Asymmetrical fault under network type grid-connected converter transient synchronization stability analysis method

Abstract

This invention discloses a method for analyzing the transient synchronization stability of grid-connected converters under asymmetrical faults. This method constructs unified expressions for the positive-sequence current and negative-sequence voltage of grid-connected converters under different types of asymmetrical faults, and considers the requirements of grid connection rules for negative-sequence reactive current. It derives the relationship expression between the negative-sequence current and positive-sequence voltage of grid-connected converters under asymmetrical faults. Furthermore, it constructs the swing equation for grid-connected converters under asymmetrical faults, constructs the energy function of the grid-connected converter system under asymmetrical faults based on the swing equation, and further derives the transient synchronization stability criterion, calculating the attraction domain of the grid-connected converter under asymmetrical faults. This invention achieves accurate quantitative analysis of the transient synchronization stability of grid-connected converters under asymmetrical faults, providing a precise and efficient transient stability assessment tool for practical power engineering.

Description

Technical Field

[0001] This invention relates to the field of new energy power generation technology, and in particular to a method for analyzing the transient synchronization stability of a grid-connected converter under asymmetric fault conditions. Background Technology

[0002] With the integration of high proportions of new energy sources and power electronic equipment, the grid structure and operation mode of power systems are undergoing profound changes. Grid-connected converters can autonomously construct voltage, compensate for the inertia and damping deficiencies of power electronic power systems, and possess active voltage and frequency support capabilities under fault conditions, making them one of the mainstream solutions for grid-connected converters in new power systems. However, grid-connected converters are prone to transient synchronous instability under large disturbances, seriously threatening system stability and the reliable absorption of new energy sources. Moreover, in actual power grids, the frequency of asymmetrical faults such as single-phase-to-ground short circuits, two-phase-to-ground short circuits, and two-phase short circuits far exceeds that of three-phase symmetrical short circuit faults. Therefore, the transient synchronous stability analysis method for grid-connected converters under asymmetrical faults is crucial for assessing the instability risk and ensuring the safe and stable operation of power systems.

[0003] Furthermore, national standards such as GB / T 19963.1-2021 "Technical Specifications for Wind Farm Access to Power Systems Part 1: Onshore Wind Power" and GB / T 19964-2024 "Technical Specifications for Photovoltaic Power Station Access to Power Systems" require converters to inject negative-sequence reactive current under asymmetrical faults to reduce voltage imbalance. The injection of both positive and negative sequence currents under asymmetrical faults can trigger complex inter-sequence coupling effects, altering the original synchronization mechanism of grid-connected converters and affecting their transient synchronization stability. Moreover, existing methods focus more on the transient stability analysis of grid-connected converters under symmetrical faults, lacking analysis methods for asymmetrical faults. Therefore, how to achieve transient synchronization stability analysis of grid-connected converters under asymmetrical faults while fully considering inter-sequence coupling effects is a pressing technical challenge and a key to ensuring the safe and stable operation of new power systems. Summary of the Invention

[0004] The purpose of this invention is to address the shortcomings of existing technologies by providing a method for analyzing the transient synchronization stability of grid-connected converters under asymmetric faults. This method enables quantitative analysis of the transient synchronization stability of grid-connected converters under asymmetric faults, taking into account inter-sequence coupling. It can solve the problems of unclear transient instability mechanisms and difficulties in quantitative analysis of grid-connected converters under asymmetric faults in current power systems.

[0005] To achieve the above objectives, this invention provides a method for analyzing the transient synchronization stability of a grid-connected converter under asymmetric fault conditions, comprising the following steps:

[0006] Step 1: Construct a unified expression for the positive sequence current and negative sequence voltage of grid-connected converters under different types of faults;

[0007] Step 2: Consider the grid connection guidelines' requirements for negative sequence reactive current, and based on the expressions for positive sequence current and negative sequence voltage, construct the relationship expression between negative sequence current and positive sequence voltage of the grid-connected converter under asymmetrical fault conditions;

[0008] Step 3: Based on the expression for positive sequence current, the relationship between negative sequence current and positive sequence voltage, construct the swing equation for the grid-connected converter under asymmetrical fault conditions;

[0009] Step 4: Based on the swing equation, construct the energy function of the grid-connected converter under asymmetric fault conditions by integration. The energy function is a scalar function of the system phase deviation and angular frequency deviation.

[0010] Step 5: Based on the energy function, construct the transient synchronization stability criterion for grid-connected converters under asymmetric faults, and calculate the attraction domain of grid-connected converters under asymmetric faults.

