CN210404741U

CN210404741U - Power transmission system

Info

Publication number: CN210404741U
Application number: CN201920698397.4U
Authority: CN
Inventors: 葛菁; 于弘洋; 赵国亮; 邓占锋; 蔡林海; 戴朝波; 宋洁莹; 尉志勇; 祁欢欢
Original assignee: State Grid Corp of China SGCC; Global Energy Interconnection Research Institute
Current assignee: State Grid Corp of China SGCC; Global Energy Interconnection Research Institute
Priority date: 2019-05-15
Filing date: 2019-05-15
Publication date: 2020-04-24
Anticipated expiration: 2029-05-15

Abstract

The utility model discloses a power transmission system, include: the at least two offshore platforms are used for forming a first alternating current system to provide electric energy for electric equipment in an offshore area after alternating current networking is carried out; a second communication system connected to each offshore platform for providing electrical energy to each offshore platform; each first AC/AC frequency conversion device is connected with the corresponding offshore platform and is used for converting the first frequency three-phase voltage of the offshore platform into a second frequency three-phase voltage, and the first frequency is greater than the second frequency; and the power transmission cable enables the first alternating current system and the second alternating current system to be connected through each first AC/AC frequency conversion device. The utility model provides a transmission capacity of circuit is promoted at double through the transmission of electricity frequency that adopts to be less than first frequency between each offshore platform of transmission system has improved transmission distance.

Description

Power transmission system

Technical Field

The utility model relates to an electric power system transmission of electricity field, concretely relates to transmission of electricity system.

Background

Energy is an important material basis for economic and social development, and the establishment of a safe, reliable, economic, efficient, clean and environment-friendly modern energy supply system is accelerated, so that the energy becomes a common strategic target of all countries in the world. In order to effectively solve the problems of energy exhaustion and environmental pollution, the development of new energy has become three challenges to energy safety, environmental pollution and climate change, and a necessary way for sustainable development of human society is realized. Wind power generation is one of the most mature and scaled power generation modes of a power generation device in new energy power generation technology. In some areas, wind power resource distribution and load centers are reversely distributed, and optimal allocation of resources is required to be realized through large-capacity long-distance power transmission.

At present, in the existing power transmission system, because the power transmission energy of an offshore platform is limited, alternating current on land needs to be transmitted to a plurality of remote offshore platforms for energy supply, but cables are generally adopted between the offshore platforms and an alternating current system on land for power transmission, the cables have obvious capacitance effect, the reactive loss can be greatly increased, the effective load capacity of the cables is reduced, meanwhile, the reactive compensation is difficult to be carried out in the midway of cable lines, and the transmission distance of the longer-distance cable lines in alternating current transmission is limited.

SUMMERY OF THE UTILITY MODEL

Therefore, the embodiment of the present invention provides a transmission distance between the offshore platforms in the prior art is limited when the ac transmission is adopted.

Therefore, the embodiment of the utility model provides a following technical scheme:

an embodiment of the utility model provides a power transmission system, include:

the at least two offshore platforms are used for forming a first alternating current system to provide electric energy for electric equipment in an offshore area after alternating current networking is carried out;

a second communication system connected to each offshore platform for providing electrical energy to said each offshore platform;

each first AC/AC frequency conversion device is connected with the corresponding offshore platform and is used for converting the first frequency three-phase voltage of the offshore platform into a second frequency three-phase voltage, and the first frequency is greater than the second frequency;

and the power transmission cable enables the first alternating current system and the second alternating current system to be connected through each first AC/AC frequency conversion device.

Optionally, each first AC/AC frequency conversion device comprises a first AC/AC frequency converter and a first switch group, wherein an input end of the first AC/AC frequency converter is connected to the AC power grid, and an output end of the first AC/AC frequency converter is connected to the power transmission cable through the first switch group.

Optionally, the first AC/AC frequency converter includes at least one set of frequency conversion modules, the frequency conversion modules include three frequency conversion units, an input end of the frequency conversion unit is connected to the AC power grid, and an output end of the frequency conversion unit is connected to the first switch set.

