Network construction type doubly-fed wind turbine generator set second-order model and order reduction method

Abstract

The invention discloses a second-order model and an order reduction method of a network construction type doubly-fed wind turbine generator, the established second-order model is subjected to order reduction processing based on a 17-order full-order model of the network construction type doubly-fed wind turbine generator, the 17-order model is firstly reduced to a 5-order model, and the wind speed and the line impedance are given; a five-order model is used for defining the rotating speed, the rotor current initial value, the virtual synchronous speed deviation initial value and the virtual power angle initial value in a two-order model, meanwhile, the integral link in the rotor side outer ring control transient state process is ignored, and in the transient state stability process of network construction control, the dynamic change process of a wind turbine is ignored, and an active reference value refers to the initial rotating speed. According to the second-order model of the network construction type doubly-fed wind turbine generator, the transient characteristics of the network construction type doubly-fed wind turbine generator can be accurately depicted and used for researching the transient energy problem, and the second-order model has the beneficial effects that the dynamic characteristics of the network construction type doubly-fed wind turbine generator in the transient process can be accurately represented, and the transient analysis difficulty is reduced.

Description

Technical Field

[0001] The present invention belongs to the technical field of transient stability of a grid-oriented doubly-fed induction wind generator (GFM-DFIG), and in particular relates to a second-order model of a grid-oriented doubly-fed wind turbine generator set and an order reduction method. Background Art

[0002] As more and more wind turbines are connected to the power grid, the safety and stability of wind turbines have received widespread attention from countries around the world. Grid-following doubly-fed wind turbines that are synchronized with the grid voltage have experienced a large number of transient synchronization instability problems under weak grid voltage fluctuations. In order to improve the stable operation capability of doubly-fed wind turbines in weak grids or island conditions, grid-fed doubly-fed wind turbines with power synchronization (GF-DFIG) have attracted widespread attention from scholars.

[0003] Due to the existence of maximum power tracking control, grid-following wind turbines are difficult to respond to changes on the grid side and cannot support the grid frequency and voltage well. However, grid-building wind turbines have changed the characteristics of power electronic equipment that have almost no rotational inertia and are difficult to interact with the grid, overcoming the shortcomings of distributed renewable energy generators that have insufficient ability to actively support voltage and frequency.

[0004] By simulating the operating mode of the synchronous generator, the grid-connected wind turbine directly controls the amplitude and phase of its output voltage, can autonomously establish a stable terminal voltage, achieve autonomous synchronization with the power grid, and actively support the frequency and voltage stability of the power grid, providing inertia and damping support for the system.

[0005] Compared with traditional wind turbines, grid-connected wind turbines are particularly suitable for local high-proportion wind power systems and weak grid environments, and even independent networking and island operation scenarios. Summary of the invention

[0006] The invention discloses a second-order model of a grid-type doubly-fed wind turbine generator set and an order reduction method, analyzes the transient stability problem of the grid-type DFIG, and can accurately represent the dynamic characteristics of the grid-type DFIG in the transient stability process.

[0007] GFM-DFIG was used as the research object. Figure 1 As shown in the figure, the GFM-DFIG unit is mainly composed of a wind wheel, a gearbox, a doubly-fed generator, a rotor-side converter, a DC bus, a grid-side converter, an export filter and other links.

[0008] In the actual synchronous dq coordinate system of the power grid, the grid-connected voltage of the doubly fed wind turbine can be expressed as:

[0009]

[0010]

[0011] The wind turbine grid-connected voltage model is:

[0012]

[0013] Where R s is the stator resistance, L s , L r , L m are the stator self-inductance, rotor self-inductance and mutual inductance of the doubly-fed wind turbine, R g is the line resistance, L g is the line inductance, s=(ω s -ω r ) / ω s is the slip rate of the doubly fed fan, ω s is the synchronous speed of the power grid, ω r is the fan speed.

[0014] Different from the grid-type doubly-fed wind turbine, the grid-type doubly-fed wind turbine uses the virtual rotor motion equation shown in formula (4) to control the terminal voltage phase:

[0015]

[0016] The virtual excitation equation shown in equation (5) and the rotor-side converter control equation shown in equation (6) are used to control the voltage amplitude at the machine end:

[0017] U ref =E0+k q (Q ref -Q e ) (5)

[0018]

[0019] In addition, in order to realize stator voltage oriented vector control, it is necessary to use the phase information generated by equation (4) to transform the coordinates of variables including voltage and current in the grid coordinate system. Therefore, there are three dq coordinate systems in the wind power system: the grid voltage coordinate system, the machine terminal voltage coordinate system and the grid control system coordinate system.

