Transient voltage stability control method for single wind power plant grid-connected system during power grid fault recovery period

Abstract

The invention discloses a transient voltage stability control method for a single wind power plant grid-connected system during a power grid fault recovery period, which comprises the following steps of: during a short-circuit fault recovery period of a power grid, firstly calculating the HLiR value of each load node in the wind power grid-connected system, and further sorting the load nodes according to the HLiR values; and raising the load node voltage with the maximum HLiR value by using the output current of the wind power plant so as to improve the power angle characteristic of the system during the fault period. In addition, when the control mode of the wind power plant is switched, the active power constraint of the wind power plant is set, the instability risk of the wind power plant is avoided, and therefore the transient voltage instability risk of the system is reduced. According to the method, the influence factors of the output current of the wind power plant on the transient voltage stability of the system during the fault recovery period are considered, and the transient power angle characteristic of the synchronous machine of the system is improved by utilizing the output capability of the wind power plant, so that 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 single wind farm grid-connected system during power grid fault recovery. During power grid fault recovery, if the wind farm enters a low voltage state again, the output current of the wind farm can be used to improve the power angle characteristics during the transient period, and an active power constraint strategy is used to maintain the stability of the wind farm, thereby reducing the risk of transient voltage instability of the wind power grid-connected system. Background Art

[0002] In recent years, with the continuous expansion of the installed capacity of wind power in the new power system, the transient voltage stability problems faced by traditional power systems will show new characteristics, which pose a major challenge to the safe and stable operation of the power grid. Due to the large-scale grid-connected operation of wind farms, the degree of power electronics on the power supply side of the system is continuously improved, the operation characteristics dominated by synchronous machines in the traditional sense are weakened, and the dynamic characteristics of power electronic devices have a deeper impact on the transient voltage of the system, further exacerbating the risk of transient voltage instability of the system, which poses a major challenge to the safe and stable operation of the power system. Therefore, it is necessary to conduct in-depth research on the transient voltage stability control strategy of the wind power grid-connected system during power grid fault recovery. At present, the relevant research carried out by domestic and foreign scholars mainly focuses on the high-voltage ride-through and transient overvoltage suppression strategies of wind farm grid-connected systems, such as the following publicly disclosed documents:

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

[0004] [2] Changping Zhou, Zhen Wang, Ping Ju, et al. High-voltage Ride Through Strategy 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] studied the influence relationship between the rotor current of a doubly-fed wind farm and the grid voltage after fault excision, proposed a method to suppress the overcurrent of the rotor, and on this basis improved the traditional control method of the grid-side converter, and proposed an improved high-voltage ride-through strategy, which can effectively improve the high-voltage ride-through ability of the doubly-fed wind farm. Reference [2] proposed a coordinated high-voltage control strategy for a doubly-fed wind farm based on the combination of Q-V control and P-V load shedding control. This strategy reduces the output power of the wind farm during fault recovery by combining the voltage state of the wind power grid connection point with the sensitivity of the Q-V curve to increase the reactive power margin of the wind farm, so as to use the reactive power regulation ability of the wind farm to suppress the transient overvoltage during fault recovery. Summarizing the voltage stability strategies of the wind power grid connection systems proposed in the above references, they mainly focus on the transient voltage level of the grid connection node of the wind farm during fault recovery, and suppress the transient overvoltage during fault recovery through their own characteristics or coordinated cooperation with reactive power compensation devices to improve the high-voltage ride-through ability of the wind farm after fault excision. However, the above research mainly considers the transient voltage stability of the wind farm itself and lacks consideration of the impact of the wind farm on the system transient voltage during the fault. Summary of the Invention

[0006] Aiming at the above deficiencies of the existing technology, the object of the present invention is to propose a transient voltage stability control method for a single wind farm grid connection system during grid fault recovery. During fault recovery, this method considers the interactive influence of the output current of the wind farm on the synchronous machine and load in the system, and uses the output ability of the wind farm to improve the power angle characteristics of the synchronous machine during fault recovery. In addition, during the switching of the wind farm control mode, the active power constraint strategy is used to improve the transient stability of the wind farm itself, thereby reducing the risk of system transient voltage instability.

