DC broadband oscillation suppression method and system for flexible DC power transmission system

Abstract

The invention provides a direct-current broadband oscillation suppression method and system for a flexible direct-current power transmission system. The method comprises the following steps: carrying out oscillation suppression on a direct-current side of the flexible direct-current power transmission system at low, medium and high frequency bands by adopting direct-current damping control; obtaining an oscillation risk frequency point at a high-frequency-band intersection point based on impedance amplitude curves of two pre-established subsystem direct-current impedance models; performing oscillation suppression on the oscillation risk frequency point of the direct current side of the flexible direct current power transmission system by adopting resonance control; in the two subsystems, one subsystem is a direct-current impedance model of a converter station in the selected flexible direct-current power transmission system, and the other subsystem is an equivalent aggregation direct-current impedance model of other parts in the flexible direct-current power transmission system. Damping of the flexible DC power transmission system is increased, and stable operation capability and harmonic suppression capability of the flexible DC power transmission system are improved.

Description

Technical Field

[0001] This invention patent application belongs to the technical field of flexible DC power transmission of modular multilevel converters (MMC), and specifically relates to a method and system for suppressing DC broadband oscillations in a flexible DC power transmission system. Background Art

[0002] Flexible DC power transmission technology based on voltage source converters, especially modular multilevel converters, has developed rapidly due to its highly controllable power characteristics. It has significant advantages in aspects such as large-scale renewable energy consumption, connection of weak grids and asynchronous grids, and construction of DC grids, and is an important part of the future new power system with new energy as the main body. Currently, more than 20 offshore wind power DC grid connection projects planned and built in Europe, as well as projects in China such as Shanghai Nanhui, Nanao three-terminal, Zhoushan five-terminal, Xiamen, Yubei back-to-back, Zhangbei DC grid, and Jiangsu Rudong offshore wind power grid connection, all adopt flexible DC power transmission technology.

[0003] The high-voltage large-capacity converters in the flexible DC transmission system belong to power electronic equipment, which has characteristics such as low inertia, diverse harmonic spectra, and rapid control actions. They exhibit different impedance characteristics in a wide frequency range, and the impedance characteristics in some frequency bands may show negative damping, thus causing oscillation problems in the flexible DC transmission system. In recent years, wide-band oscillation problems have frequently occurred in flexible DC projects, which can be mainly divided into three categories: sub / supersynchronous oscillations induced by the interaction between wind farms and flexible DC, DC oscillations between flexible DC converter stations, and high-frequency oscillations induced by the interaction between flexible DC and AC systems. Among them, the DC side oscillations of the flexible DC transmission system are affected by converters, DC transmission lines, and control systems, and the interaction mechanism is complex. However, existing methods only focus on the oscillation phenomena in specific frequency ranges such as the low-frequency band (1 Hz - 100 Hz) or the high-frequency band (500 Hz and above), and their research is separate and fragmented. For example, when studying the low-frequency band, due to the interaction between the arm inductance in the DC transmission system and the switching of the energy storage module capacitors, the DC side impedance of the converter station will have a point with a relatively small impedance amplitude in the low-frequency band. If other parts of the DC transmission system (such as DC lines) or modulation links cannot provide sufficient positive damping, it is very likely to induce medium-low frequency DC oscillations. To address the above problems, the DC damping control method is generally adopted to enhance the damping characteristics of the DC system in the medium-low frequency band. However, considering the link delay of the control loop, it may introduce negative damping characteristics in the high-frequency band. Therefore, the current research cannot reveal the DC oscillation characteristics in a wide frequency range (here, the wide frequency generally refers to the range of 1 Hz - 5000 Hz, including all frequency bands such as the low-frequency band, the medium-frequency band (100 Hz - 500 Hz), and the high-frequency band). Therefore, the proposed oscillation suppression strategy cannot effectively solve the DC oscillation problem of the flexible DC transmission system, and may even deteriorate the stability of the system, seriously affecting the stable operation of the power grid and the safe operation of the equipment. Summary of the Invention

[0004] To overcome the deficiencies of the above-mentioned prior art, this patent application for invention proposes a method for suppressing DC wide-band oscillations in a flexible DC transmission system, including:

[0005] Adopt DC damping control to suppress oscillations on the DC side of the flexible DC transmission system in the low, medium, and high-frequency bands;

[0006] Obtain the oscillation risk frequency point based on the intersection point of the impedance amplitude curves of the impedance models of two subsystems established in advance in the high-frequency band;

[0007] Adopt resonance control to suppress oscillations at the oscillation risk frequency point on the DC side of the flexible DC transmission system;

[0008] The two subsystems are divided based on a pre-established DC impedance model of a flexible DC transmission system. One subsystem is the DC impedance model of the converter station in the selected flexible DC transmission system, and the other subsystem is the equivalent aggregated DC impedance model of the other parts in the flexible DC transmission system.

[0009] Preferably, the oscillation risk frequency points are obtained from the intersection points of the impedance magnitude curves of the two pre-established DC impedance models of the subsystems in the high-frequency band, including:

[0010] Based on the impedance magnitude curves of the two pre-established DC impedance models of the subsystems, obtain the intersection points of the two impedance magnitude curves in the high-frequency band;

[0011] Based on the impedance phase difference of the two DC impedance models of the subsystems at each intersection point, determine whether there is an oscillation risk at each intersection point and obtain the oscillation risk frequency points.

[0012] Preferably, the method of determining whether there is an oscillation risk at each intersection point and obtaining the oscillation risk frequency points based on the impedance phase difference of the two DC impedance models of the subsystems at each intersection point includes:

[0013] Based on the difference in the impedance phases of the two DC impedance models of the subsystems at the intersection point, obtain the judgment difference;

[0014] Based on the magnitude relationship between the judgment difference and the set phase margin threshold, determine whether there is an oscillation risk at the intersection point.

[0015] Preferably, the method of determining whether there is an oscillation risk at the intersection point based on the magnitude relationship between the judgment difference and the set phase margin threshold includes:

[0016] When the judgment difference is greater than the set phase margin threshold, it is determined that there is no oscillation risk at the intersection point;

[0017] When the judgment difference is not greater than the set phase margin threshold, it is determined that there is an oscillation risk at the intersection point.

[0018] Preferably, the judgment difference is calculated by the following calculation formula:

[0019]

[0020] In the formula, is the judgment difference, ∠Z sys1 is the impedance phase of the impedance model of the subsystem where the impedance model of the converter station in the selected flexible DC transmission system is located at the intersection point, ∠Z sys2 is the impedance phase of the impedance model of the subsystem where the impedance models of other parts except the selected converter station in the flexible DC transmission system are located at the intersection point.

