Energy support device

Abstract

An energy support device (1) for a high voltage direct current (HVDC) transmission system (100) is provided. The HVDC transmission system includes a first HVDC converter (51) and a second HVDC converter (52) connected to each other via an HVDC link (60). The energy support device includes a plurality of cells configured to be connected in series to the HVDC link, a resistor (20) electrically connected between the plurality of cells and a reference potential (5), and a bypass switch (30) configured to allow the resistor to be bypassed. The cells include a full bridge arrangement (15) of power switches (11) and an energy storage device (12) electrically connected to the full bridge arrangement. The energy support device is configured to operate in the following modes: a charging mode, in which the bypass switch is in a closed state to allow the resistor to be bypassed, and in which the energy storage device of at least one of the plurality of cells receives electric energy from the HVDC link; a dissipation mode, in which the bypass switch is in an open state to allow the resistor to dissipate electric energy from the HVDC link, and in which the energy storage device of at least one of the plurality of cells is bypassed; and an energy release mode, in which the energy storage device of at least one of the plurality of cells discharges electric energy to the HVDC link.

Description

Technical Field

[0001] The present disclosure generally relates to the field of power system support. More specifically, the present disclosure relates to an energy support device for a high voltage direct current (HVDC) transmission system. Background Art

[0002] The existing power system has been operating robustly and reliably, primarily because most power plants are based on conventional synchronous generators, which are buffered by their natural mechanical mass (i.e., inertia). However, with growing interest in renewable energy, many conventional power plants are being replaced by renewable energy sources such as wind and solar.

[0003] Unlike traditional power plants, renewable energy sources primarily use power electronic converters to transmit generated energy into the power system. These converters have different characteristics than traditional synchronous generators. Therefore, as more and more traditional power plants are replaced by renewable energy sources, the impact of these different characteristics on the power system will be felt. This impact will create significant challenges for maintaining power system stability and recovering from faults and / or disturbances within the power system.

[0004] The natural capacity traditionally provided by synchronous generators (thereby contributing to stability within the power system) will have to be provided by other means.

[0005] In view of the above circumstances, there is a need for power system support that can improve the stability of the power system. Summary of the Invention

[0006] To at least partially meet the above requirements, the present disclosure seeks to provide improved support for HVDC transmission systems.

[0007] To achieve this, an energy support device and a method for providing energy support are provided as defined in the independent claims. Further embodiments of the disclosure are provided in the dependent claims.

[0008] According to a first aspect, an energy support device is provided. The energy support device is configured for use in a high-voltage direct current (HVDC) transmission system. The HVDC transmission system includes a first HVDC converter and a second HVDC converter interconnected via an HVDC link. The energy support device includes a plurality of cells configured to be connected in series to the HVDC link. The cells include a full-bridge arrangement of power switches and an energy storage device electrically connected to the full-bridge arrangement. The energy support device also includes a resistor electrically connected between the plurality of cells and a reference potential. The energy support device also includes a bypass switch configured to allow the resistor to be bypassed. The energy support device is configured to operate in the following modes:

[0009] a charging mode in which the bypass switch is in a closed state to allow the resistor to be bypassed and in which the energy storage device of at least one of the plurality of cells receives electrical energy from the HVDC link,

[0010] a dissipation mode in which the bypass switch is in an open state to allow the resistor to dissipate power from the HVDC link, and in which the energy storage device of at least one of the plurality of cells is bypassed, and

[0011] An energy release mode in which the energy storage device of at least one of the plurality of cells provides electrical energy to the HVDC link.

[0012] According to a second aspect, an HVDC transmission system is provided. The HVDC transmission system comprises a first HVDC converter and a second HVDC converter connected to each other via an HVDC link. The HVDC transmission system comprises at least one energy support device according to the first aspect. The energy support device is connected to the HVDC link.

