Method and system for maximum estimation of attraction domain of VSC access weak network system

Abstract

The invention discloses a maximum estimation attraction domain method and system for a VSC access weak network system, and relates to the technical field of VSC grid-connected access, and the method comprises the steps: simplifying the VSC access weak network system comprising a frequency limiter, and building a second-order mathematical model suitable for phase-locked loop synchronous transient stability analysis; the difference between the PLL output angular frequency and the rated angular frequency and the upper limit and the lower limit of the frequency limiter are defined; constructing a maximum estimation attraction domain of the VSC access weak network system without the frequency limiter based on a linear matrix inequality optimization method, and determining an optimal Lyapunov energy function corresponding to the attraction domain and a critical level value of the optimal Lyapunov energy function; the constructed maximum estimation attraction domain is limited in a region determined by the upper limit and the lower limit of the frequency limiter, the maximum estimation attraction domain of the VSC access weak network system containing the frequency limiter is obtained, and the influence on the transient synchronization stability of the system is evaluated by comparing the maximum estimation attraction domain areas under different frequency limiting values. According to the invention, protection is provided for VSC grid-connected line relay.

Description

Technical Field

[0001] The present invention relates to the technical field of VSC grid-connected access, in particular to a method and system for the maximum estimated region of attraction of a VSC accessing a weak grid system. Background Art

[0002] In recent years, with the rapid development of renewable energy power generation, the voltage source converter (VSC), as the grid-connected interface of renewable energy power generation units, has played an increasingly important role in the power system. Different from the traditional synchronous motor that maintains synchronization through a rotating rotor, the VSC usually maintains synchronization with the AC grid through a phase-locked loop (PLL). The dynamic performance of the PLL highly depends on the voltage characteristics of the grid connection point. However, under weak grid conditions, the voltage at the grid connection point is vulnerable to disturbances, thus deteriorating the dynamic characteristics of the PLL and causing instability risks. In addition, with the increasing proportion of new energy power generation, in order to ensure the safe and reliable operation of the power grid and prevent large-scale disconnection accidents, relevant national standards have been introduced to clearly define the fault ride-through characteristics of wind and solar power sources, requiring the converter to have the ability to withstand grid faults and maintain power continuity under certain fault conditions without arbitrary blocking. Therefore, it is particularly important to study the transient synchronization stability problem of a PLL-synchronized VSC accessing a weak grid system. Summary of the Invention

[0003] The purpose of this part is to outline some aspects of the embodiments of the present invention and briefly introduce some preferred embodiments. Simplifications or omissions may be made in this part, as well as in the abstract and title of the present application, to avoid obscuring the purpose of this part, the abstract, and the title, and such simplifications or omissions shall not be used to limit the scope of the present invention.

[0004] In view of the above existing problems, the present invention is proposed.

[0005] Therefore, the present invention provides a method and system for the maximum estimated region of attraction of a VSC accessing a weak grid system, which can solve the problems mentioned in the background art.

[0006] To solve the above technical problems, the present invention provides the following technical solutions:

[0007] In a first aspect, the present invention provides a method for the maximum estimated region of attraction of a VSC accessing a weak grid system, which includes simplifying the VSC accessing a weak grid system with a frequency limiter, establishing a second-order mathematical model applicable to the transient stability analysis of PLL synchronization, and defining the difference between the output angular frequency of the PLL and the rated angular frequency, as well as the upper and lower limits of the frequency limiter.

[0008] Construct the maximum estimated region of attraction of the VSC-connected weak grid system without a frequency limiter based on the linear matrix inequality optimization method, and determine the corresponding optimal Lyapunov energy function and its critical level value of this region of attraction;

[0009] Limit the constructed maximum estimated region of attraction within the region determined by the upper and lower limits of the frequency limiter to obtain the maximum estimated region of attraction of the VSC-connected weak grid system with a frequency limiter, and evaluate the impact on the transient synchronization stability of the system by comparing the areas of the maximum estimated regions of attraction under different frequency limit values.

