A method and device for protecting an offshore wind power AC line based on parameter identification

Abstract

The present invention provides a method and device for protecting an offshore wind power AC line based on parameter identification. The method is applied to an offshore wind power grid-connected system via a flexible DC link. The method includes: obtaining the flexible DC side current in real time; when the start criterion is met, obtaining the flexible DC side voltage at the current moment; calculating the fault distance corresponding to the current moment according to the flexible DC side voltage at the current moment and the flexible DC current at the current moment; wherein, the fault distance is the distance from the fault resistance to the flexible DC side; the fault resistance is connected to the wind farm side and the flexible DC side; and outputting a corresponding protection signal according to the comparison result between the fault distance and the protection criterion. Compared with the prior art, by obtaining parameters such as the flexible DC side voltage and current and taking the fault resistance as an unknown quantity to solve the fault distance, the influence of the variable of the fault resistance on the fault distance is avoided, and the fault resistance tolerance of the distance protection is improved.

Description

Technical Field

[0001] The present invention relates to the field of offshore wind power, and in particular to a protection method and device for an offshore wind power AC line based on parameter identification Background Art

[0002] Offshore wind power has the advantages of stable wind speed and small land occupation area, so it has gradually become a research hotspot in the new energy field. And the connection of offshore wind power to the grid through a flexible DC link is a transmission method suitable for the transmission of far - sea wind power

[0003] For the AC line between an offshore wind farm and a flexible DC converter station, common protection methods include distance protection, differential protection, zero - sequence current protection, etc. These traditional protection technologies require a large fault current as the action criterion (generally more than 10 times the rated current). When a fault occurs in the AC line of offshore wind power, once the current of the flexible DC converter station exceeds 2 times the rated current of the power device, the converter will take the way of blocking for self - protection, and at this time, the flexible DC converter station will no longer provide fault current. At the same time, due to the large number of power electronic devices in the offshore wind farm on the opposite side, the fault current provided by it is also very small, resulting in a situation where the short - circuit currents provided on both sides of the AC line are very small. Therefore, it may cause the AC line protection to refuse to operate under faults. In addition, single - phase grounding faults of AC lines generally contain transition resistances, and the traditional power - frequency protection principle has low tolerance to transition resistances, and the protection performance will also be affected Summary of the Invention

[0004] The present invention provides a protection method and device for an offshore wind power AC line based on parameter identification to solve the technical problem of how to improve the protection performance when the short - circuit currents on both sides of the AC line are small

[0005] To solve the above - mentioned technical problem, an embodiment of the present invention provides a protection method for an offshore wind power AC line based on parameter identification, which is applied to an offshore wind power system connected to the grid through a flexible DC link. The offshore wind power system connected to the grid through a flexible DC link includes a wind farm side and a flexible DC side. The protection method for the offshore wind power AC line includes:

[0006] Obtain the current of the flexible DC side in real time

[0007] When the current of the flexible DC side meets the starting criterion, send a voltage acquisition instruction to the flexible DC side to obtain the voltage of the flexible DC side at the current moment

[0008] Calculate the fault distance corresponding to the current moment according to the voltage of the flexible DC side at the current moment and the current of the flexible DC at the current moment; wherein, the fault distance is the distance from the fault resistance to the flexible DC side; the fault resistance connects the wind farm side and the flexible DC side

[0009] Output corresponding protection signals according to the comparison result of the fault distance and the protection criterion.

[0010] As a preferred solution, the startup criterion is specifically:

[0011] |i a (t) + i b (t) + i c (t)| - |i a (t - T) + i b (t - T) + i c (t - T)| > 0.1I N ;

[0012] Wherein, i a (t), i b (t) and i c (t) are the three-phase current sampling values on the VSC side at time t, i a (t - T), i b (t - T) and i c (t - T) are the three-phase current sampling values on the VSC side at time t - T, and I N is the rated current.