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

[0008] A transient voltage stability control method for a single wind farm grid connection system during grid fault recovery, the specific steps are as follows:

[0009] A1) During fault recovery, based on the magnitude relationship between the wind farm node voltage U w and the voltage threshold of 0.8 p.u., it is judged whether the wind farm re-enters the low-voltage ride-through state during fault recovery; when the wind farm node voltage U w <0.8 p.u., the wind farm will re-enter the low-voltage ride-through state during fault recovery;

[0010] A2) During fault recovery, if the wind farm re-enters the low-voltage ride-through state, the following definitions are made for the wind power grid connection system:

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

[0012]

[0013] Where: Z' Gi and Z' GG are the mutual impedance between the system synchronous machine node and the load node i and the self-impedance of the synchronous machine node after the fault is cleared, respectively;

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

[0015]

[0016] Where: Z' Wi and Z' WW are the mutual impedance between the system wind farm node and the load node i and the self-impedance of the wind farm node after the fault is cleared, respectively;

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

[0018]

[0019] Where: P Li0 represents the initial active power of the load node i, and P L0max is the maximum value of the initial active power of all load nodes;

[0020] Define the optimal load-lifting node index H of the wind farm during the fault recovery period Li_R Expression:

[0021]

[0022] Where H Li_R The larger it is, the greater the interaction effect of the load node i on the synchronous machine node during the fault recovery period;

[0023] A3) During the fault recovery period, calculate the H Li_R value of each load node according to the network impedance parameters and load parameters in the single wind farm grid-connected system in step A2), and use the output current of the wind farm that re-enters the low voltage ride-through state to lift the voltage of the load node i with the largest H Li_R value, so as to realize the stable control of the transient voltage of the single wind farm grid-connected system during the power grid fault recovery period; The wind farm output current expression is as follows:

[0024]

[0025] Where: I w is the current amplitude output by the wind farm to the system; I maxis the maximum output current amplitude of the wind farm; and are respectively the argument of the mutual impedance between the wind farm node and the load node i with the largest H Li_R value and the phase angle of the voltage of the load node i with the largest H Li_R value; θ w is the phase angle of the output current of the wind farm.

[0026] Furthermore, the following steps are also included.

[0027] A4) During the fault recovery period, if the control loop of the wind farm switches from current control to power control mode, the active power output by the wind farm is constrained as follows;

[0028]

[0029] In the formula: P ref is the initial active power reference value during the steady state of the wind farm; P max is the maximum active power that the wind farm can maintain stable when the control strategy switches; P max is determined as follows according to different types of wind farms.

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

[0031]

[0032] In the formula: U g is the wind farm node voltage and U g = 0.8 p.u.; I gmax is the maximum output current of the grid-side converter of the permanent magnet wind farm;

[0033] For a doubly-fed wind farm, only the stator-side power controllable operation region needs to be analyzed. If the initial reactive power of the stator reference is Q s_DFIG , then the maximum stable active power P s_DFIGmax of the stator side of the doubly-fed wind farm is:

[0034]

[0035] In the formula: I rmax is the maximum value of the rotor current under rotor constraints; R s , X s are the stator resistance and reactance of the doubly-fed wind farm.

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

[0037] The present invention takes into account the influencing factors of the output current of a wind farm on the transient voltage stability of the system during fault recovery, and uses the output capacity of the wind farm to improve the transient power angle characteristics of the system synchronous machine. In addition, during the switching of the wind farm control mode, an active power constraint strategy is further used to reduce the instability risk of the wind farm itself during fault recovery, thereby improving the transient voltage stability of the system. Description of the Drawings

[0038] Figure 1 It is a schematic diagram of the topological structure of the IEEE 3-machine 9-bus single wind farm grid-connected system.

[0039] Figure 2 is Figure 1 When a three-phase symmetrical short-circuit fault occurs on bus No. 9 of the power grid in [reference], the loads in the system are mainly constant-impedance loads. During fault recovery, when the wind farm adopts the traditional control strategy and the control strategy proposed in the present invention, the transient voltage and power angle characteristic curves of each node bus in the power system are shown.

[0040] Figure 3 is Figure 1 When a three-phase symmetrical short-circuit fault occurs on bus No. 5 of the power grid in [reference], the loads in the system are mainly constant-impedance loads. During fault recovery, when the wind farm adopts the traditional control strategy and the control strategy proposed in the present invention, the transient voltage curves of each node bus in the power system are shown. Detailed Embodiment

[0041] The following describes the detailed embodiment of the present invention in conjunction with the drawings.

[0042] The present invention is used to reduce the transient voltage instability risk of a single wind farm grid-connected system during power grid fault recovery. Figure 1 It is a schematic diagram of the topological structure of a single wind farm grid-connected system. During the recovery of the power grid short-circuit fault, first calculate the H Li_R value of each load node in the wind power grid-connected system, and further sort the load nodes according to the magnitude of the H Li_R value. Use the output current of the wind farm to lift the voltage of the load node with the largest H Li_R value to improve the power angle characteristics of the system during the fault. In addition, when the control mode of the wind farm is switched, set the active power constraint of the wind farm to avoid the instability risk of the wind farm itself, thereby reducing the transient voltage instability risk of the system.

