HVDC solution

Abstract

The invention relates to an offshore HVDC substation for transmission of electrical energy to an onshore substation comprising: a. one jacket substructure (204); b. a first self-sufficient HVDC module (200) mounted on the jacket substructure (204); c. a second self-sufficient HVDC module (201) mounted on the jacket substructure side by side with the first HVDC module (200), wherein the first HVDC module (200) and the second HVDC module (201) comprises a mirrored HVDC poleIt is also disclosed a method for assembling an offshore HVDC substation at least comprising the steps of: a. transporting a jacket (204) to a plant site; b. launching the jacket (204); c. transporting a first HVDC module (200) to the plant site; d. installing the first HVDC module to the top of the jacket (204); e. transporting a second HVDC module (201) to the plant site; and f. installing the second HVDC module (201) to the top of the jacket (204).

Description

HVDC solutionTechnical field

[0001] The disclosure relates an offshore HVDC substation for transmission of electrical energy to an onshore station.Background

[0002] HVDC systems interconnect Alternating Current (AC) networks, converting AC voltage to DC voltage, and DC voltage to AC voltage utilizing power electronics technology. They are ideal for supporting existing AC transmission systems or for building new power highways. HVDC systems enable the transfer of power, interconnection of grids, integration of renewables, and maximize grid performance.

[0003] High Voltage Direct Current (HVDC) transmission is necessary to transmit large quantities of electricity over long distances. The world's largest offshore wind farms, that is installed far from shore, utilize this technology to enable transmission, minimise transmission losses and enable feasible grid connection.

[0004] A HVDC pole is a set of HVDC equipment for receiving AC power, increase the voltage, rectify the AC power to Direct Current.

[0005] A design that mitigates the increased weight and dimension of HVDC substation is required to enable cost efficient project execution.

[0006] Most existing offshore HVDC substations are currently of a symmetrical monopole design and typically at +-320kV voltage levels. Examples are Aibel AS provided projects such as Dolwin 2, Dolwin 5, Doggerbank A / B / C, and Hornsea 3 with power levels ranging from 900MW to around 1300MW.

[0007] Tennet 2GW 525kV design is a new standardized bi-pole design with 13 projects currently in EPC phase on the German and Dutch sector. The Tennet design is implemented in one large topside which inherently becomes less flexible during execution compared to smaller modules.

[0008] Mcdermott, Patent application US 2021 / 0273421 Al, is a prior art document that is attempting to solve the weight and dimension challenge through use of modularization. The patent describes a modularization where two rectifier modules and one utility module are placed on a common jacket structure. The modules are then adjoined to the substructure and remaining cooling-, electrical- and control connections between the modules are put in place and final commissioning executed. It is claimed that this modularization reduces the overall weight of the topside. A challenge with this modularization split is the offshore installation of a larger number of interfaces resulting in offshore work exposing equipment and personnel to harsh offshore environments over extended periods.

[0009] It is the goal of the present invention to address and solve the problems related to the prior art mentioned above.Summary of the invention

[0010] In a first aspect of the invention, the invention provides an offshore HVDC substation for transmission of electrical energy to an onshore substation comprising: a) one jacket substructure; b) a first self-sufficient HVDC module mounted on the jacket substructure; c) a second self-sufficient HVDC module mounted on the jacket substructure side by side with the first HVDC module,

[0011] wherein the first HVDC module and the second HVDC module comprises an HVDC pole. The structural design of the first HVDC module and the second HVDC module can be self-supporting and has jacket interfaces integrated without module support frame. The main steel structural design of the first HVDC module and the second HVDC module can be based on "plated structure design", to transfer global loads as in-plane membrane action in the plate panels forming a water and EMC tight enclosure with weight efficient strength.

[0012] The side of the first self-sufficient HVDC module that faces a space between the first HVDC module and the second HVDC module is mirrored relative to the opposite side of the second HVDC module that faces the space between the first HVDC module and the second HVDC module.

[0013] A HVDC substation may have the capability to operate in a wide range of input AC voltage, the same voltage flexibility applies to the DC-output. In one configuration the HVDC substation can operate on 66 or 132kV input AC voltage and outputs typically between ±500kV and ±1500 kV HVDC. In one particular embodiment the HVDC substation outputs ±525 kV.

