Direct current line pilot protection method and system based on special frequency measurement impedance property

Abstract

The invention discloses a direct current line pilot protection method and system based on a special frequency measurement impedance property, and the method comprises the steps: 1, calculating a wavelet transformation modulus maximum value of a line mode voltage through employing a wavelet transformation method, and starting protection when the wavelet transformation modulus maximum value meets a protection starting criterion; step 2, the rectification side protection unit and the inversion side protection unit calculate fault components of the line mode voltage and the line mode current in each data window; 3, calculating a real part and an imaginary part of the measured impedance under two specific frequencies; step 4, determining the fault direction by using the fault direction discrimination characteristic quantity and the fault direction criterion; step 5, when the fault directions determined by the rectification side protection unit and the inversion side protection unit are consistent, determining that an internal fault occurs, otherwise, determining that an external fault occurs, and returning to the step 1; and step 6, when the occurrence of the internal fault is judged, calculating a fault pole selection characteristic quantity, and judging a fault type and a fault line according to a fault pole selection criterion, thereby solving the problems that pilot protection is slow in action, high in synchronization requirement and influenced by distributed capacitance.

Description

Technical Field

[0001] The present invention relates to the technical field of relay protection for conventional high-voltage direct current transmission lines in power systems, and in particular to a method and system for pilot protection of high-voltage direct current transmission lines based on measuring impedance properties at a specific frequency. Background Art

[0002] The distance between my country's power load center and the western energy center is as far as 800 to 3,000 km. DC transmission technology plays an important role in cross-regional energy allocation, optimal resource allocation, and regional economic coordination.

[0003] High-voltage direct current transmission lines are usually thousands of kilometers long, and the geographical environment of the areas they pass through is complex, with a high probability of failure. In the prior art, the protection configuration scheme of the direct current line of the direct current transmission project has the following problems: as the main protection, the traveling wave protection is a single-ended protection, which has the problem of easy refusal to operate due to high-resistance faults at the end and low protection sensitivity; as the backup protection, the sudden change protection and low voltage protection have the same problem as the main protection, while the current differential protection has a high delay problem of up to 1.1s, which may lead to the situation where the pole control protection takes precedence and causes DC locking. In addition, there are as many as 25 high-voltage direct current transmission lines of more than 1,000 km in the direct current transmission project of my country, which makes the traveling wave protection and backup protection face more severe action situations. The longitudinal protection is usually used as a backup protection to clear high fault resistance faults. Therefore, for the direct current line faults of the AC / DC system, it is necessary to study the longitudinal protection with low synchronization demand, fast response and no influence of distributed capacitance as the backup protection to ensure the stable operation of the system.

[0004] In summary, the existing longitudinal protection technology has the problems of slow action speed, high synchronization requirements and distributed capacitance. At present, there is an urgent need for a protection solution that can correctly distinguish the fault section, has a reasonable cost, is easy to implement and has strong applicability. Summary of the invention

[0005] In order to solve the deficiencies in the prior art, the present invention provides a DC line longitudinal protection method and system based on measuring impedance properties in a specific frequency band. Based on the difference in fault characteristics inside and outside the measured impedance zone, the present invention solves the problems of slow action speed, high synchronization requirements and influence of distributed capacitance in the existing longitudinal protection technology.

[0006] The present invention adopts the following technical solution.

[0007] The present invention proposes a DC line pilot protection method for special frequency impedance measurement, comprising:

[0008] Step 1, obtaining the positive pole voltage and current, negative pole voltage and current of the DC transmission line; using the wavelet transform method to calculate the wavelet transform modulus maximum of the line mode voltage, and when the wavelet transform modulus maximum meets the protection start criterion, start the protection; wherein the phase mode transformation method is used to obtain the line mode voltage according to the positive and negative pole voltages of the DC transmission line;

[0009] Step 2, the rectifier side protection unit and the inverter side protection unit respectively intercept the data before and after the protection start time with a data window of a set length, and calculate the fault component of the line mode voltage and the fault component of the line mode current corresponding to each data window;

[0010] Step 3, using the fault components of the line mode voltage and the line mode current corresponding to each data window, determine the real part and the imaginary part of the measured impedance at the protection installation at the set first frequency and the second frequency;

[0011] Step 4: The rectifier-side protection unit and the inverter-side protection unit both use the ratio of the imaginary part to the real part of the measured impedance at the protection installation location at the first frequency in the data window and the sum of the ratio of the imaginary part to the real part of the measured impedance at the protection installation location at the second frequency as the fault direction discrimination feature quantity; and use the fault direction discrimination feature quantity and the fault direction criterion to determine the fault direction;

[0012] Step 5: When the fault direction determined by the rectifier side protection unit is consistent with the fault direction determined by the inverter side protection unit, it is determined that the DC line has an internal fault; otherwise, it is determined that the DC line has an external fault, and the process returns to step 1;

[0013] Step 6: When it is determined that a DC line fault occurs in the area, the fault pole selection characteristic quantity is calculated, and the fault type and the fault line are determined according to the fault pole selection criterion to determine the restart control scheme of the fault line.

[0014] Preferably, the general model of the phase transformation method satisfies the following relationship:

[0015]

[0016] Where F0 is the zero mode signal, F1 is the line mode signal, and F p is the positive signal of the DC transmission line, F n It is the negative pole signal of the DC transmission line.

[0017] Preferably, after the wavelet transform coefficients of the line mode voltage are calculated using a quadratic B-spline wavelet transform with a first-order vanishing moment, the wavelet transform modulus maximum is obtained.

[0018] Preferably, the line mode voltage is calculated by wavelet transform method to obtain multiple wavelet transform module maxima, and the maximum value is compared with the protection start threshold value. When the protection start criterion is met, the protection is started; wherein the protection start criterion satisfies the following relationship:

[0019] WTMM>Δ set (2)

[0020] Where WTMM is the wavelet transform modulus maximum of the line mode voltage, Δ set To protect the startup threshold, it is set to 0.5.

[0021] Preferably, the set length of the data window is not less than 1 ms and not more than 8 ms.

[0022] Preferably, the fault component Δu1 of the line mode voltage and the fault component Δi1 of the line mode current corresponding to the data window satisfy the following relationship:

[0023]

[0024] Where u1(k) and i1(k) are the line mode voltage and line mode current at sampling point k, respectively. T ) and i1(kN T ) are kN at each sampling point in the time period before the protection start time T The line mode voltage and line mode current of the protection start time are both steady-state values. In the embodiment, the number of sampling points before the protection start time is N T It is not less than 500, corresponding to 10ms before the protection starts.

