Power transmission line protection method suitable for boundless flexible direct-current looped network

Abstract

The invention discloses a power transmission line protection method suitable for a boundless flexible direct-current looped network. The power transmission line protection method specifically comprises the following steps: step 1, calculating a line mode voltage forward wave; step 2, performing minimum-maximum normalization on the line mode voltage forward wave in the step 1; step 3, setting a sliding time window T according to the normalized line mode voltage forward wave obtained in the step 2, and searching a voltage drop point meeting a condition for the normalized forward wave by using the sliding time window; and 4, comparing the voltage drop point with a preset intra-region priority action threshold value kset1 and an extra-region priority action threshold value kset2, and identifying and judging an intra-region fault and an extra-region fault according to a comparison result. By adopting the method provided by the invention, the protection of the borderless flexible direct-current looped network power transmission line can be realized.

Description

Technical Field

[0001] The invention belongs to the technical field of electric power systems and relates to a power transmission line protection method suitable for a borderless flexible DC ring network. Background Art

[0002] Flexible DC transmission line protection methods have therefore become particularly important. They can not only ensure the effective grid connection of renewable energy and the stable operation of the power system, but also improve the system's adaptability to renewable energy fluctuations and ensure the continuity and safety of power supply. This 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, considering that the installation of current limiting reactors will increase system losses and reduce the overall efficiency of the transmission system, and the installation and maintenance of current limiting reactors in offshore or coastal areas is difficult, installing current limiting reactors at the line outlet may affect the stability and response speed of the system. The "bounded power grid" is bound to transform into a "boundless power grid" with current limiting reactors installed at the outlet of the converter station.

[0004] For the "boundaryless grid", the traditional boundary grid protection principle that uses components such as DC reactors as boundaries and combines the differences in high-frequency fault components inside and outside the fault boundary is no longer applicable. Boundaryless components will result in the differentiation of high-frequency fault components being no longer significant, which is also one of the natural defects based on the single-ended protection principle. Therefore, a boundaryless flexible DC ring network transmission line protection solution that does not rely on boundary components is urgently needed. Summary of the invention

[0005] The purpose of the present invention is to provide a power transmission line protection method applicable to a borderless flexible DC ring network, by which the power transmission line of the borderless flexible DC ring network can be protected.

[0006] The technical solution adopted by the present invention is a transmission line protection method applicable to a borderless flexible DC ring network, which specifically comprises the following steps:

[0007] Step 1, calculating the line mode voltage forward wave;

[0008] Step 2, performing minimum-maximum normalization on the line mode voltage forward wave of step 1;

[0009] Step 3: according to the normalized line mode voltage forward wave obtained in step 2, a sliding time window T is set, and the normalized forward wave is used to search for a voltage drop point that meets the conditions;

[0010] Step 4: compare the voltage drop point with the preset zone priority action threshold k set1 And the priority action threshold k outside the area set2 Compare and identify the faults inside and outside the area based on the comparison results.

[0011] The present invention is also characterized in that:

[0012] The specific process of step 1 is:

[0013] Step 1.1: After the protection start element is started, the time range from 0.5ms before start to 1.5ms after start is set as the protection time window. Within this window, read the positive voltage u at the installation location of the protection device. p (k), negative electrode voltage u n (k), positive electrode current i p (k), negative electrode current i n (k), respectively, and the average positive voltage u within 10ms before the protection is started p0 , negative electrode voltage average u n0 , average positive current i p0 , mean negative electrode current i n0 Subtract, through the following formula (1) the positive voltage fault component Δu p (k), negative pole voltage fault component Δu n (k), positive current fault component Δi p (k) and the negative current fault component Δi n (k):

[0014]

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

[0016] 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 using the following formula (2):

[0017]

[0018] Step 1.3, calculate the line mode voltage forward wave u in the time window by the following formula (3): f (k):

[0019]

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

[0021] The specific process of step 2 is:

[0022] According to the following formula (4), the line mode voltage forward wave u 0.5 ms before and 1.5 ms after the fault element is started is calculated as f (k) Normalized voltage ufN (k):

[0023]

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

[0025] The specific process of step 3 is:

[0026] Step 3.1, set the sliding time window T, and the normalized voltage u obtained in step 2 fN (k) Find the minimum point in the sliding time window, denoted as (k min ,u fmin ), where k min represents the time series of the minimum point, u fmin Represents the voltage amplitude at the minimum point;

