Method, system and device for judging stability condition of grid-following type converter, medium and product

Abstract

The invention discloses a method, system and device for judging the stability condition of a grid-following type converter, a medium and a product, and relates to the technical field of power grid detection, and the method comprises the steps: determining the initial value of each state variable based on each operation data after a power grid breaks down; based on the initial value of each state variable, solving the fourth-order nonlinear transient model within a preset time period after the initial prediction moment according to a preset time step, and obtaining a prediction value of each state variable at each prediction moment within the preset time period; drawing a power angle time-varying curve of the grid-following type converter based on the predicted value of the power angle of the grid-following type converter at each prediction moment; the transverse axis of the power angle time-varying curve of the following grid type converter is the prediction moment, and the longitudinal axis is the prediction value of the power angle of the following grid type converter; and based on the power angle time-varying curve of the grid-following type converter, determining the stability condition of the grid-following type converter in a preset time period. According to the invention, judgment of the stability condition of the grid-following type converter is realized.

Description

Technical Field

[0001] The present application relates to the field of power grid detection technology, and in particular to a method, system, device, medium and product for determining the stability of a grid-following converter. Background Art

[0002] As the earliest, most deeply studied and most widely used new energy equipment, grid-following converters have attracted extensive attention for their transient stability. Since grid-following converters isolate the machine side from the power grid, wind turbines have almost no inertia in the power grid. Therefore, wind turbines need to rely on phase-locked loops (PLLs) to achieve synchronization with the power grid. When the power grid fault is too serious, the phase oscillation of the phase-locked loop output becomes unstable, and the wind turbine will lose synchronization with the power grid. Most existing studies believe that the time constant of the current control loop is much smaller than the time constant of the phase-locked loop, thus ignoring the current dynamic process after the fault. However, for high-power converters used in large wind turbines, the bandwidth of the current control loop with a lower switching frequency cannot be designed too high, so the coupling effect between the current control loop and the phase-locked loop must be considered at this time. In addition, the voltage clamping effect of pulse width modulation (PWM) at the moment of fault will also cause a sudden change in current, so the initial condition of the fault should be corrected. However, the frequency mutation caused by the significant current mutation under severe faults will further complicate the setting of the initial fault conditions, making it difficult to judge the stability of the grid-connected converter. Summary of the invention

[0003] The purpose of the present application is to provide a method, system, device, medium and product for judging the stability of a grid-following converter, so as to solve the problem that the stability of a grid-following converter is difficult to judge.

[0004] To achieve the above objectives, this application provides the following solutions:

[0005] In a first aspect, the present application provides a method for determining the stability of a grid-connected converter, comprising:

[0006] During the operation of the power grid, the operation data of each sampling moment is acquired in real time; the operation data includes: power grid voltage, power grid line inductance, filter inductance, d-axis component of output current measurement value, d-axis component of output current reference value, q-axis component of output current reference value, power grid frequency, phase-locked loop PI parameters and current control loop PI parameters; the phase-locked loop PI parameters include: proportional parameters and integral parameters of the phase-locked loop, and the current control loop PI parameters include: proportional parameters and integral parameters of the current control loop;

[0007] After a power grid fault occurs, the initial values ​​of various state variables are determined based on various operating data of the power grid in a steady state before the fault occurs and various operating data at the first sampling moment after the power grid fault occurs; the state variables include: a power angle of the grid-following converter, a first-order derivative of the power angle of the grid-following converter, a d-axis component difference, and a first-order derivative of the d-axis component difference, wherein the d-axis component difference is a difference between a d-axis component of a calculated output current of the grid-following converter and a reference output current;

[0008] Taking the time when the fault occurs as the initial prediction time, based on the initial values ​​of each state variable, and according to the preset time step, the fourth-order nonlinear transient model is solved within a preset period after the initial prediction time to obtain the predicted value of each state variable at each prediction time within the preset period; the fourth-order nonlinear transient model is an ordinary differential equation about the state variable and the operating data;

[0009] Based on the predicted value of the power angle of the grid-type converter at each predicted time, a power angle time-varying curve of the grid-type converter is drawn; the horizontal axis of the power angle time-varying curve of the grid-type converter is the predicted time, and the vertical axis is the predicted value of the power angle of the grid-type converter;

[0010] Based on the power angle time-varying curve of the grid-type converter, the stability of the grid-type converter within the preset time period is determined; the grid-type converter is connected to the power grid, and the stability is stable or unstable.

[0011] Optionally, the fourth-order nonlinear transient model includes:

[0012]

[0013] Among them, x1 is the first state variable; x2 is the second state variable; x3 is the third state variable; x4 is the fourth state variable; δ is the power angle of the grid-following converter; is the first-order derivative of the power angle of the grid-type converter; ΔI d is the d-axis component difference; is the first-order derivative of the difference of the d-axis component; is the first-order derivative of x1; is the first-order derivative of x2; is the first-order derivative of x3; is the first-order derivative of x4; K p is the proportional parameter of the phase-locked loop; U f is the grid voltage after the fault occurs; ω n is the grid frequency; L g is the inductance of the power grid line; I dref is the d-axis component of the output current reference value; K i is the integral parameter of the phase-locked loop; Kcp is the proportional parameter of the current control loop; K ci is the integral parameter of the current control loop; L is the total inductance, L = L g +L f , L f For the filter inductor.

[0014] Optionally, after a power grid fault occurs, the initial value of each state variable is determined based on each operation data of the power grid in a steady state before the fault occurs and each operation data at the first sampling time after the power grid fault occurs, including:

[0015] Determine the initial value of the power angle of the grid-following converter based on the grid frequency, the grid impedance when the grid is in a steady state before the fault occurs, the d-axis component of the output current measurement value, and the grid voltage;

[0016] Determine the initial value of the first-order derivative of the power angle of the grid-following converter based on the phase-locked loop PI parameters, the grid frequency, the q-axis component and the d-axis component of the total voltage difference before and after the fault occurs, the grid line inductance and the filter inductance at the first sampling moment after the fault occurs, the grid voltage after the fault occurs, the grid voltage before the fault occurs, and the d-axis component and the grid line inductance of the output current measurement value at the last sampling moment before the fault occurs;

[0017] Determine the initial value of the d-axis component difference based on the d-axis component of the total voltage difference before and after the fault occurs, the grid frequency, the initial value of the first-order derivative of the power angle of the grid-following converter, and the d-axis component of the grid line inductance, the filter inductance and the output current reference value at the first sampling moment after the fault occurs;

[0018] An initial value of a first-order derivative of the d-axis component difference is determined based on a proportional parameter of the current control loop and an initial value of the d-axis component difference.

[0019] Optionally, determining an initial value of the power angle of the grid-following converter based on the grid frequency, the grid impedance when the grid is in a steady state before the fault occurs, the d-axis component of the output current measurement value, and the grid voltage includes:

[0020] The power angle calculation formula of the grid-following converter is used to calculate the initial value of the power angle of the grid-following converter according to the grid frequency, the grid impedance when the grid is in a steady state before the fault occurs, the d-axis component of the output current measurement value, and the grid voltage; the power angle calculation formula of the grid-following converter includes:

[0021]

[0022] Among them, δ0 is the initial value of the power angle of the grid-following converter; ω n is the grid frequency; L' gI' is the grid impedance when the grid is in a steady state before the fault occurs; d The d-axis component of the output current measurement value when the power grid is in a steady state before 0; U' g It is the grid voltage when the grid is in steady state before the fault occurs.

[0023] Optionally, determining an initial value of the first-order derivative of the power angle of the grid-following converter based on a phase-locked loop PI parameter, a grid frequency, a q-axis component and a d-axis component of a total voltage difference before and after a fault occurs, a grid line inductance and a filter inductance at the first sampling moment after the fault occurs, a grid voltage after the fault occurs, a grid voltage before the fault occurs, and a d-axis component and a grid line inductance of an output current measurement value at the last sampling moment before the fault occurs includes:

[0024] The initial value of the first-order derivative of the power angle of the grid-following converter is calculated by using the calculation formula of the first-order derivative of the power angle of the grid-following converter, according to the phase-locked loop PI parameters, the grid frequency, the q-axis component and the d-axis component of the total voltage difference before and after the fault occurs, the grid line inductance and the filter inductance at the first sampling time after the fault occurs, the grid voltage after the fault occurs, the grid voltage before the fault occurs, and the d-axis component and the grid line inductance of the output current measurement value at the last sampling time before the fault occurs; the calculation formula of the first-order derivative of the power angle of the grid-following converter includes:

[0025]

[0026] in, is the initial value of the first-order derivative of the power angle of the grid-type converter; A and B are both intermediate variables; ΔU q_0 is the q-axis component of the total voltage difference before and after the fault occurs; L g+ is the grid line inductance at the first sampling moment after the fault occurs; L + is the total inductance at the first sampling moment after the fault occurs, L + =L g+ +L f+ , L f+ is the filter inductance at the first sampling moment after the fault occurs; U g is the grid voltage before the fault occurs; I d- is the d-axis component of the output current measurement value at the last sampling moment before the fault occurs; L g- is the grid line inductance at the last sampling moment before the fault occurs; ΔU d_0 It is the d-axis component of the total voltage difference before and after the fault occurs.

