Multi-level direct current sending end small disturbance stability evaluation method, device, equipment and medium

Abstract

The present application relates to the technical field of power system, discloses a kind of multi-level DC sending end small signal stability evaluation method, device, equipment and medium, the present application considers the coupling influence between different heterogeneous devices, to network type new energy converter, network type energy storage converter, network type load converter and modular multi-level high voltage direct current converter station are carried out frequency domain impedance modeling, and based on the equivalent impedance model of each device, the frequency domain impedance network of all-electric system is constructed, the small signal stability of frequency domain impedance network is analyzed using generalized Nyquist stability criterion, the Nyquist diagram of impedance ratio on the two sides of voltage source and current source in frequency domain impedance network is determined The small signal stability of all-electric system under AC / DC hybrid scene, so as to provide an effective evaluation means for the stable operation of AC / DC hybrid all-electric system.

Description

Technical Field

[0001] This invention relates to the field of power system technology, and in particular to a method, apparatus, equipment, and medium for evaluating the stability of a multi-level DC sending end under small disturbances. Background Technology

[0002] Currently, energy storage and high-voltage direct current (HVDC) transmission have been widely developed to address the volatility of new energy power generation and the spatial inverse distribution of power generation and consumption. New energy sources, energy storage, and HVDC are all connected to the grid through power electronic devices. The complexity of control and the coupling between heterogeneous devices pose significant challenges to the stability of the AC / DC hybrid system they form.

[0003] Within a given region, complex coupling relationships may exist between different types of equipment and between different control links, posing potential resonant stability risks over a wide frequency band. After a large number of power electronic devices are connected to the grid, the coupling interaction of multiple devices can trigger multimodal oscillation problems. Furthermore, the internal dynamics of a Modular Multilevel Converter (MMC) are complex; the steady-state value of the variable is no longer a single DC component, but rather a mixture of multiple frequencies, making simple dq modeling inaccurate.

[0004] For wide-band, small-disturbance stability analysis, current methods typically simplify the impedance of an infinite power grid to an ideal inductance. This differs significantly from the equivalent impedance of various types of equipment in actual operation. Particularly in fully power electronic equipment systems, this discrepancy leads to biases in stability analysis results, resulting in considerable limitations. Summary of the Invention

[0005] In view of this, in order to solve the above-mentioned technical problems, the present invention provides a method, apparatus, equipment and medium for evaluating the stability of small disturbances at a multi-level DC sending end.

[0006] The first aspect of this invention provides a method for evaluating the stability of a multi-level DC sending end under small disturbances, applicable to all-electric systems in AC / DC hybrid scenarios. The all-electric system includes a grid-connected renewable energy converter, a grid-connected energy storage converter, a grid-connected load converter, and a modular multi-level high-voltage DC converter station. The method includes:

[0007] Frequency domain impedance modeling was performed on the grid-connected new energy converter, the grid-connected energy storage converter, the grid-connected load converter, and the modular multilevel high-voltage DC converter station respectively to obtain the equivalent impedance models corresponding to the grid-connected new energy converter, the grid-connected energy storage converter, the grid-connected load converter, and the modular multilevel high-voltage DC converter station respectively.

[0008] Based on the aforementioned equivalent impedance models, a frequency domain impedance network for the all-electric system is constructed.

[0009] The small-disturbance stability of the frequency domain impedance network is analyzed using the generalized Nyquist stability criterion. The small-disturbance stability of the all-electric system in the AC / DC hybrid scenario is determined by the Nyquist plot of the impedance ratio on both sides of the voltage and current sources in the frequency domain impedance network.

[0010] Preferably, the equivalent impedance model of the grid-connected new energy converter is expressed as:

[0011]

[0012] In the formula, Y gfl_vdc and Z gfl_vdc These are the equivalent admittance and equivalent impedance of the grid-connected new energy converter, respectively. , Let be the voltage and current disturbance vectors of the system at the grid connection point port, respectively, and I be the identity matrix. Let be the transfer function for transforming the grid-connected point voltage in the controller coordinate system and the system coordinate system, wherein the controller coordinate system is a coordinate system referenced to the control process of the converter itself, and the system coordinate system is a common reference coordinate system based on the entire electric system. Let be the transfer function of the grid-connected point current in the controller coordinate system and the system coordinate system. G is the transfer function of the output voltage transformation in the controller coordinate system and the system coordinate system. ci1 Let G be the transfer function from the disturbance at the outer current loop output to the disturbance at the grid connection point voltage. dei1 H is the transfer function from current disturbance to grid connection point voltage disturbance. vdc (s) is the controller of the voltage control loop, G dcv Let G be the transfer function from system voltage disturbance to DC voltage disturbance. dcvo G is the transfer function from the DC-side voltage disturbance to the grid connection point voltage disturbance. dci Z is the transfer function from system current disturbance to DC voltage disturbance. l1 This is the impedance matrix of the filter inductor along the dq axis;

[0013] in,

[0014]

[0015]

[0016]

[0017]

[0018]

[0019]

[0020]

[0021]

[0022] In the formula, , These represent the q-axis voltage component and d-axis voltage component of the grid connection point in steady state, respectively, in the system coordinate system. Let be the transfer function of the grid connection point voltage disturbance to the phase angle. , These represent the q-axis voltage component and d-axis voltage component of the grid connection point in steady state, respectively, in the system coordinate system. , These represent the q-axis output voltage component and the d-axis output voltage component of the grid-connected point in steady state, respectively, in the system coordinate system. This is the DC-side reference voltage. The controller is for the current control loop. Angular velocity, For filtering inductors, This refers to the discharge current output from the DC-side capacitor to the converter. For the Laplace operator, , These represent the equivalent capacitance on the DC side and the voltage at the DC converter port, respectively; where:

[0023]

[0024] In the formula, This is the transfer function of the phase-locked loop.

[0025] Preferably, the equivalent impedance model of the grid-type energy storage converter is:

[0026]

[0027] In the formula, The equivalent impedance of a grid-type energy storage converter. , These are the voltage and current disturbance vectors at the grid connection port of the grid-connected energy storage converter, respectively. Let G be the transfer function from the disturbance at the outer current loop output to the disturbance at the grid connection point voltage. v Let G be the transfer function from voltage disturbance in the controller coordinate system to current disturbance in the controller coordinate system. UU Let G be the transfer function of the voltage in the system coordinate system and the voltage in the control coordinate system. IU To control the transfer function of current in the control coordinate system and voltage in the system coordinate system, G UILet G be the transfer function of current in the system coordinate system and voltage in the control coordinate system. VoI Let G be the transfer function of the output voltage in the system coordinate system and the current in the system coordinate system. II To control the transfer function of the current in the control coordinate system and the current in the system coordinate system, G VoV Let G be the transfer function between the output voltage in the system coordinate system and the voltage in the system coordinate system. dei2 Let I be the transfer function from current disturbance to grid-connected point voltage disturbance, and let I be the identity matrix. This is the impedance matrix of the filter inductor along the dq axis;

[0028] in, , , , , , , , ,

[0029]

[0030]

[0031] In the formula, The controller for the current control loop of a grid-type energy storage converter. For filtering inductors, Angular velocity, Let be the transfer function of phase angle disturbance from the system coordinate system to the voltage in the controller coordinate system. Let be the transfer function from the current disturbance to the phase angle disturbance in the controller coordinate system. Let be the transfer function from voltage disturbance to phase angle disturbance in the controller coordinate system. Let be the transfer function from the phase angle disturbance in the system coordinate system to the current disturbance in the controller coordinate system. Let be the transfer function from the phase angle disturbance in the system coordinate system to the voltage disturbance at the output terminal in the system coordinate system. The controller is for the AC voltage control loop; where:

[0032] , , ,

[0033] ,

[0034] In the formula, , These represent the q-axis steady-state voltage component and the d-axis steady-state voltage component of the grid-connected energy storage converter in the dq coordinate system during steady-state operation. , These represent the q-axis steady-state voltage component and the d-axis steady-state current component of the grid-connected energy storage converter in the dq coordinate system during steady-state operation. , These represent the q-axis and d-axis output voltages of the grid-type energy storage converter in the system coordinate system. Let be the transfer function from power disturbance to phase angle disturbance. , Let be the transfer functions of voltage and current to power disturbance in the controller coordinate system, respectively; where:

[0035] , ,

[0036] In the formula, For virtual inertia coefficients, This is the virtual droop coefficient.

[0037] Preferably, the equivalent impedance model of the grid-connected load converter is:

[0038]

[0039] To match the equivalent admittance of the load section of the grid-type load converter, , These are the system voltage and current disturbance vectors at the grid connection point port of the grid-connected load converter, respectively. , These are the transfer functions from system coordinate system voltage and current disturbances to port voltages, respectively. It is the identity matrix. The line impedance is the line impedance connected to the grid load; where:

[0040]

[0041]

[0042] In the formula, G pllv3 G plli3 G pllvo3 These are the transfer functions for voltage, current, and output voltage transformations in the controller coordinate system and the system coordinate system, respectively. Let G be the transfer function of the current decoupling part. ci3 Let G be the transfer function of the current loop. PQ G is the transfer function of the proportional-integral controller in the power loop. SU and G SI Let be the transfer functions of voltage and current disturbances to the power detection disturbance, respectively; where:

[0043]

[0044]

[0045] In the formula, The controller for the current control loop of the grid-type load converter. For filtering inductors, The transfer function for the proportional-integral controller in the outer loop of power control. Angular velocity, For the Laplace operator, , These are the reference values ​​for the inner loop currents d and q, respectively. , These are the d-axis and q-axis voltage components in the controller, respectively. Let be the transfer function of the phase-locked loop in power control mode. , These are the d-axis and q-axis voltage components of the output voltage, respectively; where:

[0046]

[0047] In the formula, This is the transfer function of the phase-locked loop.

