A Time-Domain Distance Protection Method and System for the Transmission Line of a Doubly Fed Wind Farm

Abstract

The present invention relates to a time-domain distance protection method and system for a double-fed wind farm outgoing line, belonging to the technical field of power system relay protection, and solves the problems of poor accuracy in discriminating internal faults of existing double-fed wind farm outgoing lines and susceptibility to interference. A time-domain distance protection method for a double-fed wind farm outgoing line includes: after a fault occurs, collecting the fault voltage and fault current on the wind farm side of the outgoing line and determining the fault type; obtaining the short-circuit current corresponding to each sampling point based on the fault voltage at each sampling point; determining the virtual fault location of the outgoing line based on the fault type, as well as the short-circuit current and fault voltage at all sampling points; obtaining the fault similarity factor based on the virtual fault location, as well as the fault voltage and fault current at all sampling points; identifying whether an internal fault has occurred on the outgoing line based on the fault similarity factor, and if so, starting the fault protection of the outgoing line.

Description

Technical Field

[0001] The present invention relates to the technical field of relay protection in power systems, and particularly to a time-domain distance protection method and system for the outgoing line of a doubly-fed wind farm. Background Art

[0002] The protection of the outgoing line of a wind farm can be divided into frequency-domain information protection and time-domain information protection.

[0003] Frequency-domain protection mainly constructs a direction element by comparing the phase jump of the current before and after a fault to isolate the faulty line. At present, the improvements for the frequency-domain protection of the outgoing line include the following two methods: First, according to the frequency change characteristics of the short-circuit current of the doubly-fed wind turbine and the power grid, a line protection principle is constructed using the current frequency difference; Second, a protection scheme based on the peak-valley value difference is constructed by using the characteristic that the attenuation degree of the fault current amplitude is different between the wind farm side and the system side. However, when a fault occurs on the outgoing line, the accuracy of the Fourier algorithm will be affected by the frequency components of the wind turbine speed. Research shows that it contains a large amount of non-power frequency current components, which have a significant impact on the Fourier algorithm, and it is difficult to ensure the performance of the protection element that relies on power frequency quantities.

[0004] In the protection principle scheme based on time-domain information, line differential protection mainly relies on the fault differential current and differential voltage, and equivalent the internal and external faults to different inductance circuit models, and proposes a protection principle based on parameter identification for model recognition. For the improved distance protection scheme, on the one hand, a distance protection scheme based on the waveform correlation analysis of the voltage at the protection installation point and the calculated setting point is proposed; on the other hand, aiming at the problem that the main frequencies of the voltage and current of the outgoing line are different, a distance protection algorithm based on the time-domain model of the wind power outgoing line is proposed. However, the existing time-domain protection schemes do not conduct a detailed analysis of the internal model of the wind farm, nor fully consider the influence of the coupling effect between the internal parts of the wind farm, and are easily affected by the change of the internal control strategy of the wind farm. Summary of the Invention

[0005] In view of the above analysis, the embodiments of the present invention aim to provide a time-domain distance protection method and system for the outgoing line of a doubly-fed wind farm to solve the problems such as poor accuracy in discriminating internal faults and susceptibility to interference of the existing outgoing line of the doubly-fed wind farm.

[0006] On the one hand, the present invention discloses a time-domain distance protection method for the outgoing line of a doubly-fed wind farm, including:

[0007] After a fault occurs, collect the fault voltage and fault current on the wind farm side of the outgoing line and determine the fault type;

[0008] Based on the fault voltage at each sampling point, obtain the short-circuit current at the corresponding sampling point;

[0009] Based on the fault type, as well as the short-circuit current and fault voltage at all sampling points, determine the virtual fault location of the outgoing line;

[0010] Based on the virtual fault location, as well as the fault voltage and fault current at all sampling points, obtain the fault similarity factor;

[0011] Based on the fault similarity factor, identify whether an in-zone fault has occurred on the outgoing line. If so, activate the fault protection of the outgoing line.

[0012] On the basis of the above solutions, the present invention has also made the following improvements:

[0013] Further, at each sampling point, obtain the short-circuit current i at the corresponding sampling point according to the following formula M_t :

[0014]

[0015] In the formula, i ma 、i mb 、i mc are respectively the three-phase short-circuit currents on the wind farm side of the outgoing line; θ is the angle by which the d-axis of the voltage synchronous rotating coordinate system on the wind farm side of the outgoing line leads the a-axis; L -1 (·) represents the inverse Laplace transform; i M (s) represents the short-circuit current at the current sampling point in the complex frequency domain, which is obtained by processing the fault voltage at the current sampling point.

[0016] Further,

[0017]

[0018] In the formula, t1 is the fault occurrence time, [I n -X(s)Z M -1 (i,:) represents the i-th row of the matrix [I n -X(s)Z M -1 , where i takes values from 1 to n; n represents the total number of wind turbine units connected to the collector bus on the wind farm side of the outgoing line; I n is the n-order identity matrix; ψ s0i represents the initial value of the stator flux linkage of the i-th wind turbine unit; u M (s) is the value of the fault voltage at the current sampling point in the complex frequency domain;

[0019]

[0020]

[0021] ​​

[0022]

[0023]

[0024]

[0025]

[0026] Among them, s represents the complex frequency domain independent variable, and ω s represents the synchronous speed; ω slipi represents the slip angular velocity of the i-th wind turbine unit; L si 、L mi 、L ri respectively represent the stator inductance, equivalent excitation inductance, and rotor inductance of the i-th wind turbine unit; τ sni represents the stator time constant of the i-th wind turbine unit, and R si 、R ri respectively represent the stator resistance and rotor resistance of the i-th wind turbine unit; σ i is the generator leakage magnetic coefficient of the i-th wind turbine unit, is the reference value of the rotor current given in the low voltage ride-through control mode of the i-th wind turbine unit; i rdi * is the d-axis component of the reference value of the rotor current given in the low voltage ride-through control mode of the i-th wind turbine unit; ψ sdi 、ψ sqi respectively represent the d- and q-axis components of the stator flux linkage of the i-th wind turbine unit; u sdi represents the d-axis component of the stator voltage of the i-th wind turbine unit; k pi 、k ii are respectively the inner loop proportional coefficient and integral coefficient of the rotor side converter of the i-th wind turbine unit; i rd0i 、i rq0i are respectively the d- and q-axis components of the initial value of the rotor current before the fault of the i-th wind turbine unit; R li 、L li 、L ti respectively represent the resistance, inductance, and box transformer inductance of the collector line connected to the i-th wind turbine unit; X T is the equivalent reactance of the main transformer of the outgoing line.

