Complex distribution line over-current protection accurate setting method

Abstract

The invention discloses a method for accurately setting overcurrent protection of a complex distribution line, which comprises the following specific steps of: correcting a system impedance value according to an actual metallic short circuit or arc light short circuit fault type; constructing an optimal function of a system impedance value, and performing repeated iteration check by using historical fault current to obtain an optimal accurate value of the system impedance; according to the system impedance optimal accurate value and the branch line impedance value, short-circuit current accurate values at the tail end of each branch line and the section switch protection installation position are calculated respectively; different setting principles are set according to the type of the distribution network switch, and the overcurrent protection constant value is set in a graded mode. The over-current protection setting accuracy of the distribution network line is improved, over-current protection override action caused by distribution transformer low-voltage side faults is reliably prevented, the fault power failure time is greatly shortened, the power sale quantity is increased, the user satisfaction degree is improved, and the economic benefits and social images of power supply enterprises are improved.

Description

Technical Field

[0001] The present invention relates to the technical field of relay protection for power systems, and in particular to a method for accurately setting overcurrent protection for complex distribution lines. Background Art

[0002] Distribution lines are one of the most prone to failures due to their long lines, many branches, many components, complex network structure, and susceptibility to external forces and the natural environment. The current distribution line overcurrent protection setting calculation principles and deficiencies are as follows: 1. The instantaneous current quick-break protection is used as the overcurrent I section, and its current setting value: 1) It is set according to the maximum three-phase short-circuit current of the end of the line or the lower-level section (branch) circuit breaker, and the reliability factor is not less than 1.3; 2) It is set according to the adjacent transformer excitation surge current, taking (3~8)*Ie; Existing deficiencies: The instantaneous current quick-break protection of the distribution line branch (section) switch often has a small protection range or no protection range. In addition, considering the complex topology of the distribution network, incomplete electrical parameters and other factors may cause protection malfunction, the setting calculation is often retreated. 1. The instantaneous current quick-break protection is sacrificed, sacrificing the speed of relay protection; 2. The time-limited current quick-break protection and overcurrent protection are combined into overcurrent stage II, and its current setting value: 1) It is set according to the two-phase metallic short-circuit at the end of the line, and the sensitivity coefficient is not less than 1.5; 2) It is set according to the maximum load current of the line including the self-starting current, and the reliability coefficient is not less than 1.35; The shortcomings are: when the protection sensitivity is set, the protection range may extend beyond the low-voltage side of the distribution transformer, which may easily cause the distribution transformer to trip due to low-voltage faults. When the load is set, due to the frequent load changes of the distribution network line, it is also easy to cause false operation or refusal of protection, affecting the selectivity and reliability of relay protection. Summary of the invention

[0003] The technical problem to be solved by the present invention is that the existing setting method of overcurrent protection of distribution lines has problems such as unclear protection range, poor speed and insufficient sensitivity. The purpose is to provide a method for accurate setting of overcurrent protection of complex distribution lines. The system bus impedance is reversely checked multiple times through actual fault cases, and the electrical impedance of each key point of the distribution line is accurately collected and calculated. The short-circuit current at the end of each branch line and the short-circuit current at the installation point of each section boundary switch are accurately calculated. Different setting principles are set according to the type of distribution network switch, and the overcurrent protection value is set in a graded manner to improve the input rate of overcurrent I stage protection, ensure instantaneous removal in the event of a fault, shorten the fault time, and ensure equipment safety; ensure the selectivity of overcurrent II stage protection, prevent over-tripping, and improve power supply safety and reliability.

[0004] The present invention is achieved through the following technical solutions:

[0005] The present invention provides a method for accurately setting overcurrent protection of a complex distribution line, comprising the following specific steps:

[0006] Correct the system impedance value according to the actual metallic short circuit or arc short circuit fault type;

[0007] Construct the optimal function of system impedance value, use historical fault current to iterate and check repeatedly, and obtain the optimal and accurate value of system impedance;

[0008] According to the optimal and accurate value of system impedance and branch line impedance, calculate the accurate value of short-circuit current at the end of each branch line and at the installation location of the sectionalized switch protection;

[0009] Different setting principles are set according to the type of distribution network switch, and the overcurrent protection setting value is adjusted in stages.

