-side forward converter 111CP and the negative-side forward converter 111CN according to the monitoring results stored in the memory unit 11. Specifically, the power transmission distribution setting unit 16A calculates a correction value so that the total return power value 110C is distributed to the positive-side inverter 111IP and the negative-side inverter 111IN, which are operating in a droop control state.

[0063] Furthermore, the power transmission distribution setting unit 16A calculates the ratio between the sum of the gains of the positive-side inverter 111IP and the negative-side inverter 111IN, which are operating in a droop-controlled state, and the gains of the positive-side inverter 111IP and the negative-side inverter 111IN, which are operating in a droop-controlled state, based on the gain values ​​stored in the memory unit 11. Based on this ratio, it calculates a correction value so that the total return power 110C is distributed to the positive-side inverter 111IP and the negative-side inverter 111IN, which are operating in a droop-controlled state, so that the total return power 110C approaches the reference value. Details of the method for calculating the correction value will be described later.

[0064] The reference value for the total return power of 110C is determined by information transmitted from an information processing device (not shown) connected via a network (not shown), or by information prepared in advance by the system administrator, etc. The baseline value for the total return power of 110C is, for example, zero. The baseline value for the total return power of 110C may be a value other than zero.

[0065] The memory unit 11 is a functional unit for storing various data such as parameters necessary for controlling the DC power transmission system 100 and the control results of the DC power transmission system 100 by the higher-level control device 10. For example, the memory unit 11 stores the transmission-side converter station information 110A, the receiving-side converter station information 110B, and the total return line power value 110C, as described above.

[0066] The transmission-side converter station information 110A includes information about the forward converter 111C that has been stored in the memory unit 11 in advance, and information included in the monitoring signal input from the converter control device 120C. For example, the transmission-side converter station information 110A includes, but is not limited to, information about the rated capacity and operating status of the forward converter 111C, as well as information about the positive electrode transmission power and negative electrode transmission power.

[0067] The power transmission side converter station information 110A stored in the memory unit 11 may include information transmitted from an information processing device (not shown) connected via a network (not shown), or information prepared in advance by a system administrator or the like.

[0068] The receiving-side converter station information 110B includes information about the inverse converter 111I that has been stored in the memory unit 11 in advance, and information included in the monitoring signal input from the converter control device 120I. For example, the receiving-side converter station information 110B includes, but is not limited to, information about the rated capacity and operating status of the inverse converter 111I, as well as the positive electrode power received and the negative electrode power received. The operating state of the inverse converter 111I includes the droop control gain values ​​set for the positive inverse converter 111IP and the negative inverse converter 111IN, whose operating state is in droop control mode.

[0069] The power receiving converter information 110B stored in the memory unit 11 may include information transmitted from an information processing device (not shown) connected via a network (not shown), or information prepared in advance by a system administrator or the like.

[0070] The total return line power value 110C includes the power values ​​for each return line 40G connected to each receiving-side converter station 30, as measured by the return line power measurement unit 13, and the total return line power value calculation unit 14, which calculates the sum of the power of each return line 40G.

[0071] <<Method for calculating correction values, method for calculating control variables>> Figure 5 shows the controlled objects of a DC power transmission system. Figure 6 shows the control block of a DC power transmission system. Figure 7 is a flowchart showing a method for controlling the distribution of power in a DC power transmission system by a higher-level control device. Figure 8 is a graph showing an example of how the power that a converter that has stopped operating was receiving up until that moment is distributed to the positive and negative electrode converters that are still in operation. Figure 9 is a graph showing the change in the operating point of the inverter when the distribution of the received power to the inverter is changed by the DC power transmission system.

[0072] Next, we will describe in detail the method by which the correction value calculation unit 16 calculates the correction value, and the method by which the control amount calculation unit 12 corrects the command value of the control amount using the correction value calculated by the correction value calculation unit 16, calculates the control amount for each forward converter 111C, and assigns the control amount to each forward converter 111C.

[0073] First, we will describe the configuration of the power conversion system 500 and the controlled objects of the DC power transmission system 100 in Figure 5. The power conversion system 500 in Figure 5 differs from the power conversion system 500 in Figure 1 in that it comprises one transmitting AC system 200, four receiving AC systems 300, and a DC transmission system 100. The DC transmission system 100 comprises one transmitting converter station 20, four receiving converter stations 30, four positive main lines 40P, four negative main lines 40N, and four return lines 40G. Specifically, the power conversion system 500 in Figure 5 comprises one transmitting AC line 200, one positive forward converter 111CP, one negative forward converter 111CN, four receiving AC lines 300A, 300B, 300C, and 300D, four positive reverse converters 111IPA, 111IPB, 111IPC, and 111IPD, four negative reverse converters 111INA, 111INB, 111INC, and 111IND, four positive main lines 40PA, 40PB, 40PC, and 40PD, four negative main lines 40NA, 40NB, 40NC, and 40ND, and four return lines 40GA, 40GB, 40GC, and 40GD.

[0074] In this description, the DC power transmission system 100 controls the following: the positive power transmission power PPS and negative power transmission power PNS transmitted by the transmitting AC system 200; the positive power reception power PPRA and negative power reception power PNRA received by the receiving AC system 300A; the positive power reception power PPRB and negative power reception power PNRB received by the receiving AC system 300B; the positive power reception power PPRC and negative power reception power PNRC received by the receiving AC system 300C; and the positive power reception power PPRD and negative power reception power PNRD received by the receiving AC system 300D. The sum of the positive electrode transmission power PPS and the negative electrode transmission power PNS is the transmission power P.

[0075] Next, the specific rated capacities and operating conditions of the forward converter 111C and the reverse converter 111I will be described. The rated capacities of the forward converters 111C are 1700 MW for the positive forward converter 111CP and 1700 MW for the negative forward converter 111CN. Furthermore, the rated capacities of the inverse converters 111I are as follows: positive inverse converters 111IPA and 111IPB have a capacity of 250 MW, positive inverse converter 111IPC has a capacity of 600 MW, positive inverse converter 111IPD has a capacity of 1000 MW, negative inverse converters 111INA and 111INB have a capacity of 250 MW, negative inverse converter 111INC has a capacity of 600 MW, and negative inverse converter 111IND has a capacity of 1000 MW.

