A method for maximizing voltage support capability of a doubly-fed wind turbine generator considering rotor side constraints

Abstract

This invention discloses a control method for maximizing the voltage support capability of a doubly-fed induction generator (DFIG) wind turbine considering rotor-side constraints. First, an analytical model of the system's dual-sequence voltage and current, incorporating the stator inductance and magnetizing inductance of the DFIG, is established. Then, under the premise of strictly satisfying the overcurrent and overvoltage constraints of the rotor-side converter, the optimal positive-sequence and negative-sequence rotor current injection angles are analytically derived with the goal of maximizing positive-sequence voltage support and maximizing negative-sequence voltage suppression. The key innovation of this invention lies in revealing that the optimal injection angle depends not only on the grid impedance parameters but also significantly on the inherent inductance parameters of the DFIG. Simulation results show that the current command generated using this method can significantly improve the positive-sequence voltage at the point of common coupling during faults and reduce the negative-sequence voltage, thereby reducing voltage imbalance and achieving effective grid support.

Description

Technical Field

[0001] This invention belongs to the field of wind power generation control technology, specifically relating to a control method for maximizing the voltage support capability of a doubly fed wind turbine considering rotor-side constraints. Background Technology

[0002] Among the many types of wind turbine generators, the Doubly Fed Induction Generator (DFIG) dominates the current wind power market due to its advantages such as converter capacity being only 20%-30% of the rated capacity, low cost, ability to achieve variable speed constant frequency operation, and decoupled control of active and reactive power.

[0003] However, wind farms are typically located at the end of the power grid, making their grid connection environment relatively weak and highly susceptible to grid faults. Among power system faults, asymmetrical faults (such as single-phase-to-ground short circuits and two-phase-to-phase short circuits) are the most common type. When an asymmetrical fault occurs in the grid, the voltage at the point of common coupling (PCC) exhibits severe three-phase imbalance, decomposing into positive-sequence and negative-sequence voltage components. A significant drop in positive-sequence voltage weakens the system's synchronous stability, while the resulting negative-sequence voltage leads to generator torque pulsation, overheating, and malfunctions of protection devices.

[0004] To maintain the safe and stable operation of power systems, grid connection guidelines in various countries impose stringent requirements on the fault ride-through capability of wind turbines. Specifically, DFIGs not only need to maintain grid connection during faults (i.e., low-voltage ride-through), but are also required to inject reactive current into the grid to provide dynamic voltage support. The ideal control strategy should be to support positive-sequence voltage recovery while suppressing negative-sequence voltage as much as possible, thereby reducing the voltage unbalance factor (VUF).

[0005] Existing voltage control methods for asymmetrical faults mostly draw on the control theories of full-power converters or conventional static var generators. These traditional methods generally assume that, in order to maximize voltage support, the optimal phase angle of the injected current mainly depends on the impedance characteristics of the grid side (i.e., the ratio of grid inductance to grid resistance).

[0006] However, the physical topology of a DFIG differs fundamentally from that of a full-power converter. In a DFIG, the stator is directly connected to the grid, while the rotor is connected via a back-to-back converter, resulting in complex magnetic coupling between the stator and rotor. Existing control strategies often neglect the inherent electrical parameters within the DFIG unit—particularly the influence of stator inductance and magnetizing inductance on the stator-side voltage response. This neglect prevents the theoretically optimal injection angle from being obtained when calculating the rotor current reference command.

[0007] Furthermore, the rotor-side converter (RSC) of a DFIG is subject to strict physical constraints in actual operation, primarily including the maximum rotor current amplitude constraint determined by the thermal limits of the semiconductor devices and the maximum rotor voltage amplitude constraint determined by the DC bus voltage. If the coupling effects of the motor's internal parameters and the physical limitations on the rotor side are not accurately considered in the control strategy, the unit will be unable to fully utilize its capacity potential during faults, making it difficult to maximize voltage support capability. Therefore, there is an urgent need for a voltage support optimization control method that can comprehensively consider the DFIG's internal inductance parameters and rotor-side operating constraints. Summary of the Invention

[0008] This invention aims to address the problem that existing doubly-fed induction generator (DFIG) wind turbines cannot fully utilize their voltage support capacity during grid asymmetric faults due to neglecting the influence of internal generator parameters and rotor-side physical constraints. This invention provides a control method for maximizing the voltage support capacity of DFIG wind turbines that considers rotor-side constraints. This method can accurately calculate the optimal rotor current injection angle, thereby maximizing positive-sequence voltage recovery and minimizing negative-sequence voltage imbalance while ensuring equipment safety.

[0009] A method for maximizing the voltage support capability of a doubly-fed induction generator (DFIG) considering rotor-side constraints includes the following steps:

[0010] S1: Collect grid impedance and parameters such as stator and excitation inductance of the doubly fed wind turbine to establish a dual-sequence analytical model of PCC point voltage and rotor current that can accurately reflect the internal coupling effect of the motor; at the same time, based on the hardware limits of the converter, set rotor current amplitude constraints and rotor voltage constraints to prevent over-modulation, and establish the system operating boundary.

[0011] S2, based on the analytical model, introduces a phase correction angle φ, which is jointly determined by the stator inductance and the magnetizing inductance. k For the two independent objectives of maximizing positive-sequence voltage support and maximizing negative-sequence voltage suppression, the optimal injection angle φ of the positive-sequence rotor current after internal parameter correction is analytically derived. r+ and the optimal injection angle φ for negative sequence rotor current r- ;

[0012] S3: Substitute the calculated optimal injection angle into the constraints set in S1 for verification. If the verification result exceeds the limit, reduce the current amplitude proportionally while keeping the optimal angle unchanged. Finally, generate a current command that satisfies all physical constraints to drive the rotor-side converter to inject dual-sequence current and achieve the theoretical maximization of the grid voltage support capability.

[0013] Furthermore, the complex field expression of the established dual-order support model is as follows:

[0014]

[0015]

[0016] Among them, U s+ and U s- These are the positive and negative sequence voltages of PCC, U g+ and U g- These are the positive and negative sequence voltages of the power grid, I r+ and I r- These are the positive and negative sequence currents of the rotor, K. + and K - These are the positive and negative sequence coupling impedance coefficients, respectively. g and L g These are the mains resistance and inductance, L s and L m Let ω1 and ω1 represent the stator inductance and magnetizing inductance of the DFIG, respectively, and j be the imaginary unit.

[0017] Furthermore, the mathematical expression for the system coupling impedance coefficient includes coupling terms composed of the grid impedance and the internal parameters of the motor:

[0018]

[0019]

[0020] Furthermore, the formula for calculating the optimal injection angle described in S2 is as follows:

[0021] Optimal injection angle for positive sequence rotor current:

[0022]

[0023] Optimal injection angle for negative sequence rotor current:

[0024]

[0025] In the formula φ g φ is the power grid impedance angle. k The correction angle can be expressed as follows:

[0026]

[0027] Furthermore, before performing constraint verification in S3, the required rotor voltage vector must be calculated based on the current operating conditions. The calculation formula takes into account the influence of slip:

[0028]

[0029] Among them, I s+ and I s- These are the stator positive and negative sequence currents, respectively; s is the generator slip; L r This is the rotor inductance.

[0030] Furthermore, the constraint verification described in S3 must satisfy the following inequalities:

[0031] Current amplitude constraint:

[0032]

[0033] Voltage amplitude constraint:

[0034]

[0035] I rmax U is the maximum allowable current of the converter. rmax This is the maximum allowable voltage in the linear modulation region of the converter.

[0036] Furthermore, if the verification result exceeds the limit, the specific operation of proportionally reducing the current amplitude is as follows: calculate the reduction coefficient λ, and use this coefficient to update the positive and negative sequence current amplitude reference values:

[0037]

[0038] Where λ is taken as the current constraint reduction factor. i and voltage constraint reduction factor λ u The smaller value in the formula is used to ensure that the final instruction satisfies both current and voltage constraints.

[0039] This invention addresses the problem that existing control strategies fail to adequately consider the influence of DFIG internal parameters under asymmetrical fault conditions, leading to insufficient utilization of voltage support capabilities. To address this, a novel optimized control method is proposed. This method first establishes a system dual-sequence voltage and current analytical model incorporating the DFIG stator inductance and magnetizing inductance. Then, under strict constraints on rotor-side converter overcurrent and overvoltage, the optimal positive-sequence and negative-sequence rotor current injection angles are analytically derived with the goal of maximizing positive-sequence voltage support and maximizing negative-sequence voltage suppression. The key innovation of this invention lies in revealing that the optimal injection angle depends not only on the grid impedance parameters but also significantly on the inherent inductance parameters of the DFIG. Simulation results show that the current command generated using this method can significantly increase the positive-sequence voltage at the point of common coupling during faults and reduce the negative-sequence voltage, thereby reducing voltage imbalance and achieving effective grid support. Attached Figure Description

[0040] Figure 1 This is a flowchart of the method for maximizing the voltage support capability of a doubly fed wind turbine considering rotor-side constraints, as described in this invention.

[0041] Figure 2 This is a schematic diagram of the structure and control principle of the doubly fed wind turbine grid connection system provided in an embodiment of the present invention.

[0042] Figure 3 This is the steady-state equivalent circuit diagram of a doubly fed induction generator under asymmetrical fault conditions.

