Impedance Model Establishment Method for Grid-Connected Offshore Wind Farm Systems with Grid Connection

Abstract

This invention discloses a method for establishing an impedance model for a grid-connected offshore wind farm system that integrates with and connects to a grid. Belonging to the field of offshore wind power grid connection technology, the method includes: establishing full-order admittance small-signal models of converters for grid-connected and grid-connected wind turbines in a local PCC point coordinate system; uniformly transforming the full-order admittance small-signal models to the global PCC point coordinate system to obtain admittance models of converters for grid-connected and grid-connected wind turbines in the global PCC point coordinate system; dividing the wind turbines into grid-connected and grid-connected wind power clusters to obtain the equivalent impedance of these clusters aggregated to the global PCC point; treating the grid-connected and grid-connected wind power clusters and the AC grid as a three-node system, constructing equivalent node impedance matrices and equivalent node branch admittance matrices; and establishing an impedance model based on the harmonic balance relationship of the connecting lines at each node, combined with the equivalent node impedance matrix and equivalent node branch admittance matrix.

Description

Technical Field

[0001] This invention relates to the field of offshore wind power grid connection technology, specifically to a method for establishing an impedance model for an offshore wind farm grid-connected system that is integrated with the grid. Background Technology

[0002] As the installed capacity of offshore wind power continues to increase, the impedance coupling between multiple wind turbines, the collection network, and the grid side within a wind farm can easily induce broadband oscillations. To accurately characterize the grid-connected interaction characteristics and support stability analysis and oscillation suppression, it is necessary to establish a small-signal impedance model suitable for offshore wind farm grid-connection scenarios. Compared to state-space modeling methods that rely on the complete system structure and all parameters, impedance analysis can divide the system at the grid connection point and construct impedance models for each component, ultimately using the generalized Nyquist stability criterion to evaluate the stability of the interconnected system. Furthermore, when local topology or control parameters change, the impedance method only needs to update the corresponding subsystem model, facilitating engineering applications. In addition, in scenarios where multiple manufacturers and models of converters coexist and internal parameters are difficult to obtain, the impedance model can be obtained through impedance frequency sweeping technology, providing a feasible approach for stability assessment of offshore wind farms.

[0003] With the promotion of grid-based control strategies, large-scale grid-connected wind farms have become an established trend in the future development of offshore wind power. However, existing impedance modeling techniques for offshore wind farms have certain shortcomings in terms of modeling accuracy and computational complexity. Specifically: 1. Modeling methods that use the impedance of single or multi-unit aggregated wind turbines to replace the impedance of the wind farm often ignore differences in turbine control parameters, operating points, and the topology distribution characteristics of the collector lines to varying degrees, leading to accumulated deviations in impedance characteristics over a wide frequency band and making it easy to misjudge system oscillation stability; 2. For wind farm impedance modeling methods that consider the specific topology of the wind farm and the impedance models of each link and simplify them into aggregated impedance using circuit theory, equivalent modeling is usually required based on the control type of the wind turbine during the aggregation process. That is, grid-connected wind turbines need to be equivalent to the parallel admittance form of a controlled current source, while grid-connected wind turbines are equivalent to the series impedance form of a controlled voltage source. However, the above two equivalent premises contradict existing aggregation rules, making existing methods often only suitable for aggregation modeling of wind turbines with a single control type. When both grid-connected and grid-connected wind turbines coexist within a wind farm, using a uniform aggregation path or forcibly adopting the same equivalent form will inevitably introduce equivalent deviations in multi-turbine hybrid aggregation, leading to inconsistencies between the aggregated impedance characteristics and the actual wind farm, thus affecting the accuracy of broadband oscillation stability assessment. 3. To address the difficulty of actively equivalencing the impedance network in grid-connected and grid-connected hybrid systems, some scholars have proposed methods for establishing impedance models for grid-connected and grid-connected hybrid systems. However, such methods typically require treating each wind turbine within the wind farm as an independent node and constructing a high-dimensional diagonal impedance matrix based on the impedance models of each node's wind turbines. Simultaneously, it is necessary to construct the interconnection admittance matrix between nodes based on the network topology relationships of collector lines, transformers, etc., between each independent node. Due to the large number of turbines and the massive node scale in actual offshore wind farms, the resulting system impedance / admittance matrix will encounter the curse of dimensionality, significantly increasing the computational load of matrix operations and inversions, resulting in high model solution complexity, which is detrimental to stability assessment and engineering applications.

[0004] Therefore, how to provide a method for establishing an impedance model of an offshore wind farm grid-connected system that is integrated with the grid, so as to effectively reduce the computational load of the model while maintaining high accuracy, is a technical problem that urgently needs to be solved by those skilled in the art. Summary of the Invention

[0005] Therefore, this invention provides a method for establishing an impedance model for an offshore wind farm grid-connected system to solve the problems existing in the prior art.

