Fault current calculation method and device for IIDG under asymmetric fault condition

Abstract

The invention provides a fault current calculation method and device of an IIDG under an asymmetric fault condition, and relates to the technical field of power system fault analysis and protection. The method comprises the following steps: firstly, constructing a weak dynamic operation model of a weak alternating current system under an IIDG asymmetric fault condition, wherein the weak dynamic operation model comprises a PCC voltage weak dynamic part, a PLL directional weak dynamic part, an IIDG control loop weak dynamic part and a weak alternating current system and transmission line weak dynamic part; based on a weak dynamic operation model, a weak network side weak dynamic transmission equation is obtained, then IIDG control characteristics are considered and considered, a machine-network closed-loop interaction weak dynamic model is constructed, and then the fault current of the IIDG under the asymmetric fault condition is calculated. According to the method, the current calculation accuracy of the IIDG in the weak alternating current system under the asymmetric fault can be effectively improved.

Description

Technical Field

[0001] This invention relates to the field of power system fault analysis and protection technology, specifically to a method and apparatus for calculating fault current under asymmetrical fault conditions in IIDG (Integrated Inverterless Gate) faults in weak AC power grids. Background Technology

[0002] The widespread integration of inverter-interfaced distributed generation (IIDG) sources, such as wind and solar power, into the power grid has profoundly impacted the fault current characteristics of traditional power systems dominated by synchronous generators. The fault current characteristics of synchronous generator-dominated power systems are determined by the dynamic characteristics of the synchronous generators; due to the high inertia of the rotor supporting system frequency and voltage, the influence of fast-response devices such as power electronics can be neglected. However, with the increasing penetration rate of IIDGs, the system inertia level decreases rapidly. Because IIDGs possess characteristics such as fast fault response speed, wide time scale, and complex control loops, and because different control strategies can lead to drastically different fault characteristics, traditional methods for calculating fault currents in AC power grids are no longer applicable.

[0003] In recent years, the fault current output characteristics and equivalent models of IIDGs have been extensively studied. Existing research, when calculating the steady-state current of an asymmetrical fault system, typically assumes that the IIDG fault steady-state current equals a set current reference value, with all transient components decaying. For IIDGs with negative-sequence current control, the post-fault steady-state IIDG has a negative-sequence current output equal to the given negative-sequence reference value. Otherwise, the asymmetrical fault steady-state IIDG only outputs positive-sequence current. However, as the proportion of IIDGs connected increases, the AC-side grid strength further weakens, the AC-side equivalent impedance increases, and the multi-stage, multi-scale interaction between the weak grid and the IIDG intensifies. During grid faults, the IIDG fault behavior no longer transitions from a transient state to a steady state, but rather operates with weak dynamics continuously near the steady state. (For example, in a weak AC system, the PLL (phase-locked loop) needs to reduce its bandwidth to maintain system stability, but this also reduces the dynamic performance of the PLL. In unbalanced conditions, the PCC (Point of Common Coupling) voltage vector is in a state of frequency doubling fluctuation due to the superposition of positive and negative sequence voltage vectors, and the PLL output phase will continuously fluctuate and have a time-varying error in the true value.) This leads to a large error when using the steady-state solution of the differential equation system or the current reference value as the steady-state value of the fault current. Summary of the Invention

[0004] The purpose of this invention is to provide a method and apparatus for calculating the fault current of an IIDG under asymmetrical fault conditions, which solves the problem that existing technologies use steady-state solutions of differential equations or current reference values ​​as steady-state values ​​of fault current with large errors, and can effectively improve the accuracy of current calculation of an IIDG under asymmetrical fault conditions in weak AC systems.

[0005] To achieve the above objectives, in a first aspect, the present invention provides a method for calculating the fault current of an IIDG under asymmetrical fault conditions, comprising: A weak dynamic operation model of a weak AC system under IIDG asymmetric fault conditions is constructed, including the weak dynamic part of PCC voltage, the weak dynamic part of PLL orientation, the weak dynamic part of IIDG control loop, and the weak dynamic part of the weak AC system and transmission line. Based on the weak dynamic operation model, the weak dynamic transmission equation of the weak network side is obtained. Then, taking into account the control characteristics of IIDG, a machine-network closed-loop interactive weak dynamic model is constructed, and the fault current of IIDG under asymmetrical fault conditions is calculated.

