Method for lightning stroke recognition of flexible HVDC transmission line

Abstract

The application discloses a lightning stroke identification method for a flexible direct current transmission line, and specifically comprises the following steps: step 1, calculating line mode voltage forward wave u f (k); step 2, detecting a first valley point k G after protection starting, if no valley point is detected within n-2 sampling points, k G is n; step 3, detecting a half valley point, if k G is n in step 2, k L is n, if no k L is collected within n points after protection starting, k L is n; step 4, performing quadratic fitting on line mode voltage forward wave data between the 0th sampling point k0 and the half valley point k L , calculating a quadratic term coefficient a of a fitting function, and judging whether the line is subjected to a lightning stroke. The application can quickly and accurately identify whether the line is subjected to a lightning stroke after protection starting of the flexible direct current transmission line, and provides an important basis for correct action of a protection device of the flexible direct current transmission line.

Description

Technical Field

[0001] This invention belongs to the field of power system technology and relates to a method for identifying lightning strikes on flexible DC transmission lines. Background Technology

[0002] Against the backdrop of the "dual carbon" target, my country's power grid is undergoing profound changes, with the large-scale application of ultra-high voltage (UHV) transmission technology, the integration of a high proportion of renewable energy, and the widespread use of power electronic equipment. Compared with traditional DC transmission technology, flexible DC transmission technology does not suffer from commutation failure or reactive power compensation issues, and can provide active and reactive power to the AC system. It has significant advantages in renewable energy grid integration, enabling large-scale, multi-form renewable energy aggregation and complementarity, improving energy utilization efficiency, and enhancing power supply reliability.

[0003] Flexible DC power grids are "low-inertia systems." Once a fault occurs, the fault current amplitude increases rapidly and quickly spreads throughout the entire DC grid. Therefore, line protection systems need to complete fault detection within 3ms. Traveling wave protection, due to its rapid response advantage, is widely used in DC transmission projects currently in operation.

[0004] Due to the poor technical and economic efficiency of DC cables, most long-distance flexible DC transmission projects use overhead lines. Overhead lines increase the probability of DC line faults, with lightning strikes accounting for the vast majority of these faults. The high-frequency components in lightning currents often cause malfunctions in traveling wave protection systems. In the context of the rapid development of new power systems, lightning strike identification is of great significance for the safe and stable operation of the power grid.

[0005] Lightning strikes on power transmission systems are categorized into backflashover and side-flashover, and many studies have neglected backflashover in their identification criteria. Some research employs time-frequency analysis to extract high-frequency components of lightning current to distinguish whether a line has been struck by lightning; however, time-frequency analysis lacks sufficient physical theoretical support, its algorithms are complex, and it cannot guarantee the reliability of protection actions under the constraint of a 3ms short time window. This paper conducts an in-depth analysis of the transient traveling wave characteristics of flexible DC transmission systems under different types of lightning strikes, and proposes a lightning strike identification method that balances speed and reliability. This method accurately determines whether the system has been struck by lightning and is of significant value for the development of relay protection for new power systems. Summary of the Invention

[0006] The purpose of this invention is to provide a method for identifying lightning strikes on flexible DC transmission lines. This method can quickly and accurately determine whether a transmission line has been struck by lightning, providing an important basis for the correct operation of the protection device for flexible DC transmission lines.

[0007] The technical solution adopted in this invention is a method for identifying lightning strikes on flexible DC transmission lines, which specifically includes the following steps:

[0008] Step 1: After the protection starting element starts, record the fault starting point k0 as the 0th sampling point, and take the voltage and current data of the next n sampling points to calculate the positive voltage fault component Δu. p (k) Negative electrode voltage fault component Δu n (k), Positive current fault component Δi p (k) and the negative electrode current fault component Δi n (k), and then calculate the line-mode voltage fault component Δu1(k) and the line-mode current fault component Δi1(k) from this, and finally calculate the line-mode voltage traveling wave u. f (k);

[0009] Step 2: Calculate the traveling wave u of the line-mode voltage after protection activation. f (k) The first valley point k G ;

[0010] Step 3, calculate the traveling wave u of the line-mode voltage. f (k) The amplitude first exceeds h after the trough point. G The half-valley point k of / 2 L ;

[0011] Step 4, for k0 to k L The data between the two points is fitted twice, the coefficient of the quadratic term of the fitted function is calculated and denoted as a, the threshold value th1 of the lightning strike identification criterion is set and compared with a, and the transmission line is judged to have been struck by lightning based on the comparison result.

