New wind power plant outgoing line distance protection method based on transition resistance correction

Abstract

The invention discloses a novel wind power plant outgoing line distance protection method based on transition resistance correction. The method comprises the following steps: collecting parameters of a wind power plant and outgoing lines and monitoring terminal voltage and current of the wind power plant in real time; when a fault occurs, constructing a fault option factor and a fault type judgment factor by using the correction voltage break variable to determine a fault phase and a fault type; and constructing a fault differential equation under a corresponding fault by using the three-phase fault voltage equation under the pi model, solving the fault differential equation through a least square algorithm to obtain a fault distance, and finally determining whether the circuit breaker trips according to the stable fault distance. Through the technical scheme of the invention, accurate fault phase selection of the weak feed source outgoing line can be realized, and the transition resistance is corrected while the distributed capacitance of the outgoing line is considered, so that the error generated by adopting single-ended quantity to carry out fault distance calculation is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of power system relay protection, and in particular to a new method for wind farm transmission line distance protection based on transition resistance correction. Background Art

[0002] Clean energy has become an important direction for future energy development. Wind farms have become a research hotspot due to their mature technology and high conversion rate. Therefore, it is of great significance to study the impact of wind farm grid connection on the power system.

[0003] As an important component of clean energy, wind farms have very different fault transient characteristics from thermal power plants. When the terminal voltage drops slightly, the rotor-side converter will start the negative sequence current suppression strategy, resulting in the limitation of the fault current amplitude and a large number of harmonics. When the terminal voltage drops deeply, the crowbar protection circuit will be activated to protect the rotor-side converter. At this time, the frequency of the short-circuit current will shift in the range of 35 to 65 Hz. The vector extraction based on the power frequency quantity is no longer accurate. At this stage, the performance of the directional elements, phase selection elements and distance elements based on the power frequency vector deteriorates, and the fault current of some outgoing lines, such as Figure 4 (a)-(d) in Figure 1. Furthermore, the wind farm's LVRT strategy ensures that its positive- and negative-sequence impedances are significantly greater than its zero-sequence impedance. Therefore, when a ground fault occurs, the zero-sequence current accounts for a significant proportion, while the positive- and negative-sequence components account for a very small proportion. Furthermore, as the distance of the transmission line increases, its distributed capacitance becomes significant, making traditional distance protection inapplicable on wind farm transmission lines.

[0004] The unique fault characteristics of wind farms have a variety of impacts on traditional distance protection. However, many current distance protection configurations for high-voltage transmission lines of renewable energy power plants do not take into account the unique fault characteristics of their grid-connected systems and still use traditional distance protection schemes, resulting in failure to operate or malfunction when faults occur. Therefore, it is very necessary to construct a new distance protection method for transmission lines. Summary of the Invention

[0005] To address the problem of existing distance protection systems struggling to operate correctly when faults occur on wind farm transmission lines, this invention provides a new method for wind farm transmission line distance protection that accounts for the effects of distributed capacitance while maintaining transition resistance correction. This method eliminates the effects of non-power frequency fault currents, unequal positive and negative sequence fault impedances, and distributed capacitance on wind farm transmission line distance protection. Furthermore, by correcting for transition resistance, this method mitigates the drawback of distance protection systems that typically replaces two-terminal measurements with single-terminal equivalents.

[0006] To achieve the above object, the technical solution of the present invention is:

[0007] S1: Collect wind farm and transmission line parameters;

[0008] S2: Real-time monitoring of voltage and current at the wind farm terminal;

[0009] S3: Use the real-time voltage mutation at the wind farm end as a protection starting element;

[0010] S4: After the protection is started, the fault voltage equation is established based on the π model;

[0011] S5: When a fault occurs, construct a fault phase judgment factor based on the fault voltage characteristics use The size of the fault phase can be preliminarily determined;

[0012] S6: Using the fault phase judgment factor Construct the fault type judgment factor θ and judge the fault type according to the value of θ;

[0013] S7: Consider the distributed capacitance of the line and correct the transition resistance to establish the differential equation for type A fault;

[0014] S8: Consider the distributed capacitance of the line and correct the transition resistance to establish the differential equation for type B fault;

[0015] S9: Solve the classification fault differential equation based on the partial differential least squares optimization algorithm. According to the differential equation type, construct the corresponding matrix equation to solve the fault distance.

[0016] S10: Determine the stability of the fault distance. If the fault distance is unstable, return to S9 to calculate the next fault distance value. When the fault distance is stable and the protection occurs within the zone, the zone fault protection trips. When the fault distance is stable and the protection does not occur within the zone, the zone fault protection exits.

