Broadband harmonic type analysis and propagation path tracking method for flexible DC power grid system

Abstract

The invention discloses a broadband harmonic type analysis and propagation path tracking method for a flexible DC power grid system, and the method comprises the following steps: determining the number and positions of DC network harmonic monitoring points according to an electrical main wiring diagram of the flexible DC power grid system, and the harmonic limit values of different types and frequencies in each DC network harmonic monitoring point; determining the sampling frequency and interval of each converter station harmonic monitoring point, and carrying out harmonic online monitoring on the DC side bus current of each converter station of the DC power grid; when a harmonic amplitude exceeding event occurs in the direct-current power grid, determining the type of the exceeding harmonic and the frequency or frequency band of the exceeding harmonic needing to be tracked, and issuing a monitoring instruction to all monitoring points; and according to the amplitude relation of the harmonic waves needing to be tracked by each harmonic wave monitoring point in the same time period, rapid tracking of the harmonic waves in a direct current transmission network propagation path is realized by taking the frequency as the characteristic. According to the invention, rapid tracking of propagation paths of harmonic waves with different frequencies or frequency bands in the flexible DC power grid system is realized.

Description

Technical Field

[0001] The present invention belongs to the technical field of flexible DC power grids, and particularly relates to a method for analyzing the wide-frequency harmonic types and tracking the propagation paths of a flexible DC power grid system. Background Art

[0002] A flexible DC power grid interconnects multiple voltage source converter stations through multiple DC transmission lines to form a ring / mesh topology. Among them, the ring topology can not only enhance the redundancy and transmission reliability of the DC transmission system, but also improve the flexibility and security of the operation control of the DC transmission system, and realize the efficient transmission and optimal distribution of large-capacity electric energy in a wide area. When multiple converter stations in a flexible DC power grid system operate normally, they will emit a large amount of high-frequency harmonics to the AC side and DC side networks. Especially, the harmonic emission characteristics under the multi-level converter topology and sine pulse width modulation (PWM) have ultra-high order (2 - 150 kHz) and wide-frequency domain characteristics. The mesh DC transmission line structure in the flexible DC power grid system will cause the high-frequency harmonics emitted by different converter stations to interact and superimpose on the DC transmission network, resulting in complex and variable propagation characteristics of high-frequency harmonics in the DC network, showing a globalized feature, which affects the safe and stable operation of the flexible DC power grid system.

[0003] Currently, cases where the DC system is forced to shut down due to excessive high-frequency harmonic amplitudes have been reported in multiple flexible DC transmission projects. Tracking the propagation path of high-frequency harmonics in a flexible DC power grid system is also the basis for locating harmonic disturbance sources and dividing harmonic responsibilities.

[0004] Currently, the acquisition of high-frequency harmonic data for a flexible DC power grid system is mainly carried out for the AC side busbars and lines of converter stations. The utilization of harmonic acquisition data is mostly limited to basic statistical analysis, and can only simply understand the daily / monthly / quarterly time-varying characteristics and over-standard situations of the power quality indicators at harmonic monitoring points, which is difficult to meet the tracking of the harmonic propagation path in a flexible DC power grid system, especially the harmonic path tracking on the DC side. Multiple converter stations in a flexible DC power grid emit high-frequency harmonics to the DC transmission network. These high-frequency harmonics are intertwined and superimposed, not only making the high-frequency harmonic disturbance of the DC power grid globalized; at the same time, the harmonic emission characteristics of each converter station will also change dynamically with the operation mode and the magnitude of power conversion, making it very difficult to trace the high-frequency harmonics in a flexible DC power grid system. Summary of the Invention

[0005] The present invention is proposed to solve the problems existing in the prior art, and its purpose is to provide a method for analyzing the wide-frequency harmonic types and tracking the propagation paths of a flexible DC power grid system.

