Relay protection short-circuit current calculation method and device of photovoltaic-energy storage combined system

Abstract

The invention discloses a relay protection short-circuit current calculation method and device of a photovoltaic-energy storage combined system. The method comprises the steps that an inverter equivalent mathematical model of the photovoltaic-energy storage combined power generation system is established, and the inverter equivalent mathematical model enables an inverter in a fault state to be equivalent to a positive sequence current source; based on the inverter equivalent mathematical model, in combination with a low voltage ride-through control strategy, determining a function relationship between the output of the positive sequence current source and the positive sequence voltage of the grid-connected point, and forming a fault current calculation expression; and obtaining positive sequence voltage information of the grid-connected point when the power grid fails, substituting the positive sequence voltage information into the fault current calculation expression, and calculating to obtain the short-circuit current provided by the photovoltaic-energy storage combined system.

Description

Technical Field

[0001] This invention relates to the fields of new energy power systems and relay protection technology, and more specifically, to a method and apparatus for calculating short-circuit current of a photovoltaic-energy storage combined system for relay protection. Background Technology

[0002] As the penetration rate of photovoltaic power generation continues to increase, the guidelines for grid connection of new energy sources are beginning to require grid-connected new energy units to have low voltage ride-through (LVRT) capabilities. This means that during a certain period of time when a low voltage fault occurs in the grid, the unit is not only not allowed to disconnect from the grid, but also needs to provide a certain capacity of reactive power support to the grid to help restore the grid voltage. With the grid's increasing demands on the characteristics of new energy units, photovoltaic-storage / wind-storage / wind-photovoltaic-storage microgrid technologies are gradually emerging. The combination of new energy and energy storage can achieve smooth output of new energy power generation and regulate the changes in voltage, frequency, and phase in the photovoltaic-storage microgrid caused by power fluctuations. This will help break through the bottleneck of new energy power generation access.

[0003] Current research on relay protection models for photovoltaic (PV) and other new energy sources mainly focuses on the fault characteristics of the PV systems themselves and their impact on the power grid. However, similar to conventional power sources, grid connection of PV and other new energy sources is a complex system engineering project. Collector lines connect dispersed PV panels, which are then collected by collector lines to unit step-up transformers, then connected to the busbar, and finally connected to the power grid via the step-up transformers and grid connection lines. With the application of energy storage, the fault characteristics and short-circuit current calculation methods of PV-energy storage combined systems have gradually become a research focus. However, current research in this area is limited, and methods for calculating short-circuit current in PV-energy storage relay protection are lacking.

[0004] In summary, in the scenario of photovoltaic-energy storage combined system connected to the grid, how to realize the calculation of short-circuit current of relay protection of photovoltaic-energy storage combined system and ensure the accurate calculation of protection setting value is a key problem that urgently needs to be solved in the current technical field. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a method and apparatus for calculating short-circuit current in relay protection of a photovoltaic-energy storage combined system.

[0006] According to one aspect of the present invention, a method for calculating the short-circuit current of a photovoltaic-energy storage combined system for relay protection is provided, comprising:

[0007] An equivalent mathematical model of the inverter in a photovoltaic-energy storage combined power generation system is established. The equivalent mathematical model of the inverter in the fault state is equivalent to a positive sequence current source.

[0008] Based on the equivalent mathematical model of the inverter and combined with the low voltage ride-through control strategy, the functional relationship between the output of the positive sequence current source and the positive sequence voltage at the grid connection point is determined, and the fault current calculation expression is formed.

[0009] By obtaining the positive sequence voltage information of the grid connection point during a grid fault and substituting it into the fault current calculation expression, the short-circuit current provided by the photovoltaic-energy storage combined system can be calculated.

[0010] Optionally, an equivalent mathematical model of the inverter for the photovoltaic-energy storage combined power generation system is established, including:

[0011] Ignoring the transient component in the inverter output fault current, the AC side output current of the inverter after the fault is regarded as a steady-state current containing only positive sequence components, and an equivalent mathematical model of the inverter is established.

[0012] Optionally, the expression for calculating the fault current is:

[0013]

[0014] In the formula, I DG For the fault current of the photovoltaic-energy storage combined system; I N This is the inverter's rated current; The phase angle between the d-axis current and the q-axis current output by the inverter; The initial phase angle of the fundamental frequency components of the current and voltage; U is the positive sequence voltage at the grid connection point; N This is the inverter's rated voltage; For the d-axis current after the fault, This represents the q-axis current after the fault.

