Topological structure of bipolar composite offshore converter station and control method thereof

Abstract

The application discloses a topology structure and a control method of a bipolar composite offshore converter station, belongs to the technical field of offshore power transmission and distribution, and aims at solving the problems of large volume, complex structure and poor system reliability of the existing offshore wind power. The bipolar composite offshore converter station comprises a positive pole and a negative pole; the positive pole adopts a half-bridge modular multilevel converter topology structure, the negative pole adopts a diode rectifier topology structure, the two topology structures are connected in parallel at an alternating current side and are connected in series at a direct current side, and a direct current connection point is used as a grounding pole to form a symmetric bipolar structure. The topology structure of a shore converter station comprises: the positive pole and the negative pole both adopt a half-bridge modular multilevel converter topology structure. The control method comprises the following steps: when a direct current of the grounding pole increases from zero, an alternating current voltage rated value of the offshore converter station is added with a fine adjustment amount, the added value is used as a given value of a d-axis voltage ring of a wind turbine, the direct current of the grounding pole is kept as zero, power balance control of the positive pole and the negative pole is realized, and the application is used for the offshore converter station.

Description

Technical Field

[0001] This invention relates to a topology of an offshore converter station and its control method, belonging to the field of offshore power transmission and distribution technology. Background Technology

[0002] Against the backdrop of climate change and energy security concerns, offshore wind power technology has developed rapidly in recent years. The key to the development of offshore wind power is to improve system reliability and solve the problem of wind power grid connection.

[0003] For solving the grid connection problem of wind power, DC transmission grid connection is a promising method for offshore wind power grid connection. Currently, high-voltage direct current (HVDC) transmission based on modular multilevel converters (MMC) is the mainstream technology for long-distance transmission of offshore wind power. However, MMC converters are usually expensive, bulky, and limited by the offshore environment, requiring expensive offshore platforms. To improve the compactness of offshore converter stations and reduce the initial investment cost, existing technologies have also proposed HVDC transmission schemes based on diode rectifiers (DR), which can reduce the volume of offshore converter stations by 80%, weight by 65%, and total cost by 30%. However, this approach has a problem: when offshore wind farms use DR HVDC technology for grid connection, the voltage and frequency control of the offshore wind farm's AC collection network is difficult due to the uncontrollability of the diode rectifier. To address this, a distributed control strategy for offshore wind turbines has been proposed, in which each turbine contains a phase-locked loop (PLL) to achieve frequency control of the offshore AC grid and synchronization between turbines. In addition to the traditional inner current loop, each wind turbine is also equipped with an intermediate AC voltage loop and an outer power loop, enabling the turbine to simultaneously control its output power and the voltage of the offshore AC grid. However, this grid-shaping control strategy differs significantly from the traditional grid-following strategy of wind turbines, requiring substantial modifications to the turbine control system.

[0004] To address the above problems, the existing topology methods mainly include:

[0005] The first type is a composite topology where the MMC and DR are connected in series on the DC side, and the existing three-wire high-voltage AC (HVAC) cable is used as a DC cable for power transmission. By connecting one of the cables in parallel with the positive or negative cable, the power transmission capacity can be increased by 37% compared with the traditional symmetrical bipolar topology. However, this topology requires a full-bridge MMC and auxiliary thyristors to realize DC current modulation and line switching.

[0006] The second type is a multi-voltage DC network based on diode rectifiers. At sea, MMC and DR are connected in series on the DC side, and on land, MMC is connected in series. DC cables are used to connect the series connection points at sea and on land to form a multi-voltage DC network. However, the advantages of using multiple voltage levels for power transmission are not obvious, and additional DC cables are required.

[0007] Regarding how to solve the reliability problem of the system, considering that most of the faults in offshore DC cables are permanent and the system usually takes a long time to restore normal operation, sometimes up to several months, these factors place stringent requirements on the reliability of offshore wind power systems.

