A method and system for single-phase grounding protection of a double-fed wind farm transmission line

Abstract

The present invention relates to a method and system for single-phase grounding protection of a doubly-fed wind farm transmission line, the method comprising: collecting electrical data including zero-mode voltage on the wind power side and the system side of the doubly-fed wind farm transmission line; calculating the reference voltage of single-phase grounding faults in and / or out of the wind power side of the transmission line and the reference voltage of single-phase grounding faults in and / or out of the system side based on the zero-mode component equivalent network model; when making a grounding fault judgment, comparing the difference between the actually collected zero-mode voltage and the reference voltage, judging whether a preset longitudinal protection action criterion is met; if the action criterion is met, it is judged as an in-zone fault and a trip signal is immediately sent; if the action criterion is not met, it is judged as an out-of-zone fault and no trip signal is sent. The present invention can avoid the influence of the rotation speed frequency component on the protection criterion in the case of a frequency offset of the fault current in the doubly-fed wind farm, and realize accurate identification of in-zone and out-zone faults of the doubly-fed wind farm transmission line.

Description

Technical Field

[0001] The invention belongs to the technical field of relay protection, and in particular relates to a single-phase grounding protection method and system for a double-fed wind farm transmission line. Background Art

[0002] As wind power continues to be connected to the grid, the natural randomness of wind energy makes the relay protection problem of wind farms more and more prominent. There are significant differences in the electromagnetic transient characteristics of wind power concentrated access to regional power grids and traditional power grids during faults. Research on the grid protection principle applicable to wind farm access is of great significance to system safety.

[0003] At present, the protection of wind power transmission lines still adopts the conventional transmission line protection configuration, equipped with longitudinal protection as the main line protection, without considering the special problems of wind power transmission. After the external fault of the doubly fed wind turbine unit, the fault current at the machine end is approximately composed of three parts: steady-state AC component, attenuated DC component and attenuated AC component. Due to the influence of the speed frequency component on the accuracy of power frequency phase extraction, the protection using the full power frequency structure faces the problem of reduced action performance and even false refusal when applied to the transmission line of the doubly fed wind farm. Summary of the invention

[0004] In view of the above analysis, the present invention aims to disclose a single-phase grounding protection method and system for a doubly-fed wind farm transmission line, so as to solve the problem that it is difficult to accurately identify faults in wind power transmission lines.

[0005] The present invention discloses a single-phase grounding protection method for a double-fed wind farm transmission line, comprising the following steps:

[0006] Collect electrical data including zero-mode voltage on the wind power side and system side of the double-fed wind farm transmission line;

[0007] Based on the zero-mode component equivalent network model, calculate the reference voltage of the single-phase grounding fault inside and / or outside the wind power area on the transmission line, and the reference voltage of the single-phase grounding fault inside and / or outside the system area;

[0008] When making a ground fault judgment, compare the actually collected zero-mode voltage on the wind power side of the transmission line with the reference voltage of single-phase ground faults within and / or outside the wind power area, and compare the zero-mode voltage on the system side with the reference voltage of single-phase ground faults within and / or outside the system area; judge whether the comparison result meets the preset longitudinal protection action criteria; if the action criteria are met, it is judged as a single-phase ground fault within the area, and a trip signal is sent immediately; if the action criteria are not met, it is judged as a single-phase ground fault outside the area, and no trip signal is sent.

[0009] Furthermore, the reference voltage for single-phase ground fault in the wind power side area is Where, L w0is the equivalent zero-mode inductance of the wind farm output transformer; i w0 is the zero-mode current on the wind power side, and t is the time.

[0010] Furthermore, the reference voltage for single-phase ground fault outside the wind power side is In the formula, R s0 , L s0 are the equivalent zero-mode resistance and zero-mode inductance of the AC system respectively; R l0 , L l0 are the zero-mode resistance and zero-mode inductance of the wind power transmission line respectively; i w0 is the zero-mode current on the wind power side, and t is the time.

