Electromagnetic transient modeling and simulation method for internal fault of doubly-fed induction generator

Abstract

The invention discloses an electromagnetic transient modeling and simulation method for internal faults of a doubly-fed induction generator, and relates to the technical field of power system modeling and simulation. The method comprises the following steps: S1, considering a stator-rotor turn ratio, and reducing the rotor quantity to a stator side to obtain an original mathematical equation; s2, establishing mathematical models of the doubly-fed induction generator under different fault types, correcting a basic equation of the generator according to fault features, and focusing resistance and inductance matrix elements needing to be updated; s3, selecting a flux linkage as a state variable, performing discretization equivalence on the mathematical equation by using a backward Euler integral method, unifying different faults to obtain an equivalent circuit equation, and constructing an equivalent circuit of the generator; s4, the equivalent circuit of the doubly-fed induction generator is connected with an external electrical network, and the equivalent model carries out node admittance matrix iterative solution on the whole circuit based on an EMTDC solver; and updating other concerned electrical information such as the stator current, the rotor current, the electromagnetic torque, the rotating speed, the rotor angle and the like until the simulation is finished.

Description

Technical Field

[0001] The present invention belongs to the technical field of power system modeling and simulation, and in particular relates to an electromagnetic transient modeling and simulation method for doubly-fed induction generators that can reflect various types of internal faults. Background Art

[0002] With the increasing global demand for renewable energy, wind power, as a clean and sustainable energy source, has become a key driver of global energy transformation. Currently, long-distance, large-capacity offshore wind farms face challenges such as difficult fault repairs and significant downtime losses. Doubly-fed induction generators (DFIGs), with their excellent variable speed constant frequency characteristics, are a core component of doubly-fed wind turbines. Therefore, timely detection of DFIG internal faults and preventing them from escalating are crucial for ensuring the long-term stable operation of wind turbines.

[0003] Analyzing the complex internal fault conditions of DFIGs requires a large amount of fault data under various fault types. However, obtaining such data in actual operation is difficult and costly. Simulation, on the other hand, can effectively reflect DFIG fault characteristics and cost-effectively cover a wide range of operating conditions. By adjusting key parameters through parametric modeling, it is possible to effectively simulate the physical structural changes caused by faults, thereby simulating internal fault scenarios of varying types and severity. Furthermore, the high-frequency switching of the converter results in microsecond-scale transients, necessitating small-step electromagnetic transient modeling and simulation to accurately simulate these high-frequency switching characteristics. Currently, mainstream commercial electromagnetic transient simulation software such as Matlab / Simulink, PSCAD / EMTDC, and RTDS all provide DFIG models. However, these gray-box models provide only a basic mathematical model without considering internal faults, resulting in a lack of overall sophistication. Existing basic DFIG models typically ignore internal faults and use current as the state variable. Their modeling relies on the inductance matrix as a constant. However, when internal faults occur, the inductance matrix exhibits strong time-varying and nonlinear characteristics, making such methods difficult to apply. However, fault modeling for generator body simulation mainly focuses on the local characteristics of the motor body and cannot directly form the node equivalent circuit form required for electromagnetic transient simulation. It also has problems such as large computational complexity and low accuracy.

[0004] In response to the above problems, the present invention proposes an electromagnetic transient modeling and simulation method for a doubly-fed induction generator that can reflect various types of internal faults. Summary of the Invention

[0005] (1) Technical problems to be solved by the present invention:

[0006] The purpose of the present invention is to provide an electromagnetic transient modeling and simulation method for internal faults of a doubly-fed induction generator to solve the problems raised in the background art.