[0008] Furthermore, in traditional diode-based solutions, the unidirectional energy flow characteristic of diode topologies presents a significant challenge to the startup of offshore wind turbines. Existing technologies propose an auxiliary AC cable connected in parallel with the DR-HVDC converter, but this increases system cost. Another approach uses a full-bridge MMC and DR connected in series on the DC side as the offshore converter station, with the onshore converter station employing a line commutated converter (LCC). This leverages the DC current regulation capability of the LCC and the DC voltage regulation capability of the full-bridge MMC to achieve system self-starting and flexible adjustment under various operating conditions. However, the design of the full-bridge MMC's power capacity is not discussed. Summary of the Invention

[0009] The purpose of this invention is to address the problems of large size, complex structure, and poor system reliability in existing offshore wind power systems, and to provide a topology and control method for a bipolar composite offshore converter station.

[0010] The topology of the bipolar composite offshore converter station described in this invention includes:

[0011] The positive electrode adopts a half-bridge modular multilevel converter topology, and the negative electrode adopts a diode rectifier topology. The half-bridge modular multilevel converter topology and the diode rectifier topology are connected in parallel on the AC side and in series on the DC side, with the DC connection point serving as the ground electrode, forming a symmetrical bipolar structure.

[0012] Preferably, the diode rectifier topology is a 12-pulse diode rectifier topology, and has the same power rating as the half-bridge modular multilevel converter topology.

[0013] Preferably, the diode rectifier topology has a filter and a capacitor connected on the AC side.

[0014] Preferably, the connection point between the half-bridge modular multilevel converter topology and the diode rectifier topology is connected to the grounding point of the onshore positive and negative half-bridge modular multilevel converter topology via a DC cable.

[0015] Preferably, it also includes the topology of the onshore converter station, specifically including: both the positive and negative poles adopt a half-bridge modular multilevel converter topology.

[0016] Preferably, the operating mode of the offshore converter station topology is controlled AC voltage mode;

[0017] The onshore converter station operates in controlled DC voltage mode.

[0018] Preferably, both the half-bridge modular multilevel converter topology and the diode rectifier topology have AC circuit breakers connected in series on their AC sides.

[0019] The control method for the topology of the bipolar composite offshore converter station described in this invention includes:

[0020] The cables for the positive, negative, and ground poles have the same resistance R. Therefore, the imbalance rate k for obtaining the power transmission between the positive and negative poles is:

[0021]

[0022] Among them, P MMC P represents the active power transferred at the positive terminal. DR I represents the active power transmitted through the negative terminal. dcMMC I represents the direct current at the positive terminal. dcDR Indicates the direct current at the negative terminal;

[0023] When the transmission power of the offshore converter station remains constant, the sum of the DC currents at the positive and negative terminals, I... t Unchanged, therefore:

[0024] I t =I dcMMC +I dcDR ;

[0025] Therefore, we obtain:

[0026]

[0027] I dcNeu This represents the DC current at the grounding electrode;

[0028] The DC current I of the grounding electrode dcNeu As the controlled variable, I dcNeu Reference value I d * cNeu Set to zero;

[0029] When the DC current I of the grounding electrode dcNeu When increasing from zero, the rated AC voltage of the offshore converter station is superimposed with a fine-tuning amount Δv. d The superimposed value is used as the given value for the d-axis voltage loop of the wind turbine, so that the DC current I of the grounding electrode... dcNeu Keep it at zero to achieve power balance control between the positive and negative electrodes.

[0030] Preferably, the control method further includes:

[0031] When a positive ground fault occurs, the topology of the half-bridge modular multilevel converter at the positive pole of both the offshore and onshore converter stations is locked, and a disconnection command is sent to the circuit breaker on the AC side. The offshore converter station sends a positive ground fault signal to the wind turbine, which will bypass the phase-locked loop and operate in a fixed frequency mode. The output phase of the wind turbine is adjusted through low-speed communication to achieve stable control of the frequency of the offshore AC power grid.

