A fault location method for high voltage direct current transmission lines based on HHT unified iteration

Abstract

The present invention proposes a fault distance measurement method for high-voltage direct current transmission lines based on HHT unified iteration. Aiming at the problem that the frequency is too high and the wave head misjudgment point is easily generated in the traveling wave distance measurement method, a method based on HHT unified iteration is provided to eliminate the influence of the wave head misjudgment point; the three-terminal traveling wave method is used to measure the transmission line fault distance, which solves the problem that the wave speed is difficult to calculate in the traditional traveling wave method and improves the positioning accuracy; at the same time, the method proposed by the present invention is compared with the positioning method of wavelet transform, and it is found that the unified time-amplitude diagram formed by HHT unified iteration is used for wave head calibration, which has higher stability and smaller error, and the method proposed by the present invention is basically not affected by factors such as transition resistance and fault distance, and has higher positioning accuracy.

Description

Technical Field

[0001] The present invention relates to a method for fault location of a high-voltage direct current transmission line based on HHT unified iteration. Specifically, the present invention is a method of combining Clarke transform, Hilbert-Huang transform (HHT) and unified iteration and applying it to fault location of a high-voltage direct current transmission line, belonging to the technical field of transmission line fault location. Background Art

[0002] High-voltage direct current transmission lines are an indispensable part of the power system. However, the occurrence of transmission line faults will pose a huge threat to the safe operation of the power system. Therefore, rapid detection of the fault location is of great significance for shortening the power outage maintenance time and maintaining the stability of the power system. At present, the fault distance measurement methods include impedance method, natural frequency method, and traveling wave method. The impedance method is a common fault detection method and has been widely used in power systems. However, the solution process of this method is complicated and cannot eliminate the influence of pseudo roots. The natural frequency method is difficult to accurately locate line faults due to the problem of frequency confusion. The traveling wave method is a method that needs to capture the head of the fault traveling wave and realize the fault distance measurement by the time difference of the head arrival and the speed of the traveling wave. This method has been widely used in high-voltage transmission lines because it is almost unaffected by external factors and its own line structure.

[0003] In the field of traveling wave method research, the determination of wave velocity is one of the difficulties in its research, especially the wave velocity characteristics caused by the dispersion effect have not been well solved. If a certain empirical wave velocity is simply selected and the influence of frequency characteristics on wave velocity is ignored, it will cause a large error in the ranging result. In addition, the calibration of the traveling wave head is also very important. Common wave head calibration methods mainly include: wavelet transform (WT), mathematical morphology, and Hilbert Huang Transform (HHT). Wavelet transform requires the selection of decomposition scale and wavelet basis in advance, and it is impossible to calibrate the wave head more accurately; mathematical morphology is difficult to select due to the shape and size of its structure, and it is also difficult to obtain a more ideal result when analyzing fault signals; Hilbert Huang transform has problems such as over-envelope, negative frequency and when the frequency is too high, it is easy to produce wave head misjudgment points, which leads to large ranging errors. Summary of the invention

[0004] The purpose of the present invention is to provide a three-terminal traveling wave fault distance measurement method based on HHT unified iteration to solve the problems that the wave velocity is difficult to determine and the wave head misjudgment point is easy to be generated due to the high frequency in the traveling wave fault distance measurement method. While ensuring a high positioning accuracy, the influence of the wave velocity on the fault distance measurement result is eliminated, and the interference of the wave head misjudgment point is reduced.

[0005] In order to realize the above invention, the present invention provides a high voltage direct current transmission line fault location method based on HHT unified iteration.

[0006] The steps are as follows, and the flowchart is shown in Figure 1 :

[0007] 1. Use the high-voltage DC transmission model built by Matlab / Simulink to collect high-frequency current signals.

[0008] Connect the transmission line to the voltage source, ground fault module and other equipment modules. Use the voltage and current module to collect high-frequency current signals under power-on conditions, and set the transmission line fault type to single-pole ground fault in the ground fault module. Use an oscilloscope to continuously save the high-frequency current signal within four cycles as the analysis object.

[0009] 2. Clarke transform is used to decouple the collected high-frequency current signal to obtain two line-mode components and one zero-mode component of the high-frequency current, and one of the line-mode components of the high-frequency current is selected for analysis.

[0010] 3. Perform empirical mode decomposition (EMD) on the high-frequency current line mode components to obtain a series of IMF (inherent mode function) components from high frequency to low frequency.

