Flexible dc power grid line fault identification method based on voltage traveling wave refractive index

Abstract

The application relates to a flexible direct current power grid line fault recognition method based on a voltage wave refraction coefficient, which is characterized in that: 1-mode wave impedance of a line is analyzed to obtain a relatively accurate fault component 1-mode voltage wave; Peterson equivalent circuit is used to quantitatively analyze the voltage wave refraction coefficient when a positive direction fault occurs, and the voltage wave refraction coefficient under a reverse direction fault is obtained through qualitative analysis; a fault recognition criterion is constructed according to the difference of the voltage wave refraction coefficient, and judgment is made. The fault recognition method can recognize the power grid fault type only by calculating the voltage wave refraction coefficient, has strong adaptability to the primary topology of the power grid, does not need a large amount of data transmission, has a low requirement on a sampling frequency, and has strong engineering applicability.

Description

Technical Field

[0001] This invention belongs to the field of power grid line fault identification technology, and in particular to a method for identifying faults in flexible DC power grid lines based on the voltage traveling wave refractive index. Background Technology

[0002] Flexible DC transmission technology based on modular multilevel converters (MMC), especially flexible DC grids, is considered one of the effective technical means to solve the problem of renewable energy consumption in different regions. It also has broad application prospects in areas such as new energy grid integration, large-capacity long-distance power transmission, and the construction of new urban DC distribution networks. However, due to the low damping and low inertia of flexible DC grids, DC faults propagate at extremely high speeds, causing serious damage to the entire high-voltage DC grid within milliseconds. Therefore, researching line protection systems that can quickly identify faults and provide selective protection is essential for DC grids.

[0003] Existing traveling wave protection principles are mainly based on the traveling wave reflection coefficient and its variations, without exploring the possibility of applying the traveling wave refractive index in the protection field.

[0004] Dual-ended quantity protection also has some problems, such as: 1) poor adaptability to the primary system structure, which is reflected in two aspects: some longitudinal protection relies too much on line boundaries; existing analyses only discuss one case—whether the line boundary exists or not—ignoring the other case. 2) It requires the transmission of large amounts of data, placing high demands on the communication channel. Currently, no good solution has been found to address these problems. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of the prior art and propose a fault identification method for flexible DC power grid lines based on the voltage traveling wave refractive index. This method can maintain the stable operation of the power grid, quickly and accurately provide the optimal load transfer scheme, and greatly improve the work efficiency of dispatchers.

[0006] The technical problem solved by this invention is achieved through the following technical solution:

[0007] A fault identification method for flexible DC power grid lines based on voltage traveling wave refractive index includes the following steps:

[0008] Step 1: Determine the power grid line and calculate the fault component 1-mode voltage traveling wave at the beginning and end of the power grid line;

[0009] Step 2: Under the condition that Peterson's law holds, calculate the refractive index of the voltage traveling wave at the beginning and end of the power grid line based on the fault component 1-mode voltage traveling wave obtained in Step 1, and obtain the relationship between the refractive index of the voltage traveling wave at the beginning and end of the power grid line and the fault type.

[0010] Step 3: Construct a data window and sample data within the data window;

[0011] Step 4: Based on the data sampled in the data window in Step 3, and the relationship between the traveling wave refraction coefficient of the voltage at the beginning and end of the power grid line and the fault type obtained in Step 2, construct the fault identification factor.

[0012] Step 5: Calculate the fault identification factors at the beginning and end of the power grid line, and determine the fault type of the flexible DC power grid line.

[0013] Furthermore, the specific implementation method of step 1 is as follows: obtain the 1-mode voltage traveling wave impedance at the beginning and end of the high-frequency power grid line, and extract the high-frequency voltage component Δu1 and the high-frequency voltage component Δi1 at this frequency band using a digital filter. Based on the calculation formulas for the forward and reverse traveling waves, obtain the fault component 1-mode voltage traveling wave at the beginning and end of the power grid line.

[0014]

[0015] Where u q For the fault component 1-mode voltage traveling wave at a point on the power grid line, u f Z represents the fault component of the modulo-1 voltage reverse traveling wave at a point on the power grid line. c1 The traveling wave impedance of the line is the 1-mode voltage.