[0015] Optionally, the control logic for the fault current calculation expression includes:

[0016] When the grid connection point voltage is higher than the preset critical value, the inverter is controlled to increase the reactive current output according to the low voltage ride-through requirement.

[0017] When the grid connection point voltage is lower than the preset critical value and the inverter output current reaches the current limit limit, the output current amplitude is kept constant.

[0018] Optionally, the method further includes:

[0019] Based on the calculated short-circuit current, the relay protection settings are adjusted and verified.

[0020] According to another aspect of the present invention, a relay protection short-circuit current calculation device for a photovoltaic-energy storage combined system is provided, comprising:

[0021] A module is established to build an equivalent mathematical model of the inverter in a photovoltaic-energy storage combined power generation system. The equivalent mathematical model of the inverter in the fault state is equivalent to a positive sequence current source.

[0022] The module is used to determine the functional relationship between the output of the positive sequence current source and the positive sequence voltage at the grid connection point based on the equivalent mathematical model of the inverter and in combination with the low voltage ride-through control strategy, and to form the fault current calculation expression.

[0023] The calculation module is used to obtain the positive sequence voltage information of the grid connection point when the grid is faulty and substitute it into the fault current calculation expression to calculate the short-circuit current provided by the photovoltaic-energy storage combined system.

[0024] Optionally, modules are created, including:

[0025] A submodule is established to ignore the transient component in the inverter output fault current and treat the AC side output current of the inverter after the fault as a steady-state current containing only the positive sequence component, and an equivalent mathematical model of the inverter is established.

[0026] Optionally, the expression for calculating the fault current is:

[0027]

[0028] In the formula, I DG For the fault current of the photovoltaic-energy storage combined system; I N This is the inverter's rated current; The phase angle between the d-axis current and the q-axis current output by the inverter; The initial phase angle of the fundamental frequency components of the current and voltage; U is the positive sequence voltage at the grid connection point; N This is the inverter's rated voltage; For the d-axis current after the fault, This represents the q-axis current after the fault.

[0029] Optionally, the control logic for the fault current calculation expression includes:

[0030] When the grid connection point voltage is higher than the preset critical value, the inverter is controlled to increase the reactive current output according to the low voltage ride-through requirement.

[0031] When the grid connection point voltage is lower than the preset critical value and the inverter output current reaches the current limit limit, the output current amplitude is kept constant.

[0032] Optionally, the device further includes:

[0033] The verification module is used to set and verify the relay protection settings based on the calculated short-circuit current.

[0034] According to another aspect of the present invention, a computer-readable storage medium is provided, the storage medium storing a computer program for performing the methods described in any of the above aspects of the present invention.

[0035] According to another aspect of the present invention, an electronic device is provided, the electronic device comprising: a processor; a memory for storing executable instructions of the processor; the processor being configured to read the executable instructions from the memory and execute the instructions to implement the method described in any of the preceding aspects of the present invention.

[0036] Therefore, this invention provides a method for calculating the short-circuit current of a photovoltaic-energy storage combined system's relay protection. By constructing an equivalent circuit model of the photovoltaic-energy storage combined system and combining fault boundary conditions with inverter control strategies, the short-circuit current is solved in stages. In the transient fault phase, the initial value of the short-circuit current is calculated based on the Thevenin equivalent principle, reflecting the characteristics of the inrush current at the moment of the fault. In the dynamic adjustment phase, a differential equation model including the control loops of the photovoltaic and energy storage units is established, considering the impact of voltage dips on the current limiting circuit, simulating the inverter's output response under current constraints. Through a piecewise analytical method combined with numerical iteration, the time-varying characteristics of the short-circuit current amplitude and phase are accurately obtained, ensuring high-precision calculations under different fault types and operating conditions. This provides a reliable basis for relay protection setting, effectively avoiding protection maloperation and failure to operate due to short-circuit current estimation errors, and significantly improving the system's safe and stable operation under complex conditions. Attached Figure Description

[0037] Exemplary embodiments of the present invention can be more fully understood by referring to the following figures:

[0038] Figure 1 This is a flowchart illustrating a method for calculating the short-circuit current of a photovoltaic-energy storage combined system under relay protection, provided in an exemplary embodiment of the present invention.

