Method and system for determining frequency points at which a flexible direct system has a risk of generating high-frequency oscillations

Abstract

The application discloses a kind of method and system for determining the reason of high-frequency oscillation of flexible direct current transmission system, method is suitable for when at least two converter units are operated in parallel, high-frequency oscillation is generated in flexible direct current system, comprising: establishing the impedance model of converter, obtaining the impedance Z MMC Of converter;Establish the impedance model of alternating current power grid, obtain the impedance Z g Of alternating current power grid;Based on Nyquist stability criterion, the phase difference of alternating current power grid impedance and converter impedance Z MMC Z g The absolute value of ratio is equal to 1 at the frequency point, the absolute value |∠Z MMC ∠Z g | of the phase difference of alternating current power grid impedance and converter impedance Z MMC More than or equal to 180 degrees, and flexible direct current transmission system will generate high-frequency oscillation;By scanning and calculating the impedance of converter, the high-frequency fluctuation of the impedance phase-frequency characteristic of converter is obtained, and the impedance phase of converter fluctuates up and down in the range close to 90 degrees by high-frequency fluctuation.Analysis shows that the high-frequency fluctuation of converter impedance phase is generated by cross-coupling component in converter model.

Description

Technical Field

[0001] This invention relates to the field of flexible direct current transmission technology, and more specifically, to a method and system for determining the frequency points in a flexible direct current system that pose a risk of high-frequency oscillations. Background Technology

[0002] Flexible direct current (DC) transmission technology effectively solves the problem of commutation in traditional DC systems that rely on external power sources, preventing commutation failures. It features flexible structure, high controllability, and low output harmonics, and has been widely used in my country's large-capacity, long-distance power transmission systems. With the increasing number of flexible DC projects in operation, the voltage and capacity levels of individual converter stations have increased from the initial ±30kV / 18MW to ±800kV / 5000MW. Converter stations have also been upgraded from connecting to 35kV distribution networks to 500kV main grids. The stability issues brought about by the integration of flexible DC systems into the grid are becoming increasingly prominent. Low-frequency and high-frequency oscillations have emerged in applications such as renewable energy integration, urban power supply, and large-scale grid interconnection.

[0003] However, after flexible DC is connected to the system, it exhibits capacitive impedance and negative damping near the system harmonics in the low-frequency band, and "inductive" impedance and negative damping near the system harmonics in the high-frequency band. This can easily induce system oscillations and cause system instability, which has already attracted attention.

[0004] The oscillation mechanism of flexible DC-DC transmission systems is still under investigation. Previous studies have not identified any problems with the oscillation phenomena observed in existing flexible DC-DC projects; these oscillations are attributed to system resonance occurring while equipment is energized and transmitting power. Different solutions have been implemented based on the observed phenomena. From this perspective, the oscillation mechanism of flexible DC-DC transmission systems is not yet fully understood, making it difficult to clearly explain the oscillation problems encountered in current projects and to clearly understand the oscillation risks of future flexible DC-DC projects. Summary of the Invention

[0005] The present invention provides a method and system for determining the frequency points at which a flexible CRT system may generate high-frequency oscillations, thereby solving the problem of how to determine the frequency points at which a flexible CRT system may generate high-frequency oscillations.

[0006] To address the aforementioned problems, this invention provides a method for determining the frequency points at which a flexible DC system is at risk of generating high-frequency oscillations. This method is applicable when at least two converter units are operating separately, and the flexible DC system generates high-frequency oscillations, including:

[0007] Establish the impedance model of the converter and obtain the converter impedance Z. MMC ;

[0008] Establish an impedance model for the AC power grid and obtain the AC power grid impedance Z. g ;

[0009] Based on the Nyquist stability criterion, the phase difference between the AC grid impedance Z MMC and the converter impedance Z g is equal to 1, the frequency point at which the AC grid impedance and the converter impedance Z MMC have a phase difference whose absolute value |∠Z g -∠Z MMC is greater than or equal to 180°, the flexible HVDC power transmission system generates high-frequency oscillation.

[0010] By scanning and calculating the impedance of the converter, the high-frequency fluctuation of the impedance phase-frequency characteristic of the converter is obtained, the high-frequency fluctuation makes the impedance phase of the converter fluctuate up and down within a preset angle range, and the high-frequency fluctuation is caused by the cross-coupling component in the converter, so that the phase difference between the AC grid impedance Z MMC and the converter impedance Z g is equal to 1, the frequency point at which the AC grid impedance and the converter impedance Z MMC have a phase difference whose absolute value |∠Z MMC -∠Z g is greater than or equal to 180°, and the flexible HVDC power transmission system generates high-frequency oscillation at these frequency points.

