A large-scale new energy grid-connected system sending line protection method and system

Abstract

The application relates to a large-scale new energy grid-connected system outgoing line protection method and system, and belongs to the technical field of power system relay protection, which solves the problem of incorrect operation due to the lack of analysis of the internal wind farm of a double-fed wind farm outgoing line protection in the prior art. The method comprises the following steps: collecting the current and voltage at the two ends of the double-fed wind farm outgoing line before and after a fault occurs, and then obtaining the current and voltage at the two ends of the outgoing line in the frequency domain; obtaining the frequency domain model matching value at the two ends of the outgoing line based on the current and voltage at the two ends of the outgoing line in the frequency domain; determining whether an internal fault of the outgoing line occurs according to the frequency domain model matching value at the two ends of the outgoing line and a fault identification criterion; and if yes, starting the protection action of the outgoing line.

Description

Technical Field

[0001] This invention relates to the field of power system relay protection technology, and in particular to a method and system for protecting the outgoing lines of a large-scale renewable energy grid-connected system. Background Technology

[0002] With the large-scale integration of new energy sources, the transient fault characteristics of traditional power grids have changed. In particular, the fault characteristics of doubly-fed induction generator (DFIG) wind farms, such as low-order harmonics, frequency shifts, and phase distortion, pose significant challenges to traditional high-voltage transmission line protection, thereby affecting the safe and stable operation of large-scale new energy transmission systems. Therefore, researching new transmission line protection principles adapted to multi-unit grid connection in DFIG wind farms is of great significance for wind farm safety.

[0003] Currently, research on the power frequency protection principle of centralized wind power transmission systems mainly uses single-unit equivalent models to describe the transient model of the wind farm, and then analyzes and improves the adaptability of the protection. However, using only single-unit equivalent models for analysis cannot fully consider the impact of phase angle shift on the output current during faults in doubly-fed wind farms. Harmonic components and phase angle shifts generated by multiple turbines connected to the grid can also lead to adaptability problems in power frequency protection. Some research and analysis have been conducted on the protection of wind farm transmission lines. However, these studies, whether in the frequency domain or time domain, have largely failed to consider the internal conditions of the wind farm. When the connections between turbines and collector lines within the wind farm are complex, it may lead to errors in the judgment of the above protection schemes.

[0004] Therefore, the existing protection systems for doubly-fed wind farm transmission lines lack analysis of the internal workings of the wind farm, posing a risk of incorrect operation. Summary of the Invention

[0005] Based on the above analysis, the embodiments of the present invention aim to provide a method and system for protecting the transmission lines of large-scale new energy grid-connected systems, in order to solve the problem that the existing protection of transmission lines of doubly fed wind farms lacks analysis of the internal workings of the wind farm and thus causes incorrect operation.

[0006] On one hand, embodiments of the present invention provide a method for protecting the transmission line of a large-scale renewable energy grid-connected system, including:

[0007] The current and voltage at both ends of the doubly fed wind farm's transmission line are collected before and after the fault occurs, and then the current and voltage at both ends of the transmission line in the frequency domain are obtained.

[0008] Based on the current and voltage at both ends of the transmission line in the frequency domain, the frequency domain model matching values ​​at both ends of the transmission line are obtained.

[0009] Based on the frequency domain model matching values ​​at both ends of the transmission line and the fault identification criteria, it is determined whether an intra-zone fault has occurred in the transmission line; if so, the protection action of the transmission line is initiated.

[0010] Furthermore, one end of the transmission line on the wind farm side is designated as the M-terminal, and the other end on the system side is designated as the N-terminal. The frequency domain model matching values ​​at both ends of the transmission line are obtained in the following manner:

[0011] Based on the low voltage ride-through control mode entered under wind turbine fault and the frequency domain response of phase-locked loop and the coupling relationship between collector lines in the doubly fed wind farm, the frequency domain short-circuit current of the doubly fed wind farm output line is obtained, and then the frequency domain equivalent model of the doubly fed wind farm under single-phase fault is obtained.

[0012] Based on the frequency domain equivalent model of the doubly fed wind farm under single-phase fault, the fault frequency domain 1-mode equivalent model of the M-end of the transmission line under three-phase fault is obtained by phase mode transformation. Then, based on the current and voltage of the M-end of the transmission line in the frequency domain, the calculated and actual values ​​of the fault frequency domain 1-mode current of the M-end of the transmission line are obtained.

[0013] Based on the current and voltage at the N-terminal of the transmitting line in the frequency domain, the calculated and actual values ​​of the fault current at the N-terminal of the transmitting line in the frequency domain are obtained.

[0014] The frequency domain model matching values ​​of the M and N terminals of the transmitting line are obtained based on the calculated and actual values ​​of the fault frequency domain mode current.

[0015] Furthermore, the frequency domain model matching values ​​at both ends of the transmitting line are:

[0016]

[0017] in,

[0018]

[0019] In the formula, V M V N These represent the frequency domain model matching values ​​at both ends of the transmitting lines M and N, respectively, where H represents the total number of discrete frequency points extracted from the samples, and s k Δu represents the k-th discrete frequency point in the frequency domain. M1 ′(s k ), u N1 ′(s k ) represent the actual values ​​of the fault frequency domain 1-mode voltage at the M and N terminals of the transmitting line extracted at the k-th discrete frequency point; Δi M1 ′(s k ), Δi N1 ′(s k ) represent the actual values ​​of the fault frequency domain 1-mode current extracted at the M and N terminals of the transmitting line at the k-th discrete frequency point; Δi M1 (s k ), Δi N1 (s k) represent the calculated frequency domain 1-mode current values ​​at the M and N terminals of the transmission line at the k-th discrete frequency point, respectively; Y WF1 (s k ) represents the fault-mode equivalent admittance of a doubly-fed wind farm at the k-th discrete frequency point; Δi WF1 (s k Y represents the equivalent current source of a doubly-fed wind farm fault in mode 1 at the k-th discrete frequency point; G (s k ) represents the frequency domain equivalent admittance of the infinite power grid at the k-th discrete frequency point at the N-end of the transmitting line.

[0020] Furthermore, the fault identification criteria include:

[0021]

[0022] In the formula, V M-set V N-set These represent the action threshold values ​​of the frequency domain model matching values ​​at the protection installation points of the M and N terminals of the transmitting line, respectively.

[0023] If the fault identification criteria are met, the fault is determined to be within the transmission line area; otherwise, it is determined to be outside the transmission line area.

[0024] Furthermore, the action threshold value V for the frequency domain model matching value at the M and N ends of the transmitting line is determined by the following method. M-set V N-set :

[0025]

[0026] In the formula, V M1max V represents the maximum frequency domain model matching value when a high-resistance grounding fault occurs on the collector busbar of the doubly fed wind farm on the back side of the M-end of the transmitting line. M2max V represents the maximum frequency domain model matching value when a high-resistance ground fault occurs at the turbine terminal of the doubly-fed wind turbine on the M-end of the transmitting line. M3max V represents the maximum frequency domain model matching value under steady-state conditions at the M-end of the transmitting line; N1max V represents the maximum frequency domain model matching value when a high-resistance ground fault occurs in the power grid system on the back side of the N-terminal of the transmitting line. N2max V represents the maximum frequency domain model matching value under steady-state conditions at the N-terminus of the transmitting line; Mmin This represents the minimum frequency domain model matching value when a high-resistance grounding fault occurs at the M-end protection installation outlet of the transmitting line; V Nmin This is the minimum frequency domain model matching value when a high-resistance grounding fault occurs at the N-terminal protection installation outlet of the transmission line.

