CN118100172A - Multi-machine system transient stability limit excision time calculation method and related device - Google Patents

Abstract

A method and a related device for calculating transient stability limit excision time of a multi-machine system are provided, wherein the method comprises the following steps: establishing a differential algebraic equation of the power electronic multi-VSC grid-connected system and obtaining a balance point of the system, wherein the differential algebraic equation comprises a multi-VSC node dynamics model equation and a network model equation; linearizing to obtain a corresponding VSC state equation and a network state equation respectively, and calculating a simultaneous state equation to obtain a state matrix of the system; calculating to obtain class 1 balance points of the system according to the balance points and the state matrix of the system; and according to the state matrix of the system and the class 1 balance points of the system, calculating to obtain a super tangent plane equation, bringing the fault track information of the system into all tangent plane equations, and taking the time corresponding to the intersection point as the estimated value of the limit cutting time of the corresponding fault when the continuous fault track and a certain tangent plane intersect first. The method effectively solves the problem that transient stability of the multi-machine system is difficult to evaluate.

Description

A method and related device for calculating transient stability limit removal time of multi-machine system

Technical Field

The present invention relates to the technical field of safe and stable operation of electric power systems, and in particular to a method for calculating transient stability limit removal time of a multi-machine system and a related device.

Background technique

The rapid development of large-scale new energy base transmission and the application of a large number of power electronic equipment has led to tremendous changes in the composition structure and operation form of the power system: the intermittent and volatile output of new energy has led to a significant increase in the uncertainty of the operation mode of the power grid; the reduction in the rotational inertia of the power source and the decline in the frequency and voltage regulation capabilities have reduced the system's anti-interference ability and active and reactive power balance capabilities; the voltage and frequency fluctuation tolerance capabilities of new energy and DC equipment are far lower than those of traditional equipment, and AC and DC faults are prone to cause large-scale chain reactions, and the transient process is complex. The transient stability analysis method based on the cognition of traditional AC system characteristics is difficult to adapt to the new grid form and operation requirements.

There are already analysis methods such as Lyapunov function method, energy function method, bifurcation analysis, etc. for transient stability of single-machine grid-connected systems of new energy. However, the equivalent damping of single-machine equipment depends on the system state. The transient stability calculation results after ignoring damping may be conservative or radical, which is not conducive to quantitative analysis. In addition, there is a strong mutual coupling between multi-machine systems, which makes it impossible to directly apply existing methods to the transient stability analysis of multi-machine systems. Some studies have also been conducted on transient stability analysis and evaluation of new energy power systems based on numerical simulation, data-driven and artificial intelligence algorithms, but they not only have certain requirements for the simulation environment, but are also very time-consuming and cannot provide indicators related to transient stability margin. In general, there is still a lack of effective quantitative evaluation and analysis methods for transient stability analysis of multi-machine grid-connected systems.

Therefore, there is an urgent need to provide a quantitative calculation method to measure the transient stability capability of a multi-machine system, effectively solve the problem of difficulty in evaluating the transient stability of a multi-machine system, quantitatively evaluate the transient stability capability and stability margin of a multi-machine system, and reveal the transient characteristics of a power system with new energy as the main body, providing an effective basis for parameter setting of protection devices and transient characteristics analysis.

Summary of the invention

To solve the above problems, the present invention provides a method for calculating the limit cut-off time of transient stability of a multi-machine system and a related device. The transient stability of the multi-machine system can be judged by calculating the limit cut-off time of the system after a fault, and the transient stability margin of the multi-machine system can be measured. It can be used to analyze typical transient instability phenomena of the multi-machine system, and can reveal the transient characteristics of the power system with renewable energy as the main body, provide an effective basis for transient stability analysis and protection parameter setting of the power system with renewable energy as the main body, and ensure the safe and stable operation of the power system with renewable energy as the main body.

