Distance protection method for flexible DC transmission lines based on the distortion coefficient of the reverse traveling wave waveform

Abstract

The present invention discloses a ranging protection method for a flexible DC transmission line based on the distortion coefficient of the reverse traveling wave waveform. S1. 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. S2. Calculate the logarithm of the absolute value of the first derivative of the reverse traveling wave of the 1-mode voltage. S3. Based on the least squares method, fit the linear relationship between the logarithm of the absolute value of the first derivative of the reverse traveling wave of the 1-mode voltage and the sampling time, and calculate the slope of the linear relationship. S4. Calculate the fault distance. S5. Discriminate the fault area. If the criterion is satisfied, it is an in-zone fault and the protection acts; 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 ranging protection method for flexible DC transmission lines based on the distortion coefficient of the reverse traveling wave waveform. Background Art

[0002] With the gradual depletion of fossil energy and the increasingly serious environmental problems, countries around the world have put forward development plans to transform from fossil energy to renewable energy. Traditional high-voltage DC transmission shoulders the heavy responsibility 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. 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.

[0004] For a flexible DC grid, the fault isolation and clearing scheme adopts "half-bridge MMC + DC circuit breaker". To reduce the influence range of the fault, the DC grid should have the ability to quickly identify and locate faults, and then achieve fault isolation. However, in the scenarios of high-resistance faults and lightning interference, the ability of the protection principle to identify and locate faults will be weakened, seriously affecting its reliability. There is an urgent need to propose a protection principle with strong tolerance to transition resistance and lightning interference. The existing research results 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, the existing principles generally have problems such as weak tolerance to transition resistance, the setting of protection thresholds depending on simulation data, high requirements for computing power, and high dependence on boundary components such as smoothing reactors. Summary of the Invention

[0005] To solve the problems existing in the above-mentioned prior art, the purpose of the present invention is to provide a ranging protection method for flexible DC transmission lines based on the distortion coefficient of the reverse traveling wave waveform. The method of the present invention has strong tolerance to transition resistance, the setting of protection thresholds does not depend on simulation data, and the protection principle does not depend on the DC line boundary composed of centralized components (such as smoothing reactors and filter capacitors).

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

[0007] A ranging protection method for flexible DC transmission lines based on the distortion coefficient of the reverse traveling wave waveform, 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 u according to Equation (1) (1) (t i ), 0-mode voltage u (0) (t i ), 1-mode current i (1) (t i ), 0-mode current i (0) (t i ),

[0010]

[0011] where t i is the i-th sampling moment, i is the sampling sequence number, N is the total number of samplings, Q is the transformation matrix, u p (t i ) is the positive pole voltage, u n (t i ) is the negative pole voltage, i p (t i ) is the positive pole current, i n (t i ) is the negative pole current;

[0012] According to Equation (2), deduce the expression of the reverse traveling wave u of the 1-mode voltage from the defining expressions of the 1-mode voltage u (1) (t i ) and the 1-mode current i (1) (t i ), as shown in Equation (3), b(1) (t i ),

[0013]

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

[0015] u b(1) (t i) = [u (1) (t i ) - i (1) (t i )·Z C / 2 (3)

[0016] According to Equation (3), the backward traveling wave of the first-mode voltage u (1) (t i ), the first-mode current i (1) (t i ) are used to calculate the backward traveling wave of the first-mode voltage u b(1) (t i );

[0017] Step 2: Calculate the logarithm of the absolute value of the first derivative of the backward traveling wave of the first-mode voltage;

[0018] According to Equation (4), the first derivative u' b(1) (t i ) of the backward traveling wave of the first-mode voltage is calculated from the backward traveling wave of the first-mode voltage u b(1) (t i );

[0019]

[0020] where T S is the sampling interval;

[0021] The logarithm of its absolute value ln(|u' b(1) (t i )|) is calculated from the first derivative u' b(1) (t i ) of the backward traveling wave of the first-mode voltage, and is denoted as Y(t i );

[0022] Step 3: Based on the least squares method, fit the linear relationship between the logarithm of the absolute value of the first derivative of the backward traveling wave of the first-mode voltage and the sampling time, and calculate the slope of the linear relationship;

[0023] The calculation formula for the slope K0 of the linear relationship is shown in Equation (5),

[0024]

[0025] where the mean value of the logarithm of the absolute value of the first derivative of the backward traveling wave of the first-mode voltage and the mean value

[0026]

[0027] of the sampling time can be calculated according to Equation (6);

[0028] According to Equation (7), calculate the reverse traveling wave waveform distortion coefficient τ from the slope K0.

