Method and system for excitation inrush current attenuation of direct drive wind turbine grid-connected system

Abstract

The application provides a kind of calculation method and equivalent system for excitation inrush attenuation of direct drive wind turbine grid-connected system, comprising: based on the attenuation characteristics of transformer excitation inrush, the variable inductance model of transformer is constructed;Neglecting the inner loop of direct drive wind turbine grid-side converter control, the transfer function of current inner loop and filter inductance is obtained, and the expression of simplified frequency domain input impedance is obtained according to the transfer function;Based on the expression of simplified frequency domain input impedance, the direct drive wind turbine grid-side converter is equivalent to the series branch composed of circuit elements RLC;According to the variable inductance model and the series branch, the system state space equation set is established, and the state space equation of state variable is obtained by state space modeling of system state space equation set;The characteristic root and participation factor of state matrix are solved by state space equation, and the excitation inrush attenuation speed of direct drive wind power grid-connected system is obtained. The accuracy of excitation inrush attenuation calculation is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of direct-drive wind power grid-connected technology, and in particular to a calculation method and system for excitation inrush current attenuation in a direct-drive wind turbine grid-connected system. Background Art

[0002] When putting an unloaded transformer into the grid, its primary impedance and excitation resistance can be ignored, and only the excitation inductance can be considered, which is regarded as a nonlinear inductor device. Here, the dynamic process of a general linear inductor being put into the grid is explained, and the attenuation characteristics of its free component are clarified to provide a reference for the dynamic analysis of the transformer putting into the grid. Taking a simple first-order circuit system as an example, its structure is as follows Figure 2 shown.

[0003] Among them, the system power supply voltage is u s =U m sin(ωt+α), α is the initial phase angle, Ls and Rs are the equivalent inductance and resistance of the system respectively, and L is the linear inductance of the grid. After the switch is closed, the system differential equation is:

[0004]

[0005] Solving the problem, we can get the general solution of current as:

[0006]

[0007] Where: is the steady-state current amplitude; is the decay time constant; is the impedance angle after closing. The current consists of a steady-state periodic component and a decaying free component. The decay process of the instantaneous current value mainly refers to the decay of the free component.

[0008] The above process can also be used to analyze the no-load operation of an actual transformer. At a certain saturation state, the transformer is treated as a linear inductor, and the time constant and time-domain curve of the free component are calculated. During the continuous desaturation process, the transformer exhibits variable inductance characteristics.

[0009] When the transformer is air-dropped, the core flux cannot change suddenly, resulting in a free component in the flux. In fact, the attenuation of this free component is related to the external circuit. Here, we only study the equivalent model of the transformer in a specific saturation state, so the attenuation of the free component is temporarily ignored. The flux expression is:

[0010] ψ=-ψ m cos(wt+α)+ψ m cosα+ψ r

[0011] Where: α is the voltage phase angle during airdrop; ψm is the steady-state flux amplitude, which is equal to the voltage amplitude in per-unit system, ψ r is the residual magnetism of the transformer; obviously, this formula is composed of the steady-state power frequency periodic component and the unattenuated free component.

[0012] The transformer core magnetization curve can be obtained from Figure 3 The two-segment curve shown is approximate, where the magnetic flux at the inflection point is generally ψ sat =1.15-1.4pu.

[0013] When the core is not saturated, the transformer behaves as a large inductor L m1 , the excitation current can be almost ignored; in the no-load input process, due to the existence of the free component of the magnetic flux ψ0, the magnetic flux ψ is higher than ψ sat , the core will reach saturation and the transformer will show a small inductance L m2 , the excitation current increases sharply. Based on the transformer's two-stage approximate magnetization curve and the above formula, the expression for the excitation inrush current in one cycle (0, 2π) can be calculated as:

[0014]

[0015] Due to L m1 is very large, so the magnetizing inrush current can also be expressed as:

[0016]

[0017] The typical excitation inrush current i can be plotted from the above formula T Waveform Figure 4 As shown, where θ1 is the discontinuity angle, i Tm is the amplitude of the magnetizing inrush current. It can be seen that the main characteristics of the magnetizing inrush current are that it is biased to one side of the time axis and has a discontinuous waveform. Using Fourier analysis, it is easy to find that in addition to the fundamental component, this current also has free components and various harmonic components.

