An additional stabilizer and its design method for suppressing low-frequency oscillations in doubly-fed wind turbines

Abstract

This invention discloses a method for designing an additional stabilizer to suppress low-frequency oscillations in a doubly-fed induction generator (DFIG) wind turbine. The method includes: constructing the input and output terminals of the additional stabilizer and designing the control loop; plotting the Bode plot of the closed-loop transfer function of the original system to determine the dominant oscillation frequency w. d and the phase lag value θ h Perform parameter tuning calculations for the phase compensation stage in the control loop; select the proportional coefficient K of the isolation stage of the additional stabilizer. e The quality coefficient ξ of the second-order bandstop filter with added stabilizer s and resonance coefficient K s Initialize the values; calculate the time δt required to suppress the disturbance and perform iterative calculations; and obtain the optimal parameter K. e ξ s and K s The values ​​are then substituted into the expressions for the isolation stage and the second-order band-stop filter. The additional stabilizer and design method for suppressing low-frequency oscillations in doubly-fed induction generator (DFIG) wind turbines provided by this invention can effectively suppress low-frequency oscillations in DFIG grid-connected systems and significantly enhance the system's stability under small disturbances.

Description

Technical Field

[0001] This invention belongs to the field of new energy power generation technology, and more specifically, relates to an additional stabilizer and design method for suppressing low-frequency oscillations in doubly-fed wind turbines. Background Technology

[0002] To reduce the pressure from fossil fuels, my country has proposed the goal of "peak carbon and carbon neutrality" and is vigorously developing new energy power generation technologies. my country's traditional power system, which is dominated by thermal power, is shifting to a new power system based on new energy sources. Currently, the more mature technologies are wind power and photovoltaic power generation. Among them, doubly-fed induction generators (DFIGs), as the mainstream new energy power generation equipment, face many challenges in terms of stability.

[0003] Doubly fed induction generators (DFIGs) contain energy storage elements of varying capacities, exhibiting different response speeds to disturbances in the power system, resulting in multi-timescale characteristics. At the electromechanical scale, DFIGs frequently experience low-frequency oscillations. Especially in weak power grids, these low-frequency oscillations may escalate into divergent oscillations, severely threatening the safe and stable operation of the system. Extensive research and analysis have revealed that this is primarily caused by shaft oscillations. Further analysis is needed to identify the source of negative damping in the control system. Therefore, complex torque analysis is commonly used to identify the source of negative damping in DFIGs, as it offers greater physical meaning and a clearer mechanistic description compared to eigenvalue analysis using state-space methods.

[0004] There are three main approaches to improving low-frequency oscillation suppression strategies in power systems: First, improving the hardware structure of the equipment. By adjusting the hardware configuration, system stability can be enhanced. Second, adjusting the controller parameters of the equipment. This involves adjusting parameters without changing the hardware structure, resulting in low cost. However, this approach may not be fully applicable to systems with unclear instability mechanisms, strong nonlinearity, and strong coupling. Third, improving the control strategy. Using the complex torque method, new branches are introduced to adjust the negative damping introduced in the original circuit to positive damping, thereby optimizing the system's damping characteristics and further improving its small disturbance stability, thus effectively suppressing low-frequency oscillations. Therefore, this invention introduces an additional stabilizer into the original system through the design and parameter tuning to suppress low-frequency oscillations and improve the system's disturbance stability. Summary of the Invention

[0005] In view of the shortcomings and improvement needs of the existing technology, the present invention provides a solution, the purpose of which is to

[0006] To achieve the above objectives, according to one aspect of the present invention, a method for designing an additional stabilizer to suppress low-frequency oscillations in a doubly-fed wind turbine is provided, characterized by comprising:

[0007] S1, construct the input and output terminals of the additional stabilizer and design the control circuit;

[0008] S2, Plot the Bode plot of the closed-loop transfer function of the original system to determine the dominant oscillation frequency w. d and the phase lag value θ h ;

[0009] S3, Perform parameter tuning calculations for the phase compensation stage in the control loop;

[0010] S4, Select the isolation ratio coefficient K of the additional stabilizer. e The band-stop filter quality coefficient ξ of the additional stabilizer s and resonance coefficient K s The initial value;

[0011] S5, calculate the time δt required to suppress the disturbance and perform iterative calculations; and

[0012] S6, Obtain the optimal parameter K e ξ s and K s And substitute them into the expressions of the isolation stage and the second-order band-stop filter.

[0013] Preferably, the control circuit further includes an inverter, wherein the isolation circuit is connected to the input terminal, and the isolation circuit, the phase compensation circuit, the band-stop filter and the inverter are cascaded in sequence, wherein the output terminal of the inverter is the output terminal of the additional stabilizer.