[0027] Furthermore, the equivalent admittance Y of the doubly fed wind farm fault mode 1 at the kth discrete frequency point is... WF1 (sk ) is represented as:

[0028]

[0029] In the formula, m represents the total number of collector wires in a doubly-fed wind farm, and n i I represents the total number of wind turbine units on the i-th collector line, where I represents (m×n) i Z is an identity matrix of order 1. M The impedance matrix X(s) represents the coupling between the conductor impedance, the transformer substation inductance, and the output transformer impedance. k θ represents the matrix representing the terminal voltage response of each wind turbine at the k-th discrete frequency point. PLLij (s k ) represents the characteristic frequency point of the phase angle offset of the j-th wind turbine on the i-th collector line at the k-th discrete frequency point, and ij,: indicates taking the i-th row and j-th column of the matrix.

[0030] Furthermore, the characteristic frequency point θ of the phase angle offset of the j-th wind turbine on the i-th collector line at the k-th discrete frequency point... PLLij (s k ) is represented as:

[0031]

[0032] In the formula, U represents the phase-locked loop proportional and integral coefficients of the j-th fan on the i-th collector line, respectively. ij (s k Let represent the terminal voltage of the j-th wind turbine on the i-th collector line at the k-th discrete frequency point. d0ij L represents the initial value of the d-axis current of the j-th fan on the i-th collector line before the fault. tij L represents the equivalent inductance of the transformer connected to the j-th wind turbine on the i-th collector line. lij U represents the equivalent impedance of the collector line from the j-th wind turbine to the collector bus on the i-th collector line. m0ij θ represents the initial voltage at the port of the j-th fan on the i-th collector line before the fault. pcc This indicates the phase angle offset of the collector bus after a fault, ω s This indicates the synchronous angular velocity.

[0033] Furthermore, the equivalent current source Δi of the doubly fed wind farm fault mode 1 at the kth discrete frequency point... WF1 (s k ) is represented as:

[0034]

[0035] In the formula, H(s) kM(s) represents the matrix of unit internal parameters and control parameter responses at the k-th discrete frequency point. k N(s) represents the DC component matrix at the k-th discrete frequency point. k ) represents the initial flux linkage response matrix of each wind turbine at the k-th discrete frequency point, t1 represents the fault time, and ψ s0ij_1 Let i represent the initial flux linkage 1-mode component of the j-th fan on the i-th collector line. M0_1 This represents the initial value of the output current at terminal M of the transmitting line, modulo 1.

[0036] On the other hand, embodiments of the present invention provide a protection system for the transmission line of a large-scale renewable energy grid-connected system, comprising:

[0037] The data acquisition module is used to collect the current and voltage at both ends of the doubly fed wind farm's transmission line before and after the fault occurs, and then obtain the current and voltage at both ends of the transmission line in the frequency domain.

[0038] The frequency domain model matching value calculation module is used to obtain the frequency domain model matching value at both ends of the transmission line based on the current and voltage at both ends of the transmission line in the frequency domain.

[0039] The in-area fault identification module is used to determine whether an in-area fault has occurred in the transmission line based on the frequency domain model matching value at both ends of the transmission line and the fault identification criterion.

[0040] The outgoing line protection module is used to activate the protection action of the outgoing line when a fault occurs within the zone.

[0041] Furthermore, the frequency domain model matching values ​​at both ends of the transmitting line are:

[0042]

[0043] in,

[0044]

[0045] In the formula, V M V N These represent the frequency domain model matching values ​​at both ends of the transmitting lines M and N, respectively, where H represents the total number of discrete frequency points extracted by sampling, and s k Δu represents the k-th discrete frequency point in the frequency domain. M1 ′(s k ), u N1 ′(s k ) represent the actual values ​​of the fault frequency domain 1-mode voltage at the M and N terminals of the transmitting line extracted at the k-th discrete frequency point; Δi M1 ′(s k ), Δi N1 ′(s k) represent the actual values ​​of the fault frequency domain 1-mode current extracted at the M and N terminals of the transmitting line at the k-th discrete frequency point; Δi M1 (s k ), Δi N1 (s k ) represent the calculated frequency domain 1-mode current values ​​at the M and N terminals of the transmission line at the k-th discrete frequency point, respectively; Y WF1 (s k ) represents the fault-mode equivalent admittance of a doubly-fed wind farm at the k-th discrete frequency point; Δi WF1 (s k Y represents the equivalent current source of a doubly-fed wind farm fault in mode 1 at the k-th discrete frequency point; G (s k ) represents the frequency domain equivalent admittance of the infinite power grid at the k-th discrete frequency point at the N-end of the transmitting line.

[0046] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects:

[0047] 1. By collecting data after a fault, the frequency domain model matching value of the wind farm's transmission line is calculated, and the faults inside and outside the area are accurately identified based on the calculation results. The action speed is fast, and only single-end electrical quantities are needed to calculate the frequency domain model matching value of this end. There is no need for electrical quantity information exchange, so it has low dependence on communication equipment and strong resistance to transition resistance. It effectively solves the problem of incorrect operation of the protection of the transmission line of the doubly fed wind farm.

[0048] 2. The transient characteristics of multi-unit grid-connected doubly-fed wind farms are accurately described by the analytical formula of fault transient current. The unit response model, phase-locked loop phase angle deviation and coupling effect between collector lines are fully considered in the low voltage ride-through control mode after the fault.

[0049] In this invention, the above-described technical solutions can be combined with each other to achieve more preferred combinations. Other features and advantages of this invention will be set forth in the following description, and some advantages may become apparent from the description or be learned by practicing the invention. The objects and other advantages of this invention can be realized and obtained from what is particularly pointed out in the description and drawings. Attached Figure Description

[0050] The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. Throughout the drawings, the same reference numerals denote the same parts.

[0051] Figure 1 This is a flowchart illustrating a method for protecting the transmission line of a large-scale renewable energy grid-connected system according to Embodiment 1 of the present invention.

[0052] Figure 2This is a topology diagram of a doubly fed wind turbine provided in Embodiment 1 of the present invention;

[0053] Figure 3 This is a schematic diagram of a doubly fed wind farm provided in Embodiment 1 of the present invention;

[0054] Figure 4 This is a schematic diagram of the frequency domain equivalent model of a doubly fed wind farm provided in Embodiment 1 of the present invention;

[0055] Figure 5 This is a schematic diagram of a system in which a grounding fault occurs within a zone, as provided in Embodiment 1 of the present invention.

[0056] Figure 6 This is a schematic diagram of a system where a grounding fault occurs on the back side of the M end of the transmitting line according to Embodiment 1 of the present invention;

[0057] Figure 7 This is a schematic diagram of a system where a grounding fault occurs on the back side of the N-terminal of the transmitting line, as provided in Embodiment 1 of the present invention.

[0058] Figure 8 (a), (b), (c), and (d) are the frequency domain model matching values ​​of phases M and N in the transmission line area of ​​embodiment 3 of the present invention for grounding faults of phases A and AB through different transition resistors.

[0059] Figure 9 (a) and (b) are the frequency domain model matching values ​​of M and N ends when the ABC phase outside the transmission line area is grounded through different transition resistors in Embodiment 3 of the present invention.

[0060] Figure 10 (a), (b), (c), and (d) represent the frequency domain model matching values ​​of M and N ends when a two-phase ground fault (AB phase) occurs at different locations within the transmission line area in Embodiment 3 of the present invention. Detailed Implementation

[0061] Preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings, which form part of this application and are used together with the embodiments of the present invention to illustrate the principles of the present invention, but are not intended to limit the scope of the present invention.

