A practical calculation method of short-circuit current of power system considering influence of photovoltaic power station

Abstract

This invention discloses a practical method for calculating short-circuit current in power systems considering the impact of photovoltaic (PV) power plants. The method includes: performing short-circuit fault analysis on the PV power plant; calculating the PV grid-connected node voltage based on the short-circuit current output by the PV power plant under different scenarios; calculating the mean PV grid-connected node voltage as an estimated value of the grid-connected point voltage after the fault; calculating the equivalent impedance value in the Thevenin equivalent model based on the estimated voltage of the grid-connected point after the fault; connecting the PV fault equivalent branch at the PV power plant grid-connected point in the positive-sequence network of the system; and obtaining the short-circuit current distribution of the entire network using short-circuit fault calculation. The practical method for calculating short-circuit current in power systems provided by this invention does not require detailed model parameters of the PV power plant. It uses a constant potential source series impedance branch to characterize the impact of PV power plant grid connection on the system short-circuit current, avoiding the iterative calculation process in the theoretical solution of short-circuit current. It offers fast calculation speed, strong feasibility, and no convergence issues.

Description

Technical Field

[0001] This invention relates to the technical field of power system short-circuit calculation and analysis, and in particular to a practical calculation method for power system short-circuit current considering the influence of photovoltaic power plants. Background Technology

[0002] When a photovoltaic (PV) power station is connected to the grid, it possesses a certain low-voltage ride-through capability, allowing it to remain connected to the grid for a certain period during grid faults. Specifically, when the grid connection point voltage U... pcc When the voltage drops below 0.9 pu, the photovoltaic power station should output a certain reactive current to support grid stability. Because the fault characteristics of photovoltaic power stations are affected by the grid-connected inverter control strategy, the output current and grid connection point voltage exhibit a strong nonlinear relationship, differing from the characteristics of traditional synchronous machines. Therefore, establishing a photovoltaic power station model suitable for practical short-circuit current calculation and inventing a short-circuit current calculation method that considers the impact of photovoltaic power station grid connection are of great significance.

[0003] When considering the impact of fault currents from photovoltaic (PV) power plants on the power system, existing simulation software such as PSCAD / EMTDC and Matlab / Simulink require building detailed models of the PV power plants. In engineering calculations, obtaining detailed model parameters is difficult, and simulations incur significant time overhead, failing to meet practical engineering needs. In existing theoretical analyses of short-circuit currents in power systems with PV power plant connections, the inverter control strategy causes the PV power plant to exhibit nonlinear current source characteristics controlled by the grid connection voltage. The calculation of currents in each branch of the system requires iterative solutions, which contradicts traditional non-iterative calculation methods, and most current short-circuit current calculation software does not support this. Furthermore, because the fault output voltage-current characteristic curves of PV power plants have non-differentiable points, using iterative algorithms for continuously differentiable nonlinear equations may result in non-convergence. In summary, existing equivalent models of PV power plants cannot match traditional models already present in tuning software, making them difficult to implement in engineering applications and hindering the rapid and effective acquisition of the entire grid's current distribution. Summary of the Invention

[0004] The purpose of this section is to outline some aspects of embodiments of the present invention and to briefly describe some preferred embodiments. Simplifications or omissions may be made in this section, as well as in the abstract and title of this application, to avoid obscuring the purpose of these documents; however, such simplifications or omissions should not be construed as limiting the scope of the invention.

[0005] In view of the aforementioned existing problems, the present invention is proposed.

[0006] Therefore, the technical problem solved by this invention is that existing software simulation calculation models are complex, and theoretical iterative methods suffer from convergence issues, failing to meet the requirements for engineering calculation speed and efficiency.

[0007] To solve the above-mentioned technical problems, the present invention provides the following technical solution, including:

[0008] Short-circuit fault analysis is performed on photovoltaic power plants, and the grid-connected node voltage of photovoltaic power plants is calculated based on the short-circuit current output by the photovoltaic power plants under different scenarios.

[0009] The mean value of the photovoltaic grid-connected node voltage is calculated as the estimated value of the grid-connected node voltage after the fault;

[0010] Calculate the equivalent impedance value in the Thevenin equivalent model based on the estimated voltage at the grid connection point after the fault.

[0011] A photovoltaic fault equivalent branch is connected at the grid connection point of the photovoltaic power station in the positive sequence network of the system, and the short-circuit current distribution of the entire network is obtained by short-circuit fault calculation.