[0076] The operating state of the inverse converters 111I is such that the positive inverse converter 111IPA and the negative inverse converter 111INA are in a constant power control state, while the positive inverse converters 111IPB, 111IPC, and 111IPD, and the negative inverse converters 111INB, 111INC, and 111IND are in a droop control state.

[0077] The gains set for the positive-side inverse converter 111IP are as follows: the gain KdPB set for the positive-side inverse converter 111IPB is 1, the gain KdPC set for the positive-side inverse converter 111IPC is 2, and the gain KdPD set for the positive-side inverse converter 111IPD is 3. Furthermore, the gains set for the negative inverse converter 111IN are as follows: the gain KdNB set for the negative inverse converter 111INB is 1, the gain KdNC set for the negative inverse converter 111INC is 2, and the gain KdND set for the negative inverse converter 111IND is 3.

[0078] Next, we will describe the specific positive electrode transmission power, negative electrode transmission power, positive electrode reception power, and negative electrode reception power of the forward converter 111C and the reverse converter 111I. The positive terminal transmission power of the positive-side forward converter 111CP is 1500 MW, given that the positive terminal transmission power PPS transmitted by the transmission-side AC system 200 is 1500 MW, and the rated capacity of the positive-side forward converter 111CP is 1700 MW. The negative terminal power transmitted by the negative forward converter 111CN is 1500 MW, given that the negative terminal power PNS transmitted by the transmission AC system 200 is 1500 MW, and the rated capacity of the negative forward converter 111CN is 1700 MW.

[0079] The positive terminal power received by the positive-side inverter 111IPA is 250 MW, because the positive terminal power received by the receiving AC system 300 A is 250 MW, and the rated capacity of the positive-side inverter 111IPA is 250 MW. Furthermore, the positive terminal power received by the positive-side inverter 111IPB is 200 MW, given that the positive terminal power received by the receiving AC system 300B is 200 MW, and the rated capacity of the positive-side inverter 111IPB is 250 MW. Furthermore, the positive terminal power received by the positive-side inverter 111IPC is 300MW, given that the positive terminal power received by the receiving AC system 300C is 300MW, and the rated capacity of the positive-side inverter 111IPC is 600MW. Furthermore, the positive terminal power received by the positive-side inverter 111IPD is 750 MW, given that the positive terminal power received by the receiving AC system 300D is 750 MW, and the rated capacity of the positive-side inverter 111IPD is 1000 MW.

[0080] The negative terminal power received by the negative inverter 111INA is 250 MW, since the negative terminal power PNRA received by the receiving AC system 300 A is 250 MW, and the rated capacity of the negative inverter 111INA is 250 MW. Furthermore, the negative terminal power received by the negative inverter 111INB is 200 MW, given that the negative terminal power received by the receiving AC system 300B is 200 MW, and the rated capacity of the negative inverter 111INB is 250 MW. Furthermore, the negative electrode power received by the negative inverter 111INC is 300 MW, given that the negative electrode power received by the receiving AC system 300C is 300 MW, and the rated capacity of the negative inverter 111INC is 600 MW. Furthermore, the negative electrode power received by the negative inverter 111IND is 750 MW, given that the negative electrode power received by the receiving AC system 300D is 750 MW, and the rated capacity of the negative inverter 111IND is 1000 MW.

[0081] Next, we will describe the control block 400 of the DC power transmission system shown in Figure 6. In the control block 400 of the DC power transmission system 100 shown in Figure 6, a positive value of the total return power value 110C is input to the correction value calculation unit 16. The correction value calculation unit 16 includes an initial input value 161 and a PI controller 162. The initial input value 161 has a value of zero. The PI controller 162 receives a value obtained by adding the negative value of the initial input value 161 and the total return power value 110C. The PI controller 162 controls the system so that the difference between the initial input value 161 and the total return power value 110C approaches zero. The total return power value 110C is zero when all inverters 111I are in a powered state. Therefore, when all inverters 111I are in a powered state, the PI controller 162 does not perform any special control.

[0082] However, if some of the inverters 111I stop due to an accident or inspection, return power will flow through at least one of the return lines 40GA, 40GB, 40GC, or 40GD, resulting in a positive or negative return power value 110C. At this time, the PI controller 162 performs calculations to bring the total return power value 110C closer to zero.

[0083] The control variable calculation unit 12 distributes the correction values ​​calculated by the PI controller 162 to the positive forward converter 111CP and the negative forward converter 111CN, and calculates the control variable based on the distributed correction values. Specifically, the control variable calculation unit 12 calculates the corrected positive electrode transmission power 402P as the control variable by adding the value obtained by inverting the sign of the correction value calculated by the PI controller 162 to the uncorrected positive electrode transmission power 401P via the positive / negative inversion block 121, and then provides this corrected positive electrode transmission power 402P to the positive-side forward converter 111CP. Furthermore, the control variable calculation unit 12 provides the corrected negative electrode transmission power 402N, which is calculated by adding the correction value calculated by the PI controller 162 to the uncorrected negative electrode transmission power 401N, to the negative side forward converter 111CN as the control variable.

[0084] Next, we will explain the operation of each functional unit when, for example, the positive-side inverter 111IPB changes from a powered state (droop control state) to a stopped state due to a fault in the operating state described above.

[0085] If the positive-side inverter 111IPB changes from a power-receiving state (droop-controlled state) to a stopped state due to a fault, a change occurs in the destination of the positive terminal power PPRB that the power-receiving AC system 300B had been receiving. In other words, the 200MW positive terminal power PPRB that the positive-side inverter 111IPB had been receiving (converting the transmitted power transmitted from the forward converter 111C into AC power) will now be received by the positive-side inverters 111IPC and 111IPD, which are still in a droop-controlled state.

[0086] At this time, the magnitude of the positive terminal power PPRC received by the receiving AC system 300C and the positive terminal power PPRD received by the receiving AC system 300D become greater than before the positive-side inverter 111IPB changed to the stopped state. Therefore, a difference arises between the positive terminal power PPRC received by the receiving AC system 300C and the negative terminal power PNRC received by the receiving AC system 300C, causing return power to flow into the return line 40GC. At the same time, a difference arises between the positive terminal power PPRD received by the receiving AC system 300D and the negative terminal power PNRD received by the receiving AC system 300D, causing return power to flow into the return line 40GD.

