Transmission line protection methods applicable to boundaryless flexible DC ring networks

Abstract

This invention discloses a transmission line protection method applicable to a boundaryless flexible DC ring network, specifically including the following steps: Step 1, calculating the line-mode voltage traveling wave; Step 2, performing minimum-maximum normalization on the line-mode voltage traveling wave obtained in Step 1; Step 3, based on the normalized line-mode voltage traveling wave obtained in Step 2, setting a sliding time window T, and using the sliding time window to find voltage drop points that meet the conditions of the normalized traveling wave; Step 4, comparing the voltage drop points with a preset intra-area priority action threshold k. set1 and the priority action threshold k outside the zone set2 A comparison is made, and the faults within and outside the designated area are identified based on the comparison results. The method provided by this invention can achieve protection for boundaryless flexible DC ring network transmission lines.

Description

Technical Field

[0001] This invention belongs to the field of power system technology and relates to a transmission line protection method applicable to boundaryless flexible DC ring networks. Background Technology

[0002] The protection method for flexible DC transmission lines has therefore become particularly important. It can not only ensure the effective grid connection of new energy sources and the stable operation of the power system, but also improve the system's adaptability to fluctuations in new energy sources, and ensure the continuity and security of power supply. It is of great significance for promoting the development of the power system towards a more efficient and environmentally friendly direction.

[0003] However, with the development of new energy sources, considering that the installation of current-limiting reactors will increase system losses and reduce the overall efficiency of the transmission system, and that the installation and maintenance of current-limiting reactors in offshore or coastal areas are more difficult, adding current-limiting reactors at the line outlet may actually affect the stability and response speed of the system. The "boundary grid" will inevitably transform into a "boundaryless grid" where current-limiting reactors are installed at the converter station outlet.

[0004] For "boundaryless power grids," the traditional boundary-based power grid protection principle, which uses components such as DC reactors as boundaries and considers the differences in high-frequency fault components inside and outside the fault boundary, is no longer applicable. The absence of boundary components leads to a loss of significant distinguishability of high-frequency fault components, which is one of the inherent defects of single-ended quantity protection principles. Therefore, there is an urgent need for a boundaryless flexible DC ring network transmission line protection scheme that does not rely on boundary components. Summary of the Invention

[0005] The purpose of this invention is to provide a transmission line protection method applicable to boundless flexible DC ring networks, which can achieve protection of transmission lines in boundless flexible DC ring networks.

[0006] The technical solution adopted in this invention is a transmission line protection method applicable to boundaryless flexible DC ring networks, specifically including the following steps:

[0007] Step 1: Calculate the traveling wave of the line-mode voltage;

[0008] Step 2: Perform minimum-maximum normalization on the line-mode voltage traveling wave from Step 1;

[0009] Step 3: Based on the normalized line-mode voltage traveling wave obtained in Step 2, set a sliding time window T, and use the sliding time window to find the voltage drop point that meets the conditions for the normalized traveling wave.

[0010] Step 4: Compare the voltage drop point with the preset intra-region priority action threshold k. set1 and the priority action threshold k outside the zone set2 The comparison is performed, and the faults within and outside the zone are identified and judged based on the comparison results.

[0011] The invention is further characterized by:

[0012] The specific process of step 1 is as follows:

[0013] Step 1.1: After the protection starting element is activated, a protection time window is set from 0.5ms before activation to 1.5ms after activation. Within this window, the positive voltage u at the installation location of the protection device is read. p (k) Negative electrode voltage u n (k), positive current i p (k), negative electrode current i n (k) is respectively compared with the average positive voltage u within 10ms before protection activation. p0 Average negative electrode voltage u n0 Average positive current i p0 Average negative current i n0 Subtraction, using the following formula (1) 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):

[0014]

[0015] In the formula, k is the time sequence number and the fault occurrence time is defined as k = 0, 0 ≤ k ≤ N-1; N is the number of sampling points from 0.5 ms before the faulty component starts to 1.5 ms after the start.

