Method for constructing dual-sequence injected-current feasible region of VSC under asymmetric faults

Abstract

The present invention relates to a method for constructing a dual-sequence injected-current feasible region of a weakly interconnected VSC system under asymmetric faults. The method comprises the following steps: on the basis of boundary conditions of a weakly interconnected VSC system under asymmetric-fault types, forming a composite sequence network of the system; constructing a nonlinear reduced-order mathematical model of the weakly interconnected VSC system that is adapted for transient synchronization stability analysis; performing Taylor expansion on trigonometric functions in the constructed nonlinear mathematical model of the weakly interconnected VSC system, and using a linear matrix inequality optimization method to construct the largest estimated domain of attraction of the system under asymmetric faults; forming an injected-current set; constructing a corresponding largest estimated domain of attraction for each injected current in the current set; screening out from the injected-current set all injected currents capable of ensuring the transient synchronization stability of the system, thereby forming a dual-sequence injected-current feasible region of the weakly interconnected VSC system; and formulating positive-sequence and negative-sequence injected-current control strategies of a VSC under the asymmetric faults.

Description

A method for constructing feasible region of double-sequence injection current for VSC under asymmetric faults Technical Field The present invention belongs to the field of transient stability analysis of power electronic power systems. Aiming at the problem that inappropriate positive-sequence and negative-sequence injection current control strategies under asymmetric faults may cause transient synchronous instability in weakly interconnected VSC systems, a method for constructing a feasible domain of double-sequence injection current of the system is proposed. Based on the feasible domain of double-sequence injection current, a positive-sequence and negative-sequence injection current control strategy of VSC under asymmetric faults can be formulated. Background Art As a high proportion of renewable energy generation replaces traditional synchronous generators, the forced inertia and short-circuit capacity of the power grid are gradually weakened, resulting in the power electronic interface power supply mainly based on voltage source converters (VSCs) facing the operation scenario of connecting to weak grid. [1] A large number of studies have shown that weakly interconnected VSC systems are prone to transient instability of phase-locked loop synchronization under large disturbances such as faults. [2] Current research focuses on the transient stability of phase-locked loop synchronization under symmetrical faults in weakly interconnected VSC systems. Considering that asymmetrical faults are more common in actual power systems, [3] However, the models and analysis methods proposed in the above studies cannot solve the problem of transient stability of phase-locked loop synchronization under asymmetric faults. [4] . Currently, only a few references such as [5]-[7] and [8] analyze the transient stability of phase-locked loop synchronization of weakly interconnected VSC systems under asymmetric faults while ignoring the influence of positive and negative sequence coupling. Reference [5] established a nonlinear reduced-order mathematical model of weakly interconnected VSC systems under asymmetric faults and derived analytical criteria for static synchronization stability of positive and negative sequence systems during faults. However, satisfying the static stability condition alone cannot guarantee the stable operation of weakly interconnected VSC systems, which is also related to the transient process of phase-locked loop synchronization dominated by the phase-locked loop. Reference [6] completely ignored the influence of the negative sequence network and used the phase trajectory method to analyze the transient synchronization of the positive sequence phase-locked loop of the weakly interconnected VSC system under asymmetric faults. Reference [7] did not consider the control dynamic characteristics of the negative sequence phase-locked loop, but directly used the opposite of the positive sequence phase-locked loop output phase angle as the reference phase angle of the negative sequence control link. This control method will cause the weakly interconnected VSC to be unable to accurately track the negative sequence voltage phase angle of the power grid and cannot achieve precise control of the negative sequence current. Reference [8] points out that when the fault point is far away from the grid connection point of the weakly interconnected VSC, the positive and negative sequence coupling has little effect on the synchronous transient stability of the phase-locked loop. Then, from the perspective of potential energy and kinetic energy conversion, an analytical criterion for the weakly interconnected VSC to meet the positive and negative sequence phase-locked loop synchronous transient stability is proposed. However, when the grid fault is close to the grid connection point of the weakly interconnected VSC, the positive and negative sequence coupling will have a significant impact on the synchronous transient stability of its phase-locked loop. Therefore, without loss of generality, the influence of positive and negative sequence network coupling should be taken into account for the phase-locked loop synchronous transient stability problem of the weakly interconnected VSC system under asymmetric faults. Currently there are only [9]-