[0087] Here, if the return power flowing through the return lines 40GC and 40GD is set to 0, it becomes possible to prevent power loss due to return power. Therefore, the higher-level control device 10 controls the forward converter 111C so that the return power flowing through the return lines 40GC and 40GD is set to 0. The processing flow by the higher-level control device 10 will be explained below with reference to Figure 7. Figure 7 is a flowchart showing the processing flow by the higher-level control device 10.

[0088] First, the return line power measurement unit 13 measures the power passing through the return lines 40GC and 40GD (step S1). Next, the return line power total calculation unit 14 calculates the return line total power value 110C, which is the sum of the power passing through the return lines 40GC and 40GD measured by the return line power measurement unit 13 (step S2). The return line total power value 110C is 200 MW, which is the same as the positive terminal power PPRB that the positive side inverter 111IPB was receiving.

[0089] If the total return power value 110C is zero (step S3: YES), the correction value calculation unit 16 does not calculate a correction value. In this case, the control amount calculation unit 12 calculates the control amount without correcting the command values ​​of the control amounts for the positive forward converter 111CP and the negative forward converter 111CN (step S11). The control amount calculation unit 12 provides the control amount calculated in step S11 to the negative inverse converter (step S12). That is, the control amount calculation unit 12 calculates the same control amounts for the positive forward converter 111CP and the negative forward converter 111CN as before, and provides the same control amounts to the positive forward converter 111CP and the negative forward converter 111CN as before.

[0090] However, in this case, since the total return power value of 110C is not zero (step S3: NO), the correction value is calculated according to the contents of steps S4 to S10.

[0091] Based on the monitoring results of the converter information monitoring unit 15, the correction value calculation unit 16 extracts the inverse converter 111I, which is in a droop control state, from among the positive inverse converters 111IPA, 111IPB, 111IPC, 111IPD and the negative inverse converters 111INA, 111INB, 111INC, 111IND (step S4).

[0092] The power transmission distribution setting unit 16A refers to the gain values ​​KdPC, KdPD, KdNB, KdNC, and KdND set for the inverse converter 111I, which is in droop control state (step S5).

[0093] Here, the power transmission distribution setting unit 16A checks whether the gain KdPC of the positive inverse converter 111IPC and the gain KdNC of the negative inverse converter 111INC are equal, assuming that both the positive inverse converter 111IP and the negative inverse converter 111IN are in a droop control state (step S6). The power transmission distribution setting unit 16A also checks whether the gain KdPD of the positive inverse converter 111IPD and the gain KdND of the negative inverse converter 111IND are equal, assuming that both the positive inverse converter 111IP and the negative inverse converter 111IN are in a droop control state (step S6).

[0094] If either the gain KdPC of the positive inverse converter 111IPC and the gain KdNC of the negative inverse converter 111INC are not equal, or the gain KdPD of the positive inverse converter 111IPD and the gain KdND of the negative inverse converter 111IND are not equal (Step S6: NO), then the power transmission distribution setting unit 16A adjusts the gains KdPC and KdNC so that the gains KdPC of the positive inverse converter 111IPC and KdNC of the negative inverse converter 111INC match (Step S7). Also, the power transmission distribution setting unit 16A adjusts the gains KdPD and KdND so that the gains KdPD of the positive inverse converter 111IPD and KdND of the negative inverse converter 111IND match (Step S7).

[0095] In this case, the gain KdPC of the positive inverse converter 111IPC and the gain KdNC of the negative inverse converter 111INC are equal to 2, and the gain KdPD of the positive inverse converter 111IPD and the gain KdND of the negative inverse converter 111IND are equal to 3 (Step S6: YES). Therefore, the power transmission distribution setting unit 16A proceeds to the next step without adjusting the gains KdPC, KdNC, KdPD, and KdND.

[0096] Next, the correction value calculation unit 16 calculates a correction value to correct the controlled amount so that the total return power value 110C approaches zero (step S10). Specifically, the correction value calculation unit 16 detects the difference between the initial input value 161 and the total return power value 110C, and calculates a correction value such that the difference between the initial input value 161 and the total return power value 110C approaches zero.

[0097] Next, the control variable calculation unit 12 calculates the control variables for the positive-side forward converter 111CP and the negative-side forward converter 111CN based on the correction value calculated by the correction value calculation unit 16 (step S11). Specifically, the control amount calculation unit 12 allocates the correction value calculated by the correction value calculation unit 16 to the pre-correction positive electrode transmission power 401P and the pre-correction negative electrode transmission power 401N.

[0098] More specifically, the control amount calculation unit 12 adds the value obtained by inverting the sign of the correction value calculated by the correction value calculation unit 16 (PI controller 162) to the uncorrected positive electrode transmission power 401P via the positive / negative inversion block 121, thereby calculating the corrected positive electrode transmission power 402P. Furthermore, the control amount calculation unit 12 adds the correction value calculated by the PI controller 162 to the uncorrected negative electrode transmission power 401N to calculate the corrected negative electrode transmission power 402N.

[0099] Next, the control variable calculation unit 12 provides the calculated control variable to the positive-side forward converter 111CP and the negative-side forward converter 111CN (step S12).

[0100] Specifically, the control quantity calculation unit 12 provides the calculated corrected positive electrode transmission power 402P as the control quantity to the positive side forward converter 111CP. Furthermore, the control variable calculation unit 12 provides the calculated corrected negative electrode transmission power 402N as a control variable to the negative side forward converter 111CN.

[0101] Through the above steps, the control amount calculation unit 12 provides control amounts based on the corrected positive electrode transmission power 402P and the corrected negative electrode transmission power 402N to the inverse converter 111I. As a result, the total return power value 110C is distributed to the positive side inverse converters 111IPC, 111IPD, 111INB, 111INC, and 111IND, which are in a droop control state, based on the ratio of gains KdPC, KdPD, KdNB, KdNC, and KdND (step S13).

[0102] Specifically, the total return power 110C to be distributed to each inverter 111I is distributed based on a ratio obtained by dividing the gains KdPC, KdPD, KdNB, KdNC, and KdND set for each inverter 111I by the sum of the gains KdPC, KdPD, KdNB, KdNC, and KdND of each inverter 111I.

[0103] More specifically, the quotient obtained by dividing the total return power value 110C, which is 200MW, by the total gain value of the return power value 110C, which is 11, is 18.18 (≒200 / 11)MW. This quotient is then multiplied by the gain KdPC set for the positive inverse converter 111IPC, which is 2, resulting in a product of 36.36MW, which is then allocated to the positive inverse converter 111IPC.