[0025] Preferably, the rectifier side protection unit intercepts the data before and after the protection start time with a data window of a set length, and calculates the fault component of the line mode voltage corresponding to each data window of the rectifier side protection unit and the fault component of the line mode current The inverter side protection unit intercepts the data before and after the protection start time with a data window of set length, and calculates the fault component of the line mode voltage corresponding to each data window of the inverter side protection unit and the fault component of the line mode current

[0026] Preferably, step 3 comprises:

[0027] Step 3.1, the generalized S transform method is used to extract the real part ΔU of the line mode voltage fault component Δu1 corresponding to each data window at the set first frequency f1 and second frequency f2. R (f1), ΔU R (f2) and the imaginary part ΔU X(f1), ΔU X (f2), the generalized S transform method is used to extract the real part ΔI of the line mode current fault component Δi1 corresponding to each data window at the set first frequency f1 and second frequency f2 R (f1), ΔI R (f2) and the imaginary part ΔI X (f1), ΔI X (f2);

[0028] Step 3.2, using ΔU R (f1), ΔU R (f2), ΔU X (f1), ΔU X (f2), ΔI R (f1), ΔI R (f2), ΔI X (f1), ΔI X (f2) Calculate the real part of the measured impedance R at the protection installation m (f1), R m (f2) and the imaginary part X m (f1), X m (f2).

[0029] Preferably, step 3.1 comprises:

[0030] Step 3.1.1, adopting the generalized S transform method, using the fault component Δu1 of the line mode voltage and the fault component Δi1 of the line mode current corresponding to each data window to determine the complex time-frequency matrix;

[0031] Among them, the discrete model obtained by the generalized S transform method satisfies the following relationship:

[0032]

[0033] In the formula, The generalized S-transform with a frequency of m / NT is obtained by using adjustment factors η and β for the time series x[kT] with a sampling interval of T, where k = 0, 1, ..., N-1, N is the total number of sampling points, η and β are adjustment factors, both are positive numbers, and the adjustment factors η = 1 and β = 2 are selected; X[n / NT] is the discrete Fourier transform of the time series x[kT], and n is the discrete point corresponding to the sampling point k; is the Fourier transform of the discrete Gaussian sequence g[m / NT, kT] using the adjustment factor η, where m is the discrete point corresponding to the sampling point k;

[0034] Based on formula (4), let the frequency change from 1 / NT to 1 / 2T, and the time change from 0 to (N-1)T, and calculate the generalized S transform of the time series x[kT], the result is a complex time-frequency matrix with (N / 2+1) rows and N columns;

[0035] Step 3.1.2, the elements of the m+1th row in the complex time-frequency matrix correspond to the fault components of the line-mode voltage and the line-mode current at the frequency m / NT; according to the set first frequency f1 and second frequency f2, the real part ΔU of the fault component Δu1 of the line-mode voltage at the set first frequency f1 and second frequency f2 is extracted from the corresponding rows in the complex time-frequency matrix respectively. R (f1), ΔU R (f2) and the imaginary part ΔU X (f1), ΔU X (f2), the real part ΔI of the fault component Δi1 of the line mode current at the set first frequency f1 and the second frequency f2 R (f1), ΔI R (f2) and the imaginary part ΔI X (f1), ΔI X (f2);

[0036] Among them, the first row of the complex time-frequency matrix is ​​information with a frequency of 0, which is the DC component; the frequency corresponding to the m+1th row is m / NT.

[0037] Preferably, under the set first frequency f1 and second frequency f2, the rectifier side protection unit uses the fault component of the line mode voltage corresponding to each data window and the fault component of the line mode current Determine the real part of the measured impedance at the protection installation corresponding to the rectifier side protection unit With the imaginary part Under the set first frequency f1 and second frequency f2, the inverter side protection unit uses the fault component of the line mode voltage corresponding to each data window and the fault component of the line mode current Determine the real part of the measured impedance at the protection installation corresponding to the inverter side protection unit With the imaginary part

[0038] Preferably, in step 4, the rectifier side protection unit calculates the imaginary part of the measured impedance at the protection installation at the first frequency With real part ITR C (f1), calculate the imaginary part of the measured impedance of the DC transmission line at the second frequency With real part ITR C (f2), based on the ITR in the data window C(f1) and ITR C The sum of (f2) As the fault direction discrimination characteristic quantity of the rectifier side protection unit; the inverter side protection unit calculates the imaginary part of the measured impedance at the protection installation at the first frequency With real part ITR I (f1), calculate the imaginary part of the measured impedance at the protection installation at the second frequency With real part ITR I (f2); ITR within the data window I (f1) and ITR I The sum of (f2) As the fault direction discrimination feature of the inverter side protection unit.

[0039] Preferably, based on the complex time-frequency matrix obtained by formula (4), the fault direction discrimination feature quantity satisfies the following relationship:

[0040]

[0041] Where m1 and m2 are the number of rows of the complex time-frequency matrix corresponding to the first frequency f1 and the second frequency f2, respectively. s is the sampling point at the protection start time in the data window, N is the total number of sampling points, is the modulus of the imaginary part of the element in the mth row and kth column of the complex time-frequency matrix, is the modulus of the real part of the element in the mth row and kth column of the complex time-frequency matrix.

[0042] Preferably, the fault direction criterion includes:

[0043] When it is judged as a forward fault, the fault direction flag is S dir Set to 1;

[0044] When it is judged as a reverse fault, the fault direction flag is S dir Set to 0;

[0045] Among them, k set is the set direction discrimination threshold, k set The value should be no less than 30.

[0046] Preferably, The fault direction flag of the rectifier side protection unit Set to 1, otherwise, the fault direction flag Set to 0; The fault direction flag of the inverter side protection unit Set to 1, otherwise, the fault direction flag Set to 0; when and When the values ​​of are all 1, it is determined that the DC line has an internal fault; otherwise, it is determined that the DC line has an external fault.

[0047] Preferably, specifically, the fault pole selection characteristic quantity satisfies the following relationship:

[0048]

[0049] In the formula, λ is the fault selection characteristic quantity, is the positive voltage fault component at sampling point n, is the negative pole voltage fault component of sampling point n, and N is the total number of sampling points.

[0050] Preferably, the fault pole selection criteria include:

[0051] λ ≥ τ set When , it is determined to be a positive grounding fault;

[0052] λ≤-τ set When , it is determined to be a negative grounding fault;

[0053] -τ set <λ<τ set When , it is determined to be a bipolar fault.

