Improvements in or relating to chain-link modules for voltage source converters

Abstract

In the field of chain-link modules for voltage source converters, there is a need for an improved chain-link module. A chain-link module (10) for connection in series with other chain-link modules to form a chain-link converter selectively operable to provide a step-variable voltage source within a voltage source converter, the chain-link module (10) comprising a first pair (12) of series-connected switching elements (14A, 14B) separated by a first connection terminal (16) and connected in parallel with first and second series-connected energy storage devices (18, 20). The chain-link module (10) further comprises a second pair (22) of series-connected switching elements (14C, 14D) separated by a second connection terminal (24) and connected in parallel with one or other of the first and second energy storage devices (18, 20). In use, switching of the switching elements (14A, 14B, 14C, 14D) selectively: (i) directs current (I) through the first and second energy storage devices (18, 20), whereby the chain-link module (10) provides a positive voltage across the first and second connection terminals (16, 24); (ii) bypasses current (I) around the first and second energy storage devices (18, 20), whereby the chain-link module (10) provides a zero voltage; and (iii) directs current (I) through one of the first and second energy storage devices (18, 20) connected in parallel with the second pair (22) of switching elements (14C, 14D), whereby the chain-link module (10) provides a negative voltage across the first and second connection terminals (14, 24).

Description

TECHNICAL FIELD

[0001] The present invention relates to a chain-link module for connection in series with other chain-link modules to form a chain-link converter which is selectively operable to provide a step-variable voltage source within a voltage source converter, and to such a voltage source converter. BACKGROUND

[0002] In HVDC power transmission networks, AC power is typically converted to DC power for transmission via overhead lines, subsea cables and / or underground cables. This conversion removes the need to compensate for the AC capacitive load effects imposed by the power transmission medium (i.e. the transmission line or cable) and reduces the cost per kilometre of line and / or cable and is therefore 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] Conversion between DC and AC power is used where it is necessary to interconnect DC and AC networks. 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. SUMMARY

[0004] One type of power converter is a voltage source converter, although other types of power converter are possible.

[0005] Such a voltage source converter comprises first and second DC terminals between which at least one converter limb extends, and typically three converter limbs, each of which corresponds to a given phase of a three-phase electrical power system.

[0006] The or each converter limb comprises first and second limb portions separated by an AC terminal.

[0007] In use, the first and second DC terminals are connected to a DC network and the or each AC terminal is connected to a corresponding phase of an AC network.

[0008] Each branch portion comprises a chain-link converter extending between an associated AC terminal and a corresponding one of the first or second DC terminals. Each chain-link converter comprises a plurality of chain-link modules connected in series, and each chain-link module comprises a plurality of switching elements connected in parallel with an energy storage device, typically in the form of a capacitor. However, other types of energy storage device could be used, i.e. any device capable of storing and releasing energy to selectively provide a voltage, such as a fuel cell or a battery.

[0009] The provision of a plurality of chain-link modules means that it is possible to build up a combined voltage across each chain-link converter via the energy storage devices (i.e. capacitors) of the plurality of chain-link modules (where each chain-link module provides its own voltage), which is higher than the voltage available from each individual chain-link module.

[0010] Thus, each of the chain-link modules works together to permit the chain-link converter to provide a step-variable voltage source. This permits the generation of a voltage waveform across each chain-link converter using a step approximation. Thus, each chain-link converter is able to provide a wide range of composite waveforms.

[0011] For example, the operation of each chain-link converter in the aforementioned manner can be used to generate an AC voltage waveform at the or each AC terminal, and thereby enable the voltage source converter to provide the aforementioned power transfer functionality between the AC and DC networks.

[0012] According to a first aspect of the application, there is provided a chain-link module for connection in series with other chain-link modules to form a chain-link converter selectively operable to provide a step-variable voltage within a voltage source converter, the chain-link module comprising:

[0013] a first pair of series-connected switching elements separated by a first connection terminal and connected in parallel with a first and a second series-connected energy storage device; and

[0014] a second pair of series-connected switching elements separated by a second connection terminal and connected in parallel with one or other of the first and second energy storage devices;

[0015] In use, switching of the switching elements selectively:

[0016] (i) directs current through the first and second energy storage devices, whereby the chain-link module provides a positive voltage across the first and second connection terminals;

[0017] (ii) bypassing the first and second energy storage devices with current, whereby the chain-link module provides zero voltage; and

[0018] (iii) directing current through the one of the first and second energy storage devices connected in parallel with the second pair of switching elements, whereby the chain-link module provides negative voltage across the first and second connection terminals.

