Method and device for analyzing effectiveness of power system series branch resonance suppression strategy

Abstract

The present application relates to the technical field of resonance analysis, and in particular to a method and device for analyzing effectiveness of a resonance suppression strategy for a series branch of a power system. In the present application, the method realizes effective evaluation of resonance suppression of the series branch by constructing a resonance suppression evaluation index and a corresponding feasible region for resonance suppression of the series branch, and can solve the problems of limited application scenarios of current resonance suppression methods and incompatibility between resonance scenarios of the series branch and resonance suppression methods.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of resonance analysis, and particularly relates to a method and device for analyzing effectiveness of a series branch resonance suppression strategy of a power system. BACKGROUND

[0002] The resonance suppression strategy is designed based on specific series / parallel resonance scenarios such as filter resonance, device interaction resonance, interaction resonance between a device and a power grid, and grid-connected resonance of new energy. Since the resonance characteristics have multi-dimensional information such as amplitude, frequency, and phase, the suppression effect of the same resonance suppression strategy may be different in different cases, and improper strategy may cause resonance amplification frequency shift, that is, amplifying the resonance peak at a new resonance frequency. However, the existing research pays less attention to the range of scenarios applicable to the suppression strategy and effectiveness, and is mostly directed to traditional parallel resonance, and lacks a method for analyzing the effectiveness of the resonance suppression strategy for series branch resonance problems. Therefore, in order to form a better correspondence between the resonance suppression strategy and the scenario, it is necessary to study the effectiveness evaluation method of the series branch resonance suppression strategy in different scenarios, which helps to promote the application and optimization of the series branch resonance suppression method and has far-reaching significance for the research of the series branch resonance suppression method. SUMMARY

[0003] Therefore, the present application provides a method and device for analyzing effectiveness of a series branch resonance suppression strategy of a power system, to solve the problem that the existing technology lacks a method for analyzing the effectiveness of the resonance suppression strategy for series branch resonance problems.

[0004] In a first aspect, the present application provides a method for analyzing effectiveness of a series branch resonance suppression strategy of a power system, which comprises: determining a disturbance injection node, a resonance branch to be governed, and a resonance suppression strategy action node based on a branch method; determining a resonance suppression evaluation index based on the change in harmonic current amplitude of the disturbance injection node, the resonance branch to be governed, and the resonance suppression strategy action node before and after resonance suppression, the change in harmonic current amplitude being determined based on a node admittance matrix and a node impedance matrix; determining a series branch resonance suppression feasible region based on a concerned frequency band of the resonance suppression strategy, an equivalent impedance, and the resonance suppression evaluation index; determining a series branch resonance suppression ideal feasible region based on the intersection of the series branch resonance suppression feasible region and a quadrant IV part; and evaluating the effectiveness of the resonance suppression strategy based on the relationship between the equivalent impedance of the resonance suppression strategy and the series branch resonance suppression feasible region and the series branch resonance suppression ideal feasible region.

[0005] In the present application, the method realizes the effectiveness evaluation of the series branch resonance suppression by constructing the resonance suppression evaluation index and the corresponding series branch resonance suppression feasible region, and can solve the problems of limited application scenarios of the resonance suppression method and the inadaptation of the series branch resonance scenario to the resonance suppression method.

[0006] In the present application, the ideal feasible region of series branch resonance suppression is determined based on the intersection of the feasible region of series branch resonance suppression and a four-quadrant part, so that the ideal feasible region constructed not only considers the resonance suppression effect, but also further considers the problem that when the resonance suppression strategy equivalent impedance is located in the second three-quadrant, negative impedance is introduced to the system, the system damping is reduced, and the system becomes unstable. Therefore, the ideal feasible region is used to determine the resonance suppression scheme selection and parameter optimization suggestion, which can reduce the risk of system instability.

[0007] In an optional embodiment, the method further comprises: determining a plurality of resonance suppression scheme selections and parameter optimization suggestions based on the effectiveness evaluation result of the resonance suppression strategy.

[0008] In an optional embodiment, the branch method is used to determine the disturbance injection node, the resonance branch to be treated, and the action node of the resonance suppression strategy, comprising: constructing a series branch resonance analysis matrix based on the relationship between the node voltage and the injected current in the power system, the admittance matrix of the series branch in the power system, and the node impedance matrix; performing frequency scanning on the series branch resonance analysis matrix to obtain the series branch resonance distribution, and determining the disturbance injection node, the resonance branch to be treated, and the action node of the resonance suppression strategy.

[0009] In the present application, the current is analyzed by the branch method, and a series branch resonance analysis matrix is constructed therefrom, which provides a data basis for series branch resonance analysis. At the same time, it also provides a data basis for resonance suppression effect evaluation based on the corresponding element value in the series branch resonance analysis matrix, i.e. the series branch harmonic current amplitude.

[0010] In an optional embodiment, the resonance suppression evaluation index is expressed by the following formula:

[0011]

[0012] In the formula, D represents the admittance matrix of the series branch, C represents the correlation matrix, Z 0f , and Z f respectively represent the system node impedance matrix before and after resonance suppression at frequency f, m represents the resonance branch to be treated, p represents the disturbance injection node, and τ is a threshold value.

[0013] In the present application, the above formula is used as an evaluation index, which provides a data basis for accurate evaluation of the resonance suppression effect.

[0014] In an alternative embodiment, the resonance suppression strategy is based on the frequency band of interest, the equivalent impedance, and the resonance suppression evaluation index to determine the series branch resonance suppression feasible region, including: determining the impedance relationship before and after resonance suppression according to the branch addition method and the equivalent impedance of the resonance suppression strategy; substituting the impedance relationship into the resonance suppression evaluation index to obtain the relationship between the equivalent impedance and the impedance after resonance suppression; determining the series branch resonance suppression feasible region at a single frequency based on the relationship between the equivalent impedance and the impedance after resonance suppression; and determining the series branch resonance suppression feasible region within the frequency band of interest based on the frequency band of interest of the resonance suppression strategy and the series branch resonance suppression feasible region at a single frequency.

[0015] In the present application, after determining the series branch resonance suppression feasible region at a single frequency, the series branch resonance suppression feasible region within the frequency band of interest is further determined according to the frequency band of interest of the resonance suppression strategy, so that the constructed feasible region takes into account the resonance frequency shift phenomenon, thereby making the resonance suppression strategy effectiveness evaluation based on the feasible region more accurate.

