CN110042819A - A kind of marine converter station for flexible HVDC transmission system - Google Patents

Abstract

The invention provides an offshore converter station for a flexible direct current transmission system, which includes a closed superstructure and a basic structure for supporting the superstructure; the superstructure includes a large-space main powerhouse and a small-space auxiliary structure arranged side by side. Factory building; the main factory building is divided into upper and lower layers, of which the upper layer is equipped with current limiting reactor, DC field equipment, converter transformer, high-voltage switchgear, power system and emergency diesel engine, and the lower layer is equipped with converter valve and bridge arm reactor; The auxiliary workshop has six floors. The first floor is equipped with seawater pump room and valve cooling equipment, the second floor is equipped with seawater treatment equipment, the third floor is equipped with valve control room, the fourth floor is equipped with fire fighting equipment, the fifth floor is equipped with positive pressure ventilation equipment, and the sixth floor is equipped with Station control room; the superstructure is generally symmetrically arranged, the basic structure is the jacket and the passage for the barge to enter and exit is reserved in the middle. The technical scheme is compact and reasonable in layout, safe and reliable in operation, and adapts to marine environmental conditions and offshore construction characteristics.

Description

An offshore converter station for flexible DC transmission system

technical field

The invention relates to an offshore converter station for a flexible direct current transmission system.

Background technique

Offshore wind power generation is an emerging industry, which has developed rapidly in recent years. However, the offshore wind farms in my country are generally within 30km of the coast. Therefore, the electrical energy of offshore wind power in my country is currently output through AC submarine cables. With the development of offshore wind power to a larger scale and a longer distance, in general, when the offshore wind farm is more than 60km away from the shore, it is obvious that the traditional AC transmission method is no longer economical. Direct current transmission method of electrical energy transmission. With the increasing demand for transmission distance and transmission capacity, DC transmission will play an important role in the development and utilization of offshore wind farms. Offshore wind farms using DC transmission need to build offshore converter stations. The converter station is a device that collects the electric energy of the offshore wind farm with an AC line and then converts it into a DC output.

The field of onshore HVDC power transmission in my country has been developed for many years, and several onshore converter stations have been built. Conventional onshore converter stations are generally equipped with converter transformers, high-voltage switches, bridge arm reactors, converter valves, current limiting reactors, DC fields and other equipment according to the process flow of AC to DC. The converter transformers are generally arranged Outdoors, the rest of the equipment is set up in a row of single-storey workshops. However, due to the differences in the construction method and environment of the offshore converter station, it is necessary to consider such as the corrosion of seawater and sea breeze, the cooling problem of electrical equipment, the area and load-bearing capacity of the converter station, the convenience of equipment handling and maintenance, and the Problems such as the installation method of the station, etc., lead to different layout plans, structural types, cooling methods, etc. Therefore, the onshore converter station has been unable to provide inspiration.

In the existing technical solutions of fixed offshore converter stations, some problems in the space design and placement of various equipment make it difficult to transport converter transformers, high-voltage switches and other equipment, so a large number of transport channels are set up, resulting in a waste of space. , The overall area of the converter station is large, and the maintenance of the converter transformer is relatively inconvenient. At the same time, the technology is to connect the outermost row of columns with the foundation, the connection stiffness is too small, and the structural span is too large. There are also some, although the layout of the equipment has been improved to reduce the cost, but the space accommodates less equipment and electrical appliances, resulting in a single function of the converter station, which cannot be dealt with in the event of a failure, and the continuous operating conditions are poor. More importantly, none of the above-mentioned technical solutions explicitly or properly consider the problem of structural weight symmetry, and each layer structure is arranged in a rectangular shape on the horizontal plane. In these two schemes, if the converter transformers, high-voltage switches and other HVAC and water supply and drainage equipment are arranged in detail, it will inevitably lead to the structure that can only achieve no eccentricity in the direction of the short side of the horizontal plane. In the long-side direction, the center of gravity will be seriously deviated to the side with more interlayers or higher floors (the total weight of the upper block of the offshore converter station is about 20,000 tons, and the eccentricity of the above scheme can reach at least 5m, then under static conditions The overturning moment will reach 100,000 tons), which will increase the design and construction difficulty of the pile foundation, jacket and upper block frame under the load conditions of the marine environment, significantly increase the project cost, and have no feasibility of practical application.