[0021] Preferably, the DC impedance model of the flexible DC transmission system includes the high-frequency DC impedance models of each converter station and the DC line impedance models of each DC line.

[0022] Preferably, the establishment process of the DC line impedance model includes:

[0023] Determining the equivalent number of segments of the DC line based on the parameters of the DC line;

[0024] Establishing the DC line impedance model based on the lumped parameters and the equivalent number of segments of the DC line.

[0025] Preferably, the parameters of the DC line include the length of the DC line, the traveling wave speed, and the set precision frequency.

[0026] Preferably, the equivalent number of segments of the DC line is calculated by the following formula:

[0027]

[0028] where N line is the equivalent number of segments of the DC line, L line is the length of the DC line, υ is the traveling wave speed, f max is the set precision frequency, and Ceil(·) is the rounding function.

[0029] Preferably, the high-frequency DC impedance model of the converter station includes the following expression:

[0030]

[0031] where s is the Laplace operator, L arm is the arm inductance value of the converter station, R vir is the virtual resistance coefficient of the converter station, T d is the link delay of the control system of the converter station, is the high-frequency DC impedance of the converter station.

[0032] Preferably, suppressing the oscillation of the DC side of the flexible DC transmission system in the low, medium, and high frequency bands by using DC damping control includes:

[0033] Eliminating the DC component in the DC current of the flexible DC transmission system through a DC blocking link, and generating an additional control signal in combination with a virtual resistance;

[0034] Superimposing the additional control signal on the converter arm voltage reference value to obtain a modulated voltage through a modulation link;

[0035] Using the modulated voltage as the converter arm voltage to complete the oscillation suppression of the DC side of the flexible DC transmission system in the low, medium, and high frequency bands.

[0036] Preferably, the oscillation suppression of the oscillation risk frequency points on the DC side of the flexible DC transmission system by using resonance control includes:

[0037] Calculating the high-frequency damping voltage corresponding to each oscillation risk frequency point based on the frequency of each oscillation risk frequency point in the flexible DC transmission system;

[0038] Eliminating the DC component in the DC current of the flexible DC transmission system through a DC blocking link, and combining a virtual resistor and the high-frequency damping voltage corresponding to each oscillation risk frequency point to generate an additional control signal;

[0039] Superimposing the additional control signal on the converter bridge arm voltage reference value, and obtaining a modulated voltage through a modulation link;

[0040] Using the modulated voltage as the converter bridge arm voltage to complete the oscillation suppression of the oscillation risk frequency points on the DC side of the flexible DC transmission system.

[0041] Preferably, the damping is calculated by the following calculation formula:

[0042]

[0043] In the formula, is the high-frequency damping voltage corresponding to the nth oscillation risk frequency point, f n is the frequency of the nth oscillation risk frequency point, K R is the resonance proportionality coefficient, ξ is the damping ratio, ω n is the center frequency of the nth oscillation risk frequency point, and s is the Laplace operator.

[0044] Based on the same inventive concept, the present invention patent application also proposes a flexible DC transmission system DC broadband oscillation suppression system, including:

[0045] A broadband oscillation suppression module for suppressing oscillations in the low, medium, and high frequency bands on the DC side of the flexible DC transmission system by using DC damping control;

[0046] A risk point determination module for obtaining oscillation risk frequency points based on the intersection points of the impedance amplitude curves of two pre-established DC impedance models of the subsystem in the high frequency band;

[0047] A risk point oscillation suppression module for suppressing oscillations of the oscillation risk frequency points on the DC side of the flexible DC transmission system by using resonance control;

[0048] The two subsystems are divided based on a pre-established DC impedance model of a flexible DC transmission system. One of the subsystems is the DC impedance model of the converter station in the selected flexible DC transmission system, and the other subsystem is the equivalent aggregated DC impedance model of other parts in the flexible DC transmission system.

[0049] Preferably, the risk point determination module is specifically configured to:

[0050] Based on the impedance magnitude curves of the two pre-established DC impedance models of the subsystems, obtain each intersection point of the two impedance magnitude curves in the high-frequency band;

[0051] Based on the impedance phase difference of the two DC impedance models of the subsystems at each intersection point, determine whether there is an oscillation risk at each intersection point and obtain the oscillation risk frequency point.

[0052] Preferably, when the risk point determination module determines whether there is an oscillation risk at each intersection point based on the impedance phase difference of the two DC impedance models of the subsystems at each intersection point and obtains the oscillation risk frequency point, it includes:

[0053] Based on the difference in the impedance phases of the two DC impedance models of the subsystems at the intersection point, obtain a judgment difference;

[0054] Based on the magnitude relationship between the judgment difference and the set phase margin threshold, determine whether there is an oscillation risk at the intersection point.

[0055] Preferably, when the risk point determination module determines whether there is an oscillation risk at the intersection point based on the magnitude relationship between the judgment difference and the set phase margin threshold, it includes:

[0056] When the judgment difference is greater than the set phase margin threshold, it is determined that there is no oscillation risk at the intersection point;

[0057] When the judgment difference is not greater than the set phase margin threshold, it is determined that there is an oscillation risk at the intersection point.

[0058] Preferably, the judgment difference in the risk point determination module is calculated by the following calculation formula:

[0059]

[0060] In the formula, is the judgment difference, ∠Z sys1 is the impedance phase of the impedance model of the subsystem where the impedance model of the converter station in the selected flexible DC transmission system is located at the intersection point, ∠Z sys2 is the impedance phase of the impedance model of the subsystem where the impedance models of other parts except the selected converter station in the flexible DC transmission system are located at the intersection point.

[0061] Preferably, the DC impedance model of the flexible DC power transmission system includes: the high-frequency DC impedance models of each converter station and the DC line impedance models of each DC line.

[0062] Preferably, the establishment process of the DC line impedance model includes:

[0063] Determining the equivalent number of segments of the DC line based on the parameters of the DC line;

[0064] Establishing the DC line impedance model based on the lumped parameters and the equivalent number of segments of the DC line.

[0065] Preferably, the parameters of the DC line include: the length of the DC line, the traveling wave speed, and the set precision frequency.

[0066] Preferably, the equivalent number of segments of the DC line is calculated by the following formula:

[0067]

[0068] where N line is the equivalent number of segments of the DC line, L line is the length of the DC line, υ is the traveling wave speed, f max is the set precision frequency, and Ceil(·) is the rounding function.

