Improvements in or relating to converter stations

Abstract

A converter station (10) for a bipolar power transmission scheme comprises a first voltage source converter (12) and a second source converter (32). Each converter has a first terminal (14, 16) as a transmission conduit and a second terminal (36, 34) as a return conduit. The second terminal (16) of the return conduit of the first converter is electrically connected as the same terminal to the first terminal (36) of the transmission conduit of the second converter. The voltage source converters (12, 32) both further comprise at least one converter limb (18A, 18B, 18C) extending between the first and second terminals of each converter. The or each converter limb comprises first and second limb portions separated by corresponding first and second AC terminals for connection to respective phases (A, B, C) of a first or second AC network.

Description

[0001] This invention relates to a converter station for bipolar power transmission schemes, and to a method of operating such a converter station.

[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 capacitive load effect of the AC 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. One type of power converter is a voltage source converter.

[0004] Meanwhile, the most suitable HVDC power delivery scheme in an HVDC power delivery network varies depending on the application and scheme characteristics. One type of such scheme is a two-stage power delivery scheme.

[0005] According to a first aspect of the invention, a converter station for a bipolar power transmission scheme is provided, the converter station comprising:

[0006] A first voltage source converter has: a first terminal for connection to a first transmission pipe; a second terminal for connection to a return pipe; and at least one converter branch extending between the first and second terminals, said or each converter branch including first and second branch portions separated by corresponding first AC terminals for connection to a corresponding phase of a first AC network, said or each first branch portion including a first chain link converter extending between the associated first AC terminal and the first terminal, said or each second branch portion including a second chain link converter extending between the associated first AC terminal and the second terminal, and each chain link converter including a plurality of chain link modules connected in series, each chain link module having a plurality of switching elements connected in parallel with an energy storage device, thereby each chain link converter being controllable to provide a step variable voltage source; and

[0007] A second voltage source converter includes: a third terminal for connection to a second transmission pipe; a fourth terminal for connection to a return pipe; and at least one converter branch extending between the third and fourth terminals, said or each converter branch comprising third and fourth branch portions separated by corresponding second AC terminals for connection to a corresponding phase of a second AC network, said or each third branch portion comprising a third chain link converter extending between the associated second AC terminal and the third terminal, said or each fourth branch portion comprising a fourth chain link converter extending between the associated second AC terminal and the fourth terminal, and each chain link converter comprising a plurality of chain link modules connected in series, each chain link module having a plurality of switching elements connected in parallel with an energy storage device, thereby enabling each chain link converter to be controlled to provide a step-variable voltage source.

[0008] Each of the second and third chain link converters includes a first number of serially connected chain link modules, and each of the first and fourth chain link converters includes a second number of serially connected chain link modules, the second number being greater than the first number.

[0009] Providing each of the first and fourth chain link converters with a second, larger number of cascaded chain link modules allows the combined first and second voltage source converters to withstand worst-case transient fault conditions in terms of voltage stress on individual energy storage devices, i.e., internal AC phase-to-return pipe, such as ground, failure.

[0010] Meanwhile, limiting the number of cascaded chain links included in each of the second and third chain link converters to a lower first number provides a given level of converter performance, but reduces the total number of chain link modules in each voltage source converter, and thus results in a significant reduction in the capital cost of the converter station.

[0011] Furthermore, the reduced total number of chain link modules provides the option to reduce the overall structure and footprint of each chain link converter, as well as reduce the switching losses associated with each chain link converter. This allows for the use of smaller cooling equipment to service the chain link converters, and thus still provides additional options for reducing the overall footprint of the associated converter stations and therefore reducing the cost of the associated converter stations.

[0012] The reduced total number of chain link modules also reduces the maintenance burden and allows operators to carry fewer spare chain link modules for use in case of failure.

[0013] Preferably, each of the second and third chain link converters includes the same first number of cascaded chain link modules.

[0014] Each of the first and fourth chain link converters may include the same second number of cascaded chain link modules.

[0015] The above features help simplify the overall control of each voltage source converter, and also simplify the control of the converter station as a whole.

[0016] Optionally, each chain link module has the same topology.

[0017] Chain link modules with the same topology further assist in simplifying the control of each voltage source converter and provide economies of scale in the production of individual chain link modules.

[0018] In a preferred embodiment of the invention, each chain link module includes a first pair of switching elements, which are arranged in a half-bridge configuration and connected in parallel with the energy storage device to define a 2-quadrant unipolar module.

[0019] Utilizing 2-quadrant unipolar modules throughout each voltage source converter helps to achieve maximum cost savings by reducing the total number of chain link modules required to provide a given level of converter performance.

