Transient stability margin evaluation method for grid-forming type direct-driven wind power grid-connected system

Abstract

The invention discloses a transient stability margin evaluation method for a grid-forming type direct-drive wind power grid-connected system, and the method comprises the steps: collecting a three-phase voltage signal of a machine end and a voltage signal of a fault point, and calculating a voltage amplitude of the machine end and a voltage amplitude of the fault point; calculating a transient energy function, transient instability boundary energy and a transient stability margin evaluation index of the system in sequence; based on the calculated transient stability margin evaluation index, the method can quantify the transient stability margin of the grid-forming type direct-driven wind power grid-connected system during the short-circuit fault period of the power grid, and judges whether transient synchronous instability occurs in the system or not.

Description

Technical Field

[0001] The present invention relates to a method for evaluating the transient stability margin of a grid-forming direct-drive wind power grid-connected system, which is applicable to the grid-forming direct-drive wind power grid-connected system under symmetrical short-circuit faults of an AC power grid. This method can quantitatively evaluate the transient synchronous stability of the grid-forming direct-drive wind power grid-connected system under symmetrical short-circuit faults of the AC power grid. Background Art

[0002] With the gradual access of large-scale new energy generating units such as wind power to the power system, the power system dominated by traditional synchronous generators is gradually evolving into a new power system dominated by power electronic converters. The low inertia and weak damping effects are becoming more and more obvious, and the dynamic interaction between the wind power grid-connected system and the new power system, as well as the transient synchronous stability problem of the wind power grid-connected system, are becoming more and more complex and serious. Under such circumstances, the grid-forming direct-drive wind power grid-connected system that can simulate the inertia and damping characteristics of traditional synchronous machines and has the ability to actively support the power grid has become one of the current research hotspots. Therefore, proposing a method to quantitatively evaluate the transient stability margin of the grid-forming direct-drive wind power grid-connected system during grid short-circuit faults has become a key issue in the current development of wind power generation technology. At present, scholars at home and abroad have carried out relevant research, such as the following published literatures:

[0003] [1] P. Ge, C. Tu, F. Xiao, Q. Guo and J. Gao. Design-Oriented Analysis and Transient Stability Enhancement Control for a Virtual Synchronous Generator [J]. IEEE Transactions on Industrial Electronics, 2023, 70(3): 2675-2684.

[0004] [2] S. Chen, Y. Sun, X. Hou, H. Han, S. Fu and M. Su. Quantitative Parameters Design of VSG Oriented to Transient Synchronization Stability [J]. IEEE Transactions on Power Systems, 2023, 38(5): 4978-4981.

[0005] Reference [1] proposed a transient stability assessment method considering the system damping parameter based on the accelerating and decelerating areas of the grid-forming system during grid faults. This method can quantitatively evaluate the transient stability margin of the grid-forming system. However, since the damping term of the system is a non-linear term, a fifth-order numerical fitting is required for it, which leads to more parameters involved in this assessment method and more complex tuning, making it difficult to implement. Reference [2] used the Lyapunov transient energy function method to judge the transient stability of the grid-forming system and studied the influence of the damping coefficient and the inertia time constant on the transient behavior of the grid-forming system. The constructed energy function can quickly and quantitatively evaluate the transient stability margin of the system. However, this assessment method is mainly for the grid-forming system using active power as the grid-forming control scheme and is not applicable to the grid-forming direct-drive wind power grid-connected system using the DC voltage signal as the grid-forming control scheme. Direct use may lead to misjudgment of the system's transient stability. Summary of the Invention

[0006] Aiming at the above deficiencies of the existing technology, the purpose of the present invention is to propose a transient stability margin assessment method applicable to the grid-forming direct-drive wind power grid-connected system, which can judge whether the grid-forming direct-drive wind power grid-connected system will undergo transient instability during grid short-circuit faults and quantitatively evaluate the transient stability margin of the grid-forming direct-drive wind power grid-connected system.

