Self-adaptive damping controller and wind power grid-connected system oscillation suppression method thereof

Abstract

The invention discloses a self-adaptive damping controller and a wind power grid-connected system oscillation suppression method thereof, and the method comprises the steps: building a wind power grid-connected system simulation model, obtaining a system closed-loop transfer function, and connecting the damping controller between a wind power plant side and a power grid side; calculating a logarithmic derivative, analyzing impedance characteristics, judging whether the wind power grid-connected system has an unstable oscillation mode or not, and if so, calculating oscillation frequency; carrying out adaptive parameter setting on the damping controller according to the oscillation frequency, wherein the adaptive parameter setting comprises setting of parameters of a band elimination filter, setting of parameters of a band pass filter, setting of the capacity of the damping controller and setting of parameters of a phase shift link; a current reference signal is generated through the subsynchronous damping calculator and then is converted into a three-phase current through the subsynchronous current generator, the three-phase current is injected into a power grid, the impedance characteristic under the oscillation frequency is adjusted, and system oscillation is restrained. Parameters can be adjusted according to the oscillation frequency, complexity is low, implementation is easy, and high efficiency and accuracy of oscillation suppression are met at the same time.

Description

Technical Field

[0001] The invention belongs to the technical field of broadband oscillation suppression, and more specifically, relates to an adaptive damping controller and a method for suppressing oscillations in a wind power grid-connected system. Background Art

[0002] The problem of energy shortage brought about by global development has become a key issue faced by the national economic development of various countries in recent years. Accelerating the process of energy structure transformation and realizing the low-carbonization of the energy structure are gradually becoming a global consensus. The renewable energy power generation system based on wind energy has the advantages of wide resource distribution, high energy utilization efficiency, and mature equipment technology, and its status in the power system is changing from an auxiliary power source to a main power source. However, the long-distance or DC transmission of wind power has led to the characteristics of the power grid connected to wind power evolving from a single-machine infinite system to a weak grid characteristic or a power electronics characteristic. The interaction between the wind power system and the power grid will jointly affect the stability of the power system.

[0003] The frequent occurrence of low-frequency oscillation and sub / supersynchronous oscillation accidents has further attracted the attention of researchers to the problem of high-frequency interaction between wind turbines and the power grid. Accurately and quickly detecting the system oscillation phenomenon and taking oscillation suppression measures are the basis for the safe operation of the power grid; however, there are still some practical problems in the current research on oscillation suppression technology that have not been considered.

[0004] The oscillation frequency has a time-varying characteristic. For example, under different shunt compensation capacitors of the power grid, the system oscillation frequency may shift, and the existing oscillation suppression technology can only improve the damping for the designed frequency points. Although the frequency can be obtained through online frequency detection or an adaptive frequency-locked loop, traditional online frequency detection includes Fourier algorithms, wavelet analysis methods, adaptive window function methods, etc., which have problems such as large computational complexity and poor detection following performance. There is a contradiction between accuracy and rapidity in the dynamic following of the resonant frequency detection based on an adaptive notch filter frequency-locked loop.

[0005] In addition, most of the existing oscillation suppression technologies are based on single-machine cases for the design and research of controllers. On the one hand, the oscillation suppressor needs to be redesigned for different units during actual use, and on the other hand, the unit parameters may have offsets or errors in actual engineering, which may lead to poor actual operation effects of oscillation suppression. The oscillation suppression technology with grid state adaptability is a key issue that has not been focused on in current research and is also a key problem for ensuring the performance of high-frequency oscillation suppression technology in actual engineering applications. Therefore, high-performance oscillation suppression technology still needs to be broken through, and the operating performance of the wind power grid-connected system under different grid conditions still needs to be verified. Summary of the Invention

[0006] To address the deficiencies in the existing technology, the present invention aims to improve the safe grid connection and stable operation of wind turbines. It designs an oscillation suppression method with grid state adaptability, comprehensively considering the economy, environmental friendliness, energy efficiency, and safety and reliability of the wind power grid connection system. An adaptive damping controller is designed to adapt to the change of oscillation frequency by real-time identifying the oscillation frequency and online adjusting the control parameters.

[0007] The present invention adopts the following technical solutions.

[0008] The first aspect of the present invention provides an oscillation suppression method for a wind power grid connection system based on an adaptive damping controller, including the following steps:

[0009] Step 1: Build a simulation model of the wind power grid connection system to obtain the closed-loop transfer function of the wind power grid connection system. Connect a damping controller between the wind farm side and the grid side. The damping controller includes a subsynchronous damping calculator and a subsynchronous current generator. The subsynchronous damping calculator includes a band-stop filter, a band-pass filter, a proportional phase-shifting link, and a limiting link. The subsynchronous current generator includes a controller, a cascaded converter, and a step-up transformer;

[0010] Step 2: Calculate the logarithmic derivative of the closed-loop transfer function of the wind power grid connection system. Analyze the impedance characteristics of the wind power grid connection system according to the logarithmic derivative to determine whether there is an unstable oscillation mode in the wind power grid connection system. If so, calculate the oscillation frequency corresponding to the unstable oscillation mode and proceed to Step 3; otherwise, end the step;

[0011] Step 3: Perform adaptive parameter tuning on the damping controller according to the oscillation frequency, including tuning the parameters of the band-stop filter, tuning the parameters of the band-pass filter, tuning the capacity of the damping controller, and tuning the parameters of the phase-shifting link;

[0012] Step 4: Generate a current reference signal through the subsynchronous damping calculator, and then convert it into three-phase current through the subsynchronous current generator and inject it into the grid to adjust the impedance characteristics of the wind power grid connection system at the oscillation frequency calculated in Step 2 and suppress the oscillation of the wind power grid connection system.