[0007] (2) In order to achieve the above-mentioned purpose, the present invention adopts the following technical solutions:

[0008] An electromagnetic transient modeling and simulation method for an internal fault of a doubly-fed induction generator (DFIG) comprises the following steps:

[0009] S1. Consider the stator-rotor turns ratio, convert the rotor quantity to the stator side, and use Park transformation to map the original equation of the abc coordinate system to the dq0 coordinate system to obtain the voltage equation, flux equation, and motion equation under the non-fault state;

[0010] S2. Determine fault characteristics based on different fault types and establish electromagnetic transient and electromechanical transient mathematical models for the doubly fed induction generator (DFIG) under different fault conditions, focusing on the changes in the structure of the voltage equation and flux equation as well as the resistance matrix and inductance matrix, and clarify the parameter correction rules;

[0011] S3, select flux as the state variable, use the backward Euler integration method to discretize the flux-voltage equation in the dq0 coordinate system, and obtain the relationship between the stator and rotor currents and voltages;

[0012] S4. Separate the stator equation from the rotor equation, write the relationship between the stator and rotor currents and voltages in block matrix form, and obtain a unified stator and rotor current expression; at the same time, discretize the motion equation obtained in S1 to obtain the speed expression and rotor angle expression;

[0013] S5. Based on the expression obtained in S4, an equivalent model of the doubly fed induction generator under internal fault is constructed, including an electromagnetic transient equivalent circuit and an electromechanical transient model. A decoupling equivalent circuit of the doubly fed induction generator (DFIG) under internal fault state is represented by a controlled current source.

[0014] S6. Input the equivalent circuit obtained in S5 into the electromagnetic transient simulation software to perform iterative solution of the node admittance moment; update the generator stator and rotor current, electromagnetic torque, rotor speed, and rotor angle information according to the external terminal voltage.

[0015] Preferably, the voltage equation, flux equation, and motion equation in the non-fault state in S1 are expressed as follows:

[0016] Voltage equation:

[0017]

[0018] Magnetic flux equation:

[0019]

[0020] Equations of motion:

[0021]

[0022] Wherein, the subscripts ds, qs, and 0s represent stator windings, and dr, qr, and 0r represent rotor windings; ω r is the rotor angular velocity, ω1 is the synchronous rotation angular velocity of the stator magnetic field, ω s is the angular velocity of the rotor relative to the stator magnetic field, which is (ω1-ω r ); e is the velocity electromotive force column vector; L s and L' r are the stator and rotor self-inductance of DFIG respectively, and L s =L ls +L m , L' r =L' lr +L m ;T m is the mechanical torque; T e is the electromagnetic torque of the generator; K D is the mechanical damping coefficient; J is the moment of inertia of the rotor.

[0023] Preferably, the S3 specifically includes the following contents:

[0024] S3.1. Use the backward Euler method to discretize the voltage equation shown in equation (1) and obtain:

[0025] Ψ(t)-Ψ(t-Δt)=Δt[U(t)-RI(t)-e(t)] (4)

[0026] Among them, U is the voltage column vector, R is the resistance matrix, I is the current column vector, and Ψ is the flux column vector;

[0027] S3.2. Substitute the flux equation shown in formula (2) into formula (4), and treat the velocity electromotive force column vector e as a historical value, that is, e(t)≈e(t-Δt), and then obtain:

[0028] L(t)I(t)-L(t-Δt)I(t-Δt)=Δt[U(t)-RI(t)-e(t-Δt)] (5)

[0029] S3.3. Convert the equation to the form I = YU + J and get:

[0030] (L(t)+Δt·R)I(t)=ΔtU(t)-Δte(t-Δt)+L(t-Δt)I(t-Δt) (6)

[0031] S3.4. Use Parker transformation to transform the electrical quantities on the dq0 axis into the abc coordinate system to establish the electrical interface between the generator and the external electrical network. Define the stator and rotor side Parker transformation matrix as T:

[0032]

[0033] Among them, P s represents the stator side Parker transformation matrix, and its transformation angle is P r represents the rotor side Parker transformation matrix, and its transformation angle is

[0034] S3.5. After introducing Parker transformation into equation (6) and converting it into the abc coordinate system, multiply both sides by P T -1 (L(t)+Δt·R) -1 The relationship between the stator and rotor current and voltage is obtained:

[0035]

[0036] Preferably, the stator and rotor current expressions, speed expressions, and rotor angle expressions in S4 are:

[0037] I abcs (t) = G abcs (t)U abcs (t)+I abcsEQ (t-Δt)

[0038] I abcr (t) = G abcr (t)U abcr (t)+I abcrEQ (t-Δt)