[0027] Furthermore, the determination of the virtual fault location of the outgoing line includes:

[0028] Determine the expressions of each short-circuit parameter in the virtual fault location calculation equation according to the fault category;

[0029] Based on the short-circuit current and fault voltage at each sampling point, calculate the values of the expressions of each short-circuit parameter in the virtual fault location calculation equation, and use them as the short-circuit parameter value groups corresponding to the sampling points;

[0030] Based on the short-circuit parameter value groups of all sampling points, fit to obtain the virtual fault location and transition resistance in the virtual fault location calculation equation.

[0031] Furthermore, the virtual fault location calculation equation is:

[0032] p c +p R R g +p b x + p a x 2 =0 (3)

[0033] In the formula, x represents the virtual fault location, and R g represents the transition resistance;

[0034] When the fault type is single-phase ground fault, p c =p1, p R =p2, p b =p3, p a =p4;

[0035] When the fault type is two-phase interphase fault, p c =p5, p R =p6, p b =p7, p a =p8;

[0036] When the fault type is two-phase ground fault, p c =p5, p R =p9, p b =p7, p a =p8;

[0037] When the fault type is three-phase symmetrical fault, the fault between any two phases in the three-phase symmetrical fault can be regarded as a two-phase ground fault. Select any two phases and calculate p c , p R , p b and p a ;

[0038]

[0039]

[0040] p9=2p6 (6)

[0041] Among them, i mφDenotes the short - circuit current during φ - phase fault, u mφ Denotes the fault voltage during φ - phase fault, where φ takes a, b, or c; i mρ Denotes the short - circuit current of the ρ - line during two - phase fault, u mφ Denotes the fault voltage of the ρ - line during two - phase fault, where ρ takes ab, bc, or ac; u m0 、i m0 Denote the zero - sequence fault voltage and zero - sequence short - circuit current during φ - phase fault respectively, R l 、L l Are the positive - sequence resistance and inductance per unit length of the AC line respectively; R s0 、L s0 Are the zero - sequence equivalent resistance and inductance on the system side of the outgoing line respectively, R l0 、L l0 Are the zero - sequence equivalent resistance and inductance of the outgoing line respectively; Compensation coefficient Δu mρ =u mρ -u mρ0 ,Δi mρ =i mρ -i mρ0 ,u mρ0 、i mρ0 Are the steady - state component values of the line voltage and line current on the fan side of the outgoing line respectively, where ρ takes ab, bc, or ac.

[0042] Furthermore, the obtaining of the fault similarity factor includes:

[0043] Determine the expressions of each fault parameter in the virtual fault location calculation equation according to the fault category;

[0044] Based on the fault current and fault voltage at each sampling point, calculate the values of the expressions of each fault parameter in the virtual fault location calculation equation as the fault parameter value group corresponding to each sampling point;

[0045] Based on the fault parameter value groups of all sampling points and the virtual fault location and transition resistance obtained by fitting, obtain the fault similarity factor.

[0046] Furthermore,

[0047] When the fault category is single - phase - to - ground fault, p c =p1′, p R =p2′, p b =p3′, p a =p4′; When the fault category is two - phase - to - phase fault, p c =p5′, p R =p6′, p b =p7′, pa = p8'; When the fault type is a two-phase grounding fault, p c = p5', p R = p9', p b = p7', p a = p8';

[0048] When the fault type is a three-phase symmetrical fault, the fault between any two phases in the three-phase symmetrical fault can be regarded as a two-phase grounding fault. Select any two phases and calculate p according to the calculation method for a two-phase grounding fault c , p R , p b and p a ;

[0049]

[0050]

[0051] p9' = 2p6' (9)

[0052] In the formula, i mφ ' represents the fault current during a φ-phase fault; i mρ ' represents the fault current of the ρ line during a two-phase fault; i m0 ' represents the zero-sequence fault current during a φ-phase fault, Δi mρ ' = i mρ ' - i mρ0 , and ρ takes ab, bc, or ac.

[0053] Furthermore, the fault-like factor S can be expressed as:

[0054]

[0055] In the formula, K is the number of sampling points within 1 ms, p c (k), p R (k), p b (k), p a (k) are respectively calculated based on the fault current and fault voltage at the kth sampling point to obtain p c , p R , p b , p a .

[0056] Furthermore, based on the fault-like factor, identify whether a fault has occurred within the sending line area, including:

[0057] If the fault-like factor is greater than the set operating threshold value, a fault has occurred within the sending line area; otherwise, a fault has occurred outside the sending line area.

[0058] On the other hand, the present invention also provides a time-domain distance protection system for the outgoing line of a doubly-fed wind farm, including:

[0059] A data acquisition module, configured to collect the fault voltage and fault current on the wind farm side of the outgoing line after a fault occurs, and determine the fault type;

[0060] A short-circuit current acquisition module, configured to acquire the short-circuit current corresponding to each sampling point based on the fault voltage at each sampling point;

[0061] A virtual fault location determination module, configured to determine the virtual fault location of the outgoing line based on the fault type, as well as the short-circuit current and fault voltage at all sampling points;

[0062] A fault similarity factor acquisition module, configured to acquire a fault similarity factor based on the virtual fault location, as well as the fault voltage and fault current at all sampling points;

[0063] A fault protection module, configured to identify whether an in-zone fault occurs on the outgoing line based on the fault similarity factor, and if so, initiate the fault protection of the outgoing line.

[0064] Compared with the prior art, the present invention can at least achieve one of the following beneficial effects:

[0065] First, the wind farm fault current response model adopts an instantaneous value model, which can accurately describe the output current after a wind farm fault and construct an accurate fault model of the doubly-fed wind farm after the fault.

[0066] Second, incorporating the detailed fault model of the doubly-fed wind farm into the system fault model description eliminates the influence of the internal parameter coupling of the doubly-fed wind farm on protection, and can correctly and reliably implement the protection action.

[0067] Finally, the protection scheme only uses single-ended measured electrical quantities for fault location, is not affected by the transition resistance, fault location, and the opposite-side system, and has a low dependence on communication equipment.

