Power support arrangement for a power grid comprising at least three groups of phase arms

Abstract

A power support arrangement (10) for an electrical grid (24) and comprising a DC network (16) comprising a first DC line (DCL1) having a first DC potential, a second DC line (DCL2) having a second DC potential and an energy storage system comprising a first energy storage unit (18) between the first and second DC lines; a first set of Y-shaped phase arms (12) between the electrical grid and the first DC line (DCL1); a second set of Y-shaped phase arms (14) between the electrical grid (22) and the second DC line (DCL2), wherein the first and second sets of phase arms are controllable as voltage source converters for supporting the electrical grid with active power from the energy storage system; and a third set of phase arms (24) connected to the electrical grid in a Y-shaped configuration, having a neutral point and controllable to support the electrical grid with reactive power.

Description

Technical Field

[0001] This invention generally relates to power grid support. More specifically, this invention relates to a power grid support arrangement. Background Technology

[0002] Due to the increased use of renewable energy and the retirement of conventional power generation, grid operators are facing grid systems with low inertia and low short-circuit levels. Low inertia manifests as a high rate of change of frequency (ROCOF), and some grid specifications have proposed fast frequency and synthetic inertia response services to mitigate this issue.

[0003] Power electronic interface energy storage systems are strong candidates for providing these new services. Typical energy storage systems use electrochemical (e.g., batteries), dielectric (e.g., supercapacitors), or kinetic energy (e.g., machines with or without flywheels).

[0004] To provide fast frequency and synthetic inertial response services, energy storage media are required. However, some of these (such as electrochemical and dielectric storage) are expensive. This leads grid operators to only request the actual rated power, which may be less than the required apparent power. Therefore, a power support arrangement is needed that can increase the apparent power to allow additional reactive power to be supplied from the power support arrangement. Summary of the Invention

[0005] The present invention aims to provide a power support arrangement capable of providing additional reactive power and having an increased apparent power rating.

[0006] This objective is achieved through the power grid's power support arrangements, which include:

[0007] The DC network includes a first DC line having a first DC potential, a second DC line having a second DC potential, and an energy storage system, the energy storage system including a first energy storage unit connected in a branch between the first DC line and the second DC line.

[0008] The first set of phase arms is connected between the power grid and the first DC line in a Y-configuration.

[0009] The second set of phase arms, connected in a Y-configuration between the power grid and the second DC line, is controllable and advantageously can be jointly controlled, serving as a voltage source converter for utilizing active power from the energy storage system to support the power grid.

[0010] The third set of phase arms is connected to the grid in a Y-configuration, has a neutral point, and can be controlled to support the grid using reactive power.

[0011] The neutral point of the third set of phase arms can be disconnected from the DC network. Alternatively, it can be connected to the DC network. In both cases, the neutral point can be grounded or floating. If the third set of phase arms is connected to the DC network, the neutral point of that set can be connected to the connection point of the DC network, which can be a ground connection point.

[0012] The energy storage system may also include a second energy storage unit. The second energy storage unit may be connected in series with the first energy storage unit. More specifically, it may be connected in a branch including the first energy storage unit between the first DC line and the second DC line. In this case, the neutral point of the third set of phase arms may also be connected to the junction between the first and second energy storage units.

[0013] The power support arrangement may additionally include a fourth set of phase arms connected in a Y-shape between the connection points of the power grid and the DC network. If the third set of phase arms is connected to the DC network, the connection point to which the neutral point of the fourth set of phase arms is connected may be the same as the connection point used by the third set of phase arms.

[0014] According to some variations, the first and second phase arms include half-bridge elements, while the third phase arm includes full-bridge elements. More specifically, the first and second phase arms may consist of or consist only of half-bridge elements. The third phase arm may again consist of or consist only of full-bridge elements. In the case of a fourth phase arm, it may also include full-bridge elements.

[0015] A DC network may include a third DC line having a third DC potential, which may be the same as the second DC potential.

[0016] In some variations, the neutral point of the third set of phase arms is connected to the third DC line.

[0017] The energy storage system may additionally include a third energy storage unit. If the second energy storage unit is connected in series with the first energy storage unit in a branch between the first DC line and the second DC line, then in this case the third set of phase arms may be connected to the first end of the third energy storage unit via the third DC line, and the third energy storage unit may have a second end connected to the junction between the first energy storage unit and the second energy storage unit.