[0010] As a preferred solution of the method for the maximum estimated region of attraction of the VSC-connected weak grid system described in the present invention, wherein: the simplification of the VSC-connected weak grid system with a frequency limiter includes,

[0011] Ignore the electromagnetic transient processes of the LC filter of the VSC and the AC network;

[0012] Assume that the current inner loop bandwidth is much higher than the PLL bandwidth, and determine that the d-axis and q-axis currents I td and I tq track the set values I tdref and I tqref in real time;

[0013] Define the PLL with the q-axis component V tq of the grid-connected point voltage as the input signal, and after passing through a proportional-integral controller, the output is the locked-phase frequency ω pll and the phase angle θ pll , and a limiter is added after the PI control link of the PLL;

[0014] Determine the equilibrium point x e =(θ pll,e ,0), where arcsin((X F I tdref -R F I tqref ) / V F ).

[0015] As a preferred solution of the method for the maximum estimated region of attraction of the VSC-connected weak grid system described in the present invention, wherein: the linear matrix inequality optimization method includes,

[0016] Construct the optimal Lyapunov energy function V(x) of the VSC-connected weak grid system without a frequency limiter, and determine the constant c;

[0017] Solve the optimization problem with equality constraints to obtain the critical level values V cr1 and V cr2 ;

[0018] Form the maximum estimated region of attraction of the VSC-connected weak grid system without a frequency limiter, and the calculation formula is as follows:

[0019]

[0020] where, Ω FL is a strict subset of the true region of attraction of the WG-VSC_FL system.

[0021] As a preferred solution of the method for the maximum estimated region of attraction of the VSC-connected weak grid system according to the present invention, wherein: the solution of the optimization problem with equality constraints includes,

[0022] V cr1 = min{V(x)}

[0023] s.t. Δω = Δω m

[0024] V cr2 = min{V(x)}

[0025] s.t. Δω = -Δω m

[0026] That is, find the point that maximizes V(x) within the upper boundary F up and the lower boundary F low of the set F.

[0027] As a preferred solution of the method for the maximum estimated region of attraction of the VSC-connected weak grid system according to the present invention, wherein: the limitation of the constructed maximum estimated region of attraction to the region determined by the upper and lower limits of the frequency limiter includes,

[0028] Analyze the maximum estimated regions of attraction of the VSC-connected weak grid system with and without a frequency limiter when the voltage at the fault point drops to 0.09 pu, the active injection current is 0 pu, and the reactive injection current is 1 pu;

[0029] Determine the positional relationship between the equilibrium points X e,0 and X e,1 of the VSC-connected weak grid system with a frequency limiter.

[0030] As a preferred solution of the method for the maximum estimated region of attraction of the VSC-connected weak grid system according to the present invention, wherein: the comparison of the maximum estimated regions of attraction under different frequency limits includes,

[0031] Use the PSCAD / EMPDC platform to build a full-order switching model of the VSC-connected weak grid system with and without a frequency limiter.

[0032] As a preferred solution of the method for the maximum estimated attraction region of the VSC-connected weak grid system according to the present invention, wherein: comparing the maximum estimated attraction regions under different frequency limits further includes observing the change of the maximum estimated attraction region of the system when the frequency limit increases, and using it to evaluate the influence of different frequency limits on the transient synchronization stability of the system.

[0033] In a second aspect, the present invention provides a system for the maximum estimated attraction region of a VSC-connected weak grid system, which includes: a model construction module, an attraction region construction module, and an attraction region comparison module;

[0034] The model construction module is used to simplify the VSC-connected weak grid system with a frequency limiter, establish a second-order mathematical model suitable for the transient stability analysis of the phase-locked loop synchronization, and define the difference between the output angular frequency of the PLL and the rated angular frequency, as well as the upper and lower limits of the frequency limiter.

[0035] The attraction region construction module is used to construct the maximum estimated attraction region of the VSC-connected weak grid system without a frequency limiter based on the linear matrix inequality optimization method, and determine the corresponding optimal Lyapunov energy function and its critical level value of the attraction region.

[0036] The attraction region comparison module is used to limit the constructed maximum estimated attraction region within the region determined by the upper and lower limits of the frequency limiter, obtain the maximum estimated attraction region of the VSC-connected weak grid system with a frequency limiter, and evaluate the influence on the transient synchronization stability of the system by comparing the areas of the maximum estimated attraction regions under different frequency limits.

[0037] In a third aspect, the present invention provides a computer device, including a memory and a processor, where the memory stores a computer program, and wherein: when the processor executes the computer program, the steps of the method for the maximum estimated attraction region of the VSC-connected weak grid system are implemented.

