Fault current analysis of direct-drive wind turbine and optimization method of its parameters

Abstract

This invention relates to the field of power technology and its method for analyzing fault currents and optimizing parameters in direct-drive wind turbines. To address the problems of complex modeling, low simulation efficiency, and the inability of existing fault current analytical formulas to accurately reflect the relationship between control parameters and fault current in direct-drive wind turbine fault characteristic analysis, this invention first establishes accurate and detailed models of the main circuit, steady-state controller, and fault ride-through controller. Then, through mathematical model derivation, a detailed analytical formula for the fault current of the direct-drive wind turbine is obtained. Finally, using a differential evolution intelligent algorithm, based on field-measured fault current waveforms of the direct-drive wind turbine, the analytical formula for the fault current of the direct-drive wind turbine is optimized. This invention is used for fault current analysis in direct-drive wind turbines.

Description

Technical Field

[0001] This invention belongs to the field of power technology, specifically relating to a method for calculating fault current of a direct-drive wind turbine. Background Technology

[0002] With the large-scale grid connection of wind turbines in my country, wind turbines have become a major force in power supply in new power systems. Among them, direct-drive wind turbines have become the mainstream model due to their excellent stable operation. Unlike traditional synchronous generators, the steady-state operating characteristics and fault characteristics of direct-drive wind turbines are controlled by time-varying power electronic switching devices and controllers, making it difficult to obtain analytical expressions for their fault currents. Protection setting and safety and stability analysis of power systems dominated by new energy sources rely on complex electromagnetic modeling and numerical solutions under fault conditions, which significantly affects the efficiency of power system analysis and cannot meet the safety and stability analysis requirements of power grids with a high proportion of new energy sources. Therefore, there is an urgent need for analytical expressions for fault currents that can accurately characterize the fault currents of direct-drive wind turbines and optimization methods for their parameters.

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

[0004] 1. The article "Transient Analysis of Wind and Solar Power Faults and Practical Calculation of Peak Fault Current" by Jia Ke et al. was first published online. This article analyzes the transient fault processes of photovoltaic, direct-drive wind turbines, and doubly-fed induction generator (DFIG) wind turbines. For direct-drive turbines, the article calculates the peak fault current from the steady-state current values ​​before and after the fault. Through comparison of waveform data, the method demonstrates high accuracy. However, the proposed expression for calculating the peak fault current is disconnected from the control parameters and cannot accurately reflect the relationship between the actual parameters and the analytical expression for the fault current.

[0005] 2. Song Guobing et al. published "Analysis of Three-Phase Fault Current Characteristics of Direct-Drive Wind Turbines" Journal of Xi'an Jiaotong University, 2015, 49(10):1-7. This article analyzes the mathematical expression and operating characteristics of the grid-side converter of a direct-drive wind turbine, and concludes that the grid-side converter is a power balance system under control. After reasonable order reduction and simplification, the influence of each parameter of the grid-side controller is sorted out, and an approximate analytical expression for the three-phase fault current is obtained.

[0006] 3. Kuang Xiaoyun et al. published "A Full-Time Domain Fault Current Calculation Method Applicable to Networks Containing New Energy Inverter Power Sources," Electric Power Automation Equipment, 2020, 40(05):113-122. This article analyzes two different time scales: steady state and transient state. In the steady-state calculation method, the new energy inverter power source is regarded as a controlled current source, which solves the shortcomings of traditional methods in considering the inverter power source control strategy and nonlinear characteristics. This method corrects the output power of the inverter power source by iteratively calculating the voltage of each node in the network, thereby indirectly considering the mutual influence of each power source in the network. However, it is not fully applicable to the fault current analysis of direct-drive wind turbines.

[0007] In summary, existing methods have not derived a true relationship between control parameters and fault current analytical expressions for direct-drive wind turbines. The simplified calculation results obtained from these methods negatively impact the accuracy and applicability of the approximate analytical expressions for three-phase fault currents. Therefore, there is an urgent need for an accurate and efficient analytical expression for fault currents in direct-drive wind turbines and a method for optimizing its parameters. Summary of the Invention

[0008] The purpose of this invention is to solve the problems of existing direct-drive wind turbine fault characteristic analysis relying on commercial software and having complex modeling and low simulation efficiency, as well as the problem that the fault current analytical formula cannot reflect the accurate relationship between control parameters and fault current.