[0013] According to a third aspect, a method for providing energy support for an HVDC transmission system is provided. The HVDC transmission system includes a first HVDC converter and a second HVDC converter interconnected via an HVDC link. Energy support is provided by an energy support device. The energy support device includes a plurality of cells connected in series to the HVDC link. The cells include a full-bridge arrangement of power switches and an energy storage device electrically connected to the full-bridge arrangement. The energy support device also includes a resistor electrically connected between the plurality of cells and a reference potential, and a bypass switch configured to allow the resistor to be bypassed. The method includes the following steps:

[0014] dissipating excess electrical energy from the HVDC link by opening a bypass switch of the energy support device and transferring the excess electrical energy to a resistor of the energy support device, and

[0015] The electrical energy is released to the HVDC link by discharging the electrical energy from an energy storage device of at least one of the plurality of cells.

[0016] Energy support devices can provide energy storage and release on the DC side of existing or future HVDC transmission systems. By providing energy support devices, the HVDC transmission system can provide grid formation and / or virtual synchronous machine (VSM) capabilities. In other words, an HVDC transmission system having at least one energy support device connected to its HVDC link can provide grid formation and / or VSM capabilities. Thus, the energy support device can adapt the grid formation and / or VSM capabilities independently of the conditions of the AC network to which the HVDC transmission system is connected.

[0017] The energy support device according to the first aspect may allow, for example, an offshore wind farm to be connected to an onshore grid to absorb energy from an HVDC transmission system onshore or offshore grid when the need arises (such as when the onshore grid is over-frequently charged). In other words, the energy support device may allow electrical energy to be absorbed or dissipated when the onshore grid has excess electrical energy. The dissipation of electrical energy may be performed by the energy support device transmitting electrical energy from the grid through a resistor of the energy support device. Therefore, the dissipation of electrical energy may be performed much faster than when the electrical energy is absorbed by a conventional energy reservoir (such as a battery). Further, the dissipation of electrical energy may be performed at a higher current and / or voltage than when a conventional energy reservoir is used. The energy support device may therefore provide some inertia to the grid by being able to absorb electrical energy quickly. Therefore, the energy support device may increase the stability of the grid(s) and the ability to recover from faults and / or disturbances within the grid(s), which may allow a larger portion or share of renewable energy to be connected to the onshore grid.

[0018] The energy support device can also allow the HVDC transmission system to release (such as feed, generate, discharge or transmit) energy from the energy storage device to the onshore power grid, regardless of the level of electrical energy production at the offshore power grid (such as the wind speed at the offshore wind farm). Additionally, the consumption of electrical energy in the onshore power grid may suddenly increase, and renewable energy sources may not be able to provide such a sudden increase. The energy support device can discharge to the onshore power grid. In other words, the energy support device can be configured to act as an energy buffer for the onshore power grid. Therefore, when there is excess electrical energy in the power grid, the energy support device can dissipate electrical energy from the power grid and release electrical energy to the power grid when additional electrical energy is needed, such as when the power supply source accidentally trips or when the renewable energy sources cannot produce enough electrical energy.

[0019] While in the charging mode, the energy support device may be configured to absorb a relatively small amount of electrical energy from the HVDC link, wherein the relatively small amount may be less than 5% or 1% of the electrical energy transmitted between the first HVDC converter and the second HVDC converter via the HVDC link. Thus, the energy support device may be charged over a relatively long period of time without any significant impact on the transmission of electrical energy between the first HVDC converter and the second HVDC converter.

[0020] The HVDC transmission system may include multiple HVDC converters connected in series and / or in parallel. The first converter and the second converter may include multiple interconnected converters. The HVDC transmission system may be configured to transmit electrical energy from, for example, a power plant, a wind farm, a solar power plant, or another energy generation source to a power grid. Alternatively, the HVDC transmission system may be configured to transmit electrical energy between two power grids, such as between two AC power grids.

[0021] The power switch may include a power semiconductor switch. The power switch may include an insulated gate bipolar transistor (IGBT). For example, the IGBT may be an IGBT with an anti-parallel diode or a dual-mode insulated gate transistor (BIGT).

[0022] A full-bridge arrangement of power switches can alternatively be understood as a plurality of power switches, such as four power switches, arranged in a full-bridge configuration. A full-bridge arrangement can be understood as, for example, an H-bridge arrangement. A full-bridge arrangement of a cell can be configured to control the polarity, i.e., the direction, of the current applied to the cell's energy storage device. Furthermore, the full-bridge arrangement can be configured to allow bypassing of the energy storage device.

