Triple-circulation cooling system of offshore flexible direct-current power transmission platform

Abstract

The invention discloses a triple-circulation cooling system for an offshore flexible direct-current power transmission platform, which is characterized by comprising a deionized water cooling circulation system, a fresh water cooling circulation system and a seawater cooling circulation system, deionized water, fresh water and seawater are used as cooling media in the deionized water cooling circulation system, the fresh water cooling circulation system and the seawater cooling circulation system respectively. A fresh water cooling circulation system is additionally arranged between the two circulation cooling systems, and cooling is achieved through two times of heat exchange. A closed system is adopted for fresh water cooling circulation. The fresh water cooling circulation system is internally and stably pressurized, so that the pressure in the system is greater than that of the seawater cooling circulation system. When the heat exchanger is damaged, seawater can be prevented from entering the fresh water circulating system through the micro pressure difference design. In addition, the deep-water-taking and shallow-water-discharging and mixed-flow water discharging design is adopted, the warm water discharging problem of seawater discharging can be solved, and the water taking effect is improved.

Description

Technical Field

[0001] This invention relates to the field of offshore equipment cooling technology, specifically to a three-cycle cooling system for an offshore flexible DC power transmission platform. Background Technology

[0002] Against the backdrop of global energy transition, offshore wind power continues to develop, showing trends of increasing single-unit capacity, expanding farm scale, and extending into distant waters. Offshore wind power transmission methods are divided into AC transmission and DC transmission. As wind farms are located further offshore, DC technology is more economical for long-distance transmission. Among DC transmission technologies, flexible DC transmission is one of the most advanced technologies currently available and has been applied in numerous offshore power generation projects.

[0003] The three main cooling components of an offshore flexible DC transmission platform are the converter valves, transformers, and HVAC systems. The converter valves are used for AC / DC conversion and power control. The transformers convert the AC system voltage to the commutation voltage required by the converter. The HVAC system primarily controls the temperature and humidity in the valve hall.

[0004] Existing offshore flexible DC transmission platforms typically employ a two-stage cooling system, consisting of a converter valve cooling system and a seawater cooling system. This two-stage cooling system involves primary heat exchange via heat exchangers and has a simple structure, but it carries the risk of seawater seeping into the converter valve cooling system through the heat exchangers. Summary of the Invention

[0005] In view of this, embodiments of the present invention provide a three-cycle cooling system for offshore flexible DC transmission platforms, which achieves cooling through two heat exchanges, solving the problem of seawater immersion in the converter valve cooling system.

[0006] This invention provides a three-cycle cooling system for an offshore flexible DC transmission platform, comprising a deionized water cooling circulation system, a fresh water cooling circulation system, and a seawater cooling circulation system; wherein the deionized water cooling circulation system, the fresh water cooling circulation system, and the seawater cooling circulation system use deionized water, fresh water, and seawater as cooling media, respectively. The deionized water cooling circulation system is connected to the wind power converter station through a converter valve. Heat exchange is performed through the converter valve using deionized water to cool the wind power converter station. The freshwater cooling circulation system is connected to the deionized water cooling circulation system through a first heat exchanger. In the first heat exchanger, the deionized water is cooled by heat exchange with freshwater. The seawater cooling circulation system is connected to the freshwater cooling circulation system through a second heat exchanger. The freshwater is cooled by heat exchange through seawater in the second heat exchanger. The pressure of the freshwater in the freshwater cooling circulation system is higher than the pressure of the seawater in the seawater cooling circulation system. The seawater cooling circulation system has an inlet and an outlet; the inlet and outlet are respectively connected to the sea to allow seawater to flow in and out, and the vertical height of the inlet is lower than that of the outlet.

[0007] Furthermore, the deionized water cooling circulation system includes a deionized water cooling circulation loop; the deionized water cooling circulation loop includes a first circulation pump, a three-way valve, a first filter, and a first degassing tank; the first circulation pump is used to drive the deionized water to circulate within the deionized water cooling circulation loop; the three-way valve is installed on both sides of the first heat exchanger and is used to adjust the heat exchange efficiency of the first heat exchanger; the first filter is used to filter the deionized water; and the first degassing tank is used to reduce the pressure of the deionized water.

