A unidirectional isolation type energy router and control strategy suitable for an offshore AC-DC hybrid wind power system

Abstract

A one-way isolation type energy router suitable for offshore AC / DC hybrid wind power system and a control strategy, belong to offshore wind power transmission technical field, the present application is to solve the problem of high insulation stress, large MMC capacity demand and poor system economy caused by lack of electrical isolation in existing offshore wind power AC / DC hybrid system. The present application comprises: small capacity modular multilevel converter MMC and full capacity diode rectifier unit DRU, the AC side of the two is connected face to face through an isolation transformer; MMC is provided with a medium voltage DC port and a medium voltage AC port, which is connected to the DC and AC wind turbine cluster respectively; DRU is provided with a high voltage DC port for external high voltage DC transmission line. The AC / DC cascade network control strategy includes outer ring DC network control and inner layer AC network control, the medium voltage AC port voltage is adjusted according to the medium voltage DC voltage deviation, the diode rectifier unit output power is changed to match the wind farm power fluctuation, and the stable control of medium voltage DC voltage is realized.

Description

Technical Field

[0001] This invention belongs to the field of offshore wind power transmission technology, specifically relating to a unidirectional isolated energy router suitable for offshore AC / DC hybrid wind power systems and its AC / DC cascaded network control strategy. Background Technology

[0002] Offshore wind power has become an important development direction for global energy transition due to its abundant resources, high utilization hours, and proximity to load centers. Currently, high-voltage direct current (HVDC) transmission technology based on modular multilevel converters (MMCs) is widely used for grid connection and transmission from offshore wind farms due to its modular structure, ease of expansion to high-voltage levels, and excellent harmonic performance. However, offshore MMC converter stations suffer from excessive size and weight, leading to high construction costs for offshore platforms.

[0003] To improve the economics of offshore wind power transmission systems, HVDC transmission systems based on diode rectifier units (DRUs) have been proposed to reduce the cost of offshore converter stations. Compared to MMC-based solutions, the DRU approach can significantly reduce the size of offshore platforms and substantially lower overall costs. However, due to the passive and uncontrollable nature of DRUs, they cannot establish AC voltage for offshore wind farms. To address this issue, existing technologies have proposed control strategies for wind turbines to autonomously establish offshore AC voltage, but this requires specially designed grid-connected wind turbines and has poor compatibility with existing mature grid-connected wind turbines.

[0004] To ensure compatibility with mature grid-connected wind turbines, the hybrid MMC-DRU topology has been extensively studied. For example, existing schemes propose a marine converter station consisting of a large-capacity DRU and a small-capacity MMC connected in parallel. The DRU handles most of the power transmission, while the MMC establishes the offshore AC voltage. However, this MMC still needs to withstand the entire HVDC voltage, resulting in a relatively high capacity requirement. Another approach involves connecting the DRU and MMC in series on the DC side and in parallel on the AC side. This structure reduces the capacity of the auxiliary MMC but requires a full-bridge (FB) submodule-based MMC to achieve variable HVDC voltage operation, inevitably increasing losses and costs at the onshore converter station.

[0005] As the aggregation range and installed capacity of offshore wind farms continue to expand, the use of medium-voltage AC (MVAC) technology for wind power aggregation has gradually revealed numerous problems, including high power loss, high cable costs, and limitations in aggregation range and capacity due to the large amount of reactive power consumed by AC cables. In contrast, the all-DC technology using medium-voltage DC (MVDC) for wind power aggregation can solve the reactive power problem, significantly expand the aggregation range, and reduce losses. Existing technologies include connecting DC wind turbines in series to directly boost the voltage to HVDC levels, thus avoiding the construction of offshore converter stations. However, this subjectes the wind turbines to extremely high insulation stress and generates severe overvoltages when wind speeds are uneven. The auxiliary circuits introduced to address voltage imbalance further increase hardware costs and conduction losses. Other solutions, such as integrating energy storage devices into the wind turbines or using new DC / DC converters, respectively present problems such as increased operation and maintenance difficulty, complex control, and high component costs.

[0006] To integrate the advantages of both AC and DC wind turbines, hybrid offshore wind power systems have been proposed. For example, multiple multi-phase control units (MMCs) are connected in series on the DC side to boost the voltage to HVDC levels. AC wind turbines are connected to the AC terminals of the MMCs, while DC wind turbines are connected in series to the DC terminals of the intermediate MMC. However, this series-connected MMC still needs to be designed for full system capacity. Further improvements use DRUs (Dynamic Units) to replace the high-voltage side MMCs and combine them with onshore hybrid MMCs to achieve black start. However, this topology still lacks electrical isolation between the MVDC aggregation network and the HVDC transmission lines, resulting in high insulation stress on the MVDC system and DC wind turbines. Furthermore, when the output power of the series-connected DC wind turbines is unbalanced, the high-power units may experience overvoltage problems.

[0007] In summary, existing offshore wind power collection and transmission technologies still require further improvement in terms of system economy, electrical isolation, compatibility with AC and DC wind turbines, and collection network losses. In particular, there is a lack of a topology and control method that can achieve electrical isolation between high and low voltage sides while taking into account the advantages of both AC and DC wind turbines, reducing offshore platform costs, and providing stable voltage support. Summary of the Invention

[0008] To address the problems of high insulation stress, large MMC capacity requirements, and poor system economy caused by the lack of electrical isolation in existing offshore wind power AC / DC hybrid systems, this invention provides a unidirectional isolated energy router and control strategy suitable for offshore AC / DC hybrid wind power systems.

[0009] In a first aspect, the present invention provides a unidirectional isolated energy router suitable for offshore AC / DC hybrid wind power systems, comprising:

[0010] Small-capacity modular multilevel converter and full-capacity diode rectifier unit;

[0011] The AC side of the small-capacity modular multilevel converter is connected to the AC side of the full-capacity diode rectifier unit through an isolation transformer to achieve electrical isolation between the high-voltage side and the medium-voltage side of the equipment.

[0012] The small-capacity modular multilevel converter is equipped with a medium-voltage DC port and a medium-voltage AC port. The medium-voltage DC port is connected to a DC wind turbine cluster, and the medium-voltage AC port is connected to an AC wind turbine cluster.

[0013] The full-capacity diode rectifier unit is equipped with a high-voltage DC port, which is used to connect to an external high-voltage DC transmission line.

[0014] Preferably, the full-capacity diode rectifier unit is composed of two six-pulse diode rectifiers forming a twelve-pulse rectifier structure.

[0015] Preferably, the small-capacity modular multilevel converter adopts a half-bridge sub-module.

[0016] Preferably, an AC filter is installed at the medium-voltage AC port to suppress harmonics and compensate for reactive power.

[0017] Preferably, the rated capacity of the small-capacity modular multilevel converter matches the total rated power of the DC wind turbine cluster; the rated capacity of the full-capacity diode rectifier unit matches the total rated power of the AC wind turbine cluster and the DC wind turbine cluster.

[0018] Secondly, the AC / DC cascaded network control strategy of a unidirectional isolated energy router according to the present invention includes an outer ring DC network control link and an inner layer AC network control link.

[0019] The outer loop DC grid control link collects the measured voltage of the medium-voltage DC port, calculates the voltage deviation in combination with the voltage reference value, and generates the AC voltage adjustment amount based on the voltage deviation.

[0020] The inner layer AC network control circuit corrects the voltage reference value of the medium-voltage AC port according to the AC voltage adjustment amount and maintains a stable AC voltage output from the medium-voltage AC port.

[0021] When an increase in the medium-voltage DC port voltage is detected, the inner layer AC network control circuit raises the medium-voltage AC port voltage, increasing the output power of the full-capacity diode rectifier unit; when a decrease in the medium-voltage DC port voltage is detected, the inner layer AC network control circuit lowers the medium-voltage AC port voltage, reducing the output power of the full-capacity diode rectifier unit.

[0022] Preferably, the outer loop DC grid control link inputs the medium-voltage DC voltage deviation signal to the proportional-integral controller, and the proportional-integral controller outputs the AC voltage adjustment amount;

[0023] The inner-layer AC network control loop superimposes the rated AC voltage reference value with the AC voltage adjustment amount to obtain the corrected d-axis voltage reference value, while setting the q-axis voltage reference value to zero and generating the AC voltage reference phase angle based on the system's rated frequency integration.

[0024] Preferably, the parameters of the integral controller in the outer loop DC grid control loop are tuned as follows: the equivalent gain of the system is obtained by combining the power-voltage coupling characteristics of the full-capacity diode rectifier unit, the open-loop transfer function of the system is constructed by combining the equivalent capacitance of the DC side of the small-capacity modular multilevel converter and the rated voltage of the medium-voltage DC, and the closed-loop transfer function is obtained from the open-loop transfer function of the system. The natural frequency and damping ratio of the control system are obtained by solving the characteristic equation of the closed-loop transfer function, and the system damping ratio is tuned to 0.7.

[0025] Preferably, under normal power transmission conditions, the voltage operating range of the medium-voltage AC port is limited to the conduction threshold voltage of the full-capacity diode rectifier unit to the rated AC voltage of the system; the conduction threshold voltage is determined by the rated voltage of the high-voltage DC transmission line and the turns ratio of the isolation transformer at the diode rectifier unit interface; when the voltage of the medium-voltage AC port is lower than the conduction threshold voltage, the full-capacity diode rectifier unit is in reverse blocking state.

[0026] Preferably, when the overall output power of the offshore hybrid wind farm is zero, the control strategy adjusts the medium-voltage AC port voltage to no higher than the conduction threshold voltage, so that the full-capacity diode rectifier unit remains in reverse blocking.

[0027] The beneficial effects of this invention: The Unidirectional Isolated Energy Router (UIEH) and its cascaded network control strategy proposed in this invention address the core contradictions of lack of electrical isolation and poor system economy in existing technologies, starting from the inherent mechanism of power transmission and voltage support. By connecting a small-capacity MMC and a full-capacity DRU face-to-face through an isolation transformer, the UIEH achieves electrical isolation between the MVDC aggregation network and the HVDC transmission line in its physical structure. Based on this structure, this invention further utilizes the inherent voltage-power nonlinear coupling characteristics of the DRU to design an AC / DC cascaded network control—dynamically mapping the MVDC voltage deviation to the adjustment amount of the MVAC port voltage. When the wind farm power fluctuates, this control strategy does not rely on fast communication; it can change the DC output voltage and commutation voltage drop of the DRU simply by adaptively raising or lowering the MVAC voltage, thereby precisely controlling its transmitted power, automatically balancing the instantaneous power of the MVDC network, and stabilizing the MVDC voltage near its rated value. This mechanism achieves multiple beneficial effects in principle: First, electrical isolation avoids the insulation stress of HVDC voltage levels on the DC collection side, allowing the use of lower MVDC voltage levels and significantly reducing the manufacturing costs of DC wind turbines and submarine cables. Second, since the MMC only needs to handle the power of the DC wind turbine (rather than the total system power), its capacity requirement is greatly reduced. Combined with the low-cost, full-capacity DRU handling the main power transmission, the economics of the offshore converter station are improved. Third, this method simultaneously establishes a stable AC voltage for the MVAC port, allowing cost-effective AC wind turbines to be connected at the near end, while DC collection is used at the far end to reduce line losses, thus achieving complementary advantages and synergistic aggregation of AC and DC wind turbines on the same platform. Finally, during dynamic processes, the MVAC voltage experiences only minor and controllable fluctuations, ensuring the system has excellent damping characteristics and disturbance rejection capabilities. In summary, this invention, starting from the synergistic innovation of topology and control principles, systematically solves the comprehensive technical challenges of hybrid offshore wind power systems in terms of insulation safety, equipment cost, operational flexibility, and dynamic stability. Attached Figure Description

[0028] Figure 1 This is a topology diagram of an AC / DC hybrid offshore wind power system based on a unidirectional isolated energy router provided in an embodiment of the present invention.

[0029] Figure 2 This is a control block diagram of the AC / DC cascaded network control strategy provided in an embodiment of the present invention;

[0030] Figure 3 Equivalent circuit diagram of an AC / DC hybrid offshore wind power system based on a unidirectional isolated energy router provided in an embodiment of the present invention;

[0031] Figure 4Bode plot of the closed-loop transfer function of DC network control in the AC / DC cascaded network control strategy provided in this embodiment of the invention, wherein Figure 4 (a) is the amplitude-frequency response curve. Figure 4 (b) is the phase frequency response curve;

[0032] Figure 5 The simulation waveform diagram under rated operating conditions provided in the embodiments of the present invention, wherein Figure 5 (a) is a waveform diagram of the output power of AC wind turbine, DC wind turbine and unidirectional isolated energy router. Figure 5 (b) is a waveform diagram of the three-phase AC voltage at the medium-voltage AC port. Figure 5 (c) is a waveform diagram of the effective value of the voltage at the medium-voltage AC port and the valve side of the diode rectifier unit. Figure 5 (d) shows the voltage waveforms of the medium-voltage DC port and the high-voltage DC port of a unidirectional isolated energy router. Figure 5 (e) is a waveform diagram of the average capacitor voltage of the modular multilevel converter submodule in a unidirectional isolated energy router;

[0033] Figure 6 This is a simulation waveform diagram of wind power fluctuation conditions provided in an embodiment of the present invention, wherein... Figure 6 (a) is a waveform diagram of the output power of AC and DC wind turbine clusters. Figure 6 (b) is a waveform diagram of the power transmission between the modular multilevel converter and the diode rectifier unit in a unidirectional isolated energy router. Figure 6 (c) is a waveform diagram of the medium-voltage DC port voltage. Figure 6 (d) is a waveform diagram of the effective value of the medium-voltage AC port voltage. Figure 6 (e) is a waveform diagram of high voltage DC voltage. Figure 6 (f) is a waveform diagram of the bridge arm current of the modular multilevel converter in a unidirectional isolated energy router. Figure 6 (g) is a waveform diagram of the average capacitor voltage of the modular multilevel converter submodule in a unidirectional isolated energy router;

[0034] Figure 7 This is a simulation waveform diagram of an AC wind turbine under zero-output operating conditions provided in an embodiment of the present invention, wherein... Figure 7 (a) is a waveform diagram of the active power output of the AC and DC wind turbine clusters. Figure 7 (b) is a waveform diagram of the total output power of a unidirectional isolated energy router. Figure 7 (c) is a waveform diagram of the effective value of the medium-voltage AC port voltage. Figure 7 (d) is a waveform diagram of medium-voltage DC voltage;

[0035] Figure 8 This is a simulation waveform diagram of a DC wind turbine under zero-output operating conditions provided in an embodiment of the present invention, wherein... Figure 8(a) is a waveform diagram of the active power output of the AC and DC wind turbine clusters. Figure 8 (b) is a waveform diagram of the power transmission between the modular multilevel converter and the diode rectifier unit in a unidirectional isolated energy router. Figure 8 (c) is a waveform diagram of the effective value of the medium-voltage AC port voltage. Figure 8 (d) is a waveform diagram of medium-voltage DC voltage. Detailed Implementation

[0036] To make the above-mentioned objects, features, and advantages of the present invention more apparent and understandable, specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings. Although some embodiments of the present invention are shown in the drawings, it should be understood that the present invention can be implemented in various forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided to provide a more thorough and complete understanding of the present invention. It should be understood that the accompanying drawings and embodiments of the present invention are for illustrative purposes only and are not intended to limit the scope of protection of the present invention.

[0037] It should be understood that the various steps described in the method embodiments of the present invention may be performed in different orders and / or in parallel. Furthermore, the method embodiments may include additional steps and / or omit the steps shown. The scope of the present invention is not limited in this respect.

[0038] The term "comprising" and its variations as used herein are open-ended, meaning "including but not limited to"; the term "based on" means "at least partially based on"; and the term "one embodiment" means "at least one embodiment". Definitions of other terms will be given in the following description. It should be noted that the concepts of "first," "second," etc., mentioned in this invention are used only to distinguish different devices, modules, or units, and are not intended to limit the order of functions performed by these devices, modules, or units or their interdependencies.

[0039] It should be noted that the terms "a" and "a plurality of" used in this invention are illustrative rather than restrictive. Those skilled in the art should understand that, unless otherwise expressly indicated in the context, they should be understood as "one or more".

[0040] The names of the messages or information exchanged between the multiple devices in the embodiments of the present invention are for illustrative purposes only and are not intended to limit the scope of these messages or information.

[0041] In the field of offshore wind power transmission technology, offshore wind power collection and transmission systems have significant technical and economic shortcomings when dealing with large-scale deep-sea wind power development. Existing solutions are mainly divided into two categories: one is a high-voltage direct current (HVDC) transmission system based on modular multilevel converters, and the other is a high-voltage direct current (HVDC) transmission system based on diode rectifier units.

[0042] MMC-based HVDC transmission systems are widely used in grid connection of offshore wind farms due to their modular structure, ease of expansion to high-voltage levels, and excellent harmonic performance. However, offshore MMC converter stations face the problem of excessive size and weight, resulting in high construction costs for offshore platforms. In principle, the MMC needs to withstand the entire HVDC voltage, and each arm consists of numerous sub-modules connected in series. The sub-module capacitor banks account for the majority of the converter station's volume and weight. When the HVDC voltage level increases to ±320kV or even ±500kV, the number of MMC sub-modules increases exponentially, leading to a sharp increase in the load-bearing capacity and floor space of the offshore platform, further driving up foundation construction and installation costs. Furthermore, the MMC requires maintaining balanced voltage control of the sub-module capacitors during operation, making the control system complex and further increasing the difficulty of operation and maintenance.

[0043] A DRU-based HVDC transmission system has been proposed to reduce the cost of offshore converter stations. Compared to MMC-based solutions, the DRU approach can reduce the size of the offshore platform by approximately 80% and lower the overall cost by about 30%. However, DRUs are composed of uncontrollable diodes, and their conduction depends on the amplitude of the AC side voltage, making it impossible to actively establish AC voltage and frequency for offshore wind farms. This means that if DRUs are directly used as offshore converter stations, offshore wind turbines need to establish their own grid-connected voltage, requiring specially designed grid-connected wind turbines. Conventional grid-connected wind turbines rely on the external power grid for voltage reference and cannot be directly used in the DRU approach. This limitation restricts the compatibility of the DRU approach with mature wind turbines and increases the cost of retrofitting wind turbines.

[0044] To ensure compatibility with mature grid-connected wind turbines, the hybrid MMC-DRU topology has been extensively studied. Existing technologies propose offshore converter station structures consisting of a full-capacity DRU and a small-capacity MMC connected in parallel. The DRU handles most of the power transmission, while the MMC establishes the voltage and frequency of the offshore AC system. However, this MMC still needs to withstand the entire HVDC voltage, and its insulation design and the number of submodules are not reduced despite the smaller capacity, resulting in a still high MMC size and cost. Another approach is to connect the DRU and MMC in series on the DC side and in parallel on the AC side. This structure utilizes the MMC to construct the AC voltage while reducing the capacity requirements of the auxiliary MMC. However, this series topology requires the use of MMCs based on full-bridge submodules to achieve variable HVDC voltage operation. Compared to half-bridge submodules, full-bridge submodules require more switching devices and have higher conduction losses, inevitably increasing the losses and cost of the onshore converter station.

[0045] As the aggregation range and installed capacity of offshore wind farms continue to expand, the selection of the power collection network has become a key factor affecting the system's economics. The use of medium-voltage AC technology for wind power aggregation has gradually revealed numerous problems, including significant power losses, high cable costs, and limitations in aggregation range and capacity due to the large amount of reactive power consumed by AC cables. AC submarine cables charge and discharge distributed capacitance along the line while transmitting power, generating reactive power. As cable length increases, reactive power also increases, leading to higher voltage at the end, increased line losses, and limiting the effective transmission distance. In contrast, all-DC technology using medium-voltage DC for wind power aggregation can solve the reactive power problem, significantly expand the aggregation range, and reduce system losses and cable costs.

[0046] For all-DC convergence schemes, existing technologies propose connecting DC wind turbines in series to directly boost voltage to HVDC levels, thus avoiding the construction of expensive offshore converter stations. However, this structure subjectes the wind turbines to extremely high insulation stress and generates severe overvoltage under uneven wind speed conditions. In a series structure, each wind turbine has the same output current, but its output power is determined by its respective wind speed. When wind speeds are inconsistent, the output voltage of the low-wind-speed turbines decreases, while the output voltage of the high-wind-speed turbines is forced to increase to maintain voltage balance in the series circuit, leading to overvoltage risks for the high-wind-speed turbines. To address this issue, existing technologies have designed novel voltage balancing circuits to suppress voltage imbalance in series-connected DC wind turbines under uneven wind conditions. However, these auxiliary circuits require extensive interconnection with submarine cables, significantly increasing hardware costs and conduction losses. Another approach proposes a series-parallel all-DC offshore wind farm structure, integrating energy storage devices in the wind turbine nacelles to cope with output voltage fluctuations. However, the introduction of the energy storage system significantly increases the nacelle volume and mechanical load, thereby increasing the difficulty and cost of subsequent operation and maintenance. Another approach uses a parallel all-DC topology and proposes a novel machine-side DC / DC converter. Although this converter has a high boost ratio and flexible control capabilities, its switching coordination is complex and the components are expensive.

[0047] To integrate the advantages of both AC and DC wind turbines, existing technologies propose hybrid offshore wind power systems. In this approach, three converter generators (MMCs) are connected in series on the DC side to boost the voltage to HVDC levels for transmission; AC wind turbines are connected to the AC terminals of the MMCs, while DC wind turbines are connected in series to the DC terminals of the intermediate MMC. However, this series-connected MMC still requires design for the full system capacity, resulting in high converter station costs. Further improvements employ DRUs (Dynamic Unit Units) to replace the high-voltage side MMCs and combine them with onshore hybrid MMCs to achieve step-down HVDC operation under black-start conditions. However, all of these hybrid topologies suffer from a lack of electrical isolation between the MVDC aggregation network and the HVDC transmission lines, subjecting the MVDC system and DC wind turbines to high insulation stress. This lack of electrical isolation means the MVDC system's potential is directly coupled to the HVDC potential, requiring the insulation of the MVDC cables and DC wind turbines to be designed according to HVDC voltage levels, significantly increasing equipment costs. Furthermore, when the output power of the series-connected DC wind turbines is unbalanced, the high-power DC wind turbines may experience severe overvoltage problems, further complicating insulation design.

[0048] In summary, existing offshore wind power collection and transmission technologies suffer from comprehensive technical deficiencies in terms of system economy, electrical isolation, compatibility with AC and DC wind turbines, and collection network losses. Specifically, how to achieve electrical isolation between the medium-voltage collection network and high-voltage transmission lines to reduce DC-side insulation costs, while simultaneously leveraging the advantages of both AC and DC wind turbines, reducing the capacity requirements of offshore converter stations, and providing stable voltage support, is a pressing technical problem that needs to be solved in this field.

[0049] To address the aforementioned problems, this embodiment provides a unidirectional isolated energy router suitable for offshore AC / DC hybrid wind power systems and its AC / DC cascaded network control strategy.

[0050] Combination Figure 1 As shown, this embodiment of the invention provides a unidirectional isolated energy router suitable for offshore AC / DC hybrid wind power systems, comprising:

[0051] Small-capacity modular multilevel converter and full-capacity diode rectifier unit;

[0052] The AC side of the small-capacity modular multilevel converter is connected to the AC side of the full-capacity diode rectifier unit through an isolation transformer to achieve electrical isolation between the high-voltage side and the medium-voltage side of the equipment.

[0053] The small-capacity modular multilevel converter is equipped with a medium-voltage DC port and a medium-voltage AC port. The medium-voltage DC port is connected to a DC wind turbine cluster, and the medium-voltage AC port is connected to an AC wind turbine cluster.

[0054] The full-capacity diode rectifier unit is equipped with a high-voltage DC port, which is used to connect to an external high-voltage DC transmission line.

[0055] Specifically, this embodiment addresses the core contradiction of lack of electrical isolation and high cost of converter stations in existing technologies from a topology perspective. By connecting the small-capacity MMC and the full-capacity DRU face-to-face via an isolation transformer, electrical isolation between the MVDC aggregation network and the HVDC transmission line is achieved in terms of physical structure. The small-capacity MMC undertakes the dual task of establishing both MVAC and MVDC voltages, while the full-capacity DRU handles the main power transmission. The introduction of the isolation transformer completely decouples the MVDC and HVDC sides electrically. The insulation design of the MVDC cables and DC wind turbines only needs to be based on the MVDC voltage level, without needing to withstand HVDC voltage stress, thus significantly reducing insulation costs. Simultaneously, the small-capacity MMC only needs to handle the power generated by the DC wind turbines (not the total system power), significantly reducing its capacity requirements. Compared to typical all-DC offshore wind power systems where the MMC needs to be configured according to the full power of the entire wind farm, this effectively improves the economics of the offshore converter station. This embodiment of the UIEH achieves unified aggregation of AC and DC wind energy, and transmits the aggregated energy to the HVDC transmission line via a low-cost DRU. The onshore converter station is responsible for maintaining the stability of the HVDC voltage. AC wind turbines close to the offshore converter station are connected to the MVAC port of the UIEH via AC submarine cables, avoiding reactive power problems caused by long-distance AC aggregation; wind turbines at greater distances are connected to the MVDC port using DC technology, reducing aggregation line losses. Through the above topology, this embodiment solves the two major technical problems of electrical isolation and converter station capacity requirements at the physical level.

[0056] Furthermore, the full-capacity diode rectifier unit is composed of two six-pulse diode rectifiers forming a twelve-pulse rectifier structure.

[0057] Specifically, this embodiment uses a twelve-pulse rectifier structure to replace a single six-pulse rectifier. The six-pulse rectifier generates characteristic subharmonics (mainly 6k±1), causing harmonic pollution to the power grid. Two six-pulse diode rectifiers are connected to the AC side via phase-shift transformers, forming a 30° phase difference to combine and constitute a twelve-pulse rectifier structure. The characteristic harmonics they generate cancel each other out, significantly reducing grid-side current harmonics, with the lowest characteristic harmonic becoming 12k±1. This design significantly reduces the AC filter capacity requirement, lowers filtering costs, and improves power quality, enabling the UIEH to meet more stringent grid-connected harmonic requirements.

[0058] Furthermore, the small-capacity modular multilevel converter adopts a half-bridge sub-module.

[0059] Specifically, this embodiment uses half-bridge submodules to construct the MMC. Compared to full-bridge submodules, half-bridge submodules have fewer switching devices (each submodule reduces two IGBTs and two diodes), significantly reducing both conduction and switching losses. In UIEH, the MMC only needs to implement voltage support and power regulation functions, without requiring DC fault ride-through capability (fault ride-through is handled by the onshore converter station or DC circuit breaker). Therefore, using half-bridge submodules maximizes economic efficiency while meeting functional requirements. In this embodiment, the MMC has 40 submodules per arm, with a submodule capacitor voltage of 2.5kV, which meets the operating requirements of medium-voltage DC 100kV.

[0060] Furthermore, an AC filter is installed at the medium-voltage AC port to suppress harmonics and compensate for reactive power.

[0061] Specifically, in this embodiment, an AC filter is configured at the MVAC port. During rectification, the DRU generates characteristic subharmonics that are injected into the AC network, while the distributed capacitance of the AC submarine cable generates reactive power during charging. The AC filter provides a low-impedance path for the characteristic subharmonics, preventing harmonic pollution from affecting the normal operation of the AC wind turbine; it also provides reactive power compensation to maintain voltage stability at the MVAC port. By rationally designing the filter parameters, this embodiment controls the total harmonic distortion (THD) of the MVAC port voltage to within 5%, meeting the grid connection requirements of the wind turbine.

[0062] Furthermore, the rated capacity of the small-capacity modular multilevel converter matches the total rated power of the DC wind turbine cluster; the rated capacity of the full-capacity diode rectifier unit matches the total rated power of the AC wind turbine cluster and the DC wind turbine cluster.

[0063] Specifically, the capacity configuration principle in this embodiment is based on the power allocation mechanism: in the UIEH, the MMC only transmits the power generated by the DC wind turbine cluster, while the DRU transmits all AC and DC power. Therefore, the rated capacity of the MMC is configured according to the total rated power on the DC side, and the rated capacity of the DRU is configured according to the total rated power on both the AC and DC sides. Taking the simulation parameters of this embodiment as an example, the AC wind turbine cluster capacity is 1.5GW, and the DC wind turbine cluster capacity is 0.5GW, then the MMC capacity is configured as 0.5GW, and the DRU capacity is configured as 2.0GW. Compared with the existing technology where the MMC needs to be configured to full capacity, this embodiment reduces the MMC capacity requirement by 75%, significantly improving the economics of the offshore converter station.

[0064] Combination Figure 2 As shown, this embodiment of the invention also provides an AC / DC cascaded network control strategy for a unidirectional isolated energy router, including an outer-loop DC network control link and an inner-loop AC network control link;

[0065] The outer loop DC grid control link collects the measured voltage of the medium-voltage DC port, calculates the voltage deviation in combination with the voltage reference value, and generates the AC voltage adjustment amount based on the voltage deviation.

[0066] The inner layer AC network control circuit corrects the voltage reference value of the medium-voltage AC port according to the AC voltage adjustment amount and maintains a stable AC voltage output from the medium-voltage AC port.

[0067] When an increase in the medium-voltage DC port voltage is detected, the inner layer AC network control circuit raises the medium-voltage AC port voltage, increasing the output power of the full-capacity diode rectifier unit; when a decrease in the medium-voltage DC port voltage is detected, the inner layer AC network control circuit lowers the medium-voltage AC port voltage, reducing the output power of the full-capacity diode rectifier unit.

[0068] Specifically, this embodiment addresses the problems of voltage instability caused by power fluctuations and difficulties in the coordinated aggregation of multi-source energy in existing technologies from the perspective of control mechanism. Utilizing the inherent voltage-power nonlinear coupling characteristics of the DRU, an AC / DC cascaded network control is designed—dynamically mapping the MVDC voltage deviation to an adjustment amount of the MVAC port voltage. When wind farm power fluctuates, this control strategy does not rely on fast communication; it can change the DC output voltage and commutation voltage drop of the DRU simply by adaptively raising or lowering the MVAC voltage, thereby precisely regulating its transmitted power and automatically balancing the instantaneous power of the MVDC network. Specifically, when the output power of the AC wind turbine suddenly increases, in the initial stage of the power step change, the MVAC port voltage of the UIEH remains unchanged, the DRU output power remains essentially unchanged, and the resulting surplus power is injected into the MVDC network through the MMC, causing the MVDC voltage to rise. To suppress the rise in MVDC voltage, the outer-loop DC network control inputs the MVDC voltage deviation into the PI controller to generate an AC voltage adjustment amount; the inner-loop AC network control superimposes the rated AC voltage reference value with this adjustment amount, moderately raising the MVAC voltage. Based on the voltage-power coupling characteristics of the DRU, an increase in the MVAC voltage will raise the DC-side voltage of the DRU, thereby increasing the DRU output power until it is rebalanced with the wind power output power, and the MVDC voltage returns to near its rated value. Conversely, when the total output power decreases, the control strategy lowers the MVAC port voltage, reducing the DRU output power, thus achieving power balance. Through the above control logic, this embodiment achieves adaptive and stable control of the MVDC voltage, ensuring that the MVAC voltage fluctuates only slightly and remains within the design-allowed operating range.

[0069] In a preferred embodiment, combined with Figure 2As shown, the outer loop DC grid control inputs the deviation signal of the medium-voltage DC voltage into the proportional-integral controller, and its output is used as the AC voltage adjustment amount. After being superimposed with the rated AC reference value, it forms the d-axis voltage reference value; the q-axis voltage reference value is set to zero so that the MVAC port voltage vector is aligned with the d-axis; the reference phase angle is obtained by integrating the rated frequency.

[0070] The outer loop DC grid control inputs the deviation signal of the medium-voltage DC voltage into the proportional-integral controller, and the output of the proportional-integral controller is used as the AC voltage adjustment amount; the inner layer AC grid control superimposes the rated AC voltage reference value with the AC voltage adjustment amount to form a corrected d-axis voltage reference value, and sets the q-axis voltage reference value to zero, while obtaining the reference phase angle based on the integration of the rated frequency.

[0071] Specifically, the cascaded control structure in this embodiment implements closed-loop correction of the voltage reference value. The formula for calculating the d-axis voltage reference value is:

[0072] (1)

[0073] in, This is the corrected d-axis voltage reference value. This is the reference value for rated AC voltage. For the Laplace operator, and These are the proportional coefficient and integral coefficient of the proportional-integral controller, respectively. and These represent the reference and measured values ​​of the medium-voltage DC voltage, respectively. This formula maps the MVDC voltage deviation to an adjustment amount for the AC voltage d-axis reference value via a PI controller, achieving cascaded control of the DC voltage on the AC voltage. (q-axis voltage reference value) Set to zero, and adjust the q-axis voltage via a PI controller. Align the MVAC port voltage vector with the d-axis; reference phase angle From the rated frequency =Integrated at 50Hz, ensuring the frequency stability of the AC voltage.

[0074] The AC grid connection control not only establishes AC voltage for the AC wind turbines but also provides AC-side voltage support for the internal DRUs of the UIEH. Under rated operating conditions, when both offshore AC and DC wind turbines are running at full power, the adjustment amount of the outer-loop DC grid connection control output... The voltage is zero; at this time, UIEH maintains the offshore AC voltage at its rated value. When wind power fluctuates, the outer-loop DC grid control will dynamically adjust the voltage reference value of the AC grid control. Therefore, the MVAC voltage established by UIEH will vary with the output power of the AC / DC hybrid offshore wind farm. The voltage fluctuates slightly due to changes in voltage, thereby adjusting the power output from the DRU to the HVDC transmission line, so that the MVDC voltage remains stable near its rated value.

[0075] The proposed control strategy utilizes the MMC in UIEH to adaptively adjust the offshore AC voltage, enabling the system to adapt to changes in AC and DC wind power and provide stable voltage support for the offshore MVDC and MVAC collection networks, thereby achieving coordinated aggregation of AC and DC wind turbine clusters.

[0076] Under steady-state operating conditions, the AC side current of the DRU inside the UIEH With DC side voltage It can be represented as:

[0077] (2)

[0078] in, Indicates high voltage direct current. This represents the effective value of the AC line voltage on the DRU valve side. This refers to the leakage reactance of the DRU interface transformer.

[0079] Considering the voltage drop on the HVDC submarine cable, the DC side voltage of the DRU DC side voltage of onshore converter station The following relationship exists between them:

[0080] (3)

[0081] in, This represents the equivalent resistance of the HVDC power transmission cable.

[0082] Because the onshore MMC uses a constant DC voltage control mode, The voltage stabilizes at the rated value. Therefore, according to equation (3), the active power output from the hybrid offshore wind farm to the HVDC transmission line via UIEH is mainly determined by the HVDC side voltage of UIEH, and its expression is:

[0083] (4)

[0084] In the formula, This refers to the active power output by the DRU.

[0085] Neglecting system power loss, the AC voltages inside the offshore UIEH satisfy the following proportional relationship:

[0086] (5)

[0087] in, This refers to the MVAC port voltage of the UIEH. This indicates the AC voltage on the MMC valve side inside the UIEH. The turns ratio of the DRU interface transformer. This refers to the turns ratio of the MMC interface transformer.

[0088] Substituting equations (3) and (5) into equation (2), we can obtain the DC port voltage of the DRU. Regarding the MVAC port voltage of UIEH Functional relationship:

[0089] (6)

[0090] Because the onshore converter station operates in constant DC voltage mode, the DC port voltage of the DRU... Will vary with MVAC port voltage The HVDC current changes with the change, therefore It can be represented as:

[0091] (7)

[0092] Substituting equations (6) and (7) into equation (4), we can obtain the expression for the active power output of the UIEH's internal DRU:

[0093] (8)

[0094] According to the voltage-power coupling characteristics described by equation (8), the output power of the UIEH is determined by the voltage at its MVAC port. The decision was made, and it exhibits high sensitivity to this voltage. Through the analysis of... With minor adjustments, the DRU output power can be effectively controlled. This allows it to dynamically match the power generation of the hybrid wind farm, thereby achieving stable control of the MVDC voltage.

[0095] The power fluctuations of both AC and DC wind turbines will be reflected in MVDC voltage. The dynamic changes. Assuming the output power of the AC wind turbine... A sudden increase, in the initial stage of a power step change, the MVAC port voltage of the UIEH... The output power of the UIEH internal DRU remains unchanged. According to equation (8), the UIEH internal DRU output power... Determined by its internal AC voltage, it remains essentially constant for a short period. The resulting excess power is injected into the MVDC network through the MMC in the UIEH, thus affecting the MVDC voltage. rise:

[0096] (9)

[0097] in, This represents the equivalent capacitance on the DC side of the MMC inside the UIEH. This refers to the output power of the AC wind turbine. This refers to the output power of a DC wind turbine. This represents the total output power of the AC and DC wind turbine generator sets.

[0098] To suppress The proposed DC cascaded network control strategy appropriately increases the MVAC voltage to improve the DRU output power. This regulation can restore the internal energy balance of the UIEH, allowing the MVDC network voltage to dynamically stabilize near its rated value under the control of the proposed strategy.

[0099] Conversely, when the total output power of AC and DC wind turbines When the voltage is reduced, the MMC in the UIEH will actively reduce the MVAC port voltage, thereby reducing the output power of the DRU until the wind power output power and the DRU external power are rebalanced, thus achieving stable control of the offshore MVDC voltage near the rated value. This indicates the output power of the DC wind turbine.

[0100] Regardless of whether power fluctuations originate from AC or DC wind turbines, the proposed DC grid-connected control effectively stabilizes the MVDC voltage. By adaptively adjusting the MVAC port voltage, the controller can quickly drive the system to a new overall power balance state. Throughout the dynamic process, the MVAC port voltage experiences only minor fluctuations and remains within the design-allowed operating range.

[0101] Combination Figure 3 As shown, to analyze the operational characteristics of the proposed scheme, based on Figure 1 The equivalent circuit of the AC / DC hybrid offshore wind power system shown is established. Figure 3 In the equivalent circuit shown, the onshore MMC converter station adopts a constant DC voltage control mode, which can be equivalent to an ideal DC voltage source. ,in This represents the current flowing through the HVDC transmission line. According to the electrical characteristics of the DRU, its HVDC port voltage is affected by both the AC side voltage amplitude and the commutation voltage drop. On the DC side, it can be equivalent to a controlled voltage source connected in series with an equivalent impedance.

[0102] (10)

[0103] In the formula, This is the equivalent commutation reactance of the DRU.

[0104] In UIEH, the MMC can be equivalent to a controlled current source on the DC side. Because it has the ability to maintain the MVAC voltage at sea, it can be equivalent to a controlled voltage source on the AC side. Based on this AC power grid, the AC wind turbine can be represented as a controlled voltage source on the AC side. Similarly, thanks to the stable MVDC voltage established by the MMC, the DC wind turbine injects power into the DC network, which is equivalent to a controlled current source. Its amplitude is determined by the output power of the DC wind turbine. Decide.

[0105] exist Figure 3 middle, This indicates the leakage reactance of the MMC interface transformer; and These represent the equivalent impedances of AC cables and DC cables, respectively. and These are the phase angles of the MMC output voltage and the MVAC port voltage in the UIEH, respectively. This refers to the phase angle of the output voltage of the AC wind turbine.

[0106] Depend on Figure 3 As shown in the equivalent circuit, the offshore UIEH achieves unified aggregation of AC and DC wind energy and transmits the aggregated energy to the HVDC transmission line. The power generated by the DC wind turbine is injected into the UIEH through the MMC, and then merged with the output power of the AC wind turbine at the MVAC port. After rectification by the DRU, the power is sent to the HVDC system. Ignoring system power losses, the current relationship between the HVDC and MVDC ports of the offshore UIEH can be expressed as:

[0107] (11)

[0108] in, This represents the active power flowing through the MMC in the UIEH. This indicates the MVDC port current. Output power of AC and DC wind turbines. and It can be represented as:

[0109] (12)

[0110] In the formula, This indicates the output voltage of the AC wind turbine.

[0111] The MMC in the UIEH not only establishes the offshore MVAC voltage for AC wind turbines, but also achieves MVDC voltage regulation by fine-tuning its internal AC voltage to support DC wind turbine integration. UIEH internal AC voltage With MVDC voltage The following relationship exists between them:

[0112] (13)

[0113] in, For AC modulation ratio, This is a medium-voltage DC voltage. Therefore, the medium-voltage DC (MVDC) voltage... MVAC port voltage AC voltage on the DRU valve side The quantitative relationship between them can be further expressed as:

[0114] (14)

[0115] Because the onshore converter station stabilizes the DC port voltage to the rated value. ,according to Figure 3 The equivalent circuit shown can be used to derive the MVAC port voltage threshold required for DRU to turn on. :

[0116] (15)

[0117] Substituting equations (4) and (5) into equation (2), we can obtain the DC port voltage of the DRU. expression:

[0118] (16)

[0119] Substituting equation (4) into equation (3) and combining it with... Figure 3 The equivalent circuit shown can be obtained satisfy:

[0120] (17)

[0121] Therefore, the turns ratio of the internal isolation transformer of UIEH can be further derived as follows:

[0122] (18)

[0123] in, The rated DC voltage for the onshore converter station. The rated power of the system, This is the rated MVAC port voltage.

[0124] Furthermore, according to equation (8), the active power transmitted by the DRU can be controlled by adjusting the MVAC port voltage of the UIEH. Therefore, the rated MVAC port voltage It can be represented as:

[0125] (19)

[0126] Equation (18) gives the turns ratio design principle of the UIEH internal isolation transformer. When all AC and DC wind turbines are operating at rated power, the MMC in the UIEH maintains the MVAC port voltage at the rated value. This ensures that the DRU can transmit rated power. Therefore, when the output power of a hybrid offshore wind farm fluctuates, the MVAC port voltage of the UIEH remains constant. It will dynamically change within the following design operating range:

[0127] (20)

[0128] Based on the voltage range specified in equation (20), the voltage range can be used for... Figure 2 The output limiting of the AC network control loop shown is designed reasonably to ensure that the voltage of the offshore MVAC collector network is always maintained within the allowable operating range.

[0129] Furthermore, the parameters of the proportional-integral controller are tuned as follows: the equivalent gain of the system is obtained by combining the power-voltage coupling characteristics of the full-capacity diode rectifier unit, and the open-loop transfer function of the system is constructed by combining the equivalent capacitance of the DC side of the small-capacity modular multilevel converter and the rated voltage of the medium-voltage DC. The natural frequency and damping ratio of the control system are then solved, and the system damping ratio is tuned to 0.7.

[0130] Specifically, the parameter tuning method in this embodiment ensures the dynamic stability of the system from the perspective of control theory.

[0131] In a preferred embodiment, firstly, the nonlinear dynamic characteristics of the DRU inside the UIEH are analyzed at the steady-state operating point. Linearization is performed in the vicinity. Combining equations (2) and (4), the characteristic of the total transmission power of UIEH relative to the AC voltage can be derived. Gain of the change relationship :

[0132] (twenty one)

[0133] d-axis voltage AC voltage on the DRU valve side The following relationship must be satisfied:

[0134] (twenty two)

[0135] in, This refers to the deviation in DRU output power; For equivalent gain, ; This represents the deviation of the d-axis voltage.

[0136] Ignoring wind power disturbances, i.e. assuming Then the dynamic equation of the MVDC voltage can be expressed as:

[0137] (twenty three)

[0138] In the formula, It is the rated voltage of medium-voltage DC. This refers to the deviation of the medium-voltage DC voltage.

[0139] The transfer function of the DRU system can be obtained from equations (22) and (23). :

[0140] (twenty four)

[0141] In the formula, This is the Laplace transform of the medium-voltage DC voltage deviation. This is the Laplace transform of the d-axis voltage deviation.

[0142] Since the dynamic response speed of the inner AC voltage control loop is much faster than that of the outer DC voltage control loop, the actual AC voltage deviation can be considered as... Deviation from reference voltage Same, that is: .

[0143] According to equation (1), the small-signal linearization equation for the proposed cascaded network control can be obtained:

[0144] (25)

[0145] In the formula, This represents the deviation from the reference value of the medium-voltage DC voltage.

[0146] Substituting equation (24) into equation (25), the open-loop transfer function of the proposed control strategy can be derived. :

[0147] (26)

[0148] Its closed-loop transfer function can be obtained. :

[0149] (27)

[0150] Therefore, the system's natural frequency With damping ratio satisfy:

[0151] (28)

[0152] By properly adjusting the proportional gain With integral gain This can effectively optimize the dynamic performance of the system.

[0153] Figure 4 Bode plots of the proposed cascaded network control loop are presented under appropriate parameter tuning conditions. In the low-frequency range, the system gain remains unity gain (i.e., 0 dB), indicating that the proposed scheme can enable the offshore MVDC network voltage to accurately track its reference value, thus effectively supporting the connection of DC wind turbines. Furthermore, only a slight resonant peak of approximately 1.1 dB appears at 6.13 Hz, indicating that the system has good damping characteristics, with a damping ratio of approximately [missing value]. The system phase angle is approximately 0.7. It remains essentially 0° at low frequencies, gradually decreasing and approaching -90° as the frequency increases. This frequency domain characteristic not only ensures the system's ability to quickly track the reference value but also effectively suppresses high-frequency resonant oscillations, thereby comprehensively improving the stability and dynamic performance of the proposed AC / DC cascaded network control strategy.

[0154] Furthermore, the voltage operating range of the medium-voltage AC port is defined as between the conduction threshold voltage of the full-capacity diode rectifier unit and the rated AC voltage of the system; the conduction threshold voltage is determined by the rated voltage of the high-voltage DC transmission line and the turns ratio of the isolation transformer; when the voltage of the medium-voltage AC port is lower than the conduction threshold voltage, the full-capacity diode rectifier unit is in reverse blocking state.

[0155] Specifically, the voltage operating range design in this embodiment ensures the controllable turn-off capability of the DRU from the circuit principle level. The DRU is composed of an uncontrollable diode, and its conduction requires meeting the AC side voltage amplitude condition, as shown in equation (15) for the conduction threshold voltage. When the MVAC port voltage U B When the voltage is above this threshold, the DRU is in the on state, and power flows from the AC side to the DC side; when U B When the voltage drops below this threshold, the DRU enters a reverse blocking state, and the transmitted power drops to zero. In this embodiment, the operating range of the MVAC port voltage is set between the turn-on threshold voltage and the rated voltage, as shown in equation (20). This ensures that the DRU can transmit power under normal operating conditions and reliably shut down under zero-power output conditions, avoiding the risk of system runaway. Taking the simulation parameters of this embodiment as an example: HVDC rated voltage 1000kV, isolation transformer turns ratio... =391.75kV / 66kV≈5.94, the calculated conduction threshold voltage is approximately 62.49kV, the rated voltage is 66kV, and the operating range is 62.49kV to 66kV.

[0156] Furthermore, when the overall output power of the offshore hybrid wind farm is zero, the control strategy adjusts the medium-voltage AC port voltage to below the conduction threshold voltage, so that the full-capacity diode rectifier unit remains in reverse blocking.

[0157] Specifically, the zero-power operating condition handling strategy in this embodiment ensures reliable operation under extreme conditions from a system safety perspective. When the overall output power of the offshore hybrid wind farm is zero (e.g., no wind at night or all wind turbines are shut down), the control strategy in this embodiment actively adjusts the MVAC port voltage below the conduction threshold voltage upon detecting zero wind power output, putting the DRU in a reverse blocking state, thereby cutting off the energy path between the HVDC line and the offshore side. At this time, the MMC in the UIEH maintains MVDC voltage stability through the outer loop DC grid control, but since the DRU is blocked, the system only consumes minimal no-load losses. When wind power recovers, the control strategy raises the MVAC port voltage above the conduction threshold, and the DRU automatically resumes power transmission. This design enables the system to have self-recovery capabilities.

[0158] The technical effects of this embodiment are verified by specific simulations below.

[0159] To verify the effectiveness of the proposed solution, based on Figure 1 The AC / DC hybrid offshore wind power system shown is modeled in the PSCAD / EMTDC simulation environment. Detailed system parameters are listed in Table 1.

[0160] Table 1 Simulation Parameters of Hybrid Offshore Wind Power System

[0161]

[0162] Figure 5 Simulation results under rated operating conditions are presented. Figure 5 (a) It can be seen that the DC and AC wind turbines operate at rated operating conditions of 0.5GW and 1.5GW respectively, with a total power of approximately 2GW transmitted to the HVDC transmission line via UIEH. When the onshore MMC stabilizes the HVDC port voltage at the rated value of 1000kV, the HVDC port voltage of UIEH is stably maintained at around 1007.8kV. Figure 5 As shown in (d). Figure 5 (b) and Figure 5 (c) It can be seen that the MVAC port voltage of UIEH can be stably maintained at the rated value, with a peak phase voltage of 53.89kV, consistent with the theoretical analysis result of equation (19). Furthermore, to achieve rated power transmission to the onshore converter station, the effective value of the AC voltage on the DRU valve side reaches 391.75kV, verifying the correctness of the interface transformer turns ratio design given in equation (18). Figure 5 (d) and Figure 5(e) As can be seen, the MVDC port voltage of the UIEH can be stably controlled at 100kV, and the capacitor voltage of the MMC submodule fluctuates stably around 2.5kV, with its voltage ripple limited to within 9.4%. These results demonstrate that the proposed AC / DC cascaded grid control strategy can simultaneously establish stable DC and AC voltages for the DC and AC wind turbine clusters, ensuring the stable and reliable operation of the AC / DC hybrid offshore wind power system under its design rated operating conditions.

[0163] Figure 6 Simulation results under wind power fluctuation conditions are presented. Initially, both AC and DC wind turbine clusters operate at their rated power. At t=3s, the output power of the AC wind turbines decreases from 1.5GW to 0.75GW, while the DC wind turbines maintain their rated power of 0.5GW. Figure 6 As shown in (a). In the initial stage of power reduction, the total power output from the UIEH to the HVDC system remains essentially constant. The resulting power imbalance causes the MVDC network to discharge, causing the MVDC voltage to briefly drop to 99.7kV, as shown in (a). Figure 6 As shown in (c). To compensate for the power deficit, the proposed cascaded grid controller appropriately reduces the MVAC port voltage from 66kV to 64.7kV, as follows. Figure 6 As shown in (d), the reduction in MVAC voltage further decreases the transmission power of the DRU from 2GW to approximately 1.25GW, thereby quickly restoring the MVDC voltage to near its 100kV rating, as... Figure 6 (b) and Figure 6 As shown in (c), at t=5s, the output power of the DC wind turbine decreased from 0.5GW to 0.4GW, while the output of the AC wind turbine remained unchanged at 0.75GW. Figure 6 As shown in (a), the MVDC voltage experiences a slight drop to 99.9 kV, as Figure 6 As shown in (c). The cascaded grid controller further reduces the MVAC port voltage to 64.53kV, causing the DRU's external power to decrease to 1.15GW, and the MVDC voltage to recover to 100kV, as shown. Figure 6 (b) to Figure 6 As shown in (d), at t=7s, the output power of the AC wind turbine gradually recovered to the rated value of 1.5GW, while the output of the DC wind turbine remained at 0.4GW, as... Figure 6 As shown in (a), the MVDC voltage transiently rises to 100.3kV, and the controller actively increases the MVAC port voltage to 65.85kV to enhance the system's external power output. Figure 6 (b) to Figure 6As shown in (d). At t=9s, the DC wind turbine output recovered to 0.5GW, and the system returned to its rated operating state of 2GW. The controller quickly restored the MVAC port voltage to the rated value of 66kV, as shown in (d). Figure 6 (a) Figure 6 (b) and Figure 6 As shown in (d). Throughout the entire wind power fluctuation process, the HVDC voltage is consistently regulated by the onshore MMC, with only very small transient fluctuations, such as... Figure 6 As shown in (e). The above results demonstrate that, regardless of whether the power fluctuation originates from AC or DC wind turbines, the proposed DC grid control can effectively stabilize the MVDC voltage and rapidly drive the system to a new overall power balance state by adaptively adjusting the MVAC port voltage.

[0164] Figure 7 Simulation results for the AC wind turbine under zero-output condition are presented. In the initial stage, the DC wind turbine operates at 0.5GW rated power, while the AC wind turbine output power remains zero. Figure 7 As shown in (a), the proposed cascaded grid control strategy regulates the MVAC port voltage of UIEH to 63.38kV, enabling the transfer of 0.5GW of DC wind power through the MMC and DRU, as shown in (a). Figure 7 (b) and Figure 7 As shown in (c), at t=4s, the output power of the DC wind turbine rapidly drops to zero within 20ms, as... Figure 7 As shown in (a), the control strategy further reduces the MVAC port voltage from 63.38kV to 62.49kV, as... Figure 7 As shown in (c). When the MVAC port voltage drops to 62.49kV, the DRU in the UIEH enters the reverse blocking state, which is consistent with the conduction threshold defined by equation (15). The total transmission power of the UIEH drops from 0.5GW to zero, and the MVDC port voltage remains stable at around 100kV, as shown in (c). Figure 7 (b) and Figure 7 As shown in (d), at t=6s, the output power of the DC wind turbine recovers to 0.5GW within 20ms, and the system gradually returns to the initial steady-state operating state.

[0165] Figure 8 Simulation results of a DC wind turbine under zero-output conditions are presented. Initially, the AC wind turbine operates at a rated power of 1.5GW, such as... Figure 8 As shown in (a), the proposed cascaded grid controller regulates the MVAC port voltage of the UIEH to 65.2kV, enabling the transmission of 1.5GW of wind power. Since the MMC in the UIEH is only responsible for transmitting DC wind turbine power, its transmitted power is always close to zero, as... Figure 8(a) to Figure 8 As shown in (c). At t=4s, the output power of the AC wind turbine rapidly drops to zero within 20ms. The UIEH then reduces the internal AC voltage from 65.2kV to 62.49kV, as... Figure 8 (a) and Figure 8 As shown in (c), the reduced AC voltage causes the DRU to enter reverse blocking mode, and the transmission power of the UIEH rapidly drops from 1.5GW to zero. The MVDC port voltage stabilizes at around 100kV. Figure 8 (b) and Figure 8 As shown in (d), at t=6s, the output power of the AC wind turbine recovered to 1.5GW within 20ms, and the system returned to its initial operating state. Throughout the entire power transient process, the MVDC port voltage of the UIEH remained stable at around 100kV, with only slight fluctuations.

[0166] Based on the simulation results, the UIEH-based AC / DC hybrid offshore wind power system can maintain stable and reliable operation under extreme operating scenarios, including rated steady-state conditions, dynamic fluctuations in wind power, and zero-power output of AC or DC wind turbines. Simulation results verify the effectiveness of the proposed DC cascaded grid control strategy: through the cascaded coordination of outer-loop DC grid control and inner-loop AC grid control, the system can adaptively adjust the MVAC port voltage and change the power output of the DRU to match the power fluctuations of the hybrid wind farm, thereby stabilizing the MVDC voltage near its rated value. Throughout the dynamic process, the MVAC port voltage experiences only minor fluctuations and remains within the design-allowed operating range, fully demonstrating the excellent robustness and operational flexibility of the proposed scheme.

[0167] In summary, the unidirectional isolated energy router and its AC / DC cascaded network control strategy provided by this invention achieve electrical isolation between the high-voltage and medium-voltage sides through a topology structure in which a small-capacity MMC and a full-capacity DRU are connected face-to-face via an isolation transformer, significantly reducing the insulation cost of DC-side equipment. The AC / DC cascaded network control strategy enables adaptive and stable control of the MVDC voltage and coordinated aggregation of AC / DC wind turbine clusters. Furthermore, reasonable capacity configuration and parameter tuning significantly reduce the construction cost of offshore converter stations. The technical solution of this invention demonstrates significant technical and economic advantages in large-scale offshore wind power grid-connected applications.

[0168] While the invention has been described herein with reference to specific embodiments, it should be understood that these embodiments are merely examples of the principles and applications of the invention. Therefore, it should be understood that many modifications can be made to the exemplary embodiments, and other arrangements can be designed without departing from the spirit and scope of the invention as defined by the appended claims. It should be understood that different dependent claims and features described herein can be combined in ways different from those described in the original claims. It is also understood that features described in conjunction with individual embodiments can be used in other described embodiments.

Claims

1. A unidirectional isolated energy router suitable for offshore AC / DC hybrid wind power systems, characterized in that, include: Small-capacity modular multilevel converter and full-capacity diode rectifier unit; The AC side of the small-capacity modular multilevel converter is connected to the AC side of the full-capacity diode rectifier unit through an isolation transformer to achieve electrical isolation between the high-voltage side and the medium-voltage side of the equipment. The small-capacity modular multilevel converter is equipped with a medium-voltage DC port and a medium-voltage AC port. The medium-voltage DC port is connected to a DC wind turbine cluster, and the medium-voltage AC port is connected to an AC wind turbine cluster. The full-capacity diode rectifier unit is equipped with a high-voltage DC port, which is used to connect to an external high-voltage DC transmission line.

2. The unidirectional isolated energy router for offshore AC / DC hybrid wind power systems according to claim 1, characterized in that, The full-capacity diode rectifier unit consists of two six-pulse diode rectifiers combined to form a twelve-pulse rectifier structure.

3. The unidirectional isolated energy router for offshore AC / DC hybrid wind power systems according to claim 1, characterized in that, The small-capacity modular multilevel converter adopts a half-bridge sub-module.

4. The unidirectional isolated energy router for offshore AC / DC hybrid wind power systems according to claim 1, characterized in that, An AC filter is installed at the medium-voltage AC port to suppress harmonics and compensate for reactive power.

5. The unidirectional isolated energy router for offshore AC / DC hybrid wind power systems according to claim 1, characterized in that, The rated capacity of the small-capacity modular multilevel converter matches the total rated power of the DC wind turbine cluster; the rated capacity of the full-capacity diode rectifier unit matches the total rated power of the AC wind turbine cluster and the DC wind turbine cluster.

6. A control strategy for AC / DC cascaded networking of a unidirectional isolated energy router, implemented based on the unidirectional isolated energy router described in any one of claims 1 to 5, characterized in that, This includes the outer ring DC grid control system and the inner layer AC grid control system; The outer loop DC grid control link collects the measured voltage of the medium-voltage DC port, calculates the voltage deviation in combination with the voltage reference value, and generates the AC voltage adjustment amount based on the voltage deviation. The inner layer AC network control circuit corrects the voltage reference value of the medium-voltage AC port according to the AC voltage adjustment amount and maintains a stable AC voltage output from the medium-voltage AC port. When an increase in the medium-voltage DC port voltage is detected, the inner layer AC network control circuit raises the medium-voltage AC port voltage, increasing the output power of the full-capacity diode rectifier unit; when a decrease in the medium-voltage DC port voltage is detected, the inner layer AC network control circuit lowers the medium-voltage AC port voltage, reducing the output power of the full-capacity diode rectifier unit.

7. The AC / DC cascaded network control strategy for a unidirectional isolated energy router according to claim 6, characterized in that, The outer loop DC grid control link inputs the medium-voltage DC voltage deviation signal into the proportional-integral controller, and the proportional-integral controller outputs the AC voltage adjustment amount. The inner-layer AC network control loop superimposes the rated AC voltage reference value with the AC voltage adjustment amount to obtain the corrected d-axis voltage reference value, while setting the q-axis voltage reference value to zero and generating the AC voltage reference phase angle based on the system's rated frequency integration.

8. The AC / DC cascaded network control strategy for a unidirectional isolated energy router according to claim 7, characterized in that, The parameters of the integral controller in the outer loop DC grid control loop are tuned as follows: the equivalent gain of the system is obtained by combining the power-voltage coupling characteristics of the full-capacity diode rectifier unit, the equivalent capacitance of the DC side of the small-capacity modular multilevel converter and the rated voltage of the medium-voltage DC are used to construct the open-loop transfer function of the system, and the closed-loop transfer function is obtained from the open-loop transfer function of the system. The natural frequency and damping ratio of the control system are obtained by solving the characteristic equation of the closed-loop transfer function, and the system damping ratio is tuned to 0.

7.

9. The AC / DC cascaded network control strategy for a unidirectional isolated energy router according to claim 6, characterized in that, Under normal power transmission conditions, the voltage operating range of the medium-voltage AC port is limited to the conduction threshold voltage of the full-capacity diode rectifier unit to the rated AC voltage of the system; the conduction threshold voltage is determined by the rated voltage of the high-voltage DC transmission line and the turns ratio of the isolation transformer at the diode rectifier unit interface; when the voltage of the medium-voltage AC port is lower than the conduction threshold voltage, the full-capacity diode rectifier unit is in reverse blocking state.

10. The AC / DC cascaded network control strategy for a unidirectional isolated energy router according to claim 9, characterized in that, When the overall output power of the offshore hybrid wind farm is zero, the control strategy will adjust the medium-voltage AC port voltage to no higher than the conduction threshold voltage, so that the full-capacity diode rectifier unit remains in reverse blocking.

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