Flexible DC power transmission line lightning stroke interference identification method based on voltage traveling wave distribution characteristics

Abstract

The invention discloses a flexible DC power transmission line lightning stroke interference identification method based on voltage traveling wave distribution characteristics, and the method specifically comprises the following steps: 1, solving a line mode voltage fault component and a line mode current fault component, and carrying out the calculation according to the components to obtain a line mode voltage reverse traveling wave; step 2, carrying out minimum-maximum normalization on a line mode voltage reverse traveling wave from the starting moment to 1.5 ms after starting; 3, according to the waveform distribution condition of the line mode voltage reverse traveling wave time domain curve obtained in the step 2, the distribution number Q1 of the sampling points with the line mode voltage values located at (0.1, 1) and the distribution number Q2 of the sampling points with the line mode voltage values located at [0, 0.1] are extracted, and the ratio lambda between the Q1 and Q2 is calculated; and 4, judging whether lightning interference occurs or not according to the sampling distribution point number ratio lambda. By adopting the method provided by the invention, the lightning interference can be accurately identified.

Description

Technical Field

[0001] The invention belongs to the technical field of electric power systems and relates to a method for identifying lightning disturbance of flexible direct current transmission lines based on voltage traveling wave distribution characteristics. Background Art

[0002] Flexible DC transmission technology has important applications in power systems, but it faces great safety challenges when it is disturbed by lightning. Lightning interference not only causes instantaneous overvoltage, but may also cause false operation of the protection system, leading to more serious failures or power outages. Existing lightning interference identification methods are mostly based on current traveling waves, waveform analysis and other means, but they have significant defects in accuracy and real-time performance, especially under complex working conditions, which can easily lead to misjudgment or omission of lightning interference, and thus fail to effectively avoid the occurrence of protection false operation.

[0003] Although the traditional current traveling wave method can capture the transient signal caused by lightning strikes, it is difficult to accurately distinguish lightning strikes from other current fluctuation events due to their similar characteristics, which can easily lead to false operations. The waveform analysis method has difficulty in dealing with multiple lightning strikes or high-noise environments when faced with complex interference, and its signal resolution ability is poor, especially under low signal-to-noise ratio conditions, the recognition effect is significantly reduced. Although the time-frequency domain processing method can obtain relatively rich signal spectrum information, it has high computational complexity and is difficult to apply in real time in actual engineering. Summary of the invention

[0004] The purpose of the present invention is to provide a method for identifying lightning interference of flexible DC transmission lines based on the voltage traveling wave distribution characteristics. The method utilizes the fact that when a ground fault or a lightning fault occurs, the line mode voltage reverse traveling wave will not drop to near 0 subsequently. When lightning interference occurs, the line mode voltage reverse traveling wave will subsequently attenuate to 0. Even if subsequent wave head superposition occurs, there is still an obvious attenuation characteristic, thereby realizing accurate identification of lightning interference.

[0005] The technical solution adopted by the present invention is a method for identifying lightning interference of flexible direct current transmission lines based on the distribution characteristics of voltage traveling waves, which specifically includes the following steps:

[0006] Step 1: Calculate the line mode voltage fault component Δu1(k) and the line mode current fault component Δi1(k), and finally calculate the line mode voltage reverse wave u based on them. b (k);

[0007] Step 2, taking the line mode voltage reverse wave from the start time to 1.5ms after the start time for minimum-maximum normalization;

[0008] Step 3, according to the waveform distribution of the line mode voltage reverse wave time domain curve obtained in step 2, extract the number of sampling points Q1 where the line mode voltage value is located at (0.1, 1) and the number of sampling points Q2 where the line mode voltage value is located at [0, 0.1], and calculate the ratio λ between the two;

[0009] Step 4: Determine whether lightning interference occurs according to the sampling distribution point ratio λ calculated in step 3.

