Doubly-fed fan system stability improving method and system considering direct current dynamic characteristics

Abstract

The invention belongs to the technical field of power system operation and control, and discloses a double-fed fan system stability improving method considering direct current dynamic characteristics. In order to solve the problem of low-frequency-band prediction distortion caused by neglect of DC bus voltage fluctuation in a traditional model, an independent DC dynamic admittance matrix is constructed by deducing a transfer function from DC power to DC voltage, and a high-precision system total impedance model is obtained through parallel superposition. Based on the model, the key effect of the power grid resistive component on the system stability is revealed, and it is confirmed that damping increasing is beneficial for improving the system phase margin. Therefore, a stability improving measure for introducing a virtual resistor or a physical damping device is provided, and the problem of subsynchronous oscillation of the doubly-fed wind turbine generator under the weak power grid is effectively solved.

Description

Stability Improvement Method and System for Doubly Fed Wind Turbine Systems Considering DC Dynamic Characteristics Technical Field

[0001] This invention belongs to the field of power system operation and control technology, specifically relating to a method and system for improving the stability of a doubly fed wind turbine system that takes into account DC dynamic characteristics. Background Technology

[0002] Doubly-fed induction generator (DFIG) wind turbines are currently the mainstream type of wind power generation. With the expansion of wind power grid connection, wind farms are often connected to the end of the grid, facing a weak grid environment with a low short-circuit ratio (SCR). Engineering practice and theoretical research show that, in addition to the short-circuit ratio, the grid's resistance-to-inductance ratio (R / X) is also a key factor determining system stability. In grids with low R / X (high inductance), subsynchronous oscillations are highly likely to occur.

[0003] Problems with existing technology:

[0004] (1) Model prediction distortion: Traditional impedance modeling methods usually assume that the DC bus voltage is constant and ignore the dynamic response of the DC bus voltage under disturbances (DC-Link Dynamics). However, in the low frequency band, DC-side fluctuations can significantly affect the AC-side current through modulation. Ignoring this feature leads to inaccurate impedance predictions in the low frequency band and makes it impossible to accurately identify the oscillation risk caused by DC coupling.

[0005] (2) Lack of guidance for improvement measures: Although existing research indicates that increasing damping is beneficial to stability, there is a lack of models that can accurately describe the dynamic characteristics of DC to verify this law. Engineers often have to act based on experience and lack theoretical support based on high-precision model analysis, which makes it impossible to formulate targeted damping improvement schemes when facing low R / X weak grid oscillations. Summary of the Invention

[0006] The purpose of this invention is to provide a method and system for improving the stability of a doubly-fed induction generator (DFIG) wind turbine system that takes into account DC dynamic characteristics. This invention aims to establish a high-precision model, reveal the influence of grid resistive components on stability, and propose specific stability improvement measures accordingly. The core technical solution of this invention is as follows: First, this invention constructs a high-precision impedance model incorporating DC dynamic characteristics through virtual decoupling; then, it uses this model to analyze the influence of grid resistive components on the system's phase margin; finally, based on this analysis, it improves system stability by introducing virtual resistors or physical damping devices.

[0007] The specific steps include:

[0008] Baseline Modeling: Establish sequence impedance models for the grid-side and turbine-side converters, neglecting DC dynamics. This mainly includes grid-side converter modeling and turbine-side converter impedance modeling. Under vector control, if the DC dynamic characteristics of the unit are temporarily ignored and the DC bus is considered as a constant DC voltage source, then the factors causing frequency coupling in the grid-side converter of the doubly-fed induction generator (DFIG) wind turbine include phase-locked loop (PLL) control and dq-axis asymmetrical current control. Based on the grid-side control block diagram of the DFIG wind turbine shown, linearization models of the PLL control, current control, and modulation stages of the grid-side converter are performed, and the analytical model expression of the admittance matrix considering various causes of frequency coupling can be obtained. The overall approach to turbine-side impedance modeling is similar to that of grid-side converter impedance modeling. For the turbine side of the DFIG, its control is usually performed in a rotating coordinate system. Therefore, it is also necessary to perform coordinate transformation on the rotor current in the stationary coordinate system based on the output angle of the PLL control and the motor angle, and then inversely transform the obtained modulation signal to the three-phase stationary coordinate system. Therefore, for the generator side of a doubly-fed induction generator (DFIG), frequency coupling characteristics still exist due to the presence of components such as phase-locked loop (PLL) control. The impact of these frequency coupling characteristics needs to be considered when modeling the generator-side impedance of a DFIG.

