Bipolar hvdc power transmission system configurable for monopolar operation

Abstract

A bipolar power transfer scheme includes a first converter station and first and second transfer conduits. The first converter station includes a first power converter. The first converter station also includes a second power converter. The bipolar power transfer scheme also includes a converter station controller programmed to selectively transition the bipolar power transfer scheme to an asymmetric unipolar configuration while maintaining power delivery from first and second power sources by: electrically interconnecting the first and second power sources; configuring one power converter connected to a transfer conduit intended to continue transferring power in the unipolar configuration as a master converter operating in a grid forming mode; configuring another power converter connected to a transfer conduit intended to stop transferring power in the unipolar configuration as a slave converter operating in a grid following mode; and controlling the slave converter to reduce power being fed into the transfer conduit connected with the slave converter to zero.

Description

Technical Field

[0001] The present invention relates to a bipolar power transfer scheme (i.e., a bipolar power transfer device) and a method of operating such a scheme. Background Technology

[0002] In high-voltage direct current (HVDC) power transmission networks, AC power is typically converted to DC power for transmission via overhead lines, submarine cables, and / or underground cables. This conversion eliminates the need to compensate for the AC capacitive load effect imposed by the power transmission medium (i.e., transmission lines or cables) and reduces the cost per kilometer of line and / or cable, thus becoming cost-effective when power needs to be transmitted over long distances. DC power can also be transmitted directly from offshore wind farms to onshore AC power transmission networks.

[0003] The conversion between DC and AC power is used where DC and AC networks must be interconnected. In any such power transmission network, a converter (i.e., a power converter) is required at each interface between AC and DC power to achieve the required conversion from AC to DC or from DC to AC.

[0004] The selection of the most suitable HVDC power transmission scheme (i.e., power transmission device) within an HVDC power transmission network varies depending on the application and scheme characteristics. One type of such scheme is the bipolar power transmission scheme, i.e., the bipolar power transmission device. Summary of the Invention

[0005] According to a first aspect of the invention, a bipolar power transmission scheme is provided, comprising: a first converter station located remotely from a second converter station during use; and a first transmission channel and a second transmission channel for interconnecting the first converter station and the second converter station during use, thereby allowing the first converter station to transmit power to the second converter station. The first converter station includes: A first power converter electrically interconnects a first transmission channel with a first variable power source and, in use, transfers power from the first power source to the first transmission channel; and The second power converter electrically interconnects the second transmission channel with the second variable power source and transfers power from the second power source to the second transmission channel during use; The bipolar transmission scheme further includes a converter station controller programmed to selectively convert the bipolar power transmission scheme to an asymmetric unipolar configuration while maintaining power delivery from both the first and second power sources: Electrically interconnecting the first and second power sources; A power converter connected to a transmission pipeline intended to continue transmitting power in a unipolar configuration is configured as a main converter operating in a grid formation mode; Configure another power converter connected to the transmission pipe intended to stop power transmission in a unipolar configuration as a slave converter operating in grid follower mode; and Control the converter to reduce the power being fed into the transmission pipe connected to the converter to zero.

[0006] Such a converter station controller, programmed in this way, allows the transition to an asymmetric unipolar configuration while avoiding interruptions in power delivery from both power sources, and therefore does not cause highly undesirable power interruptions in any downstream power delivery networks associated with it, which are being supplied by the first and second power sources.

[0007] In addition, the converter station controller that performs the above steps includes, in this invention, causing power from the two power sources to pass only through the main converter and enter the transmission pipeline connected to it, but still entering the downstream power transmission network associated with it in use, thereby avoiding power interruption in the downstream power transmission network.

[0008] The converter station controller can be further programmed to disconnect from the first and second power sources electrically interconnected with the converter once the power being fed into the transmission pipe connected to the converter reaches zero.

[0009] This additional step electrically isolates the converter, and therefore also the transmission line connected to the converter, thus allowing safe operation on that transmission line, for example, for planned maintenance or repair.