[0011] Preferably, in step one, the positive sequence current of the grid-connected converter under different types of faults... and negative sequence voltage The unified expression is:

[0012]

[0013]

[0014] in: This refers to the grid voltage. This refers to the positive sequence voltage of a grid-connected converter. The conjugate of the negative sequence current output by the grid-connected converter; and To transfer admittance coefficient, , and For the transmission coefficient, This is the transmission impedance coefficient.

[0015] Preferably, in step two, the grid-connected conductor controls the negative sequence reactive current. The requirements are:

[0016]

[0017] in: This is the negative sequence component of the fault point voltage; This is the gain of the negative sequence reactive current.

[0018] As a preferred embodiment, the relationship between the negative sequence current and the positive sequence voltage of a grid-connected converter under asymmetrical fault conditions is expressed as follows:

[0019]

[0020] in: It is the conjugate of the negative sequence impedance on the machine side after the fault.

[0021] As a preferred option, in step three, the swing equation for the grid-connected converter under asymmetrical fault conditions is:

[0022]

[0023] in: J and D These are virtual inertia and damping, respectively; and These are the positive-sequence phase difference and angular frequency difference between the terminal voltage of the grid-connected inverter and the grid voltage, respectively. for The second derivative; P 0 represents the active power reference value; For power grid synchronization; It is a self-synchronizing item; This is an inter-sequence coupling synchronization term.

[0024] As a preferred option, power grid synchronization items Self-synchronization items Inter-order coupling synchronization terms The corresponding expression is:

[0025]

[0026] in: , , and The resistivity coefficient, and _ The sensitivity coefficient, This represents the amplitude of the positive sequence voltage. This represents the amplitude of the grid voltage.

[0027] Preferably, the expressions for the resistivity coefficient and the inductance coefficient are as follows:

[0028] ,

[0029] ,

[0030]

[0031]

[0032] in: For extracting the real part; This is for extracting the imaginary part.

[0033] Preferably, in step four, the energy function of the grid-connected converter system under asymmetric fault conditions is:

[0034]

[0035] in: The work angle is the angle of the stable equilibrium point.

[0036] As a preferred option, in step five, the transient synchronization stability criterion for grid-connected converters under asymmetrical fault conditions is:

[0037]

[0038] in: The work angle is the angle of the unstable equilibrium point.

[0039] Compared with existing technologies, the beneficial effects of this invention are as follows: This invention constructs unified expressions for the positive-sequence current and negative-sequence voltage of grid-connected converters under different types of asymmetrical faults, and considers the requirements of grid connection guidelines for negative-sequence reactive current. It then constructs the swing equation for grid-connected converters under asymmetrical faults, further constructs the energy function of the grid-connected converter system under asymmetrical faults, and derives the transient synchronization stability criterion, accurately characterizing the attraction domain of grid-connected converters under different types of asymmetrical faults. The method of this invention achieves quantitative analysis of the transient synchronization stability of grid-connected converters under asymmetrical faults, which is more consistent with actual power grid fault scenarios, providing a precise and efficient transient stability assessment tool for practical power engineering. Attached Figure Description

[0040] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0041] Figure 1 A flowchart illustrating the transient synchronization stability analysis method for grid-connected converters under asymmetric faults provided in this invention example;

[0042] Figure 2 The control structure and circuit topology diagram of the grid-connected converter provided in the embodiments of the present invention;

[0043] Figure 3 This is the sequence domain equivalent circuit diagram under a single-phase ground fault provided in an embodiment of the present invention;

[0044] Figure 4 This is a sequence domain equivalent circuit diagram provided by an embodiment of the present invention under a two-phase ground fault;

[0045] Figure 5 This is a sequence domain equivalent circuit diagram provided by an embodiment of the present invention under a two-phase short-circuit fault;

[0046] Figure 6 This is an attraction domain diagram of a grid-connected converter system under a single-phase ground fault, provided in an embodiment of the present invention.

[0047] Figure 7 This is an attraction domain diagram of a grid-connected converter system under a two-phase ground fault provided in an embodiment of the present invention.

[0048] Figure 8 This is an attraction domain diagram of a grid-connected converter system under a two-phase short-circuit fault provided in an embodiment of the present invention.

[0049] Figure 9 The figure shows the simulation results under a single-phase ground fault provided in the embodiments of the present invention.

