A Distance Protection Method for Flexible DC Transmission Lines Based on Standard Inverse Traveling Wave Area

Abstract

The present invention discloses a distance protection method for a flexible DC transmission line based on the standard reverse traveling wave area. S1. Sample the voltages and currents of the positive and negative poles of the DC transmission line, calculate the voltages and currents of the mode 1 and mode 0, and calculate the reverse traveling wave of the mode 1 voltage. S2. Identify the parameters of the reverse traveling wave of the mode 1 voltage based on the AMLM algorithm. S3. Calculate the standard reverse traveling wave of the mode 1 voltage based on the parameters identified by the AMLM algorithm. S4. Calculate the area of the standard reverse traveling wave of the mode 1 voltage. S5. Determine the fault area. If the criterion is satisfied, it is an in-zone fault and the protection operates; otherwise, return to S1. The method of the present invention can quickly detect whether a fault occurs in the DC line, which has important practical significance for the rapid isolation of faults and ensuring the safe and stable operation of the power system.

Description

Technical Field

[0001] The present invention belongs to the field of power systems, relates to the technical field of relay protection for DC transmission lines, and particularly relates to a distance protection method for a flexible DC transmission line based on the area of a standard reverse traveling wave. Background Art

[0002] Facing the increasingly severe energy crisis and climate change, countries around the world have put forward development plans to transform from fossil energy to renewable energy, and have clarified that renewable energy power generation is the future development direction. Traditional high-voltage DC transmission shoulders the important task of transmitting electric energy between energy production areas and load centers in the power system, and has obvious advantages in applications such as long-distance and large-capacity electric energy transmission and asynchronous grid interconnection. With the development of power electronic devices and control technologies, flexible DC transmission technology has been realized, breaking through the inherent bottlenecks of traditional DC transmission technologies such as commutation failure and reactive power compensation, and is applicable to scenarios such as clean energy grid connection, offshore platform power supply, urban asynchronous grid interconnection, and island power supply.

[0003] However, a flexible DC transmission system is a "low-inertia" system. After a fault, the current rises rapidly and has a large amplitude, leaving a short response time for the protection device. If the fault is not removed in time, it will quickly affect the entire system. Therefore, fast and reliable line protection is the key to ensuring its safe and stable operation. The DC grid should have the ability to quickly identify and locate faults, and then achieve fault isolation.

[0004] For a flexible DC grid, the existing research results on fault discrimination can be summarized into four categories, which are respectively based on the frequency-domain characteristics of line fault electrical quantities, the time-domain characteristics of line fault electrical quantities, artificial intelligence algorithms, and the characteristics of line boundaries (wave impedance discontinuity points). However, for DC transmission scenarios without obvious concentrated capacitive and inductive components, the numerical value of the boundary element decreases; for multi-terminal DC transmission scenarios, the position of the boundary element changes, moving from both ends of the line to the bus connecting multiple lines. For protection principles based on frequency-domain characteristics and time-domain characteristics, they strongly rely on the high-frequency filtering or attenuation characteristics of the line boundary, and generally have poor adaptability to scenarios lacking high-frequency boundaries. At the same time, in high-resistance fault and lightning interference scenarios, the ability of the protection principle to identify and locate faults will be weakened, seriously affecting its reliability. Therefore, it is urgent to propose a protection principle with strong tolerance to transition resistance and lightning interference and applicable to flexible DC transmission scenarios lacking high-frequency boundaries. Summary of the Invention

[0005] To solve the problems existing in the above-mentioned prior art, the object of the present invention is to provide a flexible DC transmission line distance protection method based on the standard reverse traveling wave area. The method of the present invention has strong tolerance to transition resistance, and the protection principle does not depend on the DC line boundary composed of centralized components (such as smoothing reactors, filter capacitors, etc.), and is applicable to the flexible DC transmission line protection scenario under the condition of missing high-frequency boundaries.

[0006] To achieve the above object, the technical solution adopted by the present invention is as follows:

[0007] A flexible DC transmission line distance protection method based on the standard reverse traveling wave area, comprising the following steps:

[0008] Step 1: Sample the voltages and currents of the positive and negative poles of the flexible DC transmission line, calculate the voltages and currents of the 1-mode and 0-mode, and calculate the reverse traveling wave of the 1-mode voltage.