[0013] Preferably, in Step 1, the simulation model of the wind power grid connection system takes the current vectors of each port of the DC transmission network as the output and the voltage vectors of each port as the input. The closed-loop transfer function of the wind power grid connection system is expressed by the following formula:

[0014] G(s) = Y dc Y C / (Y C + Y dc )

[0015] In the formula:

[0016] Z Cis a diagonal matrix, Z C Each diagonal element is the DC impedance of the converter corresponding to the port;

[0017] Y dc is the grid-side admittance matrix.

[0018] Preferably, in step 2, the logarithmic derivative of the closed-loop transfer function of the wind power grid-connected system is calculated by taking the difference in the frequency domain, and is expressed by the following formula,

[0019]

[0020] In the formula:

[0021] G(ω + Δω) is the value of the closed-loop transfer function at frequency ω + Δω;

[0022] G(ω) is the value of the closed-loop transfer function at frequency ω;

[0023] Δω is the selected frequency interval.

[0024] Preferably, in step 2, it is judged whether there is a dominant zero point from the real part and imaginary part curves of the logarithmic derivative. If there is an extreme point on the real part curve and the slope of the imaginary part curve at the frequency corresponding to the extreme point is negative, then the extreme point corresponds to a dominant zero point. Among them, the imaginary part of the dominant zero point is the extreme point frequency, and the real part of the dominant zero point is the negative reciprocal of the extreme value; it is judged whether there is an unstable oscillation mode in the wind power grid-connected system according to the dominant zero point, and the oscillation frequency corresponding to the unstable oscillation mode is calculated.

[0025] Preferably, judging whether there is an unstable oscillation mode in the wind power grid-connected system according to the dominant zero point and calculating the oscillation frequency corresponding to the unstable oscillation mode include:

[0026] If there is a dominant zero point with a real part greater than 0, then there is an unstable oscillation mode in the wind power grid-connected system. The negative of the real part of the dominant zero point is used as the damping of the unstable oscillation mode, and the imaginary part of the dominant zero point is used as the oscillation frequency of the unstable oscillation mode; if the real parts of the obtained dominant zero points are all less than 0, then the wind power grid-connected system is stable.

[0027] Preferably, in step 2, analyzing the impedance characteristics of the wind power grid-connected system includes:

[0028] Calculating the total impedance Z ∑ when the damping controller is not connected to the wind power grid-connected system, and is expressed by the following formula,

[0029] Z ∑ = Z W + Z N

[0030] In the formula:

[0031] Z Wis the equivalent impedance on the wind farm side, including the total impedance Z of the wind turbines WTG , the transformer impedance Z T and the wind farm line impedance Z NL ;

[0032] Z N is the equivalent impedance on the grid side, including the series compensation line impedance Z CL and the total impedance Z of the receiving system SYS ;

[0033] Calculate the total impedance Z′ ∑ after connecting the damping controller, which is expressed by the following formula

[0034]

[0035] wherein:

[0036] H i and H u respectively represent the transfer functions of the current and voltage input signals

[0037] Preferably, in step 3, according to the principle of retaining the amplitude of the oscillation signal and making the phase of the oscillation signal have no difference at the oscillation frequency, select the cut-off frequencies of the band-stop filter and the band-pass filter

[0038] Preferably, in step 3, the capacity of the damping controller is set according to the principle of avoiding amplitude limiting and reducing economic costs at the oscillation frequency, which is expressed by the following formula

[0039]

[0040] wherein:

[0041] I max represents the sub-synchronous current amplitude under the worst condition

[0042] U rms represents the effective voltage value

[0043] k m is the margin coefficient

[0044] Preferably, in step 3, the setting of the phase-shifting link parameters includes

[0045] adopting a second-order lead-lag link to adjust the phase shift, which is expressed by the following formula

[0046]

[0047] wherein:

[0048] K u is the gain of the phase-shifting link

[0049] T u is the time constant of the phase-shifting link;

[0050] According to the oscillation frequency f o calculate the time constant T of the phase-shifting link u , which is expressed by the following formula

[0051]

[0052] In the formula:

[0053] θ e is the selected desired impedance angle within the range of 0 to 90°;

[0054] θ f is the sum of the phase shifts of the adaptive damping controller at the oscillation frequency f o .

[0055] The second aspect of the present invention provides an adaptive damping controller for implementing the oscillation suppression method of the wind power grid-connected system based on the adaptive damping controller, including: a sub-synchronous damping calculator, a sub-synchronous current generator, an oscillation frequency identification module, and a phase-shifting parameter calculation module;

[0056] The sub-synchronous damping calculator includes a band-stop filter, a band-pass filter, a proportional phase-shifting link, and a limiting link, and is used to filter the power frequency component in the input signal and generate a current reference signal for suppressing sub-synchronous oscillation according to the oscillation component in the input signal

[0057] The sub-synchronous current generator includes a controller, a cascaded converter, and a step-up transformer, and is used to generate three-phase current i Gref and inject it into the power grid according to the current reference signal provided by the sub-synchronous damping calculator;

[0058] The oscillation frequency identification module is used to judge whether there is an unstable oscillation mode in the wind power grid-connected system, calculate the oscillation frequency corresponding to the unstable oscillation mode, and tune the parameters of the band-stop filter and the band-pass filter;

[0059] The phase-shifting parameter calculation module is used to tune the parameters of the phase-shifting link.