[0039]

[0040] θ(t)=θ(t-Δt)+Δtω r (t)

[0041] Among them, I abcs (t) represents the stator current; I abcr (t) represents the rotor current; ω r(t) represents the speed; θ(t) represents the rotor angle; the subscript abcs represents the stator winding in the abc coordinate system, and the subscript abcr represents the rotor winding in the abc coordinate system; G abcs (t) and G abcr (t) are the stator and rotor admittance matrices respectively; I abcsEQ (t-Δt) and I abcrEQ (t-Δt) are the column vectors of the historical values ​​of the stator and rotor equivalent current sources respectively; U abcs (t) is the stator voltage column vector; U abcr (t) is the stator voltage column vector; J is the angular moment of inertia; K D is the mechanical damping coefficient; T m is the mechanical torque; T e is the electromagnetic torque of the generator.

[0042] Preferably, the admittance matrix in the equation of the decoupling equivalent circuit of the doubly fed induction generator (DFIG) under an internal fault state in S5 is asymmetric, and the doubly fed induction generator (DFIG) is equivalent in the form of a controlled current source. The size of the controlled current source is calculated by a discretized mathematical equation under different fault types and is related to the fault type.

[0043] (3) The beneficial effects of the present invention include:

[0044] The present invention can accurately reflect fault characteristics, providing a model basis for the research on internal fault detection and protection strategies of doubly fed induction generators (DFIGs). It also supports direct coupling with external components such as converters and power grids, reflecting internal fault characteristics to wind turbines, and further providing a reliable simulation tool for the research on the fault evolution mechanism of wind turbines and grid interaction under complex working conditions. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Figure 1 Schematic diagram of the structure of a doubly-fed induction generator (DFIG) proposed in Example 2 of the present invention;

[0046] Figure 2 This is a schematic diagram of an electrical fault proposed in Example 2 of the present invention;

[0047] Figure 3 This is a schematic diagram of an eccentricity fault proposed in Example 2 of the present invention;

[0048] Figure 4 Schematic diagram of an equivalent circuit of a doubly-fed induction generator (DFIG) proposed in Example 2 of the present invention;

[0049] Figure 5 Schematic diagram of the modeling process of the doubly-fed induction generator (DFIG) proposed in Example 2 of the present invention. DETAILED DESCRIPTION

[0050] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0051] This invention proposes a method for electromagnetic transient modeling and simulation of a doubly-fed induction generator (DFIG) that can reflect various types of internal faults. This model regresses rotor quantities to the stator side, obtaining the original mathematical equations under non-fault conditions. Based on the fault characteristics of different fault types, parameter correction rules are defined to modify the voltage and flux equations, effectively simulating the physical changes caused by the fault. The mathematical equations are discretized using the backward Euler method, and a unified equivalent circuit for the DFIG is obtained using flux as the state variable. This results in an equivalent model for the DFIG that covers five types of faults, including stator and rotor winding faults and eccentricity faults. This model has a unified modeling process and accurately reflects fault characteristics, providing a foundation for research on DFIG internal fault detection and protection strategies. It also supports direct coupling with external components such as converters and power grids, reflecting internal fault characteristics to wind turbines, further providing a reliable simulation tool for studying wind turbine fault evolution mechanisms and grid interactions under complex operating conditions. The proposed electromagnetic transient modeling and simulation method for internal faults in DFIGs is described below with reference to the accompanying figures and specific examples.

[0052] Example 1:

[0053] The present invention proposes an electromagnetic transient modeling and simulation method for internal faults of a doubly-fed induction generator, which includes the following contents:

[0054] Step 1:

[0055] Considering the stator-rotor turns ratio, the rotor quantity is converted to the stator side, and the Park transformation is used to map the original equation of the abc coordinate system to the dq0 coordinate system to obtain the voltage equation, flux equation, and motion equation under the non-fault state.

[0056] Step 2:

[0057] According to different fault types, the fault characteristics are determined, and the electromagnetic transient and electromechanical transient mathematical models of the doubly fed induction generator under each fault type are obtained, focusing on the changes in the structure of the voltage equation and the flux equation as well as the resistance matrix and inductance matrix.