[0019] It is desirable to have a chain-link module capable of providing positive voltage, as it allows the chain-link converter (in which the module is positioned) to generate a regular voltage of the type required for normal power transmission by the associated voltage source converter. Such positive voltage can also be used to counter fault currents flowing from the first connection terminal to the second connection terminal.

[0020] In addition, arranging the second pair of series-connected switching elements in parallel with only one of the energy storage devices and separating them from the second connection terminal means that when such positive voltage is being provided (i.e. during normal power transmission by the associated voltage source converter), current can be directed through both energy storage devices as required via only two switching elements.

[0021] Consequently, a conduction ratio of one switching element per energy storage device is achieved. This results in the chain-link module of the present invention having lower conduction losses compared to other chain-link modules having a greater equivalent number of switching elements per energy storage device conducting in series, for example, when providing positive voltage for normal power transmission by the corresponding associated voltage source converter.

[0022] Furthermore, arranging the second pair of switching elements in the aforementioned manner permits selective switching of those switching elements while the chain-link module is additionally providing positive voltage during normal power transmission to vary the ratio of energy stored by each of the first and second energy storage devices. Such selective switching of switching elements can be used to rebalance the energy stored by each energy storage device, for example, which is desirable in some operating environments.

[0023] At the same time, the ability of the chain-link module to selectively provide a negative voltage is advantageous, as it can be used to combat DC fault currents flowing from the second connection terminal through the chain-link module to the first connection terminal, and thus in combination with the aforementioned positive voltage ability enables the chain-link converter (in which the module is positioned in use) (and thus also the associated voltage source converter comprising such a chain-link converter) to provide full DC fault clearing functionality, i.e. the ability to clear DC fault currents flowing between the first and second connection terminals in either direction. Thus, the need for an additional, expensive and space consuming DC circuit breaker is avoided.

[0024] A further benefit is that such DC fault clearing functionality can be provided using a plurality of identical chain-link modules of the invention within the associated chain-link converter. This simplifies the manufacturing and subsequent control of such chain-link converters compared to, for example, chain-link converters incorporating chain-link modules that are different from each other.

[0025] Furthermore, the ability to selectively provide a negative voltage also allows the first and second chain-link converters (with one or more chain-link modules of the invention in them) to work together in the converter limbs of the associated voltage source converter to overmodulate, i.e. to generate a positive converter limb voltage that is greater than the DC operating voltage of the corresponding DC network to which the voltage source converter is connected, or to generate a negative converter limb voltage.

[0026] Preferably, at least one of the energy storage devices has a selectively operable fast discharge circuit connected thereto.

[0027] The inclusion of one or more such fast discharge circuits advantageously permits the removal of excess stored energy from the corresponding energy storage device, for example in an emergency situation.

[0028] For example, when a negative voltage is provided to combat DC fault currents flowing from the second connection terminal to the first connection terminal, the energy stored by the one of the first and second energy storage devices (to which the second pair of switching elements is connected in parallel) will instantaneously rise due to the energy stored in suppressing the fault current, and the triggering (i.e. selective operation) of the fast discharge circuit connected to said storage device affected by the additional induced energy can be used to reduce the amount of energy stored.

[0029] In a preferred embodiment of the invention, each energy storage device has a selectively operable fast discharge circuit connected thereto.

[0030] As noted above, providing each energy storage device with a fast discharge circuit allows the amount of energy stored by each energy storage device to be reduced. However, in addition, each energy storage device having a fast discharge circuit also permits the relative ratio of the energy stored by the first and second energy storage devices to be varied, for example to rebalance the energy so stored.

[0031] Optionally, at least one of the switching elements in the second pair of switching elements has a lower voltage rating than one or both of the switching elements in the first pair of switching elements.

[0032] The positive voltage provided by the chain-link module has a first magnitude comparable to the combined energy stored by the first and second energy storage devices, whereas the negative voltage provided has a second magnitude comparable only to the energy stored by the one of the first and second energy storage devices connected in parallel with the second pair of switching elements. Thus, the second magnitude is lower than the first magnitude, and therefore the voltage which must be supported by the second pair of switching elements is also lower than the voltage which has to be supported by the first pair of switching elements.

[0033] This in turn can be exploited by including one or more switching elements having a lower voltage rating in the second pair, which advantageously reduces both the cost and power losses associated with the or each said lower voltage rated switching element.

[0034] The chain-link module can also include a module controller programmed to cause the first and second energy storage devices to store different amounts of energy.

