Method for accurately calculating short-circuit current of complex power distribution network

Abstract

The invention discloses a method for accurately calculating short-circuit current of a complex power distribution network. The method comprises the following steps: determining a system no-load voltage initial value according to basic data; short-circuit current during fault is collected, and a fault loop impedance initial value is determined in combination with a system no-load voltage initial value; calculating a system impedance correction value based on the fault loop impedance initial value; obtaining a short-circuit current error, constructing an error between a system impedance correction value and an actual value based on the short-circuit current error, and optimizing the system impedance correction value to obtain an accurate value of the system impedance; the method comprises the following steps: acquiring basic impedance values at the tail ends of branch lines and section switches of the distribution network, determining short-circuit current of the distribution line and short-circuit current of the low-voltage side of the distribution transformer by combining with an accurate value of system impedance, and accurately calculating short-circuit current at the tail ends of the branch lines of the distribution network, the boundary switch protection parts of the sections and the low-voltage side of the distribution transformer. And the accuracy of short-circuit current calculation is improved.

Description

Technical Field

[0001] The invention relates to the field of power system analysis and relay protection, and in particular to a method for accurately calculating short-circuit current of a complex distribution network. Background Art

[0002] The current calculation method of short-circuit current of distribution network lines and its shortcomings are as follows: First, the short-circuit current is calculated using the equivalent system impedance calculated by the upper-level power grid, which simplifies the upper-level power grid into an equivalent impedance and ignores its internal structure and characteristics. When certain assumptions such as power grid operation mode and generator status change, the accuracy of the equivalent impedance will be affected, thereby affecting the short-circuit calculation; second, due to the long lines, many branches, many components and complex network structure, the electrical parameters of the distribution network lines are often not fully collected and updated in a timely manner, which will lead to inaccurate calculation of short-circuit current; third, the existing short-circuit calculation method only considers metallic faults, but according to actual operating experience, it is found that distribution network line faults are not only simple metallic short circuits, but are often accompanied by arc short circuits. The arc resistance generated by arc short circuits will also have a certain impact on the short-circuit current. Summary of the invention

[0003] The technical problem to be solved by the present invention is that the accuracy of equivalent impedance is poor and the short-circuit current calculation is inaccurate. The purpose is to provide a method for accurately calculating the short-circuit current of a complex distribution network, and to reversely check the system impedance using the short-circuit current obtained from existing historical fault cases, thereby providing an accurate equivalent impedance and proposing a method for calculating the arc short-circuit resistance. At the same time, the electrical impedance of each key node 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 of the distribution network and at each segment boundary switch protection point and at the low-voltage side of the distribution transformer, thereby improving the accuracy of the short-circuit current calculation and providing strong data support for subsequent distribution network fault analysis and location, relay protection setting calculation, engineering design, system planning and other related applications.

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

[0005] The present invention provides a method for accurately calculating short-circuit current in a complex distribution network, comprising the following specific steps:

[0006] Collect basic data of complex distribution networks and determine the initial value of the system no-load voltage;

[0007] Collect the short-circuit current during the fault and determine the initial value of the fault loop impedance in combination with the initial value of the system no-load voltage;

[0008] Based on the initial value of the fault loop impedance, calculate the system impedance correction value;

[0009] Obtain the short-circuit current error, construct the error between the system impedance correction value and the actual value based on the short-circuit current error, optimize the system impedance correction value, and obtain the accurate value of the system impedance;

[0010] The basic impedance value of the line from the busbar to the end of each distribution network branch line and the section switch is collected, and combined with the precise value of the system impedance, the short-circuit current of the distribution line and the short-circuit current on the low-voltage side of the distribution transformer are determined.

[0011] Furthermore, the step of determining the initial value of the system no-load voltage comprises:

[0012] Obtain the initial value of system impedance, including initial resistance value and initial reactance value;

[0013] According to the bus voltage and load current before the fault, combined with the initial value of the system impedance, the initial value of the system no-load voltage is calculated:

[0014] Us0=Ubus+If(Rs_raw+jXs_raw)

[0015] Among them, Us0 represents the initial value of the system no-load voltage, Ubus represents the bus voltage before the fault, If represents the load current, Rs_raw represents the initial resistance value, and Xs_raw represents the initial reactance value.

[0016] Furthermore, the determining of the initial value of the fault loop impedance comprises the following specific steps:

[0017] Collect the short-circuit current during the fault and calculate the initial value of the fault loop impedance based on the system no-load voltage;

[0018] Combined with the impedance value of the low-voltage distribution transformer, determine whether the obtained loop impedance exceeds the limit. If so, report a low-voltage over-limit fault of the distribution transformer, and re-collect short-circuit fault data for calculation.

[0019] Furthermore, the calculation of the initial value of the fault loop impedance comprises the following specific steps:

[0020] Two-phase short circuit:

[0021]

[0022] Three-phase short circuit:

[0023]

[0024] Among them, Zd_raw represents the initial value of the fault loop impedance, Us0 represents the initial value of the system no-load voltage, and Ida, Idb, and Idc represent the three-phase short-circuit current.

[0025] Furthermore, based on the initial value of the fault loop impedance, a system impedance correction value is calculated, and the specific steps include:

[0026] Collect the short-circuit point location and line parameters during the fault, and obtain the impedance from the short-circuit point to the busbar;

[0027] Obtaining a fault type, wherein the fault type includes: a metallic short circuit fault and an arc short circuit fault;

[0028] According to the fault type, calculate the system impedance correction value respectively.

[0029] Further, the respectively calculating the system impedance correction value specifically includes:

[0030] For metallic short circuit fault:

[0031]

[0032] Wherein, Zs0 represents the system impedance correction value, Rdt represents the resistance value from the short-circuit point to the bus, Xdt represents the reactance value from the short-circuit point to the bus, 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.

[0033] Furthermore, for arc short circuit faults:

[0034]

[0035] RF=1443.57*Larc / Id_fact

[0036] 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.

[0037] Furthermore, the step of calculating the precise value of the system impedance specifically includes:

[0038] Taking the system impedance correction value as the initial value, calculate the system no-load voltage again and obtain the theoretical value of the fault current;

[0039] Obtain the error between the theoretical value of the fault current and the actual value of the fault current;

[0040] Taking the error between the theoretical value of the fault current and the actual value of the fault current as the target, 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.

[0041] Furthermore, the specific calculation steps of the short-circuit current of the distribution line include:

[0042] Obtain the basic impedance value at the end of each distribution network branch line and at the section switch;

[0043] According to the precise value of system impedance, branch line impedance and system no-load voltage, calculate the precise value of short-circuit current at the end of each branch line and at the section switch:

[0044] Metallic three-phase short-circuit current:

[0045]

[0046] Arc two-phase short-circuit current:

[0047]

[0048] Among them, Id3-metal represents metallic three-phase short-circuit current, Id2-arc represents arc two-phase short-circuit current, U0 represents system no-load voltage, Zs represents the exact value of system impedance, ZL represents the basic impedance value of the line from the busbar to the end of the branch line and at the section switch, and RF represents arc resistance.

[0049] Furthermore, the specific calculation steps of the short-circuit current on the low-voltage side of the distribution transformer include:

[0050] Three-phase metal short-circuit current:

[0051]

[0052] Two-phase metal short-circuit current:

[0053]

[0054] Among them, Id3-trans represents the three-phase metal short-circuit current, Id2-trans represents the two-phase metal short-circuit current, U0 represents the system no-load voltage, Zs represents the exact value of the system impedance, ZT represents the typical impedance value of the distribution transformer, and ZL represents the impedance value from the high-voltage side of the distribution transformer to the busbar.

[0055] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0056] The short-circuit current obtained from the existing historical fault cases reversely checks the system impedance, provides an accurate system equivalent impedance, and proposes a method for calculating the short-circuit resistance during an arc short circuit. At the same time, the electrical impedance of each key node 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 of the distribution network and the switch protection of each segment boundary, as well as the low-voltage side of the distribution transformer, thereby improving the accuracy of the short-circuit current calculation;

[0057] It helps to reverse the location of the fault through the short-circuit current when the system fails, realize fault analysis and location, and identify the low-voltage fault of the distribution transformer; it helps to calculate the accuracy of relay protection setting, so that the protection equipment can act in time when a fault occurs, improving the safety of system operation; it helps to select electrical equipment during power engineering design and evaluate system capacity and stability during power system planning. BRIEF DESCRIPTION OF THE DRAWINGS

[0058] 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:

[0059] Figure 1 A method for accurately calculating short-circuit current in an embodiment of the present invention;

[0060] Figure 2 A system no-load voltage calculation vector diagram in an embodiment of the present invention;

[0061] Figure 3 It is a system impedance calculation vector diagram in an embodiment of the present invention. DETAILED DESCRIPTION

[0062] 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.

