Self-synchronous decoupling control method for improving active support capability of inertia of hybrid flexible system

Abstract

The present application relates to a kind of self-synchronous decoupling control methods of promoting the active support ability of flexible direct current system inertia, belong to the grid-connected technology field of flexible direct current transmission system.By analogy rotor motion equation of synchronous unit, proposed considering the energy margin of each energy storage element in flexible direct current system converter station self-synchronous control method, while realizing no phase-locked loop self-synchronization, actively support receiving end power grid inertia.Analyze the available energy of flexible direct current system inertia support, quantitatively analyze the feasibility of using energy margin to improve the inertia level of receiving end power grid.On this basis, considering the influence of DC voltage safety constraint on the utilization of capacitor energy margin, proposed the decoupling control method and parameter design method of flexible direct current system DC voltage and submodule capacitor voltage, by self-adapting adjustment submodule input quantity and capacitor voltage reference value, realize the full use of energy margin of energy storage element in flexible direct current system, effectively improve the inertia level and frequency stability of receiving end power grid.

Description

Technical Field

[0001] This invention relates to the field of grid connection technology for flexible DC transmission systems, and particularly to an active energy support control method, especially a self-synchronization decoupling control method that enhances the active inertia support capability of flexible DC systems, thereby ensuring the frequency stability of the receiving-end power grid. Background Technology

[0002] Modular multilevel converter-based high voltage direct current (MMC-HVDC) technology features fast control speed, flexible operation mode, and high power supply reliability. It has become an important technical means for centralized transmission of new energy, asynchronous grid interconnection, weak grid and island power supply. The most typical example of this type of operation mode is the Zhangbei Flexible DC Demonstration Project currently in operation in my country.

[0003] In modern power systems, power electronic equipment, represented by voltage source converters, is rapidly penetrating the system and gradually replacing traditional electromechanical energy conversion equipment. Grid-connected equipment in power electronic systems lacks the mechanical equipment or rotating parts of synchronous generators, and consequently, cannot support power deficits during dynamic processes. The lack of a physical structure to represent the rotor inertia of a synchronous generator in grid-connected equipment reduces system inertia, seriously threatening its safe and stable operation. In existing virtual inertia control methods for flexible DC systems, the total number of submodules engaged in the upper and lower arms at any given time is always N, resulting in a proportional relationship between DC voltage and submodule capacitor voltage. The allowable fluctuation range of DC voltage directly limits the submodule capacitor voltage, preventing the full development of the inertia support capability of the flexible DC system. Therefore, it is necessary to explore new control methods for converter stations to actively support system inertia, participate in frequency stability regulation, and maintain system safety and stability. Summary of the Invention

[0004] The purpose of this invention is to provide a self-synchronization decoupling control method to enhance the active inertia support capability of flexible DC systems, solving the problem of insufficient development of converter station inertia support capability in existing technologies. This invention analyzes the available energy for inertia support in flexible DC systems and quantitatively analyzes the feasibility of utilizing its energy margin to improve the inertia level of the receiving-end power grid. Analogous to the rotor motion equations of synchronous generators, a converter station self-synchronization control strategy considering the energy margin of each energy storage element within the flexible DC system is proposed, actively supporting the inertia of the receiving-end power grid while achieving phase-locked loop-free self-synchronization. Based on this, considering the impact of DC voltage safety constraints on capacitor energy margin utilization, a decoupling control strategy and parameter design method for the DC voltage and submodule capacitor voltage of the flexible DC system are proposed. By adaptively adjusting the number of submodules and the capacitor voltage reference value, the energy margin of energy storage elements within the flexible DC system is fully utilized, effectively improving the inertia level and frequency stability of the receiving-end power grid.