[0104] Similarly, the quotient obtained by dividing the total return power value 110C, which is 200MW, by the total gain of the total return power value 110C, which is 11, is 18.18 (≒200 / 11)MW. This quotient is then multiplied by the gain KdPD, which is 3, set for the positive inverse converter 111IPD, resulting in a product of 54.54MW, which is then allocated to the positive inverse converter 111IPD.

[0105] Similarly, the quotient obtained by dividing the total return power value 110C, which is 200MW, by the total gain of the total return power value 110C, which is 11, is 18.18 (≒200 / 11)MW. This quotient is then multiplied by the gain KdNB, which is 1, set for the negative inverse converter 111INB, resulting in a product of 18.18MW, which is then allocated to the negative inverse converter 111INB.

[0106] Similarly, the quotient obtained by dividing the total return power value 110C (200MW) by the total gain of the total return power value 110C, which is 18.18 (≒200 / 11)MW, is multiplied by the gain KdNC of 2 set for the negative inverter 111INC, resulting in a product of 36.36MW which is allocated to the negative inverter 111INC.

[0107] Similarly, the quotient obtained by dividing the total return power value 110C (200MW) by the total gain of the total return power value 110C, which is 18.18 (≒200 / 11)MW, is multiplied by the gain KdND of 3 set for the negative inverse converter 111IND, resulting in a product of 54.54MW, which is then allocated to the negative inverse converter 111IND.

[0108] As a result, the positive electrode power PPRC of the positive-side inverse converter 111IPC is 336.36 MW. Similarly, the positive terminal power PPRD of the positive-side inverse converter 111IPD is 804.54 MW. Similarly, the power received at the negative terminal of the negative inverse converter 111INB, PNRB, is 218.18 MW. Similarly, the power received at the negative terminal of the negative inverse converter 111INC, PNRC, is 336.36 MW. Similarly, the negative electrode power PNRD, which is the controlled variable of the negative inverse converter 111IND, is 804.54 MW.

[0109] Following the steps in the flowchart above, for each receiving-side converter station 30 where both the positive-side inverse converter 111IP and the negative-side inverse converter 111IN are in a droop-controlled state, the total return power 110C can be distributed to the forward converter 111C such that the power received by the positive-side inverse converter 111IP and the power received by the negative-side inverse converter 111IN are equal, and the total return power 110C approaches zero.

[0110] Next, we will explain the process by which the total return power value of 110C becomes zero, using the graph shown in Figure 8. In Figure 8, the horizontal axis represents time (s), and the vertical axis represents the change (absolute value) (MW) of the positive terminal transmission power PPS and the negative terminal transmission power PNS from the reference value.

[0111] Curve 601A in Figure 8 shows the change in the total return power value 110C from time 0 to time t. Curve 601B in Figure 8 also shows the change in the amount of positive electrode transmission power PPS and negative electrode transmission power PNS from time 0 to time t, starting from the state immediately after the positive side inverter 111IPB changed from the power receiving state (droop control state) to the stopped state due to a fault.

[0112] The standard values ​​for positive electrode power transmission power PPS and negative electrode power transmission power PNS shall both be 1500 MW.

[0113] At time t0, immediately after the positive-side inverter 111IPB changes from a powered state (droop-controlled state) to a stopped state due to an accident, it has not yet been achieved that the total return power value 110C can be distributed to the inverter 111I, which is in a droop-controlled state. At this time, the positive terminal's transmission power PPS is 1300 MW, which is the sum of the positive terminal's receiving power PPRA of 250 MW, the positive terminal's receiving power PPRC of 300 MW, and the positive terminal's receiving power PPRD of 750 MW. Furthermore, the negative terminal transmission power PNS is 1500 MW, which is the sum of the negative terminal reception power PNRA (250 MW), the negative terminal reception power PNRB (200 MW), the negative terminal reception power PNRC (300 MW), and the negative terminal reception power PNRD (750 MW).

[0114] In other words, at time t0, curve 601A is 200 MW. Similarly, curve 601B is 0 MW.

[0115] Following the steps in the flowchart described above, when the total return power 110C is distributed to the inverse converter 111I, which is in droop control state, at time t1, the positive terminal transmission power PPS will be the sum of the positive terminal received power PPRA received by the receiving AC system 300A, the positive terminal received power PPRC received by the receiving AC system 300C, and the positive terminal received power PPRD received by the receiving AC system 300D. Similarly, at time t1, the negative terminal transmission power PNS is the sum of the negative terminal received power PNRA received by the receiving AC system 300A, the negative terminal received power PNRB received by the receiving AC system 300B, the negative terminal received power PNRC received by the receiving AC system 300C, and the negative terminal received power PNRD received by the receiving AC system 300D.

[0116] At this time, since the positive-side inverter 111IPA and the negative-side inverter 111INA continue to maintain a constant power control state, the positive terminal power PPRA and the negative terminal power PNRA received by the receiving AC system 300A continue to be 250MW each. Therefore, the positive terminal's transmitted power PPS at time t1 is 1390.9 MW, which is the sum of the positive terminal's received power PPRA of 250 MW, the positive terminal's received power PPRC of 336.36 MW, and the positive terminal's received power PPRD of 804.54 MW. Furthermore, the negative terminal transmission power PNS at time t1 is 1609.1 MW, which is the sum of the negative terminal reception power PNRA (250 MW), the negative terminal reception power PNRB (218.18 MW), the positive terminal reception power PPRC (336.36 MW), and the positive terminal reception power PPRD (804.54 MW).

[0117] At time t1, the positive terminal transmission power PPS is 1390.9MW, which is a change of 109.1MW in the negative direction from the reference value of 1500MW. Also, at time t1, the negative terminal transmission power PNS is 1609.1MW, which is a change of 109.1MW in the positive direction from the reference value of 1500MW. As a result, as shown in Figure 8, curve 601A is 0 MW at time t1. Similarly, curve 601B is 109.1 MW at time t1.

[0118] In other words, the total return power value 110C is distributed by the higher-level control device 10 to the positive-side forward converter 111CP and the negative-side forward converter 111CN by applying a control amount corrected by the correction value calculation unit 16, so that for each receiving-side converter station 30 where both the positive-side reverse converter 111IP and the negative-side reverse converter 111IN are in a droop control state, the power received by the positive-side reverse converter 111IP and the power received by the negative-side reverse converter 111IN are equal, and the total return power value 110C is zero.