[0016] Furthermore, the digital filter is a high-pass filter constructed using the Turkey window function, with a cutoff frequency greater than 100Hz and sampling frequencies of 50kHz, 20kHz, and 10kHz. The impulse response sequence of the digital filter is as follows:

[0017] h1(n)={-0.0068,-0.0179,-0.0199,-0.0199,0.9777,-0.0199,-0.0199,-0.0179,-0.0068}

[0018] h2(n)={-0.2083,-0.7083,0.7083,0.2083}

[0019] h3(n)={-0.0196,0.9068,-0.0196}

[0020] Where h1(n) is the impulse response sequence with a sampling frequency of 50kHz, h2(n) is the impulse response sequence with a sampling frequency of 20kHz, and h3(n) is the impulse response sequence with a sampling frequency of 10kHz.

[0021] The high-frequency voltage components Δu1 and Δi1 in this frequency band are extracted by convolving the impulse response sequence of the digital filter with the original signal.

[0022] Furthermore, the specific implementation method of step 2 is as follows: Under the condition that Peterson's law holds, the refractive index of the voltage traveling wave at the measurement point of this line is the ratio of the forward traveling wave of the fault component I-mode voltage at the adjacent measurement point on the opposite side of the line to the reverse traveling wave of the fault component I-mode voltage at the measurement point of this line:

[0023]

[0024] Where α(t) is the voltage traveling wave refraction coefficient at the measurement point of this line, u q (t) represents the fault component 1-mode voltage traveling wave at adjacent measurement points on the reverse side of the line, u f (t) represents the fault component 1-mode voltage reverse traveling wave at the measurement point of this line.

[0025] Furthermore, the relationship between the traveling wave refraction coefficient of the voltage at the beginning and end of the power grid line and the fault type in step 2 is as follows:

[0026]

[0027] Where, α s (t) is the voltage traveling wave refraction coefficient at the beginning of the power grid line, α m (t) is the voltage traveling wave refraction coefficient at the end of the line.

[0028] Furthermore, the data window constructed in step 3 must meet the following constraints:

[0029] Constraint 1: On a path where refraction occurs, the reflected wave generated at the end of the path has not yet reached the beginning of the path.

[0030]

[0031] Where T is the length of the time window, l1, l2, l3, l4…l y These are the actual lengths of the 1st, 2nd, 3rd, 4th, and yth protected lines, respectively, where y is the number of protected lines and v1 is the wave velocity of the 1-mode voltage traveling wave.

[0032] Constraint 2: Simultaneously, the data window must satisfy the following: the MMC converter station is equivalent to an RLC series circuit during the time before latch-up, and the latch-up time of the MMC is 3-5 ms.

[0033] T≤t MMCb = (3~5)ms

[0034] Among them, t MMCb This refers to the interlocking time of the MMC converter station.

[0035] Furthermore, the construction failure identification factor constructed in step 4 is:

[0036]

[0037] Where N is the number of sampling points within the data window; u q (t) represents the fault component 1-mode voltage reverse traveling wave at the measurement point of this line; u f (t) represents the fault component 1-mode voltage traveling wave at adjacent measurement points on the reverse side of the line.

[0038] Furthermore, the determination relationship in step 5 is as follows:

[0039]

[0040] Among them, X s X is the fault identification factor at the beginning of the line. m The fault identification factor at the end of the line is used to identify the fault type of the flexible DC power grid line by calculating the value of the fault identification factor within a time window.

[0041] The advantages and positive effects of this invention are:

[0042] This invention obtains a relatively accurate fault component, the mode-1 traveling voltage wave, by analyzing the mode-1 impedance of the line. It uses the Peterson equivalent circuit to quantitatively analyze the refractive index of the voltage traveling wave under forward faults, and qualitatively analyzes it to obtain the refractive index under reverse faults. Based on the difference in the voltage traveling wave refractive index, a fault identification criterion is constructed and a judgment is made. This fault identification method identifies the type of power grid fault simply by calculating the voltage traveling wave refractive index. It is highly adaptable to the primary topology of the power grid, requires no large amount of data transmission, has low requirements for sampling frequency, and has strong engineering applicability. Attached Figure Description

[0043] Figure 1 This is a primary structure diagram of a current-limiting reactor installed at both ends of a line.

[0044] Figure 2 This is a primary structure diagram of a current-limiting reactor installed at the converter station outlet.

