Fault current analysis and parameter optimization methods and systems for double-fed induction generator (DFIG) wind turbines

Abstract

This invention relates to a method and system for analyzing fault currents and optimizing parameters in doubly-fed induction generator (DFIG) wind turbines, belonging to the field of power technology. To address the shortcomings of existing analytical formulas for DFIG fault currents, including insufficient analysis of control factors in DFIG controllers and poor practicality, this invention first establishes a mathematical model of the DFIG fault current generator (DFIG). Then, it analyzes the roles of RSC, GSC, and Crowbar protection and establishes a fault ride-through controller model. Through the derivation of the mathematical model, a detailed analytical formula for the DFIG fault current is obtained. Finally, using a differential evolution intelligent algorithm based on field-measured DFIG fault current waveforms, the analytical formula is optimized. This invention is used for fault current analysis in doubly-fed induction generator (DFIG) wind turbines.

Description

Technical Field

[0001] This invention belongs to the field of power technology, specifically relating to a fault current analysis method for a doubly fed wind turbine. Background Technology

[0002] Doubly fed induction generator (DFIG) wind turbines have advantages such as lower converter capacity requirements and lower overall cost, leading to their widespread application in power systems. The stator of a DFIG wind turbine is directly connected to the grid, and the rotor current is controlled via an excitation converter, indirectly regulating the output power on the stator side. This design cannot effectively isolate the wind turbine from the grid, making it highly sensitive to grid faults. Furthermore, under fault conditions, the low-power excitation converter's control capability over the entire unit is limited, resulting in low fault ride-through capability. Therefore, researching accurate fault current analysis and parameter optimization methods for DFIG wind turbines is crucial for ensuring the stability and reliability of power systems.

[0003] Researchers have developed several solutions for analyzing fault currents in wind turbines, such as:

[0004] 1. Wang Zengping et al., “Research on the Impact of Fault Characteristics of Large-Scale Wind Farms on Protection”, Electrical Measurement & Instrumentation, 2018, 55(4):9-13,22. This article focuses on relay protection analysis, establishing mathematical models of the machine-side converter (RSC) and grid-side converter (GSC) in the stator and rotor reference frames respectively, and then quickly and easily iteratively calculating the fault current output by the converter. Through simulation comparison, the correctness of the conclusions of the article is confirmed. However, this method is a theoretical derivation for relay protection, and the fault current calculation expression obtained is disconnected from the control parameters, which cannot reflect the accurate relationship between the real parameters and the analytical expression of the fault current.

[0005] 2. Zhang Yibo et al. published "Simplified Calculation of Short-Circuit Current of Doubly Fed Motor Based on Sensitivity Analysis" Power Grid and Clean Energy, 2017, 33(5):6-12. Based on the sensitivity analysis of various parameters of the motor, this article derives the calculation formula for the maximum value of the three-phase short-circuit current under Crowbar operation. However, this method is based on the analytical expression of the short-circuit current obtained by multiple linear regression, which only considers the influence of some parameters, resulting in a large error and only an approximate expression.

[0006] 3. Gan Junwen et al. published "An Adaptive Short-Circuit Current Calculation Method for Doubly Fed Wind Turbines Based on Terminal Voltage Drop Depth" (Journal of Electric Power Science and Technology, 2018, 33(3):3-9). This article analyzes the stator current characteristics of doubly fed wind turbine units under two conditions: Crowbar in operation and non-operation, and derives an adaptive short-circuit current calculation method suitable for doubly fed wind turbines. This method fails to consider the Crowbar's action delay and ignores the effect of rotor current on stator current, thus having certain limitations.

[0007] In summary, existing work on practical calculations does not consider the impact of Crowbar action delay and control strategy, and generally neglects the effect of rotor current on stator short-circuit current, as well as the contribution of GSC to short-circuit current. Therefore, a method for optimizing the analytical expression for fault current and its parameters in doubly-fed induction generator (DFIG) wind turbines is needed, which combines accuracy and practicality. Summary of the Invention

[0008] The purpose of this invention is to solve the problems of insufficient analytical expression for fault current of doubly fed wind turbines and insufficient control factors of doubly fed wind turbine controllers, poor practicality of the analytical expression for fault current of doubly fed wind turbines, and insufficient accuracy of the analytical expression due to the lack of consideration for the optimization of corresponding parameters.

[0009] A method for fault current analysis and parameter optimization of doubly-fed induction generator (DFIG) wind turbines is proposed. DFIG wind turbines employ two back-to-back converters connected via a DC link for AC excitation. The grid-side converter controller is abbreviated as GSC, and the turbine-side converter controller is abbreviated as RSC. The method includes steps for acquiring fault data based on the functions of RSC, GSC, and Crowbar protection, and steps for analyzing and optimizing the fault current and parameters of the DFIG wind turbine.

[0010] The steps for obtaining fault data based on the functions of RSC, GSC, and Crowbar protection include:

[0011] The command values ​​for rotor voltage and rotor current are denoted as u. r,ref and i r,ref Construct the stator short-circuit current affected by RSC, where the d-axis and q-axis components of the rotor current command value are denoted as i. rq,ref i rd,ref The command values ​​of the d-axis and q-axis components of the GSC voltage and current are denoted as u. gd,ref u gq,ref and i gd,ref i gq,ref To construct the GSC output current during power grid faults;

[0012] Determine the reactive current i during the fault period in the case of a low voltage fault. rqL and active current irdL And the active current i restored after the low-voltage fault is cleared. rdre i rqL i rdL i during the corresponding fault period rq,ref i rd,ref i rdre i during the recovery period after fault clearance rd,ref For situations involving high-voltage faults, determine the corresponding reactive current i during the fault period. rqH i rqH i during the corresponding fault period rq,ref Control logic for maintaining steady-state active power during a fault;

[0013] The subscript 'r' in the parameters indicates the parameters corresponding to the rotor side, which is the parameters corresponding to the machine side; the subscript 're' in the parameters indicates the parameters corresponding to the recovery process; the subscript 'ref' in the parameters indicates that the corresponding parameter is the reference value of the corresponding command value, which is the reference value of the corresponding parameter.

[0014] The steps for analyzing and optimizing the fault current of a doubly-fed induction generator (DFIG) wind turbine include:

[0015] S1. Perform voltage drop and / or voltage rise fault tests, and record the three-phase voltage and three-phase current at the fan outlet.

[0016] S2. Fault ride-through controller parameter determination:

[0017] Based on the three-phase voltage and three-phase current at the wind turbine outlet, the fundamental positive-sequence voltage and fundamental positive-sequence reactive current under voltage drop conditions are extracted, and the fundamental positive-sequence reactive current is mapped to i. rqL This leads to the low-voltage ride-through reactive power support coefficient k. qL Extract the changing active power during the fault power recovery process under voltage dip conditions, and determine the corresponding active power recovery rate k. id1 and k id2 Alternatively, extract the fundamental positive-sequence voltage and fundamental positive-sequence reactive current under voltage rise conditions, and assign the fundamental positive-sequence reactive current to i. rqH This leads to the high-voltage ride-through reactive power support coefficient k. qH ;

[0018] S3. Substitute the PI controller parameters into the analytical formula for the fault current of the doubly fed fan and analyze the current:

[0019] First, let k pr k ir k p_P k i_P k pv k iv k pg kig Substitute into the analytical formula for the fault current of a doubly-fed induction generator; where k pr and k ir k is the proportional-integral coefficient of the inner loop PI control element of the RSC current; p_P and k i_P k represents the proportional-integral coefficient of the RSC active power outer loop controller. pv and k iv k represents the proportional-integral coefficient of the outer loop control of the GSC voltage system. pg and k ig The proportional-integral coefficient of the GSC current inner loop control loop;

[0020] Assume the fault occurs at time t1 and the Crowbar activates at time t2;

[0021] When the Crowbar is not activated (i.e., t1≤t≤t2), if a low-voltage fault ride-through occurs, the fault ride-through controller parameters determined in step S2 are substituted into the fault ride-through controller model to obtain the corresponding active current i during the fault period. rdL and reactive current i rqL And the active current i recovered after the low-voltage fault is cleared. rdre i rqL i rdL i during the corresponding fault period rq,ref i rd,ref i rdre i during the recovery period after fault clearance rd,ref The machine-side current command value i in the fault current analysis formula of the doubly fed wind turbine is respectively connected. r,ref Obtain the analytical expression for the fault current of the doubly fed wind turbine corresponding to the low voltage fault and analyze the current.

[0022] When the Crowbar is not activated (i.e., t1≤t≤t2), if a high-voltage fault ride-through occurs, the fault ride-through controller parameters determined in step S2 are substituted into the fault ride-through controller model to obtain i during the fault period. rqH , change i rqH In the analytical expression for the fault current of a doubly fed wind turbine, i rq,ref ; Control logic for maintaining steady-state active power during faults; Substitute the corresponding active and reactive currents into the machine-side current command value i in the fault current analytical formula of the doubly-fed induction generator (DFIG) r,ref Obtain the analytical expression for the fault current of the doubly fed wind turbine corresponding to the high voltage fault and analyze the current.

[0023] During the fault ride-through when the Crowbar does not activate, substituting i into the analytical expression for the fault current of the doubly-fed induction generator (DFIG) r,ref The specific values ​​are determined by the actual stage in which the fault occurs.

[0024] When the Crowbar is activated, i.e., t > t2, the Crowbar resistor short-circuits the rotor winding, allowing the DFIG to continue operating as a squirrel-cage induction generator. At this time, the corresponding controller parameters and steady-state operating data are substituted into the fault current analysis formula of the doubly-fed induction generator to form the fault current analysis formula of the doubly-fed induction generator after the Crowbar is activated and the current is analyzed.

[0025] S4. Under three-phase voltage dip or rise conditions, convert the three-phase current and three-phase voltage into dq-axis DC quantities, and set the objective function. Where a i For measured short-circuit current data, b i The result of the fault current analytical expression is given, where i is the number of sampling points and n is the total number of sampling points. The differential evolution intelligent algorithm is used to optimize the steady-state controller parameters. The optimization objective is to minimize the value of the objective function E, thus obtaining the final analytical expression describing the fault current of the doubly-fed wind turbine generator set.

[0026] Furthermore, the analytical formula for the fault current of the doubly-fed wind turbine is as follows:

[0027]

[0028] Where k is the difference between the stator voltage after the fault and the voltage u before the fault. s0 The ratio; U s0 The stator-side steady-state voltage u before the fault s0 The amplitude; j represents the imaginary number; ω1 is the synchronous speed; T1 = L s / R s R s L s These are the resistance and inductance of the generator stator circuit, respectively; T2 = σL r / R ra R ra =R r +R a R r L r These are the resistance and inductance of the generator rotor circuit, R. a Crowbar resistor; L m σ represents the mutual inductance between the generator stator and rotor; t represents time; σ is the leakage magnetic coefficient of the fan; k pv k iv The proportional-integral coefficients of the outer loop control of the GSC voltage are respectively associated with control loop P and control loop I; ω r i is the rotor angular velocity; r0 The current for steady-state rotor operation; i g0 This refers to the grid-side current in steady state before the fault. i g,refThis is the commanded value of the GSC current in the three-phase stationary coordinate system; p is a differential operator.

