A method for calculating maximum short-circuit capacity of a power system considering access of a photovoltaic power station

Abstract

The application discloses a kind of power system maximum short-circuit capacity calculation method considering photovoltaic power station access, belongs to the technical field of power system short-circuit calculation and analysis, solve the influence problem of photovoltaic power station grid connection to short-circuit current calculation, the application is not considered when the access of photovoltaic power station, using node injection current method to calculate the voltage of each point in system and fault point three-phase short-circuit current;When considering the access of photovoltaic power station, according to the constraint relationship between phase angle and the maximum short-circuit current provided by photovoltaic power station at short-circuit time, the short-circuit current provided by each photovoltaic power station is calculated respectively, then added to the short-circuit current calculated when not considering the access of photovoltaic power station, then the short-circuit current of fault branch is corrected, finally the short-circuit capacity calculation result is output;The method of the application avoids the iteration calculation process in the step of theoretically solving short-circuit current, has fast calculation speed, strong realizability and no convergence problem, according to actual power system, has higher short-circuit capacity calculation accuracy.

Description

Technical Field

[0001] This invention belongs to the field of power system short-circuit calculation and analysis technology, and relates to a method for calculating the maximum short-circuit capacity of a power system considering the connection of photovoltaic power plants. Background Technology

[0002] In a power network, a sudden increase in current caused by various connections or contact is called a short circuit. When a short circuit fault occurs in a power system, the faulty section needs to be quickly disconnected so that the remaining sections can continue operating. Short-circuit capacity refers to the apparent power of a power system under specified operating conditions when a three-phase short circuit occurs at the fault point. It is a characteristic parameter characterizing the power supply capacity of the power system, and its magnitude is equal to the product of the short-circuit current and the rated voltage of the short circuit. The purpose of short-circuit calculation is to correctly select and verify electrical equipment, accurately set the protection devices of the power supply and distribution system, avoid damage to electrical equipment under the action of short-circuit current, and ensure that the protection devices can operate reliably when a short circuit occurs in the power supply and distribution system.

[0003] In recent years, with the continuous expansion of grid-connected photovoltaic (PV) power generation, the impact of PV power plants on the power grid has begun to attract much attention. The impact of large-scale PV grid connection on the short-circuit current of the power grid is mainly reflected in the following aspects: First, both substations and PV power plants inject short-circuit current into the grid short-circuit point, making the short-circuit current at the short-circuit point larger than when no PV power plant is connected; second, the short-circuit current at the fault point is directly proportional to the PV grid connection capacity; third, according to the national standard GB / T 19939—2005 "Technical Requirements for Grid Connection of Photovoltaic Systems", the maximum short-circuit current supplied by PV power sources when a short circuit occurs in the distribution network shall not exceed 1.5 times the rated current of the PV; fourth, under the condition of symmetrical grid faults, the inverter distributed power sources only output positive-sequence current, and there is no negative-sequence or zero-sequence current. Therefore, based on the ordinary grid short-circuit current calculation method, it is of great significance to study the short-circuit current calculation method that considers the impact of PV power plant grid connection. Summary of the Invention

[0004] The purpose of this invention is to design a method for calculating the maximum short-circuit capacity of a power system that takes into account the grid connection of photovoltaic power plants, so as to solve the problem of the impact of grid connection of photovoltaic power plants on the calculation of short-circuit current.

[0005] The present invention solves the above-mentioned technical problems through the following technical solutions:

[0006] A method for calculating the maximum short-circuit capacity of a power system considering photovoltaic power plant integration includes the following steps:

[0007] S1. Import the node parameter matrix, line parameter matrix, fault parameter matrix, and photovoltaic parameter matrix in the specified format;

[0008] S2. Without considering the connection of photovoltaic power stations, the nodal injection current method is used to calculate the voltage at each point in the system and the three-phase short-circuit current at the fault point.

[0009] S3. Considering the phase angle at the moment of short circuit, according to the different phase angles at the moment of short circuit, select 1 to 1.5 times the rated current of each photovoltaic power station as the maximum short circuit current that it can provide, and add it to the short circuit current calculated in step S2 without considering the photovoltaic power source to obtain the short circuit current after considering the power system connected to the photovoltaic power station.

[0010] The phase angle β at the moment of short circuit and the maximum short-circuit current I provided by the photovoltaic power station T The constraints between them are:

[0011]

[0012] Among them, I N This refers to the rated output current of the photovoltaic power station.

