Method for obtaining fault resistance range corresponding to starting setting value of small-resistance grounding

Abstract

The present invention relates to the technical field of distribution network fault detection and protection, and discloses a method for obtaining a fault resistance range corresponding to a starting setting value of a small-resistance grounding, including: quantifying the mathematical representation relationship between the starting setting value of the zero-sequence voltage in the distribution network during small-resistance grounding and the corresponding fault resistance range; establishing an optimization model based on the mathematical representation relationship; solving the optimization model to obtain the size of the fault resistance range. By means of the present invention, the detection adaptability of the single-phase grounding line selection device for medium-high resistance faults is improved, and accurate identification and effective disposal of grounding faults are realized.

Description

Technical Field

[0001] The present invention relates to the technical field of distribution network fault detection and protection, and particularly relates to a method for obtaining a fault resistance range corresponding to a starting setting value of a small-resistance grounding. Background Art

[0002] In a distribution network, the probability of a single-phase grounding fault is extremely high, and the fault inducing factors are complex and changeable, often making such faults extremely good at disguising and difficult to be fully exposed in the analysis vision of existing various algorithms for judgment. The reason is mainly that the single-phase grounding fault scenarios are diverse, such as tree lines, falling to the ground, and melting, etc. The characteristics of the zero-sequence voltage of the system after the fault for grounding fault detection vary in different scenarios, while the starting setting values of actual various grounding line selection devices, that is, the zero-sequence voltage parameters, are mostly set directly according to the different grounding methods of the system by avoiding the maximum unbalanced voltage under normal operation. Therefore, such setting values cannot fully cover all working conditions and there must be a lower limit, and the grounding faults below this lower limit will fall into the blind area. In practice, the setting of this lower limit value is mostly based on experience or directly avoiding the maximum unbalanced voltage under the maximum expected working condition of the system. Obviously, the parameter setting lacks effective theoretical support and cannot scientifically and reasonably guide the fault diagnosis and disposal. Summary of the Invention

[0003] Therefore, the present invention provides a method for obtaining a fault resistance range corresponding to a starting setting value of a small-resistance grounding, constructs a bridge between the starting setting value and the upper and lower limits of the grounding fault that can be effectively monitored from the perspective of mathematical derivation, and provides a directional reference for the reasonable setting of parameters, the effective start of judgment, and the scientific disposal of faults. In addition, the present invention will also construct a fault start detection method based on phasors to improve the sensitivity of fault detection. Considering the previous work of the research team of the present invention, the present invention will take the small-resistance grounding system as the key research object, and propose a judgment method for quantitatively evaluating the zero-sequence voltage starting setting value and the fault resistance identification range, which can quantitatively judge the ability of the starting setting value to identify the upper limit of the transition resistance, and is used in production to set scientific starting setting values in combination with the vegetation, trees, etc. covering conditions of the line passing through forests and grasslands, and reliably control the first "start" before fault judgment.

[0004] The present invention is realized by the following technical solutions:

[0005] A method for obtaining a fault resistance range corresponding to a starting setting value of a small-resistance grounding, comprising:

[0006] Quantifying the mathematical representation relationship between the starting setting value of the zero-sequence voltage in the distribution network during small-resistance grounding and the corresponding fault resistance range;

[0007] Establishing an optimization model based on the mathematical representation relationship;

[0008] Solve the optimization model to obtain the size of the fault resistance range.

[0009] As an optimization, the mathematical representation relationship between the starting setting value of the zero-sequence voltage and the corresponding fault resistance range

[0010]

[0011]

[0012] Among them, represents the asymmetry during normal operation of the distribution network, that is, the initial unbalance degree of the distribution network; d al is the fault damping rate of phase A, where phase A is the fault phase; d r is the damping rate associated with the small neutral resistance; S is the set threshold of the starting setting value of the zero-sequence voltage; is the voltage of phase A, d r is the damping rate associated with the small neutral resistance; r al is the fault resistance; u i represents the imaginary part of the asymmetry.

