A power distribution network and microgrid fault characteristic analysis method during microgrid fault

Abstract

A method for analyzing the fault characteristics of distribution networks and microgrids during microgrid faults includes the following steps: When a short-circuit fault occurs within the distribution network line area, constructing an expression for the fault current within the area; obtaining the fault current characteristics within the area based on the expression for the fault current within the area; When a short-circuit fault occurs outside the distribution network line area, constructing an expression for the fault current outside the area; obtaining the fault current characteristics outside the area based on the expression for the fault current outside the area; When a short-circuit fault occurs within the target area of ​​the microgrid line, constructing the internal fault current characteristics of the microgrid; When a short-circuit fault occurs outside the target area of ​​the microgrid line, constructing the external fault current characteristics of the microgrid; When a short-circuit fault occurs in a microgrid line operating in grid-connected mode, constructing the grid-connected fault voltage characteristics of the microgrid; When a short-circuit fault occurs in a microgrid line operating in off-grid mode, constructing the off-grid fault voltage characteristics of the microgrid. The analysis method has high practicality and accuracy.

Description

Technical Field

[0001] This invention relates to a method for analyzing the fault characteristics of a distribution network including a microgrid, and more specifically, to a method for analyzing the fault characteristics of a distribution network and a microgrid during a microgrid fault. Background Technology

[0002] Since the beginning of the 21st century, with the energy crisis, environmental pollution, and the rise of green energy, the development of microgrids has become one of the hottest research directions in the international power system field. A microgrid is a small power system that integrates multiple energy supply devices and load devices within a local area and uses modern control technology for unified management. Maintaining stable and reliable operation is crucial for microgrids, and handling faults is also a major challenge and a popular research area.

[0003] The method for analyzing the fault characteristics of distribution networks and microgrids under on-grid / off-grid operation and in-region / out-of-region faults is a proposed solution to the fault problems existing in microgrids. Microgrid fault problems mainly include two categories: distribution network line short-circuit faults and microgrid line short-circuit faults. For distribution network line short-circuit faults, traditional methods are usually based on equivalent circuit models, but handling microgrid line short-circuit faults is more difficult.

[0004] While microgrid technology has made significant progress in current research, it still faces many challenges and problems due to its inherent complexity and flexibility. Several previous technical solutions have been proposed to address short-circuit faults in distribution network lines and microgrid lines. However, existing microgrid fault analysis techniques require more complex and advanced mathematical tools and software support, as well as more in-depth and detailed technical research and practical application verification. Summary of the Invention

[0005] The purpose of this invention is to address the shortcomings of the existing technology and provide a method for analyzing the fault characteristics of distribution networks and microgrids. This method can analyze the fault characteristics of distribution networks and microgrids under conditions of microgrid operation in parallel / off-grid mode and under conditions of faults within / outside the grid. By constructing different equivalent circuits, this invention proposes a method for analyzing the fault characteristics of distribution networks and microgrids under conditions of faults within / outside the grid in microgrid operation. This method has high practicality and accuracy, providing power system engineers with a scientific basis for accurately determining the location and type of microgrid faults and for quickly and effectively handling faults.

[0006] A method for analyzing the fault characteristics of distribution networks and microgrids during microgrid faults includes the following steps:

[0007] 1) When a short-circuit fault occurs in the distribution network line of the DN-MGs distribution network containing microgrids, and the fault occurs within the zone, construct the expression for the fault current within the zone based on the equivalent circuit EC-1 of the distribution network containing microgrids under the condition of no fault and the additional equivalent circuit EC-2 when the distribution network line experiences a short-circuit fault within the zone.

[0008] 2) When a short-circuit fault occurs in a distribution network line of a distribution network containing microgrids (DN-MGs), and the fault occurs within the zone, the fault current vector relationship within the zone is constructed based on the fault current expression within the zone to obtain the fault current characteristics within the zone.

[0009] 3) When a short-circuit fault occurs in a distribution line of a distribution network containing microgrids (DN-MGs), and the fault occurs outside the distribution area, construct an expression for the fault current outside the distribution area based on the equivalent circuit EC-1 and the additional equivalent circuit EC-3 when the distribution line experiences a short-circuit fault outside the distribution area.

[0010] 4) When a short-circuit fault occurs in a distribution network line in a distribution network containing microgrids (DN-MGs), and the fault occurs outside the distribution area, the vector relationship of the fault current outside the distribution area is constructed based on the expression of the fault current outside the distribution area, and the characteristics of the fault current outside the distribution area are obtained.

[0011] 5) When a short-circuit fault occurs in a microgrid line in a distribution network DN-MGs line containing a microgrid, if the fault occurs within the target area, construct the internal fault current characteristics of the microgrid under grid-connected or off-grid operation based on the fault equivalent circuit EC-4 of the distribution network DN-MGs under grid-connected operation and the fault equivalent circuit EC-5 of the distribution network DN-MGs under off-grid operation.

[0012] 6) When a short-circuit fault occurs in a microgrid line in a distribution network DN-MGs line containing a microgrid, if the fault occurs outside the target area, construct the external fault current characteristics of the microgrid in the grid-connected or off-grid operation state based on the fault equivalent circuit EC-4 of the distribution network DN-MGs in the grid-connected operation state and the fault equivalent circuit EC-5 of the distribution network DN-MGs in the off-grid operation state.

[0013] 7) When a short-circuit fault occurs in a microgrid line in a distribution network DN-MGs line containing a microgrid, and the microgrid is in grid-connected operation, construct the grid-connected fault voltage characteristics of the microgrid when the fault occurs inside or outside the zone, based on the fault equivalent circuit EC-4.

[0014] 8) When a short-circuit fault occurs in a microgrid line in a distribution network containing microgrids (DN-MGs), and the microgrid is in an off-grid operation state, construct the off-grid fault voltage characteristics of the microgrid when the fault occurs within or outside the zone, based on the fault equivalent circuit EC-5.

[0015] In step 1), when a short-circuit fault occurs in the distribution network line of the DN-MGs distribution network containing the microgrid, and the fault occurs within the zone, the expression for the fault current within the zone is constructed based on the equivalent circuit EC-1 of the distribution network containing the microgrid under the condition of no fault and the additional equivalent circuit EC-2 when the distribution network line experiences a short-circuit fault within the zone. Specifically:

[0016] The positive sequence component method is used to analyze various short-circuit faults in the distribution network containing microgrids. Based on the location of the fault point, the distribution network is divided into two subsystems, denoted as System Side 1 and System Side 2, respectively. The end nodes of the two subsystems are denoted as A and B. In the non-fault state, the two subsystems are connected by line AB.