[0009] Fault current analysis and parameter optimization methods for direct-drive wind turbines include:

[0010] Direct-drive wind turbines utilize a set of back-to-back converters to inject wind power into the AC grid. The back-to-back converters decouple the permanent magnet synchronous motor from the grid. The grid-side converter controller is abbreviated as the grid-side controller, and a corresponding steady-state controller model is constructed. The grid-side active and reactive current reference values ​​in the steady-state controller model are denoted as i. gdref and i gqref i gdref i gqref These are the grid-side current reference values ​​i gref The real and imaginary parts; simultaneously, a fault-crossing controller model is constructed. During the construction of the fault-crossing controller model...

[0011] Determine the reactive current i during the fault period in the case of a low voltage fault. gqL and active current i gpL And the active current i restored after the low-voltage fault is cleared. gpre i gqL i gpL i during the corresponding fault period gqref i gdref i gpre i after corresponding fault clearance gdrefFor situations involving high-voltage faults, determine the corresponding reactive current i during the fault period. gqH i gqH i during the corresponding fault period gqref The control logic for maintaining steady-state active power during a fault; the reactive power recovers to steady-state level instantaneously after the high-voltage fault is cleared.

[0012] Then, based on the closed-loop transfer function of the inner current loop and the analytical expression of the fault current of the direct-drive fan, current analysis and parameter optimization are performed:

[0013] S1. Perform voltage drop and voltage rise fault tests on actual direct-drive 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.

[0014] S2. Fault ride-through controller parameter determination:

[0015] Extract the fundamental positive-sequence voltage and fundamental positive-sequence reactive current under voltage sag conditions, and assign the fundamental positive-sequence reactive current to i. gqL This leads to the low-voltage ride-through reactive power support coefficient k. qL ;or,

[0016] 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. gqH This leads to the high-voltage ride-through reactive power support coefficient k. qH ;

[0017] S3. Determination of steady-state controller parameters:

[0018] Using classical engineering tuning methods, the closed-loop transfer function of the inner current loop is tuned to a typical Type I system, resulting in... Where ξ=(R g +k p ) / (2ω n L g ) is the damping ratio. L is the undamped natural oscillation angular frequency. g and R g The inductance and resistance of the filter in the inner current loop control after feedforward compensation; K pwm For the equivalent gain of pulse width modulation, T s The carrier period;

[0019] S4. The k determined in step S3... p and k i Substitute the analytical formula for the fault current of the direct-drive fan;

[0020] When 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 i. gpL and i gpre , change i gpL and i gpre The grid-side current reference value i in the fault current analysis formula of the direct-drive wind turbine is respectively connected. gref The analytical expression for the fault current of the direct-drive fan corresponding to the low-voltage fault is obtained; under full-power, three-phase voltage drop operating conditions, based on i gqL Obtain the maximum current I max ;

[0021] When 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. gqH , change i gqH In the analytical expression for fault current of a direct-drive fan, i gref We obtain the analytical expression for the fault current of the direct-drive fan corresponding to the high-voltage fault;

[0022] S5. Under three-phase voltage sag conditions, convert the three-phase current and three-phase voltage into dq-axis DC quantities, and set the objective function. Where a i b is the measured grid-side current data. i The result of the fault current analytical expression is given by i, 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 fault current analytical expression describing the direct-drive wind turbine generator.

[0023] Furthermore, the steady-state controller model is as follows:

[0024] The grid-side controller uses grid voltage-oriented vector control for its inner and outer loop controllers. The d-axis maintains the stability of the DC voltage to achieve power balance on both sides of the converter, while the q-axis controls the reactive power exchange between the unit and the grid.

[0025] The frequency domain model of the converter output voltage is as follows:

[0026]

[0027] In the formula, k p and k i These are the proportional and integral coefficients of the inner loop controller of the network-side controller, i. gdref and i gqref These are the reference values ​​for active and reactive currents on the grid side, respectively; s is the Laplace operator in the frequency domain; the inner and outer loops contain the d and q axes, with the d-axis having an outer loop and the q-axis not; the active current command for the d-axis is i. gdref .

[0028] Furthermore, in the steady-state state prior to the fault, the active current command along the d-axis in the steady-state controller model is as follows:

[0029]

[0030] In the formula, k pv and k iv V represents the proportional and integral coefficients of the outer loop controller of the network-side controller. bus_ref and V bus These are the reference and actual values ​​for DC voltage.

[0031] Furthermore, in the case of a low-voltage fault, the corresponding reactive current i during the fault period is... gqL as follows:

[0032]

[0033] In the formula, k qL For low voltage ride-through reactive power support factor; I n The rated current of the unit, u s It is the fundamental positive sequence voltage.

