External flexible controllable converter transformer and control method

Abstract

This invention discloses an external flexible controllable converter transformer and its control method, including a power frequency converter transformer, a series transformer, a series converter, a parallel hybrid converter, an energy extraction transformer, an energy dissipation device, and a control unit. The control unit is configured to control the output compensation voltage of the series converter to adjust the valve-side voltage, control the parallel hybrid converter to maintain the DC bus voltage stability, and control the energy dissipation device to smooth DC bus power fluctuations. This invention uses the parallel hybrid converter to control the DC bus voltage and perform reactive power compensation, while the series converter adjusts the LCC valve-side voltage. For different fault types such as receiving-end grid fluctuations, voltage dips, and single-phase grounding, positive-sequence dual closed-loop control and positive-negative-sequence decoupled dual closed-loop control strategies are adopted respectively. Combined with the parallel hybrid converter's DRU natural clamping voltage regulation and the energy dissipation device's power regulation, precise compensation of the valve-side commutation voltage is achieved.

Description

Technical Field

[0001] This invention belongs to the field of high voltage direct current transmission technology, specifically relating to an external flexible controllable converter transformer and control method for suppressing commutation failure and actively regulating voltage on the inverter side of a high voltage direct current (LCC-HVDC) system with a power grid commutator. Background Technology

[0002] High-voltage direct current (HVDC) transmission technology based on grid-commutated converters is widely used worldwide due to its advantages such as large transmission capacity, long transmission distance, and low overall cost. The LCC on its inverter side consists of thyristors connected in series, which can only be controlled to turn on. It requires a reverse voltage from the receiving-end grid to reliably turn off. Therefore, when the AC voltage at the receiving end drops or an AC system fault occurs, the thyristors that should be turned off may turn back on due to insufficient reverse voltage, causing commutation failure. This leads to a surge in DC current and a rapid and significant loss of DC power, causing a severe power surge to the system. In severe cases, it may lead to a chain reaction of commutation failures on multiple DC lines, posing a serious challenge to the safe and stable operation of the power grid.

[0003] Furthermore, with the large-scale grid connection of new energy power generation (wind power, photovoltaic, etc.), its inherent intermittent, volatile and random characteristics have further aggravated the voltage fluctuations of the receiving-end AC grid. Frequent switching of converter transformer taps has brought additional challenges to the commutation process of LCC-HVDC, further reducing the stability of the LCC commutation process and significantly increasing the probability of commutation failure.

[0004] To address the issue of commutation failure in DC transmission, optimization is currently being carried out in three main areas: optimizing the control strategy of the DC transmission system, adding reactive power compensation devices, and improving the converter topology.

[0005] Firstly, regarding the optimization of control strategies for DC transmission systems, the focus is mainly on improving commutation failure predictive control (CFPREV) and low-voltage current limiting control (VDCOL). This is achieved by introducing methods such as sine and cosine component detection and current prediction to enhance fault detection sensitivity, thereby increasing commutation margin and suppressing continuous commutation failures.

[0006] Secondly, regarding the installation of reactive power compensation devices, synchronous condensers or static var generators (SVG) are configured in the converter station to utilize their dynamic reactive power support capabilities to increase the commutation voltage during faults.

[0007] Third, in terms of improving converter topology, the early capacitor commutated converter (CCC) has been developed into enhanced capacitor commutated converter (ECCC) and series voltage commutated converter (SVCC), and flexible control of capacitor voltage is achieved by introducing fully controlled devices.

[0008] The above three methods improve the system's ability to resist commutation failure from the perspectives of control, compensation, and topology modification, respectively. Although they help alleviate the commutation failure problem to some extent, they still have certain shortcomings: the optimization of control strategy is limited by detection speed and parameter tuning, making it difficult to completely avoid commutation failure; the suppression effect of reactive power compensation device is limited by capacity and response speed; and topology modification faces problems such as high cost and difficulty in engineering modification.

[0009] In summary, the existing technology has the following core defects: (1) Traditional converter transformers have no flexible voltage regulation capability and cannot actively compensate for the commutation voltage on the valve side, resulting in a weak ability to cope with grid faults; (2) The compensation point of the compensation device is far from the valve side, and is greatly affected by the line impedance. The compensation accuracy is insufficient and the effect of suppressing commutation failure is limited. (3) The cost of retrofitting a fully electric electronic converter is extremely high, and it is not compatible with existing DC projects, making it difficult to promote the project. (4) The existing control strategy relies on a large number of operating signals inside the converter valve, resulting in a complex control architecture, insufficient voltage regulation capability, and high difficulty in on-site debugging and maintenance. Summary of the Invention

[0010] The purpose of this invention is to address the shortcomings of the aforementioned background technology by providing an external flexible controllable converter transformer and its control method. This method can actively regulate the commutation voltage on the valve side, thereby suppressing the failure of conventional DC commutation in LCCs and adjusting voltage fluctuations caused by new energy switching and the power market.