[0119] Next, we will explain the change in the operating point of the inverter 111I when the distribution of the received power to the inverter 111I is changed by the DC power transmission system 100, using the graph shown in Figure 9.

[0120] Figure 9(a) is a graph showing the operating points of the positive-side inverters 111IPB, 111IPC, and 111IPD before the positive-side inverter 111IPB changed from a powered state (droop-controlled state) to a stopped state due to a fault. Figure 9(b) is a graph showing the state after the operating points of the positive inverse converters 111IPB, 111IPC, and 111IPD have been changed by the control variable calculation unit 12. Figure 9(c) is a graph showing the operating points of the negative inverters 111INB, 111INC, and 111IND before the positive inverter 111IPB changes from the powered state (droop control state) to the stopped state due to a fault. Figure 9(d) is a graph showing the state after the operating points of the negative inverse converters 111INB, 111INC, and 111IND have been changed by the control variable calculation unit 12. In Figures 9(a) to 9(d), the horizontal axis represents the ratio (pu) of the magnitude of the received power of the inverse converter 111I to the reference value using the unit method, and the vertical axis represents the ratio (pu) of the magnitude of the DC voltage of the inverse converter 111I to the reference value using the unit method. In Figures 9(a) to 9(d), the positive direction (rightward) of the horizontal axis represents the ratio (pu) of the magnitude of the transmitted power to the reference value, and the negative direction (leftward) represents the ratio (pu) of the magnitude of the received power to the reference value. In Figures 9(a) to 9(d), the standard values ​​for both the magnitude of transmitted power and the magnitude of received power are set at 1000 MW. The standard value for the magnitude of DC voltage is set at 500 kV.

[0121] In Figures 9(a) and 9(b), curve 602P shows the droop control characteristics of the positive inverse converter 111IPB. Curve 603P shows the droop control characteristics of the positive inverse converter 111IPC. Curve 604P shows the droop control characteristics of the positive inverse converter 111IPD.

[0122] In Figure 9(a), operating point 602PO1 indicates the operating point of the positive-side inverter 111IPB before it changes from the powered state (droop control state) to the stopped state due to a fault. Operating point 603PO1 indicates the operating point of the positive-side inverter 111IPC before it changes from the powered state (droop control state) to the stopped state due to a fault. Operating point 604PO1 indicates the operating point of the positive-side inverter 111IPD before it changes from the powered state (droop control state) to the stopped state due to a fault.

[0123] In Figure 9(b), the operating point 603PO2 indicates the operating point of the positive-side inverse converter 111IPC after the operating point has been changed by the control variable calculation unit 12. The operating point 604PO2 indicates the operating point of the positive-side inverse converter 111IPD after the operating point has been changed by the control variable calculation unit 12.

[0124] In Figures 9(c) and 9(d), curve 602N shows the droop control characteristics of the negative inverse converter 111INB. Curve 603N shows the droop control characteristics of the negative inverse converter 111INC. Curve 604N shows the droop control characteristics of the negative inverse converter 111IND.

[0125] In Figure 9(c), operating point 602NO1 indicates the operating point of the negative inverter 111INB before the positive inverter 111IPB changes from the powered state (droop control state) to the stopped state due to a fault. Operating point 603NO1 indicates the operating point of the negative inverter 111INC before the positive inverter 111IPB changes from the powered state (droop control state) to the stopped state due to a fault. Operating point 604NO1 indicates the operating point of the negative inverter 111IND before the positive inverter 111IPB changes from the powered state (droop control state) to the stopped state due to a fault.

[0126] In Figure 9(d), operating point 602NO2 indicates the operating point of the negative inverse converter 111INB after the operating point has been changed by the control variable calculation unit 12. Operating point 603NO2 indicates the operating point of the negative inverse converter 111INC after the operating point has been changed by the control variable calculation unit 12. Also, operating point 604NO2 indicates the operating point of the negative inverse converter 111IND after the operating point has been changed by the control variable calculation unit 12.

[0127] In Figure 9(a), since the positive-side inverter 111IPB is receiving 200 MW of power, the operating point 602PO1 is at (-0.20, 1.00). Similarly, since the positive-side inverter 111IPC is receiving 300 MW of power, the operating point 603PO1 is at (-0.30, 1.00). Similarly, since the positive-side inverter 111IPD is receiving 750 MW of power, the operating point 604PO1 is at (-0.75, 1.00).

[0128] Here, when the positive-side inverter 111IPB changes from a powered state (droop-controlled state) to a stopped state due to a fault, and the control amount calculation unit 12 changes the control amount of the positive-side inverter 111IPC, the positive-side inverter 111IPC is receiving 336.36 MW of power, so the operating point 603PO2 is at (-0.34, 1.0018) according to the droop characteristics. Similarly, when the positive-side inverter 111IPB changes from a powered state (droop-controlled state) to a stopped state due to a fault, and the control amount calculation unit 12 changes the control amount of the positive-side inverter 111IPD, the positive-side inverter 111IPD is receiving 804.54 MW of power, so the operating point 604PO2 is at (-0.80, 1.0018) according to the droop characteristics.

[0129] In Figure 9(c), since the negative inverter 111INB is receiving 200MW of power, the operating point 602NO1 is at (-0.20, 1.00). Similarly, since the negative inverter 111INC is receiving 300 MW of power, the operating point 603NO1 is at (-0.30, 1.00). Similarly, since the negative inverter 111IND receives 750 MW of power, the operating point 604NO1 is at (-0.75, 1.00).

[0130] Here, when the positive inverter 111IPB changes from a powered state (droop control state) to a stopped state due to a fault, and the control amount calculation unit 12 changes the control amount of the negative inverter 111INB, the negative inverter 111INB is receiving 218.18 MW of power, so the operating point 602NO2 is at (-0.22, 1.0018) according to the droop characteristics. Similarly, when the positive inverter 111IPB changes from a powered state (droop-controlled state) to a stopped state due to a fault, and the control amount calculation unit 12 changes the control amount of the negative inverter 111INC, the operating point 603NO2 is at (-0.34, 1.0018) according to the droop characteristics, since the negative inverter 111INC is receiving 336.36 MW of power. Similarly, when the positive inverter 111IPB changes from a powered state (droop control state) to a stopped state due to a fault, and the control amount calculation unit 12 changes the control amount of the negative inverter 111IND, the operating point 604NO2 is at (-0.80, 1.0018) according to the droop characteristics, since the negative inverter 111IND is receiving 804.54 MW of power.

