Direct current micro-grid attraction domain estimation method based on mixed potential function theory

Abstract

The invention provides a direct current micro-grid attraction domain estimation method based on a mixed potential function theory. The invention firstly provides a processing method for controlling dynamic states (namely a droop ring and a voltage ring), the droop ring and the voltage ring of a converter are converted into a first circuit, an RLC network is formed according to a topological structure of a direct-current micro-grid, and attraction domain estimation is carried out by applying a mixed potential function theory based on the RLC network. The invention also provides a stability analysis method of the DC micro-grid composed of the Boost converter in the case of constant power load sudden change on the DC bus. The method provides a theoretical basis for system planning and design, operation control and fault location and isolation.

Description

Technical Field

[0001] The invention belongs to the field of electrical engineering, and in particular relates to a method for estimating a DC microgrid attraction domain based on a mixed potential function theory. Background Art

[0002] DC microgrids, with their high efficiency, flexibility and reliability, have shown great potential in distributed generation and renewable energy access. However, due to the lack of inertial support such as synchronous generators in traditional power systems, high-proportion power electronic equipment makes DC microgrids exhibit low inertia and weak damping, and the stability of their operation has gradually become a research hotspot.

[0003] The existing stability analysis research on DC microgrids can be divided into two categories: small signal (small disturbance) and large signal (large disturbance) analysis. Small signal stability analysis mainly studies small disturbances near the equilibrium point, such as small fluctuations in generator output power, parameter changes caused by charging and discharging of energy storage systems, and line parameter disturbances, to evaluate the impact of small disturbances on the system. Common small signal analysis methods are mainly eigenvalue trajectory method and impedance analysis method. Small signal stability can only ensure the asymptotic stability of the system near the equilibrium point. When the operating state of the system deviates far from the equilibrium point, such as constant power load mutation and power generation unit removal, the small signal model obtained by local linearization is often invalid, so it is necessary to establish a large signal model for the system. Large signal stability analysis is usually based on the Lyapunov direct method, using the lower level set of the Lyapunov function to characterize the attraction domain of the system, but it can only obtain sufficient conditions for system stability, that is, the attraction domain estimation is conservative. The more appropriate the Lyapunov function is, the less conservative the attraction domain estimation is, and the closer the estimated attraction domain is to the real attraction domain. Therefore, constructing a suitable Lyapunov function is the key to analyzing the large signal stability problem of the system. The hybrid potential function theory proposed by Brayton and Moser provides a method for constructing Lyapunov functions and the corresponding stability theorem, and has received widespread attention in the stability analysis of systems containing constant power loads. However, there are still many shortcomings in existing research: in terms of applicability, most existing studies only derive stability criteria for specific topologies, or only consider DC microgrid systems composed of specific converters, which are difficult to generalize to other systems; in terms of analysis results, most studies only consider circuit dynamics and ignore control dynamics, and provide analytical stability criteria but can only guarantee the asymptotic stability of the equilibrium point, and fail to provide an estimate of the attraction domain. Summary of the invention

[0004] The purpose of the present invention is to provide a method for estimating the attraction domain of a DC microgrid based on a mixed potential function theory in view of the defects existing in the prior art.

[0005] One of the purposes of the present invention is to provide a method for processing control dynamics (i.e., droop loop and voltage loop), converting the droop loop and voltage loop of the converter into a first circuit and forming an RLC network based on the topological structure of the DC microgrid, and then applying the mixed potential function theory based on the RLC network to estimate the attraction domain. The second purpose is to provide a stability analysis method for a DC microgrid composed of converters when facing a constant power load mutation on the DC bus. The present invention provides a theoretical basis for system planning and design, operation control, and fault location and isolation.

