Frequency conversion flexible tuning device, half-wavelength AC power transmission system and control method thereof

Abstract

The present invention provides a variable-frequency flexible tuning device, a half-wavelength AC power transmission system, and a control method thereof. The device comprises a first MMC module and a second MMC module, each of which includes three parallel-connected phase units. The phase units include two series-connected bridge arms, each of which includes a series-connected reactor and a power module unit. The power module unit includes multiple series-connected power submodules, each of which is a half-bridge structure power submodule or a full-bridge structure power submodule including fully controlled power electronic devices. Compared with the prior art, the variable-frequency flexible tuning device, the half-wavelength AC power transmission system, and the control method thereof provided by the present invention can flexibly adjust the output signal frequency, voltage amplitude, and phase of the variable-frequency flexible tuning device, thereby achieving flexible tuning and flexible grid connection of the half-wavelength AC power transmission system, adjusting its system parameters, suppressing overvoltage and backflow current in the half-wavelength AC transmission line, and achieving fault isolation.

Description

Technical Field

[0001] The present invention relates to the technical field of half-wavelength alternating current (AC) transmission, and in particular to a variable frequency flexible tuning device, a half-wavelength AC transmission system and a control method thereof. Background Art

[0002] The development of technologies such as photovoltaics and wind power has promoted intensive and large-scale development in areas with less than optimal resource conditions, making the transmission and implementation of ultra-long-distance, high-power electricity of broad significance and practical value.

[0003] Half-wavelength AC transmission (HWACT) refers to ultra-long-distance, three-phase AC transmission over an electrical distance approaching half the wavelength of the power frequency, or 3,000 km (50 Hz) or 2,600 km (60 Hz). A lossless half-wavelength AC line behaves like an ideal transformer with a transformation ratio of -1.0, where the voltage at the head and tail terminals are equal in magnitude and opposite in phase. With the increasing demand for ultra-long-distance, high-power transmission, half-wavelength AC transmission technology, especially ultra-high voltage (UHV) half-wavelength AC transmission, has once again attracted significant attention and research. One advantage of HWACT is its relatively high power factor. For transmission distances equal to or slightly greater than half a wavelength, its structure is simpler than existing ultra-long-distance AC and DC transmission systems. Furthermore, for developing countries, the manufacture of AC transmission equipment is simpler and more economical than the introduction, operation, and maintenance of converters.

[0004] The distances between China's western energy bases (such as coal-fired power plants in Xinjiang) and eastern load centers (such as the Pearl River Delta) can reach up to 3,000 km in some cases, necessitating enormous transmission capacity. In the future, it is also possible to develop power from neighboring countries like Russia and Mongolia and transmit it to China, North Korea, and other countries, also over distances exceeding 3,000 km. This specialized, ultra-long-distance power transmission system makes ultra-high voltage, half-wavelength AC transmission technology a promising option for China's future power transmission. Currently, vast countries like Brazil and Canada are also advancing research on half-wavelength AC transmission technology to meet the increasingly urgent demand for ultra-long-distance, high-capacity power transmission. Therefore, the application prospects of half-wavelength AC transmission are very broad.

[0005] Half-wavelength AC transmission is limited by objective conditions, and the natural length of the actual line is rarely exactly half a wavelength. When the line length is insufficient or too long, it is necessary to use a tuning circuit or compensation circuit to artificially compensate the electrical length of the transmission line to achieve the purpose of artificially half-wavelength AC transmission line. At present, the existing half-wavelength AC transmission compensation scheme is based on passive network transmission line tuning, as shown in the attached figure. Figure 1 The π-type tuning circuit / device shown, with Figure 2The T-type tuning circuit / device shown. These two tuning schemes have affected the development and application of half-wavelength AC transmission technology to a certain extent. This is because:

[0006] (1) Passive tuning circuits have relatively poor adaptability to changes in system structure, operating mode, or parameters, and are prone to losing their half-wavelength characteristics;

[0007] (2) The passive tuning circuit has a relatively simple function, only solving the tuning problem, and cannot adapt to the development requirements of flexible and intelligent power grids;

[0008] (3) When a short circuit occurs in a passive tuned circuit or a half-wavelength AC transmission line, resonance is easily induced, generating an overvoltage with a very high amplitude. This requires that the lines and equipment have higher insulation requirements, or that corresponding overvoltage limiting measures are installed to suppress the overvoltage of the line. Summary of the Invention

[0009] In order to meet the needs of the prior art, the present invention provides a variable frequency flexible tuning device, a half-wavelength AC power transmission system and a control method thereof.

