Multi-branch hydro-generator stator grounding fault active arc extinction method

Abstract

The invention discloses a multi-branch hydro-generator stator grounding fault active arc extinction method, which comprises the following steps: S1, constructing an analytic function of a double-frequency fault potential by using a neutral point and a generator end voltage after a generator fault and combining a winding connection sequence; s2, deriving a linear constraint relation between a third harmonic fault potential real part and an imaginary part; s3, according to the linear constraint relation, in combination with the potential expression of each branch winding of the third harmonic, constructing a target function for fault solution, and solving a solution set based on the third harmonic characteristic quantity; s4, according to the fundamental wave fault characteristic equation, constructing a fundamental wave difference coefficient multi-solution screening criterion, and obtaining a unique solution of the double-frequency fault potential; s5, aiming at the multi-branch hydro-generator, constructing a neutral point dual-frequency power supply injection amount calculation formula; and correctly adjusting the injection amount of the neutral point double-frequency controllable power supply by using the screened double-frequency fault potential unique solution, and suppressing the voltage of the fault point to be 0 to realize arc extinction. According to the invention, reliable arc extinction of the multi-branch hydraulic generator can be realized.

Description

Technical Field

[0001] This invention relates to the field of safety protection technology for large generators, specifically to an active arc suppression method for stator grounding faults in multi-branch hydro-generators. Background Technology

[0002] Single-phase ground faults in the stator winding are among the most common faults in large generators. The resulting ground fault current can easily lead to serious consequences such as stator insulation breakdown and core burnout, threatening the safe operation of the generator. Arc suppression methods for generator stator winding ground faults are mainly divided into passive and active methods. Currently, large generator sets generally use passive arc suppression methods with the neutral point grounded via an arc suppression coil or high-resistance grounding. However, this passive arc suppression method can only compensate for the residual reactive current at the fault point and cannot be dynamically adjusted, making it unsuitable for the complex and varied operating conditions of generators and the diverse ground fault environments. Practical experience shows that existing generator neutral point grounding methods cannot achieve reliable arc suppression under different fault scenarios. Active arc suppression methods, on the other hand, inject a source at the neutral point to regulate the voltage at the fault point to zero, thereby eliminating the ground fault current and achieving voltage reduction and arc suppression at the fault point.

[0003] Due to the spatial dispersion of the stator winding potential in generators, the required compensation potential varies depending on the location of the fault, necessitating online calculation of the fault potential. Existing research focuses only on generator units with single-branch or overlapping branch potentials. For such generators, the fault potential is the same when a ground fault occurs at the same fault turns ratio in different branches. However, for multi-branch hydro-generators, their unique winding structure characteristics result in incomplete overlap of branch potentials, rendering the above methods inapplicable.

[0004] Due to the numerous branches and dense potential distribution, multiple solutions are prone to occur in fault potential calculations, affecting the calculation of compensation quantities for arc suppression devices. Using an incorrect fault potential for arc suppression control can increase the voltage at the fault point and even exacerbate the arcing, causing serious damage to the generator. Therefore, it is necessary to combine effective multiple-solution screening methods to improve the reliability and universality of the arc suppression strategy. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide an active arc suppression method for stator grounding faults of multi-branch hydro-generators, so as to solve the problem that existing arc suppression methods cannot solve the problem of multiple solutions of fault potential in arc suppression of multi-branch hydro-generators and cannot achieve reliable arc suppression.

[0006] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows.

