Fault positioning method for offshore wind power direct current collection line based on traveling wave multi-scale gradient analysis

Abstract

The invention discloses an offshore wind power direct current collection line fault positioning method based on traveling wave multi-scale gradient analysis. The method comprises the steps that S1, a protection element judges that a fault occurs; s2, sampling the voltage and current of the positive electrode and the negative electrode of the bus side of the fault collection line, and calculating a 1-mode voltage reverse traveling wave; s3, filtering the 1-mode voltage reverse traveling wave based on a mathematical morphology algorithm, and calculating a filtering signal; s4, calculating a multi-scale gradient of a filtering signal, identifying a transient state abrupt change point of a waveform, and extracting arrival time and wave head polarity information of a fault voltage traveling wave; and S5, calculating a fault distance. According to the method, fault positioning is realized by using the fault voltage data within a short time after the fault, the specific fault position of the collection line can be quickly determined, and the method is of great significance in accelerating fault clearing and guaranteeing safe and stable operation of an offshore wind power system.

Description

Technical Field

[0001] The present invention belongs to the field of electric power systems, relates to the technical field of relay protection of offshore wind power DC collection lines, and specifically relates to a fault locating method for offshore wind power DC collection lines based on traveling wave multi-scale gradient analysis. Background Art

[0002] Faced with the increasingly severe energy crisis and climate change, countries around the world have proposed development plans to transform from fossil energy to renewable energy, and have made it clear that renewable energy generation is the future development direction. As one of the most mature and widely used renewable energy sources, wind energy is expected to become a pillar energy industry in my country. Offshore wind power has the advantages of high wind energy quality, large single-unit capacity, no land resource occupation, and can be consumed nearby, so it has broad development prospects. Nowadays, due to the increasing shortage of offshore wind farm sites, in order to adapt to its deep sea development trend, the offshore wind power full DC transmission system based on DC collection and DC transmission technology has emerged, which reduces the number of system commutations and effectively reduces system energy consumption, which is of great research significance.

[0003] Most of the line faults in the offshore wind power all-DC transmission system are permanent faults. In order to restore the normal operation of the system and power transmission, it is necessary to timely troubleshoot and clear the faults. According to the principle of fault location, the existing DC power grid fault location methods can be divided into three categories: 1) Transient characteristic method: By modeling the fault line, the mathematical relationship between the fault location and the fault characteristics after the fault occurs is obtained to achieve fault location. Among them, the single-terminal method has the advantages of good economy and fast ranging speed, but it is difficult to eliminate the influence of transition resistance on the fault ranging result; the double-terminal method has the advantages of reliable principle and high ranging accuracy, but it is also restricted by communication conditions and costs; 2) Active injection method: inject detection signals into the fault circuit through additional circuits or control power electronic converters, and realize fault ranging according to the collected signal characteristics. This type of method requires additional signal injection equipment, which increases the construction cost of the DC distribution network; 3) Traveling wave method: When a DC line fails, a fault traveling wave propagating to both sides of the line will be generated at the fault point. The fault location is achieved by extracting the amplitude, polarity, frequency and other characteristics of the transient traveling wave. However, for short lines, the difference in wave head arrival time is small, which makes it more difficult to identify the wave head, and puts forward extremely high requirements on the wave head identification algorithm. In summary, the sending end of the offshore wind power full DC system is a multi-terminal parallel network of distributed converter strings. The converter equivalent capacitance is connected in series with the line impedance, forming a complex and uneven line topology, which makes it difficult to locate the fault of the collection line. The existing positioning method has poor adaptability. It is urgent to propose a high-precision collection line fault positioning method to determine the specific fault location of the collection line, speed up the fault clearance, and ensure the safe and stable operation of the offshore wind power system. Summary of the invention

[0004] In order to solve the problems existing in the above-mentioned prior art, the purpose of the present invention is to provide a method for locating faults in offshore wind power DC collection lines based on traveling wave multi-scale gradient analysis. The method of the present invention does not have communication synchronization problems. At the same time, the proposed fault locating method has high positioning accuracy, strong robustness to noise and interference, and good engineering application value.