[0017] Optionally, in step S5, when a fault occurs, a fault phase judgment factor is constructed according to the fault voltage characteristics. use The initial judgment of the size of the fault phase includes:

[0018] Taking phase A as the benchmark, the relationship between the voltage mutation at the protection installation location and the sequence voltage at the fault occurrence point is as follows:

[0019]

[0020] In the formula, Δu mA , Δu mB , Δu mC is the phase voltage mutation amount; u f1 、u f2 、u f0 are the positive, negative and zero sequence voltages of phase A at the fault location; C1, C2 and C3 are the sequence voltage distribution coefficients; α is the rotation factor;

[0021] The voltage mutation in the above equation is corrected by the dynamic mutation correction mechanism based on the voltage distribution coefficient shown in the following equation to obtain the corrected voltage mutation for each phase:

[0022]

[0023] In the formula, Δu mA·comp , Δu mB·comp , Δu mC·comp is the phase correction current mutation amount; k is the dynamic correction factor, k = C1 / C2;

[0024] The following fault phase judgment factor is constructed based on the amplitude of the corrected voltage mutation of each phase and the inter-phase corrected voltage mutation of the other two phases:

[0025]

[0026] In the formula, is the corrected voltage mutation of one of the three phases; is the interphase correction voltage mutation of the other two phases

[0027] Sort the three fault phase judgment factors corresponding to the three phases from large to small, and the maximum value is recorded as K max , the minimum value is recorded as K min , the middle value is recorded as K mid , at this time K max The corresponding phase is one of the fault phases; if it is a single-phase fault, K max The corresponding phase is the fault phase. If it is a two-phase fault, K max and K mid The corresponding phase is the fault phase.

[0028] Optionally, the step S6 uses the fault phase judgment factor Construct a fault type judgment factor θ. According to the value of θ, the fault type is judged as follows:

[0029] The fault phase judgment factor is used to construct the following fault type judgment factor θ:

[0030]

[0031] Combining the value range of the fault type factor θ and the zero-sequence voltage under different faults, the following fault type judgment criterion is constructed:

[0032]

[0033] Where u 0·zd is the zero-sequence voltage setting value;

[0034] Single-phase grounding faults and two-phase short-circuit grounding faults are classified as Type A faults, and two-phase short-circuit faults and three-phase short-circuit faults are classified as Type B faults. When a Type A fault occurs, step S7 is executed. When the fault is a Type B fault, step S7 is skipped and step S8 is executed.

[0035] Optionally, the step S7 considers the distributed capacitance of the line and corrects the transition resistance, and establishes the type A fault differential equation including:

[0036] When a single-phase grounding fault occurs, the fault voltage equation for single-phase grounding is established by taking the A-phase grounding fault as an example, and the transition resistance correction is performed to form the fault distance equation for single-phase grounding.

[0037] According to the fault voltage equation based on the π model in step S4, the fault voltage equation at this time is:

[0038]

[0039] In the formula, r1 and r0 are the zero-sequence resistance and positive-sequence resistance of the line per unit length; l1 and l0 are the zero-sequence positive-sequence inductance and zero-sequence inductance of the line per unit length;

[0040] Correct the transition resistance, and the relationship between the fault point voltage and the transition resistance is:

[0041] u fa =i fa R f =3(i ma0 -i Cm0 -i Cf0 )NR f ;

[0042] Where R f is the transition resistance; N is the coefficient of the difference between the fault current used in the calculation and the true fault current. Since there is a phase difference between the currents on both sides of the fault point, N is a complex number, that is, N=N1+jN2, then:

[0043] NR f =(N1+jN2)R f =(R' f +jωL');

[0044] Where R' f and L' f is the corrected transition resistance and transition inductance;

[0045] Combining the above formula, the fault differential equation is as follows:

[0046] u ma =A x1 d+A x2 d2 +A x3 R' f +A x4 dR' f +A x5 d 2 R' f +A x6 L' f +A x7 dL' f +A x8 d 2 L' f ;

[0047] Where A xi is a coefficient composed of line parameters and monitoring quantities. The specific expression is as follows:

[0048]

[0049] When a two-phase short-circuit grounding fault occurs, the fault voltage equation for the two-phase short-circuit grounding is established by taking the short-circuit grounding fault of phase BC as an example, and the transition resistance correction is performed to form the fault distance equation for the two-phase short-circuit grounding.

[0050] According to the fault voltage equation based on the π model in step S4, the fault voltage equation at this time is:

[0051]

[0052] Correction of transition resistance:

[0053]

[0054] Where N is the zero-sequence current coefficient. Since there is a phase difference between the zero-sequence currents on both sides of the fault point, N can be regarded as a complex number, that is, N = N1 + jN2, then:

[0055] 2NR f =2(N1+jN2)R f =(R' f +jωL');

[0056] Combining the above formula, the fault differential equation is as follows:

[0057] u mb +u mc =A y1 d+A y2 d 2 +A y3 R' f +A y4 dR' f +A y5 d 2 R'f +A y6 L' f +A y7 dL' f +A y8 d 2 L' f ;

[0058] Where A yi is a coefficient composed of line parameters and monitoring quantities. The specific expression is as follows:

[0059]

[0060] After constructing the type A fault differential equation using the system parameters and the monitoring variables, step S8 is skipped and step S9 is executed to solve the fault distance.

