Bipolar direct current sending-out system based on DRU-MMC and black start method of bipolar direct current sending-out system

Abstract

The invention relates to a DRU-MMC-based bipolar direct current sending-out system and a black start method thereof. The invention is suitable for the technical field of offshore wind power direct current transmission. The technical problem to be solved by the invention is to provide a DRU-MMC-based bipolar direct current sending-out system and a black start method thereof. According to the technical scheme, the offshore converter platform comprises an offshore positive electrode DRU converter station, an offshore negative electrode MMC converter station and an intermediate grounding electrode between the positive electrode and the negative electrode; the ashore current conversion platform comprises an ashore positive electrode MMC converter station, an ashore negative electrode MMC converter station and a middle grounding electrode between the positive electrode and the negative electrode; the offshore positive electrode DRU converter station and the offshore negative electrode MMC converter station are respectively connected with an offshore alternating current power grid, and the shore positive electrode MMC converter station and the shore negative electrode MMC converter station are respectively connected with a shore alternating current power grid; the offshore positive electrode DRU converter station is connected with the shore positive electrode MMC converter station through a direct current cable, and the offshore negative electrode MMC converter station is connected with the shore negative electrode MMC converter station through a direct current cable.

Description

Technical Field

[0001] This invention relates to a bipolar DC transmission system based on DRU-MMC and its black-start method. It is applicable to the field of offshore wind power DC transmission technology. Background Technology

[0002] Currently, flexible DC transmission based on modular multilevel converters (MMCs) is the most common solution for offshore wind power grid connection. However, as transmission capacity continues to increase, the size and weight of MMC converter stations will also increase; furthermore, as offshore converter platforms are located further from shore, the difficulty of installation, transportation, and operation and maintenance will also increase. This will pose significant economic and technical challenges to the overall construction of offshore wind power DC transmission systems. Achieving lightweight and low-cost design has become the first pressing issue to be addressed in deep-sea, large-capacity wind power DC transmission systems.

[0003] Furthermore, as transmission capacity continues to increase, the reliability requirements for offshore wind power DC transmission systems are also gradually rising. Currently, all operational offshore DC transmission projects adopt a symmetrical single-pole connection scheme. Under this connection method, a fault could lead to a complete system outage, which would have a significant impact on the power grid. How to improve system reliability and reduce impact has become the second urgent problem to be solved for deep-sea, large-capacity wind power DC transmission systems. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide a bipolar DC transmission system based on DRU-MMC and its black start method, in view of the above-mentioned problems.

[0005] The technical solution adopted in this invention is: a bipolar DC transmission system based on DRU-MMC, comprising: The offshore converter platform includes an offshore positive DRU converter station and an offshore negative MMC converter station, as well as an intermediate grounding electrode between the positive and negative electrodes; The onshore converter platform includes an onshore positive MMC converter station and an onshore negative MMC converter station, as well as an intermediate grounding electrode between the positive and negative electrodes; The offshore positive DRU converter station and the offshore negative MMC converter station are respectively connected to the offshore AC power grid, and the onshore positive MMC converter station and the onshore negative MMC converter station are respectively connected to the onshore AC power grid. The offshore positive DRU converter station is connected to the onshore positive MMC converter station via a DC cable, and the offshore negative MMC converter station is connected to the onshore negative MMC converter station via a DC cable.

[0006] The offshore negative MMC converter station adopts a half-bridge submodule and the control strategy is a grid-type control; the onshore negative MMC converter station adopts a half-bridge submodule and the control strategy is constant DC voltage control.

[0007] The onshore positive MMC converter station adopts a half-bridge sub-module and the control strategy is constant DC voltage control.

[0008] The offshore positive electrode DRU converter station uses a 12-pulse rectifier bridge, which is composed of two 6-pulse rectifiers connected in series.

[0009] A transformer is installed between the two rectifiers of the offshore AC power grid and the offshore positive DRU converter station. The transformer is connected in star and delta configurations, respectively.