[0027] Optionally, determining an initial value of the d-axis component difference based on the d-axis component of the total voltage difference before and after the fault occurs, the grid frequency, the initial value of the first-order derivative of the power angle of the grid-following converter, and the d-axis component of the grid line inductance, the filter inductance, and the output current reference value at the first sampling moment after the fault occurs includes:

[0028] The initial value of the d-axis component difference is calculated using the d-axis component difference calculation formula according to the d-axis component of the total voltage difference before and after the fault occurs, the grid frequency, the initial value of the first-order derivative of the power angle of the grid-following converter, and the d-axis component of the grid line inductance, the filter inductance, and the output current reference value at the first sampling moment after the fault occurs; the d-axis component difference calculation formula includes:

[0029] ΔI d_0 =I d_0 -I dref+ ;

[0030]

[0031] Among them, ΔI d_0 is the initial value of the d-axis component difference; I d_0 is the d-axis component of the calculated output current at the first sampling moment after the fault occurs; I dref+ It is the d-axis component of the output current reference value at the first sampling moment after the fault occurs.

[0032] Optionally, determining an initial value of a first-order derivative of the d-axis component difference based on a proportional parameter of the current control loop and an initial value of the d-axis component difference comprises:

[0033] The initial value of the first-order derivative of the d-axis component difference is calculated using the calculation formula of the first-order derivative of the d-axis component difference according to the proportional parameter of the current control loop and the initial value of the d-axis component difference; the calculation formula of the first-order derivative of the d-axis component difference includes:

[0034]

[0035] in, is the initial value of the first derivative of the d-axis component difference.

[0036] In a second aspect, the present application provides a stability judgment system for a grid-connected converter, comprising:

[0037] An operation data acquisition module is used to obtain the operation data of each sampling moment in real time during the operation of the power grid; the operation data includes: power grid voltage, power grid line inductance, filter inductance, d-axis component of output current measurement value, d-axis component of output current reference value, q-axis component of output current reference value, power grid frequency, phase-locked loop PI parameters and current control loop PI parameters; the phase-locked loop PI parameters include: proportional parameters and integral parameters of the phase-locked loop, and the current control loop PI parameters include: proportional parameters and integral parameters of the current control loop;

[0038] An initialization module is used to determine the initial values ​​of various state variables after a power grid fault occurs, based on various operating data of the power grid in a steady state before the fault occurs and various operating data at the first sampling time after the power grid fault occurs; the state variables include: a power angle of the grid-following converter, a first-order derivative of the power angle of the grid-following converter, a d-axis component difference, and a first-order derivative of the d-axis component difference, wherein the d-axis component difference is the difference between the d-axis component of the calculated output current of the grid-following converter and the output current reference value;

[0039] A solution module is used to solve the fourth-order nonlinear transient model within a preset time period after the initial prediction time based on the initial value of each state variable and the preset time step, taking the fault occurrence time as the initial prediction time, and obtain the predicted value of each state variable at each prediction time within the preset time period; the fourth-order nonlinear transient model is an ordinary differential equation about the state variable and the operating data;

[0040] A curve drawing module, used for drawing a power angle time-varying curve of the grid-type converter based on the predicted value of the power angle of the grid-type converter at each predicted time; the horizontal axis of the power angle time-varying curve of the grid-type converter is the predicted time, and the vertical axis is the predicted value of the power angle of the grid-type converter;

[0041] A stability determination module is used to determine the stability of the grid-type converter within the preset time period based on the power angle time-varying curve of the grid-type converter; the grid-type converter is connected to the power grid, and the stability is stable or unstable.

[0042] Optionally, the fourth-order nonlinear transient model includes:

[0043]

[0044] Among them, x1 is the first state variable; x2 is the second state variable; x3 is the third state variable; x4 is the fourth state variable; δ is the power angle of the grid-following converter; is the first-order derivative of the power angle of the grid-type converter; ΔI d is the d-axis component difference; is the first-order derivative of the difference of the d-axis component; is the first-order derivative of x1; is the first-order derivative of x2; is the first-order derivative of x3; is the first-order derivative of x4; K p is the proportional parameter of the phase-locked loop; U f is the grid voltage after the fault occurs; ω n is the grid frequency; L g is the inductance of the power grid line; I dref is the d-axis component of the output current reference value; K i is the integral parameter of the phase-locked loop; K cp is the proportional parameter of the current control loop; K ci is the integral parameter of the current control loop; L is the total inductance, L = L g +L f , L f For the filter inductor.

[0045] Optionally, the initialization module includes:

[0046] A first initialization unit is used to determine an initial value of the power angle of the grid-following converter based on the grid frequency, the grid impedance when the grid is in a steady state before the fault occurs, the d-axis component of the output current measurement value, and the grid voltage;

[0047] a second initialization unit, for determining an initial value of a first-order derivative of a power angle of the grid-following converter based on a phase-locked loop PI parameter, a grid frequency, a q-axis component and a d-axis component of a total voltage difference before and after a fault occurs, a grid line inductance and a filter inductance at a first sampling time after the fault occurs, a grid voltage after the fault occurs, a grid voltage before the fault occurs, and a d-axis component and a grid line inductance of an output current measurement value at a last sampling time before the fault occurs;

[0048] a third initialization unit, for determining an initial value of a d-axis component difference based on a d-axis component of a total voltage difference before and after a fault occurs, a grid frequency, an initial value of a first-order derivative of a power angle of a grid-following converter, and a d-axis component of a grid line inductance, a filter inductance, and an output current reference value at a first sampling moment after the fault occurs;

[0049] The fourth initialization unit is used to determine an initial value of the first-order derivative of the d-axis component difference based on the proportional parameter of the current control loop and the initial value of the d-axis component difference.

[0050] Optionally, determining an initial value of the power angle of the grid-following converter based on the grid frequency, the grid impedance when the grid is in a steady state before the fault occurs, the d-axis component of the output current measurement value, and the grid voltage includes:

[0051] The power angle calculation formula of the grid-following converter is used to calculate the initial value of the power angle of the grid-following converter according to the grid frequency, the grid impedance when the grid is in a steady state before the fault occurs, the d-axis component of the output current measurement value, and the grid voltage; the power angle calculation formula of the grid-following converter includes:

[0052]

[0053] Among them, δ0 is the initial value of the power angle of the grid-following converter; ω n is the grid frequency; L' g I' is the grid impedance when the grid is in a steady state before the fault occurs; d The d-axis component of the output current measurement value when the power grid is in a steady state before 0; U' g It is the grid voltage when the grid is in steady state before the fault occurs.

[0054] Optionally, determining an initial value of the first-order derivative of the power angle of the grid-following converter based on a phase-locked loop PI parameter, a grid frequency, a q-axis component and a d-axis component of a total voltage difference before and after a fault occurs, a grid line inductance and a filter inductance at the first sampling moment after the fault occurs, a grid voltage after the fault occurs, a grid voltage before the fault occurs, and a d-axis component and a grid line inductance of an output current measurement value at the last sampling moment before the fault occurs includes:

[0055] The initial value of the first-order derivative of the power angle of the grid-following converter is calculated by using the calculation formula of the first-order derivative of the power angle of the grid-following converter, according to the phase-locked loop PI parameters, the grid frequency, the q-axis component and the d-axis component of the total voltage difference before and after the fault occurs, the grid line inductance and the filter inductance at the first sampling time after the fault occurs, the grid voltage after the fault occurs, the grid voltage before the fault occurs, and the d-axis component and the grid line inductance of the output current measurement value at the last sampling time before the fault occurs; the calculation formula of the first-order derivative of the power angle of the grid-following converter includes:

[0056]

[0057] in, is the initial value of the first-order derivative of the power angle of the grid-type converter; A and B are both intermediate variables; ΔU q_0 is the q-axis component of the total voltage difference before and after the fault occurs; L g+ is the grid line inductance at the first sampling moment after the fault occurs; L + is the total inductance at the first sampling moment after the fault occurs, L + =L g+ +L f+ , L f+ is the filter inductance at the first sampling moment after the fault occurs; U gis the grid voltage before the fault occurs; I d- is the d-axis component of the output current measurement value at the last sampling moment before the fault occurs; L g- is the grid line inductance at the last sampling moment before the fault occurs; ΔU d_0 It is the d-axis component of the total voltage difference before and after the fault occurs.

