Ulcc type dc power transmission system based on flexible controllable converter transformer and control method thereof

Abstract

This invention discloses a ULCC-type DC transmission system and its control method based on a flexible controllable converter transformer, belonging to the field of smart grid technology. The ULCC-type DC transmission system includes a unified power regulation unit that deeply integrates a grid-based phase-commutation converter, a flexible controllable converter transformer, and a reactive power compensation filter. The flexible controllable converter transformer achieves continuous adjustable control of the amplitude and phase of the AC power output from the conventional DC converter valve group, while simultaneously completing voltage matching and electrical isolation on the AC and DC sides, and suppressing characteristic harmonics generated by the converter valve, effectively supporting the converter process and improving the system's stable operation. This invention enables continuous and smooth adjustment of wide-range voltage and voltage phase angle. This wide-range precise control capability can significantly improve the DC system's ability to suppress commutation failures, enabling the DC transmission system to effectively cope with the drastic voltage fluctuations caused by large-scale renewable energy integration.

Description

Technical Field

[0001] This invention belongs to the field of smart grid technology, and more specifically, relates to a ULCC type DC transmission system based on a flexible controllable converter transformer and its control method. Background Technology

[0002] Direct current (DC) transmission is the core means of realizing large-scale, long-distance transmission and grid connection of new energy sources. Among them, high-voltage DC transmission systems based on grid-commutated converters have become a key core technology for building new power systems due to their significant advantages such as mature technology, low loss, and large capacity. However, grid-commutated converters are essentially current-source converters, highly dependent on the AC grid for commutation. This leads to limitations in the sending-end system due to transient overvoltages, and there is a coupling constraint between the transmitted power and the grid connection capacity of new energy sources. Problems such as receiving-end voltage stability and insufficient short-circuit ratios are becoming increasingly prominent.

[0003] As a core component of conventional DC transmission, converter transformers have long been limited by traditional electromagnetic theory and mechanical tap changers, exhibiting a distinctly "rigid" regulation characteristic. Their regulation response time is typically greater than 10 seconds, their voltage regulation range is narrow and discontinuous, and they completely lack phase regulation capability. With the high proportion of renewable energy integration, this "rigid" equipment struggles to cope with large and frequent fluctuations in grid voltage and cannot meet the flexible demands of the electricity spot market for minute-level DC power regulation. To improve the performance of grid commutation technology, various improvement approaches have emerged in the industry. HVDC transmission systems based on adaptive commutation converters install static var generators between the converter valve and the converter transformer, utilizing their reactive power compensation and AC filtering functions to reduce reactive power interaction during disturbances and eliminate the need for AC filters. Controllable device commutation technology employs an "LCC+IGBT" scheme, using controllable devices to achieve forced commutation and avoid commutation failure. Furthermore, hybrid commutation converter technology replaces the LCC with an IGCT fully controlled device to achieve active shutdown. However, the above solutions mainly focus on topology improvements or adding external compensation devices on the DC converter valve side, with less emphasis on electromagnetic mechanism innovation on the converter transformer side.

[0004] Therefore, when a near-area AC fault occurs in the receiving-end power grid, conventional DC systems, lacking fast and effective voltage support, are prone to large-scale commutation failures or even DC blocking, seriously threatening the operational safety of the power grid. Summary of the Invention

[0005] In view of the above-mentioned defects or improvement needs of the existing technology, the present invention provides a ULCC type DC transmission system based on a flexible controllable converter transformer and its control method, the purpose of which is to solve the technical problem that the existing DC transmission system lacks the ability to quickly adjust amplitude and phase.

[0006] To achieve the above objectives, according to one aspect of the present invention, a ULCC-type DC transmission system based on a flexible controllable converter transformer is provided, comprising: A DC line, connected to the power supply, is used for long-distance DC power transmission; A filter and smoothing reactor is connected to the DC line to filter DC power transmitted over long distances. A conventional DC converter valve group is connected to the DC line via the filter and smoothing reactor to convert the received DC power into AC power. A flexible and controllable converter transformer is connected to the conventional DC converter valve group to continuously and adjustablely control the voltage amplitude and phase of the AC power output by the conventional DC converter valve group, and to achieve voltage matching and electrical isolation between the AC and DC sides. The receiving-end AC bus is connected to the flexible controllable converter transformer to receive the converted AC power and feed it into the receiving-end AC grid, while also serving as the commutation voltage reference. A reactive power compensation filter is connected to the receiving-end AC bus to absorb the characteristic harmonics of the AC power on the receiving-end AC bus and provide fundamental reactive power compensation.

