Multi-level multi-topology series-parallel offshore wind power transmission system

Abstract

The invention discloses a multi-level multi-topology series-parallel offshore wind power transmission system which adopts a symmetrical single-pole power transmission scheme, a converter adopts a converter valve based on modular multi-level, and a medium-voltage direct-current power transmission main system is used for meeting remote electric energy transmission of various functional requirements. A symmetric bipolar power transmission system is adopted, and a converter valve based on the cascade three-level technology is adopted as a converter. By the adoption of the power transmission system, electric energy can be obtained from the alternating current side of the offshore wind power high-voltage direct current power transmission system 100 km away from the shore or above, then the medium-voltage direct current power transmission system is adopted, the power transmission system can be used for solving the problem that the power consumption load is 150 km away from the shore or above, high reliability and economical efficiency are achieved, and the power transmission system has important significance on multi-scene multifunctional development of the open sea.

Description

Technical Field

[0001] This invention relates to offshore wind power DC transmission system technology, and in particular to a multi-level, multi-topology hybrid offshore wind power transmission system. Background Technology

[0002] With the continuous development of marine resources, deep-sea development has become a future trend. Beyond the energy sector, more and more fields, including marine aquaculture, tourism development, and scientific research, are expanding into the ocean. However, the development and utilization of these functions far from land rely heavily on electricity; therefore, the power supply for deep-sea areas has become a pressing technical challenge.

[0003] A major difference between deep-sea power supply systems and conventional onshore power supply systems lies in the significant mismatch between their transmission distance and power load. This mismatch manifests in two main aspects: First, the longer the transmission distance, the greater the line loss, requiring a higher voltage level to reduce losses. However, a higher voltage level means higher economic costs. Using higher economic costs to solve the power supply problem for the low-capacity power load demand in the deep sea is somewhat counterproductive, as the low load demand is mismatched with the high cost of power supply. Second, deep-sea power supply often requires a certain level of reliability. Insufficient reliability can lead to power outages, resulting in malfunctions of electrical functions or even endangering personnel and equipment safety. Therefore, even with low loads, a sufficiently high level of safety and reliability is still required. However, ensuring safety and reliability necessitates redundancy or high-specification equipment, which still places high demands on economic efficiency.

[0004] Under current technological conditions, utilizing existing offshore wind power is a practical and feasible solution. Extracting energy from offshore wind power converter stations / step-up substations / wind turbines and then transmitting it to the required offshore load points effectively utilizes existing energy sources and saves on some of the economic costs associated with energy extraction. Therefore, a multi-tiered power transmission system formed by offshore wind power transmission systems and integrated power supply systems will be one of the main solutions for future offshore power development.

[0005] For offshore wind power development, flexible DC transmission technology offers numerous advantages, including system stability, decoupling between AC and DC systems, independent control of active and reactive power, flexible regulation methods, and minimal land occupation. It is particularly suitable for grid connection and power transmission to long-distance, large-capacity offshore wind farms. While my country has already constructed and put into operation several onshore flexible DC technology demonstration projects, flexible DC transmission from offshore wind farms is still in its early stages domestically. A key challenge is how to establish a multi-level, coordinated power supply system between the offshore wind power transmission system and the integrated energy supply system for comprehensive development, thereby resolving the contradiction between low-load power supply and high-cost development in integrated functional development.

[0006] Based on the above, this invention proposes a multi-level, multi-topology hybrid offshore wind power transmission system that can ensure the high reliability of the power supply system while also being economically efficient, thus addressing the low-load functional requirements of offshore areas. Summary of the Invention

[0007] To address the problems existing in the background technology, the present invention provides a multi-level, multi-topology hybrid offshore wind power transmission system.

[0008] The present invention adopts the following technical solution:

[0009] A multi-level, multi-topology hybrid offshore wind power transmission system is proposed, suitable for offshore power loads with a load of 5MW or less and an offshore route distance of more than 150km. It includes a high-voltage direct current (HVDC) transmission system and a medium-voltage direct current (MTDC) transmission system. The HVDC transmission system adopts a symmetrical monopolar transmission structure, and the converter uses a modular multilevel converter (MMC) based converter valve. The MTDC system adopts a symmetrical bipolar transmission structure, and the converter uses a cascaded three-level converter valve (C-NPC). The transmission topology, converter topology, and voltage level of the HVDC and MTDC transmission systems are all different from those of the HVDC system, coupled to a 66kV AC system, thus solving the power supply problem for low-load offshore power needs.