Optionally, the frequency conversion unit includes three frequency conversion bridge arms, each of the frequency conversion bridge arms includes an inductor and an H-bridge, a first end of the inductor is connected with a first end of the H-bridge, a second end of the inductor is used as an input end of the frequency conversion bridge arm, and a second end of the H-bridge is used as an output end of the frequency conversion bridge arm;

the input ends of the three frequency conversion bridge arms are respectively connected with the phase A, the phase B and the phase C of the alternating current power grid, and the output ends of the three frequency conversion bridge arms are connected with the first switch group.

Optionally, the H-bridge comprises at least one fully-controlled H-bridge.

Optionally, the fully-controlled H-bridge includes two sets of power electronic device bridge arms and a dc capacitor, the two sets of power electronic device bridge arms are connected in parallel, each power electronic device bridge arm includes two power electronic devices connected in series, and the dc capacitor is connected in parallel with the power electronic device bridge arm.

Optionally, the power electronic device comprises an insulated gate bipolar transistor and an anti-parallel diode connected in parallel with the insulated gate bipolar transistor.

Optionally, the first switch group is a circuit breaker and disconnectors disposed at two ends of the circuit breaker.

Optionally, the power transmission system further includes:

and the second AC/AC frequency conversion device is connected with the power transmission cable and the second alternating current system and is used for converting the third frequency three-phase voltage of the second alternating current system into the first frequency three-phase voltage.

Optionally, the power transmission system further includes:

each transformer is connected with the corresponding first AC/AC frequency conversion device and the offshore platform and is used for adjusting the output voltage of the offshore platform; and/or the presence of a gas in the gas,

each transformer is connected with the corresponding second AC/AC frequency conversion device and the second AC system and is used for adjusting the input voltage of the second AC system.

The embodiment of the utility model provides a technical scheme has following advantage:

Drawings

In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the embodiments or the technical solutions in the prior art will be briefly described below, and it is obvious that the drawings in the following description are some embodiments of the present invention, and for those skilled in the art, other drawings can be obtained according to these drawings without creative efforts.

Fig. 1 is a schematic view of a first configuration of a power transmission system according to an embodiment of the invention;

fig. 2 is a schematic view of a second structure of a power transmission system according to an embodiment of the present invention;

fig. 3 is a schematic view of a third structure of a power transmission system according to an embodiment of the present invention;

fig. 4 is a fourth schematic structural diagram of a power transmission system according to an embodiment of the present invention;

fig. 5 is a fifth structural schematic diagram of a power transmission system according to an embodiment of the present invention;

fig. 6 is a sixth schematic structural view of a power transmission system according to an embodiment of the present invention;

fig. 7 is a seventh schematic structural diagram of a power transmission system according to an embodiment of the present invention;

fig. 8 is an eighth schematic structural diagram of a power transmission system according to an embodiment of the present invention;

fig. 9 is a ninth structural schematic diagram of a power transmission system according to an embodiment of the present invention;

fig. 10 is a schematic view of a tenth configuration of a power transmission system according to an embodiment of the present invention;

fig. 11 is an eleventh structural schematic diagram of a power transmission system according to an embodiment of the present invention;

fig. 12 is a twelfth structural schematic diagram of a power transmission system according to an embodiment of the present invention.

Reference numerals:

1-a first communication system; 11-an offshore platform; 2-a first AC/AC frequency conversion device;

21-a first AC/AC frequency converter; 211-a frequency conversion unit; 2111-inductance;

2112-H bridge; 22-a first switch set; 221-a switch;

3-a power transmission cable; 4-a second communication system; 5-a second AC/AC frequency conversion device;

5-a second AC/AC frequency conversion device; 51-a second switch set; 52-a second AC/AC frequency converter;

6-transformer.

Detailed Description

The technical solutions of the embodiments of the present invention will be described clearly and completely with reference to the accompanying drawings, and it is to be understood that the described embodiments are only some embodiments, but not all embodiments, of the present invention. Based on the embodiments in the present invention, all other embodiments obtained by a person skilled in the art without creative work belong to the protection scope of the present invention.

