High-frequency oscillation suppression method and system for flexible direct-current power grid interface converter station

Abstract

The invention provides a high-frequency oscillation suppression method and system for a flexible direct-current power grid interface converter station, and is applied to the technical field of flexible direct-current power transmission. The method comprises the steps of sampling operation data of an alternating current side of a flexible direct current power grid interface converter station, and determining whether the flexible direct current power grid generates high-frequency oscillation based on frequency domain characteristics of the operation data; if the flexible DC power grid generates high-frequency oscillation, determining a high-frequency oscillation point based on the operation data; based on the incidence relation between the filtering parameter of the filtering branch and the impedance characteristic of the interface converter station and the high-frequency oscillation mechanism of the flexible direct-current power grid, screening out a target filtering parameter which enables the interface converter station of the flexible direct-current power grid to have a positive damping characteristic at the high-frequency oscillation point; and controlling the filtering branch to operate in the phase-locked loop based on the target filtering parameter so as to suppress high-frequency oscillation of the flexible DC power grid interface converter station. The problem that the suppression effect of high-frequency oscillation of the flexible direct-current power grid is poor is solved.

Description

Technical Field

[0001] The present invention relates to the technical field of flexible direct current power transmission, and in particular to a method and system for suppressing high-frequency oscillation of a flexible direct current grid interface converter station. Background Art

[0002] With the rise of multi-terminal flexible DC transmission technology, DC transmission networks have become increasingly complex. The flexible DC grid formed by this can achieve power transmission of multiple AC systems with fewer converters and has high operational flexibility and reliability. At present, the Zhangbei Flexible DC Grid Project has been completed and put into operation. However, with the widespread application of flexible DC grid projects, its potential problems have gradually emerged, such as high-frequency oscillation problems. Since there are multiple capacitive frequency intervals in the AC system, high-frequency oscillations are easily induced. For example, the high-frequency oscillations of about 1500Hz that occur on the AC side of the Zhangbei Flexible DC Grid Project, if not suppressed, may cause the converter station to lock and the equipment to stop operating, and then cause a power outage, which seriously threatens the normal operation of the flexible DC grid.

[0003] At present, the suppression measures for high-frequency oscillation of flexible DC grids are mostly based on the inner-loop control link, and their suppression effect is limited, which cannot guarantee the suppression effect of high-frequency oscillation of flexible DC grids. Summary of the invention

[0004] In order to overcome the problem of poor suppression effect of high-frequency oscillation in the above-mentioned flexible DC grid, the present invention provides a method and system for suppressing high-frequency oscillation in a flexible DC grid interface converter station.

[0005] In one aspect, the present invention provides a method for suppressing high-frequency oscillation of a flexible DC grid interface converter station, wherein the interface converter station comprises a phase-locked loop, wherein the phase-locked loop comprises a filter branch connected in parallel with a PI controller, and wherein the method comprises:

[0006] Sampling operation data of the AC side of the flexible DC grid interface converter station, and determining whether high-frequency oscillation occurs in the flexible DC grid based on frequency domain characteristics of the operation data;

[0007] If high-frequency oscillation occurs in the flexible DC grid, determining a high-frequency oscillation point based on the operating data;

[0008] Based on the correlation between the filter parameters of the filter branch and the impedance characteristics of the interface converter station and the high-frequency oscillation mechanism of the flexible DC grid, select target filter parameters that make the flexible DC grid interface converter station have positive damping characteristics at the high-frequency oscillation point;

[0009] Based on the target filtering parameter, the filtering branch is controlled to operate in the phase-locked loop to suppress high-frequency oscillation of the flexible DC grid interface converter station.

[0010] Optionally, before sampling the operation data of the AC side of the flexible DC grid interface converter station, the method includes:

[0011] Obtaining an impedance characteristic curve of the AC system of the interface converter station and the flexible DC power grid after removing the filter branch;

[0012] Based on the impedance method stability criterion and the impedance characteristic curves of the interface converter station and the AC system after the filter branch is removed, the impedance characteristics of the interface converter station at the oscillation frequency are analyzed to obtain the high-frequency oscillation mechanism of the flexible DC grid.

[0013] Optionally, the impedance method stability criterion is expressed as:

[0014]

[0015] Among them, Z mmc represents the impedance of the flexible DC grid interface converter station, Z ac represents the impedance of the AC system, f represents the frequency, and ∠ represents the phase;

[0016] The high-frequency oscillation mechanism of the flexible DC grid is that high-frequency oscillation is generated when the impedance phase difference between the flexible DC grid interface converter station and the AC system at the amplitude intersection frequency is outside the interval (-180°, 180°).

[0017] Optionally, the obtaining of an impedance characteristic curve of an AC system of the interface converter station and the flexible DC power grid after the filter branch is removed includes:

[0018] The impedance characteristic curves of the interface converter station and the AC system after the filter branch is removed are obtained by an impedance sweep method.

[0019] Optionally, the obtaining of an impedance characteristic curve of an AC system of the interface converter station and the flexible DC power grid after the filter branch is removed includes:

[0020] Building an AC line model of the flexible DC grid based on multiple equivalent π models and obtaining impedance characteristics of the AC line of the flexible DC grid;

[0021] The impedance model of the AC line is obtained by calculation based on the impedance characteristics of the AC line and the series-parallel relationship of each element in a plurality of equivalent π models corresponding to the AC line model;

[0022] Connecting the impedance model of the AC line and the equivalent impedance of the equivalent power source of the flexible DC grid in series to obtain an impedance model of the AC system, and obtaining an impedance characteristic curve of the AC system based on the impedance model of the AC system;

[0023] An impedance model characterizing the impedance characteristics of the interface converter station is obtained by performing impedance calculation based on a linearization method, and an impedance characteristic curve of the interface converter station is obtained based on the impedance model of the interface converter station.

[0024] Optionally, the impedance model of the interface converter station is expressed as:

[0025]

[0026] in,

[0027]

[0028] In the formula, is the high frequency positive sequence impedance of the flexible DC grid interface converter station, is the high frequency negative sequence impedance of the flexible DC grid interface converter station, U sN is the rated phase voltage on the AC side of the flexible DC grid interface converter station, L eq is the equivalent inductance of the AC side of the flexible DC grid interface converter station, G de is the link delay, G ffw is the voltage feedforward transfer function, G i is the inner loop controller, s is the Laplace operator, ω1 is the fundamental angular frequency, j is the imaginary number sign, k pPLL is the proportional coefficient of the phase-locked loop, k iPLL is the integral coefficient of the phase-locked loop, F pll is the phase-locked loop transfer function.

[0029] Optionally, the operation data includes voltage data or current data, and determining whether high-frequency oscillation occurs in the flexible DC grid based on frequency domain characteristics of the operation data includes:

[0030] Performing fast Fourier transform on the voltage data or current data on the AC side of the interface converter station to obtain corresponding frequency domain features;

[0031] It is detected whether the frequency domain feature contains a high-frequency component. If the frequency domain feature contains a high-frequency component, it is determined that the flexible direct current grid has a high-frequency oscillation.

[0032] Optionally, the correlation between the filtering parameters of the filtering branch and the impedance characteristics of the flexible DC grid interface converter station is the corresponding relationship between the filtering parameters of the filtering branch and the high-frequency negative damping characteristics of the interface converter station. Based on the correlation between the filtering parameters of the filtering branch and the impedance characteristics of the interface converter station and the high-frequency oscillation mechanism of the flexible DC grid, the target filtering parameters that enable the flexible DC grid interface converter station to have positive damping characteristics at the high-frequency oscillation point are screened out, including:

[0033] Based on the correspondence between the filtering parameters of the filtering branch and the high-frequency negative damping characteristics of the interface converter station and the high-frequency oscillation mechanism of the flexible DC grid, the filtering parameters that stabilize the flexible DC grid and maximize the reduction of the high-frequency negative damping frequency range are selected as the target filtering parameters.