[0032] When a negative ground fault occurs, the half-bridge modular multilevel converter topology of the negative pole of the onshore converter station is locked, and a disconnection command is sent to the circuit breakers on the negative AC side of both the offshore and onshore converter stations.

[0033] Preferably, when a positive ground fault occurs and the power output of the wind turbine exceeds the overcapacitance capacity x of the diode rectifier topology, the upper limit of the given d-axis current of the wind turbine is reduced from x to reduce the power output of the wind turbine, thereby preventing the diode rectifier topology from experiencing overcurrent.

[0034] Advantages of the present invention: The bipolar composite offshore converter station topology proposed in this invention adopts a half-bridge modular multilevel converter topology and a diode rectifier topology for the positive and negative poles, respectively. It has a simple structure, small size, flexible grid connection, and high reliability.

[0035] Based on this, the present invention also proposes a positive and negative power balance control method for offshore converter stations, which controls the grounding electrode current to near zero, thereby reducing the loss generated by the high voltage DC cable by 67.7%.

[0036] In addition, this invention also realizes DC asymmetric fault protection, and can actively limit the power output of the wind farm through low-speed communication, thus preventing the risk of overcurrent in the converter station. Attached Figure Description

[0037] Figure 1 This is a schematic diagram of the topology of the bipolar composite offshore converter station described in this invention.

[0038] Figure 2This is a block diagram illustrating the principle of power balance control method for the positive and negative poles of a bipolar composite offshore converter station topology.

[0039] Figure 3 This is a block diagram illustrating the principle of the positive and negative pole grounding fault control method for the topology of a bipolar composite offshore converter station.

[0040] Figure 4 This is a simulation result of the active power of the positive and negative pole power balance control method for the topology of a bipolar composite offshore converter station.

[0041] Figure 5 The figure shows the DC current simulation results of the positive and negative pole power balance control method for the topology of a bipolar composite offshore converter station.

[0042] Figure 6 The figure shows the simulation results of the offshore AC voltage amplitude of the positive and negative pole power balance control method of the topology of the bipolar composite offshore converter station.

[0043] Figure 7 The figure shows the simulation results of the power loss of the high-voltage DC cable in the topology of the bipolar composite offshore converter station, which is based on the positive and negative pole power balance control method.

[0044] Figure 8 This is a simulation result diagram of the positive DC voltage of the control method for a positive ground fault.

[0045] Figure 9 This is a simulation result diagram of the active power of the control method for a positive ground fault.

[0046] Figure 10 This is a DC current simulation result diagram of the control method for a positive ground fault.

[0047] Figure 11 This is a simulation result diagram of the three-phase current of the upper arm of the bridge arm for the control method of a positive ground fault.

[0048] Figure 12 This is a simulation result diagram of the three-phase current of the lower arm of the control method for a positive ground fault.

[0049] Figure 13 This is a simulation result diagram of the negative DC voltage of the control method for a negative ground fault.

[0050] Figure 14 This is a simulation result diagram of the active power of the control method for a negative ground fault.

[0051] Figure 15 This is a simulation result diagram of the DC current of the control method for a negative ground fault.

[0052] Figure 16This is a simulation result diagram of the three-phase current of the upper arm of the control method for a negative ground fault.

[0053] Figure 17 This is a simulation result diagram of the three-phase current of the lower arm of the control method for a negative ground fault. Detailed Implementation

[0054] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0055] It should be noted that, unless otherwise specified, the embodiments and features described in the present invention can be combined with each other.

[0056] The present invention will be further described below with reference to the accompanying drawings and specific embodiments, but this is not intended to limit the scope of the invention.

[0057] Example 1:

[0058] The following is combined Figure 1 This embodiment describes the topology of the bipolar composite offshore converter station, which includes:

[0059] The positive electrode adopts a half-bridge modular multilevel converter topology, and the negative electrode adopts a diode rectifier topology. The half-bridge modular multilevel converter topology and the diode rectifier topology are connected in parallel on the AC side and in series on the DC side, with the DC connection point serving as the ground electrode, forming a symmetrical bipolar structure.