[0006] To achieve the above objectives, the present invention provides the following technical solution: The method for establishing the impedance model of an offshore wind farm grid-connected system that is hybrid with a grid includes the following steps: Step S1: In the local PCC point coordinate system of each wind turbine, establish the full-order admittance small-signal model of the converter of the grid-connected and grid-connected wind turbines; Step S2: Transform the full-order admittance small-signal models of grid-connected and grid-connected wind turbine converters to the global PCC point coordinate system to obtain the admittance models of grid-connected and grid-connected wind turbine converters in the global PCC point coordinate system. Step S3: Divide the wind turbines into grid-connected wind power clusters and grid-connected wind power clusters. Based on the admittance models of the converters of the grid-connected and grid-connected wind turbines in the grid-connected and grid-connected wind power clusters, and combine the admittance of the collector line nodes in the grid-connected and grid-connected wind power clusters to perform interconnection equivalence and inverse calculation, so as to obtain the equivalent impedance of the grid-connected and grid-connected wind power clusters aggregated to the global PCC point. Step S4: Treat the grid-connected wind power cluster, the grid-connected wind power cluster, and the AC grid as a three-node system, obtain the equivalent impedance in the grid-connected wind power cluster, the grid-connected wind power cluster, and the AC grid respectively, and construct the equivalent node impedance matrix and the equivalent node branch admittance matrix. Step S5: Based on the harmonic balance relationship of the connecting lines of each node, and combined with the equivalent node impedance matrix and the equivalent node branch admittance matrix, establish the impedance model of the grid-connected offshore wind farm system.

[0007] Furthermore, step S1 specifically includes: Step S101: Build physical models of the converters for each grid-connected and grid-connected wind turbine unit respectively; Step S102: Based on the physical models of grid-connected and grid-connected wind turbine converters, convert each component in the grid-connected and grid-connected wind turbine converters into a small-signal model, and obtain the full-order small-signal model of the grid-connected and grid-connected wind turbine converters based on multiple small-signal models. Step S103: Based on the full-order small-signal model established in step S102, simplify according to the impedance definition to obtain the full-order admittance small-signal model of the grid-following and grid-connected wind turbine converters in the local PCC point coordinate system.

[0008] Furthermore, step S2 specifically includes: Step S201: Using the phase angle measurement unit, collect the phase angle of the local PCC point voltage of the wind turbine and the phase angle of the global PCC point voltage of the wind farm provided by each grid-connected and grid-connected wind turbine converter, and calculate the phase angle difference corresponding to each grid-connected and grid-connected wind turbine converter to construct the corresponding rotation matrix of the grid-connected and grid-connected wind turbine converter. Step S202: The full-order admittance small-signal model of the grid-following and grid-connected wind turbine converters described in step S1 is transformed by the corresponding rotation matrix to obtain the admittance model of the grid-following and grid-connected wind turbine converters in the global PCC point coordinate system.

[0009] Furthermore, in step S3, based on the connection method of wind turbines in the actual topology of the wind farm, all grid-connected wind turbine links are divided into grid-connected wind power clusters, and all grid-connected wind turbine links are divided into grid-connected wind power clusters, and a start / stop state function S is introduced. i (t) and S mj (t) represents the start-up and shutdown states of the i-th grid-connected wind turbine converter and the j-th grid-connected wind turbine converter at time t, respectively; In step S3, the equivalent impedance Z aggregated to the global PCC point of the grid-type wind power cluster GFL,C for: ; Among them, Y netGFL For the admittance matrix of the grid-connected wind power cluster nodes; Y 0,0 Y0 is the self-admittance between the grid-connected wind power cluster and the global PCC point; Y0 is the matrix formed by the mutual admittance between node 0 and other nodes of the grid-connected wind power cluster; T represents the matrix transpose operation. In step S3, the equivalent impedance Z of the grid-type wind power cluster aggregated to the global PCC point GFM,C for: ; Among them, Y netGFM It is the admittance matrix of a grid-type wind power cluster node; Y m0,0 It is the self-admittance between the grid-type wind power cluster and the global PCC point; Y m0 It is the matrix formed by the mutual admittance between node 0 and other nodes in the grid-type wind power cluster, and T represents the matrix transpose operation.

[0010] Furthermore, step S4 specifically includes: Step S401: Treat the equivalent impedances of AC grid nodes, grid-connected wind power clusters, and grid-connected wind power clusters as small-signal impedances in the dq domain of the three-node system, and place them on the diagonal of the 3×3 node matrix in the order of nodes. Set the elements on the off-diagonal to 0 to obtain the equivalent node impedance matrix. Step S402: The connection lines between AC grid nodes, grid-connected wind power cluster nodes and grid-connected wind power cluster nodes are uniformly equivalent to the inter-node branch admittances. The impedances of each branch are summarized according to the branch number, and an equivalent node branch admittance matrix is ​​constructed based on the impedance connection relationship between nodes.

[0011] Furthermore, step S5 will, based on the harmonic balance relationship of the connecting lines at each node, express the equivalent node impedance matrix and the equivalent node branch admittance matrix using the relationship between the harmonic voltage response and the harmonic current response at each port, and simultaneously solve these relationships to obtain the impedance model of the grid-connected offshore wind farm system; wherein, the impedance model Z sys (s) is: ; Among them, Z nod (s) is the equivalent nodal impedance matrix, Y b (s) is the equivalent nodal branch admittance matrix, I is the identity matrix, and v inj Δi is the virtual injected voltage vector, and Δi is the harmonic current response vector.

[0012] Furthermore, the grid-connected wind turbine converter consists of a phase-locked loop, a power outer loop control loop, a current inner loop control loop, and a coordinate transformation loop; the grid-connected wind turbine converter consists of a power synchronization loop, a reactive power-voltage droop control loop, a voltage outer loop control loop, a current inner loop control loop, and a coordinate transformation loop.

