Voltage stiffness-based method for evaluating strength of power grid having high proportion of vscs, and system

Abstract

Disclosed in the present invention are a voltage stiffness-based method for evaluating the strength of a power grid having a high proportion of VSCs and a system. The method comprises: acquiring in real time first data corresponding to a power grid to be analyzed, and processing the first data to generate first admittance matrix data corresponding to said power grid; processing the first admittance matrix data to generate impedance data of each VSC device corresponding to said power grid and second admittance matrix data; deleting rows and columns which correspond to the second admittance matrix data and in which VSC nodes are located, and generating corresponding third admittance matrix data; processing the third admittance matrix data to generate second data and third data which correspond to each VSC node; and, on the basis of the third data combined with the second data, generating power grid strength evaluation data corresponding to said power grid. Voltage stiffness fully considers the characteristic difference between the VSCs and synchronous generators, thereby overcoming the defect that it is traditionally difficult for a short-circuit ratio to represent a VSC voltage support effect.

Description

A voltage stiffness-based grid strength evaluation method and system for a high proportion of VSCs TECHNICAL FIELD

[0001] The present application belongs to the technical field of power system evaluation and processing, and particularly relates to a voltage stiffness-based grid strength evaluation method and system for a high proportion of VSCs. BACKGROUND

[0002] At present, in the traditional power system dominated by synchronous generators, voltage strength is usually represented by short-circuit ratio. The short-circuit ratio is defined as the ratio of three-phase short-circuit capacity to device capacity. The short-circuit current of a synchronous generator when a three-phase metal short circuit is provided is several tens of times the normal operating current. However, due to current limiting, the short-circuit current that can be provided by a voltage source converter (VSC) is almost the same as its operating current. Since the VSC can maintain a constant terminal voltage, the limited short-circuit current will seriously underestimate the contribution of the VSC, and the short-circuit ratio is difficult to accurately evaluate the voltage support effect of the VSC. With the increase in the proportion of renewable energy sources, the number of VSCs has also increased significantly, and how to evaluate the voltage strength of the power system with VSC access has become a problem to be solved.

[0003] In order to solve this problem, scholars have made several attempts in recent years, and various evaluation methods such as weighted short-circuit ratio, composite short-circuit ratio, and generalized short-circuit ratio have been derived. The American Power Reliability Council and General Electric respectively proposed the weighted short-circuit ratio and the composite short-circuit ratio to analyze the stability of the wind turbine cluster area system. However, the above indicators still need to use short-circuit current, and the problem of insufficient consideration of VSC still exists. Based on the singularly induced bifurcation theory, a generalized short-circuit ratio is also proposed. On this basis, a lot of in-depth research has been carried out. However, due to the complexity of the derivation and processing process, the practicality of the generalized short-circuit ratio is limited. With the increase in the penetration rate of VSCs, the applicability of the short-circuit ratio gradually decreases.

[0004] Therefore, in view of the above technical problems and defects, it is urgent to design and develop a voltage stiffness-based grid strength evaluation method and system for a high proportion of VSCs.

[0005] SUMMARY

[0006] In order to overcome the deficiencies and difficulties of the prior art, the purpose of the present application is to provide a voltage stiffness-based grid strength evaluation method and system for a high proportion of VSCs, which proposes a definition and calculation method of voltage stiffness to evaluate the voltage strength of the system. Moreover, the voltage stiffness fully considers the characteristic differences between VSCs and synchronous generators, overcoming the shortcomings of the traditional short-circuit ratio in representing the voltage support effect of VSCs. In addition, the calculation method is easy to implement and more suitable for engineering practice.

[0007] The first object of the present application is to provide a voltage stiffness-based VSC high-occupancy power grid strength evaluation method; and the second object of the present application is to provide a voltage stiffness-based VSC high-occupancy power grid strength evaluation system.

[0008] The first object of the present application is achieved in that the method comprises the following steps:

[0009] Real-time acquisition of first data corresponding to the power grid to be analyzed, and processing of first admittance matrix data corresponding to the power grid to be analyzed according to the first data; wherein the first data is basic planning data of the power grid to be analyzed; and the first admittance matrix data is initial node admittance matrix data;

[0010] Respective processing of VSC device impedance data and second admittance matrix data corresponding to the power grid to be analyzed according to the first admittance matrix data; wherein the second admittance matrix data is corrected admittance matrix data of the initial node;

[0011] Deletion of rows and columns corresponding to the second admittance matrix data and in which VSC nodes are located, and generation of corresponding third admittance matrix data; wherein the third admittance matrix data is admittance matrix data after deletion of the rows and columns in which the VSC nodes are located from the second admittance matrix data;

[0012] Respective processing of second data and third data corresponding to each VSC node according to the third admittance matrix data; wherein the second data is Thevenin equivalent impedance data of the external system of each VSC node; and the third data is voltage stiffness data of each VSC node;

[0013] Generation of power grid strength evaluation data corresponding to the power grid to be analyzed based on the third data and in combination with the second data.

[0014] Further, the real-time acquisition of first data corresponding to the power grid to be analyzed, and the processing of first admittance matrix data corresponding to the power grid to be analyzed according to the first data further comprise:

[0015] Acquisition of original node admittance matrix network data corresponding to the power grid to be analyzed;

[0016] Respective acquisition of each synchronous generator position data, each synchronous generator rated capacity data, and each synchronous generator sub-transient reactance data corresponding to the power grid to be analyzed;

[0017] Respective acquisition of each VSC position data, each VSC rated capacity data, and each VSC control mode data corresponding to the power grid to be analyzed;

[0018] Obtain initial bus voltage amplitude data and apparent power data of each LCC corresponding to the power grid to be analyzed.

[0019] Further, the processing of generating VSC device impedance data and the second admittance matrix data according to the first admittance matrix data, further comprises:

[0020] The VSC device impedance data is calculated according to the following formula:

[0021] Wherein: represents the device impedance of the VSC, U sys represents the grid voltage at the grid-connected point of the VSC, represents the conjugate of the apparent power of the VSC.