SUMMARY OF THE INVENTION

The purpose of the present invention is to overcome the shortcomings in the prior art, and to provide an offshore converter station for a flexible DC power transmission system that is compact and reasonable in arrangement, safe and reliable in operation, and adaptable to marine environmental conditions and offshore construction characteristics.

In order to achieve the above object, the technical scheme adopted in the present invention is as follows:

An offshore converter station for a flexible direct current transmission system includes a closed superstructure and a base structure for supporting the superstructure; the superstructure includes a large-space main powerhouse and a small-space auxiliary powerhouse arranged side by side; the main powerhouse It is divided into upper and lower layers. The upper layer is equipped with current-limiting reactors, DC field equipment, converter transformers, high-voltage switchgear, plant power systems and emergency diesel engines, and the lower layer is equipped with converter valves and bridge arm reactors. There are six auxiliary workshops. The first floor is equipped with seawater pump room and valve cooling equipment, the second floor is equipped with seawater treatment equipment, the third floor is equipped with valve control room, the fourth floor is equipped with fire-fighting equipment, the fifth floor is equipped with positive pressure ventilation equipment, and the sixth floor is equipped with a station control room; The upper structure is generally symmetrically arranged, the base structure is a jacket and a passage for the barge to enter and exit is reserved in the middle.

The layout of the auxiliary powerhouse in this technical scheme comprehensively considers the unification of usability and structure, and the reasons are as follows: (1) The seawater treatment and cooling equipment are arranged on the first and second floors, which is convenient for collecting seawater near the sea; (2) Valves The control room is arranged on the lower floor, which can ensure the shortest line with the valve hall and can effectively reduce the line loss; (3) The fire equipment room is arranged on the middle deck layer of the whole converter station, which is convenient to cover the fire protection needs of the whole station; (4) ) The positive pressure ventilation equipment room is set on the middle layer (five layers) of the upper layer, which can maximize the heat dissipation effect on the main substation equipment such as transformers; (5) The station control room with the smallest volume and the best robustness It is arranged on the highest floor and will not affect other equipment function rooms; (6) In particular, according to the layout scheme of the present invention, the weight of the rooms of the auxiliary factory building is gradually reduced from bottom to top, which is especially beneficial to structural earthquake resistance and vibration reduction.

Moreover, through this innovative space layout division, not only the necessary structural rigidity and weight symmetry are guaranteed, but also the function of the offshore converter station is maximized; it can also realize the two horizontal dimensions in the plane of the offshore converter station. It is as close as possible to the square; and the valve hall is placed on the lower level, which can greatly benefit the construction organization and the design of foundations, components, nodes, etc., and improve the economy of the project in an all-round way.

Further, the lower layer of the superstructure is separated into two symmetrical and large-span valve halls and two symmetrical bridge arm reactor chambers through the wall plates. The converter valve and the bridge arm reactor are respectively placed in the corresponding chambers. A steel pestle frame is arranged on the upper layer; wherein, the valve hall is adjacent to the auxiliary workshop, and a cable joint room is also provided at the junction of the valve hall and the bridge arm reactor room.

Further, the upper layer of the superstructure is divided into a first area adjacent to the auxiliary factory building and a second area and a third area opposite to each other by the wall plate and the T-shaped aisle; the first area is divided into two symmetrical current limiting in a cross shape. The reactor room and two symmetrical DC rooms; the second area and the third area are similar in pattern, and they are divided into an electrical room with a larger area on one side and three compartments with a smaller area on the other side. Converter transformers are arranged in all areas; 220V high-voltage switch is placed in the electrical room of the second area, and emergency diesel engines and backup compensation reactors are placed in the other two compartments respectively; the electrical room in the third area is placed with the factory power system, and the other two compartments are placed with 320V High voltage switches and temporary equipment.