[0011] Preferably, the impedance phase-frequency characteristic of the converter has a high-frequency fluctuation, and the small-amplitude high-frequency fluctuation decreases with the increase of frequency.

[0012] Preferably, the preset angle range includes: greater than -180° and less than 180°.

[0013] Preferably, the establishment of the impedance model of the converter, the acquisition of the converter impedance Z MMC further includes:

[0014] The amplitude-frequency characteristic and the phase-frequency characteristic of the converter impedance are obtained by frequency scanning;

[0015] The mathematical model of the converter impedance is obtained by mathematical fitting of the impedance characteristic;

[0016] The amplitude-frequency characteristic and the phase-frequency characteristic of the converter impedance Z MMC are obtained by mathematical calculation;

[0017] Preferably, the establishment of the impedance model of the AC grid, the acquisition of the AC grid impedance Z g further includes:

[0018] The amplitude-frequency characteristic and the phase-frequency characteristic of the AC grid impedance are obtained by frequency scanning;

[0019] The mathematical model of the AC power grid impedance is obtained by mathematical fitting of the impedance characteristics.

[0020] The amplitude-frequency characteristics and phase-frequency characteristics of the AC power grid impedance Z g are obtained by mathematical calculation.

[0021] Preferably, the ratio calculation and phase difference calculation of the converter impedance Z MMC and the AC power grid impedance Z g are performed, and then the Nyquist stability criterion is used to determine the frequency points at which the HVDC system has the risk of generating high-frequency oscillation.

[0022] Preferably, the above method can be used to find out the reason for the high-frequency oscillation of the two-converter parallel operation system.

[0023] Based on another aspect of the present application, the present application provides a system for determining the frequency points at which the HVDC system has the risk of generating high-frequency oscillation, which is suitable for determining the reason for the high-frequency oscillation of the HVDC system when at least two converter units are operated in parallel, and comprises:

[0024] An initial unit is configured to establish the impedance model of the converter to obtain the converter impedance Z MMC , and establish the impedance model of the AC power grid to obtain the AC power grid impedance Z g .

[0025] A judgment unit is configured to determine, based on the Nyquist stability criterion, that the HVDC system generates high-frequency oscillation when the ratio of the converter impedance Z MMC to the AC power grid impedance Z g equals 1, the absolute value of the phase difference between the AC power grid impedance and the converter impedance Z MMC |∠Z g -∠Z MMC | is greater than or equal to 180°.

[0026] A result unit is configured to obtain the high-frequency fluctuation of the phase-frequency characteristics of the converter impedance by scanning and calculating the converter impedance, wherein the high-frequency fluctuation causes the phase of the converter impedance to fluctuate up and down by 90°, and the high-frequency fluctuation is caused by the cross-coupling component in the converter, resulting in the absolute value of the phase difference between the AC power grid impedance and the converter impedance Z MMC |∠Z g -∠Z MMC | being greater than or equal to 180°, and the HVDC system generating high-frequency oscillation.

[0027] Preferably, the phase-frequency characteristics of the converter impedance have high-frequency fluctuation, wherein the small-amplitude high-frequency fluctuation decreases with the increase of the frequency.

[0028] Preferably, the impedance phase range of the converter comprises: greater than -180° and less than 180°.

[0029] Preferably, the initial unit is configured to establish an impedance model of the converter, and obtain an impedance Z of the converter MMC , and is further configured to:

[0030] obtain an impedance characteristic of the converter through frequency scanning;

[0031] obtain a mathematical model of the impedance of the converter through fitting of the impedance characteristic;

[0032] obtain a frequency characteristic of the impedance Z of the converter through theoretical calculation based on the mathematical model. MMC

[0033] Preferably, the initial unit is configured to establish an impedance model of the AC power grid, and obtain an impedance Z of the AC power grid g , and is further configured to:

[0034] obtain an impedance characteristic of the converter through frequency scanning;

[0035] obtain a mathematical model of the impedance of the AC power grid through fitting of the impedance characteristic;

[0036] obtain a frequency characteristic of the impedance Z of the AC power grid through theoretical calculation based on the mathematical model. g

[0037] Preferably, the judging unit is further configured to perform ratio calculation and phase difference calculation of the impedance Z of the converter MMC and the impedance Z of the AC power grid g , and determine, according to Nyquist stability criterion, frequency points at which the flexible DC system has a risk of generating high-frequency oscillation.