[0010] The present invention is also characterized in that:

[0011] The specific process of step 1 is:

[0012] Step 1.1: After the protection start element is started, the time range from the start time to 1.5ms after the start is set as the protection time window. Within this window, read the positive voltage u at the installation location of the protection device. p (k), negative electrode voltage u n (k), positive electrode current i p (k), negative electrode current i n (k) These data are compared with the average positive voltage u within 10ms before the protection is activated. p0 , negative electrode voltage average u n0 , average positive current i p0 , mean negative electrode current i n0 Subtract to calculate the positive voltage fault component Δu p (k), negative pole voltage fault component Δu n (k), positive current fault component Δi p (k) and the negative current fault component Δi n (k), the calculation formula is shown in formula (1):

[0013]

[0014] Step 1.2, calculate the line mode voltage fault component Δu1(k) and the line mode current fault component Δi1(k) within the time window. The calculation formula is shown in formula (2):

[0015]

[0016] Step 1.3, calculate the line mode voltage reverse wave u in the time window b (k), the calculation formula is shown in formula (3):

[0017]

[0018] In the formula, Z c1 is the line mode wave impedance of the line.

[0019] The specific process of step 2 is:

[0020] Step 2.1: for the line mode voltage reverse traveling wave data u in the time window b (k) By comparing with formula (4), when the sampled voltage amplitude is greater than 2 times the system rated voltage, the voltage value of the sampling point is recorded as 2 times the system rated voltage, as shown in the following formula (4):

[0021] u b (k)≥2U N (4)

[0022] Where: k is the time sequence number, 0≤k≤N-1; N is the number of sampling points from the time the faulty component starts to 1.5ms after starting; U N is the system rated voltage;

[0023] Step 2.2: According to the following formula (5), the line mode voltage 1.5ms after the fault element is started is reversed into u b (k) Normalized voltage u bMN (k):

[0024]

[0025] In the formula: min(u b (k)) is the minimum value in the time window; max(u b (k)) is the maximum value within the time window.

[0026] The specific process of step 4 is:

[0027] If λ>λ set , it is judged that a fault has occurred; if λ<λ set When , it is judged as lightning interference, λ set is the threshold value.

[0028] The beneficial effect of the present invention is that the present invention identifies lightning interference based on the distribution characteristics of the line mode voltage reverse wave waveform. The method normalizes the line mode voltage reverse wave, reduces the influence of the transition resistance on the identification method, and extracts the distribution characteristics in a way that enables the method to identify quickly and has strong anti-noise interference capabilities. It can still identify lightning interference well under the conditions of 0Ω and 500Ω transition resistance and 10dB noise interference. At the same time, due to its lightning identification principle constructed by reverse waves, it can be used in both bounded flexible DC power grids and unbounded flexible DC power grids. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 It is a flow chart of a method for identifying lightning interference of flexible direct current transmission lines based on voltage traveling wave distribution characteristics of the present invention;

[0030] Figure 2It is a simulation model diagram of a two-terminal true bipolar MMC flexible DC transmission system;

[0031] FIG3(a) is a simulation verification stage, in which the method for identifying lightning disturbances in flexible DC transmission lines based on voltage traveling wave distribution characteristics of the present invention is used to normalize the line mode voltage reverse traveling wave of a ground fault in a bipolar flexible DC system at 250km and a transition resistance of 500Ω, as well as the waveform distribution diagram;

[0032] Figure 3(b) shows the number of distribution points of the normalized waveform of the line-mode voltage reverse traveling wave in each amplitude range for the ground fault in the area at 250km of the bipolar flexible DC system and the transition resistance of 500Ω;

[0033] Figure 4 This is a simulation model diagram of a four-terminal MMC flexible DC transmission network;

[0034] FIG5(a) is a simulation verification stage, in which the line mode voltage reverse traveling wave for lightning interference shielding failure discrimination in the 100km area of ​​the four-terminal flexible DC power grid is normalized and the waveform distribution diagram is shown using the flexible DC transmission line lightning interference identification method based on voltage traveling wave distribution characteristics of the present invention;

[0035] Figure 5(b) shows the number of distribution points of the line mode voltage reverse traveling wave normalized waveform in each amplitude range for lightning interference shielding failure judgment in the four-terminal flexible DC power grid at 100km;

[0036] Figure 6 This is a simulation model diagram of a four-terminal "mesh type" MMC flexible DC transmission network;

[0037] FIG7(a) is a diagram of the simulation verification stage, in which the line mode voltage reverse traveling wave of the four-terminal "mesh type" flexible DC transmission line lightning interference identification method based on the voltage traveling wave distribution characteristics of the present invention is used to normalize the line mode voltage reverse traveling wave of the lightning fault shielding failure identification in the 100km area of ​​the four-terminal "mesh type" flexible DC system and the waveform distribution diagram;

[0038] Figure 7(b) shows the distribution points of the normalized waveform of the line-mode voltage reverse traveling wave in each amplitude range for lightning fault shielding judgment in the four-terminal "mesh-type" flexible DC system at 100km. DETAILED DESCRIPTION

[0039] The present invention is described in detail below with reference to the accompanying drawings and specific embodiments.