[0029]

[0030] According to Equation (8), calculate the fault distance l from the reverse traveling wave waveform distortion coefficient τ.

[0031]

[0032] Where τ0 is the reverse traveling wave waveform distortion constant.

[0033] Step Five: Determine the fault area. If the criterion is met, it is an in-zone fault and the protection operates; otherwise, return to Step One.

[0034] The fault area criterion is shown in Equation (9).

[0035] l < ε (9)

[0036] Where ε is the threshold, and its setting principle is shown in Equation (10).

[0037] ε = rel·l0 (10)

[0038] Where rel is the reliability coefficient for threshold setting, usually taking a value of 0.8 - 0.85, and l0 is the length of the protected line.

[0039] Compared with the prior art, the present invention has the following advantages:

[0040] Since the transition resistance only affects the amplitude attenuation coefficient of the reverse traveling wave and does not affect its waveform distortion coefficient, the protection principle of the present invention has strong tolerance to transition resistance; since the fault distance can be analytically expressed, the threshold setting of the present invention can eliminate the dependence on simulation data; since the calculation of the fault distance is only based on the propagation characteristics of the reverse 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 the safe and stable operation of the power system. Brief Description of the Drawings

[0041] Figure 1 is a DC system topology diagram applicable to the method of the present invention.

[0042] Figure 2 is a flowchart of the method of the present invention.

[0043] Figure 3(a) is the waveform of the positive pole voltage of the line under a DC line fault.

[0044] Figure 3(b) is the waveform of the negative pole voltage of the line under a DC line fault.

[0045] Figure 3(c) is the waveform of the positive-pole current of the line under a DC line fault.

[0046] Figure 3(d) is the waveform of the negative-pole current of the line under a DC line fault.

[0047] Figure 4(a) is the waveform of the mode-1 backward traveling wave of the line under a DC line fault.

[0048] Figure 4(b) is the waveform of the derivative of the mode-1 backward traveling wave of the line under a DC line fault.

[0049] Figure 4(c) is the waveform of the logarithm of the absolute value of the derivative of the mode-1 backward traveling wave of the line under a DC line fault.

[0050] Figure 5 is the protection operation signal discriminated based on the method of the present invention. Detailed implementation manners

[0051] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0052] As Figure 1 shown, at 0.4 ms, a positive-pole metallic ground fault (denoted as fault F1) occurs at a distance of 100 km from converter station M on the positive pole of transmission line MN.

[0053] After the occurrence of fault F1, the voltage and current waveforms measured by the sampling element are as shown in Figures 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 5 shown.

[0054] A ranging protection method for a flexible DC transmission line based on the distortion coefficient of the backward traveling wave waveform, the process of which is as Figure 2 shown, includes the following steps:

[0055] Step 1: Sample the positive-pole voltage u p (t i ), the negative-pole voltage u n (t i ), the positive-pole current i p (t i ), and the negative-pole current i n (t i ), and the results are respectively as shown in Figures 3(a), 3(b), 3(c) and 3(d).

[0056] Calculate the mode-1 voltage u (1) (t i ), the mode-0 voltage u (0) (t i ), the mode-1 current i (1) (t i ), and the mode-0 current i(0) (t i )。

[0057] According to Equation (3), from the first-mode voltage u (1) (t i ), the first-mode current i (1) (t i ), calculate the first-mode voltage backward traveling wave u b(1) (t i ), and the result is shown in Figure 4(a).

[0058] Step 2: According to Equation (4), calculate the first derivative u′ b(1) (t i ) of the first-mode voltage backward traveling wave u b(1) (t i ), and the result is shown in Figure 4(b).

