Phase-locked loop subsynchronous oscillation suppression method based on wind storage virtual short circuit impedance coupling

Abstract

The invention belongs to the wind power technology, and provides a phase-locked loop subsynchronous oscillation suppression method based on wind storage virtual short circuit impedance coupling. Comprising the steps of original wind storage system construction, elastic coupling two-degree-of-freedom model construction, stability analysis, state space expression construction, inference judgment, critical point determination, optimal frequency ratio determination, equivalent coupling impedance calculation between wind storage and damping parameter optimization. The virtual synchronization unit and the virtual short-circuit impedance control unit are arranged in the original wind storage system, so that the problems that an existing wind storage system is long in electrical distance and insufficient in short-circuit ratio are solved, and the stability and the short-circuit capacity of the system are improved; through equivalent coupling impedance between wind storage, optimal setting of the short-circuit ratio of the system is realized, structural parameters of the system are optimized, and the suppression capability of the system on subsynchronous oscillation of the phase-locked loop is improved.

Description

Technical Field

[0001] The present invention relates to the field of wind power technology, and in particular to a phase-locked loop subsynchronous oscillation suppression method based on wind storage virtual short-circuit impedance coupling. Background Art

[0002] The construction of wind power bases and load centers are inversely distributed, resulting in a long-distance transmission mode for wind power transmission systems. Considering the long electrical distance, the wind farm grid-connected coupling point is in a weak connection mode, the short-circuit ratio is insufficient, and the system's support for the wind farm is weak. After the weak connection mode is formed, the wind turbine phase-locked loop is coupled with the grid impedance, and the risk of inducing subsynchronous oscillation of the system increases significantly. Therefore, it is urgent to improve the grid-connected stability of wind turbines. At present, there are application scenarios for wind farms connected to the grid with energy storage configurations, and wind and storage combined grid connection will become a necessary measure to improve system stability in the future. The energy storage device adopts virtual synchronous machine control, which has the function of regulating the grid frequency and damping, has power-frequency response characteristics similar to traditional synchronous machines, and can provide short-circuit capacity.

[0003] Unlike wind turbines, energy storage participating in oscillation suppression generally adopts additional damping control. However, due to the lack of coupling analysis with wind turbines and system impedance, the damping oscillation effect and parameter design need to be improved. Summary of the invention

[0004] In order to overcome the shortcomings of the prior art, the purpose of the present invention is to provide a method for suppressing sub-synchronous oscillations of a phase-locked loop based on virtual short-circuit impedance coupling of wind storage. By setting a virtual synchronization unit and a virtual short-circuit impedance control unit in the original wind storage system, the problems of long electrical distance and insufficient short-circuit ratio of the existing wind storage system are solved, and the stability and short-circuit capacity of the system are improved. By using equivalent coupling impedances between wind storages, the optimal setting of the system short-circuit ratio is achieved, the structural parameters of the system are optimized, and the system's ability to suppress sub-synchronous oscillations of the phase-locked loop is improved.

[0005] To achieve the above object, the present invention provides the following solutions:

[0006] A method for suppressing subsynchronous oscillation of a phase-locked loop based on wind energy storage virtual short-circuit impedance coupling, comprising:

[0007] Constructing an original wind storage system; the original wind storage system comprises: a power grid end, a wind turbine unit, an energy storage unit, a phase-locked loop unit, a virtual synchronization unit and a virtual short-circuit impedance control unit;

[0008] Constructing an elastically coupled two-degree-of-freedom model according to the original wind-storage system;

[0009] Analyzing the solution of the elastically coupled two-degree-of-freedom model to obtain stability inference;

[0010] Construct state space expressions;

[0011] Using the state space expression to obtain a zero-pole distribution diagram of the system, and using the zero-pole distribution diagram to judge the stability inference to obtain a judgment result;

[0012] When the judgment result is that the inference is correct, determining the critical point according to the elastic coupling two-degree-of-freedom model;

[0013] determining an optimal frequency ratio according to the critical point;

[0014] The equivalent coupling impedance between the wind and the storage is calculated according to the optimal frequency ratio and the optimal short-circuit ratio requirements;

[0015] The damping parameters of the original wind-storage system are optimized according to the equivalent coupling impedance between the wind and storage units to obtain a wind-storage system with oscillation suppression.

[0016] Preferably, the elastically coupled two-degree-of-freedom model is:

[0017]

[0018] Among them, θ W is the output phase of the phase-locked loop unit; U S K is the grid connection point voltage amplitude of the phase-locked loop unit; P is the first control parameter of the phase-locked loop unit; M is the virtual inertia; U ES is the energy storage voltage of the energy storage unit; U g is the voltage at the grid end; U W B is the voltage of the wind turbine unit; 23 is the admittance between the energy storage unit and the grid terminal; θ ES0 is the initial energy storage phase of the energy storage unit; △P t is the disturbance amount; D is the virtual damping coefficient; △B 21 is the virtual short-circuit admittance; B 210 is the initial admittance between the wind turbine unit and the energy storage unit; △θ W Outputting a phase change amount to the phase-locked loop unit; Outputting a first-order derivative of a phase change amount for the phase-locked loop unit; is the second-order derivative of the phase change output by the phase-locked loop unit; △θ ES is the phase change of the energy storage unit; is the first-order derivative of the phase change of the energy storage unit; is the second-order derivative of the phase change of the energy storage unit.