[0005] To achieve the above object, the technical solution adopted by the present invention is as follows:

[0006] A method for locating faults in an offshore wind power DC collection line based on traveling wave multi-scale gradient analysis comprises the following steps:

[0007] Step 1: The protection element detects that a fault has occurred;

[0008] After a fault occurs in the offshore wind power DC collection network, the protection element is activated to identify the faulty collection line and the fault type;

[0009] Step 2: Sample the voltage and current of the positive and negative poles on the bus side of the fault collection line, and calculate the 1-mode voltage reverse wave;

[0010] Calculate the 1-mode voltage u1(k) and 1-mode current i1(k) according to formula (1);

[0011]

[0012] In the formula, k is the sampling number, N is the total number of samples in the sampling window, and u p (k) is the positive electrode voltage, u n (k) is the negative electrode voltage, i p (k) is the positive electrode current, i n (k) is the negative electrode current;

[0013] According to formula (2), calculate the 1-mode voltage reverse wave u b1 (k);

[0014] u b1 (k)=[u1(k)-i1(k)·Z C ] / twenty two)

[0015] In the formula, Z C is the wave impedance;

[0016] Step 3: Filter the 1-mode voltage reverse traveling wave based on the mathematical morphology algorithm and calculate the filtered signal;

[0017] Select a flat structural element g(x), and let its domain be D(g) = {0, 1, ..., P};

[0018] According to formula (3), the expansion operation and corrosion operation are performed on the 1-mode voltage reverse traveling wave;

[0019]

[0020] Where, the reverse traveling wave of mode 1 voltage u b1 The domain of (k) is D(u b1 )={0,1,…,N}, represents the expansion operation, ! represents the erosion operation;

[0021] According to formula (4), open and close operations are performed;

[0022]

[0023] In the formula, represents an opening operation, and represents a closing operation;

[0024] According to formula (5), the filtered signal Filter(u b1 (k));

[0025]

[0026] In the formula, OC and CO represent the open-close operation and the close-open operation respectively, and the results are calculated according to formula (6);

[0027]

[0028]

[0029] Step 4: Calculate the multi-scale gradient of the filtered signal, identify the transient mutation point of the waveform, and extract the arrival time and wave head polarity information of the fault voltage traveling wave;

[0030] Based on the Laplace pyramid algorithm, the filtered signal is divided into multiple resolution scales, denoted by Filter(u b1 (k)) is the bottom signal G0 of the Gaussian pyramid. The lth layer signal G of the Gaussian pyramid decomposition is calculated according to formula (7): l ;

[0031]

[0032] Where N g is the number of the top layer of the Gaussian pyramid, m is the independent variable of the window function, and its value is m=-2,-1,0,1,2, is a window function,

[0033] According to formula (8), the l-th layer extension sequence is calculated

[0034]

[0035] In the formula,

[0036] According to formula (9), the l-th layer signal LP of Laplace pyramid decomposition is calculated l and the top Laplacian pyramid signal

[0037]

[0038] In the formula, is the l+1th layer extension sequence, is the top signal of the Gaussian pyramid;

[0039] According to formula (10), the gradient SG of each layer of the Laplace pyramid sequence of the 1-mode voltage inverse traveling wave is calculated: l ;

[0040] SG l (k)=(LP l *ker)(k) (10)

[0041] Wherein, the convolution kernel of the Sobel gradient operator ker = [-1 0 1];

[0042] Calculate the multi-scale gradient MSG(k) of the 1-mode voltage reverse traveling wave according to formula (11);

[0043]

[0044] In the formula, Expand represents the upsampling operation;

[0045] According to the multi-scale gradient calculation results, the transient mutation point of the voltage waveform is identified, and the arrival time sampling sequence number of the measured initial traveling wave is extracted, which is recorded as k r0 ; Extract the arrival time sampling number of the second traveling wave. If the polarity of the second traveling wave head is the same as that of the initial traveling wave, it is recorded as k r1 If the polarity of the second traveling wave is opposite to that of the initial traveling wave, it is recorded as k t1 ;

[0046] Step 5: Calculate the fault distance;

[0047] Calculate the fault distance x according to formula (12);

[0048]

[0049] Where v is the speed of the traveling wave, L is the total length of the fault collection line, and the initial arrival time difference Δt between two consecutive traveling waves of the same polarity is same and the initial arrival time difference Δt between two consecutive traveling waves of opposite polarity diff Calculated according to formula (13);

[0050]

[0051] Where, T s is the sampling step length;

[0052] Preferably, the total number of sampling points N in the sampling window is 500, and the number of the top layer of the Gaussian pyramid is N. g The value is 2, the sampling step is T s The value is 1e-6.