[0011] A small number of literatures have analyzed the influence of positive and negative sequence coupling from the aspects of static stability and small disturbance stability of weakly interconnected VSC systems. Literatures [9]-

[0010] only analyzed the influence of positive and negative sequence coupling terms on the existence of equilibrium points of weakly interconnected VSC systems under asymmetric faults. Under the premise of the existence of equilibrium points, literature

[0011] explored the influence of positive and negative sequence coupling terms on the small disturbance stability of equilibrium points of weakly interconnected VSC systems based on the transfer function method. However, it should be pointed out that even if a weakly interconnected VSC system has an equilibrium point that satisfies small disturbance stability after an asymmetric fault, that is, a feasible equilibrium point, it may still experience transient instability of phase-locked loop synchronization. References [1]X.Li, Z.Tian, [2]D.Dong,B.Wen,D.Boroyevich,and P.Mattavelli,“Analysis of phase-locked loop low-frequency stability in three-phase grid-connected power converters considering impedance interactions,”IEEE Trans.Industrial Electronics,vol.62,no.1,pp.310-321,Jan.2015. [3]NERC,“1,200 MW fault induced solar photovoltaic resource interruption disturbance report–southern California 8 / 16 / 2016 event,”Jun.2017,[Online].Available:www.nerc.com. [4] R.Teodorescu,C.L.Bak,F.Iov and P.C. “Impact of wind power plant reactive current injection during asymmetrical grid faults,”IET Renewable Power Generation,vol.7,no.5,pp.484–492,Sep.2013. [5]M.G.Taul,S.Golestan,X.Wang,et al,“Modeling of converter synchronization stability under grid faults:the general case,”IEEE Journal of Emerging and Selected Topics in Power Electronics,2020. [6]H. Wu, X. Wang, "Design-Oriented Transient Stability Analysis of PLL-Synchronized Voltage-Source Converters," IEEE Trans. Power Electronics, vol. 35, no. 4, pp. 3573-3589, Apr 2020. [7]Y. Li, C. Lin, J. Hu, et al, "PLL Synchronization Stability of Grid-Connected VSCs Under Asymmetric AC Faults," IEEE Trans. Energy Conversion, vol. 37, no. 4, pp. 2438-2448, Dec. 2022. [8]X. Li, Z. Wang, L. Zhu, et al, “Analytical Dual-Sequence Current Injections Feasible Region of Weak-Grid Connected VSC Under Asymmetric Grid Faults,” IEEE Trans. Power Systems. 2022, (Early Access). [9]P. Hackl, Z. zhang, R. Schuerhuber, "Unsymmetrical fault behavior of phase-locked loop based grid-connected converters," 2022 24th European Conference on Power Electronics and Applications (EPE'22 ECCE Europe), Hanover, Germany, 2022.pp. 1-10.

[0010] X. He, C. He, S. Pan, et al, "Synchronization Instability of Inverter-Based Generation During Asymmetrical Grid Faults," IEEE Trans. Power Systems, vol. 37, no. 2, pp. 1018-1031, March 2022.