[0060] In this embodiment, the MMC and DR converters are connected in parallel on the AC side via an isolation transformer and in series on the DC side to improve the transmission voltage level. Their DC connection points serve as grounding electrodes and are connected to the shore via DC cables, thus forming a symmetrical bipolar structure. Since most DC cable faults at sea are permanent single-pole grounding faults, the bipolar structure proposed in this embodiment ensures that even in the event of a single-pole grounding fault, the offshore wind farm can still transmit power to the shore through the other pole, reducing the impact of power loss on the AC grid and significantly improving system reliability.

[0061] Furthermore, the diode rectifier topology is a 12-pulse diode rectifier topology, and has the same power rating as the half-bridge modular multilevel converter topology.

[0062] In this embodiment, to reduce the size, weight and cost of the offshore converter station, the negative electrode adopts a 12-pulse diode rectifier topology, which has the same power rating as the positive electrode MMC.

[0063] Furthermore, the diode rectifier topology has a filter and a capacitor connected on the AC side.

[0064] In this embodiment, filters and capacitors are configured on the AC side of the diode rectifier topology to filter out harmonics generated by diode commutation and provide reactive power compensation.

[0065] Furthermore, the connection point between the half-bridge modular multilevel converter topology and the diode rectifier topology serves as the grounding electrode, and is connected to the positive and negative grounding point of the onshore half-bridge modular multilevel converter topology via a DC cable.

[0066] In this embodiment, the current capacity of the grounding electrode cable is the same as that of the positive and negative electrode cables to ensure continuous power transmission and reduce electromagnetic pollution to the environment when a positive or negative grounding fault occurs.

[0067] Furthermore, it also includes the topology of the onshore converter station, specifically: both the positive and negative poles adopt a half-bridge modular multilevel converter topology.

[0068] Furthermore, the operating mode of the offshore converter station's topology is controlled AC voltage mode;

[0069] The onshore converter station operates in controlled DC voltage mode.

[0070] In this embodiment, the operating mode of the offshore converter station's topology is AC voltage control mode, which controls the voltage and frequency of the offshore AC power grid near the rated values. The operating mode of the onshore converter station's topology is DC voltage control mode, which controls the positive and negative voltages at the rated values ​​respectively.

[0071] Furthermore, both the half-bridge modular multilevel converter topology and the diode rectifier topology have AC circuit breakers connected in series on the AC side.

[0072] In this embodiment, AC circuit breakers are configured on the AC side of each MMC and DR to achieve AC / DC fault protection for the system.

[0073] Example 2:

[0074] The following is combined Figure 2 and Figure 3 This embodiment describes a control method for the topology of a bipolar composite offshore converter station, which includes:

[0075] The cables for the positive, negative, and ground poles have the same resistance R. Therefore, the imbalance rate k for obtaining the power transmission between the positive and negative poles is:

[0076]

[0077] Among them, P MMC P represents the active power transferred at the positive terminal. DR I represents the active power transmitted through the negative terminal. dcMMC I represents the direct current at the positive terminal. dcDR Indicates the direct current at the negative terminal;

[0078] When the transmission power of the offshore converter station remains constant, the sum of the DC currents at the positive and negative terminals, I... t Unchanged, therefore:

[0079] I t =I dcMMC +I dcDR ;

[0080] Therefore, we obtain:

[0081]

[0082] I dcNeu This represents the DC current at the grounding electrode;

[0083] The DC current I of the grounding electrode dcNeu As the controlled variable, I dcNeu Reference value Set to zero;

[0084] When the DC current I of the grounding electrode dcNeu When increasing from zero, the rated AC voltage of the offshore converter station is superimposed with a fine-tuning amount Δv. d The superimposed value is used as the given value for the d-axis voltage loop of the wind turbine, so that the DC current I of the grounding electrode... dcNeu Keep it at zero to achieve power balance control between the positive and negative electrodes.

