True bipolar flexible low-frequency power transmission system

Abstract

The invention discloses a true bipolar flexible low-frequency power transmission system. An existing flexible low-frequency power transmission system mainly adopts a three-phase power transmission structure which is the same as that of a power frequency power transmission system, and popularization of the flexible low-frequency power transmission system is restricted. The system comprises two frequency conversion stations, a low-frequency positive electrode line and a low-frequency negative electrode line, each frequency conversion station comprises an anode frequency converter and a cathode frequency converter; one side of the anode frequency converter is connected with a power frequency AC power grid through a three-phase transformer, and the other side is connected with a low-frequency transmission line through a single-phase three-winding transformer. One side of the cathode frequency converter is connected with the power frequency AC power grid through the three-phase transformer, and the other side is connected with the low-frequency transmission line through the single-phase three-winding transformer. The low-frequency positive electrode circuit and the low-frequency negative electrode circuit have the same voltage amplitude, the same frequency and a phase difference of 180 degrees. According to the invention, interconnection and mutual assistance among different three-phase alternating current systems are realized through a two-phase power transmission structure, a new system structure is expanded for low-frequency power transmission, and the line construction cost is reduced.

Description

Technical Field

[0001] The present invention belongs to the field of flexible low-frequency power transmission systems, and particularly relates to a true bipolar flexible low-frequency power transmission system. Background Art

[0002] There are problems in power frequency AC power transmission such as reactive power for charging long-distance submarine cables and voltage / angle difference of long overhead lines, and the transmission capacity is severely limited. The cost of the offshore converter platform for flexible DC power transmission is high and it is difficult to form a network. There is an urgent need to innovate the large-scale new energy efficient collection and transmission methods. Flexible low-frequency power transmission technology is a new type of AC power transmission technology that uses power electronic technology to reduce the frequency below 50 Hz for power transmission. It has advantages such as small line charging power, zero-crossing breaking, easy voltage transformation, etc., and can directly output low-frequency electrical energy using wind turbines without the need for an offshore converter platform, having advantages in scenarios such as new energy transmission, flexible grid interconnection, or long-distance power supply.

[0003] Currently, the domestic and foreign research on flexible low-frequency power transmission systems mainly focuses on the three-phase power transmission structure with the same structure as the power frequency power transmission system. However, with the increase in the transmission distance, especially in scenarios such as medium and long-distance offshore wind power transmission and large-scale new energy transmission, the high construction cost of the three-phase power transmission structure restricts the popularization of flexible low-frequency power transmission systems. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to overcome the defects of the above-mentioned existing technologies, and provide a true bipolar flexible low-frequency power transmission system, which realizes the interconnection and mutual assistance between different three-phase AC systems through a two-phase power transmission structure, expands a new system structure for low-frequency power transmission, reduces the line construction cost, and constructs a highly reliable energy transmission channel.

[0005] To this end, the technical solution adopted by the present invention is as follows: A true bipolar flexible low-frequency power transmission system, which includes a first frequency conversion station, a second frequency conversion station, a low-frequency positive line, and a low-frequency negative line; The first frequency conversion station includes a first positive frequency converter and a first negative frequency converter; one side of the first positive frequency converter is connected to the first power frequency AC grid through a three-phase transformer, and the other side is connected to the low-frequency positive line through a first positive single-phase three-winding transformer; one side of the first negative frequency converter is connected to the first power frequency AC grid through a three-phase transformer, and the other side is connected to the low-frequency negative line through a first negative single-phase three-winding transformer; The second frequency conversion station includes a second positive frequency converter and a second negative frequency converter; one side of the second positive frequency converter is connected to the second power frequency AC grid through a three-phase transformer, and the other side is connected to the low-frequency positive line through a second positive single-phase three-winding transformer; one side of the second negative frequency converter is connected to the second power frequency AC grid through a three-phase transformer, and the other side is connected to the low-frequency negative line through a second negative single-phase three-winding transformer; The voltage amplitudes of the low-frequency positive line and the low-frequency negative line are the same, the frequencies are the same, and the phases are 180° out of phase with each other.

[0006] The true bipolar flexible low-frequency power transmission system of the present invention is essentially a two-phase system with phases 180° out of phase with each other. Like the true bipolar flexible DC power transmission system, it has the advantage of significantly reducing the construction cost of transmission lines. After a fault occurs in a low-frequency transmission line, it can operate monopolarly without causing the problem of three-phase asymmetry on the power frequency side.