[0019] Preferably, analyzing the solution of the elastically coupled two-degree-of-freedom model to obtain stability inference includes:

[0020] Determining a solution equation for the elastically coupled two-degree-of-freedom model;

[0021] Draw an amplitude-frequency response diagram according to the solved equation;

[0022] The stability inference is determined based on the magnitude-frequency response plot.

[0023] Preferably, the state space expression is:

[0024]

[0025] in, △ω ES K is the energy storage virtual speed; I is the second control parameter of the phase-locked loop unit; Δω W is the change in angular frequency of the wind turbine unit of the wind turbine generator set; is the first-order derivative of the change in angular frequency of the wind turbine unit of the wind turbine generator set; is the first-order derivative of the change in the energy storage angular frequency of the energy storage unit; B2′1 is the equivalent coupling impedance between the wind and storage.

[0026] Preferably, when the judgment result is that the inference is correct, determining the critical point according to the elastic coupling two-degree-of-freedom model includes:

[0027] The vertical coordinate of the critical point is calculated using a coordinate calculation formula; the coordinate calculation formula is:

[0028]

[0029] Where ζ=D / (2Mω mv ); g = ω d / ω n θ d is the phase amplitude amplification parameter; ω n 2 =K I U S ;ω mv 2 =S1 / M; S1=U ES U W B2′1cos(θ ES0 -θ W0 );ω d is the frequency of system disturbance; g A is the vibration ratio of the first critical point; g B is the vibration ratio of the second critical point.

[0030] The critical point is determined according to the ordinate and the amplitude-frequency response diagram.

[0031] Preferably, the calculation formula of the optimal frequency ratio is:

[0032]

[0033] Among them, SCR W =B 21 +B E ;f a is the optimal frequency ratio; B E is the equivalent coupling impedance between the wind turbine unit and the grid terminal.

[0034] Preferably, the calculation formula of the equivalent coupling impedance between the wind and the storage is:

[0035]

[0036] Among them, B2′1 is the equivalent coupling impedance between the wind and storage.

[0037] Preferably, the calculation formula for damping parameter optimization is:

[0038]

[0039] The present invention discloses the following technical effects:

[0040] The present invention provides a method for suppressing sub-synchronous oscillation of a phase-locked loop based on virtual short-circuit impedance coupling of wind storage. By setting a virtual synchronization unit and a virtual short-circuit impedance control unit in the original wind storage system, the problems of long electrical distance and insufficient short-circuit ratio of the existing wind storage system are solved, and the stability and short-circuit capacity of the system are improved. By using equal-value coupling impedance between wind storages, the problem of poor suppression ability of the existing system on sub-synchronous oscillation of the phase-locked loop is solved, and the optimal setting of the system short-circuit ratio and the optimization of structural parameters are achieved. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative labor.

[0042] Figure 1 A schematic diagram of a phase-locked loop subsynchronous oscillation suppression process based on wind storage virtual short-circuit impedance coupling provided by an embodiment of the present invention;

[0043] Figure 2 A wind-storage combined power generation grid-connected structure diagram provided in an embodiment of the present invention;

[0044] Figure 3A phase-locked loop control structure diagram of a wind turbine generator system provided by an embodiment of the present invention;

[0045] Figure 4 A control structure diagram of a virtual synchronous machine of an energy storage device provided by an embodiment of the present invention;

[0046] Figure 5 The amplitude-frequency response diagram of the wind turbine phase under different equivalent coupling impedances provided by the embodiment of the present invention;

[0047] Figure 6 A distribution diagram of system poles and zeros under different equivalent coupling impedances provided by an embodiment of the present invention;

[0048] Figure 7 The amplitude-frequency response diagram of the fan phase under different damping parameters provided by the embodiment of the present invention;

[0049] Figure 8 A control structure diagram of a wind energy storage system after adding a virtual short-circuit impedance according to an embodiment of the present invention;

[0050] Fig. 9 A modal root locus diagram of the system when the equivalent coupling impedance between the wind and the storage changes provided by the embodiment of the present invention;

[0051] Fig.10 The power and phase-locked loop output phase response diagram of the wind turbine generator set under different control conditions provided by the embodiment of the present invention, Fig.10 (a1) is the simulation diagram of wind turbine unit output power without additional control. Fig.10 (a2) is the output phase simulation diagram of the phase-locked loop unit without additional control. Fig.10 (a3) is the Fourier spectrum analysis diagram without additional control, Fig.10 (b1) is the simulation diagram of wind turbine unit output power under the control of virtual synchronous unit. Fig.10 (b2) is the output phase simulation diagram of the phase-locked loop unit under the control of the virtual synchronization unit. Fig.10 (b3) is the Fourier spectrum analysis diagram under the control of the virtual synchronization unit. Fig.10 (c1) is the simulation diagram of wind turbine unit output power under additional virtual short-circuit impedance control. Fig.10 (c2) is the output phase simulation diagram of the phase-locked loop unit under the additional virtual short-circuit impedance control. Fig.10 (c3) is the Fourier spectrum analysis diagram under additional virtual short-circuit impedance control. DETAILED DESCRIPTION

[0052] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0053] The purpose of the present invention is to provide a method for suppressing sub-synchronous oscillation of a phase-locked loop based on virtual short-circuit impedance coupling of wind storage. By setting a virtual synchronization unit and a virtual short-circuit impedance control unit in the original wind storage system, the problems of long electrical distance and insufficient short-circuit ratio of the existing wind storage system are solved, and the stability and short-circuit capacity of the system are improved. By using equivalent coupling impedance between wind storages, the optimal setting of the system short-circuit ratio is achieved, the structural parameters of the system are optimized, and the system's ability to suppress sub-synchronous oscillation of the phase-locked loop is improved.