[0007] The technical solution of the present invention is realized as follows:

[0008] A transient stability margin assessment method for a grid-forming direct-drive wind power grid-connected system, used to quantitatively evaluate the transient stability of the grid-forming direct-drive wind power grid-connected system under grid short-circuit faults, and calculate the transient energy function V t and the transient instability boundary energy V cr of the grid-forming direct-drive wind power grid-connected system respectively. If the transient energy function V t ≤ the transient instability boundary energy V cr , then it is transiently stable, otherwise it is transiently unstable.

[0009] The specific steps of the present invention are as follows:

[0010] A1) The grid-forming direct-drive wind power grid-connected system adopts the generator convention, collects the three-phase voltage signals at the machine terminal of the system and the three-phase voltage signals at the fault point, and calculates the system machine terminal voltage amplitude U t and the fault voltage amplitude U g .

[0011] A2) On the basis of step A1), calculate the transient energy function V t of the grid-forming direct-drive wind power grid-connected system under grid short-circuit faults according to the following formula:

[0012]

[0013] where x1 and x2 are both state variables; J dc and D dc are respectively the equivalent inertia coefficient and damping coefficient of the DC voltage; k p is the synchronous gain of the DC voltage; R L and X L respectively represent the resistance and reactance of the transmission line; G eq and B eq respectively represent the equivalent conductance and susceptance of the transmission line; P MPPT is the active power of the system maximum power point tracking, i.e., the MPPT power; δ s is the stable equilibrium point of the system;

[0014] A3) According to step A2), calculate the transient instability boundary energy V of the grid-forming direct-drive wind power grid-connected system according to the following formula cr :

[0015]

[0016] where δ u is the unstable equilibrium point of the system;

[0017] A4) Based on steps A2) and A3), quantify the evaluation index γ of the transient stability margin of the grid-forming direct-drive wind power grid-connected system under grid short-circuit faults according to the following formula:

[0018] γ = V cr - V t

[0019] A5) Use the following formula as the transient stability criterion of the grid-forming direct-drive wind power grid-connected system:

[0020]

[0021] where, when the grid-forming direct-drive wind power grid-connected system meets the above stability criterion, it can smoothly reach and stably operate in the low voltage ride-through control mode during grid short-circuit faults.

[0022] Compared with the prior art, the present invention has the following beneficial effects:

[0023] The present invention can quantitatively calculate the transient stability margin value of the grid-forming direct-drive wind power grid-connected system according to the grid fault degree, the control characteristics and operating conditions of the grid-forming direct-drive wind power grid-connected system, and the line equivalent impedance, which can be used for the transient instability criterion and transient stability margin evaluation of the grid-forming direct-drive wind power grid-connected system under grid short-circuit faults, and provides guiding suggestions for the safe and stable operation of the grid-forming direct-drive wind power grid-connected system. Description of the Drawings

[0024] Figure 1It is the topological structure diagram of a grid-forming direct-drive wind power grid-connected system.

[0025] Figure 2 It is the block diagram of the grid-forming control link of a grid-forming direct-drive wind power grid-connected system.

[0026] Figure 3 For the DC voltage equivalent inertia coefficient J dc and the damping coefficient D dc When they are 0.05 and 0.6 respectively, it is the time-domain simulation result of the grid-forming direct-drive wind power grid-connected system.

[0027] Figure 4 For the DC voltage equivalent inertia coefficient J dc and the damping coefficient D dc When they are 0.2 and 1.5 respectively, it is the time-domain simulation result of the grid-forming direct-drive wind power grid-connected system. Specific implementation manner

[0028] The following describes the specific implementation of the present invention in detail with reference to the accompanying drawings.