[0011] Furthermore, the reference voltage for single-phase ground fault in the system side area is In the formula, R s0 , L s0 are the equivalent zero-mode resistance and zero-mode inductance of the AC system respectively; i s0 is the zero-mode current on the system side, and t is the time.

[0012] Furthermore, the reference voltage of the single-phase ground fault outside the system side is In the formula, R l0 , L l0 are the zero-mode resistance and zero-mode inductance of the wind power transmission line, L w0 is the equivalent zero-mode inductance of the wind farm output transformer; i w0 is the zero-mode current on the wind power side, and t is the time.

[0013] Further, a zero-mode model similarity factor is used to measure the difference between the zero-mode voltage on the wind power side and the reference voltage of the single-phase grounding fault within the area and / or outside the area, and the difference between the zero-mode voltage on the system side and the reference voltage of the single-phase grounding fault within the area and / or outside the area;

[0014] The similarity factor calculation formula is:

[0015] In the formula, k w is the zero-mode model similarity factor of the wind power side of the transmission line; k s is the zero mode model similarity factor of the system side of the transmission line; u w0 is the vector of the actual measured value of the zero-mode voltage on the wind power side, u' w0 is the reference voltage vector of the wind power transmission line on the wind power side, u s0 is the vector of the actual sampling values ​​of the zero-mode voltage on the system side, u' s0 It is the reference voltage vector on the wind power transmission line system side.

[0016] Furthermore, the reference voltage vector of the wind power transmission line on the wind power side i' w0 is the actual measured value of the zero-mode current corresponding to the actual measured value of the zero-mode voltage on the wind power side;

[0017] The reference voltage vector of the wind power transmission line system side i′ s0 is the actual measured value of the zero-mode current corresponding to the actual measured value of the zero-mode voltage on the system side.

[0018] Furthermore, the reference voltage vector of the wind power transmission line on the wind power side i' w0 is the actual measured value of the zero-mode current corresponding to the actual measured value of the zero-mode voltage on the wind power side;

[0019] The reference voltage vector of the wind power transmission line system side i′ s0 is the actual measured value of the zero-mode current corresponding to the actual measured value of the zero-mode voltage on the system side.

[0020] Furthermore, the protection action criterion is: Where k set It is the preset protection action threshold.

[0021] The present invention also discloses a double-fed wind farm transmission line single-phase grounding protection system, including a data acquisition module, a protection calculation module, and a protection action module;

[0022] The data acquisition module is used to collect electrical data including zero-mode voltage on the wind power side and the system side of the double-fed wind farm transmission line, and transmit the electrical data to the protection calculation module;

[0023] The protection calculation module is used to calculate the reference voltage of the single-phase grounding fault in the wind power side area and / or outside the area of ​​the transmission line, and the reference voltage of the single-phase grounding fault in the system side area and / or outside the area; and calculate the zero-mode voltage similarity factor of the wind power side and the system side of the transmission line according to the input electrical data, and pass it to the protection action module;

[0024] The protection action module is used to receive the zero-mode voltage similarity factor on both sides of the line, and determine whether the size of the similarity factor meets the longitudinal protection action criterion. If the action criterion is met, it is determined to be an in-zone fault and a tripping signal is sent immediately; if the action criterion is not met, it is determined to be an out-of-zone fault and no tripping signal is sent.

[0025] The present invention can achieve at least one of the following beneficial effects:

[0026] The single-phase grounding protection method and system for the doubly-fed wind farm transmission line of the present invention can avoid the influence of the rotational speed frequency component on the protection criterion in the case of a doubly-fed wind farm fault current frequency offset based on the model difference characteristics when faults occur at different locations inside and outside the area, thereby achieving accurate identification of doubly-fed wind farm transmission line faults inside and outside the area. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] The drawings are only for the purpose of illustrating particular embodiments and are not to be considered limiting of the present invention. Like reference symbols denote like components throughout the drawings.