[0010] Furthermore, the correction of the system impedance value comprises the following specific steps:

[0011] The short-circuit impedance obtained based on the actual fault case is the fault loop impedance value;

[0012] Based on the fault loop impedance value, the fault point to bus impedance value, and combined with different actual fault types, the system impedance value is corrected.

[0013] For metallic short circuit fault:

[0014]

[0015] Wherein, Zs0 represents the system impedance correction value, Rdt represents the resistance value from the short-circuit point to the busbar, Xdt represents the reactance value from the short-circuit point to the busbar, Φ=270-(90-δ)-θ, δ represents the line impedance angle at the actual fault point, θ represents the phase angle of the initial value of the system impedance, and Zd_raw represents the initial value of the fault loop impedance;

[0016] For arc short circuit fault:

[0017]

[0018] RF=1443.57*Larc / Id_fact

[0019] Wherein, RF represents arc resistance, Larc represents arc length, Id_fact represents actual value of fault current, Φ=270-(90-δ′)-θ, δ′ is the impedance angle between line resistance at actual fault point and line reactance at actual fault point after arc resistance is superimposed.

[0020] Furthermore, the step of obtaining the optimal and accurate value of the system impedance comprises:

[0021] Taking the system impedance correction value as the initial value, calculate the system no-load voltage and obtain the theoretical value of the fault current;

[0022] Obtain the error between the theoretical value of the fault current and the actual value of the fault current;

[0023] Taking the minimum current error as the goal, a target optimization function of current and impedance is constructed based on the system impedance correction value. The system impedance correction value is iteratively checked based on the target function to obtain the accurate value of the system impedance.

[0024] Furthermore, the calculation of the precise value of the short-circuit current at the end of each branch line and at the installation location of the sectionalized switch protection specifically includes:

[0025] Metallic three-phase short-circuit current:

[0026]

[0027] Arc two-phase short-circuit current:

[0028]

[0029] Among them, Id3-metal represents metallic three-phase short-circuit current, Id2-arc represents arc two-phase short-circuit current, U0 represents voltage, Zs represents the exact value of system impedance, ZL represents branch line impedance value, and RF represents arc resistance.

[0030] Furthermore, different setting principles are set according to the types of distribution network switches, and the hierarchical setting of overcurrent protection settings includes: the last-stage switch protection setting, the intermediate switch protection setting and the substation-end switch protection setting.

[0031] Furthermore, the final switch protection setting specifically includes:

[0032] Overcurrent protection current setting value of stage I: set according to the fault at the end of the line;

[0033] Overcurrent II stage protection current setting: set according to the bus fault on the low-voltage side of the transformer.

[0034] Furthermore, the intermediate switch protection setting specifically includes:

[0035] Overcurrent protection current setting value of stage I: set according to the fault of the next level protection installation location;

[0036] Overcurrent II protection current setting: set according to the sensitivity of the installation location of the next level protection or other branch line ends or according to the bus fault on the low-voltage side of the transformer.

[0037] Furthermore, the substation-side switch protection setting specifically includes:

[0038] Overcurrent protection current setting value of stage I: set according to the fault of the next level protection installation location;

[0039] Overcurrent II protection current setting value: set according to the installation location of the first-level protection and the sensitivity of the end of other branch lines or according to the busbar fault on the low-voltage side of the transformer;

[0040] Overcurrent protection current setting of stage III: It is set according to the maximum load current of the line including the self-starting current or according to the sensitivity of the protection line fault.