[0007] An active arc suppression method for stator grounding faults in multi-branch hydro-generators includes the following steps: S1. Using the neutral point and terminal voltage after a generator fault, combined with the winding connection sequence, the winding turn potential amplitude is calculated online, and an analytical function of the dual-frequency fault potential is constructed. S2. Calculate the third harmonic ground fault current online, and derive the linear constraint relationship between the real and imaginary parts of the third harmonic fault potential by combining the fault characteristic equation; S3. Based on the above linear constraint relationship and combined with the potential expression of each branch winding of the third harmonic, construct the objective function for fault solution; by solving the minimum value of the objective function on each branch, select the solution in the minimum value where the objective function is less than a certain threshold as the solution set that conforms to the characteristic quantity of the third harmonic; S4. Based on the fundamental fault characteristic equation, construct a multiple solution screening criterion for the fundamental difference coefficient; by substituting the multiple solutions and corresponding parameters in the above solution set into the criterion, calculate the corresponding fundamental difference coefficient, and select the solution with the smallest difference coefficient as the correct fault location to obtain the unique solution of the dual-frequency fault potential. S5. For multi-branch hydro-generators, based on the principle of voltage reduction and arc suppression, a formula for calculating the injection amount of the neutral point dual-frequency power supply is constructed; using the unique solution of the selected dual-frequency fault potential, the injection amount of the neutral point dual-frequency controllable power supply is correctly adjusted to suppress the fault point voltage to 0 and achieve arc suppression.

[0008] Preferably, step S1 specifically includes: S11. Calculate the winding turn potential amplitude online, specifically as follows: Based on the winding connection sequence, the fundamental KVL equations corresponding to the branch windings are listed; using the measured values ​​of the neutral point and terminal voltage after a generator fault, the fundamental turn potential amplitude of the windings is calculated online. :

[0009] in, U n1 This is the fundamental voltage of the neutral point after a generator failure; U s1 This is the fundamental voltage at the generator terminals after a generator failure. The phase angle corresponding to the fundamental wave potential of each turn; Similarly, the amplitude of the third harmonic turn potential of the winding is obtained. :

[0010] in, U n3 The third harmonic voltage at the neutral point after a generator failure; U s3 This refers to the third harmonic voltage at the generator terminals after a generator failure. The phase angle corresponding to the third harmonic turn potential of each turn; S12. Construct the analytical function for the dual-frequency fault potential, as follows: Analytical function of fundamental fault potential:

[0011] in, E 1( α ) represents the fundamental fault potential; α The fault turns ratio; This refers to the number of turns in each branch winding. The fundamental winding induced electromotive force corresponding to the first turn of the winding; For the front The fundamental winding induced electromotive force corresponding to the turn winding; Similarly, the analytical function of the third harmonic fault potential is obtained:

[0012] in, E 3( α The third harmonic fault potential is represented by the voltage. The induced electromotive force of the third harmonic winding corresponding to the first turn of the winding; For the front The induced electromotive force of the third harmonic winding corresponding to the turn winding.

[0013] Preferably, the linear constraint relationship between the real and imaginary parts of the third harmonic fault potential in step S2 is specifically as follows: The expression for the third harmonic fault potential is:

[0014] in, I f3 This refers to the third harmonic ground fault current. R g For grounding transition resistance; make , , Substituting into the above equation and separating the real and imaginary parts of the phasor equation, we get:

[0015] in, This represents the real part of the third harmonic fault potential; This represents the imaginary part of the third harmonic fault potential; This represents the real part of the third harmonic ground fault current. b This represents the imaginary part of the third harmonic ground fault current. c This represents the real part of the third harmonic voltage at the neutral point after a generator failure. d This represents the imaginary part of the third harmonic voltage at the neutral point after a generator failure; j is the imaginary unit. Eliminating the grounding transition resistance in the above formula yields the third harmonic fault potential. E 3( α The real and imaginary parts of satisfy a linear constraint relationship: .

[0016] Preferably, step S3 specifically comprises: By utilizing the fault potential to simultaneously satisfy the linear constraint relationship of step S2 and the analytic function of step S1, and based on the third harmonic component, the objective function is characterized by the distance formula from the fault potential point to the straight line. The objective function for fault solution is... as follows:

[0017] Substituting the fault potentials of each branch into the equation, we solve for the minimum value of the objective function from 0 to 1 for the fault turns ratio. We select all fault locations with objective function values ​​less than 1 from the minimum values ​​as the solution set. Substituting the obtained third harmonic fault potentials corresponding to the fault locations into the following equation, we obtain the corresponding grounding transition resistance: .