[0062] Example 1

[0063] A specific embodiment of the present invention discloses a method for protecting the transmission line of a large-scale renewable energy grid-connected system, such as... Figure 1 As shown, it includes:

[0064] S1. Collect the current and voltage at both ends of the doubly fed wind farm's transmission line before and after the fault occurs, and then obtain the current and voltage at both ends of the transmission line in the frequency domain.

[0065] Specifically, the current and voltage at both ends of the doubly fed wind farm transmission line are collected using the current transformer at the protection installation point, and then the discrete frequency information is extracted using the existing technology of discrete wavelet transform to obtain the current and voltage at both ends of the transmission line in the frequency domain.

[0066] S2. Based on the current and voltage at both ends of the transmission line in the frequency domain, obtain the frequency domain model matching value at both ends of the transmission line.

[0067] In implementation, in step S2, one end of the transmission line on the wind farm side is designated as end M, and the other end on the system side is designated as end N. The frequency domain model matching values ​​at both ends of the transmission line are obtained in the following manner:

[0068] Based on the low voltage ride-through control mode entered under wind turbine fault and the frequency domain response of phase-locked loop and the coupling relationship between collector lines in the doubly fed wind farm, the frequency domain short-circuit current of the doubly fed wind farm output line is obtained, and then the frequency domain equivalent model of the doubly fed wind farm under single-phase fault is obtained.

[0069] Based on the frequency domain equivalent model of the doubly fed wind farm under single-phase fault, the fault frequency domain 1-mode equivalent model of the M-end of the transmission line under three-phase fault is obtained by phase mode transformation. Then, based on the current and voltage of the M-end of the transmission line in the frequency domain, the calculated and actual values ​​of the fault frequency domain 1-mode current of the M-end of the transmission line are obtained.

[0070] Based on the current and voltage at the N-terminal of the transmitting line in the frequency domain, the calculated and actual values ​​of the fault current at the N-terminal of the transmitting line in the frequency domain are obtained.

[0071] The frequency domain model matching values ​​of the M and N terminals of the transmitting line are obtained based on the calculated and actual values ​​of the fault frequency domain mode current.

[0072] In specific implementation, the frequency domain model matching values ​​at both ends of the transmitting line are:

[0073]

[0074] in,

[0075]

[0076] In the formula, V M V N These represent the frequency domain model matching values ​​at both ends of the transmitting lines M and N, respectively, where H represents the total number of discrete frequency points extracted from the samples, and s k Δu represents the k-th discrete frequency point in the frequency domain. M1 ′(s k ), Δu N1 ′(s k ) represent the actual values ​​of the fault frequency domain 1-mode voltage at the M and N terminals of the transmitting line extracted at the k-th discrete frequency point; Δi M1 ′(sk ), Δi N1 ′(s k ) represent the actual values ​​of the fault frequency domain 1-mode current extracted at the M and N terminals of the transmitting line at the k-th discrete frequency point; Δi M1 (s k ), Δi N1 (s k ) represent the calculated frequency domain 1-mode current values ​​at the M and N terminals of the transmission line at the k-th discrete frequency point, respectively; Y WF1 (s k ) represents the fault-mode equivalent admittance of a doubly-fed wind farm at the k-th discrete frequency point; Δi WF1 (s k Y represents the equivalent current source of a doubly-fed wind farm fault in mode 1 at the k-th discrete frequency point; G (s k ) represents the frequency domain equivalent admittance of the infinite power grid at the k-th discrete frequency point at the N-end of the transmitting line.

[0077] Specifically, the equivalent admittance Y of the fault mode 1 in the doubly fed wind farm at the kth discrete frequency point... WF1 (s k ) is represented as:

[0078]

[0079] In the formula, m represents the total number of collector wires in a doubly-fed wind farm, and n i I represents the total number of wind turbine units on the i-th collector line, where I represents (m×n) i Z is an identity matrix of order 1. M X(s) represents the impedance matrix that integrates the impedance of the transmission line, the inductance of the transformer substation, and the impedance coupling of the transmitting transformer. k θ represents the matrix representing the terminal voltage response of each wind turbine at the k-th discrete frequency point. PLLij (s k ) represents the characteristic frequency point of the phase angle offset of the j-th wind turbine on the i-th collector line at the k-th discrete frequency point, and ij,: represents taking the i-th row and j-th column of the matrix.

[0080] More specifically, the characteristic frequency point θ of the phase angle offset of the j-th wind turbine on the i-th collector line at the k-th discrete frequency point. PLLij (s k ) is represented as:

[0081]

[0082] In the formula, U represents the phase-locked loop proportional and integral coefficients of the j-th fan on the i-th collector line, respectively. ij (s kLet represent the terminal voltage of the j-th wind turbine on the i-th collector line at the k-th discrete frequency point. d0ij L represents the initial value of the d-axis current of the j-th fan on the i-th collector line before the fault. tij L represents the equivalent inductance of the transformer connected to the j-th wind turbine on the i-th collector line. lij U represents the equivalent impedance of the collector line from the j-th wind turbine to the collector bus on the i-th collector line. m0ij θ represents the initial voltage at the port of the j-th fan on the i-th collector line before the fault. pcc This indicates the phase angle offset of the collector bus after a fault, ω s This indicates the synchronous angular velocity.

[0083] Specifically, the equivalent current source Δi of the doubly fed wind farm fault mode 1 at the kth discrete frequency point... WF1 (s k ) is represented as:

[0084]

[0085] In the formula, H(s) k M(s) represents the matrix of unit internal parameters and control parameter responses at the k-th discrete frequency point. k N(s) represents the DC component matrix at the k-th discrete frequency point. k ) represents the initial flux linkage response matrix of each wind turbine at the k-th discrete frequency point, t1 represents the fault time, and ψ s0ij_1 Let i represent the initial flux linkage 1-mode component of the j-th fan on the i-th collector line. M0_1 This represents the initial value of the output current at terminal M of the transmitting line, modulo 1.

[0086] S3. Based on the frequency domain model matching values ​​at both ends of the transmission line and the fault identification criteria, determine whether an intra-zone fault has occurred in the transmission line; if so, initiate the protection action of the transmission line.

[0087] In implementation, the fault identification criteria include:

[0088]

[0089] In the formula, V M-set V N-set These represent the action threshold values ​​of the frequency domain model matching values ​​at the protection installation points of the M and N terminals of the transmitting line, respectively.

[0090] If the fault identification criteria are met, the fault is determined to be within the transmission line area; otherwise, it is determined to be outside the transmission line area.

[0091] Specifically, the action threshold value V for the frequency domain model matching value at the M and N ends of the transmitting line is determined by the following method. M-set VN-set :

[0092]

[0093] In the formula, V M1max V represents the maximum frequency domain model matching value when a high-resistance grounding fault occurs on the collector busbar of the doubly fed wind farm on the back side of the M-end of the transmitting line. M2max V represents the maximum frequency domain model matching value when a high-resistance ground fault occurs at the turbine terminal of the doubly-fed wind turbine on the M-end of the transmitting line. M3max V represents the maximum frequency domain model matching value under steady-state conditions at the M-end of the transmitting line; N1max V represents the maximum frequency domain model matching value when a high-resistance ground fault occurs in the power grid system on the back side of the N-terminal of the transmitting line. N2max V represents the maximum frequency domain model matching value under steady-state conditions at the N-terminus of the transmitting line; Mmin This represents the minimum frequency domain model matching value when a high-resistance grounding fault occurs at the M-end protection installation outlet of the transmitting line; V Nmin This is the minimum frequency domain model matching value when a high-resistance grounding fault occurs at the N-terminal protection installation outlet of the transmission line.