[0063] As a possible implementation, Figure 1As shown, this embodiment provides a method for accurately calculating the short-circuit current of a complex distribution network, including the following specific steps: collecting basic data of the complex distribution network to determine the initial value of the system no-load voltage; collecting the short-circuit current during a fault, and determining the initial value of the fault loop impedance in combination with the initial value of the system no-load voltage; calculating the system impedance correction value based on the initial value of the fault loop impedance; obtaining the short-circuit current error, constructing the error between the system impedance correction value and the actual value based on the short-circuit current error, optimizing the system impedance correction value, and obtaining the precise value of the system impedance; collecting the basic impedance values ​​at the ends of each distribution network branch line and at the section switch, and determining the distribution line short-circuit current and the short-circuit current on the low-voltage side of the distribution transformer in combination with the precise value of the system impedance. This embodiment reversely checks the system impedance through the short-circuit current obtained from existing historical fault cases, provides accurate equivalent impedance, proposes a method for calculating the short-circuit resistance during an arc short circuit, and simultaneously accurately collects and calculates the electrical impedance of each key node of the distribution line, so as to accurately calculate the short-circuit current at the end of each branch line of the distribution network and the protection points of each segment boundary switch and the low-voltage side of the distribution transformer, thereby improving the accuracy of the short-circuit current calculation and providing strong data support for subsequent distribution network fault analysis and location, relay protection setting calculation, engineering design, system planning and other related applications.

[0064] In some possible implementations, determining the initial value of the system no-load voltage includes the following specific steps:

[0065] Obtain the initial value of system impedance Zs_raw, including the initial resistance value Rs_raw and the initial reactance value Xs_raw;

[0066] like Figure 2 As shown in the figure, according to the bus voltage Ubus and load current If before the fault, combined with the initial value of the system impedance, the initial value of the system no-load voltage is calculated:

[0067] Us0=Ubus+If(Rs_raw+jXs_raw)

[0068] Among them, Us0 represents the initial value of the system no-load voltage, Ubus represents the bus voltage before the fault, If represents the load current, Rs_raw represents the initial resistance value, and Xs_raw represents the initial reactance value.

[0069] In some possible implementations, determining the initial value of the fault loop impedance includes the following specific steps:

[0070] Collect the short-circuit current Id_fact (Ida, Idb, Idc) during the fault, and calculate the initial value of the fault loop impedance Zd_raw based on the system no-load voltage:

[0071] Two-phase short circuit:

[0072]

[0073] Three-phase short circuit:

[0074]

[0075] Among them, Zd_raw represents the initial value of the fault loop impedance, Us0 represents the initial value of the system no-load voltage, and Ida, Idb, and Idc represent the three-phase short-circuit current;

[0076] Combined with the impedance value of the low-voltage distribution transformer, determine whether the obtained loop impedance exceeds the limit. If so, report a low-voltage over-limit fault of the distribution transformer, and re-collect short-circuit fault data for calculation.

[0077] In some possible implementations, based on the initial value of the fault loop impedance, the system impedance correction value is calculated, and the specific steps include:

[0078] Collect the short-circuit point location and line parameters during the fault, and obtain the impedance from the short-circuit point to the busbar;

[0079] Get the fault type, which includes metallic short circuit fault and arc short circuit fault;

[0080] According to the fault type, calculate the system impedance correction value separately, including:

[0081] like Figure 3 As shown, for a metallic short circuit fault:

[0082]

[0083] 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;

[0084] like Figure 3 As shown, for an arc short circuit fault:

[0085]

[0086] RF=1443.57*Larc / Id_fact

[0087] 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.

[0088] In some possible implementations, the step of calculating the precise value of the system impedance specifically includes:

[0089] 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:

[0090]

[0091] The error between the theoretical value of the fault current Id_theory and the actual value of the fault current Id_fact is △I;

[0092] 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.

[0093] In some possible implementations, the specific steps of calculating the short-circuit current of the distribution line include:

[0094] Obtain the basic impedance value ZL at the end of each distribution network branch line and the section switch;

[0095] According to the precise value of system impedance, branch line impedance and system no-load voltage, calculate the precise value of short-circuit current at the end of each branch line and at the section switch:

[0096] Metallic three-phase short-circuit current:

[0097]

[0098] Arc two-phase short-circuit current:

[0099]

[0100] Among them, Id3-trans represents the metallic three-phase short-circuit current, Id2-trans represents the arc two-phase short-circuit current, U0 represents the system no-load voltage, Zs represents the precise value of the system impedance, ZL represents the line basic impedance value from the busbar to the end of the branch line and the section switch, and RF represents the arc resistance. For the calculation of the above arc two-phase short-circuit current, from theoretical analysis and calculation results of multiple fault examples, it can be seen that at the end of the long line, since the short-circuit current is relatively small, no arc effect will be generated when a fault occurs, and the above arc current calculation results do not converge. At this time, only the metallic short-circuit fault is considered in the effective calculation.

[0101] In some possible implementations, the specific steps of calculating the short-circuit current on the low-voltage side of the distribution transformer include:

[0102] Three-phase metal short-circuit current:

[0103]

[0104] Two-phase metal short-circuit current:

[0105]

[0106] Among them, Id3-trans represents the three-phase metal short-circuit current on the low-voltage side of the distribution transformer, Id2-trans represents the two-phase metal short-circuit current on the low-voltage side of the distribution transformer, U0 represents the system no-load voltage, Zs represents the exact value of the system impedance, ZT represents the typical impedance value of the distribution transformer, and ZL represents the impedance value from the high-voltage side of the distribution transformer to the busbar.