[0061] Optionally, the step S8 considers the distributed capacitance of the line and corrects the transition resistance, and establishes the type B fault differential equation including:

[0062] If the fault is a two-phase short circuit fault, taking the BC phase short circuit fault as an example, the fault voltage equation for the two-phase short circuit is established, and the transition resistance correction is performed to form the fault distance equation for the two-phase short circuit;

[0063] According to the equation in step S4, the two-phase short-circuit fault voltage equation at this time is:

[0064]

[0065] Correction of transition resistance:

[0066]

[0067] Combining the above formula, the fault differential equation is as follows:

[0068] u mb -u mc =B x1 d+B x2 d 2 +B x3 R' f +B x4 dR' f +B x5 d 2 R' f ;

[0069] Where B xi is a coefficient composed of line parameters and monitoring quantities. The specific expression is as follows:

[0070]

[0071] If it is a three-phase short circuit fault, the fault voltage equation for the three-phase short circuit is established, and the transition resistance correction is performed to form the fault distance equation for the three-phase short circuit;

[0072] According to the equation in step S4, the fault voltage equation for the three-phase fault at this time is:

[0073]

[0074] Correction of transition resistance:

[0075]

[0076] Combining the above formula, the fault differential equation is as follows:

[0077] u ma =B y1 d+B y2 d 2 +B y3 R' f +B y4 dR' f +B y5 d 2 R' f ;

[0078] Where B yi is a coefficient composed of line parameters and monitoring quantities. The specific expression is as follows:

[0079]

[0080] Optionally, step S9 solves the classified fault differential equation based on the partial differential least squares optimization algorithm, constructs a corresponding matrix equation according to the differential equation type, and solves the fault distance, including:

[0081] When a type A fault occurs, three fault differential equations are formed with three sets of sampling data and the following type A matrix differential equation is constructed:

[0082] W u =A1d+A2d 2 +A3R' f +A4dR' f +A5d 2 R' f +A6L' f +A7dL' f +A8d 2 L' f ;

[0083] Where W u is the fault voltage matrix; A i Because Axi or A yi The coefficient matrix composed of

[0084] The following type A objective function is constructed using matrix differential equations:

[0085] O A =(W u -(A1d+A2d 2 +A3R' f +A4dR' f +A5d 2 R' f +A6L' f +A7dL' f +A8d 2 L' f )) 2 ;

[0086] The three partial derivative equations of the objective function are used together to calculate the fault distance d:

[0087]

[0088] Solving the differential equation of type B fault on the transmission line based on the least squares optimization algorithm;

[0089] When a Type B fault occurs, two fault differential equations are formed with two sets of sampling data and the following Type B matrix differential equation is constructed:

[0090] W u =B1d+B2d 2 +B3R' f +B4dR' f +B5d 2 R' f ;

[0091] Where W u is the fault voltage matrix; B i Because B xi or B yi The coefficient matrix composed of

[0092] The following type B objective function is constructed using matrix differential equations:

[0093] O B =(W u -(B1d+B2d 2 +B3R' f +B4dR' f +B5d 2 R' f )) 2 ;

[0094] The three partial derivative equations of the objective function are used together to calculate the fault distance d:

[0095]

[0096] Optionally, the step S10 of judging the stability of the fault distance and deciding whether to trip includes:

[0097] If the fault distance calculation values for four consecutive times satisfy the following formula, the fault distance is considered stable;

[0098]

[0099] In the formula, d k is the calculated value of the fault distance; when the above formula is not satisfied, the fault distance d is not stable, the window is moved to the next sampling point and the process returns to step S9 to calculate the next fault distance d k+1 When the above equation is satisfied, the fault distance d reaches a stable state, and the following equation can be used to determine whether the fault occurs within the protection zone.

[0100] 0<d k <d set ;

[0101] Where, d set is the critical value of the protection interval; when the above formula is satisfied, it means that the fault occurs in the zone and the circuit breaker trips; when the above formula is not satisfied, the protection is exited.

[0102] Compared with the prior art, the technical solution of the present invention has the following beneficial technical effects:

[0103] This invention improves the distance protection of wind farm transmission lines to adapt them to the fault characteristics of renewable energy transmission lines. In terms of fault phase selection, a phase selection factor is constructed by compensating for voltage mutations, eliminating phase selection failures caused by unequal positive and negative sequence impedances of renewable energy sources. In terms of fault distance calculation, a π model is used to establish the fault differential equation to mitigate the impact of distributed capacitance on distance protection. Transition resistance correction is also performed to account for the current phase difference on both sides of the fault point, ensuring proper operation of wind farm transmission line protection. BRIEF DESCRIPTION OF THE DRAWINGS

[0104] Figure 1 Flowchart for the implementation of the present invention;

[0105] Figure 2 This is the model diagram of the wind farm transmission line;

[0106] Figure 3 This is the equivalent circuit of the three-phase π model under the single-phase grounding fault of the wind farm transmission line;

[0107] Figure 4 is the fault current waveform on the wind farm side, Figure 4(a) is the A phase ground short-circuit current waveform; Figure 4 (b) is the AB two-phase ground short-circuit current waveform; Figure 4 (c) AB two-phase short-circuit current waveform; Figure 4 (d Three-phase short-circuit current waveform. DETAILED DESCRIPTION

[0108] The present invention will be further described below in conjunction with the accompanying drawings. The following examples are only used to more clearly illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention.

[0109] Reference Figure 1 The embodiment of the present invention provides a new method for wind farm transmission line distance protection taking into account the influence of distributed capacitance under transition resistance correction, including the following steps:

[0110] Step S1: Collect the parameters of the wind farm and transmission line; let the self-resistance and self-inductance per unit length of the transmission line be r s and l s ; The mutual resistance and mutual inductance per unit length of the line are r m and l m ;The unit line capacitance to ground is c0.

