Marine-land integrated direct current power transmission system and method based on unidirectional direct current transformer

Abstract

The application provides a sea-land integrated direct current power transmission system and method based on a unidirectional direct current transformer, the system comprising an offshore wind farm side converter station, a unidirectional direct current transformer composed of a low-voltage side converter station, an alternating current side single converter transformer and a high-voltage side DRU converter, and a land receiving end setting converter station; the DRU is used to replace the traditional active MMC converter unit at the high-voltage side, and only one converter transformer is used at the alternating current side, so as to reduce the number of high-voltage side fully-controlled devices and the control and protection configuration thereof, and reduce the number of converter equipment and transformer equipment; meanwhile, the MMC converter V / F control strategy with additional direct current voltage control is combined, the indirect control of the DRU transmission power is realized through the adjustment of the voltage amplitude of the intermediate alternating current side of the direct current transformer, so that the overall construction cost of the system is reduced on the basis of considering the system control performance.

Description

Technical Field

[0001] This invention belongs to the field of high-voltage direct current transmission and offshore wind power grid connection technology, specifically relating to an integrated land-sea direct current transmission system and method based on a unidirectional DC transformer. Background Technology

[0002] As offshore wind power develops towards deeper waters, larger capacities, and longer transmission distances, the transmission distance between offshore wind farms and onshore load centers is constantly increasing. Employing high-voltage direct current (HVDC) transmission technology to transmit offshore wind power has become the mainstream development direction. For integrated onshore and offshore HVDC transmission systems, how to ensure stable system operation while reducing equipment investment costs has become a key issue that needs to be addressed in engineering applications.

[0003] Among existing DC transformer schemes, a mainstream technical approach is to adopt a face-to-face MMC (Modular Multilevel Converter) DC transformer structure, as seen in the literature [Wang Xinying, Tang Guangfu, He Zhiyuan. Research on DC / DC Converter Topology for DC Collection in Offshore Wind Farms. Proceedings of the CSEE, 2017, 37(03):837-848]. The basic idea is to first convert low-voltage DC power to AC power via a low-voltage MMC, then perform voltage transformation via an intermediate AC connection link or isolation transformer, and finally rectify it into high-voltage DC power by a high-voltage MMC, thereby achieving energy transfer and power control between different DC voltage levels. The main characteristics of this type of scheme are strong control capability, high operational flexibility, and the ability to achieve bidirectional power transfer, DC voltage regulation, electrical isolation, and a certain degree of fault protection. However, its control capability and functional completeness mainly rely on fully controlled MMC converters on both sides, which also leads to higher hardware costs and control complexity. Meanwhile, face-to-face MMC DC transformers require MMCs to be configured on both the low-voltage and high-voltage sides. Since the MMC consists of a large number of sub-modules, fully controlled power devices, sub-module capacitors, bridge arm reactors, valve control systems, cooling systems, and protection and control systems, the entire DC transformer has a large number of devices, large equipment size, complex control system, and high construction cost.

[0004] The second mainstream technical approach is the autotransformer MMC DC transformer scheme, as seen in the literature [Zhao Chengyong, Li Luyao, Zhai Xiaomeng. Novel Modular High-Voltage High-Power DC-DC Converter. Automation of Electric Power Systems, 2014, 38(04):72-78]. The autotransformer MMC DC transformer is a type of high-voltage, high-capacity DC / DC converter constructed based on modular multilevel converter technology. Its basic idea is similar to that of an AC autotransformer, that is, the low-voltage side DC port and the high-voltage side DC port share part of the bridge arm or part of the submodule, and the voltage conversion and power transmission between different DC voltage levels are realized through the voltage division effect of the MMC bridge arm. This type of topology can be further developed into an autotransformer DC transformer with DC fault current blocking capability. Through bridge arm submodule control, full-bridge submodule configuration, or specific fault control strategies, DC fault current can be suppressed or blocked to a certain extent. However, autotransformer MMC DC transformers typically lack inherent electrical isolation capabilities. Due to shared bridge arms, shared submodules, or direct electrical coupling between their high-voltage and low-voltage sides, fault voltage and inrush current can be transmitted to the low-voltage side through the autotransformer structure when a ground fault, inter-electrode fault, or transient overvoltage occurs on the high-voltage side. Secondly, autotransformer MMC DC transformers are more suitable for small to medium turns ratio applications. When applied to high turns ratio scenarios, the current stress on the low-voltage side bridge arms increases significantly due to their reliance on voltage division. This necessitates increasing the number of parallel bridge arms, improving the current-carrying capacity of components, or configuring large-capacity filter inductors to share the current. This leads to increased equipment size, weight, and cost, diminishing the economic advantage originally derived from reducing the number of submodules.