[0035] Causing the first and second energy storage devices to store different amounts of energy advantageously allows the associated DC fault clearing functionality to be tuned.

[0036] For example, causing the energy storage device connected in parallel with the second pair of switching elements to store more energy than the other energy storage device means that the negative voltage provided by the chain-link module is greater than would be the case if both energy storage devices stored the same amount of energy. The result of being able to provide such a greater negative voltage is that it enables faster DC fault clearing to be achieved.

[0037] According to a second aspect of the application, there is provided a voltage source converter comprising at least one chain-link converter having a plurality of chain-link modules connected in series, at least one of the chain-link modules being a chain-link module as described above.

[0038] The voltage source converter of the application shares the benefits of the corresponding features of the chain-link module of the application.

[0039] It will be understood that the terms “first” and “second” used in this application specification are intended only to help distinguish similar features (e.g., first and second energy storage devices connected in series) and are not intended to indicate the relative importance of one feature to 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 various 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. Attached Figure Description

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

[0042] Figure 1 A chain link module according to an embodiment of the present invention is shown; and

[0043] Figures 2(a) to 2(d) illustrate the passage Figure 1 The different current flow paths of the chain link module shown in the diagram enable the chain link module to provide positive voltage, zero voltage, and negative voltage. Detailed Implementation

[0044] The chain link module according to the first embodiment of the present invention is generally designated by reference numeral 10.

[0045] The chain link module 10 includes a first pair of 12 serially conducted switching elements, namely, first and second switching elements 14A and 14B connected in series, which are separated by a first connection terminal 16, and are also connected in parallel with first and second series-connected energy storage devices 18 and 20.

[0046] The chain link module 10 also includes a second pair of 22 series-connected switching elements, namely, third and fourth switching elements 14C and 14D connected in series, which are separated by the second connection terminal 24.

[0047] Compared to the first pair of 12 switching elements 14A, 14B, the second pair of 22 switching elements 14C, 14D is connected in parallel with only one of the first and second energy storage devices 18, 20. In the illustrated embodiment, this is the second energy storage device 20, although in other embodiments of the invention (not shown), the second pair of switching elements may instead be connected in parallel with only the first energy storage device.

[0048] The first switching element 14A comprises a first semiconductor device in the form of a first IGBT 26A connected in parallel with a first anti-parallel diode 28A. The second, third and fourth switching elements 14B, 14C, 14D similarly comprise corresponding second, third and fourth IGBTs 26B, 26C, 26D connected in parallel with corresponding second, third or fourth anti-parallel diodes 28B, 28C, 28D.

[0049] The inclusion of such self-commutated semiconductor devices (i.e. IGBTs) in each switching element 14A, 14B, 14C, 14D means that each said switching element 14A, 14B, 14C, 14D can be considered to be an active switching element in that it can be turned on and off at will by a control signal. This is in contrast to, for example, a separate passive current sensing element which instead requires a bias voltage to be applied across its conduction terminals in order for it to allow or block current flow. It is not always possible to provide such a bias voltage and therefore such passive elements cannot be turned on and off at will.

[0050] Returning to the illustrated embodiment, each of the switching elements 14A, 14B, 14C, 14D share the same configuration in that each switching element comprises an IGBT 26A, 26B, 26C, 26D arranged in parallel with a corresponding anti-parallel diode 28A, 28B, 28C, 28D, although this need not necessarily be the case in other embodiments of the invention.

[0051] For example, it is possible to use one or more other types of self-commutated semiconductor device (such as a gate turn-off thyristor (GTO), a field effect transistor (FET), a metal oxide semiconductor field effect transistor (MOSFET), an injection enhanced gate transistor (IEGT), an integrated gate-commutated thyristor (IGCT), a bi-mode insulated gate transistor (BIGT) or any other self-commutated switching device) in place of the IGBTs. Additionally, one or more of the semiconductor devices can instead comprise a wide bandgap material such as, but not limited to, silicon carbide, boron nitride, gallium nitride and aluminium nitride.

[0052] The number of semiconductor devices in each switching element 14A, 14B, 14C, 14D can vary depending on the voltage and current ratings required of that switching element 14A, 14B, 14C, 14D.

[0053] Alternative passive current sensing elements (i.e. alternatives to the anti-parallel diodes 28A, 28B, 28C, 28D) can also be included in other embodiments of the present application, such as any alternative element capable of limiting current flow in only one direction. The number of passive current sensing elements in each switching element 14A, 14B, 14C, 14D can also vary depending on the voltage and current ratings required for that switching element 14A, 14B, 14C, 14D.