To achieve the above object, the present invention provides a method for calculating the transient stability limit removal time of a multi-machine system, comprising the following steps:

S1: establishing a differential algebraic equation of a power electronic multi-VSC grid-connected system and obtaining a balance point of the system, wherein the differential algebraic equation includes a multi-VSC node dynamic model equation and a network model equation;

S2: linearizing the multi-VSC node dynamic model equation and the network model equation to obtain the corresponding VSC state equation and network state equation respectively, and combining the VSC state equation and the network state equation through coordinate transformation and input-output interface relationship to calculate the state matrix of the system;

S3: According to the equilibrium point obtained in step S1 and the state matrix of the system obtained in step S2, a type 1 equilibrium point of the system is calculated;

S4: Based on the state matrix of the system obtained in step S2 and the type 1 equilibrium point of the system obtained in step S3, the hypertangent plane equation is calculated, and the fault trajectory information of the system is brought into all the tangent plane equations. When the continuous fault trajectory intersects with a certain tangent plane first, the time corresponding to this intersection is used as the estimated value of the limit removal time of the corresponding fault.

Furthermore, step S1 specifically includes: establishing a multi-VSC node dynamic model. When only the phase-locked loop dynamics are considered, the VSC node dynamic model is represented by a second-order dynamic differential equation:

Wherein, _utq,i represents the q-axis component of the terminal voltage ut,i under the PLL control dq coordinate of the i-th VSC, _xpll,i is the output of the phase-locked loop integrator of the i-th VSC, _φi represents the angle difference between the phase-locked loop output of the i-th VSC and the reference coordinate, kp_pll _,i and _kipll,i represent the proportional coefficient and integral coefficient of the PLL respectively;

Establish a network model, ignore the dynamics of the AC current time scale, describe the network with a node admittance matrix, and represent the load with a constant admittance. Import the node admittance matrix to obtain the network model equation:

Where, I _s and I _vsc represent the injected current vectors of the balancing node and the VSC node, respectively, U _s and U _vsc represent the node voltage vectors of the balancing node and the VSC node, respectively, and the network node mixing matrix M is expressed as follows:

Among them, Yra, Yrb, Yrc, and Yrd are the four components of the network node mixing matrix considering the load impedance;

By combining equations (1.1) and (1.2), we get the differential algebraic equation of the power electronic multi-VSC grid-connected system. Let the right side of the differential algebraic equation be zero, and then solve it to get the equilibrium point of the system.

Further, step S2 specifically includes: using a block connection method to modularize each VSC, linearizing each VSC at a balance point, and selecting appropriate input and output variables to form a VSC state equation of the VSC submodule system;

Linearize the network model equation, select the same input and output variables as the VSC, and form the network state equation of the network submodule system;

Through coordinate transformation, the input and output variables of the VSC submodule system and the network submodule system are transformed into a unified coordinate system. According to the input and output interface relationship, the VSC state equation and the network state equation of each VSC submodule system are combined to calculate the state matrix of the system.

Furthermore, step S3 specifically includes: calculating the characteristic root of the equilibrium point obtained in step S1 according to the state matrix of the system obtained in step S2, and selecting the equilibrium point with only one positive characteristic root as a type 1 equilibrium point of the system.

Further, step S4 specifically includes: obtaining the hypertangent plane equation of the system according to the state matrix of the system obtained in step S2 and the type 1 equilibrium point of the system obtained in step S3:

Where φ is the phase-locked loop angle difference, x _pll is the output of the phase-locked loop integrator, φ ₁ is the steady-state value of φ at the equilibrium point, y ₁ is the coefficient of the hypertangent plane equation, and y ₁ ^T is the transpose of y ₁ ;

According to the state matrix J of the system, the coefficients of the hypertangent plane equation are calculated:

Among them, μ is the only positive eigenvalue root in the type 1 equilibrium point, J ^T is the transpose of the state matrix J;

Substitute the specific value of the fault trajectory into the hypertangent plane equation of formula (1.5). When the sign of the value of the function F changes, it is considered that the system fault trajectory crosses the hypertangent plane and satisfies

Where Δt is the time step of the system numerical simulation, and the time t corresponding to this intersection is taken as the estimated value of the limit removal time of the corresponding fault.