This optimized space layout method can ensure that the plane layout plan is basically symmetrical in the Y direction. In the X direction, the heavier converter transformer room, reactor room and DC room are arranged as close to the plane centroid as possible. The eccentricity of the structure is effectively reduced; and the equipment function rooms and their supporting rooms are arranged in the functional sequence from AC to DC conversion, and the routing of cables and other pipelines is the shortest.

Further, the first and second floors of the auxiliary powerhouse are divided into four compartments arranged in sequence, the two compartments in the middle of the first floor are valve cooling rooms, the other two end rooms are seawater pump rooms, and the two compartments in the middle of the second floor are the valve cooling rooms. One compartment is a cooling pool and the other two are water treatment chambers, so that the converter valve has an internal cooling system cooled by cooling liquid and an external cooling system relying on seawater circulation; the third floor is divided into two compartments, which are valve control. Room and ventilator room; the fourth floor is divided into battery room, secondary equipment room, fire pool and fire pump room in sequence; the fifth floor is divided into diesel oil tank room, foam fire extinguishing room and ventilator room; the sixth floor is divided into diesel power generation Engine room, station control room and other equipment room.

Further, the lower layer of the main factory building corresponds to the first to third floors of the auxiliary factory building, and the upper layer of the main factory building corresponds to the fourth to sixth floors of the auxiliary factory building; there is also a cable interlayer between the upper layer and the lower layer of the main factory building.

Further, the upper structure adopts a space truss structure, in which three steel trusses are arranged longitudinally, and multiple steel trusses are also arranged horizontally; the chord adopts a strong H-shaped or box-shaped structure.

Further, each longitudinal truss at the bottom of the superstructure is provided with a number of supports during the operation period, and each longitudinal truss inside the support during the operation period is provided with a support during the transportation period; the supports during the operation period and inside the supports during the transportation period are provided with Rubber or/and gravel.

Further, the upper structure is provided with a handling passage around the upper structure, and the end of the handling passage is provided with a heavy lift that lifts up and down; hanging objects are provided on the roof of the high-voltage switchgear and the converter transformer.

Further, the closing material of the upper structure adopts the combination of steel plate and fireproof cotton.

The present invention also solves another problem, and provides an installation method suitable for the structure, the installation steps of which include:

First, install the basic structure of the offshore converter station on the site, and reserve a passage for the barge to enter and leave the middle of the basic structure. The position of the basic structure corresponds to the support during the operation period of the offshore converter station; The superstructure is processed and manufactured onshore as a whole, the high-voltage switch and converter transformer are hoisted into the superstructure, other equipment is installed through the handling channel, and finally transported to the site by barge; again, the superstructure of the barge-loaded offshore converter station enters the The channel in the middle of the base structure, and make the position of the support and the base structure correspond one-to-one during operation; and use the ebb tide or the barge to sink, connect the superstructure to the base structure; again, separate the transport barge from the superstructure, The barge goes out of the channel in the middle of the base structure, and the weight of the superstructure is completely transferred from the support to the base structure; finally, the superstructure and the base structure are welded to complete the overall installation.

Beneficial effects of the present invention:

(1) Through the optimization of space separation and the rationalization of plane layout, the offshore converter station of the present invention occupies a small area and can also accommodate a variety of other functional equipment in addition to the basic equipment. The structure is compact and suitable for the converter station. The long-term stability and reliability of the offshore environment are guaranteed; and the overall symmetrical design is conducive to the subsequent installation by the float-over method, which not only solves the problem that the overall weight of the offshore converter station exceeds the lifting capacity of the crane vessel, but also realizes the offshore The overall installation of the converter station also ensures the stability of the transportation period and the operation period.

(2) The closed layout is adopted, and the combination of steel plate and fireproof cotton can not only isolate the corrosive gas from the outside world, prevent the electrical equipment from being corroded by the marine environment, but also can prevent fire and heat insulation, and help ensure the reliability and durability of the long-term operation of the equipment.

(3) This technical solution makes full use of the favorable environment of a large amount of seawater around the offshore converter station, extracts seawater as a heat exchange medium, and combines the traditional cooling liquid cooling technology to effectively achieve the purpose of efficient cooling in a closed environment and ensure the normal operation of the equipment. .