[0011] A.Akhavan, S.Golestan, JCVasquez, et al, "Control and Stability Analysis of Current-Controlled Grid-Connected Inverters in Asymmetrical Grids," IEEE Trans.Power Electronics, vol.37, no.12, pp.14252-14264, Dec.2022. Summary of the invention The present invention is aimed at the positive-sequence and negative-sequence injection current control of a voltage source converter (VSC) under an asymmetric fault, and provides a method for constructing a feasible domain of dual-sequence injection current of a weakly interconnected VSC system. The present invention first constructs a nonlinear reduced-order mathematical model of a weakly interconnected VSC system suitable for transient synchronous stability analysis. Subsequently, the maximum estimated attraction domain of the weakly interconnected VSC system under an asymmetric fault is constructed using a linear matrix inequality optimization method. The maximum estimated attraction domain can be used to quantitatively analyze the transient stability of the weakly interconnected VSC system under different positive-sequence and negative-sequence injection control strategies. On this basis, a feasible domain of dual-sequence injection current is constructed to ensure the transient synchronous stability of the system. Based on this feasible domain, a positive-sequence and negative-sequence injection current control strategy for the VSC under an asymmetric fault can be formulated. The technical solution is as follows: A method for constructing a feasible region of double-sequence injection current of a weakly interconnected VSC system under an asymmetric fault comprises the following steps: Step 1: Based on the boundary conditions of the weakly interconnected VSC system under asymmetric fault types, a composite sequence network of the system is formed; fast time scale dynamics including current inner loop control and positive and negative sequence separation control are ignored, the control dynamics of the positive and negative sequence phase-locked loops of the VSC, and the influence of the positive and negative sequence synchronous dynamic coupling are considered, and a nonlinear reduced-order mathematical model of the weakly interconnected VSC system suitable for transient synchronous stability analysis is constructed; Step 2: Taylor expand the trigonometric functions in the constructed nonlinear mathematical model of the weakly interconnected VSC system, and use the linear matrix inequality optimization method to construct the maximum estimated attraction domain of the system under asymmetric faults. Based on the maximum estimated attraction domain, the criterion for evaluating the transient synchronization stability of the system is determined; Step 3: List the VSC positive-sequence and negative-sequence injection currents that meet the maximum current constraint to form an injection current set; construct a corresponding maximum estimated attraction domain for each injection current in the current set, and determine whether the system can maintain transient synchronization stability under the injection current based on the transient synchronization stability criterion; Step 4: Filter out all the injected currents that can ensure the transient synchronous stability of the system in the injected current set to form the feasible domain of double-sequence injected current of the weakly interconnected VSC system; Step 5: Based on the feasible domain of dual-sequence injection current determined in step 4, formulate the positive-sequence and negative-sequence injection current control strategy of VSC under asymmetric fault. Furthermore, the control part of the converter of the weakly interconnected VSC system adopts a dual second-order generalized integrator phase-locked loop DSOGI-PLL and a current control based on proportional resonance PR in a two-phase stationary αβ coordinate system. The DSOGI-PLL is used to track the voltage phase angle of the positive and negative sequence PCCs and output the angular frequency ω of the positive and negative sequence voltages at the grid connection point. pll + ,ω pll - and phase angle θ pll + ,θ pll - ; Current control based on proportional resonance PR in two-phase stationary αβ coordinate system is used to realize the VSC dual-sequence injection current to the positive and negative sequence dq axis current reference value I tdqref + and I tdqref It is believed that the front end of DSOGI-PLL can separate the positive and negative sequence components of PCC voltage in real time, and that the VSC injection current can track the positive and negative sequence current reference values. Furthermore, the nonlinear reduced-order mathematical model of the weakly interconnected VSC system constructed in the first step is expressed as follows: Where δ + and δ - Respectively represent the phase angle difference between the positive sequence and negative sequence phase-locked loop output phase angle and the infinite grid voltage phase angle, x pll + and x pll - Respectively represent the q-axis integral voltage of the positive and negative sequence grid connection points; k p and k i are the proportional coefficient and integral coefficient of the phase-locked loop proportional-integral controller, V g Represents the infinite grid terminal voltage amplitude; |I t - |、|I t - | and θ I + ,θ I - are the amplitude and phase angle of the