[0053] Compared with the prior art, the present invention has the following advantages:

[0054] (1) Since fault location can be achieved based on the fault voltage data at the collecting bus, it is only necessary to install a traveling wave acquisition device on one side of the collecting line, so the fault location method of the present invention has lower communication and measurement costs;

[0055] (2) Since a linear filtering operation based on mathematical morphology is performed before extracting the traveling wave information, Gaussian noise and impulse noise that have a great influence on the identification of signal mutation points can be effectively filtered out, so the fault location method of the present invention has a strong anti-interference ability;

[0056] (3) Since the multi-scale gradient algorithm retains the rich details of the sequence mutation points on a small scale while taking into account the advantages of strong anti-noise interference ability on a large scale, the fault location method of the present invention has a high positioning accuracy;

[0057] In summary, the present invention can quickly determine the specific fault location of the collecting line and accelerate the fault recovery speed, which is of great significance to ensuring the safe and stable operation of the offshore wind power system. BRIEF DESCRIPTION OF THE DRAWINGS

[0058] Figure 1 The present invention is a topological diagram of an offshore wind power DC collection and transmission system suitable for the method of the present invention.

[0059] Figure 2 is a flow chart of the method of the present invention.

[0060] FIG3( a ) is a line positive voltage signal under a collection line fault.

[0061] FIG3( b ) is a line negative pole voltage signal under a collection line fault.

[0062] FIG3( c ) is a line positive current signal under a collection line fault.

[0063] FIG3( d ) is a negative pole current signal of a line under a collection line fault.

[0064] Figure 4 It is a reverse traveling wave waveform of the line 1 mode voltage under a collection line fault.

[0065] Figure 5 It is the filtered 1-mode voltage reverse traveling wave signal.

[0066] Figure 6 It is the result of multi-scale gradient calculation. DETAILED DESCRIPTION

[0067] The present invention is further described in detail below with reference to the accompanying drawings and embodiments.

[0068] like Figure 1 As shown, at 0 ms, a positive grounding fault (recorded as fault F) occurred at the positive pole of the collecting line A at a distance of 7.5 km from the collecting busbar, and the fault resistance was set to 0.1 Ω.

[0069] When fault F occurs, the line voltage and current waveforms measured by the sampling element are shown in Figure 3(a), Figure 3(b), Figure 3(c) and Figure 3(d).

[0070] A method for locating faults in offshore wind power DC collection lines based on traveling wave multi-scale gradient analysis, the process is as follows: Figure 2 As shown, the following steps are included:

[0071] Step 1: After the fault occurs in the offshore wind power DC collection network, the protection element starts and determines that the faulty collection line is line A and the fault type is a single-pole fault.

[0072] Step 2: Sampling the positive voltage u of the circuit p (k), negative electrode voltage u n (k), positive electrode current i p (k), negative electrode current i n (k), as shown in Figure 3(a), Figure 3(b), Figure 3(c) and Figure 3(d), respectively.

[0073] Calculate the 1-mode voltage u1(k) and 1-mode current i1(k) according to equation (1).

[0074] According to formula (2), calculate the reverse traveling wave u of mode 1 voltage b1 (k), the result is Figure 4 shown.

[0075] Step 3: Filter the 1-mode voltage reverse wave based on the mathematical morphology algorithm, and calculate the filtered signal according to formula (5). The result is as follows: Figure 5 as shown.

[0076] Step 4: Calculate the multi-scale gradient of the filtered voltage signal according to formula (11), and the result is shown in Figure (6).

[0077] According to the multi-scale gradient calculation results, the transient mutation point of the voltage waveform is identified, and the arrival time and wave head polarity information of the fault voltage traveling wave are extracted. The arrival sampling number corresponding to the initial traveling wave is 63, and the arrival sampling number of the second traveling wave with the same polarity is 185.

[0078] Step 5: Calculate the initial arrival time difference of two consecutive traveling waves of the same polarity as 122 μs according to formula (13);

[0079] The traveling wave velocity v is 122.89 km / ms. The fault distance x is calculated according to formula (12), and the result is 7.496 km.

[0080] It can be seen from the data of step five that the protection method of the present invention can reliably and quickly identify the fault location with high positioning accuracy.