[0069] Preferably, the high-frequency DC impedance model of the converter station includes the following expression:

[0070]

[0071] where s is the Laplace operator, L arm is the arm inductor value of the converter station, R vir is the virtual resistance coefficient of the converter station, T d is the control system link delay of the converter station, is the high-frequency DC impedance of the converter station.

[0072] Preferably, the broadband oscillation suppression module is specifically configured to:

[0073] Eliminate the DC component in the DC current of the flexible DC power transmission system through a DC blocking link, and generate an additional control signal in combination with a virtual resistance;

[0074] Superimpose the additional control signal on the converter arm voltage reference value, and obtain a modulated voltage through a modulation link;

[0075] Use the modulated voltage as the converter arm voltage to complete the oscillation suppression of the low, medium, and high frequency bands on the DC side of the flexible DC power transmission system.

[0076] Preferably, the risk point oscillation suppression module is specifically configured to:

[0077] Calculate the high-frequency damping voltage corresponding to each oscillation risk frequency point based on the frequency of each oscillation risk frequency point in the flexible DC transmission system;

[0078] Eliminate the DC component in the DC current of the flexible DC transmission system through a DC blocking link, and combine a virtual resistor and the high-frequency damping voltage corresponding to each oscillation risk frequency point to generate an additional control signal;

[0079] Superimpose the additional control signal on the converter bridge arm voltage reference value, and obtain a modulated voltage through a modulation link;

[0080] Use the modulated voltage as the converter bridge arm voltage to complete the oscillation suppression of the oscillation risk frequency points on the DC side of the flexible DC transmission system.

[0081] Preferably, the damping in the risk point oscillation suppression module is calculated by the following calculation formula:

[0082]

[0083] In the formula, is the high-frequency damping voltage corresponding to the nth oscillation risk frequency point, f n is the frequency of the nth oscillation risk frequency point, K R is the resonance proportionality coefficient, ξ is the damping ratio, ω n is the center frequency of the nth oscillation risk frequency point, and s is the Laplace operator.

[0084] Based on the same inventive concept, this patent application for invention further provides a computer device, including: one or more processors; a memory for storing one or more programs;

[0085] When the one or more programs are executed by the one or more processors, the above-mentioned method for suppressing DC broadband oscillation in a flexible DC transmission system is implemented.

[0086] Based on the same inventive concept, this patent application for invention further provides a computer-readable storage medium, on which a computer program is stored. When the computer program is executed, the above-mentioned method for suppressing DC broadband oscillation in a flexible DC transmission system is implemented.

[0087] Compared with the closest prior art, the beneficial effects of this patent application for invention are as follows:

[0088] The present invention patent application provides a method and system for suppressing DC broadband oscillations in a flexible DC transmission system, including: using DC damping control to suppress oscillations in the low, medium, and high frequency bands on the DC side of the flexible DC transmission system; obtaining the oscillation risk frequency point based on the intersection point of the impedance amplitude curves of the impedance models of two subsystems in the high frequency band in a pre-established DC impedance model; using resonance control to suppress oscillations at the oscillation risk frequency point on the DC side of the flexible DC transmission system; the two subsystems are divided based on a pre-established DC impedance model of the flexible DC transmission system, where one subsystem is the DC impedance model of the converter station in the selected flexible DC transmission system, and the other subsystem is the equivalent aggregated DC impedance model of other parts in the flexible DC transmission system. The present invention patent application can effectively evaluate the oscillation risk of the flexible DC transmission system in the broadband range, increase the damping of the flexible DC transmission system, and improve the stable operation ability and harmonic suppression ability of the flexible DC transmission system by finding the high-frequency oscillation risk frequency point after suppressing broadband oscillations and performing directional oscillation suppression on the oscillation risk frequency point. Description of the Drawings

[0089] Figure 1 It is a schematic flowchart of a method for suppressing DC broadband oscillations in a flexible DC transmission system proposed in the present invention patent application;

[0090] Figure 2 It is a control schematic diagram of DC damping control in the present invention patent application;

[0091] Figure 3 It is a schematic diagram of broadband DC damping control in the present invention patent application;

[0092] Figure 4 It is a schematic diagram of the DC impedance model of the Xiamen dual-terminal flexible DC project considering DC cables in the present invention patent application;

[0093] Figure 5 It is a schematic diagram of the impedance amplitude and phase characteristic curves of the DC impedance (without DC damping control) of two subsystems in the present invention patent application;

[0094] Figure 6 It is a schematic diagram of the DC current waveform (without DC damping control) in the present invention patent application;

[0095] Figure 7 In the present invention patent application, for the DC impedance of two subsystems (after adding Figure 2 DC damping control), it is a schematic diagram of the impedance amplitude and phase characteristic curves;

[0096] Figure 8 In the present invention patent application, for the DC current waveform (after adding Figure 2 DC damping control), it is a schematic diagram;

[0097] Figure 9 Schematic diagrams of the impedance magnitude and phase characteristics of the DC impedance (with broadband DC damping control added) of two subsystems in this invention patent application; Figure 3

[0098] Figure 10 Schematic diagram of the DC current waveform (with broadband DC damping control added) in this invention patent application; Figure 3

[0099] Figure 11 Schematic diagram of the structure of a DC broadband oscillation suppression system for a flexible DC transmission system proposed in this invention patent application. Specific implementation manners

[0100] The following further elaborates on the specific implementation manners of this invention patent application in conjunction with the accompanying drawings.

[0101] Embodiment 1:

[0102] This invention patent application provides a method for suppressing DC broadband oscillations in a flexible DC transmission system, as Figure 1 shown, including the following steps:

[0103] Step 1: Use DC damping control to suppress oscillations in the DC side of the flexible DC transmission system in the low, medium, and high frequency bands;

[0104] Step 2: Obtain the oscillation risk frequency points based on the intersection points of the impedance magnitude curves of the impedance models of two pre-established subsystems in the high frequency band;

[0105] Step 3: Use resonance control to suppress oscillations at the oscillation risk frequency points in the DC side of the flexible DC transmission system;

[0106] The two subsystems are divided based on a pre-established DC impedance model of the flexible DC transmission system. One subsystem is the DC impedance model of the converter station in the selected flexible DC transmission system, and the other subsystem is the equivalent aggregated DC impedance model of other parts of the flexible DC transmission system.

[0107] In Step 1, use DC damping control to suppress oscillations in the DC side of the flexible DC transmission system in the low, medium, and high frequency bands. The following methods are included:

[0108] ◎ Eliminate the DC component in the DC current of the flexible DC transmission system through a DC blocking link, and generate an additional control signal in combination with a virtual resistor;

[0109] ◎ Superimpose the additional control signal on the converter bridge arm voltage reference value, and obtain the modulated voltage through the modulation link;

[0110] ​​◎Use the modulation voltage as the converter arm voltage to complete the oscillation suppression of the low, medium, and high frequency bands on the DC side of the flexible DC transmission system.