[0048] Preferably, the equivalent impedance model of the modular multilevel high-voltage DC converter station is as follows:

[0049]

[0050] In the formula, The dq-axis admittance of the fourth harmonic of a modular multilevel high-voltage DC converter station. Here is the Park transformation matrix. The admittance of a modular multilevel high-voltage DC converter station under positive and negative sequence conditions. For imaginary units; where:

[0051]

[0052] In the formula, k mmc Y is the transformer turns ratio. sys Let be the admittance matrix. , , , All are the middle elements of the admittance under positive and negative sequence of modular multilevel high-voltage DC converter stations; where:

[0053]

[0054] In the formula, , , , Let be the transfer functions from voltage input to system state variable output for systems with dimensions of 52×13, 52×8, 29×13, and 29×8, respectively; where:

[0055]

[0056] In the formula, For dimension × The transfer function from the system voltage input to the system state variable output. The values ​​are (52×13), (52×8), (29×13), or (29×8), where s is the Laplace operator and I is the identity matrix. Let A be the state space matrix. s The Toplitz matrix, M is the state space matrix c The Toplitz matrix, B is the state space matrix ac The Toplitz matrix, Let I be a diagonal matrix with diagonal terms of -jnw0I. , These are the transfer functions from the harmonicized state variables and voltage variables to the duty cycle disturbance components, respectively.

[0057] Preferably, constructing the frequency domain impedance network of the all-electric system based on each of the equivalent impedance models includes:

[0058] The coordinate system of the grid connection point of the grid-connected energy storage converter is used as the global coordinate system of the system. Based on the global coordinate system, the coordinate system of the equivalent admittance in the equivalent impedance model of the grid-connected new energy converter, the grid-connected load converter, and the modular multilevel high voltage DC converter station is unified.

[0059] The grid-connected new energy converter, the grid-connected load converter, and the modular multilevel high-voltage DC converter station, after unifying the coordinate system, are connected in parallel as current source devices. The total equivalent impedance of the current source devices is obtained by performing parallel calculations based on the equivalent impedance models corresponding to the grid-connected new energy converter, the grid-connected load converter, and the modular multilevel high-voltage DC converter station, respectively.

[0060] The grid-type energy storage converter is used as a voltage source device, and the equivalent impedance of the voltage source device is determined according to the equivalent impedance model of the grid-type energy storage converter.

[0061] The equivalent circuits are connected in series based on the total equivalent impedance of the current source device and the equivalent impedance of the voltage source device to form the frequency domain impedance network of the all-electric system.

[0062] Preferably, the step of performing small-perturbation stability analysis on the frequency domain impedance network using the generalized Nyquist stability criterion, and determining the small-perturbation stability of the all-electric system in an AC / DC hybrid scenario through the Nyquist plot of the impedance ratios on both sides of the voltage and current sources within the frequency domain impedance network, includes:

[0063] Based on the frequency domain impedance network of the all-electric system, and combining the total equivalent impedance of the current source device and the equivalent impedance of the voltage source device, the impedance ratio of the all-electric system is determined.

[0064] The Nyquist plot is drawn based on the impedance ratio of the all-electric system, and the Nyquist curve of the impedance ratio of the all-electric system is obtained.

[0065] Determine whether the Nyquist curve intersects the negative real axis;

[0066] If the Nyquist curve intersects with the negative real axis, then the all-electric system is determined to have a risk of small disturbance instability.

[0067] If the Nyquist curve does not intersect with the negative real axis, then the all-electric system is determined to be free from small disturbance instability risk.

[0068] Secondly, the present invention also provides a multi-level DC sending-end small disturbance stability assessment device, applied to a full-electric system in an AC / DC hybrid scenario. The full-electric system includes a grid-connected new energy converter, a grid-connected energy storage converter, a grid-connected load converter, and a modular multi-level high-voltage DC converter station. The device includes:

[0069] The equivalent impedance construction module is used to perform frequency domain impedance modeling on the grid-connected new energy converter, the grid-connected energy storage converter, the grid-connected load converter, and the modular multilevel high-voltage DC converter station, respectively, to obtain the equivalent impedance models corresponding to the grid-connected new energy converter, the grid-connected energy storage converter, the grid-connected load converter, and the modular multilevel high-voltage DC converter station.

[0070] An impedance network construction module is used to construct the frequency domain impedance network of the all-electric system based on the equivalent impedance models described above.

[0071] The stability assessment module is used to perform small disturbance stability analysis on the frequency domain impedance network using the generalized Nyquist stability criterion. By using the Nyquist plot of the impedance ratio on both sides of the voltage source and current source in the frequency domain impedance network, the small disturbance stability of the all-electric system in the AC / DC hybrid scenario is determined.

[0072] Thirdly, the present invention also provides an electronic device, the electronic device including a memory and a processor, the memory storing a computer program, the computer program being executed by the processor causing the processor to perform the steps of the multi-level DC sending end small disturbance stability assessment method as described in the first aspect.

[0073] Fourthly, the present invention also provides a computer-readable storage medium having a computer program stored thereon, which, when executed, implements the steps of the multi-level DC transmitter small disturbance stability assessment method as described in the first aspect.

[0074] As can be seen from the above technical solutions, this invention considers the coupling effects between different heterogeneous devices, performs frequency domain impedance modeling for grid-connected new energy converters, grid-connected energy storage converters, grid-connected load converters, and modular multilevel high-voltage DC converter stations, and constructs a frequency domain impedance network for the entire electric system based on the equivalent impedance models of each device. It then uses the generalized Nyquist stability criterion to perform small-disturbance stability analysis on the frequency domain impedance network. By using the Nyquist plot of the impedance ratios on both sides of the voltage and current sources within the frequency domain impedance network, the small-disturbance stability of the entire electric system in AC / DC hybrid scenarios is determined. This provides an effective evaluation method for the stable operation of AC / DC hybrid electric systems. Simultaneously, it removes the assumption of an infinitely large ideal power grid, reducing the bias in the stability analysis results. It is applicable to different types of AC / DC hybrid electric systems and has good versatility. Attached Figure Description

[0075] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0076] Figure 1 This is an application environment diagram of a multi-level DC transmitter small disturbance stability evaluation method provided in an embodiment of the present invention;

[0077] Figure 2 A flowchart of a method for evaluating the stability of a multi-level DC transmitter under small disturbances, provided in an embodiment of the present invention;

[0078] Figure 3 A schematic diagram of the structure of a full-electric system in a typical AC / DC hybrid scenario in the Shago desert region;

[0079] Figure 4 A schematic diagram of a typical new energy transmission topology in the Shago Desert region;

[0080] Figure 5The control block diagram for a grid-connected renewable energy converter;

[0081] Figure 6 This is a control block diagram of a grid-type energy storage converter;

[0082] Figure 7 Here is the control block diagram for a grid-type load converter;

[0083] Figure 8 This is a control block diagram for MMC;

[0084] Figure 9 A diagram showing the relationship between different coordinate systems in the system;

[0085] Figure 10 Frequency domain equivalent circuit diagram for the Shago desert region;

[0086] Figure 11 This is the equivalent diagram of the frequency domain impedance network;

[0087] Figure 12 The Nyquist curve for the voltage loop proportional coefficient of a grid-type energy storage system varies;

[0088] Figure 13 A schematic diagram of a multi-level DC transmitter small disturbance stability evaluation device provided in an embodiment of the present invention;

[0089] Figure 14 This is a schematic diagram of the structure of an electronic device provided in an embodiment of the present invention. Detailed Implementation

[0090] To enable those skilled in the art to better understand the present invention, the technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and 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.

[0091] The multi-level DC transmitter small disturbance stability evaluation method provided in this application embodiment can be applied to, for example... Figure 1In the application environment shown, terminal 101 communicates with server 102 via a network. A data storage system can store the data that server 102 needs to process. The data storage system can be integrated onto server 102 or placed on the cloud or other network servers. Terminal 101 or server 102 obtains the node connection information and branch information of the distribution network to be acquired through the communication network. Based on the node connection information and branch information, it constructs a node-branch correlation matrix of the distribution network, obtains the root current flowing through the root node and the first current flowing through the first node, then obtains the first ratio corresponding to the root current and the first current, and the switching change coefficient corresponding to the first node. If the relationship between the first ratio and the switching change coefficient changes, the node-branch correlation matrix is ​​updated to obtain the updated node-branch correlation matrix. Based on the updated node-branch correlation matrix, a breadth-first search is performed to obtain the network topology of the distribution network.

[0092] Terminal 101 can be, but is not limited to, various personal computers, laptops, smartphones, and tablets.

[0093] Server 102 can be a standalone physical server, a server cluster or distributed system consisting of multiple physical servers, or a cloud server that provides cloud computing services.

[0094] This application provides a method for evaluating the stability of a multi-level DC power transmission end under small disturbances, applicable to all-electric systems in AC / DC hybrid scenarios. The all-electric system includes grid-connected renewable energy converters, grid-connected energy storage converters, grid-connected load converters, and modular multi-level high-voltage DC converter stations. For example... Figure 2 As shown, this method is applied to Figure 1 Taking terminal 101 or server 102 as an example, the method includes the following steps S1 to S3. Specifically, the multi-level DC transmitter small disturbance stability assessment method provided in this application embodiment includes:

[0095] Step S1: Perform frequency domain impedance modeling for grid-connected new energy converters, grid-connected energy storage converters, grid-connected load converters, and modular multilevel high-voltage DC converter stations respectively to obtain the equivalent impedance models corresponding to the grid-connected new energy converters, grid-connected energy storage converters, grid-connected load converters, and modular multilevel high-voltage DC converter stations.

[0096] Among them, grid-synchronized new energy converters refer to new energy power generation equipment converters that can track the voltage and frequency of the power grid. They adjust their output power by monitoring the voltage and frequency information of the power grid in real time to achieve synchronous operation with the grid. Grid-based energy storage converters have the ability to build and maintain the voltage and frequency of the power grid, playing a role in stabilizing the power grid in the whole electric system. Grid-synchronized load converters, as key equipment connecting loads and the power grid, also have a significant impact on the stability of the power grid. Modular multilevel high-voltage direct current converter stations have features such as multiple output levels and modular structure, enabling efficient and reliable operation of high-voltage direct current transmission. When modeling their frequency domain impedance, factors such as their topology, control method, and energy loss during the conversion process must be comprehensively considered to ensure that the constructed equivalent impedance model can accurately reflect their electrical characteristics in the whole electric system.

[0097] Step S2: Based on each equivalent impedance model, construct the frequency domain impedance network of the all-electric system.

[0098] When constructing the frequency domain impedance network of the entire electric system, it is important to clarify that this network is composed of the equivalent impedance models of various devices (grid-connected new energy converters, grid-connected energy storage converters, grid-connected load converters, and modular multilevel high-voltage DC converter stations) through a specific connection method. This connection method manifests in the frequency domain as a series or parallel impedance relationship, depending on the actual connection of the devices in the power grid.