[0016] Step 1.2, calculate the line-mode voltage fault component Δu1(k) and line-mode current fault component Δi1(k) within the time window using the following formula (2):

[0017]

[0018] Step 1.3, calculate the line-mode voltage traveling wave u within the time window using the following formula (3). f (k):

[0019]

[0020] In the formula, Z c1 The impedance of the line mode wave is given.

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

[0022] The traveling wave u of the line-mode voltage 0.5ms before and 1.5ms after the faulty component starts is calculated according to the following formula (4). f (k) Normalized voltage ufN (k):

[0023]

[0024] In the formula, min(u b (k) is the minimum value within the time window; max(u) is the minimum value within the time window. b (k) is the maximum value within the time window.

[0025] The specific process of step 3 is as follows:

[0026] Step 3.1: Set a sliding time window T, and apply the normalized voltage u obtained in Step 2. fN (k) Find the minimum point within the sliding time window, denoted as (k min u fmin ), where k min The time series representing the minimum point, u fmin This represents the voltage amplitude at the minimum point.

[0027] Step 3.2: Find the maximum value point within the range of sampling times less than the minimum value point within the sliding window of step 3.1, denoted as (k). max u fmax ), where k max The time series representing the maximum point, u fmax This represents the voltage amplitude at the maximum value point;

[0028] Step 3.3, calculate the maximum value u from step 3.2 according to the following formula (5). fmax Compared with the minimum value u in step 3.1 fmin The difference is greater than the set threshold u. set At this time, k max If this is the voltage drop point, proceed to step 4; otherwise, proceed to steps 3 and 4.

[0029] (u fmax -u fmin )>u set (5)

[0030] In the formula, u set To identify the voltage drop threshold value;

[0031] Step 3.4: After the sliding window T has traversed all the data within the time window, k is still not found. max When k is defined max If the maximum sampling point is found within the time window, proceed to step 4. If the sliding window has not traversed all the data within the time window, slide the sliding window forward by one sampling point and return to step 3.1.

[0032] The specific process of step 4 is as follows: k max Compared with the preset priority action threshold k within the areaset1 and the priority action threshold k outside the zone set2 Comparison, when k max ≥k set1 When k is identified as an intra-zone fault; max ≤k set2 When k is identified as an external fault; set1 >k max >k set2 At that time, the fault type is determined based on the signal from the directional element device at the opposite end of the protection measuring point.

[0033] In step 4, when a fault is identified within the zone, an action signal is sent to the DC circuit breaker, and the DC circuit breaker operates after receiving the protection action signal.

[0034] In step 4, when an external fault is identified, the protection is reset.

[0035] In step 4, based on the signal from the directional element device at the opposite end of the protection measuring point, when the directional element at the opposite end determines that the fault occurs in the direction of the protected line, it will send signal 1, which can then be identified as an internal fault and send an action signal to the DC circuit breaker. The DC circuit breaker will then operate after receiving the protection action signal. When the directional element device at the opposite end determines that the fault occurs in the opposite direction of the protected line, it will send signal 0, which can then be identified as an external fault.

[0036] The beneficial effects of this invention are that it proposes a protection method applicable to non-boundary flexible DC ring network transmission lines. It constructs a single-ended quantity protection method by utilizing the distance characteristics of the voltage drop time of the traveling wave leading to the fault mode voltage in a non-boundary flexible DC ring network. The method has rapid identification, strong resistance to transition resistance, and can distinguish between internal and external faults in the traveling wave at various points in the entire network. Attached Figure Description

[0037] Figure 1 This is a flowchart of the transmission line protection method applicable to the boundaryless flexible DC ring network of the present invention;

[0038] Figure 2 This is a topology diagram of a bounded flexible DC power grid model;

[0039] Figure 3 This is a simulation model diagram of a boundless flexible DC power grid;

[0040] Figure 4 In the simulation verification stage, the voltage drop point identification diagram of the normalized waveform of the traveling wave of the line mode voltage with a positive grounding fault at 150km in the boundless flexible DC grid area and a transition resistance of 500Ω is obtained by using the transmission line protection method of the present invention applicable to the boundless flexible DC ring network.