[0006] The technical solution of the present invention is: a method for analyzing the broadband harmonic types and tracking the propagation paths in a flexible DC power grid system, including the following steps:

[0007] A. According to the electrical main wiring diagram of the flexible DC power grid system, determine the number and locations of the DC network harmonic monitoring points, as well as the harmonic limits of different types and frequencies at each DC network harmonic monitoring point;

[0008] B. Determine the sampling frequency and interval of each converter station harmonic monitoring point, and conduct online harmonic monitoring on the DC side bus currents of each converter station in the DC power grid;

[0009] C. When a harmonic amplitude exceeding standard event occurs in the DC power grid, determine the type of the exceeding standard harmonic and the frequency or frequency band for which the exceeding standard harmonic needs to be tracked, and issue monitoring instructions to all monitoring points;

[0010] D. According to the harmonic amplitude relationship of the harmonics to be tracked at each harmonic monitoring point within the same time period, quickly track the propagation path of the harmonics in the DC transmission network with frequency as the characteristic.

[0011] Furthermore, the specific process of determining the number and locations of the DC network harmonic monitoring points in step A is as follows:

[0012] First, obtain the electrical main wiring diagram of the DC side of the involved flexible DC power grid system, and number all the converter stations and DC lines in the flexible DC power grid system;

[0013] Then, according to the electrical main wiring diagram of the flexible DC power grid system and the operating modes of each converter station, determine the number and locations of the harmonic monitoring points in the DC transmission network of the flexible DC power grid.

[0014] Furthermore, the specific process of determining the number and locations of the harmonic monitoring points in the DC transmission network of the flexible DC power grid is as follows:

[0015] For the flexible DC converter station, harmonic monitoring points are arranged at both the DC side output port and the DC bus, and corresponding numbers are assigned according to the number of the flexible DC converter station;

[0016] For the DC transmission lines in the flexible DC power grid system, one harmonic monitoring point needs to be set for each DC transmission line; and the sampling data of each DC transmission line harmonic monitoring point is stored nearby in the data center of the converter station closest to the sampling point.

[0017] Furthermore, the specific process of determining the harmonic limits of different types and frequencies at each DC network harmonic monitoring point in step A is as follows:

[0018] First, obtain the AC side and DC side voltage levels of the flexible DC power grid system, and the capacities of each converter station and line;

[0019] Then, combining the above data with the national standards for power quality, determine the limits of each harmonic, interharmonic, and total harmonic distortion rate at the harmonic monitoring points on the DC side ports of each converter station in the flexible DC power grid;

[0020] Finally, store according to the numbers of each harmonic monitoring point.

[0021] Furthermore, in step B, determine the sampling frequency and interval of the harmonic monitoring points of each converter station. The specific process is as follows:

[0022] First, according to the topological structure and modulation mode of each converter station in the flexible DC power grid system, determine the switching frequency of the power electronic devices of each converter station;

[0023] Then, select and determine the sampling frequency and sampling interval of the electrical quantity signals of each harmonic monitoring point in the flexible DC power grid system according to the above switching frequency.

[0024] Furthermore, in step B, perform online harmonic monitoring on the DC side bus currents of each converter station in the DC power grid. The specific process is as follows:

[0025] First, when the flexible DC power grid system is operating normally, only send monitoring instructions to the harmonic monitoring points at the DC buses of each converter station. These harmonic monitoring points perform online sampling and data processing on the current signals at the DC buses of each converter station according to the specified sampling frequency; while the other harmonic monitoring points in the DC power grid system do not perform signal sampling actions;

[0026] Then, perform denoising processing and fast Fourier series analysis on the time series data collected by the harmonic monitoring points at the DC buses of each converter station to obtain the harmonic current frequencies and amplitudes at the DC buses of each converter station in the same time period;

[0027] Finally, according to the limits of the harmonic current amplitudes of each harmonic monitoring point, determine whether there is an over - standard situation in the harmonic current amplitude or the total harmonic distortion rate of the current at the DC bus of the converter station. If there is no over - standard situation, no processing is required.