[0068] In the present invention, the above technical solutions can also be combined with each other to achieve more preferred combination schemes. Other features and advantages of the present invention will be described in the subsequent specification, and some advantages can be made obvious from the specification, or understood by implementing the present invention. The objectives and other advantages of the present invention can be realized and obtained through the content specifically pointed out in the specification and the drawings. Description of the Drawings

[0069] The drawings are only for the purpose of showing specific embodiments, and are not considered to be a limitation of the present invention. Throughout the drawings, the same reference signs denote the same components.

[0070] Figure 1Flowchart of the time-domain distance protection method for the outgoing line of a doubly-fed wind farm provided for the embodiment;

[0071] Figure 2 Main structure topology diagram of the DFIG provided for the embodiment;

[0072] Figure 3 Schematic diagram of a doubly-fed wind farm provided for the embodiment;

[0073] Figure 4 Schematic diagram of the single-phase grounding fault system of the outgoing line provided for the embodiment;

[0074] Figure 5 Zero-sequence fault equivalent network diagram of the outgoing line provided for the embodiment;

[0075] Figure 6 Schematic diagram of the two-phase interphase fault of the outgoing line provided for the embodiment;

[0076] Figure 7 Two-phase interphase fault component diagram of the outgoing line provided for the embodiment;

[0077] Figure 8 Schematic diagram of the two-phase grounding fault of the outgoing line provided for the embodiment;

[0078] Figure 9 Schematic diagram of the three-phase symmetrical fault of the outgoing line provided for the embodiment;

[0079] Figure 10 Schematic diagram of the structure of the time-domain distance protection system for the outgoing line of a doubly-fed wind farm provided for the embodiment. Detailed implementation manners

[0080] Next, the preferred embodiments of the present invention will be specifically described with reference to the accompanying drawings. The accompanying drawings form a part of this application and are used together with the embodiments of the present invention to explain the principle of the present invention, rather than to limit the scope of the present invention.

[0081] A specific embodiment of the present invention discloses a time-domain distance protection method for the outgoing line of a doubly-fed wind farm. The flowchart is as Figure 1 shown and includes the following steps:

[0082] Step S1: After a fault occurs, collect the fault voltage and fault current on the wind farm side of the outgoing line and determine the fault type;

[0083] Step S2: Based on the fault voltage at each sampling point, obtain the short-circuit current at the corresponding sampling point;

[0084] At each sampling point, obtain the short-circuit current i at the corresponding sampling point according to the following formula M_t :

[0085]

[0086] In the formula, i ma , i mb , i mc are respectively the three-phase short-circuit currents on the wind farm side of the outgoing line; θ is the angle by which the d-axis of the voltage synchronous rotating coordinate system on the wind farm side of the outgoing line leads the a-axis; L -1 (·) represents the inverse Laplace transform; i M (s) represents the short-circuit current at the current sampling point in the complex frequency domain, which is obtained by processing the fault voltage at the current sampling point.

[0087]

[0088] In the formula, t1 is the fault occurrence time, [I n -X(s)Z M -1 (i,:) represents the i-th row of the matrix [I n -X(s)Z M -1 , where i takes values from 1 to n; n represents the total number of wind turbine units connected to the collector bus on the wind farm side of the outgoing line; I n is the n-order identity matrix; ψ s0i represents the initial value of the stator flux linkage of the i-th wind turbine unit; u M (s) is the value of the fault voltage at the current sampling point in the complex frequency domain;

[0089]

[0090]

[0091]

[0092]

[0093]

[0094]

[0095]

[0096] Among them, s represents the independent variable in the complex frequency domain, ω s represents the synchronous speed; ω slipi represents the slip angular velocity of the i-th wind turbine unit; L si , L mi , L ri respectively represent the stator inductance, equivalent excitation inductance, and rotor inductance of the i-th wind turbine unit; τ sni ​​Represents the stator time constant of the i-th wind turbine unit, R si and R ri respectively represent the stator resistance and rotor resistance of the i-th wind turbine unit; σ i is the generator leakage magnetic coefficient of the i-th wind turbine unit, is the reference value of the rotor current given in the low voltage ride-through control mode of the i-th wind turbine unit; i rdi * is the d-axis component of the reference value of the rotor current given in the low voltage ride-through control mode of the i-th wind turbine unit; ψ sdi and ψ sqi respectively represent the d and q axis components of the stator flux linkage of the i-th wind turbine unit; u sdi represents the d-axis component of the stator voltage of the i-th wind turbine unit; k pi and k ii are respectively the inner loop proportional coefficient and integral coefficient of the rotor side converter of the i-th wind turbine unit; i rd0i and i rq0i are respectively the d and q axis components of the initial value of the rotor current before the fault of the i-th wind turbine unit; R li and L li and L ti respectively represent the resistance, inductance of the collector line and the inductance of the box transformer connected to the i-th wind turbine unit; X T is the equivalent reactance of the main transformer of the outgoing line.

[0097] Step S3: Based on the fault type, as well as the short-circuit current and fault voltage of all sampling points, determine the virtual fault location of the outgoing line; specifically, it includes:

[0098] Step S31: According to the fault category, determine the expressions of each short-circuit parameter in the virtual fault location calculation equation;

[0099] Step S32: Based on the short-circuit current and fault voltage of each sampling point, calculate the values of the expressions of each short-circuit parameter in the virtual fault location calculation equation, and use them as the short-circuit parameter value groups of the corresponding sampling points;

[0100] Step S33: Based on the short-circuit parameter value groups of all sampling points, fit to obtain the virtual fault location and transition resistance in the virtual fault location calculation equation.