[0045] Figure 3 The frequency-varying characteristics of the 0-mode voltage traveling wave and the 1-mode voltage traveling wave of this invention are shown in the diagram.

[0046] Figure 4 This is the equivalent circuit diagram of the primary structure of the current-limiting reactor of the present invention installed at both ends of the line;

[0047] Figure 5 This is the equivalent circuit diagram of the primary structure of the current-limiting reactor of the present invention installed at the outlet of the converter station;

[0048] Figure 6 This is a diagram of the refractive index under a primary structural positive direction fault according to the present invention. Detailed Implementation

[0049] The present invention will be further described in detail below with reference to the accompanying drawings.

[0050] In a power grid, the primary structure of the system can be divided into two categories, such as... Figure 1 The image shows a current-limiting reactor installed at both ends of a DC line. Figure 2 The diagram shows a current-limiting reactor installed at the converter station outlet. P1–P8 are measurement points at the beginning or end of the line, and Line 1–Line 4 are DC overhead transmission lines. For measurement point P1, f1 and f3 represent forward line faults, f2 represents a forward converter station outlet fault, f4 represents a reverse line fault, and f5 represents a reverse converter station outlet fault. Due to the symmetry of the primary structure of the power grid, the calculation method is the same; the following calculations all use measurement point P1 as an example.

[0051] A fault identification method for flexible DC power grid lines based on voltage traveling wave refractive index includes the following steps:

[0052] Step 1: Determine the power grid line and calculate the fault component 1-mode voltage traveling wave at the beginning and end of the power grid line.

[0053] like Figure 3 As shown, in the low-frequency range, the impedance 1-mode voltage traveling wave Z c1 The amplitude of the impedance Z varies drastically with frequency; however, in the high-frequency range (greater than 100Hz), the impedance Z... c1 The amplitude is basically stable around a certain specific value. At the same time, the amplitude of the zero-mode voltage traveling wave impedance changes significantly, which is not conducive to the calculation of the zero-mode voltage traveling wave. Therefore, the high-frequency band of the one-mode voltage traveling wave is selected for calculation.

[0054] The first-mode voltage traveling wave impedance at the beginning and end of the high-frequency power grid line is obtained, and the high-frequency voltage components Δu1 and Δi1 in this frequency band are extracted using a digital filter. Based on the calculation formulas for forward and reverse traveling waves, the fault component first-mode voltage traveling wave at the beginning and end of the power grid line is obtained:

[0055]

[0056] Among them, u qFor the fault component 1-mode voltage traveling wave at a point on the power grid line, u f Z represents the fault component of the modulo-1 voltage reverse traveling wave at a point on the power grid line. c1 Let Δu1 be the traveling wave impedance of the line in mode 1, Δi1 be the high-frequency component of the voltage in this frequency band, and Δi1 be the high-frequency component of the voltage in this frequency band.

[0057] To obtain a more accurate voltage traveling wave refractive index, the digital filter is a high-pass filter constructed using the Turkey window function, with a cutoff frequency greater than 100Hz and sampling frequencies of 50kHz, 20kHz, and 10kHz. The impulse response sequence of the digital filter is as follows:

[0058] h1(n)={-0.0068,-0.0179,-0.0199,-0.0199,0.9777,-0.0199,-0.0199,-0.0179,-0.0068}

[0059] h2(n)={-0.2083,-0.7083,0.7083,0.2083}

[0060] h3(n)={-0.0196,0.9068,-0.0196}

[0061] Where h1(n) is the impulse response sequence with a sampling frequency of 50kHz, h2(n) is the impulse response sequence with a sampling frequency of 20kHz, and h3(n) is the impulse response sequence with a sampling frequency of 10kHz; the high-frequency voltage component Δu1 and the high-frequency voltage component Δi1 in this frequency band are extracted by convolving the impulse response sequence of the digital filter with the original signal.

[0062] Step 2: Under the condition that Peterson's law holds, calculate the refractive index of the voltage traveling wave at the beginning and end of the power grid line based on the fault component 1-mode voltage traveling wave obtained in Step 1, and obtain the relationship between the refractive index of the voltage traveling wave at the beginning and end of the power grid line and the fault type.