Another aspect of the present invention provides a device for calculating transient stability limit removal time of a multi-machine system, comprising:

A first processor is used to establish a differential algebraic equation of a power electronic multi-VSC grid-connected system and obtain a balance point of the system, wherein the differential algebraic equation includes a multi-VSC node dynamic model equation and a network model equation;

A second processor is used to linearize the multi-VSC node dynamic model equation and the network model equation to obtain corresponding VSC state equations and network state equations, and to calculate the state matrix of the system by combining the VSC state equation and the network state equation through coordinate transformation and input-output interface relationship;

A third processor is used to calculate a type 1 equilibrium point of the system according to the equilibrium point obtained by the first processor and the state matrix of the system obtained by the second processor;

The fourth processor is used to calculate the hypertangent plane equation based on the state matrix of the system obtained by the second processor and the type 1 equilibrium point of the system obtained by the third processor, and to bring the fault trajectory information of the system into all the tangent plane equations. When the continuous fault trajectory intersects with a certain tangent plane first, the time corresponding to the intersection is used as the estimated value of the limit resection time of the corresponding fault.

1. Further, the first processor is specifically used to: establish a multi-VSC node dynamic model. When only the phase-locked loop dynamics are considered, the VSC node dynamic model is represented by a second-order dynamic differential equation:

Wherein, _utq,i represents the q-axis component of the terminal voltage ut,i under the PLL control dq coordinate of the i-th VSC, _xpll,i is the output of the phase-locked loop integrator of the i-th VSC, _φi represents the angle difference between the phase-locked loop output of the i-th VSC and the reference coordinate, kp_pll _,i and _kipll,i represent the proportional coefficient and integral coefficient of the PLL respectively;

Establish a network model, ignore the dynamics of the AC current time scale, describe the network with a node admittance matrix, and represent the load with a constant admittance. Import the node admittance matrix to obtain the network model equation:

Where, I _s and I _vsc represent the injected current vectors of the balancing node and the VSC node, respectively, U _s and U _vsc represent the node voltage vectors of the balancing node and the VSC node, respectively, and the network node mixing matrix M is expressed as follows:

Among them, Yra, Yrb, Yrc, and Yrd are the four components of the network node mixing matrix considering the load impedance;

By combining equations (1.1) and (1.2), we get the differential algebraic equation of the power electronic multi-VSC grid-connected system. Let the right side of the differential algebraic equation be zero, and then solve it to get the equilibrium point of the system:

2. Further, the second processor is specifically used to: adopt a block connection method to modularize each VSC, linearize each VSC at the equilibrium point, and select appropriate input and output variables to form a VSC state equation of the VSC submodule system;

Linearize the network model equation, select the same input and output variables as the VSC, and form the network state equation of the network submodule system;

Through coordinate transformation, the input and output variables of the VSC submodule system and the network submodule system are transformed into a unified coordinate system. According to the input and output interface relationship, the VSC state equation and the network state equation of each VSC submodule system are combined to calculate the state matrix of the system.

Furthermore, the third processor is specifically used to calculate the characteristic roots of the equilibrium point obtained by the first processor according to the state matrix of the system obtained by the second processor, and select the equilibrium point with only one positive characteristic root as a type 1 equilibrium point of the system.

3. Further, the fourth processor is specifically used to obtain the hypertangent plane equation of the system according to the state matrix of the system obtained by the second processor and the type 1 equilibrium point of the system obtained by the third processor:

Where φ is the phase-locked loop angle difference, x _pll is the output of the phase-locked loop integrator, φ ₁ is the steady-state value of φ at the equilibrium point, y ₁ is the coefficient of the hypertangent plane equation, and y ₁ ^T is the transpose of y ₁ ;

According to the state matrix J of the system, the coefficients of the hypertangent plane equation are calculated:

Among them, μ is the only positive eigenvalue root in the type 1 equilibrium point, J ^T is the transpose of the state matrix J;

Substitute the specific value of the fault trajectory into the hypertangent plane equation of formula (1.5). When the sign of the value of the function F changes, it is considered that the system fault trajectory crosses the hypertangent plane and satisfies

Where Δt is the time step of the system numerical simulation, and the time t corresponding to this intersection is taken as the estimated value of the limit removal time of the corresponding fault.

