Power instruction optimization control method for network-building type grid-connected converter under asymmetric fault of power grid

Abstract

The invention provides a power instruction optimization control method for a network-building type grid-connected converter under an asymmetric fault of a power grid, and the method comprises the steps: employing a positive and negative sequence system initial power instruction when a system is in a normal operation state; when the system is in a fault state, a self-adaptive power instruction designed by the invention is adopted; according to the method, the negative-sequence component and the negative-sequence network during the asymmetric fault period are considered, the positive-sequence active power instruction value and the negative-sequence active power instruction value during the fault period are given, and the transient synchronization stability of the positive-sequence system and the negative-sequence system is greatly improved. Meanwhile, according to the method, a power adjusting instruction is adaptively adjusted according to the voltage drop depth of the grid-connected point, complex calculation and algorithm design do not need to be carried out, and the practicability and operability of a control algorithm are greatly improved.

Description

TECHNICAL FIELD

[0001] The application relates to a power instruction optimization control method for a grid-forming grid-connected converter under an asymmetric fault of a power grid, and the method is beneficial to increasing an equivalent deceleration area of the system and improving grid-connected stability of the grid-forming grid-connected converter system by reducing a power instruction value of the grid-forming grid-connected converter system, and belongs to the technical field of new energy power generation. BACKGROUND

[0002] Under the background of renewable energy grid connection, the grid-forming grid-connected converter, as the key equipment connecting renewable energy and the power grid, its stability and reliability are very important for the normal operation of the whole system. However, the actual power grid often encounters various complex fault conditions, among which asymmetric short-circuit fault is particularly common and has a significant impact on system stability. When asymmetric fault occurs, the voltage and current waveforms of the power grid will be obviously distorted, which not only affects the normal operation of the grid-connected converter, but also may lead to power fluctuation and instability of the system. The traditional power instruction control method is usually designed based on symmetric fault or ideal grid conditions, and cannot effectively cope with the challenges brought by asymmetric fault. Therefore, it is of great significance to study a method that can adapt to asymmetric fault conditions and realize adaptive control of power instruction, for improving the fault ride-through capability of the grid-forming grid-connected converter and the stability of the power grid. In addition, with the continuous development of renewable energy grid connection technology, the performance requirements for grid-connected converters are also getting higher and higher. Under asymmetric fault conditions, how to realize accurate tracking and rapid response of power instruction is one of the key points of grid-connected converter control strategy research. Through adaptive control method, the power instruction can be adjusted in real time according to the asymmetric fault drop depth of the power grid, to ensure that the grid-connected converter can still maintain stable power output under fault conditions. At present, scholars at home and abroad have made certain research results in the adaptive control of grid-connected converter power instruction, as shown in the following published literatures:

[0003] (1) G. Wang, L. Fu, Q. Hu, et al. Transient Synchronization Stability of Grid-Forming Converter During Grid Fault Considering Transient Switched Operation Mode[J]. IEEE Transactions on Sustainable Energy, 2023, 14(3): 1504-1515.

[0004] (2) H. Wu and X. Wang. A Mode-Adaptive Power-Angle Control Method for Transient Stability Enhancement of Virtual Synchronous Generators[J]. IEEE Journal of Emerging and Selected Topics in Power Electronics, 2020, 8(2):1034-1049.

[0005] Document (1) proposes a grid symmetrical short-circuit fault based on power instruction switching grid type system transient stability enhancement control method. The method reduces the active power instruction during the grid short-circuit fault, reduces the equivalent acceleration area of the system, increases the deceleration area, and improves the transient stability of the system. However, the method does not give the value range of the active power instruction value during the fault. Document (2) proposes a power instruction value switching control method. In this method, the positive feedback mode of the power angle control of the grid type system is detected and adaptively switched to the negative feedback mode after a large disturbance, thereby reducing the active power instruction value during the fault, which can reduce the instability risk of the grid type system during the fault. However, this method needs to detect the deviation of active power and the change rate of active power in real time, and accurately adjust the power instruction through a complex algorithm, which not only increases the calculation burden, but also may cause control delay and affect real-time performance. SUMMARY

[0006] In order to solve the problems existing in the prior art, the purpose of the present application is to propose a grid asymmetric fault grid type grid-connected converter power instruction optimization control method. This method considers the negative sequence component and negative sequence network during asymmetric fault, gives the positive sequence active power instruction value and negative sequence active power instruction value during the fault, greatly improves the transient synchronous stability of the positive and negative sequence system. At the same time, this method adaptively adjusts the power instruction according to the voltage drop depth of the grid-connected point, without complex calculation and algorithm design, greatly improving the practicality and operability of the control algorithm.