[0039] Figure 2 This is a schematic diagram of a photovoltaic-storage combined power generation system provided in an exemplary embodiment of the present invention;

[0040] Figure 3 This is a schematic diagram of the internal structure of a photovoltaic-energy storage power generation unit provided in an exemplary embodiment of the present invention;

[0041] Figure 4 This is a schematic diagram of the structure of a relay protection short-circuit current calculation device for a photovoltaic-energy storage combined system provided in an exemplary embodiment of the present invention;

[0042] Figure 5 This is the structure of an electronic device provided in an exemplary embodiment of the present invention. Detailed Implementation

[0043] Hereinafter, exemplary embodiments according to the present invention will be described in detail with reference to the accompanying drawings. Obviously, the described embodiments are merely some embodiments of the present invention, and not all embodiments of the present invention. It should be understood that the present invention is not limited to the exemplary embodiments described herein.

[0044] It should be noted that, unless otherwise specifically stated, the relative arrangement, numerical expressions, and values ​​of the components and steps described in these embodiments do not limit the scope of the invention.

[0045] Those skilled in the art will understand that the terms "first," "second," etc., in the embodiments of the present invention are only used to distinguish different steps, devices, or modules, and do not represent any specific technical meaning, nor do they indicate a necessary logical order between them.

[0046] It should also be understood that in the embodiments of the present invention, "multiple" can refer to two or more, and "at least one" can refer to one, two or more.

[0047] It should also be understood that any component, data or structure mentioned in the embodiments of the present invention can generally be understood as one or more unless explicitly defined or given contrary instructions in the context.

[0048] Furthermore, the term "and / or" in this invention is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, or B existing alone. Additionally, the character " / " in this invention generally indicates that the preceding and following related objects have an "or" relationship.

[0049] It should also be understood that the description of the various embodiments in this invention emphasizes the differences between the various embodiments, and the similarities or similarities can be referred to each other. For the sake of brevity, they will not be described in detail.

[0050] At the same time, it should be understood that, for ease of description, the dimensions of the various parts shown in the accompanying drawings are not drawn according to actual scale.

[0051] The following description of at least one exemplary embodiment is merely illustrative and is in no way intended to limit the invention or its application or use.

[0052] Techniques, methods, and equipment known to those skilled in the art may not be discussed in detail, but where appropriate, they should be considered part of the specification.

[0053] It should be noted that similar labels and letters in the following figures indicate similar items; therefore, once an item is defined in one figure, it does not need to be discussed further in subsequent figures.

[0054] The embodiments of this invention can be applied to electronic devices such as terminal devices, computer systems, and servers, and can operate together with a wide range of other general-purpose or special-purpose computing system environments or configurations. Well-known examples of terminal devices, computing systems, environments, and / or configurations suitable for use with electronic devices such as terminal devices, computer systems, and servers include, but are not limited to: personal computer systems, server computer systems, thin clients, thick clients, handheld or laptop devices, microprocessor-based systems, set-top boxes, programmable consumer electronics, network PCs, minicomputer systems, mainframe computer systems, and distributed cloud computing environments including any of the above systems, etc.

[0055] Electronic devices such as terminal devices, computer systems, and servers can be described in the general context of computer system executable instructions (such as program modules) executed by a computer system. Typically, program modules can include routines, programs, object programs, components, logic, data structures, etc., which perform specific tasks or implement specific abstract data types. Computer systems / servers can be implemented in distributed cloud computing environments, where tasks are executed by remote processing devices linked through communication networks. In distributed cloud computing environments, program modules can reside on local or remote computing system storage media, including storage devices.

[0056] Exemplary methods

[0057] Figure 1 This is a flowchart illustrating a method for calculating the short-circuit current of a photovoltaic-energy storage combined system under relay protection, provided in an exemplary embodiment of the present invention. This embodiment can be applied to electronic devices, such as… Figure 1 As shown, the method 100 for calculating the short-circuit current of the relay protection of a photovoltaic-energy storage combined system includes the following steps:

[0058] Step 101: Establish the equivalent mathematical model of the inverter in the photovoltaic-energy storage combined power generation system. The equivalent mathematical model of the inverter in the fault state is equivalent to a positive sequence current source.

[0059] Step 102: Based on the equivalent mathematical model of the inverter and combined with the low voltage ride-through control strategy, determine the functional relationship between the output of the positive sequence current source and the positive sequence voltage at the grid connection point, and form the fault current calculation expression.

[0060] Step 103: Obtain the positive sequence voltage information of the grid connection point when the grid fails and substitute it into the fault current calculation expression to calculate the short-circuit current provided by the photovoltaic-energy storage combined system.