[0041] Furthermore, the fault setting at the end of the line is avoided, including:

[0042] I1=Krel*Id2

[0043] Id2=Id2-arc

[0044] Among them, Krel represents the reliability coefficient, Id2-arc represents the two-phase arc short-circuit current at the end of the branch line where the protection is installed, and I1 represents the overcurrent I stage current;

[0045] According to the busbar fault setting on the low-voltage side of the transformer, it includes:

[0046] I2=Krel*Id3max

[0047] Among them, Id3max represents the maximum three-phase short-circuit current on the low-voltage side of the distribution transformer, and I2 represents the overcurrent II stage current;

[0048] According to the fault setting of the next level protection installation, it includes:

[0049] I1=Krel*Id3

[0050] Id3=Id3-metal

[0051] Among them, Id3-metal represents the maximum three-phase metal short-circuit current at the next level protection installation;

[0052] According to the sensitivity setting of the primary protection installation or other branch line end, it includes:

[0053] I2=Id2 / Klim

[0054] Id2=Id2-arc

[0055] Among them, Klim represents the sensitivity coefficient, Id2-arc represents the minimum two-phase arc short-circuit current at the next level protection installation or the minimum two-phase arc short-circuit current at the end of other branch lines;

[0056] The maximum load current of the line including the self-starting current is set, including:

[0057] I3=Krel*If-max

[0058] Among them, If-max represents the maximum load current, and I3 represents the overcurrent stage III current;

[0059] According to the fault of the maintenance line, the sensitivity setting includes:

[0060] I3=Id2 / Klim

[0061] Id2=Id2-arc

[0062] Among them, Id2-arc is the minimum arc short-circuit current of the entire line.

[0063] Furthermore, after the substation-side switch protection is set, the sensitivity check of the adjacent transformer low-voltage side is also performed, including:

[0064] I3=Id2max / Klim

[0065] Among them, Klim represents the sensitivity coefficient, Id2max represents the maximum short-circuit current of the two phases on the low-voltage side of the transformer, and I3 represents the overcurrent stage III current.

[0066] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0067] According to metallic short-circuit faults and arc short-circuit faults, the impedance value of the branch line is determined. The system bus impedance is reversely checked multiple times, and the electrical impedance of each key point of the distribution line is accurately collected and calculated. The short-circuit current at the end of each branch line and the short-circuit current at the installation point of each section boundary switch are accurately calculated. Different setting principles are set according to the type of distribution network switch, and the overcurrent protection setting value is adjusted in stages to increase the input rate of the overcurrent I stage protection, ensure instantaneous removal of faults, shorten the fault time, and ensure equipment safety; ensure the selectivity of the overcurrent II stage protection, prevent over-tripping, and improve the safety and reliability of power supply. BRIEF DESCRIPTION OF THE DRAWINGS

[0068] In order to more clearly illustrate the technical solutions of the exemplary embodiments of the present invention, the following briefly introduces the drawings required for use in the embodiments. It should be understood that the following drawings only illustrate certain embodiments of the present invention and should not be regarded as limiting the scope. For those of ordinary skill in the art, other relevant drawings can be obtained based on these drawings without creative work. In the drawings:

[0069] Figure 1 is a setting method in an embodiment of the present invention;

[0070] Figure 2 The figure is a wiring diagram of a power distribution network in an embodiment of the present invention. DETAILED DESCRIPTION

[0071] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with embodiments and drawings. The exemplary embodiments of the present invention and their description are only used to explain the present invention and are not intended to limit the present invention.

[0072] As a possible implementation, Figure 1 As shown, the present embodiment provides a method for accurate setting of overcurrent protection of complex distribution lines, comprising the following specific steps: correcting the system impedance value according to the actual metallic short circuit or arc short circuit fault type; constructing an optimal function of the system impedance value, and using historical fault currents to iterate and verify repeatedly to obtain the optimal and accurate value of the system impedance; calculating the precise values ​​of the short-circuit current at the end of each branch line and at the installation location of the section switch protection according to the optimal and accurate value of the system impedance and the branch line impedance value; setting different setting principles according to the type of distribution network switch, and setting the overcurrent protection constant in stages.