[0111] Step S2: Monitor the voltage and current at the wind farm end protection installation in real time, and record the three-phase voltage and current as u mj andi mj (j=a,b,c).

[0112] Step S3: Using the real-time voltage mutation at the wind farm end as a fault location starting element, when the voltage mutation exceeds a threshold, start the transmission line fault location, as shown in the following formula; and record the fault time;

[0113] u mj·k -u mj·(k-M) >u Δ·zd

[0114] Where u mj·k is the voltage value at the current moment; u mj·(k-M) The voltage value before one power frequency cycle; u Δ·zd is the protection start setting value; M is the number of sampling points in one power frequency cycle.

[0115] Step S4: After the protection is started, the fault voltage equation is established based on the π model;

[0116] Reference Figure 3 The π-type equivalent model of the doubly-fed wind farm transmission line is shown in the figure, and the voltage equation from the protection installation point to the fault point is established as shown in the following formula:

[0117]

[0118] Where u fa 、u fb 、u fc is the three-phase voltage at the fault point; c s =1 / 2c0, c0 is the ground capacitance of the transmission line per unit length; d is the line length from the protection installation to the fault point.

[0119] Step S5: When a fault occurs, construct a fault phase judgment factor based on the fault voltage characteristics use The size of the fault phase can be preliminarily determined;

[0120] Taking phase A as the benchmark, the relationship between the voltage mutation at the protection installation location and the sequence voltage at the fault occurrence point is as follows:

[0121]

[0122] In the formula, Δu mA , Δu mB , Δu mC is the phase voltage mutation amount; u f1 、u f2 、u f0 are the positive, negative and zero sequence voltages of phase A at the fault location; C1, C2 and C3 are the sequence voltage distribution coefficients; α is the rotation factor.

[0123] The voltage mutation in the above equation is corrected by the dynamic mutation correction mechanism based on the voltage distribution coefficient shown in the following equation to obtain the corrected voltage mutation for each phase:

[0124]

[0125] In the formula, Δu mA·comp , Δu mB·comp , Δu mC·comp is the phase correction current mutation amount; k is the dynamic correction factor, k = C1 / C2.

[0126] The following fault phase judgment factor is constructed based on the amplitude of the corrected voltage mutation of each phase and the inter-phase corrected voltage mutation of the other two phases:

[0127]

[0128] In the formula, is the corrected voltage mutation of one of the three phases; is the inter-phase correction voltage mutation of the other two phases,

[0129] Sort the three fault phase judgment factors corresponding to the three phases from large to small, and the maximum value is recorded as K max , the minimum value is recorded as Kmin , the middle value is recorded as K mid ; If it is a single-phase fault, K max The corresponding phase is the fault phase. If it is a two-phase fault, K max and K mid The corresponding phase is the fault phase; the specific fault type is determined by step B S6.

[0130] Step S6: Using the fault phase judgment factor Construct a fault type judgment factor θ and determine the fault type based on the value of θ;

[0131] The fault phase judgment factor is used to construct the following fault type judgment factor θ:

[0132]

[0133] Combining the value range of the fault type factor θ and the zero-sequence voltage under different faults, the following fault type judgment criterion is constructed:

[0134]

[0135] Where u 0·zd is the zero-sequence voltage setting value.

[0136] Single-phase grounding faults and two-phase short-circuit grounding faults are classified as Type A faults, while two-phase short-circuit faults and three-phase short-circuit faults are classified as Type B faults. When a Type A fault occurs, the process proceeds to step S7. When the fault is a Type B fault, step S7 is skipped and the process proceeds to step S8.

[0137] Step S7: Considering the distributed capacitance of the line and correcting the transition resistance, a differential equation for type A fault is established;

[0138] When a single-phase grounding fault occurs, the fault voltage equation for single-phase grounding is established by taking the A-phase grounding fault as an example, and the transition resistance correction is performed to form the fault distance equation for single-phase grounding.

[0139] According to the equation in step S4, the fault voltage equation at this time is:

[0140]

[0141] In the formula, R1 and R0 are the zero-sequence resistance and positive-sequence resistance of the line per unit length; L1 and L0 are the zero-sequence positive-sequence inductance and zero-sequence inductance of the line per unit length.

[0142] Correct the transition resistance, and the relationship between the fault point voltage and the transition resistance is:

[0143] u fa =i fa R f=3(i ma0 -i Cm0 -i Cf0 )NR f

[0144] Where R f is the transition resistance; N is the coefficient of the difference between the fault current used in the calculation and the true fault current. Since there is a phase difference between the currents on both sides of the fault point, N is a complex number, that is, N=N1+jN2, then:

[0145] NR f =(N1+jN2)R f =(R' f +jωL')

[0146] Where R' f and L' f are the corrected transition resistance and transition inductance.