[0099] Step S3: Perform small disturbance stability analysis on the frequency domain impedance network using the generalized Nyquist stability criterion. Determine the small disturbance stability of the all-electric system in the AC / DC hybrid scenario by using the Nyquist plot of the impedance ratio on both sides of the voltage source and current source in the frequency domain impedance network.

[0100] The generalized Nyquist stability criterion is a stability assessment method based on frequency domain analysis. It accurately determines the stability of a hybrid AC / DC system under small disturbances by analyzing the Nyquist plot of the system's impedance ratio. Specifically, this method first calculates the impedance ratios on both sides of the voltage and current sources in the frequency domain impedance network. These impedance ratios reflect the dynamic interaction characteristics between different parts of the system. Then, a Nyquist plot is plotted based on the calculated impedance ratios; this is a graph describing the system's frequency response characteristics on the complex plane. By observing whether the Nyquist curve encircles a critical point (such as the (-1,0) point) and the intersection of the curve with the negative real axis, it can be determined whether the system is at risk of instability under small disturbances.

[0101] It should be noted that, by considering the coupling effects between different heterogeneous devices, this application performs frequency domain impedance modeling for grid-connected new energy converters, grid-connected energy storage converters, grid-connected load converters, and modular multilevel high-voltage DC converter stations. Based on the equivalent impedance models of each device, a frequency domain impedance network of the entire electric system is constructed. The generalized Nyquist stability criterion is used to perform small-disturbance stability analysis on the frequency domain impedance network. By using the Nyquist plot of the impedance ratios on both sides of the voltage and current sources within the frequency domain impedance network, the small-disturbance stability of the entire electric system in the AC / DC hybrid scenario is determined. This provides an effective evaluation method for the stable operation of AC / DC hybrid electric systems. At the same time, the assumption of an infinite ideal power grid is removed, reducing the bias of the stability analysis results. It is applicable to different types of AC / DC hybrid electric systems and has good versatility.

[0102] In some embodiments, the equivalent impedance model of a grid-connected renewable energy converter is expressed as:

[0103]

[0104] In the formula, Y gfl_vdc and Z gfl_vdc These are the equivalent admittance and equivalent impedance of the grid-connected new energy converter, respectively. , Let be the voltage and current disturbance vectors of the system at the grid connection point port, respectively, and I be the identity matrix. Let be the transfer function for transforming the grid-connected point voltage in the controller coordinate system and the system coordinate system. The controller coordinate system is a coordinate system referenced to the converter's own control process, while the system coordinate system is a common reference coordinate system based on the entire electric system. Let be the transfer function of the grid-connected point current in the controller coordinate system and the system coordinate system. G is the transfer function of the output voltage transformation in the controller coordinate system and the system coordinate system. ci1 Let G be the transfer function from the disturbance at the outer current loop output to the disturbance at the grid connection point voltage. dei1 H is the transfer function from current disturbance to grid connection point voltage disturbance. vdc (s) is the controller of the voltage control loop, G dcv Let G be the transfer function from system voltage disturbance to DC voltage disturbance. dcvo G is the transfer function from the DC-side voltage disturbance to the grid connection point voltage disturbance. dci Z is the transfer function from system current disturbance to DC voltage disturbance. l1 This is the impedance matrix of the filter inductor along the dq axis;

[0105] in,

[0106]

[0107]

[0108]

[0109]

[0110]

[0111]

[0112]

[0113]

[0114] In the formula, , These represent the q-axis voltage component and d-axis voltage component of the grid connection point in steady state, respectively, in the system coordinate system. Let be the transfer function of the grid connection point voltage disturbance to the phase angle. , These represent the q-axis voltage component and d-axis voltage component of the grid connection point in steady state, respectively, in the system coordinate system. , These represent the q-axis output voltage component and the d-axis output voltage component of the grid-connected point in steady state, respectively, in the system coordinate system. This is the DC-side reference voltage. The controller is for the current control loop. Angular velocity, For filtering inductors, This refers to the discharge current output from the DC-side capacitor to the converter. For the Laplace operator, , These represent the equivalent capacitance on the DC side and the voltage at the DC converter port, respectively; where:

[0115]

[0116] In the formula, This is the transfer function of the phase-locked loop.

[0117] In some embodiments, the equivalent impedance model of a grid-type energy storage converter is:

[0118]

[0119] In the formula, The equivalent impedance of a grid-type energy storage converter. , These are the voltage and current disturbance vectors at the grid connection port of the grid-connected energy storage converter, respectively. Let G be the transfer function from the disturbance at the outer current loop output to the disturbance at the grid connection point voltage. v Let G be the transfer function from voltage disturbance in the controller coordinate system to current disturbance in the controller coordinate system. UU Let G be the transfer function of the voltage in the system coordinate system and the voltage in the control coordinate system. IU To control the transfer function of current in the control coordinate system and voltage in the system coordinate system, G UI Let G be the transfer function of current in the system coordinate system and voltage in the control coordinate system. VoI Let G be the transfer function of the output voltage in the system coordinate system and the current in the system coordinate system. II To control the transfer function of the current in the control coordinate system and the current in the system coordinate system, G VoV Let G be the transfer function between the output voltage in the system coordinate system and the voltage in the system coordinate system. dei2 Let I be the transfer function from current disturbance to grid-connected point voltage disturbance, and let I be the identity matrix. This is the impedance matrix of the filter inductor along the dq axis;

[0120] in, , , , , , , , ,

[0121]

[0122]

[0123] In the formula, The controller for the current control loop of a grid-type energy storage converter. For filtering inductors, Angular velocity, Let be the transfer function of phase angle disturbance from the system coordinate system to the voltage in the controller coordinate system. Let be the transfer function from the current disturbance to the phase angle disturbance in the controller coordinate system. Let be the transfer function from voltage disturbance to phase angle disturbance in the controller coordinate system. Let be the transfer function from the phase angle disturbance in the system coordinate system to the current disturbance in the controller coordinate system. Let be the transfer function from the phase angle disturbance in the system coordinate system to the voltage disturbance at the output terminal in the system coordinate system. The controller is for the AC voltage control loop; where:

[0124] , , ,

[0125] ,

[0126] In the formula, , These represent the q-axis steady-state voltage component and the d-axis steady-state voltage component of the grid-connected energy storage converter in the dq coordinate system during steady-state operation. , These represent the q-axis steady-state voltage component and the d-axis steady-state current component of the grid-connected energy storage converter in the dq coordinate system during steady-state operation. , These represent the q-axis and d-axis output voltages of the grid-type energy storage converter in the system coordinate system. Let be the transfer function from power disturbance to phase angle disturbance. , Let be the transfer functions of voltage and current to power disturbance in the controller coordinate system, respectively; where:

[0127] , ,

[0128] In the formula, For virtual inertia coefficients, This is the virtual droop coefficient.

[0129] In some embodiments, the equivalent impedance model of the grid-connected load converter is:

[0130]

[0131] To match the equivalent admittance of the load section of the grid-type load converter, , These are the system voltage and current disturbance vectors at the grid connection point port of the grid-connected load converter, respectively. , These are the transfer functions from system coordinate system voltage and current disturbances to port voltages, respectively. It is the identity matrix. The line impedance is the line impedance connected to the grid load; where:

[0132]

[0133]

[0134] In the formula, G pllv3 G plli3 G pllvo3 These are the transfer functions for voltage, current, and output voltage transformations in the controller coordinate system and the system coordinate system, respectively. Let G be the transfer function of the current decoupling part. ci3 Let G be the transfer function of the current loop. PQ G is the transfer function of the proportional-integral controller in the power loop. SU and G SI Let be the transfer functions of voltage and current disturbances to the power detection disturbance, respectively; where:

[0135]

[0136]

[0137] In the formula, The controller for the current control loop of the grid-type load converter. For filtering inductors, The transfer function for the proportional-integral controller in the outer loop of power control. Angular velocity, For the Laplace operator, , These are the reference values ​​for the inner loop currents d and q, respectively. , These are the d-axis and q-axis voltage components in the controller, respectively. Let be the transfer function of the phase-locked loop in power control mode. , These are the d-axis and q-axis voltage components of the output voltage, respectively; where:

[0138]

[0139] In the formula, This is the transfer function of the phase-locked loop.

[0140] In some embodiments, the equivalent impedance model of a modular multilevel high-voltage direct current converter station is:

[0141]

[0142] In the formula, The dq-axis admittance of the fourth harmonic of a modular multilevel high-voltage DC converter station. Here is the Park transformation matrix. The admittance of a modular multilevel high-voltage DC converter station under positive and negative sequence conditions. For imaginary units; where:

[0143]

[0144] In the formula, k mmc Y is the transformer turns ratio. sys Let be the admittance matrix. , , , All are the middle elements of the admittance under positive and negative sequence of modular multilevel high-voltage DC converter stations; where:

[0145]

[0146] In the formula, , , , Let be the transfer functions from voltage input to system state variable output for systems with dimensions of 52×13, 52×8, 29×13, and 29×8, respectively; where:

[0147]

[0148] In the formula, For dimension × The transfer function from the system voltage input to the system state variable output. The values ​​are (52×13), (52×8), (29×13), or (29×8), where s is the Laplace operator and I is the identity matrix. Let A be the state space matrix. s The Toplitz matrix, M is the state space matrix c The Toplitz matrix, B is the state space matrix ac The Toplitz matrix, Let I be a diagonal matrix with diagonal terms of -jnw0I. , These are the transfer functions from the harmonicized state variables and voltage variables to the duty cycle disturbance components, respectively.

