Flexible dc grid single-ended line protection method based on time-domain voltage traveling wave

Abstract

The present application relates to the flexible DC power grid single-ended line protection method based on time domain voltage traveling wave, according to the difference of voltage traveling wave reflection coefficient under the positive and negative direction short circuit fault or lightning disturbance, the direction criterion is constituted, and the protection misoperation under the reverse direction fault or lightning is avoided, second, the function model that the first traveling wave of protection fault voltage satisfies under the short circuit fault in the area and the positive direction non-area short circuit fault is analyzed by using voltage traveling wave transfer function, according to the different matching degree between the first traveling wave of fault and the reference function model, the complete line single-ended protection method is formed by combining the starting criterion and the pole selection criterion.The present application builds the flexible DC power grid model on the PSCAD platform to verify the protection performance, and a large number of simulation results show that the proposed protection has high reliability and can tolerate 500Ω transition resistance.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of power grid line protection, in particular to a flexible DC power grid single-ended line protection method based on time-domain voltage traveling wave. BACKGROUND

[0002] The flexible DC power grid based on modular multilevel converter has become one of the development trends of future smart grid due to its low harmonic content, no commutation failure problem, independent control of reactive power and active power, and low operating loss. At present, overhead lines have become the main power transmission mode of flexible DC power grid, and the probability of DC line fault is high. Due to the small inertia and low damping of flexible DC power grid, the fault current has large amplitude and fast rising speed, which will endanger the entire power grid within a few milliseconds, which puts extremely strict requirements on the speed of line protection. The protection needs to complete the export within 3ms to ensure the safe and stable operation of the entire system. Therefore, it is an urgent need for the development of DC power grid to study line protection that can quickly and accurately identify faults.

[0003] According to whether a communication channel is needed, flexible DC power grid line protection can be divided into two categories. One is double-ended protection, which needs to exchange information at both ends of the line to realize fault identification. The prior art proposes a flexible DC power grid pilot protection based on linear regression, which uses the regression coefficient of the current at both ends of the line within a short time window to identify faults and select poles. The prior art proposes a DC transmission line pilot protection based on F-test of low-frequency fault component, which uses the difference in the change trend of fault currents at both ends of the line under internal and external faults to identify faults. The prior art proposes a high-voltage DC line pilot protection based on transient energy ratio of DC filter link, which uses the energy ratio value on both sides of the line boundary to distinguish internal and external faults. However, in long-distance power transmission, the large communication delay will significantly reduce the action speed of double-ended protection. At this time, only single-ended protection is expected to meet the requirements of flexible DC power grid on protection speed.

[0004] Another type is single-ended protection, which only needs line single-ended information to realize fault identification. The prior art uses the difference between the Hilbert energy amplitude and the waveform information under internal and external faults to construct a fault identification criterion, but this method is for ultra-high voltage direct current transmission systems and is difficult to apply in flexible direct current grids; the prior art designs a time-domain single-ended protection based on the concave-convex of the voltage integral of the current limiting reactor according to the significant difference in the integral of the line mode voltage under internal and external faults, but this protection is for bipolar flexible direct current transmission systems and its adaptability in flexible direct current grids remains to be verified; the prior art proposes a line single-ended protection based on the first traveling wave curvature according to the difference between the first traveling wave curvature under internal and external faults, which has improved the transition resistance resistance, but the protection may malfunction under reverse direction faults; the prior art uses the attenuation effect of the current limiting reactor on the high-frequency component of the transient voltage to construct an internal and external fault identification method based on high-frequency transient energy, which can tolerate a transition resistance of 200Ω, but does not consider the influence of lightning interference; the prior art proposes a new fault identification method based on the model identification idea according to the matching degree of the internal and external fault models and the measured electrical quantities, but this protection does not consider the influence of lightning interference. The above prior arts have studied and improved the single-ended protection of direct current lines, but have not well solved the problems of internal remote high-resistance fault rejection and lightning interference malfunction. SUMMARY

[0005] The purpose of the present application is to overcome the shortcomings of the prior art and propose a flexible direct current grid single-ended line protection method based on time-domain voltage traveling waves, which forms a complete single-ended line protection scheme through a starting criterion and a pole selection criterion.