[0054] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0055] Figure 1 A schematic diagram of a phase-locked loop subsynchronous oscillation suppression process based on wind storage virtual short-circuit impedance coupling provided by an embodiment of the present invention is shown in FIG. Figure 1 As shown, the present invention provides a phase-locked loop subsynchronous oscillation suppression method based on wind storage virtual short-circuit impedance coupling, comprising:

[0056] Step 100: constructing an original wind storage system; the original wind storage system includes: a grid end, a wind turbine unit, an energy storage unit, a phase-locked loop unit, a virtual synchronization unit, and a virtual short-circuit impedance control unit;

[0057] Step 200: constructing an elastically coupled two-degree-of-freedom model according to the original wind-storage system;

[0058] Step 300: Analyze the solution of the elastic coupling two-degree-of-freedom model to obtain stability inference;

[0059] Step 400: construct a state space expression;

[0060] Step 500: using the state space expression to obtain a zero-pole distribution diagram of the system, and using the zero-pole distribution diagram to judge the stability inference to obtain a judgment result;

[0061] Step 600: When the judgment result is that the inference is correct, determining a critical point according to the elastic coupling two-degree-of-freedom model;

[0062] Step 700: determining an optimal frequency ratio according to the critical point;

[0063] Step 800: Calculate the equivalent coupling impedance between the wind turbine and the storage tank according to the optimal frequency ratio and the optimal short-circuit ratio requirement;

[0064] Step 900: Optimize the damping parameters of the original wind-storage system according to the equivalent coupling impedance between the wind and storage systems to obtain an oscillation-suppressed wind-storage system.

[0065] Preferably, the elastically coupled two-degree-of-freedom model is:

[0066]

[0067] Among them, θ W is the output phase of the phase-locked loop unit; U S K is the grid connection point voltage amplitude of the phase-locked loop unit; P is the first control parameter of the phase-locked loop unit; M is the virtual inertia; U ES is the energy storage voltage of the energy storage unit; U g is the voltage at the grid end; U W B is the voltage of the wind turbine unit; 23 is the admittance between the energy storage unit and the grid terminal; θ ES0 is the initial energy storage phase of the energy storage unit; △P t is the disturbance amount; D is the virtual damping coefficient; △B 21 is the virtual short-circuit admittance; B 210 is the initial admittance between the wind turbine unit and the energy storage unit; △θ W Outputting a phase change amount to the phase-locked loop unit; Outputting a first-order derivative of a phase change amount for the phase-locked loop unit; is the second-order derivative of the phase change output by the phase-locked loop unit; △θ ES is the phase change of the energy storage unit; is the first-order derivative of the phase change of the energy storage unit; is the second-order derivative of the phase change of the energy storage unit.

[0068] Specifically, the solution of the elastically coupled two-degree-of-freedom model is analyzed to obtain stability inference, including:

[0069] Determining a solution equation for the elastically coupled two-degree-of-freedom model;

[0070] Draw an amplitude-frequency response diagram according to the solved equation;

[0071] The stability inference is determined based on the magnitude-frequency response plot.

[0072] Preferably, the state space expression is:

[0073]

[0074] in, △ω ES K is the energy storage virtual speed; I is the second control parameter of the phase-locked loop unit; Δω W is the change in angular frequency of the wind turbine unit of the wind turbine generator set; is the first-order derivative of the change in angular frequency of the wind turbine unit of the wind turbine generator set; is the first-order derivative of the change in the energy storage angular frequency of the energy storage unit; B2′1 is the equivalent coupling impedance between the wind and storage.

[0075] Furthermore, B2′1 does not need to be distinguished. In the stability analysis, the value of this variable parameter is substituted into the state space expression to obtain the corresponding zero-pole distribution diagram to determine whether the system is stable. This can be used to verify the influence of the value of this variable on the stability of the system during its change.

[0076] Specifically, when the judgment result is that the inference is correct, determining the critical point according to the elastic coupling two-degree-of-freedom model includes:

[0077] The vertical coordinate of the critical point is calculated using a coordinate calculation formula; the coordinate calculation formula is:

[0078]

[0079] Where ζ=D / (2Mω mv ); g = ω d / ω n θ d is the phase amplitude amplification parameter; ω n 2 =K I U S ;ω mv 2 =S1 / M; S1=U ES U W B2′1cos(θ ES0 -θ W0 );ω d is the frequency of system disturbance; g A is the vibration ratio of the first critical point; g B is the vibration ratio of the second critical point.

[0080] The critical point is determined according to the ordinate and the amplitude-frequency response diagram.

[0081] Preferably, the calculation formula of the optimal frequency ratio is:

[0082]

[0083] Among them, SCR W =B 21 +B E ;f a is the optimal frequency ratio; B E is the equivalent coupling impedance between the wind turbine unit and the grid terminal.

