Improvements in or relating to a bipolar power transfer scheme

Abstract

A bipolar power transfer scheme (10) includes a first converter station (12) located remotely from a second converter station (14) during use. The first converter station (12) includes a first power converter (30) electrically connected to a first variable power source (38). The first power converter (30) also has a first converter controller (44) programmed to control the delivery of a first power feed (46) from the first power source (38) to the first transmission channel (20) in the form of a first power feed (76). Additionally, the first converter station (12) also includes a second power converter (48) electrically connected to a second variable power source (50). The second power converter (48) also has a second converter controller (54) programmed to control the delivery of a second power feed (56) from the second power source (50) to the second transmission channel (22) in the form of a second power feed (86). During normal operation of the bipolar power transmission scheme (10), the first power converter (30) and the associated first power source (38) operate independently of the second power converter (48) and the associated second power source (50), thereby the first power feed (46) to the first transmission channel (20) and the second power feed (56) to the second transmission channel (22) are independent of each other. During operation of the bipolar power transfer scheme (10) under abnormal conditions when the return pipe (24) fails and a return current path cannot be provided, each converter controller (44, 54) is programmed to monitor the first power feed (76) in the first transmission pipe (20) and the second power feed (86) in the second transmission pipe (22), and if the first power feed (76) in the first transmission pipe (20) and the second power feed (86) in the second transmission pipe (22) are different from each other, at least one converter controller (44, 54) modifies the power feed (46, 56) from its corresponding power source (38, 50) to reduce the difference between the first power feed (76) and the second power feed (86).

Description

Technical Field

[0001] This invention relates to a bipolar power transfer scheme 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 most suitable HVDC power transfer scheme in an HVDC power transfer network varies depending on the application and scheme characteristics. One type of such scheme is the bipolar power transfer scheme. Summary of the Invention

[0005] According to a first aspect of the invention, 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 conduit, a second transmission conduit, and a return conduit 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.

[0006] The first converter station includes:

[0007] A first power converter has a first DC terminal connected to the first transmission channel, a second DC terminal connected to the return channel, and at least one AC terminal electrically connected to a first variable power source. The first power converter also has a first converter controller programmed to control the transmission of a first power infeed from the first power source into the first transmission channel in the form of a first power feed.

[0008] The second power converter has a first DC terminal connected to the second transmission channel, a second DC terminal connected to the return channel, and at least one AC terminal electrically connected to the second variable power source. The second power converter also has a second converter controller programmed to control the delivery of a second power feed from the second power source into the second transmission channel in the form of a second power feed.

[0009] During normal operation of the bipolar power transfer scheme, the first power converter and its associated first power source operate independently of the second power converter and its associated second power source, thereby ensuring that the first power feed into the first transmission channel is independent of the second power feed into the second transmission channel.

[0010] During operation of the bipolar power transfer scheme in an abnormal situation where the return pipe fails and a return current path cannot be provided, each converter controller is programmed to monitor a first power feed in the first transmission pipe and a second power feed in the second transmission pipe, and if the first power feed in the first transmission pipe and the second power feed in the second transmission pipe are different from each other, at least one converter controller modifies the power feed from its corresponding power source to reduce the difference between the first power feed and the second power feed.

[0011] In operation, when the return conduit fails and cannot provide a return current path, any imbalance (i.e., difference) in the corresponding power feed in each of the first and second transmission conduits results in a current imbalance between the first and second transmission conduits. In the absence of a functioning return conduit, this current imbalance must instead be transmitted through ground. Such transmission is undesirable, especially due to concerns about adverse effects on, for example, marine life and corrosion of the pipes underground.

[0012] The system includes at least one converter controller that modifies the power feed from its corresponding power source to reduce the difference in power carried by each transmission channel (i.e., reducing the difference between the first and second power feeds), thereby advantageously reducing the magnitude of the unbalanced current that would otherwise have to flow through ground. This, in turn, advantageously reduces the adverse effects of such current flow while allowing each power source to continue supplying power, and thus avoids highly undesirable interruptions to power transmission from the first converter station to the second converter station during use.

