Ground fault line selection method suitable for flexible DC power distribution network

Abstract

The invention discloses a grounding fault line selection method suitable for a flexible DC power distribution network, and relates to the field of flexible DC power distribution network relay protection. The method comprises the following steps: firstly, reading and recording voltage data of a direct-current line, judging whether a fault occurs or not according to a fault starting criterion, if so, inputting an active injection additional control strategy in the MMC, and injecting a preset sine characteristic signal into the line; determining a fault occurrence pole according to a fault pole selection criterion; and finally, by calculating the Pearson correlation coefficient of the one-mode current magnitude change rate on the two sides of the line and comparing the Pearson correlation coefficient with a set threshold value, after the types of faults inside and outside the area are judged, a fault signal is sent out. According to the method, a large number of power electronic devices connected into the power distribution network are utilized, characteristic signals are actively injected, additional devices are not needed, the short injection time is set, the original fault characteristics can be strengthened while the influence on the power grid is small, and the reliability and sensitivity of line selection are improved; and fault line selection can be realized only by calculating the correlation coefficient of the one-mode current magnitudes at the two sides of the line.

Description

Technical Field

[0001] The invention relates to the field of relay protection for flexible DC distribution networks, and in particular to a ground fault line selection method suitable for flexible DC distribution networks. Background Art

[0002] The "double high" and "double low" characteristics of the new power system pose a challenge to the safe and stable operation of the power grid, and increase the requirements for relay protection. "Double high" refers to a high proportion of new energy grid connection and a high proportion of power electronic devices access, and "double low" refers to low inertia and weak damping. Compared with the traditional AC distribution network, the flexible DC distribution network has the advantages of large transmission capacity, high power quality, low line loss, flexible operation and adjustment methods, and is suitable for distributed power sources, energy storage equipment and various types of DC load access. However, when a fault occurs in the flexible DC distribution network, the fault current rises quickly, the peak value is large, and the fault develops very quickly. Therefore, in order to prevent damage to the line and equipment, the flexible DC distribution network protection device needs to quickly identify and effectively remove the fault within 3 to 5 ms, and with the development of the new power system, the requirements for protection selectivity and speed (fault identification within 1 ms) are more stringent.

[0003] The complex fault characteristics of the new power system will greatly affect the applicability of the above protection principles, but its "double high" characteristics also provide a new direction for the research of protection schemes. The access of a high proportion of power electronic devices has promoted the development of passive detection technology to active injection detection methods. By utilizing the high controllability of power electronic devices, characteristic signals are actively injected without additional devices, and new fault identification criteria are constructed by analyzing the response characteristics of characteristic signals during faults. The active injection method can be divided into two types according to its implementation method: setting additional control strategies to directly generate and inject characteristic signals, and controlling the operation mode of the converter to achieve characteristic signal injection.

[0004] Regarding the method of setting additional control strategies to generate characteristic signals, some scholars use the square wave characteristic signal injected by the full-bridge MMC to judge the nature of the fault, thereby achieving efficient and fast reclosing, but its scope of application is limited and the half-bridge MMC flexible direct current system is not applicable. There are also schemes that set additional control strategies in the MMC, which can actively inject sinusoidal signals into the line after the fault, realize fault line selection based on the model recognition principle, and introduce the Spearman correlation coefficient to analyze the model differences of faults inside and outside the area, but the protection scheme based on the model recognition principle needs to extract multiple fault characteristic quantities and the calculation is complex, which affects the protection speed.

[0005] Regarding the method of controlling the operation mode of the converter to realize the characteristic signal injection, some scholars use the fault start-up judgment criteria and the coordinated control between the converters to control the converter into a frequency-controllable signal injection source, thereby selecting the harmonic injection of the appropriate frequency, and accurately locating the fault by calculating the difference in harmonic impedance, but the protection scheme has high requirements for communication. There are also schemes that use inter-station control to short-term unlock the DC / DC converter station that is locked after the fault. Since the fault current drops to 0 after locking, an injection current signal will be generated after short-term unlocking, and the fault type and location are determined by calculating the high-frequency impedance, but its short-term operation with fault has certain risks.

[0006] It can be seen that the existing ground fault line selection method based on the active injection method has limitations in different scenarios. Therefore, it is of great significance to design a fast and reliable ground fault line selection method suitable for flexible DC distribution network under active injection of characteristic signals. Summary of the invention

[0007] In order to solve the problem that the ground fault line selection method constructed based on the active injection method in the prior art has limitations in different scenarios, the present invention provides a ground fault line selection method under active injection of characteristic signals suitable for a flexible DC distribution network.

[0008] When there is a fault outside the line area, the first-mode current on both sides of the DC line is negatively correlated; when there is a fault inside the area, the first-mode current on both sides of the DC line is positively correlated. The present invention uses the first-mode current on both sides of the DC line after the characteristic signal is actively injected, calculates the Pearson correlation coefficient to describe the relationship between the first-mode current on both sides, and thus constructs a ground fault line selection method. Therefore, the present invention is achieved through the following technical solutions:

[0009] A ground fault line selection method applicable to a flexible DC distribution network comprises the following steps:

[0010] S1: Use MMC to construct additional control strategies, pre-set reasonable characteristic signal parameters, and inject characteristic signals into the line after a fault; the details are as follows:

[0011] Inject sinusoidal characteristic signal u injection as follows:

[0012]

[0013] In the formula, A represents the amplitude of the characteristic signal, ω injection Represents the angular frequency of the characteristic signal, ω injection =2πf, f is the frequency of the characteristic signal, Indicates the initial phase of the characteristic signal; based on the analysis of the actual operation of the system, the characteristic signal parameters are selected as follows: frequency f=200Hz, amplitude A=0.3kV, signal injection time Δt=10ms.

[0014] During normal operation, the positive and negative voltage change rates are opposite; when a single-pole grounding fault occurs in the DC line, the positive and negative voltage change rates are the same. After the MMC detects this feature, the additional control strategy switches to the ctrl=1 control mode and injects a characteristic signal into the line through the MMC.

[0015] S2: In order to avoid the influence of the coupling of the positive and negative lines on the fault characteristics after the fault, the positive and negative lines are decoupled, the fault current modulus is extracted, and the Pearson correlation coefficient of the modulus current on both sides of the line during the fault is calculated and analyzed; the details are as follows:

[0016] The calculation formula of the first mode current I1 after decoupling is as follows:

[0017]

[0018] In the formula, I P ,I N Respectively represent the positive and negative currents of the DC line;

[0019] When different faults occur in the system, the relationship between the single-mode currents on both sides of the line is as follows: when an out-of-zone grounding fault occurs in the system, the single-mode currents on both sides of the line have equal amplitudes, opposite polarities, and opposite rates of change; when an in-zone grounding fault occurs in the system, the single-mode current on one side of the line will increase dramatically, but the rate of change is the same.

[0020] The difference in fault characteristics inside and outside the zone is characterized by the Pearson correlation coefficient: when a fault occurs outside the zone, the change rates of the single-mode current on both sides are opposite, showing a negative correlation, and the calculated value of the Pearson correlation coefficient is close to -1; when a fault occurs inside the zone, the change rates of the single-mode current on both sides are the same, showing a positive correlation, and the calculated value of the Pearson correlation coefficient is close to 1.

[0021] S3: Design the fault line selection criteria based on the difference in correlation coefficients of the first-mode current on both sides of the line when the fault occurs inside the area and outside the area, and implement the fault line selection, as follows:

[0022] S3-1: Fault start-up and fault pole selection are realized based on equations (3) and (4):

[0023]

[0024] Where U P is the positive line voltage; U N is the negative line voltage; U dcNis the rated DC voltage, and F represents the fault selection criterion result: when the positive pole fault criterion is met, F=1; when the negative pole fault criterion is met, F=-1; if both results are not met, F=0, and a bipolar short circuit fault or other unknown fault types may occur;

[0025] S3-2: Fault line selection method based on the correlation of the single-mode current on both sides of the line: Calculate the Pearson correlation coefficient of the single-mode current on both sides of the line to identify the faults inside and outside the area. The judgment criteria are as follows:

[0026]

[0027] Where P is the calculated value of the Pearson correlation coefficient of the single-mode current on both sides of the line, P set is the correlation coefficient threshold; if the correlation coefficient is greater than the threshold, it is determined to be an in-zone fault; if the correlation coefficient is less than the threshold, it is determined to be an out-of-zone fault.

[0028] Preferably, the correlation coefficient threshold P set The setting of meets the following conditions: set The setting avoids the minimum Pearson correlation coefficient of the single-mode current on both sides of the line when the fault occurs in the area, and takes into account the influence of the error factors of line parameters, voltage, and current transformer transmission, and adjusts it according to the operating status of the system in the actual project, and then adjusts the correlation coefficient threshold according to a certain setting coefficient; further, the correlation coefficient threshold P set Set to 0.63.

[0029] Compared with the prior art, the present invention has the following beneficial effects: the present invention provides a ground fault line selection method applicable to a flexible DC distribution network, which is a ground fault line selection method for a flexible DC distribution network based on the correlation of a single-mode current under active characteristic signal injection, which can effectively solve the problem of lack of obvious available fault current characteristics when a single-pole ground fault occurs in a flexible DC distribution network under a pseudo-bipolar wiring mode, and makes good use of the MMC construction characteristic signal injection control strategy, without the need for additional devices, and sets a shorter injection time, which has a small impact on the power grid while strengthening the original fault characteristics, thereby improving the reliability and sensitivity of the protection scheme. At the same time, the decoupling algorithm is used to extract the fault single-mode current quantity characteristics, and the fault line selection can be achieved by only calculating the single-mode current quantity correlation coefficient on both sides of the line, and the calculation amount is small. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 It is a schematic diagram of the topological structure of the flexible DC distribution network involved in a specific embodiment of the present invention.

[0031] Figure 2 This is the current flow path when a ground fault occurs under the pseudo bipolar connection involved in the specific embodiment of the present invention.

[0032] Figure 3 It is a schematic diagram of an MMC converter of an additional characteristic signal injection control strategy involved in a specific embodiment of the present invention.

[0033] Figure 4 It is a schematic diagram of characteristic signal injection involved in a specific embodiment of the present invention.

[0034] Figure 5 It is a timing diagram of active injection of characteristic signals involved in a specific embodiment of the present invention.

[0035] Figure 6 It is a relationship diagram of the first mode current when an out-of-area fault occurs on the line involved in the specific embodiment of the present invention.

[0036] Figure 7 It is a relationship diagram of a mode current when a fault occurs in a line area involved in a specific embodiment of the present invention.

[0037] Figure 8 It is a flow chart of a ground fault line selection method involved in a specific embodiment of the present invention. DETAILED DESCRIPTION

[0038] The present invention is further described below in conjunction with specific embodiments.

[0039] Taking the flexible DC distribution network as an example, the typical radial flexible DC distribution network system structure is as follows: Figure 1 As shown. The left side is a 35kV AC power supply, and the right side is a DC distribution line including photovoltaic, DC load and AC power feed. The DC bus adopts a pseudo-bipolar wiring method, including two voltage levels: medium voltage ±10kV and low voltage ±375V. The AC and DC sides realize the AC / DC conversion function through MMC converters, and the DC medium and low voltage sides realize the DC / DC conversion function through DAB transformers.

[0040] The following is an analysis of the grounding fault under the pseudo-bipolar wiring mode: Under the pseudo-bipolar wiring mode, when a single-pole grounding fault occurs in the flexible DC distribution network, there is only one grounding point at the fault pole, and there is no fault discharge loop. Therefore, the system current changes very little, and there is a lack of obvious available fault current characteristics. When the fault occurs, the current flow path of the AC side and the MMC submodule capacitor is as follows: Figure 2 As shown. From the above analysis, it can be seen that in the flexible DC distribution network with pseudo bipolar connection mode, the fault characteristics are not obvious and the subsequent protection scheme cannot be constructed. Therefore, the present invention proposes a ground fault line selection method based on active injection of characteristic signals, which can construct a corresponding fault line selection method on the premise of strengthening the original fault characteristics.

[0041] The specific verification and implementation process of this embodiment is as follows Figure 8 As shown, the following steps are included:

[0042] S1: Use MMC to construct additional control strategies, pre-set reasonable characteristic signal parameters, and inject characteristic signals into the line after a fault; the details are as follows:

[0043] S1-1: Principle of characteristic signal generation and injection:

[0044] The schematic diagram of the active injection method is as follows Figure 3 to Figure 5 In normal operation, the positive and negative voltage change rates are opposite; when a single-pole grounding fault occurs in the DC line, the positive and negative voltage change rates are the same. After the MMC detects this feature, the additional control strategy switches to the ctrl=1 control mode, and the sinusoidal characteristic signal u is injected into the line through the MMC. injection The injected sinusoidal voltage signal will induce an injected current on the line, injecting a sinusoidal characteristic signal u injection as follows:

[0045]

[0046] In the formula, A represents the amplitude of the characteristic signal, ω injection Represents the angular frequency of the characteristic signal, ω injection =2πf, f is the frequency of the characteristic signal, Indicates the initial phase of the characteristic signal.

[0047] Depend on Figure 5 After a fault occurs, the characteristic signal is injected after a short delay, and the required injection duration is set. fault Indicates the time when the fault occurs; Δt0 indicates a short delay after the fault occurs, after which a characteristic signal with a duration of Δt is injected, and fault detection and identification are performed during this period; t1 indicates that fault identification has been completed.

[0048] S1-2: The characteristic signal parameters in this embodiment are selected as follows:

[0049] ① Characteristic signal frequency selection:

[0050] The selection of characteristic signal frequency should take into account the influence of many factors, the specific analysis is as follows:

[0051] First, the characteristic signal frequency should be less than the sampling frequency at the protection device. In this embodiment, the sampling frequency is set to 20kHz, so the characteristic signal frequency should be less than 10kHz; secondly, the characteristic signal frequency should be less than the MMC submodule switching frequency. In the distribution network model of the present invention, the MMC submodule switching frequency is 2.18kHz, so the characteristic signal frequency should be less than 2.18kHz; finally, considering that the characteristic signal amplitude will be affected by the line length, the longer the line length, the more serious the attenuation, and the signal attenuation rate in the high-frequency band is faster, so the low-frequency characteristic signal should be selected, and the characteristic signal frequency of the frequency band below 300Hz should be selected.

[0052] Based on the above analysis, considering leaving a certain margin, it is appropriate to select a frequency between 150 and 250 Hz as the characteristic signal frequency. The present invention selects 200 Hz as the characteristic signal frequency.

[0053] ②Selection of characteristic signal amplitude

[0054] In general, considering the accuracy of sampling and measurement calculation, the larger the characteristic signal amplitude is, the better; considering the interference and impact on the power grid, the smaller the characteristic signal amplitude is, the better.

[0055] Specifically, the measurement device error requirement for the ±10kV medium voltage DC distribution network is less than 0.75%, that is, the characteristic signal amplitude must be greater than 0.75% U dcN ; Refer to GB / T 35727-2017 "Guidelines for Medium and Low Voltage DC Distribution Voltages", the voltage deviation of ±10kV medium voltage DC distribution network is -10% to +5%, and the characteristic signal amplitude should not exceed 5%U dcN , within the allowable deviation range of the voltage, the characteristic signal has little interference to the power grid and will not bring any impact.

[0056] Based on the above analysis, the characteristic signal amplitude A∈(0.75%~5%)U dcN , the present invention selects 3% U dcN As the characteristic signal amplitude, that is 0.3kV.

