CN110556831A

CN110556831A - Stability evaluation method and system for multi-machine multi-node power generation system

Info

Publication number: CN110556831A
Application number: CN201910916002.8A
Authority: CN
Inventors: 张旸; 孙龙庭; 陈新; 龚春英; 陈杰
Original assignee: Nanjing University of Aeronautics and Astronautics; Electric Power Research Institute of State Grid Jiangsu Electric Power Co Ltd
Current assignee: Nanjing University of Aeronautics and Astronautics; Electric Power Research Institute of State Grid Jiangsu Electric Power Co Ltd
Priority date: 2019-09-26
Filing date: 2019-09-26
Publication date: 2019-12-10
Anticipated expiration: 2039-09-26
Also published as: CN110556831B

Abstract

the invention discloses a stability evaluation method and a system of a multi-machine multi-node power generation system, which comprises the steps of carrying out load flow calculation, and calculating a steady-state control signal of a converter connected with each node and the steady-state harmonic content of an electric parameter according to the voltage amplitude, the phase and the power of each node in a steady-state operation state; establishing impedance of each converter considering the frequency coupling effect, substituting the impedance into a steady-state control signal and the steady-state harmonic content of the electrical parameter to obtain the impedance considering the frequency coupling effect under a specific working condition; connecting the impedance considering the frequency coupling effect under all specific working conditions and the impedance of the passive element according to network topology to obtain a network node admittance matrix, and further obtaining equivalent network impedance aggregated to a set node; and applying a stability criterion to the equivalent network impedance aggregated to the set node to evaluate the system stability. The method can analyze the generation mechanism of the harmonic oscillation of the multi-machine multi-node power generation system and accurately evaluate the stability of the multi-machine multi-node power generation system.

Description

Stability evaluation method and system for multi-machine multi-node power generation system

Technical Field

the invention relates to the technical field of power generation systems, in particular to a stability evaluation method and system for a multi-machine multi-node power generation system.

background

almost all renewable energy sources realize the access of an alternating current power grid through a grid-connected inverter, and a dynamic interconnection system is formed between various converters and the power grid, however, unexpected dynamic interaction effects may exist at the public power grid connection part of the interconnection system.

For an ideal power generation system, all parameters in the power generation system are fixed, and according to reasonable design, a control system of a generator set can have higher stability, and the phenomenon of harmonic oscillation does not exist in the power generation system. However, in an actual power grid, due to the influence of parameters such as a transmission line, the impedance of the power grid cannot be ignored, and harmonic oscillation will be caused by the interaction between various converters (a fan inverter and a static reactive power compensation device) in a power generation system and the power grid, so that the power quality and the stability of the new energy power generation system are reduced. With the expansion of the scale of the power system and the increase of the power generation amount, the stability problem of the power system is more and more prominent. At present, most of power generation systems are multi-machine multi-node power generation systems, namely multi-machine power systems. In order to optimize the design of a multi-machine multi-node power generation system and improve the stability of the multi-machine multi-node power generation system, the generation mechanism of harmonic oscillation of the multi-machine multi-node power generation system needs to be analyzed, the stability of the multi-machine multi-node power generation system needs to be accurately judged, and the frequency of system oscillation under an unstable condition needs to be obtained.

The basis of the stability judgment method belongs to common knowledge in the power electronics profession, such as a series of stability criteria of generalized Nyquist, Nyquist and the like. But such criteria are originally applicable to simpler systems. And for more complex systems such as a multi-machine multi-node power generation system, the stability evaluation cannot be directly carried out by adopting a basic stability criterion. Because there is no stability evaluation method for a multi-machine multi-node power generation system based on an impedance network, there is an urgent need in the art for a method and a system capable of analyzing a generation mechanism of harmonic oscillation of the multi-machine multi-node power generation system and accurately evaluating the stability of the multi-machine multi-node power generation system.

Disclosure of Invention

The invention aims to provide a stability evaluation method and a system of a multi-machine multi-node power generation system, which can analyze the generation mechanism of harmonic oscillation of the multi-machine multi-node power generation system and accurately evaluate the stability of the multi-machine multi-node power generation system.

In order to achieve the purpose, the invention provides the following scheme:

a stability evaluation method for a multi-machine multi-node power system comprises the following steps:

Carrying out load flow calculation on the multi-machine multi-node power generation system to obtain the voltage amplitude, the phase and the power of each node of the multi-machine multi-node power generation system in a steady state operation state;

According to the voltage amplitude, the phase and the power of the nodes, calculating the steady state control signal of the converter connected with each node and the steady state harmonic content of the electric parameter to obtain the steady state control signal of each converter and the steady state harmonic content of the electric parameter;

establishing impedance of each converter considering the frequency coupling effect, substituting the steady-state control signal of each converter and the steady-state harmonic content of the electric parameter into the impedance of the corresponding converter considering the frequency coupling effect to obtain the impedance of each converter considering the frequency coupling effect under a specific working condition;

connecting impedance considering frequency coupling effect and passive element impedance under the specific working condition of all converters in the multi-machine multi-node power generation system network according to the network topology of the multi-machine multi-node power generation system to obtain a network node admittance matrix of the multi-machine multi-node power generation system;

Obtaining equivalent network impedance aggregated to a set node according to the network node admittance matrix;

And applying a generalized Nyquist stability criterion or a Nyquist stability criterion to the equivalent network impedance aggregated to the set nodes to evaluate the stability of the multi-machine multi-node power system.

Optionally, the calculating, according to the voltage amplitude, the phase and the power of the node, the steady-state control signal of the converter connected to each node and the steady-state harmonic content of the electrical parameter to obtain the steady-state control signal of each converter and the steady-state harmonic content of the electrical parameter specifically includes:

Establishing a power circuit steady-state frequency domain equation of each converter according to the power circuit topology of each converter;

And substituting the voltage amplitude, the phase and the power of the nodes and various parameters of the converter connected with each node into a power circuit steady-state frequency domain equation of the converter to carry out iterative solution, and calculating steady-state control signals of the converters and the steady-state harmonic content of the electric parameters.

optionally, the establishing of the impedance of each converter considering the frequency coupling effect, and substituting the steady-state control signal of each converter and the steady-state harmonic content of the electrical parameter into the impedance of the corresponding converter considering the frequency coupling effect to obtain the impedance of each converter considering the frequency coupling effect under the specific working condition specifically includes:

Establishing an impedance of a transformer that accounts for frequency coupling effectsWhen the frequency of disturbance f_pfirst disturbance voltagein positive sequence, the coupling frequency is f_p-2f₁，Y_p(s) is the grid-connected current of the converter at disturbance frequency f_pFirst current response offor the first disturbance voltageTransfer function of, Y_c(s) is the grid-connected current in-coupling of the converterResultant frequency f_p-2f₁Second current response offor the first disturbance voltageThe transfer function of (a) is selected,Is a second current responseFor the second disturbance voltageThe transfer function of, the second disturbance voltageCurrent response of converter grid-connected current at coupled frequency for considering grid impedanceThe disturbance voltage of the coupling frequency generated after flowing through the network impedance,Is a first current responseFor the second disturbance voltageThe transfer function of (a);

Substituting the steady state control signal of the converter and the steady state harmonic content of the electrical parameter into the impedance of the converter taking into account the frequency coupling effectobtaining frequency coupling effects under specific conditions of the convertersimpedance.