[0010] Furthermore, the main high-voltage direct current transmission system includes: wind turbines from wind farms are collected via 66kV AC submarine cables to the 66kV AC bus of the ±500kV offshore converter station; AC power is collected and rectified at the offshore converter station and then connected to the onshore converter station via DC submarine cables; after inversion at the onshore converter station, it is connected to the system station via a 500kV AC line.

[0011] Furthermore, the medium-voltage DC transmission main system is used to meet the long-distance power transmission needs of various energy / functional requirements. It adopts a symmetrical bipolar transmission system: energy is taken from the 66kV AC bus of the offshore converter station, and led to the cascaded three-level rectifier converter valve through a multi-winding transformer. The rectified power is transmitted to the receiving station via DC submarine cable, and led to the 400V bus through the cascaded three-level inverter converter valve and the multi-winding transformer for use by various functional loads.

[0012] Furthermore, the rectifier and inverter units of the medium-voltage cascaded three-level DC transmission system both include bipolar CNPC converter valves. The submarine transmission cable is composed of one three-core submarine cable (one positive core, one negative core, and one neutral core) or three single-core submarine cables connecting the sea and land stations. Due to the use of symmetrical bipolar transmission, the core equipment such as converter valves and DC submarine cables have redundant configurations, making this wiring scheme highly reliable: when any pole of the rectifier or inverter fails, the other pole can continue to operate without interrupting power transmission.

[0013] Furthermore, both the positive and negative poles of the medium-voltage DC transmission main system include a soft-start circuit, a multi-winding transformer, a C-NPC unit cascaded with multi-level three-level NPC sub-modules, and pole line reactors.

[0014] Furthermore, the neutral pole system of the medium-voltage DC transmission main system includes neutral line reactance and grounding resistance. Each stage of NPC three-level converter and LC filter forms an NPC sub-unit. After isolation by multi-winding connected transformers, each NPC sub-unit is connected in series on the DC side to jointly support the DC voltage of the system; and connected in parallel on the AC side to jointly share the transmission power of the system. The grounding point of the transmission system is located at the neutral line loop on the receiving end load side.

[0015] Furthermore, the sending-end converter and the receiving-end converter adopt the same maximum rated voltage and increase the cross-section of the submarine cable to prevent insulation breakdown under varying transmission power conditions.

[0016] Furthermore, the NPC three-level converter wiring includes a filter circuit, a converter circuit, and a protection circuit. Each phase contains four switching transistors and two clamping diodes. Each switching transistor contains a parallel circuit of IGBT components and diodes. The clamping diodes are used for level clamping. C1 and C2 are three-level DC-side capacitors that provide two identical DC voltages to the inverter. The voltage at the midpoint of the capacitors is zero level.

[0017] Furthermore, the converter valve system based on cascaded three-level circuits adopts grid-connected mode control and islanded mode control. The grid-connected mode control strategy is applied to the side near the converter station / booster station of the offshore wind farm, while the islanded mode is applied to the side near the functional application load.

[0018] This invention provides a multi-level, multi-topology hybrid offshore wind power transmission system, which addresses the problem of a severe mismatch between the "transmission distance" and the "power load" for offshore power loads with a capacity of 5MW or less and located more than 150km away from the power supply center.

[0019] The power is extracted from offshore wind power converter stations more than 100km offshore and then transmitted through a medium-voltage DC transmission system. Considering transmission losses, it can be applied to scenarios where the power load is more than 150km offshore, which is of great significance for the multi-scenario and multi-functional development in the open sea.

[0020] The power source of electricity requires virtually no investment cost, as it can be completed using the AC busbars of offshore converter stations. Only the power supply circuit needs to be built, thus the power supply circuit for the electrical load has high economic value.