[0020] Define the grid voltage angle δ g =0, then the angle difference between the terminal voltage coordinate system and the grid voltage coordinate system is the terminal power angle δ of the wind turbine s The angle difference between the control system coordinate system and the grid voltage coordinate system is the virtual power angle δ generated by the rotor motion equation v =θ v -θ g , the transformation relationship between the two coordinate systems can be expressed as:

[0021]

[0022] Where: T is a variable such as voltage and current; the superscript c represents the controller dq coordinate system.

[0023] In addition, it also includes wind turbine model, DC bus model and grid-side converter model, forming a full-order model of grid-connected doubly-fed wind turbine.

[0024] In order to study the transient characteristics of the grid-connected double-fed wind turbine during fault ride-through, the 17th-order model is reduced to the 5th-order model, and the following reduction measures are taken:

[0025] The wind turbine model adopts a single mass block model; the voltage of the doubly-fed asynchronous motor, the resistance term and the differential term of the flux equation are ignored; the inner loop of the rotor-side control current is ignored, and it is assumed that the rotor current of the wind turbine can track the current command value during the transient process; the DC bus dynamics are ignored, and the grid-side converter is regarded as a current source. The current injected into the grid is related to the slip rate.

[0026] After taking the above reduction measures, the terminal voltage U s It can be expressed as:

[0027]

[0028] Where, X g , X s , X m They are line, stator, and mutual inductance reactance respectively.

[0029]

[0030] The electromagnetic active power P of the fifth-order transient model is obtained e and reactive power Q e As shown by the formula:

[0031]

[0032] In the formula, the power angle δ s =arcsin(U sq / U s ).

[0033] Based on the equilibrium operation point of the fifth-order transient model, a second-order transient model that accurately describes the electromagnetic power at the machine end is established. In order to study the transient characteristics of the grid-connected double-fed wind turbine during fault ride-through, the following descending measures are taken:

[0034] 1) Given wind speed v w , and the line impedance X g , we can obtain the fan speed ω at the steady-state equilibrium point of the current fifth-order transient model r0 , initial value of rotor current Virtual synchronous speed deviation initial value Δωv0 , the initial value of virtual power angle δ v0 ;

[0035] 2) Ignore the integral link in the transient process of the rotor side outer loop control, replace the original integral loop with the initial value of the rotor current, and the reduced-order rotor side control model can be expressed as:

[0036]

[0037] 3) In the process of analyzing the transient stability of the network control, the dynamic change process of the wind turbine is ignored, and the active reference value refers to the initial speed ω r0 Given.

[0038] Through the above order reduction methods, the second-order transient model of the grid-type doubly-fed wind turbine can be obtained. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 Schematic diagram of the second-order transient model of a fictitious grid-type doubly-fed wind turbine provided in this application. DETAILED DESCRIPTION

[0040] The scheme proposed by the present invention is further described in detail below in conjunction with the accompanying drawings and specific implementation methods. According to the following description, the advantages and features of the present invention will be clearer. It should be noted that the accompanying drawings are in a very simplified form and use non-precise proportions, which are only used to conveniently and clearly assist in explaining the purpose of the implementation method of the present invention. In order to make the purposes, features and advantages of the present invention more obvious and easy to understand, please refer to the accompanying drawings. It should be noted that the structures, proportions, sizes, etc. illustrated in the accompanying drawings of this specification are only used to match the contents disclosed in the specification for people familiar with this technology to understand and read, and are not used to limit the limiting conditions for the implementation of the present invention, so they have no technical substantive significance. Any structural modification, change in proportional relationship or adjustment of size, without affecting the effects that the present invention can produce and the purposes that can be achieved, should still fall within the scope of the technical content disclosed by the present invention.

[0041] The grid-type doubly-fed wind turbine sets control the phase and amplitude of the wind turbine terminal voltage by entering the virtual rotor motion equation and the excitation equation, thereby realizing the control of the active and reactive power of the wind turbine sets, so that the wind turbines have inertia and damping characteristics similar to those of synchronous units, thereby improving the transient synchronization stability of the grid-connected system. However, unlike synchronous machines, the response time scales of the active and reactive control links in the grid-type equipment are similar, resulting in strong coupling between active and reactive power, terminal voltage phase and amplitude, and even transient instability under power grid faults. The full-order model of the grid-type doubly-fed wind turbine set is a 17th-order system with a complex structure, making it difficult to perform transient characteristic analysis. If the transient stability of the system is analyzed from the perspective of energy, it can only be used for low-order systems. Therefore, the present invention proposes a 2nd-order model of a grid-type doubly-fed wind turbine set and an order reduction method.

[0042] like Figure 1 As shown, an embodiment of the present invention provides a schematic diagram of a second-order model of a grid-type doubly-fed wind turbine generator set. The method divides the GFM-DFIG into different parts according to the control module for reduced-order modeling.