[0063] like Figure 4 As shown, under the condition that Peterson's rule holds, Figure 1 The equivalent circuit corresponding to the primary structure. When a positive line fault f1, f2 or a converter station outlet fault f3 occurs, the propagation path of the fault component 1 mode voltage traveling wave is as follows: Figure 4 As shown in (a), if the positive direction of the current is defined as the flow from the busbar to the line, then the measuring point P1 will first experience a reverse traveling wave u from the fault point. f1 (incident wave u) r Then, reflection and refraction occur, generating a traveling wave u at measurement point P1. q1 (reflected wave u)f A traveling wave u is generated at P8. q8 (refracted wave u) z The Peterson equivalent circuit is obtained as follows: Figure 4 (b)

[0064] Z MMC Z is the equivalent impedance of the converter station, which can be equivalent to a second-order series RLC circuit before blocking; L is the inductance of the current-limiting reactor; Z is the equivalent impedance of the converter station. c1 The impedance of line mode 1; u f1 The anti-traveling wave sensed at point P1; u q8 The forward wave felt at point P8.

[0065] The voltage traveling wave refractive index at point P1 is:

[0066]

[0067] When the incident wave u r Or the anti-traveling wave u at P1 f1 When the value is 1, the voltage traveling wave refractive index α is equal to the forward traveling wave u at P8. q8 Its physical meaning is: the refractive index (α) of the voltage traveling wave is equal to the unit step excitation (u). r or u f1 ) response (u q8 ):

[0068]

[0069] Solving the Peterson equivalent circuit in the complex frequency domain yields the image function U of the unit step response at P8. q8 (s), which is the image function A'(s) of the voltage traveling wave refraction coefficient at P1:

[0070]

[0071] The voltage traveling wave refractive index at point P1 in the time domain can be calculated using the inverse Laplace transform.

[0072] α'(t)=L -1 [A' q8 (s)].

[0073] like Figure 5 As shown, under the condition that Peterson's rule holds, Figure 2 The equivalent circuit corresponding to the primary structure, and the propagation diagram of the traveling wave in the positive fault component 1-mode network are shown below. Figure 5 As shown in (a), the Peterson equivalent circuit is as follows: Figure 5As shown in (b). Based on the above calculation method, the image function of the unit step response at P8 can be obtained, which is also the image function of the voltage traveling wave refractive index at P1.

[0074]

[0075]

[0076] The voltage traveling wave refractive index at point P1 in the time domain can be calculated using the inverse Laplace transform.

[0077] α (t) = L -1 [A” q8 (s)]

[0078] Among them, the parameters of the converter station, current-limiting reactor, and line in the complex frequency domain can be queried from the power grid, and the relationship between the voltage traveling wave refractive index and time for the two topologies under forward fault conditions can be calculated, such as... Figure 6 As shown.

[0079] exist Figure 6 In the case of a voltage traveling wave, the refractive index changes continuously over time due to the presence of numerous dynamic components at the boundary. During a forward fault, regardless of the primary structure of the flexible DC power grid, the absolute value of the voltage traveling wave refractive index at measurement point P1 is always less than 1 during the time Peterson's law holds.

[0080]

[0081] Furthermore, the voltage traveling wave refractive index is related to the line wave impedance, the current-limiting reactor, and the equivalent impedance of the converter station, but not to the transition resistance.

[0082] When a reverse fault occurs, regardless of the structure of the flexible DC power grid (whether line boundaries exist), the absolute value of the voltage traveling wave refraction coefficient at point P1 is always much greater than 1 during the time Peterson's law holds.

[0083]

[0084] Based on the above calculation method for the voltage traveling wave refractive index, extending from the detection point P1 to all measurement points, the voltage traveling wave refractive index is equal to the fault component 1-mode voltage forward traveling wave (i.e., the refracted wave u) at the adjacent measurement points on the reverse side of the line. z The fault component at the measurement point of this line is the 1-mode voltage reverse traveling wave (i.e., the incident wave u). r The ratio of ) to:

[0085]

[0086] The incident wave at the measurement point is equal to the fault component of the 1-mode voltage reverse traveling wave at that measurement point.

[0087] The relationship between the voltage traveling wave refraction coefficient at the beginning and end of a power grid line and the fault type is as follows: when the absolute value of the voltage traveling wave refraction coefficient at both measuring points at both ends of the line is less than 1, that is, both ends of the line are positive direction faults, then it is judged as an intra-zone fault; otherwise, it is judged as an extra-zone fault.