[0061] Specifically, this invention discloses a method for calculating short-circuit current in relay protection of a photovoltaic-energy storage combined system. This method relates to the fields of new energy power systems and relay protection technology. It establishes a typical photovoltaic-energy storage combined system, conducts theoretical analysis of the system, establishes a corresponding mathematical model, analyzes and simplifies the mathematical model, and derives short-circuit current calculation formulas applicable to different operating conditions. Combining the system's equivalent circuit and fault boundary conditions, and considering the dynamic response characteristics of the photovoltaic and energy storage units, it achieves accurate short-circuit current calculation, providing a reliable basis for relay protection setting. This method comprehensively considers the fluctuation of photovoltaic output, the charging and discharging characteristics of energy storage, and the transient process at the moment of fault. It accurately characterizes the amplitude and phase changes of the fault current through staged modeling. In the initial stage of the fault, the initial value of the short-circuit current is solved based on the Thevenin equivalent circuit. In the dynamic response stage, differential equations are introduced to describe the current regulation effect of the inverter control strategy, and the output current limiting mechanism of the photovoltaic and energy storage units is determined by combining the voltage drop degree. Simulation results demonstrate that this method effectively reflects actual short-circuit characteristics under various fault types and operating modes, with a calculation error of less than 5%, significantly improving the reliability and sensitivity of protection actions. This method can adapt to the short-circuit current calculation needs of photovoltaic-energy storage systems under various operating conditions such as grid connection, islanding, and mode switching. It effectively solves the problem of insufficient accurate basis for existing relay protection setting, improves the fault response capability and safe operation level of microgrids, and provides technical support for the large-scale integration of new energy sources.

[0062] This invention provides a method for calculating the short-circuit current of a photovoltaic-energy storage combined system for relay protection. It addresses the challenge of calculating the short-circuit current of a photovoltaic-energy storage combined power generation system in grid-connected scenarios, ensuring that the calculation results reliably reflect the short-circuit current situation during grid faults.

[0063] In view of the above problems, this invention provides a method for calculating the short-circuit current of a photovoltaic-energy storage combined system's relay protection. By constructing an equivalent circuit model of the photovoltaic-energy storage combined system and combining fault boundary conditions with inverter control strategies, the short-circuit current is solved in stages. In the transient fault stage, the initial value of the short-circuit current is calculated based on the Thevenin equivalent principle, reflecting the characteristics of the inrush current at the moment of the fault. In the dynamic adjustment stage, a differential equation model including the control loops of the photovoltaic and energy storage units is established, considering the impact of voltage dips on the current limiting circuit, and simulating the inverter's output response under current constraints. Through a piecewise analytical method combined with numerical iteration, the time-varying characteristics of the short-circuit current amplitude and phase are accurately obtained, ensuring high-precision calculations under different fault types and operating conditions. This provides a reliable basis for relay protection setting, effectively avoiding protection maloperation and failure to operate due to short-circuit current estimation errors, and significantly improving the system's safe and stable operation under complex conditions.

[0065] This invention provides a distance protection setting method and system based on electromagnetic transient simulation data, which solves the technical problem in the prior art of how to achieve dynamic calculation of distance protection settings in complex and ever-changing new power system scenarios, and ensure that the calculation results reliably reflect the changes in measured impedance state and setting values ​​during power grid faults.

[0066] like Figure 2 As shown, this invention provides a distance protection setting method based on electromagnetic transient simulation data, including:

[0067] First, the topology and component parameters of the photovoltaic-energy storage system are obtained, and a detailed mathematical model including inverter control strategies is established. Taking a photovoltaic-energy storage combined power generation system as an example, its typical structure connected to the power grid through a step-up transformer is as follows: Figure 2 As shown.

[0068] In this structure, multiple photovoltaic and energy storage units within the power station are each connected in parallel to the collecting busbar in the combiner station via a single-stage step-up transformer. The photovoltaic and energy storage units within the station are connected to the step-up transformer via the collecting busbar, and the high-voltage side of the step-up transformer is connected to the high-voltage transmission network. Typically, the low-voltage side of the unit transformer is Y-connected with its neutral point directly grounded, while the high-voltage side is d-connected; the step-up transformer typically has a Y-connection on the high-voltage side, a d-connection on the low-voltage side, and a directly grounded neutral point on the high-voltage side.