[0043] In the transient change process of the second-order model, the expression of the rotor current in the dq coordinate system of the control system can be obtained by equations (7), (8) and (11):

[0044]

[0045] Where K1 = K d X s , K2=K q X s , K3=K q (X s +(1-s)X g ), X1=K d (1-s)X g X m +X s +(1-s)X g , X2=K q (1-s)X g X m +X s +(1-s)X g , X3=X s +(1-s)X g .

[0046] Considering the rotor current limiting link, we have:

[0047]

[0048] Substituting equation (13) into equation (8), we get U s and δ s About Delta v The expression is:

[0049]

[0050]

[0051] Finally, substitute equations (14) and (15) into equation (10) to obtain the electromagnetic power P e About Delta v The power angle characteristic expression is:

[0052]

[0053] At this point, a complete second-order transient model can be obtained. In this model, all voltage and current variables can be expressed by the virtual power angle δ v To express.

[0054] The embodiments described above are only used to illustrate the technical solutions of the present application, rather than to limit them. Although the present application has been described in detail with reference to the aforementioned embodiments, a person skilled in the art should understand that the technical solutions described in the aforementioned embodiments may still be modified, or some of the technical features may be replaced by equivalents. Such modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present application, and should all be included in the protection scope of the present application.

Claims

1. A second-order model and order reduction method for a grid-connected doubly-fed wind turbine generator system, characterized in that: The grid-type doubly-fed wind turbine generator system can be reduced to the second order and the transient stability problem can be analyzed, which can accurately characterize the transient stability characteristics of the grid-type doubly-fed wind turbine generator system; The grid-type doubly-fed wind turbine second-order model and order reduction method are composed of a grid-type doubly-fed wind turbine fifth-order model and order reduction method and a grid-type doubly-fed wind turbine second-order model and order reduction method.

2. The 5th order reduction method of the grid-type doubly-fed wind turbine generator set according to claim 1 is characterized in that: The wind turbine model adopts a single mass block model; the voltage of the doubly-fed asynchronous motor, the resistance term and the differential term of the flux equation are ignored; the inner loop of the rotor-side control current is ignored, and it is assumed that the rotor current of the wind turbine can track the current command value during the transient process; the DC bus dynamics are ignored, and the grid-side converter is regarded as a current source. The current injected into the grid is related to the slip rate: I d,q =(1-s)I sd,sq (3) 3. The fifth-order model of the grid-type doubly-fed wind turbine generator set according to claim 1 is characterized in that: Terminal voltage U of a doubly-fed wind turbine under transient model s It can be expressed as: Where, X g , X s , X m They are line, stator, and mutual inductance reactance respectively; Ignoring the integral link in the transient process of the rotor side outer loop control, the rotor side control model after the original integral loop is reduced by replacing the initial value of the rotor current can be expressed as: The rotor motion equation is given by the reference phase θ v becomes to provide a virtual power angle δ v : The electromagnetic active power P of the fifth-order transient model is obtained e and reactive power Q e As shown by the formula: In the formula, the power angle δ s =arcsin(U sq / U s ).

4. The second-order reduction method of a grid-type doubly-fed wind turbine generator set according to claim 1, characterized in that: Based on the equilibrium operation point of the fifth-order transient model, a second-order transient model that accurately describes the electromagnetic power at the machine end is established, and the following downgrade measures are taken: 1) Given wind speed v w , and the line impedance X g , we can obtain the fan speed ω at the steady-state equilibrium point of the current fifth-order transient model r0 , initial value of rotor current Virtual synchronous speed deviation initial value Δω v0 , the initial value of virtual power angle δ v0 ; 2) Ignore the integral link in the transient process of the rotor side outer loop control, and replace the original integral loop with the initial value of the rotor current; 3) In the process of analyzing the transient stability of the network control, ignore the dynamic change process of the wind turbine, and the active reference value refers to the initial speed ω r0 Given.

5. According to the second-order model of the grid-type doubly-fed wind turbine generator set in claim 1, the expression of the rotor current in the dq coordinate system of the control system during the transient change of the second-order model is: where K1 = K d X s , K2 = K q X s , K3 = K q (X s +(1 - s)X g ), X1 = K d (1 - s)X g X m +X s +(1 - s)X g , X2 = K q (1 - s)X g X m +X s +(1 - s)X g , X3 = X s +(1 - s)X g ; Considering the rotor current limiting link, we have: Substituting equation (9) into equation (5), we get U s and δ s About Delta v The expression is: Finally, substitute equations (10) and (11) into equation (7) to obtain the electromagnetic power P e About Delta v The power angle characteristic expression is: At this point, a complete second-order transient model can be obtained. In this model, all voltage and current variables can be expressed by the virtual power angle δ v To express.

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