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

[0006] A self-synchronization decoupling control method for improving the active inertia support capability of flexible CRT systems includes the following steps:

[0007] Step (1) Self-synchronization control method for the receiving-end converter station of the flexible DC system:

[0008] By analogy with the inertial response equation of a synchronous machine and the characteristic equations of how various physical quantities change with power in a flexible DC system:

[0009]

[0010] In the formula: J v The effective rotational inertia of the flexible DC transmission system; ω MMC d is the output angular frequency of the receiving-end converter station; d is the differential sign; N is the sum of the number of submodules engaged in the upper and lower arms of the converter station; C SM The capacitance value for a single submodule; U SM The average voltage of the submodule capacitors; L eq The equivalent inductance of a flexible DC system; I dc It is direct current;

[0011] The coupling relationship between the output frequency of the flexible DC transmission system and the capacitor voltage of the submodule and the current of the DC transmission line was calculated.

[0012]

[0013] This involves establishing a coupling relationship between the virtual rotational speed (capacitor voltage and DC current) and the output frequency of the converter station, enabling the receiving-end converter station to possess the inertial response characteristics of a synchronous machine; and through parameter design, realizing the rational utilization of different types of energy during the self-synchronization grid connection of the power system.

[0014] Step (2) Adaptive modulation method to enhance the inertia support capability of flexible DC system: By simultaneously adjusting the reference value of submodule capacitor voltage and the number of submodules in operation, the control dimension is increased from the traditional 2 dimensions to 3 dimensions, realizing independent adjustment of DC voltage and submodule capacitor voltage. The process of flexible DC system actively supporting system inertia can be mainly divided into three stages:

[0015] 1) Energy absorption stage: Adjust the reference value of the capacitor voltage of the submodule and the number of submodules put into operation according to the change of the grid frequency, and make full use of the energy storage capacity of the capacitors and inductors in the flexible DC system to quickly provide inertia support.

[0016] 2) System adjustment phase: The frequency regulation resources of the receiving end power grid gradually play their role, so that the frequency gradually recovers to stability, and the frequency change rate of multiple time windows is used to judge the state of the system.

[0017] 3) Energy release phase: After the grid frequency stabilizes, adjust the capacitor voltage reference value and the number of sub-modules to the level before the disturbance, and the flexible DC system will resume normal operation.

[0018] The beneficial effects of this invention are as follows: In terms of self-synchronization control, the output angular frequency of the receiving-end converter station is adjusted according to the capacitor voltage of the submodule and the DC line current in the flexible DC system. Compared with the existing traditional vector control of converter stations, this invention achieves phase-locked loop-free self-synchronization between the receiving-end converter station and the power grid, and the energy in the capacitor and inductor responds autonomously to the frequency changes of the receiving-end system like synchronous kinetic energy.

[0019] In the adaptive modulation method, the reference value of the submodule capacitor voltage and the number of submodules are adaptively adjusted according to the receiving end grid frequency, realizing the decoupled control of the submodule capacitor voltage and the DC bus voltage. Compared with the inertia support energy of the converter station under the fixed modulation method, the adaptive decoupling control method increases the fluctuation range of the capacitor voltage from 0.95-1.05pu to 0.95-1.5pu, and adjusts the DC current to regulate the energy stored in the inductor, maximizing the utilization of the energy margin of the flexible DC system and effectively suppressing the rate of change and deviation of the grid frequency in the early stage of disturbance. Attached Figure Description

[0020] 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.

[0021] Figure 1 This is a diagram illustrating the structure and control block of the flexible straight-line system of the present invention.

[0022] Figure 2 This is a schematic diagram showing the distribution of the flexible DC energy storage elements of the present invention;

[0023] Figure 3 This is a flowchart of the adaptive modulation method of the present invention. Detailed Implementation

[0024] 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.

[0025] See Figures 1 to 3 As shown, the self-synchronization decoupling control method for improving the active inertia support capability of flexible DC systems of the present invention is a self-synchronization decoupling control method that can maximize the inertia support capability, and fully utilizes the energy margin of the flexible DC system while achieving active inertia support. First, the inertial energy characteristics of the flexible DC system are analyzed, the available energy of the MMC-HVDC system during the inertia response process is analyzed, and its inertia support energy margin is revealed in conjunction with engineering practice. Considering the energy sources and response differences of the flexible DC system's inertia support, a self-synchronization control method for the converter station is designed with sub-module capacitor voltage and DC current as control variables, realizing phase-locked loop-free self-synchronization grid connection of the converter station and the rational utilization of different types of energy. Based on the adaptive adjustment of the number of sub-modules put into operation and the reference value of capacitor voltage, the decoupling control of the flexible DC system sub-module capacitor voltage and DC bus voltage is realized to fully utilize the available energy of the flexible DC system and effectively improve the inertia support capability. The method includes the following steps:

[0026] 1. DC system structure and inertia support energy analysis:

[0027] The structure and control scheme of flexible DC transmission system, such as Figure 1 As shown, a symmetrical single-pole connection is adopted. The sending-end converter station uses constant active and 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 invention, which can achieve self-synchronization grid connection of the converter station without phase-locked loop, thus exhibiting voltage source characteristics and actively supporting the frequency and voltage of the receiving-end AC system. However, the energy that the flexible DC system can provide under the fixed modulation mode is limited. When the backup plan of traditional synchronous generator sets cannot meet the grid frequency regulation capacity and speed requirements, it may lead to problems such as large grid frequency change rate and frequency exceeding limits. Therefore, it is urgent to further explore the regulation potential of the flexible DC system, enabling it to actively support system inertia, participate in frequency stability regulation, and improve the safety and stability of the system.

[0028] The rotor kinetic energy of a synchronous machine is the primary source of inertia in modern power systems. However, for flexible DC systems, which contain various energy storage elements, energy is stored in capacitors and inductors in the form of potential energy and magnetic field energy, and possesses a certain energy margin, making it the main source of energy supporting inertia. For Figure 1 The distribution diagram of the energy storage elements in the flexible DC system shown is as follows: Figure 2 As shown, it mainly includes bridge arm reactors, current-limiting reactors, equivalent inductance of DC lines, and capacitors of the converter station submodules at both ends.

[0029] When the receiving-end AC grid is subjected to active power disturbance, the flexible DC system can actively support the inertia of the receiving-end grid by absorbing or releasing energy through energy storage elements. The specific energy change can be expressed as:

[0030]

[0031] Where ΔE is the energy provided by the flexible direct current system during the inertial response process; ΔE C The potential energy provided by the capacitors at both ends of the converter station; ΔE L The magnetic field energy provided by the equivalent inductance; N is the sum of the number of submodules put into operation on 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 submodule capacitors. SM0 The voltage of the submodule capacitor during steady-state operation; L eq The equivalent inductance of a flexible DC system; L l L is the equivalent inductance value of a DC line. r L is the inductance value of the DC-side current-limiting reactor. arm I represents the equivalent inductance of the bridge arm reactance at both ends of the converter station on the DC side. dc It is direct current; I dc0 This is the DC current during steady-state operation.

[0032] The magnetic field energy stored in a flexible DC system is mainly determined by the DC current and equivalent inductance. During the inertia response phase, with the input power at the sending end remaining constant, the DC current can be indirectly changed by adjusting the DC voltage, thereby absorbing or releasing magnetic field energy and actively supporting the system's inertia. The allowable fluctuation range for DC voltage deviation is typically ±0.05U. dcN The allowable fluctuation range of DC current deviation will also be limited to ±0.05I. dc0 Therefore, the inertial support energy that can be provided by storing magnetic field energy based on equivalent inductance is limited.

[0033] The potential energy stored in a flexible DC system is determined by the capacitors in the submodules and their voltages. In practical engineering, when selecting submodule capacitors, their overvoltage capability requirements are considered, and their rated voltage U... SMN Typically higher than the rated operating voltage U SM0Large, meaning it leaves a certain margin. Taking a domestic offshore wind power project connected to a flexible DC power grid as an example, its rated voltage U... SMN Rated operating voltage U SM0 1.4 times (U SMN =1.4U SM0 This means a voltage margin of 0.4 times is provided. Furthermore, the submodule capacitors have a certain withstand voltage capability beyond their rated voltage; according to IEC 61071 standard, the submodule capacitors can withstand 1.15U for an extended period. SMN Overvoltage. At this time, the theoretical maximum voltage that the submodule capacitor in the project can withstand for a long time is U. SMmax =1.61U SM0 (=1.15×1.4U SM0 The rated voltage of the power devices in the submodule is typically higher than the maximum short-term withstand voltage of the capacitor. Furthermore, in non-blocking mode, the software overvoltage protection activates at approximately 1.6 times the rated operating voltage of the submodule capacitor, triggering module blocking. Therefore, the maximum short-term withstand voltage of the submodule capacitor can be set to 1.5 times the rated operating voltage. During inertia support, 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 capacitor and power devices.