[0022] Further, the processing of generating VSC device impedance data and the second admittance matrix data according to the first admittance matrix data, further comprises:

[0023] The second admittance matrix data corresponding to the power grid to be analyzed is calculated according to the LCC, load and synchronous machine sub-transient reactance data, and the specific calculation formula is as follows:

[0024] Y exp = Y org + Y aux (2)

[0025] Wherein, Y org is the initial node admittance matrix for power flow calculation; Y exp is the modified node admittance matrix; Y aux is the additional matrix.

[0026] The calculation formula of the additional matrix is as follows:

[0027] Wherein, diag(·) represents the transformation from a vector to a diagonal matrix; in Yaux, only the diagonal elements of the nodes connected to the LCC, load and synchronous machine are not 0, and the rest are 0.

[0028] Further, the processing of deleting the row and column corresponding to the VSC node in the second admittance matrix data and generating the corresponding third admittance matrix data, further comprises:

[0029] Obtain network type VSC node data and fixed AC bus voltage type VSC node data;

[0030] The deletion processing deletes the row and column where the VSC node is located in the second admittance matrix data, and generates corresponding third admittance matrix data.

[0031] Further, the deletion processing is performed on the row and column where the VSC node is located in the second admittance matrix data, and generates corresponding third admittance matrix data, and further includes:

[0032] The third admittance matrix data corresponding to the power grid to be analyzed is calculated, and the specific calculation formula is as follows:

[0033] Y final＝ L delete (Y exp ,k) (4)

[0034] Wherein, Y final is the final obtained node admittance matrix, k is the node number of the fixed alternating current bus voltage VSC, L delete represents a function of deleting the kth row and kth column in the matrix Y final .

[0035] Further, the second data and the third data corresponding to each VSC node are respectively processed and generated based on the third admittance matrix data, and further include:

[0036] The Thevenin equivalent impedance data of the external system is calculated and generated, and the specific calculation formula is as follows:

[0037] Wherein, the Thevenin equivalent impedance of the external system is the hth diagonal element of the inverse matrix of Y final .

[0038] The voltage stiffness data of each VSC node is calculated and generated, and the specific calculation formula is as follows:

[0039] Wherein, K vtg is the voltage stiffness of each VSC node, and are the device impedance of the VSC node and the Thevenin equivalent impedance of the external system, respectively.

[0040] Further, the power grid strength evaluation data corresponding to the power grid to be analyzed is generated based on the third data and in combination with the second data, and further includes:

[0041] According to the generated power grid strength evaluation data, it is determined in real time whether the external system corresponding to the power grid to be analyzed is a strong system, and if the calculated voltage stiffness K vtg is greater than 0.95, it is determined that the system is a strong system.

[0042] A second object of the present application is achieved in that the system is applied to the power grid strength evaluation method, and the system comprises:

[0043] a first data generation unit configured to acquire first data corresponding to the power grid to be analyzed in real time, and process and generate first admittance matrix data corresponding to the power grid to be analyzed according to the first data; wherein the first data is basic planning data of the power grid to be analyzed; and the first admittance matrix data is initial node admittance matrix data;

[0044] a second data generation unit configured to process and generate, respectively, each VSC device impedance data and second admittance matrix data corresponding to the power grid to be analyzed according to the first admittance matrix data; wherein the second admittance matrix data is corrected admittance matrix data of the initial node;

[0045] a third data generation unit configured to delete a row and a column in which a VSC node is located corresponding to the second admittance matrix data, and generate corresponding third admittance matrix data; wherein the third admittance matrix data is admittance matrix data after deleting the row and the column in which the VSC node is located from the second admittance matrix data;

[0046] a fourth data generation unit configured to process and generate, respectively, second data and third data corresponding to each VSC node according to the third admittance matrix data; wherein the second data is Thevenin equivalent impedance data of an external system of each VSC node; and the third data is voltage stiffness data of each VSC node;

[0047] a fifth data generation unit configured to generate power grid strength evaluation data corresponding to the power grid to be analyzed based on the third data and in combination with the second data.

[0048] Further, the first data generation unit further comprises:

[0049] a first data acquisition module configured to acquire original node admittance matrix network data corresponding to the power grid to be analyzed;

[0050] a second data acquisition module configured to acquire, respectively, each synchronous generator position data, each synchronous generator rated capacity data and each synchronous generator sub-transient reactance data corresponding to the power grid to be analyzed;

[0051] a third data acquisition module configured to acquire, respectively, each VSC position data, each VSC rated capacity data and each VSC control mode data corresponding to the power grid to be analyzed;

[0052] a fourth data acquisition module configured to acquire, respectively, each LCC initial bus voltage amplitude data and each LCC apparent power data corresponding to the power grid to be analyzed;

[0053] And / or, the second data generation unit further comprises:

[0054] The first calculation generation module is used for calculating and generating the impedance data of each VSC device corresponding to the power grid to be analyzed; and the specific calculation formula is as follows:

[0055] Wherein: represents the device impedance of the VSC, U sys represents the grid voltage of the VSC grid connection point after the VSC is connected, represents the conjugate of the apparent power of the VSC;

[0056] And / or, the second data generation unit further comprises:

[0057] The second calculation generation module is used for calculating and generating the second admittance matrix data corresponding to the power grid to be analyzed in combination with the LCC, load and synchronous machine sub-transient reactance data; and the specific calculation formula is as follows:

[0058] Y exp = Y org + Y aux (2)

[0059] Wherein, Y org is the initial node admittance matrix for power flow calculation; Y exp is the corrected node admittance matrix; and Y aux is an additional matrix;

[0060] The calculation formula of the additional matrix is as follows:

[0061] Wherein, diag(·) represents the transformation from a vector to a diagonal matrix; in Yaux, only the diagonal elements of the nodes connected with the LCC, load and synchronous machine are not 0, and the rest of the elements are 0;

[0062] And / or, the third data generation unit further comprises:

[0063] The fifth data acquisition module is used for acquiring the network-constructed VSC node data and the fixed AC bus voltage network-following VSC node data respectively;

[0064] The first processing generation module is used for deleting the row and column in the second admittance matrix data and generating the corresponding third admittance matrix data in real time;

[0065] And / or, the third data generation unit further comprises:

[0066] The third calculation generation module is used for calculating and generating the third admittance matrix data corresponding to the power grid to be analyzed; and the specific calculation formula is as follows:

[0067] Y final＝ L delete (Y exp ,k) (4)

[0068] where Y final is the final obtained node admittance matrix, k is the node number of the fixed AC bus voltage VSC, L delete represents a function of deleting the kth row and the kth column in the matrix Y final ;

[0069] And / or, the fourth data generation unit further comprises:

[0070] A fourth calculation generation module for calculating the Thevenin equivalent impedance data of the external system; The specific calculation formula is as follows:

[0071] Wherein, the Thevenin equivalent impedance of the external system is the hth diagonal element of Y final inverse matrix;

[0072] A fifth calculation generation module for calculating the voltage stiffness data of each VSC node; The specific calculation formula is as follows:

[0073] Wherein, K vtg is the voltage stiffness of each VSC node, and are the device impedance of the VSC node and the Thevenin equivalent impedance of the external system respectively;

[0074] And / or, the fifth data generation unit further comprises:

[0075] A first generation determination module for determining whether the external system corresponding to the power grid to be analyzed is a strong system in real time according to the generated power grid strength evaluation data.

[0076] The application acquires first data corresponding to the power grid to be analyzed in real time by a method, and generates first admittance matrix data corresponding to the power grid to be analyzed according to the first data; wherein the first data is basic planning data of the power grid to be analyzed; the first admittance matrix data is initial node admittance matrix data; according to the first admittance matrix data, each VSC device impedance data and second admittance matrix data corresponding to the power grid to be analyzed are respectively generated; wherein the second admittance matrix data is corrected admittance matrix data of the initial node; the row and column where the VSC node is located corresponding to the second admittance matrix data are deleted, and corresponding third admittance matrix data is generated; wherein the third admittance matrix data is admittance matrix data after deleting the row and column where the VSC node is located in the second admittance matrix data; according to the third admittance matrix data, second data and third data corresponding to each VSC node are respectively generated; wherein the second data is Thevenin equivalent impedance data of the external system of each VSC node; the third data is voltage stiffness data of each VSC node; based on the third data, and combined with the second data, power grid strength evaluation data corresponding to the power grid to be analyzed is generated; and a system corresponding to the method is proposed, which defines and calculates the processing method of voltage stiffness, so as to evaluate the voltage strength of the system. Moreover, the voltage stiffness fully considers the characteristic difference between the VSC and the synchronous generator, overcomes the shortcoming that the traditional short-circuit ratio is difficult to represent the VSC voltage support effect. In addition, the calculation method is easy to implement, and is more suitable for engineering practice. BRIEF DESCRIPTION OF DRAWINGS

[0077] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the drawings needed to be used in the embodiment description will be briefly introduced. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative labor.

[0078] Fig. 1 is a schematic diagram of Thevenin equivalent principle of any device in a VSC high-occupancy power grid strength evaluation method based on voltage stiffness;

[0079] Fig. 2 is a schematic diagram of calculation principle of Thevenin equivalent impedance in a VSC high-occupancy power grid strength evaluation method based on voltage stiffness;

[0080] Fig. 3 is a schematic diagram of a simple high-voltage direct-current transmission system in a VSC high-occupancy power grid strength evaluation method based on voltage stiffness;

[0081] Fig. 4 is a schematic diagram of simulation results of changing series impedance in a VSC high-occupancy power grid strength evaluation method based on voltage stiffness;

[0082] Fig. 4(a) is a schematic diagram of active power of LCC after changing series impedance in a voltage stiffness-based grid strength evaluation method for VSC high proportion power grid according to the present application;

[0083] Fig. 4(b) is a schematic diagram of AC voltage of LCC after changing series impedance in a voltage stiffness-based grid strength evaluation method for VSC high proportion power grid according to the present application;

[0084] Fig. 4(c) is a schematic diagram of DC voltage of LCC after changing series impedance in a voltage stiffness-based grid strength evaluation method for VSC high proportion power grid according to the present application;

[0085] Fig. 4(d) is a schematic diagram of DC current of LCC after changing series impedance in a voltage stiffness-based grid strength evaluation method for VSC high proportion power grid according to the present application;

[0086] Fig. 5 is a schematic diagram of simulation waveform under short-circuit fault of HVDC power transmission system in a voltage stiffness-based grid strength evaluation method for VSC high proportion power grid according to the present application;

[0087] Fig. 6 is a schematic diagram of a part of 500kv provincial power grid structure in a voltage stiffness-based grid strength evaluation method for VSC high proportion power grid according to the present application;

[0088] Fig. 7 is a schematic diagram of flow of a voltage stiffness-based grid strength evaluation method for VSC high proportion power grid according to the present application;

[0089] Fig. 8 is a schematic diagram of system architecture of a voltage stiffness-based grid strength evaluation system for VSC high proportion power grid according to the present application;

[0090] The purposes, functional features and advantages of the present application will be further explained with reference to the accompanying drawings in conjunction with embodiments. DETAILED DESCRIPTION

[0091] In order to better understand the purposes, technical solutions and advantages of the present application, the present application will be further explained below in conjunction with the accompanying drawings and specific embodiments, and those skilled in the art can easily understand other advantages and effects of the present application from the content disclosed in the present specification.

[0092] The present application can also be implemented or applied through other different specific examples, and each detail in the present specification can be modified and changed in various ways without departing from the spirit of the present application.

[0093] It should be noted that if the embodiments of the present application involve directional indications (such as up, down, left, right, front, back, etc.), the directional indications are only used to explain the relative position relationship, motion condition, etc. between components in a certain specific posture (as shown in the drawings), and if the specific posture changes, the directional indications will also change accordingly.

[0094] In addition, if the embodiments of the present application involve descriptions such as "first", "second", etc., the descriptions of "first", "second", etc. are only for description purposes and cannot be understood as indicating or implying the relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined as "first", "second" can explicitly or implicitly include at least one of the features. Secondly, the technical solutions of various embodiments can be combined with each other, but it must be based on the fact that a person skilled in the art can realize it, and when the combination of technical solutions contradicts each other or cannot be realized, it should be considered that the combination of technical solutions does not exist and is not within the protection scope required by the present application.

