A novel modular multilevel converter integrated with distributed integrated reactors

Abstract

This invention relates to the field of power engineering technology and discloses a novel modular multilevel converter with integrated distributed reactors. When the distributed reactors are integrated at the bridge arm level, they are connected in series on both sides or one side of the bridge arm; when integrated at the valve tower level, they are integrated on both sides or one side of each valve tower; when integrated at the valve section level, they are integrated on both sides or one side of each valve section; and when integrated at the submodule level, they are located at any of the positive polarity ports A, B, or C of the submodule. This topology integrates at least one distributed reactor at the bridge arm, valve tower, valve section, or submodule level of the modular multilevel converter, which can limit the rate of rise of fault current and ensure reliable IGBT lockout.

Description

Technical Field

[0001] This invention relates to the field of power engineering technology, specifically to a novel modular multilevel converter that integrates distributed integrated reactors. Background Technology

[0002] Modular multilevel converters (MMCs) are an advanced high-voltage / high-power voltage source converter (VSC) topology. Their core topology feature is that each AC arm is composed of a large number of structurally identical, independently controllable submodules (SMs) (typically half-bridge structures, but full-bridge or other topologies are also possible) connected in series, forming a modular and scalable structure. The submodules integrate power electronic switching devices such as IGBTs and energy storage capacitors. By controlling the on / off states of the submodules, multilevel AC voltage waveforms can be synthesized to meet the demands of high-voltage, high-power transmission.

[0003] Currently, flexible DC transmission based on modular multilevel converters is the only option for transmitting offshore wind power. In an era where offshore wind power grid connection prices are gradually approaching parity, flexible DC transmission technology faces specific challenges, primarily in terms of high reliability and lightweight design.

[0004] To suppress circulating current in the bridge arms and limit short-circuit current, a bridge arm reactor is centrally located in each bridge arm of the converter. However, with the increasing DC capacity, the size and weight of centralized bridge arm reactors have increased significantly, sometimes requiring multiple coils connected in series to meet design requirements, placing great pressure on the compact layout of valve halls on offshore platforms. Furthermore, existing underwater flexible DC converter valve designs cannot handle two typical short-circuit faults: one is a direct short circuit across the converter valve, and the other is a short circuit between the converter valve and the bridge arm reactor. Both types of faults involve only negligible stray inductance in the circuits, lacking effective current limiting devices, resulting in extremely high fault current rise rates, far exceeding the tolerance of power devices such as IGBTs. Ultimately, this causes the devices to fail to reliably lock out, inevitably leading to severe damage to the converter valve.

[0005] Therefore, there is an urgent need for a new MMC topology that can reduce the capacity of centralized bridge arm reactors while improving the intrinsic safety of the converter under fault conditions. Summary of the Invention

[0006] In view of this, the present invention provides a novel modular multilevel converter integrating distributed integrated reactors to solve the problems of large capacity of centralized bridge arm reactors and lack of effective current limiting devices during short-circuit faults.

[0007] This invention provides a novel modular multilevel converter with integrated distributed reactors, comprising a three-phase six-arm bridge, each arm being composed of several cascaded sub-modules, wherein at least one distributed integrated reactor is integrated at at least one level in the bridge arm stage, valve tower stage, valve section stage, or sub-module stage of the converter. When the distributed integrated reactor is integrated at the bridge arm level, the distributed integrated reactor is connected in series on both sides or one side of the bridge arm of the converter, and the bridge arm is composed of several valve towers connected in series. When the distributed integrated reactor is integrated at the valve tower level, the distributed integrated reactor is integrated on both sides or one side of each valve tower, and each valve tower is composed of several valve sections connected in series. When the distributed integrated reactor is integrated at the valve segment level, the distributed integrated reactor is integrated on both sides or one side of each valve segment, and each valve segment is composed of several cascaded sub-modules; When the distributed integrated reactor is integrated at the submodule level, the distributed integrated reactor is configured at any of the positive polarity ports A, B, or C of the submodule.