[0040] The present invention provides a method for identifying lightning interference of flexible direct current transmission lines based on voltage traveling wave distribution characteristics, which specifically comprises the following steps:

[0041] Step 1: After the protection start element is started, the protection time window is set from the start time to 1.5ms after the start. In this time window, read the voltage and current data. Calculate the positive voltage fault component Δu p(k), positive current fault component Δi p (k), negative pole voltage fault component Δu n (k) and the negative current fault component Δi n (k), and then calculate the line mode voltage fault component Δu1(k) and line mode current fault component Δi1(k) based on these components. Finally, the line mode voltage reverse wave u is calculated based on this. b (k);

[0042] The specific process of step 1 is:

[0043] Step 1.1, after the protection start element is started, the time range from the start time to 1.5ms after the start is set as the protection time window. In this window, read the positive voltage u at the installation point of the protection device p (k), negative electrode voltage u n (k), positive electrode current i p (k), negative electrode current i n (k) These data are compared with the average positive voltage u within 10ms before the protection is activated. p0 , negative electrode voltage average u n0 , average positive current i p0 , mean negative current i n0 Subtract to calculate the positive voltage fault component Δu p (k), negative pole voltage fault component Δu n (k), positive current fault component Δi p (k) and the negative current fault component Δi n (k), the calculation formula is shown in formula (1):

[0044]

[0045] Step 1.2, calculate the line mode voltage fault component Δu1(k) and the line mode current fault component Δi1(k) within the time window. The calculation formula is shown in formula (2):

[0046]

[0047] Step 1.3, calculate the line mode voltage reverse wave u in the time window b (k), the calculation formula is shown in formula (3):

[0048]

[0049] In the formula, Z c1 is the line mode wave impedance of the line.

[0050] Step 2: Take the line mode voltage reverse traveling wave from the start-up time to 1.5 ms after the start-up and perform minimum-maximum normalization to obtain the corresponding normalized line mode voltage reverse traveling wave time domain curve.

[0051] Step 2.1: for the line mode voltage reverse traveling wave data u in the time window b (k) By comparing with formula (4), when the sampled voltage amplitude is greater than 2 times the system rated voltage, the voltage value at the sampling point is recorded as 2 times the system rated voltage.

[0052] u b (k)≥2U N (4)

[0053] Where: k is the time sequence number, 0≤k≤N-1; N is the number of sampling points from the time the faulty component starts to 1.5ms after starting; U N is the rated voltage of the system.

[0054] Step 2.2: According to the following formula (5), the line mode voltage 1.5ms after the fault element is started is reversed into u b (k) Normalized voltage u bMN (k).

[0055]

[0056] Where: k is the time sequence number, 0≤k≤N-1; N is the number of sampling points from the start-up time of the faulty component to 1.5ms after the start-up; min(u b (k)) is the minimum value in the time window; max(u b (k)) is the maximum value within the time window.

[0057] Step 3, determine the waveform distribution of the line mode voltage reverse wave time domain curve obtained in step 2; extract the number of sampling points Q1 with line mode voltage values ​​located at (0.1, 1) and the number of sampling points Q2 located at [0, 0.1), and calculate the ratio λ between the two.

[0058] Step 3.1, the normalized voltage u obtained in step 2 bMN (k) Draw the time domain distribution diagram of the line mode voltage reverse traveling wave: when u bMN (k)>0.1, then the line mode voltage of the sampling point is located at (0.1,1], and the number of sampling points located at (0.1,1] in the time window is recorded as Q1. bMN (k)≤0.1, the line-mode voltage of the sampling points is distributed in [0,0.1], and the number of sampling points located in [0,0.1] in the time window is recorded as Q2.

[0059] Step 3.2, calculate the ratio λ between Q1 and Q2 according to the following formula (6).