[0013] S4. Calculate the short-circuit capacity and output the calculation result. The formula for calculating the short-circuit capacity is as follows:

[0014]

[0015] In the formula, S f For three-phase short-circuit capacity, U N I represents the effective value of the rated line voltage before the fault point. f This represents the effective value of the line current at the fault point.

[0016] Further, the node parameter matrix described in step S1 is composed of: node number (bus#), node voltage (volt), node voltage phase angle (ang), node active power (p), node reactive power (q), node type (bus_type), node unit judgment (if_set), and generator node subtransient reactance (Xd”); the line parameter matrix is ​​composed of: line start (b#1), line end (b#2), line resistance (R), line reactance (X), line conductance (G), and line susceptance (B / 2); the fault parameter matrix is ​​composed of: fault branch start node (fault_b#1), fault branch end node (fault_b#2), distance from start (represented by 0-1) (fault_dis), and fault phase angle (fault_phase); the photovoltaic parameter matrix is ​​composed of: access point number (pv_bus#), photovoltaic capacity (pv_p), and rated current (pv_i).

[0017] Furthermore, the steps in step S2 for calculating the voltage at each point in the system and the three-phase short-circuit current at the fault point using the nodal injection current method are as follows:

[0018] S21. The node admittance matrix of the system is formed by the aforementioned line parameter matrix;

[0019] S22. Correct the nodal admittance matrix to obtain the corrected nodal admittance matrix;

[0020] S23. Determine the node injection current during a fault.

[0021] S24. Solve for the node voltage based on the node injection current;

[0022] S25. Calculate the short-circuit current of each branch to obtain the three-phase short-circuit current at the fault point.

[0023] Further, the method for forming the node admittance matrix of the system described in step S21 is as follows: Since the order n of the node admittance matrix Y of the network is equal to the number of nodes in the power network, an n-order zero matrix is ​​first created. The admittance of each transmission line is obtained through the line parameter matrix in step (1). According to the definition of self-admittance and mutual admittance, the calculated admittance value is used to replace the zero element at the corresponding position in the zero matrix to obtain the node admittance matrix of the distribution network.

[0024] Furthermore, the method for correcting the nodal admittance matrix in step S22 is as follows:

[0025] The correction of the node admittance matrix includes three aspects: generator node correction, fault point correction, and load node correction.

[0026] The generator node, as an active branch, is represented by the potential source Ei and the impedance Z. i For series branches, the equivalent current source I is used in short-circuit current calculations. i With ground-guided y i The transformation process is as follows:

[0027]

[0028] After the circuit equivalent transformation, generator node i has an additional ground-connected branch. Therefore, the i-th element on the diagonal of the node admittance matrix needs to be corrected. The correction formula is as follows:

[0029] fixY ii =fixY ii +1 / jX d "

[0030] Among them, fixY ii To correct the i-th diagonal element of the admittance matrix;

[0031] The photovoltaic power station is connected to the grid by an inverter, and the inverter has no subtransient reactance, so there is no need to make any modifications to the photovoltaic power station.

[0032] In practical applications, the impact of the load on the short-circuit current is usually ignored, so there is no need to correct the load.

[0033] When a three-phase short-circuit fault occurs in a power system, the structure of the power grid changes, so the node admittance matrix also needs to be corrected. Three-phase short-circuit faults in power systems are divided into two types: node three-phase short-circuit faults and branch three-phase short-circuit faults.

[0034] The two cases are distinguished based on the third column of the fault matrix, "fault distance":

[0035] When a three-phase short circuit occurs at node j, the impedance to ground is infinitely small, therefore the admittance to ground is infinitely large. Here, we change this to 999999, as shown in the following equation:

[0036] fixY jj =999999+j999999

[0037] When the faulty node is connected to more nodes, the correction method is similar;

[0038] When a three-phase short circuit occurs in branch ij, the power grid structure will also change. The node admittance matrix is ​​modified sequentially as follows, assuming a fault occurs in the middle of the branch:

[0039] fixy ii =fixy ii -y ij fixy jj =fixy jj -y ij fixy ji =fixy ji =0

[0040] fixy ii =fix ii +y ij / 0.5,fixy jj =fixy jj +y ij / 0.5

[0041] The 0.5 in the formula is determined by the "distance from the head" in the fault parameters.