[0084] Specifically, the calculation formula of the equivalent coupling impedance between the wind and the storage is:

[0085]

[0086] Among them, B2′1 is the equivalent coupling impedance between the wind and storage.

[0087] Preferably, the calculation formula for damping parameter optimization is:

[0088]

[0089] refer to Figure 2 The doubly-fed wind turbine uses a phase-locked loop to track the grid connection point for synchronization control and runs in maximum power tracking mode to ensure wind energy utilization. The energy storage is supplemented with virtual synchronous machine control to improve the friendliness of wind storage and grid connection by simulating the power support capacity of the synchronous machine. In addition, under the control of the virtual synchronous machine, after the system is short-circuited, the energy storage can provide short-circuit capacity to the system and even has the possibility of adjusting the grid-connected short-circuit ratio.

[0090] refer to Figure 3 , the grid-following wind turbine uses a phase-locked loop to track the grid connection point voltage, and its controller structure is as follows Figure 3 The phase-locked loop transforms the three-phase coordinate axis by inputting the grid-connected point voltage, and obtains the phase signal through the integral link after PI control. Its mathematical model is as follows:

[0091]

[0092] In the formula, The state variable introduced for the phase-locked loop unit; U q For U s Projection on the q-axis of the dq rotating coordinate system; U s is the voltage amplitude at the grid connection point; θ s is the phase of the wind turbine grid connection point; θ w is the phase-locked loop output phase; K I , K P It is the control parameter of the phase-locked loop PI control link.

[0093] Furthermore, the input voltage of the phase-locked loop of the doubly-fed wind turbine is a pure sinusoidal signal. According to the basic working principle of the phase-locked loop, under small disturbance conditions, due to θ s -θw The sinusoidal signal deviation can be equivalent to the phase deviation, and the q-axis voltage output by the phase-locked loop of the doubly fed wind turbine is:

[0094] U q =U s ()θ s -θ w

[0095] The second-order differential equation of the phase-locked loop is:

[0096]

[0097] After linearization, the small perturbation increment equation of the phase-locked loop is:

[0098]

[0099] From the above, it can be seen that when the phase-locked loop does not couple with the grid impedance, it conforms to the motion characteristics of the second-order dynamic model, and the dynamic stability of the phase-locked loop can be guaranteed by reasonably setting the parameters of the proportional and integral controllers. Figure 2 It can be seen that there is power transfer between wind, storage and grid-connected systems, and phase-locked loop modeling needs to consider the impact of impedance coupling between wind storage and wind grid on its dynamic characteristics.

[0100] refer to Figure 4 In this embodiment, the energy storage device is controlled by a virtual synchronous machine. Figure 4 As shown, the grid-connected point angular frequency signal ω is introduced g , the virtual speed ω is obtained through virtual damping and virtual inertia ES , after the integration link, it is input to the energy storage converter. Therefore, the energy storage has a power-frequency response characteristic similar to that of the synchronous generator. Under the control of the virtual synchronous machine, the energy storage has the function of regulating the grid frequency and damping, which is more in line with the current grid-connected support requirements of energy storage configuration. Its motion equation is:

[0101]

[0102] Where P mw is the virtual mechanical power; P ES is the virtual electromagnetic power; ω n is the rated angular frequency of the system; ω g is the grid connection point angular frequency; ω ES is the virtual speed of energy storage; D is the virtual damping; M is the virtual inertia.

[0103] From the above formula, the second-order differential equation of the energy storage virtual synchronous machine is:

[0104]

[0105] Where △P is the energy storage power deviation.

[0106] Further, according to Figure 2 It can be seen that there is power interaction between the doubly-fed wind turbine, energy storage and the power grid. The energy storage outputs active power P ES for:

[0107] P ES =U ES U g B 23 sin(θ ES -θ g )+U ES U w B 21 sin(θ ES -θ w )

[0108] Where U ES is the voltage at the energy storage terminal; U g is the grid voltage; U w B is the fan terminal voltage; 23 B is the admittance between the energy storage and the grid after star-delta conversion; 21 is the admittance between the energy storage and the wind turbine; B 13 is the admittance between the wind turbine and the grid; θ ES is the energy storage phase; θ g is the grid phase; θ w Output phase of the wind turbine phase-locked loop.

[0109] Among them, the admittance B between the energy storage and the wind turbine 21 It can be expressed as:

[0110]

[0111] Where B1 is the admittance between the wind turbine and the common coupling point, B2 is the admittance between the energy storage device and the common coupling point, B3 is the admittance between the grid and the common coupling point, and X 21 is the impedance between the energy storage device and the wind turbine, X1 is the impedance between the wind turbine and the common coupling point; X2 is the impedance between the energy storage device and the common coupling point; X3 is the impedance between the grid and the common coupling point.

[0112] After linearizing the calculation formula of energy storage output active power, we can get:

[0113] P ES0 =U ES U g B 23 cosθ ES0 △θ ES +U ES U w B 21 cos(θES0 -θ w0 )(△θ ES -△θ w )

[0114] In the formula, θ w0 The initial phase of the wind turbine phase-locked loop output; θ ES0 is the initial phase of energy storage; θ ES is the energy storage phase; θ w Output phase of the wind turbine phase-locked loop.