[0005] Against this backdrop, if DRUs (Diode Rectifier Units) can replace some of the fully controlled converter units on the high-voltage side of the DC transformer, the advantages of DRUs—simple structure, no need for complex valve control systems, low conduction losses, and low equipment costs—can be leveraged to reduce the need for fully controlled power devices and their control and protection configurations on the high-voltage side, thereby improving the economic efficiency of DC transformers in unidirectional offshore wind power transmission scenarios. However, DRUs are uncontrolled rectifier units and lack active voltage build-up, active voltage regulation, and flexible power regulation capabilities. Direct application in integrated onshore and offshore DC transmission systems can easily lead to insufficient power regulation, difficulty in coordinating and stabilizing DC-side voltage, and power mismatch under fluctuating wind power output. Therefore, it is still necessary to configure converters with active regulation capabilities on the low-voltage side of the DC transformer and design a reasonable coordinated control strategy for DC and AC voltages. This involves indirectly adjusting the transmitted power by regulating the AC voltage on the valve side of the DRU to ensure stable DC voltage and coordinated power operation under fluctuating wind power output conditions.

[0006] Therefore, how to retain the advantages of DRU's simple structure and low cost while making up for its shortcomings of uncontrollable and difficult-to-actively adjust power, and making it suitable for DC transformer scenarios where low-voltage DC is collected and high-voltage DC is sent out, is a problem that existing technologies still need to solve. Summary of the Invention

[0007] Given that existing DC transformer solutions mostly employ face-to-face MMC DC transformer structures, which suffer from problems such as a large number of fully controlled devices, complex control systems, high converter equipment costs, and numerous AC-side electrical equipment configurations, while autotransformers also have limitations such as lacking inherent electrical isolation capabilities and being unsuitable for high-turn-ratio scenarios, this invention provides a land-sea integrated DC transmission system and method based on a unidirectional DC transformer. This system aims to meet the requirements of electrical isolation and high turn-ratio while ensuring stable system operation and power coordination control capabilities, thereby reducing the overall system construction cost.

[0008] An integrated onshore-offshore DC power transmission system based on a unidirectional DC transformer includes an offshore wind farm, a DC-DC transformer, an onshore converter station, and a receiving-end power grid. The offshore wind farm comprises multiple wind turbines. The output of the wind turbines is converted into DC power by a wind farm-side converter and then collected via submarine cables to the low-voltage side of the DC-DC transformer. The DC-DC transformer is composed of a low-voltage side converter, a converter transformer, and a high-voltage side converter connected in sequence. The low-voltage side converter converts the collected low-voltage DC power into AC power, which is then stepped up by the converter transformer. The high-voltage side converter converts the stepped-up AC power into high-voltage DC power, which is then transmitted to the onshore converter station via an overhead cable. The onshore converter station converts the high-voltage DC power into AC power and injects it into the receiving-end power grid. The wind farm-side converter, the low-voltage side converter, and the onshore converter station all employ MMC (Multi-mode Converter), while the high-voltage side converter employs DRU (Digital-Receiving Unit).

[0009] Furthermore, the wind turbine is a grid-connected wind turbine, which uses a wind farm-side converter to control the amplitude and frequency of the AC voltage on the offshore wind farm side, providing voltage support for the wind farm.

[0010] Furthermore, the low-voltage side converter is used to control the DC voltage on the submarine cable side and the amplitude and frequency of the AC voltage on the AC side.

[0011] Furthermore, the DC-DC transformer is a unidirectional DC transformer used to realize DC voltage level conversion and unidirectional energy transmission. The high-voltage side converter uses a DRU with high-power diodes as uncontrollable devices. Therefore, the onshore converter station maintains the DC voltage stability of the overhead cable and undertakes the reactive power regulation function of the system.

[0012] The control method for the aforementioned integrated land-sea DC transmission system includes: The wind farm-side converter adopts a V / F (voltage-frequency ratio) control strategy to establish the AC voltage on the offshore wind farm side; The low-voltage side converter adopts a V / F control strategy with d-axis additional DC voltage control. This control strategy inputs the deviation between the actual DC side voltage of the low-voltage side converter and the DC voltage command value to the PI (proportional-integral) control loop. Then, the output of the PI control loop is used as the outer loop reference value of the d-axis voltage to control the DC voltage on the submarine cable side. At the same time, by adjusting the amplitude and frequency of the AC side voltage in the middle of the DC-DC transformer, the transmission power of the DRU is controlled to achieve unidirectional rectification and output on the high-voltage side. The onshore converter station adopts a constant DC voltage and constant reactive power control strategy to control the DC voltage of the overhead cable and regulate the reactive power of the system.

[0013] This control method, under the condition that the DC voltage on the high-voltage side of the onshore converter station is kept stable, adjusts the AC voltage amplitude in the middle of the DC-DC transformer through the low-voltage side converter to control the transmission power of the DRU, thereby realizing the power transmission and coordinated control of offshore wind power to the onshore receiving-end grid via the DC-DC transformer.

[0014] Furthermore, when the DC voltage of the DRU is constant, its transmission power is proportional to the voltage amplitude on the valve side. By adjusting the voltage amplitude on the intermediate AC side through the low-voltage side converter, the transmission power of the DRU can be controlled. The specific expression is as follows: , , in: U DRU This is the DC-side voltage of the DRU. U v This represents the effective value of the valve-side line voltage of the DRU. X u For commutation reactance, I DRU This is the DC current of the DRU. φ The power factor angle, P DRU The output active power of the DRU. Q DRU The reactive power absorbed by the DRU.

[0015] A computer device includes a memory and a processor, wherein the memory stores a computer program and the processor executes the computer program to implement the control method of the above-described integrated land-sea DC power transmission system.

[0016] A computer-readable storage medium storing a computer program, which, when executed by a processor, implements the control method for the aforementioned integrated land-sea DC power transmission system.

[0017] Based on the above technical solution, the present invention has the following beneficial technical effects: 1. This invention uses a DRU instead of a traditional active MMC converter station on the high-voltage side. By taking advantage of the simple structure of the diode rectifier unit and the fact that it does not require a complex control system, the fully controlled power devices and their control and protection configurations on the high-voltage side are reduced, which helps to reduce system equipment costs and control costs.

[0018] 2. This invention uses only one converter transformer on the AC side. Compared with the scheme of configuring multiple transformers on the AC side, it can reduce the number of transformer bodies and their supporting equipment, reduce equipment investment, installation costs and land requirements, thereby further improving the system economy.

[0019] 3. This invention employs a V / F control strategy with d-axis additional DC voltage control in the low-voltage side converter. Under the condition that the high-voltage side DRU is uncontrollable, it indirectly controls the power transmitted by the DRU by adjusting the voltage amplitude of the intermediate AC side, while simultaneously fixing the DC voltage on the submarine cable side to ensure that the system has good power coordination control capability.

[0020] 4. This invention establishes AC voltage support on the offshore wind farm side through wind farm-side converters and maintains DC voltage stability through low-voltage-side converters and onshore converter stations, so that the system can maintain good DC voltage stability and operational stability even under fluctuating wind power output conditions. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the topology of an integrated land-sea DC power transmission system based on a unidirectional DC transformer in an embodiment of the present invention.

[0022] Figure 2 This is a block diagram of the control strategy for the low-voltage side converter MMC2 in an embodiment of the present invention.

[0023] Figure 3 This is a simulation waveform diagram of the DC voltage on the submarine cable side under the power output fluctuation of the wind turbine in an embodiment of the present invention.

[0024] Figure 4 This is a simulation waveform diagram of the effective value of the AC side voltage of a unidirectional DC transformer under the power output fluctuation of the wind turbine in an embodiment of the present invention.

[0025] Figure 5 This is a simulation waveform diagram of the active power transmitted by a unidirectional DC transformer under the power output fluctuation of the wind turbine in an embodiment of the present invention. Detailed Implementation

[0026] To describe the present invention in more detail, the technical solution of the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0027] This embodiment provides an integrated land-sea DC power transmission system based on a unidirectional DC transformer and its control method. Figure 1 The topology of this integrated onshore-offshore DC transmission system is demonstrated, including the offshore wind farm-side converter station MMC1, a unidirectional DC-DC transformer, and the onshore receiving-end converter station MMC3. The unidirectional DC-DC transformer comprises the low-voltage side converter station MMC2, a single AC-side converter transformer, and a high-voltage side DRU converter station. The offshore wind farm connects to the low-voltage side DC network via MMC1. Voltage level transformation and unidirectional energy transmission are achieved through the unidirectional DC-DC transformer consisting of MMC2, the single converter transformer, and the DRU. Finally, the energy is transmitted to the onshore receiving-end converter station MMC3 via a high-voltage side DC cable and connected to the onshore receiving-end AC grid. In this structure, the high-voltage side uses a DRU instead of a traditional active converter, and the AC side only has one converter transformer. This reduces the number of fully controlled power devices and transformers, thereby lowering system equipment investment costs and control complexity.

[0028] In this embodiment, the offshore wind farm adopts grid-connected wind turbines. MMC1 uses V / F control to stabilize the AC voltage amplitude and frequency on the offshore wind farm side, providing voltage support for the wind farm. MMC2 adds a DC voltage control loop to the traditional V / F control. The difference between the actual DC side voltage of MMC2 and the DC voltage command value is adjusted by a PI controller and used as the outer loop d-axis voltage reference value, achieving the goal of stabilizing the DC voltage on the submarine cable side and the AC side voltage amplitude and frequency. Since the high-power diodes used in the DRU are uncontrollable devices, the onshore grid-side MMC3 maintains the DC voltage stability of the high-voltage overhead line and undertakes the reactive power regulation function of the system.

[0029] In this embodiment, the specific control method of the system is as follows: MMC1 adopts V / F control to establish the AC voltage amplitude and frequency on the offshore wind farm side, providing stable AC voltage support for the offshore wind farm; MMC2 adopts a V / F control strategy with d-axis additional DC voltage control, such as... Figure 2As shown, this control strategy introduces a PI control loop on the basis of traditional V / F control. The deviation between the actual DC-side voltage of MMC2 and the DC voltage command value is input to the PI regulator, and the PI regulation output is used as the outer loop reference value of the d-axis voltage to adjust the amplitude and frequency of the AC-side voltage of the DC transformer. This also solves the problem that MMC1 on the offshore wind farm side cannot stabilize the DC voltage on the submarine cable side. MMC3 is used to control the DC line voltage on the high-voltage side and the reactive power of the system. Since MMC3 stabilizes the DC voltage on the high-voltage side, the DRU transmission power is only related to the amplitude of its valve-side AC voltage. Therefore, by adjusting the amplitude of the intermediate AC-side voltage through MMC2, the power transmitted by the DRU can be controlled, thereby achieving coordinated regulation of the power of the entire transmission system.

[0030] According to the operating principle of DRU, when the DC voltage of DRU is constant, its transmission power is proportional to the amplitude of the AC voltage on the valve side, as shown in the following formula: , , , In the formula: U DRU This refers to the DC-side voltage of the DRU converter. U v This represents the effective value of the line voltage on the DRU valve side. X u For commutation reactance, I DRU This refers to the DC current of the DRU converter. The power factor angle, μ This is the commutation overlap angle. u k This represents the per-unit value of the commutation voltage drop caused by the leakage reactance of the DRU converter transformer. P DRU To transfer active power to the DRU converter, Q DRU The reactive power absorbed by the DRU converter.

[0031] Therefore, by adjusting the voltage amplitude of the intermediate AC side through MMC2, the power transmitted by the DRU can be controlled; thus, the power transmitted by offshore wind power can be coordinated and adjusted while maintaining the stability of the DC voltage on the high-voltage side.

[0032] The following is based on Figure 1The system topology shown was used to establish an electromagnetic transient simulation model on the PSCAD / EMTDC simulation platform to verify the effectiveness of the control strategy of this invention. After the system reached stable operation, a wind turbine output fluctuation condition was set for the wind farm input, causing the output power of the offshore wind turbine to change over time. The dynamic changes of the DC voltage on the submarine cable side, the effective value of the AC voltage on the unidirectional DC transformer side, and the transmitted active power were observed.

[0033] Under rated operating conditions, the wind speed is 12 m / s. At this time, each wind turbine outputs 1000 MW of active power, for a total output of 3000 MW of active power. The system reaches the steady state of rated operating conditions at 3.5 seconds. The wind speed is set to change from 12 m / s to 11 m / s at 4 seconds, from 11 m / s to 10 m / s at 6 seconds, from 10 m / s to 9 m / s at 8 seconds, from 9 m / s to 8 m / s at 10 seconds, from 8 m / s to 7 m / s at 12 seconds, from 7 m / s to 6 m / s at 14 seconds, and from 6 m / s to 5 m / s at 16 seconds.

[0034] The simulated waveform of the DC voltage on the submarine cable side under the fluctuation of wind turbine output is as follows: Figure 3 As shown, during the initial system startup, the DC voltage on the submarine cable side quickly established itself after a brief transient adjustment and stabilized near the target DC voltage. During subsequent fluctuations in wind turbine output, the DC voltage on the submarine cable side only experienced minor fluctuations and quickly recovered to near its stable value, without significant instability or sustained deviation. This indicates that the additional DC voltage control strategy employed by MMC2 can effectively suppress DC voltage fluctuations on the submarine cable side under fluctuating wind turbine output conditions, ensuring the stability of the DC voltage on the low-voltage side of the DC transformer.

[0035] The simulated waveform of the effective value of the AC side voltage of a unidirectional DC transformer under the power output fluctuation of the wind turbine is as follows: Figure 4 As shown, as the wind turbine output gradually changes, the effective value of the AC side voltage of the DC transformer shows a corresponding trend, generally decreasing step by step as the wind turbine output decreases. This phenomenon occurs because when the wind turbine output decreases, the power delivered by the wind farm is less than the power transmitted by the DC transformer, causing the DC side capacitor to release energy and resulting in a drop in DC voltage. The MMC2 can dynamically adjust the AC side voltage amplitude based on the DC side voltage deviation. When the DC side voltage decreases, it actively reduces the AC side voltage, altering the AC voltage conditions on the DRU valve side, reducing the transmission power, and restoring the DC voltage on the submarine cable side to its rated value. Since the DRU is an uncontrolled rectifier unit, its transmission power is affected by the amplitude of the valve side AC voltage; therefore, the change in the effective value of the AC side voltage directly forms the basis for adjusting the DRU's transmission power.

[0036] Simulation waveform of active power transmission by a unidirectional DC transformer under wind turbine output fluctuations is as follows: Figure 5As shown, in the initial stage of system startup, the transmitted active power is rapidly established; as the wind turbine output gradually decreases, the transmitted active power of the unidirectional DC transformer also shows a gradual decreasing trend, and... Figure 4 The effective value of the AC side voltage remains consistent. This indicates that, under the condition that the DC voltage on the high-voltage side is kept stable by MMC3, the amplitude of the intermediate AC side voltage can be effectively controlled by adjusting the voltage of the intermediate AC side through MMC2, thereby realizing the coordinated transmission of offshore wind power to the onshore receiving-end grid.

[0037] comprehensive Figures 3-5 It can be seen that under fluctuating wind turbine output conditions, although the wind turbine output power changes, the DC voltage on the submarine cable side remains within a stable range due to the V / F control strategy with additional DC voltage control. Simultaneously, the AC voltage of the DC transformer is dynamically adjusted according to the system operating status, causing the DRU transmission power to change accordingly. This ensures that the system can maintain stable operation under different wind turbine output levels, verifying the effectiveness of this control method for unidirectional DC transformers.

[0038] Furthermore, from a system economic perspective, this embodiment uses a DRU instead of a traditional active MMC converter station on the high-voltage side, avoiding the need for a large number of fully controlled power devices and complex valve control systems on the high-voltage side. Simultaneously, only one converter transformer is used on the AC side, reducing the number of transformers and their auxiliary equipment. Therefore, while meeting the requirements for stable system operation, it helps to reduce equipment investment costs, installation costs, and operation and maintenance costs. As a low-cost transmission topology, the DRU has the characteristics of requiring no complex control system, relying on natural diode commutation, and passive operation. Therefore, this invention possesses both good technical feasibility and engineering economics.

[0039] The above description of the embodiments is provided to enable those skilled in the art to understand and apply the present invention. Those skilled in the art can readily make various modifications to the above embodiments and apply the general principles described herein to other embodiments without creative effort. Therefore, the present invention is not limited to the above embodiments, and any improvements and modifications made to the present invention by those skilled in the art based on the disclosure thereof should be within the scope of protection of the present invention.

Claims

1. A land-sea integrated DC transmission system based on a unidirectional DC transformer, characterized in that, The system includes an offshore wind farm, a DC-DC converter, an onshore converter station, and a receiving-end power grid. The offshore wind farm consists of multiple wind turbines. The output of the wind turbines is converted into DC power by a wind farm-side converter and then collected via submarine cables to the low-voltage side of the DC-DC converter. The DC-DC converter is composed of a low-voltage side converter, a converter transformer, and a high-voltage side converter connected in sequence. The low-voltage side converter converts the collected low-voltage DC power into AC power, which is then stepped up by the converter transformer. The high-voltage side converter converts the stepped-up AC power into high-voltage DC power, which is then transmitted to the onshore converter station via an overhead cable. The onshore converter station converts the high-voltage DC power back into AC power and injects it into the receiving-end power grid. The wind farm-side converter, the low-voltage side converter, and the onshore converter station all use MMC (Multi-Mechanical Control Unit), while the high-voltage side converter uses DRU (Digital-Receiving Unit).

2. The integrated land-sea DC transmission system based on a unidirectional DC transformer according to claim 1, characterized in that: The wind turbine is a grid-connected type, which uses a wind farm-side converter to control the amplitude and frequency of the AC voltage on the offshore wind farm side, providing voltage support for the wind farm.

3. The integrated land-sea DC transmission system based on a unidirectional DC transformer according to claim 1, characterized in that: The low-voltage side converter is used to control the DC voltage on the submarine cable side and the amplitude and frequency of the AC voltage on the AC side.

4. The integrated land-sea DC transmission system based on a unidirectional DC transformer according to claim 1, characterized in that: The DC-DC transformer is a unidirectional DC transformer used to realize DC voltage level conversion and unidirectional energy transmission. The high-voltage side converter uses a DRU with high-power diodes as uncontrollable devices. Therefore, the onshore converter station is responsible for maintaining the DC voltage stability of the overhead cable and undertaking the reactive power regulation function of the system.

5. A control method for an integrated land-sea DC transmission system based on a unidirectional DC transformer as described in any one of claims 1 to 4, characterized in that: The wind farm-side converter adopts a V / F control strategy to establish the AC voltage on the offshore wind farm side; The low-voltage side converter adopts a V / F control strategy with d-axis additional DC voltage control. This control strategy inputs the deviation between the actual DC side voltage of the low-voltage side converter and the DC voltage command value to the PI control loop, and then uses the output of the PI control loop as the outer loop reference value of the d-axis voltage to control the DC voltage on the submarine cable side. At the same time, by adjusting the amplitude and frequency of the AC side voltage in the middle of the DC-DC transformer, the transmission power of the DRU is controlled to achieve unidirectional rectification and output on the high-voltage side. The onshore converter station adopts a constant DC voltage and constant reactive power control strategy to control the DC voltage of the overhead cable and regulate the reactive power of the system.

6. The control method according to claim 5, characterized in that: When the DC voltage of the DRU is constant, its transmission power is proportional to the voltage amplitude on the valve side. By adjusting the voltage amplitude on the intermediate AC side through the low-voltage side converter, the transmission power of the DRU can be controlled. The specific expression is as follows: ， ， in: U DRU This is the DC-side voltage of the DRU. U v This represents the effective value of the valve-side line voltage of the DRU. X u For commutation reactance, I DRU This is the DC current of the DRU. The power factor angle, P DRU The output active power of the DRU. Q DRU The reactive power absorbed by the DRU.

7. A computer device comprising a memory and a processor, wherein the memory stores a computer program, characterized in that: The processor is used to execute the computer program to implement the control method as described in claim 5 or 6.

8. A computer-readable storage medium storing a computer program, characterized in that: When the computer program is executed by the processor, it implements the control method as described in claim 5 or 6.

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