[0014] The first HVDC module and the second HVDC module may at least comprise one of: a) a HVDC pole comprising of converter valves, b) one or more transformers, c) switchgear, d) reactors, e) sea cable interfaces and f) a set of utilities such as cooling water systems, HVA / C, firefighting and control systems.

[0015] In a second aspect of the invention, the invention provides a method for assembling an offshore HVDC substation at least comprising the steps of: a) transporting a jacket to a site where the assembled HVDC substation shall be established; b) launching the jacket from a barge, or installing the jacket with a heavy lift vessel; c) transporting a first self-sufficient HVDC module on a barge or heavy transport vessel to the site where the assembled HVDC substation shall be established; d) lifting the first self-sufficient HVDC module to the top of the jacket and one of steps e - i or step j - n, or by float-over operation installing the first self-sufficient HVDC module directly onto the jacket from either the barge or the heavy transport vessel and one of steps e - i or step j - n;e) connecting the first HVDC module to AC-suppliers and to an onshore substation; f) energising and operating the first HVDC module as an independent self-sufficient HVDC module; g) transporting a second self-sufficient HVDC module on a barge or heavy transport; vessel to the site where the assembled HVDC substation shall be established, h) lifting the second self-sufficient HVDC module to the top of the jacket, or by float-over operation installing the second self-sufficient HVDC module directly onto the jacket from either the barge or the heavy transport vessel; i) connecting the second HVDC module to AC-suppliers and to an onshore substation; j) transporting a second self-sufficient HVDC module on a barge or heavy transport; vessel to the site where the assembled HVDC substation shall be established, k) lifting the second self-sufficient HVDC module to the top of the jacket, or by float-over operation installing the second self-sufficient HVDC module directly onto the jacket from either the barge or the heavy transport vessel; l) connecting the second HVDC module to AC-suppliers and to an onshore substation; m) connecting the first HVDC module and the second HVDC module to AC-suppliers and to an on-shore substation; n) energising and operating the first HVDC and the second HVDC module as an independent self-sufficient HVDC module. In one aspect of the method the steps are carried out sequentially.

[0016] The method may include a longer stay in time between item d and item e of the method described above. In one example the first self-sufficientHVDC module and the second self-sufficient HVDC module each comprises at least one of: a) an HVDC pole including converter valves, b) utilities c) one or more transformers, d) switchgear, e) reactors and f) sea cable interface.

[0017] Other advantageous features will be apparent from the accompanying claims.Brief description of the drawings

[0018] To make the invention more readily understandable, the discussion that follows will refer to the accompanying drawings, in which:

[0019] Figure 1 shows an embodiment where the installation of a second module is installed by an offshore installation vessel (205), and

[0020] Figure 2 shows a top view of a HVDC substation according to the present invention.

[0021] Figure 3 shows a view of a HVDC substation, visualising the mirrored modules according to the present invention.Detailed description of the Invention

[0022] In the following, general embodiments as well as exemplary embodiments of the invention will be described. References and possible numerals will be made to the accompanying drawings. It shall be noted, however, that the drawings are exemplary embodiments only, and that other features and embodiments may well be within the scope of the invention as described.

[0023] A HVDC module in the context of this document, is a structural topside module containing a HVDC pole.

[0024] A self-sufficient HVDC module is a module that consist of a HVDC pole in addition to cooling, ventilation and auxiliary power utilities within the module.

[0025] The increase in size of the wind farms incentivise higher voltages to increase the amount of power transferred per transmission link to shore. Due to the large size of HVDC equipment the size of the offshore constructions to be installed is a concern. The introduction of 525kV bipole HVDC transmission technology offshore significantly increase the transmission capacity of a single link but also increases the weight and dimensions HVDC topside which in turn poses a challenge to the contractors designing, constructing, transporting and installing offshore topsides where HVDC equipment has been integrated. In the context of the invention a bipole is a HVDC transmission link combining two HVDC poles on one substructure and that shares one set of export cables to an onshore substation.

