A hybrid MMC flexible DC conversion structure based on a shared bridge arm

Abstract

This application discloses a hybrid MMC flexible DC-DC converter structure based on a shared bridge arm, comprising a three-phase main circuit. Each phase main circuit includes an upper bridge arm DC switch group, a lower bridge arm DC switch group, an upper bridge arm AC switch group, a lower bridge arm AC switch group, and a shared bridge arm composed of multiple cascaded full-bridge submodules. One end of the shared bridge arm in any phase main circuit is connected to the first end of the upper bridge arm DC switch group and the upper bridge arm AC switch group, and the other end of the shared bridge arm is connected to the first end of the lower bridge arm DC switch group and the lower bridge arm AC switch group. The second ends of the upper and lower bridge arm DC switch groups serve as DC ports for connecting to the DC bus. The second ends of the upper and lower bridge arm AC switch groups are connected to form the AC port of each phase main circuit. This invention can significantly reduce the number of submodules required by the system and enables lightweight operation of the converter valve.

Description

Technical Field

[0001] This application relates to the field of power electronics technology, specifically to a hybrid MMC flexible DC-DC converter structure based on a shared bridge arm. Background Technology

[0002] Flexible DC converter valves are key components of high-voltage direct current (HVDC) transmission systems, primarily based on the Modular Multilevel Converter (MMC) topology. MMCs are characterized by high modularity, ease of expansion, and good output voltage waveforms. They can quickly achieve independent decoupling control of active and reactive power, and facilitate multi-terminal systems without communication between converter stations, making them particularly suitable for medium- and high-voltage, high-power applications. However, with increasing voltage levels and system capacity, the weight, size, and cost of converter valves increase dramatically, limiting the widespread application of flexible DC transmission technology.

[0003] Traditional MMC converter valves have the same number of sub-modules in both the upper and lower arms, and their parameters need to be tuned with reference to the DC bus voltage. The large number of sub-modules required is one of the main reasons for the large size and weight of the converter valve. Furthermore, the equivalent capacitance of the sub-modules in the upper and lower arms exhibits a split capacitor topology relative to the AC side, which causes differential-mode power frequency pulsation in the voltage of the sub-module capacitors in the upper and lower arms. Large capacitors are required to meet the ripple standard. In addition, the current in each arm of the converter valve contains a DC component, requiring bulky DC reactors.

[0004] Researching key technologies for lightweight converter valves can not only effectively reduce valve body size and weight, hardware load, and construction costs of offshore platforms, but also improve the engineering feasibility of high-pressure, high-capacity converter valves and meet the future development requirements of smart grids and global energy interconnection; it can also improve the technical and economic efficiency of new energy access systems and accelerate the construction of a low-carbon and environmentally friendly energy society. Summary of the Invention

[0005] To address at least one deficiency or improvement requirement of the prior art, the present invention provides a hybrid MMC flexible DC converter structure based on shared bridge arms. By using a shared bridge arm structure design and configuring compensation capacitors with appropriate capacitance values, the number of sub-modules required by the system can be significantly reduced, enabling lightweight operation of the converter valve.

[0006] To achieve the above objectives, according to the first aspect of the present invention, a hybrid MMC flexible DC-DC converter structure based on a shared bridge arm is provided, which includes a three-phase main circuit; wherein, each phase main circuit includes an upper bridge arm DC switch group, a lower bridge arm DC switch group, an upper bridge arm AC switch group, a lower bridge arm AC switch group, and a shared bridge arm composed of multiple cascaded full-bridge submodules.

[0007] One end of the shared bridge arm in any phase of the main circuit is connected to the first end of the upper bridge arm DC switch group and the upper bridge arm AC switch group, respectively; the other end of the shared bridge arm is connected to the first end of the lower bridge arm DC switch group and the lower bridge arm AC switch group, respectively; the second ends of the upper bridge arm DC switch group and the lower bridge arm DC switch group serve as DC ports for connecting to the DC bus; the second ends of the upper bridge arm AC switch group and the lower bridge arm AC switch group are connected to form the AC port of each phase of the main circuit.

[0008] The flexible DC-DC converter structure provided by this invention controls the switching states of the DC and AC switch groups, enabling the shared bridge arm to operate in the positive and negative half-cycles of the output voltage, respectively. This is functionally equivalent to the upper and lower bridge arms of a traditional full-bridge submodule MMC, thus realizing the topology sharing function of the shared bridge arm.

[0009] Furthermore, the aforementioned hybrid MMC flexible DC converter structure also includes a three-phase converter transformer; the primary winding of the three-phase converter transformer is connected to the AC port of each phase main circuit, and the primary winding adopts a Y-type structure.

[0010] Furthermore, the aforementioned hybrid MMC flexible DC converter structure also includes a compensation capacitor;

[0011] One end of the compensation capacitor is connected to the neutral point of the Y-shaped structure of the three-phase converter transformer, and the other end is connected to the voltage midpoint of the DC bus.

[0012] Furthermore, in the above-mentioned hybrid MMC flexible DC-DC converter structure, the capacitance value of the compensation capacitor is designed as follows:

[0013]

[0014] Where L1 represents the leakage inductance of the primary winding of the three-phase converter transformer, L2 represents the leakage inductance of the secondary winding of the three-phase converter transformer, k represents the turns ratio of the three-phase converter transformer, and ω represents the angular frequency of the AC voltage.

[0015] The flexible DC converter structure provided by this invention uses a compensation capacitor with an appropriate capacitance value to offset the adverse effects of the three-phase converter transformer on the balance control of the shared bridge arm, thereby reducing the voltage range required for the shared bridge arm to output, and further reducing the number of sub-modules required by the system.

[0016] Furthermore, in the above-mentioned hybrid MMC flexible DC converter structure, each phase main circuit also includes an AC side inductor; one end of the AC side inductor is connected to the AC port, and the other end is connected to the corresponding phase of the primary winding of the three-phase converter transformer.

[0017] Furthermore, in the above-mentioned hybrid MMC flexible DC converter structure, the line voltage of the AC port of each phase main circuit is a three-phase voltage with a phase difference of 2π / 3.

[0018] Furthermore, in the above-mentioned hybrid MMC flexible DC converter structure, the upper bridge arm DC switch group and the lower bridge arm DC switch group are each composed of multiple power switches connected in the same direction in series.

[0019] Furthermore, in the above-mentioned hybrid MMC flexible DC converter structure, the upper arm AC switch group and the lower arm AC switch group are respectively composed of multiple power switches connected in the same direction and multiple power switches connected in opposite directions connected in series.

[0020] Furthermore, in the above-mentioned hybrid MMC flexible DC-DC converter structure, the power switching transistor is an IGBT with a reverse diode.

[0021] According to a second aspect of the present invention, a flexible DC transmission system is also provided, which includes the hybrid MMC flexible DC converter structure described in any of the preceding claims.

[0022] In summary, compared with the prior art, the above-described technical solutions conceived by this invention can achieve the following beneficial effects:

[0023] (1) The hybrid MMC flexible DC converter structure based on shared bridge arm provided by the present invention achieves the sharing of the topology structure of the shared bridge arm through the structural design of the DC switch group of the upper and lower bridge arm and the AC switch group of the upper and lower bridge arm, thereby reducing the number of sub-modules required by the system; compared with the traditional full-bridge sub-module MMC, the number of sub-modules required by this topology is about one-quarter of that of the full-bridge sub-module MMC.

[0024] (2) The hybrid MMC flexible DC converter structure based on shared bridge arm provided by the present invention benefits from the reasonable control of shared bridge arm and compensation capacitor. The required submodule capacitance value of this topology is about half that of the full-bridge submodule MMC.

[0025] (3) The hybrid MMC flexible DC converter structure based on shared bridge arm provided by the present invention can greatly reduce the volume and weight of DC transmission converter station, and is suitable for applications such as offshore wind power that have high requirements for the volume and weight of converter station. Attached Figure Description

[0026] To more clearly illustrate the technical solutions in the embodiments of this application, the accompanying drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0027] Figure 1This embodiment provides a schematic diagram of the circuit structure of a hybrid MMC flexible DC-DC converter based on a shared bridge arm.

[0028] Figure 2 This is a switch structure diagram of the upper bridge arm DC switch group and the lower bridge arm DC switch group provided in this embodiment;

[0029] Figure 3 This is a switch structure diagram of the upper bridge arm AC switch group and the lower bridge arm AC switch group provided in this embodiment;

[0030] Figure 4 This is a schematic diagram of the structure of the full-bridge submodule provided in this embodiment;

[0031] Figure 5 This is a waveform diagram of the DC switch group, AC switch group, and shared bridge arm provided in this embodiment;

[0032] Figure 6 This embodiment shows the capacitor voltage waveform of a submodule in a shared bridge arm.

[0033] Figure 7 This is the output voltage waveform on the secondary side of the three-phase converter transformer provided in this embodiment;

[0034] Figure 8 A comparison diagram of bridge arm voltage waveforms for different MMC topologies provided in this embodiment. Detailed Implementation

[0035] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.

[0036] The terms "first," "second," "third," etc., in the specification, claims, and accompanying drawings of this application are used to distinguish different objects, not to describe a specific order. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover non-exclusive inclusion. For example, a process, method, system, product, or apparatus that includes a series of steps or units is not limited to the listed steps or units, but may optionally include steps or units not listed, or may optionally include other steps or units inherent to these processes, methods, products, or apparatuses.

[0037] Furthermore, to avoid obscuring the understanding of the invention by those skilled in the art, well-known or widely used techniques, elements, structures, and processes may not be described or shown in detail. Although the accompanying drawings illustrate exemplary embodiments of the invention, the drawings are not necessarily drawn to scale, and specific features may be enlarged or omitted to better illustrate and explain the invention.

[0038] Figure 1 This is a circuit diagram of a hybrid MMC flexible DC-DC converter structure based on a shared bridge arm provided in this embodiment. The hybrid MMC flexible DC-DC converter structure includes a three-phase main circuit. The topology of each phase main circuit includes a DC switch group near the DC side, an AC switch group near the AC side, and a shared bridge arm. Please refer to [link to relevant documentation]. Figure 1 Each phase of the main circuit includes an upper bridge arm DC switch group DS1 and a lower bridge arm DC switch group DS2, and an AC switch group includes an upper bridge arm AC switch group AS2 and a lower bridge arm AC switch group AS1; the shared bridge arm SAA is composed of multiple full-bridge sub-modules SM1 to SMn cascaded together.

[0039] In this circuit, one end of the shared bridge arm SAA in any phase main circuit is connected to the first end of the upper bridge arm DC switch group DS1 and the upper bridge arm AC switch group AS2, respectively. The other end of the shared bridge arm SAA is connected to the first end of the lower bridge arm DC switch group DS2 and the lower bridge arm AC switch group AS1, respectively. The second ends of the upper bridge arm DC switch group DS1 and the lower bridge arm DC switch group DS2 serve as DC ports for connecting to the DC bus. In a specific example, the second end of the upper bridge arm DC switch group DS1 serves as a DC port connected to the positive terminal of the DC bus, and the second end of the lower bridge arm DC switch group DS2 serves as a DC port connected to the negative terminal of the DC bus. The second ends of the upper bridge arm AC switch group AS2 and the lower bridge arm AC switch group AS1 are connected to form the AC port of each phase main circuit.

[0040] The flexible DC-DC converter structure provided in this embodiment controls the switching states of the DC and AC switch groups, enabling the shared bridge arm to operate in the positive and negative half-cycles of the output voltage, respectively. This is functionally equivalent to the upper and lower bridge arms of a traditional full-bridge submodule MMC, achieving topology sharing of the shared bridge arm and thus reducing the number of submodules required by the system.

[0041] Please continue reading. Figure 1 The hybrid MMC flexible DC-DC converter structure provided in this embodiment uses AC ports corresponding to the three-phase main circuits to connect to the primary windings of the three-phase converter transformer; the line voltage of the AC port of each phase main circuit is a three-phase voltage with a phase difference of 2π / 3. In a specific example, the primary winding of the three-phase converter transformer adopts a Y-type connection structure, and the secondary winding adopts a delta connection structure.

[0042] In a more preferred embodiment, the hybrid MMC flexible DC-DC converter structure provided in this embodiment also includes a compensation capacitor C. cp The compensation capacitor C cp One end of the capacitor is connected to the neutral point O of the Y-type structure of the three-phase converter transformer, and the other end is connected to the voltage midpoint G of the DC bus. The capacitance value of the compensation capacitor is designed as follows:

[0043]

[0044] Where L1 represents the leakage inductance of the primary winding of the three-phase converter transformer, L2 represents the leakage inductance of the secondary winding of the three-phase converter transformer, k represents the turns ratio of the three-phase converter transformer, and ω represents the angular frequency of the AC voltage.

[0045] The flexible DC converter structure provided in this embodiment uses a compensation capacitor with an appropriate capacitance value to offset the adverse effects of the three-phase converter transformer on the balance control of the shared bridge arm, thereby reducing the voltage range required for the shared bridge arm to output and thus reducing the number of sub-modules required by the system.

[0046] In an optional embodiment, in the above-described hybrid MMC flexible DC converter structure, each phase main circuit further includes an AC-side inductor L, one end of which is connected to the AC port of the corresponding phase main circuit, and the other end is connected to the corresponding phase of the primary winding of the three-phase converter transformer.

[0047] Figure 2 The following is a diagram of the switching structure of the upper arm DC switch group and the lower arm DC switch group provided in this embodiment, as shown below. Figure 2 As shown, in the hybrid MMC flexible DC-DC converter structure provided in this embodiment, the upper bridge arm DC switch group and the lower bridge arm DC switch group are respectively composed of N1 power switches connected in the same direction in series, where N1 is a natural number greater than 1.

[0048] Figure 3 The following is a diagram of the switching structure of the upper arm AC switch group and the lower arm AC switch group provided in this embodiment, as shown below. Figure 3 As shown, in the hybrid MMC flexible DC-DC converter structure provided in this embodiment, the upper arm AC switch group and the lower arm AC switch group are respectively composed of N2 power switches connected in the same direction and N3 power switches connected in opposite directions in series; where N2 and N3 are natural numbers greater than 1.

[0049] In a preferred example, the power switch described above is an IGBT with a reverse diode.

[0050] Conventional single-phase IGBT switches have anti-parallel diodes, and there is still uncontrollable current flow through the diodes after turn-off. The series structure of multiple power switches connected in the same direction and multiple power switches connected in opposite directions used in this embodiment can achieve complete control of the current path, which is the key to realizing the circuit function.

[0051] The shared bridge arm SAA consists of N4 cascaded full-bridge submodules. Each full-bridge submodule can be controlled independently. During operation, a faulty submodule is quickly isolated by a fast bypass switch, thus not affecting the operation of the DC system. N4 is a natural number greater than 1.

[0052] Figure 4 This is a schematic diagram of the structure of the full-bridge submodule provided in this embodiment. Each full-bridge submodule includes four power switches S1 to S4 and a floating capacitor C. One end of power switch S1 is connected to one end of power switch S3 and one end of floating capacitor C, respectively. One end of power switch S2 is connected to one end of power switch S4 and the other end of floating capacitor C, respectively. The other end of power switch S1 is connected to the other end of power switch S2, serving as one output port A of the full-bridge submodule. The other end of power switch S3 is connected to the other end of power switch S4, serving as another output port B of the full-bridge submodule.

[0053] In the hybrid MMC flexible DC-DC converter structure provided in this embodiment, the number of switching devices in the DC switch group and AC switch group, as well as the number of sub-modules in the shared bridge arm, are designed according to the following formula:

[0054]

[0055] Among them, V dc U is the DC bus voltage, m is the system modulation ratio, and U is the DC bus voltage. cm The given value for the capacitor voltage of the submodule is V. cell These are the rated voltage parameters of the switching devices.

[0056] The following explanation uses the A-phase main circuit as an example:

[0057] Let the given phase voltage at the AC port of phase A main circuit be:

[0058] v a (t)=V a sinωt (3) Let the given phase current at the AC port of phase A main circuit be:

[0059]

[0060] Among them, V a I is the phase voltage amplitude. a Let ω be the phase current amplitude, and ω be the angular frequency of the AC voltage. This represents the phase difference between the phase voltage and the phase current.

[0061] Figure 5 This embodiment provides the operating waveform diagrams of the DC switch group, AC switch group, and shared bridge arm, as shown below. Figure 5 As shown, let the drive control signals for the upper bridge arm DC switch group DS1 and the lower bridge arm AC switch group AS1 be:

[0062]

[0063] The drive control signals for the lower bridge arm DC switch group DS2 and the upper bridge arm AC switch group AS2 are as follows:

[0064]

[0065] For a shared arm SAA, power balance is achieved by injecting a third harmonic. Let the output voltage and current of the shared arm SAA be:

[0066]

[0067] in,

[0068]

[0069] Among them, I m To inject the third harmonic voltage amplitude, V dc The DC bus voltage, u arma For the output voltage of the shared bridge arm of phase A, i arma This refers to the bridge arm current shared by phase A.

[0070] from Figure 5 As can be seen, the bridge arm voltage waveform of the shared bridge arm SAA does not contain harmonic components introduced by the injected third harmonic, but the bridge arm current of the shared bridge arm SAA still contains harmonic components caused by the third harmonic current. Figure 6 The waveform of the capacitor voltage of the submodule in the shared bridge arm shows that the capacitor voltage ripple mainly contains the second harmonic component, and the amplitude of the capacitor voltage ripple can be significantly reduced. Figure 7 The output voltage waveform on the secondary side of the three-phase converter transformer is shown below. Figure 7 As can be seen, the voltage waveform is a sine wave, and the output waveform quality is good.

[0071] Figure 8 The image shows a comparison of bridge arm voltage waveforms for different MMC topologies. Figure 8 (a) shows the bridge arm voltage waveforms corresponding to the traditional full-bridge submodule MMC topology. Figure 8(b) shows the shared arm voltage waveform of the commutation structure topology proposed in this invention without compensation capacitors. Figure 8 (c) The shared arm voltage waveform of the proposed commutation topology with compensation capacitors. Based on this, Table 1 shows a comparison of the number of devices under different MMC topologies. Combined with... Figure 8 As can be seen from Table 1, compared with the traditional full-bridge submodule MMC topology, the hybrid MMC flexible DC converter structure provided in this embodiment requires only a small number of submodules, regardless of whether compensation capacitors are provided.

[0072] Table 1 Comparison of device count under different MMC topologies

[0073]

[0074] Meanwhile, by adding a compensation capacitor to the commutation topology provided in this embodiment, the number of submodules required by the system can be further reduced by 28% compared to the form without a compensation capacitor, demonstrating a significant improvement. Compared to the traditional MMC topology based on full-bridge submodules, the hybrid MMC topology with compensation capacitors provided in this embodiment reduces the number of submodules in the shared bridge arm from 4*N4 to N4, resulting in a substantial reduction in the number of submodules.

[0075] The hybrid MMC flexible DC converter structure provided in this embodiment can be applied to various flexible DC transmission systems, combining pole control, valve control, and submodule-level control to achieve high-voltage DC transmission. The pole control system generates modulation commands based on AC voltage and current. The valve control system receives and parses the modulation commands, generating switching commands for each sub-full-bridge module in the shared bridge arm. The submodule-level control system controls the switching on and off of the IGBT devices in each submodule, realizing the connection or disconnection of DC capacitors, and fitting AC voltages with different amplitudes and phase angles.

[0076] The foregoing description is merely an exemplary embodiment of this disclosure and should not be construed as limiting the scope of this disclosure. Any equivalent changes and modifications made in accordance with the teachings of this disclosure shall still fall within the scope of this disclosure. Those skilled in the art will readily conceive of embodiments of this disclosure upon considering the specification and practicing the disclosure herein. This application is intended to cover any variations, uses, or adaptations of this disclosure that follow the general principles of this disclosure and include common knowledge or customary techniques in the art not described herein. The specification and embodiments are to be considered exemplary only, and the scope and spirit of this disclosure are defined by the claims.

[0077] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0078] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A hybrid MMC flexible DC-DC converter structure based on a shared bridge arm, characterized in that, It includes a three-phase main circuit, a three-phase converter transformer, and a compensation capacitor; wherein, each phase main circuit includes an upper bridge arm DC switch group, a lower bridge arm DC switch group, an upper bridge arm AC switch group, a lower bridge arm AC switch group, and a shared bridge arm composed of multiple full-bridge sub-modules cascaded together. One end of the shared bridge arm in any phase of the main circuit is connected to the first end of the upper bridge arm DC switch group and the upper bridge arm AC switch group, respectively; the other end of the shared bridge arm is connected to the first end of the lower bridge arm DC switch group and the lower bridge arm AC switch group, respectively; the second ends of the upper bridge arm DC switch group and the lower bridge arm DC switch group serve as DC ports for connecting to the DC bus; the second ends of the upper bridge arm AC switch group and the lower bridge arm AC switch group are connected to form the AC port of each phase of the main circuit. The primary winding of the three-phase converter transformer is connected to the AC port of each phase main circuit, and the primary winding adopts a Y-type structure. One end of the compensation capacitor is connected to the neutral point of the Y-type structure of the three-phase converter transformer, and the other end is connected to the voltage midpoint of the DC bus; the capacitance value of the compensation capacitor is designed as follows: Where L1 represents the leakage inductance of the primary winding of the three-phase converter transformer, L2 represents the leakage inductance of the secondary winding of the three-phase converter transformer, and k represents the turns ratio of the three-phase converter transformer. It represents the angular frequency of alternating current voltage.

2. The hybrid MMC flexible DC-DC converter structure as described in claim 1, characterized in that, Each phase of the main circuit also includes an AC-side inductor; one end of the AC-side inductor is connected to the AC port, and the other end is connected to the corresponding phase of the primary winding of the three-phase converter transformer.

3. The hybrid MMC flexible DC-DC converter structure as described in claim 1 or 2, characterized in that, The line voltage at the AC port of each phase of the main circuit is phase-differential. The three-phase voltage.

4. The hybrid MMC flexible DC-DC converter structure as described in claim 1 or 2, characterized in that, The upper bridge arm DC switch group and the lower bridge arm DC switch group are each composed of multiple power switching transistors connected in the same direction in series.

5. The hybrid MMC flexible DC-DC converter structure as described in claim 1 or 2, characterized in that, The upper bridge arm AC switch group and the lower bridge arm AC switch group are each composed of multiple power switches connected in the same direction and multiple power switches connected in opposite directions connected in series.

6. The hybrid MMC flexible DC-DC converter structure as described in claim 4 or 5, characterized in that, The power switch is an IGBT with a reverse diode.

7. A flexible DC transmission system, characterized in that, Includes the hybrid MMC flexible DC converter structure as described in any one of claims 1 to 6.

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