[0147] Combining the above formula, the fault differential equation is as follows:

[0148] u ma =A x1 d+A x2 d 2 +A x3 R' f +A x4 dR' f +A x5 d 2 R' f +A x6 L' f +A x7 dL' f +A x8 d 2 L' f

[0149] Where A xi is a coefficient composed of line parameters and monitoring quantities. The specific expression is as follows:

[0150]

[0151] When a two-phase short-circuit grounding fault occurs, the fault voltage equation for the two-phase short-circuit grounding is established by taking the short-circuit grounding fault of phase BC as an example, and the transition resistance correction is performed to form the fault distance equation for the two-phase short-circuit grounding.

[0152] According to the equation in step S4, the fault voltage equation at this time is:

[0153]

[0154] Correction of transition resistance:

[0155]

[0156] Where N is the zero-sequence current coefficient. Since there is a phase difference between the zero-sequence currents on both sides of the fault point, N can be regarded as a complex number, that is, N = N1 + jN2, then:

[0157] 2NR f =2(N1+jN2)R f =(R' f +jωL')

[0158] Combining the above formula, the fault differential equation is as follows:

[0159] u mb +u mc =A y1 d+A y2 d 2 +A y3 R' f +A y4 dR' f +A y5 d 2 R' f +A y6 L' f +A y7 dL' f +A y8 d 2 L' f

[0160] Where A yi is a coefficient composed of line parameters and monitoring quantities. The specific expression is as follows:

[0161]

[0162] After constructing the type A fault differential equation using the system parameters and monitoring variables, step S8 is skipped and the process goes to step S9 to solve the fault distance.

[0163] Step S8: Considering the distributed capacitance of the line and correcting the transition resistance, a differential equation for type B fault is established;

[0164] If the fault is a two-phase short circuit fault, taking the BC phase short circuit fault as an example, the fault voltage equation for the two-phase short circuit is established, and the transition resistance correction is performed to form the fault distance equation for the two-phase short circuit.

[0165] According to the equation in step S4, the two-phase short-circuit fault voltage equation at this time is:

[0166]

[0167] Correction of transition resistance:

[0168]

[0169] Combining the above formula, the fault differential equation is as follows:

[0170] u mb -u mc =B x1 d+B x2 d 2 +B x3 R' f +B x4 dR' f +B x5 d 2 R' f

[0171] Where B xi is a coefficient composed of line parameters and monitoring quantities. The specific expression is as follows:

[0172]

[0173] If it is a three-phase short circuit fault, the fault voltage equation for the three-phase short circuit is established, and the transition resistance correction is performed to form the fault distance equation for the three-phase short circuit.

[0174] According to the equation in step S4, the fault voltage equation for the three-phase fault at this time is:

[0175]

[0176] Correction of transition resistance:

[0177]

[0178] Combining the above formula, the fault differential equation is as follows:

[0179] u ma =B y1 d+B y2 d 2 +B y3 R' f +B y4 dR' f +B y5 d 2 R' f

[0180] Where B yi is a coefficient composed of line parameters and monitoring quantities. The specific expression is as follows:

[0181]

[0182] Step S9: Solve the classification fault differential equation based on the partial differential least squares optimization algorithm, construct the corresponding matrix equation according to the differential equation type, and solve the fault distance;

[0183] When a type A fault occurs, three fault differential equations are formed by sampling three sets of data and forming the following type A matrix differential equation:

[0184] W u =A1d+A2d 2 +A3R' f +A4dR' f +A5d 2 R' f +A6L' f +A7dL' f +A8d 2 L' f

[0185] Where W u is the fault voltage matrix; A i Because A xi or A yi The coefficient matrix composed of .

[0186] The following type A objective function is constructed using matrix differential equations:

[0187] O A =(W u -(A1d+A2d 2 +A3R' f +A4dR' f +A5d 2 R' f +A6L' f +A7dL' f +A8d 2 L' f )) 2

[0188] The three partial derivative equations of the objective function are used together to calculate the fault distance d:

[0189]

[0190] Solving the differential equation of type B fault on the transmission line based on the least squares optimization algorithm;

[0191] When a type B fault occurs, two fault differential equations are formed with two sets of data sampling and the following type B matrix differential equation is constructed:

[0192] W u =B1d+B2d 2+B3R' f +B4dR' f +B5d 2 R' f

[0193] Where W u is the fault voltage matrix; B i Because B xi or B yi The coefficient matrix composed of .

[0194] The following type B objective function is constructed using matrix differential equations:

[0195] O B =(W u -(B1d+B2d 2 +B3R' f +B4dR' f +B5d 2 R' f )) 2

[0196] The three partial derivative equations of the objective function are used together to calculate the fault distance d:

[0197]

[0198] Step S10: Determine the stability of the fault distance. If the fault distance is unstable, return to S9 to calculate the next fault distance value. When the fault distance is stable and the protection occurs within the zone, the zone fault protection trips. When the fault distance is stable and the protection does not occur within the zone, the zone fault protection exits.

[0199] If the fault distance calculation values for four consecutive times satisfy the following formula, the fault distance is considered stable;

[0200]

[0201] In the formula, d k is the calculated value of the fault distance; when the above formula is not satisfied, the fault distance d is not stable, the window is moved to the next sampling point and the process returns to step S9 to calculate the next fault distance d k+1 When the above equation is satisfied, the fault distance d reaches a stable state, and the following equation can be used to determine whether the fault occurs within the protection zone.

