Method for obtaining three-phase short-circuit current of double-fed wind turbine considering grid-side converter (GSC) feedback current

Abstract

The application discloses a method for obtaining three-phase short-circuit current of a doubly-fed fan considering GSC outgoing current of a grid-side converter, and when three-phase short-circuit occurs in a power grid, power fluctuation exists between RSC and GSC under the condition of RSC control; the DC bus voltage fluctuation between RSC and GSC is solved from the power fluctuation of the DC side; the reference value of the grid-side converter current can be obtained according to the DC bus voltage fluctuation; the outgoing current of the grid-side converter can be obtained according to the d and q axis current reference values. The stator short-circuit current in the stationary coordinate system considering RSC control is added to the outgoing current of the GSC, so that the three-phase short-circuit current of the doubly-fed fan considering the outgoing current of the GSC is obtained. Through the application, the transient characteristics of the DFIG can be more accurately analyzed, the short-circuit characteristic analysis of the DFIG is improved, and a basis and reference are provided for the fault characteristic analysis of the DFIG.

Description

TECHNICAL FIELD

[0001] The application relates to a method for obtaining three-phase short-circuit current of a doubly-fed fan considering GSC outgoing current of a grid-side converter, and belongs to the technical field of fault analysis of a wind power generation system. BACKGROUND

[0002] The statements in this section merely provide background information related to the application and do not necessarily constitute prior art.

[0003] With the proposal of the "carbon peak and carbon neutral" goal and under the new situation of "building a new power system mainly based on new energy", new energy is showing a new trend of accelerated development, and new energy power generation represented by wind power technology is highly valued. The doubly-fed induction generator has become the most widely used in wind farms due to its good operating characteristics, active and reactive decoupling control, and small converter capacity.

[0004] According to the wind power grid connection technology regulation, all wind turbines connected to the grid must have low voltage ride through (LVRT) capability. The low voltage ride through capability of existing wind turbines can be divided into two types: one is to rely on hardware circuit to realize LVRT, and the other is to rely on converter control strategy to realize LVRT. The former includes pry bar circuit, unloading circuit, and GSC double parallel, which have been studied. For the latter, the fault characteristics of DFIG are influenced by the excitation control of the converter and the electromagnetic process of the generator, showing strong nonlinear coupling characteristics. Therefore, the application is aimed at the case where the rotor-side converter still plays a regulating role when the three-phase symmetrical voltage at the generator terminal occurs.

[0005] The fault current of the doubly-fed wind turbine is composed of two parts: the stator-side short-circuit current provided by the stator side and the short-circuit current provided by the grid-side converter. Current research mainly focuses on the stator-side short-circuit current, and less on the short-circuit current provided by the grid side. In the case of non-locking of the rotor side, there is a power imbalance between the rotor side and the grid side during the fault, which will cause the current fluctuation of the GSC side. Therefore, if the GSC side current is ignored, it will inevitably cause errors in the calculation results of the DFIG short-circuit current. Therefore, it is of great significance to study the three-phase short-circuit current characteristics of the DFIG considering the GSC outgoing current. SUMMARY

[0006] The application provides a method for obtaining three-phase short-circuit current of a doubly-fed fan considering GSC outgoing current of a grid-side converter, which realizes the obtaining of three-phase short-circuit current of a doubly-fed fan considering GSC outgoing current of a grid-side converter through stator three-phase short-circuit current and grid-side converter three-phase outgoing current in a three-phase stationary coordinate system.

[0007] The technical scheme of the present application is:

[0008] According to a first aspect of the present application, a method for obtaining three-phase short-circuit current of a doubly-fed wind turbine considering grid-side converter GSC outgoing current is provided, comprising the following steps: when three-phase short-circuit occurs in the power grid, considering the case that the rotor-side converter RSC is controlled, there is power fluctuation between the rotor-side converter RSC and the grid-side converter GSC, the power fluctuation between the rotor-side converter RSC and the grid-side converter GSC is solved; according to the power fluctuation between the rotor-side converter RSC and the grid-side converter GSC, the DC bus voltage fluctuation between the rotor-side converter RSC and the grid-side converter GSC is solved; according to the DC bus voltage fluctuation, the reference value of the grid-side converter current based on the d, q synchronous rotating coordinate system is obtained; wherein the reference value of the grid-side converter current based on the d, q synchronous rotating coordinate system includes the reference value of the grid-side converter d-axis current and the q-axis current reference value; according to the grid-side converter d-axis and q-axis current reference value based on the d, q synchronous rotating coordinate system, the grid-side converter outgoing current based on the d, q synchronous rotating coordinate system is obtained; wherein the grid-side converter outgoing current based on the d, q synchronous rotating coordinate system includes the d, q axis components of the grid-side converter outgoing current; the stator three-phase short-circuit current and the grid-side converter three-phase outgoing current in the three-phase stationary coordinate system are obtained by converting the stator short-circuit current and the grid-side converter outgoing current based on the d, q synchronous rotating coordinate system; the stator three-phase short-circuit current and the grid-side converter three-phase outgoing current in the three-phase stationary coordinate system are added to obtain the three-phase short-circuit current of the doubly-fed wind turbine considering the grid-side converter GSC outgoing current.

[0009] Further, when the power grid has a three-phase short-circuit fault at t=t1, the approximate value ΔP' of the power fluctuation between the rotor-side converter RSC and the grid-side converter GSC is obtained; the approximate value ΔP' of the power fluctuation between the rotor-side converter RSC and the grid-side converter GSC is taken as the power fluctuation ΔP between the rotor-side converter RSC and the grid-side converter GSC, and the specific expression is as follows:

[0010]

[0011] Wherein, P g represents the grid-side power, P r represents the rotor-side power; s is the slip ratio; k is the amplitude of the terminal voltage drop after the power grid fails; U s0 is the amplitude of the terminal voltage in steady state operation; i rq is the q-axis component of the rotor current; e represents a natural constant; τ1 represents the stator transient flux decay constant of the doubly-fed wind turbine.