[0043] Figure 4 This is a schematic diagram showing the vector relationships between the two-sequence components of voltage and current in a system under asymmetric fault conditions.

[0044] Figure 5 This is a vector schematic diagram of positive sequence voltage support maximization control.

[0045] Figure 6 This is a vector principle diagram of negative sequence voltage suppression maximization control. Detailed Implementation

[0046] To describe the present invention in more detail, the technical solution of the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0047] The present invention provides a method for maximizing the voltage support capability of a doubly-fed induction generator (DFIG) wind turbine, taking into account rotor-side constraints. This method comprises the following steps:

[0048] Step S1: Collect grid impedance and parameters such as stator and excitation inductance of the doubly fed wind turbine, and establish a dual-sequence analytical model of PCC point voltage and rotor current that can accurately reflect the internal coupling effect of the motor; at the same time, based on the hardware limits of the converter, set rotor current amplitude constraints and rotor voltage constraints to prevent over-modulation, and establish the system operating boundary.

[0049] like Figure 2 As shown, this invention is applied to a doubly-fed induction generator (DFIG) wind power generation system. The system mainly consists of a wind turbine, gearbox, doubly-fed induction generator (DFIG), rotor-side converter (RSC), grid-side converter (GSC), and DC bus capacitor. The stator side is directly connected to the point of common coupling (PCC), and the rotor side is connected to the PCC through the converter. When an asymmetrical fault occurs at the PCC point, the control method described in this invention is executed in the RSC controller.

[0050] Furthermore, the complex field expression of the established dual-order support model is as follows:

[0051]

[0052]

[0053] Among them, U s+ and U s- These are the positive and negative sequence voltages of PCC, U g+ and U g- These are the positive and negative sequence voltages of the power grid, I r+ and I r- These are the positive and negative sequence currents of the rotor, K. + and K - These are the positive and negative sequence coupling impedance coefficients, respectively. g and L g These are the mains resistance and inductance, L s and L m Let ω1 and ω1 represent the stator inductance and magnetizing inductance of the DFIG, respectively, and j be the imaginary unit.

[0054] The mathematical expression for the system coupling impedance coefficient includes coupling terms composed of the grid impedance and the internal parameters of the motor:

[0055]

[0056]

[0057] Step S2: Based on the analytical model, a phase correction angle φ, jointly determined by the stator inductance and the magnetizing inductance, is introduced. k For the two independent objectives of maximizing positive-sequence voltage support and maximizing negative-sequence voltage suppression, the optimal injection angle φ of the positive-sequence rotor current after internal parameter correction is analytically derived. r+ and the optimal injection angle φ for negative sequence rotor current r- ;

[0058] Based on the model in step S1, in order to maximize the voltage support capability, the optimal current phase needs to be determined.

[0059] Define φ g The impedance angle of the power grid can be expressed as follows:

[0060]

[0061] This invention introduces a correction angle, which physically represents the phase shift caused by leakage inductance and mutual inductance within the motor. The calculation formula is as follows:

[0062]

[0063] like Figure 5 As shown, the positive sequence voltage increment generated by the rotor positive sequence current can be expressed as:

[0064]

[0065] To ensure that the voltage increment generated by the rotor current is in the same direction as the positive-sequence voltage of the power grid, the optimal positive-sequence injection angle is:

[0066]

[0067] At this point, the positive sequence voltage magnitude is increased to the maximum.

[0068] like Figure 6 As shown, the negative sequence voltage increment generated by the rotor negative sequence current can be expressed as:

[0069]

[0070] To ensure that the voltage increment generated by the rotor current is opposite to the negative sequence voltage of the power grid, the optimal negative sequence injection angle is:

[0071]

[0072] At this point, the magnitude of the negative sequence voltage is weakened to the greatest extent.

[0073] Step S3: Substitute the calculated optimal injection angle into the constraints set in S1 for verification. If the verification result exceeds the limit, reduce the current amplitude proportionally while keeping the optimal angle unchanged. Finally, generate a current command that satisfies all physical constraints to drive the rotor-side converter to inject dual-sequence current and achieve the theoretical maximization of the grid voltage support capability.

[0074] In actual control, it is necessary to ensure the safe operation of RSC, to ensure that the rotor current does not exceed the limit to prevent overcurrent from burning out the device, and at the same time, the rotor voltage does not exceed the limit to prevent over-modulation and runaway.

[0075] Before performing constraint verification in S3, the required rotor voltage vector must be calculated based on the current operating conditions. The calculation formula takes into account the influence of slip rate.

[0076]

[0077] Among them, I s+ and I s- These are the stator positive and negative sequence currents, respectively; s is the generator slip; L r This is the rotor inductance.

[0078] The control logic is as follows: First, allocate the positive and negative sequence current amplitude ratios according to the requirements of the power grid guidelines; second, apply the optimal angle calculated in step S2; then check the rotor constraint inequality: if the constraints are met, directly output the current reference command to the RSC current inner loop; if the constraints are not met, keep the optimal angle unchanged (ensure the direction is optimal), and reduce the current amplitude proportionally until all constraints are met.

[0079] The constraint verification described in S3 must satisfy the following inequalities:

[0080] Current amplitude constraint:

[0081]

[0082] Voltage amplitude constraint:

[0083]

[0084] I rmax U is the maximum allowable current of the converter. rmax This is the maximum allowable voltage in the linear modulation region of the converter.

[0085] If the verification result exceeds the limit, the specific operation of proportionally reducing the current amplitude is as follows: calculate the reduction coefficient λ, and use this coefficient to update the positive and negative sequence current amplitude reference values:

[0086]

[0087] Where λ is taken as the current constraint reduction factor. i and voltage constraint reduction factor λ u The smaller value in the formula is used to ensure that the final instruction satisfies both current and voltage constraints.

[0088] The above description of the embodiments is provided to enable those skilled in the art to understand and apply the present invention. Those skilled in the art can readily make various modifications to the above embodiments and apply the general principles described herein to other embodiments without creative effort. Therefore, the present invention is not limited to the above embodiments, and any improvements and modifications made to the present invention by those skilled in the art based on the disclosure thereof should be within the scope of protection of the present invention.

Claims

1. A control method for maximizing the voltage support capability of a doubly-fed induction generator considering rotor-side constraints, characterized in that, Includes the following steps: S1: Collect grid impedance and stator and excitation inductance parameters of the doubly fed wind turbine, establish a dual-sequence analytical model of PCC point voltage and rotor current that can accurately reflect the internal coupling effect of the motor; based on the hardware limits of the converter, set rotor current amplitude constraints and rotor voltage constraints to prevent over-modulation, and establish the system operating boundary. S2, based on the analytical model, introduces a phase correction angle φ, which is jointly determined by the stator inductance and the magnetizing inductance. k For the two independent objectives of maximizing positive-sequence voltage support and maximizing negative-sequence voltage suppression, the optimal injection angle φ of the positive-sequence rotor current after internal parameter correction is analytically derived. r+ and the optimal injection angle φ for negative sequence rotor current r- ; S3: Substitute the calculated optimal injection angle into the constraints set in S1 for verification. If the verification result exceeds the limit, reduce the current amplitude proportionally while keeping the optimal angle unchanged. Finally, generate a current command that satisfies all physical constraints to drive the rotor-side converter to inject dual-sequence current and achieve the theoretical maximization of the grid voltage support capability.

2. The method according to claim 1, characterized in that, The complex field expression of the analytical model established in S1 is: Among them, U s+ and U s- These are the positive and negative sequence voltages of PCC, U g+ and U g- These are the positive and negative sequence voltages of the power grid, I r+ and I r- These are the positive and negative sequence currents of the rotor, K. + and K - These are the positive and negative sequence coupling impedance coefficients, respectively; R g and L g These are the mains resistance and inductance, L s and L m Let ω1 and ω1 represent the stator inductance and magnetizing inductance of the DFIG, respectively, and j be the imaginary unit.

3. The method according to claim 2, characterized in that, The mathematical expression for the system coupling impedance coefficient includes coupling terms composed of the grid impedance and the internal parameters of the motor: 。 4. The method according to claim 1, characterized in that, The formula for calculating the optimal injection angle in S2 is: Optimal injection angle for positive sequence rotor current: Optimal injection angle for negative sequence rotor current: In the formula φ g φ is the power grid impedance angle. k The correction angle is expressed as follows: 。 5. The method according to claim 1, characterized in that, Before performing constraint verification in S3, the required rotor voltage vector must be calculated based on the current operating conditions. The calculation formula takes into account the influence of slip rate. Among them, I s+ and I s- These are the stator positive and negative sequence currents, respectively; s is the generator slip; L r This is the rotor inductance.

6. The method according to claim 1, characterized in that, The constraint verification described in S3 must satisfy the following inequalities: Current amplitude constraint: Voltage amplitude constraint: I rmax U is the maximum allowable current of the converter. rmax This is the maximum allowable voltage in the linear modulation region of the converter.

7. The method according to claim 6, characterized in that, If the verification result exceeds the limit in S3, the specific operation of proportionally reducing the current amplitude is as follows: calculate the reduction coefficient λ, and use this coefficient to update the positive and negative sequence current amplitude reference values: Where λ is taken as the current constraint reduction factor. i and voltage constraint reduction factor λ u The smaller value in the formula is used to ensure that the final instruction satisfies both current and voltage constraints.

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