[0009] Calculate the 1-mode voltage u1(t), 0-mode voltage u0(t), 1-mode current i1(t), and 0-mode current i0(t) according to Equation (1),

[0010]

[0011] where t is the sampling time, Q is the transformation matrix, u p (t) is the positive pole voltage, u n (t) is the negative pole voltage, i p (t) is the positive pole current, i n (t) is the negative pole current;

[0012] According to Equation (2), deduce the expression of the reverse traveling wave u b1 (t) of the 1-mode voltage from the definition formulas of the 1-mode voltage u1(t) and 1-mode current i1(t), as shown in Equation (3),

[0013]

[0014] where u f1 (t) is the forward traveling wave of the 1-mode voltage, u b1 (t) is the reverse traveling wave of the 1-mode voltage, i f1 (t) is the forward traveling wave of the 1-mode current, i b1 (t) is the reverse traveling wave of the 1-mode current, Z C is the wave impedance;

[0015] u b1 (t) = [u1(t) - i1(t)·Z C / 2 (3)

[0016] According to Equation (3), calculate the reverse traveling wave u b1(t);

[0017] Step 2: Identify the parameters of the 1-mode voltage backward traveling wave based on the Adaptive Multi-step Levenberg-Marquart algorithm (AMLM algorithm).

[0018] The target to identify the 1-mode voltage backward traveling wave u b1 (t) is expressed as shown in Equation (4);

[0019]

[0020] where A1, A2, and A3 are the amplitude coefficients of the 1-mode voltage backward traveling wave, and T1 and T2 are the time constants;

[0021] According to Equation (5), define the fitting error Error(t) of the AMLM algorithm and determine the fitting target min||Error(t)|| 2 ;

[0022]

[0023] Perform AMLM algorithm iteration according to the iteration format shown in Equation (6);

[0024]

[0025] where k is the iteration number, all appearing in subscript form, G k is the approximate Jacobian matrix, Δ k is the iteration step size, ε k is the LM factor, and I is the identity matrix;

[0026] Update the sampling time t according to Equation (7);

[0027]

[0028] where ratio k is the error ratio of the k-th iteration, and p0 is a preset parameter;

[0029] Calculate the error ratio ratio according to Equation (8) k ;

[0030]

[0031] where Act k is the actual error, and Pre k is the predicted error;

[0032] Update the approximate Jacobian matrix G according to Equation (9);

[0033]

[0034] In the formula, J k is the accurate Jacobian matrix, n is the number of iterations using the same approximate Jacobian matrix, and n max is the maximum number of iterations that can use the same approximate Jacobian matrix, and p1 is a preset parameter;

[0035] Update the LM factor ε according to Equation (10);

[0036]

[0037] In the formula, η is the error coefficient, and δ is the preset number of error times;

[0038] Update the error coefficient η according to Equation (11);

[0039]

[0040] In the formula, both c1 and c2 are preset parameters, and η min is the preset minimum error coefficient, and both p2 and p3 are preset parameters;

[0041] Step 3: Calculate the standard mode 1 voltage backward traveling wave based on the parameters identified by the AMLM algorithm;

[0042] Define the calculation formula for the standard mode 1 voltage backward traveling wave u b1,s (t), as shown in Equation (12),

[0043]

[0044] Step 4: Calculate the area of the standard mode 1 voltage backward traveling wave;

[0045] Define the calculation formula for the area S of the standard mode 1 voltage backward traveling wave, as shown in Equation (13),

[0046]

[0047] In the formula, T is the fixed time window, and u b1,s (t) is the standard mode 1 voltage backward traveling wave;

[0048] Step 5: Discriminate the fault area. If the criterion is met, it is an in-zone fault and the protection operates; otherwise, return to Step 1;

[0049] The fault area criterion is as shown in Equation (14),

[0050] S > S set (14)

[0051] In the formula, S set is the threshold, and its setting principle is as shown in Equation (15),

[0052] S set = rel·S L (15)

[0053] where rel is the reliability coefficient for threshold setting, and S L is the area of the standard mode-1 voltage backward traveling wave at the end of the protected line.

[0054] In the AMLM algorithm, the preset parameter c1 is set to 4, c2 is set to 0.25, p0 is set to 0.0001, p1 is set to 0.5, p2 is set to 0.25, p3 is set to 0.75, the initial value of the error coefficient η1 is set to 10 -5 , and the preset minimum error coefficient η min is set to 10 -8 , the maximum number of iterations n using the same approximate Jacobian matrix max is set to 10; the fixed time window T is set to 0.5 ms, and the reliability coefficient rel for threshold setting is set to 1.02.