[0060] Compared with the prior art, the beneficial effects of the present invention at least include:

[0061] (1) The present invention adopts the logarithmic derivative method, and approximately calculates the logarithmic derivative of the system closed-loop transfer function by taking the difference in the frequency domain, reducing the calculation amount and improving the oscillation frequency identification efficiency.

[0062] (2) The adaptive damping controller of the present invention can make full use of measurement data by introducing an oscillation frequency identification module and a phase shift parameter calculation module, adjust the parameters of the control link according to the change of the vibration frequency, and does not need to know the system model or oscillation frequency in advance.

[0063] (3) The adaptive damping controller of the present invention can adapt to the change of the system operating point, realize the adaptive suppression of subsynchronous oscillation, has a clear physical meaning and is easy to implement, and is expected to provide a new control device for solving the oscillation problem of the wind power grid-connected system in engineering practice. Brief Description of the Drawings

[0064] Figure 1 is a schematic diagram of the connection position of the adaptive damping controller

[0065] Figure 2 is a flowchart of the oscillation suppression of the wind power grid-connected system;

[0066] Figure 3 is a schematic diagram of the structure of the adaptive damping controller;

[0067] Figure 4 is a flowchart of the oscillation frequency identification;

[0068] Figure 5 is an equivalent circuit diagram after connecting the parallel adaptive damping controller;

[0069] Figure 6 (1) is the Bode diagram of the wind turbine and the flexible DC under voltage disturbance from 1 to 1000 Hz;

[0070] Figure 6 (2) is the Bode diagram of the wind turbine and the flexible DC under voltage disturbance from 40 to 60 Hz;

[0071] Figure 7 is the Bode diagram of the band-pass filter;

[0072] Figure 8 is the curve of the real part of the eigenvalue changing with different impedance amplitudes and impedance angles. Detailed Embodiment

[0073] To make the objectives, technical solutions and advantages of the present invention clearer, the technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. The embodiments described in this application are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the spirit of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative work belong to the protection scope of the present invention.

[0074] Embodiment 1 of the present invention provides a method for suppressing oscillation of a wind power grid-connected system based on an adaptive damping controller, including the following steps:

[0075] Step 1: Obtain the grid structure information and equipment conditions of the wind power grid-connected system, build a simulation model of the wind power grid-connected system to count the oscillation conditions in different frequency ranges of the wind power grid-connected system. Obtain the closed-loop transfer function of the wind power grid-connected system according to the simulation model of the wind power grid-connected system, and connect a damping controller between the wind farm side and the grid side, where the damping controller includes a subsynchronous damping calculator (SDC) and a subsynchronous current generator (SCG). The subsynchronous damping calculator includes a band-stop filter, a band-pass filter, a proportional phase-shifting link, and a limiting link. The subsynchronous current generator includes a controller, a cascaded converter, and a step-up transformer.

[0076] Preferably, represent the DC side of the wind power grid-connected system as a multi-input multi-output system (MIMO), and the input-output relationship of the system is as follows:

[0077]

[0078] In the formula:

[0079] I dc and U dc are vectors composed of the currents and voltages of each port of the DC transmission network respectively;

[0080] E is the identity matrix;

[0081] Z C is a diagonal matrix, and the diagonal elements of the matrix are the DC impedance models of the converters corresponding to the ports;

[0082]

[0083] Y dc is the grid-side admittance matrix.

[0084] According to the input-output relationship of the system, the closed-loop transfer function of the wind power grid-connected system is obtained as G(s) = Y dc Y C / (Y C +Y dc ), and the denominator determinant is:

[0085] |G| = det(Y C +Y dc )

[0086] Step 2: Calculate the logarithmic derivative D L (G) of the closed-loop transfer function G(s) of the wind power grid-connected system, analyze the impedance characteristics of the wind power grid-connected system according to the logarithmic derivative, judge whether there is an unstable oscillation mode in the wind power grid-connected system. If so, calculate the oscillation frequency corresponding to the unstable oscillation mode, and enter Step 3; otherwise, end the step.

[0087] Those skilled in the art can understand that by solving the zeros of the closed-loop transfer function G(s) of the wind power grid-connected system, that is, the poles of the system, the oscillation modes can be obtained and their stability can be determined. To meet the needs of adaptive damping control and achieve fast and accurate identification of the oscillation frequency, and to avoid the problem of high computational complexity in solving the zeros of the high-order closed-loop transfer function, the logarithmic derivative method is adopted to identify the dominant zeros of the transfer function G(s) from the logarithmic derivative curve, that is, the dominant oscillation modes of the system. By calculating the changes in the dominant oscillation modes under different parameters, the implicit relationship between the parameters and the damping and oscillation frequency of the oscillation modes can be qualitatively obtained.

[0088] In a preferred but non-limiting embodiment of the present invention, step 2 specifically includes:

[0089] Step 2.1, to reduce the amount of calculation, the logarithmic derivative D L (G) of the transfer function G(s) is approximately calculated by taking the difference in the frequency domain, which is expressed by the following formula:

[0090]

[0091] where:

[0092] G(ω + Δω) is the value of the closed-loop transfer function at the frequency ω + Δω;

[0093] G(ω) is the value of the closed-loop transfer function at the frequency ω;

[0094] Δω is the selected frequency interval.