[0038] In a fourth aspect, the present invention provides a computer-readable storage medium, on which a computer program is stored, and wherein: when the computer program is executed by a processor, the steps of the method for the maximum estimated attraction region of the VSC-connected weak grid system are implemented.

[0039] Compared with the prior art, the beneficial effects of the present invention are as follows: a method for constructing the maximum estimated attraction domain of a VSC-connected weak grid system with a frequency limiter is proposed. Based on this maximum estimated attraction domain, the transient synchronization stability analysis of the VSC grid-connected system with a frequency limiter can be realized; by using the method for constructing the maximum estimated attraction domain of a VSC-connected weak grid system with a frequency limiter disclosed in this patent, the maximum estimated attraction domain of the system under the frequency limit value can be obtained, and the influence of different frequency limit values on the transient synchronization stability of the system can be intuitively analyzed; by using the method for constructing the maximum estimated attraction domain of a phase-locked loop synchronized VSC-connected weak grid system with a frequency limiter disclosed in the present invention, the critical clearing time after the system suffers a ground short-circuit fault can be calculated efficiently and quickly, so as to play a certain reference role in the setting of relay protection for the phase-locked loop synchronized VSC grid-connected line with a frequency limiter. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following will briefly introduce the drawings required for the description of the embodiments. Obviously, the following drawings are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0041] Figure 1 Topological structure diagram of a method and system for the maximum estimated attraction domain of a VSC-connected weak grid system provided by an embodiment of the present invention;

[0042] Figure 2 Internal structure diagram of a computer device of a method and system for the maximum estimated attraction domain of a VSC-connected weak grid system provided by an embodiment of the present invention;

[0043] Figure 3 Equivalent circuit model diagram of a method and system for the maximum estimated attraction domain of a VSC-connected weak grid system provided by an embodiment of the present invention;

[0044] Figure 4 Set schematic diagram of a method and system for the maximum estimated attraction domain of a VSC-connected weak grid system provided by an embodiment of the present invention;

[0045] Figure 5 Horizontal value schematic diagram of a method and system for the maximum estimated attraction domain of a VSC-connected weak grid system provided by an embodiment of the present invention;

[0046] Figure 6 Comparison analysis diagram of a method and system for the maximum estimated attraction domain of a VSC-connected weak grid system provided by an embodiment of the present invention;

[0047] Figure 7Electromagnetic transient simulation verification diagram of a method and system for the maximum estimated attraction region of a VSC-connected weak grid system provided by an embodiment of the present invention;

[0048] Figure 8 Comparison diagram of the maximum estimated attraction region of a VSC-connected weak grid system under different frequency limits for a method and system for the maximum estimated attraction region of a VSC-connected weak grid system provided by an embodiment of the present invention. Detailed implementation manners

[0049] To make the above objects, features, and advantages of the present invention more comprehensible, the following will describe the detailed implementation manners of the present invention with reference to the accompanying drawings of the specification. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all of them. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0050] In the following description, many specific details are set forth to fully understand the present invention. However, the present invention can also be implemented in other ways different from those described herein. Those skilled in the art can make similar extensions without departing from the connotation of the present invention. Therefore, the present invention is not limited by the specific embodiments disclosed below.

[0051] Secondly, the so-called "one embodiment" or "embodiment" herein refers to a specific feature, structure, or characteristic that can be included in at least one implementation manner of the present invention. The "in one embodiment" that appears in different places in this specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment that excludes other embodiments.

[0052] Embodiment 1, referring to Figures 1-6 , which is the first embodiment of the present invention. This embodiment provides a method for the maximum estimated attraction region of a VSC-connected weak grid system, including:

[0053] This application provides a solution that can effectively solve the above-mentioned problems. Next, multiple embodiments will be used to elaborate in detail how to implement the method for the maximum estimated attraction region of a VSC-connected weak grid system;

[0054] Figure 1 The flowchart of a method for the maximum estimated attraction region of a VSC-connected weak grid system and system is shown, including:

[0055] S1: Simplify the VSC-connected weak grid system with a frequency limiter, establish a second-order mathematical model suitable for the synchronous transient stability analysis of the phase-locked loop, and define the difference between the PLL output angular frequency and the rated angular frequency, as well as the upper and lower limits of the frequency limiter;

[0056] Furthermore, Figure 1Topology and control structure of the WG-VSC (WG-VSC with frequency limiter, WG-VSC_FL) system studied in this paper; C is the DC capacitor, u dc is the DC capacitor voltage; the VSC is connected to the infinite grid through an LC filter, L f and C f are the filter reactance and capacitance respectively; E, V t and V F represent the VSC output voltage, grid connection point voltage and fault point voltage respectively; Z grid is the equivalent impedance from the fault point to the infinite grid; R F and L F are the equivalent resistance and reactance from the grid connection point to the fault point F respectively, which can take into account the influence of the weak connection characteristics under severe symmetrical faults on the synchronous transient stability of the phase-locked loop (PLL).