[0018] Preferably, step S4 specifically comprises: Fundamental ground fault current I f1 The expression is:

[0019] in, I f1 For the fundamental ground fault current; Δ U n1 This represents the change in the fundamental voltage at the generator neutral point before and after the fault. ω Angular frequency; This is the sum of the generator stator winding capacitance to ground; R n This is the equivalent resistance value of the generator neutral point grounding short-circuit impedance referred to the primary side; X k This is the equivalent reactance value of the neutral point grounding short-circuit impedance referred to the primary side; The expression for the fundamental fault potential is:

[0020] Combining the above two equations, we obtain the multiple-solution screening criterion for the fundamental frequency difference coefficient. d h :

[0021] The above multiple solutions and the corresponding fundamental fault potentialE 1( α and grounding transition resistance R g Substitute each value into the criterion, calculate the corresponding fundamental frequency difference coefficient, select the solution with the smallest difference coefficient as the correct fault location, and screen out the unique solution of the dual-frequency fault potential.

[0022] Preferably, the formula for calculating the neutral point dual-frequency power injection amount in step S5 is as follows:

[0023]

[0024] in, I i1 This refers to the injection amount of the fundamental frequency at the neutral point. I i3 The third harmonic injection amount at the neutral point; m is the number of branches per phase; This is the equivalent capacitance of a single-turn coil to ground. C t This refers to the ground capacitance of the generator terminal direct connection system. The third harmonic potential of the i-th turn coil of the j-th branch of phase A; E A3 The phase potential of the third harmonic A-phase winding is given.

[0025] Due to the adoption of the above technical solutions, the technical progress achieved by this invention is as follows.

[0026] Based on the grounding fault characteristics of multi-branch hydro-generators, and considering the problem of multiple solutions to the fault potential that are prone to occur during the arc suppression process, this invention proposes a dual-frequency coordinated fault potential calculation method. This method provides a unique and correct dual-frequency fault potential for calculating the arc suppression injection amount, avoids the influence of incorrect fault potential on the arc suppression effect, and achieves reliable arc suppression.

[0027] This invention employs a method for solving third harmonic fault potentials based on linear constraint relationships, which can calculate all fault solution sets that meet the requirements; and a multi-solution screening method for fault potentials based on the fundamental frequency difference coefficient, which can reliably screen out the unique correct solution.

[0028] This invention addresses the issue of non-overlapping branch potentials in multi-branch hydro-generators by considering the differences in branch-to-ground capacitance currents. It provides a formula for calculating the neutral point dual-frequency power injection based on the voltage reduction arc suppression mechanism, thereby improving the reliability and universality of arc suppression. Attached Figure Description

[0029] Figure 1 This is a flowchart of the present invention; Figure 2 This is the equivalent circuit diagram of a single-phase ground fault in the generator stator winding according to the present invention; Figure 3 This is the equivalent circuit diagram of the fundamental zero-sequence frequency of a single-phase ground fault in the generator stator winding according to the present invention. Figure 4 The figure shows the solution results for the fault scenario in Case 1 of this invention; Figure 5 The figure shows the solution results for the fault scenario of Case 2 in this invention; Figure 6 The figure shows the solution results for the fault scenario of Case 3 in this invention; Figure 7 The figure shows the solution results for the fault scenario of Case 4 in this invention; Figure 8 This is a diagram showing the arc suppression effect of the alternative fault point (A1, 0.5715) in Case 3 of the present invention. Figure 9 This is a diagram showing the arc suppression effect of the alternative fault point (A1, 0.7327) in Case 3 of the present invention. Figure 10 This is a diagram showing the arc suppression effect of the alternative fault point (A3, 0.7435) in Case 3 of the present invention. Figure 11 This is a diagram showing the arc suppression effect of the alternative fault point (A8, 0.6590) in Case 3 of the present invention. Detailed Implementation

[0030] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0031] An active arc suppression method for stator grounding faults in multi-branch hydro-generators, combined with Figure 1 As shown, it includes the following steps: S1. Using the neutral point and terminal voltage after a generator fault, combined with the winding connection sequence, the winding turn potential amplitude is calculated online, and an analytical function of the dual-frequency fault potential is constructed.