[0007] Furthermore, the flexible controllable converter transformer is also used to respond rapidly to the state of the power grid connected to the receiving-end AC bus and provide emergency reactive power and voltage support to the receiving-end AC bus to suppress commutation failure of the conventional DC converter valve group.

[0008] Furthermore, the flexible controllable converter transformer is an external structure, comprising: The first connecting transformer is used to transform the AC voltage on the valve side to the AC bus voltage at the receiving end; A parallel transformer, the first end of which is connected to the receiving end AC busbar for drawing electrical energy; The first AC / AC power electronic conversion module has its rectifier side connected to the second end of the parallel transformer, and is used to realize the conversion of voltage and current amplitude and phase; A series transformer has one end connected to the inverter side of the first AC / AC power electronic conversion module and the other end connected to the low-voltage side of the grid winding of the first series transformer, with its neutral point grounded. The series transformer is used to superimpose the voltage injected by the first AC / AC power electronic conversion module with the electromotive force of the grid winding to jointly regulate the grid voltage.

[0009] Furthermore, the flexible controllable converter transformer has a built-in structure, including: The second connecting transformer has a grid-side winding, a valve-side winding, an energy extraction winding, and a magnetically controlled voltage regulating winding integrated on its core; the valve-side winding and the grid-side winding are respectively wound on different core columns of the second connecting transformer; the energy extraction winding and the valve-side winding are wound on the same core column; the magnetically controlled voltage regulating winding is wound separately on another core column of the second connecting transformer; The second AC / AC power electronic conversion module has its rectifier side connected to the energy harvesting winding and its inverter side connected to the magnetically controlled voltage regulating winding, which is used to realize the active magnetically controlled voltage regulation function.

[0010] Furthermore, the valve-side winding includes two sets, one set using a star connection and the other set using a delta connection; the grid-side winding uses a star connection and is connected to the receiving-end AC power grid.

[0011] Furthermore, when adopting a unified AC / DC coordinated control architecture, unified and coordinated control is achieved through the rapid voltage regulation control of the flexible controllable converter transformer, the firing angle adjustment of the conventional DC converter valve group, and the switching of the reactive power compensation filter.

[0012] According to another aspect of the present invention, a control method for a ULCC-type DC transmission system based on a flexible controllable converter transformer is provided, comprising: A1: Based on the actual operating status of the receiving-end grid, the voltage amplitude and phase of the AC power output by the conventional DC converter valve group are continuously adjusted in real time using a flexible and controllable converter transformer. A2: After the adjustment action is performed, the actual operating status after adjustment is continuously monitored, and the actual operating status is fed back to the input terminal for comparison with the control target command value. Feedback adjustment is performed based on the comparison result to form a closed-loop control system.

[0013] Furthermore, the control method also includes: when adopting an AC / DC unified coordinated control architecture, responding rapidly according to the state of the power grid connected to the receiving-end AC bus, and providing emergency reactive power and voltage support to the receiving-end AC bus to suppress the commutation failure of the conventional DC converter valve group.

[0014] Furthermore, A1 includes: real-time monitoring of the amplitude and phase of the voltage on the receiving end of the power grid, determining whether it fluctuates within the normal operating range; if so, activating the quasi-steady-state optimization strategy under the unified AC / DC coordinated control architecture to continuously adjust the voltage amplitude and phase in real time; the quasi-steady-state optimization strategy includes: calculating the compensation voltage vector required to maintain the constant valve-side voltage based on real-time dispatch instructions and the fluctuation of new energy output, and adjusting according to the compensation voltage vector.

[0015] Furthermore, A1 includes: real-time identification of whether there is a voltage drop or severe phase jump in the AC bus that exceeds the normal operating range; if so, switching to a transient emergency support strategy under the AC / DC unified coordinated control architecture; the transient emergency support strategy includes: calculating an emergency compensation voltage vector that can provide maximum voltage support and forced phase correction based on the current commutation failure risk; and using the millisecond-level response speed of the flexible controllable converter transformer to perform compensation in a very short time.