Compared with the prior art, the present invention has the following beneficial effects:

Aiming at the problem that the transient stability capability and stability margin of the new power system with renewable energy as the main body are difficult to quantitatively calculate and analyze, a transient stability limit cut-off time calculation method based on the super-tangent plane of the stability domain is provided, which can calculate the limit cut-off time of the multi-machine system after a fault. The transient stability capability of the multi-machine system can be evaluated and the stability margin of the multi-machine system can be quantitatively measured through the limit cut-off time, which effectively solves the problem that the transient stability of the multi-machine system is difficult to evaluate, and provides an effective basis for the transient stability analysis and protection parameter setting of the new power system with renewable energy as the main body.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1 is a flow chart of a method for calculating transient stability limit removal time of a multi-machine system according to an embodiment of the present invention;

FIG2 is a schematic diagram of the topological structure of a three-machine nine-node system according to an embodiment of the present invention.

Detailed ways

In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

As shown in FIG1 , a first embodiment of the present invention provides a method for calculating transient stability limit removal time of a multi-machine system, comprising the following steps:

S1: establishing a differential algebraic equation of a power electronic multi-VSC grid-connected system and obtaining a balance point of the system, wherein the differential algebraic equation includes a multi-VSC node dynamic model equation and a network model equation;

Step S1 specifically includes: establishing a multi-VSC node dynamic model. When only the phase-locked loop dynamics are considered, the VSC node dynamic model can be expressed by a second-order dynamic differential equation:

Wherein, _utq,i represents the q-axis component of the terminal voltage ut,i under the PLL-controlled dq coordinate of the i-th VSC, _xpl,i is the output of the phase-locked loop integrator of the i-th VSC, _φi represents the angle difference between the phase-locked loop output of the i-th VSC and the reference coordinate, _{kp_pll,i} and _kipll,i represent the proportional coefficient and integral coefficient of the PLL, respectively.

For the balanced nodes in the system, an infinite voltage source is used to simplify the analysis.

Establish a network model, ignore the dynamics of the AC current time scale, the network can be described by the node admittance matrix, the load is represented by a constant admittance, and the network model equation can be obtained by importing the node admittance matrix:

Where, I _s and I _vsc represent the injected current vectors of the balancing node and the VSC node, respectively, U _s and U _vsc represent the node voltage vectors of the balancing node and the VSC node, respectively, and the network node mixing matrix M can be expressed as follows:

Among them, Yra, Yrb, Yrc, and Yrd are the four components of the network node mixing matrix considering the load impedance.

By combining equations (1.1) and (1.2), we get the differential algebraic equation of the power electronic multi-VSC grid-connected system. Let the right side of the differential algebraic equation be zero, and then solve it to get the equilibrium point of the system.

S2: Linearize the multi-VSC node dynamic model equations and network model equations to obtain the corresponding state equations. Through coordinate transformation and input-output interface relationship, the VSC state equation and the network state equation are combined to calculate the system state matrix J;

Step S2 specifically includes: adopting a block connection method to modularize each VSC, linearizing each VSC at a balance point, and selecting appropriate input and output variables to form a VSC state equation of the VSC submodule system;

Linearize the network model equation, select the same input and output variables as the VSC, and form the network state equation of the network submodule system;

Through coordinate transformation, the input and output variables of the VSC submodule system and the network submodule system are transformed into a unified coordinate system. According to the input and output interface relationship, the state equations of each VSC submodule (VSC state equation and network state equation) are combined to calculate the state matrix J of the system.

S3: According to the equilibrium point obtained in step S1 and the state matrix of the system obtained in step S2, a type 1 equilibrium point of the system is calculated;

Step S3 specifically includes: calculating the characteristic root of the equilibrium point obtained in step S1 according to the state matrix of the system obtained in step S2, and selecting the equilibrium point with only one positive characteristic root, which is the type 1 equilibrium point of the system.