[0027] Step 3.2: Find the maximum value within the range of the sampling time less than the minimum value in the sliding window of step 3.1, and record it as (k max ,u fmax ), where k max represents the time series of the maximum point, u fmax Represents the voltage amplitude at the maximum point;

[0028] Step 3.3, calculate the maximum value u of step 3.2 according to the following formula (5): fmax The minimum value u in step 3.1 fmin The difference is greater than the set threshold u set At this time, k max This is the voltage drop point, go to step 4, otherwise go to step 3.4:

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

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

[0031] Step 3.4: After sliding window T has traversed the data in the time window, it still does not find k max When k max is the maximum sampling point in the time window, and the process goes to step 4. When the sliding window has not traversed all the data in the time window, the sliding window slides back one sampling point and returns to step 3.1.

[0032] The specific process of step 4 is: max The preset priority action threshold k in the areaset1 And the priority action threshold k outside the area set2 For comparison, when k max ≥k set1 When k max ≤k set2 When k set1 >k max >k set2 When the protection point is in fault condition, 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 it is identified as an internal fault, an action signal is sent to the DC circuit breaker, and the DC circuit breaker is actuated after receiving the protection action signal.

[0034] In step 4, when it is identified as an out-of-zone fault, the protection is restored.

[0035] In step 4, according to the signal of 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 protection line, it will send signal 1. At this time, it can be identified as an internal fault and an action signal is sent to the DC circuit breaker. The DC circuit breaker is actuated 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 protection line, it sends signal 0 and identifies it as an external fault.

[0036] The beneficial effect of the present invention is that the present invention proposes a protection method suitable for borderless flexible DC ring network transmission lines, and constructs a single-ended protection method by utilizing the distance characteristics of the voltage drop time of the front-traveling wave of the fault line mode voltage of the borderless flexible DC ring network. The method has rapid recognition, strong transition resistance resistance capability, and can distinguish internal and external faults of fault traveling waves in various parts of the entire network. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 It is a flow chart of a power transmission line protection method applicable to a borderless flexible DC ring network of the present invention;

[0038] Figure 2 It is a topological diagram of a boundary flexible DC power grid model;

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

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

[0041] Figure 5In the simulation verification stage, the transmission line protection method of the present invention applicable to the borderless flexible DC ring network is used to obtain a voltage drop point identification diagram after the normalized waveform of the line mode voltage forward wave with a positive grounding fault at the exit of the forward MMC2 converter station outside the borderless flexible DC grid area and a transition resistance of 0Ω;

[0042] Figure 6 In the simulation verification stage, the transmission line protection method suitable for the borderless flexible DC ring network of the present invention is used to obtain a voltage drop point identification diagram after the normalized waveform of the line mode voltage forward wave with a positive pole grounding fault at the head end of the reverse lower-level line outside the borderless flexible DC grid area and a transition resistance of 0Ω. DETAILED DESCRIPTION

[0043] The present invention is described in detail below with reference to the accompanying drawings and specific embodiments.

[0044] Example 1

[0045] The present invention is applicable to a transmission line protection method for a borderless flexible DC ring network, and the process is as follows: Figure 1 As shown, the specific steps include:

[0046] Step 1: Take the 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), and then calculate the line mode voltage forward wave u based on this. f (k);

[0047] Step 2, performing minimum-maximum normalization on the line mode voltage forward wave of step 1;

[0048] Step 3: According to the normalized line-mode voltage forward wave obtained in step 2, set the sliding time window T. Find the minimum point of the normalized forward wave in the sliding time window, denoted as (k min ,u fmin ), k min represents the time series of the minimum point, u fmin represents the voltage amplitude at the minimum value point, and then find the maximum value point within the sliding window within the range where the sampling time is less than the minimum value point, which is recorded as (k max ,u fmax ), k max represents the time series of 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 That is the voltage drop point, the sliding time window stops sliding, if the maximum value point k is not found in the time window max , then k max Denote as the time window length;

[0049] Step 4: k max The preset priority action threshold k in the area set1 And the priority action threshold k outside the area set2 For comparison, when k max ≥k set1 When k max ≤k set2 When k set1 >k max >k set2 When , go to step 5;

[0050] According to the theoretical analysis results of the voltage forward wave when a fault occurs and the simulation test results, the priority threshold value k in the area is defined set1 The theoretical voltage drop point where the fault occurs at 95% of the protected line is defined as the out-of-zone priority threshold k. set2 It is the theoretical voltage drop point when the fault occurs outside the zone and is 5% of the positive voltage drop point of the downstream line.