[0054] Among them, τ set is the fault type discrimination threshold, which is set to 20.

[0055] The present invention also proposes a DC line pilot protection system with special frequency impedance measurement, comprising:

[0056] A fault component acquisition module is used to obtain the positive pole voltage and current, negative pole voltage and current of the DC transmission line; the wavelet transform method is used to calculate the wavelet transform modulus maximum of the line mode voltage, and when the wavelet transform modulus maximum meets the protection start criterion, the protection is started; wherein, the phase mode transformation method is used to obtain the line mode voltage according to the positive and negative pole voltages of the DC transmission line; the rectifier side protection unit and the inverter side protection unit respectively intercept the data before and after the protection start time with a data window of a set length, and calculate the fault component of the line mode voltage and the fault component of the line mode current corresponding to each data window;

[0057] A fault judgment module is used to determine the real part and imaginary part of the measured impedance at the protection installation under the set first frequency and second frequency by using the fault component of the line mode voltage and the fault component of the line mode current corresponding to each data window; the rectifier side protection unit and the inverter side protection unit both use the ratio of the imaginary part to the real part of the measured impedance at the protection installation under the first frequency and the sum of the ratio of the imaginary part to the real part of the measured impedance at the protection installation under the second frequency as the fault direction discrimination characteristic quantity; the fault direction is determined by using the fault direction discrimination characteristic quantity and the fault direction criterion; when the fault direction determined by the rectifier side protection unit is consistent with the fault direction determined by the inverter side protection unit, it is determined that the DC line has an in-zone fault, otherwise it is determined that the DC line has an out-of-zone fault; when it is determined that the DC line has an in-zone fault, the fault pole selection characteristic quantity is calculated, and the fault type and the fault line are determined according to the fault pole selection criterion to determine the restart control plan of the fault line.

[0058] The present invention is also a terminal, comprising a processor and a storage medium; the storage medium is used to store instructions; the processor is used to operate according to the instructions to execute the steps of the method.

[0059] The present invention is also a computer-readable storage medium having a computer program stored thereon, which implements the steps of the method when executed by a processor.

[0060] The beneficial effects of the present invention are that, compared with the prior art, at least the method proposed by the present invention can correctly identify faults within 23ms for a high-voltage direct current transmission line as long as 2000km, has a transition resistance capability of up to 900Ω, is not affected by distributed capacitance, has low requirements for synchronization of voltage and current data on both sides, has a certain noise resistance capability, can be used as a fast backup protection for direct current transmission lines, and can clear high-resistance faults in direct current lines. BRIEF DESCRIPTION OF THE DRAWINGS

[0061] Figure 1 The present invention is a flowchart of a method for longitudinal protection of a high-voltage direct current transmission line based on special-frequency measurement of impedance properties. DETAILED DESCRIPTION

[0062] In order to make the purpose, technical scheme and advantages of the present invention clearer, the technical scheme of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. The embodiments described in this application are only embodiments of a part of the present invention, rather than all embodiments. Based on the spirit of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work belong to the protection scope of the present invention.

[0063] The present invention proposes a method for pilot protection of high voltage direct current transmission lines based on special frequency measurement impedance properties, such as Figure 1 As shown, the following steps are included:

[0064] Step 1: Obtain the positive voltage u of the DC transmission line p and current i p , negative electrode voltage u n and current i n ; The wavelet transform method is used to calculate the wavelet transform modulus maximum of the line mode voltage u1. When the wavelet transform modulus maximum meets the protection start criterion, the protection is started; wherein, the phase mode transformation method is used to obtain the line mode voltage according to the positive and negative pole voltages of the DC transmission line.

[0065] Specifically, the general model of the phase transformation method satisfies the following relationship:

[0066]

[0067] Where F0 is the zero mode signal, F1 is the line mode signal, and F p is the positive signal of the DC transmission line, F n It is the negative pole signal of the DC transmission line.

[0068] When the positive voltage u of the DC transmission line is used p and negative voltage u n According to formula (1), the line mode voltage u1 is obtained.

[0069] Specifically, the wavelet transform coefficients of the line mode voltage are calculated using a quadratic B-spline wavelet transform with a first-order vanishing moment, and then the wavelet transform modulus maximum is obtained; the wavelet transform method used in this embodiment is a non-restrictive and preferred choice, and technical personnel in this field can choose different wavelet transform algorithms according to the calculation accuracy and speed requirements.

[0070] Specifically, the line mode voltage u1 is calculated by wavelet transform method to obtain multiple wavelet transform module maxima, and the maximum value is compared with the protection start threshold value. When the protection start criterion is met, the protection is started; wherein the protection start criterion satisfies the following relationship:

[0071] WTMM>Δ set (2)

[0072] Where WTMM is the wavelet transform modulus maximum of the line mode voltage, Δ set To protect the startup threshold, it is set to a value that can distinguish between a fault and normal operation, and in the embodiment of the present invention, it is set to 0.5.

[0073] In the embodiment, the wavelet transform modulus maximum is calculated by using a scaling filter and a wavelet filter.

[0074] Step 2: The rectifier side protection unit and the inverter side protection unit respectively intercept the data before and after the protection start time with a data window of set length, and calculate the fault component Δu1 of the line mode voltage and the fault component Δi1 of the line mode current corresponding to each data window.

[0075] In the embodiment, the set length of the data window is not less than 1ms and not more than 8ms, and the general sampling time window is 4ms before and after the fault. The data window cannot be too short, which will lead to insufficient frequency resolution in the frequency domain. The frequency resolution must be sufficient to obtain information of two set frequencies; and the data window should also ensure that only the reverse wave can be felt for faults outside the measurement point area.

[0076] Specifically, the fault component Δu1 of the line mode voltage and the fault component Δi1 of the line mode current corresponding to the data window satisfy the following relationship:

[0077]

[0078] Where u1(k) and i1(k) are the line mode voltage and line mode current at sampling point k, respectively. T ) and i1(kN T ) are the sampling points k to N in the time period before the protection start time. T The line mode voltage and line mode current of the protection start time are both steady-state values. In the embodiment, the number of sampling points before the protection start time is N T is not less than 500, corresponding to 10 before the protection start time ms .

[0079] In the embodiment, the rectifier side protection unit intercepts the data before and after the protection start time with a data window of a set length, and calculates the fault component of the line mode voltage corresponding to each data window of the rectifier side protection unit and the fault component of the line mode current The inverter side protection unit intercepts the data before and after the protection start time with a data window of set length, and calculates the fault component of the line mode voltage corresponding to each data window of the inverter side protection unit and the fault component of the line mode current

[0080] Step 3: using the fault component Δu1 of the line mode voltage and the fault component Δi1 of the line mode current corresponding to each data window, determine the real part and the imaginary part of the measured impedance at the protection installation at the set first frequency and the second frequency.