[0111] In the frequency range of 150Hz, the impedance characteristics of the DC side of the flexible DC transmission system are jointly determined by the arm inductance, the converter valve sub-module capacitance, and the control system. The flexible DC transmission system may adopt different control architectures, so it is difficult to derive a theoretical analytical model of the DC side impedance. Generally, the impedance characteristic curves of the AC and DC sides of the flexible DC transmission system are obtained by impedance measurement based on the electromagnetic transient simulation model.

[0112] Due to the interaction between the arm inductance and the switching of sub-modules, there will be a point with a small impedance amplitude in the low frequency band of the DC side impedance of the flexible DC converter station. If other parts of the flexible DC transmission system (such as the DC line) cannot provide sufficient positive damping, it is very likely to induce medium and low frequency DC oscillations. Although this harmonic oscillation mode will not diverge unstably, it still has a negative impact on the operation performance of the flexible DC transmission system, such as the 25Hz oscillation of the DC current in the Xiamen project and the 35Hz oscillation of the DC current in the Chongqing-Hubei project.

[0113] To solve the above problems, DC damping control is adopted to improve the harmonic stability of the DC side of the flexible DC transmission system. The general process is as follows: The DC current signal i input to the DC side of the converter valve in the flexible DC transmission system measured dc Passes through a DC blocking link to eliminate the DC component in the current, and is multiplied by the virtual resistance R vir To generate an additional control signal Then the additional control signal is superimposed on the voltage reference value of the converter, and finally sent to the modulation link. The control block diagram is as Figure 2 shown, where R vir is the virtual resistance coefficient, T w is the time constant of the DC blocking link, and T d is the time delay of the control system link.

[0114] In step 2, based on the intersection points of the impedance amplitude curves of the two pre-established DC impedance models of the subsystems in the high frequency band, the oscillation risk frequency points are obtained, including the following methods:

[0115] ◎Based on the impedance amplitude curves of the two pre-established DC impedance models of the subsystems, obtain each intersection point of the two impedance amplitude curves in the high frequency band;

[0116] ◎Based on the impedance phase difference of the two subsystems' DC impedance models at each intersection point, determine whether there is an oscillation risk at each intersection point and obtain the oscillation risk frequency points.

[0117] Among them, judging whether there is an oscillation risk at each intersection based on the impedance phase difference of the two subsystem DC impedance models at each intersection, and obtaining the oscillation risk frequency points includes:

[0118] ※ Obtaining a judgment difference based on the difference in the impedance phases of the two subsystem DC impedance models at the intersection;

[0119] ※ Judging whether there is an oscillation risk at the intersection based on the magnitude relationship between the judgment difference and a set phase margin threshold.

[0120] Figure 2 The damping control strategy shown can effectively enhance the damping on the DC side of the converter valve in the medium and low frequency bands and suppress the possible weakly damped oscillation phenomenon. However, there is a large link delay in the actual flexible DC control loop, resulting in the phase of the additional damping control signal superimposed on the bridge arm not matching the expectation, and negative damping may be generated in the high frequency band and form positive feedback, thereby inducing system oscillation divergence. In order to accurately evaluate the possible high frequency oscillation risk, it is necessary to establish a DC impedance model for the flexible DC transmission system. In the high frequency band, the DC impedance of the converter valve is mainly affected by the bridge arm inductance and the additional damping control, and its simplified expression is:

[0121]

[0122] In the formula, s is the Laplace operator, L arm is the bridge arm inductance value of the converter station, R vir is the virtual resistance coefficient of the converter station, T d is the control system link delay of the converter station, is the high frequency DC impedance of the converter station.

[0123] In this invention patent application, the DC impedance model of the flexible DC transmission system includes: the high frequency DC impedance models of each converter station and each DC line impedance model.

[0124] Among them, the establishment process of the DC line impedance model includes:

[0125] ※ Determining the equivalent number of segments of the DC line based on the parameters of the DC line;

[0126] ※ Establishing a DC line impedance model based on the lumped parameters and the equivalent number of segments of the DC line.

[0127] The parameters of the DC line include: the length of the DC line, the traveling wave speed, and the set precision frequency.

[0128] The impedance characteristics of the DC line cannot be ignored in the DC side high frequency harmonic stability analysis. The DC line can be equivalent to a multi - stage cascaded T - type (or PI - type) lumped circuit in impedance modeling. In order to accurately simulate the frequency fmax The amplitude and phase characteristics of the DC line impedance within, for a DC line with length L line For the equivalent number of segments of a DC line, at least N line segments of T-type (or PI-type) lumped circuits are required, and the calculation formula is as follows:

[0129]

[0130] In the formula, N line is the equivalent number of segments of the DC line, L line is the length of the DC line, υ is the traveling wave speed, f max is the set precision frequency, and Ceil(·) is the rounding function.

[0131] For a two-terminal flexible DC transmission system or a multi-terminal DC grid, the high-frequency DC impedance model of each converter station can be established according to Equation (1), and then the equivalent number of segments and lumped parameters of each DC line can be determined according to Equation (2) (the equivalent number of segments and lumped parameters of each DC line are used to determine the DC line impedance model). Select the DC side of a certain converter station as the analysis section, and divide the DC system into two subsystems: the impedance Z sys1 of Subsystem 1 is the DC high-frequency impedance of the selected converter station, and the impedance Z sys2 of Subsystem 2 can be obtained by equivalent aggregation of the remaining converter stations and the DC line impedance model according to the DC system topology. The impedance analysis model of the DC subsystem can be compared and verified with the results of impedance simulation measurement.

[0132] Adopt the impedance-based stability analysis method, that is, verify whether the impedance ratio Z sys1 and Z sys2 meet the Nyquist stability criterion to determine the stability of the DC system. There may be multiple intersections between the amplitude curves of the two subsystems Z sys1 and Z sys2 in the high-frequency band. Equation (3) is used to determine whether there is a risk of weakly damped or negatively damped oscillation at the intersection frequency. is the set phase margin threshold, which can be set to 0° - 0.5°.

[0133]

[0134] That is, based on the magnitude relationship between the judgment difference and the set phase margin threshold, it is judged whether there is an oscillation risk at the intersection point, including:

[0135] * When the judgment difference is greater than the set phase margin threshold, it is determined that there is no oscillation risk at the intersection point;

[0136] * When the judgment difference is not greater than the set phase margin threshold, it is determined that there is an oscillation risk at the intersection point.