[0095] Step 2.2, determine whether there are dominant zeros from the real and imaginary part curves of the logarithmic derivative D L (G), and then determine whether there are unstable oscillation modes in the wind power grid-connected system, and calculate the oscillation frequency corresponding to the unstable oscillation mode.

[0096] Further preferably, if there is an extreme point on the real part curve and the slope of the imaginary part curve at the frequency of the extreme point is negative, then this extreme point corresponds to a dominant zero. The imaginary part of the dominant zero is the frequency ω z of the extreme point, and the real part α z of the dominant zero is the negative reciprocal of the extreme value, which is expressed by the following formula:

[0097]

[0098] If there is a dominant zero with a real part greater than 0, then this dominant zero corresponds to an unstable oscillation mode. The negative of its real part corresponds to the damping of the oscillation mode, and the imaginary part corresponds to the oscillation frequency f o = ω z ; if the real parts of the obtained zeros are all less than 0, then the system is stable.

[0099] Step 2.3: Analyze the impedance characteristics of the wind power grid-connected system so that after the damping controller is connected, the impedance characteristics of the system at the sub-synchronous frequency are optimized.

[0100] Equivalent the damping controller to a controllable impedance Z with the bus voltage u of the wind farm and the line current i as input signals. c . It shows impedance in the sub-synchronous frequency range and an open circuit in the power frequency range. Based on the oscillation frequency f identified in Step 2.2 o Connect the damping controller and adjust the damping characteristics of the wind power grid-connected system at the oscillation frequency point to achieve the purpose of suppressing sub-synchronous oscillation.

[0101] As Figure 1 shown, when the damping controller is connected between the wind farm side and the grid side, the equivalent impedance Z of the wind farm side W includes the total impedance Z of the wind turbines WTG , the transformer impedance Z T and the wind farm line impedance Z NL . The equivalent impedance Z of the grid side N includes the series-compensated line impedance Z CL and the total impedance Z of the receiving-end system SYS .

[0102] Among them, the wind turbines include squirrel-cage induction generators (SEIG), doubly-fed induction generators (DFIG) and permanent magnet synchronous generators (PMSG), and the corresponding impedance models are:

[0103] Z SEIG = [r r s(s - jω r ) -1 + sL r / / (sL m ) + R s sL s

[0104]

[0105] Z PMSG = R PMSG + jX PMSG

[0106] The impedance models of the wind farm line, series-compensated line, transformer and receiving-end system are:

[0107] Z NL = sL NL + R NL

[0108] Z CL = sL CL + R CL + 1 / (sC)

[0109] Z T = sL T + R T

[0110] Z SYS = sL SYS + R SYS

[0111] When the damping controller is not connected to the system, i.e., in the open-loop state, the total impedance of the system can be expressed as Z ∑ = Z W + Z N . If the wind farm exhibits negative resistance at the oscillation frequency f o , the wind power grid-connected system will be unstable and may undergo subsynchronous oscillation.

[0112] When the damping controller is connected, the total impedance Z' ∑ is expressed as:

[0113]

[0114] In the formula:

[0115] H i and H u respectively represent the transfer functions of the current and voltage input signals i and u.

[0116] By the action of the damping controller, the impedance characteristics of the system at the subsynchronous frequency are changed, so that the real part α z of the dominant zero changes from positive to negative, and the system changes from unstable to stable at f o , thus achieving the purpose of suppressing subsynchronous oscillation.

[0117] Step 3: Perform adaptive parameter tuning on the damping controller according to the oscillation frequency to automatically update the parameters after detecting the oscillation, including the tuning of the band-stop filter parameters, the tuning of the band-pass filter parameters, the tuning of the damping controller capacity, and the tuning of the phase-shifting link.

[0118] In the preferred but non-limiting embodiment of the present invention, Step 3 specifically includes:

[0119] Step 3.1: Tuning of the band-stop filter and band-pass filter parameters: There are three major principles for extracting the oscillation signal, that is, to retain the amplitude of the oscillation signal as much as possible at the oscillation frequency, to make the phase of the oscillation signal have no difference, and to have a suitable bandwidth range. Based on this, the cut-off frequency of the filter can be selected.

[0120] Step 3.2: Tuning of the damping controller capacity: The impedance Z cThe amplitude reflects the gain of the damping control loop, i.e., the device capacity of the adaptive damping controller. The greater the gain, the larger the output current, the smaller the corresponding impedance, and the stronger the suppression ability. In practical applications, it is necessary to consider avoiding control limit and reducing economic costs at the oscillation frequency to reasonably design the device capacity, and the design can be carried out with reference to the following formula:

[0121]

[0122] In the formula:

[0123] I max represents the amplitude of the subsynchronous current under the worst conditions;

[0124] U rms represents the effective voltage value;

[0125] k m is the margin coefficient, which can be set considering future power grid development, such as the expansion demand of wind farms, etc.