[0018] Alternatively, if the branch between the first DC line and the second DC line includes only the first energy storage unit, the second energy storage unit can be connected in an additional branch extending between the second DC line and the third DC line. In this case, the additional branch can include only the second energy storage unit.

[0019] As an alternative to the first and second group of phase arms including half-bridge elements, all groups of phase arms may include full-bridge elements. Therefore, all groups including the first and second groups may include full-bridge elements. They may additionally all include only full-bridge elements.

[0020] The power support arrangement may additionally include control equipment configured to control the first, second, and third phase arms. If a fourth phase arm exists, the control equipment may also be configured to control that fourth phase arm.

[0021] The control performed by the control equipment can be a joint control of the first and second sets of phase arms, such that at least one of these sets supplies active power to the grid, while the other set supplies reactive power. This control can also ensure that only the first set supplies active power and only the second set supplies reactive power. The control can additionally include control of a third set of phase arms to utilize reactive power to support the grid. The control of the third set can be separate from the control of the first and second sets. However, if a fourth set of phase arms exists, the control can be a joint control of the first, second, third, and fourth sets of phase arms, wherein the first and fourth sets are controlled to supply active power, while the second and third sets are controlled to supply reactive power. In this case, only the first and fourth sets can supply active power, and only the second and third sets can supply reactive power.

[0022] Alternatively, the first and second sets of phase arms can be controlled jointly, allowing both to supply active and reactive power to the grid. If a third and fourth set of phase arms are connected to the DC network, both can also be controlled to supply both active and reactive power to the grid. In cases where the neutral point of the third set of phase arms is connected to the junction between the first and second energy storage units, and one set of the first and second sets of phase arms fails while the other is healthy, the third set of phase arms can be controlled jointly with the healthy set of phase arms to supply active and reactive power to the grid.

[0023] Each phase arm in the first group may be part of an upper branch connecting the first DC line to the corresponding AC phase of the power grid, and each phase arm in the second group may be part of a lower branch connecting the second DC line to the corresponding phase of the power grid. Additionally, each phase arm in the fourth group may be part of an upper branch, and each phase arm in the third group may be part of a lower branch.

[0024] Control may nominally involve a first branch providing a first portion of the DC grid voltage and a second branch providing a second portion of the DC grid voltage, wherein the sum of these portions is the total DC grid voltage, which may be the difference between the first and second DC potentials. The first portion may be equal to the second portion, and the second portion may be half of the DC grid voltage.

[0025] Then, the control performed by the control device may include, for each phase of the power grid, adding a DC offset to one of the branches and subtracting a DC offset from the other branch, wherein the DC offset may be set as a portion of the DC voltage nominally provided by the branch from which the subtraction is performed.

[0026] The control may additionally include injecting a fundamental frequency circulating current component into the upper and lower branches, the circulating current component being configured to utilize the reactive power component formed in the added offset shielded branch and the active power component formed in the subtracted offset shielded branch.

[0027] The phase angle of the steady-state current through the branch with the added offset can be 0° or 180°, the phase angle of the current through the branch with the subtracted offset can be 90° or -90°, and the absolute value of the phase angle of the combined current of the upper and lower branches can be between 60° and 120°.

[0028] This invention has many advantages. It can provide additional reactive power, and therefore the rated apparent power of the power-supported arrangement can be higher than the rated active power, which allows for limitations on the size of the energy storage system. Attached Figure Description

[0029] The invention will now be described with reference to the accompanying drawings, in which:

[0030] Figure 1 A first form of a first embodiment of a power support arrangement connected to the power grid is schematically shown, wherein the power support arrangement includes first, second, and third sets of phase arms and an energy storage system.

[0031] Figure 2 A second embodiment of a power support arrangement connected to the power grid is schematically illustrated.

[0032] Figure 3 A third embodiment of a power support arrangement connected to the power grid is schematically illustrated.

[0033] Figure 4 A fourth embodiment of a power support arrangement connected to the power grid is schematically illustrated.

[0034] Figure 5 A fifth embodiment, similar to the second embodiment, is schematically illustrated.

[0035] Figure 6 A second form of the first embodiment of the power support arrangement connected to the power grid is schematically shown, and

[0036] Figure 7 Several steps in a method for controlling the upper and lower branches of the first and second phase arms, which include a power-supported arrangement, are shown. Detailed Implementation

[0037] A detailed description of preferred embodiments of the present invention will be given below.