[0017] When a fault occurs at point f1 within section AB, the system's normal-state response current is calculated using the equivalent circuit EC-1 of the distribution network including the microgrid under normal operating conditions, i.e., when no fault occurs:

[0018]

[0019] In the formula: The voltages at nodes A and B are respectively; Z 1AB The line impedance between nodes A and B; These are the normal response currents of line AB, terminal A, and terminal B, respectively.

[0020] The additional response current for faults within the zone is calculated using the equivalent circuit EC-2 when a short-circuit fault occurs in the distribution network line within the zone during the fault operation state:

[0021]

[0022]

[0023]

[0024]

[0025] In the formula: U F The fault equivalent electromotive force; These are the additional fault currents at nodes A and B, respectively; Z 1ΣA Z 1ΣB Z1' represents the generalized loop impedance as seen from the fault voltage into the network. A =Z 1A +Z 1AF Z1' B =Z 1B +Z 1BF Z represents the equivalent impedance on the sides where nodes A and B are located, respectively; 1AF Z1BF Let Z be the line impedances between nodes A and B and the fault point, respectively, and Z be the line impedances between nodes A and B and the fault point. 1AF +Z 1BF =Z 1AB Z 1A Z 1B Let ΔZ be the impedance between nodes A and B and their respective power sources; F This is the equivalent grounding impedance at the fault point;

[0026] According to the superposition theorem for linear circuits, the expression for the fault current within the region is constructed by superimposing the normal state response current of the system and the additional response current of the fault within the region:

[0027]

[0028]

[0029] In the formula: These are the positive-sequence fault currents at nodes A and B, respectively, and it is specified that when they are compared with the normal operating current... When they are consistent, the current value is positive.

[0030] In step 2), when a short-circuit fault occurs in a distribution network line within a DN-MGs distribution network containing a microgrid, and the fault occurs within the zone, the zone fault current vector relationship is constructed based on the zone fault current expression to obtain the zone fault current characteristics, specifically:

[0031] According to equations (1)-(7), in the equivalent circuit EC-1 and the supplementary equivalent circuit EC-2, if the power flow direction of the distribution network before the fault occurs is from A to B, then the following definition applies. They are respectively and and The phase difference is the change in the phase of the positive-sequence current phasor caused by the short-circuit fault. According to the phase relationship, That is, the phase change of the positive sequence current at point A is negative. That is, the change in the phase of the positive-sequence current at point B is positive, where arg(*) is the argument of the phasor within the parentheses; if, before the fault occurred, the power flow direction of the distribution network was from B to A, then For short-circuit faults occurring inside the line, the positive sequence current phase angles at both ends of the faulty line always change in opposite directions.

[0032] In step 3), when a short-circuit fault occurs in a distribution line within a distribution network containing microgrids (DN-MGs), and the fault occurs outside the distribution area, an expression for the external fault current is constructed based on the equivalent circuit EC-1 and the additional equivalent circuit EC-3 when an external short-circuit fault occurs in the distribution network line. Specifically:

[0033] When a fault occurs at point f2 outside section AB, the normal state response current of the system is calculated from the equivalent circuit EC-1 of the distribution network including the microgrid under normal operating conditions, i.e., when no fault occurs:

[0034]

[0035] The additional response current for external faults can be calculated from the equivalent circuit EC-3 when an external short-circuit fault occurs in the distribution network line under fault operating conditions:

[0036]

[0037] In the formula: Z 1Bx Let be the impedance between node B and the fault point.

[0038] According to the superposition theorem for linear circuits, the expression for the external fault current is constructed by superimposing the normal state response current of the system and the additional response current of the external fault:

[0039]

[0040]

[0041] In step 4), when a short-circuit fault occurs in a distribution network line within a distribution network containing microgrids (DN-MGs), and the fault occurs outside the designated area, the external fault current vector relationship is constructed based on the external fault current expression to obtain the external fault current characteristics, specifically:

[0042] According to equations (8)-(10), in the equivalent circuit EC-1 and the supplementary equivalent circuit EC-3, assuming that before the fault occurs, the power flow direction of the distribution network is from A to B, we define... They are respectively and and The phase difference, according to the phase relationship, That is, the change in the phase of the positive sequence current at point A is negative. That is, the change in the phase of the positive sequence current at point B is negative; similarly, if the power flow direction before the fault was from B to A, then: For short-circuit faults occurring outside the line, the direction of change of the positive sequence current phase angle at both ends of the faulted line is always the same.

[0043] The criteria for determining the location of short-circuit faults in the distribution network are shown in equation (11).

[0044]

[0045] In step 5), when a short-circuit fault occurs in a microgrid line within a distribution network DN-MGs line containing a microgrid, if the fault occurs within the target area, the internal fault current characteristics of the microgrid under grid-connected or off-grid operation are constructed based on the fault equivalent circuit EC-4 of the distribution network DN-MGs under grid-connected operation and the fault equivalent circuit EC-5 of the distribution network DN-MGs under off-grid operation. Specifically:

[0046] When the microgrid is in grid-connected operation, all connected distributed generation (DG) generators use PQ control; when the microgrid is in off-grid operation, all connected DG generators use V / f control. DG generators using PQ control can be considered equivalent to current sources controlled by the grid connection point voltage, while DG generators using V / f control can be considered equivalent to voltage sources controlled by the grid connection point output current. When a fault occurs, the current detection threshold is:

[0047]

[0048] In the formula: I dec,in This is the detection threshold for the DG fault current when the fault occurs within the target area. This refers to the rated operating current of the DG when the fault occurs inside the target area.

[0049] For line MN and M-terminal protection QF1, when the microgrid is connected to the grid, the fault current flowing through protection QF1 is jointly provided by its downstream generation units; when the microgrid is operating independently, the fault current flowing through protection QF1 is only provided by the generation units of the parallel branches. Furthermore, due to the current limiting effect of the power electronic converter, the fault current flowing through protection QF1 under both independent and grid-connected operation conditions should satisfy the following relationship:

[0050] I off,in <I on,in (13)

[0051] In the formula: I off,in When a fault occurs within the target area, the fault current flowing through protection QF1 during independent operation of the microgrid is I. on,in When a fault occurs within the target area, the fault current flowing through protection QF1 during grid-connected operation of the microgrid;

[0052] When a fault occurs at point f1 within the microgrid line MN, the current at both ends of bus M and N will be... and The magnitudes are variable, but the directions are the same; all flows from the busbar to the line. Ideally, they are in the same phase with a phase difference of 0°.

[0053]

[0054] In the formula: arg(*) is the argument of the phasor within the parentheses.

