High-voltage direct-current pseudo bipolar-true bipolar conversion system and control method thereof

Abstract

The invention discloses a high-voltage direct-current pseudo bipolar-true bipolar conversion system and a control method thereof.The system comprises two direct-current voltage conversion devices connected with a pseudo bipolar direct-current system and a true bipolar direct-current system, each voltage shifter comprises two MMC which are connected in series in a positive direct-current line and a negative direct-current line of a pseudo bipolar respectively, and the two MMC voltage conversion devices are connected with the true bipolar direct-current system. AC sides are connected in parallel to realize power exchange, two voltage shifters are adopted to realize conversion from pseudo bipolar to true bipolar, and grounding points of the two voltage shifters are connected to form a true bipolar symmetrical structure. In + / -400 kV / + / -800 kV application, each MMC only bears 400 kV voltage, and compared with a traditional high-voltage large-capacity direct-current transformer scheme, the number of sub-modules is reduced to 66.7%. The invention provides an innovative technical scheme for efficient interconnection of an offshore wind power pseudo bipolar direct current system and an onshore true bipolar direct current system, has the advantages of small number of sub-modules, low cost, high flexibility and the like, and has important engineering application value and economic benefit.

Description

Technical Field

[0001] This invention belongs to the field of high voltage direct current transmission technology in power systems, specifically relating to a high voltage direct current pseudo-bipolar to true bipolar conversion system and its control method. Background Technology

[0002] With the large-scale development of offshore wind power, high-voltage direct current (HVDC) transmission technology has become an important technical means for grid connection of offshore wind power. Offshore wind power HVDC transmission systems typically adopt a pseudo-bipolar structure, where the positive and negative poles operate independently, using seawater or metal return lines as the return path, and the neutral point is grounded. This structure offers advantages such as fewer devices, simpler structure, and lower cost on offshore platforms. In contrast, onshore HVDC transmission networks typically employ a true bipolar structure, where the positive and negative poles are insulated from the ground, and the two poles are connected through a load center. This structure offers advantages such as high power supply reliability, minimal impact from single-pole faults, and symmetrical potential to ground.

[0003] Currently, existing technologies for achieving pseudo-bipolar and true bipolar DC interconnection in high-voltage DC scenarios typically use cascaded submodule-type isolated DC / DC converters as the basic unit. Through a topology with parallel inputs and series outputs, they convert unipolar DC voltage into balanced, symmetrical positive and negative outputs. Submodule cascading technology based on Modular Multilevel Converters (MMCs) employs a directly coupled autotransformer topology, eliminating the bulky intermediate frequency isolation transformer and removing the "DC-AC-DC" conversion process. It directly achieves the conversion from unipolar input to symmetrical positive and negative bipolar outputs on the circuit through specific submodule control logic. The literature [Guo Lingyu, Yao Gang, Yin Zhizhu. Bipolar output non-isolated autotransformer suitable for DC interconnection of offshore wind farms [J]. Power System Technology, 2020, 44(1): 174-184] improves the traditional face-to-face structure. It adopts a set of multiplexed converter modules on the unipolar side (inverter side), while the bipolar side (rectifier side) is connected through two independent transformers and rectifier branches. This scheme is an isolated scheme. Its innovation lies in the fact that the inverter part on the unipolar side is shared, and two rectifier branches are led out through two isolation transformers, which correspond to the two polarities of true bipolarity respectively.

[0004] However, the connection scheme between the aforementioned offshore wind power pseudo-bipolar DC system and the onshore true bipolar DC system mainly faces the following problems: 1. Voltage level mismatch. Offshore wind power typically uses relatively low DC voltage levels, such as ±320kV or ±400kV, for economic and technological maturity considerations; while onshore long-distance DC transmission often uses higher voltage levels, such as ±500kV, ±800kV, or even higher, to reduce transmission losses and the width of DC line corridors. There is a significant voltage level difference between the two, which requires voltage level conversion.