[0058] Step 3:

[0059] Selecting flux as the state variable, the backward Euler integration method is used to discretize the flux-voltage equation in the dq0 coordinate system, resulting in the relationship between the stator and rotor currents and voltages. This method is characterized by the fact that the winding inductance distribution changes, and the inductance matrix in the dq0 coordinate system is not a constant matrix. If current is used as the state variable, complex time-varying partial derivatives of the inductance matrix exist. A discretization and decoupling method using flux as the state variable is proposed, which, to a certain extent, avoids the calculation of the time-varying partial derivatives of the inductance matrix.

[0060] Step 4:

[0061] Separate the stator equation from the rotor equation, write the relationship between the stator and rotor currents and voltages in block matrix form, and obtain a unified stator and rotor current expression. To facilitate the interface between DFIG and the external electrical network, convert it to the abc coordinate system. At the same time, discretize the motion equation in step 1 to obtain the speed expression and rotor angle expression. It is characterized in that: the stator current I abcs (t), rotor current I abcr (t), speed ω r The expressions for (t) and rotor angle θ(t) are as follows:

[0062] I abcs (t) = G abcs (t)U abcs (t)+I abcsEQ (t-Δt)

[0063] I abcr (t) = G abcr (t)U abcr (t)+I abcrEQ (t-Δt)

[0064]

[0065] θ(t)=θ(t-Δt)+Δtω r (t)

[0066] Where G abcs (t) and G abcr (t) are the stator and rotor admittance matrices respectively; I abcsEQ (t-Δt) and I abcrEQ (t-Δt) are the column vectors of the historical values ​​of the stator and rotor equivalent current sources respectively; U abcs (t) is the stator voltage column vector; U abcr (t) is the stator voltage column vector; J is the angular moment of inertia; K D is the mechanical damping coefficient; T m is the mechanical torque; T e is the electromagnetic torque of the generator.

[0067] Step 5:

[0068] Based on the expression described in step 4, an equivalent model of the doubly-fed induction generator under internal fault conditions can be obtained, including an electromagnetic transient equivalent circuit and an electromechanical transient model. The decoupling equivalent circuit under internal fault conditions of the doubly-fed induction generator is represented by a controlled current source. This method is characterized in that the admittance matrix in the equivalent circuit equation under internal fault conditions of the doubly-fed induction generator is asymmetric, a controlled current source is used to represent the doubly-fed induction generator, and the magnitude of the controlled current source is calculated using discretized mathematical equations for different fault types and is dependent on the fault type.

[0069] Step 6:

[0070] The equivalent circuit of the doubly-fed induction generator obtained in step 5 is placed into the EMTDC solution system to iteratively solve the node admittance moment. Based on the external terminal voltage, the generator's stator and rotor currents, electromagnetic torque, rotor speed, and rotor angle are updated.

[0071] Example 2:

[0072] Based on Example 1, but different in that, this example proposes an electromagnetic transient modeling and simulation method for a doubly-fed induction generator that can reflect various types of internal faults. The modeling steps of the present invention will be further described in detail below with reference to the accompanying drawings:

[0073] Step 1:

[0074] The DFIG structure diagram is as attached. Figure 1 Assume that the direction of voltage and current follows the convention of electric motors. DFIG is an AC motor. The core structure includes stator, rotor and auxiliary system. The DFIG rotor adopts three-phase AC winding. AC excitation current is injected from the outside to achieve stator-rotor magnetic field coupling.

[0075] The voltage-flux equations of DFIG are shown in equations (1) and (2):

[0076]

[0077] Where, the subscript abcs represents the stator winding in the abc coordinate system, and the subscript abcr represents the rotor winding in the abc coordinate system; U is the voltage column vector, R is the resistance matrix, I is the current column vector, and Ψ is the flux column vector; L m is the magnetizing inductance, L ls and L lr are the leakage inductance of the stator and rotor respectively.