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

[0044] A1) During fault recovery, according to the relationship between the wind farm node voltage U w and the voltage threshold of 0.8 p.u., determine whether the wind farm enters the low voltage ride-through state again during fault recovery; when the wind farm node voltage U wWhen it is <0.8 p.u., the wind farm will enter the low voltage ride-through state again during the fault recovery period;

[0045] A2) During the fault recovery period, if the wind farm enters the low voltage ride-through state again, the following definitions are made for the wind power grid-connected system:

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

[0047]

[0048] In the formula: Z' Gi and Z' GG are respectively the mutual impedance between the system synchronous machine node and the load node i and the self-impedance of the synchronous machine node after the fault is removed;

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

[0050]

[0051] In the formula: Z' Wi and Z' WW are respectively the mutual impedance between the system wind farm node and the load node i and the self-impedance of the wind farm node after the fault is removed;

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

[0053]

[0054] In the formula: P Li0 represents the initial active power of the load node i, and P L0max is the maximum value of the initial active power of all load nodes;

[0055] Define the optimal load-lifting node index H of the wind farm during the fault recovery period Li_R Expression:

[0056]

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

[0058] A3) During the fault recovery period, calculate the H Li_R value of each load node according to the network impedance parameters and load parameters in the single wind farm grid-connected system in step A2), and use the output current of the wind farm that enters the low voltage ride-through state again to lift H Li_RThe voltage of the load node i with the largest value is achieved, thereby realizing the stable control of the transient voltage of the single-wind farm grid-connected system during the power grid fault recovery; the expression of the wind farm output current is as follows:

[0059]

[0060] In the formula: I w is the amplitude of the current output from the wind farm to the system; I max is the maximum amplitude of the output current of the wind farm; and are respectively the argument of the mutual impedance between the wind farm node and the load node i with the largest H Li_R value and the phase angle of the voltage of the load node i with the largest H Li_R value; θ w is the phase angle of the wind farm output current.

[0061] Furthermore, the present invention further includes the following steps,

[0062] A4) During the fault recovery period, if the control loop of the wind farm switches from current control to power control mode, then the active power output from the wind farm is constrained as follows;

[0063]

[0064] In the formula: P ref is the initial active power reference value during the steady state of the wind farm; P max is the maximum active power that the wind farm can maintain stable when the control strategy switches.

[0065] Among them, P max is determined as follows according to different types of wind farms,

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

[0067]

[0068] In the formula: U g is the wind farm node voltage and U g = 0.8 p.u.; I gmax is the maximum output current of the grid-side converter of the permanent magnet wind farm;

[0069] For a doubly-fed wind farm, only the stator-side power controllable operation region needs to be analyzed. If the initial reactive power of the stator reference is Q s_DFIG , then the maximum stable active power P s_DFIGmax of the stator side of the doubly-fed wind farm is:

[0070]

[0071] In the formula: I rmax is the maximum value of the rotor current under rotor constraints; R s , X s are the stator resistance and reactance of the doubly-fed wind farm.

[0072] Description of the effects of the present invention:

[0073] Taking the IEEE 3-machine 9-bus power system as an example to illustrate the effectiveness of the proposed method. Figure 2 When a three-phase symmetrical short circuit occurs at bus 9 of the power grid, the fault duration is 0.3 s, and the loads in the system are mainly constant-impedance loads, the transient simulation waveform diagram of the system is given. Figure 2 (a) is the transient voltage curve of the wind power grid-connected system when using the traditional control strategy. It can be seen from the figure that during the fault recovery period, the system voltage continuously drops and then starts to oscillate. At this time, it is considered that the system has experienced transient voltage instability. Figure 2 (b) is the transient power angle curve of the wind power grid-connected system when using the traditional control strategy. It can be seen from the figure that during the fault, the power angle difference between synchronous machine No. 2 and synchronous machine No. 1 is continuously increasing, and the power angle difference exceeds 180° at about 1.42 s. This indicates that during the transient period, the power angle of the synchronous machine has experienced power angle instability during the swinging process, which in turn leads to voltage instability. Figure 2 (d) is the transient power angle curve of the wind power grid-connected system when using the proposed control strategy during the fault recovery period. It can be seen from the figure that after adopting the improved strategy during the fault, the maximum value of the power angle difference between synchronous machine No. 2 and synchronous machine No. 1 during the transient period is 78°, which is much less than 180°. The power angle characteristic of the system is improved, and the system does not experience power angle instability. From Figure 2 (c), it can be seen that after adopting the proposed control strategy during the fault recovery period, the system voltage recovers to stability within 3 s.

[0074] Figure 3 When a three-phase symmetrical short circuit occurs at bus 5 of the power grid, the fault duration is 0.3 s, and the loads in the system are mainly constant-impedance loads, the transient simulation waveform diagram of the system is given. Figure 3 (a) is the transient voltage curve of the wind power grid-connected system when using the traditional control strategy. It can be seen from the figure that during the fault recovery period, the system voltage drops to the lowest value at 1.5 s, and then the voltage starts to slowly rise. At 1.7 s, the voltage of the wind farm recovers to the voltage threshold of 0.8 p.u., and the control loop of the wind farm switches from the current loop control during low-voltage ride-through to the power outer loop control. At this time, the system voltage oscillates. This is because when the wind farm switches to the power loop, due to unreasonable power command setting, the output oscillates, resulting in voltage oscillation. Figure 3 (b) is the transient voltage curve of the wind power grid-connected system when using the control strategy proposed in the present invention. From 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 stably within 5 seconds.