[0029] Figure 1 It is the topological structure diagram of a grid-forming direct-drive wind power grid-connected system. Among them, Z g is the line impedance between the short-circuit fault point and the power grid, U g is the fault voltage amplitude of the short-circuit fault point, Z L is the grid-connected line impedance, T1 and T2 both represent step-up transformers, L f and C f are the filter inductor and capacitor respectively, C dc and U dc are the DC bus capacitor and DC voltage respectively. Figure 2 It is the block diagram of the grid-forming control link of a grid-forming direct-drive wind power grid-connected system. Among them, U dc0 is the rated value of the DC voltage, is the difference between the square of the actual value of the DC voltage and the square of its rated value, ω0 is the rated angular frequency of the power grid, and δ is the output phase angle of the system, that is, the equivalent power angle.

[0030] The present invention is a method for evaluating the transient stability margin of a grid-forming direct-drive wind power grid-connected system, which is used to judge the transient stability of the grid-forming direct-drive wind power grid-connected system under grid short-circuit faults. The transient energy function V t and the transient instability boundary energy V cr of the grid-forming direct-drive wind power grid-connected system under grid short-circuit faults are calculated respectively. If the transient energy function V t ≤ the transient instability boundary energy V cr , then it is transiently stable, otherwise it is transiently unstable.

[0031] The specific implementation steps are as follows:

[0032] A1) The grid-forming type direct-drive wind power grid-connected system adopts the generator convention, collects the three-phase voltage signals at the machine terminal of the system and the three-phase voltage signals at the fault point, and calculates the amplitude U of the voltage at the machine terminal of the system t and the amplitude U of the fault voltage g ;

[0033] A2) On the basis of step A1), calculate the transient energy function V of the grid-forming type direct-drive wind power grid-connected system under the grid short-circuit fault according to the following formula t :

[0034]

[0035] In the formula, both x1 and x2 are state variables; J dc and D dc are the equivalent inertia coefficient and damping coefficient of the DC voltage respectively; k p is the synchronous gain of the DC voltage; R L and X L represent the resistance and reactance of the transmission line respectively; G eq and B eq represent the equivalent conductance and susceptance of the transmission line respectively; P MPPT is the active power of the maximum power point tracking of the system, that is, the MPPT power; δ s is the stable equilibrium point of the system;

[0036] A3) According to step A2), calculate the transient instability boundary energy V of the grid-forming type direct-drive wind power grid-connected system according to the following formula cr :

[0037]

[0038] In the formula, δ u is the unstable equilibrium point of the system;

[0039] A4) On the basis of steps A2) and A3), quantify the evaluation index γ of the transient stability margin of the grid-forming type direct-drive wind power grid-connected system under the grid short-circuit fault according to the following formula:

[0040] γ = V cr -V t

[0041] A5) Use the following formula as the transient stability criterion of the grid-forming type direct-drive wind power grid-connected system:

[0042]

[0043] Among them, when the grid-forming type direct-drive wind power grid-connected system meets the above stability criterion, it can smoothly reach and stably operate in the low voltage ride-through control mode during the grid short-circuit fault.

[0044] Description of the effects of the present invention:

[0045] Figure 3 The equivalent inertia coefficient J of the DC voltage is given dc and the damping coefficient D dc When they are 0.05 and 0.6 respectively, the time-domain simulation results of the grid-forming direct-drive wind power grid-connected system under the operating condition that the grid voltage level U g drops to 0.4 p.u. and the line equivalent impedance Z L is 0.5 p.u. are shown. Table 1 shows the transient stability evaluation results of the system under the current operating conditions, where γ1 and γ2 are the transient stability margin evaluation indexes proposed by the present invention and the traditional transient stability margin evaluation indexes respectively. The traditional transient stability margin evaluation method does not consider the damping term in the evaluation method proposed by the present invention. From Figure 3 and Table 1, it can be seen that compared with the traditional method, the evaluation index proposed by the present invention can more accurately reflect the transient stability margin of the system.