[0115] Furthermore, under small disturbance conditions, the phase change of the energy storage under the control of the virtual synchronous machine is approximately equal to the grid connection point detected by the phase-locked loop. In order to analyze the coupling relationship between the wind turbine phase-locked loop, energy storage and grid-connected system, the wind-storage coupling dynamic vibration model can be obtained as follows:

[0116]

[0117] In the formula, θ w0 The initial phase of the wind turbine phase-locked loop output; θ ES0 is the initial phase of energy storage; θ w is the phase-locked loop output phase; θ ES is the energy storage phase; K P , K I They are the control parameters of the phase-locked loop PI control link; △P t is the disturbance encountered by the system; M is the virtual inertia; D is the damping coefficient.

[0118] From the above formula, it can be seen that there is a phase difference coupling relationship between the power flow between wind and energy storage. The phase change of the energy storage device is closely related to the wind and energy storage terminal voltage and the impedance between the wind storage and the power grid. Its value can determine the power transfer caused by the relative phase displacement between wind and energy storage, and between wind and energy storage and the power grid. If the impedance parameter B between wind and energy storage is changed, 21 ,By adjusting the equivalent electrical distance between the wind and storage, that is, changing the short-circuit ratio of the wind and storage grid, the vibration coupling characteristics between the phase-locked loop and the energy storage and the grid can be affected.

[0119] Preferably, the short-circuit ratio is used to measure the electrical strength of the new energy grid connection, reflecting the electrical distance between the new energy and the system. After the energy storage device adopts virtual synchronous machine control, it can provide a short-circuit capacity similar to that of the traditional synchronous machine. If the energy storage equivalent impedance is taken into account, the short-circuit ratio of the wind-storage grid connection system can be improved. Figure 2 As shown, ignoring the influence of the system equivalent resistance, the short-circuit ratio SCR of the wind turbine grid access point can be obtained. w for:

[0120]

[0121] Where 1 / X E=B E =B1B3 / (B1+B2+B3) is the equivalent coupling impedance between the wind turbine and the grid-connected system. It can be seen that the coupling impedance B between the wind turbine and the storage 21 It is closely related to the grid-connected short-circuit ratio and will affect the oscillation characteristics of the phase-locked loop coupling system. However, under the current virtual synchronous machine control, energy storage does not yet have the ability to efficiently suppress subsynchronous oscillations induced by the phase-locked loop in the weak connection mode. When the energy storage is in synchronous operation, the phase difference with the synchronous generator is not considered. At this time, the oscillation model induced by the phase-locked loop of the wind turbine is a single-degree-of-freedom oscillation form. Under a single degree of freedom, there is no power transfer between wind and storage during the control design process. In theory, energy storage cannot transfer oscillation energy. In order to construct a new power transfer path between wind and storage and improve the oscillation characteristics of the phase-locked loop, this embodiment adds a new virtual short-circuit impedance control link in the grid-type energy storage control system to change the impedance coupling relationship between wind and storage. The equivalent coupling impedance between wind and storage can be expressed as:

[0122]

[0123] Where, X 210 , B 210 are the initial impedance and admittance of the wind-storage room respectively; △X 21 , △B 21 They are defined as virtual short-circuit impedance and admittance respectively.

[0124] Then the energy storage active power output can be expressed as:

[0125] P ES1 =U ES U g B 23 cosθ ES0 △θ ES +U ES U w B2′1cos(θ ES0 -θ w0 )(△θ ES -△θ w )

[0126] From the above formula, it can be seen that there is a new power transfer change between the wind and the storage. Considering that after the system parameters are set, the initial values ​​of the wind and storage terminal voltages and phase angles can be obtained, and the virtual short-circuit impedance △X is adjusted. 21 (i.e. △B 21 ) can generate a new phase deviation between the wind and the storage, thereby establishing a new coupling relationship, so that there is a new power transfer path between the two. In this mode, a new controllable two-degree-of-freedom vibration system is formed between the wind and the storage.

[0127] Specifically, the two-degree-of-freedom model of the wind-storage system elastic coupling can be expressed as:

[0128]

[0129] After adding a virtual short-circuit impedance to the energy storage virtual synchronous machine control system, the energy storage not only establishes a new phase deviation coupling relationship with the wind turbine phase-locked loop, but the oscillation induced by the phase-locked loop has a new energy transfer path. By equivalently changing the system short-circuit ratio, it has the function of adjusting the grid-connected stability of the wind turbine phase-locked loop.

[0130] Furthermore, in order to analyze the influence of the additional virtual short-circuit impedance on the subsynchronous oscillation induced by the phase-locked loop of the wind-storage system, the variables of the wind-storage system motion model are replaced. In order to facilitate the analysis and calculation, the following variable parameters are introduced to simplify the elastic coupling two-degree-of-freedom model of the wind-storage system into the standard form of the second-order differential equation:

[0131]

[0132] In the formula, ω n 2 =K I U s ,ω mv 2 =S1 / M,ω sv 2 =R / M,ζ=D / (2Mω mv ), S1=U ES U w B' 21 cos(θ ES0 -θ w0 ), R=U ES U g B 23 cosθ ES0 .