[0012] The invention is further characterized by:

[0013] The specific process of step 1 is as follows:

[0014] Step 1.1: After the protection starting element is activated, read the positive voltage u at the protection installation location. p (k) Negative electrode voltage u n (k), positive current i p (k), negative electrode current i n (k), respectively compared with the average positive voltage u in the 10ms before protection activation. p0 Average negative electrode voltage u n0 Average positive current i p0 Average negative current i n0 Subtraction, positive voltage fault component Δu p (k) Negative electrode voltage fault component Δu n (k), Positive current fault component Δi p (k) and the negative electrode current fault component Δi n (k), the calculation formula is shown in equation (1):

[0015]

[0016] Step 1.2: Calculate the line-mode voltage fault component Δu1(k) and the line-mode current fault component Δi1(k) within the data window. The calculation formula is shown in equation (2):

[0017]

[0018] Step 1.3: Calculate the traveling wave u of the line-mode voltage within the data window. f (k), the calculation formula is shown in equation (3):

[0019]

[0020] In the formula, Z c This is the line wave impedance.

[0021] The specific process of step 2 is as follows:

[0022] The forward wave u of the line-mode voltage after protection activation is calculated according to the following formula (4). f (k) is the first valley point that appears and is denoted as k. G ,k G ∈[1,n-2], and record the amplitude at the valley point as h. G If no valley value is collected within n-2 points after the protection is started, then k G The value is n;

[0023]

[0024] The specific process of step 3 is as follows:

[0025] The traveling wave of the line-mode voltage u f (k) The amplitude first exceeds h after the trough point. G The sampling point of / 2 is denoted as the half-valley point k. L If k in step 2 G Let n be the number of elements, then k L Let n be the number of points. If k is not collected within n points after the protection is activated... L Then k L Let n be the value of n.

[0026] In step 4, if the set threshold value th1 and a satisfy equation (5), then it is determined that the transmission line has been struck by lightning; otherwise, it is determined that the line has not been struck by lightning.

[0027] a>th1 (5).

[0028] In step 4, the least squares method is used for the second-order fitting, from the starting point k0 to the half-valley point k. L Total k L The formula for calculating the coefficients of the quadratic term of the fitted function is as follows: +1 points are used for quadratic fitting.

[0029]

[0030]

[0031]

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[0034]

[0035]

[0036]

[0037] In the formula, x i For sampling point k i i = 0, 1, 2, ..., k L ;y i The traveling wave amplitude u of the line-mode voltage corresponding to each sampling point f (k i ).

[0038] The beneficial effects of this invention are that the proposed method for identifying lightning strikes on flexible DC transmission lines identifies whether a transmission line has been struck by lightning based on the traveling wave waveform characteristics of a single-end measured line modulus. This method can accurately determine whether the protection device is activated due to a lightning strike or other reasons, and can distinguish between different types of lightning strikes (such as lightning strikes on towers, lightning strikes on transmission lines, induced lightning, etc.) and ordinary short-circuit faults (including faults within and outside the protection zone with different transition resistances and different fault types). The identification method provided by this invention requires only a 0.3ms data window for discrimination when the number of sampling points n is 30, meeting the requirements of flexible DC transmission systems for rapid protection. Attached Figure Description

[0039] Figure 1 This is a flowchart of the lightning strike identification method for flexible DC transmission lines proposed in this invention;

[0040] Figure 2 This is a simulation model diagram of a true bipolar MMC flexible DC transmission system at both ends;

[0041] Figure 3 This is the simulation verification stage, using the lightning strike identification method for flexible DC transmission lines proposed in this invention to determine the lightning current backflash interference at a distance of 150km.