[0058] Optionally, determining an initial value of the d-axis component difference based on the d-axis component of the total voltage difference before and after the fault occurs, the grid frequency, the initial value of the first-order derivative of the power angle of the grid-following converter, and the d-axis component of the grid line inductance, the filter inductance, and the output current reference value at the first sampling moment after the fault occurs includes:

[0059] The initial value of the d-axis component difference is calculated using the d-axis component difference calculation formula according to the d-axis component of the total voltage difference before and after the fault occurs, the grid frequency, the initial value of the first-order derivative of the power angle of the grid-following converter, and the d-axis component of the grid line inductance, the filter inductance, and the output current reference value at the first sampling moment after the fault occurs; the d-axis component difference calculation formula includes:

[0060] ΔI d_0 =I d_0 -I dref+ ;

[0061]

[0062] Among them, ΔI d_0 is the initial value of the d-axis component difference; I d_0 is the d-axis component of the calculated output current at the first sampling moment after the fault occurs; I dref+ It is the d-axis component of the output current reference value at the first sampling moment after the fault occurs.

[0063] Optionally, determining an initial value of a first-order derivative of the d-axis component difference based on a proportional parameter of the current control loop and an initial value of the d-axis component difference comprises:

[0064] The initial value of the first-order derivative of the d-axis component difference is calculated using the calculation formula of the first-order derivative of the d-axis component difference according to the proportional parameter of the current control loop and the initial value of the d-axis component difference; the calculation formula of the first-order derivative of the d-axis component difference includes:

[0065]

[0066] in, is the initial value of the first derivative of the d-axis component difference.

[0067] In a third aspect, the present application provides a computer device, comprising: a memory, a processor, and a computer program stored in the memory and executable on the processor, wherein the processor executes the computer program to implement the method for determining the stability of a grid-following converter as described in any one of the above items.

[0068] In a fourth aspect, the present application provides a computer-readable storage medium having a computer program stored thereon, which, when executed by a processor, implements the method for determining the stability of a grid-connected converter as described in any one of the above items.

[0069] In a fifth aspect, the present application provides a computer program product, including a computer program, which, when executed by a processor, implements the method for determining the stability of a grid-connected converter as described in any one of the above items.

[0070] According to the specific embodiments provided in this application, this application discloses the following technical effects:

[0071] The present application discloses a method, system, device, medium and product for judging the stability of a grid-following converter. First, a fourth-order nonlinear transient model is constructed; the fourth-order nonlinear transient model is an ordinary differential equation about state variables and operating data; the state variables include: the power angle of the grid-following converter, the first-order derivative of the power angle of the grid-following converter, the d-axis component difference and the first-order derivative of the d-axis component difference, the d-axis component difference is the difference between the output current calculated value of the grid-following converter and the d-axis component of the output current reference value, the operating data include: grid voltage, grid line inductance, filter inductance, d-axis component of the output current measurement value, d-axis component of the output current reference value, q-axis component of the output current reference value, grid frequency, phase-locked loop PI parameters and current control loop PI parameters; the phase-locked loop PI parameters include: proportional parameters and integral parameters of the phase-locked loop, and the current control loop PI parameters include: proportional parameters and integral parameters of the current control loop; then, during the operation of the grid, During the process, the operation data of each sampling moment is obtained in real time; secondly, after the power grid fails, the initial value of each state variable is determined based on the operation data of the power grid in a steady state before the failure and the operation data at the first sampling moment after the power grid failure occurs; then, taking the time of the failure as the initial prediction moment, based on the initial value of each state variable, according to the preset time step, the fourth-order nonlinear transient model is solved within a preset time period after the initial prediction moment to obtain the predicted value of each state variable at each prediction moment within the preset time period; thirdly, based on the predicted value of the power angle of the grid-following converter at each prediction moment, the power angle time-varying curve of the grid-following converter is drawn; the horizontal axis of the power angle time-varying curve of the grid-following converter is the prediction moment, and the vertical axis is the predicted value of the power angle of the grid-following converter; finally, based on the power angle time-varying curve of the grid-following converter, the stability of the grid-following converter within the preset time period is determined; the grid-following converter is connected to the power grid, and the stability is stable or unstable. This application uses the solver ode45 to iteratively solve the fourth-order nonlinear transient model, simulates and predicts the state variables, and realizes the judgment of the stability of the grid-following converter. BRIEF DESCRIPTION OF THE DRAWINGS

[0072] In order to more clearly illustrate the technical solutions in the embodiments of the present application or related technologies, the drawings required for use in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0073] Figure 1 A schematic flow chart of a method for determining the stability of a grid-connected converter provided in one embodiment of the present application;

[0074] Figure 2 A schematic diagram of the connection between the grid-following converter and the power grid;

[0075] Figure 3 The equivalent model diagram of VSC considering the coupling between SRF-PLL and current control loop;

[0076] Figure 4 This is the quasi-static large signal model diagram of the PLL;

[0077] Figure 5 It is a schematic diagram of the equal area criterion without considering the current mutation;

[0078] Figure 6 Schematic diagram of the equal area criterion considering current mutation;

[0079] Figure 7 It is a schematic diagram of the current control loop structure;

[0080] Figure 8 Schematic diagram of the synchronization model considering the current dynamics;

[0081] Fig. 9 This is a simulation comparison chart between the fourth-order nonlinear transient model and the traditional second-order model;

[0082] Fig.10 A schematic diagram of the structure of a computer device provided in one embodiment of the present application. DETAILED DESCRIPTION

[0083] The following will be combined with the drawings in the embodiments of the present application to clearly and completely describe the technical solutions in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of this application.

[0084] The purpose of the present application is to provide a method, system, device, medium and product for judging the stability of a grid-following converter, aiming to realize the judgment of the stability of the grid-following converter.

[0085] In order to make the above-mentioned objects, features and advantages of the present application more obvious and easy to understand, the present application is further described in detail below in conjunction with the accompanying drawings and specific implementation methods.

[0086] Before judging the stability of the grid-following converter, the following analysis process is performed.

[0087] 1. Analyze the synchronous stability mechanism of grid-type converter.

[0088] Figure 2 For the control structure of the grid-type converter, Figure 2In the synchronous reference frame phase-locked loop (SRF-PLL), it is used to detect the phase of the common connection point (PCC). dc is the DC side voltage, U c is the terminal voltage, L f is the filter inductor, U pcc is the voltage at PCC, I pcc is the current injected into the grid by the grid-following converter at the PCC, Z g is the grid impedance, U g is the grid voltage, δ pll is the power angle of PLL, ω pll is the angular frequency of the phase-locked loop. The grid-following converter for achieving stable current control can be regarded as a controlled current source directed by the PLL.

[0089] The purpose of the grid-following converter is to generate a terminal voltage U c The d-axis component U cq * and the q-axis component U cd * Change the d-axis component U of the original terminal voltage cq and the q-axis component U cd To control the d-axis component of the output current I d and the q-axis component I q Tracking reference value I dref ,I qref . Normally, the evaluation of PLL stability only needs to consider the fault voltage of the PCC. However, when evaluating the synchronization stability, the state of the power grid and the impedance of the transmission line should also be taken into account. This is because the line impedance will have a negative impact on the synchronization stability, and ignoring it may lead to erroneous results. In most high-voltage systems, the grid impedance is dominated by inductance. Therefore, for the sake of simplicity, this application mainly performs stability analysis under purely inductive grid impedance. Furthermore, the purely inductive grid impedance system is the worst operating condition that affects the transient synchronization stability of the converter, because the current at this time appears as pure active current. In summary, in order to ensure the transient synchronization stability of the grid-following converter under various conditions, this application studies the worst case, that is, ignoring the resistance when considering the line impedance.

[0090] In order to more intuitively describe and express the influence of the current control loop and PLL on the transient synchronization process of VSC, Figure 2 The configuration in is equivalent to Figure 3 The Thevenin circuit in the figure is given in detail, and the structure diagram of the current control loop and PLL is given in detail. The controlled current source corresponding to the grid-type converter is composed of I pcc And power angle δ: I pcc Determined by the current control loop, the power angle δ is generated by the phase-locked loop.

[0091] Figure 3 middle, I dq_ref is the dq axis component of the output current reference value; K cp is the proportional parameter of the current control loop; s is a complex variable in the complex plane; K ci is the integral parameter of the current control loop; U gdq is the dq component of the grid voltage; L is the total inductance; I dq is the dq axis component of the calculated output current; (x, y) is the component representation of the current in the dq coordinate system, x is d, y is q; (r, θ) is the representation of the current in the polar coordinate system, r is the amplitude, θ is the phase angle; θ I is the current vector angle in the dq coordinate system, θ I =tan -1 (I q / I d );I q is the d-axis component of the calculated output current; I d is the q-axis component of the calculated output current; e is the natural exponent; j is the imaginary unit; Z g is the line impedance; U pccd For U pcc The d-axis component of U pccq For U pcc The q-axis component of ; δ is the power angle of the grid-type converter; is the first-order derivative of the power angle of the grid-type converter; ω n is the grid frequency.