[0006] The present application solves its technical problems by adopting the following technical solutions:

[0007] The flexible direct current grid single-ended line protection method based on time-domain voltage traveling waves comprises the following steps:

[0008] Step 1, measuring the positive voltage, negative voltage and current data at the protection measuring point, and calculating the fault component line mode voltage;

[0009] Step 2, constructing a starting criterion, and determining whether the fault component line mode voltage meets the starting criterion, if yes, proceeding to step 3, otherwise returning to step 1;

[0010] Step 3, calculating the forward traveling wave and the reverse traveling wave of the fault component line mode voltage;

[0011] Step 4, calculating the direction discrimination factor according to the forward traveling wave and the reverse traveling wave;

[0012] Step 5, constructing a direction criterion, and determining whether the direction discrimination factor is a positive direction short-circuit fault, if yes, proceeding to step 6, otherwise it is a reverse direction short-circuit fault or lightning interference, and resetting the protection;

[0013] Step 6, calculating fitting error;

[0014] Step 7, constructing identification criterion, judging whether the fitting error meets the identification criterion, if yes, it is short-circuit fault in area, and proceeding to Step 8, otherwise, it is positive direction non-short-circuit fault in area;

[0015] Step 8, calculating polar selection factor;

[0016] Step 9, constructing polar selection criterion, judging whether it is positive pole fault, bipolar fault or negative pole fault according to the relationship between the polar selection factor and the polar selection criterion.

[0017] Moreover, the calculation method of the fault component line mode voltage in Step 1 is:

[0018]

[0019] Wherein, u pij , u nij are the positive pole voltage and the negative pole voltage measured at the protection measuring point P ij , U0 is the line voltage (500 kV) when the system is normally operated, Δu pij , Δu nij are the positive pole fault voltage component and the negative pole fault voltage component at P ij , Δu 0ij , Δu 1ij are the ground mode fault voltage component and the line mode fault voltage component at P ij , which can be obtained according to the positive pole fault voltage component and the negative pole fault voltage component u pij , u nij at the protection measuring point, and the fault component line mode voltage Δu 1ij at the protection measuring point.

[0020] Moreover, the starting criterion constructed in Step 2 is:

[0021] |Δu 1ij (i)|>Δ set

[0022] Wherein, Δu 1ij is the fault component line mode voltage at the protection measuring point P ij , and Δ set is the starting criterion setting value, which is set according to the maximum unbalanced fault component line mode voltage occurring during normal operation.

[0023] Moreover, the calculation method of the direction discrimination factor in Step 4 is:

[0024]

[0025] Wherein, W is the direction discrimination factor, and Δu qijTo protect the voltage forward wave of measuring point P ij fij To protect the voltage reverse wave of measuring point P ij ; N is the number of sampling points of voltage forward wave in 0.5 ms time window after starting, the calculation formula of forward and reverse wave is:

[0026]

[0027] Where, i pij , i nij are the positive and negative currents measured at the protection measuring point P ij ; I 0ij is the current at the protection measuring point P ij under normal system operation, Δi pij , Δi nij are the positive and negative fault current components at P ij ; Δi 0ij , Δi 1ij are the ground mode and line mode fault current components at P ij ; Δu 1ij is the line mode fault voltage component at P ij ; Z c1 is the 1-mode voltage forward wave impedance of the line, Δu qij , Δu fij are the forward and reverse wave of 1-mode fault voltage component at P ij .

[0028] Moreover, the direction criterion constructed in step 5 is:

[0029]

[0030] Where, W is the direction discrimination factor, W set is the direction criterion setting value.