[0008] This invention provides a novel modular multilevel converter with integrated distributed integrated reactors. This topology integrates at least one distributed integrated reactor in the bridge arm stage, valve tower stage, valve section stage, or sub-module stage of the modular multilevel converter (MMC). By replacing some or all of the traditional bridge arm reactors with distributed current limiting, the fault current rise rate can be limited, ensuring reliable IGBT lockout. At the same time, the reactor size and weight are significantly reduced, making it suitable for the lightweight requirements of offshore platforms.

[0009] In one alternative implementation, when the distributed integrated reactor is configured on one side of the bridge arm, the distributed integrated reactor and the original bridge arm reactor are located on the DC side and AC side of the bridge arm, respectively.

[0010] In one alternative implementation, when the distributed integrated reactor is configured on one side of the valve tower, the distributed integrated reactor and the original bridge arm reactor are located on opposite sides of the valve tower.

[0011] In one alternative implementation, when the distributed integrated reactor is configured on one side of the valve section, the distributed integrated reactor and the original bridge arm reactor are located on opposite sides of the valve section.

[0012] In one alternative implementation, if the original bridge arm reactor is located on the DC side of the converter, the distributed integrated reactor is configured at the positive polarity port B of the submodule.

[0013] In one alternative implementation, if the original bridge arm reactor is located on the AC side of the converter, the distributed integrated reactor is configured at the positive polarity port A of the submodule.

[0014] In one alternative implementation, the inductance value of the distributed integrated reactor... Lc Satisfy the following formula:

[0015] in, Uc This represents the capacitor voltage of the submodule. ρ This represents the maximum permissible rate of rise of the fault current for the converter. n The number of distributed integrated reactors in a single bridge arm. N This represents the total number of submodules in a single bridge arm. Attached Figure Description

[0016] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0017] Figure 1 This is a schematic diagram of a typical MMC topology in existing technologies; Figure 2 This is a schematic diagram of the fault path when a short circuit occurs at both ends of the valve in the existing MMC topology. Figure 3 This is a schematic diagram of the integrated reactor on both sides (or one side) of the converter valve bridge arm according to an embodiment of the present invention; Figure 4 This is a schematic diagram of the integrated reactors on both sides of each valve tower (or one side) within the converter valve bridge arm according to an embodiment of the present invention; Figure 5 This is a schematic diagram of the integrated reactors on both sides of each valve section (or one side) within the converter valve bridge arm according to an embodiment of the present invention; Figure 6 This is a schematic diagram of the integrated reactor in each submodule within the converter valve bridge arm according to an embodiment of the present invention. Detailed Implementation

[0018] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0019] In the description of this invention, it should be noted that the terms "center," "upper," "lower," "left," "right," "vertical," "horizontal," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are used only for the convenience of describing the invention and for simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the invention. Furthermore, the terms "first," "second," and "third" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0020] In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "installation," "connection," and "linking" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can also refer to the internal connection of two components; and they can refer to a wireless connection or a wired connection. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0021] Furthermore, the technical features involved in the different embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0022] like Figure 1 The diagram shows a typical topology of a modular multilevel converter (MMC) in the prior art. Its core feature is that the MMC adopts a three-phase six-arm architecture, with each arm consisting of several cascaded sub-modules (SMs) and connected in series with an arm reactor L. Figure 1 The configuration shown is the current mainstream solution. In actual applications, the bridge arm reactor can also be adjusted to be arranged on the AC side according to the system design requirements to adapt to the installation and operation requirements of different scenarios.

[0023] like Figure 2 As shown, under extreme conditions such as inter-terminal faults in the converter valve bridge arm or inter-layer faults within the valve tower, the DC capacitors of the converter valve submodules will discharge rapidly. Because there are almost no current-limiting measures within the bridge arm except for a small amount of stray inductance, the fault current rise rate is extremely high, preventing power devices from properly latching off, thus causing equipment damage. Therefore, to address this safety shortcoming of the existing topology, it is urgent to explore and adopt more reliable fault protection measures.

[0024] Therefore, the present invention provides a novel modular multilevel converter with integrated distributed integrated reactors, wherein at least one distributed integrated reactor is integrated in at least one level of the converter, such as the bridge arm level, valve tower level, valve section level or sub-module level.

[0025] This invention provides a novel modular multilevel converter with integrated distributed integrated reactors. This topology integrates at least one distributed integrated reactor in the bridge arm stage, valve tower stage, valve section stage, or sub-module stage of the modular multilevel converter (MMC). By replacing some or all of the traditional bridge arm reactors with distributed current limiting, the fault current rise rate can be limited, ensuring reliable IGBT lockout. At the same time, the reactor size and weight are significantly reduced, making it suitable for the lightweight requirements of offshore platforms.

[0026] like Figure 3 As shown, this is the first optimization scheme proposed in this embodiment, in which at least one distributed integrated reactor is integrated in the bridge arm stage of the converter, wherein the bridge arm is composed of several valve towers connected in series.

[0027] Specifically, when a distributed integrated reactor is integrated at the bridge arm level, the distributed integrated reactor is connected in series on both sides or one side of the bridge arm of the converter. When the distributed integrated reactor is configured on one side of the bridge arm, the distributed integrated reactor and the original bridge arm reactor are located on the DC side and AC side of the bridge arm, respectively.

[0028] In this embodiment of the invention, a distributed integrated reactor is integrated on both sides (or one side) of the bridge arm to address the current limiting requirements of the converter valve bridge arm. This distributed integrated reactor can immediately suppress sudden changes in the bridge arm current (such as a sudden current surge triggered by a short-circuit fault), thereby effectively reducing the fault current rise rate and preventing the current increase from exceeding the device's tolerance range.

[0029] Specifically, this solution significantly increases the total inductance of the converter valve bridge arm by connecting the aforementioned distributed integrated reactor in series. This increased inductance directly slows the rise rate of the short-circuit current, providing sufficient time for reliable latch-up of power devices such as IGBTs. This fundamentally solves the core problem in existing technologies where the rapid rise of fault current prevents device latch-up. Simultaneously, the integrated design of the distributed integrated reactor fully considers the dynamic response performance during normal system operation. Through precise inductance parameter optimization, it ensures that the reactor does not negatively impact the voltage waveform quality, circulating current suppression effect, or load regulation capability of the converter, achieving a performance balance between fault protection and normal operation.

[0030] Among the two configuration options of two-sided integration and single-sided integration, this embodiment preferably adopts the single-sided integration scheme. The core configuration feature of this preferred scheme is that the original bridge arm reactor and the newly added distributed integrated reactor in this embodiment are respectively arranged on both sides of the bridge arm. Specifically, if the original bridge arm reactor is configured on the DC side of the converter valve bridge arm, the distributed integrated reactor is configured on the AC side of the bridge arm; if the original bridge arm reactor is located on the AC side, the distributed integrated reactor is arranged on the DC side. This symmetrical arrangement logic can fully ensure that there are effective current limiting measures in the fault circuit under any fault scenario, completely eliminate the current limiting blind zone, and maximize the intrinsic safety level of the converter.

[0031] Experimental results show that the bridge arm-level integration scheme can significantly improve the operational reliability of the system under extreme conditions such as inter-terminal faults and inter-layer faults, providing a more efficient fault protection scheme for modular multilevel converters (MMC) and effectively reducing the risk of equipment damage.

[0032] like Figure 4 As shown, this is the second optimization scheme proposed in this embodiment, in which at least one distributed integrated reactor is integrated in the valve tower stage of the converter, wherein each valve tower is composed of several valve sections connected in series.

[0033] Specifically, when a distributed integrated reactor is integrated at the valve tower level, the distributed integrated reactor is integrated on both sides or one side of each valve tower. When the distributed integrated reactor is configured on one side of the valve tower, the distributed integrated reactor and the original bridge arm reactor are located on opposite sides of the valve tower.

[0034] In this embodiment of the invention, to address the current limiting requirements of the converter valve bridge arm, a technical solution is adopted that integrates distributed integrated reactors on both sides (or one side) of the valve tower. Since each converter valve bridge arm is composed of several valve towers connected in series, based on this structure, this embodiment integrates distributed integrated reactors on both sides or one side of each valve tower to achieve downward coverage of the current limiting measures to the valve tower unit.

[0035] like Figure 4 As shown, taking a typical scenario where the bridge arm includes two valve towers as an example, in this configuration, distributed integrated reactors can be further adapted and integrated into each valve tower, specifically arranged on both sides or one side of the valve tower. Consistent with the design logic of bridge arm-level integration, the optimized configuration in this embodiment is single-sided integration. Its core configuration principle is also the same: the distributed integrated reactor and the original bridge arm reactor are arranged on both sides of the valve tower respectively. Through this side-by-side arrangement, it is ensured that when a fault occurs at the valve tower level, an effective current limit can be formed in the circuit immediately, while taking into account the inductance parameter matching during normal system operation, avoiding negative impacts on the dynamic performance of the converter.

[0036] like Figure 5As shown, this is the third optimization scheme proposed in this embodiment, in which at least one distributed integrated reactor is integrated at the valve section level of the converter, wherein each valve section is composed of several cascaded sub-modules.

[0037] Specifically, when a distributed integrated reactor is integrated at the valve section level, the distributed integrated reactor is integrated on both sides or one side of each valve section. When the distributed integrated reactor is configured on one side of a valve section, the distributed integrated reactor and the original bridge arm reactor are located on opposite sides of the valve section.

[0038] In this embodiment of the invention, a technical solution is adopted to integrate distributed integrated reactors on both sides (or one side) of the valve section to meet the current limiting requirements of the converter valve bridge arm. From a structural perspective, each valve tower consists of several valve sections connected in series, and each valve section is composed of several cascaded sub-modules. Based on this multi-level architecture, this embodiment aims to integrate distributed integrated reactors into the valve section to achieve more precise fault current limiting coverage.

[0039] like Figure 5 As shown, regarding the aforementioned structural relationship between valve section, valve tower, and submodule, this embodiment proposes two specific configuration approaches. The first is to integrate distributed integrated reactors on both sides of the valve section, and the second is to integrate distributed integrated reactors on one side of the valve section. Consistent with the design logic of bridge arm-level integration, the optimized configuration method in this embodiment is single-side integration. Its core configuration principle is also clear: the distributed integrated reactor and the original bridge arm reactor are respectively arranged on both sides of the valve section. Through this split-side arrangement, it is possible to ensure that effective current limiting can be immediately formed in the fault circuit when a fault occurs at the valve section level, and to avoid negative impacts on the dynamic response performance of the converter during normal operation through precise inductance parameter matching, thus achieving a balance between fault protection and system stability.

[0040] like Figure 6 As shown, this is the fourth optimization scheme proposed in this embodiment, in which at least one distributed integrated reactor is integrated at the sub-module level of the converter.

[0041] Specifically, when a distributed integrated reactor is integrated at the submodule level, the distributed integrated reactor is configured at any of the positive polarity ports A, B, or C of the submodule. If the original bridge arm reactor is located on the DC side of the converter, the distributed integrated reactor is configured at the positive polarity port B of the submodule; if the original bridge arm reactor is located on the AC side of the converter, the distributed integrated reactor is configured at the positive polarity port A of the submodule.

[0042] In this embodiment of the invention, to address the current-limiting requirements of the converter valve bridge arm, a technical solution is adopted that integrates a distributed integrated reactor into the sub-module. For example... Figure 6 As shown, this solution provides three specific structural forms for the configuration of distributed integrated reactors: such as Figure 6As shown in (a), the distributed integrated reactor is configured at the positive terminal A of the submodule; as Figure 6 As shown in (b), the distributed integrated reactor is configured at the positive terminal B of the submodule; as Figure 6 As shown in (c), the distributed integrated reactor is configured at the positive polarity port C of the submodule.

[0043] In practical applications, when configuring the ports of distributed integrated reactors at the submodule ports, the principle is to select them as far as possible on either side of the bridge arm reactor within the submodule. That is, if the original bridge arm reactor is configured on the DC side of the converter valve, then it is preferable to place it on either side of the submodule. Figure 6 In the scheme shown in (b), the distributed integrated reactor is placed at the positive terminal B of the submodule; conversely, if the original bridge arm reactor is configured on the AC side of the converter valve, then the preferred configuration is... Figure 6 The scheme shown in (a) places the distributed integrated reactor at the positive terminal A of the submodule. This design, which separates the submodule integrated reactor from the bridge arm reactor, ensures that an effective current-limiting path can be formed in the fault circuit under any fault scenario, maximizing the fault protection function of the distributed integrated reactor, while avoiding interference with the normal charging and discharging of the submodule and the overall dynamic response of the converter.

[0044] In one optional implementation, any two, three, or all four levels can be selected from the four levels of bridge arm level, valve tower level, valve section level, and submodule level for combination configuration. Each distributed integrated reactor is connected to the corresponding circuit in series to limit the rate of rise of fault current, improve the intrinsic safety of the converter, and at the same time reduce the size and weight of the bridge arm reactor to achieve lightweight layout of the offshore platform.

[0045] Specifically, when selecting any two-stage combination configuration, taking the bridge arm stage and valve tower stage combination as an example, at least one distributed integrated reactor is connected in series on both sides or one side of the bridge arm, and at least one distributed integrated reactor is integrated on both sides or one side of each valve tower. The two stages of reactors are distributed and work together to limit current. This combination can significantly improve the fault tolerance of the bridge arm as a whole and the valve tower unit, and is suitable for medium-capacity offshore wind power flexible DC converter systems.

[0046] When any three-level combination configuration is selected, taking the bridge arm level, valve tower level, and valve section level combination as an example, distributed integrated reactors are connected in series in the bridge arm, each valve tower, and each valve section to form a three-level gradient distributed current limiting structure. This combination can comprehensively cover faults such as valve end short circuit, valve internal short circuit, valve section short circuit, and valve tower inter-short circuit, with significant current limiting effect, and can greatly reduce or even eliminate the need for centralized bridge arm reactors.

[0047] When the full four-level configuration is selected, distributed integrated reactors are connected in series in the bridge arm, each valve tower, each valve section, and each submodule, forming a distributed current-limiting topology covering the entire level, the entire link, and the entire domain. The four-level reactors work together to minimize the rate of rise of fault current, completely covering the most severe fault scenarios such as short circuits at both ends of the valve, short circuits inside the valve, and short circuits between the valve and the reactor, truly achieving the intrinsic safety of the converter. At the same time, the capacity of the bridge arm reactor can be significantly reduced, and with optimized design, the bridge arm reactor can be eliminated, greatly reducing the volume and weight of the valve hall and improving the economics of the offshore platform.

[0048] The novel modular multilevel converter topology proposed in this invention integrates distributed reactors. In fact, it proposes a novel topology at different levels: bridge arm, valve tower, valve section, and submodule. Specifically, it integrates reactors in the bridge arm, valve tower, valve section, and submodule, aiming to match different application requirements and cost considerations. This improves the intrinsic safety of the converter valve, reduces or even eliminates bridge arm reactors, and improves the economy and cost-effectiveness of offshore platforms. Its beneficial effects are mainly reflected in the following aspects: First, it significantly improves the intrinsic safety of the converter, covering extreme faults such as valve-end faults, internal valve faults, and faults between the valve and reactor; second, it reduces the capacity and size of the bridge arm reactors, lowering their dimensions, volume, and weight. With sufficient optimization, bridge arm reactors can be eliminated, further reducing valve hall size and achieving lightweighting of offshore platforms.

[0049] In one alternative implementation, the primary purpose of the distributed integrated reactor is to limit the rate of rise of the fault current. The parameter selection principle is as follows: if each bridge arm contains N sub-modules, the sub-module capacitor voltage is Uc, and a total of n distributed integrated reactors are configured within that bridge arm (e.g., n=1 (single-sided configuration) or n=2 (both sides configuration)), and the system's maximum allowable rate of rise of the fault current is... ρ The inductance value of the distributed integrated reactor Lc Satisfy the following formula:

[0050] in, Uc This represents the capacitor voltage of the submodule. ρ This represents the maximum permissible rate of rise of the fault current for the converter. n The number of distributed integrated reactors in a single bridge arm. N This represents the total number of submodules in a single bridge arm.

[0051] Although embodiments of the invention have been described in conjunction with the accompanying drawings, those skilled in the art can make various modifications and variations without departing from the spirit and scope of the invention, and such modifications and variations all fall within the scope defined by the appended claims.

Claims

1. A novel modular multilevel converter integrating distributed integrated reactors, comprising a three-phase six-arm bridge, each arm being composed of several cascaded sub-modules, characterized in that, At least one distributed integrated reactor is integrated in at least one level of the converter, including the bridge arm stage, valve tower stage, valve section stage, or sub-module stage. When the distributed integrated reactor is integrated at the bridge arm level, the distributed integrated reactor is connected in series on both sides or one side of the bridge arm of the converter, and the bridge arm is composed of several valve towers connected in series. When the distributed integrated reactor is integrated at the valve tower level, the distributed integrated reactor is integrated on both sides or one side of each valve tower, and each valve tower is composed of several valve sections connected in series. When the distributed integrated reactor is integrated at the valve segment level, the distributed integrated reactor is integrated on both sides or one side of each valve segment, and each valve segment is composed of several cascaded sub-modules; When the distributed integrated reactor is integrated at the submodule level, the distributed integrated reactor is configured at any of the positive polarity ports A, B, or C of the submodule.

2. The novel modular multilevel converter with integrated distributed reactor as described in claim 1, characterized in that, When the distributed integrated reactor is configured on one side of the bridge arm, the distributed integrated reactor and the original bridge arm reactor are located on the DC side and AC side of the bridge arm, respectively.

3. The novel modular multilevel converter with integrated distributed reactor as described in claim 1, characterized in that, When the distributed integrated reactor is configured on one side of the valve tower, the distributed integrated reactor and the original bridge arm reactor are located on opposite sides of the valve tower.

4. The novel modular multilevel converter with integrated distributed reactor as described in claim 1, characterized in that, When the distributed integrated reactor is configured on one side of the valve section, the distributed integrated reactor and the original bridge arm reactor are located on opposite sides of the valve section.

5. The novel modular multilevel converter with integrated distributed reactor according to claim 1, characterized in that, If the original bridge arm reactor is located on the DC side of the converter, the distributed integrated reactor is configured at the positive polarity port B of the submodule.

6. The novel modular multilevel converter with integrated distributed reactor according to claim 5, characterized in that, If the original bridge arm reactor is located on the AC side of the converter, the distributed integrated reactor is configured at the positive polarity port A of the submodule.

7. The novel modular multilevel converter with integrated distributed reactor according to claim 1, characterized in that, The inductance value of the distributed integrated reactor Lc Satisfy the following formula: in, Uc This represents the capacitor voltage of the submodule. ρ This represents the maximum permissible rate of rise of the fault current for the converter. n The number of distributed integrated reactors in a single bridge arm. N This represents the total number of submodules in a single bridge arm.

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