[0041] Figure 5In the simulation verification stage, the voltage drop point identification diagram is obtained by normalizing the forward wave of the line-mode voltage at the positive grounding fault at the outlet of the MMC2 converter station outside the boundaryless flexible DC grid area and with a transition resistance of 0Ω using the transmission line protection method of this invention applicable to the boundaryless flexible DC ring network.

[0042] Figure 6 In the simulation verification stage, the voltage drop point identification diagram is obtained by normalizing the forward wave of the line-mode voltage after the positive pole grounding fault at the first end of the reverse downstream line outside the boundaryless flexible DC grid and the transition resistance is 0Ω, using the transmission line protection method of this invention applicable to the boundaryless flexible DC ring network. Detailed Implementation

[0043] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments.

[0044] Example 1

[0045] This invention relates to a transmission line protection method for boundaryless flexible DC ring networks, the process of which is as follows: Figure 1 As shown, the specific steps include the following:

[0046] Step 1: Take data from 0.5ms before startup to 1.5ms after startup to calculate the line-mode voltage fault component Δu1(k) and the line-mode current fault component Δi1(k). Then, calculate the line-mode voltage traveling wave u based on this data. f (k);

[0047] Step 2: Perform minimum-maximum normalization on the line-mode voltage traveling wave from Step 1;

[0048] Step 3: Based on the normalized line-mode voltage traveling wave obtained in Step 2, set a sliding time window T. Find the minimum point of this normalized traveling wave within the sliding time window, denoted as (k... min u fmin ), k min The time series representing the minimum point, u fmin The voltage amplitude represents the minimum point. Then, the maximum point is found within the range of sampling times within the sliding window that are less than the minimum point, denoted as (k). max u fmax ), k max The time series representing the maximum point, u fmax Represents the voltage amplitude at the maximum point, calculate u. fmax with u fmin The difference is greater than the set threshold u. set At this time, k max This is the voltage drop point; the sliding time window stops sliding if the maximum value point k is not found within the time window. max Then k max This is denoted as the length of the time window;

[0049] Step 4, set k max Compared with the preset priority action threshold k within the area set1 and the priority action threshold k outside the zone set2 Comparison, when k max ≥k set1 When a fault is identified within the zone, an action signal is sent to the DC circuit breaker, proceeding to step 6. When k max ≤k set2 When k is identified as an external fault, the protection resets; set1 >k max >k set2 Then proceed to step 5;

[0050] Based on the theoretical analysis results of the voltage traveling wave at the time of the fault, and the simulation test results, the priority threshold value k in the region is defined. set1 Define the external priority threshold value k as the theoretical voltage drop point where the fault occurs at 95% of the protected line. set2 This is the theoretical voltage drop point where the fault occurs 5% of the voltage drop along the downstream line outside the zone.

[0051] Step 5: Based on the signal from the directional element device at the opposite end of the protection measuring point, when the directional element at the opposite end determines that the fault occurs in the direction of the protected line, it will send signal 1, which can then be identified as an internal fault and send an action signal to the DC circuit breaker, proceeding to step 6; when the directional element device at the opposite end determines that the fault occurs in the opposite direction of the protected line, it will send signal 0, which can then be identified as an external fault.

[0052] Step 6: The DC circuit breaker activates after receiving the protection action signal.

[0053] Example 2

[0054] The specific process of step 1 is as follows:

[0055] Step 1.1: After the protection starting element is activated, a protection time window is set from 0.5ms before activation to 1.5ms after activation. Within this window, the positive voltage u at the installation location of the protection device is read. p (k) Negative electrode voltage u n (k), positive current i p (k), negative electrode current i n (k) Compare these data with the average positive voltage u within 10ms before protection activation. p0 Average negative electrode voltage u n0 Average positive current i p0 Average negative current i n0 Subtract 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), the calculation formula is shown in equation (1):

[0056]

[0057] In the formula, k is the time sequence number and the fault occurrence time is defined as k = 0, 0 ≤ k ≤ N-1; N is the number of sampling points from 0.5 ms before the faulty component starts to 1.5 ms after the start.