[0012] As a preferred embodiment of the practical calculation method for short-circuit current of power systems considering the impact of photovoltaic power plants described in this invention, the short-circuit fault analysis of photovoltaic power plants includes:

[0013] The low-voltage ride-through control strategy adopted when considering grid-connected inverter faults in photovoltaic power plants is expressed as follows:

[0014]

[0015]

[0016] Among them, I f-dref I is the reference value for output active current during a fault. f-qref I is the reference value for output reactive current during a fault. N I is the rated current of the photovoltaic power station. max =K×I N To allow the maximum current to flow through the grid-connected inverter, typically K = 1.1-2, U pcc This is the positive sequence voltage at the grid connection point.

[0017] As a preferred embodiment of the practical calculation method for power system short-circuit current considering the impact of photovoltaic power plants described in this invention, the approximate relationship between the output short-circuit current of the photovoltaic power plant and the positive-sequence voltage at the grid connection point includes:

[0018] The approximate relation is expressed as:

[0019]

[0020] Among them, I pv For the output short-circuit current of the photovoltaic power station, If-dref I f-qref This provides reference values ​​for output active and reactive current during a fault; U pcc This is the positive sequence voltage at the grid connection point.

[0021] As a preferred embodiment of the practical calculation method for short-circuit current of a power system considering the impact of photovoltaic power plants as described in this invention, the calculation of the photovoltaic grid-connected node voltage includes:

[0022] When the influence of the short-circuit current output by the grid-connected photovoltaic power plant is ignored, the grid-connected node voltage of the photovoltaic power plant is expressed as:

[0023]

[0024] in, Let be the voltage at any node i in the power network. Z represents the normal voltage at fault point f before a short circuit in the power network. if Z represents the mutual impedance between node i and faulty node f in a power network. ff Let i be the self-impedance of the faulty node f in the power network, when node i is a grid connection point.

[0025] If all photovoltaic power stations in the system output the maximum short-circuit current, and this short-circuit current is reactive current and does not contain active components, the photovoltaic grid-connected node voltage is expressed as:

[0026]

[0027] in, The voltage at grid connection point p of the photovoltaic power station after the fault. Z represents the grid connection voltage of the photovoltaic power station before the fault. pp Z is the self-impedance of node p. ff Z is the self-impedance of node f. pf Let z be the mutual impedance between nodes p and f in the nodal impedance matrix. f For the transition resistance at fault point f, when node p is a grid connection point,

[0028] As a preferred embodiment of the practical calculation method for power system short-circuit current considering the impact of photovoltaic power plants described in this invention, wherein:

[0029] The average voltage of the photovoltaic grid-connected node is calculated as an estimated value of the grid-connected node voltage after a fault, including:

[0030] The estimated voltage at the grid connection point after the fault is expressed as follows:

[0031]

[0032] Among them, Upcc-pvmin U is the grid-connected photovoltaic node voltage when the output current is zero. pcc-pvmax The maximum output current is the voltage of the photovoltaic grid-connected node.

[0033] As a preferred embodiment of the practical calculation method for short-circuit current of a power system considering the impact of photovoltaic power plants as described in this invention, the calculation of the equivalent impedance value based on the estimated voltage at the grid connection point after the fault includes:

[0034] The equivalent impedance value is expressed as:

[0035]

[0036] Among them, z pv U is the equivalent impedance of the photovoltaic model, E is the value of the constant potential source, usually 1, and U pcc-est I is the estimated voltage at the grid connection point of the photovoltaic power plant. N I is the rated current of the photovoltaic power station. max The maximum current allowed to flow through the grid-connected inverter.

[0037] As a preferred embodiment of the practical calculation method for short-circuit current of power systems considering the impact of photovoltaic power plants described in this invention, the Thevenin equivalent model includes: an impedance and a voltage source in series; the photovoltaic branch is equivalent to a series branch of a constant potential source and an impedance, and the circuit is processed by unified calculation.

[0038] As a preferred embodiment of the practical calculation method for short-circuit current of power system considering the impact of photovoltaic power plants described in this invention, the short-circuit fault calculation method includes: calculating the short-circuit single current using the node impedance matrix based on three-phase symmetrical faults and asymmetrical faults.

