A method for transient voltage stability control of a single wind farm grid-connection system during power grid fault recovery

Abstract

The application discloses a kind of single wind farm grid-connected system transient voltage stability control methods during power grid fault recovery, during power grid short-circuit fault recovery, first calculate the load node in wind power grid-connected system H Li_R Value size, further according to H Li_R Value size is sorted to load node, using the output current of wind farm is lifted H Li_R The voltage of the load node with the maximum value, to improve the power angle characteristic of system during fault. In addition, when the control mode of wind farm is switched, the active power constraint of wind farm is set to avoid the instability risk of wind farm itself, thereby reducing the transient voltage instability risk of system. The application considers the influencing factors of the output current of wind farm on the transient voltage stability of system during fault recovery, uses the output capacity of wind farm to improve the transient power angle characteristic of synchronous machine of system, thereby improves the transient voltage stability of system.

Description

Technical Field

[0001] This invention relates to a transient voltage stability control method for a single wind farm grid-connected system during grid fault recovery. If the wind farm re-enters a low-voltage state during grid fault recovery, the method can utilize the output current of the wind farm to improve the power angle characteristics during the transient period, and use an active power constraint strategy to maintain the stability of the wind farm, thereby reducing the risk of transient voltage instability in the wind power grid-connected system. Background Technology

[0002] In recent years, with the continuous expansion of wind power installed capacity in new power systems, the transient voltage stability problem faced by traditional power systems will exhibit new characteristics, posing a significant challenge to the safe and stable operation of the power grid. Due to the large-scale grid connection of wind farms, the degree of power electronics on the power supply side of the system is constantly increasing, weakening the traditional synchronous machine-dominated operating characteristics. The dynamic characteristics of power electronic equipment have a deeper impact on system transient voltage, further exacerbating the risk of system transient voltage instability, which poses a significant challenge to the safe and stable operation of the power system. Therefore, it is necessary to conduct in-depth research on transient voltage stability control strategies for wind power grid-connected systems during grid fault recovery. Current research by domestic and international scholars mainly focuses on high-voltage ride-through and transient overvoltage suppression strategies for wind farm grid-connected systems, as exemplified by the following published literature:

[0003] [1] Zou Le, Wu Xueguang, Kou Longze, et al. Research on improved control strategy of doubly fed wind power generation system under symmetrical voltage surge in power grid [J]. Power System Technology, 2020, 44(04): 1360-1367.

[0004] [2] Changping Zhou, Zhen Wang, Ping Ju, et al. High-voltage Ride ThroughStrategy for DFIG Considering Converter Blocking of HVDC System [J]. Journal of Modern Power Systems and Clean Energy, 2020, 8(3): 491-498.

[0005] Reference [1] proposed a method to suppress rotor overcurrent by studying the influence relationship between rotor current and grid voltage of doubly-fed wind farm after fault clearing. Based on this, it improved the traditional control method of grid-side converter and proposed an improved high-voltage ride-through strategy, which can effectively improve the high-voltage ride-through capability of doubly-fed wind farm. Reference [2] proposed a high-voltage coordinated control strategy for doubly-fed wind farm based on the combination of QV control and PV load shedding control. During fault recovery, this strategy reduces the output power of wind farm by combining the voltage state of wind farm grid connection point with the sensitivity of QV curve, so as to improve the reactive power margin of wind farm and thus use the reactive power regulation capability of wind farm to suppress transient overvoltage during fault recovery. In summary, the voltage stability strategies of wind farm grid connection system proposed in the above literature mainly focus on the transient voltage level of wind farm grid connection node during fault recovery. They suppress transient overvoltage during fault recovery through their own characteristics or coordination with reactive power compensation device, thereby improving the high-voltage ride-through capability of wind farm after fault clearing. However, the above studies mainly consider the transient voltage stability of the wind farm itself, and lack consideration of the impact of the wind farm on 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 single wind farm grid-connected system during grid fault recovery. This method considers the interactive influence of the wind farm's output current on the synchronous motor and loads in the system during fault recovery, utilizing the wind farm's output capacity to improve the synchronous motor's power angle characteristics during this period. Furthermore, during wind farm control mode switching, an active power constraint strategy is employed to enhance the wind farm's transient stability, thereby reducing the risk of system transient voltage instability.