[0020] Preferably, at least one voltage source converter includes a corresponding voltage source converter controller programmed to control a step variable voltage source provided by the corresponding voltage source converter, and when doing so, to balance the total energy stored by the energy storage device in one branch of the voltage source converter with the total energy stored by the energy storage device in the other corresponding branches of the voltage source converter.

[0021] Balancing the total energy stored in the opposing limb portions helps ensure that only minimal circulation flows between the limb portions.

[0022] At least one voltage source converter controller is programmable to cause the chain link modules in a branch having a higher second number of chain link modules to store a proportionally lower average amount of individual energy than the average amount of individual energy stored by each chain link module in the other corresponding branch.

[0023] This characteristic makes it easy to achieve the desired energy balance between corresponding, opposing branches.

[0024] According to a second aspect of the present invention, a method for operating a converter station is provided, the converter station comprising:

[0025] A first voltage source converter has: a first terminal for connection to a first transmission pipe; a second terminal for connection to a return pipe; and at least one converter branch extending between the first and second terminals, said or each converter branch including first and second branch portions separated by corresponding first AC terminals for connection to a corresponding phase of a first AC network, said or each first branch portion including a first chain link converter extending between the associated first AC terminal and the first terminal, said or each second branch portion including a second chain link converter extending between the associated first AC terminal and the second terminal, and each chain link converter including a plurality of chain link modules connected in series, each chain link module having a plurality of switching elements connected in parallel with an energy storage device, thereby each chain link converter being controllable to provide a step variable voltage source; and

[0026] A second voltage source converter includes: a third terminal for connection to a second transmission pipe; a fourth terminal for connection to a return pipe; and at least one converter branch extending between the third and fourth terminals, said or each converter branch comprising third and fourth branch portions separated by corresponding second AC terminals for connection to a corresponding phase of a second AC network, said or each third branch portion comprising a third chain link converter extending between the associated second AC terminal and the third terminal, said or each fourth branch portion comprising a fourth chain link converter extending between the associated second AC terminal and the fourth terminal, and each chain link converter comprising a plurality of chain link modules connected in series, each chain link module having a plurality of switching elements connected in parallel with an energy storage device, thereby enabling each chain link converter to be controlled to provide a step-variable voltage source.

[0027] The method includes the following steps:

[0028] Each of the second and third chain link converters is provided with a first number of cascaded chain link modules; and

[0029] Each of the first and fourth chain link converters is provided with a second number of cascaded chain link modules, the second number being greater than the first number.

[0030] This method shares the benefits of the corresponding features of the converter station of the present invention.

[0031] 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 and second transmission channels, and first and second voltage source converters), and not to indicate the relative importance of one feature over another, unless otherwise specified.

[0032] 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 be subordinate to any other claim and / or incorporated into any feature of any other claim, although not originally claimed in that manner.

[0033] 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:

[0034] Figure 1 A schematic view of a converter station according to a first embodiment of the present disclosure is shown; and

[0035] Figure 2 The diagram is formed by Figure 1 The balance of total energy stored in the energy storage device in the corresponding branch of the voltage source converter of a part of the converter station shown in the figure.

[0036] like Figure 1 As shown in the figure, the converter station according to the first embodiment of the present invention is generally designated by reference numeral 10.

[0037] The converter station 10 is used in a bipolar power transfer scheme (not shown), which will typically include an additional converter station, which, depending on the nature of the scheme, may be located close to or far from the converter station 10 of the present invention, and which is interconnected with the converter station 10 of the present invention via first and second transmission channels (i.e., a first “pole” and a second “pole” (therefore constituting a scheme of “bipolar” scheme)), which allow power transfer between the converter stations.

[0038] Each of the first and second transmission conduits may be or includes a submarine cable; however, one or more transmission conduits may be or include an underground cable, an overhead line, or a mixture of such cables and lines.

[0039] In use, the converter stations are also interconnected via return pipes, which are normally or include dedicated metal return components, typically in the form of additional submarine cables, though alternative forms of electrical conductors can be used, along with (though less than ideal) earth patches.

[0040] In the illustrated embodiment, the converter station 10 includes a first voltage source converter 12 having a first terminal 14 that is connected in use to the aforementioned first transmission channel.

[0041] The first voltage source converter 12 also includes a second terminal 16 connected in use to the aforementioned return conduit, which may be in the form of a ground diameter as indicated.

[0042] Three converter branches 18A, 18B, and 18C extend between the first and second terminals 14 and 16; however, other embodiments of the invention may include fewer or more than three converter branches.

