Negative-sequence current component pilot protection method and system for outgoing line of doubly-fed wind farm

Abstract

The present invention discloses a longitudinal protection method and system for negative sequence current components of the outgoing line of a doubly-fed wind farm, including: obtaining the sudden change of three-phase current of the outgoing line of the doubly-fed wind farm, and determining whether a fault occurs in the outgoing line of the doubly-fed wind farm according to the sudden change of three-phase current; judging the type of fault according to the existence of negative sequence current; for an asymmetrical fault, calculating the adaptive amplitude ratio of negative sequence current on both sides under the asymmetrical fault; for a symmetrical fault, calculating the adaptive amplitude ratio of negative sequence current on both sides under the symmetrical fault; and performing corresponding protection actions according to the action values of asymmetrical faults and symmetrical faults. The present invention can solve the problem that the protection action performance deteriorates due to the frequency deviation of the current on the wind farm side when a fault occurs in the outgoing line of the doubly-fed wind farm, has a good ability to withstand transition resistance, can respond to all types of faults, and can effectively improve the reliability of the protection of the outgoing line of the doubly-fed wind farm, with good effects.

Description

Technical Field

[0001] The present invention relates to a pilot protection method for negative sequence current components of outgoing lines of a doubly-fed wind farm, belonging to the technical field of relay protection of power systems. Background Art

[0002] As a clean and renewable energy source, wind energy is widely distributed and has rich exploitable resources. Wind power generation has the advantages of mature power generation technology, stable power generation cost, and no environmental costs such as carbon emissions; compared with solar energy and tidal energy, the wind energy industry has a good foundation, obvious economic advantages, and less environmental pollution.

[0003] Among many wind turbines, doubly-fed wind turbines have become one of the mainstream models in wind farms due to their mature technology, small required variable frequency capacity, and small volume. With the increase in the grid-connected installed capacity of wind power, the grid's requirements for its grid connection are constantly improving, and some problems have also emerged in doubly-fed wind turbine generators during grid short-circuit faults. During a grid short-circuit fault, the converter on the rotor side of the fan will experience overcurrent, causing the fan to trip off the grid, which is not conducive to the safe and stable operation of the grid. When a fault occurs in the outgoing line of a doubly-fed wind farm and causes a slight voltage dip at the machine terminal, the current sensed by the rotor side circuit of the fan is small, and the fan relies on the RSC control strategy to achieve low-voltage ride-through without the need to input a crowbar circuit. At this time, the short-circuit current characteristics provided by the doubly-fed wind turbines on the wind farm side are closely related to the control strategy of the RSC. When a fault in the outgoing line causes a severe voltage dip at the machine terminal, the current sensed by the rotor side circuit of the fan can be too large. At this time, in order to protect the rotor side converter from being damaged, a Crowbar protection circuit is input to protect the rotor side converter. This stage is short-lived, generally lasting 3 - 5 ms, and the outgoing line protection is too late to act.

[0004] In addition, most wind farms are connected in a large-scale centralized manner, and electric energy is sent out to the system through the outgoing line after passing through the main transformer. At present, conventional line protection is used for the outgoing lines of wind farms, and its principle is to form protection based on the power frequency voltage and current on both sides of the line. Since the control technology and grid connection method of wind turbines are different from those of traditional synchronous generators, there are significant differences in the electromagnetic transient characteristics of the short-circuit current provided by the wind farm side during a fault on the outgoing line, and the relay protection action performance based on the fault transient response characteristics of traditional power systems cannot be guaranteed.

[0005] Therefore, in order to cope with the impact brought by the access of a large number of doubly-fed wind turbine generators to the grid and ensure the correct operation of relay protection on the outgoing line, it is of great significance to design a pilot protection method for negative sequence current applicable to the outgoing lines of wind farms. Summary of the Invention

[0006] The present invention provides a longitudinal protection method for negative-sequence current components of the outgoing line of a doubly-fed wind farm, which is used to realize the fault judgment of the outgoing line of the doubly-fed wind farm, further identify different fault types, and then give the protection action judgment basis for each fault type to realize longitudinal protection.

[0007] The technical solution of the present invention is: a longitudinal protection method for negative-sequence current components of the outgoing line of a doubly-fed wind farm, including:

[0008] Obtain the sudden change amount of the three-phase current of the outgoing line of the doubly-fed wind farm, and determine whether a fault occurs in the outgoing line of the doubly-fed wind farm according to the sudden change amount of the three-phase current; when no fault occurs, the longitudinal protection of the negative-sequence current component does not start, and when a fault occurs, the longitudinal protection of the negative-sequence current component starts;

[0009] After the protection starts, judge the fault type according to whether there is negative-sequence current. If there is negative-sequence current, it is an asymmetric fault, otherwise it is a symmetric fault;

[0010] For asymmetric faults, calculate the negative-sequence current components on both sides of the outgoing line of the doubly-fed wind farm under asymmetric faults, and calculate the adaptive coefficient under asymmetric faults according to the phase difference of the negative-sequence current on both sides of the outgoing line of the doubly-fed wind farm under asymmetric faults, and then calculate the adaptive amplitude ratio of the negative-sequence current on both sides under asymmetric faults as the action value for asymmetric faults;

[0011] For symmetric faults, first construct the negative-sequence current component by using the current value of phase A before the fault and the current values of phases B and C after the fault, and calculate the adaptive coefficient under symmetric faults according to the phase difference of the negative-sequence current on both sides of the outgoing line of the doubly-fed wind farm under symmetric faults, and then calculate the adaptive amplitude ratio of the negative-sequence current on both sides under symmetric faults as the action value for symmetric faults;

[0012] Based on the action value for asymmetric faults and the action value for symmetric faults, construct the protection criteria for longitudinal protection under symmetric faults and asymmetric faults;

[0013] Based on the protection criteria for longitudinal protection under symmetric faults and asymmetric faults, perform corresponding protection actions.