[0023] The resistor electrically connected between the plurality of cells and the reference potential may include a plurality of resistors. The plurality of resistors may be connected in series and / or in parallel. The reference potential may be understood to be, for example, ground potential or zero potential. The plurality of cells may include a first terminal and a second terminal. The first terminals of the plurality of cells may be connected to the HVDC link. A bypass switch may be connected in parallel with the resistor between the second terminals of the plurality of cells and the reference potential.

[0024] In the charging mode, the energy storage device of at least one of the plurality of cells may be charged by electrical energy from the HVDC link. Alternatively, the dissipation mode may be understood as a rapid energy absorption mode. Alternatively, the energy release mode may be understood as a discharge mode.

[0025] The energy support device may also be configured to operate in the following modes:

[0026] An idle mode in which the full bridge device of at least one cell of the plurality of cells is configured to prevent electrical energy from the HVDC link from passing through the energy support device.

[0027] In idle mode, at least some or all of the power switches of the full-bridge device may be turned off. Thus, the full-bridge device may be in a non-conducting state or configuration. Idle mode may alternatively be understood as a blocking mode. In idle mode, there may not be any current exchange between the energy support device and the HVDC link.

[0028] By providing energy support configured to operate in and switch to and from an idle mode, the energy support device may be substantially disconnected from the HVDC link. Thus, a higher degree of control and / or stability may be achieved with the connected energy support device, which is capable of switching from the idle mode to one of the other modes when it is necessary to absorb energy from, dissipate energy from, or release energy to the HVDC link.

[0029] A full bridge arrangement of cells in multiple cells can be configured as:

[0030] In charging mode, the energy storage device of the unit is allowed to receive electrical energy,

[0031] In dissipative mode, the unit's energy storage device is allowed to be bypassed, and

[0032] In the energy release mode, the energy storage device of the cell is allowed to discharge electrical energy.

[0033] The full-bridge device can be configured to arrange multiple switches in at least three different configurations, states, or operating modes. The full-bridge device can be configured to arrange multiple switches in three, four, five, or more different configurations, states, or operating modes. Different configurations can correspond to different modes of the energy support device. Therefore, multiple switches can be in a charging configuration, a dissipation configuration, an energy release configuration, and / or an idle configuration. It should be understood that the term "arranging switches" means that the switches can be arranged in a closed state or an open state, which corresponds to an on state or an off state. In the charging mode, the multiple switches can be arranged so that a charging current is applied to the energy storage element, thereby charging the energy storage element. In the energy release mode, the multiple switches can be arranged so that a discharge current with a direction opposite to the charging current flowing through the energy storage element when in the charging mode is applied to the energy storage element. In the dissipation mode, the multiple switches can be arranged so that no current flows through the energy storage element.

[0034] The full-bridge arrangement of cells in the plurality of cells may be configured to, in an idle mode, prevent electrical energy from the HVDC link from passing through the energy support device. In the idle mode, the plurality of switches may be arranged such that a voltage potential across the plurality of cells is higher than a voltage of the HVDC link. In the idle mode, the plurality of switches of the full-bridge arrangement may be in an open state.

[0035] A cell of the plurality of cells may further comprise a DC / DC converter.The energy storage device of the cell may be connected to the full-bridge arrangement via the DC / DC converter.

[0036] The units in the plurality of units may further include a capacitor connected between the DC / DC converter and the full-bridge device. The capacitor may be connected in parallel with the DC / DC converter and the full-bridge device. The DC / DC converter may be configured to convert a voltage and / or current of electrical energy from the full-bridge device, wherein the converted voltage and / or current may be optimized with respect to charging, discharging, or releasing energy to or from the electrical storage device. Thus, the DC / DC converter may improve the efficiency of the energy support device.

[0037] The energy storage devices of the cells of the plurality of cells may include supercapacitors and / or batteries.

[0038] The energy storage device may include one or more batteries and / or one or more supercapacitors. The one or more batteries and / or one or more supercapacitors may be connected in series and / or in parallel.