[0058] Step 1.2: Calculate the line-mode voltage fault component Δu1(k) and the line-mode current fault component Δi1(k) within the time window. The calculation formula is shown in equation (2):

[0059]

[0060] Step 1.3: Calculate the traveling wave u of the line-mode voltage within the time window. f (k), the calculation formula is shown in equation (3):

[0061]

[0062] In the formula, Z c1 The impedance of the line mode wave is given.

[0063] Example 3

[0064] The specific process of step 2 is as follows:

[0065] The traveling wave u of the line-mode voltage 0.5ms before and 1.5ms after the faulty component starts is calculated according to the following formula (4). f (k) Normalized voltage u fN (k):

[0066]

[0067] In the formula, min(u b (k) is the minimum value within the time window; max(u) is the minimum value within the time window. b (k) is the maximum value within the time window.

[0068] Example 4

[0069] The specific process of step 3 is as follows:

[0070] Step 3.1: Set the sliding time window T = 0.2 ms, and apply the normalized voltage u obtained in Step 2... fN (k) Find the minimum point within the sliding time window, denoted as (k min u fmin );k minThe time series representing the minimum point, u fmin This represents the voltage amplitude at the minimum point.

[0071] Step 3.2: Find the maximum value point within the range of sampling times less than the minimum value point within the sliding window of step 3.1, denoted as (k). max u fmax ), k max The time series representing the maximum point, u fmax This represents the voltage amplitude at the maximum value point;

[0072] Step 3.3, calculate the maximum value u from step 3.2 according to the following formula (5). fmax Compared with the minimum value u in step 3.1 fmin The difference is greater than the set threshold u. set At this time, k max If the voltage drop point is found, proceed to step 4; otherwise, proceed to steps 3 and 4.

[0073] (u fmax -u fmin )>u set (5)

[0074] In the formula, u set To identify the voltage drop point threshold and avoid errors in voltage drop point identification due to voltage waveform fluctuations caused by factors such as traveling wave reflections and noise interference, an identification threshold value u is defined. set =0.2.

[0075] Step 3.4: After the sliding window T has traversed all the data within the time window, k is still not found. max When k is defined max If the maximum sampling point is found within the time window, proceed to step 4. If the sliding window has not traversed all the data within the time window, slide the sliding window forward by one sampling point and return to step 3.1.

[0076] As attached Figure 2 , Figure 3 As shown, Figure 2 For a boundary-bounded flexible DC power grid, current-limiting reactors are installed at the line outlet. Figure 3 In a boundaryless flexible DC power grid, current-limiting reactors are installed at the converter station outlet. Due to the disappearance of boundary elements, the traveling wave propagating in the opposite direction along the line at the fault point causes a voltage drop in the traveling wave waveform of the line-mode voltage. The time it takes for the traveling wave propagating in the opposite direction to reach the protection measurement point is directly related to the fault distance. Therefore, based on the distance characteristics of the voltage drop phenomenon, a single-ended quantity protection scheme for the boundaryless power grid is constructed.

[0077] As attached Figure 3The figure shows a simulation model of a boundaryless flexible DC power grid. In the figure, the parameters of current-limiting reactors L1 to L4 are all 0.2H. MMC1, MMC2, MMC3, and MMC4 are modular converters. P1 is the line boundary protection measuring point, and the direction of the protection measuring point towards the protected line is defined as the positive direction. The figure shows f... MMC1 f MMC2 f MMC3 f MMC4 These represent faults occurring at the output of the corresponding converter station, f Line11 f Line12 f Line13 These represent faults occurring at the beginning, middle, and end of Line 1, respectively. Line21 f Line22 f Line23 These represent faults occurring at the beginning, middle, and end of Line 2, respectively. Line31 f Line32 f Line33 These represent faults occurring at the beginning, middle, and end of Line 3, respectively. Line41 f Line42 f Line43 These represent faults occurring at the beginning, section, and end of Line 4, respectively. Note that at the line boundaries, the point closer to the protection measuring point P1 is considered the beginning, and the point farther from the protection measuring point is considered the end, regardless of the actual distance between the line boundaries and the protection measuring points. This system has a rated voltage of ±500kV, a rated transmission capacity of 3000MVA, and a total length of 207.9km for Line 1. It uses an overhead line frequency-varying parameter model, and its line surge impedances are Z... c0 =320Ω, Z c1 =260Ω. During system simulation, the sampling frequency was 100kHz, and a 1.2 / 50μs standard lightning current model was used to simulate a lightning strike. The entire text uses Line 1, measuring point P1 as an example; the fault distance is the distance from the fault location to measuring point P1, as shown in the attached figure. Figure 1 The identification process.