[0018] The above analysis shows that existing methods for calculating inrush current attenuation primarily include simulations using electromagnetic transient software and theoretical calculations using a constant transformer inductance model. The former relies on time-domain simulations, making it difficult to reveal the attenuation mechanism of the inrush current. The latter ignores transformer nonlinearity, resulting in significant calculation errors. Summary of the Invention

[0019] In response to the above technical problems, the present invention provides a method for calculating the excitation inrush current attenuation of a direct-drive wind turbine grid-connected system, comprising:

[0020] Determine the attenuation characteristics of the transformer's magnetizing inrush current based on the transformer's flux linkage and the free component of the current; and construct a variable inductance model of the transformer based on the attenuation characteristics.

[0021] Ignoring the inner control loop of the transformer-side converter of the direct-drive wind turbine grid, obtaining the transfer function of the inner current loop and the filter inductor, and obtaining an expression of a simplified frequency-domain input impedance based on the transfer function; based on the expression of the simplified frequency-domain input impedance, the transformer-side converter of the direct-drive wind turbine grid is equivalent to a series branch composed of circuit elements RLC;

[0022] Establishing a system state space equation group including an equivalent RLC circuit of a wind farm based on the series branch formed by the variable inductance model of the transformer and the basic circuit element RLC, and obtaining a state space equation of a state variable by performing state space modeling on the system state space equation group;

[0023] The characteristic roots and participation factors of the state matrix are obtained by the state space equation, and the attenuation speed of the excitation inrush current of the direct-drive wind power grid-connected system is obtained.

[0024] Furthermore, before the step of determining the attenuation characteristic of the transformer excitation inrush current based on the transformer flux linkage and the free component of the current, the method further includes:

[0025] It is determined that the flux linkage and current of nonlinear inductance and linear inductance components satisfy different relationships, as shown in the following equations:

[0026]

[0027] Wherein, the linear inductance L1 is a constant whose value is equal to L, ψ is the inductor flux, i is the inductor current, h(i) is a nonlinear function of current-flux, and the nonlinear inductance L2 is a variable, which is a function of the inductor current i, that is, L2 = f(i).

[0028] Furthermore, based on the attenuation characteristics, a variable inductance model of the transformer is constructed, including:

[0029] The variable inductance model of the transformer is:

[0030]

[0031] Where L0 and L0(n) are the representations of the variable inductance in the continuous time domain and discrete domain, respectively, ψ0 is the free component in the flux, Ι0 is the free component in the current, n is a positive integer, and h(I0) is the nonlinear function of current-flux in the discrete domain.

[0032] Furthermore, it also includes:

[0033] According to the influence of the initial closing angle change on the saturation degree of the transformer, the residual magnetism ψ r is equal to 0, then the magnetic flux can be expressed as

[0034] ψ=-ψ m cos(ωt+α)+ψ mcosα (3)

[0035] Where, ψ m is the flux amplitude, t is the time variable, ω is the angular frequency, and α is the initial phase angle of closing. By performing Fourier analysis on Equations (2) and (3), the discrete parameters of the variable inductance model of the transformer can be obtained.

[0036] Furthermore, ignoring the inner control loop of the grid-side converter of the direct-drive wind turbine, the transfer function of the inner current loop and the filter inductor is obtained. Based on the transfer function, the expression of the simplified frequency domain input impedance is obtained, including:

[0037] The specific equation of the transfer function of the current inner loop and the filter inductor is:

[0038]

[0039] Where H i (s) is the transfer function of the inner current loop, K p is the proportional gain, K i is the integral gain, v t is the converter output voltage, v is the common connection point voltage, L t is the filter inductor, s is the Laplace operator, i ref is the current reference value.

[0040] According to the transfer function, the simplified frequency domain input impedance Z(s) expression is derived as follows:

[0041]

[0042] Where K i is the current inner loop control proportional gain, T i It is the integral gain of the current inner loop control.

[0043] Furthermore, based on the simplified frequency-domain input impedance expression, the grid-side converter of the direct-drive wind turbine is equivalent to a series branch composed of circuit elements RLC, including:

[0044] Based on the series branch composed of circuit elements RLC, its frequency domain impedance Z1(s) is expressed as:

[0045]

[0046] Where R is the series resistance, L is the series inductance, and C is the series capacitance.

[0047] Furthermore, the system state space equations including the wind farm equivalent RLC circuit are specifically:

[0048]

[0049] Where, L T is the input inductance, L P is the equivalent inductance of the fan branch, C P is the equivalent capacitance of the fan branch, L s is the equivalent inductance of the AC system, i P is the branch current of the new energy converter, i Tq 、i Td is the rotating current of the inductor branch, i sd 、i sq is the rotating current of the system power branch, u C is the equivalent capacitance voltage of the fan branch, and w0 is the reference angular frequency.