[0029] Furthermore, the doubly-fed asynchronous generator model of the electrical part of the doubly-fed wind turbine is as follows:

[0030] A mathematical model of a doubly-fed asynchronous generator is established in a two-phase rotating dq coordinate system, and its voltage and flux linkage equations are as follows:

[0031]

[0032] In the formula: R s L s The resistance and inductance of the generator stator circuit; R r L r For the resistance and inductance of the generator rotor circuit; L m For the mutual inductance between the stator and rotor of the generator; ψ s,dq ψ r,dq For the stator and rotor dq shaft flux linkages of the generator; u s,dq u r,dq For the stator and rotor dq-axis voltages of the generator; i s,dq i r,dq ω represents the stator and rotor dq-axis currents of the generator; ω1 represents the synchronous speed; ω represents the slip speed.

[0033] Then the DFIG mathematical model in the two-phase rotating coordinate system is transformed into the three-phase stationary coordinate system.

[0034] Furthermore, the equations for the DFIG voltage and flux linkage after the Crowbar action are as follows:

[0035]

[0036] Furthermore, the machine-side converter controller, i.e., RSC, is as follows:

[0037] RSC employs stator flux linkage-oriented vector control. The voltage command expression for RSC control is:

[0038] u r,ref =k pr (i r,ref -i r,dq )+k ir ∫(i r,ref -i r,dq )dt+jωσL r i r,dq (3)

[0039] In the formula: u r,ref i r,refThese are the command values ​​for rotor voltage and rotor current, respectively; k pr and k ir σ is the proportional-integral coefficient of the PI control loop; σ is the leakage flux coefficient of the fan.

[0040] When the mains voltage drops and the Crowbar does not operate, the rotor current is controlled by RSC; the stator current is as follows:

[0041]

[0042] Stator steady-state flux linkage ψ before the fault s0 and stator flux linkage ψ after the fault sF as follows:

[0043]

[0044]

[0045] In the formula, ψ s0 and ψ sF The values ​​are in a three-phase stationary coordinate system; τ1 is the decay time constant, τ1 = R s / L s ω1 = 2πf = 100π (rad / s); t represents time; ψ sF The subscript F represents the stator flux linkage ψ in the three-phase stationary coordinate system under fault conditions. s ;

[0046] The relationship between rotor flux linkage and rotor current after a fault is as follows:

[0047]

[0048] ψ rF F (where F represents fault) represents the rotor flux linkage ψ in the three-phase stationary coordinate system under fault conditions. r .

[0049] Based on the relationship between rotor flux and rotor current after the fault,

[0050]

[0051] In the formula, f s (ψ s ) represents the stator flux linkage ψ s A function, f s (ψ s )=(pψ sF +jωψ sF )L m / L s p represents the differential operator;

[0052] Solving equation (8) yields the expression for the faulted rotor current. Combining the stator flux linkage and rotor current expressions, we obtain the stator current expression for a non-deep voltage drop:

[0053]

[0054] In the formula, i r0 The current is the rotor current in steady state; the values ​​of λ1, λ2, β1, and β2 are respectively... i sF Let i be the fault state represented by F. s,dq .

[0055] Furthermore, the relationship between rotor flux linkage and rotor current after the fault is as follows:

[0056] Furthermore, the grid-side converter controller, i.e., GSC, is as follows:

[0057] The voltage outer loop control equation for GSC is:

[0058] i gd =k pv (u dc -u dc,ref )+k iv ∫(u dc -u dc,ref )dt (10)

[0059] Among them, i gd For the d-axis component of the GSC current, u dc u dc,ref These are the DC voltage value and DC voltage command value of the outer loop of the grid-side converter voltage control, respectively;

[0060] The inner loop control equation for the current is:

[0061]

[0062] In the formula, L g For the filter equivalent inductance, u dc i is the DC bus voltage. gq For the q-axis component of the GSC current, u s For the grid-side fundamental positive sequence voltage, u gq,ref =S(t)u dc S(t) is the switching function of GSC;

[0063] The power balance equation for GSC is:

[0064] P-1.5u gd i gd =u dc C buspu dc (12)

[0065] In the formula, C bus P is the DC bus capacitor, P is the power value of the wind turbine, and p is the differential operator;

[0066] Combining equations (10) to (12), the current during a symmetrical fault is obtained as follows:

[0067]

[0068] i g Let i be the value of the GSC current in the three-phase stationary coordinate system. gd0 This refers to the d-axis component of the GSC current during steady-state operation.

[0069] The basic voltage equation for GSC is:

[0070] u g =R r i g +L g (pi g +jω1i g )+u s (14)

[0071] Among them, u g This represents the GSC voltage in a three-phase stationary coordinate system.

[0072] Using equations (11) and (14), the current differential equation for the GSC is obtained:

[0073]

[0074] Among them, R g For the filter equivalent resistance, L g p is the equivalent inductance for filtering; 2 Indicates the second derivative;

[0075] The analytical expression for the output current of the GSC is obtained by solving:

[0076]

[0077] In the formula, i g,ref The commanded value of the GSC current in the three-phase stationary coordinate system is given by i. gd,ref and i gq,ref It is obtained by transformation matrix transformation from two-phase rotating dq coordinate system to three-phase stationary ABC coordinate system; i g0 This represents the GSC current in steady state before the fault.

[0078] Furthermore, the mathematical model of the fault ride-through controller is as follows:

[0079] A. When a low-voltage fault occurs in the power grid at time t1, the wind turbine detects that the system voltage has dropped below the threshold and switches to the low-voltage ride-through control logic. This means that the power outer loop of the rotor-side controller is disconnected, and its control structure is switched to the fault control structure, followed by the current inner loop. After the low-voltage ride-through fault occurs, the wind turbine injects reactive current into the power grid to support the recovery of the grid voltage. The dynamic reactive current output by the wind farm to the power grid is:

[0080] i sqL =k qL ·(0.9-u s )I N (17)

[0081] Reference value of rotor-side reactive current i during the fault period rqL as follows:

[0082]

[0083] In equations (17) and (18), k qL The low-voltage ride-through reactive power support factor is determined by the grid connection standard; I N The rated current of the unit; u s This is the fundamental positive sequence voltage on the grid side;

[0084] During a fault, the wind turbine effectively generates reactive power to support the grid voltage. The active current is limited by the reactive current output. The reference value i for the rotor-side active current during a fault is... rdL for:

[0085]

[0086] In the formula, i rdL This is a reference value for the active current during the fault; i drN I represents the steady-state value of the active current before the fault occurred. rmax The maximum rotor current; k p_P k is the proportional gain of the active power outer loop controller. i_P P is the integral coefficient of the active power outer loop controller; ref P is the active power reference value, where P is the power value of the wind turbine.

[0087] When the power grid is at t b When the low-voltage fault is cleared, the voltage returns to its pre-fault level; at this time, the reactive power instantly recovers to its steady-state level after the fault is cleared; the rotor-side active current reference value i recovers at a certain recovery rate. rdre :

[0088] irdre =k p_P (P re,ref -P)+k i_P ∫(P re,ref -P)dt (20)

[0089] In the formula, P re,ref This is a reference value for active power during fault recovery;

[0090] During fault recovery, P re,ref The expression is:

[0091]

[0092] Where, k id1 ,k id2 These represent two different active power recovery rates; P fault t represents the steady-state active power generated by the DFIG during a fault. b t represents the fault clearing time; c P represents the starting point of the active power recovery phase with the second slope; eF P represents the DFIG active power value during fault recovery. eN This refers to the steady-state active power generated before the DFIG fault.

[0093] B. When a high-voltage fault occurs in the power grid at time t1, the wind turbine detects that the system voltage has risen above the threshold and switches to high-voltage ride-through control logic. After a high-voltage ride-through fault occurs, the wind turbine absorbs reactive current from the power grid to reduce the impact of the voltage rise. The reactive current needs to be adjusted according to the degree of voltage rise. During the fault, the dynamic reactive current absorbed by the wind farm from the power grid is:

[0094] i sqH =k qH ·(u s -1.1)I N (twenty two)

[0095] Reference value of rotor-side reactive current i during the fault period rqH as follows:

[0096]

[0097] In equations (22) and (23), k qH The high-voltage ride-through reactive power support factor is determined by the grid connection standard; I N This refers to the rated current of the unit;

[0098] Control logic for maintaining steady-state active power during a fault;

[0099] When the power grid is at t bThe control logic ensures that the high-voltage fault is cleared and the voltage returns to normal; the active power remains in a steady state, and the reactive power instantly returns to a steady-state level after the fault is cleared.

[0100] Furthermore, the stator short-circuit current under the action of Crowbar protection is as follows:

[0101] The expression for the stator current at times t1 to t2 is the same as that in equation (9);

[0102]

[0103] In the formula, ψ sF2 Let F2 represent the stator flux linkage ψ in the three-phase stationary coordinate system under the second-stage fault condition. s ;

[0104] After the Crowbar is triggered, the differential equation concerning the rotor flux linkage is obtained as follows:

[0105]

[0106] Solve the differential equation (25), where the unknown is ψ. r,dq The solution is denoted as ψ. ra The rotor flux linkage expression is obtained by solving for:

[0107]

[0108] In the formula, τ2 is the attenuation constant, τ2=R ra / σL r Combined with the initial state expression of the flux linkage during Crowbar operation (24), the initial value C1 of the DC component with τ2 as the decay time constant is obtained. in

[0109] Then, the stator current i after Crowbar operation is obtained from the flux linkage equation. sF2 :

[0110]

[0111] In the formula,

[0112] A fault current analysis and parameter optimization system for doubly-fed induction generator (DFIG) wind turbines. The DFIG wind turbines use two back-to-back converters connected via a DC link for AC excitation. The grid-side converter controller is abbreviated as GSC; the turbine-side converter controller is abbreviated as RSC. The system includes:

[0113] Fault data acquisition unit: acquires fault data based on the functions of RSC, GSC, and Crowbar protection. The specific process includes:

[0114] The command values ​​for rotor voltage and rotor current are denoted as u. r,ref and i r,ref Construct the stator short-circuit current affected by RSC, where the d-axis and q-axis components of the rotor current command value are denoted as i. rq,ref i rd,ref The command values ​​of the d-axis and q-axis components of the GSC voltage and current are denoted as u. gd,ref u gq,ref and i gd,ref i gq,ref To construct the GSC output current during power grid faults;

[0115] Determine the reactive current i during the fault period in the case of a low voltage fault. rqL and active current i rdL And the active current i restored after the low-voltage fault is cleared. rdre i rqL i rdL i during the corresponding fault period rq,ref i rd,ref i rdre i during the recovery period after fault clearance rd,ref For situations involving high-voltage faults, determine the corresponding reactive current i during the fault period. rqH i rqH i during the corresponding fault period rq,ref Control logic for maintaining steady-state active power during a fault;

[0116] The subscript 'r' in the parameters indicates the parameters corresponding to the rotor side, which is the parameters corresponding to the machine side; the subscript 're' in the parameters indicates the parameters corresponding to the recovery process; the subscript 'ref' in the parameters indicates that the corresponding parameter is the reference value of the corresponding command value, which is the reference value of the corresponding parameter.