[0038] The technical scheme of the present application provides a method and system for determining frequency points at which a flexible DC system has a risk of generating high-frequency oscillation. The method and system are aimed at system high-frequency oscillation occurring in the two converter units of the south channel of the Chongqing-Emergency flexible DC project. The method and system clarify the reason for the high-frequency oscillation occurring in the two converter units in a split operation mode, and determine that the high-frequency oscillation is caused by cross-coupling components of the converter. The technical scheme of the present application provides technical support for solving the system oscillation occurring in the two converter units of the flexible DC system in a split operation (independent operation) mode. BRIEF DESCRIPTION OF DRAWINGS

[0039] The exemplary embodiments of the present application can be more completely understood by referring to the following drawings:

[0040] Figure 1 FIG. 1 is a flowchart of a method for determining frequency points at which a flexible DC system has a risk of generating high-frequency oscillation according to a preferred embodiment of the present application;

[0041] ​​Figure 2 Impedance phase frequency characteristic diagram obtained by frequency scanning according to the preferred embodiment of the present application;

[0042] Figure 3 Impedance phase frequency characteristic diagram obtained by simulation calculation according to the preferred embodiment of the present application; and

[0043] Figure 4 System structure diagram for determining a frequency point at which a flexible direct current system exists a risk of generating high-frequency oscillation according to the preferred embodiment of the present application. DETAILED DESCRIPTION

[0044] Reference will now be made to the drawings to describe the exemplary embodiments of the present application in greater detail. The present application can be variously embodied and is not limited to the embodiments described herein, which are provided for the purpose of fully and completely disclosing the present application and to convey the full scope of the present application to those skilled in the art. The terms used in the exemplary embodiments of the present application shown in the drawings are terms that are used to describe the present application and are not intended to limit the present application. In the drawings, the same elements are denoted by the same reference numerals.

[0045] Unless otherwise defined, the terms (including technical terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art. In addition, it is to be understood that the terms defined by commonly used dictionaries are to be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and are not to be interpreted in an idealized or overly formal sense unless expressly so defined herein.

[0046] Figure 1 Flow chart of a method for determining a frequency point at which a flexible direct current system exists a risk of generating high-frequency oscillation according to the preferred embodiment of the present application. The present application proposes a method for determining a frequency point at which a flexible direct current system exists a risk of generating high-frequency oscillation, which is a conclusion obtained by analyzing the mechanism of high-frequency oscillation in the Yueside of the Yueshan flexible direct current south passage. The method can provide a reference for analyzing the cause of high-frequency oscillation in other flexible direct current projects with the same operation mode.

[0047] The engineering background of the present application is that, during the Yueshan back-to-back flexible direct current south passage system debugging, when the Yueside section loss-of-field test was performed, the in-line switch at Shizhou station was opened, forming two busbars each carrying two converter units and operating separately. Thus, two converter units were open-loop operated in the Yueside, and system oscillation with a 13 times frequency of the main frequency, i.e., about 665 Hz, as the main frequency occurred. In order not to affect the operation of the Yueshan south passage and the system debugging of the project, it was determined through research that, in the two converter unit open-loop operation mode, if system oscillation is detected, the direct current control system blocks one unit, and the other unit continues to operate.

[0048] The present invention provides a method for determining the frequency points at which a flexible CRT system has the risk of generating high-frequency oscillations, providing technical support for analyzing and solving the problem of high-frequency resonance in flexible CRT systems.

[0049] To solve the above problems, such as Figure 1 As shown, this invention provides a method for determining the frequency points at which a flexible DC system has the risk of generating high-frequency oscillations. The method is applicable when at least two converter units are operating separately, causing the flexible DC system to generate high-frequency oscillations. The method includes:

[0050] Step 101: Establish the impedance model of the converter and obtain the converter impedance Z. MMC Preferably, an impedance model of the converter is established to obtain the converter impedance Z. MMC It also includes:

[0051] The amplitude-frequency and phase-frequency characteristics of the converter impedance are obtained by frequency scanning;

[0052] A mathematical model of the converter impedance is derived by mathematical fitting of the impedance characteristics.