(4) The combination of roof hoisting large equipment and common equipment transportation channel handling, on the one hand, solves the problem that due to the high weight and large size of high-voltage switches and converter transformers, traditional horizontal transportation is very difficult, and the required transportation channels are also large. On the other hand, horizontal and vertical transportation channels are conducive to the convenience of daily maintenance and transportation of equipment.

(5) This technology adopts the technical scheme of the space truss structure, and the support during the operation period is set on the outside of the offshore converter station. The double support is used for the support during the operation period, and the support during the transportation period is set on the inner side of the support during the operation period. . Since the barge channel is reserved in the middle of the offshore converter, there is generally a large span of about 50m. This technology can effectively solve the problem of the large span of the structure. At the same time, the support during operation can be double-supported to help ensure the structural rigidity during operation, so that the overall structure can effectively resist horizontal forces such as wind, waves and earthquakes during operation.

(6) The features of symmetry, compact space and small occupation are combined with the floating-over installation technology, which effectively solves the problem that the overall weight of the offshore converter station exceeds the lifting capacity of the crane vessel, and realizes the overall installation of the offshore converter station.

Description of drawings

FIG. 1 is a plan view of the lower floor of the main factory building of the present invention.

FIG. 2 is a plan view of the upper floor of the main factory building of the present invention.

Figure 3 is a schematic diagram of a longitudinal section of the present invention.

4 is a schematic longitudinal cross-sectional view of the front view of the present invention.

FIG. 5 is a spatial plan view of the first floor of the superstructure of the present invention.

FIG. 6 is a plan view of the second floor space of the auxiliary factory building of the present invention.

FIG. 7 is a spatial plan view of the third floor of the auxiliary factory building of the present invention.

FIG. 8 is a spatial plan view of the fourth floor of the superstructure of the present invention.

Fig. 9 is a spatial plan view of the fifth floor of the superstructure of the present invention.

FIG. 10 is a spatial plan view of the sixth floor of the superstructure of the present invention.

Figure 11 is an elevation view of the present invention.

Detailed ways

The present invention will be further described below in conjunction with the accompanying drawings.

Refer to attached drawings.

This example is an offshore converter station with a scale of 1200MW, and its AC end is connected to three offshore wind farms of 400MW each. After the flow, the 220kV AC is converted into ±320kV DC, and sent out through a ±320kV DC submarine cable. The plane size of this embodiment is 92m×72m, the main workshop is arranged on the second floor, the total height is 36m, and the total weight of the entire converter station is about 19,000 tons.

Refer to attached drawings. The present invention is provided with a closed superstructure 1 and a base structure 2 for supporting the superstructure. Wherein, the upper structure 1 is in the shape of a rectangular parallelepiped as a whole, and the overall pattern and equipment arrangement are generally left-right symmetrical. The superstructure 1 is divided into a large-space main plant 10 and a small-space sub-plant 11 arranged side by side. The main building 10 is divided into upper and lower layers, wherein the lower layer is separated into two left-right symmetrical and large-span valve halls 100 and two left-right symmetrical bridge arm reactor chambers 101 adjacent to the valve halls, the converter valve 20 and the bridge The arm reactors 21 are respectively placed in the corresponding rooms, and the upper layer of the valve hall is provided with a steel pestle frame 200 to ensure the large-span structure of the valve hall 100 is stable. The valve hall 100 is adjacent to the auxiliary workshop 11 , and a cable joint chamber 102 is also provided at the junction of the valve hall 100 and the bridge arm reactor chamber 101 .