[0016] In an alternative embodiment, when τ>0, the boundary of the series branch resonance suppression is a circle with (α,β) as the center and r as the radius, and the internal region of the circle is the series branch resonance suppression feasible region; when τ=0, the boundary of the resonance suppression or amplification is a straight line, and the series branch resonance suppression feasible region is one side of the straight line.

[0017] In the present application, the boundary of the series branch resonance suppression feasible region is limited, so that the determined feasible region is more intuitive.

[0018] In an alternative embodiment, the effectiveness of the resonance suppression strategy is evaluated based on the relationship between the equivalent impedance of the resonance suppression strategy and the series branch resonance suppression feasible region and the ideal series branch resonance suppression feasible region, including: comparing the equivalent impedance of a plurality of resonance suppression schemes with the overlap of the series branch resonance suppression feasible region within the frequency band of interest and the ideal series branch resonance suppression feasible region to evaluate the effectiveness of the resonance suppression strategy.

[0019] In a second aspect, the present application provides a device for analyzing effectiveness of a series branch resonance suppression strategy of a power system, the device comprising: a node determination module configured to determine a disturbance injection node, a resonance branch to be treated, and a node affected by the resonance suppression strategy based on a branch method; an evaluation index determination module configured to determine a resonance suppression evaluation index based on a change in harmonic current amplitude before and after the resonance suppression of the disturbance injection node, the resonance branch to be treated, and the node affected by the resonance suppression strategy, the change in harmonic current amplitude being determined based on a node admittance matrix and a node impedance matrix; a feasible region determination module configured to determine a series branch resonance suppression feasible region based on a frequency band of interest of the resonance suppression strategy, an equivalent impedance, and the resonance suppression evaluation index; an ideal feasible region determination module configured to determine a series branch resonance suppression ideal feasible region based on an intersection of the series branch resonance suppression feasible region and a quadrant IV part; and an evaluation module configured to evaluate effectiveness of the resonance suppression strategy based on a relationship between the equivalent impedance of the resonance suppression strategy and the series branch resonance suppression feasible region and the series branch resonance suppression ideal feasible region.

[0020] In a third aspect, the present application provides a computer device, comprising: a memory and a processor, which are in communication connection with each other, the memory stores computer instructions, and the processor executes the computer instructions to perform the power system series branch resonance suppression strategy effectiveness analysis method of the first aspect or any of the corresponding embodiments thereof.

[0021] In a fourth aspect, the present application provides a computer readable storage medium, which stores computer instructions, and the computer instructions are used to make a computer perform the power system series branch resonance suppression strategy effectiveness analysis method of the first aspect or any of the corresponding embodiments thereof.

[0022] In a fifth aspect, the present application provides a computer program product, which comprises computer instructions, and the computer instructions are used to make a computer perform the power system series branch resonance suppression strategy effectiveness analysis method of the first aspect or any of the corresponding embodiments thereof. BRIEF DESCRIPTION OF DRAWINGS

[0023] In order to more clearly illustrate the specific embodiments of the present application or the technical solutions in the prior art, the drawings needed in the specific embodiments or the prior art description will be briefly introduced as follows. Obviously, the drawings in the following description are some embodiments of the present application, and those skilled in the art can also obtain other drawings according to these drawings without creative labor.

[0024] Figure 1 is a flowchart of the power system series branch resonance suppression strategy effectiveness analysis method according to an embodiment of the present application;

[0025] Figure 2 FIG. 7(a) and FIG. 7(b) are schematic diagrams of current amplitude-frequency curves of a series branch before and after the resonance suppression according to an embodiment of the present application;

[0026] FIG. 3(a) and FIG. 3(b) are schematic diagrams of boundaries of a feasible region of series branch resonance suppression when the threshold τ takes different values according to an embodiment of the present application;

[0027] Figure 4 FIG. 1 is a schematic diagram of a simple network structure of a power system according to an embodiment of the present application;

[0028] FIG. 5(a), FIG. 5(b) and FIG. 5(c) are current amplitude-frequency curves of a series branch 1, a series branch 2 and a series branch 3 according to an embodiment of the present application;

[0029] FIG. 6(a) and FIG. 6(b) are schematic diagrams of distributions of existing resonance suppression strategies in a feasible region when the resonance suppression strategies act on different nodes according to an embodiment of the present application;

[0030] FIG. 7(a) and FIG. 7(b) are schematic diagrams of current amplitude-frequency curves of a series branch before and after the resonance suppression according to an embodiment of the present application;

[0031] Figure 8 FIG. 2 is a structural block diagram of an effectiveness analysis device of a series branch resonance suppression strategy of a power system according to an embodiment of the present application;

[0032] Figure 9 FIG. 8 is a schematic diagram of a hardware structure of a computer device according to an embodiment of the present application. DETAILED DESCRIPTION

[0033] In order to make the objectives, technical solutions and advantages of embodiments of the present application clearer, the following will be combined with the accompanying drawings for the embodiments of the present application to clearly and completely describe the technical solutions in the embodiments of the present application. Obviously, the described embodiments are some but not all of the embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative work fall within the scope of the present application.

[0034] According to an embodiment of the present application, there is provided an effectiveness analysis method of a series branch resonance suppression strategy of a power system. It should be noted that the steps shown in the flowchart of the accompanying drawings can be executed in a computer system such as a set of computer executable instructions, and although a logical order is shown in the flowchart, in some cases, the steps shown or described can be executed in an order different from that shown here.

[0035] The application provides a method for analyzing effectiveness of a series branch resonance suppression strategy of a power system, which can be used for electronic devices such as computers, mobile phones and tablet computers, Figure 1 The method for analyzing effectiveness of the series branch resonance suppression strategy of the power system according to the embodiment of the application is shown in a flowchart as Figure 1 The flowchart comprises the following steps:

[0036] In step S101, a disturbance injection node, a resonance branch to be treated and a node for implementing a resonance suppression strategy are determined based on a branch method.

[0037] Specifically, the basic idea of the branch method is to establish a relationship between a series branch current and a node injection current. In a power system, the node injection current refers to a current flowing into a node from outside, and the series branch current refers to a current flowing through a series branch (a branch composed of inductors, capacitors and resistors in series). Then, the relationship between the series branch current and the node injection current is recalculated by changing the frequency of an input signal through frequency scanning, and the relationship between the series branch current and the node injection current varying with frequency is obtained. When the series branch current appears a peak value at a certain frequency, it means that the circuit occurs series branch resonance at the frequency.