[0010] Optionally, the first power source and the second power source are temporarily interconnected at a common coupling point.

[0011] Using a common coupling point where the first and second power sources are located provides a convenient way to achieve such an interconnection, especially since such a common coupling point is often located within the first converter station and thus close to each power converter.

[0012] In another preferred embodiment of the invention, before interconnecting the first power source and the second power source, the converter station controller is programmed to synchronize the voltages and frequencies of the first power source and the second power source with each other.

[0013] Synchronizing the voltage and frequency of the first power source with the voltage and frequency of the second power source is intended to reduce the risk of damage to the first and second power converters, as well as the first converter station, should such an interconnection occur.

[0014] Preferably, droop control is initially applied to both power converters after the first and second power sources are interconnected.

[0015] Applying such droop control to two power converters prevents any interaction between the respective converter controllers of the power converters, while still allowing power to be shared between the first and second power sources.

[0016] In another preferred embodiment of the invention, the main converter operating in the grid formation mode maintains the voltage and frequency of the power source connected to the main converter.

[0017] In another preferred embodiment of the invention, the slave converter, operating in grid follow mode, measures the voltage and frequency of a power source connected to the slave converter so that the output voltage of the slave converter is synchronized with the power source.

[0018] The above steps are intended to allow a power converter (i.e., a power converter configured as a master converter) to assume overall control over the first and second power sources electrically interconnected, and to allow another power converter (i.e., another power converter configured as a slave converter) to modify the output of the other power converter according to the voltage and frequency of the first and second power sources interconnected.

[0019] Therefore, the voltage and frequency set and maintained by the master converter can serve as a reference that can be followed by the slave converter, and thus the master converter can exhibit the necessary level of control over the slave converter without requiring a dedicated communication channel between the converters.

[0020] Optionally, the first converter station is an offshore converter station, and at least one of the first power source and the second power source is an offshore wind farm.

[0021] The motivation to reduce carbon emissions implies an increased demand for renewable energy sources, and therefore the applicability of this invention to offshore wind farms is highly advantageous.

[0022] A bipolar power transmission scheme may include a second converter station interconnected with a first converter station via a first transmission pipe and a second transmission pipe.

[0023] Including such a second converter station provides the opportunity for end-to-end control of the entire bipolar power transfer scheme.

[0024] According to a second aspect of the invention, a method for operating a bipolar power transfer scheme is provided, the bipolar power transfer scheme comprising: a first converter station located remotely from a second converter station during use; and a first transmission channel and a second transmission channel for interconnecting the first converter station and the second converter station during use, thereby allowing the first converter station to transfer power to the second converter station. The first converter station includes: A first power converter electrically interconnects a first transmission channel with a first variable power source and, in use, transfers power from the first power source to the first transmission channel; and The second power converter electrically interconnects the second transmission channel with the second variable power source and transfers power from the second power source to the second transmission channel during use; The bipolar power transmission scheme also includes a converter station controller, and The method includes the following steps: enabling the converter station controller to selectively convert a bipolar power transfer scheme to an asymmetric unipolar configuration while maintaining power transfer from both the first and second power sources: Electrically interconnecting the first and second power sources; A power converter connected to a transmission pipeline intended to continue transmitting power in a unipolar configuration is configured as a main converter operating in a grid formation mode; Configure another power converter connected to the transmission pipe intended to stop power transmission in a unipolar configuration as a slave converter operating in grid follower mode; and Control the converter to reduce the power being fed into the transmission pipe connected to the converter to zero.

[0025] The method of the present invention shares the benefits of the corresponding features of the bipolar power transfer scheme of the present invention.

[0026] It will be understood that the use of terms such as “first” and “second” in this patent specification is intended only to help distinguish similar features (e.g., first transmission pipe and second transmission pipe, and first power converter and second power converter) and not to indicate the relative importance of one feature over another, unless otherwise specified.