[0081] Specifically, step 3 includes:

[0082] Step 3.1, the generalized S transform method is used to extract the real part of the fault component Δu1 of the line mode voltage corresponding to each data window at the set first frequency f1 and second frequency f2 Δ U R (f1), Δ U R (f2) and the imaginary part ΔU X (f1), ΔU X (f2), the generalized S transform method is used to extract the real part ΔI of the line mode current fault component Δi1 corresponding to each data window at the set first frequency f1 and second frequency f2 R (f1), ΔI R (f2) and the imaginary part ΔI X (f1), ΔI X (f2); including:

[0083] Step 3.1.1, adopting the generalized S transform method, using the fault component Δu1 of the line mode voltage and the fault component Δi1 of the line mode current corresponding to each data window to determine the complex time-frequency matrix;

[0084] Among them, the discrete model obtained by the generalized S transform method satisfies the following relationship:

[0085]

[0086] In the formula, A generalized S transform with a frequency of m / NT is obtained by using adjustment factors η and β for a time series x[kT] with a sampling interval of T, wherein k=0, 1, ..., N-1, N is the total number of sampling points, η and β are both adjustment factors, both are positive numbers, and in the embodiment of the present invention, adjustment factors η=1 and β=2 are selected; X[n / NT] is a discrete Fourier transform of the time series x[kT], and n is a discrete point corresponding to the sampling point k; is the Fourier transform of the discrete Gaussian sequence g[m / NT, kT] using the adjustment factor η, where m is the discrete point corresponding to the sampling point k;

[0087] Based on formula (4), by changing the frequency from 1 / Nf to 1 / 2T and the time from 0 to (N-1)T, the discrete S transform of the time series x[kT] can be calculated, and the result is a complex time-frequency matrix with (N / 2+1) rows and N columns.

[0088] Step 3.1.2, the elements of the m+1th row in the complex time-frequency matrix correspond to the fault components of the line-mode voltage and the line-mode current at the frequency m / NT; according to the set first frequency f1 and second frequency f2, the real part ΔU of the fault component Δu1 of the line-mode voltage at the set first frequency f1 and second frequency f2 is extracted from the corresponding rows in the complex time-frequency matrix respectively. R (f1), ΔUR (f2) and the imaginary part ΔU X (f1), ΔU X (f2), the real part ΔI of the fault component Δi1 of the line mode current at the set first frequency f1 and the second frequency f2 R (f1), ΔI R (f2) and the imaginary part ΔI X (f1), ΔI X (f2);

[0089] The first row of the complex time-frequency matrix contains information with a frequency of 0, i.e., the DC component. Therefore, the frequency corresponding to the m+1th row is m / NT.

[0090] Step 3.2, using ΔU R (f1), ΔU R (f2), ΔU X (f1), ΔU X (f2), ΔI R (f1), ΔI R (f2), ΔI X (f1), ΔI X (f2) Calculate the real part of the measured impedance R at the protection installation m (f1), R m (f2) and the imaginary part X m (f1), X m (f2).

[0091] In the embodiment, under the set first frequency f1 and second frequency f2, the rectifier side protection unit uses the fault component of the line mode voltage corresponding to each data window and the fault component of the line mode current Determine the real part of the measured impedance at the protection installation corresponding to the rectifier side protection unit With the imaginary part Under the set first frequency f1 and second frequency f2, the inverter side protection unit uses the fault component of the line mode voltage corresponding to each data window and the fault component of the line mode current Determine the real part of the measured impedance at the protection installation corresponding to the inverter side protection unit With the imaginary part

[0092] In step 4, the rectifier side protection unit and the inverter side protection unit both use the ratio of the imaginary part to the real part of the measured impedance at the protection installation at the first frequency and the sum of the ratio of the imaginary part to the real part of the measured impedance at the protection installation at the second frequency as the fault direction discrimination characteristic quantity; and use the fault direction discrimination characteristic quantity and the fault direction criterion to determine the fault direction.

[0093] In the embodiment, the rectifier side protection unit calculates the imaginary part of the measured impedance at the protection installation at the first frequency With real part The ratio of TR C (f1), calculate the imaginary part of the measured impedance at the protection installation at the second frequency With real part ITR C (f2), based on the ITR in the data window C (f1) and ITR C The sum of (f2) As the fault direction discrimination characteristic quantity of the rectifier side protection unit; the inverter side protection unit calculates the imaginary part of the measured impedance at the protection installation at the first frequency With real part ITR I (f1), calculate the imaginary part of the measured impedance at the protection installation at the second frequency With real part ITR I (f2); ITR within the data window I (f1) and ITR I The sum of (f2) As the fault direction discrimination feature of the inverter side protection unit.

[0094] Based on the complex time-frequency matrix obtained by formula (4), the fault direction discrimination feature quantity satisfies the following relationship:

[0095]

[0096] Where m1 and m2 are the number of rows of the complex time-frequency matrix corresponding to the first frequency f1 and the second frequency f2, respectively. s is the sampling point at the protection start time in the data window, N is the total number of sampling points, is the modulus of the imaginary part of the element in the mth row and kth column of the complex time-frequency matrix, is the modulus of the real part of the element in the mth row and kth column of the complex time-frequency matrix.

[0097] Fault direction criteria include:

[0098] When it is judged as a forward fault, the fault direction flag is S dir Set to 1;

[0099] When it is judged as a reverse fault, the fault direction flag is S dir Set to 0;

[0100] Among them, k setis the set direction discrimination threshold. In the embodiment, since tan(89°)=57, considering the influence of noise and leaving a certain margin, k set The value should be no less than 30.

[0101] Step 5: When the fault direction determined by the rectifier side protection unit is consistent with the fault direction determined by the inverter side protection unit, it is determined that the DC line has an in-zone fault; otherwise, it is determined that the DC line has an out-of-zone fault, and the process returns to step 1.

[0102] In the embodiment, The fault direction flag of the rectifier side protection unit Set to 1, otherwise, the fault direction flag Set to 0; The fault direction flag of the inverter side protection unit Set to 1, otherwise, the fault direction flag Set to 0; when and When the values ​​of are all 1, it is determined that the DC line has an internal fault; otherwise, it is determined that the DC line has an external fault.