[0006] The technical solution adopted by the present invention is specifically as follows:

[0007] A method for estimating the attraction domain of a DC microgrid based on a hybrid potential function theory comprises:

[0008] Obtaining the topological structure and parameters of a DC microgrid; wherein the DC microgrid includes a plurality of units, each of which is connected to a DC bus through a converter;

[0009] Convert the DC microgrid into an RLC network, specifically: convert the constant power unit of the DC microgrid into a constant power load , the resistive unit is equivalent to a resistive load , converting the droop loop and voltage loop of the converter into a first circuit and forming an RLC network according to the topological structure of the DC microgrid; the first circuit consists of a resistor ,resistance and inductance ,inductance Composition, among which, the resistor and inductance Resistors in series and parallel and inductance One end connected, parallel resistor and inductance The other end is connected to a voltage source with a DC voltage rating of voltage; where, ; are the proportional coefficient and integral coefficient of the voltage loop PI controller corresponding to the kth converter in the DC microgrid, is the variable coefficient, r k is the droop coefficient of the kth converter, represents the line resistance between the converter k and the DC bus, represents the line inductance between the converter k and the DC bus;

[0010] The hybrid potential function theory is used to estimate the attraction domain of the RLC network based on the DC microgrid.

[0011] Furthermore, the constant power load includes a photovoltaic system operating in MPPT mode, an energy storage system with constant power control, and a DC charging pile.

[0012] Furthermore, when the voltage across the constant power load is less than the minimum DC voltage, it is equivalent to a constant current source, and when the constant power load current is less than or equal to the minimum output current, it is equivalent to a constant voltage source.

[0013] Furthermore, the RLC network based on the DC microgrid applies the mixed potential function theory to estimate the attraction domain, including:

[0014] Write the hybrid potential function based on the RLC network of the DC microgrid;

[0015] Construct a Lyapunov function based on the obtained mixed potential function;

[0016] The resulting Lyapunov function contains the variable parameter , defined when The subproblem of estimating the attraction domain when it is fixed;

[0017] The lower level set of the Lyapunov function is used to characterize the obtained attraction domain estimation, and the attraction domain estimation problem is obtained;

[0018] Solve the attraction domain estimation subproblem, obtain the corresponding solution, and get the final attraction domain estimation.

[0019] Furthermore, the subproblem of attractor estimation is solved and the corresponding solution is obtained, which is:

[0020] Will Discretization, for each discretized Calculate the attraction domain estimation subproblem and obtain the corresponding solution.

[0021] A transient stability analysis method for a DC microgrid, comprising:

[0022] When the system faces a large disturbance, the attraction domain of the equilibrium point before the disturbance is first calculated based on the DC microgrid attraction domain estimation method based on the hybrid potential function theory, and then it is verified whether the state after the disturbance is within the attraction domain. If it is within the attraction domain, the transient state is stable, otherwise it is unstable.

[0023] Furthermore, the large disturbance includes a sudden load change and disconnection of distributed power sources.

[0024] An electronic device comprises a memory, a processor and a computer program stored in the memory and executable on the processor. When the processor executes the computer program, a DC microgrid attraction domain estimation method based on a mixed potential function theory is implemented.

[0025] A storage medium containing computer executable instructions, which, when executed by a computer processor, can implement a DC microgrid attraction domain estimation method based on mixed potential function theory.

[0026] A computer program product includes a computer program / instruction, which, when executed by a processor, implements the steps of a method for estimating a DC microgrid attraction domain based on a mixed potential function theory.

[0027] The beneficial effects of the present invention are:

[0028] This paper proposes a novel attraction domain estimation method based on hybrid potential function theory. This method takes into account the control dynamics of the droop loop and the voltage loop, and aims to more comprehensively evaluate the transient stability of the DC microgrid, providing theoretical support and technical guarantee for the safe and stable operation of the system.

[0029] The attraction domain estimation obtained by the present invention can guide the design of DC microgrid circuit parameters and converter control parameters. According to the attraction domain estimation results, the direction of the influence of system circuit parameters and control parameters on stability can be determined, thereby greatly reducing the workload in the simulation stage and providing guarantee for system operation control. Guide the planning and design of DC microgrids. According to the attraction domain estimation results, the scale of constant power sources such as photovoltaics and energy storage systems, as well as the power level and number of ports of electrical equipment such as DC charging piles, are set. Guide system fault diagnosis and troubleshooting. According to the relationship between the system state after the fault and the attraction domain, the minimum fault troubleshooting time is given to provide a theoretical basis for system fault diagnosis and troubleshooting. Provide a theoretical basis for system optimization operation. On the basis of meeting the stability criterion, optimize the energy management strategy of the DC microgrid, including power distribution and energy storage system operation strategy, to improve system performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 is a DC microgrid and control block diagram of an embodiment of the present invention; in the control block diagram, R s,k , L s,k represent the resistance and inductance in the kth converter respectively;