[0092] It can be seen that the phase-locked loop passes U pccq Calculate the output frequency and power angle to keep the frequency of the grid-following converter consistent with the grid frequency and ensure the power angle is stable. In order to obtain stable current control during steady-state operation, U pccq Should be equal to zero. And when U pccq = 0 phase lock, the active and reactive currents of the grid-type converter are decoupled. It is worth noting that due to the presence of harmonics, U pccq In practice, it is impossible to be exactly zero. However, the focus of this application is baseband synchronization, so it is assumed that due to the effect of PLL pccq In steady state, it is zero. Figure 4 As shown in Figure 2, the synchronous transient model can be divided into a grid synchronization loop and a self-synchronization loop (assuming that the power angle of the PCC is the reference angle, the phase of the grid voltage is -δ pll ). Among them, the grid synchronization loop is a negative feedback, which is used to offset the positive feedback effect of the self-synchronization loop. For example, when a disturbance occurs in the system and causes over-U pccq ≠0 is not 0 and causes the change in the angular frequency of the phase-locked loop Δω pll When δ is positive, pllIf the phase-locked loop can gradually converge to a steady state, it means that Δω in the self-synchronizing loop pll L g I d The increase and the reduction of the power grid synchronization ring can offset each other, thus making the total U pccq is adjusted to zero. However, if δ pll When the synchronous ring effect has not been absorbed, Δω pll Still positive, δ pll It will further increase and fail to converge, causing the system to become unstable.

[0093] In addition, according to Figure 3 It can also be found that the synchronization process is not only related to the PLL, but also to the current dynamics, that is, there is a coupling effect between the phase-locked loop and the current control loop. When the current control loop bandwidth is much larger than the phase-locked loop bandwidth, that is, the dynamic response of the current control loop is much faster than the PLL, it can indeed be considered that the current can always accurately track the current reference value within the phase-locked loop time scale. However, under some special working conditions, the premise that the current control loop bandwidth is much larger than the PLL bandwidth cannot be met, and the current cannot quickly track the reference value after a fault occurs. At this time, the grid-following converter cannot be regarded as a controlled current source directed by the PLL, and the existing current mutation will affect the VSC synchronization process.

[0094] 2. Construct an improved transient synchronization fourth-order model (fourth-order nonlinear transient model) that takes into account the dynamics of the current control loop and frequency jumps.

[0095] Depend on Figure 2 It can be seen that the voltage of PCC in steady state is expressed as:

[0096]

[0097] Among them, φ c is the line impedance angle; θ g is the grid voltage phase angle; θ c For I pcc The phase angle, θ c =θ I +θ pll .

[0098] Before the fault occurs, the system is stable. pcc = 0 and the reactive current reference is 0 in steady state, that is, I pcc =I d =I dref_ According to formula (1), at the steady-state equilibrium point U pcc with U g The angle difference, that is, the power angle δ0 of the grid-type converter is:

[0099]

[0100] And the power angle cannot change suddenly before and after the fault, that is: δ0=δ + =δ _ .

[0101] Among them, δ + is the power angle of the grid-connected converter at the instant after the fault occurs (i.e. the first sampling moment); δ _ It is the power angle of the grid-connected converter just before the fault occurs (i.e. the last sampling moment).

[0102] U pccq The q-axis component of is:

[0103]

[0104] Among them, U f is the grid voltage after the fault occurs; L g is the grid line inductance.

[0105] In fact, there is a coupling effect between the current control loop and the PLL during the transient synchronization process of the VSC. When the bandwidth of the current control loop is much larger than the bandwidth of the PLL, it can be considered that the synchronization process is dominated by the PLL alone. The traditional second-order model can better describe the transient synchronization process. When the bandwidth of the current control loop is not large enough, the transient synchronization process of the VSC is also affected by the d-axis current dynamic ΔI d The influence of the second-order model makes it no longer applicable, so it is upgraded to the fourth-order model.

[0106]

[0107] Among them, U pccq_4 is the q-axis component of the PCC voltage under the fourth-order model; I dref is the d-axis component of the output current reference value.

[0108] When performing large signal analysis on the nonlinear model of the grid-following converter during transient synchronization, the transient synchronization process of the grid-following converter is similar to the movement of the synchronous generator (SG) rotor. By analogy with the equivalent inertia coefficient H and equivalent damping coefficient D in the SG swing equation, a large signal model similar to the SG rotor swing equation is established. The synchronization process can be described as:

[0109]

[0110] in, is the second-order derivative of the power angle of the grid-following converter; K p is the proportional parameter of the phase-locked loop; K i is the integral parameter of the phase-locked loop.

[0111] Similar to the Equal-Area Criterion (EAC) analysis of synchronous generators, ω n L g (ΔI d +I dref ) is the input of VSC, U f sinδ is the output of VSC. Combining the input and output curves, the improved EAC is as follows Figure 5 and Figure 6 shown. Figure 5 and Figure 6 In the figure, point A is the stable working point before the fault, and point C is the stable working point after the fault. After the fault occurs, the equivalent input ω n L g (ΔI d +I dref ) and equivalent output U f The deviation between sinδ drives the acceleration or deceleration of the PLL output angular frequency. The area where the equivalent input is greater than the equivalent output is the acceleration area, and the area where the equivalent input is less than the equivalent output is the deceleration area.

[0112] When current mutation is not considered: if S1>S2, that is, the acceleration area is larger than the deceleration area, the excess energy that the system cannot absorb will be converted into kinetic energy, causing instability; if S1<S2, that is, the acceleration area is smaller than the deceleration area, the system can stabilize at the new equilibrium point C after the fault occurs. When current mutation is considered, at the moment of fault I d It will increase rapidly and then gradually decay to the reference value. The sudden change in current will increase the input of the VSC within a certain period of time, resulting in an increase in the acceleration area represented by S3 and a decrease in the deceleration area represented by S4. This means that the phase-locked loop is more likely to become unstable after considering the dynamic effect of the current control loop, and the system can only be stable after the fault when S3<S4 is satisfied. Compared with the EAC analysis corresponding to the traditional second-order model, it is obvious that considering the d-axis component difference ΔI d The equivalent model is less conservative and more accurate. In order to solve this problem, a fourth-order nonlinear transient model considering the current dynamics caused by the current control loop is derived and established.

[0113] To U pccq_4 The derivative is:

[0114]

[0115] in, For U pccq_4 The first derivative of ; is the first derivative of the difference of the d-axis components.

[0116] According to the working principle of the phase-locked loop, the calculation formula of the power angle is:

[0117] δ=∫(K p Upccq +K i U pccq dt+ω n )dt(8)

[0118] By taking the derivative of formula (7) twice, we can get:

[0119]

[0120] in, For U pccq The first derivative of .

[0121] When the current control loop bandwidth is not large enough to ignore the current dynamic behavior compared to the phase-locked loop bandwidth, the coupling between the current control loop and the phase-locked loop must be considered. Compared with the assumption of a fixed current in previous methods, this transient current leads to a larger self-synchronization loop Δω in the quasi-static model of the PLL. pll L g I d The introduction of greater positive feedback leads to a δ pll Increase is not conducive to δ pll Convergence before reaching 90° reduces the stability of the phase-locked loop. In other words, transient current may cause synchronization instability, which cannot be described and reflected by the second-order model. Since the current mutation is caused by the grid voltage drop, the grid voltage disturbance is used as input to solve the current control loop response. The current control loop structure block diagram is shown in Figure 7 As shown, the transfer function between the output current and the input voltage disturbance of the current control loop is as follows:

[0122]

[0123] This application mainly studies the case where the line resistance is ignored, so the state variable ΔI is added after considering the coupling effect between the current control loop and the PLL. d and ΔI d =I d -I dref .

[0124] By transforming equation (9), we can get equations (10) and (11).

[0125]

[0126] in, is the second-order derivative of the difference of the d-axis component; For U f The first derivative of the q-axis component of .

[0127] In an exemplary embodiment, Figure 1As shown, a method for judging the stability of a grid-following converter is provided, comprising:

[0128] Step 1: During the operation of the power grid, the operation data at each sampling time is obtained in real time.

[0129] Among them, the operating data includes: grid voltage, grid line inductance, filter inductance, d-axis component of output current measurement value, d-axis component of output current reference value, q-axis component of output current reference value, grid frequency, phase-locked loop PI parameters and current control loop PI parameters; the phase-locked loop PI parameters include: proportional parameters and integral parameters of the phase-locked loop, and the current control loop PI parameters include: proportional parameters and integral parameters of the current control loop.