[0013] Furthermore, the full-order admittance small-signal model of the grid-connected wind turbine converter in the local PCC point coordinate system is as follows: ; in, This represents the full-order admittance small-signal model of the i-th grid-connected wind turbine converter; , It is the small-signal component matrix of voltage and current at the local PCC point of the i-th grid-connected wind turbine converter; , , , , , , , , , The transfer function matrix is ​​the intermediate variable used in the process of obtaining the admittance of the i-th grid-connected wind turbine converter; I is the identity matrix. The full-order admittance small-signal model of the grid-type wind turbine converter in the local PCC point coordinate system is as follows: ; in, This represents the full-order admittance small-signal model of the converter of the j-th grid-connected wind turbine unit; , It is the matrix of voltage and current small-signal components at the local PCC point of the converter of the j-th grid-type wind turbine unit; , , , , , , , , , , , , , , , The intermediate variable transfer function matrix is ​​used in the process of obtaining the admittance of the converter of the j-th grid-type wind turbine; I is the identity matrix.

[0014] Furthermore, the rotation matrix of the grid-connected and grid-connected wind turbine converters is as follows: ; Among them, T i Let T be the rotation matrix of the i-th grid-connected wind turbine converter. mj Let Δθ be the rotation matrix of the j-th grid-connected wind turbine converter. i Δθ represents the difference between the local PCC point voltage phase angle and the global PCC point voltage phase angle at the converter of the i-th grid-connected wind turbine unit. mj The difference between the local PCC point voltage phase angle and the global PCC point voltage phase angle at the converter of the j-th grid-type wind turbine unit; The admittance models for grid-following and grid-connected wind turbine converters in the global PCC point coordinate system are as follows: ; in, and These are the admittance models of the i-th grid-connected wind turbine converter and the j-th grid-connected wind turbine converter in the global PCC point coordinate system, respectively. and Let T represent the full-order admittance small-signal models of the i-th grid-type and j-th grid-type wind turbine converters, respectively. i Let T be the rotation matrix of the i-th grid-connected wind turbine converter. mj Let be the rotation matrix of the j-th grid-type wind turbine converter.

[0015] Furthermore, the equivalent node impedance matrix Z in step S4 nod (s) is: ; Among them, Z GFL,C The equivalent impedance of the grid-connected wind power cluster aggregated to the global PCC point; Z GFM,CZ represents the equivalent impedance of a grid-type wind power cluster aggregated to the global PCC point. g It is the impedance of the AC power grid node, and its value is determined by the external power grid. Equivalent nodal branch admittance matrix Y b (s) is: ; Among them, Y b11 (s) to Y b33 (s) is the equivalent nodal branch admittance matrix Y b In (s), the elements on the non-main diagonal represent the mutual admittance between two node numbers, and the elements on the main diagonal represent the self admittance of the node. The subscripts 1, 2, and 3 represent the nodes of the grid-connected wind power cluster, the nodes of the grid-connected wind power cluster, and the nodes of the AC grid, respectively.

[0016] The present invention has the following advantages: 1. Compared with existing methods for establishing impedance models of offshore wind farm grid-connected systems, this invention can effectively solve the equivalent deviation problem caused by multi-unit hybrid aggregation of grid-connected and grid-connected wind turbines, and can effectively improve the accuracy of the impedance model of hybrid wind farm grid-connected systems. 2. Compared with existing methods for establishing impedance models of grid-connected and hybrid grid systems, this invention can effectively reduce the dimensionality of the system impedance model by using the method of clustering the impedance of units of the same category to form equivalent cluster impedance, thereby reducing the amount of computation in the model solving process. Attached Figure Description

[0017] To more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are merely exemplary, and those skilled in the art can derive other embodiments based on the provided drawings without creative effort.

[0018] The structures, proportions, sizes, etc. illustrated in this specification are only for the purpose of assisting those skilled in the art in understanding and reading the content disclosed herein, and are not intended to limit the conditions under which the present invention can be implemented. Therefore, they have no substantial technical significance. Any modifications to the structure, changes in the proportions, or adjustments to the size, without affecting the effects and objectives that the present invention can produce, should still fall within the scope of the technical content disclosed in the present invention.

[0019] Figure 1 A flowchart of the impedance model establishment method provided by the present invention; Figure 2The topology diagram of the hybrid offshore wind farm provided by this invention; Figure 3 The grid-connected wind turbine topology and its control block diagram provided by this invention; Figure 4 The grid-type wind turbine topology and its control block diagram provided by this invention; Figure 5 This is a full-order small-signal model diagram of the grid-connected wind turbine converter constructed in this invention; Figure 6 This is a full-order small-signal model diagram of the grid-type wind turbine converter constructed in this invention. Figure 7 This is a schematic diagram of the equivalent impedance aggregation of global PCC points for grid-connected and grid-connected wind power clusters constructed in this invention. Figure 8 This is a schematic diagram of the impedance model for solving the three-node system of the grid-connected wind power cluster, the grid-connected wind power cluster, and the AC power grid constructed in this invention. Detailed Implementation

