Direct current power flow controller current distribution analysis method based on asymmetric current control

Abstract

The invention provides a DC power flow controller current distribution analysis method based on asymmetric current control, and the method comprises the steps: S1, building an asymmetric current control three-wire DC power flow controller modal space model, and carrying out the dynamic segmentation of the model, and obtaining an autonomous system of the model; s2, constructing a transfer function mathematical model for the autonomous system, and representing transient oscillation conditions of the direct current power flow controller under different current reference instructions by using a maximum singular value frequency sweeping result; and S3, according to the influence of the setting of the control instruction of the controlled power transmission line on transient oscillation, obtaining the optimal current distribution with the optimal transient state. According to the method, a mathematical model of the asymmetric current control direct current power flow controller is constructed, the transient oscillation rule of the direct current power flow controller under different current reference instructions is researched based on singular value analysis, an optimal current distribution analysis method is provided, and reference is provided for avoiding unreasonable current instruction values of the direct current power flow controller.

Description

Technical Field

[0001] The present invention relates to the technical field of electrical engineering, and in particular, to an optimal current distribution analysis method for a three-line DC power flow controller based on asymmetric current control. Background Art

[0002] With the strong promotion of the construction of a new power system, the source-load characteristics targeted by traditional power system regulation strategies are no longer applicable. At the same time, the proportion of distributed resources such as distributed energy storage, electric transportation vehicles, and distributed generation accessing the power grid has increased rapidly, and the new power system presents characteristics such as strong randomness, high flexibility, and active power. The flexible DC transmission technology has the characteristics of low harmonic level, no commutation failure, and can realize power supply to a passive system, which can better alleviate the randomness problem of the new power system.

[0003] The voltage source type flexible DC transmission based on modular multilevel converters, two-level converters, and three-level converters can realize the decoupled control of active and reactive power in the power system, further improving the flexibility of the new power system. However, there is an unreasonable phenomenon in the power flow distribution of the voltage source type flexible DC transmission, which causes overload of DC transmission lines, leading to inefficient operation of the DC grid and even serious safety hazards.

[0004] Therefore, introducing a DC power flow controller applicable to a multi-terminal DC system can rationalize the power flow distribution of DC transmission lines and prevent line overload. Aiming at the non-fault switching transient optimization problem of the three-line DC power flow controller, the optimal current distribution analysis method for the three-line DC power flow controller based on asymmetric current control proposed in this solution can provide a reference for reducing the influence of the control command setting of the three-line DC power flow controller on transient oscillation.

[0005] In the Chinese patent document with the publication number CN116799809A, a hierarchical control method and system based on a DC power flow controller are disclosed. This method performs small-signal modeling on the DC power flow controller, writes out the small-signal state space equation; designs PI control parameters based on the established small-signal modeling and the transfer function of the n-line modular capacitor type DC power flow controller to control the DC power flow controller; the centralized control layer performs power flow analysis to obtain the analysis result. This method does not involve non-fault transient oscillation caused by current control commands.

[0006] Based on the three-line DC power flow controller with asymmetric current control, this solution proposes an optimal current distribution analysis method based on non-fault transient oscillation optimization, providing a reference for reducing non-fault transient oscillation caused by current control commands. Summary of the Invention

[0007] Aiming at the defects in the prior art, the purpose of the present invention is to provide a method for analyzing the current distribution of a DC power flow controller based on asymmetric current control.

[0008] A method for analyzing the current distribution of a DC power flow controller based on asymmetric current control provided by the present invention includes:

[0009] Step S1: Establish a modal space model of a three-line DC power flow controller with asymmetric current control, and dynamically partition the model to obtain the autonomous system of the model;

[0010] Step S2: Construct a transfer function mathematical model for the autonomous system, and characterize the transient oscillation condition of the DC power flow controller under different current reference commands with the maximum singular value sweep result;

[0011] Step S3: Obtain the optimal current distribution with the best transient performance according to the influence of the control command setting of the controlled transmission line on the transient oscillation.

[0012] Preferably, in the step S1:

[0013] The establishment of the modal space model of the three-line DC power flow controller considering asymmetric current control includes: establishing a differential equation for the system dynamics of the three-line power flow controller, establishing a complete state space model, and completing the modal space modeling;

[0014] The dynamic partitioning of the model to obtain the autonomous system of the model includes: defining the state variables directly related to the PI control loop as the controllable variables Xc, and the variables indirectly related to the PI control loop as the autonomous variables Xo.