[0126] Step 3.3, tuning of the phase-shifting link parameters: The phase angle of the impedance Z c corresponds to the total phase shift of the damping control loop. From the phase angle change, it can be seen that when the parallel impedance is a positive resistor and an inductor, and the corresponding phase angle is in the range of 0 to 90°, the real part of the eigenvalue can be negative to maintain system stability. The adaptive damping controller uses a second-order lead-lag link as the proportional phase-shifting link to realize the adjustment of the phase shift:

[0127]

[0128] In the formula: K u is the gain of the phase-shifting link, and T u is the time constant of the phase-shifting link.

[0129] Although it is difficult to obtain the equivalent circuit of the system for analysis in actual operation, the desired impedance angle θ e can be selected in the range of 0 to 90°. Then, considering the phase shift of the filtering link of the adaptive damping controller, the phase-shifting link parameters are modified according to the oscillation frequency so that the total phase shift remains consistent at different oscillation frequencies. According to the oscillation frequency f o the expression for calculating the time constant T u of the phase-shifting link is as follows:

[0130]

[0131] In the formula: θ e is the desired impedance angle selected in the range of 0 to 90°, and θ f is the sum of the phase shifts of the filter at f o .

[0132] Step 4: Generate a current reference signal through a subsynchronous damping calculator, and then convert it into three-phase currents through a subsynchronous current generator and inject them into the power grid to adjust the impedance characteristics of the wind power grid-connected system at the oscillation frequency calculated in Step 2 and suppress the oscillation of the wind power grid-connected system.

[0133] Preferably, the DC voltage U dc is the most significant influencing factor in the subsynchronous oscillation phenomenon. In the present invention, U dc is used as the voltage input signal u of the damping controller.

[0134] In a preferred but non-limiting embodiment of the present invention, Step 4 specifically includes:

[0135] Step 4.1: After the input signal is fed into the subsynchronous damping calculator, use the band-stop filter F s (s) and the band-pass filter F p (s) to filter the power frequency components in the input signal to achieve signal extraction, process the signal using a proportional phase-shifting link, and synthesize the required current reference signal through an adder The relationship between the output and the input signal of the subsynchronous damping calculator is:

[0136]

[0137] In the formula:

[0138] H i (s) = F s (s)F p (s)k i ∠θ i , H u (s) = F s (s)F p (s)k u ∠θ u ;

[0139] k u and k i respectively represent the gains of the voltage and current subsynchronous frequency signals;

[0140] θ u and θ i respectively represent the phase shifts of the voltage and current subsynchronous frequency signals.

[0141] Step 4.2: The current reference signal of the subsynchronous damping calculator is sent to the subsynchronous current generator through optical fiber communication, and the subsynchronous current generator converts the current reference signal into the current actually injected into the power grid.

[0142] Further preferably, the optical fiber communication data format is based on the FT3 communication protocol of the IEC60870-5-1 standard.

[0143] It is understandable that the present invention can adapt to the change of oscillation frequency through the rapid identification of oscillation characteristics and the online adjustment of parameters, and can effectively suppress oscillation under different working conditions. Compared with the existing oscillation suppression methods, it can adjust parameters according to the oscillation frequency, with lower complexity, easier to implement, and at the same time meet the high efficiency and accuracy of oscillation suppression.

[0144] Embodiment 2 of the present invention provides an adaptive damping controller, including: a subsynchronous damping calculator (SDC), a subsynchronous current generator (SCG), an oscillation frequency identification module, and a phase-shifting parameter calculation module;

[0145] The adaptive damping controller is an oscillation suppression device installed on the grid side of the wind farm collection bus. The parameters of the filtering link and the phase-shifting link in the adaptive damping controller come from the oscillation frequency identification module and the phase-shifting parameter calculation module respectively. Among them, the oscillation frequency identification module adopts an oscillation frequency identification method based on the logarithmic derivative method. The SDC uses the oscillation component in the input signal to generate a reference current signal, and the SCG tracks the reference current signal to inject three-phase current into the point of common coupling of the wind farm, enhancing the damping of the system within a specific frequency range and realizing the suppression of subsynchronous oscillation by the adaptive damping controller.

[0146] Different from the damping control with fixed parameters, the adaptive damping controller adopts the method of "identifying and controlling immediately", adapts to the change of oscillation frequency by real-time identifying the oscillation frequency and online adjusting the control parameters, and effectively suppresses oscillation under diverse system operation modes.

[0147] Preferably, the subsynchronous damping calculator includes a band-stop filter, a band-pass filter, a proportional phase-shifting link, and a limiting link, which are used to filter the power frequency component in the input signal and generate a current reference signal for suppressing subsynchronous oscillation according to the oscillation component in the input signal

[0148] Preferably, the subsynchronous current generator includes a controller, a cascaded converter, and a step-up transformer, which are used to generate three-phase current i according to the current reference signal provided by the subsynchronous damping calculator through the cascaded converter Gref Inject into the power grid;

[0149] It should be noted that the role of the SCG controller is to convert the reference signal provided by the subsynchronous damping calculator SDC into the modulation wave signal required by the cascaded converter. Different from the traditional power frequency cascaded converter, the components of the cascaded converter for subsynchronous frequency need special design, and special attention should be paid to the selection of DC capacitors and connection reactances to ensure that the response speed is fast enough and the DC voltage fluctuation is as small as possible.

[0150] Preferably, the oscillation frequency identification module is used to determine whether there is an unstable oscillation mode in the wind power grid-connected system, calculate the oscillation frequency corresponding to the unstable oscillation mode, and set the parameters of the band-stop filter and the band-pass filter;

[0151] Preferably, the phase-shift parameter calculation module is used for the phase-shift link parameters.