[0034] Optionally, controlling the filter branch to operate in the phase-locked loop based on the target filter parameter to suppress high-frequency oscillation of the flexible DC grid interface converter station includes:

[0035] Controlling the filtering branch to perform high-pass filtering of the input voltage signal of the phase-locked loop based on the target filtering parameter to obtain a filtered signal;

[0036] The filtered signal is superimposed on the proportional integral result of the PI controller to equivalently increase the proportional coefficient of the phase-locked loop and suppress high-frequency oscillation of the flexible DC grid interface converter station.

[0037] Optionally, after determining whether the flexible DC grid has high-frequency oscillation, the method further includes:

[0038] If it is determined that the flexible DC grid does not have high-frequency oscillation, the filter branch of the phase-locked loop is cut off.

[0039] On the other hand, the present invention further provides a high-frequency oscillation suppression system for a flexible DC grid interface converter station, wherein the interface converter station comprises a phase-locked loop, wherein the phase-locked loop comprises a filter branch connected in parallel with a PI controller, and wherein the high-frequency oscillation suppression system comprises:

[0040] A high-frequency oscillation determination module is used to sample the operation data of the AC side of the flexible DC grid interface converter station, and determine whether the flexible DC grid has high-frequency oscillation based on the frequency domain characteristics of the operation data; if the flexible DC grid has high-frequency oscillation, determine the high-frequency oscillation point based on the operation data;

[0041] A parameter screening module, for screening target filtering parameters that make the flexible DC grid interface converter station have positive damping characteristics at the high-frequency oscillation point based on the correlation between the filtering parameters of the filtering branch and the impedance characteristics of the interface converter station and the high-frequency oscillation mechanism of the flexible DC grid;

[0042] A control suppression module is used to control the filter branch to operate in the phase-locked loop based on the target filter parameter to suppress high-frequency oscillation of the flexible DC grid interface converter station.

[0043] Optionally, the system further comprises:

[0044] An impedance characteristic acquisition module, used to acquire an impedance characteristic curve of the AC system of the interface converter station and the flexible DC power grid after the filter branch is removed;

[0045] The oscillation mechanism analysis module is used to analyze the impedance characteristics of the interface converter station at the oscillation frequency based on the impedance method stability criterion and the impedance characteristic curve of the interface converter station and the AC system after the filter branch is removed, so as to obtain the high-frequency oscillation mechanism of the flexible DC power grid.

[0046] Optionally, the impedance method stability criterion is expressed as:

[0047]

[0048] Among them, Z mmc represents the impedance of the flexible DC grid interface converter station, Z ac represents the impedance of the AC system, f represents the frequency, and ∠ represents the phase;

[0049] The high-frequency oscillation mechanism of the flexible DC grid is that high-frequency oscillation is generated when the impedance phase difference between the flexible DC grid interface converter station and the AC system at the amplitude intersection frequency is outside the interval (-180°, 180°).

[0050] Optionally, the impedance characteristic acquisition module includes:

[0051] The impedance test submodule is used to obtain the impedance characteristic curve of the interface converter station and the AC system after the filter branch is removed through the impedance sweep method.

[0052] Optionally, the impedance characteristic acquisition module includes an impedance calculation submodule, and the impedance calculation submodule is used to:

[0053] Building an AC line model of the flexible DC grid based on multiple equivalent π models and obtaining impedance characteristics of the AC line of the flexible DC grid;

[0054] The impedance model of the AC line is obtained by calculation based on the impedance characteristics of the AC line and the series-parallel relationship of each element in a plurality of equivalent π models corresponding to the AC line model;

[0055] Connecting the impedance model of the AC line and the equivalent impedance of the equivalent power source of the flexible DC grid in series to obtain an impedance model of the AC system, and obtaining an impedance characteristic curve of the AC system based on the impedance model of the AC system;

[0056] An impedance model characterizing the impedance characteristics of the interface converter station is obtained by performing impedance calculation based on a linearization method, and an impedance characteristic curve of the interface converter station is obtained based on the impedance model of the interface converter station.

[0057] Optionally, the impedance model of the interface converter station is expressed as:

[0058]

[0059] in,

[0060]

[0061] In the formula, is the high frequency positive sequence impedance of the flexible DC grid interface converter station, is the high frequency negative sequence impedance of the flexible DC grid interface converter station, U sN is the rated phase voltage on the AC side of the flexible DC grid interface converter station, L eq is the equivalent inductance of the AC side of the flexible DC grid interface converter station, G de is the link delay, G ffw is the voltage feedforward transfer function, G i is the inner loop controller, s is the Laplace operator, ω1 is the fundamental angular frequency, j is the imaginary number sign, k pPLL is the proportional coefficient of the phase-locked loop, k iPLL is the integral coefficient of the phase-locked loop, F pll is the phase-locked loop transfer function.

[0062] Optionally, the operation data includes voltage data or current data, and the high-frequency oscillation determination module includes:

[0063] A time-frequency transformation submodule, used for performing a fast Fourier transform on the voltage data or current data on the AC side of the interface converter station to obtain corresponding frequency domain features;

[0064] The oscillation judgment submodule is used to detect whether the frequency domain feature contains a high-frequency component, and if the frequency domain feature contains a high-frequency component, determine that the flexible direct current grid has a high-frequency oscillation.

[0065] Optionally, the correlation between the filtering parameters of the filtering branch and the impedance characteristics of the flexible DC grid interface converter station is the corresponding relationship between the filtering parameters of the filtering branch and the high-frequency negative damping characteristics of the interface converter station, and the parameter screening module is specifically used for:

[0066] Based on the correspondence between the filtering parameters of the filtering branch and the high-frequency negative damping characteristics of the interface converter station and the high-frequency oscillation mechanism of the flexible DC grid, the filtering parameters that stabilize the flexible DC grid and maximize the reduction of the high-frequency negative damping frequency range are selected as the target filtering parameters.

[0067] Optionally, the control suppression module includes:

[0068] A control filter submodule, used for controlling the filter branch to perform high-pass filtering on the input voltage signal of the phase-locked loop based on the target filter parameter to obtain a filtered signal;

[0069] The signal superposition submodule is used to superimpose the filtered signal with the proportional integral result of the PI controller to equivalently increase the proportional coefficient of the phase-locked loop and suppress the high-frequency oscillation of the flexible DC grid interface converter station.

[0070] Optionally, the system further comprises:

[0071] The branch removal module is used to remove the filter branch of the phase-locked loop if it is determined that the flexible DC grid does not have high-frequency oscillation.

[0072] On the other hand, the present invention also provides an electronic device, comprising: at least one processor and a memory; the memory and the processor are connected via a bus;

[0073] The memory is used to store one or more programs;

[0074] When the one or more programs are executed by the at least one processor, the high-frequency oscillation suppression method of the flexible DC grid interface converter station described in any one of the above items is implemented.

[0075] On the other hand, the present invention further provides a readable storage medium having an execution program stored thereon, and when the execution program is executed, the high-frequency oscillation suppression method of the flexible DC grid interface converter station described in any one of the above items is implemented.

[0076] Compared with the prior art, the present invention has the following beneficial effects:

[0077] The present invention provides a method and system for suppressing high-frequency oscillation of a flexible DC grid interface converter station. On the one hand, by adding a filter branch connected in parallel with a PI controller in a phase-locked loop of the interface converter station, the high-frequency impedance characteristics of the interface converter station are reshaped to achieve the effect of suppressing high-frequency oscillation of the flexible DC grid. On the other hand, based on the frequency domain characteristics of the operation data of the AC side of the flexible DC grid interface converter station, it is determined whether high-frequency oscillation occurs, and the high-frequency oscillation point is determined based on the operation data; based on the correlation between the filter parameters of the pre-constructed filter branch and the impedance characteristics of the interface converter station and the high-frequency oscillation mechanism of the flexible DC grid, the target filter parameters that make the flexible DC grid interface converter station have positive damping characteristics at the high-frequency oscillation point are screened out; based on the target filter parameters, the filter branch is controlled to be put into the phase-locked loop for operation, and the target filter parameters are screened based on the high-frequency oscillation mechanism of the flexible DC grid, which can ensure the suppression effect of high-frequency oscillation and achieve effective suppression of high-frequency oscillation of the flexible DC grid. BRIEF DESCRIPTION OF THE DRAWINGS

[0078] Figure 1 This is one of the flow diagrams of a method for suppressing high-frequency oscillation of a flexible DC grid interface converter station of the present invention;

[0079] Figure 2 This is a schematic diagram of a main circuit of a flexible DC power grid according to an example of the present invention;

[0080] Figure 3 It is a schematic diagram of the control implementation process of high-frequency oscillation suppression of the flexible DC grid interface converter station of the present invention;

[0081] Figure 4 The second flowchart of the method for suppressing high-frequency oscillation of a flexible DC grid interface converter station of the present invention;

[0082] Figure 5 It is a schematic structural diagram of the electronic device of the present invention. DETAILED DESCRIPTION

[0083] The specific implementation modes of the present invention are further described in detail below with reference to the accompanying drawings.

[0084] Embodiment 1:

[0085] The present invention provides a method for suppressing high-frequency oscillation of a flexible DC grid interface converter station, as shown in the schematic diagram Figure 1 As shown, including:

[0086] Step S110, sampling the operation data of the AC side of the flexible DC grid interface converter station, and determining whether high-frequency oscillation occurs in the flexible DC grid based on the frequency domain characteristics of the operation data;

[0087] Step S120, if high-frequency oscillation occurs in the flexible DC grid, determining a high-frequency oscillation point based on the operation data;

[0088] Step S130, based on the correlation between the filter parameters of the filter branch and the impedance characteristics of the interface converter station and the high-frequency oscillation mechanism of the flexible DC grid, select target filter parameters that make the flexible DC grid interface converter station have positive damping characteristics at the high-frequency oscillation point;

[0089] Step S140, based on the target filtering parameter, the filter branch is controlled to operate in a phase-locked loop to suppress high-frequency oscillation of the flexible DC grid interface converter station.

[0090] In this example implementation, the flexible DC grid may include an AC system and a converter station side, the AC system includes an equivalent power source, an equivalent impedance and an AC line, and the converter station side includes an interface converter station and a converter transformer, as shown in FIG. Figure 2 The figure shows the main circuit diagram of the flexible DC grid. Figure 2 In the middle, use L gIndicates the equivalent impedance of the AC system, L t Represents the equivalent impedance of the converter transformer. The DC grid interface converter station is connected to the PCC point (Point of Common Coupling) through the converter transformer. The flexible DC grid may be a flexible DC grid scenario based on a modular multilevel converter or a two-level VSC (Voltage-Source-Converter). The interface converter station may be a modular multilevel converter or a two-level VSC converter, etc. The operating data on the AC side of the flexible DC grid interface converter station may be the current data or voltage data on the AC side, which may be obtained by measuring the operating data on the AC side at the PCC point. Frequency domain features may include high-frequency components, low-frequency components, etc. The high-frequency oscillation frequency point may be directly obtained through the three-phase current or voltage based on electromagnetic transient simulation, or the high-frequency oscillation frequency point may be determined by observing the frequency domain data. The filter branch is located in the phase-locked loop and is connected in parallel with the PI controller in the phase-locked loop, such as Figure 3 As shown. The relationship between different filter parameters of the filter branch and the impedance characteristics of the interface converter station can be established in advance, and the influence of different filter parameters on the impedance characteristics of the interface converter station can be analyzed. The filter branch can adopt a first-order high-pass filter, and the filter parameters can include the gain and cut-off frequency of the first-order high-pass filter. By analyzing the impedance characteristics of the interface converter station without a filter branch, the mechanism of high-frequency oscillation of the flexible DC grid is analyzed, and the filter parameters that stabilize the flexible DC grid are screened out as target filter parameters based on the current high-frequency oscillation point. Based on the target filter parameters, the filter branch is controlled to run in the phase-locked loop to suppress the high-frequency oscillation of the flexible DC grid interface converter station.

[0091] Since the phase-locked loop can provide additional positive resistance for the converter station in the high frequency band, and the additional positive resistance is positively correlated with the proportional coefficient of the phase-locked loop, that is, as the proportional coefficient of the phase-locked loop increases, the additional positive resistance also increases accordingly. Therefore, if the proportional coefficient of the phase-locked loop can be equivalently increased, the additional positive resistance provided by the phase-locked loop will increase, and the high-frequency negative damping effect of the interface converter station will be weakened accordingly, thereby improving the high-frequency stability of the flexible DC power grid. Based on the above analysis, it is considered to connect a filter branch in parallel on the basis of the phase-locked loop PI controller, and the filter branch can equivalently increase the proportional coefficient of the phase-locked loop in the high frequency band. On the one hand, the present invention reshapes the high-frequency impedance characteristics of the interface converter station by adding a filter branch in parallel with the PI controller in the phase-locked loop of the interface converter station, so as to achieve the effect of suppressing the high-frequency oscillation of the flexible DC power grid. On the other hand, based on the frequency domain characteristics of the operating data on the AC side of the flexible DC grid interface converter station, it is determined whether high-frequency oscillation occurs, and the high-frequency oscillation point is determined based on the operating data; based on the correlation between the filtering parameters of the pre-constructed filtering branch and the impedance characteristics of the interface converter station and the high-frequency oscillation mechanism of the flexible DC grid, the target filtering parameters that make the flexible DC grid interface converter station have positive damping characteristics at the high-frequency oscillation point are screened out; based on the target filtering parameters, the filtering branch is controlled to operate in the phase-locked loop, and the target filtering parameters are screened based on the high-frequency oscillation mechanism of the flexible DC grid, which can ensure the suppression effect of high-frequency oscillation and achieve effective suppression of high-frequency oscillation in the flexible DC grid.

[0092] In an exemplary implementation, before sampling the operation data of the AC side of the flexible DC grid interface converter station in S110, the following process is included:

[0093] Obtaining an impedance characteristic curve of the AC system of the interface converter station and the flexible DC power grid after removing the filter branch;

[0094] Based on the impedance method stability criterion and the impedance characteristic curves of the interface converter station and the AC system after the filter branch is removed, the impedance characteristics of the interface converter station at the oscillation frequency are analyzed to obtain the high-frequency oscillation mechanism of the flexible DC grid.

[0095] In this example implementation, the gain of the filter branch can be set to 0 by controlling the impedance characteristics of the interface converter station of the flexible DC grid when the filter branch is not considered. Then, based on the impedance characteristic curve of the interface converter station and the AC system when there is no filter branch (after the filter branch is removed), the impedance method stability criterion is used to study the mechanism of high-frequency oscillation in the flexible DC grid, and analyze the impedance characteristics of the interface converter station at the oscillation frequency, including inductance and capacitance characteristics, positive resistance, negative resistance characteristics, etc., to provide ideas for the selection of filtering parameters of the filter branch. Exemplarily, the mechanism of high-frequency oscillation in the flexible DC grid is studied, that is, to check whether the phase difference at the intersection of the amplitudes of the interface converter station and the AC system is within 180°. By analyzing the impedance characteristics of the interface converter station at the oscillation frequency, it can be known that the addition of the filter branch is to make the interface converter station of the flexible DC grid present a positive damping characteristic at the amplitude intersection frequency. Among them, the expression of the impedance method stability criterion is:

[0096]

[0097] Among them, Z mmc represents the impedance of the flexible DC grid interface converter station, Z ac Represents the impedance of the AC system, f represents the frequency, and ∠ represents the phase. The high-frequency oscillation mechanism of the flexible DC grid is that high-frequency oscillation is generated when the impedance phase difference between the flexible DC grid interface converter station and the AC system at the amplitude intersection frequency is outside the interval (-180°, 180°). From the expression of the impedance method stability criterion, it can be seen that at the point where the impedance amplitude of the flexible DC grid interface converter station is equal to the impedance amplitude of the AC system, if the phase difference between the two is outside the ±180 degree interval, the flexible DC grid is unstable. Therefore, when there is no intersection between the impedance amplitude of the flexible DC grid interface converter station and the AC system or the phase at the intersection is within the ±180 degree interval, the flexible DC grid is stable.

[0098] For example, when analyzing the high-frequency oscillation mechanism, the impedance characteristic curve of the flexible DC grid interface converter station and the impedance characteristic curve of the AC system can be drawn in the same coordinate system, wherein the impedance characteristic curve includes an impedance amplitude characteristic curve and an impedance phase characteristic curve. According to the impedance stability criterion, if the impedance phase difference between the flexible DC grid interface converter station and the AC system is outside the ±180 degree range, the system is unstable. On the contrary, if the phase difference is within the ±180 degree range, the system is stable. According to the impedance stability criterion, it can be found that the high-frequency oscillation of the system is the result of the interaction between the inductive negative resistance of the flexible DC grid interface converter station and the capacitive characteristics of the AC system. Since the AC system impedance presents a passive nature, the impedance phase of the AC system must be within the ±90 degree range. Therefore, in order to ensure the high-frequency stability of the system, it can be seen from the impedance stability criterion that at the amplitude intersection frequency, the impedance phase of the flexible DC grid interface converter station must also be within the ±90 degree range, otherwise the impedance phase difference between the two will be outside the ±180 degree range, and the system will be unstable. That is to say, at the amplitude intersection frequency, the flexible DC grid interface converter station must exhibit positive damping characteristics to stabilize the system.

[0099] Exemplarily, the obtaining of the impedance characteristic curve of the AC system of the interface converter station and the flexible DC grid after the filter branch is removed includes: obtaining the impedance characteristic curve of the interface converter station and the AC system after the filter branch is removed by an impedance sweep method.

[0100] In this example implementation, when the internal structures and parameters of the interface converter station and the flexible DC grid AC system are unknown, impedance sweeping can be used to obtain the impedance characteristic curves of the interface converter station and the AC system.

[0101] In other examples, when the internal structures and parameters of the interface converter station and the flexible DC grid AC system are known, the impedance calculation method can be used to obtain the corresponding impedance characteristic curve, which specifically includes the following process:

[0102] Building an AC line model of the flexible DC grid based on multiple equivalent π models and obtaining impedance characteristics of the AC line of the flexible DC grid;

[0103] The impedance model of the AC line is obtained by calculation based on the impedance characteristics of the AC line and the series-parallel relationship of each element in a plurality of equivalent π models corresponding to the AC line model;

[0104] Connecting the impedance model of the AC line and the equivalent impedance of the equivalent power source of the flexible DC grid in series to obtain an impedance model of the AC system, and obtaining an impedance characteristic curve of the AC system based on the impedance model of the AC system;

[0105] An impedance model characterizing the impedance characteristics of the interface converter station is obtained by performing impedance calculation based on a linearization method, and an impedance characteristic curve of the interface converter station is obtained based on the impedance model of the interface converter station.

[0106] In this example implementation, impedance calculation relies on a linearization method. The AC line model can be divided into lumped parameter, distributed parameter, and frequency-dependent response models. This example only describes the distributed parameter model, and uses multiple equivalent π models to obtain the impedance characteristics of the AC line. The impedance model of the AC line can be calculated by the series-parallel relationship of each element in multiple equivalent π models. The AC system impedance model is composed of the equivalent impedance of the AC line and the equivalent power supply in series.

[0107] The expression of the impedance model of the interface converter station is:

[0108]

[0109] in,

[0110]

[0111] In the formula, is the high frequency positive sequence impedance of the flexible DC grid interface converter station, is the high frequency negative sequence impedance of the flexible DC grid interface converter station, U sN is the rated phase voltage on the AC side of the flexible DC grid interface converter station, L eq is the equivalent inductance of the AC side of the flexible DC grid interface converter station, G de is the link delay, G ffw is the voltage feedforward transfer function, G i is the inner loop controller, s is the Laplace operator, ω1 is the fundamental angular frequency, j is the imaginary number sign, k pPLL is the proportional coefficient of the phase-locked loop, k iPLL is the integral coefficient of the phase-locked loop, F pll is the phase-locked loop transfer function. Based on the above formula, the high-frequency positive and negative sequence impedance characteristic curve of the flexible DC grid interface converter station can be drawn. When the phase-locked loop transfer function F pll ≠0, the impedance characteristics of the phase-locked loop parallel filter branch are considered. If F pll = 0 is the impedance characteristic under normal conditions when there is no filter branch. The impedance characteristic curves of the flexible DC grid interface converter station and the AC system can be drawn in the same coordinate system, including the amplitude characteristic curve and the phase characteristic curve, so as to judge the high-frequency stability of the flexible DC grid by the impedance method.

[0112] In some example implementations, the determining whether the flexible DC grid has high-frequency oscillation based on the frequency domain characteristics of the operating data in step S110 above includes:

[0113] Performing fast Fourier transform on the voltage data or current data on the AC side of the interface converter station to obtain corresponding frequency domain features;

[0114] It is detected whether the frequency domain feature contains a high-frequency component. If the frequency domain feature contains a high-frequency component, it is determined that the flexible direct current grid has a high-frequency oscillation.

[0115] In this example implementation, the operation data includes voltage data or current data, and the voltage or current on the AC side of the flexible DC grid interface converter station at the PCC point is sampled and a fast Fourier transform FFT analysis is performed on it to detect whether the analysis result contains a high-frequency component, thereby determining whether the flexible DC grid has high-frequency oscillation. When the flexible DC grid has high-frequency oscillation, the phase-locked loop additional parallel filter branch can be put into operation.

[0116] In some example implementations, the step S130 described above, based on the correlation between the filter parameters of the filter branch and the impedance characteristics of the interface converter station and the high-frequency oscillation mechanism of the flexible DC grid, screening out target filter parameters that make the flexible DC grid interface converter station have positive damping characteristics at the high-frequency oscillation point includes:

[0117] Based on the correspondence between the filtering parameters of the filtering branch and the high-frequency negative damping characteristics of the interface converter station and the high-frequency oscillation mechanism of the flexible DC grid, the filtering parameters that stabilize the flexible DC grid and maximize the reduction of the high-frequency negative damping frequency range are selected as the target filtering parameters.

[0118] In this example implementation, the correlation between the filter parameters of the filter branch and the impedance characteristics of the interface converter station can be obtained by analyzing the change trend of the impedance characteristics of the flexible DC grid interface converter station under different first-order high-pass filter cutoff frequencies and gains. Specifically, it may include changes in the high-frequency negative damping frequency range, the correspondence between different filter parameters and the high-frequency negative damping characteristics of the interface converter station, etc. According to the high-frequency oscillation mechanism, in order to suppress high-frequency oscillations, the flexible DC grid interface converter station must be guaranteed to exhibit positive damping characteristics at the amplitude intersection frequency in order to stabilize the system. According to the above analysis, it can be concluded that the connection between impedance characteristics and parameter selection is: parameters that can make the flexible DC grid interface converter station exhibit positive damping characteristics at the amplitude intersection frequency need to be selected. This high-frequency oscillation mechanism will need to be used when selecting parameters later. By analyzing the influence of the first-order high-pass filter parameters on the impedance characteristics of the flexible DC grid interface converter station, the high-frequency oscillation mechanism is used to select parameters that can make the flexible DC grid interface converter station exhibit positive damping characteristics at the amplitude intersection frequency. In other words, parameters are selected to ensure that the impedance phase difference between the flexible DC grid interface converter station and the AC system at the amplitude intersection frequency is within the range of ±180 degrees.

[0119] In some example implementations, the step S140 of controlling the filter branch to operate in the phase-locked loop based on the target filter parameter to suppress high-frequency oscillation of the flexible DC grid interface converter station includes:

[0120] Controlling the filtering branch to perform high-pass filtering of the input voltage signal of the phase-locked loop based on the target filtering parameter to obtain a filtered signal;

[0121] The filtered signal is superimposed on the proportional integral result of the PI controller to equivalently increase the proportional coefficient of the phase-locked loop and suppress high-frequency oscillation of the flexible DC grid interface converter station.

[0122] In this example implementation, the filter branch is connected in parallel with the PI controller of the phase-locked loop, so that the filter branch can equivalently increase the proportional coefficient of the phase-locked loop in the high frequency band, so that the additional positive resistance provided by the phase-locked loop increases, and the high-frequency negative damping effect of the interface converter station is weakened, thereby improving the high-frequency stability of the flexible DC grid. The signal processing control flow of the interface converter is as follows: Figure 3 As shown, Figure 3 In the example, the filter branch is a first-order high-pass filter, and the voltage u at the PCC point is measured by a voltage sensor. sabc , that is, the phase voltage u on the AC side at the PCC point sa 、u sb 、u sc The phase voltage is transformed into the dq coordinate system by the coordinate transformation module (abc / dq) and is obtained as u sd and u sq ,u sq Input to the phase-locked loop and parallel to the PI controller (k ppll ), filter branch Integrator (k ipll ) is processed by superposition and fusion of the processed signals. The superposition and fusion result is processed by the baseband angular frequency ω1 and then processed by the inverse of the Laplace operator s to obtain the output angle θ of the phase-locked loop. pll When the system is stable, this angle is the PCC point u sa The phase, θ pll Used as the angle required in the coordinate transformation module (abc / dq). k ppll and k ipll are the proportional coefficient and integral coefficient of the phase-locked loop, k hpf is the gain coefficient of the first-order high-pass filter, ω hpf =2πf hpf , f hpf is the cutoff frequency of the first-order high-pass filter, ω1 is the fundamental angular frequency, s is the Laplace operator, θ pllis the phase-locked loop output angle. The transfer function of the filter branch is

[0123] In some example implementations, after determining whether the flexible DC grid has high-frequency oscillation, the method further includes: if it is determined that the flexible DC grid has not had high-frequency oscillation, cutting off a filter branch of the phase-locked loop.

[0124] In this example implementation, the filtering branch of the phase-locked loop may be removed by setting the gain of the filtering branch, i.e., the first-order high-pass filter, to 0, or by directly removing the filtering branch (e.g., by isolating it through a switch, etc.). When it is detected that the flexible DC grid does not have high-frequency oscillation, if the operating conditions of the flexible DC grid change, and the system can be kept stable even without the phase-locked loop additional parallel filtering branch, the filtering branch may be removed.

[0125] In the present invention, the parameters of the filter that can achieve the positive damping characteristics of the converter station at the oscillation point can be selected according to the optimization effect of the negative damping area of ​​the converter station under different filtering parameters and the determined frequency oscillation point. Under different working conditions, such as changes in active power, the correlation relationship between the filtering parameters of the filter branch and the impedance characteristics of the interface converter station is reconstructed. The parameter range designed for the filter branch of the present invention needs to achieve the phase of the converter station at the oscillation point within ±90°, that is, to achieve the positive damping characteristics of the converter station at this frequency point.

[0126] The present invention can also verify the suppression effect of high-frequency oscillation of the flexible DC grid through electromagnetic transient simulation, thereby illustrating the correctness of the selected parameters.

[0127] The following is a specific example to illustrate the specific steps of a high-frequency oscillation suppression method for a flexible DC grid interface converter station of the present invention. Figure 4 As shown, the following processes may be included:

[0128] (1) Obtain the impedance characteristic curves of the flexible DC grid interface converter station and the AC system. The impedance characteristic curves of the flexible DC grid interface converter station and the AC system can be obtained through impedance sweep and impedance calculation. Among them, the impedance sweep does not require the knowledge of the internal structure and parameters of the device, while the impedance calculation relies on the linearization method to obtain the impedance model expression that can characterize the impedance characteristics of the device through calculation. It is necessary to know the internal structure and parameters of the device in advance, and the impedance sweep result can be used to verify whether the calculated impedance expression is correct. Obtain the impedance characteristic curve of the flexible DC grid interface converter station when considering the additional parallel filter branch of the phase-locked loop, and draw the impedance characteristic curves of the flexible DC grid interface converter station and the AC system in the same coordinate system, including the amplitude characteristic curve and the phase characteristic curve, so as to judge the high-frequency stability of the flexible DC grid by the impedance method.

[0129] (2) Analyze the high-frequency oscillation mechanism of the flexible DC grid. By setting the gain of the phase-locked loop filter branch to 0, the impedance characteristics of the flexible DC grid interface converter station can be obtained without considering the filter branch. Then, based on the impedance characteristic curve of the interface converter station and the AC system without the filter branch, the impedance method stability criterion is used to study the mechanism of high-frequency oscillation of the flexible DC grid, and analyze the impedance characteristics of the interface converter station at the oscillation frequency, including the inductance and capacitance characteristics and the positive and negative resistance characteristics, etc., to provide ideas for the parameter selection of the filter.

[0130] (3) Analyze the influence of the first-order high-pass filter parameters on the impedance characteristics of the flexible DC grid interface converter station. The additional filter branch is connected in parallel with the phase-locked loop PI controller to use a first-order high-pass filter. The change trend of the impedance characteristics of the flexible DC grid interface converter station under different first-order high-pass filter cutoff frequencies and gains is analyzed, including the change in the high-frequency negative damping frequency range. The correlation or correspondence between different filter parameters and the impedance characteristics of the flexible DC grid interface converter station is established.

[0131] (4) Determine whether the flexible DC grid has high-frequency oscillation. By sampling the voltage or current on the AC side of the flexible DC grid interface converter station online and performing FFT analysis on it, it is detected whether the analysis result contains high-frequency components, thereby determining whether the flexible DC grid has high-frequency oscillation and the high-frequency oscillation points. When the flexible DC grid has high-frequency oscillation, the phase-locked loop can be added with a parallel filter branch. The impedance method can be used to determine the high-frequency stability of the flexible DC grid under different parameter conditions.

[0132] (5) Determine the cutoff frequency and gain of the first-order high-pass filter. Based on the influence of the first-order high-pass filter parameters obtained in step (3) on the impedance characteristics of the flexible DC grid interface converter station, select the cutoff frequency and gain that can stabilize the flexible DC grid and reduce the high-frequency negative damping frequency range as the target filter parameters.

[0133] (6) Control and suppress high-frequency oscillation of the flexible DC grid. Based on the cutoff frequency and gain (target filter parameters) of the first-order high-pass filter that can stabilize the high frequency of the flexible DC grid determined in step (5), control the input of the additional parallel filter branch of the phase-locked loop.

[0134] The present invention analyzes the high-frequency oscillation problem of the flexible DC power grid. The problem is related to the link delay of the interface converter, the inner loop control, the phase-locked loop and other links. The high-frequency oscillation suppression measures can be based on the inner loop control link, for example, by adding a low-pass filter in the voltage feedforward link or by additional active damping control to reshape the high-frequency impedance characteristics of the interface converter, so as to achieve the purpose of suppressing high-frequency oscillation. However, the phase-locked loop also has a certain degree of influence on the high-frequency oscillation of the system, and can provide a wide range of positive damping characteristics in the high frequency band. Therefore, from the perspective of the phase-locked loop, high-frequency oscillation suppression measures can be proposed, especially for oscillation types with a high degree of phase-locked loop participation, the effect is more obvious.

[0135] In order to suppress the high-frequency oscillation of the flexible DC grid, the present invention reshapes the high-frequency impedance characteristics of the flexible DC grid interface converter by connecting a filter branch (first-order high-pass filter) in parallel to the phase-locked loop, thereby suppressing the high-frequency oscillation of the flexible DC grid. The control structure of the present invention is simple and can effectively suppress the high-frequency oscillation of the flexible DC grid; it is not only applicable to modular multi-level converters, but also to two-level VSC converters. The method of the present invention can be implemented in a control system without generating additional active power loss, reactive power compensation, etc., with low cost and good economy.

[0136] Embodiment 2:

[0137] Based on the same inventive concept, the present invention also provides a high-frequency oscillation suppression system for a flexible DC grid interface converter station, wherein the interface converter station comprises a phase-locked loop, wherein the phase-locked loop comprises a filter branch connected in parallel with a PI controller, and wherein the high-frequency oscillation suppression system comprises:

[0138] A high-frequency oscillation determination module is used to sample the operation data of the AC side of the flexible DC grid interface converter station, and determine whether the flexible DC grid has high-frequency oscillation based on the frequency domain characteristics of the operation data; if the flexible DC grid has high-frequency oscillation, determine the high-frequency oscillation point based on the operation data;

[0139] A parameter screening module, for screening target filtering parameters that make the flexible DC grid interface converter station have positive damping characteristics at the high-frequency oscillation point based on the correlation between the filtering parameters of the filtering branch and the impedance characteristics of the interface converter station and the high-frequency oscillation mechanism of the flexible DC grid;

[0140] A control suppression module is used to control the filter branch to operate in the phase-locked loop based on the target filter parameter to suppress high-frequency oscillation of the flexible DC grid interface converter station.

[0141] In a possible implementation, the system further includes:

[0142] An impedance characteristic acquisition module, used to acquire an impedance characteristic curve of the AC system of the interface converter station and the flexible DC power grid after the filter branch is removed;

[0143] The oscillation mechanism analysis module is used to analyze the impedance characteristics of the interface converter station at the oscillation frequency based on the impedance method stability criterion and the impedance characteristic curve of the interface converter station and the AC system after the filter branch is removed, so as to obtain the high-frequency oscillation mechanism of the flexible DC power grid.

[0144] In a possible implementation manner, the impedance method stability criterion is expressed as:

[0145]

[0146] Among them, Z mmc represents the impedance of the flexible DC grid interface converter station, Z ac represents the impedance of the AC system, f represents the frequency, and ∠ represents the phase;

[0147] The high-frequency oscillation mechanism of the flexible DC grid is that high-frequency oscillation is generated when the impedance phase difference between the flexible DC grid interface converter station and the AC system at the amplitude intersection frequency is outside the interval (-180°, 180°).

[0148] In a possible implementation manner, the impedance characteristic acquisition module includes:

[0149] The impedance test submodule is used to obtain the impedance characteristic curve of the interface converter station and the AC system after the filter branch is removed through the impedance sweep method.

[0150] In a possible implementation manner, the impedance characteristic acquisition module includes an impedance calculation submodule, and the impedance calculation submodule is used to:

[0151] Building an AC line model of the flexible DC grid based on multiple equivalent π models and obtaining impedance characteristics of the AC line of the flexible DC grid;

[0152] The impedance model of the AC line is obtained by calculation based on the impedance characteristics of the AC line and the series-parallel relationship of each element in a plurality of equivalent π models corresponding to the AC line model;

[0153] Connecting the impedance model of the AC line and the equivalent impedance of the equivalent power source of the flexible DC grid in series to obtain an impedance model of the AC system, and obtaining an impedance characteristic curve of the AC system based on the impedance model of the AC system;

[0154] An impedance model characterizing the impedance characteristics of the interface converter station is obtained by performing impedance calculation based on a linearization method, and an impedance characteristic curve of the interface converter station is obtained based on the impedance model of the interface converter station.

[0155] In a possible implementation manner, the impedance model of the interface converter station is expressed as:

[0156]

[0157] in,

[0158]

[0159] In the formula, is the high frequency positive sequence impedance of the flexible DC grid interface converter station, is the high frequency negative sequence impedance of the flexible DC grid interface converter station, U sN is the rated phase voltage on the AC side of the flexible DC grid interface converter station, L eq is the equivalent inductance of the AC side of the flexible DC grid interface converter station, G de is the link delay, G ffw is the voltage feedforward transfer function, G i is the inner loop controller, s is the Laplace operator, ω1 is the fundamental angular frequency, j is the imaginary number sign, k pPLL is the proportional coefficient of the phase-locked loop, k iPLL is the integral coefficient of the phase-locked loop, F pll is the phase-locked loop transfer function.

[0160] In a possible implementation manner, the operating data includes voltage data or current data, and the high-frequency oscillation determination module includes:

[0161] A time-frequency transformation submodule, used for performing a fast Fourier transform on the voltage data or current data on the AC side of the interface converter station to obtain corresponding frequency domain features;

[0162] The oscillation judgment submodule is used to detect whether the frequency domain feature contains a high-frequency component. If the frequency domain feature contains a high-frequency component, it is determined that the flexible direct current grid has a high-frequency oscillation.

[0163] In a possible implementation manner, the correlation relationship between the filtering parameters of the filtering branch and the impedance characteristics of the flexible DC grid interface converter station is the corresponding relationship between the filtering parameters of the filtering branch and the high-frequency negative damping characteristics of the interface converter station, and the parameter screening module is specifically used for:

[0164] Based on the correspondence between the filtering parameters of the filtering branch and the high-frequency negative damping characteristics of the interface converter station and the high-frequency oscillation mechanism of the flexible DC grid, the filtering parameters that stabilize the flexible DC grid and maximize the reduction of the high-frequency negative damping frequency range are selected as the target filtering parameters.

[0165] In a possible implementation, the control suppression module includes:

[0166] A control filter submodule, used for controlling the filter branch to perform high-pass filtering on the input voltage signal of the phase-locked loop based on the target filter parameter to obtain a filtered signal;

[0167] The signal superposition submodule is used to superimpose the filtered signal with the proportional integral result of the PI controller to equivalently increase the proportional coefficient of the phase-locked loop and suppress the high-frequency oscillation of the flexible DC grid interface converter station.

[0168] In a possible implementation, the system further includes:

[0169] The branch removal module is used to remove the filter branch of the phase-locked loop if it is determined that the flexible DC grid does not have high-frequency oscillation.

[0170] Embodiment 3:

[0171] like Figure 5 As shown, the present invention also provides an electronic device, which may be a computer device, a single-chip device, an intelligent mobile device, etc. The electronic device in this embodiment may include a processor, a memory, a transceiver component, etc. The memory, the processor, and the transceiver component are connected via a bus; the memory may be used to store an execution program, and an exemplary execution program may include instructions; the processor is used to execute the instructions stored in the memory. The memory may also be used to store data, which may be called and / or modified when the instructions are executed.

[0172] The processor may be a central processing unit (CPU), or other general-purpose processors, digital signal processors (DSP), application-specific integrated circuits (ASIC), field-programmable gate arrays (FPGA) or other programmable logic devices, discrete gates or transistor logic devices, discrete hardware components, etc. It is the computing core and control core of the terminal, which is suitable for implementing one or more instructions, and is specifically suitable for loading and executing one or more instructions in a storage medium to implement a corresponding method flow or a corresponding function, so as to implement the steps of a high-frequency oscillation suppression method for a flexible DC grid interface converter station in the above-mentioned embodiment.

[0173] Example 4

[0174] Based on the same inventive concept, the present invention also provides a readable storage medium, specifically an electronic device readable storage medium (Memory), which is a memory device in an electronic device for storing programs and data. It can be understood that the storage medium here can include both built-in storage media in electronic devices and, of course, extended storage media supported by electronic devices. The storage medium provides a storage space, which stores the operating system of the terminal. In addition, one or more instructions suitable for being loaded and executed by a processor are also stored in the storage space, and these instructions can be one or more execution programs (including program codes). It should be noted that the storage medium here can be a high-speed RAM memory or a non-volatile memory, such as at least one disk memory. The processor loads and executes one or more instructions stored in the storage medium, which can implement the steps of a high-frequency oscillation suppression method for a flexible DC grid interface converter station in the above embodiment.

[0175] Those skilled in the art will appreciate that embodiments of the present invention may be provided as methods, systems, or computer program products. Therefore, the present invention may take the form of a complete hardware embodiment, a complete software embodiment, or an embodiment combining software and hardware. Moreover, the present invention may 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.

[0176] The present invention is described with reference to flowcharts and / or block diagrams of methods, devices (systems), and computer program products according to embodiments of the present invention. It should be understood that each process and / or block in the flowchart and / or block diagram, as well as the combination of processes 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 a processor of a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing device to produce a machine, so that the instructions executed by the processor of the computer or other programmable data processing device generate instructions for implementing the processes in the flowchart and / or block diagram. Figure 1 A process or multiple processes and / or boxes Figure 1 A device that provides the functions specified in a block or multiple blocks.

[0177] These computer program instructions may also be stored in a computer-readable memory capable of directing a computer or other programmable data processing device to operate in a specific manner, so that the instructions stored in the computer-readable memory produce an article of manufacture comprising an instruction device, which implements the process Figure 1 A process or multiple processes and / or boxes Figure 1 A function specified in one or more boxes.

[0178] These computer program instructions can also be loaded onto a computer or other programmable data processing device so that a series of operating steps are executed on the computer or other programmable device to produce a computer-implemented process, thereby providing instructions for implementing the process. Figure 1 A process or multiple processes and / or boxes Figure 1 The steps for the functions specified in one or more boxes.

[0179] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit its protection scope. Although the present invention has been described in detail with reference to the above embodiments, ordinary technicians in the field should understand that after reading the present invention, those skilled in the art can still make various changes, modifications or equivalent substitutions to the specific implementation methods of the application, but these changes, modifications or equivalent substitutions are all within the protection scope of the claims to be approved.

Claims

1. A method for suppressing high-frequency oscillation of a flexible DC grid interface converter station, characterized in that: The interface converter station includes a phase-locked loop, the phase-locked loop includes a filter branch connected in parallel with a PI controller, and the method includes: Sampling operation data of the AC side of the flexible DC grid interface converter station, and determining whether high-frequency oscillation occurs in the flexible DC grid based on frequency domain characteristics of the operation data; If high-frequency oscillation occurs in the flexible DC grid, determining a high-frequency oscillation point based on the operating data; Based on the correlation between the filter parameters of the filter branch and the impedance characteristics of the interface converter station and the high-frequency oscillation mechanism of the flexible DC grid, select target filter parameters that make the flexible DC grid interface converter station have positive damping characteristics at the high-frequency oscillation point; Based on the target filtering parameter, the filtering branch is controlled to operate in the phase-locked loop to suppress high-frequency oscillation of the flexible DC grid interface converter station.

2. The method according to claim 1, characterized in that: Before sampling the AC side operation data of the flexible DC grid interface converter station, including: Obtaining an impedance characteristic curve of the AC system of the interface converter station and the flexible DC power grid after removing the filter branch; Based on the impedance method stability criterion and the impedance characteristic curves of the interface converter station and the AC system after the filter branch is removed, the impedance characteristics of the interface converter station at the oscillation frequency are analyzed to obtain the high-frequency oscillation mechanism of the flexible DC grid.

3. The method according to claim 2, characterized in that The expression of the stability criterion of the impedance method is: Among them, Z mmc represents the impedance of the flexible DC grid interface converter station, Z ac represents the impedance of the AC system, f represents the frequency, and ∠ represents the phase; The high-frequency oscillation mechanism of the flexible DC grid is that high-frequency oscillation is generated when the impedance phase difference between the flexible DC grid interface converter station and the AC system at the amplitude intersection frequency is outside the interval (-180°, 180°).

4. The method according to claim 2, characterized in that: The obtaining of the impedance characteristic curve of the AC system of the interface converter station and the flexible DC power grid after the filter branch is removed includes: The impedance characteristic curves of the interface converter station and the AC system after the filter branch is removed are obtained by an impedance sweep method.

5. The method according to claim 2, characterized in that: The obtaining of the impedance characteristic curve of the AC system of the interface converter station and the flexible DC power grid after the filter branch is removed includes: Building an AC line model of the flexible DC grid based on multiple equivalent π models and obtaining impedance characteristics of the AC line of the flexible DC grid; The impedance model of the AC line is obtained by calculation based on the impedance characteristics of the AC line and the series-parallel relationship of each element in a plurality of equivalent π models corresponding to the AC line model; Connecting the impedance model of the AC line and the equivalent impedance of the equivalent power source of the flexible DC grid in series to obtain an impedance model of the AC system, and obtaining an impedance characteristic curve of the AC system based on the impedance model of the AC system; An impedance model characterizing the impedance characteristics of the interface converter station is obtained by performing impedance calculation based on a linearization method, and an impedance characteristic curve of the interface converter station is obtained based on the impedance model of the interface converter station.

6. The method according to claim 5, characterized in that The impedance model of the interface converter station is expressed as: in, In the formula, is the high frequency positive sequence impedance of the flexible DC grid interface converter station, is the high frequency negative sequence impedance of the flexible DC grid interface converter station, U sN is the rated phase voltage on the AC side of the flexible DC grid interface converter station, L eq is the equivalent inductance of the AC side of the flexible DC grid interface converter station, G de is the link delay, G ffw is the voltage feedforward transfer function, G i is the inner loop controller, s is the Laplace operator, ω1 is the fundamental angular frequency, j is the imaginary number sign, k pPLL is the proportional coefficient of the phase-locked loop, k iPLL is the integral coefficient of the phase-locked loop, F pll is the phase-locked loop transfer function.

7. The method according to claim 2, characterized in that The operation data includes voltage data or current data, and determining whether high-frequency oscillation occurs in the flexible DC grid based on frequency domain characteristics of the operation data includes: Performing fast Fourier transform on the voltage data or current data on the AC side of the interface converter station to obtain corresponding frequency domain features; It is detected whether the frequency domain feature contains a high-frequency component. If the frequency domain feature contains a high-frequency component, it is determined that the flexible direct current grid has a high-frequency oscillation.

