A multi-branch hydro-generator stator ground fault active arc extinguishing method

Abstract

The application discloses a kind of multi-branch hydroelectric generator stator ground fault active arc extinguishing method, comprising the following steps: S1.utilize the voltage of generator fault after neutral point and machine end, combine winding connection order, construct the analytic function of double-frequency fault potential;S2.deduce the linear constraint relationship between the real part and imaginary part of third harmonic fault potential;S3.according to linear constraint relationship, combine third harmonic each branch winding potential expression, construct the objective function of fault solution, solve the solution set based on third harmonic characteristic quantity;S4.according to the fundamental fault characteristic equation, construct the multiple solution screening criterion of fundamental difference coefficient, obtain the unique solution of double-frequency fault potential;S5.for multi-branch hydroelectric generator, construct neutral point double-frequency power injection amount calculation formula;Using the unique solution of double-frequency fault potential screened out, the injection amount of neutral point double-frequency controllable power is correctly adjusted, the fault point voltage is suppressed to 0, and arc extinguishing is realized.The application can realize the reliable arc extinguishing of multi-branch hydroelectric generator.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of large generator safety protection, and particularly relates to a multi-branch hydro-generator stator grounding fault active arc extinguishing method. BACKGROUND

[0002] Stator winding single-phase grounding fault is one of the most common faults of large generators, and the generated grounding fault current is easy to cause serious consequences such as stator insulation breakdown and core burnout, threatening the safe operation of the generator. The arc extinguishing method for the grounding fault of the stator winding of the generator mainly includes passive arc extinguishing method and active arc extinguishing method. At present, the passive arc extinguishing method of neutral point through arc suppression coil or high resistance grounding is generally used for large generator sets. However, this passive arc extinguishing method can only compensate for the residual power frequency reactive current at the fault point, and cannot be dynamically adjusted, and cannot adapt to complex and variable generator operating conditions and different grounding fault environments. Practical operation experience shows that the existing neutral point grounding methods of the generator cannot realize reliable fault arc extinguishing under different fault scenarios. The active arc extinguishing method controls the voltage at the fault point to be 0 by injecting a source into the neutral point, thereby eliminating the grounding fault current and realizing voltage reduction arc extinguishing at the fault point.

[0003] Due to the spatial dispersion characteristics of the stator winding potential of the generator, different positions need to be compensated for different potentials, and the fault potential needs to be calculated online. The existing related researches only focus on single-branch or branch potential coinciding generators. For such generators, the fault potentials are the same when the grounding fault occurs at the same fault turn ratio of different branches. However, for multi-branch hydro-generators, the unique winding structure characteristics cause the branch potentials not to completely coincide, and the above method is no longer applicable.

[0004] Due to the large number of branches and dense potential distribution, multiple solutions may occur in the calculation of the fault potential, affecting the compensation amount calculation of the arc extinguishing device. If the wrong fault potential is used for arc extinguishing control, the voltage at the fault point will be increased, and even the arc burning degree will be aggravated, thereby causing serious damage to the generator. Therefore, an effective multiple solution screening method needs to be combined to improve the reliability and universality of the arc extinguishing strategy. SUMMARY

[0005] The technical problem to be solved by the present application is to provide a multi-branch hydro-generator stator grounding fault active arc extinguishing method, so as to solve the problem that the existing arc extinguishing method cannot solve the multiple solution problem of the fault potential in the arc extinguishing of the multi-branch hydro-generator, and cannot realize reliable arc extinguishing.

[0006] To solve the above technical problems, the technical solutions adopted by the present application are as follows.

[0007] A multi-branch hydro-generator stator grounding fault active arc extinguishing method, comprising the following steps:

[0008] S1. Using the generator neutral point and terminal voltage after fault, combined with the winding connection sequence, the winding turn potential amplitude is calculated online, and the analytical function of the double-frequency fault potential is constructed;

[0009] S2. The third harmonic ground fault current is calculated online, and the linear constraint relationship between the real part and the imaginary part of the third harmonic fault potential is derived combined with the fault characteristic equation;

[0010] S3. According to the above linear constraint relationship, combined with the expression of the third harmonic potential of each branch winding, the objective function of fault solving is constructed; by solving the minimum value of the objective function on each branch, the solution with the objective function less than a certain threshold is selected as the solution set that meets the third harmonic characteristic quantity;