[0031] Moreover, the calculation method of fitting error in step 6 is:

[0032]

[0033]

[0034] Where, X is the fitting error and also the identification factor, the size of which is the 2-norm of residual (Δu fij -y); Δu fij is the fault first forward wave in T time with the time t1 when the reverse wave first arrives at the protection as the starting point; T is the length of time window, t1 is the time when the reverse wave first arrives, t2 is the time when the second arrives, recorded as; y=y(t) is the fitting function obtained by least square method.

[0035] ​And, the step 7 constructs the identification criterion as:

[0036]

[0037] Wherein, X is fitting error and identification factor; X set Is identification criterion setting value, which is set according to the maximum value of residual 2 norm when short-circuit fault in the zone is avoided.

[0038] And, the calculation method of the pole selection factor in the step 8 is:

[0039]

[0040] Wherein, Y is pole selection factor, Δu pij Is positive fault component voltage at the protection measuring point P ij , Δu nij Is negative fault component voltage at the protection measuring point P ij , and N is the number of voltage wave sampling points in the 0.5ms time window after starting.

[0041] And, the pole selection criterion constructed in the step 9 is:

[0042]

[0043] Wherein, PGF is positive fault, PNF is bipolar fault, NGF is negative fault, and Y set Is setting value of the pole selection criterion.

[0044] The advantages and positive effects of the present application are:

[0045] The present application constructs the direction criterion according to the difference between voltage wave reflection coefficient under positive and reverse direction short-circuit fault or lightning disturbance, to avoid protection misoperation under reverse direction fault or lightning; secondly, the function model satisfied by the first wave of fault voltage at the protection under in-zone short-circuit fault and positive direction non-in-zone short-circuit fault is analyzed by using voltage wave transfer function, and the complete line single-ended quantity protection method is formed by combining the starting criterion and the pole selection criterion according to the difference between the matching degree between the first wave of fault and the reference function model. The present application builds flexible HVDC power grid model on PSCAD platform to verify protection performance, and a large number of simulation results show that the proposed protection is suitable for line main protection, meets the requirement of HVDC power grid on protection speed, has strong reliability and selectivity, can accurately identify when short-circuit fault or lightning disturbance occurs at various positions, can tolerate 500Ω transition resistance, and theoretical analysis shows that the present application has strong adaptability to power grid operation mode. BRIEF DESCRIPTION OF DRAWINGS

[0046] Figure 1 The flow chart of the present application;

[0047] Figure 2 The structure diagram of the flexible DC power grid simulation model built for the embodiment of the present application is shown in the figure;

[0048] Figure 3 The schematic diagram of the DC line model used in the embodiment of the present application is shown in the figure;

[0049] Figure 4 The schematic diagram of the simulation result of the reverse direction fault or lightning bipolar fault in the embodiment of the present application is shown in the figure;

[0050] Figure 5 The schematic diagram of the simulation result of the reverse direction fault or lightning interference in the embodiment of the present application is shown in the figure;

[0051] Figure 6 The schematic diagram of the simulation result of the positive direction out-of-zone fault bipolar fault in the embodiment of the present application is shown in the figure;

[0052] Figure 7 The schematic diagram of the simulation result of the positive direction lightning interference in-zone lightning interference in the embodiment of the present application is shown in the figure;

[0053] Figure 8 The schematic diagram of the simulation result of the bipolar short circuit fault in-zone bipolar fault in the embodiment of the present application is shown in the figure;

[0054] Figure 9 The schematic diagram of the simulation result of the single pole grounding fault in-zone positive pole fault in the embodiment of the present application is shown in the figure. DETAILED DESCRIPTION

[0055] The present application is further described in detail below with reference to the accompanying drawings.

[0056] The flexible DC power grid single-end line protection method based on time domain voltage traveling wave, as shown in the figure, comprises the following steps: Figure 1

[0057] Step 1, measuring the positive pole voltage, negative pole voltage and current data at the protection measuring point, and calculating the fault component line mode voltage.