[0055] Compared with the prior art, the present invention has the following advantages:

[0056] Since the standard mode-1 voltage backward traveling wave is defined as the ratio of the mode-1 voltage backward traveling wave to its amplitude coefficient, the influence of the transition resistance is eliminated, and the protection principle of the present invention has a strong ability to withstand the transition resistance; since the calculation of the area of the standard mode-1 voltage backward traveling wave is only based on the propagation characteristics of the backward traveling wave on the DC line, the protection principle of the present invention can eliminate the dependence on the boundary characteristics of the DC line. In summary, the present invention has important practical significance for the rapid isolation of faults and ensuring the safe and stable operation of the power system. Description of the Drawings

[0057] Figure 1 is a topology diagram of a DC system applicable to the method of the present invention.

[0058] Figure 2 is a flowchart of the method of the present invention.

[0059] Figure 3(a) is the voltage waveform of the positive pole of the line under a DC line fault.

[0060] Figure 3(b) is the voltage waveform of the negative pole of the line under a DC line fault.

[0061] Figure 3(c) is the current waveform of the positive pole of the line under a DC line fault.

[0062] Figure 3(d) is the current waveform of the negative pole of the line under a DC line fault.

[0063] Figure 4 is the waveform of the mode-1 voltage backward traveling wave of the line under a DC line fault.

[0064] Figure 5 It is the waveform of the standard mode 1 voltage backward traveling wave under a DC line fault.

[0065] Figure 6 It is the protection operation signal discriminated based on the method of the present invention. Detailed implementation manners

[0066] The present invention will be further described in detail below with reference to the drawings and embodiments.

[0067] As Figure 1 shown, at 0.3 ms, a positive pole metallic grounding fault (denoted as fault F) occurs at a distance of 100 km from converter station M on the outgoing line MN at the positive pole.

[0068] After the occurrence of fault F, the voltage and current waveforms measured by the sampling element are as shown in FIGS. 3(a), 3(b), 3(c), and 3(d), and the protection element operation signal discriminated based on the method of the present invention is as Figure 6 shown.

[0069] A flexible DC transmission line distance protection method based on the standard backward traveling wave area, the process of which is as Figure 2 shown, includes the following steps:

[0070] Step 1: Sample the positive pole voltage u p (t), the negative pole voltage u n (t), the positive pole current i p (t), and the negative pole current i n (t), and the results are respectively shown in FIGS. 3(a), 3(b), 3(c), and 3(d).

[0071] Calculate the mode 1 voltage u1(t), mode 0 voltage u0(t), mode 1 current i1(t), and mode 0 current i0(t) according to Equation (1).

[0072] According to Equation (3), calculate the mode 1 voltage backward traveling wave u b1 (t) from the mode 1 voltage u1(t) and mode 1 current i1(t), and the result is as Figure 4 shown.

[0073] Step 2: Identify the mode 1 backward traveling wave amplitude coefficient A1 according to the AMLM algorithm, and the result is -0.515 p.u.

[0074] Step 3: Calculate the standard mode 1 voltage backward traveling wave u b1,s (t) according to Equation (12), and the result is as Figure 5 shown.

[0075] Step 4: Calculate the standard mode 1 voltage backward traveling wave area S according to Equation (13), and the result is 9.12 p.u.

[0076] Step 5: Set the threshold S according to Equation (15) set , where the reliability coefficient rel is taken as 1.02, and the area S of the standard mode 1 voltage backward traveling wave at the end of the protected line L is 8.54 p.u., and S set is set to 8.71 p.u.

[0077] Judge the fault area according to Equation (14). If the criterion meets the internal fault, the protection trips and the process ends. It can be seen from Figure 6 that: the protection method of the present invention can reliably identify faults and issue correct action signals, and the protection action time is short.

