CN111999595B - MMC-HVDC power transmission line fault judging method - Google Patents
MMC-HVDC power transmission line fault judging method Download PDFInfo
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R31/00—Arrangements for testing electric properties; Arrangements for locating electric faults; Arrangements for electrical testing characterised by what is being tested not provided for elsewhere
- G01R31/08—Locating faults in cables, transmission lines, or networks
- G01R31/088—Aspects of digital computing
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- G—PHYSICS
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- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R31/00—Arrangements for testing electric properties; Arrangements for locating electric faults; Arrangements for electrical testing characterised by what is being tested not provided for elsewhere
- G01R31/08—Locating faults in cables, transmission lines, or networks
- G01R31/081—Locating faults in cables, transmission lines, or networks according to type of conductors
- G01R31/085—Locating faults in cables, transmission lines, or networks according to type of conductors in power transmission or distribution lines, e.g. overhead
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R31/00—Arrangements for testing electric properties; Arrangements for locating electric faults; Arrangements for electrical testing characterised by what is being tested not provided for elsewhere
- G01R31/08—Locating faults in cables, transmission lines, or networks
- G01R31/081—Locating faults in cables, transmission lines, or networks according to type of conductors
- G01R31/086—Locating faults in cables, transmission lines, or networks according to type of conductors in power transmission or distribution networks, i.e. with interconnected conductors
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- H—ELECTRICITY
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- H02J3/00—Circuit arrangements for ac mains or ac distribution networks
- H02J3/36—Arrangements for transfer of electric power between ac networks via a high-tension dc link
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J2203/00—Indexing scheme relating to details of circuit arrangements for AC mains or AC distribution networks
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- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
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Abstract
The invention relates to a fault judging method of an MMC-HVDC power transmission line, which is technically characterized by comprising the following steps of: the invention adopts discrete wavelet analysis to extract the effective values of the high-frequency voltage at the direct current side and the converter side of the line smoothing reactor for comparison so as to judge faults in and out of the area. And then adopting a K-means clustering algorithm, utilizing the voltage and current data of the single-side protection unit acquired by using different fault types and different fault positions and a shorter time window after the fault of the direct current line occurs, carrying out clustering analysis on the voltage and current data, acquiring the centroid and the threshold value of the voltage and current data of the single-side protection unit, and determining corresponding protection criteria through training to realize fault pole selection. The method can quickly and accurately detect the faults of the MMC-HVDC power transmission line without being influenced by the fault positions and the transition resistance, and identify the fault types.
Description
Technical Field
The invention belongs to the technical field of power transmission lines, and particularly relates to a fault judging method of an MMC-HVDC power transmission line.
Background
In recent years, china quickens the development and utilization of renewable energy sources represented by wind energy and solar energy, and the clean energy source ratio is continuously improved. Compared with the traditional direct current transmission technology, the flexible direct current transmission system can independently adjust active power and reactive power, has no commutation failure problem, and is very suitable for large-scale and long-distance transmission of renewable energy sources.
Converters employed in flexible dc systems include two-level or three-level voltage source converters based on pulse width modulation PWM techniques and modular multilevel converters (modular multilevel converter, MMC) based on Yu Jieti wave modulation techniques. The two-level or three-level converter has higher switching frequency, larger switching loss, poorer voltage waveform and more application to lower voltage class. At present, in a flexible direct current power grid, particularly a direct current power grid with higher voltage level, a modularized multi-level converter is widely applied.
The fault characteristics of an MMC-HVDC system are closely related to the primary system structure and the control system, and the electrical quantity, such as current and voltage, of a direct current transmission line presents different characteristics when different types of systems have faults. The MMC-HVDC system which is put into operation at present adopts a symmetrical monopole main wiring mode, and a natural bipolar system is formed by a single converter. Grounding modes are divided into an alternating-current side grounding mode and a direct-current side grounding mode through a clamping resistor.