[0058] The calculation method of the fault component line mode voltage is as follows:

[0059]

[0060] Wherein, u pij , u nij are the positive pole voltage and negative pole voltage measured at the protection measuring point P ij , U0 is the line voltage (500 kV) when the system is normally operated, Δu pij , Δu nij are the positive pole fault voltage component and negative pole fault voltage component at P ij , Δu 0ij , Δu 1ij are the positive pole fault voltage component and negative pole fault voltage component at P ij ​The fault component line mode voltage Δu pij , u nij at the protection measuring point P 1ij is obtained according to the positive and negative fault voltage components u 1ij , u set at the protection measuring point P 1ij .

[0061] Step 2, a starting criterion is constructed, and it is judged whether the fault component line mode voltage satisfies the starting criterion, if yes, step 3 is performed, otherwise, step 1 is returned.

[0062] The starting criterion is:

[0063] |Δu 1ij (i)|>Δ set

[0064] wherein, Δu 1ij is the fault component line mode voltage at the protection measuring point P ij ; Δ set is the starting criterion setting value, which is set according to the maximum unbalanced fault component line mode voltage appearing in normal operation.

[0065] Step 3, the forward and reverse traveling waves of the fault component line mode voltage are calculated.

[0066] Step 4, the direction discrimination factor is calculated according to the forward and reverse traveling waves.

[0067] The calculation method of the direction discrimination factor is:

[0068]

[0069] wherein, W is the direction discrimination factor, Δu qij is the voltage forward traveling wave at the protection measuring point P ij , Δu fij is the voltage reverse traveling wave at the protection measuring point P ij ; N is the sampling point number of the voltage traveling wave in the 0.5 ms time window after starting. The calculation formula of the forward and reverse traveling waves is:

[0070]

[0071] wherein, i pij , i nij are the positive and negative currents measured at the protection measuring point P ij , I 0ij is the current at the protection measuring point P ij in normal operation of the system, Δi pij , Δi nij are the positive and negative fault current components at P ij , Δi 0ij , Δi 1ij are the positive and negative current differences at P ijThe fault current component at the place of the line mode, Δu 1ij For P ij The fault voltage component at the place of the line mode, Z c1 The 1-mode voltage traveling wave impedance of the line, Δu qij , Δu fij For P ij The fault component 1-mode voltage forward and reverse traveling wave at the place.

[0072] Step 5, construct the direction criterion, and judge whether the direction discrimination factor is a positive direction short-circuit fault, if it is a positive direction short-circuit fault, proceed to step 6, otherwise it is a reverse direction short-circuit fault or lightning disturbance, and reset the protection.

[0073] The direction criterion is:

[0074]

[0075] Wherein, W is the direction discrimination factor, W set is the direction criterion setting value.

[0076] Step 6, calculate the fitting error.

[0077] The calculation method of the fitting error is:

[0078]

[0079]

[0080] Wherein, X is the fitting error and also the identification factor, and its size is the 2-norm of the residual (Δu fij -y); Δu fij is the fault first traveling wave within T time with the time t1 when the reverse traveling wave first arrives at the protection as the starting point; T is the time window length, t1 is the time when the reverse traveling wave first arrives, t2 is the time when the second arrives, recorded as; y=y(t) is the fitting function obtained by the least square method.

[0081] Step 7, construct the identification criterion, judge whether the fitting error meets the identification criterion, if it meets, it is an intra-zone short-circuit fault, and proceed to step 8, otherwise it is a positive direction non-intra-zone short-circuit fault.

[0082] The identification criterion is:

[0083]

[0084] Wherein, X is the fitting error and also the identification factor; X set is the identification criterion setting value, which is set according to the maximum value of the 2-norm of the residual when the intra-zone short-circuit fault is avoided.

[0085] Step 8, calculate the pole selection factor.