[0013] Such a modification to the power feed can also be made to return to operation as soon as the pipe becomes inactive, and thus helps to minimize the time during which large unbalanced currents may otherwise flow.

[0014] Additionally, a converter controller having at least one such controller capable of functioning solely based on monitoring a first power feed in a first transmission pipe and a second power feed in a second transmission pipe provides an arrangement in which the device can operate in isolation without any other external input, such as from a second converter station, and thus avoids the need for dedicated (and typically expensive) long-distance communication between converter stations.

[0015] Preferably, a converter controller that modifies the power feed from its corresponding power source to reduce the difference between the first power feed and the second power feed is programmed to modify its power feed such that the power feeds in each transmission pipe are matched to each other.

[0016] During abnormal operation of a bipolar power transfer scheme, i.e., in the absence of a working return pipe, it is desirable to have such a matched power feed in each transmission pipe, as it virtually eliminates the current imbalance between transmission pipes and thus essentially avoids current flow through ground, while allowing each power source to continue providing power and continuing to transmit that power to the second converter station in use.

[0017] In a preferred embodiment of the invention, each power source includes at least one power controller that adjusts the power feed provided by the power source according to the operating frequency of the power source and its associated power converter, and each converter controller is programmed to modify the power feed from its corresponding power source by modifying the operating frequency at which its corresponding power converter operates.

[0018] In this way, the operating frequency of a given power converter and its associated power source can be used to deliver power demand according to the overall operating conditions of the bipolar power transfer scheme, i.e., the power required by the power source.

[0019] In another preferred embodiment of the invention, if the first power feed in the first transmission channel is greater than the second power feed in the second transmission channel, the first converter controller is programmed to reduce the first power feed from the first power source, and if the second power feed in the second transmission channel is greater than the first power feed in the first transmission channel, the second converter controller is programmed to reduce the second power feed from the second power source.

[0020] Having a first and second converter controller programmed in this way is beneficial for certain types of power sources, such as those that more directly throttle or otherwise reduce the power feed they provide, as opposed to those that are easy to increase the power feed.

[0021] The first converter station may be an offshore converter station, and at least one of the first power source and the second power source may be an offshore wind farm.

[0022] Optionally, the offshore wind farm includes a plurality of wind turbines, each of the plurality of wind turbines including a power controller in the form of an individual turbine power controller, the individual turbine power controller adjusting the power supplied by the individual wind turbine according to the operating frequency of the wind turbine.

[0023] 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 and individual wind turbines is highly advantageous.

[0024] Furthermore, the stand-alone mode (in which the invention can operate) is particularly suitable for such devices because it avoids the need for any input from a second converter station (e.g., an onshore converter station), which may be located extremely far from the first converter station, and thus makes any communication between the two converter stations extremely difficult and / or unreliable.

[0025] Preferably, at least one wind turbine includes a power dissipation element that can be selectively operated by a corresponding turbine power controller to temporarily dissipate the power feed provided by the individual wind turbine.

[0026] Including such power dissipation elements provides an opportunity to temporarily reduce the power supplied by the corresponding wind turbine. Such temporary reduction can then be used by the turbine power controller to accommodate the limited amount of time required to mechanically adjust the pitch of the turbine blades in order to modify the actual power output provided by the turbine, i.e., after the turbine power controller observes a change in the operating frequency of the associated power converter (which indicates a need to modify the power output of the wind turbine), for example, to help temporarily reduce the total power feed in from the associated offshore wind farm.

[0027] The bipolar power transfer scheme may further include a first excess power absorber electrically connected between a first transmission pipe and a return pipe, and a second excess power absorber electrically connected between a second transmission pipe and a return pipe, wherein a corresponding excess power absorber in the excess power absorber is used to absorb power from one of the first and second transmission pipes or the other transmission pipe while a change in power feed is occurring in the first or second transmission pipe.