[0032] Specifically, suppose the connection sequence of a certain branch winding is as follows:

[0033] This branch consists of n turns of winding connected in series. The slot number is for 2n slot conductors, where U represents the upper side conductor in the slot and L represents the lower side conductor in the slot.

[0034] If the induced potential of the conductor in slot 0 is selected as the reference phase 0, then the induced potential of each conductor in slot can be obtained as follows: ,in, E k The amplitude of the induced electromotive force in the slot conductor; Let be the phase angle of the induced electromotive force in the slot conductor, and , The slot number corresponds to each slot conductor. Let be the slot pitch electrical angle. From this, the fundamental turn potential of the first turn winding of this branch can be obtained as:

[0035] in, The phase angle of the induced electromotive force in the conductor of the first slot; The phase angle is the induced electromotive force of the conductor in slot number two.

[0036] Similarly, by determining the winding connection sequence, the fundamental turn potential of each winding can be calculated. After unifying the reference phase, the fundamental KVL equations satisfied by each branch winding can be written:

[0037] in, U n1 and U s1 These are the fundamental voltages at the neutral point and the generator terminals after a generator failure, respectively. Let be the phase angle corresponding to the fundamental wave potential of each turn. Therefore, the amplitude of the fundamental wave potential of the winding can be calculated online. :

[0038] Similarly, the amplitude of the third harmonic turn potential of the winding can be calculated online. :

[0039] in, U n3 and U s3 These are the third harmonic voltages at the neutral point and the generator terminals after a generator failure; This represents the phase angle corresponding to the third harmonic turn potential of each turn. Based on this, combined with formula... The fundamental fault potential can be obtained from the winding connection sequence. Compared with the fault turns ratio The analytic function between them:

[0040] in, E 1( α ) represents the fundamental fault potential; α The fault turns ratio; This refers to the number of turns in each branch winding. The fundamental winding induced electromotive force corresponding to the first turn of the winding; For the front The fundamental winding induced electromotive force corresponding to the turn winding.

[0041] Similar to the fundamental frequency, the analytical function relating the third harmonic fault potential to the fault turns ratio is obtained:

[0042] in, E 3( α The third harmonic fault potential is represented by the voltage. The induced electromotive force of the third harmonic winding corresponding to the first turn of the winding; For the front The induced electromotive force of the third harmonic winding corresponding to the turn winding.

[0043] S2. Calculate the third harmonic ground fault current online, and derive the linear constraint relationship between the real and imaginary parts of the third harmonic fault potential by combining the fault characteristic equation.

[0044] Specifically, according to Figure 2 The equivalent circuit diagram of a single-phase ground fault in the generator stator winding is shown. The formula for calculating the third harmonic ground fault current is as follows:

[0045] in, The third harmonic ground fault current; j is the imaginary unit; ω Angular frequency; This is the sum of the generator stator winding capacitance to ground; This represents the change in the third harmonic voltage at the generator neutral point before and after the fault. R n This is the equivalent resistance value of the generator neutral point grounding short-circuit impedance referred to the primary side; X k This is the equivalent reactance value of the neutral point grounding short-circuit impedance referred to the primary side.

[0046] Combination Figure 2 From the KVL of the grounding branch, the expression for the third harmonic fault potential can be derived:

[0047] in, R g This is the grounding transition resistor.

[0048] make , , Substituting into the above equation and separating the real and imaginary parts of the phasor equation, we get:

[0049] in, This represents the real part of the third harmonic fault potential; This represents the imaginary part of the third harmonic fault potential; This represents the real part of the third harmonic ground fault current. b This represents the imaginary part of the third harmonic ground fault current. c This represents the real part of the third harmonic voltage at the neutral point after a generator failure. d This represents the imaginary part of the third harmonic voltage at the neutral point after a generator failure.