[0034] In summary, the magnetic field energy and electric potential energy stored within the flexible DC system can both serve as energy sources for its inertia support. However, in existing control methods, the DC voltage is strongly coupled with the submodule capacitor voltage. The safety constraints of the DC voltage make it difficult to fully utilize the energy margin of the flexible DC system, thus failing to maximize its inertia support capability. The following section will focus on a self-synchronizing decoupling control method for the flexible DC system to fully explore its control potential and improve the system's frequency stability.

[0035] 2. Self-synchronization control method for receiving-end converter station of flexible DC system:

[0036] The inertial response process of a synchronous machine can be described as follows:

[0037]

[0038] In the formula: J is the moment of inertia of the synchronous machine; ω is the angular frequency of the synchronous machine; d is the differential sign; P M P represents mechanical power. E This refers to electromagnetic power.

[0039] The characteristic equations for the changes of various physical quantities with power in a flexible DC system are shown in equation (2-2-2), which can be used to describe the dynamic changes of capacitor voltage and DC current caused by unbalanced power.

[0040]

[0041] In the formula: P in P is the input power of the flexible DC system. out This refers to the output power of the receiving-end converter station.

[0042] To enable the receiving-end converter station to possess the inertial response characteristics of a synchronous machine, equation (2-2-2) can be obtained by analogy with the rotor motion equation of a synchronous machine.

[0043]

[0044] Taking definite integrals on both sides of equation (2-2-3) and simplifying, we can obtain...

[0045]

[0046] In the formula: ω MMC and ω MMC0 These are the actual output angular frequency and the rated angular frequency of the converter station, respectively; J v This is the effective moment of inertia of the converter station.

[0047] The above equation demonstrates the coupling relationship between the output angular frequency of the receiving-end converter station in a flexible DC system and the state variables of the energy storage elements. Therefore, the output angular frequency can be directly changed according to the changes in capacitor voltage and inductor current during power imbalance, thereby adjusting the output power of the receiving-end converter station and enabling it to possess inertial response characteristics similar to a synchronous machine. Therefore, this invention, combining the coupling relationship established by equation (2-2-4), designs a self-synchronization control method for the receiving-end converter station of a flexible DC system. The specific control block diagram is shown below. Figure 1 As shown in the red dashed box, it exhibits voltage source characteristics externally, actively supporting the system's inertia.

[0048] Self-synchronization control requires designing control parameters based on the actual conditions of the flexible DC transmission system.

[0049] (1) Effective moment of inertia J v

[0050] The rotor of a synchronous generator is directly electromechanically coupled to the power grid, and its inertial response process is a physical process. In contrast, the inertial response provided by a converter station based on self-synchronization control is a control process, and the energy sources for inertial support differ between the two. From the perspective of energy conservation, the relationship between the energy provided by the flexible DC system, the effective rotational inertia, and the grid frequency during the inertial support stage can be established as shown in equation (2-2-5).

[0051]

[0052] In the formula: ω g ω is the angular frequency of the AC system. g0 The rated operating angular frequency of the AC system.

[0053] Since the inertial support energy of a flexible DC system mainly comes from the potential energy of the capacitor and the magnetic field energy of the inductor, in the design of J... v When considering the safety constraints of submodule capacitor voltage and DC line current, the energy margin should be fully utilized while ensuring equipment safety. Combining equations (2-1-1) and (2-2-5), we can obtain...

[0054]

[0055] Where: ΔE Cm Provides the maximum potential energy for the capacitors at both converter stations; ΔE Lm Provides the maximum magnetic field energy for inductive components in flexible DC systems; ω gm U SMm I dcm These are the limits for grid frequency, submodule capacitor voltage, and DC current during the inertia response phase, respectively.

