Built-in flexible controllable converter transformer and control method

Abstract

The application discloses a built-in flexible controllable converter transformer and a control method, which comprises a transformer body, a series converter, a parallel converter, a power taking transformer and a control unit. The transformer body comprises a valve side winding, a grid side winding and a series winding integrated in the inside of an iron core. The direct current side of the series converter is connected with the direct current side of the parallel converter to form a common direct current bus. The alternating current side of the parallel converter is connected with a receiving end alternating current grid through the power taking transformer. The control unit is configured to control the series converter to output a compensation voltage to adjust the valve side voltage fluctuation and control the parallel converter to stabilize the direct current bus voltage. The application can realize fast, accurate and stable regulation and control of the commutation voltage of the LCC-HVDC system inverter side, suppresses the commutation failure and improves the safe and stable operation ability of the high voltage direct current transmission system.

Description

Technical Field

[0001] This invention belongs to the field of high voltage direct current transmission technology, specifically relating to a built-in flexible controllable converter transformer and control method, which is applied to the suppression of commutation failure and active voltage regulation on the inverter side of a grid commutator high voltage direct current (LCC-HVDC) system. Background Technology

[0002] LCC-HVDC (Limited-Chip Capacitor-based High Voltage Direct Current) transmission systems, with their advantages of large transmission capacity, long transmission distance, low loss, and high technological maturity, have become core technologies and equipment for cross-regional energy optimization and large-capacity transmission of new energy, and are widely used in ultra-high voltage direct current transmission projects. The inverter side of the LCC-HVDC system uses semi-controlled thyristors as the core converter device. Its commutation process relies entirely on the natural commutation conditions provided by the AC system, and it is highly sensitive to the amplitude, phase, and waveform of the AC voltage.

[0003] When the inverter-side AC system experiences short-circuit faults, voltage dips, reactive power fluctuations, or harmonic disturbances, the receiving-end AC bus voltage drops rapidly, the inverter-side commutation margin angle decreases sharply, and the thyristors cannot restore their forward blocking capability within the specified time, leading to commutation failure. Commutation failure is the most frequent and damaging type of fault in LCC-HVDC systems. It directly causes a surge in DC current, a collapse in DC voltage, and violent oscillations in transmitted power. In severe cases, it can lead to bipolar blocking of the DC system, unplanned outages, and even cascading faults, threatening the safe and stable operation of the receiving-end AC grid and causing the risk of large-scale power outages. Furthermore, with the large-scale grid connection of new energy power generation such as wind power and photovoltaics, their inherent intermittent, volatile, and random characteristics further exacerbate AC system voltage fluctuations, leading to frequent switching of converter transformer taps. This poses additional challenges to the LCC-HVDC commutation process, reduces commutation stability, and significantly increases the probability of commutation failure.

[0004] Existing technical solutions for suppressing commutation failure on the inverter side of LCC-HVDC mainly fall into four categories: The first category consists of passive and active reactive power compensation schemes on the AC side, including devices such as Static Var Compensators (SVCs), Static Synchronous Compensators (STATCOMs), and Static Var Generators (SVGs). These devices indirectly improve commutation conditions by rapidly injecting reactive power during faults to raise the AC bus voltage. However, these devices can only provide indirect voltage support and cannot directly regulate the valve-side commutation voltage, thus having limited ability to suppress deep voltage sags.

[0005] The second category is the converter valve control strategy optimization scheme, which improves commutation robustness by increasing the turn-off angle, early triggering, commutation failure prediction control, and DC current auxiliary adjustment. However, this type of method is limited by the physical characteristics of the thyristor and the switching frequency of the device, and the adjustment range is limited. Excessively increasing the turn-off angle will significantly reduce the system operating efficiency, increase the converter valve loss and heat generation, and cannot fundamentally eliminate the risk of commutation failure.

[0006] The third type is the external series compensation and auxiliary commutation device, which achieves active regulation of the valve-side voltage by connecting independent transformers and modular multilevel converters, etc., in series on the grid side of the converter transformer. However, this type of scheme adopts a separate structure for the main transformer and the control unit, and the magnetic circuits are independent of each other, resulting in defects such as high losses and complex coordination control; the power electronic converter is installed externally, with weak anti-interference ability and insufficient fault ride-through capability, and does not fully utilize the integration potential of the transformer magnetic circuit.

[0007] The fourth type is the traditional converter transformer, which only has voltage transformation, electrical isolation and excitation functions, but no active voltage regulation capability. When the AC system experiences a voltage dip, it cannot actively raise the commutation voltage on the valve side, and cannot prevent commutation failure from the source.

[0008] In summary, existing technologies generally suffer from problems such as low magnetic circuit integration, slow response speed, insufficient control precision, significant impact of line impedance, and large amount of engineering modifications, making it difficult to meet the operational requirements of the new generation of ultra-high voltage LCC-HVDC systems for high reliability, low loss, and strong fault ride-through capability. Summary of the Invention

[0009] The purpose of this invention is to address the shortcomings of the aforementioned background technology and provide a built-in flexible controllable converter transformer and control method that can achieve rapid, accurate, and stable regulation of the commutation voltage on the inverter side of the LCC-HVDC system, suppress commutation failure, and improve the safe and stable operation capability of the high-voltage direct current transmission system.

[0010] A first aspect of this invention provides a built-in flexible controllable converter transformer applied to the inverter side of an LCC-HVDC system for regulating valve-side voltage and suppressing commutation failure. The transformer includes a transformer body, a series converter, a parallel converter, an energy extraction transformer, and a control unit. The transformer body includes a valve-side winding, a grid-side winding, and a series winding integrated inside the core. The valve-side winding is used to connect to the LCC inverter-side converter valve, and the grid-side winding is used to connect to the receiving-end AC power grid. The series winding includes a primary winding and a secondary winding. The primary winding is connected in series with the grid-side winding, and the secondary winding is connected in series with the AC side of the series converter. The series winding is used to couple the compensation voltage output by the series converter into the grid-side winding. The DC side of the series converter and the DC side of the parallel converter are connected to form a common DC bus; the series converter is used to output compensation voltage. The AC side of the parallel converter is connected to the receiving-end AC power grid through the energy harvesting transformer, and the energy harvesting transformer provides power to the parallel converter. The control unit is configured to control the output compensation voltage of the series converter to regulate valve-side voltage fluctuations and control the DC bus voltage of the parallel converter to stabilize the DC bus voltage.