[0039] The energy support device may further include a control unit configured to control power switches and bypass switches of the full-bridge arrangement of the plurality of cells.

[0040] The energy support device may include one or more control units. The control unit may be configured to control the power switches of the full-bridge device of one or more of the plurality of cells. For example, the energy support device may include a single control unit. Alternatively, the energy support device may include multiple control units configured to control corresponding one or more of the plurality of cells.

[0041] The control unit may be communicatively connected to the full-bridge arrangements and bypass switches of the plurality of cells to control the switch states of the switches. The control unit may also be configured to sense current and / or voltage at the full-bridge arrangements, bypass switches, and / or resistors of the cells in the plurality of cells, for example, using a sensor arrangement. The sensor arrangement may be configured to sense the voltage across each cell to allow for regulation of the current and cell voltage.

[0042] The energy support device may further include an impedance disposed between the plurality of cells and the bypass switch.

[0043] The impedance may include a resistor and / or an inductor. The impedance may be configured to regulate the current in the energy support device. For example, in charging mode, some of the energy storage devices of the cells in the plurality of cells may receive electrical energy from the HVDC link, while other cells in the plurality of cells may be bypassed. The resistor may be used to fine-tune the current through the energy support device, which may otherwise be too sensitive to be controlled. By providing an impedance between the plurality of cells and the bypass switch, the current may be regulated or determined to a more appropriate level. Thus, the impedance may increase the stability and / or controllability of the energy support device.

[0044] A cell in the plurality of cells may include a cell bypass switch configured to allow the cell to be bypassed. The cell bypass switch may be configured to bypass the cell in response to determining that a fault has occurred in the cell. Thus, the cell bypass switch may provide an increased level of protection.

[0045] The first and second HVDC converters of the HVDC transmission system may comprise a plurality of HVDC converters, wherein the first and second transmission lines may be arranged between respective HVDC converters of the first and second HVDC converters. The first and second transmission lines may comprise transmission lines and / or cables.

[0046] The HVDC transmission system may be configured as a symmetrical monopolar HVDC transmission system or a bipolar HVDC transmission system.

[0047] The HVDC transmission system may comprise at least two energy support devices according to the first aspect. A first one of the energy support devices may be connected to a first transmission line of the HVDC link, and a second one of the energy support devices may be connected to a second transmission line of the HVDC link.

[0048] The HVDC transmission system may further include a control system configured to control power switches and bypass switches of the full-bridge arrangement of the plurality of cells. The HVDC transmission system may further include at least one sensor configured to sense the current and / or voltage of the HVDC link. The at least one sensor may be communicatively coupled to the control system. The control system may further be configured to cause the energy support device to adopt one of the modes based on the received current and / or voltage of the HVDC link.

[0049] The control system can be configured to control the power switches and bypass switches of the full-bridge arrangement of the multiple units via one or more control units of the energy support device. The control performed by the control system can be based on a support request from the AC network to which the HVDC transmission system is connected and / or based on sensed conditions at the HVDC link. The control system can be configured as a master device, and the control unit(s) can be configured as slave devices. In other words, the control system can be configured to perform higher-level control, and the control unit(s) can be configured to perform lower-level control that can be subordinate to the higher-level control.

[0050] The method may further comprise the step of absorbing electrical energy from the HVDC link by closing a bypass switch of the energy support device and transferring the electrical energy to an energy storage device of at least one of the plurality of units of the energy support device.

[0051] Thus, absorbing electrical energy from the HVDC link allows charging the energy storage device(s) of at least one of the plurality of units of the energy support device.By being able to charge the energy storage device, the method may allow performing the step of releasing or discharging electrical energy to the HVDC link.

[0052] The step of absorbing electrical energy may also include transferring the electrical energy to a certain number of electrical storage devices of the plurality of units. The number may be based on the sensed charge level of the electrical storage devices. Thus, a plurality of electrical storage devices may be charged simultaneously.