[0101] The virtual fault location calculation equation is:

[0102] p c +p R R g +p b x + p a x 2 = 0 (3)

[0103] In the formula, x represents the virtual fault location, and R g represents the transition resistance;

[0104] When the fault type is single-phase-to-ground fault, p c = p1, p R = p2, p b = p3, p a = p4;

[0105] When the fault type is two-phase phase-to-phase fault, p c = p5, p R = p6, p b = p7, p a = p8;

[0106] When the fault type is two-phase-to-ground fault, p c = p5, p R = p9, p b = p7, p a = p8;

[0107] When the fault type is three-phase symmetrical fault, the fault between any two phases in the three-phase symmetrical fault can be regarded as a two-phase-to-ground fault. Arbitrarily select two phases and calculate p according to the calculation method when it is a two-phase-to-ground fault c , p R , p b and p a ;

[0108]

[0109]

[0110] p9 = 2p6 (6)

[0111] Among them, i mφ represents the short-circuit current during the fault of phase φ, and u mφ represents the fault voltage during the fault of phase φ, where φ takes a, b or c; i mρ represents the short-circuit current of line ρ during a two-phase fault, and u mφ represents the fault voltage of line ρ during a two-phase fault, where ρ takes ab, bc or ac; u m0 , i m0 respectively represent the zero-sequence fault voltage and zero-sequence short-circuit current during the fault of phase φ, R l , L l are respectively the positive-sequence resistance and inductance per unit length of the AC line; R s0 , L s0 are respectively the zero-sequence equivalent resistance and inductance on the system side of the outgoing line, R l0, L l0 are the zero-sequence equivalent resistance and inductance of the outgoing line respectively; compensation factor Δu mρ = u mρ - u mρ0 , Δi mρ = i mρ - i mρ0 , u mρ0 , i mρ0 are the steady-state component values of the line voltage and line current on the fan side of the outgoing line respectively, and ρ takes ab, bc or ac.

[0112] Step S4: Based on the virtual fault location, and the fault voltages and fault currents of all sampling points, obtain the fault similarity factor; specifically, the obtaining of the fault similarity factor includes:[[]]

[0113] Step S41: According to the fault type, determine the expressions of each fault parameter in the virtual fault location calculation equation;

[0114] Step S42: Based on the fault current and fault voltage of each sampling point, calculate the values of the expressions of each fault parameter in the virtual fault location calculation equation, and use them as the fault parameter value groups of the corresponding sampling points;

[0115] When the fault type is single-phase ground fault, p c = p1′, p R = p2′, p b = p3′, p a = p4′; when the fault type is two-phase interphase fault, p c = p5′, p R = p6′, p b = p7′, p a = p8′; when the fault type is two-phase ground fault, p c = p5′, p R = p9′, p b = p7′, p a = p8′:

[0116] When the fault type is three-phase symmetrical fault, the fault between any two phases in the three-phase symmetrical fault can be regarded as a two-phase ground fault. Select any two phases and calculate p c , p R , p b and p a ;

[0117]

[0118]

[0119] p9′ = 2p6′ (9)

[0120] Wherein, i mφ ′ represents the fault current during a φ-phase fault; i mρ ′ represents the fault current of the ρ line during a two-phase fault; i m0 ′ represents the zero-sequence fault current during a φ-phase fault, Δi mρ ′ = i mρ ′ - i mρ0 , and ρ takes ab, bc or ac.

[0121] Step S43: Based on the fault parameter value sets of all sampling points, as well as the fitted virtual fault location and transition resistance, obtain the fault similarity factor.

[0122] The fault similarity factor S can be expressed as:

[0123]

[0124] Wherein, K is the number of sampling points within 1 ms, p c (k), p R (k), p b (k), p a (k) are respectively p calculated based on the fault current and fault voltage of the k-th sampling point c , p R , p b , p a .

[0125] Step S5: Based on the fault similarity factor, identify whether an in-zone fault occurs on the outgoing line. If so, activate the fault protection of the outgoing line.

[0126] If the fault similarity factor is greater than the set action threshold value, an in-zone fault occurs on the outgoing line; otherwise, an out-of-zone fault occurs on the outgoing line.

[0127] Exemplarily, the action threshold value can be set through test data. In this embodiment, the action threshold value is set to 5.

[0128] The solution in this embodiment is obtained based on the following theoretical analysis:

[0129] First, according to the situation when the DFIG enters the low-voltage ride-through control mode after a fault, derive the DFIG single-machine current instantaneous response model.

[0130] The doubly-fed induction generator mainly consists of mechanical parts such as a gearbox, an asynchronous generator, and power electronic devices such as converters. The stator of the DFIG is directly connected to the power grid, and the rotor is connected to the power grid via two converters. The main structure is asFigure 2 as shown

[0131] Taking Figure 2 the DFIG model shown as an example, assuming that a grid fault occurs at time \(t = t_1\), without considering phase jumps, the terminal voltage of the DFIG drops from \(u\) s0 to \(u\) s . When the terminal voltage is less than 0.9 p.u., the DFIG switches to the low-voltage ride-through control mode. In the low-voltage ride-through control mode, the rotor voltage is provided by the rotor-side converter (RSC). Analyzing in the synchronous rotating coordinate system, the stator, rotor, and flux linkage equations of the DFIG under the motor convention are:

[0132]

[0133]

[0134]

[0135]

[0136] where \(u\) sd , \(u\) sq represent the d-axis and q-axis components of the stator voltage respectively; \(u\) rd , \(u\) rq represent the d-axis and q-axis components of the rotor voltage respectively; \(i\) sd , \(i\) sq represent the d-axis and q-axis components of the stator current respectively; \(i\) rd , \(i\) rq represent the d-axis and q-axis components of the rotor current respectively; \(\psi\) sd , \(\psi\) sq represent the d-axis and q-axis components of the stator flux linkage respectively; \(\psi\) rd , \(\psi\) rq represent the d-axis and q-axis components of the rotor flux linkage respectively; \(R\) s , \(R\) r represent the stator and rotor resistances respectively, and \(L\) s , \(L\) m , \(L\) r represent the stator inductance, equivalent excitation inductance, and rotor inductance respectively. \(\omega\) s represents the synchronous speed, and \(\omega\) slip represents the slip angular velocity.

[0137] Since the DFIG immediately starts the low-voltage ride-through control mode after the grid fault occurs, therefore, assuming that only the inner loop of the RSC participates in the control, its control equation can be written as:

[0138]

[0139] where \(k\) p, k i are the proportionality coefficient and integral coefficient of the RSC inner loop respectively; σ is the generator leakage magnetic coefficient, are the d-axis and q-axis components of the given rotor current reference value under the low voltage ride-through control mode of the wind turbine respectively.

[0140] According to the requirements of the Technical Regulations for Wind Farms Connected to the Power System GB / T 19963.1-2021, the wind turbine should have dynamic reactive power support ability during the low voltage ride-through process after the terminal voltage drops. That is, the unit should calculate the current reference value according to the terminal voltage at the moment of the voltage drop and output reactive current, and the rotor current reference value is:

[0141]

[0142] In the formula, U s is the voltage amplitude after the terminal voltage drops, K d is the reactive power gain coefficient, generally K d ≥1.5, I max is the amplitude of the maximum output current of the wind turbine.

