Discrete time domain modal analysis method suitable for alternating current-direct current hybrid system containing direct current multi-port element and related device

Abstract

The invention provides a discrete time domain modal analysis method suitable for an alternating-current and direct-current hybrid system containing multi-port direct current and a related device, which are used for evaluating small disturbance stability and oscillation propagation characteristics of the system. The method comprises the following steps: firstly, constructing a whole-network discrete state matrix, and calculating a characteristic mode to judge a stability and instability mode serial number; determining a main influence area of oscillation through a node voltage observability matrix; and finally, determining an oscillation propagation path in the region by using the branch current observability matrix. According to the method, the limitation that a traditional method is only suitable for a two-port direct current system is solved, the method is expanded to a multi-port direct current scene, and support is provided for oscillation positioning, traceability and stability control strategy formulation of a complex system.

Description

Technical Field

[0001] This invention relates to the field of small-disturbance stability modal analysis of power systems, specifically a discrete-time domain modal analysis method and related apparatus applicable to AC / DC hybrid systems containing DC multi-port components. Background Technology

[0002] In recent years, with the large-scale application of new energy power generation and high-voltage direct current (HVDC) transmission technologies, power systems are rapidly evolving towards a hybrid AC / DC structure with multiple DC feeds. Against this backdrop, the number of power electronic devices in the system has surged, and their rapid control dynamics differ significantly from those of traditional synchronous generators. This can easily induce interactions between control components, leading to broadband oscillation instability risks. Such oscillation problems can cause equipment disconnection or even large-scale power outages. Therefore, small-disturbance stability analysis and oscillation propagation characteristics research have become core aspects of ensuring the safe operation of new power systems.

[0003] However, stability analysis of large-scale AC / DC hybrid systems faces severe challenges due to their complex model structures and high order. Traditional state-space modeling methods based on differential-algebraic equations require manual derivation of matrix equations of thousands of orders, resulting in low modeling efficiency and susceptibility to errors. Existing research proposes a discretized state-space modeling and time-domain modal analysis method, which uniformly transforms common power system components into discrete state-space models, improving modeling efficiency. It also constructs node voltage observability matrices and branch current observability matrices to achieve visualized analysis of oscillation source location and propagation paths. However, this technique still has limitations: its modeling framework is only compatible with traditional AC components and two-port DC topologies. With the development of UHVDC transmission technology, multi-port DC systems (such as hierarchically connected UHVDC) have been widely used in engineering. These systems connect to the AC bus in parallel through multiple converter stations at the sending and receiving ends of the power grid, forming a complex multi-port network structure. Summary of the Invention

[0004] To address the aforementioned deficiencies in existing technologies, this invention provides a discrete-time domain modal analysis method and related apparatus applicable to AC / DC hybrid systems containing DC multi-port components. This extends the discretized state-space modeling method to complex AC / DC hybrid systems with multi-port DC components, thereby resolving the blind spots in oscillation and instability analysis in new power grid configurations and providing theoretical support for the safe and stable operation of systems with high proportions of renewable energy integration.

[0005] A discrete-time modal analysis method applicable to AC / DC hybrid systems with multi-port DC transmission includes the following steps:

[0006] S1. Construct the discrete state matrix of the entire network and calculate the instability modes:

[0007] Based on the discrete equivalent circuit model of multi-port DC, the discretized state-space model of multi-port DC is written. Combined with the discrete state equations of AC system components, the discrete state matrix of the entire AC-DC hybrid system containing multi-port DC is constructed. By analyzing the discrete state matrix of the entire network, the characteristic modes of the system are obtained, the unstable modes are screened out and their mode numbers are recorded.

[0008] S2. Construct the node voltage observability matrix and determine the main influence regions:

[0009] Based on the mode number of the unstable mode obtained in step S1, the discrete state matrix of the entire network is diagonalized to generate mutually decoupled modal quantities; combined with the node voltage constraints of the multi-port DC quantity, a node voltage observability matrix is ​​constructed, the response components of the unstable mode in each node voltage are calculated, and the main influence area corresponding to the oscillation is determined according to the node voltage distribution coefficient.

[0010] S3. Construct the branch current observability matrix and determine the oscillation propagation path:

[0011] Based on the main influence area determined in step S2, and combined with the branch current constraint of the multi-port DC quantity, a branch current observability matrix is ​​constructed. The response components of the unstable mode in each branch current within the main influence area are calculated, and the oscillation propagation path within the area is determined according to the amplitude and phase of the branch current.