[0034] Furthermore, in the case of a low-voltage fault, the corresponding active current i during the fault period... gpL as follows:

[0035]

[0036] In the formula, i gpL This is a reference value for the active current during the fault; i pnormal I represents the steady-state value of the active current before the fault occurred. max This is the maximum current.

[0037] Furthermore, in the event of a low-voltage fault, the active current i restored after the low-voltage fault is cleared is... gpre as follows:

[0038]

[0039] In the formula, i p_fault t represents the steady-state active current during the fault; t represents time; t2 represents the moment when the low-voltage fault is cleared; t3 represents the moment when the first stage of active power increase after the low-voltage fault is cleared ends, and the slope of the corresponding active power is k1; t3 represents the moment when the second stage of active power increase after the low-voltage fault is cleared ends, that is, the moment when it returns to the level before the fault, and the slope of the corresponding active power is k2; the first stage and the second stage of active power increase are distinguished according to the slope of the active power.

[0040] Furthermore, in the case of a high-voltage fault, the corresponding reactive current i during the fault period is... gqH as follows:

[0041]

[0042] k qH For high voltage ride-through reactive power support factor; I n This is the rated current of the unit.

[0043] Furthermore, the closed-loop transfer function of the inner current loop is as follows:

[0044]

[0045] Among them, i gd For the d-axis current of the grid-side controller; i gdref The grid-side active current reference value is given; s is the Laplace operator in the frequency domain; k p and k i These are the proportional and integral coefficients of the grid-side controller's inner loop current controller, respectively; L g and R g The inductance and resistance of the filter in the inner current loop control after feedforward compensation.

[0046] Furthermore, S3 tunes the closed-loop transfer function of the inner current loop to fit a typical Type I system, and the tuned form is as follows:

[0047]

[0048] In the formula, Let ξ represent the angular frequency of undamped natural oscillation, and ξ = (R g +k p ) / (2ω n L g ) indicates the damping ratio.

[0049] Furthermore, the analytical formula for the fault current of a direct-drive fan is as follows:

[0050]

[0051] Where i0 is the current before the fault.

[0052] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0053] Improved analytical efficiency and independence: By establishing a mathematical model, this invention eliminates reliance on commercial software, thereby improving the efficiency and flexibility of fault characteristic analysis for direct-drive wind turbines. This independent analysis method allows for faster and more sensitive assessment of various situations, especially in real-world operating environments requiring rapid response.

[0054] More accurate control parameter reflection: This invention, through precise and detailed mathematical models of the main circuit, steady-state controller, and fault ride-through controller, can more accurately reflect the relationship between control parameters and fault current. This accuracy represents a significant improvement over existing technologies, providing more precise fault characteristic analysis and stronger support for the design and operation of wind turbine units.

[0055] Intelligent Algorithm Optimization: By using a differential evolution intelligent algorithm to optimize the fault current analytical formula of the direct-drive wind turbine based on field measurement data, this invention can more accurately simulate and predict fault current conditions in actual operation. The application of this intelligent algorithm makes the model not only theoretically accurate but also more effective and applicable in practical applications.

[0056] Enhancing the application value of field data: By utilizing field-measured fault current waveform data of direct-drive wind turbines, this invention strengthens the connection between theory and practical operation. This method, which combines measured data, improves the applicability and reliability of the theoretical model, making the analysis results closer to actual operating conditions.

[0057] Overall, this invention provides a more efficient, accurate, and practical method for fault characteristic analysis of direct-drive wind turbines, which plays an important role in promoting the technological development and practical application of wind power. Attached Figure Description

[0058] 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.

[0059] Figure 2 This is a block diagram of the inner and outer loop control of the grid-side converter of the present invention, where ω s For grid synchronization speed, PLL is a phase-locked loop, SPWM is sinusoidal PWM modulation, dq-abc is the transformation between the synchronization coordinate axis and the three-phase coordinate axis, u ga u gb u gc For the converter output voltage modulation wave, i ga i gb i gc θ is the sampled value of the grid current. r The phase angle of the power grid locked by the PLL.

[0060] Figure 3 This is the power response curve for the entire low-voltage fault ride-through process of the present invention.

[0061] Figure 4 For the low-pass control logic of the present invention, i in the figure pnormal i represents the steady-state active current value. qnormalThis is the steady-state reactive current value.

[0062] Figure 5 This is the power response curve for the entire high-voltage fault ride-through process of the present invention.

[0063] Figure 6 This is the high-penetration control logic of the present invention.

[0064] Figure 7 This is a schematic diagram of the active power recovery process of the present invention.

[0065] Figure 8 This is a block diagram of the current inner loop control of the present invention.