[0014] Preferably, the input terminal is the rotor speed change signal, and the output terminal is connected to the mid-voltage change signal in the system.

[0015] Preferably, the dominant oscillation frequency w d Equal to the angular frequency of the original system's Bode plot at the phase transition, where the original system's Bode plot at w d The phase lag value is θ h .

[0016] Preferably, the expression for the phase compensation stage is as follows:

[0017]

[0018] The tuning formulas for parameters α and T are as follows:

[0019]

[0020] Wherein, parameter α represents the proportional coefficient of the phase compensation element, and parameter T represents the lag time constant of the phase compensation element.

[0021] Preferably, the expression for the isolation step is as follows:

[0022]

[0023] Among them, T w K is the time constant of the isolation process. e This is the ratio coefficient for the isolation process.

[0024] Preferably, the expression for the band-stop filter is as follows:

[0025]

[0026] Among them, w s Let ω be the notch filter frequency. s =ω d ξ s K is the quality coefficient. s is the resonance coefficient.

[0027] Preferably, the isolation ratio coefficient K in the isolation process is verified. e And based on the suppression results, update the isolation link proportional coefficient K. e .

[0028] Preferably, the ξ in the band-stop filter is verified. s Quality factor, the resonance coefficient K s And based on the suppression results, update the ξ s The quality coefficient and the resonance coefficient K s .

[0029] The present invention also discloses an additional stabilizer for suppressing low-frequency oscillations in a doubly-fed wind turbine, characterized in that it includes the aforementioned isolation stage, phase compensation stage, band-stop filter, and inverter, wherein the isolation stage, the phase compensation stage, the band-stop filter, and the inverter are cascaded in sequence.

[0030] In summary, by introducing an additional stabilizer into the system through the above-described technical solutions conceived in this invention, the original system is transformed from low-frequency oscillation instability to small-disturbance stability.

[0031] This invention proposes an additional stabilizer and design method to suppress low-frequency oscillations in doubly-fed induction generator (DFIG) wind turbines. This stabilizer can effectively suppress low-frequency oscillations in DFIG grid-connected systems and effectively enhance the stability of the system under small disturbances. Attached Figure Description

[0032] Figure 1This is a control block diagram of the electromechanical scale model of a doubly-fed wind turbine in the prior art;

[0033] Figure 2 The figure shows a design drawing of an additional stabilizer for suppressing low-frequency oscillations in a doubly-fed wind turbine, according to an embodiment of the present invention.

[0034] Figure 3 A flowchart illustrating the design of an additional stabilizer for suppressing low-frequency oscillations in a doubly-fed wind turbine, according to an embodiment of the present invention;

[0035] Figure 4 Bode plot of the closed-loop transfer function of the system before introducing an additional stabilizer to suppress low-frequency oscillations of the doubly-fed wind turbine, according to an embodiment of the present invention;

[0036] Figure 5 A time-domain simulation comparison diagram of the system before and after introducing an additional stabilizer to suppress low-frequency oscillations of a doubly-fed wind turbine is provided according to an embodiment of the present invention. Detailed Implementation

[0037] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.

[0038] In this invention, the terms "first," "second," etc. (if present) in the invention and the accompanying drawings are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence.

[0039] In doubly-fed induction generator (DFIG) wind turbine grid-connected systems, low-frequency oscillations exist. Under weak grid conditions, these low-frequency oscillations may develop into divergent oscillations, threatening the safe and stable operation of the power system. To suppress these low-frequency oscillations and ensure the stable and safe operation of the power system, this invention discloses an additional stabilizer and its design method for suppressing low-frequency oscillations in DFIG wind turbines. This additional stabilizer effectively suppresses the low-frequency oscillation phenomenon.

[0040] Figure 1 The control block diagram of the electromechanical scale model of a doubly-fed wind turbine in the prior art is shown below. Figure 1 As shown, the existing electromechanical control block diagram of a doubly-fed induction generator (DFIG) includes: a phase-locked loop (PLL) stage, a speed control stage, a terminal voltage control stage, and a rotor dynamic stage. In the embodiment of this invention, the rotor dynamic stage adopts a single-mass block model, P ref For a constant input mechanical power, P tLet H be the electromagnetic output power, and H be the known rotor inertia coefficient. Point A in the speed control loop represents the speed difference, i.e., the speed ω. r With reference value of rotational speed ω rref The difference between them, also known as the rotor speed change signal, is used by the speed control system based on the speed difference (ω). r -ω rref Generating active current i rd Point B in the terminal voltage control loop represents the terminal voltage difference, which is the terminal voltage U. t With terminal voltage reference value U tref The difference between them is also called the terminal voltage change signal, and the terminal voltage control loop is based on the terminal voltage difference (U). t -U tref Generate reactive current i rq The phase-locked loop (PLL) circuit acquires the voltage U at the acquisition terminal. t Projection u on the q-axis tq =U t sin(θ t -θ pll ), to provide phase information θ pll The current in the dq coordinate system is transformed to the common coordinate system, and finally the control part generates the internal potential E. s It is then integrated into the power grid.