[0038] In the formula, the series branch determined by the branch method when the series branch resonance occurs is taken as the resonance branch to be treated. The node for implementing the resonance suppression strategy is a position for implementing the resonance suppression measure, that is, the resonance suppression measure is implemented at the position to change the electrical parameters of the node, so as to change the electrical characteristics of the resonance branch to be treated, and the purpose of suppressing resonance is achieved. In a specific implementation, the selection of the node for implementing the resonance suppression strategy is usually related to the resonance branch to be treated, and the suppression strategy is usually taken at a node connected to the resonance branch to be treated or at a node capable of effectively changing the electrical parameters of the branch. For example, if a series branch occurs resonance, a measure can be taken at two end nodes connected to the branch, such as adding a damping resistor at the node, installing a reactor or a capacitor to change the electrical parameters of the branch.

[0039] The disturbance injection node is usually used to inject a disturbance signal to observe the response (such as the change of the series branch current and the fluctuation of the node voltage) of the system to detect whether the system has resonance and the characteristics of the resonance. That is, the disturbance injection node is mainly used for diagnosing the resonance of the system. Therefore, the disturbance injection node is usually selected at a position capable of effectively affecting the state of the system and facilitating the monitoring of the response of the system. In the branch method, the disturbance can be injected at a node close to the series branch or a representative node in the system. For example, in a power system, if a plurality of series branches may occur resonance, a disturbance signal such as a small-amplitude voltage pulse or a current pulse can be injected at a common bus node connected to the series branches.

[0040] Step S102, determine the resonance suppression evaluation index based on the change of harmonic current amplitude before and after the resonance suppression of the perturbation injection node, the resonance to-be-governed branch and the resonance suppression strategy action node. The change of harmonic current amplitude is determined based on the node admittance matrix and the node impedance matrix. Specifically, according to the analysis in step S101, the series branch resonance is determined by the change relationship between the series branch current and the injected current with the frequency, so the resonance suppression evaluation index can be determined by the change of current amplitude before and after the resonance, which can be understood as the evaluation of the resonance suppression effect of the implemented resonance suppression strategy. Specifically, when the current amplitude change before and after the resonance meets certain conditions such as threshold, it is considered that the implemented resonance suppression evaluation strategy achieves the resonance suppression effect. The change of harmonic current amplitude is related to the change of node admittance matrix and node impedance matrix, so the resonance suppression evaluation index can be further determined by the change of node admittance matrix and node impedance matrix.

[0041] Step S103, determine the series branch resonance suppression feasible region based on the concerned frequency band of the resonance suppression strategy, the equivalent impedance and the resonance suppression evaluation index. Specifically, when the resonance suppression strategy is implemented at the node in the power system, the equivalent impedance will change, and the change of equivalent impedance is not the same after the implementation of different resonance suppression strategies, so the different resonance suppression strategies can be represented by the equivalent impedance in the resonance suppression evaluation index. Then the corresponding feasible region is obtained by combining the change of node impedance in the evaluation index. The feasible region can be understood as a region representing the effect of the resonance suppression strategy.

[0042] Since the resonance suppression strategy may cause resonance frequency shift phenomenon (i.e. the current at the resonance frequency decreases and the current at the surrounding frequency increases), the resonance suppression domain near the resonance frequency range should be analyzed, and thus the corresponding concerned frequency band is determined for each resonance suppression strategy, and the feasible region is further adjusted in combination with the concerned frequency band to obtain the series branch resonance suppression feasible region in the concerned frequency band. The frequency range in the concerned frequency band is [f r -f, f r +f], where f r is the resonance peak frequency, and the frequency band width Δf can be designed flexibly according to application requirements.

[0043] Step S104, determining a series branch resonance suppression ideal feasible region based on the intersection of the series branch resonance suppression feasible region and a four-quadrant part; the series branch resonance suppression ideal feasible region is determined based on the intersection of the series branch resonance suppression feasible region and the four-quadrant part, so that the ideal feasible region constructed not only considers the resonance suppression effect, but also further considers the problem that when the resonance suppression strategy equivalent impedance is located in the second three-quadrant, negative impedance is introduced to the system, the system damping is reduced, and the system is unstable, so that the ideal feasible region is used to determine a plurality of resonance suppression scheme selection and parameter optimization suggestions, and the system instability risk can be reduced.

[0044] Step S105, evaluating the effectiveness of the resonance suppression strategy based on the relationship between the equivalent impedance of the resonance suppression strategy and the series branch resonance suppression feasible region and the series branch resonance suppression ideal feasible region. Specifically, the effectiveness of the resonance suppression scheme can be evaluated by the distribution of the equivalent impedance of the resonance suppression scheme in the feasible region.

[0045] The effectiveness analysis method of the series branch resonance suppression strategy of the power system provided by the embodiment of the present application can realize the effectiveness evaluation of the series branch resonance suppression by constructing the resonance suppression evaluation index and the corresponding series branch resonance suppression feasible region, and can solve the problems that the application scene of the resonance suppression method is limited, and the series branch resonance scene is not adapted to the resonance suppression method.

[0046] In the embodiment, an effectiveness analysis method of a series branch resonance suppression strategy of a power system is provided, as shown in Figure 2 The method comprises the following steps:

[0047] Step S201, determining a disturbance injection node, a resonance to be governed branch and a resonance suppression strategy action node based on a branch method.

[0048] Specifically, the above step S201 comprises:

[0049] Step S2011, constructing a series branch resonance analysis matrix based on the relationship between the node voltage and the injected current in the power system, the admittance matrix of the series branch in the power system and the node impedance matrix.

[0050] Step S2012, performing frequency scanning on the series branch resonance analysis matrix to obtain a series branch resonance distribution, and determining the disturbance injection node, the resonance to be governed branch and the resonance suppression strategy action node.

[0051] For an n-node system, the relationship between the node voltage and the injected current is as follows:

[0052]

[0053] U=Y -1I (2)

[0054] where Y is the node admittance matrix, U is the node voltage vector, and I is the injected node current vector.