[0010] A black-start method for the bipolar DC transmission system, comprising: S1. Start the onshore positive MMC converter station and the onshore negative MMC converter station. Both adopt constant DC voltage control. After startup, the AC power of the onshore AC grid is converted into DC power, and the DC side voltage of the system is gradually established. S2. Start the offshore negative MMC converter station, using grid-type control. After startup, the offshore AC side voltage is gradually established based on the system DC side voltage and stabilized at the first AC voltage, which is less than the rectification threshold voltage of the offshore positive DRU converter station. S3. Unlock the offshore wind turbine. The onshore AC grid provides the active and reactive power required for black start to the offshore wind turbine through the onshore negative MMC converter station and the offshore negative MMC converter station, and starts the offshore wind turbine. S4. The offshore AC side voltage is raised to the second AC voltage through the offshore negative MMC converter station. This second AC voltage is greater than the rectification threshold voltage of the offshore positive DRU converter station, so that the offshore positive DRU converter station is turned on. S5. The power generated by the offshore wind turbine is collected at the offshore AC power grid's collection bus, and the power output by the offshore wind turbine is transmitted to the onshore AC power grid through the offshore positive DRU converter station and the offshore negative MMC converter station, respectively.

[0011] The beneficial effects of the present invention are as follows: The offshore converter platform of the present invention includes an offshore positive pole DRU converter station and an offshore negative pole MMC converter station. It adopts a DRU-MMC hybrid topology structure, which makes full use of the characteristics of DRU converter station such as low loss, light weight, small footprint and low construction cost, and makes up for the disadvantages of MMC such as large size and high cost.

[0012] This invention fully utilizes the flexible control and bidirectional power flow characteristics of MMC converter stations, enabling the construction of offshore AC voltage and system black start without altering traditional wind turbine control. It overcomes the disadvantages of DRU's uncontrollability and single power flow direction, and eliminates the limitations of DRU on wind turbine control types.

[0013] This invention adopts a bipolar wiring method, with the offshore positive DRU converter station and the offshore negative MMC converter station operating independently, allowing for different power and voltage levels and high operational flexibility. Furthermore, the system has a certain fault ride-through capability; when a unipolar fault occurs, power can continue to be transmitted through the non-faulty pole, reducing fault losses and impact on the onshore AC power grid, thus improving system reliability. Attached Figure Description

[0014] Figure 1 This is a system topology diagram for an example.

[0015] Figure 2 The diagram below shows the grid control block diagram of the offshore negative electrode MMC converter station in the embodiment.

[0016] Figure 3 This is a schematic diagram of the simulation waveform of the system during the startup phase when using the black start method proposed in the embodiment. Detailed Implementation

[0017] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, 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, 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.

[0018] Example 1: As Figure 1 As shown, this embodiment is a bipolar DC transmission system based on DRU-MMC, including: offshore wind farm, offshore converter platform, DC cable and onshore converter platform, etc.

[0019] In this example, the offshore converter platform includes an offshore positive DRU converter station and an offshore negative MMC converter station, as well as an intermediate grounding electrode between the positive and negative poles; the onshore converter platform includes an onshore positive MMC converter station and an onshore negative MMC converter station, as well as an intermediate grounding electrode between the positive and negative poles.

[0020] In this embodiment, the offshore positive DRU converter station and the offshore negative MMC converter station are connected to the offshore AC power grid, and the onshore positive MMC converter station and the onshore negative MMC converter station are connected to the onshore AC power grid. The offshore positive DRU converter station is connected to the onshore positive MMC converter station via a DC cable, and the offshore negative MMC converter station is connected to the onshore negative MMC converter station via a DC cable.

[0021] The power generated by the offshore wind farm is collected at the offshore AC power grid's collection bus. After being boosted and rectified by the offshore converter platform, it is converted into DC power and transmitted over long distances to the onshore converter platform via DC cables. Finally, it is inverted into AC power and fed into the onshore AC power grid.

[0022] In some specific embodiments, the offshore positive DRU converter station uses a 12-pulse rectifier bridge, which consists of two 6-pulse rectifiers connected in series. A transformer is installed between the offshore AC power grid and the two rectifiers of the offshore positive DRU converter station. In order to reduce the voltage and current harmonic distortion of the offshore AC system caused by the DRU, the transformer is connected in star and delta configurations, respectively.

[0023] In some specific embodiments, the offshore negative electrode MMC converter station adopts a half-bridge submodule, and the control strategy is a network-based control (e.g., Figure 2 (As shown); both the onshore positive MMC converter station and the onshore negative MMC converter station adopt half-bridge sub-modules, and the control strategy is constant DC voltage control.

[0024] In some specific embodiments, the offshore wind turbines in the offshore wind farm are direct-drive permanent magnet synchronous wind turbines, and the control strategy is a general constant power control.

[0025] Example 2: This example is a black start method based on the bipolar DC transmission system in Example 1, specifically including the following steps: S1. Start the onshore positive MMC converter station and the onshore negative MMC converter station. Both adopt constant DC voltage control. After startup, the AC power of the onshore AC grid is converted into DC power, and the DC side voltage of the system is gradually established.