[0013] As an optimization, the specific process of quantifying the mathematical representation relationship between the starting setting value of the zero-sequence voltage and the corresponding fault resistance range is as follows:

[0014] Combined with Kirchhoff's law, at the neutral point, combined with the law of conservation of the inflow and outflow of node currents, deduce the mathematical representation relationship of the initial zero-sequence voltage of the distribution network:

[0015]

[0016] Among them, represents the initial zero-sequence voltage; Y a 、Y b 、Y c and Y0 are the ground conductances and susceptances of phases A, B, and C to the ground and the reciprocal of the small neutral resistance respectively; e is an operator, is the voltage of phase A; d r is the damping rate associated with the small neutral resistance, and where w is the angular frequency, w = 2πf, f represents the distribution network frequency, g r represents the reciprocal of the small neutral resistance, C a 、C b and C c are the charging capacitances of phases A, B, and C to the ground of the phase voltages respectively; represents the asymmetry during normal operation of the distribution network, that is, the initial unbalance degree of the distribution network;

[0017] After a ground fault occurs in phase A, the specific expression of the fault zero-sequence voltage of the distribution network is:

[0018]

[0019] Wherein, is the zero-sequence fault voltage of the distribution network after a grounding fault occurs, that is, the system zero-sequence voltage under the fault; is the initial unbalance degree of the distribution network, d r is the damping ratio associated with the small resistance of the neutral point; d al is the fault damping ratio of phase A; According to the fault zero-sequence voltage is greater than the protection startup setting value the following formula is derived:

[0020] Wherein, S is the set threshold of the zero-sequence voltage startup setting value;

[0021] Combined with the following formula is derived:

[0022]

[0023] Wherein, u r represents the real part of the asymmetry degree, u i represents the imaginary part of the asymmetry degree;

[0024] Combined with the damping ratio under the fault the mathematical relationship between the fault resistance and the zero-sequence voltage startup setting value is further obtained:

[0025]

[0026] Wherein, g al represents the reciprocal of the fault resistance, r al represents the fault resistance, X c represents the capacitive reactance, U a0 represents the phase A voltage of the three-phase voltage in the distribution network, I c represents the maximum capacitive current when a grounding fault occurs in the distribution network.

[0027] As an optimization, the specific expression of the asymmetry degree is:

[0028]

[0029] As an optimization, the optimization model is specifically:

[0030] obj: 2u i -2S 2 d r

[0031] subject to u i 2 +ur 2 = u 00 2

[0032] Among them, obj represents the objective function, and subject to represents the constraint condition.

[0033] As an optimization, the method for solving the optimization model includes solving by an optimization software, and the optimization software includes Gurobi or Cplex.

[0034] The present invention also discloses a fault start detection method based on phasors. The detection criterion is set using the parameters obtained by the method for obtaining the fault resistance range corresponding to the start setting value of a small-resistance grounded system as described above. After the detection criterion is satisfied, the distribution network starts fault detection. Among them, the detection criterion includes a start detection criterion based on the zero-sequence voltage amplitude and / or a start detection criterion based on the sudden change in the zero-sequence voltage amplitude and / or a start detection criterion based on the sudden change in the zero-sequence voltage phasor.

[0035] As an optimization, the specific expression of the start detection criterion based on the zero-sequence voltage amplitude is:

[0036]

[0037] Among them, is the fault zero-sequence voltage after the grounding fault occurs in the distribution network; u 00 is the initial unbalance degree of the distribution network; d al is the fault damping ratio of phase A; d r is the damping ratio associated with the small resistance at the neutral point; is the phase A voltage.

[0038] As an optimization, the specific expression of the start detection criterion based on the sudden change in the zero-sequence voltage amplitude is:

[0039]

[0040] Among them, Δ|U| represents the sudden change in the zero-sequence voltage amplitude; is the fault zero-sequence voltage after the grounding fault occurs in the distribution network; represents the initial zero-sequence voltage; u 00 is the initial unbalance degree of the distribution network; d al is the fault damping ratio of phase A; d r is the damping ratio associated with the small resistance at the neutral point; is the phase A voltage.