[0057] Furthermore, simplify the phase-locked loop synchronous voltage source converter (VSC) with a frequency limiter connected to a weak grid system, establish a second-order mathematical model suitable for the analysis of the synchronous transient stability of the PLL, and define the difference between the PLL output angular frequency and the rated angular frequency, as well as the upper and lower limits of the frequency limiter.

[0058] Specifically, the control part on the grid-connected converter side mainly includes the current inner loop control and the phase-locked loop synchronous control; the PLL takes the q-axis component V tq of the grid connection point voltage as the input signal, and then passes through a proportional-integral (PI) controller (k p and k i are the proportional and integral coefficients respectively) and outputs the locked phase frequency ω pl and the phase angle θ pl ; it should be noted that θ pl uses the angular frequency ω s corresponding to the infinite grid voltage frequency as the reference axis; to limit the accumulation of errors in the PI controller and avoid the operating frequency of the VSC deviating too much from the rated value and improve the power quality output by the VSC, a limiter is usually added after the PI control link of the PLL, its upper limit is Δω m , the lower limit is -Δω m , the corresponding frequency upper limit Δf m = Δω m / (2*π), the frequency lower limit is -Δf m ; the current inner loop adopts a common PI control structure, I td and I tq are the d-axis and q-axis currents injected by the VSC into the grid respectively, I tdref and I tqrefThey are the current loop d-axis and q-axis current set values respectively; during normal operation, the current setting switch is at position 1, I tdref and I tqref are generated by the power outer loop control; during a severe grid fault, to avoid overcurrent in the system, the current setting switch is switched to position 1, I tdref and I tqref are directly given according to the grid standard under low voltage ride through (LVRT).

[0059] Furthermore, based on a large number of existing relevant studies on PLL synchronous transient stability, the modeling process of the second-order mathematical model of the present invention includes,

[0060] ignoring the LC filter of the VSC and the electromagnetic transients of the AC network, because the time scale of such electromagnetic transients is much faster than the PLL synchronous transient time scale concerned in this article; the current loop bandwidth is usually much higher than the PLL bandwidth, so the dynamic process of the current inner loop is ignored, that is, it is considered that I td and I tq approximately track the current set values I tdref and I tqref in real time;

[0061] Based on the above simplification conditions and the topology and control structure of the WG-VSC_FL system, the equivalent circuit model of the VSC grid-connected system during grid faults is as Figure 2 shown.

[0062] Furthermore, combining the Figure 3 equivalent circuit model shown and the Figure 1 control structure of the phase-locked loop in the calculation formula of the reduced-order model

[0063]

[0064] Δω=-k p (R F I tqref -X F I tdref +V F sinx 1 )-k i x 2

[0065] where, x 1 represents θ pl , x 2 represents the integral voltage ∫V tq dt, V tq represents the q-axis component of the grid connection point voltage, k p and k idenote the proportional coefficient and integral coefficient of the PLL proportional-integral (PI) controller, V s denote the infinite grid terminal voltage amplitude, R F and X F respectively denote the equivalent resistance and reactance from the grid connection point to the fault point, I tdref and I tdref respectively denote the d-axis reference current and q-axis reference current, Δω denotes the difference between the output angular frequency of the phase-locked loop and the rated angular frequency, Δω m and -Δω m respectively denote the upper and lower limits of the frequency limiter; the calculation formula of the function Sat(g(x), a, b) is as follows:

[0066]

[0067] where, g(x) represents the actual signal input to the saturation function, which may be the value of a certain variable in the system, such as the angular frequency deviation of the PLL, a represents the lower limit of the saturation function, and b is the upper limit of the saturation function.

[0068] Furthermore, the equilibrium point of the WG-VSC_FL system is determined as x e =(θ pl,e , 0), θ pl,e is the phase angle component of the PLL, and the expression is arcsin((X F I tdref -R F I tqref ) / V F ).