[0103] Step 6: When it is determined that a DC line fault occurs in the area, the fault pole selection characteristic quantity is calculated, and the fault type and the fault line are determined according to the fault pole selection criterion to determine the restart control scheme of the fault line.

[0104] Specifically, the fault selection characteristic quantity satisfies the following relationship:

[0105]

[0106] In the formula, λ is the fault selection characteristic quantity, is the positive voltage fault component at sampling point n, is the negative voltage fault component at sampling point n;

[0107] Fault selection criteria include:

[0108] λ ≥ τ set When , it is determined to be a positive grounding fault;

[0109] λ≤-τ set When , it is determined to be a negative grounding fault;

[0110] -τ set <λ<τ set When , it is determined to be a bipolar fault.

[0111] Among them, τ set is the fault type discrimination threshold, which is set to 100×lg1.6=20. The fault pole selection criterion is established based on electromagnetic coupling theory and is applicable to different DC transmission systems.

[0112] In order to solve the problems of slow action speed, high synchronization requirements and influence of distributed capacitance of existing longitudinal protection, the present invention proposes a longitudinal protection method for high-voltage direct current transmission lines based on special frequency measurement impedance properties. The longitudinal protection method for high-voltage direct current transmission lines based on special frequency measurement impedance properties proposed by the present invention has a transition resistance capacity of up to 900Ω, can assume the role of fast backup protection of direct current lines, and identify terminal high-resistance faults that cannot be identified by the main protection; the protection principle proposed by the present invention has a fast action speed. For high-voltage direct current transmission lines up to 2000km, the proposed protection method can correctly identify faults within 23ms; the protection principle proposed by the present invention has low synchronization requirements for voltage and current data on both sides, and only needs to transmit the direction flag bits on both sides, and the communication volume is low; the protection principle proposed by the present invention has anti-interference ability to noise to a certain extent; the protection principle proposed by the present invention uses the difference in the measured impedance properties under the specific frequency band of the fault inside and outside the area to construct the criterion for determining the direction, which is not affected by the distributed capacitance; the protection principle proposed by the present invention has a simple setting method, no need to determine the threshold through simulation, and is not affected by the line boundary parameters. It has universality and can be applied to different conventional direct current transmission systems.

[0113] In a typical AC / DC hybrid power grid structure, the DC transmission system is a ±800kV LCC-HVDC system, 12 / 24 and 2 / 39 DC filters are respectively configured on both sides of the DC transmission line, and 150mH smoothing reactors are respectively configured on the neutral bus and the pole bus. The control strategy of the rectifier station adopts constant current control, minimum trigger angle control and low-voltage current limiting control; the control strategy of the inverter station adopts a strategy of matching fixed shutdown angle control and constant current control, and low-voltage current limiting control and current deviation control are configured at the same time. The method proposed in the present invention is implemented by simulation means in a typical AC / DC hybrid power grid. When the amplitude-frequency characteristics and phase-frequency characteristics of the equivalent impedance on the back side of the measuring point are considered, it is found that when the frequency is between 700Hz and 1200Hz, the equivalent impedance on the back side of the measuring point is purely capacitive. The method proposed in the present invention is implemented by simulation in a ±800kV high-voltage direct current transmission line. The frequency characteristics of the real and imaginary parts of the line-mode wave impedance are considered. It is found that when the frequency exceeds 100Hz, the imaginary part of the line-mode wave impedance is much smaller than the real part. Therefore, the two specific frequencies selected are 875Hz and 1000Hz respectively.

[0114] The protection units at both ends of the rectifier side and the inverter side intercept a 4ms (N=400) data window before and after the protection start time. When the sampling frequency is 50kHz and the data window is used, the frequency resolution is 125Hz. The fault components of the line mode voltage and current in the intercepted data window are obtained. The fault component Δu1 of the line mode voltage and the fault component Δi1 of the line mode current corresponding to the data window satisfy the following relationship:

[0115]

[0116] The real and imaginary parts of the measured impedance at 875Hz and 1000Hz in the 4ms data window are extracted, and the ratio of the imaginary part to the real part is calculated, and then the fault judgment is made.

[0117] Fault A1: When a ±800kV high-voltage direct current transmission line has an in-region fault, a positive grounding fault through a 600Ω transition resistor occurs at a distance of 1200km from the measuring point of the rectifier side protection unit. After calculation, the fault direction discrimination characteristic quantity of the rectifier side protection unit is 1260.811, which is greater than the direction discrimination threshold, and the flag is set to 1; the fault direction discrimination characteristic quantity of the inverter side protection unit is 1053.121, which is greater than the direction discrimination threshold, and the flag is set to 1.

[0118] Fault A2: A negative pole grounding fault occurs through a 600Ω transition resistor at a distance of 1200km from the measuring point of the rectifier side protection unit. After calculation, the fault direction discrimination characteristic quantity of the rectifier side protection unit is 13713.000, which is greater than the direction discrimination threshold, and the flag is set to 1; the fault direction discrimination characteristic quantity of the inverter side protection unit is 997.551, which is greater than the direction discrimination threshold, and the flag is set to 1.

[0119] Fault A3: A bipolar fault through a 600Ω transition resistor occurred at a distance of 1200km from the measuring point on the rectifier side. After calculation, the fault direction discrimination characteristic quantity of the rectifier side protection unit is 1188.693, which is greater than the direction discrimination threshold, and the flag is set to 1; the fault direction discrimination characteristic quantity of the inverter side protection unit is 994.863, which is greater than the direction discrimination threshold, and the flag is set to 1.

[0120] Fault B: A metallic grounding fault occurs outside the inverter-side smoothing reactor of the ±800kV high-voltage DC transmission line. After calculation, the fault direction discrimination characteristic quantity of the rectifier-side protection unit is 107.754, which is greater than the direction discrimination threshold, and the flag is set to 1; the fault direction discrimination characteristic quantity of the inverter-side protection unit is 1.692, which is less than the direction discrimination threshold, and the flag is set to 0.

[0121] Fault C: A grounding fault occurs through a 300Ω transition resistor on the outside of the smoothing reactor on the rectifier side of the ±800kV high-voltage DC transmission line. After calculation, the fault direction discrimination characteristic quantity of the rectifier side protection unit is 0.836, which is less than the direction discrimination threshold, and the flag is set to 0; the fault direction discrimination characteristic quantity of the inverter side protection unit is 866.161, which is greater than the direction discrimination threshold, and the flag is set to 1.