[0085] In this embodiment, the transmission loss P generated by the positive and negative poles and the grounding pole cable loss for:

[0086]

[0087] Therefore, when k = 1, P loss Take the minimum value:

[0088]

[0089] Therefore, when the positive and negative converters of a bipolar composite topology transmit power in a balanced manner, P MMC =P DRThe power transmission cable generates the least loss.

[0090] Therefore, when designing the turns ratio of the AC-side transformer in a diode rectifier, it is essential to ensure a balance between the output power of the MMC and the diodes under rated operating conditions in the composite topology. Specifically, when all offshore wind turbines are operating at full power, the MMC should control the offshore AC voltage at its rated value (V). d0 Furthermore, the output power of both the MMC and the diode should be half of the rated power P0 of the composite topology.

[0091]

[0092] This ensures that the current flowing through the grounding cable under rated operating conditions is near zero, thereby reducing transmission cable losses. Based on this principle, the turns ratio n of the diode AC side transformer can be expressed as:

[0093]

[0094] Among them, V dc0 Indicates the rated DC voltage of the positive and negative terminals, V d0 This indicates the rated phase voltage amplitude of the offshore AC power grid.

[0095] Power balance control is achieved through PI regulation at the rated AC voltage V at sea. d0 On top of this, a fine adjustment Δv is added. d As the d-axis voltage loop setpoint, this is used to reduce the offshore AC voltage, thereby reducing the output power of the negative DR converter and increasing the power transmitted by the positive MMC, until the power transmitted by the positive and negative converters is restored to balance, and the grounding current I... dcNeu The voltage is gradually controlled to near zero. Due to the low impedance of the DR-HVDC system, fine-tuning of the offshore AC voltage allows for full-range adjustment of the negative DR transmission power from its rated value to zero. Therefore, the AC voltage fine-tuning amount Δv of the power balance control output is... d The lower limit is set to -0.1 pu.

[0096] When a positive or negative ground fault occurs, the power generated by the wind farm is transmitted through the non-faulty pole, and the power of the faulty pole drops to zero. Therefore, in the case of DC asymmetric fault, the power balance control of the positive and negative poles needs to be shielded.

[0097] Furthermore, the control method also includes:

[0098] When a positive ground fault occurs, the topology of the half-bridge modular multilevel converter at the positive pole of both the offshore and onshore converter stations is locked, and a disconnection command is sent to the circuit breaker on the AC side. The offshore converter station sends a positive ground fault signal to the wind turbine, which will bypass the phase-locked loop and operate in a fixed frequency mode. The output phase of the wind turbine is adjusted through low-speed communication to achieve stable control of the frequency of the offshore AC power grid.

[0099] When a negative ground fault occurs, the half-bridge modular multilevel converter topology of the negative pole of the onshore converter station is locked, and a disconnection command is sent to the circuit breakers on the negative AC side of both the offshore and onshore converter stations.

[0100] In this embodiment, when a positive ground fault occurs, both the offshore and onshore positive MMCs are locked to prevent overcurrent in the converters. Simultaneously, a command is sent to the AC-side circuit breaker to disconnect, thereby cutting off the fault current fed from the AC side to the DC side. The offshore converter station sends a positive ground fault signal to the wind turbines. Each wind turbine bypasses its phase-locked loop and operates in a fixed-frequency mode. Low-speed communication is used to adjust the output phase of the wind turbines, thereby achieving synchronization between the turbines and stable control of the offshore AC grid frequency. Under a positive ground fault, the offshore MMC is disconnected, and the power generated by the offshore wind farm is transmitted through the negative DR converter.

[0101] When a negative ground fault occurs, the onshore negative MMC is locked to prevent overcurrent in the converter. Simultaneously, it sends a command to disconnect the AC-side circuit breaker on the shore and the AC-side circuit breaker on the offshore DR, thereby cutting off the fault current fed into the AC side. Under a negative ground fault, the DR is disconnected, and the power generated by the offshore wind farm is transmitted by the positive MMC converter.