[0028] Including such an excess power absorber provides an alternative component that adapts to the finite amount of time that may be required to change the actual power provided by a given first or second power source.

[0029] The bipolar power transmission scheme may also include a second converter station interconnected with the first converter station via a first transmission pipe, a second transmission pipe, and a return pipe.

[0030] Including such a second converter station provides the opportunity for end-to-end control of the entire bipolar power transfer scheme.

[0031] 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, a second transmission channel, and a return 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.

[0032] The first converter station includes:

[0033] A first power converter has: a first DC terminal connected to the first transmission channel; a second DC terminal connected to the return channel; and at least one AC terminal electrically connected to a first variable power source. The first power converter further has a first converter controller programmed to control the transfer of a first power feed from the first power source into the first transmission channel in the form of a first power feed.

[0034] A second power converter has: a first DC terminal connected to the second transmission pipe; a second DC terminal connected to the return pipe; and at least one AC terminal electrically connected to a second variable power source. The second power converter also has a second converter controller programmed to control the delivery of a second power feed from the second power source into the second transmission pipe in the form of a second power feed.

[0035] The method includes the following steps:

[0036] Under normal conditions, the bipolar power transfer scheme is operated as follows: the first power converter and its associated first power source operate independently of the second power converter and its associated second power source, thereby making the first power feed into the first transmission channel independent of the second power feed into the second transmission channel, and

[0037] During abnormal conditions when the return pipe fails and no return current path can be provided, the bipolar power scheme is operated as follows: each converter controller monitors the first power feed in the first transmission pipe and the second power feed in the second transmission pipe, and if the first power feed in the first transmission pipe and the second power feed in the second transmission pipe are different from each other, at least one converter controller modifies the power feed from its corresponding power source to reduce the difference between the first power feed and the second power feed.

[0038] The method of the present invention shares the benefits of the corresponding features of the bipolar power transfer scheme of the present invention.

[0039] 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.

[0040] 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

[0041] The following is a brief description of preferred embodiments of the invention by way of non-limiting example with reference to the accompanying drawings, wherein:

[0042] Figure 1 A schematic diagram of a bipolar power transfer scheme according to a first embodiment of the present invention is shown; and

[0043] Figure 2 The diagram is used to form Figure 1 Possible control algorithms for the converter controller as part of the bipolar power transfer scheme shown. Detailed Implementation

[0044] 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.

[0045] 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 and 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.

[0046] 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 its associated downstream power transfer network, and the bipolar power transfer scheme of the present invention is intended to interoperate with such a second converter station and associated power transfer network.

[0047] Returning to the illustrated embodiment, the first converter station 12 and the second converter station 14 (i.e., the offshore converter station and the onshore converter station 16, 18) are interconnected via a first transmission pipe 20 and a second transmission pipe 22 (i.e., the first 'pole' and the second 'pole' (thus constituting a 'bipolar' scheme)) and a return pipe 24, which allows the first offshore converter station 16 to transmit power to the second onshore converter station 18.

[0048] Each of the first transmission conduit 20 and the second transmission conduit 22 is a submarine cable 26 or includes a submarine cable 26; however, in other embodiments of the invention, one or more transmission conduits may be or include underground cables, overhead lines, or a mixture of such cables and lines.

[0049] Meanwhile, the return pipe 24 is or includes a dedicated metal return element 28, which is typically in the form of another submarine cable 26, but may also use some other form of electrical conductor.

[0050] The first offshore converter station 16 includes a first power converter 30 having a first DC terminal 32 connected to a first transmission conduit 20 and a second DC terminal 34 connected to a return conduit 24. The return conduit 24 is connected to ground 84, which is intentionally used as a reference to which the first DC terminal 32 and the second DC terminal 34 can be compared.