[0006] According to the present invention, a method for calculating the fault current of an IIDG under asymmetrical fault conditions includes a weak dynamic part of the PCC voltage, comprising: The phase-locked loop function of the PLL is implemented based on PI control. q Axis voltage components v q Its reference value v qref =0 static error-free tracking; voltage sequence component transformation to dq The expression for calculating a rotating coordinate system is: (1) In formula (1) v q =0, obtaining the target phase of PLL control during the voltage fault. θ pcc for: (2) Where ω0 is the steady-state value of the system's angular frequency; t is time; V 1. V 2 represents the amplitude of the positive-sequence and negative-sequence voltages, respectively; θ 1. θ 2 represents the phase of the positive-sequence and negative-sequence voltages, respectively; Substituting equation (2) into equation (1), we obtain the PCC voltage amplitude as follows: (3) Linearizing equations (2) and (3), we obtain the weak dynamic equation of the PCC voltage as follows: (4) Among them, A coup This is the dynamic coefficient matrix of the PCC sequence components; ΔV1, ΔV2, Δθ1, and Δθ2 represent small perturbations in the positive-sequence voltage amplitude, negative-sequence voltage amplitude, positive-sequence voltage phase, and negative-sequence voltage phase, respectively; Δ θ pcc For small perturbations in the phase of the PCC voltage; Δ V pcc This represents a small disturbance in the PCC voltage amplitude. According to the fault current calculation method of IIDG under asymmetrical fault conditions provided by the present invention, the expression of each element of the dynamic coefficient matrix of PCC sequence component is as follows: (5) Among them, V 10 V 20 θ 10 θ 20 , V pcc0 These represent the steady-state values ​​of the positive-sequence voltage, negative-sequence voltage, positive-sequence voltage phase, negative-sequence voltage phase, and PCC voltage amplitude, respectively. According to the present invention, a method for calculating the fault current of an IIDG under asymmetrical fault conditions includes a PLL directional weak dynamic component comprising: PLL phase orientation error Δ θ pll By influencing the feedforward voltage of the inner current loop v pccd , v pccq This affects the IIDG port voltage; the expression for calculating the feedforward voltage is: (6) in, V pcc PCC voltage, θ pll Indicates the PLL output phase; PLL phase transfer function expression T pll ( s )for: (7) in, K p pll , K i pll These are the proportional and integral parameters of the PI controller in the PLL, respectively; s is the Laplace operator. Linearizing equation (6) and substituting equation (7) into it, we obtain the weak dynamic equation of the PLL as follows: (8) Where, Δ v pccd Δ v pccq Feedforward voltage v pccd , v pccq Small perturbation, H PLL This is the PLL weak dynamic characteristic matrix; θ pll0 , θ pcc0 These are the steady-state phase value of the PLL output and the steady-state phase value of the PLL control target, respectively. According to the present invention, a method for calculating the fault current of an IIDG under asymmetrical fault conditions is provided, wherein the weak dynamic part of the IIDG control loop includes: IIDG feedback active power P reactive power Q Compared with reference active power P ref reactive power Q ref The reference current is jointly regulated by the IIDG power outer loop. i dref , i qref The change, calculated as follows: (9) in, K p P , K i P These are the proportional and integral coefficients of the IIDG power outer loop PI regulator, respectively; Δ P ref With Δ Q ref These represent small disturbances in the reference active power and reactive power, respectively; Δ P Δ Q These are the feedback active power and reactive power small disturbances, respectively. Disturbances caused by asymmetrical faults affect the phase of the PCC voltage, and are then altered by the weak dynamic characteristics of the PLL output. dq The transformed reference phase ultimately affects the feedforward voltage. v pccd , v pccq With feedback current i d ,i q ; and reference current i dref , i qref The IIDG port voltage is constructed using the inner current loop as follows: (10) in, v invd and v invq These are the d-axis and q-axis components of the IIDG port voltage, respectively. K p I , K i I These are the proportional and integral parameters of the inner loop PI regulator, respectively, and L is the filter inductance. ω The system angular frequency; Combining equations (9) and (10) and linearizing them at the operating point, we obtain the IIDG control loop equations as follows: (11) Among them, H I H P H Id These are the current inner loop matrix, the power outer loop matrix, and the current inner loop decoupling matrix, respectively.