positive and negative sequence currents, respectively, and |I t + |={(I td + )2 +(I tq + ) 2} 1 / 2 ,|I t + |={(I td - ) 2 +(I tq - ) 2} 1 / 2 θ I + =arctan(I tq + / I td + ),θ I - =arctan(I tq - / I td - );I td + ,I tq + and I td - ,I tq - represents the positive and negative sequence d-axis and q-axis injection currents of the VSC, respectively, which are directly given by the asymmetric fault ride-through control strategy; Z e + =|Z e + |∠φ e + =Z g + K 1 +Z L + ,Z e - =|Z e - |∠φ e - =Z g - K 3 +Z L - ,Z c =|Z c |∠φ c =Z g + K 2 ; K i =|K i |∠φ i,i=1,2,3, K under different types of asymmetric faults i The expression of is shown in the following table. F Represents the grounding resistance, Z L and Z g Respectively represent the equivalent impedance between PCC and the fault point and between the fault point and the power supply point; the superscripts "+", "-" and "0" represent the positive, negative and zero-sequence impedances respectively: Furthermore, in the second step, the trigonometric functions in the constructed nonlinear mathematical model of the weakly interconnected VSC system are Taylor expanded to the eighth order. Furthermore, in the second step, the maximum estimated attraction domain Ω of the system under asymmetric faults is constructed using the linear matrix inequality optimization method. V It is expressed as follows: Ω V ={(δ + ,x pll + ,δ - ,x pll - )|V≤V cr} Where V is the Lyapunov function of the system, V cr is the critical energy value; Let X 0 is the operating point before the system failure, based on the maximum estimated attraction domain Ω V The following criteria for evaluating the transient synchronization stability of the system are determined: V(X 0 )≤V cr If the above equation is true, it means that the system can maintain transient synchronous stability; if the above equation is not true, it means that the system may be transiently unstable. Furthermore, in the third step, the injected current set is described as follows: Ψ={(I t + ,θ I + ,I t - ,θ I - )|I tmax ≤I m ,I tmax =max{I ta ,I tb ,I tc}} Where I ta ,I tb and I tcThey represent the maximum values ​​of the current injected into the a, b and c phases by the VSC, respectively, and are expressed as follows, where the subscript x represents the a, b and c phases; a , γ b and γ c are equal to 0, -2π / 3 and 2π / 3m respectively, δ e + and δ e - Respectively represent δ + and δ - The steady-state value of The beneficial effects of the present invention are as follows: 1) Considering the dynamic coupling of positive and negative sequence control, a method for constructing the maximum estimated attraction domain of the weakly interconnected VSC system under asymmetric faults is proposed. Based on the maximum estimated attraction domain, the transient stability of the weakly interconnected VSC system under different positive and negative sequence injection control strategies can be quantitatively analyzed, and the feasible domain of the dual-sequence injection current of the weakly interconnected VSC system can be constructed. 2) By using the dual-sequence injection current feasible domain constructed by the present invention, a VSC positive-sequence and negative-sequence injection current control strategy can be formulated to ensure transient synchronous stability of the system under asymmetric faults. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 Schematic diagram of the topology and control structure of the weakly interconnected VSC system Figure 2 Equivalent circuit of weakly interconnected VSC system Fig. 3 Feasible region of double-sequence injection current in weakly interconnected VSC system Fig. 4 Electromagnetic transient simulation results. Specific implementation methods The present invention is described below in conjunction with the accompanying drawings and embodiments. 1) Reduced-order mathematical model of phase-locked loop synchronous VSC connected to weak grid system under asymmetric fault The topological structure and control block diagram of the VSC access weak network system involved in the embodiment of the present invention are shown in FIG1. abc 、v tabc 、v Fabc and v gabc Respectively represent the VSC outlet voltage, grid connection point voltage, fault point voltage and infinite grid voltage; i tabc The current injected into the grid by the VSC; L f and C f is the inductance and capacitance of the LC filter on the AC side; Z L and Z gRespectively represent the equivalent impedance between PCC and power source and between fault point and power source; the control part of the converter is mainly composed of a double second-order generalized integrator PLL (DSOGI-PLL) and a proportional-resonant (PR) current control in a two-phase stationary αβ coordinate system, as shown in Figure 1. DSOGI-PLL realizes the tracking of the voltage phase angle of the positive and negative sequence PCC, and outputs the angular frequency ω of the positive and negative sequence voltage at the grid connection point. pll + ,ω pll - and phase angle θ pll + ,θ pll - The current control based on PR in the two-phase stationary αβ coordinate system realizes the VSC dual-sequence injection current to the positive and negative sequence dq axis current reference value I tdqref + and I tdqref tracking. It should be pointed out that under asymmetric fault, the electrical quantity x abc contains positive and negative sequence