[0005] 2. Differences in topology. Pseudo-bipolar and true bipolar systems differ fundamentally in grounding methods, neutral point potential, fault characteristics, and protection configurations. Pseudo-bipolar systems allow for asymmetrical voltages between the two poles and ground, while true bipolar systems require strictly symmetrical voltages between the two poles and ground. Direct connection between the two poles can lead to problems such as grounding current and potential distribution.

[0006] 3. Traditional solutions are costly and inefficient. In existing technologies, high-voltage, high-capacity DC voltage level conversion mainly adopts DC transformer technology based on modular multilevel converters. A typical solution uses two independent MMC DC transformers, which are connected to the positive and negative poles of the pseudo-bipolar system respectively. The DC side of each MMC DC transformer needs to bear the entire output voltage, resulting in a huge number of sub-modules required.

[0007] Therefore, there is an urgent need to propose a new type of DC voltage conversion device and system scheme that can effectively connect offshore wind power pseudo-bipolar DC systems with onshore true bipolar DC systems with fewer sub-modules, lower cost, and greater flexibility. Summary of the Invention

[0008] In view of the above, the present invention provides a high-voltage DC pseudo-bipolar to true bipolar conversion system and its control method, which has the advantages of fewer sub-modules, lower cost, and higher flexibility, and has significant engineering application value and economic benefits.

[0009] A high-voltage DC pseudo-bipolar to true bipolar conversion system includes a pseudo-bipolar DC system, a true bipolar DC system, and a DC voltage conversion device connecting the two. The DC voltage conversion device is used for voltage level conversion and topology conversion between the two and consists of two voltage shifters. The voltage shifter is a series-type DC voltage shifting device, which includes two MMCs. The AC sides of the two MMCs are connected in parallel to the same AC system, and the DC sides of the two MMCs are connected in series in two independent DC lines, respectively, to boost or reduce the voltage of the two DC lines. The two MMCs realize the voltage shifting function of the two DC lines through power exchange on the AC side.

[0010] Furthermore, the positive and negative DC terminals of the MMC in the voltage regulator are respectively connected to two different potential points of the DC line. The DC side voltages of the two MMCs are equal or can be controlled independently, and power balance is achieved through AC side connection. The voltage regulation amplitude of the two MMCs on their respective lines is equal, and internal power balance is maintained through AC side power exchange.

[0011] Furthermore, the DC voltage conversion device includes a first voltage shifter and a second voltage shifter. The first voltage shifter includes two MMCs, namely MMC1 and MMC2, and the second voltage shifter includes two MMCs, namely MMC3 and MMC4. The negative DC terminal of MMC1 is connected to the positive DC line of the pseudo-bipolar DC system, and the positive DC terminal is connected to the positive DC line of the true bipolar DC system. The negative DC terminal of MMC2 is connected to the negative DC line of the pseudo-bipolar DC system, and the positive DC terminal is grounded. The positive DC terminal of MMC3 is connected to the positive DC line of the pseudo-bipolar DC system, and the negative DC terminal is grounded. The positive DC terminal of MMC4 is connected to the negative DC line of the pseudo-bipolar DC system, and the negative DC terminal is connected to the negative DC line of the true bipolar DC system.

[0012] Furthermore, the voltage levels of the positive and negative DC lines of the pseudo-bipolar DC system are +U1 and -U1, respectively, and the return path is earth or a metallic return line; the voltage levels of the positive and negative DC lines of the true bipolar DC system are +U2 and -U2, respectively, and the two poles are insulated from the ground; where U2 > U1.

[0013] Furthermore, the first voltage shifter is used to boost the positive and negative potentials of the DC line of the pseudo-bipolar DC system from (+U1, -U1) to (+U2, 0) respectively, and the second voltage shifter is used to depress the positive and negative potentials of the DC line of the pseudo-bipolar DC system from (+U1, -U1) to (0, -U2) respectively. The grounding points of the two voltage shifters are physically connected together to form the neutral point of the true bipolar DC system, ensuring the strict symmetry of the voltage between the two poles and ground of the true bipolar DC system.