[0050] Furthermore, by performing state space modeling on the system state space equations, the state space equations of the state variables are obtained, including:

[0051] The state space equations of the system are modeled in a synchronous coordinate system, and the state space equations of the system are organized into a state space equation containing 6 state variables, specifically:

[0052]

[0053] Where A is the state matrix, B is the input matrix, and u is the input variable.

[0054] Furthermore, the characteristic roots and participation factors of the state matrix are obtained by the state space equation to obtain the excitation inrush current attenuation rate of the direct-drive wind power grid-connected system, including:

[0055] Obtain the characteristic roots and participation factors of the state matrix, select the oscillation mode related to the transformer branch current,

[0056] Assuming the function M is selected, the dominant characteristic root can be obtained as:

[0057] λ=M(λ i ,p i ) (9)

[0058] Where λ i 、p i (i=1,2,3) are the characteristic roots and corresponding participation factors respectively;

[0059] The imaginary part of the characteristic root represents the frequency of the oscillation component. By converting this frequency from the synchronous coordinate system to the stationary coordinate system, we can obtain the oscillation attenuation frequency of the instantaneous value of the current.

[0060] When a characteristic root appears in the system as a negative real number, its absolute value represents the decay rate of the free component of the branch.

[0061] The present invention also provides a calculation system for attenuation of excitation inrush current in a direct-drive wind turbine grid-connected system, comprising:

[0062] A variable inductance model building module is used to determine the attenuation characteristics of the transformer excitation inrush current based on the transformer flux linkage and the free component of the current; based on the attenuation characteristics, a variable inductance model of the transformer is built;

[0063] A series branch composition module is configured to ignore the inner control loop of the direct-drive wind turbine grid-side converter, obtain a transfer function of the current inner loop and the filter inductor, and obtain an expression of a simplified frequency-domain input impedance based on the transfer function; based on the simplified frequency-domain input impedance expression, the direct-drive wind turbine grid-side converter is equivalent to a series branch composed of circuit elements RLC;

[0064] a state-space equation acquisition module, configured to establish a system state-space equation group including an equivalent RLC circuit of a wind farm based on a variable inductance model of the transformer and a series branch composed of basic circuit elements RLC, and to obtain state-space equations of state variables by performing state-space modeling on the system state-space equation group;

[0065] The attenuation speed determination module is used to obtain the characteristic roots and participation factors of the state matrix through the state space equation to obtain the attenuation speed of the excitation inrush current of the direct-drive wind power grid-connected system.

[0066] The present invention provides a calculation method and system for the attenuation of the excitation inrush current of a direct-drive wind turbine grid-connected system. The direct-drive wind turbine is equivalent to an RLC series circuit model and the transformer is equivalent to a variable inductance circuit model, thereby revealing the attenuation mechanism of the excitation inrush current of the direct-drive wind turbine grid-connected system with a small calculation error. BRIEF DESCRIPTION OF THE DRAWINGS

[0067] Figure 1 1 is a flow chart of a method for calculating excitation inrush current attenuation of a direct-drive wind turbine grid-connected system provided by an embodiment of the present invention;

[0068] Figure 2 This is a simple system structure of a first-order circuit type involved in an embodiment of the present invention;

[0069] Figure 3 This is a two-stage approximate magnetization curve involved in the embodiment of the present invention;

[0070] Figure 4 is a typical excitation inrush current waveform involved in the embodiment of the present invention;

[0071] Figure 5 is a transformer flux-current free component characteristic curve involved in an embodiment of the present invention;

[0072] Figure 6is a fitting result of a transformer variable inductance-current free component characteristic curve involved in an embodiment of the present invention;

[0073] Figure 7 This is a direct-drive wind turbine grid-connected system according to an embodiment of the present invention;

[0074] Figure 8 This is an equivalent circuit of a transformer-driven direct-drive wind power grid-connected system according to an embodiment of the present invention;

[0075] Figure 9 is a comparison diagram of three-phase current free components involved in an embodiment of the present invention;

[0076] Figure 10 The present invention provides a schematic diagram of a calculation system for attenuating excitation inrush current in a direct-drive wind turbine grid-connected system. DETAILED DESCRIPTION

[0077] The following description sets forth numerous specific details to facilitate a thorough understanding of the present invention. However, the present invention can be implemented in many other ways than those described herein, and those skilled in the art can make similar generalizations without departing from the scope of the present invention. Therefore, the present invention is not limited to the specific implementations disclosed below.