[0073] In this embodiment, the system bus impedance is reverse-checked multiple times through actual fault cases, and the electrical impedance of each key point of the distribution line is accurately collected and calculated, so as to accurately calculate the short-circuit current at the end of each branch line and the short-circuit current at the installation point of each section boundary switch. Different setting principles are then set according to the type of distribution network switch, and the overcurrent protection constant is adjusted in a graded manner to ensure rapid and accurate fault removal, shorten the fault time, prevent over-tripping, and improve the safety and reliability of power supply.

[0074] In some possible embodiments, the system impedance value is corrected, and the specific steps include:

[0075] The short-circuit impedance obtained based on the actual fault case is the fault loop impedance value;

[0076] Based on the fault loop impedance value, the fault point to bus impedance value, and combined with different actual fault types, the system impedance value is corrected.

[0077] For metallic short circuit fault:

[0078]

[0079] Wherein, Zs0 represents the system impedance correction value, Rdt represents the resistance value from the short-circuit point to the busbar, Xdt represents the reactance value from the short-circuit point to the busbar, Φ=270-(90-δ)-θ, δ represents the line impedance angle at the actual fault point, θ represents the phase angle of the initial value of the system impedance, and Zd_raw represents the initial value of the fault loop impedance;

[0080] For arc short circuit fault:

[0081]

[0082] RF=1443.57*Larc / Id_fact

[0083] Wherein, Zs0 represents the system impedance correction value, Rdt represents the resistance value from the short-circuit point to the busbar, RF represents the arc resistance, Larc represents the arc length, Id_fact represents the actual value of the fault current, Xdt represents the reactance value from the short-circuit point to the busbar, Φ=270-(90-δ′)-θ, δ′ is the impedance angle between the line resistance at the actual fault point and the line reactance at the actual fault point after the arc resistance is superimposed, and Zd_raw represents the initial value of the fault loop impedance.

[0084] In some possible embodiments, an optimal function of the system impedance value is constructed to obtain an optimal and accurate value of the system impedance. The specific steps include:

[0085] Taking the system impedance correction value Zs0 as the initial value, calculate the system no-load voltage U0 again, and obtain the theoretical value of the fault current:

[0086]

[0087] The error between the theoretical value of the fault current Id_theory and the actual value of the fault current Id_fact is △I;

[0088] Multiple short-circuit fault instances are collected, and the error between the theoretical value of the barrier current and the actual value of the fault current is taken as the target. Based on the system impedance correction value, the target optimization function of current and impedance △I=f(△Z) is constructed. Based on the target function, the system impedance correction value is iteratively checked to obtain the accurate value of the system impedance.

[0089] In some possible embodiments, calculating the precise value of the short-circuit current at the end of each branch line and at the location where the sectionalizer protection is installed specifically includes:

[0090] Metallic three-phase short-circuit current:

[0091]

[0092] Arc two-phase short-circuit current:

[0093]

[0094] Among them, Id3-metal represents metallic three-phase short-circuit current, Id2-arc represents arc two-phase short-circuit current, U0 represents voltage, Zs represents the exact value of system impedance, ZL represents branch line impedance value, and RF represents arc resistance.

[0095] In some possible embodiments, different setting principles are set according to the types of distribution network switches, and the specific contents of the hierarchical setting of overcurrent protection settings include: the last-stage switch protection setting, the intermediate switch protection setting and the substation-end switch protection setting.