[0050] Figure 10 The figure shows the simulation results under a two-phase-to-ground short-circuit fault provided in the embodiment of the present invention.

[0051] Figure 11 The simulation results for a two-phase short-circuit fault provided in an embodiment of the present invention are shown in the figure. Detailed Implementation

[0052] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0053] In this document, the term "comprising" is intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitation, an element defined by the phrase "comprising..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0054] Reference Figure 1 , Figure 1 A flowchart illustrating the transient synchronization stability analysis method for a grid-connected converter under asymmetric fault conditions provided in this invention example.

[0055] like Figure 1 As shown, this embodiment provides a method for transient synchronization stability analysis of a grid-connected converter under asymmetric fault conditions, including:

[0056] Step 1: Construct a unified expression for the positive sequence current and negative sequence voltage of grid-connected converters under different types of asymmetrical faults.

[0057] For step one, the following are included:

[0058] Reference Figure 2 , Figure 2 The control structure and circuit topology of the grid-connected converter provided in the embodiments of the present invention are shown in the figure.

[0059] like Figure 2 As shown, in the main circuit section, the converter, filter circuit, and transformer are connected in sequence, and the transformer is connected to the power grid. This is the grid voltage. This refers to the terminal voltage of the grid-type converter; For filtering inductors, C f For filtering capacitors; Z L The equivalent impedance on the machine side after the fault. Z g This represents the equivalent impedance on the grid side after a fault. The grid-based control section mainly includes an active power control loop, a positive-sequence voltage and current inner loop, a negative-sequence current loop, a positive-negative-sequence separation stage, and a PWM stage. The inner loop bandwidth is much larger than the power outer loop bandwidth; therefore, the equivalent gain of the inner loop is considered to be 1.

[0060] Reference Figures 3 to 5 , Figures 3 to 5 The following are sequence domain equivalent circuit diagrams provided by embodiments of the present invention under different types of asymmetric faults; wherein: Figure 3 This is the sequence domain equivalent circuit diagram under a single-phase-to-ground short-circuit fault. Figure 4 This is the sequence domain equivalent circuit diagram under a two-phase-to-ground short-circuit fault. Figure 5 This is the sequence domain equivalent circuit diagram under a two-phase short-circuit fault.

[0061] like Figures 3 to 5 Based on Kirchhoff's laws, the equivalent circuit diagram of the sequence domain shown in the figure yields the unified expressions for the positive-sequence current and negative-sequence voltage of the grid-connected converter under different types of asymmetrical faults:

[0062] (1)

[0063] (2)

[0064] in: This refers to the grid voltage. This refers to the positive sequence voltage of a grid-connected converter. The conjugate of the negative sequence current output by the grid-connected converter; and To transfer admittance coefficient, , and For the transmission coefficient, This is the transmission impedance coefficient.

[0065] In this embodiment, the key coefficients in the expressions for the positive-sequence current and negative-sequence voltage of the grid-type converter include , , , , and Its mathematical expression is shown in Table 1.

[0066] Table 1

[0067]

[0068] Step 2: Consider the requirements of the grid connection guidelines for negative sequence reactive current, and based on the expressions for positive sequence current and negative sequence voltage, construct the relationship expression between negative sequence current and positive sequence voltage of the grid-connected converter under asymmetrical fault conditions.

[0069] For step two, the methods include:

[0070] According to national standards GB / T 19963.1-2021 "Technical Specifications for Wind Farm Connection to Power System Part 1: Onshore Wind Power" and GB / T 19964-2024 "Technical Specifications for Photovoltaic Power Station Connection to Power System", the requirements for negative sequence reactive current injected by grid-connected converters under asymmetrical faults are as follows:

[0071] (3)

[0072] in: This is the negative sequence component of the fault point voltage; This is the gain for negative-sequence reactive current. It's important to note that grid-connected converters do not generate negative-sequence active current during asymmetrical faults, therefore... .

[0073] Combining equations (1), (2), and (3), the relationship between the negative sequence current and the positive sequence voltage of a grid-connected converter under asymmetrical fault conditions is expressed as follows:

[0074] (4)

[0075] in: It is the conjugate of the negative sequence impedance on the machine side after the fault.

[0076] Step 3: Based on the expression for positive sequence current, the relationship between negative sequence current and positive sequence voltage, construct the swing equation for the grid-connected converter under asymmetrical fault conditions.