[0011] The first aspect of this invention provides an external flexible controllable converter transformer, applied to the inverter side of an LCC-HVDC system, for regulating valve-side voltage fluctuations and suppressing commutation failure. It includes a power frequency converter transformer, a series transformer, a series converter, a parallel hybrid converter, an energy harvesting transformer, an energy dissipation device, and a control unit. The grid-side winding of the power frequency converter transformer is used to connect to the receiving-end AC power grid, and the valve-side winding is used to connect to the AC side of the LCC inverter bridge. The series transformer is arranged independently of the power frequency converter transformer. The primary side of the series transformer is connected in series between the grid-side winding and the ground wire, and the secondary side is connected to the AC port of the series converter. It is used to couple the compensation voltage output by the series converter into the grid-side winding. The DC port of the series converter and the DC port of the parallel hybrid converter are connected to form a common DC bus, and the series converter is used to output compensation voltage. The parallel hybrid converter includes a parallel diode rectifier unit and an auxiliary voltage regulator MMC. The AC port of the parallel hybrid converter is connected to the receiving end AC power grid through the energy harvesting transformer. The energy harvesting transformer is used to provide a stable AC operating power supply for the parallel hybrid converter. The energy-consuming device is connected in parallel to the common DC bus to absorb active power and smooth out DC bus power fluctuations. The control unit is configured to control the output compensation voltage of the series converter to adjust the valve-side voltage, control the parallel hybrid converter to maintain the DC bus voltage stability, and control the energy-consuming device to smooth DC bus power fluctuations.

[0012] A second aspect of this invention provides a control method based on the above-described external 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 and negative sequence components are obtained by performing positive and negative sequence decomposition and dq transformation 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.

[0013] This invention adopts an external structure, requiring no modification to the original converter transformer, converter valve's main structure and wiring, resulting in low modification costs. Through an innovative grid-side coupling compensation arrangement, it achieves precise compensation of the valve-side commutation voltage, significantly improving the inverter-side fault ride-through capability of the LCC-HVDC system, while also possessing voltage regulation functionality, adapting to the upgrade and modification needs of DC projects at various voltage levels.

[0014] This invention utilizes a parallel hybrid converter to control the DC bus voltage and perform reactive power compensation, while a series converter regulates the LCC valve-side voltage. It eliminates the need to collect operating signals such as internal current, commutation angle, and firing angle of the LCC converter valve; instead, it uses only the receiving-end grid voltage and the electrical quantities at the series converter ports as input. For different fault types in the receiving-end grid, such as symmetrical voltage sags, swells, and single-phase grounding, it employs positive-sequence dual-closed-loop control and positive-negative-sequence decoupled dual-closed-loop control strategies. Combined with the parallel hybrid converter's DRU natural clamping voltage regulation and power regulation of the energy-consuming device, it achieves precise compensation of the valve-side commutation voltage, suppresses thyristor commutation failures, and ensures the inverter-side fault ride-through capability and continuous power transmission capability of the LCC-HVDC system.

[0015] The indirect control mode adopted in this invention relies solely on external grid electrical quantities to achieve closed-loop control, eliminating the need for internal signals from the converter valve. It is compatible with upgrades and retrofits of existing LCC-HVDC projects and features "plug-and-play" functionality. Separate control strategies are implemented for symmetrical and asymmetrical faults, providing precise compensation and rapid response, completely eliminating commutation failure causes. The parallel-side DRU clamping control is simple, reliable, and has low operating losses. The entire system boasts strong voltage regulation and excellent anti-interference capabilities, comprehensively enhancing the inverter-side voltage regulation and fault ride-through capabilities of the LCC-HVDC system, ensuring continuous and stable operation of the DC transmission system.

[0016] The beneficial effects of this invention are as follows: (1) External topology modification is convenient: The present invention adopts an external structure with grid-side series access, which completely preserves the original converter transformer body and does not require modification of the main equipment of the converter station. It is suitable for existing LCC-HVDC projects and can modify existing converter transformers.

[0017] (2) Low cost and good economic efficiency: The present invention retains the original converter transformer and only adds an external flexible unit, which significantly reduces the cost compared to converting the entire station to MMC; the parallel side uses a large capacity DRU in parallel with a small capacity MMC, and the main power is borne by the low cost DRU. The MMC has a small capacity and low cost, and the overall cost performance is high.

[0018] (3) Achieve wide-range flexible amplitude and phase voltage regulation: Under steady-state conditions, the present invention can suppress voltage fluctuations caused by new energy and the power market, and avoid frequent switching of taps; under fault conditions, it can accurately compensate the commutation voltage on the valve side and suppress commutation failure. Attached Figure Description

[0019] 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.

[0020] Figure 1 This is a schematic diagram of the topology of an external flexible controllable converter transformer (LCC twelve-pulse) applied to an LCC-HVDC system, as provided in Embodiment 1 of the present invention.

[0021] Figure 2 A schematic diagram of the external flexible controllable converter transformer provided in Embodiment 1 of the present invention applied to the topology of an LCC-HVDC system (LCC 24 pulses).

[0022] Figure 3 This is a schematic diagram of the wiring between the external flexible controllable converter transformer and the grid-side structure in Embodiment 1 of the present invention.

[0023] Figure 4 This is a schematic diagram of the single-phase structure of the three-phase four-column power frequency transformer in Embodiment 1 of the present invention.

[0024] Figure 5 This is a schematic diagram of the single-phase structure of the three-phase five-limb power frequency transformer in Embodiment 1 of the present invention.