[0026] Bi-pole HVDC equipment integrated in one single topside module can result in sizes and weights which significantly reduces the number of yards capable to fabricate, it reduces the number of vessels capable to transport and reduces the number of installation vessels capable of lifting such heavy topsides onto their substructures. For the largest topsides, the only capable heavy lift vessel is Pioneering Spirit.

[0027] These weight and volume challenges can add significant cost to the project but also add significant risk. If the lifting vessel for some reason is unavailable, the complete execution plan can fail since there are no good backup. This could be caused by other clients booking vessel for extended periods of time, the vessel operating on another continent, failure of vessel or similar.

[0028] The following describes a solution for efficiently integrating bi-pole HVDC technology specifically related to wind energy transmission but generally can apply to any offshore transmission solution integrating bi-pole HVDC, by bi-pole is meant two distinct HVDC rectification poles with opposite polarity and with shared neutral point. In one concrete embodiment the HVDC substation operates on 66 or 132kV input AC voltage and outputs +-525kV HVDC. In other embodiments the HVDC output can be in the range of ±500kV and ±1500 kV.

[0029] The current is converted from alternating current to direct current by a rectifier offshore, while the current is converted from direct current to alternating current with an inverter on the land side, so that the transmission line connects two asynchronous alternating current systems. The offshore rectifier comprises several components such as converter unit, converter valves, converter transformers, filters, reactive power source, HVDC system pole etc. As indicated at the beginning, it is very demanding to install HVDC equipment offshore. In the present invention, the problem is solved by modularisation, which will at least comprise a jacket with several legs as well as two modules each comprising an offshore rectifier. Each of the two modules can be installed offshore by an offshore heavy lift vessel, or by float-over operation where each of the modules are directly installed onto the jacket from either a barge or heavy transport vessel by ballasting down the transport vessel.

[0030] Each of the two modules (DC poles) are physically integrated in two physically separate modules where the supporting auxiliary systems per pole are to a practical extent able to operate autonomously. Each modules structural integrity is developed to be physically independent. Each physically separated module can be installed separately, and at a different time, and the auxiliary systems support operation as a monopole. The autonomy of each module allows for minimized offshore hookup work since there are very few interfaces between the modules, completed onshore commissioning, and it allows for lower installation weights (pr module). The lower installation weigth provides flexibility with respect to capable heavy lifting vessels, and their availability in the market, serving as a significant advantage in project execution.

[0031] Each of the two modules contain High Voltage AC (HVAC), HVDC, auxiliary systems and utility systems with sufficient capacity and with sufficient space to support all operations.

[0032] This invention utilizes modularization to reduce the cost and risk associated with transport and installation. Splitting the topside into two separate modules decreases the required size of vessels and it istherefore more vessels capable of transporting and lifting the topside onto the jacket.

[0033] One embodiment of the invention is shown in Figure 2. The embodiment splits the HVDC bipole topside into two modules that are to the practical extent independent of each other. A bipolar link has two distinct HVDC rectification poles with opposite polarity and with shared neutral point. Three cables connect the offshore and onshore stations on the positive, negative and neutral voltage levels.

[0034] The most significant advantage of the bipolar link is that if any of their links stop operating, the link can be converted into monopolar mode because of the ground return system. The half of the system continues supplies the power. Such types of links are commonly used in the HVDC systems.

[0035] The independence of the two modules is achieved by each module containing a complete HVDC pole comprising AC-DC rectifying valves, transformers, switchgear, reactors and sea cable interface and a complete set of required utilities such as cooling water systems, HVA / C, firefighting and control systems. It could also include AC harmonic filters. All utility systems in each pole are also redundant such that in principle no single fault can cause a trip of HVDC operation. The modules are independent to the degree that a first module could be installed and put into operation months or years before a second module. This independence between modules enables a staged deployment and therefore better capital utilization. It also enables a high degree of onshore commissioning, and it minimizes the offshore hookup work required for a functioning HVDC substation.

[0036] The only hookup required for the first module is to connect to sea cables, lower seawater pumps, installation of necessary secondary structures such as walk-ways and structural adjoining to the substructure. The second module have the same hookup with the addition of a neutral cable between the two modules allowing them to be series connected.