[0095] The present application will be described in further detail below with reference to the accompanying drawings. As shown in FIGS. 1-7, the present application provides a voltage stiffness-based VSC high-occupancy grid strength evaluation method, which comprises the following steps:

[0096] S1, real-time acquisition of first data corresponding to the grid to be analyzed, and processing of first admittance matrix data corresponding to the grid to be analyzed according to the first data; wherein the first data is basic planning data of the grid to be analyzed; and the first admittance matrix data is initial node admittance matrix data;

[0097] S2, processing of respective VSC device impedance data and second admittance matrix data corresponding to the grid to be analyzed according to the first admittance matrix data; wherein the second admittance matrix data is corrected admittance matrix data of the initial node;

[0098] S3, deletion of the row and column corresponding to the second admittance matrix data and the VSC node, and generation of corresponding third admittance matrix data; wherein the third admittance matrix data is the admittance matrix data after deleting the row and column of the VSC node in the second admittance matrix data;

[0099] S4, processing of second data and third data corresponding to respective VSC nodes according to the third admittance matrix data; wherein the second data is Thevenin equivalent impedance data of the external system of each VSC node; and the third data is voltage stiffness data of each VSC node;

[0100] S5, generating power grid strength evaluation data corresponding to the power grid to be analyzed based on the third data and in combination with the second data.

[0101] The real-time acquisition of the first data corresponding to the power grid to be analyzed and the processing of the first admittance matrix data corresponding to the power grid to be analyzed according to the first data further include:

[0102] S11, acquiring original node admittance matrix network data corresponding to the power grid to be analyzed;

[0103] S12, acquiring respective synchronous generator position data, respective synchronous generator rated capacity data and respective synchronous generator sub-transient reactance data corresponding to the power grid to be analyzed;

[0104] S13, acquiring respective VSC position data, respective VSC rated capacity data and respective VSC control mode data corresponding to the power grid to be analyzed;

[0105] S14, acquiring respective LCC initial bus voltage amplitude data and respective LCC apparent power data corresponding to the power grid to be analyzed.

[0106] The processing of the respective VSC device impedance data and the second admittance matrix data corresponding to the power grid to be analyzed according to the first admittance matrix data further includes:

[0107] S21, calculating the respective VSC device impedance data corresponding to the power grid to be analyzed, and the specific calculation formula is as follows:

[0108] Wherein: represents the device impedance of the VSC, U sys represents the grid voltage of the VSC grid connection point after the VSC is connected, represents the conjugate of the apparent power of the VSC.

[0109] The processing of the respective VSC device impedance data and the second admittance matrix data corresponding to the power grid to be analyzed according to the first admittance matrix data further includes:

[0110] S22, calculating the second admittance matrix data corresponding to the power grid to be analyzed in combination with the LCC, the load and the synchronous machine sub-transient reactance data, and the specific calculation formula is as follows:

[0111] Y exp = Y org + Y aux (2)

[0112] Wherein, Y org is the initial node admittance matrix used for power flow calculation; Y expY is the modified node admittance matrix; aux Yaux is the additional matrix;

[0113] The calculation formula of the additional matrix is as follows:

[0114] Wherein, diag(·) represents the transformation from a vector to a diagonal matrix; in Yaux, only the diagonal elements of the nodes connected with LCC, load and synchronous machine are not 0, and the rest of the elements are 0.

[0115] The deletion processing corresponds to the second admittance matrix data and the row and column where the VSC node is located, and generates corresponding third admittance matrix data, and further comprises:

[0116] S31, respectively acquiring network type VSC node data and fixed AC bus voltage type VSC node data;

[0117] S32, deleting the row and column where the second admittance matrix data is located, and generating corresponding third admittance matrix data in real time.

[0118] The deletion processing corresponds to the second admittance matrix data and the row and column where the VSC node is located, and generates corresponding third admittance matrix data, and further comprises:

[0119] S33, calculating and generating third admittance matrix data corresponding to the power grid to be analyzed, and the specific calculation formula is as follows:

[0120] Y final＝ L delete (Y exp ,k) (4)

[0121] Wherein, Y final is the final obtained node admittance matrix, k is the node number of the fixed AC bus voltage VSC, L delete represents the function of deleting the kth row and kth column in the matrix Y final .

[0122] The third admittance matrix data is used to respectively process and generate second data and third data corresponding to each VSC node, and further comprises:

[0123] S41, calculating and generating Thevenin equivalent impedance data of the external system, and the specific calculation formula is as follows:

[0124] Wherein, the Thevenin equivalent impedance of the external system is the hth diagonal element of the inverse matrix of Y final .

[0125] S42, the voltage stiffness data of each VSC node is calculated, and the specific calculation formula is as follows:

[0126] Wherein, K vtg is the voltage stiffness of each VSC node, And The device impedance of the VSC node and the Thevenin equivalent impedance of the external system are respectively.

[0127] The third data is generated based on the second data, and the power grid strength evaluation data corresponding to the power grid to be analyzed is generated.

[0128] S51, according to the generated power grid strength evaluation data, it is determined in real time whether the external system corresponding to the power grid to be analyzed is a strong system, if the calculated voltage stiffness K vtg is greater than 0.95, then the system is determined to be a strong system.

[0129] Specifically, in the embodiment of the application, a power grid strength evaluation method of VSC high proportion power system based on voltage stiffness index is provided, which comprises the following steps: (1) obtaining the basic planning data of the power grid to be analyzed, calculating the initial node admittance matrix Y org .(2) calculating the device impedance of each VSC of the power grid to be analyzed.(3) calculating the node admittance matrix Y exp of the power grid to be analyzed after correction.(4) deleting the row and column where the specific VSC node is located in Y exp , to obtain the final node admittance matrix Y final .(5) using the matrix Y final to calculate the Thevenin equivalent impedance of the external system of each VSC node (6) calculating the voltage stiffness K vtg of each VSC node, and evaluating the power grid strength.

[0130] The planning data in step (1) mainly includes four parts: 1) obtaining the network data of the original node admittance matrix; 2) obtaining the position, rated capacity and sub-transient reactance of each synchronous generator in the power grid to be studied; 3) obtaining the position, rated capacity and control mode of each VSC; 4) obtaining the initial bus voltage amplitude and apparent power of each LCC.