[0043] Each converter branch 18A, 18B, 18C includes first and second branch portions 20A, 20B, 20C, 22A, 22B, 22C separated by corresponding first AC terminals 24A, 24B, 24C, each of which is connected in use to the corresponding phase A, B, C of a first three-phase AC network.

[0044] Each first branch portion 20A, 20B, 20C includes a first chain link converter 26 extending between associated first AC terminals 24A, 24B, 24C and first terminal 14, while each second branch portion 22A, 22B, 22C includes a second chain link converter 28 extending between associated first AC terminals 24A, 24B, 24C and second terminal 16.

[0045] Each of the first and second chain link converters 26, 28 further includes a plurality of chain link modules 30 connected in series (only one is shown schematically for clarity). Each chain link module 30 has a plurality of switching elements connected in parallel with the energy storage device, thereby each corresponding chain link converter 26, 28 can be controlled to provide a step variable voltage source.

[0046] More specifically, each chain link module 30 has the same topology, and still more specifically, each chain link module 30 includes a first pair of switching elements connected in parallel with the energy storage device in a known half-bridge arrangement to define a 2-quadrant unipolar module. The switching of the switching elements selectively directs current through the energy storage device or causes current to bypass the energy storage device, such that each chain link module 30 can provide zero voltage or positive voltage and can conduct current in both directions.

[0047] The energy storage device takes the form of a capacitor; however, other types of energy storage devices (i.e., any device capable of storing and releasing energy to selectively provide voltage, such as fuel cells or battery packs) can also be used. More than one energy storage device can be used in each chain module.

[0048] In addition, each switching element includes a semiconductor device that typically takes the form of an insulated gate bipolar transistor (IGBT).

[0049] However, it is possible to use other types of self-commutating semiconductor devices, such as gate-off thyristors (GTOs), field-effect transistors (FETs), metal-oxide-semiconductor field-effect transistors (MOSFETs), injection-enhanced gate transistors (IEGTs), integrated gate-commutated thyristors (IGCTs), dual-mode insulated-gate transistors (BIGTs), or any other self-commutating switching devices. Furthermore, one or more of the semiconductor devices may be replaced with wide-bandgap materials, such as, but not limited to, silicon carbide, boron nitride, gallium nitride, and aluminum nitride.

[0050] The number of semiconductor devices in each switching element can vary depending on the rated voltage and current required by that switching element.

[0051] Each switching element also includes a passive current sensing element connected in anti-parallel to the corresponding semiconductor device. Each passive current sensing element may include at least one passive current sensing device. Each passive current sensing device may be any device capable of limiting current flow in only one direction, such as a diode. The number of passive current sensing devices in each passive current sensing element may vary depending on the rated voltage and current required by that passive current sensing element.

[0052] Other embodiments of the invention may include different exemplary chain link modules, which include first and second pairs of switching elements and capacitors connected in a known full-bridge arrangement to define a 4-quadrant bipolar module. In a manner similar to the chain link module 30 described above, the switching of the switching elements again selectively directs current through the capacitor or causes current to bypass the capacitor, enabling other exemplary chain link modules to provide zero voltage, positive voltage, or negative voltage and to conduct current in both directions.

[0053] In another embodiment of the invention, each of the first and second chain link converters 26, 28 may include only other exemplary chain link modules, or a combination of chain link module 20 and other exemplary chain link modules.

[0054] Returning to the illustrated embodiment, the converter station 10 also includes a second voltage source converter 32, which has a third terminal 34 connected in use to the aforementioned second transmission channel and a fourth terminal 36 connected in use to the aforementioned return channel.

[0055] Similarly, the second voltage source converter 32 also includes three converter branches 18A, 18B, and 18C extending between the third and fourth terminals 34 and 36. Again, similarly, each converter branch 18A, 18B, and 18C includes third and fourth branch portions 38A, 38B, 38C, 40A, 40B, and 40C separated by corresponding second AC terminals 42A, 42B, and 42C, which are connected in use to the corresponding phases A, B, and C of the second three-phase AC network.

[0056] Each third branch portion 38A, 38B, 38C includes a third chain link converter 44 extending between the associated second AC terminals 42A, 42B, 42C and the third terminal 34, while each fourth branch portion 40A, 40B, 40C includes a fourth chain link converter 46 extending between the associated second AC terminals 42A, 42B, 42C and the fourth terminal 36.