Claims

1. A method for locating faults in offshore wind power DC collection lines based on traveling wave multi-scale gradient analysis, characterized in that: The steps include: Step 1: The protection element detects that a fault has occurred; After a fault occurs in the offshore wind power DC collection network, the protection element is activated to identify the faulty collection line and the fault type; Step 2: Sample the voltage and current of the positive and negative poles on the bus side of the fault collection line, and calculate the 1-mode voltage reverse wave; Calculate the 1-mode voltage u1(k) and 1-mode current i1(k) according to formula (1); In the formula, k is the sampling number, N is the total number of samples in the sampling window, and u p (k) is the positive electrode voltage, u n (k) is the negative electrode voltage, i p (k) is the positive electrode current, i n (k) is the negative electrode current; According to formula (2), calculate the 1-mode voltage reverse wave u b1 (k); at b1 (k)=[u1(k)-i1(k)·Z C ] / 2 (2) In the formula, Z C is the wave impedance; Step 3: Filter the 1-mode voltage reverse traveling wave based on the mathematical morphology algorithm and calculate the filtered signal; Select a flat structural element g(x), and let its domain be D(g) = {0, 1, ..., P}; According to formula (3), the expansion operation and corrosion operation are performed on the 1-mode voltage reverse traveling wave; (u b1 ! g)(k)=min{f(k+x)-g(k)|(k+x)∈D(u b1 ),k∈D(g)}where, the 1-mode voltage reverse wave u b1 The domain of (k) is D(u b1 )={0,1,…,N}, represents the expansion operation, ! represents the erosion operation; According to formula (4), open and close operations are performed; In the formula, represents an opening operation, and represents a closing operation; According to formula (5), the filtered signal Filter(u b1 (k)); In the formula, OC and CO represent the open-close operation and the close-open operation respectively, and the results are calculated according to formula (6); Step 4: Calculate the multi-scale gradient of the filtered signal, identify the transient mutation point of the waveform, and extract the arrival time and wave head polarity information of the fault voltage traveling wave; Based on the Laplace pyramid algorithm, the filtered signal is divided into multiple resolution scales, denoted by Filter(u b1 (k)) is the bottom signal G0 of the Gaussian pyramid. The lth layer signal G of the Gaussian pyramid decomposition is calculated according to formula (7): l ; Where N g is the number of the top layer of the Gaussian pyramid, m is the independent variable of the window function, and its value is m=-2,-1,0,1,2, is a window function, According to formula (8), the l-th layer extension sequence is calculated In the formula, According to formula (9), the l-th layer signal LP of Laplace pyramid decomposition is calculated l and the top Laplacian pyramid signal In the formula, is the l+1th layer extension sequence, is the top signal of the Gaussian pyramid; According to formula (10), the gradient SG of each layer of the Laplace pyramid sequence of the 1-mode voltage inverse traveling wave is calculated: l ; SG l (k)=(LP l *ker)(k) (10) Wherein, the convolution kernel of the Sobel gradient operator ker = [-1 0 1]; Calculate the multi-scale gradient MSG(k) of the 1-mode voltage reverse traveling wave according to formula (11); In the formula, Expand represents the upsampling operation; According to the multi-scale gradient calculation results, the transient mutation point of the voltage waveform is identified, and the arrival time sampling sequence number of the measured initial traveling wave is extracted, which is recorded as k r0 ; Extract the arrival time sampling number of the second traveling wave. If the polarity of the second traveling wave head is the same as that of the initial traveling wave, it is recorded as k r1 If the polarity of the second traveling wave is opposite to that of the initial traveling wave, it is recorded as k t1 ; Step 5: Calculate the fault distance; Calculate the fault distance x according to formula (12); or Where v is the speed of the traveling wave, L is the total length of the fault collection line, and the initial arrival time difference Δt between two consecutive traveling waves of the same polarity is same and the initial arrival time difference Δt between two consecutive traveling waves of opposite polarity diff Calculated according to formula (13); Where, T s is the sampling step length.

2. The method for locating faults in offshore wind power DC collection lines based on traveling wave multi-scale gradient analysis according to claim 1 is characterized in that: The total number of sampling points N in the sampling window is 500, and the number of the top layer of the Gaussian pyramid is N g The value is 2, the sampling step is T s The value is 1e-6.

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