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

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

[0009] A1) During fault recovery, based on the wind farm node voltage U w The relationship between the voltage threshold of 0.8 pu and the value of the voltage threshold is used to determine whether the wind farm re-enters a low-voltage ride-through state during fault recovery; when the wind farm node voltage U w If the voltage is less than 0.8 pu, the wind farm will re-enter the low voltage ride-through state during the fault recovery period;

[0010] A2) If the wind farm re-enters a low-voltage ride-through state during fault recovery, the wind power grid-connected system is defined as follows:

[0011] Define the influence factor A of the synchronous machine node on the load node i during fault recovery. LiNormalized expression:

[0012]

[0013] In the formula: Z' Gi and Z' GG These are the mutual impedance between the system's synchronous node and load node i, and the self-impedance of the synchronous node, respectively, after the fault is cleared.

[0014] Define the influence factor B of the wind farm on load node i during fault recovery. Li Normalized expression:

[0015]

[0016] In the formula: Z' Wi and Z' WW These are the mutual impedance between the wind farm node and load node i in the system after fault clearance, and the self-impedance of the wind farm node, respectively.

[0017] Define the load magnitude C Li Normalized expression:

[0018]

[0019] In the formula: P Li0 P represents the initial active power of load node i. L0max This represents the initial maximum active power of all load nodes.

[0020] Define the optimal load-lifting node index H for wind farms during fault recovery. Li_R expression:

[0021]

[0022] In the formula H Li_R The larger the value, the greater the interaction between load node i and the synchronous machine node during fault recovery;

[0023] A3) During fault recovery, follow the steps in A2) to calculate the H of each load node based on the network impedance parameters and load parameters of the single wind farm grid-connected system. Li_R The value is used to boost H by utilizing the wind farm's output current as it re-enters the low-voltage ride-through state. Li_R The voltage at the load node i with the largest value is used to achieve stable control of transient voltage in a single wind farm grid-connected system during grid fault recovery; the expression for the wind farm output current is as follows:

[0024]

[0025] In the formula: I w I represents the amplitude of the current output from the wind farm to the system. maxThis represents the maximum output current amplitude of the wind farm. and These are wind farm nodes and H. Li_R The mutual impedance argument angle and H between the load nodes i with the largest values Li_R The phase angle of the voltage at the load node i with the largest value; θ w This represents the phase angle of the wind farm's output current.

[0026] Furthermore, it also includes the following steps,

[0027] A4) During fault recovery, if the wind farm's control loop switches from current control to power control mode, the following constraints shall be applied to the active power output of the wind farm.

[0028]

[0029] In the formula: P ref P represents the initial active power reference value during the steady-state period of the wind farm. max To ensure that the wind farm maintains a stable maximum active power output during control strategy switching; P max The determination is made according to the different types of wind farms as follows.

[0030] For a permanent magnet wind farm, if the initial reactive power is Q PMSG Then its stable maximum active power P PMSGmax for:

[0031]

[0032] In the formula: U g The node voltage of the wind farm and U g =0.8pu; I gmax This represents the maximum output current of the grid-side converter in a permanent magnet wind farm.

[0033] For a doubly-fed induction generator (DFIG) wind farm, only the stator-side power controllable operating domain needs to be analyzed. If the initial reactive power referenced by the stator is Q... s_DFIG The maximum active power P stable on the stator side of the doubly-fed wind farm is then determined. s_DFIGmax for:

[0034]

[0035] In the formula: I rmax R represents the maximum value of the rotor current under rotor constraints. s X s This refers to the stator resistance and reactance of a doubly fed wind farm.