[0012] Furthermore, in step S1, the process of constructing the discrete state matrix of the entire network includes:

[0013] Ignoring the characteristics of DC components having multiple historical current sources and no branch voltage, we treat DC components as models with a single historical current source and a single branch, and uniformly write the discrete state-space equations of the entire system.

[0014] By combining the discretized state-space model of multi-port DC, the coefficient matrices (BdLt, (-Lt)TDd) related to DC in the discrete state-space equation of the whole system are eliminated to obtain the discrete state matrix of the whole network containing multi-port DC.

[0015] Furthermore, in step S2, the process of constructing the node voltage observability matrix includes:

[0016] Based on the mode number of the unstable mode obtained in step S1, the node voltage is represented as a linear combination of each characteristic mode through the modal quantity, and a preliminary node voltage observability matrix is ​​generated.

[0017] Elimination is performed on the DC-related coefficients ((-Lt)TDd) in the preliminary node voltage observability matrix to obtain a node voltage observability matrix containing multi-port DC.

[0018] Based on the response components of each node voltage in the node voltage observability matrix, the node voltage distribution coefficient is calculated, and nodes with distribution coefficients within 1 / 10 of the maximum value are designated as the main influence areas.

[0019] Furthermore, in step S3, the process of constructing the branch current observability matrix includes:

[0020] Based on the main influence area determined in step S2, a preliminary branch current observability matrix is ​​generated using the relationship between branch current and modal quantity.

[0021] Elimination is performed on the DC-related coefficients (YbLt, Dd) in the preliminary branch current observability matrix to obtain the branch current observability matrix containing multi-port DC.

[0022] Based on the response components of each branch current in the branch current observability matrix, combined with the current amplitude and phase, the oscillation propagation path within the main influence area is determined.

[0023] A discrete-time modal analysis device suitable for AC / DC hybrid systems containing multi-port DC, comprising:

[0024] Discrete State Matrix Construction Module: Based on the discrete equivalent circuit model of multi-port DC, this module writes the discretized state-space model of multi-port DC, constructs the discrete state matrix of the entire AC / DC hybrid system containing multi-port DC, calculates the characteristic modes of the system, and outputs the mode numbers of the system's instability modes.

[0025] Node voltage observability matrix construction module: used to receive the mode number of the unstable mode, construct the node voltage observability matrix of the AC-DC hybrid system with multi-port DC, calculate the response components of the oscillation mode in each node voltage, and output the main influence region corresponding to the oscillation.

[0026] Branch current observability matrix construction module: used to receive the main influence area, construct the branch current observability matrix of the AC-DC hybrid system containing multi-port DC, calculate the response components of the oscillation mode in each branch current, and output the oscillation propagation path in the main influence area.

[0027] Furthermore, the process of constructing the discrete state matrix of the entire network includes:

[0028] Ignoring the characteristics of DC components having multiple historical current sources and no branch voltage, we treat DC components as models with a single historical current source and a single branch, and uniformly write the discrete state-space equations of the entire system.

[0029] By combining the discretized state-space model of multi-port DC, the coefficient matrices (BdLt, (-Lt)TDd) related to DC in the discrete state-space equation of the whole system are eliminated to obtain the discrete state matrix of the whole network containing multi-port DC.

[0030] Furthermore, based on the mode index of the unstable mode, the node voltage is represented as a linear combination of each characteristic mode through the modal quantity, generating a preliminary node voltage observability matrix;

[0031] Elimination is performed on the DC-related coefficients ((-Lt)TDd) in the preliminary node voltage observability matrix to obtain a node voltage observability matrix containing multi-port DC.

[0032] Based on the response components of each node voltage in the node voltage observability matrix, the node voltage distribution coefficient is calculated, and nodes with distribution coefficients within 1 / 10 of the maximum value are designated as the main influence areas.

[0033] Furthermore, based on the main affected area, a preliminary branch current observability matrix is ​​generated through the relationship between branch current and modal quantity;

[0034] Elimination is performed on the DC-related coefficients (YbLt, Dd) in the preliminary branch current observability matrix to obtain the branch current observability matrix containing multi-port DC.

[0035] Based on the response components of each branch current in the branch current observability matrix, combined with the current amplitude and phase, the oscillation propagation path within the main influence area is determined.