[0020] The following specific embodiments illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0021] Methods for establishing impedance models for grid-connected offshore wind farm systems that are hybrid with grids, such as... Figure 1 The process, as shown, includes the following steps: Step S1: In the local PCC point coordinate system of each wind turbine, establish the full-order admittance small-signal model of the converter of the grid-connected and grid-connected wind turbines; Step S2: Transform the full-order admittance small-signal models of grid-connected and grid-connected wind turbine converters to the global PCC point coordinate system to obtain the admittance models of grid-connected and grid-connected wind turbine converters in the global PCC point coordinate system. Step S3: Divide the wind turbines into grid-connected wind power clusters and grid-connected wind power clusters. Based on the admittance models of the converters of the grid-connected and grid-connected wind turbines in the grid-connected and grid-connected wind power clusters, and combine the admittance of the collector line nodes in the grid-connected and grid-connected wind power clusters to perform interconnection equivalence and inverse calculation, so as to obtain the equivalent impedance of the grid-connected and grid-connected wind power clusters aggregated to the global PCC point. Step S4: Treat the grid-connected wind power cluster, the grid-connected wind power cluster, and the AC grid as a three-node system, obtain the equivalent impedance in the grid-connected wind power cluster, the grid-connected wind power cluster, and the AC grid respectively, and construct the equivalent node impedance matrix and the equivalent node branch admittance matrix. Step S5: Based on the harmonic balance relationship of the connecting lines of each node, and combined with the equivalent node impedance matrix and the equivalent node branch admittance matrix, establish the impedance model of the grid-connected offshore wind farm system.

[0022] Step S1 specifically includes: Step S101: Build physical models of the converters for each grid-connected and grid-connected wind turbine unit respectively; Step S102: Based on the physical models of grid-connected and grid-connected wind turbine converters, convert each component in the grid-connected and grid-connected wind turbine converters into a small-signal model, and obtain the full-order small-signal model of the grid-connected and grid-connected wind turbine converters based on multiple small-signal models. Step S103: Based on the full-order small-signal model established in step S102, simplify according to the impedance definition to obtain the full-order admittance small-signal model of the grid-following and grid-connected wind turbine converters in the local PCC point coordinate system.

[0023] like Figure 2 The diagram shows the topology of a grid-connected offshore wind farm. In this grid-connected system, the wind turbine feeders are connected to the AC power grid via an equivalent impedance. (For example...) Figure 3 The diagram shows the topology and control block diagram of a grid-connected wind turbine. The converter of this grid-connected wind turbine consists of a phase-locked loop (PLL), an outer power control loop, an inner current control loop, and a coordinate transformation loop. Figure 4 The diagram shows the topology and control block diagram of a grid-connected wind turbine. The grid-connected wind turbine converter consists of a power synchronization stage, a reactive power-voltage droop control stage, a voltage outer loop control stage, a current inner loop control stage, and a coordinate transformation stage. In the topology, each grid-connected and grid-connected wind turbine converter is connected via the filter inductor L of the grid-connected wind turbine converter. fci and filter capacitor C fci The filter inductor L of the grid-connected wind turbine converter fmj and filter capacitor C fmj Connect to the power collection system.

[0024] Based on the physical models of grid-connected and grid-connected wind turbine converters, each component is converted into a small-signal model and then plotted as follows. Figure 5 and Figure 6 The full-order small-signal models of grid-connected and grid-connected wind turbine converters are shown, and the full-order admittance small-signal models are simplified according to the impedance definition to obtain the full-order admittance small-signal models.

[0025] The full-order admittance small-signal model of the grid-connected wind turbine converter in the local PCC point coordinate system is as follows: ; in, This represents the full-order admittance small-signal model of the i-th grid-connected wind turbine converter; , It is the small-signal component matrix of voltage and current at the local PCC point of the i-th grid-connected wind turbine converter; , , , , , , , , , The transfer function matrix is ​​the intermediate variable used in the process of obtaining the admittance of the i-th grid-connected wind turbine converter; I is the identity matrix. The specific expressions for each transfer function matrix involved are shown below: ; Where s is the Laplace variable; ω0 is the rated angular frequency corresponding to 50Hz; and These are the d-axis and q-axis components of the local PCC point voltage of the i-th grid-connected wind turbine; and These are the d-axis and q-axis components of the local PCC point current of the i-th grid-connected wind turbine; K kPQi and K iPQi These are the proportional and integral coefficients of the controller in the outer loop power control of the i-th grid-connected wind turbine converter; K pplli and K iplli These are the proportional and integral coefficients of the phase-locked loop controller of the i-th grid-connected wind turbine converter; K pi and K ii These are the proportional and integral coefficients of the i-th grid-connected wind turbine's inner current control loop; ω LPF It is the cutoff frequency of the first-order low-pass filter used. odi I oqi These are the d-axis and q-axis components of the steady-state current at the local PCC point of the i-th grid-connected wind turbine converter; cdi I cqi These are the d-axis and q-axis components of the steady-state current of the converter arm of the i-th grid-connected wind turbine unit; V cdi V cqi These are the d-axis and q-axis components of the steady-state voltage of the converter arm of the i-th grid-connected wind turbine unit; Vodi V oqi These are the d-axis and q-axis components of the steady-state voltage at the local PCC point of the i-th grid-connected wind turbine converter; L fci and C fci For the filter inductor and filter capacitor of the grid-connected wind turbine converter; G PQi (s), G LPF (s), G plli (s) is the transfer function of the intermediate variable in the impedance calculation of the i-th grid-connected wind turbine converter; the symbol “Δ” represents the small signal component.

[0026] The full-order admittance small-signal model of the grid-type wind turbine converter in the local PCC point coordinate system is as follows: ; in, This represents the full-order admittance small-signal model of the converter of the j-th grid-connected wind turbine unit; , It is the matrix of voltage and current small-signal components at the local PCC point of the converter of the j-th grid-type wind turbine unit; , , , , , , , , , , , , , , , The intermediate variable transfer function matrix is ​​used in the process of obtaining the admittance of the converter of the j-th grid-type wind turbine; I is the identity matrix.

