A doubly-fed wind turbine subsynchronous oscillation additional damping controller based on capacitance current compensation

Abstract

The application discloses a double-fed fan subsynchronous oscillation additional damping controller based on capacitor current compensation and relates to the field of novel power system stability analysis and control; solves the problems that the existing controller is relatively complex in determining control parameters, lacks universality and operation parameters cannot be modified at will; comprises a small signal model structure based on double-fed induction fan and weak power grid grid connection, and a general phase lead compensation link controller; phase compensation controller parameters are determined through double-fed induction fan voltage and current frequency response curves; the original system control structure can be not changed, the original control parameters of the system do not need to be adjusted, direct DC current signals are obtained from the DC capacitor part of the double-fed fan, control currents are injected in the form of controlled current sources, the rotor side control and the grid side control are decoupled, and the subsynchronous oscillation phenomenon caused by the interaction between the double-fed fan phase-locked loop and the weak power grid is suppressed.

Description

Technical Field

[0001] This invention relates to the field of novel power system stability analysis and control, specifically to a doubly fed wind turbine subsynchronous oscillation additional damping controller based on capacitor current compensation. Background Technology

[0002] Doubly fed induction generators (DFIGs) have been widely used in industrial production due to their low cost, small size, and ability to achieve variable speed constant frequency operation and independent active and reactive power control. However, since wind farms are usually located far from load centers, the high capacity and long distance of power transmission pose challenges to the stability of grid-connected wind power generation systems.

[0003] With the increase in wind power generation capacity, the decrease in short-circuit ratio, and the weakening of the grid structure, oscillations between doubly-fed induction generators (DFIGs) and weak power grids frequently occur. In recent years, subsynchronous oscillation accidents have occurred in several wind farms worldwide, such as in Texas, USA in 2009 and in Guyuan Wind Farm, Hebei, China in 2012, leading to the tripping of several wind turbine generators. Since the occurrence of oscillation accidents between DFIG wind farms and weak power grids, research on modeling, principle analysis, and damping control strategies for wind farms based on weak power grids and series-compensated DFIGs has received increasing attention. Currently, methods such as eigenvalue analysis, impedance analysis, and dynamic time-domain simulation are being used to study the mechanism and characteristics of subsynchronous oscillation phenomena.

[0004] Regarding oscillation suppression, existing controllers primarily address it through additional hardware control, software configuration, and parameter modification. Among these, the subsynchronous oscillation suppression method based on a phase lag / lead combined controller is widely adopted due to its simple structure. However, determining its control parameters is relatively complex. Typically, phase compensation parameters are adjusted using test signals or field testing. However, the compensation phase determined by these methods only applies to specific operating conditions and lacks universality for different operating states. Furthermore, the gain factor is usually determined through simulation or experimentation. While modifying the controller parameters of the inherent system structure through methods such as eigenvalue analysis can improve the system's damping, the turbine parameters in actual production cannot be arbitrarily modified. Therefore, designing a low-cost, easily adjustable, and non-intrusive additional damping control device to suppress subsynchronous oscillations in weak power grid interactions is a pressing research issue in industrial production. Summary of the Invention

[0005] The purpose of this invention is to solve the problems of existing controllers having relatively complex control parameters, lack of universality, and inability to arbitrarily modify operating parameters, and to propose a doubly fed wind turbine subsynchronous oscillation additional damping controller based on capacitor current compensation.

[0006] The objective of this invention can be achieved through the following technical solution: The specific steps for using this damping controller include:

[0007] S1: Based on the small-signal model structure of doubly fed induction wind turbine and weak grid connection, a controller with general phase lead compensation link is configured;

[0008] S2: Determine the phase compensation controller parameters using the voltage, current, and frequency response curves of the doubly-fed induction fan; specifically:

[0009] A lead controller is installed in the rotor grid-side power coupling channel, and its transfer function is: The phase compensation parameters of the controller are determined by the interaction between the port voltage, current, and impedance functions of the doubly-fed induction generator (DFIG) and the power grid, including:

[0010] S21: Plot the impedance interaction curves between the doubly-fed induction fan and the AC weak grid. Divide the impedance characteristics of the doubly-fed induction fan into inductive and capacitive characteristics based on the phase lag characteristics. At the same time, analyze the influence of the rotor-side control module channel, the grid-side control module channel, and the rotor-grid-side power coupling channel on the frequency response characteristics of the total impedance of the system.