[0027] Within the scope of this application, it is expressly intended that the various aspects, embodiments, examples, and alternatives set forth in the foregoing paragraphs and claims and / or the following description and drawings, and in particular their individual features, may be employed independently or in any combination. That is, all embodiments and all features of any embodiment may be combined in any manner and / or combination unless such features are incompatible. The applicant reserves the right to accordingly modify any originally filed claim or to file any new claim, including the right to modify any originally filed claim to depend on any feature of any other claim and / or incorporate any feature of any other claim, although not originally claimed in that manner. Attached Figure Description

[0028] The following is a brief description of preferred embodiments of the invention by way of non-limiting example with reference to the accompanying drawings, in which: Figure 1 A schematic diagram of a bipolar power transfer scheme operating under normal conditions according to a first embodiment of the present invention is shown; Figure 2 Show Figure 1 The diagram shows the transformation of the bipolar power transfer scheme into an asymmetric unipolar configuration; and Figure 3 Show Figure 1 The diagram shown is a schematic representation of a bipolar power transfer scheme with asymmetric unipolar operation. Detailed Implementation

[0029] like Figure 1 As shown, the bipolar power transfer scheme according to the first embodiment of the present invention is generally specified by reference numeral 10.

[0030] The bipolar power transfer scheme 10 includes a first converter station 12 located remotely from the second converter station 14. In the illustrated embodiment, the first converter station 12 is an offshore converter station 16, while the second converter station 14 is an onshore converter station 18. However, in other embodiments of the invention, this is not necessarily the case; for example, both converter stations 12 and 14 can be onshore converter stations.

[0031] Furthermore, in the illustrated embodiment, the bipolar power transfer scheme 10 of the present invention includes a second converter station 14, i.e., an onshore converter station 18. However, other embodiments of the present invention may include only the first converter station 12, for example, only an offshore converter station. Such other embodiments of the present invention are also applicable when a third party owns or manages the second converter station and the downstream power transfer network associated with the second converter station, and the bipolar power transfer scheme of the present invention is intended to interoperate with such a second converter station and the associated power transfer network.

[0032] Returning to the illustrated embodiment, the first converter station 12 and the second converter station 14 (i.e., offshore converter station 16 and onshore converter station 18) are interconnected via a first transmission pipe 20 and a second transmission pipe 22 (i.e., a first 'pole' and a second 'pole' (thus constituting a 'bipolar' scheme)), which allows the first offshore converter station 16 to transmit power to the second onshore converter station 18.

[0033] Each of the first transmission conduit 20 and the second transmission conduit 22 is or includes a submarine cable 24; however, in other embodiments of the invention, one or more transmission conduits may be or may include underground cables, overhead lines, or a mixture of such cables and lines.

[0034] The first converter station 12 and the second converter station 14 are also interconnected via a return conduit 26, which is or includes a dedicated metal return element 28, which is typically in the form of another submarine cable 24, but may also use some other form of electrical conductor.

[0035] The first offshore converter station 16 includes a first power converter 30 that electrically interconnects the first transmission channel 20 with the first variable power source 32.

[0036] In the illustrated embodiment, the first variable power source 32 is a first offshore wind farm 34 comprising a plurality of wind turbines (not shown). The first offshore wind farm 34 is configured as an offshore AC grid that provides AC power infeed to the first power converter 30; however, this is not necessarily the case in other embodiments of the invention.

[0037] In addition to the above, the first power converter 30 also includes its own first converter controller 36, which is programmed to control the power from the first power source 32 (i.e., the first offshore wind farm 34) to be delivered to the first transmission duct 20 in the form of DC power feed.

[0038] The first power converter 30 shown is a voltage source converter; however, other types of power converters may also be used.

[0039] The voltage source converter shown has a known configuration comprising three converter branches, each of which is divided into a first branch section and a second branch section, and each of the branch sections includes a chain link converter. Each chain link converter includes multiple chain link modules connected in series, and each chain link module includes multiple switching elements connected in parallel with an energy storage device in the form of a capacitor (although, however, other types of energy storage devices can also be used, i.e., any device capable of storing and releasing energy to selectively provide voltage, such as a fuel cell or a battery).

[0040] Supplying multiple chain link modules means that it is possible to accumulate a combined voltage across each chain link converter via an energy storage device (i.e., a capacitor) that inserts multiple chain link modules (each of which provides its own voltage), the combined voltage being higher than the voltage available from each individual chain link module.

[0041] Therefore, each of the chain link modules works together to allow the chain link converter to provide a step-variable voltage source. This allows for the generation of voltage waveforms using stepwise approximations across each chain link converter. Thus, each chain link converter is able to provide a wide range of complex waveforms, and thereby enables the voltage source converter to provide the aforementioned power transfer functionality, i.e., transferring power from the first power source 32 (i.e., the first offshore wind farm 34) to the first transmission duct 20.

[0042] The first offshore converter station 16 also includes a second power converter 38, which is also a voltage source converter and is configured in substantially the same manner as the first power converter 30, as described above.

[0043] However, the second power converter 38 instead electrically interconnects the second transmission pipe 22 with the second variable power source 40.

[0044] In the illustrated embodiment, the second variable power source 40 is a second offshore wind farm 42, which similarly includes a plurality of wind turbines (not shown).

[0045] Similarly, the second power converter 38 also includes its own second converter controller 44, which is programmed to control the power from the second power source 40 (i.e., the second offshore wind farm 42) to be delivered to the second transmission duct 22 in the form of DC power feed.

[0046] Like the first offshore wind farm 34, the second offshore wind farm 42 is similarly configured to provide an offshore AC grid that feeds AC power into the second power converter 38; however, this need not be the case in other embodiments of the invention.

[0047] Meanwhile, in the illustrated embodiment, the second shore converter station 18 includes a third power converter 48 and a fourth power converter 50, which also function as voltage source converters, configured substantially in the same manner as the first power converter 30 and the second power converter 38 in the first offshore converter station 16, although in a mirror configuration. However, it is also possible to include other types of power converters and other configurations of power converters in the second shore converter station 18 instead.

[0048] The third power converter 48 and the fourth power converter 50 are configured to feed DC power from the corresponding first transmission pipe 20 or the second transmission pipe 22 to the corresponding first AC network 52 or the second AC network 54, and do so in response to the DC voltage of the corresponding first transmission pipe 20 or the second transmission pipe 22. However, in other embodiments of the invention, the third power converter and the fourth power converter may instead be configured to feed DC power from the first pipe and the second pipe to a single AC network. In any case, the third power converter 48 and the fourth power converter 50 operate under DC voltage control, and they continue to do so throughout the operation of the bipolar power transfer scheme 10 of the invention, as described below.

[0049] In addition to the above, the bipolar transmission scheme 10 also includes a converter station controller 46, which, in the illustrated embodiment, takes the form of a station controller located within the first converter station 12. However, in other embodiments of the invention, the converter station controller may take the form of a station controller located remotely from the first converter station (e.g., in or near the second converter station). In still other embodiments of the invention, the converter station controller may take the form of one of the first and second converter controllers or the other converter controller, or each such converter controller may define a converter station controller.

[0050] In use, and while the bipolar power transfer scheme 10 is operating normally, the first converter controller 36 is programmed to control the first power converter 30 in a first grid formation mode 56. In this mode, the first power converter 30 maintains the first power source 32 (i.e., the first offshore wind farm 34) at a first voltage 58 and a first frequency 60; that is, the first power converter 30 regulates both the instantaneous AC frequency 60 and the AC voltage 58 of the first offshore wind farm 34. The first power converter 30 is also capable of providing reactive current equal to any steady-state rated current during AC faults within the first offshore wind farm 34.