S4: Based on the state matrix of the system obtained in step S2 and the type 1 equilibrium point of the system obtained in step S3, the hypertangent plane equation of the system is calculated, and the fault trajectory information of the system is brought into all the tangent plane equations. When the continuous fault trajectory intersects with a certain tangent plane first, the time corresponding to the intersection point can be used as the estimated value of the limit removal time of the corresponding fault.

Step S4 specifically includes: according to the state matrix of the system obtained in step S2 and the type 1 equilibrium point of the system obtained in step S3, the hypertangent plane equation of the system can be obtained:

Where φ is the phase-locked loop angle difference, x _pll is the output of the phase-locked loop integrator, φ ₁ is the steady-state value of φ at the equilibrium point, y ₁ is the coefficient of the hypertangent plane equation, and y ₁ ^T is the transpose of y ₁ ;

According to the state matrix J of the system, the coefficients of the hypertangent plane equation can be calculated:

Where μ is the only positive eigenvalue in a type 1 equilibrium point, and J ^T is the transpose of the state matrix J.

Substitute the specific value of the fault trajectory into the hypertangent plane equation of formula (1.5). When the sign of the value of the function F changes, it is considered that the system fault trajectory crosses the hypertangent plane, that is, it satisfies

Wherein Δt is the time step of the system numerical simulation, and the time t corresponding to this intersection is taken as the estimated value of the limit removal time of the corresponding fault. It is worth noting that since the stability domain is usually convex near the type 1 equilibrium point, the intersection between the fault trajectory and the hyper-cut plane is often outside the stability domain, that is, the calculation result of the method proposed in the present invention is always slightly larger than the actual value. The calculation result has good conservatism when the system fault is removed and the protection device parameters are set, which can ensure the stable operation of the system.

In a specific embodiment, the three-machine nine-node system shown in Figure 2 is built in MATLAB/Smulink, and the transient stability limit cut-off time of the built three-machine nine-node system is calculated according to a method for calculating the transient stability limit cut-off time of a multi-machine system provided by an embodiment of the present invention.

Table 1 is a comparison table of the results of calculating the limit fault clearing time by the method proposed in the present invention and the simulation results of MATLAB/Smulink under different voltage drop levels at bus 1 of the three-machine nine-bus system.

Table 1 Limit cut-off time under different voltage drop levels

It can be seen that for faults when the voltage drops to 0.4pu, 0.3pu, 0.2pu, 0.1pu and 0, the method proposed in the present invention can effectively calculate the limit clearing time under different faults of the multi-machine system, and the relative error of the calculated limit fault clearing time is kept within 7%, which has good calculation accuracy and can effectively measure the transient stability margin of the multi-machine system.

In summary, the calculation method proposed in the first embodiment of the present invention can obtain the limit cut-off time of the multi-machine system under different voltage drop degrees, has good calculation accuracy, can quantitatively evaluate the transient stability capability of the multi-machine system, and effectively measure the transient stability margin of the multi-machine system.

Embodiment 2 of the present invention provides a device for calculating transient stability limit removal time of a multi-machine system, comprising the following steps:

A first processor is used to establish a differential algebraic equation of a power electronic multi-VSC grid-connected system and obtain a balance point of the system, wherein the differential algebraic equation includes a multi-VSC node dynamic model equation and a network model equation;

A second processor is used to linearize the multi-VSC node dynamic model equation and the network model equation to obtain corresponding VSC state equations and network state equations, and to calculate the state matrix of the system by combining the VSC state equation and the network state equation through coordinate transformation and input-output interface relationship;

A third processor is used to calculate a type 1 equilibrium point of the system according to the equilibrium point obtained by the first processor and the state matrix of the system obtained by the second processor;

The fourth processor is used to calculate the hypertangent plane equation based on the state matrix of the system obtained by the second processor and the type 1 equilibrium point of the system obtained by the third processor, and to bring the fault trajectory information of the system into all the tangent plane equations. When the continuous fault trajectory intersects with a certain tangent plane first, the time corresponding to the intersection is used as the estimated value of the limit resection time of the corresponding fault.