[0055] In step 6), when a short-circuit fault occurs in a microgrid line within a distribution network DN-MGs line containing a microgrid, if the fault occurs outside the target area, the external fault current characteristics of the microgrid under grid-connected or off-grid operation are constructed based on the fault equivalent circuit EC-4 of the distribution network DN-MGs under grid-connected operation and the fault equivalent circuit EC-5 of the distribution network DN-MGs under off-grid operation. Specifically:

[0056] When the microgrid is in grid-connected operation, all connected distributed generation (DG) systems employ PQ control; when the microgrid is in off-grid operation, all connected DG systems employ V / f control. A DG system using PQ control can be considered equivalent to a current source controlled by the grid connection point voltage, while a DG system using V / f control can be considered equivalent to a voltage source controlled by the grid connection point output current. When a fault occurs, the current detection threshold is:

[0057]

[0058] In the formula: I dec,out This is the detection threshold for the DG fault current when the fault occurs outside the target area. This refers to the rated operating current of the DG when the fault occurs outside the target area.

[0059] For line MN and M-terminal protection QF1, when the microgrid is connected to the grid, the fault current flowing through protection QF1 is jointly provided by its downstream generation units; when the microgrid is operating independently, the fault current flowing through protection QF1 is only provided by the generation units of the parallel branches. Furthermore, due to the current limiting effect of the power electronic converter, the fault current flowing through protection QF1 under both independent and grid-connected operation conditions should satisfy the following relationship:

[0060] I off,out <I on,out (16)

[0061] In the formula: I off,out When a fault occurs within the target area, the fault current flowing through protection QF1 during independent operation of the microgrid is I. on,out When a fault occurs within the target area, the fault current flowing through protection QF1 during grid-connected operation of the microgrid;

[0062] When a fault occurs at point f2 or f3 outside the microgrid line MN, the current at both ends of bus M and N will be affected. and They are equal in size but opposite in direction, flowing from the busbar to the line at one end and from the line to the busbar at the other, with a phase difference of 180°.

[0063]

[0064] In the formula: arg(*) is the argument of the phasor within the parentheses.

[0065] In step 7), when a short-circuit fault occurs in the microgrid line of the distribution network DN-MGs line containing the microgrid, and the microgrid is in grid-connected operation, the grid-connected fault voltage characteristics of the microgrid under fault conditions within or outside the zone are constructed based on the fault equivalent circuit EC-4, specifically as follows:

[0066] When a microgrid is connected to the grid, it lacks the support of the main grid after a fault, and the capacity of the distributed generation (DG) is limited. Therefore, the voltage at the DG output terminal on the faulty line side will drop significantly.

[0067]

[0068] In the formula, These represent the voltages at the DG output terminals on the faulty line side before and after the fault.

[0069] In step 8), when a short-circuit fault occurs in the microgrid line of the distribution network DN-MGs line containing the microgrid, and the microgrid is in an off-grid operation state, the off-grid fault voltage characteristics of the microgrid under fault conditions within or outside the zone are constructed according to the fault equivalent circuit EC-5, specifically as follows:

[0070] When a fault occurs in an independently operating microgrid, the power electronic converter in the distributed generation (DG) is forced to limit the current, resulting in a significant voltage drop at the DG output.

[0071]

[0072] In the formula, These represent the voltages at the DG output terminals on the faulty line side before and after the fault.

[0073] In summary, the technical solutions conceived in this invention have the following beneficial effects compared with the prior art:

[0074] 1) This method can perform characteristic analysis on faults that occur in microgrids under grid-connected / off-grid operation, helping power system engineers to quickly and effectively determine the location and type of faults.

[0075] 2) This method constructs different equivalent circuits to analyze the fault characteristics of microgrids, distinguishes between faults inside and outside the area, and classifies and analyzes the faults of microgrids in grid-connected / off-grid operation, which has high practicality and accuracy.

[0076] 3) This method analyzes the fault current and voltage characteristics of microgrids in grid-connected / off-grid operation, which can provide power system engineers with a reference for fault handling, reduce fault handling time, and enhance grid security and stability.

[0077] In summary, this invention patent proposes a method for analyzing the fault characteristics of distribution networks and microgrids under on-grid / off-grid operation conditions when faults occur within / outside the grid. This method has high practicality and accuracy, providing power system engineers with a scientific basis for accurately determining the location and type of microgrid faults and for quickly and effectively handling faults. Attached Figure Description

[0078] Figure 1 This is a flowchart of the method of the present invention;

[0079] Figure 2 This is a structural diagram of a distribution network (DN-MGs) including microgrids, according to an embodiment of the present invention.

[0080] Figure 3 This is the equivalent circuit (EC-1) of the DN-MGs in normal operating state in an embodiment of the present invention;

[0081] Figure 4 This is the equivalent circuit (EC-2) of the distribution network line in the DN-MGs of the present invention when a short-circuit fault occurs in the area;

[0082] Figure 5 This is a schematic diagram of a DN-MGs line failure in an embodiment of the present invention;

[0083] Figure 6 This is an electrical quantity vector diagram of the distribution network line in the DN-MGs of the present invention when a short-circuit fault occurs in the area;

[0084] Figure 7 This is the equivalent circuit (EC-3) of the distribution network line in the DN-MGs of the present invention when a short-circuit fault occurs outside the area;

[0085] Figure 8 This is an electrical quantity vector diagram of the distribution network line in the DN-MGs of the present invention when a short-circuit fault occurs outside the zone;

[0086] Figure 9 This is a schematic diagram of a microgrid in DN-MGs, an embodiment of the present invention;

[0087] Figure 10 This is the equivalent circuit (EC-4) of the microgrid in DN-MGs when a short-circuit fault occurs in the microgrid line during grid-connected operation in the microgrid of this invention embodiment;

[0088] Figure 11 This is the equivalent circuit (EC-5) of the microgrid in DN-MGs when a short-circuit fault occurs in the microgrid line during off-grid operation in an embodiment of the present invention;

[0089] Figure 12 This refers to the current flowing through protection QF1 when a short-circuit fault occurs in the microgrid line of the DN-MGs during grid-connected operation in the microgrid of this invention embodiment.

[0090] Figure 13 This refers to the current flowing through protection QF1 when a short-circuit fault occurs in the microgrid lines of the DN-MGs in the off-grid operation state of the microgrid in this embodiment of the invention.

[0091] Figure 14 This refers to the voltage at node DG1 when a short-circuit fault occurs in the microgrid line of the DN-MGs during grid-connected operation in the microgrid of this invention embodiment.

[0092] Figure 15 This refers to the voltage at node DG1 when a short-circuit fault occurs in the microgrid line of the DN-MGs in the off-grid operation state of the microgrid in this embodiment of the invention.