[0014] The determination of whether a fault occurs in the outgoing line of the doubly-fed wind farm according to the sudden change amount of the three-phase current includes:

[0015] When the sudden change amount of the phase current of any one of the three phases A, B, and C is greater than the protection startup threshold value I d it is judged that a fault has occurred, and the longitudinal protection of the negative-sequence current component starts. On the contrary, it is judged that there is no fault, and the longitudinal protection of the negative-sequence current component does not start.

[0016] The sudden change amount of the current is calculated by the formula:

[0017]

[0018] Where: Are the sudden change amounts of the phase currents of phases A, B, and C at the k-th sampling point respectively; Are the phase current values of phases A, B, and C at the k-th sampling point respectively; Are the phase current values of phases A, B, and C in the previous cycle respectively; Are the phase current values of phases A, B, and C in the previous two cycles respectively; N is the number of sampling points in one power frequency cycle; k is the k-th sampling point; I d Is the starting threshold value; I n Is the rated current of the line.

[0019] The judgment basis for judging the fault type according to the presence or absence of negative sequence current is: if the negative sequence current is greater than the negative sequence current setting value I set2 , then it is an asymmetric fault, otherwise, it is a symmetric fault.

[0020] The calculation formula for the adaptive amplitude ratio of the negative sequence currents on both sides under the asymmetric fault is:

[0021]

[0022] Where: Are the negative sequence current components on both sides of the outgoing line MN of the doubly-fed wind farm under the asymmetric fault respectively; Are the phase current values of phases A, B, and C on the M side of the outgoing line of the doubly-fed wind farm under the asymmetric fault respectively; Are the phase current values of phases A, B, and C on the N side of the outgoing line of the doubly-fed wind farm under the asymmetric fault respectively; a, a 2 Is the phase-to-sequence conversion coefficient; μ 1 Is the adaptive coefficient; θ MN1 Is the phase difference between the negative sequence current components on both sides of the outgoing line MN of the doubly-fed wind farm under the asymmetric fault; K 1 Is the adaptive amplitude ratio of the negative sequence currents on both sides of MN under the asymmetric fault.

[0023] The calculation formula for the adaptive amplitude ratio of the negative sequence currents on both sides under the symmetric fault is:

[0024]

[0025] Where: Are the negative sequence current components on both sides of the outgoing line MN of the doubly-fed wind farm under the symmetric fault respectively; Is the phase current value of phase A before the fault on both sides of the outgoing line MN of the doubly-fed wind farm; They are the phase current values of phases B and C on the M side of the outgoing line of the doubly-fed wind farm under symmetrical faults respectively; They are the phase current values of phases B and C on the N side of the outgoing line of the doubly-fed wind farm under symmetrical faults respectively; a, a 2 is the phase-to-sequence conversion coefficient; μ 2 is the adaptive coefficient; θ MN2 is the phase difference of the negative-sequence current components on both sides of the outgoing line MN of the doubly-fed wind farm under symmetrical faults; K 2 is the adaptive amplitude ratio of the negative-sequence currents on both sides of MN under symmetrical faults.

[0026] The protection criterion expressions of the pilot protection under the above-mentioned symmetrical faults and asymmetrical faults are:

[0027]

[0028] In the formula: K 1 is the adaptive amplitude ratio of the negative-sequence currents on both sides of MN under asymmetrical faults, K 2 is the adaptive amplitude ratio of the negative-sequence currents on both sides of MN under symmetrical faults, K set is the setting value for the pilot protection to operate.

[0029] According to the protection criterion of the pilot protection under symmetrical faults and asymmetrical faults, corresponding protection actions are carried out. Specifically: when the action value under asymmetrical faults is greater than the setting value, and / or when the action value under symmetrical faults is greater than the setting value, the protection acts; otherwise, the protection does not act.

[0030] According to another aspect of the present invention, there is also provided a pilot protection system for the negative-sequence current components of the outgoing line of a doubly-fed wind farm, including:

[0031] The first judgment module is used to obtain the sudden change of the three-phase current of the outgoing line of the doubly-fed wind farm, and determine whether a fault occurs in the outgoing line of the doubly-fed wind farm according to the sudden change of the three-phase current; when no fault occurs, the pilot protection for the negative-sequence current components does not start, and when a fault occurs, the pilot protection for the negative-sequence current components starts;

[0032] The second judgment module is used to judge the type of fault according to whether there is negative-sequence current after the protection starts. If there is negative-sequence current, it is an asymmetrical fault, otherwise it is a symmetrical fault;

[0033] The first calculation module is used for asymmetrical faults, calculating the negative-sequence current components on both sides of the outgoing line of the doubly-fed wind farm under asymmetrical faults, and calculating the adaptive coefficient under asymmetrical faults according to the phase difference of the negative-sequence currents on both sides of the outgoing line of the doubly-fed wind farm under asymmetrical faults, and then obtaining the adaptive amplitude ratio of the negative-sequence currents on both sides under asymmetrical faults as the action value under asymmetrical faults;

[0034] The second calculation module is used for a symmetrical fault. First, it constructs a negative sequence current component by using the current value of phase A before the fault and the current values of phases B and C after the fault, and calculates the adaptive coefficient under the symmetrical fault according to the phase difference of the negative sequence currents on both sides of the outgoing line of the doubly-fed wind farm under the symmetrical fault, and then obtains the adaptive amplitude ratio of the negative sequence currents on both sides under the symmetrical fault, which is used as the action value for the symmetrical fault.

[0035] The construction module is used to construct the protection criteria for pilot protection under symmetrical and asymmetrical faults based on the action values of asymmetrical faults and symmetrical faults.

[0036] The execution module is used to perform corresponding protection actions according to the protection criteria for pilot protection under symmetrical and asymmetrical faults.