[0038] Figure 1 A first variant of a first embodiment of a power support arrangement 10 connected to an alternating current (AC) grid 22 is shown. The power support arrangement 10 includes first and second sets of phase arms 12 and 14 and a direct current (DC) network DCN 16 including an energy storage system (ESS), wherein the energy storage system includes a first energy storage unit 18A. The DC network also includes a first DC line DCL1 and a second DC line DCL2. The first energy storage unit 18A is connected in a branch between the first and second DC lines DCL1 and DCL2.

[0039] Phase arms 12 in the first group are Y-connected, with each phase arm connected at its first end to a corresponding phase of the power grid 22 and at its second end to a first DC line DCL1, which has a first DC potential. The second ends are interconnected, forming the neutral point of the first group in the Y-connection. This neutral point is connected to the first DC line DCL1. Similarly, phase arms 14 in the second group are also Y-connected, with each phase arm's first end connected to a corresponding phase of the power grid 22 and its second end connected to a second DC line DCL2, which has a second DC potential. The second ends are interconnected, forming the neutral point of the second group in the Y-connection. This neutral point is connected to the second DC line DCL2. The first group of phase arms is connected between the power grid and the first DC line in a Y-configuration.

[0040] The second DC potential can be zero, or equal in magnitude to the first DC potential but with opposite polarity. The first and second sets of phase arms can additionally form a voltage source converter (VSC) together. Due to the phase arm implementation, this voltage source converter can be considered a dual-Y converter having an AC side formed by the first ends of the first and second sets of phase arms 12 and 14 and a DC side formed by the second ends of the first and second sets of phase arms 12 and 14. This is because the first and second phase arms can together form the AC waveform of the power grid 22. For this reason, each phase arm can also include a unit, and in a first variant of the first embodiment, the units in the first and second sets of phase arms are full-bridge units. Due to the use of units, the converter can also be considered a dual-Y modular multilevel converter (MMC).

[0041] The first and second sets of phase arms can be jointly controlled as a VSC to support the power grid, and this support may include an energy storage system that supports the power grid using active power. The control may additionally include joint control of the first and second sets of phase arms, such that at least one set supplies active power to the power grid, while the other set supplies reactive power. In a first variant of the first embodiment, both the first and second sets supply active power to the power grid.

[0042] As is known in the art, the AC side may additionally be connected to the power grid 22 via a transformer (not shown).

[0043] As described above, the DC network 16 also includes a first energy storage unit 18, wherein the first energy storage unit 18 is connected in a branch extending between the first and second DC lines DCL1 and DCL2. In the first embodiment, the branch includes only the first energy storage unit 18, and therefore a first end of the first energy storage unit 18 is connected to the first DC line DCL1, and a second end of the first energy storage unit 18 is connected to the second DC line DCL2. The energy storage system may be an electrochemical, kinetic, and / or dielectric energy storage system, and therefore the first energy storage unit may include a flywheel, a supercapacitor, and / or a battery.

[0044] The converter, comprising first and second sets of phase arms, includes power electronic devices. The converter is used to connect an energy storage system (ESS) to the power grid 22. Therefore, a converter with an energy storage system can also be referred to as a dual-Y power electronic interface energy storage system (PE-ESS).

[0045] The power support arrangement 10 also includes a third set of Y-connected phase arms 24, which are typically also composed of full-bridge units. Thus, the first end of each phase arm is connected to the corresponding phase of the power grid 22, while the second ends are interconnected to form a neutral point. This neutral point can be grounded. Alternatively, it can be floating. In this case, the neutral point is also isolated from the DC network. The third set of phase arms 24 can form a first auxiliary voltage source converter, which can also be an MMC. This converter can also be referred to as a parallel converter because it is essentially connected to the power grid in parallel with the PE-ESS. The auxiliary converter can also be considered as a static VAR compensator (STATCOM).

[0046] The power support arrangement 10 may additionally include a control device 20 configured to control a converter, the control being control of the first and second sets of phase arms 12 and 14 forming a double Y-shaped VSC to support the grid 22 with active and / or reactive power, and control of the third set of phase arms 24 forming a first auxiliary converter to support the grid 22 with reactive power.

[0047] The requirement is that the actual and apparent rated power of the electrical support layout should not be equivalent. Therefore, as Figure 1 As shown, adding a parallel Y-type converter to a dual Y-type PE-ESS can be cost-effective. Therefore, the reactive power supply capacity of the power support arrangement can be increased without increasing the active power supply capacity. Consequently, the power support arrangement can have an apparent power rating higher than the active power rating. This increased apparent power rating is also achieved without increasing the capacity of the energy storage system.