[0133] in, Assume that the solution of the equation is of the form:

[0134]

[0135] Substituting the solution into the standard form of the second-order differential equation, the target phase calculation formula can be obtained:

[0136]

[0137] In the formula, f a =ω mv / ω n , f b =ω sv / ω n , g = ω d / ω n .

[0138] From the above formula, we can see that f a It is strongly related to the equivalent coupling impedance between the wind and the storage, and ζ is strongly related to the system damping. The phase amplitude amplification parameter θ is introduced d , used to determine the degree of phase oscillation. |△θ w / θ d | is the amplitude ratio. The above formula can be used to analyze the amplitude-frequency response characteristics of the wind-storage system under different parameter settings. After adding the virtual short-circuit impedance, the electrical distance between the wind-storage system can be equivalently changed, and the short-circuit ratio of the wind turbine grid access point changes accordingly, such as Figure 5 Obviously, the fan phase amplitude frequency response curve has a resonance peak in the area around 12Hz, and as the frequency ratio f a During this process, another resonance peak appeared in the area around 3Hz, and the amplitude decreased with the frequency ratio f. a The risk of system oscillation increases with the increase of , and the risk of system oscillation increases significantly. From the perspective of short-circuit ratio, when the short-circuit ratio is at a small value, the system is in a weak connection mode and there is a greater risk of subsynchronous oscillation. If an additional virtual short-circuit impedance is introduced, the electrical distance between the wind and storage is equivalently changed by coupling the power grid, which plays a role in increasing the short-circuit ratio. After the short-circuit ratio increases, the subsynchronous oscillation induced by the phase-locked loop is suppressed, the system tends to be stable, and the oscillation amplitude decreases. However, if the short-circuit ratio continues to increase, the damping on the grid-type energy storage side will be weakened, and the system stability will deteriorate. Therefore, according to this characteristic, there should be an optimal value for the equivalent coupling impedance between wind and storage.

[0139] Preferably, in order to study the stability of the two-degree-of-freedom system after adding the virtual short-circuit impedance, △θ is selected ES , △θ w , △ω ES , △ω w As a state variable, the root locus method is used to analyze the change of the system poles when the equivalent coupling impedance between the wind and the storage changes. The state space expression of the system is:

[0140]

[0141] in,

[0142] refer to Figure 6 , with B' 21 The poles first keep moving away from the imaginary axis. The system damping increases, which is beneficial to the stability of the system, but at B' 21 In the process of continuous increase, the pole has an inflection point and keeps moving to the right close to the imaginary axis. At this time, the system damping decreases, which is not conducive to the stability of the system. This verifies the conclusion drawn from the amplitude-frequency response characteristic curve.

[0143] Furthermore, the optimal frequency ratio fa can be calculated based on the phase-amplitude-frequency response characteristic curve of the wind turbine phase-locked loop and the short-circuit ratio requirement of the wind turbine grid-side access point can be determined. The two-degree-of-freedom system will have two resonance peaks and the phase-amplitude-frequency response characteristic curve will pass through two points A (the first critical point) and B (the second critical point) when the damping changes (reference Figure 7 ). Substituting the two critical cases of ζ=0 and ζ=∞ into the target phase calculation formula, we can get the oscillation frequency ratio at points A and B. Let the two be equal (what's the point of substituting them separately), and then substituting the frequency ratio of the two points into the target phase calculation formula, we can calculate the ordinates of points A and B, and thus get the following formula:

[0144]

[0145] The optimal frequency ratio f can be calculated a And the optimal short-circuit ratio requirement range, the optimal frequency ratio can be expressed as:

[0146]

[0147] According to the optimal frequency ratio and the optimal short-circuit ratio requirement range, the equivalent coupling impedance B' between wind and storage can be calculated. 21 It is expressed as:

[0148]

[0149] The above formula provides a basis for the design of equivalent coupling impedance parameters between wind and storage. Under this condition, the amplitude of the main vibration system reaches the minimum value and the system obtains the best vibration suppression effect.

[0150] Furthermore, the equivalent coupling impedance B' between wind and storage 21 After determination, the damping parameters are optimized. When the curvature of the curve at points A and B is 0, that is, the highest amplitude point, the main system can obtain the minimum amplitude. Therefore, the calculation formula of the equivalent coupling impedance between the wind and the storage is substituted into the target phase calculation formula to make the equation optimally tuned, and the derivative is taken to make the slope equal to 0 at points A and B. In the process of designing parameters, the average value of the two is taken as the optimal damping ratio and damping parameter, which are expressed as:

[0151]

[0152] Optimize the design of equivalent coupling impedance B' between wind and storage 21 The damping D not only improves the system's oscillation suppression effect, but also effectively avoids the resonance risk caused by the introduction of additional control.