[0011] Assume that the high-frequency current line mode component is i1(t). For any time series i1(t), the decomposition steps of EMD are as follows:

[0012] (1) Determine all extreme points of the high-frequency current line mode component i1(t)

[0013] (2) Use the spline function to find the upper and lower envelopes of i1(t), y1(t), y2(t), and let their mean be m1(t)

[0014]

[0015] (3) Subtract i1(t) from m1(t), that is

[0016] h1(t)=i1(t)-m1(t) (2)

[0017] Determine whether h1(t) satisfies the two conditions of the intrinsic mode function. If so, treat h1(t) as the first IMF and continue processing. If not, treat h1(t) as the new i1(t) and repeat the above steps (1) and (2) until the conditions are met.

[0018] (4) Let p1(t) = h1(t), then p1(t) is the first IMF component, and let i1(t) be the difference with p1(t), we get

[0019] r1(t)=i1(t)-p1(t) (3)

[0020] Repeat the above process n times to obtain the second IMF component p2(t), the third IMF component p3(t) and the nth IMF component p n (t), r n (t) = r n-1 (t)-p n (t). where r n (t) is the residual term

[0021] (5) Repeat the above steps until p n (t) or r n (t) Stop when the given termination condition is met. The signal obtained after decomposition is expressed as follows

[0022]

[0023] 4. Perform Hilbert transform and unified iteration on the IMF component to obtain the unified time-amplitude diagram and determine the time when the fault traveling wave reaches the three measurement ends.

[0024] (1) Select the first IMF component p1(t), f1(t) is the Hilbert transform of p1(t), p1(t), f1(t) are expressed as follows

[0025]

[0026]

[0027] Define the analytical signal as Z1(t):

[0028] Z1(t)=p1(t)+jf1(t)=a1(t)e jθ1(t) (7)

[0029] Where a1(t) is the instantaneous amplitude obtained after Hilbert transform.

[0030] (2) Perform a normalized iterative process on the instantaneous amplitude a1(t), determine the iteration order k, obtain a normalized time-amplitude diagram, and determine the time when the traveling wave reaches the three measurement ends.

[0031] Normalize a1(t) to get the first normalized component with an amplitude range of (0,1], denoted as u1(t). The normalized component after multiple k-order iterations is denoted as S1(k), and the iteration order is denoted as k:

[0032]

[0033] The component with amplitude (0,1) approaches 0 after k iterations, while the component with amplitude 1 maintains its original amplitude at 1 after k iterations. The corresponding time-amplitude diagram is drawn from S1(k) after multiple iterations, and the time when the traveling wave head arrives at the three measurement ends is determined by the unique mutation point in the diagram.

[0034] 5. Using the above-determined time for the wave head to arrive at the measuring end, combined with the three-terminal traveling wave ranging formula, the fault distance of the transmission line is measured.

[0035] High voltage direct current transmission system Figure 2 Shown

[0036] When a fault occurs between MPs, the time of fault occurrence is T F , assuming that the time when the fault traveling wave reaches the three measurement ends is T M 、T P 、T N . D MF , D FN They are the distance from the rectifier side M to the fault point F and the distance from the fault point F to the inverter side N. d is the distance between the rectifier side M and the measuring point P (d can be set arbitrarily), L is the total length of the line, and v is the wave speed.

[0037] The equation is established as follows:

[0038] D MF -D FN =v×[(T M -T F )-(T N -T F )] (9)

[0039] Ld=v×[(T N -T M )-(T p -T M )] (10)

[0040] At the same time, the total length between MN is L, so:

[0041] D MF +D FN =L (11)

[0042] From the above three formulas, the fault traveling wave distance calculation formula is derived as follows:

[0043]

[0044] When the fault occurs between PNs, the fault traveling wave distance calculation formula can be obtained by the same logic:

[0045]

[0046] Compared with the existing inventions, the advantages of the present invention are

[0047] The present invention solves the problem that the traveling wave frequency is too high and easily generates the wave head misjudgment point by using the HHT unified iteration method; uses the three-terminal traveling wave method to perform fault distance measurement, solves the problem that the wave velocity is difficult to calculate in the traditional traveling wave method, and improves the positioning accuracy; at the same time, the method proposed by the present invention is compared with the positioning method of wavelet transform, and it is found that the unified time-amplitude diagram formed by the HHT unified iteration is used for wave head calibration, which has higher stability and smaller error, and the method proposed by the present invention is basically not affected by factors such as transition resistance and fault distance, and has higher positioning accuracy. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] Figure 1 Fault location flowchart