[0013] As a preferred solution, calculating the fault distance corresponding to the current time according to the voltage on the VSC side at the current time and the VSC current at the current time is specifically:

[0014] Obtain the fault distance at the current time according to the solution result of the following ranging formula:

[0015]

[0016] Wherein, i0 is the zero-sequence current on the VSC side, k r , k L , r MMC , r WF , L MMC and L WF are all intermediate variables used to simplify the operation. Specifically:

[0017]

[0018]

[0019]

[0020] r MMC = r0d + r TMMC0 ;

[0021] r WF = r0(l - d) + r TWF0 ;

[0022] L MMC = L0d + L TMMC0 ;

[0023] L WF = L0(l - d) + L TWF0 ;

[0024] Wherein, i MMC(A) , i MMC(B) and i MMC(C) are the three-phase phase currents on the VSC side, u MMC(B) is the phase A voltage on the VSC side, R f is the fault resistance, i f is the current flowing through the fault resistance, d is the distance from the fault resistance to the VSC side, l is the total length of the line between the main transformer on the wind farm side and the converter transformer on the VSC side, r1 is the positive sequence resistance per unit length of the line, r0 is the zero sequence resistance per unit length of the line, L1 is the positive sequence inductance per unit length of the line, L0 is the zero sequence inductance per unit length of the line, r TMMC0 is the equivalent resistance of the converter transformer on the VSC side, L TMMC0 is the equivalent leakage inductance of the converter transformer on the VSC side, r TWF0 is the equivalent resistance of the main transformer on the wind farm side, L TWF0 is the equivalent leakage inductance of the main transformer on the wind farm side, and i0' is the zero sequence current on the wind farm side.

[0025] As a preferred solution, the solution result is obtained by the following method: solving the ranging formula by replacing the differential component with a difference component.

[0026] As a preferred solution, according to the comparison result of the fault distance and the protection criterion, corresponding protection signals are output, specifically:

[0027] The protection criterion includes a preset distance threshold value;

[0028] When the fault distance is less than the preset distance threshold value, a protection trip signal is output; otherwise, a protection reset signal is output.

[0029] Correspondingly, an embodiment of the present invention further provides a protection device for an offshore wind power AC line based on parameter identification, which is applied to an offshore wind power integrated grid system through a VSC. The offshore wind power integrated grid system through a VSC includes a wind farm side and a VSC side. The protection device for the offshore wind power AC line includes:

[0030] An acquisition module, configured to acquire the current on the VSC side in real time;

[0031] A judgment module, configured to send a voltage acquisition instruction to the VSC side when the current on the VSC side meets the start criterion, and acquire the voltage on the VSC side at the current moment;

[0032] A calculation module, configured to calculate a fault distance corresponding to the current moment according to the voltage on the VSC side at the current moment and the VSC current at the current moment; wherein, the fault distance is the distance from the fault resistance to the VSC side; the fault resistance connects the wind farm side and the VSC side.

[0033] A protection module, configured to output a corresponding protection signal according to the comparison result between the fault distance and a protection criterion.

[0034] As a preferred solution, the startup criterion is specifically:

[0035] |i a (t)+i b (t)+i c (t)|-|i a (t-T)+i b (t-T)+i c (t-T)|>0.1I N ;

[0036] Wherein, i a (t), i b (t) and i c (t) are the three-phase current sampling values on the VSC side at the moment t, and i a (t-T), i b (t-T) and i c (t-T) are the three-phase current sampling values on the VSC side at the moment t-T, and I N is the rated current.

[0037] As a preferred solution, the calculation module calculates the fault distance corresponding to the current moment according to the voltage on the VSC side at the current moment and the VSC current at the current moment, specifically:

[0038] The calculation module obtains the fault distance at the current moment according to the solution result of the following ranging formula:

[0039]

[0040] Wherein, i0 is the zero-sequence current on the VSC side, and k r , k L , r MMC , r WF , L MMC and L WF are all intermediate variables used to simplify the operation. Specifically:

[0041]

[0042]

[0043]

[0044] r MMC = r0d + r TMMC0 ;

[0045] r WF = r0(l - d) + r TWF0 ;

[0046] L MMC = L0d + L TMMC0 ;

[0047] L WF = L0(l - d) + L TWF0 ;

[0048] wherein, i MMC(A) , i MMC(B) and i MMC(C) are the three-phase phase currents on the VSC side, u MMC(B) is the phase A voltage on the VSC side, R f is the fault resistance, i f is the current flowing through the fault resistance, d is the distance from the fault resistance to the VSC side, l is the total length of the line between the main transformer on the wind farm side and the converter transformer on the VSC side, r1 is the positive-sequence resistance per unit length of the line, r0 is the zero-sequence resistance per unit length of the line, L1 is the positive-sequence inductance per unit length of the line, L0 is the zero-sequence inductance per unit length of the line, r TMMC0 is the equivalent resistance of the converter transformer on the VSC side, L TMMC0 is the equivalent leakage inductance of the converter transformer on the VSC side, r TWF0 is the equivalent resistance of the main transformer on the wind farm side, L TWF0 is the equivalent leakage inductance of the main transformer on the wind farm side, and i0' is the zero-sequence current on the wind farm side.