[0015] Preferably, the state space model includes:

[0016]

[0017] Where, Δ represents that the variable is a small signal perturbation, A represents the system matrix of the three-line DC power flow controller with asymmetric current control, B represents the input matrix of the system, C represents the output matrix of the system, X represents the state variable of the system, represents the differential of the system state variable, u represents the input variable of the system, Y represents the output variable of the system. X1 and X2 are vectors composed of the first 8 variables and the last 8 variables of X respectively, and are respectively the vectors composed of the first 8 variables and the last 8 variables of, O 8×8 represents an 8×8 zero matrix. A 11 、A 12 、A 21 and A 22 are four 8×8 block matrices inside the A matrix, B11 , B 12 , B 21 and B 22 are four 8-row 2-column submatrices inside the B matrix, C 11 , C 22 are two 8-row 8-column submatrices inside the C matrix. i 14ref is the current control reference value for line line 14 , i 24ref is the current control reference value for line line 24 .

[0018] Preferably, the step S2 includes:

[0019] Based on the Laplace transform, the transfer function of the autonomous system of the three-line DC power flow controller is:

[0020] G o = C oo (sI - A o ) -1 B oo

[0021] where s is the Laplace operator, I is an 8-row 8-column identity matrix, and G o is the transfer function of the autonomous system of the inter-line DC power flow controller;

[0022] For an n×m order matrix G o , there exist an n×n order matrix U and an m×m order matrix V satisfying:

[0023] G o (s) = U(s)S(s)V T (s)

[0024] where V T is the transpose of the orthogonal matrix V, S is an m×n diagonal matrix and also the singular value function matrix, and its diagonal elements are the singular values of the matrix G o , arranged in descending order; the column vectors of U(s) are called left singular vectors, representing the output direction; the column vectors of V(s) are called right singular vectors, representing the input direction;

[0025] Since the largest singular value can reflect the oscillation amplitude of the output variable caused by the input variable of the autonomous system of the asymmetric current control three-line DC power flow controller; considering the singular situation of S(s) at any oscillation mode s0 is equivalent to its largest singular value, that is:

[0026]

[0027] In the formula, det(·) represents the determinant of the matrix, Denote the maximum singular value. Let the function of the maximum singular value of S(s) with respect to s be the dominant transfer function of the system and satisfy:

[0028]

[0029] Obtain the dominant transfer function through sweep frequency analysis the corresponding amplitude change, at different frequency points the magnitude of the amplitude represents the magnitude of the transient oscillation of the autonomous system, and complete the transient oscillation analysis of the system.

[0030] Preferably, the step S3 includes: studying the transient oscillation characteristics of the three-line DC power flow controller with asymmetric current control under different current reference commands, and determining the optimal current distribution; the parameters in the current reference command include: the current distribution ratio between the controlled DC transmission lines, and the sum of the total controlled current reference values.

[0031] According to a DC power flow controller current distribution analysis system based on asymmetric current control provided by the present invention, it includes:

[0032] Module M1: Establish a modal space model of the three-line DC power flow controller with asymmetric current control, and dynamically partition the model to obtain the autonomous system of the model;

[0033] Module M2: Construct a transfer function mathematical model for the autonomous system, and characterize the transient oscillation condition of the DC power flow controller under different current reference commands with the sweep result of the maximum singular value;

[0034] Module M3: Obtain the optimal current distribution of the transient optimum according to the influence of the setting of the control command of the controlled transmission line on the transient oscillation.

[0035] Preferably, in the module M1:

[0036] The establishment of the modal space model of the three-line DC power flow controller considering asymmetric current control includes: establishing a differential equation for the system dynamics of the three-line power flow controller, establishing a complete state space model, and completing the modal space modeling;

[0037] The dynamic partitioning of the model to obtain the autonomous system of the model includes: defining the state variables directly related to the PI control loop as the controllable variables Xc, and the variables indirectly related to the PI control loop as the autonomous variables Xo.

[0038] Preferably, the state space model includes:

[0039]

[0040] Among them, Δ represents that the variable is a small-signal perturbation, A represents the system matrix of the asymmetric current control three-line DC power flow controller, B represents the input matrix of the system, C represents the output matrix of the system, X represents the state variable of the system, represents the differential of the system state variable, u represents the input variable of the system, and Y represents the output variable of the system. X1 and X2 are vectors composed of the first 8 variables and the last 8 variables of X respectively, and are respectively the vectors composed of the first 8 variables and the last 8 variables of 8×8 denotes an 8×8 zero matrix. A 11 , A 12 , A 21 and A 22 are four 8×8 block matrices inside the A matrix, B 11 , B 12 , B 21 and B 22 are four 8×2 block matrices inside the B matrix, C 11 , C 22 are two 8×8 block matrices inside the C matrix. i 14ref is the current control reference value of line 14 , i 24ref is the current control reference value of line 24 .

[0041] Preferably, the module M2 includes:

[0042] Based on the Laplace transform, the transfer function of the three-line DC power flow controller autonomous system is:

[0043] G o = C oo (sI - A o ) -1 B oo

[0044] Among them, s is the Laplace operator, I is an 8×8 identity matrix, and G o is the transfer function of the line-to-line DC power flow controller autonomous system;

[0045] For an n×m matrix G o , there exist an n×n matrix U and an m×m matrix V that satisfy:

[0046] G o (s) = U(s)S(s)V T (s)

[0047] Among them, V Tis the transpose of the orthogonal matrix V, and S is an m×n diagonal matrix, which is also the singular value function matrix, and its diagonal elements are the singular values of matrix G o The column vectors of U(s) are called left singular vectors, representing the output direction; the column vectors of V(s) are called right singular vectors, representing the input direction;

[0048] Since the largest singular value can reflect the oscillation amplitude of the output variable caused by the input variable of the autonomous system of the three-phase DC power flow controller with asymmetric current control; considering that at any oscillation mode s0 of the system, the singular situation of S(s) is equivalent to its largest singular value, that is:

[0049]

[0050] In the formula, det(·) represents the determinant of the matrix, represents the largest singular value. Let the function of the largest singular value of S(s) with respect to s be the dominant transfer function of the system, and satisfy:

[0051]

[0052] The dominant transfer function is obtained through sweep frequency analysis The corresponding amplitude change, at different frequency points The amplitude size characterizes the transient oscillation of the autonomous system, and the transient oscillation analysis of the system is completed.

[0053] Preferably, the module M3 includes: studying the transient oscillation characteristics of the three-phase DC power flow controller with asymmetric current control under different current reference commands, and determining the optimal current distribution; the parameters in the current reference command include: the current distribution ratio between the controlled DC transmission lines, and the sum of the total controlled current reference values.

[0054] Compared with the prior art, the present invention has the following beneficial effects:

[0055] 1. The present invention constructs a mathematical model of the DC power flow controller with asymmetric current control, studies the transient oscillation law of the DC power flow controller under different current reference commands based on singular value analysis, and proposes an optimal current distribution analysis method, providing a reference for avoiding unreasonable current command values of the DC power flow controller.

[0056] 2. The present invention applies the dynamic segmentation method to segment the modal space model of the DC power flow controller with asymmetric current control to obtain a mathematical model of the autonomous system highly related to the transient oscillation.

[0057] 3. For the transient oscillation characterization of the non-fault switching of the power flow controller, the present invention uses the singular value analysis method, which has stronger adaptability than the traditional transient oscillation analysis method, to accurately characterize the transient oscillation of the transient state variables of the autonomous system.

[0058] 4. The present invention innovatively discusses the influence of control commands on the transient oscillation of the multi-line power flow controller, adjusts the sum of the current distribution ratio and the current reference value, calculates the amplitude of the maximum singular value of the autonomous system under different current distributions, and then obtains the optimal current distribution with the best transient performance.

[0059] 5. By optimizing the current distribution ratio between the controlled HVDC transmission lines and the sum of the total controlled current reference values, the present invention effectively reduces the transient oscillation caused by unreasonable current control commands, thereby improving the stability and reliability of the flexible HVDC transmission system. It can reduce the problem of inefficient operation of the DC power grid caused by inappropriate current distribution, make the power transmission more efficient, and reduce energy losses.

[0060] 6. Based on the application of advanced mathematical tools such as modal space modeling and dynamic segmentation, transfer function construction, and singular value analysis, the present invention provides a solid theoretical basis and scientific guidance for the design of DC power flow controllers, which helps engineers to design more accurate control systems.

[0061] 7. By calculating the amplitude of the maximum singular value of the autonomous system under different current distributions, the present invention obtains the current distribution law with the best transient performance, simplifies the parameter setting process in actual engineering projects, and provides a convenient operation guide for practical applications. BRIEF DESCRIPTION OF THE DRAWINGS

[0062] Other features, objects, and advantages of the present invention will become more apparent by reading the following detailed description of non-limiting embodiments with reference to the accompanying drawings:

[0063] Figure 1 is the topology diagram of the three-line DC power flow controller with asymmetric current control in the present invention;

[0064] Figure 2 is the schematic diagram of working mode 1 of the three-line DC power flow controller with asymmetric current control in the present invention;

[0065] Figure 3 is the schematic diagram of working mode 2 of the three-line DC power flow controller with asymmetric current control in the present invention;

[0066] Figure 4 is the schematic diagram of working mode 3 of the three-line DC power flow controller with asymmetric current control in the present invention;

[0067] Figure 5 is the maximum singular value in the present invention Schematic diagram of corresponding amplitude change;

[0068] Figure 6 Under different current distribution ratio conditions in the present invention Three-dimensional amplitude diagram;

[0069] Figure 7 Under different conditions of the sum of current reference values in the present invention Three-dimensional amplitude diagram;

[0070] Figure 8 Schematic diagram of the four-terminal loop network HVDC transmission system in the present invention;

[0071] Figure 9 For the common inductance current i under current distribution 1, current distribution 2, and current distribution 3 in the present invention L Transient waveform diagram;

[0072] Figure 10 For the transient waveform diagram of the voltage u3 at the VSC3 port under current distribution 1, current distribution 2, and current distribution 3 in the present invention;

[0073] Figure 11 For the common inductance current i under current distribution 2, current distribution 4, and current distribution 5 in the present invention L Transient waveform diagram;

[0074] Figure 12 For the transient waveform diagram of the voltage u3 at the VSC3 port under current distribution 2, current distribution 4, and current distribution 5 in the present invention. Specific embodiments

[0075] The present invention will be described in detail below in conjunction with specific embodiments. The following embodiments will help those skilled in the art to further understand the present invention, but do not limit the present invention in any form. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present invention, several changes and improvements can still be made. These all belong to the protection scope of the present invention.

[0076] The present invention discloses an optimal current distribution analysis method for a three-line DC power flow controller based on asymmetric current control. The optimal current distribution analysis method for the DC power flow controller mainly includes modal space modeling and dynamic segmentation of the asymmetric current control power flow controller, construction of a mathematical model of the transfer function of the autonomous system and transient oscillation analysis, and analysis of the optimal current distribution of the power flow controller. Based on the modal space model of the three-line DC power flow controller considering asymmetric current control, dynamic segmentation is performed and a mathematical model of the transfer function of the autonomous system obtained after segmentation is constructed. The transient oscillation condition of the DC power flow controller is characterized by the maximum singular value sweep result, and then the influence of the setting of the control commands of the two controlled transmission lines on the transient oscillation is explored, and the optimal current distribution law of the transient optimum is given. The specific implementation means are as follows:

[0077] A. Modal space modeling and dynamic segmentation of the asymmetric current control power flow controller

[0078] The research object of this solution is the topology of a three-line common inductance type DC power flow controller with asymmetric current control, as Figure 1 shown. In the complete working cycle of the three-line DC power flow controller with asymmetric current control, there are a total of three working modes, as Figure 2 , Figure 3 and Figure 4 shown. Differential equations are established for the system dynamics of the three-line power flow controller, and a complete state space model is established as shown in Equations (1) and (2), and then modal space modeling is completed.

[0079]

[0080] Among them, Δ represents that the variable is a small signal perturbation, A represents the system matrix of the three-line DC power flow controller with asymmetric current control, B represents the input matrix of the system, C represents the output matrix of the system, X represents the state variable of the system, represents the differential of the system state variable, u represents the input variable of the system, and Y represents the output variable of the system. X1 and X2 are vectors composed of the first 8 variables and the last 8 variables of X respectively, and are respectively vectors composed of the first 8 variables and the last 8 variables of, O 8×8 represents an 8×8 zero matrix. A 11 , A 12 , A 21 and A 22 are four 8×8 sub-block matrices inside the A matrix, B 11 , B 12 , B 21 and B 22 are four 8×2 sub-block matrices inside the B matrix, C 11 , C22 are two 8×8 sub - matrices within the C matrix. i 14ref is the line line 14 current control reference value, i 24ref is the line line 24 current control reference value. X1 and X2 can be written in the forms of Equation (3) and Equation (4).

[0081] ΔX1 = [Δi L Δi 12 Δi 14 Δi 23 Δi 24 Δi 34 Δu C1 Δu C2 T (3)

[0082] ΔX2 = [Δu C3 Δu1 Δu2 Δu3 Δξ1 Δξ2 Δξ3 Δξ4 ] T (4)

[0083] where, i L represents the common - use inductor current, i 12 represents the line line 12 current, i 14 represents the line line 14 current, i 23 represents the line line 23 current, i 24 represents the line line 24 current, i 34 represents the line line 34 current. u C1 represents the voltage across capacitor C1, u C2 represents the voltage across capacitor C2, u C3 represents the voltage across capacitor C3. u1 represents the port voltage of voltage - source converter VSC1, u2 represents the port voltage of voltage - source converter VSC2, u3 represents the port voltage of voltage - source converter VSC3. ξ1 represents the PI current - loop differential term for controlling i 14 the PI voltage - loop differential term for controlling i 14 the PI current - loop differential term for controlling i 24 the PI current - loop differential term for controlling i 24 the PI voltage - loop differential term for controlling i. The four sub - matrices within the A matrix can be expanded as in Equations (5) - (8):

[0084]

[0085] ​

[0086] Among them, D1 and D2 respectively represent the duty cycle steady-state values of switches Q A1A and Q A2A . R 12 is the line resistance of line 12 , and L 12 is the line inductance of line 12 . R 14 is the line resistance of line 14 , and L 14 is the line inductance of line 14 . R 23 is the line resistance of line 23 , and L 23 is the line inductance of line 23 . R 24 is the line resistance of line 24 , and L 24 is the line inductance of line 24 . R 34 is the line resistance of line 34 , and L 34 is the line inductance of line 34 . C s1 is the output capacitor of VSC1, C s2 is the output capacitor of VSC2, C s3 is the output capacitor of VSC3. k pC1 and k pC2 are respectively the proportional coefficient of the control line 14 current PI loop and the proportional coefficient of the control line 24 current PI loop. P1, P2, and P3 are respectively the output powers of VSC1, VSC2, and VSC3. k iC1 and k iC2 are respectively the proportional coefficient of the control line 14 current PI loop and the integral coefficient of the control line 24 current PI loop. The four block matrices in the B matrix can be expanded as shown in Eqs. (9) to (12):

[0087]

[0088]

[0089] Among them, k pV1 and k pV2 are respectively the proportional coefficient of the control u C1 voltage PI loop and the proportional coefficient of the control u C2 voltage PI loop. The block matrix C 11 and C 22It can be expanded as shown in Equation (13):

[0090]

[0091] Thus, the modal space modeling of the asymmetric current-controlled power flow controller is completed.

[0092] Next, dynamic partitioning is carried out. The state variables directly related to the PI control loop are defined as the controllable variables X c , and the variables indirectly related to the PI control loop are defined as the autonomous variables X o . Then, the state variable vectors of the controllable part and the autonomous part of the asymmetric current-controlled three-line DC power flow controller are shown in Equations (14) and (15):

[0093] ΔX c =[Δi 14 Δu C1 Δi 24 Δu C2 Δξ1 Δξ2 Δξ3 Δξ4] T (14)

[0094] ΔX o =[Δi L Δi 12 Δi 23 Δi 34 Δu C3 Δu1 Δu2 Δu3] T (15)

[0095] The state space equations of the controllable system and the autonomous system can be respectively expressed as shown in Equations (16) and (17):

[0096]

[0097] A c represents the system matrix of the controllable system, u c represents the input variable vector of the controllable system, B c represents the input matrix corresponding to the input variable vector u c in the controllable system, u oc represents the input variable vector generated by the autonomous system and input into the controllable system, B oc represents the input matrix corresponding to the input variable vector u oc ; A o represents the system matrix of the autonomous system, B co represents the input matrix corresponding to the input variable vector u co in the autonomous system, u o represents the input variable vector of the autonomous system, B oRepresents the input matrix corresponding to the input variable vector u2. The input variable vectors are shown in Equations (18) to (20) as follows:

[0098] Δu c =[Δd1 Δd2] T (18)

[0099] Δu oc =[Δi L Δi 12 Δi 23 Δi 34 Δu C3 Δu1 Δu2 Δu3] T (19)

[0100] Δu o =[Δi 14 Δu C1 Δi 24 Δu C2 T (20)

[0101] Since in the autonomous system, the six main input variables are Δd1, Δd2, Δi 14 , Δu C1 , Δi 24 and Δu C2 , these six variables can be unified into the input variable vector Δu oo of the autonomous system, as shown in Equation (21):

[0102] Δu oo =[Δd1 Δd2 Δi 14 Δu C1 Δi 24 Δu C2 T (21)

[0103] Therefore, the state-space equation of the autonomous system can be re-expressed as shown in Equations (22) to (25):

[0104]

[0105] Y o =C oo X o (23)

[0106]

[0107] where B oo represents the input matrix corresponding to the input variable vector u oo , Y o represents the output variable vector of the autonomous system, C​​oo Output matrix representing the autonomous system. U C1 、U C2 and U C3 are the steady-state values of the capacitor voltages, and I L is the steady-state value of the common inductor current.

[0108] B. Construction of the mathematical model of the transfer function of the autonomous system and transient oscillation analysis

[0109] Based on the Laplace transform, the transfer function of the autonomous system of the three-line DC power flow controller is:

[0110] G o = C oo (sI - A o ) -1 B oo (26)

[0111] where s is the Laplace operator, I is the 8×8 identity matrix, and G o is the transfer function of the autonomous system of the inter-line DC power flow controller.

[0112] For the 8×6 matrix G o , there exist a 6×6 matrix U and an 8×8 matrix V that satisfy Equation (27):

[0113] G o (s) = U(s)S(s)V T (s) (27)

[0114] where V T is the transpose of the orthogonal matrix V, S is a 6×8 diagonal matrix and also a singular value function matrix, and its diagonal elements are the singular values of the matrix G o , arranged in descending order. The column vectors of U(s) are called left singular vectors, representing the output direction; the column vectors of V(s) are called right singular vectors, representing the input direction.

[0115] Since the largest singular value can reflect the magnitude of the oscillation of the output variable caused by the input variable of the autonomous system of the asymmetric current-controlled three-line DC power flow controller. Considering the system at any oscillation mode s0, the singular situation of S(s) is equivalent to its largest singular value, that is

[0116]

[0117] where det(·) represents the determinant of the matrix, represents the largest singular value. Let the function of the largest singular value of S(s) with respect to s be the dominant transfer function of the system and satisfy Equation (29):

[0118]

[0119] The dominant transfer function can be obtained through sweep frequency analysis The corresponding amplitude change is as Figure 5 shown, and the amplitudes at different frequency points characterize the magnitude of the transient oscillation of the autonomous system, thus completing the analysis of the system's transient oscillation.

[0120] C. Optimal current distribution analysis of the power flow controller

[0121] The setting of the current reference value of the controlled HVDC line has a significant impact on the transient oscillation characteristics of the three-line DC power flow controller for asymmetric current control.

[0122] Let the current reference value of line 14 be i 14ref and the current reference value of line 24 be i 24ref The sum i 14ref +i 24ref is kept constant. Define the current distribution ratio as i 14ref / i 24ref . Gradually increase i 14 , calculate the amplitudes under different current distribution ratio conditions , and a three-dimensional graph as shown in Figure 6 can be plotted. From Figure 6 , it can be obtained that the larger the current distribution ratio, the smaller the amplitude, indicating that the transient oscillation amplitude of the overall autonomous system state variables is smaller. On the contrary, the smaller the current distribution ratio, the larger the amplitude, indicating that the transient oscillation amplitude of the overall autonomous system state variables is larger. Therefore, it can be concluded that when the total controlled current magnitude is determined, the larger the control current ratio, that is, the larger i 14ref , the smaller the system transient oscillation, which is the optimal current distribution corresponding to the determined total controlled current magnitude.

[0123] Let the current distribution ratio i 14ref / i 24ref be constantly equal to 1, gradually increase the sum of the current reference values i 14ref +i 24ref , calculate the amplitudes under different conditions of the sum of the current reference values , and a three-dimensional graph as shown in Figure 7 can be plotted. From Figure 7 , it can be obtained that the larger the sum of the current reference values i 14ref +i 24ref , the larger the amplitude, indicating that the transient oscillation amplitude of the overall autonomous system state variables is larger. On the contrary, the smaller the sum of the current reference values i 14ref +i 24ref , The smaller the amplitude, the smaller the transient oscillation amplitude of the state variables of the overall autonomous system. Therefore, it can be concluded that when the magnitude of the current distribution ratio is determined, the smaller the sum of the control current reference values, that is, i 14ref +i 24ref the smaller, the smaller the system transient oscillation, which is the optimal current distribution corresponding to the determined magnitude of the current distribution ratio.

[0124] The following further elaborates on the solution of the present invention in conjunction with the accompanying drawings and specific embodiments.

[0125] Embodiment Example 1:

[0126] In the four-terminal DC power transmission system as Figure 8 shown, verify the transient oscillation law analysis method of the double-loop PI controlled DC power flow controller based on the singular value decomposition technique. As Figure 8 shown, configure the DC power flow controller near VSC4 of the four-terminal DC power transmission system, and use the double-loop PI controller of the power flow controller to control the line 14 and line 24 line currents to achieve active regulation of the DC power flow of line 14 and line 24 and passive regulation of line 34 . Set five groups of current distribution parameters as shown in Table 2. The combinations of current distribution 1, current distribution 2, and current distribution 3 are used to verify the influence of the sum of current reference values on the system transient oscillation, and the combinations of current distribution 2, current distribution 4, and current distribution 5 are used to verify the influence of the current distribution ratio on the system transient oscillation. The initial system parameters are shown in Table 1.

[0127] Table 1: System parameters of the asymmetric current control three-line DC power flow controller

[0128]

[0129] Table 2: Experimental parameters of the contribution degree of different current distribution conditions to oscillation

[0130]

[0131]

[0132] Set the current distribution ratio i 14ref / i 24ref = 1, and conduct the experiment on the contribution degree of the sum of current reference values to oscillation. First, set the line current reference values i 14ref = i 24ref = 3A, and gradually increase the sum of current reference values i 14ref +i 24ref . From Figure 9 and Figure 10It can be seen that when the sum of the current reference values gradually increases, the main state variables i L and u3 transient oscillations gradually become larger, and the oscillation time gradually becomes longer, indicating that when the current distribution ratio is determined, the smaller the sum of the controlled current reference values, that is, i 14ref + i 24ref is smaller, the smaller the system transient oscillation. This is the optimal current distribution corresponding to the determined current distribution ratio.

[0133] Set the sum of the current reference values i 14ref + i 24ref = 7A, and conduct an experiment on the contribution degree of the current distribution ratio to the oscillation. First, set the line current distribution i 14ref / i 24ref = 0.75, and gradually increase the current distribution ratio. From Figure 11 and Figure 12 it can be seen that when the current distribution ratio gradually increases, the main state variables i L and u3 transient oscillations gradually decrease, and the oscillation time gradually becomes shorter, indicating that when the sum of the current reference values is determined, the larger the controlled current distribution ratio, the smaller the system transient oscillation. This is the optimal current distribution corresponding to the determined sum of the current reference values.

[0134] Those skilled in the art know that in addition to implementing the system and its various devices, modules, and units provided by the present invention in the form of pure computer-readable program code, the method steps can be logically programmed to enable the system and its various devices, modules, and units provided by the present invention to be implemented in the form of logic gates, switches, application-specific integrated circuits, programmable logic controllers, and embedded microcontrollers to achieve the same functions. Therefore, the system and its various devices, modules, and units provided by the present invention can be considered as a hardware component, and the devices, modules, and units included therein for implementing various functions can also be regarded as the structure within the hardware component; the devices, modules, and units for implementing various functions can also be regarded as either software modules for implementing the method or the structure within the hardware component.

[0135] In the description of the present application, it should be understood that the orientation or positional relationship indicated by the terms "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", etc. is based on the orientation or positional relationship shown in the drawings, and is only for the convenience of describing the present application and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be construed as a limitation to the present application.

[0136] The specific embodiments of the present invention have been described above. It should be understood that the present invention is not limited to the above specific embodiments, and those skilled in the art can make various changes or modifications within the scope of the claims, which do not affect the essence of the present invention. Without conflict, the embodiments of the present application and the features in the embodiments can be combined with each other arbitrarily.

Claims

1. A current distribution analysis method for a DC power flow controller based on asymmetric current control, characterized in that: include: Step S1: establishing a modal space model of an asymmetric current controlled three-line DC power flow controller, and dynamically segmenting the model to obtain an autonomous system of the model; Step S2: constructing a transfer function mathematical model of the autonomous system, and using the maximum singular value frequency sweep result to characterize the transient oscillation state of the DC power flow controller under different current reference instructions; Step S3: Obtaining the transient optimal current distribution according to the influence of the setting of the control instruction of the controlled transmission line on the transient oscillation.

2. The current distribution analysis method of a DC power flow controller based on asymmetric current control according to claim 1 is characterized in that: In step S1: The establishment of a modal space model of a three-line DC power flow controller considering asymmetric current control includes: establishing differential equations for the system dynamics of the three-line DC power flow controller, establishing a complete state space model, and completing modal space modeling; The method of dynamically segmenting the model to obtain the autonomous system of the model includes: defining the state variables directly related to the PI control loop as controllable variables Xc, and the variables indirectly related to the PI control loop as autonomous variables Xo.

3. The current distribution analysis method of a DC power flow controller based on asymmetric current control according to claim 1, characterized in that: The state space model includes: Among them, Δ indicates that the variable is a small signal disturbance, A indicates the system matrix of the asymmetric current controlled three-line DC power flow controller, B indicates the input matrix of the system, C indicates the output matrix of the system, X indicates the state variable of the system, represents the differential of the system state variable, u represents the input variable of the system, and Y represents the output variable of the system. X1 and X2 are vectors composed of the first 8 variables and the last 8 variables of X respectively. and They are The vector consisting of the first 8 variables and the last 8 variables of 8×8 A represents a zero matrix with 8 rows and 8 columns. 11 , A 12 , A 21 and A 22 There are four 8-row and 8-column block matrices inside the A matrix, and B 11 , B 12 , B 21 and B 22 There are four 8-row and 2-column block matrices inside the B matrix, C 11 , C 22 There are two 8-row and 8-column block matrices inside the C matrix. 14ref For line 14 Current control reference value, i 24ref For line 24 Current control reference value.

4. The current distribution analysis method of a DC power flow controller based on asymmetric current control according to claim 1, characterized in that: The step S2 comprises: Based on Laplace transform, the transfer function of the three-line DC power flow controller autonomous system is: G o =C oo (sI-A o ) -1 B oo Among them, s is the Laplace operator, I is the identity matrix of 8 rows and 8 columns, G o is the transfer function of the autonomous system of the line DC power flow controller, B oo Represents the input matrix corresponding to the input variable vector, C oo represents the output matrix of the autonomous system, A o A system matrix representing an autonomous system; For a matrix G of order n×m o , there exist n×n matrix U and m×m matrix V that satisfy: G o (s)=U(s)S(s)V T (s) Among them, V T is the transpose of the orthogonal matrix V, S is an m×n diagonal matrix and also a singular value function matrix, whose diagonal elements are the matrix G o The singular values ​​of are arranged in descending order; the column vector of U(s) is called the left singular vector, indicating the output direction; the column vector of V(s) is called the right singular vector, indicating the input direction; Since the maximum singular value can reflect the oscillation amplitude of the output variable caused by the input variable of the autonomous system of the asymmetric current controlled three-line DC power flow controller; considering the system at any oscillation mode s0, the singular state of S(s) is equivalent to its maximum singular value, that is: In the formula, det(·) represents the determinant of the matrix, Denotes the maximum singular value. Let the maximum singular value of S(s) be a function of s is the dominant transfer function of the system and satisfies: Obtaining the dominant transfer function through frequency sweep analysis Corresponding amplitude changes at different frequency points The amplitude of represents the size of transient oscillation of the autonomous system, completing the transient oscillation analysis of the system.

5. The current distribution analysis method of a DC power flow controller based on asymmetric current control according to claim 1, characterized in that: The step S3 includes: studying the transient oscillation characteristics of the three-line DC power flow controller with asymmetric current control under different current reference instructions, and determining the optimal current distribution; the parameters in the current reference instruction include: the current distribution ratio between the controlled DC transmission lines, and the sum of the total controlled current reference values.

6. A current distribution analysis system for a DC power flow controller based on asymmetric current control, characterized in that: include: Module M1: Establish the modal space model of the asymmetric current controlled three-line DC power flow controller, and dynamically segment the model to obtain the autonomous system of the model; Module M2: Construct a mathematical model of the transfer function of the autonomous system, and use the maximum singular value frequency sweep results to characterize the transient oscillation of the DC power flow controller under different current reference instructions; Module M3: Obtain the optimal current distribution with the best transient state according to the influence of the setting of the control command of the controlled transmission line on the transient oscillation.

7. The current distribution analysis system of a DC power flow controller based on asymmetric current control according to claim 1, characterized in that: In the module M1: The establishment of a modal space model of a three-line DC power flow controller considering asymmetric current control includes: establishing differential equations for the system dynamics of the three-line DC power flow controller, establishing a complete state space model, and completing modal space modeling; The method of dynamically segmenting the model to obtain the autonomous system of the model includes: defining the state variables directly related to the PI control loop as controllable variables Xc, and the variables indirectly related to the PI control loop as autonomous variables Xo.

8. The current distribution analysis system of a DC power flow controller based on asymmetric current control according to claim 1, characterized in that: The state space model includes: Among them, Δ indicates that the variable is a small signal disturbance, A indicates the system matrix of the asymmetric current controlled three-line DC power flow controller, B indicates the input matrix of the system, C indicates the output matrix of the system, X indicates the state variable of the system, represents the differential of the system state variable, u represents the input variable of the system, and Y represents the output variable of the system. X1 and X2 are vectors composed of the first 8 variables and the last 8 variables of X respectively. and They are The vector consisting of the first 8 variables and the last 8 variables of 8×8 A represents a zero matrix with 8 rows and 8 columns. 11 , A 12 , A 21 and A 22 There are four 8-row and 8-column block matrices inside the A matrix, and B 11 , B 12 , B 21 and B 22 There are four 8-row and 2-column block matrices inside the B matrix, C 11 , C 22 There are two 8-row and 8-column block matrices inside the C matrix. 14ref For line 14 Current control reference value, i 24ref For line 24 Current control reference value.

9. The current distribution analysis system of a DC power flow controller based on asymmetric current control according to claim 1, characterized in that: The module M2 comprises: Based on Laplace transform, the transfer function of the three-line DC power flow controller autonomous system is: G o =C oo (sI-A o ) -1 B oo Among them, s is the Laplace operator, I is the identity matrix of 8 rows and 8 columns, G o is the transfer function of the autonomous system of the line DC power flow controller, B oo Represents the input matrix corresponding to the input variable vector, C oo represents the output matrix of the autonomous system, A o A system matrix representing an autonomous system; For a matrix G of order n×m o , there exist n×n matrix U and m×m matrix V that satisfy: G o (s)=U(s)S(s)V T (s) Among them, V T is the transpose of the orthogonal matrix V, S is an m×n diagonal matrix and also a singular value function matrix, whose diagonal elements are the matrix G o The singular values ​​of are arranged in descending order; the column vector of U(s) is called the left singular vector, indicating the output direction; the column vector of V(s) is called the right singular vector, indicating the input direction; Since the maximum singular value can reflect the oscillation amplitude of the output variable caused by the input variable of the autonomous system of the asymmetric current controlled three-line DC power flow controller; considering the system at any oscillation mode s0, the singular state of S(s) is equivalent to its maximum singular value, that is: In the formula, det(·) represents the determinant of the matrix, Denotes the maximum singular value. Let the maximum singular value of S(s) be a function of s is the dominant transfer function of the system and satisfies: Obtaining the dominant transfer function through frequency sweep analysis Corresponding amplitude changes at different frequency points The amplitude of represents the size of transient oscillation of the autonomous system, completing the transient oscillation analysis of the system.

10. The current distribution analysis system of a DC power flow controller based on asymmetric current control according to claim 1, characterized in that: The module M3 includes: studying the transient oscillation characteristics of the three-line DC power flow controller with asymmetric current control under different current reference instructions, and determining the optimal current distribution; the parameters in the current reference instruction include: the current distribution ratio between the controlled DC transmission lines, and the sum of the total controlled current reference values.

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