[0130] Step 2: After a power grid fault occurs, the initial value of each state variable is determined based on each operating data of the power grid in a steady state before the fault occurs and each operating data at the first sampling time after the power grid fault occurs.

[0131] Among them, the state variables include: the power angle of the grid-following converter, the first-order derivative of the power angle of the grid-following converter, the d-axis component difference and the first-order derivative of the d-axis component difference. The d-axis component difference is the difference between the d-axis component of the calculated output current of the grid-following converter and the output current reference value.

[0132] As an optional implementation, step 2 includes:

[0133] Step 21: Determine an initial value of the power angle of the grid-following converter based on the grid frequency, the grid impedance when the grid is in a steady state before the fault occurs, the d-axis component of the output current measurement value, and the grid voltage.

[0134] As an optional implementation, step 21 includes:

[0135] The power angle calculation formula of the grid-following converter is used to calculate the initial value of the power angle of the grid-following converter according to the grid frequency, the grid impedance when the grid is in a steady state before the fault occurs, the d-axis component of the output current measurement value, and the grid voltage; the power angle calculation formula of the grid-following converter includes:

[0136]

[0137] Among them, δ0 is the initial value of the power angle of the grid-following converter; ω n is the grid frequency; L' g I' is the grid impedance when the grid is in a steady state before the fault occurs; d The d-axis component of the output current measurement value when the power grid is in a steady state before 0; U' g It is the grid voltage when the grid is in steady state before the fault occurs.

[0138] Step 22: Determine the initial value of the first-order derivative of the power angle of the grid-following converter based on the phase-locked loop PI parameters, the grid frequency, the q-axis component and the d-axis component of the total voltage difference before and after the fault occurs, the grid line inductance and the filter inductance at the first sampling moment after the fault occurs, the grid voltage after the fault occurs, the grid voltage before the fault occurs, and the d-axis component and the grid line inductance of the output current measurement value at the last sampling moment before the fault occurs.

[0139] As an optional implementation, step 22 includes:

[0140] The calculation formula of the first-order derivative of the power angle of the grid-following converter is used to calculate the initial value of the first-order derivative of the power angle of the grid-following converter according to the phase-locked loop PI parameters, the grid frequency, the q-axis component and the d-axis component of the total voltage difference before and after the fault occurs, the grid line inductance and the filter inductance at the first sampling time after the fault occurs, the grid voltage after the fault occurs, the grid voltage before the fault occurs, and the d-axis component and the grid line inductance of the output current measurement value at the last sampling time before the fault occurs. The calculation formula of the first-order derivative of the power angle of the grid-following converter includes:

[0141]

[0142]

[0143] in, is the initial value of the first-order derivative of the power angle of the grid-type converter; A and B are both intermediate variables; ΔU q_0 is the q-axis component of the total voltage difference before and after the fault occurs; L g+ is the grid line inductance at the first sampling moment after the fault occurs; L + is the total inductance at the first sampling moment after the fault occurs, L + =L g+ +L f+ , L f+ is the filter inductance at the first sampling moment after the fault occurs; U g is the grid voltage before the fault occurs; I d- is the d-axis component of the output current measurement value at the last sampling moment before the fault occurs; L g- is the grid line inductance at the last sampling moment before the fault occurs; ΔU d_0 It is the d-axis component of the total voltage difference before and after the fault occurs.

[0144] Step 23: Determine the initial value of the d-axis component difference based on the d-axis component of the total voltage difference before and after the fault occurs, the grid frequency, the initial value of the first-order derivative of the power angle of the grid-following converter, and the d-axis component of the grid line inductance, filter inductance and output current reference value at the first sampling moment after the fault occurs.

[0145] As an optional implementation, step 23 includes:

[0146] The d-axis component difference calculation formula is used to calculate the initial value of the d-axis component difference according to the d-axis component of the total voltage difference before and after the fault occurs, the grid frequency, the initial value of the first-order derivative of the power angle of the grid-following converter, and the d-axis component of the grid line inductance, filter inductance and output current reference value at the first sampling moment after the fault occurs; the d-axis component difference calculation formula includes:

[0147] ΔI d_0 =I d_0 -I dref+ (16)

[0148]

[0149] Among them, ΔI d_0 is the initial value of the d-axis component difference; I d_0 is the d-axis component of the calculated output current at the first sampling moment after the fault occurs; I dref+ It is the d-axis component of the output current reference value at the first sampling moment after the fault occurs.

[0150] Step 24: Determine an initial value of the first-order derivative of the d-axis component difference based on the proportional parameter of the current control loop and the initial value of the d-axis component difference.

[0151] As an optional implementation, step 24 includes:

[0152] The calculation formula of the first-order derivative of the d-axis component difference is used to calculate the initial value of the first-order derivative of the d-axis component difference according to the proportional parameter of the current control loop and the initial value of the d-axis component difference; the calculation formula of the first-order derivative of the d-axis component difference includes:

[0153]

[0154] in, is the initial value of the first derivative of the d-axis component difference.

[0155] Specifically, the derivation process of the calculation formula of each state variable includes:

[0156] At the moment of a serious fault, the terminal voltage of the grid-following converter cannot change in steps but remains fixed until the next PWM modulation (i.e., PWM voltage clamping), and the grid voltage drops suddenly. The voltage difference between the two will cause a significant current mutation. Under the action of the PLL proportional link, the current mutation will be reflected in the frequency mutation. In the actual circuit, the inductance of the line causes the current and frequency to be coupled. Therefore, this part is analyzed in detail theoretically and the current and frequency jump values ​​at the moment of the fault are given to correct the system state change at the moment of a serious fault. The frequency jump value at the moment of the fault is expressed as It can be decomposed into proportional part and integral part as

[0157]

[0158] Among them, U pccq_4_0 For U pccq_4 The initial value of .

[0159] At the moment of fault, the voltage at the VSC terminal cannot change in a step but remains fixed until the next PWM modulation. The current changes due to the combined effects of the VSC terminal voltage that cannot change suddenly, the faulty grid voltage, and the line impedance after the fault.

[0160]

[0161] ΔU d_0 +ΔU q_0 j=U cd+ +U cq+ j-(U f sinδ + +U f cosδ + j)(23)

[0162] is the initial value of the first-order derivative of the power angle of the grid-type converter; A and B are both intermediate variables; ΔU q_0 is the q-axis component of the total voltage difference before and after the fault occurs; L g+ is the grid line inductance at the first sampling moment after the fault occurs; L + is the total inductance at the first sampling moment after the fault occurs, L + =L g+ +L f+ , L f+ is the filter inductance at the first sampling moment after the fault occurs; U g is the grid voltage before the fault occurs; I d- is the d-axis component of the output current measurement value at the last sampling moment before the fault occurs; L g- is the grid line inductance at the last sampling moment before the fault occurs; ΔU d_0It is the d-axis component of the total voltage difference before and after the fault occurs.

[0163] After considering the frequency jump, the fault instantaneous current I 4ord_0 The expression is:

[0164]

[0165] The phase-locked loop integration requires time to ignore the influence of the last two terms of the integral part of the frequency jump variable at the moment of the fault, while the first term has a derivative and it is known that the q-axis component of the current value of the current control loop before the fault is 0. The value after integration is It can be expressed as:

[0166]

[0167] After considering the PWM clamping effect, the q-axis component of the PCC voltage at the moment of fault is written as:

[0168]

[0169] By combining equations (19) and (27), we get the fourth-order nonlinear transient model.

[0170] Step 3: Taking the time when the fault occurs as the initial prediction time, based on the initial values ​​of each state variable, and according to the preset time step, solve the fourth-order nonlinear transient model within the preset time period after the initial prediction time to obtain the predicted values ​​of each state variable at each prediction time within the preset time period.

[0171] Among them, the fourth-order nonlinear transient model is an ordinary differential equation about state variables and operating data.

[0172] Specifically, the fourth-order nonlinear transient model is solved using the ode45 solver of Matlab.

[0173] As an optional implementation, Figure 8 As shown, the fourth-order nonlinear transient model includes:

[0174]

[0175]

[0176] Among them, x1 is the first state variable; x2 is the second state variable; x3 is the third state variable; x4 is the fourth state variable; δ is the power angle of the grid-following converter; is the first-order derivative of the power angle of the grid-type converter; ΔI d is the d-axis component difference; is the first-order derivative of the difference of the d-axis component; is the first-order derivative of x1; is the first-order derivative of x2; is the first-order derivative of x3; is the first-order derivative of x4; K p is the proportional parameter of the phase-locked loop; U f is the grid voltage after the fault occurs; ω n is the grid frequency; L g is the inductance of the power grid line; I dref is the d-axis component of the output current reference value; K i is the integral parameter of the phase-locked loop; K cp is the proportional parameter of the current control loop; K ci is the integral parameter of the current control loop; L is the total inductance, L = L g +L f , L f For the filter inductor.