[0049] As a preferred solution, the solution result is obtained by the following method: solving the ranging formula by replacing the differential component with a difference component.

[0050] As a preferred solution, the protection module outputs a corresponding protection signal according to the comparison result between the fault distance and the protection criterion, specifically:

[0051] The protection criterion includes a preset distance threshold;

[0052] When the fault distance is less than the preset distance threshold, the protection module outputs a protection trip signal; otherwise, the protection module outputs a protection reset signal.

[0053] Compared with the prior art, the embodiments of the present invention have the following beneficial effects:

[0054] An embodiment of the present invention provides a method and device for protecting an AC line of an offshore wind farm based on parameter identification. The method is applied to an offshore wind power grid-connected system via a flexible DC link. The offshore wind power grid-connected system via a flexible DC link includes a wind farm side and a flexible DC link side. The method includes: obtaining the current on the flexible DC link side in real time; when the current on the flexible DC link side satisfies the startup criterion, sending a voltage acquisition instruction to the flexible DC link side to obtain the voltage on the flexible DC link side at the current moment; calculating the fault distance corresponding to the current moment according to the voltage on the flexible DC link side at the current moment and the current on the flexible DC link side at the current moment; where the fault distance is the distance from the fault resistance to the flexible DC link side; the fault resistance connects the wind farm side and the flexible DC link side; outputting a corresponding protection signal according to the comparison result between the fault distance and the protection criterion. Compared with the prior art, by obtaining parameters such as the voltage and current on the flexible DC link side and taking the fault resistance as an unknown quantity to solve the fault distance, the influence of the variable of the fault resistance on the fault distance is avoided, and the fault resistance tolerance of the distance protection is improved. Description of the Drawings

[0055] Figure 1 : It is a schematic flowchart of an embodiment of the method for protecting an AC line of an offshore wind farm provided by the present invention based on parameter identification.

[0056] Figure 2 : It is a schematic topology diagram of an embodiment of the offshore wind power grid-connected system via a flexible DC link provided by the present invention.

[0057] Figure 3 : It is an equivalent schematic diagram of a single-phase grounding fault of an AC line through a fault resistance provided by the present invention.

[0058] Figure 4 : It is a schematic diagram of the zero-sequence component network of a single-phase grounding fault of an AC line through a fault resistance provided by the present invention.

[0059] Figure 5 : It is a schematic structural diagram of an embodiment of the device for protecting an AC line of an offshore wind farm provided by the present invention based on parameter identification. Detailed Embodiments

[0060] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0061] Embodiment 1:

[0062] Please refer to Figure 1 , Figure 1A method for protecting an offshore wind power AC line based on parameter identification provided by an embodiment of the present invention includes steps S1 to S4, where

[0063] Step S1: Obtain the current on the VSC side in real time.

[0064] Step S2: When the current on the VSC side meets the starting criterion, send a voltage acquisition instruction to the VSC side to obtain the voltage on the VSC side at the current moment.

[0065] Step S3: Calculate the fault distance corresponding to the current moment according to the voltage on the VSC side and the current on the VSC side at the current moment; where the fault distance is the distance from the fault resistance to the VSC side; the fault resistance connects the wind farm side and the VSC side.

[0066] Step S4: Output a corresponding protection signal according to the comparison result between the fault distance and the protection criterion.

[0067] In this embodiment, referring to Figures 2 to 4 , Figure 2 is a topological schematic diagram of an offshore wind power system connected to the grid through a VSC. Figure 3 is an equivalent diagram of a single-phase grounding fault of an AC line through a fault resistance. Figure 4 is a zero-sequence component network diagram of a single-phase grounding fault of an AC line through a fault resistance provided by the present invention. Where label 1 is the wind farm side, 2 is the VSC side, the main transformer on the wind farm side and the converter transformer on the VSC side are respectively represented as main transformer and converter transformer. In addition, R in the figure f is the fault resistance, and both the VSC side and the wind farm side are the installation locations of measurement devices to obtain relevant parameters.