[0078] The present invention provides a calculation method for the excitation inrush current attenuation of a direct-drive wind turbine grid-connected system. The direct-drive wind turbine is equivalent to an RLC series circuit model and the transformer is equivalent to a variable inductance circuit model, thereby revealing the excitation inrush current attenuation mechanism of the direct-drive wind turbine grid-connected system with a small calculation error. The specific steps are as follows: Figure 1 As shown, the following steps are included:

[0079] Step S101 : determining the attenuation characteristics of the transformer's magnetizing inrush current according to the transformer's flux linkage and the free component of the current; and constructing a variable inductance model of the transformer based on the attenuation characteristics.

[0080] It is determined that the flux linkage and current of nonlinear inductance and linear inductance components satisfy different relationships, as shown in the following equations:

[0081]

[0082] Where, the linear inductance L1 is a constant whose value is equal to L, ψ is the flux, i is the inductor current, h(i) is the nonlinear function of current-flux, and the nonlinear inductance L2 is a variable, which is a function of the inductor current i, that is, L2 = f(i).

[0083] In the transient phase of a linear inductor being connected to the grid, the attenuation of its current is mainly determined by its free component. When an unloaded transformer is connected to an infinite system at a certain residual magnetism and angle, the flux contains a constant free component. When connected to an actual grid, due to the presence of electrical components in the external circuit, the free component of the transformer flux gradually decays over time, accompanied by the desaturation of the transformer. Therefore, the free component ψ0 in the flux is closely related to the saturation level of the transformer, and the free component Ι0 in the current can also better characterize the attenuation characteristics of the current during the desaturation process. From an analogy with the process of linear inductor connection, we can also try to use the free components of the flux and current to analyze the attenuation characteristics of the transformer excitation inrush current. To this end, a variable inductance model of the transformer is proposed here, which can be described as:

[0084]

[0085] Where L0 and L0(n) are the representations of the variable inductance in the continuous time domain and discrete domain, respectively, ψ0 is the free component in the flux, Ι0 is the free component in the current, n is a positive integer, and h(I0) is the current-flux function in the discrete domain.

[0086] Since the residual magnetism of the transformer and the size of the initial closing phase angle will affect the free component of the flux linkage, this section is only to illustrate the influence of the change of the free component of the flux linkage on the saturation degree of the transformer, and does not distinguish which factor causes this change. In order to simplify the analysis, only the influence of the change of the initial closing angle on the saturation degree of the transformer is considered in the model establishment stage. It can be set as ψ r is equal to 0, then the magnetic flux can be expressed as

[0087] ψ=-ψmcos(ωt+α)+ψ m cosα (3)

[0088] Where, ψ m is, ω is the angular frequency, α is the initial phase angle, and the discrete parameters of the variable inductance model of the transformer can be obtained by performing Fourier analysis on equations (2) and (3).

[0089] Through equations (2) and (3) and Fourier analysis, the discrete parameters of the transformer variable inductance model can be obtained. To obtain the flux linkage-current free component curve in the transient operation range of the transformer, it is necessary to set different initial voltage phase angles in sequence, simulate different saturation levels of the transformer, draw the flux linkage-current free component curve point by point, and calculate the discrete parameters of the variable inductance model. The specific calculation steps are:

[0090] Step 1: Set different initial voltage phase angles α in turn, calculate the transformer flux free component ψ0 and the excitation inrush current free flow component I0, and plot I0-ψ0 point by point. Typical data are as follows: Figure 5 shown.

[0091] Step 2: By using the method of obtaining the variable inductance in the discrete domain of formula (2), the equivalent inductance values ​​of the transformer corresponding to different saturation levels can be obtained. Then, by taking into account the equivalent inductance values ​​at all saturation levels, the discretization curve of the transformer variable inductance changing with the free component of the current can be obtained.

[0092] Step 3: Fit the discretized curve using the second-order Rational curve fitting method to obtain the time domain function model of the transformer variable inductance, that is,

[0093]

[0094] Where a1, a2, b1, and b2 are fitting parameters. Typical fitting results are as follows: Figure 6 As shown, it can be seen that the function model can fit the original discrete data curve well. From the curve shape, in the saturation stage of the transformer, the equivalent inductance gradually increases during the decay process of the current free component from large to small, and the change process shows a strong nonlinear characteristic.