An MMC-HVDC power transmission line with the dc side grounded via a clamping resistor is constituted by half-bridge submodules as shown in fig. 2. The system mainly comprises an MMC rectifying station, an inversion station, a direct current transmission line and smoothing reactors at two ends of the line, wherein an M side is a rectifying side, an N side is an inversion side, and each bridge arm of the MMC consists of a plurality of sub-modules and a bridge arm reactance. The topology structure of the three-phase MMC is shown in FIG. 3, and comprises 6 bridge arms, wherein the upper bridge arm and the lower bridge arm of each phase are combined together to form a phase unit.
The working state of the submodule can be changed by controlling the on and off of the IGBT in the submodule, and the analysis can know that the submodule has 3 working states and 6 working modes. In actual operation of the MMC, the voltage of the direct current side needs to be kept constant, and the number of submodules in an input state at each moment in each phase unit is required to be equal and kept unchanged; the three-phase ac voltage needs to be output on the ac side, and is generally realized by a pulse width modulation PWM method and a step wave modulation method. Along with the increase of the number of submodules of the modularized multi-level converter, the latest level approach modulation mode can output quite ideal alternating voltage waveforms, and the method is widely applied.
Wherein, the faults of MMC-HVDC transmission line can be divided into AC system faults and DC system faults. When the AC system faults occur, the positive sequence voltage of the AC power grid drops greatly, and the current of the DC system drops; if the fault is an asymmetry fault, double frequency fluctuation of direct current voltage and current can be caused, and the fluctuation can be eliminated and restrained by improving a control strategy.
For dc system faults, it can be classified into monopolar ground faults and bipolar short-circuit faults. For a unipolar grounding fault, due to the existence of a clamping resistor in a direct current system, the voltage drop of a fault pole pair is zero, and the voltage of a non-fault pole pair is doubled. The bridge arm current rises to some extent, but the overcurrent flow is not large, the direct current can quickly recover to be normal, and the direct current voltage can fluctuate in a short time due to the charging and discharging processes of the direct current circuit. For bipolar ground short circuit, the direct current voltage is rapidly reduced to zero, the direct current is rapidly increased and can be increased to several times of rated current, and finally, the voltage is stable. Therefore, the bipolar grounding short-circuit fault is the most serious fault type and has great influence on the safe, reliable and stable operation of the power system.
However, for the current MMC-HVDC power transmission line, a method for accurately identifying a fault area and judging a fault type is not found, so that a huge problem is caused in maintenance and overhaul of the MMC-HVDC power transmission line, and the development of a flexible direct current power transmission system is restrained.
Disclosure of Invention
The invention aims to overcome the defects of the prior art, and provides a fault judging method for an MMC-HVDC power transmission line, which can accurately judge a fault area and judge a fault type.
The invention solves the technical problems by adopting the following technical scheme:
the MMC-HVDC power transmission line fault judging method comprises the following steps:
step 1, monitoring the high-frequency direct-current voltage change rate of the direct-current side of the smoothing reactor of the MMC-HVDC power transmission line in real time, judging whether the direct-current voltage change rate is larger than a direct-current voltage change rate threshold value, if so, performing step 2, otherwise, repeatedly performing step 1;
step 2, extracting a high-frequency direct-current voltage effective value of a smoothing reactor direct-current side of the MMC-HVDC power transmission line and a high-frequency direct-current voltage effective value of a converter side through discrete wavelet analysis, and carrying out fault judgment according to the extracted effective values, wherein the judgment result is divided into a fault, an out-of-zone fault or no fault of the direct-current line;
step 3, continuously collecting positive and negative voltage and current of the MMC-HVDC rectifying side or positive and negative voltage and current of the inversion side through a K-means clustering algorithm by adopting a time window, performing per unit processing on the positive and negative voltage and current of the MMC-HVDC rectifying side or the positive and negative voltage and current of the inversion side, and summing to obtain a data processing result;
step 4, taking the data processing result as a vector X, and calculating the vector X and each fault optimal centroid n m And determining the selected vector X and the best centroid n of each fault m And judging whether the minimum value of the minimum distance is smaller than or equal to a threshold value corresponding to the fault, if so, judging that the MMC-HVDC power transmission line is corresponding to the fault, and if not, judging that the MMC-HVDC power transmission line is out-of-zone fault or has no fault.