[0137] The above judgment difference is calculated by the following calculation formula:

[0138]

[0139] In the formula, is the judgment difference, ∠Z sys1 is the impedance phase at the intersection of the impedance model of the subsystem where the impedance model of the converter station in the selected flexible DC transmission system is located, ∠Z sys2 is the impedance phase at the intersection of the impedance model of the subsystem where the impedance models of other components in the flexible DC transmission system except the selected converter station are located.

[0140] In step 3, resonance control is used to suppress the oscillation risk frequency points on the DC side of the flexible DC transmission system, including the following methods:

[0141] ◎ Based on the frequencies of the oscillation risk frequency points in the flexible DC transmission system, calculate the high-frequency damping voltages corresponding to the oscillation risk frequency points;

[0142] ◎ Eliminate the DC component in the DC current of the flexible DC transmission system through a DC-blocking link, and combine the virtual resistance and the high-frequency damping voltages corresponding to the oscillation risk frequency points to generate an additional control signal;

[0143] ◎ Superimpose the additional control signal on the reference value of the converter bridge arm voltage, and obtain the modulated voltage through the modulation link;

[0144] ◎ Use the modulated voltage as the converter bridge arm voltage to complete the oscillation suppression of the oscillation risk frequency points on the DC side of the flexible DC transmission system.

[0145] Among them, the damping is calculated by the following calculation formula:

[0146]

[0147] In the formula, is the high-frequency damping voltage corresponding to the nth oscillation risk frequency point, f n is the frequency of the nth oscillation risk frequency point, K R is the resonance proportionality coefficient, ξ is the damping ratio, usually set to 0.707, ω n is the center frequency of the nth oscillation risk frequency point, s is the Laplace operator.

[0148] The above-mentioned weak damping or negative damping resonance frequency points existing on the DC side are assumed to be f1, f2,..., f n . To avoid possible oscillation instability phenomena, a resonance control strategy with good tracking effect for specific frequency signals can be considered. At Figure 2On the basis of the DC damping control shown above, a resonance control link for the frequency points with the risk of oscillation instability is further superimposed, effectively improving the damping characteristics at the resonance frequency points. The transfer function of the resonance control link is as shown in the calculation formula of the damping described above. The improved wide-band DC damping control strategy is as Figure 3 shown.

[0149] In summary, it is possible to locate the potential oscillation frequency points on the DC side of the system, improve the damping characteristics of the entire frequency band on the DC side of the flexible DC transmission system, effectively solve the wide-band oscillation problem on the DC side, and improve the stable operation ability of the system.

[0150] Applying the method proposed in this invention patent application has successfully solved the 25Hz and 520Hz oscillation problems that occurred on the DC side of the Xiamen flexible DC project. The DC impedance model of the Xiamen flexible DC project is as Figure 4 shown. The DC impedance model is divided into two subsystems: Subsystem 1 ( Figure 4 Subsystem1 in it) includes the DC impedance Z mmc1 (s) of the MMC1 converter station and the smoothing reactor L s ; Subsystem 2 ( Figure 4 Subsystem2 in it) contains the DC impedance Z mmc2 (s) of the MMC2 converter station, the smoothing reactor L s , and the impedance models of DC Line 1 and DC Line 2. The two DC line models can be represented by a single T-type lumped circuit through the calculation of formula (2), and the lumped circuit parameters are L dc1 , C dc1 and L dc2 , C dc2 . Without adding DC damping control, the impedance magnitude and phase characteristic curves of the two DC subsystems are as Figure 5 shown, Figure 5 where Frequency is the frequency, Hz is the unit of frequency hertz, Subsystem1 represents Subsystem 1, Subsystem2 represents Subsystem 2, Magnitude is the magnitude intensity, Ohm is the unit of importance degree, Phase is the phase, and DEG is the angle unit of the phase. There is a point with a small impedance magnitude at 25Hz, resulting in a large 25Hz oscillation component in the DC current, as Figure 6 shown, Figure 6 where Time is the time, the unit s is second, DCCurrent is the DC current, and the unit A is ampere.

[0151] Adding Figure 2After the DC damping control shown, the impedance amplitude at 25 Hz can be effectively increased, and the oscillation phenomenon at this frequency can be suppressed. However, due to the link delay of the control loop, high-frequency oscillations around 520 Hz are excited. The impedance amplitude and phase characteristic curves of the DC subsystem after adding the damping control are as Figure 7 shown, Figure 7 where Frequency is the frequency, Hz is the unit of frequency Hertz, Subsystem1 represents Subsystem 1, Subsystem2 represents Subsystem 2, Magnitude is the magnitude intensity, Ohm is the unit of importance Ohm, Phase is the phase, and DEG is the angular unit of the phase. The waveform of the DC current oscillation instability is as Figure 8 shown, Figure 8 where Time is the time, the unit s is seconds, DC Current is the DC current, and the unit A is amperes.

[0152] After adding Figure 3 the improved broadband DC damping control shown, the impedance amplitude at 25 Hz and the damping characteristics at around 520 Hz can be improved simultaneously, realizing the stable operation of the DC system. The impedance amplitude and phase characteristic curves of the DC subsystem after adding the improved broadband DC damping control are as Figure 9 shown, Figure 9 where Frequency is the frequency, Hz is the unit of frequency Hertz, Subsystem1 represents Subsystem 1, Subsystem2 represents Subsystem 2, Magnitude is the magnitude intensity, Ohm is the unit of importance Ohm, Phase is the phase, and DEG is the angular unit of the phase. The steady-state waveform of the DC current is as Figure 10 shown, Figure 10 where Time is the time, the unit s is seconds, DC Current is the DC current, and the unit A is amperes.

[0153] In summary, the method proposed in this invention patent application can effectively evaluate the oscillation risk in the broadband range of the system, increase the damping of the DC system, and improve the stable operation ability and harmonic suppression ability of the system; at the same time, no additional hardware circuit or power equipment is required, and only the corresponding control algorithm needs to be added to the flexible DC pole control protection or valve base control protection, and there is no excessive algorithm operation process, which has the advantage of being easy to implement; in addition, no start trigger signal is required and it can be always put into operation along with the operation of the control system.