[0009] DC Feature Extraction: Research on the DC dynamic characteristics of doubly-fed induction generator (DFIG) wind turbines requires analysis of the DC dynamic mechanism, including how dynamic fluctuations in the DC bus voltage occur under small disturbances, and how these fluctuations affect the impedance characteristics of the wind turbine. Under small disturbances, the characteristics of a DFIG wind turbine can be described by the impedance characteristics between the AC grid connection point port voltage disturbance and the current response. Based on small-signal analysis, the closed-loop transfer function from DC power fluctuations to DC voltage fluctuations is derived. .

[0010] Closed-loop transfer function The expression is as follows:

[0011] ;

[0012] Where s is the Laplace operator; V dc0 and C dc These represent the steady-state values ​​of the DC bus voltage and the DC capacitances g1~g6, respectively. 2*1 and B 2*1 The expression is:

[0013] ;

[0014] ;

[0015] ;

[0016] ;

[0017] ;

[0018] ;

[0019] in, Indicates conjugate. This represents the fundamental vector of the grid-side converter current. The equivalent stator disturbance current is defined during the derivation process. Let j be the fundamental steady-state vector of the grid voltage, where j is the imaginary unit. For small signal perturbation frequency, The fundamental frequency of the power grid. For the stator self-inductance of the induction motor, For the mutual inductance between the stator and the rotor, This represents the fundamental steady-state vector of the rotor current. Stator / rotor turns ratio / voltage matching factor This refers to the stator resistance of the induction motor. For the stator self-inductance of the induction motor, The equivalent stator disturbance current is defined during the derivation process. The rotor electrical frequency;

[0020] ;

[0021] ;

[0022] ;

[0023] ;

[0024] in, For the PWM modulation and delay loop of the grid-side converter, This is the transfer function of the DC voltage outer loop controller. This is the transfer function of the grid-side GSC current inner loop controller. This represents the fundamental steady-state vector of the modulation signal from the grid-side converter. For grid-side current control, the cross-decoupling coefficient is... The common frequency angular frequency, For the rotor resistance of the induction motor, The denominator for the characteristic impedance of the motor at the positive-sequence disturbance frequency is... For the self-inductance of the rotor of the induction motor, This refers to the PWM modulation and delay stage of the rotor-side converter. The transfer function of the rotor-side RSC current inner loop controller is given. For the cross-decoupling coefficient of rotor-side current control, The denominator for the characteristic impedance of the motor at the positive-sequence disturbance frequency is... This is the fundamental steady-state vector of the modulation signal of the rotor-side converter.

[0025] DC branch construction: Based on the derived DC dynamic characteristic expression, the equivalent parallel impedance caused by the DC dynamic characteristics is determined. This can be described by the following 2x2 admittance matrix. This matrix corrects the model's characteristics in the low-frequency range.

[0026] ;

[0027] in, For equivalent parallel impedance, This represents the current disturbance group item. This represents the coupling term between grid-side voltage and current. This represents the coupling term between the rotor-side current and the stator-side flux linkage. Grid-side converter control matrix This is the impedance matrix of the induction motor.

[0028] Model synthesis: A parallel superposition method is used to combine... By combining with the baseline model, a total system impedance model considering DC dynamic characteristics is obtained. .

[0029] ;

[0030] in, This indicates the newly added impedance model.

[0031] Model Equivalent Transformation: To facilitate the stability problem study below, the obtained 2*2 admittance matrix is ​​transformed into an equivalent SISO impedance using the following conversion. It should be noted that the SISO impedance obtained by this equivalent transformation still contains the characteristics of frequency coupling.

[0032] ;

[0033] Where Y11, Y12, Y21 and Y22 are the four elements in the 2*2 admittance matrix, and Zgp2 is the grid impedance at the coupling frequency.