[0143] According to the stator flux linkage conservation law, the stator flux linkage response formula after the grid fault is:

[0144]

[0145] In the formula, τ sn = 1 / τ s +jω s , the stator time constant τ s = σL s / R s . ψ s represents the stator flux linkage vector, u s represents the stator voltage vector, t represents the current moment, and t1 represents the moment when the fault occurs.

[0146] Taking the rotor currents i rd0 , i rq0 before the fault as the initial values and the initial change rate of the rotor current as 0, according to formulas (1)-(17), the rotor current response equation is solved simultaneously as:

[0147]

[0148] In the formula,

[0149] According to the stator fault flux linkage response and rotor current response, the frequency domain expression of the short-circuit current of the wind turbine in the DFIG single-machine current instantaneous response model can be obtained as:

[0150]

[0151] where u s (s) represents the stator voltage in the complex frequency domain, represents the reference value of the rotor current, ψ s0 represents the initial value of the stator flux linkage.

[0152] Second, according to the coupling relationship between the resistance and inductance of different feeders, establish the equation between the outgoing current and voltage of the wind farm, and derive the short-circuit current of the outgoing line of the doubly-fed wind farm.

[0153] Each wind turbine unit in the doubly-fed wind farm is connected to the collector bus through a box transformer and a collector line, and then sent to the high-voltage outgoing line by the main transformer of the outgoing line. The structure diagram is as Figure 3 shown.

[0154] As Figure 3 shown, u si represents the terminal voltage of the i-th wind turbine unit, where i ranges from 1 to n, and n represents the total number of wind turbine units; R li , L li , L ti respectively represent the collector line resistance, inductance, and box transformer inductance connected to the i-th wind turbine unit; X T is the equivalent reactance of the main transformer of the outgoing line, and i M is the outgoing current of the doubly-fed wind farm.

[0155] According to Figure 3 , the frequency-domain expression of the short-circuit current of each unit in the doubly-fed wind farm can be obtained as:

[0156]

[0157] In the formula, u M (s) is the voltage at the M terminal in the complex frequency domain, and u i (s) is the terminal voltage of the i-th wind turbine unit in the complex frequency domain,

[0158]

[0159] According to Equation (17), the frequency-domain expression of the response current of the doubly-fed wind turbine unit can be obtained as:

[0160]

[0161] In the formula, is the initial stator flux linkage matrix of the wind turbine unit, ψ s0iIt represents the initial value of the stator magnetic flux of the i-th wind turbine unit, which is a fixed value. X(s) represents the matrix of the terminal voltage response of each wind turbine unit, H(s) represents the matrix of the response of the internal parameters and control parameters of the unit, M(s) represents the DC component matrix, and N(s) represents the matrix of the initial magnetic flux response of each unit. For the specific formula, please refer to the foregoing description.

[0162] By combining Equation (20) and Equation (21) and eliminating the terminal voltage, the frequency-domain expression of the short-circuit current of each unit in the doubly-fed wind farm can be obtained as follows:

[0163]

[0164] In the formula, I n is the n-order identity matrix, and X(s)Z M represents the parameter coupling matrix between different feeders.

[0165] According to Equation (19), the short-circuit current of the outgoing line of the doubly-fed wind farm with n wind turbine units in parallel is:

[0166]

[0167] In the formula, [I n -X(s)Z M -1 (i,:) represents the i-th row of the matrix [I n -X(s)Z M -1 the i-th row.

[0168] By transforming Equation (23) into the time domain and reducing it to the three-phase static coordinate system, the expression of the instantaneous value of the short-circuit current of the outgoing line of the doubly-fed wind farm in the time domain is obtained as:

[0169]

[0170] In the formula, θ is the angle by which the d-axis of the M-terminal voltage synchronous rotating coordinate system leads the a-axis, and L -1 (·) represents the inverse Laplace transform.

[0171] Thirdly, based on the voltage sampling value at the protection installation location and the short-circuit current, calculate the virtual fault location of the outgoing line. The calculation methods of the virtual fault location of the line under different fault conditions are different, which are described as follows:

[0172] (1) Calculation of the virtual fault location of the line under single-phase grounding fault

[0173] As Figure 4 shown, a single-phase grounding fault of phase A occurs at the x position of the outgoing line M-N of the wind farm, and the transition resistance is R g ​​For example, the value range of \(x\) is \(0 - 1\), which represents the ratio of the fault occurrence position to the total length of the outgoing line. When \(x = 0\), it corresponds to the M side; when \(x = 1\), it corresponds to the N side. From the figure, the zero-sequence fault equivalent network of the outgoing line is as Figure 5 shown.

[0174] As Figure 4 shown, \(i\) ma , \(i\) mb , \(i\) mc are the three-phase short-circuit currents at the M end, and \(u\) ma , \(u\) mb , \(u\) mc are the three-phase fault voltages at the M end; \(i\) na , \(i\) nb , \(i\) nc are the three-phase fault currents at the N end, and \(u\) na , \(u\) nb , \(u\) nc are the three-phase fault voltages at the N end. Figure 5 In it, \(d\) is the total length of the outgoing line M - N, \(R\) s0 , \(L\) s0 are the zero-sequence equivalent resistance and inductance of the opposite-side AC power supply respectively, \(R\) l0 , \(L\) l0 are the zero-sequence equivalent resistance and inductance of the outgoing line M - N respectively, \(i\) f0 is the zero-sequence short-circuit current flowing through the transition resistance, \(u\) m0 , \(u\) n0 are the zero-sequence fault voltages at the M end and N end respectively, \(i\) m0 , \(i\) n0 are the zero-sequence short-circuit currents at the M end and N end respectively.

[0175] According to Figure 4 , the expression of the voltage \(u\) fa of phase A at the fault point can be written as:

[0176]

[0177] In the formula, \(R\) l , \(L\) l are the positive-sequence equivalent resistance and inductance per unit length of the AC line M - N respectively, \(i\) m0 is the zero-sequence current at the M end, and the compensation coefficient

[0178] Referring to Figure 4 , the voltage equation of phase A at the fault point can be written according to the zero-sequence currents at both ends of M and N as:

[0179] 3(\(i\) m0 +\(i\) n0 )\(R\) g =\(u\) fa (26)

[0180] Wherein, u fa is the phase A voltage at the fault point, and i n0 is the zero-sequence current at the N terminal.