[0131] In other words, the higher-level control device 10 calculates a correction value so that the total return power value 110C approaches zero, corrects the command value of the control amount using the calculated correction value, and then applies the control amount to the positive-side forward converter 111CP and the negative-side forward converter 111CN, thereby making the power received by the positive-side reverse converter 111IP at a value below the rated capacity and the power received by the negative-side reverse converter 111IN at a value below the rated capacity of the receiving-side converter station 30, where the operating states of both the positive-side reverse converter 111IP and the negative-side reverse converter 111IN are in a droop control state, equal.

[0132] As described above, in the DC power transmission system 100 according to the embodiment, the higher-level control device 10 calculates a correction value to correct the control amount so that the total return power value 110C approaches zero, and calculates the control amounts for the positive-side forward converter 111CP and the negative-side forward converter 111CN by correcting the command value of the control amount with the correction value.

[0133] According to this, in the DC power transmission system 100, the control amount given to the positive-side forward converter 111CP and the negative-side forward converter 111CN is controlled to distribute the total return power value 110C to the positive-side forward converter 111CP and the negative-side forward converter 111CN so that the power received by the positive-side reverse converter 111IP and the power received by the negative-side reverse converter 111IN at the receiving-side converter station 30, where the operating states of both the positive-side reverse converter 111IP and the negative-side reverse converter 111IN are equal. Therefore, the difference in power received by the positive-side inverter 111IP and the negative-side inverter 111IN can be reduced, thereby reducing power loss in a bipolar DC power transmission system.

[0134] Furthermore, in the DC power transmission system 100 according to this embodiment, the total return power value 110C is distributed to the positive-side inverter 111IP and the negative-side inverter 111IN, which are operating in a droop control state.

[0135] According to this, within the rated capacity range of the inverter 111I, the total return power 110C is distributed to the positive inverter 111IP and the negative inverter 111IN.

[0136] Furthermore, in the DC power transmission system 100 according to this embodiment, the total return power value 110C is distributed to the positive-side inverter 111IP and negative-side inverter 111IN, which are operating in a droop control state, based on the gain values ​​of the droop control stored in the memory unit 11, so as to approach a reference value based on the ratio between the sum of the gains of the positive-side inverter 111IP and negative-side inverter 111IN, which are operating in a droop control state, and the gains of the positive-side inverter 111IP and negative-side inverter 111IN, which are operating in a droop control state.

[0137] In other words, the total return power value 110C is divided by the total gain value of the return power value 110C based on the gain value set for each inverter 111I. The quotient obtained by multiplying this quotient by the gains KdPB, KdPC, KdPD, KdNB, KdNC, and KdND set for each inverter 111I is distributed to each inverter 111I whose operating state is in droop control state according to the ratio of the products obtained.

[0138] ≪Second Embodiment≫ Figure 10 is a diagram showing the functional block configuration of a higher-level control device according to a second embodiment of the present invention.

[0139] The higher-level control device 10A according to the second embodiment has a gain changing unit 17 in addition to the functional blocks of the higher-level control device 10 according to the first embodiment. This functional block is realized through the cooperation of hardware resources and software constituting the higher-level control device 10A, similar to the higher-level control device 10 according to the first embodiment.

[0140] The gain changing unit 17 is a functional unit that, for a power receiving converter station 30 where either the positive-side inverter 111IP or the negative-side inverter 111IN is in a stopped state and the other is in a droop-controlled state, changes the gain of the positive-side inverter 111IP or the negative-side inverter 111IN so that the positive-side inverter 111IP or the negative-side inverter 111IN in the droop-controlled state receives power at a constant power level, and updates the gain value stored in the memory unit 11.

[0141] Constant power includes the state in which the positive-side inverter 111IP or the negative-side inverter 111IN is receiving power in a constant power control state. The state in which the positive-side reverse converter 111IP or the negative-side reverse converter 111IN is receiving power under constant power control includes the state in which the positive-side reverse converter 111IP or the negative-side reverse converter 111IN is receiving power at its rated capacity, and (if it is less than the rated capacity) the state in which it is receiving power at the sum of the transmitted power of the positive-side forward converter 111CP or the sum of the transmitted power of the negative-side forward converter 111CN.

[0142] When the positive-side inverter 111IP or the negative-side inverter 111IN is receiving power in a constant power control state, the received power is a value determined by information transmitted from an information processing device (not shown) connected via a network (not shown), or by information prepared in advance by a system administrator or the like.

[0143] Figure 11 is a graph showing the change in droop control characteristics before and after changing the gain of the inverse converter. In Figure 11, the horizontal axis represents the change in power received by the inverse converter 111I (MW), and the vertical axis represents the change in DC voltage of the inverse converter 111I (V).

[0144] In Figure 11, curve 701 shows the characteristics of the droop control of the inverter 111I in the droop control state before the gain change at the receiving-side converter station 30, where either the positive-side inverter 111IP or the negative-side inverter 111IN is in a stopped state and the other is in a droop control state. Curve 702 shows the characteristics of the droop control of the inverter 111I in the droop control state at the receiving-side converter station 30, where either the positive-side inverter 111IP or the negative-side inverter 111IN is in a stopped state and the other is in a droop control state.

[0145] As shown in Figure 11, when the gain of the droop-controlled inverter 111I in the receiving-side converter station 30 is changed, and either the positive-side inverter 111IP or the negative-side inverter 111IN is stopped while the other is in a droop-controlled state, the droop-controlled characteristics change from curve 701 to curve 702. As a result, the inverter 111I in the droop-controlled state with the changed gain changes to characteristics close to constant-power control. That is, the power received by the inverter 111I in the droop-controlled state with the changed gain hardly changes from the control amount before correction, even when a control amount is given by the control amount calculation unit 12 with a correction value added. Therefore, the power received by the inverter 111I in the droop-controlled state with the changed gain does not change if some of the inverters 111I change from a powered state to a stopped state due to an accident or inspection.

[0146] Figure 12 is a flowchart showing a method for controlling the distribution of transmitted and received power in a DC power transmission system by a higher-level control device according to a second embodiment of the present invention.