[0011] A second aspect of this invention provides a control method based on the above-described built-in flexible controllable converter transformer, including adjusting the valve-side voltage: Calculate the voltage deviation based on the actual voltage and reference voltage of the receiving-end AC power grid; The positive-sequence component and the negative-sequence component are obtained by performing positive-sequence decomposition and dq decoupling on the voltage deviation, the output voltage and current of the series converter. A pulse drive signal is generated based on the positive and negative sequence components using a dual closed-loop control strategy, which drives the series converter to output a compensation voltage to regulate the valve-side voltage fluctuation.

[0012] The beneficial effects of this invention are as follows: (1) The present invention adopts a multi-winding transformer structure, which integrates the series windings on the traditional transformer core and shares the magnetic circuit with the valve side and grid side windings, shortens the power transmission path, reduces line impedance and loss, improves space utilization and energy conversion efficiency, reduces cost, and adapts to the upgrade requirements of UHVDC projects.

[0013] (2) The present invention integrates the series converter with the transformer body. In case of fault, the double closed-loop control is used to inject compensation voltage into the series winding based on the voltage drop command, adjust the valve side voltage, compensate for the commutation voltage time and area loss, ensure sufficient turn-off time of thyristors, and suppress commutation failure caused by symmetrical and asymmetrical faults.

[0014] (3) The present invention connects the DC side of the series and parallel converters, stabilizes the DC bus voltage of the parallel converter, and quickly releases energy to provide power support for the series side in case of a fault, thereby achieving optimized matching of voltage compensation and energy supply, dynamically adjusting the active power flow direction, ensuring continuous high power compensation capability, and taking into account both voltage stability and reactive power support.

[0015] (4) Based on the positive and negative sequence decoupling dual closed-loop architecture, the present invention enables positive sequence control for symmetrical faults and simultaneously enables positive and negative sequence independent loops for asymmetrical faults, accurately compensates for positive sequence drops and suppresses negative sequence components, and enhances the adaptability to complex working conditions. Attached Figure Description

[0016] To more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0017] Figure 1 This is a schematic diagram illustrating the application scenario of the built-in flexible controllable converter transformer connected to the twelve-pulse converter valve as described in Embodiment 1 of the present invention.

[0018] Figure 2 This is a schematic diagram illustrating the application scenario of the built-in flexible controllable converter transformer connected to the dual twelve-pulse converter valve as described in Embodiment 1 of the present invention.

[0019] Figure 3 This is a schematic diagram of the wiring structure of the single-phase four-column transformer with built-in flexible controllable converter transformer as described in Embodiment 1 of the present invention.

[0020] Figure 4 This is a schematic diagram of the single-phase five-limb transformer structure with built-in flexible controllable converter transformer as described in Embodiment 1 of the present invention.

[0021] Figure 5 This is a schematic diagram of the positive and negative sequence dual closed-loop control structure used in the series converter described in Embodiment 1 of the present invention.

[0022] Figure 6 This is a schematic diagram of the dual closed-loop control structure for stabilizing DC bus voltage used in the parallel converter described in Embodiment 1 of the present invention.

[0023] Figures 7(a1)-7(a4) are schematic diagrams of the simulation results of AC receiving-end grid voltage, series converter compensation voltage, inverter-side converter valve voltage, and DC line voltage under single-phase ground fault conditions without the installation of a flexible control transformer.

[0024] Figures 7(b1) to 7(b4) are schematic diagrams of the simulation results of AC receiving-end grid voltage, series converter compensation voltage, inverter-side converter valve voltage, and DC line voltage under single-phase ground fault conditions with a flexible control transformer.

[0025] Figures 8(a1)-8(a4) are schematic diagrams of the simulation results of AC receiving-end grid voltage, series converter compensation voltage, inverter-side converter valve voltage, and DC circuit voltage under three-phase symmetrical voltage sag conditions without a flexible control transformer.

[0026] Figures 8(b1)-8(b4) are schematic diagrams of the simulation results of AC receiving-end grid voltage, series converter compensation voltage, inverter-side converter valve voltage, and DC circuit voltage under the three-phase symmetrical voltage sag condition with a flexible control transformer.

[0027] In the diagram, 1-Sending-end AC power grid; 2-Sending-end converter transformer; 3-Thyristor converter valve rectifier; 4-DC transmission line; 5-Thyristor converter valve inverter; 6-Built-in flexible controllable converter transformer; 7-Receiving-end AC power grid; 8-Transformer body; 8.1-Valve-side Y-connected winding; 8.2-Valve-side D-connected winding; 8.3-Grid-side winding; 8.4-Primary winding; 8.5-Secondary winding; 9-Series converter; 10-Parallel converter; 11-Power extraction transformer. Detailed Implementation

[0028] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments to facilitate a clear understanding of the present invention, but these descriptions do not constitute a limitation on the present invention.

[0029] It should be understood that, when used in this specification and the appended claims, the term "comprising" indicates the presence of the described features, integrals, steps, operations, elements and / or components, but does not exclude the presence or addition of one or more other features, integrals, steps, operations, elements, components and / or collections thereof.

[0030] Furthermore, references to "one embodiment" or "some embodiments" in this specification mean that one or more embodiments of the invention include the specific features, structures, or characteristics described in connection with that embodiment. Therefore, phrases such as "in one embodiment," "in some embodiments," "in other embodiments," and "in still other embodiments" appearing in different parts of this specification do not necessarily refer to the same embodiment, but rather mean "one or more, but not all, embodiments," unless otherwise specifically emphasized.