[0117] Current analysis and parameter optimization unit: This unit analyzes and optimizes the fault current of doubly-fed induction generator (DFIG) wind turbines, including:

[0118] Three-phase voltage and current acquisition module: Based on voltage drop and / or voltage rise fault tests, obtain the three-phase voltage and three-phase current at the fan outlet;

[0119] Fault ride-through controller parameter determination module: Based on the three-phase voltage and three-phase current at the wind turbine outlet, extract the fundamental positive-sequence voltage and fundamental positive-sequence reactive current under voltage drop conditions, and map the fundamental positive-sequence reactive current to i. rqL This leads to the low-voltage ride-through reactive power support coefficient k. qL Extract the changing active power during the fault power recovery process under voltage dip conditions, and determine the corresponding active power recovery rate k. id1 and k id2Alternatively, extract the fundamental positive-sequence voltage and fundamental positive-sequence reactive current under voltage rise conditions, and assign the fundamental positive-sequence reactive current to i. rqH This leads to the high-voltage ride-through reactive power support coefficient k. qH ;

[0120] Doubly fed wind turbine fault current analysis module: First, k pr k ir k p_P k i_P k pv k iv k pg k ig Substitute into the analytical formula for the fault current of a doubly-fed induction generator; where k pr and k ir k is the proportional-integral coefficient of the inner loop PI control element of the RSC current; p_P and k i_P k represents the proportional-integral coefficient of the RSC active power outer loop controller. pv and k iv k represents the proportional-integral coefficient of the outer loop control of the GSC voltage system. pg and k ig The proportional-integral coefficient of the GSC current inner loop control loop;

[0121] Assume the fault occurs at time t1 and the Crowbar activates at time t2;

[0122] When the Crowbar is not activated (i.e., t1≤t≤t2), if a low-voltage fault ride-through occurs, the fault ride-through controller parameters determined in step S2 are substituted into the fault ride-through controller model to obtain the corresponding active current i during the fault period. rdL and reactive current i rqL And the active current i recovered after the low-voltage fault is cleared. rdre i rqL i rdL i during the corresponding fault period rq,ref i rd,ref i rdre i during the recovery period after fault clearance rd,ref The machine-side current command value i in the fault current analysis formula of the doubly fed wind turbine is respectively connected. r,ref Obtain the analytical expression for the fault current of the doubly fed wind turbine corresponding to the low voltage fault and analyze the current.

[0123] When the Crowbar is not activated (i.e., t1≤t≤t2), if a high-voltage fault ride-through occurs, the fault ride-through controller parameters determined in step S2 are substituted into the fault ride-through controller model to obtain i during the fault period. rqH , change i rqH In the analytical expression for the fault current of a doubly fed wind turbine, irq,ref ; Control logic for maintaining steady-state active power during faults; Substitute the corresponding active and reactive currents into the machine-side current command value i in the fault current analytical formula of the doubly-fed induction generator (DFIG) r,ref Obtain the analytical expression for the fault current of the doubly fed wind turbine corresponding to the high voltage fault and analyze the current.

[0124] During the fault ride-through when the Crowbar does not activate, substituting i into the analytical expression for the fault current of the doubly-fed induction generator (DFIG) r,ref The specific values ​​are determined by the actual stage in which the fault occurs.

[0125] When the Crowbar is activated, i.e., t > t2, the Crowbar resistor short-circuits the rotor winding, allowing the DFIG to continue operating as a squirrel-cage induction generator. At this time, the corresponding controller parameters and steady-state operating data are substituted into the fault current analysis formula of the doubly-fed induction generator to form the fault current analysis formula of the doubly-fed induction generator after the Crowbar is activated and the current is analyzed.

[0126] Optimization module: Under three-phase voltage dip or rise conditions, convert the three-phase current and three-phase voltage into dq-axis DC quantities, and set the objective function. Where a i For measured short-circuit current data, b i The result of the fault current analytical expression is given, where i is the number of sampling points and n is the total number of sampling points. The differential evolution intelligent algorithm is used to optimize the steady-state controller parameters. The optimization objective is to minimize the value of the objective function E, thus obtaining the final analytical expression describing the fault current of the doubly-fed wind turbine generator set.

[0127] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0128] Improving the accuracy and practicality of analytical expressions: This invention comprehensively considers different grid voltage drop depths and RSC and GSC controls, and derives short-circuit current expressions that include Crowbar based on actual fault conditions and the effects of Crowbar protection actions and their delays. This makes the control of influencing factors more comprehensive, resulting in more accurate expressions that are more suitable for practical applications.

[0129] Providing more accurate control parameters: This invention constructs a simulation model of a doubly-fed induction generator (DFIG) wind turbine through detailed and precise electromagnetic transient modeling, which can more accurately reflect the relationship between the wind turbine's control parameters and fault current. This accuracy is based on the refinement of the simulation model's simulation step size and the accuracy of the simulation parameters, providing a more accurate basis for the fault characteristic analysis of DFIG wind turbines.

[0130] Intelligent Algorithm Optimization: This invention optimizes the analytical expression for the fault current of doubly-fed induction generator (DFIG) wind turbines based on field measurement data. By using a differential evolution intelligent algorithm, it can more accurately simulate and predict the fault current of wind turbines under actual operating scenarios. The application of the intelligent algorithm makes the analytical expression for the fault current of DFIG wind turbines more accurate in theory and more widely applicable in practical applications.

[0131] Effective use of actual field data: This invention effectively utilizes the fault current waveform data of doubly fed wind turbines measured on-site, and optimizes the control parameters of wind turbine units based on this data. The method of combining theory and practice improves the accuracy and practicality of analytical expressions, making the error between theoretical prediction results and actual field results smaller.

[0132] In short, this invention provides a more accurate and practical analytical expression for fault current analysis of doubly-fed wind turbines, which is of positive significance for ensuring the stability and reliability of power systems. Attached Figure Description

[0133] Figure 1 It consists of two sets of six IGBTs and anti-parallel diodes forming a two-level three-phase converter, and the two sets of converters form a back-to-back converter system.

[0134] Figure 2 This is a block diagram of the generator-side converter control of the present invention, where ω is the differential speed, ω1 is the synchronous angular velocity, PLL is the phase-locked loop, SVPWM is the space vector modulation, coordinate transformation is the transformation between the synchronous coordinate axis and the three-phase coordinate axis, θ1 is the grid phase angle locked by the PLL, and U sabc I sabc For grid voltage and grid current.

[0135] Figure 3 This is a block diagram of the inner and outer loop control of the grid-side converter.

[0136] Figure 4 The power response curve is shown for the entire low-voltage fault ride-through process.

[0137] Figure 5 This is for low-voltage control logic.

[0138] Figure 6 This is the power response curve for the entire high-voltage fault ride-through process.

[0139] Figure 7 This is for high-level control logic.

[0140] Figure 8 This is a schematic diagram of the active power recovery process during low voltage ride-through.

[0141] Figure 9 This is a schematic diagram of the reactive power recovery process during high voltage ride-through.

[0142] Figure 10 A flowchart of the method for optimizing fault current parameters of a doubly-fed wind turbine. Detailed Implementation

[0143] This invention first establishes accurate and detailed models of the main circuit, steady-state controller, and fault ride-through controller. Then, through the derivation of the mathematical model, a detailed analytical expression for the fault current of the doubly-fed induction generator (DFIG) is obtained. Finally, the analytical expression for the fault current of the DFIG is optimized based on the field-measured fault current waveform of the DFIG using a differential evolution intelligent algorithm.

[0144] First, let's explain the subscripts of the parameters: s (stator) represents the stator side, r (rotor) represents the rotor side, d and q represent the d-axis and q-axis respectively, and ref represents the command value, which is the corresponding reference value.

[0145] Meanwhile, the d-axis controls active power and the q-axis controls reactive power. Therefore, the d-axis current is often referred to as active current and the q-axis current as reactive current. When using subscripts, the active and reactive components are represented by d and q respectively, that is, d represents the active component (i.e., the d-axis component) and q represents the reactive component (i.e., the q-axis component).

[0146] In this invention, the stator of the doubly-fed induction generator (DFIG) wind turbine is connected to the power grid, and the rotor is connected to an AC excitation converter. Both the stator and rotor participate in power feeding, hence the name "doubly-fed" wind turbine. The DFIG wind turbine uses two back-to-back converters connected via a DC link for AC excitation, thereby achieving variable-speed constant-frequency operation and maximum energy tracking control. The grid-side converter controller is abbreviated as Grid-Side Controller (GSC); the turbine-side converter controller is abbreviated as Turbine-Side Controller (RSC). The short-circuit current of the DFIG wind turbine is calculated in two parts: the short-circuit current of the turbine stator winding during a grid fault, and the output current of the GSC in response to a grid fault.

[0147] The command values ​​for rotor voltage and rotor current are denoted as u. r,ref and i r,ref The d-axis and q-axis components of the rotor current command value are denoted as i. rq,ref i rd,ref The proportional-integral coefficient of the inner current loop PI control element on the RSC side is denoted as k. pr and k ir The leakage flux coefficient of the fan is denoted as σ, and a stator short-circuit current analysis considering the influence of RSC is constructed.

[0148] The command values ​​of the d-axis and q-axis components of the GSC voltage and current are denoted as u. gd,ref u gq,ref and i gd,ref i gq,refSimultaneously, an analysis of the GSC output current during power grid faults was constructed.

[0149] Determine the reactive current i during the fault period in the case of a low voltage fault. rqL and active current i rdL And the active current i restored after the low-voltage fault is cleared. rdre i rqL i rdL i during the corresponding fault period rq,ref i rd,ref i rdre i during the recovery period after fault clearance rd,ref ;

[0150] Determine the reactive current i during the high-voltage fault period. rqH i rqH i during the corresponding fault period rq,ref The control logic for maintaining steady-state active power during a fault; the reactive power recovers to steady-state level instantaneously after a high-voltage fault is cleared.