[0036] A discrete-time modal analysis system suitable for AC / DC hybrid systems with multi-port DC, comprising: a computer-readable storage medium and a processor;

[0037] The computer-readable storage medium is used to store executable instructions;

[0038] The processor is used to read executable instructions stored in the computer-readable storage medium and execute the discrete-time domain modal analysis method applicable to AC / DC hybrid systems with multi-port DC.

[0039] A non-transitory computer-readable storage medium storing a computer program that, when executed by a processor, implements the discrete-time domain modal analysis method applicable to AC / DC hybrid systems with multi-port DC.

[0040] The present invention has the following beneficial effects:

[0041] 1) By calculating the eigenvalues ​​of the discrete state matrix of the entire AC / DC hybrid system with multi-port DC, the oscillation modes of complex systems under multiple scenarios can be calculated, the oscillation risk of the system can be determined, and oscillation risk warning can be provided for the power system.

[0042] 2) By calculating the node voltage observability matrix of a hybrid AC / DC system with multi-port DC, the main influence area corresponding to the oscillation is determined, providing a reference for the location selection of power system oscillation measurement and suppression devices.

[0043] 3) By calculating the observability matrix of branch currents in a hybrid AC / DC system with multi-port DC, the oscillation propagation path in the main oscillation area can be determined, which facilitates system personnel to rehearse the oscillation propagation path under different fault scenarios and thus implement targeted stability control strategies.

[0044] 4) It solves the problem that the traditional discrete-time modal analysis method can only analyze the broadband oscillation characteristics of AC systems with simple two-port DC, and extends the modal analysis method to multi-port DC, which is in line with the current trend of DC multi-port. Attached Figure Description

[0045] Figure 1 This is the discretized equivalent circuit diagram of the AC / DC hybrid system containing multi-port DC of the present invention;

[0046] Figure 2 This is a schematic diagram of the discrete state variable coefficient matrix form of the present invention;

[0047] Figure 3 This is a schematic diagram of the discrete state variable-branch voltage coefficient matrix form of the present invention;

[0048] Figure 4 This is a schematic diagram of the node injection current-discrete state variable coefficient matrix form of the present invention;

[0049] Figure 5 This is a schematic diagram of the branch-node association matrix form of the present invention;

[0050] Figure 6 This is a flowchart illustrating a discrete-time modal analysis method for AC / DC hybrid systems with multi-port DC connections, according to an embodiment of the present invention. Detailed Implementation

[0051] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0052] Please see Figure 6 This invention provides a discrete-time modal analysis method for AC / DC hybrid systems with multi-port DC, comprising the following steps:

[0053] Step 1. Construct the discrete state matrix of the entire network and calculate the instability modes.

[0054] According to such Figure 1 The discrete equivalent circuit model of the multi-port DC circuit is shown. The discretized state-space model of the multi-port DC circuit is then described, with the specific equations as shown in equation [equation missing]. As shown:

[0055]

[0056] In the formula, h h (t), h h (t-Δt) represent the values ​​of the discretized state variables of DC at times t and t-Δt, respectively. ixy (t), i ixy (t)(i=1,…,n) represent the input voltage and output current of the i-th AC bus connected to the DC transmission line at time t in the unified xy coordinate system of the entire network; where i=1,…,m are the AC buses at the DC sending end, and i=m+1~n are the AC buses at the DC receiving end. g ii (i=1,…,n) represents the equivalent self-conductance of the i-th AC bus, g ij (i,j=1,…,n) represents the equivalent mutual conductance between the i-th AC bus and the j-th AC bus. A d-h B is the coefficient matrix of the discrete state variables for DC. d-hi (i,j=1,…,n) represent the discrete state variables of the DC model – the nodal voltage coefficients; D d-hi (i=1,…,n) are the coefficients related to the historical current term in the DC discrete model.

[0057] Ignoring the characteristics of DC components having multiple historical current sources and no branches, and assuming that DC components are treated as AC components with a single historical current source and a single branch, we can unify the form, smoothly arrange all components, and write the discrete state-space equations of the entire system, as shown in the following equation. As shown.