Optionally, the connecting impedance considering the frequency coupling effect and the passive element impedance under the specific working condition of all the converters in the multi-machine multi-node power generation system network according to the network topology of the multi-machine multi-node power generation system to obtain the network node admittance matrix of the multi-machine multi-node power generation system specifically includes:

Calculating the ground admittance of each node and the admittance between any two nodes according to the network topology of the multi-node power generation system and the impedance of all the transformers under the specific working condition by considering the frequency coupling effect, the impedance of all the transformers and the impedance of all the lines;

And connecting the ground admittance of each node with the admittance between any two nodes according to the network topology of the multi-machine multi-node power generation system to form an impedance network, and numbering the impedance network to obtain a network node admittance matrix of the multi-machine multi-node power generation system.

Optionally, the obtaining, according to the network node admittance matrix, an equivalent network impedance aggregated to a set node specifically includes:

Establishing a set node voltage equation according to the network node admittance matrix by adopting kirchhoff current law and ohm law;

Establishing a set node disturbance current equation by adopting a kirchhoff current law according to the network node admittance matrix;

and substituting the set node voltage equation into the set node disturbance current equation to obtain the equivalent network impedance aggregated to the set node.

in order to achieve the above purpose, the invention also provides the following scheme:

A stability evaluation system of a multi-machine multi-node electrical system, comprising:

the load flow calculation module is used for carrying out load flow calculation on the multi-machine multi-node power generation system to obtain the voltage amplitude, the phase and the power of each node of the multi-machine multi-node power generation system in a steady state operation state;

The steady-state harmonic content calculation module of the steady-state control signal and the electric parameter is used for calculating the steady-state harmonic content of the steady-state control signal and the electric parameter of the converter connected with each node according to the voltage amplitude, the phase and the power of the node to obtain the steady-state harmonic content of the steady-state control signal and the electric parameter of each converter;

The impedance establishing module is used for establishing the impedance of each converter considering the frequency coupling effect, substituting the steady-state control signal of each converter and the steady-state harmonic content of the electric parameter into the impedance of the corresponding converter considering the frequency coupling effect to obtain the impedance of each converter considering the frequency coupling effect under the specific working condition;

the network node admittance matrix establishing module is used for connecting impedance considering frequency coupling effect and passive element impedance under the specific working condition of all converters in the multi-machine multi-node power generation system network according to the network topology of the multi-machine multi-node power generation system to obtain a network node admittance matrix of the multi-machine multi-node power generation system;

The aggregation module is used for obtaining equivalent network impedance aggregated to a set node according to the network node admittance matrix;

And the evaluation module is used for applying a generalized Nyquist stability criterion or a Nyquist stability criterion to the equivalent network impedance aggregated to the set nodes to evaluate the stability of the multi-machine multi-node power system.

Optionally, the steady-state harmonic content calculation module of the steady-state control signal and the electrical parameter specifically includes:

The power circuit steady-state frequency domain equation establishing unit is used for establishing a power circuit steady-state frequency domain equation of each converter according to the power circuit topology of each converter;

and the iterative solution unit is used for substituting the voltage amplitude, the phase and the power of the nodes and various parameters of the converter connected with each node into a power circuit steady-state frequency domain equation of the converter to carry out iterative solution, and calculating steady-state control signals and steady-state harmonic content of electric parameters of each converter.

Optionally, the impedance establishing module specifically includes:

Impedance taking into account frequency coupling effectsa setting unit for setting up the impedance of the converter taking into account the frequency coupling effectWhen the frequency of disturbance f_pFirst disturbance voltageIn positive sequence, the coupling frequency is f_p-2f₁，Y_p(s) is the grid-connected current of the converter at disturbance frequency f_pFirst current response offor the first disturbance voltageTransfer function of, Y_c(s) is the grid-connected current of the converter at the coupling frequency f_p-2f₁Second current response offor the first disturbance voltageThe transfer function of (a) is selected,is a second current responseFor the second disturbance voltageThe transfer function of, the second disturbance voltagecurrent response of converter grid-connected current at coupled frequency for considering grid impedancethe disturbance voltage of the coupling frequency generated after flowing through the network impedance,Is a first current responseFor the second disturbance voltageThe transfer function of (a);

an impedance establishing unit considering frequency coupling effect under specific working condition for substituting the steady state control signal of the converter and the steady state harmonic content of the electric parameter into the impedance considering frequency coupling effect of the converterAnd obtaining the impedance considering the frequency coupling effect under the specific working condition of each converter.

Optionally, the network node admittance matrix establishing module specifically includes:

the device comprises a ground admittance and internode admittance calculating unit, a frequency coupling effect calculating unit and a frequency coupling effect calculating unit, wherein the ground admittance and internode admittance calculating unit is used for calculating the admittance between each node and any two nodes according to the network topology of the multi-machine multi-node power generation system and the impedance of all the transformers under the specific working condition in consideration of the frequency coupling effect, the impedance of all the transformers and the impedance of all the lines;

And the network node admittance matrix establishing unit is used for connecting the admittance between each node and the ground admittance and any two nodes according to the network topology of the multi-machine multi-node power generation system to form an impedance network, and numbering the impedance network to obtain the network node admittance matrix of the multi-machine multi-node power generation system.

optionally, the aggregation module specifically includes:

The voltage equation establishing unit is used for establishing a set node voltage equation according to the network node admittance matrix by adopting kirchhoff current law and ohm law;

The current equation establishing unit is used for establishing a set node disturbance current equation according to the network node admittance matrix by adopting a kirchhoff current law;

And the aggregation unit is used for substituting the set node voltage equation into the set node disturbance current equation to obtain the equivalent network impedance aggregated to the set node.