[0010] In a first aspect, a technical solution of a variable frequency flexible tuning device in the present invention is:

[0011] The variable frequency flexible tuning device includes a first MMC module and a second MMC module; the first MMC module and the second MMC module are connected to each other on the DC side, and the AC side is the input / output end of the variable frequency flexible tuning device;

[0012] The first MMC module and the second MMC module each include three parallel phase units; the phase unit includes two bridge arms connected in series, each bridge arm includes a reactor and a power module unit connected in series; the series connection point of the bridge arms in each phase unit is the AC terminal of the AC side, and the parallel connection point of each phase unit is the DC terminal of the DC side;

[0013] The power module unit includes a plurality of power sub-modules connected in series, and the power sub-modules are half-bridge structure power sub-modules or full-bridge structure power sub-modules including fully controlled power electronic devices.

[0014] In a second aspect, a technical solution of a half-wavelength AC power transmission system in the present invention is:

[0015] The half-wavelength AC power transmission system includes a sending-end AC system, a half-wavelength AC transmission line, and a receiving-end AC system connected in sequence. The half-wavelength AC power transmission system includes two of the above-mentioned variable frequency flexible tuning devices; one of the variable frequency flexible tuning devices is installed between the sending-end AC system and the half-wavelength AC transmission line, and the other of the variable frequency flexible tuning devices is installed between the receiving-end AC system and the half-wavelength AC transmission line.

[0016] In a third aspect, a technical solution of a control method for the above-mentioned half-wavelength AC power transmission system in the present invention is:

[0017] The control method includes adjusting the operating mode of the variable frequency flexible tuning device to flexibly tune and flexibly connect the half-wavelength AC power transmission system to the grid, thereby adjusting the system parameters of the half-wavelength AC power transmission system; suppressing overvoltage and backfeed current in the half-wavelength AC transmission line and achieving fault isolation; the system parameters include the power factor and bus voltage of the sending-end AC system, and the power factor and bus voltage of the receiving-end AC system.

[0018] Compared with the closest prior art, the beneficial effects of the present invention are:

[0019] 1. The present invention provides a variable frequency flexible tuning device, comprising a first MMC module and a second MMC module, each of which includes a plurality of power submodules composed of fully-controlled power electronic devices. Thus, by controlling the conduction and blocking of the fully-controlled power electronic devices and changing the input / output signals of the first MMC module and the second MMC module, electrical parameters such as the frequency, voltage amplitude, and phase of the output signal of the variable frequency flexible tuning device can be flexibly adjusted.

[0020] 2. The present invention provides a half-wavelength AC power transmission system, wherein variable frequency flexible tuning devices are installed at both ends of the half-wavelength AC transmission line. These devices can tune the half-wavelength AC transmission line to maintain its half-wavelength characteristics after changes in the line structure, line parameters, or operating mode of the half-wavelength AC transmission line.

[0021] 3. The present invention provides a control method for a half-wavelength AC transmission system. By changing the operating mode of the variable-frequency flexible tuning device, the half-wavelength AC transmission system is flexibly tuned and flexibly connected to the grid, and the system parameters of the half-wavelength AC transmission system are adjusted. The overvoltage and backfeed current of the half-wavelength AC transmission line are suppressed and fault isolation is achieved. The operation is flexible and simple. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 : Schematic diagram of the π-type tuning circuit / device structure;

[0023] Figure 2 : Schematic diagram of T-type tuning circuit / device structure;

[0024] Figure 3 : A structural diagram of a variable frequency flexible tuning device in an embodiment of the present invention;

[0025] Figure 4 : Schematic diagram of the half-bridge power submodule structure;

[0026] Figure 5 : Schematic diagram of the full-bridge power submodule structure;

[0027] Figure 6 : Schematic diagram of the structure of a half-wavelength AC transmission system in an embodiment of the present invention. DETAILED DESCRIPTION

[0028] To make the objectives, technical solutions, and advantages of the embodiments of the present invention more clear, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts shall fall within the scope of protection of the present invention.