[0068] In step S1, the current on the VSC side is mainly the current sampling value obtained by a measurement device. This embodiment performs continuous and real-time acquisition. After obtaining the current parameter on the VSC side, it is possible to judge whether the starting criterion of the protection is met based on this current parameter.

[0069] In the above step S2, the starting criterion is:

[0070] |i a (t)+i b (t)+i c (t)|-|i a (t - T)+i b (t - T)+i c (t - T)|>0.1I N ;

[0071] where, i a (t), i b (t) and i c(t) is the three-phase current sampling value of the VSC-HVDC side at time t, i a (t - T), i b (t - T), and i c (t - T) are the three-phase current sampling values of the VSC-HVDC side at time t - T, I N is the rated current.

[0072] After determining that the VSC-HVDC side current satisfies the starting criterion, a voltage acquisition instruction is sent to the VSC-HVDC side to obtain the voltage of the VSC-HVDC side at the current moment. According to Figure 4 and Kirchhoff's voltage law, we can get:

[0073]

[0074] Among them, i0 is the zero-sequence current of the VSC-HVDC side, k r , k L , r MMC , r WF , L MMC , and L WF are all intermediate variables used to simplify the operation. Specifically:

[0075]

[0076]

[0077] r MMC = r0d + r TMMC0 ;

[0078] r WF = r0(l - d)+ r TWF0 ;

[0079] L MMC = L0d + L TMMC0 ;

[0080] L WF = L0(l - d)+ L TWF0 ;

[0081] Among them, i MMC(A) , i MMC(B) , and i MMC(C) are the three-phase phase currents of the VSC-HVDC side, u MMC(B) is the phase A voltage of the VSC-HVDC side, R f is the fault resistance, i fLet \(I\) be the current flowing through the fault resistance, \(d\) be the distance from the fault resistance to the VSC side (i.e., the fault distance to be solved in this embodiment), \(l\) be the total length of the line between the main transformer on the wind farm side and the converter transformer on the VSC side, \(r_1\) be the positive-sequence resistance per unit length of the line, \(r_0\) be the zero-sequence resistance per unit length of the line, \(L_1\) be the positive-sequence inductance per unit length of the line, \(L_0\) be the zero-sequence inductance per unit length of the line, \(r\) TMMC0 is the equivalent resistance of the converter transformer on the VSC side, \(L\) TMMC0 is the equivalent leakage inductance of the converter transformer on the VSC side, \(r\) TWF0 is the equivalent resistance of the main transformer on the wind farm side, \(L\) TWF0 is the equivalent leakage inductance of the main transformer on the wind farm side, and \(i_0'\) is the zero-sequence current on the wind farm side.

[0082] Furthermore, the fault-phase voltage of the line can be expressed as an expression of the fault-phase current with zero-sequence compensation and the fault-point voltage. According to the grounding method of the transformer, the fault-point voltage using the zero-sequence component network can be expressed as a function of the zero-sequence current on one side. Therefore, the single-ended voltage and current quantities can be used to calculate the fault distance.

[0083] According to Kirchhoff's current law:

[0084] u f =i f ·R f ;

[0085] And

[0086] It can be obtained that:

[0087]

[0088] Therefore, in step S3, calculating the fault distance corresponding to the current moment according to the voltage on the VSC side at the current moment and the current on the VSC side at the current moment, specifically:

[0089] Obtain the fault distance at the current moment according to the solution result of the following ranging formula:

[0090]

[0091] This ranging formula is derived from Kirchhoff's voltage law and the above formula.

[0092] Preferably, as an example of this embodiment, the solution result is obtained by the following method: solving the ranging formula by replacing the differential quantity with the difference quantity.

[0093] The specific replacement of the differential quantity with the difference quantity is:

[0094]

[0095] Among them, u(k) is the voltage sampling value at time k, i(k) is the current sampling value at time k, u(k - 1) is the voltage sampling value at time k - 1, i(k - 1) is the current sampling value at time k - 1, u(k + 1) is the voltage sampling value at time k + 1, i(k + 1) is the current sampling value at time k + 1, and △t is the sampling time interval. Thus, the solution result can be obtained.