[0077] For step three, the following are included:

[0078] according to Figure 2 Based on the control structure shown and equations (1) and (3), the swing equation of the grid-connected converter under asymmetric fault conditions is obtained as follows:

[0079] (5)

[0080] in: J and D These are virtual inertia and damping, respectively; and These are the positive-sequence phase difference and angular frequency difference between the terminal voltage of the grid-connected inverter and the grid voltage, respectively. for The second derivative; P 0 represents the active power reference value; For power grid synchronization; It is a self-synchronizing item; This is an inter-sequence coupling synchronization term.

[0081] In this embodiment, the power grid synchronization item Self-synchronization items Inter-order coupling synchronization terms The corresponding expression is:

[0082] (6)

[0083] in: , , and The resistivity coefficient, and _ The sensitivity coefficient, This represents the amplitude of the positive sequence voltage. This represents the amplitude of the grid voltage.

[0084] Their expressions are:

[0085] ,

[0086] ,

[0087]

[0088]

[0089] in: For extracting the real part; This is for extracting the imaginary part.

[0090] Step 4: Based on the swing equation, construct the energy function of the grid-connected converter under asymmetric fault conditions by integration. The energy function is a scalar function of the system phase deviation and angular frequency deviation.

[0091] For step four, the following are included:

[0092] Integrating equation (5), we obtain the energy function of the grid-connected converter under asymmetrical fault conditions as follows:

[0093] (7)

[0094] in: The work angle is the angle of the stable equilibrium point.

[0095] Step 5: Based on the energy function, construct the transient synchronization stability criterion for grid-connected converters under asymmetric faults, and calculate the attraction domain of grid-connected converters under asymmetric faults.

[0096] For step five, the following are included:

[0097] According to Lyapunov's direct method, the transient synchronization stability criterion for grid-connected converters under asymmetric faults is as follows:

[0098] (8)

[0099] in: The work angle is the angle of the unstable equilibrium point.

[0100] Reference Figures 6 to 8 , Figures 6 to 8 These are attraction domain diagrams for different types of asymmetric faults provided in embodiments of the present invention; wherein: Figure 6 This is a diagram of the attraction domain of a grid-connected converter system under a single-phase-to-ground short-circuit fault. Figure 7 This is a diagram of the attraction domain of a grid-connected converter system under a two-phase-to-ground short-circuit fault. Figure 8 The attraction domain diagram of a grid-connected converter system under a two-phase short-circuit fault.

[0101] In the specific application of this embodiment, the parameters of the grid-connected converter system are shown in Table 2.

[0102] Table 2

[0103]

[0104] In the specific application of this embodiment, such as Figures 6 to 8 As shown:

[0105] Figure 6 The diagram shows that after a single-phase-to-ground short-circuit fault, the phase trajectory of the grid-connected converter system indicates that the system can stabilize at a new stable equilibrium point. The attraction domain obtained by the method proposed in this embodiment under a single-phase ground fault surrounds the initial equilibrium point of the system. The system can be determined to remain stable, consistent with the actual operating conditions of the system.

[0106] Figure 7 It is shown that when a two-phase ground fault occurs, the phase trajectory of the grid-connected system of the grid-connected converter indicates that the system has lost synchronous stability. The calculation method proposed in this embodiment shows that there is no attraction domain in the system under this fault condition, that is, the system is determined to be unstable, which is the same as the actual operation of the system.

[0107] Figure 8 The diagram shows that after a two-phase short-circuit fault occurs, the phase trajectory of the grid-connected converter system indicates that the system has lost synchronous stability. The attraction domain obtained by the method proposed in this embodiment under a two-phase short-circuit fault does not enclose the initial equilibrium point. This means that the system is determined to be unstable, which is the same as the actual operating condition of the system.

[0108] based on Figures 6 to 8 Based on the analysis, the attraction domain obtained by the method proposed in this embodiment enables accurate evaluation of the transient synchronization stability of grid-connected converter systems under different types of asymmetric faults.

[0109] Reference Figures 9 to 11 , Figures 9 to 11 These are simulation results diagrams for different types of asymmetric faults provided in embodiments of the present invention; wherein, Figure 9 The figure shows the simulation results under a single-phase ground fault. Figure 10 The figure shows the simulation results under a two-phase-to-ground short-circuit fault. Figure 11 The figure shows the simulation results under a two-phase short-circuit fault.