[0007] The technical scheme of the present application is as follows:

[0008] A grid asymmetric fault grid type grid-connected converter power instruction optimization control method, this method involves adaptive control of the positive sequence power instruction and negative sequence power instruction of the grid type grid-connected converter system; the specific calculation steps are as follows:

[0009] A1) During normal operation of the grid type grid-connected converter system, the positive sequence power instruction and negative sequence power instruction is

[0010]

[0011] wherein, is the positive sequence system initial power instruction; is the negative sequence system initial power instruction; is the actual value of grid-connected point voltage during system operation; U N is the rated voltage value of system grid-connected point;

[0012] A2) During the fault, the positive sequence power instruction is calculated according to the grid voltage drop degree as follows and negative sequence power instruction

[0013]

[0014] A3) Based on the positive sequence power instruction obtained in step A2) and negative sequence power instruction The positive sequence power instruction and the negative sequence power instruction of the grid-forming grid-connected converter system during the fault are automatically adjusted, thereby improving the transient synchronous stability of the system.

[0015] Compared with the prior art, the present application has the following beneficial effects:

[0016] The present application proposes a grid-forming grid-connected converter power instruction optimization control method under the condition of asymmetric short-circuit of the grid. The method can adjust the power instruction in real time according to the actual operation condition of the grid and the state of the grid-forming converter, thereby adapting to various complex grid environments and operation conditions. This high adaptability enables the converter to maintain stable performance under different working scenarios, improving the reliability and efficiency of the system. BRIEF DESCRIPTION OF DRAWINGS

[0017] Figure 1 is a schematic diagram of the positive and negative sequence system power ring control structure of the grid-forming grid-connected converter.

[0018] Figure 2 is a block diagram of the power instruction adaptive control algorithm of the grid-forming grid-connected converter.

[0019] Figure 3 is a comparison diagram of simulation waveforms of the grid-forming grid-connected converter system under the condition of single-phase ground fault, using the traditional control strategy and the control method of the present application. DETAILED DESCRIPTION

[0020] The specific embodiments of the present application are described in detail below with reference to the accompanying drawings.

[0021] Figure 1 A schematic diagram of the power loop control structure for the positive and negative sequence system of a grid-connected converter.

[0022] An optimized power command control method for grid-connected converters under asymmetrical grid faults is proposed to improve the transient synchronization stability of grid-connected converter systems under asymmetrical grid faults and address the transient instability problem of grid-connected converters during asymmetrical short-circuit faults. This method involves optimizing the positive-sequence power command of the grid-connected converter system. and negative sequence power command Adaptive control; the specific calculation steps are as follows:

[0023] A1) During normal operation of a network-type system, the positive sequence power command and negative sequence power command for

[0024]

[0025] In the formula, This is the initial power command for the positive-sequence system. This is the initial power command for the negative sequence system; This represents the actual voltage at the grid connection point during system operation; U N This refers to the rated voltage value at the system's grid connection point.

[0026] A2) During a fault, the positive-sequence system power command can be obtained based on the degree of grid voltage drop. and negative sequence system power command for

[0027]

[0028] A3) Based on the grid voltage sag depth, the positive sequence power command can be obtained from step A2). and negative sequence power command This allows for automatic adjustment of the positive-sequence power command and negative-sequence power command of the networked system during faults, which can significantly improve the transient synchronization stability of the system.

[0029] Figure 2 This is a block diagram of the power command adaptive control algorithm for a grid-connected converter. When When the power output is greater than 0.9 pu, the system is in normal operation, and during normal operation, the positive and negative sequence system initial power command designed in this invention is used. When When the power consumption is less than or equal to 0.9 PU, the system is in a fault state. During the fault period, the adaptive power command designed in this invention is used, i.e.