[0039] The node impedance matrix is ​​expressed as:

[0040]

[0041] Among them, Z ij (i,j=1,2,…,n and i≠j) represents the mutual impedance between nodes i and j in the nodal impedance matrix, Z i Let be the self-impedance of node i. Let be the voltage deviation at node i. Injected current for photovoltaic grid-connected nodes, Inject current into the fault point.

[0042] As a preferred embodiment of the practical calculation method for short-circuit current of a power system considering the impact of photovoltaic power plants as described in this invention, the calculation of short-circuit current using the node impedance matrix for three-phase symmetrical faults includes:

[0043] The short-circuit current of any branch is expressed as:

[0044]

[0045] Among them, z pq Let be the branch impedance between node p and node q in the power network. Let be the voltage at any node p in the power network. Let be the voltage at any node q in the power network. Let be the branch current between nodes p and q in the power network, and k be the turns ratio of the transformer branch. For non-transformer branches, let k = 1.

[0046] As a preferred embodiment of the practical calculation method for short-circuit current of power systems considering the impact of photovoltaic power plants described in this invention, the calculation of short-circuit current using the node impedance matrix for asymmetrical faults includes:

[0047] The network is decomposed into positive-sequence, negative-sequence, and zero-sequence networks using the symmetric component method. For power systems connected to photovoltaic power plants, voltage estimation is performed only at the grid connection point in the positive-sequence network to obtain the parameters of the Thevenin equivalent model. The positive-sequence network is modified by adding the calculated photovoltaic equivalent synchronous machine model to the positive-sequence network without considering the impact of photovoltaic access. The traditional asymmetric fault short-circuit calculation method is used to calculate and correct the short-circuit current distribution of the system connected to the photovoltaic power plant.

[0048] The beneficial effects of this invention are as follows: The practical calculation method for short-circuit current in power systems provided by this invention reasonably estimates the grid-connected voltage of photovoltaic power plants after a fault, calculates equivalent model parameter values, corrects the short-circuit current distribution, and provides clear and straightforward calculation steps. It is compatible with existing calculation methods in setting software, requires less time, has a fast calculation speed, and is highly feasible for engineering applications. It does not require specific photovoltaic power plant parameters or the construction of a detailed photovoltaic power plant model, thus improving calculation efficiency. It avoids the iterative process and convergence problems in theoretical calculations and analyses, and the calculation error is within the allowable range for engineering applications. It can verify or re-set the relay protection setting values ​​of systems with photovoltaic power plants connected to the grid, and plan and guide the capacity of photovoltaic power plants connected to the system. Attached Figure Description

[0049] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort. Wherein:

[0050] Figure 1 This is a flowchart illustrating a practical calculation method for short-circuit current in a power system considering the impact of photovoltaic power plants, as described in one embodiment of the present invention.

[0051] Figure 2 This is a schematic diagram of the symmetrical fault short-circuit analysis calculation principle of a practical calculation method for short-circuit current of a power system considering the influence of photovoltaic power plants, as described in one embodiment of the present invention.

[0052] Figure 3 This is a schematic diagram of the IEEE 33-node system structure of a practical calculation method for short-circuit current of a power system considering the impact of photovoltaic power plants, as described in an embodiment of the present invention.

[0053] Figure 4 The Thevenin equivalent model diagram is shown in one embodiment of the present invention for a practical calculation method of short-circuit current of power system considering the influence of photovoltaic power plants;

[0054] Figure 5 This is a schematic diagram illustrating the asymmetric fault short-circuit analysis calculation principle of a practical calculation method for power system short-circuit current considering the influence of photovoltaic power plants, as described in one embodiment of the present invention. Detailed Implementation

[0055] To make the above-mentioned objects, features, and advantages of the present invention more apparent and understandable, specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the protection scope of the present invention.

[0056] Many specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways different from those described herein, and those skilled in the art can make similar extensions without departing from the spirit of the invention. Therefore, the invention is not limited to the specific embodiments disclosed below.

[0057] Secondly, the term "one embodiment" or "embodiment" as used herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the present invention. The phrase "in one embodiment" appearing in different places in this specification does not necessarily refer to the same embodiment, nor is it a single or selective embodiment that is mutually exclusive with other embodiments.

[0058] This invention is described in detail with reference to the schematic diagrams. When detailing the embodiments of this invention, for ease of explanation, the cross-sectional views illustrating the device structure may be partially enlarged, not adhering to the usual scale. Furthermore, the schematic diagrams are merely examples and should not be construed as limiting the scope of protection of this invention. In actual fabrication, the three-dimensional spatial dimensions of length, width, and depth should be included.

