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

Abstract

The utility model 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] The utility model relates to an offshore wind power direct current transmission system technology, in particular to a multi-level and multi-topology hybrid offshore wind power transmission system. Background Art

[0002] With the continuous development of marine resources, deep-sea development has become a future development trend. In addition to the energy field, more and more fields, including marine aquaculture, tourism development, scientific research, etc., are developing in the depths of the ocean. However, the development and utilization of these functions far from land cannot be separated from electricity. Therefore, the supply of electricity in the deep sea has become a technical problem that needs to be solved urgently.

[0003] A major difference between the deep-sea energy supply system and the conventional onshore energy supply system is that the "transmission distance" and "power load" are extremely mismatched, which is mainly reflected in two aspects: first, the longer the transmission distance, the greater the line loss, and a higher voltage level needs to be matched to reduce the loss. However, the higher the voltage level means the higher the economic cost. It is a bit "putting the cart before the horse" to use a high economic cost to solve the energy supply problem for the low-capacity power load demand in the distant sea. The low load demand for electricity does not match the high cost of power supply; second, deep-sea power supply often needs to have a certain degree of reliability. If the energy supply reliability is insufficient, then when the distant sea load loses power, the power function cannot operate normally at the least, and it may endanger the safety of personnel and equipment at the worst. Therefore, although it is a low-load function, it still requires sufficiently high safety and reliability guarantees; however, the guarantee of safety and reliability means the need for redundancy or the support of high-specification equipment, which still puts forward high requirements on economy.

[0004] Under the existing technical conditions, using the existing power source of offshore wind power is a relatively feasible technical solution. Taking energy from the converter station / boosting station / wind turbine of offshore wind power and sending it to the required offshore load points can effectively utilize existing energy and save some economic costs in terms of power extraction. Therefore, the multi-level power transmission system formed by the offshore wind power transmission system and the comprehensive development energy supply system will be one of the main solutions for the future development of offshore power sources.

[0005] For offshore wind power development, flexible DC transmission technology has many advantages, such as system stability, decoupling of AC and DC systems, independent control of active and reactive power, flexible adjustment methods, and small footprint. It is especially suitable for grid-connected power transmission of long-distance, large-capacity deep offshore wind farms. Currently, several land-based flexible DC technology demonstration projects have been built and put into operation, but the flexible DC transmission of offshore wind farms is still in its infancy in China. How to form a multi-level coordinated energy supply system between the offshore wind power transmission system and the energy supply system for comprehensive functional development, and solve the contradiction between low-load power supply and high-cost development for comprehensive functional development, is a difficulty that needs to be addressed.

[0006] Based on the above situation, the utility model proposes a multi-level and multi-topology hybrid offshore wind power transmission system, which can ensure the high reliability of the energy supply system while having the characteristics of better economy, and solve the low-load function requirements in the open sea. Utility Model Content

[0007] In order to solve the problems existing in the background technology, the utility model provides a multi-level and multi-topology hybrid offshore wind power transmission system.

[0008] The utility model adopts the following technical solutions:

[0009] A multi-level and multi-topology hybrid offshore wind power transmission system is suitable for offshore power loads with a power load of 5MW or less and an offshore route distance of more than 150km, including a high-voltage direct current transmission system and a medium-voltage direct current transmission system: wind turbines in wind farms are collected via 66kV AC submarine cables to the 66kV AC busbar of a ±500kV offshore converter station, the AC power is collected and rectified at the offshore converter station and then connected to the onshore converter station via a DC submarine cable, and then inverted at the onshore converter station and connected to the system station via an AC 500kV line, and the converter adopts a converter valve based on modular multi-level; the medium-voltage direct current functional system adopts a symmetrical bipolar transmission structure, and the converter adopts a converter valve based on a cascaded three-level, and the transmission topology, converter topology and voltage level of the high-voltage direct current transmission system and the medium-voltage direct current transmission system are coupled with different direct current transmission systems through AC 66kV.

[0010] Furthermore, the medium voltage DC transmission main system is used to meet the long-distance power transmission of various energy / functional requirements, and adopts a symmetrical bipolar transmission system: energy is taken from the 66kV AC bus of the offshore converter station, and connected to the cascaded three-level rectifier converter valve via a multi-winding transformer. The rectified electric energy is transmitted to the receiving station via a DC submarine cable, passes through a cascaded three-level inverter converter valve, and is connected to the 400V bus via a multi-winding transformer for use by loads of various functional developments.