[0069] S2: Construct the maximum estimated attraction domain of the VSC-connected weak grid system without a frequency limiter based on the linear matrix inequality optimization method, and determine the corresponding optimal Lyapunov energy function and its critical level value of this attraction domain;

[0070] Furthermore, considering that when the limiter is not triggered, that is, when the output angular frequency of the PLL is within the set the WG-VSC_NFL system and the WG-VSC_FL system are exactly the same;

[0071] Then Theorem 1 is given. Theorem 1 includes,

[0072] If there exists a set satisfying the following Condition 1 and Condition 2, then the set is a strict subset of the true attraction domain of the WG-VSC_FL system;

[0073] Condition 1 includes where is the maximum estimated attraction domain of the WG-VSC_NFL system, and V(x) is the Lyapunov energy function of the WG-VSC_NFL system;

[0074] Condition 2 includes the set any point x a trajectory does not exceed the set F, that is

[0075] Define the following set Ω FL , and according to the given Theorem 1, the calculation formula is as follows:

[0076]

[0077] where, V cr is defined as

[0078] Furthermore, the method for constructing the maximum estimated attraction domain of the WG-VSC_FL system includes

[0079] Construct the optimal Lyapunov energy function V(x) of the WG-VSC_NFL system based on the linear matrix inequality optimization method and determine the constant c;

[0080] Solve the optimization problem Determine the critical level value V of V(x) cr1 , and then the maximum estimated attraction domain (Ω FL ) of the WG-VSC_FL system can be obtained, as Figure 3 shown, where and are the upper and lower boundaries of the set F respectively.

[0081] Furthermore, constructing the maximum estimated attraction domain of the VSC access weak grid system without a frequency limiter includes

[0082] From Figure 4 it can be seen that the set F has upper and lower boundaries, so the critical level value V of V(x) cr is expressed as follows:

[0083]

[0084] where, V cr1 and V cr2 respectively represent the horizontal set of V(x) and the upper boundary of the set F and the lower boundary when they are exactly tangent, corresponding to the horizontal value of V(x), as Figure 5 shown;

[0085] Vcr1 and V cr2 It is obtained by solving an optimization problem with equality constraints. It should be noted that in the present invention, the optimization problems with equality constraints can all be solved based on the Lagrange multiplier method, which will not be elaborated in the present invention. The optimization problem with equality constraints is as follows:

[0086] V cr1 = min{V(x)}

[0087] s.t. Δω = Δω m

[0088] V cr2 = min{V(x)}

[0089] s.t. Δω = -Δω m

[0090] Based on the above analysis, the present invention proposes the following construction algorithm for the maximum estimated region of attraction of the WG-VSC_FL system (1) referred to as Algorithm 1, that is,

[0091]

[0092] S3: Limit the constructed maximum estimated region of attraction within the region determined by the upper and lower limits of the frequency limiter to obtain the maximum estimated region of attraction of the VSC-connected weak grid system with a frequency limiter, and evaluate the impact on the transient synchronous stability of the system by comparing the areas of the maximum estimated regions of attraction under different frequency limit values.

[0093] Furthermore, Figure 6 gives the maximum estimated regions of attraction of the VSC-connected weak grid system with and without a frequency limiter when the fault point voltage drops to 0.09 pu, the active injection current is 0 pu, and the reactive injection current is 1 pu. It should be noted that X e,1 is the feasible equilibrium point of the VSC-connected weak grid system during the fault, and X e,0 is the feasible equilibrium point of the VSC-connected weak grid system before the fault. It can be seen from the figure that the maximum estimated region of attraction with a frequency limiter is smaller, indicating that the frequency limiter will reduce the PLL synchronous transient stability of the VSC grid-connected system. In addition, it can be seen from the figure that when there is no frequency limiter, X e,0 is located within the maximum estimated region of attraction of the VSC-connected weak grid system, indicating that when there is no frequency limiter, the system can stably transition from X e,0 to X e,1 , that is, the VSC grid-connected system can maintain PLL synchronous transient stability during the transient process. However, when there is a frequency limiter, X e,0 is located outside the maximum estimated region of attraction of the VSC-connected weak grid system, indicating that when there is a frequency limiter, the system may not be able to stably transition from Xe,0 Transition X e,1 , that is, the VSC may experience PLL synchronization transient instability

