Transient stability boundary acquisition and stability analysis method for new energy unit grid-connected system

Abstract

The invention discloses a new energy unit grid-connected system transient stability boundary acquisition and stability analysis method, and the method comprises the steps: solving a phase trajectory of a system under different parameters through an ode23tb solver in Matlab for boundary acquisition, judging a parameter value, exceeding a set range, of the trajectory to be unstable, obtaining a point set enabling the system to be transient-stable, and carrying out the transient stability of the system; and fitting the boundary of the region by using Matlab, and drawing an attraction domain or / and a parameter feasible domain capable of ensuring transient stability of the system. According to the stability analysis, whether the system has a balance point or not and whether a stable balance point and a state variable are located in an attraction domain or not are judged, and a system transient stability analysis result is obtained. According to the method, the attraction domain of the system and the parameter feasible domain capable of ensuring the transient stability of the grid-connected system can be quickly solved, a new thought is provided for new energy grid-connected transient stability analysis, and the defects of existing new energy grid-connected transient stability analysis and research are overcome.

Description

Technical Field

[0001] The present invention relates to the technical field of new energy grid-connected power generation, and in particular to a method for obtaining a transient stability boundary of a new energy grid-connected inverter system under fault conditions. Background Art

[0002] Compared with traditional fossil energy, new energy sources dominated by wind power and photovoltaics are highly efficient and environmentally friendly. Moreover, new energy sources such as wind energy and light energy are inexhaustible and can effectively cope with energy crises and environmental pollution problems. Therefore, new energy power generation technology has been highly valued and widely used by countries around the world.

[0003] As a large number of new energy units are connected to the grid through inverters, a new energy grid-connected power generation system has been formed. This grid-connected mode has become an important part of the modern power supply system. However, new energy power generation has significant intermittent and fluctuating characteristics. For example, solar power generation depends on sunshine conditions, and wind power generation is subject to wind speed changes. This makes the operating characteristics of the new energy grid-connected system more complex than the traditional fossil fuel power generation grid-connected system.

[0004] In the operation of power systems, transient stability is a crucial aspect. The transient process refers to the process in which the system transitions from one stable operating state to another stable operating state or unstable state after being subjected to large disturbances such as short circuit faults and sudden load changes. For new energy grid-connected systems, ensuring that they can maintain stable operation under transient events is the key to ensuring power supply reliability and grid security.

[0005] However, it is not easy to accurately analyze the transient stability of renewable energy grid-connected systems. On the one hand, renewable energy units usually use grid-connected inverters for grid-connected control, and their control strategies and dynamic characteristics are relatively complex, which has a significant impact on the transient stability of the system. On the other hand, traditional methods for analyzing the transient stability of power systems are often difficult to accurately capture the various behaviors of the system in the transient process when facing objects with special operating characteristics such as renewable energy grid-connected systems, and cannot meet the needs of accurate analysis of their transient stability.

[0006] Therefore, accurate parameter design plays a decisive role in improving the transient stability and operational reliability of renewable energy grid-connected systems. However, there is still a lack of rapid transient stability assessment methods for renewable energy grid-connected systems, which makes it difficult to provide a scientific and reliable basis for parameter design, thus limiting the improvement of the overall performance of renewable energy grid-connected systems. Summary of the invention

[0007] In view of this, the purpose of the present invention is to provide a grid-connected transient stability analysis and parameter design method based on Matlab solver to solve the technical problem of accurately evaluating the transient stability performance of the grid-connected system of the new energy grid-following inverter and providing effective guidance for parameter design.

[0008] The method for obtaining the transient stability boundary of the grid-connected system of a new energy generator set based on the Matlab / ode solver of the present invention comprises the following steps: traversing the parameter values ​​of the grid-connected system of the new energy generator set, using the ode23tb solver in Matlab to obtain the phase trajectory of the system under different parameters, and converting the trajectory beyond (-π-δ e ,π-δ e ) range is judged as unstable, and the point set that makes the system transiently stable is obtained, and the parameter value range that satisfies the transient stability of the full-order model of the new energy unit grid-connected system is drawn; then the boundary of the point set area is fitted by Matlab, and the attraction domain and / or parameter feasible domain that can ensure the transient stability of the system are drawn; the aforementioned δ e The expression is as follows:

[0009]

[0010] Where U g is the grid voltage, R g is the equivalent resistance between the grid connection point and the grid, L g is the equivalent inductance between the grid connection point and the grid, ω PLL is the oscillation frequency of the phase-locked loop in the grid-connected system of new energy units, I odref It is the d-axis setting value of the current loop in the grid-connected system of the new energy unit.