[0029] A variable frequency flexible tuning device provided by an embodiment of the present invention is described below with reference to the accompanying drawings.

[0030] Figure 3 This is a structural schematic diagram of a variable frequency flexible tuning device in an embodiment of the present invention. As shown in the figure, the variable frequency flexible tuning device in this embodiment includes a first MMC module 1 and a second MMC module 2. The DC sides of the first MMC module 1 and the second MMC module 2 are connected, and the AC side is the input / output end of the variable frequency flexible tuning device.

[0031] The first MMC module 1 and the second MMC module 2 each include three parallel-connected phase units, each of which includes two series-connected bridge arms, each of which includes a series-connected reactor and power module unit. The series connection point of the bridge arms in each phase unit is the AC terminal on the AC side, and the parallel connection point of each phase unit is the DC terminal on the DC side. That is, the series connection point of the bridge arms in each phase unit of the first MMC module 1 is the AC terminal on the AC side, and the parallel connection point of each phase unit is the DC terminal on the DC side; the series connection point of the bridge arms in each phase unit of the second MMC module 2 is the AC terminal on the AC side, and the parallel connection point of each phase unit is the DC terminal on the DC side.

[0032] The power module unit includes multiple power sub-modules (SM) connected in series, and the power sub-module is a half-bridge structure power sub-module (HBSM) or a full-bridge structure power sub-module (FBSM) including fully controlled power electronic devices. Figure 4 This is a schematic diagram of the half-bridge power submodule structure. Figure 5 This is a schematic diagram of the full-bridge power submodule structure.

[0033] In this embodiment, the variable frequency flexible tuning device includes a first MMC module 1 and a second MMC module 2. The first MMC module 1 and the second MMC module 2 each include a plurality of power submodules composed of fully-controlled power electronic devices. Therefore, by controlling the conduction and blocking of the fully-controlled power electronic devices and changing the input / output signals of the rectifier module and the inverter module, the frequency, voltage amplitude, phase and other electrical parameters of the output signal of the variable frequency flexible tuning device can be flexibly adjusted.

[0034] Furthermore, in this embodiment, the two bridge arms of the phase unit can be connected in the following two ways: the two bridge arms are connected through their respective reactors, or through their respective power module units. That is, depending on the actual installation position of the variable frequency flexible tuning device, the reactor can be installed at the AC terminal of the phase unit, or the reactor can be installed at the DC terminal of the phase unit. Figure 3 As shown, in this embodiment, the two bridge arms of each phase unit are connected through respective reactors. At the same time, a line is drawn out from each AC terminal to form a three-phase AC line A, B, and C.

[0035] Furthermore, in this embodiment, the power submodules of the first MMC module 1 are of the same structure, and the reactors are of the same structure. Simultaneously, the power submodules of the second MMC module 2 are also of the same structure, and the reactors are of the same structure.

[0036] Furthermore, in this embodiment, the power submodule of the first MMC module 1 and the power submodule of the second MMC module 2 may have the same or different structures, and the reactor of the first MMC module 1 and the reactor of the second MMC module 2 may have the same or different structures. That is, the structures of the first MMC module 1 and the second MMC module 2 may be the same or different.

[0037] Furthermore, in this embodiment, the parameters of the components included in each power submodule within the first MMC module 1 are identical, and the reactance values ​​of each reactor are identical, ensuring the consistency of each bridge arm of the first MMC module 1. The parameters of the components included in each power submodule within the second MMC module 2 are identical, and the reactance values ​​of each reactor are identical, ensuring the consistency of each bridge arm of the second MMC module 2.

[0038] The present invention also provides a half-wavelength AC power transmission system and gives a specific embodiment.

[0039] Figure 6This is a schematic diagram of the structure of a half-wavelength AC power transmission system according to an embodiment of the present invention. As shown in the figure, the half-wavelength AC power transmission system in this embodiment includes a sending-end AC system, a half-wavelength AC transmission line, a receiving-end AC system, and the two aforementioned variable-frequency flexible tuning devices. One variable-frequency flexible tuning device is installed between the sending-end AC system and the half-wavelength AC transmission line, and the other variable-frequency flexible tuning device is installed between the receiving-end AC system and the half-wavelength AC transmission line. As shown in the figure, the variable-frequency flexible tuning device connected to the sending-end AC system includes a first MMC module 11 and a second MMC module 12, while the variable-frequency flexible tuning device connected to the receiving-end AC system includes a first MMC module 21 and a second MMC module 22.