In the description of the embodiments of the present invention, it should be noted that the terms "center", "upper", "lower", "left", "right", "vertical", "horizontal", "inner", "outer", and the like indicate orientations or positional relationships based on the orientations or positional relationships shown in the drawings, which are only for convenience of describing the embodiments of the present invention and simplifying the description, but do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and thus, should not be construed as limiting the present invention. Furthermore, the terms "first," "second," and "third" are used for descriptive purposes only and are not to be construed as indicating or implying relative importance.

In the description of the embodiments of the present invention, it should be noted that unless explicitly stated or limited otherwise, the terms "mounted," "connected," and "connected" are to be construed broadly, and may be, for example, fixedly connected, detachably connected, or integrally connected; can be mechanically or electrically connected; the two elements may be directly connected or indirectly connected through an intermediate medium, or may be communicated with each other inside the two elements, or may be wirelessly connected or wired connected. The specific meaning of the above terms in the present invention can be understood in specific cases to those skilled in the art.

Furthermore, the technical features mentioned in the different embodiments of the invention described below can be combined with each other as long as they do not conflict with each other.

Example 1

An embodiment of the utility model provides a power transmission system is applied to electric power system alternating current transmission, as shown in fig. 1, include: two offshore platforms 11, a first AC/AC conversion device 2, a power transmission cable 3 and a second AC system 4.

The two offshore platforms 11 form a first alternating current system 1 for alternating current networking, and then provide electric energy for electric equipment in an offshore area. The offshore platform 11 is mainly disposed on each island or island for performing production operations or other activities, such as: the offshore platform 11 can be used for exploiting oil on the island through oil drilling, and the offshore platform 11 can be used for exploiting coal resources on the island, so that the offshore platform 11 needs to provide electric energy for electric equipment for production operation. Because the islands on the sea are usually distributed in a relatively dispersed manner, and because a certain distance exists between the islands, in order to realize remote power transmission among the offshore platforms 11 on the islands, the offshore platforms 11 can be powered to perform low-frequency power transmission transformation, so that the transmission capacity can be increased, the transmission distance is increased, the transformation cost is saved, and the construction difficulty is reduced, therefore, a plurality of offshore platforms 11 are required to perform low-frequency networking. The offshore platform 11 in this embodiment may be one, forming the first ac system 1, for supplying power to the electrical equipment in the offshore area. In this embodiment, there are two offshore platforms 11, and after the two offshore platforms 11 form the first ac system 1 and perform ac networking, the two offshore platforms provide electric energy to electric equipment in the offshore area. As other alternative embodiments, the number of the offshore platforms 11 in this embodiment may also be three, four, five, or the like, that is, three or more offshore platforms 11 form the first ac system 1 for low frequency networking. As shown in fig. 2, after a first communication system is formed for three offshore platforms 11, communication networking is performed.

A second ac system 4 connected to each offshore platform 11 for providing electrical power to said each offshore platform 11. The second ac system 4 is mainly a distribution station and/or a power plant which are installed on the land, and once the power of the offshore platform 11 is not enough, the power can be obtained from the first ac system 1 on the land for supplying so as not to affect the production operation of the offshore platform 11.

As shown in fig. 1, because two offshore platforms 11 are used for AC networking, two first AC/AC frequency conversion devices 2 also need to be correspondingly arranged, and each first AC/AC frequency conversion device 2 is connected to its corresponding offshore platform 11 and is used for converting a first frequency three-phase voltage of the offshore platform 11 into a second frequency three-phase voltage, where the first frequency is greater than the second frequency. As another alternative, when there are a plurality of offshore platforms 11 for AC networking, the number of the corresponding first AC/AC frequency conversion devices 2 is also multiple, that is, the number of the first AC/AC frequency conversion devices 2 is the same as the number of the offshore platforms 11. The power transmission system is applied to large-capacity long-distance power transmission, the range of the first frequency and the range of the second frequency are determined according to practical application, the preferred range of the first frequency is less than or equal to 75Hz, the second frequency is only required to be less than the first frequency, for example, the first frequency is 60Hz, and the second frequency can be any frequency less than 60 Hz. In the embodiment, because the power frequency of the power grid in China is 50Hz, the first frequency is set to be 50 Hz;the second frequency was set to 50/3 Hz; with the reduction of the transmission frequency, the current of the cable is increased, the insulation performance is reduced, the 50/3Hz low-frequency transmission frequency is adopted after the influence of various factors such as insulation, cost and the like is comprehensively considered, and therefore the transmission capacity can be increased by 3 times, the line impedance is reduced, and the transmission distance is increased. In the present embodiment, as shown in fig. 3, the three-phase voltages of the first ac system 11 are respectively represented as a-phase voltages V_AVoltage V of phase B_BAnd a C-phase voltage V_CThe phase difference of the three-phase voltage is 120 degrees; the converted three-phase voltages are respectively expressed as first voltages V_MAA second voltage V_MBAnd a third voltage V_MCThe phase difference of the three-phase voltage is 120 degrees. In this embodiment, the first ac system 11 is the ac power output by the offshore platform 11, such as ac networking, and the number of the offshore platforms 11 may be set reasonably as required.