[0057] ③Selection of characteristic signal injection duration

[0058] The selection of the characteristic signal injection time takes into account the effectiveness of extracting fault information, and the injection time cannot be too short; at the same time, considering the impact on the sound line, the injection time should not be too long. Considering the influence of characteristic signal generation delay, control strategy delay, measurement equipment delay and simulation step length, the injection time is selected as 10ms, and a short delay of 0.5ms is set before injection.

[0059] S2: Active injection of next-mode current fault feature analysis: In order to avoid the influence of positive and negative line coupling on fault features after the fault, the positive and negative lines are decoupled, the first-mode fault current is extracted, and the Pearson correlation coefficient of the first-mode current on both sides of the line during the fault is calculated and analyzed; the details are as follows:

[0060] S2-1: Positive and negative current decoupling:

[0061] Based on the principle of modulus decomposition, the influence of coupling between positive and negative lines on fault characteristics is eliminated by Karrenbauer transformation. The specific decoupling matrix is ​​as follows:

[0062]

[0063] Where S is the Karrenbauer decoupling matrix, S -1 is the decoupled inverse matrix; X P , X N They represent the positive and negative electrical quantities of the DC line respectively, and X0 and X1 represent the zero-mode and one-mode electrical quantities after decoupling respectively.

[0064] From equations (2) and (3), it can be seen that the first-mode current after modulus decomposition is the first-mode current I1 after decoupling. The calculation formula is as follows:

[0065]

[0066] In the formula, I P ,I N They represent the positive and negative currents of the DC line respectively; I1 represents the first-mode current after decoupling.

[0067] S2-2: Fault feature analysis:

[0068] When an out-of-area fault occurs on the line, taking the positive pole grounding fault as an example, the fault network diagram is as follows: Figure 6 As shown, the first-mode current on the m and n sides of the positive and negative lines is:

[0069]

[0070] Analysis shows that I 1m =-I 1n , that is, when a positive ground fault occurs outside the area, the amplitude of the first-mode current on both sides of the line is equal and the polarity is opposite. Similarly, when a negative ground fault occurs outside the area, I 1m =-I 1n .

[0071] When a fault occurs in the line, take the positive grounding fault as an example, the fault network diagram is as follows: Figure 7 As shown, the first-mode current on the m and n sides of the positive and negative lines is:

[0072]

[0073] Analysis shows that I 1m < 1n And the change rate is the same, that is, when a positive grounding fault occurs in the area, the first-mode current on the m side of the line is much smaller than the first-mode current on the n side of the line and has the same change rate. Similarly, when a negative grounding fault occurs in the area, the first-mode current on the m and n sides of the positive and negative lines is:

[0074]

[0075] Analysis shows that I 1m >>I 1n ​And the change rate is the same, that is, when a negative pole grounding fault occurs in the area, the first-mode current on the line m side is much larger than the first-mode current on the line n side and has the same change rate.

[0076] Based on the above analysis, we can get the relationship shown in Table 1 between the first-mode current on both sides of the line when there are faults inside and outside the area.

[0077] Table 1 Fault characteristics of single-mode current on both sides of the line

[0078]

[0079] According to the relationship between the single-mode current on both sides of the line in Table 1, it can be concluded that when an internal or external fault occurs, the relationship between the single-mode current on both sides of the line is: when the system has an external ground fault, the single-mode current on both sides of the line has the same amplitude, opposite polarity, and opposite rate of change; when the system has an internal ground fault, the single-mode current on one side of the line will increase dramatically, but the rate of change is the same. Therefore, the correlation algorithm can be used to describe the relationship between the rate of change of the single-mode current on both sides of the line and construct the protection criterion.

[0080] The calculation results of Pearson correlation analysis of the rate of change of the first-mode current on both sides of the line are as follows:

[0081] Table 2 Correlation characteristics of the first mode current on both sides of the line

[0082]

[0083] Therefore, combining the above description and fault characteristics, the correlation coefficient relationship can be obtained as follows: when the fault occurs outside the zone, the change rates of the single-mode current on both sides are opposite, showing a negative correlation, and the calculated value of P (Pearson correlation coefficient) is close to -1; when the fault occurs within the zone, the change rates of the single-mode current on both sides are the same, showing a positive correlation, and the calculated value of P is close to 1.

[0084] S3: Design the fault line selection criteria based on the correlation difference between the first mode current on both sides of the line when the fault occurs inside the area and outside the area, and realize the fault line selection, as follows:

[0085] S3-1: Fault start criteria design:

[0086] The ground fault start criterion constructed according to the voltage change rate is as follows:

[0087]

[0088] Where U P is the positive line voltage; U Nis the negative line voltage. In normal operation, the positive and negative voltage change rates are opposite; when a single-pole grounding fault occurs, the fault pole voltage drops to 0, and the non-fault pole voltage becomes twice the original, and their change rates are both greater than 0. The sampling frequency is 20kHz, and when the start criterion is met for three consecutive sampling points, it is considered that a grounding fault has occurred.

[0089] S3-2: Design of fault selection criteria:

[0090] When the positive pole is grounded, the positive pole voltage U Pn →0, negative voltage U Nn →2U Nn ; When the negative pole is grounded, the negative pole voltage U Nn →0, non-fault pole voltage U Pn →2U Pn , U Pn , U Nn The rated voltages of the positive and negative electrodes are respectively, and the inter-electrode voltage remains almost unchanged. According to the change of the positive and negative electrode voltages during a ground fault, the ground fault pole selection criterion is constructed as follows:

[0091]

[0092] Where U dcN is the rated DC voltage, F represents the fault pole selection criterion result, and the value of F is specified as follows: when the positive pole fault criterion is met, F=1; when the negative pole fault criterion is met, F=-1; if both results are not met, F=0, and a bipolar short circuit fault or other fault types may occur.

[0093] S3-3: Design of fault line selection criteria:

[0094] Fault line selection method based on the correlation of the single-mode current on both sides of the line: Calculate the Pearson correlation coefficient of the single-mode current on both sides of the line to identify the faults inside and outside the area. The judgment criteria are as follows:

[0095]

[0096] Where P is the calculated value of the Pearson correlation coefficient of the single-mode current on both sides of the line, P set is the correlation coefficient threshold; if the correlation coefficient is greater than the threshold, it is determined to be an in-zone fault; if the correlation coefficient is less than the threshold, it is determined to be an out-of-zone fault.

[0097] In this embodiment, the correlation coefficient threshold P set The selection meets the following conditions: setThe setting avoids the minimum Pearson correlation coefficient of the single-mode current on both sides of the line when the fault occurs in the area, and takes into account the influence of the error factors of line parameters, voltage, and current transformer transmission, and adjusts it according to the state of the circuit in the actual project, and then adjusts the correlation coefficient threshold according to a certain setting coefficient; in this embodiment, the correlation coefficient threshold P set Set to 0.63.

[0098] According to the principle described in this embodiment, the fault start criterion, the fault pole selection criterion and the fault line selection criterion are combined on the basis of active injection of characteristic signals to form an overall protection scheme as follows: Figure 6 The protection start-up criterion is started 0.1ms after the fault occurs, and the characteristic signal is injected after a short delay of 0.5ms. After the fault pole selection is completed, the Pearson correlation coefficient of the single-mode current on both sides of the line is calculated, and the calculation result is compared with the set threshold to realize the fault line selection.

[0099] The scope of protection claimed by the present invention is not limited to the above specific embodiments. For those skilled in the art, the present invention may have various deformations and changes. Any modifications, improvements and equivalent substitutions made within the concept and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A ground fault line selection method applicable to a flexible DC distribution network, characterized by: The steps include: S1: Use MMC to construct additional control strategies, pre-set characteristic signal parameters, and inject characteristic signals into the line after a fault; S2: In order to avoid the influence of the coupling of the positive and negative lines on the fault characteristics after the fault, the positive and negative lines are decoupled, the fault current modulus is extracted, and the Pearson correlation coefficient of the modulus current on both sides of the line during the fault is calculated and analyzed; S3: Design the fault line selection criteria based on the difference in correlation coefficients of the single-mode currents on both sides of the line when there are faults inside the area and outside the area, and realize fault line selection.

2. A ground fault line selection method applicable to a flexible DC distribution network according to claim 1, characterized in that: Step S1 is specifically as follows: Inject sinusoidal characteristic signal u injection as follows: （1） In the formula, A represents the amplitude of the characteristic signal, ω injection Represents the angular frequency of the characteristic signal, ω injection =2πf, f is the frequency of the characteristic signal, φ injection Indicates the initial phase of the characteristic signal; according to the analysis of the actual operation of the system, the characteristic signal parameters are selected as follows: frequency f=200Hz, amplitude A=0.3kV, signal injection time Δt=10ms; During normal operation, the positive and negative voltage change rates are opposite; when a single-pole grounding fault occurs in the DC line, the positive and negative voltage change rates are the same. After the MMC detects this feature, the additional control strategy switches to the ctrl=1 control mode and injects a characteristic signal into the line through the MMC.

3. A ground fault line selection method applicable to a flexible DC distribution network according to claim 2, characterized in that: Step S2 is specifically as follows: The calculation formula of the first mode current I1 after decoupling is as follows: （2） In the formula, I P ,I N Respectively represent the positive and negative currents of the DC line; When different faults occur in the system, the relationship between the single-mode current on both sides of the line is as follows: when the system has an out-of-area grounding fault, the single-mode current on both sides of the line has the same amplitude, opposite polarity, and opposite rate of change; when the system has an in-area grounding fault, the single-mode current on one side of the line will increase dramatically, but the rate of change is the same; The difference in fault characteristics inside and outside the zone is characterized by the Pearson correlation coefficient: when a fault occurs outside the zone, the change rates of the single-mode current on both sides are opposite, showing a negative correlation, and the calculated value of the Pearson correlation coefficient is close to -1; when a fault occurs inside the zone, the change rates of the single-mode current on both sides are the same, showing a positive correlation, and the calculated value of the Pearson correlation coefficient is close to 1.

4. The method for selecting a ground fault line applicable to a flexible DC distribution network according to claim 3, characterized in that: Step S3 is as follows: S3-1: Fault start-up and fault pole selection are realized based on equations (3) and (4): （3） （4） Where U P is the positive line voltage; U N is the negative line voltage; U dcN is the rated DC voltage, and F represents the fault selection criterion result: when the positive pole fault criterion is met, F=1; when the negative pole fault criterion is met, F=-1; if both results are not met, F=0, and a bipolar short circuit fault or other unknown fault types may occur; S3-2: Fault line selection method based on the correlation of the single-mode current on both sides of the line: Calculate the Pearson correlation coefficient of the single-mode current on both sides of the line to identify the faults inside and outside the area. The judgment criteria are as follows: （5） Where P is the calculated value of the Pearson correlation coefficient of the single-mode current on both sides of the line, P set is the correlation coefficient threshold; if the correlation coefficient is greater than the threshold, it is determined to be an in-zone fault; if the correlation coefficient is less than the threshold, it is determined to be an out-of-zone fault.

5. The method for selecting a ground fault line applicable to a flexible DC distribution network according to claim 4, characterized in that: Correlation coefficient threshold P set The setting of meets the following conditions: set The setting avoids the minimum Pearson correlation coefficient of the single-mode current on both sides of the line when there is a fault in the area, while taking into account the influence of the error factors of line parameters, voltage, and current transformer transmission, and adjusts it according to the operating status of the system in the actual project, and then adjusts the correlation coefficient threshold according to a certain setting coefficient.

6. A ground fault line selection method applicable to a flexible DC distribution network according to claim 4, characterized in that: Correlation coefficient threshold P set Set to 0.63.

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