[0177] Step 4: Based on the predicted value of the power angle of the grid-following converter at each prediction time, draw a power angle time-varying curve of the grid-following converter; the horizontal axis of the power angle time-varying curve of the grid-following converter is the prediction time, and the vertical axis is the predicted value of the power angle of the grid-following converter.

[0178] Step 5: Based on the power angle time-varying curve of the grid-following converter, determine the stability of the grid-following converter within a preset time period; the grid-following converter is connected to the power grid, and the stability is stable or unstable.

[0179] Specifically, when the power angle time-varying curve of the grid-following converter fluctuates near the steady-state value, it is converged and the stability is stable; otherwise, the stability is or is unstable. If the power angle fluctuates after a fault but eventually reaches stability, it means that the grid-following converter can maintain transient synchronous stability under this condition; conversely, if the power angle fluctuation continues to intensify, it means that the grid-following converter cannot maintain stability under this condition and loses synchronization with the grid. Therefore, the power angle variation parameters obtained above can be used to judge the synchronous stability of the power grid system to solve the problem of transient synchronous stability of the power grid.

[0180] Furthermore, in order to verify the effectiveness of the strategy proposed in this application, the following simulation model was constructed in the MATLAB / Simulink simulation platform: Figure 2 The simplified model of the wind turbine grid-connected system is shown in FIG. 1 , and the parameters used in the model are shown in Table 1.

[0181] Table 1 Simplified model of wind turbine grid-connected system

[0182]

[0183] Set the grid voltage U of the wind turbine grid-connected system gThe voltage drops to 100V, and the waveform comparison of the power angle δ before and after the Simulink simulation and the improved transient synchronization fourth-order model considering the dynamics of the current control loop and frequency jump and the traditional second-order model is shown in the figure below. Fig. 9 As shown. The green curve represents the Simulink simulation result, and the yellow and red curves represent the simulation results of the improved transient synchronization fourth-order model considering the dynamics of the current control loop and the frequency jump and the traditional second-order model, respectively. It can be seen that the improved transient synchronization fourth-order model considering the dynamics of the current control loop and the frequency jump can more accurately fit the transient synchronization process at the moment of the fault, verifying the correctness of the theoretical results of this application.

[0184] In an exemplary embodiment, a stability judgment system for a grid-connected converter is provided, comprising:

[0185] The operation data acquisition module is used to obtain the operation data at each sampling moment in real time during the operation of the power grid; the operation data includes: power grid voltage, power grid line inductance, filter inductance, d-axis component of output current measurement value, d-axis component of output current reference value, q-axis component of output current reference value, power grid frequency, phase-locked loop PI parameters and current control loop PI parameters; the phase-locked loop PI parameters include: proportional parameters and integral parameters of the phase-locked loop, and the current control loop PI parameters include: proportional parameters and integral parameters of the current control loop.

[0186] An initialization module is used to determine the initial values ​​of various state variables after a power grid fault occurs, based on various operating data of the power grid in a steady state before the fault occurs and various operating data at the first sampling moment after the power grid fault occurs; the state variables include: the power angle of the grid-following converter, the first-order derivative of the power angle of the grid-following converter, the d-axis component difference and the first-order derivative of the d-axis component difference, and the d-axis component difference is the difference between the d-axis component of the calculated output current of the grid-following converter and the output current reference value.

[0187] The solution module is used to solve the fourth-order nonlinear transient model within a preset time period after the initial prediction time based on the initial value of each state variable and the preset time step, taking the fault occurrence time as the initial prediction time, and obtain the predicted value of each state variable at each prediction time within the preset time period; the fourth-order nonlinear transient model is an ordinary differential equation about the state variables and operating data.

[0188] The curve drawing module is used to draw the power angle time-varying curve of the grid-following converter based on the predicted value of the power angle of the grid-following converter at each predicted time; the horizontal axis of the power angle time-varying curve of the grid-following converter is the predicted time, and the vertical axis is the predicted value of the power angle of the grid-following converter.

[0189] The stability determination module is used to determine the stability of the grid-following converter within a preset time period based on the power angle time-varying curve of the grid-following converter; the grid-following converter is connected to the power grid, and the stability is stable or unstable.

[0190] As an optional implementation, the initialization module includes:

[0191] The first initialization unit is used to determine the initial value of the power angle of the grid-following converter based on the grid frequency, the grid impedance when the grid is in a steady state before the fault occurs, the d-axis component of the output current measurement value, and the grid voltage.

[0192] The second initialization unit is used to determine the initial value of the first-order derivative of the power angle of the grid-following converter based on the phase-locked loop PI parameters, the grid frequency, the q-axis component and the d-axis component of the total voltage difference before and after the fault occurs, the grid line inductance and the filter inductance at the first sampling moment after the fault occurs, the grid voltage after the fault occurs, the grid voltage before the fault occurs, and the d-axis component and the grid line inductance of the output current measurement value at the last sampling moment before the fault occurs.

[0193] The third initialization unit is used to determine the initial value of the d-axis component difference based on the d-axis component of the total voltage difference before and after the fault occurs, the grid frequency, the initial value of the first-order derivative of the power angle of the grid-following converter, and the d-axis component of the grid line inductance, filter inductance and output current reference value at the first sampling moment after the fault occurs.

[0194] The fourth initialization unit is used to determine an initial value of the first-order derivative of the d-axis component difference based on the proportional parameter of the current control loop and the initial value of the d-axis component difference.

[0195] In an exemplary embodiment, a computer device is provided, including: a memory, a processor, and a computer program stored in the memory and executable on the processor, wherein the processor executes the computer program to implement a method for determining the stability of a grid-following converter.

[0196] In an exemplary embodiment, a computer-readable storage medium is provided, on which a computer program is stored. When the computer program is executed by a processor, a method for determining the stability of a grid-following converter is implemented.

[0197] In an exemplary embodiment, a computer program product is provided, including a computer program, which implements a method for determining a stability condition of a grid-following converter when executed by a processor.

[0198] In an exemplary embodiment, a computer device is provided. The computer device may be a server or a terminal. The internal structure diagram thereof may be as follows: Fig.10As shown. The computer device includes a processor, a memory, an input / output interface (Input / Output, referred to as I / O) and a communication interface. The processor, the memory and the input / output interface are connected through a system bus, and the communication interface is connected to the system bus through the input / output interface. The processor of the computer device is used to provide computing and control capabilities. The memory of the computer device includes a non-volatile storage medium and an internal memory. The non-volatile storage medium stores an operating system, a computer program and a database. The internal memory provides an environment for the operation of the operating system and the computer program in the non-volatile storage medium. The input / output interface of the computer device is used to exchange information between the processor and an external device. The communication interface of the computer device is used to communicate with an external terminal through a network connection. When the computer program is executed by the processor, a method for determining the stability of a grid-following converter is implemented.

[0199] Those skilled in the art will understand that Fig.10 The structure shown in the figure is only a block diagram of a part of the structure related to the solution of the present application, and does not constitute a limitation on the computer device to which the solution of the present application is applied. The specific computer device may include more or fewer components than those shown in the figure, or combine certain components, or have a different arrangement of components.

[0200] It should be noted that the user information (including but not limited to user device information, user personal information, etc.) and data (including but not limited to data used for analysis, stored data, displayed data, etc.) involved in this application are all information and data authorized by the user or fully authorized by all parties, and the collection, use and processing of relevant data must comply with relevant regulations.

[0201] Those of ordinary skill in the art can understand that all or part of the processes in the above-mentioned embodiment methods can be completed by instructing the relevant hardware through a computer program, and the computer program can be stored in a non-volatile computer-readable storage medium. When the computer program is executed, it can include the processes of the embodiments of the above-mentioned methods. Among them, any reference to the memory, database or other medium used in the embodiments provided in the present application can include at least one of non-volatile and volatile memory. Non-volatile memory can include read-only memory (ROM), magnetic tape, floppy disk, flash memory, optical memory, high-density embedded non-volatile memory, resistive random access memory (ReRAM), magnetic random access memory (MRAM), ferroelectric random access memory (FRAM), phase change memory (PCM), graphene memory, etc. Volatile memory can include random access memory (RAM) or external cache memory, etc. By way of illustration and not limitation, RAM may be in various forms, such as static random access memory (SRAM) or dynamic random access memory (DRAM).

[0202] The database involved in each embodiment provided in this application may include at least one of a relational database and a non-relational database. The non-relational database may include a distributed database based on blockchain, etc., but is not limited thereto. The processor involved in each embodiment provided in this application may be a general-purpose processor, a central processing unit, a graphics processor, a digital signal processor, a programmable logic device, a data processing logic device based on quantum computing, etc., but is not limited thereto.