In the present embodiment, as shown in fig. 3, each of the first AC/AC frequency conversion devices 2 includes a first AC/AC frequency converter 21 and a first switch group 22, wherein an input end of the first AC/AC frequency converter 21 is connected to the first alternating current system 1, and an output end of the first AC/AC frequency converter 21 is connected to the power transmission cable 3 through the first switch group 22. The first switch group 22 is arranged between the output end of the first AC/AC frequency converter 21 and the power transmission cable 3, so that on one hand, networking between the alternating current grids of the first alternating current system 1 is facilitated, and the switches in the first switch group 22 are turned on to enable the first alternating current system 1 to be connected to the power transmission system; on the other hand, when the power transmission cable 3 has a fault, the power transmission cable 3 is disconnected from the first ac system 1 by disconnecting the switches in the first switch group 22, which is convenient for maintenance and repair of the power transmission cable 3. The first switch group 22 is distributed according to the requirements of system protection, maintenance and the like. In this embodiment, first switch group 22 includes at least a set of switch, and each group switch includes three switch, and each switch wherein all includes the circuit breaker and sets up in the isolator at circuit breaker both ends, and the input and an isolator of circuit breaker are connected promptly, and the output of circuit breaker is connected with an isolator again, and the concrete number of switch is according to actual need reasonable setting can. In the present embodiment, as shown in fig. 5, the first switch group 22 includes three groups of switches, which are respectively referred to as a first group of switches, a second group of switches and a third group of switches, wherein, the three-phase voltage output end of the first AC/AC frequency converter 21 is connected with the first end of the first group of switches, the second end of the first group of switches is respectively connected with the first end of the second group of switches and the first end of the third group of switches, the second group of switches and the third group of switches are connected in parallel, the second end of the second group of switches is connected with one offshore platform 11 through the first AC/AC frequency converter 21, the second end of the third group of switches is also connected with the other offshore platform 11 through the first AC/AC frequency converter 21, in fig. 5, the first AC system 1 composed of three offshore platforms 11 is AC networked by the corresponding first AC/AC frequency converter 21 to ensure the same frequency. Of course, in other embodiments, the first set of switches shown in fig. 5 may be omitted and may be arranged as appropriate as desired.

The first AC/AC frequency converter 21 comprises at least one set of frequency conversion modules, the frequency conversion modules comprise three frequency conversion units 211, the input ends of the frequency conversion units 211 are connected with the first AC system 1, and the output ends of the frequency conversion units 211 are connected with the first switch set 22. In this embodiment, as shown in fig. 4, the first AC/AC frequency converter 21 includes a set of frequency conversion modules, each of the frequency conversion modules includes three frequency conversion units 211, each of the frequency conversion units 211 includes three frequency conversion bridge arms, each of the frequency conversion bridge arms includes an inductor 2111 and an H bridge 2112, a first end of the inductor 2111 is connected to a first end of the H bridge 2112, a second end of the inductor 2111 serves as an input end of the frequency conversion bridge arm, and a second end of the H bridge 2112 serves as an output end of the frequency conversion bridge arm; the input ends of the three frequency conversion bridge arms are respectively connected with the phase A, the phase B and the phase C of the first alternating current system 1, and the output ends of the three frequency conversion bridge arms are connected with the first switch group 22. The first AC/AC frequency converter 21 contains 9 legs consisting of cascaded H-bridge 2112 modules, from which a three-phase low-frequency alternating current is drawn from the neutral point.