[0122] Fault D: A metallic fault of phase A outside the inverter-side commutation busbar of the ±800kV high-voltage DC transmission line occurs. After calculation, the fault direction discrimination characteristic quantity of the rectifier-side protection unit is 37.523, which is greater than the direction discrimination threshold, and the flag is set to 1; the fault direction discrimination characteristic quantity of the inverter-side protection unit is 0.920, which is less than the direction discrimination threshold, and the flag is set to 0.

[0123] Fault E: A metallic fault of phase A outside the rectifier side commutation busbar of the ±800kV high-voltage DC transmission line occurs. After calculation, the fault direction discrimination characteristic quantity of the rectifier side protection unit is 1.485, which is less than the direction discrimination threshold, and the flag is set to 0; the fault direction discrimination characteristic quantity of the inverter side protection unit is 236.984, which is greater than the direction discrimination threshold, and the flag is set to 1.

[0124] Fault F: A phase metallic fault occurs at the midpoint of Hui I line in the receiving end AC line of the ±800kV high-voltage DC transmission line. After calculation, the fault direction discrimination characteristic quantity of the rectifier side protection unit is 33.920, which is greater than the direction discrimination threshold, and the flag is set to 1; the fault direction discrimination characteristic quantity of the inverter side protection unit is 3.520, which is less than the direction discrimination threshold, and the flag is set to 0.

[0125] As the longitudinal protection, select the optical cable with strong anti-interference and fast transmission speed to exchange information. The two ends exchange the direction flags and determine the fault interval according to the direction flags; for fault A1, fault A2 and fault A3, the direction flags are all 1, and the protection correctly determines that they are faults within the zone; for fault B, fault D and fault F, the direction flag on the inverter side is 0, and the protection correctly determines that they are faults outside the zone and returns to step 1; for fault C and fault E, the direction flag on the rectifier side is 0, and the protection correctly determines that they are faults outside the zone and returns to step 1. .

[0126] After fault A1 is determined to be an intra-zone fault, the fault pole selection characteristic value on the rectifier side is calculated to be 53.493, which is greater than 20. The fault pole selection element selects the positive line as the fault line, and the rectifier side implements the positive line restart strategy; the fault pole selection characteristic value on the inverter side is 63.545, which is greater than 20. The fault pole selection element selects the positive line as the fault line, and the inverter side implements the positive line restart strategy.

[0127] After fault A2 is determined to be an intra-zone fault, the fault pole selection characteristic value on the rectifier side is calculated to be -65.692, which is less than -20. The fault pole selection element selects the negative line as the fault line, and the rectifier side implements the negative line restart strategy; the fault pole selection characteristic value on the inverter side is -75.254, which is less than -20. The fault pole selection element selects the negative line as the fault line, and the inverter side implements the negative line restart strategy.

[0128] After fault A3 is determined to be an intra-zone fault, the fault pole selection characteristic value on the rectifier side is calculated to be -0.081, which is greater than -20 and less than 20. The fault pole selection element selects the positive and negative pole lines as fault lines, and the rectifier side implements the positive and negative pole line restart strategy; the fault pole selection characteristic value on the inverter side is 0.063, which is greater than -20 and less than 20. The fault pole selection element selects the positive and negative pole lines as fault lines, and the inverter side implements the positive and negative pole line restart strategy.

[0129] The present invention also proposes a DC line pilot protection system based on special frequency measurement impedance properties, comprising:

[0130] A fault component acquisition module is used to obtain the positive pole voltage and current, negative pole voltage and current of the DC transmission line; the wavelet transform method is used to calculate the wavelet transform modulus maximum of the line mode voltage, and when the wavelet transform modulus maximum meets the protection start criterion, the protection is started; wherein, the phase mode transformation method is used to obtain the line mode voltage according to the positive and negative pole voltages of the DC transmission line; the rectifier side protection unit and the inverter side protection unit respectively intercept the data before and after the protection start time with a data window of a set length, and calculate the fault component of the line mode voltage and the fault component of the line mode current corresponding to each data window;

[0131] A fault judgment module is used to determine the real part and imaginary part of the measured impedance at the protection installation under the set first frequency and second frequency by using the fault component of the line mode voltage and the fault component of the line mode current corresponding to each data window; the rectifier side protection unit and the inverter side protection unit both use the ratio of the imaginary part to the real part of the measured impedance at the protection installation under the first frequency and the sum of the ratio of the imaginary part to the real part of the measured impedance at the protection installation under the second frequency as the fault direction discrimination characteristic quantity; the fault direction is determined by using the fault direction discrimination characteristic quantity and the fault direction criterion; when the fault direction determined by the rectifier side protection unit is consistent with the fault direction determined by the inverter side protection unit, it is determined that the DC line has an in-zone fault, otherwise it is determined that the DC line has an out-of-zone fault; when it is determined that the DC line has an in-zone fault, the fault pole selection characteristic quantity is calculated, and the fault type and the fault line are determined according to the fault pole selection criterion to determine the restart control plan of the fault line.

[0132] The present disclosure may be a system, a method and / or a computer program product. The computer program product may include a computer-readable storage medium carrying computer-readable program instructions for causing a processor to implement various aspects of the present disclosure.

[0133] A computer-readable storage medium may be a tangible device that can hold and store instructions used by an instruction execution device. A computer-readable storage medium may be, for example, but not limited to, an electrical storage device, a magnetic storage device, an optical storage device, an electromagnetic storage device, a semiconductor storage device, or any suitable combination of the foregoing. More specific examples of computer-readable storage media (a non-exhaustive list) include: a portable computer disk, a hard disk, a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or flash memory), a static random access memory (SRAM), a portable compact disk read-only memory (CD-ROM), a digital versatile disk (DVD), a memory stick, a floppy disk, a mechanical encoding device, such as a punch card or a raised structure in a groove on which instructions are stored, and any suitable combination of the foregoing. As used herein, a computer-readable storage medium is not to be interpreted as a transient signal per se, such as a radio wave or other freely propagating electromagnetic wave, an electromagnetic wave propagating through a waveguide or other transmission medium (e.g., a light pulse through a fiber optic cable), or an electrical signal transmitted through a wire.

[0134] The computer-readable program instructions described herein can be downloaded from a computer-readable storage medium to each computing / processing device, or downloaded to an external computer or external storage device via a network, such as the Internet, a local area network, a wide area network, and / or a wireless network. The network can include copper transmission cables, optical fiber transmissions, wireless transmissions, routers, firewalls, switches, gateway computers, and / or edge servers. The network adapter card or network interface in each computing / processing device receives the computer-readable program instructions from the network and forwards the computer-readable program instructions for storage in the computer-readable storage medium in each computing / processing device.