[0094] Furthermore, this embodiment also provides a maximum estimated attraction domain system for a VSC accessing a weak grid system, including: a model construction module, an attraction domain construction module, and an attraction domain comparison module;

[0095] The model construction module is used to simplify the VSC accessing weak grid system with a frequency limiter, establish a second-order mathematical model suitable for phase-locked loop synchronization transient stability analysis, and define the difference between the PLL output angular frequency and the rated angular frequency, as well as the upper and lower limits of the frequency limiter;

[0096] The attraction domain construction module is used to construct the maximum estimated attraction domain of the VSC accessing weak grid system without a frequency limiter based on the linear matrix inequality optimization method, and determine the corresponding optimal Lyapunov energy function and its critical level value of the attraction domain;

[0097] The attraction domain comparison module is used to limit the constructed maximum estimated attraction domain within the area determined by the upper and lower limits of the frequency limiter to obtain the maximum estimated attraction domain of the VSC accessing weak grid system with a frequency limiter, and evaluate the impact on the transient synchronization stability of the system by comparing the areas of the maximum estimated attraction domains under different frequency limit values.

[0098] This embodiment also provides a computer device, which can be a terminal, and its internal structure diagram can be as Figure 2 shown. The computer device includes a processor, a memory, a communication interface, a display screen, and an input device connected through a system bus. Among them, the processor of the computer device is used to provide computing and control capabilities. The memory of the computer device includes a non-volatile storage medium and an internal memory. The non-volatile storage medium stores an operating system and a computer program. The internal memory provides an environment for the operation of the operating system and the computer program in the non-volatile storage medium. The communication interface of the computer device is used to communicate with an external terminal in a wired or wireless manner, and the wireless manner can be implemented through WIFI, operator network, NFC (near field communication) or other technologies. When the computer program is executed by the processor, it realizes a method for the maximum estimated attraction domain of a VSC accessing a weak grid system. The display screen of the computer device can be a liquid crystal display screen or an electronic ink display screen, and the input device of the computer device can be a touch layer covered on the display screen, or a button, a trackball or a touchpad set on the shell of the computer device, or an external keyboard, a touchpad or a mouse, etc.

[0099] This embodiment also provides a computer-readable storage medium, on which a computer program is stored. When the computer program is executed by a processor, the following steps are implemented:

[0100] Simplify the VSC-connected weak grid system with a frequency limiter, establish a second-order mathematical model suitable for the transient stability analysis of PLL synchronization, and define the difference between the output angular frequency of the PLL and the rated angular frequency, as well as the upper and lower limits of the frequency limiter;

[0101] Based on the linear matrix inequality optimization method, construct the maximum estimated attraction domain of the VSC-connected weak grid system without a frequency limiter, and determine the corresponding optimal Lyapunov energy function and its critical level value of this attraction domain;

[0102] Limit the constructed maximum estimated attraction domain within the region determined by the upper and lower limits of the frequency limiter to obtain the maximum estimated attraction domain of the VSC-connected weak grid system with a frequency limiter, and evaluate the impact on the transient synchronization stability of the system by comparing the areas of the maximum estimated attraction domains under different frequency limit values.

[0103] Example 2, refer to Figure 7 - Figure 8 , which is the second embodiment of the present invention. This embodiment provides a method for the maximum estimated attraction domain of a VSC-connected weak grid system. In order to verify the beneficial effects of the present invention, scientific demonstration is carried out through economic benefit calculation and simulation experiments.

[0104] In order to verify the effectiveness of the above theoretical analysis, in this paper, full-order switching models of the VSC-connected weak grid system with and without a frequency limiter are respectively built on the PSCAD / EMPDC platform. It should be noted that the upper limit of the frequency limiter is 3Hz and the lower limit is -3Hz. Other system parameters are shown in Table 1.

[0105] Table 1 Simulation model parameters of the VSC-connected weak grid system

[0106]

[0107]

[0108] Figure 7 When the voltage at the fault point drops to 0.09 pu, the active injection current is 0 pu, and the reactive injection current is 1 pu, the electromagnetic transient simulation waveforms of the system with and without a frequency limiter are given. It can be seen from the figure that under the same disturbance, when there is no frequency limiter, the VSC-connected weak grid system can maintain PLL transient synchronization stability. However, after adding a frequency limiter, the PLL transient synchronization of the VSC-connected weak grid system becomes unstable. The above shows that the simulation results are consistent with Figure 5 the theoretical analysis results shown, verifying the effectiveness of the maximum estimated attraction domain with a frequency limiter constructed in this paper, and also indicating that the frequency limiter will reduce the transient synchronization stability of the VSC-connected weak grid system during the fault ride-through process.