[0014] Furthermore, the rated DC-side voltages of MMC1, MMC2, MMC3, and MMC4 are all U2-U1. Compared to the traditional DC transformer scheme using a rated DC-side voltage of U2, the number of sub-modules required by the system of the present invention is reduced to (U2-U1) / U2 of the traditional scheme.

[0015] Furthermore, the pseudo-bipolar DC system is connected to an offshore wind farm, and the true bipolar DC system is connected to an onshore AC power grid.

[0016] The control method for the above-mentioned high-voltage direct current pseudo-bipolar to true bipolar conversion system is as follows: The DC voltage of MMC1 is controlled to be U2-U1, which boosts the positive DC line of the pseudo-bipolar DC system from +U1 to +U2; the DC voltage of MMC2 is controlled to be U2-U1, which boosts the negative DC line of the pseudo-bipolar DC system from -U1 to 0; the DC voltage of MMC3 is controlled to be U2-U1, which drops the positive DC line of the pseudo-bipolar DC system from +U1 to 0; the DC voltage of MMC4 is controlled to be U2-U1, which drops the negative DC line of the pseudo-bipolar DC system from -U1 to -U2; at the same time, the DC voltage stability and power balance of each MMC are maintained through the AC side power control of the MMC.

[0017] Furthermore, when a unipolar fault occurs in a pseudo-bipolar DC system, the faulty pole is isolated and the healthy pole is derated by adjusting the DC voltage and power distribution of the MMC in the corresponding voltage shifter; when a unipolar fault occurs in a true bipolar DC system, the faulty pole is isolated and the system continues to operate by coordinating the control of the two voltage shifters.

[0018] Based on the above technical solution, the present invention has the following beneficial technical effects: 1. Significantly reduced equipment cost and wear, improving system economy. Because this invention employs a series-type DC voltage shifting structure, its MMC no longer needs to bear the full DC voltage on the output side like traditional parallel-type DC transformers, but only the voltage difference between the pseudo-bipolar and true bipolar systems. Based on causal deduction, the decrease in the rated DC voltage directly leads to a proportional reduction in the number of sub-modules required in series for each bridge arm, thereby significantly reducing the construction cost, footprint, and switching losses of the converter valve during operation. In typical offshore wind boosting scenarios, the reduction in the number of sub-modules greatly improves the engineering feasibility of high-voltage, high-capacity DC-DC conversion schemes.

[0019] 2. This invention achieves decoupling switching between pseudo-bipolar and true bipolar topologies, ensuring the symmetry and stability of the power grid operation. Through voltage reorganization of the four MMC modules in the first and second voltage shifters, this invention cleverly transforms the asymmetrical positive and negative voltages relative to ground potential in the pseudo-bipolar system into the symmetrical potentials required by the true bipolar system. By grounding the DC positive terminal of MMC2 and the DC negative terminal of MMC3 together, a zero-potential neutral point of the true bipolar system is forcibly constructed at the physical level. This solves the problems of grounding current disturbance, neutral point displacement, and uneven potential distribution that may occur when a pseudo-bipolar system is directly connected to an onshore true bipolar power grid, thus ensuring the operational safety of the onshore main grid.

[0020] 3. Excellent scalability and voltage matching capability. Because the voltage shifter is connected to the line in series, the voltage gain at both ends can be dynamically fine-tuned according to the construction needs of different stages of the offshore wind farm. Even if the output voltage of the offshore wind power system fluctuates, or the onshore power grid has adjustment requirements for the DC voltage level, a wide range of voltage matching can be achieved simply by changing the modulation ratio of the MMC, without the need to replace large-capacity transformers or main primary equipment, which significantly improves the flexibility of DC transmission network interconnection. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the topology of the series-type DC voltage shifter in this invention.

[0022] Figure 2 This is a schematic diagram of the topology of the high-voltage DC pseudo-bipolar to true bipolar conversion system of the present invention.