[0147] The flowchart according to the second embodiment has the same processing steps as the flowchart according to the first embodiment, specifically steps S1 to S7 and S10 to S13. On the other hand, the flowchart according to the second embodiment has steps S8 to S9 in addition to the processing steps of the flowchart according to the first embodiment.

[0148] In step S7, the power transmission power distribution setting unit 16A adjusts the gains KdPC and KdNC so that the gain KdPC of the positive inverter 111IPC matches the gain KdNC of the negative inverter 111INC, and adjusts the gains KdPD and KdND so that the gain KdPD of the positive inverter 111IPD matches the gain KdND of the negative inverter 111IND, before proceeding to step S8.

[0149] Specifically, the converter station information monitoring unit 15 checks whether the other positive-side inverse converter 111IP or negative-side inverse converter 111IN (the positive-side inverse converter 111IP or negative-side inverse converter 111IN that is in a power-receiving state) of the power-receiving converter station 30, where either the positive-side inverse converter 111IP or the negative-side inverse converter 111IN is in a stopped state, is in a droop control state (step S8).

[0150] If either the positive-side inverter 111IP or the negative-side inverter 111IN of the receiving-side converter station 30 is in a stopped state, and the other positive-side inverter 111IP or negative-side inverter 111IN is in a droop-controlled state (step S8: YES), the gain changing unit 17 changes the gain set for the positive-side inverter 111IP or negative-side inverter 111IN of the receiving-side converter station 30 that is in a droop-controlled state, so that it has characteristics close to constant power control.

[0151] Changing the gain to achieve characteristics close to constant power control means bringing the gain set for the positive inverse converter 111IP or the negative inverse converter 111IN, which are in a droop control state, closer to zero.

[0152] If either the positive-side inverter 111IP or the negative-side inverter 111IN of the receiving-side converter station 30 is in a stopped state, and the other positive-side inverter 111IP or negative-side inverter 111IN is not in a droop control state (is in a constant power control state) (step S8: NO), the converter station information monitoring unit 15 does not perform any further processing, and the higher-level control device 10 proceeds to step S10 or later.

[0153] In step S13, the power transmission power distribution setting unit 16A excludes the positive-side inverter 111IP or negative-side inverter 111IN, whose gain has been changed (changed to zero) by the gain changing unit 17 to have characteristics close to constant power control, from the allocation target of the total return power value 110C. Then, the total return power value 110C is allocated to the positive-side inverter 111IP and negative-side inverter 111IN, which are in a droop control state, based on the gain ratio.

[0154] In the DC power transmission system 100A according to the embodiment described above, the gain changing unit 17 of the higher-level control device 10 changes the gain of the positive-side inverter 111IP or the negative-side inverter 111IN so that the positive-side inverter 111IP or the negative-side inverter 111IN in the droop-controlled state receives power at a constant level in the receiving-side converter station 30 where either the positive-side inverter 111IP or the negative-side inverter 111IN in the droop-controlled state is stopped and the other is in a droop-controlled state.

[0155] According to this, the inverter 111I in the droop control state with the gain changed changes to characteristics close to constant power control (the gain becomes zero). Therefore, the inverter 111I in the droop control state with the gain changed is treated as not being allocated the total return power 110C, and the total return power 110C is allocated only to the inverter 111I in the droop control state when the operating state without the gain changed is the droop control state. In other words, if either the positive-side inverter 111IP or the negative-side inverter 111IN is in a stopped state, and the other is in a droop-controlled state, the power passing through the return line 40G connected to the receiving-side converter station 30 can be set to zero. Therefore, it becomes possible to further reduce power losses in bipolar DC power transmission systems.

[0156] <<Extension of the Embodiment>> Although the present inventors have described the invention in detail based on embodiments, it goes without saying that the present invention is not limited thereto and can be modified in various ways without departing from its essence.

[0157] The flowcharts described above are examples and are not limited to the processing procedures shown in Figures 7 and 12. For example, other processes may be inserted between each step shown in Figures 7 and 12, or some processes may be parallelized.

[0158] For example, if both the positive-side inverter 111IP and the negative-side inverter 111IN are in a droop-controlled state at the same power receiving station 30, the gains of the positive-side inverter 111IP and the negative-side inverter 111IN are usually set to the same value. Therefore, steps S6 and S7 may be omitted in the flowcharts shown in Figures 7 and 12.

[0159] For example, the return line power total calculation unit 14 calculates the return line power total value 110C by subtracting the negative electrode power total value (the sum of power flowing through the negative electrode main line 40N between the negative electrode forward converter 111CN and the negative electrode reverse converter 111IN) from the positive electrode power total value, which is the sum of power flowing through the positive electrode main line 40P between the positive electrode forward converter 111CP and the positive electrode reverse converter 111IP, but is not limited to this. The return line power total calculation unit 14 may also calculate the return line power total value 110C by subtracting the positive electrode power total value (the sum of power flowing through the positive electrode main line 40P, which is the sum of power flowing through the negative electrode main line 40N between the positive electrode forward converter 111CP and the positive electrode reverse converter 111IP) from the negative electrode power total value (the sum of power flowing through the negative electrode main line 40N between the negative electrode forward converter 111CN and the negative electrode reverse converter 111IN).

[0160] At this time, the control variable calculation unit 12 calculates the corrected negative pole transmission power 402N as the control variable by adding the value obtained by inverting the sign of the correction value calculated by the PI controller 162 to the uncorrected negative pole transmission power 401N via the positive / negative inversion block 121, and provides this corrected negative pole transmission power 402N to the negative side forward converter 111CN. Furthermore, the control variable calculation unit 12 provides the corrected positive electrode transmission power 402P, which is calculated by adding the correction value calculated by the PI controller 162 to the uncorrected positive electrode transmission power 401P, to the positive-side forward converter 111CP as the control variable.