[0011] S4. According to the fundamental fault characteristic equation, the base difference coefficient multi-solution screening criterion is constructed; by substituting the multi-solutions in the above solution set and the corresponding parameters into the criterion, the corresponding base difference coefficients are calculated, and the solution with the smallest difference coefficient is selected as the correct fault location, and the unique solution of the double-frequency fault potential is obtained;

[0012] S5. For multi-branch hydro-generator, based on the principle of voltage reduction and arc extinction, the formula for calculating the injection amount of neutral point double-frequency power source is constructed; using the unique solution of the double-frequency fault potential screened out, the injection amount of the neutral point double-frequency controllable power source is correctly adjusted to suppress the fault point voltage to 0, and arc extinction is realized.

[0013] Preferably, the step S1 specifically comprises:

[0014] S11. Online calculation of winding turn potential amplitude, specifically:

[0015] According to the connection sequence of the winding, the base KVL equation corresponding to the branch winding is listed; the winding base turn potential amplitude is calculated online using the measured values of the generator neutral point and terminal voltage after fault :

[0016]

[0017] wherein, U n1 is the base voltage of the generator neutral point after fault; U s1 is the base voltage of the generator terminal after fault; is the phase angle corresponding to each turn base turn potential;

[0018] Similarly, the winding third harmonic turn potential amplitude :

[0019]

[0020] wherein,U n3 the third harmonic voltage of the neutral point after the generator fault; U s3 the third harmonic voltage of the machine terminal after the generator fault; the phase angle corresponding to the third harmonic turn potential of each turn;

[0021] S12. Constructing an analytical function of the double-frequency fault potential, specifically as follows:

[0022] Analytical function of the fundamental frequency fault potential:

[0023]

[0024] wherein, E 1( α ) is the fundamental frequency fault potential; α is the fault turn ratio; is the number of turns of each branch winding; is the fundamental frequency winding induced potential corresponding to the first turn winding; is the fundamental frequency winding induced potential corresponding to the first turn winding;

[0025] Similarly, the analytical function of the third harmonic fault potential is obtained as follows:

[0026]

[0027] wherein, E 3( α ) is the third harmonic fault potential; is the third harmonic winding induced potential corresponding to the first turn winding; is the third harmonic winding induced potential corresponding to the first turn winding;

[0028] Preferably, the linear constraint relationship between the real part and the imaginary part of the third harmonic fault potential in the step S2 is specifically:

[0029] The expression of the third harmonic fault potential is:

[0030]

[0031] wherein, I f3 is the third harmonic ground fault current; R g is the grounding transition resistance;

[0032] Let , , , substitute the above formula, and separate the real and imaginary parts of the above phasor equation to obtain: ​​

[0033]

[0034] wherein, is the real part of the third harmonic fault potential; is the imaginary part of the third harmonic fault potential; is the real part of the third harmonic ground fault current; b is the imaginary part of the third harmonic ground fault current; c is the real part of the third harmonic voltage at the neutral point after the generator fault; d is the imaginary part of the third harmonic voltage at the neutral point after the generator fault; j is the imaginary unit;

[0035] The real part and the imaginary part of the third harmonic fault potential E 3( α ) satisfy a linear constraint relationship:

[0036] .

[0037] Preferably, the step S3 is specifically:

[0038] Using the fault potential to simultaneously satisfy the linear constraint relationship of the step S2 and the analytical function of the step S1, according to the third harmonic component, the objective function is described by the distance formula from the fault potential point to the straight line, and the objective function of the fault solving is as follows:

[0039]

[0040] Each branch fault potential is substituted into the formula, the minimum value of the objective function from 0 to 1 is solved, all fault positions corresponding to the objective function value less than 1 in the minimum value are selected as the solution set, the third harmonic fault potential corresponding to the fault position obtained is brought into the following formula, and the corresponding ground transition resistance is obtained:

[0041] .

[0042] Preferably, the step S4 is specifically:

[0043] The expression of the fundamental harmonic ground fault current I f1 is as follows:

[0044]

[0045] wherein, I f1 is the fundamental harmonic ground fault current; Δ U n1 is the change of the fundamental harmonic voltage at the neutral point of the generator before and after the fault; ωω is the angular frequency; ω is the angular frequency; R n ω is the angular frequency; X k ω is the angular frequency;

[0046] The expression of the fundamental fault potential is:

[0047]

[0048] By combining the above two formulas, the multiple solution screening criterion of the fundamental difference coefficient is obtained d h :

[0049]

[0050] The above multiple solutions and the corresponding fundamental fault potentials E 1( α ) and the ground transition resistance R g are respectively substituted into the criterion, the corresponding fundamental difference coefficients are calculated, the solution with the minimum difference coefficient is selected as the correct fault location, and the unique solution of the double-frequency fault potential is screened out.