[0054] In the particular embodiment shown, each of the switching elements in the second pair 20 of switching elements (i.e. each of the third and fourth switching elements 14C, 14D) has a lower voltage rating than both of the switching elements in the first pair 18 of switching elements (i.e. than both the first and second switching elements 14A, 14B). As an example, each of the first and second switching elements 14A, 14B can be rated at 6.5kV, while the third and fourth switching elements 14C, 14D can be less expensive and less lossy 3.3kV rated switching elements.

[0055] At the same time, the first and second energy storage devices 18, 20 take the form of corresponding first and second capacitors 30, 32, although other types of energy storage devices could be used, i.e. other types of devices capable of storing and releasing energy to selectively provide a voltage, such as a fuel cell or a battery.

[0056] In addition to the foregoing, each energy storage device 18, 20 (i.e. each of the first and second capacitors 30, 32) also has connected thereto, and more particularly connected in parallel thereto, a corresponding first or second selectively operable fast discharge circuit 34, 36.

[0057] The first fast discharge circuit 34 is formed by a further fifth switching element 14E, which includes a fifth IGBT 26E connected in parallel with a fifth anti-parallel diode 28E, which in turn is connected in series with an energy dissipating element in the form of a first resistor 38.

[0058] The second fast discharge circuit 36 is similarly formed by a sixth switching element 14F, which includes a sixth IGBT 26F connected in parallel with a sixth anti-parallel diode 28F, which is arranged in series with a second resistor 40.

[0059] The fifth and sixth switching elements 14E, 14F can have the same ratings as the third and fourth switching elements 14C, 14D, such as 3.3kV, although this need not necessarily be the case.

[0060] The fifth and sixth switching elements and the anti-parallel diodes 14E, 14F, 28E, 28F can also be different in the same manner as described above, and other forms of energy dissipation elements can also be used in other embodiments of the invention (not shown).

[0061] In use, the first to fourth switching elements 14A, 14B, 14C, 14D are selectively switched, for example by a module controller 42 programmed to control such switches, so as to guide current I through the chain link module 10 along different current flow paths so that the chain link module 10 provides positive voltage, zero voltage, and negative voltage.

[0062] More specifically, as schematically shown in Figure 2(a), turning on the first and fourth switching elements 14A, 14D (i.e., turning on the corresponding first and fourth IGBTs 26A, 26D therein) directs current I in the first direction 44 through the chain module 10 from the second connection terminal 24 to the first connection terminal 16, and through both of the first and second energy storage devices 18, 20, i.e., through both of the first and second capacitors 30, 32. This causes the chain module 10 to provide a positive voltage with a first amplitude equivalent to the combined energy stored by the first and second capacitors 30, 32, i.e., equal to the sum of the voltages.

[0063] Such a positive voltage helps to provide a stable voltage by including a chain link converter 10 within it during use, and thereby facilitates normal power transfer through an associated voltage source converter, which is in turn incorporated within the associated voltage source converter.

[0064] Additionally, such a positive voltage can be used to counteract and subsequently extinguish the DC fault current I flowing from the first connection terminal 16 to the second connection terminal 24 via the chain link module 10 in the second direction 46, opposite to the first direction 44. F .

[0065] Meanwhile, as schematically shown in Figure 2(b), turning on only the second switching element 14B (i.e., the second IGBT 26B therein) causes the current I flowing through the chain link module 10 in the second direction 46 to bypass the two capacitors 30, 32, thereby causing the chain link module 10 to provide zero voltage.

[0066] Similarly, as schematically shown in Figure 2(c), turning on only the fourth switching element 14D (i.e., the fourth IGBT 26D therein) causes the current I flowing through the chain link module 10 in the first direction 44 to bypass the two capacitors 30, 32, and thereby again causes the chain link module to provide zero voltage.

[0067] Simultaneously, the turn-on current I of the second and third switching elements 14B, 14C passes through the chain link module 10 (i.e., from the first terminal 16 to the second terminal 24) in the second direction 46 and only through the second capacitor 32, as schematically shown in FIG2(d). This causes the chain link module to provide a negative voltage with a second amplitude, which is equivalent to the energy stored only by the second capacitor 32, i.e., equal to the voltage stored only by the second capacitor 32.

[0068] However, such a negative voltage can still be used to counteract and subsequently extinguish the DC fault current I flowing from the second terminal 24 to the first terminal 16 via the chain link module 10 in the first direction 44. F .