[0050] Eliminating the grounding transition resistance in the above formula yields the third harmonic fault potential. The real and imaginary parts satisfy a linear constraint relationship: .

[0051] S3. Based on the above linear constraint relationship and combined with the potential expression of each branch winding of the third harmonic, construct the objective function for fault solution; by solving the minimum value of the objective function on each branch, select the solution in the minimum value where the objective function is less than a certain threshold as the solution set that conforms to the characteristic quantity of the third harmonic.

[0052] This step utilizes a fault potential solution method based on linear constraints, specifically a third harmonic fault potential solution method based on linear constraint relationships. Specifically, it uses the data obtained after the fault occurs... I f3 and U n3 The straight line containing the third harmonic component of the fault potential can be determined. Combining the mathematical mapping of the fault potential, the potential curves of each branch winding are plotted on the complex plane. The fault location should be at the intersection of the potential curves of each branch winding and this straight line. Therefore, utilizing the fault potential's simultaneous satisfaction of the linear constraint relationship in step S2 and the analytic function in step S1, and based on the third harmonic component, the objective function is characterized by the distance from the fault potential point to this straight line. The objective function for fault solution is... as follows:

[0053] Because the winding electromotive force is continuous, the closer to the fault point, the more likely it is to occur. The smaller the value, the more likely the fault point is. At the minimum value, and the corresponding value is sufficiently small (the threshold is set to 1 considering the noise influence in generator operation). Therefore, substituting the fault potential of each branch into the equation, we solve for the minimum value of the objective function from 0 to 1 for the fault turns ratio. We select all fault locations where the corresponding objective function value is less than 1 from the minimum values ​​as the solution set. Substituting the obtained third harmonic fault potential corresponding to the fault location into the following formula, we can obtain the corresponding grounding transition resistance: .

[0054] S4. Based on the fundamental fault characteristic equation, construct a multiple solution screening criterion for the fundamental difference coefficient; by substituting the multiple solutions and corresponding parameters in the above solution set into the criterion, calculate the corresponding fundamental difference coefficient, and select the solution with the smallest difference coefficient as the correct fault location to obtain the unique solution of the dual-frequency fault potential.

[0055] Based on step S3, this invention employs a dual-frequency coordinated fault potential multi-solution screening method. Specifically, this step, based on the fundamental frequency difference coefficient, yields a similar expression for the fundamental frequency ground fault current, similar to the third harmonic method.

[0056] in, I f1 For the fundamental ground fault current; Δ U n1 This represents the change in the fundamental voltage at the generator neutral point before and after the fault.

[0057] according to Figure 3 The equivalent circuit of the fundamental zero-sequence fault in the stator winding of the generator shown is given. By writing the KVL equations for the fundamental fault branch, the expression for the fundamental fault potential can be obtained:

[0058] By combining the above two equations, we can obtain the multiple-solution screening criterion for the fundamental frequency difference coefficient. d h :

[0059] The above multiple solutions and the corresponding fundamental fault potential E 1( α and grounding transition resistance R g Substitute each value into the criterion and calculate the corresponding fundamental frequency difference coefficient. Select the solution with the smallest difference coefficient as the correct fault location, and a unique solution for the dual-frequency fault potential can be found.

[0060] S5. For multi-branch hydro-generators, based on the principle of voltage reduction and arc suppression, a formula for calculating the injection amount of the neutral point dual-frequency power supply is constructed; by using the unique solution of the selected dual-frequency fault potential, the injection amount of the neutral point dual-frequency controllable power supply is correctly adjusted to suppress the fault point voltage to 0, thereby achieving reliable arc suppression.

[0061] Specifically, considering the non-overlapping potentials of the branches of a multi-branch hydro-generator and the differences in the branch-to-ground capacitance currents, when calculating the third harmonic injection with zero-sequence characteristics, it is necessary to calculate the ground capacitance current of each branch separately. Therefore, the specific formula for calculating the current injected by the neutral-point dual-frequency current source is as follows:

[0062]

[0063] in, I i1 This refers to the injection amount of the fundamental frequency at the neutral point. I i3 The third harmonic injection amount at the neutral point; m is the number of branches per phase; This is the equivalent capacitance of a single-turn coil to ground. C t This refers to the ground capacitance of the generator terminal direct connection system. The third harmonic potential of the i-th turn coil of the j-th branch of phase A; E A3 The phase potential of the third harmonic A-phase winding is given.

[0064] The effectiveness of the proposed method is verified through simulation below.

[0065] This embodiment uses a multi-branch hydro-generator as a prototype to establish a quasi-distributed parameter model for simulation analysis. The generator has a rated voltage of 20kV, each phase contains 8 branch windings, the stator winding resistance is 9.864mΩ / phase, the stator winding leakage inductance is 11.072mH / phase, the stator winding capacitance is 3.846μF / phase, the number of stage pairs is 40, the total number of slots is 840, the slot pitch electrical angle is 17.14286°, and the direct-connected system capacitance to ground is 0.405μF / phase.

[0066] To verify the effectiveness of the proposed method at different fault locations, taking phase A as an example, ground faults with a grounding transition resistance of 100Ω were set at different branches and different fault turns ratios. The four fault locations are (A1, 0.1429), (A4, 0.4286), (A1, 0.5714), and (A8, 0.8571), and the corresponding fault potential parameters are shown in Table 1.

[0067] Table 1

[0068] 1) For the four fault scenarios mentioned above, the proposed method for solving the third harmonic fault potential based on linear constraint relationships is used. The preliminary solution results for the third harmonic are as follows: Figures 4 to 7 As shown in Table 2, the calculation results using the proposed fault potential multi-solution screening method based on the fundamental frequency difference coefficient are presented.

[0069] Table 2

[0070] Figure 4 The solution results for the fault scenario in Case 1 are shown in the figure. Figure 5 The solution results for the fault scenario in Case 2 are shown in the figure. Figure 6 The solution results for the fault scenario in Case 3 are shown in the figure. Figure 7 The solution results for Case 4, the fault scenario, are shown in the figure. The objective function corresponding to each point on each branch is also shown. The values ​​are mapped using color coding, with colors ranging from blue to yellow. As the value increases, all solutions lie in At the local minimum. From Figures 4 to 7 It can be seen that there are multiple fault solutions obtained by using only the third harmonic, and the solution set contains the correct fault location, indicating that the proposed method can effectively calculate the fault solution set; in Table 2, the fundamental difference coefficient corresponding to the correct fault location is much smaller than that of other false solutions, indicating that the proposed method can effectively screen out the unique fault solution.

[0071] 2) For the above four fault scenarios, the calculated fundamental and third harmonic fault potentials under different grounding transition resistances are shown in Table 3.

[0072] Table 3

[0073] Comparing the calculation results in Table 3 with the theoretical data in Table 1, it can be seen that the proposed method can accurately calculate the fundamental and third harmonic components of the fault potential under different fault locations and different ground fault transition resistances, which can provide a reliable prerequisite for the arc suppression of ground faults.

[0074] 3) At 0.2s, the above Case 3 (A1, 0.5714) occurs. R g Taking a ground fault with a current of 100Ω as an example, the arc suppression injection quantity corresponding to four alternative fault solutions under the above fault scenario is calculated. At 0.25s, an arc suppression current is injected through a neutral point parallel current source, and the resulting arc suppression effect is as follows. Figures 8 to 11 As shown.