[0093] Figure 16 This refers to the current phase difference between the two ends of bus M and N when a short-circuit fault occurs at point f1 in DN-MGs during grid-connected operation of the microgrid in this embodiment of the invention.

[0094] Figure 17 This refers to the current phase difference between the two ends of bus M and N when a short-circuit fault occurs at point f2 in DN-MGs during grid-connected operation of the microgrid in this embodiment of the invention.

[0095] Figure 18 This refers to the current phase difference between the M and N ends of the bus when a short-circuit fault occurs at point f3 in the DN-MGs during grid-connected operation of the microgrid in this embodiment of the invention.

[0096] Figure 19 This refers to the current phase difference between the two ends of bus M and N when a short-circuit fault occurs at point f1 in DN-MGs during off-grid operation of the microgrid in this embodiment of the invention.

[0097] Figure 20 This refers to the current phase difference between the two ends of bus M and N when a short-circuit fault occurs at point f2 in DN-MGs during off-grid operation of the microgrid in this embodiment of the invention.

[0098] Figure 21 This refers to the current phase difference between the M and N ends of the bus when a short-circuit fault occurs at point f3 in the DN-MGs during off-grid operation of the microgrid in this embodiment of the invention.

[0099] Figure 22 This refers to the voltage of the node where DG1 is located when a short-circuit fault occurs at point f1 in DN-MGs during the grid-connected / off-grid operation of the microgrid in this embodiment of the invention.

[0100] Figure 23 This refers to the voltage of the node where DG1 is located when a short-circuit fault occurs at point f2 in DN-MGs during the grid-connected / off-grid operation of the microgrid in this embodiment of the invention.

[0101] Figure 24 This refers to the voltage of the node where DG1 is located when a short-circuit fault occurs at point f3 in DN-MGs during the grid-connected / off-grid operation of the microgrid in this invention embodiment. Detailed Implementation

[0102] The technical solutions of this invention will be clearly and completely described below with reference to the accompanying drawings and specific embodiments. For example... Figure 1 As shown, the method for analyzing the fault characteristics of distribution networks and microgrids during microgrid faults according to the present invention includes the following steps:

[0103] 1) When a short-circuit fault occurs in the distribution network line of the DN-MGs distribution network containing microgrids, and the fault occurs within the zone, construct the expression for the fault current within the zone based on the equivalent circuit EC-1 of the distribution network containing microgrids under the condition of no fault and the additional equivalent circuit EC-2 when the distribution network line experiences a short-circuit fault within the zone.

[0104] 2) When a short-circuit fault occurs in a distribution network line of a distribution network containing microgrids (DN-MGs), and the fault occurs within the zone, the fault current vector relationship within the zone is constructed based on the fault current expression within the zone to obtain the fault current characteristics within the zone.

[0105] 3) When a short-circuit fault occurs in a distribution line of a distribution network containing microgrids (DN-MGs), and the fault occurs outside the distribution area, construct an expression for the fault current outside the distribution area based on the equivalent circuit EC-1 and the additional equivalent circuit EC-3 when the distribution line experiences a short-circuit fault outside the distribution area.

[0106] 4) When a short-circuit fault occurs in a distribution network line in a distribution network containing microgrids (DN-MGs), and the fault occurs outside the distribution area, the vector relationship of the fault current outside the distribution area is constructed based on the expression of the fault current outside the distribution area, and the characteristics of the fault current outside the distribution area are obtained.

[0107] 5) When a short-circuit fault occurs in a microgrid line in a distribution network DN-MGs line containing a microgrid, if the fault occurs within the target area, construct the internal fault current characteristics of the microgrid under grid-connected or off-grid operation based on the fault equivalent circuit EC-4 of the distribution network DN-MGs under grid-connected operation and the fault equivalent circuit EC-5 of the distribution network DN-MGs under off-grid operation.

[0108] 6) When a short-circuit fault occurs in a microgrid line in a distribution network DN-MGs line containing a microgrid, if the fault occurs outside the target area, construct the external fault current characteristics of the microgrid in the grid-connected or off-grid operation state based on the fault equivalent circuit EC-4 of the distribution network DN-MGs in the grid-connected operation state and the fault equivalent circuit EC-5 of the distribution network DN-MGs in the off-grid operation state.

[0109] 7) When a short-circuit fault occurs in a microgrid line in a distribution network DN-MGs line containing a microgrid, and the microgrid is in grid-connected operation, construct the grid-connected fault voltage characteristics of the microgrid when the fault occurs inside or outside the zone, based on the fault equivalent circuit EC-4.

[0110] 8) When a short-circuit fault occurs in a microgrid line in a distribution network containing microgrids (DN-MGs), and the microgrid is in an off-grid operation state, construct the off-grid fault voltage characteristics of the microgrid when the fault occurs within or outside the zone, based on the fault equivalent circuit EC-5.

[0111] like Figure 2 As shown, a distribution network including microgrids (DN-MGs) comprises a feeder node S, four distributed generation sources (DGs), two microgrids (MGs) consisting of generators and loads, and five protection devices 1-5. Under fault-free conditions, an equivalent circuit EC-1 is established for the normal operation of the distribution network including microgrids (DN-MGs), as shown below. Figure 3 ;

[0112] To meet the characteristic analysis requirements of various types of short-circuit faults, the more universal positive-sequence component method is used to analyze distribution network faults. The following section combines... Figure 2 The principle of the short-circuit fault topology network shown is explained. The operating current when the system is in a non-fault state is...

[0113]

[0114] In the formula: The voltages at nodes A and B are respectively; Z 1AB Let EC-1 be the line impedance between nodes A and B. The equivalent circuit under normal operating conditions is abbreviated as EC-1. Figure 3 As shown.

[0115] like Figure 4 As shown, when a short-circuit fault occurs in a distribution network line and the fault is located within the line area, an additional equivalent circuit EC-2 for DN-MGs is established.

[0116] like Figure 5 As shown, the positive sequence component method is used to analyze various short-circuit faults in the distribution network containing microgrids. According to the location of the fault point, the distribution network is divided into two subsystems, denoted as system side 1 and system side 2, respectively. The end nodes of the two subsystems are denoted as A and B. In the non-fault state, the two subsystems are connected by line AB.

[0117] When a fault occurs at point f1 in section AB, according to the superposition theorem of linear circuits, the fault response of the network can be decomposed into the superposition of two states: the normal state response and the fault-addition response.