Compared with the prior art, the method and system for calculating the transient stability limit removal time of a multi-machine system provided by the embodiment of the present invention have the following beneficial effects:

Aiming at the problem that the transient stability capability and stability margin of the new power system with renewable energy as the main body are difficult to quantitatively calculate and analyze, a transient stability limit cut-off time calculation method based on the super-tangent plane of the stability domain is provided, which can calculate the limit cut-off time of the multi-machine system after a fault. The transient stability capability of the multi-machine system can be evaluated and the stability margin of the multi-machine system can be quantitatively measured through the limit cut-off time, which effectively solves the problem that the transient stability of the multi-machine system is difficult to evaluate, and provides an effective basis for the transient stability analysis and protection parameter setting of the new power system with renewable energy as the main body.

Another aspect of the present invention provides a transient stability limit cut-off time calculation system for a multi-machine system based on a stability domain hypercut plane, comprising: a computer-readable storage medium and a processor;

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

The processor is used to read the executable instructions stored in the computer-readable storage medium to execute the method for calculating the transient stability limit cut-off time of a multi-machine system based on the stability domain hypercut plane described in the first aspect.

On the other hand, the present invention provides a non-transitory computer-readable storage medium having a computer program stored thereon. When the computer program is executed by a processor, the method for calculating the transient stability limit cut-off time of a multi-machine system based on the stability domain hypercut plane described in the first aspect is implemented.

Those skilled in the art will appreciate that the embodiments of the present application may be provided as methods, systems, or computer program products. Therefore, the present application may adopt the form of a complete hardware embodiment, a complete software embodiment, or an embodiment combining software and hardware. Moreover, the present application may adopt the form of a computer program product implemented on one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) containing computer-usable program codes.

The present application is described with reference to the flowcharts and/or block diagrams of the methods, devices (systems), and computer program products according to the embodiments of the present application. It should be understood that each process and/or box in the flowchart and/or block diagram, as well as the combination of the processes and/or boxes in the flowchart and/or block diagram, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing device to generate a machine, so that the instructions executed by the processor of the computer or other programmable data processing device generate a device for implementing the functions specified in one or more processes in the flowchart and/or one or more boxes in the block diagram.

These computer program instructions may also be stored in a computer-readable memory that can direct a computer or other programmable data processing device to operate in a specific manner, so that the instructions stored in the computer-readable memory produce a manufactured product including an instruction device that implements the functions specified in one or more processes in the flowchart and/or one or more boxes in the block diagram.

These computer program instructions may also be loaded onto a computer or other programmable data processing device so that a series of operational steps are executed on the computer or other programmable device to produce a computer-implemented process, whereby the instructions executed on the computer or other programmable device provide steps for implementing the functions specified in one or more processes in the flowchart and/or one or more boxes in the block diagram.

Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, ordinary technicians in the relevant field should understand that the specific implementation methods of the present invention can still be modified or replaced by equivalents. Any modification or equivalent replacement that does not depart from the spirit and scope of the present invention should be covered within the scope of protection of the claims of the present invention.

Claims (10)