[0016] In summary, compared with the prior art, the above-described technical solutions conceived by this invention can achieve the following beneficial effects: (1) This invention provides a ULCC-type DC transmission system based on a flexible controllable converter transformer, which deeply integrates the grid-commutated converter (LCC), the flexible controllable converter transformer, and the reactive power compensation filter to form a unified power regulation unit. The flexible controllable converter transformer achieves continuous adjustable control of the amplitude and phase of the AC power output from the conventional DC converter valve group, while completing voltage matching and electrical isolation on the AC and DC sides, and suppressing the characteristic harmonics generated by the converter valve, effectively supporting the converter process and improving the system's stable operation capability. This solution flexibly modifies the "rigid" converter transformer to give it a similar amplitude and phase decoupling control function to UPFC, enabling continuous and smooth adjustment of wide-range voltage and voltage phase angle. This wide-range precise control capability allows the LCC DC transmission system to not only effectively cope with the drastic voltage fluctuations caused by large-scale new energy access, but also meet the demand of the electricity spot market for minute-level large-scale DC power adjustment, greatly improving the flexibility and operating efficiency of conventional DC transmission in new power systems.

[0017] (2) This scheme realizes unified and coordinated control of AC and DC, which significantly improves the DC system’s ability to suppress commutation failure, effectively solves the problem of voltage fluctuation support caused by high proportion of new energy access, and meets the demand for large-scale regulation of DC minute-level active power in the electricity spot market environment.

[0018] (3) Based on the flexible transformation method, this scheme can divide the flexible controllable converter transformer into external and internal types. The external flexible controllable converter transformer does not modify the original connected transformer body structure, but only changes the grid-side winding connection method. For the external topology scheme, the auxiliary flexible control component is connected in series between the conventional converter transformer grid-side winding and the grounding point, which can realize the flexible upgrade and transformation of the existing LCC project without changing the main structure of the existing converter station. With the electromagnetic optimization design using high-end electrical steel, this scheme can reduce the transformer no-load loss while providing flexible amplitude and phase adjustment function, taking into account the economic efficiency and energy-saving and environmental protection requirements of the project transformation.

[0019] (4) The built-in flexible controllable converter transformer in this scheme requires modification of the original connected transformer body structure to wind the energy harvesting winding and the magnetically controlled voltage regulating winding. For the built-in topology scheme, through the deep integration design of the power electronic converter and the converter transformer winding, direct active control of electromagnetic energy is realized, which is suitable for newly built flexible control converter stations, and can optimize the internal electromagnetic distribution to the maximum extent and reduce the system integration cost. Utilizing its active magnetically controlled voltage regulation theory, the core flux path can be reconstructed in real time, and while achieving wide-range voltage regulation, excitation inrush current and DC bias are suppressed, thereby improving the operational reliability and high availability of the equipment.

[0020] (5) This scheme achieves an overall improvement in the performance of the AC / DC system through the deep coupling of LCC converter valve firing angle adjustment and flexible voltage regulation of the flexible controllable converter transformer. By optimizing the distribution strategy of reactive power between the converter and the traditional filter, this scheme reduces the impact of frequent switching of passive filters on the power grid, unifies the operating characteristics under different connection methods, reduces the types of backup equipment, and effectively improves the engineering economy and operation and maintenance convenience of large converter stations. Furthermore, by unifying and rapidly coordinating the flexible controllable converter transformer, conventional DC converter valve group and reactive power compensation filter, it makes up for the defects of slow response, narrow voltage regulation range and no phase regulation capability of traditional converter transformer mechanical tap changers. Utilizing the rapid response characteristics of the flexible controllable converter transformer, this scheme can achieve unified coordinated control of AC and DC in the steady state and transient process of the system, provide precise amplitude and phase regulation and dynamic reactive power support in the event of AC side faults, significantly improve the voltage stability of the receiving end power grid, effectively reduce the risk of commutation failure of LCC converter stations and accelerate power recovery after faults. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of a ULCC type DC transmission system based on a flexible controllable converter transformer provided in an embodiment of this application; Figure 2 This is a schematic diagram of a DC transmission system topology based on an externally mounted flexible controllable converter transformer, provided in one embodiment of this application. Figure 3 This is a schematic diagram of a DC transmission system topology based on a built-in flexible controllable converter transformer provided in an embodiment of this application; Figure 4 This is a schematic diagram illustrating the amplitude and phase modulation principle of a flexible controllable converter transformer according to an embodiment of this application; Figure 5 This is a flowchart illustrating the implementation of the amplitude and phase adjustment function of a flexible controllable converter transformer according to an embodiment of this application. Detailed Implementation

[0022] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.