[0049] Figure 2 High Voltage Direct Current Transmission System

[0050] Figure 3 HVDC Transmission Model

[0051] Figure 4 Unified time-width diagram of three measurement terminals DETAILED DESCRIPTION

[0052] The present invention is specifically described below through examples. It is necessary to point out that the examples are only used to further explain the present invention and cannot be understood as limiting the scope of the present invention. Those skilled in the art can make some non-essential improvements and adjustments based on the above contents of the present invention.

[0053] In conjunction with the examples, the specific steps of the method of the present invention are as follows:

[0054] 1. Use the high-voltage DC transmission model built by Matlab / Simulink to collect high-frequency current signals.

[0055] This example is built as Figure 3 The HVDC transmission model shown is used to simulate a single-pole grounding fault under high frequency conditions.

[0056] Simulation parameters: power supply voltage level is 230kV, simulation time is 0-0.2s, fixed length between MP is d=400km, total length of line is 1200km. Specific parameters of transmission line are shown in Table 1 below

[0057] Table 1

[0058] Parameter Type R / (Ω / km) L / (H / km) C / (F / km) Positive sequence parameter 0.07899 <![CDATA[0.4098×10 -3 ]]> <![CDATA[0.1791×10 -6 ]]> Zero sequence parameters 0.2234 <![CDATA[0.1237×10 -2 ]]> <![CDATA[0.186×10 -6 ]]>

[0059] The fault points are set at 100km, 200km, 300km, 600km, 800km, and 1000km from the rectifier side, and the transition resistances are 5Ω, 60Ω, 100Ω, 160Ω, and 200Ω, respectively. A ground fault occurs at 0.08s. (Sampling period 10 -5 s)

[0060] 2. Extract high-frequency current line mode components

[0061] The high-frequency current components of the three measuring ends 300 km away from the rectifier side M end, with a transition resistance of 200Ω and a fault type of single-pole grounding fault, are selected for Clarke transformation to extract the high-frequency current line mode components of the three measuring ends.

[0062] 3. Perform EMD decomposition on the high-frequency current line mode component to obtain the corresponding IMF component

[0063] The high-frequency current line mode components at the three measurement ends are decomposed by EMD using formula (1) to formula (4) to obtain a series of IMF components from high frequency to low frequency, and the first IMF component of the high-frequency current line mode components is extracted for analysis.

[0064] 4. Perform Hilbert transform and iterative transformation on the first IMF component to determine the time when the wave head reaches the rectifier side M, the measurement point P, and the inverter side N. Figure 4 shown.

[0065] The first IMF component extracted is subjected to Hilbert transform and iterative processing using formula (5) to (8), the iteration order k is determined, and a time-amplitude diagram is drawn. The time when the traveling wave head arrives at the three measurement ends is determined by the unique mutation point in the diagram.

[0066] 5. Implementation of fault distance measurement

[0067] Using the time when the wave head reaches the three measuring ends determined above, the fault distance is calculated by formula (12), as shown below:

[0068]

[0069] The relative error is 0.10% and the absolute error is 0.287km.

[0070] 6. Comparison of the positioning accuracy of the present invention and wavelet transform (WT)

[0071] Under different fault distance conditions, the positioning accuracy of the HHT iterative method proposed in the present invention is compared with that of the wavelet transform (WT). The former uses the unique mutation point of the first IMF component normalized time-amplitude diagram for determination, while the wavelet transform uses the first wavelet coefficient modulus maximum point of the first scale for judgment. The positioning results are shown in Table 2, where the wavelet transform uses the Haar wavelet basis and the decomposition scale is 4.

[0072] Table 2

[0073]

[0074] Comparing the positioning accuracy of the above two different fault location methods, the HHT unified iteration method proposed in the present invention is better than the wavelet transform.

[0075] 7. Comparison of the accuracy and stability of the method proposed in the present invention

[0076] In order to verify the adaptability and accuracy of the method proposed in the present invention, tests were carried out under different transition resistance and fault distance conditions. The results are shown in Tables 3 and 4.

[0077] Table 3

[0078]

[0079] Table 4

[0080]

[0081] It can be seen from Tables 3 and 4 that the method proposed in the present invention has strong adaptability, high positioning accuracy, and is almost unaffected by transition resistance and fault distance.