[0131] The device impedance of each VSC of the power grid to be analyzed in step (2) can be calculated by the following formula:

[0132] Wherein: Indicates the device impedance of the VSC, U sys Indicates the grid voltage of the VSC grid connection point after the VSC is connected, Conjugate of the apparent power of the VSC.

[0133] The step (3) node admittance matrix is modified to include the sub-transient reactance of LCC, load and synchronous machine as follows:

[0134] Y exp = Y org + Y aux (2)

[0135] Where Y org is the initial node admittance matrix for power flow calculation; Y exp is the modified node admittance matrix; Y aux is the additional matrix, whose expression is:

[0136] Where diag(·) denotes the transformation from vector to diagonal matrix. In Y exp aux, only the diagonal elements of the nodes connected to LCC, load and synchronous machine are not zero, and the rest of the elements are zero.

[0137] The specific VSC nodes in the step (4) are the grid-forming VSC nodes and the fixed AC bus voltage following VSC nodes. In Y final , the expression of Y final＝ is obtained by deleting the row and column of the specific VSC nodes:

[0138] Y delete L exp (Y final ,k) (4)

[0139] Where Y delete is the final node admittance matrix, k is the node number of the fixed AC bus voltage VSC, and L final denotes the function of deleting the kth row and kth column in the matrix Y final .

[0140] The Thevenin equivalent impedance of the external system of the hth VSC node in the step (5) is calculated as follows:

[0141] That is, the Thevenin equivalent impedance of the external system is the hth diagonal element of the inverse matrix of Y final .

[0142] The formula for calculating the voltage stiffness of each VSC node in the step (6) is as follows:

[0143] Where K vtg is the voltage stiffness of each VSC node, and The device impedance of the VSC node and the Thevenin equivalent impedance of the external system, respectively. When the calculated voltage stiffness K vtg is greater than 0.95, the system is considered to be a strong system.

[0144] That is, the voltage strength is described by the ability to maintain the voltage amplitude close to the no-load voltage after the device is connected, named voltage stiffness, denoted as:

[0145] where K vtg is the voltage stiffness, U sys is the voltage amplitude after the element is turned on, and U sys0 is the no-load voltage amplitude.

[0146] The value range of K vtg is [0, 1]. If K vtg is close to or even equal to 1, it means that the connection state of the device has little effect on the voltage amplitude, indicating that the voltage strength is strong. On the contrary, if K vtg is far from 1, the device connection will significantly reduce the voltage amplitude. Next, the voltage stiffness is introduced from the perspective of impedance using the Thevenin equivalent circuit. When studying the voltage strength, the object of study is the positive sequence network at the fundamental frequency. According to the Thevenin equivalence principle, the external system at any point can be represented by a Thevenin equivalent circuit, as shown in Figure 1.

[0147] In Figure 1, is the voltage phasor of the device, is the Thevenin equivalent impedance phasor, is the equivalent electromotive force phasor. Among them, can be expressed as,

[0148] From the perspective of impedance, the voltage stiffness can be written as (13). According to (13), it is obtained that the voltage stiffness only needs the Thevenin equivalent impedance of the system and the device impedance.

[0149]

[0150] Fig. 2 introduces the calculation principle of Thevenin equivalent impedance. Place every independent power source in the system to zero, then the equivalent impedance of the fundamental frequency positive sequence network from the equipment is the Thevenin equivalent impedance to be solved. Generally speaking, the GFM controlled VSC and the GFL VSC with PV control have the external characteristics of variable voltage source, while the GFL VSC with PQ control is a variable current source. Therefore, the GFM and PV type GFL VSC are regarded as short circuit, while the PQ type GFL VSC is regarded as open circuit. For the synchronous generator, short circuit through the sub-transient reactance is adopted. In addition, the load and LCC are replaced by constant impedance.

[0151] Next, the grid strength is evaluated according to the method proposed in the application.

[0152] (1) Obtain the basic planning data of the grid to be analyzed, calculate the initial node admittance matrix Y org (2) Calculate the equipment impedance of each VSC in the grid to be analyzed. (3) Calculate the node admittance matrix Y exp of the grid to be analyzed after correction. (4) Delete the row and column where the VSC node is located in Y exp , to obtain the final node admittance matrix Y final (5) Use the matrix Y final to solve the Thevenin equivalent impedance of the external system of each VSC node (6) Calculate the voltage stiffness K vtg of each VSC node and evaluate the grid strength.

[0153] The planning data in step (1) mainly includes four parts: 1) Obtain the network data of the original node admittance matrix; 2) Obtain the position, rated capacity and sub-transient reactance of each synchronous generator in the grid to be studied; 3) Obtain the position, rated capacity and control mode of each VSC; 4) Obtain the initial bus voltage amplitude and apparent power of each LCC.

[0154] The equipment impedance of each VSC in the grid to be analyzed in step (2) can be calculated by the following formula:

[0155] Wherein: represents the equipment impedance of the VSC, U sys represents the grid voltage at the grid connection point of the VSC after connection, represents the conjugate of the apparent power of the VSC.

[0156] Further, it is characterized in that: the node admittance matrix in step (3) is corrected according to the following method to include the sub-transient reactance of the LCC, load and synchronous machine.

[0157] Y exp = Y org + Y aux (2)

[0158] where Y org is the initial nodal admittance matrix for power flow calculation; Y exp is the modified nodal admittance matrix; Y aux is the additional matrix, whose expression is:

[0159] where diag(·) denotes the transformation from a vector to a diagonal matrix. In Yaux, only the diagonal elements of the nodes connected with LCC, load and synchronous machine are not zero, and the other elements are zero.

[0160] The specific VSC nodes in the step (4) are the grid-forming VSC nodes and the fixed AC bus voltage-following VSC nodes, and the expression of Y exp after deleting the rows and columns where the VSC nodes are located is: final

[0161] Y final＝ L delete (Y exp ,k) (4)

[0162] where Y final is the final obtained nodal admittance matrix, k is the node number of the fixed AC bus voltage VSC, and L delete denotes the function of deleting the kth row and the kth column in the matrix Y final .

[0163] The Thevenin equivalent impedance of the external system of the hth VSC node in the step (5) is calculated as:

[0164] that is, the Thevenin equivalent impedance Z of the external system is the hth diagonal element of the inverse matrix of Y final .