[0153] Preferably, when wind turbines and energy storage are connected to the grid, the initial impedance parameters between the wind turbines and energy storage are not controllable. Although the impedance parameters between the wind turbines and energy storage are coupled, they are rigidly connected. In order to improve the short-circuit ratio and establish a controllable elastic coupling relationship between wind turbines and energy storage, this embodiment adds a virtual short-circuit impedance control link to the virtual synchronous machine control to achieve the expected goal. The initial power transfer amount U between wind turbines and energy storage can be calculated based on the initial phase of the wind turbine and energy storage and the voltage of the wind and energy storage terminals. ES U w cos(θ ES0 -θ w0 )B 210 (△θ ES -△θ w ), and then calculate the optimal wind-storage equivalent coupling impedance B' 21 , we can further calculate the optimal power transfer between wind and storage U ES U w cos(θ ES0 -θ w0 )B' 21 (△θ ES -△θ w ), the power transfer between wind and energy storage provided under the optimal virtual short-circuit impedance can be obtained through the deviation between the initial power transfer and the optimal power transfer. After adding the virtual short-circuit impedance, the output power of energy storage changes, which can be expressed as:

[0154] △P ew =S△B 21 (θ ES -θ ES0 )-S△B 21 (θ w -θ w0 )

[0155] Where S = U ES U w cos(θ ΕS0 -θ ω0 ).

[0156] The controller structure with additional virtual short-circuit impedance is as follows: Figure 8 As shown. Among them, P vr is the reference value of active power, P cot is the energy storage correction power, P mw is the output virtual mechanical power, P ES为 Virtual electromagnetic power, △P ew A new power delivery path.

[0157] Furthermore, in order to verify the suppression effect of additional virtual short-circuit impedance control on the oscillation caused by the phase-locked loop of the wind turbine in the wind-storage combined grid-connected system, this paper uses DIgSILENT / PowerFactory software to build a wind-storage grid-connected simulation system. The simulation system adopts the IEEE 3-machine 9-node system, including a wind farm (50 × 2MW double-fed wind turbines), and a 10MW / 39kAh (10kV) energy storage battery is configured in coordination. The capacities of the three synchronous generators (G1, G2, and G3) are 40MW, 30MW, and 24MW, respectively. The wind-storage combined power generation system transmits power through the double-circuit line Line11-7. The simulation system assumes that the wind speed is always maintained at 8m / s. The parameters of the double-fed wind turbine are detailed in Table 1.

[0158] Table 1

[0159]

[0160] In order to analyze the impact of additional virtual short-circuit impedance on system stability, the equivalent coupling impedance B' between wind and storage is 21 The root loci in modal analysis when the Fig. 9 As shown in Figure 1, the main participating factor of mode 1 is the wind turbine phase-locked loop; the main participating factor of mode 2 is the energy storage control; and the main participating factor of mode 3 is the wind turbine shaft system. Fig. 9 It can be seen that when the wind-storage equal coupling impedance B' 21 As B' increases from 0 to 10, the system mode 3 remains almost unchanged, but the system modes 1 and 2 have significant changes. 21 As B' increases, it gradually moves away from the imaginary axis, and the system oscillation is significantly suppressed. 21 As B' increases, the damping ratio increases, and the equivalent electrical distance between the wind and storage is changed, the short-circuit ratio is improved, and the subsynchronous oscillation caused by the phase-locked loop is significantly suppressed, and the corresponding root trajectory moves away from the imaginary axis. 21 The continuous increase of , the root locus of mode 2 significantly shifts to the left, even crossing the imaginary axis, causing the system to lose stability. The modal simulation results are consistent with the theoretical analysis. Therefore, the equivalent coupling impedance B' between the wind and the storage 21 The design needs to be optimized so that it can reduce the risk of oscillation in mode 1 while avoiding the risk of oscillation in mode 2.

[0161] Specifically, the simulation sets the three-phase short circuit of the transmission line 7 for 10s, the voltage drops to 0.42pu, and the system equivalent total impedance value x is set. e =0.5pu, taking into account the equivalent reactance of the wind turbine, the SCR at this time can be obtained w =1.5, the short-circuit capacity provided by the system is about 75MW. Fig.10 (a1) to Fig.10 (c3) It can be seen that when there is no additional control of energy storage, the system damping is insufficient, the wind turbine power and phase produce continuous oscillations and generate amplification oscillations after a certain period of time; when the energy storage adopts virtual synchronous machine control, although the sub-synchronous oscillation caused by the phase-locked loop can be suppressed, because the system is in a weak connection mode, a long recovery time is required and small-amplitude continuous oscillations cannot be eliminated, and the suppression effect is not good; after the energy storage adopts additional virtual short-circuit impedance control, the equivalent electrical distance between the wind and the storage is changed, and the short-circuit ratio is equivalently improved. The system achieves the best oscillation suppression effect and significantly improves the safe and stable operation capability of wind turbines connected to the grid under weak power grids. In addition, it can be verified through spectrum analysis that after the additional virtual short-circuit impedance control, the frequency oscillation amplitude of about 10Hz is significantly reduced.

[0162] The beneficial effects of the present invention are as follows:

[0163] The present invention solves the problems of long electrical distance and insufficient short-circuit ratio of the existing wind-storage system by arranging a virtual synchronization unit and a virtual short-circuit impedance control unit in the original wind-storage system, thereby improving the stability and short-circuit capacity of the system; through the equivalent coupling impedance between wind-storage systems, the optimal setting of the system short-circuit ratio is achieved, the structural parameters of the system are optimized, and the system's ability to suppress sub-synchronous oscillations of the phase-locked loop is improved.

[0164] The various embodiments in this specification are described in a progressive manner, and each embodiment focuses on the differences from other embodiments. The same or similar parts between the various embodiments can be referenced to each other.