[0042] Furthermore, the method for determining the node injection current during a fault, as described in step S23, is as follows: first, the voltage source is equivalently transformed into a current source, and then the node injection current is calculated. This is an n-dimensional column vector, where n is the number of nodes in the power system. After transforming the generator nodes from potential sources to current sources, the injected current of the generator nodes during a fault is calculated, thus generating a column vector of node injected currents. The formula is: In practical calculations of short-circuit current, it is approximated that the potential of all generator nodes is 1. Therefore, the formula is transformed as follows:

[0043] Further, the method for solving the node voltage based on the node injection current in step S24 is as follows: After obtaining the modified node admittance matrix and the node injection current column vector, the node voltage is obtained by solving a system of linear equations, as shown in the formula: in, This is the corrected nodal admittance matrix.

[0044] Furthermore, the method for determining the short-circuit current of each branch in step S25 is as follows:

[0045] After obtaining the node voltage, the solution for the short-circuit current of non-fault-related branches is the same as that for power flow calculation. In order to facilitate the calculation of the short-circuit fault point current of the node, the branch connected to the fault node can be treated as a non-fault-related branch for calculation. For power systems without photovoltaic access, the final short-circuit current result can be directly calculated.

[0046] The short-circuit current calculation result is represented by a (nl+1)×4 matrix, where nl is the number of branches in the original data; the first two columns of the matrix are the first and last node numbers, the third column is the short-circuit current phasor, and the fourth column is the effective value of the short-circuit current; the first nl rows of the matrix are the branches corresponding to the line matrix in the original data and their short-circuit current values; the last row of the matrix is ​​the short-circuit current value at the fault point.

[0047] The formula for the short-circuit current at the fault point of a three-phase short circuit at a node is:

[0048]

[0049] Formula for short-circuit current at the fault point of a three-phase short circuit in a branch (taking a fault distance of 0.5 as an example):

[0050]

[0051] This allows us to obtain the three-phase short-circuit current at the fault point without considering the photovoltaic power station.

[0052] The advantages of this invention are:

[0053] The method of this invention calculates the voltage at each point in the system and the three-phase short-circuit current at the fault point using the nodal injection current method when the photovoltaic power station is not considered. When the photovoltaic power station is considered, the short-circuit current provided by each photovoltaic power station is calculated separately according to the constraint relationship between the phase angle at the time of the short circuit and the maximum short-circuit current provided by the photovoltaic power station. These are accumulated and added to the short-circuit current calculated when the photovoltaic power station is not considered. Then, the short-circuit current of the fault branch is corrected, and finally the short-circuit capacity calculation result is output. The method of this invention avoids the iterative calculation process in the theoretical solution of the short-circuit current. It has a fast calculation speed, strong feasibility, and no convergence problem. The calculation based on the actual power system can verify that the method has a high accuracy in short-circuit capacity calculation and can lay the foundation for the protection and setting of power systems containing photovoltaic power stations. Attached Figure Description

[0054] Figure 1 This is a flowchart illustrating the method for calculating the maximum short-circuit capacity of a power system considering photovoltaic power plant access, as described in an embodiment of the present invention.

[0055] Figure 2 This is a schematic diagram of the access topology of a single photovoltaic power station in a power system.

[0056] Figure 3 This is a schematic diagram of the topology for multiple photovoltaic power stations connected to a certain power system.

[0057] Figure 4 Equivalent circuit diagram of generator node;

[0058] Figure 5 Equivalent circuit diagram for node short-circuit fault;

[0059] Figure 6 Equivalent circuit diagram for a branch short-circuit fault;

[0060] Figure 7 A comparison chart of the short-circuit capacity values ​​calculated by the method of this invention under different photovoltaic access conditions and different fault conditions;

[0061] Figure 8 A comparison chart showing the short-circuit capacity values ​​calculated by the method of this invention under different photovoltaic access conditions and different short-circuit time phase angles. Detailed Implementation

[0062] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0063] The technical solution of the present invention will be further described below with reference to the accompanying drawings and specific embodiments:

[0064] Example 1

[0065] like Figure 1 As shown, an embodiment of the present invention provides a method for calculating the maximum short-circuit capacity of a power system considering photovoltaic power plant access, comprising the following steps:

[0066] 1. Import node parameter matrix (bus matrix), line parameter matrix (ine matrix), fault parameter matrix (fault matrix), and photovoltaic parameter matrix (PV matrix) data.