[0036] Compared with the prior art, the present invention has the following beneficial effects:

[0037] This invention considers the impact of wind farm output current on system transient voltage stability during fault recovery, and utilizes the wind farm's output capacity to improve the transient power angle characteristics of the system's synchronous machine. Furthermore, during wind farm control mode switching, an active power constraint strategy is further employed to reduce the wind farm's own instability risk during fault recovery, thereby improving the system's transient voltage stability. Attached Figure Description

[0038] Figure 1 This is a schematic diagram of the topology of an IEEE 3-machine 9-node single wind farm grid-connected system.

[0039] Figure 2 for Figure 1 A three-phase symmetrical short-circuit fault occurred on bus 9 of the China Power Grid, and the loads in the system were mainly constant impedance loads. During the fault recovery period, the transient voltage and power angle characteristic curves of each node bus in the power system were plotted when the wind farm adopted both the traditional control strategy and the control strategy proposed in this invention.

[0040] Figure 3 for Figure 1 A three-phase symmetrical short-circuit fault occurred on busbar 5 of the China Power Grid, and the loads in the system were mainly constant impedance loads. During the fault recovery period, the transient voltage curves of each busbar in the power system were shown when the wind farm adopted both the traditional control strategy and the control strategy proposed in this invention. Detailed Implementation

[0041] The specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

[0042] This invention is used to reduce the risk of transient voltage instability in a single wind farm grid-connected system during grid fault recovery. Figure 1 This is a schematic diagram of the topology of a single wind farm grid-connected system. During grid short-circuit fault recovery, the load nodes H of each load node in the wind power grid-connected system are first calculated. Li_R The value is determined further based on H. Li_R The load nodes are sorted by value, and the output current of the wind farm is used to increase H. Li_R The maximum load node voltage is set to improve the system's power angle characteristics during faults. Furthermore, when switching wind farm control modes, active power constraints are set for the wind farm to avoid its own instability risks, thereby reducing the risk of system transient voltage instability.

[0043] The specific implementation steps of this invention are as follows:

[0044] A1) During fault recovery, based on the wind farm node voltage U w The relationship between the voltage threshold of 0.8 pu and the value of the voltage threshold is used to determine whether the wind farm re-enters a low-voltage ride-through state during fault recovery; when the wind farm node voltage U wIf the voltage is less than 0.8 pu, the wind farm will re-enter the low voltage ride-through state during the fault recovery period;

[0045] A2) If the wind farm re-enters a low-voltage ride-through state during fault recovery, the wind power grid-connected system is defined as follows:

[0046] Define the influence factor A of the synchronous machine node on the load node i during fault recovery. Li Normalized expression:

[0047]

[0048] In the formula: Z' Gi and Z' GG These are the mutual impedance between the system's synchronous node and load node i, and the self-impedance of the synchronous node, respectively, after the fault is cleared.

[0049] Define the influence factor B of the wind farm on load node i during fault recovery. Li Normalized expression:

[0050]

[0051] In the formula: Z' Wi and Z' WW These are the mutual impedance between the wind farm node and load node i in the system after fault clearance, and the self-impedance of the wind farm node, respectively.

[0052] Define the load magnitude C Li Normalized expression:

[0053]

[0054] In the formula: P Li0 P represents the initial active power of load node i. L0max This represents the initial maximum active power of all load nodes.