[0051] Step 5: According to the signal of 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 protection line, it will send signal 1. At this time, it can be identified as an internal fault and an action signal is sent to the DC circuit breaker to enter step 6; when the directional element device at the opposite end determines that the fault occurs in the opposite direction of the protection line, it sends signal 0 and identifies it as an external fault.

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

[0053] Example 2

[0054] The specific process of step 1 is:

[0055] Step 1.1: After the protection start element is started, the time range from 0.5ms before start to 1.5ms after start is set as the protection time window. Within this window, read the positive voltage u at the installation location of the protection device. p (k), negative electrode voltage u n (k), positive electrode current i p (k), negative electrode current i n (k) These data are compared with the average positive voltage u within 10ms before the protection is activated. p0 , negative electrode voltage average u n0 , average positive current i p0 , mean negative current i n0 Subtract to calculate the positive voltage fault component Δu p (k), negative pole voltage fault component Δu n(k), positive current fault component Δi p (k) and the negative current fault component Δi n (k), the calculation formula is shown in formula (1):

[0056]

[0057] Wherein, k is the time sequence number and defines the fault occurrence time k=0, 0≤k≤N-1; N is the number of sampling points from 0.5ms before the fault component starts to 1.5ms 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 formula (2):

[0059]

[0060] Step 1.3, calculate the line mode voltage forward wave u in the time window f (k), the calculation formula is shown in formula (3):

[0061]

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

[0063] Example 3

[0064] The specific process of step 2 is:

[0065] According to the following formula (4), the line mode voltage forward wave u 0.5 ms before and 1.5 ms after the fault element is started is calculated as f (k) Normalized voltage u fN (k):

[0066]

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

[0068] Example 4

[0069] The specific process of step 3 is:

[0070] Step 3.1, set the sliding time window T = 0.2ms, and calculate the normalized voltage u obtained in step 2 fN (k) Find the minimum point in the sliding time window, denoted as (k min ,u fmin );k minrepresents the time series of the minimum point, u fmin Represents the voltage amplitude at the minimum point;

[0071] Step 3.2: Find the maximum value within the range of the sampling time less than the minimum value in the sliding window of step 3.1, and record it as (k max ,u fmax ), k max represents the time series of the maximum point, u fmax Represents the voltage amplitude at the maximum point;

[0072] Step 3.3, calculate the maximum value u of step 3.2 according to the following formula (5): fmax The minimum value u in step 3.1 fmin The difference is greater than the set threshold u set At this time, k max That is the voltage drop point, go to step 4, otherwise go to step 3.4;

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

[0074] In the formula, u set In order to identify the voltage drop point threshold value and avoid the voltage drop point identification error caused by voltage waveform fluctuations caused by wave refraction reflection, noise interference and other factors, the identification threshold value u is defined set =0.2.

[0075] Step 3.4: After sliding window T has traversed the data in the time window, it still does not find k max When k max is the maximum sampling point in the time window, and the process goes to step 4. When the sliding window has not traversed all the data in the time window, the sliding window slides back one sampling point and returns to step 3.1.

[0076] As attached Figure 2 , Figure 3 As shown, Figure 2 For a flexible DC grid with boundaries, the current limiting reactor is installed at the line outlet. Figure 3 For a borderless flexible DC grid, the current limiting reactor is installed at the exit of the converter station. Due to the disappearance of boundary elements, the traveling wave propagating in the reverse direction along the line at the fault point will cause a voltage drop in the waveform of the line mode voltage. The time for the traveling wave propagating in the reverse direction to reach the protection measurement point is directly related to the distance of the fault. Therefore, based on the distance characteristics of the voltage drop phenomenon, a single-ended protection scheme for a borderless grid is constructed.