[0037] According to another aspect of the present invention, a terminal device is further provided, including a memory, a processor, and a program stored on the memory and executable by the processor. When the processor executes the program, the steps of the negative sequence current component pilot protection method for the outgoing line of the doubly-fed wind farm described in any one of the above are implemented.

[0038] The beneficial effects of the present invention are as follows:

[0039] (1) When a fault occurs and the protection is started, for different fault types, the present invention obtains the adaptive negative sequence current amplitude ratio by comparing the amplitude ratio of the negative sequence currents at the protection installation points on both sides of the outgoing line with the adaptive coefficient formed by the phase difference of the negative sequence angles at both ends, and compares the adaptive negative sequence current amplitude ratio with the setting value. If it is greater than the setting value, the protection acts; otherwise, the protection does not act. Based on the above analysis, it can be seen that the present invention can quickly and reliably distinguish internal and external faults, improving the sensitivity of the protection.

[0040] (2) The present invention constructs a negative sequence current component by using the pre-fault phase current of phase A and the post-fault phase currents of phases B and C, effectively solving the problem that the negative sequence component can only reflect asymmetrical fault types. Therefore, after starting the sudden change protection, the present invention adopts different negative sequence current calculation modules according to the existence of the negative sequence current component, enabling the negative sequence current pilot protection to reflect different fault types.

[0041] (3) The present invention uses the adaptive negative sequence current amplitude ratio as the protection criterion, which can overcome the problem of the decline in the protection action performance caused by the frequency offset of the short-circuit current on the wind farm side of the outgoing line and has a good ability to withstand the transition resistance.

[0042] In summary, the present invention can solve the problem of the decline in the protection action performance caused by the frequency offset of the current on the wind farm side during the fault of the outgoing line of the doubly-fed wind farm and has a good ability to withstand the transition resistance, and can reflect all fault types, effectively improving the reliability of the protection of the outgoing line of the doubly-fed wind farm, with good effects. Description of the Drawings

[0043] Figure 1 It is a flowchart of the pilot protection of the negative-sequence current of the outgoing line of a doubly-fed wind farm;

[0044] Figure 2 It is a schematic diagram of the simulation model of the grid-connected system of a doubly-fed wind turbine;

[0045] Figure 3 It is the phase angle difference of the negative-sequence currents on both sides of MN after considering the crowbar input during the A-phase ground short-circuit fault at the midpoint of the outgoing line;

[0046] Figure 4 It is the amplitude ratio of the negative-sequence currents on both sides of MN after considering the crowbar input during the A-phase ground short-circuit fault at the midpoint of the outgoing line;

[0047] Figure 5 It is the adaptive amplitude ratio of the negative-sequence currents on both sides of MN after considering the crowbar input during the A-phase ground short-circuit fault at the midpoint of the outgoing line. Detailed Implementation Manner

[0048] The invention will be further described below in conjunction with the drawings and embodiments, but the content of the invention is not limited to the described scope.

[0049] Embodiment 1:

[0050] As Figures 1 - 5 shown, a method for pilot protection of the negative-sequence current component of the outgoing line of a doubly-fed wind farm includes: obtaining the sudden change of the three-phase current of the outgoing line of the doubly-fed wind farm, and determining whether a fault occurs in the outgoing line of the doubly-fed wind farm according to the sudden change of the three-phase current; when no fault occurs, the pilot protection of the negative-sequence current component does not start, and when a fault occurs, the pilot protection of the negative-sequence current component starts; after the protection starts, judge the fault type according to whether there is negative-sequence current. If there is negative-sequence current, it is an asymmetric fault, otherwise it is a symmetric fault; for an asymmetric fault, calculate the negative-sequence current components on both sides of the outgoing line of the doubly-fed wind farm under the asymmetric fault, and calculate the adaptive coefficient under the asymmetric fault according to the phase difference of the negative-sequence currents on both sides of the outgoing line of the doubly-fed wind farm under the asymmetric fault, and then calculate the adaptive amplitude ratio of the negative-sequence currents on both sides under the asymmetric fault as the action value for the asymmetric fault; for a symmetric fault, first construct the negative-sequence current component by using the current value of phase A before the fault and the current values of phases B and C after the fault, and calculate the adaptive coefficient under the symmetric fault according to the phase difference of the negative-sequence currents on both sides of the outgoing line of the doubly-fed wind farm under the symmetric fault, and then calculate the adaptive amplitude ratio of the negative-sequence currents on both sides under the symmetric fault as the action value for the symmetric fault; based on the action value for the asymmetric fault and the action value for the symmetric fault, construct the protection criterion for the pilot protection under the symmetric fault and the asymmetric fault; according to the protection criterion for the pilot protection under the symmetric fault and the asymmetric fault, perform the corresponding protection action.

[0051] Optionally, determining whether a fault occurs in the outgoing line of the doubly-fed wind farm according to the sudden change of three-phase current includes:

[0052] When the sudden change of phase current in any one of the three phases A, B, and C is greater than the protection startup threshold I d , it is determined that a fault has occurred, and the longitudinal protection of the negative-sequence current component starts. Otherwise, it is determined that there is no fault, and the longitudinal protection of the negative-sequence current component does not start.

[0053] Optionally, the calculation formula for the sudden change of current is as follows:

[0054]

[0055] In the formula: are the sudden changes of phase current in phases A, B, and C at the k-th sampling point respectively; are the phase current values in phases A, B, and C at the k-th sampling point respectively; are the phase current values in phases A, B, and C in the previous cycle respectively; are the phase current values in phases A, B, and C in the previous two cycles respectively; N is the number of sampling points in a power frequency cycle; k is the k-th sampling point; I d is the startup threshold; I n is the rated current of the line.

[0056] Optionally, the judgment basis for judging the fault type according to the presence of negative-sequence current is: if the negative-sequence current is greater than the negative-sequence current setting value I set2 , it is an asymmetrical fault. Otherwise, it is a symmetrical fault. The negative-sequence current setting value I set2 in this embodiment takes 0.2 times the rated current.