[0109] Figure 8 The comparison of the maximum estimated attraction domain of the VSC accessing the weak grid system under different frequency limits is given. It can be seen from the figure that the larger the frequency limit is, the larger the maximum estimated attraction domain of the system is. However, when the frequency limit exceeds a certain value, if the frequency limit continues to increase, the maximum estimated attraction domain of the system will remain unchanged.

[0110] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit them. Although the present invention has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that the technical solutions of the present invention can be modified or equivalently replaced without departing from the spirit and scope of the technical solutions of the present invention, and they should all be covered within the scope of the claims of the present invention.

[0111] Those skilled in the art should understand that the embodiments of the present application can be provided as methods, systems, or computer program products. Therefore, the present application can take the form of a complete hardware embodiment, a complete software embodiment, or an embodiment combining software and hardware aspects. Moreover, the present application can take 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 code. The solutions in the embodiments of the present application can be implemented in various computer languages. For example, object-oriented programming languages such as Java and interpreted scripting languages such as JavaScript.

[0112] The present application is described with reference to the flowcharts and / or block diagrams of methods, devices (systems), and computer program products according to the embodiments of the present application. It should be understood that each process and / or block in the flowchart and / or block diagram can be implemented by computer program instructions, and the combination of the processes and / or blocks in the flowchart and / or block diagram can also be implemented by computer program instructions. These computer program instructions can be provided to the processor of a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing devices to generate a machine, so that the instructions executed by the processor of the computer or other programmable data processing devices generate means for implementing the functions specified in Figure 1 one process or multiple processes and / or blocks Figure 1 one block or multiple blocks.

[0113] These computer program instructions can also be stored in a computer-readable memory that can direct a computer or other programmable data processing device to work in a specific manner, so that the instructions stored in the computer-readable memory generate a manufactured product including instruction means, and the instruction means implements the functions specified in Figure 1 one process or multiple processes and / or blocks Figure 1 one block or multiple blocks.

[0114] These computer program instructions can also be loaded onto a computer or other programmable data processing apparatus, so that a series of operation steps are executed on the computer or other programmable apparatus to produce a computer-implemented process, and thus the instructions executed on the computer or other programmable apparatus provide steps for implementing the functions specified in one process or a plurality of processes and / or blocks Figure 1 one process or a plurality of processes and / or blocks Figure 1 or steps for implementing the functions specified in a plurality of blocks or blocks.

[0115] Although the preferred embodiments of the present application have been described, those skilled in the art can make additional changes and modifications to these embodiments once they learn the basic creative concepts. Therefore, the appended claims are intended to be construed to include the preferred embodiments as well as all changes and modifications that fall within the scope of the present application.

[0116] Obviously, those skilled in the art can make various changes and modifications to the present application without departing from the spirit and scope of the present application. Thus, if these modifications and variations of the present application fall within the scope of the claims of the present application and their equivalent technologies, the present application is also intended to include these modifications and variations.

Claims

1. A method for maximum estimated attraction region of VSC access to weak network system, characterized by: Including, simplifying the VSC access weak grid system with frequency limiter, establishing a second-order mathematical model suitable for phase-locked loop synchronous transient stability analysis, and defining the difference between the PLL output angular frequency and the rated angular frequency as well as the upper and lower limits of the frequency limiter; Based on the linear matrix inequality optimization method, the maximum estimated attraction domain of the VSC connected to the weak network system without frequency limiter is constructed, and the optimal Lyapunov energy function and its critical level value corresponding to the attraction domain are determined. The constructed maximum estimated attraction domain is limited to the area determined by the upper and lower limits of the frequency limiter, and the maximum estimated attraction domain of the VSC connected to the weak network system with a frequency limiter is obtained. The impact on the transient synchronization stability of the system is evaluated by comparing the maximum estimated attraction domain areas under different frequency limits.