[0051] Preferably, the neutral point double-frequency power injection amount calculation formula in the step S5 is specifically:

[0052]

[0053]

[0054] wherein, I i1 is the neutral point fundamental injection amount; I i3 is the neutral point third harmonic injection amount; m is the number of each phase branches; is the equivalent capacitance of a single-turn coil to ground; C t is the ground capacitance of the generator terminal direct connection system; is the third harmonic potential of the i-turn coil of the j-branch of the A phase; E A3 is the third harmonic A phase winding phase potential.

[0055] Thanks to the above technical solutions, the technical progress achieved by the present application is as follows.

[0056] The application is based on the grounding fault characteristics of the multi-branch hydro-generator, considers the fault potential multi-solution problem prone to occur in the arc extinction process, and proposes a fault potential calculation method of double frequencies in cooperation, so as to provide a unique correct double-frequency fault potential for calculating the arc extinction injection quantity, avoid the influence of the incorrect fault potential on the arc extinction effect, and realize reliable arc extinction.

[0057] The application can calculate all required fault solution sets by the third harmonic fault potential solving method based on the linear constraint relationship, and reliably selects the unique correct solution by the fault potential multi-solution screening method based on the fundamental difference coefficient.

[0058] The application gives the neutral point double-frequency power injection quantity calculation formula based on the voltage reduction arc extinction mechanism according to the characteristics that the branch potentials of the multi-branch hydro-generator do not coincide, and improves the reliability and universality of arc extinction. BRIEF DESCRIPTION OF DRAWINGS

[0059] Figure 1 The flowchart of the application;

[0060] Figure 2 The equivalent circuit diagram of the single-phase grounding fault of the generator stator winding of the application;

[0061] Figure 3 The fundamental wave zero sequence equivalent circuit diagram of the single-phase grounding fault of the generator stator winding of the application;

[0062] Figure 4 The solving result diagram under the Case1 fault scenario of the application;

[0063] Figure 5 The solving result diagram under the Case2 fault scenario of the application;

[0064] Figure 6 The solving result diagram under the Case3 fault scenario of the application;

[0065] Figure 7 The solving result diagram under the Case4 fault scenario of the application;

[0066] Figure 8 The arc extinction effect diagram of the alternative fault point (A1, 0.5715) of Case3 of the application;

[0067] Figure 9 The arc extinction effect diagram of the alternative fault point (A1, 0.7327) of Case3 of the application;

[0068] Figure 10 The arc extinction effect diagram of the alternative fault point (A3, 0.7435) of Case3 of the application;

[0069] Figure 11Arc extinguishing effect diagram of the alternative fault point (A8, 0.6590) of Case 3 of the application. DETAILED DESCRIPTION

[0070] The application will be further described in detail below in combination with the drawings and specific embodiments.

[0071] A multi-branch hydro-generator stator ground fault active arc extinguishing method, in combination with Figure 1 As shown in the drawings, the method comprises the following steps:

[0072] S1. Using the neutral point voltage and terminal voltage after the generator fault, in combination with the winding connection sequence, the winding turn potential amplitude is calculated online to construct an analytical function of the double-frequency fault potential.

[0073] Specifically, assuming that the connection sequence of a branch winding is:

[0074]

[0075] wherein the branch is composed of n turn windings in series, is the slot number of the 2n slot conductors, U represents the upper layer side conductor in the slot, and L represents the lower layer side conductor in the slot.

[0076] If the induced potential of the 0th slot conductor is selected as the reference 0 phase, the induced potential of each slot conductor can be obtained as wherein, E k is the amplitude of the slot conductor induced potential; is the phase angle of the slot conductor induced potential, and , is the slot number corresponding to each slot conductor, is the slot pitch electrical angle. Thus, the fundamental turn potential of the first turn winding of the branch can be obtained as:

[0077]

[0078] wherein, is the phase angle of the induced potential of the 1st slot conductor; is the phase angle of the induced potential of the 2nd slot conductor.

