Transient voltage stability control method for multi-wind-power-plant grid-connected system during power grid fault duration

Abstract

The invention discloses a transient voltage stability control method for a multi-wind-power-plant grid-connected system during a power grid fault duration period. The method comprises the following steps: firstly, determining a synchronous machine which is most prone to instability during a transient period; and then determining the optimal lifting load node indexes H < w >, L < n >, H < w >, L < n >, H < w >, L < n > = DL < n > + WL < n > + GL < n > of the wind power node w during the fault period based on the influence factor DL < n > of the synchronous machine node most prone to instability on the load node, the influence factor WL < n > of the wind power node on the load node and the load degree GL < n > of the load node in the grid-connected system. And during the fault duration, each wind power node boosts the load node voltage with the maximum optimal boosted load node index in the respective set through the output current, that is, the transient voltage stability control of the grid-connected system during the fault duration is realized. According to the method, the transient power angle characteristic of the system synchronous machine is improved by utilizing the output capacity of the wind power plant, so that the voltage fluctuation of the system during the fault period is optimized, and the transient voltage stability of the system is improved.

Description

Technical Field

[0001] The present invention relates to a transient voltage stability control method for a multi-wind farm grid-connected system during a power grid fault. The method 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. During a short-circuit fault, the output current of each wind turbine can be used to improve the transient power angle characteristics of the system, thereby improving the transient voltage stability of the system. Background Art

[0002] In recent years, with the continuous expansion of wind power installed capacity in new power systems, the interaction between wind farms and power systems has continued to increase, and the voltage stability problem of the system will show new characteristics, which poses a major challenge to the safe and stable operation of the power grid. In addition, since my country's wind power stations are often located in remote areas and are weakly connected to the power grid, the voltage support capacity is poor. Moreover, compared with traditional synchronous power plants, wind farms have relatively weak reactive power output capabilities. After large-scale wind power is connected to the grid, the grid's ability to withstand disturbances and load fluctuations is reduced, and the risk of transient voltage instability is increased, which seriously threatens the safe and stable operation of the power system. Therefore, improving the transient voltage stability of large-scale wind power grid-connected systems during faults is a key issue in the current development of wind power. At present, the relevant research carried out by scholars at home and abroad mainly focuses on the transient voltage stability control strategy of multi-wind farm grid-connected systems, such as the following published documents:

[0003] [1] Tang Guanjun, Chen Yonghua, Luo Jianbo, et al. Research on reactive power and voltage emergency control technology in grid-connected wind farm cluster areas [J]. Power System 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-Event Recovery of Large-Scale Wind Farm[J]. IEEE Transactions on Sustainable Energy, 2021, 12(3): 1549-1560.

[0005] Reference [1] proposed a voltage situation judgment zone based on the voltage level of the wind power cluster during the transient period, and adopted different reactive equipment compensation control strategies according to the voltage zone and stage, so as to achieve rapid control of transient voltage problems such as voltage undervoltage and voltage overlimit. Reference [2] proposed a transient voltage coordinated control of drop control and adaptive mode predictive control, which calculated the sensitivity relationship between each wind turbine controller and the terminal voltage during the transient period, and used the voltage sensitivity relationship to coordinate the reactive output of each wind turbine to optimize the voltage state of the wind power grid connection point during the fault period. The voltage stability 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, and optimize the transient voltage level of the wind power node through its own characteristics or coordination with the reactive 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 in the system during the fault period, and lack the consideration of the wind power node's ability to coordinate the transient voltage of the system during the fault period. Summary of the Invention

[0006] In view of the above-mentioned deficiencies in the prior art, the purpose of the present invention is to propose a method for transient voltage stability control of a multi-wind farm grid-connected system during a grid fault. The method is based on the interactive influence of the output of the wind power node on the system synchronous machine power supply and load during the fault period. The output current of the wind power node is used to improve the transient power angle characteristics of the system during the fault period, thereby improving the transient voltage stability of the system.

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

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

[0009] A1) Identify the synchronous machine most likely to become unstable during transient conditions;

[0010] A2) The following definition is given for a multi-wind farm grid-connected system:

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

[0012]

[0013] Where: Z GG is the self-impedance of the synchronous machine node that is most prone to instability; Z Gn is the mutual impedance between the synchronous machine node that is most prone to instability and the load node n;

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

[0015]

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

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

[0018]

[0019] Where: P Ln0 represents the initial active power of load node n, P L0max is the maximum initial active power of all load nodes;

[0020] Define the optimal load node index H of wind power node w during fault period w,Ln expression:

[0021]

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

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

[0024]

[0025] The wth row of the H matrix is ​​the optimal load node index set H for wind power node w to all load nodes. w , H w ={H w,L1 ,H w,L2 …,H w,Ln …,H w,Lq During the fault period, each wind power node w raises its own aggregate H by outputting current. w The voltage of the load node with the largest load node index is optimally raised, that is, transient voltage stability control of the multi-wind farm grid-connected system is achieved during the duration of the grid fault; the output current instruction of the wind power node w is as follows:

[0026]

[0027] Where: I w is the output current amplitude of wind power node w; I max is the maximum output current amplitude of wind power node w during fault period; θ wn 、 They are the output current angle of wind power node w, H w,Ln The voltage angle of the maximum load node, the wind power node w and H w,Ln The mutual impedance angle between the maximum load nodes n.