[0058]

[0059] Where h is the state vector of the entire system; U branch U is the branch voltage of the component; node This represents the node voltage of the component. A dThe system's discrete state variable coefficient matrix is ​​formed by filling the discrete state variable coefficient matrices of each component of the system into a diagonal matrix in sequence, as shown in the following form. Figure 2 As shown in Figure B. d The system's discrete state variables and branch voltages correspond to the coefficient matrix, in the form of: Figure 3 As shown. The diagonal matrix corresponding to the AC components represents the coefficients in the local discrete state space of each component in the system; for DC components, the concept of branchless operation is ignored, and the diagonal matrix corresponding to the DC component is set to I2. t The branch-node incidence matrix is ​​in the form of: Figure 4 As shown. Elements at the nodes connected to single-port elements are set to the identity matrix I2; elements at the two nodes connected to AC two-port elements are set to I2 and -I2 respectively; for DC elements, the concept of no branches is ignored, and elements at the nodes connected to DC are all set to I2; all other elements are set to 0. G is the node conductance matrix of the discrete circuit network. The diagonal element values ​​g of the conductance matrix G are... ii Equal to the sum of the equivalent conductances of all elements connected to node i, and the off-diagonal element g ij It is equal to the negative of the equivalent conductance of the element directly connected between nodes i and j. D d Inject the current-discrete state variable coefficient matrix into the nodes of the system, in the form of: Figure 5 As shown. For AC components, the discrete state variable coefficient matrices of each component in the system are filled into a diagonal matrix in sequence; for DC components, the multiple independent historical current sources are ignored, and the diagonal matrix corresponding to the DC component is set to I2.

[0060] This process ignores the characteristics of DC components having multiple historical current sources at input and no branch voltage, and requires adjustment to B. d L t D d The relevant transformations are then processed subsequently. Combined formula The characteristics of DC components, for formula China B d L t 、(-L t ) T D d The coefficients related to DC are eliminated. For ease of description, it is assumed that the branch where the DC transmission line is located is numbered k, and the AC bus node is numbered n. i (i=1,2,…,n), the specific processing of the coefficient matrix in the discrete model of DC components is as follows:

[0061] 1) Matrix B d L t The k-th row and the n-th row iThe column elements are represented by the coefficient matrix B in the discrete state-space model of DC components. d-hi Replacement, written as BL;

[0062] 2) Transform the matrix (-L) t ) T D d The nth i The elements in row and k-th column are represented by the coefficient matrix -D in the discrete state-space model of DC components. d-hi Replacement, written as LD.

[0063] The discrete state matrix A of the entire AC / DC hybrid system containing multi-port DC can be obtained by following the above steps. D , as shown As shown, solving for it can determine the instability scenarios and corresponding instability modes of complex systems. If A D If the real parts of the eigenvalues ​​in the eigenvalue analysis results are all negative, then the system remains stable. If A D If the real part of the eigenvalue analysis result is positive, it indicates that the damping of the eigenvalue is positive, and the system may oscillate or become unstable. The mode number i of the positive eigenvalue is recorded for subsequent analysis of the main influence region and oscillation propagation path.

[0064]

[0065] Step 2. Construct the node voltage observability matrix and determine the main influence regions.

[0066] Discretize the state-space equations and state matrix A of the entire system calculated in step 1. D Diagonalization is performed to obtain the right eigenvector W of the discrete eigenvalues. Based on W, the discrete state variable h(t) is linearized to generate the decoupled modal variables Z(t).

[0067]

[0068]

[0069] Joint Japanese style The relationship between system node voltages and modal quantities is obtained:

[0070]

[0071] In the formula, S is the node voltage observability matrix, and the i-th column of matrix S characterizes the response of the i-th mode in each node voltage.

[0072] This process neglects the characteristics of DC components, and considers the equation (-L) t ) T D d The coefficients related to DC are eliminated, that is, (-L) t ) T D d Change to LD.

[0073] The node voltage observability matrix of the AC / DC hybrid system containing multi-port components can be obtained by following the above steps, as shown in the equation. As shown.

[0074]

[0075] The i-th column of matrix S characterizes the response of the i-th mode on the x / y axes of the node voltages. Since the node voltages and branch currents are transformed into a unified xy rotating coordinate system for the entire network during state-space modeling, the x-axis and y-axis components need to be superimposed to obtain the total response of the oscillating components in the voltages of each node. That is, the voltage oscillation component of the i-th mode at node p. for:

[0076]

[0077] To compare the relative amplitudes of the oscillating components in the voltage at each node, the node voltage distribution coefficient is defined as follows:

[0078]

[0079] Based on the calculated voltage distribution coefficients of each node, the power grid section consisting of nodes with relatively large amplitudes can be designated as the main oscillation risk area of ​​the system. Generally, nodes with voltage distribution coefficients within 1 / 10 of their maximum values ​​are selected as the main oscillation risk area, and measuring devices need to be installed in this area to monitor the stability of the system.