[0031] Example 1 like Figure 1 , Figure 2 The diagram shown is a schematic of the magnetically integrated built-in flexible controllable converter transformer described in this invention applied to a high-voltage direct current transmission system. Its overall topology, from left to right, consists of: sending-end AC grid 1, sending-end converter transformer 2, thyristor converter valve rectifier 3, DC transmission line 4, and thyristor converter valve inverter (…). Figure 1 The middle one is a twelve-pulse converter valve. Figure 2 5. The middle part is a double twelve-pulse converter valve) 6. The built-in flexible controllable converter transformer (hereinafter referred to as the flexible control converter transformer) 7. The receiving end AC power grid.

[0032] Specifically, the sending-end AC grid is connected to a rectifier composed of thyristor converter valves via a sending-end converter transformer. This rectifier converts the AC power to DC power, which is then transmitted over long distances to the inverter side via DC transmission lines. On the inverter side, the thyristor converter valve inverter converts the DC power back to AC power, which is then connected to the receiving-end AC grid via a flexible-controlled converter transformer. The flexible-controlled converter transformer is used to suppress voltage fluctuations caused by new energy fluctuations and the electricity market. It actively regulates the voltage and is connected between the inverter-side converter valves and the AC receiving-end grid. It is used to raise the valve-side voltage when a voltage dip occurs in the AC receiving-end grid or a single-phase ground fault occurs on the AC receiving-end grid side, thereby suppressing commutation failure of the inverter-side converter valves and improving the operational stability of the LCC-HVDC system.

[0033] like Figure 3 As shown in the figure, this embodiment provides a topology of a magnetically integrated built-in flexible controllable converter transformer, including: a transformer body 8, a series converter 9, a parallel converter 10, an energy harvesting transformer 11, and a control unit (not shown in the figure). The connection relationship and functions of each device are described below.

[0034] The transformer body mainly realizes voltage transformation, electrical isolation and provides magnetic circuit coupling. The transformer body 10 includes a valve-side winding, a grid-side winding and a series winding integrated inside the iron core. The valve-side winding is used to connect the LCC inverter-side converter valve, and the grid-side winding is used to connect the receiving-end AC power grid. The converter valve includes, but is not limited to, a twelve-pulse converter valve, a double twelve-pulse converter valve, a double twenty-four-pulse converter valve, etc.

[0035] The series winding includes a primary winding 8.4 and a secondary winding 8.5. The primary winding 8.4 is connected in series with the grid-side winding, that is, one end of the primary winding is connected to one end of the grid-side winding and the other end is grounded. The secondary winding 8.5 is connected in series with the AC side of the series converter, that is, one end of the three secondary windings is respectively connected to the three-phase port of the AC side of the series converter, and the other end of the three secondary windings is Y-connected. The series winding is used to couple the compensation voltage output by the series converter into the grid-side winding.

[0036] The DC side of the series converter and the DC side of the parallel converter are connected to form a common DC bus. The series converter is used to output compensation voltage, and the parallel converter is used to stabilize the DC bus voltage. The AC side of the parallel converter is connected to the receiving-end AC power grid through the energy extraction transformer. The energy extraction transformer obtains electrical energy from the receiving-end AC power grid to provide operating power for the parallel converter, while achieving electrical isolation and ensuring the safe operation of the converter.

[0037] The control unit is configured to coordinate the control of the entire flexible converter transformer, mainly based on the line structure parameters to control the output compensation voltage of the series converter to adjust the valve side voltage and control the parallel converter to stabilize the DC bus voltage.

[0038] When a three-phase symmetrical voltage sag or a single-phase ground fault occurs on the receiving-end grid, the magnetically integrated built-in flexible controllable converter transformer of this invention, based on the line voltage information related to the voltage sag and single-phase ground fault point of the receiving-end grid, rapidly compensates the valve-side voltage through a series converter and rapidly releases energy through a parallel converter, providing reliable DC power support for the series converter and ensuring the voltage compensation capability of the series converter. The two work together to suppress or eliminate the occurrence of commutation failure on the inverter side.

[0039] Specifically, the series converter possesses rapid response capabilities, regulating voltage fluctuations under non-fault conditions and avoiding frequent tap switching that could lead to voltage fluctuations and affect power supply quality and system stability. It can rapidly output compensation voltage instantaneously in the event of a voltage dip or single-phase ground fault in the AC receiving-end grid, achieving rapid compensation of the valve-side commutation voltage, effectively suppressing commutation failures, and ensuring the continuous and stable operation of the LCC-HVDC system.

[0040] Parallel converters possess four-quadrant operation capability, enabling independent control of active and reactive power. During steady-state operation, they stabilize the common DC bus voltage, inject reactive power into the receiving-end AC system to stabilize the bus voltage, detect and compensate for harmonic currents on the receiving-end grid side, and improve power quality. Under fault conditions, they rapidly release energy, suppress fault harmonics, provide reliable DC power support for series converters, ensure the voltage compensation capability of series converters, and achieve coordinated and efficient operation of the two converters.

[0041] In this invention, the series winding is integrated inside the core of the transformer body. The series winding (primary side + secondary side) is coupled to the main magnetic flux path through the core. Its primary winding is connected in series with the grid-side winding, which is responsible for obtaining or injecting power from the grid side. The secondary winding is connected in series with the AC side of the series converter, which serves as the interface for voltage regulation of the series converter. This connection method enables the series converter to compensate or regulate the valve-side voltage through electromagnetic coupling.