[0007] Furthermore, the topologies of the first positive-frequency converter, the first negative-frequency converter, the second positive-frequency converter, and the second negative-frequency converter are the same, all being modular multilevel converters. Each modular multilevel converter includes three phase units. Each phase unit consists of an upper arm and a lower arm. Each arm is composed of a plurality of full-bridge sub-modules and an arm inductor connected in series. The three-phase ports on the input side of the modular multilevel converter are connected to the midpoints of the three phase units. The output sides of the three upper arms of the modular multilevel converter are connected in parallel and then connected to the first winding of the single-phase three-winding transformer. The output sides of the three lower arms of the modular multilevel converter are connected in parallel and then connected to the second winding of the single-phase three-winding transformer.

[0008] Furthermore, the three-phase voltages on the input side of the frequency converter are symmetrical, and the amplitudes of the two-phase voltages on the output side are the same, the frequencies are the same, and the phases are 180° out of phase with each other.

[0009] Furthermore, the single-phase three-winding transformer includes a first winding, a second winding, and a third winding, and the turns ratio is 1:1: K T ; The same-name terminal of the first winding is connected to the upper-arm port of the frequency converter, and the opposite-name terminal of the first winding is grounded; the opposite-name terminal of the second winding is connected to the lower-arm port of the frequency converter, and the same-name terminal of the second winding is grounded; the same-name terminal of the third winding of the positive single-phase three-winding transformer is connected to the low-frequency positive line, and the opposite-name terminal of the third winding of the positive single-phase three-winding transformer is grounded; the same-name terminal of the third winding of the negative single-phase three-winding transformer is grounded, and the opposite-name terminal of the third winding of the negative single-phase three-winding transformer is connected to the low-frequency negative line.

[0010] Furthermore, determine the state variables of the frequency converter control strategy based on the arm currents. Select three AC currents on the power frequency side, one AC current on the low frequency side, and the circulating currents of the two phase units of the frequency converter as six state variables. Define the internal electromotive forces corresponding to the six state variables as control quantities. The dynamic equations of the internal electromotive forces and the state variables satisfy the decoupling relationship. According to the decoupling relationship between the internal electromotive forces and the state variables, use the following inner loop controller for the frequency converter: The reference value of the internal electromotive force on the power frequency side is obtained through the power frequency side AC current controller or the power frequency side AC voltage controller; the input command of the power frequency side AC current controller is the reference value of the power frequency side AC current, which is obtained from the outer loop controller of the frequency converter; the input commands of the power frequency side AC voltage controller are the reference value of the power frequency side voltage amplitude and the reference value of the power frequency side voltage frequency, which are obtained from the outer loop controller; the reference value of the internal electromotive force on the low frequency side is obtained through the low frequency side AC current controller or the low frequency side AC voltage controller; the input command of the low frequency side AC current controller is the reference value of the low frequency side AC current, which is obtained from the outer loop controller; the input commands of the low frequency side AC voltage controller are the reference value of the low frequency side voltage amplitude and the reference value of the low frequency side voltage frequency, which are obtained from the outer loop controller; the reference value of the internal electromotive force of the circulating current is obtained through the circulating current controller, and the input command of the circulating current controller is the reference value of the circulating current, which is obtained from the outer loop controller.

[0011] Furthermore, use the following outer loop controller for the frequency converter: For the control objectives on the power frequency side, there are three control modes: U dc / Q Mode, control the capacitor voltages of each sub-module of the arm to be constant and control the reactive power of the power frequency side AC port to be constant; P / Q Mode, control the active power of the power frequency side AC port to be constant and control the reactive power of the power frequency side AC port to be constant; V / f Mode, control the voltage amplitude of the power frequency side AC port to be constant and control the voltage frequency of the power frequency side AC port to be constant; For the control objectives on the low frequency side, there are three control modes: U dc / Q Mode, control the capacitor voltages of each sub-module of the arm to be constant and control the reactive power of the low frequency side AC port to be constant; P / Q Mode, control the active power of the low frequency side AC port to be constant and control the reactive power of the low frequency side AC port to be constant; V / f Mode, control the voltage amplitude of the low frequency side AC port to be constant and control the voltage frequency of the low frequency side AC port to be constant; For the control objective of the circulating current, control the sum of the capacitor energies of the sub-modules of the six arms to achieve energy balance among the six arms.