[0028] Furthermore, in step C, when a harmonic amplitude over - standard event occurs in the DC power grid, if there is an over - standard situation in the harmonic current amplitude or the total harmonic distortion rate of the harmonic current at the harmonic monitoring points in the involved flexible DC power grid system, immediately report to the system - level control center of the flexible DC power grid system. At the same time, it is also necessary to report the frequency / band and amplitude of the over - standard harmonics.

[0029] Furthermore, in step C, determine the type of the over - standard harmonics and the frequency or band that needs to track the over - standard harmonics, and send monitoring instructions to all monitoring points. The specific process is as follows:

[0030] First, the system-level control center of the flexible DC grid determines the type of the excessive harmonic in the DC grid system according to the reported excessive harmonic frequency or frequency band.

[0031] Then, the system-level control center of the flexible DC grid determines the harmonic frequency or frequency band to be tracked according to the harmonic type and spectral characteristics.

[0032] Next, the system-level control center of the flexible DC grid determines the sampling frequency and sampling interval of the electrical quantity signals of all harmonic monitoring points in the DC grid according to the harmonic frequency or frequency band to be tracked.

[0033] Finally, the system-level control center of the flexible DC grid issues monitoring instructions and the sampling frequency of the electrical quantity signals to all harmonic monitoring points.

[0034] Furthermore, D. According to the relationship of the harmonic amplitudes to be tracked at each harmonic monitoring point within the same time period, the rapid tracking of the propagation path of the harmonic in the DC transmission network is realized with frequency as the characteristic. The specific process is as follows:

[0035] First, when the flexible DC grid system involved reaches a steady state, all harmonic monitoring points collect the current signals at this point according to the issued monitoring instructions and the sampling frequency of the electrical quantity signals within the same time period.

[0036] Then, all harmonic monitoring points in the DC grid store and analyze the collected data to obtain the harmonic current amplitudes to be tracked at each harmonic monitoring point; and upload the harmonic current amplitude data to be tracked to the system-level control center of the flexible DC grid.

[0037] Next, the system-level control center of the flexible DC grid, according to the harmonic amplitude data of the frequency or frequency band to be tracked reported by each harmonic monitoring point, and in combination with the electrical main wiring diagram of the flexible DC grid system, uses the frequency to be tracked as the characteristic quantity, and determines the propagation path diagram of the harmonic to be tracked according to the harmonic amplitude relationship at different harmonic monitoring points within the same time period, so as to realize the rapid tracking of the harmonic propagation path in the flexible DC grid system.

[0038] Finally, according to the amplitude relationship of the harmonic to be tracked at different harmonic monitoring points, the quantification of the harmonic responsibility for each converter station is determined.

[0039] The beneficial effects of the present invention are as follows:

[0040] The present invention can quickly analyze and judge whether harmonic over-standard occurs in the flexible DC grid system and the type of the over-standard harmonic. The present invention can meet the monitoring requirements of the flexible DC grid system for different types and frequencies of harmonics.

[0041] The present invention realizes the rapid tracking of the propagation paths of harmonics with different frequencies or frequency bands in a flexible DC power grid system. The present invention realizes the quantitative calculation of harmonic responsibilities of different converter stations in a flexible DC power grid system. Description of the Drawings

[0042] Figure 1 is the flowchart of the method of the present invention. Detailed Embodiments

[0043] Hereinafter, the present invention will be described in detail with reference to the drawings and embodiments:

[0044] As Figure 1 shown, the method for analyzing the wide - band harmonic types and tracking the propagation paths in a flexible DC power grid system includes the following steps:

[0045] A. According to the electrical main connection diagram of the flexible DC power grid system, determine the number and positions of DC network harmonic monitoring points, as well as the harmonic limits of different types and frequencies in each DC network harmonic monitoring point;

[0046] B. Determine the sampling frequencies and intervals of harmonic monitoring points at each converter station, and conduct on - line harmonic monitoring of the DC - side bus currents of each converter station in the DC power grid;

[0047] C. When a harmonic amplitude over - standard event occurs in the DC power grid, determine the type of the over - standard harmonic and the frequency or frequency band for which the over - standard harmonic needs to be tracked, and issue monitoring instructions to all monitoring points;

[0048] D. According to the relationship of the harmonic amplitudes to be tracked at each harmonic monitoring point within the same time period, achieve the rapid tracking of the propagation path of harmonics in the DC transmission network with frequency as the feature.