[0055] The voltage of branch m between nodes i and j is:

[0056] U' m = U i - U j (3)

[0057] The connection relationship and coupling degree between nodes can be obtained from the elements of matrix Y. Assuming that there are r independent series branches, the branches are numbered in the order of the initial node from small to large, and there are:

[0058]

[0059] U' = CU (5)

[0060] where C is the incidence matrix. Assuming that branch m is between node i and node j, and y ij ≠ 0, then:

[0061]

[0062] The current of series branch m is J m = - U' m y ij , d m = - y ij .

[0063] The admittance matrix of the series branch is:

[0064] D = diag (d1, d2, K, d r ) (7)

[0065] The branch current vector J = [J1, J2, K, J r ] T is:

[0066] J = DCU (8)

[0067] Combining equation (2) and equation (8) gives:

[0068] J = HI (9)

[0069] H = DCY -1 = DCZ (10)

[0070] where: Z is the node impedance matrix.

[0071] The unit harmonic current is injected at node i, and the current of each series branch in the power grid is shown in equation (11):

[0072]

[0073] At this time, the change of each element in the matrix H with frequency can represent the change rule of the harmonic current amplification of any series branch with frequency. When the harmonic current amplification frequency is the same as the resonance frequency of the system series branch (i.e., when each element in H has a maximum point), the system series branch resonance is the cause of the peak value of the series branch harmonic current, so the change of the series branch current with the injected current with frequency can reflect the system series branch resonance, and the matrix H is a series branch resonance analysis matrix. By analyzing the sweep results of each element in the matrix H, the system series branch resonance distribution is obtained, and information such as the resonance branch to be treated and the node to be acted on by the suppression method is determined. At the same time, the resonance suppression effect can be evaluated by the effect of suppressing the amplitude of the series branch harmonic current (i.e., the amplitude of the corresponding element in the series branch resonance analysis matrix H).

[0074] In step S202, the resonance suppression evaluation index is determined based on the change of the harmonic current amplitude of the perturbation injection node, the resonance branch to be treated, and the node to be acted on by the resonance suppression strategy before and after the resonance suppression, and the change of the harmonic current amplitude is determined based on the node admittance matrix and the node impedance matrix.

[0075] Specifically, if the perturbation injection node is p, the resonance branch to be treated is m, and the resonance suppression strategy is applied to node s. The series branch resonance suppression effect of the resonance suppression strategy can be reflected by the change of the amplitude of the corresponding element in the series branch resonance analysis matrix H before and after the suppression. The corresponding elements in the matrix H before and after the resonance suppression at frequency f are H 0f (m,p) and H f (m,p), respectively. When the change of the corresponding element is greater than a threshold value, it is considered that the suppression of the series branch resonance of branch m is achieved, that is:

[0076] |H 0,f (m,p)| 2 -|H f (m,p)| 2 >τ (12)

[0077] In the formula, τ is a threshold value, τ≥0. The value of τ can be flexibly selected according to the actual needs of the project.

[0078] As can be seen from equations (5), (7), and (10), the resonance suppression strategy only affects the node admittance matrix after the action, so equation (12) can be transformed into:

[0079]

[0080] In the formula, Z 0f , Zf respectively represent the system node impedance matrix before and after the resonance suppression at frequency f.

[0081] Further arrangement is obtained:

[0082] |Z 0,f (i,p)-Z 0,f (j,p)| 2 -|Z f (i,p)-Z f (j,p)| 2 >τ|z l,m | 2 (14)

[0083] In the formula: i, j are nodes at both ends of branch m respectively, z l,m is the impedance of branch m, and || represents the amplitude.

[0084] Step S203, determining the series branch resonance suppression feasible region based on the concerned frequency band of the resonance suppression strategy, the equivalent impedance, and the resonance suppression evaluation index;

[0085] Specifically, the above step S203 includes:

[0086] Step S2031, determining the impedance relationship before and after the resonance suppression according to the branch addition method and the equivalent impedance of the resonance suppression strategy.

[0087] Step S2032, substituting the impedance relationship into the resonance suppression evaluation index to obtain the relationship between the equivalent impedance and the impedance after the resonance suppression.

[0088] Step S2033, determining the series branch resonance suppression feasible region at a single frequency based on the relationship between the equivalent impedance and the impedance after the resonance suppression.

[0089] Step S2034, determining the series branch resonance suppression feasible region within the concerned frequency band based on the concerned frequency band of the resonance suppression strategy and the series branch resonance suppression feasible region at a single frequency.

[0090] Wherein, the branch addition method is a method of gradually constructing a circuit model and solving the node impedance matrix. In circuit analysis, a simple basic circuit may be started initially, and then a complete circuit model is constructed by gradually adding branches (such as branches composed of resistors, inductors, capacitors, etc.). Each added branch will have a corresponding impact on the node impedance matrix, and the node impedance matrix is updated through a series of matrix operations. Therefore, the resonance suppression strategy implemented in the power system is equivalent to adding a branch with impedance δ to ground (i.e. a branch with one end connected to a system node and the other end connected to ground). Therefore, in formula (14), Z f (i,p), Z f (j,p) can be transformed into:

[0091]

[0092] Where δ represents the equivalent impedance corresponding to the resonance suppression strategy. To simplify the notation, the subscript f will be omitted in the following text.

[0093] Substituting equations (15) and (16) into equation (14) and further sorting out, we can obtain:

[0094]

[0095] Where: A=(z is -z js )z sp , B=z ip -z jp , C=AB * . z is is the mutual impedance between system nodes i and s before resonance suppression, z js 、z sp 、z ip 、z jp Similarly. ss,r 、z ss,i is the real and imaginary part of the self-impedance of the system node s before resonance suppression. The impedance of each node in the system can be obtained by inverting the system node admittance matrix before suppression. * is the conjugate of B, δ r , δ i Represent the real and imaginary parts of δ, C r 、C i Represent the real and imaginary parts of C respectively. Whether the threshold in Equation (17) is 0 will affect the final form of the criterion. Further deduction shows that the series branch resonance suppression feasible domain (SRSD) is:

[0096] Ω SRS ={(δ r ,δ i )ε1(δ r ,δ i )<0} (18)

[0097]

[0098] α=[(|B| 2 -τ|z l,m | 2 )z ss,r -C r ] / (τ|z l,m | 2 ) (20)

[0099] β=[(|B| 2 -τ|zl,m | 2 )z ss,i -C i ] / (τ|z l,m | 2 ) (21)

[0100]

[0101] From formula (18)-(22), when τ>0, the boundary of series branch resonance suppression is a circle with (a, b) as the center and r as the radius, and the internal region of the circle is the series branch resonance suppression feasible region; when τ=0, the boundary of resonance suppression / amplification is a straight line, and the series branch resonance suppression feasible region is one side of the straight line.