[0135] The computer program instructions for performing the operation of the present disclosure may be assembly instructions, instruction set architecture (ISA) instructions, machine instructions, machine-related instructions, microcode, firmware instructions, state setting data, or source code or object code written in any combination of one or more programming languages, including object-oriented programming languages, such as Smalltalk, C++, etc., and conventional procedural programming languages, such as "C" language or similar programming languages. Computer-readable program instructions may be executed completely on a user's computer, partially on a user's computer, as an independent software package, partially on a user's computer, partially on a remote computer, or completely on a remote computer or server. In the case of a remote computer, the remote computer may be connected to the user's computer via any type of network, including a local area network (LAN) or a wide area network (WAN), or may be connected to an external computer (e.g., using an Internet service provider to connect via the Internet). In some embodiments, an electronic circuit, such as a programmable logic circuit, a field programmable gate array (FPGA), or a programmable logic array (PLA), may be customized by utilizing the state information of the computer-readable program instructions, and the electronic circuit may execute the computer-readable program instructions, thereby realizing various aspects of the present disclosure.

[0136] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, ordinary technicians in the relevant field should understand that the specific implementation methods of the present invention can still be modified or replaced by equivalents, and any modifications or equivalent replacements that do not depart from the spirit and scope of the present invention should be covered within the scope of protection of the claims of the present invention.

Claims

1. A DC line pilot protection method based on special frequency measurement impedance properties, characterized in that: include: Step 1, obtaining the positive pole voltage and current, negative pole voltage and current of the DC transmission line; using the wavelet transform method to calculate the wavelet transform modulus maximum of the line mode voltage, and when the wavelet transform modulus maximum meets the protection start criterion, start the protection; wherein the phase mode transformation method is used to obtain the line mode voltage according to the positive and negative pole voltages of the DC transmission line; Step 2, the rectifier side protection unit and the inverter side protection unit respectively intercept the data before and after the protection start time with a data window of a set length, and calculate the fault component of the line mode voltage and the fault component of the line mode current corresponding to each data window; Step 3, using the fault components of the line mode voltage and the line mode current corresponding to each data window, extracting the real part and the imaginary part of the measured impedance at the protection installation under the set first frequency and the second frequency; Step 4: The rectifier-side protection unit and the inverter-side protection unit both use the ratio of the imaginary part to the real part of the measured impedance at the protection installation location at the first frequency in the data window and the sum of the ratio of the imaginary part to the real part of the measured impedance at the protection installation location at the second frequency as the fault direction discrimination feature quantity; and use the fault direction discrimination feature quantity and the fault direction criterion to determine the fault direction; Step 5: When the fault direction determined by the rectifier side protection unit is consistent with the fault direction determined by the inverter side protection unit, it is determined that the DC line has an internal fault; otherwise, it is determined that the DC line has an external fault, and the process returns to step 1; Step 6: When it is determined that a DC line fault occurs in the area, the fault pole selection characteristic quantity is calculated, and the fault type and the fault line are determined according to the fault pole selection criterion to determine the restart control scheme of the fault line.

2. The DC line pilot protection method based on special frequency measurement impedance property according to claim 1 is characterized in that: The general model of the phase transformation method satisfies the following relationship: Where F0 is the zero mode signal, F1 is the line mode signal, and F p is the positive signal of the DC transmission line, F n It is the negative pole signal of the DC transmission line.

3. The DC line pilot protection method based on special frequency measurement impedance property according to claim 2 is characterized in that: The wavelet transform coefficients of the line mode voltage are calculated by using the quadratic B-spline wavelet transform with the first-order vanishing moment, and then the maximum value of the wavelet transform modulus is obtained.

4. The DC line pilot protection method based on special frequency measurement impedance property according to claim 3 is characterized in that: The wavelet transform method is used to calculate the line mode voltage to obtain multiple wavelet transform module maxima, and the maximum value is compared with the protection start threshold value. When the protection start criterion is met, the protection is started; wherein, the protection start criterion satisfies the following relationship: WTMM>Δ set (2) Where WTMM is the wavelet transform modulus maximum of the line mode voltage, Δ set To protect the startup threshold, it is set to 0.

5.

5. The DC line pilot protection method based on special frequency measurement impedance property according to claim 1, characterized in that: The set length of the data window is not less than 1ms and not more than 8ms.

6. The DC line pilot protection method based on special frequency measurement impedance property according to claim 5 is characterized in that: The fault component Δu1 of the line mode voltage and the fault component Δi1 of the line mode current corresponding to the data window satisfy the following relationship: Where u1(k) and i1(k) are the line mode voltage and line mode current at sampling point k, respectively. T ) and i1(kN T ) are kN at each sampling point in the time period before the protection start time T The line mode voltage and line mode current of the protection start time are both steady-state values. In the embodiment, the number of sampling points before the protection start time is N T It is not less than 500, corresponding to 10ms before the protection starts.

7. The DC line pilot protection method based on special frequency measurement impedance property according to claim 6 is characterized in that: The rectifier side protection unit intercepts the data before and after the protection start time with a data window of set length, and calculates the fault component of the line mode voltage corresponding to each data window of the rectifier side protection unit and the fault component of the line mode current The inverter side protection unit intercepts the data before and after the protection start time with a data window of set length, and calculates the fault component of the line mode voltage corresponding to each data window of the inverter side protection unit and the fault component of the line mode current 8. The DC line pilot protection method based on special frequency measurement impedance property according to claim 7 is characterized in that: Step 3 includes: Step 3.1, the generalized S transform method is used to extract the real part ΔU of the line mode voltage fault component Δu1 corresponding to each data window at the set first frequency f1 and second frequency f2. R (f1), ΔU R (f2) and the imaginary part ΔU X (f1), ΔU X (f2), the generalized S transform method is used to extract the real part ΔI of the line mode current fault component Δi1 corresponding to each data window at the set first frequency f1 and second frequency f2 R (f1), ΔI R (f2) and the imaginary part ΔI X (f1), ΔI X (f2); Step 3.2, using ΔU R (f1), ΔU R (f2), ΔU X (f1), ΔU X (f2), ΔI R (f1), ΔI R (f2), ΔI X (f1), ΔI X (f2) Calculate the real part of the measured impedance R at the protection installation m (f1), R m (f2) and the imaginary part X m (f1), X m (f2).