[0055] Define the optimal load-lifting node index H for wind farms during fault recovery. Li_R expression:

[0056]

[0057] In the formula H Li_R The larger the value, the greater the interaction between load node i and the synchronous machine node during fault recovery;

[0058] A3) During fault recovery, follow the steps in A2) to calculate the H of each load node based on the network impedance parameters and load parameters of the single wind farm grid-connected system. Li_R The value is used to boost H by utilizing the wind farm's output current as it re-enters the low-voltage ride-through state. Li_RThe voltage at the load node i with the largest value is used to achieve stable control of transient voltage in a single wind farm grid-connected system during grid fault recovery; the expression for the wind farm output current is as follows:

[0059]

[0060] In the formula: I w I represents the amplitude of the current output from the wind farm to the system. max This represents the maximum output current amplitude of the wind farm. and These are wind farm nodes and H. Li_R The mutual impedance argument angle and H between the load nodes i with the largest values Li_R The phase angle of the voltage at the load node i with the largest value; θ w This represents the phase angle of the wind farm's output current.

[0061] Furthermore, the present invention also includes the following steps:

[0062] A4) During fault recovery, if the wind farm's control loop switches from current control to power control mode, the following constraints shall be applied to the active power output of the wind farm.

[0063]

[0064] In the formula: P ref P represents the initial active power reference value during the steady-state period of the wind farm. max To ensure that the wind farm can maintain a stable maximum active power when the control strategy is switched.

[0065] Among them, P max The determination is made according to the different types of wind farms as follows.

[0066] For a permanent magnet wind farm, if the initial reactive power is Q PMSG Then its stable maximum active power P PMSGmax for:

[0067]

[0068] In the formula: U g The node voltage of the wind farm and U g =0.8pu; I gmax This represents the maximum output current of the grid-side converter in a permanent magnet wind farm.

[0069] For a doubly-fed induction generator (DFIG) wind farm, only the stator-side power controllable operating domain needs to be analyzed. If the initial reactive power referenced by the stator is Q... s_DFIG The maximum active power P stable on the stator side of the doubly-fed wind farm is then determined. s_DFIGmax for:

[0070]

[0071] In the formula: I rmax R represents the maximum value of the rotor current under rotor constraints. s X s This refers to the stator resistance and reactance of a doubly fed wind farm.

[0072] Description of the effects of this invention:

[0073] The effectiveness of the proposed method is illustrated using the IEEE 3-machine 9-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 9 of the power grid, the fault duration is 0.3 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, the system voltage continues to drop during the fault recovery period and then begins to oscillate. At this time, it is considered that the system has experienced 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 Synchronous Machine No. 2 and Synchronous Machine No. 1 is increasing during the fault period. At about 1.42 seconds, the power angle difference exceeds 180°. This indicates that the power angle of the synchronous machine is unstable during the swing during the transient period, which in turn leads to voltage instability. Figure 2 (d) The transient power angle curve of the wind power grid-connected system during the fault recovery period using the proposed control strategy. As shown in the figure, after adopting the improved strategy during the fault period, the maximum power angle difference between synchronous generator No. 2 and synchronous generator No. 1 during the transient period was 78°, far less than 180°, indicating improved power angle characteristics and no power angle instability occurred. Figure 2 (c) It can be seen that after adopting the proposed control strategy during the fault recovery period, the system voltage stabilized within 3 seconds.

[0074] Figure 3 The transient simulation waveforms of the system are given when a three-phase symmetrical short circuit occurs on bus 5 of the power grid, the fault duration is 0.3 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 using a traditional control strategy. The graph shows that during fault recovery, the system voltage drops to its lowest value in 1.5 seconds, then slowly rises, recovering to the voltage threshold of 0.8 pu at 1.7 seconds. The wind farm's control loop is controlled by the current loop during low-voltage ride-through and the outer loop for switching success rate control. At this time, the system voltage oscillates. This is because the wind farm's output oscillates due to an unreasonable power command during the switching success rate loop, causing the voltage to oscillate accordingly. Figure 3 (b) is a transient voltage curve of the wind power grid-connected system when the control strategy proposed in this invention is adopted. Figure 3(b) It can be seen that after the wind farm adopts the power constraint strategy during the fault recovery process, the system voltage oscillation disappears and the system voltage recovers to stability within 5 seconds.