[0007] According to the present invention, a method for calculating fault current of an IIDG under asymmetrical fault conditions is provided, wherein the weak dynamic part of the weak AC system and transmission line includes: The dynamic equations of a weak AC system in a rotating coordinate system are: (12) in, L g and L f These are the equivalent inductance of the AC power grid and the filter inductance, respectively. v gd , v gq These are the d-axis and q-axis components of the grid-side voltage, respectively. R g The equivalent resistance of the AC power grid; Linearizing equation (12) at the running point and performing a Laplace transform, we obtain the weak dynamic equation for line transmission in a weak AC system as follows: (13) Among them, Z g Z is the transmission matrix for grid-connected lines, and Z is the transmission matrix for filtered lines; Δ vinvd Δ v invq These are the small perturbations of the d-axis and q-axis components of the IIDG port voltage, respectively; Δ i d Δ i q Feedback current i d , i q Small perturbations; IIDG feedback active power P reactive power Q The linearized power equation is calculated as follows: (14) Among them, i dq0 v dq0 These are the steady-state value matrices for current and voltage, respectively. According to the present invention, a method for calculating the fault current of an IIDG under asymmetrical fault conditions is provided. Based on a weak dynamic operation model, the weak dynamic transmission equations on the weak network side are obtained. Then, considering the control characteristics of the IIDG, a machine-grid closed-loop interactive weak dynamic model is constructed, and the fault current of the IIDG under asymmetrical fault conditions is calculated, including: Substituting the PCC voltage weak dynamic equation (4), PLL weak dynamic equation (8), and weak AC system line transmission weak dynamic equation (13) into the power equation (14), we obtain the weak dynamic transmission equation on the weak network side as follows: (15) Among them, H grid The open-loop power transmission matrix on the network side is expressed as: (16) Among them, Z g -1 It is the inverse matrix of the grid-connected transmission matrix; Considering the IIDG control characteristics, a machine-network closed-loop interactive weak dynamic model is constructed; the IIDG control loop equation (11) and the weak dynamic equation (13) of the weak AC system line transmission are combined, and the intermediate variable Δ is eliminated through the PLL weak dynamic equation (8). i d Δ i q After simplification, the open-loop power transfer equation on the IIDG side can be obtained as follows: (17) Among them, H inv The open-loop power transfer matrix on the IIDG side is expressed as follows: (18) in, It is a 2×2 unit diagonal matrix; Therefore, the formula for calculating the fault current considering the dynamic operation of a weak AC system is: (19) Among them, A 12 A 21 The dynamic coefficient matrix A of the PCC order components are respectively coup The column vector is expressed as: (20).

[0008] According to the present invention, a method for calculating fault current of IIDG under asymmetrical fault conditions is provided, wherein the short-circuit current ratio of the weak AC system is less than 3. According to the present invention, a method for calculating fault current of IIDG under asymmetrical fault conditions is provided. Asymmetrical faults include imbalance in the amplitude and phase of three-phase voltage or current.

[0009] In a second aspect, the present invention provides a fault current calculation device for an IIDG under asymmetrical fault conditions, comprising: The modeling unit is used to construct a weak dynamic operation model of a weak AC system under IIDG asymmetric fault conditions, including the weak dynamic part of PCC voltage, the weak dynamic part of PLL orientation, the weak dynamic part of IIDG control loop, and the weak dynamic part of weak AC system and transmission line. The calculation unit is used to obtain the weak dynamic transmission equations on the weak network side based on the weak dynamic operation model, and then, taking into account the control characteristics of the IIDG, construct a machine-network closed-loop interactive weak dynamic model, and then calculate the fault current of the IIDG under asymmetrical fault conditions.

[0010] This invention has at least the following technical effects: This invention provides a method and apparatus for calculating the fault current of an IIDG (Integrated Induction Generator) under asymmetrical fault conditions. First, a weak dynamic operating model of the weak AC system under IIDG asymmetrical fault conditions is constructed, including a weak dynamic component of the PCC (Physical Control Center) voltage, a weak dynamic component of the PLL (Programmable Logic Controller), a weak dynamic component of the IIDG control loop, and a weak dynamic component of the weak AC system and transmission lines. Based on the weak dynamic operating model, the weak dynamic transmission equations on the weak network side are obtained. Then, considering the IIDG control characteristics, a machine-network closed-loop interactive weak dynamic model is constructed, and the fault current of the IIDG under asymmetrical fault conditions is calculated. This invention can effectively improve the accuracy of IIDG current calculation under asymmetrical fault conditions in weak AC systems. Attached Figure Description