[0086] The method for calculating the polarity selection factor is as follows:

[0087]

[0088] Where Y is the polarity selection factor, Δu pij To protect measuring point P ij The positive voltage of the fault component at the location, Δu nij To protect measuring point P ij The fault component is the negative voltage at the location, and N is the number of sampling points of the voltage traveling wave within a 0.5ms time window after startup.

[0089] Step 9: Construct the polarity selection criterion and determine whether the fault is positive, bipolar, or negative based on the relationship between the polarity selection factor and the polarity selection criterion.

[0090] The selection criterion is:

[0091]

[0092] Among them, PGF represents positive electrode fault, PNF represents bipolar fault, NGF represents negative electrode fault, and Y represents positive electrode fault. set This is the setpoint value for the polarity selection criterion.

[0093] Based on the aforementioned flexible DC grid single-ended line protection method based on time-domain voltage traveling waves, a system is built in PSCAD as follows: Figure 2 The simulation model of the flexible DC power grid shown is based on P. 11 Taking the protection as an example, the performance of the proposed single-ended quantity protection is verified by adopting the following method: Figure 3 The circuit model shown is configured to have a short-circuit fault or lightning strike occurring at "time 0", with a sampling frequency of 50kHz.

[0094] Due to the nonlinearity of DC systems, it is generally difficult to set the DC protection settings through simple calculations. Digital simulation is typically used to determine the settings for various protections. Table 1 shows the setting values ​​for each protection criterion after traversing relevant short-circuit faults or lightning interference.

[0095] Table 1 Protection Criterion Setting Values

[0096]

[0097] 1. Short-circuit fault analysis outside the zone

[0098] for Figure 2 Installed in P 11 For protection purposes, non-zone short-circuit faults mainly include: reverse short-circuit faults (f5~f7) or lightning interference (f5~f7), forward-direction out-of-zone short-circuit faults (f2~f4), and forward-direction lightning interference (f1~f4). In these cases, the protection should reliably not operate. According to... Figure 1The protection process shown above examines the protection performance under the above-mentioned conditions.

[0099] Lightning interference is considered in the case of a common lightning strike to the positive line, and the widely accepted 1.2 / 50µs double-exponential lightning current is used for simulation.

[0100] 1.1 Reverse direction fault or lightning strike

[0101] When a metallic double-pole short-circuit fault occurs at point f5 (midpoint of line 4), the protection measuring point P... 11 Voltage at the location, such as Figure 4 As shown in (a), the traveling wave quantity is as follows: Figure 4 As shown in (b).

[0102] Depend on Figure 4 It can be seen that the fault component line-mode voltage is greater than the set value of 10kV at 0.16ms, triggering the protection. Using a data window of 0.5ms following the start time of 0.16ms, the calculated direction discrimination factor W is 15.60, which is greater than 1.2. Therefore, according to... Figure 1 If the protection system determines that the fault is in the opposite direction or that a lightning strike has occurred, no identification or polarity selection is required.

[0103] When a lightning strike occurs at point f5 (midpoint of line 4), the protection measuring point P... 11 Voltage at the location, such as Figure 5 As shown in (a), the traveling wave quantity is as follows: Figure 5 As shown in (b), the protection starts at 0.18ms, the direction discrimination factor W is 201.92, and the protection reliably does not operate.

[0104] When other types of short circuits or lightning strikes occur in the opposite direction, the direction discrimination factor W at the protection installation location is greater than the set value of 1.2, indicating that it is a short circuit or lightning strike interference in the opposite direction, and the protection reliably does not operate. The corresponding simulation results are shown in Table 2.

[0105] Table 2 Simulation results of reverse faults or lightning strikes

[0106]

[0107] 1.2. Fault outside the positive direction zone

[0108] When a metallic double-pole short-circuit fault occurs at point f4, the fault point is located at the beginning of line 2, and the protection measuring point P... 11 The fault component line-mode voltage of the protection is as follows Figure 6 As shown in (a), the protection measuring point P 11 The fault component of the protection line-mode voltage before and after the reverse traveling wave, such as Figure 6 As shown in (b), the protection measuring point P 11 The fault component line-mode voltage initial reverse traveling wave and curve fitting of the protection are as follows: Figure 6 As shown in (c).