[0151] Then, taking into account the functions of RSC, GSC, and Crowbar protection, the fault current of the doubly-fed wind turbine is analyzed and its parameters are optimized. The invention will now be described in detail with reference to specific implementation methods. Specific implementation method one:

[0153] This embodiment is a method for analyzing fault current and optimizing parameters of a doubly-fed wind turbine, including the following steps:

[0154] First, a doubly-fed induction generator (DFIG) wind turbine comprises mechanical and electrical components. The mechanical components are existing technology and will not be described in this invention. The electrical components typically include three main components: a doubly-fed induction generator (DFIG), a back-to-back converter, and a control unit. The stator of the DFIG wind turbine is directly connected to the power grid, while the rotor achieves AC excitation through a back-to-back converter. Electrical power can be exchanged with the power grid through dual channels on both the stator and rotor. The back-to-back converter (e.g., Figure 1 As shown, the two PWM converters can be referred to as grid-side converters and machine-side converters respectively, based on their positions. Because their stators are directly connected to the grid, they have the advantages of small converter capacity, low cost and low manufacturing cost, and are widely used in wind power generation.

[0155] 1. Mathematical Model of Electrical Components

[0156] Like a typical three-phase AC motor, a doubly-fed asynchronous wind power system (DFIG) using three-phase phase variables is a high-order, multivariable, nonlinear, and strongly coupled time-varying system, making it difficult to directly implement operation control and perform system analysis and design. DFIG control primarily focuses on power control. To achieve effective (decoupled) control of its active and reactive power, vector transformation control technology from AC speed-regulating drives can be applied to DFIG wind power system control. This involves decomposing the rotor current into active and reactive components through coordinate transformation, and then controlling these two rotor current components to independently control the active and reactive power of the DFIG, thereby achieving the control objective of variable-speed constant-frequency power generation. Vector transformation control is a control technology implemented using coordinate transformation; therefore, DFIG operation analysis and control strategy research should begin with establishing mathematical models of the DFIG in different coordinate systems.

[0157] When establishing the mathematical model of the DFIG, both its stator and rotor windings adopt the conventions of electric motors. The mathematical model of the doubly-fed asynchronous generator is established in a two-phase rotating dq coordinate system, and its voltage and flux linkage equations are as follows:

[0158]

[0159] In the formula: R s L s The resistance and inductance of the generator stator circuit; R r L r For the resistance and inductance of the generator rotor circuit; L m For the mutual inductance between the stator and rotor of the generator; ψ s,dq ψ r,dq For the stator and rotor dq shaft flux linkages of the generator; u s,dq u r,dq For the stator and rotor dq-axis voltages of the generator; i s,dq i r,dq ω is the stator and rotor dq-axis current of the generator; ω1 is the synchronous speed, ω1=2πf=100π(rad / s); ω is the slip speed.

[0160] After the Crowbar operates, the Crowbar resistor is connected in series with the rotor circuit. At this time, the equivalent resistance R of the rotor circuit is... ra Transformed into rotor resistance and Crowbar resistance R a The sum, i.e., R ra =R r +R a The electromagnetic relationship of DFIG after the Crowbar action is shown in equation (2).

[0161]

[0162] According to the coordinate transformation theory of AC motors, the transformation matrix from the two-phase rotating dq coordinate system to the three-phase stationary ABC coordinate system is denoted as C. dq / ABC The transformation from the two-phase rotating dq coordinate system to the three-phase stationary coordinate system is shown in equation (3):

[0163]

[0164] in, Let X be the phase angle between the d-axis and the phase reference axis, typically taken as the phase angle of phase A voltage; where X... A X B X C Let A, B, and C represent the values ​​of the variable to be transformed in the three-phase stationary coordinate system, respectively. Let X represent the variable to be transformed, with subscripts A, B, and C representing phases A, B, and C in the three-phase stationary coordinate system, respectively. Similarly, X... d X q These are the quantities of the variable to be transformed along the d-axis and q-axis in the two-phase rotating coordinate system, respectively. Using equation (3), the DFIG mathematical model in the two-phase rotating coordinate system can be transformed into the three-phase stationary coordinate system.

[0165] The mathematical model for the electrical components provides a model for implementing subsequent processes.

[0166] 2. Stator short-circuit current analysis considering the influence of RSC

[0167] The RSC employs stator flux-oriented vector control, with an outer loop for active and reactive power control and an inner loop for current control. The inner current loop incorporates feedforward compensation to eliminate cross-coupling between active and reactive power; that is, the dq-axis components of the AC voltage on the RSC side are only related to the corresponding rotor current components. The model control logic of the machine-side converter is shown in [link to model control logic]. Figure 2 The control structure can be seen in Table 1.

[0168] The voltage command expression for RSC control is:

[0169] u r,ref =k pr (i r,ref -i r,dq )+k ir ∫(i r,ref -i r,dq )dt+jωσL r i r,dq (4)

[0170] In the formula: u r,ref i r,ref These are the command values ​​for rotor voltage and rotor current, respectively; k pr and k ir σ is the proportional-integral coefficient of the PI control loop; σ is the leakage flux coefficient of the fan.

[0171] When the grid voltage experiences a slight dip, the Crowbar does not activate, and the rotor current is controlled by the RSC. Assuming a fault occurs at time t1, neglecting voltage phase transitions, let the stator voltage after the fault be equal to the stator steady-state voltage u before the fault. s0 The ratio is k. The stator current is determined by equation (5).

[0172]

[0173] Ignore stator circuit resistance R s The corresponding stator steady-state flux linkage ψ before the fault s0 And not neglecting the stator circuit resistance R s The corresponding stator flux linkage ψ after the fault sF The expression is:

[0174]

[0175]

[0176] In the formula, ψ s0 and ψ sF The values ​​are in a three-phase stationary coordinate system; τ1 is the decay time constant, τ1 = R s / L s ; t represents time; ψ sF F (where F represents fault) represents the stator flux linkage ψ in the three-phase stationary coordinate system under fault conditions. s , ψ s ψ s,dq Substitute into equation (3) and transform to obtain the result.

[0177] The relationship between rotor flux linkage and rotor current after a fault is as follows:

[0178]

[0179] ψ rF F (where F represents fault) represents the rotor flux linkage ψ in the three-phase stationary coordinate system under fault conditions. r , where ψ r ψ r,dq Substitute into equation (3) and transform to obtain the result.

[0180] Substituting equation (8) into the rotor voltage equation in formula (1), we get:

[0181]

[0182] In the formula, f s (ψ s ) represents the stator flux linkage ψ sA function, f s (ψ s )=(pψ sF +jωψ sF )L m / L s ;ψ s ψ s,dq Substituting into equation (3) and transforming, we obtain that p represents the differential operator.

[0183] Solving equation (9) yields the expression for the fault rotor current i. r,dq Combining the stator flux linkage and rotor current expressions in formula (1), the stator current expression for a slight voltage drop can be obtained (which is actually solving for i). s,dq ):

[0184]

[0185] In the formula, i r0 The current is the rotor current in steady state; the values ​​of λ1, λ2, β1, and β2 are respectively... i sF Let i be the fault state represented by F. s,dq .

[0186] Table 1. Introduction to the Control Structure of the Doubly Fed Wind Turbine Machine-Side Converter

[0187]

[0188]

[0189] 3. Analysis of GSC output current during power grid faults

[0190] The GSC employs terminal voltage-oriented control to achieve decoupling control of active and reactive power. When the Crowbar protection fails to operate under normal wind turbine operation or fault conditions, the GSC connects to the RSC via the DC bus, and its operating principle is similar to that of the RSC. The model control logic of the grid-side converter is shown in [link to model control logic]. Figure 3 The control structure can be seen in Table 2.

[0191] The voltage outer loop control equation for GSC is:

[0192] i gd =k pv (u dc -u dc,ref )+k iv ∫(u dc -u dc,ref )dt (11)

[0193] Among them, i gd For the d-axis component of the GSC current, udc u dc,ref These are the DC voltage value and DC voltage reference value of the outer loop of the grid-side converter voltage control, respectively.

[0194] The inner loop control equation for the current is:

[0195]

[0196] In the formula, L g For the filter equivalent inductance, i gq For the q-axis component of the GSC current, u s For the grid-side fundamental positive sequence voltage, u gq,ref =S(t)u dc S(t) is the switching function of GSC.

[0197] Furthermore, under the grid voltage-oriented control mode, the GSC controls the AC side through the d-axis current i gd The flow of active power is controlled to maintain the DC bus voltage within a specific range. While the converter remains connected to the rotor circuit, the effect of the inner current loop is ignored, and the GSC current is assumed to be equal to its reference value (i.e., i...). gd =i gd,ref The current of a GSC is mainly related to its power balance. The power balance equation of a GSC is:

[0198] P-1.5u gd i gd =u dc C bus pu dc (13)

[0199] In the formula, C bus denoted as DC bus capacitor, P as the power value of the wind turbine generator, and p as the differential operator.

[0200] Solve equations (11) to (13) simultaneously to find i. gd i gq The current under symmetrical fault conditions is obtained as follows:

[0201]

[0202] Among them, i g by i gd and i gq i is obtained by transformation through equation (3). g Let i be the value of the GSC current in the three-phase stationary coordinate system. gd0 This refers to the d-axis component of the GSC current during steady-state operation.

[0203] Currently, wind turbines typically employ grid voltage-oriented vector control. Here, it is assumed that the grid connection point voltage is not affected by GSC control, and only the current inner loop control is considered during fault processes.

[0204] The basic voltage equation for GSC is:

[0205] u g =R r i g +L g (pi g +jω1i g )+u s (15)

[0206] Among them, u g This represents the GSC voltage in a three-phase stationary coordinate system.

[0207] Using equations (12) and (15), the current differential equation for GSC is obtained:

[0208]

[0209] Among them, R g For the filter equivalent resistance, L g p is the equivalent inductance for filtering; 2 It represents the second derivative.

[0210] Solve the differential equation (16), where the unknown is i. g After solving, denote it as i. g,out The analytical expression for the output current of the GSC is obtained as follows:

[0211]

[0212] In the formula, i g,ref The command value of the GSC current in the three-phase stationary coordinate system can be obtained from i gd,ref and i gq,ref Obtained by transformation via equation (3); i g0 This represents the GSC current in steady state before the fault.

[0213] Table 2. Introduction to the Control Structure of the Doubly Fed Wind Turbine Grid-Side Converter

[0214]

[0215] 4. Mathematical Model of Fault Transit Controller

[0216] When a voltage fault occurs in the power grid, wind turbines need to generate or absorb a certain amount of reactive power and limit the output of active power if necessary, depending on the severity of the voltage drop or rise.