The upper layer of the superstructure 1 is provided with a current-limiting reactor 28 , a DC field device 22 , a converter transformer 23 , a high-voltage switchgear, a factory power system 26 and an emergency diesel engine 27 . Specifically, the upper floor is divided into a first area adjacent to the auxiliary factory building and a second area and a third area opposite to each other at intervals by the wall panels and the T-shaped aisle. The first area is divided into two symmetrical current limiting reactor chambers 103 and two left and right symmetrical DC chambers 104 in a cross shape. Therefore, the positive and negative current limiting reactors 28 are separated and symmetrically arranged on the plane. On the one hand, it is beneficial to Reducing the interference between each other, on the other hand, is beneficial to the symmetry of the overall structure, which makes the converter station run stably for a long time. The second area and the third area are similar in structure, and are divided into an electrical room 105 with a larger area on one side and three compartments 106 with a smaller area on the other side, and the three small compartments 106 are adjacent to the DC room 104 . Among them, converter transformers are arranged in the middle compartment; 220V high-voltage switch 24 is placed in the electrical room of the second area, and emergency diesel engines 27 and backup compensation reactors are placed in the other two compartments respectively; the electrical room of the third area is placed with factory The electricity system 26, and the other two compartments are respectively placed with 320V high-voltage switches 25 and temporary equipment. There is also a transport channel 108 between the electrical room in the third area and the three cubicles.

The auxiliary factory building 11 has six floors, of which the first to third floors correspond to the first floor of the main factory building, and the fourth to sixth floors correspond to the second floor of the main factory building. The first and second floors of the auxiliary powerhouse are divided into four compartments arranged in sequence. The two compartments in the middle of the first floor 31 are the valve cooling room 110 and the other two end rooms are the sea water pump room 111. Inside the valve cooling room 110 Valve cooling equipment is installed, and the seawater pump house 111 is used to extract seawater as a heat exchange medium. The two compartments in the middle of the second floor 32 are the cooling pools 120 and the other two are the water treatment chambers 121. The water treatment chambers 121 are provided with seawater treatment equipment for removing impurities such as marine organisms and solid wastes in the seawater. . Therefore, the design can take advantage of the favorable environment of a large amount of seawater around the offshore converter station, and extract seawater as a heat exchange medium to achieve the purpose of efficient cooling. Combined with coolant cooling technology, two independent systems of inner cooling and outer cooling are formed. Specifically, a heat exchanger is provided between the inner cooling system and the outer cooling system. The internal cooling system uses pure coolant, which enters the valve body to take the heat out of the valve body and transport it to the heat exchange device. A heat exchanger is set on one side of the exchange valve, and the cooling liquid of the exchange valve and the heat exchange seawater are connected to the heat exchanger. After the heat exchange is realized in the heat exchanger, the heated seawater is discharged into the sea. Of course, other large heating equipment in the station, such as converter transformers, are also cooled by seawater cooling technology, thereby improving the heat dissipation effect in the converter station and reducing the cooling cost while being environmentally friendly.

The third floor 33 of the auxiliary factory building is divided into two compartments of the same size, namely the valve control room 130 and the ventilation machine room 131. The ventilation machine room 131 adopts a positive pressure ventilation system to ensure the air circulation and air quality in the enclosed space. The plenum system can adopt the prior art such as disclosed in patent ZL201410553826.0. The fourth floor 34 is divided into a battery room 140, a secondary equipment room 141, a fire pool 142 and a fire pump room 143 in parallel in order to prepare the converter station for potential fire hazards and improve operational safety. The fifth floor 35 is sequentially divided into a diesel oil tank room 150, a foam fire extinguishing room 151 and a ventilation machine room 152. The arrangement of multiple ventilation machine rooms can help improve the air flow in the converter station. Sixth floor 36 is divided into diesel generator room 160 , station control room 161 and other equipment room 162 . The battery and generator settings can be used for emergency power outages.

A cable interlayer 107 is also provided between the upper and lower parts of the main building. The cable interlayer 107 is mainly arranged under the second area and the third area on the upper floor of the main building, so as to make full use of the space and not cause space crowding for the placement of large-volume equipment. The cable interlayer 107 can communicate with the cable joint room 102 on the lower floor of the main building.

In order to reduce the corrosion of the equipment in the offshore environment, it is necessary to seal the offshore converter station around, and the sealing material is a combination of steel plate and fireproof cotton, which can not only isolate the corrosive gas from the outside, but also prevent fire and heat insulation.