[0094] Compared with existing technologies, this embodiment provides a protection method for the outgoing lines of a large-scale renewable energy grid-connected system. It calculates the frequency domain model matching value of the wind farm's outgoing lines by collecting data after a fault, and accurately identifies faults inside and outside the area based on the calculation results. It has a fast response speed, only requires single-end electrical quantities to calculate the local frequency domain model matching value, does not require electrical quantity information exchange, has low dependence on communication equipment, and has strong resistance to transition resistance. It effectively solves the problem of incorrect operation of the protection of the outgoing lines of doubly-fed wind farms. It accurately describes the transient characteristics of multi-unit grid connection of doubly-fed wind farms through the analytical formula of fault transient current, and fully considers the unit response model, phase-locked loop phase angle deviation, and coupling effect between collector lines under the low voltage ride-through control mode after a fault.

[0095] To facilitate a better understanding of the formation process of the solution in this embodiment by those skilled in the art, the following will be used as an example. Figure 2 The doubly fed wind turbine topology diagram and Figure 3 Taking the doubly-fed wind farm schematic diagram shown as an example, the working principle of the transmission line protection method for a large-scale renewable energy grid-connected system provided in this embodiment is explained as follows:

[0096] by Figure 2 Taking the DFIG model shown as an example, when a grid fault occurs at t = t1, neglecting phase transition, the DFIG terminal voltage changes from u s0 Falling to u sAssuming the DFIG switches to low-voltage ride-through control mode at the moment of the fault, and the rotor voltage is provided by the rotor-side converter RSC, the analysis is performed using a synchronous rotating coordinate system. The conventional DFIG stator, rotor, and flux linkage equations for the motor are as follows:

[0097]

[0098]

[0099]

[0100]

[0101] In the formula, u sd u sq These represent the d-axis and q-axis components of the stator voltage, respectively; i sd i sq ψ represents the d-axis and q-axis components of the stator current, respectively; sd ψ sq Represent the d-axis and q-axis components of the stator flux linkage, respectively; R s Represents stator resistance; ω s Indicates synchronous angular velocity; u rd u rq These represent the d-axis and q-axis components of the rotor voltage, respectively; i rd i rq ψ represents the d-axis and q-axis components of the rotor current, respectively. rd ψ rq Represent the d-axis and q-axis components of the rotor flux linkage, respectively; R r Represents rotor resistance; ω slip L represents the slip angular velocity; s L m L r These are the stator inductance, equivalent magnetizing inductance, and rotor inductance, respectively.

[0102] Since the DFIG immediately initiates low-voltage ride-through mode after a grid fault, it is assumed that only the inner loop of the RSC participates in the control, and its control equation can be written as:

[0103]

[0104] in,

[0105] In the formula, k p k q These represent the proportional coefficient and integral coefficient of the inner loop of RSC, respectively; σ represents the d-axis and q-axis components of the rotor current reference value given in the low voltage ride-through control mode of the wind turbine, respectively; σ represents the generator leakage flux coefficient.

[0106] According to the low-voltage ride-through requirements, wind turbines should have dynamic reactive power support capability during the low-voltage ride-through process after the terminal voltage drops. Therefore, the rotor current reference value is:

[0107]

[0108] In the formula, U represents the d-axis and q-axis components of the rotor current reference value, respectively. s K represents the magnitude of the voltage drop after the terminal voltage decreases. d K represents the reactive power gain coefficient, preferably... d ≥1.5; I max This indicates the maximum output current amplitude of the fan.

[0109] According to the law of conservation of stator flux linkage, the stator flux linkage response after a power grid fault is as follows:

[0110]

[0111] Where, τ sn =1 / τ s +jω s , τ s =σL s / R s .

[0112] In the formula, ψ s Represents the stator flux linkage vector; u s τ represents the stator voltage vector; s t represents the stator time constant; t represents the current time, t1 represents the fault time; j represents the imaginary number in the complex number expression.

[0113] After a wind turbine starts and reaches a steady state, its electrical quantities remain stable, and the rotor current remains consistent throughout steady-state operation. Therefore, the rotor current i before the fault is used as the reference value. rd0 i rq0 Using the initial value and assuming the initial rate of change of the rotor current is 0, by combining equations (1)-(7), the rotor current response equation can be solved as follows:

[0114]

[0115] in,

[0116]

[0117]

[0118]

[0119]

[0120]

[0121]

[0122] Based on the stator fault flux linkage response and rotor current response, the frequency domain expression for the fan short-circuit current can be obtained as follows:

[0123]

[0124] in,

[0125] In the formula, i s (s) represents the stator current in the frequency domain, where s represents a complex variable, and ψ s0 This indicates the initial flux linkage value of the stator before the fault. This indicates the reference value for the rotor-side current.

[0126] In a doubly-fed induction generator (DFIG) wind farm, each wind turbine is cascaded to a collector line via a box-type substation. Multiple collector lines converge at the collector busbar, and then the power is transmitted to the high-voltage transmission line via the main transformer. A transient characteristic analysis is performed using an example of m chain-structured collector lines, as shown in the structural diagram below. Figure 3 As shown. Where, u sij This represents the terminal voltage of the j-th wind turbine unit on the i-th collector line, where i ranges from 1 to m and j ranges from 1 to n. i m represents the total number of collectors in a doubly-fed wind farm, and n i R represents the total number of wind turbine units on the i-th collector line; l L l L t These represent the equivalent resistance per unit length, inductance per unit length, and transformer inductance of the conductors connected to each wind turbine unit, respectively; l represents the length of the conductors connected to each unit; X T i represents the equivalent reactance of the main transformer of the transmitting line. M U represents the output current of a doubly-fed wind farm. M This indicates the voltage at the protective installation point at the wind farm outlet.

[0127] The short-circuit current of the j-th fan on the i-th collector line is represented by i. sij It is stated that, according to Figure 3 The frequency domain expression for the short-circuit current of each unit in a doubly-fed wind farm can be obtained as follows:

[0128]

[0129] in,

[0130]

[0131]

[0132]

[0133] y = 1, 2, ..., m

[0134] In the formula, This indicates the nth collector wire of the y-th type. yi The lengths of the collector cables connected to each wind turbine unit are yi=1,2,…,y.

[0135] It should be noted that the "-" in equation (10) has no numerical meaning and is only used for matrix partitioning.

[0136] Based on the phase-locked loop frequency domain response, the phase angle offset of the doubly-fed wind turbine phase-locked loop is:

[0137]

[0138] In the formula, θ PLLij (s) represents the characteristic frequency point of the phase angle offset of the j-th fan on the i-th collector line at the k-th discrete frequency point. U represents the phase-locked loop proportional and integral coefficients of the j-th fan on the i-th collector line, respectively. ij (s) represents the terminal voltage of the j-th fan on the i-th collector line at the k-th discrete frequency point. d0ij L represents the initial value of the d-axis current of the j-th fan on the i-th collector line before the fault. tij L represents the equivalent inductance of the transformer connected to the j-th wind turbine on the i-th collector line. lij U represents the equivalent impedance of the collector line from the j-th wind turbine to the collector bus on the i-th collector line. m0ij θ represents the initial voltage at the port of the j-th fan on the i-th collector line before the fault. pcc This indicates the phase angle shift of the collector bus after a fault.