[0217] The low-voltage fault ride-through response curves of active and reactive power of wind turbine units are as follows: Figure 4 As shown.

[0218] analyze Figure 4 It can be seen that the wind turbine operates under steady-state conditions during the 0-t1 period. The reactive power injected into the grid by the wind turbine is generally maintained at around 0, while the active power operates under steady-state control strategies such as speed control, maximum power point tracking control, and power control, depending on the wind speed.

[0219] A. When a low-voltage fault occurs in the power grid at time t1, the wind turbine detects that the system voltage has dropped below the threshold and switches to low-voltage ride-through control logic. This means the outer power loop of the rotor-side controller is disconnected, and its control structure is switched to the fault-fault control structure, followed by the inner current loop. Therefore, the reference value for the rotor-side current loop will change accordingly. According to my country's grid connection standards, after a low-voltage ride-through fault, the wind turbine needs to have the ability to inject reactive current into the grid to support grid voltage recovery. The dynamic reactive current output from the wind farm to the grid is:

[0220] i sqL =k qL ·(0.9-u s )I N (18)

[0221] Reference value of rotor-side reactive current i during the fault period rqL as follows:

[0222]

[0223] At this point, i in the formula will be... rqL The value assigned to i rq,ref In equations (18) and (19), k qL The low-voltage ride-through reactive power support factor is determined by the grid connection standard; I N The rated current of the unit; u s This is the fundamental positive sequence voltage on the grid side.

[0224] To ensure that wind turbines can effectively generate reactive power to support grid voltage during faults, most mainstream units employ reactive power priority control. This means that active current is limited by reactive current output, and the reference value i of the rotor-side active current during a fault is... rdL for:

[0225]

[0226] Reference value i of rotor-side active current rdL This is the reference value for the d-axis current. At this point, i in the formula... rdL The value assigned to i rd,ref In the formula, i rdL This is a reference value for the active current during the fault; i drN I represents the steady-state value of the active current before the fault occurred.rmax The maximum rotor current; k p_P k is the proportional gain of the active power outer loop controller. i_P P is the integral coefficient of the active power outer loop controller; ref P is the active power reference value, and P is the power value of the wind turbine.

[0227] When the power grid is at t b When the low-voltage fault is cleared, the voltage returns to its pre-fault level. At this time, reactive power mostly recovers to steady-state levels instantaneously after the fault is cleared. Meanwhile, the rotor-side active current reference value i recovers at a certain recovery rate. rdre :

[0228] i rdre =k p_P (P re,ref -P)+k i_P ∫(P re,ref -P)dt (21)

[0229] At this point, i in the formula will be... rdre Value assigned to i rd,ref In the formula, P re,ref This is a reference value for active power during fault recovery; the superscript "re" stands for "recovery," indicating recovery.

[0230] Its active power recovery is as follows Figure 8 As shown. During fault recovery, P re,ref The expression is:

[0231]

[0232] Where, k id1 ,k id2 These represent two different active power recovery rates; P fault t represents the steady-state active power generated by the DFIG during a fault. b t represents the fault clearing time; c P represents the starting point of the active power recovery phase with the second slope; eF P represents the DFIG active power value during fault recovery. eN This represents the steady-state active power generated before the DFIG fault.

[0233] Based on the above discussion, the low-voltage fault control logic can be obtained as follows: Figure 5 .

[0234] B. When a high-voltage fault occurs in the power grid at time t1, the wind turbine detects that the system voltage has risen above the threshold and switches to high-voltage ride-through control logic. According to my country's grid connection standards, after a high-voltage ride-through fault occurs, the wind turbine needs to absorb reactive current from the grid to reduce the impact of the voltage rise. The reactive current needs to be adjusted according to the degree of voltage rise. During the fault, the dynamic reactive current absorbed by the wind farm from the grid is:

[0235] i sqH =k qH ·(u s -1.1)I N (twenty three)

[0236] Reference value of rotor-side reactive current i during the fault period rqH as follows:

[0237]

[0238] At this point, i in the formula will be... rqH Value assigned to i rq,ref In equations (23) and (24), k qH The high-voltage ride-through reactive power support factor is determined by the grid connection standard; I N This is the rated current of the unit.

[0239] During a fault, the active power generally does not change, maintaining the control logic in steady state.

[0240] When the power grid is at t b The high-voltage fault was cleared immediately, and the voltage returned to normal.

[0241] The high-voltage fault ride-through response curves of the active and reactive power of the wind turbine are as follows: Figure 6 As shown. Analysis Figure 6 It can be seen that the wind turbine operates under steady-state conditions during the 0-t1 period. The reactive power injected into the grid by the wind turbine is generally maintained at around 0, while the active power operates under steady-state control strategies such as speed control, maximum power point tracking control, and power control, depending on the wind speed. Figure 9 for Figure 6 The reactive power recovery curve during medium- and high-voltage ride-through, at t b When a high-voltage fault is cleared, the reactive power returns to a steady-state level instantly after the voltage returns to normal.

[0242] Based on the above discussion, the high-voltage fault control logic can be obtained as follows: Figure 7 As shown.

[0243] During the limiting control phase, a specific active current control strategy is implemented to protect the inverter. In this phase, the active current setting directly depends on the set limit value and the reactive current setpoint. This ensures that the converter will not be damaged by exceeding its current limit under high load or abnormal conditions.

[0244] 5. Stator short-circuit current analysis considering Crowbar

[0245] When the grid voltage drops sharply, considering the delay of Crowbar triggering, assuming the fault occurs at time t1 and the Crowbar operates at time t2, the transient characteristics before its operation are mainly determined by the wind turbine parameters and RSC. The transient process analysis during the time period t1 to t2 is the same as the "Stator short-circuit current analysis considering the influence of RSC" process in Part 2 above. The stator flux linkage at time t2 is shown in Equation (25), and the stator current expression at time t1 to t2 is the same as Equation (10).

[0246]

[0247] In the formula, ψ sF2 F2 represents the stator flux linkage ψ in the three-phase stationary coordinate system under the second-stage fault state (i.e., Crowbar triggering). s That is, ψ sF2 It is the value when t = t2 in formula (7).

[0248] After the Crowbar is triggered, by combining equations (5) and (2), we can obtain the differential equation concerning the rotor flux linkage as follows:

[0249]

[0250] Solve the differential equation (26), where the unknown is ψ. r,dq The solution is denoted as ψ. ra (Subscript r indicates rotor, ra takes the same value as R) ra With the same subscript (referring to the state after Crowbar triggering), the rotor flux linkage expression can be obtained as follows:

[0251]

[0252] In the formula, τ2 is the attenuation constant, τ2=R ra / σL r Combining this with the initial state expression of the flux linkage during Crowbar operation (25), the initial value C1 of the DC component with τ2 as the decay time constant can be obtained. in

[0253] The stator current i after Crowbar operation can then be obtained from the flux linkage equation. sF2Substitute (26) and (27) into (2) to solve for i. s,dq The result is denoted as i. sF2 :

[0254]

[0255] In the formula,

[0256] i sF2 The second-stage fault state is indicated by the subscript F2, where F stands for fault, and 2 is used to distinguish it from formula (10).

[0257] 6. Analytical Formula for Fault Current of Doubly Fed Fans

[0258] Taking into account the functions of RSC, GSC, and Crowbar protection, the analytical formula for the fault current of the wind turbine can be calculated. short (t)=i g (t)+i s (t).

[0259] Assume the voltage drops sharply at time t1 and the Crowbar is inserted into the rotor circuit at time t2. In the short-circuit current expression, the stator short-circuit current affecting RSC is represented by a piecewise function composed of equations (10) and (28) with t2 as the dividing point, and the current change law of GSC output current is characterized by equation (17).

[0260] For t1≤t≤t2, i short (t)=i g (t)+i s (t)=i g,out (t)+i sF (t); for t>t2, i short (t)=i g (t)+i s (t)=i g,out (t)+i sF2 (t). The analytical formula for the fault current of a doubly fed wind turbine is as follows (29):

[0261]

[0262] In the formula, T1 = L s / R s ;U s0 The stator-side steady-state voltage u before the fault s0 The amplitude; T2=σL r / R ra ; ω r ω is the rotor angular velocity.

[0263] It should be noted that the time t2 of the Crowbar action may vary depending on the specific unit and manufacturer settings; t2 may occur between t1 and t2. a It is also possible that it will happen in t b Afterwards; although formula (29) is based on the form of t1 and t2, for the case where the crowbar does not move (there is no time point t2), the expression involving t1 is substituted in; in addition, i substituted into (29) r,ref The values ​​of the parameters are calculated based on the actual fault stage and substituted into (29), that is: from t1 onwards, i is calculated according to the specific fault stage. r,ref The parameter values ​​are used for calculation. Although t is not used directly... b t c The expression involved, but it is actually implicitly included in substituting the expression for fault crossing into i. r,ref The parameters.

[0264] See Figure 10 The fault current optimization method for doubly-fed wind turbines described in this embodiment includes:

[0265] S1. Perform voltage drop (e.g., 20% three-phase voltage drop) and voltage rise (e.g., 130% three-phase voltage rise) fault tests on actual doubly-fed wind turbine generator sets, manufacturer's black box model, and actual unit controller semi-physical simulation model, and record the three-phase voltage and three-phase current at the wind turbine outlet.

[0266] S2. Fault ride-through controller parameter determination:

[0267] Based on the three-phase voltage and three-phase current at the wind turbine outlet, the fundamental positive-sequence voltage and fundamental positive-sequence reactive current under voltage drop conditions are extracted and substituted into formula (18). The fundamental positive-sequence voltage corresponds to u in formula (18). s The fundamental positive sequence reactive current corresponds to i in formula (18) sqL The low-voltage ride-through reactive power support coefficient k can then be calculated. qL Extract the active power that changes during the fault power recovery process under voltage drop conditions, and substitute it into the corresponding formula (22) to determine the corresponding active power recovery rate k. id1 and k id2 .

[0268] or,

[0269] Based on the three-phase voltage and three-phase current at the wind turbine outlet, the fundamental positive-sequence voltage and rotor-side reactive current under voltage rise conditions are extracted and substituted into formula (23). The fundamental positive-sequence voltage corresponds to u in formula (23). s The fundamental positive sequence reactive current corresponds to i in formula (23) gqHThe high-voltage ride-through reactive power support coefficient k can then be calculated. qH .

[0270] S3. Substitute the PI controller parameters into the analytical expression for the fault current of the doubly fed fan.