[0181] Substituting Equation (26) into Equation (24) gives:

[0182]

[0183] According to Figure 4 it can be obtained that the voltage derivation from both ends M and N to the fault point should be the same, that is:

[0184]

[0185] By combining Equation (24) and Equation (25) and eliminating the influence of the current and voltage at the N terminal, we can get:

[0186] p1 + p2R g + p3x + p4x 2 = 0 (29)

[0187] The coefficients in Equation (29) are as follows:

[0188]

[0189] When a single-phase fault occurs, based on each set of sampling points, a set of p1 - p4 can be calculated and substituted into Equation (29) to obtain an equation with R g and x as unknowns; multiple sets of sampling points can obtain multiple equations, and by fitting these multiple equations, R g and x can be determined.

[0190] In addition, it should be noted that when single-phase grounding faults occur in phases B and C, the derivation process is similar. Therefore, based on Equation (30), a general expression for single-phase grounding fault parameters is sorted out and described as Equation (4).

[0191] (2) Calculation of the virtual fault location of the line in the case of a two-phase interphase fault

[0192] Taking the AB phase interphase fault occurring at the x position of the wind farm outgoing line M - N with a transition resistance of R g as an example, as Figure 6 shown. It can be seen from the figure that the fault component network of the outgoing line is as Figure 7 shown.

[0193] According to Figure 5 the line voltage at the fault point can be obtained as:

[0194]

[0195] Wherein, u mab 、imab They are the fault voltage of line AB at the M terminal and the line short-circuit current respectively.

[0196] From Figure 5 it can be seen that the line voltage at the fault point obtained from the line currents at both the M and N terminals is:

[0197]

[0198] In the formula, i nab is the line current of line AB at the N terminal.

[0199] Substituting Equation (32) into Equation (31) gives:

[0200]

[0201] According to Figure 6 it can be obtained that the voltages calculated from both the M and N terminals to the fault point are equal, that is:

[0202]

[0203] In the formula, R W , L W are the equivalent resistance and inductance of the system voltage at the N terminal respectively, Δu mab is the line fault voltage at the M terminal of the fault component network, Δi mab , Δi nab are the line short-circuit currents at the M terminal and the N terminal of the fault component network respectively;

[0204] According to the superposition theorem, it can be obtained that:

[0205] i mab +i nab =Δi mab +Δi nab (35)

[0206] Substituting Equation (35) into Equation (34) gives:

[0207]

[0208] By combining Equation (36) and Equation (33), we can get:

[0209] p5 + p6R g + p7x + p8x 2 = 0 (37)

[0210] The coefficients in Equation (37) are as follows:

[0211]

[0212] When a two-phase interphase fault occurs, based on each set of sampling points, a set of p5 - p8 can be calculated and substituted into formula (38) to obtain an equation with R g and x as unknowns; multiple sets of sampling points can obtain multiple equations, and by fitting these multiple equations, R g and x can be determined.

[0213] In addition, it should be noted that when BC and AC two-phase interphase faults occur, the derivation process is similar. Therefore, based on formula (38), a general expression for two-phase interphase fault parameters is sorted out and described as formula (5).

[0214] (3) Calculation of the virtual fault location of the line under two-phase grounding fault

[0215] Taking the AB phase grounding fault occurring at the x position of the wind farm outgoing line M - N as an example, as Figure 8 shown.

[0216] According to Figure 8 the phase voltages of the fault points A and B can be obtained as:

[0217]

[0218] where R F is the fault grounding resistance.

[0219] From Figure 7 the line voltage of the fault point can be obtained as:

[0220] u fab =(i mab +i nab )R g (40)

[0221] Substituting equation (40) into equation (39) gives

[0222]

[0223] Similar to the two-phase interphase fault, a fault component network can also be obtained for the two-phase grounding fault. By analogy with the two-phase interphase fault, the virtual position equation for the two-phase grounding fault can be obtained as:

[0224] p5 + p9R g + p7x + p8x 2 = 0 (42)

[0225] where p9 = 2p6.

[0226] (4) Calculation of the virtual fault location of the line under ABC three-phase symmetrical fault

[0227] Taking the ABC three-phase symmetrical fault occurring at the x position of the wind farm outgoing line M - N as an example, asFigure 9 as shown

[0228] According to Figure 9 it can be seen that the fault between any two phases in a three-phase symmetrical fault can be regarded as a two-phase grounding fault. Therefore, the virtual fault location equation for a three-phase symmetrical fault is the same as that for a two-phase grounding fault.

[0229] In summary, when various types of faults occur in the outgoing line, the general formula for the virtual fault location calculation equation is as follows:

[0230] p c +p R R g +p b x + p a x 2 = 0 (43)

[0231] When the fault type is a single-phase grounding fault, p c = p1, p R = p2, p b = p3, p a = p4;

[0232] When the fault type is a two-phase interphase fault, p c = p5, p R = p6, p b = p7, p a = p8;

[0233] When the fault type is a two-phase grounding fault, p c = p5, p R = p9, p b = p7, p a = p8;

[0234] When the fault type is a three-phase symmetrical fault, the fault between any two phases in the three-phase symmetrical fault can be regarded as a two-phase grounding fault. Select any two phases and calculate p c , p R , p b and p a ;

[0235] Step S3: Based on the virtual fault location of the outgoing line, calculate the fault similarity factor using the current sampling value at the protection installation location to identify whether the fault is an in-zone fault.

[0236] When a fault occurs in the outgoing line of the doubly-fed wind farm, the short-circuit current fed into the outgoing line of the doubly-fed wind farm calculated according to Equation (24) is consistent with the fault current of the wind farm, that is, the i ma , i mb , i mcIf the fault current is the busbar fault current at the M terminal collector line, the virtual fault location calculated by Equation (43) is consistent with the actual fault location. At this time, Equation (43) holds, that is:

[0237] p c +p R R g +p b x + p a x 2 =0 (44)

[0238] When a fault occurs inside the doubly-fed wind farm, since Equation (21) is the short-circuit current of the wind farm derived from the external fault of the doubly-fed wind farm, the i ma , i mb , i mc calculated according to Equation (21) is inconsistent with the busbar fault current at the M terminal collector line, and the virtual fault location calculated by Equation (43) will deviate from the actual fault location. At this time, Equation (43) does not hold, that is:

[0239] p c +p R R g +p b x + p a x 2 ≠0 (45)

[0240] When a fault occurs in the lower-level line of the outgoing line M-N, the virtual fault location calculated according to Equation (43) is still consistent with the actual fault location, but at this time the virtual fault location exceeds the protection range and the protection does not start.

[0241] To sum up, when a fault occurs in the protected area of the outgoing line, the virtual fault location calculated according to Equation (43) is consistent with the actual fault location. When a fault occurs inside the wind farm on the back side of the M terminal of the outgoing line, the virtual fault location calculated according to Equation (43) is inconsistent with the actual fault location. To quantify the consistency between the calculated virtual fault location and the actual fault location, in this embodiment, the fault similarity factor S is defined as:

[0242]

[0243] In the formula, K is the number of sampling points within 1 ms, p c (k), p R (k), p b (k), p a (k) are the p calculated according to the fault current and fault voltage at the kth sampling point respectively c , p R , p b , p a .

[0244] If the fault resemblance factor is greater than the set operation threshold value, it indicates that a fault has occurred within the sending line M-N area; otherwise, the fault has occurred outside the sending line M-N area.

[0245] Embodiment 2

[0246] Embodiment 2 of the present invention discloses a time-domain distance protection system for a sending line of a doubly-fed wind farm. The schematic structural diagram of the system is as Figure 10 shown and includes:

[0247] A data acquisition module, configured to acquire the fault voltage and fault current on the wind farm side of the sending line after a fault occurs, and determine the fault type;

[0248] A short-circuit current acquisition module, configured to acquire the short-circuit current corresponding to each sampling point based on the fault voltage at each sampling point;

[0249] A virtual fault location determination module, configured to determine the virtual fault location of the sending line based on the fault type, as well as the short-circuit current and fault voltage at all sampling points;

[0250] A fault resemblance factor acquisition module, configured to acquire the fault resemblance factor based on the virtual fault location, as well as the fault voltage and fault current at all sampling points;

[0251] A fault protection module, configured to identify whether a fault has occurred within the sending line based on the fault resemblance factor. If so, start the fault protection of the sending line.

[0252] The above method embodiments and system embodiments are implemented based on the same principle, and their relevant parts can be mutually referred to and can achieve the same technical effects. For the specific implementation process of the system embodiment of the present invention, refer to the above method embodiments, and this embodiment will not be elaborated here. Since the system embodiment of the present invention has the same principle as the above method embodiment, the system also has the corresponding technical effects of the above method embodiment.

[0253] Those skilled in the art can understand that all or part of the processes for implementing the above method embodiments can be completed by instructing relevant hardware through a computer program, and the program can be stored in a computer-readable storage medium. Among them, the computer-readable storage medium is a magnetic disk, an optical disk, a read-only memory, or a random access memory, etc.

[0254] The above are only the preferred specific implementation manners of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed by the present invention should be covered by the protection scope of the present invention.

Claims

1. A time-domain distance protection method for the outgoing line of a doubly-fed wind farm, characterized in that Including: After a fault occurs, collect the fault voltage and fault current on the wind farm side of the outgoing line, and determine the fault type; Based on the fault voltage at each sampling point, obtain the short-circuit current at the corresponding sampling point; Based on the fault type, as well as the short-circuit current and fault voltage at all sampling points, determine the virtual fault location of the outgoing line; Based on the virtual fault location, as well as the fault voltage and fault current at all sampling points, obtain the fault similarity factor; Based on the fault similarity factor, identify whether an in-zone fault has occurred on the outgoing line. If so, initiate the fault protection of the outgoing line; The determining the virtual fault location of the outgoing line includes: According to the fault category, determine the expressions of each short-circuit parameter in the virtual fault location calculation equation; Based on the short-circuit current and fault voltage at each sampling point, calculate the values of the expressions of each short-circuit parameter in the virtual fault location calculation equation, and use them as the short-circuit parameter value groups at the corresponding sampling points; Based on the short-circuit parameter value groups at all sampling points, fit to obtain the virtual fault location and transition resistance in the virtual fault location calculation equation; The obtaining the fault similarity factor includes: According to the fault category, determine the expressions of each fault parameter in the virtual fault location calculation equation; Based on the fault current and fault voltage at each sampling point, calculate the values of the expressions of each fault parameter in the virtual fault location calculation equation, and use them as the fault parameter value groups at the corresponding sampling points; Based on the fault parameter value groups at all sampling points, as well as the virtual fault location and transition resistance obtained by fitting, obtain the fault similarity factor.

2. The time-domain distance protection method for the outgoing line of a doubly-fed wind farm according to claim 1, wherein At each sampling point, the short-circuit current i at the corresponding sampling point is obtained according to the following formula M_t :[[]]END]] In the formula, i ma 、i mb 、i mc are respectively the three-phase short-circuit currents on the wind farm side of the outgoing line; θ is the angle by which the d-axis of the voltage synchronous rotating coordinate system on the wind farm side of the outgoing line leads the a-axis; L- 1 (·) represents the inverse Laplace transform; i M (s) represents the short-circuit current at the current sampling point in the complex frequency domain, which is obtained by processing the fault voltage at the current sampling point.