[0012] Furthermore, in the true bipolar flexible low-frequency power transmission system, the power is transmitted from the sending end to the receiving end, that is, the power flows from the first industrial-frequency AC grid to the second industrial-frequency AC grid, or from the second industrial-frequency AC grid to the first industrial-frequency AC grid; for the sending-end frequency converter station, usually the U dc / Q mode is adopted on the industrial-frequency side, and the V / f mode is adopted on the low-frequency side; for the receiving-end frequency converter station, usually the P / Q mode is adopted on the industrial-frequency side, and the U dc / Q mode is adopted on the low-frequency side.

[0013] Furthermore, define the relationship between these six current state variables and the six arm currents as: , wherein, 、 、 are the AC currents of phase a, phase b, and phase c on the industrial-frequency side respectively, is the current of the third winding of the single-phase three-winding transformer, , are the circulating current components of the upper and lower arms of phase a and the upper and lower arms of phase b of the frequency converter respectively, , , are the currents of the upper arms of phase a, phase b, and phase c respectively, , , are the currents of the lower arms of phase a, phase b, and phase c respectively.

[0014] Even further, the relationship between the internal potential and the arm voltage is:

[0015] wherein, , , are the voltages of the upper arms of phase a, phase b, and phase c respectively; , , are the voltages of the lower arms of phase a, phase b, and phase c respectively; , , , , , is the internal potential, corresponding to 、 、 and 、 and ; L s is the arm inductor; is the leakage inductance of the single-phase three-winding transformer converted to the modular multilevel converter side, that is, the half-through inductance from the first winding or the second winding to the third winding.

[0016] Furthermore, in the true bipolar flexible low-frequency power transmission system of the present invention, when one of the low-frequency power transmission lines (such as the low-frequency positive line) fails, the circuit breakers at both ends of the line trip, and the frequency converter is blocked, and the power is continued to be transmitted through the other low-frequency power transmission line (such as the low-frequency negative line).

[0017] The beneficial effects of the present invention are as follows: The true bipolar flexible low-frequency power transmission system of the present invention provides a new topological structure for the energy transmission between AC power grids, realizing the interconnection and mutual assistance between different three-phase AC systems.

[0018] In the true bipolar flexible low-frequency power transmission system of the present invention, there are only the positive low-frequency line and the negative low-frequency line, a total of two-phase power transmission lines. Compared with the conventional three-phase power transmission system, the construction cost can be significantly reduced. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 is the topological structure diagram of the true bipolar flexible low-frequency power transmission system of the present invention; Figure 2 is the topological structure diagram of the frequency converter in the true bipolar flexible low-frequency power transmission system of the present invention; Figure 3 is the simulation waveform diagram of the port voltage of the true bipolar flexible low-frequency power transmission system of the present invention; Figure 4 is the structure diagram of the inner loop controller of the frequency converter in the true bipolar flexible low-frequency power transmission system of the present invention; Figure 5 is the structure diagram of the outer loop controller of the frequency converter in the true bipolar flexible low-frequency power transmission system of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0020] The present invention will be described in detail below with reference to the accompanying drawings and specific embodiments. This embodiment is implemented on the premise of the technical solution of the present invention, and gives the detailed implementation manner and specific operation process, but the protection scope of the present invention is not limited to the following embodiments.

[0021] Such as Figure 1As shown in the figure, the present invention is a true bipolar flexible low-frequency power transmission system, which consists of two frequency conversion stations (the first frequency conversion station and the second frequency conversion station), a low-frequency positive line and a low-frequency negative line. Each frequency conversion station includes two frequency converters (a positive frequency converter and a negative frequency converter). Each frequency converter is connected to the industrial frequency AC power grid through a three-phase transformer on one side and to the low-frequency transmission line through a single-phase three-winding transformer on the other side.

[0022] Specifically, the first frequency conversion station includes a first positive frequency converter and a first negative frequency converter; one side of the first positive frequency converter is connected to the first industrial frequency AC power grid through a three-phase transformer, and the other side is connected to the low-frequency positive line through a first positive single-phase three-winding transformer; one side of the first negative frequency converter is connected to the first industrial frequency AC power grid through a three-phase transformer, and the other side is connected to the low-frequency negative line through a first negative single-phase three-winding transformer.