[0075] Therefore, it is evident that the proposed coordinated control strategy can effectively improve the power angle characteristics of the synchronous machine during fault recovery, reducing the risk of voltage instability caused by power angle instability in the power system. Furthermore, when the wind farm adopts a power constraint strategy during fault recovery, it can effectively avoid voltage oscillations caused by unreasonable reference power commands during control mode switching, thereby improving the transient voltage stability of the system.

[0076] 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 single wind farm grid-connected system during grid fault recovery, characterized in that: The specific steps are as follows: A1) During fault recovery, based on the wind farm node voltage U w The relationship between the voltage threshold of 0.8 pu and the value of the voltage threshold is used to determine whether the wind farm re-enters a low-voltage ride-through state during fault recovery; when the wind farm node voltage U w If the voltage is less than 0.8 pu, the wind farm will re-enter the low voltage ride-through state during the fault recovery period; A2) If the wind farm re-enters a low-voltage ride-through state during fault recovery, the wind power grid-connected system is defined as follows: Define the influence factor A of the synchronous machine node on the load node i during fault recovery. Li Normalized expression: In the formula: Z' Gi and Z' GG These are the mutual impedance between the system's synchronous node and load node i, and the self-impedance of the synchronous node, respectively, after the fault is cleared. Define the influence factor B of the wind farm on load node i during fault recovery. Li Normalized expression: In the formula: Z' Wi and Z' WW These are the mutual impedance between the wind farm node and load node i in the system after fault clearance, and the self-impedance of the wind farm node, respectively. Define the load magnitude C Li Normalized expression: In the formula: P Li0 P represents the initial active power of load node i. L0max This represents the initial maximum active power of all load nodes. Define the optimal load-lifting node index H for wind farms during fault recovery. Li_R expression: In the formula H Li_R The larger the value, the greater the interaction between load node i and the synchronous machine node during fault recovery; A3) During fault recovery, follow the steps in A2) to calculate the H of each load node based on the network impedance parameters and load parameters of the single wind farm grid-connected system. Li_R The value is used to boost H by utilizing the wind farm's output current as it re-enters the low-voltage ride-through state. Li_R The voltage at the load node i with the largest value is used to achieve stable control of transient voltage in a single wind farm grid-connected system during grid fault recovery; the expression for the wind farm output current is as follows: In the formula: I w I represents the amplitude of the current output from the wind farm to the system. max This represents the maximum output current amplitude of the wind farm. and These are wind farm nodes and H. Li_R The mutual impedance argument angle and H between the load nodes i with the largest values Li_R The phase angle of the voltage at the load node i with the largest value; θ w This represents the phase angle of the wind farm's output current.

2. The transient voltage stabilization control method for a single wind farm grid-connected system during grid fault recovery as described in claim 1, characterized in that: It also includes the following steps, A4) During fault recovery, if the wind farm's control loop switches from current control to power control mode, the following constraints shall be applied to the active power output of the wind farm. In the formula: P ref P represents the initial active power reference value during the steady-state period of the wind farm. max To ensure that the wind farm maintains a stable maximum active power output during control strategy switching; P max The determination is made according to the different types of wind farms as follows. For a permanent magnet wind farm, if the initial reactive power is Q PMSG Then its stable maximum active power P PMSGmax for: In the formula: U g The node voltage of the wind farm and U g =0.8pu; I gmax This represents the maximum output current of the grid-side converter in a permanent magnet wind farm. For a doubly-fed induction generator (DFIG) wind farm, only the stator-side power controllable operating domain needs to be analyzed. If the initial reactive power referenced by the stator is Q... s_DFIG The maximum active power P stable on the stator side of the doubly-fed wind farm is then determined. s_DFIGmax for: In the formula: I rmax R represents the maximum value of the rotor current under rotor constraints. s X s This refers to the stator resistance and reactance of a doubly fed wind farm.

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