[0088]

[0089] Where, α s (t) is the voltage traveling wave refraction coefficient at the beginning of the power grid line, α m (t) is the voltage traveling wave refraction coefficient at the end of the line.

[0090] Step 3: Construct a data window and sample data within the data window.

[0091] To ensure the relationship between the traveling wave refraction coefficient at the beginning and end of the power grid line and the fault type holds true, the data window must satisfy the following constraints:

[0092] Constraint 1: On a path where refraction occurs, the reflected wave generated at the end of the path has not yet reached the beginning of the path.

[0093]

[0094] Where T is the length of the time window, l1, l2, l3, l4…l y y represents the actual length of the protected line in milliseconds (ms); y represents the number of protected lines in kilometers (km); v1 represents the wave velocity of the mode 1 voltage traveling wave in kilometers per second (km / s). The mode 1 wave velocity of overhead lines is close to the speed of light, approximately 300 km / s.

[0095] Constraint 2: When performing time-domain and complex frequency-domain calculations using the Peterson equivalent circuit, the converter station must be equivalent to a second-order series RLC circuit. Simultaneously, the MMC converter station must be equivalent to an RLC series circuit before the latch-up time, which is 3–5 ms. Therefore, the data window length must also satisfy the following:

[0096] T≤t MMCb = (3~5)ms

[0097] Among them, t MMCb This refers to the interlocking time of the MMC converter station.

[0098] Step 4: Based on the data sampled in the data window in Step 3, and the relationship between the traveling wave refraction coefficient of the power grid line head and tail voltage and the fault type obtained in Step 2, construct the fault identification factor.

[0099] For measurement points at the beginning or end of the line, during faults within the designated area, the refracted wave amplitude will always be less than the incident wave amplitude at any given moment within the data window; the opposite is true for faults outside the designated area. To amplify the difference, the refracted and incident wave amplitudes within the data window are summed to construct a fault identification factor:

[0100]

[0101] Where N is the number of sampling points within the data window; u r (t) represents the fault component 1-mode voltage reverse traveling wave at the measurement point of this line; u z (t) represents the fault component 1-mode voltage traveling wave at adjacent measurement points on the reverse side of the line.

[0102] Step 5: Calculate the fault identification factors at the beginning and end of the power grid line, and determine the fault type of the flexible DC power grid line:

[0103]

[0104] Among them, X s X is the fault identification factor at the beginning of the line. m The fault identification factor at the end of the line is used to identify the fault type of the flexible DC power grid line by calculating the value of the fault identification factor within a time window. After the protection is started, the value of the fault identification factor within a specified time window is calculated, and then the fault is identified according to the discrimination relationship in step 5. Only the logical signal of the fault identification factor needs to be transmitted, which has low requirements for the communication channel.

[0105] It should be emphasized that the embodiments described in this invention are illustrative rather than limiting. Therefore, this invention includes, but is not limited to, the embodiments described in the specific implementation. Any other implementations derived by those skilled in the art based on the technical solutions of this invention are also within the scope of protection of this invention.

Claims

1. A method for flexible HVDC grid line fault identification based on voltage traveling wave refractive index, characterized in that: The method comprises the following steps: Step 1, determining a power grid line and calculating fault component 1 mode voltage traveling wave at the head and tail of the power grid line; Step 2, under the Peterson rule, calculating voltage traveling wave refraction coefficients at the head and tail of the power grid line according to the fault component 1 mode voltage traveling wave at the head and tail of the power grid line obtained in step 1, and obtaining the relationship between the voltage traveling wave refraction coefficients at the head and tail of the power grid line and the fault type; The specific implementation method of step 2 is that, under the Peterson rule, the voltage traveling wave refraction coefficient at the measurement point of the line is the ratio of the fault component 1 mode voltage forward wave at the adjacent measurement point of the line on the opposite side to the fault component 1 mode voltage backward wave at the measurement point of the line: wherein, a(t) is the voltage traveling wave refraction coefficient of the measuring point of the line, u q (t) is the forward traveling wave of the fault component 1-mode voltage at the adjacent measuring point of the opposite side line, u f (t) is the backward traveling wave of the fault component 1-mode voltage at the measuring point of the line; Step 3, constructing a data window and sampling data in the data window; Step 4, constructing a fault identification factor according to the data sampled in the data window in step 3 and the relationship between the voltage traveling wave refraction coefficients at the head and tail of the power grid line and the fault type obtained in step 2; Step 5, calculating the fault identification factors at the head and tail of the power grid line and identifying the fault type of the flexible HVDC power grid line.