[0149] For example, such as Figure 3 The diagram shows a typical renewable energy transmission topology in the Shago Desert region. The Shago Desert system includes at least a grid-connected renewable energy converter, a grid-connected energy storage converter, a grid-connected load converter, and a modular multilevel high-voltage DC converter station. The simplified circuit model for this region is as follows: Renewable Energy Converter G FL_Vdc Energy storage converter G FM_PV Load converter G FL_PQ After filtering inductor L f1 L f2 L f3 And the transformer is connected to the power grid, such as Figure 4 As shown. Since the leakage reactance of a step-up transformer is generally small, a simplification is made, directly converting the per-unit values ​​of new energy sources, energy storage, and loads to the high-voltage side for simulation. The Modular Multilevel Converter (MMC) passes through a transformer and R... gr L grIn the grid structure, transformers in high-voltage direct current (HVDC) transmission equipment typically have large leakage reactance (0.1~0.2 pu), which is retained in the simulation. On the other hand, the DC side of energy storage and loads usually has bidirectional DC / DC converters, whose DC-side voltage can remain constant over a large frequency range. Therefore, voltage variations on the DC side of energy storage and loads are ignored, and voltage sources are used for simulation. For new energy sources with an approximate constant current source on the DC side, their dynamic characteristics are simulated by a DC capacitor and a constant current source. In addition to constant power loads, there are also constant impedance loads in the system, represented by Rload in the circuit topology. Since there is no infinite power grid, the frequency in this region is mainly regulated by grid-connected energy storage and MMC (Multi-mode Capacitor). By changing the power command of the energy storage and MMC, the frequency in this region is stabilized at 50Hz.

[0150] The control block diagram of the grid-connected new energy converter is as follows: Figure 5 As shown, the control loop of a grid-connected renewable energy converter mainly includes a phase-locked loop (PLL), a current loop, and a voltage control outer loop. The PLL is responsible for tracking the phase of the grid-connected voltage, providing the phase angle for the controller's sampling and output. The current loop is responsible for controlling the output voltage V. odl and V oql This enables precise control of the current. The outer loop of the DC voltage control is responsible for adjusting the output current to ensure that the voltage on the DC side remains constant.

[0151] The control equations for the phase-locked loop are as follows:

[0152] (1)

[0153] In the formula, H pll1 (s) is the controller of the phase-locked loop, k ppll1 and k ipll1 It is H pll1 The proportional and integral coefficients of (s), U gfl_q1 This is the value of the q-axis voltage in the controller. The control equations for the voltage controller and the current controller are:

[0154] (2)

[0155] (3)

[0156] In the formula, H vdc (s) is the controller of the voltage control loop, H i1 (s) is the controller of the current control loop, k pdc k idc and k pc1 k ic1 It is H vdc (s) and H i The proportional and integral coefficients of (s), Idref_vdc I qref_vdc It is the inner loop reference dq-axis current output from the outer voltage loop, I gfl_d1 I gfl_q1 It is the real-time dq-axis current in the converter, U gfl_d1 U gfl_q1 It is the real-time dq-axis voltage in the converter, V odl and V oql These are the output voltages of the d-axis and q-axis.

[0157] To address the dynamic characteristics of the phase-locked loop (PLL), the current loop and DC voltage loop are modeled. First, the impact of the PLL on the controller coordinate system is considered. Since the dynamic tracking capability of the PLL is limited, disturbances will cause a phase angle disturbance between the controller coordinate system and the system coordinate system of the grid connection point voltage. The magnitude of this disturbance... for:

[0158] (4)

[0159] In the formula, H represents the grid connection point voltage disturbance in the controller coordinate system. pll (s) is the phase-locked loop transfer function.

[0160] There is a certain angular deviation between the controller coordinate system and the system coordinate system, and the relationship is as follows:

[0161] (5)

[0162] In the formula, M c M represents the disturbance quantity M in the controller coordinate system. s The disturbance M represents the system's steady-state coordinate system. Due to differences in dynamic performance, there is a dynamic transformation matrix between the two. Under small perturbations, ≈0, so cos =1, sin = The relationship between voltage disturbances in the controller coordinate system and voltage disturbances in the system coordinate system is as follows:

[0163] (6)

[0164] in, and It is the voltage disturbance at the grid connection point in the system coordinate system. and This is the voltage value in steady state. =0. Here, variables with the superscript 's' represent variables in the corresponding system coordinate system. Combining (4) and (6), the relationship between phase angle disturbance and voltage disturbance in the system coordinate system can be written as:

[0165] (7)

[0166] In the formula, G pll_vdc It is the transfer function from grid connection point voltage disturbance to phase angle disturbance.

[0167] Substituting equation (7) into (5), the relationship between the variables in the controller coordinate system and the variables in the system coordinate system is as follows:

[0168] (8)

[0169] In the formula, and

[0170] The relationship between DC-side voltage disturbance and AC-side voltage and current can be obtained from the capacitor voltage expression and the power balance on both sides:

[0171] (9)

[0172] If we ignore the changes in the current of new energy power generation, we can consider that... Combining the two equations in (9), we can obtain the relationship between the DC side voltage and the system voltage and current disturbances as follows:

[0173] (10)

[0174] (11)

[0175] Linearizing (2) and (3) yields the small perturbation transfer functions for the current loop and voltage control loop:

[0176] (12)

[0177] The relationship between the output voltage, the grid connection point voltage, and the current in the filter inductor in the system coordinate system is as follows:

[0178] (13)

[0179] By combining (8) and (11)-(13), the relationship between the grid-connected port system voltage and current can be obtained, that is, the equivalent admittance of the new energy converter is:

[0180] (14)

[0181] The control block diagram of a grid-type energy storage converter is as follows: Figure 6As shown, the synchronization of the grid-type energy storage converter mainly relies on the Virtual Synchronous Generator (VSG) module, which mimics the second-order rotor equation of a synchronous generator and possesses virtual inertia and damping characteristics. Its synchronization equation is as follows:

[0182] (15)

[0183] Among them, P refgfm It is the active power command value, used to simulate the input power of a synchronous machine, P. gfm It is the output power detected by the converter. This is the rated speed of the virtual synchronizer. The system's active power P is 314 prad / s. gfm The calculation method is as follows:

[0184] (16)

[0185] In the formula, U gfm_d U gfm_q , I gfm_d, I gfm_q These are sampled values ​​in the controller coordinate system. The system's voltage control includes a voltage control loop and a current control loop, and its control equations are:

[0186] (17)

[0187] (18)

[0188] Where, k pv2 k iv2 and k pc2 k ic2 It is H v_gfm (s) and H i2 The proportional and integral coefficients of (s). U ref This is the specified value for the system's AC voltage measurement; the specified voltage for the q-axis is 0. k dei This is the decoupling coefficient for current decoupling, typically set to 1. In the modulation section, since the DC-side circuit is simplified to a constant voltage source, V... dc It is always equal to V dcref That is, V always exists dc / V dcref =1. The equations for the circuit section are:

[0189] (19)

[0190] In the formula, V od2 and V oq2These represent the output voltages of the converter ports along the d-axis and q-axis in the system coordinate system, respectively.

[0191] Similar to the relationship between the controller coordinate system and the system coordinate system variables in the modeling of grid-type new energy converters, the variables in the controller coordinate system and the system coordinate system of grid-type energy storage also have a transformation relationship as shown in (5). The active power disturbance detected in the controller is:

[0192] (20)

[0193] Linearizing (15) yields the phase angle disturbance and active power disturbance as follows:

[0194] (twenty one)

[0195] In the formula, For angular velocity perturbation, This is a phase angle disturbance.

[0196] By combining the equations, we can obtain:

[0197] (twenty two)

[0198] The relationship between voltage and current disturbances in the controller coordinate system and voltage and current disturbances in the system coordinate system is as follows:

[0199] (twenty three)

[0200] Solve equations (20), (21), and (22) simultaneously, and substitute the result into U. s gfm_q =0, we can find:

[0201] (twenty four)

[0202] Will Substituting the result into (23), we can simplify it to:

[0203] (25)

[0204] Linearizing (17) and (18) yields the disturbance in the controller coordinate system, where voltage and current disturbances affect the output voltage:

[0205] (26)

[0206] (27)

[0207] In the formula, k dei This represents the decoupling coefficient of the current loop.

[0208] Combining (25), (26), and (27) with the circuit equations, we can finally obtain the equivalent impedance of the grid-type energy storage converter as follows:

[0209] (28)

[0210] The control block diagram of the grid-type load converter is as follows: Figure 7 As shown, the control method and topology of the load converter are similar to those of grid-connected renewable energy converters. The main difference is that the DC side is a constant voltage source, and its dynamic characteristics are negligible. Simultaneously, the primary objective of this controller is to adjust the power absorbed by the converter from the grid to meet load requirements; therefore, its outer loop uses a proportional-integral (PI) controller with PQ control to achieve error-free power regulation. The control equations for the phase-locked loop, current loop, and power loop in the system are as follows:

[0211] (29)

[0212] (30)

[0213] (31)

[0214] In the formula, P refPQ Q refPQ P is the set reference value for active and reactive power. gfl_PQ Q gfl_PQ I represents the real-time active and reactive power in the converter. dref_PQ I qref_PQ I is the reference value for the inner loop current dq output from the outer power loop. gfl_d3 I gfl_q3 For the inner loop current dq reference value, U gfl_d3 U gfl_q3 It is the value of the dq axis voltage in the controller, V od3 and V oq3 It is the dq-axis voltage at the output port, H pll3 It is the transfer function of the phase-locked loop, H PQ H represents the proportional-integral controller transfer function of the outer loop of power control. i3 This represents the transfer function of the inner current loop, and their control parameters are k. ppll3 ,k ipll3 ,k ppq , k ipq ,k pc3 ,k ic3 The current loop employs voltage feedforward and current decoupling to reduce the coupling effect of the dq term. The definitions of the remaining parameters are similar to those described above, except the object is now the grid load. The power calculation method in the controller is as follows:

[0215] (32)

[0216] Impedance modeling of load converters is similar to that of new energy converters. First, the relationship between the disturbance variables in the controller coordinate system and the disturbance variables in the system coordinate system during phase-locked loop disturbances is determined. Then, each control element is linearized, and finally, a simultaneous solution is performed to obtain the equivalent admittance of the system. The specific derivation steps are as follows:

[0217] (33)

[0218] (34)

[0219] Among them, G pllv3 G plli3 G pllvo3 It is a transfer function that transforms voltage, current, and output voltage in two coordinate systems, while the other variables are disturbances in the corresponding coordinate systems.