[0080] Step 3. Construct the branch current observability matrix and determine the oscillation propagation path:

[0081] Calculate the current in each branch based on the nodal equations of the AC system, as shown in the equation. As shown:

[0082]

[0083] Among them, I b Y is the column vector of branch currents; bIt is a diagonal block matrix composed of discrete equivalent conductances of all AC components, and is called the branch conductance matrix.

[0084] Joint Japanese style The relationship between the current in each branch and the modal quantities is obtained as shown in the following equation:

[0085]

[0086] Q is the branch current observability matrix. The i-th column of the branch current observability matrix Q represents the distribution of the i-th characteristic mode in the branch currents of the system.

[0087] This process neglects the characteristics of DC components, and considers the influence of Y in the equation. b L t 、-(L t ) T D d and D d The coefficients related to DC are eliminated. For ease of description, it is assumed that the branch where the DC transmission line is located is numbered 'k', and the AC bus node is numbered n. i (i=1,2,…,n), the specific processing of the coefficient matrix in the discrete model of DC components is as follows:

[0088] 1) Transform matrix Y b L t The k-th row and the n-th row i Replace the element in column with [g 1i… g ni ] T , the k-th row, the n-th row j Replace the element at column with [-g 1j … -g nj ] T , written as YL.

[0089] 2) (-L) t ) T D d Change to LD.

[0090] 3) D d Replace the element at row k and column k with [D] d-h1… D d-hn ] T , written as D.

[0091] The above steps yield the branch current observability matrix of the AC / DC hybrid system containing multi-port components, as shown in the equation. As shown.

[0092]

[0093] The i-th column element of the branch current observability matrix Q characterizes the distribution of the i-th characteristic mode along the x / y axes of the branch currents in the system. From the equation... calculate Determine the magnitude of the total current oscillation response of the i-th characteristic mode in branch k, where 'k' represents the branch number and the subscript (2k, i) represents the index number of the element in the Q matrix.

[0094]

[0095] pass The propagation path of oscillating current within the main affected area is determined, with amplitude representing the magnitude of the oscillating current and phase representing its direction. This result allows system operators to intuitively understand the distribution and propagation patterns of oscillating modes within the large power grid.

[0096] This invention has the following features and effects:

[0097] 1. Expanding the scope of modal analysis: By constructing a discrete state matrix of the entire network containing multi-port DC, the limitation of traditional discrete-time domain modal analysis methods being applicable only to simple two-port DC systems is solved, realizing the wideband oscillation characteristic analysis of multi-port DC AC-DC hybrid systems, which is in line with the development trend of DC multi-port.

[0098] 2. Precisely locate the oscillation-affected area: Based on the node voltage observability matrix, the response components of the oscillation mode in each node voltage can be quantified, the main oscillation-affected area can be identified, and a scientific basis can be provided for the location selection of oscillation measurement and suppression devices.

[0099] 3. Clarify the oscillation propagation path: By using the branch current observability matrix, the oscillation propagation path in the main affected area can be determined, supporting system personnel to rehearse the oscillation diffusion process under fault scenarios, and laying the foundation for formulating targeted stability control strategies.

[0100] Support system stability analysis and early warning: By calculating the characteristic modes of the discrete state matrix of the entire network, the stability and instability modes of the system can be effectively judged, providing oscillation risk early warning for complex AC-DC hybrid systems with multi-port DC, and ensuring the safe operation of the new power system.

[0101] This invention also provides a discrete-time modal analysis device suitable for AC / DC hybrid systems containing multi-port DC, comprising:

[0102] Discrete State Matrix Construction Module: Based on the discrete equivalent circuit model of multi-port DC, this module writes the discretized state-space model of multi-port DC, constructs the discrete state matrix of the entire AC / DC hybrid system containing multi-port DC, calculates the characteristic modes of the system, and outputs the mode numbers of the system's instability modes.

[0103] Node voltage observability matrix construction module: used to receive the mode number of the unstable mode, construct the node voltage observability matrix of the AC-DC hybrid system with multi-port DC, calculate the response components of the oscillation mode in each node voltage, and output the main influence region corresponding to the oscillation.