[0152] To further clearly introduce the technical solution of the present invention and the beneficial technical effects brought by it, the following introduces application examples. In a certain wind power grid-connected system, the oscillation suppression method for the wind power grid-connected system based on the adaptive damping controller provided in Embodiment 1 of the present invention is applied, as Figure 2 shown, and the implementation process is described in detail as follows:

[0153] First, analyze the grid structure and equipment conditions of the wind power grid-connected system, and establish an equivalent model of the system. Represent the DC side of the flexible DC transmission system as a multi-input multi-output system, and the closed-loop transfer function of the system is Y dc Y C / (Y C +Y dc ).

[0154] Next, install an adaptive damping control device on the grid side of the wind farm collection bus. The structure of the adaptive damping controller is as Figure 3 shown. Figure 3 Among them, K p_out and K i_out are the outer-loop proportional gain and integral gain; K p_in and K i_in are the inner-loop proportional gain and integral gain; I gd and I gd_ref are the grid-side d-axis current component and its given value; I gq is the grid-side q-axis current component; U gd and U gd_ref are the grid-side d-axis voltage component and its given value; ω g is the grid synchronous rotating electrical angle; L f is the grid-side filter inductor; U dc_ref is the DC voltage given value. Filter the input signal to obtain the oscillation component, adjust the amplitude and phase through the proportional phase-shift link to generate the current reference signal i Gref . Introduce the oscillation frequency identification module and the phase-shift parameter calculation module, and the parameters of the filtering link and the phase-shift link come from the oscillation frequency identification and the phase-shift parameter calculation respectively.

[0155] The flow chart of the oscillation frequency identification is as Figure 4As shown. By solving the zeros of G(s), that is, the system poles, the oscillation modes are obtained and their stability is determined. The dominant zeros of the transfer function G(s) are identified using the logarithmic derivative method. By calculating the changes in the dominant oscillation modes under different parameters, the implicit relationship between the parameters and the oscillation damping and frequency is obtained. The logarithmic derivative D L (G):

[0156]

[0157] where: Δω is the selected frequency interval.

[0158] From the real and imaginary part curves of the logarithmic derivative D L (G), it is judged whether there are dominant zero-poles. If there is an extreme point on the real part curve and the slope of the imaginary part curve at the frequency of the extreme point is negative, then this extreme point corresponds to a dominant zero. The imaginary part of the dominant zero is the frequency ω z of the extreme point, and the real part of the dominant zero is the negative reciprocal of the extreme value.

[0159]

[0160] If there is a zero with a real part greater than 0, then this zero corresponds to an unstable oscillation mode. The negative of its real part corresponds to the oscillation mode damping, and the imaginary part corresponds to the oscillation frequency; if the real parts of all the obtained zeros are less than 0, then the system is stable.

[0161] The RLC equivalent circuit can explain the mechanism of subsynchronous oscillation in a wind farm. Figure 5 is the equivalent circuit after the parallel adaptive damping controller, and a parallel impedance representing the adaptive damping controller is introduced. The adaptive damping controller uses the DC voltage as the input signal to generate a subsynchronous current, which can be equivalent to a controllable impedance in the subsynchronous frequency range. At the subsynchronous oscillation frequency, the wind farm side is equivalent to a negative resistance R w and an inductor L w , and the grid side is equivalent to a positive resistance R g and a capacitor C g . Assume that the adaptive damping controller is a resistive-inductive branch R c and L c .

[0162] After that, a disturbance is applied between the wind turbine and the three-phase voltage source, the voltage and current after the disturbance are analyzed, their impedance is extracted, and the Bode diagram is plotted, as Figure 6 shown, and the stability analysis is carried out using the frequency domain analysis method.

[0163] From Figure 6It can be seen that the system has an oscillation risk at 45 Hz. Based on the frequency shift characteristic of the dq transformation, for the subsynchronous frequency signal, the 45 Hz signal is transformed by the dq transformation in the αβ coordinate system, and the signal frequency becomes 5 Hz. The 45 Hz disturbance of the three-phase voltage at the generator terminal is converted into a 5 Hz signal in the DC voltage signal. Therefore, suppressing the 5 Hz oscillation signal in dc the DC voltage U

[0164] selects a high-pass filter with a cut-off frequency of 2 Hz and a low-pass filter with a cut-off frequency of 12 Hz. Then the transfer function G band (s) of the band-pass filter is as follows:

[0165]

[0166] In the formula: G high (s) and G low (s) are the transfer functions of the high-pass filter and the low-pass filter respectively.

[0167] Perform input-output tests on the band-pass filter. From Figure 7 it can be obtained that the band-pass filtering is 1.57 - 15.6 Hz, the amplitude at 5 Hz is -1.38 dB, the amplitude of the oscillation signal is greatly retained, the phase angle at 5 Hz is -0.388°, and there is basically no phase difference, which well ensures the accurate filtering of the target signal.

[0168] Based on the equivalent circuit shown in Figure 5 , design the parameters of the adaptive damping controller. When there is no adaptive damping controller, the eigenvalues of the second-order circuit are:

[0169]

[0170] In the formula: R w +R g <0, the real part of the eigenvalue is positive, indicating that the system has an unstable oscillation mode, and the imaginary part of the eigenvalue is the oscillation angular frequency.