components, which can be expressed as follows: In the formula and The electrical quantities x abc The positive and negative sequence rotating phasors of x + =x d + +jx q + , x - =x d - +jx q - ; In addition, x αβ + and x αβ - Respectively represent x α + +jx β + and x α - +jx β - ;x dq + and x dq - Respectively represent x d + +jxq + and x d - +jx q - The present invention focuses on the transient stability problem of PLL synchronization when VSC is connected to a weak network system, so the following assumptions are considered to simplify the analysis: 1) Ignore LC filters and AC network dynamics; 2) Ignoring the SOGI-QSG (second-order generalized integrator quadrature signal generator) dynamics and current control dynamics, it can be assumed that the front end of the DSOGI-PLL can separate the positive and negative sequence components of the PCC voltage in real time, and that the VSC injection current can approximately track the positive and negative sequence current reference values ​​in real time; Based on the above assumptions, the equivalent circuit model of the system is shown in FIG2 . The double-sequence current model in FIG2 is shown in formula (2): Where I t + and I t - In polar coordinates, it can also be expressed as I t + =|I t + |∠θ I + ,I t - =|I t - |∠θ I - ; Based on the symmetrical component method and the equivalent circuit model shown in Figure 2, the positive and negative sequence PCC voltages under three common asymmetrical faults (SLG, DLG and LL) are and It can be uniformly expressed as follows:. In the above formula, “*” represents the conjugation symbol, K i =|K i |∠φ i ,i=1,2,3, K under different fault types i The expression of is shown in the following table. Table 1 K under different fault types i The expression Transform equations (3) and (4) to PLL + and PLL - In the reference coordinate system, the q component V of the positive and negative sequence PCC voltage can be obtained tq + and V tq - . Then combined with PLL + and PLL - The control structure of VSC is a double-order reduced-order mathematical model for transient synchronous stability analysis of VSC connected to weak grid under asymmetric faults. It can be expressed as follows, where x = (δ + ,x pll + ,δ - ,x pll - ). In the above formula, x pll + =∫V tq + dt,x pll - =∫V tq - dt, It should be pointed out that there is a local asymptotically stable equilibrium point, i.e., a feasible equilibrium point, during a fault period when the VSC is connected to a weak network system, which is a prerequisite for the transient synchronization stability of the system. Existing studies have conducted in-depth research on the conditions for the existence of a feasible equilibrium point for the VSC to access a weak network system. Therefore, the present invention assumes that during a fault period, there is a feasible equilibrium point X for the VSC to access a weak network system. e =(δ e + ,0,δ e - ,0), and δ e + and δ e - They are denoted as X e1 and X e3 . Normally, the positive-sequence impedance is the same as the negative-sequence impedance, and the zero-sequence impedance is greater than the positive-sequence impedance. To simplify the analysis, the positive-sequence, negative-sequence, and zero-sequence impedances are considered equal in subsequent theoretical analysis and simulation verification. 2) Construction of the maximum estimated attraction domain For the weakly interconnected VSC system considering positive and negative sequence coupling, the trigonometric Taylor expansion of its nonlinear mathematical model is firstly carried out to the eighth order. Then, the maximum estimated attraction domain is constructed based on the linear matrix inequality optimization method, which is described as follows: Constructing the maximum estimated attraction domain of the weakly interconnected VSC system under asymmetric faults is equivalent to solving the optimization problem shown in the following equation: Where Q = diag (Q 1 ,Q 2 ), Λ, Q 1 and Q 2 They are v(x) and q 1 (x) and q 2 The SMR matrix of (x), q 1 (x),q 2 (x) is an auxiliary polynomial function, usually obtained in the process of solving the optimization problem (B); P(c,Q,Λ)+L(α) is the complete SMR matrix of p(x), trace(Λ) represents the trace of the matrix Λ, trace(Q 1 ) represents the matrix Q 1 The trace in the constraint (Q 1 )=1 is to normalize the variables in (B) and minimize trace(Λ) / V cr This is equivalent to maximizing the area Ω α The volume of the polynomial function. The SMR matrix and the complete SMR matrix are defined as follows: Basis vectors of a polynomial: If for any polynomial h(x) whose highest power is not greater than m (m is a positive integer), there exists a vector h with m+1 rows satisfying: h(x) = h T b pol (x,m), then b pol (x,m) is the basis vector of the polynomial whose highest power is not greater than m. SMR matrix: For a polynomial function h(x) with a highest power less than or equal to 2m, if there is a symmetric matrix H that satisfies: f(x) = b pol (x,m) T H pol (x,m), then H is called h(x) with respect to b pol The SMR matrix of (x,m). pol (x,m) are the basis vectors of the polynomial whose highest power is not greater than d. Complete SMR matrix: For a polynomial function h(x) with a highest power less than or equal to 2m. If there are symmetric matrices H and L(α) satisfying h(x) = b pol (x,m) T (H+L(α))b pol (x,m), then the matrix H+L(α) is called the complete SMR matrix of h(x), where α is an arbitrary constant. The above optimization problem contains bilinear matrix inequalities, which can be solved by iteration. The specific steps are as follows: Step 1: Select a positive quadratic function as the initial Lyapunov function of the system; Step 2: Fix the Lyapunov function of the system to solve the above optimization problem and obtain the matrix Q and constant c; Step 3: Fix the matrix Q and constant c obtained in the second step and solve the above optimization problem to update the Lyapunov function of the system. Step 4: Repeat the second and third steps until the objective function has no significant change, end the iteration, and obtain the maximum estimated attraction domain Ω of the system V , which is expressed as follows: Ω V ={(δ + ,x pll + ,δ - ,x pll - )|V≤V cr} (7) Let X 0 is the operating point before the system failure. Based on the maximum estimated attraction domain, the following criterion for evaluating the transient synchronization stability of the system is determined: V(X 0 )≤V cr (8) If the above equation is true, it means that the system can maintain transient synchronous stability; if the above equation is not true, it means that the system may be transiently unstable. 3) Constructing the feasible region of double-sequence injection current of weakly interconnected VSC system based on the maximum estimated attraction region The core idea of ​​constructing the dual-sequence CIFR in the present invention is that the VSC positive-sequence and negative-sequence injection currents that satisfy the maximum current constraint constitute an injection current set. The dual-sequence injection current set can be described as follows: Ψ={(I t + ,θ I + ,I t - ,θ I - )|I tmax ≤I m ,I tmax =max{I ta ,I tb ,I tc}} (9) Where I ta ,I tb and I tcThey represent the maximum values ​​of the current injected into the a, b and c phases by the VSC, respectively, and are expressed as follows, where the subscript x represents the a, b and c phases; a , γ b and γ c are equal to 0, -2π / 3 and 2π / 3m respectively, δ e + and δ e - Respectively represent δ + and δ - The steady-state value of For each injected current in the current set, the corresponding maximum estimated attraction domain is constructed, and based on the transient synchronization stability criterion, it is determined whether the system can maintain transient synchronization stability under the injected current. All injected currents in the injected current set that can ensure the transient synchronization stability of the system are screened out to form the feasible domain of the double-sequence injected current of the weakly interconnected VSC system. 4) Formulate the positive and negative sequence injection current control strategy of VSC under asymmetric fault based on the feasible region of dual sequence injection current Based on the dual-sequence injection current feasible domain constructed by the present invention, the positive-sequence and negative-sequence injection currents of the VSC are set within or on the boundary of the feasible domain, and a positive-sequence and negative-sequence injection current control strategy of the VSC under an asymmetric fault is obtained. Attached Figure 3 shows the projection of the feasible region of the double-sequence injection current in the negative-sequence injection current space under a two-phase grounding fault (the amplitude of the positive-sequence injection current is 0.4 pu and the phase angle is 90°). It should be noted that other parameters of the system are shown in Table 2. Table 2 System parameters In this patent, the positive-sequence and negative-sequence injection currents of the VSC are set at the boundary point C of the feasible domain, that is, the positive-sequence and negative-sequence injection current control strategies of the VSC are: the positive-sequence injection current is set to 0.4∠90°, and the negative-sequence injection current is set to 0.375∠0°. To verify the effectiveness of the above theoretical analysis, a detailed model of the weakly interconnected VSC system as shown in Figure 1 was built on the PSCAD / EMTDC platform. Figure 4 shows the electromagnetic transient simulation results of the weakly interconnected VSC system under the positive-sequence and negative-sequence injection current control strategies formulated using the dual-sequence injection current feasible domain. As can be seen from FIG4, the positive-sequence and negative-sequence injection current control strategies formulated using the dual-sequence injection current feasible region can enable the system to maintain transient synchronous stability, verifying the effectiveness and practicality of the dual-sequence injection current feasible region constructed by this patent. The principles and implementation methods of the present invention are described in this article using specific examples. The description of the above embodiments is only used to help understand the method and core idea of ​​the present invention. At the same time, for those skilled in the art, according to the idea of ​​the present invention, there will be changes in the specific implementation methods and application scope. In summary, the content of this specification should not be understood as limiting the present invention.