Claims

1. A distance protection method for flexible DC transmission lines based on the standard reverse traveling wave area, characterized in that: It includes the following steps: Step 1: Sample the voltages and currents of the positive and negative poles of the flexible DC transmission line, calculate the voltages and currents of the 1-mode and 0-mode, and calculate the backward traveling wave of the 1-mode voltage; Calculate the 1-mode voltage u1(t), 0-mode voltage u0(t), 1-mode current i1(t), and 0-mode current i0(t) according to Equation (1). where t is the sampling time, Q is the transformation matrix, u p (t) is the positive electrode voltage, u n (t) is the negative electrode voltage, i p (t) is the positive electrode current, i n (t) is the negative electrode current; According to Equation (2), the expression of the 1-mode voltage backward wave u b1 (t) is derived from the defining equations of the 1-mode voltage u1(t) and the 1-mode current i1(t), as shown in Equation (3). where, u f1 (t) is the forward traveling wave of the mode-1 voltage, u b1 (t) is the backward traveling wave of the mode-1 voltage, i f1 (t) is the forward traveling wave of the mode-1 current, i b1 (t) is the backward traveling wave of the mode-1 current, Z C is the wave impedance; u b1 (t) = [u1(t) - i1(t)·Z C / 2 (3) According to Equation (3), the reverse traveling wave u of the first-mode voltage is calculated from the first-mode voltage u1(t) and the first-mode current i1(t). b1 (t); Step 2: Identify the parameters of the backward traveling wave of the 1-mode voltage based on the adaptive multi-step Levenberg–Marquardt AMLM algorithm; Target identification 1-mode voltage backward traveling wave u b1 (t) is expressed as shown in Equation (4); In the formula, A1, A2, and A3 are the amplitude coefficients of the backward traveling wave of the 1-mode voltage, and T1 and T2 are the time constants; According to Equation (5), define the fitting error Error(t) of the AMLM algorithm and determine the fitting objective min||Error(t)|| 2 ; Perform AMLM algorithm iteration according to the iteration format shown in Equation (6); where k is the number of iterations, all appearing in subscript form, G k is the approximate Jacobian matrix, Δ k is the iteration step size, ε k is the LM factor, and I is the identity matrix; Update the sampling time t according to Equation (7); where ratio k is the error ratio of the k-th iteration, and p0 is a preset parameter; Calculate the error ratio ratio according to Equation (8). k ; where Act k is the actual error, and Pre k is the predicted error; Update the approximate Jacobian matrix G according to Equation (9); where J k is the accurate Jacobian matrix, n is the number of iterations using the same approximate Jacobian matrix, and n max is the maximum number of iterations that can use the same approximate Jacobian matrix, and p1 is a preset parameter; Update the LM factor ε according to Equation (10); In the formula, η is the error coefficient and δ is the preset number of error times; Update the error coefficient η according to Equation (11); wherein, c1 and c2 are both preset parameters, and η min is a preset minimum error coefficient, and p2 and p3 are both preset parameters; Step 3: Calculate the standard backward traveling wave of the 1-mode voltage based on the parameters identified by the AMLM algorithm; Define the calculation formula of the standard 1-mode voltage backward traveling wave u b1,s (t), as shown in Equation (12). Step 4: Calculate the area of the standard backward traveling wave of the 1-mode voltage; Define the calculation formula for the area S of the standard backward traveling wave of the 1-mode voltage as shown in Equation (13). In the formula, T is the fixed time window; Step 5: Discriminate the fault area. If the criterion is satisfied, it is an in-zone fault and the protection operates. Otherwise, return to Step 1; The fault area criterion is shown in Equation (14). S > S set (14) where S set is the threshold value, and its setting principle is shown in Equation (15). S set = rel·S L (15) where rel is the reliability coefficient for threshold setting, and S L is the area of the standard mode 1 voltage backward traveling wave at the end of the protected line.

2. The distance protection method for a flexible DC transmission line based on the standard reverse traveling wave area according to claim 1, characterized in that: In the AMLM algorithm, the preset parameter c1 is set to 4, c2 is set to 0.25, p0 is set to 0.0001, p1 is set to 0.5, p2 is set to 0.25, p3 is set to 0.75, and the initial value of the error coefficient η1 is set to 10 -5 , and the preset minimum error coefficient η min is set to 10 -8 , and the maximum number of iterations n max using the same approximate Jacobian matrix is set to 10.

3. The distance protection method for a flexible DC transmission line based on the standard reverse traveling wave area according to claim 1, characterized in that: The value of the fixed time window T is 0.5 ms, and the reliability coefficient rel for threshold setting is 1.02.

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