[0057] Each of the third and fourth chain link converters 44, 46 similarly includes a plurality of chain link modules 30 connected in series, having the same topology and construction as the chain link modules 30 in each of the first and second chain link converters 26, 28, i.e., adopting a known half-bridge arrangement connected in parallel with capacitors to define the first pair of switching elements (in the form of IGBTs with anti-parallel diodes) of a 2-quadrant unipolar module.

[0058] Each of the second and third chain link converters 28 and 44 includes the same first number of cascaded chain link modules 30, and each of the first and fourth chain link converters 26 and 46 includes the same second number of cascaded chain link modules 30, wherein the second number is greater than the first number.

[0059] More specifically, the second quantity is typically about 5% to 10% larger than the first quantity.

[0060] In other embodiments of the invention (not shown), each of the second and third chain link converters may have a first number of chain link modules that are different from each other, and each of the first and fourth chain link converters may have a second number of chain link modules that are different from each other, but the different second number is still greater than the different first number.

[0061] The first voltage source converter 12 includes a corresponding first voltage source converter controller 48, and the second voltage source converter 32 includes a corresponding second voltage source converter controller 50.

[0062] In use, the first controller 48 is programmed to control the step variable voltage source provided by the first voltage source converter 12, and when doing so, to balance the total energy stored by the energy storage device 30 in one branch 20A, 20B, 20C of the first voltage source converter 12 with the total energy stored by the energy storage device 30 in the other corresponding branches 22A, 22B, 22C of the first voltage source converter 12.

[0063] More specifically, such as Figure 2 The diagram schematically illustrates that the first controller 48 is programmed to cause the chain link modules 30 in the first branch sections 20A, 20B, 20C, which have a higher second number of chain link modules 30 (i.e., modules of the second number 52), to store an average individual energy 54 that is proportionally lower than the average individual energy 56 stored by each chain link module in the other corresponding second branch sections 22A, 22B, 22C, which have a lower first number of chain link modules 30 (i.e., modules of the first number 58). However, other ways of balancing the energy stored in opposing branch sections are also possible.

[0064] Similarly, the second controller 50 is programmed in use to control the step variable voltage source provided by the second voltage source converter 32, and when doing so, to balance the total energy stored by the energy storage devices 30 in one branch 38A, 38B, 38C of the second voltage source converter 38 with the total energy stored by the energy storage devices 30 in the other corresponding branches 40A, 40B, 40C of the second voltage source converter 32. The second controller 50 may similarly achieve this by causing the chain link modules 30 in the third branch 38A, 38B, 38C to store a proportionally higher average amount of individual energy than the average amount of individual energy stored by each of the higher number of chain link modules 30 in the other corresponding fourth branches 40A, 40B, 40C.

Claims

1. A converter station (10) for a bipolar power transmission scheme, comprising: a first voltage source converter (12) having a first terminal (14) for connection to a first transmission conduit, a second terminal (16) for connection to a return conduit, and at least one converter limb (18A, 18B, 18C) extending between the first and second terminals (14, 16), each converter limb (18A, 18B, 18C) comprising first and second limb portions (20A, 20B, 20C, 22A, 22B, 22C) separated by a corresponding first AC terminal (24A, 24B, 24C) for connection to a respective phase (A, B, C) of a first AC network, each first limb portion (20A, 20B, 20C) comprising a first chain-link converter (26) extending between the associated first AC terminal (24A, 24B, 24C) and the first terminal (14), each second limb portion (22A, 22B, 22C) comprising a second chain-link converter (28) extending between the associated first AC terminal (24A, 24B, 24C) and the second terminal (16), and each chain-link converter (26, 28) comprising a plurality of series-connected chain-link modules (30) each having a plurality of switching elements connected in parallel with an energy storage device, whereby each chain-link converter (26, 28) is controllable to provide a step- variable voltage source; and a second voltage source converter (32) having a third terminal (34) for connection to a second transmission conduit, a fourth terminal (36) for connection to the return conduit, and at least one converter limb (18A, 18B, 18C) extending between the third and fourth terminals (34, 36), each converter limb (18A, 18B, 18C) comprising third and fourth limb portions (38A, 38B, 38C, 40A, 40B, 40C) separated by a corresponding second AC terminal (42A, 42B, 42C) for connection to a respective phase (A, B, C) of a second AC network, each third limb portion (38A, 38B, 38C) comprising a third chain-link converter (44) extending between the associated second AC terminal (42A, 42B, 42C) and the third terminal (34), each fourth limb portion (40A, 40B, 40C) comprising a fourth chain-link converter (46) extending between the associated second AC terminal (42A, 42B, 42C) and the fourth terminal (36), and each chain-link converter (44, 46) comprising a plurality of series-connected chain-link modules (30) each having a plurality of switching elements connected in parallel with an energy storage device, whereby each chain-link converter (44, 46) is controllable to provide a step- variable voltage source, ​ Each of the second and third chain-link converters (28, 44) includes a first number (58) of series-connected chain-link modules (30), and each of the first and fourth chain-link converters (26, 46) includes a second number (52) of series-connected chain-link modules (30), the second number (52) being greater than the first number (58).