[0042] In the above scheme, the transformer body is a grouped three-phase transformer composed of three single-phase multi-column transformers. The iron core columns on both sides of the single-phase multi-column transformers are used for the main magnetic flux to pass through, serving as the return path of the magnetic flux and ensuring the magnetic circuit is closed. The iron core column in the middle is used for magnetoelectric conversion, that is, as the carrier of the main winding, realizing the conversion between electrical energy and magnetic energy. This functional division helps to concentrate magnetic field energy, reduce leakage flux, and improve magnetic energy conversion efficiency. Figure 3 The transformer body in the middle consists of three single-phase four-limb transformers. Figure 4 The middle section is a single-phase five-column transformer structure, and the number of core columns can be set according to needs in practical applications.

[0043] The valve-side winding and the grid-side winding are wound together on the same central iron core column, achieving magnetic circuit coupling and electromagnetic isolation. The valve-side winding includes a valve-side Y-connected winding 8.1 and a valve-side D-connected winding 8.2. The valve-side Y-connected winding 8.1 has a star (Y) connection structure, and the valve-side D-connected winding 8.2 has a delta (D) connection structure. The grid-side winding 8.3 has a star (Y) connection structure and is wound together with the valve-side Y-connected winding 8.1 and the valve-side D-connected winding 8.2 on the same iron core column. This arrangement enables efficient magnetic circuit coupling, thereby ensuring effective energy transmission. When the series winding is injected with compensation voltage, since the grid-side and valve-side windings are on the same magnetic column, the compensation energy can be transferred to the valve side most directly and with minimal loss. This reduces leakage flux between windings and lowers leakage inductance, resulting in higher waveform quality and lower response delay of the compensation voltage. As a result, it can keep up with voltage changes more quickly when the grid voltage drops, ensuring the efficiency, speed, and stability of valve-side voltage regulation.

[0044] The primary and secondary windings are wound together on the same newly added iron core column in the middle, which improves the tightness of their magnetic coupling. This tight coupling reduces the leakage inductance of the series winding, enabling more precise and faster response to control commands when regulating voltage through the series converter. It reduces voltage drops or phase lag caused by leakage inductance, thereby improving the accuracy and dynamic response speed of valve-side voltage regulation. Furthermore, the newly added iron core column acts as an independent magnetic source, and the magnetic flux it generates can be adjusted independently of the original magnetic flux of the main transformer. This ensures that the energy injected by the series converter will not disturb the original magnetomotive force balance of the main transformer, thus achieving magnetic circuit decoupling and independent control. It avoids interference with the main magnetic flux and provides a more stable and controllable voltage injection environment for the series converter, thereby more effectively suppressing valve-side voltage fluctuations and enhancing the system's ability to suppress commutation failures. At the same time, since the series winding has its own magnetic circuit, its turns ratio can be designed more freely, thereby flexibly adjusting the amplitude range of the compensation voltage without being limited by the original turns ratio constraints of the main transformer.

[0045] The structural design of the transformer body in this invention improves the integration, efficiency and reliability of the built-in flexible controllable converter transformer, solves the problems of magnetic circuit optimization and spatial integration under complex coupling of multiple windings, and provides a solid hardware foundation for the stable operation of the inverter side of the LCC-HVDC system.

[0046] In the above scheme, both the series converter and the parallel converter adopt an MMC (Modular Multilevel Converter) topology. The MMC topology adopts a three-phase six-arm structure, with each phase containing two series branches: an upper arm and a lower arm. Each arm contains several half-bridge submodules (HBSMs), and the number of submodules can be selected according to requirements. Each half-bridge submodule (HBSM) consists of two insulated-gate bipolar transistors (IGBTs), two anti-parallel freewheeling diodes, and one submodule capacitor connected in series. The three-phase AC output terminals are led out from the midpoints of the upper and lower arms of each phase, respectively. The DC side is formed by the common upper end of the three-phase upper arms and the common lower end of the three-phase lower arms, which constitutes a DC bus, realizing bidirectional AC-DC power conversion.

[0047] Compared to traditional two-level or three-level converters, the converter with the MMC topology of this invention can achieve higher voltage levels, lower switching frequencies, and lower harmonic distortion, thereby reducing losses and improving power quality. The MMC topology offers excellent modularity, scalability, and redundancy, easily expanding to higher voltage levels, making it particularly suitable for high-power applications such as HVDC transmission. Furthermore, the series combination of half-bridge submodules not only disperses voltage stress but also provides flexible voltage control capabilities, enabling the series converter to more accurately and quickly regulate the valve-side voltage, effectively suppressing commutation failures. For parallel converters, the MMC topology helps to more stably control the DC bus voltage, improving the system's adaptability to grid disturbances. This design also enhances system redundancy and fault ride-through capability; even if some submodules fail, a certain level of operational capability can still be maintained, thereby improving the overall operational reliability and availability of the built-in flexible controllable converter transformer.

[0048] As a preferred technical solution, this embodiment also provides a control method for the above-mentioned magnetically integrated built-in flexible controllable converter transformer. Both the series converter and the parallel converter adopt dual closed-loop control. Based on the voltage drop value of the receiving end grid or the DC bus voltage value, multiple PI controllers are used to achieve the purpose of regulating the valve side voltage and stabilizing the DC bus voltage.

[0049] The steps for adjusting the valve-side voltage include: The voltage deviation is calculated based on the actual voltage and reference voltage of the receiving-end AC grid. The reference voltage is the rated voltage of the receiving-end grid before the fault. The voltage deviation reflects the voltage fluctuation that needs to be compensated by the series converter. The voltage deviation, the output voltage and current of the series converter are decomposed into positive and negative sequence components and decoupled into dq components to obtain positive and negative sequence components. The positive and negative sequence decomposition separates the three-phase AC quantities into symmetrical positive and negative sequence components, so that different sequence components can be independently controlled in the future, which is suitable for asymmetrical fault conditions. A pulse drive signal is generated using a dual closed-loop control strategy based on the positive-sequence and negative-sequence components, driving the series converter to output a compensation voltage to regulate the valve-side voltage. Specifically, when a symmetrical fault occurs in the receiving-end power grid, a positive-sequence pulse drive signal is generated using a positive-sequence dual closed-loop control strategy based on the positive-sequence component; when an asymmetrical fault occurs in the receiving-end power grid, a positive-sequence voltage reference value is generated using a positive-sequence dual closed-loop control strategy based on the positive-sequence component, and a negative-sequence voltage reference value is generated using a negative-sequence dual closed-loop control strategy based on the negative-sequence component. The positive-sequence and negative-sequence voltage reference values ​​are superimposed to form a modulation voltage signal, and a composite pulse drive signal is generated based on the modulation voltage signal.