[0102] Furthermore, when a positive ground fault occurs, if the power output of the wind turbine exceeds the overcapacitance capacity x of the diode rectifier topology, the upper limit of the given d-axis current of the wind turbine is reduced from x, thereby reducing the power output of the wind turbine and preventing the diode rectifier topology from experiencing overcurrent.

[0103] In this embodiment, the overcapacity of the DR converter is set to 1.1 pu. After a positive ground fault, when the power output of the wind farm exceeds the rated power of the DR by 1.1 pu, the power limiting control actively sets the upper limit I of the d-axis current of the wind turbine through low-speed communication. dmax Reduced from 1.1 pu to limit the fan d-axis current setpoint This reduces the power output of the wind turbine and ensures that the DR converter does not experience overcurrent. Considering the communication delay between the offshore converter station and the wind turbine, a delay element e is introduced into the controller. -sT A delay of 100ms is used, meaning that only 10Hz low-speed communication is required between the offshore converter station and the wind turbine. A low-pass filter is used in the control loop to filter out low-frequency harmonics, with its characteristic frequency set to 10Hz. Under normal operating conditions, the DR transmission power P... DR When the power setpoint is below 1.1 pu, the difference between the power setpoint 1.1 pu and the feedback value is limited to zero by the dead zone circuit, and the power limit control output is zero, thus not affecting the operation of the fan under normal working conditions.

[0104] In this embodiment, to prevent overcurrent in the positive MMC after a negative grounding fault, the power limiting control changes the control variable from DR power P. DR Switching to marine MMC power P MMC Similar to a positive ground fault, during a negative ground fault, when the wind farm's output power exceeds 1.1 pu of the offshore MMC's rated power, the power limiting control actively reduces the upper limit of the turbine's d-axis current I via low-speed communication. dmax This reduces the power output of the fan and ensures that the MMC converter does not experience overcurrent.

[0105] In this embodiment, since most DC cable faults at sea are permanent single-pole grounding faults, protection and control methods are proposed for the positive and negative grounding faults of the system. Power limiting control of the offshore converter station based on low-speed communication is proposed to improve the DC fault ride-through capability of the system.

[0106] In this invention, the proposed bipolar composite offshore converter station topology uses a positive-pole MMC to control the voltage and frequency of the offshore AC grid without requiring excessive modifications to the wind turbine control strategy; the wind turbine can operate in a standard power control mode. Furthermore, the controllable pole composed of MMCs enables the onshore AC grid to supply power to the offshore wind farm, thus solving the wind turbine startup problem. This proposed topology combines the advantages of flexible MMC control and small DR size, significantly reducing the size and cost of the offshore converter station without affecting system performance.

[0107] In this invention, a simulation model was built using PSCAD / EMTDC. The same parameters were used for both offshore and onshore MMC converters, as shown in Table 1.

[0108] Table 1

[0109]

[0110] Simulation results of positive and negative power balance control are as follows: Figures 4-7 As shown.

[0111] Before t=2s, the system operates at its rated operating state, and power balance control is not enabled. Even without power balance control, the composite topology can still transmit 1GW of active power at both the positive and negative poles under rated conditions. The DC current at both poles is equal, around 2.5kA, therefore the grounding current is near zero. Figure 4 and Figure 5 As shown.

[0112] At t=2s, the power output of the offshore wind farm gradually decreases from the rated value of 2GW to 1GW in a ramp manner within 0.2s. At this time, most of the power generated by the wind farm is transmitted through the negative electrode DR, and the positive electrode MMC power is close to zero. Figure 4 and Figure 6 As shown, the negative DC current is much larger than the positive DC current, and the difference flows through the grounding electrode, reaching up to 1.84 kA. Figure 5 As shown, this increases the transmission loss caused by the DC cable resistance, reaching up to 16.7MW. Figure 7 As shown.