[0069] In an environment where the first and second capacitors 30, 32 (i.e., the first and second energy storage devices 18, 20) store the same amount of energy (i.e., are charged to the same voltage), the first amplitude of the positive voltage is twice the second amplitude of the negative voltage.

[0070] In other embodiments, for example, the module controller 42 is programmed to cause the first and second energy storage devices 18, 20 (i.e., the first and second capacitors 30, 32) to store different amounts of energy, such that the first amplitude of the positive voltage is not exactly twice the second amplitude of the negative voltage.

[0071] For example, the first capacitor 30 can be controlled to store a voltage of 1.8 kV, while the second capacitor 32 can be controlled to store a larger voltage of 2.2 kV. In such an environment, the first amplitude of the positive voltage will remain the sum of the voltages stored by the first and second capacitors 30 and 32, for example, 4.0 kV, while the second amplitude of the negative voltage will be greater than half of the positive voltage, that is, it will be equal to the voltage stored by the second capacitor 32, for example, 2.2 kV.

[0072] In addition to the foregoing, the third switching element 14C (i.e., the third IGBT 26C therein) can also be turned on to guide current I in the second direction 46 from the first connection terminal 16, through the first anti-parallel diode 28, through the first capacitor 30 only, and through the third IGBT 26C to the second connection terminal 24.

[0073] Alternatively, the first switching element 14A (i.e., the first IGBT 26A therein) can be turned on to guide current I in the first direction 44 from the second connection terminal 24, through the third anti-parallel diode 28C, through the first capacitor 30 only, and through the first IGBT 26A to the first connection terminal.

[0074] The guide current I flowing through the first capacitor 30 in any of the aforementioned ways can be used for a short period of time to regulate the ratio of energy stored by the first and second capacitors 30, 32.

[0075] It can also be used (perhaps for only a few microseconds at a time) to provide a transition between a fully bypassed chain link module 10 providing zero voltage and a fully bypassed chain link module 10 providing a fully positive voltage of the first magnitude, for example, to reduce the size of the voltage step applied to the associated external system.

[0076] In addition, during use, one or both of the fast discharge circuits 34 and 36 can be triggered, i.e., triggered by switching on the corresponding fifth or sixth switching elements 14E and 14F, such as IGBTs 26E and 26F, to remove energy, i.e., voltage, from the energy storage devices 18 and 20 (i.e., the capacitors 30 and 32 connected thereto).

[0077] The voltage source converter (not shown) according to a second embodiment of the present invention includes at least one chain link converter having a plurality of chain link modules connected in series, at least one of which is the chain link module 10 as described above.

Claims

1. A chain link module (10) for series connection with other chain link modules to form a chain link converter selectively operable to provide a step variable voltage within a voltage source converter, said chain link module comprising: The first pair (12) of series-connected switching elements (14A, 14B) are separated by the first connection terminal (16) and connected in parallel with the first and second series-connected energy storage devices (18, 20); and The second pair (22) of series-connected switching elements (14C, 14D) are separated by the second connection terminal (24) and connected in parallel with one or the other of the first and second energy storage devices (18, 20); In use, the switching elements (14A, 14B, 14C, 14D) are selectively switched as follows: (i) A guiding current (I) passes through the first and second energy storage devices (18, 20), thereby providing a positive voltage across the first connection terminal and the second connection terminal (16, 24) of the chain link module (10); (ii) Bypass the first and second energy storage devices (18, 20) with current (I), thereby providing zero voltage to the chain link module (10); as well as (iii) A guiding current (I) passes through one of the energy storage devices (18, 20) connected in parallel with the second pair (22) of switching elements (14C, 14D), thereby providing a negative voltage across the first and second connection terminals (16, 24) of the chain link module (10). The chain link module further includes a module controller (42) which is programmed to enable the first and second energy storage devices (18, 20) to store different amounts of energy.

2. The chain link module (10) according to claim 1, wherein, At least one of the energy storage devices (18, 20) has a fast discharge circuit (34, 36) that can be selectively operated thereto.

3. The chain link module (10) according to claim 2, wherein, Each energy storage device (18, 20) has a selectively operable fast discharge circuit (34, 36) connected to it.

4. The chain link module (10) according to any one of claims 1-3, wherein, At least one of the switching elements (14C, 14D) in the second pair (22) of switching elements has a lower voltage rating than one or both of the switching elements (14A, 14B) in the first pair (12) of switching elements.

5. A voltage source converter including at least one chain link converter having a plurality of chain link modules connected in series, at least one of the chain link modules being a chain link module (10) according to any of the preceding claims.

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