[0053] The converter impedance Z is obtained through mathematical calculation. MMC The amplitude-frequency and phase-frequency characteristics.

[0054] This invention first establishes a converter impedance model, and then obtains the converter impedance Z through theoretical calculations. MMC Then, it is compared with the converter impedance obtained by frequency scanning, and then corrected to obtain the accurate converter impedance Z. MMC .

[0055] Step 102: Establish the impedance model of the AC power grid and obtain the AC power grid impedance Z. g ;

[0056] Preferably, an impedance model of the AC power grid is established to obtain the AC power grid impedance Z. g It also includes:

[0057] The amplitude-frequency and phase-frequency characteristics of the AC power grid impedance are obtained by frequency scanning;

[0058] A mathematical model of AC power grid impedance is obtained by mathematical fitting of impedance characteristics;

[0059] The AC power grid impedance Z is obtained through mathematical calculation. g The amplitude-frequency and phase-frequency characteristics.

[0060] Step 103: Based on the Nyquist stability criterion, at the converter impedance Z... MMC With AC power grid impedance Z g At a frequency point where the ratio equals 1, the AC grid impedance and the converter impedance Z MMCthe absolute value of the phase difference of Z g -∠Z MMC | is greater than or equal to 180°, the flexible DC power transmission system generates high-frequency oscillation;

[0061] The present application applies the Nyquist stability criterion to the Z g / Z MMC The frequency point at which the amplitude of Z g -∠Z MMC | is greater than or equal to 180°, the system generates high-frequency oscillation, otherwise the system does not generate high-frequency oscillation.

[0062] Step 104: Obtain the high-frequency fluctuation of the impedance phase-frequency characteristic of the converter by scanning and calculating the impedance of the converter. In some frequency bands, the high-frequency fluctuation makes the impedance phase of the converter fluctuate up and down at 90°. According to the derived mathematical model expression of the converter impedance, it can be judged that the high-frequency fluctuation is caused by the cross-coupling component of the converter. The phase fluctuation greater than 90° deteriorates the frequency characteristic of the converter, and makes the phase difference between the AC power grid impedance and the converter impedance Z MMC the absolute value of the phase difference of Z g -∠Z MMC | is greater than or equal to 180°, if there is a frequency point at which the amplitude of Z g / Z MMC equals 1, the flexible DC power transmission system generates high-frequency oscillation. The application occasion frequency band in the present application is 750Hz or below, and the specific frequency band range is adjusted according to different occasions.

[0063] Preferably, the impedance phase-frequency characteristic of the converter has high-frequency fluctuation, wherein the small-amplitude high-frequency fluctuation decreases with the increase of frequency.

[0064] Preferably, the preset angle range includes: greater than -180° and less than 180°.

[0065] The present application can thus determine the reason for high-frequency oscillation of two converter units operating in separate columns: flexible DC control loop link delay, feedforward voltage and cross-coupling component.

[0066] This invention provides a method for determining the frequency points at which high-frequency oscillations are likely to occur in a flexible DC system. The method includes: obtaining the converter impedance and the connected AC grid impedance through frequency scanning and theoretical calculations, and then obtaining mathematical expressions for the amplitude-frequency and phase-frequency characteristics of the converter impedance and the AC line, facilitating impedance characteristic analysis. Based on the amplitude-frequency and phase-frequency characteristics of the two converter impedances and the AC line, this invention derives the impedance ratio and phase difference at the high-frequency resonance point of the flexible DC system. Through the amplitude-frequency and phase-frequency characteristics, as well as the impedance ratio and phase difference, this invention shows that the converter impedance phase-frequency characteristic exhibits high-frequency fluctuations, and these small high-frequency fluctuations gradually weaken as the frequency increases. This fluctuation causes the converter impedance phase to fluctuate around 90°, resulting in intermittent deterioration of the phase-frequency characteristics of the converter impedance in this frequency band. The mathematical model expression of the converter impedance determines that the cause of the deterioration in the converter impedance characteristics is the cross-coupling component of the converter, providing technical support for analyzing and solving the high-frequency resonance problem in flexible DC systems.