[0109] from Figure 6 (a) It can be seen that the fault component line-mode voltage satisfies the start-up criterion at 0.64ms, and the protection starts; from Figure 6 (b) uses data from 0.5ms after startup to calculate the direction discrimination factor W as 0.46, which is determined to be a short circuit in the positive direction or a lightning strike; Figure 6 (c) Based on the data from 0.3ms before startup and 0.5ms after startup, the identification criterion data window is determined to be 0.5ms (starting from 0.62ms) using wavelet transform modulus maxima, and curve fitting is performed. The identification factor X is calculated to be 17.68, which is greater than the identification criterion tuning value X. set , to protect against action.

[0110] When a short-circuit fault occurs at other locations outside the positive direction zone, the simulation results are shown in Table 3. It is easy to see from this that the protection reliably does not operate for various faults outside the positive direction zone.

[0111] Table 3 Simulation results of faults outside the positive direction zone

[0112]

[0113]

[0114] 1.3. Forward lightning interference

[0115] When a lightning strike occurs at the midpoint (f1) of line 1, the protection measuring point P... 11 The fault component line-mode voltage is as follows Figure 7 As shown in (a), the protection measuring point P 11 Before the fault component line-mode voltage, the reverse traveling wave is as follows Figure 7 As shown in (b), the protection measuring point P 11 The initial reverse traveling wave and curve fitting of the fault component line-mode voltage are as follows: Figure 7 As shown in (c), the protection starts at 0.32ms. The direction discrimination factor W is calculated to be 0.47 based on the data within 0.5ms after startup. The identification criterion time window length is 0.5ms obtained by wavelet transform modulus maxima method. Curve fitting is performed using the data within this time period, and the identification factor X is 125.6. According to the action logic, the protection is reliable and does not operate.

[0116] When lightning strikes occur at other locations in the positive direction, the corresponding simulation results are shown in Table 4. Based on the protection's operating logic, the judgment results are all short-circuit faults outside the positive direction zone, meaning that the protection reliably does not operate under positive direction lightning strike interference.

[0117] Table 5 Simulation results of forward-direction lightning interference

[0118]

[0119] From the above analysis, the proposed protection will not misoperate under lightning disturbance, reverse direction short circuit and positive direction out-of-zone short circuit.

[0120] 2. In-zone short circuit fault analysis

[0121] When line 1 has a short circuit fault, the corresponding protection should act reliably. The following analyzes the protection action from two aspects of bipolar short circuit fault and single pole grounding fault.

[0122] 2.1. Bipolar short circuit fault

[0123] Assume that a metallic bipolar short circuit fault occurs in the zone, the distance from the fault point to P 11 is 25% of the full length of line 1, then the fault component line mode voltage measured at the protection is as shown in Figure 8 (a), the initial reverse traveling wave of the fault component line mode voltage measured at the protection is as shown in Figure 8 (b), the initial reverse traveling wave of the fault component line mode voltage measured at the protection and the curve fitting are as shown in Figure 8 (c), and the positive and negative pole voltages of the fault component measured at the protection are as shown in Figure 8 (d).

[0124] From Figure 8 (a), the absolute value of the fault component line mode voltage is greater than 10 kV 0.16 ms after the fault occurs, which meets the starting criterion, and the protection starts; the data window of 0.5 ms after starting (0.16-0.66 ms), the direction discrimination factor W is 0.58, which is judged as a positive direction fault or lightning, as shown in Figure 8 (b); Figure 8 (c), in which the data from 0.3 ms before starting to 0.5 ms after starting (-0.14-0.66 ms), the fault voltage reverse traveling wave arrival time t1=0.14 ms, t2=0.44 ms is obtained by the wavelet transform modulus maximum value method, the recognition criterion time window length T=0.3 ms (0.14-0.44 ms), and the recognition factor X=3.41 is calculated by curve fitting, and the protection judges that it is an in-zone short circuit fault; according to Figure 8 (d), the positive and negative pole voltages of the fault component in the data window of 0.5 ms after starting (0.16-0.66 ms) are used to obtain the pole selection factor Y=1.00, which is judged as a bipolar short circuit fault, and the protection sends a trip command to the circuit breaker. The above description is a complete protection process.

