A power coordination recovery control method for inhibiting repeated low voltage ride through of a doubly-fed wind turbine

Abstract

The application discloses a power coordination recovery control method for inhibiting repeated low voltage ride through of a doubly-fed wind turbine. The application proposes to coordinate the recovery slope of active current and reactive current aiming at the repeated low voltage ride through problem of the doubly-fed wind turbine after fault clearance. Firstly, a phase-locked loop second-order underdamped model is established to estimate the peak time and valley time in the voltage recovery process; then, the current recovery expression is substituted into the grid connection point voltage equation to construct a voltage constraint inequality, so that the peak time does not trigger high voltage ride through and the valley time does not trigger low voltage ride through; finally, the feasible range of the active recovery slope and the reactive recovery slope is determined according to the constraint inequality, the optimal active and reactive current recovery slope is determined by combining the current limiting or recovery time matching constraint, and the current reference instruction is input to the rotor side converter. The application can effectively inhibit voltage oscillation when exiting the low voltage ride through stage and improve the transient stable operation ability of the wind turbine under weak grid conditions.

Description

Technical Field

[0001] This invention relates to the field of wind power generation technology, and in particular to a power coordination recovery control method for suppressing repeated low voltage ride-throughs in doubly-fed induction generator (DFIG) wind turbines. Background Technology

[0002] With the increasing penetration of renewable energy, the low voltage ride-through (LVRT) capability of wind turbines has become a basic requirement of grid guidelines. However, actual operation shows that during the recovery phase after a fault is cleared, the voltage at the grid connection point may drop again due to the surge in active power and the rapid recovery of reactive power, causing the unit to repeatedly oscillate near the low voltage threshold and enter and exit the LVRT state multiple times.

[0003] The existing technical solutions mainly include:

[0004] Based on the active power-voltage curve of a wind power grid-connected system, this paper analyzes the mechanism of voltage fluctuations caused by repeated entry and exit of the LVRT (Low Voltage Retrieval System) and proposes a strategy to suppress oscillations by increasing the LVRT start-up threshold. However, this method is mainly applicable to the operating conditions during the fault duration and cannot effectively solve the voltage oscillation problem during the fault recovery phase. Furthermore, it fails to provide specific control parameter tuning principles.

[0005] Under high-wind conditions, by coordinating with hybrid grid-connected and grid-built renewable energy power stations, grid-built energy storage can be switched to charging mode, and combined with voltage thresholds and active power absorption criteria, repeated low-voltage ride-throughs can be suppressed. However, this method requires the configuration of grid-built energy storage devices at the power station and relies on inter-station communication and coordinated control, making it mainly suitable for specific scenarios where power is transmitted via flexible DC islanding. For conventional wind farms lacking energy storage or containing only doubly-fed induction generators, adding additional power electronic devices will inevitably increase the operating costs of the wind turbines, making this method difficult to apply.

[0006] Another study, based on a small-signal model and current closed-loop control function for converter grid connection, has analyzed in depth the influence mechanism of grid strength and converter control parameters on repeated low-voltage ride-through (LVRT) behavior and provided directions for optimizing control parameters. However, this study still does not provide specific quantitative values ​​for control parameters to suppress repeated LVRT. Especially for doubly-fed induction generators (DFIGs), whose stators are directly connected to the AC grid, the power relationship during fault recovery is more complex. Summary of the Invention

[0007] This invention addresses the problems of existing technologies by providing a power coordination and recovery control method for doubly-fed wind turbines during fault recovery.

[0008] The basic principle of this invention is as follows: In weak grid scenarios, the absolute values ​​of the partial derivatives of the grid connection point voltage with respect to active and reactive power increase significantly, leading to more severe voltage fluctuations with the same power change. During the fault recovery phase, the wind turbine rapidly withdraws reactive current and increases active power. This process causes a momentary drop in the terminal voltage within tens to hundreds of milliseconds, easily triggering secondary low-voltage ride-through. This invention, through a coordinated recovery strategy, smoothly increases active power and recovers reactive power. The system can smoothly absorb the new power, avoiding severe voltage surges and ensuring that the operating point follows a stable path. Under constraints, the grid connection point voltage does not exceed limits at either peak or trough, ultimately converging to a stable operating point, thus fundamentally avoiding repeated low-voltage ride-throughs.

[0009] In a first aspect, the present invention provides a power coordination recovery control method for suppressing repeated low-voltage ride-throughs in doubly-fed wind turbines, comprising the following steps:

[0010] S1: Obtain the system parameters of the doubly fed wind turbine grid-connected system, detect the voltage amplitude at the grid connection point in real time, and lock the initial values ​​of the stator active current and stator reactive current at the current moment when the grid fault is cleared and the voltage at the grid connection point recovers to above the low voltage crossover exit threshold.