[0053] The step of dissipating excess power may further comprise dissipating excess power from the HVDC link if the level of excess power in the HVDC link is above a predetermined threshold, for example indicating the occurrence of power peaks and / or power transients in the HVDC link. Thus, this embodiment allows handling, for example, faults, trips or sudden imbalances between power grids to which the HVDC transmission is connected.

[0054] The method may further include dissipating excess power in response to the first support request. The method may further include releasing excess power in response to the second support request. The first support request and / or the second support request may be sent from an AC network to which the HVDC transmission system is connected and / or from a sensor configured to sense a condition at the HVDC link.

[0055] It should be noted that other embodiments can be envisioned that utilize all possible combinations of the features listed in the above embodiments. Therefore, the present disclosure also relates to all possible combinations of the features mentioned herein. Any embodiment described herein can be combined with other embodiments also described herein, and the present disclosure relates to all combinations of features. BRIEF DESCRIPTION OF THE DRAWINGS

[0056] Exemplary embodiments will now be described with reference to the accompanying drawings, in which:

[0057] Figure 1 It is a schematic diagram of the energy support device.

[0058] Figure 2a and Figure 2b is a schematic diagram of a unit of an energy support device.

[0059] Figure 3 is a schematic diagram of the HVDC transmission system.

[0060] All the figures are schematic, not necessarily to scale, and generally show only parts which are necessary in order to elucidate embodiments of the invention, wherein other parts may be omitted or merely suggested. Like reference numerals refer to like elements throughout. DETAILED DESCRIPTION

[0061] The present invention will now be described hereinafter with reference to the accompanying drawings, in which exemplary embodiments are shown. However, the inventive concept can be implemented in many different forms and should not be construed as limited to the embodiments set forth herein; rather, as will be readily appreciated by those skilled in the art, these embodiments are provided by way of example so that this disclosure will convey the scope of the invention as defined by the appended claims. In the drawings, the same reference numerals denote the same or similar components having the same or similar functions, unless otherwise specifically stated.

[0062] Figure 1 is a schematic diagram of the energy support device 1.

[0063] The energy support device 1 comprises a plurality of units 10 connected in series. Figure 1 Three units 10 are shown connected in series, but the energy support device 1 may comprise any number of units 10. This is further emphasized by the dashed line between two of the units 10. The units 10 are connected in series to an HVDC link of an HVDC transmission system (not shown; see for example Figure 3 ) transmission lines 61, 62. Thus, the first terminals of the plurality of cells 10 connected in series are connected to the transmission lines 61, 62 of the HVDC link. The first terminals of the plurality of cells 10 may be connected to the transmission lines 61, 62 directly or indirectly via a disconnector. The transmission lines 61, 62 may be the DC poles of the HVDC link.

[0064] Energy support device 1 includes a resistor 20 and a bypass switch 30. Resistor 20 and bypass switch 30 are electrically connected between the second terminals of the plurality of cells 10 connected in series and a reference potential 5. In some examples, reference potential 5 can be understood as ground potential or zero potential. Bypass switch 30 is configured to allow resistor 20 to be bypassed. Therefore, if bypass switch 30 is in a closed state, current from the plurality of cells 10 can be transferred to the reference potential via bypass switch 30 rather than through resistor 20.

[0065] The energy support device 1 may further include an impedance 25. The impedance 25 is disposed between the plurality of cells 10 and the bypass switch 30. In other words, the impedance 25 may be electrically connected between the plurality of cells 10 and the bypass switch 30.

[0066] The energy support device 1 may further comprise a control unit 45. The control unit 45 is configured to control the power switches (not shown; see, for example, FIG. 2 ) and the bypass switches 30 of the full-bridge arrangement of the plurality of cells 10. The energy support device 1 may comprise a communication device configured to allow the control unit 45 to switch the power switches and the bypass switches 30 of the full-bridge arrangement of the plurality of cells 10, such as Figure 1The communication device may be configured as a cable electrically connected to the power switch and the bypass switch.

[0067] The energy support device 1 may be operable in a charging mode, a dissipating mode, an energy releasing mode, and / or an idle mode.