[0078] The number of turns and parameters of the DFIG stator and rotor windings are different, so the rotor parameters are first converted to the stator side to ensure the consistency of the model. Assume that the stator windings have the same effective number of turns N s , the rotor winding has the same number of turns Nr , define the turns ratio N s / N r =n, the calculation rule is shown in formula (3):

[0079]

[0080] The superscript ' indicates the reduced variable.

[0081] The voltage equation and flux equation after reduction are shown in equations (4) and (5):

[0082]

[0083] Since the equations in the abc coordinate system have time-varying, strongly coupled nonlinear differential characteristics, Park transformation is used to map them to the dq0 coordinate system. The time-varying inductance matrix in the original model is converted into a constant coefficient matrix. Consider the stator winding equation and the reduced rotor winding equation:

[0084] Voltage equation:

[0085]

[0086] Magnetic flux equation:

[0087]

[0088] Equations of motion:

[0089]

[0090] Wherein, the subscripts ds, qs, and 0s represent stator windings, and dr, qr, and 0r represent rotor windings. r is the rotor angular velocity, ω1 is the synchronous rotation angular velocity of the stator magnetic field, ω s is the angular velocity of the rotor relative to the stator magnetic field, which is (ω1-ω r ). e is the velocity electromotive force column vector, L s and L' r are the stator and rotor self-inductance of DFIG respectively, and L s =L ls +L m , L' r =L' lr +L m . T m is the mechanical torque, T e is the electromagnetic torque of the generator, K D is the mechanical damping coefficient, and J is the moment of inertia of the rotor.

[0091] Step 2:

[0092] According to different fault types, the fault characteristics are determined, and the electromagnetic transient and electromechanical transient mathematical models of the doubly fed induction generator under each fault type are obtained. Focus on the changes in the structure of the voltage equation and the flux equation as well as the resistance matrix and the inductance matrix, and clarify the parameter correction rules. The electrical winding fault model is shown in the attached figure. Figure 2 The eccentricity fault model is shown in the attached Figure 3 shown.

[0093] The DFIG high resistance connection occurs in the stator winding and the rotor winding. It is only necessary to add an additional resistor R to the fault phase winding. add As attached Figure 2 As shown in (a), the fault phase resistance in the voltage equation changes, the flux equation remains unchanged, and the corresponding resistance matrix changes.

[0094]

[0095] For the winding open circuit fault, when the two phases of the motor have an open circuit fault, the motor will stop immediately, and the fault modeling can be achieved by constraining the current to zero; in the case of a single-phase open circuit fault, the fault phase winding is disconnected, and the three phases of the motor are unbalanced. Figure 2 (b) shown.

[0096] The equivalent circuit of a single-phase grounding fault of the winding is shown in the attached figure. Figure 2 As shown in (a), R g is the grounding resistance, i f is the fault current under single-phase grounding fault; α represents the fault location, and the fault range is between 0 and 1. f Flowing through the transition resistor R g The voltage equation and flux equation are shown in Equations (10) and (11). The influence of the grounding branch on the stator-rotor mutual inductance needs to be considered, and the corresponding resistance matrix and inductance matrix change.

[0097]

[0098]

[0099] The equivalent circuit under inter-turn short circuit is shown in the attached figure. Figure 2 As shown in (b), a fault loop is introduced, and the fault current i f Flowing through the short-circuit resistor R g , the ratio of the number of short-circuit coil turns to the total number of turns of a phase winding is defined as the short-circuit ratio μ. Figure 2 For the winding circuit structure shown in (b), when a stator inter-turn short circuit fault occurs, the voltage equations of the DFIG fault branch and fault ring are shown in Equations (12) and (13), and the voltage equation of the non-fault branch remains unchanged.

[0100]

[0101] Depend on Figure 2(b) The inductance distribution can be used to obtain the flux equations of the fault branch, non-fault branch and fault loop under the DFIG inter-turn short-circuit fault, as shown in Equations (14)-(17).