[0051] The first power converter 30 also includes three AC terminals 36A, 36B, and 36C, wherein each of the three AC terminals 36A, 36B, and 36C corresponds to a corresponding phase A, B, or C of the first variable power source 38 electrically connected to the AC terminals 36A, 36B, and 36C.

[0052] In the illustrated embodiment, the first variable power source 38 is a first offshore wind farm 40 comprising a plurality of wind turbines 42. Each wind turbine 42 includes a power controller in the form of an individual turbine power controller (not shown), which may be in the form of a local phase-locked loop (PLL) controller, but in any case regulates the power supplied by the individual wind turbine 42 according to the operating frequency of the wind turbine 42.

[0053] In addition to the above, the first power converter 30 also includes its own first converter controller 44, which is programmed to control the transmission of the first power feed 46 from the first power source 38 (i.e., the first offshore wind farm 40) to the first transmission duct 20 in the form of a first power feed 76, which is a DC power feed.

[0054] Each wind turbine 42 also includes a power dissipation element (not shown), such as a dynamic braking resistor, which can be used to temporarily dissipate the individual power supplied to each wind turbine 42. These individual power contributions are otherwise combined to define, as a whole, a first power feed 46 from the first offshore wind farm 40. More specifically, the first offshore wind farm 40 is configured to an offshore AC grid, and the first power feed 46 takes the form of an AC power feed; however, in other embodiments of the invention, such a configuration and type of power feed is not necessarily required. Additionally, the first converter controller 44 controls the first power converter 30 in such a way that it supplies voltage to the offshore AC grid (i.e., to the first offshore wind farm 40).

[0055] The first power converter 30 shown is a voltage source converter; however, other types of power converters may also be used.

[0056] The voltage source converter shown has a known configuration, which includes three converter branches extending between a first DC terminal 32 and a second DC terminal 34 and corresponding to respective phases A, B, and C of the first power source 38. Each converter branch includes a first branch portion and a second branch portion separated by corresponding AC terminals 36A, 36B, and 36C.

[0057] Each branch section includes a chain link converter extending between associated AC terminals 36A, 36B, 36C and a corresponding DC terminal of either the first DC terminal 32 or the second DC terminal 34. 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 may 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).

[0058] Supplying multiple chain link modules means that it is possible to build up 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.

[0059] 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. Consequently, each chain link converter is capable of providing a wide range of complex waveforms.

[0060] For example, the operation of each chain link converter in the manner described above can be used to generate AC voltage waveforms at each AC terminal 36A, 36B, thereby enabling the voltage source converter to provide the aforementioned power transfer functionality, namely, the transfer of the first power feed 46 from the first power source 38 (i.e., the first offshore wind farm 40) to the first transmission duct 20.

[0061] The first offshore converter station 16 also includes a second power converter 48, which is also a voltage source converter and is configured in substantially the same manner as the first power converter 30, as described above.

[0062] However, the second power converter 48 does have a first DC terminal 32 connected to the second transmission pipe 22, while its second DC terminal 34 is similarly connected to the return pipe 24.

[0063] Each of the three AC terminals 36A, 36B, and 36C of the second power converter 48 is electrically connected to the second variable power source 50.

[0064] In the illustrated embodiment, the second variable power source 50 is a second offshore wind farm 52, which similarly includes a plurality of wind turbines 42. Each wind turbine 42 further includes a power controller in the form of an individual turbine power controller (not shown), such as a local phase-locked loop (PLL) controller, which adjusts the power supplied by the individual wind turbine 42 according to the operating frequency of the wind turbine 42.

[0065] Similarly, the second power converter 48 also includes its own second converter controller 54, which is programmed to control the second power feed 56 from the second power source 50 (i.e., the second offshore wind farm 52) to the second transmission duct 22 in the form of a second power feed 86, which is similarly a DC power feed.