In order to improve the strength of the converter station, the upper structure 1 adopts a space truss structure, in which three steel trusses are arranged longitudinally, and multiple steel trusses are also arranged horizontally; the chord adopts a strong H-shaped or box-shaped structure to achieve large span span. This structure is not only convenient to construct, but also has economical materials, beautiful appearance and no difficulty, which can effectively reduce the cost of offshore engineering. In particular, each longitudinal truss at the bottom of the upper structure is provided with four supports 41 during operation, which are symmetrically arranged to ensure the rigidity of the longitudinal structure during operation. Each longitudinal truss on the inner side of the support during operation is provided with two supports 42 during transportation, which are symmetrically arranged to ensure overall stability during transportation and operation. The inside of the support 41 during operation and the support 42 during transportation are also provided with buffer materials such as rubber or sandstone, which are used for buffering and vibration isolation during installation and docking.

In order to facilitate the transportation and daily maintenance of large-scale equipment, a handling channel 108 is provided around the upper structure, and the end of the handling channel 108 is provided with a heavy lift that lifts up and down to form a horizontal and vertical combined handling channel 108; The roof of the current transformer is provided with hanging holes, and these devices can be lifted directly from the hanging holes with a hoist, saving the problem of horizontal transportation of large equipment.

Since the upper structure of the technical solution is symmetrical as a whole, and the structure is compact and occupies less space, the float-over installation technology can be used to solve the problems of overall transportation and installation. Among them, the basic structure is a jacket and a passage for the barge to enter and exit is reserved in the middle. The specific transportation plan is as follows: ① First, install the base structure 2 of the offshore converter station on the site, and reserve a passage for the barge to enter and exit in the middle of the base structure 2. The position of the base structure 2 corresponds to the support during the operation period of the offshore converter station. . ②The superstructure 1 of the offshore converter station is processed and manufactured onshore as a whole and transported to the site by barge. ③ The upper structure of the barge-loaded offshore converter station enters the channel in the middle of the base structure, and the positions of the supports and the base structure are in one-to-one correspondence during operation. ④Using ebb tide or flooding the barge to sink, connect the superstructure to the base structure. ⑤Separate the transport barge from the superstructure, drive the barge out of the channel in the middle of the base structure, and the weight of the superstructure 1 is completely transferred from the support to the base structure. ⑥Weld the upper structure and the base structure to complete the overall installation.

Therefore, the technical solution of the present invention is not only conducive to the adaptation of the converter station to the offshore environment, but also occupies a small space as a whole, and provides advantageous conditions for the installation of the float-over method, which solves the problem from equipment transportation to converter station construction to Various problems occurred in the whole process of the operation of the converter station.

It should be noted that the foregoing describes embodiments of the present invention. However, those skilled in the art should understand that the present invention is not limited by the above-mentioned embodiments, and the descriptions in the above-mentioned embodiments are only to illustrate the principle of the present invention. Variations and improvements are intended to fall within the scope of the claimed invention.

Claims (10)