[0079] Through the winding connection sequence, the fundamental turn potential of each turn winding can be calculated in the same way. After uniform reference to the phase, the fundamental KVL equation satisfied by each branch winding can be written as:

[0080]

[0081] wherein, U n1 and U s1are the fundamental voltages at the neutral point and the generator terminal after the generator fault, respectively; is the phase angle corresponding to the fundamental turn potential of each turn. Thus, the amplitude of the fundamental turn potential of the winding can be calculated online :

[0082]

[0083] Similarly, the amplitude of the third harmonic turn potential of the winding can be calculated online :

[0084]

[0085] wherein, U n3 and U s3 are the third harmonic voltages at the neutral point and the generator terminal after the generator fault, respectively; is the phase angle corresponding to the third harmonic turn potential of each turn. On this basis, combined with the winding connection order in formula , the analytical function between the fundamental fault potential and the fault turn ratio can be obtained:

[0086]

[0087] wherein, E 1( α ) is the fundamental fault potential; α is the fault turn ratio; is the number of turns of each branch winding; is the fundamental winding induced potential corresponding to the first turn winding; is the fundamental winding induced potential corresponding to the first turn winding.

[0088] Similarly to the fundamental, the analytical function between the third harmonic fault potential and the fault turn ratio is obtained:

[0089]

[0090] wherein, E 3( α ) is the third harmonic fault potential; is the third harmonic winding induced potential corresponding to the first turn winding; is the third harmonic winding induced potential corresponding to the first turn winding.

[0091] S2. Online calculation of the third harmonic ground fault current, combined with the fault characteristic equation, to derive the linear constraint relationship between the real part and the imaginary part of the third harmonic fault potential.

[0092] Specifically, according toFigure 2 The equivalent circuit diagram of the single-phase ground fault of the generator stator winding is shown, and the formula for calculating the third harmonic ground fault current is:

[0093]

[0094] wherein, is the third harmonic ground fault current; j is the imaginary unit; ω is the angular frequency; is the sum of the ground capacitances of the generator stator winding; is the change in the third harmonic voltage at the generator neutral point before and after the fault; R n is the equivalent resistance value of the ground short-circuit impedance of the generator neutral point through the transformer, which is calculated to the primary side; X k is the equivalent reactance value of the ground short-circuit impedance of the neutral point through the transformer, which is calculated to the primary side.

[0095] Combined with the KVL of the ground branch, Figure 2 the expression of the third harmonic fault potential can be derived as follows:

[0096]

[0097] wherein, R g is the ground transition resistance.

[0098] Let , , , substitute into the above formula, and separate the real and imaginary parts of the phasor equation of the above formula to obtain:

[0099]

[0100] wherein, is the real part of the third harmonic fault potential; is the imaginary part of the third harmonic fault potential; is the real part of the third harmonic ground fault current; b is the imaginary part of the third harmonic ground fault current; c is the real part of the third harmonic voltage at the neutral point of the generator after the fault; d is the imaginary part of the third harmonic voltage at the neutral point of the generator after the fault.

[0101] Eliminating the ground transition resistance in the above formula can obtain the linear constraint relationship between the real part and the imaginary part of the third harmonic fault potential

[0102] .

[0103] ​S3. According to the linear constraint relationship, combined with the potential expression of each branch winding of the third harmonic, the objective function for fault solving is constructed; by solving the minimum value of the objective function on each branch, the solution in which the objective function is less than a certain threshold is selected as the solution set of the third harmonic characteristic quantity.

[0104] This step is a linear constraint-based fault potential solving method, specifically a third harmonic fault potential solving method based on linear constraint relationship. Specifically, the fault occurred after the fault is obtained I f3 and U n3 The straight line in which the third harmonic component of the fault potential is located can be determined. Combined with the mathematical mapping mode of the fault potential, the potential curve of each branch winding is drawn on the complex plane, and the fault position should be at the intersection of the potential curve of each branch winding and the straight line. Thus, by using the fault potential which satisfies the linear constraint relationship of step S2 and the analytical function of step S1, according to the third harmonic component, the objective function is described by the distance from the fault potential point to the straight line, and the objective function for fault solving is as follows:

[0105]

[0106] Since the winding electromotive force has continuity, the closer to the fault point, the smaller the . Therefore, the fault point must be at the minimum value of , and the corresponding value is small enough (considering the noise in the generator working condition, the threshold is set to 1). Thus, the fault potential of each branch is substituted into the objective function, and the minimum value of the objective function from 0 to 1 is solved, and all fault positions in which the corresponding objective function value is less than 1 are selected as the solution set. The third harmonic fault potential corresponding to the obtained fault position is brought into the following formula, and the corresponding grounding transition resistance is obtained:

[0107] .

[0108] S4. According to the fundamental fault characteristic equation, a fundamental difference coefficient multi-solution screening criterion is constructed; by substituting the multi-solution in the above solution set and the corresponding parameters into the criterion, the corresponding fundamental difference coefficient is calculated, and the solution with the smallest difference coefficient is selected as the correct fault position, and the unique solution of the double-frequency fault potential is obtained.

[0109] On the basis of step S3, the invention is a fault potential multi-solution screening method based on double-frequency cooperation. In this step, the fault potential multi-solution screening method based on the fundamental difference coefficient is constructed, specifically, similar to the third harmonic, the expression of the fundamental grounding fault current can also be obtained:

[0110]

[0111] wherein, I f1 is the fundamental frequency ground fault current; Δ U n1 is the change of fundamental frequency voltage at the generator neutral point before and after the fault.

[0112] According to Figure 3 the single-phase ground fault fundamental frequency zero sequence equivalent circuit of the generator stator winding shown in FIG. 1, the KVL equation of the fundamental frequency fault branch is written, and the expression of the fundamental frequency fault potential can be obtained:

[0113]

[0114] By combining the above two formulas, the multi-solution screening criterion of the fundamental frequency difference coefficient can be obtained d h :

[0115]

[0116] Substitute the above multi-solution and the corresponding fundamental frequency fault potential E 1( α ) and the ground transition resistance R g into the criterion respectively, and calculate the corresponding fundamental frequency difference coefficient. Select the solution with the smallest difference coefficient as the correct fault location, and the unique solution of the double-frequency fault potential can be screened out.

[0117] S5. For a multi-branch hydro-generator, based on the principle of voltage reduction arc extinction, a neutral point double-frequency source injection amount calculation formula is constructed; using the unique solution of the double-frequency fault potential screened out, the injection amount of the neutral point double-frequency controllable power source is adjusted correctly, the fault point voltage is suppressed to 0, and reliable arc extinction is realized.

[0118] Specifically, considering the difference of branch-to-ground capacitance current for the characteristics of the branch potential of the multi-branch hydro-generator not coinciding, the branch-to-ground capacitance current of each branch needs to be calculated respectively when calculating the third harmonic injection amount with zero sequence property. Therefore, the amount calculation formula of the current injected by the neutral point double-frequency current source is specifically:

[0119]

[0120]

[0121] wherein, I i1 is the neutral point fundamental frequency injection amount; I i3 is the neutral point third harmonic injection amount; m is the number of branches per phase; is the equivalent capacitance of a single-turn coil to ground; C tCapacitance of the direct connected system to ground; Third harmonic voltage of the i-th coil of the j-th branch of phase A; E A3 Third harmonic phase voltage of phase A winding.

[0122] The effectiveness of the proposed method is verified by simulation.

[0123] A multi-branch hydro-generator is taken as a prototype to establish a quasi-distributed parameter model for simulation analysis. The rated voltage of the generator is 20 kV, each phase contains 8 branch windings, the stator winding resistance is 9.864 mΩ / phase, the stator winding leakage inductance is 11.072 mH / phase, the stator winding capacitance is 3.846 μF / phase, the pole pair number is 40, the total slot number is 840, the slot pitch electrical angle is 17.14286°, and the capacitance of the direct connected system to ground is 0.405 μF / phase.

[0124] To verify the effectiveness of the proposed method at different fault locations, the ground fault with a transition resistance of 100 Ω is set at different branches and different fault turn ratios of phase A. The four fault locations are (A1, 0.1429), (A4, 0.4286), (A1, 0.5714), and (A8, 0.8571), and the corresponding fault voltage parameters are shown in Table 1.

[0125] Table 1

[0126]

[0127] 1) For the above four fault scenarios, the third harmonic preliminary solution results are shown in Table 1 using the proposed third harmonic fault voltage solving method based on linear constraint relationship. The calculation results of the proposed fault voltage multiple solution screening method based on the fundamental difference coefficient are shown in Table 2. Figures 4 to 7

[0128] Table 2

[0129]

[0130] Figure 4 The solution results for Case 1 fault scenario; Figure 5 The solution results for Case 2 fault scenario; Figure 6 The solution results for Case 3 fault scenario; Figure 7 The solution results for Case 4 fault scenario. The target function values of each point on each branch are mapped using color coding, and the color corresponds to the increase of from blue to yellow. All solutions are located at the minimum value of Figures 4 to 7 ​​​​It can be seen that there are multiple fault solutions using only the third harmonic, and the correct fault location is included in the solution set, which shows 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 the other false solutions, which shows that the proposed method can effectively filter out the unique fault solution.

[0131] 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.

[0132] Table 3

[0133]

[0134] 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 grounding fault transition resistances, which can provide a reliable premise for the implementation of grounding fault arc suppression.

[0135] 3) At 0.2s, the above Case3 (A1, 0.5714) occurs R g =100Ω grounding fault, calculate the arc injection amount corresponding to the four alternative fault solutions in the above fault scenarios. At 0.25s, the arc current is injected through the neutral point parallel current source, and the arc suppression effect is shown in Figures 8 to 11 .

[0136] As shown in Figures 8 to 11 , the horizontal axis t represents time in seconds, and the vertical axis represents the fault current in A, represents the fault potential in kV. Figure 8 is the arc suppression effect diagram of Case3 alternative fault point (A1, 0.5715); Figure 9 is the arc suppression effect diagram of Case3 alternative fault point (A1, 0.7327); Figure 10 is the arc suppression effect diagram of Case3 alternative fault point (A3, 0.7435); Figure 11 is the arc suppression effect diagram of Case3 alternative fault point (A8, 0.6590), where Figure 8 is the arc suppression effect corresponding to the correct fault solution, and the rest Figure 9 , Figure 10 and Figure 11 are the arc suppression effects corresponding to the false fault solutions. Comparing the two, using the correct fault potential to adjust the injection amount, after injecting the arc current at the neutral point, the voltage at the fault point to ground can be quickly regulated to close to 0, and the fault voltage to ground can be suppressed within the safety limit. For fault potentials that have not been filtered by the fundamental, the arc suppression effect is poor, and the grounding fault current is still large, exceeding the safety threshold.

[0137] 4) For the above four grounding fault scenarios, the arc extinction simulation results after multi-solution screening under different grounding transition resistances are shown in Table 4.

[0138] Table 4

[0139]

[0140] In Table 4, under different fault scenarios, the proposed method can clamp the fault grounding current to a low level to achieve voltage reduction and arc extinction. It is shown that the proposed method can reliably achieve fault arc extinction at the grounding point under different grounding fault location conditions, and the method is not affected by the grounding transition resistance.

Claims

1. A method for active arc extinction of a multi-branch hydrogenerator stator ground fault, characterized in that: The method comprises the following steps: S1. Using the post-fault neutral point and terminal voltage of the generator, combined with the winding connection sequence, the winding turn potential amplitude is calculated online to construct an analytical function of the double-frequency fault potential; S2. The third harmonic ground fault current is calculated online, combined with the fault characteristic equation, the linear constraint relationship between the real part and the imaginary part of the third harmonic fault potential is derived; S3. According to the linear constraint relationship, combined with the expression of the third harmonic potential of each branch winding, the objective function for solving the fault is constructed; by solving the minimum value of the objective function on each branch, the solution in which the objective function is less than a certain threshold is selected as the solution set that meets the third harmonic characteristic quantity; S4. According to the fundamental fault characteristic equation, a base wave difference coefficient multi-solution screening criterion is constructed; by substituting the multi-solutions in the solution set and the corresponding parameters into the criterion, the corresponding base wave difference coefficients are calculated, and the solution with the smallest difference coefficient is selected as the correct fault location to obtain a unique solution of the double-frequency fault potential; The step S4 is specifically: Fundamental ground fault current I f1 The expression is: wherein, I f1 is the fundamental frequency ground fault current; Δ U n1 is the change of the fundamental frequency voltage at the generator neutral point before and after the fault; ω is the angular frequency; is the sum of the stator winding-to-ground capacitances of the generator; R n is the equivalent resistance value of the generator neutral point through transformer grounding short circuit impedance reduced to the primary side; X k is the equivalent reactance value of the neutral point through transformer grounding short circuit impedance reduced to the primary side; j is the imaginary unit; The expression of the fundamental fault potential is: wherein, U n1 is the fundamental voltage at the neutral point after the generator fault; R g is the grounding transition resistance. Combining the above two equations, the screening criterion for the multiple solutions of the fundamental difference coefficient is obtained d h : The above multiple solutions and corresponding fundamental fault potential E 1( α ) and ground transition resistance R g , respectively into the criterion, calculate the corresponding fundamental difference coefficient, select the solution with the smallest difference coefficient as the correct fault location, and screen out the unique solution of the double-frequency fault potential; S5. For a multi-branch hydro-generator, based on the voltage reduction arc extinction principle, a neutral point double-frequency power injection amount calculation formula is constructed; the unique solution of the double-frequency fault potential selected is used to correctly adjust the injection amount of the neutral point double-frequency controllable power supply, so that the fault point voltage is suppressed to 0, and arc extinction is realized; The neutral point double-frequency power injection amount calculation formula in the step S5 is specifically: wherein, I i1 is the neutral point fundamental injection amount; I i3 is the neutral point third harmonic injection amount; m is the number of branches per phase; is the number of turns per branch winding; is the equivalent capacitance of a single turn coil to ground; C t is the generator terminal direct system to ground capacitance; is the third harmonic potential of the A phase jth branch ith turn coil; E 3( α ) is the third harmonic fault potential; E A3 is the third harmonic A phase winding phase potential.

2. A method for active arc extinction of ground fault in a multi-branch hydro-generator stator according to claim 1, characterized in that: The step S1 specifically comprises: S11. The winding turn potential amplitude is calculated online, specifically: According to the connection sequence of the winding, the fundamental KVL equation corresponding to the branch winding is listed; the winding fundamental turn potential amplitude is calculated on-line by using the measured values of the neutral point and the terminal voltage of the generator after the fault : wherein, U n1 is the fundamental voltage at the neutral point after generator fault; U s1 is the fundamental voltage at the machine terminal after generator fault; is the phase angle corresponding to the fundamental turn potential of each turn. By the same token, the amplitude of the third harmonic winding potential is obtained : wherein, U n3 is the third harmonic voltage at the neutral point after generator fault; U s3 is the third harmonic voltage at the machine terminal after generator fault; is the phase angle corresponding to the third harmonic turn potential of each turn. S12. The analytical function of the double-frequency fault potential is constructed, specifically as follows: The analytical function of the fundamental fault potential is: wherein, E 1( α ) is the fundamental fault potential; α is the fault turn ratio; is the number of turns of each branch winding; is the fundamental winding induced potential corresponding to the first 1 turn winding; is the fundamental winding induced potential corresponding to the first turn winding; Similarly, the analytical function of the third harmonic fault potential is obtained: wherein, E 3( α ) is the third harmonic fault potential; is the third harmonic winding induced potential corresponding to the first 1 turn winding; is the third harmonic winding induced potential corresponding to the first turn winding.

3. The method of claim 2, wherein the method further comprises: determining the faulted phase of the multi-branch hydro-generator stator; and determining the faulted phase of the multi-branch hydro-generator stator. The linear constraint relationship between the real part and the imaginary part of the third harmonic fault potential in the step S2 is specifically: The expression of the third harmonic fault potential is: wherein, I f3 is the third harmonic ground fault current; R g is the ground transition resistance; Let , , , substituting the above equation, separating the real and imaginary parts of the above equation phasor equation can be obtained: wherein, is the real part of the third harmonic fault potential; is the imaginary part of the third harmonic fault potential; is the real part of the third harmonic ground fault current; b is the imaginary part of the third harmonic ground fault current; c is the real part of the third harmonic neutral voltage after generator fault; d is the imaginary part of the third harmonic neutral voltage after generator fault; j is the imaginary unit; Eliminating the ground transition resistance from the above equation gives the third harmonic fault potential E 3( α ) and the imaginary part of the complex impedance satisfy a linear constraint relationship: 。 4. The method of claim 3, wherein the method further comprises: The step S3 is specifically: The target function of the fault solving is described according to the third harmonic component through the distance formula from the fault potential point to the straight line, by using the fault potential to simultaneously satisfy the linear constraint relationship of the step S2 and the analytical function of the step S1 As follows: Each branch fault potential is substituted into the formula to solve the minimum value of the objective function from 0 to 1, and all fault positions in which the corresponding objective function value is less than 1 are selected as the solution set; the third harmonic fault potential corresponding to the fault position obtained is substituted into the following formula to obtain the corresponding ground transition resistance: 。

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