Lcc-clcc direct current system cooperative control method, device and equipment considering frequency and voltage active support and medium

Abstract

The application provides an LCC-CLCC direct-current system cooperative control method, device, equipment and medium considering frequency and voltage active support, and relates to the technical field of direct-current system control. The method comprises the following steps: selecting a small disturbance or a large disturbance control mode according to a comparison result of a rectification side direct-current voltage and a threshold value; in the small disturbance mode, adjusting the direct-current voltage according to a frequency deviation, and determining a voltage-current droop coefficient K 2 so as to maintain constant reactive power consumption of a converter, and adjusting a direct-current according to the voltage-current droop coefficient K 2; in the large disturbance mode, designing a voltage regulation dead zone according to a voltage regulation capacity, calculating a trigger angle additional amount according to an alternating-current voltage deviation to correct the trigger angle, and switching the voltage-current droop coefficient from K 3 to adjust the direct-current. The application can improve the frequency stability and the voltage stability of a receiving end power grid simultaneously.

Description

Technical Field

[0001] This invention relates to the field of DC system control technology, and in particular to a collaborative control method, apparatus, equipment and medium for an LCC-CLCC DC system that takes into account both frequency and voltage active support. Background Technology

[0002] With the rapid growth of renewable energy installed capacity and the continuous decline in the proportion of traditional synchronous generator units, the inertia and reactive power support capabilities of the power system have been significantly weakened, and the problems of system frequency and voltage stability have become increasingly prominent. High-voltage direct current (HVDC) transmission technology, with its advantages of large transmission capacity, long transmission distance, and low operating losses, plays a core role in large-scale renewable energy consumption and inter-regional power exchange. However, traditional LCC-HVDC transmission systems based on grid-commutated converters are constrained by their own commutation characteristics, have high reactive power consumption, and insufficient active support capabilities, making it difficult to adapt to the flexibility and stability requirements of high-proportion renewable energy power systems. In contrast, controllable grid-commutated converters (CLCCs) have controllable shutdown capabilities, effectively avoiding commutation failure problems, and can provide short-term power support for the power system, which is of great significance for improving the safe and stable operation of the power system.

[0003] To reveal the working mechanism, stress characteristics, and controllability of controllable commutated converters, scholars both domestically and internationally have conducted extensive research from multiple dimensions, including simulation modeling, experimental verification, and control strategy design. The literature "Chen Hao, Li Jianhua, Huang Zhiguang, et al. Simplified electromagnetic transient modeling method for controllable grid commutated converters [J]. Power System Technology, 2024, 48(8): 3467-3475" proposes a simplified electromagnetic transient modeling method. This model can support harmonic characteristic analysis of controllable commutated converters, simulation verification of control strategies, and analysis of external bridge arm fault evolution laws. Corresponding low-voltage energization test results further confirm that controllable commutated converters possess good controllable regulation performance under low-voltage load conditions. Existing research employs multi-wave resonant power supply superposition analysis to analyze the transient characteristics of the entire AC fault current interruption process of controllable grid commutated converters in commutation failure scenarios. Based on a single controllable converter valve high-current turn-off test platform, related experimental conclusions verify the withstand limit of the converter valve under transient electrical stress. Other studies have systematically reviewed and deeply analyzed the key electrical parameters, overall structural design scheme, and complete type test procedures of controllable grid commutated converters. However, existing control strategies for controllable grid commutated converters fail to fully exploit their inherent power regulation potential under AC fault conditions. (Reference: Lian Panjie, Pang Bohan, Liu Wenzhuo, et al. Research on Fault Characteristics and Control Optimization of Novel Controllable Grid Commutated Converters [J]). The Proceedings of the CSEE, 2025, 45(2): 679-690, compares the dynamic power response characteristics of controllable grid commutator converters and traditional grid commutator converters during AC fault transients at the receiving end of the grid. Based on this, a control optimization scheme with widening the maximum firing angle as the core is proposed, which effectively improves the transient power support level of controllable grid commutator converters during faults. On this basis, some scholars have proposed an energy consumption control mode to further enhance the system power support effect under AC faults at the receiving end, alleviate the power impact of the fault on the sending end of the grid, and ensure the stable operation of the power systems at both the sending and receiving ends. Other studies have confirmed that controllable grid commutator converters can achieve autonomous shutdown without reverse voltage within the arc extinction angle range, and can still operate stably for a short time even if the arc extinction angle is zero or negative. Reference: Xin Yechun, Ma Zhiqiang, Wang Tuo, et al. Analysis of CLCC multi-quadrant power regulation capability and short-time reactive power support control strategy [J / OL]. The Proceedings of the Chinese Society for Electrical Engineering systematically studied the four-quadrant power regulation characteristics of controllable grid phase-commutation converters under AC fault conditions, and proposed a reactive power support strategy based on firing angle compensation. Through simulation and experimental verification, this strategy can effectively improve the system's transient voltage support capability while maintaining the stability of DC transmission power.

[0004] Currently, most research on the coordinated support control of sending and receiving end converter stations focuses on the frequency support control of high-voltage direct current transmission systems with traditional grid phase-commutation converters. The literature "Li R, Bozhko S, Asher G. Frequency control design for offshore wind farm grid with LCC-HVDC link connection[J]. IEEE Transactions on Power Electronics, 2008, 23(3): 1085-1092" and "Miao Z, FanL, Osborn D, et al. Wind farms with HVdc delivery in inertial response and primary frequency control[J]. IEEE Transactions on Energy Conversion, 2010, 25(4): 1171-1178" employ a frequency-DC current (P / Idc) droop control strategy, which achieves frequency regulation of the sending-end grid by adjusting the DC current reference value on the rectifier side. Based on this, the paper "Rakhshani E, Mehrjerdi H, Remon D, et al. Frequency control of HVDC interconnected system considering derivative based inertia emulation[C] / / 2016 IEEE Power and Energy Society General Meeting (PESGM). IEEE, 2016: 1-5" introduces a frequency differential element in P / Idc droop control to achieve inertia support for the sending-end grid. The paper "Liu Y, Song Y, Wang Z, et al. Optimal emergency frequency control based on coordinated droop in multi-infeed hybrid AC-DC system[J]. IEEE Transactions on Power Systems, 2021, 36(4): 3305-3316" designs a frequency signal filtering unit that can dynamically select the frequency signal of the sending or receiving end grid to participate in frequency modulation, ensuring that only the droop control signal with the best response is called under emergency frequency modulation conditions.However, the signal screening process increases the selection delay of control commands, which may reduce the response sensitivity of high-voltage DC frequency regulation. To address the limitation of traditional strategies that can only provide frequency support for a single-end grid, the literature "Liu Y, Shen C, Wang Z, et al. Incentive mechanism design for emergency frequency control in multi-infeed hybrid AC-DC system[J]. IEEE Transactions on Power Systems, 2023, 39(1): 1867-1880" uses the frequency difference between the sending and receiving end grids as the control input, achieving bidirectional frequency support for both grids. However, none of the above studies have modeled and analyzed the AC voltage variation characteristics during the high-voltage DC frequency regulation process. The paper "Kwon DH, Kim YJ, Moon S I. Modeling and analysis of an LCC HVDC system using DC voltage control to improve transient response and short-term power transfer capability[J]. IEEE Transactions on Power Delivery, 2018, 33(4): 1922-1933" achieves high-voltage DC power regulation by coordinating the control of DC current on the inverter side and DC voltage on the rectifier side, providing frequency support for the sending-end power grid. The paper "Lee GS, Kwon DH, Moon SI, et al. A coordinated control strategy for LCC HVDC systems for frequency support with suppression of AC voltage fluctuations[J]. IEEE Transactions on Power Systems, 2020, 35(4): 2804-2815" proposes a control method that does not require communication and is more compatible with traditional high-voltage DC receiving-end frequency regulation. It achieves frequency information transmission from the inverter side to the rectifier side by introducing DC voltage deviation, and can effectively suppress AC voltage fluctuations during the frequency regulation process. During frequency regulation in the high-voltage DC system of the grid commutator, the dynamic adjustment of active power will cause the reactive power of the converter to fluctuate, which in turn will lead to AC grid voltage oscillation.The paper "Chen M, Xue Y, Chen Y, et al. Adaptive AC Voltage Control Using LCC-HVDC Based on LinearizedReactive Power-AC Voltage Characteristic[J]. IEEE Transactions on Power Systems, 2025" utilizes the reactive power-AC voltage linearization characteristic (LGM) to achieve adaptive regulation of AC voltage fluctuations, effectively suppressing AC voltage deviation in high-voltage direct current systems. The paper "Lee GS, Kwon DH, Kim YK, et al. Anew communication-free grid frequency and AC voltage control of hybrid LCC-VSC-HVDC systems for offshore wind farm integration[J]. IEEE Transactions on Power Systems, 2022, 38(2): 1309-1321" combines droop control technology to achieve stable AC voltage control by synchronously regulating DC voltage and DC current. For asynchronously interconnected power systems, the paper "Li Z, Xue Y, Chen Y, et al. Bilateral primary frequency support and AC voltage regulation of LCC-HVDC systems for asynchronously interconnected systems[J]. IEEE Transactions on Power Systems, 2024, 40(3): 2600-2612" proposes a collaborative control scheme for grid-commutated HVDC systems without inter-station communication. This scheme can simultaneously achieve primary frequency regulation and AC voltage stability of the grids at both ends, and the analytical design of control parameters is completed based on a low-order frequency model.