[0096] In some possible embodiments, Figure 2 The protection setting of the circuit breaker DL3, i.e. the final switch, specifically includes:

[0097] Overcurrent I stage protection current setting: set according to the fault at the end of the line, including:

[0098] I1=Krel*Id2

[0099] Id2=Id2-arc

[0100] Among them, Krel represents the reliability coefficient. It can be seen from the operating example data that the fault location error of the short-circuit current obtained by this method is within 0.1km. After obtaining the precise value of the short-circuit current, in order to improve the protection range and investment rate of the current quick-break protection and ensure that the faults at the end of the line and the high-voltage side of the distribution transformer can be instantly removed, the reliability coefficient Krel is appropriately reduced to 0.95; Id2-arc is the two-phase arc short-circuit current at the end of the branch line at the protection installation. If the arc current does not converge, Id2=0.866*Id3-meta, that is, the two-phase metal short-circuit current at the end of the branch line at the protection installation.

[0101] Overcurrent II protection current setting: set according to the bus fault on the low-voltage side of the transformer, including:

[0102] I2=Krel*Id3max

[0103] Among them, Krel represents the reliability coefficient, which is 1.1, and Id3max represents the maximum three-phase short-circuit current on the low-voltage side of the distribution transformer.

[0104] In some possible embodiments, Figure 2 The protection setting of the circuit breaker DL2, i.e. the intermediate switch, specifically includes:

[0105] Overcurrent protection current setting value of stage I: set according to the fault of the next level protection installation, including:

[0106] I1=Krel*Id3

[0107] Id3=Id3-metal

[0108] Among them, Krel represents the reliability coefficient, which is 1.05, and Id3-metal represents the maximum three-phase metal short-circuit current at the next level of protection installation;

[0109] Overcurrent II protection current setting value: set according to the sensitivity of the installation location of the first-level protection or other branch line ends and set according to the busbar fault on the low-voltage side of the transformer:

[0110] According to the sensitivity setting of the primary protection installation or other branch line end, it includes:

[0111] I2=Id2 / Klim

[0112] Id2=Id2-arc

[0113] Wherein, Klim represents the sensitivity coefficient, which is 1.05, and Id2-arc represents the minimum two-phase arc short-circuit current at the next level protection installation or the minimum two-phase arc short-circuit current at the end of other branch lines. If the arc current does not converge, Id2=0.866*Id3-meta is taken, which is the two-phase metal short-circuit current;

[0114] According to the busbar fault setting on the low-voltage side of the transformer, it includes:

[0115] I2=Krel*Id3max

[0116] Among them, Krel represents the reliability coefficient, which is 1.1, and Id3max represents the maximum three-phase short-circuit current on the low-voltage side of the distribution transformer.

[0117] In some possible embodiments, Figure 2 The circuit breaker DL1 in the substation is set to protect the switch, including:

[0118] Overcurrent I stage protection current setting: set according to the fault of the next level protection installation, including:

[0119] I1=Krel*Id3

[0120] Id3=Id3-metal

[0121] Among them, Krel represents the reliability coefficient, which is 1.05, and Id3-metal represents the maximum three-phase metal short-circuit current at the next level of protection installation;

[0122] Overcurrent II protection current setting value: set according to the installation location of the first-level protection and the sensitivity of other branch line ends and set according to the busbar fault on the low-voltage side of the transformer;

[0123] According to the sensitivity setting of the primary protection installation or other branch line end, it includes:

[0124] I2=Id2 / Klim

[0125] Id2=Id2-arc

[0126] Wherein, Klim represents the sensitivity coefficient, which is 1.05, and Id2-arc represents the minimum two-phase arc short-circuit current at the next level protection installation or the minimum two-phase arc short-circuit current at the end of other branch lines. If the arc current does not converge, Id2=0.866*Id3-meta is taken, which is the two-phase metal short-circuit current;

[0127] According to the busbar fault setting on the low-voltage side of the transformer, it includes:

[0128] I2=Krel*Id3max

[0129] Among them, Krel represents the reliability coefficient, which is 1.1, and Id3max represents the maximum three-phase short-circuit current on the low-voltage side of the distribution transformer;

[0130] Overcurrent protection current setting of stage III: set according to the maximum load current of the line including the self-starting current, set according to the sensitivity of the protection line fault and perform sensitivity check on the low-voltage side of the adjacent transformer:

[0131] The maximum load current of the line including the self-starting current is set, including:

[0132] I3=Krel*If-max

[0133] Among them, Krel represents the reliability coefficient, which is 1.35 to 2, and If-max represents the maximum load current. Considering the safety of the equipment, If-max cannot be greater than the rated primary current of the CT and the maximum allowable current carrying capacity of the line;

[0134] According to the fault of the maintenance line, the sensitivity setting includes:

[0135] I3=Id2 / Klim

[0136] Id2=Id2-arc

[0137] Among them, Klim represents the sensitivity coefficient, which is 1.05, and Id2-arc is the minimum arc short-circuit current of the entire line.

[0138] The overcurrent stage III of the substation-end switch is used as the overall backup protection for the entire line. After setting all the previous currents, it also includes sensitivity verification of the low-voltage side of the adjacent transformer:

[0139] I3=Id2max / Klim

[0140] Among them, Klim represents the sensitivity coefficient, and Id2max represents the maximum short-circuit current of the two phases on the low-voltage side of the transformer.

[0141] The specific implementation methods described above further illustrate the objectives, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above description is only a specific implementation method of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for accurate setting of overcurrent protection of complex distribution lines, characterized in that: The specific steps include: Correct the system impedance value according to the actual metallic short circuit or arc short circuit fault type; Construct the optimal function of system impedance value, use historical fault current to iterate and check repeatedly, and obtain the optimal and accurate value of system impedance; According to the optimal and accurate value of system impedance and branch line impedance, calculate the accurate value of short-circuit current at the end of each branch line and at the installation location of the sectionalized switch protection; Different setting principles are set according to the type of distribution network switch, and the overcurrent protection setting value is adjusted in stages.

2. The method for accurate setting of overcurrent protection of complex distribution lines according to claim 1 is characterized in that: The specific steps of correcting the system impedance value include: The short-circuit impedance obtained based on the actual fault case is the fault loop impedance value; Based on the fault loop impedance value, the fault point to bus impedance value, and the actual different fault types, the system impedance value is corrected; For metallic short circuit fault: Wherein, Zs0 represents the system impedance correction value, Rdt represents the resistance value from the short-circuit point to the busbar, Xdt represents the reactance value from the short-circuit point to the busbar, Φ=270-(90-δ)-θ, δ represents the line impedance angle at the actual fault point, θ represents the phase angle of the initial value of the system impedance, and Zd_raw represents the initial value of the fault loop impedance; For arc short circuit fault: RF=1443.57*Larc / Id_fact Wherein, Zs0 represents the system impedance correction value, Rdt represents the resistance value from the short-circuit point to the busbar, RF represents the arc resistance, Larc represents the arc length, Id_fact represents the actual value of the fault current, Xdt represents the reactance value from the short-circuit point to the busbar, Φ=270-(90-δ′)-θ, δ′ is the impedance angle between the line resistance at the actual fault point and the line reactance at the actual fault point after the arc resistance is superimposed, and Zd_raw represents the initial value of the fault loop impedance.

3. The method for accurate setting of overcurrent protection of complex distribution lines according to claim 1 is characterized in that: The optimal function of the system impedance value is constructed to obtain the optimal and accurate value of the system impedance, and the specific steps include: Taking the system impedance correction value as the initial value, calculate the system no-load voltage and obtain the theoretical value of the fault current; Obtain the error between the theoretical value of the fault current and the actual value of the fault current; Taking the minimum current error as the goal, a target optimization function of current and impedance is constructed based on the system impedance correction value. The system impedance correction value is iteratively checked based on the target function to obtain the accurate value of the system impedance.

4. The method for accurate setting of overcurrent protection of complex distribution lines according to claim 1 is characterized in that: The calculation of the precise value of the short-circuit current at the end of each branch line and at the installation location of the sectionalized switch protection specifically includes: Metallic three-phase short-circuit current: Arc two-phase short-circuit current: Among them, Id3-metal represents metallic three-phase short-circuit current, Id2-arc represents arc two-phase short-circuit current, U0 represents voltage, Zs represents the exact value of system impedance, ZL represents branch line impedance value, and RF represents arc resistance.