The H-bridge 2112 includes at least one fully-controlled H-bridge 2112, in this embodiment, as shown in fig. 4, the H-bridge 2112 includes one fully-controlled H-bridge 2112, the fully-controlled H-bridge 2112 includes two sets of power electronic device bridge arms and a dc capacitor, the two sets of power electronic device bridge arms are connected in parallel, each power electronic device bridge arm includes two power electronic devices connected in series, and the dc capacitor is connected in parallel with the power electronic device bridge arms; the power electronic device includes an Insulated Gate Bipolar Transistor (IGBT) and an anti-parallel diode connected in parallel with the IGBT. Of course, in other embodiments, the power electronic device may also be a metal-oxide-semiconductor (MOS) field effect Transistor (MOS) or a Bipolar Junction Transistor (BJT), and the like, and may be reasonably arranged according to needs. Since one fully-controlled H-bridge 2112 can bear a limited voltage level and the voltage of the ac grid is high, a plurality of fully-controlled H-bridges 2112 are required to be cascaded, as shown in fig. 5, the H-bridge 2112 includes two fully-controlled H-bridges 2112, and in other embodiments, the number of cascaded fully-controlled H-bridges 2112 may be set reasonably as required.

A set of frequency conversion modules can convert the first frequency three-phase voltage into a second frequency three-phase voltage, and the second frequency three-phase voltage is connected to another offshore platform 11 through the first AC/AC frequency converter 21 via the first switch group 22, that is, in fig. 4, the two offshore platforms 11 are networked through the first AC/AC frequency converter 21 corresponding to each other at the end of the low frequency, so that the frequencies of the two offshore platforms 11 are the same. When the first switch group 22 connected with the frequency conversion modules comprises a group of switches, the group of frequency conversion modules is connected with one offshore platform 11 through the first AC/AC frequency converter 21, so that the two offshore platforms 11 form the first AC system 1 to perform low-frequency power transmission through AC networking, and the same frequency can be ensured. When the first switch group 22 connected with the frequency conversion modules comprises at least two groups of switches, one group of frequency conversion modules is connected with at least two offshore platforms 11, and the first AC/AC frequency converter 21 is connected with at least two offshore platforms 11, so that one AC power grid can be connected with a plurality of offshore platforms 11 after networking; as shown in fig. 5, the first switch group 22 includes three groups of switches, and one group of frequency conversion modules is connected to two offshore platforms 11, so that the first AC/AC frequency converter 21 is connected to two offshore platforms 11 in parallel, and a corresponding first AC/AC frequency converter 21 is disposed beside each offshore platform.

Of course, in other embodiments, the first AC/AC inverter 21 may comprise a plurality of sets of inverter modules, and one AC grid may be connected to a plurality of sets of inverter modules. For example, the first AC/AC frequency converter 21 comprises two sets of frequency conversion modules, i.e. two sets of frequency conversion modules are connected in parallel. When the first switch group 22 connected to each frequency conversion module comprises a group of switches, each group of frequency conversion modules is connected to one offshore platform 11, and the first AC/AC frequency converter 2121 is connected to two offshore platforms 11, so that an AC power grid after networking can be connected to a plurality of offshore platforms 11, and a corresponding first AC/AC frequency converter 21 is disposed beside each offshore platform, as shown in fig. 6. When the first switch group 22 connected with each frequency conversion module comprises at least two groups of switches, each group of frequency conversion module is connected with at least two offshore platforms 11, so that one alternating current power grid can be connected with a plurality of offshore platforms 11 after networking; as shown in fig. 7, the first switch group 22 includes three groups of switches, so that each group of frequency conversion modules is connected to two offshore platforms 11, the first AC/AC frequency converter 21 is connected to four offshore platforms 11, and a corresponding first AC/AC frequency converter 21 is disposed beside each offshore platform. The specific networking form of the power transmission system can be reasonably set according to the actual conditions of an alternating current power grid and an offshore platform 11, as shown in fig. 8, the power transmission system comprises a looped network formed by four first AC/AC frequency conversion devices 2, and the specific connection relationship is shown in the attached fig. 8 in detail.