[0031] Figure 2 is a schematic diagram of converting a converter into a first circuit when considering droop loop and voltage loop control according to an embodiment of the present invention;

[0032] Figure 3 is a schematic diagram of converting the entire DC microgrid into an RLC network according to an embodiment of the present invention;

[0033] Figure 4 It is a supplementary schematic diagram of the definition domain of the constant power load of the present invention;

[0034] Figure 5It is a flow chart of a method for estimating the attraction domain of a DC microgrid based on a mixed potential function theory of the present invention;

[0035] Figure 6 This is a diagram of an attraction domain estimation result obtained by applying the method of the present invention according to an embodiment of the present invention. DETAILED DESCRIPTION

[0036] Exemplary embodiments will be described in detail herein, examples of which are shown in the accompanying drawings. When the following description refers to the drawings, the same numbers in different drawings represent the same or similar elements unless otherwise indicated. The implementations described in the following exemplary embodiments do not represent all implementations consistent with the present application. Instead, they are merely examples of devices and methods consistent with some aspects of the present application as detailed in the appended claims.

[0037] The terminology used in the present application is for the purpose of describing particular embodiments only and is not intended to be limiting of the present application.

[0038] As used in this application and the appended claims, the singular forms "a", "an", "said", and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It should also be understood that the term "and / or" as used herein refers to and encompasses any and all possible combinations of one or more of the associated listed items.

[0039] It should be understood that although the terms first, second, third, etc. may be used in the present application to describe various information, these information should not be limited to these terms. These terms are only used to distinguish the same type of information from each other. For example, without departing from the scope of the present application, the first information may also be referred to as the second information, and similarly, the second information may also be referred to as the first information. Depending on the context, the word "if" as used herein may be interpreted as "at the time of" or "when" or "in response to determining".

[0040] The present invention proposes a DC microgrid attraction domain estimation method based on mixed potential function theory, such as Figure 5 As shown, the following steps are included:

[0041] Step 1: Obtain the topology and parameters of the DC microgrid;

[0042] Figure 1 FIG. 1 is a DC microgrid and a control block diagram of an embodiment; Figure 1 As shown, the DC microgrid includes several units, each of which is connected to the DC bus through a converter. In the present invention, the DC microgrid is converted into an RLC network. Therefore, the parameters to be obtained include the resistance, voltage, power, and DC bus capacitance C of each unit. dc, the droop coefficient of the converter corresponding to each unit, the line resistance between the converter and the DC bus, the line inductance between the converter and the DC bus, and real-time operating parameters such as the DC side output current, DC voltage, and corresponding unit voltage of each converter.

[0043] Step 2: Convert the DC microgrid into an RLC network:

[0044] The present invention converts and replaces the circuit elements of each unit according to the characteristics of each unit. Figure 1 As shown, for constant power units, including photovoltaic systems operating with MPPT, energy storage systems operating with constant power, and DC charging piles, they are equivalent to constant power loads. ; The resistive unit is regarded as a resistive load, then The converter adopts voltage and current double closed-loop droop control, ignoring the current loop dynamics. For the converter, the droop loop calculation method is:

[0045]

[0046] In the formula is the DC voltage rating, , and are the DC voltage reference value, droop coefficient and DC side output current of the kth droop-controlled converter respectively. The voltage loop calculation method is:

[0047]

[0048] In the formula are the proportional coefficient and integral coefficient of the voltage loop PI controller respectively, and are the inductor current reference value and DC voltage of the kth droop control converter respectively. Ignoring the current loop dynamics, i.e., the inductor current ,thereby:

[0049]

[0050] in is the unit voltage corresponding to the kth converter. Converted into the frequency domain model, we get:

[0051]

[0052] In the formula is the variable coefficient, s is the Laplace operator, Furthermore, based on the above-obtained out-of-frequency characteristics, we know

[0053]

[0054] Y is the admittance, from which we can know that the voltage loop corresponding to the kth converter can be converted into a resistor ,inductance The circuit formed by parallel connection, combined with the converter droop ring and other connection structures with the DC bus, the kth converter can finally be converted into a resistor ,resistance and inductance ,inductance The first circuit is composed of Figure 2 As shown. Among them, the resistance and inductance Resistors in series and parallel and inductance One end connected, parallel resistor and inductance The other end is connected to a voltage source with a DC voltage rating of voltage, . represents the line resistance between the converter k and the DC bus, represents the line inductance between the converter k and the DC bus. The corresponding inductor current is equal to the DC side output current of the converter k, that is: . The inductor The corresponding inductor current is then a virtual current that describes the controller dynamics.