[0040] In this embodiment, the variable frequency flexible tuning device is installed at both ends of the half-wavelength AC transmission line. After the line structure, line parameters or operating mode of the half-wavelength AC transmission line changes, the half-wavelength AC transmission line can be tuned so that it does not lose its half-wavelength characteristics.

[0041] Furthermore, in this embodiment, the variable frequency flexible tuning device connected to the sending end AC system is as follows: Figure 6 As shown, the first MMC module 11 is connected to the sending-end AC system, and the second MMC module 12 is connected to the half-wavelength AC transmission line. The variable frequency flexible tuning device connected to the receiving-end AC system has the second MMC module 22 connected to the receiving-end AC system, and the first MMC module 21 connected to the half-wavelength AC transmission line.

[0042] The present invention also provides a control method for the above-mentioned half-wavelength AC power transmission system and gives a specific embodiment.

[0043] In this embodiment, the control method for a half-wavelength AC transmission system includes adjusting the operating mode of a variable-frequency flexible tuning device to perform flexible tuning and grid connection of the half-wavelength AC transmission system, adjust system parameters of the half-wavelength AC transmission system, suppress overvoltage and backfeed current in the half-wavelength AC transmission line, and achieve fault isolation. System parameters include the power factor and bus voltage of the sending-end AC system and the power factor and bus voltage of the receiving-end AC system. In this embodiment, by adjusting the operating mode of the variable-frequency flexible tuning device, the half-wavelength AC transmission system is flexibly tuned and grid-connected, adjusts system parameters of the half-wavelength AC transmission system, suppresses overvoltage and backfeed current in the half-wavelength AC transmission line, and achieves fault isolation. The control method is flexible and simple. Flexible tuning, flexible grid connection, adjusting system parameters, suppressing overvoltage and backfeed current, and achieving fault isolation are described in detail below.

[0044] 1. Flexible tuning

[0045] In this embodiment, the flexible tuning of the half-wavelength AC power transmission system can be performed according to the following steps, specifically:

[0046] (1) When the electrical distance of the half-wavelength AC transmission line is less than one and a half wavelength corresponding to the grid frequency, the operating frequency of the half-wavelength AC transmission line is increased by controlling the output signal frequency of the variable frequency flexible tuning device, and the increased operating frequency is used for AC transmission until the electrical distance is equal to one and a half wavelength corresponding to the increased operating frequency.

[0047] (2) When the electrical distance of the half-wavelength AC transmission line is greater than one and a half wavelength corresponding to the grid frequency, the operating frequency of the half-wavelength AC transmission line is reduced by controlling the output signal frequency of the variable frequency flexible tuning device, and the reduced operating frequency is used for AC transmission until the electrical distance is equal to one and a half wavelength corresponding to the reduced operating frequency.

[0048] (3) When the line structure, line parameters or operating mode of the half-wavelength AC transmission line changes, the operating frequency of the half-wavelength AC transmission line is adjusted by controlling the output signal frequency of the variable frequency flexible tuning device, and AC power transmission is carried out using the adjusted operating frequency until the electrical distance of the half-wavelength AC transmission line after the line structure, line parameters or operating mode changes is equal to one and a half wavelength corresponding to the changed operating frequency.

[0049] During flexible tuning of the half-wavelength AC transmission system, the second MMC module 12 can be equivalent to a voltage source, providing voltage to the half-wavelength AC transmission line, and the first MMC module 21 can be equivalent to a current source, i.e., a resistive load. Furthermore, in this embodiment, the first MMC module 1 can operate in either a rectifying or an inverting state, and the second MMC module 2 can operate in either a rectifying or an inverting state. Therefore, the sending-end AC system and the receiving-end AC system can be interchanged based on actual operating conditions. Accordingly, the first MMC module 21 can be equivalent to a voltage source, providing voltage to the half-wavelength AC transmission line, and the second MMC module 12 can be equivalent to a current source, i.e., a resistive load.

