Double-fed wind power plant sending-out line single-end time domain distance protection method based on arc fault model

Abstract

The invention relates to a double-fed wind power plant sending-out line single-end time domain distance protection method based on an arc fault model, and belongs to the technical field of power system relay protection. Firstly, transient voltage and transient current of a wind power plant side are collected, a time domain distance equation containing three unknown numbers of fault distance, resistance and arc voltage is formed through a fault boundary equation obtained by establishing an arc fault model and a fault voltage equation obtained by establishing a line fault model, and the time domain distance equation is converted into a matrix form; and solving a fault distance according to a least square algorithm, if the fault distance is greater than or equal to 0 and less than or equal to the full length of the line, determining an internal fault and a protection action, otherwise, determining an external fault and a protection non-action. According to the method, the situation that traditional distance protection acts incorrectly due to the fact that errors exist in power frequency quantity extraction caused by the frequency deviation characteristic of the wind power plant is avoided, opposite-end system impedance parameters do not need to be input, and the internal and external faults of the line can be accurately and sensitively recognized under various fault working conditions.

Description

Technical Field

[0001] The invention relates to a single-end time domain distance protection method for a double-fed wind farm transmission line based on an arc fault model, and belongs to the technical field of power system relay protection. Background Art

[0002] After a fault occurs in the transmission line of a wind farm, if the configured protection can act quickly and accurately, it can prevent the fault from expanding and causing more serious impacts, and can improve the reliability of power system operation. Since the short-circuit characteristics of a doubly-fed wind farm are quite different from those of a traditional synchronous generator, when a line fails, the fault current on the wind farm side will have a frequency offset. Traditional protection will be affected by the frequency offset characteristics, resulting in errors when extracting the power frequency quantity, which may cause the protection to fail to act accurately and fail to meet the requirements of rapidity. The protection based on traditional power frequency quantities will be challenged. Summary of the invention

[0003] The purpose of the present invention is to provide a single-ended time domain distance protection method for a doubly-fed wind farm transmission line based on an arc fault model, aiming to solve the technical problem that the traditional distance protection does not operate correctly due to the error in extracting the power frequency quantity caused by the frequency offset characteristics of the wind farm.

[0004] To achieve the above object, the technical solution of the present invention is: a single-ended time domain distance protection method for a double-fed wind farm transmission line based on an arc fault model, the specific steps are:

[0005] Step 1: Collect transient voltage and transient current on the wind farm side;

[0006] Step 2: Establish an arc fault model and obtain the fault boundary equation;

[0007] Step 3: Establish the fault voltage equation through the line fault model;

[0008] Step 4: Establish a time domain distance equation containing three unknowns: fault distance, resistance and arc voltage;

[0009] Step 5: Convert the time domain distance equation into a matrix form;

[0010] Step 6: Solve the fault distance according to the least squares algorithm;

[0011] Step 7: If the fault distance is greater than or equal to 0 and less than or equal to the total length of the line, it is determined to be an internal fault and the protection is activated. Otherwise, it is determined to be an external fault and the protection is not activated.

[0012] The Step 2 is specifically as follows:

[0013] As far as the fault arc current is concerned, the nonlinear behavior of the arc has little effect on it, so the arc is represented by the segmented arc voltage-current characteristics. According to the arc characteristics, the fault boundaries can be listed as:

[0014] (1)

[0015] In the formula, u f (t) represents the fault voltage, u arc (t) represents the arc voltage, R arc represents the arc resistance, i arc (t) represents the arc current, where:

[0016] (2)

[0017] (3)

[0018] In the formula, i f (t) represents the fault current, i0(t) represents the zero-sequence current. In the high-voltage transmission system, the reactance of the line and the system is much greater than the resistance. The current i flowing through the equivalent resistance on both sides of the fault point is f1 and i f2 The phases are close, that is, K≈i f2 / i f1 , K is a constant, U arc represents the arc voltage amplitude, assuming that the transient current at the measuring end is in phase with the fault current, where sgn(·) represents the sign function:

[0019] (4)

[0020] From the above formula, the fault voltage can be obtained as:

[0021] (5).