[0021] The power is obtained from the AC bus of the offshore converter station, so the safety and reliability of the power supply can be guaranteed. In addition, the low-load power circuit has little impact on the operation of the wind farm, so it is a highly reliable solution. Attached Figure Description

[0022] Figure 1 This is the overall topology diagram of the present invention;

[0023] Figure 2 This is the wiring diagram for a cascaded three-level CNPC converter system;

[0024] Figure 3 This is the wiring diagram for a single NPC three-level converter;

[0025] Figure 4 This is a schematic diagram of the working state of a single-phase bridge arm;

[0026] Figure 5 This is a schematic diagram of the bypass unit's operating status. Detailed Implementation

[0027] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0028] As attached Figure 1As shown, a multi-level, multi-topology hybrid offshore wind power transmission system is suitable for offshore power loads with an electricity demand of 5MW or less and an offshore route distance of more than 150km. It includes a high-voltage direct current (HVDC) transmission system and a medium-voltage direct current (MTDC) transmission system. The HVDC transmission system adopts a symmetrical monopolar transmission structure, and the converter uses a modular multilevel converter (MMC) based converter valve. The MTDC system adopts a symmetrical bipolar transmission structure to improve the safety and reliability of power supply, and the converter uses a cascaded three-level converter valve (C-NPC). The HVDC and MTDC transmission systems have different transmission topologies, converter topologies, and voltage levels, and are coupled via 66kV AC to solve the power supply problem for low-load offshore power demand.

[0029] As shown in the figure, the high-voltage direct current transmission is a typical offshore wind power transmission system, which adopts a symmetrical single-pole transmission scheme. The main system for offshore wind power transmission is as follows: the wind turbine 100 of the wind farm is collected via a 66kV AC submarine cable 200 to the 66kV AC bus 330 of the ±500kV offshore converter station 300. All the power is rectified by the offshore converter station and then connected to the onshore converter station 600 via two DC submarine cables 800. After being inverted by the onshore converter station, it is connected to the system station 700 via two 500kV lines.

[0030] The medium-voltage DC transmission main system is used to meet the long-distance power transmission needs of various functions. It adopts a symmetrical bipolar transmission system: energy is taken from the 66kV AC bus 330 of the offshore converter station, and led to the cascaded three-level rectifier converter valve 350 through the multi-winding transformer 340. The rectified power is transmitted to the receiving station 500 through the DC submarine cable 400, and led to the 400V bus 530 through the cascaded three-level inverter converter valve 510 and the multi-winding transformer 520, for use by loads of various functions.

[0031] The system power supply comes from the 66kV AC bus 330 of the offshore converter station 300. Since the offshore converter station adopts a control strategy of constant AC bus voltage and AC bus frequency, its 66kV bus 330 has sufficient stability to meet the needs of the medium-voltage DC functional system.

[0032] The high-voltage DC power transmission system employs a symmetrical bipolar transmission method to enhance the safety and reliability of power supply. Typically, both the rectifier 350 and inverter 510 include bipolar CNPC converter valves. The submarine transmission cable connects the sea and land stations via either a three-core submarine cable (one positive core, one negative core, and one neutral core) or three single-core submarine cables. Due to the symmetrical bipolar transmission, core equipment such as converter valves and DC submarine cables have redundant configurations. This wiring scheme is characterized by extremely high reliability; if either pole of the rectifier 350 or inverter 510 fails, the other pole can continue operating without interrupting power transmission.

[0033] Coordinated control among the converter stations in the system is the core of control and protection. To maintain DC voltage and system stability, based on the basic operating principles of flexible DC transmission, the operating range of each converter station should meet the following constraints:

[0034]

[0035] -P imax ≤P i ≤P imax ;

[0036] -Q imax ≤Q i ≤Q imax ;

[0037] S imin (P i Q i )≤S i (P i Q i )≤S imax (P i Q i );

[0038] U dcimin ≤U dci ≤U dcimax ;

[0039] In the above formula, the converter station numbers are i = 1 to n, where i = 1 is the offshore converter station, i = 2 is the onshore converter station, and i = 3 to n are various functional load stations; P i Q i S i U dci These represent the active power, reactive power, apparent power, and DC voltage of converter station i, respectively; P imax Q imax S imax U dcimax These represent the maximum permissible values ​​of active power, reactive power, apparent power, and DC voltage for converter station i; S imin U dcimin P represents the apparent power and minimum allowable DC voltage of converter station i, respectively; loss This represents the losses in the DC system. When the converter station injects active power into the DC network, the active power P is positive; when the converter station draws active power from the DC network, the active power P is negative.