[0050] Furthermore, in order to control the variable frequency flexible tuning device to output a higher frequency voltage / current signal at the same switching frequency, before controlling the output signal frequency of the variable frequency flexible tuning device in the above-mentioned case (1), the number of power sub-modules put into operation in the first MMC module 1 and the second MMC module 2 directly connected to the half-wavelength AC transmission line in the variable frequency flexible tuning device can also be controlled to be greater than the number of power sub-modules put into operation in the first MMC module 1 and the second MMC module 2 not directly connected to the half-wavelength AC transmission line. That is, the number of power sub-modules put into operation in the second MMC module 12 is controlled to be greater than the number of power sub-modules put into operation in the first MMC module 11, and the number of power sub-modules put into operation in the first MMC module 21 is controlled to be greater than the number of power sub-modules put into operation in the second MMC module 22.

[0051] 2. Flexible grid connection

[0052] This embodiment includes two types of grid connection: one is that the sending-end AC system is connected to a half-wavelength AC transmission line and then connected to the receiving-end AC system; the other is that the receiving-end AC system is connected to a half-wavelength AC transmission line and then connected to the sending-end AC system. The flexible grid connection methods for these two types of grid connection are described in detail below.

[0053] (1) When the sending-end AC system is connected to the half-wavelength AC transmission line and then connected to the receiving-end AC system, the following steps can be followed:

[0054] ①: Control the first MMC module 11 of the variable frequency flexible tuning device on the sending-end AC system side to be in a rectification state and establish a corresponding DC voltage, control the second MMC module 12 to be in an inversion state and generate an AC voltage corresponding to the preset operating frequency of the half-wavelength AC transmission line, and transmit the AC voltage to the variable frequency flexible tuning device on the receiving-end AC system side through the half-wavelength AC transmission line.

[0055] ②: Control the first MMC module 21 of the variable frequency flexible tuning device on the receiving AC system side to be in the rectification state and generate the corresponding DC voltage, and control the second MMC module 22 to be in the inverter state and track the voltage frequency, amplitude, phase sequence and phase of the receiving AC system.

[0056] ③: When the voltage frequency, amplitude, phase sequence, and phase of the AC-side output signal are identical to those of the receiving AC system, i.e., when the grid-connection conditions of the power system are met, the grid-connection mechanical switch between the second MMC module 22 and the receiving AC system is closed to complete the grid connection. The grid-connection conditions of the power system include: the two systems must have the same frequency, the same voltage, the same phase sequence, and the same voltage phase.

[0057] (2) When the receiving-end AC system is connected to the half-wavelength AC transmission line and then connected to the sending-end AC system, the following steps can be followed:

[0058] ①: Control the second MMC module 22 of the variable frequency flexible tuning device on the receiving-end AC system side to be in a rectification state and establish a corresponding DC voltage, control the first MMC module 21 to be in an inversion state and generate an AC voltage corresponding to the preset operating frequency of the half-wavelength AC transmission line, and transmit the AC voltage to the variable frequency flexible tuning device on the sending-end AC system side through the half-wavelength AC transmission line.

[0059] ②: Control the second MMC module 12 of the variable frequency flexible tuning device on the sending-end AC system side to be in the rectification state and generate the corresponding DC voltage, control the first MMC module 11 to be in the inverter state and track the voltage frequency, amplitude, phase sequence and phase of the sending-end AC system.

[0060] ③: When the voltage frequency, amplitude, phase sequence and phase of the AC side output signal are respectively the same as the voltage frequency, amplitude, phase sequence and phase of the sending-end AC system, that is, when the grid-connected conditions of the power system are met, the grid-connected mechanical switch between the first MMC module 11 and the sending-end AC system is closed to complete the grid connection.

[0061] 3. Adjust the power factor

[0062] In this embodiment, the first MMC module 11 in the variable frequency flexible tuning device on the sending-end AC system can be controlled to operate at a unity power factor. The first MMC module 11 acts as a resistive load, thereby improving the power factor of the sending-end AC system. Specifically, the power factor of the first MMC module 11 is corrected to unity, and the first MMC module 11 is controlled to operate at the corrected power factor.

[0063] In this embodiment, the second MMC module 22 in the variable frequency flexible tuning device on the receiving AC system side can be controlled to operate at a unity power factor. The second MMC module 22 acts as a resistive load, thereby improving the power factor of the receiving AC system. Specifically, the power factor of the second MMC module 22 is corrected to 1, and the second MMC module 22 is controlled to operate at the corrected power factor.