2. The method for voltage-based traveling wave refraction coefficient based HVDC line fault identification as claimed in claim 1, wherein: The specific implementation method of step 1 is that, the 1 mode voltage traveling wave impedance at the head and tail of the power grid line in a high frequency band is obtained, and the voltage high frequency component Δu1 in the frequency band and the voltage high frequency component Δi1 in the frequency band are extracted through a digital filter, and the fault component 1 mode voltage traveling wave at the head and tail of the power grid line is obtained according to the calculation formula of the forward wave and the backward wave: where u q is the forward traveling wave of the fault component 1-mode voltage at a point on the power grid line, u f is the backward traveling wave of the fault component 1-mode voltage at a point on the power grid line, Z c1 is the 1-mode voltage traveling wave impedance of the line.

3. The method for voltage-based traveling wave refraction coefficient based HVDC line fault identification as claimed in claim 2, wherein: The digital filter is a high-pass filter constructed through a Turkey time window function, the cut-off frequency of which is greater than 100 Hz, the sampling frequencies are 50 kHz, 20 kHz and 10 kHz respectively, and the impulse response sequence of the digital filter is: h1(n) = {-0.0068, -0.0179, -0.0199, -0.0199, 0.9777,-0.0199,-0.0199,-0.0179,-0.0068} h2(n) = {-0.2083, -0.7083, 0.7083, 0.2083} h3(n) = {-0.0196, 0.9068, -0.0196} wherein h1(n) is the impulse response sequence of the sampling frequency of 50 kHz, h2(n) is the impulse response sequence of the sampling frequency of 20 kHz, and h3(n) is the impulse response sequence of the sampling frequency of 10 kHz; The voltage high frequency component Δu1 in the frequency band and the voltage high frequency component Δi1 in the frequency band are extracted by convolution of the impulse response sequence of the digital filter and the original signal.

4. The voltage- based traveling wave refraction coefficient method for flexible HVDC grid line fault identification of claim 1, wherein: The relationship between the voltage traveling wave refraction coefficients at the head and tail of the power grid line and the fault type in step 2 is: wherein α s (t) is the voltage traveling wave refraction coefficient at the end of the line. m (t) is the voltage traveling wave refraction coefficient at the end of the line.

5. The voltage- wave- travel- refraction- coefficient-based flexible HVDC grid line fault identification method of claim 1, wherein: The data window constructed in step 3 needs to meet the constraint conditions: Constraint condition 1, on the line where refraction occurs, the reflected wave generated at the tail of the line has not reached the head of the line: Wherein, T is the length of the time window, l1, l2, l3, l4…l y Respectively, the actual length of the first, second, third, fourth, the yth protected line, y is the number of protected lines, v1 is the wave speed of the 1-mode voltage traveling wave; Constraint condition 2, meanwhile, the data window meets that the MMC converter station is equivalent to an RLC series circuit within the time before the MMC is locked, and the locking time of the MMC is 3-5 ms: T≤t MMCb = (3-5) ms Wherein, t MMCb is the blocking time of the MMC converter station.

6. The voltage- wave- travel- refraction- coefficient-based flexible DC power grid line fault identification method according to claim 1, characterized in that: The fault identification factor constructed in step 4 is: where N is the number of sampling points in the data window; u q (t) is the fault component 1-mode voltage backward traveling wave at the measurement point of the current line; u f (t) is the fault component 1-mode voltage forward traveling wave at the adjacent measurement point of the opposite direction backside line.

7. The voltage- wave- travel- refraction- coefficient-based flexible HVDC grid line fault identification method of claim 1, wherein: The determination relationship of step 5 is: wherein X s is a fault identification factor of the line head, X m is a fault identification factor of the line tail, and the type of the flexible DC power grid line fault is identified by calculating the value of the fault identification factor within the time window.

Images