A hybrid converter valve topology and control method suitable for offshore wind power direct current transmission

Abstract

This invention discloses a hybrid converter valve topology and control method suitable for DC transmission from offshore wind power, belonging to the field of new energy power generation and DC transmission technology. This topology connects a three-phase adjustable reverse voltage cascaded bridge arm in series with a three-phase diode rectifier bridge arm. This invention employs a half-cycle energy balance overmodulation control strategy, enabling the system to operate stably at unity power factor and eliminating voltage ripple on the DC side. The topology is simple in structure, requires no additional filtering stage, and significantly improves power density and semiconductor device utilization while reducing engineering implementation difficulty.

Description

Technical Field

[0001] This invention discloses a hybrid converter valve topology and control method applicable to DC transmission of offshore wind power, belonging to the technical field of power generation, transformation or distribution. Background Technology

[0002] To address the demands for lightweight, low-cost, and high-reliability converter equipment in current offshore wind power DC transmission scenarios, academia and industry have increasingly shifted their research focus in recent years to hybrid topologies combining diode uncontrolled rectifier units (DRUs) and modular multilevel converters (MMCs). This approach aims to integrate the advantages of DRUs—simple structure, high power density, and low investment cost—with the characteristics of MMCs—controllable voltage, good waveform quality, and strong fault handling capabilities—to improve system operational flexibility while ensuring efficient power transmission. In existing hybrid topology research, parallel combinations of DRUs and MMCs have become a primary research direction. Parallel DRU-MMC schemes, using small-capacity MMCs to provide voltage support and reactive power compensation for the DRUs, can improve the harmonic characteristics of the DRUs to some extent. However, parallel structures still require additional isolation or coupling elements, increasing system integration complexity. On the other hand, some research attempts to functionally reconfigure the MMC and DRU arms, reducing the proportion of fully controlled components through arm reuse or hybrid structures. For example, a topology employing an auxiliary MMC bridge arm working in conjunction with a DRU can achieve harmonic suppression and reactive power regulation under certain operating conditions. However, its energy balance mechanism is complex, often requiring additional voltage or power equalization control strategies, which limits its applicability across a wide power operating range. In summary, while existing hybrid DRU and MMC topologies combine the technical advantages of both approaches to some extent, there is still room for optimization in terms of the number of submodules, energy self-balancing capability, fault isolation mechanism, and system simplification. For applications such as offshore wind power, which have stringent requirements for the compactness, economy, and reliability of converter equipment, there is an urgent need to propose a hybrid converter valve topology with a simpler structure, more efficient control, and inherent fault isolation capability. This would enable efficient reuse of power devices and half-cycle self-balancing of energy, thereby reducing system costs while improving overall operating performance. Summary of the Invention

[0003] The technical problem to be solved by this invention is to overcome the shortcomings of the prior art and provide a hybrid converter valve topology and control method suitable for DC transmission of offshore wind power. By using a hybrid topology structure with cascaded bridge arms and diode rectifier bridge arms connected in series, the invention aims to significantly reduce the number of sub-modules while achieving energy self-balancing during half-power frequency cycle and self-isolation of DC side faults. This solves the technical problems of complex integration and complex energy balance mechanism in existing hybrid topology systems.

[0004] To achieve the above-mentioned objectives, the present invention employs the following technical solution:

[0005] A hybrid converter valve topology suitable for DC transmission of offshore wind power is provided, consisting of a j-phase cascaded bridge arm and a j-phase diode rectifier bridge arm. The j-phase cascaded bridge arm is composed of cascaded sub-modules of fully controlled devices. The positive terminal of the j-phase cascaded bridge arm is connected to the j-phase AC voltage, and the negative terminal of the j-phase cascaded bridge arm is connected to the midpoint of the j-phase diode rectifier bridge arm. The diode rectifier bridge arms of each phase are connected in parallel and then connected to the DC side. J can be a, b, or c.

[0006] As a further optimization scheme for the hybrid converter valve topology suitable for DC transmission of offshore wind power, the j-phase cascaded bridge arm is formed by cascading at least one of the following sub-modules: a full-bridge sub-module composed of fully controlled devices, a diode clamping sub-module, and a clamping dual capacitor sub-module.

[0007] As a further optimization scheme for the hybrid converter valve topology applicable to DC transmission of offshore wind power, the j-phase diode rectifier bridge arm includes: a j-phase first uncontrolled device and a j-phase second uncontrolled device. The anode of the j-phase first uncontrolled device and the cathode of the j-phase second uncontrolled device are connected as the midpoint of the j-phase diode rectifier bridge arm and connected to the negative terminal of the j-phase cascaded bridge arm. The cathode of the j-phase first uncontrolled device is connected to the positive terminal of the DC side, and the anode of the j-phase second uncontrolled device is connected to the negative terminal of the DC side.

[0008] A control method for a hybrid converter valve topology suitable for DC transmission of offshore wind power is proposed. By overmodulating the AC side voltage amplitude, the control signals of the first uncontrolled device and the second uncontrolled device of phase j are obtained when the energy fluctuation of the cascaded bridge arm of phase j is zero within half a power frequency cycle.

[0009] As a further optimization scheme for the control method of the hybrid converter valve topology applicable to DC transmission of offshore wind power, the energy fluctuation W of the j-phase cascaded bridge arm within half the power frequency cycle is... j The expression that is zero is: ,in, The energy fluctuation of the j-phase cascaded bridge arm within the positive half-power frequency cycle. The modulation voltage of the bridge arm of the j-phase cascaded submodule is... For phase j alternating current, DC side voltage The modulation ratio, , This represents the maximum value of the AC side voltage. This represents the maximum value of the AC side current. Angular frequency, for Phase.

[0010] As a further optimization of the control method for hybrid converter valve topology applicable to DC transmission from offshore wind power, when the energy fluctuation of the j-phase cascaded bridge arm is zero within half a power frequency cycle, the control signals of the first uncontrolled device and the second uncontrolled device of j-phase are: ,in, , These are the control signals for the first uncontrolled device and the second uncontrolled device in phase j.

[0011] The present invention, by adopting the above technical solution, has the following beneficial effects:

[0012] 1. The topology proposed in this invention integrates the AC valve string with the DRU by connecting the three-phase cascaded bridge arm and the three-phase diode rectifier bridge arm in series. The cascaded bridge arm and the three-phase diode rectifier bridge arm work together to shape the output voltage within the power frequency cycle, thereby achieving a wide range of voltage input and output.

[0013] 2. In the topology proposed in this invention, the cascaded bridge arms are composed of adjustable reverse pressure submodules. By blocking AC fault paths through AC side valve series and self-isolating DC fault paths through diode bridge arms, AC and DC fault isolation can be effectively and reliably achieved, thereby improving operational reliability.

[0014] 3. The control method proposed in this invention utilizes an overmodulation control strategy to achieve unity power factor operation, thereby achieving energy self-balancing in half a power frequency cycle without the need for additional control strategies. This results in more flexible operation control, no DC-side pulsation issues, and the cascaded bridge arms are reused for half a power frequency cycle during topology operation, significantly reducing the number of cascaded bridge arm sub-modules, thereby reducing the topology size and increasing the topology power density.

[0015] In summary, the hybrid converter valve topology and control method proposed in this invention reduces the complexity of system integration and energy balance mechanism through a simple topology and concise control method, and has good prospects for engineering applications. Attached Figure Description

[0016] Figure 1 This is a hybrid converter valve topology diagram for offshore wind power DC transmission provided in an embodiment of the present invention.

[0017] Figure 2 This is a specific hybrid converter valve topology diagram for offshore wind power DC transmission provided in an embodiment of the present invention.

[0018] Figure 3 This is a schematic diagram of the basic overmodulation control method in an embodiment of the present invention.

[0019] Explanation of the labels in the diagram: D j1 For the first uncontrolled device in phase j, D j2 For the second uncontrolled device in phase j, v j Let i be the phase j AC voltage.j For phase j alternating current, V jref The modulation voltage of the bridge arm of the j-phase cascaded submodule is denoted as . Detailed Implementation

[0020] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this 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 only some, not all, of the embodiments of this invention.

[0021] This invention proposes a hybrid converter valve topology suitable for DC transmission from offshore wind power, as follows: Figure 1 As shown, it includes: a phase A cascaded bridge arm, a phase B cascaded bridge arm, a phase C cascaded bridge arm, and a three-phase diode rectifier bridge arm. In the topology proposed in this application, the three-phase diode rectifier includes six groups of uncontrolled devices, and the j-phase diode rectifier bridge arm is composed of the first uncontrolled device D in phase j. j1 and the second uncontrolled device D in phase j j2 The series connection is used, where j is a, b, or c. The AC side three-phase cascaded bridge arms select adjustable reverse voltage submodules such as full-bridge submodules, diode-clamped submodules, and clamped dual-capacitor submodules. Topology principle analysis is conducted based on the above selections, and the principle analysis of other devices constructing the topology is similar. This topology structure achieves efficient reuse of power devices and energy self-balancing during the half-power frequency cycle by introducing a three-phase diode rectifier bridge arm between the three-phase cascaded bridge arms and the DC side. The name "A Hybrid Converter Valve Topology Suitable for Offshore Wind Power DC Transmission" precisely anchors the core characteristics of the topology from three aspects: application scenario, structural essence, and technical advantages. First, "suitable for offshore wind power DC transmission" clarifies its scenario positioning. Addressing the stringent requirements of offshore wind power for converter equipment to be "lightweight, low-cost, and highly reliable," this topology, through designs such as reducing the number of submodules by approximately 75% and eliminating large-capacity reactive power compensation devices, adapts to the operational requirements of limited space on offshore platforms, large-scale external transmission, and complex marine environments, distinguishing it from general-purpose converter topologies. Second, "hybrid converter valve" highlights... In essence, the core of the topology is composed of a hybrid structure of a three-phase diode rectifier bridge arm consisting of uncontrolled devices and an adjustable reverse-voltage cascaded bridge arm consisting of fully controlled devices. The diode rectifier bridge undertakes the basic functions of natural current commutation and fixing the polarity of the DC side voltage, while the cascaded bridge arm provides controllable voltage support and fault isolation capability. Finally, the "topology" has the advantages of modular voltage synthesis and strong scalability, but the introduction of the diode rectifier bridge significantly reduces the number of fully controlled devices, achieving an innovative breakthrough in device structure and control method, and accurately reflecting its technical advantages of low cost, high reliability and easy expansion in new energy DC transmission.

[0022] Figure 2 In the topology shown, the three phases are symmetrical. Taking phase A as an example, let i be the alternating current of phase a. aAs the reference phase, the AC voltage of phase a With current The expression is:

[0023] (1)

[0024] In equation (1), It is the maximum value of the AC side current. It is the maximum value of the AC side voltage. It is the power factor angle. ω is the angular frequency.

[0025] The expression for modulation intensity m is defined as follows:

[0026] (2)

[0027] In equation (2), This is the DC side voltage.

[0028] The working principle of this topology is that energy balance can be achieved in both arm cycles of the half-power frequency cycle:

[0029] (3)

[0030] When phase A is in the upper arm cycle: the phase A cascaded arm current is i, which is the phase a AC current i. a Through the first uncontrolled device D in phase a a1 When phase A is in the lower arm cycle: the cascaded bridge arm current i of phase A a Through the second uncontrolled device D in phase a a2 ;

[0031] Right now:

[0032] (4)

[0033] The modulation voltage V of the cascaded bridge arm of phase a can be obtained by using the switching device expression in equation (4) and Kirchhoff's voltage law. aref The expression is:

[0034] (5)

[0035] Depend on Figure 2 It can be known that the current in the cascaded bridge arm of phase A is the AC side current i. a Instantaneous power is the product of voltage and current, and the bridge arm energy fluctuation is the integral of instantaneous power. Given the bridge arm modulation voltage and bridge arm current from the above analysis, taking the upper bridge arm cycle of phase A as an example, the cascaded bridge arm energy fluctuation W of phase A within the upper bridge arm cycle is... a It must be zero to obtain:

[0036] (6)

[0037] In equation (6), V aref The modulation voltage of the bridge arm of the cascaded submodule in phase a. for Phase.

[0038] Solving equation (6) allows us to derive m and The relation is:

[0039] (7)

[0040] Equation (7) is defined as the energy balance equation, where the modulation index m and the power factor angle are... When the above condition is met, the energy of the cascaded bridge arms is balanced during half-cycle power frequency. From the above formula, it can be seen that the modulation index m is always greater than 1, meaning this modulation method is an overmodulation method. The same logic applies to the bridge arm cycle under phase A, and the analysis of the other two phases is also the same.

[0041] This embodiment takes phase A as an example to analyze the two modes in which phase A exists. The other two phases are similar, and the results can be obtained. Figure 3 The AC current i provided in phase A under overmodulation conditions a Phase A AC voltage v a The overmodulation voltage V of the A-phase cascaded bridge arm aarm Waveform diagram.

[0042] Derivation of the selection of the number of sub-modules in a cascaded bridge arm.

[0043] Taking phase A as an example. Since the energy of the cascaded bridge arm of phase A is balanced over half a power frequency cycle, only half a power frequency cycle needs to be analyzed. The modulation voltage V of the cascaded bridge arm of phase A. aref for:

[0044] (8)

[0045] There is also phase A AC voltage v a :

[0046] (9)

[0047] Where V m =mV dc / 2, the number of sub-modules n in a single bridge arm of a traditional MMC can be expressed as:

[0048] (10)

[0049] Therefore, the number of sub-modules required for the cascaded bridge arm is n. c This can be used to determine the answer.

[0050] (11)

[0051] Considering that renewable energy transmission primarily involves pure active power output, i.e., unity power factor, substituting into the above formula reveals that only 0.5n of the cascaded bridge arm submodules are required. The derivation based on the number of cascaded bridge arm submodules demonstrates that this invention solves the problems of large number of traditional MMC submodules, large converter station footprint, and low power density, while also leveraging the advantages of the DRU solution—small size and low cost. It ensures large-scale DC power transmission capability while improving the lightweight nature of converter equipment and optimizing fault isolation and operational reliability. Thus, it adapts to the stringent requirements of large-scale renewable energy bases such as offshore wind power for converter equipment while reducing the number of semiconductor devices.

[0052] The embodiments of the present invention have been described in detail above with reference to the accompanying drawings. However, the present invention is not limited to the above embodiments. Within the scope of knowledge possessed by those skilled in the art, various changes can be made without departing from the spirit of the present invention.

Claims

1. A hybrid converter valve topology suitable for DC transmission from offshore wind power, characterized in that, It consists of a j-phase cascaded bridge arm and a j-phase diode rectifier bridge arm. The j-phase cascaded bridge arm is formed by cascading sub-modules composed of fully controlled devices. The positive terminal of the j-phase cascaded bridge arm is connected to the j-phase AC voltage, and the negative terminal of the j-phase cascaded bridge arm is connected to the midpoint of the j-phase diode rectifier bridge arm. The diode rectifier bridge arms of each phase are connected in parallel and then connected to the DC side. j is a, b, or c.

2. The hybrid converter valve topology suitable for DC transmission from offshore wind power as described in claim 1, characterized in that, The j-phase cascaded bridge arm is formed by cascading at least one of the following sub-modules: a full-bridge sub-module composed of fully controlled devices, a diode clamping sub-module, and a clamping dual-capacitor sub-module.

3. The hybrid converter valve topology suitable for DC transmission from offshore wind power as described in claim 2, characterized in that, The j-phase diode rectifier bridge arm includes a j-phase first uncontrolled device and a j-phase second uncontrolled device. The anode of the j-phase first uncontrolled device and the cathode of the j-phase second uncontrolled device are connected together as the midpoint of the j-phase diode rectifier bridge arm and connected to the negative terminal of the j-phase cascaded bridge arm. The cathode of the j-phase first uncontrolled device is connected to the positive terminal of the DC side, and the anode of the j-phase second uncontrolled device is connected to the negative terminal of the DC side.

4. The control method for a hybrid converter valve topology suitable for offshore wind power DC transmission as described in claim 3, characterized in that, By overmodulating the AC side voltage amplitude, the control signals of the first uncontrolled device and the second uncontrolled device of phase j are obtained when the energy fluctuation of the cascaded bridge arm of phase j is zero within half a power frequency cycle.

5. The control method for a hybrid converter valve topology suitable for offshore wind power DC transmission according to claim 4, characterized in that, The energy fluctuation W of the j-phase cascaded bridge arm within half a power frequency cycle j The expression that is zero is: ,in, The energy fluctuation of the j-phase cascaded bridge arm within the positive half-power frequency cycle. The modulation voltage of the bridge arm of the j-phase cascaded submodule is... For phase j alternating current, DC side voltage The modulation ratio, , This represents the maximum value of the AC side voltage. This represents the maximum value of the AC side current. Angular frequency, for Phase.

6. The control method for a hybrid converter valve topology suitable for offshore wind power DC transmission according to claim 5, characterized in that, When the energy fluctuation of the j-phase cascaded bridge arm is zero within the half-power frequency cycle, the control signals of the first uncontrolled device and the second uncontrolled device of j-phase are: ,in, , These are the control signals for the first uncontrolled device and the second uncontrolled device in phase j.

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