9. The DC line pilot protection method based on special frequency measurement impedance property according to claim 8, characterized in that: Step 3.1 includes: Step 3.1.1, adopting the generalized S transform method, using the fault component Δu1 of the line mode voltage and the fault component Δi1 of the line mode current corresponding to each data window to determine the complex time-frequency matrix; Among them, the discrete model obtained by the generalized S transform method satisfies the following relationship: In the formula, The generalized S-transform with a frequency of m / NT is obtained by using adjustment factors η and β for the time series x[kT] with a sampling interval of T, where k = 0, 1, ..., N-1, N is the total number of sampling points, η and β are adjustment factors, both are positive numbers, and the adjustment factors η = 1 and β = 2 are selected; X[n / NT] is the discrete Fourier transform of the time series x[kT], and n is the discrete point corresponding to the sampling point k; is the Fourier transform of the discrete Gaussian sequence g[m / NT, kT] using the adjustment factor η, where m is the discrete point corresponding to the sampling point k; Based on formula (4), let the frequency change from 1 / NT to 1 / 2T, and the time change from 0 to (N-1)T, and calculate the generalized S transform of the time series x[kT], the result is a complex time-frequency matrix with (N / 2+1) rows and N columns; Step 3.1.2, extracting the real part and the imaginary part at the first frequency f1 and the second frequency f2 set in the data window from the complex time-frequency matrix; Among them, the first row of the complex time-frequency matrix is ​​information with a frequency of 0, which is the DC component; the frequency corresponding to the m+1th row is m / NT.

10. The DC line pilot protection method based on special frequency measurement impedance property according to claim 9, characterized in that: Under the set first frequency f1 and second frequency f2, the rectifier side protection unit uses the fault component of the line mode voltage corresponding to each data window to and the fault component of the line mode current Determine the real part of the measured impedance at the protection installation corresponding to the rectifier side protection unit With the imaginary part Under the set first frequency f1 and second frequency f2, the inverter side protection unit uses the fault component of the line mode voltage corresponding to each data window and the fault component of the line mode current Determine the real part of the measured impedance at the protection installation corresponding to the inverter side protection unit With the imaginary part 11. The DC line pilot protection method based on special frequency measurement impedance property according to claim 10, characterized in that: In step 4, the rectifier side protection unit calculates the imaginary part of the measured impedance at the protection installation at the first frequency With real part ITR C (f1), calculate the imaginary part of the measured impedance at the protection installation at the second frequency With real part ITR C (f2), based on the ITR in the data window C (f1) and ITR C The sum of (f2) As the fault direction discrimination characteristic quantity of the rectifier side protection unit; the inverter side protection unit calculates the imaginary part of the measured impedance at the protection installation at the first frequency With real part The ratio of ITRI(f1) to the imaginary part of the measured impedance at the protection installation at the second frequency is calculated. With real part ITR I (f2); ITR within the data window I (f1) and ITR I The sum of (f2) As the fault direction discrimination feature of the inverter side protection unit.

12. The DC line pilot protection method of special frequency impedance measurement according to claim 11, characterized in that: Based on the complex time-frequency matrix obtained by formula (4), the fault direction discrimination feature quantity satisfies the following relationship: Where m1 and m2 are the number of rows of the complex time-frequency matrix corresponding to the first frequency f1 and the second frequency f2, respectively. s is the sampling point at the protection start time in the data window, N is the total number of sampling points, is the modulus of the imaginary part of the element in the mth row and kth column of the complex time-frequency matrix, is the modulus of the real part of the element in the mth row and kth column of the complex time-frequency matrix.

13. The DC line pilot protection method based on special frequency measurement impedance property according to claim 12, characterized in that: Fault direction criteria include: When it is judged as a forward fault, the fault direction flag is S dir Set to 1; When it is judged as a reverse fault, the fault direction flag is S dir Set to 0; Among them, k set is the set direction discrimination threshold, k set The value should be no less than 30.

14. The DC line pilot protection method of special frequency impedance measurement according to claim 13, characterized in that: The fault direction flag of the rectifier side protection unit Set to 1, otherwise, the fault direction flag Set to 0; The fault direction flag of the inverter side protection unit Set to 1, otherwise, the fault direction flag Set to 0; when and When the values ​​of are all 1, it is determined that the DC line has an internal fault; otherwise, it is determined that the DC line has an external fault.

15. The DC line pilot protection method based on special frequency measurement impedance property according to claim 1, characterized in that: Specifically, the fault selection characteristic quantity satisfies the following relationship: In the formula, λ is the fault selection characteristic quantity, is the positive voltage fault component at sampling point n, is the negative voltage fault component of sampling point n, and N is the total number of sampling points.

16. The DC line pilot protection method based on special frequency measurement impedance property according to claim 15, characterized in that: Fault selection criteria include: λ ≥ τ set When , it is determined to be a positive grounding fault; λ≤-τ set When , it is determined to be a negative grounding fault; -τ set <λ<τ set When , it is determined to be a bipolar fault. Among them, τ set is the fault type discrimination threshold, which is set to 20.

17. A DC line pilot protection system based on special frequency measurement impedance properties, characterized in that: include: A fault component acquisition module is used to obtain the positive pole voltage and current, negative pole voltage and current of the DC transmission line; the wavelet transform method is used to calculate the wavelet transform modulus maximum of the line mode voltage, and when the wavelet transform modulus maximum meets the protection start criterion, the protection is started; wherein, the phase mode transformation method is used to obtain the line mode voltage according to the positive and negative pole voltages of the DC transmission line; the rectifier side protection unit and the inverter side protection unit respectively intercept the data before and after the protection start time with a data window of a set length, and calculate the fault component of the line mode voltage and the fault component of the line mode current corresponding to each data window; A fault judgment module is used to determine the real part and imaginary part of the measured impedance at the protection installation under the set first frequency and second frequency by using the fault component of the line mode voltage and the fault component of the line mode current corresponding to each data window; the rectifier side protection unit and the inverter side protection unit both use the ratio of the imaginary part to the real part of the measured impedance at the protection installation under the first frequency in the data window and the sum of the ratio of the imaginary part to the real part of the measured impedance of the DC transmission line under the second frequency as the fault direction discrimination characteristic quantity; the fault direction is determined by using the fault direction discrimination characteristic quantity and the fault direction judgment criterion; when the fault direction determined by the rectifier side protection unit is consistent with the fault direction determined by the inverter side protection unit, it is determined that the DC line has an in-zone fault, otherwise it is determined that the DC line has an out-of-zone fault; when it is determined that the DC line has an in-zone fault, the fault pole selection characteristic quantity is calculated, and the fault type and the fault line are determined according to the fault pole selection criterion to determine the restart control scheme of the fault line.

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