[0022] The Step 3 is specifically as follows:

[0023] Transient voltage measured at the wind farm measurement end for:

[0024] (6)

[0025] Among them, R1 and L1 are the positive sequence resistance and positive sequence inductance per unit length of the line, and x f Indicates the distance between the fault location and the wind farm measurement end, i N is the transient current measured at the wind farm side, u f is the fault voltage;

[0026] The transient voltage of phase A measured at the wind farm measurement end for:

[0027] (7)

[0028] (8)

[0029] In the formula, K R , K L are the zero-sequence compensation coefficients of the resistance and inductance components, respectively. R1 and R0 are the positive-sequence resistance and zero-sequence resistance per unit length of the transmission line, respectively. L1 and L0 are the positive-sequence inductance and zero-sequence inductance per unit length of the transmission line, respectively. i A is the transient current of phase A measured by the wind farm measurement end, i0 is the zero-sequence current measured by the wind farm measurement end, let δ be the difference of i(t) at point n, i(t) represents the current changing with time, n represents the number of sampling points, then the discrete form of the transient voltage of phase A is obtained for:

[0030] (9)

[0031] In the above formula,

[0032] (10)

[0033] i A (n) represents the discrete form of the A-phase transient current measured at the wind farm measurement end, i0(n) represents the discrete form of the zero-sequence current measured at the wind farm measurement end, u fA (n) represents the discrete form of the fault voltage of phase A, k R , k L is the zero-sequence compensation coefficient of the resistance and inductance components in discrete form.

[0034] The Step 4 is specifically as follows:

[0035] Let R ′ =(K+1)R arc ,get:

[0036] (11)

[0037] Where U arc (n) represents the discrete form of arc voltage.

[0038] The Step 5 is specifically as follows:

[0039] Convert the time domain distance equation into matrix form:

[0040] (12)

[0041] Where U AThe matrix form represents the transient voltage of phase A at the measuring end of the wind farm side, where A represents the calculation matrix and T represents the transpose of the matrix.

[0042] The Step 6 is specifically as follows:

[0043] Solve the fault distance x according to the least squares algorithm f , we can get:

[0044] (13)

[0045] in:

[0046] (14)

[0047] (15)

[0048] In the formula, u A (n+1),u A (n+2),…,u A (n+m) represents the n+1, n+2, ..., n+mth sampling points of the A-phase transient voltage at the measurement end on the wind farm side; i0(n+1), i0(n+2), ..., i0 (n+m) represents the n+1, n+2, ..., n+mth sampling points of the zero-sequence current at the measurement end on the wind farm side, i A (n+1), i A (n+2),…,i A (n+m) represents the n+1th, n+2th, …, n+mth sampling points of the A-phase transient current at the measuring end on the wind farm side.

[0049] The principle of the present invention is: starting from the perspective of time domain distance algorithm, combined with the characteristics of arc faults in high-voltage transmission lines, an arc fault model is established, the boundary equation required for single-ended quantity protection is constructed, and then the solution equation containing the fault distance is established in combination with the line fault model, and then the fault distance is calculated according to the least squares algorithm. When an in-zone fault occurs, the obtained fault distance will be between 0 and the full length of the line, and the protection will be activated. When an out-of-zone fault occurs, the obtained fault distance will be less than 0 or greater than the full length of the line, and the protection will not be activated, thereby realizing single-ended time domain distance protection of the transmission line of a doubly fed wind farm.

[0050] The beneficial effects of the present invention are:

[0051] (1) Compared with the traditional power frequency protection, the present invention overcomes the adverse effect of the frequency deviation characteristics of the doubly-fed wind farm on the protection;

[0052] (2) The present invention is less affected by the fault location and transition resistance;

[0053] (3) The data window length of the present invention is only 5 ms, which can quickly identify faults inside and outside the area, thereby improving the sensitivity of wind farm transmission line protection. BRIEF DESCRIPTION OF THE DRAWINGS

[0054] Figure 1 It is a topological diagram of the simulation model of the present invention;

[0055] Figure 2 is an arc fault model established by the present invention;

[0056] Figure 3 is a schematic diagram of a line fault model of the present invention;