[0055] Further combining the units and topological structure of the DC microgrid, the DC microgrid can be converted into a plurality of first circuits connected in parallel and connected to the DC bus capacitor C dc , constant power load P CPL , resistive load R L One end is connected to the DC capacitor C dc , constant power load P CPL , resistive load R L The other end of the RLC network is grounded, such as Figure 3 shown.

[0056] In addition, Figure 3 The volt-ampere characteristics of the medium constant power load are supplemented, such as Figure 4 As shown. After supplementation, the volt-ampere characteristic of the constant power load is:

[0057]

[0058] in , , , The turning points are supplemented to represent the maximum and minimum values ​​of DC voltage and the maximum and minimum values ​​of output current. When the current is constant, the current value is , when the output current is less than or equal to When it shows constant voltage characteristic, the voltage value is When the output current is and When the DC voltage is between and ), it exhibits a constant power characteristic.

[0059] That is, when the voltage across the constant power load is low, it is equivalent to a constant current source, and when the current of the constant power load is low, it is equivalent to a constant voltage source. This treatment is reasonable because the attraction domain is a closed set, and there are upper and lower bounds of voltage and current.

[0060] Step 3: Based on the simplified circuit model of the RLC network of the DC microgrid, the hybrid potential function theory is used to estimate the attraction domain; this step can adopt a conventional method, and in a specific implementation scheme, specifically includes the following sub-steps:

[0061] Step 3.1: Write the hybrid potential function based on the RLC network of the DC microgrid;

[0062] by Figure 3 Take the RLC network shown in the figure as an example, and write Figure 3 The mixed potential function of the RLC network circuit is as follows:

[0063]

[0064]

[0065]

[0066]

[0067] Where N is the number of converters. For all non-energy storage components in the circuit (i.e. ) is the voltage potential function, is the voltage potential function of constant power load and resistive load, is the energy in the capacitor. middle, and are the starting point and end point of integration, is the DC voltage at any time, starting point is the DC voltage at the equilibrium point (i.e. the DC voltage in steady state), is the resistive load resistance, Indicates the constant power load power, is the final mixed potential function, i and v are variables, representing the current and voltage in the RLC network circuit. is the mixed potential function of the equilibrium point, that is, .

[0068] Step 3.2: Construct the Lyapunov function based on the obtained mixed potential function. First, define the following matrix:

[0069]

[0070] in Refers to The diagonal matrix composed of Represents the DC bus capacitance.

[0071] definition For the matrix The minimum eigenvalue of is defined as follows :

[0072]

[0073] Finally, the Lyapunov function of the system is:

[0074]

[0075] in Given by the following formula

[0076]

[0077]

[0078] is the energy in all the inductors in the circuit.

[0079] Step 3.3: The resulting Lyapunov function Include , so it includes variable parameters , defined when The subproblem of estimating the attraction domain when fixed:

[0080]

[0081] in, for Estimation of the domain of attraction when fixed. is a scalar used to represent a set , that is, using the Lyapunov function The lower level set of is used to characterize the resulting attraction domain estimate: .

[0082] Step 3.4: Solve the attraction domain estimation subproblem, obtain the corresponding solution, and obtain the final attraction domain estimation, as follows:

[0083] The constant power load definition domain supplemented above is:

[0084]

[0085] because , where the DC source voltage is a fixed value, so according to The range is obtained Range:

[0086]

[0087] and Respectively represent the variable coefficients corresponding to each converter The minimum and maximum values ​​of .

[0088] Will Discretize and get

[0089]

[0090] M stands for The number of discretization intervals, M is a positive integer. The larger M is, the less conservative the obtained attraction domain is.