[0220] Substituting the controller variables into the control equations, we can finally obtain the relationship between the output voltage and the system-side voltage and current disturbances as follows:

[0221] (35)

[0222] in:

[0223] (36)

[0224] Combining (35) with the circuit equations, we finally obtain the expression for the equivalent admittance of the load component:

[0225] (37)

[0226] The control block diagram of MMC is as follows: Figure 8 As shown, the MMC contains a large number of harmonics, and the multiplication of these harmonics generates even higher-order harmonics, leading to harmonic coupling effects. Therefore, the conventional dq modeling method is no longer applicable, and a harmonic state-space modeling method is required. Each arm of the MMC contains n submodules, and the capacitance of each submodule is nC. M By adjusting the number of sub-modules in operation, the output voltage can be effectively controlled. In addition to the sub-modules, each bridge arm also includes a bridge arm inductor L. m and bridge arm resistance R m After the currents from the upper and lower bridge arms converge, the system is connected to the point of common coupling (PCC) through a transformer. In high-voltage direct current transmission equipment, the transformer typically performs certain protection functions, therefore its leakage reactance L... tmmc The value is relatively large and cannot be ignored in the modeling. On the DC side, the equivalent impedance Z... dcThis includes not only the resistance and inductance of the DC transmission line, but also the impedance of the smoothing inductor, which typically has a large impedance value. Furthermore, to simplify the impact of the inverter-side network on the rectifier-side, the inverter-side HVDC, whose control objective is to maintain a constant DC-side voltage, is simplified to a DC voltage source with a constant amplitude. Wherein, i ua i ub i uc i la i lb i lc V represents the current in phases a, b, and c of the upper and lower bridge arms, respectively. cuabc ,v clabc This represents the capacitor voltage in the upper and lower bridge arms, n. uabc and n labc This indicates the duty cycle of the upper and lower bridge arms. The voltages of the upper and lower bridge arm submodules are respectively v. uabc , v labc The current on the low-voltage side of the transformer is i. gabcl The voltage is v mabcl The high-voltage side current is i gabch The voltage is v Mabch The voltage on the DC side is v Mdc .

[0227] The control section of the MMC mainly consists of four parts: a phase-locked loop (PLL), a current loop, a power and voltage outer loop, and a circulating current control loop. The PLL is used to track the voltage at the grid connection point to obtain the phase angle of the grid, transferring the system variables from the abc coordinate system to the dq coordinate system. Its parameter is k. ppll4 and k ipll4 The current loop controls the output voltage and current values ​​of the system. This loop uses voltage feedforward and current decoupling control to achieve better decoupling. The proportional-integral controller parameter of the current loop is k. pc4 and k ic4 The command value for the current loop is given by the outer power voltage control loop, where the reference current i along the d-axis is... dref_mmc Used to regulate active power, the q-axis reference current i qref_mmc Used to change reactive power to achieve AC voltage measurement of a specified value V acref P in the system mmc and V ac_mmc The detection method is as follows:

[0228] (38)

[0229] The time parameter T of the first-order filter for active power sampling s =0.01s, its proportional-integral control loop parameter is k pp and k ip The proportional-integral control parameter for the reactive power component is k.pac and k iac .

[0230] To suppress circulating current between the bridge arms within the MMC and prevent increased system losses and shortened lifespan, a circulating current controller is included in the control strategy. The main component of the circulating current in the system is the second harmonic component. A PR controller is set up to suppress the variable at this frequency; its transfer function is:

[0231] (39)

[0232] Where, k pcc and k rcc These are the proportional gain coefficient and the resonant gain coefficient, respectively. It is its damping ratio, It is its natural frequency. The current i before the PR controller... Mabch First, it passes through a second-order high-pass filter, whose transfer function is:

[0233] (40)

[0234] In the formula, For cutoff frequency, This is the damping ratio of the filter. The duty cycle of the current control section output is transformed to the abc coordinate system after inverse dq transformation and multiplied by 2 / V. dc0 Become m abc1 In modulation with m abc2 Together they form the final duty cycle n of the upper and lower bridge arms. uabc and n labc Its expression is:

[0235] (41)

[0236] Since the modeling of MMC is relatively complex, we will first perform HSS modeling on the circuit part of the system. Let the potential of the DC side midpoint of the MMC relative to ground be v. n The three-phase circuit equations for MMC can be written as:

[0237] (42)

[0238] In the formula, L m and R m These are the bridge arm inductance and bridge arm resistance, respectively, L ac It is the equivalent inductance on the AC side, which includes the bridge arm inductance L. m and the leakage reactance L on the transformer tmmc v n The DC side neutral point voltage, v Mdc v uabc v labci uabc i labc v Mabcl The three-phase voltage and current in matrix form are expressed as follows:

[0239] (43)

[0240] In the formula, the magnitude of the DC side voltage is v. Mdc The three-phase voltages on the low-voltage side of the transformer are respectively v Mal ,v Mbl ,v Mcl vua,v ub ,v uc ,v la ,v lb ,v lc The voltages in phases a, b, and c of the upper and lower bridge arms are represented respectively, and i ua i ub i uc i la i lb i lc These represent the currents in phases a, b, and c of the upper and lower bridge arms, respectively. The voltages of the upper and lower bridge arm submodules are V and V, respectively. uabc ,v labc The currents of the upper and lower bridge arm submodules are i, respectively. uabc i labc .

[0241] According to the circuit topology, the circulating current i on the DC side cabc and the output current i on the AC side gabc This can be expressed as:

[0242] (44)

[0243] By combining (42) and (44), the circulating current i can be obtained. cabc and AC output current i gabc Differential equation:

[0244] (45)

[0245] For each bridge arm, the relationship between the total voltage of its upper and lower bridge arms and the capacitor voltage of each submodule is as follows:

[0246] (46)

[0247] In the formula, v cuabc , v clabc This represents the capacitor voltage in the upper and lower bridge arms. uabc and n labc These represent the duty cycles of the upper and lower bridge arms, respectively, and their expressions are as follows:

[0248] (47)

[0249] In the formula, n ua , n ub , n uc These are the duty cycles of the three phases a, b, and c of the upper bridge arm, respectively, and n la , n lb , n lc These are the duty cycles of the three phases a, b, and c of the lower bridge arm.

[0250] The differential equation for the capacitor voltage is:

[0251] (48)

[0252] In the formula, C m The capacitor in each submodule of each bridge arm. Let i in the equation... uabc and i labc Replace with i cabc and i gabc (48) can be converted to:

[0253] (49)

[0254] After combining (45), (46), and (49) and linearizing them, we can obtain the state-space model of the MMC circuit part in the abc coordinate system as follows:

[0255] (50)

[0256] In the formula, x abc N represents the disturbance of variable x in the abc coordinate system. labc N uabc V is the modulation ratio of the upper and lower bridge arms under steady state. labc V uabc I represents the three-phase upper and lower bridge arm voltages under steady-state conditions. labc I uabc Let represent the three-phase upper and lower bridge arm currents under steady state. To simplify the analysis and more easily determine the coupling relationship between harmonics, the state-space model is transformed from a three-phase model to a positive and negative sequence coordinate system. The transformation matrix is:

[0257] (51)

[0258] In the formula, M a M b M c M is any variable in the three-phase coordinate system. p M nM0 is its value when transformed to the pn0 coordinate system. P represents the transformation matrix from three-phase to positive-negative sequence.

[0259] v n Its function is to force the AC output current of the system to have no zero-sequence component, where n = labc, gabc, uabc, cabc, cuabc, etc., and their variable relationships are as follows:

[0260] (52)

[0261] In the formula, v abc For DC side neutral point voltage disturbance, i g0 R represents the zero-sequence component of the AC output current of the system. m L is the bridge arm resistance. ac It is the equivalent inductance on the AC side, which includes the bridge arm inductance L. m and the leakage reactance L on the transformer tmmc .

[0262] Combining (50) and (52), eliminate P v n The state-space equations under positive and negative order are:

[0263] (53)

[0264] In the formula, all subscripts containing the symbol pn represent variables in the pn coordinate system, while the specific physical quantities remain unchanged.

[0265] The H2 matrix is ​​a 3x3 matrix, and its expression is: This eliminates the zero-sequence current on the AC side.

[0266] The entire state-space equations were simplified. According to the circuit equations, the relationship between the circulating current disturbance and the DC-side voltage disturbance is as follows:

[0267] (54)

[0268] In the formula, Z dc The DC-side equivalent impedance includes not only the resistance and inductance of the DC transmission line, but also the impedance of the smoothing inductor, which is usually quite large. i c0 For zero-sequence circulating current disturbance, G cdc This represents the disturbance of the circulating current to the DC-side voltage. The zero-sequence circulating current disturbance multiplied by the DC-side impedance equals the DC-side voltage disturbance. The state-space equation in matrix form is:

[0269] (55)

[0270] In the formula, x pn0 Let x be the disturbance of the state variable x in the positive and negative coordinate system. Let x be the differential of the disturbance of the state variable x in the positive and negative order coordinate system. n pn0 The modulation ratio perturbation of the upper and lower bridge arms. v Mpnol For the port voltage in pn coordinates, the variables in the system are in the following forms:

[0271] (56)

[0272] The expressions for each state-space matrix are:

[0273] (57)

[0274] The modulation of the duty cycle is mainly related to the grid voltage, the AC output current, and the circulating current within the system. The disturbance of the modulation ratio can be expressed as a variable of the relevant disturbance as follows:

[0275] (58)

[0276] In the formula, n pn0 This represents the disturbance in the modulation ratio. Its structure is as follows:

[0277] (59)

[0278] G nCC This indicates that the circulation disturbance is 0.5 times the m. pn02 The disturbance, G ngi G ngvi This indicates that the AC side current and voltage disturbance is 0.5 times m. pn01 The disturbance, m pn01 and m pn02 It is m abc1 and m abc2 The components in the positive and negative order coordinate systems. Combining (55) and (58), the state-space equations of the entire system can be written as:

[0279] (60)

[0280] For any time-varying signal x(t) with period T0, it can be decomposed into several harmonic components by Fourier transform:

[0281] (61)

[0282] X nFor the amplitude of the multiple harmonic components, this formula is a Fourier transform expression.

[0283] Taking the derivative with respect to x(t), we can obtain:

[0284] (62)

[0285] Similarly, performing Fourier decomposition on the right side of (55) yields the following result:

[0286] (63)

[0287] Representing it as a matrix, we get:

[0288] (64)

[0289] In the formula, X pn0 yes x pn0 The state variables after harmonicization V Mpn0l yes v Mpn0l The harmonicized state variables. Q is a diagonal matrix with diagonal terms of -jnw0I. G[] indicates that this matrix is ​​a Topplitz matrix. Considering that the modeling accuracy of MMC can be guaranteed after the fourth harmonic, therefore, , , The expression for Q can be written as:

[0290] (65)

[0291] The expressions for each submatrix can be written as:

[0292] (66)

[0293] The steady-state variable N in the formula n uabc , N n labc V n cuabc V n clabc The general form of the expression for the nth harmonic component of the modulation ratio of the upper and lower bridge arms and the capacitor voltage of the upper and lower bridge arms is:

[0294] (67)

[0295] In the formula, n represents the nth harmonic component, and M k and kThis represents the amplitude of the M component corresponding to k and its phase angle. It's important to note that at negative frequencies, the measured phase angle is the phase angle at positive frequencies, and e needs to be converted to a more balanced value. j k Change to e -j k .