[0011] S2: Establish a phase-locked loop dynamic model, and estimate the peak voltage and valley voltage times during the voltage recovery process based on the phase-locked loop control parameters in the system parameters.

[0012] S3: Set the stator active current and stator reactive current to recover linearly from the initial values ​​using the active current recovery slope and the reactive current recovery slope, respectively;

[0013] S4: Establish a linearized relationship between the grid connection point voltage and the stator active current and stator reactive current. Combine the voltage constraints at the voltage peak time and voltage valley time to determine the feasible range of the combined slope of the active current recovery slope and the reactive current recovery slope. Then, determine the optimal combined slope from the feasible range based on the symmetric optimization criterion.

[0014] S5: Based on the optimal combination slope, and combined with the current limiting constraint or recovery time matching constraint, determine the optimal active current recovery slope and the optimal reactive current recovery slope respectively.

[0015] S6: Generate a current reference command based on the optimal active current recovery slope, the optimal reactive current recovery slope, and the initial value, and control the rotor-side converter to adjust the rotor excitation voltage based on the current reference command so that the actual stator current tracks the current reference command.

[0016] In a second aspect, the present invention provides an electronic device, including a memory and a processor, wherein the memory is used to store one or more computer instructions, wherein when the one or more computer instructions are executed by the processor, a power coordination recovery control method for suppressing repeated low voltage ride-throughs of a doubly-fed wind turbine is implemented.

[0017] Thirdly, the present invention provides a computer-readable storage medium storing a computer program, which, when executed by a processor, is used to implement a power coordination recovery control method for suppressing repeated low-voltage ride-throughs in doubly-fed wind turbines.

[0018] Compared with the prior art, the present invention has the following beneficial effects:

[0019] 1. Avoid secondary voltage overshoot: By coordinating and controlling the recovery slope of active and reactive currents, the voltage threshold constraints of high voltage ride-through (1.1 pu) and low voltage ride-through (0.9 pu) are met simultaneously, thus fundamentally preventing repeated voltage oscillations during the recovery phase.

[0020] 2. Quantitative calculation without repeated trial and error: Based on the second-order underdamped phase-locked loop model, the peak and valley times are accurately estimated, and a set of slope constraint inequalities is derived. The power recovery slope that minimizes voltage fluctuations within the safe and feasible region can be directly calculated, avoiding the blindness of traditional methods that rely on experience-based debugging.

[0021] 3. High versatility: This method only requires adding a slope calculation step to the existing LVRT control logic, without the need for additional hardware, which greatly reduces costs and is easy to implement in engineering. Attached Figure Description

[0022] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort. Wherein:

[0023] Figure 1 is an equivalent circuit diagram of a single-unit grid-connected doubly-fed wind turbine system according to an embodiment of the present invention;

[0024] Figure 2 is a block diagram of the phase-locked loop structure used in the doubly fed wind turbine according to an embodiment of the present invention;

[0025] Figure 3 is a structural block diagram of the doubly fed wind turbine low voltage ride-through control system according to an embodiment of the present invention;

[0026] Figure 4 is a flowchart of the power coordination recovery control method provided in an embodiment of the present invention;

[0027] Figure 5 shows the simulation results when the power recovery slope does not meet the constraints in the embodiments of the present invention;

[0028] Figure 6 shows the simulation results when the power recovery slope satisfies the constraints in the embodiment of the present invention. Detailed Implementation

[0029] To make the above-mentioned objects, features, and advantages of the present invention more apparent and understandable, specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the protection scope of the present invention.

[0030] like Figure 1 As shown, the doubly-fed induction generator (DFIG) single-unit grid-connected system involved in this embodiment includes a DFIG generator, a step-up transformer at the generator end, transmission lines, and an infinite power grid. The stator-side grid connection point (PCC) and the equivalent reactance of the grid are marked in the figure. and grid voltage Active component of stator current and reactive components Since line reactance affects grid connection voltage, coordinated control of the recovery process of active and reactive currents during the fault recovery phase is key to suppressing grid connection voltage oscillations.

[0031] like Figure 2 As shown, the structure of a phase-locked loop (PLL) includes: the input is a three-phase voltage transformed by Clark. Output phase error The loop filter uses a proportional-integral (PI) controller with a proportional gain of 1. The integral coefficient is Output phase .

[0032] like Figure 3 As shown, the low-voltage ride-through control system includes a rotor-side converter (RSC), an inner current loop controller, an outer power loop controller, and a low-voltage ride-through logic module. The inner current loop receives d-axis and q-axis current reference commands. The rotor voltage command is output through the PI regulator to control the rotor-side converter to generate excitation voltage, so that the actual stator current tracks the reference value.

[0033] like Figure 4 As shown, the power coordination and recovery control method of this embodiment includes the following steps, which are described in detail below.

[0034] Step 1: Obtain system parameters and check exit conditions

[0035] First, the key parameters of the doubly-fed induction generator (DFIG) grid-connected system must be obtained. In this embodiment, at least the following parameters need to be obtained:

[0036] Equivalent reactance of power grid (Unit: pu);

[0037] Rated voltage at grid connection point (Typically 1.0 pu);

[0038] The proportional gain of a phase-locked loop (PLL) and integral coefficient ;

[0039] Converter current limiting value (For example, it is usually taken as 1.2 pu).

[0040] These parameters can be obtained through actual measurements or parameter identification at the wind farm, and can be determined by those skilled in the art based on the specific system configuration.

[0041] Real-time monitoring of grid connection point voltage amplitude When the power grid fault is cleared and When the voltage recovers to above 0.9 pu, the low-voltage ride-through exit condition is deemed met. At this point, the initial value of the stator d-axis active current at the current moment is locked. Initial value of stator q-axis reactive current (Units are all pu). These two initial values ​​will serve as the starting point for subsequent linear recovery.

[0042] Step 2: Establish a dynamic model of the phase-locked loop and estimate the peak and trough times of the voltage.

[0043] Because the bandwidth of the inner current loop is designed to be more than 5 times that of the phase-locked loop, the current tracking error is negligible, and the recovery process of the grid connection point voltage can be approximately dominated by the underdamped second-order dynamics of the phase-locked loop.

[0044]

[0045] This refers to the phase error of the phase-locked loop. For the damping ratio, This is the natural oscillation frequency.

[0046] Establish a second-order underdamped model of the phase-locked loop (PLL). Assume the proportional gain of the PLL is... The integral coefficient is The voltage amplitude of the power grid is (Per unit value). Its closed-loop transfer function can be equivalent to a standard second-order system:

[0047]

[0048] The natural oscillation frequency Damping ratio For actual wind farms, phase-locked loops are typically designed to be underdamped (…). Therefore, the voltage response will exhibit overshoot and oscillation.

[0049] Based on the step response characteristics of a second-order system, the peak time during the voltage recovery process... Valley time They are estimated as follows:

[0050]

[0051] Step 3: Define the current linear recovery expression.

[0052] To avoid voltage surges caused by sudden current changes during the recovery process, this embodiment employs a linear recovery strategy. Let the d-axis active current after fault clearance be... and q-axis reactive current It changes according to the following linear pattern:

[0053]

[0054] in The active current recovery slope (unit: pu / s). The reactive current recovery slope (unit: pu / s). This is the time counted from the moment the fault was cleared.

[0055] Step 4: Determining the Combined Slope Using Voltage Constraints and Symmetrical Optimization

[0056] Grid connection point voltage The relationship between the stator current of the doubly-fed induction generator (DFIG) and the stator current is determined by the equivalent circuit of the grid-connected system. Based on the phasor diagram or power circle equation, neglecting resistance, the expression is as follows:

[0057]

[0058] In the initial stage of fault recovery, the current variation range is small, and the above equation can be linearized near the operating point. This embodiment uses a four-equal interval division. Related nonlinear terms Perform a linear approximation. Let... Its domain is Divide the interval into four equal parts to obtain five discrete points. Take the first segment (i.e., Linearization is performed to approximate the circular arc using a two-point form of the straight line equation. The calculation yields:

[0059] coefficient Substituting the above approximation into the original voltage equation, we obtain the linearized expression:

[0060] remember The linearized grid connection point voltage equation is then:

[0061]

[0062] in The coupling coefficients obtained by linearizing the system ( ), For equivalent reactance, The voltage is the grid voltage. This linearized voltage equation has high accuracy over a small current variation range. Those skilled in the art can also use other piecewise linearization methods based on actual system parameters, but this embodiment uses a four-eight-segment first segment as an example.

[0063] The linear recovery expression for current , Substituting, we get:

[0064] To avoid triggering repeated low-voltage ride-throughs, at peak times... Valley value time Must meet:

[0065]

[0066] The above inequality determines the combined slope. The feasible range.

[0067] To strike a balance between these two constraints and ensure a smooth voltage recovery process, this embodiment makes the peak and valley voltages symmetrical relative to the rated voltage. This symmetry prevents the voltage recovery trajectory from biasing to one side, thereby reducing the risk of repeated crossovers. This symmetry is used as the combined slope. The basis for determination is:

[0068]

[0069] Solving for:

[0070]

[0071] like If the value falls within the feasible range, then that value is used; otherwise, the nearest boundary value is used. This determines the combined slope. .

[0072] Step 5: Determine the slope using current limiting or recovery time matching. and

[0073] exist Given that, we have Further confirmation is needed. and Their respective values.

[0074] Current limiting method:

[0075] The converter current is limited by hardware and must meet the following requirements:

[0076] Expanded and organized as follows:

[0077] in,

[0078]

[0079] Since the maximum current amplitude during the recovery process usually occurs at the initial moment or near the peak / valley moment, it is possible to focus only on a few key time points (such as...). , , Constraints are applied to approximately ensure that the entire process does not exceed limits. This embodiment uses a more precise method: the square of the current amplitude is considered as... The quadratic function, its maximum point .like Then only verification is needed. The limit condition is met; if The maximum value is at From this, we obtain... This is a quadratic inequality. Those skilled in the art can solve this inequality by substituting actual parameters to obtain... The feasible range. Solving this inequality yields... feasible range In this embodiment, the maximum value is taken within this interval. As the active current recovery slope, and let The optimal reactive current recovery slope was obtained.

[0080] Recovery time matching method:

[0081] To restore both active and reactive currents to the commanded values ​​simultaneously, we can set:

[0082]

[0083] in This is the target value for active current (usually the value before the fault). The amount of reactive current that needs to be reduced (because) (Set to a positive and relatively large value). Combined with... Solving for:

[0084]

[0085] Recalculate , the results , Each is used as a final reference recovery slope , This method ensures the synchronization of active and reactive power recovery processes.

[0086] During project implementation, any of the above methods can be selected according to actual needs. This embodiment will use method one as an example for subsequent explanation.

[0087] Step 6: Generate current reference command and execute control.

[0088] Based on the final determined slope , and initial value , Generate current reference command:

[0089]

[0090] These two commands are input to the inner current loop controller of the rotor-side converter. The controller adjusts the rotor excitation voltage to make the actual stator current track the reference value. Because the slope is optimized, the voltage will not touch the 0.9 pu and 1.1 pu boundaries during the recovery process, thus avoiding repeated low voltage ride-throughs.

[0091] Based on the same concept as the above method, this application embodiment also provides an electronic device, including a memory and a processor, wherein the memory is used to store one or more computer instructions, wherein when the one or more computer instructions are executed by the processor, a power coordination recovery control method for suppressing repeated low voltage ride-through of a doubly-fed wind turbine is implemented.

[0092] Based on the same concept as the above method, this application embodiment also provides a computer-readable storage medium storing a computer program, which, when executed by a processor, is used to implement a power coordination recovery control method for suppressing repeated low voltage ride-throughs in doubly-fed wind turbines.

[0093] Numerical simulation example:

[0094] Taking a doubly-fed induction generator (DFIG) grid-connected system as an example, a per-unit system is adopted. The grid equivalent reactance is... pu, mains voltage pu. Damping ratio of phase-locked loop parameters Natural oscillation frequency .

[0095] Locking the initial current during fault clearing pu, PU, converter current limiting pu, coupling coefficient Calculate the constants The combined slope is obtained from the symmetry criterion. Using the current limiting method, in Next, solve the minimum constraint of the quadratic function of the current amplitude squared, and obtain... Take the maximum value of the feasible interval. pu / s, and thus pu / s. Verification voltages are all within... Within the specified range; therefore, this method can effectively suppress repeated low-voltage ride-throughs. Simulation verification is consistent with calculations, such as... Figure 5 and Figure 6 As shown. Figure 5 The simulated waveform for the power recovery slope that does not meet the constraints in the embodiments of the present invention is characterized by repeated triggering of low voltage ride-through control during the recovery phase, resulting in oscillation. Figure 6 The grid-connected point voltage waveform is shown in the embodiment of the invention when the power recovery slope meets the constraints. It is characterized by the voltage returning to normal after the fault is cleared, without repeated oscillations.

[0096] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A power coordination recovery control method for suppressing repeated low-voltage ride-through in doubly-fed wind turbines, characterized in that, Includes the following steps: S1: Obtain the system parameters of the doubly fed wind turbine grid-connected system, detect the voltage amplitude at the grid connection point in real time, and lock the initial values ​​of the stator active current and stator reactive current at the current moment when the grid fault is cleared and the voltage at the grid connection point recovers to above the low voltage crossover exit threshold. S2: Establish a phase-locked loop dynamic model, and estimate the peak voltage and valley voltage times during the voltage recovery process based on the phase-locked loop control parameters in the system parameters. S3: Set the stator active current and stator reactive current to recover linearly from the initial values ​​using the active current recovery slope and the reactive current recovery slope, respectively; S4: Establish a linearized relationship between the grid connection point voltage and the stator active current and stator reactive current. Combine the voltage constraints at the voltage peak time and voltage valley time to determine the feasible range of the combined slope of the active current recovery slope and the reactive current recovery slope. Then, determine the optimal combined slope from the feasible range based on the symmetric optimization criterion. S5: Based on the optimal combination slope, and combined with the current limiting constraint or recovery time matching constraint, determine the optimal active current recovery slope and the optimal reactive current recovery slope respectively. S6: Generate a current reference command based on the optimal active current recovery slope, the optimal reactive current recovery slope, and the initial value, and control the rotor-side converter to adjust the rotor excitation voltage based on the current reference command so that the actual stator current tracks the current reference command.

2. The power coordination recovery control method for suppressing repeated low-voltage ride-through in a doubly-fed induction generator (DFIG) according to claim 1, characterized in that, The system parameters include at least one of the following: grid equivalent reactance, grid connection point rated voltage, wind turbine rated current, converter current loop PI control parameters, and phase-locked loop second-order control parameters.

3. The power coordination recovery control method for suppressing repeated low-voltage ride-through in a doubly-fed induction generator (DFIG) according to claim 1, characterized in that, The phase-locked loop dynamic model is a second-order underdamped model, and the voltage peak time and the voltage trough time are determined according to the damping ratio and natural oscillation frequency of the phase-locked loop.

4. A power coordination recovery control method for suppressing repeated low-voltage ride-through in a doubly-fed induction generator (DFIG) according to claim 1 or 3, characterized in that, In step S3, the linear recovery expressions for the stator active current and the stator reactive current are respectively: ， in for The stator active current at a given moment. for Stator reactive current at any given moment The initial value of the stator active current. The initial value of the stator reactive current is given. The active current recovery slope is given. The reactive current recovery slope is... This is the time counted from the moment the fault was cleared.

5. The power coordination recovery control method for suppressing repeated low-voltage ride-through in a doubly-fed induction generator (DFIG) according to claim 1, characterized in that, In S4, the linearization relationship is: in, for The grid connection point voltage at any given time, This is the grid voltage. For equivalent reactance, The coupling coefficients are obtained by linearizing the system. Substituting the linear recovery expression, we obtain the expression for the change of grid connection point voltage over time: in, .

6. The power coordination recovery control method for suppressing repeated low-voltage ride-through in a doubly-fed induction generator (DFIG) according to claim 5, characterized in that: In S4, the voltage constraint is: in This refers to the peak voltage moment. This refers to the time of the voltage valley. The symmetry optimization criterion is: to make the peak voltage and valley voltage symmetrical with respect to the rated voltage, that is: The optimal combination slope is obtained by solving: like If the value falls within the feasible range, then that value is used; otherwise, the nearest boundary value within the feasible range is used.

7. The power coordination recovery control method for suppressing repeated low-voltage ride-through in a doubly-fed induction generator (DFIG) according to claim 6, characterized in that: In step S5, determining the optimal active current recovery slope and the optimal reactive current recovery slope based on the current limiting constraint is specifically as follows: In the optimal combination of slope Once determined, by Establish the relationship between the active current recovery slope and the reactive current recovery slope; Combined with converter current limiting constraints: Solve for the active current recovery slope. The feasible range; The maximum value within the feasible range is selected as the optimal active current recovery slope. ,make The optimal reactive current recovery slope is obtained. .

8. The power coordination recovery control method for suppressing repeated low-voltage ride-through in a doubly-fed induction generator (DFIG) according to claim 6, characterized in that: In step S5, determining the optimal active current recovery slope and the optimal reactive current recovery slope based on the recovery time matching constraint specifically involves: Make the active current recovery time equal to the reactive current recovery time: in, The target value for active current. This is the amount of reactive current that needs to be reduced. Combination Solving for: The results , These are respectively used as the optimal active current recovery slope. and the optimal reactive current recovery slope .

9. An electronic device, characterized in that, The system includes a memory and a processor, the memory being used to store one or more computer instructions, wherein the one or more computer instructions, when executed by the processor, implement the method as described in any one of claims 1-8.

10. A computer-readable storage medium, characterized in that, The computer-readable storage medium contains a computer program that, when executed by a processor, is used to implement the method as described in any one of claims 1-8.

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