[0005] In summary, existing technologies mainly focus on the topology and engineering applications of CLCC converters, and are limited to the analysis of the active and reactive power regulation capabilities of the CLCC converter itself. They have not yet considered the impact of disturbances in the LCC-CLCC DC system on the grid frequency, and the coordinated support of frequency and voltage under different disturbance conditions is still lacking. Summary of the Invention

[0006] This invention provides a method, apparatus, device, and medium for coordinated control of an LCC-CLCC DC system that takes into account both frequency and voltage active support, in order to achieve at least one of the following objectives: 1) Based on the forced commutation characteristics of CLCC, and combined with relevant control methods, the multi-quadrant operation capability of CLCC was analyzed, revealing the key influencing factors of CLCC providing short-term reactive power support; 2) For small disturbance conditions, by co-optimizing the droop coefficient and dynamically correcting the voltage-current droop coefficient according to the frequency deviation, the reactive power consumption of the CLCC converter is kept approximately constant during the active power regulation process, thereby suppressing AC voltage fluctuations while achieving active power regulation. 3) For large disturbance conditions, the CLCC transient frequency and voltage are supported by adjusting the DC current and firing angle simultaneously; the voltage regulation dead zone is designed according to the control mode characteristics under the 180° firing angle condition, and the adaptive switching of the compensation strategy under different disturbance conditions is realized.

[0007] In a first aspect, the present invention provides a coordinated control method for an LCC-CLCC DC system that takes into account both frequency and active voltage support, the method comprising: Based on the topology and quasi-steady-state operation mechanism of the CLCC converter, a quasi-steady-state mathematical model of the CLCC converter is established, and the quantitative relationship between the reactive power and firing angle of the CLCC converter is determined based on the quasi-steady-state mathematical model. The rectifier-side DC voltage detection value is obtained, and the rectifier-side DC voltage detection value is compared with a preset voltage threshold. When the rectifier-side DC voltage detection value is greater than or equal to the voltage threshold, a small disturbance control mode is executed; when the rectifier-side DC voltage detection value is less than the voltage threshold, a large disturbance control mode is executed. In the small disturbance control mode, based on the frequency deviation Δ measured on the inverter side f and frequency-voltage droop factor K 1. Calculate the DC voltage deviation Δ on the inverter side. U di According to the DC voltage deviation Δ on the inverter side U di Adjust the reference value of the DC voltage on the inverter side; with the goal of maintaining constant reactive power consumption of the CLCC converter during frequency regulation, based on the measured frequency deviation Δ on the inverter side. f Determine the voltage-current droop coefficient under small disturbance conditions K The current value of 2; based on the determined K 2 and the DC voltage deviation Δ on the inverter side U di The DC current deviation Δ is calculated. I dc According to the DC current deviation ΔI dc Adjust the reference value of the DC current; Under the large disturbance control mode, based on the AC voltage deviation Δ U ac With AC voltage dead zone value U acH Based on the comparison results, the additional adjustment amount Δ of the firing angle is calculated. α i Additional adjustment Δ based on the firing angle α i The firing angle reference value is corrected to achieve additional constant AC voltage control; simultaneously, the voltage-current droop coefficient is adjusted from... K 2. Voltage-current droop coefficient when switching to fault mode K 3. Based on the switched K 3 and the DC voltage deviation Δ on the inverter side U di The improved DC current command value is calculated.

[0008] In one possible design, the quasi-steady-state mathematical model of the CLCC converter is determined as follows: For a 6-pulse converter, based on the commutation process of the converter, the phase angle difference between the single-phase AC voltage and AC current... Represented as: (1) in, μ This is the commutation overlap angle. λ The extinguishing arc angle, α For trigger angle; According to the phase angle difference With DC power P To obtain the reactive power consumed by the inverter Q Represented as: (2) Based on the quasi-steady-state operation mechanism of the CLCC converter, the quasi-steady-state mathematical model of the CLCC converter is determined as follows: (3) (4) in, U aci This is the effective value of the AC line voltage. I dc It is direct current; U di This is the DC voltage of the inverter station; N It is the extreme logarithm; k This refers to the converter transformer voltage ratio; Xr This refers to the equivalent commutation reactance on the secondary side of the converter transformer. P d This is DC power.

[0009] In one possible design, under the small disturbance control mode, the frequency deviation Δ measured on the inverter side is... f and frequency-voltage droop factor K 1. Calculate the DC voltage deviation Δ on the inverter side. U di ,include: Based on the frequency deviation Δ measured on the inverter side f and frequency-voltage droop factor K 1. Determine the DC voltage on the inverter side using the following formula. U di : (5) in, U diref This is the reference value for the DC voltage on the inverter side of the CLCC; Δ f = f - f N To measure the frequency deviation on the inverter side; f The measurement frequency is on the inverter side; f N Rated frequency; Based on the DC voltage on the inverter side U di The deviation Δ of the DC voltage on the inverter side is determined by the following formula. U di : (6) According to the determined K 2 and the DC voltage deviation Δ on the inverter side U di The DC current deviation Δ is calculated. I dc ,include: Determine the DC voltage on the rectifier side U dr DC voltage of inverter station U di The relationship is represented as: (7) in, I dc The steady-state DC current is expressed as: (8) in, I dcrefThis is the reference value for DC current. U dr The DC voltage measured at the rectifier. R dc The resistance of the DC transmission line; Substituting equation (7) into equation (8), we obtain the DC current deviation Δ. I dc The expression is as follows: (9) in, I dc0 This is a reference value for DC current. exist K 2 R dc Neglecting the issue, the DC current deviation Δ I dc The expression is as follows: (10)

[0010] In one possible design, the frequency-voltage droop coefficient K 1. Determined by the following formula: (11) Where, Δ U di,max Δ is the maximum allowable DC voltage deviation on the inverter side of the system. f max This represents the maximum frequency deviation.

[0011] In one possible design, the goal is to maintain a constant reactive power consumption of the CLCC converter during frequency regulation, based on the frequency deviation Δ measured on the inverter side. f Determine the voltage-current droop coefficient under small disturbance conditions K The current values ​​of 2 include: Based on the quasi-steady-state mathematical model of the CLCC converter and the inverter-side DC voltage U di The calculation formula is used to obtain the CLCC inverter side firing angle. α Frequency deviation Δ from inverter side measurement f Functional relationship between them: (12) The reactive power of the CLCC converter Q Expressed as a function of DC voltage, DC current, firing angle, and commutation overlap angle: (13) an( α + μ / 2) is defined as λ (Δ f ), to reduce the steady-state reactive power consumption before the disturbance. Q Using 0 as the control target, the reactive power can be obtained by inversely solving equation (13). Q Q 0 o'clock K The voltage-current droop coefficient under small disturbance conditions is expressed by the following formula. K Determining the current value of 2: (14) in, U d0 This is the rated DC voltage on the rectifier side.

[0012] In one possible design, under the large disturbance control mode, based on the AC voltage deviation Δ U ac With AC voltage dead zone value U acH Based on the comparison results, the additional adjustment amount Δ of the firing angle is calculated. α i The process is represented as: (15) in, U acH This refers to the dead zone value of the AC voltage. x i , x i,max These are the state variables and their limiting values ​​for the LCC-CLCC DC system, respectively. K p and K i These are the coefficients for the proportional and integral components, respectively; Δ α i Δ α max These are the additional adjustment amount and upper limit value of the firing angle after the AC voltage at the receiving end drops beyond the dead zone; α ref For; trigger angle reference value α ref0 This is the initial reference value for the trigger angle.