[0041] As an optimization, the specific expression of the start detection criterion based on the sudden change in the zero-sequence voltage phasor is:

[0042]

[0043] Among them, is the sudden change quantity of zero-sequence voltage phasor; is the fault zero-sequence voltage after the grounding fault occurs in the distribution network; represents the initial zero-sequence voltage; u 00 is the initial unbalance degree of the distribution network; d al is the fault damping rate of phase A; d r is the damping rate associated with the small neutral resistance; is the voltage of phase A.

[0044] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0045] In view of the fact that current grounding line selection devices under the small-resistance grounding method mostly set the starting detection value by directly avoiding the maximum unbalance voltage during system operation, which is too rough and difficult to adapt to the diverse working conditions of a large number of distribution lines with different operation modes, the present invention proposes a method for obtaining the fault resistance interval corresponding to the starting value under the small-resistance grounding method. mainly through Kirchhoff's current law, a mathematical representation model of the zero-sequence voltage of the system before and after the fault is constructed, and the upper and lower limits of the fault resistance detection associated with the starting value are deduced. On this basis, three starting detection criteria based on the zero-sequence voltage amplitude, the sudden change quantity of the zero-sequence voltage amplitude, and the sudden change quantity of the zero-sequence voltage phasor are proposed, aiming to improve the detection adaptability of the single-phase grounding line selection device for medium and high resistance faults and achieve accurate identification and effective disposal of grounding faults. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] The drawings described herein are used to provide a further understanding of the embodiments of the present invention, form a part of this application, and do not limit the embodiments of the present invention. In the drawings:

[0047] Figure 1 is a flowchart of a method for obtaining the fault resistance interval corresponding to the starting value of a small-resistance grounding according to the present invention;

[0048] Figure 2 is a schematic diagram of the fault interval for reliable starting detection under different starting values;

[0049] Figure 3 is a schematic diagram of the upper limit of the fault for reliable starting detection under different starting values;

[0050] Figure 4 is a schematic diagram of the lower limit of the fault for reliable starting detection under different starting values;

[0051] Figure 5 is the fault detection sensitivity curve (large interval) associated with three starting methods;

[0052] Figure 6 The fault detection sensitivity curves (small intervals) associated with three startup methods. Specific implementation mode

[0053] To make the objectives, technical solutions and advantages of the present invention clearer and more understandable, the present invention will be further described in detail below in conjunction with embodiments and drawings. The illustrative embodiments of the present invention and their descriptions are only used to explain the present invention and are not intended to limit the present invention.

[0054] Embodiment 1 of the present invention provides a method for obtaining a fault resistance interval corresponding to a startup setting value for grounding through a small resistor, as Figure 1 shown, including:

[0055] S1. Quantify the mathematical representation relationship between the startup setting value of the zero-sequence voltage in the distribution network when grounding through a small resistor and the corresponding fault resistance interval;

[0056] A mathematical relationship for quantifying the startup setting value of the zero-sequence voltage and its adapted fault resistance detection interval belongs to three links:

[0057] S11. When the system is grounded through a small resistor under normal conditions, the system unbalance degree will change slightly. To quantitatively describe this relationship, Kirchhoff's law is combined with the law of conservation of node current inflow and outflow at the neutral point here to derive the mathematical representation relationship of the initial system zero-sequence voltage:

[0058]

[0059] In the formula: Y a 、Y b 、Y c and Y0 are the ground conductances and susceptances of phases A, B, and C, respectively, and the reciprocal of the small resistor at the neutral point. e is an operator, and its value is is the voltage of phase A and d r is the damping ratio associated with the small resistor at the neutral point, and where w is the angular frequency, w = 2πf, C a 、C b and C c are the three-phase ground charging capacitors.

[0060] S12. Assume that phase A is the special fault phase. After a ground fault occurs on it, the system zero-sequence voltage will have the following form:

[0061]

[0062] In the formula: is the initial unbalance degree of the system, d al is the fault damping ratio, d r is the damping ratio associated with the small resistor at the neutral point, rai is the fault resistance, g r is the reciprocal sum of the neutral point small resistances is the system zero-sequence voltage under the fault.