[0012] Furthermore, based on the power fluctuation ΔP between the rotor-side converter RSC and the grid-side converter GSC, the DC bus voltage fluctuation ΔU between the rotor-side converter RSC and the grid-side converter GSC is calculated. dc The expression is:

[0013]

[0014] Among them, C dc ω1 is the DC-side capacitor; ω1 is the synchronous speed; s is the slip; k is the magnitude of the voltage drop at the generator terminals after a grid fault; U s0 i represents the amplitude of the terminal voltage during steady-state operation. rq U represents the q-axis component of the rotor current; e represents the natural constant; τ1 represents the stator transient flux decay constant of the doubly-fed induction generator; t1 represents the moment when a three-phase short-circuit fault occurs; U dc1 U dc2 U dc3 U dc4 m dc η1, η2, Indicates a mnemonic.

[0015] Furthermore, the reference value of the d-axis current of the grid-side converter based on the d- and q synchronous rotating coordinate system... The expression is:

[0016]

[0017] Where A0, A1, A2, A3, A4, η1, and η2 represent mnemonics; e represents the natural constant; j represents the imaginary number; ω1 is the synchronous speed; t represents time; and τ1 represents the stator transient flux decay constant of the doubly-fed wind turbine.

[0018] Without considering the reactive power support of the grid-side converter GSC, the reference value of the q-axis current of the grid-side converter based on the d-q synchronous rotating coordinate system. It is 0.

[0019] Furthermore, the expression for the grid-side converter feed-out current based on the d-q synchronous rotating coordinate system is:

[0020]

[0021] Among them, i gd i gq These are the d-axis and q-axis components of the grid-side converter feedout current, respectively. This indicates the reference value of the d-axis current of the grid-side converter. The values ​​represent the reference values ​​for the q-axis current of the grid-side converter; c1, c2, c3, c4, r1, and r2 are mnemonic symbols; e represents the natural constant; and t represents time.

[0022] Further, the stator short-circuit current i s , the expression is:

[0023]

[0024] Wherein, B1, B2, B3, Indicate the mnemonic; e indicates the natural constant; j indicates the imaginary number; ω1 is the synchronous speed; t indicates the time; τ1 indicates the double-fed wind turbine stator transient flux linkage decay constant.

[0025] Further, in the three-phase stationary coordinate system, the three-phase short-circuit current of the double-fed wind turbine, taking phase A as an example, the expression is:

[0026] i DFIGa = i sa + i ga ;

[0027]

[0028] Wherein: i DFIGa is the double-fed wind turbine A-phase short-circuit current; i sa is the stator A-phase short-circuit current in the three-phase stationary coordinate system, i ga is the grid-side converter A-phase feeding current in the three-phase stationary coordinate system; e indicates the natural constant; j indicates the imaginary number; ω1 is the synchronous speed; t indicates the time; τ1 indicates the double-fed wind turbine stator transient flux linkage decay constant; A0, A1, A2, A3, A4, A5, A6, η1, η2, r1, r2, B1, B2, B3 indicate the mnemonic; Re() indicates the real part.

[0029] According to the second aspect of the present application, a double-fed wind turbine three-phase short-circuit current acquisition device considering the grid-side converter GSC feeding current is provided, which comprises a module for executing the double-fed wind turbine three-phase short-circuit current acquisition method considering the grid-side converter GSC feeding current as described in any one of the above.

[0030] According to the third aspect of the present application, a terminal device is provided, which comprises a memory, a processor, and a program stored in the memory and executable by the processor, and the processor executes the program to realize the double-fed wind turbine three-phase short-circuit current acquisition method considering the grid-side converter GSC feeding current as described in any one of the above.

[0031] According to a fourth aspect of the present application, there is provided a computer readable storage medium comprising a stored program, wherein the computer readable storage medium is caused to perform the method for obtaining three-phase short-circuit current of a doubly-fed wind turbine considering GSC outgoing current of a grid-side converter when the program is run.

[0032] The present application has the following advantages:

[0033] The present application is based on the disturbance of DC bus voltage caused by power imbalance between RSC and GSC when a fault occurs, thereby affecting the d-axis current command value of the voltage outer loop of the grid-side converter, and the GSC outgoing current can be obtained according to the command value. Further, the DFIG output current is obtained by adding the stator-side short-circuit current. According to the proposed short-circuit current calculation method, the transient characteristics of the DFIG can be more accurately analyzed, the short-circuit characteristic analysis of the DFIG is improved, and a basis for the fault characteristic analysis of the DFIG is provided. BRIEF DESCRIPTION OF DRAWINGS

[0034] Figure 1 Equivalent circuit of the doubly-fed wind turbine in dq coordinate system;

[0035] Figure 2 Structure of the doubly-fed wind turbine system;

[0036] Figure 3 Control block diagram of the GSC;

[0037] Figure 4 Control block diagram of the RSC;

[0038] Figure 5 Rotor q-axis current of the doubly-fed wind turbine;

[0039] Figure 6 Fluctuation of the DC bus voltage;

[0040] Figure 7 Outgoing current of the grid-side converter of the doubly-fed wind turbine in phase A;

[0041] Figure 8 Short-circuit current of the stator in phase A of the doubly-fed wind turbine;

[0042] Figure 9 Short-circuit current of the doubly-fed wind turbine in phase A. DETAILED DESCRIPTION

[0043] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. It should be noted that, unless otherwise specified, the embodiments and features in the embodiments of this application can be arbitrarily combined with each other.

