Hybrid DC power transmission system nonlinear mathematical model establishment method considering discontinuous characteristics and related device

Abstract

The invention provides a method for establishing a nonlinear mathematical model of a hybrid direct-current power transmission system considering discontinuous characteristics and a related device. The method comprises the following steps: representing a dynamic process of discontinuous links in a main loop and a control loop of the hybrid direct-current power transmission system by utilizing a step function; performing continuous approximation on the step function by using a linear combination of hyperbolic tangent functions to obtain a function expression of continuous expression; modular nonlinear differential equations of a rectification side network, an inversion side network and an AC / DC side network are obtained, and discontinuous dynamic parts involved in the equations are correspondingly replaced by adopting a continuously expressed function expression; all the modules are interconnected according to the input and output relation, and a complete hybrid direct current power transmission system nonlinear mathematical model is obtained. According to the method, a step function and hyperbolic tangent function continuous approximation method is adopted, a nonlinear mathematical model of the hybrid direct current power transmission system capable of accurately reflecting discontinuous dynamic characteristics is established, and therefore the accuracy and practicability of system modeling are improved.

Description

Technical Field

[0001] The invention belongs to the technical field of power system modeling, and in particular relates to a method for establishing a nonlinear mathematical model of a hybrid direct current transmission system taking discontinuous characteristics into consideration and a related device. Background Art

[0002] In order to solve the energy consumption problem caused by the reverse distribution of primary energy bases and actual load demand and realize large-capacity and long-distance power transmission, high-voltage direct current transmission technology has been fully developed. At present, high-voltage direct current transmission mainly adopts two types: line commutated converter (LCC) and voltage source converter (VSC). LCC uses semi-controlled semiconductor devices as the core component of the converter, and there is a problem of commutation failure, but because of its mature technology, relatively low cost and large transmission capacity, it will still be widely used in the future. VSC has gone through the development process from two-level and three-level converters to modular multilevel converters (MMC). MMC uses fully controlled semiconductor devices and has great advantages in controllability, adaptability and harmonic level. The hybrid direct current transmission system takes into account the advantages of both, and has great advantages and broad development potential and application prospects.

[0003] However, with the continuous deepening of the power electronics process, various instability problems of DC transmission systems have become increasingly prominent. In existing high-voltage DC projects, many unstable oscillation events have occurred. The analysis and treatment of the above problems rely on the establishment of accurate mathematical models.

[0004] For the modeling problem of hybrid DC transmission system, the existing technical solutions are usually carried out under the continuous nonlinear time-invariant framework, characterizing the dynamic behavior of LCC and MMC in an average manner. However, such methods still have the following problems:

[0005] First, the inherent discontinuous switching characteristics of power electronic devices such as thyristors in LCC and submodules in MMC cannot be taken into account.

[0006] Second, it is impossible to characterize the impact of discrete processes such as limiting and multi-controller switching in actual control systems on the system dynamic process.

[0007] Third, it is unable to reveal the typical periodic time-varying behavior of the hybrid DC transmission system. Summary of the invention

[0008] In view of this, the present invention provides a method and related devices for establishing a nonlinear mathematical model of a hybrid DC power transmission system taking into account discontinuous characteristics, so as to solve the above-mentioned problems existing in the existing modeling methods of hybrid DC power transmission systems.

[0009] In order to achieve the above object, the technical solution provided by the present invention is as follows:

[0010] In a first aspect, the present invention provides a method for establishing a nonlinear mathematical model of a hybrid DC power transmission system considering discontinuous characteristics, comprising the following steps:

[0011] The step function is used to represent the dynamic process of discontinuous links in the main loop and control loop of the hybrid DC transmission system;

[0012] The step function is approximated continuously using the linear combination of hyperbolic tangent functions to obtain a continuous expression of the function.

[0013] Obtain the modular nonlinear differential equations of the rectifier side, inverter side and AC / DC side networks, and replace the discontinuous dynamic parts involved in the modular nonlinear differential equations with continuous function expressions;

[0014] The modules are interconnected according to the input-output relationship to obtain a complete nonlinear mathematical model of the hybrid DC transmission system considering discontinuous characteristics.

[0015] Furthermore, the discontinuous link in the main circuit is a thyristor, and a step function is used to represent the dynamic process of the discontinuous link in the main circuit of the hybrid DC transmission system, including:

[0016] Using S th Characterizing the switching function of the thyristor, we have:

[0017]

[0018] In the formula, S f and S i They respectively indicate whether the trigger signal arrives and whether the forward current is less than the holding current, and are expressed as:

[0019]

[0020] In the formula, is the trigger angle, φ is the synchronous phase, i is the forward current, I hold is the maintaining current; and the step function H(x) is expressed as:

[0021] .

[0022] Furthermore, the discontinuous links in the control loop are the limiting link and the switching link. The step function is used to represent the dynamic process of the discontinuous links in the control loop of the hybrid DC transmission system, including:

[0023] The following formulas are used to characterize the discontinuous characteristics of the limiting link and the switching link, respectively:

[0024]

[0025]

[0026] In the formula, α in / min / max They represent the trigger angle input value / output lower limit value / output upper limit value of the constant current limiting link respectively; u dc / min / max Respectively represent the DC voltage / DC voltage lower limit / DC voltage upper limit of the low voltage current limiting unit input; I ord is the output current command value; They represent the lower limit / upper limit of the output current command respectively; and the step function H(x) is expressed as:

[0027] .

[0028] Furthermore, the step function is continuously approximated using a linear combination of hyperbolic tangent functions, as follows:

[0029]

[0030] Where h(x) is the linear combination of the hyperbolic tangent function, tanh is the hyperbolic tangent function, and k is an adjustable coefficient.

[0031] Furthermore, for the main circuit module on the rectifier side, the discontinuous dynamic part involved in the modular nonlinear differential equation is replaced by a continuous function expression, including:

[0032] The mathematical model of the differential equation between the port voltage / current of the main circuit module on the rectifier side is determined according to the thyristor switch characteristics, as follows:

[0033]

[0034] Where, L t is the transformer inductance, i a / b / c is the A / B / C phase current, , , and are the switching functions of the upper bridge arm and the lower bridge arm of phase j respectively;

[0035] The bridge arm switching function that has been continuously approximated by the hyperbolic tangent function is used to replace the original bridge arm switching function. and , and obtain a new differential equation mathematical model of the main circuit module on the rectifier side.

[0036] Furthermore, for the constant current control loop on the rectifier side, the discontinuous dynamic part involved in the modular nonlinear differential equation is replaced by a continuous function expression, including:

[0037] Determine the differential equation mathematical model of the control loop as follows:

[0038]

[0039] In the formula, x 1,dc and x 2,dc is the state variable, T m and G m is the time constant and proportional coefficient of the first-order inertial filter link, i dc is the DC current on the rectifier side, k i is the integral coefficient in the proportional-integral link, I ord is the current command value;

[0040] The current command value that has been continuously approximated by the hyperbolic tangent function is used to replace the original current command value I ord ;

[0041] The trigger angle α is determined based on the current command value of the continuous approximation process and continuously approximated, as follows:

[0042]

[0043] In the formula, α in / min / max They represent the trigger angle input value / output lower limit value / output upper limit value of the constant current limiting link respectively, h(x) is the linear combination of the hyperbolic tangent function, k p is the proportional coefficient in the proportional-integral link.

[0044] Furthermore, the expression of the nonlinear mathematical model of the complete hybrid DC transmission system is as follows:

[0045]

[0046] In the formula, x is the system state variable, that is, the differential in the modular nonlinear differential equation, and N is the total number of system state variables; is a nonlinear vector field.

[0047] In a second aspect, the present invention provides a device for establishing a nonlinear mathematical model of a hybrid DC power transmission system taking into account discontinuous characteristics, comprising:

[0048] The first processing module is used to represent the dynamic process of discontinuous links in the main loop and the control loop of the hybrid DC power transmission system by using a step function;

[0049] The second processing module is used to perform continuous approximation on the step function by using a linear combination of hyperbolic tangent functions to obtain a continuous expression of the function;

[0050] The third processing module is used to obtain the modular nonlinear differential equations of the rectifier side, the inverter side and the AC and DC side networks, and replace the discontinuous dynamic parts involved in the modular nonlinear differential equations with continuous function expressions;

[0051] The model building module is used to interconnect the modules according to the input-output relationship to obtain a complete nonlinear mathematical model of the hybrid DC transmission system taking into account the discontinuous characteristics.

[0052] In a third aspect, the present invention provides a computer device, the device comprising a processor and a memory:

[0053] The memory is used to store the computer program and send the instructions of the computer program to the processor;

[0054] The processor executes the method for establishing a nonlinear mathematical model of a hybrid direct current power transmission system taking into account discontinuous characteristics as in the first aspect according to the instructions of the computer program.

[0055] In a fourth aspect, the present invention provides a computer-readable storage medium having a computer program stored thereon, which, when executed by a processor, implements a method for establishing a nonlinear mathematical model of a hybrid DC transmission system taking into account discontinuous characteristics as in the first aspect.

[0056] In summary, the present invention provides a method and related device for establishing a nonlinear mathematical model of a hybrid DC transmission system considering discontinuous characteristics, including using a step function to represent the dynamic process of discontinuous links in the main loop and control loop of the hybrid DC transmission system; using a linear combination of hyperbolic tangent functions to continuously approximate the step function to obtain a continuously expressed function expression; obtaining modular nonlinear differential equations of the rectifier side, inverter side and AC / DC side networks, and replacing the discontinuous dynamic parts involved in the modular nonlinear differential equations with continuously expressed function expressions; interconnecting each module according to the input-output relationship to obtain a complete nonlinear mathematical model of the hybrid DC transmission system considering discontinuous characteristics. The present invention establishes a nonlinear mathematical model of a hybrid DC transmission system that can accurately reflect discontinuous dynamic characteristics by using a method of continuous approximation of step functions and hyperbolic tangent functions, thereby improving the accuracy and practicality of system modeling. BRIEF DESCRIPTION OF THE DRAWINGS

[0057] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0058] Figure 1 A flowchart of a method for establishing a nonlinear mathematical model of a hybrid DC power transmission system taking into account discontinuous characteristics provided by an embodiment of the present invention;

[0059] Figure 2 A topological structure diagram of a hybrid direct current transmission system provided by an embodiment of the present invention;

[0060] Figure 3 A comparison diagram of the rectifier-side AC current, DC current, and DC voltage during a process in which the DC current command value of the constant current controller provided by an embodiment of the present invention drops suddenly from 1pu to 0.5pu;

[0061] Figure 4 A comparison diagram of the inverter side AC current, DC current and DC voltage during a process in which the DC current command value of the constant current controller provided by an embodiment of the present invention drops suddenly from 1pu to 0.5pu;

[0062] Figure 5 A block diagram of a device for establishing a nonlinear mathematical model of a hybrid DC power transmission system taking into account discontinuous characteristics provided by an embodiment of the present invention;

[0063] Figure 6 A block diagram of a computer device provided in an embodiment of the present invention. DETAILED DESCRIPTION

[0064] In order to make the purpose, features and advantages of the present invention more obvious and easy to understand, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the embodiments described below are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0065] See also Figure 1 This embodiment provides a method for establishing a nonlinear mathematical model of a hybrid DC power transmission system considering discontinuous characteristics, comprising the following steps:

[0066] S1: Use step functions to represent the dynamic process of discontinuous links in the main loop and control loop of the hybrid DC transmission system.

[0067] It should be noted that a step function is a discontinuous function, usually used to indicate a sudden change in a signal or system state. It jumps from one value to another at a certain point, and remains constant before and after this point.

[0068] Some components in hybrid DC transmission systems, such as thyristor switches, submodule input / removal, etc., have obvious switching characteristics. Step functions can simulate such instantaneous changes very well. For example, a step function of 1 can be used to represent the conduction of a thyristor, and vice versa, a step function of 0 can be used to represent the turn-off of a thyristor. By using step functions, these discontinuous processes can be transformed into part of the mathematical model.

[0069] S2: Use the linear combination of hyperbolic tangent functions to make a continuous approximation of the step function and obtain a continuous expression of the function.

[0070] It should be noted that the hyperbolic tangent function (tanh(x)) is a smooth transition function, and its graph presents an S-shaped curve. The linear combination of the hyperbolic tangent function 0.5·[1+tanh(k·x)] approaches 0 and 1 in the extreme case, but is continuous and smooth in the middle.

[0071] Although the step function can accurately describe discontinuities, it is not conducive to simulation, numerical solution, and mechanism analysis of system dynamic behavior. To overcome this problem, the hyperbolic tangent function can be used to approximate the step function. Specifically, by adjusting the parameter k in the linear combination of the hyperbolic tangent function, its shape can be close to the step function while retaining good mathematical properties, such as differentiability and continuity. The function obtained in this way can not only numerically approximate the behavior of the step function, but also ensure stability and accuracy during the calculation process.

[0072] S3: Obtain the modular nonlinear differential equations of the rectifier side, inverter side and AC / DC side networks, and replace the discontinuous dynamic parts involved in the modular nonlinear differential equations with continuous function expressions.

[0073] It should be noted that modular nonlinear differential equations refer to decomposing the entire system into several subsystems (modules), each of which has its own nonlinear differential equations to describe its dynamic behavior. Discontinuous dynamics refers to non-smooth changes in the system, such as transient changes in the vector field caused by switching actions.

[0074] For different parts of the hybrid DC transmission system, such as the rectifier side, the inverter side, and the AC / DC network, their nonlinear differential equations are established respectively. Then, in these equations, all the places involving discontinuous dynamics represented by step functions are replaced by the continuous hyperbolic tangent function expression obtained in step S2. This not only ensures that the dynamic behavior within each subsystem is accurately described, but also makes the overall system model more mathematically consistent and easy to handle.

[0075] S4: Interconnect each module according to the input-output relationship to obtain a complete nonlinear mathematical model of the hybrid DC transmission system considering discontinuous characteristics.

[0076] It should be noted that based on the existing modular nonlinear differential equations, after all discontinuous parts have been processed continuously, the next step is to define the input-output relationship between the modules according to the actual physical connection. This step involves determining which variables are shared and how to correctly pass the output of one module to the next module as input. The final result is a complete system-level nonlinear mathematical model that can reflect the actual working conditions of the hybrid DC transmission system, including all its discontinuous characteristics.

[0077] This embodiment provides a method for establishing a nonlinear mathematical model of a hybrid DC transmission system taking into account discontinuous characteristics. By adopting a method of continuous approximation of a step function and a hyperbolic tangent function, a nonlinear mathematical model of a hybrid DC transmission system that can accurately reflect discontinuous dynamic characteristics is established, thereby improving the accuracy and practicability of system modeling.

[0078] In one embodiment, the discontinuous link in the main circuit is a thyristor, and two situations need to be considered: turning on due to the arrival of a trigger signal and turning off due to the forward current being less than the holding current. th Characterizing the switching function of the thyristor, we have:

[0079] (1)

[0080] In the formula, S f and S i They respectively indicate whether the trigger signal arrives and whether the forward current is less than the holding current, and are expressed as:

[0081] (2)

[0082] In the formula, is the trigger angle, φ is the synchronous phase, i is the forward current, I hold is the maintaining current; and the step function H(x) is expressed as:

[0083] .

[0084] In one embodiment, the discontinuous links in the control loop are limiting and switching. For the limiting link (taking the output limiting of the constant current controller as an example) and the switching link (taking the low-voltage current limiting unit as an example) in the control loop, the following equations (3) and (4) are used to characterize their discontinuous characteristics respectively:

[0085] (3)

[0086] (4)

[0087] In the formula, α in / min / max They represent the trigger angle input value / output lower limit value / output upper limit value of the constant current limiting link respectively; u dc / min / max Respectively represent the DC voltage / DC voltage lower limit / DC voltage upper limit of the low voltage current limiting unit input; I ord is the output current command value; Respectively represent the lower limit / upper limit value of the output current command.

[0088] In one embodiment, the step function is continuously approximated using a linear combination of hyperbolic tangent functions, and the approximation accuracy is adjusted by the approximation coefficient k. Define the adjustable coefficient k, and note that when x≠0, , so equation (5) can be used to approximate equations (2) to (4) and ensure the continuity of mathematical description:

[0089] (5)

[0090] Substituting the relevant parts of equations (2) to (4) as the independent variables of the hyperbolic tangent function into equation (5) yields the corresponding continuous expression of the function.

[0091] In one embodiment, for the main circuit module on the rectifier side, the following port voltage / current differential equation mathematical model can be written according to the thyristor switch characteristics:

[0092] (6)

[0093] in , . is the j-phase upper (lower) bridge arm switching function (i.e., S in the above embodiment) th , and processed using the continuous approximation method in step S2). For example, for the upper bridge arm of phase A , whose expression is:

[0094] (7)

[0095] For the inverter side main circuit module, the following port voltage / current differential equation mathematical model can be written based on the averaged sub-module dynamic process:

[0096] (8)

[0097] in Represents the sum of the capacitance voltages of the upper (lower) bridge arm submodules of phase j; equivalent capacitance C eq = The submodule capacitance divided by the number of bridge arm submodules; the equivalent inductance L eq =L t +0.5L arm , L t is the transformer inductance, L arm is the bridge arm inductance; u nN Indicates the voltage difference between the neutral points of AC and DC; i j represents the j-phase current at the common coupling point, represents the circulating current in the j-phase bridge arm; Represents the bridge arm differential mode voltage; is the j-phase upper (lower) bridge arm modulation signal.

[0098] In one embodiment, for the control loop module, the rectifier side constant current control loop is taken as an example. Since the control loop includes a first-order inertia filter link and a proportional-integral link, there are:

[0099] (9)

[0100] where x 1,dc and x 2,dc is the state variable; T m and G m is the time constant and proportional coefficient of the first-order inertial filter; i dc is the DC current on the rectifier side; k i I is the integral coefficient in the proportional-integral link; ord is the current command value (i.e., I ord , and processed using the continuous approximation method in step S2). Then the trigger angle a output by the control system after processing in step S2 is written as:

[0101] (10)

[0102] where k p is the proportional coefficient in the proportional-integral link. The main circuit module of the AC and DC side network is composed of inductance, capacitance and resistance, and the differential equation can be directly written.

[0103] In one embodiment, the modules are combined to finally obtain a nonlinear mathematical model of a hybrid DC transmission system taking into account discontinuous characteristics, which can be written as follows:

[0104] (11)

[0105] Where x is the system state variable, i.e., the differential on the left side of the equation in equations (6)-(9), and N is the total number of system state variables; is a nonlinear vector field. For example, in equation (6) a As the first state variable x1 in equation (11), f1(x) is written as:

[0106] (12)

[0107] All variables appearing on the right side of equation (12) are related to other state variables (such as equation (9) in the control system module).

[0108] Through the above steps, a nonlinear mathematical model of the hybrid DC transmission system with extremely high precision can be effectively established to accurately characterize the system's periodic time variation and switching, limiting, and switching processes, laying a solid foundation for stability and bifurcation analysis.

[0109] The above data and process can be used for simulation verification:

[0110] In PSCAD / EMTDC, Figure 2 The simplified simulation model of an actual hybrid DC transmission system shown in the figure has a rated power of 1250MW. The LCC on the rectifier side adopts constant current control and takes into account the role of the low-voltage current limiting unit; the MMC on the inverter side adopts constant voltage / constant reactive power outer loop and inner loop vector current control, and is equipped with circulating current suppression control.

[0111] Take the case where the DC current command value of the constant current controller on the rectifier side drops suddenly from 1pu to 0.5pu in 2 seconds. Figure 3 and Figure 4 The PSCAD / EMTDC simulation results and the solution results of the mathematical model proposed in this patent in MATLAB are given respectively. Figure 3 Give the waveforms of AC current, DC current and DC voltage on the rectifier side; Figure 4 The waveforms of AC current, DC current and DC voltage on the inverter side are given. It can be seen that the solution results of the nonlinear mathematical model of the hybrid DC transmission system proposed in this patent are highly consistent with the simulation results.

[0112] Based on the same inventive concept, the embodiment of the present application also provides a device for establishing a nonlinear mathematical model of a hybrid DC transmission system considering discontinuous characteristics for implementing the method for establishing a nonlinear mathematical model of a hybrid DC transmission system considering discontinuous characteristics involved above. The implementation scheme for solving the problem provided by the system is similar to the implementation scheme recorded in the above method, so the specific limitations in the embodiment of the device for establishing a nonlinear mathematical model of a hybrid DC transmission system considering discontinuous characteristics provided below can refer to the limitations of the method for establishing a nonlinear mathematical model of a hybrid DC transmission system considering discontinuous characteristics above, and will not be repeated here.

[0113] See also Figure 5 The embodiment of the present invention further provides a device for establishing a nonlinear mathematical model of a hybrid DC power transmission system taking into account discontinuous characteristics, comprising:

[0114] A first processing module is used to represent the dynamic process of discontinuous links in the main loop and the control loop of the hybrid DC power transmission system using a step function;

[0115] The second processing module is used to perform continuous approximation on the step function by using a linear combination of hyperbolic tangent functions to obtain a continuous expression of the function;

[0116] The third processing module is used to obtain the modular nonlinear differential equations of the rectifier side, the inverter side and the AC and DC side networks, and replace the discontinuous dynamic parts involved in the modular nonlinear differential equations with continuous function expressions;

[0117] The model building module is used to interconnect the modules according to the input-output relationship to obtain a complete nonlinear mathematical model of the hybrid DC transmission system taking into account the discontinuous characteristics.

[0118] Those skilled in the art can clearly understand that, for the convenience and simplicity of description, only the division of the above-mentioned functional units and modules is used as an example for illustration. In practical applications, the above-mentioned function allocation can be completed by different functional units and modules as needed, that is, the internal structure of the system can be divided into different functional units or modules to complete all or part of the functions described above. The functional units and modules in the embodiment can be integrated in a processing unit, or each unit can exist physically separately, or two or more units can be integrated in one unit. The above-mentioned integrated unit can be implemented in the form of hardware or in the form of software functional units. In addition, the specific names of the functional units and modules are only for the convenience of distinguishing each other, and are not used to limit the scope of protection of this application. The specific working process of the units and modules in the above-mentioned system can refer to the corresponding process in the aforementioned method embodiment, which will not be repeated here.

[0119] Reference Figure 6 An embodiment of the present invention further provides a computer device, comprising: a memory and a processor and a computer program stored in the memory. When the computer program is executed on the processor, the method for establishing a nonlinear mathematical model of a hybrid DC transmission system considering discontinuous characteristics as described in any one of the above methods is implemented.

[0120] The computer device may be a desktop computer, a notebook, a PDA, a cloud server or other computing device. The computer device may include, but is not limited to, a processor and a memory. Those skilled in the art will understand that Figure 6 It is only an example of a computer device and does not constitute a limitation of the computer device. It may include more or fewer components than shown in the figure, or a combination of certain components, or different components. For example, it may also include input and output devices, network access devices, etc.

[0121] The processor may be a central processing unit (CPU), other general-purpose processors, digital signal processors (DSP), application-specific integrated circuits (ASIC), field-programmable gate arrays (FPGA) or other programmable logic devices, discrete gate or transistor logic devices, discrete hardware components, etc. A general-purpose processor may be a microprocessor or any conventional processor, etc.

[0122] In some embodiments, the memory may be an internal storage unit of the computer device, such as a hard disk or memory of the computer device. In other embodiments, the memory may also be an external storage device of the computer device, such as a plug-in hard disk, a smart media card (SMC), a secure digital (SD) card, a flash card, etc. equipped on the computer device. Further, the memory may include both an internal storage unit and an external storage device of the computer device. The memory is used to store an operating system, an application program, a boot loader (BootLoader), data, and other programs, such as the program code of the computer program. The memory may also be used to temporarily store data that has been output or is to be output.

[0123] An embodiment of the present invention further provides a computer-readable storage medium having a computer program stored thereon. When the computer program is executed by a processor, the method for establishing a nonlinear mathematical model of a hybrid direct current transmission system taking into account discontinuous characteristics as described in any one of the above methods is implemented.

[0124] In this embodiment, if the integrated unit is implemented in the form of a software functional unit and sold or used as an independent product, it can be stored in a computer-readable storage medium. Based on this understanding, the present application implements all or part of the processes in the above-mentioned embodiment method, which can be completed by instructing the relevant hardware through a computer program. The computer program can be stored in a computer-readable storage medium. When the computer program is executed by the processor, the steps of the above-mentioned various method embodiments can be implemented. Among them, the computer program includes computer program code, and the computer program code can be in source code form, object code form, executable file or some intermediate form. The computer-readable medium can at least include: any entity or device that can carry the computer program code to the camera / terminal device, recording medium, computer memory, read-only memory (ROM, Read-Only Memory), random access memory (RAM, Random Access Memory), electric carrier signal, telecommunication signal and software distribution medium. For example, USB flash drive, mobile hard disk, disk or optical disk. In some jurisdictions, according to legislation and patent practice, computer-readable media cannot be electric carrier signals and telecommunication signals.

[0125] In the above embodiments, the description of each embodiment has its own emphasis. For parts that are not described or recorded in detail in a certain embodiment, reference can be made to the relevant descriptions of other embodiments.

[0126] Those of ordinary skill in the art will appreciate that the units and algorithm steps of each example described in conjunction with the embodiments disclosed herein can be implemented in electronic hardware, or a combination of computer software and electronic hardware. Whether these functions are performed in hardware or software depends on the specific application and design constraints of the technical solution. Professional and technical personnel can use different methods to implement the described functions for each specific application, but such implementation should not be considered to be beyond the scope of this application.

[0127] In the embodiments disclosed in the present application, it should be understood that the disclosed devices / terminal equipment and methods can be implemented in other ways. For example, the device / terminal equipment embodiments described above are only schematic. For example, the division of the modules or units is only a logical function division. There may be other division methods in actual implementation, such as multiple units or components can be combined or integrated into another system, or some features can be ignored or not executed. Another point is that the mutual coupling or direct coupling or communication connection shown or discussed can be through some interfaces, indirect coupling or communication connection of devices or units, which can be electrical, mechanical or other forms.

[0128] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit the same. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that the technical solutions described in the aforementioned embodiments may still be modified, or some of the technical features may be replaced by equivalents. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for establishing a nonlinear mathematical model of a hybrid DC transmission system considering discontinuous characteristics, characterized in that: The steps include: The step function is used to represent the dynamic process of discontinuous links in the main loop and control loop of the hybrid DC transmission system; The step function is continuously approximated by using a linear combination of hyperbolic tangent functions to obtain a continuous function expression; Obtaining modular nonlinear differential equations of the rectifier side, inverter side and AC / DC side networks, and replacing the discontinuous dynamic parts involved in the modular nonlinear differential equations with the function expressions expressed in a continuous manner; The modules are interconnected according to the input-output relationship to obtain a complete nonlinear mathematical model of the hybrid DC transmission system considering discontinuous characteristics.

2. The method for establishing a nonlinear mathematical model of a hybrid DC power transmission system considering discontinuous characteristics according to claim 1, characterized in that: The discontinuous link in the main circuit is the thyristor. The step function is used to represent the dynamic process of the discontinuous link in the main circuit of the hybrid DC transmission system, including: Using S th Characterizing the switching function of the thyristor, we have: In the formula, S f and S i They respectively indicate whether the trigger signal arrives and whether the forward current is less than the holding current, and are expressed as: In the formula, is the trigger angle, φ is the synchronous phase, i is the forward current, I hold is the maintaining current; and the step function H(x) is expressed as: 。 3. The method for establishing a nonlinear mathematical model of a hybrid DC power transmission system considering discontinuous characteristics according to claim 1, characterized in that: The discontinuous links in the control loop are the limiting link and the switching link. The step function is used to represent the dynamic process of the discontinuous links in the control loop of the hybrid DC transmission system, including: The discontinuous characteristics of the limiting link and the switching link are characterized by the following formulas respectively: In the formula, α in / min / max They represent the trigger angle input value / output lower limit value / output upper limit value of the constant current limiting link respectively; u dc / min / max Respectively represent the DC voltage / DC voltage lower limit / DC voltage upper limit of the low voltage current limiting unit input; I ord is the output current command value; They represent the lower limit / upper limit of the output current command respectively; and the step function H(x) is expressed as: 。 4. The method for establishing a nonlinear mathematical model of a hybrid DC power transmission system considering discontinuous characteristics according to claim 2 or 3, characterized in that: The step function is continuously approximated using a linear combination of hyperbolic tangent functions, as follows: Where h(x) is the linear combination of the hyperbolic tangent function, tanh is the hyperbolic tangent function, and k is an adjustable coefficient.

5. The method for establishing a nonlinear mathematical model of a hybrid DC power transmission system considering discontinuous characteristics according to claim 1, characterized in that: For the main circuit module on the rectifier side, the discontinuous dynamic part involved in the modular nonlinear differential equation is replaced by the function expression expressed in a continuous manner, including: The mathematical model of the differential equation between the port voltage / current of the main circuit module on the rectifier side is determined according to the thyristor switch characteristics, as follows: Where, L t is the transformer inductance, i a / b / c is the A / B / C phase current, , , and are the switching functions of the upper bridge arm and the lower bridge arm of phase j respectively; The bridge arm switching function which has been continuously approximated by the hyperbolic tangent function is used to replace the original bridge arm switching function. and , and obtain a new differential equation mathematical model of the rectifier side main loop module.

6. The method for establishing a nonlinear mathematical model of a hybrid DC power transmission system considering discontinuous characteristics according to claim 1, characterized in that: For the rectifier-side constant current control loop, the discontinuous dynamic part involved in the modular nonlinear differential equation is replaced by the function expression expressed in a continuous manner, including: Determine the differential equation mathematical model of the control loop as follows: In the formula, x 1,dc and x 2,dc is the state variable, T m and G m is the time constant and proportional coefficient of the first-order inertial filter link, i dc is the DC current on the rectifier side, k i is the integral coefficient in the proportional-integral link, I ord is the current command value; The current command value that has been continuously approximated by the linear combination of the hyperbolic tangent function is substituted for the original current command value I ord ; The trigger angle α is determined based on the current command value of the continuous approximation process and continuously approximated, as follows: In the formula, α in / min / max They represent the trigger angle input value / output lower limit value / output upper limit value of the constant current limiting link respectively, h(x) is the linear combination of the hyperbolic tangent function, k p is the proportional coefficient in the proportional-integral link.

7. The method for establishing a nonlinear mathematical model of a hybrid DC power transmission system considering discontinuous characteristics according to claim 1, characterized in that: The expression of the nonlinear mathematical model of the complete hybrid DC transmission system is as follows: Wherein, x is the system state variable, i.e., the differential in the modular nonlinear differential equation, and N is the total number of system state variables; is a nonlinear vector field.

8. A device for establishing a nonlinear mathematical model of a hybrid DC transmission system considering discontinuous characteristics, characterized in that: include: A first processing module is used to represent the dynamic process of discontinuous links in the main loop and the control loop of the hybrid DC power transmission system using a step function; A second processing module is used to perform continuous approximation on the step function using a linear combination of hyperbolic tangent functions to obtain a continuous expression of the function; A third processing module is used to obtain modular nonlinear differential equations of the rectifier side, the inverter side and the AC / DC side networks, and replace the discontinuous dynamic part involved in the modular nonlinear differential equation with the function expression expressed in a continuous manner; The model building module is used to interconnect the modules according to the input-output relationship to obtain a complete nonlinear mathematical model of the hybrid DC transmission system taking into account the discontinuous characteristics.

9. A computer device, characterized in that: The device comprises a processor and a memory: The memory is used to store a computer program and send instructions of the computer program to the processor; The processor executes the method for establishing a nonlinear mathematical model of a hybrid direct current transmission system considering discontinuous characteristics according to any one of claims 1 to 7 according to the instructions of the computer program.

10. A computer-readable storage medium, characterized in that: The computer-readable storage medium stores a computer program, and when the computer program is executed by the processor, the method for establishing a nonlinear mathematical model of a hybrid direct current transmission system taking into account discontinuous characteristics according to any one of claims 1 to 7 is implemented.

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