[0064] 4. Adjust bus voltage

[0065] In this embodiment, the bus voltage refers to the bus voltage between the sending-end AC system and its connected variable frequency flexible tuning device, and the bus voltage between the receiving-end AC system and its connected variable frequency flexible tuning device.

[0066] (1) In this embodiment, the bus voltage of the sending-end AC system can be adjusted according to the following steps. Specifically, when the bus voltage is low, the first MMC module 11 of the variable frequency flexible tuning device is controlled to output capacitive reactive power to the sending-end AC system; when the bus voltage is high, the first MMC module 11 is controlled to output inductive reactive power to the sending-end AC system.

[0067] (2) In this embodiment, the bus voltage of the receiving AC system can be adjusted according to the following steps. Specifically, when the bus voltage is low, the second MMC module 22 of the variable frequency flexible tuning device is controlled to output capacitive reactive power to the receiving AC system; when the bus voltage is high, the second MMC module 22 is controlled to output inductive reactive power to the receiving AC system.

[0068] 5. Suppress overvoltage and latent current

[0069] In this embodiment, when the half-wavelength AC power transmission system operates normally, the second MMC module 22 connected to the variable frequency flexible tuning device of the sending-end AC system can function as a voltage source, outputting AC voltage to the half-wavelength AC transmission line. If a fault occurs in the half-wavelength AC transmission line, the AC voltage output by the second MMC module 22 can be rapidly reduced, suppressing overvoltage and backflow current, and achieving fault isolation.

[0070] Furthermore, in this embodiment, in order to avoid the problems of line overvoltage and back-feed current suppression caused by the half-wavelength AC transmission line, the half-wavelength AC transmission line can be made equivalent to a conventional AC line by adjusting the operating mode of the variable frequency flexible tuning device. Specifically, by controlling the output signal frequency of the variable frequency flexible tuning device, the operating frequency of the half-wavelength AC transmission line is reduced until the electrical distance of the half-wavelength AC transmission line is much larger than one and a half wavelength corresponding to before the operating frequency is reduced.

[0071] Furthermore, in this embodiment, in order to meet the demand for DC transmission under actual working conditions, the half-wavelength AC transmission line can be equivalent to a DC line by adjusting the operating mode of the variable frequency flexible tuning device. Specifically, by controlling the output signal frequency of the variable frequency flexible tuning device, the operating frequency of the half-wavelength AC transmission line is reduced to 0 Hz.

[0072] Obviously, those skilled in the art may make various changes and modifications to the present invention without departing from the spirit and scope of the present invention. Thus, if such changes and modifications fall within the scope of the claims and their equivalents, the present invention is intended to include such changes and modifications.

Claims

1. A control method for a half-wavelength AC power transmission system, characterized in that: The control method includes adjusting the operating mode of the variable frequency flexible tuning device to flexibly tune and connect the half-wavelength AC power transmission system to the grid, adjusting system parameters of the half-wavelength AC power transmission system, suppressing overvoltage and backfeed current in the half-wavelength AC transmission line, and achieving fault isolation; the system parameters include the power factor and bus voltage of the sending-end AC system, and the power factor and bus voltage of the receiving-end AC system; Wherein, the half-wavelength AC power transmission system comprises a sending-end AC system, a half-wavelength AC transmission line and a receiving-end AC system connected in sequence; The half-wavelength AC power transmission system further comprises two variable frequency flexible tuning devices; one variable frequency flexible tuning device is installed between the sending-end AC system and the half-wavelength AC transmission line, and the other variable frequency flexible tuning device is installed between the receiving-end AC system and the half-wavelength AC transmission line; The variable frequency flexible tuning device includes a first MMC module and a second MMC module; the first MMC module and the second MMC module are connected to each other on the DC side, and the AC side is the input / output end of the variable frequency flexible tuning device; The first MMC module and the second MMC module each include three parallel phase units; the phase unit includes two bridge arms connected in series, each bridge arm includes a reactor and a power module unit connected in series; the series connection point of the bridge arms in each phase unit is the AC terminal of the AC side, and the parallel connection point of each phase unit is the DC terminal of the DC side; The power module unit includes a plurality of power sub-modules connected in series, and the power sub-modules are half-bridge structure power sub-modules or full-bridge structure power sub-modules including fully controlled power electronic devices.