[0044] Example 1: As Figures 1-9 As shown, according to a first aspect of the present invention, a method for obtaining the three-phase short-circuit current of a doubly-fed induction generator (DFIG) considering the feed current of the grid-side converter (GSC) is provided, comprising the following steps: when a three-phase short circuit occurs in the power grid, considering the control of the rotor-side converter (RSC), there is a power fluctuation between the rotor-side converter (RSC) and the grid-side converter (GSC), and the power fluctuation between the rotor-side converter (RSC) and the grid-side converter (GSC) is calculated; based on the power fluctuation between the rotor-side converter (RSC) and the grid-side converter (GSC), the DC bus voltage fluctuation between the rotor-side converter (RSC) and the grid-side converter (GSC) is calculated; based on the DC bus voltage fluctuation, a reference value of the grid-side converter current based on the d, q synchronous rotating coordinate system is obtained; wherein, the grid-side converter current based on the d, q synchronous rotating coordinate system is... The reference values ​​for the grid-side converter current include the reference values ​​for the d-axis current and q-axis current of the grid-side converter. Based on the reference values ​​for the d-axis and q-axis currents of the grid-side converter in the d- and q-synchronous rotating coordinate system, the grid-side converter feed-out current in the d- and q-synchronous rotating coordinate system is obtained. The grid-side converter feed-out current in the d- and q-synchronous rotating coordinate system includes the d- and q-axis components of the grid-side converter feed-out current. The stator short-circuit current and grid-side converter feed-out current in the d- and q-synchronous rotating coordinate system are converted into the stator three-phase short-circuit current and grid-side converter three-phase feed-out current in the three-phase stationary coordinate system. The stator three-phase short-circuit current in the three-phase stationary coordinate system is added to the grid-side converter three-phase feed-out current to obtain the doubly-fed induction generator (DFIG) three-phase short-circuit current, taking into account the grid-side converter GSC feed-out current.

[0045] Furthermore, when a three-phase short-circuit fault occurs in the power grid at time t = t1, the approximate power fluctuation value ΔP' between the rotor-side converter RSC and the grid-side converter GSC is obtained. This approximate power fluctuation value ΔP' is used as the power fluctuation value ΔP between the rotor-side converter RSC and the grid-side converter GSC, and its specific expression is as follows:

[0046]

[0047] Among them, the grid-side power P g ≈3kU s0 si rq, rotor-side power s is the slip, s = 1 - ω r / ω1, ω r denotes the rotor speed; ω1 is the synchronous speed; k is the degree of the terminal voltage drop after the grid fault; U s0 is the amplitude of the terminal voltage in steady state; i rq is the q-axis component of the rotor current; e denotes the natural constant; τ1 denotes the stator transient flux decay constant of the doubly-fed wind turbine, τ1 = R s / L s +jω1, L s = L ls +L m ; L s , L m are the equivalent stator inductance and mutual inductance, respectively; L ls is the stator leakage inductance of the doubly-fed machine, R s denotes the equivalent stator resistance.

[0048] Further, the DC bus voltage fluctuation ΔU dc between the rotor-side converter RSC and the grid-side converter GSC is solved according to the power fluctuation between the rotor-side converter RSC and the grid-side converter GSC, and the expression is:

[0049]

[0050] wherein C dc is the DC side capacitor; ω1 is the synchronous speed; U dc1 , U dc2 , U dc3 , U dc4 , m dc , η1, η2, denotes the mnemonic, σ is the leakage coefficient, L s , L r , L m are the equivalent stator inductance, rotor inductance and mutual inductance between the stator and rotor, respectively; β1 = (R p +k p ) / (σL r ); β2 = k i / (σL r ); k p is the rotor-side current inner loop proportional coefficient, k i is the rotor-side current inner loop integral coefficient; is the q-axis forced component of the rotor current; i rqm is the q-axis transient DC component of the rotor current; irq1 , i rq2 is the rotor current q-axis damping speed component; i r0 is the rotor current in steady state; R r is the rotor resistance.

[0051] Further, the grid-side converter d-axis current reference value based on the d, q synchronous rotating coordinate system is The expression is:

[0052]

[0053] Wherein, A0, A1, A2, A3, A4, η1, η2 represent mnemonics; e represents a natural constant; j represents an imaginary number; t represents time; τ1 represents a double-fed wind turbine stator transient flux linkage damping constant;

[0054] In the case of not taking into account the reactive power support of the grid-side converter GSC, the grid-side converter q-axis current reference value based on the d, q synchronous rotating coordinate system is 0.

[0055] The mnemonics involved in the above are explained as follows: A2 = (2k vp τ1-k vi )U dc2 / 2τ1; A 01 = 2U dc1 + U dc2 + 2U dc3 + 2U dc4 ; A 02 = 2U dc1 + U dc2 + 2U dc3 ; A 03 = 2U dc1 + U dc2 + 2U dc4 ; A 04 = 2U dc1 + U dc2 ; k vp represents the grid-side converter voltage outer loop proportional coefficient; k vi represents the grid-side converter voltage outer loop integral coefficient; U dc1 , U dc2 , U dc3 , U dc4 , represent mnemonics, and the expression is the same as the foregoing.

[0056] Further, the grid-side converter feed-out current based on the d, q synchronous rotating coordinate system has the expression:

[0057]

[0058] where i gd , i gq are the d, q axis components of the grid-side converter feeding current respectively; id* represents the d axis current reference of the grid-side converter, iq* represents the q axis current reference of the grid-side converter; c1, c2, c3, c4, r1, r2 represent the mnemonics;

[0059] The mnemonics involved in the above are explained as follows: m1 = (R g +k pg )ω1 / L g ; i gd0 , i gq0 are the d, q axis components of the grid-side converter feeding current respectively in steady state; ω1 is the synchronous speed; R g , L g are the resistance and inductance of the grid-side and machine-side respectively; k pg , k ig are the proportional and integral parameters of the inner current loop of the grid-side converter; u sq represents the q axis component of the stator voltage.

[0060] Further, the stator short-circuit current i s under the d, q synchronous rotating coordinate system is expressed as:

[0061]

[0062] where B1, B2, B3, represent the mnemonics; e represents the natural constant; j represents the imaginary number; t represents time; τ1 represents the transient stator flux decay constant of the doubly-fed wind turbine;

[0063] The mnemonics involved in the above are explained as follows: β1 = (R p +k p )L s / L D ; β2 = k i L s / L D ; L s = L ls + L m ; L r = L lr + L m ; L s , Lr , L m are the equivalent stator inductance, rotor inductance and mutual inductance between stator and rotor respectively; k is the voltage drop degree of the terminal voltage after the power grid fault; u s represents the stator voltage; represents the rotor current reference value; R r is the rotor resistance; k p is the rotor-side current inner loop proportional coefficient, k i is the rotor-side current inner loop integral coefficient; σ is the leakage coefficient, L ls is the stator leakage inductance of the doubly-fed motor; L lr is the rotor leakage inductance of the doubly-fed motor; τ1 represents the stator transient flux linkage decay constant, τ1 = R s / L s +jω1; R s represents the equivalent stator resistance.