[0066] Similarly, each wind turbine 42 in the second offshore wind farm 52 also includes a power dissipation element (not shown) that can be used to temporarily dissipate the individual power supplied to each wind turbine 42, and such individual power contributions are otherwise combined to define a second power feed 56 from the second offshore wind farm 52 as a whole. Like the first offshore wind farm 40, the second offshore wind farm 52 is similarly configured to an offshore AC grid, and the second power feed 56 is in the form of an AC power feed. However, in other embodiments of the invention, such a configuration and type of power feed is not necessarily required. Additionally, the second converter controller 54 similarly controls the second power converter 48 in such a way that it supplies voltage to the offshore AC grid (i.e., to the second offshore wind farm 52).

[0067] In addition to the above, the bipolar power transfer scheme 10 also includes a first excess power absorber 58 electrically connected between the first transmission pipe 20 and the return pipe 24, and a second excess power absorber 60 electrically connected between the second transmission pipe 22 and the return pipe 24. In the illustrated embodiment, both excess power absorbers 58 and 60 are located within the second converter station 14, i.e., within the onshore converter station 18, but this is not necessarily the case.

[0068] Each excess power absorber 58, 60 is preferably a dynamic braking system; however, other types of power absorbers may also be used. In any case, a given excess power absorber 58, 60 is configured such that it can be used to absorb power from a corresponding transmission pipe connected to it in the first and second transmission pipes 20, 22.

[0069] Meanwhile, in the illustrated embodiment, the second shore converter station 18 includes a third power converter and a fourth power converter 68, 70, which are also voltage source converters, configured substantially in the same manner as the first power converter and the second power converters 30, 48 in the first offshore converter station 16, although configured in a mirror manner. However, other types of power converters and other configurations of power converters may be included in the second shore converter station 18 instead. The third power converter 68 and the fourth power converter 70 are configured to deliver the corresponding first power feed 76 or second power feed 86 (i.e., the corresponding DC power feed) from the corresponding first transmission pipe 20 or second transmission pipe 22 to the corresponding first AC network 72 or second AC network 74. However, in other embodiments of the invention, the third power converter and the fourth power converter may be configured to deliver the first power feed and the second power feed (i.e., the DC power feed from the first pipe and the second pipe) to a single AC network.

[0070] During normal operation of the bipolar power transfer scheme 10, the first power converter 30 and its associated first power source 38 operate independently of the second power converter 48 and its associated second power source 50, such that the first power feed 46 into the first transmission duct 20 and the second power feed 56 into the second transmission duct 22 are completely independent of each other. In other words, individual offshore wind farms 40, 52 can independently feed power into each transmission duct 20, 22.

[0071] In contrast, during operation of the bipolar power transfer scheme 10 under abnormal conditions—in the context of this invention, the abnormal conditions being when the return pipe 24 fails and cannot provide a return current path (or is damaged or inoperable to the extent that it must be disconnected from scheme 10)—each of the converter controllers in the first converter controller 44 and the second converter controller 54 is programmed to monitor the first power feed 76 in the first transmission pipe 20 and the second power feed 86 in the second transmission pipe 22.

[0072] The first converter controller 44 and the second converter controller 54 can monitor each of the first power feeds 76 and 86 in the corresponding transmission channels 20 and 22 by: directly measuring or otherwise calculating the corresponding power feeds 76 and 86; or indirectly by measuring or otherwise calculating the corresponding power feeds 46 and 56 from the associated first power source 38 or second power source 50 and determining the actual or expected power feeds 76 and 86 in the corresponding first transmission channel 20 or second transmission channel 22, for example, based on the operating parameters of the corresponding first power converter 30 or second power converter 48.

[0073] More specifically, the converter controllers 44, 54 are further programmed such that if the first power feed 76 in the first transmission channel 20 is different from the second power feed 86 in the second transmission channel 22, at least one converter controller 44, 54 modifies the power feeds 46, 56 from their corresponding power sources 38, 50 to reduce the difference between the first power feed 76 and the second power feed 86.

[0074] Furthermore, whichever converter controller 44, 54 is being modified to reduce the difference between the first power feed 76 and the second power feed 86 from its corresponding power sources 38, 50, is programmed to modify the power feeds 46, 56 such that the power feeds 76, 86 in each transmission channel 20, 22 are matched to each other.