[0023] Example 1 This embodiment provides a ULCC type DC transmission system based on a flexible controllable converter transformer, including: a DC line, a filter and smoothing reactor, a conventional DC converter valve group, a flexible controllable converter transformer, a receiving-end AC bus, and a reactive power compensation filter.

[0024] The system includes: a DC line connected to the supply end for long-distance DC power transmission; a filter and smoothing reactor connected to the DC line for filtering the transmitted DC power; a conventional DC converter valve group connected to the DC line via the filter and smoothing reactor for converting the received DC power into AC power; a flexible controllable converter transformer connected to the conventional DC converter valve group for continuously adjustable control of the voltage amplitude and phase of the AC power output from the conventional DC converter valve group, and for achieving voltage matching and electrical isolation between the AC and DC sides; a receiving-end AC bus connected to the flexible controllable converter transformer for receiving the converted AC power and feeding it into the receiving-end AC grid, while also serving as a commutation voltage reference; and a reactive power compensation filter connected to the receiving-end AC bus for absorbing characteristic harmonics of the AC power on the receiving-end AC bus and providing fundamental reactive power compensation.

[0025] like Figure 1 As shown in the figure, this application provides a schematic diagram of a ULCC-type DC transmission system based on a flexible controllable converter transformer, which is an improvement scheme for conventional DC transmission systems, such as... Figure 1As shown, the DC line is used for transmitting large-capacity, long-distance DC power to the receiving-end conventional DC converter valve group. The filter and smoothing reactor, connected between the DC line and the conventional DC converter valve group's LCC, primarily suppresses transient overvoltages during system start-up and shutdown and weakens DC-side harmonic voltages during operation. The conventional DC converter valve group converts the DC power received from the DC line into AC power, feeding the converted AC power into the receiving-end AC bus via a flexible controllable converter transformer. The reactive power compensation filter absorbs a large number of specific frequency harmonics generated by the conventional DC converter valve group during commutation, ensuring the power quality fed into the AC grid and providing fundamental reactive power compensation for the conventional DC converter valve group. The flexible controllable converter transformer, available in external and internal types, continuously and adjustablely controls the amplitude and phase of the AC power output from the conventional DC converter valve group, while simultaneously performing voltage matching and electrical isolation on the AC and DC sides, suppressing characteristic harmonics generated by the converter valves, effectively supporting the converter process and improving system stability.

[0026] As an optional implementation, the flexible controllable converter transformer is also used to respond quickly to the state of the power grid connected to the receiving-end AC bus and provide emergency reactive power and voltage support to the receiving-end AC bus to suppress commutation failure of the conventional DC converter valve group.

[0027] As an optional implementation, the flexible controllable converter transformer is an external structure, comprising: a first connecting transformer, a parallel transformer, a first AC / AC power electronic conversion module, and a series transformer. Specifically, the first connecting transformer is used to convert the valve-side AC voltage to the receiving-end AC bus voltage. The parallel transformer has its first end connected to the receiving-end AC bus for drawing electrical energy. The first AC / AC power electronic conversion module has its rectifier side connected to the second end of the parallel transformer for converting the voltage and current amplitude and phase. The series transformer has one end connected to the inverter side of the first AC / AC power electronic conversion module and the other end connected to the low-voltage side of the grid-side winding of the first connecting transformer, with its neutral point grounded. The series transformer is used to superimpose the voltage injected by the first AC / AC power electronic conversion module with the electromotive force of the grid-side winding to jointly regulate the grid voltage.