[0051] Simultaneously, the second converter controller 44 is similarly programmed to control the second power converter 38 in the second grid formation mode 62, in which the second power converter 38 maintains the second power source 40 at the second voltage 64 and the second frequency 66; that is, the second power converter 38 regulates the instantaneous AC frequency 66 and AC voltage 64 of the second offshore wind farm 42. Similarly, the second power converter 38 is also capable of providing reactive current equal to any steady-state rated current during AC faults within the second offshore wind farm 42.

[0052] The first grid formation mode 56 and the second grid formation mode 62 are completely independent of each other. In other words, individual offshore wind farms 34 and 42 can independently feed power to each transmission line 20 and 22 via corresponding power converters 30 and 38.

[0053] Additionally, the converter station controller 46 is programmed to selectively convert the bipolar power transfer scheme 10 to an asymmetric unipolar configuration, such as... Figure 3 As shown in the diagram. Furthermore, the converter station controller achieves this conversion while maintaining the transfer of power from the first power source 32 and the second power source 40.

[0054] In the illustrated embodiment, the converter station controller 46 selectively converts the bipolar power transfer scheme 10 into an asymmetric unipolar configuration by electrically interconnecting the first power source 32 and the second power source 40 (i.e., by electrically interconnecting the first offshore wind farm 34 and the second offshore wind farm 42), such as... Figure 2 As shown in the image.

[0055] More specifically, the first offshore wind farm 34 and the second offshore wind farm 42 are temporarily interconnected at a common coupling point 68, which in the illustrated embodiment is via an AC busbar (connected via a bus coupler) shared by the two power converters 30, 38 to form a combined power source 90.

[0056] Before interconnecting the first offshore wind farm 34 and the second offshore wind farm 42, the converter station controller 46 is programmed to synchronize the voltages 58, 64 and frequencies 60, 66 of the first offshore wind farm 34 and the second offshore wind farm 42 (i.e., the first power source 32 and the second power source 40). Additionally, similar to synchronizing the voltage amplitudes 58, 64 and frequencies 60, 66, the phase angles of the first power source 32 and the second power source 40 are also synchronized.

[0057] For example, the converter station controller 46 can cause the second converter station controller 44 to synchronize the first voltage 58 and frequency 60 of the first offshore wind farm 34 with the second voltage 64 and frequency 66 of the second offshore wind farm 42. This can be accomplished by sending a measurement of the second voltage 64 to the first converter controller 36, which in turn modifies the first voltage 58 and frequency 60 accordingly. More specifically, in the illustrated embodiment, the measured voltages include instantaneous three-phase voltages, from which the amplitude, phase angle, and frequency of the three-phase voltages can be determined.

[0058] Following the electrical interconnection of the first offshore wind farm 34 and the second offshore wind farm 42, i.e., to form a combined power source 90, the first power converter 30 and the second power converter 38 initially continue to operate in their respective first grid formation mode 56 and second grid formation mode 62. However, while the second converter controller 44 continues to control the second power converter 38 to maintain the second power source 40 (i.e., the second offshore wind farm 42) at the second voltage 64 and the second frequency 66, the first converter controller 36 modifies the first grid formation mode 56 of the first power converter 30 such that the first power converter 30 is configured to operate the first power source 32 (i.e., the first offshore wind farm 34) at the second voltage 64 and the second frequency 66.

[0059] In other embodiments of the invention (not shown), the second voltage and frequency of the second offshore wind farm may be synchronized to match the first voltage and first frequency of the first offshore wind farm.

[0060] Additionally, although the first power converter 30 and the second power converter 38 initially continue to operate in the first grid formation mode 56 and the second grid formation mode 62 described above, droop control is applied to the two power converters 30, 38. This is to prevent any interaction between the corresponding converter controllers 36, 44 of the first power converter 30 and the second power converter 38.

[0061] Subsequently, the converter station controller 46 is programmed to continue the process of converting the bipolar power transmission scheme 10 to an asymmetric unipolar configuration by configuring one power converter 30, 38 connected to the transmission lines 20, 22 intended to continue transmitting power in the unipolar configuration as a master converter operating in grid formation mode; and configuring another power converter 30, 38 connected to the transmission lines 20, 22 intended to stop transmitting power in the unipolar configuration as a slave converter operating in grid follower mode controlled by the master converter.

[0062] For example, and continuing the example above, the converter station controller 46 can be programmed to configure the second power converter 38 as the master converter 70 (note that the second transmission pipe 22 is thus intended to continue transmitting power in a unipolar configuration), and to configure the first power converter 30 as the slave converter 72 (similarly, note that the first transmission pipe 20 is thus intended to stop transmitting power in a unipolar configuration), as... Figure 2 As shown in the image.

[0063] As indicated above, the main converter 70 (e.g., the second power converter 38) operates in a grid formation mode. In practice, this means that the second power converter 38 operates in a third grid formation mode 74, in which the second power converter 38 maintains the now electrically interconnected first power source 32 and second power source 40 (i.e., combined power source 90) at a third voltage 76 and a third frequency 78. The third voltage 76 and third frequency 78 may differ from the second voltage 64 and second frequency 66, in which the second power converter 38 previously maintained the second power source 40 at the second voltage 64 and second frequency 66 and indirectly, through voltage synchronization, also maintained the first power source 32 at the second voltage 64 and second frequency 66.

[0064] Meanwhile, converter 72 (e.g., first power converter 30) is now configured to operate in grid follower mode 80.

[0065] More specifically, a third voltage 76 and a third frequency 78 (i.e., voltage and frequency at common coupling point 68) of the combined power source 90 are measured from converter 72 (e.g., first power converter 30), and thereafter the output voltage of converter 72 is synchronized with the output voltage of the combined power source 90, that is, the output voltage of converter 72 is synchronized with the third voltage 76 and the third frequency 78.

[0066] In other words, the slave converter 72 is locked to the third voltage 76 generated by the master converter 70, making the slave converter 72 now considered to be under power control. Therefore, the grid-following converter (i.e., the slave converter 72) is matched to the AC voltage 76 and frequency 78 of the combined power source 90. The slave converter 72 is also capable of providing reactive current equal to the steady-state rated current during AC faults within the combined power source 90.

[0067] Finally, the converter station controller 46 is programmed to complete the process of converting the bipolar power transmission scheme 10 into an asymmetric unipolar configuration by controlling the slave converter 72 (e.g., the first power converter 30) to reduce the power being fed to the transmission pipe (e.g., the first transmission pipe 20) connected to the slave converter 72 to zero.

[0068] Therefore, in the example embodiment described herein, the converter station controller 46 enables the second converter controller 44 to complete the transformation by reducing the power being fed into the first transmission pipe 20 by the first power converter 30 to zero.

[0069] Once the power being fed into the first transmission duct 20 reaches zero, each of the first power converter 30 and the third power converter 48 is blocked, and the converter station controller 46 disconnects the converter 72 (e.g., the first power converter 30) from the combined power source 90. The converter station controller 46 also disconnects the return duct 26 from the first power converter 30 and the third power converter 48, and disconnects the third power converter 48 from the first AC network 52, thereby allowing maintenance and / or repair of the corresponding first transmission duct 20.

[0070] Simultaneously, power from the combined power source 90 (i.e., power from the two offshore wind farms 34, 42) passes solely through the main converter 70 (e.g., the second power converter 38) (provided the power rating of the second power converter 38 is not exceeded) and enters the second transmission duct 22 connected to the main converter 70, and thus proceeds forward into the second AC network 52, thereby avoiding interruption of power delivery from the first power source 32 and the second power source 40. Meanwhile, voltage and / or frequency regulation of the combined power source 90 by the main converter 70 (i.e., the second power converter 38) can help ensure that the power rating of the second power converter 38 is not exceeded. Furthermore, in embodiments where the converter station controller 46 communicates directly and remotely with one or more of the first offshore wind farm 34 and the second offshore wind farm 42, this can also be used to limit the power of the combined power source 90 to below the rating of the second power converter 38.