[0139] Based on the above derivation, the frequency domain expression for the short-circuit current of a doubly-fed wind farm, taking into account the phase-locked loop deviation and the transient characteristics of a single unit, is as follows:

[0140]

[0141] In the formula, X(s) represents the matrix of the terminal voltage response of each wind turbine, H(s) represents the matrix of the response of the internal parameters and control parameters of the turbine, M(s) represents the DC component matrix, and N(s) represents the initial flux linkage response matrix of each wind turbine.

[0142] Specifically, the formulas for each matrix are as follows:

[0143]

[0144]

[0145]

[0146]

[0147] in,

[0148]

[0149]

[0150]

[0151]

[0152]

[0153] τ snij =1 / τ sij +jω s

[0154] τ sij =σ ij L sij / R sij

[0155]

[0156] In the formula, L sij L mij L rij Let ω represent the stator inductance, equivalent magnetizing inductance, and rotor inductance of the j-th fan on the i-th collector line, respectively; slipij R represents the slip angular velocity of the j-th fan on the i-th collector line; sij R rij Represents the stator resistance and rotor resistance of the j-th fan on the i-th collector line; i rd0 i rq0 These represent the initial values ​​of the rotor current before the fault; k pij k qij Let RSC represent the inner loop proportional coefficient and integral coefficient of the j-th fan on the i-th collector line, respectively; This represents the reference value of the rotor-side current of the j-th fan on the i-th collector line.

[0157] From equation (12), the frequency domain expression of the short-circuit current in a doubly-fed wind farm can be obtained as follows:

[0158]

[0159] In the formula, I represents (m×n) i Z is an identity matrix of order 1. MX(s) represents the coupling matrix between parameters of different feeders and parameters within the wind turbine unit, ψ s0ij Let represent the initial flux linkage of the j-th fan on the i-th collector line, and ij,: denote taking the i-th row and j-th column of the matrix.

[0160] Based on the above derivations, considering the low-voltage ride-through control mode under wind turbine faults and the frequency domain response of the phase-locked loop in a doubly-fed wind farm, as well as the coupling relationship between the collector wires, the frequency domain equivalent model of a doubly-fed wind farm is expressed as follows:

[0161] Δi M (s)=Y WF (s)·Δu M (s)+Δi WF (s) (18)

[0162]

[0163] In the formula, i M0 This indicates the output current at terminal M of the sending line before the fault, Y WF (s) represents the fault-mode equivalent admittance matrix of a doubly fed wind farm in the frequency domain, Δi WF (s) represents the equivalent current source of a fault in a doubly fed wind farm in the frequency domain.

[0164] From equations (18) and (19), a schematic diagram of the frequency domain equivalent model of a doubly-fed wind farm can be obtained, as follows: Figure 4 As shown.

[0165] The frequency domain equivalent model of the doubly-fed wind farm above is based on the analysis of single-phase fault characteristics. However, actual line faults occur on three-phase lines. To eliminate the electromagnetic coupling effects between the three-phase lines, phase mode transformation is used to decouple the three-phase lines. According to the definition of Clark transform, the calculation formula is as follows:

[0166]

[0167] In the formula, X1, X2, and X0 represent the electrical quantities of mode 1, mode 2, and mode 0 after phase-mode transformation, respectively. a X b X c These represent the three-phase electrical quantities.

[0168] Since the model obtained from the first-mode electrical quantities can contain all the fault quantity information when a ground fault occurs, according to equation (20), the frequency domain first-mode equivalent model of the M-end of the doubly fed wind farm transmission line is as follows:

[0169] Δi M1 (s)=Y WF1 (s)·Δu M1 (s)+Δi WF1(s) (21)

[0170]

[0171] In the formula, ψ s0ij_1 Let i represent the initial flux linkage 1-mode component of the j-th fan on the i-th collector line. M0_1 This represents the initial value of the output current at terminal M, modulo 1.

[0172] In a doubly-fed induction generator (DFIG) wind farm transmission system, assuming the opposite end of the transmission line is the system grid, the frequency domain 1-mode equivalent model of the N-end of the transmission line is obtained as follows:

[0173] i N1 (s)=-Y G ·Δu N1 (s) (23)

[0174] In the formula, Y G Δu represents the frequency domain equivalent admittance of an infinite power grid. N1 (s), Δi N1 (s) represent the frequency domain modulo 1 components of the fault voltage and current at the N-terminal of the transmitting line, respectively.

[0175] Based on the fault type of the transmitting line, the frequency domain 1-mode equivalent model of the M and N terminals of the transmitting line is analyzed to obtain the relationship between the fault type and the frequency domain 1-mode equivalent model, and then the frequency domain model matching value and fault identification criterion are given; specifically:

[0176] A ground fault occurs at position x of the wind farm's transmission line, with a transition resistance of R. g For example, Figure 5 As shown. Where d represents the total length of the outgoing line, Z... L U represents the frequency domain equivalent impedance per unit length of the transmitting line. f This represents the frequency domain fault component power supply at the fault point.

[0177] according to Figure 5 When a fault occurs within the affected area, the frequency domain model of the M-terminal of the transmitting line can be written as follows:

[0178] Δi M1 =Y WF1 ·Δu M1 +Δi WF1 (24) Similarly, according to Figure 5 When a fault occurs within the area, the frequency domain model of the N-terminal of the transmitting line is as follows:

[0179] Δi N1 =-Y G ·Δu N1 (25)

[0180] Based on the above analysis, it can be seen that when a fault occurs within the area, the fault frequency domain models detected at both ends of the sending lines M and N are the equivalent frequency domain 1-mode models on their back side.

[0181] Taking a grounding fault in the internal collector line of a doubly fed wind farm on the back side of the M-end of the transmission line as an example, such as Figure 6 As shown.

[0182] Depend on Figure 6 Therefore, when a grounding fault occurs in the collector line inside the doubly fed wind farm on the back side of the M-end of the transmitting line, the frequency domain model of the M-end of the transmitting line is:

[0183] Δi M1 =(dZ) L -1 +Y G )Δu M1 (26)

[0184] according to Figure 6 It can be seen that the frequency domain model of the N-end of the sending line is the same as that in equation (25).

[0185] Taking a ground fault occurring inside the power grid system on the back side of the N-terminal of the transmitting line as an example, such as Figure 7 As shown.

[0186] Depend on Figure 7 Therefore, when an internal ground fault occurs in the grid system behind the N-terminal of the transmitting line, the frequency domain model of the N-terminal is:

[0187]

[0188] according to Figure 7 It can be seen that the frequency domain model of the M-end of the sending line is the same as that of equation (24).

[0189] The above analysis shows that when an external fault occurs, the frequency domain model obtained on the side where the external fault occurs is different from the equivalent frequency domain 1-modulus model when an internal fault occurs, while the frequency domain model obtained on the other side is the same as the equivalent frequency domain 1-modulus model when an internal fault occurs.

[0190] Based on the above analysis of frequency domain models inside and outside the transmission line, it can be seen that there are significant differences in the fault frequency domain models obtained when the fault is inside or outside the transmission line. Therefore, the judgment of faults inside and outside the transmission line is transformed into the identification of fault frequency domain models.

[0191] In this embodiment, the actual fault frequency domain model obtained by sampling values ​​during an actual fault is compared with the calculated intra-region fault frequency domain 1-mode model obtained above. This allows for accurate determination of whether the actual fault is an intra-region fault. Furthermore, in this embodiment, the actual value of the fault frequency domain 1-mode current obtained by actual sampling is compared with the calculated value of the fault frequency domain 1-mode current obtained by equations (21) and (23) for specific quantitative analysis.

[0192] When the actual fault occurs within the transmission line area, the actual fault frequency domain model satisfies equations (21) and (23), that is, the calculated and actual values ​​of the fault frequency domain 1-mode current at the M and N ends of the transmission line are equal:

[0193]

[0194] In the formula, Δi M1 ′、Δi N1 ′ represents the actual value of the fault current extracted from the M and N terminals of the transmitting line in the 1-mode frequency domain.

[0195] When the actual fault occurs outside the transmission line area, the actual fault frequency domain model is different from the calculated fault frequency domain 1-mode model within the area, and does not satisfy equation (28), that is, the calculated value and actual value of the fault frequency domain 1-mode current at the M and N ends of the transmission line are not equal.

[0196] In summary, when a fault occurs within the transmission line, the actual fault frequency domain model is the same as the calculated fault frequency domain 1-mode model within the transmission line. That is, the calculated and actual values ​​of the fault frequency domain 1-mode current at the M and N ends of the transmission line are equal. Equation (28) shows that the result at the M and N ends of the transmission line is 0. When a fault occurs outside the transmission line, the actual fault frequency domain model is very different from the calculated fault frequency domain 1-mode model within the transmission line. That is, the calculated and actual values ​​of the fault frequency domain 1-mode current at the M and N ends of the transmission line are not equal. Equation (28) shows that the result at the M and N ends of the transmission line is not 0.

[0197] Therefore, in this embodiment, the matching degree between the fault frequency domain mode 1 model calculated at both ends of the line and the actual fault frequency domain model is quantified by the frequency domain model matching value, that is, the matching degree between the calculated and actual values ​​of the fault frequency domain mode 1 current at the M and N ends of the line. Fault identification is performed using the frequency domain model matching value, and the frequency domain model matching value V at both ends of the line is sent out. M V N They are represented as follows:

[0198]

[0199] in,

[0200]

[0201] In the formula, H represents the total number of discrete frequency points extracted by sampling, and Δu M1 ′(s k ), Δu N1 ′(s k ) represent the actual values ​​of the fault frequency domain 1-mode voltage at the M and N terminals of the transmitting line extracted at the k-th discrete frequency point; Δi M1 ′(s k ), ΔiN1 ′(s k ) represent the actual values ​​of the fault frequency domain 1-mode current extracted at the M and N terminals of the transmitting line at the k-th discrete frequency point; Δi M1 (s k ), Δi N1 (s k ) represent the calculated frequency domain 1-mode current values ​​at the M and N terminals of the transmission line at the k-th discrete frequency point, respectively; Y WF1 (s k ) represents the fault-mode equivalent admittance of a doubly-fed wind farm at the k-th discrete frequency point; Δi WF1 (s k Y represents the equivalent current source of a doubly-fed wind farm fault in mode 1 at the k-th discrete frequency point; G (s k ) represents the frequency domain equivalent admittance of an N-terminal infinite power grid at the k-th discrete frequency point.

[0202] Considering the influence of uncertainties such as measurement errors, the frequency domain model of the fault in the MN zone of the transmitting line does not strictly satisfy equation (28). Therefore, the action threshold value of the frequency domain model matching value at the protection installation points at both ends of the transmitting line M and N is set by the following formula:

[0203]

[0204] In the formula, V M1max V represents the maximum frequency domain model matching value when a high-resistance grounding fault occurs on the collector busbar of the M-end backside doubly fed wind farm. M2max V represents the maximum frequency domain model matching value when a high-resistance ground fault occurs at the M-end backside doubly-fed wind turbine generator terminal. M3max V represents the maximum frequency domain model matching value under steady-state conditions at the M-end; N1max V represents the maximum frequency domain model matching value when a high-resistance ground fault occurs in the N-terminal back-side power grid system. N2max V represents the maximum frequency domain model matching value under steady-state conditions at the N-terminal end. Mmin This is the minimum frequency domain model matching value when a high-resistance grounding fault occurs at the N-terminal protection outlet of the transmitting line; V Nmin This is the minimum frequency domain model matching value when a high-resistance grounding fault occurs at the M-end protection installation outlet of the transmission line. It should be noted that the above frequency domain model matching values ​​can be obtained according to equation (30) under different fault types.

[0205] Therefore, the fault identification criteria include:

[0206]

[0207] In the formula, V M-set V represents the action threshold value of the M-terminal frequency domain model matching value. N-setThis represents the action threshold value for matching the N-terminal frequency domain model.

[0208] If the fault identification criteria are met, the fault is determined to be within the sending line area; otherwise, it is determined to be outside the sending line area. Specifically, if the fault is within the sending line area, a trip signal is issued.

[0209] Furthermore, in this embodiment, considering the influence of factors such as calculation error and noise drying, and based on the limitation of formula (30), the action threshold value of the protection at both ends is set to 50.

[0210] Therefore, after a fault occurs, the fault identification and judgment are performed based on the frequency domain model matching values ​​at both ends of the sending lines M and N. If the frequency domain model matching values ​​at both ends of the sending lines M and N are greater than the corresponding action threshold values, then an intra-zone fault occurs, and the protection at both ends of the sending lines transmits the fault direction as "intra-zone fault" and sends a trip signal to the protection device of the faulty line; otherwise, it is judged that an extra-zone fault occurs and no action is taken.

[0211] Example 2

[0212] A specific embodiment 2 of the present invention provides a protection system for the transmission line of a large-scale renewable energy grid-connected system, comprising:

[0213] The data acquisition module is used to collect the current and voltage at both ends of the doubly fed wind farm's transmission line before and after the fault occurs, and then obtain the current and voltage at both ends of the transmission line in the frequency domain.

[0214] The frequency domain model matching value calculation module is used to obtain the frequency domain model matching value at both ends of the transmission line based on the current and voltage at both ends of the transmission line in the frequency domain.

[0215] The in-area fault identification module is used to determine whether an in-area fault has occurred in the transmission line based on the frequency domain model matching value at both ends of the transmission line and the fault identification criterion.

[0216] The outgoing line protection module is used to activate the protection action of the outgoing line when a fault occurs within the zone.

[0217] In practice, the frequency domain model matching values ​​at both ends of the transmitting line are:

[0218]

[0219] in,

[0220]

[0221] In the formula, V M V N These represent the frequency domain model matching values ​​at both ends of the transmitting lines M and N, respectively, where H represents the total number of discrete frequency points extracted from the samples, and sk Δu represents the k-th discrete frequency point in the frequency domain. M1 ′(s k ), u N1 ′(s k ) represent the actual values ​​of the fault frequency domain 1-mode voltage at the M and N terminals of the transmitting line extracted at the k-th discrete frequency point; Δi M1 ′(s k ), Δi N1 ′(s k ) represent the actual values ​​of the fault frequency domain 1-mode current extracted at the M and N terminals of the transmitting line at the k-th discrete frequency point; Δi M1 (s k ), Δi N1 (s k ) represent the calculated frequency domain 1-mode current values ​​at the M and N terminals of the transmission line at the k-th discrete frequency point, respectively; Y WF1 (s k ) represents the fault-mode equivalent admittance of a doubly-fed wind farm at the k-th discrete frequency point; Δi WF1 (s k Y represents the equivalent current source of a doubly-fed wind farm fault in mode 1 at the k-th discrete frequency point; G (s k ) represents the frequency domain equivalent admittance of the infinite power grid at the k-th discrete frequency point at the N-end of the transmitting line.

[0222] The specific implementation process of this invention can be found in the above method embodiments, and will not be repeated here.