[0057] Optionally, the calculation formula for the adaptive amplitude ratio of the negative-sequence current on both sides under the asymmetrical fault is:

[0058]

[0059] In the formula: are the negative-sequence current components on both sides of the outgoing line MN of the doubly-fed wind farm under the asymmetrical fault respectively; are the phase current values in phases A, B, and C on the M side of the outgoing line of the doubly-fed wind farm under the asymmetrical fault respectively; are the phase current values in phases A, B, and C on the N side of the outgoing line of the doubly-fed wind farm under the asymmetrical fault respectively; a, a 2 are the phase-to-sequence conversion coefficients, and the values are a = e j120° , a 2 = ej240° ; μ 1 is the adaptive coefficient; θ MN1 is the phase difference of the negative sequence current components on both sides of the outgoing line MN of the doubly-fed wind farm under asymmetric faults; K 1 is the adaptive amplitude ratio of the negative sequence current on both sides of MN under asymmetric faults, are the amplitudes of the negative sequence current components on the M side and N side of the outgoing line of the doubly-fed wind farm under symmetrical faults, respectively.

[0060] Optionally, the calculation formula for the adaptive amplitude ratio of the negative sequence current on both sides under symmetrical faults is:

[0061]

[0062] In the formula: are the negative sequence current components on both sides of the outgoing line MN of the doubly-fed wind farm under symmetrical faults, respectively; is the phase current value of phase A before the fault on both sides of the outgoing line MN of the doubly-fed wind farm; are the phase current values of phases B and C on the M side of the outgoing line of the doubly-fed wind farm under symmetrical faults, respectively; are the phase current values of phases B and C on the N side of the outgoing line of the doubly-fed wind farm under symmetrical faults, respectively; a, a 2 are the phase-to-sequence conversion coefficients; μ 2 is the adaptive coefficient; θ MN2 is the phase difference of the negative sequence current components on both sides of the outgoing line MN of the doubly-fed wind farm under symmetrical faults; K 2 is the adaptive amplitude ratio of the negative sequence current on both sides of MN under symmetrical faults, are the amplitudes of the negative sequence current components on the M side and N side of the outgoing line of the doubly-fed wind farm under symmetrical faults, respectively.

[0063] Among the above, the phase current value can be calculated using FFT.

[0064] Optionally, the protection criterion expression of the pilot protection under symmetrical and asymmetric faults is:

[0065]

[0066] In the formula: K 1 is the adaptive amplitude ratio of the negative sequence current on both sides of MN under asymmetric faults, K 2 is the adaptive amplitude ratio of the negative sequence current on both sides of MN under symmetrical faults, K set is the setting value for the pilot protection to operate.

[0067] Optionally, according to the protection criteria of pilot protection under symmetrical faults and asymmetrical faults, corresponding protection actions are taken. Specifically: when the action value under asymmetrical faults is greater than the setting value, and / or when the action value under symmetrical faults is greater than the setting value, the protection acts; otherwise, the protection does not act. That is, when any one of the expressions in the protection criteria of pilot protection under symmetrical faults and asymmetrical faults satisfies the action condition, the protection acts; when neither of the two expressions in the protection criteria of pilot protection under symmetrical faults and asymmetrical faults is satisfied, the protection does not act, which indicates an external fault.

[0068] Furthermore, the sensitivity coefficient is calculated according to the following formula:

[0069]

[0070] In the formula: K sen is the sensitivity coefficient of the pilot protection action; X represents K 1 or K 2 .

[0071] The specific principle of the present invention is as follows:

[0072] (1) Negative-sequence current characteristics of doubly-fed wind turbines and their influence on the negative-sequence sudden change protection of outgoing lines.

[0073] When a fault occurs in the outgoing line of a doubly-fed wind farm, resulting in a slight voltage dip at the machine terminal, the current sensed by the rotor-side circuit of the wind turbine is small. The wind turbine relies on the RSC control strategy to achieve low-voltage ride-through without the need to engage the crowbar circuit. At this time, the short-circuit current characteristics provided by the doubly-fed wind turbines on the wind farm side are closely related to the control strategy of the RSC. When a fault in the outgoing line causes a severe voltage dip at the machine terminal, the current sensed by the rotor-side circuit of the wind turbine can be too large. At this time, in order to protect the rotor-side converter RSC from being damaged, the Crowbar protection circuit is engaged to protect the rotor-side converter. This stage is short-lived, generally lasting 3 - 5 ms, and the outgoing line protection has no time to act. Under asymmetrical faults, the expressions for the negative-sequence components of the short-circuit current of doubly-fed wind turbines considering RSC control and crowbar engagement are shown in Equations (1) and (2).

[0074]

[0075]

[0076] In the formula, the superscript "-" represents the reverse synchronous rotating coordinate system; the subscripts s2 and sc2 represent the negative sequence after considering RSC control and crowbar engagement, respectively; are the negative-sequence components of the short-circuit current of doubly-fed wind turbines considering RSC control and crowbar engagement, respectively; t is time; are the fundamental frequency component coefficients considering RSC control and crowbar engagement, respectively; They are the decaying DC component coefficients considering the RSC control and crowbar input, respectively. They are the decaying speed component coefficients considering the RSC control and crowbar input, respectively. Their magnitudes are related to the parameters of the doubly-fed wind turbine and the degree of voltage dip. ω 1 is the synchronous speed; τ s is the time decay constant on the stator side; τ r is the time decay constant on the rotor side.

[0077] It can be seen from Equations (1) and (2) that the negative-sequence component of the stator current consists of three parts: the steady-state fundamental frequency component, the decaying transient DC component, and the decaying speed frequency component. When the voltage drops deeply, the negative-sequence component of the stator current is mainly composed of the decaying speed frequency component, and its frequency depends on the slip ratio. The variation range of the slip ratio of the doubly-fed wind turbine is generally -0.3 to 0.3. Therefore, when the voltage drops deeply, the negative-sequence current output at the machine terminal under different operating conditions is mainly composed of the decaying speed frequency component and will vary in the range of 35 - 65 Hz.