[0025] Figure 6 This is a block diagram of the positive and negative sequence double closed-loop control of the external flexible controllable converter transformer in Embodiment 1 of the present invention.

[0026] Figures 7(a) and 7(b) are voltage vector adjustment diagrams of grid-side unity power factor and grid-side non-unity power factor when the external flexible controllable converter transformer actively adjusts the voltage in Embodiment 1 of the present invention.

[0027] Figures 8(a) and 8(b) are schematic diagrams of power flow when the external flexible controllable converter transformer experiences a fault crossing and a voltage drop or rise on the grid side, respectively, in Embodiment 1 of the present invention.

[0028] Figures 9(a1)-9(a4) are simulation results of grid-side voltage, series transformer compensation voltage, valve-side voltage, and DC line voltage when an external flexible controllable converter transformer is set up to simulate a three-phase voltage drop fault on the grid side in Embodiment 1 of the present invention.

[0029] Figures 9(b1)-9(b4) are simulation results of grid-side voltage, series transformer compensation voltage, valve-side voltage, and DC line voltage when simulating a three-phase voltage drop fault on the grid side without an external flexible controllable converter transformer in Embodiment 1 of the present invention.

[0030] Figures 10(a1)-10(a4) are simulation results of grid-side voltage, series transformer compensation voltage, valve-side voltage, and DC line voltage when an external flexible controllable converter transformer is set up to simulate a single-phase non-metallic grounding fault on the grid side in Embodiment 2 of the present invention.

[0031] Figures 10(b1)-10(b4) are simulation results of grid-side voltage, series transformer compensation voltage, valve-side voltage, and DC line voltage when simulating a single-phase non-metallic grounding fault on the grid side without an external flexible controllable converter transformer in Embodiment 2 of the present invention.

[0032] 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-External flexible controllable converter transformer; 7-Receiving-end AC power grid; 8-Power frequency converter transformer; 9-Series transformer; 10-Series converter; 11-Parallel hybrid converter; 12-Energy extraction transformer; 13-Energy consumption device. Detailed Implementation

[0033] 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.

[0034] 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.

[0035] It should also be understood that the term “and / or” as used in this specification and the appended claims refers to any combination of one or more of the associated listed items and all possible combinations, and includes such combinations.

[0036] Example 1 like Figure 1 , Figure 2 The diagram shows the application of the external flexible controllable converter transformer of this invention in a high-voltage direct current transmission system. Its overall topology, from left to right, consists of: 1. Sending-end AC grid; 2. Sending-end converter transformer; 3. Thyristor converter valve rectifier; 4. DC transmission line; and 5. 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. an external flexible controllable converter transformer (hereinafter referred to as flexible control transformer) and the receiving end AC power grid.

[0037] 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 a bipolar DC transmission line. 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 connected between the inverter-side converter valves and the receiving-end AC grid. In steady state, it is used to suppress voltage fluctuations caused by new energy fluctuations and the electricity market. When voltage dips or swells occur on the receiving-end AC grid, or when a single-phase ground fault occurs on the receiving-end AC grid side, it is used to regulate the valve-side voltage to suppress commutation failures in the inverter-side converter valves, thereby improving the operational stability of the LCC-HVDC system.

[0038] This invention employs a flexible and controllable converter transformer to actively regulate the commutation voltage on the valve side. The flexible and controllable converter transformer adds a power electronic converter to the traditional power frequency transformer. The power frequency transformer transmits the main power, while the power electronic converter only performs power electronic conversion and regulation on a portion of the power. This leverages the high flexibility and controllability of the power electronic device while also possessing the advantages of high reliability, low loss, and low cost of the traditional power frequency transformer. This achieves active regulation and control of the commutation voltage, effectively expanding the critical voltage range of the LCC converter to withstand commutation failure. It also suppresses conventional DC commutation failure in LCC converters and regulates voltage fluctuations caused by new energy switching and the power market.

[0039] like Figure 1-3 As shown in the figure, the topology of an external flexible controllable converter transformer provided in this embodiment includes a power frequency converter transformer 8, a series transformer 9, an MMC series converter 10, a parallel hybrid converter of DRU + small capacity MMC 11, an energy harvesting transformer 12, an energy dissipation device 13, and a control unit (not shown in the figure). The connection relationship and function of each device are described below.

[0040] The power frequency converter transformer, as a basic voltage transformation unit, is a group of three-phase transformers consisting of three single-phase multi-limb transformers, including but not limited to three-limb transformers (such as...). Figure 3 As shown), four-column transformer (such as Figure 4 (as shown), five-limb transformer (such as) Figure 5 As shown in the figure, the number of core columns can be selected according to needs in practical applications. The windings on each single-phase transformer include a grid-side Y-connected winding 8.1, a valve-side Y-connected winding 8.2, and a valve-side D-connected winding 8.3 wound on the same core column. The grid-side Y-connected winding is connected to the 345kV receiving-end AC power grid, and the valve-side winding is connected to the AC side of the LCC inverter bridge to realize the functions of power frequency voltage level conversion, electrical isolation, and short-circuit impedance matching.