[0063] Since the three-phase topology is symmetric; if the B-phase fails, the B-phase can be renamed as the A-phase, the original C-phase becomes the B-phase, and the A-phase becomes the C-phase; the same applies to the C-phase fault. Therefore, from the perspective of analysis, only the A-phase fault needs to be considered, which is universal. Assume that the protection starting setting value is set to SU a0 , then the range of the grounding fault that can be started satisfies:

[0064]

[0065] In the formula: The variable S is the set zero-sequence voltage starting setting threshold. According to traditional experience, this value needs to avoid the initial system asymmetry during normal system operation.

[0066] After simplification, the above formula can be transformed into the following form:

[0067]

[0068] Asymmetry is a phasor, and its direction is determined by the mutual magnitude relationship of the three-phase-to-ground capacitances. Obviously, it is different for different distribution systems. For the same distribution system, with the changes in the network structure, operation mode, and meteorological conditions, the three-phase-to-ground capacitances also change. Moreover, when faults occur in the ABC three phases, the phase relationship between the fault admittance and is also significantly different.

[0069] Therefore, for the above formula, after defining: , it can be simplified to the following form:

[0070]

[0071] Combined with the damping ratio under the fault, the mathematical relationship between the fault resistance and the zero-sequence voltage starting setting value can be further obtained as follows:

[0072]

[0073] In the formula: I c is the system capacitive current.

[0074] S2. Establish an optimization model based on the above mathematical characterization relationship;

[0075] S3. Solve the optimization model to obtain the size of the fault resistance interval.

[0076] For the mathematical relationship between the zero-sequence voltage starting setting value described in S1 and the fault transition resistance, it is transformed into an optimization problem that can be solved with the help of optimization software. Optimization software such as Gurobi and Cplex can be used for solving, and the upper and lower regions of the identifiable fault transition resistance under the zero-sequence voltage starting setting value can be further obtained. In other words, r al The key to obtaining the maximum and minimum values lies in 2u i -2S 2 d r whether the value of is the minimum or the maximum.

[0077] obj: 2u i -2S 2 d r

[0078] subject to u i 2 +u r 2 =u 00 2

[0079] Among them, obj represents the objective function, and subject to represents the constraint condition.

[0080] The present invention is a method for obtaining the starting setting value of the grounding fault in the distribution network. In the distribution network, the system characteristics after the grounding fault are related not only to the grounding resistance at the fault point but also to the system grounding method.

[0081] The system grounding method is divided into small-current grounding and large-current grounding. Among them, small-current grounding mainly includes ungrounded and grounded through an arc suppression coil. The present invention patent discusses the method for obtaining the starting setting value under the large-current grounding method.

[0082] Embodiment 2 also discloses a fault starting detection method based on phasors. The detection criterion is set using the parameters obtained by the method for obtaining the fault resistance interval corresponding to the starting setting value of small-resistance grounding in Embodiment 1. After the detection criterion is satisfied, the distribution network is started for fault detection. Among them, the detection criterion includes a starting detection criterion based on the zero-sequence voltage amplitude and / or a starting detection criterion based on the sudden change in the zero-sequence voltage amplitude and / or a starting detection criterion based on the sudden change in the zero-sequence voltage phasor.

[0083] Embodiment 2 mainly proposes an analysis model for quantifying the starting detection sensitivity of the zero-sequence voltage amplitude, the sudden change in the zero-sequence voltage amplitude, and the sudden change in the zero-sequence voltage phasor. Mainly combined with the system zero-sequence voltage before and after the fault, the starting detection criteria in the three ways can be deduced as follows:

[0084] Starting detection criterion based on the zero-sequence voltage amplitude:

[0085]

[0086] Starting detection criterion based on sudden change in zero-sequence voltage amplitude:

[0087]

[0088] Starting detection criterion based on sudden change in zero-sequence voltage phasor:

[0089]

[0090] Among them, is the fault zero-sequence voltage after the grounding fault occurs in the distribution network; Δ|U| represents the sudden change in zero-sequence voltage amplitude; is the sudden change in zero-sequence voltage phasor; u 00 is the initial unbalance degree of the distribution network; d al is the fault damping rate of phase A; d r is the damping rate associated with the small neutral-point resistance; is the phase A voltage.

[0091] When the above equation is satisfied, fault detection starts.

[0092] Next, the present invention will be described with a specific case.