[0165] The formula for calculating the voltage stiffness of each VSC node in the step (6) is:

[0166] where K vtg is the voltage stiffness of each VSC node, and are the device impedance of the VSC node and the Thevenin equivalent impedance of the external system, respectively. When the calculated voltage stiffness K vtg is greater than 0.95, the system is considered to be a strong system. ​​

[0167] Example 1:

[0168] First, a simple high-voltage direct current transmission system was built on PSCAD / EMTDC. The system structure is shown in Figure 3, and the system parameters are shown in Table 1.

[0169] Table 1. Main parameters of the simple HVDC infeed system.

[0170] First, simulations were conducted by changing the series impedance of the system to model the stability characteristics of the system under different voltage stiffness conditions. At 2.0 s, the voltage stiffness changed from 0.99 to 0.93, 0.94, and 0.95, respectively, and the simulation results are shown in Figure 4. Figure 4 shows that the system becomes unstable only when the voltage stiffness is less than 0.93.

[0171] Then, with a voltage stiffness of 0.95, a three-phase metallic short-circuit fault was applied to the inverter side, and the simulation results are shown in Figure 5. Figure 5 shows that the system can operate stably after the fault is cleared.

[0172] Example 2:

[0173] Simulations were conducted on a provincial power grid. Partial structure of the 500kV grid is shown in Figure 6. Simulations were performed under two scenarios: Scenario 1: both VSCs use constant reactive power control; Scenario 2: both VSCs use constant AC voltage control. The red line was lengthened to reduce the AC system strength relative to LCC2. The operating point where LCC2 is in a critical voltage instability state was found. The critical short-circuit ratio was 0.907 in Scenario 1 and 0.903 in Scenario 2. In both scenarios, a voltage stiffness of 0.9 as the stability boundary closely matches the simulation results.

[0174] To achieve the above objectives, the present invention also provides a power grid strength assessment system based on voltage stiffness and high VSC ratio, as shown in Figure 8. The system is applied to the aforementioned power grid strength assessment method and includes:

[0175] The first data generation unit is used to acquire first data corresponding to the power grid to be analyzed in real time, and process and generate first admittance matrix data corresponding to the power grid to be analyzed based on the first data; wherein, the first data is the basic planning data of the power grid to be analyzed; and the first admittance matrix data is the initial node admittance matrix data.

[0176] The second data generation unit is configured to generate, respectively, each VSC device impedance data and second admittance matrix data corresponding to the power grid to be analyzed according to the first admittance matrix data; wherein the second admittance matrix data is the initial node corrected admittance matrix data;

[0177] The third data generation unit is configured to delete the row and column where the VSC node is located in the second admittance matrix data, and generate corresponding third admittance matrix data; wherein the third admittance matrix data is the admittance matrix data after deleting the row and column where the VSC node is located in the second admittance matrix data;

[0178] The fourth data generation unit is configured to generate, respectively, second data and third data corresponding to each VSC node according to the third admittance matrix data; wherein the second data is the Thevenin equivalent impedance data of the external system of each VSC node; and the third data is the voltage stiffness data of each VSC node.

[0179] The fifth data generation unit is configured to generate grid strength evaluation data corresponding to the power grid to be analyzed based on the third data and in combination with the second data.

[0180] The first data generation unit further comprises:

[0181] The first data acquisition module is configured to acquire original node admittance matrix network data corresponding to the power grid to be analyzed.

[0182] The second data acquisition module is configured to acquire, respectively, each synchronous generator position data, each synchronous generator rated capacity data and each synchronous generator sub-transient reactance data corresponding to the power grid to be analyzed.

[0183] The third data acquisition module is configured to acquire, respectively, each VSC position data, each VSC rated capacity data and each VSC control mode data corresponding to the power grid to be analyzed.

[0184] The fourth data acquisition module is configured to acquire, respectively, each LCC initial bus voltage amplitude data and each LCC apparent power data corresponding to the power grid to be analyzed.

[0185] And / or, the second data generation unit further comprises:

[0186] The first calculation generation module is configured to calculate and generate each VSC device impedance data corresponding to the power grid to be analyzed; and the specific calculation formula is as follows:

[0187] Wherein: represents the device impedance of the VSC, U sys represents the grid voltage of the VSC grid-connected point after the VSC is connected, The conjugate of the apparent power of the VSC;

[0188] And / or, the second data generation unit further includes:

[0189] The second calculation and generation module is used to combine LCC, load, and synchronous machine subtransient reactance data to calculate and generate the second admittance matrix data corresponding to the power grid to be analyzed; the specific calculation formula is as follows:

[0190] Y exp =Y org +Y aux (2)

[0191] Among them, Y org Initial nodal admittance matrix used for power flow calculation; Y exp Y is the corrected nodal admittance matrix; aux For additional matrices;

[0192] The formula for calculating the additional matrix is ​​as follows:

[0193] Where diag(·) represents the transformation from a vector to a diagonal matrix; in Yax, only the diagonal elements of the nodes connected to the LCC, load, and synchronizing machine are not 0, and all other elements are 0;

[0194] And / or, the third data generation unit further includes:

[0195] The fifth data acquisition module is used to acquire VSC node data of network type and constant AC bus voltage data of network type VSC node respectively;

[0196] The first processing and generation module is used to delete the rows and columns in the second admittance matrix data and generate the corresponding third admittance matrix data in real time.

[0197] And / or, the third data generation unit further includes:

[0198] The third calculation and generation module is used to calculate and generate the third admittance matrix data corresponding to the power grid to be analyzed; the specific calculation formula is as follows:

[0199] Y final＝ L delete (Y exp ,k) (4)

[0200] Among them, Y final This is the final node admittance matrix, where k is the node number where the constant AC bus voltage VSC is located, and L... delete In matrix Y final A function to delete the k-th row and k-th column;

[0201] And / or, the fourth data generation unit further comprises:

[0202] A fourth calculation generation module for calculating the Thevenin equivalent impedance data of the external system, and the specific calculation formula is as follows:

[0203] Wherein, the Thevenin equivalent impedance of the external system Y final The hth diagonal element of the inverse matrix;

[0204] A fifth calculation generation module for calculating the voltage stiffness data of each VSC node, and the specific calculation formula is as follows:

[0205] Wherein, K vtg The voltage stiffness of each VSC node, And The device impedance of the VSC node and the Thevenin equivalent impedance of the external system, respectively;

[0206] And / or, the fifth data generation unit further comprises:

[0207] A first generation determination module for determining whether the external system corresponding to the power grid to be analyzed is a strong system in real time according to the generated power grid strength evaluation data.