[0069] The internal structure of the photovoltaic-energy storage power generation unit is as follows: Figure 2 As shown in the figure. (u) dc This refers to the DC bus voltage; i ga i gb i gc and u ga u gb u gc These represent the three-phase current and voltage at the inverter output; R and L are the filter resistor and inductor, respectively. The photovoltaic array in the photovoltaic power generation unit adopts a two-stage grid-connected structure, connected to the DC bus after being boosted by a unidirectional DC / DC converter; the energy storage battery in the energy storage power generation unit is boosted by a bidirectional DC / DC converter and then connected in parallel to the DC bus. The inverter converts the DC power into three-phase AC power, which is then connected to the grid via a box-type transformer.

[0070] Distributed power sources can be classified into two types based on interface type: rotating and inverter-type. Among them, the new energy-storage combined generation system belongs to the inverter-type power source. It is connected to the grid via an inverter, and its output exhibits strong nonlinearity. A large-capacity capacitor is typically connected between the generator unit and the inverter in a new energy-storage combined generation system. During a fault, the DC-side voltage remains essentially constant, so the dynamic response of the generator unit can be ignored. Therefore, the output characteristics of the new energy-storage combined generation system are primarily determined by the inverter control strategy.

[0071] Before the voltage drop, the inverter was in steady-state operation and running at unity power factor, with its d-axis and q-axis currents i d0 i q0 The following relationship must be satisfied:

[0072]

[0073] In the formula, This represents the d-axis current before the fault.

[0074] Therefore, the three-phase current i output by the inverter before the voltage drop ga0 i gb0 i gc0 It can be represented as:

[0075]

[0076] In the formula: ω1 is the synchronous angular velocity; θ i The initial phase of the fundamental frequency component of the d-axis current; The initial phase angle is the fundamental frequency component of the current.

[0077] Because an inverter interface is used, the inertial time constant is very small, and the transient component in the inverter output fault current can be ignored. It can be approximated that the AC output current of the inverter under grid fault conditions does not contain attenuated DC components or fundamental frequency free components, and the inverter directly enters fault steady-state operation. Therefore, the d-axis and q-axis components of the fault current output by the inverter after a voltage drop can be expressed as:

[0078]

[0079] In the formula: i d0 (t) — d-axis current before the fault; Δi df (t) — Change in d-axis current after the fault; —D-axis current after fault; i q0 (t) — q-axis current before the fault; Δi qf (t) — Change in q-axis current after the fault; —q-axis current after the fault;

[0080] Therefore, the three-phase fault current output by the inverter can be obtained as follows:

[0081]

[0082] In the formula: These are the commanded values ​​for the d-axis and q-axis currents after the fault. ω1—synchronous angular velocity; θ i —Initial phase of the fundamental frequency component of the d-axis current; —Initial phase angle of the fundamental frequency component of the current.

[0083] Because the inverter employs a grid voltage-oriented vector control strategy and maintains unity power factor during grid connection, its active current is oriented towards the grid connection point voltage under normal operating conditions. During fault periods, the increased reactive current output will cause the output current to lag behind the grid connection point voltage.

[0084] Meanwhile, the relationship between the fault output current of the photovoltaic-storage inverter and the grid connection point voltage can also be expressed as:

[0085]

[0086] In the formula: I d —Inverter output d-axis current; I d —Inverter output q-axis current; —The phase angle of the grid connection point voltage Ug.

[0087] As mentioned earlier, when a grid fault occurs, the fault current output by the inverter can be considered to contain only the positive-sequence current component. Therefore, in the event of a grid fault, the inverter can be treated as an equivalent positive-sequence current source for fault analysis. Furthermore, this positive-sequence current source can be obtained from the expression based on the LVRT control strategy. With the positive sequence voltage of the grid connection point The functional relationship between them, that is, the equivalent mathematical model of the inverter, can be expressed in the form of equation (6), that is, the short-circuit current calculation expression of the photovoltaic-energy storage combined system is as follows:

[0088]

[0089] In the formula, I DG —Fault current in photovoltaic-energy storage combined system;

[0090] I N —Inverter rated current; —The phase angle between the d-axis current and the q-axis current output by the inverter; —Initial phase angle of the fundamental frequency components of current and voltage; —Positive sequence voltage at grid connection point; U N —Inverter rated voltage; —d-axis current after fault —q-axis current after the fault;

[0091] In summary, when a grid-side fault occurs, the voltage at the grid connection point decreases, and the output current of the photovoltaic-storage combined generation system increases compared to before the fault. However, due to the inverter's current limiting constraint, when the voltage at the grid connection point drops to a certain level and the inverter's steady-state short-circuit current reaches its upper limit, the inverter's output current remains constant at 1.05 times the rated output current. As mentioned earlier, after a fault occurs in the photovoltaic-storage combined generation system, an equivalent positive-sequence current source model is typically used for fault analysis.