[0048] However, auxiliary converters have no actual power capability and typically need to be controlled separately.

[0049] Therefore, the controllability of the power support layout can be difficult to implement, especially in terms of stability. For example, a double-Y PE-ESS may be determined by oscillation equations, and the auxiliary converter needs to be synchronized with the PLL.

[0050] To improve this situation and enhance the use of the third set of phase arms for forming the auxiliary converter, aspects of the invention relate to connecting the third set of phase arms to the DC network 16. Therefore, the neutral point of the third set of phase arms 24 can be connected to the DC network 16. Thus, the third set of phase arms can be integrated into the PE-ESS to create a triple-Y PE-ESS. Several different ways in which this can be achieved are shown below.

[0051] Figure 2 A second embodiment of the power support arrangement 10 in which this has been implemented is schematically shown. Here, the energy storage system also includes a second energy storage unit 26, which in this embodiment is also connected in a branch between the first and second DC lines DCL1 and DCL2, including the first energy storage unit 18. In this case, the second energy storage unit 26 is also connected in series with the first energy storage unit 18 in the branch. More specifically, the first energy storage unit 18 has a first end connected to the first DC line DCL1 and a second end connected to the first end of the second energy storage unit 26, wherein the second end of the second energy storage unit 26 is connected to the second DC line DCL2. Thus, the first and second energy storage units 18 and 26 are connected in series between the first and second DC lines DCL1 and DCL2. In this case, the neutral point of the third set of phase arms 24 is connected to the DC network. More specifically, its connection point to the DC network is formed by a junction between the two energy storage units 18 and 26, which, in some variations, may be at ground potential, thus making the junction a grounded junction. Alternatively, the junction may be floating.

[0052] Here, control may also include controlling the first and second phase arms as VSCs, which can be the joint control of the first and second sets of phase arms, such that at least one set of these sets supplies active power to the grid, while the other set supplies reactive power. In this embodiment, both the first and second sets supply active power to the grid. In this case, the third set of phase arms 24 may not contribute any active power output. However, it can be used as an auxiliary VSC for reactive power support. The third set of phase arms 24 also has the advantage of providing redundancy in the event that one of the energy storage units must be disconnected. For example, if the first energy storage unit 18 fails, the second and third sets of phase arms 14 and 24 can be controlled as VSCs to allow the second energy storage unit 26 to support the grid 22, while the first and third sets of phase arms 12 and 24 can be controlled as VSCs to allow the first energy storage unit 18 to support the grid 22 if the second energy storage unit 26 is unavailable. Thus, it can be seen that if one of the first and second sets of phase arms fails, while the other set is healthy, the third set of phase arms can be jointly controlled with the healthy set of phase arms to supply both active and reactive power to the grid.

[0053] Figure 3 A third embodiment is shown, in which the third set of phase arms 24 is part of the PE ESS, and it also includes a second energy storage unit 26 connected in series with the first energy storage unit 18 in a branch between the first and second DC lines DCL1 and DCL2. Therefore, in this case, the first energy storage unit 18 still has a first end connected to the first DC line DCL1 and a second end connected to the first end of the second energy storage unit 26, wherein the second end of the second energy storage unit 26 is connected to the second DC line DCL2. The energy storage system also includes a third energy storage unit 28, which has a first end connected to the third DC line DCL3 of the DC network 16 and a second end connected to the junction between the first and second energy storage units 18 and 26. Therefore, the neutral point of the third set of phase arms is connected to the first end of the third energy storage unit 28 via the third DC line DCL3, and the second end of the third energy storage unit 28 is connected to the junction between the first and second energy storage units 18 and 26. Additionally, in this case, the neutral point of the third set of phase arms 24 is connected to the third DC line DCL3. In this case, the third DC line DCL3 may have a third potential that is the same as the first potential. According to this embodiment, the first and third energy storage units 18 and 28 can operate in parallel with the second energy storage unit 26 to support the power grid 22 using both active and reactive power. Redundancy is also possible.

[0054] Figure 4A fourth embodiment is shown, which includes a second energy storage unit 26 and a third set of phase arms 24 as part of the PE ESS. In this case, the neutral point of the third set of phase arms 24 is still connected to the third DC line DCL3. The second end of the second energy storage unit is still connected to the second DC line DCL2. However, in this case, the first end of the second energy storage unit 26 is connected to the third DC line DCL3. Therefore, the DC network 16 includes two parallel DC subsystems that can be used to support the power grid 22. Thus, the branch between the first and second DC lines DCL1 and DCL2 includes only the first energy storage unit 18, while the second energy storage unit 26 is connected in an additional branch extending between the second and third DC lines DCL2 and DCL3, wherein this additional branch includes only the second energy storage unit 26.

[0055] for Figure 3 and Figure 4 The configuration shown allows all three phase arms to generate active power. These configurations increase controllability because the three Y-connected phase arms can be controlled as a PE-ESS, and their response will be determined by the oscillation equation. Parallel operation using droop or relative inertia constants is possible. Furthermore, redundancy is possible in the event that the ES unit must be disconnected.

[0056] In the embodiments described so far, all phase arm groups include full-bridge units. Therefore, all groups including the first, second, and third groups may include full-bridge units. Additionally, they may all consist solely of full-bridge units. Furthermore, the first and second phase arm groups are jointly controlled as VSCs, and both supply active and reactive power to the grid. In the third and fourth embodiments, where the third phase arm group is connected to the DC network, this group may also be controlled together with the second phase arm group to supply active and reactive power to the grid.

[0057] The main advantage of the aspects described in this article is that they can be used in systems of unequal reactive and active power, especially when the reactive power rating is higher than the active power rating.

[0058] As above Figure 1 As seen in the diagram, if high reactive power requirements are needed, the auxiliary power converter can be connected in parallel with the PE-ESS. This is also cost-effective because it avoids the need for additional energy storage components, such as additional batteries and additional supercapacitors.

[0059] Instead of a parallel setup, the auxiliary power converter can be integrated into the PE-ESS. This leads to several different system configurations, such as... Figure 2 , Figure 3 and Figure 4As shown. These different configurations increase the redundancy of the energy storage system, and coordinated control of the entire system can lead to system optimization and other benefits.

[0060] It should be understood that the embodiments given above are merely examples and may vary. For example, more converters may be connected in parallel with the PE-ESS, or more sets of phase arms may be included as part of the PE-ESS. For example, it may be possible to... Figure 3 and Figure 4 The structure adds one or more additional phase arm groups, each of which has a neutral point connected to a corresponding additional DC line connected to a first end of a corresponding additional energy storage unit, wherein the second end of each of these additional energy storage units is connected to a junction between the first and second energy storage units (third embodiment) or to a second DC line (fourth embodiment).

[0061] In the above configuration, all phase arm groups consist of full-bridge units. It should be understood that a combination of half-bridge and full-bridge units can be used to achieve the power support arrangement.

[0062] Figure 5 The figure illustrates one form of this situation, showing a fifth embodiment as a variation of the second embodiment. Here, the neutral point of the first group of phase arms 12 is connected to the first DC line DCL1, and the neutral point of the second group of phase arms 14 is connected to the second DC line DCL2. Each phase arm of the first group 12 is connected at one end to the corresponding AC phase via the first AC line ACL1, and the other ends are interconnected and connected to the first DC line DCL1. Each phase arm of the second group 14 is connected at one end to the corresponding AC phase via the second AC line ACL2, and the other ends are interconnected and connected to the second DC line DCL2. In this case, there is also a third group of phase arms 24, whose first end is connected to the AC phase of the power grid (also via the second AC line ACL2), and whose second end is interconnected and grounded. As shown, there is also a fourth group of Y-connected phase arms 30, in which each phase arm includes a first end connected to the corresponding AC phase of the power grid via the first AC link ACL1, and the other ends are interconnected and grounded. It can be seen that the phase arms are shown as voltage sources. It can also be seen that the connection point of the DC network connected to the neutral point of the fourth phase arm 30 is the same as the connection point used by the third phase arm 24.

[0063] In this configuration, the first, second, third, and fourth groups of phase arms 12, 14, 24, and 30 are jointly controlled as voltage source converters to support the power grid. The first and fourth groups of phase arms 12 and 30 can operate together to form an AC waveform on the first AC link ACL1, and the second and third groups of phase arms 14 and 24 can operate together to form an AC waveform on the second AC link ACL2. Phase arms of the first and fourth groups 12 and 30 connected to the same phase of the first AC link ACL1 form the upper branch or positive branch indicated by p, while phase arms of the second and third groups 14 and 24 connected to the same phase of the second AC link ACL2 form the lower branch or negative branch indicated by n. Therefore, each phase arm in the first group is part of the upper branch connecting the first DC line to the corresponding AC connection of the power grid, and each phase arm in the second group is part of the lower branch connecting the second DC line to the corresponding phase of the power grid. Additionally, each phase arm in the fourth group is part of the upper branch, and each phase arm in the third group is part of the lower branch.