[0026] In this embodiment, both the onshore positive and negative MMC converter stations adopt constant DC voltage control. Before startup, all MMCs are in a locked state. After the onshore MMCs are unlocked, the onshore AC power grid converts AC power to DC power through the inverter function of the MMCs, gradually establishing the DC side voltage of the system and stabilizing it at 1.0 pu (rated DC voltage), providing stable DC side voltage support for the subsequent startup of the offshore converter station.

[0027] S2. Start the offshore negative MMC converter station, using grid-type control. After startup, the offshore AC side voltage is gradually established based on the system DC side voltage and stabilized at the first AC voltage, which is less than the rectification threshold voltage of the offshore positive DRU converter station.

[0028] When DC side voltage U dc After the voltage reaches 1.0 PU and stabilizes, the offshore negative electrode MMC converter station is started, and the offshore AC side voltage is established through grid-type control. U s Furthermore, to prevent the DRU from turning on (if it does, the DC-side voltage will be forcibly clamped, disrupting the construction process of the offshore AC voltage, and since the wind turbines are not running and there is no power support, voltage collapse is likely to occur), the offshore AC voltage must be maintained. U s ≤0.86pu (rectified threshold voltage of DRU), controlling the AC voltage at sea. U s =0.8pu (first AC voltage).

[0029] In this example, the grid-type control does not rely on an external AC power supply and can autonomously output a stable amplitude (0.8 pu) and frequency, providing a qualified AC-side environment for the wind turbine startup.

[0030] S3. Unlock the offshore wind turbine. The onshore AC grid provides the active and reactive power required for black start to the offshore wind turbine through the onshore negative MMC converter station and the offshore negative MMC converter station, thus starting the offshore wind turbine.

[0031] When the sea AC voltage U s Once the voltage reaches 0.8 PU and stabilizes, the offshore wind turbines in the offshore wind farm are unlocked, and the onshore AC grid can supply the active and reactive power required for black start to the offshore wind farm through the offshore negative MMC converter station.

[0032] In this embodiment, the command value for the active power output of the offshore wind turbine is first set. P ref =0, offshore wind farms can start offshore wind turbines normally under conventional control.

[0033] S4. The offshore AC side voltage is raised to the second AC voltage through the offshore negative MMC converter station. This second AC voltage is greater than the rectification threshold voltage of the offshore positive DRU converter station, so that the offshore positive DRU converter station is turned on.

[0034] After the offshore wind turbine has started up, set the command value for the active power output of the offshore wind turbine. P ref=1.0pu, while the offshore AC voltage is boosted by the offshore MMC converter station. U s Gradually increase to the rated voltage (second AC voltage), when the offshore AC voltage U s When the voltage is greater than 0.86 pu, the DC side voltage of the offshore positive DRU converter station is greater than the DC side voltage of the onshore positive MMC converter station, and the offshore positive DRU converter station begins to transmit active power.

[0035] This embodiment utilizes the controllability of the MMC to precisely boost the voltage, achieving "impact-free turn-on" of the DRU and avoiding voltage / power surges when uncontrollable components are connected. The system then forms a "bipolar power transmission path"—positive DRU (rectified transmission) + negative MMC (rectified transmission), fully leveraging the advantages of the DRU's low loss and the MMC's controllability.

[0036] S5. The power generated by the offshore wind turbine is collected at the offshore AC power grid's collection bus, and the power output by the offshore wind turbine is transmitted to the onshore AC power grid through the offshore positive DRU converter station and the offshore negative MMC converter station, respectively.

[0037] When offshore wind farms generate active power P wt After reaching 1.0 pu, the system transmits the active power output from the offshore wind farm to the onshore AC power grid through the offshore positive DRU converter station and the offshore negative MMC converter station, respectively, to realize the system's power transmission.

[0038] In this embodiment, the positive path is: offshore AC power grid → DRU converter station (rectifier) ​​→ DC cable → onshore positive MMC (inverter) → onshore AC power grid; the negative path is: offshore AC power grid → MMC converter station (rectifier) ​​→ DC cable → onshore negative MMC (inverter) → onshore AC power grid.

[0039] The following is an example to illustrate this: Using the black-start method described in the embodiments, a simulation model was built in PSCAD. The main parameters of the simulation system are shown in Table 1.