[0060]

[0061] Step 4: If λ>λ set , it is judged that a fault has occurred; if λ<λ set When , it is judged as lightning interference.

[0062] The specific process of step 4 is:

[0063] Step 4.1, if λ>λ set , it is judged that a fault has occurred; if λ<λ set When , it is judged as lightning interference. The obtained ratio λ and the threshold value λ set By contrast, as shown in formula (5), the relationship between λ and the threshold value is used as the criterion for lightning disturbance identification.

[0064]

[0065] Step 4.2: Select the threshold value λ according to the difference in voltage reverse wave waveforms when a fault occurs and when a lightning strike occurs, as well as the simulation test results. set =5.

[0066] As attached Figure 2 As shown in the figure, it is a simulation model diagram of a double-terminal flexible DC transmission system. In the figure, the current limiting reactor L = 0.2H, MMC1 and MMC2 are modular converters, P1 and P2 are line boundary protection measurement points, f1 is the disturbance location, the rated voltage of this system is ±500kV, the rated transmission capacity is 3000MVA, the total length of the transmission line is 500km, and the overhead line frequency variable parameter model is adopted. The line wave impedance is Z c0 =320Ω、Z c1 =260Ω. When simulating the system, the sampling frequency is 100kHz, and the 1.2 / 50μs standard lightning current model is used to simulate lightning strikes. The full text takes the line Line1, P1 measuring point as an example, and the fault distance is the distance from the fault point to the measuring point P1. Figure 1 The identification process is used to identify lightning interference.

[0067] Example 1

[0068] As shown in Figure 3(a), Figure 2In the double-terminal flexible DC transmission system shown in the figure, when a positive grounding fault with a transition resistance of 500Ω occurs 250km away from the protection measuring point P1, the line mode voltage reverse wave simulation value measured at the protection measuring point. After the protection element is started, the line mode voltage reverse wave data of 150 sampling points after the start-up are read and normalized. The λ value is calculated, and it is determined to be a fault, and the protection is activated. As shown in Figure 3(b), the point distribution of the normalized line mode voltage reverse wave in each amplitude range proves the feasibility of the protection scheme.

[0069] As attached Figure 4 As shown in the figure, it is a simulation model diagram of a four-terminal MMC flexible DC transmission network. In the figure, the current limiting reactor L = 0.2H, MMC1, MMC2, MMC3, MMC4 are modular converters, P1 and P2 are line boundary protection measurement points, f1 is the disturbance location, the rated voltage between MMC1 and MMC2 is ±500kV, the rated transmission capacity is 3000MVA, the total length of the transmission line is 207.9km, and the overhead line frequency variable parameter model is adopted. The line wave impedance is Z c0 =310Ω、Z c1 =250Ω. When simulating the system, the sampling frequency is 100kHz, and the 1.2 / 50μs standard lightning current model is used to simulate lightning strikes. The structure of the four-terminal flexible DC power grid is highly symmetrical. The full text takes Line 1, P1 measuring point as an example. The fault distance is the distance from the fault point to the measuring point P1. Figure 1 The identification process is used to identify lightning interference.

[0070] Example 2

[0071] As shown in Figure 5(a), Figure 4 In the four-terminal MMC flexible DC transmission network shown in the figure, when a lightning strike interference occurs 100 km away from the protection installation, the line mode voltage reverse wave simulation value measured at the protection measurement point. After the protection element is started, the line mode voltage reverse wave data of 150 sampling points after the start-up are read and normalized. The λ value is calculated, and it is determined to be a lightning interference, and the protection is returned. As shown in Figure 5(b), the point distribution of the normalized line mode voltage reverse wave in each amplitude range proves the feasibility of the protection scheme.

[0072] As attached Figure 6 As shown in the figure, it is a simulation model diagram of a four-terminal "mesh type" MMC flexible DC transmission network. In the figure, the current limiting reactor L = 0.2H, MMC1, MMC2, MMC3, MMC4 are modular converters, P1 and P2 are line boundary protection measurement points, f1 is the disturbance location, the rated voltage between MMC1 and MMC2 is ±500kV, the rated transmission capacity is 3000MVA, the total length of the transmission line is 207.9km, and the overhead line frequency variable parameter model is adopted. The line wave impedance is Z c0 =310Ω、Zc1 =250Ω. When simulating the system, the sampling frequency is 100kHz, and the 1.2 / 50μs standard lightning current model is used to simulate lightning strikes. The four-terminal "mesh-type" flexible DC power grid structure is highly symmetrical. Taking the line Line1, P1 measuring point as an example, the fault distance is the distance from the fault point to the measuring point P1. According to the attached Figure 1 The identification process is used to identify lightning faults.