[0057] Figure 4 is a flowchart of specific steps of an embodiment of the present invention;

[0058] Figure 5 is a curve diagram for solving a phase A ground short circuit fault occurring 35 km away from the wind farm in Example 1 of the present invention;

[0059] Figure 6 is a curve diagram for solving the problem when an out-of-zone fault occurs in Embodiment 1 of the present invention;

[0060] Figure 7 is a fault distance curve diagram of Example 2 of the present invention when an AB two-phase grounding short circuit fault occurs 30 km away from the wind farm and the transition resistance is 100 Ω;

[0061] Figure 8 This is a fault distance curve diagram when a three-phase short circuit fault occurs 30 km away from the wind farm and the transition resistance is 100 Ω in Example 2 of the present invention. DETAILED DESCRIPTION

[0062] The present invention will be further described below in conjunction with the accompanying drawings and specific implementation methods.

[0063] Example 1: Large-scale wind farm grid-connected system simulation model Figure 1 As shown in the figure, the total installed capacity of the wind farm is 300MW, the low voltage ride-through modes of the wind turbines are divided into the use of crowbars and continuous excitation by the inverter, the total length of the transmission line is 65km, and the voltage level is 220 kV. The fault is set to occur 35 km from the wind farm side of the line, and the fault types are set to A phase grounding short circuit fault and out-of-zone fault, the transition resistance is set to 0.01 Ω, and the sampling rate is 10 kHz. The arc fault model established by the present invention is shown in the attached figure. Figure 2 As shown, Figure 3 It is a schematic diagram of a line fault model of the present invention.

[0064] like Figure 4 As shown, the specific steps are:

[0065] Step 1: Collect transient voltage and transient current on the wind farm side;

[0066] Step 2: Establish an arc fault model and obtain the fault boundary equation;

[0067] Specifically, the arc is represented by the segmented arc voltage-current characteristics. According to the arc characteristics, the fault boundaries are listed as follows:

[0068] (1)

[0069] In the formula, u f (t) represents the fault voltage, u arc (t) represents the arc voltage, R arc represents the arc resistance, i arc (t) represents the arc current, where:

[0070] (2)

[0071] (3)

[0072] In the formula, i f (t) represents the fault current, i0(t) represents the zero-sequence current, K≈i f2 / i f1 , K is a constant, i f1 and i f2 is the current flowing through the equivalent resistance on both sides of the fault point, U arc represents the arc voltage amplitude, assuming that the transient current at the measuring end is in phase with the fault current, where sgn(·) represents the sign function:

[0073] (4)

[0074] From the above formula, the fault voltage can be obtained as:

[0075] (5).

[0076] Step 3: Establish the fault voltage equation through the line fault model;

[0077] Specifically, the transient voltage u measured at the wind farm measurement end is N for:

[0078] (6)

[0079] Among them, R1 and L1 are the positive sequence resistance and positive sequence inductance per unit length of the line, and x f Indicates the distance between the fault location and the wind farm measurement end, i N is the transient current measured at the wind farm side, u f is the fault voltage;

[0080] The transient voltage of phase A measured at the wind farm measurement end for:

[0081] (7)

[0082] (8)

[0083] In the formula, K R , K L are the zero-sequence compensation coefficients of the resistance and inductance components, respectively. R1 and R0 are the positive-sequence resistance and zero-sequence resistance per unit length of the transmission line, respectively. L1 and L0 are the positive-sequence inductance and zero-sequence inductance per unit length of the transmission line, respectively. i A is the transient current of phase A measured by the wind farm measurement end, i0 is the zero-sequence current measured by the wind farm measurement end, let δ be the difference of i(t) at point n, i(t) represents the current changing with time, n represents the number of sampling points, then the discrete form of the transient voltage of phase A is obtained for:

[0084] (9)

[0085] In the above formula,

[0086] (10)

[0087] i A (n) represents the discrete form of the A-phase transient current measured at the wind farm measurement end, i0(n) represents the discrete form of the zero-sequence current measured at the wind farm measurement end, u fA (n) represents the discrete form of the fault voltage of phase A, k R , k L is the zero-sequence compensation coefficient of the resistance and inductance components in discrete form.

[0088] Step 4: Establish a time domain distance equation containing three unknowns: fault distance, resistance and arc voltage;

[0089] Specifically, let R ′ =(K+1)R arc ,get:

[0090] (11)

[0091] Where U arc (n) represents the discrete form of arc voltage.

[0092] Step 5: Convert the time domain distance equation into a matrix form;

[0093] (12)

[0094] Where U A The matrix form represents the transient voltage of phase A at the measuring end of the wind farm side, where A represents the calculation matrix and T represents the transpose of the matrix.

[0095] Step 6: Solve the fault distance according to the least squares algorithm;

[0096] (13)

[0097] in:

[0098] (14)

[0099] (15)

[0100] In the formula, u A (n+1),u A (n+2),…,u A (n+m) represents the n+1, n+2, ..., n+mth sampling points of the A-phase transient voltage at the measurement end on the wind farm side; i0(n+1), i0(n+2), ..., i0 (n+m) represents the n+1, n+2, ..., n+mth sampling points of the zero-sequence current at the measurement end on the wind farm side, i A (n+1), i A (n+2),…,i A (n+m) represents the n+1th, n+2th, …, n+mth sampling points of the A-phase transient current at the measuring end on the wind farm side.

[0101] Step 7: If the fault distance x f If it is greater than or equal to 0 and less than or equal to the total length of the line l, it is judged as an internal fault and the protection is activated. Otherwise, it is judged as an external fault and the protection is not activated. The judgment criteria are:

[0102] (16)

[0103] like Figure 5 This is the solution curve diagram for a phase A ground short circuit fault 35 km away from the wind farm; Figure 6 The fault distance x obtained by solving the fault outside the fault zone is f is 34.71 km, which is greater than 0 and less than the total line length of 65 km. It can be determined that the fault is within the area and the protection is actuated. The fault distance x obtained under the condition of an out-of-area fault is f It is 76.13 km, which is longer than the total line length of 65 km. It is determined to be an out-of-area fault and the protection does not operate.

[0104] Example 2: Large-scale wind farm grid-connected system simulation model Figure 1 As shown in the figure, the total installed capacity of the wind farm is 300MW, the low voltage ride-through modes of the wind turbines are divided into the use of crowbars and continuous excitation by the inverter, the total length of the transmission line is 65km, and the voltage level is 220kV. The fault is set to occur 30km away from the wind farm side of the line, and the fault types are set to AB two-phase grounding short circuit fault and three-phase short circuit fault, the transition resistance is set to 200Ω, and the sampling rate is 10kHz.

[0105] The specific implementation steps are:

[0106] Step 1: Collect transient voltage and transient current on the wind farm side;

[0107] Step 2: Establish an arc fault model and obtain the fault boundary equation;

[0108] Specifically, the arc is represented by the segmented arc voltage-current characteristics. According to the arc characteristics, the fault boundaries are listed as follows:

[0109] (1)

[0110] In the formula, u f (t) represents the fault voltage, u arc (t) represents the arc voltage, R arc represents the arc resistance, i arc (t) represents the arc current, where:

[0111] (2)

[0112] (3)

[0113] In the formula, i f (t) represents the fault current, i0(t) represents the zero-sequence current, K≈i f2 / i f1 , K is a constant, i f1 and i f2 is the current flowing through the equivalent resistance on both sides of the fault point, U arc represents the arc voltage amplitude, assuming that the transient current at the measuring end is in phase with the fault current, where sgn(·) represents the sign function:

[0114] (4)

[0115] From the above formula, the fault voltage can be obtained as:

[0116] . (5)

[0117] Step 3: Establish the fault voltage equation through the line fault model;

[0118] Specifically, the transient voltage u measured at the wind farm measurement end is N for:

[0119] (6)

[0120] Among them, R1 and L1 are the positive sequence resistance and positive sequence inductance per unit length of the line, and x f Indicates the distance between the fault location and the wind farm measurement end, i N is the transient current measured at the wind farm side, u f is the fault voltage;

[0121] The transient voltage of phase A measured at the wind farm measurement end for:

[0122] (7)

[0123] (8)

[0124] In the formula, K R , K L are the zero-sequence compensation coefficients of the resistance and inductance components, respectively. R1 and R0 are the positive-sequence resistance and zero-sequence resistance per unit length of the transmission line, respectively. L1 and L0 are the positive-sequence inductance and zero-sequence inductance per unit length of the transmission line, respectively. i A is the transient current of phase A measured by the wind farm measurement end, i0 is the zero-sequence current measured by the wind farm measurement end, let δ be the difference of i(t) at point n, i(t) represents the current changing with time, n represents the number of sampling points, then the discrete form of the transient voltage of phase A is obtained for:

[0125] (9)

[0126] In the above formula,

[0127] (10)

[0128] i A (n) represents the discrete form of the A-phase transient current measured at the wind farm measurement end, i0(n) represents the discrete form of the zero-sequence current measured at the wind farm measurement end, u fA (n) represents the discrete form of the fault voltage of phase A, k R , k L is the zero-sequence compensation coefficient of the resistance and inductance components in discrete form.

[0129] Step 4: Establish a time domain distance equation containing three unknowns: fault distance, resistance and arc voltage;

[0130] Specifically, let R ′ =(K+1)R arc ,get:

[0131] (11)

[0132] Where U arc (n) represents the discrete form of arc voltage.

[0133] Step 5: Convert the time domain distance equation into a matrix form;

[0134] (12)

[0135] Where U A The matrix form represents the transient voltage of phase A at the measuring end of the wind farm side, where A represents the calculation matrix and T represents the transpose of the matrix.

[0136] Step 6: Solve the fault distance according to the least squares algorithm;

[0137] (13)

[0138] in:

[0139] (14)

[0140] (15)

[0141] In the formula, u A (n+1),u A (n+2),…,u A (n+m) represents the n+1, n+2, ..., n+mth sampling points of the A-phase transient voltage at the measurement end on the wind farm side; i0(n+1), i0(n+2), ..., i0 (n+m) represents the n+1, n+2, ..., n+mth sampling points of the zero-sequence current at the measurement end on the wind farm side, i A (n+1), i A (n+2),…,i A (n+m) represents the n+1th, n+2th, …, n+mth sampling points of the A-phase transient current at the measuring end on the wind farm side.

[0142] Step 7: If the fault distance x f If it is greater than or equal to 0 and less than or equal to the total length of the line l, it is judged as an internal fault and the protection is activated. Otherwise, it is judged as an external fault and the protection is not activated. The judgment criteria are:

[0143] (16)

[0144] like Figure 7 The figure shows the fault distance curve when a two-phase AB grounding short circuit fault occurs 30 km away from the wind farm and the transition resistance is 100 Ω. Figure 8 is the fault distance curve when a three-phase short circuit occurs 30 km away from the wind farm and the transition resistance is 100 Ω. The fault distance x obtained by solving the AB two-phase grounding short circuit fault f is 29.03 km, which is greater than 0 and less than the total line length of 65 km. It can be determined as an internal fault, and the protection is activated. The fault distance x obtained under the three-phase short-circuit fault f It is 29.01 km, which is greater than 0 and less than the total line length of 65 km. It can be determined as an internal fault and the protection is activated.

[0145] Therefore, the method can accurately operate under various fault types and when the fault passes through a high transition resistance of 200 Ω.

[0146] The specific implementation modes of the present invention are described in detail above in conjunction with the accompanying drawings, but the present invention is not limited to the above implementation modes, and various changes can be made within the knowledge scope of ordinary technicians in this field without departing from the purpose of the present invention.

Claims

1. A single-ended time domain distance protection method for a double-fed wind farm transmission line based on an arc fault model, characterized in that: The method comprises: Step 1: Collect transient voltage and transient current on the wind farm side; Step 2: Establish an arc fault model and obtain the fault boundary equation; Step 3: Establish the fault voltage equation through the line fault model; Step 4: Establish a time domain distance equation containing three unknowns: fault distance, resistance and arc voltage; Step 5: Convert the time domain distance equation into a matrix form; Step 6: Solve the fault distance according to the least squares algorithm; Step 7: If the fault distance is greater than or equal to 0 and less than or equal to the total length of the line, it is determined to be an internal fault and the protection is activated. Otherwise, it is determined to be an external fault and the protection is not activated.

2. The single-ended time domain distance protection method for a double-fed wind farm transmission line based on an arc fault model according to claim 1 is characterized in that: The Step 2 is specifically as follows: The arc is represented by the segmented arc voltage-current characteristics. According to the arc characteristics, the fault boundaries are listed as follows: （1）； In the formula, u f (t) represents the fault voltage, u arc (t) represents the arc voltage, R arc represents the arc resistance, i arc (t) represents the arc current, where: （2）； （3）； In the formula, i f (t) represents the fault current, i0(t) represents the zero-sequence current, K≈i f2 / i f1 , K is a constant, i f1 and i f2 is the current flowing through the equivalent resistance on both sides of the fault point, U arc represents the arc voltage amplitude, assuming that the transient current at the measuring end is in phase with the fault current, where sgn(·) represents the sign function: （4）； From the above formula, the fault voltage can be obtained as: （5）。 3. The single-ended time domain distance protection method for a double-fed wind farm transmission line based on an arc fault model according to claim 1 is characterized in that: The Step 3 is specifically as follows: Transient voltage measured at the wind farm measurement end for: （6）； Among them, R1 and L1 are the positive sequence resistance and positive sequence inductance per unit length of the line, and x f Indicates the distance between the fault location and the wind farm measurement end, i N is the transient current measured at the wind farm side, u f is the fault voltage; The transient voltage of phase A measured at the wind farm measurement end for: （7）； （8）； In the formula, K R , K L are the zero-sequence compensation coefficients of the resistance and inductance components, respectively. R1 and R0 are the positive-sequence resistance and zero-sequence resistance per unit length of the transmission line, respectively. L1 and L0 are the positive-sequence inductance and zero-sequence inductance per unit length of the transmission line, respectively. i A is the transient current of phase A measured by the wind farm measurement end, i0 is the zero-sequence current measured by the wind farm measurement end, let δ be the difference of i(t) at point n, i(t) represents the current changing with time, n represents the number of sampling points, then the discrete form of the transient voltage of phase A is obtained for: （9）； In the above formula, （10）； i A (n) represents the discrete form of the A-phase transient current measured at the wind farm measurement end, i0(n) represents the discrete form of the zero-sequence current measured at the wind farm measurement end, u fA (n) represents the discrete form of the fault voltage of phase A, k R , k L is the zero-sequence compensation coefficient of the resistance and inductance components in discrete form.

4. The single-ended time domain distance protection method for a double-fed wind farm transmission line based on an arc fault model according to claim 1 is characterized in that: The Step 4 is specifically as follows: Let R ′ =(K+1)R arc ,get: （11）； Where U arc (n) represents the discrete form of arc voltage.

5. The single-ended time domain distance protection method for a double-fed wind farm transmission line based on an arc fault model according to claim 1 is characterized in that: The Step 5 is specifically as follows: Convert the time domain distance equation into matrix form: （12）； Where U A The matrix form represents the transient voltage of phase A at the measuring end of the wind farm side, where A represents the calculation matrix and T represents the transpose of the matrix.

6. The single-ended time domain distance protection method for a double-fed wind farm transmission line based on an arc fault model according to claim 1 is characterized in that: The Step 6 is specifically as follows: Solve the fault distance x according to the least squares algorithm f , we can get: （13）； in: （14）； （15）； In the formula, u A (n+1),u A (n+2),…,u A (n+m) represents the n+1th, n+2th, ..., n+mth sampling points of the A-phase transient voltage at the measuring end on the wind farm side; i0(n+1), i0(n+2), …, i0 (n+m) represent the n+1, n+2, …, n+mth sampling points of the zero-sequence current at the wind farm side measurement end, i A (n+1), i A (n+2),…,i A (n+m) represents the n+1th, n+2th, …, n+mth sampling points of the A-phase transient current at the measuring end on the wind farm side.

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