A method for controlling transient voltage stability of a grid-connected system during a power grid fault duration of a plurality of wind farms

Abstract

This invention discloses a transient voltage stability control method for a multi-wind farm grid-connected system during a grid fault. First, the synchronous machine most prone to instability during the transient period is identified. Then, the method is based on the influence factor of the synchronous machine node most prone to instability on the load node. D Ln Influence factors of wind power nodes on load nodes W Ln The magnitude of the load at load nodes in a grid-connected system G Ln Determine wind power nodes during the fault period w Optimal load increase node index H w,Ln , H w,Ln = D Ln + W Ln + G Ln During the fault duration, each wind farm node outputs current to raise the voltage of the load node with the highest optimal load node rise index in its respective aggregate, thus achieving transient voltage stability control of the grid-connected system during the fault duration. This invention utilizes the output capacity of the wind farm to improve the transient power angle characteristics of the system's synchronous machine, thereby optimizing the system's voltage fluctuations during the fault and improving the system's transient voltage stability.

Description

Technical Field

[0001] This invention relates to a transient voltage stability control method for a multi-wind farm grid-connected system during a grid fault. It is applicable to a large-scale wind farm grid-connected system based on phase-locked synchronous control under a symmetrical short-circuit fault in the power grid. This strategy can utilize the output current of each wind turbine to improve the transient power angle characteristics of the system during a short-circuit fault, thereby improving the transient voltage stability of the system. Background Technology

[0002] In recent years, with the continuous expansion of wind power installed capacity in new power systems, the interaction between wind farms and the power system has been continuously strengthened, and the voltage stability problem of the system will exhibit new characteristics, posing a significant challenge to the safe and stable operation of the power grid. Furthermore, because wind farms in my country are often located in remote areas with weak connections to the power grid, their voltage support capacity is relatively poor. Moreover, compared to traditional synchronous turbine power plants, wind farms have relatively weak reactive power output capacity. After large-scale wind power grid connection, the grid's ability to withstand disturbances and load fluctuations decreases, exacerbating the risk of transient voltage instability and seriously threatening the safe and stable operation of the power system. Therefore, improving the transient voltage stability of large-scale wind power grid-connected systems during fault periods is a key issue in the current development of wind power. Current research by domestic and international scholars mainly focuses on transient voltage stability control strategies for multi-wind farm grid-connected systems, as illustrated in the following published literature:

[0003] [1] Tang Guanjun, Chen Yonghua, Luo Jianbo, et al. Research on emergency control technology of reactive voltage in grid-connected areas of clustered wind farms [J]. Power Grid and Clean Energy, 2017, 33(1): 107-114.

[0004] [2]Wei Juan, Cao Yijia, Wu Qiuwei, et al. Coordinated Droop Control and Adaptive Model Predictive Control for Enhancing HVRT and Post-EventRecovery of Large-Scale Wind Farm[J]. IEEE Transactions on Sustainable Energy, 2021, 12(3): 1549-1560.

[0005] Reference [1] proposes a voltage status criterion zone based on the voltage level of the wind power cluster during the transient period. Different reactive power equipment compensation control strategies are adopted in stages according to the voltage zone to achieve rapid control of transient voltage problems such as low voltage and voltage exceeding the limit. Reference [2] proposes a transient voltage coordination control of drop control and adaptive mode predictive control. During the transient period, the sensitivity relationship between each wind turbine controller and the terminal voltage is calculated, and the reactive power output of each wind turbine is coordinated and controlled using the voltage sensitivity relationship to optimize the voltage state of the wind power grid connection point during the fault period. The voltage stabilization strategies of the wind power grid connection system proposed in the above references mainly focus on the voltage state of the wind farm grid connection node during the fault period. The transient voltage level of the wind power node is optimized by its own characteristics or by coordination with the reactive power compensation device. However, most of the above studies ignore the impact of the output current of the wind power node on other power sources and loads of the system during the fault period, and lack consideration of the ability of the wind power node to coordinate and control the transient voltage of the system during the fault period. Summary of the Invention

[0006] To address the aforementioned shortcomings of existing technologies, the present invention aims to propose a transient voltage stability control method for a multi-wind farm grid-connected system during a grid fault. This method is based on the interactive influence of wind turbine node output on the system's synchronous power supply and load during the fault period. During the fault period, the output current of the wind turbine nodes is used to improve the transient power angle characteristics of the system, thereby enhancing the transient voltage stability of the system.