[0011] To more clearly illustrate the technical solutions in this invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of this invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0012] In the attached diagram: Figure 1 This is a schematic diagram of the interactive signal flow of the weak dynamic operation model of the present invention; Figure 2 The transfer function block diagram for the machine-network closed-loop weak dynamic interaction of this invention is shown below. Figure 3 This is a PCC current sequence component curve diagram of a specific embodiment of the present invention; Figure 4 This is a flowchart of the fault current calculation method of the IIDG under asymmetrical fault conditions according to the present invention. Detailed Implementation

[0013] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of this invention. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0014] The following detailed description of some embodiments of the present invention will be provided in conjunction with the accompanying drawings. Unless otherwise specified, the following embodiments and features can be combined with each other.

[0015] First, the technical terms used in this invention will be explained.

[0016] A weak AC system refers to an AC power grid system with low short-circuit capacity and relatively high equivalent impedance. In weak AC systems (generally considered to have a short-circuit ratio (SCR) of less than 3), the significant increase in the penetration rate of new energy power generation (such as photovoltaic and wind power) has led to a decrease in the proportion of synchronous generators in the power grid, resulting in weakened grid inertia, reduced dynamic regulation capability, and decreased system strength.

[0017] Inverter-type distributed generation systems use inverters to convert direct current (DC) generated by distributed generators (such as photovoltaic modules or wind turbines) into alternating current (AC) and connect it to the power grid. This technology not only improves energy efficiency but also effectively integrates renewable energy into the grid, supporting the construction and stable operation of smart grids.

[0018] An asymmetrical fault refers to a type of fault in a power system where the amplitude or phase of the three-phase voltage or current is unbalanced. It is commonly seen in scenarios such as single-phase grounding and two-phase short circuits. Its characteristic is that there are significant negative-sequence or zero-sequence components at the fault point, resulting in an imbalance of electrical quantities in the system.

[0019] Please see Figure 4 This invention provides a method for calculating the fault current of an IIDG (Inverter-Inverter Generator) under asymmetrical fault conditions, applicable to weak AC systems. It primarily considers the multi-stage interaction between the converter (IIDG) and the weak power grid, especially the continuous orientation error of the phase-locked loop (PLL) under asymmetrical fault conditions. The fault current calculation method includes: Step 1: Construct a weak dynamic operation model of the weak AC system under IIDG asymmetrical fault conditions, including the weak dynamic part of PCC voltage, the weak dynamic part of PLL orientation, the weak dynamic part of IIDG control loop, and the weak dynamic part of the weak AC system and transmission line. Specifically, such as Figure 1 As shown, based on the sequential response characteristics of the system after an asymmetrical voltage fault, this weak dynamic operation model is divided into four stages to discuss the weak dynamic process of the system: "PCC voltage weak dynamic - PLL directional weak dynamic - IIDG control loop weak dynamic - weak AC system and transmission line weak dynamic".

[0020] PCC voltage weak dynamic range.

[0021] The phase-locked loop function of the PLL is implemented based on PI control. q Axis voltage components v q Its reference value v qref Static error-free tracking with =0. Considering the transformation of voltage sequence components to... dq The rotating coordinate system has the following expression: (1) In formula (1) v q =0, which gives the target phase of PLL control during voltage faults. θ pcc for: (2) Where ω0 is the steady-state value of the system's angular frequency; t is time; V 1. V 2 represents the magnitude of the positive-sequence and negative-sequence voltage vectors. θ 1. θ 2 represents the phase of the positive-sequence and negative-sequence voltage vectors, respectively.

[0022] Substituting equation (2) into equation (1), we obtain the PCC voltage amplitude as shown in equation (3): (3) Linearizing equations (2) and (3), we obtain the weak dynamic equation of the PCC voltage as follows: (4) Where Δ represents the small perturbation of the physical quantity. ΔV1, ΔV2, Δθ1, and Δθ2 represent the small perturbations of the positive-sequence voltage amplitude, negative-sequence voltage amplitude, positive-sequence voltage phase, and negative-sequence voltage phase, respectively; Δ θ pcc For small perturbations in the phase of the PCC voltage; Δ V pcc This represents a small disturbance in the PCC voltage amplitude. A coup This is the dynamic coefficient matrix of the PCC order components.