[0059] Furthermore, in the description of this invention, it should be noted that the terms "upper," "lower," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. These terms are used solely for the convenience of describing the invention and for simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the invention. In addition, the terms "first," "second," or "third" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0060] Unless otherwise explicitly specified and limited, the terms "installation," "connection," and "joining" in this invention should be interpreted broadly. For example, they can refer to fixed connections, detachable connections, or integral connections; similarly, they can refer to mechanical connections, electrical connections, or direct connections, or indirect connections through an intermediate medium, or internal connections between two components. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0061] Example 1

[0062] Reference Figures 1-5 This is the first embodiment of the present invention, which provides a practical calculation method for short-circuit current of a power system considering the impact of photovoltaic power plants, including:

[0063] S1: Perform short-circuit fault analysis on photovoltaic power plants and calculate the grid-connected voltage of photovoltaic nodes based on the short-circuit current output by the photovoltaic power plants under different scenarios.

[0064] Furthermore, short-circuit fault analysis of photovoltaic power plants includes:

[0065] The low-voltage ride-through control strategy adopted when considering grid-connected inverter faults in photovoltaic power plants is expressed as follows:

[0066]

[0067]

[0068] Among them, I f-dref I is the reference value for output active current during a fault. f-qref I is the reference value for output reactive current during a fault. N I is the rated current of the photovoltaic power station. max =K×I N To allow the maximum current to flow through the grid-connected inverter, typically K = 1.1-2, U pcc This is the positive sequence voltage at the grid connection point.

[0069] It should be noted that the photovoltaic power station is connected to the grid through a full-power inverter. When a fault occurs, the control strategy of the grid-connected inverter determines the output characteristics of the photovoltaic power station. Low voltage ride-through control can ensure that when the grid fails, the voltage at the photovoltaic power station's grid connection point decreases, allowing it to continue generating electricity without immediately disconnecting from the grid.

[0070] Furthermore, the approximate relationship between the output short-circuit current of a photovoltaic power station and the positive-sequence voltage at the grid connection point includes:

[0071] The approximate relation is expressed as:

[0072]

[0073] Among them, I pv For the output short-circuit current of the photovoltaic power station, I f-dref I f-qref This provides reference values ​​for output active and reactive current during a fault; U pcc This is the positive sequence voltage at the grid connection point.

[0074] It should be noted that the output current is given by the low-voltage control strategy of the grid-connected photovoltaic power station, exhibiting nonlinear voltage-controlled current source characteristics; under short-circuit faults, there is an approximate relationship between the output short-circuit current of the photovoltaic power station and the positive sequence voltage at the grid connection point, indicating that the short-circuit current provided by the photovoltaic is related to the voltage at its connection point.

[0075] Furthermore, the calculation of the photovoltaic grid-connected node voltage includes:

[0076] When the influence of the short-circuit current output by the grid-connected photovoltaic power plant is ignored, the grid-connected node voltage of the photovoltaic power plant is expressed as:

[0077]

[0078] in, Let be the voltage at any node i in the power network. Z represents the normal voltage at fault point f before a short circuit in the power network. if Z represents the mutual impedance between node i and faulty node f in a power network. ff Let i be the self-impedance of the faulty node f in the power network, when node i is a grid connection point.

[0079] It should be noted that when performing short-circuit fault analysis and calculation, the influence of the photovoltaic power station output current on the system short-circuit current is initially ignored. The grid connection point voltage U is obtained in the setting calculation software using the traditional short-circuit current calculation method. pcc-pvmin Because the photovoltaic power station outputs a certain amount of reactive current during a fault, which provides some support for the system voltage, the calculated voltage value is lower than the actual value.

[0080] If all photovoltaic power stations in the system output the maximum short-circuit current, and this short-circuit current is reactive current and does not contain active components, the photovoltaic grid-connected node voltage is expressed as:

[0081]

[0082] in, The voltage at grid connection point p of the photovoltaic power station after the fault. Z represents the grid connection voltage of the photovoltaic power station before the fault. pp Z is the self-impedance of node p. ff Z is the self-impedance of node f. pf Let z be the mutual impedance between nodes p and f in the nodal impedance matrix. f For the transition resistance at fault point f, when node p is a grid connection point,

[0083] It should be noted that, for the fault component network of a photovoltaic power station, the impact of the photovoltaic power station on the system is an increase of magnitude I at the photovoltaic grid-connected nodes in the fault component network. max The injected reactive current is assumed to be the maximum current output by all photovoltaic power stations, and all of them are reactive currents. In this case, the calculated voltage value is higher than the actual value.