[0011] The transient stability analysis method of the new energy unit grid connection of the present invention comprises: after a transient event occurs in the new energy unit grid connection system, the following judgment is made:

[0012] 1) Determine whether the system equilibrium point still exists. If so, proceed to step 2). Otherwise, the system is transiently unstable.

[0013] 2) Determine whether there is a stable equilibrium point in the system equilibrium point. If so, proceed to step 3). Otherwise, the system is transiently unstable.

[0014] 3) The attraction domain of the system is obtained through the transient stability boundary acquisition method of the new energy unit grid-connected system based on the Matlab / ode solver, and it is judged whether the state variables of the system are within the attraction domain. If so, the system is transiently stable, otherwise the system is transiently unstable.

[0015] Furthermore, in judgment 1), the condition for the existence of the system equilibrium point is δ e Existence, δ e The condition for existence is |Rg i oqref +ω PLL L g i odref | g .

[0016] Furthermore, in judgment 2), by changing the parameters of the system, the eigenvalues ​​of the Jacobian matrix of the system at the system equilibrium point are solved. If each eigenvalue has a negative real part, there is a stable equilibrium point, otherwise there is no stable equilibrium point.

[0017] Beneficial effects of the present invention:

[0018] 1. The present invention provides a method for obtaining transient stability boundaries of a grid-connected system of a new energy generator set based on a Matlab / ode solver. Based on this method, the attraction domain of the system and the feasible domain of parameters that can ensure the transient stability of the grid-connected system can be quickly obtained.

[0019] By determining the parameter domain, a scientific basis can be provided for parameter design, thereby effectively improving the stability and reliability of the grid-connected system of new energy units.

[0020] 2. The transient stability analysis method of the new energy unit connected to the grid of the present invention provides a new idea for the transient stability analysis of the new energy connected to the grid, and makes up for the shortcomings of the existing research on transient stability analysis of the new energy connected to the grid. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 It is the structure diagram of the grid-connected system of new energy units, where U dc is the DC side voltage of the inverter, E is the inverter port voltage, U o is the grid voltage, U g is the grid voltage, I o is the grid-connected current, I L is the inductor current, L f and C f is the filter inductance and filter capacitance of the LC filter, R g and L g is the equivalent resistance and equivalent inductance between the grid connection point and the grid, θ PLL The phase angle determined by the phase-locked loop, ω PLL is the oscillation frequency of the phase-locked loop, [i od ,i oq ] and [u od ,u oq ] are the dq axis components of the voltage and current at the grid connection point, respectively, [I odref ,I oqref ] are the dq axis components of the current loop setting value, GVF is the voltage feedforward coefficient, [u d ,u q ] and [e​d ,e q ] are the dq axis components of the current loop output before and after considering the control delay;

[0022] Figure 2 It is the distribution of characteristic value trajectory under the change of voltage amplitude and current reference, where (a) is the characteristic value trajectory diagram under the voltage drop (1p.u.→0.4pu) event; (b) is the characteristic value trajectory diagram under the reference current increase (1p.u.→0.4pu) event;

[0023] Figure 3 It is a flow chart of grid-connected small signal parameter domain solution based on Matlab / ode solver;

[0024] Figure 4 It is a process diagram for obtaining the current loop PI parameter domain that ensures small signal stability;

[0025] Figure 5 is the PI small signal parameter domain diagram of the current loop, where (a) is U g PI small signal parameter domain diagram with different values, (b) is I dref Take different values ​​of the PI small signal parameter domain;

[0026] Figure 6 is the PI small signal parameter domain diagram of the phase-locked loop, where (a) is U g PI small signal parameter domain diagram with different values, (b) is I dref Take different values ​​of the PI small signal parameter domain;

[0027] Figure 7 It is a flow chart for solving transient attraction domain or parameter domain;

[0028] Figure 8 It is the response curve diagram under the voltage drop (1p.u.→0.45pu) event;

[0029] Fig. 9 It is the response curve diagram under the voltage drop (1p.u.→0.42pu) event;

[0030] Fig.10 is the transient stability domain diagram of the phase-locked loop PI, where (a) is U g Taking different values ​​of PI transient stability region diagram, (b) is I dref Take the transient stability region diagram of PI with different values;

[0031] Fig.11 is the transient stability domain diagram of the current loop PI, where (a) is U g Taking different values ​​of PI transient stability region diagram, (b) is I dref Take different values ​​of PI transient stability domain diagram. DETAILED DESCRIPTION

[0032] The present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0033] like Figure 1 As shown, the new energy unit grid-connected system in this embodiment includes a grid-connected inverter, a DC power supply connected to the DC side of the grid-connected inverter, an LC filter circuit connected to the AC side of the grid-connected inverter, a power grid connected to the LC filter circuit, an SVPWM modulation module for controlling the grid-connected inverter, and a phase-locked loop and a current loop for controlling the input signal of the SVPWM modulation module.