[0042] Figure 4This is the simulation verification stage, using the lightning strike identification method for flexible DC transmission lines proposed in this invention to determine the positive short-circuit grounding fault at a distance of 350km.

[0043] Figure 5 This is the simulation verification stage, where the lightning strike identification method for flexible DC transmission lines proposed in this invention is used to determine the positive short-circuit grounding fault outside the zone.

[0044] Figure 6 This is the simulation verification stage, using the lightning strike identification method for flexible DC transmission lines proposed in this invention to determine the lightning current backflash fault at a distance of 50km. Detailed Implementation

[0045] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments.

[0046] This invention relates to a method for identifying lightning strikes on flexible DC transmission lines. By calculating the traveling wave of the line-mode voltage after a fault occurs on the flexible DC transmission line, and based on the peak characteristics of the lightning current waveform, it can distinguish between ordinary short-circuit faults and lightning strikes. The process is as follows: Figure 1 As shown, please follow these steps:

[0047] Step 1: After the protection starting element starts, record the fault starting point k0 as the 0th sampling point, and take the voltage and current data of the next n sampling points to calculate the positive voltage fault component Δu. p (k) Negative electrode voltage fault component Δu n (k), Positive current fault component Δi p (k) and the negative electrode current fault component Δi n (k), and then calculate the line-mode voltage fault component Δu1(k) and the line-mode current fault component Δi1(k) from this, and finally calculate the line-mode voltage traveling wave u. f (k); where step 1 includes the following steps:

[0048] Step 1.1 After the protection starting element is activated, read the positive voltage u at the protection installation location. p (k) Negative electrode voltage u n (k), positive current i p (k), negative electrode current i n (k), respectively compared with the average positive voltage u in the 10ms before protection activation. p0 Average negative electrode voltage u n0 Average positive current i p0 Average negative current i n0 Subtraction, positive voltage fault component Δu p (k) Negative electrode voltage fault component Δu n (k), Positive current fault component Δi p(k) and the negative electrode current fault component Δi n (k), the calculation formula is shown in equation (1):

[0049]

[0050] Step 1.2: Calculate the line-mode voltage fault component Δu1(k) and the line-mode current fault component Δi1(k) within the data window. The calculation formula is shown in equation (2):

[0051]

[0052] Step 1.3: Calculate the traveling wave u of the line-mode voltage within the data window. f (k), the calculation formula is shown in equation (3):

[0053]

[0054] In the formula Z c This is the line wave impedance.

[0055] Step 2: Calculate the forward traveling wave u of the line-mode voltage after protection activation according to equation (4). f (k) is the first valley point that appears and is denoted as k. G ,k G ∈[1,n-2], and record the amplitude at the valley point as h. G If no valley value is collected within n-2 points after the protection is started, then k G The value is n;

[0056]

[0057] Step 3, calculate the traveling wave u of the line-mode voltage. f (k) The amplitude first exceeds h after the trough point. G The sampling point is denoted as / 2, and this point is called the half-valley point k. L If k in step 2 G Let n be the number of elements, then k L Let n be the value of k. If k is not collected within n points after the protection is activated... L Then k L Let n be the value of n;

[0058] Step 4, for k0 to k L The data between the two points is subjected to a second fitting, the coefficient of the quadratic term of the fitting function is calculated and denoted as a, the threshold value th1 of the lightning strike identification criterion is set and compared with a, if equation (5) is satisfied:

[0059] a>th1 (5);

[0060] If the signal is positive, the transmission line is determined to have been struck by lightning; otherwise, the line is determined not to have been struck by lightning.

[0061] In step 4, the second-order fitting uses the least squares method, applying the data from the starting point k0 (the 0th sampling point) to the half-valley point k. L Total k L The formula for calculating the coefficients of the quadratic term of the fitted function is as follows: +1 points are used for quadratic fitting.