[0011] S22: Based on the interaction curves of the doubly fed induction fan and the AC weak grid, calculate the phase difference at the intersection of the amplitude response. If it exceeds 180°, then based on the angle exceeded and the corresponding phase stability margin, obtain the phase compensation parameters of the lead transfer function at the rotor grid-side power coupling channel.

[0012] S3: Controller equivalent transformation based on transfer function transformation, which transforms the general phase lead compensation controller mentioned in S1 into a practical additional damping controller.

[0013] In a preferred embodiment of the present invention, in S1, the doubly-fed induction turbine model is separated from the weak grid model, wherein the doubly-fed induction turbine model is further divided, and its phase lag relationship is analyzed based on signal transmission, including:

[0014] S11: The doubly-fed induction generator (DFIG) model is divided into a rotor control module and a grid-side control module. The rotor control module takes the grid-side three-phase voltage as input and outputs the motor stator current and rotor output power. The grid-side control module takes the DC capacitor-side power and grid-side voltage as input. In this case, the DFIG model is composed of two sub-modules that are cross-connected and are mutually dual. Among the two sub-modules, the rotor control module is a single-input two-output system, and the grid-side control module is a two-input single-output system. In the small-signal model, the input and output channels are replaced by equivalent transfer functions. In this case, the DFIG impedance model consists of three parts: the rotor-side control module channel, the grid-side control module channel, and the rotor-grid-side power coupling channel. The transfer function of the rotor-side control module channel is T1(s), the transfer function of the grid-side control module channel is T2(s), and the transfer function of the rotor-grid-side power coupling channel is T3(s)*T4(s). Plot the impedance frequency response curves that reflect the voltage and current of the DFIG.

[0015] S12: The impedance relationship between the AC weak current grid side and its response voltage and current is represented by a transfer function. The transfer function can be T. l (s), plot its frequency response curve;

[0016] S13: Based on the AC weak grid impedance curve and the doubly fed induction fan input-output curve, the stability and stability margin of the system are analyzed using the generalized Nyquist standard.

[0017] In a preferred embodiment of the present invention, the general phase lead compensation circuit is converted into a practical additional damping controller by utilizing the equivalent transformation of the transfer function, including:

[0018] S31: Based on the equivalent transformation of the transfer function, the controller form is as follows:

[0019]

[0020] Where a and T are the control parameters of the phase lead compensation stage, U dc C is the voltage across the DC capacitor. bus The DC bus capacitor, abbreviated as C, is further converted to:

[0021]

[0022]

[0023]

[0024]

[0025] Where P g and P rLet the power values ​​be the grid-side and rotor-side power, respectively. As can be seen from the final equation, the added control is equivalent to adding a compensation power term to the power side of the original capacitor control module.

[0026]

[0027] S32: Further converts the compensation power term into current form, since it is added to the DC capacitor module of the doubly-fed induction fan, based on the DC capacitor C. bus The current on both sides is controlled:

[0028]

[0029] This is the compensation current. As you can see, when the DC voltage is stable, the compensation current is 0, and the compensation controller will not function.

[0030] Compared with the prior art, the beneficial effects of the present invention are:

[0031] 1. The controller of this invention can obtain DC current signals directly from the DC capacitor part of the doubly fed wind turbine without changing the original control structure of the system or adjusting the original control parameters of the system. It injects control current in the form of a controlled current source, thereby decoupling the rotor-side control from the grid-side control and suppressing the subsynchronous oscillation phenomenon caused by the interaction between the doubly fed wind turbine phase-locked loop and the weak power grid.