[0028] like Figure 2 As shown, S8, S9, and S10 are the externally mounted connection transformer bodies; externally mounted converter transformers do not require modification of the main body. S11, S12, and S13 are valve-side windings (three-phase), one with a star (Y) connection and the other with a delta (△) connection, used to transmit conventional DC converter valve groups to the AC grid via electromagnetic coupling. S14, S15, and S16 are grid-side windings (three-phase), serving as the direct connection terminals between the converter transformer and the external AC grid, used to carry ultra-high voltage level electrical energy input. and These represent the three-phase voltages (a, b, and c) of the grid-side windings. S17 is an external active magnetically controlled power supply (neutral point grounded), comprising a series transformer, a parallel transformer, and an AC / AC power electronic conversion module. One end of the series transformer is connected in series with the grid-side winding of the parallel transformer, and the other end is connected to the inverter side of the AC / AC power electronic conversion module. One end of the parallel transformer is connected to the grid for energy extraction, and the other end is connected to the rectifier side of the AC / AC power electronic conversion module. The AC / AC power electronic conversion module is used to convert the voltage and current amplitude and phase. The external active magnetically controlled power supply and the grid-side winding voltage together constitute the grid-side voltage of the receiving-end AC grid. and .

[0029] As an optional implementation, the flexible controllable converter transformer has a built-in structure, including: a second connecting transformer and a second AC / AC power electronic conversion module. The second connecting transformer has a grid-side winding, a valve-side winding, an energy extraction winding, and a magnetically controlled voltage regulating winding integrated on its core. The valve-side winding and the grid-side winding are wound on different core columns of the second connecting transformer, respectively. The energy extraction winding and the valve-side winding are wound on the same core column. The magnetically controlled voltage regulating winding is wound separately on another core column of the second connecting transformer. The second AC / AC power electronic conversion module has its rectifier side connected to the energy extraction winding and its inverter side connected to the magnetically controlled voltage regulating winding, used to realize active magnetically controlled voltage regulation. Further, the valve-side winding includes two sets, one set using a star connection and the other using a delta connection. The grid-side winding uses a star connection and is connected to the receiving-end AC power grid.

[0030] like Figure 3 As shown in the figure, this application provides a schematic diagram of a DC transmission system topology based on a built-in flexible controllable converter transformer. The built-in solution integrates voltage regulation functionality into the core through deep reconfiguration of the transformer body, making it suitable for new construction projects. S18, S19, and S20 represent the connecting transformer body; external converter transformers require body modification to achieve flexibility. S21, S22, and S23 represent built-in active magnetically controlled power supplies. The energy harvesting winding and the grid-side winding are wound on the same connecting transformer core column, while the magnetically controlled voltage regulating winding is wound separately on another core column of the connecting transformer. The energy harvesting winding and the magnetically controlled voltage regulating winding are connected via an AC / AC power electronic conversion module. The energy harvesting winding directly draws the energy required for active voltage regulation from the internal electromagnetic field. Based on the theory of active magnetically controlled voltage regulation, the AC / AC power electronic conversion module adjusts the current in the magnetically controlled voltage regulating winding in real time, thereby changing the magnetic flux path distribution in the core. This method can achieve rapid and smooth adjustment of valve-side voltage without changing the physical turns ratio, and has better electromagnetic distribution and lower losses.

[0031] As an optional implementation, when adopting an AC / DC unified coordinated control architecture, unified and coordinated control is achieved through the rapid voltage regulation control of the flexible controllable converter transformer, the firing angle adjustment of the conventional DC converter valve group, and the switching of the reactive power compensation filter.

[0032] Example 2 This implementation provides a control method for a ULCC-type DC transmission system based on a flexible controllable converter transformer, including: A1: Real-time continuous adjustment of the voltage amplitude and phase of the AC power output from the conventional DC converter valve group using a flexible controllable converter transformer, based on the actual operating state of the receiving-end grid. A2: After executing the adjustment action, continuously monitoring the actual operating state after adjustment, feeding the actual operating state back to the input terminal for comparison with the control target command value, and performing feedback adjustment based on the comparison result to form a closed-loop control system. Further, the control method also includes: When using a unified AC / DC control architecture, rapidly responding to the state of the grid connected to the receiving-end AC bus, and providing emergency reactive power and voltage support to the receiving-end AC bus to suppress commutation failure of the conventional DC converter valve group.

[0033] like Figure 4 As shown in the embodiments of this application, the amplitude and phase modulation mechanism of a flexible controllable converter transformer under different operating conditions is illustrated through the principle of vector synthesis. Its core logic lies in: by sensing the state changes of the receiving-end AC grid in real time, dynamically adjusting the compensation voltage vector injected by the power electronic conversion module, so that it works in conjunction with the grid-side voltage and leakage reactance voltage to always maintain the constant and stable valve-side voltage.