[0067] The data formats for the bus matrix, line matrix, fault matrix, and PV matrix are as follows:

[0068] The node parameter matrix (bus matrix) consists of: node number (bus#), node voltage (volt), node voltage phase angle (ang), node active power (p), node reactive power (q), node type (bus_type), node unit determination (if_set), and generator node subtransient reactance (Xd”).

[0069] The line parameter matrix consists of: line start (b#1), line end (b#2), line resistance (R), line reactance (X), line conductance (G), and line susceptance (B / 2).

[0070] The fault parameter matrix consists of: the first node of the faulty branch (fault_b#1), the last node of the faulty branch (fault_b#2), the distance from the first node (represented by 0-1) (fault_dis), and the fault phase (fault_phase);

[0071] The photovoltaic parameter matrix (PV matrix) consists of: access point number (PV_bus#), photovoltaic capacity (PV_p), and rated current (PV_i).

[0072] The system data above is in .xls format and was read into the workspace using the xlsread function built into the Matlab system.

[0073] 2. Without considering the grid connection of photovoltaic power plants, the nodal injection current method is used to calculate the voltage at each point in the system and the three-phase short-circuit current at the fault point.

[0074] like Figure 2 As shown, the steps for calculating the voltage at each point in the system and the three-phase short-circuit current at the fault point using the nodal injection current method are as follows:

[0075] 2.1. Using the line parameter matrix from step 1, the nodal admittance matrix of the system is formed as follows:

[0076] The diagonal element Y of the nodal admittance matrix ii (i = 1, 2, ..., n) is called the self-admittance. Self-admittance Y ii Numerically, this is equivalent to the current injected into the network through node i when a unit voltage is applied to node i and all other nodes are grounded. In a specific power system network, the self-admittance Y of node i... ii Numerically, it is equal to the sum of the admittances of all branches directly connected to that node.

[0077] The off-diagonal element Y of the nodal admittance matrix ji (j = 1, 2, ..., n; i = 1, 2, ..., n; j ≠ i) are called mutual admittance. Mutual admittance Y ji Numerically, this is equivalent to the current injected into the network through node j when a unit voltage is applied to node i and all other nodes are grounded. In a specific power system network, the mutual admittance Y between nodes j and i is... ji Numerically, this is equal to the negative of the admittance of the branch connecting nodes j and i. Clearly, Y ji The identity is equal to Y ij Furthermore, if there is no direct connection between nodes j and i, Y ji =Y ij =0.

[0078] Since the order n of the node admittance matrix Y of the network is equal to the number of nodes in the power network, an n-order zero matrix is ​​first created in Matlab. Using the line parameter matrix from step 1, the admittance of each transmission line can be obtained. According to the definitions of self-admittance and mutual admittance, the calculated admittance values ​​are used to replace the corresponding zero elements in the zero matrix to obtain the node admittance matrix of the distribution network. To simplify the calculation, the transformer turns ratio can be approximated as 1, and a simple branch equivalence method is used, i.e., ignoring the real part of all branch impedances and only taking their imaginary parts, and ignoring all nodes' ground-connected branches.

[0079] The above process is written as a Ymatrix function and encapsulated. The Ymatrix function is used to calculate the node and line information extracted in step 1 to obtain the uncorrected node admittance matrix Y.

[0080] 2.2. The nodal admittance matrix is ​​corrected to obtain the corrected nodal admittance matrix. The specific correction method is as follows:

[0081] The correction of the node admittance matrix mainly includes three aspects: generator node correction, fault point correction, and load node correction.

[0082] The correction method for generator nodes is as follows: Figure 4As shown, a generator node, as an active branch, is typically represented by a potential source Ei and an impedance Z. i (i.e. jX) d In the short-circuit current calculation, the equivalent current source I is used for the series branch (the "branch" between terminal node i and the zero potential point). i With ground-guided y i The transformation process is as follows:

[0083]

[0084] After the circuit equivalent transformation, generator node i has an additional ground-connected branch. Therefore, the i-th element on the diagonal of the node admittance matrix needs to be corrected. The correction formula is as follows:

[0085] fixY ii =fixY ii +1 / jX d "

[0086] Among them, fixY ii This is to correct the i-th diagonal element of the admittance matrix.

[0087] Photovoltaic power plant installations are typically connected to the grid via inverters. Since inverters do not have subtransient reactances, no modifications are needed for the photovoltaic power plant installations. In practical applications, the impact of the load on the short-circuit current is usually ignored, therefore no load modifications are required.