[0271] Let k be the proportional-integral coefficient of the inner loop PI control element of the RSC current. pr and k ir Let k be the proportional-integral coefficient of the RSC active power outer loop controller. p_P and k i_P Let k be the proportional-integral coefficient of the outer loop control of the GSC voltage. pv and k iv Let k be the proportional-integral coefficient of the GSC current inner loop control loop. pg and k ig ;

[0272] k pr k ir k p_P k i_P k pv k iv k pg k ig Substitute the analytical formula for the short-circuit current of the doubly fed fan into (29);

[0273] When the Crowbar is not activated, i.e., t1≤t≤t2, if a low-voltage fault ride-through occurs, the fault ride-through controller parameters determined in step S2 are substituted into the fault ride-through controller model to obtain the corresponding formulas (19), (20), and (21). Formula (19) represents i during the fault period. rq,ref Formula (20) represents i during the fault period rd,ref Formula (21) represents i during the fault recovery period. rd,ref During the fault period, formulas (19) and (20) are respectively connected to i in formula (29). r,ref The fault current of a low-voltage doubly-fed wind turbine is calculated using an analytical formula. During the fault recovery process, formula (22) is substituted into formula (21) to obtain the corresponding i during the fault recovery process. rd,ref Connect it to formula (29)i r,ref The analytical formula for the fault current of a low-voltage doubly-fed wind turbine is derived.

[0274] When the Crowbar does not activate, i.e., t1≤t≤t2, if a high-voltage fault ride-through occurs, the fault ride-through controller parameters determined in step S2 are substituted into the fault ride-through controller model to obtain the corresponding formula (24). Formula (24) represents i during the fault period. rq,refDuring the fault, the control logic for maintaining steady-state active power incorporates formula (24) and the rotor-side active current during the fault (control logic for maintaining steady-state) into formula (29). r,ref The analytical formula for the fault current of a high-voltage doubly-fed wind turbine is derived.

[0275] During the fault pass where the Crowbar does not move, substituting i into formula (29) r,ref The specific value depends on the specific stage of the actual fault crossing.

[0276] When the Crowbar activates (i.e., t > t2), the Crowbar resistor short-circuits the rotor winding, allowing the DFIG to continue operating as a squirrel-cage induction generator. At this time, the required controller parameter k in equation (29) is... pr k ir k p_P k i_P k pv k iv k pg k ig and steady-state operating data i g0 , u s0 Substituting into formula (29), we can form the analytical expression for the fault current of the doubly fed fan after the Crowbar action.

[0277] S4. Under three-phase voltage dip or rise conditions, convert the three-phase current and three-phase voltage into dq-axis DC quantities, and set the objective function. Where a i b represents the measured short-circuit current data from actual doubly-fed induction generator (DFIG) wind turbine generators, manufacturer's black-box models, or semi-physical simulation models of actual generator controllers. i The result is the analytical expression for the fault current of the doubly-fed induction generator (DFIG), where i is the number of sampling points and n is the total number of sampling points. A differential evolutionary intelligent algorithm is used to optimize the steady-state controller parameters, with the optimization objective being to minimize the value of the objective function E. This yields the final analytical expression for the fault current of the DFIG wind turbine, which accurately describes the DFIG-type wind turbine generator set. Specific Implementation Method Two:

[0279] This implementation method is a fault current analysis and parameter optimization system for doubly-fed wind turbine generators. The doubly-fed wind turbine generator uses two back-to-back converters connected by a DC link for AC excitation. The grid-side converter controller is abbreviated as GSC; the turbine-side converter controller is abbreviated as RSC.

[0280] The fault current analysis and parameter optimization system for doubly-fed wind turbines described in this embodiment includes:

[0281] Fault data acquisition unit: acquires fault data based on the functions of RSC, GSC, and Crowbar protection. The specific process includes:

[0282] The command values ​​for rotor voltage and rotor current are denoted as u. r,ref and i r,ref Construct the stator short-circuit current affected by RSC, where the d-axis and q-axis components of the rotor current command value are denoted as i. rq,ref i rd,ref The command values ​​of the d-axis and q-axis components of the GSC voltage and current are denoted as u. gd,ref u gq,ref and i gd,ref i gq,ref To construct the GSC output current during power grid faults;

[0283] Determine the reactive current i during the fault period in the case of a low voltage fault. rqL and active current i rdL And the active current i restored after the low-voltage fault is cleared. rdre i rqL i rdL i during the corresponding fault period rq,ref i rd,ref i rdre i during the recovery period after fault clearance rd,ref For situations involving high-voltage faults, determine the corresponding reactive current i during the fault period. rqH i rqH i during the corresponding fault period rq,ref Control logic for maintaining steady-state active power during a fault;

[0284] The subscript 'r' in the parameters indicates the parameters corresponding to the rotor side, which is the parameters corresponding to the machine side; the subscript 're' in the parameters indicates the parameters corresponding to the recovery process; the subscript 'ref' in the parameters indicates that the corresponding parameter is the reference value of the corresponding command value, which is the reference value of the corresponding parameter.

[0285] Current analysis and parameter optimization unit: This unit analyzes and optimizes the fault current of doubly-fed induction generator (DFIG) wind turbines, including:

[0286] Three-phase voltage and current acquisition module: Based on voltage drop and / or voltage rise fault tests, obtain the three-phase voltage and three-phase current at the fan outlet;

[0287] Fault ride-through controller parameter determination module: Based on the three-phase voltage and three-phase current at the wind turbine outlet, extract the fundamental positive-sequence voltage and fundamental positive-sequence reactive current under voltage drop conditions, and map the fundamental positive-sequence reactive current to i. rqL This leads to the low-voltage ride-through reactive power support coefficient k. qLExtract the changing active power during the fault power recovery process under voltage dip conditions, and determine the corresponding active power recovery rate k. id1 and k id2 Alternatively, extract the fundamental positive-sequence voltage and fundamental positive-sequence reactive current under voltage rise conditions, and assign the fundamental positive-sequence reactive current to i. rqH This leads to the high-voltage ride-through reactive power support coefficient k. qH ;

[0288] Doubly fed wind turbine fault current analysis module: First, k pr k ir k p_P k i_P k pv k iv k pg k ig Substitute into the analytical formula for the fault current of a doubly-fed induction generator; where k pr and k ir k is the proportional-integral coefficient of the inner loop PI control element of the RSC current; p_P and k i_P k represents the proportional-integral coefficient of the RSC active power outer loop controller. pv and k iv k represents the proportional-integral coefficient of the outer loop control of the GSC voltage system. pg and k ig The proportional-integral coefficient of the GSC current inner loop control loop;

[0289] Assume the fault occurs at time t1 and the Crowbar activates at time t2;

[0290] When the Crowbar is not activated (i.e., t1≤t≤t2), if a low-voltage fault ride-through occurs, the fault ride-through controller parameters determined in step S2 are substituted into the fault ride-through controller model to obtain the corresponding active current i during the fault period. rdL and reactive current i rqL And the active current i recovered after the low-voltage fault is cleared. rdre i rqL i rdL i during the corresponding fault period rq,ref i rd,ref i rdre i during the recovery period after fault clearance rd,ref The machine-side current command value i in the fault current analysis formula of the doubly fed wind turbine is respectively connected. r,ref Obtain the analytical expression for the fault current of the doubly fed wind turbine corresponding to the low voltage fault and analyze the current.

[0291] When the Crowbar is not activated (i.e., t1≤t≤t2), if a high-voltage fault ride-through occurs, the fault ride-through controller parameters determined in step S2 are substituted into the fault ride-through controller model to obtain i during the fault period. rqH , change i rqH In the analytical expression for the fault current of a doubly fed wind turbine, i rq,ref ; Control logic for maintaining steady-state active power during faults; Substitute the corresponding active and reactive currents into the machine-side current command value i in the fault current analytical formula of the doubly-fed induction generator (DFIG) r,ref Obtain the analytical expression for the fault current of the doubly fed wind turbine corresponding to the high voltage fault and analyze the current.

[0292] During the fault ride-through when the Crowbar does not activate, substituting i into the analytical expression for the fault current of the doubly-fed induction generator (DFIG) r,ref The specific values ​​are determined by the actual stage in which the fault occurs.

[0293] When the Crowbar is activated, i.e., t > t2, the Crowbar resistor short-circuits the rotor winding, allowing the DFIG to continue operating as a squirrel-cage induction generator. At this time, the corresponding controller parameters and steady-state operating data are substituted into the fault current analysis formula of the doubly-fed induction generator to form the fault current analysis formula of the doubly-fed induction generator after the Crowbar is activated and the current is analyzed.

[0294] Optimization module: Under three-phase voltage dip or rise conditions, convert the three-phase current and three-phase voltage into dq-axis DC quantities, and set the objective function. Where a i For measured short-circuit current data, b i The result of the fault current analytical expression is given, where i is the number of sampling points and n is the total number of sampling points. The differential evolution intelligent algorithm is used to optimize the steady-state controller parameters. The optimization objective is to minimize the value of the objective function E, thus obtaining the final analytical expression describing the fault current of the doubly-fed wind turbine generator set.

[0295] The analytical formula for the fault current of the doubly fed wind turbine is as follows:

[0296]

[0297] Where k is the difference between the stator voltage after the fault and the voltage u before the fault. s0 The ratio; U s0 The stator-side steady-state voltage u before the fault s0 The amplitude; j represents the imaginary number; ω1 is the synchronous speed; T1 = L s / R s R s L s These are the resistance and inductance of the generator stator circuit, respectively; T2 = σL r / R ra Rra =R r +R a R r L r These are the resistance and inductance of the generator rotor circuit, R. a Crowbar resistor; L m σ represents the mutual inductance between the generator stator and rotor; t represents time; σ is the leakage magnetic coefficient of the fan; k pv k iv The proportional-integral coefficients of the outer loop control of the GSC voltage are respectively associated with control loop P and control loop I; ω r i is the rotor angular velocity; r0 The current for steady-state rotor operation; i g0 This refers to the grid-side current in steady state before the fault. i g,ref This is the commanded value of the GSC current in the three-phase stationary coordinate system; p is a differential operator.

[0298] The fault current analysis and parameter optimization system for doubly-fed wind turbines described in this embodiment is actually the computer system corresponding to Specific Embodiment 1. It should be understood that computer program products, software, or computerized methods, and the instructions of computer program products, software, or computerized methods can be used to program a computer system and run on a computer or other electronic device.

[0299] The above examples of the present invention are merely illustrative of the computational model and process of the present invention, and are not intended to limit the implementation of the present invention. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is impossible to exhaustively list all possible implementations here. Any obvious variations or modifications derived from the technical solutions of the present invention are still within the scope of protection of the present invention.

Claims

1. A method for fault current analysis and parameter optimization of a doubly-fed induction generator (DFIG) wind turbine, wherein the DFIG wind turbine uses two back-to-back converters connected via a DC link for AC excitation; the grid-side converter controller is abbreviated as GSC; the turbine-side converter controller is abbreviated as RSC; characterized in that... The method includes the steps of acquiring fault data based on the functions of RSC, GSC and Crowbar protection, and the steps of analyzing the fault current and optimizing the parameters of the doubly fed wind turbine. The steps for obtaining fault data based on the functions of RSC, GSC, and Crowbar protection include: The command values ​​for rotor voltage and rotor current are denoted as u. r,ref and i r,ref Construct the stator short-circuit current affected by RSC, where the d-axis and q-axis components of the rotor current command value are denoted as i. rq,ref i rd,ref The command values ​​of the d-axis and q-axis components of the GSC voltage and current are denoted as u. gd,ref u gq,ref and i gd,ref i gq,ref To construct the GSC output current during power grid faults; Determine the reactive current i during the fault period in the case of a low voltage fault. rqL and active current i rdL And the active current i restored after the low-voltage fault is cleared. rdre i rqL i rdL i during the corresponding fault period rq,ref i rd,ref i rdre i during the recovery period after fault clearance rd,ref For situations involving high-voltage faults, determine the corresponding reactive current i during the fault period. rqH i rqH i during the corresponding fault period rq,ref Control logic for maintaining steady-state active power during a fault; The subscript 'r' in the parameters indicates the parameters corresponding to the rotor side, which is the parameters corresponding to the machine side; the subscript 're' in the parameters indicates the parameters corresponding to the recovery process; the subscript 'ref' in the parameters indicates that the corresponding parameter is the reference value of the corresponding command value, which is the reference value of the corresponding parameter. The steps for analyzing and optimizing the fault current of a doubly-fed induction generator (DFIG) wind turbine include: S1. Perform voltage drop and / or voltage rise fault tests, and record the three-phase voltage and three-phase current at the fan outlet. S2. Fault ride-through controller parameter determination: Based on the three-phase voltage and three-phase current at the wind turbine outlet, the fundamental positive-sequence voltage and fundamental positive-sequence reactive current under voltage drop conditions are extracted, and the fundamental positive-sequence reactive current is mapped to i. rqL This leads to the low-voltage ride-through reactive power support coefficient k. qL Extract the changing active power during the fault power recovery process under voltage dip conditions, and determine the corresponding active power recovery rate k. id1 and k id2 Alternatively, extract the fundamental positive-sequence voltage and fundamental positive-sequence reactive current under voltage rise conditions, and assign the fundamental positive-sequence reactive current to i. rqH This leads to the high-voltage ride-through reactive power support coefficient k. qH ; S3. Substitute the PI controller parameters into the analytical formula for the fault current of the doubly fed fan and analyze the current: First, let k pr k ir k p_P k i_P k pv k iv k pg k ig Substitute into the analytical formula for the fault current of a doubly-fed induction generator; where k pr and k ir k is the proportional-integral coefficient of the inner loop PI control element of the RSC current; p_P and k i_P k represents the proportional-integral coefficient of the RSC active power outer loop controller. pv and k iv k represents the proportional-integral coefficient of the outer loop control of the GSC voltage system. pg and k ig The proportional-integral coefficient of the GSC current inner loop control loop; Assume the fault occurs at time t1 and the Crowbar activates at time t2; When the Crowbar is not activated (i.e., t1≤t≤t2), if a low-voltage fault ride-through occurs, the fault ride-through controller parameters determined in step S2 are substituted into the fault ride-through controller model to obtain the corresponding active current i during the fault period. rdL and reactive current i rqL And the active current i recovered after the low-voltage fault is cleared. rdre i rqL i rdL i during the corresponding fault period rq,ref i rd,ref i rdre i during the recovery period after fault clearance rd,ref The machine-side current command value i in the fault current analysis formula of the doubly fed wind turbine is respectively connected. r,ref Obtain the analytical expression for the fault current of the doubly fed wind turbine corresponding to the low voltage fault and analyze the current. When the Crowbar is not activated (i.e., t1≤t≤t2), if a high-voltage fault ride-through occurs, the fault ride-through controller parameters determined in step S2 are substituted into the fault ride-through controller model to obtain i during the fault period. rqH , change i rqH In the analytical expression of fault current of a doubly fed wind turbine, i rq,ref ; Control logic for maintaining steady-state active power during faults; Substitute the corresponding active and reactive currents into the machine-side current command value i in the fault current analytical formula of the doubly-fed induction generator (DFIG) r,ref Obtain the analytical expression for the fault current of the doubly fed wind turbine corresponding to the high voltage fault and analyze the current. During the fault ride-through when the Crowbar does not activate, substituting i into the analytical expression for the fault current of the doubly-fed induction generator (DFIG) r,ref The specific values ​​are determined by the actual stage in which the fault occurs. When the Crowbar is activated, i.e., t > t2, the Crowbar resistor short-circuits the rotor winding, allowing the DFIG to continue operating as a squirrel-cage induction generator. At this time, the corresponding controller parameters and steady-state operating data are substituted into the fault current analysis formula of the doubly-fed induction generator to form the fault current analysis formula of the doubly-fed induction generator after the Crowbar is activated and the current is analyzed. S4. Under three-phase voltage dip or rise conditions, convert the three-phase current and three-phase voltage into dq-axis DC quantities, and set the objective function. Where a i For measured short-circuit current data, b i The result of the fault current analytical expression is given, where i is the number of sampling points and n is the total number of sampling points. The differential evolution intelligent algorithm is used to optimize the steady-state controller parameters. The optimization objective is to minimize the value of the objective function E, thus obtaining the final analytical expression describing the fault current of the doubly-fed wind turbine generator set.

2. The method for fault current analysis and parameter optimization of doubly-fed wind turbine generators according to claim 1, characterized in that, The analytical formula for the fault current of the doubly fed wind turbine is as follows: Where k is the difference between the stator voltage after the fault and the voltage u before the fault. s0 The ratio; U s0 The stator-side steady-state voltage u before the fault s0 The amplitude; j represents the imaginary number; ω1 is the synchronous speed; T1 = L s / R s R s L s These are the resistance and inductance of the generator stator circuit, respectively; T2 = σL r / R ra R ra =R r +R a R r L r These are the resistance and inductance of the generator rotor circuit, R. a Crowbar resistor; L m σ represents the mutual inductance between the generator stator and rotor; t represents time; σ is the leakage magnetic coefficient of the fan; k pv k iv The proportional-integral coefficients of the outer loop control of the GSC voltage are respectively associated with control loop P and control loop I; ω r i is the rotor angular velocity; r0 The current for steady-state rotor operation; i g0 This refers to the grid-side current in steady state before the fault. i g,ref This is the commanded value of the GSC current in the three-phase stationary coordinate system; p is a differential operator.

3. The method for fault current analysis and parameter optimization of doubly-fed wind turbine generators according to claim 2, characterized in that, The doubly-fed asynchronous generator model of the electrical part of the doubly-fed wind turbine is as follows: A mathematical model of a doubly-fed asynchronous generator is established in a two-phase rotating dq coordinate system, and its voltage and flux linkage equations are as follows: In the formula: R s L s The resistance and inductance of the generator stator circuit; R r L r For the resistance and inductance of the generator rotor circuit; L m For the mutual inductance between the generator stator and rotor; ψ s,dq ψ r,dq For the stator and rotor dq shaft flux linkages of the generator; u s,dq u r,dq For the stator and rotor dq-axis voltages of the generator; i s,dq i r,dq ω represents the stator and rotor dq-axis currents of the generator; ω1 represents the synchronous speed; ω represents the slip speed. Then the DFIG mathematical model in the two-phase rotating coordinate system is transformed into the three-phase stationary coordinate system.

4. The method for fault current analysis and parameter optimization of doubly-fed wind turbine generators according to claim 3, characterized in that, The equations for the DFIG voltage and flux linkage after the Crowbar action are as follows:

5. The method for fault current analysis and parameter optimization of doubly-fed wind turbine generators according to claim 4, characterized in that, The machine-side converter controller, i.e., RSC, is as follows: RSC employs stator flux linkage-oriented vector control. The voltage command expression for RSC control is: u r,ref =k pr (i r,ref -i r,dq )+k ir ∫(i r,ref -i r,dq )dt+jωσL r i r,dq (3) In the formula: u r,ref i r,ref These are the command values ​​for rotor voltage and rotor current, respectively; k pr and k ir σ is the proportional-integral coefficient of the PI control loop; σ is the leakage flux coefficient of the fan. When the mains voltage drops and the Crowbar does not operate, the rotor current is controlled by RSC; the stator current is as follows: Stator steady-state flux linkage ψ before the fault s0 and stator flux linkage ψ after the fault sF as follows: In the formula, ψ s0 and ψ sF The values ​​are in a three-phase stationary coordinate system; τ1 is the decay time constant, τ1 = R s / L s ω1 = 2πf = 100π (rad / s); t represents time; ψ sF The subscript F represents the stator flux linkage ψ in the three-phase stationary coordinate system under fault conditions. s ; The relationship between rotor flux linkage and rotor current after a fault is as follows: ψ rF F (where F represents fault) represents the rotor flux linkage ψ in the three-phase stationary coordinate system under fault conditions. r； Based on the relationship between rotor flux and rotor current after the fault, In the formula, f s (ψ s ) represents the stator flux linkage ψ s A function, f s (ψ s )=(pψ sF +jωψ sF )L m / L s p represents the differential operator; Solving equation (8) yields the expression for the faulted rotor current. Combining the stator flux linkage and rotor current expressions, we obtain the stator current expression for a non-deep voltage drop: In the formula, i r0 The current is the rotor current in steady state; the values ​​of λ1, λ2, β1, and β2 are respectively... i sF Let i be the fault state represented by F. s,dq .

6. The method for fault current analysis and parameter optimization of doubly-fed wind turbine generators according to claim 5, characterized in that, The relationship between rotor flux linkage and rotor current after a fault is as follows:

7. The method for fault current analysis and parameter optimization of doubly-fed wind turbine generators according to claim 5, characterized in that, The grid-side converter controller, i.e., GSC, is as follows: The voltage outer loop control equation for GSC is: and gd =k pv (in dc -in dc,ref )+k iv ∫(in dc -in dc,ref )dt (10) Among them, i gd For the d-axis component of the GSC current, u dc u dc,ref These are the DC voltage value and DC voltage command value of the outer loop of the grid-side converter voltage control, respectively; The inner current control equation is: In the formula, L g For the filter equivalent inductance, u dc i is the DC bus voltage. gq For the q-axis component of the GSC current, u s For the grid-side fundamental positive sequence voltage, u gq,ref =S(t)u dc S(t) is the switching function of GSC; The power balance equation for GSC is: P-1.5u gd i gd =u dc C bus pu dc (12) In the formula, C bus P is the DC bus capacitor, P is the power value of the wind turbine, and p is the differential operator; Combining equations (10) to (12), the current during a symmetrical fault is obtained as follows: i g Let i be the value of the GSC current in the three-phase stationary coordinate system. gd0 This refers to the d-axis component of the GSC current during steady-state operation. The basic voltage equation for GSC is: u g =R r i g +L g (pi g +jω1i g )+u s (14) Among them, u g This represents the GSC voltage in a three-phase stationary coordinate system. Using equations (11) and (14), the current differential equation for the GSC is obtained: Among them, R g For the filter equivalent resistance, L g p is the equivalent inductance for filtering; 2 Indicates the second derivative; The analytical expression for the output current of the GSC is obtained by solving: In the formula, i g,ref The commanded value of the GSC current in the three-phase stationary coordinate system is given by i. gd,ref and i gq,ref It is obtained by transformation matrix transformation from two-phase rotating dq coordinate system to three-phase stationary ABC coordinate system; i g0 This represents the GSC current in steady state before the fault.

8. The method for fault current analysis and parameter optimization of a doubly-fed induction generator (DFIG) wind turbine according to claim 7, characterized in that, The mathematical model of the fault ride-through controller is as follows: A. When a low-voltage fault occurs in the power grid at time t1, the wind turbine detects that the system voltage has dropped below the threshold and switches to the low-voltage ride-through control logic. This means that the power outer loop of the rotor-side controller is disconnected, and its control structure is switched to the fault control structure, followed by the current inner loop. After the low-voltage ride-through fault occurs, the wind turbine injects reactive current into the power grid to support the recovery of the grid voltage. The dynamic reactive current output by the wind farm to the power grid is: i sqL =k qL ·(0.9-u s )I N (17) Reference value of rotor-side reactive current i during the fault period rqL as follows: In equations (17) and (18), k qL The low-voltage ride-through reactive power support factor is determined by the grid connection standard; I N The rated current of the unit; u s This is the fundamental positive sequence voltage on the grid side; During a fault, the wind turbine effectively generates reactive power to support the grid voltage. The active current is limited by the reactive current output. The reference value i for the rotor-side active current during a fault is... rdL for: In the formula, i rdL This serves as a reference value for the active current during a fault. i drN This represents the steady-state value of the active current before the fault occurred. I rmax The maximum rotor current; k p_P k is the proportional gain of the active power outer loop controller. i_P P is the integral coefficient of the active power outer loop controller; ref P is the active power reference value, where P is the power value of the wind turbine. When the power grid is at t b When the low-voltage fault is cleared, the voltage returns to its pre-fault level; at this time, the reactive power instantly recovers to its steady-state level after the fault is cleared; the rotor-side active current reference value i recovers at a certain recovery rate. rdre : i rdre =k p_P (P re,ref -P)+k i_P ∫(P re,ref -P)dt (20) In the formula, P re,ref This is a reference value for active power during fault recovery; During fault recovery, P re,ref The expression is: Where, k id1 ,k id2 These represent two different active power recovery rates; P fault t represents the steady-state active power generated by the DFIG during a fault. b t represents the fault clearing time; c P represents the starting point of the active power recovery phase with the second slope; eF P represents the active power value of the DFIG during fault recovery. eN This refers to the steady-state active power generated before the DFIG fault. B. When a high-voltage fault occurs in the power grid at time t1, the wind turbine detects that the system voltage has risen above the threshold and switches to high-voltage ride-through control logic. After a high-voltage ride-through fault occurs, the wind turbine absorbs reactive current from the power grid to reduce the impact of the voltage rise. The reactive current needs to be adjusted according to the degree of voltage rise. During the fault, the dynamic reactive current absorbed by the wind farm from the power grid is: i sqH =k qH ·(u s -1.1)I N (22) Reference value of rotor-side reactive current i during the fault period rqH as follows: In equations (22) and (23), k qH The high-voltage ride-through reactive power support factor is determined by the grid connection standard; I N This refers to the rated current of the unit; Control logic for maintaining steady-state active power during a fault; When the power grid is at t b The control logic ensures that the high-voltage fault is cleared and the voltage returns to normal; the active power remains in a steady state, and the reactive power instantly returns to a steady-state level after the fault is cleared.

9. The method for fault current analysis and parameter optimization of a doubly-fed induction generator (DFIG) wind turbine according to claim 8, characterized in that, The stator short-circuit current under the action of Crowbar protection is as follows: The expression for the stator current at times t1 to t2 is the same as that in equation (9); In the formula, ψ sF2 Let F2 represent the stator flux linkage ψ in the three-phase stationary coordinate system under the second-stage fault condition. s ; After the Crowbar is triggered, the differential equation concerning the rotor flux linkage is obtained as follows: Solve the differential equation (25), where the unknown is ψ. r,dq The solution is denoted as ψ. ra The rotor flux linkage expression is obtained by solving for: In the formula, τ2 is the attenuation constant, τ2=R ra / σL r Combined with the initial state expression of the flux linkage during Crowbar operation (24), the initial value C1 of the DC component with τ2 as the decay time constant is obtained. in Then, the stator current i after Crowbar operation is obtained from the flux linkage equation. sF2 : In the formula, 10. A fault current analysis and parameter optimization system for doubly-fed induction generator (DFIG) wind turbines, wherein the DFIG wind turbine uses two back-to-back converters connected via a DC link for AC excitation; the grid-side converter controller is abbreviated as GSC; the turbine-side converter controller is abbreviated as RSC; characterized in that... The system includes: Fault data acquisition unit: acquires fault data based on the functions of RSC, GSC, and Crowbar protection. The specific process includes: The command values ​​for rotor voltage and rotor current are denoted as u. r,ref and i r,ref Construct the stator short-circuit current affected by RSC, where the d-axis and q-axis components of the rotor current command value are denoted as i. rq,ref i rd,ref The command values ​​of the d-axis and q-axis components of the GSC voltage and current are denoted as u. gd,ref u gq,ref and i gd,ref i gq,ref To construct the GSC output current during power grid faults; Determine the reactive current i during the fault period in the case of a low voltage fault. rqL and active current i rdL And the active current i restored after the low-voltage fault is cleared. rdre i rqL i rdL i during the corresponding fault period rq,ref i rd,ref i rdre i during the recovery period after fault clearance rd,ref For situations involving high-voltage faults, determine the corresponding reactive current i during the fault period. rqH i rqH i during the corresponding fault period rq,ref Control logic for maintaining steady-state active power during a fault; The subscript 'r' in the parameters indicates the parameters corresponding to the rotor side, which is the parameters corresponding to the machine side; the subscript 're' in the parameters indicates the parameters corresponding to the recovery process; the subscript 'ref' in the parameters indicates that the corresponding parameter is the reference value of the corresponding command value, which is the reference value of the corresponding parameter. Current analysis and parameter optimization unit: This unit analyzes and optimizes the fault current of doubly-fed induction generator (DFIG) wind turbines, including: Three-phase voltage and current acquisition module: Based on voltage drop and / or voltage rise fault tests, obtain the three-phase voltage and three-phase current at the fan outlet; Fault ride-through controller parameter determination module: Based on the three-phase voltage and three-phase current at the wind turbine outlet, extract the fundamental positive-sequence voltage and fundamental positive-sequence reactive current under voltage drop conditions, and map the fundamental positive-sequence reactive current to i. rqL This leads to the low-voltage ride-through reactive power support coefficient k. qL Extract the changing active power during the fault power recovery process under voltage dip conditions, and determine the corresponding active power recovery rate k. id1 and k id2 Alternatively, extract the fundamental positive-sequence voltage and fundamental positive-sequence reactive current under voltage rise conditions, and assign the fundamental positive-sequence reactive current to i. rqH This leads to the high-voltage ride-through reactive power support coefficient k. qH ; Doubly fed wind turbine fault current analysis module: First, k pr k ir k p_P k i_P k pv k iv k pg k ig Substitute into the analytical formula for the fault current of a doubly-fed induction generator; where k pr and k ir k is the proportional-integral coefficient of the inner loop PI control element of the RSC current; p_P and k i_P k represents the proportional-integral coefficient of the RSC active power outer loop controller. pv and k iv k represents the proportional-integral coefficient of the outer loop control of the GSC voltage system. pg and k ig The proportional-integral coefficient of the GSC current inner loop control loop; Assume the fault occurs at time t1 and the Crowbar activates at time t2; When the Crowbar is not activated (i.e., t1≤t≤t2), if a low-voltage fault ride-through occurs, the fault ride-through controller parameters determined in step S2 are substituted into the fault ride-through controller model to obtain the corresponding active current i during the fault period. rdL and reactive current i rqL And the active current i recovered after the low-voltage fault is cleared. rdre i rqL i rdL i during the corresponding fault period rq,ref i rd,ref i rdre i during the recovery period after fault clearance rd,ref The machine-side current command value i in the fault current analysis formula of the doubly fed wind turbine is respectively connected. r,ref Obtain the analytical expression for the fault current of the doubly fed wind turbine corresponding to the low voltage fault and analyze the current. When the Crowbar is not activated (i.e., t1≤t≤t2), if a high-voltage fault ride-through occurs, the fault ride-through controller parameters determined in step S2 are substituted into the fault ride-through controller model to obtain i during the fault period. rqH , change i rqH In the analytical expression of fault current of a doubly fed wind turbine, i rq,ref ; Control logic for maintaining steady-state active power during faults; Substitute the corresponding active and reactive currents into the machine-side current command value i in the fault current analytical formula of the doubly-fed induction generator (DFIG) r,ref Obtain the analytical expression for the fault current of the doubly fed wind turbine corresponding to the high voltage fault and analyze the current. During the fault ride-through when the Crowbar does not activate, substituting i into the analytical expression for the fault current of the doubly-fed induction generator (DFIG) r,ref The specific values ​​are determined by the actual stage in which the fault occurs. When the Crowbar is activated, i.e., t > t2, the Crowbar resistor short-circuits the rotor winding, allowing the DFIG to continue operating as a squirrel-cage induction generator. At this time, the corresponding controller parameters and steady-state operating data are substituted into the fault current analysis formula of the doubly-fed induction generator to form the fault current analysis formula of the doubly-fed induction generator after the Crowbar is activated and the current is analyzed. Optimization module: Under three-phase voltage dip or rise conditions, convert the three-phase current and three-phase voltage into dq-axis DC quantities, and set the objective function. Where a i For measured short-circuit current data, b i The result of the fault current analytical expression is given, where i is the number of sampling points and n is the total number of sampling points. The differential evolution intelligent algorithm is used to optimize the steady-state controller parameters. The optimization objective is to minimize the value of the objective function E, thus obtaining the final analytical expression describing the fault current of the doubly-fed wind turbine generator set.

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