According to the specific embodiment provided by the invention, the invention discloses the following technical effects:

the invention discloses a stability evaluation method and a system of a multi-machine multi-node electric system, which are used for evaluating the stability of a more complex system such as the multi-machine multi-node electric system by considering the frequency coupling effect of a converter in the multi-machine multi-node system so that interactive influence exists among the converters, therefore, the impedance of all the converters considering the frequency coupling effect under the actual working condition of the converters needs to be obtained, the relevance of the interactive influence between the converters and the frequency coupling effect is found by researching the influence of the frequency coupling effect, and the interactive influence is expressed and simplified in the form of an impedance network, so that accurate analysis can be carried out, and the stability evaluation method is an important precondition for ensuring the accuracy of the stability evaluation method of the multi-machine multi-node electric system. In addition, after the accurate impedance of each single machine is obtained, the network impedance needs to be constructed according to the node and the system structure, then the network impedance is aggregated to one of the nodes, the impedance equivalent to one side of the node is obtained, and the stability evaluation can be carried out by adopting the basic stability criterion. And then, the harmonic oscillation generation mechanism can be analyzed by combining with an actual system, or the parameter optimization design is carried out.

Drawings

in order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings needed to be used in the embodiments will be briefly described below, and it is obvious that the drawings in the following description are only some embodiments of the present invention, and it is obvious for those skilled in the art to obtain other drawings without inventive exercise.

FIG. 1 is a flowchart of an embodiment of a method for evaluating stability of a multi-node electrical power system according to the present invention;

FIG. 2 is a block diagram of an embodiment of a stability evaluation system for a multi-node electrical system according to the present invention;

FIG. 3 is a diagram of a new energy power generation system of a certain wind farm in the west of China;

FIG. 4 is a flowchart of a stability evaluation method of a new energy power generation system of a wind farm;

FIG. 5 is a typical cascaded H-bridge SVG circuit and control diagram;

FIG. 6 is an impedance equivalent circuit diagram considering frequency coupling effect under a specific working condition of a cascade H bridge SVG;

FIG. 7 is an equivalent circuit diagram of a network node admittance matrix of the wind farm new energy power generation system;

FIG. 8 is an impedance equivalent circuit diagram of the wind power plant new energy power generation system when SVG impedance is converted to the wind power plant side;

FIG. 9 is an equivalent circuit diagram of an impedance network, using bus #4 as an example;

FIG. 10 is a graph of different stability analysis results of the wind farm new energy power generation system obtained by obtaining a characteristic root of a bus #4 impedance network according to a generalized Nyquist stability criterion and adjusting impedance characteristics of a wind turbine inverter by modifying control parameters of the wind turbine inverter;

FIG. 11 is a graph of voltage and current time domain simulation waveforms and Fourier analysis results of a bus #4 when a wind farm new energy power generation system is unstable;

Fig. 12 is a voltage-current time-domain simulation waveform diagram of the bus #4 when the new energy power generation system of the wind farm is stable.

Detailed Description

the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.

In order to make the aforementioned objects, features and advantages of the present invention comprehensible, embodiments accompanied with figures are described in detail below.

example 1

Fig. 1 is a flowchart of an embodiment of a stability evaluation method for a multi-node electrical system according to the present invention. Referring to fig. 1, the method for evaluating the stability of the multi-machine multi-node power system comprises the following steps:

Step 101: and carrying out load flow calculation on the multi-machine multi-node power generation system to obtain the voltage amplitude, the phase and the power of each node of the multi-machine multi-node power generation system in a steady state operation state.

Step 102: and calculating the steady-state control signal of the converter and the steady-state harmonic content of the electric parameter connected with each node according to the voltage amplitude, the phase and the power of the node to obtain the steady-state control signal of each converter and the steady-state harmonic content of the electric parameter.

The step 102 specifically includes:

Step 103: establishing impedance of each converter considering the frequency coupling effect, substituting the steady-state control signal and the steady-state harmonic content of the electrical parameter of each converter into the impedance of the corresponding converter considering the frequency coupling effect, and obtaining the impedance of each converter considering the frequency coupling effect under the specific working condition.

The step 103 specifically includes:

establishing an impedance of a transformer that accounts for frequency coupling effectswhen the frequency of disturbance f_pfirst disturbance voltagein positive sequence, the coupling frequency is f_p-2f₁，Y_p(s) is the grid-connected current of the converter at disturbance frequency f_pFirst current response ofFor the first disturbance voltagetransfer function of, Y_c(s) is the grid-connected current of the converter at the coupling frequency f_p-2f₁Second current response offor the first disturbance voltagethe transfer function of (a) is selected,is a second current responseFor the second disturbance voltageThe transfer function of, the second disturbance voltageCurrent response of converter grid-connected current at coupled frequency for considering grid impedanceThe disturbance voltage of the coupling frequency generated after flowing through the network impedance,is a first current responseFor the second disturbance voltageThe transfer function of (a);

Substituting the steady state control signal of the converter and the steady state harmonic content of the electrical parameter into the impedance of the converter taking into account the frequency coupling effectAnd obtaining the impedance considering the frequency coupling effect under the specific working condition of each converter.

The impedance method for establishing the converter considering the frequency coupling effect is to use the frequency f as the disturbance frequency_pFirst disturbance voltageFor positive sequence, an impedance method of the converter is established that takes into account the effect of frequency coupling. When the frequency of disturbance f_pfirst disturbance voltagein negative sequence, the coupling frequency is f_p+2f₁the impedance method of the converter considering the frequency coupling effect is the same as above, and is not described herein again. Since the relationship between the two cases (positive and negative) is symmetrical, it is generally only discussed from one case, both cases conclude similarly, and the analysis method is the same.

Step 104: and connecting the impedance considering the frequency coupling effect and the passive element impedance under the specific working condition of all the converters in the multi-machine multi-node power generation system network according to the network topology of the multi-machine multi-node power generation system to obtain the network node admittance matrix of the multi-machine multi-node power generation system.

The step 104 specifically includes:

Step 105: and obtaining the equivalent network impedance aggregated to the set node according to the network node admittance matrix.

The step 105 specifically includes:

step 106: and applying a generalized Nyquist stability criterion or a Nyquist stability criterion to the equivalent network impedance aggregated to the set nodes to evaluate the stability of the multi-machine multi-node power system.

Example 2

FIG. 2 is a block diagram of an embodiment of a stability evaluation system for a multi-node electrical system according to the invention. Referring to fig. 2, the stability evaluation system of the multi-machine multi-node power generation system comprises:

The load flow calculation module 201 is configured to perform load flow calculation on the multi-machine multi-node power generation system to obtain a voltage amplitude, a phase, and a power of each node in the multi-machine multi-node power generation system in a steady-state operation state.