[0068] When the energy support device 1 is operating in the charging mode, the full bridge device of at least one of the plurality of cells 10 is configured to allow the energy storage device of the at least one cell 10 to receive electrical energy from the transmission lines 61, 62, and the resistor 20 is bypassed via the bypass switch 30. The cell(s) 10 that do not receive electrical energy from the transmission lines 61, 62 may be configured to allow the energy storage device(s) of the cell(s) 10 to be bypassed.

[0069] When the energy support device 1 operates in the dissipation mode, the full bridge arrangement of the plurality of cells 10 is configured to bypass the energy storage device of the plurality of cells 10 , and the bypass switch 30 is in an open state for allowing the resistor 20 to dissipate electrical energy from the transmission lines 61 , 62 .

[0070] When the energy support device 1 is operated in the energy release mode, the full bridge device of at least one of the plurality of cells 10 is configured to allow the energy storage device of the at least one cell 10 to discharge electrical energy to the transmission lines 61, 62. The cell(s) 10 that do not discharge electrical energy from the transmission lines 61, 62 may be configured to allow the energy storage device of the cell(s) 10 to be bypassed.

[0071] When the energy support device 1 operates in the idle mode, the full bridge arrangement of the plurality of cells 10 is configured to block current flow through the cells 10 .

[0072] The energy support device 1 may further comprise a housing 20. A plurality of units 10, a resistor 20, a bypass switch 30, a reference potential 5, an impedance and a control unit 45 may be arranged inside the housing 20. It should be understood that one or more of the components comprised by the energy support device 1 may be arranged outside the housing 2. Alternatively, the reference potential 5 may be arranged outside the housing 2, for example on the ground or on a structure. Further, the control unit 40 may be arranged outside the housing 2, and a communication device of the control unit 40 may be arranged to pass through the housing 2. The units 10 connected to the transmission lines 61, 62 are connected via the housing 2.

[0073] Figure 2a is an energy support device (not shown; see e.g. Figure 1 or Figure 3 ) is a schematic diagram of unit 10.

[0074] The cell 10 includes a full bridge arrangement 15 of four power switches 11a-11d and an energy storage device 12 electrically connected to the full bridge arrangement 15. The four power switches 11a-11d include a first power switch 11a, a second power switch 11b, a third power switch 11c, and a fourth power switch 11d.

[0075] The unit 10 comprises a first connection 19a and a second connection 19b. The unit can be connected to an HVDC link (not shown; see e.g. Figure 1 or Figure 3 ) or another unit of the energy support device (not shown; see e.g. Figure 1 ) and is connected via a second connection 19b to a (further) unit of the energy support device or to a resistor and a bypass switch of the energy support device (not shown; see for example Figure 1 ).

[0076] First connector 19a, first power switch 11a, and third power switch 11c share a common coupling point. Second connector 19b, second power switch 11b, and fourth power switch 11d share a common coupling point. First power switch 11a, second power switch 11b, and one pole of energy storage device 12 share a common coupling point. Third power switch 11c, fourth power switch 11d, and the other pole of energy storage device 12 share a common coupling point.

[0077] Thus, when the energy support device is in charging mode, the energy storage device 12 can receive electrical energy when the first power switch 11a and the fourth power switch 11d are in the closed state. When the energy support device is in dissipation mode, if the first power switch 11a and the second power switch 11b are in the closed state, or if the third power switch 11c and the fourth power switch 11d are in the closed state, the energy storage device 12 is bypassed. When the energy support device is in idle mode, all power switches 11a to 11d are in the open state, so that no current can be transmitted through the unit 10.

[0078] Cell 10 may also include a cell bypass switch 17. The cell bypass switch 17 is disposed between first connector 19a and second connector 19b. When cell bypass switch 17 is closed, cell 10 is bypassed. In other words, when cell bypass switch 17 is closed, current flows between first connector 19a and second connector 19b via cell bypass switch 17, bypassing full-bridge device 15.

[0079] Figure 2b is an energy support device (not shown; see e.g. Figure 1 or Figure 3 ) is a schematic diagram of unit 10.

[0080] Apart from Figure 2b The unit 10 shown in FIG. 1 also includes a DC / DC converter 13 and a capacitor 14. Figure 2b The unit 10 shown in FIG. 1 may be similar to Figure 2a The unit shown in .