[0027] The specific expressions for each transfer function matrix involved are shown below:

[0028] ; Where s is the Laplace variable; K pj and K qj These are the proportional coefficients of the power synchronization link and the reactive power-voltage droop control link of the converter of the j-th grid-type wind turbine unit, respectively. and These are the d-axis and q-axis components of the local PCC point voltage of the j-th grid-type wind turbine; and These are the d-axis and q-axis components of the local PCC point current of the j-th grid-connected wind turbine; D j and Hj These are the damping coefficient and inertia coefficient of the converter of the j-th grid-connected wind turbine, respectively; K pvmj and K ivmj These are the proportional and integral coefficients of the controller in the outer loop voltage control of the j-th grid-connected wind turbine converter; K pimj and K iimj These are the proportional and integral coefficients of the controller in the inner loop current control loop of the j-th grid-connected wind turbine converter; I omdj I omdj These are the d-axis and q-axis components of the steady-state current at the local PCC point of the j-th grid-connected wind turbine converter; cmdj I cmqj These are the d-axis and q-axis components of the steady-state current of the converter arm of the j-th grid-connected wind turbine unit; V cmdj V cmqj These are the d-axis and q-axis components of the steady-state voltage of the converter arm of the j-th grid-connected wind turbine unit; V omdj V omqj These are the d-axis and q-axis components of the steady-state voltage at the local PCC point of the j-th grid-connected wind turbine converter; L fmj and C fmj For the filter inductor and filter capacitor of the grid-connected wind turbine converter; G kpj (s) is the intermediate variable transfer function in the impedance calculation of the converter of the j-th grid-type wind turbine.

[0029] Step S2 specifically includes: Step S201: Using the phase angle measurement unit (PMU), collect the phase angle of the local PCC point voltage of the wind turbine and the phase angle of the global PCC point voltage of the wind farm provided by each grid-connected and grid-connected wind turbine converter, and calculate the phase angle difference corresponding to each grid-connected and grid-connected wind turbine converter to construct the corresponding rotation matrix of the grid-connected and grid-connected wind turbine converter. Step S202: The full-order admittance small-signal models of the grid-following and grid-connected wind turbine converters obtained in Step S1 are transformed by the corresponding rotation matrix to obtain the admittance models of the grid-following and grid-connected wind turbine converters in the global PCC point coordinate system.

[0030] The rotation matrix of the converter for grid-connected and grid-connected wind turbine units is as follows: ; Among them, T i Let T be the rotation matrix of the i-th grid-connected wind turbine converter. mj Let Δθ be the rotation matrix of the j-th grid-connected wind turbine converter. iΔθ represents the difference between the local PCC point voltage phase angle and the global PCC point voltage phase angle at the converter of the i-th grid-connected wind turbine unit. mj The difference between the local PCC point voltage phase angle and the global PCC point voltage phase angle at the converter of the j-th grid-type wind turbine unit; The admittance models of grid-following and grid-connected wind turbine converters in the global PCC point coordinate system are as follows: ; in, and These are the admittance models of the i-th grid-connected wind turbine converter and the j-th grid-connected wind turbine converter in the global PCC point coordinate system, respectively. and Let T represent the full-order admittance small-signal models of the i-th grid-type and j-th grid-type wind turbine converters, respectively. i Let T be the rotation matrix of the i-th grid-connected wind turbine converter. mj Let S be the rotation matrix of the converter of the j-th grid-type wind turbine. Step S3 specifically includes: Step S301: Considering that all wind turbines on the same wind turbine link within a hybrid wind farm use the same control method, and that each wind turbine link will be connected to the same global PCC point via collector lines, we can start from the global PCC point and, based on the actual wind turbine link connection method in the wind farm's topology, divide all grid-connected wind turbine links into a grid-connected wind power cluster and grid-connected wind turbine links into a grid-connected wind power cluster according to the different wind turbine link control methods. Since wind turbines on the same wind turbine link may have different start-stop states, a start-stop state function S can be introduced. i (t) and S mj Let (t) represent the start-up and shutdown states of the i-th grid-connected wind turbine converter and the j-th grid-connected wind turbine converter, respectively; the start-up and shutdown state function can be expressed by the following formula: ; Here, 1 represents the on state and 0 represents the off state.

[0031] Step S302: Based on the external characteristics of the converter under different control modes, the impedance circuit of the grid-connected wind turbine converter can be represented by a current source and output admittance in parallel according to the Norton equivalent principle; the impedance circuit of the grid-connected wind turbine converter can be represented by a voltage source and output admittance in series according to the Thevenin equivalent principle. The collector lines can be modeled using the loss consistency method, taking into account the actual line parameters of the offshore wind farm.