1. A method for calculating transient stability limit removal time of a multi-machine system, characterized in that it comprises the following steps: S1: establishing a differential algebraic equation of a power electronic multi-VSC grid-connected system and obtaining a balance point of the system, wherein the differential algebraic equation includes a multi-VSC node dynamic model equation and a network model equation; S2: linearizing the multi-VSC node dynamic model equation and the network model equation to obtain the corresponding VSC state equation and network state equation respectively, and combining the VSC state equation and the network state equation through coordinate transformation and input-output interface relationship to calculate the state matrix of the system; S3: According to the equilibrium point obtained in step S1 and the state matrix of the system obtained in step S2, a type 1 equilibrium point of the system is calculated; S4: Based on the state matrix of the system obtained in step S2 and the type 1 equilibrium point of the system obtained in step S3, the hypertangent plane equation is calculated, and the fault trajectory information of the system is brought into all the tangent plane equations. When the continuous fault trajectory intersects with a certain tangent plane first, the time corresponding to this intersection is used as the estimated value of the limit removal time of the corresponding fault. 2. The method for calculating the transient stability limit removal time of a multi-machine system according to claim 1, characterized in that: step S1 specifically comprises: establishing a multi-VSC node dynamic model, when only the phase-locked loop dynamics are considered, the node dynamic model of the VSC is represented by a second-order dynamic differential equation: Wherein, _utq,i represents the q-axis component of the terminal voltage ut,i under the PLL control dq coordinate of the i-th VSC, _xpll,i is the output of the phase-locked loop integrator of the i-th VSC, _φi represents the angle difference between the phase-locked loop output of the i-th VSC and the reference coordinate, kp_pll _,i and _kipll,i represent the proportional coefficient and integral coefficient of the PLL respectively; Establish a network model, ignore the dynamics of the AC current time scale, describe the network with a node admittance matrix, and represent the load with a constant admittance. Import the node admittance matrix to obtain the network model equation: ; Where, I _s and I _vsc represent the injected current vectors of the balancing node and the VSC node, respectively, U _s and U _vsc represent the node voltage vectors of the balancing node and the VSC node, respectively, and the network node mixing matrix M is expressed as follows: ; Among them, Yra, Yrb, Yrc, and Yrd are the four components of the network node mixing matrix considering the load impedance; By combining equations (1.1) and (1.2), we get the differential algebraic equation of the power electronic multi-VSC grid-connected system. Let the right side of the differential algebraic equation be zero, and then solve it to get the equilibrium point of the system. 3. The method for calculating the transient stability limit cut-off time of a multi-machine system according to claim 1, characterized in that: step S2 specifically comprises: adopting a block connection method, modularizing each VSC, linearizing each VSC at a balance point, and selecting appropriate input and output variables to form a VSC state equation of the VSC submodule system; Linearize the network model equation, select the same input and output variables as the VSC, and form the network state equation of the network submodule system; Through coordinate transformation, the input and output variables of the VSC submodule system and the network submodule system are transformed into a unified coordinate system. According to the input and output interface relationship, the VSC state equation and the network state equation of each VSC submodule system are combined to calculate the state matrix of the system. 4. The method for calculating the transient stability limit cut-off time of a multi-machine system as described in claim 1 is characterized in that: step S3 specifically includes: calculating the characteristic root of the equilibrium point obtained in step S1 according to the state matrix of the system obtained in step S2, and selecting the equilibrium point with only one positive characteristic root as a type 1 equilibrium point of the system. 5. The method for calculating transient stability limit removal time of a multi-machine system according to claim 1, characterized in that: step S4 specifically comprises: obtaining the hypertangent plane equation of the system according to the state matrix of the system obtained in step S2 and the type 1 equilibrium point of the system obtained in step S3: ; Where φ is the phase-locked loop angle difference, x _pll is the output of the phase-locked loop integrator, φ ₁ is the steady-state value of φ at the equilibrium point, y ₁ is the coefficient of the hypertangent plane equation, and y ₁ ^T is the transpose of y ₁ ; According to the state matrix J of the system, the coefficients of the hypertangent plane equation are calculated: ; Among them, μ is the only positive eigenvalue root in the type 1 equilibrium point, J ^T is the transpose of the state matrix J; Substitute the specific value of the fault trajectory into the hypertangent plane equation of formula (1.5). When the sign of the value of the function F changes, it is considered that the system fault trajectory crosses the hypertangent plane and satisfies ; Where Δt is the time step of the system numerical simulation, and the time t corresponding to this intersection is taken as the estimated value of the limit removal time of the corresponding fault. 