[0078] Example 5

[0079] like Figure 4 As shown, in Figure 3 In the boundless flexible DC power grid shown, when a positive ground fault with a transition resistance of 500Ω occurs 150km from the protection measuring point P1, the normalized waveform of the simulated line-mode voltage traveling wave measured at the protection measuring point is shown. After the protection element is activated, the line-mode voltage traveling wave data from 50 sampling points before activation and 150 sampling points after activation are read and normalized. k is calculated. max Value, k max >k set1 The fault was determined to be within the designated area.

[0080] Example 6

[0081] like Figure 5 As shown, in Figure 3 In the boundaryless flexible DC grid shown, when a positive ground fault with a transition resistance of 0Ω occurs at the outlet of the MMC2 converter station outside the zone, the normalized waveform of the simulated line-mode voltage traveling wave measured at the protection measuring point is shown. After the protection element is activated, the line-mode voltage traveling wave data from 50 sampling points before activation and 150 sampling points after activation are read and normalized. k is calculated. max Value, k set1 >k max >k set2 Based on the signal 0 from the opposite direction element, it is determined to be an external fault.

[0082] Example 7

[0083] like Figure 6 As shown, in Figure 3 In the boundaryless flexible DC power grid shown, when a positive ground fault with a transition resistance of 0Ω occurs at the beginning of Line 4, the reverse downstream line outside the protection zone, the normalized waveform of the simulated line-mode voltage traveling wave measured at the protection measuring point is shown. After the protection element is activated, the line-mode voltage traveling wave data from 50 sampling points before activation and 150 sampling points after activation are read and normalized. k is calculated. max Value, k max <k set2 The fault was determined to be outside the designated area.

[0084] To comprehensively verify the impact of fault distance, lightning strike, transition resistance, and system type on the discrimination results, a four-terminal boundaryless flexible DC ring network was used as an example. Lightning faults, grounding faults with transition resistances of 0Ω and 500Ω were respectively set, and the lightning interference identification method was verified based on the simulation results. set1 The threshold value is set to 89, k set2 The set threshold is 73.

[0085] Table 1 provides simulation data for a four-terminal, boundaryless flexible DC ring network at different fault distances, where all faults are metallic grounding faults. The data shows that faults inside and outside the fault zone can be distinguished at different fault distances.

[0086] Table 1 Simulation verification results for different fault distances

[0087]

[0088] Table 2 shows the simulation data for different types of faults in the four-terminal boundless flexible DC ring network. The data shows that faults inside and outside the zone can be distinguished under different fault types.

[0089] Table 2 Simulation verification results for different fault types

[0090]

[0091]

[0092] This invention normalizes the line-mode voltage traveling wave and distinguishes between internal and external faults by extracting the voltage drop point of the line-mode voltage traveling wave. Simultaneously, this method has the advantages of low computational complexity and fast response speed, meeting the real-time and high-precision requirements of flexible DC transmission systems. This invention utilizes the characteristic that the voltage drop of the voltage traveling wave changes with the fault distance to normalize the line-mode voltage reverse traveling wave, reducing the impact of transition resistance on the identification method, exhibiting strong anti-interference capabilities, and solving the defect of maloperation in the original method of constructing single-ended quantity protection using boundary conditions in boundaryless power grids.