[0032] like Figure 7The left half of the diagram shows the equivalent topology of a hybrid offshore wind farm connected to a grid. Within the grid-connected and grid-connected wind farm clusters, the nodes are sequentially numbered 1, 2, ..., i, ..., n and 1, 2, ..., j, ..., m, respectively. Considering the similarity of the equivalent impedance aggregation process for the two types of clusters, the equivalent connection of the grid-connected wind farm cluster is used as an example for explanation. First, the node admittance matrix Y of the grid-connected wind farm cluster is established according to the node order. netGFL This matrix can be expressed by the formula: ; The top right corner indicates the node number. Y netGFL The elements on the main diagonal of the matrix represent the self-admittance of the nodes. Assuming node i is the middle node in the wind turbine link, let's take node i as an example. The self-admittance... It can be expressed by the following formula: ; in, and These are the equivalent admittance of the collector line impedance and the equivalent admittance of the capacitor between nodes i and i+1, respectively. and These are the equivalent admittance of the collector line impedance and the equivalent admittance of the capacitor between nodes i-1 and i, respectively. Elements off-diagonal represent the mutual admittance between two node indices. Taking nodes i and i+1 as an example... The mutual admittance can be expressed by the following formula: ; Subsequently, based on the constructed admittance matrix of the grid-connected wind power cluster nodes, the node voltage equations of the grid-connected wind power cluster are constructed. Let the global PCC point be node 0, then the node voltage equations of the grid-connected wind power cluster can be expressed as: ; Where v0 is the small-signal voltage component of node 0; Y0 is the matrix formed by the mutual admittance between node 0 and other nodes in the grid-connected wind power cluster; V is the column vector composed of the small-signal voltages of each node in the grid-connected wind power cluster. The above two transfer function matrices can be expressed by the following formula: ; ; Where T in the upper right corner represents the matrix transpose operation; Y 0,1 To Y 0,n It is the mutual admittance between node 0 and nodes 1 through n; v1 to v n It is the small signal voltage component from node 1 to node n.

[0033] Subsequently, applying the KCL theorem to node 0, the small-signal current component i0 injected into the grid-connected wind power cluster at node 0 can be expressed as: ; Among them, Y 0,0 It is the self-admittance between the grid-type wind power cluster and the global PCC point.

[0034] Simplifying the above two equations by substitution, we obtain the equivalent impedance Z of the grid-type wind power cluster aggregated to the global PCC point. GFL,C It can be expressed by the following formula: ; Among them, Y netGFL For the admittance matrix of the grid-connected wind power cluster nodes; Y 0,0 Y0 is the self-admittance between the grid-connected wind power cluster and the global PCC point; Y0 is the matrix formed by the mutual admittance between node 0 and other nodes of the grid-connected wind power cluster; T represents the matrix transpose operation. Using the same impedance aggregation method as grid-connected wind power clusters, the equivalent impedance Z of grid-connected wind power clusters is aggregated to the global PCC point. GFM,C It can be expressed by the following formula: ; Among them, Y netGFM It is the admittance matrix of a grid-type wind power cluster node; Y m0,0 It is the self-admittance between the grid-type wind power cluster and the global PCC point; Y m0 It is the matrix formed by the mutual admittance between node 0 and other nodes in the grid-type wind power cluster, and T represents the matrix transpose operation.

[0035] Step S4 specifically includes: Step S401: Treat the equivalent impedances of AC grid nodes, grid-connected wind power clusters, and grid-connected wind power clusters as small-signal impedances in the dq domain of the three-node system, and place them on the diagonal of the 3×3 node matrix in the order of nodes. Set the elements on the off-diagonal to 0 to obtain the equivalent node impedance matrix. Wherein, the equivalent nodal impedance matrix Z nod (s) is: ; Among them, Z GFL,C The equivalent impedance of the grid-connected wind power cluster aggregated to the global PCC point; Z GFM,C Z represents the equivalent impedance of a grid-type wind power cluster aggregated to the global PCC point. g It is the impedance of the AC power grid node, and its value is determined by the external power grid. Step S402: The connection lines (including cables, transformers and equivalent series elements) between AC grid nodes, grid-connected wind power cluster nodes and grid-connected wind power cluster nodes are uniformly equivalent to the inter-node branch admittance. The impedance of each branch is summarized according to the branch number, and the equivalent node branch admittance matrix is ​​constructed based on the impedance connection relationship between nodes.

[0036] Among them, the equivalent nodal branch admittance matrix Y b (s) is: ; Among them, Y b11 (s) to Y b33 (s) is the equivalent nodal branch admittance matrix Y b In (s), the elements on the non-main diagonal represent the mutual admittance between two node numbers, and the elements on the main diagonal represent the self admittance of the node. The subscripts 1, 2, and 3 represent the nodes of the grid-connected wind power cluster, the nodes of the grid-connected wind power cluster, and the grid node, respectively.

[0037] The elements off the main diagonal represent the mutual admittance between the two node numbers, where the mutual admittance Y is... bkl (s) can be expressed by the following formula: ; in, It is the equivalent admittance of the collector line impedance between nodes k and l; Elements on the diagonal represent the self-admittance of a node, where self-admittance Y is the self-admittance. bkk (s) can be expressed by the following formula: ; in, It is the equivalent admittance of the collector line capacitance at node k.

[0038] Step S5 will express the equivalent node impedance matrix and the equivalent node branch admittance matrix based on the harmonic balance relationship of the connecting lines of each node using the relationship between the harmonic voltage response and the harmonic current response of each port, and solve them together to obtain the impedance model of the grid-connected offshore wind farm system.

[0039] like Figure 8 The figure shows a method for solving the impedance model of a grid-connected offshore wind farm system based on harmonic balance relationships. In the figure, (j is the node number, j=1, 2, 3) represents the virtual injection voltage Δv for each node. j and Δi j This represents the harmonic voltage and harmonic current response at the j-th node caused by the virtual injection voltage. Based on the definition of impedance, the impedance relationships at each node can be uniformly expressed by the following formula: ; Where Δv is the harmonic voltage response vector, and Δi is the harmonic current response vector, it can be expressed by the following formula: ; Wherein, Δv1 to Δv3 are the harmonic voltage responses of nodes 1 to 3; Δi1 to Δi3 are the harmonic current responses of nodes 1 to 3.

[0040] Subsequently, based on the harmonic current response balance relationship of the connecting lines at each node, the harmonic current response can be expressed by the following formula: ; Among them, v inj It is a virtual injected voltage vector, which can be expressed by the following formula: ; in, to It is the virtual injection voltage from node 1 to node 3.

[0041] By combining the impedance relationships of each node and the harmonic balance expression, the impedance model Z of the grid-connected offshore wind farm system can be obtained. sys (s), which can be expressed by the following formula: ; Among them, Z nod (s) is the equivalent nodal impedance matrix, Y b (s) is the equivalent nodal branch admittance matrix, I is the identity matrix, and v inj Δi is the virtual injected voltage vector, and Δi is the harmonic current response vector.

[0042] The impedance model Z of the grid-connected offshore wind farm system obtained by solving the problem is as follows: sys (s) can be used to determine the small-disturbance stability of the system through eigenvalue analysis, or to analyze the small-disturbance stability of the system by plotting a generalized Nyquist curve from the perspective of frequency response. On the one hand, it can effectively solve the equivalent deviation problem caused by the multi-unit hybrid aggregation of grid-connected and grid-connected wind turbines, and can effectively improve the accuracy of the impedance model of the grid-connected hybrid power station system; on the other hand, by adopting the method of aggregating the impedance of similar units to form a cluster equivalent impedance, the dimensionality of the system impedance model can be effectively reduced, thereby reducing the amount of computation in the model solution process.

[0043] Although the present invention has been described in detail above with general descriptions and specific embodiments, modifications or improvements can be made to it, which will be obvious to those skilled in the art. Therefore, all such modifications or improvements made without departing from the spirit of the present invention fall within the scope of protection claimed by the present invention.

Claims

1. A method for establishing an impedance model for a grid-connected offshore wind farm system, characterized in that, Includes the following steps: Step S1: In the local PCC point coordinate system of each wind turbine, establish the full-order admittance small-signal model of the converter of the grid-connected and grid-connected wind turbines; Step S2: Transform the full-order admittance small-signal models of grid-connected and grid-connected wind turbine converters to the global PCC point coordinate system to obtain the admittance models of grid-connected and grid-connected wind turbine converters in the global PCC point coordinate system. Step S3: Divide the wind turbines into grid-connected wind power clusters and grid-connected wind power clusters. Based on the admittance models of the converters of the grid-connected and grid-connected wind turbines in the grid-connected and grid-connected wind power clusters, and combine the admittance of the collector line nodes in the grid-connected and grid-connected wind power clusters to perform interconnection equivalence and inverse calculation, so as to obtain the equivalent impedance of the grid-connected and grid-connected wind power clusters aggregated to the global PCC point. Step S4: Treat the grid-connected wind power cluster, the grid-connected wind power cluster, and the AC grid as a three-node system, obtain the equivalent impedance in the grid-connected wind power cluster, the grid-connected wind power cluster, and the AC grid respectively, and construct the equivalent node impedance matrix and the equivalent node branch admittance matrix. Step S5: Based on the harmonic balance relationship of the connecting lines of each node, and combined with the equivalent node impedance matrix and the equivalent node branch admittance matrix, establish the impedance model of the grid-connected offshore wind farm system. Step S2 specifically includes: Step S201: Using the phase angle measurement unit, collect the phase angle of the local PCC point voltage of the wind turbine and the phase angle of the global PCC point voltage of the wind farm provided by each grid-connected and grid-connected wind turbine converter, and calculate the phase angle difference corresponding to each grid-connected and grid-connected wind turbine converter to construct the corresponding rotation matrix of the grid-connected and grid-connected wind turbine converter. Step S202: The full-order admittance small-signal model of the grid-following and grid-connected wind turbine converters described in step S1 is transformed by the corresponding rotation matrix to obtain the admittance model of the grid-following and grid-connected wind turbine converters in the global PCC point coordinate system. In step S3, based on the connection method of wind turbines in the actual topology of the wind farm, all grid-connected wind turbine links are divided into grid-connected wind power clusters, and all grid-connected wind turbine links are divided into grid-connected wind power clusters. A start / stop state function S is then introduced. i (t) and S mj (t) represents the start-up and shutdown states of the i-th grid-connected wind turbine converter and the j-th grid-connected wind turbine converter at time t, respectively; In step S3, the equivalent impedance Z aggregated to the global PCC point of the grid-type wind power cluster GFL,C for: ； Among them, Y netGFL For the admittance matrix of the grid-connected wind power cluster nodes; Y 0,0 Y0 is the self-admittance between the grid-connected wind power cluster and the global PCC point; Y0 is the matrix formed by the mutual admittance between node 0 and other nodes of the grid-connected wind power cluster; T represents the matrix transpose operation. In step S3, the equivalent impedance Z of the grid-type wind power cluster aggregated to the global PCC point GFM,C for: ； Among them, Y netGFM It is the admittance matrix of a grid-type wind power cluster node; Y m0,0 It is the self-admittance between the grid-type wind power cluster and the global PCC point; Y m0 It is the matrix formed by the mutual admittance between node 0 and other nodes in the grid-type wind power cluster, and T represents the matrix transpose operation.

2. The method for establishing the impedance model of an offshore wind farm grid-connected system as described in claim 1, characterized in that, Step S1 specifically includes: Step S101: Build physical models of the converters for each grid-connected and grid-connected wind turbine unit respectively; Step S102: Based on the physical models of grid-connected and grid-connected wind turbine converters, convert each component in the grid-connected and grid-connected wind turbine converters into a small-signal model, and obtain the full-order small-signal model of the grid-connected and grid-connected wind turbine converters based on multiple small-signal models. Step S103: Based on the full-order small-signal model established in step S102, simplify according to the impedance definition to obtain the full-order admittance small-signal model of the grid-following and grid-connected wind turbine converters in the local PCC point coordinate system.

3. The method for establishing the impedance model of an offshore wind farm grid-connected system as described in claim 1, characterized in that, Step S4 specifically includes: Step S401: Treat the equivalent impedances of AC grid nodes, grid-connected wind power clusters, and grid-connected wind power clusters as small-signal impedances in the dq domain of the three-node system, and place them on the diagonal of the 3×3 node matrix in the order of nodes. Set the elements on the off-diagonal to 0 to obtain the equivalent node impedance matrix. Step S402: The connection lines between AC grid nodes, grid-connected wind power cluster nodes and grid-connected wind power cluster nodes are uniformly equivalent to the inter-node branch admittances. The impedances of each branch are summarized according to the branch number, and an equivalent node branch admittance matrix is ​​constructed based on the impedance connection relationship between nodes.

4. The method for establishing the impedance model of an offshore wind farm grid-connected system as described in claim 1, characterized in that, Step S5 will, based on the harmonic balance relationship of the connecting lines at each node, express the equivalent node impedance matrix and the equivalent node branch admittance matrix using the relationship between the harmonic voltage response and harmonic current response at each port, and solve them simultaneously to obtain the impedance model of the grid-connected offshore wind farm system; wherein, the impedance model Z sys (s) is: ； Among them, Z nod (s) is the equivalent nodal impedance matrix, Y b (s) is the equivalent nodal branch admittance matrix, I is the identity matrix, and v inj Δi is the virtual injected voltage vector, and Δi is the harmonic current response vector.

5. The method for establishing the impedance model of an offshore wind farm grid-connected system as described in claim 2, characterized in that, The grid-connected wind turbine converter consists of a phase-locked loop, a power outer loop control loop, a current inner loop control loop, and a coordinate transformation loop; the grid-connected wind turbine converter consists of a power synchronization loop, a reactive power-voltage droop control loop, a voltage outer loop control loop, a current inner loop control loop, and a coordinate transformation loop.

6. The method for establishing the impedance model of an offshore wind farm grid-connected system as described in claim 2, characterized in that, The full-order admittance small-signal model of the grid-following wind turbine converter in the local PCC point coordinate system is as follows: ； in, This represents the full-order admittance small-signal model of the i-th grid-connected wind turbine converter; , It is the small-signal component matrix of voltage and current at the local PCC point of the i-th grid-connected wind turbine converter; , , , , , , , , , The transfer function matrix is ​​the intermediate variable used in the process of obtaining the admittance of the i-th grid-connected wind turbine converter; I is the identity matrix. The full-order admittance small-signal model of the grid-type wind turbine converter in the local PCC point coordinate system is as follows: ； in, This represents the full-order admittance small-signal model of the converter of the j-th grid-connected wind turbine unit; , It is the matrix of voltage and current small-signal components at the local PCC point of the converter of the j-th grid-type wind turbine; , , , , , , , , , , , , , , , The intermediate variable transfer function matrix is ​​used in the process of obtaining the admittance of the j-th grid-type wind turbine converter; I is the identity matrix.

7. The method for establishing the impedance model of an offshore wind farm grid-connected system as described in claim 1, characterized in that, The rotation matrix of the grid-connected and grid-connected wind turbine converters is as follows: ； Among them, T i Let T be the rotation matrix of the i-th grid-connected wind turbine converter. mj Let Δθ be the rotation matrix of the j-th grid-connected wind turbine converter. i Δθ represents the difference between the local PCC point voltage phase angle and the global PCC point voltage phase angle at the converter of the i-th grid-connected wind turbine unit. mj The difference between the local PCC point voltage phase angle and the global PCC point voltage phase angle at the converter of the j-th grid-type wind turbine unit; The admittance models for grid-following and grid-connected wind turbine converters in the global PCC point coordinate system are as follows: ； in, and These are the admittance models of the i-th grid-connected wind turbine converter and the j-th grid-connected wind turbine converter in the global PCC point coordinate system, respectively. and Let T represent the full-order admittance small-signal models of the i-th grid-type and j-th grid-type wind turbine converters, respectively. i Let T be the rotation matrix of the i-th grid-connected wind turbine converter. mj Let be the rotation matrix of the j-th grid-type wind turbine converter.

8. The method for establishing the impedance model of an offshore wind farm grid-connected system as described in claim 3, characterized in that, The equivalent node impedance matrix Z in step S4 nod (s) is: ； Among them, Z GFL,C The equivalent impedance of the grid-connected wind power cluster aggregated to the global PCC point; Z GFM,C Z represents the equivalent impedance of a grid-type wind power cluster aggregated to the global PCC point. g It is the impedance of the AC power grid node, and its value is determined by the external power grid. Equivalent nodal branch admittance matrix Y b (s) is: ； Among them, Y b11 (s) to Y b33 (s) is the equivalent nodal branch admittance matrix Y b In (s), the elements on the non-main diagonal represent the mutual admittance between two node numbers, and the elements on the main diagonal represent the self admittance of the node. The subscripts 1, 2, and 3 represent the nodes of the grid-connected wind power cluster, the nodes of the grid-connected wind power cluster, and the nodes of the AC grid, respectively.

Images