[0075] As indicated above, each power source 38, 50 includes multiple power controllers, i.e., individual turbine power controllers for each wind turbine 42, which together regulate the power feeds 46, 56 provided by the power sources 38, 50 and their associated power converters 30, 48 according to their operating frequencies. That is, they regulate the power provided by each individual wind turbine 42 according to the operating frequency, at which all wind turbines 42 within the offshore wind farm 40, 52 are operated.

[0076] Furthermore, each converter controller 44, 54 is programmed to modify the power feed 46, 56 from its corresponding power source 38, 50 by modifying the operating frequency at which its corresponding power converter 30, 48 operates, and which in turn modifies the operating frequency at which all wind turbines 42 in the corresponding offshore wind farm 40, 52 are forced to operate.

[0077] In other words, during the operation of the bipolar power transfer scheme 10 under the aforementioned abnormal conditions, such as Figure 2 The schematic diagram shows that each converter controller 44, 54 is programmed as follows:

[0078] - Monitor the first power feed 76 and the second power feed 86 in the corresponding first transmission pipe 20 and second transmission pipe 22; and

[0079] -If the first power feed 76 and the second power feed 86 are different from each other;

[0080] Then a converter controller 44, 54 determines the required operating frequency change 62 so that the nominal operating frequency 64 of the corresponding power converter 30, 48 currently in operation is changed to a new operating frequency 66, which in turn will adjust the power feed 46, 56 provided by the corresponding first power source 38 or second power source 50 to the desired level, so that the first power feed 76 and the second power feed 86 are matched with each other.

[0081] More specifically, if the first power feed 76 in the first transmission pipe 20 is greater than the second power feed 86 in the second transmission pipe 22, then the first converter controller 44 is programmed to reduce the first power feed 46 from the first power source 38, and if the second power feed 86 in the second transmission pipe 22 is greater than the first power feed 76 in the first transmission pipe 20, then the second converter controller 54 is programmed to reduce the second power feed 56 from the second power source 50. This ensures that in each instance, the first power feed 76 and the second power feed 86 are matched to each other.

[0082] In practice, when each turbine power controller detects a change in the operating frequency of the power converters 30, 56 (to which each turbine power controller is feeding power), and thus detects a change in the operating frequency at which the associated wind turbine 42 is required to operate, the corresponding turbine power controller changes the power reference for the wind turbine 42, and subsequently mechanically adjusts the pitch of the turbine blades of the wind turbine 42 to achieve the required reduction in the power output of the wind turbine 42.

[0083] Although each turbine power controller can respond very quickly (typically within 100 milliseconds or less) to changes in the operating frequency of the associated wind turbine 42, changing the pitch of each turbine blade takes much longer. Therefore, there is a considerable finite amount of time before the corresponding power feeds 46, 56 can be reduced, in order to further reduce the difference in power feeds 76, 86 in transmission ducts 20, 22, and ultimately eliminate that difference.

[0084] Similarly, when unbalanced currents flow through the ground due to imbalances in the power feeds 76, 86 in each of the transmission pipes 20, 22, accompanied by adverse effects on, for example, marine life and pipe corrosion, this will otherwise remain for a limited period of time after the failure of the return pipe 24 and the bipolar power transmission scheme 10 begins to operate under abnormal conditions.

[0085] In order to eliminate such effects while the corresponding power feeds 76, 86 are reduced (i.e., during the mechanical response time of the individual wind turbines 42), a corresponding first excess power absorber 58 or a second excess power absorber 60 (e.g., a corresponding dynamic braking system) connected to the transmission ducts 20, 22 with larger power feeds 76, 86 can be operated to temporarily absorb the amount of power necessary to temporarily balance the first power feed 76 and the second power feed 86 in the first and second transmission ducts 20, 22.