[0171] After connecting the parallel adaptive damping controller, the state matrix of the third-order circuit is:

[0172]

[0173] Change the amplitude |Z c | and the impedance angle of the adaptive damping controller, substitute them into this matrix, and calculate the real part of the eigenvalue of the oscillation mode. The results are as shown in Figure 8 .

[0174] The impedance Z cThe amplitude reflects the gain of the damping control loop, that is, the capacity of the adaptive damping controller. The greater the gain, the larger the output current, the smaller the corresponding impedance, and the stronger the suppression ability. Considering avoiding control clipping and reducing economic costs, the device capacity is reasonably designed and designed according to the following formula:

[0175]

[0176] In the formula: I max represents the amplitude of the subsynchronous current under the worst conditions; U rms represents the effective voltage value; k m is the margin coefficient, which can be set considering future grid development, such as the expansion demand of wind farms, etc.

[0177] The phase angle of the impedance Z c corresponds to the total phase shift of the damping control loop. From the phase angle change, it can be seen that when the parallel impedance is a positive resistor and an inductor, and the corresponding phase angle is in the range of 0-90°, the real part of the eigenvalue can be negative to maintain system stability. The controller uses a second-order lead-lag link to adjust the phase shift:

[0178]

[0179] Select the desired impedance angle θ e in the range of 0-90°. Then consider the phase shift of the filtering link and modify the parameters of the phase shift link according to the oscillation frequency so that the total phase shift remains consistent at different oscillation frequencies. Calculate the time constant T o of the phase shift link according to the oscillation frequency f u The expression is as follows:

[0180]

[0181] In the formula: θ f is Figure 6 the sum of the phase shifts of the filter at f o .

[0182] After detecting the oscillation, the adaptive damping controller automatically updates the parameters and outputs a suppression current signal, realizing the oscillation suppression of the wind power grid-connected system at 45 Hz.

[0183] In addition, in each embodiment of the present invention, each functional unit can be integrated into a processing unit, or each unit can exist physically alone, or two or more units can be integrated into one unit. The above-mentioned integrated unit can be implemented in the form of hardware or in the form of a software functional unit. In the form of hardware, at the lowest level, an FPGA is used to implement signal acquisition, filtering, proportional phase shift, etc. for generating reference current signals with high real-time requirements. The real-time processor communicates with the host computer through an Ethernet port. The host computer includes oscillation frequency identification, parameter calculation, and a human-machine interaction interface.

[0184] Compared with the prior art, the beneficial effects of the present invention at least include:

[0185] (1) The present invention adopts the logarithmic derivative method, and approximately calculates the logarithmic derivative of the system closed-loop transfer function by taking differences in the frequency domain, reducing the calculation amount and improving the oscillation frequency identification efficiency.

[0186] (2) The adaptive damping controller of the present invention can make full use of measurement data by introducing an oscillation frequency identification module and a phase shift parameter calculation module, and adjust the parameters of the control link according to the change of the vibration frequency without prior knowledge of the system model or the oscillation frequency.

[0187] (3) The adaptive damping controller of the present invention can adapt to the change of the system operating point, realize the adaptive suppression of subsynchronous oscillation, has a clear physical meaning and is easy to implement, and is expected to provide new control equipment for solving the oscillation problem of the wind power grid-connected system in engineering practice.

[0188] The present disclosure can be a system, a method, and / or a computer program product. The computer program product can include a computer-readable storage medium having computer-readable program instructions thereon for causing a processor to implement various aspects of the present disclosure.

[0189] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to the above embodiments, those of ordinary skill in the art should understand that: modifications or equivalent replacements can still be made to the specific embodiments of the present invention, and any modification or equivalent replacement that does not depart from the spirit and scope of the present invention should be covered by the protection scope of the claims of the present invention.

Claims

1. A method for suppressing oscillation of a wind power grid-connected system based on an adaptive damping controller, characterized in that: The following steps are involved: Step 1, build a wind power grid-connected system simulation model, obtain the closed-loop transfer function of the wind power grid-connected system, and connect a damping controller between the wind farm side and the grid side, wherein the damping controller includes a sub-synchronous damping calculator and a sub-synchronous current generator, the sub-synchronous damping calculator includes a band-stop filter, a band-pass filter, a proportional phase shift link and a limiting link, and the sub-synchronous current generator includes a controller, a cascade converter and a step-up transformer; Step 2, calculate the logarithmic derivative of the closed-loop transfer function of the wind power grid-connected system, analyze the impedance characteristics of the wind power grid-connected system according to the logarithmic derivative, and determine whether the wind power grid-connected system has an unstable oscillation mode. If so, calculate the oscillation frequency corresponding to the unstable oscillation mode and proceed to step 3, otherwise end the step; Step 3, adaptively adjusting the parameters of the damping controller according to the oscillation frequency, including adjusting the parameters of the band-stop filter, the band-pass filter, the capacity of the damping controller and the parameters of the phase shift link; Step 4, generate a current reference signal through a sub-synchronous damping calculator, and then convert it into a three-phase current through a sub-synchronous current generator and inject it into the power grid to adjust the impedance characteristics of the wind power grid-connected system at the oscillation frequency calculated in step 2 to suppress the oscillation of the wind power grid-connected system.