[0075] like Figures 8 to 11 As shown, the horizontal axis t Represents time, in seconds, vertical axis This represents the fault current, in amperes (A). This represents the fault potential, expressed in kV. Figure 8 Arc suppression effect diagram for the alternative fault point (A1, 0.5715) in Case 3; Figure 9 Arc suppression effect diagram for the alternative fault point (A1, 0.7327) in Case 3; Figure 10 Arc suppression effect diagram for the alternative fault point (A3, 0.7435) in Case 3; Figure 11 The arc suppression effect diagram for the alternative fault point (A8, 0.6590) in Case 3 is shown. Figure 8 The arc extinguishing effect corresponds to the correct fault solution, the rest Figure 9 , Figure 10 and Figure 11 The arc suppression effect is compared to that of incorrect fault solutions. It can be seen that using the correct fault potential adjustment injection amount, after injecting arc suppression current at the neutral point, can quickly regulate the fault-to-ground voltage to near zero, suppressing the fault-to-ground voltage within safe limits. However, for fault potentials that have not been fundamentally screened, the arc suppression effect is poor, and the ground fault current remains large, exceeding the safety threshold.

[0076] 4) For the above four grounding fault scenarios, the arc suppression simulation results after multiple solution screening under different grounding transition resistances are shown in Table 4.

[0077] Table 4

[0078] Table 4 shows that under different fault scenarios, the proposed method can clamp the fault ground current to a lower level, achieving voltage reduction and arc suppression. This indicates that the proposed method can reliably achieve fault arc suppression at the grounding point under different ground fault location conditions, and the method is not affected by the grounding transition resistance.

Claims

1. An active arc suppression method for stator grounding faults in multi-branch hydro-generators, characterized in that: Includes the following steps: S1. Using the neutral point and terminal voltage after a generator fault, combined with the winding connection sequence, the winding turn potential amplitude is calculated online, and an analytical function of the dual-frequency fault potential is constructed. S2. Calculate the third harmonic ground fault current online, and derive the linear constraint relationship between the real and imaginary parts of the third harmonic fault potential by combining the fault characteristic equation; S3. Based on the above linear constraint relationship and combined with the potential expression of each branch winding of the third harmonic, construct the objective function for fault solution; by solving the minimum value of the objective function on each branch, select the solution in the minimum value where the objective function is less than a certain threshold as the solution set that conforms to the characteristic quantity of the third harmonic; S4. Based on the fundamental fault characteristic equation, construct a multiple solution screening criterion for the fundamental difference coefficient; by substituting the multiple solutions and corresponding parameters in the above solution set into the criterion, calculate the corresponding fundamental difference coefficient, and select the solution with the smallest difference coefficient as the correct fault location to obtain the unique solution of the dual-frequency fault potential. S5. For multi-branch hydro-generators, based on the principle of voltage reduction and arc suppression, a formula for calculating the injection amount of the neutral point dual-frequency power supply is constructed; using the unique solution of the selected dual-frequency fault potential, the injection amount of the neutral point dual-frequency controllable power supply is correctly adjusted to suppress the fault point voltage to 0 and achieve arc suppression.

2. The active arc suppression method for stator grounding faults of a multi-branch hydro-generator according to claim 1, characterized in that: Step S1 specifically includes: S11. Calculate the winding turn potential amplitude online, specifically as follows: Based on the winding connection sequence, the fundamental KVL equations corresponding to the branch windings are listed; using the measured values ​​of the neutral point and terminal voltage after a generator fault, the fundamental turn potential amplitude of the windings is calculated online. : in, U n1 This is the fundamental voltage of the neutral point after a generator failure; U s1 This is the fundamental voltage at the generator terminals after a generator failure. The phase angle corresponding to the fundamental wave potential of each turn; Similarly, the amplitude of the third harmonic turn potential of the winding is obtained. : in, U n3 The third harmonic voltage at the neutral point after a generator failure; U s3 This refers to the third harmonic voltage at the generator terminals after a generator failure. The phase angle corresponding to the third harmonic turn potential of each turn; S12. Construct the analytical function for the dual-frequency fault potential, as follows: Analytical function of fundamental fault potential: in, E 1( α ) represents the fundamental fault potential; α The fault turns ratio; This refers to the number of turns in each branch winding. The fundamental winding induced electromotive force corresponding to the first turn of the winding; For the front The fundamental winding induced electromotive force corresponding to the turn winding; Similarly, the analytical function of the third harmonic fault potential is obtained: in, E 3( α The third harmonic fault potential is represented by the voltage. The induced electromotive force of the third harmonic winding corresponding to the first turn of the winding; For the front The induced electromotive force of the third harmonic winding corresponding to the turn winding.