[0008] Furthermore, the deionized water cooling circulation system also includes a deionized water replenishment branch and a deionized water nitrogen pressurization branch; the deionized water replenishment branch and the deionized water nitrogen pressurization branch are connected to the deionized water cooling circulation loop; The deionized water replenishment branch includes a first replenishment pump and an ion exchanger; the first replenishment pump is used to replenish the deionized water cooling circulation loop with deionized water; the ion exchanger is used to filter out ionic impurities from the deionized water; The deionized water nitrogen pressurization branch includes a nitrogen cylinder and a first expansion tank; the nitrogen cylinder is used to provide nitrogen supply; the first expansion tank is used to increase the pressure of deionized water with nitrogen; the pressure regulation of the deionized water cooling circulation loop is achieved through the cooperation of the first expansion tank and the first degassing tank.

[0009] Furthermore, the freshwater cooling circulation system includes a freshwater cooling circulation loop; the freshwater cooling circulation loop includes a second circulation pump, a second filter, and a second degassing tank; the second circulation pump is used to drive freshwater to circulate within the freshwater cooling circulation loop; the second filter is used to filter the freshwater; and the second degassing tank is used to reduce the pressure of the freshwater.

[0010] Furthermore, the freshwater cooling circulation system also includes a freshwater replenishment branch and a freshwater nitrogen pressurization branch; the freshwater replenishment branch and the freshwater nitrogen pressurization branch are connected to the freshwater cooling circulation loop; The freshwater replenishment branch includes a second replenishment pump and an ion exchanger; the second replenishment pump is used to provide freshwater replenishment to the freshwater cooling circulation loop; the ion exchanger is used to filter out ionic impurities in the freshwater. The freshwater nitrogen pressurization branch includes a nitrogen cylinder and a second expansion tank; the nitrogen cylinder is used to provide nitrogen supply; the second expansion tank is used to increase the pressure of the freshwater through nitrogen; the pressure regulation of the freshwater cooling circulation loop is achieved through the cooperation of the second expansion tank and the second degassing tank.

[0011] Furthermore, the freshwater cooling circulation system also includes a water quality sensor; the water quality sensor is used to determine whether seawater has entered the freshwater cooling circulation loop.

[0012] Furthermore, the freshwater cooling circulation system also includes a third heat exchanger; the third heat exchanger is connected to the transformer's cooling circuit; and freshwater is used in the third heat exchanger to perform heat exchange operations and cool the transformer's cooling medium.

[0013] Furthermore, the seawater cooling circulation system includes a seawater pump, a third filter, a filter screen, and an electrolysis device. The seawater pump is installed at the inlet and outlet of the seawater cooling circulation system to drive seawater into the system from the inlet and to extract seawater at the outlet to neutralize heat and flow out. The third filter is used to filter the seawater in the seawater cooling circulation system. The filter screen is installed at the inlet and outlet of the seawater cooling circulation system to prevent marine organisms from entering the system. The electrolysis device is used to electrolyze the seawater to generate chlorine gas, which is then applied to the filter screen to repel marine organisms.

[0014] Furthermore, the seawater cooling circulation system also includes a fourth heat exchanger; the third heat exchanger is connected to the cooling circuit of the HVAC system; and the fourth heat exchanger performs heat exchange operations through seawater to cool the cooling medium of the HVAC system.

[0015] Furthermore, the seawater cooling circulation system also includes a freshwater preparation device and a fire-fighting system; the freshwater preparation device is used to prepare freshwater for supply to the freshwater cooling circulation system; the fire-fighting system is used to draw seawater to extinguish fires when a fire occurs on the offshore flexible DC transmission platform.

[0016] The embodiments of the present invention have the following beneficial effects: The three-cycle cooling system for offshore flexible DC transmission platforms of the present invention adds a freshwater cooling cycle system between the two existing cooling cycles, achieving cooling through two heat exchanges. The freshwater cooling cycle of the present invention adopts a closed-loop system. The freshwater cooling cycle system uses internal stable pressurization to ensure that the pressure within the system is greater than that of the seawater cooling cycle system. When the heat exchanger fails, a micro-pressure differential design can prevent seawater from entering the freshwater cycle system. Furthermore, the present invention also features a deep-intake, shallow-discharge and mixed-flow drainage design, which can solve the problem of warm discharge from seawater and improve water intake efficiency.

[0017] Additional aspects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description or may be learned by practice of the invention. Attached Figure Description

[0018] To more clearly illustrate the technical solutions in the embodiments of this application, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0019] Figure 1 This is a schematic diagram of the overall structure of a three-cycle cooling system for a flexible DC power transmission platform at sea according to the present invention; Figure 2 This is a schematic diagram of the deionized water cooling circulation system of the present invention; Figure 3 This is a schematic diagram of the freshwater cooling circulation system of the present invention; Figure 4 This is a schematic diagram of the seawater cooling circulation system of the present invention. Detailed Implementation

[0020] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application.

[0021] like Figure 1 As shown, this embodiment of the invention provides a three-cycle cooling system for an offshore flexible DC transmission platform, including a deionized water cooling circulation system, a fresh water cooling circulation system, and a seawater cooling circulation system; the deionized water cooling circulation system, the fresh water cooling circulation system, and the seawater cooling circulation system use deionized water, fresh water, and seawater as cooling media, respectively. The deionized water cooling circulation system is connected to the wind power converter station through a converter valve. Heat exchange is carried out through the converter valve using deionized water to cool the wind power converter station. The freshwater cooling circulation system is connected to the deionized water cooling circulation system through the first heat exchanger. In the first heat exchanger, the deionized water is cooled by heat exchange with freshwater. The seawater cooling circulation system is connected to the freshwater cooling circulation system through a second heat exchanger. In the second heat exchanger, seawater is used for heat exchange to cool the freshwater. The pressure of the freshwater in the freshwater cooling circulation system is higher than that of the seawater in the seawater cooling circulation system. The seawater cooling circulation system has an inlet and an outlet; the inlet and outlet are respectively connected to the sea to allow seawater to flow in and out, and the vertical height of the inlet is lower than that of the outlet.

[0022] This invention, through a unique three-loop structure design, adds an intermediate freshwater cooling loop to the traditional two-loop system, achieving reliable heat transfer through two heat exchanges. This invention eliminates the risk of seawater intrusion into the precision systems of offshore flexible DC transmission platforms in the event of a seawater heat exchanger rupture, thus improving the safety and reliability of cooling for flexible DC transmission platform equipment.

[0023] On the other hand, traditional methods typically involve directly drawing water from the warmer sea surface and discharging the hot wastewater into nearby waters. This can easily cause a significant increase in local sea temperature, resulting in warm wastewater discharge and causing continuous adverse effects on the marine ecosystem. In contrast, this invention places the inlet below the outlet and draws seawater from the outlet to further dilute the temperature, thereby controlling the temperature of the discharged seawater at a lower level and overcoming the warm wastewater discharge problem. In some embodiments, the inlet can also be designed with adjustable height, allowing it to dynamically adjust its height according to tides and ocean currents to stably obtain cool, deep seawater with lower temperature and less sediment content, preventing backflow.

[0024] The implementation process of each part of this invention is described in detail below: Deionized water cooling circulation system: such as Figure 2 As shown, the deionized water cooling circulation system includes a deionized water cooling circulation loop; the deionized water cooling circulation loop includes a first circulation pump, a three-way valve, a first filter, and a first degassing tank; the first circulation pump is used to drive the deionized water to circulate within the deionized water cooling circulation loop; the three-way valve is installed on both sides of the first heat exchanger to adjust the heat exchange efficiency of the first heat exchanger; the first filter is used to filter the deionized water; and the first degassing tank is used to reduce the pressure of the deionized water.

[0025] In this embodiment of the invention, the circulating loop devices in the deionized water circulation system are redundantly configured to meet the high reliability requirements of the converter valve cooling system. When one set of circulating loop devices fails, the system can switch to other circulating loop devices for cooling circulation, ensuring the normal operation of the flexible DC transmission platform.

[0026] In this embodiment of the invention, the deionized water cooling circulation system further includes a deionized water replenishment branch and a deionized water nitrogen pressurization branch; the deionized water replenishment branch and the deionized water nitrogen pressurization branch are connected to the deionized water cooling circulation loop; the deionized water replenishment branch includes a first replenishment pump and an ion exchanger; the first replenishment pump is used to replenish deionized water to the deionized water cooling circulation loop; the ion exchanger is used to filter out ionic impurities in the deionized water; the deionized water nitrogen pressurization branch includes a nitrogen cylinder and a first expansion tank; the nitrogen cylinder is used to provide nitrogen supply; the first expansion tank is used to increase the pressure of the deionized water by using nitrogen; the pressure regulation of the deionized water cooling circulation loop is achieved through the cooperation of the first expansion tank and the first degassing tank.

[0027] In this embodiment of the invention, the design of the ionized water replenishment branch and the deionized water nitrogen pressurization branch enables the deionized water to maintain high purity and low conductivity, and prevents air from entering through pressure regulation.

[0028] Freshwater cooling circulation system: such as Figure 3 As shown, the freshwater cooling circulation system includes a freshwater cooling circulation loop; the freshwater cooling circulation loop includes a second circulation pump, a second filter, and a second degassing tank; the second circulation pump is used to drive the freshwater to circulate within the freshwater cooling circulation loop; the second filter is used to filter the freshwater; and the second degassing tank is used to reduce the pressure of the freshwater.

[0029] In this embodiment of the invention, the freshwater cooling circulation system, in addition to employing a redundant design, further adopts a micro-pressure differential gradient design. By actively establishing and precisely maintaining a stable pressure gradient where the freshwater side pressure is always higher than the seawater side pressure, the risk of seawater backflowing into the freshwater system and even the upstream deionized water system is prevented when the seawater heat exchanger breaks down due to corrosion or damage. This also avoids the corrosion and contamination of the precision equipment of the flexible DC transmission platform by high-salinity seawater.

[0030] In this embodiment of the invention, the freshwater cooling circulation system further includes a third heat exchanger; the third heat exchanger is connected to the cooling circuit of the transformer; and the transformer's cooling medium is cooled by heat exchange through freshwater in the third heat exchanger.

[0031] In this embodiment of the invention, the freshwater cooling circulation system also includes a freshwater replenishment branch and a freshwater nitrogen pressurization branch; the freshwater replenishment branch and the freshwater nitrogen pressurization branch are connected to the freshwater cooling circulation loop; The freshwater makeup branch includes a second makeup water pump and an ion exchanger; the second makeup water pump is used to provide freshwater to the freshwater cooling circulation loop; the ion exchanger is used to filter out ionic impurities in the freshwater. The freshwater nitrogen pressurization branch includes a nitrogen cylinder and a second expansion tank; the nitrogen cylinder is used to provide nitrogen supply; the second expansion tank is used to increase the pressure of the freshwater through nitrogen; the pressure regulation of the freshwater cooling circulation loop is achieved through the cooperation of the second expansion tank and the second degassing tank.

[0032] In this embodiment of the invention, the freshwater supply of the freshwater cooling circulation system is obtained through seawater separation, which can provide sufficient freshwater as a cooling medium on the offshore platform, while saving the consumption of deionized water.

[0033] In some embodiments, the freshwater cooling circulation system also includes a water quality sensor; the water quality sensor determines whether seawater has entered the freshwater cooling circulation loop. Through a micro-pressure differential design combined with the water quality sensor, multiple protective measures are implemented to prevent seawater intrusion into the freshwater system, ensuring the long-term operational reliability and maintenance economy of the offshore flexible DC transmission platform in a marine environment.