[0023] PCC Order Component Dynamic Coefficient Matrix A coup The calculation expressions for each element are shown in equation (5). The dynamic coefficient matrix A of the PCC order components. coup The dynamic mechanism of PCC voltage vector amplitude and phase fluctuations induced at different operating points is described quantitatively.

[0024] (5) Among them, V 10 V 20 θ 10 θ 20 , V pcc0 These represent the steady-state values ​​of the positive-sequence voltage, negative-sequence voltage, positive-sequence voltage phase, negative-sequence voltage phase, and PCC voltage amplitude, respectively.

[0025] PLL directional weak dynamic part.

[0026] PLL phase orientation error Δ θ pll By influencing the feedforward voltage of the inner current loop v pccd , v pccq This affects the IIDG port voltage. The expression for calculating the feedforward voltage is as follows: (6) in, V pcc PCC voltage, θ pll This indicates the output phase of the PLL.

[0027] Considering the phase transfer function expression of the PLL Tpll ( s As shown in equation (7): (7) in, K p pll , K i pll ... (8) Where, Δ v pccd Δ v pccq Feedforward voltage v pccd , v pccq Small perturbation, H PLL This is the PLL weak dynamic characteristic matrix; θ pll0 , θ pcc0 These are the steady-state phase value of the PLL output and the steady-state phase value of the PLL control target, respectively.

[0028] Weak dynamic part of the IIDG control loop.

[0029] IIDG feedback active power P reactive power Q Compared with reference active power P ref reactive power Q ref The reference current is jointly regulated by the IIDG power outer loop. i dref , i qref The change and calculation expression are shown in equation (9): (9) in, K p P , K i P For the proportional and integral coefficients of the IIDG power outer loop PI regulator; Δ P ref With Δ Q ref These represent small disturbances in the reference active power and reactive power, respectively; Δ P ΔQ These are the feedback active power and reactive power small disturbances, respectively.

[0030] Disturbances caused by asymmetrical faults affect the phase of the PCC voltage, and are then altered by the weak dynamic characteristics of the PLL output. dq The transformed reference phase ultimately affects the feedforward voltage. v pccd , v pccq With feedback current i d , i q They are related to the reference current. i dref , i qref The IIDG port voltage is constructed together in the inner current loop as shown in equation (10): (10) in, v invd and v invq These are the d-axis and q-axis components of the IIDG port voltage, respectively. K p I , K i I These are the proportional and integral parameters of the inner current loop PI regulator, respectively, and L is the filter inductance.

[0031] Combining equations (9) and (10) and linearizing them at the operating point, we can obtain the IIDG control loop equations, as shown in equation (11): (11) Among them, H I H P H Id These are the current inner loop matrix, the power outer loop matrix, and the current inner loop decoupling matrix, respectively.

[0032] Weak AC systems and the weak dynamic components of transmission lines.

[0033] The dynamic equations of the weak AC system in the rotating coordinate system are shown in equation (12): (12) in, L g and L f These are the equivalent inductance of the AC power grid and the filter inductance, respectively. v gd , v gqThese are the d-axis and q-axis components of the grid-side voltage, respectively. R g This is the equivalent resistance of the AC-side power grid.

[0034] Linearizing equation (12) at the running point and performing a Laplace transform on both sides of the equation, we obtain the weak dynamic equation for line transmission in a weak AC system as shown in equation (13): (13) Among them, Z g Z is the line transmission characteristic matrix, and Z is the filtered line transmission matrix; Δ v invd Δ v invq These are the small perturbations of the d-axis and q-axis components of the IIDG port voltage, respectively; Δ i d Δ i q Feedback current i d , i q Small perturbation.

[0035] IIDG feedback active power P reactive power Q The linearized power equation is calculated as follows: (14) Among them, i dq0 v dq0 This is a matrix of steady-state values ​​for current and voltage.

[0036] Step 2: Based on the weak dynamic operation model, obtain the weak dynamic transmission equation on the weak network side. Then, taking into account the IIDG control characteristics, construct the machine-network closed-loop interactive weak dynamic model, and then calculate the fault current of IIDG under asymmetrical fault conditions.