[0041] Figure 2 The figure shows a design diagram of an additional stabilizer for suppressing low-frequency oscillations in a doubly-fed wind turbine, according to an embodiment of the present invention. Figure 2 Combining Figure 1 Describe, such as Figure 2 As shown, the input terminal of the auxiliary stabilizer is point A of the speed control loop, and the output terminal is point B of the voltage control loop. From left to right, the auxiliary stabilizer consists of an isolation loop, a phase compensation loop, a second-order band-stop filter, and an inverter, cascaded in sequence. In an embodiment of the invention, the expression for the isolation loop is: T w The time constant for the isolation process is typically given a value between 3 and 5. e K is the isolation ratio coefficient. In the embodiments of the present invention, K e Taking into account the values ​​of various parameters in the actual system, the oscillation suppression effect is compared through repeated iterative calculations until the final isolation link proportional coefficient K is selected. e .

[0042] Furthermore, the expression for the phase compensation stage is as follows: Parameter α represents the proportional coefficient of the phase compensation stage, and parameter T represents the lag time constant of the phase compensation stage. The expression for a second-order band-stop filter is: Where w sThe notch filter frequency, in a low-frequency oscillation environment, has a value between 0 and 1. For example, selecting w... s =w d ξ s K is the quality coefficient, with a value ranging from 1 to 5. s K is the resonance coefficient. In an embodiment of the present invention, K... s Taking into account the values ​​of various parameters of the actual system, the oscillation suppression effect is compared through repeated iterations until the final resonance coefficient K is selected. s .

[0043] Figure 3 A flowchart illustrating the design of an additional stabilizer for suppressing low-frequency oscillations in a doubly-fed wind turbine, according to an embodiment of the present invention. Figure 3 Combining Figure 2 and Figure 1 Describe, such as Figure 3 As shown, the design steps for an additional stabilizer to suppress low-frequency oscillations in a doubly-fed wind turbine are as follows:

[0044] Step S1: Design the input, output, and internal control circuit of the additional stabilizer. Specifically, the input of the additional stabilizer is point A of the speed control circuit in the existing system, the output is point B of the terminal voltage control circuit, and the internal control circuit is... Figure 2 The isolation circuit, phase compensation circuit, second-order band-stop filter, and inverter cascade are shown.

[0045] Step S2: Plot the Bode plot of the closed-loop transfer function of the original system to determine the dominant oscillation frequency w. d And the phase lag value is θ h Specifically, the frequency at the phase transition point of the Bode plot of the closed-loop transfer function in the original system is the dominant oscillation frequency w. d The magnitude of the phase jump in the Bode plot of the closed-loop transfer function corresponds to the phase lag value θ. h .

[0046] Step S3: Perform parameter tuning calculations for the phase compensation stage of the additional stabilizer. Specifically, the formula for calculating the parameter tuning of the phase compensation stage is as follows:

[0047]

[0048] Among them, the dominant oscillation frequency w d And the phase lag value is θ h Obtained in step S2.

[0049] Step S4: Select the isolation ratio coefficient K of the additional stabilizer. e Quality coefficient ξ s and resonance coefficient K s The initial value, specifically, Ke The value range is generally 1-10, ξ s The value range is generally 1-5, K s The value range is generally 1-5, with the three parameters taking values ​​from smallest to largest.

[0050] Step S5: Calculate the time δt required to suppress the disturbance and perform iterative calculation. Specifically, given a given disturbance condition, the disturbance time is denoted as t0, and the time when the waveform converges to the stable point is denoted as t. c The time required for the disturbance to converge to a stable state is denoted as δt = t c -t0, optimize δt, and iterate through the isolation link proportional coefficient K. e Quality coefficient ξ s resonance coefficient K s Calculate the corresponding δt and obtain the final iterative parameter K. e ξ and K s The optimal solution.

[0051] Step S6: Obtain the optimal parameter K e ξ s and K s Substitute this into the expressions for the isolation stage and the second-order band-stop filter. Then, apply the optimal parameter K... e ξ s and K s After substitution, the expression for the internal control loop of the additional stabilizer is determined.

[0052] Figure 4 Bode plot of the closed-loop transfer function of the system before introducing an additional stabilizer to suppress low-frequency oscillations of the doubly-fed wind turbine, according to an embodiment of the present invention. Figure 4 As shown, the lag phase θ corresponding to the phase transition point in the Bode plot of the closed-loop transfer function is... h The dominant oscillation frequency w d =2πf d , where f d ω represents electrical frequency, measured in Hz; d This represents angular frequency, measured in rad / s.