[0028] Figure 1 This is a flow chart of the single-phase grounding protection method for the doubly-fed wind farm transmission line in the first embodiment;

[0029] Figure 2 This is a schematic diagram of an equivalent zero-mode network model for a wind farm transmission line fault in the first embodiment;

[0030] Figure 3 This is a schematic diagram of the principle of the single-phase grounding protection system for the output line of a doubly-fed wind farm in the first embodiment;

[0031] Figure 4 This is a schematic diagram of a wind power transmission system in the second embodiment;

[0032] Figure 5 is a zero-mode voltage similarity factor curve diagram when a single-phase grounding fault occurs in the outgoing line area of ​​a wind farm in the second embodiment;

[0033] Figure 6 is a zero-mode voltage similarity factor curve diagram when a single-phase grounding fault occurs in the AC system at the wind power receiving end in the second embodiment;

[0034] Figure 7 This is a zero-mode voltage similarity factor curve diagram when a single-phase grounding fault occurs inside a wind farm in the second embodiment. DETAILED DESCRIPTION

[0035] The preferred embodiments of the present invention will be described in detail below in conjunction with the accompanying drawings, wherein the accompanying drawings constitute a part of this application and are used to illustrate the principles of the present invention together with the embodiments of the present invention.

[0036] Embodiment 1

[0037] An embodiment of the present invention discloses a method for single-phase grounding protection of a double-fed wind farm transmission line. Figure 1 As shown, the following steps are included:

[0038] Step S1, collecting electrical data on the wind power side and the system side of the double-fed wind farm transmission line;

[0039] The collected measured electrical data include zero-mode voltage and zero-mode current on the wind power side of the double-fed wind farm transmission line and zero-mode voltage and zero-mode current on the system side.

[0040] Step S2: Based on the zero-mode component equivalent network model, the virtual zero-mode voltage on the wind power side and the system side of the transmission line is calculated as a reference voltage for judging faults at different locations inside and outside the area;

[0041] Specifically, the calculated virtual zero-mode voltage includes a reference voltage of a single-phase grounding fault in a zone and / or outside a zone on the wind power side of the transmission line, and a reference voltage of a single-phase grounding fault in a zone and / or outside a zone on the system side;

[0042] The zero-mode component equivalent network model of the wind power transmission system of this embodiment is as follows: Figure 2 As shown in the figure. Since the wind farm output transformer adopts the neutral point direct grounding connection method, the zero-sequence equivalent inductance of the wind farm only includes the zero-sequence inductance of the output transformer. L w0 is the equivalent zero-mode inductance of the wind farm output transformer; R s0 , L s0 are the equivalent zero-mode resistance and zero-mode inductance of the AC system respectively; R l0 , L l0 are the zero-mode resistance and zero-mode inductance of the wind power transmission line respectively; i w0 、i s0 are the zero-mode currents on the wind power side and the system side respectively; u w0 、u s0 are the zero-mode voltages on the wind power side and the system side, respectively.

[0043] The reference voltage of single-phase ground fault in the wind power side area is calculated according to the zero-mode component equivalent network model. Where, L w0 is the equivalent zero-mode inductance of the wind farm output transformer; i w0 is the zero-mode current on the wind power side, and t is the time.

[0044] Therefore, when a fault occurs in the protection zone on the wind power side, the measured value of the zero-mode voltage at the protection installation is the voltage drop on the back inductor, that is, the zero-mode voltage at this time reflects the characteristics of the equivalent network model of the zero-mode component on the back side of the protection.

[0045] According to the zero-mode component equivalent network model, the reference voltage of the single-phase grounding fault outside the wind power side is calculated. In the formula, R s0 , L s0 are the equivalent zero-mode resistance and zero-mode inductance of the AC system respectively; R l0 , L l0 are the zero-mode resistance and zero-mode inductance of the wind power transmission line respectively; i w0 is the zero-mode current on the wind power side, and t is the time.