[0050] Through the above technical solution, this invention can accurately detect and quantify voltage fluctuations in the receiving-end AC power grid. The application of positive and negative sequence decomposition and dq decoupling enables the control unit to adopt a decoupled control method, thereby effectively dealing with symmetrical and asymmetrical voltage disturbances. Based on these decomposed and decoupled parameters, the dual closed-loop control strategy can achieve precise and dynamic regulation of the valve-side voltage. This comprehensive control mechanism enhances the ability to suppress valve-side voltage fluctuations, thereby improving the overall stability and reliability of the LCC-HVDC system under various power grid fault conditions and effectively reducing the risk of commutation failure.

[0051] In the above scheme, the dual closed-loop control strategy includes: Voltage outer loop: The d and q axis components of the positive and negative sequence components of the voltage deviation are respectively subtracted from the d and q axis components of the positive and negative sequence components of the output voltage of the series converter. The difference is adjusted by the PI controller and output as the d and q axis components of the current inner loop reference value. Inner current loop: The d and q axis components of the inner current loop reference value are subtracted from the d and q axis components of the positive and negative sequence components of the output current of the series converter, respectively. The difference is adjusted by the PI controller and then output as a modulated voltage signal. Generate driving signal: Generate pulse driving signal by performing near-level modulation or carrier phase shift modulation on the modulated voltage signal.

[0052] Through the above technical solution, this invention enables precise control of the output compensation voltage of a series converter. The outer voltage loop macroscopically tracks the valve-side voltage target, eliminates voltage deviations through a PI controller, and provides an accurate current reference for the inner current loop. The inner current loop, based on this, precisely controls the output current of the series converter through a PI controller, ensuring its rapid response and tracking of the outer loop commands. Finally, the pulse drive signal generated by modulating the voltage regulation signal with the nearest-level modulation or carrier phase-shift modulation can efficiently and accurately drive the series converter, enabling its output compensation voltage to quickly and stably regulate the valve-side voltage. This layered and coordinated control method achieves high-precision, rapid-response, and zero-steady-state-error regulation of the valve-side voltage. Especially when symmetrical or asymmetrical faults occur, precise voltage and current control improves the commutation margin on the inverter side of the LCC-HVDC system, effectively suppressing commutation failures and enhancing system stability and reliability.

[0053] In the above scheme, under the condition of a single-phase ground fault on the receiving end of the power grid, a positive and negative sequence decoupled dual closed-loop control strategy is adopted to independently control the positive and negative sequence components, thereby achieving voltage compensation and commutation failure suppression under asymmetrical faults. This method solves the complexity of voltage compensation under asymmetrical faults by separating the control of the positive and negative sequence components, ensuring the reliability of phase commutation of each phase by the inverter-side converter valve during a single-phase ground fault. Figure 5 As shown, the specific implementation process is as follows: Voltage command based on the three-phase drop in the receiving-end power grid U mj With the voltage command before the drop U m Calculate the three-phase voltage drop command Δ U j = U m - U mj The drop instruction contains both positive and negative sequence components.

[0054] Collect the port voltage of the series converter U mi and port current I mi Based on port voltage U mi and port current I mi The three-phase voltage drop command is decomposed into positive and negative sequence components. Due to the asymmetry of the three-phase voltage caused by a single-phase ground fault, both the positive and negative sequence components Δ are obtained simultaneously. U j + Δ U j -This positive and negative sequence decomposition achieves the symmetric decomposition of asymmetric electrical quantities, laying the foundation for independent control; by decoupling the positive and negative sequence components using dq, the positive sequence d-axis component Δ is obtained. U j + d Positive sequence q-axis component Δ U j + q Negative d-axis component Δ U j - d Negative q-axis component Δ U j - q Together, they serve as the reference command set for outer loop voltage control. This dq decoupling is performed separately in the positive and negative sequence dual synchronous rotating coordinate system, realizing the decoupling control of the positive and negative sequence components.

[0055] Port voltage U mi and port current I mi Positive and negative sequence decomposition is performed to obtain positive and negative sequence components; the positive and negative sequence components are then decoupled by dq to obtain the positive-sequence d-axis, positive-sequence q-axis, negative-sequence d-axis, and negative-sequence q-axis components of voltage and current, respectively. U mi + d , U mi + q , U mi - d , U mi - q , I mi + d , I mi + q , I mi - d , I mi - q .

[0056] Execute the positive-sequence dual closed-loop control loop: The d-axis component Δ of the positive-sequence voltage reference command... Uj + d d-axis component of positive sequence voltage feedback U mi + d The difference is the q-axis component Δ of the positive sequence voltage reference command. U j + q q-axis component of positive sequence voltage feedback U mi + q The difference between the positive sequence voltage errors is input into the proportional-integral controller for adjustment, generating inner-loop current reference commands for the positive sequence d-axis and q-axis. This positive sequence control loop is mainly used to compensate for the positive sequence voltage drop in the grid voltage, maintain the amplitude and phase of the positive sequence component of the valve-side voltage, and ensure that the thyristor converter valve inverter can still obtain a stable positive sequence commutation voltage under three-phase imbalance conditions. The positive sequence d-axis inner-loop current reference command and the d-axis component of the positive sequence current feedback are then compared. I mi + d The difference between the positive-sequence q-axis inner loop current reference command and the q-axis component of the positive-sequence current feedback. I mi + q The difference is calculated, and the resulting positive sequence current error is input into a proportional-integral controller for adjustment, generating a positive sequence pulse width modulation wave.