And the steady-state harmonic content calculation module 202 for the steady-state control signals and the electrical parameters is used for calculating the steady-state harmonic content of the steady-state control signals and the electrical parameters of the converters connected with each node according to the voltage amplitude, the phase and the power of the node to obtain the steady-state harmonic content of the steady-state control signals and the electrical parameters of each converter.

the steady-state harmonic content calculation module 202 for the steady-state control signal and the electrical parameter specifically includes:

The impedance establishing module 203 is configured to establish an impedance of each converter considering the frequency coupling effect, and substitute the steady-state control signal and the steady-state harmonic content of the electrical parameter of each converter into the impedance of the corresponding converter considering the frequency coupling effect to obtain the impedance of each converter considering the frequency coupling effect under the specific working condition.

The impedance establishing module 203 specifically includes:

an impedance establishing unit for establishing an impedance of the transformer considering the frequency coupling effectWhen the frequency of disturbance f_pfirst disturbance voltageIn positive sequence, the coupling frequency is f_p-2f₁，Y_p(s) is the grid-connected current of the converter at disturbance frequency f_pFirst current response offor the first disturbance voltageTransfer function of, Y_c(s) is the grid-connected current of the converter at the coupling frequency f_p-2f₁second current response ofFor the first disturbance voltageThe transfer function of (a) is selected,is a second current responsefor the second disturbance voltagethe transfer function of, the second disturbance voltageCurrent response of converter grid-connected current at coupled frequency for considering grid impedanceThe disturbance voltage of the coupling frequency generated after flowing through the network impedance,Is a first current responseFor the second disturbance voltagethe transfer function of (a);

And the network node admittance matrix establishing module 204 is used for connecting the impedance considering the frequency coupling effect and the passive element impedance under the specific working condition of all the converters in the multi-machine multi-node power generation system network according to the network topology of the multi-machine multi-node power generation system to obtain the network node admittance matrix of the multi-machine multi-node power generation system.

The network node admittance matrix establishing module 204 specifically includes:

And an aggregation module 205, configured to obtain, according to the network node admittance matrix, a network impedance model aggregated to the set node.

the aggregation module 205 specifically includes:

And the evaluation module 206 is used for applying a generalized Nyquist stability criterion or a Nyquist stability criterion to the equivalent network impedance aggregated to the set nodes to evaluate the stability of the multimachine multi-node power system.

example 3

FIG. 3 is a diagram of a new energy power generation system of a wind farm in the west of China. Referring to fig. 3, the new energy power generation system of the wind power plant comprises a polymerization fan inverter 1, an SVG (static var compensator) 2, a transmission line equivalent impedance 3, a public large power grid 4, buses # 1-5 at all levels and transformers T1-T3 at all levels. A plurality of converters exist in the wind power plant new energy power generation system.

FIG. 4 is a flowchart of a stability evaluation method of the wind farm new energy power generation system. Referring to fig. 4, the stability evaluation method of the wind farm new energy power generation system includes:

Step 401: and carrying out load flow calculation on the wind power plant new energy power generation system to obtain the voltage amplitude, the phase and the power of each node of the wind power plant new energy power generation system in a steady-state operation state.

calculating the electric quantity of the wind power plant new energy power generation system in a steady state operation state according to the wiring mode, the parameters and the operation conditions of the wind power plant new energy power generation system; the electrical quantities include voltage amplitude, phase and power of each node.

in the process of analyzing the stability of the wind power plant new energy power generation system, the trend analysis needs to be carried out on the wind power plant new energy power generation system under the specified working condition. The specific algorithm of the network load flow calculation is various, such as gauss seidel method, newton raphson method, PQ decomposition method, and the like. The tidal current calculation is a calculation for determining steady state operation state parameters of each part of the power system according to parameters and initial states of given elements such as a power grid structure, a generator, various power electronic equipment, lines, loads and the like. And carrying out iterative computation according to the initial state to obtain the information of each node.

The control of a converter and the admittance of each node in the new energy power generation system of the wind power plant are considered, if the control target of the converter is a machine side filter inductor, the filter capacitor of the converter needs to be considered as the node self-admittance, and if the control target is a network side current, the action of the filter capacitor does not need to be considered; considering different converter operating modes (i.e. converter control targets, different control targets of the same converter in power systems with different structures, such as common PQ control, V/f control, reactive power control, etc.), different node types are also used for performing power flow analysis (when power flow calculation is performed, the types of all nodes need to be considered, the node types are such as PQ nodes and balancing nodes, and the types of the nodes are generally determined by the control targets of the access converters); considering the equivalent circuit of the transformer, different types of transformers have amplitude and phase changes, and the node voltages on two sides of the transformer are corrected after the load flow calculation is completed (the transformer has delta and star connection (Y), when the two sides of the transformer are respectively connected by Y-delta, the voltage, current and amplitude on two sides are related to turn ratio, and the phase changes. And (3) integrating the factors (the factors needing to be considered in power system load flow calculation), carrying out load flow analysis on the wind power plant new energy power generation system under the specified working condition, and obtaining voltage amplitude, phase and power information of each node of the wind power plant new energy power generation system, namely the voltage amplitude, phase and power information of each level of buses # 1-5.

Step 402: and calculating the steady-state control signal of the converter and the steady-state harmonic content of the electric parameter connected with each node according to the voltage amplitude, the phase and the power of the node to obtain the steady-state control signal of each converter and the steady-state harmonic content of the electric parameter.

A converter, i.e. a power electronic converter, usually includes two major parts, namely a controller and a power circuit. And iteratively solving through a power circuit steady-state frequency domain equation according to voltage amplitude, phase and power information of each node obtained through network power flow analysis to obtain steady-state control signals of the converter and steady-state harmonic content (components) of the electric parameters. The steady harmonic refers to specific values of various electric quantities of the converter when the converter operates in a steady state, such as alternating current and direct current voltages and currents, but the harmonic generally contains components at several frequencies, and is called the steady harmonic. The steady state control signal, i.e. the control signal given by the control part of the converter (the control signal given by the controller for controlling the power circuit to perform a certain function), may also comprise components at a plurality of frequencies. The load flow information is the working state of the converter accessed to the power grid, and the steady state harmonic content of the converter steady state control signal and the electric parameter can be obtained by iterative solution by substituting the load flow information into a power circuit steady state frequency domain equation of the converter. Because the converter contains a wide variety, and the steady-state harmonic wave of the converter contains all the electric quantities in one converter, the variety is also large, so that the steady-state frequency domain equations of the power circuits of different converters are different. And establishing a power circuit steady-state frequency domain equation of each converter according to the power circuit topology of each converter and parameters of the converters connected with each node. Because the power electronic converter has two main parts, namely a power circuit and a controller, various parameters of the converter are parameters of devices and controllers in the power circuit.