[0095] In step S102, the control inner loop of the grid-side converter of the direct-drive wind turbine is ignored, and the transfer function of the current inner loop and the filter inductor is obtained. Based on the transfer function, an expression of a simplified frequency-domain input impedance is obtained; based on the expression of the simplified frequency-domain input impedance, the grid-side converter of the direct-drive wind turbine is equivalent to a series branch composed of circuit elements RLC.

[0096] Due to the complex control structure of the converter, it is often simplified to a port equivalent model while minimizing the model order. Unlike the general frequency-domain impedance method, this approach uses basic electrical components for equivalence, allowing for better integration with dynamic circuit analysis. The converter control system is simplified here, considering only the inner current control loop. Therefore, based on frequency-domain impedance modeling, this section ignores the outer grid-side converter control loop and derives the transfer functions of the inner current loop and filter inductor. Both components are linear, and the specific equations are:

[0097]

[0098] Where H i (s) is the transfer function of the inner current loop, K p is the proportional gain, K i is the integral gain, v t is the converter output voltage, v is the common connection point voltage, L t is the filter inductor, s is the Laplace operator, i ref is the current reference value.

[0099] According to the transfer function, the simplified frequency domain input impedance Z(s) expression is derived as follows:

[0100]

[0101] Where K i is the current inner loop control proportional gain, T i It is the integral gain of the current inner loop control.

[0102] It can be seen that the impedance of the converter consists of three parts: a proportional link in the form of a resistor, a filter inductor in the form of an inductor, and an integral link in the form of a capacitor.

[0103] Based on the series branch composed of circuit elements RLC, its frequency domain impedance Z1(s) is expressed as:

[0104]

[0105] Where R is the series resistance, L is the series inductance, and C is the series capacitance.

[0106] It is not difficult to find that the expressions of Equation (5) and Equation (6) are unified, so a simplified dynamic analysis model is proposed here, that is, the grid-side converter of the direct-drive wind turbine is represented by a set of series-connected "resistance-inductance-capacitance" (abbreviated as RLC) models, where the resistance R P Equal to K i , inductance L P Equal to L t , capacitor C P Equal to T i .

[0107] Step S103: Based on the series branch composed of the variable inductance model of the transformer and the basic circuit element RLC, a system state space equation group including the wind farm equivalent RLC circuit is established, and the state space equation of the state variable is obtained by performing state space modeling on the system state space equation group.

[0108] In the vicinity of a wind farm, the magnetizing inrush current mainly comes from the no-load closing process of the station's step-up transformer or the nearby extra-high voltage / ultra-high voltage transformer. Considering the example system built in this typical station, Figure 7 As shown, it includes direct-drive wind turbines, AC systems, no-load transformers, station step-up transformers, box transformers, filter inductors and other equipment.

[0109] According to the above, the wind turbine grid-side converter is equivalent to an RLC series branch, the transformer is equivalent to a variable inductor, and the AC system is equivalent to a Thevenin equivalent circuit. The equivalent circuit model of the transformer no-load direct-drive wind power grid-connected system can be obtained, as shown in the figure: Figure 8 As shown:

[0110] Among them, i P is the direct drive wind turbine branch current, i T is the transformer branch current, i sis the system power branch current, u s is the equivalent voltage source of the system.

[0111] There are many methods for analyzing high-order dynamic circuits, such as finding poles using transfer functions, finding characteristic roots using characteristic equations, and finding characteristic roots using state-space equations. Since we need to find not only characteristic roots but also to identify which characteristic roots the state variables of the transformer branch are related to, so as to analyze the current attenuation characteristics of the branch, we use the state-space equation to find characteristic roots and participation factors. To simplify the analysis of the problem, we use the linear inductor element L T Taking the new energy grid-connected system as an example, the system state space equations including the wind farm equivalent RLC circuit are established, as shown in Equation (7):

[0112]

[0113] Step S104 , obtaining the characteristic roots and participation factors of the state matrix through the state space equation to obtain the attenuation speed of the excitation inrush current of the direct-drive wind power grid-connected system.

[0114] It is worth noting that since all electrical quantities are sinusoidal, the state space equations of the system are modeled in a synchronous coordinate system, and the final oscillation frequency needs to be converted to a stationary coordinate system. The state space equations of the system are organized into a state space equation containing 6 state variables, specifically:

[0115]

[0116] Where A is the state matrix, B is the input matrix, and u is the input variable.