[0107] 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 accurately calculating short-circuit current in a complex distribution network, characterized in that: The specific steps include: Collect basic data of complex distribution networks and determine the initial value of the system no-load voltage; Collect the short-circuit current during the fault and determine the initial value of the fault loop impedance in combination with the initial value of the system no-load voltage; Based on the initial value of the fault loop impedance, calculate the system impedance correction value; Obtain the short-circuit current error, construct the error between the system impedance correction value and the actual value based on the short-circuit current error, optimize the system impedance correction value, and obtain the accurate value of the system impedance; The basic impedance value of the line from the busbar to the end of each distribution network branch line and the section switch is collected, and combined with the precise value of the system impedance, the short-circuit current of the distribution line and the short-circuit current on the low-voltage side of the distribution transformer are determined.

2. The method for accurately calculating short-circuit current in a complex distribution network according to claim 1, characterized in that: The specific steps of determining the initial value of the system no-load voltage include: Obtain the initial value of system impedance, including initial resistance value and initial reactance value; According to the bus voltage and load current before the fault, combined with the initial value of the system impedance, the initial value of the system no-load voltage is calculated: Us0=Ubus+If(Rs_raw+jXs_raw) Among them, Us0 represents the initial value of the system no-load voltage, Ubus represents the bus voltage before the fault, If represents the load current, Rs_raw represents the initial resistance value, and Xs_raw represents the initial reactance value.

3. The method for accurately calculating short-circuit current in a complex distribution network according to claim 1, characterized in that: The specific steps of determining the initial value of the fault loop impedance include: Collect the short-circuit current during the fault and calculate the initial value of the fault loop impedance based on the system no-load voltage; Combined with the impedance value of the low-voltage distribution transformer, determine whether the obtained loop impedance exceeds the limit. If so, report a low-voltage over-limit fault of the distribution transformer, and re-collect short-circuit fault data for calculation.

4. The method for accurately calculating short-circuit current in a complex distribution network according to claim 3 is characterized in that: The specific steps of calculating the initial value of the fault loop impedance include: Two-phase short circuit: Three-phase short circuit: Among them, Zd_raw represents the initial value of the fault loop impedance, Us0 represents the initial value of the system no-load voltage, and Ida, Idb, and Idc represent the three-phase short-circuit current.

5. The method for accurately calculating short-circuit current in a complex distribution network according to claim 1, characterized in that: Based on the initial value of the fault loop impedance, the system impedance correction value is calculated. The specific steps include: Collect the short-circuit point location and line parameters during the fault, and obtain the impedance from the short-circuit point to the busbar; Obtaining a fault type, wherein the fault type includes: a metallic short circuit fault and an arc short circuit fault; According to the fault type, calculate the system impedance correction value respectively.

6. The method for accurately calculating short-circuit current in a complex distribution network according to claim 5, characterized in that: The respectively calculating the system impedance correction values ​​specifically includes: 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 bus, Xdt represents the reactance value from the short-circuit point to the bus, 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.

7. The method for accurately calculating short-circuit current in a complex distribution network according to claim 5, characterized in that: 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.

8. The method for accurately calculating short-circuit current in a complex distribution network according to claim 1, characterized in that: The steps for calculating the precise value of the system impedance specifically include: Taking the system impedance correction value as the initial value, calculate the system no-load voltage again 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 error between the theoretical value of the fault current and the actual value of the fault current as the target, 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.

9. The method for accurately calculating short-circuit current in a complex distribution network according to claim 1, characterized in that: The specific calculation steps of the short-circuit current of the distribution line include: Obtain the basic impedance value at the end of each distribution network branch line and at the section switch; According to the precise value of system impedance, branch line impedance and system no-load voltage, calculate the precise value of short-circuit current at the end of each branch line and at the section switch: 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 system no-load voltage, Zs represents the exact value of system impedance, ZL represents the basic impedance value of the line from the busbar to the end of the branch line and at the section switch, and RF represents arc resistance.

10. The method for accurately calculating short-circuit current in a complex distribution network according to claim 1, characterized in that: The specific calculation steps of the short-circuit current on the low-voltage side of the distribution transformer include: Three-phase metal short-circuit current: Two-phase metal short-circuit current: Among them, Id3-trans represents the three-phase metal short-circuit current on the low-voltage side of the distribution transformer, Id2-trans represents the two-phase metal short-circuit current on the low-voltage side of the distribution transformer, U0 represents the system no-load voltage, Zs represents the exact value of the system impedance, ZT represents the typical impedance value of the distribution transformer, and ZL represents the impedance value from the high-voltage side of the distribution transformer to the busbar.

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