[0034] This implementation takes a 2MW grid-connected inverter with typical parameters as an example for simulation research. The structure diagram is shown in Figure 1 The unit parameters are shown in Table 1.

[0035] Table 1 Main parameters of the system:

[0036]

[0037]

[0038] In this embodiment, the method for obtaining the transient stability boundary of the grid-connected system of the new energy generator set based on the Matlab / ode solver includes: by traversing the parameter values ​​of the grid-connected system of the new energy generator set, using the ode23tb solver in Matlab to obtain the phase trajectory of the system under different parameters, and converting the trajectory beyond (-π-δ e ,π-δ e ) range is judged as unstable, and the point set that makes the system transiently stable is obtained, and the parameter value range that satisfies the transient stability of the full-order model of the new energy unit grid-connected system is drawn; then the boundary of the point set area is fitted by Matlab, and the attraction domain and / or parameter feasible domain that can ensure the transient stability of the system are drawn; the aforementioned δ e The expression is as follows:

[0039]

[0040] Where U g is the grid voltage, R g is the equivalent resistance between the grid connection point and the grid, L g is the equivalent inductance between the grid connection point and the grid, ω PLL is the oscillation frequency of the phase-locked loop in the grid-connected system of new energy units, I odref It is the d-axis setting value of the current loop in the grid-connected system of the new energy unit.

[0041] The full-order model establishment process of the new energy unit grid-connected system in this embodiment is as follows:

[0042] The system phase-locked loop equation is established as:

[0043]

[0044] In the formula is the first-order derivative of the phase-locked deviation angle with respect to time, Δω is the output of the phase-locked loop PI controller, is the first-order derivative of the phase-locked loop PI controller output with respect to time, u oq is the q-axis component of the grid-connected point voltage, is the first-order derivative of the q-axis component of the grid-connected point voltage with respect to time, k p is the proportional parameter of the phase-locked loop PI controller, k i is the integral parameter of the phase-locked loop PI controller.

[0045] The system current loop equation is established as:

[0046]

[0047] In the formula, u d To consider the d-axis component of the current loop output before the control delay, u q To consider the q-axis component of the current loop output before the control delay, k pc is the proportional coefficient of the current loop PI controller, k ic is the integral coefficient of the current loop PI controller, i odref is the d-axis component of the current loop setting value, i oqref is the q-axis component of the current loop set value, GVF is the voltage feedforward coefficient, u od is the d-axis component of the grid-connected point voltage, ω PLL is the phase-locked loop angular frequency, L f is the filter inductance of the LC filter, i gd is the d-axis component of the grid current, i gq is the q-axis component of the grid current.

[0048] Establish the system delay control equation:

[0049]

[0050] Where e d To consider the d-axis component of the current loop output after the control delay, e q To consider the q-axis component of the current loop output after the control delay, for e d The first derivative with respect to time, for e q The first derivative with respect to time,

[0051] T d is the delay coefficient, for u d The first derivative with respect to time, for u q The first derivative with respect to time.

[0052] Establish the system voltage and current equation:

[0053]

[0054] In the formula is the second-order derivative of the d-axis component of the inductor current with respect to time, is the second-order derivative of the d-axis component of the grid current with respect to time, i Lq is the q-axis component of the inductor current, is the first-order derivative of the d-axis component of the grid-connected point voltage with respect to time, U g is the grid voltage, R d is the internal resistance of the filter inductor, R g is the equivalent resistance between the grid connection point and the grid, L g is the equivalent inductance between the grid connection point and the grid, C f is the filter capacitor of the LC filter, i Ld is the axis component of the filter output current d; i Lq is the q-axis component of the filter output current.

[0055] Take the system state variable as x full =[x1,x2,…,x 12 ] T =[δ,Δω,u d ,u q ,e d ,e q ,i Ld ,i Lq ,u od ,u oq ,i od ,i oq ] T , the full-order model represented by the above equations (1) to (4) is arranged into a standard form to obtain a set of 12-order nonlinear differential equations, which is the nonlinear full-order model of the new energy grid-connected inverter system:

[0056]

[0057] Where δ is the phase angle θ of the phase-locked loop output in the system controller coordinate system PLL The phase angle θ with the actual coordinate system of the power grid g The deflection angle between PLL is the phase-locked loop angular frequency ω PLL and grid angular frequency ω g difference.

[0058] This embodiment is based on the above-mentioned method for obtaining the transient stability boundary of the new energy generator grid-connected system. The method includes the following judgments after a transient event occurs in the new energy generator grid-connected system:

[0059] 1) Determine whether the system equilibrium point still exists. If so, proceed to step 2). Otherwise, the system is transiently unstable.

[0060] By setting the derivative of the state variable in the full-order model described by formula (5) to 0, we can obtain the equilibrium point x full_e =[x 1e ,x 2e ,…x 12e ]:

[0061]

[0062] In judgment 1), the condition for the existence of the system equilibrium point is δ e Existence, δ e The condition for existence is |R g i oqref +ω PLL L g i odref | g .

[0063] 2) Under the premise that the system equilibrium point exists, determine whether there is a stable equilibrium point in the system equilibrium point. If so, proceed to step 3), otherwise the system is transiently unstable. In judgment 2), by changing the system parameters, solve the eigenvalues ​​of the Jacobian matrix of the system at the system equilibrium point. If each eigenvalue has a negative real part, there is a stable equilibrium point, otherwise there is no stable equilibrium point.

[0064] Next, we use small signal stability analysis to determine whether the equilibrium point is stable.

[0065] The full-order model is linearized as:

[0066]

[0067] Where J is x full In x full_e The Jacobian matrix of the linear model expanded at 11 ~A 22 for:

[0068]

[0069] From this we can get J 12×12 In x full_e The eigenvalue at , and then determine each eigenvalue λ i ​Do (i=1,2,…,12) all have negative real parts?

[0070] Figure 2 (a) and (b) are U g Decrease from 1p.u. to 0.4pu and I odref J increased from 0 p.u. to 2.5 pu 12×12 Eigenvalue trajectory distribution (U g Change I odref Take 1 p.u., I odref When U changes g Take 1p.u). It can be seen that U g When it drops to 0.4pu, it intersects with the imaginary axis, that is, at U g =0.4pu, the system loses small signal stability. g becomes smaller, the system characteristic root moves to the right half plane, that is, the equilibrium point under this parameter cannot maintain small signal stability, making the system unstable, thus the transient stability of the system can be quickly judged. Figure 2 (b) It can be seen that as I odref The setting value of increases gradually, and the dominant characteristic value of the system moves toward the imaginary axis continuously. odref When it increases to 2.5pu, it intersects with the imaginary axis and the system loses its small signal stability, that is, the system is transiently unstable.

[0071] The feasible domain of parameters of grid-connected small signal is solved based on Matlab / ode solver, the flow chart is as follows Figure 3 As shown, the process of obtaining Figure 4 As shown. The parameters in the blue area can ensure that all characteristic roots are negative, that is, the parameters meet the small signal stability, and the red area is the unstable area. Based on this method, the current loop and the phase-locked loop are drawn in U g and I odref The PI parameter domain when changing is as follows Figure 5 , Figure 6 It can be seen that the parameters of the phase-locked loop have a more obvious impact on the small signal stability of the system than the current loop parameters.

[0072] 3) If only small signal stability is satisfied, the system will still be transiently unstable, so it is also necessary to determine whether the system state variables are within the attraction domain. The attraction domain of the system is obtained by the transient stability boundary acquisition method of the grid-connected system of new energy units based on the Matlab / ode solver. The solution process of the attraction domain is as follows: Figure 7 As shown, it is determined whether the state variables of the system are within the attraction domain. If so, the system is transiently stable, otherwise, the system is transiently unstable.

[0073] Taking voltage drop as an example, draw the system at U gThe phase trajectory diagram of δ and Δω and their attraction domain when the voltage drops from 1p.u. to 0.45pu and from 1p.u. to 0.42pu, as shown in the figure Figure 8 , Fig. 9 shown.

[0074] Depend on Figure 8 It can be seen that when U g When U drops from 1p.u. to 0.45pu, g =1p.u. The initial point is within the plotted DOA1 and converges to the first steady-state point A under this condition. After the transient event occurs, U g =0.45pu, at this time the system's attraction domain becomes DOA2, and due to the change in conditions, the system is no longer balanced, and the operating point will migrate to a new steady-state point. It can be seen from the figure that when the transient event occurs, the initial value of the system is point A, and A is located at U g =0.45pu in the attraction domain DOA2, so after the transient event, the system can converge to the next stable operating point B, that is, the system is in U g It is transiently stable when it drops from 1p.u. to 0.45pu.

[0075] Depend on Fig. 9 It can be seen that in U g When the drop depth increases further and drops to 0.42pu, the situation changes. It can be seen that after the transient event occurs, the attraction domain DOA2 when Ug=0.42pu no longer contains the initial operating point A of the system after the transient event. Therefore, after the transient event occurs, the system diverges, and the boundary value of the voltage drop that the system can withstand is 0.45pu.

[0076] Draw U separately g and I odref The transient parameter domain of the phase-locked loop PI when changing is as follows Fig.10 As shown, it can be seen that the grid voltage amplitude U g It is positively correlated with the transient parameter domain of the phase-locked loop PI. The larger the grid voltage amplitude, the larger the transient parameter domain of the phase-locked loop PI. odref It is negatively correlated with the transient parameter domain of the phase-locked loop PI, I odref The larger the setting, the smaller the transient parameter range of the phase-locked loop PI.

[0077] Draw U separately g and I odref The transient parameter domain of the current loop PI when changing is as follows Fig.11 As shown, it can be seen that the grid voltage drop amplitude is related to the grid-connected d-axis setting value I odref When it changes, the current loop PI parameter domain does not change much.

[0078] By changing the line inductance, grid voltage, reference current and phase-locked loop parameters, the influence of the above parameters on the transient synchronization stability of the system can be obtained as shown in Table 2. Therefore, the attraction domain can be expanded by changing the parameters, thereby improving the transient stability of the system.

[0079] Table 2 Effect of parameters on system transient synchronization stability

[0080]

[0081] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solution of the present invention can be modified or replaced by equivalents without departing from the purpose and scope of the technical solution of the present invention, which should be included in the scope of the claims of the present invention.

Claims

1. A method for obtaining transient stability boundary of a grid-connected system of a new energy generator set based on Matlab / ode solver, characterized in that: This includes traversing the parameter values ​​of the grid-connected system of new energy units, using the ode23tb solver in Matlab to obtain the phase trajectory of the system under different parameters, and converting the trajectory beyond (-π-δ e ,π-δ e ) range is judged as unstable, and the point set that makes the system transiently stable is obtained. Then, the boundary of the point set area is fitted using Matlab to draw the attraction domain and / or parameter feasible domain that can ensure the transient stability of the system; the aforementioned δ e The expression is as follows: Where U g is the grid voltage, R g is the equivalent resistance between the grid connection point and the grid, L g is the equivalent inductance between the grid connection point and the grid, ω PLL is the oscillation frequency of the phase-locked loop in the grid-connected system of new energy units, I odref It is the d-axis setting value of the current loop in the grid-connected system of the new energy unit.

2. A method for analyzing transient stability of a new energy generator grid-connected system according to the method for obtaining transient stability boundary of a new energy generator grid-connected system according to claim 1, characterized in that: After a transient event occurs in the grid-connected system of a new energy unit, the following judgments are made: 1) Determine whether the system equilibrium point still exists. If so, proceed to step 2). Otherwise, the system is transiently unstable. 2) Determine whether there is a stable equilibrium point in the system equilibrium point. If so, proceed to step 3). Otherwise, the system is transiently unstable. 3) The attraction domain of the system is obtained through the transient stability boundary acquisition method of the new energy unit grid-connected system based on the Matlab / ode solver, and it is judged whether the state variables of the system are within the attraction domain. If so, the system is transiently stable, otherwise the system is transiently unstable.

3. The method for analyzing the transient stability of a new energy generator set connected to the grid according to claim 2 is characterized by: In judgment 1), the condition for the existence of the system equilibrium point is δ e Existence, δ e The condition for existence is |R g i oqref +ω PLL L g i odref | g .​ 4. The method for analyzing transient stability of grid-connected new energy units according to claim 2 is characterized in that: In judgment 2), by changing the parameters of the system, the eigenvalues ​​of the Jacobian matrix of the system at the system equilibrium point are solved. If each eigenvalue has a negative real part, there is a stable equilibrium point, otherwise there is no stable equilibrium point.

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