[0125] By changing the transition resistance and fault location, the transition resistance of bipolar short circuit fault will not be very large, here 0 Ω and 50 Ω are considered, the fault location is expressed in percentage, the protection performance under in-zone bipolar short circuit fault is verified, and the simulation results are shown in Table 5.

[0126] Table 5 Simulation results of bipolar faults within the region.

[0127]

[0128] As can be seen from Table 5, the protection system can make accurate judgments and operate reliably for all types of intra-zone bipolar short-circuit faults.

[0129] 2.2 Single-pole grounding fault

[0130] To verify the performance of the proposed protection against a high-resistance fault at the end of the line, a positive ground fault is simulated at the end of line 1. The transition resistance is set to 500Ω, and the fault component line-mode voltage at the protection measuring point is as follows: Figure 8 As shown in (a), the fault component line-mode voltage and the reverse traveling wave at the protection measuring point are as follows: Figure 8 As shown in (b), the initial reverse traveling wave and curve fitting of the fault component line-mode voltage at the protection measuring point are as follows: Figure 8 As shown in (c), the positive and negative voltages of the fault component at the protection measuring point are as follows: Figure 8 As shown in (d).

[0131] from Figure 4 It can be seen that P 11 The protection is activated at 0.62ms. The time window lengths for direction, recognition, and pole selection criteria are all 0.5ms. The direction discrimination factor W is 0.41, the recognition factor X is 4.80, and the pole selection factor Y is 5.50. ​ The protection logic determines that the fault is a positive short circuit fault within the zone, and the protection will not fail to operate under a high-resistance fault at the far end of the zone.

[0132] To verify the impact of transition resistance and fault location on protection performance, the transition resistance was set to 0Ω, 300Ω, and 500Ω, and the fault location was also expressed as a percentage. The simulation results are shown in Table 6.

[0133] Table 7 Simulation results of unipolar faults within the region

[0134]

[0135] It is easy to see from Table 6 that the protection can still accurately identify when the transition resistance reaches 500Ω.

[0136] 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 single-ended line protection method for flexible DC power grids based on time-domain voltage traveling waves, characterized in that: Includes the following steps: Step 1: Measure the positive voltage, negative voltage, and current data at the protection measuring point, and calculate the fault component line-mode voltage; Step 2: Construct the start-up criterion and determine whether the fault component line-mode voltage meets the start-up criterion. If it does, proceed to Step 3; otherwise, return to Step 1. Step 3: Calculate the forward and reverse traveling waves of the fault component line-mode voltage; Step 4: Calculate the direction discrimination factor based on the traveling wave and the anti-traveling wave; Step 5: Construct direction criteria and determine whether the direction discrimination factor is a positive direction short circuit fault. If it is a positive direction short circuit fault, proceed to step 6; otherwise, if it is a reverse direction short circuit fault or lightning interference, perform protection reset. Step 6: Calculate the fitting error; Step 7: Construct identification criteria and determine whether the fitting error meets the identification criteria. If it does, it is an intra-zone short circuit fault, and proceed to Step 8; otherwise, it is a positive-direction non-intra-zone short circuit fault. Step 8: Calculate the polarity selection factor; Step 9: Construct the polarity selection criterion and determine whether the fault is positive, bipolar, or negative based on the relationship between the polarity selection factor and the polarity selection criterion.