[0046] Therefore, when a single-phase grounding fault occurs outside the zone, the zero-mode voltage measurement value at the protection installation is the voltage drop on the equivalent model of the transmission line and the wind power receiving system, that is, the zero-mode voltage at this time reflects the characteristics of the zero-mode component equivalent network model of the opposite system.

[0047] According to the zero-mode component equivalent network model, the reference voltage of the single-phase grounding fault in the system side area In the formula, R s0 , L s0 are the equivalent zero-mode resistance and zero-mode inductance of the AC system respectively; i s0 is the zero-mode current on the system side, and t is the time.

[0048] Therefore, when a single-phase grounding fault occurs in the area, the zero-mode voltage measurement value at the protection installation location is the voltage drop on the wind power receiving system model on the back side, that is, the zero-mode voltage at this time reflects the characteristics of the zero-mode component equivalent network model of the AC system of the wind power receiving end on the back side of the protection.

[0049] According to the zero-mode component equivalent network model, the reference voltage of the single-phase grounding fault outside the system side area In the formula, R l0 , L l0 are the zero-mode resistance and zero-mode inductance of the wind power transmission line, L w0 is the equivalent zero-mode inductance of the wind farm output transformer; i w0 is the zero-mode current on the wind power side, and t is the time.

[0050] Therefore, when an out-of-zone fault occurs, the zero-mode voltage measurement value at the protection installation location is the voltage drop on the transmission line and the equivalent model of the wind farm. That is, the zero-mode voltage at this time reflects the characteristics of the zero-mode component equivalent network model of the opposite system.

[0051] Step S3, when making a ground fault judgment, compare the difference between the actually collected zero-mode voltage and the reference voltage to determine whether the preset longitudinal protection action criterion is met; if the action criterion is met, it is determined to be an in-zone fault and a trip signal is sent immediately; if the action criterion is not met, it is determined to be an out-of-zone fault and no trip signal is sent.

[0052] Specifically, when making a ground fault judgment, the zero-mode voltage on the wind power side of the transmission line actually collected is compared with the reference voltage of the single-phase ground fault in the wind power area and / or outside the area, and the zero-mode voltage on the system side is compared with the reference voltage of the single-phase ground fault in the system area and / or outside the area;

[0053] When a fault occurs in the transmission line area, the zero-mode voltage at the protection installation on the wind power side and the system side reflects the zero-mode voltage drop of the back-side system. When a fault occurs outside the transmission line area, the zero-mode voltage at the protection installation on the fault side reflects the zero-mode voltage drop of the opposite side system. The virtual zero-mode voltage drop is solved using the back-side system parameters, and this voltage is used as the reference voltage. The difference between the measured voltage and the reference voltage is used to determine the difference in the zero-mode model.

[0054] Furthermore, since the zero-mode voltage value fluctuates greatly under different fault conditions, in order to facilitate setting, ensure the sensitivity of the protection criterion, and reduce the impact of the error introduced by the differential calculation on the result, in this embodiment, the zero-mode model similarity factor of the measured voltage on the wind power side and the system side of the transmission line and the reference voltage waveform is used to measure the difference between the measured voltage and the reference voltage;

[0055] The similarity factor calculation formula is: In the formula, k w is the zero-mode model similarity factor of the wind power side of the transmission line; k s is the zero mode model similarity factor of the system side of the transmission line; u w0 is the vector of the actual measured value of the zero-mode voltage on the wind power side, u' w0 is the reference voltage vector of the wind power transmission line on the wind power side, u s0 is the vector of the actual sampling values ​​of the zero-mode voltage on the system side, u' s0 It is the reference voltage vector on the wind power transmission line system side.

[0056] More specifically, the reference voltage vector u' on the wind power side of the wind power transmission line w0 and the reference voltage vector u' on the wind power transmission line system side s0 There are two options:

[0057] Solution 1:

[0058] The reference voltage vector of the wind power transmission line on the wind power side i' w0 is the actual measured value of the zero-mode current corresponding to the actual measured value of the zero-mode voltage on the wind power side;

[0059] The reference voltage vector of the wind power transmission line system side i′ s0 is the actual measured value of the zero-mode current corresponding to the actual measured value of the zero-mode voltage on the system side.

[0060] Option 2:

[0061] The reference voltage vector of the wind power transmission line on the wind power side i' w0is the actual measured value of the zero-mode current corresponding to the actual measured value of the zero-mode voltage on the wind power side;

[0062] The reference voltage vector of the wind power transmission line system side i′ s0 is the actual measured value of the zero-mode current corresponding to the actual measured value of the zero-mode voltage on the system side.

[0063] The reference voltage vector u' on the wind power side of the wind power transmission line using Scheme 1 w0 and the reference voltage vector u′ on the wind power transmission line system side s0 Analyze the similarity factor.

[0064] When a fault occurs in the transmission line area, the zero-mode voltage measurement value at the protection installation is the voltage drop on the back system model, u w0 =u' w0 ,u s0 =u' s0 , so the measured voltage on both sides has the same waveform as the reference voltage, k w =1, k s =1.

[0065] When there is an internal fault in the wind farm, the zero-mode voltage measurement value at the system-side protection installation is the voltage drop on the AC model of the wind power receiving end on the back side, u s0 =u' s0 , so the measured voltage on the system side has the same waveform as the reference voltage k s =1. The zero-mode voltage measurement value at the wind power side protection installation is the voltage drop on the opposite side system model, u w0 ≠u' w0 Therefore, the measured voltage on the wind power side is different from the reference voltage waveform, k w <1.

[0066] When the wind power receiving system fails, the zero-mode voltage measurement value at the wind power side protection installation is the voltage drop on the back inductor, u w0 =u' w0 Therefore, the measured voltage on the wind power side has the same waveform as the reference voltage. w = 1. The zero-mode voltage measurement value at the system-side protection installation is the voltage drop on the opposite-side system model, u s0 ≠u' s0 Therefore, the actual voltage measured on the system side is different from the reference voltage waveform, k s <1.

[0067] This embodiment is simulated through simulation data and it is found that when there is an internal fault in the wind farm, 0.5 <k w <1; when the wind power receiving system fails, 0.5 <k s <1.

[0068] The reference voltage vector u' on the wind power side of the wind power transmission line using Scheme 2 w0 and the reference voltage vector u′ on the wind power transmission line system side s0 Analyzing the similarity factor can produce similar results as above.

[0069] Therefore, the protection action criterion constructed according to the above analysis results is: Where k set is the preset protection action threshold, k set =0.5.

[0070] When determining a ground fault, if the calculated k w or k s Exceeds the preset protection action threshold k set =0.5, it is judged as an internal fault and a trip signal is sent immediately; if the action criterion is not met, it is judged as an external fault and no trip signal is sent.

[0071] Another solution of this embodiment discloses a double-fed wind farm transmission line single-phase grounding protection system, such as Figure 3 As shown, it includes a data acquisition module, a protection calculation module, and a protection action module;

[0072] It includes data acquisition module, protection calculation module and protection action module;

[0073] The data acquisition module is used to collect electrical data including zero-mode voltage on the wind power side and the system side of the double-fed wind farm transmission line, and transmit the electrical data to the protection calculation module;

[0074] The protection calculation module is used to calculate the reference voltage of the single-phase grounding fault in the wind power side area and / or outside the area of ​​the transmission line, and the reference voltage of the single-phase grounding fault in the system side area and / or outside the area; and calculate the zero-mode voltage similarity factor of the wind power side and the system side of the transmission line according to the input electrical data, and pass it to the protection action module;

[0075] The protection action module is used to receive the zero-mode voltage similarity factor on both sides of the line, and determine whether the size of the similarity factor meets the longitudinal protection action criterion. If the action criterion is met, it is determined to be an in-zone fault and a tripping signal is sent immediately; if the action criterion is not met, it is determined to be an out-of-zone fault and no tripping signal is sent.

[0076] The technical details in this solution are the same as those in the previous solution, so they will not be described in detail here.

[0077] In summary, through the scheme of this embodiment, according to the model difference characteristics of faults at different locations inside and outside the area, in the case of fault current frequency offset in the doubly fed wind farm, the influence of the rotation speed frequency component on the protection criterion can be avoided, and accurate identification of faults inside and outside the doubly fed wind farm transmission line area can be achieved.

[0078] Embodiment 2

[0079] This embodiment adopts a parallel equivalent model of doubly-fed wind turbines to simulate the entire wind farm, and uses the single-phase grounding protection method and system for the doubly-fed wind farm transmission line in the first embodiment to determine the single-phase grounding fault.

[0080] like Figure 4 As shown in the wind power transmission system diagram, the wind farm consists of 300 identical double-fed asynchronous wind turbines connected in parallel, each with a rated capacity of 1.5MW, and operates at rated output; the wind farm first passes through a 0.69 / 20kV on-site transformer, then through a 20 / 230kV transformer and through the wind power transmission line to send electricity to the system. The main parameters of the wind power transmission system are shown in Table 1.

[0081] Table 1 Main parameters of wind power transmission system

[0082]

[0083] The scenario set in this embodiment is: at t=1.4s, a single-phase grounding fault occurs inside the wind farm transmission line, the wind farm receiving end AC system and the wind farm respectively, the fault lasts for 0.1s, and the transition resistance is 0Ω.

[0084] The judgment result of the method of Example 1 is:

[0085] Measure the zero-mode current and zero-mode voltage values ​​on the wind power side and the system side in the wind power transmission system, and calculate the zero-mode voltage similarity factor at both ends, such as Figure 5 , Figure 6 , Figure 7 As shown. Among them, according to Figure 5 It can be seen that when there is a fault in the area, the zero-mode voltage similarity factors on the wind power side and the system side are close to 1, which is greater than the protection threshold value of 0.5, and the protection operates correctly. Figure 6 , Figure 7 As shown in the figure, when the AC system at the wind power receiving end fails, the zero-mode voltage similarity factor on the system side reaches a maximum value of -0.848 at t=1.43s, which is much smaller than the protection threshold value of 0.5. When there is an internal fault in the wind farm, the zero-mode voltage similarity factor on the wind farm side reaches a maximum value of 0.166 at t=1.4s, which is much smaller than the protection threshold value. Therefore, when there is an out-of-area fault, the protection is reliable and does not operate.

[0086] The results in Example 2 prove that the method and system for single-phase grounding protection of the doubly-fed wind farm transmission line in Example 1 can avoid the influence of the rotation speed frequency component on the protection criterion and realize accurate identification of faults inside and outside the doubly-fed wind farm transmission line area.

[0087] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by any technician familiar with the technical field within the technical scope disclosed by the present invention should be covered within the protection scope of the present invention.