[0053] Figure 5 A time-domain simulation comparison diagram of the system before and after introducing an additional stabilizer to suppress low-frequency oscillations of a doubly-fed wind turbine is provided according to an embodiment of the present invention. Figure 5 As shown, the addition stabilizer disclosed in this invention effectively suppresses low-frequency oscillation instability in doubly-fed wind turbines, maintaining safe and stable operation. The variable θ... pll For the phase-locked loop phase, the variable ω r The horizontal axis represents the rotor speed, and the horizontal axis represents time t.

[0054] In existing doubly-fed induction generator (DFIG) wind turbine systems connected to the grid, a small voltage drop in the grid at 1 second can cause low-frequency oscillations and instability, especially under weak grid conditions. According to the supplementary stabilizer and parameter tuning calculation disclosed in this invention for suppressing low-frequency oscillations in DFIG wind turbines, introducing the supplementary stabilizer into the system transforms the original system from low-frequency oscillation instability to small-disturbance stability.

[0055] In summary, this invention proposes an additional stabilizer and design method to suppress low-frequency oscillations in doubly-fed induction generator (DFIG) wind turbines. This effectively suppresses low-frequency oscillations in DFIG grid-connected systems and enhances the stability of the system under small disturbances.

[0056] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for designing an additional stabilizer to suppress low-frequency oscillations in a doubly-fed wind turbine, characterized in that, include: S1, construct the input and output terminals of the additional stabilizer and design the control circuit; S2. Plot the Bode plot of the closed-loop transfer function of the original system to determine the dominant oscillation frequency ω. d and phase lag value θ h ; S3, Perform parameter tuning calculations for the phase compensation stage in the control loop, wherein the expression for the phase compensation stage is as follows: The tuning formulas for parameters α and T are as follows: Wherein, parameter α represents the proportional coefficient of the phase compensation element, and parameter T represents the lag time constant of the phase compensation element; S4, Select the isolation ratio coefficient of the additional stabilizer. K e The quality coefficient of the second-order bandstop filter of the additional stabilizer ξ s and resonance coefficient K s The initial value of the isolation element is given by the following expression: in, T w The time constant of the isolation process, K e This refers to the ratio coefficient for the isolation process; S5, Calculate the time required to suppress the disturbance. δt And perform iterative calculations; and S6, Obtain the optimal parameters K e , ξ s as well as K s Substitute these values ​​into the expressions for the isolation stage and the second-order band-stop filter, where the expression for the second-order band-stop filter is as follows: Where, ω s Let be the notch filter frequency, and take... ω s = ω d , ξ s For quality coefficient, K s is the resonance coefficient.

2. The method for designing an additional stabilizer to suppress low-frequency oscillations in a doubly-fed wind turbine according to claim 1, characterized in that, The control circuit also includes an inverter, wherein the isolation circuit is connected to the input terminal, and the isolation circuit, the phase compensation circuit, the band-stop filter and the inverter are cascaded in sequence, wherein the output terminal of the inverter is the output terminal of the additional stabilizer.

3. The method for designing an additional stabilizer to suppress low-frequency oscillations in a doubly-fed wind turbine according to claim 1, characterized in that, The input terminal is the rotor speed change signal, and the output terminal is connected to the mid-voltage change signal in the system.

4. The method for designing an additional stabilizer to suppress low-frequency oscillations in a doubly-fed wind turbine according to claim 1, characterized in that, The dominant oscillation frequency ω d Equal to the angular frequency of the original system's Bode plot at the phase transition, where the original system's Bode plot at... ω d The phase lag value is θ h .

5. The method for designing an additional stabilizer to suppress low-frequency oscillations in a doubly-fed wind turbine according to claim 1, characterized in that, Verify the isolation ratio coefficient in the isolation process. K e And based on the suppression results, update the isolation link ratio coefficient. K e .

6. The method for designing an additional stabilizer to suppress low-frequency oscillations in a doubly-fed wind turbine according to claim 1, characterized in that, Verify the second-order bandstop filter described above. ξ s Quality coefficient, the resonance coefficient K s And update the [data / method] based on the suppression results. ξ s Quality coefficient and the resonance coefficient K s .

7. An additional stabilizer for suppressing low-frequency oscillations in a doubly-fed wind turbine, characterized in that, It includes the isolation stage, phase compensation stage, band-stop filter, and inverter as described in claims 1-6, wherein the isolation stage, the phase compensation stage, the band-stop filter, and the inverter are cascaded in sequence.

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