Step 403: establishing impedance of each converter considering the frequency coupling effect, substituting the steady-state control signal and the steady-state harmonic content of the electrical parameter of each converter into the impedance of the corresponding converter considering the frequency coupling effect, and obtaining the impedance of each converter considering the frequency coupling effect under the specific working condition.

If the cascade H-bridge SVG positive sequence admittance coefficient matrix, namely the single-machine impedance of the SVG (without considering the influence and working condition of the frequency coupling effect), is expressed as Y_s(Y_s2n +1 order matrix, n being the order number considering steady state harmonics):

Y_s＝[(U_s+Y_lM_sZ_cM_s)+Y_l(M_sZ_cI_s+V_c)·(U_s-EZ_cI_s)^-1(Q+EZ_cM_s)]^-1Y_l·[U_s-(M_sZ_cI_s+V_c)(U_s-EZ_cI_s)^-1P]

fig. 5 is a typical cascaded H-bridge SVG circuit and control diagram. Referring to fig. 5, the cascaded H-bridge SVG positive sequence admittance coefficient matrix equation can be expressed as:

FIG. 5(a) shows a schematic diagram of a cascaded H-bridge SVG three-phase circuit, wherein each phase only comprises a single bridge arm, N sub-modules (SM) adopting an H-bridge topology are cascaded in each bridge arm, and the capacitor in each sub-module is C_m. (b) H in (e) denotes a PI controller, and the subscripts are used to distinguish the different control loops. K represents a constant related to the power circuit or controller design, with subscripts to distinguish between different loops.

Wherein:

U＝diag[1,1,1,1,1,1,1]

Q_i＝diag[0,H_i(j2π(f_p-f₁))+jK_id,0,H_i(j2π(f_p-f₁))-jK_id,0 0,0]

Each matrix corresponds to each control loop, steady state signal, and power circuit. The representation of each matrix is modeled by the controller structure and the power circuit structure in fig. 5.

Since the actual single-machine impedance is usually a complex expression or matrix, different transformers can be modeled by using different methods (establishing single-machine impedance), but the impedances of different transformers have no commonality, so that different modeling methods can also cause different expression forms of the impedances.

therefore, a transfer function (original output admittance) of the response of the cascade H-bridge SVG grid-connected current at the disturbance frequency to the disturbance voltage and a transfer function (frequency coupling term) of the response of the grid-connected current at the coupling frequency to the disturbance voltage can be obtained, wherein the transfer functions are respectively as follows:

Y_p(s)＝Y_s(4,4)

Y_c(s)＝Y_s(2,4)

wherein, Y_s(4,4)And Y_s(2,4)are respectively admittance matrix Y_sthe (4,4) and (2,4) elements of (1).

The relationship between the disturbance voltage and the response current can be understood as follows: an additional voltage signal (disturbance voltage) is applied to a system, which generates a current signal (current response) after passing through the power circuit and the control circuit. The expression of the perturbation voltage is artificially defined. And the form of the current response can be obtained by mathematical modeling of the actual system.

grid-connected current of converter in wind power plant new energy power generation system at disturbance frequency f_presponse to (2)For disturbance voltageTransfer function (raw output admittance)) Is marked as Y_p(s). Grid-connected current at coupling frequency f_p-2f₁response to (2)For disturbance voltageThe transfer function (frequency coupling term) of (2), denoted as Y_c(s)。

response of grid-connected current at coupled frequency taking into account grid impedanceGenerating disturbance voltage of coupling frequency after flowing through network impedanceCurrent responseFor disturbance voltageHas a transfer function ofWhile the current is in responseFor disturbance voltagehas a transfer function of

Therefore, the impedance of the cascaded H-bridge SVG considering the frequency coupling effect can be defined as:

Second order matrixis a transformer impedance taking into account the frequency coupling effect, and an impedance not taking into account the frequency coupling effecty in (1)_p(s) only after taking frequency coupling effect into accountOf the other several components. Coupling is for example a change in a and b, which in turn affects a, the coupling between the quantities being expressed mathematically in the form of a matrix. The impedance of the transformer taking into account the frequency coupling effect is a matrix. Y is_p(s) and Y_c(s) is already present in the single-machine impedance and can be extracted from the matrix. After taking into account the impedance of the networkandThe same holds true for the stand-alone impedance. When there is no grid impedance present,Andcan be extracted directly from the single-machine impedance. But when there is a grid impedance present,Andwill affect the small signal harmonic of the converter, thereby affecting the Y in the single machine impedance_p(s) and Y_c(s) have an influence, so that when there is a grid impedance (this)where the grid impedance refers to the impedance connected to the converter, e.g. external equipment impedance or line impedance, etc.), it is necessary to consider Y_p(s) and Y_c(s) influence of the reaction. In the invention, the impedance considering the frequency coupling effect, namely Y is considered_p(s) and Y_c(s), but the prior art does not consider the frequency coupling effect and does not consider the grid impedance to Y_p(s) and Y_c(s) influence of the reaction. The invention is based on the basic principle of impedance modeling: assuming that a voltage disturbance is injected at the output side, then the current response is observed at the same point, the division of the two is the impedance, and the impedance considering the frequency coupling effect is establishedThe basic theory is consistent with the difference in establishing a single-machine impedance, in that the factors and angles considered are different. The current response of the coupling frequency, taking into account the network impedance, produces a voltage disturbance at the coupling frequency on the network impedance, which disturbance acts on the frequency coupling term impedance of the converter, producingAndThe existing few impedance modeling methods (harmonic linearization method, multi-harmonic linearization method and the like) can analyze and obtain the frequency coupling term in the impedance, but the influence of the existing stability analysis methods is not considered, and the frequency coupling effect can cause interaction influence between equipment and a power grid and has great influence on the conclusion of stability analysis.