3. The active arc suppression method for stator grounding faults of a multi-branch hydro-generator according to claim 2, characterized in that: The linear constraint relationship between the real and imaginary parts of the third harmonic fault potential in step S2 is as follows: The expression for the third harmonic fault potential is: in, I f3 This refers to the third harmonic ground fault current. R g For grounding transition resistance; make , , Substituting into the above equation and separating the real and imaginary parts of the phasor equation, we get: in, This represents the real part of the third harmonic fault potential; This represents the imaginary part of the third harmonic fault potential; This represents the real part of the third harmonic ground fault current. b This represents the imaginary part of the third harmonic ground fault current. c This represents the real part of the third harmonic voltage at the neutral point after a generator failure. d This represents the imaginary part of the third harmonic voltage at the neutral point after a generator failure; j is the imaginary unit. Eliminating the grounding transition resistance in the above formula yields the third harmonic fault potential. E 3( α The real and imaginary parts of satisfy a linear constraint relationship: 。 4. The active arc suppression method for stator grounding faults of a multi-branch hydro-generator according to claim 3, characterized in that: Step S3 specifically involves: By utilizing the fault potential to simultaneously satisfy the linear constraint relationship of step S2 and the analytic function of step S1, and based on the third harmonic component, the objective function is characterized by the distance formula from the fault potential point to the straight line. The objective function for fault solution is... as follows: Substituting the fault potentials of each branch into the equation, we solve for the minimum value of the objective function from 0 to 1 for the fault turns ratio. We select all fault locations with objective function values ​​less than 1 from the minimum values ​​as the solution set. Substituting the obtained third harmonic fault potentials corresponding to the fault locations into the following equation, we obtain the corresponding grounding transition resistance: 。 5. The active arc suppression method for stator grounding faults of a multi-branch hydro-generator according to claim 4, characterized in that: Step S4 specifically involves: Fundamental ground fault current I f1 The expression is: in, I f1 For the fundamental ground fault current; Δ U n1 This represents the change in the fundamental voltage at the generator neutral point before and after the fault. ω Angular frequency; This is the sum of the generator stator winding capacitance to ground; R n This is the equivalent resistance value of the generator neutral point grounding short-circuit impedance referred to the primary side; X k This is the equivalent reactance value of the neutral point grounding short-circuit impedance referred to the primary side; The expression for the fundamental fault potential is: Combining the above two equations, we obtain the multiple-solution screening criterion for the fundamental frequency difference coefficient. d h : The above multiple solutions and the corresponding fundamental fault potential E 1( α and grounding transition resistance R g Substitute each value into the criterion, calculate the corresponding fundamental frequency difference coefficient, select the solution with the smallest difference coefficient as the correct fault location, and screen out the unique solution of the dual-frequency fault potential.

6. The active arc suppression method for stator grounding faults of a multi-branch hydro-generator according to claim 5, characterized in that: The specific formula for calculating the neutral point dual-frequency power injection amount in step S5 is as follows: in, I i1 This refers to the injection amount of the fundamental frequency at the neutral point. I i3 The third harmonic injection amount at the neutral point; m is the number of branches per phase; This is the equivalent capacitance of a single-turn coil to ground. C t This refers to the ground capacitance of the generator terminal direct connection system. The third harmonic potential of the i-th turn coil of the j-th branch of phase A; E A3 The phase potential of the third harmonic A-phase winding is given.

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