[0034] In quasi-steady-state operation scenarios, due to frequent electricity market transactions and random fluctuations in renewable energy output, the grid-side voltage of the receiving-end power grid often exhibits persistent amplitude drift or phase fluctuations. Flexible controllable converter transformers (VCTs) rapidly sense the shift in grid-side voltage and actively inject a specific compensation voltage vector, enabling real-time vector synthesis with the fluctuating grid-side voltage and system leakage reactance voltage drop. This compensation process ensures that the synthesized valve-side voltage vector maintains both amplitude and phase constancy in the spatial coordinate system. This allows conventional DC transmission systems to readily cope with the operational challenges brought by large-scale renewable energy integration and meets the urgent demand of the electricity spot market for flexible DC power adjustment, fundamentally improving the regulation efficiency and response level of conventional DC in new power systems.

[0035] Under transient fault conditions in the power grid, the receiving-end AC bus voltage typically experiences a sharp amplitude drop or phase jump, which can easily lead to commutation failure of conventional converter valve groups due to insufficient commutation voltage. In this situation, the flexible controllable converter transformer utilizes its rapid vector reconfiguration capability to generate a wide-range and precise compensation voltage vector at the instant of the fault, performing real-time electromagnetic repair on the grid-side voltage after the jump. Through this voltage compensation mechanism, the valve-side voltage is maintained within the safe operating range throughout the entire transient process, providing the necessary commutation voltage support for the converter valves. This significantly reduces the probability of commutation failure and effectively suppresses the risk of DC blockage caused by consecutive commutation failures, greatly accelerating the power recovery speed of the system after fault clearance.

[0036] As an optional implementation, A1 includes: real-time monitoring of the amplitude and phase of the voltage on the receiving end of the power grid, and determining whether it fluctuates within the normal operating range. If so, the quasi-steady-state optimization strategy under the unified AC / DC coordinated control architecture is activated to continuously adjust the voltage amplitude and phase in real time. The quasi-steady-state optimization strategy includes: calculating the compensation voltage vector required to maintain a constant valve-side voltage based on real-time dispatch instructions and fluctuations in new energy output, and adjusting according to the compensation voltage vector.

[0037] like Figure 5 As shown, the main integrated functions of the ULCC type DC transmission system based on the flexible controllable converter transformer are: quasi-steady-state operation and transient fault operation.

[0038] Specifically, the implementation process of the quasi-steady-state operating mode is as follows: First, the amplitude and phase of the voltage on the receiving end of the power grid are monitored in real time through a data acquisition system to determine whether it fluctuates within the normal operating range. Second, when it is confirmed that the fluctuation is within the quasi-steady-state range (such as being affected by random fluctuations in renewable energy or dispatch instructions), the UDAC quasi-steady-state optimization strategy is activated. The specific implementation principle is as follows: The UDAC strategy calculates the precise compensation voltage vector required to maintain a constant valve-side voltage based on the principle of vector synthesis, according to real-time dispatch instructions and renewable energy output fluctuations. Compared with the discrete adjustment of the converter transformer tap in traditional LCC systems, this application utilizes the continuous adjustment characteristics of flexible and controllable converter transformers to achieve the primary operational goals of improving efficiency and promoting power consumption while ensuring the stability of the valve-side voltage.

[0039] As an optional implementation, A1 includes: real-time identification of whether there is a voltage drop or severe phase jump on the AC bus that exceeds the normal operating range. If so, switching to a transient emergency support strategy under the UDAC architecture. The transient emergency support strategy includes: calculating an emergency compensation voltage vector that can provide maximum voltage support and forced phase correction based on the current commutation failure risk. Compensation is performed in a very short time using the millisecond-level response speed of the flexible controllable converter transformer.

[0040] Specifically, the implementation process of the transient fault operation mode is as follows: First, the high-speed detection unit identifies in real time whether there is a voltage drop or severe phase jump on the AC bus that exceeds the normal operating range. Second, when a transient fault is detected on the AC side, the system immediately switches to the UDAC transient emergency support strategy. The execution logic of this mode is as follows: The UDAC strategy prioritizes assessing the current commutation failure risk and quickly calculates the emergency compensation voltage vector that can provide maximum voltage support and forced phase correction. Utilizing the millisecond-level response speed of the flexible and controllable link, the physical repair of the voltage vector is completed in a very short time. The primary goal is to ensure the safety and stable commutation of the switching process, effectively reducing the risk of DC blocking caused by grid disturbances.