[0066] Figure 9 This is a flowchart of the fault current parameter optimization method for direct-drive wind turbines according to the present invention. Detailed Implementation

[0067] 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 direct-drive wind turbine is obtained. Finally, based on the measured fault current waveforms of the direct-drive wind turbine in the field, the analytical expression for the fault current of the direct-drive wind turbine is optimized using a differential evolution intelligent algorithm. The invention will be described in detail below with reference to specific implementation methods. Specific implementation method one:

[0069] This embodiment describes a method for analyzing fault currents and optimizing parameters in direct-drive wind turbine generators, including the following steps:

[0070] First, direct-drive wind turbines utilize a set of back-to-back converters to inject wind power into the AC grid. This configuration offers significant advantages compared to doubly-fed induction generators (DFIGs) or other partially power-converter wind turbines. Because direct-drive wind turbines have a lower synchronous speed, they can be directly connected to the motor rotor, eliminating the need for a gearbox for speed matching. This design simplifies the drivetrain, significantly improves system efficiency, reduces mechanical noise, and drastically reduces turbine failures caused by gearbox issues. Furthermore, the amount and cost of post-failure maintenance are greatly reduced, effectively improving system reliability and extending turbine lifespan.

[0071] Direct-drive wind turbines consist of 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 permanent magnet synchronous generator (PMSG), a back-to-back converter, and a control unit. The back-to-back converter includes a machine-side converter and a grid-side converter, whose main function is to convert DC voltage and current into AC voltage and current, enabling bidirectional energy flow, including rectification mode (from AC side to DC side) and inverter mode (from DC side to AC side). The permanent magnet synchronous generator is connected to the machine-side converter to convert AC to DC, and the grid-side converter converts DC to AC and connects it to the power grid. Figure 1 As shown. Through these sophisticated converter technologies, direct-drive wind turbines can effectively improve converter efficiency and reliability while reducing system costs.

[0072] 1. Mathematical Model of Electrical Components

[0073] A permanent magnet synchronous generator (PMSG) is a type of motor that differs from an asynchronous motor. It gets its name from the fact that the rotational speed of the electromagnetic field generated by its stator windings is synchronized with the speed of the rotor. Therefore, when a synchronous motor is running, if the stator's power supply frequency *f* remains constant, the rotor's steady-state speed is constant and independent of the load. PMSGs offer advantages such as small size, light weight, high efficiency, no rotor overheating, no need for external excitation, and simple control. Furthermore, the rapid development of control theory and power electronics technology in recent years has provided a foundation for the control of synchronous motors, leading to their widespread attention both domestically and internationally.

[0074] The stator of a permanent magnet synchronous motor has three-phase symmetrical windings, and the rotor is excited by permanent magnet poles. The electromagnetic field generated by the stator windings and the magnetic field generated by the rotor permanent magnets are coupled through the air gap. The two have relative motion and have a very complex electromagnetic relationship.

[0075] The mathematical model for the permanent magnet synchronous generator is as follows:

[0076]

[0077] In the formula: L sq L sd L0—Self-inductance of generator stator d, q, and 0-axis coils; ψ sd ψ sq ψ s0 For the stator d, q, and 0 axis flux linkages of the generator; i sd i sq i s0 For the generator stator d, q, and 0-axis currents; ψ f It is a permanent magnet flux linkage.

[0078] Stator voltage equations in dq coordinate system:

[0079]

[0080] In the formula: U sd and U sq R represents the d-axis and q-axis voltages of the generator stator. s For stator current, ω e The value is the rotational speed.

[0081] Substituting formula (1) into formula (2) yields:

[0082]

[0083] As can be seen from formula (3), the mathematical model of the motor is greatly simplified in the two-phase rotating coordinate system, which provides great convenience for analyzing the control problem of permanent magnet synchronous generator.

[0084] Equation of electromagnetic torque in rotating coordinate system:

[0085]

[0086] In the formula: p is the pole logarithm.

[0087] According to the instantaneous power correlation theory, the generator outputs instantaneous active power P in the d-q rotating coordinate system. s and reactive power Q s It can be calculated that:

[0088]

[0089] The mathematical model between the grid-side converter output voltage and the grid voltage can be expressed as:

[0090]

[0091] In the formula, u gd u gq These are the d-axis and q-axis voltages at the converter output, respectively, R g For the filter resistor, L g For the filter resistor, i gd i gq These are the d-axis and q-axis currents of the grid-side controller, respectively. gd v gq These are the d-axis and q-axis voltages of the power grid, respectively, ω s To achieve grid synchronization speed.