[0118] set up These are the positive-sequence fault currents at nodes A and B, respectively, and it is specified that when they are compared with the normal operating current... When the current is constant, it is positive. The equivalent impedances on the sides of nodes A and B are defined as Z1'. A =Z 1A +Z 1AF Z1' B =Z 1B +Z 1BF And Z 1AF +Z 1BF =Z 1AB Z 1AF Z 1BF Z represents the line impedance between nodes A and B and the fault point, respectively. 1A Z 1B Let be the impedance between nodes A and B and their respective power sources. Define ΔZ. F It is the equivalent grounding impedance at the fault point, and its magnitude is determined by the fault type and the grounding material.

[0119] When a fault occurs at point f1 within section AB, the system's normal-state response current is calculated using the equivalent circuit EC-1 of the distribution network including the microgrid under normal operating conditions, i.e., when no fault occurs:

[0120]

[0121] In the formula: The voltages at nodes A and B are respectively; Z 1AB The line impedance between nodes A and B; These are the normal response currents of line AB, terminal A, and terminal B, respectively.

[0122] The additional response current for faults within the zone is calculated using the equivalent circuit EC-2 when a short-circuit fault occurs in the distribution network line within the zone during the fault operation state:

[0123]

[0124]

[0125] in:

[0126]

[0127] In the formula: U F The fault equivalent electromotive force; These are the normal response currents of nodes A and B, respectively. These are the additional fault currents at nodes A and B, respectively; Z 1ΣA Z 1ΣB These are the generalized loop impedances seen from the fault voltage into the network; Z 1ΣA Z 1ΣB The impedance angle;

[0128] According to the superposition principle, the fault current within the region consists of the normal response current component and the fault-addition current component:

[0129]

[0130]

[0131] In the formula: These are the positive-sequence fault currents at nodes A and B, respectively, and it is specified that when they are compared with the normal operating current... When the current is consistent, the current value is positive. According to equations (2)-(7), when a short circuit fault occurs inside line AB, the equivalent circuit with nodes A and B as the center (i.e. Figure 3 and Figure 4 In the diagram, the phasor relationships of the electrical quantities are as follows: Figure 6 As shown.

[0132] Assuming that before the fault occurred, the power flow direction in the distribution network was from A to B, define... They are respectively and and The phase difference is the change in the phase of the positive-sequence current phasor caused by a short-circuit fault. As shown in the figure, That is, the phase change of the positive sequence current at point A is negative. That is, the change in phase of the positive-sequence current at point B is positive, where arg(*) is the argument of the phasor within the parentheses. If the power flow direction before the fault was from B to A, then... Contrary to the previous conclusion, in summary, for short-circuit faults occurring inside the line, regardless of the specific direction of power flow on the line before the fault, the direction of change of the positive sequence current phase angle at both ends of the faulted line is always opposite.

[0133] When a short-circuit fault occurs in the distribution network lines of DN-MGs, and the fault occurs outside the zone, an additional equivalent circuit EC-3 is constructed for DN-MGs, such as... Figure 7 ;

[0134] When a fault occurs at point f2 outside section AB, the fault response is still decomposed into the superposition of the normal state response and the fault-related additional response according to the superposition theorem of linear circuits.

[0135] When a fault occurs at point f2 outside section AB, the equivalent network's normal operating state is as follows: Figure 3 As shown,

[0136] The system's normal-state response current is calculated from the equivalent circuit EC-1 of the distribution network including the microgrid under normal operating conditions, i.e., when no faults occur:

[0137]

[0138] The additional response current for external faults can be calculated from the equivalent circuit EC-3 when an external short-circuit fault occurs in the distribution network line under fault operating conditions:

[0139]

[0140] In the formula: Z 1Bx The impedance between node B and the fault point;

[0141] According to the superposition theorem for linear circuits, the expression for the external fault current is constructed by superimposing the normal state response current of the system and the additional response current of the external fault:

[0142]

[0143]

[0144] When a short-circuit fault occurs in the distribution network line of DN-MGs, and the fault occurs outside the zone, the fault current vector relationship is constructed according to the fault current expression to obtain the fault current characteristics.

[0145] Based on equations (8)-(10), the vector relationships of various electrical quantities during an external line fault can be drawn as follows: Figure 8 As shown.

[0146] Assuming that before the fault occurred, the power flow direction in the distribution network was from A to B, and still defining... They are respectively and and The phase difference. As can be seen from the figure, That is, the change in the phase of the positive sequence current at point A is negative. That is, the change in the phase of the positive sequence current at point B is negative. Similarly, if the power flow direction before the fault was from B to A, then: Contrary to the previous conclusion, in summary, for short-circuit faults occurring outside the line, regardless of the specific direction of power flow on the line before the fault, the direction of change of the positive sequence current phase angle at both ends of the faulted line is always the same.

[0147] The criteria for determining the location of short-circuit faults in the distribution network are shown in equation (13).

[0148]

[0149] microgrid structure such as Figure 9 As shown, when the microgrid is in grid-connected operation, all connected distributed generation (DG) uses PQ control; when the microgrid is in off-grid operation, all connected DG uses V / f control. When a three-phase short-circuit fault occurs at point f1, the grid-connected fault equivalent circuit EC-4 is as follows: Figure 10 As shown in the figure. (E) S I g1 I g2 I g3 I m I n U g These are the system power supply, fault current output of DG1, fault current output of DG2, fault current output of DG3, fault current at the location of circuit breaker QF1, fault current at the location of circuit breaker QF2, and the equivalent voltage of V / f controlling DG3; Z S Z2, Z3, Z M1 Z N1 These are the system equivalent impedance, the line impedance of line L2, the line impedance of line L3, the line impedance from bus M to fault point f1, and the line impedance from bus N to fault point f1, respectively.

[0150] When a microgrid fails, a DG controlled by PQ can be equivalent to a current source controlled by the grid connection point voltage.

[0151] microgrid structure such as Figure 9 As shown, when the microgrid is in grid-connected operation, all connected distributed generation (DG) uses PQ control; when the microgrid is in off-grid operation, all connected DG uses V / f control. When a three-phase short-circuit fault occurs at point f1, the off-grid fault equivalent circuit EC-5 is as follows: Figure 11 As shown in the figure. (E) S I g1 I g2 I g3 I m I n U gThese are the system power supply, fault current output of DG1, fault current output of DG2, fault current output of DG3, fault current at the location of circuit breaker QF1, fault current at the location of circuit breaker QF2, and the equivalent voltage of V / f controlling DG3; Z S Z2, Z3, Z M1 Z N1 These are the system equivalent impedance, the line impedance of line L2, the line impedance of line L3, the line impedance from bus M to fault point f1, and the line impedance from bus N to fault point f1, respectively.

[0152] When a microgrid experiences a fault, a distributed generation (DG) controlled by V / f can be equivalent to a voltage source controlled by the output current at the grid connection point. To enhance the fault ride-through capability of the microgrid, the fault current detection threshold of the DG is set to 3 to 5 times the rated current when a fault occurs.