[0041] Specifically, the closed-loop control logic of the above-mentioned regulation mode is as follows: by actively changing the equivalent electromagnetic relationship between the grid side and the valve side, the voltage amplitude and phase on the valve side are adjusted in real time. After the system executes the adjustment action, it continuously monitors the actual operating effect after adjustment and feeds the result back to the input terminal in real time for comparison, thereby forming a closed-loop control system that ensures the system's accurate control and optimization of the overall AC / DC state.

[0042] In summary, the ULCC-type DC transmission system based on a flexible and controllable converter transformer constructed in this application fundamentally breaks through the long-standing "rigid" bottleneck of isolated control of converter valves and converter transformers in conventional DC transmission systems by introducing a flexible and controllable element with continuous amplitude and phase adjustment capabilities. This system relies on a unified AC / DC coordinated control strategy, enabling the electromagnetic transmission characteristics of the transformer to adapt to the triggering state of the converter valve, achieving physical-level coupling and dynamic coordination in the AC / DC energy conversion process, and significantly improving the response speed and operational resilience of the DC system under complex operating conditions.

[0043] To address different engineering application needs, the two implementation schemes proposed in this application each possess unique technical advantages. The external topology scheme focuses on the flexible upgrade of existing converter stations. This scheme, by injecting an active voltage vector in series into the grid-side circuit of the connecting transformer, endows traditional converter stations with flexible voltage / phase regulation capabilities without altering the existing converter transformer structure. This allows the system to switch to flexible voltage regulation, flexible phase shifting, or transient recovery modes according to grid demand, reducing the risk of commutation failure caused by grid disturbances at the receiving end in a highly economical manner and significantly improving the operational safety of existing projects.

[0044] For the built-in topology scheme, the focus is on the deep functional integration of new projects. This scheme directly injects voltage regulation energy into the internal magnetic field by reconstructing the core magnetic circuit of the connecting transformer, and realizes the physical integration of voltage transformation and flexible control using active magnetically controlled voltage regulation theory. This design avoids the redundant configuration of large-scale external compensation equipment and significantly optimizes the system integration and spatial layout of the converter station. At the same time, by optimizing the internal magnetic flux distribution and loss design, this scheme reduces operating losses while providing rapid voltage regulation support, achieving a balance between high efficiency and high reliability.

[0045] Furthermore, the ULCC system proposed in this application effectively reduces the backup redundancy of equipment and daily operation and maintenance costs in the station by unifying the transformer wiring form and multi-scale collaborative protection logic, providing a global optimization solution that balances flexibility and economy for DC transmission technology under the background of high proportion of new energy access.

[0046] The technical features of the embodiments described above can be combined arbitrarily. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as the combination of these technical features does not contradict each other, it should be considered within the scope of this specification. It should be noted that the terms "in one embodiment," "for example," and "again" in this invention are intended to illustrate the invention and are not intended to limit the invention.

[0047] The embodiments described above are merely examples of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the patent application. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention.

Claims

1. A ULCC-type DC transmission system based on a flexible controllable converter transformer, characterized in that, include: A DC line, connected to the power supply, is used for long-distance DC power transmission; A filter and smoothing reactor is connected to the DC line to filter DC power transmitted over long distances. A conventional DC converter valve group is connected to the DC line via the filter and smoothing reactor to convert the received DC power into AC power. A flexible and controllable converter transformer is connected to the conventional DC converter valve group to continuously and adjustablely control the voltage amplitude and phase of the AC power output by the conventional DC converter valve group, and to achieve voltage matching and electrical isolation between the AC and DC sides. The receiving-end AC bus is connected to the flexible controllable converter transformer to receive the converted AC power and feed it into the receiving-end AC grid, while also serving as the commutation voltage reference. A reactive power compensation filter is connected to the receiving-end AC bus to absorb the characteristic harmonics of the AC power on the receiving-end AC bus and provide fundamental reactive power compensation.

2. The ULCC type DC transmission system based on a flexible controllable converter transformer as described in claim 1, characterized in that, The flexible controllable converter transformer is also used to respond quickly to the state of the power grid connected to the receiving-end AC bus and provide emergency reactive power and voltage support to the receiving-end AC bus to suppress commutation failure of the conventional DC converter valve group.