The nonlinear control method of the power electronic equipment is characterized in that the nonlinearity of a control circuit of the power electronic equipment (partial devices in the power circuit are nonlinear), the nonlinearity of the control circuit of the power electronic equipment generally comprises two parts, namely 1, nonlinearity of a controller, the nonlinearity of a PI controller generally exists in the controller, the integral part of the PI controller is nonlinear 2, nonlinearity of a control structure, and a phase-locked loop structure and a structure for converting a three-phase rotating coordinate system into a two-phase rotating coordinate system are generally adopted in three-phase grid-connected equipmentSuch a structure has a condition of information extraction and omission in the transformation process, and is called non-linearity), the transformer has f at the applied frequency_pwill generate a frequency f different from the disturbance frequency_pCurrent response (i.e. generation of coupling frequency f)_p-2f₁Current response of (f)₁At the fundamental frequency, i.e., 50Hz line frequency), a phenomenon known as frequency coupling effects. The coupled frequency current response will produce a coupled frequency voltage across the grid impedance, which in turn will produce a disturbance frequency current response. Thus, considering the network impedance, the frequency coupling effect is equivalent to connecting an additional impedance in parallel to the original impedance of the transformer. And in the same way, the impedance of the wind generating set in the wind power plant considering frequency coupling under the stable operation condition can be obtained. Fig. 6 is an impedance equivalent circuit diagram considering the frequency coupling effect under the specific working condition of the cascaded H-bridge SVG. The impedance considering the frequency coupling effect under the specific working condition of the cascaded H-bridge SVG is the impedance of the converter considering the network impedance. The specific working condition is the actual working condition, and the impedance under the specific working condition can be obtained by substituting the steady-state control signal and the steady-state harmonic content of the electric parameter into the impedance. According to fig. 6, an impedance network equivalent circuit on a certain side of a node of a wind farm grid-connected system (wind farm new energy power generation system) can be obtained. Applying kirchhoff's law according to fig. 6, it can be obtained that the admittance of the converter becomes when the network impedance is consideredIn the formula (I), the compound is shown in the specification,for grid admittance at a coupling frequency, withThe frequency coupling effect of the converter will generate an extra response when the converter is connected to the power grid considering the network impedance, which is equivalent to the original impedance of the converter in parallel, and further may affect the stability of the network.The superscript asterisk of (a) indicates that the conjugation is being solved,And Y_g(s) are conjugate to each other, but the former is the admittance at the coupling frequency, the latter is the admittance at the disturbance frequency, the frequencies are different. In FIG. 6AndAnd the identity of the two is identical,AndEquivalent,(s) represents that under the s domain,Is the grid impedance at the coupling frequency.

the transformer impedance modeling theory adopts a harmonic linearization impedance modeling method, and the basic idea is that a disturbance voltage with a certain frequency is injected at the output side of the transformer, then the current response under the frequency is observed, and the division of the disturbance voltage and the current response is the impedance of the transformer at the frequency. Due to the frequency coupling effect, the same disturbance voltage can generate current response at another frequency after passing through the impedance of the converter. After synthesis, there is a positive order admittance coefficient matrix Y_s. Some corresponding items in the matrix are extracted to form a matrix Y(s).

step 404: and connecting the impedance considering the frequency coupling effect and the passive element impedance under the specific working condition of all the converters in the multi-machine multi-node power generation system network according to the network topology of the multi-machine multi-node power generation system to obtain the network node admittance matrix of the multi-machine multi-node power generation system.

FIG. 7 is an equivalent circuit diagram of a network node admittance matrix of the wind farm new energy power generation system. The network node admittance matrix is the impedance network. After impedance of a frequency coupling effect is considered under a specific working condition (actual working condition) of the cascaded H bridge SVG, the ground admittance of one node and the admittance between the nodes can be analyzed by combining the structure of the wind power plant new energy power generation system. Namely, the impedance of the frequency coupling effect needs to be substituted into the cascade H-bridge SVG under a specific working condition (actual working condition) when an impedance network matrix (network node admittance matrix) is constructed. The admittance between the nodes is the line impedance and the impedance characteristic of the transformer.

after the converter and the line impedance in the wind farm new energy power generation system suitable for network analysis are obtained, the ground admittance and the inter-node admittance of each node can be respectively calculated, and an impedance network on a certain side of the node in the system is obtained according to the calculated admittance. Generally, the impedance network equivalent circuit of the disturbance frequency and the coupling frequency has the same number of nodes as the actual power generation system, which are respectively numbered as 0, 1, 2 … n (wherein the node connected with the source side admittance is numbered as 0, that is, the node connected with the large power grid side admittance is numbered as 0, and is sequentially numbered away from the large power grid side), the converter impedance response of the i-th node (the converter impedance response connected to the power grid system is represented in the form of a matrix, but each element in the matrix has a correlation, which is represented in the form of a controlled current source connected in parallel with the original output admittance as shown in fig. 6) is represented in the form of a controlled current source connected in parallel with the converter admittance, the current source is a power source outputting a fixed current, the controlled current source is a current source controlled by other factors, which can be defined according to the actual system as shown in fig. 7, the current response to express the coupling frequency has an effect on the impedance characteristics of the converter connected to the power grid, and the admittance between the ith node and the jth node is expressed as the line impedance. The transformers in the network are also denoted as pi-type equivalent circuits, and are respectively included in the admittance of each node to the ground and the admittance between the nodes.

The wind farm new energy power generation system is equivalent according to the method, as can be seen in fig. 7, so that the admittance of the ith node to the ground can be defined in a 2 × 2 matrix form, and is related to the impedance of the node access converter:

Similarly, the admittance between the ith node and the jth node is defined as a 2 × 2 matrix:

In the formula, Y_ij(s) is disturbance frequency voltageAnd current responseRatio of (A) to (B), Y_ij(s-2jω₁) Perturbing voltage for coupling frequencyAnd coupled frequency current responseThe ratio of (a) to (b), i.e. the line impedance to the impedance representation of the transformer. When the transformer and cable are represented as pi-type (pi-type) equivalent circuits, then there is both ground and inter-node admittance to the connected nodes.

Y_NW(s) is the network node admittance matrix:

main diagonal element Y in the matrix_iiRepresenting the sum of the admittance of the branches connected to the ith node. Non-principal diagonal element Y_ijand the negative value of the admittance between the ith node and the jth node is obtained, and if the branch admittance does not exist, the zero matrix is 2 x 2. I.e. Y_i(s) and Y_ij(s) combining to obtain a network node admittance matrix of the target network.

Step 405: and obtaining the equivalent network impedance aggregated to the set node according to the network node admittance matrix.

The equivalent network impedance aggregated to the set node is a mathematical model including impedance information of all devices and elements on the set node side. The system network impedance is aggregated by building the system network impedance, and the aggregated impedance still comprises all information in the system, including node connection, impedance of each device and element and other information, and the aggregation simplifies the form, but does not simplify the content, so the stability of the system can be reflected by the result of a certain node. In brief, if the power generation system is stable, it indicates that the states of all nodes in the system are stable, and when the system is unstable, all nodes in the power generation system cannot stably operate, the stability conclusion of a certain node in the system is the same as the stability conclusion of the system, that is, if the node is stable, the system is stable, and if the node is unstable, the system is also unstable. The setting node may be any one of nodes 0 to n, and for convenience of calculation, the setting node is the 0 th node in this embodiment.