[0223] Since this embodiment is based on the same principle as the above-described method embodiments, this system also has the corresponding technical effects of the above-described method embodiments.

[0224] Example 3

[0225] To verify the correctness of Embodiments 1 and 2 of the present invention, this embodiment conducts experimental verification of the schemes in the above embodiments. The main parameters of the doubly-fed induction generator (DFIG) wind farm transmission system in this embodiment are shown in Table 1; the main parameters of the internal feeders of the DFIG wind farm are shown in Table 2; the main parameters of feeder unit 1 of the DFIG wind farm are shown in Table 3; the main parameters of feeder unit 2 of the DFIG wind farm are shown in Table 4; and the main parameters of feeder unit 3 of the DFIG wind farm are shown in Table 5. In this embodiment, the frequency domain model matching threshold value at both ends of the transmission line is set to 50.

[0226] Scenario 1 in this implementation is as follows: when a phase A ground fault and a two-phase AB ground fault occur at 50% of the output line MN, the transition resistance varies from 0 to 300Ω.

[0227] Depend on Figure 8 (a) and Figure 8(b) It can be seen that when a ground fault occurs in phase A through different transition resistances, as the transition resistance increases, the frequency domain model matching value V of the M and N terminals under the same time section also increases. M V N The value gradually decreases; when a phase A ground fault occurs and the grounding resistance is constant, the frequency domain model matching value fluctuates little with the time window. Specifically, at t = 1.05 ms and the transition resistance is 300 Ω, the frequency domain model matching value reaches its minimum of 364.52. Figure 8 (a) shown) and 678.4 ( Figure 8 (b) shows that the value is much greater than the set threshold value of 50, which indicates that a fault has occurred in the area.

[0228] Table 1 Main parameters of the doubly-fed wind turbine and wind farm

[0229]

[0230] Table 2 Main parameters of the internal feeder in a doubly-fed wind farm

[0231]

[0232] Table 3 Main parameters of feeder unit 1 in a doubly fed wind farm

[0233]

[0234] Table 4 Main parameters of feeder unit 2 in doubly fed wind farm

[0235]

[0236] Table 5 Main parameters of feeder line 3 unit in the doubly fed wind farm

[0237]

[0238] Depend on Figure 8 (c) and Figure 8 (d) It can be seen that when a ground fault occurs between phases A and B through different resistances, as the transition resistance increases, the frequency domain model matching value V of the M and N terminals at the same time section also increases. M V N The value gradually decreases; when the transition resistance is constant, the fluctuation range of the frequency domain model matching value with the sliding of the time window is not significant. Similarly, if the frequency domain model matching value is much larger than the set threshold value, it is judged that an in-zone fault has occurred.

[0239] The frequency domain model matching values ​​for both fault types mentioned above are much greater than the threshold value. Therefore, it can be determined that a fault has occurred in the MN zone of the transmitting line, and the protection system operates correctly. From the above analysis, it can be seen that the method in Example 1 and the system in Example 2 can accurately identify faults occurring in the AC line zone with different transition resistances, unaffected by the transition resistance or fault type, demonstrating high sensitivity and rapid fault identification capability.

[0240] Scenario 2 in this implementation is as follows: two-phase ground faults (A and B) occur at different locations within the AC line area, with a transition resistance of 200Ω.

[0241] Depend on Figure 9 (a) and Figure 9 (b) It can be seen that when a two-phase ground fault (AB phase grounding) occurs at different locations within the transmission line area, the matching value at the same time section gradually decreases as the actual fault location increases from the protection installation location. Among these, the value is determined by... Figure 9 (a) It can be seen that when t = 2.41 ms and the actual fault location is 90% of the distance from terminal M, the minimum frequency domain impedance model matching value is 274.45, which is still much larger than the set threshold value; Figure 9 (b) It can be seen that when t = 2.74 ms and the fault location is 80% away from the N end, the minimum value of the frequency domain impedance model matching value is 682.9, which is much greater than the set threshold value.

[0242] The above analysis shows that the protection criteria are not affected by the location and type of the fault, and still have high sensitivity when a fault occurs at the end of the line.

[0243] Scenario 3 in this implementation is as follows: a ground fault occurs at the back side of the collector bus outside the M end of the transmission line area and at the back side of the N end outside the transmission line area, respectively, and the transition resistance varies from 0 to 300Ω.

[0244] Depend on Figure 10 (a) It can be seen that when a three-phase ground fault occurs at the back side of terminal M through different transition resistances, the fluctuation amplitude of the frequency domain impedance model matching value at different time sections is small as the transition resistance increases; among them, when the transition resistance is 300Ω and t=6.03ms, the frequency domain impedance model matching value has a maximum value of 1.13, which is much smaller than the set threshold value. Figure 10 (b) It can be seen that when a three-phase ground fault occurs at the back side of terminal M through different transition resistances, the fluctuation amplitude of the frequency domain model matching value under different time sections is small as the transition resistance increases. According to Figure 10 (a) and Figure 10 (b) shows that the frequency domain model matching value does not meet the protection operation conditions, that is, no fault has occurred within the protection range of the sending line, and the protection is reliable and does not operate.

[0245] Depend on Figure 10 (c) It can be seen that when a three-phase fault occurs at the back side of the N terminal through different transition resistances, as the transition resistance increases, the fluctuation amplitude of the frequency domain model matching value at the M terminal under different time sections is small, but still much larger than the threshold value. Figure 10 (d) shows that when a three-phase ground fault occurs at the back side of the N terminal through different transition resistances, the maximum value of the frequency domain model matching value at the N terminal is taken when t = 6.35 ms and the transition resistance is 0 Ω. The maximum value is 0.74 and is much smaller than the threshold value. According to Figure 10 (c) and Figure 10 (d) The results show that the frequency domain model matching value is much smaller than the set threshold value, which does not meet the protection action conditions, that is, no fault has occurred within the protection range of the sending line, and the protection is reliable and does not operate.

[0246] Based on the above analysis, it can be seen that the protection schemes proposed in Embodiments 1 and 2 can correctly distinguish between faults on the back side of the transmission line and faults outside the protection range, and have a strong ability to quickly identify faults.

[0247] Those skilled in the art will understand that all or part of the processes of the methods described in the above embodiments can be implemented by a computer program instructing related hardware, and the program can be stored in a computer-readable storage medium. The computer-readable storage medium may be a disk, optical disk, read-only memory, or random access memory, etc.

[0248] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for protecting the transmission line of a large-scale renewable energy grid-connected system, characterized in that, include: The current and voltage at both ends of the doubly fed wind farm's transmission line are collected before and after the fault occurs, and then the current and voltage at both ends of the transmission line in the frequency domain are obtained. Based on the current and voltage at both ends of the transmitting line in the frequency domain, the frequency domain model matching values ​​at both ends of the transmitting line are obtained; the frequency domain model matching values ​​at both ends of the transmitting line are: ； in, ； In the formula, , These represent the frequency domain model matching values ​​at the M and N ends of the transmitting lines, respectively. H This represents the total number of discrete frequency points extracted from the samples. In the frequency domain, the first A discrete frequency point; , They represent the first time. The actual values ​​of the fault frequency domain 1-mode voltage at the M and N terminals of the transmitting line extracted from discrete frequency points; , They represent the first time. The actual values ​​of the fault frequency domain 1-mode current at the M and N ends of the transmitting line extracted from discrete frequency points; , They represent the first time. Calculated values ​​of fault frequency domain mode current at the M and N terminals of discrete frequency transmission lines; Indicates the first One-mode equivalent admittance of a discrete frequency doubly fed wind farm fault; Indicates the first A discrete frequency point doubly fed wind farm fault mode 1 equivalent current source; This indicates that the infinite power grid at the Nth end of the transmission line is in the... Frequency domain equivalent admittance at discrete frequency points; Based on the frequency domain model matching values ​​at both ends of the transmission line and the fault identification criteria, determine whether an intra-zone fault has occurred in the transmission line; if so, initiate the protection action of the transmission line. The fault identification criteria include: ； In the formula, , These represent the action threshold values ​​of the frequency domain model matching values ​​at the protection installation locations at the M and N ends of the transmitting line, respectively. If the fault identification criteria are met, the fault is determined to be within the transmission line area; otherwise, it is determined to be outside the transmission line area.