[0007] The technical solution of this invention is implemented as follows:

[0008] A method for transient voltage stability control of a multi-wind farm grid-connected system during a grid fault, comprising the following steps:

[0009] A1) Identify the synchronous machine most prone to instability during the transient period;

[0010] A2) The following definition applies to multi-wind farm grid-connected systems:

[0011] Define the influence factor D of the synchronous machine node most prone to instability on the load node n. Ln Normalized expression:

[0012]

[0013] In the formula: Z GG Z represents the self-impedance of the synchronous machine node most prone to instability. Gn The mutual impedance between the synchronous machine node and the load node n, which is most prone to instability;

[0014] Define the influence factor W of wind power node w on load node n. Ln Normalized expression:

[0015]

[0016] In the formula: Z wn Z represents the mutual impedance between wind turbine node w and load node n; ww S is the self-impedance of wind turbine node w; w and S w_max These represent the capacity of wind power node w and the maximum capacity of all wind power nodes, respectively.

[0017] Define the load magnitude G of load node n in a multi-wind farm grid-connected system. Ln Normalized expression:

[0018]

[0019] In the formula: P Ln0 P represents the initial active power of load node n. L0max This represents the initial maximum active power of all load nodes;

[0020] Define the optimal load-lifting node index H for wind power node w during a fault. w,Ln expression:

[0021]

[0022] In the formula H w,Ln The larger the value, the greater the impact of load node n on the active power output of the synchronous machine node most prone to instability during the fault period;

[0023] A3) In a multi-wind farm grid-connected system, there are m wind turbine nodes w and q load nodes n, where w = {1, 2, ..., m} and n = {1, 2, ..., q}. During a fault, based on A2), the optimal load node boosting index H matrix for each wind turbine node w relative to each load node n in the system is obtained, and its expression is as follows:

[0024]

[0025] The w-th row of matrix H represents the set of optimal load-lifting node indices for wind node w across all load nodes. w H w ={ H w,L1 H w,L2 …, H w,Ln …, H w,Lq During the fault, each wind node w raises its respective aggregate H by outputting current. w The optimal load node voltage is the one that maximizes the load node's performance index, thus achieving transient voltage stability control of the multi-wind farm grid-connected system during grid fault periods. The wind node w output current command is shown below:

[0026]

[0027] In the formula: I w The output current amplitude of the wind turbine node w; I max This represents the maximum output current amplitude during a fault at wind power node w. , , These are the output current angles of wind turbine node w and H, respectively. w,Ln Voltage angle at the maximum load node, wind node w and H w,Ln The mutual impedance angle between nodes n with the maximum load.

[0028] Further, in step A1), the synchronizing machine most prone to instability during the transient period is determined as follows;

[0029] During a grid fault, the power angle stability index A of each synchronous machine in the grid-connected system is calculated using the following formula. The synchronous machine with the smallest power angle stability index A is the one most prone to instability during the transient period. The normalized expression for the power angle stability index A is as follows:

[0030]

[0031] Where △δ max This represents the maximum value of the power angle difference between each synchronizing machine and the balancing node synchronizing machine during the fault period.

[0032] Compared with the prior art, the present invention has the following beneficial effects:

[0033] This invention considers the influence of the wind farm's output current on the system's transient voltage stability during a fault, and utilizes the wind farm's output capacity to improve the transient power angle characteristics of the system's synchronous machine, thereby optimizing the system's voltage fluctuations during a fault and improving the system's transient voltage stability. Attached Figure Description

[0034] Figure 1 This is a schematic diagram of the topology of an IEEE 10-machine, 39-node multi-wind farm grid-connected system.

[0035] Figure 2 A three-phase symmetrical short-circuit fault occurred on bus 18 of the power grid. The load in the system was mainly constant impedance load. When the wind turbine adopts the traditional control strategy and the control strategy proposed in this invention, the transient voltage and power angle characteristic curves of each node bus in the power system are shown.

[0036] Figure 3 A three-phase symmetrical short-circuit fault occurred on bus 24 of the power grid. The load in the system was mainly constant impedance load. When the wind turbine adopts the traditional control strategy and the control strategy proposed in this invention, the transient voltage and power angle characteristic curves of each node bus in the power system are shown. Detailed Implementation

[0037] The specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

[0038] This invention is used to improve the transient voltage stability of large-scale wind power grid-connected systems during grid faults. Figure 1 This is a schematic diagram of the topology of a multi-wind farm grid-connected system in a certain embodiment. During a grid short-circuit fault, the optimal index H matrix for each wind turbine node w relative to each load node n in the system is first calculated. Then, the optimal load node voltage rise index set H for each wind turbine node w relative to each load node n is further determined. w H w ={ H w,L1 H w,L2 …, H w,Lq Finally, each wind node w only needs to raise its respective set H. w The load node voltage with the largest median value can improve the power angle characteristics of the system during a fault, thereby reducing the risk of transient voltage instability.