[0034] Stability analysis: Through simulation analysis, as R / X increases, the frequency of the intersection of the amplitudes of the doubly fed wind turbine impedance and the grid impedance remains almost unchanged, while the corresponding phase angle difference gradually decreases. This indicates that as R / X increases, the system phase margin increases and the stability improves.

[0035] Stability Enhancement: When analysis indicates that the current grid R / X level poses a risk of oscillation, virtual resistance control or physical damping devices can be implemented at the grid connection point. By artificially increasing the resistive component at the grid connection point, the physical characteristic of resistance dissipating oscillation energy is utilized to effectively improve system stability.

[0036] The beneficial effects of this invention are:

[0037] Precise Diagnosis: Introduction Subsequently, the model accurately reproduced the negative damping characteristics in the low-frequency band, solving the problem of the traditional model being "inaccurate".

[0038] The underlying principle is clear: This invention uses a high-precision model to qualitatively verify the principle that the larger the power grid's R / X ratio, the more stable the system, providing a solid theoretical basis for engineering measures.

[0039] Effective measures: The proposed damping compensation measures (such as STATCOM virtual resistors) directly target the cause of insufficient damping, and increase the system's energy dissipation capacity through physical or control means, thereby effectively suppressing subsynchronous oscillations. Attached Figure Description

[0040] The above and other objects, features, and advantages of exemplary embodiments of this application will become readily understood by reading the following detailed description with reference to the accompanying drawings. In the drawings, several embodiments of this application are illustrated by way of example and not limitation, and the same or corresponding reference numerals denote the same or corresponding parts, wherein:

[0041] Figure 1 is a flowchart of the stability improvement method for a doubly fed wind turbine system considering DC dynamic characteristics provided by the present invention.

[0042] Figure 2 is the system impedance Bode plot provided by the present invention. Detailed Implementation

[0043] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of this application. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0044] It should be understood that the terms "comprising" and "including" used in the specification and claims of this application indicate the presence of the described features, integrals, steps, operations, elements and / or components, but do not exclude the presence or addition of one or more other features, integrals, steps, operations, elements, components and / or collections thereof.

[0045] It should also be understood that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the application. As used in this specification and claims, the singular forms “a,” “an,” and “the” are intended to include the plural forms unless the context clearly indicates otherwise. It should also be understood that the term “and / or” as used in this specification and claims refers to any combination and all possible combinations of one or more of the associated listed items, and includes such combinations.

[0046] As used in this specification and claims, the term "if" may be interpreted, depending on the context, as "when," "once," "in response to determination," or "in response to detection." Similarly, the phrase "if determined" or "if [described condition or event] is detected" may be interpreted, depending on the context, as "once determined," "in response to determination," "once [described condition or event] is detected," or "in response to detection of [described condition or event]."

[0047] The specific embodiments of this application will now be described in detail with reference to the accompanying drawings.

[0048] This application provides a stability improvement method for a doubly-fed induction generator (DFIG) wind turbine system that takes into account DC dynamic characteristics. The system impedance modeling and stability improvement method are shown in Figure 1, and include the following steps:

[0049] S101, establish sequence impedance models for the grid-side and turbine-side converters, neglecting DC dynamics. This mainly includes grid-side converter modeling and turbine-side converter impedance modeling. Under vector control, if the DC dynamic characteristics of the unit are temporarily ignored and the DC bus is considered as a constant DC voltage source, then the factors causing frequency coupling in the grid-side converter of the doubly-fed induction generator (DFIG) wind turbine include phase-locked loop (PLL) control and dq-axis asymmetrical current control. Based on the grid-side control block diagram of the DFIG wind turbine shown, linearization models of the PLL control, current control, and modulation stages of the grid-side converter are performed, and the analytical model expression of the admittance matrix considering various causes of frequency coupling can be obtained. The overall approach to turbine-side impedance modeling is similar to that of grid-side converter impedance modeling. For the turbine side of the DFIG, its control is usually performed in a rotating coordinate system. Therefore, it is also necessary to perform coordinate transformation on the rotor current in the stationary coordinate system based on the output angle of the PLL control and the motor angle, and then inversely transform the obtained modulation signal to the three-phase stationary coordinate system. Therefore, for the generator side of a doubly-fed induction generator (DFIG), frequency coupling characteristics still exist due to the presence of components such as phase-locked loop (PLL) control. The impact of these frequency coupling characteristics needs to be considered when modeling the generator-side impedance of a DFIG.

[0050] S102. The study of DC dynamic characteristics of doubly-fed induction generator (DFIG) wind turbines requires analysis of the DC dynamic mechanism, including how dynamic fluctuations in the DC bus voltage are generated under small disturbances, and how these fluctuations affect the impedance characteristics of the wind turbine. Under small disturbances, the characteristics of a DFIG wind turbine can be described by the impedance characteristics between the AC grid connection point port voltage disturbance and the current response. Based on small-signal analysis, the closed-loop transfer function from DC power fluctuations to DC voltage fluctuations is derived. Closed-loop transfer function The expression is as follows:

[0051] ;

[0052] Where s is the Laplace operator; V dc0 and C dc These represent the steady-state values ​​of the DC bus voltage and the DC capacitances g1~g6, respectively. 2*1 and B 2*1 The expression is:

[0053] ;

[0054] ;

[0055] ;

[0056] ;

[0057] ;

[0058] ;

[0059] in, Indicates conjugate. This represents the fundamental vector of the grid-side converter current. The equivalent stator disturbance current is defined during the derivation process. Let j be the fundamental steady-state vector of the grid voltage, where j is the imaginary unit. For small signal perturbation frequency, The fundamental frequency of the power grid. For the stator self-inductance of the induction motor, For the mutual inductance between the stator and the rotor, This represents the fundamental steady-state vector of the rotor current. Stator / rotor turns ratio / voltage matching factor This refers to the stator resistance of the induction motor. For the stator self-inductance of the induction motor, The equivalent stator disturbance current is defined during the derivation process. The rotor electrical frequency;

[0060] ;

[0061] ;

[0062] ;

[0063] ;

[0064] in, For the PWM modulation and delay loop of the grid-side converter, This is the transfer function of the DC voltage outer loop controller. This is the transfer function of the grid-side GSC current inner loop controller. This represents the fundamental steady-state vector of the modulation signal from the grid-side converter. For grid-side current control, the cross-decoupling coefficient is... The common frequency angular frequency, For the rotor resistance of the induction motor, The denominator for the characteristic impedance of the motor at the positive-sequence disturbance frequency is... For the self-inductance of the rotor of the induction motor, This refers to the PWM modulation and delay stage of the rotor-side converter. The transfer function of the rotor-side RSC current inner loop controller is given. For the cross-decoupling coefficient of rotor-side current control, The denominator for the characteristic impedance of the motor at the positive-sequence disturbance frequency is... This is the fundamental steady-state vector of the modulation signal of the rotor-side converter.

[0065] S103, DC Branch Construction: Based on the derived DC dynamic characteristic expression, the equivalent parallel impedance caused by the DC dynamic characteristics... This can be described by the following 2x2 admittance matrix. This matrix corrects the model's characteristics in the low-frequency range.

[0066] ;

[0067] in, For equivalent parallel impedance, This represents the current disturbance group item. This represents the coupling term between grid-side voltage and current. This represents the coupling term between the rotor-side current and the stator-side flux linkage. Grid-side converter control matrix This is the impedance matrix of the induction motor.

[0068] S104, Model Synthesis: Using a parallel stacking method, By combining with the baseline model, a total system impedance model considering DC dynamic characteristics is obtained. The derived doubly fed impedance model considering frequency coupling characteristics was verified through time-domain simulation.

[0069] ;

[0070] in, This indicates the newly added impedance model.

[0071] S105, Model Equivalent Transformation: To facilitate the stability problem study below, the obtained 2*2 admittance matrix is ​​transformed into an equivalent SISO impedance through a conversion. It should be noted that the SISO impedance obtained by this equivalent transformation still contains the characteristics of frequency coupling.

[0072] ;

[0073] S106, Stability Analysis: The system impedance Bode plot obtained through simulation analysis of the grid's R / X ratio is shown in Figure 2. The figure reveals that when R / X = 0, i.e., when the doubly-fed induction generator (DFIG) is connected to a purely inductive grid, the DFIG impedance and grid impedance have an amplitude intersection point around 80Hz, with a corresponding phase angle difference of approximately 180°, meaning the system phase margin (PM) is approximately 0°. As R / X increases, the frequency of the amplitude intersection point between the DFIG impedance and grid impedance remains almost unchanged, while the corresponding phase angle difference gradually decreases. This indicates that as R / X increases, the system phase margin increases, and stability improves.