[0203] The technical features of the above embodiments may be combined arbitrarily. To make the description concise, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0204] This article uses specific examples to illustrate the principles and implementation methods of this application. The description of the above embodiments is only used to help understand the method and core ideas of this application. At the same time, for those skilled in the art, according to the ideas of this application, there will be changes in the specific implementation methods and application scope. In summary, the content of this specification should not be understood as limiting this application.

Claims

1. A method for judging the stability of a grid-connected converter, characterized in that: include: During the operation of the power grid, the operation data of each sampling moment is obtained in real time; The operation data includes: grid voltage, grid line inductance, filter inductance, d-axis component of output current measurement value, d-axis component of output current reference value, q-axis component of output current reference value, grid frequency, phase-locked loop PI parameters and current control loop PI parameters; the phase-locked loop PI parameters include: proportional parameters and integral parameters of the phase-locked loop, and the current control loop PI parameters include: proportional parameters and integral parameters of the current control loop; After a power grid fault occurs, the initial values ​​of various state variables are determined based on various operating data of the power grid in a steady state before the fault occurs and various operating data at the first sampling moment after the power grid fault occurs; the state variables include: a power angle of the grid-following converter, a first-order derivative of the power angle of the grid-following converter, a d-axis component difference, and a first-order derivative of the d-axis component difference, wherein the d-axis component difference is a difference between a d-axis component of a calculated output current of the grid-following converter and a reference output current; Taking the time when the fault occurs as the initial prediction time, based on the initial values ​​of each state variable, and according to the preset time step, the fourth-order nonlinear transient model is solved within a preset period after the initial prediction time to obtain the predicted value of each state variable at each prediction time within the preset period; the fourth-order nonlinear transient model is an ordinary differential equation about the state variable and the operating data; Based on the predicted value of the power angle of the grid-type converter at each predicted time, a power angle time-varying curve of the grid-type converter is drawn; the horizontal axis of the power angle time-varying curve of the grid-type converter is the predicted time, and the vertical axis is the predicted value of the power angle of the grid-type converter; Based on the power angle time-varying curve of the grid-type converter, the stability of the grid-type converter within the preset time period is determined; the grid-type converter is connected to the power grid, and the stability is stable or unstable.

2. The method for determining the stability of a grid-connected converter according to claim 1, characterized in that: The fourth-order nonlinear transient model includes: Among them, x1 is the first state variable; x2 is the second state variable; x3 is the third state variable; x4 is the fourth state variable; δ is the power angle of the grid-following converter; is the first-order derivative of the power angle of the grid-type converter; ΔI d is the d-axis component difference; is the first-order derivative of the difference of the d-axis component; is the first-order derivative of x1; is the first-order derivative of x2; is the first-order derivative of x3; is the first-order derivative of x4; K p is the proportional parameter of the phase-locked loop; U f is the grid voltage after the fault occurs; ω n is the grid frequency; L g is the inductance of the power grid line; I dref is the d-axis component of the output current reference value; K i is the integral parameter of the phase-locked loop; K cp is the proportional parameter of the current control loop; K ci is the integral parameter of the current control loop; L is the total inductance, L = L g +L f , L f For the filter inductor.

3. The method for determining the stability of a grid-connected converter according to claim 2, characterized in that: After a power grid fault occurs, the initial value of each state variable is determined based on each operation data of the power grid in a steady state before the fault occurs and each operation data at the first sampling time after the power grid fault occurs, including: Determine the initial value of the power angle of the grid-following converter based on the grid frequency, the grid impedance when the grid is in a steady state before the fault occurs, the d-axis component of the output current measurement value, and the grid voltage; Determine the initial value of the first-order derivative of the power angle of the grid-following converter based on the phase-locked loop PI parameters, the grid frequency, the q-axis component and the d-axis component of the total voltage difference before and after the fault occurs, the grid line inductance and the filter inductance at the first sampling moment after the fault occurs, the grid voltage after the fault occurs, the grid voltage before the fault occurs, and the d-axis component and the grid line inductance of the output current measurement value at the last sampling moment before the fault occurs; Determine the initial value of the d-axis component difference based on the d-axis component of the total voltage difference before and after the fault occurs, the grid frequency, the initial value of the first-order derivative of the power angle of the grid-following converter, and the d-axis component of the grid line inductance, the filter inductance and the output current reference value at the first sampling moment after the fault occurs; An initial value of a first-order derivative of the d-axis component difference is determined based on a proportional parameter of the current control loop and an initial value of the d-axis component difference.

4. The method for determining the stability of a grid-connected converter according to claim 3, characterized in that: Based on the grid frequency, the grid impedance when the grid is in a steady state before the fault occurs, the d-axis component of the output current measurement value and the grid voltage, the initial value of the power angle of the grid-following converter is determined, including: The power angle calculation formula of the grid-following converter is used to calculate the initial value of the power angle of the grid-following converter according to the grid frequency, the grid impedance when the grid is in a steady state before the fault occurs, the d-axis component of the output current measurement value, and the grid voltage; the power angle calculation formula of the grid-following converter includes: Among them, δ0 is the initial value of the power angle of the grid-following converter; ω n is the grid frequency; L' g I' is the grid impedance when the grid is in a steady state before the fault occurs; d The d-axis component of the output current measurement value when the power grid is in a steady state before 0; U' g It is the grid voltage when the grid is in steady state before the fault occurs.

5. The method for determining the stability of a grid-connected converter according to claim 4, characterized in that: Based on the phase-locked loop PI parameters, the grid frequency, the q-axis component and the d-axis component of the total voltage difference before and after the fault occurs, the grid line inductance and the filter inductance at the first sampling time after the fault occurs, the grid voltage after the fault occurs, the grid voltage before the fault occurs, and the d-axis component and the grid line inductance of the output current measurement value at the last sampling time before the fault occurs, the initial value of the first-order derivative of the power angle of the grid-following converter is determined, including: The initial value of the first-order derivative of the power angle of the grid-following converter is calculated by using the calculation formula of the first-order derivative of the power angle of the grid-following converter, according to the phase-locked loop PI parameters, the grid frequency, the q-axis component and the d-axis component of the total voltage difference before and after the fault occurs, the grid line inductance and the filter inductance at the first sampling time after the fault occurs, the grid voltage after the fault occurs, the grid voltage before the fault occurs, and the d-axis component and the grid line inductance of the output current measurement value at the last sampling time before the fault occurs; the calculation formula of the first-order derivative of the power angle of the grid-following converter includes: in, is the initial value of the first-order derivative of the power angle of the grid-type converter; A and B are both intermediate variables; ΔU q_0 is the q-axis component of the total voltage difference before and after the fault occurs; L g+ is the grid line inductance at the first sampling moment after the fault occurs; L + is the total inductance at the first sampling moment after the fault occurs, L + =L g+ +L f+ , L f+ is the filter inductance at the first sampling moment after the fault occurs; U g is the grid voltage before the fault occurs; I d- is the d-axis component of the output current measurement value at the last sampling moment before the fault occurs; L g- is the grid line inductance at the last sampling moment before the fault occurs; ΔU d_0 It is the d-axis component of the total voltage difference before and after the fault occurs.

6. The method for determining the stability of a grid-connected converter according to claim 5, characterized in that: Based on the d-axis component of the total voltage difference before and after the fault occurs, the grid frequency, the initial value of the first-order derivative of the power angle of the grid-following converter, and the d-axis component of the grid line inductance, the filter inductance, and the output current reference value at the first sampling moment after the fault occurs, the initial value of the d-axis component difference is determined, including: The initial value of the d-axis component difference is calculated using the d-axis component difference calculation formula according to the d-axis component of the total voltage difference before and after the fault occurs, the grid frequency, the initial value of the first-order derivative of the power angle of the grid-following converter, and the d-axis component of the grid line inductance, the filter inductance, and the output current reference value at the first sampling moment after the fault occurs; the d-axis component difference calculation formula includes: ΔI d_0 =I d_0 -I dref+ ; Among them, ΔI d_0 is the initial value of the d-axis component difference; I d_0 is the d-axis component of the calculated output current at the first sampling moment after the fault occurs; I dref+ It is the d-axis component of the output current reference value at the first sampling moment after the fault occurs.

7. The method for determining the stability of a grid-connected converter according to claim 6, characterized in that: Based on the proportional parameter of the current control loop and the initial value of the d-axis component difference, an initial value of the first-order derivative of the d-axis component difference is determined, including: The initial value of the first-order derivative of the d-axis component difference is calculated using the calculation formula of the first-order derivative of the d-axis component difference according to the proportional parameter of the current control loop and the initial value of the d-axis component difference; the calculation formula of the first-order derivative of the d-axis component difference includes: in, is the initial value of the first derivative of the d-axis component difference.