[0023] Specifically, the second frequency conversion station includes a second positive frequency converter and a second negative frequency converter; one side of the second positive frequency converter is connected to the second industrial frequency AC power grid through a three-phase transformer, and the other side is connected to the low-frequency positive line through a second positive single-phase three-winding transformer; one side of the second negative frequency converter is connected to the second industrial frequency AC power grid through a three-phase transformer, and the other side is connected to the low-frequency negative line through a second negative single-phase three-winding transformer.

[0024] The low-frequency positive line and the low-frequency negative line form a true bipolar system, and their voltage amplitudes are the same, frequencies are the same, and phases are 180° out of phase with each other.

[0025] As Figure 2 shown, the topological structures of the first positive frequency converter, the first negative frequency converter, the second positive frequency converter, and the second negative frequency converter are the same, and they are all modular multilevel converters (MMC), which include three phase units. Each phase unit consists of an upper arm and a lower arm. Each arm is composed of multiple full-bridge sub-modules SM and an arm inductor L connected in series. The input-side three-phase ports of the MMC are connected to the midpoints of the three phase units. The output sides of the three upper arms of the MMC are connected in parallel and then connected to the first winding of the single-phase three-winding transformer. The output sides of the three lower arms of the MMC are connected in parallel and then connected to the second winding of the single-phase three-winding transformer. The three-phase voltages on the input side of the frequency converter are symmetrical, and the amplitudes of the two-phase voltages on the output side are the same, frequencies are the same, and phases are 180° out of phase with each other.

[0026] Figure 2 The connection manner of the MMC in Figure 1 is as K T , KT The value range of [[ ]] has no special limitation and is usually taken as 0.5 - 5. The same - named terminal of the first winding is connected to the upper - bridge - arm port of the frequency converter, and the different - named terminal of the first winding is grounded; the different - named terminal of the second winding is connected to the lower - bridge - arm port of the frequency converter, and the same - named terminal of the second winding is grounded; the same - named terminal of the third winding of the positive - pole single - phase three - winding transformer is connected to the low - frequency positive - pole line, and the different - named terminal of the third winding of the positive - pole single - phase three - winding transformer is grounded; the same - named terminal of the third winding of the negative - pole single - phase three - winding transformer is grounded, and the different - named terminal of the third winding of the negative - pole single - phase three - winding transformer is connected to the low - frequency negative - pole line.

[0027] As Figure 3 shown is the AC voltage waveform of the first frequency - conversion station. The 50Hz three - phase voltage on the power - frequency side is symmetrical, with an amplitude of 1kV. The positive - pole line voltage and the negative - pole line voltage on the low - frequency side have equal amplitudes and opposite phases, with an amplitude of 1kV. Then the amplitude of the inter - pole voltage of the low - frequency transmission line is 2kV.

[0028] For Figure 1 any one of the MMC frequency converters in the shown true - bipolar flexible low - frequency power - transmission system, the control method of internal - potential decoupling can be adopted. According to Figure 2 the frequency - converter topology in [[ ]], the dynamic equations of the six bridge arms can be expressed as: (1) Among them, , , are the voltages of phase a, phase b, and phase c of the power - frequency AC power grid respectively, , , are the currents of the upper - bridge arms of phase a, phase b, and phase c respectively; , , are the currents of the lower - bridge arms of phase a, phase b, and phase c respectively; , , are the voltages of the upper - bridge arms of phase a, phase b, and phase c respectively; , , are the voltages of the lower - bridge arms of phase a, phase b, and phase c respectively; is the bridge - arm inductance, , are the voltages of the first winding and the second winding of the single - phase three - winding transformer.

[0029] The voltage - current ratio relationship of the three windings of the single - phase three - winding transformer is: (2) Among them, is the voltage of the third winding of the single - phase three - winding transformer, , , are the currents of the first winding, the second winding, and the third winding of the single-phase three-winding transformer.

[0030] The dynamic equations among the three windings of the single-phase three-winding transformer are as follows: (3) where, is the leakage inductance of the single-phase three-winding transformer referred to the MMC (Modular Multilevel Converter) side, that is, the half-through inductance from the first winding or the second winding to the third winding.