The first AC/AC frequency conversion device 2 can realize the change of the multi-terminal low-frequency networking operation mode by controlling the three-phase low-frequency switch, including the change of the networking mode and the realization of the fault isolation function. In the multi-terminal low-frequency network, the power flow in the network can be regulated by controlling the first AC/AC frequency converter 21, so that the power flow control function is realized.

And the power transmission cable 3 is connected with the first AC/AC frequency conversion device 2 and the second AC system 4 and is used for networking a plurality of offshore platforms 11 to form an AC power grid of the first AC system 1. In this embodiment, the second ac system 4 may be a transmission line, a transformer substation, a power transformation line, a distribution substation, or an electric device on the land, and may be set reasonably as required. In this embodiment, the power transmission cable 33 is a cable, but in other embodiments, it may be an overhead wire or the like, and may be appropriately arranged as needed.

In the power transmission system, the first frequency three-phase voltage of the alternating current power grid in the first alternating current system 1 is converted into the second frequency three-phase voltage through the first AC/AC frequency conversion device 2, and the first frequency is greater than the second frequency. The transmission system improves the transmission capacity of the line by doubling the transmission frequency lower than the first frequency, and improves the transmission distance.

In order to increase the networking convenience and the operation flexibility of the power transmission system, on the basis of the power transmission system, as shown in fig. 9, the power transmission system further includes: and the second AC/AC frequency conversion device 5 is connected with the power transmission cable 3 and the second alternating current system 4 and is used for converting the third frequency three-phase voltage of the alternating current power grid in the second alternating current system 41 into the first frequency three-phase voltage. In this embodiment, since the frequency of the power grid in China is 50Hz, the third frequency is set to be 50Hz, and certainly, in other embodiments, the third frequency can be reasonably set according to the requirement of the offshore platform 11, and if the frequency required by the offshore platform 11 is 75Hz, the third frequency is set to be 75 Hz; the desired frequency for the offshore platform 11 is 30Hz and the third frequency is set to 30 Hz. In the present embodiment, as shown in fig. 10, the second AC/AC frequency conversion device 5 includes a second switch group and a second AC/AC frequency converter, the power transmission cable 3 is connected to an input end of the second AC/AC frequency converter through the second switch group, and an output end of the second AC/AC frequency converter is connected to the second AC system 4. The specific circuit structure of the second AC/AC frequency converter is similar to that of the first AC/AC frequency converter 21, and the second AC/AC frequency converter can be obtained by interchanging the input end and the output end of the first AC/AC frequency converter 21, so the circuit structure of the second AC/AC frequency converter is not described herein again; the second switch set has a similar circuit structure to the first switch set 22, and is not described herein again. A first AC/AC frequency conversion device 2 is additionally arranged on the side (a transmission end) of an alternating current network, three-phase first frequency voltage is converted into low-frequency voltage with frequency lower than the first frequency for transmission, the low-frequency voltage reaches a second alternating current system 4 (a receiving end) and is converted into three-phase first frequency voltage through a second AC/AC conversion device for load power supply, and therefore low-frequency multi-end alternating current networking convenient for an offshore platform 11 is formed. Power is supplied among the offshore platforms 11 on the islands for transformation, so that the transmission capacity can be increased, the line loss is reduced, the transmission distance is increased, the transformation cost is saved, and the construction difficulty is reduced.

In order to satisfy the adjustment between different voltage levels, on the basis of the above power transmission system, as shown in fig. 11, the power transmission system further includes:

at least two transformers 6, each transformer 6 is connected with the corresponding first AC/AC frequency conversion device 2 and the offshore platform 11 and is used for adjusting the output voltage of the offshore platform 11; and/or each transformer 6 is connected with its corresponding second AC/AC frequency conversion device 5 and second AC system 4 for regulating the input voltage of the second AC system 4. Each transformer 6 is connected to the first AC/AC frequency conversion means 2 and the first AC system 1 for regulating the output voltage of the offshore platform 11. In the present embodiment, the number of the first AC/AC frequency conversion devices 2 is two, so the number of the transformers 6 is also two; if the voltage of an alternating current power grid is 220V, the three-phase alternating current voltage of 220V is boosted to 10kV through the transformer 6, so the transformer 6 is the step-up transformer 6, then the three-phase voltage is converted, a high-voltage power transmission line is adopted for low-frequency transmission, and the high-voltage power transmission reduces the heat loss caused by current and reduces the material cost of long-distance power transmission. Of course, in other embodiments, the transformer 6 may also be boosted to different voltages, such as 500kV or 750kV, and may be set reasonably as required; the transformer 6 can also step down to different voltage values, for example, the voltage value of the three-phase alternating-current voltage is 220kV, and the voltage can be reduced to 110kV, 35kV, 10kV or 220V through the transformer 6, and the voltage can be reasonably set according to needs. In the embodiment, the transformer 6 is arranged between the first AC/AC frequency conversion device 2 and the first AC system 1, and the arrangement makes the transformer 6 smaller in size and lower in cost; of course, in other embodiments, the transformer 6 may also be disposed between the first AC/AC frequency conversion device 2 and the power transmission cable 3, in this case, the transformer 6 is a low-frequency transformer 6, and the lower the frequency, the larger the volume of the transformer 6, and the higher the cost, so that the transformer can be disposed reasonably as required. If the voltage of the power transmission system is consistent with the applicable voltage of the first AC/AC frequency converter 21, the transformer 6 at the end of the system can be omitted, thereby greatly reducing the cost.

In order to meet the different voltage levels required by the offshore platform 11 to form the first AC system 1, on the basis of the power transmission system, as shown in fig. 12, a transformer 6 may be further disposed between the second AC/AC frequency conversion device 5 and the second AC system 4 for adjusting the value of the AC voltage transmitted by the AC power grid to the second AC system 4; this arrangement allows the transformer 6 to be small and inexpensive. Of course, in other embodiments, the transformer 6 may also be arranged between the power transmission cable 3 and the second AC/AC frequency conversion device 5, as appropriate. If the voltages applied to the second AC system 4 and the second AC/AC inverter are the same, the transformer 6 at the end of the system can be omitted, thereby further reducing the cost.

It should be understood that the above examples are only for clarity of illustration and are not intended to limit the embodiments. Other variations and modifications will be apparent to persons skilled in the art in light of the above description. And are neither required nor exhaustive of all embodiments. And obvious variations or modifications can be made without departing from the scope of the invention.

Claims

1. A power transmission system, comprising:

2. The power transmission system according to claim 1, characterized in that each first AC/AC frequency conversion device comprises a first AC/AC frequency converter and a first set of switches, wherein an input of the first AC/AC frequency converter is connected to the first alternating current system and an output of the first AC/AC frequency converter is connected to the power transmission cable via the first set of switches.

3. The power transmission system according to claim 2, characterized in that the first AC/AC frequency converter comprises at least one set of frequency conversion modules, which frequency conversion modules comprise three frequency conversion units, the input of which frequency conversion units are connected to the first AC system and the output of which frequency conversion units are connected to the first set of switches.

4. The power transmission system according to claim 3, wherein the frequency conversion unit comprises three frequency conversion legs, each of the frequency conversion legs comprises an inductor and an H-bridge, a first end of the inductor is connected with a first end of the H-bridge, a second end of the inductor serves as an input end of the frequency conversion leg, and a second end of the H-bridge serves as an output end of the frequency conversion leg;

the input ends of the three frequency conversion bridge arms are respectively connected with the phase A, the phase B and the phase C of the first alternating current system, and the output ends of the three frequency conversion bridge arms are connected with the first switch group.

5. The power transmission system of claim 4, wherein the H-bridge comprises at least one fully-controlled H-bridge.

6. The power transmission system of claim 5, wherein the fully-controlled H-bridge comprises two sets of power electronics legs connected in parallel, each of the power electronics legs comprising two power electronics in series, and a DC capacitor connected in parallel with the power electronics legs.

7. The power transmission system of claim 6, wherein the power electronics device comprises an insulated gate bipolar transistor and an anti-parallel diode connected in parallel with the insulated gate bipolar transistor.

8. The power transmission system of claim 7, wherein the first switch set is a circuit breaker and a disconnector disposed across the circuit breaker.

9. The power transmission system of claim 8, further comprising:

10. The power transmission system of claim 9, further comprising: