Medium-voltage direct-current distribution cable fault identification method based on decoupling modulus

Abstract

The invention provides a medium-voltage direct-current distribution cable fault identification method based on decoupling modulus, and the method comprises the steps: judging an operation mode of a direct-current cable, obtaining a conductor ground voltage uC (t), a conductor current iC (t) and a ground wire current iG (t) if the operation mode is a single-pole operation mode, and calculating the modal signals of the current and the voltage; acquiring time t0, and judging whether a fault occurs or sudden decrease or sudden increase of the load occurs; if the operation mode is a bipolar operation mode, relevant parameters are obtained, and modal signals of current and voltage are calculated; acquiring time t1, judging whether a fault occurs or not, and if the fault occurs, continuing to judge a specific fault type; and if no fault occurs, judging whether load change or normal operation occurs. The method is not influenced by electric arcs and initial fault conditions, does not need priori knowledge, is short in calculation time and high in precision, and has very high effectiveness and robustness.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of direct current power distribution network fault detection, in particular to a medium voltage direct current power distribution cable fault identification method based on decoupling modulus. BACKGROUND

[0002] Compared with alternating current cables, direct current cables have the advantages of low loss, convenient maintenance and high stability. The insulation of direct current cables is prone to aging under the long-term influence of external and internal factors, which leads to the occurrence of faults. The single-stage ground fault current of direct current cables is weak, and the line-to-line short-circuit current rises very quickly, which is not conducive to the fault identification of direct current cables. Therefore, an effective and feasible fault identification method helps to identify faults and disturbances, reduce power outage time and ensure the stability and safety of the entire network.

[0003] In order to meet the construction requirements of strong smart grid and cater to the development direction of large-scale renewable energy grid connection, direct current cables, as the key carrier of direct current transmission, have been widely used in distributed energy grid connection and other occasions. At present, in the power distribution network dominated by alternating current cables, the fault identification technology has shown good performance, however, the fault identification technology of direct current cables has not been fully researched and developed. Although the fault current of direct current cables changes very quickly and is not easy to detect, it can be distinguished from load fluctuations in a very short time by calculating the decoupling signal. Therefore, the fault identification method of medium voltage direct current cables based on decoupling modulus not only has theoretical research value, but also has important significance for reducing the complexity of operation and maintenance and the workload of maintenance. SUMMARY

[0004] Therefore, it is necessary to provide a direct current cable fault identification method using the modulus signal decoupled from the cable impedance and admittance, which is suitable for 10kV medium voltage power distribution cable system, and each cable is provided with an original electric signal sampling point at the first end.

[0005] The present application is to solve the above technical problems, and the technical scheme adopted is as follows:

[0006] A medium voltage direct current power distribution cable fault identification method based on decoupling modulus, characterized in that it comprises the following steps:

[0007] S1, first determine the operation mode of the cable, if it is a single-pole operation mode, obtain the conductor-to-ground voltage u C (t), the conductor current i C (t) and the grounding line current i G (t) from the first end of the cable;

[0008] S2, obtain the phase modulus transformation matrix T MU_SP and TMI_SP , the conductor-to-ground voltage u C (t), the conductor current i C (t), and the ground wire current i G (t) are decoupled, and α-mode, β-mode, and γ-mode voltage and current signals are calculated;

[0009] S3, a first singular point of the α-mode voltage u α (t) is determined using a wavelet Teager algorithm, and a time t0 corresponding to the singular point is determined;

[0010] S4, the time t0 is substituted into a fault identification criterion formula (3) to determine whether a fault occurs;

[0011] S5, if it is determined that no fault occurs, the time t0 is substituted into criterion formulas (4) and (5) to further determine whether the load suddenly increases or suddenly decreases;

[0012] S6, if the operating mode of the cable is a bipolar operating mode, the positive conductor current i PC (t), the positive conductor-to-ground voltage u PC (t), the ground wire current i PG (t) of the positive cable, the negative conductor current i NC (t), the negative conductor-to-ground voltage u NC (t), and the ground wire current i NG (t) of the negative cable are collected;

[0013] S7, a phase-mode transformation matrix T MU_DP and T MI_DP of the cable in the bipolar operating mode are obtained; PC (t), the positive conductor-to-ground voltage u PC (t), the ground wire current i PG (t) of the positive cable, the negative conductor current i NC (t), the negative conductor-to-ground voltage u NC (t), and the ground wire current i NG (t) of the negative cable are decoupled, and χ-mode, κ-mode, δ-mode, ε-mode, ξ-mode, and η-mode voltage and current signals are calculated;

[0014] S8, a first singular point of the χ-mode voltage signal u χ (t) is determined using a wavelet Teager algorithm, and a time t1 corresponding to the singular point is determined;

[0015] S9, the time t1 is substituted into a bipolar cable fault identification criterion formula (9) to determine whether a fault occurs; if it is determined that a fault occurs, fault classification criterion formulas (10) and (11) are used to determine a fault type;

[0016] S10, if no fault occurs, t1 is substituted into the criterion formula (12) to further determine whether a load mutation occurs.

[0017] Further, the step S2 is to obtain the phase-mode transformation matrix T MU_SP and T MI_SP of the cable in the single-pole operation mode C (t), the conductor current i C (t) and the grounding wire current i G (t) are decoupled to obtain the alpha-mode, beta-mode and gamma-mode voltage and current signals, and the calculation formula is:

[0018]

[0019]

[0020] Wherein, i α (t), i β (t) and i γ (t) represent the alpha-mode, beta-mode and gamma-mode current signals; u α (t), u β (t), u γ (t) represent the alpha-mode, beta-mode, gamma-mode voltage signals; i C (t), i S (t), i A (t) and i G (t) are the current signals flowing in the wire, the metal sheath, the armor layer and the grounding wire respectively; u C (t), u S (t), u A (t) and u G (t) represent the ground voltage of the wire, the metal sheath, the armor layer and the grounding wire; R G represents the ground resistance.

[0021] Further, the step S4 is to substitute t0 into the fault identification criterion formula (3), if the criterion is met, it is judged that a fault occurs; otherwise, it is judged that no fault occurs;

[0022]

[0023] Wherein, the functions max() and min() are used to determine the maximum and minimum values of the modal signals; the absolute value of the modal signal is taken by ||; u α_LV (t) represents the alpha-mode voltage signal obtained when the most severe load mutation occurs; T is an alternating current power frequency time interval; U α_LV represents the maximum value of u α_LV (t) in the time interval [t0+0.5T, t0+1.5T]; Rα_LV This indicates that within the time intervals [t0-1.5T, t0-0.5T] and [t0+0.5T, t0+1.5T], u α_LV The ratio of the maximum value to the minimum value of (t).

[0024] Furthermore, step S5 is to substitute t0 into criterion (4) and (5) after determining that no fault has occurred in step S4, and further determine whether the load has suddenly increased or decreased; if either criterion (4) or criterion (5) is satisfied, it is determined that the load has suddenly decreased, otherwise it is determined that the load has suddenly increased.

[0025]

[0026]

[0027] Where Δt is the sampling interval.

[0028] Furthermore, step S7 is based on the phase mode transformation matrix T of the cable in bipolar operation mode. MU_DP and T MI_DP For the positive conductor current i PC (t), voltage u of positive conductor to ground PC (t) Grounding current i of the positive cable PG (t), negative conductor current i NC (t), Voltage u of negative conductor to ground NC (t) and the grounding current i of the negative cable NG (t) Decoupling is performed, and the χ-mode, κ-mode, δ-mode, ε-mode, ξ-mode, and η-mode voltage and current signals are calculated using the following formulas:

[0029]

[0030]

[0031]

[0032] Among them, i χ (t), i κ (t), i δ (t), i ε (t), i ζ (t) and i η (t) represents the χ-mode, κ-mode, δ-mode, ε-mode, ζ-mode, and η-mode current signals; u χ (t), u κ (t), u δ (t), u ε (t), u ζ (t) and u η(t) represents the χ-mode, κ-mode, δ-mode, ε-mode, ζ-mode, and η-mode voltage signals; i P (t), i PS (t), i PA (t) and i PG (t) is the current signal flowing through the positive conductor, the metal sheath, and the grounding wire; i N (t), i NS (t), i NA (t) and i NG (t) Current signal flowing through the negative conductor, metal sheath, and grounding wire; u P (t), u PS (t) and u PA (t) is the grounding voltage of the positive conductor, metal sheath, and armor layer; u N (t),u NS (t) and u NA (t) is the grounding voltage of the negative conductor, metal sheath, and armor layer; R PG and R NG This indicates the grounding resistance of the positive and negative cables.

[0033] Further, step S9 is to substitute t1 into the bipolar cable fault identification criterion (9). If the criterion is satisfied, it is determined that a fault has occurred. The fault classification criterion (10) and (11) are used to determine the fault type. If either (10) or (11) is satisfied, it is determined that a bipolar fault has occurred. Otherwise, it is determined that a single-pole direct / indirect grounding fault has occurred.

[0034]

[0035] Among them, u ε_LV (t) represents the ε-mode voltage signal obtained when the most severe load change occurs; U ε_LV Indicate u ε_LV (t) is the maximum value of t over the time interval [t0+0.5T, t0+1.5T]; R ε_LV This indicates that u is within the time intervals [t0-1.5T, t0-0.5T] and [t0+0.5T, t0+1.5T]. ε_LV The ratio of the maximum to the minimum value of (t);

[0036]

[0037] Among them, u χ_HIF (t) and i χ_HIF (t) represents the χ-mode voltage and current signal (u) when a high-impedance fault occurs. χ (t),i χ (t)).

[0038] Furthermore, step S10 is to substitute t1 into criterion (12) after it is determined in step S9 that no fault has occurred to further determine whether a load change has occurred; if the criterion is met, it is determined that there is a load change, otherwise it is determined that the cable is in normal operation.

[0039]

[0040] The advantages of this invention are:

[0041] This invention decouples the original signal from the cable head end, and identifies medium-voltage DC cable faults by calculating the modal signals and constructing criteria. It provides a solution to the current predicament of medium-voltage DC cable fault identification, and is not affected by electric arcs or initial fault conditions, nor does it require prior knowledge. This method can effectively identify cable faults, has a short calculation time, high accuracy, and has important practical engineering application value. Attached Figure Description

[0042] Figure 1 This is a flowchart of the medium-voltage DC cable fault identification method of the present invention;

[0043] Figure 2 The simulation results of the modulus in Case 1 and Case 2 are shown in the figure.

[0044] Figure 3 The simulation results of the modulus for Cases 3, 4 and 5 are shown in the figure. Detailed Implementation

[0045] The technical solutions in the embodiments of this patent will be clearly and completely described below. Obviously, the described embodiments are only a part of the embodiments of this patent, and not all of them. Based on the embodiments of this patent, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this patent.

[0046] Example

[0047] S1. First, determine the cable's operating mode. If it is a single-pole operating mode, obtain the conductor-to-ground voltage u from the cable's beginning. C (t), conductor current i C (t) and grounding current i G (t);

[0048] S2. Obtain the phase mode transformation matrix T of the cable in unipolar operation mode. MU_SP and T MI_SP According to the phase mode transformation matrix T of the cable in unipolar operation mode MU_SP and T MI_SP For conductor-to-ground voltage u C (t), conductor current iC (t) and grounding current i G (t) is decoupled to obtain α-mode, β-mode, and γ-mode voltage and current signals, and their calculation formulas are as follows:

[0049]

[0050]

[0051] Among them, i α (t), i β (t) and i γ (t) represents the α-mode, β-mode, and γ-mode current signals; u α (t), u β (t), u γ (t) represents the α-mode, β-mode, and γ-mode voltage signals; i C (t), i S (t), i A (t) and i G (t) represent the current signals flowing in the conductor, metal sheath, armor layer, and grounding wire, respectively; u C (t), u S (t), u A (t) and u G (t) represents the voltage to ground of the conductor, metal sheath, armor layer, and grounding wire; R G Indicates resistance to ground;

[0052] S3. Use the wavelet Teager algorithm to determine the α-mode voltage u. α The time t0 corresponding to the first singularity of (t);

[0053] S4. Substitute t0 into the fault identification criterion (3). If the criterion is satisfied, it is determined that a fault has occurred; otherwise, it is determined that no fault has occurred.

[0054]

[0055] The functions max() and min() are used to determine the maximum and minimum values ​​of the modal signal; || is used to take the absolute value of the modal signal; u α_LV (t) represents the α-mode voltage signal obtained during the most severe load change; T is the AC power frequency time interval; U α_LV Indicate u α_LV (t) is the maximum value of t within the time interval [t0+0.5T, t0+1.5T]; R α_LV This indicates that within the time intervals [t0-1.5T, t0-0.5T] and [t0+0.5T, t0+1.5T], u α_LV The ratio of the maximum to the minimum value of (t);

[0056] S5. After determining that no fault has occurred in step S4, substitute t0 into the criteria (4) and (5) to further determine whether the load has suddenly increased or decreased. If either (4) or (5) is satisfied, it is determined that the load has suddenly decreased; otherwise, it is determined that the load has suddenly increased.

[0057]

[0058]

[0059] Where Δt is the sampling interval;

[0060] S6. If the cable operates in bipolar mode, then the positive conductor current i is sampled from the beginning of the cable. PC (t), voltage u of positive conductor to ground PC (t) Grounding current i of the positive cable PG (t), collecting the negative conductor current i NC (t), Voltage u of negative conductor to ground NC (t) Grounding current i of the negative cable NG (t);

[0061] S7. Obtain the phase mode transformation matrix T of the cable in bipolar operation mode. MU_DP and T MI_DP Based on the phase mode transformation matrix T of the cable in bipolar operation mode MU_DP and T MI_DP For the positive conductor current i PC (t), voltage u of positive conductor to ground PC (t) Grounding current i of the positive cable PG (t), negative conductor current i NC (t), Voltage u of negative conductor to ground NC (t) and the grounding current i of the negative cable NG (t) Decoupling is performed, and the χ-mode, κ-mode, δ-mode, ε-mode, ξ-mode, and η-mode voltage and current signals are calculated using the following formulas:

[0062]

[0063]

[0064]

[0065] Among them, i χ (t), i κ (t), i δ (t), i ε (t), i ζ(t) and i η (t) represents the χ-mode, κ-mode, δ-mode, ε-mode, ζ-mode, and η-mode current signals; u χ (t), u κ (t), u δ (t), u ε (t), u ζ (t) and u η (t) represents the χ-mode, κ-mode, δ-mode, ε-mode, ζ-mode, and η-mode voltage signals; i P (t), i PS (t), i PA (t) and i PG (t) is the current signal flowing through the positive conductor, the metal sheath, and the grounding wire; i N (t), i NS (t), i NA (t) and i NG (t) Current signal flowing through the negative conductor, metal sheath, and grounding wire; u P (t), u PS (t) and u PA (t) is the grounding voltage of the positive conductor, metal sheath, and armor layer; u N (t),u NS (t) and u NA (t) is the grounding voltage of the negative conductor, metal sheath, and armor layer; R PG and R NG This indicates the resistance of the grounding wire of the positive and negative cables;

[0066] S8. Use the wavelet Teager algorithm to determine the χ-mode voltage signal u. χ The time t1 corresponds to the first singularity of (t);

[0067] S9. Substitute t1 into the bipolar cable fault identification criterion (9). If the criterion is satisfied, it is determined that a fault has occurred. Then use the fault classification criterion (10) and (11) to determine the fault type. If either (10) or (11) is satisfied, it is determined that a bipolar fault has occurred. Otherwise, it is determined that a single-pole direct / indirect grounding fault has occurred.

[0068]

[0069] Among them, u ε_LV (t) represents the ε-mode voltage signal obtained when the most severe load change occurs; U ε_LV Indicate u ε_LV (t) is the maximum value of t over the time interval [t0+0.5T, t0+1.5T]; R ε_LVThis indicates that u is within the time intervals [t0-1.5T, t0-0.5T] and [t0+0.5T, t0+1.5T]. ε_LV The ratio of the maximum to the minimum value of (t);

[0070]

[0071]

[0072] Among them, u χ_HIF (t) and i χ_HIF (t) represents the χ-mode voltage and current signal (u) when a high-impedance fault occurs. χ (t),i χ (t));

[0073] S10. If it is determined that no fault has occurred, substitute t1 into the criterion (12) to further determine whether a load change has occurred; if the criterion is met, it is determined to be a load change, otherwise, it is determined that the cable is in normal operation.