[0093] I. Setting the initial unbalance degree of the system The system capacitive current Ic = 60A, the neutral-point small resistance R = 10Ω, and the unit ratio of the initial zero-sequence voltage of the system after the action of the small resistance is 0.001493. The variation relationship curve of the upper and lower limits of fault detection with the starting setting value can be drawn, as shown in Figures 2 - 4 shown. Observing Figures 2 - 4 it can be known that: 1) Adopting the small-resistance grounding method, the upper limit curve of the detected fault resistance shows a steep downward trend with the increase of the starting setting value, while the lower limit of the reliable detection resistance is relatively slow; 2) In practical applications, the reliability of protection starting is often more concerned. Analyzing the lower limit curve of reliable fault detection here, it can be analyzed that the range of the detected fault resistance does not exceed 3000Ω. For example, when the starting setting value is set to 0.0016, slightly higher than the unit ratio of the initial zero-sequence voltage of the system under the action of the small resistance of 0.001493, the reliable detected fault resistance is 1648Ω.

[0094] II. Setting the initial unbalance degree of the system When the system capacitive current Ic = 60A and the neutral-point small resistance R = 10Ω, the unit ratio of the initial zero-sequence voltage of the system under the action of the small resistance is 0.001493. The variation relationship curves of the system zero-sequence voltage amplitude, the sudden change in zero-sequence voltage amplitude, and the sudden change in zero-sequence voltage phasor with the fault damping rate can be drawn, as shown in Figure 5 andFigure 6 As shown. Observe Figures 5 - 6 , it can be known that: 1) After adopting a small resistance, the three starting methods constructed by using zero-sequence voltage all show linear detection characteristics, which is beneficial for setting the starting value without being restricted by the comprehensive system operating conditions and grounding methods; 2) Compared with the other two, the starting detection criterion constructed by using the zero-sequence voltage phasor has a slight detection advantage, which can be verified from the fact that the change amount of the zero-sequence voltage phasor after a fault is higher than its amplitude amount and amplitude change amount; 3) When the setting values of the three starting methods are all set to about 0.0016 (zero-sequence voltage amplitude: 0.001612, zero-sequence voltage amplitude mutation: 0.001524, and zero-sequence voltage phasor mutation: 0.001602), they are all slightly higher than the system initial zero-sequence voltage unit ratio of 0.001493 under the action of a small resistance, and the detectable fault resistances are 3200Ω, 2200Ω, and 6200Ω respectively (the larger the detectable fault resistance, the better the detection effect). Obviously, the starting detection criterion constructed by using the zero-sequence voltage phasor has more advantages.

[0095] The specific implementation manners described above further elaborate on the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above is only the specific implementation manner of the present invention and is not used to limit the protection scope of the present invention. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for obtaining a fault resistance interval corresponding to a starting setting value of a low resistance grounding, characterized in that: include: Quantify the mathematical relationship between the starting value of the zero-sequence voltage in the distribution network and the corresponding fault resistance interval when there is a small resistance ground fault; The mathematical relationship between the starting setting of the zero-sequence voltage and the corresponding fault resistance interval is expressed as: in, It indicates the asymmetry degree of the distribution network during normal operation, that is, the initial imbalance degree of the distribution network; is the fault damping rate of phase A, where phase A is the fault phase; is the damping rate associated with the small resistance at the neutral point; It is the set zero-sequence voltage starting fixed value threshold; is the A phase voltage, is the damping rate associated with the small resistance at the neutral point; is the fault resistance; represents the imaginary part of the asymmetry; Establishing an optimization model based on the mathematical representation relationship; The optimization model is solved to obtain the size of the fault resistance interval.