2. The method for suppressing oscillation of a wind power grid-connected system based on an adaptive damping controller according to claim 1, characterized in that: In step 1, the wind power grid-connected system simulation model uses the current vector of each port of the DC transmission network as output and the voltage vector of each port as input. The closed-loop transfer function of the wind power grid-connected system is expressed by the following formula: G(s)=Y dc AND C / (AND C +Y dc ) Where: Z C is a diagonal matrix, Z C Each diagonal element is the DC impedance of the converter corresponding to the port; Y dc is the network side admittance matrix.

3. The method for suppressing oscillation of a wind power grid-connected system based on an adaptive damping controller according to claim 1, characterized in that: In step 2, the logarithmic derivative of the closed-loop transfer function of the wind power grid-connected system is calculated by taking a difference in the frequency domain, which is expressed as follows: Where: G(ω+Δω) is the closed-loop transfer function value at frequency ω+Δω; G(ω) is the closed-loop transfer function value at frequency ω; Δω is the selected frequency interval.

4. The method for suppressing oscillation of a wind power grid-connected system based on an adaptive damping controller according to claim 1, characterized in that: In step 2, the real and imaginary curves of the logarithmic derivative are used to determine whether there is a dominant zero point. If the real curve has an extreme point and the slope of the imaginary curve is negative at the frequency corresponding to the extreme point, then the extreme point corresponds to a dominant zero point, where the imaginary part of the dominant zero point is the extreme point frequency, and the real part of the dominant zero point is the negative reciprocal of the extreme value. Based on the dominant zero point, it is determined whether there is an unstable oscillation mode in the wind power grid-connected system, and the oscillation frequency corresponding to the unstable oscillation mode is calculated.

5. The method for suppressing oscillation of a wind power grid-connected system based on an adaptive damping controller according to claim 4 is characterized in that: Judging whether there is an unstable oscillation mode in the wind power grid-connected system based on the dominant zero point and calculating the oscillation frequency corresponding to the unstable oscillation mode include: If there is a dominant zero point with a real part greater than 0, there is an unstable oscillation mode in the wind power grid-connected system. The negative number of the real part of the dominant zero point is used as the damping of the unstable oscillation mode, and the imaginary part of the dominant zero point is used as the oscillation frequency of the unstable oscillation mode. If the real parts of the dominant zero points are all less than 0, the wind power grid-connected system is stable.

6. The method for suppressing oscillation of a wind power grid-connected system based on an adaptive damping controller according to claim 1, characterized in that: In step 2, the impedance characteristics of the wind power grid-connected system are analyzed including: Calculate the total impedance Z when the damping controller is not connected to the wind power grid system ∑ , expressed as the following formula, WITH ∑ =Z W +Z N Where: Z W is the equivalent impedance of the wind farm side, including the total impedance of the wind turbine group Z WTG , Transformer impedance Z T And the wind farm line impedance Z NL ; Z N is the equivalent impedance on the grid side, including the series compensation line impedance Z CL and the total impedance of the receiving system Z SYS ; Calculate the total impedance Z′ after connecting the damping controller ∑ , expressed as the following formula, Where: H i and H u Represent the transfer functions for current and voltage input signals respectively.

7. The method for suppressing oscillation of a wind power grid-connected system based on an adaptive damping controller according to claim 1, characterized in that: In step 3, the cutoff frequencies of the band-stop filter and the band-pass filter are selected according to the principle of retaining the amplitude of the oscillation signal at the oscillation frequency and making the phase of the oscillation signal invariant.

8. The method for suppressing oscillation of a wind power grid-connected system based on an adaptive damping controller according to claim 1, characterized in that: In step 3, the damping controller capacity is adjusted according to the principle of avoiding limiting at the oscillation frequency and reducing economic costs, which is expressed as follows: Where: I max Represents the subsynchronous current amplitude under the worst case; U rms Represents the effective value of voltage; k m is the margin factor.

9. The method for suppressing oscillation of a wind power grid-connected system based on an adaptive damping controller according to claim 1, characterized in that: In step 3, the setting of phase shifting link parameters includes: The second-order lead-lag link is used to adjust the phase shift, which is expressed by the following formula: Where: K u is the gain of the phase shift link; T u is the time constant of the phase shift link; According to the oscillation frequency f o Calculate the time constant T of the phase shifter u , expressed as the following formula, Where: θ e Select the desired impedance angle within the range of 0 to 90°; θ f is the adaptive damping controller at the oscillation frequency f o The sum of the phase shifts.

10. An adaptive damping controller, used to implement the method for suppressing oscillation of a wind power grid-connected system based on an adaptive damping controller as claimed in any one of claims 1 to 9, characterized in that: include: A subsynchronous damping calculator, a subsynchronous current generator, an oscillation frequency identification module and a phase shift parameter calculation module; The subsynchronous damping calculator includes a band-stop filter, a band-pass filter, a proportional phase shift link and a limiter link, which are used to filter the power frequency component in the input signal and generate a current reference signal to suppress subsynchronous oscillation according to the oscillation component in the input signal. The sub-synchronous current generator includes a controller, a cascade converter and a step-up transformer, which is used to generate a three-phase current i through the cascade converter according to a current reference signal provided by the sub-synchronous damping calculator. Gref Injection into the grid; The oscillation frequency identification module is used to determine whether there is an unstable oscillation mode in the wind power grid-connected system, calculate the oscillation frequency corresponding to the unstable oscillation mode, and adjust the parameters of the band-stop filter and the band-pass filter; The phase-shift parameter calculation module is used to adjust the phase-shift link parameters.

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