[0057] Execute the negative-sequence dual closed-loop control loop: The d-axis component Δ of the negative-sequence voltage reference command... U j - d d-axis component of negative sequence voltage feedback U mi - d The difference is calculated using the q-axis component Δ of the negative sequence voltage reference command. U j - q q-axis component of negative sequence voltage feedback U mi - q The difference between the negative sequence voltage errors is input into a proportional-integral controller for adjustment, generating inner-loop current reference commands for both the negative sequence d-axis and q-axis. This negative sequence control loop primarily suppresses the negative sequence voltage component introduced by single-phase ground faults by injecting negative sequence compensation voltage into the series windings to offset the influence of the grid's negative sequence voltage on the valve-side voltage. The negative sequence d-axis inner-loop current reference commands are then compared with the d-axis component of the negative sequence current feedback. Imi - d The difference between the negative-sequence q-axis inner loop current reference command and the q-axis component of the negative-sequence current feedback. I mi - q The difference is calculated, and the resulting negative sequence current error is input into the proportional-integral controller for adjustment, generating a negative sequence pulse width modulation wave. This negative sequence compensation reduces the imbalance of the valve-side voltage of the thyristor converter inverter and suppresses commutation failure caused by asymmetric faults.

[0058] The positive-sequence pulse width modulation wave and the negative-sequence pulse width modulation wave are vector superimposed to generate the final composite pulse width modulation drive signal, which controls the switching timing of each phase half-bridge submodule of the series converter, realizing the coordinated control of positive-sequence voltage compensation and negative-sequence voltage suppression. The positive and negative sequence decoupled dual closed-loop control uses independent positive-sequence loops and negative-sequence loops to accurately control the symmetrical and asymmetrical components respectively, avoiding cross-coupling between positive and negative sequence components, and improving the dynamic response speed and steady-state compensation accuracy of the control system under asymmetrical faults.

[0059] In the above scheme, under the condition of a symmetrical voltage dip in the three-phase grid at the receiving end, a positive-sequence dual closed-loop control strategy is adopted to achieve active voltage rise on the valve side and suppression of commutation failure. This method ensures that the inverter-side LCC converter valve maintains stable commutation conditions during the symmetrical voltage drop in the grid, thereby guaranteeing the continuous power transmission capability of the DC transmission system through precise positive-sequence voltage compensation. The specific implementation process is as follows: Voltage command based on the three-phase drop in the receiving-end power grid U mj With the voltage command before the drop U m Calculate the three-phase voltage drop command Δ U j Δ U j = U m - U mj This voltage drop command characterizes the depth and phase information of the grid voltage sag, providing a target reference for subsequent voltage compensation control. Collect port voltage and port current of series converter U mi , I mi The three-phase voltage drop command is decomposed into positive and negative sequences to obtain Δ. U j + Δ U j -Since the negative sequence component is zero under a three-phase voltage symmetrical sag fault, the positive sequence component Δ is obtained. U j + The positive-sequence components are decoupled by dq to obtain the d-axis components Δ. U j + d With q-axis component Δ U j + q As a reference command for outer loop voltage control, the dq decoupling process realizes the transformation of three-phase AC quantities into a two-phase rotating coordinate system, simplifying the control complexity of AC quantities.

[0060] The acquired port voltage and port current of the series converter are decomposed into positive and negative sequences to obtain the positive sequence components. U mi + , I mi + Furthermore, dq decoupling is performed to obtain the d-axis and q-axis components of voltage and current. U mi + d , U mi + q , I mi + d , I mi + q。

[0061] Execute outer loop voltage control, and set the d-axis component Δ of the voltage reference command to... U j + d d-axis component of voltage feedback U mi + d The difference is calculated using the q-axis component Δ of the voltage reference command. U j + q q-axis component of voltage feedback U mi + qThe difference is calculated and input into the proportional-integral controller for adjustment, generating inner-loop current reference commands for the d-axis and q-axis. This outer-loop voltage control achieves zero steady-state error tracking of the amplitude and phase of the valve-side voltage. The proportional-integral controller parameters are tuned according to the system's dynamic response characteristics and stability requirements to ensure that the voltage outer loop has appropriate bandwidth and damping characteristics, balancing response speed and overshoot suppression.

[0062] Execute inner-loop current control, and combine the generated d-axis inner-loop current reference command with the d-axis component of the current feedback. I mi + d The difference between the q-axis inner loop current reference command and the q-axis component of the current feedback. I mi + q The difference in current is calculated, and the resulting current error is input to a proportional-integral controller for adjustment, generating a pulse width modulation wave to drive the power devices. This inner-loop current control achieves rapid limiting protection of the converter output current, preventing overcurrent damage to the power devices. Through space vector modulation or carrier phase-shift modulation strategies, the switching state of the half-bridge submodule is controlled, allowing the series converter to inject a compensation voltage with controllable amplitude and phase into the series winding of the flexible transformer. The positive-sequence dual closed-loop control achieves precise tracking of the valve-side voltage amplitude and phase through the voltage outer loop, and achieves rapid response and limiting protection of the converter output current through the current inner loop. Thus, during the three-phase voltage symmetrical sag, the valve-side voltage of the thyristor converter valve inverter is actively raised to compensate for the voltage time area loss caused by the grid voltage drop, ensuring that the thyristor converter valve obtains sufficient commutation voltage time area during commutation and suppressing commutation failure.

[0063] In the above scheme, the steps for stabilizing the DC bus voltage include: Obtain the actual DC bus voltage and the output current of the parallel converter; Based on the actual DC bus voltage and the output current of the parallel converter, a pulse signal is generated using a dual closed-loop control method to drive the parallel converter to stabilize the DC bus voltage.