[0091] For each Calculate the attraction domain estimation subproblem and obtain the corresponding solution

[0092]

[0093] for The Lyapunov function when Then The solution obtained by solving the above attraction domain estimation subproblem is .

[0094] Finally, the final attraction domain estimation is obtained:

[0095]

[0096] When the system faces a large disturbance, such as a sudden load change or the removal of distributed power sources, the attraction domain of the equilibrium point before the disturbance is calculated first, and then it is verified whether the state after the disturbance is within the attraction domain. If it is within the attraction domain, the transient state is stable, otherwise it is unstable.

[0097] The present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be emphasized that the above drawings and the following description are only exemplary and are not intended to limit the scope of the present invention and its application.

[0098] Embodiment of a DC microgrid attraction domain estimation method considering control dynamics based on a hybrid potential function:

[0099] A DC microgrid attraction domain estimation method based on mixed potential function theory in an embodiment of the present invention is as follows: Figure 5 As shown, first, the fixed power unit is equivalent to a constant power load, and the droop control unit is converted into the first circuit, so as to obtain a simplified circuit model of the DC microgrid, that is, the RLC network model. Then, the volt-ampere characteristics of the constant power load are supplemented, and the mixed potential function and Lyapunov function of the circuit are listed to obtain the attraction domain estimation problem. Finally, discrete attraction domain estimation sub-problems are obtained, and the sub-problems are solved respectively and the intersection is obtained as the final attraction domain estimation.

[0100] The present invention is further described in conjunction with an embodiment. The DC microgrid studied in the embodiment includes a constant power load, a resistive load and a Boost circuit with droop control, such as Figure 1 As shown (ie, N=1 in the figure). Table 1 shows the parameters of the DC microgrid of the embodiment. For ease of reading, Table 1 only lists the parameters used in the method of the present invention.

[0101] Table 1 Parameters of the embodiment

[0102]

[0103] To facilitate the display of results, we take M=1 when solving the problem, that is, , using the fminsearch function in Matlab to solve, the estimated attraction domain is as follows Figure 6 Furthermore, when analyzing the transient stability problem of a system subjected to a large disturbance, it is possible to verify whether the state after the disturbance is Figure 6 In the attraction domain shown, if , the method of the present invention can ensure transient stability.

[0104] Corresponding to the aforementioned embodiment of a DC microgrid attraction domain estimation method based on a mixed potential function theory, the present invention also provides an electronic device, comprising one or more processors, for implementing a DC microgrid attraction domain estimation method based on a mixed potential function theory in the above embodiment.

[0105] The device embodiment can be implemented by software, hardware or a combination of software and hardware. Taking software implementation as an example, as a device in a logical sense, it is formed by the processor of any device with data processing capability in which it is located to read the corresponding computer program instructions in the non-volatile memory into the memory for execution. From the hardware level, it mainly includes a processor, memory, network interface, and non-volatile memory. In addition, any device with data processing capability in which the device in the embodiment is located can also include other hardware according to the actual function of the device with data processing capability, which will not be described in detail.

[0106] The implementation process of the functions and effects of each unit in the above-mentioned device is specifically described in the implementation process of the corresponding steps in the above-mentioned method, and will not be repeated here.

[0107] For the device embodiment, since it basically corresponds to the method embodiment, the relevant parts can refer to the partial description of the method embodiment. The device embodiment described above is only schematic, wherein the units described as separate components may or may not be physically separated, and the components displayed as units may or may not be physical units, that is, they may be located in one place, or they may be distributed on multiple network units. Some or all of the modules may be selected according to actual needs to achieve the purpose of the scheme of the present invention. Ordinary technicians in this field can understand and implement it without paying creative work.

[0108] An embodiment of the present invention further provides a computer-readable storage medium having a program stored thereon. When the program is executed by a processor, a method for estimating a DC microgrid attraction domain based on a mixed potential function theory in the above embodiment is implemented.

[0109] The computer-readable storage medium may be an internal storage unit of any device with data processing capability described in any of the aforementioned embodiments, such as a hard disk or a memory. The computer-readable storage medium may also be any device with data processing capability, such as a plug-in hard disk, a smart memory card (Smart Media Card, SMC), an SD card, a flash card (Flash Card), etc. equipped on the device. Furthermore, the computer-readable storage medium may also include both an internal storage unit of any device with data processing capability and an external storage device. The computer-readable storage medium is used to store the computer program and other programs and data required by any device with data processing capability, and may also be used to temporarily store data that has been output or is to be output.

