Self-synchronous decoupling control method for adaptive inertia and damping simulation of hvdc system

Abstract

The present application relates to a kind of adaptive inertia and damping simulation of flexible system self-synchronous decoupling control method, belong to flexible HVDC transmission technical field.First, the available energy of flexible HVDC system inertia support and its feasibility of active support system inertia are analyzed, and the self-synchronous control strategy of receiving end converter station considering capacitor energy margin is proposed, while realizing the self-synchronous of phase-locked loop, actively support receiving end grid inertia.On this basis, considering the influence of DC voltage constraint on capacitor energy margin utilization, by the adaptive adjustment of the number of sub-modules of receiving end converter station and rotational inertia, further utilize capacitor energy margin to improve its inertia support capability;While sending end converter station introduces damping control link, realizes frequency deviation adjustment and improves the stability of system.The present application can be applied to asynchronous grid based on flexible HVDC interconnection, can further improve the inertia support and damping capability of flexible HVDC system, effectively suppress the frequency variation rate of system.

Description

Technical Field

[0001] This invention relates to the field of flexible DC transmission technology, and particularly to a self-synchronizing decoupling control method for adaptive inertia and damping simulation of a flexible DC system. Specifically, it relates to an adaptive inertia simulation decoupling control strategy for the receiving-end converter station and an inertia and damping simulation decoupling control strategy for the sending-end converter station, thereby improving the system's inertia support and damping capability. Background Technology

[0002] Modular multilevel converter-based high voltage direct current (MMC-HVDC) technology boasts advantages such as fast control speed, flexible operation, and high power supply reliability, making it a crucial technology for centralized transmission of new energy sources, asynchronous grid interconnection, weak grids, and isolated power supply. However, with the increasing prevalence of power electronics on both the source, grid, and load sides, system inertia is gradually decreasing, and frequency stability issues are becoming increasingly prominent.

[0003] Domestic and international scholars have conducted extensive research on improving the inertia support capability of flexible DC systems. Currently, the control methods are mainly divided into grid-connected control and grid-connected control. Most converter stations currently use grid-connected control, which utilizes phase-locked loops to measure the phase information of the grid connection point to achieve synchronization with the grid. However, it exhibits current source characteristics externally and cannot actively support system inertia. This not only weakens the system's inertia response capability but also, due to DC asynchrony, prevents the sharing of inertia responses between interconnected areas, accelerating frequency changes under disturbances to some extent. Furthermore, normal operation requires a voltage source in the system to construct voltage at the grid connection point. As the proportion of synchronous machines in the system gradually decreases and grid strength declines, the lack of a voltage source to construct voltage directly affects the normal operation of the system. Moreover, as the number of grid-connected converters connected to the grid increases, even small load fluctuations can trigger excessive responses in the converters, leading to large-scale disconnection of grid-connected energy storage systems and even triggering a chain reaction that could cause power system collapse. Grid-based control employs a power synchronization strategy similar to that of synchronous generators, achieving synchronization without phase-locked loops (PLLs). It utilizes energy storage elements or reserved backup capacity to provide virtual inertia and damping for the system. However, given that the system already has a steady-state operating point, achieving synchronization with the grid by controlling the DC voltage introduces two challenges. First, it adds a dynamic element from active power to DC voltage that is detrimental to system stability to the mathematical model. Second, the flexibility in selecting control parameters complicates stability analysis. Furthermore, the grid-based converter has a lower limit on its overcurrent capacity, requiring current-limited operation under fault conditions, which presents new challenges to transient stability analysis. Therefore, it is necessary to further explore the control potential of flexible DC systems, design control strategies to enhance the inertia support and damping capabilities of flexible DC systems, and assume the responsibility of supporting system frequency. Summary of the Invention