[0074] S107, Implementation of Stability Improvement Measures: When encountering oscillation problems caused by low R / X weak power grids in actual engineering projects, based on the rules derived in step two, this invention proposes a strategy of increasing damping to improve stability. Specifically, this includes the following two optional measures:

[0075] Measure A (Virtual Resistance Enhancement Based on STATCOM): If the grid connection point is equipped with a Static Synchronous Compensator (STATCOM), introduce virtual resistance control into the control strategy. The control logic is as follows: acquire the grid connection point voltage, and control the STATCOM to output an active current component that is in phase with the voltage. Principle: This control makes the STATCOM exhibit resistive characteristics in its electrical ports, thereby effectively increasing the R / X level of the grid connection point and increasing system damping without changing the physical wiring.

[0076] Measure B (Improved Physical Damping): A high-pass filter damping branch (e.g., a branch consisting of a capacitor and a resistor in series) is installed at the wind farm's collector outlet. Principle: The capacitor isolates the power frequency voltage (reducing losses), and the resistor dissipates high-frequency / oscillating frequency currents. This is equivalent to physically increasing the grid resistance R in the oscillation frequency band, directly improving system stability.

[0077] The flowcharts and / or block diagrams of the methods and systems of embodiments of this application have been described above by way of example, and related aspects have been described. It should be understood that each block or combination thereof in the flowcharts and / or block diagrams can be implemented by computer program instructions, by dedicated hardware performing a specified function or action, or by a combination of dedicated hardware and computer instructions. When implemented in hardware, it can be, for example, an electronic circuit, an application-specific integrated circuit (ASIC), appropriate firmware, a plug-in, a function card, etc.; when implemented in software, it is a program or code segment used to perform the required task. The program or code segment can be stored in memory or transmitted over a transmission medium or communication link via data signals carried in a carrier wave. The code segment can be downloaded via a computer network such as the Internet, an intranet, etc.

[0078] While this application has shown and described numerous embodiments, it will be apparent to those skilled in the art that such embodiments are provided by way of example only. Many modifications, alterations, and alternatives will arise for those skilled in the art without departing from the spirit and intent of this application. It should be understood that various alternatives to the embodiments of this application described herein may be employed in the practice of this application. The appended claims are intended to define the scope of protection of this application and therefore cover equivalents or alternatives within the scope of these claims.

Claims

1. A method for improving the stability of a doubly-fed induction generator (DFIG) wind turbine system considering DC dynamic characteristics, characterized in that, Includes the following steps: Based on the vector control principle, reference impedance models for the grid-side converter and the generator-side converter are established, respectively, neglecting the influence of DC bus voltage fluctuations. Based on the power balance equation of the DC bus capacitor, a small-signal dynamic model of the DC link of the doubly-fed induction generator (DFIG) is established. Combined with the outer-loop control equation of the grid-side converter voltage, the closed-loop transfer function from DC power fluctuations to DC bus voltage fluctuations is derived. Based on the aforementioned DC dynamic closed-loop transfer function Construct a DC mode admittance matrix independent of the AC control loop. The The independent contributions of DC bus voltage fluctuations to the positive and negative sequence current responses on the AC side were characterized; the DC mode admittance matrix was superimposed in parallel to characterize the DC mode admittance matrix. The system impedance model is obtained by adding the reference impedance models of the grid-side converter and the machine-side converter to the reference impedance model of the generator side. The total system impedance model taking into account DC dynamic coupling is obtained.

2. The method according to claim 1, characterized in that, The construction process of the reference sequence impedance model includes: linearizing the control systems of the grid-side converter and the machine-side converter respectively; the control system includes a current inner loop controller, a phase-locked loop (PLL) controller, and a system delay element; in the stationary coordinate system, the response characteristics of the above control elements are converted into a 2*2 transfer function matrix form to obtain the reference sequence impedance model.