[0032] 2. This invention has low cost and minimal impact on the original system's inherent control structure and parameters. It can be added to a doubly fed wind turbine as an additional controller. At the same time, the signals used by the controller and the control implementation signals are located in the same position in the system, thus avoiding packet loss and delay problems caused by the large-scale transmission of system signals.

[0033] 3. The control mechanism of this invention is clear, which can ensure that the addition of the device will not cause new types of oscillation interaction phenomena, thereby ensuring the stable operation of the system. Attached Figure Description

[0034] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments or the prior art 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.

[0035] Figure 1 This is a schematic diagram of the controller structure of the present invention;

[0036] Figure 2 This is a diagram of the internal structure of the compensation controller of the present invention;

[0037] Figure 3This is a flowchart illustrating the implementation of the damping controller of the present invention;

[0038] Figure 4 This is a transfer function structure diagram of the doubly fed induction fan system of the present invention;

[0039] Figure 5 The system pole distribution diagrams were plotted after the controller of the present invention was added and different control parameters were selected.

[0040] Figure 6 This is a waveform diagram before the implementation of the controller in this invention;

[0041] Figure 7 The waveform diagram after adding the controller in this invention;

[0042] Figure 8 This is a current diagram output by the controller of the present invention. Detailed Implementation

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

[0044] Please see Figure 1 As shown, a subsynchronous oscillation additional damping controller for a doubly-fed induction generator (DFIG) based on capacitor current compensation includes a machine-side converter 1, a controlled current source 2, ammeter 3, ammeter 4, DC capacitor 5, DC capacitor 6, a voltmeter 7, a compensation controller 8, and a grid-side converter 9. The positive terminal of the machine-side converter 1 is connected to one end of the controlled current source 2 and then connected to the positive terminal of ammeter 3. The negative terminal of ammeter 3 is connected to one end of DC capacitor 5 and then connected to the positive terminal of ammeter 4. The other end is connected in parallel with one end of DC capacitor 6 and then grounded. The other end of DC capacitor 6 is connected in parallel with the negative terminal of the machine-side converter 1, the other end of the controlled current source 2, and the negative terminal of voltmeter 7 and then connected to the negative terminal of the grid-side converter 9. The positive terminal of voltmeter 7 is connected to the negative terminal of ammeter 4 and then connected to the positive terminal of the grid-side converter 9. Ammeter 3 and ammeter 4 are respectively connected to the irsc and igsc terminals of the compensation controller 8. The compensation controller 8 is also connected to the controlled current source 2.

[0045] Please see Figure 2 As shown, the internal components of the compensation controller 8 include rotor-side converter current 81, grid-side converter current 82, controller gain 1 83, controller gain 2 84, controller transfer function 85, adder 86, current limiting circuit 87, and switching switch 88.

[0046] Please see Figures 3-8 As shown, a subsynchronous oscillation damping controller for a doubly-fed induction generator (DFIG) based on capacitor current compensation is described. Its oscillation suppression steps include:

[0047] S1. Controller settings for general advance compensation circuit based on the small-signal model structure of doubly fed induction wind turbine and weak grid connection;

[0048] S2. Determination of controller parameters based on phase compensation: The phase compensation controller parameters are determined by using the voltage, current and frequency response curves of the doubly fed induction fan.

[0049] S3. Based on the equivalent transformation of the controller using transfer function transformation, the general phase lead compensation element mentioned in S1 is transformed into a practical additional damping controller.