[0165] The principles and implementation methods of the present invention are described in this article using specific examples. The description of the above embodiments is only used to help understand the method and core idea of ​​the present invention. At the same time, for those skilled in the art, according to the idea of ​​the present invention, there will be changes in the specific implementation methods and application scope. In summary, the content of this specification should not be understood as limiting the present invention.

Claims

1. A method for suppressing subsynchronous oscillation of a phase-locked loop based on wind energy storage virtual short-circuit impedance coupling, characterized in that: include: Constructing an original wind storage system; the original wind storage system comprises: a power grid end, a wind turbine unit, an energy storage unit, a phase-locked loop unit, a virtual synchronization unit and a virtual short-circuit impedance control unit; Constructing an elastically coupled two-degree-of-freedom model according to the original wind-storage system; Analyzing the solution of the elastically coupled two-degree-of-freedom model to obtain stability inference; Construct state space expressions; Using the state space expression to obtain a zero-pole distribution diagram of the system, and using the zero-pole distribution diagram to judge the stability inference to obtain a judgment result; When the judgment result is that the inference is correct, determining the critical point according to the elastic coupling two-degree-of-freedom model; determining an optimal frequency ratio according to the critical point; The equivalent coupling impedance between the wind and the storage is calculated according to the optimal frequency ratio and the optimal short-circuit ratio requirements; The damping parameters of the original wind-storage system are optimized according to the equivalent coupling impedance between the wind and storage units to obtain a wind-storage system with oscillation suppression.

2. According to claim 1, a phase-locked loop subsynchronous oscillation suppression method based on wind storage virtual short-circuit impedance coupling is characterized in that: The elastic coupling two-degree-of-freedom model is: Among them, θ W is the output phase of the phase-locked loop unit; U S K is the grid connection point voltage amplitude of the phase-locked loop unit; P is the first control parameter of the phase-locked loop unit; M is the virtual inertia; U ES is the energy storage voltage of the energy storage unit; U g is the voltage at the grid end; U W B is the voltage of the wind turbine unit; 23 is the admittance between the energy storage unit and the grid terminal; θ ES0 is the initial energy storage phase of the energy storage unit; △P t is the disturbance amount; D is the virtual damping coefficient; △B 21 is the virtual short-circuit admittance; B 210 is the initial admittance between the wind turbine unit and the energy storage unit; △θ W Outputting a phase change amount to the phase-locked loop unit; Outputting a first-order derivative of a phase change amount for the phase-locked loop unit; is the second-order derivative of the phase change output by the phase-locked loop unit; △θ ES is the phase change of the energy storage unit; is the first-order derivative of the phase change of the energy storage unit; is the second-order derivative of the phase change of the energy storage unit.

3. According to claim 2, a phase-locked loop subsynchronous oscillation suppression method based on wind storage virtual short-circuit impedance coupling is characterized in that: The solution of the elastically coupled two-degree-of-freedom model is analyzed to obtain stability inferences, including: Determining a solution equation for the elastically coupled two-degree-of-freedom model; Draw an amplitude-frequency response diagram according to the solved equation; The stability inference is determined based on the magnitude-frequency response plot.

4. According to claim 3, a method for suppressing subsynchronous oscillation of a phase-locked loop based on wind energy storage virtual short-circuit impedance coupling is characterized in that: The state space expression is: in, △ω ES K is the energy storage virtual speed; I is the second control parameter of the phase-locked loop unit; Δω W is the change in angular frequency of the wind turbine unit of the wind turbine generator set; is the first-order derivative of the change in angular frequency of the wind turbine unit of the wind turbine generator set; is the first-order derivative of the change in the energy storage angular frequency of the energy storage unit; B2′1 is the equivalent coupling impedance between the wind and storage.

5. According to claim 4, a method for suppressing subsynchronous oscillation of a phase-locked loop based on wind energy storage virtual short-circuit impedance coupling is characterized in that: When the judgment result is that the inference is correct, determining the critical point according to the elastic coupling two-degree-of-freedom model includes: The vertical coordinate of the critical point is calculated using a coordinate calculation formula; the coordinate calculation formula is: Where ζ=D / (2Mω mv ); g = ω d / ω n θ d is the phase amplitude amplification parameter; ω n 2 =K I U S ;ω mv 2 =S1 / M; S1=U ES U W B′ 21 cos(θ ES0 -θ W0 );ω d is the frequency of system disturbance; g A is the vibration ratio of the first critical point; g B is the vibration ratio of the second critical point. The critical point is determined according to the ordinate and the amplitude-frequency response diagram.

6. A method for suppressing subsynchronous oscillation of a phase-locked loop based on wind energy storage virtual short-circuit impedance coupling according to claim 5, characterized in that: The calculation formula of the optimal frequency ratio is: Among them, SCR W =B 21 +B E ;f a is the optimal frequency ratio; B E is the equivalent coupling impedance between the wind turbine unit and the grid terminal.

7. A method for suppressing subsynchronous oscillation of a phase-locked loop based on wind energy storage virtual short-circuit impedance coupling according to claim 6, characterized in that: The calculation formula of the equivalent coupling impedance between the wind and the storage is: Among them, B2′1 is the equivalent coupling impedance between the wind and storage.

8. The method for suppressing subsynchronous oscillation of a phase-locked loop based on wind energy storage virtual short-circuit impedance coupling according to claim 7 is characterized in that: The calculation formula for damping parameter optimization is:

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