2. The method for single-ended line protection of flexible DC power grid based on time-domain voltage traveling wave as described in claim 1, characterized in that: The method for calculating the fault component line-mode voltage in step 1 is as follows: Among them, u pij u nij To protect measuring point P ij The positive and negative voltages measured at the point, U0 is the line voltage when the system is operating normally, Δu pij , Δu nij For P ij The positive and negative fault voltage components at the point, Δu 0ij , Δu 1ij For P ij The ground and line fault voltage components at the location are determined based on the positive and negative fault voltage components u at the protection measuring point. pij u nij The fault component line-mode voltage Δu at this location is obtained. 1ij .

3. The method for single-ended line protection of flexible DC power grid based on time-domain voltage traveling wave as described in claim 1, characterized in that: The startup criterion constructed in step 2 is as follows: |Due 1ij (i)|>D set Where, Δu 1ij To protect measuring point P ij The fault component line-mode voltage at the location; Δ set The start-up criterion setting value is set based on the line-mode voltage of the component that avoids the maximum unbalanced fault that occurs during normal operation.

4. The method for single-ended line protection of flexible DC power grid based on time-domain voltage traveling wave as described in claim 1, characterized in that: The method for calculating the direction discrimination factor in step 4 is as follows: Where W is the direction discrimination factor, Δu qij To protect measuring point P ij The voltage traveling wave at point Δu fij To protect measuring point P ij The voltage reverse traveling wave at the point; N is the number of sampling points of the voltage traveling wave within a 0.5ms time window after startup. The calculation formulas for the forward and reverse traveling waves are: Among them, i pij i nij To protect measuring point P ij The positive and negative currents measured at the point, I 0ij To protect measuring point P during normal system operation ij The current at point Δi pij , Δi nij For P ij The positive and negative fault current components at the point, Δi 0ij , Δi 1ij For P ij Ground mode and line mode fault current components, Δu 1ij For P ij The line-mode fault voltage component at point Z c1 The traveling wave impedance of the line's mode 1 voltage, Δu qij , Δu fij For P ij The fault component at the location is the forward and reverse traveling wave of the first modulus voltage.

5. The method for single-ended line protection of flexible DC power grid based on time-domain voltage traveling wave as described in claim 1, characterized in that: The direction criterion constructed in step 5 is as follows: Where W is the direction discrimination factor, W set This is the setting value for the direction criterion.

6. The method for single-ended line protection of flexible DC power grid based on time-domain voltage traveling wave as described in claim 1, characterized in that: The method for calculating the fitting error in step 6 is as follows: Where X is the fitting error and also the identification factor, its magnitude is the residual (Δu) fij -y) 2-norm; Δu fij Let t1 be the time window of the first fault wave arriving at the protection point, and t2 be the time window of the second arrival of the reverse wave. y = y(t) is the fitting function obtained by the least squares method.

7. The method for single-ended line protection of flexible DC power grid based on time-domain voltage traveling wave as described in claim 1, characterized in that: The identification criterion constructed in step 7 is as follows: Where X is the fitting error and also the identification factor; X set To identify the criterion setting value, it is set according to the maximum value of the residual 2-norm that occurs when avoiding short-circuit faults within the zone.

8. The method for single-ended line protection of flexible DC power grid based on time-domain voltage traveling wave according to claim 1, characterized in that: The method for calculating the polarity selection factor in step 8 is as follows: Where Y is the polarity selection factor, Δu pij To protect measuring point P ij The positive voltage of the fault component at the location, Δu nij To protect measuring point P ij The fault component is the negative voltage at the location, and N is the number of sampling points of the voltage traveling wave within a 0.5ms time window after startup.

9. The method for single-ended line protection of flexible DC power grid based on time-domain voltage traveling wave as described in claim 8, characterized in that: The pole selection criterion constructed in step 9 is as follows: Among them, PGF represents positive electrode fault, PNF represents bipolar fault, NGF represents negative electrode fault, and Y represents positive electrode fault. set This is the setpoint value for the polarity selection criterion.

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