[0077] As attached Figure 3As shown in the figure, it is a simulation model diagram of a certain borderless flexible DC power grid. The parameters of the current limiting reactors L1~L4 in the figure are all 0.2H, MMC1, MMC2, MMC3, and MMC4 are modular converters, P1 is the line boundary protection measurement point, and it is stipulated that the protection measurement point is facing the protected line in the positive direction. MMC1 、f MMC2 、f MMC3 、f MMC4 They represent the faults occurring at the corresponding converter station exits, f Line11 、f Line12 、f Line13 Respectively represent the faults at the beginning, section, and end of Line 1. Line21 、f Line22 、f Line23 Respectively represent the faults at the beginning, section, and end of Line2, f Line31 、f Line32 、f Line33 They represent the faults at the beginning, section, and end of Line 3, respectively. Line41 、f Line42 、f Line43 They represent the faults at the beginning, section, and end of Line 4. Note that the line boundary is close to the protection measuring point P1 in the topological structure as the beginning, and far away from the protection measuring point as the end. It has nothing to do with the actual distance between the two ends of the line boundary and the protection measuring point. The rated voltage of this system is ±500kV, the rated transmission capacity is 3000MVA, the total length of the transmission line Line 1 is 207.9km, and the overhead line frequency variable parameter model is adopted. The line wave impedance is Z c0 =320Ω、Z c1 =260Ω. When simulating the system, the sampling frequency is 100kHz, and the 1.2 / 50μs standard lightning current model is used to simulate lightning strikes. The full text takes the line Line1, P1 measuring point as an example, and the fault distance is the distance from the fault point to the measuring point P1. Figure 1 identification process.

[0078] Example 5

[0079] like Figure 4 As shown, in Figure 3 In the borderless flexible DC grid shown in the figure, when a positive ground fault with a transition resistance of 500Ω occurs 150km away from the protection measuring point P1, the line mode voltage forward wave simulation value measured at the protection measuring point is normalized. After the protection element is started, the line mode voltage forward wave data of the 50 sampling points before the start and the 150 sampling points after the start are read for normalization. Calculate k max Value, k max >k set1 It is determined to be an in-zone fault.

[0080] Example 6

[0081] like Figure 5 As shown, in Figure 3 In the borderless flexible DC grid shown in the figure, when a positive grounding fault with a transition resistance of 0Ω occurs at the exit of the forward MMC2 converter station outside the zone, the normalized waveform of the line mode voltage forward wave simulation value measured at the protection measuring point. After the protection element is started, the line mode voltage forward wave data of the 50 sampling points before the start and the 150 sampling points after the start are read for normalization. Calculate k max Value, k set1 >k max >k set2 , combined with the opposite end direction element signal 0, it is judged to be an out-of-zone fault.

[0082] Example 7

[0083] like Figure 6 As shown, in Figure 3 In the borderless flexible DC grid shown in the figure, when a positive grounding fault with a transition resistance of 0Ω occurs at the head end of the reverse lower-level line Line4 outside the area, the normalized waveform of the line mode voltage forward wave simulation value measured at the protection measuring point. After the protection element is started, the line mode voltage forward wave data of the 50 sampling points before the start and the 150 sampling points after the start are read for normalization. Calculate k max Value, k max <k set2 , it is judged as an out-of-zone fault.

[0084] In order to fully verify the influence of fault distance, lightning strike, transition resistance and system type on the identification results, a four-terminal borderless flexible DC ring network is taken as an example, and lightning strike fault, transition resistance of 0Ω and grounding fault of 500Ω are set respectively, and the lightning strike interference identification method is verified according to the simulation results. set1 The setting threshold is 89, k set2 The setting threshold is 73.

[0085] From Table 1, we can get the simulation data of the four-terminal borderless flexible DC ring network at different fault distances, where all faults are metallic grounding faults. From the data, we can see that: at different fault distances, the internal and external faults can be distinguished.

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

[0087]

[0088] From Table 2, we can get the simulation data of the four-terminal borderless flexible DC ring network under different types of faults. From the data, we can get that: under different fault types, internal and external faults can be distinguished.

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

[0090]

[0091]

[0092] The present invention normalizes the line-mode voltage forward wave, and distinguishes the faults inside and outside the zone by extracting the voltage drop point of the line-mode voltage forward wave. At the same time, this method has the advantages of low computational complexity and fast response speed, and can meet the requirements of flexible direct current transmission system for real-time and high precision. The present invention utilizes the characteristic that the voltage drop phenomenon of the voltage forward wave changes with the fault distance, normalizes the line-mode voltage reverse wave, reduces the influence of the transition resistance on the identification method, has strong anti-interference ability, and solves the defect of false operation of the original single-ended quantity protection method constructed by using boundary conditions in the borderless power grid.