[0013] In one possible design, the AC voltage dead zone value U acH The design method and the design method of voltage-current droop coefficient K3 during faults are as follows: AC voltage U aci Represented as: (16) Substituting the firing angle of 180° into equation (16) yields... U acH The expression is: (17) Voltage-current droop coefficient during faults K 3. Select a rated DC current that is a multiple of the set value. I dc,max To determine the upper limit of the current, select a rated voltage that is a multiple of the set value. U dc,max The voltage upper limit is set according to the aforementioned current upper limit and voltage upper limit. K The maximum value of 3: (18) Based on the DC voltage detection results on the rectifier side, the voltage-current droop coefficient is switched according to the following logic: (19) in, U dr The DC voltage measured at the rectifier. K i This represents the equivalent droop coefficient actually applied under fault conditions. U dr0 This is the rated DC voltage on the rectifier side.

[0014] Secondly, the present invention provides a coordinated control device for an LCC-CLCC DC system that takes into account both frequency and voltage active support, the device comprising: The characteristic analysis module is configured to establish a quasi-steady-state mathematical model of the CLCC converter based on its topology and quasi-steady-state operating mechanism, and to determine the quantitative relationship between the reactive power and firing angle of the CLCC converter based on the quasi-steady-state mathematical model. The mode selection module is configured to acquire the rectifier-side DC voltage detection value, compare the rectifier-side DC voltage detection value with a preset voltage threshold; when the rectifier-side DC voltage detection value is greater than or equal to the voltage threshold, execute the small disturbance control mode; when the rectifier-side DC voltage detection value is less than the voltage threshold, execute the large disturbance control mode. The first control module is configured to, under the small disturbance control mode, determine the frequency deviation Δ measured on the inverter side. f and frequency-voltage droop factor K 1. Calculate the DC voltage deviation Δ on the inverter side. U di According to the DC voltage deviation Δ on the inverter side U diAdjust the reference value of the DC voltage on the inverter side; with the goal of maintaining constant reactive power consumption of the CLCC converter during frequency regulation, based on the measured frequency deviation Δ on the inverter side. f Determine the voltage-current droop coefficient under small disturbance conditions K The current value of 2; based on the determined K 2 and the DC voltage deviation Δ on the inverter side U di The DC current deviation Δ is calculated. I dc According to the DC current deviation Δ I dc Adjust the reference value of the DC current; The second control module is configured to, under the large disturbance control mode, determine the AC voltage deviation Δ U ac With AC voltage dead zone value U acH Based on the comparison results, the additional adjustment amount Δ of the firing angle is calculated. α i Additional adjustment Δ based on the firing angle α i The firing angle reference value is corrected to achieve additional constant AC voltage control; simultaneously, the voltage-current droop coefficient is adjusted from... K 2. Voltage-current droop coefficient when switching to fault mode K 3. Based on the switched K 3 and the DC voltage deviation Δ on the inverter side U di The improved DC current command value is calculated.

[0015] Thirdly, embodiments of the present invention provide an electronic device, including: at least one processor and a memory; the memory stores computer-executable instructions; the at least one processor executes the computer-executable instructions stored in the memory, causing the at least one processor to execute the LCC-CLCC DC system collaborative control method that takes into account both frequency and voltage active support as described in the first aspect and various possible designs of the first aspect.

[0016] Fourthly, embodiments of the present invention provide a computer-readable storage medium storing computer-executable instructions. When a processor executes the computer-executable instructions, it implements the LCC-CLCC DC system collaborative control method that balances frequency and voltage active support as described in the first aspect and various possible designs of the first aspect.

[0017] Fifthly, embodiments of the present invention provide a computer program product, including a computer program, which, when executed by a processor, implements the LCC-CLCC DC system collaborative control method that balances frequency and voltage active support as described in the first aspect and various possible designs of the first aspect.

[0018] The LCC-CLCC DC system coordinated control method, apparatus, equipment, and medium provided by this invention, which takes into account both frequency and voltage active support, have at least the following beneficial effects: 1) For scenarios with small perturbations, the droop coefficient is optimized through coordination. K 1. K 2. Furthermore, the reactive power response of the CLCC converter is adaptively adjusted based on the difference between the firing angle and the DC current. K 2. While achieving rapid frequency support at the receiving end, the converter maintains a basically constant reactive power consumption during frequency regulation, effectively suppressing AC voltage fluctuations and improving the coordinated stability of system frequency and voltage.

[0019] 2) For scenarios with large disturbances, the voltage regulation dead zone and control parameters are designed based on the constant DC voltage characteristics under the condition of 180° firing angle. In case of a fault, the DC current is increased by switching the droop coefficient, realizing the adaptive switching of the regulation function under different disturbance levels. By simultaneously adjusting the DC current and firing angle, the reactive power support capability of CLCC is further increased, and the active power support level of LCC-CLCC system is improved. This significantly improves the voltage recovery and frequency support effect of the receiving end grid and enhances the transient stability level of the receiving end grid. Attached Figure Description

[0020] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with the invention and, together with the description, serve to explain the principles of the invention.

[0021] Figure 1 A flowchart illustrating the overall process of a collaborative control method for an LCC-CLCC DC system that balances frequency and voltage active support, provided in an embodiment of the present invention. Figure 2 The LCC-CLCC system structure and control block diagram provided in the embodiments of the present invention; Figure 3 A schematic diagram illustrating the three-dimensional relationship between frequency variation and voltage and current variation provided in an embodiment of the present invention; Figure 4 A flowchart illustrating the implementation of coordinated control of an LCC-CLCC DC system provided in this embodiment of the invention; Figure 5Simulation results of different control methods under small disturbance conditions provided in the embodiments of the present invention are shown in the figure; wherein, (a) receiving end grid frequency; (b) DC voltage; (c) DC current; (d) transmitted active power; (e) AC voltage; (f) reactive power generated by CLCC; Figure 6 Simulation results of different control methods under minor faults provided in the embodiments of the present invention are shown in the figure; wherein, (a) receiving end grid frequency; (b) transmitted active power; (c) DC voltage; (d) DC current; (e) AC voltage; (f) reactive power generated by CLCC; (g) inverter side firing angle; Figure 7 The following are simulation results of different control methods under severe faults provided in the embodiments of the present invention; wherein, (a) inverter side firing angle; (b) AC voltage; (c) reactive power generated by CLCC; (d) receiving end grid frequency; (e) transmitted active power; (f) DC current; (g) DC voltage; Figure 8 This is a structural diagram of an LCC-CLCC DC system collaborative control device that takes into account both frequency and voltage active support, provided in an embodiment of the present invention.

[0022] The accompanying drawings have illustrated specific embodiments of the present invention, which will be described in more detail below. These drawings and descriptions are not intended to limit the scope of the invention in any way, but rather to illustrate the concept of the invention to those skilled in the art through reference to specific embodiments. Detailed Implementation

[0023] Exemplary embodiments will now be described in detail, examples of which are illustrated in the accompanying drawings. When the following description relates to the drawings, unless otherwise indicated, the same numerals in different drawings denote the same or similar elements. The embodiments described in the following exemplary embodiments do not represent all embodiments consistent with the present invention. Rather, they are merely examples of apparatuses and methods consistent with some aspects of the invention as detailed in the appended claims.

[0024] The collection, storage, use, processing, transmission, provision, and disclosure of relevant data and information in the technical solution of this invention all comply with the provisions of relevant laws and regulations and do not violate public order and good morals.

[0025] It should be noted that in the embodiments of the present invention, certain software, components, models and other existing solutions in the industry may be mentioned. These should be regarded as exemplary and are only intended to illustrate the feasibility of implementing the technical solution of the present invention. However, they do not mean that the inventor has used or necessarily used the solution.

[0026] The technical solution of the present invention and how the technical solution of the present invention solves the above-mentioned technical problems are described in detail below with specific embodiments. These specific embodiments can be combined with each other, and the same or similar concepts or processes may not be described again in some embodiments. The embodiments of the present invention will now be described with reference to the accompanying drawings.

[0027] Controllable commutated converters (CLCCs) possess controllable turn-off capabilities, providing short-term power support to the system while avoiding commutation failure, which is of great significance for improving system safety and stability. Based on this, this invention provides a coordinated control method for LCC-CLCC DC systems that balances active frequency and voltage support. First, the method analyzes the multi-quadrant operating characteristics of CLCCs, revealing the response mechanism and key influencing factors for providing short-term reactive power support. Next, it proposes a coordinated control strategy for LCC-CLCC frequency support based on the approximate decoupling of active and reactive power, effectively suppressing AC voltage fluctuations while improving frequency stability under small disturbance conditions. Then, it proposes a transient frequency and voltage coordinated support strategy based on the coordinated adjustment of DC current and firing angle, effectively improving the active and reactive power support capabilities of LCC-CLCCs under large disturbance conditions. Furthermore, it designs a voltage regulation dead zone value and related control parameters to achieve adaptive switching of the regulation function under different disturbance conditions. Finally, a simulation example was conducted based on the PSCAD / EMTDC model to verify the proposed collaborative control strategy. Compared with the traditional method, the proposed strategy fully leverages the regulation capability of the CLCC and can simultaneously improve the frequency stability and voltage stability of the receiving-end power grid.

[0028] Specifically, such as Figure 1 As shown, the LCC-CLCC DC system coordinated control method that takes into account both frequency and voltage active support includes the following steps S10-S40 during implementation.

[0029] S10: Based on the topology and quasi-steady-state operation mechanism of the CLCC converter, establish a quasi-steady-state mathematical model of the CLCC converter, and determine the quantitative relationship between the reactive power and firing angle of the CLCC converter based on the quasi-steady-state mathematical model.

[0030] Step S10 aims to reveal the essence of power regulation in CLCC converters from a mechanistic perspective, providing a mathematical model basis for subsequent control strategies.

[0031] In one specific embodiment, the LCC-CLCC DC system structure is as follows: Figure 2 As shown. The sending-end LCC uses constant DC current control to ensure stable power output from the sending-end system; the receiving-end CLCC uses constant DC voltage control to ensure stable DC voltage in the LCC-CLCC DC system.

[0032] Each phase commutator arm of a CLCC consists of a main branch and an auxiliary branch connected in parallel. The main branch includes a high-voltage thyristor valve and an IGBT-based fully controlled valve. The auxiliary branch consists of a low-current thyristor valve, an IGBT fully controlled valve, a damping circuit connected in parallel with each sub-valve, and a surge arrester. CLCC has two commutation modes: natural commutation and forced commutation. In natural commutation mode, the startup logic of CLCC is essentially the same as that of LCC. Forced commutation mode is activated when the arm voltage cannot meet the natural commutation conditions at the commutation moment. It transfers current to the auxiliary branch by turning off the main branch devices. Once the main branch thyristors regain their blocking capability, the auxiliary branch is turned off to create a reverse voltage, forcing the current to transfer to the branch of the target conducting arm, thus achieving inter-arm commutation and avoiding commutation failure from a topological perspective.

[0033] For a 6-pulse converter, the phase angle difference between single-phase AC voltage and AC current is... for: (1) in, μ This is the commutation overlap angle. λ The extinguishing arc angle, α The trigger angle.

[0034] Let the DC power be P Then the reactive power consumed by the inverter is: (2) From equation (2), it can be seen that when the DC power is constant, the reactive power consumed by the inverter is... Q Depending on the trigger angle α and commutation overlap angle μ The LCC inverter is constrained by the thyristor turn-off characteristics. An increase in the firing angle will reduce the turn-off angle and easily lead to commutation failure, resulting in limited firing angle adjustment and large reactive power consumption. Since the CLCC commutation voltage is formed by the superposition of AC line voltage and auxiliary branch surge arrester residual voltage, even if the AC voltage is negative, the surge arrester residual voltage can make up the voltage gap to maintain the positive effective commutation voltage. It can operate at zero turn-off angle or even negative turn-off angle for a short time. By flexibly adjusting the firing angle, reactive power absorption and generation can be achieved.

[0035] The mathematical model of CLCC under quasi-steady state (i.e., the quasi-steady-state mathematical model of the CLCC converter) is as follows: (3) (4) in, U aci This is the effective value of the AC line voltage. I dc It is direct current; U diThis is the DC voltage of the inverter station; N It is the extreme logarithm; k This refers to the converter transformer voltage ratio; X r This refers to the equivalent commutation reactance on the secondary side of the converter transformer. P d This is DC power.

[0036] From the above formula, it can be seen that when the CLCC firing angle α As the voltage increases, the DC voltage rises, thus increasing the active power delivered to the AC system. The firing angle increases to the point where the phase difference between the voltage and current increases. When the angle is greater than 180°, the CLCC generates inductive reactive power.

[0037] When the DC current increases, the reactive power consumed by the CLCC will increase accordingly. However, since the sensitivity of the firing angle to reactive power regulation is much higher than that of the DC current, the overall reactive power consumption of the CLCC still shows a downward trend when the firing angle and DC current are increased simultaneously. At the same time, increasing the DC current during AC faults will increase the commutation overlap angle, which will help to further reduce the reactive power consumption of the CLCC.

[0038] Based on the above analysis, it can be seen that increasing the DC current and the inverter-side firing angle can significantly improve the active power amplitude and reactive power support capability of the LCC-CLCC system. By designing a reasonable control strategy, its support capability can be fully utilized, thereby improving the stability of the LCC-CLCC system.

[0039] S20: Obtain the DC voltage detection value on the rectifier side and compare it with the preset voltage threshold. When the DC voltage detection value on the rectifier side is greater than or equal to the voltage threshold, execute the small disturbance control mode. When the DC voltage detection value on the rectifier side is less than the voltage threshold, execute the large disturbance control mode.

[0040] Step S20 aims to achieve rapid identification and automatic switching between the two control modes, enabling the system to adaptively invoke the corresponding support strategy based on the severity of the disturbance. This step uses the degree of DC voltage drop on the rectifier side as the criterion for determining the disturbance condition. When the voltage drop is not severe, a small disturbance control mode is triggered to balance frequency support and voltage stability. When the voltage drop is severe, a large disturbance control mode is triggered to fully utilize the short-term power support capability of the CLCC. This hierarchical switching mechanism ensures that the active and reactive power regulation capabilities of the CLCC can be rationally utilized under different disturbance conditions, effectively avoiding the problem of insufficient applicability of a single control strategy in a wide range of disturbance scenarios.

[0041] S30: In small disturbance control mode, based on the frequency deviation Δ measured on the inverter side f and frequency-voltage droop factor K1. Calculate the DC voltage deviation Δ on the inverter side. U di According to the DC voltage deviation Δ on the inverter side U di Adjust the reference value of the DC voltage on the inverter side; aiming to maintain constant reactive power consumption of the CLCC converter during frequency regulation, based on the measured frequency deviation Δ on the inverter side. f Determine the voltage-current droop coefficient under small disturbance conditions K The current value of 2; based on the determined K 2 and the DC voltage deviation Δ on the inverter side U di The DC current deviation Δ is calculated. I dc According to the DC current deviation Δ I dc Adjust the reference value for the DC current.

[0042] In LCC-CLCC DC transmission systems, CLCC converters exhibit significant active-reactive power coupling characteristics during steady-state operation. When the system participates in grid frequency regulation, the adjustment of DC active power triggers synchronous changes in reactive power, leading to fluctuations in AC voltage at the converter station side. This is detrimental to the stable operation of both the converter station and the receiving-end grid. To address this issue, this embodiment designs a frequency support coordinated control strategy for LCC-CLCC systems under small disturbance conditions (i.e., the small disturbance control mode executed in step S30). Its overall control architecture is as follows: Figure 2 As shown, the frequency support coordinated control strategy of this LCC-CLCC system first adjusts the reference values ​​of DC voltage and DC current in coordination based on the system frequency deviation signal and the droop control principle. At the same time, it adaptively adjusts the voltage-current droop coefficient according to the frequency deviation. While realizing rapid adjustment of active power, it effectively smooths the dynamic fluctuations of reactive power consumption, thereby suppressing AC voltage fluctuations.

[0043] In one specific implementation, the inverter-side DC voltage U di Adjusted to its reference value by the inverter U diref Therefore, during small perturbations U di It can be represented as: (5) in, U diref This is the reference value for the DC voltage on the inverter side of the CLCC; Δ f = f - f N To measure the frequency deviation on the inverter side; fThe measurement frequency is on the inverter side; f N The rated frequency; K 1 is f - U Sag coefficient.

[0044] The deviation Δ of the DC voltage on the inverter side U di It can be represented as: (6) As shown in equation (6), the DC voltage deviation on the inverter side can directly reflect the frequency deviation.

[0045] Theoretically, adjusting only the DC voltage can achieve the active power frequency regulation target within a certain range. However, adjusting the DC voltage alone will cause a significant change in the converter firing angle, which in turn will cause fluctuations in reactive power consumption and result in voltage deviation on the AC side. Therefore, this embodiment adopts a coordinated adjustment strategy of DC voltage and DC current. While enhancing the frequency regulation capability, it utilizes the opposite characteristics of DC current and firing angle in their effects on reactive power consumption, and makes the reactive power effects of the two cancel each other out by reasonably matching the adjustment ratio.

[0046] rectifier side DC voltage U dr and U di The relationship can be represented as: (7) in, I dc The steady-state DC current is expressed as: (8) in, I dcref This is the reference value for DC current. K 2 is U / I Sag coefficient, U dr The DC voltage measured at the rectifier. R dc This is the resistance of the DC transmission line.

[0047] Analogous to the analysis logic of the inverter side, substituting equation (7) into equation (8), we obtain the DC current deviation Δ I dc The expression is as follows: (9) in, I dc0 This is the reference value for DC current.

[0048] In this embodiment,R dc It is considered relatively small. Therefore, in equation (9) K 2 R dc The term can be ignored; DC current deviation Δ I dc It can be represented as: (10) In summary, as shown in equations (5) and (10), the DC voltage is based on the grid frequency and the droop coefficient. K 1. Control is performed, while the DC current is determined based on the change in DC voltage and the droop coefficient. K 2. To achieve adjustment.

[0049] The droop characteristics of inverters and rectifiers are as follows Figure 3 As shown, when the system frequency increases, it can be seen from equation (5) that the DC voltage reference value decreases, causing the firing angle to decrease; combined with equation (10), the DC current also decreases accordingly, realizing active power adjustment. At the same time, the decrease in firing angle will increase the absorption of reactive power by the CLCC, while the decrease in active power transmission amplitude caused by the decrease in DC current will suppress the increase in reactive power demand to a certain extent. Therefore, when the grid frequency increases, by simultaneously reducing the DC voltage and DC current, the system can effectively suppress the fluctuation of reactive power of the converter while realizing the reduction of active power to support the frequency.

[0050] In summary, K 1 and K The value of 2 is crucial for achieving frequency support and suppressing AC voltage fluctuations. In order to ensure equipment safety while achieving the desired strategy effect, the control parameters need to be designed reasonably.

[0051] against K 1. To ensure that the DC voltage does not exceed the system's allowable safe operating range, the maximum allowable inverter-side DC voltage deviation Δ is set as the control boundary. U dimax The droop coefficient is designed to represent the maximum voltage regulation allowed by the control. K The maximum value of 1 is used to ensure that the frequency deviation reaches Δ. f max The DC voltage before the circuit does not exceed its allowable upper and lower limits. The control parameters are designed as shown in equation (10).

[0052] (11) Voltage-current droop coefficient K 2 is a key factor in achieving AC voltage fluctuation suppression. If a constant is used throughout the frequency modulation process... K2. The coupling effect of active power and reactive power often causes the reactive power consumption of CLCC to deviate from the original steady-state value, thereby causing additional fluctuations in AC bus voltage.

[0053] To suppress reactive power fluctuations in the CLCC during frequency modulation, this embodiment maintains the frequency-voltage droop coefficient. K Under the premise that 1 remains unchanged, the voltage-current droop coefficient is... K 2. Perform adaptive correction. The basic idea is: utilize... K 2. Adjust the response amplitude of DC current to DC voltage deviation to change the amount of active power transmission change, so as to compensate for the reactive power fluctuation caused by the firing angle change, and keep the reactive power consumption of CLCC as close as possible to the original steady state value when participating in frequency regulation.

[0054] To establish K The adaptive adjustment law of 2 is first analyzed by examining the functional relationship between the firing angle and the frequency deviation. Combining equations (3) and (5), the firing angle of the CLCC inverter side can be obtained. α The expression is (12) Equation (12) shows that under small disturbance conditions, the system frequency change can be directly transmitted to the firing angle of the CLCC inverter side through the control link, making it an important intermediate variable affecting the change of reactive power.

[0055] After further considering the disturbance, based on the steady-state operation mechanism of the CLCC converter, its reactive power... Q It can be expressed as a function of variables such as DC voltage, DC current, firing angle, and commutation overlap angle, i.e. (13) Considering that the research object in this embodiment is a small disturbance condition, and the system operating point offset is finite, it can be approximately assumed that the change in the commutation overlap angle before and after the disturbance is small. In the steady-state analysis, it is regarded as constant, and tan( α + μ / 2) is defined as λ (Δ f At this point, if we consider the steady-state reactive power consumption before the disturbance... Q Using 0 as the control target, and substituting the known operating quantities such as voltage and current under steady-state conditions into equation (14), we can inversely obtain the result that satisfies the reactive power requirement. Q Q 0 o'clock K 2. Expression. Therefore, we can conclude that... K 2 can essentially be written as a function of frequency deviation: (14) From equation (14), we can see that, KEssentially, this can be written as a function of frequency deviation. That is, under different frequency deviations, different voltage-current droop coefficients should be used to coordinate the relationship between DC current changes and firing angle changes, thereby suppressing reactive power offset.

[0056] In summary, to address the coordinated control requirements of frequency support and AC voltage stability in the receiving-end system under small disturbances, this embodiment addresses this by adjusting the droop coefficient. K 1、 K The coordinated optimization design of 2, while achieving rapid frequency support, also optimizes the sag coefficient. K 2. Adaptive adjustment is performed to keep the reactive power consumption of the converter as constant as possible during frequency regulation, thereby improving the frequency dynamic performance and effectively suppressing the fluctuation of AC side voltage.

[0057] S40: In large disturbance control mode, based on AC voltage deviation Δ U ac With AC voltage dead zone value U acH Based on the comparison results, the additional adjustment amount Δ of the firing angle is calculated. α i Additional adjustment Δ based on the firing angle α i The firing angle reference value is corrected to achieve additional constant AC voltage control; simultaneously, the voltage-current droop coefficient is adjusted from... K 2. Voltage-current droop coefficient when switching to fault mode K 3. Based on the switched K 3 and the DC voltage deviation Δ on the inverter side U di The improved DC current command value is calculated.

[0058] After an AC fault occurs, the AC bus voltage drops significantly, reducing the active power transmission capacity on the receiving end AC side, leading to an active power deficit and frequency sag in the system. To provide effective voltage support to the AC system during faults of varying severity and to suppress transient frequency sags in the receiving end grid, this embodiment proposes an LCC-CLCC transient frequency and voltage coordinated support strategy that is oriented towards large disturbance conditions and takes into account the voltage regulation dead zone. This strategy is implemented through step S40, i.e., the large disturbance control mode.

[0059] In one specific implementation, a voltage adaptive control strategy is proposed to meet the AC voltage support requirements during a fault. The principle expression of the control strategy is shown in Equation (15).

[0060] (15) in, U acH This refers to the dead zone value of the AC voltage. x i , x i,max These are the state variables and their limiting values ​​for the LCC-CLCC DC system, respectively. K p and K i These are the coefficients for the proportional and integral components, respectively; Δ α i Δ α max These are the additional adjustment amount and upper limit value of the firing angle after the AC voltage at the receiving end drops beyond the dead zone; α ref This is a reference value for the trigger angle; α ref0 This is the initial reference value for the trigger angle.

[0061] When Δ U ac > U acH At that time, the controller corrects the firing angle within the integral and firing angle limit range to stabilize the voltage deviation within the dead zone value; when Δ U ac < U acH At that time, the compensation strategy will automatically exit.

[0062] Regarding AC voltage support, U acH This directly determines the timing of the control strategy's implementation and is crucial for achieving adaptive voltage regulation. Therefore, based on the output characteristics of constant DC voltage control, a method is proposed... U acH The design method is to meet the adaptive voltage active support requirements mentioned above.

[0063] During a DC voltage control failure, the DC voltage is increased by increasing the firing angle. However, as the failure worsens, the DC voltage drops significantly. The DC voltage control continuously increases the firing angle to compensate for the voltage drop. When the angle reaches 180°, further increases actually decrease the DC voltage, rendering the basic DC voltage control compensation ineffective and unable to continue increasing the firing angle to meet the AC voltage recovery requirements. Therefore, this embodiment uses... α Using 180° as the boundary condition, the minimum AC side voltage required to restore the DC voltage to the steady-state reference value when the firing angle of the constant DC voltage control reaches its limit is derived, and this voltage is defined as the dead zone threshold. U acH .

[0064] AC voltageU aci It can be represented as: (16) Substituting the firing angle of 180° into equation (16) yields... U acH The expression is: (17) It can be seen that, through the analysis of U acH The well-designed system enables adaptive switching of adjustment functions under different disturbance conditions. When a minor fault occurs in the system, adjustment can be completed solely by the basic control strategy without the need for additional compensation. When the fault worsens or the constant DC voltage control fails, the control strategy will further increase the firing angle, enhance the reactive power output capability of the CLCC converter, and effectively strengthen the system's transient voltage support effect.