[0102] As can be seen from formula (19), the series branch resonance suppression feasible region decouples the resonance suppression strategy equivalent impedance and the series branch resonance analysis matrix H, and the distribution position of the equivalent impedance of the resonance suppression strategy in the feasible region can be used to judge the rationality of the resonance suppression strategy and to optimize it. The distribution of the series branch resonance suppression feasible region when τ is 0 can be used to judge the effect of the suppression strategy on the resonance (i.e., whether the resonance is reduced or amplified); the resonance suppression feasible region when τ>0 can be used to judge the suppression effect of the suppression strategy on the resonance (i.e., whether the resonance is reduced) and to guide the design of the resonance suppression strategy.

[0103] The series branch resonance suppression feasible region determined by the above-mentioned manner is the series branch resonance suppression feasible region at a single frequency. However, since the resonance usually presents in the form of a resonance peak, the frequency band around the resonance frequency will also have a large current distribution, that is, in addition to paying attention to the distribution of the feasible region at the resonance frequency, attention should also be paid to the frequency band around the resonance frequency. Therefore, the resonance frequency band range and frequency interval should be given, and the series branch resonance suppression feasible region in the frequency band should be drawn one by one, and the attention frequency band should be designed according to the analysis and suppression requirements under different scenes and resonance characteristics.

[0104] The distribution of the series branch resonance suppression feasible region boundary at the resonance frequency and at different frequency points in the frequency band is shown in FIG. 3(a) and FIG. 3(b), wherein FIG. 3(a) represents the series branch resonance suppression feasible region boundary distribution schematic diagram when τ=0, and FIG. 3(b) represents the series branch resonance suppression feasible region boundary distribution schematic diagram when τ=0.5. When the threshold value is 0, the series branch resonance suppression feasible region boundary at a certain frequency is a straight line, and the two sides of the straight line are the resonance amplification region and the suppression region, respectively, wherein the dashed line corresponds to the resonance suppression feasible region boundary when the resonance peak frequency, and the solid line corresponds to the resonance suppression feasible region boundary at other frequencies in the attention frequency band; when the threshold value is not 0, the internal region of any circular boundary in the figure is the resonance suppression feasible region of its equivalent impedance.

[0105] In order to ensure that the resonance suppression effect is achieved within a certain resonance frequency band, the equivalent impedance of the resonance suppression strategy must be simultaneously within the feasible domain of resonance suppression at each frequency.

[0106] Step S204 determines the ideal feasible region for series branch resonance suppression based on the intersection of the feasible region for series branch resonance suppression and the first four-quadrant region. Specifically, when the equivalent impedance of the resonance suppression strategy is in the second and third quadrants, negative impedance is introduced into the system, reducing system damping and causing instability. Therefore, the ideal feasible region for series branch resonance suppression is determined based on the intersection of the feasible region for series branch resonance suppression and the first four-quadrant region, corresponding to the gray-shaded area in the figure. A sufficient condition for safe and effective resonance suppression is that the admittance change remains consistently within the ideal feasible region within the frequency band.

[0107] Step S205, based on the relationship between the equivalent impedance of the resonance suppression strategy and the feasible domain of series branch resonance suppression and the ideal feasible domain of series branch resonance suppression, the effectiveness of the resonance suppression strategy is evaluated. Specifically, the existing resonance suppression strategies mainly include active damping control, installation of resonance suppression equipment, change of system parameters, etc. The impact of the access of the resonance suppression strategy on the system operation state can be reflected by its equivalent impedance. By comparing the overlap of the equivalent impedance distribution under different resonance suppression strategies and parameters with the feasible domain of series branch resonance suppression in the frequency band, the effectiveness of the resonance suppression strategy can be judged. Among them, when judging, if the equivalent impedance of the resonance suppression strategy is simultaneously located in the feasible domain of resonance suppression of each frequency, it means that the resonance suppression effect is played in the frequency band of interest for resonance suppression.

[0108] Step S206: Determine multiple resonance suppression options and parameter optimization recommendations based on the effectiveness evaluation results of the resonance suppression strategy. Specifically, after determining the ideal feasible region for series branch resonance suppression, the resonance suppression strategy selection and parameter optimization recommendations can be provided by comprehensively considering the resonance suppression strategy effectiveness analysis conclusions from step S205 and combining the degree of overlap between the resonance suppression strategy and the ideal feasible region.

[0109] As a specific application example of the present invention, in order to verify the effectiveness analysis method of the proposed power system series branch resonance suppression strategy, Figure 4 The model is the analysis object. It includes three series branches. Each branch includes a voltage source converter (VSC) composed of an LCL filter (including two inductors and one capacitor) and a switching element. The impedance between the VSC and the grid connection point is represented by Z l Indicates that the grid connection point to the grid u g The impedance between Z gThe figure shows that nodes 2, 3, and 4 represent the output positions of the VSC, and node 1 represents the bus position connected by the three branches. In this embodiment, nodes 1, 2, 3, and 4 are taken as the disturbance injection nodes, and I1, I2, I3, and I4 represent the current vectors of the corresponding injection nodes, and J1, J2, and J3 represent the current vectors of the three branches. Figure 4 The parameter settings of the model are shown in Table 1 below.

[0110] Table 1

[0111]

[0112]

[0113] Establishment Figure 4 The node admittance matrix of the mathematical model is Y, the admittance matrix D of the series branch, and the incidence matrix C. The series branch resonance analysis matrix H is established according to formula (10). The specific modeling process is omitted here. The system series branch resonance distribution is obtained by analyzing the sweep frequency results of each element in the matrix H, as shown in FIGS. 5(a), 5(b), and 5(c). FIG. 5(a) shows the current amplitude-frequency curve of the series branch 1, FIG. 5(b) shows the current amplitude-frequency curve of the series branch 2, and FIG. 5(c) shows the current amplitude-frequency curve of the series branch 3. Branch 1 and branch 2 are branches with the same parameters, both of which have two resonance points at 625 Hz and 742 Hz. When the disturbance injection node is not the node between the two branches, the 625 Hz resonance point will have a slight frequency shift. Among the two resonance points of branch 1, the 742 Hz harmonic current amplification is most serious when the disturbance injection nodes are 2 and 3. Branch 2 is similar. Branch 3 only has one obvious 742 Hz resonance point, and the harmonic current amplification is most serious when the disturbance injection node is 4.