Claims

1. A power transmission line protection method applicable to a borderless flexible DC ring network, characterized in that: The specific steps include: Step 1, calculating the line mode voltage forward wave; Step 2, performing minimum-maximum normalization on the line mode voltage forward wave of step 1; Step 3: according to the normalized line mode voltage forward wave obtained in step 2, a sliding time window T is set, and the normalized forward wave is used to search for a voltage drop point that meets the conditions; Step 4: compare the voltage drop point with the preset zone priority action threshold k set1 And the priority action threshold k outside the area set2 Compare and identify the faults inside and outside the area based on the comparison results.

2. The power transmission line protection method applicable to a borderless flexible DC ring network according to claim 1 is characterized in that: The specific process of step 1 is as follows: Step 1.1: After the protection start element is started, the time range from 0.5ms before start to 1.5ms after start is set as the protection time window. Within this window, read the positive voltage u at the installation location of the protection device. p (k), negative electrode voltage u n (k), positive electrode current i p (k), negative electrode current i n (k), respectively, and the average positive voltage u within 10ms before the protection is started p0 , negative electrode voltage average u n0 , average positive current i p0 , mean negative electrode current i n0 Subtract, through the following formula (1) the positive voltage fault component Δu p (k), negative pole voltage fault component Δu n (k), positive current fault component Δi p (k) and the negative current fault component Δi n (k): Where k is the time sequence number and defines the fault occurrence time k=0, 0≤k≤N-1; N is the number of sampling points from 0.5ms before the fault component starts to 1.5ms after the start; 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 using the following formula (2): Step 1.3, calculate the line mode voltage forward wave u in the time window by the following formula (3): f (k): In the formula, Z c1 is the line mode wave impedance of the line.

3. The power transmission line protection method applicable to a borderless flexible DC ring network according to claim 2 is characterized in that: The specific process of step 2 is: According to the following formula (4), the line mode voltage forward wave u 0.5 ms before and 1.5 ms after the fault element is started is calculated as f (k) Normalized voltage u fN (k): In the formula, min(u b (k)) is the minimum value in the time window; max(u b (k)) is the maximum value within the time window.

4. The power transmission line protection method applicable to a borderless flexible DC ring network according to claim 3 is characterized in that: The specific process of step 3 is as follows: Step 3.1, set the sliding time window T, and the normalized voltage u obtained in step 2 fN (k) Find the minimum point in the sliding time window, denoted as (k min ,u fmin ), where k min represents the time series of the minimum point, u fmin Represents the voltage amplitude at the minimum point; Step 3.2: Find the maximum value within the range of the sampling time less than the minimum value in the sliding window of step 3.1, and record it as (k max ,u fmax ), where k max represents the time series of the maximum point, u fmax Represents the voltage amplitude at the maximum point; Step 3.3, calculate the maximum value u of step 3.2 according to the following formula (5): fmax The minimum value u in step 3.1 fmin The difference is greater than the set threshold u set At this time, k max This is the voltage drop point, go to step 4, otherwise go to step 3.4: (in fmax -in fmin )>in set (5) In the formula, u set To identify the voltage drop point threshold; Step 3.4: After sliding window T has traversed the data in the time window, it still does not find k max When k max is the maximum sampling point in the time window, and the process goes to step 4. When the sliding window has not traversed all the data in the time window, the sliding window slides back one sampling point and returns to step 3.

1.

5. The power transmission line protection method applicable to a borderless flexible DC ring network according to claim 4 is characterized in that: The specific process of step 4 is: max The preset priority action threshold k in the area set1 And the priority action threshold k outside the area set2 For comparison, when k max ≥k set1 When k max ≤k set2 When k set1 >k max >k set2 When the protection point is in fault condition, the fault type is determined based on the signal from the directional element device at the opposite end of the protection measuring point.

6. The power transmission line protection method applicable to a borderless flexible DC ring network according to claim 5 is characterized in that: In the step 4, when it is identified as an internal fault, an action signal is sent to the DC circuit breaker, and the DC circuit breaker is actuated after receiving the protection action signal.

7. The power transmission line protection method applicable to a borderless flexible DC ring network according to claim 5 is characterized in that: In step 4, when it is identified as an out-of-zone fault, the protection is restored.

8. The power transmission line protection method applicable to a borderless flexible DC ring network according to claim 5 is characterized in that: In step 4, according to the signal of 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 protection line, it will send signal 1. At this time, it can be identified as an internal fault and an action signal is sent to the DC circuit breaker. The DC circuit breaker is actuated 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 protection line, it sends signal 0 and identifies it as an external fault.

Images