[0040] Because of the low power demand of the system, the cross-section of the medium-voltage DC submarine cable 400 is usually not large, which will bring problems such as high voltage drop and high loss in long-distance power transmission.

[0041] For long-distance power transmission with high voltage drops, when the power load is running at full power, the current in the power supply circuit is high. According to the basic formula U=I×R, the voltage drop will also increase, resulting in a significant voltage difference between the sending and receiving ends. When the power load is in startup or low-power operation, the current in the power supply circuit may be close to zero, and the voltage drop will also be close to zero. Therefore, the operating voltages at the sending and receiving ends will be very similar. Thus, for medium-voltage DC systems, the sending-end converter 350 and the receiving-end converter 510 should be selected with the same maximum rated voltage and the cross-section of the submarine cable should be increased to prevent insulation breakdown under varying power transmission conditions.

[0042] like Figure 2 As shown, the offshore cascaded three-level (CNPC) DC transmission system mainly consists of a soft-start circuit 340, a multi-winding transformer 520, a C-NPC unit composed of cascaded multi-level three-level NPC submodules, and a pole line reactor 352 for each positive and negative pole. The neutral pole system mainly includes a neutral line reactor 353 and a grounding resistor 354. Each stage of the NPC three-level converter 351 and LC filter forms an NPC subunit. After isolation by multi-winding transformers, each NPC subunit is connected in series on the DC side to jointly support the DC voltage of the system; and connected in parallel on the AC side to share the transmission power of the system. The system grounding point is located on the neutral line loop on the receiving end load side.

[0043] Based on the cascaded three-level converter valve system wiring, the cascaded three-level converter is composed of multiple NPC sub-units cascaded together. From the AC side, the multiple NPC sub-units are equivalent to a parallel relationship, and from the DC side, they are equivalent to a series relationship. Each NPC sub-unit is an independent, fully functional composite integrated power module.

[0044] Based on the cascaded three-level converter valve wiring, the single-pole CNPC converter unit consists of 7 cascaded NPC sub-units, including one for hot standby. During normal operation, all 7 sub-modules are in operation; when a sub-module experiences a short-term, unrecoverable fault, the faulty sub-module is blocked and bypassed, while the remaining 6 sub-modules continue to operate stably.

[0045] like Figure 3 As shown, the NPC three-level converter wiring consists of three parts: a filter circuit, a converter circuit, and a protection circuit. Each phase includes four switching transistors S1-S4 and two clamping diodes D5 and D6. Each switching transistor S1-S4 contains a parallel circuit of IGBT components T1-T4 and diodes D1-D4. Clamping diodes D5 and D6 are used for level clamping. C1 and C2 are three-level DC-side capacitors, providing two identical DC voltages to the inverter. The voltage at the midpoint of the capacitors is zero level.

[0046] like Figure 4The diagram shows the operating state of a single-phase bridge arm. Assume its output voltage switching function is:

[0047]

[0048] Let k = α1 - α2, where the switching function α i for

[0049]

[0050] k has two possibilities: 1, 0, or -1, and the output voltage is shown in the table below:

[0051] Switching state S1 and S2 are on. S2 and S3 are on. S3 and S4 are on. K 1 0 -1 AC side voltage +Udc / 2 0 -Udc / 2

[0052] Table 1

[0053] like Figure 5 The diagram shows the working state of the bypass unit. The bypass circuit consists of four switching transistors S11, S12, S13, and S14. Each switching transistor S11 to S14 includes a parallel circuit of IGBT components T11 to T14 and diodes D11 to D14. S12 and S13 are connected in parallel with the bypass switch K. When the bypass switch K is closed, S11 and S14 are conducting, the circuit is in bypass mode, and the output voltage is 0. When the bypass switch K is open, but S12 and S13 are conducting, the DC side voltage is also 0.

[0054] The control strategies for cascaded three-level (CNPC) converters mainly employ two methods: grid-connected mode control and islanded mode control. Grid-connected mode control is generally applied near the converter / boost station of an offshore wind farm, while islanded mode control is applied near the functional load side.