[0041] The series transformer is the core unit for voltage coupling, arranged independently of the power frequency converter transformer. It is installed between the Y-connected winding and ground wire on the grid side of the power frequency converter transformer, without replacing the original converter transformer body. It is suitable for upgrading and retrofitting existing UHV / HVDC projects, including but not limited to twelve-pulse converter valves, dual twelve-pulse converter valves, and dual twenty-four-pulse converter valves. The primary side of the series transformer is connected in series between the Y-connected winding and ground wire on the grid side of the power frequency converter transformer, while the secondary side is connected to the AC port of the series converter. The compensation voltage output from the series converter can be directly coupled into the transformer's grid-side winding. Through the electromagnetic coupling effect of the transformer windings, the amplitude and phase of the commutation voltage on the LCC inverter bridge valve side are indirectly and precisely adjusted, eliminating the influence of line impedance on the compensation effect and significantly improving compensation accuracy and response speed. The series transformer can adopt a traditional two-winding transformer structure or a multi-winding transformer to provide flexible voltage injection capabilities.

[0042] The DC port of the series converter is connected to the parallel hybrid converter to form a common DC bus. The series converter, as the core execution unit of flexible compensation, adopts positive and negative sequence dual closed-loop control and can output AC compensation voltage with fully controllable amplitude, phase and frequency. Under non-fault conditions, it suppresses voltage fluctuations caused by new energy and the power market, actively adjusts grid voltage fluctuations, and avoids frequent tap switching. Under fault conditions, it dynamically compensates for voltage drops and phase distortions on the valve side caused by grid faults, ensures that the thyristors have sufficient commutation margin, and suppresses commutation failure.

[0043] The series converter adopts a modular multilevel converter (MMC) topology. The MMC topology includes a three-phase, six-arm bridge, with each arm containing n submodules. The number of submodules n can be selected according to the voltage level. The submodules are half-bridge submodules (HBSMs), each consisting of two insulated-gate bipolar transistors (IGBTs), two anti-parallel freewheeling diodes, and one submodule capacitor connected in series. The MMC topology allows for 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 distributes 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.

[0044] The parallel hybrid converter consists of a diode rectifier unit (DRU) 11.1 and an auxiliary voltage regulator modular multilevel converter (MMC) 11.2 (i.e., the subsequent small-capacity MMC) connected in parallel. Its AC port is connected in parallel to the receiving end AC grid side through an energy extraction transformer, and its DC port is connected to the series converter via a common DC bus to achieve bidirectional energy interaction. Under normal operating conditions, the DRU provides unidirectional energy feeding, and under fault conditions, the energy-consuming device and the small-capacity MMC work together to maintain the DC bus voltage stability.

[0045] The DRU employs an uncontrolled rectifier topology (i.e., a three-phase bridge uncontrolled rectifier circuit), requiring no control system. It undertakes the main power transmission, achieving unidirectional stable power feed from the receiving-end grid to the DC bus, providing the basic energy for the system's steady-state operation. The small-capacity MMC serves as an auxiliary unit, with its rated capacity designed to be x times (0 < x < 1) the rated capacity of the series converter. It assists in stabilizing the DC bus voltage while also providing partial power transmission and dynamic reactive power compensation and harmonic mitigation functions for the grid side. Preferably, x is 0.2. This capacity design allows the small-capacity MMC to focus on rapid response to grid disturbances or load changes, performing precise voltage fine-tuning without incurring the high costs and complexity associated with high-power transmission.

[0046] Parallel hybrid converters fully utilize the economy and reliability of diode rectifier circuits, as well as the flexible control capabilities of MMCs, avoiding the high cost and complexity associated with using full-capacity MMCs. This reduces the overall cost and losses of parallel hybrid converters while ensuring DC bus voltage stability, thereby improving the system's economy and operating efficiency.

[0047] The primary side of the energy extraction transformer is connected in parallel to the receiving-end AC power grid, and the secondary side is connected to the AC port of the parallel hybrid converter. This is used to provide a stable AC power supply for the parallel hybrid converter and ensure its reliable operation under all operating conditions.

[0048] The energy dissipation device is connected in parallel to the common DC bus of the series converter and the parallel hybrid converter. Under normal operating conditions, it is in a floating charge standby state, smoothing out minor power fluctuations on the DC bus. When the receiving-end grid voltage drops, it absorbs active power, smooths out power fluctuations, and prevents the DC voltage from rising due to power accumulation. The energy dissipation device is one or a combination of energy dissipation resistors, battery energy storage units, supercapacitor energy storage units, and flywheel energy storage units. It can select or combine the most suitable energy storage technology according to the specific characteristics of DC bus power fluctuations. This flexible configuration improves the adaptability and response speed of the energy dissipation device to different types of power fluctuations, ensures the stability of the common DC bus voltage, and thus guarantees the reliable operation of the entire external flexible controllable converter transformer system, effectively suppressing commutation failure.

[0049] The control unit is configured to control the output compensation voltage of the series converter to regulate the valve-side voltage, control the parallel hybrid converter to maintain DC bus voltage stability, and control the energy-consuming device to smooth DC bus power fluctuations. It includes a voltage sampling module, a compensation command calculation module, a positive and negative sequence dual closed-loop control module, a pulse modulation module, a parallel-side voltage stabilization module, and an energy-consuming coordination module.