[0034] Seawater cooling circulation system: such as Figure 4 As shown, the seawater cooling circulation system includes a seawater pump, a third filter, a filter screen, and an electrolysis device. The seawater pump is installed at the inlet and outlet of the seawater cooling circulation system to drive seawater into the system from the inlet and to extract seawater at the outlet to neutralize heat and flow out. The third filter is used to filter the seawater in the seawater cooling circulation system. The filter screen is installed at the inlet and outlet of the seawater cooling circulation system to prevent marine organisms from entering the system. The electrolysis device is used to electrolyze the seawater to generate chlorine gas, which is then applied to the filter screen to repel marine organisms.

[0035] In this embodiment of the invention, the seawater cooling circulation system uses seawater as the cooling medium. Through redundantly configured circulation loop devices combined with a self-cleaning seawater filter and heat exchanger pipeline made of highly corrosion-resistant material, it can stably transport the heat generated by the flexible DC transmission platform to the sea for a long period of time.

[0036] In this embodiment of the invention, the seawater cooling circulation system further includes a fourth heat exchanger; the third heat exchanger is connected to the cooling circuit of the HVAC system; and the fourth heat exchanger performs heat exchange operations through seawater to cool the cooling medium of the HVAC system.

[0037] In this embodiment of the invention, the seawater cooling circulation system further includes a freshwater preparation device and a fire-fighting system; the freshwater preparation device is used to prepare freshwater for supply to the freshwater cooling circulation system; the fire-fighting system is used to extract seawater to extinguish fires when a fire occurs on the offshore flexible DC transmission platform.

[0038] This invention's freshwater preparation device employs reverse osmosis technology, which can efficiently convert seawater or raw water into pure freshwater that meets the requirements of the cooling system, ensuring a stable and sufficient supply of cooling medium in the freshwater cooling circulation system. The fire-fighting system is integrated into the seawater intake stage, enabling the extraction of seawater for fire suppression in the event of a fire on the offshore flexible DC transmission platform.

[0039] In some embodiments, the present invention also includes a cooling control and protection device in the offshore flexible DC transmission platform. In these embodiments, the cooling control and protection device is used to detect, control, and record the operating status of the deionized water, freshwater / seawater cooling circulation system, as well as the converter valves, transformers, and HVAC systems. The continuous monitoring of key parameters such as pressure, flow rate, temperature, liquid level, and conductivity within the cooling control and protection device enables the proactive maintenance of a safe pressure gradient between the freshwater and seawater through a micro-differential pressure control algorithm, thereby eliminating the risk of seawater intrusion. Simultaneously, the cooling control and protection device employs a redundant configuration, with multiple independent units serving as backups for each other, to maintain reliable communication with the circulation system and the flexible DC transmission platform. This provides reliability and safety assurance for the long-term unattended operation of the entire offshore flexible DC transmission platform cooling system in harsh marine environments.

[0040] In summary, this invention provides a three-cycle cooling system for offshore flexible DC transmission platforms. This system adds a freshwater cooling cycle between the two existing cooling cycles, achieving cooling through two heat exchanges. The freshwater cooling cycle is a closed-loop system. The freshwater cooling cycle maintains a stable internal pressurization, ensuring the internal pressure is higher than that of the seawater cooling cycle. In the event of heat exchanger failure, a micro-pressure differential design prevents seawater from entering the freshwater cycle. Furthermore, this invention incorporates a deep-intake, shallow-discharge and mixed-flow drainage design, which addresses the issue of warm water discharge and improves water intake efficiency.