[0092] Therefore, this invention can adapt to the short-circuit current calculation needs of photovoltaic-energy storage systems under various operating conditions such as grid connection, islanding, and mode switching, effectively solve the problem of the lack of accurate basis for the setting of existing relay protection values, improve the fault response capability and safe operation level of microgrids, and provide technical support for the large-scale access of new energy sources.

[0093] Exemplary device

[0094] Figure 4 This is a schematic diagram of the structure of a relay protection short-circuit current calculation device for a photovoltaic-energy storage combined system provided in an exemplary embodiment of the present invention. Figure 4 As shown, the device 400 includes:

[0095] Module 410 is established to establish the equivalent mathematical model of the inverter in the photovoltaic-energy storage combined power generation system. The equivalent mathematical model of the inverter in the fault state is equivalent to a positive sequence current source.

[0096] The module 420 is used to determine the functional relationship between the output of the positive sequence current source and the positive sequence voltage at the grid connection point based on the inverter equivalent mathematical model and in combination with the low voltage ride-through control strategy, and to form a fault current calculation expression.

[0097] The calculation module 430 is used to obtain the positive sequence voltage information of the grid connection point when the grid is faulty and substitute it into the fault current calculation expression to calculate the short-circuit current provided by the photovoltaic-energy storage combined system.

[0098] Optionally, module 410 is established, including:

[0099] A submodule is established to ignore the transient component in the inverter output fault current and treat the AC side output current of the inverter after the fault as a steady-state current containing only the positive sequence component, and an equivalent mathematical model of the inverter is established.

[0100] Optionally, the expression for calculating the fault current is:

[0101]

[0102] In the formula, I DG For the fault current of the photovoltaic-energy storage combined system; I N This is the inverter's rated current; The phase angle between the d-axis current and the q-axis current output by the inverter; The initial phase angle of the fundamental frequency components of the current and voltage; U is the positive sequence voltage at the grid connection point; N This is the inverter's rated voltage; For the d-axis current after the fault, This represents the q-axis current after the fault.

[0103] Optionally, the control logic for the fault current calculation expression includes:

[0104] When the grid connection point voltage is higher than the preset critical value, the inverter is controlled to increase the reactive current output according to the low voltage ride-through requirement.

[0105] When the grid connection point voltage is lower than the preset critical value and the inverter output current reaches the current limit limit, the output current amplitude is kept constant.

[0106] Optionally, the device 400 further includes:

[0107] The verification module is used to set and verify the relay protection settings based on the calculated short-circuit current.

[0108] Exemplary electronic devices

[0109] Figure 5 This is the structure of an electronic device provided in an exemplary embodiment of the present invention. For example... Figure 5 As shown, the electronic device 50 includes one or more processors 51 and a memory 52.

[0110] The processor 51 may be a central processing unit (CPU) or other form of processing unit with data processing and / or instruction execution capabilities, and may control other components in the electronic device to perform desired functions.

[0111] The memory 52 may include one or more computer program products, which may include various forms of computer-readable storage media, such as volatile memory and / or non-volatile memory. The volatile memory may include, for example, random access memory (RAM) and / or cache memory. The non-volatile memory may include, for example, read-only memory (ROM), hard disk, flash memory, etc. One or more computer program instructions may be stored on the computer-readable storage medium, and the processor 51 may execute the program instructions to implement the methods of the software programs of the various embodiments of the present invention described above, and / or other desired functions. In one example, the electronic device may also include an input device 53 and an output device 54, these components being interconnected via a bus system and / or other forms of connection mechanisms (not shown).

[0112] In addition, the input device 53 may also include, for example, a keyboard, a mouse, etc.

[0113] The output device 54 can output various information to the outside. The output device 54 may include, for example, a display, a speaker, a printer, and a communication network and its connected remote output devices, etc.

[0114] Of course, for the sake of simplicity, Figure 5 Only some of the components of this electronic device relevant to the present invention are shown, omitting components such as buses, input / output interfaces, etc. In addition, the electronic device may include any other suitable components depending on the specific application.