[0208] In the system scheme embodiment of the application, the method steps involved in the power grid strength evaluation based on the voltage stiffness of the VSC high-occupancy power grid have been described in detail above, that is, the function modules in the system are used to realize the steps or sub-steps in the above method embodiment, which will not be described here.

[0209] The application acquires first data corresponding to the power grid to be analyzed in real time by a method, and generates first admittance matrix data corresponding to the power grid to be analyzed according to the first data; wherein the first data is basic planning data of the power grid to be analyzed; the first admittance matrix data is initial node admittance matrix data; according to the first admittance matrix data, each VSC device impedance data and second admittance matrix data corresponding to the power grid to be analyzed are respectively generated; wherein the second admittance matrix data is corrected admittance matrix data of the initial node; the row and column where the VSC node is located corresponding to the second admittance matrix data are deleted, and corresponding third admittance matrix data is generated; wherein the third admittance matrix data is admittance matrix data after deleting the row and column where the VSC node is located in the second admittance matrix data; according to the third admittance matrix data, second data and third data corresponding to each VSC node are respectively generated; wherein the second data is Thevenin equivalent impedance data of the external system of each VSC node; the third data is voltage stiffness data of each VSC node; based on the third data, and combined with the second data, power grid strength evaluation data corresponding to the power grid to be analyzed is generated; and a system corresponding to the method is proposed, which defines and calculates the processing method of voltage stiffness, to evaluate the voltage strength of the system. Moreover, the voltage stiffness fully considers the characteristic difference between the VSC and the synchronous generator, overcomes the shortcoming that the traditional short-circuit ratio is difficult to represent the VSC voltage support effect. In addition, the calculation method is easy to implement and more suitable for engineering practice.

[0210] The above-described embodiments only express several embodiments of the present application, which are described in detail and specifically, but cannot be understood as the limitation of the patent scope of the present application. It should be noted that for ordinary skilled in the art, several modifications and improvements can be made without departing from the concept of the present application, which are all within the protection scope of the present application. Therefore, the protection scope of the present application patent should be subject to the appended claims.

Claims

1. A voltage-stiffness-based VSC high-occupation grid strength evaluation method, characterized in that, The method comprises the steps of: real-time acquisition of first data corresponding to the power grid to be analyzed, and processing of first admittance matrix data corresponding to the power grid to be analyzed according to the first data; wherein the first data is basic planning data of the power grid to be analyzed; and the first admittance matrix data is initial node admittance matrix data; processing of VSC device impedance data and second admittance matrix data corresponding to the power grid to be analyzed respectively according to the first admittance matrix data; wherein the second admittance matrix data is initial node corrected admittance matrix data; deletion of rows and columns corresponding to the second admittance matrix data and located at VSC nodes, and generation of corresponding third admittance matrix data; wherein the third admittance matrix data is admittance matrix data after deletion of rows and columns located at VSC nodes from the second admittance matrix data; processing of second data and third data corresponding to each VSC node respectively according to the third admittance matrix data; wherein the second data is Thevenin equivalent impedance data of the external system of each VSC node; and the third data is voltage stiffness data of each VSC node; generation of power grid strength evaluation data corresponding to the power grid to be analyzed based on the third data and in combination with the second data.

2. The voltage-stiffness-based VSC high-occupancy grid strength evaluation method according to claim 1, characterized in that, The real-time acquisition of first data corresponding to the power grid to be analyzed, and processing of first admittance matrix data corresponding to the power grid to be analyzed according to the first data further comprises: acquisition of original node admittance matrix network data corresponding to the power grid to be analyzed; acquisition of each synchronous generator position data, each synchronous generator rated capacity data and each synchronous generator sub-transient reactance data corresponding to the power grid to be analyzed respectively; acquisition of each VSC position data, each VSC rated capacity data and each VSC control mode data corresponding to the power grid to be analyzed respectively; acquisition of each LCC initial bus voltage amplitude data and each LCC apparent power data corresponding to the power grid to be analyzed respectively.

3. The voltage-stiffness-based VSC high-occupancy grid strength evaluation method according to claim 1, characterized in that, The processing of VSC device impedance data and second admittance matrix data corresponding to the power grid to be analyzed respectively according to the first admittance matrix data further comprises: Impedance data of each VSC device corresponding to the power grid to be analyzed is calculated, and the specific calculation formula is as follows: wherein: denotes the device impedance of the VSC, U sys denotes the grid voltage at the point of grid connection of the VSC after connection of the VSC, a conjugate of apparent power of the VSC.

4. The voltage-stiffness-based VSC high-occupation grid strength evaluation method according to claim 1 or 3, characterized in that, The processing of VSC device impedance data and second admittance matrix data corresponding to the power grid to be analyzed respectively according to the first admittance matrix data further comprises: combination of LCC, load and synchronous machine sub-transient reactance data, and calculation of generation of second admittance matrix data corresponding to the power grid to be analyzed, and the specific calculation formula is as follows: Y exp = Y org + Y aux (2) wherein Y org is the initial nodal admittance matrix for the power flow calculation; Y exp is the modified nodal admittance matrix; Y aux is the additional matrix; The calculation formula of the additional matrix is as follows: wherein diag(·) represents a transformation from a vector to a diagonal matrix; in Yaux, only the diagonal elements of the nodes connected with the LCC, load and synchronous machine are not 0, and the remaining elements are all 0.

5. The voltage-stiffness-based VSC high-occupancy grid strength evaluation method according to claim 1, characterized in that, The deletion of rows and columns corresponding to the second admittance matrix data and located at VSC nodes, and the generation of corresponding third admittance matrix data further comprise: acquisition of network type VSC node data and fixed AC bus voltage network type VSC node data respectively; deletion of rows and columns located in the second admittance matrix data, and real-time generation of corresponding third admittance matrix data.