Claims

1. A method for constructing a feasible region of double-sequence injection current of a weakly interconnected VSC system under an asymmetric fault, comprising the following steps: Step 1: Based on the boundary conditions of the weakly interconnected VSC system under asymmetric fault types, a composite sequence network of the system is formed; fast time scale dynamics including current inner loop control and positive and negative sequence separation control are ignored, the control dynamics of the positive and negative sequence phase-locked loops of the VSC, and the influence of the positive and negative sequence synchronous dynamic coupling are considered, and a nonlinear reduced-order mathematical model of the weakly interconnected VSC system suitable for transient synchronous stability analysis is constructed; Step 2: Taylor expand the trigonometric functions in the constructed nonlinear mathematical model of the weakly interconnected VSC system, and use the linear matrix inequality optimization method to construct the maximum estimated attraction domain of the system under asymmetric faults. Based on the maximum estimated attraction domain, the criterion for evaluating the transient synchronization stability of the system is determined; Step 3: List the VSC positive-sequence and negative-sequence injection currents that satisfy the maximum current constraint to form an injection current set; For each injected current in the current concentration, a corresponding maximum estimated attraction domain is constructed, and based on the transient synchronization stability criterion, it is determined whether the system can maintain transient synchronization stability under the injected current; Step 4: Filter out all the injected currents that can ensure the transient synchronous stability of the system in the injected current set to form the feasible domain of double-sequence injected current of the weakly interconnected VSC system; Step 5: Based on the feasible domain of dual-sequence injection current determined in step 4, formulate the positive-sequence and negative-sequence injection current control strategy of VSC under asymmetric fault.