[0074]

[0075] Simulation verification

[0076] A 10kV medium-voltage DC distribution cable with a length of 10km was constructed in RTDS operating modes under both single-pole and bi-pole modes. The cable load was 1MW in single-pole mode and 2.95MW for both poles in bi-pole mode. The cable type was YJV22-80mm². 2 The sampling frequency was set to 1MHz. Simulations were conducted on the aforementioned cable experimental platform under various conditions to consider the impact of different scenarios on the proposed method, and a large number of experiments were carried out. Detailed experimental information is shown in Table 1, and the experimental results are shown in Table 2. Where Δf represents the percentage of load change; R1 represents the transition resistance at the fault point; x1 represents the location of the fault; P... FD This indicates the accuracy of fault identification. Five typical experiments were conducted under different conditions to address parameter variations. Details of the five experimental cases are shown in Table 3. The cable modal signals under the five conditions are as follows: Figure 2 and Figure 3 As shown, IT represents the time when the load change or fault begins.

[0077] Table 1 Experimental information under different scenarios

[0078]

[0079]

[0080] Table 2 Fault identification results under different scenarios

[0081]

[0082] Table 3 details five typical cases.

[0083]

[0084] As can be seen from the table, the method proposed in this invention has good identification accuracy for medium-voltage DC cables when faults occur and load fluctuations occur.

[0085] This patent and its embodiments have been described, but this description is not restrictive. The accompanying drawings show only one embodiment of this patent, and the actual structure is not limited to this. In short, if a person skilled in the art, inspired by this description, designs a similar structure and embodiment without departing from the inventive spirit of this patent, such design should fall within the scope of protection of this patent.

Claims

1. A method for fault identification of medium-voltage DC distribution cables based on decoupling modulus, characterized in that, Includes the following steps: S1. First, determine the cable's operating mode. If it is a single-pole operating mode, obtain the conductor-to-ground voltage u from the cable's beginning. C (t), conductor current i C (t) and grounding current i G (t); S2. Obtain the phase mode transformation matrix T of the cable in unipolar operation mode. MU_SP and T MI_SP For conductor-to-ground voltage u C (t), conductor current i C (t) and grounding current i G (t) is decoupled, and the α-mode, β-mode and γ-mode voltage and current signals are calculated; S3. Use the wavelet Teager algorithm to determine the α-mode voltage u. α The time t0 corresponding to the first singularity of (t); S4. Substitute t0 into the fault identification criterion (3) to determine whether the fault has occurred; S5. If it is determined that no fault has occurred, then substitute t0 into the criterion (4) and (5) to further determine whether the load has suddenly increased or decreased. S6. If the cable operates in bipolar mode, then the positive conductor current i is sampled from the beginning of the cable. PC (t), voltage u of positive conductor to ground PC (t) Grounding current i of the positive cable PG (t), collecting the negative conductor current i NC (t), Voltage u of negative conductor to ground NC (t) Grounding current i of the negative cable NG (t); S7. Obtain the phase mode transformation matrix T of the cable in bipolar operation mode. MU_DP and T MI_DP For the positive conductor current i PC (t), voltage u of positive conductor to ground PC (t) Grounding current i of the positive cable PG (t), negative conductor current i NC (t), Voltage u of negative conductor to ground NC (t) and the grounding current i of the negative cable NG (t) is decoupled, and the χ-mode, κ-mode, δ-mode, ε-mode, ξ-mode and η-mode voltage and current signals are calculated; S8. Use the wavelet Teager algorithm to determine the χ-mode voltage signal u. χ The time t1 corresponds to the first singularity of (t); S9. Substitute t1 into the bipolar cable fault identification criterion (9) to determine whether a fault has occurred; if a fault has occurred, continue to use the fault classification criterion (10) and (11) to determine the fault type. S10. If it is determined that no fault has occurred, then t1 is substituted into the criterion (12) to further determine whether a load change has occurred.

2. The method for fault identification of medium-voltage DC distribution cables based on decoupling modulus according to claim 1, characterized in that: Step S2 is based on the phase mode transformation matrix T of the cable in unipolar operation mode. MU_SP and T MI_SP For conductor-to-ground voltage u C (t), conductor current i C (t) and grounding current i G (t) is decoupled to obtain α-mode, β-mode, and γ-mode voltage and current signals, and their calculation formulas are as follows: Among them, i α (t), i β (t) and i γ (t) represents the α-mode, β-mode, and γ-mode current signals; u α (t), u β (t), u γ (t) represents the α-mode, β-mode, and γ-mode voltage signals; i C (t), i S (t), i A (t) and i G (t) represent the current signals flowing in the conductor, metal sheath, armor layer, and grounding wire, respectively; u C (t), u S (t), u A (t) and u G (t) represents the voltage to ground of the conductor, metal sheath, armor layer, and grounding wire; R G This represents the resistance to ground.

3. The method for fault identification of medium-voltage DC distribution cables based on decoupling modulus according to claim 1, characterized in that: Step S4 involves substituting t0 into the fault identification criterion (3). If the criterion is satisfied, it is determined that a fault has occurred; otherwise, it is determined that no fault has occurred. The functions max() and min() are used to determine the maximum and minimum values ​​of the modal signal; || is used to take the absolute value of the modal signal; u α_LV (t) represents the α-mode voltage signal obtained during the most severe load change; T is the AC power frequency time interval; U α_LV Indicate u α_LV (t) is the maximum value of t within the time interval [t0+0.5T, t0+1.5T]; R α_LV This indicates that within the time intervals [t0-1.5T, t0-0.5T] and [t0+0.5T, t0+1.5T], u α_LV The ratio of the maximum value to the minimum value of (t).

4. The method for fault identification of medium-voltage DC distribution cables based on decoupling modulus according to claim 1, characterized in that: Step S5 is to substitute t0 into the criteria (4) and (5) after it is determined in step S4 that no fault has occurred, and further determine whether the load has suddenly increased or decreased; if either (4) or (5) is satisfied, it is determined that the load has suddenly decreased, otherwise it is determined that the load has suddenly increased. Where Δt is the sampling interval.

5. The method for fault identification of medium-voltage DC distribution cables based on decoupling modulus according to claim 1, characterized in that: Step S7 is based on the phase mode transformation matrix T of the cable in bipolar operation mode. MUDP and T MIDP For the positive conductor current i PC (t), voltage u of positive conductor to ground PC (t) Grounding current i of the positive cable PG (t), negative conductor current i NC (t), Voltage u of negative conductor to ground NC (t) and the grounding current i of the negative cable NG (t) Decoupling is performed, and the χ-mode, κ-mode, δ-mode, ε-mode, ξ-mode, and η-mode voltage and current signals are calculated using the following formulas: Among them, i χ (t), i κ (t), i δ (t), i ε (t), i ζ (t) and i η (t) represents the χ-mode, κ-mode, δ-mode, ε-mode, ζ-mode, and η-mode current signals; u χ (t), u κ (t), u δ (t), u ε (t), u ζ (t) and u η (t) represents the χ-mode, κ-mode, δ-mode, ε-mode, ζ-mode, and η-mode voltage signals; i P (t), i PS (t), i PA (t) and i PG (t) is the current signal flowing through the positive conductor, the metal sheath, and the grounding wire; i N (t), i NS (t), i NA (t) and i NG (t) Current signal flowing through the negative conductor, metal sheath, and grounding wire; u P (t), u PS (t) and u PA (t) is the grounding voltage of the positive conductor, metal sheath, and armor layer; u N (t),u NS (t) and u NA (t) is the grounding voltage of the negative conductor, metal sheath, and armor layer; R PG and R NG This indicates the grounding resistance of the positive and negative cables.

6. The method for fault identification of medium-voltage DC distribution cables based on decoupling modulus according to claim 1, characterized in that: Step S9 is to substitute t1 into the bipolar cable fault identification criterion (9). If the criterion is satisfied, it is determined that a fault has occurred. The fault classification criterion (10) and (11) are used to determine the fault type. If either (10) or (11) is satisfied, it is determined that a bipolar fault has occurred. Otherwise, it is determined that a single-pole direct / indirect grounding fault has occurred. Among them, u ε_LV (t) represents the ε-mode voltage signal obtained when the most severe load change occurs; U ε_LV Indicate u ε_LV (t) is the maximum value of t over the time interval [t0+0.5T, t0+1.5T]; R ε_LV This indicates that u is within the time intervals [t0-1.5T, t0-0.5T] and [t0+0.5T, t0+1.5T]. ε_LV The ratio of the maximum to the minimum value of (t); Among them, u χ_HIF (t) and i χ_HIF (t) represents the χ-mode voltage and current signal (u) when a high-impedance fault occurs. χ (t),i χ (t)).

7. The method for fault identification of medium-voltage DC distribution cables based on decoupling modulus according to claim 1, characterized in that: Step S10 is to substitute t1 into criterion (12) after it is determined in step S9 that no fault has occurred to further determine whether a load change has occurred; if the criterion is met, it is determined that there is a load change, otherwise it is determined that the cable is in normal operation.

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