6. The voltage-stiffness-based VSC high-dominance grid strength evaluation method according to claim 1 or 5, characterized in that, The deleting processing corresponds to the second admittance matrix data and the row and column where the VSC node is located, and generates corresponding third admittance matrix data, and further comprises: The third admittance matrix data corresponding to the power grid to be analyzed is calculated, and the specific calculation formula is as follows: Y final = L delete (Y exp , k) (4) where Y final is the final obtained nodal admittance matrix, k is the number of the node where the DC bus voltage VSC is located, L delete denotes the function of deleting the kth row and the kth column in the matrix Y final .

7. The voltage-stiffness-based VSC high-occupancy grid strength evaluation method according to claim 1, characterized in that, The second data and the third data corresponding to each VSC node are respectively generated based on the third admittance matrix data, and further comprises: The data of Thevenin equivalent impedance of the external system is calculated as follows: wherein the Thevenin equivalent impedance of the external system is Y final the hth diagonal element of the inverse matrix; The voltage stiffness data of each VSC node is calculated, and the specific calculation formula is as follows: where K vtg is the voltage stiffness of each VSC node, and The device impedance of the VSC node and the Thevenin equivalent impedance of the external system.

8. The voltage-stiffness-based VSC high-occupancy grid strength evaluation method according to claim 1, characterized in that, The third data is combined with the second data to generate the power grid strength evaluation data corresponding to the power grid to be analyzed, and further comprises: According to the generated power grid strength evaluation data, it is determined in real time whether the external system corresponding to the power grid to be analyzed is a strong system. If the calculated voltage stiffness K vtg is greater than 0.95, it is determined that the system is a strong system.

9. A voltage-stiffness-based VSC high-occupancy grid strength evaluation system, characterized in that, The system is applied to the power grid strength evaluation method in any one of claims 1-8, and the system comprises: The first data generation unit is configured to acquire the first data corresponding to the power grid to be analyzed in real time, and generate the first admittance matrix data corresponding to the power grid to be analyzed based on the first data; wherein the first data is basic planning data of the power grid to be analyzed; and the first admittance matrix data is initial node admittance matrix data; The second data generation unit is configured to generate the VSC device impedance data and the second admittance matrix data corresponding to the power grid to be analyzed based on the first admittance matrix data; wherein the second admittance matrix data is corrected admittance matrix data of the initial node; The third data generation unit is configured to delete the row and column where the VSC node is located corresponding to the second admittance matrix data, and generate corresponding third admittance matrix data; wherein the third admittance matrix data is admittance matrix data after deleting the row and column where the VSC node is located in the second admittance matrix data; The fourth data generation unit is configured to generate the second data and the third data corresponding to each VSC node based on the third admittance matrix data; wherein the second data is Thevenin equivalent impedance data of the external system of each VSC node; and the third data is voltage stiffness data of each VSC node; The fifth data generation unit is configured to generate the power grid strength evaluation data corresponding to the power grid to be analyzed based on the third data and in combination with the second data. The first data generation unit further comprises:

10. The voltage-stiffness-based VSC high-penetration grid strength evaluation system of claim 9, wherein, The first data acquisition module is configured to acquire the original node admittance matrix network data corresponding to the power grid to be analyzed; The second data acquisition module is configured to acquire the position data, the rated capacity data and the sub-transient reactance data of each synchronous generator corresponding to the power grid to be analyzed; The third data acquisition module is configured to acquire the position data, the rated capacity data and the control mode data of each VSC corresponding to the power grid to be analyzed; The fourth data acquisition module is configured to acquire the initial bus voltage amplitude data and the apparent power data of each LCC corresponding to the power grid to be analyzed; And / or, the second data generation unit further comprises: The conjugate of the apparent power of the VSC; The first calculation generation module is configured to calculate and generate impedance data of each VSC device corresponding to the power grid to be analyzed, and the specific calculation formula is as follows: wherein: denotes the device impedance of the VSC, U sys denotes the grid voltage at the point of grid connection of the VSC after connection of the VSC, And / or, the second data generation unit further comprises: The conjugate of the apparent power of the VSC; The second calculation generation module is configured to generate second admittance matrix data corresponding to the power grid to be analyzed in combination with LCC, load and synchronous machine sub-transient reactance data, and a specific calculation formula is as follows: Y exp = Y org + Y aux (2) wherein Y org is the initial node admittance matrix for the power flow calculation; Y exp is the modified node admittance matrix; Y aux is the additional matrix; The calculation formula of the additional matrix is as follows: Wherein, diag(·) represents the transformation from a vector to a diagonal matrix; in Yaux, only the diagonal elements of the nodes connected with the LCC, load and synchronous machine are not 0, and the rest of the elements are 0; And / or, the third data generation unit further comprises: The fifth data acquisition module is configured to acquire network-constructing VSC node data and fixed AC bus voltage network-following VSC node data respectively; The first processing generation module is configured to delete the row and column in the second admittance matrix data and generate corresponding third admittance matrix data in real time; And / or, the third data generation unit further comprises: The third calculation generation module is configured to generate third admittance matrix data corresponding to the power grid to be analyzed; and a specific calculation formula is as follows: Y final = L delete (Y exp , k) (4) where Y final is the final node admittance matrix, k is the number of the node where the DC bus voltage VSC is located, L delete Table The function shown in matrix Y final deleting the kth row and kth column in matrix Y And / or, the fourth data generation unit further comprises: The fourth calculation generation module is configured to calculate and generate the Thevenin equivalent impedance data of the external system, and the specific calculation formula is as follows: wherein the Thevenin equivalent impedance of the external system is Y final the hth diagonal element of the inverse matrix; The fifth calculation generation module is configured to calculate and generate voltage stiffness data of each VSC node, and the specific calculation formula is as follows: where K vtg is the voltage stiffness of each VSC node, and The device impedance of the VSC node and the Thevenin equivalent impedance of the external system are respectively acquired; And / or, the fifth data generation unit further comprises: The first generation determination module is configured to determine whether the external system corresponding to the power grid to be analyzed is a strong system in real time according to the generated power grid strength evaluation data.

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