[0096] In the above step S4, the output of the corresponding protection signal according to the comparison result of the fault distance and the protection criterion is specifically as follows:

[0097] The protection criterion includes a preset distance threshold value d set ; As an example of this embodiment, the threshold value of this distance is the setting distance value at the installation location of the measuring device, and the set value d set = 0.8l.

[0098] When the fault distance is less than the preset distance threshold value, that is, the fault distance is less than d set , at this time, it is determined that the fault that occurs is an in - zone fault, and a protection trip signal is output, and the fault can be cleared by the circuit breaker; otherwise (the fault distance is greater than or equal to d set ), at this time, it is determined that the fault that occurs is an out - of - zone fault, and a protection reset signal is output. Implementing the embodiments of the present application, through the measurement and identification of parameters, a differential equation is written based on the R - L model of the transmission line for solution. The fault resistance is replaced by an unknown number in the fault loop equation and solved simultaneously with the fault distance, improving the ability of distance protection to withstand transition resistance. This method requires a short data window, has a fast action speed, and is not affected by the grid frequency fluctuation.

[0099] Correspondingly, referring to Figure 5 , the embodiments of the present invention also provide a protection device for an offshore wind power AC line based on parameter identification, which is applied to an offshore wind power system connected to the grid through a flexible DC link. The offshore wind power system connected to the grid through a flexible DC link includes a wind farm side and a flexible DC link side. The protection device for the offshore wind power AC line includes:

[0100] An acquisition module 101, configured to acquire the current of the flexible DC link side in real time;

[0101] A judgment module 102, configured to send a voltage acquisition instruction to the flexible DC link side when the current of the flexible DC link side meets the start criterion, and acquire the voltage of the flexible DC link side at the current moment;

[0102] A calculation module 103, configured to calculate the corresponding fault distance at the current moment according to the voltage of the flexible DC link side at the current moment and the current of the flexible DC link at the current moment; wherein, the fault distance is the distance from the fault resistance to the flexible DC link side; the fault resistance connects the wind farm side and the flexible DC link side;

[0103] The protection module 104 is configured to output a corresponding protection signal according to the comparison result between the fault distance and the protection criterion.

[0104] As a preferred solution, the starting criterion is specifically:

[0105] |i a (t) + i b (t) + i c (t)| - |i a (t - T) + i b (t - T) + i c (t - T)| > 0.1I N ;

[0106] Wherein, i a (t), i b (t) and i c (t) are the three-phase current sampling values on the VSC side at time t, and i a (t - T), i b (t - T) and i c (t - T) are the three-phase current sampling values on the VSC side at time t - T, and I N is the rated current.

[0107] As a preferred solution, the calculation module 103 calculates the fault distance corresponding to the current moment according to the voltage on the VSC side at the current moment and the VSC current at the current moment, specifically:

[0108] The calculation module 103 obtains the fault distance at the current moment according to the solution result of the following ranging formula:

[0109]

[0110] Wherein, i0 is the zero-sequence current on the VSC side, and k r , k L , r MMC , r WF , L MMC and L WF are all intermediate variables used to simplify the operation. Specifically:

[0111]

[0112]

[0113]

[0114] r MMC = r0d + r TMMC0 ;

[0115] rWF = r0(l - d)+ r TWF0 ;

[0116] L MMC = L0d + L TMMC0 ;

[0117] L WF = L0(l - d)+ L TWF0 ;

[0118] Wherein, i MMC(A) , i MMC(B) and i MMC(C) are the three-phase phase currents on the VSC side, u MMC(B) is the phase A voltage on the VSC side, R f is the fault resistance, i f is the current flowing through the fault resistance, d is the distance from the fault resistance to the VSC side, l is the total length of the line between the main transformer on the wind farm side and the converter transformer on the VSC side, r1 is the positive-sequence resistance per unit length of the line, r0 is the zero-sequence resistance per unit length of the line, L1 is the positive-sequence inductance per unit length of the line, L0 is the zero-sequence inductance per unit length of the line, r TMMC0 is the equivalent resistance of the converter transformer on the VSC side, L TMMC0 is the equivalent leakage inductance of the converter transformer on the VSC side, r TWF0 is the equivalent resistance of the main transformer on the wind farm side, L TWF0 is the equivalent leakage inductance of the main transformer on the wind farm side, and i0' is the zero-sequence current on the wind farm side.

[0119] As a preferred solution, the solution result is obtained by the following method: solving the ranging formula by replacing the differential component with the difference component.