[0055] Note that the above description is merely a preferred embodiment of the present invention and the technical principles employed. Those skilled in the art will understand that the present invention is not limited to the specific embodiments described herein, and various obvious changes, readjustments, and substitutions can be made without departing from the scope of protection of the present invention. Therefore, although the present invention has been described in detail through the above embodiments, the present invention is not limited to the above embodiments, and may include many other equivalent embodiments without departing from the concept of the present invention, the scope of which is determined by the scope of the appended claims.

Claims

1. A multi-level, multi-topology hybrid offshore wind power transmission system, suitable for offshore power loads with a power consumption of 5MW or less and an offshore route distance of more than 150km, characterized in that, This includes high-voltage direct current (HVDC) transmission systems and medium-voltage direct current (DC) transmission systems. The HVDC transmission system adopts a symmetrical monopolar transmission structure, and the converter uses a modular multilevel converter valve. The medium-voltage DC transmission system adopts a symmetrical bipolar transmission structure, and the converter uses a cascaded three-level converter valve. The transmission topology, converter topology, and voltage level of the HVDC transmission system and the medium-voltage DC transmission system are all coupled to different DC transmission systems via AC 66kV.

2. The multi-level, multi-topology hybrid offshore wind power transmission system according to claim 1, characterized in that, The main high-voltage direct current transmission system includes: wind turbines from wind farms are collected via 66kV AC submarine cables to the 66kV AC bus of the ±500kV offshore converter station; AC power is collected and rectified at the offshore converter station and then connected to the onshore converter station via DC submarine cables; after inversion at the onshore converter station, it is connected to the system station via a 500kV AC line.

3. The multi-level, multi-topology hybrid offshore wind power transmission system according to claim 1, characterized in that, The medium-voltage DC transmission main system adopts a symmetrical bipolar transmission system: energy is drawn from the 66kV AC bus of the offshore converter station, and then led to the cascaded three-level rectifier converter valve through a multi-winding transformer. The rectified power is transmitted to the receiving station via a DC submarine cable, and then led to the 400V bus through the cascaded three-level inverter converter valve and the multi-winding transformer to supply the loads of various functions.

4. A multi-level, multi-topology hybrid offshore wind power transmission system according to claim 1, characterized in that, The rectifier and inverter of the medium-voltage DC transmission main system both contain two-pole CNPC converter valves, and the submarine transmission cable is composed of one three-core submarine cable or three single-core submarine cables connecting the sea and land stations.

5. A multi-level, multi-topology hybrid offshore wind power transmission system according to claim 1, characterized in that, The positive and negative pole systems of the medium-voltage DC transmission main system each include a soft-start circuit, a multi-winding connected transformer, a C-NPC unit with cascaded multi-level three-level NPC sub-modules, and pole line reactors.

6. A multi-level, multi-topology hybrid offshore wind power transmission system according to claim 1, characterized in that, The neutral pole system of the medium-voltage DC transmission main system includes neutral line reactance and grounding resistance. Each stage of NPC three-level converter and LC filter forms an NPC sub-unit. After isolation by multi-winding connected transformers, each NPC sub-unit is connected in series on the DC side to jointly support the DC voltage of the system; and connected in parallel on the AC side to jointly share the transmission power of the system. The grounding point of the transmission system is located at the neutral line loop on the receiving end load side.

7. A multi-level, multi-topology hybrid offshore wind power transmission system according to claim 1, characterized in that, The sending-end converter and the receiving-end converter adopt the same maximum rated voltage and increase the cross-section of the submarine cable to prevent insulation breakdown under the condition of power transmission variation.

8. A multi-level, multi-topology hybrid offshore wind power transmission system according to claim 6, characterized in that, The NPC three-level converter wiring includes a filter circuit, a converter circuit, and a protection circuit. Each phase contains four switching transistors and two clamping diodes. Each switching transistor contains a parallel circuit of IGBT components and diodes. The clamping diodes are used for level clamping. C1 and C2 are three-level DC-side capacitors that provide two identical DC voltages to the inverter. The voltage at the midpoint of the capacitors is zero level.

9. A multi-level, multi-topology hybrid offshore wind power transmission system according to claim 3, characterized in that, The cascaded three-level converter valve adopts grid-connected mode control and islanded mode control. The grid-connected mode control strategy is applied to the side near the offshore wind farm converter station / booster station, while the islanded mode is applied to the side near the functional application load.

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