[0092] The electrical part mathematical model provides a model for implementing the subsequent process, but in fact, formulas (1)-(5) do not directly participate in the subsequent processing.

[0093] 2. Mathematical model of steady-state controller on the grid side

[0094] Back-to-back converters decouple the permanent magnet synchronous motor from the grid. Therefore, the fault current external characteristics of the direct-drive wind turbine are determined by the grid-side converter controller. The grid-side converter controller is simply referred to as the grid-side controller. The inner and outer loop controllers of the grid-side controller adopt grid voltage-oriented vector control, based on PI control and feedforward decoupling control. The d-axis maintains the stability of the DC voltage to achieve power balance on both sides of the converter, and the q-axis controls the reactive power exchange between the unit and the grid. The frequency domain model of the converter outlet voltage (inner loop controller) is shown in formula (7):

[0095]

[0096] In the formula, k p and k i These are the proportional and integral coefficients of the grid-side controller's inner loop current controller, i. gdref and i gqref These are the reference values ​​for active and reactive currents on the grid side, respectively, and s is the Laplace operator in the frequency domain. The inner and outer loops encompass the d- and q-axis; the d-axis has an outer loop, while the q-axis does not. The active current command for the d-axis is i. gdref ;

[0097] The active current command on the grid-side d-axis is generated by the outer-loop DC voltage controller to control DC voltage stability and converter power balance. Therefore, the frequency domain model of the outer-loop controller of the grid-side controller can be expressed as:

[0098]

[0099] In the formula, k pv and k iv V represents the proportional and integral coefficients of the outer loop controller of the network-side controller. bus_ref and V bus These are the reference and actual values ​​for DC voltage.

[0100] The important role of the controller is to control the generator-side converter and the grid-side converter. The controller of the grid-side converter is divided into a steady-state controller and a fault ride-through controller.

[0101] The constructed wind turbine uses a grid-side converter to control the bus voltage. The control block diagram of the grid-side converter is shown below. Figure 2 As shown. The outer control loop is the DC bus voltage loop. The deviation between the given value and the actual value of the DC bus voltage is output as the given current i through the PI regulator. gdref Then, through the inner loop control of the current, the output is set to a given voltage via a PI regulator. Reactive power control is achieved by controlling the q-axis component. The control logic in the model is visible. Figure 2 The control structure can be seen in Table 1.

[0102] Table 1. Introduction to the Control Structure of the Grid-Side Converter for Direct-Drive Wind Turbines

[0103]

[0104] 3. Mathematical model of the fault ride-through controller on the network side

[0105] 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.

[0106] The low-voltage fault ride-through response curves of active and reactive power of wind turbine units are as follows: Figure 3 As shown.

[0107] analyze Figure 3 It can be seen that during the 0-t0 period, the wind turbine operates under steady-state conditions. 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.

[0108] A. When a low-voltage fault occurs in the power grid at time t0, the wind turbine detects that the system voltage has dropped below the threshold and switches to low-voltage ride-through control logic. According to my country's grid connection standards, after a low-voltage ride-through fault occurs, the wind turbine needs to have the ability to inject reactive current into the grid to support grid voltage recovery. The reactive current needs to be adjusted according to the voltage drop depth.

[0109]

[0110] At this point, i in the formula gqL The value assigned to i gqref In the formula, k qL The low-voltage ride-through reactive power support factor is determined by the grid connection standard; u s_set2 u s_set1 The reactive voltage threshold is determined by the grid connection standard; I n The rated current of the unit, u s It is the fundamental positive sequence voltage.

[0111] According to my country's grid connection standards, the values ​​in formula (9) can be:

[0112]

[0113] To ensure that wind turbines can effectively generate reactive power to support grid voltage during faults, most mainstream turbines adopt reactive power priority control, meaning that active current is limited by reactive current output.

[0114]

[0115] At this point, i in the formula will be... gpL The value assigned to i gdrefIn the formula, i gpL This is a reference value for the active current during the fault; i pnormal I represents the steady-state value of the active current before the fault occurred. max This represents the maximum current. Based on the above discussion, the low-voltage fault control logic can be derived as follows: Figure 4 .

[0116] The high-voltage fault ride-through response curves of the active and reactive power of the wind turbine are as follows: Figure 5 As shown.

[0117] analyze Figure 5 It can be seen that during the 0-t0 period, the wind turbine operates under steady-state conditions. 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.

[0118] When the low-voltage fault is cleared at time t2, the voltage returns to its pre-fault level. At this time, reactive power mostly recovers to its steady-state level instantaneously after the fault is cleared. Active power, however, mainly recovers in two ways: instantaneously to its pre-fault level or slowly increases at a certain recovery rate. The reference value for active current recovering at a certain recovery rate can be found as follows:

[0119]

[0120] At this point, i in the formula will be... gpre Value assigned to i gdref In the formula, i p_fault The current represents the steady-state active current during the fault; t represents time; t2 represents the moment the low-voltage fault is cleared; t3 represents the moment the first stage of active power increase after the low-voltage fault is cleared ends, corresponding to an active power slope of k1; t3 represents the moment the second stage of active power increase after the low-voltage fault is cleared ends, that is, the moment when the active power returns to the pre-fault level, corresponding to an active power slope of k2. The first and second stages of active power increase are distinguished by the slope of the active power. Its power trend can be seen... Figure 7 The time intervals from t0 to t4.

[0121] In reality, the active power increase after the low voltage fault is cleared can only take one stage. At this time, it is actually equivalent to k1 and k2 being equal. Therefore, it is only necessary to make k1 and k2 equal. The above formula also applies.

[0122] B. When a high-voltage fault occurs in the power grid at time t0, 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.

[0123]

[0124] At this point, i in the formula will be... gqH Value assigned to i gqref ;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 generator set. Active power generally does not change during faults, maintaining the control logic in steady state.

[0125] When the high-voltage fault in the power grid is cleared at time t2, the voltage returns to normal. At this time, the reactive power instantly returns to the steady-state level after the fault is cleared.

[0126] Based on the above discussion, the high-voltage fault control logic can be obtained as follows: Figure 6 As shown.

[0127] 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.

[0128] 4. Analytical Formula for Fault Current of Direct-Drive Fans

[0129] Because of u in equation (7) gd (s) The switching signal needs to be modulated before being sent to the converter, therefore u in formula (7) gd (s) and u in formula (6) gd There is an inertial element between (s). To simulate this characteristic, u in formula (7) gd (s) Adding an inertial element 1 / (sT) s +1) and K pwm / (0.5sT s +1) and then substitute it into formula (6), such as Figure 8 As shown, Figure 8 The block diagram of the current inner loop control after feedforward compensation is shown. In this block diagram, the control of the current inner loop is implemented through a proportional-integral (PI) controller, where k... p and k i These represent the proportional and integral parameters of the current inner-loop controller (PI controller), respectively. The equivalent gain of PWM (Pulse Width Modulation) is represented by K. pwm It means, and T s This represents the carrier period. Furthermore, the filter's inductance and resistance are represented by L. g and R g express.

[0130] i grefThis is the grid-side current reference value, and its actual part is i. gdref The imaginary part is i gqref In dq axis control, the control method and parameters are the same.

[0131] merge Figure 8 Inertial element 1 / (sT) s +1) and K pwm / (0.5sT s +1) becomes K pwm / (1.5sT s +1), since the switching frequency modulated by the controller is relatively high, it has little impact on the external characteristics of the fan. Therefore, the dynamics of the PWM in the figure are ignored, making K... pwm =1, the closed-loop transfer function of the inner current loop becomes:

[0132]

[0133] When formula (14) is tuned to a typical Type I system, the transfer function becomes:

[0134]

[0135] In the formula, Let ξ represent the angular frequency of undamped natural oscillation, and ξ = (R g +k p ) / (2ω n L g ) indicates the damping ratio.

[0136] When a fault occurs, the current does not change abruptly at the moment of the fault, but its command value undergoes a step jump, i g- For i g The value at the moment before the fault is defined as: i g- =i0; i g+ For i g The value at a moment after the fault is defined as i. g+ =i gref .

[0137] i0 is the current before the fault; just record it. During low-voltage ride-through, i... gref The real part i gdref The value is i gpL imaginary part i gqref The value is i gqL High voltage ride-through i gref The real part i gdref The value is the active current before the fault, and the imaginary part i. gqref The value is i gqHThe current is calculated using formulas (10)-(13). To enable the current to quickly track the command value, the parameters are usually set to an underdamped form, i.e., 0 < ξ < 1. The fault component of the current can be obtained using formula (15):

[0138]

[0139] By performing a reverse pull transformation on formula (16), we can obtain the analytical expression for the fault current component. Adding this to the current before the fault gives us the analytical expression for the fault current after the fault, as shown in formula (17).

[0140]

[0141] See Figure 9 The fault current optimization method for direct-drive wind turbines described in this embodiment includes:

[0142] 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 direct-drive wind turbine generator sets, manufacturer black box models, and actual unit controller semi-physical simulation models, and record the three-phase voltage and three-phase current at the wind turbine outlet.

[0143] S2. Fault ride-through controller parameter determination:

[0144] Extract the fundamental positive sequence voltage and fundamental positive sequence reactive current under voltage drop conditions, and substitute them into formula (10). The fundamental positive sequence voltage corresponds to u in formula (10). s The fundamental positive sequence reactive current corresponds to i in formula (10) gqL The low-voltage ride-through reactive power support coefficient k can then be calculated. qL .

[0145] Extract the fundamental positive sequence voltage and fundamental positive sequence reactive current under voltage rise conditions, and substitute them into formula (13). The fundamental positive sequence voltage corresponds to u in formula (13). s The fundamental positive sequence reactive current corresponds to i in formula (13) gqH The high-voltage ride-through reactive power support coefficient k can then be calculated. qH .

[0146] S3. Determination of steady-state controller parameters:

[0147] Using classical engineering tuning methods, the closed-loop transfer function (14) of the controller is tuned to a typical Type I system, resulting in... k p and k i The damping ratio is determined by the optimal setting parameters of the Siemens regulator, and in this invention, the damping ratio is taken as 0.707.

[0148] S4. The k determined in step S3...p and k i Substitute this into the fault current analytical expression (17).

[0149] When 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 (11) and (12). Formulas (11) and (12) are then connected to the i-th parameter of formula (17). gref The analytical formula for low-voltage fault current is formed. Under full-power, three-phase voltage drop operating conditions, i can be obtained from formula (11). pnormal The value of 1 is necessarily greater than If we want the larger value, then taking the smaller of the two, formula 11 can be simplified to: Where i gpL The value of i is the fundamental positive sequence active current. gqL Substituting these values ​​into the equation allows us to calculate the maximum current I. max .

[0150] When 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 (13). Formula (13) is then connected to the i in formula (17). gref The analytical formula for high-voltage fault current is formed.

[0151] S5. Under three-phase voltage sag 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 grid-side current data from actual direct-drive wind turbine generators, manufacturer black-box models, or semi-physical simulation models of actual generator controllers. i The result is the analytical expression for the fault current, 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 that accurately describes the direct-drive wind turbine generator.

[0152] The above examples of this invention are merely illustrative of the computational model and process of this invention, and are not intended to limit the implementation of this 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 this invention are still within the scope of protection of this invention.

Claims

1. A method for analyzing fault current of a direct drive wind turbine generator system and optimizing parameters thereof, characterized in that, include: Direct-drive wind turbines utilize a set of back-to-back converters to inject wind power into the AC grid. The back-to-back converters decouple the permanent magnet synchronous motor from the grid. The grid-side converter controller is abbreviated as the grid-side controller, and a corresponding steady-state controller model is constructed. The grid-side active and reactive current reference values ​​in the steady-state controller model are denoted as follows: and , , These are the grid-side current reference values. The real and imaginary parts; simultaneously, a fault-crossing controller model is constructed. During the construction of the fault-crossing controller model... Determine the reactive current during the fault period in the case of a low voltage fault. and active current And the active current restored after the low-voltage fault is cleared. ; , During the corresponding fault period , , After the corresponding fault is cleared Determine the reactive current during the high-voltage fault period. , During the corresponding fault period ; The control logic for maintaining steady-state active power during a fault; the reactive power recovers to steady-state level instantaneously after the high-voltage fault is cleared. Then, based on the closed-loop transfer function of the inner current loop and the analytical expression of the fault current of the direct-drive fan, current analysis and parameter optimization are performed: S1. Perform voltage drop and voltage rise fault tests on actual direct-drive 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. S2. Fault ride-through controller parameter determination: Extract the fundamental positive sequence voltage and fundamental positive sequence reactive current under voltage sag conditions, and map the fundamental positive sequence reactive current to... This leads to the low-voltage ride-through reactive power support coefficient. ; or, Extract the fundamental positive sequence voltage and fundamental positive sequence reactive current under voltage rise conditions, and map the fundamental positive sequence reactive current to... This leads to the high-voltage ride-through reactive power support coefficient. ; S3. Determination of steady-state controller parameters: Using classical engineering tuning methods, the closed-loop transfer function of the inner current loop is tuned to a typical Type I system, resulting in... , ,in For the damping ratio, The frequency of undamped natural oscillation. and The inductance and resistance of the filter in the inner current loop control after feedforward compensation; This is the equivalent gain of pulse width modulation. The carrier period; S4. The steps determined in step S3 are as follows: and Substitute the analytical formula for the fault current of the direct-drive fan; When 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... and ,Will and The grid-side current reference values ​​in the fault current analysis formula of the direct-drive wind turbine are respectively connected. The analytical expression for the fault current of the direct-drive fan corresponding to the low-voltage fault is obtained; under full-power, three-phase voltage drop operating conditions, based on Get the maximum current ; When 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... ,Will In the analytical formula for fault current of direct-drive fan We obtain the analytical expression for the fault current of the direct-drive fan corresponding to the high-voltage fault; S5. Under three-phase voltage sag conditions, convert the three-phase current and three-phase voltage into dq-axis DC quantities, and set the objective function. ,in This is measured grid-side current data. This is the result of the analytical expression for the fault current. Number of sampling points The total number of sampling points is given. 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, thereby obtaining the final analytical expression for the fault current of the direct-drive wind turbine generator.