[0086] Alternatively, while reducing the corresponding power feed 76, 86, i.e., during the aforementioned mechanical response time of the individual wind turbine 42, individual power dissipation elements (e.g., dynamic braking resistors) included in each wind turbine 42 can be operated to temporarily dissipate a corresponding individual amount of power provided by each wind turbine 42, so as to temporarily reduce the combined total power feed 46, 56 as a whole from the associated power sources 38, 50, i.e., the combined total power feed 46, 56 from the associated first offshore wind farm 40 or second offshore wind farm 40, 52, to temporarily balance the first power feed 76 and the second power feed 86 in the first transmission duct 20 and the second transmission duct 22.

Claims

1. A bipolar power transmission device (10), comprising: The first converter station (12) is located far from the second converter station (14) during use; The first transmission channel (20), the second transmission channel (22), and the return channel (24) are used to interconnect the first converter station (12) with the second converter station (14) in use, and thereby allow the first converter station (12) to transmit power to the second converter station (14). The first converter station (12) includes: A first power converter (30) having: a first DC terminal (32) connected to the first transmission channel (20); a second DC terminal (34) connected to the return channel (24); and at least one AC terminal (36A, 36B, 36C) electrically connected to the first variable power source (38), the first power converter (30) further having a first converter controller (44) programmed to control the first power feed (46) from the first variable power source (38) to the first transmission channel (20) in the form of a first power feed (76); and A second power converter (48) having: a first DC terminal (32) connected to the second transmission channel (22); a second DC terminal (34) connected to the return channel (24); and at least one AC terminal (36A, 36B, 36C) electrically connected to the second variable power source (50), the second power converter (48) further having a second converter controller (54) programmed to control the transfer of a second power feed (56) from the second variable power source (50) to the second transmission channel (22) in the form of a second power feed (86). During normal operation of the bipolar power transmission device (10), the first power converter (30) and its associated first variable power source (38) operate independently of the second power converter (48) and its associated second variable power source (50), thereby making the first power feed (46) in the first transmission channel (20) and the second power feed (56) in the second transmission channel (22) independent of each other. During operation of the bipolar power transmission device (10) under abnormal conditions when the return pipe (24) fails and a return current path cannot be provided, each converter controller (44, 54) is programmed to monitor the first power feed (76) in the first transmission pipe (20) and the second power feed in the second transmission pipe (22), and if the first power feed (76) in the first transmission pipe (20) and the second power feed (86) in the second transmission pipe (22) are different from each other, at least one converter controller (44, 54) modifies the power feed from its corresponding power source (38, 50) to reduce the difference between the first power feed (76) and the second power feed (86).

2. The bipolar power transmission device (10) according to claim 1, wherein, The converter controllers (44, 54) that modify the power feeds (46, 56) from their respective power sources (38, 50) to reduce the difference between the first power feed (76) and the second power feed (86) are programmed to modify their power feeds (46, 56) such that the power feeds (76, 86) in each transmission channel (20, 22) are matched to each other.

3. The bipolar power transmission device (10) according to claim 1 or 2, wherein, Each power source (38, 50) includes at least one power controller that adjusts the power feed (46, 56) provided by the power source (38, 50) according to the operating frequency of the power source (38, 50) and its associated power converter (30, 48), and each converter controller (44, 54) is programmed to modify the power feed (46, 56) from its corresponding power source (38, 50) at the operating frequency by modifying the operating frequency.

4. The bipolar power transmission device (10) according to claim 1 or 2, wherein, The first converter controller (44) is programmed to reduce the first power feed (46) from the first variable power source (38) if the first power feed (76) in the first transmission pipe (20) is greater than the second power feed (86) in the second transmission pipe (22), and the second converter controller (54) is programmed to reduce the second power feed (56) from the second variable power source (50) if the second power feed (86) in the second transmission pipe (22) is greater than the first power feed (76) in the first transmission pipe (20).