[0067] In this invention, under the condition of two converter units operating separately in a flexible DC converter station, the converter impedance phase frequency characteristic exhibits high-frequency fluctuations. These small high-frequency fluctuations gradually weaken as the frequency increases. This fluctuation causes the converter impedance phase to fluctuate around 90°, resulting in intermittent deterioration of the converter impedance phase frequency characteristic in this frequency band. Analysis of the converter model characteristics confirms that the cause of this deterioration is the cross-coupling component. Therefore, the main reasons for high-frequency resonance in the flexible DC system are the flexible DC control loop link delay, feedforward voltage, and cross-coupling components. The high-frequency oscillations occurring on the Chongqing side of the Chongqing-Ezhou flexible DC southern corridor, where the two converter units operate separately, occurred even after considering the effects of control loop link delay and feedforward voltage and implementing corresponding control measures. Therefore, the high-frequency oscillations caused by the separate operation of the two converter units are due to high-frequency fluctuations in the converter phase frequency characteristic caused by the cross-coupling component, leading to a deterioration in the converter impedance characteristic.

[0068] The converter impedance frequency sweep and simulation results verified this conclusion. Figure 2 This is the frequency scan result. Figure 3 The results are from simulation calculations. The horizontal axis in the figure is in Hz, and the vertical axis is in phase angle.

[0069] In the established mathematical model of converter impedance, if the cross-coupling component in the converter impedance is removed, the small-amplitude fluctuation of the phase frequency characteristic of the converter disappears, and the frequency of the phase frequency characteristic fluctuation in the impedance mathematical model is consistent with the frequency of the phase frequency characteristic fluctuation of the converter obtained by frequency scanning.

[0070] The method for determining the frequency point at which the flexible direct current system has a high-frequency oscillation risk provided in the present application is to establish the impedance model of the converter and the connected alternating current grid, apply the Nyquist stability criterion, obtain the amplitude-frequency characteristic and the phase-frequency characteristic of the high-frequency resonance point of the flexible direct current system, and determine that the high-frequency fluctuation of the phase-frequency characteristic of the converter at 90 degrees causes the intermittent damping characteristic deterioration of the phase-frequency characteristic of the converter impedance in the frequency band, and the high-frequency fluctuation is caused by the cross-coupling component of the converter. Based on the engineering background of the system high-frequency oscillation occurring in the two converter units of the Yuzhou-Ezhou flexible direct current south channel in the split operation mode, it is proposed that the reason for the high-frequency oscillation of the flexible direct current system in the split operation mode of the two converter units is that the cross-coupling component of the converter control causes the high-frequency oscillation.

[0071] The Yuzhou-Ezhou ± 420 kV direct current back-to-back flexible direct current networking project is divided into a south channel and a north channel, the south channel is a Shizhou direct current, and the north channel is a Yichang direct current. Each direct current converter station comprises two converter units, and a single pole symmetric topology structure is adopted. Each converter unit has a rated capacity of 1250 MW, and the total transmission capacity of the four converter units of the south and north channels is 5000 MW. The Yuzhou-Ezhou project was put into operation in 2019.

[0072] The present application provides a method for determining the reason for high-frequency oscillation of a flexible direct current system, and clarifies the reason for the system oscillation problem in the split operation (independent operation) mode of the two converter units of the flexible direct current, thereby providing technical support for solving the system oscillation in the split operation (independent operation) mode of the two converter units of the flexible direct current.

[0073] Figure 4 It is a schematic diagram of the main circuit of the flexible direct current converter.

[0074] The operation steps of the embodiment of the present application are as follows:

[0075] Step S1: The accurate mathematical model of the converter impedance and the connected alternating current grid impedance is obtained through frequency scanning and theoretical calculation;

[0076] Step S2: When the two converter units of the Yuzhou side of the Shizhou converter station of the Yuzhou-Ezhou south channel are in the split operation mode, the amplitude-frequency characteristic and the phase-frequency characteristic of the converter impedance and the connected alternating current grid impedance are obtained through simulation, and the impedance ratio and the phase difference of the high-frequency oscillation point are determined;

[0077] Step S3: It is obtained that the phase-frequency characteristic of the converter impedance has high-frequency fluctuation, and the small-amplitude high-frequency fluctuation gradually weakens as the frequency increases, and the fluctuation causes the phase of the converter impedance to fluctuate up and down near 90 degrees, so that the phase-frequency characteristic of the converter impedance in the frequency band has intermittent damping characteristic deterioration;

[0078] Step S4: It is determined from the accurate model of the converter impedance that the reason for the deterioration of the converter impedance characteristic is the cross-coupling component.

[0079] Step S5: When the two converter units are determined to be in the partial column operation mode, the factors causing the high-frequency oscillation of the flexible DC system are: the link delay of the flexible DC control loop, the feedforward voltage and the cross-coupling component.