[0037] Figure 3 illustrates that the first 200 and the second 201 HVDC module are arranged opposite and next to each other with a space 202 between first HVDC module 200 and second HVDC module 201. To facilitate this "next to each other"- configuration the HVDC modules side facing each other are mirrored. This is illustrated with utilities 211 of the first HVDC module 200 facing the utilities 211 of the second HVDC module. Mirroring the two HVDC modules facilitates connecting them together and also to operate them. Like an entrance / exit in one HVDC module faces an entrance / exit in the neighbouring HVDC module, similarly, this applies to the connections between neighbouring modules.

[0038] The mirroring of the first self-sufficient HVDC module 200 and the second self-sufficient HVDC module 201 facilitate good material handling between the two self-sufficient HVDC modules.

[0039] Although not required, the two modules could have additional cross connections to improve availability numbers such as: a) Between incoming AC switchgear of the two modules such that in the event of unavailability of one of the modules power from the wind turbines which is usually transferred via the unavailable module can be transferred via the available module. b) Between auxiliary power systems of the two modules such that in the event of unavailability of the auxiliary power for one module this function can be served by the other module to increase availability. c) Between cooling systems of the two modules such that in the event of unavailability of the cooling systems in module this function can be served by the other module to increase availability.

[0040] The modularisation approach is not expected to reduce the total weight of the complete topside, but it decreases the risk of fabricating, transporting and installing the topside. E.g. two modules of this shape are easier to build because cranes do not need to reach so far from the outer boundary of the module and smaller modules are easier to handle in general. The modularisation approach brings the weight of each module down to alevel where there are several available installation vessels, and this is one feature that lowers project costs and risks of installation.

[0041] In one embodiment the topside structural design of each module is self- supporting and has jacket interfaces integrated in such a way that no module support frame is required. The overall main steel structural design of the topside can be based on "plated structure design", i.e. global loads are transferred as in-plane membrane action in the plate panels forming a water and EMC tight enclosure with weight efficient strength.

[0042] With reference to figure 2 the first module (200) and the second module (201) is installed on the jacket (204). Jacket is not visible between the first module and the second module in top view since the space between the first module and the second module (202) is filled with access platforms. Each module (200, 201) is independent of each other except for main export cable connection. However, a small number of interfaces can be installed (208) to increase the redundancy and therefore the availability of the bipole. Outline of jacket (204) can briefly be seen on the jacket interface locations.

[0043] After installation on the jacket substructure, each of the two modules are structurally connected only to the jacket and not to each other. The space between modules can be used for access platforms. This structural design allows relative motions between the modules.

[0044] The substructure is a traditional jacket structure with either piles or suction buckets securing it to the seabed. The jacket will always be customized to each specific site with the relevant depths and metocean conditions. However, the interface between jacket and topside modules is independent of external circumstances and can therefore be the same for several installations.

[0045] Carrying out the invention may include the steps of:1. Transporting the jacket on a barge to the site where the HVDC substation shall be established;2. Launching the jacket with a heavy lift vessel, or installing the jacket with a heavy lift vessel;After set down on seabed, it is secured in place using suction buckets or piling operations;3. Transporting a first module on a barge or heavy transport vessel to the site where the HVDC substation shall be established;4. Lifting the first module to the top of the jacket, or by float-over operation;5. Energising and operating the first HVDC module as an independent self-sufficient HVDC module;6. Transporting a second module on a barge or heavy transport; vessel to the site where the assembled HVDC substation shall be established,7. Lifting the second module to the top of the jacket, or by float-over operation where the second module is directly installed onto the jacket from either a barge or heavy transport vessel.

[0046] The lifting operation may be carried out using large heavy lift vessels. As indicated above the modules are standalone units which means that they can operate alone and dependent on each other or independent of each other. There are two main alternatives, namely:Alternative 1: a. The first module 1 is commissioned and put into operation before the second module is installed; b. The second module installed when the wind farm development has reached a size that uses the complete capacity of the first module.Alternative 2: a. The second module is installed directly following the first module; b. Commissioning of both modules simultaneously.

[0047] The installation steps 1 - 6 above still applies, however it may include a longer stay in time between item 4 above and item 5.

[0048] The lifting vessels may have lift capacities in the range of 5000 to 50.000 metric tonnes for topsides and up 25000 metric tonnes lifting capacities for jackets.