5. The method for accurate setting of overcurrent protection of complex distribution lines according to claim 1 is characterized in that: Different setting principles are set according to the type of distribution network switch, and the overcurrent protection setting value is set in stages. The setting contents include: the last-stage switch protection setting, the intermediate switch protection setting and the substation-end switch protection setting.

6. The method for accurate setting of overcurrent protection of complex distribution lines according to claim 5 is characterized in that: The final switch protection setting specifically includes: Overcurrent protection current setting value of stage I: set according to the fault at the end of the line; I1=Krel*Id2 Id2=Id2-arc Among them, Krel represents the reliability coefficient, Id2-arc represents the two-phase arc short-circuit current at the end of the branch line where the protection is installed, and I1 represents the overcurrent I stage current; Overcurrent II protection current setting value: set according to the bus fault on the low-voltage side of the transformer; I2=Krel*Id3max Among them, Id3max represents the maximum three-phase short-circuit current on the low-voltage side of the distribution transformer, and I2 represents the overcurrent stage II current.

7. The method for accurate setting of overcurrent protection of complex distribution lines according to claim 5 is characterized in that: The intermediate switch protection setting specifically includes: Overcurrent protection current setting value of stage I: set according to the fault of the next level protection installation location; I1=Krel*Id3 Id3=Id3-metal Among them, Id3-metal represents the maximum three-phase metal short-circuit current at the next level protection installation, and I1 represents the overcurrent I stage current; Overcurrent II protection current setting value: set according to the installation location of the first-level protection or the sensitivity of the end of other branch lines or according to the busbar fault on the low-voltage side of the transformer; I2=Id2 / Klim Id2=Id2-arc Among them, Klim represents the sensitivity coefficient, Id2-arc represents the minimum two-phase arc short-circuit current at the next level protection installation or the minimum two-phase arc short-circuit current at the end of other branch lines; I2=Krel*Id3max Among them, Id3max represents the maximum three-phase short-circuit current on the low-voltage side of the distribution transformer, and I2 represents the overcurrent stage II current.

8. The method for accurate setting of overcurrent protection of complex distribution lines according to claim 5 is characterized in that: The substation-side switch protection setting specifically includes: Overcurrent protection current setting value of stage I: set according to the fault of the next level protection installation location; I1=Krel*Id3 Id3=Id3-metal Among them, Id3-metal represents the maximum three-phase metal short-circuit current at the next level protection installation, and I1 represents the overcurrent I stage current; Overcurrent II protection current setting value: set according to the installation location of the first-level protection and the sensitivity of the end of other branch lines or according to the busbar fault on the low-voltage side of the transformer; I2=Id2 / Klim Id2=Id2-arc Among them, Klim represents the sensitivity coefficient, Id2-arc represents the minimum two-phase arc short-circuit current at the next level protection installation or the minimum two-phase arc short-circuit current at the end of other branch lines; I2=Krel*Id3max Among them, Id3max represents the maximum three-phase short-circuit current on the low-voltage side of the distribution transformer, and I2 represents the overcurrent II stage current; Overcurrent protection current setting of stage III: set according to the maximum load current of the line including the self-starting current or set according to the sensitivity of the protection line fault, and perform sensitivity check on the low-voltage side of the adjacent transformer; I3=Krel*If-max Where, If-max represents the maximum load current; According to the fault of the maintenance line, the sensitivity setting includes: I3=Id2 / Klim Id2=Id2-arc Among them, Id2-arc is the minimum arc short-circuit current of the entire line; I3=Id2max / Klim Among them, Klim represents the sensitivity coefficient, Id2max represents the maximum short-circuit current of the two phases on the low-voltage side of the transformer, and I3 represents the overcurrent stage III current.

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