[0037] Specifically, by substituting the PCC voltage weak dynamic equation (4), the PLL weak dynamic equation (8), and the weak AC system line transmission weak dynamic equation (13) into the power equation (14), the weak network side weak dynamic transmission equation is obtained as shown in equation (15): (15) Among them, H grid The open-loop power transmission matrix on the network side is expressed as (16): (16) Next, we consider incorporating the IIDG control characteristics, i.e., the IIDG machine-side dynamic characteristics, to construct a machine-grid closed-loop interactive weak dynamic model. We simultaneously establish the IIDG control loop equation (11) and the weak dynamic equation (13) of the weak AC system line transmission, and eliminate the intermediate variable Δ through the PLL weak dynamic equation (8). i d Δ i q After simplification, the open-loop power transfer equation on the IIDG side can be obtained, as shown in equation (17): (17) Among them, H inv This is the IIDG side open-loop power transfer matrix, which describes the changes in reference power and feedback power through the generator side to Δ. θ pcc Δ V pcc The impact of H. inv The expression is shown in equation (18): (18) In the formula, It is a 2×2 unit diagonal matrix.

[0038] From equations (15) and (17), the transfer function block diagram of the machine-network closed-loop weak dynamic interaction can be obtained as follows: Figure 2 As shown.

[0039] Therefore, the calculation expression for fault current considering the dynamic operation of a weak AC system can be derived (19). (19) In equation (19), A 12 A 21 All are composed of the dynamic coefficient matrix A of the PCC order components. coup The column vectors are formed as shown in equation (20): (20) In summary, this invention provides a method for calculating the fault current of an IIDG (Inverter-Inverter-Governed Controller) in a weak AC system under asymmetrical fault conditions. It particularly considers the error caused by the interaction between the converter and the weak AC system under asymmetrical operating conditions, which leads to the influence of the system's weak dynamic operation on the magnitude of the fault current. A weak dynamic operation model of the system is established, quantifying the magnitude of the fault current change caused by weak dynamic operation.

[0040] This invention quantifies the magnitude of fault current caused by the weak dynamic characteristics of various components of the system during asymmetrical faults in weak networks. Furthermore, it establishes an IIDG fault current calculation process that considers the weak dynamic characteristics of multiple components such as PLLs, control loops, and transmission lines, correcting the calculation error of fault current caused by the weak dynamic operation of the system and improving the calculation accuracy of asymmetrical current.

[0041] Based on the same inventive concept, another embodiment of the present invention provides a fault current calculation device for IIDG under asymmetrical fault conditions. This device corresponds to the method of the foregoing embodiment and includes: The modeling unit is used to construct a weak dynamic operation model of a weak AC system under IIDG asymmetric fault conditions, including the weak dynamic part of PCC voltage, the weak dynamic part of PLL orientation, the weak dynamic part of IIDG control loop, and the weak dynamic part of weak AC system and transmission line. The calculation unit is used to obtain the weak dynamic transmission equations on the weak network side based on the weak dynamic operation model, and then, taking into account the control characteristics of the IIDG, construct a machine-network closed-loop interactive weak dynamic model, and then calculate the fault current of the IIDG under asymmetrical fault conditions.

[0042] The following are specific embodiments of the present invention.

[0043] To verify the correctness and effectiveness of the weak dynamic operation model (SCC model) provided in this invention, a simulation system for a single-converter grid-connected system was designed. The IIDG employs a voltage vector oriented control strategy, and the grid-side short-circuit ratio is set to 2.80 for a weak AC system. At 10 seconds, the voltage of phase A on the grid side is allowed to drop by 20% of its rated voltage to simulate the disturbance caused by an asymmetrical fault on the AC side. Using the fault current calculation method proposed in this invention, the magnitudes of the positive-sequence and negative-sequence voltages and currents of the PCC after the system fault are calculated and compared with the simulation results of electromagnetic transient (EMT) in MATLAB / Simulink. Figure 3 As can be seen from the EMT current simulation curves, the error of the positive-sequence and negative-sequence current fault steady-state values ​​calculated using the method of this invention is significantly reduced.

[0044] Other embodiments of the invention will readily occur to those skilled in the art upon consideration of the specification and practice of the embodiments disclosed herein. This invention is intended to cover any variations, uses, or adaptations of the invention that follow the general principles of the invention and include common knowledge or customary techniques in the art not disclosed herein. It should be understood that the invention is not limited to the precise structures described above and shown in the accompanying drawings, and various modifications and changes can be made without departing from its scope. The scope of the invention is limited only by the appended claims.