[0296] The control strategy of MMC mainly includes phase-locked loop (PLL), current loop, and power-voltage outer loop control at the base frequency, as well as suppression control for the second harmonic component. In the modeling process, the PLL, current loop, and power-voltage outer loop are modeled first. Similar to the modeling approach for grid-connected renewable energy converters, considering the phase angle disturbance sampled by the PLL, the variable disturbance in the system controller coordinate system is expressed as:

[0297] (68)

[0298] In the formula, G pllvm G pllim G pllvmo It is the transfer function for the transformation between voltage, current, and output voltage in two coordinate systems. G pll_mmc V is the transfer function of the phase-locked loop. s mq V s md V represents the voltage along the d and q axes in the system coordinate system. s moq V s mod I represents the port voltages along the d and q axes in the system coordinate system. s mq I s md Represents the d-axis and q-axis currents in the system coordinate system. V Md , V Mq These are the voltage d-axis and q-axis perturbations. I Md , I Mq Let be the dq-axis disturbance of the current. V s od1 , V s oq1 Let be the d-axis and q-axis disturbances of the port voltages in the system coordinate system. v mod , v moq These represent the d-axis and q-axis disturbances of the port voltage.

[0299] The perturbation of the system output duty cycle in the dq coordinate system can be written as:

[0300] (69)

[0301] In the formula, G cim G represents the transfer function from the disturbance at the outer loop output of the MMC current to the disturbance at the grid connection point voltage. deim G represents the transfer function from the MMC current disturbance to the grid connection point voltage disturbance. PQm It is the transfer function of the proportional-integral controller in the MMC power loop, G vcm Let G be the proportional-integral transfer function of the voltage loop. Pim G is the transfer function from current disturbance to power calculation. Pum G is the transfer function from current disturbance to power calculation. nidq0 G ncdq0 Let be the transfer function from the dq-axis current-voltage disturbance to the duty cycle disturbance in the system coordinate system. The expressions for each matrix are as follows:

[0302] (70)

[0303] Since the dq coordinate system is a rotating coordinate system with an angular velocity of w0, the perturbation in this coordinate system will generate coupling at multiple frequencies when transferred to the pn coordinate system. For G in (69) nidq0 and G nvdq0 Its internal structure is as follows:

[0304] (71)

[0305] The transfer function of current disturbance to duty cycle in the pn coordinate system is:

[0306] (72)

[0307] Among them, G nipp (s) and G ninn (s) represents the frequency f p The positive and negative sequence currents are perturbed to a frequency of f. p of n pi , n ni The transfer function of G. ninp (s) represents f p The positive sequence current perturbation to frequency f p -2w0 n ni The transfer function of the disturbance, G nipn (s) represents f p The negative sequence current perturbs to frequency f p+2w0 n pi The transfer function.

[0308] Therefore, the expression for HGnx in (64) is:

[0309] (73)

[0310] In the formula, HG 1nx For baseband control in the system, HG 2nx For the circulating current control in the system, the matrix expression corresponding to the base frequency control is:

[0311] (74)

[0312] The expressions for each submatrix are as follows:

[0313] (75)

[0314] Similarly, construct the transfer function HG from voltage perturbation to duty cycle perturbation. nv HG nv (s). Since the circulating current control is only related to the circulating current state variable and is independent of the AC side voltage disturbance, the influence of the circulating current control does not need to be considered. Its expression is:

[0315] (76)

[0316] For circulating current control, it targets the circulating current I of the system. cabc To suppress, the expression under positive and negative order is:

[0317] (77)

[0318] In the formula, G ccabc G is the transfer function of the circulating control section in the three-phase coordinate system. ccpn0 G is the transfer function of the circulating control part in the pn coordinate system. hpsaa The transfer function of the high-pass filter, H pr This represents the transfer function of the PR controller in circulating current control.

[0319] Since the coordinate system of the circulating current control is a stationary coordinate system, the duty cycle disturbance of each frequency is only related to the circulating current disturbance of its own frequency, and there is no complex coupling between multiple frequencies. Therefore, HG 2nx The expression can be written as:

[0320] (78)

[0321] The expressions for each submatrix are:

[0322] (79)

[0323] Substitute HG into expressions (65), (73), and (78) nx HG nv , , , The transfer function from the voltage input to the system state variable output, Q, can be written as:

[0324] (80)

[0325] In the formula, G sys The transfer function representing the system's voltage input to its state variable output, when combined with the voltage and current at corresponding frequencies within the system, forms the system's admittance:

[0326] (81)

[0327] Transferring this to the high-voltage side, we can finally obtain the system's admittance in both positive and negative sequences as follows:

[0328] (82)

[0329] Using matrix A z The admittance in positive and negative order can be transferred to the dq coordinate system to finally obtain the impedance model expression of MMC considering the fourth harmonic:

[0330] (83)

[0331] In some embodiments, a frequency domain impedance network for the entire electric system is constructed based on various equivalent impedance models, including:

[0332] Step S201: Take the coordinate system of the grid connection point of the grid-connected energy storage converter as the global coordinate system of the system, and based on the global coordinate system, unify the coordinate system of the equivalent admittance in the equivalent impedance model of the grid-connected new energy converter, the grid-connected load converter and the modular multilevel high voltage DC converter station.

[0333] In impedance modeling, the obtained impedance results are stored in the system coordinate system of the grid connection point. However, due to differences in output power, there are steady-state phase angle differences between different grid connection points. Therefore, the obtained frequency domain impedance / admittance are not in a common coordinate system, making it difficult to directly divide the phases to plot the Nyquist curve. The relationship between different coordinate systems in the system is as follows: Figure 9As shown, different shades represent the steady-state coordinate systems of different grid connection points, and there is a steady-state phase angle difference between them. Due to disturbances in grid voltage and current, the controller coordinate system experiences phase angle disturbances around the steady-state coordinate system of the grid connection point. Transferring the obtained frequency domain equivalent impedance / admittance to the global common coordinate system requires angle manipulation of each variable.

[0334] Since there is no infinite bus in the system, the coordinate system of the grid-connected energy storage point is selected as the global coordinate system of the system. Therefore, the expressions for the admittances of grid-connected new energy, grid-connected load, and MMC in the global coordinate system are as follows:

[0335] (84)

[0336] In the formula, This represents the transpose matrix from the grid connection point coordinate system of converter x to the grid connection point coordinate system of the grid-connected energy storage system. For the inductive and resistive branches in the system, the transpose result is:

[0337] (85)

[0338] In the formula, Let be the transpose matrix of the impedance branch to the coordinate system of the grid-connected energy storage point, where R and L are the branch resistance and inductance. It can be observed that passive components in the system have the same expression in different coordinate systems; therefore, their expressions can be written directly without coordinate system transformation.

[0339] Step S202: Connect the grid-connected new energy converter, grid-connected load converter, and modular multilevel high-voltage DC converter station in parallel as current source devices after unifying the coordinate system. Perform parallel calculations based on the equivalent impedance models corresponding to the grid-connected new energy converter, grid-connected load converter, and modular multilevel high-voltage DC converter station respectively to obtain the total equivalent impedance of the current source devices.

[0340] Among them, such as Figure 10 The frequency domain equivalent circuit diagram of the Shago desert region shown indicates that all current source devices are connected in parallel, and their overall equivalent impedance is Z. cs The total impedance of the grid-type energy storage branch is Z. branch The expression is:

[0341] (86)

[0342] In the formula, Z l1 Z l2 Z l3 Z rlg The line impedances for grid-connected new energy, grid-connected energy storage, grid-connected load, and MMC are respectively, Z. gfm Y is the equivalent impedance of a grid-type energy storage converter.0 mmc_dq Y 0 gfl_PQ Y 0 gfl_vdc These represent the frequency admittances of MMC, grid-connected load, and grid-connected renewable energy, respectively. cs This is the overall equivalent impedance of all current source type devices.

[0343] Step S203: Treat the grid-type energy storage converter as a voltage source device, and determine the equivalent impedance of the voltage source device based on the equivalent impedance model of the grid-type energy storage converter.

[0344] The equivalent impedance of the voltage source type equipment, i.e. the total impedance Zb of the grid-type energy storage branch, is calculated by equation (86). ranch .

[0345] Step S204: Connect the equivalent circuits in series based on the total equivalent impedance of the current source device and the equivalent impedance of the voltage source device to form the frequency domain impedance network of the entire electric system.

[0346] For a power grid, the system can be divided into two subsystems—a voltage source and a current source—at any point within the grid. These two subsystems are then connected in series with equivalent loops to form the frequency domain impedance network of the entire power system. The equivalent diagram of the frequency domain impedance network is shown below. Figure 11 As shown, the impedance on the current source side is Z1, and the impedance on the voltage source side is Z2. According to the circuit equations, the expression for the current between them is:

[0347] (87)

[0348] In the formula, I(s) represents the output current of the current source, V(s) represents the voltage of the voltage source, and I(s) represents the current between the two subsystems.

[0349] In some embodiments, the generalized Nyquist stability criterion is used to perform small-disturbance stability analysis on the frequency domain impedance network. The small-disturbance stability of the all-electric system in an AC / DC hybrid scenario is determined by the Nyquist plot of the impedance ratios on both sides of the voltage and current sources within the frequency domain impedance network, including:

[0350] Step S301: Based on the frequency domain impedance network of the entire electric system, and combined with the total equivalent impedance of the current source type equipment and the equivalent impedance of the voltage source type equipment, determine the impedance ratio of the entire electric system.

[0351] Among them, by referring the load impedance to the current source side, the impedance ratio of the entire electric system can be written as:

[0352] (88)

[0353] Step S302: Plot the Nyquist plot based on the impedance ratio of the entire electric system, and obtain the Nyquist curve of the impedance ratio of the entire electric system.