[0088] Three-phase short-circuit faults in power systems are classified into two types: node-based three-phase short-circuit faults and branch-based three-phase short-circuit faults. These two types are distinguished by the third column of the fault matrix, "Fault Distance." Here, a value of 0 in the third column of the fault matrix indicates that the three-phase short circuit occurred at a node. Figure 5 As shown, when a three-phase short circuit occurs at node j, it can be understood that the impedance to ground is infinitely small, and therefore the admittance to ground is infinitely large. Here, we change this to 999999, as shown in the following equation:

[0089] fixY jj =999999+j999999

[0090] When the faulty node is connected to more nodes, the correction method is similar.

[0091] When a three-phase short circuit occurs in branch ij, such as Figure 6 As shown, the power grid structure will also change. Taking a fault occurring in the middle of a branch as an example (dis = 0.5), the node admittance matrix is ​​modified as follows:

[0092] fixy ii =fixy ii -y ij fixyjj =fixy jj -y ij fixy ji =fixy ji =0

[0093] fixy ii =fix ii +y ij / 0.5,fixy jj =fixy jj +y ij / 0.5

[0094] The 0.5 in the formula is determined by the "distance from the head" in the fault parameters.

[0095] Write the above process as a FixY function and encapsulate it. The FixY function must contain a check for the correction type. Use the FixY function to correct the nodal admittance matrix obtained in step 2.1, and obtain the corrected nodal admittance matrix, named fixY.

[0096] 2.3. The specific method for determining the node injection current during a fault is as follows:

[0097] First, the voltage source is equivalently transformed into a current source, and then the node injection current is calculated. Node injection current. This is an n-dimensional column vector, where n is the number of nodes in the power system. By transforming the generator nodes from potential sources to equivalent current sources, the injected current at the generator nodes during a fault can be calculated, thus generating a column vector of node injected currents.

[0098]

[0099] In practical calculations of short-circuit current, it can be approximated that the potential of all generator nodes is 1. Therefore, the formula can be transformed as follows:

[0100]

[0101] The above process is written as an Inode function and encapsulated. The Inode function is used to solve the corrected node admittance matrix fixY to obtain the node injection current vector, which is named Iinj.

[0102] 2.4 Solving for node voltages based on node injection currents

[0103] Based on the corrected nodal admittance matrix and the nodal injected current column vector, the nodal voltage is obtained by solving a system of linear equations, as shown in the following formula:

[0104]

[0105] Among them, section Let the point injection current column vector be... This is the corrected nodal admittance matrix.

[0106] 2.5. Calculate the short-circuit current of each branch to obtain the three-phase short-circuit current at the fault point.

[0107] The method for determining the short-circuit current of each branch is as follows:

[0108] After obtaining the node voltage, the short-circuit current of non-fault-related branches is calculated in the same way as the power flow calculation.

[0109] "Non-fault-related branches" refer to branches that are not faulty themselves. To facilitate the calculation of the short-circuit fault current at a node, branches connected to the faulty node can be treated as non-fault-related branches. For power systems without photovoltaic access, the final short-circuit current result can be calculated directly.

[0110] In this method, the short-circuit current calculation result can be represented by a (nl+1)×4 matrix, where nl is the number of branches in the original data. The first two columns of the matrix are the node numbers of the beginning and end points, the third column is the short-circuit current phasor, and the fourth column is the effective value of the short-circuit current. The first nl rows of the matrix correspond to the branches and their short-circuit current values ​​in the line matrix of the original data. The last row of the matrix is ​​the short-circuit current value at the fault point.

[0111] As attached Figure 5 As shown, the formula for the short-circuit current at the fault point of a three-phase short circuit at a node is:

[0112]

[0113] As attached Figure 6 As shown, the formula for the short-circuit current at the fault point of a three-phase short circuit in a branch (taking a fault distance of 0.5 as an example):

[0114]

[0115] The above process is written as an `Ibranch` function and encapsulated. Based on the node voltage obtained in step 2.4, the `Ibranch` function is used to calculate the short-circuit current of each branch, and the output result is a `Bcurrent` vector, which is the current value of each branch under the short-circuit fault condition. Based on the location information of the fault point, the magnitude of the short-circuit current at the fault point can be extracted from the `Bcurrent` vector.

[0116] 3. Considering the phase angle at the moment of short circuit, calculate the short-circuit current provided by each photovoltaic power station, and sum them up to the short-circuit current calculated in step 2 to obtain the maximum short-circuit current of the power system considering the photovoltaic power stations connected to the grid.

[0117] The method for calculating the system short-circuit current after considering the integration of photovoltaic power is as follows:

[0118] For a three-phase symmetrical short-circuit fault, the maximum inrush current occurs at a 90° phase angle; the further away from the 90° phase angle, the smaller the inrush current. Given that the short-circuit current provided by a photovoltaic (PV) power station is limited to 1.5 times its rated current, this invention employs a superposition principle, considering the phase angle at the moment of the short circuit. Depending on the phase angle, 1 to 1.5 times the rated current of each PV power station is selected as its maximum available short-circuit current. This maximum current is then added to the short-circuit current calculated without considering the PV power station, forming the system short-circuit current after considering the PV power station's connection. Specifically, the phase angle β at the moment of the short circuit is related to the maximum short-circuit current I provided by the PV power station. T The constraints between them are:

[0119]

[0120] Among them, I N This is the rated output current of the photovoltaic power source.

[0121] The three-phase short-circuit current at the fault point obtained in step 2 without considering the photovoltaic power station is compared with the maximum short-circuit current I provided by the photovoltaic power station after considering the phase angle at the short-circuit moment. T Adding them together gives the maximum short-circuit current of the power system considering the photovoltaic power station.

[0122] 4. Calculate the short-circuit capacity and output the calculation results.

[0123] The method for calculating the short-circuit capacity is as follows:

[0124]

[0125] In the formula, S f For three-phase short-circuit capacity, U N I represents the effective value of the rated line voltage before the fault point. f This represents the effective value of the line current at the fault point.

[0126] During the calculation, U N For the information provided in step 1, I f The product of the two values ​​is the maximum short-circuit current of the power system considering the photovoltaic power station connection obtained in step 3, which is the maximum short-circuit capacity of the power system considering the photovoltaic power station connection.

[0127] Application Example 1:

[0128] In this application example, according to Figure 1 The steps shown are for Figure 2 The short-circuit capacity of the power system containing one photovoltaic power station is calculated. Rated voltage UN Selected as 10kV, reference power S B Choosing 1MW, the current base value is 100A. Information about this power system without considering photovoltaic integration is shown in Tables 1 and 2:

[0129] Table 1 Node information of a small power distribution network system

[0130]

[0131]

[0132] Table 2. Line information of a small power distribution network system

[0133]

[0134]

[0135] First, the short-circuit current and short-circuit capacity of the above-mentioned distribution network without photovoltaic power source were calculated under different short-circuit fault conditions. The calculation results are shown in Table 3.

[0136] Table 3 Calculation results for different fault locations when no photovoltaic power source is connected.

[0137]

[0138] A photovoltaic power source is connected at node 15, and the photovoltaic information is shown in Table 4. Distribution network faults are set as shown in Tables 5 and 6, respectively, according to the appendix... Figure 1 The flowchart shown calculates the maximum short-circuit capacity of the distribution network. The "fault phase angle" is the phase difference between the zero-crossing point of the photovoltaic power source input current and the zero-crossing point of the distribution network current at the moment of the fault occurrence.

[0139] Table 4 Information on Single Photovoltaic Power Systems

[0140] Access point number (pv_bus#) Photovoltaic capacity (pv_p) Rated current (pv_i) 15 0.025 0.025

[0141] Table 5 Calculation results for different fault locations when a single photovoltaic power source is connected.

[0142]

[0143]

[0144] Table 6 Calculation results for different short-circuit angle settings when a single photovoltaic power source is connected.

[0145]

[0146] The calculations above show that connecting a single photovoltaic power source will increase the fault current at different locations in the distribution network. Furthermore, for the same fault location, the timing of the fault will also affect the magnitude of the short-circuit current.

[0147] Verification has shown that the calculation results are close to reality and have high accuracy and persuasiveness.

[0148] Application Example 2:

[0149] This application example is an extension of Application Example 1, adding more photovoltaic (PV) power sources. PV power sources are connected at nodes 2, 4, 7, 14, 15, 20, 21, 25, 26, and 27, respectively. PV information is shown in Table 7. Distribution network faults are configured as shown in Tables 8 and 9, according to... Figure 1 The procedure shown is used to calculate the maximum short-circuit capacity of the distribution network. The test method is the same as when a single photovoltaic power source is connected.