[0104] Branch current observability matrix construction module: used to receive the main influence area, construct the branch current observability matrix of the AC-DC hybrid system containing multi-port DC, calculate the response components of the oscillation mode in each branch current, and output the oscillation propagation path in the main influence area.

[0105] Another embodiment of the present invention provides a discrete-time modal analysis system suitable for AC / DC hybrid systems with multi-port DC, comprising: a computer-readable storage medium and a processor;

[0106] The computer-readable storage medium is used to store executable instructions;

[0107] The processor is used to read executable instructions stored in the computer-readable storage medium and execute the discrete-time domain modal analysis method applicable to AC / DC hybrid systems with multi-port DC.

[0108] Another embodiment of the present invention provides a non-transitory computer-readable storage medium having a computer program stored thereon, which, when executed by a processor, implements the discrete-time domain modal analysis method applicable to AC / DC hybrid systems with multi-port DC.

[0109] Those skilled in the art will understand that embodiments of the present invention can be provided as methods, systems, or computer program products. Therefore, the present invention can take the form of a completely hardware embodiment, a completely software embodiment, or an embodiment combining software and hardware aspects. Furthermore, the present invention can take the form of a computer program product embodied on one or more computer-usable storage media (including, but not limited to, disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code.

[0110] This invention is described with reference to flowchart illustrations and / or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of the invention. It will be understood that each block of the flowchart illustrations and / or block diagrams, and combinations of blocks in the flowchart illustrations and / or block diagrams, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, special-purpose computer, embedded processor, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, generate instructions for implementing the flowchart illustrations and / or block diagrams. Figure 1 One or more processes and / or boxes Figure 1 A device that provides the functions specified in one or more boxes.

[0111] These computer program instructions may also be stored in a computer-readable storage medium that can direct a computer or other programmable data processing device to function in a particular manner, such that the instructions stored in the computer-readable storage medium produce an article of manufacture including instruction means, which are implemented in a process Figure 1 One or more processes and / or boxes Figure 1 The function specified in one or more boxes.

[0112] These computer program instructions may also be loaded onto a computer or other programmable data processing equipment to cause a series of operational steps to be performed on the computer or other programmable equipment to produce a computer-implemented process, thereby providing instructions that execute on the computer or other programmable equipment for implementing the process. Figure 1 One or more processes and / or boxes Figure 1 The steps of the function specified in one or more boxes.

[0113] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the specific implementation of the present invention. Any modifications or equivalent substitutions that do not depart from the spirit and scope of the present invention should be covered within the scope of protection of the claims of the present invention.

Claims

1. A discrete-time domain modal analysis method suitable for AC / DC hybrid system with multi-port DC, characterized in that, The method comprises the following steps: S1. Constructing a full-network discrete state matrix and calculating an unstable mode: According to the discrete equivalent circuit model of the multi-port DC, the discrete state space model of the multi-port DC is written, and the discrete state equation of the AC system element is combined to construct the full-network discrete state matrix of the AC / DC hybrid system containing the multi-port DC; the characteristic mode of the system is obtained by eigenvalue analysis of the full-network discrete state matrix, and the unstable mode is screened out and the mode number is recorded; S2. Constructing a node voltage observability matrix and determining a main influence area: Based on the mode number of the unstable mode obtained in step S1, the full-network discrete state matrix is diagonalized to generate decoupled modal quantities; a node voltage observability matrix is constructed in combination with the node voltage constraint of the multi-port DC flow, the response components of the unstable mode in each node voltage are calculated, and the main influence area corresponding to the oscillation is determined according to the node voltage distribution coefficient; S3. Constructing a branch current observability matrix and determining an oscillation propagation path: Based on the main influence area determined in step S2, a branch current observability matrix is constructed in combination with the branch current constraint of the multi-port DC flow, the response components of the unstable mode in each branch current in the main influence area are calculated, and the oscillation propagation path in the main influence area is determined according to the amplitude and phase of the branch current.

2. The method of claim 1, wherein, In step S1, the construction process of the full-network discrete state matrix comprises: Ignoring the characteristics of the multi-history current source input and the absence of branch voltage of the DC element, the DC element is regarded as a model with single-history current source input and single branch, and the discrete state space equation of the whole system is uniformly written; In combination with the discrete state space model of the multi-port DC, the coefficient matrix related to the DC in the discrete state space equation of the whole system is eliminated to obtain the full-network discrete state matrix containing the multi-port DC.