[0050] In step S1, the doubly-fed induction generator (DFIG) model is separated from the weak grid model. The DFIG model is further subdivided to analyze its phase lag relationship based on signal transmission. This includes:

[0051] S11. The doubly-fed induction generator (DFIG) model is divided into a rotor control module and a grid-side control module. The rotor control module takes the grid-side three-phase voltage as input and outputs the motor stator current and rotor output power. The grid-side control module takes the DC capacitor-side power and grid-side voltage as input. Thus, the DFIG model is composed of two interconnected sub-modules, which are mutually dual. The rotor control module is a single-input, two-output system, while the grid-side control module is a two-input, single-output system. In the small-signal model, each input and output channel can be replaced by an equivalent transfer function. Therefore, the DFIG impedance model can be composed of three parts: the rotor-side control module channel (transfer function T1(s)), the grid-side control module channel (transfer function T2(s)), and the rotor-grid-side power coupling channel (transfer function T3(s)*T4(s)). The impedance frequency response curve reflecting the voltage and current of the DFIG can then be plotted.

[0052] S12. The impedance relationship between the AC weak current grid side and its response voltage can also be expressed using a transfer function. The transfer function can be T. l (s). Therefore, its frequency response curve can be plotted.

[0053] S13. Based on the AC weak grid impedance curve and the input / output curve of the doubly-fed induction generator (DFIG), the stability and stability margin of the system can be analyzed using the generalized Nyquist standard. The results show that the rotor-grid-side power coupling channel in S11 causes a further lag in the voltage-current relationship, inducing subsynchronous oscillations and leading to instability. This instability is also related to the grid-side phase-locked loop (PLL) and the DFIG output current. Adding a phase-leading element to the rotor-grid-side power coupling channel can mitigate the occurrence of subsynchronous oscillations.

[0054] In step S2, a lead control controller is set in the rotor grid-side power coupling channel, and its transfer function is: At this point, the phase compensation parameters of the controller are determined by the interaction between the port voltage, current, and impedance functions of the doubly-fed induction generator and the power grid, including:

[0055] S21. Plot the impedance interaction curves between the doubly-fed induction fan and the AC weak grid. Divide the impedance characteristics of the doubly-fed induction fan into inductive and capacitive characteristics based on the phase lag characteristics. At the same time, analyze the influence of the rotor-side control module channel, the grid-side control module channel, and the rotor-grid-side power coupling channel on the frequency response characteristics of the total impedance of the system.

[0056] S22. Based on the interaction curves of the doubly fed induction fan and the AC weak grid, calculate the phase difference at the intersection of the amplitude response. If it exceeds 180°, then based on the angle of excess and with a certain phase stability margin, obtain the phase compensation parameters of the lead transfer function at the rotor grid-side power coupling channel.

[0057] In step S3, the key feature is the use of transfer function equivalent transformation to convert the general phase lead compensation element set in S2 into a practical additional damping controller. This includes:

[0058] S31. Based on the equivalent transformation of the transfer function, the S2 controller takes the following form:

[0059]

[0060] Where a and T are the control parameters of the phase lead compensation stage, U dc C is the voltage across the DC capacitor. bus This refers to the DC bus capacitor, hereinafter abbreviated as C. It is further converted to:

[0061]

[0062]

[0063]

[0064]

[0065] Where P g and P r Let the power values ​​be the grid-side and rotor-side power, respectively. As can be seen from the final equation, the added control is equivalent to adding a compensation power term to the power side of the original capacitor control module.

[0066]

[0067] S32. Further convert the compensation power term into current form. Since it is added to the DC capacitor module of the doubly-fed induction fan, it can be based on the DC capacitor C. bus The current on both sides is controlled:

[0068]

[0069] This is the compensation current. As you can see, when the DC voltage is stable, the compensation current is 0, and the compensation controller will not function.

[0070] The preferred embodiments of the present invention disclosed above are merely illustrative of the invention. These preferred embodiments do not exhaustively describe all details, nor do they limit the invention to any specific implementation. Clearly, many modifications and variations can be made based on the content of this specification. This specification selects and specifically describes these embodiments to better explain the principles and practical applications of the invention, thereby enabling those skilled in the art to better understand and utilize the invention. The invention is limited only by the claims and their full scope and equivalents.