[0354] Specifically, based on the impedance ratio expression, impedance ratio data at different frequencies are scanned and plotted on the complex plane to obtain the Nyquist curve. This curve can intuitively reflect the impedance characteristics of the system at different frequencies.

[0355] Step S303: Determine whether the Nyquist curve intersects the negative real axis;

[0356] Step S304: If the Nyquist curve intersects with the negative real axis, it is determined that the all-electric system has a risk of small disturbance instability.

[0357] Step S305: If the Nyquist curve does not intersect with the negative real axis, then it is determined that the all-electric system does not have the risk of small disturbance instability.

[0358] Assuming the impedance on the current source side is constant when connected to an infinitely large power grid, that is, V s All poles of I(s) / Z2(s) are in the left half-plane, so the stability criterion can be transformed into determining whether I / (I+Z1(s) / Z2(s)) has poles in the right half-plane. Considering its transfer function form with unity negative feedback, we only need to determine the number of loops of Z1(s) / Z2(s) around the point (-1,0) to know the number of poles in the right half-plane of G(s). The equivalent impedance of the converter controlled by the phase-locked loop does not have zeros in the right half-plane, and the equivalent impedance of the converter controlled by the power synchronization loop does not have poles in the right half-plane. Therefore, the impedance ratio Z1(s) / Z2(s) on both sides of the system does not have open-loop poles. We only need to determine whether the Nyquist curve of the system includes (-1,0) to determine the stability of the system. In the dq coordinate system, the impedance is a 2×2 matrix, and its impedance ratio becomes a two-dimensional matrix. At this time, to determine the stability of the system, it is necessary to solve for the two eigenvalues ​​of Z1(s) / Z2(s). The system can be determined to be a stable system when the Nyquist curves corresponding to the two eigenvalues ​​do not contain (-1,0).

[0359] For example, the stability of the system can be solved by plotting two Nyquist curves of G(s). This is verified in a system where the grid-type energy storage capacity is 20% of the new energy capacity. The G(s) values ​​from -600Hz to 600Hz are calculated, their eigenvalues ​​are solved, and the Nyquist plot is shown below. Figure 12 As shown. Figure 12 In the middle, the light-colored solid line represents the proportionality coefficient k of the outer loop of the grid energy storage voltage. pv2The Nyquist curve for eigenvalue 1 is shown at 2.6 pu, while the light-colored dashed line represents the Nyquist curve for eigenvalue 2. It can be seen that the curve for eigenvalue 1 is close to the (-1,0) point, indicating a risk of instability. Reducing the proportional gain k of the voltage control loop... pv2 The system's Nyquist curve changes from light to dark. At this point, the dark blue Nyquist curve corresponding to eigenvalue 1 includes the point (-1, 0), indicating that the system has changed from stable to unstable. This is verified in the time-domain model, where k... pv2 The voltage reaches steady state with parameters of 2.6 pu. At t=10s, the parameter k of the voltage control loop is changed. pv2 With a voltage of 2.2 pu, it can be observed that the d-axis voltage in the grid-type energy storage controller gradually diverges, and at the same time, the frequency of its active power synchronization loop output also gradually diverges, transitioning from a steady state to an unstable state.

[0360] Based on the same inventive concept, this application also provides a multilevel DC transmitter small disturbance stability evaluation system for implementing the multilevel DC transmitter small disturbance stability evaluation method involved above.

[0361] The solution provided by this system is similar to the solution described in the above method. Therefore, the specific limitations of one or more multi-level DC transmitter small disturbance stability assessment system embodiments provided below can be found in the limitations of the multi-level DC transmitter small disturbance stability assessment method above, and will not be repeated here.

[0362] like Figure 13 As shown in the embodiment of this application, a multi-level DC sending-end small disturbance stability assessment device is also provided, which is applied to a full-electric system in an AC / DC hybrid scenario. The full-electric system includes a grid-connected new energy converter, a grid-connected energy storage converter, a grid-connected load converter, and a modular multi-level high-voltage DC converter station. This device includes:

[0363] The equivalent impedance construction module 100 is used to perform frequency domain impedance modeling for grid-connected new energy converters, grid-connected energy storage converters, grid-connected load converters, and modular multilevel high-voltage DC converter stations, respectively, to obtain the equivalent impedance models corresponding to the grid-connected new energy converters, grid-connected energy storage converters, grid-connected load converters, and modular multilevel high-voltage DC converter stations.

[0364] Impedance network construction module 200 is used to construct the frequency domain impedance network of the entire electric system based on various equivalent impedance models;

[0365] The stability assessment module 300 is used to perform small disturbance stability analysis on the frequency domain impedance network using the generalized Nyquist stability criterion. By using the Nyquist plot of the impedance ratio on both sides of the voltage source and current source in the frequency domain impedance network, the small disturbance stability of the all-electric system in the AC / DC hybrid scenario is determined.

[0366] like Figure 14 As shown, this application provides an electronic device. The electronic device 10 includes a memory 20 and a processor 30. The memory 20 stores a computer program. When the computer program is executed by the processor 30, the processor 30 performs the steps of the multi-level DC sending end small disturbance stability evaluation method as described in the above embodiment.

[0367] This application provides a computer-readable storage medium storing a computer program thereon. When the computer program is executed, it implements the steps of the multi-level DC transmitter small disturbance stability evaluation method as described in the above embodiments.

[0368] Those skilled in the art will clearly understand that, for the sake of convenience and brevity, the specific working processes of the systems, electronic devices, and computer storage media described above can be referred to the corresponding processes in the foregoing method embodiments, and will not be repeated here.

[0369] It should be noted that the terms "comprising" and "having" and any variations thereof in the specification, claims and accompanying drawings of this invention are intended to cover non-exclusive inclusion. For example, a process, method, system, product or device that includes a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to such process, method, product or device.

[0370] It should be understood that although the steps in the flowcharts of the embodiments described above are shown sequentially according to the arrows, these steps are not necessarily executed in the order indicated by the arrows. Unless explicitly stated herein, there is no strict order restriction on the execution of these steps, and they can be executed in other orders. Moreover, at least some steps in the flowcharts of the embodiments described above may include multiple steps or multiple stages. These steps or stages are not necessarily completed at the same time, but can be executed at different times. The execution order of these steps or stages is not necessarily sequential, but can be performed alternately or in turn with other steps or at least some of the steps or stages of other steps.

[0371] In the embodiments provided by this invention, it should be understood that the disclosed systems, electronic devices, computer storage media, and methods can be implemented in other ways. For example, the device embodiments described above are merely illustrative; for instance, the division of units is only a logical functional division, and in actual implementation, there may be other division methods. For example, multiple units or components may be combined or integrated into another system, or some features may be ignored or not executed. Furthermore, the coupling or direct coupling or communication connection shown or discussed may be an indirect coupling or communication connection between devices or units through some interfaces, and may be electrical, mechanical, or other forms.

[0372] The units described as separate components may or may not be physically separate. The components shown as units may or may not be physical units; that is, they may be located in one place or distributed across multiple network units. Some or all of the units can be selected to achieve the purpose of this embodiment according to actual needs.

[0373] Furthermore, the functional units in the various embodiments of the present invention can be integrated into one processing unit, or each unit can exist physically separately, or two or more units can be integrated into one unit. The integrated unit can be implemented in hardware or as a software functional unit.

[0374] If the integrated unit is implemented as a software functional unit and sold or used as an independent product, it can be stored in a computer-readable storage medium. Based on this understanding, the technical solution of the present invention, in essence, or the part that contributes to the prior art, or all or part of the technical solution, can be embodied in the form of a software product. This computer software product is stored in a storage medium and includes several instructions for executing all or part of the steps of the methods described in the various embodiments of the present invention through a computer device (which may be a personal computer, a server, or a network device, etc.). The aforementioned storage medium includes: USB flash drives, portable hard drives, read-only memory (ROM), random access memory (RAM), magnetic disks, optical disks, and other media capable of storing program code.

[0375] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A multi-level DC sending end small signal stability evaluation method applied to an all-electric system in an AC-DC hybrid scenario, the all-electric system comprising a grid-following new energy converter, a grid-forming energy storage converter, a grid-following load converter, and a modular multi-level high-voltage DC converter station, characterized in that, The method comprises: respectively, the grid-forming energy storage converter, the grid-following load converter and the modular multilevel high-voltage direct current converter station are subjected to frequency domain impedance modeling to obtain equivalent impedance models corresponding to the grid-following new energy converter, the grid-forming energy storage converter, the grid-following load converter and the modular multilevel high-voltage direct current converter station respectively; based on the equivalent impedance models, a frequency domain impedance network of the all-electric system is constructed, comprising: taking the coordinate system of the grid connection point of the grid-forming energy storage converter as the global coordinate system of the system, and based on the global coordinate system, the coordinate systems of the equivalent admittance in the equivalent impedance models of the grid-following new energy converter, the grid-following load converter and the modular multilevel high-voltage direct current converter station are unified; the grid-following new energy converter, the grid-following load converter and the modular multilevel high-voltage direct current converter station after the unified coordinate system are connected in parallel as current source type devices, and the grid-following new energy converter, the grid-following load converter and the modular multilevel high-voltage direct current converter station are subjected to parallel operation in combination with the equivalent impedance models corresponding to the grid-following new energy converter, the grid-following load converter and the modular multilevel high-voltage direct current converter station respectively to obtain the total equivalent impedance of the current source type devices; the grid-forming energy storage converter is taken as a voltage source type device, and the equivalent impedance of the voltage source type device is determined according to the equivalent impedance model of the grid-forming energy storage converter; the total equivalent impedance of the current source type devices and the equivalent impedance of the voltage source type device are connected in series according to the equivalent loop to form the frequency domain impedance network of the all-electric system; the frequency domain impedance network is subjected to small disturbance stability analysis by using the generalized Nyquist stability criterion, and the small disturbance stability of the all-electric system in the AC / DC hybrid scenario is determined through the Nyquist diagram of the impedance ratio between the voltage source and the current source in the frequency domain impedance network.