[0050] The system comprises several modules: a voltage sampling module that collects real-time data on the three-phase voltage of the receiving-end grid, the port voltage of the series converter, and the port current, providing fundamental data for control calculations; a compensation command calculation module that calculates sag commands based on the voltage difference before and after a fault, performing positive-negative sequence decomposition and dq decoupling to generate voltage control reference commands; a positive-negative sequence dual-closed-loop control module that executes independent positive-sequence control or decoupled positive-negative sequence control according to the fault type, generating a modulation voltage reference value, which effectively addresses three-phase voltage symmetry and asymmetry faults in the grid; a pulse modulation module that converts the generated modulation voltage reference value into PWM or SPWM pulse drive signals to drive the power devices of the series converter, outputting compensation voltage to the grid-side windings through the series transformer to maintain stable commutation voltage and suppress thyristor commutation failure; a parallel-side voltage stabilization module that clamps and stabilizes the DC bus voltage using a DRU; and an energy-consuming collaborative control unit that switches power according to the fault conditions to improve the compensation response speed. The entire system can quickly respond after a grid fault occurs, restoring the valve-side commutation voltage to over 99% of its rated value within a short time, thus suppressing commutation failure.

[0051] The control strategy of the control unit is illustrated below using the three-phase symmetrical voltage sag fault response as an example: Assuming a three-phase voltage symmetrical sag fault occurs in the receiving-end power grid, with a voltage drop of 10% and zero negative sequence component, the control unit engages a positive sequence dual closed-loop control strategy, and the responses of each module are as follows: Sampling and Command Calculation: The voltage sampling module collects the three-phase voltage of the receiving-end power grid and the port voltage U of the series converter in real time. mi With current I mi The compensation instruction calculation unit calculates the voltage drop instruction ΔU. j =U m -U mj After positive and negative order decomposition, the negative order component is 0, and only the positive order component ΔU is extracted. j + And ΔU is obtained by dq transformation. j + d ΔU j + q Used as a reference for the outer voltage loop.

[0052] Positive-sequence dual closed-loop control: The outer voltage loop calculates the difference between the positive-sequence voltage reference and the feedback component, and generates the d / q axis current reference command through the PI regulator; the inner current loop calculates the difference between the current reference and the actual feedback, and generates the modulation voltage reference value through the PI regulator.

[0053] Pulse drive and voltage compensation: The pulse modulation module drives the MMC series converter through space vector modulation, injects positive sequence compensation voltage into the series transformer, and raises the commutation voltage on the valve side through electromagnetic coupling.

[0054] Parallel and energy dissipation coordination: The DRU of the parallel hybrid converter maintains the natural clamping and stabilization of the DC bus, and the small-capacity MMC fine-tunes the bus voltage to within ±5% of the rated value; the energy dissipation device absorbs active power to prevent the DC voltage from continuously rising due to power accumulation.

[0055] Fault suppression effect: It can restore the valve-side commutation voltage to more than 99% of the rated value in a short time, make up for the commutation voltage time area gap, ensure sufficient thyristor commutation margin, prevent commutation failure, and maintain continuous and stable power transmission of DC system.

[0056] As a preferred technical solution, this embodiment also provides a control method based on the above-mentioned external flexible controllable converter transformer. The DC bus voltage is controlled and reactive power compensation is performed by a parallel hybrid converter, and the LCC valve-side voltage is adjusted by a series converter. The control method does not require the acquisition of the internal current and commutation angle signals of the converter valve, but only the external grid voltage information is used as input to achieve rapid compensation of the valve-side voltage.

[0057] 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, and the voltage deviation is the voltage drop depth on the grid side, reflecting 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 transformed by dq 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 subsequent process, 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 voltage reference value 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 the same strategy, 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 reference value, and a composite pulse drive signal is generated based on this modulation voltage reference value.

[0058] 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 system 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.

[0059] In the above scheme, 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 inner current loop reference value 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 the modulation voltage reference value. Generate driving signal: Perform coordinate transformation on the modulation voltage reference value and carrier modulation to generate pulse driving signal.

[0060] 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, by carrier modulation and coordinate transformation of the modulated voltage reference value to generate a pulse drive signal, the series converter can be driven efficiently and accurately, 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.

[0061] In application, when a three-phase voltage symmetrical sag / surge fault occurs in the receiving-end power grid, the negative sequence component is zero. A positive sequence dual-closed-loop control strategy is adopted, which uses precise positive sequence voltage compensation to actively raise the valve-side voltage, compensating for the commutation voltage time-area loss and maintaining stable commutation conditions for the LCC converter valve. The specific implementation process is as follows: First, the voltage sag command calculation is performed, based on the rated voltage command U before the receiving-end power grid fault. m The actual voltage command U after the fault mj The three-phase voltage drop command is received: ΔU j =U m -U mj The command accurately characterizes the depth and phase information of the grid voltage sag, serving as the core reference for voltage compensation. Subsequently, positive and negative sequence decomposition and dq transformation are performed. Since three-phase symmetrical sags have no negative sequence component, only the positive sequence component ΔU is extracted. j + ， The negative order component is zero, and the d-axis component ΔU is obtained after dq transformation. j + d With q-axis component ΔU j + q This serves as the reference command for the outer voltage loop control; simultaneously, it acquires the series converter port voltage U. mi Port current I mi Decomposition yields the positive-order component U mi + I mi + And dq transform to U mi + d U mi +q I mi + d I mi + q Next, voltage outer loop control is executed. The d-axis and q-axis components of the positive-sequence voltage reference command and the feedback command are subtracted, and the difference is input to the proportional-integral (PI) controller for adjustment, generating the inner loop current reference command. The voltage outer loop achieves zero steady-state error tracking of the valve-side voltage amplitude and phase. The PI parameters are tuned according to the system's dynamic response and stability, taking into account both response speed and overshoot suppression. Then, current inner loop control is executed. The difference between the current reference command and the actual current feedback component is calculated, and the difference is adjusted by the PI controller to generate a modulated voltage reference value. The current inner loop achieves rapid current limiting protection for the converter output current, preventing overcurrent damage to power devices. Finally, through space vector modulation, the series converter is controlled to inject a positive-sequence compensation voltage with controllable amplitude and phase into the series winding, actively raising the valve-side commutation voltage, compensating for the commutation voltage time area gap caused by grid voltage drops, ensuring sufficient margin in the thyristor commutation process, and suppressing commutation failure under symmetrical faults.