The calculating method for extracting the high-frequency direct-current voltage effective value of the smoothing reactor on the MMC-HVDC power transmission line and the high-frequency direct-current voltage effective value on the converter side in the step 2 is as follows:
wherein U is R Is the effective value of the direct current voltage, k is the sampling point number in a sampling time window, u tran Sample values representing the dc voltage high frequency band, i=1, 2.
The fault judging method in the step 2 is as follows: judging whether the absolute value of the difference value between the DC side high-frequency DC voltage effective value of the smoothing reactor of the MMC-HVDC power transmission line and the DC side high-frequency DC voltage effective value of the converter is larger than the set threshold value of the voltage effective value difference value, judging whether the DC side high-frequency DC voltage effective value of the smoothing reactor is larger than the DC side high-frequency DC voltage effective value of the converter, if so, judging that the DC line fails, and otherwise, judging that the DC line fails or is free of faults.
In the step 3, the method for carrying out per unit processing and summing on the positive and negative voltage and current of the MMC-HVDC rectification side or the positive and negative voltage and current of the inversion side to obtain the data processing result comprises the following steps:
wherein S is i I=1, 2,3,4, s as the calculation result of the positive and negative electrode voltage and current data on the rectifying side or the inverting side 1 S is the calculation result of the positive electrode voltage of the rectifying side or the inverting side 2 S is the calculation result of the positive electrode current of the rectifying side or the inverting side 3 S is the calculation result of the negative electrode current of the rectifying side or the inverting side 4 Calculation result of rectifying side or inverting side negative electrode current, H b,i Is the per unit value of the voltage and the current of the positive electrode and the negative electrode of the rectifying side or the inverting side, H b,1 For the per unit value of the positive current of the rectifying side or the inverting side, H b,2 Is the per unit value of the positive voltage of the rectifying side or the inverting side, H b,3 Is the per unit value of the negative voltage of the rectifying side or the inverting side, H b,4 The per unit value of the negative electrode current at the rectifying side or the inverting side is n, the number of sampling points in a time window is H i,j The sampling value of the voltage and the current of the positive electrode and the negative electrode of the rectifying side or the inverting side in the time window is H 1,j Is the sampling value of the positive voltage of the rectifying side or the inverting side in the time window, H 2,j H is the sampling value of the positive electrode current of the rectifying side or the inverting side in the time window 3,j H is the sampling value of the negative electrode voltage of the rectifying side or the inverting side in the time window 4,j Is the sampling value of the negative electrode current at the rectifying side or the inverting side in the time window.
Furthermore, the time window in the step 3 is 1ms.
Further, the vector X in the step 4 is expressed as follows:
X＝[S 1 ,S 2 ,S 3 ,S 4 ]
wherein S is 1 S is the calculation result of the positive electrode voltage of the rectifying side or the inverting side 2 S is the calculation result of the positive electrode current of the rectifying side or the inverting side 3 S is the calculation result of the negative electrode current of the rectifying side or the inverting side 4 And calculating the current of the negative electrode at the rectifying side or the inverting side.
Moreover, each fault in step 4 includes: positive ground fault, negative ground fault, and bipolar short fault.
Furthermore, the vector X and the optimal centroid n of each fault are calculated in the step 4 m And determining the selected vector X and the best centroid n of each fault m The calculation method for whether the minimum value of the minimum distance is smaller than or equal to the threshold value corresponding to the fault comprises the following steps:
wherein n is m For the best mass center of each fault, mu m For the thresholds of various faults under each fault, m=1, 2,3, and m is a fault type number, 1 is a positive electrode ground fault, 2 is a negative electrode ground fault, 3 is a bipolar short circuit fault, con (X) =1 is a corresponding fault, and Con (X) =0 is an out-of-zone fault or no fault.
The threshold value of the DC voltage change rate in the step 1 and the set threshold value of the voltage effective value difference in the step 2 are described, and the optimal mass center n of each fault in the step 3 m And threshold mu for each type of fault under each fault m The determination is made by means of a data set simulating an MMC-HVDC power transmission line.
The MMC-HVDC power transmission line adopts a mode that the direct current side is grounded through a clamping resistor, and is composed of a half-bridge submodule.
The invention has the advantages and positive effects that:
the invention adopts discrete wavelet analysis to extract the high-frequency voltage effective values of the direct current side and the converter side of the line smoothing reactor to compare so as to judge faults in and out of the region, then adopts K-means clustering algorithm, utilizes different fault types and different fault positions, collects the obtained single-side protection unit voltage and current data in a shorter time window after the direct current line fault occurs, carries out clustering analysis on the single-side protection unit voltage and current data, obtains the centroid and the threshold value of the single-side protection unit voltage and current data, and determines corresponding protection criteria through training so as to realize fault pole selection. The method can quickly and accurately detect the faults of the MMC-HVDC power transmission line without being influenced by the fault positions and the transition resistance, and identify the fault types.
Drawings
FIG. 1 is a flow chart of the present invention;
FIG. 2 is a block diagram of an MMC-HVDC system;
FIG. 3 is a MMC-HVDC system topology;
fig. 4 is a bipolar short circuit fault equivalent circuit diagram.
Detailed Description
The present invention will be described in further detail with reference to the accompanying drawings.
A fault judging method of an MMC-HVDC power transmission line, as shown in figure 1, comprises the following steps:
step 1, monitoring the high-frequency direct-current voltage change rate of the direct-current side of the smoothing reactor of the MMC-HVDC power transmission line in real time, judging whether the direct-current voltage change rate is larger than a direct-current voltage change rate threshold value, if so, performing step 2, otherwise, repeatedly executing step 1.
And 2, extracting a high-frequency direct-current voltage effective value of a direct-current side of a smoothing reactor of the MMC-HVDC power transmission line and a high-frequency direct-current voltage effective value of a converter side by discrete wavelet analysis, judging whether the absolute value of the difference value between the high-frequency direct-current voltage effective value of the smoothing reactor of the MMC-HVDC power transmission line and the high-frequency direct-current voltage effective value of the converter side is larger than a set threshold value of the voltage effective value difference value, judging whether the high-frequency direct-current voltage effective value of the smoothing reactor of the MMC-HVDC power transmission line is larger than the high-frequency direct-current voltage effective value of the converter side, if not, judging that the direct-current line fails or fails, and performing step 3 if not, judging that the direct-current line fails.
The calculation method for extracting the effective value of the high-frequency direct-current voltage at the direct-current side and the effective value of the high-frequency direct-current voltage at the converter side of the smoothing reactor of the MMC-HVDC power transmission line in the step comprises the following steps:
wherein U is R Is the effective value of the direct current voltage, k is the sampling point number in a sampling time window, u tran Sample values representing the dc voltage high frequency band, i=1, 2.
The method for judging whether the absolute value of the difference value between the DC side high-frequency direct-current voltage effective value of the smoothing reactor of the MMC-HVDC power transmission line and the DC side high-frequency direct-current voltage effective value of the converter is larger than the set threshold value of the voltage effective value difference value, and judging whether the DC side high-frequency direct-current voltage effective value of the smoothing reactor is larger than the DC side high-frequency direct-current voltage effective value of the converter comprises the following steps:
U R.dc is the effective value of the high-frequency direct-current voltage at the direct-current side of the smoothing reactor, U R.ac Is the effective value of high-frequency direct-current voltage at the converter side of the smoothing reactor, U R.set A threshold value is set for the voltage effective value difference.
And 3, continuously acquiring positive and negative voltage and current of the MMC-HVDC rectifying side or positive and negative voltage and current of the inversion side by adopting a K-means clustering algorithm through a time window of 1ms, performing per unit processing on the positive and negative voltage and current of the MMC-HVDC rectifying side or the positive and negative voltage and current of the inversion side, and summing to obtain a data processing result.
In the step, the method for carrying out per unit processing and summing on the positive and negative voltage and current of the MMC-HVDC rectifying side or the positive and negative voltage and current of the inversion side to obtain a data processing result comprises the following steps:
wherein S is i I=1, 2,3,4, s as the calculation result of the positive and negative electrode voltage and current data on the rectifying side or the inverting side 1 S is the calculation result of the positive electrode voltage of the rectifying side or the inverting side 2 S is the calculation result of the positive electrode current of the rectifying side or the inverting side 3 S is the calculation result of the negative electrode current of the rectifying side or the inverting side 4 Calculation result of rectifying side or inverting side negative electrode current, H b,i Is the per unit value of the voltage and the current of the positive electrode and the negative electrode of the rectifying side or the inverting side, H b,1 For the per unit value of the positive current of the rectifying side or the inverting side, H b,2 Is the per unit value of the positive voltage of the rectifying side or the inverting side, H b,3 Is the per unit value of the negative voltage of the rectifying side or the inverting side, H b,4 The per unit value of the negative electrode current at the rectifying side or the inverting side is n, the number of sampling points in a time window is H i,j The sampling value of the voltage and the current of the positive electrode and the negative electrode of the rectifying side or the inverting side in the time window is H 1,j Is the sampling value of the positive voltage of the rectifying side or the inverting side in the time window, H 2,j H is the sampling value of the positive electrode current of the rectifying side or the inverting side in the time window 3,j H is the sampling value of the negative electrode voltage of the rectifying side or the inverting side in the time window 4,j Is the sampling value of the negative electrode current at the rectifying side or the inverting side in the time window.
Step 4, taking the data processing result as a vector X, and calculating the vector X and each fault optimal centroid n m And determining the selected vector X and the best centroid n of each fault m Whether the minimum value of the minimum distance is smaller than or equal to the threshold value corresponding to the fault, if so, judging the MMC-HVDC power transmission line as a pairIf the failure is not satisfied, judging that the failure is out of the area or is no, wherein each failure comprises the following steps: positive ground fault, negative ground fault, and bipolar short fault.
Vector X is represented in this step as follows:
X＝[S 1 ,S 2 ,S 3 ,S 4 ]。
calculate vector X and each failure best centroid n m And determining the selected vector X and the best centroid n of each fault m The calculation method for whether the minimum value of the minimum distance is smaller than or equal to the threshold value corresponding to the fault comprises the following steps:
wherein n is m For the best mass center of each fault, mu m For the thresholds of various faults under each fault, m=1, 2,3, and m is a fault type number, 1 is a positive electrode ground fault, 2 is a negative electrode ground fault, 3 is a bipolar short circuit fault, con (X) =1 is a corresponding fault, and Con (X) =0 is an out-of-zone fault or no fault.
The threshold value of the DC voltage change rate in the step 1 and the set threshold value of the voltage effective value difference in the step 2 are described, and the optimal mass center n of each fault in the step 3 m And threshold mu for each type of fault under each fault m The MMC-HVDC power transmission line is determined by a data set simulated by the MMC-HVDC power transmission line, and the MMC-HVDC power transmission line adopts a mode that the direct current side is grounded through a clamping resistor and is composed of a half-bridge submodule.
The invention is derived by the following deduction process:
when bipolar short-circuit fault occurs at the outlet side of a DC line of a converter at one end of an MMC-HVDC power transmission line, after the fault occurs, the capacitance of a submodule discharges rapidly before the converter is not locked, and a fault equivalent circuit at the moment is shown in figure 4.
Wherein n represents the number of sub-modules of each bridge arm, C 0 Representing the capacitance of the submodule, R 0 Representing the equivalent resistance of the bridge arm, L 0 Representing bridge arm equivalentInductance, L d The reference point of zero potential is represented by O, which represents a smoothing reactor arranged at the outlet side of a direct current line. According to the average model of the MMC, the three-phase ac system may be equivalently a circuit structure as shown on the right side of fig. 4, and uf is an equivalent additional voltage of the fault point.
At this time, the DC voltage u at both ends of the smoothing reactor installed at the outlet side of the DC line d_ d c And u is equal to d_ac The ratio relation of the two in the frequency domain can be obtained:
as the frequency increases, this ratio will be much greater than 1. When the direct current line fails, the amplitude of the high-frequency direct current voltage at the direct current side of the smoothing reactor is far greater than that of the high-frequency direct current voltage at the converter side.
By the MMC-HVDC power transmission line fault judging method, a certain MMC-HVDC power transmission line fault simulation system is subjected to simulation detection, and the simulation result of PSCAD/EMTDC shows that the method is not influenced by the fault position and the transition resistance, can rapidly and accurately detect the MMC-HVDC power transmission line fault, and identifies the fault type.
It should be emphasized that the examples described herein are illustrative rather than limiting, and therefore the invention includes, but is not limited to, the examples described in the detailed description, as other embodiments derived from the technical solutions of the invention by a person skilled in the art are equally within the scope of the invention.
Claims (6)
1. A fault judging method of an MMC-HVDC power transmission line is characterized in that: the method comprises the following steps:
step 1, monitoring the high-frequency direct-current voltage change rate of the direct-current side of the smoothing reactor of the MMC-HVDC power transmission line in real time, judging whether the direct-current voltage change rate is larger than a direct-current voltage change rate threshold value, if so, repeating the step 1 for continuous monitoring, and if so, performing the step 2;
step 2, extracting a high-frequency direct-current voltage effective value of a smoothing reactor of the MMC-HVDC power transmission line and a high-frequency direct-current voltage effective value of a converter side by discrete wavelet analysis, judging whether the absolute value of the difference value between the high-frequency direct-current voltage effective value of the smoothing reactor of the MMC-HVDC power transmission line and the high-frequency direct-current voltage effective value of the converter side is larger than a set threshold value of the voltage effective value difference value, judging whether the high-frequency direct-current voltage effective value of the smoothing reactor of the MMC-HVDC power transmission line is larger than the high-frequency direct-current voltage effective value of the converter side, if not, judging that the direct-current line fails or is free of failure, and if yes, judging that the direct-current line fails and performing step 3;
the method for judging whether the absolute value of the difference value between the DC side high-frequency direct-current voltage effective value of the smoothing reactor of the MMC-HVDC power transmission line and the DC side high-frequency direct-current voltage effective value of the converter is larger than the set threshold value of the voltage effective value difference value, and judging whether the DC side high-frequency direct-current voltage effective value of the smoothing reactor is larger than the DC side high-frequency direct-current voltage effective value of the converter comprises the following steps:
is the effective value of the high-frequency direct-current voltage at the direct-current side of the smoothing reactor, +.>Is the effective value of high-frequency direct-current voltage at the converter side of the smoothing reactor, < >>Setting a threshold value for the voltage effective value difference value;
step 3, continuously collecting positive and negative voltage and current of the MMC-HVDC rectifying side or positive and negative voltage and current of the inversion side through a K-means clustering algorithm by adopting a time window of 1ms, carrying out per unit processing on the positive and negative voltage and current of the MMC-HVDC rectifying side or the positive and negative voltage and current of the inversion side, and summing to obtain a data processing result;
the calculation method for the data processing result obtained by carrying out per unit processing and summation on the positive and negative voltage and current of the MMC-HVDC rectifying side or the positive and negative voltage and current of the inversion side comprises the following steps:
wherein (1)>For the calculation result of the voltage and current data of the positive and negative poles of the rectifying side or the inverting side,/for the current data>=1,2,3,4，S 1 For the calculation result of the positive voltage on the rectifying side or on the inverting side,/->For the calculation result of the positive current on the rectifying side or on the inverting side,/->For the calculation result of the rectifying-side or inverting-side negative current, < >>Calculation result of rectifying-side or inverting-side negative electrode current, < >>Is the per unit value of the voltage and current of the positive and negative poles of the rectifying side or the inverting side, +.>For the positive current per unit value of the rectifying side or the inverting side,/->For the per unit value of the positive voltage on the rectifying side or the inverting side,/->Is the per unit value of the negative voltage of the rectifying side or the inverting side,is the per unit value of the negative current of the rectifying side or the inverting side, n is the sampling point number in the time window,/and the negative current of the rectifying side or the inverting side>Is the sampling value of the voltage and current of the positive electrode and the negative electrode of the rectifying side or the inverting side in the time window, +.>For the sampling value of the positive voltage of the rectifying side or the inverting side in the time window, +.>For the sampling value of the positive current of the rectifying side or the inverting side in the time window, +.>For the sampling value of the negative voltage on the rectifying side or on the inverting side in the time window, < >>Sampling values of the negative electrode current of the rectifying side or the inverting side in a time window;
step 4, taking the data processing result as a vector X, and calculating the vector X and the optimal mass center of each faultn m And determining the selected vector X and the best centroid of each faultn m Judging whether the minimum value of the minimum distance is smaller than or equal to a threshold value of a corresponding fault, if so, judging that the MMC-HVDC power transmission line is the corresponding fault, and if not, judging that the MMC-HVDC power transmission line is out-of-zone fault or has no fault;
the calculation method of the data processing result as the vector X comprises the following steps:
；
optimum mass center of each faultn m And thresholds for various faults under various faultsThe determination is made by means of a data set simulated for an MMC-HVDC power transmission line.
2. The MMC-HVDC power transmission line fault judgment method as set forth in claim 1, wherein: in the step 2, the calculation method of the DC side high-frequency DC voltage effective value of the smoothing reactor of the MMC-HVDC power transmission line and the DC side high-frequency DC voltage effective value of the converter is as follows:wherein (1)>Is the effective value of DC voltage, +.>For the number of sample points within the sample time window,u tran sampling values representing the high frequency band of the direct current voltage, +.>。
3. The MMC-HVDC power transmission line fault judgment method as set forth in claim 1, wherein: each fault in the step 4 includes: positive ground fault, negative ground fault, and bipolar short fault.
4. The MMC-HVDC power transmission line fault judgment method as set forth in claim 1, wherein: calculating the vector X and the best centroid of each fault in the step 4n m And determining the selected vector X and the best centroid of each faultn m The calculation method for whether the minimum value of the minimum distance is smaller than or equal to the threshold value corresponding to the fault comprises the following steps:wherein,n m for the best centroid of each type of fault under each fault +.>For the threshold value of each type of fault under each fault,m=1, 2,3 andmthe fault type is numbered, 1 is positive electrode ground fault, 2 is negative electrode ground fault, 3 is bipolar short circuit fault,Con（X）the value of =1 is that the corresponding fault occurs,Con（X）=0 is out of zone fault or no fault.
5. The MMC-HVDC power transmission line fault judgment method as set forth in claim 1, wherein: the threshold value of the direct current voltage change rate in the step 1 and the set threshold value of the voltage effective value difference in the step 2 are the optimal mass center of each fault in the step 3n m And thresholds for various faults under various faultsThe determination is made by means of a data set simulated for an MMC-HVDC power transmission line.
6. The MMC-HVDC power transmission line fault judgment method as set forth in claim 1, wherein: the MMC-HVDC power transmission line adopts a mode that a direct current side is grounded through a clamping resistor and is composed of a half-bridge submodule.
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