2. The multi-level DC sending-end small signal stability evaluation method according to claim 1, characterized in that, The equivalent impedance model of the grid-following new energy converter is represented as: In the formula, Y gfl_vdc and Z gfl_vdc These are the equivalent admittance and equivalent impedance of the grid-connected new energy converter, respectively. , Let be the voltage and current disturbance vectors of the system at the grid connection point port, respectively, and I be the identity matrix. Let be the transfer function for transforming the grid-connected point voltage in the controller coordinate system and the system coordinate system, wherein the controller coordinate system is a coordinate system referenced to the control process of the converter itself, and the system coordinate system is a common reference coordinate system based on the entire electric system. Let be the transfer function of the grid-connected point current in the controller coordinate system and the system coordinate system. Let be the transfer function for the transformation of the output voltage in the controller coordinate system and the system coordinate system. G ci1 This is the transfer function from the disturbance at the outer current loop output to the voltage disturbance at the grid connection point. G dei1 Let be the transfer function from current disturbance to grid connection point voltage disturbance. H vdc ( s () is the controller for the voltage control loop. G dcv Let be the transfer function from system voltage disturbance to DC voltage disturbance. G dcvo This is the transfer function from the DC-side voltage disturbance to the grid connection point voltage disturbance. G dci Let be the transfer function from system current disturbance to DC voltage disturbance. Z l1 This is the impedance matrix of the filter inductor along the dq axis; wherein In the formula, , These represent the q-axis voltage component and d-axis voltage component of the grid connection point in steady state, respectively, in the system coordinate system. Let be the transfer function from the grid connection point voltage disturbance to the phase angle. , These represent the q-axis current component and d-axis current component at the grid connection point in steady state, respectively, in the system coordinate system. , These represent the q-axis output voltage component and the d-axis output voltage component of the grid-connected point in steady state, respectively, in the system coordinate system. This is the DC-side reference voltage. The controller is for the current control loop. Angular velocity, For filtering inductors, This refers to the discharge current output from the DC-side capacitor to the converter. For the Laplace operator, , These are the equivalent capacitance on the DC side and the voltage at the DC converter port, respectively; where: In the formula, is the transfer function of the phase-locked loop.

3. The multi-level DC sending-end small signal stability evaluation method according to claim 1, characterized in that, The equivalent impedance model of the grid-forming energy storage converter is: wherein, is the equivalent impedance of the grid-forming energy storage converter, , are the grid point port system voltage and current disturbance vectors of the grid-forming energy storage converter, respectively, is the transfer function from the current outer loop output disturbance to the grid point voltage disturbance, G v is the transfer function from the voltage disturbance to the current disturbance in the controller coordinate system, G UU is the transfer function from the voltage in the system coordinate system to the voltage in the control coordinate system, G IU is the transfer function from the current in the control coordinate system to the voltage in the system coordinate system, G UI is the transfer function from the voltage in the system coordinate system to the voltage in the control coordinate system, G VoI is the transfer function from the output voltage in the system coordinate system to the current in the system coordinate system, G II is the transfer function from the current in the system coordinate system to the current in the control coordinate system, G VoV is the transfer function from the output voltage in the system coordinate system to the voltage in the system coordinate system, G dei2 is the transfer function from the current disturbance to the grid point voltage disturbance, I is the identity matrix, is the impedance matrix in the dq axis of the filter inductance; wherein , , , , , , , , wherein, is a controller of a current control loop of the grid-forming energy storage converter, is a filter inductance, is an angular velocity, is a transfer function of a phase angle disturbance in a system coordinate system to a voltage in a controller coordinate system, is a transfer function of a disturbance of a current in the controller coordinate system to the phase angle disturbance, is a transfer function of a disturbance of the voltage in the controller coordinate system to the phase angle disturbance, is a transfer function of the phase angle disturbance in the system coordinate system to a current disturbance in the controller coordinate system, is a transfer function of the phase angle disturbance in the system coordinate system to a voltage disturbance at an output terminal in the system coordinate system, is a controller of an alternating current control loop; wherein: ， ， ， ， In the formula, , are the q-axis and d-axis steady-state voltage components of the grid-forming energy storage converter in dq coordinate system at steady state, respectively, , are the q-axis and d-axis steady-state current components of the grid-forming energy storage converter in dq coordinate system at steady state, respectively, , are the q-axis and d-axis output voltages of the grid-forming energy storage converter in system coordinate system, respectively, is the transfer function from power disturbance to phase angle disturbance, , are the transfer functions from voltage, current to power disturbance in controller coordinate system; wherein: ， ， wherein is a virtual inertia coefficient, is a virtual droop coefficient.

4. The multi-level DC sending-end small signal stability evaluation method according to claim 1, characterized in that, The equivalent impedance model of the grid-following load converter is: Yp is the equivalent admittance of the load part of the grid-following load converter, Vp and Ip are the grid point port system voltage and current disturbance vectors of the grid-following load converter, respectively, Gv and Gi are the transfer functions from the system coordinate system voltage and current disturbances to the port voltage, respectively, I is the identity matrix, Z is the line impedance of the grid-following load; wherein:​​ wherein, G pllv3 , G plli3 , G pllvo3 are the transfer functions of the voltage, current, output voltage in the controller coordinate system and the system coordinate system respectively, is the transfer function of the current decoupling part, G ci3 is the transfer function of the current loop, G PQ is the transfer function of the proportional-integral controller in the power loop, G SU and G SI are the transfer functions of the voltage, current disturbance to the disturbance of the power detection respectively; wherein: wherein is the controller of the current control loop of the grid-connected load converter, is the filter inductance, is the proportional-integral controller transfer function of the power control outer loop, is the angular velocity, is the Laplace operator, , are the inner loop current d, q current reference values, respectively, , are the d, q axis voltage components in the controller, respectively, is the transfer function of the phase-locked loop in power control mode, , are the d, q axis voltage components of the output voltage, respectively; wherein: In the formula, is the transfer function of the phase-locked loop.

5. The multi-level DC sending-end small signal stability evaluation method according to claim 1, characterized in that, The equivalent impedance model of the modular multilevel high-voltage direct current converter station is: wherein is the dq-axis admittance of the 4th harmonic of the modular multilevel high voltage direct current converter station, is the Park transformation matrix, is the admittance of the modular multilevel high voltage direct current converter station under positive and negative sequence, is the imaginary unit; wherein: wherein k mmc is the transformation ratio of the transformer, Y sys is the admittance matrix, , , , are elements of the admittance in the positive and negative sequence of the modular multilevel high-voltage direct-current converter station; wherein: wherein , , , are the transfer functions of the system with dimensions 52x13, 52x8, 29x13, 29x8, respectively, of the voltage input to the system state variable output; wherein: wherein is the dimension × transfer function of the voltage input to the system state variable output of the system, is (52x13), (52x8), (29x13) or (29x8), s is the Laplace operator, I is the identity matrix, is the Toeplitz matrix of the state space matrix A s , is the Toeplitz matrix of the state space matrix M c , is the Toeplitz matrix of the state space matrix B ac , is a diagonal matrix with diagonal entries -jnw0I, , are the transfer functions of the state variable, voltage variable to the duty cycle disturbance component after harmonization, respectively.

6. The multi-level DC sending-end small signal stability evaluation method according to claim 1, characterized in that, The frequency domain impedance network of the all-electric system is subjected to small disturbance stability analysis by using the generalized Nyquist stability criterion, and the small disturbance stability of the all-electric system in the AC / DC hybrid scenario is determined through the Nyquist diagram of the impedance ratio between the voltage source and the current source in the frequency domain impedance network, comprising: determining the impedance ratio of the all-electric system according to the frequency domain impedance network of the all-electric system in combination with the total equivalent impedance of the current source type devices and the equivalent impedance of the voltage source type device; drawing the Nyquist diagram according to the impedance ratio of the all-electric system to obtain the Nyquist curve of the impedance ratio of the all-electric system; determining whether the Nyquist curve intersects with the negative real axis; if the Nyquist curve intersects with the negative real axis, it is determined that the all-electric system has a small disturbance instability risk; if the Nyquist curve does not intersect with the negative real axis, it is determined that the all-electric system does not have a small disturbance instability risk.

7. A multi-level DC sending terminal small signal stability evaluation device applied to an all-electric system in an AC / DC hybrid scenario, the all-electric system comprising a grid-following new energy converter, a grid-forming energy storage converter, a grid-following load converter, and a modular multi-level high-voltage DC converter station, characterized in that, The device comprises: An equivalent impedance construction module is configured to perform frequency-domain impedance modeling on the grid-following new energy converter, the grid-forming energy storage converter, the grid-following load converter, and the modular multilevel high-voltage direct current converter station respectively, to obtain equivalent impedance models corresponding to the grid-following new energy converter, the grid-forming energy storage converter, the grid-following load converter, and the modular multilevel high-voltage direct current converter station respectively; An impedance network construction module is configured to construct a frequency-domain impedance network of the all-electric system based on the equivalent impedance models. The equivalent impedance models are used to construct the frequency-domain impedance network of the all-electric system, including: Taking the coordinate system of the grid-connected point of the grid-forming energy storage converter as a global coordinate system of the system, and unifying the coordinate systems of equivalent admittances in the equivalent impedance models of the grid-following new energy converter, the grid-following load converter, and the modular multilevel high-voltage direct current converter station based on the global coordinate system; Parallelly connecting the grid-following new energy converter, the grid-following load converter, and the modular multilevel high-voltage direct current converter station as current source type devices after unifying the coordinate systems, and performing parallel operation in combination with the equivalent impedance models corresponding to the grid-following new energy converter, the grid-following load converter, and the modular multilevel high-voltage direct current converter station, to obtain a total equivalent impedance of the current source type devices; Connecting the grid-forming energy storage converter as a voltage source type device, and determining an equivalent impedance of the voltage source type device according to the equivalent impedance model of the grid-forming energy storage converter; Connecting the total equivalent impedance of the current source type devices and the equivalent impedance of the voltage source type device in series to form the frequency-domain impedance network of the all-electric system; A stability evaluation module is configured to perform small disturbance stability analysis on the frequency-domain impedance network by using a generalized Nyquist stability criterion, and to determine small disturbance stability of the all-electric system in an AC / DC hybrid scenario through Nyquist diagrams of impedance ratios on both sides of voltage sources and current sources in the frequency-domain impedance network.

8. An electronic device, comprising: The electronic device includes a memory and a processor, the memory stores a computer program, and the computer program is executed by the processor to make the processor execute the steps of the multi-level direct current sending end small disturbance stability evaluation method according to any one of claims 1-6.

9. A computer-readable storage medium having stored thereon a computer program, characterized in that, The computer program is executed to implement the steps of the multi-level direct current sending end small disturbance stability evaluation method according to any one of claims 1-6.

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