3. The method according to claim 1, characterized in that, After linearization, the closed-loop transfer function of the DC dynamic characteristics of the doubly-fed induction generator (DFIG) wind turbine is obtained. The expression is as follows: Where s is the Laplace operator; V dc0 and C dc These represent the steady-state values ​​of the DC bus voltage and the DC capacitances g1~g6, respectively. 2*1 and B 2*1 The expression is: ； ； ； ； ； ;;;in, Indicates conjugate. This represents the fundamental vector of the grid-side converter current. The equivalent stator disturbance current is defined during the derivation process. Let j be the fundamental steady-state vector of the grid voltage, where j is the imaginary unit. For small signal perturbation frequency, The fundamental frequency of the power grid. For the stator self-inductance of the induction motor, For the mutual inductance between the stator and the rotor, This represents the fundamental steady-state vector of the rotor current. Stator / rotor turns ratio / voltage matching factor This refers to the stator resistance of the induction motor. For the stator self-inductance of the induction motor, The equivalent stator disturbance current is defined during the derivation process. The rotor electrical frequency; ； ； ； ;in, For the PWM modulation and delay loop of the grid-side converter, This is the transfer function of the DC voltage outer loop controller. This is the transfer function of the grid-side GSC current inner loop controller. This represents the fundamental steady-state vector of the modulation signal from the grid-side converter. For grid-side current control, the cross-decoupling coefficient is... The common frequency angular frequency, For the rotor resistance of the induction motor, The denominator for the characteristic impedance of the motor at the positive-sequence disturbance frequency is... For the self-inductance of the induction motor rotor, This refers to the PWM modulation and delay stage of the rotor-side converter. The transfer function of the rotor-side RSC current inner loop controller is given. For the cross-decoupling coefficient of rotor-side current control, The denominator for the characteristic impedance of the motor at the positive-sequence disturbance frequency is... This is the fundamental steady-state vector of the modulation signal of the rotor-side converter.

4. The method according to claim 1, characterized in that, Based on the equivalent structure of the doubly-fed wind turbine system considering DC dynamic characteristics and the closed-loop transfer function of DC dynamic characteristics. The equivalent parallel impedance caused by the DC dynamic characteristics is described by the following 2*2 admittance matrix: ;in, For equivalent parallel impedance, This represents the current disturbance group item. This represents the coupling term between grid-side voltage and current. This represents the coupling term between the rotor-side current and the stator-side flux linkage. Grid-side converter control matrix This is the impedance matrix of the induction motor.

5. The method according to claim 1, characterized in that, The impedance model of a doubly-fed induction generator (DFIG) wind turbine considering frequency coupling characteristics is derived from the reference impedance model of the grid-side converter. Machine-side converter reference impedance model With DC mode admittance matrix The result is obtained by addition, and its expression is as follows: ;in, This indicates the newly added impedance model.

6. A stability improvement system for a doubly-fed induction generator (DFIG) wind turbine system considering DC dynamic characteristics, the system being used to implement the stability improvement method for a DFIG wind turbine system considering DC dynamic characteristics as described in claim 1, characterized in that... The system includes: using the total impedance model of the system to analyze the impact of the change in the grid resistance reactance ratio R / X on the system phase margin, and confirming the effectiveness of increasing the grid resistive component in improving system stability; based on the above analysis, implementing engineering measures to introduce virtual resistors or physical damping devices at the wind farm grid connection point to improve the system stability under low R / X weak grid conditions.

7. The system according to claim 4, characterized in that, Due to the frequency coupling characteristics, the impedance model of the doubly-fed wind turbine under vector control established above is a 2*2 admittance matrix; the obtained 2*2 admittance matrix is ​​then transformed into an equivalent SISO impedance through conversion: Where Y11, Y12, Y21 and Y22 are the four elements in the 2*2 admittance matrix, and Zgp2 is the grid impedance at the coupling frequency.

8. The system according to claim 4, characterized in that, The engineering measures include: controlling the output of the static synchronous compensator (STATCOM) at the grid connection point to have an additional current in phase with the grid connection point voltage, so that it exhibits virtual resistance characteristics; or configuring a physical damping resistor device in the grid connection line to improve the equivalent resistance reactance ratio (R / X) of the grid connection point.

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