Claims

1. A method for fault location of a high-voltage direct current transmission line based on HHT unified iteration, the specific steps are as follows: Use the high-voltage DC transmission model built with Matlab / Simulink to collect high-frequency current signals; Connect the transmission line with the voltage source, the ground fault module and the voltage and current module; use the voltage and current module to collect high-frequency current signals under power-on conditions, and set the transmission line fault type as a single-pole ground fault in the ground fault module; and use an oscilloscope to continuously save the high-frequency current signals within four cycles as analysis objects; Clarke transform is used to decouple the collected high-frequency current signal to obtain two line-mode components and one zero-mode component of the high-frequency current, and one of the line-mode components of the high-frequency current is selected for analysis; Perform empirical mode decomposition (EMD) on the high-frequency current line mode components to obtain a series of IMF (inherent mode function) components from high frequency to low frequency: Assume that the high-frequency current line mode component is i1(t). For any time series i1(t), the decomposition steps of EMD are as follows: (1) Determine all extreme points of the high-frequency current line mode component i1(t); (2) Use the spline function to find the upper and lower envelopes of i1(t), y1(t), y2(t), and let their mean be m1(t); (3) Subtract i1(t) from m1(t), that is: h1(t)=i1(t)-m1(t) (2) Determine whether h1(t) satisfies the two conditions of the intrinsic mode function. If so, treat h1(t) as the first IMF and continue processing. If not, treat h1(t) as the new i1(t) and repeat the above steps (1) and (2) until the conditions are met. (4) Let p1(t) = h1(t), then p1(t) is the first IMF component, and let i1(t) be the difference with p1(t), we get: r1(t)=i1(t)-p1(t) (3) Repeat the above process n times to obtain the second IMF component p2(t), the third IMF component p3(t) and the nth IMF component p n (t), r n (t) = r n-1 (t)-p n (t), where r n (t) is the residual term; (5) Repeat the above steps until p n (t) or r n (t) stops when the given termination condition is met. The signal obtained after decomposition is expressed as follows: Perform Hilbert transform and normalization iteration on the IMF component to obtain the normalized time-amplitude diagram and determine the time when the fault traveling wave reaches the three measurement ends: (1) Select the first IMF component p1(t), f1(t) is the Hilbert transform of p1(t), p1(t), f1(t) are expressed as follows: Define the analytical signal as Z1(t): Z1(t)=p1(t)+jf1(t)=a1(t)e jθ1(t) Where a1(t) is the instantaneous amplitude obtained after Hilbert transform (2) Perform a normalized iterative process on the instantaneous amplitude a1(t), determine the iteration order k, obtain a normalized time-amplitude diagram, and determine the time when the traveling wave reaches the three measurement ends; Normalize a1(t) to get the first normalized component with an amplitude range of (0,1], denoted as u1(t). The normalized component after multiple k-order iterations is denoted as S1(k), and the iteration order is denoted as k. The component with amplitude (0,1) approaches 0 after k iterations, while the component with amplitude 1 keeps its original amplitude unchanged at 1 after k iterations. The corresponding time-amplitude diagram is drawn from S1(k) after multiple iterations, and the time when the traveling wave head reaches the three measurement ends is determined by the unique mutation point in the diagram. The above-determined time for the wave head to arrive at the measuring end is used in combination with the three-terminal traveling wave ranging formula to perform fault ranging of the transmission line.

2. A method for fault location of a high-voltage direct current transmission line based on HHT-based iteration according to claim 1, characterized in that: When a fault occurs between MPs in a high-voltage direct current transmission system, the time of the fault is T F , assuming that the time when the fault traveling wave reaches the three measurement ends is T M , T P , T N , D MF , D FN They are the distance from the rectifier side M to the fault point F and the distance from the fault point F to the inverter side N, d is the distance between the rectifier side M and the measuring point P, d can be set arbitrarily, L is the total length of the line, v is the wave speed, and the equation is established as follows: D MF -D FN =v×[(T M -T F )-(T N -T F )] (9) L-d=v×[(T N -T M )-(T p -T M )] (10) At the same time, the total length between MN is L, so; D MF +D FN =L(11) From the above three formulas, the fault traveling wave distance calculation formula is derived as follows: When the fault occurs between PN, the fault traveling wave distance calculation formula can be obtained by the same logic:

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