[0113] To achieve power balance between the positive and negative poles, the proposed power balance control is enabled at t=3s. Figures 4-7 As can be seen, the active control reduces the offshore AC voltage from the rated value to 0.95 pu, thereby reducing the negative DR transmission power from 902 MW to 477 MW, while the positive MMC transmission power increases, achieving a power balance between the positive and negative electrodes. Figure 4 As shown, with the balance of transmitted power, the positive and negative currents gradually become consistent, thus the grounding current gradually decreases to zero. Consequently, the losses generated by the high-voltage DC cable decrease from 16.7MW to 5.4MW, a reduction of 67.7%. Figure 5 and Figure 7 As shown.

[0114] Simulation results of performance during positive ground faults are as follows: Figures 8-12 As shown, it is assumed that a permanent short-circuit fault to ground occurs at the midpoint of the positive DC cable at t=2s. After the fault is detected, the positive offshore and onshore MMC converters are shut down to avoid overcurrent damage. In the simulation, the DC fault monitoring time is set to 2ms, and the AC circuit breaker operating time is 50ms.

[0115] When a DC asymmetric fault occurs at t=2s, the positive DC voltage drops to zero, as follows: Figure 8 As shown. After a fault is detected, the positive MMC gate drive signals at both the offshore and onshore terminals are shut off to protect the converter. The positive active power drops to zero, while the negative DR transmission power increases. When the set value of 1.1 pu is reached, the proposed power limiting control is triggered. The offshore converter station sends a command to the wind farm via low-speed communication to set the upper limit of the d-axis current, actively reducing the power output of the wind farm. The power of the wind farm and DR converter is gradually controlled at 1036MW. Figure 9 As shown, this maximizes the power transmission capacity of offshore wind farms during positive short-circuit faults and effectively reduces the impact on the onshore AC power grid caused by power loss.

[0116] As AC circuit breakers B1 and B3 open at t = 2.052s, the positive DC current and the MMC three-phase bridge arm current both drop to zero, while the negative DC current gradually stabilizes at around 2.52kA. Figure 10-12 As shown.

[0117] Simulation results of performance during negative ground faults are as follows: Figures 13-17As shown, a permanent short-circuit fault to ground occurred at the midpoint of the negative DC cable at t=2s. After the fault was detected, the shore-based negative MMC converter was shut down to protect the equipment.

[0118] When a negative ground fault occurs at t=2s, the negative DC voltage drops from -400kV to zero. Figure 13 As shown. 2ms after the fault occurs, the onshore negative MMC is shut down, the active power transmitted by the negative DR drops to zero, and the positive power transmission increases. When it reaches the set value of 1.1 pu, the proposed power limiting control is triggered, thereby actively reducing the power output of the wind farm. The power of the positive MMC is gradually controlled to near the rated value, as shown. Figure 14 As shown, this ensured the continuous transmission of power from the offshore wind farm during the fault.

[0119] During the fault, the positive MMC bridge arm current and DC current oscillated. When AC circuit breakers B2 and B4 tripped at t = 2.052s and the fault was cleared, the positive MMC gradually returned to a stable operating state. Figure 15-17 As shown.

[0120] While the invention has been described herein with reference to specific embodiments, it should be understood that these embodiments are merely examples of the principles and applications of the invention. Therefore, it should be understood that many modifications can be made to the exemplary embodiments, and other arrangements can be designed without departing from the spirit and scope of the invention as defined by the appended claims. It should be understood that different dependent claims and features described herein can be combined in ways different from those described in the original claims. It is also understood that features described in conjunction with individual embodiments can be used in other described embodiments.