[0080] Step S6: Providing technical support for analyzing and solving the high-frequency resonance problem of the flexible DC system.

[0081] The present application determines the reasons for the high-frequency oscillation of the flexible DC system after the two converter units are in the partial column operation mode, and provides a method for determining the reasons for the high-frequency oscillation of the flexible DC system. The present application determines the reasons for the high-frequency oscillation of the two converter units in the partial column operation mode of the Yueside of the Nantong channel of the Yueshan flexible DC system, and provides technical support for solving the high-frequency oscillation.

[0082] Figure 4 A system structure diagram for determining the reasons for the high-frequency oscillation of the flexible DC system according to the preferred embodiment of the present application. As shown in Figure 4 , the present application provides a system for determining the reasons for the high-frequency oscillation of the flexible DC system, comprising:

[0083] An initial unit 401 is configured to establish an impedance model of the converter, obtain the impedance Z MMC of the converter, establish an impedance model of the AC power grid, and obtain the impedance Z g of the AC power grid.

[0084] Preferably, the initial unit 401 is configured to establish an impedance model of the converter, obtain the impedance Z MMC of the converter, and is further configured to:

[0085] obtain the impedance characteristics of the converter through frequency scanning;

[0086] obtain a mathematical model of the impedance of the converter through fitting of the impedance characteristics;

[0087] obtain the frequency characteristics of the impedance Z MMC of the converter through theoretical calculation based on the mathematical model.

[0088] Preferably, the initial unit 401 is configured to establish an impedance model of the AC power grid, obtain the impedance Z g of the AC power grid, and is further configured to:

[0089] obtain the impedance characteristics of the converter through frequency scanning;

[0090] obtain a mathematical model of the impedance of the AC power grid through fitting of the impedance characteristics;

[0091] obtain the frequency characteristics of the impedance Z g of the AC power grid through theoretical calculation based on the mathematical model.

[0092] Judgment unit 402 is used to determine the converter impedance Z based on the Nyquist stability criterion. MMC With AC power grid impedance Z g At a frequency point where the ratio equals 1, the AC grid impedance and the converter impedance Z MMC The absolute value of the phase difference |∠Z g -∠Z MMC When the angle is greater than or equal to 180°, the flexible DC transmission system will generate high-frequency oscillations.

[0093] Result unit 403 is used to obtain the high-frequency fluctuations of the phase frequency characteristics of the converter impedance by scanning and calculating the converter impedance. The high-frequency fluctuations cause the phase of the converter impedance to fluctuate around 90°. The high-frequency fluctuations are generated by the cross-coupling components in the converter, causing the converter to make the AC grid impedance and the converter impedance Z... MMC The absolute value of the phase difference |∠Z g -∠Z MMC When the angle is greater than or equal to 180°, the flexible DC transmission system will generate high-frequency oscillations.

[0094] Preferably, the phase frequency characteristic of the converter impedance has high-frequency fluctuations, wherein small high-frequency fluctuations weaken as the frequency increases.

[0095] Preferably, it further includes: the impedance phase range of the converter includes: greater than -180° and less than 180°.

[0096] Preferably, the judgment unit is also used to determine the converter impedance Z. MMC With AC power grid impedance Z g The ratio and phase difference are calculated, and then the Nyquist stability criterion is used to determine at which frequency points the flexible DC system is at risk of generating high-frequency oscillations.

[0097] The invention has been described with reference to a few embodiments. However, as will be known to those skilled in the art, and as defined in the appended claims, other embodiments besides those disclosed above fall equivalently within the scope of the invention.

[0098] Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless otherwise expressly defined herein. All references to “a / / the [device, component, etc.]” ​​are openly interpreted as at least one instance of the device, component, etc., unless otherwise expressly stated. The steps of any method disclosed herein are not necessarily to be performed in the exact order disclosed, unless explicitly stated otherwise.