[0041] Those skilled in the art will understand that modules in the device of the embodiments of the present invention can be adaptively modified and placed in one or more devices different from those embodiments. Modules, units, or components in the embodiments of the present invention can be combined into a single module, unit, or component, and further, they can be divided into multiple sub-modules, sub-units, or sub-components. Except where at least some of such features and / or processes or units are mutually exclusive, any combination can be used to combine all features disclosed in this specification (including the corresponding claims, abstract, and drawings) and all processes or units of any method or device so disclosed. Unless expressly stated otherwise, each feature disclosed in this specification (including the corresponding claims, abstract, and drawings) may be replaced by an alternative feature that serves the same, equivalent, or similar purpose.

[0042] Through the above description of the embodiments, those skilled in the art will understand that, for the sake of convenience and brevity, only the division of the above functional modules is used as an example. In actual applications, the above functions can be assigned to different functional modules as needed, that is, the internal structure of the device can be divided into different functional modules to complete all or part of the functions described above.

[0043] In the description of this specification, references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0044] Furthermore, the terms "first," "second," etc., used in the embodiments of this invention are for descriptive purposes only and should not be construed as indicating or implying relative importance, or implicitly specifying the number of technical features indicated in this embodiment. Therefore, features defined with terms such as "first" and "second" in the embodiments of this invention can explicitly or implicitly indicate that the embodiment includes at least one of those features. In the description of this invention, the word "multiple" means at least two or more, such as two, three, four, etc., unless otherwise explicitly specified in the embodiments.

[0045] In embodiments of the present invention, the terms "comprising," "including," or any other variations thereof are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes that element. Furthermore, components, features, and elements with the same names in different embodiments of the present invention may have the same meaning or different meanings, the specific meaning of which must be determined by its interpretation in that specific embodiment or further in conjunction with the context of that specific embodiment.

[0046] Although embodiments of the present invention have been shown and described above, it is to be understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make changes, modifications, substitutions, and variations to the above embodiments within the scope of the present invention. Other embodiments of the present invention will readily conceive of by considering the specification and practicing the invention. This application is intended to cover any variations, uses, or adaptations of the invention that follow the general principles of the invention and include common knowledge or customary techniques in the art not disclosed herein. The specification and embodiments are to be considered exemplary only, and the true scope and spirit of the invention are indicated by the following claims.

Claims

1. A three cycle cooling system for a marine HVDC platform, characterized by, The system comprises a deionized water cooling circulation system, a fresh water cooling circulation system and a sea water cooling circulation system; the deionized water cooling circulation system, the fresh water cooling circulation system and the sea water cooling circulation system use deionized water, fresh water and sea water as cooling medium respectively; The deionized water cooling circulation system is connected with the wind power converter station through a converter valve, and the deionized water cooling circulation system is cooled by heat exchange operation of deionized water in the converter valve; The fresh water cooling circulation system is connected with the deionized water cooling circulation system through a first heat exchanger, and the deionized water is cooled by heat exchange operation of fresh water in the first heat exchanger; The sea water cooling circulation system is connected with the fresh water cooling circulation system through a second heat exchanger, and the fresh water is cooled by heat exchange operation of sea water in the second heat exchanger; the pressure of the fresh water in the fresh water cooling circulation system is higher than the pressure of the sea water in the sea water cooling circulation system; The sea water cooling circulation system has a water inlet and a water outlet; the water inlet and the water outlet are respectively connected with the sea to realize the inflow and outflow of sea water, and the vertical height of the water inlet is lower than that of the water outlet.

2. A three cycle cooling system for a subsea HVDC platform according to claim 1, characterized in that, The deionized water cooling circulation system comprises a deionized water cooling circulation loop; the deionized water cooling circulation loop comprises a first circulating pump, a three-way valve, a first filter and a first degassing tank; the first circulating pump is used to drive the deionized water to circulate in the deionized water cooling circulation loop; the three-way valve is installed on both sides of the first heat exchanger and is used to adjust the heat exchange efficiency of the first heat exchanger; the first filter is used to filter the deionized water; and the first degassing tank is used to reduce the pressure of the deionized water.