Claims

1. The topology of a bipolar composite offshore converter station, characterized in that, It includes: The positive electrode adopts a half-bridge modular multilevel converter topology to build an offshore AC power grid and provide voltage and frequency support for the wind turbine. The negative electrode adopts a diode rectifier topology; The half-bridge modular multilevel converter topology and the diode rectifier topology are connected in parallel on the AC side and in series on the DC side, with the DC connection point serving as the ground electrode, forming a symmetrical bipolar structure. The DC current of the grounding electrode As the controlled variable, its reference value is... Set to zero; When the DC current of the grounding electrode When increasing from zero, the rated AC voltage of the offshore converter station is superimposed with a fine-tuning amount. The superimposed value is used as the given value for the d-axis voltage loop of the wind turbine. By adjusting the offshore AC voltage, the DC current of the grounding electrode is increased. Keep it at zero to achieve power balance control between the positive and negative electrodes.

2. The topology of the bipolar composite offshore converter station according to claim 1, characterized in that, The diode rectifier topology is a 12-pulse diode rectifier topology, and has the same power rating as the half-bridge modular multilevel converter topology.

3. The topology of the bipolar composite offshore converter station according to claim 1, characterized in that, A diode rectifier topology has a filter and a capacitor connected on the AC side.

4. The topology of the bipolar composite offshore converter station according to claim 1, characterized in that, The connection points of the half-bridge modular multilevel converter topology and the diode rectifier topology are connected to the onshore positive and negative half-bridge modular multilevel converter topology grounding points via DC cables.

5. The topology of the bipolar composite offshore converter station according to claim 1, characterized in that, It also includes the topology of onshore converter stations; Specifically, both the positive and negative terminals adopt a half-bridge modular multilevel converter topology.

6. The topology of the bipolar composite offshore converter station according to claim 5, characterized in that, The operating mode of the offshore converter station's topology is controlled AC voltage mode. The onshore converter station operates in controlled DC voltage mode.

7. The topology of the bipolar composite offshore converter station according to claim 6, characterized in that, Both the half-bridge modular multilevel converter topology and the diode rectifier topology have AC circuit breakers connected in series on the AC side.

8. A control method for the topology of a bipolar composite offshore converter station, wherein the control method is implemented based on the topology of any one of claims 1-7, characterized in that, The control method includes: The cables with positive, negative, and ground terminals have the same resistance. , Therefore, the imbalance rate of power transfer between the positive and negative electrodes is obtained. for: ； in, This represents the active power transferred at the positive terminal. This represents the active power transmitted through the negative pole. Represents the direct current at the positive terminal. Indicates the direct current at the negative terminal; When the transmission power of the offshore converter station remains constant, the sum of the DC currents at the positive and negative terminals is... Unchanged, therefore: ； Therefore, we obtain: This represents the DC current at the grounding electrode; The DC current of the grounding electrode As the controlled quantity, Reference value Set to zero; When the DC current of the grounding electrode When increasing from zero, the rated AC voltage of the offshore converter station is superimposed with a fine-tuning amount. The superimposed value is used as the given value for the d-axis voltage loop of the wind turbine, so that the DC current of the grounding electrode... Keep it at zero to achieve power balance control between the positive and negative electrodes.

9. The control method for the topology of the bipolar composite offshore converter station according to claim 8, characterized in that, The control method also includes: When a positive ground fault occurs: The positive half-bridge modular multilevel converter topology of the offshore converter station and the onshore converter station is locked, and a disconnection command is sent to the circuit breaker on the AC side. The offshore converter station sends a positive ground fault signal to the wind turbine. The wind turbine will bypass the phase-locked loop and operate in a fixed frequency mode. The output phase of the wind turbine is adjusted through low-speed communication to achieve stable control of the frequency of the offshore AC power grid. When a negative ground fault occurs: The half-bridge modular multilevel converter topology of the negative pole of the onshore converter station is locked, and a disconnection command is sent to the circuit breaker on the negative AC side of both the offshore and onshore converter stations.

10. The control method for the topology of the bipolar composite offshore converter station according to claim 9, characterized in that, When a positive ground fault occurs: The power output of the fan exceeds the overcapacitance capacity of the diode rectifier topology. At that time, the upper limit of the given d-axis current of the fan is changed from The power output of the fan is reduced, thus preventing the diode rectifier topology from overcurrent.

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