[0084] S2: The mean value of the grid-connected node voltage of the photovoltaic system is the estimated value of the grid-connected node voltage after the fault.

[0085] Furthermore, the mean voltage of the photovoltaic grid-connected node is calculated as an estimated value of the grid-connected node voltage after the fault, including:

[0086] The estimated voltage at the grid connection point after the fault is expressed as follows:

[0087]

[0088] Among them, U pcc-pvmin U is the grid-connected photovoltaic node voltage when the output current is zero. pcc-pvmax The maximum output current is the voltage of the photovoltaic grid-connected node.

[0089] S3: Calculate the equivalent impedance value in the Thevenin equivalent model based on the estimated voltage at the grid connection point after the fault;

[0090] Furthermore, the equivalent impedance value is calculated based on the estimated voltage at the grid connection point after the fault, including:

[0091] The equivalent impedance value is expressed as:

[0092]

[0093] Among them, z pvU is the equivalent impedance of the photovoltaic model, E is the value of the constant potential source, usually 1, and U pcc-est I is the estimated voltage at the grid connection point of the photovoltaic power plant. N I is the rated current of the photovoltaic power station. max The maximum current allowed to flow through the grid-connected inverter.

[0094] It should be noted that when the voltage drop is less than 0.9 pu, the fault current is included in the load current, and the impact of the short-circuit current output from the photovoltaic power station to the system is ignored; when the voltage drop is less than or equal to 0.9 pu and is severe, the output current from the photovoltaic power station to the system is considered.

[0095] Furthermore, the Davy Long equivalent model includes:

[0096] An impedance and a voltage source in series; the photovoltaic branch is equivalent to a constant potential source and an impedance in series branch, and the circuit is processed in a unified manner.

[0097] It should be noted that the photovoltaic model is a current source model, while the traditional power supply is a voltage source model. The Norton branch is converted into a Thevenant branch so that the photovoltaic power supply can be treated like the traditional power supply. The Thevenant equivalent model is completely consistent with the traditional synchronous machine equivalent model in form. The potential source is a constant value, and the impedance value is selected based on the voltage drop at the grid connection point. Reasonably selecting the equivalent impedance value ensures that the output current during a photovoltaic power station fault is consistent with the actual output current, which facilitates unified processing of all power supplies.

[0098] S4: Connect the photovoltaic fault equivalent branch at the grid connection point of the photovoltaic power station in the positive sequence network of the system, and use short-circuit fault calculation to obtain the short-circuit current distribution of the entire network.

[0099] Furthermore, traditional short-circuit fault calculation methods include: calculating the short-circuit single current using the node impedance matrix based on three-phase symmetrical faults and asymmetrical faults;

[0100] The node impedance matrix is ​​expressed as:

[0101]

[0102] Among them, Z ij (i,j=1,2,…,n and i≠j) represents the mutual impedance between nodes i and j in the nodal impedance matrix, Z i Let be the self-impedance of node i. Let be the voltage deviation at node i. Injected current for photovoltaic grid-connected nodes, Inject current into the fault point.

[0103] Furthermore, the calculation of short-circuit current using the node impedance matrix in a three-phase symmetrical fault includes:

[0104] The short-circuit current of any branch is expressed as:

[0105]

[0106] Among them, z pq Let be the branch impedance between node p and node q in the power network. Let be the voltage at any node p in the power network. Let be the voltage at any node q in the power network. Let be the branch current between nodes p and q in the power network, and k be the turns ratio of the transformer branch. For non-transformer branches, let k = 1.

[0107] Furthermore, the calculation of short-circuit current using the node impedance matrix in asymmetric faults includes:

[0108] The network is decomposed into positive-sequence, negative-sequence, and zero-sequence networks using the symmetric component method. For power systems connected to photovoltaic power plants, voltage estimation is performed only at the grid connection point in the positive-sequence network to obtain the parameters of the Thevenin equivalent model. The positive-sequence network is modified by adding the calculated photovoltaic equivalent synchronous machine model to the positive-sequence network without considering the impact of photovoltaic access. The traditional asymmetric fault short-circuit calculation method is used to calculate and correct the short-circuit current distribution of the system connected to the photovoltaic power plant.