[0049] Ref table

Claims

Claims1. An offshore HVDC substation for transmission of electrical energy to an onshore substation comprising: a. one jacket substructure (204); b. a first self-sufficient HVDC module (200) mounted on the jacket substructure (204); c. a second self-sufficient HVDC module (201) mounted on the jacket substructure side by side with the first HVDC module (200), wherein the first HVDC module (200) and the second HVDC module (201) each comprises an HVDC pole.

2. An offshore HVDC substation according to claim 1 where the structural design of the first HVDC module and the second HVDC module is self- supporting and has jacket interfaces (203) integrated without module support frame.

3. An offshore HVDC substation according to claim 1 or 2, where the side of the first self-sufficient HVDC module that faces a space between the first HVDC module and the second HVDC module is mirrored relative to the opposite side of the second HVDC module that faces the space between the first HVDC module and the second HVDC module.

4. An offshore HVDC substation according to claim 2 where the main steel structural design of the first HVDC module (200) and the second HVDC module (201) is based on "plated structure design", to transfer global loads as in-plane membrane action in the plate panels forming a water and EMC tight enclosure with weight efficient strength.

5. An offshore HVDC substation according to any of the previous claims where the first HVDC module (200) and the second HVDC module (201) both integrates bi-pole HVDC technology.

6. An offshore HVDC substation according to claim 4 or 5 where the HVDC substation operates on 66 or 132kV input AC voltage and outputs typically between ±500kV and ±1500 kV HVDC.

7. An offshore HVDC substation according to claim 6 where the HVDC substation outputs ±525 kV HVDC.

8. An offshore HVDC substation according to any of the previous claims where the first HVDC module (200) and the second HVDC module (201) at least comprises one of: a. a HVDC pole comprising converter valves, b. one or more transformers, c. switchgear, d. reactors, e. sea cable interfaces and f. a set of utilities such as cooling water systems, HVA / C, firefighting and control systems.

9. A method for assembling an offshore HVDC substation at least comprising the steps of: a. transporting a jacket (204) to a site where the HVDC substation shall be established; b. launching the jacket (204) from a barge, or with a heavy lift vessel, or installing the jacket with a heavy lift; c. transporting a first self-sufficient HVDC module (200) on a barge or heavy transport vessel to the site where the HVDC substation shall be established; d. lifting the first self-sufficient HVDC module to the top of the jacket (204) and one of steps e - i or step j - n, or by float-over operation installing the first self-sufficient HVDC module (200) directly onto the jacket from either the barge or the heavy transport vessel and one of steps e - i or step j - n; e. connecting the first HVDC module to AC-suppliers and to an on-shore substation; f. energizing and operating the first HVDC module as an independent self- sufficient HVDC module;g. transporting a second self-sufficient HVDC module (201) on a barge or heavy transport; vessel to the site where the second assembled HVDC substation shall be established, h. lifting the second self-sufficient HVDC module (201) to the top of the jacket (204), or by float-over operation installing the self-sufficient HVDC module (201) directly onto the jacket (204) from either the barge or the heavy transport vessel i. connecting the second HVDC module to AC-suppliers and to an on-shore substation; j. transporting a second self-sufficient HVDC module on a barge or heavy transport; vessel to the site where the assembled HVDC substation shall be established, k. lifting the second self-sufficient HVDC module to the top of the jacket, or by float-over operation installing the second self-sufficient HVDC module directly onto the jacket from either the barge or the heavy transport vessel; l. connecting the second HVDC module to AC-suppliers and to an on-shore substation; m. connecting the first HVDC module and the second HVDC module to AC-suppliers and to an on-shore substation; n. energising and operating the first HVDC and the second HVDC module as an independent self-sufficient HVDC module.

10. A method for assembling an offshore HVDC substation according to claim 9 where the steps are carried out sequentially.

11. A method for assembling an offshore HVDC substation according to claim10 wherein the first self-sufficient HVDC module (200) and the second self- sufficient HVDC module (201) each comprises at least one of: a. a HVDC pole including converter valves, b. utilities,c. one or more transformers d. switchgear, e. reactors and f. sea cable interface.

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