[0078] Due to the existence of the decaying speed frequency negative-sequence component and the generally small capacity of the wind farm, the performance of the negative-sequence sudden change protection deteriorates, the sensitivity is insufficient, and misjudgment may occur under extreme conditions.

[0079] (2) Principle of the longitudinal protection of the negative-sequence current component

[0080] When a fault occurs outside the outgoing line area of the wind farm, the currents at the protection installation points on both sides of MN are provided by the current on the same side, and the amplitude ratio of the two-side currents is 1; when a fault occurs inside the area, the current on the system side is provided by the system power supply, and the current on the wind farm side is provided by the wind turbines. Affected by the internal power electronic components of the wind turbines, the short-circuit current provided by the doubly-fed wind turbines is relatively small, generally not exceeding 1.5 - 2 times of their rated current. Therefore, when a fault occurs inside the area, the amplitude ratio of the current on the system side to the current on the wind farm side is relatively large. The calculation formula for the amplitude ratio of the two-side currents is shown in Equation (3).

[0081]

[0082] In the formula, K is the amplitude ratio of the currents on both sides of the outgoing line; are the amplitudes of the negative-sequence current components on both sides of the outgoing line MN, respectively.

[0083] (3) Determination of the adaptive coefficient

[0084] The adaptive coefficient is constructed based on the phase difference of the negative-sequence current components on both sides of the outgoing line, and the specific solution formula is shown in Equation (4).

[0085]

[0086] In the formula, μ is the adaptive coefficient; θ MNis the phase difference between the negative-sequence current components on both sides of the outgoing line MN. During an external fault, the currents on both sides are provided by the same current. Since the current directions are both from the line to the line, theoretically, there is θ MN = 180°, cos|θ MN | = -1, and μ approaches 0. During an internal fault, theoretically, the phase angles of the currents on both sides are the same, θ MN = 0, cos|θ MN | = 1, and μ approaches infinity. However, through the analysis of Equations (1) and (2), it can be seen that the phase angle of the negative-sequence current on the wind farm side varies between -90° and 0°. Therefore, the phase angle difference between the currents on both sides varies between 0° and 90°, and the value of μ is always much greater than 1. In the above, by using the fact that the phase angle difference between the current components on both sides is smaller during an internal fault and larger during an external fault, an adaptive coefficient is constructed, so that the adaptive coefficient is much greater than 1 during an internal fault and less than 1 during an external fault. Furthermore, the adaptive amplitude ratio can correctly distinguish between internal and external faults and improve the protection sensitivity.

[0087] (4) Calculation of negative-sequence current components

[0088] For an asymmetrical fault, the negative-sequence current components are calculated using the symmetrical component method, and the specific formula is shown in Equation (5). For a symmetrical fault, first, the negative-sequence current components are constructed by using the pre-fault phase current of phase A and the post-fault phase currents of phases B and C, similar to the BC-phase interphase fault. Then, the corresponding negative-sequence components are calculated using the symmetrical component method, and the specific formula is shown in Equation (6).

[0089]

[0090]

[0091] (5) Determination of protection operation setting values

[0092] During an external fault, ideally, the adaptive amplitude ratio of the negative-sequence current components on both sides of MN approaches 0. When considering the phase angle difference between the negative-sequence currents on both sides in actual operation, the adaptive amplitude ratio may be close to 1. During an internal fault, the adaptive amplitude ratio of the negative-sequence current components on both sides of MN is generally much greater than 10. Therefore, the protection operation setting value selected in the present invention is K set = 10.

[0093] The following details the optional specific embodiments of the present invention.

[0094] In this example, a wind farm with 6 doubly-fed wind turbines is taken as an example. The doubly-fed wind turbine grid-connected system is built using the electromagnetic transient simulation software Matlab / Simulink, and a negative-sequence current pilot protection method applicable to the outgoing line of a doubly-fed wind farm proposed in this embodiment is simulated and verified:

[0095] (1) Model building

[0096] The simulation model of the double-fed wind farm grid-connected system via the transmission line is as follows Figure 2 As shown. The wind farm has a total of 6 double-fed wind turbines (DFIG1-DFIG6), and the parameters of each wind turbine are as follows: rated voltage is 0.575kV; rated capacity is 1.5MW; power factor is 0.9. The wind turbine is connected to the 35kV collector line through the box transformer (0.575kV / 35kV), and then the voltage is increased to 110kV through the collector and main transformer (35kV / 110kV), and finally connected to the grid through the 30km transmission line. The basic parameters of the wind farm and the basic parameters of the transmission line are shown in Table 1 and Table 2.

[0097] Table 1 Basic parameters of a single 1.5MW doubly-fed wind turbine

[0098] Parameter Value Parameter Value Power factor 0.9 pu Rotor speed 1.2 pu Stator resistance 0.023 pu Stator leakage inductance 0.18 pu Rotor resistance 0.016 pu Rotor leakage inductance 0.16 pu Chopper resistance 0.05 pu Mutual inductance between stator and rotor 2.9 pu

[0099] Table 2 Basic parameters of wind farm transmission lines

[0100] Parameter Value Parameter Value Voltage level 110 kV Line length 30 km Positive - sequence resistance 0.1278 Ω / km Zero - sequence resistance 0.3834 Ω / km Positive - sequence inductance 1.124 mH / km Zero - sequence inductance 3.372 mH / km

[0101] (2) Simulation analysis

[0102] Simulation results under different control strategies

[0103] 1) Simulation results under different fault conditions considering crowbar control

[0104] In order to verify the effectiveness of the negative sequence current longitudinal protection method for the transmission line of a doubly fed wind farm proposed in this embodiment, faults of different conditions are set at point f of the transmission line, and the crowbar resistor is instantaneously put into use after the fault, and the data of the protection installation of the protection line are recorded in Tables 3-5. The simulation results of different types of metallic faults when point f is located at the midpoint of the transmission line are shown in Table 3, the simulation results of AB two-phase grounding faults with different transition resistances when point f is located at the midpoint of the transmission line are shown in Table 4, and the simulation results of three-phase short circuits when point f is located at different positions of the transmission line are shown in Table 5.