3. The time-domain distance protection method for the outgoing line of a doubly-fed wind farm according to claim 1, wherein Where, t1 is the fault occurrence time, [I n -X(s)Z M -1 (i,:) represents the i-th row of the matrix [I n -X(s)Z M -1 , where i ranges from 1 to n; n represents the total number of fan units connected to the collector bus on the wind farm side of the outgoing line; I n is the n-order identity matrix; ψ s0i represents the initial value of the stator flux linkage of the i-th fan unit; u M (s) is the value of the fault voltage at the current sampling point in the complex frequency domain;​​ where s represents the complex frequency domain independent variable, ω s represents the synchronous speed; ω slipi represents the slip angular velocity of the i-th wind turbine unit; L si and L mi and L ri respectively represent the stator inductance, equivalent excitation inductance, and rotor inductance of the i-th wind turbine unit; τ sni represents the stator time constant of the i-th wind turbine unit, R si and R ri respectively represent the stator resistance and rotor resistance of the i-th wind turbine unit; σ i is the generator leakage magnetic coefficient of the i-th wind turbine unit, is the given rotor current reference value under the low voltage ride-through control mode of the i-th wind turbine unit; is the d-axis component of the given rotor current reference value under the low voltage ride-through control mode of the i-th wind turbine unit; u sdi represents the d-axis component of the stator voltage of the i-th wind turbine unit; ψ sdi and ψ sqi respectively represent the d- and q-axis components of the stator flux linkage of the i-th wind turbine unit; k pi and k ii are respectively the inner loop proportional coefficient and integral coefficient of the rotor side converter of the i-th wind turbine unit; i rd0i and i rq0i are respectively the d- and q-axis components of the initial value of the rotor current before the fault of the i-th wind turbine unit; R li and L li and L ti respectively represent the resistance, inductance of the collector line, and inductance of the box transformer connected to the i-th wind turbine unit; X T is the equivalent reactance of the main transformer of the outgoing line.

4. The time-domain distance protection method for the outgoing line of a doubly-fed wind farm according to claim 3, characterized in that The virtual fault location calculation equation is: p c +p R R g +p b x + p a x 2 = 0 (3) where x represents the virtual fault location and R g represents the transition resistance; When the fault type is single-phase grounding fault, p c = p1, p R = p2, p b = p3, p a = p4; When the fault type is a two-phase interphase fault, p c = p5, p R = p6, p b = p7, p a = p8; When the fault type is two-phase grounding fault, p c = p5, p R = p9, p b = p7, p a = p8; When the fault type is a three-phase symmetrical fault, the fault between any two phases in the three-phase symmetrical fault can be regarded as a two-phase grounding fault. Select any two phases and calculate p according to the calculation method for two-phase grounding faults c , p R , p b and p a ; p9 = 2p6 (6) Among them, i mφ represents the short-circuit current during a φ-phase fault, and u mφ represents the fault voltage during a φ-phase fault, where φ takes a, b, or c; i mρ represents the short-circuit current of the ρ line during a two-phase fault, and u mφ represents the fault voltage of the ρ line during a two-phase fault, where ρ takes ab, bc, or ac; u m0 , i m0 respectively represent the zero-sequence fault voltage and zero-sequence short-circuit current during a φ-phase fault, R l , L l are respectively the positive-sequence resistance and inductance per unit length of the AC line; R s0 , L s0 are respectively the zero-sequence equivalent resistance and inductance on the system side of the outgoing line, and R l0 , L l0 are respectively the zero-sequence equivalent resistance and inductance of the outgoing line; the compensation coefficient Δu mρ = u mρ - u mρ0 , Δi mρ = i mρ - i mρ0 , u mρ0 , i mρ0 are respectively the steady-state component values of the line voltage and line current on the fan side of the outgoing line, where ρ takes ab, bc, or ac.

5. The time-domain distance protection method for the outgoing line of a doubly-fed wind farm according to claim 4, wherein When the fault type is single-phase grounding fault, p c = p1′, p R = p2′, p b = p3′, p a = p4′; When the fault type is a two-phase interphase fault, p c = p5′, p R = p6′, p b = p7′, p a = p8′; When the fault type is two-phase grounding fault, p c = p5′, p R = p9′, p b = p7′, p a = p8′: When the fault type is a three-phase symmetrical fault, the fault between any two phases in the three-phase symmetrical fault can be regarded as a two-phase grounding fault. Arbitrarily select two phases and calculate p according to the calculation method for two-phase grounding faults c , p R , p b and p a ; p9′ = 2p6′ (9) where, i mφ ' represents the fault current during a φ-phase fault; i mρ ' represents the fault current of the ρ line during a two-phase fault; i m0 ' represents the zero-sequence fault current during a φ-phase fault, Δi mρ ' = i mρ ' - i mρ0 , and ρ takes ab, bc, or ac.

6. The time-domain distance protection method for the outgoing line of a doubly-fed wind farm according to claim 5, characterized in that, The fault similarity factor S can be expressed as: Wherein, K is the number of sampling points within 1 ms, p c (k), p R (k), p b (k), p a (k) are the p calculated based on the fault current and fault voltage at the k-th sampling point respectively c , p R , p b , p a .

7. The time-domain distance protection method for the outgoing line of a doubly-fed wind farm according to claim 6, characterized in that, Based on the fault similarity factor, identifying whether an in-zone fault has occurred on the outgoing line includes: If the fault similarity factor is greater than the set action threshold value, an in-zone fault has occurred on the outgoing line; otherwise, an out-of-zone fault has occurred on the outgoing line.

8. A time-domain distance protection system for the outgoing line of a doubly-fed wind farm, characterized in that, Including: A data acquisition module, configured to collect the fault voltage and fault current on the wind farm side of the outgoing line after a fault occurs, and determine the fault type; A short-circuit current acquisition module, configured to obtain the short-circuit current at the corresponding sampling point based on the fault voltage at each sampling point; A virtual fault location determination module, configured to determine the virtual fault location of the outgoing line based on the fault type, as well as the short-circuit current and fault voltage at all sampling points; A fault similarity factor acquisition module, configured to obtain the fault similarity factor based on the virtual fault location, as well as the fault voltage and fault current at all sampling points; A fault protection module, configured to identify whether an in-zone fault has occurred on the outgoing line based on the fault similarity factor. If so, initiate the fault protection of the outgoing line; The determining the virtual fault location of the outgoing line includes: According to the fault category, determine the expressions of each short-circuit parameter in the virtual fault location calculation equation; Based on the short-circuit current and fault voltage at each sampling point, calculate the values of the expressions of each short-circuit parameter in the virtual fault location calculation equation, and use them as the short-circuit parameter value groups corresponding to the sampling points; Based on the short-circuit parameter value groups of all sampling points, fit to obtain the virtual fault location and transition resistance in the virtual fault location calculation equation; The obtaining of the fault similarity factor includes: According to the fault category, determine the expressions of each fault parameter in the virtual fault location calculation equation; Based on the fault current and fault voltage at each sampling point, calculate the values of the expressions of each fault parameter in the virtual fault location calculation equation, and use them as the fault parameter value groups corresponding to the sampling points; Based on the fault parameter value groups of all sampling points, as well as the virtual fault location and transition resistance obtained by fitting, obtain the fault similarity factor.

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