[0073] Example 3

[0074] As shown in Figure 7(a), Figure 6 In the four-terminal "mesh-type" MMC flexible DC transmission network shown in the figure, when a lightning shielding failure occurs 100 km away from the protection installation, the line mode voltage reverse wave simulation value measured at the protection measurement point. After the protection element is started, the line mode voltage reverse wave data of 150 sampling points after the start-up are read and normalized. The λ value is calculated, it is determined to be a fault, and the protection is activated. As shown in Figure 5(b), the point distribution of the normalized line mode voltage reverse wave in each amplitude range proves the feasibility of the protection scheme.

[0075] In order to comprehensively verify the influence of fault distance, lightning type, transition resistance and system type on the identification results, lightning faults, lightning interference and grounding faults with transition resistance of 0Ω and 500Ω are set for the two-terminal flexible DC system, four-terminal flexible DC grid and four-terminal "mesh type" flexible DC grid respectively, and the influence of 10dB noise is considered. The lightning interference identification method is verified according to the simulation results.

[0076] From Table 1, we can get the simulation data of different fault distances on the bipolar flexible DC transmission line, where all ground faults are metallic ground faults, and both lightning faults and lightning interference are shielding faults. From the data in the time window, we can get that when lightning interference occurs, λ is less than 5; when lightning faults and ground faults occur, λ is greater than 5. Lightning interference can be identified at different fault distances.

[0077] Table 1 Simulation verification results of different fault distances

[0078]

[0079]

[0080] From Table 2, we can get the simulation data of different transition resistance grounding faults on the bipolar flexible DC transmission line. From the data in the time window, we can get that when a grounding fault occurs, λ is greater than 5. The fault can be accurately identified under different transition resistance grounding faults.

[0081] Table 2 Simulation verification results of different transition resistances

[0082]

[0083] From Table 3, we can get the simulation data of the bipolar flexible DC transmission line under the conditions of shielding failure and counterattack. From the data in the time window, we can get that when lightning interference occurs, λ is less than 5; when lightning fault and ground fault occur, λ is greater than 5. Whether lightning shielding failure or lightning counterattack occurs, lightning interference can be accurately identified.

[0084] Table 3 Simulation verification results of different lightning strike types

[0085]

[0086]

[0087] From Table 4, we can get the simulation data of the bipolar flexible DC transmission line fault under 10dB noise interference. From the data in the time window, we can get: when lightning interference occurs, λ is less than 5; when lightning fault and ground fault occur, λ is greater than 5. The protection criterion can identify lightning interference under 10dB noise interference.

[0088] Table 4 Simulation verification results under the influence of 10dB noise interference

[0089]

[0090] From Table 5, we can get the simulation data of faults in three different flexible DC grids. From the data in the time window, we can get that when lightning interference occurs, λ is less than 5; when lightning fault and ground fault occur, λ is greater than 5. The protection criterion can identify lightning interference in different flexible DC grids.

[0091] Table 5 Simulation verification results of faults in three different flexible DC power grids

[0092]

[0093]

[0094] The present invention normalizes the line mode voltage reverse traveling wave, and greatly reduces the possibility of misjudgment and missed judgment by extracting the waveform distribution characteristics of the line mode voltage reverse traveling wave. At the same time, the voltage traveling wave analysis method has the advantages of low computational complexity and fast response speed, which can meet the requirements of flexible direct current transmission system for real-time and high precision, and avoid protection misoperation. The purpose of the present invention is to provide a lightning interference identification method based on the distribution characteristics of voltage traveling waves. This method utilizes that when a ground fault or a lightning fault occurs, the line mode voltage reverse traveling wave will not fall to near 0 subsequently. When a lightning interference occurs, the line mode voltage reverse traveling wave will decay to 0 subsequently, and even if subsequent wave head superposition occurs, there is still an obvious attenuation characteristic. In this way, the line mode voltage reverse traveling wave is normalized, the influence of the transition resistance on the identification method is reduced, and the way of extracting the distribution characteristics makes the method recognize quickly and has a strong anti-noise interference ability. At the same time, due to its lightning identification principle constructed by reverse traveling waves, it can be used in both bounded flexible direct current grids and unbounded flexible direct current grids.