[0154] Embodiment 2:

[0155] Based on the same inventive concept, this invention patent application provides a DC broadband oscillation suppression system for a flexible DC transmission system, as Figure 11 shown, including:

[0156] A broadband oscillation suppression module, which is used to suppress oscillations in the low, medium, and high frequency bands of the DC side of a flexible DC transmission system by using DC damping control;

[0157] A risk point determination module, which is used to obtain the oscillation risk frequency point based on the intersection point of the impedance amplitude curves of two pre-established DC impedance models of the subsystems in the high frequency band;

[0158] A risk point oscillation suppression module, which is used to suppress oscillations at the oscillation risk frequency point of the DC side of a flexible DC transmission system by using resonance control;

[0159] The two subsystems are divided based on a pre-established DC impedance model of a flexible DC transmission system. One subsystem is the DC impedance model of the converter station in the selected flexible DC transmission system, and the other subsystem is the equivalent aggregated DC impedance model of other parts in the flexible DC transmission system.

[0160] The risk point determination module is specifically used for:

[0161] Based on the impedance amplitude curves of two pre-established DC impedance models of the subsystems, obtain each intersection point of the two impedance amplitude curves in the high frequency band;

[0162] Based on the impedance phase difference of the two subsystem DC impedance models at each intersection point, determine whether there is an oscillation risk at each intersection point, and obtain the oscillation risk frequency point.

[0163] The risk point determination module determines whether there is an oscillation risk at each intersection point based on the impedance phase difference of the two subsystem DC impedance models at each intersection point, and obtains the oscillation risk frequency point, including:

[0164] Based on the difference in the impedance phases of the two subsystem DC impedance models at the intersection point, obtain a judgment difference;

[0165] Based on the magnitude relationship between the judgment difference and the set phase margin threshold, determine whether there is an oscillation risk at the intersection point.

[0166] The risk point determination module determines whether there is an oscillation risk at the intersection point based on the magnitude relationship between the judgment difference and the set phase margin threshold, including:

[0167] When the judgment difference is greater than the set phase margin threshold, it is determined that there is no oscillation risk at the intersection point;

[0168] When the judgment difference is not greater than the set phase margin threshold, it is determined that there is an oscillation risk at the intersection point.

[0169] The judgment difference in the risk point determination module is calculated by the following calculation formula:

[0170]

[0171] Wherein, is the judgment difference, ∠Z sys1 is the impedance phase at the intersection of the impedance model of the subsystem where the impedance model of the converter station in the selected flexible DC transmission system is located, ∠Z sys2 is the impedance phase at the intersection of the impedance models of the subsystems where the impedance models of other components in the flexible DC transmission system except the selected converter station are located.

[0172] The DC impedance model of the flexible DC transmission system includes: the high-frequency DC impedance models of each converter station and the DC line impedance models of each DC line.

[0173] The establishment process of the DC line impedance model includes:

[0174] Based on the parameters of the DC line, determine the equivalent number of segments of the DC line;

[0175] Based on the lumped parameters and the equivalent number of segments of the DC line, establish the DC line impedance model.

[0176] The parameters of the DC line include: the length of the DC line, the traveling wave speed, and the set precision frequency.

[0177] The equivalent number of segments of the DC line is calculated by the following calculation formula:

[0178]

[0179] Wherein, N line is the equivalent number of segments of the DC line, L line is the length of the DC line, υ is the traveling wave speed, f max is the set precision frequency, Ceil(·) is the rounding function.

[0180] The high-frequency DC impedance model of the converter station contains the following expression:

[0181]

[0182] Wherein, s is the Laplace operator, L arm is the arm inductance value of the converter station, R vir is the virtual resistance coefficient of the converter station, T d is the control system link delay of the converter station, is the high-frequency DC impedance of the converter station.

[0183] The broadband oscillation suppression module is specifically used for:

[0184] Eliminate the DC component in the DC current of the flexible DC transmission system through a DC blocking link, and generate an additional control signal in combination with a virtual resistance;

[0185] Superimpose an additional control signal on the converter arm voltage reference value, and obtain a modulation voltage through the modulation link;

[0186] Use the modulation voltage as the converter arm voltage to complete the oscillation suppression of the low, medium, and high frequency bands on the DC side of the flexible DC transmission system.

[0187] The risk point oscillation suppression module is specifically used for:

[0188] Based on the frequencies of each oscillation risk frequency point in the flexible DC transmission system, calculate the high-frequency damping voltage corresponding to each oscillation risk frequency point;

[0189] Eliminate the DC component in the DC current of the flexible DC transmission system through the DC blocking link, and combine the virtual resistance and the high-frequency damping voltage corresponding to each oscillation risk frequency point to generate an additional control signal;

[0190] Superimpose the additional control signal on the converter arm voltage reference value, and obtain a modulation voltage through the modulation link;

[0191] Use the modulation voltage as the converter arm voltage to complete the oscillation suppression of the oscillation risk frequency point on the DC side of the flexible DC transmission system.

[0192] The damping in the risk point oscillation suppression module is calculated by the following calculation formula:

[0193]

[0194] In the formula, is the high-frequency damping voltage corresponding to the nth oscillation risk frequency point, f n is the frequency of the nth oscillation risk frequency point, K R is the resonance proportionality coefficient, ξ is the damping ratio, ω n is the center frequency of the nth oscillation risk frequency point, and s is the Laplace operator.

[0195] Embodiment 3:

[0196] Based on the same inventive concept, this patent application for invention also provides a computer device, which includes a processor and a memory. The memory is used to store a computer program, and the computer program includes program instructions. The processor is used to execute the program instructions stored in the computer storage medium. The processor may be a Central Processing Unit (CPU), or may also be other general-purpose processors, Digital Signal Processors (DSPs), Application Specific Integrated Circuits (ASICs), Field-Programmable Gate Arrays (FPGAs), or other programmable logic devices, discrete gate or transistor logic devices, discrete hardware components, etc. It is the computing core and control core of the terminal, and is suitable for implementing one or more instructions. Specifically, it is suitable for loading and executing one or more instructions in the computer storage medium to implement the corresponding method flow or corresponding function, so as to implement the steps of a method for suppressing DC broadband oscillation in a flexible DC transmission system in the above embodiments.

[0197] Embodiment 4:

[0198] Based on the same inventive concept, this patent application for invention also provides a storage medium, specifically a computer-readable storage medium (Memory). The computer-readable storage medium is a memory device in a computer device and is used to store programs and data. It can be understood that the computer-readable storage medium here can include both the built-in storage medium in the computer device and, of course, the extended storage medium supported by the computer device. The computer-readable storage medium provides a storage space, and this storage space stores the operating system of the terminal. And, one or more instructions suitable for being loaded and executed by the processor are also stored in this storage space. These instructions can be one or more computer programs (including program codes). It should be noted that the computer-readable storage medium here can be a high-speed RAM memory or a non-volatile memory, such as at least one disk memory. The one or more instructions stored in the computer-readable storage medium can be loaded and executed by the processor to implement the steps of a method for suppressing DC broadband oscillation in a flexible DC transmission system in the above embodiments.