3. A three-cycle cooling system for a subsea HVDC platform according to claim 2, characterized in that, The deionized water cooling circulation system further comprises a deionized water water supplement branch and a deionized water nitrogen pressure increasing branch; the deionized water water supplement branch and the deionized water nitrogen pressure increasing branch are connected with the deionized water cooling circulation loop; The deionized water water supplement branch comprises a first water supplement pump and an ion exchanger; the first water supplement pump is used to supply deionized water to the deionized water cooling circulation loop; and the ion exchanger is used to filter ion impurities in the deionized water; The deionized water nitrogen pressure increasing branch comprises a nitrogen cylinder and a first expansion tank; the nitrogen cylinder is used to supply nitrogen; and the first expansion tank is used to increase the pressure of the deionized water by nitrogen; the pressure of the deionized water cooling circulation loop is adjusted by cooperation of the first expansion tank and the first degassing tank.

4. The three-cycle cooling system for a submarine HVDC platform of claim 1, wherein, The fresh water cooling circulation system comprises a fresh water cooling circulation loop; the fresh water cooling circulation loop comprises a second circulating pump, a second filter and a second degassing tank; the second circulating pump is used to drive the fresh water to circulate in the fresh water cooling circulation loop; the second filter is used to filter the fresh water; and the second degassing tank is used to reduce the pressure of the fresh water.

5. A three cycle cooling system for a subsea HVDC platform according to claim 4, characterized in that, The fresh water cooling circulation system further comprises a fresh water water supplement branch and a fresh water nitrogen pressure increasing branch; the fresh water water supplement branch and the fresh water nitrogen pressure increasing branch are connected with the fresh water cooling circulation loop; The freshwater replenishment branch includes a second replenishment pump and an ion exchanger; the second replenishment pump is used to provide freshwater replenishment to the freshwater cooling circulation loop; the ion exchanger is used to filter out ionic impurities in the freshwater. The freshwater nitrogen pressurization branch includes a nitrogen cylinder and a second expansion tank; the nitrogen cylinder is used to provide nitrogen supply; the expansion tank is used to increase the pressure of the freshwater through nitrogen; the pressure regulation of the freshwater cooling circulation loop is achieved through the cooperation of the second expansion tank and the second degassing tank.

6. A three-cycle cooling system for a submarine HVDC platform according to claim 4 or 5, characterized in that, The freshwater cooling circulation system also includes a water quality sensor; the water quality sensor is used to determine whether seawater has entered the freshwater cooling circulation loop.

7. The three-cycle cooling system for a submarine HVDC platform of claim 1, wherein, The freshwater cooling circulation system also includes a third heat exchanger; the third heat exchanger is connected to the transformer's cooling circuit; and freshwater is used in the third heat exchanger to perform heat exchange operations and cool the transformer's cooling medium.

8. The three-cycle cooling system for a submarine HVDC platform of claim 1, wherein, The seawater cooling circulation system includes a seawater pump, a third filter, a filter screen, and an electrolysis device. The seawater pump is installed at the inlet and outlet of the seawater cooling circulation system to drive seawater into the system from the inlet and to extract seawater to neutralize heat and flow out from the outlet. The third filter is used to filter the seawater in the seawater cooling circulation system. The filter screen is installed at the inlet and outlet of the seawater cooling circulation system to prevent marine organisms from entering the system. The electrolysis device is used to electrolyze the seawater to generate chlorine gas, which is then applied to the filter screen to repel marine organisms.

9. The three-cycle cooling system for a submarine HVDC platform of claim 1, wherein, The seawater cooling circulation system also includes a fourth heat exchanger; the third heat exchanger is connected to the cooling circuit of the HVAC system; and the fourth heat exchanger performs heat exchange operations through seawater to cool the cooling medium of the HVAC system.

10. The three-cycle cooling system for a submarine HVDC platform of claim 1, wherein, The seawater cooling circulation system also includes a freshwater preparation device and a fire-fighting system; the freshwater preparation device is used to prepare freshwater for supply to the freshwater cooling circulation system; the fire-fighting system is used to extract seawater to extinguish fires when a fire occurs on the offshore flexible DC transmission platform.

Images