3. The method of claim 1, wherein, In step S2, the construction process of the node voltage observability matrix comprises: Based on the mode number of the unstable mode obtained in step S1, the node voltage is expressed as a linear combination of each characteristic mode through the modal quantity to generate a preliminary node voltage observability matrix; The coefficient terms related to the DC in the preliminary node voltage observability matrix are eliminated to obtain the node voltage observability matrix containing the multi-port DC; According to the response components of each node voltage in the node voltage observability matrix, the node voltage distribution coefficient is calculated, and the nodes with a distribution coefficient within 1 / 10 of the maximum value are designated as the main influence area.

4. The method of claim 1, wherein, In step S3, the construction process of the branch current observability matrix comprises: Based on the main influence area determined in step S2, a preliminary branch current observability matrix is generated through the relationship between the branch current and the modal quantity; The coefficient terms related to the DC in the preliminary branch current observability matrix are eliminated to obtain the branch current observability matrix containing the multi-port DC; According to the response components of each branch current in the branch current observability matrix, the oscillation propagation path in the main influence area is determined in combination with the current amplitude and phase.

5. A discrete time domain modal analysis apparatus suitable for use in an AC / DC hybrid system containing multi-port DC, characterized in that, It comprises: The discrete state matrix construction module is configured to construct a discrete state space model of the multi-port DC according to a discrete equivalent circuit model of the multi-port DC, construct a full-network discrete state matrix of the AC / DC hybrid system containing the multi-port DC, calculate characteristic modes of the system, and output a mode sequence number of an unstable mode of the system. The node voltage observability matrix construction module is configured to receive the mode sequence number of the unstable mode, construct a node voltage observability matrix of the AC / DC hybrid system containing the multi-port DC, calculate response components of the oscillation mode in each node voltage, and output a main influence area corresponding to the oscillation. The branch current observability matrix construction module is configured to receive the main influence area, construct a branch current observability matrix of the AC / DC hybrid system containing the multi-port DC, calculate response components of the oscillation mode in each branch current, and output an oscillation propagation path in the main influence area.

6. The apparatus of claim 5, wherein, The construction process of the full-network discrete state matrix includes: The DC elements are regarded as a single-history current source input and a single-branch model by ignoring the characteristics of multi-history current source input and no branch voltage of the DC elements, and the discrete state space equations of the full system are uniformly written; The coefficient matrix related to the DC in the discrete state space equations of the full system is eliminated by combining the discrete state space model of the multi-port DC, and a full-network discrete state matrix containing the multi-port DC is obtained.

7. The apparatus of claim 5, wherein, The construction process of the node voltage observability matrix includes: Based on the mode sequence number of the unstable mode, the node voltage is expressed as a linear combination of each characteristic mode through modal quantities to generate a preliminary node voltage observability matrix; The coefficient terms related to the DC in the preliminary node voltage observability matrix are eliminated to obtain a node voltage observability matrix containing the multi-port DC; According to the response components of each node voltage in the node voltage observability matrix, the distribution coefficients of the node voltage are calculated, and the nodes with distribution coefficients within 1 / 10 of the maximum value are determined as the main influence area.

8. The apparatus of claim 5, wherein, The construction process of the branch current observability matrix includes: Based on the main influence area, a preliminary branch current observability matrix is generated through the relationship between the branch current and the modal quantity; The coefficient terms related to the DC in the preliminary branch current observability matrix are eliminated to obtain a branch current observability matrix containing the multi-port DC; According to the response components of each branch current in the branch current observability matrix, the oscillation propagation path in the main influence area is determined in combination with the current amplitude and phase.

9. A discrete-time domain modal analysis system suitable for use in an AC / DC hybrid system containing multi-port DC, comprising: A computer readable storage medium and a processor; The computer readable storage medium is configured to store executable instructions; The processor is configured to read the executable instructions stored in the computer readable storage medium and execute the discrete-time domain modal analysis method for the AC / DC hybrid system containing the multi-port DC according to any one of claims 1-4.

10. A non-transitory computer readable storage medium having stored thereon a computer program, which, when executed by a processor, implements the discrete-time domain modal analysis method for the AC / DC hybrid system containing the multi-port DC according to any one of claims 1-4.

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