With reference to the structure diagram of fig. 7, a 0 th node voltage equation is established by using the 0 th node, and the 0 th node voltage equation can be expressed as:

in the formula (I), the compound is shown in the specification,is a network node voltage vector; y is₀(s) is the admittance vector between the 0 th node and other nodes in the network;The subscript p takes the value range of node numbers 0-n as the disturbance voltage vector.

in conjunction with the block diagram of fig. 7, the current flowing into the 0 th node (0 th node disturbance current equation) can be expressed as:

in the formula (I), the compound is shown in the specification,Is the sum of the admittances of node 0, Y₀(s) is the admittance of node 0 to ground, with T in the superscript referring to the transpose of the matrix.

substituting the 0 th node voltage equation into the 0 th node disturbance current equation can obtain:

in the formula, the-1 in the superscript indicates the inverse of the matrix.

Referring to fig. 3, in the 0 th node in the wind farm new energy power generation system in the figure, when all the impedances of the devices and the power devices are equivalently aggregated together as viewed from the left, or when all the impedances are aggregated together as viewed from the right, the equivalent network impedance aggregated to the 0 th node is defined as:

in which all elements are second order matrices, so Y_wf(s) is a second order matrix, and the result is expressed asForm (a). The numbers 11, 12, 21, 22 in the subscripts refer to the elements in the second order matrix, the former being rows and the latter being columns.

will be provided within (1)Can obtainThe disturbance current is divided by the disturbance voltage to obtain the admittance. The admittance and the impedance are reciprocal.

FIG. 8 is an impedance equivalent circuit diagram of the wind farm new energy power generation system when SVG impedance is converted to the wind farm side. In the figureAndAnd the identity of the two is identical,andand the identity of the two is identical,Andand the identity of the two is identical,AndAnd the identity of the two is identical,Refers to the disturbance voltage at the network side at the disturbance frequency, due toandbetween the grid impedance Z at the disturbance frequency_g(s) thereforeAndthe specific value of the voltage is changed. Referring to fig. 8, when the stability of the wind farm PCC node (bus #4 in fig. 3) in the wind power grid-connected system (wind farm new energy power generation system) is analyzed, if the impedance of the SVG is reduced toOn the wind farm side, the admittance matrix on the wind farm side can then be expressed as:

And the impedance matrix on the large grid side is represented as:

in the formula, Z_g(s) is the grid impedance (Z) at the disturbance frequency_g(s) is Z_g(s) an element of the matrix), Z_g(s-2jω₁) For the grid impedance at the coupling frequency, ω refers to the angular velocity (frequency) and 1 refers to the fundamental (i.e., power frequency 50 Hz).

From fig. 8, the expression of the network-entering current at the disturbance frequency is:

in the formula, Y_wf(s) and Y_wf(s) is different from Y_wf(s) is a common transfer function (i.e. Y)_wf(s) one element of the matrix),Is the disturbance voltage on the grid side at the disturbance frequency.

Wherein:

In the formula, Y_g(s-2jω₁) For grid admittance at coupled frequency, Y_g(s-2jω₁) Andthe phase sequence of (1) is different, wherein the former is positive sequence and the latter is negative sequence. In the theory of impedance analysis, in the case of a positive sequence of the disturbance voltage, some of the same impedance will show a negative sequence and some will show a negative sequence at different frequenciesIt appears as a positive sequence.

Therefore, when Z is_g(s)Y_wf(s) the system stabilizes when the Nyquist stability criterion is met.

Fig. 9 is an equivalent circuit diagram of an impedance network using bus #4 as an example. Referring to fig. 9, when SVG impedance is reduced to the wind farm side, a wind farm side network impedance equivalent circuit is shown in fig. 9(a), and a grid side network impedance equivalent circuit is shown in fig. 9 (b). The wind farm side admittance matrix can be obtained from fig. 9:

Y_wf(s)＝Y_svg(s)+Y_3T3(s)+Y_1T3(s)-Y_1T3(s)(Y_wi(s)+Y_Cf(s)+Y_2T3(s)+Y_1T3(s))^-1Y_1T3(s)

z_g(s)＝Y_g(s)^-1＝(Y_2T2(s)+Y_1T2(s)-y₀(s)^TY_NW(s)^-1y₀(s))^-1

Wherein:

y₀(s)^T＝[Y_1T2(s) 0]

In the formula, Y denotes an admittance matrix, subscript SVG denotes that the admittance matrix is an admittance matrix of equipment SVG, subscript wi denotes a fan converter, subscript Cf denotes a filter capacitor output by the fan converter, subscript includes T denotes a transformer in a system, the number behind T is the serial number of the transformer in the system, and the number in front denotes that the part is a certain part in pi-type equivalence of the transformer. Y is_g(s) denotes the grid admittance matrix, Y_Line(s) represents a line admittance matrix.

Step 406: and applying a generalized Nyquist stability criterion or a Nyquist stability criterion to the equivalent network impedance aggregated to the set node to evaluate the stability of the wind power plant new energy power generation system.

the generalized Nyquist stability criterion or the Nyquist stability criterion is an inherent algorithm in the classical control theory, impedance on two sides of a set node (the node is arranged on the left side and the right side) is combined according to the requirement of the criterion, a function is obtained, the function is called a transfer function in the control theory, the variable of the function is frequency, a curve of the function is drawn under the coordinate axis of a complex function according to the frequency and is called a Nyquist curve, the number of times the curve crosses a point (-j, 0) determines the stability of the system, j in the (-j, 0) is an imaginary unit, and the square of j is equal to-1. The intersection of the curve with the unit circle reflects the stability margin of the system.