[0023] Figure 3 The diagram shows the simulated voltage waveforms of the positive and negative terminals of the pseudo-bipolar DC system and the true bipolar DC system connected to the voltage shifter of this invention. The horizontal axis represents time (in seconds), and the vertical axis represents voltage (in kV).

[0024] Figure 4 The figure shows the simulation waveforms of active and reactive power output from the voltage shifter of this invention to the connected AC system. The horizontal axis represents time (in seconds), and the vertical axis represents power (in MVar). Detailed Implementation

[0025] To describe the present invention in more detail, the technical solution of the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0026] like Figure 1 As shown, the present invention provides a series DC voltage shifter, which consists of two modular multilevel converters (MMCs). These two MMCs are connected in parallel to the same AC system on the AC side. The shifter raises or lowers the series DC branch voltage through its internal MMCs.

[0027] like Figure 2 As shown, taking pressure shifter 1 (upper pressure shifter) as an example, it includes MMC1 and MMC2.

[0028] MMC1: Responsible for voltage boosting of the positive line. Its DC negative terminal is connected to the pseudo-bipolar positive line (potential +U1), and its DC positive terminal is connected to the true bipolar positive line (potential +U2). MMC1 absorbs active power from the AC system, creating a voltage boost of U2-U1 on its DC side, thereby raising the potential of +U1 to +U2. If the DC direction is defined as flowing from the pseudo-bipolar to the true bipolar, then MMC1 operates in rectification mode.

[0029] MMC2: Responsible for voltage boosting of the negative line. Its DC negative terminal is connected to the pseudo-bipolar negative line (potential -U1), and its DC positive terminal is connected to the ground point (potential 0kV). MMC2 absorbs active power from the AC system, creating a voltage boost of 0 - (-U1) = U1 on its DC side, thereby raising the potential of -U1 to 0kV. If the DC direction is defined as flowing from the pseudo-bipolar to the true bipolar, then MMC2 operates in rectification mode.

[0030] The AC sides of MMC1 and MMC2 are connected in parallel to the same AC bus, which means that they can flexibly exchange active and reactive power to achieve their respective DC voltage control and maintain AC side power balance.

[0031] like Figure 2 As shown, the high-voltage direct current pseudo-bipolar to true bipolar conversion system of this invention mainly comprises three parts: an offshore wind power pseudo-bipolar DC system, an onshore true bipolar DC system, and a DC voltage conversion device connecting the two. The offshore wind power pseudo-bipolar DC system uses a ±400kV voltage level, including a positive line and a negative line. The positive line has a ground potential of +400kV, the negative line has a ground potential of -400kV, and the neutral point is grounded. The onshore true bipolar DC system uses a ±800kV voltage level, also including a positive line and a negative line. The positive line has a ground potential of +800kV, and the negative line has a ground potential of -800kV. The two systems are connected via a DC voltage conversion device to achieve voltage level and topology conversion.

[0032] From the perspective of the overall system potential distribution, the positive electrode potential of the true bipolar system is +800kV. After being stepped down by 400kV through MMC1, it reaches +400kV at the pseudo-bipolar positive electrode. Then, passing through the positive electrode line of the offshore wind power system, it maintains a +400kV potential at the other end of the pseudo-bipolar positive electrode. Finally, after being stepped down by 400kV through MMC3, it reaches 0kV at the grounding point. This completes the potential transition process of the positive electrode line from +800kV through +400kV to finally reaching 0kV at grounding. Similarly, the negative electrode line starts from 0kV at grounding, is stepped down by 400kV through MMC2 (actually from 0kV to -400kV; "stepped down" here refers to a decrease in potential value) to reach -400kV at the pseudo-bipolar negative electrode. After passing through the negative electrode line of the offshore wind power system, it maintains a -400kV potential at the other end of the pseudo-bipolar negative electrode. Finally, after being stepped down by 400kV through MMC4, it reaches -800kV at the true bipolar negative electrode. The grounding points of the two voltage shifters are physically connected together, forming the neutral point of the true bipolar system, which ensures the strict symmetry of the voltage between the two poles of the true bipolar system and the ground.