[0109] It should be noted that the method of the present invention does not require detailed model parameters of the photovoltaic power station. It uses the series impedance branch of the constant potential source to characterize the impact of the photovoltaic power station grid connection on the system short-circuit current, which is consistent with the traditional synchronous machine fault model. This avoids the iterative calculation process in the theoretical solution of short-circuit current, is compatible with the short-circuit current calculation method in existing software, has a fast calculation speed, and does not have convergence problems.

[0110] Example 2

[0111] Reference Figures 1-5 This is the second embodiment of the present invention. To verify the beneficial effects, scientific verification is carried out through a specific node network.

[0112] The IEEE 33-node network structure diagram is as follows: Figure 3 As shown, the system rated voltage is 10kV, the reference voltage is taken as 10.5kV, and the reference power is S. B =100MVA, and the resistance and reactance values ​​of each branch are shown in Table 1. Photovoltaic power station PV1-5 is connected at nodes 11, 20, 25, 28, and 33 respectively. The rated capacities of PV1-5 are 1MW, 4MW, 3MW, 2MW, and 2MW respectively. The power factor is 1 during normal operation. A control strategy based on positive sequence component control is adopted, and K=1.2 in the control loop.

[0113] Table 1 Branch Parameters

[0114]

[0115]

[0116] Assume a three-phase short-circuit ground fault occurs at node 7, with a transition resistance of 1Ω. All electrical quantities are per unit. Assume that the bus voltage is 1 before the fault. After the fault, to prevent the formation of an island, PV1 of the photovoltaic power station should be taken out of operation. The output characteristics of PV1 are not considered.

[0117] The voltage U of each photovoltaic grid-connected node with the output current of PV2-PV5 in the photovoltaic power station set to zero is obtained by short-circuit current calculation. pcc-pvmin ;

[0118] Modify the fault component, represented as:

[0119]

[0120] Based on the modified fault components, the voltage U of each photovoltaic grid-connected node at the maximum output current of PV2-PV5 of the photovoltaic power station is calculated using the short-circuit current. pcc-pvmax According to U pcc-pvmin and U pcc-pvmax The average voltage U is calculated. pcc-est The estimated voltage at the grid connection point after the fault and the calculated equivalent impedance are shown in Table 2.

[0121] Table 2 Grid Connection Point Voltage

[0122]

[0123]

[0124] Combination Figure 4 and Figure 5 When a photovoltaic fault equivalent branch is connected at the grid connection point of a photovoltaic power station in the positive sequence network, and a three-phase short-circuit ground fault occurs at node 7 in the IEEE 33-node network with a transition resistance of 1Ω, considering the impact of the photovoltaic power station connection on the system short-circuit current, the system short-circuit current is corrected using the traditional symmetrical or asymmetrical fault short-circuit current calculation method. The theoretical calculation results obtained by applying the accurate iterative method are shown in Table 3.

[0125] Table 3 Calculation Error

[0126]

[0127] As shown in Table 3, the corrected short-circuit current considering the photovoltaic effect of the method of the present invention is less than the short-circuit current without considering the photovoltaic grid connection effect. The smaller the result error, the higher the calculation accuracy of the Thevenin equivalent model. The Thevenin equivalent model used in the method of the present invention is consistent with the synchronous generator model in the existing setting calculation software. There is no need to add a new equivalent model. It is compatible with the existing traditional short-circuit current calculation method, which is easy to implement. The calculation result error is within the allowable range of engineering, and it has practical application value.