2. The converter station (10) of claim 1, wherein, Each of the second and third chain-link converters (28, 44) includes the same first number (58) of series-connected chain-link modules (30).

3. The converter station (10) of claim 1 or 2, wherein Each of the first and fourth chain-link converters (26, 46) includes the same second number (52) of series-connected chain-link modules (30).

4. The converter station (10) of any preceding claim, wherein, Each chain-link module (30) has the same topology.

5. The converter station (10) of claim 4, wherein, Each chain-link module (30) includes a first pair of switching elements connected in parallel with the energy storage device in a half-bridge arrangement to define a 2-quadrant unipolar module.

6. The converter station (10) of any preceding claim, wherein, At least one voltage source converter (12, 32) includes a corresponding voltage source converter controller (48, 50) programmed to control a step-variable voltage source provided by the corresponding voltage source converter (12, 32), and when so doing, to balance total energy stored by energy storage devices (30) in one limb portion (20A, 20B, 20C, 40A, 40B, 40C) of the voltage source converter (12, 32) with total energy stored by energy storage devices (30) in other corresponding limb portions (22A, 22B, 22C, 38A, 38B, 38C) of the voltage source converter (12, 32).

7. The converter station (10) of claim 6, wherein, At least one voltage source converter controller (48, 50) is programmed to cause chain-link modules (50) in a limb portion (20A, 20B, 20C, 40A, 40B, 40C) having a higher second number (52) of chain-link modules (30) to store proportionally lower average amounts (54) of individual energy than an average amount (56) of individual energy stored by each of chain-link modules (30) in other corresponding limb portions (22A, 22B, 22C, 38A, 38B, 38C).

8. A method of operating a converter station (10), the converter station comprising: a first voltage source converter (12) having a first terminal (14) for connection to a first transmission duct, a second terminal (16) for connection to a return duct, and at least one converter limb (18A, 18B, 18C) extending between the first and second terminals (14, 16), each converter limb (18A, 18B, 18C) comprising first and second limb portions (20A, 20B, 20C, 22A, 22B, 22C) separated by a corresponding first AC terminal (24A, 24B, 24C) for connection to a respective phase (A, B, C) of a first AC network, each first limb portion (20A, 20B, 20C) comprising a first chain-link converter (26) extending between the associated first AC terminal (24A, 24B, 24C) and the first terminal (14), each second limb portion (22A, 22B, 22C) comprising a second chain-link converter (28) extending between the associated first AC terminal (24A, 24B, 24C) and the second terminal (16), and each chain-link converter (26, 28) comprising a plurality of series-connected chain-link modules (30) each having a plurality of switching elements connected in parallel with an energy storage device, whereby each chain-link converter (26, 28) is controllable to provide a step-variable voltage source; and a second voltage source converter (32) having a third terminal (34) for connection to a second transmission duct, a fourth terminal (36) for connection to the return duct, and at least one converter limb (18A, 18B, 18C) extending between the third and fourth terminals (34, 36), each converter limb (18A, 18B, 18C) comprising third and fourth limb portions (38A, 38B, 38C, 40A, 40B, 40C) separated by a corresponding second AC terminal (42A, 42B, 42C) for connection to a respective phase (A, B, C) of a second AC network, each third limb portion (38A, 38B, 38C) comprising a third chain-link converter (44) extending between the associated second AC terminal (42A, 42B, 42C) and the third terminal (34), each fourth limb portion (40A, 40B, 40C) comprising a fourth chain-link converter (46) extending between the associated second AC terminal (42A, 42B, 42C) and the fourth terminal (36), and each chain-link converter (44, 46) comprising a plurality of series-connected chain-link modules (30) each having a plurality of switching elements connected in parallel with an energy storage device, whereby each chain-link converter (44, 46) is controllable to provide a step-variable voltage source, the method comprising the steps of: each of the second and third chain-link converters (28, 44) is provided with a first number (58) of series-connected chain-link modules (30); and each of the first and fourth chain-link converters (26, 46) is provided with a second number (52) of series-connected chain-link modules (30), the second number (52) being greater than the first number (58).

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