[0011] Furthermore, the rectifier and inverter of the medium-voltage cascade three-level DC transmission system both contain two-pole CNPC converter valves, and the transmission submarine cable is composed of a three-core submarine cable (1 core positive pole, 1 core negative pole cable, and 1 core neutral line) or three single-core submarine cables connecting the sea and land stations. Due to the use of symmetrical bipolar transmission, core equipment such as converter valves and DC submarine cables are redundantly configured, and this wiring scheme has a very high reliability: when any pole of the rectifier and inverter fails, the other pole can continue to operate without causing power transmission interruption.

[0012] Furthermore, both the positive system and the negative system of the medium voltage DC transmission main system include a soft start circuit, a multi-winding connected transformer, a C-NPC unit of a multi-stage three-level NPC submodule cascaded, and a pole line reactance.

[0013] Furthermore, the neutral pole system of the medium voltage DC transmission main system includes a neutral line reactance and a grounding resistor. The NPC three-level converter and LC filter at each level form an NPC subunit. The NPC subunits at each level are isolated by a multi-winding connection transformer and then connected in series on the DC side to jointly support the DC voltage of the system; they are 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 load side.

[0014] 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 when the transmission power changes.

[0015] Furthermore, the NPC three-level converter wiring includes a filtering circuit, a commutation circuit, and a protection circuit. Each phase contains 4 switching tubes and 2 clamping diodes. Each switching tube contains a parallel circuit of an IGBT component and a diode. The clamping diode is used for level clamping. C1 and C2 are three-level DC side capacitors, which provide two identical DC voltages for the inverter. The midpoint voltage of the capacitor is zero level.

[0016] The cascaded three-level converter valve system adopts a grid-connected mode control method and an island mode control method. The grid-connected mode control strategy is applied to the converter station / boost station side close to the offshore wind farm, and the island mode is applied to the functional application load side.

[0017] The utility model provides a multi-level, multi-topology hybrid offshore wind power transmission system: It solves the problem of "power transmission distance" and "power load" being extremely mismatched for offshore power loads with a capacity of 5MW or less and more than 150km away from the power supply center;

[0018] The electricity is obtained from offshore wind power converter stations more than 100km offshore, and then a medium-voltage DC transmission system is used. Taking transmission losses into consideration, it can be applied to scenarios where the power load is more than 150km offshore, which is of great significance for multi-scenario and multi-functional development in the open sea.

[0019] The power supply of electric energy basically does not require investment costs, and can be completed with the help of the AC busbar of the offshore converter station. For the power supply circuit, only the transmission system needs to be built. Therefore, it has a high economic value for the power supply circuit of the power load;

[0020] Electric energy is obtained from the AC bus of the offshore converter station, so the safety and reliability of the power supply can also 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. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 It is the overall topological diagram of the utility model;

[0022] Figure 2 This is the wiring diagram of the cascaded three-level CNPC converter system;

[0023] Figure 3 This is the wiring diagram of a single NPC three-level converter;

[0024] Figure 4 It is a schematic diagram of the working state of a single-phase bridge arm;

[0025] Figure 5 It is a schematic diagram of the bypass unit working status. DETAILED DESCRIPTION

[0026] The utility model is further described in detail below in conjunction with the accompanying drawings and specific implementation methods.

[0027] As attached Figure 1 As shown, a multi-level and multi-topology hybrid offshore wind power transmission system is suitable for offshore power loads with a power load of 5MW or less and an offshore routing distance of more than 150km, including a high-voltage direct current transmission system and a medium-voltage direct current transmission system: the high-voltage direct current transmission adopts a symmetrical unipolar transmission structure, and the converter adopts a converter valve based on a modular multi-level (MMC); the medium-voltage direct current functional system adopts a symmetrical bipolar transmission structure to improve the safety and reliability of energy supply, and the converter adopts a converter valve based on a cascaded three-level (C-NPC). The direct current transmission systems with different transmission topologies, converter topologies and voltage levels of the high-voltage direct current transmission system and the medium-voltage direct current transmission system are coupled through AC 66kV to solve the energy supply problem of low-load electricity consumption in the offshore.

[0028] As shown in the figure, high-voltage direct current transmission is a typical offshore wind power transmission system, which adopts a symmetrical monopole transmission scheme. The main offshore wind power transmission system is: wind farm wind turbines 100 are collected via 66kV AC submarine cables 200 to the 66kV AC bus 330 of the ±500kV offshore converter station 300. All electric energy is rectified at the offshore converter station and then landed via two DC submarine cables 800 to the onshore converter station 600. After being inverted at the onshore converter station, it is connected to the system station 700 via two 500kV lines.