Claims

1. A bipolar power transmission device, comprising: The first converter station is located far from the second converter station during use; And a first transmission pipe and a second transmission pipe, which are used to interconnect the first converter station and the second converter station in use, and thereby allow the first converter station to transmit power to the second converter station. The first converter station includes: A first power converter electrically interconnects the first transmission channel with a first variable power source and, in use, transfers power from the first variable power source to the first transmission channel; and A second power converter electrically interconnects the second transmission channel with a second variable power source and, in use, transfers power from the second variable power source to the second transmission channel. The bipolar power transmission device further includes a converter station controller programmed to selectively convert the bipolar power transmission device to an asymmetric unipolar configuration while maintaining power transfer from both the first and second variable power sources in the following manner: The first variable power source and the second variable power source are electrically interconnected; A power converter connected to the transmission pipeline intended to continue transmitting power in the unipolar configuration is configured as a main converter operating in grid formation mode, wherein the main converter operating in grid formation mode maintains the voltage and frequency of the power source connected to the main converter; Another power converter connected to the transmission pipe intended to stop power transmission in the unipolar configuration is configured as a slave converter operating in grid follower mode, wherein the slave converter operating in grid follower mode measures the voltage and frequency of the power source connected to the slave converter so as to synchronize the output voltage of the slave converter with the power source; and The control unit reduces the power being fed into the transmission pipe connected to the slave converter to zero.

2. The bipolar power transmission device according to claim 1, wherein, The converter station controller (46) is further programmed to disconnect the slave converter from the first and second electrically interconnected variable power sources once the power being fed into the transmission pipe connected to the slave converter reaches zero.

3. The bipolar power transmission device according to claim 1 or 2, wherein, The first variable power source and the second variable power source are temporarily interconnected at a common coupling point.

4. The bipolar power transmission device according to any one of claims 1-2, wherein, Before interconnecting the first variable power source and the second variable power source, the converter station controller is programmed to synchronize the voltages and frequencies of the first variable power source and the second variable power source with each other.

5. The bipolar power transmission device according to any one of claims 1-2, wherein, After the first and second variable power sources are interconnected, droop control is initially applied to the two power converters (30, 38).

6. The bipolar power transmission device according to any one of claims 1-2, wherein, The first converter station is an offshore converter station, and at least one of the first variable power source and the second variable power source is an offshore wind farm.

7. The bipolar power transmission device according to any one of claims 1-2 further includes a second converter station, which is interconnected with the first converter station through the first transmission pipe and the second transmission pipe.

8. A method of operating a bipolar power transmission device, the bipolar power transmission device comprising: The first converter station is located far from the second converter station during use; And a first transmission pipe and a second transmission pipe, which are used to interconnect the first converter station and the second converter station in use, and thereby allow the first converter station to transmit power to the second converter station. The first converter station includes: A first power converter electrically interconnects the first transmission channel with a first variable power source and, in use, transfers power from the first variable power source to the first transmission channel; and A second power converter electrically interconnects the second transmission channel with a second variable power source and, in use, transfers power from the second variable power source to the second transmission channel. The bipolar power transmission device also includes a converter station controller, and The method includes the following steps: enabling the converter station controller to selectively convert the bipolar power transmission device to an asymmetric unipolar configuration while maintaining power transmission from both the first variable power source and the second variable power source in the following manner: The first variable power source and the second variable power source are electrically interconnected; A power converter connected to the transmission pipeline intended to continue transmitting power in the unipolar configuration is configured as a main converter operating in grid formation mode, wherein the main converter operating in grid formation mode maintains the voltage and frequency of the power source connected to the main converter; Another power converter connected to the transmission pipe intended to stop power transmission in the unipolar configuration is configured as a slave converter operating in grid follower mode, wherein the slave converter operating in grid follower mode measures the voltage and frequency of the power source connected to the slave converter so as to synchronize the output voltage of the slave converter with the power source; and The control unit reduces the power being fed into the transmission pipe connected to the slave converter to zero.

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