[0056] (2) DC current reference value I dc0

[0057] Combining equation (2-2-4), it can be seen that the DC current reference value in the self-synchronization control loop is I during its steady-state operation. dc0 To ensure that the flexible DC system can reliably provide inertia support under different transmission power conditions, the DC current reference value I is set while neglecting the losses of the converter station and DC line. dc0 It should follow the DC transmission power P in Adaptive adjustment, as shown in equation (2-2-7), thereby fully utilizing the energy margin of the flexible DC system.

[0058]

[0059] The output power of the converter station under self-synchronization control can be expressed as:

[0060]

[0061] In the formula: U MMC θ MMC and The voltage amplitude, phase, and phase angle at the MMC grid connection point; U g θ g ω g and X represents the voltage amplitude, phase, angular frequency, and phase angle of the receiving-end AC grid; X is the sum of the grid-connected reactance, line reactance, and equivalent grid impedance; δ is the power angle.

[0062] When the power grid encounters a disturbance, the output frequency of the receiving-end converter station and the grid frequency will deviate from synchronization. Under the coupling relationship established by equation (2-2-4), taking the rise of the receiving-end AC grid frequency as an example, the self-synchronization characteristic of the converter station is reflected in the corresponding relationship of equation (2-2-9):

[0063]

[0064] When ω g During the rise, according to equation (2-2-8), the power angle δ decreases, and the power transmitted from the flexible DC system to the receiving-end grid decreases. Since the output power of the sending-end converter station remains unchanged, the flexible DC system will exhibit the dynamic characteristics of increased submodule capacitor voltage and decreased DC current under unbalanced power, as shown in equation (2-2-2). Under the designed self-synchronization control, the angular frequency of the converter station output increases until it is synchronized with the grid frequency. Based on the above analysis, it can be seen that under the traditional modulation method, the increase in submodule capacitor voltage (increased DC voltage) will lead to a decrease in DC current, which will reduce the self-synchronization speed of the receiving-end converter station and cause a decrease in the magnetic field energy stored in the inductor, weakening the inertia support capability of the converter station. Therefore, it is urgent to design a new modulation method that can fully utilize the inertia support role of the energy storage element while possessing the self-synchronization capability of the synchronous machine, thereby improving the frequency stability of the system.

[0065] 3. Adaptive modulation method to enhance the inertia support capability of flexible CRT systems:

[0066] In existing virtual inertia control methods for flexible DC systems, the total number of submodules deployed in the upper and lower arms at any given time is always N. This results in a proportional relationship between the DC voltage and the submodule capacitor voltage. The allowable fluctuation range of the DC voltage directly limits the submodule capacitor voltage, thus failing to fully develop the inertia support capability of the flexible DC system. Therefore, this invention proposes an adaptive decoupling control method for DC voltage and capacitor voltage to fully utilize the energy of the energy storage components within the flexible DC system and enhance its inertia support capability.

[0067] The self-adaptive modulation method designed in this invention is as follows: Figure 1As shown by the purple dashed line, by simultaneously adjusting the reference value of the submodule capacitor voltage and the number of submodules in operation, the control dimension is increased from the traditional 2 dimensions to 3 dimensions, realizing independent adjustment of DC voltage and submodule capacitor voltage. Taking the rising AC grid frequency at the receiving end as an example, the process of the flexible DC system actively supporting the system inertia can be divided into three stages: (1) Energy absorption stage: Adjust the reference value of the submodule capacitor voltage and the number of submodules in operation according to the change in grid frequency, and make full use of the energy storage capacity of capacitors and inductors in the flexible DC system to quickly provide inertia support. (2) System adjustment stage: The frequency regulation resources of the receiving end grid gradually play their role, so that the frequency gradually recovers to stability, and the frequency change rate of multiple time windows is used to judge the state of the system. (3) Energy release stage: After the grid frequency stabilizes, the reference value of the capacitor voltage and the number of submodules in operation are adjusted to the level before the disturbance, and the flexible DC system resumes normal operation. The following is a detailed introduction to each stage.

[0068] (1) Energy absorption stage: When the AC grid at the receiving end disconnects from the load, the system frequency rises, the output power of the receiving end converter station decreases, and the voltage of the submodule capacitor rises. Under the action of self-synchronization control, ω MMC The voltage is gradually increased to track changes in system frequency. Since the energy stored in the capacitors within the flexible DC system accounts for a large proportion of the total energy, and there is a strong coupling relationship between the output angular frequency deviation of the receiving-end converter station and the submodule capacitor voltage, the submodule capacitor voltage can reach 1.5µF to fully utilize the capacitor energy margin. SMN When ω MMC The rate of change exceeds its activation threshold RoCoF sth When the system frequency deviation is constrained, the reference value of the submodule capacitor voltage can be adjusted according to the frequency deviation, as shown in equation (2-3-1).

[0069]

[0070] In the formula: U' SM0 Δf is the reference value for the capacitor voltage of the energy absorption stage submodule; a is the control coefficient; f is the actual value of the receiving-end system frequency; f0 is the rated frequency; Δf max The maximum deviation limit for the frequency of the AC system; ΔU SM0max This represents the maximum increment of the capacitor voltage reference value during the energy absorption phase.

[0071] Meanwhile, the number of submodules put into the upper and lower arms of the converter station can be adaptively adjusted according to the deviation between the submodule capacitor voltage and the frequency, as shown in Equation (2-3-2), so as to realize the decoupling control of the submodule capacitor voltage and the DC voltage.

[0072]

[0073] Where: N newN represents the sum of the number of sub-modules deployed in the upper and lower arms of the converter station during the energy absorption phase; N represents the sum of the number of sub-modules deployed in the upper and lower arms of the converter station during steady-state operation; b and c are control coefficients; 0.05 in the numerator of b represents the per-unit value of the maximum permissible deviation of the DC bus voltage.

[0074] The number of submodules deployed at the sending-end converter station is synchronized with that at the receiving-end converter station via communication, ensuring full utilization of the energy margin of the submodule capacitors. It can be seen that by changing the number of submodules deployed at the converter station, the capacitor voltage can be increased while the DC voltage is decreased. Without changing the input power at the sending end, the DC current will also increase with the frequency, transforming the energy release of the inductors in the flexible DC system from that of traditional modulation methods into energy storage. This fully utilizes the energy storage components within the flexible DC system, further enhancing its inertia support capability.

[0075] (2) System Adjustment Phase: When the frequency of the receiving-end grid exceeds the primary frequency regulation dead zone, the synchronous generator begins primary frequency regulation to participate in the system's unbalanced power regulation. The frequency change rate gradually decreases until the system frequency reaches its maximum value after the disturbance. At this point, the energy absorption phase of the flexible DC system ends. The reference value of the capacitor voltage and the number of sub-modules in operation are kept unchanged to prevent the energy stored in the capacitors from being released in large quantities as the frequency decreases, thus slowing down the system frequency recovery speed. Under the action of primary frequency regulation, the system frequency gradually tends to stabilize. The state is judged by introducing multiple time window frequency change rates. When the absolute values ​​are all lower than the set values, the system is considered to have recovered to a new steady state, and the system adjustment phase ends.

[0076] (3) Energy release stage: After the system frequency stabilizes, in order to enable the synchronous generator to bear all the unbalanced power and restore the state of the flexible DC system to the state before the disturbance, in the energy release stage, the reference value of the sub-module capacitor voltage is restored with a fixed slope k1, and the coupling coefficient d is introduced so that the number of sub-modules put into operation is restored synchronously with a slope k2.

[0077]

[0078] In the formula: d is the coupling coefficient; m is the percentage of submodules put into operation; k1 is the recovery slope of the reference value of the capacitor voltage of the submodule; k2 is the recovery slope of the number of submodules put into operation.

[0079] When the number of submodules and the capacitor voltage reference value return to their steady-state values, the method exits operation, and the energy release phase ends. Therefore, the overall implementation flow of the adaptive modulation method can be obtained as follows: Figure 3 As shown.