[0029] The medium voltage DC transmission main system is used to meet the long-distance power transmission of various functional requirements, and adopts a symmetrical bipolar transmission system: energy is taken from the 66kV AC bus 330 of the offshore converter station, and is connected to the cascaded three-level rectifier converter valve 350 via a multi-winding transformer 340. The rectified electric energy is transmitted to the receiving station 500 via a DC submarine cable 400, and is connected to the 400V bus 530 via a multi-winding transformer 520 through a cascaded three-level inverter converter valve 510 for use by loads of various functional developments.

[0030] The system power comes from the AC 66kV bus 330 of the offshore converter station 300. Since the offshore converter station adopts a control strategy of fixed AC bus voltage and AC bus frequency, its 66kV bus 330 has sufficient stability to provide the needs of the medium voltage DC functional system.

[0031] The medium-voltage DC function system of the system adopts a symmetrical bipolar transmission method to improve the safety and reliability of energy supply. Usually, the rectifier 350 and the inverter 510 both include bipolar CNPC converter valves, and the transmission submarine cable is composed of a three-core submarine cable (1 core positive pole, 1 core negative pole submarine cable, and 1 core neutral line) 400 or three single-core submarine cables connecting the sea and land stations. Due to the use of symmetrical bipolar transmission, core equipment such as converter valves and DC submarine cables are redundantly configured. This wiring scheme is characterized by high reliability. When any pole of the rectifier 350 and the inverter 510 fails, the other pole can continue to operate without causing power transmission interruption.

[0032] The coordinated control between the converter stations in the system is the core of control and protection. In order to maintain the stability of the DC voltage and system, according to the basic operating principle of flexible DC transmission, the operating range of each converter station should meet the following constraints:

[0033]

[0034] -P imax ≤P i ≤P imax ;

[0035] -Q imax ≤Q i ≤Q imax ;

[0036] S imin (P i , Q i )≤S i (P i , Q i )≤S imax (P i , Q i );

[0037] U dcimin ≤U dci ≤U dcimax ;

[0038] In the above formula, the converter station number is i=1~n, i=1 is the offshore converter station, i=2 is the onshore converter station, and i=3~n are various functional load stations; P i , Q i , S i , U dci are the active power, reactive power, apparent power and DC voltage of converter station i respectively; P imax , Q imax , S imax , U dcimax are respectively the maximum allowable values ​​of active power, reactive power, apparent power and DC voltage of converter station i; S imin , U dcimin are the minimum allowable values ​​of apparent power and DC voltage of converter station i respectively; P loss is the DC system loss. When the converter station injects active power into the DC network, the active power P is positive, and when the converter station draws active power from the DC network, the active power P is negative.

[0039] Due to the low-load power demand of the system, the cross-section of the medium-voltage DC submarine cable 400 is usually not large, which will cause problems such as high voltage drop and high loss in long-distance power transmission.

[0040] For the high voltage drop of long-distance power transmission, when the power load is running at full power, the power supply circuit current is high. According to the basic formula of U=I×R, the voltage drop will also increase, and the operating voltages of the sending end and the receiving end will be at a higher voltage difference; when the power load is in the startup or low-power operation state, the power supply circuit current may be close to 0, and the voltage drop will also be close to 0, so the operating voltages of the sending end and the receiving end are very close. Therefore, for the medium-voltage DC functional system, the sending-end converter 350 and the receiving-end converter 510 should adopt the same selection of the highest rated voltage, increase the cross-section of the submarine cable, etc., to prevent insulation breakdown problems when the transmission power changes.

[0041] like Figure 2As shown, the offshore cascade three-level (CNPC) DC transmission system, the positive system and the negative system single pole both mainly include a soft start loop 340, a multi-winding connection transformer 520, a C-NPC unit of a multi-level three-level NPC submodule cascade, and a pole line reactance 352. The neutral pole system mainly includes a neutral line reactance 353 and a grounding resistor 354. The NPC three-level converter 351 and the LC filter of each level form an NPC subunit. After being isolated by a multi-winding connection transformer, the NPC subunits of each level are 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 system grounding point is located at the neutral line loop on the receiving load side.

[0042] Based on the cascaded three-level converter valve system connection, the cascaded three-level converter is composed of multiple NPC sub-units. 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.

[0043] Based on the cascaded three-level converter valve connection, the single-pole CNPC converter unit is cascaded by 7 NPC sub-units, including one hot standby. During normal operation, the 7 sub-modules are put into operation; when a short-term unremovable fault occurs in a sub-module, the faulty sub-module is locked and bypassed, while the remaining 6 sub-modules continue to operate stably.