Claims

1. A method for single-phase grounding protection of a doubly-fed wind farm transmission line, characterized in that: The following steps are involved: Collect electrical data including zero-mode voltage on the wind power side and system side of the double-fed wind farm transmission line; Based on the zero-mode component equivalent network model, calculate the reference voltage of the single-phase grounding fault inside and / or outside the wind power area on the transmission line, and the reference voltage of the single-phase grounding fault inside and / or outside the system area; When making a ground fault judgment, compare the actually collected zero-mode voltage on the wind power side of the transmission line with the reference voltage of the single-phase ground fault in the wind power area and / or outside the area, and compare the zero-mode voltage on the system side with the reference voltage of the single-phase ground fault in the system area and / or outside the area; judge whether the comparison result meets the preset longitudinal protection action criterion; If the action criteria are met, it is determined to be a single-phase grounding fault in the zone, and a trip signal is sent immediately; If the action criteria are not met, it is determined to be an out-of-zone single-phase grounding fault and no trip signal is sent; The zero-mode model similarity factor is used to measure the difference between the zero-mode voltage on the wind power side and the reference voltage of the single-phase grounding fault within the area and / or outside the area, and the difference between the zero-mode voltage on the system side and the reference voltage of the single-phase grounding fault within the area and / or outside the area; The similarity factor calculation formula is: In the formula, k w is the zero-mode model similarity factor of the wind power side of the transmission line; k s is the zero mode model similarity factor of the system side of the transmission line; u w0 is the vector of the actual measured value of the zero-mode voltage on the wind power side, u' w0 is the reference voltage vector of the wind power transmission line on the wind power side, u s0 is the vector of the actual sampling values ​​of the zero-mode voltage on the system side, u' s0 It is the reference voltage vector on the wind power transmission line system side; The reference voltage vector of the wind power transmission line on the wind power side i' w0 is the actual measured value of the zero-mode current corresponding to the actual measured value of the zero-mode voltage on the wind power side; The reference voltage vector of the wind power transmission line system side i s '0 is the actual measured value of zero-mode current corresponding to the actual measured value of zero-mode voltage on the system side; The reference voltage vector of the wind power transmission line on the wind power side i' w0 is the actual measured value of the zero-mode current corresponding to the actual measured value of the zero-mode voltage on the wind power side; The reference voltage vector of the wind power transmission line system side i s '0 is the actual measured value of the zero-mode current corresponding to the actual measured value of the zero-mode voltage on the system side.

2. The single-phase grounding protection method for a doubly-fed wind farm transmission line according to claim 1 is characterized in that: Reference voltage for single-phase ground fault in wind power area Where, L w0 is the equivalent zero-mode inductance of the wind farm output transformer; i w0 is the zero-mode current on the wind power side, and t is the time.

3. The single-phase grounding protection method for a doubly-fed wind farm transmission line according to claim 1 is characterized in that: Reference voltage for single-phase ground fault outside wind power area In the formula, R s0 , L s0 are the equivalent zero-mode resistance and zero-mode inductance of the AC system respectively; R l0 , L l0 are the zero-mode resistance and zero-mode inductance of the wind power transmission line respectively; i w0 is the zero-mode current on the wind power side, and t is the time.

4. The single-phase grounding protection method for a doubly-fed wind farm transmission line according to claim 1 is characterized in that: Reference voltage for single-phase ground fault in the system side area In the formula, R s0 , L s0 are the equivalent zero-mode resistance and zero-mode inductance of the AC system respectively; i s0 is the zero-mode current on the system side, and t is the time.

5. The single-phase grounding protection method for a doubly-fed wind farm transmission line according to claim 1, characterized in that: Reference voltage for single-phase ground fault outside the system side In the formula, R l0 , L l0 are the zero-mode resistance and zero-mode inductance of the wind power transmission line, L w0 is the equivalent zero-mode inductance of the wind farm output transformer; i w0 is the zero-mode current on the wind power side, and t is the time.

6. The method for single-phase grounding protection of a doubly-fed wind farm transmission line according to claim 1, characterized in that: The protection action criterion is: Where k set It is the preset protection action threshold.

7. A system for implementing the single-phase grounding protection method for a doubly-fed wind farm transmission line according to any one of claims 1 to 6, characterized in that: It includes data acquisition module, protection calculation module and protection action module; The data acquisition module is used to collect electrical data including zero-mode voltage on the wind power side and the system side of the double-fed wind farm transmission line, and transmit the electrical data to the protection calculation module; The protection calculation module is used to calculate the reference voltage of the single-phase grounding fault in the wind power side area and / or outside the area of ​​the transmission line, and the reference voltage of the single-phase grounding fault in the system side area and / or outside the area; and calculate the zero-mode voltage similarity factor of the wind power side and the system side of the transmission line according to the input electrical data, and pass it to the protection action module; The protection action module is used to receive the zero-mode voltage similarity factor on both sides of the line, and determine whether the size of the similarity factor meets the longitudinal protection action criterion. If the action criterion is met, it is determined to be an in-zone fault and a trip signal is sent immediately; if the action criterion is not met, it is determined to be an out-of-zone fault and no trip signal is sent.

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