[0028] Furthermore, in step A1), the synchronous machine most likely to become unstable during the transient period is determined according to the following method;

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

[0030]

[0031] where △δ max It is the maximum value of the power angle difference between each synchronous machine and the synchronous machine at the balance node during the fault period.

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

[0033] The present invention takes into account the impact 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 system's transient power angle characteristics of the synchronous machine, thereby optimizing the system's voltage fluctuations during the fault and improving the system's transient voltage stability. BRIEF DESCRIPTION OF THE DRAWINGS

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

[0035] Figure 2 A three-phase symmetrical short circuit fault occurs on bus No. 18 of the power grid. The load in the system is mainly constant impedance load. When the wind turbine adopts the traditional control strategy and the control strategy proposed by the present invention, the transient voltage and power angle characteristic curves of the bus at each node in the power system are shown.

[0036] Figure 3 A three-phase symmetrical short circuit fault occurs on bus No. 24 of the power grid. The load in the system is mainly constant impedance load. When the wind turbine adopts the traditional control strategy and the control strategy proposed by the present invention, the transient voltage and power angle characteristic curves of the bus at each node in the power system are shown. DETAILED DESCRIPTION

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

[0038] The present invention is used to improve the transient voltage stability of a large-scale wind power grid-connected system during a power grid fault. Figure 1 The topology diagram of a multi-wind farm grid-connected system in a certain embodiment is shown. During a grid short circuit fault, the optimal index matrix H for each wind power node w to each load node n in the system is first calculated, and then the optimal load node voltage raising index set H for each wind power node w to each load node n is further determined. w , where H w ={H w,L1 ,H w,L2 …,H w,Lq}, and finally each wind power node w only needs to raise its own set H w The load node voltage with the largest value can improve the power angle characteristics of the system during the fault, thereby reducing the risk of transient voltage instability.

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

[0040] A1) Determine the synchronous machine most likely to become unstable during transient conditions as follows:

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

[0042]

[0043] where △δ max It is the maximum value of the power angle difference between each synchronous machine and the synchronous machine at the balance node during the fault period.

[0044] A2) The following definition is given for a multi-wind farm grid-connected system:

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

[0046]

[0047] Where: Z GG is the self-impedance of the synchronous machine node that is most prone to instability; Z Gn is the mutual impedance between the synchronous machine node that is most prone to instability and the load node n;

[0048] Define the impact factor W of wind power node w on load node n Ln Normalized expression:

[0049]

[0050] Where: Z wn and Z ww is the mutual impedance between the wind power node w and the load node n, and the self-impedance of the wind power node w; S w and S w_max They represent the capacity of wind power node w and the maximum capacity of all wind power nodes respectively; this indicator includes the impact of wind power node capacity and network impedance on load nodes;

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

[0052]

[0053] Where: P Ln0 represents the initial active power of load node n, P L0max is the maximum initial active power of all load nodes;

[0054] Define the optimal load node index H of wind power node w during fault period w,Ln expression:

[0055]

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

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

[0058]

[0059] The wth row of the H matrix is ​​the optimal load node index set H for wind power node w to all load nodes. w , H w ={H w,L1 ,H w,L2 …,H w,Ln …,H w,Lq During the fault period, each wind power node w raises its own aggregate H by outputting current. w The voltage of the load node with the largest load node index is optimally raised, that is, transient voltage stability control of the multi-wind farm grid-connected system is achieved during the duration of the grid fault; the output current instruction of the wind power node w is as follows:

[0060]

[0061] Where: I w is the output current amplitude of wind power node w; I max is the maximum output current amplitude of wind power node w during fault period; θ wn 、 They are the output current angle of wind power node w, H w,Ln The voltage angle of the maximum load node, the wind power node w and H w,Ln The mutual impedance angle between the maximum load nodes n.

[0062] Effect description of the present invention:

[0063] by Figure 1 The IEEE 10-machine 39-bus power system is used as an example to illustrate the effectiveness of the proposed method. Figure 2 The transient simulation waveform of the system is given when a three-phase symmetrical short circuit occurs on bus No. 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 the transient voltage curve of the wind power grid-connected system when the traditional control strategy is adopted. It can be seen from the figure that after the fault ends, the system voltage continues to drop, and the voltage drops to the lowest point around 1.5 seconds. Then the system voltage begins to oscillate, and the system experiences transient voltage instability. Figure 2 (b) is the transient power angle curve of the wind power grid-connected system when the traditional control strategy is adopted. It can be seen from the figure that the power angle difference between the No. 2 synchronous machine and the No. 1 synchronous machine continues to increase during the fault period. At around 1.4 seconds, the power angle difference exceeds 180°, which indicates that the system power angle instability occurred during the transient period. At this time, the system voltage instability is mainly caused by the power angle instability. Figure 2 (d) is the transient power angle curve of the wind power grid-connected system when the control strategy proposed by the present invention is adopted. It can be seen from the figure that after the improved strategy is adopted during the fault period, the maximum power angle difference between the No. 2 synchronous machine and the No. 1 synchronous machine during the transient period is 131°, which does not exceed 180°. The power angle characteristics of the system are improved, and the system does not experience power angle instability. Figure 2 (c) It can be seen that during the fault period, the system voltage dropped to the lowest value at 1.2 seconds, and then the voltage began to rise slowly, and the system voltage returned to stability within 3 seconds.