2. The maximum estimated attraction region method for VSC access to a weak network system according to claim 1, characterized in that: The simplification of the VSC with frequency limiter access to the weak network system includes: Ignore the electromagnetic transient process of the VSC's LC filter and AC network; Assuming that the bandwidth of the current inner loop is much higher than the PLL bandwidth, the d-axis and q-axis currents I injected into the grid are determined. td and I tq Real-time tracking of set value I tdref and I tqref ; Define the PLL as the q-axis component of the grid voltage V tq is the input signal, and after passing through the proportional integral controller, the output is the phase-locked frequency ω pll and phase angle θ pll , and add a limiter after the PI control link of the PLL; Determine the equilibrium point x of the system e =(θ pll,e ,0), where arcsin((X F I tdref -R F I tqref ) / V F ).

3. The maximum estimated attraction region method for VSC access to a weak network system according to claim 2, characterized in that: The linear matrix inequality optimization method includes: Construct the optimal Lyapunov energy function V(x) of the VSC connected to the weak grid system without frequency limiter, and determine the constant c; Solve the optimization problem with equality constraints to obtain the critical level value V of V(x) cr1 and V cr2 ; The maximum estimated attraction domain of the VSC access to the weak network system without frequency limiter is formed, and the calculation formula is as follows: Among them, Ω FL It is a strict subset of the real attraction domain of the WG-VSC_FL system.

4. The maximum estimated attraction region method for VSC access to a weak network system according to claim 3, characterized in that: The solution to the optimization problem with equality constraints includes: V cr1 =min{V(x)} stGive=Give m V cr2 =min{V(x)} stDω=-Dω m That is, on the upper boundary F of the set F up and the lower boundary F low Find the point within which V(x) is maximum.

5. The maximum estimated attraction region method for VSC access to a weak network system according to claim 4, characterized in that: The maximum estimated attraction domain to be constructed is limited to the region determined by the upper and lower limits of the frequency limiter, including: Analyze the maximum estimated attraction domain of VSC with and without frequency limiter connected to the weak grid system when the voltage at the fault point drops to 0.09pu, the active injection current is 0pu, and the reactive injection current is 1pu; Determine the balance point X of the VSC with frequency limiter connected to the weak grid system e,0 and X e,1 position relationship.

6. The maximum estimated attraction region method for VSC access to a weak network system according to claim 5, characterized in that: The comparison of the maximum estimated attraction domain under different frequency limits includes: The full-order switching model of VSC access to weak grid system with and without frequency limiter is built using PSCAD / EMPDC platform.

7. The maximum estimated attraction region method for VSC access to a weak network system according to claim 6, characterized in that: The comparison of the maximum estimated attraction domains under different frequency limits also includes observing changes in the maximum estimated attraction domain of the system when the frequency limit increases, and using it to evaluate the impact of different frequency limits on the transient synchronization stability of the system.

8. A VSC access weak network system maximum estimated attraction domain system, based on the VSC access weak network system maximum estimated attraction domain method according to any one of claims 1 to 7, characterized in that: Including, a model building module, an attraction domain building module and an attraction domain comparison module; The model building module is used to simplify the VSC access weak network system containing a frequency limiter, establish a second-order mathematical model suitable for phase-locked loop synchronous transient stability analysis, and define the difference between the PLL output angular frequency and the rated angular frequency and the upper and lower limits of the frequency limiter; The attraction domain construction module is used to construct the maximum estimated attraction domain of the VSC access to the weak network system without a frequency limiter based on the linear matrix inequality optimization method, and determine the optimal Lyapunov energy function and its critical level value corresponding to the attraction domain; The attraction domain comparison module is used to limit the constructed maximum estimated attraction domain to the area determined by the upper and lower limits of the frequency limiter, obtain the maximum estimated attraction domain of the VSC connected to the weak network system with the frequency limiter, and evaluate the impact on the transient synchronization stability of the system by comparing the maximum estimated attraction domain areas under different frequency limits.

9. A computer device comprising a memory and a processor, wherein the memory stores a computer program, wherein: When the processor executes the computer program, the steps of the method for maximum estimated attraction domain of VSC access to a weak network system according to any one of claims 1 to 7 are implemented.

10. A computer-readable storage medium having a computer program stored thereon, characterized in that: When the computer program is executed by a processor, the steps of the method for maximum estimated attraction domain of VSC access to a weak network system according to any one of claims 1 to 7 are implemented.

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