[0115] Exemplary computer program products and computer-readable storage media

[0116] In addition to the methods and apparatus described above, embodiments of the present invention may also be computer program products, which include computer program instructions that, when executed by a processor, cause the processor to perform the steps in the methods according to various embodiments of the present invention described in the "Exemplary Methods" section above.

[0117] The computer program product can be written in any combination of one or more programming languages ​​to perform the operations of the embodiments of the present invention. The programming languages ​​include object-oriented programming languages ​​such as Java and C++, as well as conventional procedural programming languages ​​such as C or similar languages. The program code can be executed entirely on the user's computing device, partially on the user's computing device, as a standalone software package, partially on the user's computing device and partially on a remote computing device, or entirely on a remote computing device or server.

[0118] Furthermore, embodiments of the present invention may also be computer-readable storage media storing computer program instructions thereon, which, when executed by a processor, cause the processor to perform the steps of the methods according to various embodiments of the present invention described in the "Exemplary Methods" section above.

[0119] The computer-readable storage medium may be any combination of one or more readable media. A readable medium may be a readable signal medium or a readable storage medium. A readable storage medium may be, for example, an electrical, magnetic, optical, electromagnetic, infrared, or semiconductor system, device, or any combination thereof. More specific examples (a non-exhaustive list) of readable storage media include: an electrical connection having one or more wires, a portable disk, a hard disk, random access memory (RAM), read-only memory (ROM), erasable programmable read-only memory (EPROM or flash memory), optical fiber, portable compact disk read-only memory (CD-ROM), optical storage device, magnetic storage device, or any suitable combination thereof.

[0120] The basic principles of the present invention have been described above with reference to specific embodiments. However, it should be noted that the advantages, benefits, and effects mentioned in the present invention are merely examples and not limitations, and should not be considered as essential features of each embodiment of the present invention. Furthermore, the specific details disclosed above are for illustrative and facilitative purposes only, and are not limitations. These details do not limit the present invention to the necessity of employing the aforementioned specific details.

[0121] The various embodiments in this specification are described in a progressive manner, with each embodiment focusing on its differences from other embodiments. Similar or identical parts between embodiments can be referred to interchangeably. For system embodiments, since they largely correspond to method embodiments, the description is relatively simple; relevant parts can be referred to the descriptions in the method embodiments.

[0122] The block diagrams of devices, systems, devices, and systems involved in this invention are merely illustrative examples and are not intended to require or imply that they must be connected, arranged, or configured in the manner shown in the block diagrams. As those skilled in the art will recognize, these devices, systems, devices, and systems can be connected, arranged, and configured in any manner. Words such as “comprising,” “including,” “having,” etc., are open-ended terms meaning “including but not limited to,” and are used interchangeably with them. The terms “or” and “and” as used herein refer to the terms “and / or,” and are used interchangeably with them unless the context clearly indicates otherwise. The term “such as” as used herein refers to the phrase “such as but not limited to,” and is used interchangeably with it.

[0123] The methods and systems of the present invention may be implemented in many ways. For example, they may be implemented by software, hardware, firmware, or any combination of software, hardware, and firmware. The above-described order of steps for the methods is for illustrative purposes only, and the steps of the methods of the present invention are not limited to the order specifically described above unless otherwise specifically stated. Furthermore, in some embodiments, the present invention may also be implemented as a program recorded on a recording medium, the program comprising machine-readable instructions for implementing the methods according to the present invention. Thus, the present invention also covers recording media storing programs for performing the methods according to the present invention.

[0124] It should also be noted that in the systems, apparatus, and methods of the present invention, the components or steps can be disassembled and / or recombined. These disassemblies and / or recombinations should be considered equivalents of the present invention. The above description of the disclosed aspects is provided to enable any person skilled in the art to make or use the invention. Various modifications to these aspects will be readily apparent to those skilled in the art, and the general principles defined herein can be applied to other aspects without departing from the scope of the invention. Therefore, the invention is not intended to be limited to the aspects shown herein, but rather to be carried out within the widest scope consistent with the principles and novel features disclosed herein.

[0125] The above description has been given for purposes of illustration and description. Furthermore, this description is not intended to limit the embodiments of the invention to the forms disclosed herein. Although numerous exemplary aspects and embodiments have been discussed above, those skilled in the art will recognize certain variations, modifications, alterations, additions, and sub-combinations thereof.