[0046] Table 1

[0047]

[0048] Figure 4 The equivalent inertia coefficient J of the DC voltage is given dc and the damping coefficient D dc When they are 0.2 and 1.5 respectively, the time-domain simulation results of the grid-forming direct-drive wind power grid-connected system are shown, and the remaining operating conditions are set the same as Figure 3 before. Table 2 shows the transient stability evaluation results of the system under the current operating conditions. From Figure 4 and Table 2, it can be seen that compared with the traditional method, the accuracy of the transient stability evaluation results of the system obtained by the evaluation index proposed by the present invention is greatly improved.

[0049] Table 2

[0050]

[0051] The present invention proposes a transient stability margin evaluation method applicable to the grid-forming direct-drive wind power grid-connected system under grid short-circuit faults. This method can judge whether the grid-forming direct-drive wind power grid-connected system undergoes transient instability under grid short-circuit faults according to the degree of grid voltage drop, the MPPT power command value of the grid-forming direct-drive wind power grid-connected system during the fault, the DC voltage synchronization gain, the DC voltage equivalent inertia and damping coefficient, and the line equivalent impedance, and quantitatively calculate the numerical value of the transient stability margin of the grid-forming direct-drive wind power grid-connected system, providing a quantitative evaluation basis for the transient synchronous stability of the grid-forming direct-drive wind power grid-connected system.

[0052] Finally, it should be noted that the above examples of the present invention are merely illustrations given to explain the present invention, and are not limitations on the implementation modes of the present invention. Although the applicant has described the present invention in detail with reference to the preferred embodiments, for those of ordinary skill in the art, other different forms of changes and modifications can still be made based on the above description. It is impossible to enumerate all the implementation modes here. Any obvious changes or modifications derived from the technical solutions of the present invention still fall within the protection scope of the present invention.

Claims

1. A transient stability margin evaluation method for a grid-forming direct-drive wind power grid-connected system, which is used to quantitatively evaluate the transient stability of the grid-forming direct-drive wind power grid-connected system under grid short-circuit faults, and is characterized in that: Calculate the transient energy function V of the grid-forming direct-drive wind power grid-connected system under grid short-circuit faults respectively t and the transient instability boundary energy V cr . If the transient energy function V t ≤ the transient instability boundary energy V cr , then it is transiently stable; otherwise, it is transiently unstable.

2. The transient stability margin evaluation method for a grid-forming direct-drive wind power grid-connected system according to claim 1, characterized in that: The specific steps are as follows: A1) The grid-forming direct-drive wind power grid-connected system adopts the generator convention, collects the three-phase voltage signals at the machine terminal of the system and the three-phase voltage signals at the fault point, and calculates the voltage amplitude U at the machine terminal of the system t and the fault voltage amplitude U g ; A2) On the basis of step A1), calculate the transient energy function V of the grid-forming direct-drive wind power grid-connected system under the grid short-circuit fault according to the following formula t :[[]]END]] where x1 and x2 are both state variables; J dc and D dc are the equivalent inertia coefficient and damping coefficient of the DC voltage respectively; k p is the synchronization gain of the DC voltage; R L and X L represent the resistance and reactance of the transmission line respectively; G eq and B eq represent the equivalent conductance and susceptance of the transmission line respectively; P MPPT is the active power of the system maximum power point tracking; δ s is the stable equilibrium point of the system; A3) According to step A2), calculate the transient instability boundary energy V of the grid-forming direct-drive wind power grid-connected system according to the following formula cr : where δ u is the unstable equilibrium point of the system; A4) Based on steps A2) and A3), evaluate the transient stability margin index γ of the grid-forming direct-drive wind power grid-connected system under grid short-circuit faults according to the following formula: γ = V cr -V t A5) Use the following formula as the transient stability criterion for the grid-forming direct-drive wind power grid-connected system: Wherein, when the grid-forming direct-drive wind power grid-connected system meets the above stability criterion, it can smoothly reach and stably operate in the low-voltage ride-through control mode during grid short-circuit faults.

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