[0031] The first winding, the second winding of the single-phase three-winding transformer, and the arm currents also satisfy the following relationship: (4) Combining equations (1) to (4), the dynamic equations of the arm voltage and current in matrix form can be obtained, satisfying the decoupling relationship: (5) The frequency converter topology of the present invention has 6 arms, that is, 6 control degrees of freedom. Six state variables need to be established. The input currents 、 、 on the three-phase power frequency grid side can be used as 3 state variables, and the current

[0032] (6) where, , are the circulating current components of the upper and lower arms of the a-phase of the frequency converter and the circulating current components of the upper and lower arms of the b-phase, respectively.

[0033] The relationship between the current of each state variable and the arm current can be expressed in matrix form as: (7) Define the relationship between the internal potential of the six arms of the frequency converter and the arm voltage as: (8) Figure 2 The decoupled dynamic equation of the internal potential of the MMC frequency converter topology shown is: (9) When the three-phase power frequency grid voltage on the input side is symmetrical, the above equation can be written as: (10) After the internal potential is decoupled, the dynamic equations of the power frequency side current, low frequency side current, and circulating current are obtained as follows: (11) (12) (13) According to equations (11) to (13), design Figure 4 The inner loop controller structure shown in Figure 4 In the upper right corner of the variable Indicates the reference value, PR is the proportional resonant controller, and PI is the proportional integral controller. Internal potential reference value on the power frequency side It can be obtained through the AC current controller on the power frequency side or through the AC voltage controller on the power frequency side. The input command of the AC current controller on the power frequency side is the current reference value on the power frequency side. , obtained by the outer loop controller. The input command of the AC voltage controller on the power frequency side is the voltage amplitude reference value on the power frequency side And the power frequency side voltage frequency reference value , obtained by the outer loop controller. The internal potential reference value on the low frequency side It can be obtained through the low-frequency side AC current controller or the low-frequency side AC voltage controller. The input command of the low-frequency side AC current controller is the current reference value of the third winding of the single-phase three-winding transformer , obtained by the outer loop controller. The input command of the low-frequency side AC voltage controller is the low-frequency side voltage amplitude reference value and low frequency side voltage frequency reference value , obtained by the outer loop controller. The circulating current component Internal potential reference value The input command of the circulating flow controller is the circulating flow reference value. , obtained by the outer loop controller.

[0034] Figure 5 The figure shows the outer loop controller of the MMC inverter. For the control target on the power frequency side, there are three control modes: U dc / Q mode, controlling the capacitor voltage of each submodule of the bridge arm to be constant and controlling the reactive power of the AC port on the power frequency side to be constant; P / Q Mode, control the active power of the AC port on the power frequency side to be constant, and control the reactive power of the AC port on the power frequency side to be constant; V / f Mode, control the voltage amplitude of the AC port on the power frequency side to be constant, and control the voltage frequency of the AC port on the power frequency side to be constant. For the control target on the low frequency side, there are three control modes: Udc / Q Mode, controlling the capacitor voltages of each sub-module of the bridge arm to be constant and controlling the reactive power of the low-frequency side AC port to be constant; P / Q Mode, controlling the active power of the low-frequency side AC port to be constant and controlling the reactive power of the low-frequency side AC port to be constant; V / f Mode, controlling the voltage amplitude of the low-frequency side AC port to be constant and controlling the voltage frequency of the low-frequency side AC port to be constant. For the control objective of the circulating current, control the sum of the capacitor energies of the sub-modules of the six bridge arms to achieve the mutual balance of energy among the six bridge arms.

[0035] For the true bipolar flexible low-frequency power transmission system described above, its power is transmitted from the sending end to the receiving end, that is, the power flows from the first industrial-frequency AC grid to the second industrial-frequency AC grid, or from the second industrial-frequency AC grid to the first industrial-frequency AC grid. For the MMC of the sending-end frequency conversion station, usually U dc / Q Mode is adopted on the industrial-frequency side and V / f Mode is adopted on the low-frequency side. For the MMC of the receiving-end frequency conversion station, usually P / Q Mode is adopted on the industrial-frequency side and U dc / Q Mode is adopted on the low-frequency side.

[0036] The true bipolar flexible low-frequency power transmission system of the present invention provides a new topological structure for the energy transmission between AC grids, realizing the interconnection and mutual assistance between different three-phase AC systems; there are only a positive low-frequency line and a negative low-frequency line, a total of two-phase transmission lines, and compared with the conventional three-phase transmission system, it can significantly reduce the construction cost.