8. The method according to claim 3, characterized in that The correlation between the filtering parameters of the filtering branch and the impedance characteristics of the flexible DC grid interface converter station is the corresponding relationship between the filtering parameters of the filtering branch and the high-frequency negative damping characteristics of the interface converter station. Based on the correlation between the filtering parameters of the filtering branch and the impedance characteristics of the interface converter station and the high-frequency oscillation mechanism of the flexible DC grid, the target filtering parameters that make the flexible DC grid interface converter station have positive damping characteristics at the high-frequency oscillation point are screened out, including: Based on the correspondence between the filtering parameters of the filtering branch and the high-frequency negative damping characteristics of the interface converter station and the high-frequency oscillation mechanism of the flexible DC grid, the filtering parameters that stabilize the flexible DC grid and maximize the reduction of the high-frequency negative damping frequency range are selected as the target filtering parameters.

9. The method according to any one of claims 1 to 8, characterized in that: The controlling the filter branch to operate in the phase-locked loop based on the target filter parameter to suppress high-frequency oscillation of the flexible DC grid interface converter station includes: Controlling the filtering branch to perform high-pass filtering of the input voltage signal of the phase-locked loop based on the target filtering parameter to obtain a filtered signal; The filtered signal is superimposed on the proportional integral result of the PI controller to equivalently increase the proportional coefficient of the phase-locked loop and suppress high-frequency oscillation of the flexible DC grid interface converter station.

10. The method according to claim 1, characterized in that After determining whether the flexible DC grid has high-frequency oscillation, the method further includes: If it is determined that the flexible DC grid does not have high-frequency oscillation, the filter branch of the phase-locked loop is cut off.

11. A high-frequency oscillation suppression system for a flexible DC grid interface converter station, characterized in that: The interface converter station includes a phase-locked loop, the phase-locked loop includes a filter branch connected in parallel with a PI controller, and the high-frequency oscillation suppression system includes: A high-frequency oscillation determination module is used to sample the operation data of the AC side of the flexible DC grid interface converter station, and determine whether the flexible DC grid has high-frequency oscillation based on the frequency domain characteristics of the operation data; if the flexible DC grid has high-frequency oscillation, determine the high-frequency oscillation point based on the operation data; A parameter screening module, for screening target filtering parameters that make the flexible DC grid interface converter station have positive damping characteristics at the high-frequency oscillation point based on the correlation between the filtering parameters of the filtering branch and the impedance characteristics of the interface converter station and the high-frequency oscillation mechanism of the flexible DC grid; A control suppression module is used to control the filter branch to operate in the phase-locked loop based on the target filter parameter to suppress high-frequency oscillation of the flexible DC grid interface converter station.

12. The system according to claim 11, characterized in that The system further comprises: An impedance characteristic acquisition module, used to acquire an impedance characteristic curve of the AC system of the interface converter station and the flexible DC power grid after the filter branch is removed; The oscillation mechanism analysis module is used to analyze the impedance characteristics of the interface converter station at the oscillation frequency based on the impedance method stability criterion and the impedance characteristic curve of the interface converter station and the AC system after the filter branch is removed, so as to obtain the high-frequency oscillation mechanism of the flexible DC power grid.

13. The system according to claim 12, characterized in that The expression of the stability criterion of the impedance method is: Among them, Z mmc represents the impedance of the flexible DC grid interface converter station, Z ac represents the impedance of the AC system, f represents the frequency, and ∠ represents the phase; The high-frequency oscillation mechanism of the flexible DC grid is that high-frequency oscillation is generated when the impedance phase difference between the flexible DC grid interface converter station and the AC system at the amplitude intersection frequency is outside the interval (-180°, 180°).

14. The system according to claim 12, characterized in that The impedance characteristic acquisition module includes: The impedance test submodule is used to obtain the impedance characteristic curve of the interface converter station and the AC system after the filter branch is removed through the impedance sweep method.

15. The system according to claim 12, characterized in that The impedance characteristic acquisition module includes an impedance calculation submodule, and the impedance calculation submodule is used to: Building an AC line model of the flexible DC grid based on multiple equivalent π models and obtaining impedance characteristics of the AC line of the flexible DC grid; The impedance model of the AC line is obtained by calculation based on the impedance characteristics of the AC line and the series-parallel relationship of each element in a plurality of equivalent π models corresponding to the AC line model; Connecting the impedance model of the AC line and the equivalent impedance of the equivalent power source of the flexible DC grid in series to obtain an impedance model of the AC system, and obtaining an impedance characteristic curve of the AC system based on the impedance model of the AC system; An impedance model characterizing the impedance characteristics of the interface converter station is obtained by performing impedance calculation based on a linearization method, and an impedance characteristic curve of the interface converter station is obtained based on the impedance model of the interface converter station.

16. The system according to claim 15, characterized in that The impedance model of the interface converter station is expressed as: in, In the formula, is the high frequency positive sequence impedance of the flexible DC grid interface converter station, is the high frequency negative sequence impedance of the flexible DC grid interface converter station, U sN is the rated phase voltage on the AC side of the flexible DC grid interface converter station, L eq is the equivalent inductance of the AC side of the flexible DC grid interface converter station, G de is the link delay, G ffw is the voltage feedforward transfer function, G i is the inner loop controller, s is the Laplace operator, ω1 is the fundamental angular frequency, j is the imaginary number sign, k pPLL is the proportional coefficient of the phase-locked loop, k iPLL is the integral coefficient of the phase-locked loop, F pll is the phase-locked loop transfer function.

17. The system according to claim 11, characterized in that The operation data includes voltage data or current data, and the high-frequency oscillation determination module includes: A time-frequency transformation submodule, used for performing a fast Fourier transform on the voltage data or current data on the AC side of the interface converter station to obtain corresponding frequency domain features; The oscillation judgment submodule is used to detect whether the frequency domain feature contains a high-frequency component. If the frequency domain feature contains a high-frequency component, it is determined that the flexible direct current grid has a high-frequency oscillation.

18. The system according to claim 13, characterized in that The correlation between the filtering parameters of the filtering branch and the impedance characteristics of the flexible DC grid interface converter station is the corresponding relationship between the filtering parameters of the filtering branch and the high-frequency negative damping characteristics of the interface converter station. The parameter screening module is specifically used for: Based on the correspondence between the filtering parameters of the filtering branch and the high-frequency negative damping characteristics of the interface converter station and the high-frequency oscillation mechanism of the flexible DC grid, the filtering parameters that stabilize the flexible DC grid and maximize the reduction of the high-frequency negative damping frequency range are selected as the target filtering parameters.

19. The system according to any one of claims 11 to 18, characterized in that: The control suppression module comprises: A control filter submodule, used for controlling the filter branch to perform high-pass filtering on the input voltage signal of the phase-locked loop based on the target filter parameter to obtain a filtered signal; The signal superposition submodule is used to superimpose the filtered signal with the proportional integral result of the PI controller to equivalently increase the proportional coefficient of the phase-locked loop and suppress the high-frequency oscillation of the flexible DC grid interface converter station.

20. The system according to claim 11, characterized in that The system further comprises: The branch removal module is used to remove the filter branch of the phase-locked loop if it is determined that the flexible DC grid does not have high-frequency oscillation.

21. An electronic device, characterized in that: include: at least one processor and memory; The memory and the processor are connected via a bus; The memory is used to store one or more programs; When the one or more programs are executed by the at least one processor, the high-frequency oscillation suppression method for the flexible DC grid interface converter station according to any one of claims 1 to 10 is implemented.

22. A readable storage medium, characterized in that: An execution program is stored thereon, and when the execution program is executed, the high-frequency oscillation suppression method of the flexible DC grid interface converter station according to any one of claims 1 to 10 is implemented.

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