[0062] In applications, when an asymmetrical fault such as a single-phase grounding fault occurs in the receiving-end power grid, for example, a single-phase non-metallic grounding fault, the three-phase voltage becomes unbalanced and generates a negative sequence component. The control system switches to a positive and negative sequence decoupling dual closed-loop control strategy, independently adjusting the positive and negative sequence components to eliminate the influence of negative sequence voltage distortion and ensure the balanced and stable commutation voltage of each phase. Figure 6 As shown, the specific implementation process is as follows: First, calculate the voltage drop command ΔU using the same formula. j =U m -U mj This instruction contains complete positive and negative sequence fault information; it will set ΔU j Perform positive and negative order decomposition to simultaneously obtain the positive order component ΔU j + With negative order component ΔU j - Then, a dq transformation is performed to obtain ΔU. j + d ΔU j + q ΔU j - d ΔU j - qThis constitutes the outer loop voltage reference command set. Simultaneously, the port voltage and current of the series converter are decomposed into positive and negative sequences and transformed using dq transformation to obtain positive and negative sequence voltage and current feedback components, which are then fed into the positive and negative sequence control loops for independent calculation. The positive sequence control loop compensates for positive sequence voltage drops, maintains stable positive sequence commutation voltage on the valve side, and its control logic is consistent with the positive sequence dual-closed-loop control under symmetrical faults, outputting a positive sequence voltage reference value. The negative sequence control loop aims to eliminate negative sequence voltage. It subtracts the negative sequence voltage reference command from the feedback component, generates a negative sequence current command through PI regulation, and then outputs the negative sequence voltage reference value through the inner current loop. Negative sequence control can effectively offset the negative sequence voltage introduced by grid asymmetry faults, reduce valve side voltage imbalance, and prevent commutation voltage zero-crossing point deviation. Finally, the positive and negative sequence voltage reference values ​​are superimposed to form a modulation voltage reference value. Based on the modulation voltage reference value, a composite drive pulse signal is generated to control the series converter to collaboratively output positive and negative sequence compensation voltages. This decoupling control strategy completely avoids the cross-coupling of positive and negative sequence components, significantly improves the compensation accuracy and dynamic response speed under asymmetrical faults, and reliably suppresses commutation failures caused by asymmetrical faults.

[0063] In applications, the parallel hybrid converter employs a DRU natural clamping + small-capacity MMC auxiliary voltage regulation control mode. This eliminates the need for complex active control logic, achieving stable DC bus voltage output and providing reliable energy supply to the series converter. The core of the parallel hybrid converter is the diode rectifier unit (DRU). After uncontrolled rectification by the DRU, the DC voltage is naturally clamped using the unidirectional conduction characteristic of the diodes. For the upper and lower dual 12-pulse commutator valves, the effective values ​​of the DC bus voltage and the AC phase voltage satisfy a fixed proportional relationship: U dc ≈2.7U sabc (U) sabc The transformer outputs a stable DC voltage (based on the rated voltage of the secondary side of the power extraction transformer) without any adjustment during steady-state operation, significantly reducing control complexity and operating losses. The small-capacity MMC acts only as an auxiliary voltage regulator, acquiring the DC bus voltage in real time. When the bus voltage deviates from the rated value by ±5%, a simple voltage / current closed-loop regulation is activated to slightly correct the DC voltage and stabilize it within the rated range. The DRU clamping control and MMC auxiliary regulation work together, ensuring DC voltage stability while simplifying the control system architecture. There is no signal interaction with the series converter compensation control, preventing interference and improving the overall system reliability.

[0064] Simulation test To verify the effectiveness of the LCC technology based on an externally mounted flexible transformer in suppressing commutation failure proposed in this embodiment, a simulation model was built on a simulation platform, such as... Figure 1As shown, a topology modification was carried out on the inverter side of a conventional LCC-HVDC high-voltage direct current transmission system, using an external flexible control transformer. The simulation settings are as follows: the grid-side voltage drop is set to 10% at 3 seconds, and the grid-side voltage recovers at 4 seconds. During this period, the series transformer inputs compensation voltage to the grid-side winding of the line power frequency transformer to instantly compensate for the commutation voltage drop and suppress the occurrence of commutation failure.

[0065] Table 1 Simulation Parameters The simulation sets up a three-phase voltage dip condition on the grid side: The grid-side voltage drop was 10% in 3 seconds; The series transformer responds instantaneously after 3 seconds, outputting a response compensation voltage to maintain the valve-side commutation voltage constant. After 4 seconds, the grid-side voltage returned to normal, the output compensation voltage of the series transformer was 0, the valve-side commutation voltage remained unchanged, and the system did not experience commutation failure.

[0066] Figures 7(a) and 7(b) show voltage vector regulation analysis diagrams, illustrating the vector synthesis principle of amplitude and phase regulation achieved by the topology of this invention. During the simulation analysis, the receiving-end grid voltage vector is set. When voltage regulation or compensation is required, the required compensation voltage vector (adjustable amplitude and phase) is calculated and superimposed onto the grid voltage vector. Figures 7(a) and 7(b) visually demonstrate how a controllable compensation voltage vector (ΔU) is injected. j This ensures that the synthesized valve-side voltage vector remains stable (constant amplitude or according to a given reference), regardless of whether the grid voltage vector fluctuates or shifts in phase.

[0067] Figures 8(a) and 8(b) show the power flow through an external flexible transformer during a fault in the AC grid at the receiving end (such as a three-phase drop or a single-phase ground fault). Figures 8(a) and 8(b) illustrate the energy flow path during the fault: the DRU continuously draws energy (or the grid feeds energy to the DC bus through the energy-drawing transformer and the parallel MMC), the series converter draws energy from the DC bus to output a compensation voltage, and the energy-consuming device absorbs the surplus power that may be generated on the DC bus due to the fault (to prevent the DC voltage from rising due to power accumulation). This demonstrates the power balance and coordination mechanism of the system under fault conditions.

[0068] Figures 9(a1)-9(a4) and 9(b1)-9(b4) show the simulation results comparing voltage regulation with and without an external flexible transformer.

[0069] As shown in Figures 9(a1)-9(a4), when a three-phase voltage drop fault occurs, after setting an external flexible control transformer, the series converter injects compensation voltage instantaneously after 3 seconds, the valve-side commutation voltage remains unchanged (or above 99%), the DC voltage and power remain continuously stable, and no commutation failure occurs, proving that the topology can effectively suppress commutation failure and stabilize voltage under symmetrical faults.

[0070] As shown in Figures 9(b1)-9(b4), when a three-phase voltage drop fault occurs, without an external flexible control transformer, after the voltage drop, the valve-side commutation voltage decreases, the commutation angle decreases or commutation fails, the DC current surges, and the power transmission is interrupted or fluctuates violently.

[0071] Comparing Figures 9(a1)-9(a4) and 9(b1)-9(b4), the simulation results confirm that the external flexible control transformer, through the innovative arrangement of grid-side coupling compensation, achieves precise control of the valve-side commutation voltage, effectively suppresses commutation failure, and significantly improves the fault ride-through capability of the inverter side of the LCC-HVDC system.

[0072] Example 2 This embodiment provides an operational example of an external flexible controllable converter transformer operating under an asymmetrical fault on the grid side. Based on the topology of Embodiment 1, this embodiment describes the system response and execution process of the positive-negative sequence decoupling dual-closed-loop control strategy for a non-metallic ground fault in phase A of the receiving-end grid (fault resistance 10Ω, phase A voltage drop, and three-phase voltage asymmetry): When a phase A nonmetallic ground fault occurs in the receiving-end power grid, the three-phase voltage becomes unbalanced and generates a negative sequence component. The voltage on the valve side of the LCC inverter bridge faces the risk of unbalanced distortion. The entire system responds quickly: the parallel hybrid converter: the DRU continuously maintains the DC voltage natural clamping, unaffected by the asymmetrical fault, and the small-capacity MMC quickly stabilizes the DC bus voltage without fluctuations or overflow.

[0073] Energy dissipation device: After the fault detection unit triggers the signal, it is instantly connected to the DC bus to absorb active power and prevent the DC bus voltage from rising continuously.

[0074] The single-phase ground fault control strategy is executed as follows: the control system switches to a positive and negative sequence decoupling dual closed-loop control strategy, independently adjusting the positive and negative sequence components. The specific process is as follows: Fault voltage calculation: The compensation command calculation unit calculates the drop command ΔU. j =U m -U mj The positive-sequence component ΔU is obtained synchronously through positive and negative sequence decomposition. j + With negative order component ΔU j - Then, a dq transformation is performed to obtain ΔU. j+ d ΔU j + q ΔU j - d ΔU j - q .

[0075] Positive sequence control loop: Executes positive sequence dual closed-loop control to compensate for positive sequence voltage drops, maintains the stability of the amplitude and phase of the valve-side positive sequence commutation voltage, and ensures the basic commutation conditions of the thyristor.

[0076] Negative sequence control loop: With the goal of eliminating negative sequence voltage, the negative sequence voltage reference and feedback component are subtracted, and a negative sequence current command is generated through PI regulation. The negative sequence voltage reference value is output through the inner current loop to cancel the negative sequence voltage distortion introduced by the fault.