[0114] The series branch resonance suppression feasible region when τ is 0 proposed in this embodiment can be used to evaluate the effectiveness of the resonance suppression strategy. Taking the virtual impedance and the passive damping of the series connection of resistance and capacitance and the series connection of resistance and inductance as examples, for the 742 Hz series branch resonance of branch 1 when the disturbance injection node is 2 in this example, the frequency band of interest is 722 Hz-762 Hz. FIG. 6(a) shows the series branch resonance suppression feasible region when the resonance suppression strategy acts on node 2, and FIG. 6(b) shows the series branch resonance suppression feasible region when the resonance suppression strategy acts on node 3. FIGS. 6(a) and 6(b) also show the impedance changes of the passive damping of the series connection of 1 Ω-5 Ω virtual impedance, 0.01 Ω-0.05 Ω resistance, and 5 μF-25 μF capacitance, and the passive damping of the series connection of 0.01 Ω-0.05 Ω resistance and 1 mH-5 mH inductance. The impedance distributions in the three cases are compared with the boundary.

[0115] From the expressions of the feasible region of series branch resonance suppression by the series branch resonance suppression strategy, it can be seen that the distribution of the feasible region of series branch resonance suppression is related to the self-impedance of the node where the resonance suppression strategy acts and the mutual-impedance between the node where the resonance suppression strategy acts and the two end nodes of the branch. The coefficients A and z in the corresponding expressions are ss When the resonance suppression strategy acts on different nodes, the coefficients A and z are ss different, which makes the distribution of the feasible region of series branch resonance suppression different. The comparison of the distribution of the feasible region of series branch resonance suppression by the resonance suppression strategy acting on different nodes can be seen from Figure 6(a) and 6(b) It can be seen that when the resonance suppression strategy acts on node 2 (as shown in FIG. 6(a)), the ideal feasible region of series branch resonance suppression is the largest, and the virtual impedance is always located in the ideal feasible region; when the passive damping is connected, the ideal feasible region is located in the ideal feasible region when the impedance value is small (corresponding to the decrease of the capacitance and the increase of the inductance), but as the impedance value increases, the passive damping gradually enters the resonance amplification region of some frequency bands, which leads to the resonance amplification of some frequencies in the frequency band, and when the parameters further increase, the resonance peak amplification may even occur. When the resonance suppression strategy acts on node 3 (as shown in FIG. 6(b)), the ideal feasible region of series branch resonance suppression is smaller than that when the resonance suppression strategy acts on node 2, and the virtual impedance and the passive damping are not located in the ideal feasible region, i.e., they are located in the resonance amplification region of some frequencies, which leads to the resonance amplification of some frequency bands.

[0116] From the above analysis, it can be seen that when the virtual impedance resonance suppression strategy acts on node 2, it has a good suppression effect on the 742 Hz series branch resonance of series branch 1.

[0117] Taking the passive damping scheme one of the virtual impedance of 4Ω, the resistance of 0.03Ω and the capacitance of 15μF in series and the passive damping scheme two of the resistance of 0.05Ω and the inductance of 5mH in series as examples, the sweep frequency analysis of the series branch resonance is carried out to verify the accuracy of the effectiveness evaluation method of the resonance suppression strategy based on the feasible region of series branch resonance suppression. FIG. 7(a) shows the series branch current amplitude-frequency curve before and after the resonance suppression when the resonance suppression strategy acts on node 2; FIG. 7(b) shows the series branch current amplitude-frequency curve before and after the resonance suppression when the resonance suppression strategy acts on node 2. From Figure 7(a) and 7(b) It can be seen that when the virtual impedance acts on node 2 and node 3, most of the harmonic currents in the frequency range are reduced, which suppresses the resonance, while the connection of the passive damping leads to the resonance frequency shift and the amplification of the harmonic currents in the frequency band of interest. The above conclusion is consistent with the analysis results of Figure 6(a) and 6(b) .

[0118] The effectiveness evaluation of the above resonance suppression method is carried out in Figure 6(a) and 6(b)and both of FIG. 7(a) and FIG. 7(b) can be embodied, but the series branch current amplitude-frequency curve drawing in FIG. 7(a) and FIG. 7(b) needs to substitute the mathematical model of the resonance suppression method into the admittance matrix Y in the series branch resonance analysis matrix H one by one and inverse, and the calculation amount is large, and Figure 6(a) and 6(b) The feasible region analysis of the resonance suppression strategy of the power system series branch is only started from the perspective of the equivalent impedance of the resonance suppression strategy, the rationality of the resonance suppression method can be directly judged and parameter setting suggestions can be given through analyzing the distribution of the equivalent impedance in the feasible region, and the analysis result is more efficient and direct.

[0119] Through the above description, the basic functions of the effectiveness analysis method of the power system series branch resonance suppression strategy are described. The power system series branch resonance suppression feasible region analysis method provided by the present application can evaluate the effectiveness of the series branch resonance suppression strategy under different scenes and provide design guidance, avoids the inverse calculation of the complex matrix for each suppression scene, simplifies the evaluation process, has good applicability to different types of resonance suppression strategies, and has important significance for solving the problems that the application scene of the resonance suppression method is limited and the series branch resonance scene is not suitable for the resonance suppression method.

[0120] In the embodiment, an effectiveness analysis device of a power system series branch resonance suppression strategy is also provided, which is used to implement the above-mentioned embodiments and preferred embodiments, and will not be described again. As used below, the term "module" can be a combination of software and / or hardware that implements a predetermined function. Although the device described in the following embodiments is preferably implemented in software, hardware, or a combination of software and hardware is also possible and contemplated.

[0121] The embodiment provides an effectiveness analysis device of a power system series branch resonance suppression strategy, as shown in Figure 8 , comprising:

[0122] A node determination module 81 is configured to determine a disturbance injection node, a resonance branch to be treated, and a resonance suppression strategy action node based on a branch method.

[0123] An evaluation index determination module 82 is configured to determine a resonance suppression evaluation index based on the change of harmonic current amplitude of the disturbance injection node, the resonance branch to be treated, and the resonance suppression strategy action node before and after resonance suppression, and the change of the harmonic current amplitude is determined based on a node admittance matrix and a node impedance matrix.

[0124] A feasible region determination module 83 is configured to determine a series branch resonance suppression feasible region based on a frequency band of interest of the resonance suppression strategy, an equivalent impedance, and the resonance suppression evaluation index.

[0125] The ideal feasible region determination module 84 determines the series branch resonance suppression ideal feasible region based on the intersection of the series branch resonance suppression feasible region and a four-quadrant part;

[0126] The evaluation module 85 evaluates the effectiveness of the resonance suppression strategy based on the relationship between the equivalent impedance of the resonance suppression strategy and the series branch resonance suppression feasible region and the series branch resonance suppression ideal feasible region.

[0127] In an optional implementation, the device further comprises an optimization module configured to determine a plurality of resonance suppression scheme selections and parameter optimization suggestions based on the evaluation result of the effectiveness of the resonance suppression strategy.

[0128] In an optional implementation, the node determination module is specifically configured to construct a series branch resonance analysis matrix based on the relationship between the node voltage and the injected current in the power system, the admittance matrix of the series branch in the power system, and the node impedance matrix; perform frequency scanning on the series branch resonance analysis matrix to obtain a series branch resonance distribution, and determine the disturbance injection node, the resonance to-be-governed branch, and the resonance suppression strategy action node.

[0129] In an optional implementation, the resonance suppression evaluation index is expressed by the following formula:

[0130]

[0131] In the formula, D represents the admittance matrix of the series branch, C represents the correlation matrix, Z represents the system node impedance matrix before resonance suppression, Z represents the system node impedance matrix after resonance suppression, m represents the resonance to-be-governed branch, p represents the disturbance injection node, and τ is a threshold value. 0f f , respectively represent the system node impedance matrix before and after resonance suppression at the frequency f, m represents the resonance to-be-governed branch, p represents the disturbance injection node, and τ is a threshold value.

[0132] In an optional implementation, the feasible region determination module is specifically configured to: determine the impedance relationship before and after resonance suppression according to the branch addition method and the equivalent impedance of the resonance suppression strategy; substitute the impedance relationship into the resonance suppression evaluation index to obtain the relationship between the equivalent impedance and the impedance after resonance suppression; determine the series branch resonance suppression feasible region at a single frequency based on the relationship between the equivalent impedance and the impedance after resonance suppression; and determine the series branch resonance suppression feasible region within the concerned frequency band based on the concerned frequency band of the resonance suppression strategy and the series branch resonance suppression feasible region at the single frequency.

[0133] In an optional implementation, when τ>0, the boundary of the series branch resonance suppression is a circle with (α, β) as the center and r as the radius, and the internal region of the circle is the series branch resonance suppression feasible region; when τ=0, the boundary of the resonance suppression or amplification is a straight line, and the series branch resonance suppression feasible region is one side of the straight line.

[0134] ​In an optional embodiment, the evaluation module is specifically used to compare the equivalent impedances of multiple resonance suppression schemes with the overlap between the feasible domain of series branch resonance suppression and the ideal feasible domain of series branch resonance suppression in the frequency band of interest, so as to evaluate the effectiveness of the resonance suppression strategy.

[0135] The further functional description of each of the above modules is the same as that of the above corresponding embodiments and will not be repeated here.

[0136] The embodiment of the present invention also provides a computer device having the above Figure 8 The effectiveness analysis device of the power system series branch resonance suppression strategy is shown.

[0137] See also Figure 9 , Figure 9 is a structural diagram of a computer device provided by an optional embodiment of the present invention, such as Figure 9 As shown, the computer device includes: one or more processors 10, memory 20, and interfaces for connecting various components, including high-speed interfaces and low-speed interfaces. Various components utilize different buses to communicate with each other and can be installed on a common mainboard or installed in other ways as needed. The processor can process the instructions executed in the computer device, including instructions stored in the memory or on the memory to display the graphical information of the GUI on an external input / output device (such as, a display device coupled to the interface). In some optional embodiments, if necessary, multiple processors and / or multiple buses can be used together with multiple memories and multiple memories. Equally, multiple computer devices can be connected, and each device provides part of the necessary operations (for example, as a server array, a group of blade servers, or a multi-processor system). Figure 9 A processor 10 is taken as an example.

[0138] The processor 10 may be a central processing unit, a network processor, or a combination thereof. The processor 10 may further include a hardware chip. The hardware chip may be an application-specific integrated circuit, a programmable logic device, or a combination thereof. The programmable logic device may be a complex programmable logic device, a field programmable gate array, a general purpose array logic, or any combination thereof.

[0139] The memory 20 stores instructions that can be executed by at least one processor 10, so as to enable at least one processor 10 to execute the method shown in the above embodiment.

[0140] The memory 20 can include a program storage area and a data storage area. The program storage area can store an operating system, application programs required by at least one function, etc. The data storage area can store data created by the computer device according to the presentation of a small program landing page, etc. In addition, the memory 20 can include a high-speed random access memory, and can also include a non-transitory memory, such as at least one magnetic disk storage device, a flash memory device, or other non-transitory solid-state memory device. In some optional embodiments, the memory 20 can optionally include a memory disposed remotely relative to the processor 10, and these remote memories can be connected to the computer device through a network. Examples of the network include, but are not limited to, the Internet, an intranet, a local area network, a mobile communication network, and a combination thereof.

[0141] The memory 20 can include a volatile memory, such as a random access memory, and can also include a non-volatile memory, such as a flash memory, a hard disk, or a solid-state disk, and can also include a combination of the above-mentioned kinds of memories.

[0142] The computer device further includes a communication interface 30 for the computer device to communicate with other devices or a communication network.

[0143] The embodiments of the present application also provide a computer readable storage medium, and the above-mentioned method according to the embodiments of the present application can be implemented in hardware, firmware, or recorded in a storage medium, or stored in a remote storage medium or a non-transitory machine readable storage medium and stored in a local storage medium through network downloading of computer code, so that the method described herein can be processed by such software on a storage medium using a general-purpose computer, a special-purpose processor, or programmable or special-purpose hardware. The storage medium can be a magnetic disk, an optical disk, a read-only memory, a random access memory, a flash memory, a hard disk, or a solid-state disk, etc. Further, the storage medium can also include a combination of the above-mentioned kinds of memories. It can be understood that the computer, the processor, the microprocessor controller, or the programmable hardware includes a storage component that can store or receive software or computer code, when the software or computer code is accessed and executed by the computer, the processor, or the hardware, the method shown in the above embodiments is implemented.

[0144] Part of the present application can be applied as a computer program product, for example, computer program instructions, when executed by a computer, through the operation of the computer, can invoke or provide the method and / or technical solutions according to the present application. Those skilled in the art should understand that the form of computer program instructions in computer readable medium includes but is not limited to source files, executable files, installation package files and the like, and accordingly, the way of computer program instructions executed by computer includes but is not limited to: the computer directly executes the instructions, or the computer compiles the instructions and then executes the corresponding compiled program, or the computer reads and executes the instructions, or the computer reads and installs the instructions and then executes the corresponding installed program. Here, the computer readable medium can be any available computer readable storage medium or communication medium accessible to the computer.

[0145] Although the embodiments of the present application are described in conjunction with the drawings, various modifications and changes can be made by those skilled in the art without departing from the spirit and scope of the present application, and such modifications and changes fall within the scope defined by the appended claims.

Claims

1. A method for analyzing the effectiveness of a power system series branch resonance suppression strategy, characterized in that: The method comprises: Determine the disturbance injection node, the resonance control branch and the resonance suppression strategy action node based on the branch method; The resonance suppression evaluation index is determined based on the harmonic current amplitude changes of the disturbance injection node, the resonance control branch and the resonance suppression strategy action node before and after resonance suppression. The harmonic current amplitude changes are determined based on the node admittance matrix and the node impedance matrix. Determining a feasible region for series branch resonance suppression based on a frequency band of interest, equivalent impedance, and the resonance suppression evaluation index of the resonance suppression strategy; Determine the ideal feasible region for series branch resonance suppression based on the intersection of the feasible region for series branch resonance suppression and a four-quadrant part; The effectiveness of the resonance suppression strategy is evaluated based on the relationship between the equivalent impedance of the resonance suppression strategy and the feasible region and ideal feasible region of the series branch resonance suppression. The resonance suppression evaluation index is expressed by the following formula: Where D represents the admittance matrix of the series branch, C represents the correlation matrix, Z 0f 、Z f They represent the system node impedance matrix before and after resonance suppression at frequency f, m represents the branch to be controlled, p represents the disturbance injection node, is the threshold.

2. The method according to claim 1, characterized in that The method further comprises: Based on the effectiveness evaluation results of the resonance suppression strategy, multiple resonance suppression scheme selections and parameter optimization recommendations are determined.

3. The method according to claim 1, characterized in that Based on the branch method, the disturbance injection node, the resonance branch to be controlled, and the resonance suppression strategy action node are determined, including: Construct a series branch resonance analysis matrix based on the relationship between node voltage and injection current in the power system, the admittance matrix of the series branch in the power system, and the node impedance matrix; The series branch resonance analysis matrix is ​​frequency scanned to obtain the series branch resonance distribution, and the disturbance injection node, the resonance branch to be controlled, and the node where the resonance suppression strategy works are determined.

4. The method according to claim 1, wherein Determining a feasible region for series branch resonance suppression based on a frequency band of interest, equivalent impedance, and the resonance suppression evaluation index of the resonance suppression strategy includes: Determine the impedance relationship before and after resonance suppression based on the branch addition method and the equivalent impedance of the resonance suppression strategy; Substituting the impedance relationship into the resonance suppression evaluation index to obtain the relationship between the equivalent impedance and the impedance after resonance suppression; Determining a feasible region for series branch resonance suppression at a single frequency based on a relationship between the equivalent impedance and the impedance after resonance suppression; Based on the focus frequency band of the resonance suppression strategy and the feasible domain of series branch resonance suppression at a single frequency, the feasible domain of series branch resonance suppression within the focus frequency band is determined.

5. The method according to claim 4, characterized in that: when >0, the boundary of the series branch resonance suppression is a ( α , β ) is the center of the circle, r The circle with the radius of , the inner area of ​​the circle is the feasible region for suppressing the resonance of the series branch; when =0, the boundary of resonance suppression or amplification is a straight line, and the feasible region of resonance suppression of the series branch is one side of the straight line.

6. The method according to claim 4, characterized in that The effectiveness of the resonance suppression strategy is evaluated based on the relationship between the equivalent impedance of the resonance suppression strategy and the feasible region and ideal feasible region of the series branch resonance suppression, including: The effectiveness of the resonance suppression strategy is evaluated by comparing the equivalent impedances of multiple resonance suppression schemes with the overlap between the feasible domain of series branch resonance suppression and the ideal feasible domain of series branch resonance suppression within the frequency band of interest.

7. A device for analyzing the effectiveness of a power system series branch resonance suppression strategy, characterized in that: The device comprises: A node determination module is used to determine the disturbance injection node, the resonance control branch and the resonance suppression strategy action node based on the branch method; An evaluation index determination module is used to determine the resonance suppression evaluation index based on the harmonic current amplitude changes of the disturbance injection node, the resonance control branch, and the resonance suppression strategy action node before and after resonance suppression. The harmonic current amplitude changes are determined based on the node admittance matrix and the node impedance matrix; A feasible domain determination module is used to determine a feasible domain for series branch resonance suppression based on a frequency band of interest, equivalent impedance, and the resonance suppression evaluation index of the resonance suppression strategy; An ideal feasible region determination module determines an ideal feasible region for series branch resonance suppression based on an intersection of the series branch resonance suppression feasible region and a four-quadrant part; An evaluation module for evaluating the effectiveness of the resonance suppression strategy based on the relationship between the equivalent impedance of the resonance suppression strategy and the feasible region and ideal feasible region of the series branch resonance suppression; The resonance suppression evaluation index is expressed by the following formula: Where D represents the admittance matrix of the series branch, C represents the correlation matrix, Z 0f 、Z f They represent the system node impedance matrix before and after resonance suppression at frequency f, m represents the branch to be controlled, p represents the disturbance injection node, is the threshold.

8. A computer device, characterized in that: include: A memory and a processor, wherein the memory and the processor are communicatively connected to each other, the memory stores computer instructions, and the processor executes the effectiveness analysis method of the power system series branch resonance suppression strategy according to any one of claims 1 to 6 by executing the computer instructions.

9. A computer-readable storage medium, characterized in that The computer-readable storage medium stores computer instructions, and the computer instructions are used to enable a computer to execute the effectiveness analysis method of the power system series branch resonance suppression strategy according to any one of claims 1 to 6.

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