[0004] The purpose of this invention is to provide a self-synchronizing decoupling control method for adaptive inertia and damping simulation of flexible DC systems, solving the following problems in existing technologies: Flexible DC transmission technology using traditional vector control strategies decouples the frequencies of the sending and receiving AC systems, preventing active participation in system inertia support and frequency adjustment; due to the inherent coupling relationship between DC voltage and submodule capacitor voltage, the energy of the capacitors within the system cannot be fully utilized; the control method used in the receiving-end converter station relies on a phase-locked loop, requiring an internal voltage source while failing to effectively support frequency and voltage. This invention analyzes the available energy margin of flexible DC systems (flexible DC transmission systems), and, for the receiving-end converter station, establishes the coupling relationship between the output angular frequency and the state variables of the capacitor elements by analogy with the rotor motion equations of synchronous generator sets. An adaptive inertia simulation decoupling control strategy and its related control parameters, taking into account the capacitor energy margin, are designed. By flexibly adjusting the rotational inertia of the self-synchronizing control link and the number of submodules deployed, the inertia support capability of the flexible DC system is improved while ensuring its safe and stable operation. For the sending-end converter station, the DC voltage deviation is introduced into its modulation and outer-loop control loop to adaptively adjust the number of sub-modules and the active power reference value, thereby realizing the inertia support and damping simulation of the sending-end converter station. The control parameters were also designed. Ultimately, a complete self-synchronizing decoupling control method and strategy for adaptive inertia and damping simulation of flexible DC systems was formed. This strategy can fully utilize the energy margin of the flexible DC system and suppress system frequency variations.

[0005] The above-mentioned objective of this invention is achieved through the following technical solution:

[0006] A self-synchronizing decoupling control method for adaptive inertia and damping simulation of flexible-conductive systems includes the following steps:

[0007] Step (1) Inertia support energy margin analysis of flexible DC system;

[0008] Step (2) Design of self-synchronization decoupling control strategy and control parameters for adaptive inertial simulation of flexible DC system receiving-end converter station;

[0009] Step (3) Design of inertial and damping simulation control strategy and control parameters for the sending-end converter station of the flexible DC system.

[0010] The inertia support energy margin analysis of the flexible DC system described in step (1) is to analyze the main sources of inertia support energy of the flexible DC system and its margin.

[0011] The inertia support energy of the flexible DC transmission system mainly comes from the submodule capacitors. If the maximum voltage that the submodule capacitors can withstand for a short time is set to 1.5 times the rated operating voltage, the energy that can be stored during the inertia support period will reach 1.25 times the rated capacitance. This provides a considerable amount of inertia support energy while ensuring the safety of the submodule capacitors and power devices.

[0012] The self-synchronization decoupling control strategy and its control parameter design for the adaptive inertial simulation of the flexible DC system receiving-end converter station described in step (2) are as follows: Based on the analysis in step (1), the coupling relationship between the output angular frequency of the receiving-end converter station and the state quantity of the capacitor element is established. Based on this, a self-synchronization control strategy is designed to make the flexible DC system exhibit voltage source characteristics. An adaptive inertial simulation decoupling control strategy is proposed, which realizes the independent adjustment of DC voltage and sub-module capacitor voltage by simultaneously adjusting the moment of inertia and the number of sub-modules.

[0013] The coupling relationship between the converter station output angular frequency and the state variables of the capacitor elements is expressed as follows:

[0014]

[0015] In the formula: ω MMC ω is the output angular frequency of the receiving-end converter station. MMC0 The rated output angular frequency of the converter station under steady-state conditions; N is the number of sub-modules in the receiving-end MMC; C SM The capacitance value for a single submodule; U SM U represents the average voltage across the capacitors of the submodule. SM0 J is the submodule capacitor voltage in steady state; v The virtual moment of inertia of the converter station;

[0016] In the adaptive inertial simulation decoupling control strategy, the expression for the virtual rotational inertia of the receiving-end converter station is:

[0017]

[0018] In the formula: J v J represents the virtual moment of inertia of the converter station. v0 J is the controllable rotational inertia under a fixed modulation strategy. vm ω represents the maximum value of the moment of inertia. MMCm This is the maximum value of the converter station's output angular frequency, which is 1.01 times ω. MMC0 The details are as follows:

[0019]

[0020] In the formula: ω gm The limit value for the angular frequency of the AC system; ω g0 The rated operating angular frequency of the AC system;

[0021] The expression for the number of submodules put into operation at the receiving-end converter station is:

[0022]

[0023] Where: N new.r This represents the sum of the number of sub-modules deployed on the upper and lower arms of the receiving-end converter station after the disturbance; round(x) is the rounding function.

[0024] The inertial and damping simulation control strategy and its control parameters design for the sending-end converter station of the flexible DC system described in step (3) are as follows: Based on the analysis in step (2), an inertial and damping simulation decoupling control strategy is designed. By simultaneously adjusting the number of sub-modules and the output active power, the inertial support of the sending-end converter station capacitor and the damping provided by the sending-end grid to the receiving-end system are realized; according to the maximum limits of DC voltage and sub-module capacitor voltage (1.05U respectively) dc0 and 1.5U SM0 The minimum number of sub-modules N was designed for each. min And the droop coefficient K;

[0025] The expressions for the number of submodules put into operation and the reference value of active power during the modulation process of the sending-end converter station are as follows:

[0026]

[0027] Where: N new.s The sum of the number of submodules deployed on the upper and lower arms of the sending-end converter station; P ref and P ref0 These are the reference values ​​of active power at the sending-end converter station before and after the strategy is implemented; N min This represents the minimum number of sub-modules to be deployed; K is the droop coefficient between the input power and DC voltage of the sending-end converter station.

[0028] The expression for the minimum number of sub-modules to be deployed at the sending-end converter station is:

[0029]

[0030] Where: N min N represents the minimum number of converter station sub-modules to be deployed; N is the total number of converter station sub-modules to be deployed.

[0031] The expression for the droop coefficient between input power and DC voltage during the adjustment process of the sending-end converter station is:

[0032]

[0033] Where: ΔP inmin This represents the maximum reduction in input power at the sending-end converter station.

[0034] The beneficial effects of this invention are as follows: It proposes a self-synchronization control strategy for the receiving-end converter station that considers capacitor energy margin, actively supporting the inertia of the receiving-end grid while achieving phase-locked loop-free self-synchronization. Based on this, considering the impact of DC voltage constraints on the utilization of capacitor energy margin, a self-synchronization decoupling control method and parameter design strategy for adaptive inertia and damping simulation of flexible DC systems are designed. By adaptively adjusting the number of sub-modules and rotational inertia of the receiving-end converter station, the inertia support capability is further enhanced by utilizing the capacitor energy margin. Simultaneously, a damping control loop is introduced for the sending-end converter station to achieve frequency deviation adjustment and improve the stability of the control system. This invention can be applied to asynchronous grids based on flexible DC transmission interconnection, further improving the inertia support and damping capability of flexible DC systems and effectively suppressing the system's frequency variation rate, thus ensuring the stable operation of the system.

[0035] Taking into account the capacitor energy margin, the receiving-end converter station achieves phase-locked loop self-synchronization control that adaptively adjusts the number of sub-modules in operation and uses rotational inertia for inertia support; the sending-end converter station introduces a damping control loop to adjust the minimum number of sub-modules in operation and the droop coefficient, thereby achieving automatic recovery of the number of sub-modules in operation and the output power of the converter station after the system frequency stabilizes. Attached Figure Description

[0036] The accompanying drawings, which are included to provide a further understanding of the invention and form part of this application, illustrate the invention and are used to explain it, but do not constitute an undue limitation of the invention.

[0037] Figure 1 This is a block diagram of the flexible DC system structure and control of the present invention. Detailed Implementation

[0038] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0039] See Figure 1As shown, the purpose of this invention is to provide a self-synchronizing decoupling control method for adaptive inertia and damping simulation of a flexible DC system when the receiving-end grid experiences disturbances. First, the available energy margin of the flexible DC system is analyzed in detail, and the electric field energy stored in the capacitors is used as the energy source for inertia support. Specifically, for the receiving-end converter station, independent control strategies are designed for the DC voltage and the submodule capacitor voltage. When the rotational inertia of the converter station is changed, the proportional coefficient between the capacitor voltage deviation and the converter station output frequency deviation can be altered. For the same voltage deviation, the capacitor can store more energy, thereby improving its inertia support capability. When the number of submodules deployed is changed, the amplitude of the submodule capacitor voltage change changes accordingly, achieving effective decoupling between the DC voltage and the submodule capacitor voltage, ensuring that the DC voltage does not exceed limits while transmitting the receiving-end grid frequency information.

[0040] For the sending-end converter station, since the DC voltage can transmit the frequency information of the receiving-end grid, the deviation of the DC voltage is introduced into its modulation and outer-loop control. By adaptively adjusting the number of sub-modules and the active power reference value, the energy stored in the capacitors of the sending-end converter station is fully utilized while simulating the damping characteristics of the synchronous machine. Under the primary frequency regulation of the receiving-end grid synchronous machine, the system power gradually balances and the frequency gradually stabilizes. At this point, a high-pass filter is introduced to process the DC voltage signal, which can achieve frequency stability of the receiving-end system. In addition, the maximum value limits of the DC voltage and capacitor voltage are proposed, and the selection principles of the control parameters are presented. This invention effectively improves the inertia support capability of the flexible DC system and ensures the stable operation of the system.

[0041] 1. Structure and Energy Margin Analysis of Flexible DC Systems

[0042] Flexible DC system structure such as Figure 1 As shown, a symmetrical single-pole connection method is adopted. The sending-end converter station adopts constant active power control and constant reactive power control to provide constant active and reactive power to the receiving end. The receiving-end converter station adopts the self-synchronization control proposed in this paper, which can realize the self-synchronization grid connection of the converter station without phase-locked loop, that is, it exhibits voltage source characteristics to actively support the frequency and voltage of the receiving-end AC system.

[0043] Flexible DC transmission systems contain various energy storage elements. Their energy is stored in capacitors as potential energy and in inductors as magnetic field energy, which are the main sources of inertia support energy, and both have a certain energy margin. However, since the magnetic field energy stored in inductors is determined by the DC current and equivalent inductance, it is usually much smaller than the electric field energy stored in capacitors, and its inertia support effect can be approximately ignored. Therefore, when the receiving-end AC grid is subjected to active disturbances, the flexible DC system can actively support the inertia of the receiving-end grid by absorbing or releasing energy through capacitor elements. The specific energy changes are shown in equation (1-1).

[0044]

[0045] Where: ΔE C The potential energy provided by the capacitors at both ends of the converter station; N is the sum of the number of submodules connected to the upper and lower arms of the converter station; C SM The capacitance value for a single submodule; U SM U represents the average voltage across the capacitors of the submodule. SM0 This refers to the capacitor voltage of the submodule during steady-state operation.

[0046] The potential energy stored in a flexible DC system is determined by the submodule capacitors and their voltages. In practical engineering, when selecting submodule capacitors, a certain safety margin is allowed. Taking into account the overvoltage capability requirements and the capacitor voltage threshold for software overvoltage protection activation under non-blocking conditions, the maximum voltage that the submodule capacitors can withstand in a short time can be set to 1.5 times the rated operating voltage. During the inertia support period, the energy that can continue to be stored will reach 1.25 times the rated capacitor capacity, providing a considerable amount of inertia support energy while ensuring the safety of the submodule capacitors and power devices.

[0047] Based on the above analysis, it can be seen that flexible DC systems have a large capacitor energy margin, and the energy can be fully utilized by designing reasonable control strategies to maximize their inertia support capabilities.

[0048] 2. Design of decoupled control strategy based on adaptive inertia and damping simulation

[0049] In existing virtual inertia control strategies for flexible DC systems, the total number of submodules deployed in the upper and lower bridge arms at any given time is always N. This results in a proportional relationship between the DC voltage and the submodule capacitor voltage. The safety constraint on the DC voltage (allowable deviation range of ±5%) directly limits the submodule capacitor voltage, leading to a limited inertia support energy that the flexible DC system can provide. Furthermore, the power fed into the flexible DC system from the sending-end grid is a fixed value and cannot autonomously respond to frequency changes in the receiving-end grid. When traditional synchronous generator sets cannot meet the grid's frequency regulation speed requirements, this may lead to problems such as large frequency change rates and frequency exceeding limits. Therefore, simulated decoupling control strategies are designed for the sending and receiving-end converter stations respectively, fully utilizing the energy of the capacitors within the flexible DC system to enhance its inertia support capability.

[0050] For the receiving-end converter station:

[0051] In a flexible DC system, the average capacitor voltage varies with power, which can be used to describe the dynamic change characteristics of capacitor voltage caused by unbalanced power, as shown in equation (2-1).

[0052]

[0053] In the formula: P in Input power for flexible DC systems; Pout The output power of the receiving-end converter station;

[0054] In order to enable the converter station to possess the inertial response characteristics of a synchronous machine, analogous to the rotor motion equation of a synchronous machine, P in This can be considered as the mechanical power of a synchronous machine, which is assumed to be constant here, meaning only the DC capacitor is considered to provide inertia support for the system; P out If we consider the DC voltage as the electromagnetic power of a synchronous generator, then the change in DC voltage can be compared to the change in the speed of a synchronous machine. The main difference is that the DC capacitor uses its own energy to provide virtual inertia support for the system, thereby suppressing frequency abrupt changes; while the synchronous machine uses its rotor kinetic energy to provide inertia support, directly determining the system frequency. Therefore:

[0055]

[0056] In the formula: J v ω represents the virtual moment of inertia of the converter station. MMC The output angular frequency of the converter station;

[0057] Integrating and simplifying both sides of equation (2-2), we get:

[0058]

[0059] In the formula: ω MMC0 The rated output angular frequency of the converter station;

[0060] The above formula reflects the coupling relationship between the output angular frequency of the receiving-end converter station and the state quantity of the capacitor element. Therefore, the output angular frequency can be directly changed according to the change of capacitor voltage during power imbalance, thereby adjusting the output power of the receiving-end converter station and enabling it to have inertial response characteristics similar to those of a synchronous machine.

[0061] By simultaneously adjusting the moment of inertia and the number of sub-modules, the control dimension of the inverter station is increased from the traditional 2 dimensions to 3 dimensions, enabling independent adjustment of DC voltage and sub-module capacitor voltage.

[0062] In order to enable the energy stored in the capacitor to break through the limitations of the fixed modulation mode of the converter station, the controllable rotational inertia can be adaptively adjusted according to the output frequency deviation of the converter station under the constraint of the system frequency deviation, as shown in Equation (2-4), to further improve the inertia support capability of the flexible DC system.

[0063]

[0064] In the formula: J v0 J is the controllable rotational inertia under a fixed modulation strategy. vm ω represents the maximum value of the moment of inertia. MMCm This is the maximum value of the converter station's output angular frequency, which is 1.01 times ω. MMC0;

[0065] If a high-pass filter is introduced into the proportional element of equation (2-4), the output angular frequency ω of the converter station can be extracted. MMC By simulating the inertia of high-frequency components in the system, the virtual rotational inertia can be automatically restored to its pre-disturbance state after the frequency of the receiving system stabilizes.

[0066] Meanwhile, in order to maintain the DC voltage within a safe range, the modulation strategy of the converter station can be adaptively adjusted according to the deviation between the submodule capacitor voltage and the frequency, that is, the number of submodules put into the upper and lower bridge arms can be changed, as shown in Equation (2-5), to achieve decoupling control between the submodule capacitor voltage and the DC voltage, and to transmit the receiving end grid frequency information while ensuring that the DC voltage does not exceed the limit.

[0067]

[0068] Where: N new.r This represents the sum of the number of sub-modules deployed on the upper and lower arms of the receiving-end converter station after the disturbance; round(x) is the rounding function.

[0069] For the sending-end converter station:

[0070] As shown in equation (2-4), DC voltage can transmit the frequency information of the receiving-end power grid. As the power grid frequency gradually increases, the DC voltage will also increase. Therefore, the deviation of DC voltage is introduced into its modulation and outer loop control, and the number of its sub-modules and the active power reference value are adaptively adjusted, as shown in equation (2-6), which can simultaneously realize inertia support and damping simulation.

[0071]

[0072] At this time, introducing a high-pass filter at the receiving-end converter station to process the DC voltage signal can also realize the automatic recovery of the number of sub-modules put into operation and the output power of the sending-end converter station after the frequency of the receiving-end system stabilizes.

[0073] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the invention by those skilled in the art. Any modifications, equivalent substitutions, or improvements made to the present invention should be included within the scope of protection of the present invention.

Claims

1. A self-synchronizing decoupling control method for adaptive inertia and damping simulation of flexible-conductive systems, comprising the following steps: Step (1) Inertia support energy margin analysis of flexible DC system; Step (2) Self-synchronization decoupling control strategy and control parameter design for adaptive inertial simulation of flexible DC system receiving-end converter station: Based on the analysis in step (1), the coupling relationship between the output angular frequency of the receiving-end converter station and the state quantity of the capacitor element is established. Based on this, a self-synchronization control strategy is designed to make the flexible DC system exhibit voltage source characteristics. An adaptive inertial simulation decoupling control strategy is proposed, which realizes independent adjustment of DC voltage and sub-module capacitor voltage by simultaneously adjusting the moment of inertia and the number of sub-modules. The coupling relationship between the output angular frequency of the receiving-end converter station and the state variables of the capacitor elements is expressed as follows: (3-1)； In the formula: The output angular frequency of the receiving-end converter station; ω is the rated output angular frequency of the receiving-end converter station in steady state; N is the number of submodules in the receiving-end converter station; C SM The capacitance value for a single submodule; U SM U represents the average voltage across the submodule capacitors. SM0 J is the submodule capacitor voltage in steady state; v The virtual moment of inertia of the receiving-end converter station; In the adaptive inertial simulation decoupling control strategy, the expression for the virtual rotational inertia of the receiving-end converter station is: (3-2) ； In the formula: J v J represents the virtual moment of inertia of the receiving-end converter station. v0 J is the controllable rotational inertia under a fixed modulation strategy. vm This represents the maximum value of the moment of inertia. This is the maximum value of the output angular frequency of the receiving-end converter station, which is 1.01 times the maximum value. The details are as follows: (3-3)； In the formula: This is a limit value for the angular frequency of the AC system; The rated operating angular frequency of the AC system; The expression for the number of submodules put into operation at the receiving-end converter station is: (3-4)； Where: N new.r The sum of the number of sub-modules deployed on the upper and lower arms of the receiving-end converter station after the disturbance; The rated output angular frequency of the receiving-end converter station; U SMn The rated value of the capacitor voltage of the submodule; round(x) is the rounding function; Step (3) Design of inertial and damping simulation control strategy and control parameters for the sending-end converter station of the flexible DC system.

2. The self-synchronization decoupling control method for adaptive inertia and damping simulation of flexible-conductive systems according to claim 1, characterized in that: The energy margin analysis of the inertia support of the flexible DC system mentioned in step (1) is to analyze the source and margin of the energy of the inertia support of the flexible DC system. The inertia support energy of the flexible DC system mainly comes from the submodule capacitors. If the maximum voltage that the capacitors can withstand for a short time is set to 1.5 times the rated operating voltage, the energy that can be stored during the inertia support period will reach 1.25 times the rated capacitance. This provides considerable inertia support energy while ensuring the safety of the submodule capacitors and power devices.

3. The self-synchronization decoupling control method for adaptive inertia and damping simulation of flexible-conductive systems according to claim 1, characterized in that: The inertial and damping simulation control strategy and its control parameters design for the sending-end converter station of the flexible DC system described in step (3) are as follows: Based on the analysis in step (2), an inertial and damping simulation decoupling control strategy is designed. By simultaneously adjusting the number of sub-modules and the output active power, the inertial support of the capacitor of the sending-end converter station and the damping provided by the sending-end power grid to the receiving-end system are realized. Based on the maximum limits of DC voltage and submodule capacitor voltage, which are 1.05U respectively. dc0 and 1.5U SM0 The minimum number of sub-modules N was designed respectively. min And the droop coefficient K; The expressions for the number of submodules put into operation and the reference value of active power during the modulation process of the sending-end converter station are as follows: (4-1)； Where: N new.s The sum of the number of submodules deployed in the upper and lower arms of the sending-end converter station; N min The minimum number of items to be added to the submodule; U dc U is the average value of DC voltage; dc0 U is the DC voltage in steady state; dcmax P represents the maximum value of the DC voltage. ref and P ref0 These are the reference values ​​of active power of the sending-end converter station before and after the strategy is started; K is the droop coefficient between the input power and DC voltage of the sending-end converter station; The expression for the minimum number of sub-modules to be deployed at the sending-end converter station is: (4-2)； Where: N min This represents the minimum number of sub-modules deployed at the sending-end converter station; U SMmax N represents the maximum value of the capacitor voltage of the submodule; N represents the number of submodules put into operation at the sending-end converter station. The expression for the droop coefficient between input power and DC voltage during the adjustment process of the sending-end converter station is: (4-3) ； Where: ∆P inmin This represents the maximum reduction in input power at the sending-end converter station.

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