Claims

1.A method for determining the frequency point at which a HVDC system is at risk of generating high frequency oscillation, the method is applicable when at least two converter units are operating in partial column, comprising: obtaining the impedance characteristic of the converter by frequency scanning; fitting the impedance characteristic to obtain a mathematical model of the converter impedance; obtaining the impedance characteristic of the converter by frequency scanning; fitting the impedance characteristic to obtain a mathematical model of the AC grid impedance; 2.The method of claim 1, wherein the phase-frequency characteristic of the converter impedance has high frequency fluctuations, and the amplitude of the high frequency fluctuations decreases as the frequency increases. The phase range of the converter impedance includes: greater than -180° and less than 180°. An impedance model of the converter is established to obtain the impedance Z of the converter MMC and further comprising: 4.A system for determining the frequency point at which a HVDC system is at risk of generating high frequency oscillation, the system is applicable when at least two converter units are operating in partial column, comprising: 5.The system of claim 4, wherein the phase-frequency characteristic of the converter impedance has high frequency fluctuations, and the amplitude of the high frequency fluctuations decreases as the frequency increases. The phase range of the converter impedance includes: greater than -180° and less than 180°. ​ Theoretical calculation is made by the mathematical model to obtain frequency characteristics of the converter impedance Z MMC ; establishing an impedance model of the alternating current power grid to obtain an impedance Z of the alternating current power grid g and further comprising: ​ ​ Theoretically calculating the frequency characteristic of the AC power grid impedance Z g by the mathematical model; The converter impedance Z MMC The AC grid impedance Z g The ratio calculation and phase difference calculation, and according to the Nyquist stability criterion, determine which frequency points of the flexible direct system exist the risk of generating high frequency oscillation; Based on the Nyquist stability criterion, the phase difference between the AC grid impedance Z MMC and the converter impedance Z g at the frequency point where the ratio equals 1 is denoted as ∠Z MMC . g The absolute value of the phase difference |∠Z MMC | is greater than or equal to 180°, the HVDC system generates high-frequency oscillation. By scanning and calculating the impedance of the converter, the high-frequency fluctuation of the phase-frequency characteristic of the impedance of the converter is obtained, the high-frequency fluctuation makes the impedance phase of the converter fluctuate up and down at 90°, the high-frequency fluctuation is generated by the cross-coupling component in the converter, causing the phase difference between the impedance of the converter and the impedance of the alternating current grid Z MMC of the converter to be greater than or equal to 180°, and the flexible direct current power transmission system generates high-frequency oscillation. g -∠Z MMC | ​ 3. The method of claim 1, further comprising: ​ ​ An initial unit is configured to establish an impedance model of the converter, and obtain an impedance Z of the converter MMC , and is further configured to: obtain an impedance characteristic of the converter through frequency scanning; obtain a mathematical model of the impedance of the converter through fitting of the impedance characteristic; and obtain a frequency characteristic of the impedance Z of the converter through theoretical calculation based on the mathematical model MMC An initial unit is configured to establish an impedance model of the AC power grid, and obtain an impedance Z of the AC power grid g , and is further configured to: obtain an impedance characteristic of the converter through frequency scanning; obtain a mathematical model of the impedance of the converter through fitting of the impedance characteristic; and obtain a frequency characteristic of the impedance Z of the converter through theoretical calculation based on the mathematical model g An initial unit is configured to establish an impedance model of the AC power grid, and obtain an impedance Z of the AC power grid g , and is further configured to: obtain an impedance characteristic of the converter through frequency scanning; obtain a mathematical model of the impedance of the converter through fitting of the impedance characteristic; and obtain a frequency characteristic of the impedance Z of the converter through theoretical calculation based on the mathematical model g A judgment unit is used to determine the converter impedance Z based on the Nyquist stability criterion. MMC With the AC power grid impedance Z g At a frequency point where the ratio equals 1, the AC grid impedance and the converter impedance Z MMC The absolute value of the phase difference ∠Z g -∠Z MMC | When the angle is greater than or equal to 180°, the flexible DC transmission system generates high-frequency oscillations; the judgment unit is also used to determine the converter impedance Z. MMC With AC power grid impedance Z g The ratio and phase difference are calculated, and then the Nyquist stability criterion is used to determine at which frequency points the flexible DC system is at risk of generating high-frequency oscillations. The result unit is configured to obtain high-frequency fluctuation of the phase-frequency characteristic of the impedance of the converter by scanning and calculating the impedance of the converter, the high-frequency fluctuation causes the phase of the impedance of the converter to fluctuate up and down by 90°, the high-frequency fluctuation is caused by the cross-coupling component in the converter, and causes the phase difference between the impedance of the AC power grid and the impedance Z of the converter to be greater than or equal to 180°, and the HVDC power transmission system generates high-frequency oscillation. MMC -∠Z g -∠Z MMC | ​ 6. The system of claim 4, further comprising: ​

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