[0161] For example, the return line power measurement unit 13 measures the return line power, which is the power flowing through the return line 40G, when a difference occurs between the power transmitted between the positive forward converter 111CP and the positive reverse converter 111IP and the power transmitted between the negative forward converter 111CN and the negative reverse converter 111IN. The return line power total value calculation unit 14 then calculates the return line total value 110C, which is the sum of the power of each return line 40G measured by the return line power measurement unit 13. However, this is not the only way to do this. In other words, the higher-level control device 10 may include, in addition to or instead of the return line power measurement unit 13, a functional unit that measures the return line current, which is the current flowing through the return line 40G, when a difference occurs between the power transmitted between the positive-side forward converter 111CP and the positive-side reverse converter 111IP and the power transmitted between the negative-side forward converter 111CN and the negative-side reverse converter 111IN. In this case, the return line total value calculation unit 14 may calculate the return line total value 110C based on the return line current of each return line 40G measured by the functional unit that measures the return line current. [Explanation of symbols]

[0162] 10. Higher-level control unit 11 Storage section 12 Control variable calculation unit 13 Return line power measurement unit 14. Return line power total value calculation unit 15. Information Monitoring Department of the Conversion Station 16 Correction Value Calculation Unit 16A Power transmission power distribution setting unit 20 Power transmission side converter station 30 Power receiving conversion station 40 DC transmission lines 40P Positive Main Line 40N Negative electrode main line 40G return line 50 Connecting Line 100 DC power transmission systems 101 Arithmetic equipment 102 Storage device 1021 Program 1022 data 103 Input device 104 I / F device 105 Output device 106 Bus 110A Transmission Side Converter Station Information 110B Receiving Side Converter Station Information 110C Return Line Total Power Value 110D power adjustment information 111C Forward Converter 111I Inverse Converter 112 Converter Station Protection Control Panel 113 Converter control panel 200 Transmission side AC system 300 Power receiving side AC system 400 DC power transmission system control block 500 Power Conversion Systems 602 Characteristics of Droop Control of the Converter at the Second Converter Station on the Receiving Side 603 Characteristics of Droop Control of the Converter at the Third Converter Station on the Receiving Side 604 Characteristics of Droop Control of the Converter at the Fourth Converter Station on the Receiving Side 602O Operating point of the converter at the second converter station on the receiving side 603O Operating point of the converter at the third converter station on the receiving side 604O Operating point of the converter at the fourth converter station on the receiving side 701 Curve showing the characteristics of droop control before changing the gain of the inverse converter. 702 Curve showing the characteristics of droop control after changing the gain of the inverse converter. 1021 Program Power received by the positive electrode of the PPR PNR negative terminal power Power transmission from the positive electrode of the PPS PNS negative electrode power transmission

Claims

1. A power transmission station comprising a positive-side forward converter that converts AC power to positive-side DC power according to a given control amount, and a negative-side forward converter that converts AC power to negative-side DC power according to a given control amount, A plurality of receiving-side converter stations comprising a positive-side inverse converter that converts DC power transmitted from the positive-side forward converter to AC power, and a negative-side inverse converter that converts DC power transmitted from the negative-side forward converter to AC power, A DC transmission line including a positive terminal main line, a negative terminal main line, and a return line, provided corresponding to each power receiving converter station, and connecting the power transmitting converter station and the power receiving converter station, The system includes a higher-level control device that controls at least one of the power transmission side converter station and the power receiving side converter station, The aforementioned higher-level control device is Memory unit and, A control amount calculation unit calculates the control amount for the positive forward converter and the negative forward converter, and provides the control amount to the positive forward converter and the negative forward converter, A return line power measurement unit measures the power of the return line among the DC transmission lines and stores the measured results in the storage unit, A return line total power calculation unit calculates a return line total power value, which is the sum of the power of each of the return lines, and stores the calculated result in the storage unit. It includes a correction value calculation unit that calculates a correction value for correcting the control amount so that the total return power value approaches zero, The control amount calculation unit calculates the control amount by correcting the command value of the control amount with the correction value. DC power transmission system.

2. The system according to claim 1, The above-mentioned higher-level control device has a gain changing unit that changes the gain, The gain changing unit, in the case of the power receiving side converter station where either the positive-side inverter or the negative-side inverter is in a stopped state and the other is in a droop-controlled state, changes the gain of the positive-side inverter or the negative-side inverter so that the positive-side inverter or the negative-side inverter in the droop-controlled state receives power at a constant power, and updates the gain value stored in the memory unit. DC power transmission system.

3. A method for controlling a DC power transmission system, The aforementioned DC power transmission system is A power transmission station comprising a positive-side forward converter that converts AC power to positive-side DC power according to a given control amount, and a negative-side forward converter that converts AC power to negative-side DC power according to a given control amount, A plurality of receiving-side converter stations comprising a positive-side inverse converter that converts DC power transmitted from the positive-side forward converter to AC power, and a negative-side inverse converter that converts DC power transmitted from the negative-side forward converter to AC power, A DC transmission line is provided corresponding to each of the receiving-side converter stations, and includes a positive terminal main line, a negative terminal main line, and a return line, connecting the transmitting-side converter station and the receiving-side converter station. A control amount calculation step of calculating the control amount of the positive forward converter and the negative forward converter, and assigning the control amount to the positive forward converter and the negative forward converter, A return line power measurement step for measuring the power of the return line among the DC transmission lines, The process includes a correction value calculation step of calculating a correction value for correcting the control amount so that the total return line power value, which is the sum of the power of each of the return lines, approaches zero. The control amount calculation step includes a step of calculating the control amount by correcting the command value of the control amount with the correction value. method.

4. A power transmission station comprising a positive-side forward converter that converts AC power to positive-side DC power according to a given control amount, and a negative-side forward converter that converts AC power to negative-side DC power according to a given control amount, A DC power transmission system comprising: a plurality of receiving-side converter stations, each comprising a positive-side inverse converter for converting DC power transmitted from the positive-side forward converter to AC power, and a negative-side inverse converter for converting DC power transmitted from the negative-side forward converter to AC power; a DC transmission line including a positive main line, a negative main line, and a return line, provided corresponding to each receiving-side converter station and connecting the transmitting-side converter station and the receiving-side converter station; and a higher-level control device for controlling at least one of the transmitting-side converter station and the receiving-side converter station, wherein the program to be executed by the higher-level control device is as follows: A control amount calculation step of calculating the control amount of the positive forward converter and the negative forward converter, and assigning the control amount to the positive forward converter and the negative forward converter, A return line power measurement step for measuring the power of the return line among the DC transmission lines, A step to calculate the total return line power, which is the sum of the power of each of the aforementioned return lines, The process includes a correction value calculation step of calculating a correction value for correcting the controlled amount so that the total return power value approaches zero, The control amount calculation step includes a step of calculating the control amount by correcting the command value of the control amount with the correction value. program.

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