[0153] Depend on Figure 9 and Figure 10 It can be seen that for line MN and M-terminal protection QF1, the fault current flowing through protection QF1 during microgrid grid-connected operation is jointly provided by grid DG3; when microgrid is operating independently, the fault current flowing through protection QF1 is provided only by DG3. At the same time, due to the current limiting of the power electronic converter, the fault current flowing through protection QF1 during independent operation will be less than the fault current flowing through protection QF1 during grid-connected operation.

[0154] When a fault occurs at point f1 within line MN, regardless of the microgrid's operating state or power flow direction, the currents at both ends of bus M and N will remain constant. and The magnitudes are variable, but the direction is the same: all flows from the busbar to the line. Ideally, they are in the same phase, with a phase difference of 0°.

[0155]

[0156] In the formula: arg(*) is the argument of the phasor within the parentheses;

[0157] When a fault occurs at point f2 or f3, the current will flow to the fault point, regardless of the microgrid's operating state or the power flow direction during normal operation. The current at both ends of buses M and N will also be constant. and They are equal in size but opposite in direction, with one end flowing from the busbar to the line and the other end flowing from the line to the busbar, with a phase difference of 180°.

[0158]

[0159] When a microgrid is connected to the grid, regardless of which point (f1 to f3) experiences a fault, the voltage at the output terminal of the DG on the faulty line side will drop significantly due to the lack of support from the main grid and the limited capacity of the DG.

[0160]

[0161] In the formula, These represent the voltages at the DG output terminals on the faulty line side before and after the fault.

[0162] Specifically, in step 13), when a short-circuit fault occurs in the microgrid lines of the DN-MGs, and the microgrid is in an off-grid operation state, the fault voltage characteristics of the microgrid under fault conditions inside or outside the area are constructed according to EC-5, specifically as follows:

[0163] When a fault occurs in an independently operating microgrid, the voltage at the output terminal of the distributed generation (DG) will drop significantly due to the current-limiting effect of the power electronic converter.

[0164]

[0165] In the formula, These represent the voltages at the DG output terminals on the faulty line side before and after the fault.

[0166] That is, suspected faulty lines can be selected by monitoring the voltage drop at the DG output terminal, and low voltage protection can be used as starting protection.

[0167] Specifically, in this embodiment, the midpoint of the line is set as the fault point. A three-phase metallic short-circuit fault occurs 5 seconds after the system has stabilized. The fault duration is 0.2 seconds. Figure 12 This refers to the current flowing through protection QF1 when a short-circuit fault occurs in the microgrid line of the DN-MGs during grid-connected operation in the microgrid of this invention embodiment. Figure 13 This describes the current flowing through protection QF1 when a short-circuit fault occurs in the microgrid lines of the DN-MGs during off-grid operation in an embodiment of the present invention. It can be seen that the current decreases after 2 seconds; therefore, the fault current flowing through protection QF1 when the microgrid is operating independently is less than the fault current flowing through protection QF1 when the microgrid is operating in grid-connected mode.

[0168] Figure 14 This refers to the voltage at node DG1 when a short-circuit fault occurs in the microgrid line of the DN-MGs during grid-connected operation in the microgrid of this invention embodiment. Figure 15 This is the voltage at node DG1 when a short-circuit fault occurs in the microgrid line of the DN-MGs in the off-grid operation of the microgrid in this embodiment of the invention. It can be seen that the voltage decreases after 2 seconds, and the voltage at the DG output terminal will drop significantly during a microgrid fault, consistent with theoretical analysis.

[0169] Figure 16This refers to the current phase difference between the two ends of bus M and N when a short-circuit fault occurs at point f1 in DN-MGs during grid-connected operation of the microgrid in this embodiment of the invention. Figure 17 This refers to the current phase difference between the two ends of bus M and N when a short-circuit fault occurs at point f2 in DN-MGs during grid-connected operation of the microgrid in this embodiment of the invention. Figure 18 This describes the current phase difference between the M and N terminals of the microgrid in the DN-MGs when a short-circuit fault occurs at point f3 in the microgrid during grid-connected operation. It can be seen that the phase difference remains essentially unchanged after 2 seconds. When the microgrid is in grid-connected operation, if a fault occurs at point f1 within the zone, both conditions are met, and the protection at both ends of the microgrid bus M and N will operate. If faults occur at points f2 and f3 outside the zone, the current phase difference does not meet the fault criterion, and the protection at both ends of the microgrid bus M and N will not operate.

[0170] Figure 19 This refers to the current phase difference between the two ends of bus M and N when a short-circuit fault occurs at point f1 in DN-MGs during off-grid operation of the microgrid in this embodiment of the invention. Figure 20 This refers to the current phase difference between the two ends of bus M and N when a short-circuit fault occurs at point f2 in DN-MGs during off-grid operation of the microgrid in this embodiment of the invention. Figure 21 This describes the current phase difference between the M and N terminals of the microgrid in the DN-MGs when a short-circuit fault occurs at point f3 in the microgrid during off-grid operation, as described in this invention's embodiment. It can be seen that the voltage decreases after 2 seconds. When the microgrid is in off-grid operation, if a fault occurs at point f1 within the zone, both conditions are met, and the protection at the M and N terminals of the microgrid bus will operate. However, if faults occur at points f2 and f3 outside the zone, the current phase difference does not meet the fault criterion, and the protection at the M and N terminals of the microgrid bus will not operate.

[0171] Figure 22 This refers to the voltage of the node where DG1 is located when a short-circuit fault occurs at point f1 in DN-MGs during the grid-connected / off-grid operation of the microgrid in this embodiment of the invention. Figure 23 This refers to the voltage of the node where DG1 is located when a short-circuit fault occurs at point f2 in DN-MGs during the grid-connected / off-grid operation of the microgrid in this embodiment of the invention. Figure 24 This refers to the voltage at the node where DG1 is located when a short-circuit fault occurs at point f3 in DN-MGs during the grid-connected / off-grid operation of the microgrid in this embodiment of the invention. It can be seen that the voltage decreases after 2 seconds. Regardless of whether the microgrid is operating in grid-connected or independent mode, although the voltage at the DG output terminal on the faulty line side drops significantly, the current phase difference does not meet criterion 2, and the protection at both ends of the microgrid bus M and N will not operate.

[0172] Table 1 shows the simulation results of different types of short-circuit faults occurring at various fault points when a fault occurs in the distribution network. It can be seen that the simulation results demonstrate that the fault characteristic analysis method can effectively identify the fault area under different fault conditions and at different points.