2. The control method of a half-wavelength AC power transmission system according to claim 1, characterized in that: The two bridge arms in the phase unit of the variable frequency flexible tuning device are connected via respective reactors; or, The two bridge arms in the phase unit of the variable frequency flexible tuning device are connected via respective power module units.

3. The control method of a half-wavelength AC power transmission system according to claim 1, characterized in that: The power submodules of the first MMC module of the variable frequency flexible tuning device are power submodules of the same structure, and the reactors are reactors of the same structure; The power submodules of the second MMC module of the variable frequency flexible tuning device are power submodules with the same structure, and the reactors are reactors with the same structure.

4. The control method of a half-wavelength AC power transmission system according to claim 1, characterized in that: The power submodule of the first MMC module and the power submodule of the second MMC module of the variable frequency flexible tuning device have the same or different structures; The structures of the reactor of the first MMC module and the reactor of the second MMC module of the variable frequency flexible tuning device are the same or different.

5. The control method of a half-wavelength AC power transmission system according to claim 1, characterized in that: The parameters of the components contained in each power submodule in the first MMC module of the variable frequency flexible tuning device are the same, and the reactance values ​​of the reactors are the same; The parameters of the components contained in each power submodule in the second MMC module of the variable frequency flexible tuning device are all the same, and the reactance values ​​of the reactors are all the same.

6. The control method of a half-wavelength AC power transmission system according to claim 1, characterized in that: In one of the variable frequency flexible tuning devices, the first MMC module is connected to the sending-end AC system, and the second MMC module is connected to the half-wavelength AC transmission line; in the other variable frequency flexible tuning device, the second MMC module is connected to the receiving-end AC system, and the first MMC module is connected to the half-wavelength AC transmission line.

7. The control method of a half-wavelength AC power transmission system according to claim 1, characterized in that: The flexible tuning of the half-wavelength AC power transmission system includes: When the electrical distance of the half-wavelength AC transmission line is less than one and a half wavelength corresponding to the grid frequency, the operating frequency of the half-wavelength AC transmission line is increased by controlling the output signal frequency of the variable frequency flexible tuning device until the electrical distance is equal to one and a half wavelength corresponding to the increased operating frequency; When the electrical distance of the half-wavelength AC transmission line is greater than one and a half wavelength corresponding to the grid frequency, the operating frequency of the half-wavelength AC transmission line is reduced by controlling the output signal frequency of the variable frequency flexible tuning device until the electrical distance is equal to one and a half wavelength corresponding to the reduced operating frequency; When the line structure, line parameters or operating mode of the half-wavelength AC transmission line changes, the operating frequency of the half-wavelength AC transmission line is adjusted by controlling the output signal frequency of the variable frequency flexible tuning device until the electrical distance of the half-wavelength AC transmission line after the line structure, line parameters or operating mode changes is equal to one and a half wavelength corresponding to the changed operating frequency.

8. The control method of a half-wavelength AC power transmission system according to claim 7, characterized in that: When the electrical distance is less than one and a half wavelength corresponding to the grid frequency, the output signal frequency of the variable frequency flexible tuning device is controlled, which includes: The number of power submodules put into operation in the first MMC module and the second MMC module directly connected to the half-wavelength AC transmission line in the variable frequency flexible tuning device is controlled to be greater than the number of power submodules put into operation in the first MMC module and the second MMC module not directly connected to the half-wavelength AC transmission line.

9. The control method of a half-wavelength AC power transmission system according to claim 1, wherein: The flexible grid connection types include: the sending-end AC system is connected to the half-wavelength AC transmission line and then connected to the receiving-end AC system, and the receiving-end AC system is connected to the half-wavelength AC transmission line and then connected to the sending-end AC system.