1. An offshore converter station for a flexible direct current transmission system, characterized in that it comprises a closed superstructure and a base structure for supporting the superstructure; the superstructure comprises a large-space main powerhouse arranged side by side The main building is divided into upper and lower layers, of which the upper layer is equipped with current limiting reactor, DC field equipment, converter transformer, high-voltage switchgear, factory power system and emergency diesel engine, and the lower layer is equipped with converters Valves and bridge arm reactors; the auxiliary workshop has six floors, the first floor is equipped with seawater pump room and valve cooling equipment, the second floor is equipped with seawater treatment equipment, the third floor is equipped with valve control room, the fourth floor is equipped with fire fighting equipment, and the fifth floor is equipped with Positive pressure ventilation equipment is set up, and a station control room is set on the sixth floor; the upper structure is generally symmetrically set, and the basic structure is a jacket with a channel reserved in the middle for the barge to enter and exit. 2. An offshore converter station for a flexible direct current transmission system as claimed in claim 1, characterized in that the lower layer of the superstructure is separated into two symmetrical and large-span valve halls and two symmetrical valve halls by means of wall plates The bridge arm reactor room is located in the bridge arm reactor room, the converter valve and the bridge arm reactor are respectively placed in the corresponding rooms, and a steel pestle frame is arranged on the upper layer of the valve hall; wherein, the valve hall is adjacent to the auxiliary workshop, the valve hall is A cable joint room is also provided at the junction of the hall and the bridge arm reactor room. 3. An offshore converter station for a flexible direct current transmission system as claimed in claim 1, characterized in that the upper layer of the superstructure is separated into a first area adjacent to the auxiliary powerhouse and each other through a wall plate and a T-shaped aisle The second area and the third area are opposite to each other; the first area is divided into two symmetrical current limiting reactor chambers and two symmetrical DC chambers in a cross shape; the second area and the third area are similar in pattern, It is divided into an electrical room with a larger area on one side and three compartments with a smaller area on the other side, and the converter transformers are arranged in the middle compartment; the electrical room in the second area is placed with 220V high voltage switch, the emergency diesel engine and backup compensation reactor are respectively placed in the other two compartments; the electrical room in the third area is placed with the factory power system, and the other two compartments are respectively placed with 320V high-voltage switches and Temporary equipment. 4. An offshore converter station for a flexible direct current transmission system according to claim 1, characterized in that the first and second floors of the auxiliary powerhouse are divided into four compartments arranged in sequence, and the first The two compartments in the middle of the second floor are the valve cooling chambers and the other two are the seawater pump rooms, the two compartments in the middle of the second floor are the cooling pools and the remaining two are the water treatment rooms, so that the The converter valve has an internal cooling system cooled by cooling liquid and an external cooling system by seawater circulation; the three layers are divided into two compartments, which are the valve control room and the ventilation room respectively; the fourth layer is divided in parallel in order It is a battery room, a secondary equipment room, a fire fighting pool and a fire pump room; the five floors are divided into a diesel oil tank room, a foam fire extinguishing room and a ventilator room in turn; the six floors are divided into a diesel generator room, the station control room room and other equipment rooms. 5. An offshore converter station for a flexible direct current transmission system according to any one of claims 1-4, characterized in that the lower layer of the main powerhouse corresponds to the first to third floors of the auxiliary powerhouse, and the main powerhouse The upper floor of the factory building corresponds to the fourth to sixth floors of the auxiliary factory building; a cable interlayer is also provided between the upper and lower floors of the main factory building. 6. An offshore converter station for a flexible direct current transmission system according to claim 1, characterized in that the upper structure adopts a space truss structure, wherein three steel trusses are arranged longitudinally, and multiple steel trusses are also arranged horizontally at the same time. Truss; chord adopts strong H-type or box-type structure. 7. An offshore converter station for a flexible direct current transmission system according to claim 6, characterized in that each longitudinal truss at the bottom of the upper structure is provided with a number of supports during the operation period. Each longitudinal truss on the inner side of the seat is provided with a support during transportation; rubber or/and sand and gravel are provided inside the support during operation and during transportation. 8. An offshore converter station for a flexible direct current transmission system as claimed in claim 1, characterized in that a handling channel is provided around the upper structure, and an end of the handling channel is provided with a heavy duty lifting up and down Elevator; hanging object holes are arranged on the roof of the high-voltage switchgear and the converter transformer. 9. An offshore converter station for a flexible direct current transmission system according to claim 1, characterized in that the sealing material of the upper structure adopts a combination of steel plate and fireproof cotton. 10. An installation method for an offshore converter station of a flexible direct current transmission system as claimed in claim 1, characterized in that: First of all, the basic structure of the offshore converter station is installed on the site, and the passage for the barge to enter and exit is reserved in the middle of the basic structure. The position of the basic structure corresponds to the support during the operation period of the offshore converter station; Secondly, the superstructure of the offshore converter station is processed and manufactured onshore as a whole, the high-voltage switch and converter transformer are hoisted into the superstructure, other equipment is installed through the handling channel, and finally transported to the site by barge; Thirdly, the superstructure of the barge-loaded offshore converter station enters the channel in the middle of the base structure, and the positions of the supports and the base structure are matched one-to-one during operation; ; Thirdly, the transport barge is separated from the superstructure, the barge is driven out of the channel in the middle of the base structure, and the weight of the superstructure is completely transferred from the support to the base structure; Finally, the superstructure and base structure are welded to complete the overall installation.