[0150] Table 7 PV Matrix of Multi-PV Power System

[0151]

[0152]

[0153] Table 8 Calculation results for different fault locations when a single photovoltaic power source is connected.

[0154]

[0155] Table 9 Calculation results for different short-circuit angle settings when a single photovoltaic power source is connected.

[0156]

[0157] The calculations above show that connecting multiple photovoltaic (PV) power sources increases the fault current at different locations in the distribution network. Furthermore, the more PV power sources connected and the larger their rated capacity, the greater the increase in short-circuit current. For the same fault location, the timing of the fault also affects the magnitude of the short-circuit current.

[0158] Figure 7 This invention compares the short-circuit capacity values ​​calculated by the method under different photovoltaic grid connection conditions and different fault scenarios. Fault1 to Fault5 correspond to the five short-circuit fault types set in Tables 3, 5, and 8, respectively; Figure 7 It can be seen that as the number of photovoltaic power sources connected increases, the short-circuit capacity will also increase.

[0159] Figure 8 For different photovoltaic grid connection scenarios and different short-circuit timing phase angles, the short-circuit capacity values ​​calculated by the method of this invention are compared, corresponding to the four short-circuit timing phase angles set in Tables 6 and 9. Figure 8 It can be seen that, for the same fault location, the timing of the fault occurrence also affects the magnitude of the short-circuit current.

[0160] Verification has shown that the calculation results of this invention are close to reality and have high accuracy and persuasiveness.

[0161] 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 the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for calculating the maximum short-circuit capacity of a power system considering photovoltaic power plant integration, characterized in that, Includes the following steps: S1. Import the node parameter matrix, line parameter matrix, fault parameter matrix, and photovoltaic parameter matrix in the specified format; S2. Without considering the connection of photovoltaic power stations, the nodal injection current method is used to calculate the voltage at each point in the system and the three-phase short-circuit current at the fault point. S3. Considering the phase angle at the moment of short circuit, according to the different phase angles at the moment of short circuit, select 1 to 1.5 times the rated current of each photovoltaic power station as the maximum short circuit current that it can provide, and add it to the short circuit current calculated in step S2 without considering the photovoltaic power source to obtain the short circuit current after considering the power system connected to the photovoltaic power station. The phase angle β at the moment of short circuit and the maximum short-circuit current I provided by the photovoltaic power station T The constraints between them are: Among them, I N This refers to the rated output current of the photovoltaic power station. S4. Calculate the short-circuit capacity and output the calculation result. The formula for calculating the short-circuit capacity is as follows: In the formula, S f For three-phase short-circuit capacity, U N I represents the effective value of the rated line voltage before the fault point. f This represents the effective value of the line current at the fault point.

2. The method for calculating the maximum short-circuit capacity of a power system considering photovoltaic power plant access according to claim 1, characterized in that, The node parameter matrix described in step S1 consists of: node number bus#, node voltage volt, node voltage phase angle ang, node active power p, node reactive power q, node type bus_type, node unit judgment if_set, and generator node subtransient reactance Xd''; the line parameter matrix consists of: line start point b#1, line end point b#2, line resistance R, line reactance X, line conductance G, and line susceptance B / 2; the fault parameter matrix consists of: fault branch start point node fault_b#1, fault branch end point node fault_b#2, distance from the start point fault_dis (represented by 0-1), and fault phase angle fault_phase; the photovoltaic parameter matrix consists of: access point number pv_bus#, photovoltaic capacity pv_p, and rated current pv_i.

3. The method for calculating the maximum short-circuit capacity of a power system considering photovoltaic power plant access according to claim 2, characterized in that, The steps in step S2 for calculating the voltage at each point in the system and the three-phase short-circuit current at the fault point using the nodal injection current method are as follows: S21. The node admittance matrix of the system is formed by the aforementioned line parameter matrix; S22. Correct the nodal admittance matrix to obtain the corrected nodal admittance matrix; S23. Determine the node injection current during a fault. S24. Solve for the node voltage based on the node injection current; S25. Calculate the short-circuit current of each branch to obtain the three-phase short-circuit current at the fault point.