[0064] This invention enables the parallel converter to respond quickly to changes in DC bus voltage, ensuring the stable operation of both the series and parallel converters. It also provides a solid foundation for the reliability of the inverter side of the entire LCC-HVDC system, avoiding commutation failure or system collapse that may be caused by unstable DC bus voltage, thereby improving the overall performance and reliability of the entire built-in flexible controllable converter transformer.

[0065] The dual closed-loop control method includes: The voltage deviation is obtained by subtracting the actual voltage of the DC bus from the set voltage. The voltage deviation is then adjusted by the PI controller and decoupled by dq to obtain the reference value of the inner current loop. The current feedback component is obtained by decoupling the output current of the parallel converter by dq. The difference between the inner current loop reference value and the current feedback component is calculated, and the difference is adjusted by the PI controller and then generated into a pulse signal by the nearest level modulation or carrier phase shift modulation.

[0066] This invention employs a dual closed-loop control method to control the parallel converter, maintaining DC bus voltage stability and providing energy support for the series converter. This method achieves energy balance between the series and parallel sides through dynamic adjustment of active power, ensuring the continuous operation of the flexible controllable converter transformer during faults. Figure 6 As shown, the specific implementation process is as follows: First, set the DC bus voltage reference command. U dc Collect the actual voltage of the DC bus of the parallel converter. U dc Calculate the voltage deviation Δ U dc = U dc - U dc This voltage deviation reflects the energy state of the DC bus capacitor.

[0067] Secondly, the voltage deviation Δ U dc The input proportional-integral controller is used for modulation to generate active current commands, and dq decoupling is performed to obtain the d-axis current inner loop reference command. This outer loop voltage control realizes zero steady-state error regulation of DC bus voltage. The q-axis reference command is set according to reactive power compensation requirements, which can realize reactive power support for the receiving end grid.

[0068] Furthermore, the port current of the parallel converter is collected. I PC dq decoupling is performed to obtain the d-axis and q-axis components. I PCd , I PCq The inner loop reference command of the d-axis is combined with the d-axis component of the current. I PCd Difference between q-axis reference command and current q-axis component I PCqThe difference is calculated, and the resulting current error is input into the proportional-integral controller for modulation. Then, it is modulated by the nearest-level modulation or carrier phase-shift modulation to generate a pulse drive signal. This inner-loop current control realizes fast response and precise control of the port current, controls the parallel converter to exchange active power with the receiving-end grid, and maintains the DC bus capacitor voltage constant.

[0069] In this embodiment, the controller employs multiple different resonant controllers (PI), and the transfer function of the resonant controller PI... G PI ( S ) is represented as: G PI ( S )= K P + K i / s ,in, K P and K i These are the proportional and integral coefficients, respectively. s is the independent variable in the complex frequency domain.

[0070] Simulation test To verify the effectiveness of the control method based on the magnetically integrated built-in flexible controllable converter transformer in this embodiment, a simulation model was built on a simulation platform. The simulation results are shown in Figures 7(a1)-7(a4), 7(b1)-7(b4), and 8(a1)-8(a4), 8(b1)-8(b4), which are schematic diagrams comparing the simulation results of line parameters with and without the flexible control transformer under single-phase ground fault and three-phase symmetrical voltage sag conditions, respectively. The per-unit values ​​of the receiving-end grid voltage sag and the single-phase ground fault resistance, and the model parameters are shown in Table 1.

[0071] Table 1 Simulation Parameters The simulation includes a single-phase ground fault condition: At 3 seconds, a single-phase ground fault occurred in phase A; The single-phase ground fault in phase A ended at 4 seconds. The fault resistor is set to 10Ω.

[0072] As shown in Figures 7(a1)-7(a4), under single-phase ground fault conditions, when the fault resistance is 10Ω, without a converter transformer, the voltage of the converter valve on the inverter side is not compensated, which leads to commutation failure of the thyristor and collapse of the DC line voltage.

[0073] As shown in Figures 7(b1)-7(b4), under the single-phase ground fault condition, when the fault resistance is 10Ω, a converter transformer is set up, the converter valve voltage on the inverter side is compensated, the thyristor does not fail to commutate, and the DC line voltage is normal.

[0074] Comparing Figures 7(a1)-7(a4) and 7(b1)-7(b4), the difference between the LCC-HVDC system with and without a converter transformer can be clearly seen. According to the simulation results, the magnetically integrated built-in flexible controllable converter transformer added in this invention can compensate for the voltage of the converter valve on the inverter side, effectively suppress commutation failure, and improve the fault ride-through capability of the LCC-HVDC system on the inverter side.

[0075] Example 2 Based on the same inventive concept, this embodiment of the invention sets a three-phase symmetrical voltage dip in the AC receiving-end power grid. A symmetrical three-phase voltage dip occurs in 1 second and recovers in 2 seconds. During the fault, the per-unit value of the three-phase voltage becomes 0.9 pu. Since this embodiment involves a three-phase symmetrical voltage dip, there is no negative sequence component. Compared to a single-phase ground fault, the control method for the series converter does not require negative sequence control, only positive sequence control (this has been described in the previous section on positive sequence double closed-loop control). The control method for the parallel converter in this embodiment is the same as that in Embodiment 1. Since Embodiment 1 has already detailed the control methods for both series and parallel converters, the specific control method in this embodiment can be referred to the control method in Embodiment 1, and will not be repeated here.

[0076] As shown in Figures 8(a1)-8(a4), under the three-phase symmetrical voltage sag condition, the per-unit voltage becomes 0.9pu. Without a converter transformer, the voltage of the converter valve on the inverter side is not compensated, which leads to commutation failure of the thyristor and collapse of the DC line voltage. As shown in Figures 8(b1)-8(b4), under the three-phase symmetrical voltage sag condition, the per-unit voltage becomes 0.9pu. With the converter transformer installed, the voltage of the converter valve on the inverter side is compensated, the thyristor does not experience commutation failure, and the DC line voltage is normal.