[0173] Table 1 Simulation results of faults at various points in the distribution network

[0174]

[0175]

[0176] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for analyzing the fault characteristics of distribution networks and microgrids during microgrid faults, characterized in that, Includes the following steps: 1) When a short-circuit fault occurs in the distribution network line of the DN-MGs distribution network containing microgrids, and the fault occurs within the zone, construct the expression for the fault current within the zone based on the equivalent circuit EC-1 of the distribution network containing microgrids under the condition of no fault and the additional equivalent circuit EC-2 when the distribution network line experiences a short-circuit fault within the zone. 2) When a short-circuit fault occurs in a distribution network line of a distribution network containing microgrids (DN-MGs), and the fault occurs within the zone, the fault current vector relationship within the zone is constructed based on the fault current expression within the zone to obtain the fault current characteristics within the zone. 3) When a short-circuit fault occurs in a distribution line of a distribution network containing microgrids (DN-MGs), and the fault occurs outside the distribution area, construct an expression for the fault current outside the distribution area based on the equivalent circuit EC-1 and the additional equivalent circuit EC-3 when the distribution line experiences a short-circuit fault outside the distribution area. 4) When a short-circuit fault occurs in a distribution network line in a distribution network containing microgrids (DN-MGs), and the fault occurs outside the distribution area, the vector relationship of the fault current outside the distribution area is constructed based on the expression of the fault current outside the distribution area, and the characteristics of the fault current outside the distribution area are obtained. 5) When a short-circuit fault occurs in a microgrid line in a distribution network DN-MGs line containing a microgrid, if the fault occurs within the target area, construct the internal fault current characteristics of the microgrid under grid-connected or off-grid operation based on the fault equivalent circuit EC-4 of the distribution network DN-MGs under grid-connected operation and the fault equivalent circuit EC-5 of the distribution network DN-MGs under off-grid operation. 6) When a short-circuit fault occurs in a microgrid line in a distribution network DN-MGs line containing a microgrid, if the fault occurs outside the target area, construct the external fault current characteristics of the microgrid in the grid-connected or off-grid operation state based on the fault equivalent circuit EC-4 of the distribution network DN-MGs in the grid-connected operation state and the fault equivalent circuit EC-5 of the distribution network DN-MGs in the off-grid operation state. 7) When a short-circuit fault occurs in the microgrid line of the distribution network DN-MGs line containing the microgrid, and the microgrid is in grid-connected operation, construct the grid-connected fault voltage characteristics of the microgrid when the fault occurs inside or outside the area, based on the fault equivalent circuit EC-4. 8) When a short-circuit fault occurs in a microgrid line in a distribution network DN-MGs line containing a microgrid, and the microgrid is in an off-grid operation state, construct the off-grid fault voltage characteristics of the microgrid when the fault occurs inside or outside the zone, based on the fault equivalent circuit EC-5.

2. The method for analyzing the fault characteristics of distribution networks and microgrids during microgrid faults according to claim 1, characterized in that, In step 1), when a short-circuit fault occurs in the distribution network line of the DN-MGs distribution network containing the microgrid, and the fault occurs within the zone, the expression for the fault current within the zone is constructed based on the equivalent circuit EC-1 of the distribution network containing the microgrid under the condition of no fault and the additional equivalent circuit EC-2 when the distribution network line experiences a short-circuit fault within the zone. Specifically: The positive sequence component method is used to analyze various short-circuit faults in the distribution network containing microgrids. Based on the location of the fault point, the distribution network is divided into two subsystems, denoted as System Side 1 and System Side 2, respectively. The end nodes of the two subsystems are denoted as A and B. In the non-fault state, the two subsystems are connected by line AB. When in section AB When a fault occurs, the normal state response current of the system is calculated from the equivalent circuit EC-1 of the distribution network including the microgrid under normal operating conditions, i.e., when no fault occurs: （1）； In the formula: , These are the voltages at nodes A and B, respectively. The line impedance between nodes A and B; , , These are the normal response currents of line AB, terminal A, and terminal B, respectively. The additional response current for faults within the zone is calculated using the equivalent circuit EC-2 when a short-circuit fault occurs in the distribution network line within the zone during the fault operation state: （2）； （3）； （4）； （5）； In the formula: The fault equivalent electromotive force; , These are the additional fault currents at nodes A and B, respectively. , These are the generalized loop impedances seen from the fault voltage into the network; , These are the equivalent impedances on the sides where nodes A and B are located, respectively. , These are the line impedances between nodes A and B and the fault point, respectively. ; , Let A and B be the impedances between their respective power supplies and nodes A and B. This is the equivalent grounding impedance at the fault point; According to the superposition theorem for linear circuits, the expression for the fault current within the region is constructed by superimposing the normal state response current of the system and the additional response current of the fault within the region: （6）； （7）； In the formula: , These are the positive-sequence fault currents at nodes A and B, respectively, and it is specified that when they are compared with the normal operating current... When they are consistent, the current value is positive.

3. The method for analyzing the fault characteristics of distribution networks and microgrids during microgrid faults according to claim 2, characterized in that, In step 2), when a short-circuit fault occurs in a distribution network line within a microgrid DN-MGs distribution network, and the fault occurs within the zone, the zone fault current vector relationship is constructed based on the zone fault current expression to obtain the zone fault current characteristics, specifically: According to equations (1)-(7), in the equivalent circuit EC-1 and the supplementary equivalent circuit EC-2, if the power flow direction of the distribution network before the fault occurs is from A to B, then the following definition applies. , They are respectively and , and The phase difference is the change in the phase of the positive-sequence current phasor caused by the short-circuit fault. According to the phase relationship, That is, the phase change of the positive sequence current at point A is negative. That is, the change in the phase of the positive sequence current at point B is positive, where To calculate the argument of the phasor within the parentheses; If, prior to the fault, the power flow direction in the distribution network was from B to A, then , For short-circuit faults occurring inside the line, the positive sequence current phase angle changes in opposite directions at both ends of the faulty line.