2. The method for constructing a feasible region of dual-sequence injection current according to claim 1, characterized in that: The control part of the converter of the weakly interconnected VSC system adopts a dual second-order generalized integrator phase-locked loop DSOGI-PLL and a current control based on proportional resonance PR in a two-phase stationary αβ coordinate system. The DSOGI-PLL is used to track the voltage phase angle of the positive and negative sequence PCC and output the angular frequency ω of the positive and negative sequence voltage at the grid connection point. pll + ,ω pll - and phase angle θ pll + ,θ pll - ; The current control based on proportional resonance PR in the two-phase stationary αβ coordinate system is used to realize the VSC dual-sequence injection current to the positive and negative sequence dq axis current reference value I tdqref + and I tdqref It is believed that the front end of DSOGI-PLL can separate the positive and negative sequence components of PCC voltage in real time, and that the VSC injection current can track the positive and negative sequence current reference values.

3. The method for constructing a feasible region of dual-sequence injection current according to claim 1, characterized in that: The nonlinear reduced-order mathematical model of the weakly interconnected VSC system constructed in the first step is expressed as follows: Where δ + and δ - Respectively represent the phase angle difference between the positive sequence and negative sequence phase-locked loop output phase angle and the infinite grid voltage phase angle, x pll + and x pll - Respectively represent the q-axis integral voltage of the positive and negative sequence grid connection points; k p and k i are the proportional coefficient and integral coefficient of the phase-locked loop proportional-integral controller, V g Represents the infinite grid terminal voltage amplitude; |I t - |、|I t - | and θ I + ,θ I - are the amplitude and phase angle of the positive and negative sequence currents, respectively, and |I t + |={(I td + ) 2 +(I tq + ) 2 } 1 / 2 ,|I t + |={(I td - ) 2 +(I tq - ) 2 } 1 / 2 ; θ I + =arctan(I tq + / I td + ),θ I - =arctan(I tq - / I td - );I td + ,I tq + and I td - ,I tq - represents the positive and negative sequence d-axis and q-axis injection currents of the VSC, respectively, which are directly given by the asymmetric fault ride-through control strategy; Z e + =|Z e + |∠φ e + =Z g + K1+Z L + ,Z e - =|Z e - |∠φ e - =Z g - K3+Z L - ,Z c =|Z c |∠φ c =Z g + K2; K i =|K i |∠φ i ,i=1,2,3, K under different types of asymmetric faults i The expression of is shown in the following table. F Represents the grounding resistance, Z L and Z g Respectively represent the equivalent impedance between PCC and the fault point and between the fault point and the power supply point; the superscripts "+", "-" and "0" represent the positive, negative and zero-sequence impedances respectively:

4. The method for constructing a feasible region of dual-sequence injection current according to claim 2, characterized in that: In the second step, the trigonometric functions in the constructed nonlinear mathematical model of the weakly interconnected VSC system are Taylor expanded to the eighth order.

5. The method for constructing a feasible region of dual-sequence injection current according to claim 2, characterized in that: In the second step, the maximum estimated attraction domain Ω of the system under asymmetric faults is constructed using the linear matrix inequality optimization method. V It is expressed as follows: V ={(δ + ,x pll + ,δ - ,x pll - )|V≤V cr } Where V is the Lyapunov function of the system, V cr is the critical energy value; Assume X0 is the operating point before the system failure, based on the maximum estimated attraction domain Ω V The following criteria for evaluating the transient synchronization stability of the system are determined: V(X0)≤V cr If the above equation is true, it means that the system can maintain transient synchronous stability; if the above equation is not true, it means that the system may be transiently unstable.

6. The method for constructing a feasible region of dual-sequence injection current according to claim 2, characterized in that: In the third step, the injected current set is described as follows: Ψ={(I t + ,θ I + ,I t - ,θ I - )|I tmax ≤I m ,I tmax =max{I ta ,I tb ,I tc }} Where I ta ,I tb and I tc They represent the maximum values ​​of the current injected into the a, b and c phases by the VSC, respectively, and are expressed as follows, where the subscript x represents the a, b and c phases; a , γ b and γ c are equal to 0, -2π / 3 and 2π / 3m respectively, δ e + and δ e - Respectively represent δ + and δ - The steady-state value of

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