[0105] Table 3 Protection action under different fault types

[0106]

[0107] Note: In the table, AG refers to a single-phase grounding fault; AB refers to a two-phase phase-to-phase fault; ABG refers to a two-phase grounding fault; and ABG refers to a three-phase grounding fault.

[0108] Table 4 Protection action under different transition resistance

[0109]

[0110] Table 5 Protection Action Conditions under Different Fault Positions

[0111]

[0112] Note: The fault position coefficient d in the table refers to the ratio of the line length from the wind farm side (M) protection to the fault position to the total length of the outgoing line; "-" means that the sensitivity coefficient does not need to be calculated.

[0113] It can be seen from Table 3 that when the crowbar protection action is considered, no matter what type of fault occurs at the midpoint of the outgoing line, the negative-sequence current pilot protection method proposed in this embodiment for the outgoing line of the doubly-fed wind farm can correctly identify the internal fault, and the sensitivity is relatively high. The phase angle difference and amplitude ratio of the negative-sequence current on both sides of line MN are greatly affected by the fault type, and finally the adaptive amplitude ratio on both sides of MN and the sensitivity coefficient of the protection are greatly affected by the fault type. Among them, the phase angle difference, negative-sequence current amplitude ratio and negative-sequence current adaptive amplitude ratio at both ends of the A-phase ground short circuit are as Figures 3 - 5 shown.

[0114] It can be seen from Table 4 that when the crowbar protection action is considered, as the transition resistance of the fault point increases, the phase angle difference of the negative-sequence current on both sides of line MN will decrease, resulting in an increase in the adaptive coefficient, and finally the adaptive amplitude ratio on both sides of MN increases, improving the sensitivity of the protection. Therefore, the negative-sequence current pilot protection method proposed in this embodiment for the outgoing line of the doubly-fed wind farm has good ability to withstand the transition resistance.

[0115] It can be seen from Table 5 that when the crowbar protection action is considered, in the case of internal fault, as the fault position coefficient increases, both the phase angle difference and amplitude ratio of the negative-sequence current on both sides of line MN will increase, and the increase amplitude is relatively small. The negative-sequence current adaptive amplitude ratio increases and the increase amplitude is relatively small. The negative-sequence current pilot protection method proposed in this embodiment for the outgoing line of the doubly-fed wind farm can accurately identify internal and external faults.

[0116] 2) Simulation Results under Different Fault Conditions Considering RSC Control

[0117] To verify the effectiveness of the negative-sequence current pilot protection method proposed in this embodiment for the outgoing line of the doubly-fed wind farm, faults with different conditions are set at point f on the outgoing line. The crowbar is not put into operation after the fault, and the data at the protection installation location of the protected line are recorded in Tables 6 - 8. The simulation results of different types of metallic faults occurring when point f is located at the midpoint of the outgoing line are shown in Table 6, the simulation results of A-phase ground faults with different transition resistances occurring when point f is located at the midpoint of the outgoing line are shown in Table 7, and the simulation results of AB-phase ground short circuits occurring when point f is located at different positions on the outgoing line are shown in Table 8.

[0118] Table 6 Protection Action Conditions under Different Fault Types

[0119]

[0120] Note: In the table, AG refers to single-phase ground fault; AB refers to two-phase interphase fault; ABG refers to two-phase ground fault; ABG refers to three-phase ground fault.

[0121] Table 7 Operating conditions of protection under different transition resistances

[0122]

[0123] Table 8 Operating conditions of protection at different fault positions

[0124]

[0125] Note: In the table, the fault position coefficient d refers to the ratio of the line length from the wind farm side (M) protection to the fault position to the total length of the outgoing line; "-" means that the sensitivity coefficient does not need to be calculated.

[0126] In the above Tables 3 and 6, x takes the value of 1 or 2. Taking 1 represents an asymmetric fault, and taking 2 represents a symmetric fault. Additionally, it should be noted that the phase angle difference, negative sequence current amplitude ratio, and negative sequence current adaptive amplitude ratio values in Tables 3 - 8 are the rounded results calculated after computer simulation using the model constructed according to the method of the present invention.

[0127] It can be seen from Table 6 that when the RSC control is considered, no matter what type of fault occurs at the midpoint of the outgoing line, a negative sequence current pilot protection method proposed in this embodiment for the outgoing line of a doubly-fed wind farm can correctly identify internal faults, and the sensitivity is relatively high. The phase angle difference and amplitude ratio of the negative sequence current on both sides of line MN are greatly affected by the fault type, and finally the negative sequence current adaptive amplitude ratio on both sides of MN and the sensitivity coefficient of the protection are greatly affected by the fault type.

[0128] It can be seen from Table 7 that when the RSC control is considered, as the transition resistance of the fault point increases, the phase angle difference of the negative sequence current on both sides of line MN will first decrease and then increase, resulting in the adaptive coefficient increasing first and then decreasing. The amplitude ratio of the negative sequence current on both sides of line MN basically remains unchanged. Finally, the adaptive amplitude ratio on both sides of MN increases first and then decreases, but the protection can correctly identify internal faults and the sensitivity is relatively high. Therefore, a negative sequence current pilot protection method proposed in this embodiment for the outgoing line of a doubly-fed wind farm has good ability to withstand transition resistance.

[0129] As can be seen from Table 8, when the RSC control is taken into account, during an in-zone fault, as the fault location coefficient increases, the phase angle difference of the negative-sequence currents on both sides of line MN gradually decreases, and the amplitude ratio gradually increases, and the change amplitudes are all small. However, the adaptive amplitude ratio of the negative-sequence current increases and the increase amplitude is relatively large. A negative-sequence current pilot protection method applicable to the outgoing line of a doubly-fed wind farm proposed in this embodiment can accurately identify in-zone and out-of-zone faults.

[0130] Embodiment 2:

[0131] According to another aspect of the embodiments of the present invention, there is also provided a negative-sequence current component pilot protection system applicable to the outgoing line of a doubly-fed wind farm, including:

[0132] A first judgment module, configured to obtain the sudden change amount of the three-phase current of the outgoing line of the doubly-fed wind farm, and determine whether a fault occurs in the outgoing line of the doubly-fed wind farm according to the sudden change amount of the three-phase current; when no fault occurs, the negative-sequence current component pilot protection does not start, and when a fault occurs, the negative-sequence current component pilot protection starts;

[0133] A second judgment module, configured to judge the fault type according to whether there is negative-sequence current after the protection starts. If there is negative-sequence current, it is an asymmetrical fault, otherwise it is a symmetrical fault;

[0134] A first calculation module, for an asymmetrical fault, calculating the negative-sequence current components on both sides of the outgoing line of the doubly-fed wind farm under the asymmetrical fault, and calculating the adaptive coefficient under the asymmetrical fault according to the phase difference of the negative-sequence currents on both sides of the outgoing line of the doubly-fed wind farm under the asymmetrical fault, and then obtaining the adaptive amplitude ratio of the negative-sequence currents on both sides under the asymmetrical fault as the action value for the asymmetrical fault;

[0135] A second calculation module, for a symmetrical fault, first constructing the negative-sequence current component by using the current value of phase A before the fault and the current values of phases B and C after the fault, and calculating the adaptive coefficient under the symmetrical fault according to the phase difference of the negative-sequence currents on both sides of the outgoing line of the doubly-fed wind farm under the symmetrical fault, and then obtaining the adaptive amplitude ratio of the negative-sequence currents on both sides under the symmetrical fault as the action value for the symmetrical fault;

[0136] A construction module, configured to construct the protection criteria for the pilot protection under symmetrical faults and asymmetrical faults based on the action value for the asymmetrical fault and the action value for the symmetrical fault;

[0137] An execution module, configured to perform corresponding protection actions according to the protection criteria for the pilot protection under symmetrical faults and asymmetrical faults.

[0138] It should be noted that the above-mentioned various modules can be implemented by software or hardware. For example, for the latter, it can be implemented in the following manner: the above-mentioned various modules can be located in the same processor; and / or, the above-mentioned various modules are located in different processors in any combination.

[0139] Embodiment 3:

[0140] According to another aspect of the embodiments of the present invention, a terminal device is further provided, including a memory, a processor, and a program stored on the memory and executable by the processor. When the processor executes the program, the steps of the negative-sequence current component pilot protection method for the outgoing line of the doubly-fed wind farm described in any one of the above are implemented.

[0141] The serial numbers of the above embodiments of the present invention are only for description and do not represent the advantages or disadvantages of the embodiments.

[0142] In the above embodiments of the present invention, the descriptions of the respective embodiments have their own emphases. For the parts not detailed in a certain embodiment, reference may be made to the relevant descriptions of other embodiments.

[0143] The specific embodiments of the present invention have been described in detail above with reference to the accompanying drawings. However, the present invention is not limited to the above embodiments, and various changes can be made without departing from the spirit of the present invention within the scope of knowledge possessed by those of ordinary skill in the art.

Claims

1. A longitudinal protection method for negative sequence current components of the outgoing line of a doubly-fed wind farm, characterized in that: It includes: Obtain the sudden change of the three-phase current of the outgoing line of the doubly-fed wind farm, and determine whether a fault occurs in the outgoing line of the doubly-fed wind farm according to the sudden change of the three-phase current; When no fault occurs, the longitudinal protection of the negative sequence current component does not start, and when a fault occurs, the longitudinal protection of the negative sequence current component starts; After the protection starts, judge the fault type according to whether there is negative sequence current. If there is negative sequence current, it is an asymmetric fault, otherwise it is a symmetric fault; For an asymmetric fault, calculate the negative sequence current components on both sides of the outgoing line of the doubly-fed wind farm under the asymmetric fault, and calculate the adaptive coefficient under the asymmetric fault according to the phase difference of the negative sequence current on both sides of the outgoing line of the doubly-fed wind farm under the asymmetric fault, and then calculate the adaptive amplitude ratio of the negative sequence current on both sides under the asymmetric fault as the action value for the asymmetric fault; For a symmetric fault, first construct the negative sequence current component by using the current value of phase A before the fault and the current values of phases B and C after the fault, and calculate the adaptive coefficient under the symmetric fault according to the phase difference of the negative sequence current on both sides of the outgoing line of the doubly-fed wind farm under the symmetric fault, and then calculate the adaptive amplitude ratio of the negative sequence current on both sides under the symmetric fault as the action value for the symmetric fault; Based on the action value for the asymmetric fault and the action value for the symmetric fault, construct the protection criterion for the longitudinal protection under the symmetric fault and the asymmetric fault; Based on the protection criterion for the longitudinal protection under the symmetric fault and the asymmetric fault, perform the corresponding protection action.

2. The longitudinal protection method for negative sequence current of the outgoing line of a doubly-fed wind farm according to claim 1, characterized in that: The determination of whether a fault occurs in the outgoing line of the doubly-fed wind farm according to the sudden change of the three-phase current includes: When the sudden change in phase current of any one of the three phases A, B, and C is greater than the protection startup threshold value I d it is judged that a fault has occurred, and the pilot protection of negative-sequence current component starts. On the contrary, it is judged that there is no fault, and the pilot protection of negative-sequence current component does not start.