8. A stability judgment system for a grid-following converter, characterized in that: include: The operation data acquisition module is used to obtain the operation data at each sampling moment in real time during the operation of the power grid; The operation data includes: grid voltage, grid line inductance, filter inductance, d-axis component of output current measurement value, d-axis component of output current reference value, q-axis component of output current reference value, grid frequency, phase-locked loop PI parameters and current control loop PI parameters; the phase-locked loop PI parameters include: proportional parameters and integral parameters of the phase-locked loop, and the current control loop PI parameters include: proportional parameters and integral parameters of the current control loop; An initialization module is used to determine the initial values ​​of various state variables after a power grid fault occurs, based on various operating data of the power grid in a steady state before the fault occurs and various operating data at the first sampling time after the power grid fault occurs; the state variables include: a power angle of the grid-following converter, a first-order derivative of the power angle of the grid-following converter, a d-axis component difference, and a first-order derivative of the d-axis component difference, wherein the d-axis component difference is the difference between the d-axis component of the calculated output current of the grid-following converter and the output current reference value; A solution module is used to solve the fourth-order nonlinear transient model within a preset time period after the initial prediction time based on the initial value of each state variable and the preset time step, taking the fault occurrence time as the initial prediction time, and obtain the predicted value of each state variable at each prediction time within the preset time period; the fourth-order nonlinear transient model is an ordinary differential equation about the state variable and the operating data; A curve drawing module, used for drawing a power angle time-varying curve of the grid-type converter based on the predicted value of the power angle of the grid-type converter at each predicted time; the horizontal axis of the power angle time-varying curve of the grid-type converter is the predicted time, and the vertical axis is the predicted value of the power angle of the grid-type converter; A stability determination module is used to determine the stability of the grid-type converter within the preset time period based on the power angle time-varying curve of the grid-type converter; the grid-type converter is connected to the power grid, and the stability is stable or unstable.

9. The stability judgment system of the grid-connected converter according to claim 8, characterized in that: The fourth-order nonlinear transient model includes: Among them, x1 is the first state variable; x2 is the second state variable; x3 is the third state variable; x4 is the fourth state variable; δ is the power angle of the grid-following converter; is the first-order derivative of the power angle of the grid-type converter; ΔI d is the d-axis component difference; is the first-order derivative of the difference of the d-axis component; is the first-order derivative of x1; is the first-order derivative of x2; is the first-order derivative of x3; is the first-order derivative of x4; K p is the proportional parameter of the phase-locked loop; U f is the grid voltage after the fault occurs; ω n is the grid frequency; L g is the inductance of the power grid line; I dref is the d-axis component of the output current reference value; K i is the integral parameter of the phase-locked loop; K cp is the proportional parameter of the current control loop; K ci is the integral parameter of the current control loop; L is the total inductance, L = L g +L f , L f For the filter inductor.

10. The stability judgment system of the grid-connected converter according to claim 9, characterized in that: Initialization module, including: A first initialization unit is used to determine an initial value of the power angle of the grid-following converter based on the grid frequency, the grid impedance when the grid is in a steady state before the fault occurs, the d-axis component of the output current measurement value, and the grid voltage; a second initialization unit, for determining an initial value of a first-order derivative of a power angle of the grid-following converter based on a phase-locked loop PI parameter, a grid frequency, a q-axis component and a d-axis component of a total voltage difference before and after a fault occurs, a grid line inductance and a filter inductance at a first sampling time after the fault occurs, a grid voltage after the fault occurs, a grid voltage before the fault occurs, and a d-axis component and a grid line inductance of an output current measurement value at a last sampling time before the fault occurs; a third initialization unit, for determining an initial value of a d-axis component difference based on a d-axis component of a total voltage difference before and after a fault occurs, a grid frequency, an initial value of a first-order derivative of a power angle of a grid-following converter, and a d-axis component of a grid line inductance, a filter inductance, and an output current reference value at a first sampling moment after the fault occurs; The fourth initialization unit is used to determine an initial value of the first-order derivative of the d-axis component difference based on the proportional parameter of the current control loop and the initial value of the d-axis component difference.

11. The stability judgment system of the grid-connected converter according to claim 10, characterized in that: Based on the grid frequency, the grid impedance when the grid is in a steady state before the fault occurs, the d-axis component of the output current measurement value and the grid voltage, the initial value of the power angle of the grid-following converter is determined, including: The power angle calculation formula of the grid-following converter is used to calculate the initial value of the power angle of the grid-following converter according to the grid frequency, the grid impedance when the grid is in a steady state before the fault occurs, the d-axis component of the output current measurement value, and the grid voltage; the power angle calculation formula of the grid-following converter includes: Among them, δ0 is the initial value of the power angle of the grid-following converter; ω n is the grid frequency; L' g I' is the grid impedance when the grid is in a steady state before the fault occurs; d The d-axis component of the output current measurement value when the power grid is in a steady state before 0; U' g It is the grid voltage when the grid is in steady state before the fault occurs.

12. The stability judgment system of the grid-connected converter according to claim 11, characterized in that: Based on the phase-locked loop PI parameters, the grid frequency, the q-axis component and the d-axis component of the total voltage difference before and after the fault occurs, the grid line inductance and the filter inductance at the first sampling time after the fault occurs, the grid voltage after the fault occurs, the grid voltage before the fault occurs, and the d-axis component and the grid line inductance of the output current measurement value at the last sampling time before the fault occurs, the initial value of the first-order derivative of the power angle of the grid-following converter is determined, including: The initial value of the first-order derivative of the power angle of the grid-following converter is calculated by using the calculation formula of the first-order derivative of the power angle of the grid-following converter, according to the phase-locked loop PI parameters, the grid frequency, the q-axis component and the d-axis component of the total voltage difference before and after the fault occurs, the grid line inductance and the filter inductance at the first sampling time after the fault occurs, the grid voltage after the fault occurs, the grid voltage before the fault occurs, and the d-axis component and the grid line inductance of the output current measurement value at the last sampling time before the fault occurs; the calculation formula of the first-order derivative of the power angle of the grid-following converter includes: in, is the initial value of the first-order derivative of the power angle of the grid-type converter; A and B are both intermediate variables; ΔU q_0 is the q-axis component of the total voltage difference before and after the fault occurs; L g+ is the grid line inductance at the first sampling moment after the fault occurs; L + is the total inductance at the first sampling moment after the fault occurs, L + =L g+ +L f+ , L f+ is the filter inductance at the first sampling moment after the fault occurs; U g is the grid voltage before the fault occurs; I d- is the d-axis component of the output current measurement value at the last sampling moment before the fault occurs; L g- is the grid line inductance at the last sampling moment before the fault occurs; ΔU d_0 It is the d-axis component of the total voltage difference before and after the fault occurs.

13. The stability judgment system of the grid-connected converter according to claim 12, characterized in that: Based on the d-axis component of the total voltage difference before and after the fault occurs, the grid frequency, the initial value of the first-order derivative of the power angle of the grid-following converter, and the d-axis component of the grid line inductance, the filter inductance, and the output current reference value at the first sampling moment after the fault occurs, the initial value of the d-axis component difference is determined, including: The initial value of the d-axis component difference is calculated using the d-axis component difference calculation formula according to the d-axis component of the total voltage difference before and after the fault occurs, the grid frequency, the initial value of the first-order derivative of the power angle of the grid-following converter, and the d-axis component of the grid line inductance, the filter inductance, and the output current reference value at the first sampling moment after the fault occurs; the d-axis component difference calculation formula includes: ΔI d_0 =I d_0 -I dref+ ; Among them, ΔI d_0 is the initial value of the d-axis component difference; I d_0 is the d-axis component of the calculated output current at the first sampling moment after the fault occurs; I dref+ It is the d-axis component of the output current reference value at the first sampling moment after the fault occurs.

14. The stability judgment system of the grid-connected converter according to claim 12, characterized in that: Based on the proportional parameter of the current control loop and the initial value of the d-axis component difference, an initial value of the first-order derivative of the d-axis component difference is determined, including: The initial value of the first-order derivative of the d-axis component difference is calculated using the calculation formula of the first-order derivative of the d-axis component difference according to the proportional parameter of the current control loop and the initial value of the d-axis component difference; the calculation formula of the first-order derivative of the d-axis component difference includes: in, is the initial value of the first derivative of the d-axis component difference.

15. A computer device comprising: A memory, a processor, and a computer program stored in the memory and executable on the processor, wherein the processor executes the computer program to implement the method for determining the stability of a grid-following converter as described in any one of claims 1 to 7.

16. A computer-readable storage medium having a computer program stored thereon, characterized in that: When the computer program is executed by a processor, the method for determining the stability of a grid-connected converter as described in any one of claims 1 to 7 is implemented.

17. A computer program product comprising a computer program, characterized in that When the computer program is executed by a processor, the method for determining the stability of a grid-connected converter as described in any one of claims 1 to 7 is implemented.

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