2. The method for protecting the transmission line of a large-scale renewable energy grid-connected system according to claim 1, characterized in that, The wind farm side of the transmission line is the M end, and the system side is the N end; The frequency domain model matching values ​​at both ends of the transmitting line are obtained in the following way: Based on the low voltage ride-through control mode entered under wind turbine fault and the frequency domain response of phase-locked loop and the coupling relationship between collector lines in the doubly fed wind farm, the frequency domain short-circuit current of the doubly fed wind farm output line is obtained, and then the frequency domain equivalent model of the doubly fed wind farm under single-phase fault is obtained. Based on the frequency domain equivalent model of the doubly fed wind farm under single-phase fault, the fault frequency domain 1-mode equivalent model of the M-end of the transmission line under three-phase fault is obtained by phase mode transformation. Then, based on the current and voltage of the M-end of the transmission line in the frequency domain, the calculated and actual values ​​of the fault frequency domain 1-mode current of the M-end of the transmission line are obtained. Based on the current and voltage at the N-terminal of the transmitting line in the frequency domain, the calculated and actual values ​​of the fault current at the N-terminal of the transmitting line in the frequency domain are obtained. The frequency domain model matching values ​​of the M and N terminals of the transmitting line are obtained based on the calculated and actual values ​​of the fault frequency domain mode current.

3. The method for protecting the transmission line of a large-scale renewable energy grid-connected system according to claim 1, characterized in that, The action threshold values ​​for the frequency domain model matching values ​​at the M and N ends of the transmitting line are determined using the following method. , : ； In the formula, This represents the maximum frequency domain model matching value when a high-resistance grounding fault occurs on the collector busbar of the doubly fed wind farm on the M-end of the transmitting line. This represents the maximum frequency domain model matching value when a high-resistance grounding fault occurs at the turbine terminal of the doubly-fed wind turbine on the M-end of the transmitting line. This represents the maximum frequency domain model matching value under steady-state conditions at the M-end of the transmitting line; This represents the maximum frequency domain model matching value when a high-resistance ground fault occurs in the power grid system on the back side of the N-terminal of the transmitting line. This represents the maximum frequency domain model matching value under steady-state conditions at the N-end of the transmitting line; This represents the minimum frequency domain model matching value when a high-resistance grounding fault occurs at the M-end protection installation outlet of the transmitting line; This is the minimum frequency domain model matching value when a high-resistance grounding fault occurs at the N-terminal protection installation outlet of the transmission line.

4. The method for protecting the transmission line of a large-scale renewable energy grid-connected system according to claim 1, characterized in that, In the first One-mode equivalent admittance of a discrete-frequency doubly-fed wind farm fault. Represented as: ； In the formula, This indicates the total number of collector cables in a doubly-fed wind farm. Indicates the first The total number of fan units on the power line. Represents the identity matrix. This represents the impedance matrix that integrates the impedance of the transmission line, the inductance of the transformer substation, and the impedance coupling of the output transformer. Indicates the first The matrix of the terminal voltage response of each wind turbine at discrete frequency points. Indicates the first The discrete frequency point The first of the cable lines The characteristic frequency points of the phase angle offset of each wind turbine This indicates taking the first element of the matrix. Line 1 List.

5. The method for protecting the transmission line of a large-scale new energy grid-connected system according to claim 4, characterized in that, In the first The discrete frequency point The first of the cable lines Characteristic frequency points of phase angle offset of individual wind turbines Represented as: ； In the formula, , They represent the first The first of the cable lines The phase-locked loop proportional and integral coefficients of each wind turbine. To indicate the first The discrete frequency point The first of the cable lines The voltage at the terminal of each fan. Indicates the number of times before the fault occurred. The first of the cable lines Initial value of d-axis current of typhoon fan Indicates the first The first of the cable lines Equivalent inductance of the transformer box connected to the typhoon generator. Indicates the first The first of the cable lines Equivalent impedance of the collector cable passing through from the typhoon generator to the collector bus. Indicates the number of times before the fault occurred. The first of the cable lines Initial value of typhoon generator port voltage. This indicates the phase angle shift of the collector bus after the fault. This indicates the synchronous angular velocity.

6. The method for protecting the transmission line of a large-scale renewable energy grid-connected system according to claim 5, characterized in that, In the first A discrete frequency point doubly fed wind farm fault 1-mode equivalent current source Represented as: ； In the formula, Indicates the first The matrix of unit internal parameters and control parameter responses at discrete frequency points Indicates the first DC component matrix at discrete frequency points Indicates the first Initial flux linkage response matrix of each wind turbine at discrete frequency points Indicates the time of failure. Indicates the first The first of the cable lines The initial magnetic flux linkage of a single fan is a 1-mode component. This represents the initial value of the output current at terminal M of the transmitting line, modulo 1.

7. A protection system for the transmission line of a large-scale renewable energy grid-connected system, characterized in that, include: The data acquisition module is used to collect the current and voltage at both ends of the doubly fed wind farm's transmission line before and after the fault occurs, and then obtain the current and voltage at both ends of the transmission line in the frequency domain. The frequency domain model matching value calculation module is used to obtain the frequency domain model matching values ​​at both ends of the transmitting line based on the current and voltage at both ends in the frequency domain; the frequency domain model matching values ​​at both ends of the transmitting line are: ； in, ； In the formula, , These represent the frequency domain model matching values ​​at the M and N ends of the transmitting lines, respectively. H This represents the total number of discrete frequency points extracted from the samples. In the frequency domain, the first A discrete frequency point; , They represent the first time. The actual values ​​of fault frequency domain 1-mode voltage at the M and N terminals of the transmitting line extracted from discrete frequency points; , They represent the first time. The actual values ​​of fault frequency domain mode 1 current at the M and N ends of the transmitting line extracted from discrete frequency points; , They represent the first time. Calculated values ​​of fault frequency domain mode current at the M and N terminals of discrete frequency transmission lines; Indicates the first One-mode equivalent admittance of a discrete frequency doubly fed wind farm fault; Indicates the first A discrete frequency point doubly fed wind farm fault mode 1 equivalent current source; This indicates that the infinite power grid at the Nth end of the transmitting line is in the... Frequency domain equivalent admittance at discrete frequency points; The intra-area fault identification module is used to determine whether an intra-area fault has occurred on the transmission line based on the frequency domain model matching values ​​at both ends of the transmission line and fault identification criteria; the fault identification criteria include: ； In the formula, , These represent the action threshold values ​​of the frequency domain model matching values ​​at the protection installation locations at the M and N ends of the transmitting line, respectively. If the fault identification criteria are met, the fault is determined to be within the transmission line area; otherwise, it is determined to be outside the transmission line area. The outgoing line protection module is used to activate the protection action of the outgoing line when a fault occurs within the zone.

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