Claims

1. A transmission line protection method applicable to boundaryless flexible DC ring networks, characterized in that: Specifically, the steps include the following: Step 1: Calculate the traveling wave of the line-mode voltage; Step 2: Perform minimum-maximum normalization on the line-mode voltage traveling wave from Step 1; Step 3: Based on the normalized line-mode voltage traveling wave obtained in Step 2, set a sliding time window T, and use the sliding time window to find the voltage drop point that meets the conditions for the normalized traveling wave. The specific process of step 3 is as follows: Step 3.1, Set the sliding time window T The normalized voltage obtained in step 2 Find the minimum point within the sliding time window, denoted as ( k min , u fmin ), in, k min The time series representing the minimum point, u fmin This represents the voltage amplitude at the minimum point. Step 3.2: Find the maximum value point within the range of sampling times less than the minimum value point within the sliding window of step 3.1, and denote it as ( k max , u fmax ),in, k max The time series representing the maximum point, u fmax This represents the voltage amplitude at the maximum value point; Step 3.3, calculate the maximum value of step 3.2 according to the following formula (1). u fmax Minimum value in step 3.1 u fmin The difference is greater than the set threshold. u set At this time k max If this is the voltage drop point, proceed to step 4; otherwise, proceed to steps 3 and 4. （1） In the formula, u set To identify the voltage drop threshold value; Step 3.4: After the sliding window T has traversed all the data within the time window, it still has not found the data. k max When, define k max If the maximum sampling point is within the time window, proceed to step 4. If the sliding window has not traversed all the data within the time window, slide the sliding window to the next sampling point and return to step 3.

1. Step 4: Compare the voltage drop point with the preset priority action threshold within the zone. k set1 and the threshold for priority actions outside the zone k set2 Compare the faults and identify whether they are faults within or outside the zone based on the comparison results; The specific process of step 4 is as follows: ... k max With the preset priority action threshold within the area k set1 and the threshold for priority actions outside the zone k set2 When comparing, k max ≥ k set1 When, it is identified as a fault within the area; when k max ≤ k set2 When, it is identified as an external fault; when k set1 > k max > k set2 When the fault type is determined, proceed to step 5 and determine the fault type based on the signal from the directional element device at the opposite end of the protection measuring point. In step 5, the fault type is determined based on the signal from the directional element device at the opposite end of the protection measuring point. When the directional element at the opposite end determines that the fault occurs in the direction of the protected line, it will send signal 1, which can be identified as an internal fault and send an action signal to the DC circuit breaker. The DC circuit breaker will then operate after receiving the protection action signal. When the directional element device at the opposite end determines that the fault occurs in the opposite direction of the protected line, it will send signal 0, which can be identified as an external fault.

2. The transmission line protection method applicable to boundaryless flexible DC ring networks 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, a protection time window is set from 0.5ms before activation to 1.5ms after activation. Within this window, the positive voltage at the installation location of the protection device is read. Negative voltage Positive current Negative current The values ​​were compared with the average positive voltage within 10 ms before the protection was activated. Average negative electrode voltage Average positive current Average negative current Subtracting the components, the positive voltage fault component is calculated using the following formula (2). Negative voltage fault component Positive current fault component With negative current fault component : （2） In the formula, k Define the time sequence number and the time of fault occurrence. k = 0, 0≤ k ≤ N -1; N This represents the number of sampling points from 0.5 ms before the faulty component starts up to 1.5 ms after it starts up; Step 1.2, calculate the line-mode voltage fault component within the time window using the following formula (3). Linear current fault component : （3） Step 1.3, calculate the forward wave of the line-mode voltage within the time window using the following formula (4). : （4） In the formula, The impedance of the line mode wave is given.

3. The transmission line protection method applicable to boundaryless flexible DC ring networks according to claim 2, characterized in that: The specific process of step 2 is as follows: The traveling wave of the line-mode voltage 0.5 ms before and 1.5 ms after the faulty component starts is calculated according to the following formula (5). Normalized voltage : （5） In the formula, It is the minimum value within the time window; It is the maximum value within the time window.

4. The transmission line protection method applicable to boundaryless flexible DC ring networks according to claim 3, characterized in that: In step 4, when a fault is identified within the zone, an action signal is sent to the DC circuit breaker, and the DC circuit breaker operates after receiving the protection action signal.

5. The transmission line protection method applicable to boundaryless flexible DC ring networks according to claim 4, characterized in that: In step 4, when an external fault is identified, the protection is reset.

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