[0049] In step A, the process of determining the number and positions of DC network harmonic monitoring points is as follows:

[0050] First, obtain the electrical main connection diagram of the DC side of the involved flexible DC power grid system, and number all converter stations and DC lines in the flexible DC power grid system;

[0051] Then, according to the electrical main connection diagram of the flexible DC power grid system and the operation modes of each converter station, determine the number and positions of harmonic monitoring points in the DC transmission network of the flexible DC power grid.

[0052] The process of determining the number and positions of harmonic monitoring points in the DC transmission network of the flexible DC power grid is as follows:

[0053] For a flexible - to - DC converter station, harmonic monitoring points are arranged at both its DC - side output port and DC bus, and corresponding numbers are assigned according to the number of the flexible - to - DC converter station;

[0054] For the DC transmission lines in the flexible DC grid system, a harmonic monitoring point needs to be set for each DC transmission line; and the sampling data of the harmonic monitoring points of each DC transmission line is stored near the sampling point in the data center of the converter station closest to the sampling point.

[0055] In step A, the harmonic limits of different types and frequencies in each DC network harmonic monitoring point are determined, and the specific process is as follows:

[0056] First, obtain the voltage levels on the AC side and DC side of the flexible DC grid system, and the capacities of each converter station and line.

[0057] Then, combine the above data with the national standard of power quality to determine the limits of each harmonic, inter-harmonic, and total harmonic distortion rate of the harmonic monitoring points at the DC side ports of each converter station in the flexible DC grid.

[0058] Finally, store according to the numbers of each harmonic monitoring point.

[0059] Specifically, the limit of the k-th harmonic current in the harmonic monitoring point at the DC side port of the converter station The calculation formula is as follows:

[0060]

[0061] Among them, U ac , U dc are the rated voltage values on the AC side and DC side of this converter station respectively.

[0062] and are the limits of the (k - 1)-th and (k + 1)-th harmonics on the AC side of this converter station respectively.

[0063] Specifically, the formula for the limit of the total harmonic distortion rate of the current in the harmonic monitoring point at the DC side port of each converter station is as follows:

[0064]

[0065] Among them, I dc0 is the DC current value corresponding to the rated capacity of the DC side of this converter station.

[0066] In step B, the sampling frequencies and intervals of the harmonic monitoring points of each converter station are determined, and the specific process is as follows:

[0067] First, according to the topological structure and modulation mode of each converter station in the flexible DC grid system, determine the switching frequency of the power electronic devices of each converter station.

[0068] Then, select and determine the electrical quantity signal sampling frequency and sampling interval of each harmonic monitoring point in the flexible DC grid system according to the above switching frequency.

[0069] As an implementation manner, the switching frequency f of the power electronic devices in each converter station s . If the converter station adopts a low-level structure and single-polarity / double-polarity SPWM modulation, the sampling frequency and sampling interval of the electrical quantity signals at each harmonic monitoring point in the flexible DC power grid system are selected and determined according to at least twice the switching frequency.

[0070] As another implementation manner, if the converter station adopts a low-level structure and single-polarity frequency-doubling SPWM modulation, the sampling frequency and sampling interval of the electrical quantity signals at each harmonic monitoring point in the flexible DC power grid system are selected and determined according to at least four times the switching frequency.

[0071] As still another implementation manner, if the converter adopts a multi-level topology structure and the nearest level approximation modulation technique, then according to the structure and modulation method, the sampling frequency and sampling interval of the electrical quantity signals at each harmonic monitoring point in the flexible DC power grid system are selected and determined according to at least twice the switching frequency.

[0072] In step B, the harmonic online monitoring of the DC side bus current of each converter station in the DC power grid is carried out, and the specific process is as follows:

[0073] First, when the flexible DC power grid system is operating normally, monitoring instructions are only sent to the harmonic monitoring points at the DC buses of each converter station. These harmonic monitoring points perform online sampling and data processing on the current signals at the DC buses of each converter station according to the specified sampling frequency; while the other harmonic monitoring points in the DC power grid system do not perform signal sampling actions;

[0074] Specifically, due to the high sampling frequency and large amount of data of the harmonic monitoring points, the time series data collected by all harmonic monitoring points are stored in the local data storage data center of the corresponding converter station according to the sampling point numbers.

[0075] Then, the time series data collected at the harmonic monitoring points at the DC buses of each converter station are subjected to denoising processing and fast Fourier series analysis to obtain the harmonic current frequencies and amplitudes at the DC buses of each converter station in the same time period;

[0076] Finally, according to the limit values of the harmonic current amplitudes at each harmonic monitoring point, it is judged whether there is an over-standard of the harmonic current amplitude or the total harmonic distortion rate of the current at the DC bus of the converter station. If there is no over-standard, no processing is performed.

[0077] In step C, when a harmonic amplitude over-standard event occurs in the DC power grid, if there is an over-standard of the harmonic current amplitude or the total harmonic distortion rate of the harmonic at the harmonic monitoring point in the involved flexible DC power grid system, it shall immediately report to the system-level control center of the flexible DC power grid system. At the same time, the frequency / frequency band and amplitude of the over-standard harmonic also need to be reported.

[0078] In step C, determine the type of the excessive harmonic and the frequency or frequency band for which the excessive harmonic needs to be traced, and issue monitoring instructions to all monitoring points. The specific process is as follows:

[0079] First, the system-level control center of the flexible DC power grid determines the type of the excessive harmonic in the DC power grid system according to the reported frequency or frequency band of the excessive harmonic;

[0080] Then, the system-level control center of the flexible DC power grid determines the harmonic frequency or frequency band to be traced according to the harmonic type and spectral characteristics;

[0081] Next, the system-level control center of the flexible DC power grid determines the sampling frequency and sampling interval of the electrical quantity signals of all harmonic monitoring points in the DC power grid according to the harmonic frequency or frequency band to be traced;

[0082] Finally, the system-level control center of the flexible DC power grid issues monitoring instructions and the sampling frequency of the electrical quantity signals to all harmonic monitoring points.

[0083] Specifically, the process of determining the type of the excessive harmonic in the DC power grid system is as follows:

[0084] If the frequency of the excessive harmonic is an integer multiple of the switching frequency, i.e., kf s (k is a positive integer), then the type of the excessive harmonic is the harmonic caused by the switching of the power device.

[0085] If the frequency f h value satisfies 0 < f h < f 0 or f 0 < f h < 2f 0 , then the type of this harmonic is the inter-harmonic caused by the AC side of this station. Wherein, f 0 is the rated frequency of the AC power grid.

[0086] D. According to the relationship of the harmonic amplitudes to be traced at each harmonic monitoring point within the same time period, quickly trace the propagation path of the harmonic in the DC transmission network with frequency as the feature. The specific process is as follows:

[0087] First, when the flexible DC power grid system involved reaches a steady state, all harmonic monitoring points collect the current signals at this point according to the issued monitoring instructions and the sampling frequency of the electrical quantity signals within the same time period;

[0088] Then, all harmonic monitoring points in the DC power grid store and analyze the collected data to obtain the harmonic current amplitudes to be traced at each harmonic monitoring point; and upload the harmonic current amplitude data to be traced to the system-level control center of the flexible DC power grid;

[0089] Subsequently, the system-level control center of the flexible DC grid determines the propagation path diagram of the harmonics to be tracked based on the harmonic amplitude data of the frequencies or frequency bands that need to be tracked reported by each harmonic monitoring point and in combination with the electrical main wiring diagram of the flexible DC grid system. Using the required tracking frequency as a characteristic quantity and based on the harmonic amplitude relationship of different harmonic monitoring points within the same time period, the system quickly tracks the propagation path of the harmonics in the flexible DC grid system;

[0090] Finally, based on the amplitude relationship of the harmonics to be tracked at different harmonic monitoring points, the harmonic responsibilities of each converter station are quantified.

[0091] Specifically, the proportion of the harmonic responsibility of each converter station in the flexible DC grid is calculated as follows:

[0092]

[0093] In the formula, is the amplitude of the harmonic current to be tracked at the DC port of the i-th converter station in the flexible DC grid system.

[0094] The present invention can quickly analyze and judge whether the harmonics in the flexible DC grid system exceed the standard and the types of the excessive harmonics. The present invention can meet the monitoring requirements of the flexible DC grid system for different types and frequencies of harmonics.

[0095] The present invention realizes the rapid tracking of the propagation paths of harmonics with different frequencies or frequency bands in the flexible DC grid system. The present invention realizes the quantitative calculation of the harmonic responsibilities of different converter stations in the flexible DC grid system.

Claims

1. A method for analyzing broadband harmonic types and tracing propagation paths in a flexible DC power grid system, characterized by: The following steps are involved: A. According to the electrical main wiring diagram of the flexible DC grid system, determine the number and location of DC network harmonic monitoring points, as well as the harmonic limits of different types and frequencies in each DC network harmonic monitoring point; B. Determine the sampling frequency and interval of the harmonic monitoring points of each converter station, and conduct online harmonic monitoring of the DC bus current of each converter station in the DC grid; C. When a harmonic amplitude exceeds the standard in the DC power grid, determine the type of the harmonic that exceeds the standard and the frequency or frequency band of the harmonic that needs to be tracked, and issue monitoring instructions to all monitoring points; D. Based on the relationship between the harmonic amplitudes that need to be tracked at each harmonic monitoring point in the same time period, the frequency is used as a feature to achieve rapid tracking of the propagation path of harmonics in the DC transmission network.

2. The method for analyzing broadband harmonic types and tracing propagation paths of a flexible DC power grid system according to claim 1, characterized in that: In step A, the number and location of DC network harmonic monitoring points are determined. The specific process is as follows: First, the main electrical connection diagram of the DC side of the flexible DC grid system involved is obtained, and all converter stations and DC lines in the flexible DC grid system are numbered; Then, according to the electrical main wiring diagram of the flexible DC grid system and the operation mode of each converter station, the number and location of harmonic monitoring points in the DC transmission network of the flexible DC grid are determined.

3. The method for analyzing broadband harmonic types and tracing propagation paths of a flexible DC power grid system according to claim 2, characterized in that: Determine the number and location of harmonic monitoring points in the flexible DC grid DC transmission network. The specific process is as follows: For flexible DC converter stations, harmonic monitoring points are arranged at the DC side output ports and DC busbars, and are numbered accordingly according to the number of the flexible DC converter station; For the DC transmission lines in the flexible DC grid system, a harmonic monitoring point needs to be set up for each DC transmission line; and the sampling data of the harmonic monitoring point of each DC transmission line is stored in the data center of the converter station closest to the sampling point.

4. The method for analyzing broadband harmonic types and tracing propagation paths of a flexible DC power grid system according to claim 1, characterized in that: In step A, the harmonic limits of different types and frequencies in each DC network harmonic monitoring point are determined. The specific process is as follows: Firstly, the voltage levels of the AC and DC sides of the flexible DC grid system and the capacities of each converter station and line are obtained; Then, the above data are combined with the national standard for power quality to determine the limits of each harmonic, interharmonic and total harmonic distortion rate at the harmonic monitoring points of the DC side ports of each converter station in the flexible DC grid; Finally, the harmonic monitoring points are stored according to their numbers.

5. The method for analyzing broadband harmonic types and tracing propagation paths of a flexible DC power grid system according to claim 1, characterized in that: In step B, the sampling frequency and interval of the harmonic monitoring points of each converter station are determined. The specific process is as follows: Firstly, according to the topological structure and modulation mode of each converter station in the flexible DC grid system, the switching frequency of the power electronic devices of each converter station is determined; Then, the electrical quantity signal sampling frequency and sampling interval of each harmonic monitoring point in the flexible DC grid system are selected and determined according to the above switching frequency.

6. The method for analyzing broadband harmonic types and tracing propagation paths of a flexible DC power grid system according to claim 1, characterized in that: In step B, the harmonics of the DC bus current of each converter station in the DC grid are monitored online. The specific process is as follows: First, when the flexible DC grid system is working normally, it only issues monitoring instructions to the harmonic monitoring points at the DC bus of each converter station. These harmonic monitoring points perform online sampling and data processing on the current signals at the DC bus of each converter station according to the specified sampling frequency; while other harmonic monitoring points in the DC grid system do not perform signal sampling actions; Then, the time series data collected by the harmonic monitoring points at the DC bus of each converter station are subjected to denoising and fast Fourier series analysis to obtain the harmonic current frequency and amplitude of the DC bus of each converter station in the same time period; Finally, according to the limit values ​​of the harmonic current amplitude at each harmonic monitoring point, it is determined whether the harmonic current amplitude of the DC bus of the converter station exceeds the standard or the total harmonic distortion rate of the current exceeds the standard. If it does not exceed the standard, no action is taken.

7. The method for analyzing broadband harmonic types and tracing propagation paths of a flexible DC power grid system according to claim 1, characterized in that: In step C, if a harmonic amplitude exceeds the standard event in the DC power grid, and if the harmonic current amplitude exceeds the standard or the total harmonic distortion rate of the current exceeds the standard at the harmonic monitoring point of the flexible DC power grid system involved, it shall be reported immediately to the system-level control center of the flexible DC power grid system. At the same time, the frequency / frequency band and amplitude of the exceeding harmonics shall also be reported.

8. The method for analyzing broadband harmonic types and tracing propagation paths of a flexible DC power grid system according to claim 1, characterized in that: In step C, the type of excessive harmonics and the frequency or frequency band of the excessive harmonics to be tracked are determined, and monitoring instructions are issued to all monitoring points. The specific process is as follows: First, the system-level control center of the flexible DC grid determines the type of excessive harmonics in the DC grid system based on the reported excessive harmonic frequencies or frequency bands; Then, the system-level control center of the flexible DC grid determines the harmonic frequency or frequency band that needs to be tracked based on the harmonic type and spectrum characteristics; Afterwards, the system-level control center of the flexible DC grid will need to track the harmonic frequency or frequency band and determine the sampling frequency and sampling interval of the electrical quantity signals of all harmonic monitoring points in the DC grid; Finally, the system-level control center of the flexible DC grid issues monitoring instructions and electrical quantity signal sampling frequencies to all harmonic monitoring points.

9. The method for analyzing broadband harmonic types and tracing propagation paths of a flexible DC power grid system according to claim 1, characterized in that: D. According to the harmonic amplitude relationship that needs to be tracked at each harmonic monitoring point in the same time period, the frequency is used as a feature to achieve rapid tracking of the propagation path of harmonics in the DC transmission network. The specific process is as follows: First, when the flexible DC grid system involved reaches a steady state, all harmonic monitoring points collect the current signal of the point in the same time period according to the monitoring instructions issued and the sampling frequency of the electrical quantity signal; Then, all harmonic monitoring points in the DC grid store and analyze the collected data to obtain the harmonic current amplitudes required to be tracked at each harmonic monitoring point; and upload the harmonic current amplitude data required to be tracked to the system-level control center of the flexible DC grid; Then, the system-level control center of the flexible DC grid determines the propagation path diagram of the harmonics to be tracked based on the harmonic amplitude data of the frequency or frequency band to be tracked reported by each harmonic monitoring point, combined with the electrical main wiring diagram of the flexible DC grid system, and taking the required tracking frequency as the characteristic quantity, according to the harmonic amplitude relationship of different harmonic monitoring points in the same time period, so as to realize the rapid tracking of the harmonic propagation path of the flexible DC grid system; Finally, the harmonic responsibility of each converter station is quantified according to the amplitude relationship of the required tracked harmonics at different harmonic monitoring points.

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