[0064] Further, in the three-phase stationary coordinate system, the three-phase short-circuit current of the doubly-fed wind turbine is taken as an example, and the expression of the A-phase is as follows:

[0065] i DFIGa =i sa +i ga ;

[0066] wherein,

[0067] wherein, i DFIGa is the A-phase short-circuit current of the doubly-fed wind turbine; i sa are the stator A-phase short-circuit currents in the three-phase stationary coordinate system, i ga is the A-phase feeding current of the grid-side converter in the three-phase stationary coordinate system; A0, A1, A2, A3, A4, A5, A6, η1, η2, r1, r2, B1, B2, B3 represent mnemonics, and the expressions are as follows: A1 = (k vp τ1-k vi )U dc1 / τ1; A2 = (2k vp τ1-k vi )U dc2 / 2τ1; A 01 =2U dc1 +U dc2 +2U dc3 +2U dc4 ;A 02 =2U dc1 +U dc2 +2Udc3 ; A 03 = 2U dc1 + U dc2 + 2U dc4 ; A 04 = 2U dc1 + U dc2 ; τ1= R s / L s + jω1; k vp represents the grid-side converter voltage outer loop proportional coefficient; k vi represents the grid-side converter voltage outer loop integral coefficient; R r is the rotor resistance; L s , L r , L m are the equivalent stator inductance, rotor inductance and mutual inductance between stator and rotor, respectively; k is the degree of the terminal voltage drop amplitude after the power grid fault; k p is the proportional parameter of the rotor current inner loop PI controller.

[0068] According to a second aspect of the embodiments of the present application, there is provided an apparatus for obtaining three-phase short-circuit current of a doubly-fed wind turbine considering GSC outgoing current, comprising modules for performing the method for obtaining three-phase short-circuit current of a doubly-fed wind turbine considering GSC outgoing current as described in any of the above. Specifically, the apparatus comprises: a first module for, when three-phase short-circuit occurs in a power grid, solving power fluctuation between a rotor-side converter (RSC) and a grid-side converter (GSC) considering control of the RSC, in a case where power fluctuation exists between the RSC and the GSC; and solving DC bus voltage fluctuation between the RSC and the GSC according to the power fluctuation between the RSC and the GSC; a second module for obtaining reference values of GSC current in a d-q synchronous rotating coordinate system according to the DC bus voltage fluctuation; a third module for obtaining GSC outgoing current in the d-q synchronous rotating coordinate system according to the reference values of GSC d-axis and q-axis current in the d-q synchronous rotating coordinate system; and a fourth module for converting stator three-phase short-circuit current and GSC three-phase outgoing current in the d-q synchronous rotating coordinate system into stator three-phase short-circuit current and GSC three-phase outgoing current in a three-phase stationary coordinate system; and obtaining three-phase short-circuit current of a doubly-fed wind turbine considering GSC outgoing current by adding the stator three-phase short-circuit current and the GSC three-phase outgoing current in the three-phase stationary coordinate system. The term "module" as used in the above can be a combination of software and / or hardware that implements a predetermined function. Although the system described in the above embodiments is preferably implemented in software, implementation in hardware, or a combination of software and hardware, is also possible and contemplated. For parts not described in detail for each module, reference can be made to the relevant description of the embodiments.

[0069] According to a third aspect of the embodiments of the present application, there is provided a terminal device, comprising a memory, a processor, and a program stored in the memory and executable by the processor, wherein the processor implements the method for obtaining three-phase short-circuit current of a doubly-fed wind turbine considering GSC outgoing current as described in any of the above when executing the program.

[0070] According to a fourth aspect of the embodiments of the present application, a computer readable storage medium is provided, which includes a stored program, wherein the program, when executed, controls a device where the computer readable storage medium is located to perform the method for obtaining three-phase short-circuit current of a doubly-fed wind turbine considering GSC outgoing current of a grid-side converter. In an exemplary embodiment, the computer readable storage medium can include, but is not limited to, a U disk, a read-only memory (ROM), a random access memory (RAM), a mobile hard disk, a magnetic or optical disk, and various media that can store computer programs.

[0071] Obviously, those skilled in the art should understand that the modules or steps of the present application described above can be realized by general computing devices, which can be centralized on a single computing device or distributed on a network composed of multiple computing devices, and can be realized by program codes executable by the computing devices, so that they can be stored in storage devices and executed by the computing devices, and in some cases, the steps shown or described can be executed in different orders, or they can be manufactured into individual integrated circuit modules or a single integrated circuit module. Therefore, the present application is not limited to any specific combination of hardware and software.

[0072] Embodiment 2: In combination with experimental data, an alternative embodiment of the present application is described in detail as follows:

[0073] Taking a grid-connected doubly-fed wind turbine as an example, it is assumed that a three-phase short-circuit fault occurs in the power grid at 1.5s, and there is power fluctuation between the RSC and the GSC in the case of RSC control. The DC bus voltage fluctuation between the RSC and the GSC is solved according to the DC side power fluctuation. According to the DC bus voltage fluctuation, the reference values of the grid-side converter d, q-axis current can be obtained. According to the reference values of the grid-side converter d, q-axis current, the grid-side converter outgoing current d, q-axis components can be obtained. The stator short-circuit current in the stationary coordinate system is added to the grid-side converter outgoing current to obtain the three-phase short-circuit current of the doubly-fed wind turbine DFIG considering the GSC outgoing current of the grid-side converter. After the fault occurs in the power grid, the machine terminal voltage drops to a magnitude of k=0.22, and the specific parameters of the doubly-fed wind turbine are shown in Table 1:

[0074] Table 1 Specific parameters of the doubly-fed wind turbine

[0075]

[0076] S1, according to the specific parameters of the doubly-fed wind turbine in Table 1, the power fluctuation between the GSC and the RSC is calculated, and the expression is:

[0077]

[0078] where s = -0.2; k = 0.22; U s0 = 1; τ1= 0.0694; t1is the time of fault occurrence; t is time.

[0079] S2, according to the specific parameters of the doubly-fed wind turbine in Table 1, the voltage fluctuation between the GSC and the RSC is calculated, and the expression is:

[0080]

[0081] where, t1is the time of fault occurrence; t is time; is the forced component of the rotor current q-axis; i rqm is the transient direct current component of the rotor current q-axis; i rq1 , i rq2 is the decay speed component of the rotor current q-axis.

[0082] S3, according to the specific parameters of the doubly-fed wind turbine in Table 1, the d-axis current reference value of the grid-side converter is expressed as:

[0083]

[0084] where:

[0085] A0= 20.1686 A 0m ;

[0086] A 0m = 0.0048 A 01 + 45.5376 A 02 + 3.4128 A 03 + 2.4283 A 04 ;

[0087]

[0088] S4, according to the specific parameters of the doubly-fed wind turbine in Table 1, the expression of the grid-side converter output current is:

[0089]

[0090] where:

[0091] c1 = -0.2162 + j0.0102; c2 = -0.2162 - j0.0102; c3 = -0.03 + j0.0014; c4 = -0.03 - j0.0014; r1 = -1.4334 + j12.8260; r2 = -1.4334 - j12.8260.

[0092] S5, according to the specific parameters of the doubly-fed wind turbine in Table 1, the stator side three-phase current component, the expression is:

[0093]

[0094] Wherein:

[0095] B1 = -0.8474 + j0.3617; B2 = 0.0019 - j0.2174; B3 = 0.0448 + j0.0876; τ1 = 0.0694.

[0096] Based on the stator side three-phase current component, the stator short-circuit current i s = i sa + i sb + i sc .

[0097] S6, according to the specific parameters of the doubly-fed wind turbine in Table 1, taking the A-phase short-circuit current as an example, as shown in Figures 7-9 The expression of DFIG considering GSC feeding current in three-phase stationary coordinate system is:

[0098] i DFIGa = i sa + i ga ;

[0099] Wherein:

[0100]

[0101] B1 = -0.8474 + j0.3617; B2 = 0.0019 - j0.2174; B3 = 0.0448 + j0.0876;

[0102] A0 = 20.1686 A 0m ;

[0103] A 0m = 0.0048 A 01 + 45.5376 A 02 + 3.4128 A 03+2.4283A 04 ;

[0104]

[0105] c1 = -0.2162 + j0.0102; c2 = -0.2162 - j0.0102; c3 = -0.03 + j0.0014; c4 = -0.03 - j0.0014;

[0106] r1 = -1.4334 + j12.8260; r2 = -1.4334 - j12.8260.

[0107] Current research on the DFIG short-circuit current characteristics analysis can consider the influence of one or more factors on the short-circuit current. However, in actual calculation, the contribution of GSC feeding current to the short-circuit current is less considered. The present application analyzes the short-circuit current of the doubly-fed wind turbine in the scenario of slight grid voltage drop, analyzes the effect of converter (RSC and GSC) control respectively, and regards the short-circuit current as the sum of stator short-circuit current and GSC feeding current as shown in Figure 1 Therefore, the present application considers the three-phase short-circuit current characteristics of DFIG with GSC feeding current, firstly obtains the GSC feeding current and the stator side short-circuit current, and then obtains the three-phase short-circuit current of the doubly-fed wind turbine based on the above-mentioned obtained DFIG with GSC feeding current. Specifically, when the grid fails, the electromagnetic power output will change greatly, resulting in DC bus voltage disturbance, which will further affect the GSC feeding current. Therefore, the GSC feeding current solving process is as follows: when the grid is three-phase short-circuited, the power fluctuation exists between RSC and GSC under the condition of considering RSC control; the DC bus voltage fluctuation between RSC and GSC is solved from the DC side power fluctuation; the reference value of grid-side converter current can be obtained according to the DC bus voltage fluctuation; the grid-side converter feeding current can be obtained according to the d, q-axis current reference value. The stator short-circuit current in the stationary coordinate system considering RSC control and the feeding current of GSC are added, and the three-phase short-circuit current of the doubly-fed wind turbine considering GSC feeding current of DFIG is obtained. Through the present application, the transient characteristics of DFIG can be more accurately analyzed, the short-circuit characteristic analysis of DFIG is improved, and a basis for fault characteristic analysis of DFIG is provided. Further, the principle of the present application is explained as follows:

[0108] 1), the control of the grid-side converter and the calculation model of the stator current are explained as follows:

[0109] The control of the grid-side converter is specifically: the reactive power instruction value is set to zero, that is, the grid-side converter does not control the grid voltage, which is the main way of the operation of the grid-side converter. As Figure 3As shown, under its control mode, the influence of the grid-side converter on the generator rotor current is mainly transmitted through the DC bus voltage, and the degree of influence depends on the control performance of the grid-side converter on the DC voltage. In steady-state operation, the active power exchange of the two converters is balanced under the control of the grid-side converter, and the DC bus voltage is stable. When a short circuit occurs in the power grid, the invention operates in the super-synchronous state, and the voltage drop at the machine end causes the active power flowing out of the grid-side converter to decrease, while the overcurrent and overvoltage in the rotor winding cause the active power flowing out of the rotor-side converter to increase, resulting in a rapid increase in the DC voltage at the moment of short circuit. The more serious the power grid fault, the greater the difference between the two powers, and the greater the maximum value of the DC voltage. In the case of sufficient capacity of the grid-side converter, due to the increase in the voltage deviation of the DC voltage control loop, the power flowing out of the grid-side converter will also increase, so that the DC voltage gradually decreases under the control of the grid-side converter and eventually stabilizes around the DC voltage command value. As can be seen, when a three-phase short circuit occurs in the power grid, considering the control of the rotor-side converter RSC, there is power fluctuation between the rotor-side converter RSC and the grid-side converter GSC.

[0110] The calculation model of the stator short-circuit current in the d, q synchronous rotating coordinate system is as follows:

[0111] When the rotor impact voltage caused by a slight drop in the machine end voltage of the doubly-fed wind power generator during a power grid fault is not enough to trigger the crowbar protection action, the rotor-side converter can remain connected to the rotor winding and rely on the adjustment of the rotor-side converter output voltage to suppress the rotor impact current as shown in Figure 2 The stator short-circuit current of the doubly-fed wind turbine is mainly determined by the stator flux linkage and the rotor current. In steady-state operation, the synchronous rotating magnetic motive force formed by the rotor excitation current of the doubly-fed induction generator generates an induced electromotive force in the stator winding. At the moment of short circuit, due to the conservation of magnetic flux, the stator induced electromotive force remains constant while the machine end voltage drops, forming a voltage difference between the stator induced electromotive force and the machine end voltage, thereby generating a periodic short-circuit current component. During the power grid short circuit, the stator DC flux linkage causes overcurrent in the rotor winding, and when the rotor transient current is controllable, under the lagging correction of the rotor-side converter, the rotor overcurrent continuously decays and reattains stability. When the closed-loop bandwidth of the controller is large enough, the AC side voltage of the rotor-side converter can well track the command value, so the periodic component is considered constant. The stator DC flux linkage generates a stationary magnetic field in the air gap, and the rotation of the rotor cuts this magnetic field to generate a transient DC current in the rotor winding.

[0112] According to the stator short-circuit current provided by the stator side and the feeding current provided by the grid-side converter, the double-fed wind turbine short-circuit current considering the GSC feeding current is obtained. According to the power characteristics of the DFIG, the active power exchanged with the power grid under normal operation of the grid-side converter is considered as the slip times of the stator active power fluctuation, and the GSC current will also fluctuate in the fault, so if the influence of the GSC current is ignored, it will inevitably cause errors in the calculation result of the DFIG short-circuit current. Therefore, the method for obtaining the three-phase short-circuit current of the double-fed wind turbine considering the GSC feeding current of the grid-side converter is more accurate in the calculation result of the DFIG short-circuit current.

[0113] 2) Establish the mathematical model of the DFIG in the dq synchronous rotating coordinate system

[0114] According to the equivalent circuit diagram of the double-fed wind turbine (see the attached Figure 1 , the mathematical model of the DFIG in the dq coordinate system is obtained as follows:

[0115]

[0116] Wherein, u s , u r are the stator and rotor voltages in the dq synchronous rotating coordinate system; i s , i r are the stator and rotor currents in the dq synchronous rotating coordinate system; ψ s , ψ r are the stator and rotor fluxes in the dq synchronous rotating coordinate system; R s , R r are the stator and rotor resistances; ω1 is the synchronous speed, ω r is the slip angular velocity, and ω r is the rotor angular velocity; P is the differential operator; L s , L r , L m are the equivalent stator inductance, rotor inductance and mutual inductance between the stator and the rotor.

[0117] Since the rotor converter of the double-fed induction generator adopts stator flux-oriented vector control, the stator voltage dq axis components u sd = 0, u sq = U s , and the steady-state operation of the stator and rotor fluxes can be obtained as follows:

[0118]

[0119] Wherein, U s is the grid voltage amplitude; ω1 is the synchronous speed; ψ sd , ψ sq are the stator flux d and q axis components.

[0120] Since L m >> L ls ; L m >> L lr , it is approximately considered that L m = L s ; L m = L r , the rotor voltage and grid-side current in steady state can be expressed as:

[0121]

[0122] where i rq is the rotor current q-axis component; ψ s is the stator flux linkage; i gq is the grid-side current q-axis component; s is the slip.

[0123] The expression of stator flux linkage under three-phase short-circuit fault at the machine terminal is:

[0124]

[0125] where ψ' s is the stator flux linkage after short-circuit; the short-circuit fault occurs at t=t1; k is the voltage drop amplitude degree at the machine terminal after the power grid fault; τ1 represents the stator transient flux linkage decay constant, τ1=R s / L s +jω1.

[0126] 3), the specific process of solving the rotor short-circuit current of the rotor-side converter in the two-phase synchronous rotating coordinate system in the full-current mode is as follows:

[0127] According to the control principle of the rotor-side converter RSC as shown in Figure 4 , the rotor voltage u r equation in the dq rotating coordinate system can be obtained:

[0128]

[0129] where i is the rotor current reference value; i r is the rotor current; k p is the rotor-side current inner loop proportional coefficient, k i is the rotor-side current inner loop integral coefficient; σ is the leakage coefficient.

[0130] By combining equation (1) and equation (7), the second-order differential equation of the rotor current can be obtained:

[0131]

[0132] where β1=R r +kp ) / (σL D );β2=k i / (σL D );m r =L m τ1(1-s)U s0 / (σL r L s );U s0 is the amplitude of the terminal voltage in steady state.

[0133] Solving the second-order differential equation of rotor current (8) can obtain:

[0134]

[0135] where, L s =L ls +L m ;L r =L lr +L m ;L ls , L lr are the stator and rotor leakage inductance of the doubly-fed machine; i rm is the transient DC component of rotor current; i r1 , i r2 are the rotor current decay speed components; i r0 is the initial value of rotor current.

[0136] 4) The specific process for solving the power disturbance is as follows:

[0137] Neglecting the loss of the reactor and the loss of the switching power device, the input power P g of the grid-side converter and the input power P r of the rotor-side converter are as follows:

[0138] P g ≈3kU s0 si rq (10)

[0139]

[0140] At t=t1, the short-circuit fault occurs. According to the DC bus voltage balance formula , the power fluctuation at the DC side is as follows:

[0141]

[0142] where, the q-axis component of rotor current i

[0143] ​5), the specific process for solving the DC bus voltage fluctuation is:

[0144] Since the grid-side converter adopts the grid voltage oriented vector control strategy, the static gain G of the DC bus voltage pedc (jω1) can be approximately expressed as:

[0145]

[0146] where C dc is the DC side capacitor; ω1 is the synchronous speed.

[0147] From equation (12) and equation (13), the DC bus voltage fluctuation is:

[0148]

[0149] where,

[0150] The DC bus voltage fluctuation adopts Laplace transform, and the corresponding Laplace equation is:

[0151]

[0152] 6), solve the grid-side converter dq synchronous rotating coordinate system under the output current command value:

[0153] The grid-side converter reference value d-axis component Laplace equation is:

[0154]

[0155] where k vp represents the grid-side converter voltage outer loop proportional coefficient; k vi represents the grid-side converter voltage outer loop integral coefficient;

[0156] In the case of considering that the GSC does not provide reactive power support, the grid-side converter q-axis reference current based on the d, q synchronous rotating coordinate system is 0.

[0157] The circuit equation of the GSC current inner loop in the two-phase synchronous rotating coordinate system is:

[0158]

[0159] where u gd , u gq are the grid-side voltage d, q-axis components respectively; U sd , U sq are the stator voltage d, q-axis components respectively; R g , L g are the grid-side and machine-side resistances, grid-side and machine-side inductances respectively; igd , i gq are the grid-side input current d, q-axis components respectively; ω1 is the synchronous speed.

[0160] The basic control equation of the grid-side converter current inner loop is:

[0161]

[0162] In the formula, are the grid-side current reference values d, q-axis components respectively; k pg , k ig are the proportional integral parameters of the grid-side converter current inner loop.

[0163] The two sides of the voltage equation (17) are differentiated simultaneously to obtain:

[0164]

[0165] The two sides of the basic control equation (18) are differentiated simultaneously to obtain:

[0166]

[0167] The second-order differential equation of the grid-side converter output current is obtained by combining equation (19) and equation (20):

[0168]

[0169] Wherein, m1=(R g +k pg )ω1 / L g ;

[0170] The analytic expression of the grid-side converter output current in the dq synchronous rotating coordinate system is solved, and the analytic expression of the grid-side output current is the sum of the general solution of the homogeneous equation corresponding to the second-order differential equation of the grid-side current and the particular solution of the second-order differential equation of the grid-side current, that is:

[0171]

[0172] Wherein,

[0173] The d-axis component and the q-axis component of the GSC output current are substituted into the GSC short-circuit current to obtain the GSC output current in the dq rotating coordinate system:

[0174]

[0175] 7), solve the stator-side short-circuit current:

[0176] From the DFIG mathematical model in dq synchronous rotating coordinate system, the stator flux linkage equation in equation (2) is transformed into the stator short-circuit current equation:

[0177]

[0178] where ψ s is the stator flux linkage, ψ s = L s i s + L m i r ; L s = L ls + L m ; L s , L m are the equivalent stator inductance and the mutual inductance between stator and rotor, respectively; L ls is the stator leakage inductance of the DFIG; and i r is the rotor current.

[0179] Substituting the stator flux linkage equation (2) and the rotor current equation (9) into the stator short-circuit current equation, the following equation is obtained:

[0180]

[0181] where,

[0182] 8) The output current of the DFIG in three-phase stationary coordinate system considering the GSC feeding current is:

[0183] The stator short-circuit current in d, q synchronous rotating coordinate system is converted to three-phase stationary coordinate system as follows:

[0184]

[0185] where i sa , i sb , i sc are the currents of phase A, B and C in three-phase stationary coordinate system, respectively; and Re is the real part symbol.

[0186]

[0187] The GSC feeding current equation (23) in dq rotating coordinate system is converted to three-phase stationary coordinate system as follows:

[0188]

[0189] where i ga , i gb , i gcrespectively are the grid-side converter A, B, C phase currents in three-phase stationary coordinate system; Re is the symbol of taking real part;

[0190]

[0191] wherein, A1 = (k vp τ1-k vi )U dc1 / τ1; A2 = (2k vp τ1-k vi )U dc2 / 2τ1; A 01 = 2U dc1 + U dc2 + 2U dc3 + 2U dc4 ; A 02 = 2U dc1 + U dc2 + 2U dc3 ; A 03 = 2U dc1 + U dc2 + 2U dc4 ; A 04 = 2U dc1 + U dc2 ; τ1 = R s / L s + jω1; k vp represents the grid-side converter voltage outer loop proportional coefficient; k vi represents the grid-side converter voltage outer loop integral coefficient; R r is the rotor resistance; L s , L r , L m are the equivalent stator inductance, rotor inductance and mutual inductance between stator and rotor respectively; k is the degree of the terminal voltage drop amplitude after the grid fault; k p is the proportional parameter of the rotor current inner loop PI controller.

[0192] In the three-phase stationary coordinate system, taking phase a as an example, as identified, Figures 7-9 the three-phase short-circuit current of the doubly-fed wind turbine considering the GSC feeding current is:

[0193] i DFIGa = i sa + i ga (30)

[0194] wherein,

[0195] The application can know that when a mild voltage drop occurs in a power grid, a double-fed wind turbine usually relies on the adjustment of the output voltage of a rotor converter to suppress rotor impact current in order to cope with low voltage ride through (LVRT). Power exchange imbalance between the RSC and the GSC causes disturbance of the DC bus voltage, making the transient process of the DFIG more complex. Therefore, the application designs an analytical method of three-phase short-circuit current of the double-fed wind turbine considering the feeding current of the grid-side converter. The application can more accurately analyze the transient characteristics of the DFIG and perfect the short-circuit characteristic analysis of the DFIG, thereby providing a basis and reference for the fault characteristic analysis of the DFIG.

[0196] The specific embodiments of the application are described in detail above with reference to the accompanying drawings, but the application is not limited to the above-described embodiments, and various changes can be made within the knowledge of those skilled in the art without departing from the purpose of the application.

Claims

1. A method for obtaining the three-phase short-circuit current of a doubly-fed induction generator (DFIG) considering the feeder current of the grid-side converter (GSC), characterized in that, Includes the following steps: When a three-phase short circuit occurs in the power grid, considering the control of the rotor-side converter RSC, there is a power fluctuation between the rotor-side converter RSC and the grid-side converter GSC. Solve for the power fluctuation between the rotor-side converter RSC and the grid-side converter GSC; based on the power fluctuation between the rotor-side converter RSC and the grid-side converter GSC, solve for the DC bus voltage fluctuation between the rotor-side converter RSC and the grid-side converter GSC. Based on the DC bus voltage fluctuation, a reference value for the grid-side converter current in a synchronous rotating coordinate system of d and q is obtained; wherein, the reference value for the grid-side converter current in a synchronous rotating coordinate system of d and q includes a reference value for the d-axis current and a reference value for the q-axis current of the grid-side converter. Based on the reference values ​​of the d-axis and q-axis currents of the grid-side converter in the d-q synchronous rotating coordinate system, the feed current of the grid-side converter in the d-q synchronous rotating coordinate system is obtained; wherein, the feed current of the grid-side converter in the d-q synchronous rotating coordinate system includes the d-axis and q-axis components of the grid-side converter feed current. The stator short-circuit current and grid-side converter feed-out current in the d-q synchronous rotating coordinate system are transformed into the stator three-phase short-circuit current and grid-side converter three-phase feed-out current in the three-phase stationary coordinate system. The stator three-phase short-circuit current in the three-phase stationary coordinate system is added to the grid-side converter three-phase feed-out current to obtain the doubly-fed induction generator (DFIG) three-phase short-circuit current taking into account the grid-side converter GSC feed-out current.

2. The method for obtaining the three-phase short-circuit current of a doubly-fed wind turbine, taking into account the feeder current of the grid-side converter (GSC), as described in claim 1, is characterized in that... When a three-phase short-circuit fault occurs in the power grid at time t = t1, the approximate power fluctuation value ΔP' between the rotor-side converter RSC and the grid-side converter GSC is obtained. This approximate power fluctuation value ΔP' is taken as the power fluctuation ΔP between the rotor-side converter RSC and the grid-side converter GSC, and the specific expression is as follows: Among them, P g P represents grid-side power. r Represents rotor-side power; s is slip; k is the magnitude of voltage drop at the generator terminals after a grid fault; U s0 i represents the amplitude of the terminal voltage during steady-state operation. rq τ represents the q-axis component of the rotor current; e represents the natural constant; τ1 represents the stator transient flux decay constant of the doubly-fed fan.

3. The method for obtaining the three-phase short-circuit current of a doubly-fed wind turbine, taking into account the feeder current of the grid-side converter (GSC), as described in claim 1, is characterized in that... The DC bus voltage fluctuation ΔU between the rotor-side converter RSC and the grid-side converter GSC is calculated based on the power fluctuation ΔP between them. dc The expression is: Among them, C dc ω1 is the DC-side capacitor; ω1 is the synchronous speed; s is the slip; k is the magnitude of the voltage drop at the generator terminals after a grid fault; U s0 i represents the amplitude of the terminal voltage during steady-state operation. rq U represents the q-axis component of the rotor current; e represents the natural constant; τ1 represents the stator transient flux decay constant of the doubly-fed induction generator; t1 represents the moment when a three-phase short-circuit fault occurs; U dc1 U dc2 U dc3 U dc4 η1 and η2 represent mnemonics, and t represents time.

4. The method for obtaining the three-phase short-circuit current of a doubly-fed wind turbine, taking into account the feeder current of the grid-side converter (GSC), as described in claim 1, is characterized in that... The reference value of the d-axis current of the grid-side converter based on the d and q synchronous rotating coordinate system The expression is: Where A0, A1, A2, A3, A4, η1, and η2 represent mnemonics; e represents the natural constant; j represents the imaginary number; ω1 is the synchronous speed; t represents time; and τ1 represents the stator transient flux decay constant of the doubly-fed wind turbine. Without considering the reactive power support of the grid-side converter GSC, the reference value of the q-axis current of the grid-side converter based on the d-q synchronous rotating coordinate system. It is 0.

5. The method for obtaining the three-phase short-circuit current of a doubly-fed wind turbine, taking into account the feeder current of the grid-side converter (GSC), as described in claim 1, is characterized in that... The expression for the grid-side converter feed-out current based on the d and q synchronous rotating coordinate system is: Among them, i gd i gq These are the d-axis and q-axis components of the grid-side converter feedout current, respectively. This indicates the reference value of the d-axis current of the grid-side converter. The values ​​represent the reference values ​​for the q-axis current of the grid-side converter; c1, c2, c3, c4, r1, and r2 are mnemonic symbols; e represents the natural constant; and t represents time.

6. The method for obtaining the three-phase short-circuit current of a doubly-fed wind turbine, taking into account the feeder current of the grid-side converter (GSC), as described in claim 1, is characterized in that... The stator short-circuit current i based on the d and q synchronous rotating coordinate system s The expression is: Among them, B1, B2, B3, represents the mnemonic; e represents the natural constant; j represents the imaginary number; ω1 is the synchronous speed; t represents time; τ1 represents the stator transient flux decay constant of the doubly-fed wind turbine.

7. The method for obtaining the three-phase short-circuit current of a doubly-fed wind turbine, taking into account the feeder current of the grid-side converter (GSC), as described in claim 1, is characterized in that... In a three-phase stationary coordinate system, the three-phase short-circuit current of a doubly-fed induction generator (DFIG) is expressed as follows, taking phase A as an example: i DFIGa =i sa +i ga ; Where: i DFIGa This refers to the A-phase short-circuit current of the doubly-fed induction generator; i sa Let i be the stator phase A short-circuit current in a three-phase stationary coordinate system. ga Let A be the feed current of phase A of the grid-side converter in a three-phase stationary coordinate system; e represents the natural constant; j represents the imaginary number; ω1 is the synchronous speed; t represents time; τ1 represents the stator transient flux decay constant of the doubly-fed induction generator; A0, A1, A2, A3, A4, A5, A6, η1, η2. r1, r2, B1, B2, B3 represent mnemonics; Re() represents taking the real part.

8. A device for obtaining the three-phase short-circuit current of a doubly-fed wind turbine, taking into account the feeder current of the grid-side converter (GSC), characterized in that, Includes a module for performing the method for obtaining the three-phase short-circuit current of a doubly-fed wind turbine, taking into account the feed current of the grid-side converter GSC, as described in any one of claims 1-7.

9. A terminal device, characterized in that: The method includes a memory, a processor, and a program stored in the memory and executable by the processor. When the processor executes the program, it implements the method for obtaining the three-phase short-circuit current of a doubly-fed wind turbine, taking into account the feed current of the grid-side converter (GSC), as described in any one of claims 1-7.

10. A computer-readable storage medium, characterized in that: The computer-readable storage medium includes a stored program, wherein, when the program is executed, it controls the device containing the computer-readable storage medium to perform the method for obtaining the three-phase short-circuit current of a doubly-fed wind turbine, taking into account the feed current of the grid-side converter GSC, as described in any one of claims 1-7.

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