[0117] Obtain the characteristic roots and participation factors of the state matrix, select the oscillation mode related to the transformer branch current,

[0118] Assuming the function M is selected, the dominant characteristic root can be obtained as:

[0119] λ=M(λ i ,p i ) (9)

[0120] Where λ i 、p i (i=1,2,3) are the characteristic roots and corresponding participation factors respectively;

[0121] The imaginary part of the characteristic root represents the frequency of the oscillation component. By converting this frequency from the synchronous coordinate system to the stationary coordinate system, we can obtain the oscillation attenuation frequency of the instantaneous value of the current.

[0122] When a characteristic root appears in the system as a negative real number, its absolute value represents the decay rate of the free component of the branch.

[0123] Furthermore, considering the case of inputting variable inductance, the equivalent inductance value of this time step is calculated based on the current value of this time step, and then dynamic analysis is performed to obtain the current value of the next time step. The specific calculation process is as follows:

[0124] Step 1: Let k = 1, the calculation start time be t1, and set the initial value of the variable inductor current I0(k) = c.

[0125] Step 2: For the kth time step, calculate the transformer equivalent linear inductance L0(k) = f[I0(k)] based on the current free component value of the variable inductor current I0(k), and then obtain the state space equation of the system as

[0126]

[0127] Where x=[u C i P i T ] T is the state variable, A(k) and B(k) are the state matrix and input matrix at this time step. The dominant characteristic root at this time step is further obtained as:

[0128] λ(k)=M(λ i (k),p i (k))

[0129] Where λ i (k), p i (k)(i=1,2,3) are the characteristic roots and corresponding participation factors at that time step respectively.

[0130] Step 3: Calculate the free component value of the variable inductor current at the k+1th time step using the following formula:

[0131]

[0132] Among them, a k is the mode factor at this time step, a k When λ(k) is 1, it is the dominant characteristic root of complex number, a k When λ(k) is taken as 0, it is the real dominant characteristic root.

[0133] Step 4: Let k = k + 1, repeat steps 2 and 3 until the calculation of each time step is completed and the loop ends, and the free component value of the variable inductor current at each time step is output.

[0134] The present invention establishes a system model containing a saturated transformer and a new energy converter, applies and simulates the overall model and dynamic analysis method, and verifies the effect of the invention, and compares it with the existing transformer average inductance model. Figure 7 The system parameters are shown in Table 1.

[0135] Table 1 System parameters

[0136]

[0137]

[0138] When the three-phase residual magnetism of the transformer is 0 and the closing angle (taking phase a as an example) is 0 degrees, the free component of the excitation inrush current is estimated according to the above method and compared with the simulation results. Figure 9 The figure also shows the calculation results of the transformer using the average inductance model. It can be seen that the free component attenuation curve of the excitation inrush current calculated using the variable inductance model is almost consistent with the simulation results, verifying the effectiveness of the overall system model and dynamic analysis algorithm. The comparison of the two transformer models shows that the variable inductance model described in this invention has a higher fitting accuracy for the free component attenuation characteristics.

[0139] Based on the same inventive concept, the present invention also provides a calculation system 100 for attenuation of excitation inrush current in a direct-drive wind turbine grid-connected system, such as Figure 10 Shown, including:

[0140] The variable inductance model construction module 110 is used to determine the attenuation characteristics of the transformer excitation inrush current based on the transformer flux linkage and the free component of the current; and to construct a variable inductance model of the transformer based on the attenuation characteristics;

[0141] The series branch composition module 120 is configured to ignore the inner control loop of the direct-drive wind turbine grid-side converter, obtain a transfer function of the current inner loop and the filter inductor, and obtain an expression for a simplified frequency-domain input impedance based on the transfer function; based on the simplified frequency-domain input impedance expression, the direct-drive wind turbine grid-side converter is equivalent to a series branch composed of circuit elements RLC;

[0142] A state-space equation acquisition module 130 is configured to establish a system state-space equation group including an equivalent RLC circuit of a wind farm based on the variable inductance model of the transformer and the series branch composed of the basic circuit element RLC, and to obtain a state-space equation of a state variable by performing state-space modeling on the system state-space equation group;

[0143] The attenuation speed determination module 140 is used to obtain the characteristic roots and participation factors of the state matrix through the state space equation to obtain the attenuation speed of the excitation inrush current of the direct-drive wind power grid-connected system.

[0144] It will be understood by those skilled in the art that embodiments of the present invention may be provided as methods, systems, or computer program products. Thus, the present invention may take the form of an entirely hardware embodiment, an entirely software embodiment, or an embodiment combining software and hardware. Furthermore, the present invention may take the form of a computer program product implemented on one or more computer-usable storage media (including but not limited to magnetic disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code.