[0077] Composite drive and compensation: The positive and negative sequence voltage reference values ​​are used to form the modulation voltage reference values. Based on the modulation voltage reference values, a composite drive pulse signal is generated to control the MMC series converter to output positive and negative sequence compensation voltages.

[0078] Fault suppression effect: Valve-side voltage imbalance is reduced to less than 1%, commutation voltage does not drop, commutation voltage time area is sufficient, commutation failure caused by asymmetric faults is suppressed, DC system voltage and power do not fluctuate significantly, and fault ride-through capability meets the standard.

[0079] The parallel hybrid converter adopts DRU natural clamping and small-capacity MMC auxiliary voltage regulation throughout the process. It has no complicated control logic and the DC bus voltage is stable within ±5% of the rated value, providing a continuous and reliable energy supply for the series converter. The decoupled control with positive and negative sequence has no signal cross-interference, ensuring the reliability of system operation.

[0080] The simulation results comparing voltage regulation with and without external flexible transformer are shown in Figures 10(a1)-10(a4) and 10(b1)-10(b4).

[0081] As shown in Figures 10(a1)-10(a4), when a single-phase non-metallic ground fault occurs, after setting an external flexible control transformer, the series converter instantaneously injects compensation voltage after 3 seconds. The positive sequence compensation voltage drops, and the negative sequence compensation cancels the negative sequence voltage. The voltage imbalance on the valve side is reduced to less than 1%, and the commutation voltage drops almost without any drop, maintaining the rated commutation voltage. The DC system voltage and power do not fluctuate significantly, effectively suppressing the commutation failure caused by the asymmetrical fault, proving the effectiveness of the topology and control strategy under asymmetrical faults.

[0082] As shown in Figures 10(b1)-10(b4), when a single-phase non-metallic grounding fault occurs, no external flexible control transformer is installed, resulting in three-phase voltage imbalance, large negative sequence voltage, significant drop in valve-side voltage, easy phase commutation failure, and large fluctuations in the DC system.

[0083] Comparing Figures 10(a1)-10(a4) and 10(b1)-10(b4), the simulation results confirm that the external flexible control transformer, through the innovative arrangement of grid-side coupling compensation, achieves precise control of the valve-side commutation voltage, effectively suppresses commutation failure, and significantly improves the fault ride-through capability of the inverter side of the LCC-HVDC system.

[0084] 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.

[0085] 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.

[0086] 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 externally mounted flexible controllable converter transformer, characterized in that: This includes power frequency converter transformers, series transformers, series converters, parallel hybrid converters, energy extraction transformers, energy consumption devices, and control units; The grid-side winding of the power frequency converter transformer is used to connect to the receiving-end AC power grid, and the valve-side winding is used to connect to the AC side of the LCC inverter bridge. The series transformer is arranged independently of the power frequency converter transformer. The primary side of the series transformer is connected in series between the grid-side winding and the ground wire of the power frequency converter transformer, and the secondary side is connected to the AC port of the series converter, so as to couple the compensation voltage output by the series converter into the grid-side winding. The DC port of the series converter and the DC port of the parallel hybrid converter are connected to form a common DC bus. The parallel hybrid converter includes a parallel diode rectifier unit and an auxiliary voltage regulator MMC. The AC port of the parallel hybrid converter is connected to the receiving end AC power grid through the energy extraction transformer. The energy-consuming device is connected in parallel to the common DC bus; The control unit is configured to control the output compensation voltage of the series converter to adjust the valve-side voltage, control the parallel hybrid converter to maintain the DC bus voltage stability, and control the energy-consuming device to smooth DC bus power fluctuations.

2. The external flexible controllable converter transformer according to claim 1, characterized in that: The series converter adopts an MMC topology, and each arm of the MMC topology contains several half-bridge sub-modules.

3. The external flexible controllable converter transformer according to claim 1, characterized in that: The diode rectifier unit is a three-phase bridge uncontrolled rectifier circuit used to realize unidirectional power feed from the receiving end AC grid to the DC bus; the rated capacity of the auxiliary voltage regulator MMC is x times the rated capacity of the series converter, used to assist in stabilizing the DC bus voltage, where 0 < x < 1.

4. The external flexible controllable converter transformer according to claim 1, characterized in that: The energy-consuming device is any one or a combination of energy-consuming resistors, battery energy storage units, supercapacitor energy storage units, and flywheel energy storage units.

5. A control method for an external 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 and negative sequence components are obtained by performing positive and negative sequence decomposition and dq transformation 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 inner current loop reference value 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 the modulation voltage reference value. Generate driving signal: Modulate the modulation voltage reference value to generate a pulse driving signal.

9. The control method according to claim 5, characterized in that: It also includes stabilizing the DC bus voltage: maintaining the DC bus voltage naturally clamped by a diode rectifier unit, and using a dual closed-loop control method to drive the auxiliary voltage regulator MMC to fine-tune the DC bus voltage based on the DC bus voltage.

10. The control method according to claim 5, characterized in that: It also includes smoothing DC bus power fluctuations: when the receiving end grid is fault-free, the energy-consuming device is in a floating charge standby state; when the receiving end grid experiences a voltage drop, the energy-consuming device is controlled to be put into operation instantaneously to absorb active power and smooth DC bus power fluctuations.

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