[0039] The specific implementation steps of this invention are as follows:

[0040] A1) Determine the synchronous machine most prone to instability during the transient period using the following method;

[0041] During a grid fault, the power angle stability index A of each synchronous machine in the grid-connected system is calculated using the following formula. The synchronous machine with the smallest power angle stability index A is the one most prone to instability during the transient period. The normalized expression for the power angle stability index A is as follows:

[0042]

[0043] Where △δ max This represents the maximum value of the power angle difference between each synchronizing machine and the balancing node synchronizing machine during the fault period.

[0044] A2) The following definition applies to multi-wind farm grid-connected systems:

[0045] Define the influence factor D of the synchronous machine node most prone to instability on the load node n. Ln Normalized expression:

[0046]

[0047] In the formula: Z GG Z represents the self-impedance of the synchronous machine node most prone to instability. Gn The mutual impedance between the synchronous machine node and the load node n, which is most prone to instability;

[0048] Define the influence factor W of wind power node w on load node n. LnNormalized expression:

[0049]

[0050] In the formula: Z wn Z represents the mutual impedance between wind turbine node w and load node n; ww S is the self-impedance of wind turbine node w; w and S w_max These represent the capacity of wind power node w and the maximum capacity of all wind power nodes, respectively; this index includes the impact of wind power node capacity and network impedance on load nodes.

[0051] Define the load magnitude G of load node n in a multi-wind farm grid-connected system. Ln Normalized expression:

[0052]

[0053] In the formula: P Ln0 P represents the initial active power of load node n. L0max This represents the initial maximum active power of all load nodes;

[0054] Define the optimal load-lifting node index H for wind power node w during a fault. w,Ln expression:

[0055]

[0056] In the formula H w,Ln The larger the value, the greater the impact of load node n on the active power output of the synchronous machine node most prone to instability during the fault period;

[0057] A3) In a multi-wind farm grid-connected system, there are m wind turbine nodes w and q load nodes n, where w = {1, 2, ..., m} and n = {1, 2, ..., q}. During a fault, based on A2), the optimal load node boosting index H matrix for each wind turbine node w relative to each load node n in the system is obtained, and its expression is as follows:

[0058]

[0059] The w-th row of matrix H represents the set of optimal load-lifting node indices for wind node w across all load nodes. w H w ={ H w,L1 H w,L2 …, H w,Ln …, H w,Lq During the fault, each wind node w raises its respective aggregate H by outputting current. wThe optimal load node voltage is the one that maximizes the load node's performance index, thus achieving transient voltage stability control of the multi-wind farm grid-connected system during grid fault periods. The wind node w output current command is shown below:

[0060]

[0061] In the formula: I w The output current amplitude of the wind turbine node w; I max This represents the maximum output current amplitude during a fault at wind power node w. , , These are the output current angles of wind turbine node w and H, respectively. w,Ln Voltage angle at the maximum load node, wind node w and H w,Ln The mutual impedance angle between nodes n with the maximum load.

[0062] Description of the effects of this invention:

[0063] by Figure 1 The effectiveness of the proposed method is illustrated using the IEEE 10-machine 39-bus power system as an example. Figure 2 The transient simulation waveforms of the system are given when a three-phase symmetrical short circuit occurs on bus 18 of the power grid, the fault duration is 0.35 seconds, and the load in the system is mainly constant impedance load. Figure 2 (a) is a transient voltage curve of the wind power grid-connected system when using the traditional control strategy. As can be seen from the figure, after the fault ends, the system voltage continues to drop, and the voltage drops to the lowest point in about 1.5 seconds. Then the system voltage begins to oscillate, at which point the system experiences transient voltage instability. Figure 2 (b) is a transient power angle curve of the wind power grid-connected system when using the traditional control strategy. As can be seen from the figure, the power angle difference between the No. 2 synchronous machine and the No. 1 synchronous machine continued to increase during the fault period. At about 1.4 seconds, the power angle difference exceeded 180°, which indicates that the system experienced power angle instability during the transient period. At this time, the voltage instability of the system was mainly caused by power angle instability. Figure 2 (d) The transient power angle curve of the wind power grid-connected system when using the control strategy proposed in this invention. As shown in the figure, after adopting the improved strategy during the fault period, the power angle difference between synchronizer No. 2 and synchronizer No. 1 did not exceed 180° during the transient period, the power angle characteristics of the system were improved, and the system did not experience power angle instability. Figure 2 (c) It can be seen that during the fault, the system voltage gradually dropped to the lowest value, and then the voltage began to rise slowly. The system voltage returned to stability within 3 seconds.