[0037] In the true bipolar flexible low-frequency power transmission system of the present invention, the current is easy to be interrupted when passing through zero, the manufacturing difficulty and cost of the low-frequency circuit breaker are significantly lower than those of the DC circuit breaker, and the low-frequency transformer is similar to the industrial-frequency transformer, which is easy to form a low-frequency power transmission network.

[0038] For the true bipolar flexible low-frequency power transmission system of the present invention, after one circuit fails and is cut off, the power can continue to be transmitted through the other circuit, and the three-phase current on the industrial-frequency side can still remain symmetrical without additional negative-sequence control.

[0039] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that: they can still modify the technical solutions recorded in the foregoing embodiments, or perform equivalent replacements on some of the technical features; and these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the various embodiments of the present invention, and should all be included within the protection scope of the present invention.

Claims

1. A true bipolar flexible low-frequency power transmission system, characterized in that: It includes a first frequency conversion station, a second frequency conversion station, a low-frequency positive line and a low-frequency negative line; The first frequency conversion station includes a first positive frequency converter and a first negative frequency converter; one side of the first positive frequency converter is connected to the first power frequency AC power grid through a three-phase transformer, and the other side is connected to the low-frequency positive line through a first positive single-phase three-winding transformer; one side of the first negative frequency converter is connected to the first power frequency AC power grid through a three-phase transformer, and the other side is connected to the low-frequency negative line through a first negative single-phase three-winding transformer; The second frequency conversion station includes a second positive frequency converter and a second negative frequency converter; one side of the second positive frequency converter is connected to the second power frequency AC power grid through a three-phase transformer, and the other side is connected to the low-frequency positive line through a second positive single-phase three-winding transformer; one side of the second negative frequency converter is connected to the second power frequency AC power grid through a three-phase transformer, and the other side is connected to the low-frequency negative line through a second negative single-phase three-winding transformer; The low-frequency positive circuit and the low-frequency negative circuit have the same voltage amplitude, the same frequency and a phase difference of 180°.

2. The true bipolar flexible low-frequency power transmission system according to claim 1, characterized in that: The first positive inverter, the first negative inverter, the second positive inverter and the second negative inverter have the same topological structure, which are all modular multilevel converters, including three phase units, each phase unit consists of an upper bridge arm and a lower bridge arm, each bridge arm is composed of multiple full-bridge sub-modules and a bridge arm inductor connected in series, the input side three-phase port of the modular multilevel converter is connected to the midpoint of the three phase units, the output sides of the three upper bridge arms of the modular multilevel converter are connected in parallel to the first winding of the single-phase three-winding transformer, and the output sides of the three lower bridge arms of the modular multilevel converter are connected in parallel to the second winding of the single-phase three-winding transformer.

3. The true bipolar flexible low-frequency power transmission system according to claim 1, characterized in that: The three-phase voltage on the input side of the inverter is symmetrical, and the two-phase voltage on the output side has the same amplitude, frequency and a phase difference of 180°.

4. The true bipolar flexible low-frequency power transmission system according to claim 1, characterized in that: The single-phase three-winding transformer includes the first winding, the second winding and the third winding, and the transformation ratio is 1:1: K T ; The same-name end of the first winding is connected to the upper bridge arm port of the inverter, and the opposite-name end of the first winding is grounded; the opposite-name end of the second winding is connected to the lower bridge arm port of the inverter, and the same-name end of the second winding is grounded; the same-name end of the third winding of the positive single-phase three-winding transformer is connected to the low-frequency positive line, and the opposite-name end of the third winding of the positive single-phase three-winding transformer is grounded; the same-name end of the third winding of the negative single-phase three-winding transformer is grounded, and the opposite-name end of the third winding of the negative single-phase three-winding transformer is connected to the low-frequency negative line.

5. The true bipolar flexible low-frequency power transmission system according to claim 1, characterized in that: The state variables of the inverter control strategy are determined according to the bridge arm current. Three AC currents on the power frequency side, one AC current on the low frequency side, and the circulating currents of the two phase units of the inverter are selected as six state variables. The internal potential corresponding to the six state variables is defined as the control quantity. The dynamic equations of the internal potential and the state variables satisfy the decoupling relationship. According to the decoupling relationship between the internal potential and the state variables, the following inverter inner loop controller is adopted: the internal potential reference value on the power frequency side is obtained by the power frequency side AC current controller or the power frequency side AC voltage controller; the input instruction of the power frequency side AC current controller is the power frequency side AC current reference value, which is obtained by the inverter outer loop controller; The input instructions of the AC voltage controller on the power frequency side are the voltage amplitude reference value and the voltage frequency reference value on the power frequency side, which are obtained by the outer loop controller; the internal potential reference value on the low frequency side is obtained through the AC current controller on the low frequency side or the AC voltage controller on the low frequency side; the input instruction of the AC current controller on the low frequency side is the AC current reference value on the low frequency side, which is obtained by the outer loop controller; the input instructions of the AC voltage controller on the low frequency side are the voltage amplitude reference value and the voltage frequency reference value on the low frequency side, which are obtained by the outer loop controller; the internal potential reference value of the circulating current is obtained through the circulating current controller, and the input instruction of the circulating current controller is the circulating current reference value, which is obtained by the outer loop controller.

6. The true bipolar flexible low-frequency power transmission system according to claim 5, characterized in that: The following inverter outer loop controller is used: For the control target on the power frequency side, there are three control modes: U dc / Q mode, controlling the capacitor voltage of each submodule of the bridge arm to be constant and controlling the reactive power of the AC port on the power frequency side to be constant; P / Q Mode, control the active power of the AC port on the power frequency side to be constant and control the reactive power of the AC port on the power frequency side to be constant; V / f Mode, control the voltage amplitude of the AC port on the power frequency side to be constant and control the voltage frequency of the AC port on the power frequency side to be constant; for the control target on the low frequency side, there are three control modes: U dc / Q mode, controlling the capacitor voltage of each submodule of the bridge arm to be constant and controlling the reactive power of the AC port on the low-frequency side to be constant; P / Q Mode, control the active power of the AC port on the low frequency side to be constant and control the reactive power of the AC port on the low frequency side to be constant; V / f mode, control the voltage amplitude of the AC port on the low-frequency side to be constant and control the voltage frequency of the AC port on the low-frequency side to be constant; for the control target of the circulating current, control the energy sum of the sub-module capacitors of the six bridge arms to achieve mutual energy balance among the six bridge arms.

7. The true bipolar flexible low-frequency power transmission system according to claim 6, characterized in that: The power of the true bipolar flexible low-frequency transmission system is transmitted from the sending end to the receiving end, that is, the power flows from the first power frequency AC grid to the second power frequency AC grid, or from the second power frequency AC grid to the first power frequency AC grid; for the sending end frequency conversion station, the power frequency side is adopted U dc / Q mode, using the low frequency side V / f Mode; for the receiving end frequency conversion station, the power frequency side is adopted P / Q mode, using the low frequency side U dc / Q model.

8. The true bipolar flexible low-frequency power transmission system according to claim 5, characterized in that: The relationship between the six current state variables and the six bridge arm currents is defined as: ， in, 、 、 They are the AC currents of phase a, phase b, and phase c on the power frequency side. is the current of the third winding of a single-phase three-winding transformer, , They are the circulating current components of the upper and lower bridge arms of phase a and phase b of the inverter, respectively. , , They are the currents of the upper bridge arms of phases a, b, and c respectively. , , They are the currents of the lower bridge arms of phase a, phase b, and phase c respectively.

9. The true bipolar flexible low-frequency power transmission system according to claim 8, characterized in that: The relationship between the internal potential and the bridge arm voltage is: ， in, , , They are the voltages of the upper bridge arms of phase a, phase b, and phase c respectively; , , They are the voltages of the lower bridge arms of phase a, phase b, and phase c respectively; , , , , , is the internal potential, corresponding to 、 、 , 、 , ; L s is the bridge arm inductance; It is the leakage inductance of the single-phase three-winding converted to the modular multi-level converter side, that is, the half-through inductance from the first winding or the second winding to the third winding.

10. The true bipolar flexible low-frequency power transmission system according to claim 1, characterized in that: When one of the low-frequency transmission lines fails, the circuit breakers at both ends of the line trip, the inverter is locked, and power continues to be transmitted through the other low-frequency transmission line.

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