Claims

1. A method for calculating the fault current of an IIDG under asymmetrical fault conditions, characterized in that, include: A weak dynamic operation model of a weak AC system under IIDG asymmetric fault conditions is constructed, including the weak dynamic part of PCC voltage, the weak dynamic part of PLL orientation, the weak dynamic part of IIDG control loop, and the weak dynamic part of the weak AC system and transmission line. The IIDG is an inverter-type distributed power source, the PCC is a common coupling point, and the PLL is a phase-locked loop; Based on the aforementioned weak dynamic operation model, the weak dynamic transmission equations on the weak network side are obtained. Then, taking into account the control characteristics of the IIDG, a machine-network closed-loop interactive weak dynamic model is constructed, and the fault current of the IIDG under asymmetrical fault conditions is calculated.

2. The method for calculating the fault current of an IIDG under asymmetrical fault conditions according to claim 1, characterized in that, The weak dynamic portion of the PCC voltage includes: The phase-locked loop function of the PLL is implemented based on PI control. q Axis voltage components v q Its reference value v qref =0 static error-free tracking; voltage sequence component transformation to dq The expression for calculating a rotating coordinate system is: (1) In formula (1) v q =0, obtaining the target phase of PLL control during the voltage fault. θ pcc for: (2) Where ω0 is the steady-state value of the system's angular frequency; t is time; V 1. V 2 represents the amplitude of the positive-sequence and negative-sequence voltages, respectively; θ 1. θ 2 represents the phase of the positive-sequence and negative-sequence voltages, respectively; Substituting equation (2) into equation (1), we obtain the PCC voltage amplitude as follows: (3) Linearizing equations (2) and (3), we obtain the weak dynamic equation of the PCC voltage as follows: (4) Among them, A coup This is the dynamic coefficient matrix of the PCC sequence components; ΔV1, ΔV2, Δθ1, and Δθ2 represent small perturbations in the positive-sequence voltage amplitude, negative-sequence voltage amplitude, positive-sequence voltage phase, and negative-sequence voltage phase, respectively; Δ θ pcc For small disturbances in the phase of the PCC voltage; Δ V pcc This represents a small disturbance in the PCC voltage amplitude.

3. The method for calculating the fault current of an IIDG under asymmetrical fault conditions according to claim 2, characterized in that, The expressions for each element of the PCC order component dynamic coefficient matrix are as follows: （5） Among them, V 10 V 20 θ 10 θ 20 , V pcc0 These represent the steady-state values ​​of the positive-sequence voltage, negative-sequence voltage, positive-sequence voltage phase, negative-sequence voltage phase, and PCC voltage amplitude, respectively.

4. The method for calculating the fault current of an IIDG under asymmetrical fault conditions according to claim 2, characterized in that, The PLL directional weak dynamic component includes: PLL phase orientation error Δ θ pll By influencing the feedforward voltage of the inner current loop v pccd , v pccq This affects the IIDG port voltage; the expression for calculating the feedforward voltage is: （6） in, V pcc PCC voltage, θ pll Indicates the PLL output phase; PLL phase transfer function expression T pll ( s )for: （7） in, K p pll , K i pll These are the proportional and integral parameters of the PI controller in the PLL, respectively; s is the Laplace operator. Linearizing equation (6) and substituting equation (7) into it, we obtain the weak dynamic equation of the PLL as follows: （8） Where, Δ v pccd Δ v pccq Feedforward voltage v pccd , v pccq Small perturbation, H PLL This is the PLL weak dynamic characteristic matrix; θ pll0 , θ pcc0 These are the steady-state phase value of the PLL output and the steady-state phase value of the PLL control target, respectively.

5. The method for calculating the fault current of an IIDG under asymmetrical fault conditions according to claim 4, characterized in that, The weak dynamic part of the IIDG control loop includes: IIDG feedback active power P reactive power Q Compared with reference active power P ref reactive power Q ref The reference current is jointly regulated by the IIDG power outer loop. i dref , i qref The change, calculated as follows: (9) in, K p P , K i P These are the proportional and integral coefficients of the IIDG power outer loop PI regulator, respectively; Δ P ref With Δ Q ref These represent small disturbances in the reference active power and reactive power, respectively; Δ P Δ Q These are the feedback active power and reactive power small disturbances, respectively. Disturbances caused by asymmetrical faults affect the phase of the PCC voltage, and are then altered by the weak dynamic characteristics of the PLL output. dq The transformed reference phase ultimately affects the feedforward voltage. v pccd , v pccq With feedback current i d , i q ; and reference current i dref , i qref The IIDG port voltage is constructed using the inner current loop as follows: (10) in, v invd and v invq These are the d-axis and q-axis components of the IIDG port voltage, respectively. K p I , K i I These are the proportional and integral parameters of the inner loop PI regulator, respectively, and L is the filter inductance. ω The system angular frequency; Combining equations (9) and (10) and linearizing them at the operating point, we obtain the IIDG control loop equations as follows: (11) Among them, H I H P H Id These are the current inner loop matrix, the power outer loop matrix, and the current inner loop decoupling matrix, respectively.