10. The control method of a half-wavelength AC power transmission system according to claim 9, characterized in that: When the grid connection type is that the sending-end AC system is connected to the half-wavelength AC transmission line and then connected to the receiving-end AC system, the flexible grid connection of the half-wavelength AC transmission system includes: Controlling the first MMC module of the variable frequency flexible tuning device on the sending-end AC system side to be in a rectifying state and the second MMC module to be in an inverting state, and controlling the AC side of the second MMC module to generate an AC voltage corresponding to the preset operating frequency of the half-wavelength AC transmission line; The first MMC module of the variable frequency flexible tuning device on the receiving-end AC system side is controlled to be in a rectifying state and the second MMC module to be in an inverting state, and the AC-side output signal of the second MMC module is controlled to track the voltage frequency, amplitude, phase sequence and phase of the receiving-end AC system; when the voltage frequency, amplitude, phase sequence and phase of the AC-side output signal are respectively the same as the voltage frequency, amplitude, phase sequence and phase of the receiving-end AC system, the grid-connected mechanical switch between the second MMC module and the receiving-end AC system is closed to complete the grid connection.

11. The control method of a half-wavelength AC power transmission system according to claim 9, characterized in that: When the grid connection type is that the receiving-end AC system is connected to the half-wavelength AC transmission line and then connected to the sending-end AC system, the flexible grid connection of the half-wavelength AC transmission system includes: Controlling the second MMC module of the variable frequency flexible tuning device on the receiving-end AC system side to be in a rectifying state and the first MMC module to be in an inverting state, and controlling the AC side of the first MMC module to generate an AC voltage corresponding to the preset operating frequency of the half-wavelength AC transmission line; The second MMC module of the variable frequency flexible tuning device on the sending-end AC system side is controlled to be in a rectifying state and the first MMC module to be in an inverting state, and the AC side output signal of the first MMC module is controlled to track the voltage frequency, amplitude, phase sequence and phase of the sending-end AC system; when the voltage frequency, amplitude, phase sequence and phase of the AC side output signal are respectively the same as the voltage frequency, amplitude, phase sequence and phase of the sending-end AC system, the grid-connected mechanical switch between the first MMC module and the sending-end AC system is closed to complete the grid connection.

12. The control method of a half-wavelength AC power transmission system according to claim 1, wherein: The adjusting the power factor of the sending-end AC system includes: controlling the first MMC module of the variable frequency flexible tuning device on one side of the sending-end AC system to operate according to the unity power factor; The adjusting the power factor of the receiving-end AC system includes controlling the second MMC module of the variable frequency flexible tuning device on one side of the receiving-end AC system to operate according to the integral power factor.

13. The control method of a half-wavelength AC power transmission system according to claim 1, wherein: The adjusting the bus voltage of the sending-end AC system includes: when the bus voltage between the sending-end AC system and the variable frequency flexible tuning device connected thereto is low, controlling the first MMC module of the variable frequency flexible tuning device to output capacitive reactive power to the sending-end AC system; and when the bus voltage is high, controlling the first MMC module to output inductive reactive power to the sending-end AC system. The adjusting the bus voltage of the receiving-end AC system includes: when the bus voltage between the receiving-end AC system and the variable frequency flexible tuning device connected to it is low, controlling the second MMC module of the variable frequency flexible tuning device to output capacitive reactive power to the receiving-end AC system; when the bus voltage is high, controlling the second MMC module to output inductive reactive power to the receiving-end AC system.

14. The control method of a half-wavelength AC power transmission system according to claim 1, wherein: The method of suppressing overvoltage and backflow current of the half-wavelength AC transmission line includes: when a fault occurs in the half-wavelength AC transmission line, controlling the AC voltage output by the second MMC module in the variable frequency flexible tuning device on the sending-end AC system side to be rapidly reduced to suppress the overvoltage and backflow current and achieve fault isolation.

15. The control method of a half-wavelength AC power transmission system according to claim 1, wherein: The control method further includes adjusting the operating mode of the variable frequency flexible tuning device to make the half-wavelength AC transmission line equivalent to a conventional AC line, specifically: reducing the operating frequency of the half-wavelength AC transmission line by controlling the output signal frequency of the variable frequency flexible tuning device until the electrical distance of the half-wavelength AC transmission line is much greater than one half-wavelength corresponding to the operating frequency before the reduction; The control method also includes adjusting the operating mode of the variable frequency flexible tuning device to convert the half-wavelength AC transmission line into a DC line, specifically: reducing the operating frequency of the half-wavelength AC transmission line to 0 Hz by controlling the output signal frequency of the variable frequency flexible tuning device.

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