[0064] In this embodiment, the first and second sets of phase arms 12 and 14 include or consist of half-bridge units, while the third and fourth sets of phase arms 24 and 30 include or consist of full-bridge units. Additionally, the third and fourth sets of phase arms 24 and 30 can be considered as shared resources of the two MMCs. Units of the third set of phase arms 24 can help form waveforms on the second AC link ACL2, and units of the fourth set of phase arms 30 can help form waveforms on the first AC link ACL1.

[0065] It should be understood that the first and optional second energy storage units can also be connected in series between the two DC lines DCL1 and DCL2, and the neutral points of the third and fourth sets of phase arms 24 and 30 can be connected to the junction between these energy storage units.

[0066] Figure 6 A second variant of the first embodiment is shown, wherein the first and second sets of phase arms 12 and 14 are coupled with... Figure 1 and Figure 5 The phase arms 12 and 14 of the first and second sets of phase arms are connected to the phases of the power grid in the same manner as the corresponding DC lines. The phase arms 12 and 14 of the first set of phase arms are also connected to the phases of the power grid via the same first AC line ACL1. Therefore, the phase arms in the first set of phase arms form the upper branch or positive branch, and the phase arms in the second set of phase arms form the lower branch or negative branch.

[0067] In this configuration, the first and second sets of phase arms 12 and 14 can form an MMC, which is operated together to generate an AC waveform on the first AC link ACL1. The third set of phase arms 24 is still used as a separate VSC and is connected to the phase of the power grid, which is accomplished through a separate connection, i.e., a connection separate from the first AC line ACL1 and with the neutral point grounded. The difference here from the first variant of the first embodiment is that the units of the first and second sets of phase arms 12 and 14 are still half-bridge units, while the units of the third set of phase arms 24 are full-bridge units, as in the fifth embodiment.

[0068] therefore, Figure 5 and Figure 6 The first and second sets of phase arms shown in the embodiment include half-bridge units, while the third set of phase arms includes full-bridge units. More specifically, the first and second sets of phase arms are composed of or consist only of half-bridge units, while the third set of phase arms may be composed of or consist only of full-bridge units. When a fourth set of phase arms is present, it also includes full-bridge units, and more specifically, may be composed of or consist only of full-bridge units.

[0069] The power support arrangement may additionally include control equipment.

[0070] Control of the power support arrangement is performed by control device 20, which is therefore configured to control the first, second, and third phase arms. When a fourth phase arm exists, the control device also controls that fourth phase arm.

[0071] Therefore, in Figure 5 and Figure 6 A hybrid cell MMC with low power requirements and bipolar DC network compatibility was proposed. Figure 5 Compared to the configuration in the middle, Figure 6 The structure in the code combines the FB cells into a single Y-STATCOM. In the event of a lost pole, asymmetrical unipolar operation may require a ground connection.

[0072] The hybrid unit implementation of the power support arrangement can additionally have modified operations. In this case, control includes joint control of the first and second sets of phase arms, such that one set of these sets supplies active power to the grid, while the other set supplies reactive power. Control performed by the control equipment can be joint control of the first and second sets of phase arms, such that at least one set of these sets supplies active power to the grid, while the other set supplies reactive power. This control can be such that only the first set supplies active power and only the second set supplies reactive power. This control can additionally include control of a third set of phase arms to utilize reactive power to support the grid. The control of the third set can be separate from the control of the first and second sets. However, if a fourth set of phase arms exists, the control can be joint control of the first, second, third, and fourth sets of phase arms, wherein the first and fourth sets of phase arms are controlled to supply active power, while the second and third sets are controlled to supply reactive power. In this case, only the first and fourth sets can supply active power, and only the second and third sets can supply reactive power.

[0073] Compared to conventional dual-star MMCs operating with enhanced STATCOM, this control is characterized by reduced energy requirements. This reduced energy storage requirement is beneficial for the cell protection section. This is particularly important because there is ongoing activity regarding low-power STATCOM and medium-voltage DC (MVDC) MMC cells based on industrial insulated-gate bipolar transistor (IGBT) modules, where stored energy directly impacts cell bypass size.

[0074] The control may nominally involve a first branch providing a first portion of the DC grid voltage and a second branch providing a second portion of the DC grid voltage, wherein the sum of these portions is the total DC grid voltage, which may be the difference between the first and second DC potentials. The first portion may be equal to the second portion, and the second portion may be half of the DC grid voltage.

[0075] The control may then include, for each phase of the grid, adding a DC offset to one branch and subtracting a DC offset from the other branch, wherein the DC offset may be configured as a portion of the DC voltage nominally provided by the branch from which the subtraction is performed. The control may additionally include injecting a fundamental frequency circulating current component into the upper and lower branches, the circulating current component being configured to utilize the reactive power component in the branch with the added offset and the active power component formed in the branch with the subtracted offset.

[0076] As an example, an offset can be added to the upper branch and subtracted from the lower branch. For clarity, the following quantities are defined and used consistently throughout this document:

[0077] ·f g : power grid frequency

[0078] •S: Apparent power

[0079] ·V dc DC link voltage

[0080] · Grid current angle

[0081] ·k ac The ratio between peak AC grid voltage and half-DC voltage

[0082] The proposed modified hybrid unit MMC operation, performed by the control device for the converter branch, can be summarized for each phase as follows:

[0083] A DC offset of Vdc / 2 is added to the upper branch and subtracted from the lower branch, such that only the upper branch supports the entire DC voltage.

[0084]

[0085]

[0086] • By injecting a base frequency circulating current, the reactive power component is shielded in the upper branch, while the active power component is shielded in the lower branch, because the lower branch cannot exchange any active power with the DC link through the DC component.

[0087]

[0088] like Figure 7 As shown in the flowchart, the control device for the power support arrangement can therefore control a converter formed by at least the first and second sets of phase arms and optionally also by the third and fourth sets of phase arms to add a DC offset to a branch (step 30), where the DC offset can be half of the DC voltage of the DC network. The control device can also control the converter to subtract the DC offset from another branch (step 32) and inject a base frequency circulating current into the upper and lower branches (step 34). In this example, the DC offset is added to the upper branch and subtracted from the lower branch.

[0089] Therefore, the branch current is...

[0090]

[0091]

[0092] The proposed dual-Y MMC operation with DC offset and power component shielding, compared with conventional dual-Y MMC operation, demonstrates an excellent operating range of ±30° near -90° grid angle and ±10° near +90° grid angle, within which less energy storage is required compared to conventional MMC operation.

[0093] Therefore, control may include controlling the upper and lower branches to inject power at a phase angle in the range of -120 to -60 degrees relative to the grid or at a phase angle in the range of 80 to 100 degrees relative to the grid.

[0094] It should be understood that MMC operation, where the DC voltage is evenly distributed between the positive and negative branches (φ≈0), produces the minimum storage energy requirement for a high power factor, while STATCOM operation, where the DC voltage is unevenly distributed between the upper and lower branches (φ≈90°), produces the minimum storage energy requirement for a low power factor.

[0095] For load angles exceeding approximately 60°, STATCOM-like operation (with asymmetrically split DC voltage contributions for the upper and lower branches) results in reduced energy demand.

[0096] Therefore, the phase angle of the steady-state current through the branch with the added offset can be 0° or 180°, the phase angle of the current through the branch with the subtracted offset can be 90° or -90°, and the absolute value of the phase angle of the combined current of the upper and lower branches can be between 60° and 120°.

[0097] It can be seen that, according to Figure 5 and Figure 6 The power support layout and related operations have the following characteristics:

[0098] 1. A bipolar DC network with a converter including an HB unit and a grounded Y-STATCOM.

[0099] 2. Operation of FACTS devices with high Q and low P operation, featuring DC terminals and low energy storage requirements, achieved through asymmetrical sharing of the DC voltage component. Therefore, it is possible to have 33% FB cells and 66% HB cells.

[0100] Figure 5 and Figure 6 Some advantages of the converter arrangement in this configuration are:

[0101] Single-converter bipolar operation, in which the converter is potentially integrated into a valve chamber.

[0102] The merging of FB cell functions in a bipolar system leads to a reduction in the required storage energy.

[0103] • A simple modification to the standard operation of a dual-star MMC when the operating range is close to a low power factor value. The benefit is savings in semiconductor device and cell bypass circuitry requirements.

[0104] The control device can be implemented as a processor that acts according to computer instructions, such as one or more discrete components. However, it can also be implemented as a processor with accompanying program memory containing computer program code that, when run on the processor, performs the desired control function. The computer program product carrying this code can be provided as a data carrier (such as one or more CD-ROM discs or one or more memory sticks carrying the computer program code), which acts as a control unit when loaded into the processor.