[0044] like Figure 3 As shown in the figure, the NPC three-level converter wiring consists of three parts: filter circuit, commutation circuit and protection circuit. Each phase contains 4 switch tubes S1~S4 and 2 clamping diodes D5 and D6. Each switch tube 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, which provide two identical DC voltages for the inverter. The midpoint voltage of the capacitor is zero level.

[0045] like Figure 4 As shown in the figure, the schematic diagram of the working state of the single-phase bridge arm is shown, assuming that the switching function of its output voltage is:

[0046]

[0047] Let k = α1-α2, where the switching function α i for

[0048]

[0049] Then k has two possibilities: 1, 0, -1, and the output voltage is shown in the following table:

[0050] Switching tube status S1, S2 are on S2, S3 are turned on S3, S4 are on K 1 0 -1 AC side voltage +Udc / 2 0 -Udc / 2

[0051] Table 1

[0052] like Figure 5 As shown in the figure, it is a schematic diagram of the bypass unit working state. The bypass loop is composed of four switch tubes S11, S12, S13, and S14. Each switch tube S11~S14 contains a parallel circuit of IGBT components T11~T14 and diodes D11~D14. Among them, S12 and S13 are connected in parallel with the bypass switch K. When the bypass switch K is closed, S11 and S14 are turned on, the loop is in the bypass state, and the output voltage is 0; when the bypass switch K is opened, but S12 and S13 are turned on, the DC side voltage is also 0.

[0053] The cascaded three-level (CNPC) converter control strategy mainly adopts two methods: grid-connected mode control strategy and island mode control strategy. Among them, the grid-connected mode control strategy is generally used near the converter station / boosting station side of the offshore wind farm, and the island mode is used near the functional application load side.

[0054] Note that the above are only preferred embodiments of the present invention and the technical principles used. Those skilled in the art will understand that the present invention is not limited to the specific embodiments described herein, and that various obvious changes, readjustments and substitutions can be made by those skilled in the art without departing from the scope of protection of the present invention. Therefore, although the present invention is described in more detail through the above embodiments, the present invention is not limited to the above embodiments, and may include more other equivalent embodiments without departing from the concept of the present invention, and the scope of the present invention 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 load of 5MW or less and an offshore routing distance of more than 150km, characterized in that: It includes a high-voltage direct current transmission system and a medium-voltage direct current transmission system: the wind turbines of the wind farm of the high-voltage direct current transmission system are collected to the 66kV AC bus of the ±500kV offshore converter station via a 66kV AC submarine cable. The AC power is collected and rectified at the offshore converter station and then connected to the onshore converter station via a DC submarine cable. After being inverted at the onshore converter station, it is connected to the system station via an AC 500kV line; the medium-voltage direct current functional system adopts a symmetrical bipolar transmission structure. The transmission topology, converter topology and voltage level of the high-voltage direct current transmission system and the medium-voltage direct current transmission system are coupled with different direct current transmission systems through an AC 66kV.

2. A multi-level, multi-topology hybrid offshore wind power transmission system according to claim 1, characterized in that: The main medium-voltage DC transmission system is connected from the 66kV AC bus of the offshore converter station to the cascaded three-level rectifier converter valves via a multi-winding transformer. The rectified electric energy is transmitted to the receiving station via a DC submarine cable, passes through a cascaded three-level inverter converter valve, and is connected to the 400V bus via a multi-winding transformer.

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

4. A multi-level, multi-topology hybrid offshore wind power transmission system according to claim 1, characterized in that: Both the positive system and the negative system of the medium voltage DC transmission main system include a soft start circuit, a multi-winding connected transformer, a C-NPC unit of a multi-stage three-level NPC submodule cascaded, and a pole line reactance.

5. The 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. The NPC three-level converter and LC filter at each level form an NPC subunit. The NPC subunits at each level are isolated by a multi-winding connection transformer and then connected in series on the DC side. The grounding point of the transmission system is located at the neutral line loop on the load side of the receiving end.

6. A multi-level, multi-topology hybrid offshore wind power transmission system according to claim 5, characterized in that: The NPC three-level converter wiring includes a filter circuit, a commutation circuit, and a protection circuit. Each phase contains four switching tubes and two clamping diodes. Each switching tube contains a parallel circuit of an IGBT component and a diode. The clamping diode is used for level clamping. C1 and C2 are three-level DC side capacitors that provide two identical DC voltages for the inverter. The midpoint voltage of the capacitor is zero level.

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