[0202] 0<d k <d set

[0203] Where, d set is the critical value of the protection interval; when the above formula is satisfied, it means that the fault occurs in the zone and the circuit breaker trips; when the above formula is not satisfied, the protection is exited.

[0204] The following is a specific implementation case of the present invention:

[0205] Build a new energy transmission line simulation model on the MATLAB / SIMULINK platform, refer to Figure 2 The simulation sampling frequency is 10 kHz, with the N side representing the grid and the M side representing the doubly-fed wind farm. Bus M represents the renewable energy farm's export bus after converging through the collector line. The outgoing line is 100 km long, with a positive-sequence resistance per unit length of 0.0491 Ω / km, a positive-sequence inductance of 0.3867 mH / km, a grounding capacitance of 0.1686 uF / km, a zero-sequence resistance of 0.1053 Ω / km, and a zero-sequence inductance of 0.1782 mH / km. The main transformer ratio is 110 kV / 35 kV, and the system-side voltage is 330 kV. The protection range is set to 80% of the outgoing line, using the M-side protection as an example.

[0206] (1) When a phase A ground fault occurs at 25 km from the M side of the transmission line: the voltage mutation detected on the M side exceeds the threshold, the fault type judgment factor θ=34.1>10 in step S6, and K A The phase selection factor is the maximum, and it is determined to be a phase A grounding fault. The differential equation corresponding to the single-phase grounding fault in step S7 is established, and the system parameters and the monitoring value of phase A are substituted into it. The stable fault distance d is calculated to be 25.04 km through step S9. The fault occurs in the protection zone, and the protection trips.

[0207] (2) When a BC phase-to-ground fault occurs at 45 km from the M side of the transmission line: the voltage mutation detected on the M side exceeds the threshold, the fault type judgment factor in step S6 is 1<θ=2.1<10, the zero sequence voltage exceeds the set value, and K B is the maximum phase selection factor, K C The intermediate phase selection factor is used to determine that the fault is an inter-phase-to-ground fault between the BC phases. The differential equation corresponding to the inter-phase-to-ground fault in step S7 is established, and the system parameters and the BC phase monitoring data are substituted into the equation. The stable fault distance d is calculated to be 45.97 km through step S9. The fault occurs within the protection zone, and the protection trips.

[0208] (3) When a BC phase fault occurs at 55 km from the M side of the transmission line: the voltage mutation detected on the M side exceeds the threshold, the fault type judgment factor in step S6 is 1<θ=2.8<10, the zero sequence voltage is less than the set value, and K C is the maximum phase selection factor, K B The intermediate phase selection factor is used to determine that the fault is between phases BC. The differential equation corresponding to the phase fault in step S8 is established, and the system parameters and the BC phase monitoring data are substituted into the equation. The stable fault distance d is calculated to be 55.03 km in step S9. The fault occurs within the protection zone, and the protection trips.

[0209] (4) When a three-phase fault occurs on the transmission line at a distance of 85 km from the M side: the voltage mutation detected on the M side exceeds the threshold, the fault type judgment factor θ in step S6 is 0.19 < 0.5, and K C The maximum phase selection factor is used, and it is judged to be a three-phase fault. The differential equation corresponding to the three-phase fault is established in step S8, and the system parameters and the C-phase monitoring value are substituted into it. The fault distance d is calculated to be 85.13 km through step S9. The fault occurs outside the protection zone and the protection does not trip.

Claims

1. A new method for wind farm transmission line distance protection based on transition resistance correction, characterized by The steps include: S1: Collect wind farm and transmission line parameters; S2: Real-time monitoring of voltage and current at the wind farm terminal; S3: Use the real-time voltage mutation at the wind farm end as a protection starting element; S4: After the protection is started, the fault voltage equation is established based on the π model; S5: When a fault occurs, construct a fault phase judgment factor based on the fault voltage characteristics use The size of the fault phase can be preliminarily determined; S6: Using the fault phase judgment factor Construct a fault type judgment factor θ and determine the fault type based on the value of θ; S7: Consider the distributed capacitance of the line and correct the transition resistance to establish the differential equation for type A fault; S8: Consider the distributed capacitance of the line and correct the transition resistance to establish the differential equation for type B fault; S9: Solve the classification fault differential equation based on the partial differential least squares optimization algorithm. According to the differential equation type, construct the corresponding matrix equation to solve the fault distance. S10: Determine the stability of the fault distance. If the fault distance is unstable, return to S9 to calculate the next fault distance value. When the fault distance is stable and the protection occurs within the zone, the zone fault protection trips; When the fault distance is stable and protection does not occur within the zone, the out-of-zone fault protection is exited.