3. The ULCC type DC transmission system based on a flexible controllable converter transformer as described in claim 1, characterized in that, The flexible controllable converter transformer is an external structure, comprising: The first connecting transformer is used to transform the AC voltage on the valve side to the AC bus voltage at the receiving end; A parallel transformer, the first end of which is connected to the receiving end AC busbar for drawing electrical energy; The first AC / AC power electronic conversion module has its rectifier side connected to the second end of the parallel transformer, and is used to realize the conversion of voltage and current amplitude and phase; A series transformer has one end connected to the inverter side of the first AC / AC power electronic conversion module and the other end connected to the low-voltage side of the grid winding of the first series transformer, with its neutral point grounded. The series transformer is used to superimpose the voltage injected by the first AC / AC power electronic conversion module with the electromotive force of the grid winding to jointly regulate the grid voltage.

4. The ULCC type DC transmission system based on a flexible controllable converter transformer as described in claim 1, characterized in that, The flexible controllable converter transformer has a built-in structure and includes: The second connecting transformer has a grid-side winding, a valve-side winding, an energy extraction winding, and a magnetically controlled voltage regulating winding integrated on its core; the valve-side winding and the grid-side winding are respectively wound on different core columns of the second connecting transformer; the energy extraction winding and the valve-side winding are wound on the same core column; the magnetically controlled voltage regulating winding is wound separately on another core column of the second connecting transformer; The second AC / AC power electronic conversion module has its rectifier side connected to the energy harvesting winding and its inverter side connected to the magnetically controlled voltage regulating winding, which is used to realize the active magnetically controlled voltage regulation function.

5. The ULCC type DC transmission system based on a flexible controllable converter transformer as described in claim 4, characterized in that, The valve-side winding includes two sets, one set using a star connection and the other set using a delta connection; The grid-side winding adopts a star connection and is connected to the receiving-end AC power grid.

6. The ULCC type DC transmission system based on a flexible controllable converter transformer as described in claim 1, characterized in that, When an AC / DC unified coordinated control architecture is adopted, unified and coordinated control is achieved through the rapid voltage regulation control of the flexible controllable converter transformer, the firing angle adjustment of the conventional DC converter valve group, and the switching of the reactive power compensation filter.

7. A control method for a ULCC-type DC transmission system based on a flexible controllable converter transformer, characterized in that, include: A1: In the actual operating state of the receiving end grid side of the ULCC type DC transmission system based on the flexible controllable converter transformer according to claims 1-6, the voltage amplitude and phase of the AC power output by the conventional DC converter valve group are continuously adjusted in real time using the flexible controllable converter transformer. A2: After the adjustment action is performed, the actual operating status after adjustment is continuously monitored, and the actual operating status is fed back to the input terminal for comparison with the control target command value. Feedback adjustment is performed based on the comparison result to form a closed-loop control system.

8. The control method for a ULCC-type DC transmission system based on a flexible controllable converter transformer as described in claim 7, characterized in that, Also includes: When an AC / DC unified coordinated control architecture is adopted, the system responds quickly according to the state of the power grid connected to the receiving-end AC bus and provides emergency reactive power and voltage support to the receiving-end AC bus to suppress the commutation failure of the conventional DC converter valve group.

9. The control method for a ULCC-type DC transmission system based on a flexible controllable converter transformer as described in claim 8, characterized in that, The A1 includes: real-time monitoring of the amplitude and phase of the voltage on the receiving end of the power grid, determining whether it fluctuates within the normal operating range; if so, activating the quasi-steady-state optimization strategy under the unified AC / DC coordinated control architecture to continuously adjust the voltage amplitude and phase in real time; the quasi-steady-state optimization strategy includes: calculating the compensation voltage vector required to maintain the constant valve-side voltage based on real-time dispatch instructions and the fluctuation of new energy output, and adjusting according to the compensation voltage vector.

10. The control method for a ULCC-type DC transmission system based on a flexible controllable converter transformer as described in claim 8, characterized in that, The A1 includes: real-time identification of whether there is a voltage drop or severe phase jump in the AC bus that exceeds the normal operating range; if so, switching to a transient emergency support strategy under the AC / DC unified coordinated control architecture; the transient emergency support strategy includes: calculating an emergency compensation voltage vector that can provide maximum voltage support and forced phase correction based on the current commutation failure risk; and using the millisecond-level response speed of the flexible controllable converter transformer to perform compensation in a very short time.

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