[0065] AC faults can cause a significant drop in DC voltage. To maintain power delivery and improve reactive power support, it is necessary to increase the DC current under fault conditions. However, the existing voltage-current droop control will reduce the DC current in sync with the DC voltage drop, resulting in a further reduction in DC transmission power, exacerbating the active power deficit at the receiving end, and hindering fault clearance and system recovery.

[0066] Therefore, this embodiment switches the voltage-current droop coefficient under fault conditions to increase the DC current command value when the DC voltage drops, thereby increasing the DC current while avoiding overcurrent, in order to maintain DC power transmission and maximize reactive power support. (Regarding the droop coefficient under fault conditions...) K 3. Taking into account the current carrying capacity limit of the IGBT devices in the CLCC converter and the system voltage constraint, and using the safe operation of the equipment as the boundary condition, a current twice the rated DC current is selected. I dc,max As the upper limit of current, select 1.05 times the rated voltage. U dc,max This is the upper limit of the voltage; adjust accordingly. K The maximum value of 3, the parameter design expression is shown in equation (18): (18) The coefficient switching threshold is set with reference to the allowable deviation of DC voltage steady state, and 0.95 pu is taken as the switching criterion. During steady-state operation of the system, the DC voltage on the rectifier side is monitored in real time. When the voltage drops below the threshold, a system fault is determined. If the fault criterion is not triggered, the system maintains the original small disturbance frequency support mode. Based on the voltage detection results, the voltage-current droop coefficient is switched according to the following logic: (19) in, K2 represents the voltage-current droop coefficient under small disturbance conditions. K 3 represents the voltage-current droop coefficient during a fault. K i This represents the equivalent droop coefficient actually applied under fault conditions. U dr0 This is the rated DC voltage on the rectifier side.

[0067] After a receiving-end fault, the DC current command value is actively increased in transient state through fault detection and voltage-current droop coefficient switching to improve active power output to support system frequency and improve transient response. At the same time, the increase in commutation overlap angle caused by the rise in DC current can also reduce the reactive power consumption of the converter and further enhance the AC side voltage support effect.

[0068] Finally, the control logic designed in this embodiment is as follows: Figure 4 As shown, it can be summarized as follows: Real-time monitoring of the receiving end frequency and voltage; under normal operation and small disturbance conditions, the system uses the small disturbance frequency support control to suppress AC voltage fluctuations while adjusting the active power amplitude; when the voltage detection module determines that the system has entered a fault state, it decides whether to activate the additional constant AC voltage control based on whether the AC voltage is lower than the dead zone threshold, and increases the DC current command value through coefficient switching during the fault period, thereby enhancing the frequency support capability and reducing the reactive power consumption of the converter, providing additional voltage support for the AC system.

[0069] To verify the rationality and effectiveness of the coordinated control method for LCC-CLCC DC systems that takes into account both frequency and voltage active support, a simulation model of a 12-pulse 800 kV / 5kA LCC-CLCC DC transmission system was built in PSCAD / EMTDC. The specific simulation parameters are shown in Table 1 below.

[0070] Table 1 Basic Parameters of LCC-CLCC High Voltage DC System

[0071] To verify the effectiveness and superiority of the proposed method, this embodiment simulates the following control methods and compares and analyzes the response effects of different control methods. The specific control methods are set as follows: Method 1 CLCC does not use any additional control; Method 2 CLCC adopts the bilateral frequency support strategy proposed in the literature "Li Z, Xue Y, Chen Y, et al. Bilateral primaryfrequency support and AC voltage regulation of LCC-HVDC systems for asynchronously interconnected systems[J]. IEEE Transactions on Power Systems,2024, 40(3): 2600-2612"; Method 3 adopts the control strategy proposed in this embodiment; the voltage regulation dead zone threshold is calculated to be 0.2 pu; Method 4 adopts the control strategy proposed in this embodiment, and no dead zone is set.

[0072] To verify the effectiveness of the proposed cooperative control strategy under scenario one after power disturbance, t Simulation verification was performed by disconnecting 200MW of load from the receiving-end power grid at time =3s. The simulation results are as follows: Figure 5 As shown.

[0073] Simulation results show that the frequency responses of the two methods are almost identical. However, if... Figure 5 As shown in (b) and (c), Method 3 transmits frequency information by reducing the DC voltage, which contrasts with the strategy used in Method 2. Therefore, Method 3 achieves a similar frequency regulation effect while requiring a smaller DC current adjustment range, such as... Figure 5 As shown in (d). The AC voltage, CLCC reactive power, and firing angle are as follows: Figure 5 As shown in (e)-(f), the reactive power adjustment in Method 1 reaches 220 Mvar, and the reactive power change in Method 2 is 150 Mvar, both showing varying degrees of reactive power increase; while Method 3, through adjustment K 2. It achieves near-constant reactive power, and effectively suppresses AC voltage fluctuations while improving frequency stability under small disturbance conditions.

[0074] To verify the effectiveness of the proposed cooperative control strategy under scenario two after a large disturbance, a simulation analysis was first performed on a three-phase ground fault occurring on the inverter side of the LCC-CLCC DC transmission system. The fault occurred at 3.0 s, and the ground fault inductance was set to 0.4H (minor fault) and 0.1H (serious fault), respectively. The fault duration was 0.5 s. The simulation results are as follows: Figure 6 and Figure 7As shown.

[0075] The simulation results show that, Figure 6 As shown in (a)-(b), after the fault occurs, the system transmission power drops, the receiving end frequency decreases significantly, and the recovery process is accompanied by obvious oscillations. This indicates that relying solely on the original regulation resources is insufficient to promptly mitigate the impact of disturbances, and the system's dynamic stability is weak. Figure 6 As shown in (a)-(d), in Mode 3, the DC voltage and DC current are rapidly adjusted immediately upon the occurrence of the disturbance, effectively mitigating the instantaneous active power imbalance in the receiving-end system. Consequently, the power drop is significantly suppressed, and the system can recover to a stable operating state in a shorter time. The minimum receiving-end frequency increases from 49.9Hz to approximately 49.98Hz, effectively suppressing the frequency drop. This fully demonstrates that the proposed control strategy can provide effective frequency support for the system during disturbances. Simultaneously, the increase in DC current also correspondingly enhances the reactive power output of the CLCC converter, from... Figure 6 As shown in (e)-(g), the AC voltage remains above the voltage regulation dead zone threshold, therefore the AC voltage compensation circuit is not triggered. Compared to method 1, method 3 has a larger trigger angle during faults, increases the maximum reactive power compensation capacity by approximately 50 Mvar, and raises the minimum point of the receiving-end AC voltage by approximately 10 kV. These results verify that the proposed control strategy possesses excellent transient frequency and voltage coordination capabilities.

[0076] The simulation results show that, Figure 7 As shown in (a): When a serious fault occurs, the constant DC voltage control quickly reaches its upper limit, and the constant AC voltage control then starts, providing additional firing angle compensation. Figure 7 As shown in (b) to (c), the peak reactive power output of Method 3 reaches 3800 Mvar, an increase of 700 Mvar compared to Method 4. This effectively suppresses the drop in AC voltage at the receiving end, and the lowest point of the AC voltage at the receiving end is 30 kV higher than that of Method 4, and is stably maintained at the voltage regulation dead zone threshold. Figure 7 As can be seen from (f) to (g), when the firing angle is greater than 180°, the DC voltage of method 3 decreases compared to method 4, but the proposed collaborative control strategy correspondingly increases the DC current command value. For example... Figure 7 As shown in (d) to (e), improving the active power transmission capacity effectively alleviates the system power imbalance, thereby significantly reducing the drop in the receiving-end frequency. These results fully validate the transient frequency and voltage co-support performance of the proposed control strategy.

[0077] This embodiment addresses the frequency and voltage support issues of LCC-CLCC DC systems under receiving-end grid disturbances. It proposes a collaborative control strategy and parameter design method that balances active frequency and voltage support. This approach can fully leverage the CLCC's control capabilities while improving the receiving-end grid's frequency stability under small disturbances and its transient voltage and frequency support capabilities under large disturbances. Simulation analysis confirms the following beneficial effects of this method: 1) For scenarios with small perturbations, the droop coefficient is optimized through coordination. K 1. K 2. Furthermore, the reactive power response of the CLCC converter is adaptively adjusted based on the difference between the firing angle and the DC current. K 2. While achieving rapid frequency support at the receiving end, the converter maintains a basically constant reactive power consumption during frequency regulation, effectively suppressing AC voltage fluctuations and improving the coordinated stability of system frequency and voltage.

[0078] 2) For scenarios with large disturbances, the voltage regulation dead zone and control parameters are designed based on the constant DC voltage characteristics under the condition of 180° firing angle. In case of a fault, the DC current is increased by switching the droop coefficient, realizing the adaptive switching of the regulation function under different disturbance levels. By simultaneously adjusting the DC current and firing angle, the reactive power support capability of CLCC is further increased, and the active power support level of LCC-CLCC system is improved. This significantly improves the voltage recovery and frequency support effect of the receiving end grid and enhances the transient stability level of the receiving end grid.

[0079] This invention also provides a coordinated control device for an LCC-CLCC DC system that balances frequency and voltage active support, used to implement the methods described in any of the above embodiments, such as... Figure 8 As shown, the device includes: The characteristic analysis module 801 is configured to establish a quasi-steady-state mathematical model of the CLCC converter based on the topology and quasi-steady-state operation mechanism of the CLCC converter, and determine the quantitative relationship between the reactive power and the firing angle of the CLCC converter based on the quasi-steady-state mathematical model. The mode selection module 802 is configured to acquire the rectifier-side DC voltage detection value, compare the rectifier-side DC voltage detection value with a preset voltage threshold; when the rectifier-side DC voltage detection value is greater than or equal to the voltage threshold, execute the small disturbance control mode; when the rectifier-side DC voltage detection value is less than the voltage threshold, execute the large disturbance control mode. The first control module 803 is configured to, under the small disturbance control mode, measure the frequency deviation Δ on the inverter side. f and frequency-voltage droop factor K 1. Calculate the DC voltage deviation Δ on the inverter side. U diAccording to the DC voltage deviation Δ on the inverter side U di Adjust the reference value of the DC voltage on the inverter side; with the goal of maintaining constant reactive power consumption of the CLCC converter during frequency regulation, based on the measured frequency deviation Δ on the inverter side. f Determine the voltage-current droop coefficient under small disturbance conditions K The current value of 2; based on the determined K 2 and the DC voltage deviation Δ on the inverter side U di The DC current deviation Δ is calculated. I dc According to the DC current deviation Δ I dc Adjust the reference value of the DC current; The second control module 804 is configured to, under the large disturbance control mode, determine the AC voltage deviation Δ U ac With AC voltage dead zone value U acH Based on the comparison results, the additional adjustment amount Δ of the firing angle is calculated. α i Additional adjustment Δ based on the firing angle α i The firing angle reference value is corrected to achieve additional constant AC voltage control; simultaneously, the voltage-current droop coefficient is adjusted from... K 2. Voltage-current droop coefficient when switching to fault mode K 3. Based on the switched K 3 and the DC voltage deviation Δ on the inverter side U di The improved DC current command value is calculated.

[0080] This invention provides an electronic device. The electronic device may include a processor and a memory, wherein the processor and the memory can communicate; exemplarily, the processor and the memory communicate via a communication bus.

[0081] The processor executes computer execution instructions stored in memory, causing the processor to perform the scheme in the above embodiments. The processor can be a general-purpose processor, including a central processing unit (CPU), a network processor (NP), etc.; it can also be a digital signal processor (DSP), an application-specific integrated circuit (ASIC), a field-programmable gate array (FPGA), or other programmable logic devices, discrete gate or transistor logic devices, or discrete hardware components.

[0082] The communication bus can be a Peripheral Component Interconnect (PCI) bus or an Extended Industry Standard Architecture (EISA) bus, etc. The system bus can be divided into address bus, data bus, control bus, etc. Transceivers are used to enable communication between database access devices and other computers (e.g., clients, read-write libraries, and read-only libraries). Memory may include random access memory (RAM) and may also include non-volatile memory.

[0083] The electronic device provided in this embodiment of the invention can be the terminal device described in the above embodiments.

[0084] This invention also provides a computer-readable storage medium storing computer instructions. When these computer instructions are executed on a computer, the computer performs the technical solution of the LCC-CLCC DC system coordinated control method that takes into account both frequency and voltage active support as described in the above embodiments.

[0085] This invention also provides a computer program product, which includes a computer program stored in a computer-readable storage medium. At least one processor can read the computer program from the computer-readable storage medium. When the at least one processor executes the computer program, it can implement the technical solution of the LCC-CLCC DC system coordinated control method that takes into account both frequency and voltage active support in the above embodiments.

[0086] In the several embodiments provided by this invention, it should be understood that the disclosed devices and methods can be implemented in other ways. For example, the device embodiments described above are merely illustrative; for instance, the division of modules is only a logical functional division, and in actual implementation, there may be other division methods. For example, multiple modules may be combined or integrated into another system, or some features may be ignored or not executed. Furthermore, the coupling or direct coupling or communication connection shown or discussed may be indirect coupling or communication connection through some interfaces, devices, or modules, and may be electrical, mechanical, or other forms.

[0087] The modules described as separate components may or may not be physically separate. The components shown as modules may or may not be physical units; that is, they may be located in one place or distributed across multiple network units. Some or all of the modules can be selected to implement the solution of this embodiment according to actual needs.

[0088] Furthermore, the functional modules in the various embodiments of the present invention can be integrated into one processing unit, or each module can exist physically separately, or two or more modules can be integrated into one unit. The unit composed of the above modules can be implemented in hardware or in the form of hardware plus software functional units.

[0089] The integrated modules described above, implemented as software functional modules, can be stored in a computer-readable storage medium. These software functional modules, stored in a storage medium, include several instructions to cause a computer device (which may be a personal computer, server, or network device, etc.) or processor to execute certain steps of the methods of the various embodiments of the present invention.

[0090] It should be understood that the aforementioned processor can be a Central Processing Unit (CPU), or other general-purpose processors, digital signal processors (DSPs), application-specific integrated circuits (ASICs), etc. A general-purpose processor can be a microprocessor or any conventional processor. The steps of the method disclosed in this invention can be directly manifested as execution by a hardware processor, or execution by a combination of hardware and software modules within the processor.

[0091] The memory may include high-speed RAM, and may also include non-volatile storage (NVM), such as at least one disk storage device, and may also be a USB flash drive, external hard drive, read-only memory, disk or optical disc, etc.

[0092] Buses can be Industry Standard Architecture (ISA) buses, Peripheral Component Interconnect (PCI) buses, or Extended Industry Standard Architecture (EISA) buses, etc. Buses can be categorized into address buses, data buses, control buses, etc.

[0093] The aforementioned storage medium can be implemented by any type of volatile or non-volatile storage device or a combination thereof, such as static random access memory (SRAM), electrically erasable programmable read-only memory (EEPROM), erasable programmable read-only memory (EPROM), programmable read-only memory (PROM), read-only memory (ROM), magnetic storage, flash memory, magnetic disk, or optical disk. The storage medium can be any available medium that can be accessed by a general-purpose or special-purpose computer.

[0094] An exemplary storage medium is coupled to a processor, enabling the processor to read information from and write information to the storage medium. Alternatively, the storage medium can be an integral part of the processor. The processor and storage medium can reside in an Application Specific Integrated Circuit (ASIC). Alternatively, the processor and storage medium can exist as discrete components in an electronic control unit or main control device.

[0095] Those skilled in the art will understand that all or part of the steps of the above-described method embodiments can be implemented by hardware related to program instructions. The aforementioned program can be stored in a computer-readable storage medium. When executed, the program performs the steps of the above-described method embodiments; and the aforementioned storage medium includes various media capable of storing program code, such as ROM, RAM, magnetic disks, or optical disks.

[0096] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; 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 or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A collaborative control method for an LCC-CLCC DC system that balances frequency and voltage active support, characterized in that, The method includes: Based on the topology and quasi-steady-state operation mechanism of the CLCC converter, a quasi-steady-state mathematical model of the CLCC converter is established, and the quantitative relationship between the reactive power and firing angle of the CLCC converter is determined based on the quasi-steady-state mathematical model. The rectifier-side DC voltage detection value is obtained, and the rectifier-side DC voltage detection value is compared with a preset voltage threshold. When the rectifier-side DC voltage detection value is greater than or equal to the voltage threshold, a small disturbance control mode is executed; when the rectifier-side DC voltage detection value is less than the voltage threshold, a large disturbance control mode is executed. In the small disturbance control mode, based on the frequency deviation Δ measured on the inverter side f and frequency-voltage droop factor K 1. Calculate the DC voltage deviation Δ on the inverter side. U di According to the DC voltage deviation Δ on the inverter side U di Adjust the reference value of the DC voltage on the inverter side; with the goal of maintaining constant reactive power consumption of the CLCC converter during frequency regulation, based on the measured frequency deviation Δ on the inverter side. f Determine the voltage-current droop coefficient under small disturbance conditions K The current value of 2; based on the determined K 2 and the DC voltage deviation Δ on the inverter side U di The DC current deviation Δ is calculated. I dc According to the DC current deviation Δ I dc Adjust the reference value of the DC current; Under the large disturbance control mode, based on the AC voltage deviation Δ U ac With AC voltage dead zone value U acH Based on the comparison results, the additional adjustment amount Δ of the firing angle is calculated. α i Additional adjustment Δ based on the firing angle α i The firing angle reference value is corrected to achieve additional constant AC voltage control; simultaneously, the voltage-current droop coefficient is adjusted from... K 2. Voltage-current droop coefficient when switching to fault mode K 3. Based on the switched K 3 and the DC voltage deviation Δ on the inverter side U di The improved DC current command value is calculated.

2. The method according to claim 1, characterized in that, The quasi-steady-state mathematical model of the CLCC converter was determined as follows: For a 6-pulse converter, based on the commutation process of the converter, the phase angle difference between the single-phase AC voltage and AC current... Represented as: (1) in, μ This is the commutation overlap angle. λ The extinguishing arc angle, α For trigger angle; According to the phase angle difference With DC power P To obtain the reactive power consumed by the inverter Q Represented as: (2) Based on the quasi-steady-state operation mechanism of the CLCC converter, the quasi-steady-state mathematical model of the CLCC converter is determined as follows: (3) (4) in, U aci This is the effective value of the AC line voltage. I dc It is direct current; U di This is the DC voltage of the inverter station; N It is the extreme logarithm; k This refers to the converter transformer voltage ratio; X r This refers to the equivalent commutation reactance on the secondary side of the converter transformer. P d This is DC power.

3. The method according to claim 2, characterized in that, In the small disturbance control mode, based on the frequency deviation Δ measured on the inverter side f and frequency-voltage droop factor K 1. Calculate the DC voltage deviation Δ on the inverter side. U di ,include: Based on the frequency deviation Δ measured on the inverter side f and frequency-voltage droop factor K 1. Determine the DC voltage on the inverter side using the following formula. U di : (5) in, U diref This is the reference value for the DC voltage on the inverter side of the CLCC; Δ f = f - f N To measure the frequency deviation on the inverter side; f The measurement frequency is on the inverter side; f N Rated frequency; Based on the DC voltage on the inverter side U di The deviation Δ of the DC voltage on the inverter side is determined by the following formula. U di : (6) According to the determined K 2 and the DC voltage deviation Δ on the inverter side U di The DC current deviation Δ is calculated. I dc ,include: Determine the DC voltage on the rectifier side U dr DC voltage of inverter station U di The relationship is represented as: (7) in, I dc The steady-state DC current is expressed as: (8) in, I dcref This is the reference value for DC current. U dr The DC voltage measured at the rectifier. R dc The resistance of the DC transmission line; Substituting equation (7) into equation (8), we obtain the DC current deviation Δ. I dc The expression is as follows: (9) in, I dc0 This is a reference value for DC current. exist K 2 R dc Neglecting the issue, the DC current deviation Δ I dc The expression is as follows: (10) 4. The method according to claim 3, characterized in that, The frequency-voltage droop coefficient K 1. Determined by the following formula: (11) Where, Δ U di,max Δ is the maximum allowable DC voltage deviation on the inverter side of the system. f max This represents the maximum frequency deviation.

5. The method according to claim 3, characterized in that, With the goal of maintaining constant reactive power consumption of the CLCC converter during frequency regulation, the frequency deviation Δ measured on the inverter side is used as the basis for this calculation. f Determine the voltage-current droop coefficient under small disturbance conditions K The current values ​​of 2 include: Based on the quasi-steady-state mathematical model of the CLCC converter and the inverter-side DC voltage U di The calculation formula is used to obtain the CLCC inverter side firing angle. α Frequency deviation Δ from inverter side measurement f Functional relationship between them: (12) The reactive power of the CLCC converter Q Expressed as a function of DC voltage, DC current, firing angle, and commutation overlap angle: (13) tan( α + μ / 2) is defined as λ (Δ f ), to reduce the steady-state reactive power consumption before the disturbance. Q Using 0 as the control target, the reactive power can be obtained by inversely solving equation (13). Q Q 0 o'clock K The voltage-current droop coefficient under small disturbance conditions is expressed by the following formula. K Determining the current value of 2: (14) in, U d0 This is the rated DC voltage on the rectifier side.

6. The method according to claim 1, characterized in that, Under the large disturbance control mode, based on the AC voltage deviation Δ U ac With AC voltage dead zone value U acH Based on the comparison results, the additional adjustment amount Δ of the firing angle is calculated. α i The process is represented as: (15) in, U acH This refers to the dead zone value of the AC voltage. x i , x i,max These are the state variables and their limiting values ​​for the LCC-CLCC DC system, respectively. K p and K i These are the coefficients for the proportional and integral components, respectively; Δ α i Δ α max These are the additional adjustment amount and upper limit value of the firing angle after the AC voltage at the receiving end drops beyond the dead zone; α ref This is a reference value for the trigger angle; α ref0 This is the initial reference value for the trigger angle.

7. The method according to claim 6, characterized in that, The AC voltage dead zone value U acH The design method and the design method of voltage-current droop coefficient K3 during faults are as follows: AC voltage U aci Represented as: (16) Substituting the firing angle of 180° into equation (16) yields... U acH The expression is: (17) Voltage-current droop coefficient during faults K 3. Select a rated DC current that is a multiple of the set value. I dc,max To determine the upper limit of the current, select a rated voltage that is a multiple of the set value. U dc,max The voltage upper limit is set according to the aforementioned current upper limit and voltage upper limit. K The maximum value of 3: (18) Based on the DC voltage detection results on the rectifier side, the voltage-current droop coefficient is switched according to the following logic: (19) in, U dr The DC voltage measured at the rectifier. K i This represents the equivalent droop coefficient actually applied under fault conditions. U dr0 This is the rated DC voltage on the rectifier side.

8. A coordinated control device for an LCC-CLCC DC system that balances frequency and voltage active support, characterized in that, The device includes: The characteristic analysis module is configured to establish a quasi-steady-state mathematical model of the CLCC converter based on its topology and quasi-steady-state operating mechanism, and to determine the quantitative relationship between the reactive power and firing angle of the CLCC converter based on the quasi-steady-state mathematical model. The mode selection module is configured to acquire the rectifier-side DC voltage detection value, compare the rectifier-side DC voltage detection value with a preset voltage threshold; when the rectifier-side DC voltage detection value is greater than or equal to the voltage threshold, execute the small disturbance control mode; when the rectifier-side DC voltage detection value is less than the voltage threshold, execute the large disturbance control mode. The first control module is configured to, under the small disturbance control mode, determine the frequency deviation Δ measured on the inverter side. f and frequency-voltage droop factor K 1. Calculate the DC voltage deviation Δ on the inverter side. U di According to the DC voltage deviation Δ on the inverter side U di Adjust the reference value of the DC voltage on the inverter side; with the goal of maintaining constant reactive power consumption of the CLCC converter during frequency regulation, based on the measured frequency deviation Δ on the inverter side. f Determine the voltage-current droop coefficient under small disturbance conditions K The current value of 2; based on the determined K 2 and the DC voltage deviation Δ on the inverter side U di The DC current deviation Δ is calculated. I dc According to the DC current deviation Δ I dc Adjust the reference value of the DC current; The second control module is configured to, under the large disturbance control mode, determine the AC voltage deviation Δ U ac With AC voltage dead zone value U acH Based on the comparison results, the additional adjustment amount Δ of the firing angle is calculated. α i Additional adjustment Δ based on the firing angle α i The firing angle reference value is corrected to achieve additional constant AC voltage control; simultaneously, the voltage-current droop coefficient is adjusted from... K 2. Voltage-current droop coefficient when switching to fault mode K 3. Based on the switched K 3 and the DC voltage deviation Δ on the inverter side U di The improved DC current command value is calculated.

9. An electronic device, characterized in that, include: A processor, and a memory communicatively connected to the processor; The memory stores computer-executed instructions; The processor executes the computer execution instructions stored in the memory to implement the LCC-CLCC DC system collaborative control method that takes into account both frequency and voltage active support as described in any one of claims 1-7.

10. A computer-readable storage medium, characterized in that, The computer-readable storage medium stores computer-executable instructions, which, when executed by a processor, are used to implement the LCC-CLCC DC system coordinated control method that takes into account both frequency and voltage active support as described in any one of claims 1-7.

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