2. The method for fault current analysis and parameter optimization of direct-drive wind turbine generators according to claim 1, characterized in that, The steady-state controller model is as follows: The grid-side controller uses grid voltage-oriented vector control for its inner and outer loop controllers. The d-axis maintains the stability of the DC voltage to achieve power balance on both sides of the converter, while the q-axis controls the reactive power exchange between the unit and the grid. The frequency domain model of the converter output voltage is as follows: （7） In the formula, and These are the proportional and integral coefficients of the inner loop controller of the network-side controller, respectively. and These are the reference values ​​for active and reactive currents on the grid side, respectively. It is the Laplace operator in the frequency domain; the inner and outer loops include the d-q axes, the d-axis has an outer loop, and the q-axis does not; the active current command on the d-axis is... , , These are the d-axis and q-axis voltages at the converter output, respectively. , These are the d-axis and q-axis currents of the grid-side controller, respectively. , These are the d-axis and q-axis voltages of the power grid, respectively. To achieve grid synchronization speed.

3. The method for fault current analysis and parameter optimization of direct-drive wind turbine generators according to claim 2, characterized in that, In the steady-state state prior to the fault, the active current command along the d-axis in the steady-state controller model is as follows: （8） In the formula, and The proportional and integral coefficients of the outer loop controller of the network-side controller. and These are the reference and actual values ​​for DC voltage.

4. The method for fault current analysis and parameter optimization of direct-drive wind turbine generators according to claim 3, characterized in that, Reactive current during a low-voltage fault as follows: （10） In the formula, The reactive power support factor for low voltage ride-through; The rated current of the unit. It is the fundamental positive sequence voltage.

5. The method for fault current analysis and parameter optimization of direct-drive wind turbine generators according to claim 4, characterized in that, Active current during a low-voltage fault as follows: （11） In the formula, This represents the steady-state value of the active current before the fault occurred. This is the maximum current.

6. The method for fault current analysis and parameter optimization of direct-drive wind turbine generators according to claim 5, characterized in that, In the event of a low-voltage fault, the active current restored after the low-voltage fault is cleared. as follows: （12） In the formula, This refers to the steady-state active current during the fault period; Indicates time; Indicates the time when a low-voltage fault is cleared; This indicates the moment when the first stage of active power increase ends after a low-voltage fault is cleared, and the corresponding slope of the active power is... ; This indicates the end of the second phase of active power increase after a low-voltage fault is cleared, which is the moment when the active power returns to the pre-fault level. The corresponding slope of the active power is... The first and second stages of active power increase are distinguished by the slope of the active power.

7. The method for fault current analysis and parameter optimization of direct-drive wind turbine generators according to claim 6, characterized in that, The reactive current during a high-voltage fault as follows: （13） The high-voltage ride-through reactive power support factor; This is the rated current of the unit.

8. The method for fault current analysis and parameter optimization of direct-drive wind turbine generators according to any one of claims 1 to 7, characterized in that, The closed-loop transfer function of the inner current loop is as follows: in, This refers to the d-axis current of the grid-side controller. This is a reference value for the grid-side active current; It is the Laplace operator in the frequency domain; and These are the proportional and integral coefficients of the grid-side controller's inner loop current controller, respectively. and The inductance and resistance of the filter in the inner current loop control after feedforward compensation.

9. The method for fault current analysis and parameter optimization of direct-drive wind turbine generators according to claim 8, characterized in that, S3 tunes the closed-loop transfer function of the inner current loop to fit a typical Type I system, and the tuned form is as follows: （15） In the formula, This represents the angular frequency of undamped natural oscillation, while This indicates the damping ratio.

10. The method for fault current analysis and parameter optimization of direct-drive wind turbine generators according to claim 9, characterized in that, The analytical formula for the fault current of a direct-drive fan is as follows: （17） in, This represents the current before the fault.

Images