[0030] Description of the effects of this invention:

[0031] Figure 3Figures (a), (b), (c), and (d) respectively present the simulation waveform comparison of the grid-connected converter system under the single-phase ground fault condition, when the positive sequence voltage component at the grid connection point drops to 0.71 pu and the negative sequence voltage component at the grid connection point rises to 0.3 pu, using the traditional control strategy and the control method of this invention. Figure 3 (a) is a simulation waveform diagram of the positive sequence system of a grid-connected converter using a traditional control strategy; Figure 3 (b) is a simulation waveform diagram of a grid-connected converter using the positive-sequence power command adaptive control strategy of the present invention; 3(c) is a simulation waveform diagram of a negative-sequence system of a grid-connected converter using the traditional control strategy; Figure 3 (d) is a simulation waveform diagram of the negative sequence system of a grid-connected converter when the negative sequence power command adaptive control strategy of the present invention is adopted. + P - These represent the positive-sequence active power and negative-sequence active power output of a grid-connected converter system, respectively, δ. + and δ - For grid-connected converter systems, this refers to the positive-sequence and negative-sequence power angles. For example... Figure 3 As shown in (a), when the positive-sequence system adopts a traditional control strategy, the duration of the power grid short-circuit fault is as follows: The system is set to 1 pu. At this time, the system outputs active power P. + The system experiences significant fluctuations and remains unstable. The system's power angle δ + Fluctuations between -1 pu and 1 pu indicate that the system cannot maintain synchronization with the power grid and is in an unstable state. For example... Figure 3 As shown in (b), the positive-sequence power command value adaptive control strategy proposed in this invention is adopted. Set to 0.5 pu. System output active power P + After a transient process, the system stabilizes at 0.5 pu, and the system power angle δ + After a transient process, the system stabilizes. At this point, the system can maintain synchronized and stable operation. For example... Figure 3 As shown in (c), when the negative sequence system adopts the traditional control strategy, the duration of the power grid short-circuit fault is as follows: Set to 0.2 pu. The system's negative sequence active power P - It experiences significant fluctuations and remains unstable. The negative-sequence system's power angle δ - Fluctuations between -1 pu and 1 pu indicate that the system cannot maintain synchronization with the power grid and is in an unstable state. For example... Figure 3 As shown in (d), during the duration of a short-circuit fault in the power grid, the negative-sequence power command adaptive control strategy proposed in this invention is adopted. Set to 0 pu. System output active power P - After a transient process, the system stabilizes at 0pu, and the power angle δ of the negative sequence system is... -After a transient process, it becomes stable. At this point, the negative-sequence system can maintain synchronous and stable operation.

[0032] In summary, the control scheme proposed in this invention can automatically adjust the power command according to the voltage sag depth of the grid, ensuring stable operation of the grid-connected converter under various complex environments. This method not only improves the system's power generation efficiency and power quality but also reduces operating costs and maintenance difficulty. Furthermore, its rapid response characteristics help reduce instability caused by grid fluctuations, enhancing the overall system reliability. In conclusion, the adaptive power command control method for grid-connected converters, with its superior performance and broad adaptability, demonstrates enormous application potential and value in the field of grid-connected power generation.

[0033] Finally, it should be noted that the above examples of the present invention are merely illustrative and not intended to limit the implementation of the invention. Although the applicant has described the present invention in detail with reference to preferred embodiments, those skilled in the art can make other variations and modifications based on the above description. It is impossible to exhaustively list all possible implementations here. All obvious variations or modifications derived from the technical solutions of the present invention are still within the scope of protection of the present invention.

Claims

1.A method for power instruction optimization control of a grid-forming grid-connected converter under asymmetric grid faults, characterized in that: The method relates to adaptive control of positive sequence power instruction and negative sequence power instruction of a grid-forming grid-connected converter system; the specific calculation steps are as follows: A1) positive sequence power instruction and negative sequence power instruction during normal operation of the meshed grid connected converter system and are In the formula, is the positive sequence system initial power instruction; is the negative sequence system initial power instruction; is the actual value of the system during the operation of the grid-connected point voltage; U N is the rated voltage value of the system grid-connected point; A2) During the fault, the positive and negative sequence power commands are calculated according to the grid voltage drop degree, as follows and negative sequence power commands A3) obtaining positive sequence power commands and negative sequence power commands based on the positive sequence power commands obtained in step A2) and negative sequence power commands Automatically adjusting positive sequence power commands and negative sequence power commands of grid-forming grid-tie converter systems during faults, thereby improving transient synchronous stability of the system.

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