[0062]

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[0065]

[0066]

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[0068]

[0069]

[0070] In the formula: x i For sampling point k i i = 0, 1, 2, ..., k L ;y i The traveling wave amplitude u of the line-mode voltage corresponding to each sampling point f (k i );

[0071] The method for tuning the threshold value th1 in step 4 is as follows:

[0072] Simulations of different types of lightning strikes and ordinary short circuits were conducted at distances of 10%, 50%, and 90% from the beginning of the line. The quadratic coefficients of the traveling wave of the line-mode voltage were calculated based on the simulation data. The calculation results are shown in Table 1.

[0073] As shown in Table 1, when the transmission line is not struck by lightning, the quadratic coefficient of the fitted quadratic term of the line-mode voltage traveling wave within the data window is mostly less than 1, with the maximum value obtained when an inter-electrode short circuit occurs at 450km, and the fitted quadratic coefficient is 1.36. When the transmission line is struck by lightning, the quadratic coefficient of the fitted quadratic term of the line-mode voltage traveling wave within the data window is mostly greater than 10, with the minimum value obtained when a backflash fault occurs at 250km, and the fitted quadratic coefficient is 6.68. The average of the maximum fitted quadratic coefficient when the line is struck by lightning and the minimum fitted quadratic coefficient when it is not struck by lightning is taken, and the threshold value th1 is finally adjusted to 4.

[0074] Table 1. Fitting results of linear mode voltage traveling waves under different types of disturbances.

[0075]

[0076]

[0077] like Figure 2 The figure shown is a simulation model diagram of a two-terminal true bipolar MMC flexible DC transmission system. The system has a rated DC voltage of ±500kV, a rated operating current of 2.5kA, a rated capacity of 2500MW, a total transmission line length of 500km, and adopts an overhead line frequency-varying parameter model with a sampling frequency of 100kHz. Figure 1 The flowchart shown illustrates lightning strike identification.

[0078] Example 1

[0079] like Figure 3 As shown, after a lightning strike interference occurred 150km from the protection installation location, the protection element activated. Voltage and current data were read from 30 sampling points after the activation point k0, and the line-mode voltage traveling wave was calculated. A valley point was detected at the second sampling point, and a half-valley point k was detected at the seventh sampling point. L The traveling wave data between sampling points 0 and 7 were fitted twice, and the coefficient of the quadratic term a was calculated. The result was 34.21, which is greater than the setting threshold value of 4, so it was determined that the line had been struck by lightning.

[0080] Example 2

[0081] like Figure 4 As shown, after a positive ground fault occurs 350km from the protection installation location with a transition resistance of 100Ω, the protection element activates, reading voltage and current data from 30 sampling points after the activation point k0, and calculating the line-mode voltage traveling wave. A valley point is detected at the 4th sampling point, and no half-valley point is detected within the 30th sampling point. Therefore, the half-valley point k is set as... L The result is 30. The forward wave data between sampling points 0 and 30 are fitted twice, and the coefficient of the quadratic term a is calculated. The result is 0.78, which is less than the setting threshold value of 4. Therefore, it is determined that the line has not been struck by lightning.

[0082] Example 3

[0083] like Figure 5 As shown, after a positive ground fault occurs outside the zone and the transition resistance is 100Ω, the protection element activates, reading voltage and current data from 30 sampling points after the activation point k0, and calculating the line-mode voltage traveling wave. If no valley point is detected within the 30th sampling point, then the half-valley point k is set as... LThe value is 30. The forward wave data between sampling points 0 and 30 are fitted twice, and the coefficient of the quadratic term a is calculated. The result is 0.048, which is less than the setting threshold value of 4. Therefore, it is determined that the line has not been struck by lightning.

[0084] Example 4

[0085] like Figure 6 As shown, after a lightning backflash fault occurs 50km from the protection installation location, the protection element activates, reading voltage and current data from 30 sampling points after the activation point k0, and calculating the line-mode voltage traveling wave. A valley point is detected at the first sampling point, and a half-valley point k is detected within the fifth sampling point. L The traveling wave data between sampling points 0 and 5 were fitted twice, and the coefficient of the quadratic term 'a' was calculated. The result was 52.69, which is greater than the setting threshold value of 4, indicating that the line had been struck by lightning.