[0128] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A practical calculation method for short-circuit current in a power system considering the impact of photovoltaic power plants, characterized in that, include: Short-circuit fault analysis is performed on photovoltaic (PV) power plants. The voltage at the PV grid-connected node is calculated based on the short-circuit current under different output scenarios. This analysis includes considering the low-voltage ride-through control strategy employed when the PV power plant's grid-connected inverter fails, as expressed below: ； ； in, I f-dref This is the reference value for output active current during a fault. I f-qref This is the reference value for output reactive current during a fault. I N The rated current of the photovoltaic power station I max = K I N To allow the maximum current to flow through the grid-connected inverter, U pcc This is the positive sequence voltage at the grid connection point; The approximate relationship between the output short-circuit current of a photovoltaic power plant and the positive-sequence voltage at the grid connection point includes: The approximate relationship is expressed as: ； in, I pv To output short-circuit current for photovoltaic power plants, I f-dref , I f-qref Provides reference values ​​for output active and reactive current during a fault. U pcc The positive sequence voltage at the grid connection point; calculation of the photovoltaic grid-connected node voltage includes: When the influence of the short-circuit current output by the grid-connected photovoltaic power plant is ignored, the grid-connected node voltage of the photovoltaic power plant is expressed as: ； in, For any node in the power network i voltage, The point of failure before a short circuit in the power network f Normal voltage; Nodes in a power network i and faulty nodes f mutual impedance between Faulty nodes in the power network f The self-impedance of the node i When it is a grid connection point, U pcc-pvmin = ; If all photovoltaic power stations in the system output the maximum short-circuit current, and this short-circuit current is reactive current and does not contain active components, the photovoltaic grid-connected node voltage is expressed as: ； in, For the grid connection point of the photovoltaic power station after the fault p voltage, The voltage at the grid connection point of the photovoltaic power station before the fault. For nodes p Self-impedance, For nodes f Self-impedance, For nodes in the nodal impedance matrix p and f Mutual impedance between them z f Fault point f Transition resistance, when node p When it is a grid connection point, U pcc-pvmin = ; The average voltage of the photovoltaic grid-connected node is calculated as an estimated value of the grid-connected node voltage after a fault, specifically including: The estimated voltage at the grid connection point after the fault is expressed as follows: ； in, U pcc-pvmin Photovoltaic grid-connected node voltage when output current is zero ，U pcc-pvmax This is the voltage at the photovoltaic grid-connected node when the output current reaches its maximum value. The equivalent impedance value in the Thevenin equivalent model is calculated based on the estimated voltage at the grid connection point after the fault. This calculation includes: The equivalent impedance value is expressed as: ； in, z pv Here, E represents the equivalent impedance of the photovoltaic model, and E is the value of the constant potential source. U pcc-est This is the estimated voltage at the grid connection point of the photovoltaic power plant. I N The rated current of the photovoltaic power station I max The maximum current allowed to flow through the grid-connected inverter; A photovoltaic fault equivalent branch is connected at the grid connection point of the photovoltaic power station in the positive sequence network of the system, and the short-circuit current distribution of the entire network is obtained by short-circuit fault calculation.

2. The practical calculation method for short-circuit current of a power system considering the impact of photovoltaic power plants as described in claim 1, characterized in that, The Thevenin equivalent model includes: an impedance and a voltage source in series; the photovoltaic branch is equivalent to a constant potential source and an impedance in series, and the circuit is processed in a unified manner.

3. A practical calculation method for short-circuit current in a power system considering the impact of photovoltaic power plants, as described in claim 2, is characterized in that... Short-circuit fault calculation methods include: calculating the short-circuit single current using the node impedance matrix based on three-phase symmetrical faults and asymmetrical faults; The node impedance matrix is ​​expressed as: ； in, For nodes in the nodal impedance matrix and Mutual impedance between them For nodes Self-impedance, For nodes Voltage deviation Injected current for photovoltaic grid-connected nodes, Inject current into the fault point.

4. A practical calculation method for short-circuit current in a power system considering the impact of photovoltaic power plants, as described in claim 3, is characterized in that... Calculating the short-circuit current in a three-phase symmetrical fault using the aforementioned node impedance matrix includes: The short-circuit current of any branch is expressed as: ； in, Nodes in a power network and nodes Branch impedance between For any node in the power network voltage, For any node in the power network voltage, Nodes in a power network and nodes Branch currents between Let be the turns ratio of the transformer branch. For the non-transformer branch, let be... .

5. A practical calculation method for short-circuit current in a power system considering the impact of photovoltaic power plants, as described in claim 4, characterized in that... Calculating the short-circuit current using the aforementioned node impedance matrix for asymmetric faults includes: The network is decomposed into positive-sequence, negative-sequence, and zero-sequence networks using the symmetric component method. For power systems connected to photovoltaic power plants, voltage estimation is performed only at the grid connection point in the positive-sequence network to obtain the parameters of the Thevenin equivalent model. The positive-sequence network is modified by adding the calculated photovoltaic equivalent synchronous machine model to the positive-sequence network without considering the impact of photovoltaic access. The traditional asymmetric fault short-circuit calculation method is used to calculate and correct the short-circuit current distribution of the system connected to the photovoltaic power plant.

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