5. The bipolar power transmission device (10) according to claim 1 or 2, wherein, Each converter controller (44, 54) is programmed to monitor the first power feed (76) in the first transmission pipe (20) and the second power feed in the second transmission pipe (22) in such a way as follows: Directly; or Indirectly fed in via corresponding power from the associated first variable power source (38) or second variable power source (50) (46, 56).

6. The bipolar power transmission device (10) according to claim 1 or 2, wherein, The first converter station (12) is an offshore converter station (16), and at least one of the first variable power source (38) and the second variable power source (50) is an offshore wind farm (40, 52).

7. The bipolar power transmission device (10) according to claim 6, wherein, The offshore wind farm (40, 52) or each offshore wind farm (40, 52) includes a plurality of wind turbines (42), each of the plurality of wind turbines (42) including a power controller in the form of an individual turbine power controller, the individual turbine power controller adjusting the power supplied by the individual wind turbine (42) according to the operating frequency of the wind turbine (42).

8. The bipolar power transmission device according to claim 7, wherein, At least one wind turbine (42) includes a power dissipation element that can be selectively operated by a corresponding turbine power controller to temporarily dissipate the power feed provided by the individual wind turbine (42).

9. The bipolar power transmission device (10) according to claim 1 or 2, further comprising: A first excess power absorber (58) is electrically connected between the first transmission pipe (20) and the return pipe (24); And a second excess power absorber (60) electrically connected between the second transmission pipe (22) and the return pipe (24), wherein a corresponding excess power absorber in the excess power absorber (58, 60) is used to absorb power from one or the other of the first transmission pipe (20) and the second transmission pipe (22) while a change in the power feed (76, 86) in the first transmission pipe (20) or the second transmission pipe (22) is occurring.

10. The bipolar power transmission device (10) according to claim 1 or 2 further includes a second converter station (14) interconnected with the first converter station (12) via the first transmission pipe (20), the second transmission pipe (22), and the return pipe (24).

11. A method of operating a bipolar power transfer device (10), the bipolar power transfer device (10) comprising: The first converter station (12) is located far from the second converter station (14) during use; The first transmission channel (20), the second transmission channel (22), and the return channel (24) are used to interconnect the first converter station (12) with the second converter station (14) in use, and thereby allow the first converter station (12) to transmit power to the second converter station (14). The first converter station (12) includes: A first power converter (30) having: a first DC terminal (32) connected to the first transmission channel (20); a second DC terminal (34) connected to the return channel (24); and at least one AC terminal (36A, 36B, 36C) electrically connected to the first variable power source (38), the first power converter (30) further having a first controller (44) programmed to control the first power feed (46) from the first variable power source (38) to the first transmission channel (20) in the form of a first power feed (76); and A second power converter (48) having: a first DC terminal (32) connected to the second transmission pipe (22); a second DC terminal (34) connected to the return pipe (24); and at least one AC terminal (36A, 36B, 36C) electrically connected to the second variable power source (50), the second power converter (48) further having a second controller (54) programmed to control the transfer of a second power feed (56) from the second variable power source (50) to the second transmission pipe (22) in the form of a second power feed (86). The method includes the following steps: The bipolar power transmission device (10) is operated under normal conditions by making the first power converter (30) and its associated first variable power source (38) operate independently of the second power converter (48) and its associated second variable power source (50), thereby making the first power feed (46) in the first transmission channel (20) and the second power feed (56) in the second transmission channel (22) independent of each other, and During abnormal conditions when the return pipe (24) fails and a return current path cannot be provided, the bipolar power transmission device (10) is operated as follows: each controller (44, 54) monitors the first power feed (76) in the first transmission pipe (20) and the second power feed (86) in the second transmission pipe (22), and if the first power feed (76) in the first transmission pipe (20) is different from the second power feed (86) in the second transmission pipe (22), at least one controller (44, 54) modifies the power feed (46, 56) from its corresponding power source (38, 50) to reduce the difference between the first power feed (76) and the second power feed (86).

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