6. A device for calculating transient stability limit removal time of a multi-machine system, characterized by comprising: A first processor is used to establish a differential algebraic equation of a power electronic multi-VSC grid-connected system and obtain a balance point of the system, wherein the differential algebraic equation includes a multi-VSC node dynamic model equation and a network model equation; A second processor is used to linearize the multi-VSC node dynamic model equation and the network model equation to obtain corresponding VSC state equations and network state equations, and to calculate the state matrix of the system by combining the VSC state equation and the network state equation through coordinate transformation and input-output interface relationship; A third processor is used to calculate a type 1 equilibrium point of the system according to the equilibrium point obtained by the first processor and the state matrix of the system obtained by the second processor; The fourth processor is used to calculate the hypertangent plane equation based on the state matrix of the system obtained by the second processor and the type 1 equilibrium point of the system obtained by the third processor, and to bring the fault trajectory information of the system into all the tangent plane equations. When the continuous fault trajectory intersects with a certain tangent plane first, the time corresponding to the intersection is used as the estimated value of the limit resection time of the corresponding fault. 7. The device for calculating transient stability limit removal time of a multi-machine system according to claim 6, wherein: the first processor is specifically used to: establish a multi-VSC node dynamic model, when only the phase-locked loop dynamics are considered, the node dynamic model of the VSC is represented by a second-order dynamic differential equation: ; Wherein, _utq,i represents the q-axis component of the terminal voltage ut,i under the PLL control dq coordinate of the i-th VSC, _xpll,i is the output of the phase-locked loop integrator of the i-th VSC, _φi represents the angle difference between the phase-locked loop output of the i-th VSC and the reference coordinate, kp_pll _,i and _kipll,i represent the proportional coefficient and integral coefficient of the PLL respectively; Establish a network model, ignore the dynamics of the AC current time scale, describe the network with a node admittance matrix, and represent the load with a constant admittance. Import the node admittance matrix to obtain the network model equation: ; Where, I _s and I _vsc represent the injected current vectors of the balancing node and the VSC node, respectively, U _s and U _vsc represent the node voltage vectors of the balancing node and the VSC node, respectively, and the network node mixing matrix M is expressed as follows: ; Among them, Yra, Yrb, Yrc, and Yrd are the four components of the network node mixing matrix considering the load impedance; By combining equations (1.1) and (1.2), we get the differential algebraic equation of the power electronic multi-VSC grid-connected system. Let the right side of the differential algebraic equation be zero, and then solve it to get the equilibrium point of the system. 8. The device for calculating transient stability limit cut-off time of a multi-machine system according to claim 6, characterized in that: the second processor is specifically used to: adopt a block connection method to modularize each VSC, linearize each VSC at the equilibrium point, and select appropriate input and output variables to form a VSC state equation of the VSC submodule system; Linearize the network model equation, select the same input and output variables as the VSC, and form the network state equation of the network submodule system; Through coordinate transformation, the input and output variables of the VSC submodule system and the network submodule system are transformed into a unified coordinate system. According to the input and output interface relationship, the VSC state equation and the network state equation of each VSC submodule system are combined to calculate the state matrix of the system. 9. The transient stability limit cut-off time calculation device for a multi-machine system as described in claim 6 is characterized in that: the third processor is specifically used to calculate the characteristic roots of the equilibrium point obtained by the first processor based on the state matrix of the system obtained by the second processor, and select the equilibrium point with only one positive characteristic root as a type 1 equilibrium point of the system. 10. The device for calculating transient stability limit cut-off time of a multi-machine system according to claim 6, wherein the fourth processor is specifically used to obtain the hypertangent plane equation of the system according to the state matrix of the system obtained by the second processor and the type 1 equilibrium point of the system obtained by the third processor: ; Where φ is the phase-locked loop angle difference, x _pll is the output of the phase-locked loop integrator, φ ₁ is the steady-state value of φ at the equilibrium point, y ₁ is the coefficient of the hypertangent plane equation, and y ₁ ^T is the transpose of y ₁ ; According to the state matrix J of the system, the coefficients of the hypertangent plane equation are calculated: ; Among them, μ is the only positive eigenvalue root in the type 1 equilibrium point, J ^T is the transpose of the state matrix J; Substitute the specific value of the fault trajectory into the hypertangent plane equation of formula (1.5). When the sign of the value of the function F changes, it is considered that the system fault trajectory crosses the hypertangent plane and satisfies ; Where Δt is the time step of the system numerical simulation, and the time t corresponding to this intersection is taken as the estimated value of the limit removal time of the corresponding fault.