6. The method for calculating the fault current of an IIDG under asymmetrical fault conditions according to claim 5, characterized in that, The weak dynamic component of the weak AC system and transmission line includes: The dynamic equations of a weak AC system in a rotating coordinate system are: (12) in, L g and L f These are the equivalent inductance of the AC power grid and the filter inductance, respectively. v gd , v gq These are the d-axis and q-axis components of the grid-side voltage, respectively. R g The equivalent resistance of the AC power grid; Linearizing equation (12) at the running point and performing a Laplace transform, we obtain the weak dynamic equation for line transmission in a weak AC system as follows: (13) Among them, Z g Z is the transmission matrix for grid-connected lines, and Z is the transmission matrix for filtered lines; Δ v invd Δ v invq These are the small perturbations of the d-axis and q-axis components of the IIDG port voltage, respectively; Δ i d Δ i q Feedback current i d , i q Small perturbations; IIDG feedback active power P reactive power Q The linearized power equation is calculated as follows: （14） Among them, i dq0 v dq0 These are the steady-state value matrices for current and voltage, respectively.

7. The method for calculating the fault current of an IIDG under asymmetrical fault conditions according to claim 6, characterized in that, Based on the aforementioned weak dynamic operation model, the weak dynamic transmission equations on the weak network side are obtained. Then, considering the IIDG control characteristics, a machine-network closed-loop interactive weak dynamic model is constructed, and the fault current of the IIDG under asymmetrical fault conditions is calculated, including: Substituting the PCC voltage weak dynamic equation (4), PLL weak dynamic equation (8), and weak AC system line transmission weak dynamic equation (13) into the power equation (14), we obtain the weak dynamic transmission equation on the weak network side as follows: （15） Among them, H grid The open-loop power transmission matrix on the network side is expressed as: （16） Among them, Z g -1 It is the inverse matrix of the grid-connected transmission matrix; Considering the IIDG control characteristics, a machine-network closed-loop interactive weak dynamic model is constructed; the IIDG control loop equation (11) and the weak dynamic equation (13) of the weak AC system line transmission are combined, and the intermediate variable Δ is eliminated through the PLL weak dynamic equation (8). i d Δ i q After simplification, the open-loop power transfer equation on the IIDG side can be obtained as follows: （17） Among them, H inv The open-loop power transfer matrix on the IIDG side is expressed as follows: （18） in, It is a 2×2 unit diagonal matrix; Therefore, the formula for calculating the fault current considering the dynamic operation of a weak AC system is: （19） Among them, A 12 A 21 The dynamic coefficient matrix A of the PCC order components are respectively coup The column vector is expressed as: （20）。 8. The method for calculating the fault current of an IIDG under asymmetrical fault conditions according to claim 1, characterized in that, The short-circuit current ratio of the weak AC system is less than 3.

9. The method for calculating the fault current of an IIDG under asymmetrical fault conditions according to claim 1, characterized in that, The asymmetrical fault includes imbalance in the amplitude and phase of the three-phase voltage or current.

10. A fault current calculation device for IIDG under asymmetrical fault conditions, characterized in that, include: The modeling unit is used to construct a weak dynamic operation model of a weak AC system under IIDG asymmetric fault conditions, including the weak dynamic part of PCC voltage, the weak dynamic part of PLL orientation, the weak dynamic part of IIDG control loop, and the weak dynamic part of weak AC system and transmission line. The calculation unit is used to obtain the weak dynamic transmission equation on the weak network side based on the weak dynamic operation model, and then, taking into account the control characteristics of the IIDG, construct a machine-network closed-loop interactive weak dynamic model, and then calculate the fault current of the IIDG under asymmetrical fault conditions.

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