[0102]

[0103] Eccentricity refers to the inconsistency of the air gap distance between the stator and the rotor. The air gap eccentricity model is shown in the attached figure. Figure 3 As shown, by setting the eccentricity ρ m , a motor model was established that can simulate rotor eccentricity faults under different eccentricity degrees. m With the eccentric direction γ m The change of rotor eccentricity type needs to be calculated in advance. Rotor eccentricity changes the air gap distribution, which directly affects the inductance of the doubly fed induction generator, including the self-inductance part, the mutual inductance part and the additional terms caused by eccentricity.

[0104]

[0105] Where, O s is the center of the stator, O r is the center of the rotor, ρ s is the static eccentricity, ρ d is the dynamic eccentricity, and γ0 is the initial eccentricity angle.

[0106] Taking the stator inductance as an example for derivation, under eccentric fault, both the self-inductance and mutual inductance of the stator inductance matrix have additional terms caused by eccentricity, namely L abc =A0*L0+ΔL abc , where L0 is the stator inductance matrix in the abc three-phase coordinate system under normal conditions, A0 is the eccentricity additional coefficient, and if there is no eccentricity, the additional coefficient A0 = 1.

[0107]

[0108] Among them, the additional terms of self-inductance and mutual inductance, γ mi / mj and θ mi / mj (i=a,b,c;j=b,c,a) is given by equation (20):

[0109]

[0110] Where,

[0111] Different fault types manifest themselves in changes to the equation structure and the resistance and inductance matrices. High-resistance connections and open-circuit faults are addressed simply by adjusting the resistance matrix elements. Eccentricity faults are addressed by modifying the inductance matrix by introducing additional inductance. Faults such as single-phase grounding and turn-to-turn short circuits require reconstructing the equation structure by extending the grounding branch and introducing a fault loop, which alters both the inductance and resistance matrices.

[0112] Step 3:

[0113] The flux linkage is selected as the state variable to avoid the calculation of the time-varying partial derivatives of the inductance matrix. The backward Euler integration method is used to discretize the flux linkage-voltage equation in the dq0 coordinate system to obtain the relationship between the stator and rotor currents and voltages.

[0114] First, the backward Euler method is used to discretize Equation (6):

[0115] Ψ(t)-Ψ(t-Δt)=Δt[U(t)-RI(t)-e(t)] (21)

[0116] Substituting the flux equation (7) into equation (21), in order to facilitate the derivation of the discretized equation and maintain the symmetry of the admittance matrix, the velocity electromotive force column vector e is treated as a historical value, thereby avoiding the inversion and multiplication calculation of a large number of time-varying matrices when the equation is equivalent, that is, e(t)≈e(t-Δt), and the following equation (22) is obtained:

[0117] L(t)I(t)-L(t-Δt)I(t-Δt)=Δt[U(t)-RI(t)-e(t-Δt)] (22)

[0118] Transforming the equation into the form of I=YU+J, we get formula (23):

[0119] (L(t)+Δt·R)I(t)=ΔtU(t)-Δte(t-Δt)+L(t-Δt)I(t-Δt) (23)

[0120] In order to make the electrical interface between the generator and the external electrical network, the electrical quantity on the dq0 axis can be transformed into the abc coordinate system using the Park transformation. The stator and rotor side Park transformation matrix is ​​defined as T, as shown in formula (24):

[0121]

[0122] Among them, the stator side Parker transformation matrix P s The transformation angle is Rotor-side Parker transformation matrix P r The transformation angle is

[0123] After introducing Park transformation into the abc coordinate system, multiply both sides by have to:

[0124]

[0125] Step 4:

[0126] Separate the stator equation from the rotor equation, write the relationship between the stator and rotor currents and voltages in block matrix form, and process Equation (25) block by block to obtain the equivalent circuit model of DFIG, as shown in Equation (26):

[0127]

[0128] Specifically expanded as follows:

[0129]

[0130] At the same time, the motion equation in step 1 is discretized to obtain the speed expression and rotor angle expression.

[0131]

[0132] Where G abcs (t) and G abcr (t) are the stator and rotor admittance matrices respectively; I abcsEQ (t-Δt) and I abcrEQ (t-Δt) are the column vectors of the historical values ​​of the stator and rotor equivalent current sources respectively; U abcs (t) is the stator voltage column vector; U abcr (t) is the stator voltage column vector; J is the angular moment of inertia; K D is the mechanical damping coefficient; T m is the mechanical torque; T e is the electromagnetic torque of the generator.