[0080] The adaptive modulation method requires designing control parameters based on the actual conditions of the flexible DC transmission system.

[0081] (1) Maximum increment of capacitor voltage reference value ΔU SM0max

[0082] In self-synchronization control, the deviation between the actual value and the reference value of the submodule capacitor voltage directly reflects the system frequency deviation information. Considering this deviation constraint, ΔU in equation (2-3-4) SM0max The following formula can be used for calculation.

[0083]

[0084] In the formula: I dcmax U SMmax The maximum values ​​of DC current and submodule capacitor voltage under the adaptive modulation method; ΔI dcmax , ΔU SMmax The maximum difference between the actual value and its reference value; U' SM0max This represents the maximum value that the submodule capacitor voltage reference value can reach during the rise process.

[0085] (2) Recovery slope k1

[0086] To reduce the impact of the flexible DC system on the grid frequency during the recovery process, the output power increment of the flexible DC system can be controlled by adjusting the value of k1, thereby limiting the fluctuation of the grid frequency. The energy changes of the sending-end converter station and the receiving-end converter station are determined by the submodule capacitor voltage. Due to the coupling of the DC voltage of the transmission line and the transmission of modulation information, the energy changes of the two converter stations can be regarded as the same. Therefore, the value of k1 can be designed according to the constraints of the energy release rate of the flexible DC system and the frequency change rate of the receiving-end grid, as shown in equation (2-3-5).

[0087]

[0088] Where: ΔP max The system unbalanced power limit; H is the receiving-end grid inertial time constant; RoCoF max The maximum rate of change of frequency; ΔP g ΔP represents the power increment of the receiving-end power grid. S and ΔP R These represent the power changes at the sending and receiving converter stations, respectively.

[0089] 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-synchronization decoupling control method for improving the active inertia support capability of a flexible CRT system, characterized in that: Includes the following steps: Step (1) Self-synchronization control method for the receiving-end converter station of the flexible DC system: By analogy with the inertial response equation of a synchronous machine and the characteristic equations of how various physical quantities change with power in a flexible DC system: (1-2-1)； In the formula: J v The effective rotational inertia of the flexible DC transmission system; ω MMC d is the output angular frequency of the receiving-end converter station; d is the differential sign; N is the sum of the number of submodules engaged in the upper and lower arms of the converter station; C SM The capacitance value for a single submodule; U SM The average voltage of the submodule capacitors; L eq The equivalent inductance of a flexible DC system; I dc It is direct current; The coupling relationship between the output frequency of the flexible DC transmission system and the capacitor voltage of the submodule and the current of the DC transmission line was calculated. (1-2-2)； This involves establishing a coupling relationship between the virtual rotational speed and output frequency of the converter station, enabling the receiving-end converter station to possess the inertial response characteristics of a synchronous machine; and through parameter design, achieving the rational utilization of different types of energy during the self-synchronization and grid connection of the flexible DC system; where: ω MMC0 U is the rated angular frequency; SM0 This refers to the capacitor voltage of the submodule during steady-state operation. I dc0 This refers to the DC current during steady-state operation. Step (2) Adaptive modulation method to enhance the inertia support capability of flexible CRT systems: By simultaneously adjusting the reference value of the submodule capacitor voltage and the number of submodules in operation, the control dimension is increased from the traditional 2 dimensions to 3 dimensions, enabling independent adjustment of the DC voltage and the submodule capacitor voltage; the process of the flexible DC system actively supporting the system inertia is divided into three stages: 1) Energy absorption stage: Adjust the reference value of the capacitor voltage of the submodule and the number of submodules put into operation according to the change of the grid frequency, and make full use of the energy storage capacity of the capacitors and inductors in the flexible DC system to quickly provide inertia support. 2) System adjustment phase: The frequency regulation resources of the receiving-end power grid gradually come into play, so that the frequency gradually returns to stability, and the frequency change rate of multiple time windows is used to judge the state of the system; 3) Energy release phase: After the grid frequency stabilizes, adjust the capacitor voltage reference value and the number of sub-modules to the level before the disturbance, and the flexible DC system will resume normal operation.

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