2. The method according to claim 1, wherein: In step S5, when a fault occurs, a fault phase judgment factor is constructed according to the fault voltage characteristics. use The initial judgment of the size of the fault phase includes: Taking phase A as the benchmark, the relationship between the voltage mutation at the protection installation location and the sequence voltage at the fault occurrence point is as follows: In the formula, Δu mA , Δu mB , Δu mC is the phase voltage mutation amount; u f1 、u f2 、u f0 are the positive, negative and zero sequence voltages of phase A at the fault location; C1, C2 and C3 are the sequence voltage distribution coefficients; α is the rotation factor; The voltage mutation in the above equation is corrected by the dynamic mutation correction mechanism based on the voltage distribution coefficient shown in the following equation to obtain the corrected voltage mutation for each phase: In the formula, Δu mA·comp , Δu mB·comp , Δu mC·comp is the phase correction current mutation amount; k is the dynamic correction factor, k = C1 / C2; The following fault phase judgment factor is constructed based on the amplitude of the corrected voltage mutation of each phase and the inter-phase corrected voltage mutation of the other two phases: In the formula, is the corrected voltage mutation of one of the three phases; is the interphase correction voltage mutation of the other two phases Sort the three fault phase judgment factors corresponding to the three phases from large to small, and the maximum value is recorded as K max , the minimum value is recorded as K min , the middle value is recorded as K mid ; If it is a single-phase fault, K max The corresponding phase is the fault phase. If it is a two-phase fault, K max and K mid The corresponding phase is the fault phase.

3. The method according to claim 1, wherein: Step S6 uses the fault phase judgment factor Construct a fault type judgment factor θ. According to the value of θ, the fault type is judged as follows: The fault phase judgment factor is used to construct the following fault type judgment factor θ: Combining the value range of the fault type factor θ and the zero-sequence voltage under different faults, the following fault type judgment criterion is constructed: Where u 0·zd is the zero-sequence voltage setting value; Single-phase grounding faults and two-phase short-circuit grounding faults are classified as Type A faults, and two-phase short-circuit faults and three-phase short-circuit faults are classified as Type B faults. When a Type A fault occurs, step S7 is executed. When the fault is a Type B fault, step S7 is skipped and step S8 is executed.

4. The method according to claim 1, wherein: The step S7 considers the distributed capacitance of the line and corrects the transition resistance to establish the differential equation of type A fault, which includes: When a single-phase grounding fault occurs, the fault voltage equation for single-phase grounding is established by taking the A-phase grounding fault as an example, and the transition resistance correction is performed to form the fault distance equation for single-phase grounding. According to the fault voltage equation based on the π model in step S4, the fault voltage equation at this time is: In the formula, r1 and r0 are the zero-sequence resistance and positive-sequence resistance of the line per unit length; l1 and l0 are the zero-sequence positive-sequence inductance and zero-sequence inductance of the line per unit length; Correct the transition resistance, and the relationship between the fault point voltage and the transition resistance is: u fa =i fa R f =3(i ma0 -i Cm0 -i Cf0 )NR f ; Where R f is the transition resistance; N is the coefficient of the difference between the fault current used in the calculation and the true fault current. Since there is a phase difference between the currents on both sides of the fault point, N is a complex number, that is, N=N1+jN2, then: NR f =(N1+jN2)R f =(R' f +jωL'); Where R' f and L' f is the corrected transition resistance and transition inductance; Combining the above formula, the fault differential equation is as follows: u ma =A x1 d+A x2 d 2 +A x3 R' f +A x4 dR' f +A x5 d 2 R' f +A x6 L' f +A x7 dL' f +A x8 d 2 L' f ; Where A xi is a coefficient composed of line parameters and monitoring quantities. The specific expression is as follows: When a two-phase short-circuit grounding fault occurs, the fault voltage equation for the two-phase short-circuit grounding is established by taking the short-circuit grounding fault of phase BC as an example, and the transition resistance correction is performed to form the fault distance equation for the two-phase short-circuit grounding. According to the fault voltage equation based on the π model in step S4, the fault voltage equation at this time is: Correction of transition resistance: Where N is the zero-sequence current coefficient. Since there is a phase difference between the zero-sequence currents on both sides of the fault point, N can be regarded as a complex number, that is, N = N1 + jN2, then: 2NR f =2(N1+jN2)R f =(R' f +jωL'); Combining the above formula, the fault differential equation is as follows: u mb +u mc =A y1 d+A y2 d 2 +A y3 R' f +A y4 dR' f +A y5 d 2 R' f +A y6 L' f +A y7 dL' f +A y8 d 2 L' f ; Where A yi is a coefficient composed of line parameters and monitoring quantities. The specific expression is as follows: After constructing the type A fault differential equation using the system parameters and the monitoring variables, step S8 is skipped and step S9 is executed to solve the fault distance.

5. The method according to claim 1, wherein: The step S8 considers the distributed capacitance of the line and corrects the transition resistance to establish the type B fault differential equation, which includes: If the fault is a two-phase short circuit fault, taking the BC phase short circuit fault as an example, the fault voltage equation for the two-phase short circuit is established, and the transition resistance correction is performed to form the fault distance equation for the two-phase short circuit; According to the equation in step S4, the two-phase short-circuit fault voltage equation at this time is: Correction of transition resistance: Combining the above formula, the fault differential equation is as follows: u mb -u mc =B x1 d+B x2 d 2 +B x3 R' f +B x4 dR' f +B x5 d 2 R' f ; Where B xi is a coefficient composed of line parameters and monitoring quantities. The specific expression is as follows: If it is a three-phase short circuit fault, the fault voltage equation for the three-phase short circuit is established, and the transition resistance correction is performed to form the fault distance equation for the three-phase short circuit; According to the equation in step S4, the fault voltage equation for the three-phase fault at this time is: Correction of transition resistance: Combining the above formula, the fault differential equation is as follows: u ma =B y1 d+B y2 d 2 +B y3 R' f +B y4 dR' f +B y5 d 2 R' f ; Where B yi is a coefficient composed of line parameters and monitoring quantities. The specific expression is as follows:

6. The method according to claim 1, wherein: The step S9 solves the classification fault differential equation based on the partial differential least squares optimization algorithm, constructs the corresponding matrix equation according to the differential equation type, and solves the fault distance, including: When a type A fault occurs, three fault differential equations are formed with three sets of sampling data and the following type A matrix differential equation is constructed: <h2 style=";text-align:left;direction:ltr">W<h2 style=";text-align:left;direction:ltr"> u <h2 style=";text-align:left;direction:ltr"> (A1d+A2d)<h2 style=";text-align:left;direction:ltr"> 2 <h2 style=";text-align:left;direction:ltr"> +A3R'<h2 style=";text-align:left;direction:ltr"> f <h2 style=";text-align:left;direction:ltr"> +A4dR'<h2 style=";text-align:left;direction:ltr"> f <h2 style=";text-align:left;direction:ltr"> +A5d<h2 style=";text-align:left;direction:ltr"> 2 <h2 style=";text-align:left;direction:ltr"> R'<h2 style=";text-align:left;direction:ltr"> f <h2 style=";text-align:left;direction:ltr"> +A6L'<h2 style=";text-align:left;direction:ltr"> f <h2 style=";text-align:left;direction:ltr"> +A7dL'<h2 style=";text-align:left;direction:ltr"> f <h2 style=";text-align:left;direction:ltr"> +A8d<h2 style=";text-align:left;direction:ltr"> 2 <h2 style=";text-align:left;direction:ltr"> L'<h2 style=";text-align:left;direction:ltr"> f <h2 style=";text-align:left;direction:ltr"> ; Where W u is the fault voltage matrix; A i Because A xi or A yi The coefficient matrix composed of The following type A objective function is constructed using matrix differential equations: <h2 style=";text-align:left;direction:ltr">O<h2 style=";text-align:left;direction:ltr"> A <h2 style=";text-align:left;direction:ltr"> (W)<h2 style=";text-align:left;direction:ltr"> u <h2 style=";text-align:left;direction:ltr"> -(A1d+A2d<h2 style=";text-align:left;direction:ltr"> 2 <h2 style=";text-align:left;direction:ltr"> +A3R'<h2 style=";text-align:left;direction:ltr"> f <h2 style=";text-align:left;direction:ltr"> +A4dR'<h2 style=";text-align:left;direction:ltr"> f <h2 style=";text-align:left;direction:ltr"> +A5d<h2 style=";text-align:left;direction:ltr"> 2 <h2 style=";text-align:left;direction:ltr"> R'<h2 style=";text-align:left;direction:ltr"> f <h2 style=";text-align:left;direction:ltr"> +A6L'<h2 style=";text-align:left;direction:ltr"> f <h2 style=";text-align:left;direction:ltr"> +A7dL'<h2 style=";text-align:left;direction:ltr"> f <h2 style=";text-align:left;direction:ltr"> +A8d<h2 style=";text-align:left;direction:ltr"> 2 <h2 style=";text-align:left;direction:ltr"> L'<h2 style=";text-align:left;direction:ltr"> f <h2 style=";text-align:left;direction:ltr"> ))<h2 style=";text-align:left;direction:ltr"> 2 <h2 style=";text-align:left;direction:ltr"> ; The three partial derivative equations of the objective function are used together to calculate the fault distance d: Solving the differential equation of type B fault on the transmission line based on the least squares optimization algorithm; When a Type B fault occurs, two fault differential equations are formed with two sets of sampling data and the following Type B matrix differential equation is constructed: W u =B1d+B2d 2 +B3R' f +B4dR' f +B5d 2 R' f ; Where W u is the fault voltage matrix; B i Because B xi or B yi The coefficient matrix composed of The following type B objective function is constructed using matrix differential equations: O B =(W u -(B1d+B2d 2 +B3R' f +B4dR' f +B5d 2 R' f )) 2 ; The three partial derivative equations of the objective function are used together to calculate the fault distance d:

7. The method according to claim 1, wherein: Step S10 determines the stability of the fault distance. If the fault distance is unstable, the process returns to S9 to calculate the next fault distance value. When the fault distance is stable and the protection occurs within the zone, the zone fault protection trips. When the fault distance is stable and the protection does not occur within the zone, the zone fault protection exits. If the fault distance calculation values for four consecutive times satisfy the following formula, the fault distance is considered stable; In the formula, d k is the calculated value of the fault distance; when the above formula is not satisfied, the fault distance d is not stable, the window is moved to the next sampling point and the process returns to step S9 to calculate the next fault distance d k+1 When the above equation is satisfied, the fault distance d reaches a stable state, and the following equation can be used to determine whether the fault occurs within the protection zone. 0<d k <d set ; Where, d set is the critical value of the protection interval; when the above formula is satisfied, it means that the fault occurs in the zone and the circuit breaker trips; when the above formula is not satisfied, the protection is exited.

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