[0133] Step 5:

[0134] According to the expression described in step 4, the equivalent model of the doubly fed induction generator under internal fault conditions can be obtained, including the electromagnetic transient equivalent circuit and the electromechanical transient model. The decoupling equivalent circuit under internal fault conditions of the doubly fed induction generator is represented by a controlled current source. In the equivalent circuit equation under internal fault conditions of the doubly fed induction generator, the admittance matrix is ​​asymmetric. The controlled current source form is used to equate the doubly fed induction generator, and the size of the controlled current source is calculated by the discretized mathematical equation under different fault types, which is related to the fault type. The DFIG equivalent circuit is shown in the attached figure. Figure 4 shown.

[0135] Step 6:

[0136] The equivalent circuit of the doubly fed induction generator obtained in step 5 is placed in the EMTDC solution system. First, the reference values ​​used in the normalization process are calculated and the calculation results are stored in the register to prevent additional calculations for each update. Then, the fault type is determined. According to different fault types, the fault characteristics are determined, and the changes in resistance, inductance, and flux linkage terms are determined, focusing on the changes in the block matrices A / B / C / D and J(t-Δt). Secondly, the equivalent circuit parameters of the generator are calculated: that is, the admittance matrix Gabcs (t), G abcr (t) and historical current source value I abcsEQ (t-Δt), I abcrEQ (t-Δt); Then, the DFIG equivalent circuit is directly connected to the external electrical network, and a current source is injected into the external network. The EMTDC solver iteratively solves the node admittance matrix of the overall circuit of the DFIG equivalent circuit and the external electrical network. Next, the generator stator and rotor currents, electromagnetic torque, rotor angle, rotor speed and other information are iteratively updated according to the external terminal voltage. Finally, the updated parameters are stored as the historical values ​​of the next step, and then the next step simulation is entered. The modeling process under the DFIG internal fault condition is shown in the attached figure. Figure 5 shown.

[0137] In the above steps 1 to 6, the previous step is the basis for the execution of the latter step. These 6 modeling steps are closely linked to each other and executed sequentially, forming an organic and indivisible whole.

[0138] The above description is only a preferred specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any technician familiar with the technical field can make equivalent replacements or changes based on the technical solutions and improved concepts of the present invention within the technical scope disclosed by the present invention, and these changes should be covered by the scope of protection of the present invention.

Claims

1. An electromagnetic transient modeling and simulation method for internal faults of a doubly-fed induction generator, characterized in that: The steps include: S1. Consider the stator-rotor turns ratio, convert the rotor quantity to the stator side, and use Park transformation to map the original equation of the abc coordinate system to the dq0 coordinate system to obtain the voltage equation, flux equation, and motion equation under the non-fault state; S2. Determine fault characteristics based on different fault types and establish electromagnetic transient and electromechanical transient mathematical models for doubly-fed induction generators under different fault conditions, focusing on the changes in the structure of the voltage equation and flux equation as well as the resistance matrix and inductance matrix, and clarify the parameter correction rules; S3, select flux as the state variable, use the backward Euler integration method to discretize the flux-voltage equation in the dq0 coordinate system, and obtain the relationship between the stator and rotor currents and voltages; S4. Separate the stator equation from the rotor equation, write the relationship between the stator and rotor currents and voltages in block matrix form, and obtain a unified stator and rotor current expression; at the same time, discretize the motion equation obtained in S1 to obtain the speed expression and rotor angle expression; S5. Based on the expression obtained in S4, an equivalent model of the doubly fed induction generator under internal fault is constructed, including an electromagnetic transient equivalent circuit and an electromechanical transient model, and a decoupling equivalent circuit under internal fault state of the doubly fed induction generator is represented by a controlled current source; S6. Input the equivalent circuit obtained in S5 into the electromagnetic transient simulation software to perform iterative solution of the node admittance moment; update the generator stator and rotor current, electromagnetic torque, rotor speed, and rotor angle information according to the external terminal voltage.