[0064] Figure 3 The transient simulation waveform of the system is given when a three-phase symmetrical short circuit occurs on bus No. 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 the transient voltage curve of the wind power grid-connected system when the traditional control strategy is adopted. It can be seen from the figure that after the fault ends, the system voltage continues to drop, reaching the lowest point at around 1.42 seconds, and then the system voltage begins to oscillate. Figure 3 (b) is the transient power angle curve of the wind power grid-connected system when the traditional control strategy is adopted. It can be seen that during the transient period, the power angle difference between the No. 2 synchronous machine and the No. 1 and No. 6 synchronous machines continues to increase. At 1.40 seconds and 1.42 seconds, the power angle difference exceeds 180° successively, and the system becomes unstable. Figure 3 (d) is the transient power angle curve of the wind power grid-connected system when the control strategy proposed by the present invention is adopted. It can be seen from the figure that after the improved strategy is adopted during the fault period, the maximum power angle difference between the No. 2 synchronous machine and the No. 1 and No. 6 synchronous machines during the transient period is 82° and 58° respectively. The power angle difference of the synchronous machines is significantly reduced during the fault recovery period, the power angle characteristics are improved, and the system power angle is stable. Figure 3 (c) It can be seen that during the fault recovery period, when the improved control strategy is adopted, the system voltage stops decreasing at 1.25 seconds, 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] It can be seen that the coordinated control strategy proposed in the present invention can effectively improve the transient voltage stability of large-scale wind power grid-connected systems during fault periods.

[0066] Finally, it should be noted that the above examples of the present invention are merely illustrative of the present invention and are not intended to limit the embodiments of the present invention. Although the applicant has described the present invention in detail with reference to preferred embodiments, those skilled in the art will appreciate that other variations and modifications can be made based on the above description. It is not possible to enumerate all embodiments here. Any obvious variations or modifications derived from the technical solution of the present invention remain within the scope of protection of the present invention.

Claims

1. A method for controlling transient voltage stability of a multi-wind farm grid-connected system during a power grid fault, characterized by: The specific steps are as follows: A1) Identify the synchronous machine most likely to become unstable during transient conditions; A2) The following definition is given for a multi-wind farm grid-connected system: Define the influence factor D of the synchronous machine node that is most prone to instability on the load node n Ln Normalized expression: Where: Z GG is the self-impedance of the synchronous machine node that is most prone to instability; Z Gn is the mutual impedance between the synchronous machine node that is most prone to instability and the load node n; Define the impact factor W of wind power node w on load node n Ln Normalized expression: Where: Z wn and Z ww is the mutual impedance between the wind power node w and the load node n, and the self-impedance of the wind power node w; S w and S w_max They represent the capacity of wind power node w and the maximum capacity of all wind power nodes respectively; Define the load level G of load node n in the multi-wind farm grid-connected system Ln Normalized expression: Where: P Ln0 represents the initial active power of load node n, P L0max is the initial maximum active power of all load nodes; Define the optimal load node index H of wind power node w during fault period w,Ln expression: Where H w,Ln The larger the value is, the greater the impact of load node n on the active power output of the synchronous machine node that is most prone to instability during the fault period; A3) There are m wind power nodes w and q load nodes n in a multi-wind farm grid-connected system, where w = {1, 2, …, m} and n = {1, 2, …, q}. During a fault, the optimal load node index H matrix for each wind power node w to each load node n in the system is obtained according to A2), and its expression is as follows: The wth row of the H matrix is ​​the optimal load node index set H for wind power node w to all load nodes. w , H w ={H w,L1 ,H w,L2 …,H w,Ln …,H w,Lq During the fault period, each wind power node w raises its own aggregate H by outputting current. w The voltage of the load node with the largest load node index is optimally raised, that is, transient voltage stability control of the multi-wind farm grid-connected system is achieved during the duration of the grid fault; the output current instruction of the wind power node w is as follows: Where: I w is the output current amplitude of wind power node w; I max is the maximum output current amplitude of wind power node w during fault period; θ wn 、 They are the output current angle of wind power node w, H w,Ln The voltage angle of the maximum load node, the wind power node w and H w,Ln The mutual impedance angle between the maximum load nodes n.

2. The method for controlling transient voltage stability of a multi-wind farm grid-connected system during a power grid fault according to claim 1, characterized in that: In step A1), the synchronous machine most likely to become unstable 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 as follows. The synchronous machine with the smallest power angle stability index A is the one most likely to become unstable during a transient period. The normalized expression of the power angle stability index A is as follows: where △δ max It is the maximum value of the power angle difference between each synchronous machine and the synchronous machine at the balance node during the fault period.

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