[0120] As a preferred solution, the protection module 104 outputs a corresponding protection signal according to the comparison result of the fault distance and the protection criterion, specifically:

[0121] The protection criterion includes a preset distance threshold;

[0122] When the fault distance is less than the preset distance threshold, the protection module 104 outputs a protection trip signal; otherwise, the protection module 104 outputs a protection reset signal.

[0123] Compared with the prior art, the embodiments of the present invention have the following beneficial effects:

[0124] An embodiment of the present invention provides a method and device for protecting an offshore wind power AC line based on parameter identification. The method is applied to an offshore wind power grid-connected system via a flexible DC link. The offshore wind power grid-connected system via a flexible DC link includes a wind farm side and a flexible DC link side. The method includes: obtaining the current on the flexible DC link side in real time; when the current on the flexible DC link side meets the start criterion, sending a voltage acquisition instruction to the flexible DC link side to obtain the voltage on the flexible DC link side at the current moment; calculating the fault distance corresponding to the current moment according to the voltage on the flexible DC link side at the current moment and the current on the flexible DC link at the current moment; where the fault distance is the distance from the fault resistance to the flexible DC link side; the fault resistance connects the wind farm side and the flexible DC link side; outputting a corresponding protection signal according to the comparison result between the fault distance and the protection criterion. Compared with the prior art, by obtaining parameters such as the voltage and current on the flexible DC link side and taking the fault resistance as an unknown quantity to solve the fault distance, the influence of the variable of the fault resistance on the fault distance is avoided, and the fault resistance tolerance of the distance protection is improved.

[0125] In the above specific embodiments, the purpose, technical solution, and beneficial effects of the present invention have been further described in detail. It should be understood that the above are only specific embodiments of the present invention and are not used to limit the protection scope of the present invention. In particular, for those skilled in the art, any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for protecting an offshore wind power AC line based on parameter identification, characterized in that Applied to the HVDC flexible grid-connected system for offshore wind power, the HVDC flexible grid-connected system for offshore wind power includes a wind farm side and a flexible DC side. The protection method for the AC line of the offshore wind power includes: Obtain the current on the flexible DC side in real time; When the current on the flexible DC side meets the starting criterion, send a voltage acquisition command to the flexible DC side to obtain the voltage on the flexible DC side at the current moment; Calculate the fault distance corresponding to the current moment according to the voltage on the flexible DC side at the current moment and the flexible DC current at the current moment; wherein, the fault distance is the distance from the fault resistance to the flexible DC side; the fault resistance connects the wind farm side and the flexible DC side; Output corresponding protection signals according to the comparison result between the fault distance and the protection criterion; The calculation of the fault distance corresponding to the current moment according to the voltage on the flexible DC side at the current moment and the flexible DC current at the current moment is specifically: Obtain the fault distance at the current moment according to the solution result of the following ranging formula: Among them, i0 is the zero-sequence current on the flexible DC side, k r , k L , r MMC , r WF , L MMC and L WF are all intermediate variables used to simplify the calculation. Specifically: r MMC ＝r0d+r TMMC0 ； r WF ＝r0(l-d)+r TWF0 ； L MMC = L0d + L TMMC0 ; L WF = L0(l - d)+L TWF0 ; where i MMC(A) 、i MMC(B) and i MMC(C) are the three-phase phase currents on the VSC side, u MMC(B) is the phase A voltage on the VSC side, R f is the fault resistance, i f is the current flowing through the fault resistance, d is the distance from the fault resistance to the VSC side, l is the total length of the line between the main transformer on the wind farm side and the converter transformer on the VSC side, r1 is the positive sequence resistance per unit length of the line, r0 is the zero sequence resistance per unit length of the line, L1 is the positive sequence inductance per unit length of the line, L0 is the zero sequence inductance per unit length of the line, r TMMC0 is the equivalent resistance of the converter transformer on the VSC side, L TMMC0 is the equivalent leakage inductance of the converter transformer on the VSC side, r TWF0 is the equivalent resistance of the main transformer on the wind farm side, L TWF0 is the equivalent leakage inductance of the main transformer on the wind farm side.