Claims

1. A method for identifying lightning disturbance of flexible DC transmission lines based on voltage traveling wave distribution characteristics, characterized by: The specific steps include: Step 1: Calculate the line mode voltage fault component Δu1(k) and the line mode current fault component Δi1(k), and finally calculate the line mode voltage reverse wave u based on them. b (k); Step 2, taking the line mode voltage reverse wave from the start time to 1.5ms after the start time for minimum-maximum normalization; Step 3, according to the waveform distribution of the line mode voltage reverse wave time domain curve obtained in step 2, extract the number of sampling points Q1 where the line mode voltage value is located at (0.1, 1) and the number of sampling points Q2 where the line mode voltage value is located at [0, 0.1], and calculate the ratio λ between the two; Step 4: Determine whether lightning interference occurs according to the sampling distribution point ratio λ calculated in step 3.

2. The method for identifying lightning disturbance of flexible direct current transmission lines based on voltage traveling wave distribution characteristics according to claim 1 is characterized in that: The specific process of step 1 is as follows: Step 1.1: After the protection start element is started, the time range from the start time to 1.5ms after the start is set as the protection time window. Within this window, read the positive voltage u at the installation location of the protection device. p (k), negative electrode voltage u n (k), positive electrode current i p (k), negative electrode current i n (k) These data are compared with the average positive voltage u within 10ms before the protection is activated. p0 , negative electrode voltage average u n0 , average positive current i p0 , mean negative electrode current i n0 Subtract to calculate the positive voltage fault component Δu p (k), negative pole voltage fault component Δu n (k), positive current fault component Δi p (k) and the negative current fault component Δi n (k), the calculation formula is shown in formula (1): Step 1.2, calculate the line mode voltage fault component Δu1(k) and the line mode current fault component Δi1(k) within the time window. The calculation formula is shown in formula (2): Step 1.3, calculate the line mode voltage reverse wave u in the time window b (k), the calculation formula is shown in formula (3): In the formula, Z c1 is the line mode wave impedance of the line.

3. The method for identifying lightning disturbance of flexible direct current transmission lines based on voltage traveling wave distribution characteristics according to claim 2 is characterized in that: The specific process of step 2 is: Step 2.1: for the line mode voltage reverse traveling wave data u in the time window b (k) By comparing with formula (4), when the sampled voltage amplitude is greater than 2 times the system rated voltage, the voltage value of the sampling point is recorded as 2 times the system rated voltage, as shown in the following formula (4): in b (k)≥2U N (4) Where: k is the time sequence number, 0≤k≤N-1; N is the number of sampling points from the time the faulty component starts to 1.5ms after starting; U N is the system rated voltage; Step 2.2: According to the following formula (5), the line mode voltage 1.5ms after the fault element is started is reversed into u b (k) Normalized voltage u bMN (k): In the formula: min(u b (k)) is the minimum value in the time window; max(u b (k)) is the maximum value within the time window.

4. The method for identifying lightning disturbance of flexible direct current transmission lines based on voltage traveling wave distribution characteristics according to claim 3 is characterized in that: The specific process of step 3 is as follows: Step 3.1, the normalized voltage u obtained in step 2 bMN (k) Draw the time domain distribution diagram of the line mode voltage reverse traveling wave: when u bMN (k)>0.1, then the line mode voltage of the sampling point is located at (0.1,1], and the number of sampling points located at (0.1,1] in the time window is recorded as Q1. bMN (k)≤0.1, then the line mode voltage of the sampling points is distributed in [0,0.1], and the number of sampling points located in [0,0.1] in the time window is recorded as Q2; Step 3.2, calculate the ratio λ between Q1 and Q2 according to the following formula (6):

5. The method for identifying lightning disturbance of flexible direct current transmission lines based on voltage traveling wave distribution characteristics according to claim 4 is characterized in that: The specific process of step 4 is as follows: If λ>λ set , it is judged that a fault has occurred; if λ<λ set When , it is judged as lightning interference, λ set is the threshold value.

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