2. The method for obtaining the fault resistance interval corresponding to the starting setting value of a low resistance grounding according to claim 1 is characterized in that: The specific process of quantifying the mathematical representation relationship between the starting setting of the zero-sequence voltage and the corresponding fault resistance interval is: Combined with Kirchhoff's law, combined with the conservation law of node current inflow and outflow at the neutral point, the mathematical representation relationship of the initial zero-sequence voltage of the distribution network is derived: ； in, represents the initial zero-sequence voltage; , , and are the conductance and susceptance of phase A, phase B, phase C to ground and the reciprocal of the neutral point resistance; is the operator, ; is the A phase voltage; is the damping ratio associated with the small neutral resistance, and ,in is the angular frequency, , f represents the frequency of the distribution network, Represents the reciprocal of the neutral point resistance. , and They are the charging capacitance to ground of phase A, phase B and phase C voltages respectively; It indicates the asymmetry degree of the distribution network during normal operation, that is, the initial imbalance degree of the distribution network; After a ground fault occurs in phase A, the specific expression of the fault zero-sequence voltage of the distribution network is: in, is the fault zero-sequence voltage after a ground fault occurs in the distribution network, that is, the system zero-sequence voltage under the fault; is the initial imbalance of the distribution network, is the damping rate associated with the small resistance at the neutral point; is the fault damping rate of phase A; According to the fault zero sequence voltage Greater than the protection start setting The following formula is derived: ,in, It is the set zero-sequence voltage starting fixed value threshold; Combination The following formula is derived: ； in, represents the real part of the asymmetry, represents the imaginary part of the asymmetry; Combined fault damping rate The mathematical relationship between the fault resistance and the zero-sequence voltage starting setting is further obtained: in, represents the inverse of the fault resistance, represents the fault resistance, Represents capacitive reactance, Indicates the A-phase voltage of the three-phase voltage in the distribution network, Indicates the maximum capacitive current when a ground fault occurs in the distribution network.

3. The method for obtaining the fault resistance interval corresponding to the starting setting value of a low resistance grounding according to claim 1 or 2, characterized in that: The specific expression of the asymmetry is: ； in, , and They are the charging capacitance to ground of phase A, phase B and phase C voltages respectively; For the operator, .

4. The method for obtaining the fault resistance interval corresponding to the starting setting value of a low resistance grounding according to claim 1 or 2, characterized in that: The optimization model is specifically: in, represents the objective function, represents the constraints, represents the imaginary part of the asymmetry, Represents the real part of the asymmetry.

5. The method for obtaining the fault resistance interval corresponding to the starting setting value of a low resistance grounding according to claim 4 is characterized in that: The method for solving the optimization model includes solving it using optimization software, and the optimization software includes Gurobi or Cplex.

6. A phasor-based fault start detection method, characterized in that: A detection criterion is set using parameters obtained by a method for obtaining a fault resistance interval corresponding to a starting constant of a small resistance grounding as described in any one of claims 1-5, and after the detection criterion is met, fault detection of the distribution network is started, wherein the detection criterion includes a starting detection criterion based on the zero-sequence voltage amplitude and / or a starting detection criterion based on the zero-sequence voltage amplitude mutation amount and / or a starting detection criterion based on the zero-sequence voltage phasor mutation amount.

7. A phasor-based fault initiation detection method according to claim 6, characterized in that: The specific expression of the startup detection criterion based on the zero-sequence voltage amplitude is: in, is the fault zero-sequence voltage after a ground fault occurs in the distribution network; is the initial imbalance of the distribution network; is the fault damping rate of phase A; is the damping rate associated with the small resistance at the neutral point; is the A phase voltage.

8. The method for detecting a fault initiation based on a phasor method according to claim 6, characterized in that: The specific expression of the startup detection criterion based on the zero-sequence voltage amplitude mutation amount is: in, Indicates the sudden change of zero-sequence voltage amplitude; is the fault zero-sequence voltage after a ground fault occurs in the distribution network; represents the initial zero-sequence voltage; is the initial imbalance of the distribution network; is the fault damping rate of phase A; is the damping rate associated with the small resistance at the neutral point; is the A phase voltage.

9. The method for detecting a fault initiation based on a phasor method according to claim 6, characterized in that: The specific expression of the startup detection criterion based on the zero-sequence voltage phasor mutation quantity is: in, is the zero-sequence voltage phasor mutation quantity; is the fault zero-sequence voltage after a ground fault occurs in the distribution network; represents the initial zero-sequence voltage; is the initial imbalance of the distribution network; is the fault damping rate of phase A; is the damping rate associated with the small resistance at the neutral point; is the A phase voltage.

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