[0199] Those skilled in the art should understand that the embodiments of this invention patent application can be provided as a method, a system, or a computer program product. Therefore, this invention patent application can take the form of a completely hardware embodiment, a completely software embodiment, or an embodiment combining software and hardware aspects. Moreover, this invention patent application can take the form of a computer program product implemented on one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code.

[0200] This invention patent application is described with reference to the flowcharts and / or block diagrams of methods, devices (systems), and computer program products according to the embodiments of this invention patent application. It should be understood that each flow and / or block in the flowchart and / or block diagram, as well as the combination of flows and / or blocks in the flowchart and / or block diagram, can be implemented by computer program instructions. These computer program instructions can be provided to the processor of a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing devices to generate a machine, so that the instructions executed by the processor of the computer or other programmable data processing devices generate means for implementing the functions specified in Figure 1 one or more flows and / or blocks Figure 1 one or more blocks.

[0201] These computer program instructions can also be stored in a computer-readable memory that can direct a computer or other programmable data processing device to work in a specific manner, so that the instructions stored in the computer-readable memory generate a manufactured article including instruction means, and the instruction means implement the functions specified in Figure 1 one or more flows and / or blocks Figure 1 one or more blocks.

[0202] These computer program instructions can also be loaded onto a computer or other programmable data processing device, so that a series of operation steps are executed on the computer or other programmable device to generate a computer-implemented process. Thus, the instructions executed on the computer or other programmable device provide steps for implementing the functions specified in Figure 1 one or more flows and / or blocks Figure 1 one or more blocks.

[0203] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this invention patent application and not to limit its protection scope. Although the above embodiments have been described in detail with reference to this invention patent application, those of ordinary skill in the art should understand that after reading this invention patent application, various changes, modifications or equivalent substitutions can still be made to the specific implementation manners of the application. However, these changes, modifications or equivalent substitutions are all within the protection scope of the claims pending approval of the application.

Claims

1. A method for suppressing DC broadband oscillations in a flexible DC transmission system, characterized in that, Including: Adopting DC damping control to suppress oscillations in the low, medium, and high frequency bands on the DC side of a flexible DC transmission system; Obtaining the oscillation risk frequency points based on the intersection points of the impedance magnitude curves of two pre-established DC impedance models of the subsystems in the high frequency band; Adopting resonant control to suppress oscillations at the oscillation risk frequency points on the DC side of the flexible DC transmission system; The two subsystems are divided based on a pre-established DC impedance model of the flexible DC transmission system. One subsystem is the DC impedance model of the converter station in the selected flexible DC transmission system, and the other subsystem is the equivalent aggregated DC impedance model of the other parts in the flexible DC transmission system.

2. The method according to claim 1, characterized in that, The obtaining of the oscillation risk frequency points based on the intersection points of the impedance magnitude curves of two pre-established DC impedance models of the subsystems in the high frequency band includes: Based on the impedance magnitude curves of two pre-established DC impedance models of the subsystems, obtaining the intersection points of the two impedance magnitude curves in the high frequency band; Based on the impedance phase difference of the two DC impedance models of the subsystems at each intersection point, judging whether there is an oscillation risk at each intersection point and obtaining the oscillation risk frequency points.

3. The method according to claim 2, characterized in that, The judging whether there is an oscillation risk at each intersection point and obtaining the oscillation risk frequency points based on the impedance phase difference of the two DC impedance models of the subsystems at each intersection point includes: Based on the impedance phase difference of the two DC impedance models of the subsystems at the intersection point, obtaining the judgment difference; Based on the magnitude relationship between the judgment difference and the set phase margin threshold, judging whether there is an oscillation risk at the intersection point.

4. The method according to claim 3, characterized in that, The judging whether there is an oscillation risk at the intersection point based on the magnitude relationship between the judgment difference and the set phase margin threshold includes: When the judgment difference is greater than the set phase margin threshold, it is determined that there is no oscillation risk at the intersection point; When the judgment difference is not greater than the set phase margin threshold, it is determined that there is an oscillation risk at the intersection point.

5. The method according to claim 3, characterized in that, The judgment difference is calculated by the following calculation formula: Wherein, is the judgment difference, ∠Z sys1 is the impedance phase at the intersection point of the DC impedance model of the converter station in the selected flexible DC transmission system, ∠Z sys2 is the impedance phase at the intersection point of the equivalent aggregated DC impedance model of other parts in the flexible DC transmission system.

6. The method according to claim 1, characterized in that, The DC impedance model of the flexible DC transmission system includes: the high-frequency DC impedance models of each converter station and the DC line impedance models of each DC line.

7. The method according to claim 6, characterized in that, The establishment process of the DC line impedance model includes: Based on the parameters of the DC line, determining the equivalent number of segments of the DC line; Based on the lumped parameters and the equivalent number of segments of the DC line, establishing the DC line impedance model.

8. The method according to claim 7, characterized in that, The parameters of the DC line include: the length of the DC line, the traveling wave speed, and the set precision frequency.

9. The method according to claim 7, characterized in that, The equivalent number of segments of the DC line is calculated by the following calculation formula: Where N line is the equivalent number of segments of the DC line, L line is the length of the DC line, υ is the traveling wave velocity, f max is the set precision frequency, and Ceil(·) is the rounding function.

10. The method according to claim 6, characterized in that, The high-frequency DC impedance model of the converter station contains the following expression: Where s is the Laplacian operator, and L arm is the arm inductance value of the converter station, and R vir is the virtual resistance coefficient of the converter station, and T d is the control system link delay of the converter station, is the high-frequency DC impedance of the converter station.

11. The method according to claim 1, characterized in that, The adopting of DC damping control to suppress oscillations in the low, medium, and high frequency bands on the DC side of the flexible DC transmission system includes: Eliminating the DC component in the DC current of the flexible DC transmission system through a DC blocking link and generating an additional control signal in combination with a virtual resistor; Superimposing the additional control signal on the reference value of the converter bridge arm voltage and obtaining the modulated voltage through a modulation link; Using the modulated voltage as the converter bridge arm voltage to complete the suppression of oscillations in the low, medium, and high frequency bands on the DC side of the flexible DC transmission system.