[0145] The present invention is described with reference to flowcharts and / or block diagrams of methods, devices (systems), and computer program products according to embodiments of the present invention. It should be understood that each process and / or block in the flowcharts and / or block diagrams, as well as combinations of processes and / or blocks in the flowcharts and / or block diagrams, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing device to produce a machine, so that the instructions executed by the processor of the computer or other programmable data processing device generate instructions for implementing the processes in the flowcharts and / or block diagrams. Figure 1 a process or multiple processes and / or boxes Figure 1 A device that provides the functions specified in a block or multiple blocks.

[0146] These computer program instructions may also be stored in a computer readable memory that can direct a computer or other programmable data processing device to work in a specific manner, so that the instructions stored in the computer readable memory produce an article of manufacture comprising an instruction device, which implements the process Figure 1 a process or multiple processes and / or boxes Figure 1 The function specified in one or more boxes.

[0147] These computer program instructions can also be loaded onto a computer or other programmable data processing device so that a series of operational steps are executed on the computer or other programmable device to produce a computer-implemented process, thereby providing the instructions executed on the computer or other programmable device for implementing the process. Figure 1 a process or multiple processes and / or boxes Figure 1 The steps for the function specified in one or more boxes.

[0148] 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 it. Although the present invention has been described in detail with reference to the above embodiments, ordinary technicians in the field should understand that the specific implementation methods of the present invention can still be modified or replaced by equivalents. Any modifications or equivalents that do not depart from the spirit and scope of the present invention should be included in the scope of the claims of the present invention.

Claims

1. A calculation method for excitation inrush current attenuation of a direct-drive wind turbine grid-connected system, characterized in that: include: Determine the attenuation characteristics of the transformer's magnetizing inrush current based on the transformer's flux linkage and the free component of the current; and construct a variable inductance model of the transformer based on the attenuation characteristics. Ignoring the inner control loop of the transformer-side converter of the direct-drive wind turbine grid, obtaining the transfer function of the inner current loop and the filter inductor, and obtaining an expression of a simplified frequency-domain input impedance based on the transfer function; based on the expression of the simplified frequency-domain input impedance, the transformer-side converter of the direct-drive wind turbine grid is equivalent to a series branch composed of circuit elements RLC; Establishing a system state space equation group including an equivalent RLC circuit of a wind farm based on the series branch formed by the variable inductance model of the transformer and the basic circuit element RLC, and obtaining a state space equation of a state variable by performing state space modeling on the system state space equation group; The characteristic roots and participation factors of the state matrix are obtained by the state space equation to obtain the excitation inrush current attenuation rate of the direct-drive wind power grid-connected system; By performing state space modeling on the system state space equations, the state space equations of the state variables are obtained, including: The state space equations of the system are modeled in a synchronous coordinate system, and the state space equations of the system are organized into a state space equation containing 6 state variables, specifically: Where A is the state matrix, B is the input matrix, and u is the input variable; The characteristic roots and participation factors of the state matrix are obtained by the state space equation to obtain the excitation inrush current attenuation rate of the direct-drive wind power grid-connected system, including: Obtain the characteristic roots and participation factors of the state matrix, select the oscillation mode related to the transformer branch current, Assuming the function M is selected, the dominant characteristic root can be obtained as: λ=M(λ i ,p i ) (9) Where λ i 、p i (i=1,2,3) are the characteristic roots and corresponding participation factors respectively; The imaginary part of the characteristic root represents the frequency of the oscillation component. By converting this frequency from the synchronous coordinate system to the stationary coordinate system, we can obtain the oscillation attenuation frequency of the instantaneous value of the current. When a characteristic root appears in the system as a negative real number, its absolute value represents the decay rate of the free component of the branch.

2. The method according to claim 1, characterized in that Before the step of determining the attenuation characteristics of the transformer excitation inrush current based on the transformer flux linkage and the free component of the current, the step further includes: It is determined that the flux linkage and current of nonlinear inductance and linear inductance components satisfy different relationships, as shown in the following equations: Wherein, the linear inductance L1 is a constant whose value is equal to L, ψ is the inductor flux, i is the inductor current, h(i) is a nonlinear function of current-flux, and the nonlinear inductance L2 is a variable, which is a function of the inductor current i, that is, L2 = f(i).

3. The method according to claim 1, characterized in that Based on the attenuation characteristics, a variable inductance model of the transformer is constructed, including: The variable inductance model of the transformer is: Where L0 and L0(n) are the representations of the variable inductance in the continuous time domain and discrete domain, respectively, ψ0 is the free component in the flux, Ι0 is the free component in the current, n is a positive integer, and h(I0) is the nonlinear function of current-flux in the discrete domain.

4. The method according to claim 3, characterized in that Also includes: According to the influence of the initial closing angle change on the saturation degree of the transformer, the residual magnetism ψ r is equal to 0, then the magnetic flux can be expressed as ψ=-ψ m cos(ωt+α)+ψ m thing (3) Where, ψ m is the flux amplitude, t is the time variable, ω is the angular frequency, and α is the initial phase angle of closing. By performing Fourier analysis on Equations (2) and (3), the discrete parameters of the variable inductance model of the transformer can be obtained.

5. The method according to claim 1, wherein Ignoring the inner control loop of the grid-side converter of the direct-drive wind turbine, the transfer function of the inner current loop and the filter inductor is obtained. Based on the transfer function, the expression of the simplified frequency-domain input impedance is obtained, including: The specific equation of the transfer function of the current inner loop and the filter inductor is: Where H i (s) is the transfer function of the inner current loop, K p is the proportional gain, K i is the integral gain, v t is the converter output voltage, v is the common connection point voltage, L t is the filter inductor, s is the Laplace operator, i ref is the current reference value; According to the transfer function, the simplified frequency domain input impedance Z(s) expression is derived as follows: Where K i is the current inner loop control proportional gain, T i It is the integral gain of the current inner loop control.

6. The method according to claim 1, characterized in that Based on the simplified frequency-domain input impedance expression, the grid-side converter of the direct-drive wind turbine is equivalent to a series branch composed of circuit elements RLC, including: Based on the series branch composed of circuit elements RLC, its frequency domain impedance Z1(s) is expressed as: Where R is the series resistance, L is the series inductance, and C is the series capacitance.

7. The method according to claim 1, characterized in that The system state space equations including the wind farm equivalent RLC circuit are specifically: Where, L T is the input inductance, L P is the equivalent inductance of the fan branch, C P is the equivalent capacitance of the fan branch, L s is the equivalent inductance of the AC system, i P is the branch current of the new energy converter, i Tq 、i Td is the rotating current of the inductor branch, i sd 、i sq is the rotating current of the system power branch, u C is the equivalent capacitance voltage of the fan branch, and w0 is the reference angular frequency.

8. A calculation system for attenuation of excitation inrush current in a direct-drive wind turbine grid-connected system, characterized in that: include: A variable inductance model building module is used to determine the attenuation characteristics of the transformer excitation inrush current based on the transformer flux linkage and the free component of the current; based on the attenuation characteristics, a variable inductance model of the transformer is built; A series branch composition module is configured to ignore the inner control loop of the direct-drive wind turbine grid-side converter, obtain a transfer function of the current inner loop and the filter inductor, and obtain an expression of a simplified frequency-domain input impedance based on the transfer function; based on the simplified frequency-domain input impedance expression, the direct-drive wind turbine grid-side converter is equivalent to a series branch composed of circuit elements RLC; a state-space equation acquisition module, configured to establish a system state-space equation group including an equivalent RLC circuit of a wind farm based on a variable inductance model of the transformer and a series branch composed of basic circuit elements RLC, and to obtain state-space equations of state variables by performing state-space modeling on the system state-space equation group; A decay rate determination module is used to obtain the characteristic roots and participation factors of the state matrix through the state space equation to obtain the excitation inrush current decay rate of the direct-drive wind power grid-connected system; By performing state space modeling on the system state space equations, the state space equations of the state variables are obtained, including: The state space equations of the system are modeled in a synchronous coordinate system, and the state space equations of the system are organized into a state space equation containing 6 state variables, specifically: Where A is the state matrix, B is the input matrix, and u is the input variable; The characteristic roots and participation factors of the state matrix are obtained by the state space equation to obtain the excitation inrush current attenuation rate of the direct-drive wind power grid-connected system, including: Obtain the characteristic roots and participation factors of the state matrix, select the oscillation mode related to the transformer branch current, Assuming the function M is selected, the dominant characteristic root can be obtained as: λ=M(λ i ,p i ) (9) Where λ i 、p i (i=1,2,3) are the characteristic roots and corresponding participation factors respectively; The imaginary part of the characteristic root represents the frequency of the oscillation component. By converting this frequency from the synchronous coordinate system to the stationary coordinate system, we can obtain the oscillation attenuation frequency of the instantaneous value of the current. When a characteristic root appears in the system as a negative real number, its absolute value represents the decay rate of the free component of the branch.

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