[0081] The DC / DC converter 13 is electrically connected between the full-bridge device 15 and the energy storage device 12. The capacitor 14 is electrically connected in parallel between the DC / DC converter 13 and the full-bridge device 15. Thus, the energy storage device 12 is electrically connected to the full-bridge device 15 via the DC / DC converter 13.

[0082] Figure 3 1 is a schematic diagram of an HVDC transmission system 100. The HVDC transmission system 100 includes a first HVDC converter 51 and a second HVDC converter 52. The first HVDC converter 51 and the second HVDC converter 52 are connected to each other via an HVDC link 60. The HVDC link 60 includes a first transmission line 61 and a second transmission line 62.

[0083] The first HVDC converter 51 is connected to the first AC network 71 via a first inductor 81 and a first transformer 83 connected in series. The first inductor 81 and the first transformer 83 can be configured to transform the current between the first HVDC converter 51 and the first AC network 71. Accordingly, the second HVDC converter 52 is connected to the second AC network 72 via a second inductor 82 and a second transformer 84. The second inductor 82 and the second transformer 84 can be configured to transform the current between the second HVDC converter 52 and the second AC network 72. The HVDC transmission system 100 is not limited to including the inductors 81, 82 and / or transformers 83, 84 between the HVDC converters 51, 52 and the AC networks 71, 72.

[0084] The HVDC transmission system 100 is configured as a (symmetrical) unipolar HVDC transmission system. However, it should be understood that this is merely exemplary. For example, the HVDC transmission system 100 may be configured as a bipolar HVDC transmission system.

[0085] The second AC network 72 may include or be formed by a power plant based on renewable energy (such as a wind farm or a solar farm, which may be an islanded wind farm, for example). In another example, the second AC network 72 may be or form part of an AC network for a geographic region or country (such as a national AC grid).

[0086] HVDC transmission system 100 includes two energy support devices 1. Each energy support device 1 is directly or indirectly connected to a respective transmission line 61, 62 of an HVDC link 60. Transmission lines 61, 62 can be understood as comprising DC poles of HVDC transmission system 100. First converter 51 and second converter 52 may include respective pole busbars to which transmission lines 61, 62 (i.e., DC poles) are connected.

[0087] HVDC transmission system 100 may further include: a control system (not shown) configured to control power switches and bypass switches of the full-bridge devices of the plurality of cells; and at least one sensor (not shown) configured to sense the current and / or voltage of the HVDC links 60, 61, 62 and communicatively coupled to the control system. The control system may further be configured to cause energy support device 1 to adopt one of the modes based on the received current and / or voltage of the HVDC links 60, 61, 62.

[0088] Although the invention has been illustrated in the drawings and foregoing description, such illustration is to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments may be understood and effected by a person skilled in the art in practicing the claimed invention by studying the drawings, the disclosure and the appended claims. In the appended claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are set forth in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.

Claims

1. An energy support device (1) for providing grid forming and / or virtual synchronous machine (VSM) capabilities to a high voltage direct current (HVDC) transmission system (100), the high voltage direct current (HVDC) transmission system comprising a first HVDC converter (51) and a second HVDC converter (52) connected to each other via an HVDC link (60), wherein the energy support device comprises: A plurality of units (10) configured to be connected in series to the HVDC link, wherein the units comprise: a full-bridge arrangement (15) of power switches (11a-11d); and an energy storage device (12) electrically connected to the full-bridge device; a resistor (20) electrically connected between the plurality of cells and a reference potential (5); and a bypass switch (30) configured to allow the resistor to be bypassed; wherein the energy support device is configured to operate in the following modes: a charging mode in which the bypass switch is in a closed state to allow the resistor to be bypassed, and in which the energy storage device of at least one of the plurality of cells receives electrical energy from the HVDC link; a dissipation mode in which the bypass switch is in an open state to allow the resistor to dissipate electrical energy from the HVDC link, and in which the energy storage devices of the plurality of cells are bypassed, and An energy release mode in which the energy storage device of at least one unit of the plurality of units discharges electrical energy to the HVDC link, thereby providing grid forming and / or VSM capabilities.