[0075] Thus, adopting the proposed coordinated control strategy during the fault recovery period can effectively improve the power angle characteristics of the synchronous machine during the fault process and reduce the risk of voltage instability caused by power angle instability in the power system. In addition, when the wind farm adopts the power constraint strategy during the fault recovery period, it can effectively avoid the voltage oscillation caused by the unreasonable reference power command of the wind farm during the control mode switching, and improve the transient voltage stability of the system.

[0076] Finally, it should be noted that the above examples of the present invention are merely examples for illustrating 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 the preferred embodiments, those of ordinary skill in the art can make other different forms of changes and modifications based on the above description. It is impossible to list all the embodiments here. Any obvious changes or modifications derived from the technical solutions of the present invention are still within the protection scope of the present invention.

Claims

1. A transient voltage stability control method for a single wind farm grid-connected system during power grid fault recovery, characterized in that: The specific steps are as follows: A1) During the fault recovery, based on the magnitude relationship between the node voltage U of the wind farm w and the voltage threshold of 0.8 p.u., it is judged whether the wind farm enters the low voltage ride-through state again during the fault recovery; when the node voltage U of the wind farm w < 0.8 p.u., the wind farm will enter the low voltage ride-through state again during the fault recovery; A2) During fault recovery, if the wind farm enters the low voltage ride-through state again, the following definitions are made for the wind power grid connection system: Define the influence factor A of the synchronous machine node on the load node i during fault recovery Li Normalization expression: where: Z’ Gi and Z’ GG are respectively the mutual impedance between the system synchronous machine node and the load node i after fault removal and the self-impedance of the synchronous machine node; Define the influence factor B of the wind farm on load node i during fault recovery Li Normalized expression: where: Z' Wi and Z' WW are the mutual impedance between the system wind farm node and the load node i and the self-impedance of the wind farm node after fault removal, respectively; Define the load magnitude level C Li Normalization expression: Where: P Li0 represents the initial active power of load node i, and P L0max is the maximum value of the initial active power of all load nodes; Define the optimal load-lifting node index H of the wind farm during fault recovery Li_R Expression: where H Li_R The larger it is, the greater the interactive influence of load node i on the synchronous machine node during fault recovery; A3) During fault recovery, calculate the H value of each load node according to the network impedance parameters and load parameters in the single wind farm grid-connected system according to the steps in A2), and use the output current of the wind farm that re-enters the low voltage ride-through state to lift the voltage of the load node i with the largest H value, so as to realize the stable control of the transient voltage of the single wind farm grid-connected system during the power grid fault recovery; the expression of the wind farm output current is as follows: Li_R Value, and use the output current of the wind farm that re-enters the low voltage ride-through state to lift the H Li_R The voltage of the load node i with the largest value, so as to realize the stable control of the transient voltage of the single wind farm grid-connected system during the power grid fault recovery; the expression of the wind farm output current is as follows: Where: I w is the amplitude of the current output from the wind farm to the system; I max is the maximum amplitude of the output current of the wind farm; and are respectively the argument of the mutual impedance between the wind farm node and the load node i with the largest H Li_R value and the phase angle of the voltage of the load node i with the largest H Li_R value; θ w is the phase angle of the current output from the wind farm.

2. The transient voltage stability control method for a single wind farm grid-connected system during power grid fault recovery according to claim 1, wherein: It also includes the following steps A4) During fault recovery, if the control loop of the wind farm switches from current control to power control mode, the following constraints are imposed on the active power output of the wind farm; Where: P ref is the initial active power reference value during the steady state of the wind farm; P max is the maximum active power that the wind farm can maintain stability when the control strategy is switched; P max is determined as follows according to different types of wind farms For a permanent magnet wind farm, if the initial reactive power is Q PMSG , then the maximum stable active power P PMSGmax is as follows: Where: U g is the node voltage of the wind farm and U g = 0.8 p.u.; I gmax is the maximum value of the output current of the grid-side converter of the permanent magnet wind farm; For a doubly-fed wind farm, only the controllable operating region of the stator-side power needs to be analyzed. If the initial reactive power of the stator reference is Q s_DFIG , then the maximum stable active power P s_DFIGmax of the stator side of the doubly-fed wind farm is as follows: Where: I rmax is the maximum value of the rotor current under rotor constraints; R s , X s are the stator resistance and reactance of the doubly-fed wind farm.

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