Claims

1. A method for calculating short-circuit current for relay protection of a photovoltaic- energy storage combined system, characterized in that, The method comprises the following steps: An equivalent mathematical model of an inverter of a photovoltaic- energy storage combined power generation system is established, wherein the equivalent mathematical model of the inverter is equivalent to a positive sequence current source in a fault state; Based on the equivalent mathematical model of the inverter and in combination with a low voltage ride through control strategy, a function relationship between an output of the positive sequence current source and a positive sequence voltage of a grid-connected point is determined, and a fault current calculation expression is formed; Positive sequence voltage information of the grid-connected point in a power grid fault is obtained and substituted into the fault current calculation expression, and a short circuit current provided by the photovoltaic- energy storage combined system is calculated.

2. The method of claim 1, wherein, The equivalent mathematical model of the inverter of the photovoltaic- energy storage combined power generation system comprises: The transient component in the fault current output by the inverter is ignored, and the AC side output current of the inverter after the fault is regarded as a steady-state current containing only the positive sequence component, and the equivalent mathematical model of the inverter is established.

3. The method of claim 1, wherein, The fault current calculation expression is: In the formula, I DG is the fault current of the photovoltaic- energy storage combined system; I N is the rated current of the inverter; is the phase angle of the d-axis current and the q-axis current output by the inverter; is the initial phase angle of the fundamental frequency component of the current and the voltage; is the positive sequence voltage at the grid-connected point; U N is the rated voltage of the inverter; is the d-axis current after the fault, is the q-axis current after the fault.

4. The method of claim 1, wherein, The control logic of the fault current calculation expression comprises: When the voltage of the grid-connected point is higher than a preset threshold value, the inverter is controlled to increase the reactive current output according to the low voltage ride through requirement; When the voltage of the grid-connected point is lower than the preset threshold value and the output current of the inverter reaches the upper limit of current limiting, the amplitude of the output current is controlled to remain constant.

5. The method of claim 1, wherein, The method further comprises: Based on the calculated short circuit current, the setting and verification of the relay protection setting value are performed.

6. A device for calculating short-circuit current for relay protection of a photovoltaic- energy storage combined system, characterized by, The method comprises the following steps: An equivalent mathematical model of an inverter of a photovoltaic- energy storage combined power generation system is established, wherein the equivalent mathematical model of the inverter is equivalent to a positive sequence current source in a fault state; Based on the equivalent mathematical model of the inverter and in combination with a low voltage ride through control strategy, a function relationship between an output of the positive sequence current source and a positive sequence voltage of a grid-connected point is determined, and a fault current calculation expression is formed; Positive sequence voltage information of the grid-connected point in a power grid fault is obtained and substituted into the fault current calculation expression, and a short circuit current provided by the photovoltaic- energy storage combined system is calculated.

7. The apparatus of claim 6, wherein, The equivalent mathematical model of the inverter of the photovoltaic- energy storage combined power generation system comprises: The transient component in the fault current output by the inverter is ignored, and the AC side output current of the inverter after the fault is regarded as a steady-state current containing only the positive sequence component, and the equivalent mathematical model of the inverter is established.

8. The apparatus of claim 6, wherein, The fault current calculation expression is: In the formula, I DG is the fault current of the photovoltaic- energy storage combined system; I N is the rated current of the inverter; is the phase angle of the d-axis current and the q-axis current output by the inverter; is the initial phase angle of the fundamental frequency component of the current and the voltage; is the positive sequence voltage at the grid-connected point; U N is the rated voltage of the inverter; is the d-axis current after the fault, is the q-axis current after the fault.

9. The apparatus of claim 6, wherein, The control logic of the fault current calculation expression comprises: When the voltage of the grid-connected point is higher than a preset threshold value, the inverter is controlled to increase the reactive current output according to the low voltage ride through requirement; When the voltage of the grid-connected point is lower than the preset threshold value and the output current of the inverter reaches the upper limit of current limiting, the amplitude of the output current is controlled to remain constant.

10. The apparatus of claim 6, wherein, The device further comprises: A verification module is configured to perform the setting and verification of the relay protection setting value based on the calculated short circuit current.

11. A computer readable storage medium characterized by, The storage medium stores a computer program, and the computer program is configured to execute the method in any one of claims 1-5.

12. An electronic device, comprising: The electronic device comprises: A processor; A memory for storing executable instructions of the processor; The processor is configured to read the executable instructions from the memory and execute the instructions to implement the method in any one of claims 1-5.

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