[0077] Comparing Figures 8(a1)-8(a4) and 8(b1)-8(b4), the difference between the LCC-HVDC system with and without a converter transformer can be clearly seen. According to the simulation results, the magnetically integrated built-in flexible controllable converter transformer added in this invention can compensate for the voltage of the converter valve on the inverter side, effectively suppress commutation failure, and improve the fault ride-through capability of the LCC-HVDC system on the inverter side.

[0078] It is understood that those skilled in the art will clearly recognize that, for the sake of convenience and brevity, the above-described division of functional units and modules is used as an example. In practical applications, the above functions can be assigned to different functional units and modules as needed, that is, the internal structure of the device can be divided into different functional units or modules to complete all or part of the functions described above. The functional units and modules in the embodiments can be integrated into one processing unit, or each unit can exist physically separately, or two or more units can be integrated into one unit. The integrated unit can be implemented in hardware or as a software functional unit. Furthermore, the specific names of the functional units and modules are only for easy distinction and are not intended to limit the scope of protection of this invention.

[0079] Those skilled in the art will recognize that the units and algorithm steps of the various examples described in conjunction with the embodiments disclosed herein can be implemented in electronic hardware, or a combination of computer software and electronic hardware. Whether these functions are implemented in hardware or software depends on the specific application and design constraints of the technical solution. Those skilled in the art can use different methods to implement the described functions for each specific application, but such implementations should not be considered beyond the scope of this invention.

[0080] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention, and should all be included within the protection scope of the present invention. Contents not described in detail in this specification belong to the prior art known to those skilled in the art.

Claims

1. An internally flexible controllable converter transformer, characterized by: It includes the transformer body, series converter, parallel converter, energy extraction transformer, and control unit. The transformer body includes a valve-side winding, a grid-side winding, and a series winding integrated inside the core. The valve-side winding is used to connect to the LCC inverter-side converter valve, and the grid-side winding is used to connect to the receiving-end AC power grid. The series winding includes a primary winding and a secondary winding. The primary winding is connected in series with the grid-side winding, and the secondary winding is connected in series with the AC side of the series converter. The DC side of the series converter and the DC side of the parallel converter are connected to form a common DC bus; The AC side of the parallel converter is connected to the receiving-end AC power grid through the energy extraction transformer; The control unit is configured to control the output compensation voltage of the series converter to regulate valve-side voltage fluctuations and control the DC bus voltage of the parallel converter to stabilize the DC bus voltage.

2. The built-in flexible controllable converter transformer according to claim 1, characterized in that: The transformer body is a group-type three-phase transformer composed of three single-phase multi-column transformers. The iron core columns on both sides of the single-phase multi-column transformer are used for the main magnetic flux to pass through, and the iron core column in the middle is used for magnetoelectric conversion. The valve-side winding and the grid-side winding are wound together on the same iron core column.

3. The built-in flexible controllable converter transformer according to claim 1, characterized in that: The primary and secondary windings of the series winding are wound together on the same newly added iron core column.

4. The built-in flexible controllable converter transformer according to claim 1, characterized in that: Both the series converter and the parallel converter adopt the MMC topology, and each arm of the MMC topology contains several half-bridge sub-modules.

5. A control method based on the built-in flexible controllable converter transformer according to any one of claims 1-4, characterized in that, Including regulating valve-side voltage: Calculate the voltage deviation based on the actual voltage and reference voltage of the receiving-end AC power grid; The positive-sequence component and the negative-sequence component are obtained by performing positive-sequence decomposition and dq decoupling on the voltage deviation, the output voltage and current of the series converter. A pulse drive signal is generated based on the positive and negative sequence components using a dual closed-loop control strategy, which drives the series converter to output a compensation voltage to regulate the valve-side voltage.

6. The control method according to claim 5, characterized in that, The generation of pulse drive signals based on a dual closed-loop control strategy using positive-sequence and negative-sequence components includes: When a symmetrical fault occurs in the receiving-end power grid, a positive-sequence pulse drive signal is generated based on the positive-sequence component using a positive-sequence dual closed-loop control strategy.

7. The control method according to claim 5, characterized in that, The generation of pulse drive signals based on a dual closed-loop control strategy using positive-sequence and negative-sequence components includes: When an asymmetrical fault occurs in the receiving-end power grid, a positive-sequence voltage reference value is generated based on the positive-sequence component using a positive-sequence dual-closed-loop control strategy, and a negative-sequence voltage reference value is generated based on the negative-sequence component using a negative-sequence dual-closed-loop control strategy. The positive-sequence voltage reference value and the negative-sequence voltage reference value are superimposed to form a modulated voltage signal, and a composite pulse drive signal is generated based on the modulated voltage signal.

8. The control method according to claim 5, characterized in that: The dual closed-loop control strategy includes: Voltage outer loop: The difference between the voltage deviation and the positive and negative sequence components of the output voltage of the series converter is used to output the reference value of the inner loop current after the difference is adjusted by the PI controller. Inner current loop: The difference between the reference value of the inner current loop and the positive and negative sequence components of the output current of the series converter is calculated, and the difference is adjusted by the PI controller to output a modulated voltage signal. Generate driving signal: Generate pulse driving signal by performing near-level modulation or carrier phase shift modulation on the modulated voltage signal.

9. The control method according to claim 5, characterized in that, It also includes stabilizing the DC bus voltage: Obtain the actual DC bus voltage and the output current of the parallel converter; Based on the actual DC bus voltage and the output current of the parallel converter, a pulse signal is generated using a dual closed-loop control method to drive the parallel converter to stabilize the DC bus voltage.

10. The control method according to claim 9, characterized in that, The dual closed-loop control method includes: The voltage deviation is obtained by subtracting the actual voltage of the DC bus from the set voltage. The voltage deviation is then adjusted by a PI controller to obtain the reference value of the inner current loop. The difference between the inner current loop reference value and the current feedback component is calculated, and the difference is adjusted by the PI controller to generate a pulse signal.

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