Claims

1. A subsynchronous oscillation additional damping controller for a doubly-fed wind turbine based on capacitor current compensation, characterized in that, The specific oscillation suppression steps of this damping controller include: S1: Based on the small-signal model structure of doubly fed induction wind turbine and weak grid connection, a controller with general phase lead compensation link is configured; S2: Determine the phase compensation controller parameters using the voltage, current, and frequency response curves of the doubly-fed induction fan; specifically: A lead controller is installed in the rotor grid-side power coupling channel, and its transfer function is: 'a' and 'T' are the control parameters of the phase lead compensation stage. The phase compensation parameters of the controller are determined by the interaction between the port voltage, current, and impedance functions of the doubly-fed induction generator and the power grid, including: S21: Plot the impedance interaction curves between the doubly-fed induction fan and the AC weak grid. Divide the impedance characteristics of the doubly-fed induction fan into inductive and capacitive characteristics based on the phase lag characteristics. At the same time, analyze the influence of the rotor-side control module channel, the grid-side control module channel, and the rotor-grid-side power coupling channel on the frequency response characteristics of the total impedance of the system. S22: Based on the interaction curves of the doubly fed induction fan and the AC weak grid, calculate the phase difference at the intersection of the amplitude response. If it exceeds 180°, then based on the angle exceeded and the corresponding phase stability margin, obtain the phase compensation parameters of the lead transfer function at the rotor grid-side power coupling channel. S3: Controller equivalent transformation based on transfer function transformation, which transforms the general phase lead compensation controller mentioned in S1 into a practical additional damping controller.

2. The doubly-fed wind turbine subsynchronous oscillation additional damping controller based on capacitor current compensation according to claim 1, characterized in that, In S1, the doubly-fed induction generator (DFIG) model is separated from the weak grid model. The DFIG model is further subdivided, and its phase lag relationship is analyzed based on signal transmission, including: S11: The doubly-fed induction generator (DFIG) model is divided into a rotor control module and a grid-side control module. The rotor control module takes the grid-side three-phase voltage as input and outputs the motor stator current and rotor output power. The grid-side control module takes the DC capacitor-side power and grid-side voltage as input. In this case, the DFIG model is composed of two sub-modules that are cross-connected and are mutually dual. Among the two sub-modules, the rotor control module is a single-input two-output system, and the grid-side control module is a two-input single-output system. In the small-signal model, the input and output channels are replaced by equivalent transfer functions. In this case, the DFIG impedance model consists of three parts: the rotor-side control module channel, the grid-side control module channel, and the rotor-grid-side power coupling channel. The transfer function of the rotor-side control module channel is T1(s), the transfer function of the grid-side control module channel is T2(s), and the transfer function of the rotor-grid-side power coupling channel is T3(s)*T4(s). Plot the impedance frequency response curves that reflect the voltage and current of the DFIG. S12: The impedance relationship between the AC weak current grid side and its response voltage and current is represented by a transfer function. The transfer function can be: Plot its frequency response curve; S13: Based on the AC weak grid impedance curve and the doubly fed induction fan input-output curve, the stability and stability margin of the system are analyzed using the generalized Nyquist standard.

3. A doubly-fed wind turbine subsynchronous oscillation additional damping controller based on capacitor current compensation according to claim 1, characterized in that, By utilizing the equivalent transformation of the transfer function, the set general phase lead compensation element is converted into a practical additional damping controller, including: S31: Based on the equivalent transformation of the transfer function, the controller form is as follows: ; Where a and T are the control parameters of the phase lead compensation circuit. This is the voltage across the DC capacitor, abbreviated as U. For DC bus capacitors, hereinafter abbreviated as It is further transformed into: ; ; ; ; in and The power values ​​are on the grid side and rotor side, respectively. As can be seen from the final equation, the added control is equivalent to adding a compensation power term to the power side of the original capacitor control module. ; S32: Further converts the compensation power term into current form, since it is added to the DC capacitor module of the doubly-fed induction fan, based on the DC capacitor. The current on both sides is controlled: ； This is the compensation current. As you can see, when the DC voltage is stable, the compensation current is 0, and the compensation controller will not function.

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