3. The longitudinal protection method for negative sequence current of the outgoing line of a doubly-fed wind farm according to claim 2, characterized in that: The sudden change in current is calculated by the formula: Where: are the sudden change amounts of phase currents of phases A, B, and C at the k-th sampling point, respectively; are the phase current values of phases A, B, and C at the k-th sampling point, respectively; are the phase current values of phases A, B, and C in the previous cycle, respectively; are the phase current values of phases A, B, and C in the previous two cycles, respectively; N is the number of sampling points in one power frequency cycle; k is the k-th sampling point; I d is the starting threshold value; I n is the rated current of the line.

4. The longitudinal protection method for negative sequence current of the outgoing line of a doubly-fed wind farm according to claim 1, characterized in that: The judgment basis for judging the fault type according to the presence or absence of negative sequence current is as follows: if the negative sequence current is greater than the negative sequence current setting value I set2 , it is an asymmetrical fault; otherwise, it is a symmetrical fault.

5. The longitudinal protection method for negative sequence current of the outgoing line of a doubly-fed wind farm according to claim 1, characterized in that: The calculation formula for the adaptive amplitude ratio of the negative sequence current on both sides under the asymmetric fault is: Wherein: are the negative-sequence current components on both sides of the outgoing line MN of the doubly-fed wind farm under asymmetrical faults, respectively; are the phase current values of phases A, B, and C on the M side of the outgoing line of the doubly-fed wind farm under asymmetrical faults, respectively; are the phase current values of phases A, B, and C on the N side of the outgoing line of the doubly-fed wind farm under asymmetrical faults, respectively; a, a 2 is the phase-to-sequence conversion coefficient; μ 1 is the adaptive coefficient; θ MN1 is the phase difference of the negative-sequence current components on both sides of the outgoing line MN of the doubly-fed wind farm under asymmetrical faults; K 1 is the adaptive amplitude ratio of the negative-sequence currents on both sides of MN under asymmetrical faults.

6. The longitudinal protection method for negative sequence current of the outgoing line of a doubly-fed wind farm according to claim 1, characterized in that: The calculation formula for the adaptive amplitude ratio of the negative sequence current on both sides under the symmetric fault is: In the formula: are respectively the negative-sequence current components on both sides of the outgoing line MN of the doubly-fed wind farm under symmetrical faults; is the phase current value of phase A before the fault on both sides of the outgoing line MN of the doubly-fed wind farm; are respectively the phase current values of phases B and C on the M side of the outgoing line of the doubly-fed wind farm under symmetrical faults; are respectively the phase current values of phases B and C on the N side of the outgoing line of the doubly-fed wind farm under symmetrical faults; a, a 2 is the phase-to-sequence conversion coefficient; μ 2 is the adaptive coefficient; θ MN2 is the phase difference of the negative-sequence current components on both sides of the outgoing line MN of the doubly-fed wind farm under symmetrical faults; K 2 is the adaptive amplitude ratio of the negative-sequence current on both sides of MN under symmetrical faults.

7. The longitudinal protection method for negative sequence current of the outgoing line of a doubly-fed wind farm according to claim 1, characterized in that: The expression of the protection criterion for the longitudinal protection under the symmetric fault and the asymmetric fault is: Where: K 1 is the adaptive amplitude ratio of the negative sequence currents on both sides of MN under asymmetrical faults, and K 2 is the adaptive amplitude ratio of the negative sequence currents on both sides of MN under symmetrical faults, and K set is the setting value for the operation of pilot protection.

8. The longitudinal protection method for negative sequence current of the outgoing line of a doubly-fed wind farm according to claim 1, characterized in that: The corresponding protection action based on the protection criterion for the longitudinal protection under the symmetric fault and the asymmetric fault is specifically: when the action value for the asymmetric fault is greater than the setting value, and / or when the action value for the symmetric fault is greater than the setting value, the protection acts; otherwise, the protection does not act.

9. A longitudinal protection system for negative sequence current components applicable to the outgoing line of a doubly-fed wind farm, characterized in that: It includes: The first judgment module is used to obtain the sudden change of the three-phase current of the outgoing line of the doubly-fed wind farm, and determine whether a fault occurs in the outgoing line of the doubly-fed wind farm according to the sudden change of the three-phase current; When no fault occurs, the pilot protection of negative-sequence current component does not start. When a fault occurs, the pilot protection of negative-sequence current component starts; The second judgment module is used to judge the fault type according to the existence of negative-sequence current after the protection starts. If there is negative-sequence current, it is an asymmetric fault, otherwise it is a symmetric fault; The first calculation module is used for an asymmetric fault to calculate the negative-sequence current components on both sides of the outgoing line of the doubly-fed wind farm under the asymmetric fault, and calculate the adaptive coefficient under the asymmetric fault according to the phase difference of the negative-sequence currents on both sides of the outgoing line of the doubly-fed wind farm under the asymmetric fault, and then calculate the adaptive amplitude ratio of the negative-sequence currents on both sides under the asymmetric fault as the action value for the asymmetric fault; The second calculation module is used for a symmetric fault to first construct the negative-sequence current component by using the current value of phase A before the fault and the current values of phases B and C after the fault, and calculate the adaptive coefficient under the symmetric fault according to the phase difference of the negative-sequence currents on both sides of the outgoing line of the doubly-fed wind farm under the symmetric fault, and then calculate the adaptive amplitude ratio of the negative-sequence currents on both sides under the symmetric fault as the action value for the symmetric fault; The construction module is used to construct the protection criteria for the pilot protection under symmetric faults and asymmetric faults based on the action value for the asymmetric fault and the action value for the symmetric fault; The execution module is used to perform corresponding protection actions according to the protection criteria for the pilot protection under symmetric faults and asymmetric faults.

10. A terminal device Characterized in that: It includes a memory, a processor, and a program stored on the memory and executable by the processor. When the processor executes the program, it implements the steps of the method for pilot protection of negative-sequence current component for the outgoing line of the doubly-fed wind farm as described in any one of claims 1-8.

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