2. The method for protecting an offshore wind power AC line based on parameter identification according to claim 1, wherein The starting criterion is specifically: |i a (t)+i b (t)+i c (t)|-|i a (t - T)+i b (t - T)+i c (t - T)|>0.1I N ; where i a (t), i b (t) and i c (t) are the three-phase current sampling values of the VSC side at time t, and i a (t-T), i b (t-T) and i c (t-T) are the three-phase current sampling values of the VSC side at time t-T, and I N is the rated current.

3. The method for protecting an offshore wind power AC line based on parameter identification according to claim 1, characterized in that, The solution result is obtained by the following method: replacing the differential component with a difference component to solve the ranging formula.

4. A method for protecting an offshore wind power AC line based on parameter identification according to any one of claims 1 to 3, characterized in that The output of corresponding protection signals according to the comparison result between the fault distance and the protection criterion is specifically: The protection criterion includes a preset distance threshold value; When the fault distance is less than the preset distance threshold value, output a protection trip signal; otherwise, output a protection reset signal.

5. An offshore wind power AC line protection device based on parameter identification, characterized in that, Applied to the HVDC flexible grid-connected system for offshore wind power, the HVDC flexible grid-connected system for offshore wind power includes a wind farm side and a flexible DC side. The protection device for the AC line of the offshore wind power includes: An acquisition module for obtaining the current on the flexible DC side in real time; A judgment module for sending a voltage acquisition command to the flexible DC side to obtain the voltage on the flexible DC side at the current moment when the current on the flexible DC side meets the starting criterion; A calculation module for calculating the fault distance corresponding to the current moment according to the voltage on the flexible DC side at the current moment and the flexible DC current at the current moment; wherein, the fault distance is the distance from the fault resistance to the flexible DC side; the fault resistance connects the wind farm side and the flexible DC side; A protection module for outputting corresponding protection signals according to the comparison result between the fault distance and the protection criterion; The calculation by the calculation module of the fault distance corresponding to the current moment according to the voltage on the flexible DC side at the current moment and the flexible DC current at the current moment is specifically: The calculation module obtains the fault distance at the current moment according to the solution result of the following ranging formula: Among them, i0 is the zero-sequence current on the flexible DC side, k r , k L , r MMC , r WF , L MMC and L WF are all intermediate variables used to simplify the calculation. Specifically: r MMC ＝r0d+r TMMC0 ； r WF ＝r0(l-d)+r TWF0 ； L MMC = L0d + L TMMC0 ; L WF = L0(l - d)+L TWF0 ; Among them, i MMC(A) , i MMC(B) and i MMC(C) are the three-phase phase currents on the flexible DC side, u MMC(B) is the phase A voltage on the flexible DC side, R f is the fault resistance, i f is the current flowing through the fault resistance, d is the distance from the fault resistance to the flexible DC side, l is the total length of the line between the main transformer on the wind farm side and the converter transformer on the flexible DC side, r1 is the positive sequence resistance per unit length of the line, r0 is the zero sequence resistance per unit length of the line, L1 is the positive sequence inductance per unit length of the line, L0 is the zero sequence inductance per unit length of the line, r TMMC0 is the equivalent resistance of the converter transformer on the flexible DC side, L TMMC0 is the equivalent leakage inductance of the converter transformer on the flexible DC side, r TWF0 is the equivalent resistance of the main transformer on the wind farm side, L TWF0 is the equivalent leakage inductance of the main transformer on the wind farm side.

6. The offshore wind power AC line protection device based on parameter identification according to claim 5, characterized in that, The starting criterion is specifically: |i a (t)+i b (t)+i c (t)|-|i a (t - T)+i b (t - T)+i c (t - T)|>0.1I N ; Among them, i a (t), i b (t) and i c (t) are the three-phase current sampling values of the VSC side at time t, and i a (t-T), i b (t-T) and i c (t-T) are the three-phase current sampling values of the VSC side at time t-T, and I N is the rated current.

7. The offshore wind power AC line protection device based on parameter recognition according to claim 5, characterized in that, The solution result is obtained by the following method: replacing the differential component with a difference component to solve the ranging formula.

8. A protection device for an offshore wind power AC line based on parameter identification according to any one of claims 5 to 7, characterized in that, The output of corresponding protection signals by the protection module according to the comparison result between the fault distance and the protection criterion is specifically: The protection criterion includes a preset distance threshold value; When the fault distance is less than the preset distance threshold value, the protection module outputs a protection trip signal; otherwise, the protection module outputs a protection reset signal.

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