[0105] As will be apparent from the foregoing discussion, the present invention can be varied in many ways. Therefore, it should be understood that the present invention is limited only to the following claims.

Claims

1. A power support arrangement (10) for connection to a power grid (22), the power support arrangement (10) comprising: DC network (16), the DC network including a first DC line (DCL1) having a first DC potential, a second DC line (DCL2) having a second DC potential, and an energy storage system, the energy storage system including a first energy storage unit (18) in a branch connected between the first DC line and the second DC line. The first set of phase arms (12) is connected in a Y-shape between the power grid and the first DC line (DCL1). The second set of phase arms (14), connected in a Y-configuration between the power grid (22) and the second DC line (DCL2), the first set of phase arms and the second set of phase arms being controllable as voltage source converters for supporting the power grid with active power from the energy storage system, and The third set of phase arms (24), which is connected to the power grid in a Y-shaped configuration, has a neutral point and can be controlled to support the power grid using reactive power. The first set of phase arms and the second set of phase arms can be controlled together, enabling them to supply active and reactive power to the power grid. If one of the first group of phase arms and the second group of phase arms fails, while the other group is healthy, the third group of phase arms is jointly controlled with the healthy group of phase arms to supply active and reactive power to the power grid.

2. The power support arrangement (10) according to claim 1, wherein, The neutral point of the third set of phase arms (24) is connected to the DC network (16).

3. The power support arrangement (10) according to claim 1 or 2, wherein, The energy storage system includes a second energy storage unit (26).

4. The power support arrangement (10) according to claim 3, wherein, The second energy storage unit (26) is connected in series with the first energy storage unit in the branch between the first DC line and the second DC line (DCL1, DCL2).

5. The power support arrangement (10) according to claim 4, wherein, The neutral point of the third set of phase arms (24) is connected to the junction between the first energy storage unit and the second energy storage unit.

6. The power support arrangement (10) according to claim 2 further includes a fourth set of phase arms (30) connected in a Y-shape between the connection point of the power grid and the DC network.

7. The power support arrangement (10) according to claim 6, wherein, The connection point is the same as the connection point used by the third set of phase arms.

8. The power support arrangement (10) according to any one of claims 1 to 2 and 4 to 7, wherein, The neutral point of the third phase arm (24) is grounded.

9. The power support arrangement (10) according to claim 6, wherein, The first group of phase arms and the second group of phase arms include half-bridge units, while the third group of phase arms includes full-bridge units.

10. The power support arrangement (10) according to claim 9, wherein, The fourth set of phase arms includes a full-bridge unit.

11. The power support arrangement (10) according to claim 3, wherein, The neutral point of the third set of phase arms (24) is connected to the third DC line (DCL3) with a third DC potential.

12. The power support arrangement (10) according to claim 11, wherein, The energy storage system includes a third energy storage unit (28), a second energy storage unit (26) connected in series with the first energy storage unit (18) in a branch between the first DC line and the second DC line (DCL1, DCL2), a third set of phase arms (24) connected to a first end of the third energy storage unit (28), and the third energy storage unit (28) having a second end connected to a junction between the first energy storage unit and the second energy storage unit.

13. The power support arrangement (10) according to claim 11, wherein, The second energy storage unit (26) is connected in the branch between the second DC line and the third DC line (DCL2, DCL3).

14. The power support arrangement (10) according to any one of claims 1 to 2, 4 to 7, and 9 to 13, wherein, All phase arm groups include full-bridge units.

15. The power support arrangement (10) according to any one of claims 1 to 2, 4 to 7, and 9 to 13, wherein, Each phase arm in the first group (12) is part of an upper branch that connects the first DC line (DCL1) to the corresponding AC phase of the power grid (22), and each phase arm in the second group (14) is part of a lower branch that connects the second DC line (DCL2) to the corresponding phase of the power grid (22), and also includes a control device (20) configured to add a DC offset to one of the branches for each phase of the power grid and subtract the DC offset from the other branches.

16. The power support arrangement (10) according to claim 15, wherein, The control device is configured to inject a fundamental frequency circulating current component into the upper branch and the lower branch, the circulating current component being configured to shield the reactive power component formed in the branch using an added offset and to shield the active power component formed in the branch using a subtracted offset.

17. The power support arrangement (10) according to claim 16, wherein, The phase angle of the steady-state current through the branch with the added offset is 0° or 180°, the phase angle of the current through the branch with the subtracted offset is 90° or -90°, and the absolute value of the phase angle of the combined current of the upper and lower branches is between 60° and 120°.

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