[0086] To comprehensively verify the impact of disturbance distance and transition resistance on the discrimination results, different types of lightning strikes and different types of ordinary short-circuit faults (both inside and outside the zone) were simulated at distances of 50km, 150km, 250km, 350km, and 450km. For the ordinary short-circuit faults, transition resistances of 0Ω, 100Ω, and 500Ω were respectively set, with a lightning current parameter of 2.6 / 50μs. The proposed lightning strike identification method was verified based on simulation results. The verification results are shown in Table 2.

[0087] Table 2. Fitting results of linear mode voltage traveling waves under different types of disturbances.

[0088]

[0089]

[0090]

[0091] It can be seen that the lightning strike identification method proposed in this invention can accurately and quickly identify common short-circuit faults inside and outside different zones and different types of lightning strikes, and the identification results are not affected by transition resistance, fault distance, etc.

Claims

1. A method for identifying lightning strikes on flexible DC transmission lines, characterized in that, Specifically, the steps include the following: Step 1: After the protection starting element is activated, record the fault starting point. For the 0th sampling point, take the points after it. n Calculate the positive voltage fault component from the voltage and current data of each sampling point. Negative voltage fault component Positive current fault component With negative current fault component Then, the line-mode voltage fault component is calculated from this. Linear current fault component Finally, the traveling wave of the line-mode voltage was calculated based on this. ; Step 2: Calculate the forward traveling wave of the line-mode voltage after protection activation. The first valley point The specific process of step 2 is as follows: The forward wave of the line-mode voltage after protection activation is calculated according to the following formula (4). The first valley point that appears is denoted as , And record the amplitude at the valley point as . If after protection is started n If no valley value points are collected within -2 points, then Values n ; （1）； Step 3, Calculate the traveling wave of the line-mode voltage. The amplitude exceeded the trough point for the first time. Half valley point The specific process of step 3 is as follows: traveling wave of line-mode voltage The amplitude exceeded the trough point for the first time. The sampling point is denoted as the half valley point. If in step 2 Take as n ,but Take as n If no data is collected within n points after the protection is activated ,but Let n be the value of n; Step 4, for arrive Perform a quadratic fit on the data between two points, calculate the coefficients of the quadratic term of the fitted function, and denot them as follows. a Set the threshold value for lightning strike identification criteria. and a The comparison is used to determine whether the transmission line has been struck by lightning.

2. The method for identifying lightning strikes on flexible DC transmission lines according to claim 1, characterized in that, The specific process of step 1 is as follows: Step 1.1: After the protection starting element is activated, read the positive voltage at the protection installation location. Negative voltage Positive current Negative current The values ​​were compared with the average positive voltage 10 ms before protection activation. Average negative electrode voltage Average positive current Average negative current Subtraction, positive voltage fault component Negative voltage fault component Positive current fault component With negative current fault component The calculation formula is shown in equation (2): （2）； Step 1.2: Calculate the line-mode voltage fault component within the data window. Linear current fault component The calculation formula is shown in equation (3): （3）； Step 1.3: Calculate the traveling wave of the line-mode voltage within the data window. The calculation formula is shown in equation (4): （4）； In the formula, This is the line wave impedance.

3. The method for identifying lightning strikes on flexible DC transmission lines according to claim 2, characterized in that, In step 4, if the threshold value is set... and a If equation (5) is satisfied, the transmission line is determined to have been struck by lightning; otherwise, the line is determined not to have been struck by lightning. （5）。 4. The method for identifying lightning strikes on flexible DC transmission lines according to claim 3, characterized in that, In step 4, the least squares method is used for the second-order fitting, and the starting point is... to the half valley value point common The formula for calculating the coefficients of the quadratic term of the fitted function is as follows: +1 points are used for quadratic fitting. （6）； （7）； （8）； （9）； （10）； （11）； （12）； （13）； In the formula, Sampling points , =0, 1, 2, ... ; The traveling wave amplitude of the line-mode voltage corresponding to each sampling point. .

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