[0110] The above is only a preferred specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by a person skilled in the art within the technical scope disclosed by the present invention should be included in the protection scope of the present invention. Therefore, the protection scope of the present invention should be based on the protection scope of the claims.

[0111] The contents not described in detail in this specification belong to the prior art known to professional and technical personnel in this field.

Claims

1. A DC microgrid attraction domain estimation method based on mixed potential function theory, characterized in that: include: Obtaining the topological structure and parameters of a DC microgrid; wherein the DC microgrid includes a plurality of units, each of which is connected to a DC bus through a converter; Convert the DC microgrid into an RLC network, specifically: convert the constant power unit of the DC microgrid into a constant power load , the resistive unit is equivalent to a resistive load , the droop loop and voltage loop of the converter are converted into a first circuit and an RLC network is formed according to the topological structure of the DC microgrid; the first circuit is composed of a resistor ,resistance and inductance ,inductance Composition, among which, the resistor and inductance Resistors in series and parallel and inductance One end connected, parallel resistor and inductance The other end is connected to a voltage source with a DC voltage rating of voltage; where, ; are the proportional coefficient and integral coefficient of the voltage loop PI controller corresponding to the kth converter in the DC microgrid, is the variable coefficient, r k is the droop coefficient of the kth converter, represents the line resistance between the converter k and the DC bus, represents the line inductance between the converter k and the DC bus; The hybrid potential function theory is used to estimate the attraction domain of the RLC network based on the DC microgrid.

2. The method according to claim 1, characterized in that The constant power load includes a photovoltaic system operating in MPPT mode, an energy storage system with constant power control, and a DC charging pile.

3. The method according to claim 1, characterized in that When the voltage across the constant power load is less than the minimum DC voltage, it is equivalent to a constant current source. When the constant power load current is less than or equal to the minimum output current, it is equivalent to a constant voltage source.

4. The method according to claim 1, characterized in that: The RLC network based on the DC microgrid applies the mixed potential function theory to estimate the attraction domain, including: Write the hybrid potential function based on the RLC network of the DC microgrid; Construct a Lyapunov function based on the obtained mixed potential function; The resulting Lyapunov function contains the variable parameter , defined when The subproblem of estimating the attraction domain when it is fixed; The lower level set of the Lyapunov function is used to characterize the obtained attraction domain estimation, and the attraction domain estimation problem is obtained; Solve the attraction domain estimation subproblem, obtain the corresponding solution, and get the final attraction domain estimation.

5. The method according to claim 4, characterized in that Solve the attraction domain estimation subproblem and obtain the corresponding solution, which is: Will Discretization, for each discretized Calculate the attraction domain estimation subproblem and obtain the corresponding solution.

6. A transient stability analysis method for a DC microgrid, characterized in that: include: When the system faces a large disturbance, the attraction domain of the equilibrium point before the disturbance is first calculated based on the DC microgrid attraction domain estimation method based on the hybrid potential function theory as described in any one of claims 1-5, and then it is verified whether the state after the disturbance is within the attraction domain. If it is within the attraction domain, the transient state is stable, otherwise it is unstable.

7. The method according to claim 6, characterized in that The large disturbance includes load mutation and disconnection of distributed power sources.

8. An electronic device comprising a memory, a processor, and a computer program stored in the memory and executable on the processor, characterized in that: When the processor executes the computer program, the method for estimating the attraction domain of a DC microgrid based on a mixed potential function theory as described in any one of claims 1 to 5 is implemented.

9. A storage medium comprising computer executable instructions, wherein the computer executable instructions, when executed by a computer processor, implement a DC microgrid attraction domain estimation method based on a mixed potential function theory as described in any one of claims 1 to 5.

10. A computer program product comprising a computer program / instructions, characterized in that When the computer program / instruction is executed by a processor, the steps of a method for estimating the attraction domain of a DC microgrid based on a hybrid potential function theory as described in any one of claims 1 to 5 are implemented.

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