[0040] Table 1 Simulation System Parameters The simulation conditions are as follows: Before system startup, all MMCs are in a locked state. At t=0s, the onshore positive and negative MMC converter stations are simultaneously unlocked and constant DC voltage control is adopted; at t=0.5s, the offshore negative MMC converter station is unlocked and constant AC voltage amplitude and frequency control is adopted. At t=0.8s, the offshore wind turbines are unlocked and constant power control is adopted. At t=1.0s, the wind turbines start generating electricity, increasing the offshore AC voltage to the rated value. As the wind farm power climbs to the rated value, the system transmits the active power output from the offshore wind farm to the onshore AC grid through the offshore positive DRU converter station and the offshore negative MMC loop, realizing the power transmission of the entire offshore wind farm. The simulation lasts until 2.5s.

[0041] From simulation Figure 3 As can be seen, during t=0~0.5s, the onshore MMC converter station is unlocked, and charging is performed by the AC grid, with the DC side voltage controlled at 1.0pu. During t=0.5~0.8s, the offshore negative MMC converter station is unlocked, and the offshore AC voltage amplitude increases linearly from 0 to 0.8pu. At this time, the offshore AC voltage is lower than the DRU rectification threshold voltage, and the offshore positive DRU converter station is not conducting. During t=0.8~1.0s, the offshore wind turbine is unlocked, and its output power is 0. The offshore wind farm can start the offshore wind turbine normally according to conventional control. During t=1.0~2.5s, the offshore AC voltage increases from 0.8pu to 1.0pu, and the offshore positive DRU converter station begins to transmit power; the wind farm power increases linearly from 0 to 1000MW, and the power of the positive and negative poles gradually reaches equilibrium. Throughout the entire startup process, the system parameters such as wind turbine output power, offshore AC system voltage, and positive and negative pole power transmission change smoothly and all reach their rated values.

[0042] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.

Claims

1. A DRU-MMC based bipolar DC transmission system, characterized in that, include: The offshore converter platform includes an offshore positive DRU converter station and an offshore negative MMC converter station, as well as an intermediate grounding electrode between the positive and negative electrodes; The onshore converter platform includes an onshore positive MMC converter station and an onshore negative MMC converter station, as well as an intermediate grounding electrode between the positive and negative electrodes; The offshore positive DRU converter station and the offshore negative MMC converter station are respectively connected to the offshore AC power grid, and the onshore positive MMC converter station and the onshore negative MMC converter station are respectively connected to the onshore AC power grid. The offshore positive DRU converter station is connected to the onshore positive MMC converter station via a DC cable, and the offshore negative MMC converter station is connected to the onshore negative MMC converter station via a DC cable.

2. The DRU-MMC based bipolar DC transmission system of claim 1, wherein: The offshore negative MMC converter station adopts a half-bridge submodule and the control strategy is a grid-type control; the onshore negative MMC converter station adopts a half-bridge submodule and the control strategy is constant DC voltage control.

3. The DRU-MMC based bipolar DC transmission system of claim 2, wherein: The onshore positive MMC converter station adopts a half-bridge sub-module and the control strategy is constant DC voltage control.

4. The DRU-MMC based bipolar DC transmission system of claim 1, wherein: The offshore positive electrode DRU converter station uses a 12-pulse rectifier bridge, which is composed of two 6-pulse rectifiers connected in series.

5. The DRU-MMC based bipolar DC transmission system of claim 1, wherein: A transformer is installed between the two rectifiers of the offshore AC power grid and the offshore positive DRU converter station. The transformer is connected in star and delta configurations, respectively.

6. A black start method for the bipolar DC transmission system according to any one of claims 1 to 5, characterized in that, include: S1. Start the onshore positive MMC converter station and the onshore negative MMC converter station. Both adopt constant DC voltage control. After startup, the AC power of the onshore AC grid is converted into DC power, and the DC side voltage of the system is gradually established. S2. Start the offshore negative MMC converter station, using grid-type control. After startup, the offshore AC side voltage is gradually established based on the system DC side voltage and stabilized at the first AC voltage, which is less than the rectification threshold voltage of the offshore positive DRU converter station. S3. Unlock the offshore wind turbine. The onshore AC grid provides the active and reactive power required for black start to the offshore wind turbine through the onshore negative MMC converter station and the offshore negative MMC converter station, and starts the offshore wind turbine. S4. The offshore AC side voltage is raised to the second AC voltage through the offshore negative MMC converter station. This second AC voltage is greater than the rectification threshold voltage of the offshore positive DRU converter station, so that the offshore positive DRU converter station is turned on. S5. The power generated by the offshore wind turbine is collected at the offshore AC power grid's collection bus, and the power output by the offshore wind turbine is transmitted to the onshore AC power grid through the offshore positive DRU converter station and the offshore negative MMC converter station, respectively.

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