2. The energy support device of claim 1 , wherein the energy support device is further configured to operate in the following modes: An idle mode in which the full bridge device of at least one cell of the plurality of cells is configured to prevent electrical energy from the HVDC link from passing through the energy support device.

3. The energy support device according to any one of the preceding claims, wherein the full bridge arrangement of cells of the plurality of cells is configured as: In the charging mode, the energy storage device of the cell is allowed to receive electrical energy, allowing the energy storage device of the cell to be bypassed in the dissipative mode, and In the energy release mode, the energy storage device of the cell is allowed to discharge electrical energy.

4. The energy support device according to any one of claims 1-2, wherein a unit of the plurality of units further comprises a DC / DC converter (13), wherein the energy storage device of the unit is connected to the full-bridge device via the DC / DC converter.

5. The energy support device according to any one of claims 1-2, wherein the energy storage device of a cell of the plurality of cells comprises a supercapacitor (12) and / or a battery (12).

6. The energy support device according to any one of claims 1-2, further comprising a control unit (45) configured to control the power switches and the bypass switches of the full-bridge arrangement of the plurality of cells.

7. A high voltage direct current (HVDC) transmission system (100), comprising a first HVDC converter (51) and a second HVDC converter (52) connected to each other via an HVDC link (60), and comprising: At least one energy support device (1) according to claim 1, wherein the energy support device is connected to the HVDC link.

8. The HVDC transmission system according to claim 7, configured as a symmetrical monopolar HVDC transmission system or a bipolar HVDC transmission system.

9. The HVDC transmission system according to claim 8, comprising at least two energy support devices (1) according to any one of claims 1 to 7, wherein a first energy support device of the energy support devices is connected to a first transmission line (61) of the HVDC link, and a second energy support device of the energy support devices is connected to a second transmission line (62) of the HVDC link.

10. The HVDC transmission system according to claim 8, further comprising: a control system configured to control the power switches and the bypass switches of the full-bridge arrangement of the plurality of cells; as well as at least one sensor configured to sense current and / or voltage of the HVDC link and communicatively coupled to the control system; The control system is further configured to cause the energy support device to adopt one of the modes based on the received current and / or voltage of the HVDC link.

11. A method for providing a high voltage direct current (HVDC) transmission system with grid forming and / or virtual synchronous machine (VSM) capabilities, the HVDC transmission system comprising a first HVDC converter and a second HVDC converter connected to each other via an HVDC link, wherein the grid forming and / or virtual synchronous machine (VSM) capabilities are provided by means of an energy support device, the energy support device comprising: a plurality of cells connected in series to the HVDC link, wherein the cells comprise: a full-bridge arrangement of power switches; and an energy storage device electrically connected to the full-bridge device; a resistor electrically connected between the plurality of cells and a reference potential; and a bypass switch configured to allow the resistor to be bypassed; The method comprises the following steps: dissipating excess electrical energy from the HVDC link by opening a bypass switch of the energy support device and transferring the excess electrical energy to a resistor of the energy support device, and Electrical energy is released to the HVDC link by discharging electrical energy from an energy storage device of at least one of the plurality of cells, thereby providing grid forming and / or VSM capabilities.

12. The method according to claim 11, further comprising the steps of: An energy storage device absorbs electric energy from the HVDC link by closing a bypass switch of the energy support device and transmits the electric energy to at least one unit among a plurality of units of the energy support device.

13. The method of claim 12, wherein the step of absorbing electrical energy further comprises: The electrical energy is transferred to a quantity of electrical storage devices of the plurality of cells, wherein the quantity is based on the sensed charge levels of the electrical storage devices.

14. The method according to any one of claims 11 to 13, wherein the step of dissipating excess electrical energy further comprises: The excess power is dissipated from the HVDC link in the following cases The level of the excess electrical energy of the HVDC link is above a predetermined threshold.

15. The method according to any one of claims 11 to 13, comprising: In response to the first support request, performing the step of dissipating excess electrical energy; as well as In response to the second support request, the step of discharging excess electric energy is performed.

Images