4. The method for calculating the maximum short-circuit capacity of a power system considering photovoltaic power plant access according to claim 3, characterized in that, The method for forming the node admittance matrix of the system described in step S21 is as follows: Since the order n of the node admittance matrix Y of the network is equal to the number of nodes in the power network, an n-order zero matrix is ​​first created. The admittance of each transmission line is obtained through the line parameter matrix. According to the definition of self-admittance and mutual admittance, the calculated admittance value is used to replace the zero element at the corresponding position in the zero matrix to obtain the node admittance matrix of the distribution network.

5. The method for calculating the maximum short-circuit capacity of a power system considering photovoltaic power plant access according to claim 4, characterized in that, The method for correcting the nodal admittance matrix in step S22 is as follows: The correction of the node admittance matrix includes three aspects: generator node correction, fault point correction, and load node correction. The generator node, as an active branch, is represented by the potential source Ei and the impedance Z. i For series branches, the equivalent current source I is used in short-circuit current calculations. i With ground-guided y i The transformation process is as follows: → After the circuit equivalent transformation, generator node i has an additional ground-connected branch. Therefore, the i-th element on the diagonal of the node admittance matrix needs to be corrected. The correction formula is as follows: Among them, fixY ii To correct the i-th diagonal element of the admittance matrix; Indicates the subtransient reactance of the generator node; The photovoltaic power station is connected to the grid by an inverter, and the inverter has no subtransient reactance, so there is no need to make any modifications to the photovoltaic power station. In practical applications, the impact of the load on the short-circuit current is usually ignored, so there is no need to correct the load. When a three-phase short-circuit fault occurs in a power system, the structure of the power grid changes, so the node admittance matrix also needs to be corrected. Three-phase short-circuit faults in power systems are divided into two types: node three-phase short-circuit faults and branch three-phase short-circuit faults. The two cases are distinguished based on the fault distance in the third column of the fault matrix: When a three-phase short circuit occurs at node j, the impedance to ground is infinitely small, therefore the admittance to ground is infinitely large. Here, we change this to 999999, as shown in the following equation: When the faulty node is connected to more nodes, the repair method is the same; When a three-phase short circuit occurs in branch ij, the power grid structure will also change. The node admittance matrix is ​​modified sequentially according to the fault occurrence in the branch as follows: ， ， ， The 0.5 in the formula is determined by the distance from the beginning of the fault parameter.

6. The method for calculating the maximum short-circuit capacity of a power system considering photovoltaic power plant access according to claim 5, characterized in that, The method for determining the node injection current during a fault, as described in step S23, is as follows: First, the voltage source is equivalently transformed into a current source, and then the node injection current is calculated. This is an n-dimensional column vector, where n is the number of nodes in the power system. After transforming the generator nodes from potential sources to current sources, the injected current of the generator nodes during a fault is calculated, thus generating a column vector of node injected currents. The formula is: In practical calculations of short-circuit current, it is approximated that the potential of all generator nodes is 1. Therefore, the formula is transformed as follows: .

7. The method for calculating the maximum short-circuit capacity of a power system considering photovoltaic power plant access according to claim 6, characterized in that, The method for solving the node voltage based on the node injection current in step S24 is as follows: After obtaining the modified node admittance matrix and the node injection current column vector, the node voltage is obtained by solving a system of linear equations, as shown in the formula: ,in, This is the corrected nodal admittance matrix.

8. The method for calculating the maximum short-circuit capacity of a power system considering photovoltaic power plant access according to claim 7, characterized in that, The method for determining the short-circuit current of each branch in step S25 is as follows: After obtaining the node voltage, the solution for the short-circuit current of non-fault-related branches is the same as that for power flow calculation. In order to facilitate the calculation of the short-circuit fault point current of the node, the branch connected to the fault node can be treated as a non-fault-related branch for calculation. For power systems without photovoltaic access, the final short-circuit current result can be directly calculated. The short-circuit current calculation result is represented by a (nl+1)×4 matrix, where nl is the number of branches in the original data; the first two columns of the matrix are the first and last node numbers, the third column is the short-circuit current phasor, and the fourth column is the effective value of the short-circuit current; the first nl rows of the matrix are the branches corresponding to the line matrix in the original data and their short-circuit current values; the last row of the matrix is ​​the short-circuit current value at the fault point. The formula for the short-circuit current at the fault point of a three-phase short circuit at a node is: The fault distance is taken as 0.

5. The formula for the short-circuit current at the fault point of the three-phase short circuit in the branch is as follows: This allows us to obtain the three-phase short-circuit current at the fault point without considering the photovoltaic power station.

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