2. The electromagnetic transient modeling and simulation method for internal faults of a doubly-fed induction generator according to claim 1, characterized in that: The voltage equation, flux equation, and motion equation under the non-fault state described in S1 are expressed as follows: Voltage equation: Magnetic flux equation: Equations of motion: Wherein, the subscripts ds, qs, and 0s represent stator windings, and dr, qr, and 0r represent rotor windings; ω r is the rotor angular velocity, ω1 is the synchronous rotation angular velocity of the stator magnetic field, ω s is the angular velocity of the rotor relative to the stator magnetic field, which is (ω1-ω r ); e is the velocity electromotive force column vector; L s and L' r are the stator and rotor self-inductance of DFIG respectively, and L s =L ls +L m , L' r =L' lr +L m ;T m is the mechanical torque; T e is the electromagnetic torque of the generator; K D is the mechanical damping coefficient; J is the moment of inertia of the rotor.

3. The electromagnetic transient modeling and simulation method for internal faults of a doubly-fed induction generator according to claim 2, characterized in that: The S3 specifically includes the following contents: S3.

1. Use the backward Euler method to discretize the voltage equation shown in equation (1) and obtain: Ψ(t)-Ψ(t-Δt)=Δt[U(t)-RI(t)-e(t)] (4) Among them, U is the voltage column vector, R is the resistance matrix, I is the current column vector, and Ψ is the flux column vector; S3.

2. Substitute the flux equation shown in formula (2) into formula (4), and treat the velocity electromotive force column vector e as a historical value, that is, e(t)≈e(t-Δt), and then obtain: L(t)I(t)-L(t-Δt)I(t-Δt)=Δt[U(t)-RI(t)-e(t-Δt)] (5) S3.

3. Convert the equation to the form I = YU + J and get: (L(t)+Δt·R)I(t)=ΔtU(t)-Δte(t-Δt)+L(t-Δt)I(t-Δt) (6) S3.

4. Use Parker transformation to transform the electrical quantities on the dq0 axis into the abc coordinate system to establish the electrical interface between the generator and the external electrical network. Define the stator and rotor side Parker transformation matrix as T: Among them, P s represents the stator side Parker transformation matrix, and its transformation angle is P r represents the rotor side Parker transformation matrix, and its transformation angle is S3.

5. After introducing Parker transformation into equation (6) and converting it into the abc coordinate system, multiply both sides by P T -1 (L(t)+Δt·R) -1 The relationship between the stator and rotor current and voltage is obtained:

4. The electromagnetic transient modeling and simulation method for internal faults of a doubly-fed induction generator according to claim 3, characterized in that: The stator and rotor current expressions, speed expressions, and rotor angle expressions described in S4 are: I abcs (t)=G abcs (t)U abcs (t)+I abcsEQ (t-Δt) (9) I abcr (t)=G abcr (t)U abcr (t)+I abcrEQ (t-Δt) (10) θ(t)=θ(t-Δt)+Δtω r (t) (12) Among them, I abcs (t) represents the stator current; I abcr (t) represents the rotor current; ω r (t) represents the speed; θ(t) represents the rotor angle; the subscript abcs represents the stator winding in the abc coordinate system, and the subscript abcr represents the rotor winding in the abc coordinate system; G abcs (t) and G abcr (t) are the stator and rotor admittance matrices respectively; I abcsEQ (t-Δt) and I abcrEQ (t-Δt) are the column vectors of the historical values ​​of the stator and rotor equivalent current sources respectively; U abcs (t) is the stator voltage column vector; U abcr (t) is the stator voltage column vector; J is the angular moment of inertia; K D is the mechanical damping coefficient; T m is the mechanical torque; T e is the electromagnetic torque of the generator.

5. The electromagnetic transient modeling and simulation method for internal faults of a doubly-fed induction generator according to claim 4, characterized in that: The admittance matrix in the equation of the decoupling equivalent circuit of the doubly fed induction generator under an internal fault state described in S5 is asymmetric, and the doubly fed induction generator is equivalent to a controlled current source. The size of the controlled current source is calculated by discretizing mathematical equations under different fault types and is related to the fault type.

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