[0033] Each MMC in the voltage shifter adopts the standard topology of a modular multilevel converter, specifically a three-phase double-star structure, with each phase containing two arms, for a total of six arms. Each arm consists of several half-bridge submodules connected in series. The rated voltage of each submodule is 2.5kV, and each submodule contains two IGBT switching devices, two anti-parallel diodes, and one DC capacitor. To enable the MMC to withstand a DC voltage of 400kV, each arm requires 400kV divided by 2.5kV, which equals 160 submodules. Therefore, a single MMC with six arms requires a total of 960 submodules; the entire conversion system contains four MMCs, requiring a total of 3840 submodules.

[0034] In contrast to the present invention, there is a traditional DC transformer solution. The traditional solution also requires four MMCs to connect the pseudo-bipolar and true bipolar systems. However, two of the MMCs bear a 400kV DC voltage, while the other two MMCs bear the entire 800kV DC voltage of the true bipolar side. Using the same submodule rated voltage of 2.5kV, the traditional solution requires 800kV divided by 2.5kV, which equals 320 submodules per bridge arm. A single MMC requires 1920 submodules, and all four MMCs require a total of 5760 submodules.

[0035] As can be seen from the comparison, the number of sub-modules in the present invention is 3840, which is only 66.7% of the 5760 in the traditional solution. The number of sub-modules is reduced by one-third, which directly leads to a significant reduction in equipment cost, floor space, and cooling system scale.

[0036] The active power control objective of each MMC in the voltage shifter is to maintain the DC side voltage of the MMC stable at the rated value, while the reactive power control objective is to control the reactive power on the AC side. The two MMCs in the same voltage shifter automatically achieve active power balance through AC side connection.

[0037] Since the AC sides of the first voltage regulators MMC1 and MMC2 are connected in parallel, they automatically achieve active power balance through the AC bus. Even if their DC power differs slightly due to differences in line parameters, the power difference will be automatically balanced through energy exchange on the AC side. Let the DC power of MMC1 and MMC2 be respectively... P dc1 and P dc2 The AC power is respectively P ac1 and P ac2 Then we have:

[0038]

[0039] The AC bus power balance condition is:

[0040] In the formula: P ac,ext1 This represents the net power absorbed by the first voltage regulator from the external AC system. Since both MMC1 and MMC2 are controlled by DC voltage, their DC voltages will automatically adjust to the balance point, ensuring that the AC power satisfies the above equation.

[0041] Similarly, the second voltage regulators, MMC3 and MMC4, also achieve power balancing via the AC side:

[0042] If two voltage regulators are connected to the same AC system, the power balance condition for the entire system is:

[0043] That is, the power absorbed by the first voltage regulator from the AC system is equal to the power output by the second voltage regulator to the AC system; the AC system itself does not provide net power. In actual operation, due to power losses, the right side of the following equation should represent the total losses:

[0044] This portion of the loss needs to be supplied by the AC system.

[0045] In terms of reactive power control, the four MMCs can coordinate and work together to support the AC bus voltage. Let the AC bus voltage be... U ac The reactive power output of each MMC is Q ac1 , Q ac2 , Q ac3 , Q ac4 Then the total reactive power is:

[0046] By adjusting the q-axis current reference values ​​of each MMC It can control reactive power output. For example, when the AC bus voltage is lower than the rated value, the reactive power output of each MMC is increased (capacitive) to inject reactive power into the AC system to support the voltage; when the AC bus voltage is higher than the rated value, the reactive power output is reduced or reactive power is absorbed (inductive) to lower the voltage. Through this coordinated control, the voltage shifter can support the voltage of the connected AC system.

[0047] Below, we verify the series-type DC voltage shifting device of this invention in a high-voltage DC pseudo-bipolar offshore wind power transmission system and an onshore DC true bipolar system. The simulation is based on PSCAD / EMTDC software, and the system topology is as follows. Figure 2 As shown.

[0048] Figure 3 Simulated waveforms of the positive and negative voltages (U1p, U1n) of the pseudo-bipolar DC system connected to the voltage regulator and the positive and negative voltages (U2p, U2n) of the true bipolar DC system are displayed.

[0049] Pseudo-bipolar DC system voltage: Figure 3 The upper half of the image clearly shows the positive voltage U1p and negative voltage U1n of the pseudo-bipolar DC system. During the simulation period from 2.00 seconds to 4.00 seconds, U1p stabilized at approximately +400kV, while U1n stabilized at approximately -400kV. This indicates that the pseudo-bipolar system operates stably, with its voltage levels precisely maintained within the preset ±400kV range.

[0050] True bipolar DC system voltage: Figure 3 The lower half depicts the positive voltage U2p and negative voltage U2n of the true bipolar DC system after conversion by the voltage shifter; throughout the simulation period, U2p stabilizes at approximately +800kV and U2n stabilizes at approximately -800kV.

[0051] from Figure 3 As can be seen, the ±400kV voltage of the pseudo-bipolar DC system was successfully converted to the ±800kV voltage of the true bipolar DC system, and the conversion process was smooth, with the DC voltage remaining highly stable near its rated value. This strongly verifies that the voltage shifter structure proposed in this invention can effectively realize the conversion of DC voltage levels and the conversion from pseudo-bipolar to true bipolar topology, and that the output true bipolar voltage is symmetrical with respect to ground, meeting the requirements of a true bipolar HVDC system.

[0052] Figure 4 The simulated waveforms of the active power (Ps1) and reactive power (Qs1) output by the voltage shifter to the connected AC system are shown.

[0053] From 2.00 to 3.00 seconds: During this period, both Ps1 and Qs1 stabilized at levels close to zero. This indicates that in the early stages of steady-state operation, there was no significant net active or reactive power exchange between the voltage shifter and the AC system. In other words, the power balance between the various MMCs through the AC system reached a relatively static equilibrium point. Furthermore, the reactive power control of the voltage shifter prevented it from generating additional reactive power impact on the AC system, thus maintaining the stability of the AC system.

[0054] At 3.00 seconds, the reactive power command value of the voltage shifter was changed, causing Qs1 to rapidly rise from near zero and stabilize at approximately 320 Mvar, while Ps1 experienced minor fluctuations before returning to 0. This simulated a scenario of AC system voltage drop or increased reactive power demand. By compensating for reactive power, the MMC effectively supported the stability of the AC grid voltage, verifying that the MMC, as a voltage source converter, possesses strong reactive power regulation capabilities and AC voltage support functions.

[0055] The above description of the embodiments is provided to enable those skilled in the art to understand and apply the present invention. Those skilled in the art can readily make various modifications to the above embodiments and apply the general principles described herein to other embodiments without creative effort. Therefore, the present invention is not limited to the above embodiments, and any improvements and modifications made to the present invention by those skilled in the art based on the disclosure thereof should be within the scope of protection of the present invention.

Claims

1. A high-voltage direct current pseudo-bipolar to true bipolar conversion system, characterized in that: It includes a pseudo-bipolar DC system, a true bipolar DC system, and a DC voltage conversion device connecting the two. The DC voltage conversion device is used for voltage level conversion and topology conversion between the two and consists of two voltage shifters. The voltage shifter is a series-type DC voltage shifting device, which includes two MMCs. The AC sides of the two MMCs are connected in parallel to the same AC system, and the DC sides of the two MMCs are connected in series in two independent DC lines, respectively, to boost or reduce the voltage of the two DC lines. The two MMCs realize the voltage shifting function of the two DC lines through power exchange on the AC side.

2. The high-voltage direct current pseudo-bipolar to true bipolar conversion system according to claim 1, characterized in that: The DC positive and DC negative terminals of the MMC in the voltage regulator are respectively connected to two different potential points of the DC line. The DC side voltages of the two MMCs are equal or can be controlled independently, and power balance is achieved through AC side connection. The voltage regulation amplitude of the two MMCs on their respective lines is equal, and internal power balance is maintained through AC side power exchange.

3. The high-voltage direct current pseudo-bipolar to true bipolar conversion system according to claim 1, characterized in that: The DC voltage conversion device includes a first voltage shifter and a second voltage shifter. The first voltage shifter includes two MMCs, namely MMC1 and MMC2, and the second voltage shifter includes two MMCs, namely MMC3 and MMC4. The negative DC terminal of MMC1 is connected to the positive DC line of the pseudo-bipolar DC system, and the positive DC terminal is connected to the positive DC line of the true bipolar DC system. The negative DC terminal of MMC2 is connected to the negative DC line of the pseudo-bipolar DC system, and the positive DC terminal is grounded. The positive DC terminal of MMC3 is connected to the positive DC line of the pseudo-bipolar DC system, and the negative DC terminal is grounded. The positive DC terminal of MMC4 is connected to the negative DC line of the pseudo-bipolar DC system, and the negative DC terminal is connected to the negative DC line of the true bipolar DC system.

4. The high-voltage direct current pseudo-bipolar to true bipolar conversion system according to claim 3, characterized in that: The voltage levels of the positive and negative DC lines of the pseudo-bipolar DC system are +U1 and -U1, respectively, and the return path is earth or a metallic return line; the voltage levels of the positive and negative DC lines of the true bipolar DC system are +U2 and -U2, respectively, and the two poles are insulated from the ground; where U2 > U1.

5. The high-voltage direct current pseudo-bipolar to true bipolar conversion system according to claim 4, characterized in that: The first voltage shifter is used to boost the positive and negative potentials of the DC line of the pseudo-bipolar DC system from (+U1, -U1) to (+U2, 0) respectively. The second voltage shifter is used to depress the positive and negative potentials of the DC line of the pseudo-bipolar DC system from (+U1, -U1) to (0, -U2) respectively. The grounding points of the two voltage shifters are physically connected together to form the neutral point of the true bipolar DC system, ensuring the strict symmetry of the voltage between the two poles and ground in the true bipolar DC system.

6. The high-voltage direct current pseudo-bipolar to true bipolar conversion system according to claim 4, characterized in that: The rated DC voltage of MMC1, MMC2, MMC3, and MMC4 is U2-U1.

7. The high-voltage direct current pseudo-bipolar to true bipolar conversion system according to claim 1, characterized in that: The pseudo-bipolar DC system is connected to an offshore wind farm, while the true bipolar DC system is connected to an onshore AC power grid.

8. The control method for the high-voltage direct current pseudo-bipolar to true bipolar conversion system as described in any one of claims 4 to 6, characterized in that: The DC voltage of MMC1 is controlled to be U2-U1, which boosts the positive DC line of the pseudo-bipolar DC system from +U1 to +U2; the DC voltage of MMC2 is controlled to be U2-U1, which boosts the negative DC line of the pseudo-bipolar DC system from -U1 to 0; the DC voltage of MMC3 is controlled to be U2-U1, which drops the positive DC line of the pseudo-bipolar DC system from +U1 to 0; the DC voltage of MMC4 is controlled to be U2-U1, which drops the negative DC line of the pseudo-bipolar DC system from -U1 to -U2; at the same time, the DC voltage stability and power balance of each MMC are maintained through the AC side power control of the MMC.

9. The control method according to claim 8, characterized in that: When a unipolar fault occurs in a pseudo-bipolar DC system, the faulty pole is isolated and the healthy pole is derated by adjusting the DC voltage and power distribution of the MMC in the corresponding voltage shifter. When a unipolar fault occurs in a true bipolar DC system, the faulty pole is isolated and the system continues to operate by coordinating the control of the two voltage shifters.

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