12. The method according to claim 1, wherein, The adopting of resonant control to suppress oscillations at the oscillation risk frequency points on the DC side of the flexible DC transmission system includes: Calculate the high-frequency damping voltage corresponding to each oscillation risk frequency point based on the frequency of each oscillation risk frequency point in the flexible DC transmission system; Eliminate the DC component in the DC current of the flexible DC transmission system through a DC-blocking link, and combine a virtual resistor and the high-frequency damping voltage corresponding to each oscillation risk frequency point to generate an additional control signal; Superimpose the additional control signal on the converter arm voltage reference value, and obtain a modulated voltage through a modulation link; Use the modulated voltage as the converter arm voltage to complete the oscillation suppression of the oscillation risk frequency points on the DC side of the flexible DC transmission system.

13. The method according to claim 12, wherein, The high-frequency damping voltage is represented by the following calculation formula: Wherein, is the high-frequency damping voltage corresponding to the nth oscillation risk frequency point, f n is the frequency of the nth oscillation risk frequency point, K R is the resonance proportionality coefficient, ξ is the damping ratio, ω n is the center frequency of the nth oscillation risk frequency point, and s is the Laplace operator.

14. A DC broadband oscillation suppression system for a flexible DC power transmission system, wherein, Including: A broadband oscillation suppression module for suppressing oscillations in the low, medium, and high frequency bands on the DC side of the flexible DC transmission system using DC damping control; A risk point determination module for obtaining oscillation risk frequency points based on the intersection points of the impedance magnitude curves of two pre-established DC impedance models of the subsystems in the high frequency band; A risk point oscillation suppression module for suppressing oscillations at the oscillation risk frequency points on the DC side of the flexible DC transmission system using resonance control; The two subsystems are divided based on a pre-established DC impedance model of the flexible DC transmission system. One subsystem is the DC impedance model of the converter station in the selected flexible DC transmission system, and the other subsystem is the equivalent aggregated DC impedance model of other parts of the flexible DC transmission system.

15. The system according to claim 14, wherein, The risk point determination module is specifically used for: Based on the impedance magnitude curves of two pre-established DC impedance models of the subsystems, obtain each intersection point of the two impedance magnitude curves in the high frequency band; Based on the impedance phase difference of the two DC impedance models of the subsystems at each intersection point, determine whether there is an oscillation risk at each intersection point and obtain the oscillation risk frequency points.

16. The system according to claim 15, wherein, The risk point determination module determines whether there is an oscillation risk at each intersection point based on the impedance phase difference of the two DC impedance models of the subsystems at each intersection point and obtains the oscillation risk frequency points, including: Based on the difference in the impedance phases of the two DC impedance models of the subsystems at the intersection point, obtain a judgment difference; Based on the magnitude relationship between the judgment difference and a set phase margin threshold, determine whether there is an oscillation risk at the intersection point.

17. The system according to claim 16, wherein, The risk point determination module determines whether there is an oscillation risk at the intersection point based on the magnitude relationship between the judgment difference and a set phase margin threshold, including: When the judgment difference is greater than the set phase margin threshold, it is determined that there is no oscillation risk at the intersection point; When the judgment difference is not greater than the set phase margin threshold, it is determined that there is an oscillation risk at the intersection point.

18. The system according to claim 16, wherein, The judgment difference in the risk point determination module is calculated by the following calculation formula: Wherein, is the judgment difference, ∠Z sys1 is the impedance phase at the intersection point of the DC impedance model of the converter station in the selected flexible DC transmission system, ∠Z sys2 is the impedance phase at the intersection point of the equivalent aggregated DC impedance model of other parts in the flexible DC transmission system.

19. The system according to claim 14, wherein, The DC impedance model of the flexible DC transmission system includes: the high-frequency DC impedance models of each converter station and each DC line impedance model.

20. The system according to claim 19, wherein, The establishment process of the DC line impedance model includes: Based on the parameters of the DC line, determine the equivalent number of segments of the DC line; Based on the lumped parameters and the equivalent number of segments of the DC line, establish a DC line impedance model.

21. The system according to claim 20, wherein The parameters of the DC line include: the length of the DC line, the traveling wave speed, and a set precision frequency.

22. The system according to claim 20, wherein The equivalent number of segments of the DC line is calculated by the following calculation formula: Where N line is the equivalent number of segments of the DC line, L line is the length of the DC line, υ is the traveling wave velocity, f max is the set precision frequency, and Ceil(·) is the rounding function.

23. The system according to claim 19, wherein The high-frequency DC impedance model of the converter station includes the following expressions: where s is the Laplacian operator, L arm is the arm inductance value of the converter station, R vir is the virtual resistance coefficient of the converter station, T d is the control system link delay of the converter station, is the high-frequency DC impedance of the converter station.

24. The system according to claim 14, wherein The broadband oscillation suppression module is specifically configured to: Eliminate the DC component in the DC current of the flexible DC transmission system through a DC-blocking link, and generate an additional control signal in combination with a virtual resistor; Superimpose the additional control signal on the converter bridge arm voltage reference value, and obtain a modulated voltage through a modulation link; Use the modulated voltage as the converter bridge arm voltage to complete the oscillation suppression of the low, medium, and high frequency bands on the DC side of the flexible DC transmission system.

25. The system according to claim 14, wherein The risk point oscillation suppression module is specifically configured to: Calculate the high-frequency damping voltage corresponding to each oscillation risk frequency point based on the frequency of each oscillation risk frequency point in the flexible DC transmission system; Eliminate the DC component in the DC current of the flexible DC transmission system through a DC-blocking link, and generate an additional control signal in combination with a virtual resistor and the high-frequency damping voltage corresponding to each oscillation risk frequency point; Superimpose the additional control signal on the converter bridge arm voltage reference value, and obtain a modulated voltage through a modulation link; Use the modulated voltage as the converter bridge arm voltage to complete the oscillation suppression of the oscillation risk frequency points on the DC side of the flexible DC transmission system.

26. The system according to claim 25, wherein The damping in the risk point oscillation suppression module is calculated by the following calculation formula: In the formula, is the high-frequency damping voltage corresponding to the nth oscillation risk frequency point, f n is the frequency of the nth oscillation risk frequency point, K R is the resonance proportionality coefficient, ξ is the damping ratio, ω n is the center frequency of the nth oscillation risk frequency point, and s is the Laplace operator.

27. A computer device, wherein Comprising: One or more processors; A memory for storing one or more programs; When the one or more programs are executed by the one or more processors, a method for suppressing DC broadband oscillations in a flexible DC transmission system according to any one of claims 1 to 13 is implemented.

28. A computer-readable storage medium, wherein A computer program is stored thereon, and when the computer program is executed, a method for suppressing DC broadband oscillations in a flexible DC transmission system according to any one of claims 1 to 13 is implemented.

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