4. The method for analyzing the fault characteristics of distribution networks and microgrids during microgrid faults according to claim 2, characterized in that, In step 3), when a short-circuit fault occurs in the distribution network line of the distribution network containing the microgrid (DN-MGs), and the fault occurs outside the zone, the expression for the fault current outside the zone is constructed based on the equivalent circuit EC-1 and the additional equivalent circuit EC-3 when the distribution network line experiences a short-circuit fault outside the zone. Specifically: Outside of section AB When a fault occurs, the normal state response current of the system is calculated from the equivalent circuit EC-1 of the distribution network including the microgrid under normal operating conditions, i.e., when no fault occurs: （1）； The additional response current for external faults can be calculated from the equivalent circuit EC-3 when an external short-circuit fault occurs in the distribution network line under fault operating conditions: （8）； In the formula: The impedance between node B and the fault point; According to the superposition theorem for linear circuits, the expression for the external fault current is constructed by superimposing the normal state response current of the system and the additional response current of the external fault: （9）； （10）。 5. The method for analyzing the fault characteristics of distribution networks and microgrids during microgrid faults according to claim 4, characterized in that, In step 4), when a short-circuit fault occurs in a distribution network line within a distribution network containing microgrids (DN-MGs), and the fault occurs outside the designated area, the external fault current vector relationship is constructed based on the external fault current expression to obtain the external fault current characteristics, specifically: According to equations (8)-(10), in the equivalent circuit EC-1 and the supplementary equivalent circuit EC-3, assuming that before the fault occurs, the power flow direction of the distribution network is from A to B, we define... , They are respectively and , and The phase difference, according to the phase relationship, That is, the change in the phase of the positive sequence current at point A is negative. That is, the change in the phase of the positive sequence current at point B is negative; if the power flow direction before the fault was from B to A, the same conclusion can be drawn: , For short-circuit faults occurring outside the line, the positive sequence current phase angle changes in the same direction at both ends of the faulted line. The criteria for determining the location of short-circuit faults in the distribution network are shown in equation (11); （11）。 6. The method for analyzing the fault characteristics of distribution networks and microgrids during microgrid faults according to claim 1, characterized in that, In step 5), when a short-circuit fault occurs in a microgrid line within a distribution network DN-MGs line containing a microgrid, if the fault occurs within the target area, the internal fault current characteristics of the microgrid under grid-connected or off-grid operation are constructed based on the fault equivalent circuit EC-4 of the distribution network DN-MGs under grid-connected operation and the fault equivalent circuit EC-5 of the distribution network DN-MGs under off-grid operation. Specifically: When the microgrid is in grid-connected operation, all connected distributed generation (DG) generators use PQ control; when the microgrid is in off-grid operation, all connected DG generators use V / f control. DG generators using PQ control can be considered equivalent to current sources controlled by the grid connection point voltage, while DG generators using V / f control can be considered equivalent to voltage sources controlled by the grid connection point output current. When a fault occurs, the current detection threshold is: （12）； In the formula: This is the detection threshold for the DG fault current when the fault occurs within the target area. This refers to the rated operating current of the DG when the fault occurs inside the target area. For line MN and M-terminal protection QF1, when the microgrid is connected to the grid, the fault current flowing through protection QF1 is jointly provided by its downstream generation units; when the microgrid is operating independently, the fault current flowing through protection QF1 is only provided by the generation units of the parallel branches. Furthermore, due to the current limiting effect of the power electronic converter, the fault current flowing through protection QF1 under both independent and grid-connected operation conditions should satisfy the following relationship: （13）； In the formula: When a fault occurs within the target area, the fault current flows through protection QF1 during independent operation of the microgrid. When a fault occurs within the target area, the fault current flowing through protection QF1 during grid-connected operation of the microgrid; When the microgrid line MN is within When a fault occurs at point M, the current at both ends of bus M and N will be... and The magnitudes are variable, but the directions are the same; all flows from the busbar to the line. Ideally, they are in the same phase with a phase difference of 0°. （14）； In the formula: To calculate the argument of the phasor within the brackets.

7. The method for analyzing the fault characteristics of distribution networks and microgrids during microgrid faults according to claim 1, characterized in that, In step 6), when a short-circuit fault occurs in a microgrid line within a distribution network DN-MGs line containing a microgrid, if the fault occurs outside the target area, the external fault current characteristics of the microgrid under grid-connected or off-grid operation are constructed based on the fault equivalent circuit EC-4 of the distribution network DN-MGs under grid-connected operation and the fault equivalent circuit EC-5 of the distribution network DN-MGs under off-grid operation. Specifically: When the microgrid is in grid-connected operation, all connected distributed generation (DG) generators use PQ control; when the microgrid is in off-grid operation, all connected DG generators use V / f control. DG generators using PQ control can be considered equivalent to current sources controlled by the grid connection point voltage, while DG generators using V / f control can be considered equivalent to voltage sources controlled by the grid connection point output current. When a fault occurs, the current detection threshold is: （15）； In the formula: This is the detection threshold for the DG fault current when the fault occurs outside the target area. This refers to the rated operating current of the DG when the fault occurs outside the target area. For line MN and M-terminal protection QF1, when the microgrid is connected to the grid, the fault current flowing through protection QF1 is jointly provided by its downstream generation units; when the microgrid is operating independently, the fault current flowing through protection QF1 is only provided by the generation units of the parallel branches. Furthermore, due to the current limiting effect of the power electronic converter, the fault current flowing through protection QF1 under both independent and grid-connected operation conditions should satisfy the following relationship: （16）； In the formula: When a fault occurs within the target area, the fault current flows through protection QF1 during independent operation of the microgrid. When a fault occurs within the target area, the fault current flowing through protection QF1 during grid-connected operation of the microgrid; When the location is outside the microgrid line MN Point or When a fault occurs, the current at both ends of bus M and N will be... and Equal in magnitude but opposite in direction, flowing from the busbar to the line at one end and from the line to the busbar at the other, with a phase difference of [value missing]. ,Right now （17）； In the formula: To calculate the argument of the phasor within the brackets.

8. The method for analyzing the fault characteristics of distribution networks and microgrids during microgrid faults according to claim 1, characterized in that, In step 7), when a short-circuit fault occurs in the microgrid line of the distribution network DN-MGs line containing the microgrid, and the microgrid is in grid-connected operation, the grid-connected fault voltage characteristics of the microgrid under fault conditions within or outside the zone are constructed based on the fault equivalent circuit EC-4, specifically as follows: When a microgrid is connected to the grid, it lacks the support of the main grid after a fault, and the capacity of the distributed generation (DG) is limited. Therefore, the voltage at the DG output terminal on the faulty line side will drop significantly. （18）； In the formula, , These represent the voltages at the DG output terminals on the faulty line side before and after the fault.

9. The method for analyzing the fault characteristics of distribution networks and microgrids during microgrid faults according to claim 1, characterized in that, In step 8), when a short-circuit fault occurs in the microgrid line of the distribution network DN-MGs line containing the microgrid, and the microgrid is in an off-grid operation state, the off-grid fault voltage characteristics of the microgrid under fault conditions within or outside the zone are constructed according to the fault equivalent circuit EC-5, specifically as follows: When a fault occurs in an independently operating microgrid, the power electronic converter in the distributed generation (DG) is forced to limit the current, resulting in a significant voltage drop at the DG output. （19）； In the formula, , These represent the voltages at the DG output terminals on the faulty line side before and after the fault.

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