Stability criteria such as generalized Nyquist or Nyquist and the like are applied to the impedance network (the equivalent network impedance aggregated to the 0 th node) of each node in the wind power plant new energy power generation system, and then the stability of the wind power plant new energy power generation system can be evaluated. The generalized Nyquist stability criterion and the Nyquist stability criterion belong to basic stability judgment methods, but the two methods need to be combined and used in a complex system. Equivalent wind farm side impedance Y due to frequency coupling_wf(s) and large grid side impedance Z_g(s) there may be a right half-plane pole, but this does not mean that the system is unstable. When wind power plant admittance matrix Y_wf(s) impedance matrix Z with large grid side_g(s) none of the elements has a right half-plane pole, and Z_g(s)Y_wfAnd(s) when the number of poles of the right half-plane is the same as the number of times of counterclockwise enclosing (-1,0j) points by the Nyquist curve, the system is still stable. Specifically, for the first case, Y_g(s-2jω₁)+Y_wf22Number of right half-plane poles of(s) and Z_g(s)Y_wfthe Nyquist curve of(s) is the same number of times that the point (1, 0j) surrounds counterclockwise, the system is stable; for the second case, 1+ Z_g22(s)Y_wf22(s)+Z_g21(s)Y_wf12(s)、1+Z_g22(s)Y_wf22(s)+Z_g12(s)Y_wf21(s)、Y_g22(s)+Y_wf22(s) and Z_g22(s)+Z_wf22Sum of poles of right half-plane of(s) four terms and Z_g(s)Y_wfNyquist curve of(s)the system is stable when the lines wrap counterclockwise the same number of times around the (-1,0j) point. In the above formula, Z denotes impedance, Y denotes admittance, g in subscript denotes grid, wf denotes wind farm, and numbers 11, 12, 21, 22 denote elements in a second-order matrix, the former being rows and the latter being columns. All quantities referred to in this invention are indicated as a matrix when they are bold and not italic, and are an ordinary transfer function (i.e., an element in a matrix) when they are italic only.

fig. 10 is a diagram of different stability analysis results of the wind farm new energy power generation system obtained by obtaining a characteristic root of a bus #4 impedance network according to a generalized nyquist stability criterion and adjusting impedance characteristics of a wind turbine inverter by modifying control parameters of the wind turbine inverter. Where the fan inverter current loop bandwidth in fig. 10(a) is 340Hz (phase margin of 66 °), and in fig. 10(b) is 400Hz (phase margin of 45 °). The generalized Nyquist curve in FIG. 10(a) is encircled with (-1,0j), which represents the instability of the wind farm new energy power generation system, wherein the oscillation frequencies predicted from the curve are 10Hz and 90 Hz; and the generalized Nyquist curve in FIG. 10(b) does not enclose (-1,0j), which shows that the new energy power generation system of the wind farm is stable. Fig. 11 is a graph of a voltage-current time-domain simulation waveform and a fourier analysis result of a bus #4 when the wind farm new energy power generation system is unstable, it can be seen that the wind farm new energy power generation system is unstable, subsynchronous/supersynchronous oscillation exists, and the main harmonic frequencies are 10Hz and 90Hz, which are consistent with the analysis result in fig. 10 (a). Fig. 12 is a voltage-current time-domain simulation waveform diagram of the bus #4 when the new energy power generation system of the wind farm is stable, and it can be seen that the new energy power generation system of the wind farm is stable, and the system is consistent with the analysis result in fig. 10 (b).

The invention provides a stability evaluation method and a stability evaluation system for a multi-machine multi-node power generation system based on an impedance network, wherein the stability of the multi-machine multi-node power generation system is accurately evaluated by establishing a converter impedance considering a frequency coupling effect under a specific working condition. The frequency coupling effect and the specific working condition are two factors, and if the two factors are not considered, the stability of the multi-node power system of the multi-machine cannot be accurately evaluated. In engineering practice, only single-machine stability can be ensured in the design of the controllers of the converters, but the overall stability of the system cannot be ensured due to interaction in a complex system. The traditional stability design usually requires a 30-degree stability margin for the system to ensure the stability, and the conclusion is not in line with the theory, and the main reason is that the interaction effect in the system, namely the coupling effect of the actual working condition and the frequency, cannot be considered. The invention researches the influence of the frequency coupling effect, finds the relevance of the interaction influence between the frequency coupling effect and the equipment, and then expresses the interaction influence in the form of an impedance network and simplifies the interaction influence, thereby being capable of carrying out accurate system stability analysis.

The embodiments in the present description are described in a progressive manner, each embodiment focuses on differences from other embodiments, and the same and similar parts among the embodiments are referred to each other. For the system disclosed by the embodiment, the description is relatively simple because the system corresponds to the method disclosed by the embodiment, and the relevant points can be referred to the system part for description.

The principles and embodiments of the present invention have been described herein using specific examples, which are provided only to help understand the method and the core concept of the present invention; meanwhile, for a person skilled in the art, according to the idea of the present invention, the specific embodiments and the application range may be changed. In view of the above, the present disclosure should not be construed as limiting the invention.

Claims

1. A stability evaluation method of a multi-machine multi-node electric system is characterized by comprising the following steps:

2. The method for evaluating the stability of a multi-machine multi-node electrical power system according to claim 1, wherein the step of calculating the steady-state control signal of the converter and the steady-state harmonic content of the electrical parameter connected to each node according to the voltage amplitude, the phase and the power of the node to obtain the steady-state control signal of each converter and the steady-state harmonic content of the electrical parameter comprises the following steps:

3. The method for evaluating the stability of a multi-machine multi-node electrical power system according to claim 1, wherein the establishing of the impedance of each converter considering the frequency coupling effect, the substituting of the steady-state control signal and the steady-state harmonic content of the electrical parameter of each converter into the impedance of the corresponding converter considering the frequency coupling effect to obtain the impedance of each converter considering the frequency coupling effect under the specific working condition specifically comprises:

4. the method for evaluating the stability of a multi-machine multi-node electrical power generation system according to claim 1, wherein the impedance considering the frequency coupling effect and the impedance of the passive component under the specific working condition of all the converters in the multi-machine multi-node electrical power generation system network are connected according to the network topology of the multi-machine multi-node electrical power generation system to obtain the network node admittance matrix of the multi-machine multi-node electrical power generation system, specifically comprising:

5. The method for evaluating the stability of a multi-machine multi-node electrical system according to claim 1, wherein the obtaining of the equivalent network impedance aggregated to the set node according to the network node admittance matrix specifically comprises:

6. A stability evaluation system of a multi-machine multi-node electrical system, comprising:

7. The system for evaluating the stability of a multi-machine multi-node electrical system according to claim 6, wherein the module for calculating the steady-state harmonic content of the steady-state control signal and the electrical parameter comprises:

8. the system for evaluating the stability of a multi-machine multi-node electrical system of claim 6, wherein the impedance establishing module specifically comprises:

9. the system for evaluating the stability of a multi-machine multi-node electrical system according to claim 6, wherein the network node admittance matrix establishing module specifically comprises:

10. the system for evaluating the stability of a multi-machine multi-node electrical system of claim 6, wherein the aggregation module specifically comprises: