Calculation method and system for crowbar action boundary of doubly-fed wind generator

Abstract

The invention discloses a calculation method and system for a crowbar action boundary of a doubly-fed wind generator. The calculation method comprises the following steps: establishing a short-circuit current response expression of a doubly-fed wind generator set under continuous excitation control of a rotor; based on the short-circuit current response expression of the doubly-fed wind turbine generator, a rotor current dynamic analytic solution under the synchronous rotating coordinate system is obtained through simultaneous establishment of a flux linkage equation, a voltage equation and a current loop control equation; according to a rotor current dynamic analytic solution in a synchronous rotating coordinate system, determining a quantitative relation between a transient component attenuation coefficient and a current inner loop control parameter; based on the quantitative relation between the transient component attenuation coefficient and the current inner loop control parameter, obtaining a relational expression of a crowbar action boundary about the voltage drop degree and the slip rate; and if the crowbar action of the doubly-fed wind turbine generator is located above the crowbar action boundary, crowbar action is performed after the fault. The problems that in the doubly-fed wind power plant low-voltage ride-through process, the crowbar protection action boundary is not clear, and the accuracy of a field group grouping equivalent model is insufficient are solved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of power system relay protection short-circuit current calculation, and particularly relates to a calculation method and system for the action boundary of a double-fed wind generator crowbar. BACKGROUND

[0002] With the proposal of the "carbon peak and carbon neutral" goal, building a new type of power system has become a national strategy, and high proportion of new energy, high proportion of power electronic equipment and ultra / extra-high voltage long distance transmission will become the main features of China's new power system. By the end of 2024, the total installed capacity of new energy in China has exceeded 145 million kilowatts, accounting for 43.4% of the total installed capacity. Compared with traditional synchronous generators, new energy units access the grid through power electronic equipment, which is more flexible in control, but has lower inertia and weaker tolerance to voltage and frequency deviation. To meet the requirements of the "Technical Regulation for Access of Wind Farms to Power Systems" for non-withdrawal operation during faults, new energy units are designed with high / low voltage ride-through, crowbar and other control strategies, and are switched according to the operating state. Under the action of complex fault ride-through control strategies and different control parameters, the short-circuit current characteristics of wind turbines are quite different from those of traditional synchronous generators, which may degrade the performance of current power system protection devices and affect the safe and stable operation of the system.

[0003] The existing short-circuit current calculation method does not fully consider the dynamic process of wind turbine faults. The dynamic process of double-fed wind turbines DFIG after a fault includes the non-reactive stage of the converter, the stage of crowbar input-asynchronous machine, and the stage of crowbar exit-rsc control. Most existing inventions only calculate for a certain stage, making it difficult to accurately reflect the entire dynamic process of the fault of the field group. Therefore, it is essential to analyze the influence of low voltage ride-through and crowbar control during faults on short-circuit current and to establish an accurate wind farm model for the analysis of new power systems. SUMMARY

[0004] In view of the problems of unclear action boundary of the crowbar protection during the low voltage ride-through process of the double-fed wind farm and insufficient accuracy of the field group equivalent model, the present application provides a calculation method for the action boundary of a double-fed wind generator crowbar, comprising:

[0005] establishing a short-circuit current response expression of a double-fed wind turbine under continuous excitation control of the rotor;

[0006] Based on the short-circuit current response expression of the double-fed wind turbine, the dynamic analytical solution of the rotor current in the synchronous rotating coordinate system is obtained by simultaneously solving the flux linkage equation, the voltage equation and the current loop control equation;

[0007] According to the dynamic analytical solution of the rotor current in the synchronous rotating coordinate system, the quantitative relationship between the transient component attenuation coefficient and the current inner loop control parameter is determined.

[0008] Based on the quantitative relationship between the transient component attenuation coefficient and the current inner loop control parameter, a relationship between the crowbar action demarcation line and the voltage drop degree and the slip rate is obtained; if the crowbar action of the doubly-fed wind turbine is located above the crowbar action demarcation line, the crowbar action after the fault occurs.

[0009] Further, the short-circuit current response expression of the doubly-fed wind turbine under the continuous excitation control of the rotor is established, including:

[0010] The flux linkage equation expression of the doubly-fed wind turbine on the dq synchronous rotating coordinate axis is:

[0011]

[0012] In the formula, ψ s , ψ r are the stator and rotor flux linkage vectors of the doubly-fed wind turbine; i s , i r are the stator and rotor current vectors; L s , L r are the full inductances of the stator and rotor windings of the doubly-fed wind turbine.

[0013] The voltage equation expression of the doubly-fed wind turbine on the dq synchronous rotating coordinate axis is:

[0014]

[0015] In the formula, U s , U r are the stator and rotor current and voltage vectors of the doubly-fed wind turbine.

[0016] Further, based on the short-circuit current response expression of the doubly-fed wind turbine, the rotor current dynamic analytical solution under the synchronous rotating coordinate system is obtained by simultaneously solving the flux linkage equation, the voltage equation and the current loop control equation, including:

[0017] The current loop control equation of the rotor side of the doubly-fed wind turbine is written in the vector form, specifically:

[0018]

[0019] In the formula, ω p = sω s is the angular frequency of the rotor current before the coordinate transformation.

[0020] The stator flux linkage after the fault occurs is analyzed, and according to the voltage flux linkage equation of the doubly-fed wind turbine given by formula (1) and (2), the stator and rotor flux linkages are rewritten as:

[0021]

[0022] wherein is the generator leakage coefficient, and k s = L m / L s is the inductance coupling coefficient of the stator winding;

[0023] Substituting equation (4) into equation (1), a first-order differential equation of the stator voltage with respect to the stator flux linkage is solved, and an expression of the stator flux linkage of the DFIG when the stator terminal voltage suddenly changes is shown as equation (5), t0 is the time when the fault starts,

[0024]

[0025] It can be seen from equation (5) that after the three-phase short-circuit fault of the power grid occurs, according to the principle of flux linkage conservation, a direct-current decay component of the stator flux linkage is generated, and the decay coefficient is τ sn The steady-state value of the flux linkage is related to the stator voltage after the fault;

[0026] Equations (2), (3) and (5) are combined and a first-order derivative is taken simultaneously to obtain a dynamic equation of the rotor current under the symmetrical fault:

[0027]

[0028] wherein

[0029]

[0030] It can be seen from equation (6) that the rotor loop of the three-phase short-circuit of the power grid under the continuous excitation control strategy of the converter is a second-order dynamic circuit, and the analytical solution of the rotor current in the synchronous rotating coordinate system can be obtained by solving the non-homogeneous second-order differential equation as

[0031]

[0032] It can be seen from equation (7) that the rotor current of the DFIG in the synchronous rotating coordinate system during the fault contains a steady-state component with a given value i rref of the rotor current, a power frequency oscillation component with a decay coefficient τ sn , and harmonic components with decay coefficients p1 and p2, and m1, m2 and m3 are respectively amplitude coefficients of the two harmonic components and the power frequency oscillation component;

[0033] The decay coefficients p1 and p2 are respectively the reciprocals of the two roots of the characteristic equation of equation (6), as shown in equation (8):

[0034]

[0036] Further, according to the dynamic analytical solution of the rotor current in the synchronous rotating coordinate system, a quantitative relationship between the transient component attenuation coefficient and the current inner loop control parameter is determined, including:

[0037] At the moment of fault occurrence, the rotor current cannot be abruptly changed, and the initial value of the rotor current change rate will change due to the stator terminal voltage and the rotor voltage. According to the impulse function matching method, the coefficients of the transient components can be solved as:

[0038]

[0039] Combined with equations (5), (7), the expression of the DFIG stator current during the fault can be obtained as:

[0040]

[0041] Where i sf is the steady-state value of the stator current, which is generally determined by the control strategy. The coefficients m4, m5, and m6 of the transient attenuation component of the stator current are expressed as follows:

[0042]

[0043] According to equation (11), the transient response of the stator current in the stationary coordinate system is also composed of four parts, which are the steady-state component, the transient DC component with an attenuation coefficient τ sn , and the transient harmonic component with attenuation coefficients p1 and p2. According to equation (8), the attenuation coefficients p1 and p2 depend on the current inner loop control parameters of the rotor converter. When the proportional coefficient k p increases, the real parts of p1 and p2 increase, and the attenuation speed of the free component accelerates. When the integral coefficient increases, the real parts of p1 and p2 decrease, and the attenuation speed of the free component slows down. According to equation (5), the attenuation coefficient τ sn is only related to the impedance parameters of the motor.

[0044] The transient component with an attenuation coefficient τ sn is affected by the system control parameters. According to equation (11), the size of m6 depends on m3. According to equation (9), the influence of different current inner loop control parameters on the size of m3 can be obtained.

[0045] Further, based on the quantitative relationship between the transient component attenuation coefficient and the current inner loop control parameter, a relationship formula of the crowbar action dividing line with respect to the voltage drop degree and the slip rate is obtained, including:

[0046] By combining equations (5), (6), (7), (8), and (9), a relationship formula of the crowbar action dividing line with respect to the voltage drop degree k and the slip rate s can be obtained as:

[0047]

[0048] wherein

[0049]

[0050] In formula (13), the synchronous speed of the doubly-fed wind turbine ω s is 50 Hz, the rotor speed of the doubly-fed wind turbine ω r ranges from 10 Hz to 100 Hz, and the slip s ranges from 0.8 to -1.

[0051] The demarcation line of the crowbar action can be obtained according to the relationship between the voltage drop degree k and the slip s.

[0052] The application also provides a calculation system for the demarcation line of the crowbar action of a doubly-fed wind turbine, comprising:

[0053] A response expression establishment module is configured to establish a short-circuit current response expression of the doubly-fed wind turbine under continuous excitation control of the rotor;

[0054] A current dynamic analysis module is configured to obtain a dynamic analytical solution of the rotor current in the synchronous rotating coordinate system by simultaneously solving a flux linkage equation, a voltage equation and a current loop control equation based on the short-circuit current response expression of the doubly-fed wind turbine;

[0055] A quantitative relationship determination module is configured to determine a quantitative relationship between a transient component attenuation coefficient and a current inner loop control parameter according to the dynamic analytical solution of the rotor current in the synchronous rotating coordinate system;

[0056] A crowbar action module is configured to obtain a relationship formula of the demarcation line of the crowbar action with respect to the voltage drop degree and the slip based on the quantitative relationship between the transient component attenuation coefficient and the current inner loop control parameter; and if the crowbar action of the doubly-fed wind turbine is located above the demarcation line of the crowbar action, the crowbar action is performed after a fault.

[0057] Further, the response expression establishment module comprises:

[0058] A flux linkage equation submodule is configured to express the flux linkage equations of the doubly-fed wind turbine on the stator side and the rotor side in the dq synchronous rotating coordinate system as follows:

[0059]

[0060] In the formula, ψ s and ψ r are the stator flux linkage vector and the rotor flux linkage vector of the doubly-fed wind turbine; i s and i r are the stator current vector and the rotor current vector; L s and L r are the total inductance of the stator winding and the rotor winding of the doubly-fed wind turbine.

[0061] a voltage equation sub-module for expressing the stator-side and rotor-side voltage equations of the doubly-fed wind power generator in dq synchronous rotating coordinate axes as:

[0062]

[0063] wherein U s , U r are the stator current and voltage vectors of the doubly-fed wind power generator respectively.

[0064] Further, the current dynamic analysis module comprises:

[0065] a vector equation sub-module for writing the rotor-side current loop control equation of the doubly-fed wind power generator in vector form, specifically:

[0066]

[0067] wherein ω p = sω s is the angular frequency of the rotor current before coordinate transformation;

[0068] a stator-rotor flux rewriting sub-module for analyzing the stator flux after the fault occurs, rewriting the stator flux and rotor flux according to the voltage flux equations of the doubly-fed wind power generator given by equations (1) and (2) as:

[0069]

[0070] wherein is the leakage flux coefficient of the generator, and k s = L m / L s is the inductance coupling coefficient of the stator winding;

[0071] a differential equation solving sub-module for substituting equation (4) into equation (1) to solve the first-order differential equation of the stator voltage with respect to the stator flux, so as to obtain the expression of the stator flux of the DFIG when the stator terminal voltage suddenly changes, as shown in equation (5), wherein t0 is the starting time of the fault,

[0072]

[0073] According to the flux conservation principle, the stator flux has a direct-current decay component after the three-phase short-circuit fault of the power grid, and the decay coefficient is τ sn , and the steady-state value of the flux is related to the stator voltage after the fault;

[0074] a current dynamic equation sub-module for simultaneously taking the first-order derivative of equations (2), (3) and (5) to obtain the rotor current dynamic equation under the symmetrical fault:

[0075]

[0076] wherein

[0077]

[0078] Rotor current solution sub-module, for the rotor circuit of the three-phase short-circuit of the power grid under the continuous excitation control strategy of the converter is a second-order dynamic circuit, and the analytical solution of the rotor current in the synchronous rotating coordinate system can be obtained by solving the non-homogeneous second-order differential equation as

[0079]

[0080] As can be seen from equation (7), the rotor current of the DFIG in the synchronous rotating coordinate system during the fault contains a steady-state component with a given value of i rref , a power frequency oscillation component with a damping coefficient of τ sn , and harmonic components with damping coefficients of p1 and p2, and m1, m2, and m3 are respectively the amplitude coefficients of the two harmonic components and the power frequency oscillation component.

[0081] The damping coefficients p1 and p2 are respectively the reciprocals of the two roots of the characteristic equation of equation (6), as shown in equation (8):

[0082]

[0083] Further, the quantitative relationship determination module includes:

[0084] Transient component coefficient solving sub-module, for at the moment of the fault, the rotor current cannot change abruptly, and the initial value of the rotor current rate of change will change due to the stator terminal voltage and the rotor voltage, and according to the impulse function matching method, the coefficients of the transient components can be solved as:

[0085]

[0086] Combining equations (5) and (7), the expression of the stator current of the DFIG during the fault can be obtained as:

[0087]

[0088] where i sf is the steady-state value of the stator current during the fault, which is generally determined by the control strategy. The expressions of the coefficients m4, m5, and m6 of the transient damping component of the stator current are as follows:

[0089]

[0090] The first quantitative relationship submodule is used to determine that the transient response of the stator current in the stationary coordinate system is also composed of four parts according to equation (11), which are respectively a steady-state component, a damping coefficient of τ sntransient component, the transient harmonic component with attenuation coefficients p1 and p2; according to formula (8), the attenuation coefficients p1, p2 depend on the current inner loop control parameters of the rotor converter, when the proportional coefficient k p increases, the real parts of p1 and p2 increase, and the attenuation speed of the free component accelerates; when the integral coefficient increases, the real parts of p1 and p2 decrease, and the attenuation speed of the free component slows down; according to formula (5), the attenuation coefficient τ sn is only related to the motor impedance parameters;

[0091] The second quantitative relationship submodule is used for the amplitude m6 of the transient component with the attenuation coefficient τ sn is affected by the system control parameters, and according to formula (11), the size of m6 depends on m3; according to formula (9), the influence law of different current inner loop control parameters on the size of m3 can be obtained.

[0092] Further, the crowbar action module comprises:

[0093] The relationship formula determination submodule is used for simultaneously solving formula (5), (6), (7), (8) and (9), so that the relationship formula of the crowbar action demarcation line about the voltage drop degree k and the slip s can be obtained.

[0094]

[0095] wherein

[0096]

[0097] In formula (13), the synchronous speed ω s of the doubly-fed wind power generator is 50Hz, the rotor speed ω r of the doubly-fed wind power generator varies in the range of 10Hz-100Hz, and the slip s varies in the range of 0.8-1.

[0098] The action demarcation line determination submodule is used for obtaining the crowbar action demarcation line according to the relationship formula of the voltage drop degree k and the slip s.

[0099] The application provides a calculation method and system for a crowbar action boundary of a doubly-fed wind power generator, first establishes a short-circuit current response expression of the doubly-fed wind generator under rotor continuous excitation control, derives a dynamic analytical solution of rotor current in a synchronous rotating coordinate system through simultaneously solving a flux linkage equation, a voltage equation and a control equation, and reveals a quantitative relationship between a transient component attenuation coefficient and a current inner loop control parameter. On this basis, a crowbar action demarcation line analytical formula describing a mapping relationship between a voltage drop depth and a slip is proposed in combination with a crowbar action threshold condition, so that accurate judgment of whether the crowbar action is performed or not is realized under the condition that the wind speed and the voltage drop are known. The problems of unclear crowbar protection action boundary and insufficient accuracy of field group equivalent model in a low voltage ride-through process of a doubly-fed wind power plant are solved. BRIEF DESCRIPTION OF DRAWINGS

[0100] Figure 1 is a flowchart of a method for calculating the pry bar action boundary of a doubly-fed wind generator according to an embodiment of the present application;

[0101] Figure 2 is a schematic diagram of a doubly-fed wind turbine control system according to an embodiment of the present application;

[0102] Figure 3 is a schematic diagram of a DFIG vector form T equivalent circuit in a dq synchronous rotating coordinate system according to an embodiment of the present application;

[0103] Figure 4 is a schematic diagram of a grid symmetrical fault voltage amplitude according to an embodiment of the present application;

[0104] Figure 5 is a schematic diagram of the influence of control parameters on the transient direct current component amplitude coefficient m3 according to an embodiment of the present application;

[0105] Figure 6 is a schematic diagram of a pry bar action analysis demarcation according to an embodiment of the present application;

[0106] Figure 7 is a schematic diagram of a comparison between a pry bar analysis action demarcation line and a pry bar action according to an embodiment of the present application;

[0107] Figure 8 is a schematic diagram of a structure of a system for calculating the pry bar action boundary of a doubly-fed wind generator according to an embodiment of the present application. DETAILED DESCRIPTION

[0108] In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present application. However, it will be apparent to one skilled in the art that the present application can be practiced without the specific details set forth in this description. In other instances, well-known methods have not been described in detail in order to avoid obscuring the present application.

[0109] The present application derives a pry bar action demarcation line analysis formula for a doubly-fed wind generator, and under the condition of known wind speed and voltage drop, accurately judges whether the pry bar acts or not, which is of great significance to fault analysis and protection scheme design. Based on this, the present application provides a method for calculating the pry bar action boundary of a doubly-fed wind generator, as shown in Figure 1 The method comprises the following steps:

[0110] Step S101, a short-circuit current response expression of a doubly-fed wind turbine under rotor sustained excitation control is established.

[0111] A doubly-fed wind turbine DFIG control system block diagram is as shown in Figure 2as shown.

[0112] DFIG's stator and rotor windings are both motor convention, the DFIG vector form T equivalent circuit in dq synchronous rotating coordinate axis system is as shown in Figure 3 L ls , L lr DFIG stator and rotor windings self-induction, L m DFIG stator and rotor windings mutual inductance, R s , R r stator and rotor resistance respectively.

[0113] DFIG's stator and rotor side flux linkage equation expression in dq synchronous rotating coordinate axis is:

[0114]

[0115] ψ s , ψ r DFIG stator and rotor flux linkage vector; i s , i r stator and rotor current vector; L s , L r DFIG stator and rotor winding full inductance;

[0116] DFIG's stator and rotor side voltage equation expression in dq synchronous rotating coordinate axis is:

[0117]

[0118] U s , U r DFIG stator and rotor current, voltage vector respectively.

[0119] Step S102, based on the short-circuit current response expression of the DFIG, by simultaneously solving the flux linkage equation, voltage equation and current loop control equation, the dynamic analytical solution of the rotor current in the synchronous rotating coordinate system is obtained.

[0120] As shown in the DFIG control system block diagram of the doubly-fed wind power generator, the rotor side current loop control equation of the doubly-fed wind power generator is written in vector form, as shown in equation (3):

[0121]

[0122] ω p = sω s angular frequency of the rotor current before coordinate transformation;

[0123] To analyze the short-circuit current of DFIG wind turbine under grid symmetrical fault, the grid symmetrical fault is equivalent to three-phase voltage amplitude step, and the influence of phase jump on short-circuit current is not considered. The voltage amplitude during the fault is shown in FIG. 1, and K is the symmetrical drop depth of grid voltage. Figure 4

[0124] To calculate and analyze the stator and rotor short-circuit currents of DFIG after the fault occurs, the stator flux linkage after the fault occurs needs to be analyzed first. According to the voltage flux linkage equations of double-fed wind turbine given by equations (1) and (2), the stator and rotor flux linkages are rewritten as:

[0125]

[0126] wherein is the leakage coefficient of generator, and k s = L m / L s is the inductance coupling coefficient of stator winding;

[0127] Substitute equation (4) into equation (1) to solve the first-order differential equation of stator voltage about stator flux linkage, and the expression of DFIG stator flux linkage when the stator terminal voltage is suddenly changed is shown in equation (5), t0 is the starting time of fault,

[0128]

[0129] According to the flux linkage conservation principle, the stator flux linkage generates a direct current decay component after the three-phase short-circuit fault of the grid occurs, and the decay coefficient is τ sn , and the steady-state value of the flux linkage is related to the stator voltage after the fault;

[0130] Combine equations (2), (3) and (5) and take the first-order derivative to obtain the rotor current dynamic equation under symmetrical fault:

[0131]

[0132] wherein

[0133]

[0134] According to equation (6), the rotor loop of the three-phase short-circuit of the grid under the continuous excitation control strategy of the converter is a second-order dynamic circuit, and the analytical solution of the rotor current in the synchronous rotating coordinate system can be obtained by solving the non-homogeneous second-order differential equation as

[0135]

[0136] According to equation (7), the DFIG rotor current in the synchronous rotating coordinate system during the fault contains a steady-state component with a given value i rref of rotor current, and the decay coefficient is τ​sn The power frequency oscillation component, the harmonic components with attenuation coefficients p1 and p2, and m1, m2, and m3 are the amplitude coefficients of the two harmonic components and the power frequency oscillation component, respectively.

[0137] The attenuation coefficients p1 and p2 are the opposites of the two roots of the characteristic equation of equation (6), as shown in equation (8):

[0138]

[0139] Step S103: Based on the dynamic analytical solution of the rotor current in the synchronous rotating coordinate system, determine the quantitative relationship between the transient component attenuation coefficient and the inner loop control parameters of the current.

[0140] At the instant of the fault, the rotor current cannot change abruptly, but the initial value of the rotor current rate of change will change due to the stator terminal voltage and the rotor voltage. According to the impulse function matching method, the coefficients of each transient component can be obtained as follows:

[0141]

[0142] Combining equations (5) and (7), the expression for the DFIG stator current during the fault period can be obtained:

[0143]

[0144] Where i sf The steady-state value of the stator current during a fault is generally determined by the control strategy. The expressions for the coefficients m4, m5, and m6 of the transient attenuation components of the stator current are as follows:

[0145]

[0146] According to equation (11), the transient response of the stator current in the stationary coordinate system also consists of four parts: the steady-state component, the attenuation coefficient τ, and the constant-state component. sn The transient DC component and the transient harmonic components with attenuation coefficients p1 and p2; according to equation (8), the attenuation coefficients p1 and p2 depend on the current inner loop control parameters of the rotor converter, when the proportional coefficient k p When the integral coefficient increases, the real parts of p1 and p2 increase, and the decay rate of the free component accelerates; when the integral coefficient increases, the real parts of p1 and p2 decrease, and the decay rate of the free component slows down; according to equation (5), the decay coefficient τ sn It is only related to the motor impedance parameters;

[0147] The attenuation coefficient is τ sn The amplitude m6 of the transient component is affected by the system control parameters. According to equation (11), the magnitude of m6 depends on m3. According to equation (9), the influence of different current inner loop control parameters on the magnitude of m3 can be obtained, such as... Figure 5 As shown.

[0148] According to Figure 5 (a), the function image of m3 with respect to k i is not monotonic, when k i is greater than the extreme point, m3 monotonically decreases, and the fluctuation amplitude of the corresponding transient component decreases with the increase of k i ; when k i is less than the extreme point, m3 monotonically increases, resulting in the fluctuation amplitude of the corresponding transient component increasing with the increase of k i . When the ratio of the proportional coefficient and the integral coefficient of the current inner loop regulator k p / k i is larger, k i basically does not affect the size of m3. According to Figure 5 (b), the larger the proportional coefficient k p is, the smaller m3 will be. When k p / k i is smaller, the greater the influence of k p on m3 is.

[0149] In summary, the short-circuit current output by the doubly-fed wind turbine contains three transient components with different decay coefficients, the current inner loop parameters determine the decay coefficient of two transient components, and affect the amplitude of the transient component with decay coefficient τ sn . Appropriately increasing the proportional coefficient and reducing the integral coefficient of the rotor converter current inner loop is beneficial to accelerate the decay of the corresponding transient component. Increasing k p / k i can reduce the amplitude of the transient component with decay coefficient τ sn , which is beneficial to the rapid stabilization of the short-circuit current.

[0150] In step S104, based on the quantitative relationship between the transient component decay coefficient and the current inner loop control parameter, a relationship formula of the crowbar action demarcation line with respect to the voltage drop degree and the slip rate is obtained; if the crowbar action of the doubly-fed wind turbine is located above the crowbar action demarcation line, the crowbar action after the fault is performed.

[0151] Based on the rotor short-circuit current formula, the function relationship formula of the voltage drop depth K with respect to the slip rate s can be derived, which is the analytical formula of the crowbar action demarcation line.

[0152] The simultaneous equations (5), (6), (7), (8), (9) are combined. In equation (7), due to the action delay of the crowbar, the time t is 3 ms, and the action condition of the crowbar is that the rotor short-circuit current exceeds 1.5 times the rated rotor current, so the value of the rotor short-circuit current i r is 1.5 times the rated rotor current. The relationship formula of the crowbar action demarcation line with respect to the voltage drop degree k and the slip rate s is obtained:

[0153]

[0154] wherein

[0155]

[0156] In formula (13), the synchronous speed of the doubly-fed wind turbine ω s is 50 Hz, the rotor speed of the doubly-fed wind turbine ω r ranges from 10 Hz to 100 Hz. The range of the slip s is from 0.8 to -1. The greater the wind speed and power, the greater the rotor speed, and the smaller the slip;

[0157] The demarcation line of the crowbar action can be obtained according to the relationship between the voltage drop degree k and the slip s. It can be known that the voltage drop depth when the crowbar acts at any slip, as shown in formula (14). Figure 6

[0158] If (s, k) of the wind turbine is above the demarcation line of the crowbar action, the crowbar will act after the fault, otherwise the crowbar will not act.

[0159] According to the MPPT curve of the wind turbine, a one-to-one correspondence between the wind speed and the slip of the DFIG can be obtained. Therefore, the crowbar of each wind turbine in the field group can be accurately judged whether to act or not by combining the wind speed and the voltage drop depth of the wind turbine when the fault occurs, so that the group modeling in the fault stage is feasible.

[0160] In order to verify the correctness of the analytical formula of the demarcation line of the crowbar action in the foregoing theoretical derivation, a typical DFIG wind turbine model is built based on the PSCAD simulation platform. The voltage level of the transformer is 690V / 35kV, and the rated power value of the wind turbine is 2MW.

[0161] In the doubly-fed wind turbine model, the fault type is set to three-phase short circuit, the short circuit point is located on the collector line, and the voltage measuring point is located at the outlet of the box transformer. When the fault occurs, if the rotor short circuit current is greater than 1.5 times the rated rotor current, the crowbar protection is put into operation. Considering the control time delay of the DFIG control system in engineering practice, and considering the crowbar action process, the crowbar protection device is set to be activated 3ms after the fault in the simulation model, so as to simulate the real operation scene.

[0162] By adjusting the change of the slip s inside the wind turbine, the running conditions of the DFIG under different wind speeds are simulated. Under a given wind speed (or slip), the transition resistance value of the short circuit point is changed, so as to change the electrical distance between the short circuit point and the measuring point, and then change the voltage drop △U. Through a large number of simulations, the action conditions of the crowbar under different slips and different voltage drop degrees are counted, the demarcation line of the crowbar action is simulated, and the simulation comparison result is compared with the analytical demarcation line, as shown in formula (15). Figure 7 ​The simulation results based on different voltage drop degrees and different slip rates show that the analytical formula of the crowbar action demarcation line can accurately depict the actual action of the crowbar, the analytical curve is basically consistent with the action boundary in the simulation, and the error is small.

[0163] The greater the wind speed (or initial power) before short circuit and the deeper the voltage drop at the time of short circuit, the easier the crowbar action. In combination with the MPPT curve of the wind turbine, the initial wind speed and the voltage drop depth can be used to accurately determine the crowbar action.

[0164] Based on the same inventive concept, the application also provides a calculation system 800 for a crowbar action boundary of a doubly-fed wind turbine, as shown in the accompanying drawings, comprising: Figure 8

[0165] A response expression establishment module 810 is configured to establish a short-circuit current response expression of the doubly-fed wind turbine under continuous excitation control of the rotor;

[0166] A current dynamic analytical module 820 is configured to obtain a dynamic analytical solution of the rotor current in the synchronous rotating coordinate system by simultaneously solving the flux linkage equation, the voltage equation and the current loop control equation based on the short-circuit current response expression of the doubly-fed wind turbine;

[0167] A quantitative relationship determination module 830 is configured to determine a quantitative relationship between a transient component attenuation coefficient and a current inner loop control parameter according to the dynamic analytical solution of the rotor current in the synchronous rotating coordinate system;

[0168] A crowbar action module 840 is configured to obtain a relationship formula of the crowbar action demarcation line with respect to the voltage drop degree and the slip rate based on the quantitative relationship between the transient component attenuation coefficient and the current inner loop control parameter; if the crowbar action of the doubly-fed wind turbine is located above the crowbar action demarcation line, the crowbar action occurs after the fault.

[0169] Further, the response expression establishment module comprises:

[0170] A flux linkage equation sub-module is configured to express the flux linkage equations of the doubly-fed wind turbine on the stator side and the rotor side in the dq synchronous rotating coordinate axis as follows:

[0171]

[0172] In the formula, ψ s and ψ r are the stator flux linkage vector and the rotor flux linkage vector of the doubly-fed wind turbine; i s and i r are the stator current vector and the rotor current vector; L s and L r are the total inductance of the stator winding and the rotor winding of the doubly-fed wind turbine.

[0173] ​A voltage equation submodule is configured to express the stator-side voltage equation of the doubly-fed wind power generator in dq synchronous rotating coordinate axes as:

[0174]

[0175] wherein U s , U r are stator current and voltage vectors of the doubly-fed wind power generator, respectively.

[0176] Further, the current dynamic analysis module comprises:

[0177] A vector equation submodule is configured to express the rotor-side current loop control equation of the doubly-fed wind power generator in vector form, and specifically as:

[0178]

[0179] wherein ω p = sω s is the angular frequency of the rotor current before coordinate transformation.

[0180] A stator-rotor flux rewriting submodule is configured to analyze the stator flux after the fault occurs, and rewrite the stator-rotor flux according to the voltage flux equations of the doubly-fed wind power generator given by equations (1) and (2) as:

[0181]

[0182] wherein is a leakage flux coefficient of the generator, and k s = L m / L s is an inductance coupling coefficient of the stator winding.

[0183] A differential equation solving submodule is configured to substitute equation (4) into equation (1) to solve the first-order differential equation of the stator voltage with respect to the stator flux, and obtain the expression of the stator flux of the doubly-fed wind power generator when the stator terminal voltage is suddenly changed, as shown in equation (5), wherein t0 is the starting time of the fault,

[0184]

[0185] According to equation (5), after the three-phase short-circuit fault of the power grid occurs, according to the flux conservation principle, the stator flux generates a direct-current decay component, and the decay coefficient is τ sn , and the steady-state value of the flux is related to the stator voltage after the fault.

[0186] A current dynamic equation submodule is configured to simultaneously take the first-order derivative of equations (2), (3) and (5) to obtain the rotor current dynamic equation under the symmetrical fault:

[0187]

[0188] in

[0189]

[0190] The rotor current solution module is used to solve the rotor current in the synchronous rotating coordinate system. As shown in equation (6), the rotor circuit under the continuous excitation control strategy of the converter with a three-phase short circuit in the power grid is a second-order dynamic circuit. Solving this non-homogeneous second-order differential equation yields the analytical solution of the rotor current.

[0191]

[0192] As can be seen from equation (7), during the fault, the DFIG rotor current in the synchronous rotating coordinate system contains a value equal to the rotor current given value i. rref The steady-state component and the attenuation coefficient are τ sn The power frequency oscillation component, the harmonic components with attenuation coefficients p1 and p2, and m1, m2, and m3 are the amplitude coefficients of the two harmonic components and the power frequency oscillation component, respectively.

[0193] The attenuation coefficients p1 and p2 are the opposites of the two roots of the characteristic equation of equation (6), as shown in equation (8):

[0194]

[0195] Furthermore, the quantitative relationship determination module includes:

[0196] The submodule for solving the coefficients of transient components is used when, at the instant of a fault, the rotor current cannot change abruptly, but the initial value of the rotor current rate of change will change due to the stator terminal voltage and the rotor voltage. Based on the impulse function matching method, the coefficients of each transient component can be solved as follows:

[0197]

[0198] Combining equations (5) and (7), the expression for the DFIG stator current during the fault period can be obtained:

[0199]

[0200] Where i sf The steady-state value of the stator current during a fault is generally determined by the control strategy. The expressions for the coefficients m4, m5, and m6 of the transient attenuation components of the stator current are as follows:

[0201]

[0202] The first quantitative relational submodule is used to determine, according to equation (11), that the transient response of the stator current in the stationary coordinate system also consists of four parts: the steady-state component, the attenuation coefficient τ, and the constant-state component. sntransient component, and the damping coefficients p1 and p2 of the transient harmonic component; according to formula (8), the damping coefficients p1 and p2 depend on the current inner loop control parameters of the rotor converter, when the proportional coefficient k p increases, the real parts of p1 and p2 increase, and the attenuation speed of the free component accelerates; when the integral coefficient increases, the real parts of p1 and p2 decrease, and the attenuation speed of the free component slows down; according to formula (5), the damping coefficient τ sn only relates to the motor impedance parameters;

[0203] The second quantitative relationship submodule is used for determining the amplitude m6 of the transient component with the damping coefficient τ sn The amplitude m6 of the transient component with the damping coefficient τ

[0204] Further, the crowbar action module comprises:

[0205] The relationship formula determination submodule is used for simultaneously determining formula (5), (6), (7), (8), (9), and the relationship formula of the crowbar action demarcation line about the voltage drop degree k and the slip s can be obtained.

[0206]

[0207] wherein

[0208]

[0209] In formula (13), the synchronous speed ω s of the doubly-fed wind power generator is 50 Hz, the rotor speed ω r of the doubly-fed wind power generator varies in the range of 10 Hz to 100 Hz, and the slip s varies in the range of 0.8 to -1.

[0210] The action demarcation line determination submodule is used for obtaining the crowbar action demarcation line according to the relationship formula of the voltage drop degree k and the slip s.

[0211] Those skilled in the art should understand that the embodiments of the present application can be provided as a method, a system, or a computer program product. Therefore, the present application can adopt a completely hardware embodiment, a completely software embodiment, or an embodiment combining software and hardware aspects. Moreover, the present application can adopt a computer program product in the form of being implemented on one or more computer usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) containing computer usable program codes.

[0212] The computer program instructions can also be loaded onto a computer or other programmable data processing apparatus to cause a series of operational steps to be performed on the computer or other programmable apparatus to produce a computer-implemented process such that the instructions which execute on the computer or other programmable apparatus provide steps for implementing the functions specified in the flowchart block or blocks. Figure 1 one or more flow or blocks Figure 1 one or more flow or blocks

[0213] These computer program instructions can also be stored in a computer readable memory that can direct a computer or other programmable data processing apparatus to function in a particular manner, such that the instructions stored in the computer readable memory produce an article of manufacture including instructions which implement the function specified in the flowchart block or blocks. Figure 1 one or more flow or blocks Figure 1 one or more flow or blocks

[0214] The computer program instructions can also be loaded onto a computer or other programmable data processing apparatus to cause a series of operational steps to be performed on the computer or other programmable apparatus to produce a computer-implemented process such that the instructions which execute on the computer or other programmable apparatus provide steps for implementing the functions specified in the flowchart block or blocks. Figure 1 one or more flow or blocks Figure 1 one or more flow or blocks

[0215] Finally, it should be noted that the above-mentioned embodiments are merely used to illustrate the technical solutions of the present application, but are not intended to limit the present application. Although the present application is described in detail with reference to the above embodiments, those skilled in the art should understand that the specific embodiments of the present application can be modified or replaced, and any modification or replacement without departing from the spirit and scope of the present application should be covered in the scope of the claims of the present application.

Claims

1. A method for calculating the pry bar action boundary of a doubly-fed wind power generator, characterized in that, The method comprises the following steps: establishing a short-circuit current response expression of a doubly-fed wind turbine under rotor continuous excitation control; obtaining a dynamic analytical solution of rotor current in a synchronous rotating coordinate system by simultaneously solving the flux linkage equation, the voltage equation and the current loop control equation based on the short-circuit current response expression of the doubly-fed wind turbine; determining a quantitative relationship between a transient component attenuation coefficient and a current inner loop control parameter according to the dynamic analytical solution of rotor current in the synchronous rotating coordinate system; obtaining a relationship formula of a crowbar action demarcation line with respect to a voltage drop degree and a slip rate based on the quantitative relationship between the transient component attenuation coefficient and the current inner loop control parameter; if the crowbar action of the doubly-fed wind turbine is located above the crowbar action demarcation line, the crowbar action is performed after a fault.

2. The method of claim 1, wherein, The method for establishing a short-circuit current response expression of a doubly-fed wind turbine under rotor continuous excitation control comprises the following steps: the flux linkage equation expressions of the doubly-fed wind turbine on the dq synchronous rotating coordinate axis are as follows: where: ψ s , ψ r are stator and rotor flux vectors of the doubly-fed wind turbine; i s , i r are stator and rotor current vectors; L s , L r are stator and rotor winding full inductances of the doubly-fed wind turbine; the voltage equation expressions of the doubly-fed wind turbine on the dq synchronous rotating coordinate axis are as follows: where: U s , U r are the stator and rotor current, voltage vectors of the DFIG respectively.

3. The method of claim 1, wherein, the dynamic analytical solution of rotor current in the synchronous rotating coordinate system is obtained by simultaneously solving the flux linkage equation, the voltage equation and the current loop control equation based on the short-circuit current response expression of the doubly-fed wind turbine, and the solution comprises the following steps: the current loop control equation of the doubly-fed wind turbine on the rotor side is written in a vector form, and the vector form is as follows: where ω p = sω s is the angular frequency of the rotor currents before coordinate transformation; the stator flux linkage after the fault occurs is analyzed, the stator and rotor flux linkages are rewritten according to the voltage flux linkage equation of the doubly-fed wind turbine given by the formula (1) and the formula (2), and the rewritten formula is as follows: wherein is the leakage coefficient of the generator, k s = L m / L s is the inductance coupling coefficient of the stator winding; the formula (4) is substituted into the formula (1), a first-order differential equation of the stator voltage with respect to the stator flux linkage is solved, and the expression of the DFIG stator flux linkage when the stator end voltage suddenly changes is as shown in the formula (5), t0 is a starting moment of the fault, From equation (5), it can be seen that after three-phase short-circuit fault, the stator flux linkage has a DC decay component according to the flux linkage conservation principle, and the decay coefficient is τ sn , the steady-state value of the flux linkage is related to the stator voltage after the fault. the formula (2), the formula (3) and the formula (5) are simultaneously solved, and a rotor current dynamic equation under a symmetrical fault is obtained: wherein it is known from the formula (6) that the rotor loop under the three-phase short circuit of the power grid in the converter continuous excitation control strategy is a second-order dynamic circuit, and the analytical solution of the rotor current in the synchronous rotating coordinate system can be obtained by solving the non-homogeneous second-order differential equation, and the solution is as follows: From equation (7), it can be seen that the DFIG rotor current in the synchronous rotating reference frame during the fault contains a steady-state component with magnitude of the rotor current given value i rref , a power frequency oscillation component with damping coefficient τ sn , and harmonic components with damping coefficients p1 and p2, where m1, m2, and m3 are the amplitude coefficients of the two harmonic components and the power frequency oscillation component, respectively. the attenuation coefficients p1 and p2 are the reciprocals of two roots of the characteristic equation of the formula (6), and the formula is as shown in the formula (8):

4. The method of claim 1, wherein, the quantitative relationship between the transient component attenuation coefficient and the current inner loop control parameter is determined according to the dynamic analytical solution of rotor current in the synchronous rotating coordinate system, and the determination comprises the following steps: at the moment of the fault, the rotor current cannot suddenly change, and the initial value of the rotor current change rate will change due to the stator end voltage and the rotor voltage, according to the impulse function matching method, the coefficients of the transient components can be solved, and the coefficients are as follows: the formula (5) and the formula (7) are combined, and a DFIG stator current expression during the fault is obtained: where i sf is the stator current fault steady state value, generally determined by the control strategy. The coefficients m4, m5, m6of the stator current transient decay component are expressed as follows: According to formula (11), the transient response of the stator current in the stationary coordinate system is also composed of four parts, which are the steady-state component, the transient direct current component with attenuation coefficient τ sn , the transient harmonic component with attenuation coefficients p1 and p2; according to formula (8), the attenuation coefficients p1 and p2 depend on the current inner loop control parameters of the rotor converter, when the proportional coefficient k p increases, the real part of p1 and p2 increases, and the attenuation speed of the free component accelerates; when the integral coefficient increases, the real part of p1 and p2 decreases, and the attenuation speed of the free component slows down; according to formula (5), the attenuation coefficient τ sn only relates to the motor impedance parameters; The attenuation coefficient is τ sn The amplitude m6 of the transient component is affected by the system control parameters. According to equation (11), the size of m6 depends on m3. According to equation (9), the influence of different current inner loop control parameters on the size of m3 can be obtained.

5. The method of claim 1, wherein, the relationship formula of the crowbar action demarcation line with respect to the voltage drop degree and the slip rate is obtained based on the quantitative relationship between the transient component attenuation coefficient and the current inner loop control parameter, and the obtaining comprises the following steps: the formula (5), the formula (6), the formula (7), the formula (8) and the formula (9) are simultaneously solved, and the relationship formula of the crowbar action demarcation line with respect to the voltage drop degree k and the slip rate s is obtained: wherein In formula (13), the synchronous speed of the doubly-fed wind turbine ω s is 50 Hz, the rotor speed of the doubly-fed wind turbine ω r varies in the range of 10 Hz to 100 Hz, and the slip s varies in the range of 0.8 to -1. the crowbar action demarcation line is obtained according to the relationship formula of the voltage drop degree k and the slip rate s.

6. A computing system for double-fed wind power generator crowbar action boundary, characterized in that, The method comprises the following steps: The response expression establishing module is configured to establish a short-circuit current response expression of the doubly-fed wind turbine under continuous excitation control of the rotor; The current dynamic analysis module is configured to obtain a dynamic analytical solution of the rotor current in a synchronous rotating coordinate system by simultaneously solving a flux equation, a voltage equation and a current loop control equation based on the short-circuit current response expression of the doubly-fed wind turbine; The quantitative relationship determining module is configured to determine a quantitative relationship between a transient component attenuation coefficient and a current inner loop control parameter according to the dynamic analytical solution of the rotor current in the synchronous rotating coordinate system; The crowbar action module is configured to obtain a relationship formula of a crowbar action demarcation line with respect to a voltage drop degree and a slip based on the quantitative relationship between the transient component attenuation coefficient and the current inner loop control parameter; and if the crowbar action of the doubly-fed wind turbine is located above the crowbar action demarcation line, the crowbar action is performed after the fault.

7. The system of claim 6, wherein, The response expression establishing module comprises: The flux equation submodule is configured to establish flux equation expressions of the doubly-fed wind turbine on the dq synchronous rotating coordinate axis as follows: where: ψ s , ψ r are the stator and rotor flux vectors of the doubly-fed wind turbine; i s , i r are the stator and rotor current vectors; L s , L r are the stator and rotor winding full inductances of the doubly-fed wind turbine; The voltage equation submodule is configured to establish voltage equation expressions of the doubly-fed wind turbine on the dq synchronous rotating coordinate axis as follows: where: U s , U r are the stator and rotor current, voltage vectors of the DFIG respectively.

8. The system of claim 6, wherein, The current dynamic analysis module comprises: The vector equation submodule is configured to write a current loop control equation of the doubly-fed wind turbine on the rotor side into a vector form, specifically as follows: where ω p = sω s is the angular frequency of the rotor currents before coordinate transformation; The stator and rotor flux rewriting submodule is configured to analyze the stator flux after the fault occurs, and rewrite the stator and rotor fluxes according to the voltage flux equations of the doubly-fed wind turbine given by equations (1) and (2) as follows: wherein is the leakage coefficient of the generator, k s = L m / L s is the inductance coupling coefficient of the stator winding; The differential equation solving submodule is configured to substitute equation (4) into equation (1) to solve a first-order differential equation of the stator voltage with respect to the stator flux, and obtain an expression of the stator flux of the DFIG when the stator terminal voltage suddenly changes, as shown in equation (5), wherein t0 is a starting time of the fault, From equation (5), it can be seen that after three-phase short-circuit fault, the stator flux linkage has a DC decay component according to the flux linkage conservation principle, and the decay coefficient is τ sn , the steady-state value of the flux linkage is related to the stator voltage after the fault. The current dynamic equation submodule is configured to simultaneously take a first-order derivative of equations (2), (3) and (5) to obtain a rotor current dynamic equation under a symmetrical fault as follows: wherein The rotor current solving submodule is configured to know from equation (6) that the rotor loop under the converter continuous excitation control strategy is a second-order dynamic circuit under a three-phase short circuit of the power grid, and solve the non-homogeneous second-order differential equation to obtain an analytical solution of the rotor current in the synchronous rotating coordinate system as follows: From equation (7), it can be seen that the DFIG rotor current in the synchronous rotating reference frame during the fault contains a steady-state component with magnitude of the rotor current given value i rref , a power frequency oscillation component with damping coefficient τ sn , and harmonic components with damping coefficients p1 and p2, where m1, m2, and m3 are the amplitude coefficients of the two harmonic components and the power frequency oscillation component, respectively. The attenuation coefficients p1 and p2 are the reciprocals of two roots of a characteristic equation of equation (6), as shown in equation (8):

9. The system of claim 6, wherein, The quantitative relationship determining module comprises: The coefficient solving submodule of each transient component is configured to know that the rotor current cannot suddenly change at the moment of the fault, and the initial value of the rotor current change rate will change due to the stator terminal voltage and the rotor voltage; and according to an impulse function matching method, coefficients of each transient component can be solved as follows: Combined with equations (5) and (7), a DFIG stator current expression during the fault can be obtained as follows: where i sf is the stator current fault steady state value, generally determined by the control strategy. The coefficients m4, m5, m6of the stator current transient decay component are expressed as follows: The first quantitative relationship submodule is configured to know from formula (11) that the transient response of the stator current in the stationary coordinate system is also composed of four parts, which are a steady-state component, a transient direct current component with a damping coefficient τ sn , a transient harmonic component with damping coefficients p1 and p2; according to formula (8), the damping coefficients p1 and p2 depend on the current inner loop control parameters of the rotor converter, when the proportional coefficient k p increases, the real parts of p1 and p2 increase, and the attenuation speed of the free component accelerates; when the integral coefficient increases, the real parts of p1 and p2 decrease, and the attenuation speed of the free component slows down; according to formula (5), the damping coefficient τ sn only relates to the motor impedance parameters; The second quantitative relationship submodule is configured to attenuate the amplitude m6 of the transient component with the attenuation coefficient τ sn The amplitude m6 of the transient component with the attenuation coefficient τ is influenced by the system control parameters. According to formula (11), the size of m6 depends on m3. According to formula (9), the influence law of different current inner loop control parameters on the size of m3 can be obtained.

10. The system of claim 6, wherein, The crowbar action module comprises: The relationship formula determining submodule is configured to simultaneously solve equations (5), (6), (7), (8) and (9) to obtain a relationship formula of the crowbar action demarcation line with respect to a voltage drop degree k and a slip s as follows: wherein In formula (13), the synchronous speed of the doubly-fed wind turbine ω s is 50 Hz, the rotor speed of the doubly-fed wind turbine ω r ranges from 10 Hz to 100 Hz, and the slip s ranges from 0.8 to -1. The action demarcation line determining submodule is configured to obtain the crowbar action demarcation line according to the relationship formula of the voltage drop degree k and the slip s.

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