[0064] Figure 3 The transient simulation waveforms of the system are given when a three-phase symmetrical short circuit occurs on bus 24 of the power grid, the fault duration is 0.25 seconds, and the load in the system is mainly constant impedance load. Figure 3(a) is a transient voltage curve of the wind power grid-connected system when using the traditional control strategy. As can be seen from the figure, after the fault ends, the system voltage continues to drop, and the voltage drops to the lowest point at about 1.42 seconds. Then the system voltage begins to oscillate. Figure 3 (b) is a transient power angle curve of the wind power grid-connected system when using the traditional control strategy. It can be seen that during the transient period, the power angle difference between Synchronous Unit 2 and Synchronous Units 1 and 6 is continuously increasing. At 1.40 seconds and 1.42 seconds, the power angle difference exceeds 180°, and the system becomes unstable. Figure 3 (d) shows the transient power angle curves of the wind power grid-connected system when using the control strategy proposed in this invention. As can be seen from the figure, after adopting the improved strategy during the fault period, the maximum power angle difference between synchronizer No. 2 and synchronizers No. 1 and No. 6 during the transient period was 58° and 82°, respectively. The power angle difference between the synchronizers decreased significantly during the fault recovery period, the power angle characteristics were improved, and the system power angle stabilized. Figure 3 (c) It can be seen that during the fault recovery period, when the improved control strategy is adopted, the system voltage has stopped decreasing at 1.25 seconds and the voltage begins to recover. The system changes from an unstable state to a stable state, and the transient voltage stability of the system is improved.

[0065] Therefore, it can be seen that the coordinated control strategy proposed in this invention can effectively improve the transient voltage stability of large-scale wind power grid-connected systems during faults.

[0066] 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 transient voltage stability control of a multi-wind farm grid-connected system during a grid fault, characterized in that: The specific steps are as follows: A1) Identify the synchronous machine most prone to instability during the transient period; A2) The following definition applies to multi-wind farm grid-connected systems: Define the influence factor D of the synchronous machine node most prone to instability on the load node n. Ln Normalized expression: In the formula: Z GG Z represents the self-impedance of the synchronous machine node most prone to instability. Gn The mutual impedance between the synchronous machine node and the load node n, which is most prone to instability; Define the influence factor W of wind power node w on load node n. Ln Normalized expression: In the formula: Z wn Z represents the mutual impedance between wind turbine node w and load node n; ww S is the self-impedance of wind turbine node w; w and S w_max These represent the capacity of wind power node w and the maximum capacity of all wind power nodes, respectively. Define the load magnitude G of load node n in a multi-wind farm grid-connected system. Ln Normalized expression: In the formula: P Ln0 P represents the initial active power of load node n. L0max This represents the initial maximum active power of all load nodes; Define the optimal load-lifting node index H for wind node w during a fault. w,Ln expression: In the formula H w,Ln The larger the value, the greater the impact of load node n on the active power output of the synchronous machine node most prone to instability during the fault period; A3) In a multi-wind farm grid-connected system, there are m wind turbine nodes w and q load nodes n, where w = {1, 2, ..., m} and n = {1, 2, ..., q}. During a fault, based on A2), the optimal load node boosting index H matrix for each wind turbine node w relative to each load node n in the system is obtained, and its expression is as follows: The w-th row of matrix H represents the set of optimal load-lifting node indices for wind node w across all load nodes. w H w ={H w,L1 H w,L2 …, H w,Ln …, H w,Lq During the fault, each wind node w raises its respective aggregate H by outputting current. w The optimal load node voltage is the one that maximizes the load node's performance index, thus achieving transient voltage stability control of the multi-wind farm grid-connected system during grid fault periods. The wind node w output current command is shown below: In the formula: I w The output current amplitude of the wind power node w; I max This represents the maximum output current amplitude during a fault at wind power node w. , , These are the output current angles of wind turbine node w and H, respectively. w,Ln Voltage angle at the maximum load node, wind node w and H w,Ln The mutual impedance angle between nodes n with the maximum load.

2. The transient voltage stabilization control method for a multi-wind farm grid-connected system during a grid fault as described in claim 1, characterized in that: In step A1), the synchronous machine most prone to instability during the transient period is determined as follows; During a grid fault, the power angle stability index A of each synchronous machine in the grid-connected system is calculated using the following formula. The synchronous machine with the smallest power angle stability index A is the one most prone to instability during the transient period. The normalized expression for the power angle stability index A is as follows: Where △δ max This represents the maximum value of the power angle difference between each synchronizing machine and the balancing node synchronizing machine during the fault period.

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