[0059] Calculate the logarithm of its absolute value ln(|u′ b(1) (t i )|) based on the first derivative u′ b(1) (t i ), and the result is shown in Figure 4(c).

[0060] Step 3: According to Equation (5), calculate the slope K0 of the linear relationship based on the least squares method, and the result is -2.4398×10 5 .

[0061] Step 4: Calculate the waveform distortion coefficient τ according to Equation (7), and the result is 4.0987×10 -6 .

[0062] Take the waveform distortion constant τ0 as 4.06×10 -8 s / km, and according to Equation (8), calculate the fault distance l, and the result is 100.0547 km.

[0063] Step 5: Set the threshold ε according to Equation (10), where the reliability coefficient rel is taken as 0.85, the length l0 of the protected line is 300 km, and ε is set to 255 km.

[0064] Perform fault area discrimination according to Equation (9). If the criterion is met for an internal fault, the protection trips and the process ends. It can be seen from Figure 5 that this protection method can reliably identify faults and send out correct action signals, and the protection action time is short.

Claims

1. A ranging protection method for a flexible DC transmission line based on the distortion coefficient of the reverse traveling wave waveform, 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 u according to Equation (1) (1) (t i ), the 0-mode voltage u (0) (t i ), the 1-mode current i (1) (t i ), and the 0-mode current i (0) (t i ). where t i is the i-th sampling moment, i is the sampling sequence number, N is the total number of samplings, Q is the transformation matrix, u p (t i ) is the positive electrode voltage, u n (t i ) is the negative electrode voltage, i p (t i ) is the positive electrode current, i n (t i ) is the negative electrode current; According to Equation (2), from the 1-mode voltage u (1) (t i ) and the definition formula of the 1-mode current i (1) (t i ), derive the expression of the 1-mode voltage backward wave u b(1) (t i ), as shown in Equation (3). where, u f(1) (t i ) is the forward traveling wave of the voltage in mode 1, u b(1) (t i ) is the backward traveling wave of the voltage in mode 1, i f(1) (t i ) is the forward traveling wave of the current in mode 1, i b(1) (t i ) is the backward traveling wave of the current in mode 1, Z C is the wave impedance; u b(1) (t i )=[u (1) (t i )-i (1) (t i )·Z C / 2 (3) According to Equation (3), from the first-mode voltage u (1) (t i ) and the first-mode current i (1) (t i ), calculate the first-mode voltage backward traveling wave u b(1) (t i ); Step 2: Calculate the logarithm of the absolute value of the first derivative of the backward traveling wave of the 1-mode voltage; According to Equation (4), the first-order derivative u' of the 1-mode voltage backward wave u b(1) (t i ) is calculated; b(1) (t i ); where T S is the sampling interval; From the first derivative u′ of the 1-mode voltage backward traveling wave b(1) (t i ) calculate the logarithm of its absolute value ln(|u′ b(1) (t i )|), briefly denoted as Y(t i ); Step 3: Fit the linear relationship between the logarithm of the absolute value of the first derivative of the backward traveling wave of the 1-mode voltage and the sampling time based on the least squares method, and calculate the slope of the linear relationship; The calculation formula for the slope K0 of the linear relationship is shown in Equation (5). In the formula, the mean value of the logarithm of the absolute value of the first derivative of the 1-mode voltage backward traveling wave and the mean value of the sampling time are calculated according to Equation (6); Step 4: Calculate the fault distance; According to Equation (7), calculate the backward traveling wave waveform distortion coefficient τ from the slope K0; According to Equation (8), calculate the fault distance l from the backward traveling wave waveform distortion coefficient τ; In the formula, τ0 is the backward traveling wave waveform distortion constant; 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 (9). l<ε (9) In the formula, ε is the threshold, and its setting principle is shown in Equation (10). ε=rel·l0 (10) In the formula, rel is the reliability coefficient for threshold setting, and l0 is the length of the protected line.

2. A ranging protection method for a flexible DC transmission line based on the distortion coefficient of the reverse traveling wave waveform according to claim 1, characterized in that: The value of the reliability coefficient rel for threshold setting is 0.8 - 0.85.

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