[0110] like Figures 9 to 11 As shown: Figure 9 This demonstrates that when a single-phase ground fault occurs in the system, the system can recover stability, which is the same as the attraction domain determination result obtained by the method proposed in this embodiment. Figure 10 This demonstrates that when a two-phase-to-ground short-circuit fault occurs in the system, the system oscillates, indicating that the system loses synchronization and stability, which is the same as the attraction domain determination result obtained by the method proposed in this embodiment; Figure 11The diagram shows that when a two-phase short-circuit fault occurs in the system, the system oscillates, indicating that the system loses synchronization and stability, which is the same as the attraction domain determination result obtained by the method proposed in this embodiment.

[0111] based on Figures 9 to 11 The analysis and implementation cases of grid-connected converters under different types of asymmetrical faults fully verify the accuracy of the transient synchronization stability analysis method of grid-connected converters under asymmetrical faults proposed in this embodiment, and realize the accurate determination of the transient synchronization stability of grid-connected converters under asymmetrical faults.

[0112] Finally, it should be noted that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for analyzing the transient synchronization stability of a grid-connected converter under asymmetric fault conditions, characterized in that, Including the following steps: Step 1: Construct a unified expression for the positive sequence current and negative sequence voltage of grid-connected converters under different types of faults; Step 2: Consider the grid connection guidelines' requirements for negative sequence reactive current, and based on the expressions for positive sequence current and negative sequence voltage, construct the relationship expression between negative sequence current and positive sequence voltage of the grid-connected converter under asymmetrical fault conditions; Step 3: Based on the expression for positive sequence current, the relationship between negative sequence current and positive sequence voltage, construct the swing equation for the grid-connected converter under asymmetrical fault conditions; Step 4: Based on the swing equation, construct the energy function of the grid-connected converter under asymmetric fault conditions by integration. The energy function is a scalar function of the system phase deviation and angular frequency deviation. Step 5: Based on the energy function, construct the transient synchronization stability criterion for grid-connected converters under asymmetric faults, and calculate the attraction domain of grid-connected converters under asymmetric faults; In step three, the swing equation for the grid-connected converter under asymmetrical fault conditions is: in: J and D These are virtual inertia and damping, respectively; and These are the positive-sequence phase difference and angular frequency difference between the terminal voltage of the grid-connected inverter and the grid voltage, respectively. for The second derivative; P 0 represents the active power reference value; For power grid synchronization; It is a self-synchronizing item; This is an inter-sequence coupling synchronization term; Power grid synchronization items Self-synchronization items Inter-order coupling synchronization terms The corresponding expression is: in: , , and The resistivity coefficient, and _ The sensitivity coefficient, This represents the amplitude of the positive sequence voltage. This represents the amplitude of the grid voltage. The expressions for the resistivity and inductance coefficients are as follows: ， ， in: For extracting the real part; For extracting the imaginary part; and For the transfer admittance coefficient; It is the conjugate of the negative sequence impedance on the machine side after the fault; , and For the transmission coefficient; The transmission impedance coefficient; This is the gain of the negative sequence reactive current.

2. The transient synchronization stability analysis method for grid-connected converters under asymmetric faults according to claim 1, characterized in that, In step one, the positive sequence current of the grid-connected converter under different types of faults... and negative sequence voltage The unified expression is: in: This refers to the grid voltage. This refers to the positive sequence voltage of a grid-connected converter. The conjugate of the negative sequence current output by the grid-connected converter; and To transfer admittance coefficient, , and For the transmission coefficient, This is the transmission impedance coefficient.

3. The transient synchronization stability analysis method for grid-connected converters under asymmetric faults according to claim 2, characterized in that, In step two, the grid connection guideline applies to negative sequence reactive current. The requirements are: in: This is the negative sequence component of the fault point voltage; This is the gain of the negative sequence reactive current.

4. The transient synchronization stability analysis method for grid-connected converters under asymmetric faults according to claim 3, characterized in that, The relationship between the negative sequence current and positive sequence voltage of a grid-connected converter under asymmetrical fault conditions is expressed as follows: in: It is the conjugate of the negative sequence impedance on the machine side after the fault.

5. The transient synchronization stability analysis method for grid-connected converters under asymmetric faults according to claim 1, characterized in that, In step four, the energy function of the grid-connected converter system under asymmetrical fault conditions is: in: The work angle is the angle of the stable equilibrium point.

6. The transient synchronization stability analysis method for grid-connected converters under asymmetric faults according to claim 5, characterized in that, In step five, the transient synchronization stability criterion for grid-connected converters under asymmetrical fault conditions is: in: The work angle is the angle of the unstable equilibrium point.

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