Method for acquiring fault resistance interval corresponding to starting constant value of small resistor grounding

Abstract

The invention relates to the technical field of power distribution network fault detection and protection, and discloses a method for obtaining a fault resistance interval corresponding to a starting constant value of small resistor grounding, and the method comprises the steps: quantifying a mathematical representation relation between the starting constant value of zero-sequence voltage in a power distribution network and the corresponding fault resistance interval when the small resistor is grounded; establishing an optimization model based on the mathematical representation relationship; and solving the optimization model to obtain the size of the fault resistance interval. According to the invention, the detection adaptability of the single-phase grounding line selection device to 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 in particular to a method for obtaining a fault resistance interval corresponding to a starting setting of a low-resistance grounding. Background Art

[0002] The probability of single-phase grounding faults in distribution networks is extremely high, and the causes of faults are complex and changeable, which often leads to such faults being extremely well disguised and difficult to be fully exposed in the analysis field of existing algorithms. The main reason is that the single-phase grounding fault scenarios are changeable, such as tree lines, falling to the ground, and melting. The post-fault system zero-sequence voltage characteristics used for grounding fault detection in different scenarios vary, and the actual starting values ​​of various grounding line selection devices, that is, the zero-sequence voltage parameters, are mostly set according to the different system grounding methods, directly avoiding the maximum unbalanced voltage of the system under normal operation. Therefore, such values ​​cannot fully cover all working conditions, and there must be a lower limit, and the grounding faults below this lower limit fall into a blind spot. In practice, the setting of this lower limit is mostly based on experience or directly avoiding the maximum unbalanced voltage under the expected maximum working condition of the system. Obviously, the parameter setting lacks effective theoretical support and cannot scientifically and reasonably guide fault diagnosis and disposal. Summary of the invention

[0003] To this end, the present invention provides a method for obtaining the fault resistance interval corresponding to the starting constant of low-resistance grounding, and constructs a bridge between the starting constant and the upper and lower limits of the grounding fault that can be effectively monitored from the perspective of mathematical derivation, so as to provide a directional reference for the reasonable setting of parameters, the effective start-up of the judgment and the scientific disposal of the fault. In addition, the present invention will also construct a phasor-based fault start-up detection method to improve the sensitivity of fault detection. Taking into account the previous work of the team of the present invention, the present invention will focus on the low-resistance grounding system, and propose a method for quantitatively evaluating the zero-sequence voltage starting constant and the fault resistance identification interval, which can quantitatively judge the ability of the upper limit of the transition resistance that can be identified by the starting constant, and is used to produce the coverage of vegetation, trees, etc. when the line passes through the forest grassland, set a scientific starting constant, and reliably control the first pass of "start-up" before fault judgment.

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

[0005] A method for obtaining a fault resistance interval corresponding to a starting setting value of a small resistance grounding includes:

[0006] 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;

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

[0008] The optimization model is solved to obtain the size of the fault resistance interval.

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

[0010]

[0011]

[0012] in, It indicates the asymmetry degree of the distribution network during normal operation, that is, the initial imbalance 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 resistance of the neutral point; S is the set zero-sequence voltage starting fixed value threshold; is the voltage of phase A, d r is the damping rate associated with the small resistance at the neutral point; 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 of the zero-sequence voltage and the corresponding fault resistance interval is:

[0014] 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:

[0015]

[0016] in, Represents the initial zero-sequence voltage; Y a , Y b , Y c and Y0 are the conductance and susceptance of phase A, phase B, phase C to ground and the reciprocal of the small resistance of the neutral point; e is an operator, is the voltage of phase A; d r is the damping ratio 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 neutral point resistance, C a , C b and C c 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;

[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] 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 unbalance of the distribution network, d r is the damping rate associated with the small resistance at the neutral point; d al 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:

[0020] Among them, S is the set zero-sequence voltage starting constant threshold;

[0021] Combination The following formula is derived:

[0022]

[0023] Among them, u r represents the real part of the asymmetry, u i represents the imaginary part of the asymmetry;

[0024] Combined fault damping rate The mathematical relationship between the fault resistance and the zero-sequence voltage starting setting is further obtained:

[0025]

[0026] Among them, g al Represents the inverse of the fault resistance, r al represents the fault resistance, X c Represents capacitive reactance, U a0 Indicates the A-phase voltage of the three-phase voltage in the distribution network, I c Indicates the maximum capacitive current when a ground fault occurs in the distribution network.

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

[0028]

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

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

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

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

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

[0034] The present invention also discloses a phasor-based fault startup detection method, which uses the parameters obtained by the aforementioned method for obtaining the fault resistance interval corresponding to the startup constant of a small resistance grounding to set the detection criterion, and after meeting the detection criterion, starts the fault detection of the distribution network, wherein the detection criterion includes a startup detection criterion based on the zero-sequence voltage amplitude and / or a startup detection criterion based on the zero-sequence voltage amplitude mutation amount and / or a startup detection criterion based on the zero-sequence voltage phasor mutation amount.

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

[0036]

[0037] in, u is the fault zero-sequence voltage after a ground fault occurs in the distribution network; 00 is the initial imbalance of the distribution network; d al is the fault damping rate of phase A; d r is the damping rate associated with the small resistance at the neutral point; is the A phase voltage.

[0038] As an optimization, the specific expression of the startup detection criterion based on the zero-sequence voltage amplitude mutation amount is:

[0039]

[0040] Among them, Δ|U| represents 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; u 00 is the initial imbalance of the distribution network; d al is the fault damping rate of phase A; d r is the damping rate associated with the small resistance at the neutral point; is the A phase voltage.

[0041] As an optimization, the specific expression of the startup detection criterion based on the zero-sequence voltage phasor mutation quantity is:

[0042]

[0043] 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; u 00 is the initial imbalance of the distribution network; d al is the fault damping rate of phase A; d r is the damping rate associated with the small resistance at the neutral point; is the A phase voltage.

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

[0045] In view of the fact that the current grounding line selection devices under the low-resistance grounding mode mostly set the starting detection constant according to directly avoiding the maximum unbalanced voltage during system operation, which is too rough and difficult to adapt to the differentiated operating conditions of massive distribution lines with different operating modes, the present invention proposes a method for obtaining the fault resistance interval corresponding to the starting constant under the low-resistance grounding mode. Mainly through Kirchhoff's current law, a mathematical characterization model of the system zero-sequence voltage before and after the fault is constructed, and the upper and lower limit intervals of the fault resistance detection associated with the starting constant are derived. On this basis, three starting detection criteria based on the zero-sequence voltage amplitude, the zero-sequence voltage amplitude mutation amount and the zero-sequence voltage phasor mutation amount are proposed respectively, aiming to improve the detection adaptability of the single-phase grounding line selection device for medium and high resistance faults and realize the 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, constitute a part of this application, and do not constitute a limitation of the embodiments of the present invention. In the drawings:

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

[0048] Figure 2 It is a schematic diagram of the fault range for reliable start detection under different start settings;

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

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

[0051] Figure 5 Fault detection sensitivity curves associated with the three starting modes (large range);

[0052] Figure 6 Fault detection sensitivity curves (small intervals) associated with the three starting modes. DETAILED DESCRIPTION

[0053] 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.

[0054] This embodiment 1 provides a method for obtaining a fault resistance interval corresponding to a starting setting value of a small resistance grounding, such as Figure 1 As shown, including:

[0055] S1. The mathematical relationship between the starting value of the zero-sequence voltage in the distribution network and the corresponding fault resistance interval when the small resistance grounding is quantified;

[0056] A mathematical relationship between a quantified zero-sequence voltage starting setting and its adapted fault resistance detection interval includes three links:

[0057] S11. Under normal circumstances, after the system is grounded with a small resistance, the system imbalance will change slightly. In order to quantitatively describe this relationship, Kirchhoff's law is combined with the conservation law of node current inflow and outflow at the neutral point to derive the mathematical representation relationship of the initial system zero-sequence voltage:

[0058]

[0059] Where: Y a , Y b , Y c and Y0 are the conductance and susceptance of phase A, phase B and phase C to ground, and the reciprocal of the neutral point resistance, e is an operator, and its value is is the phase A voltage and d r is the damping ratio associated with the small neutral resistance, and Where w is the angular frequency, w = 2πf, C a , C b and C c Charge the capacitors three-phase to ground.

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

[0061]

[0062] Where: is the initial imbalance of the system, d al is the fault damping rate, d r is the damping rate associated with the small resistance at the neutral point, rai is the fault resistance, g r is the reciprocal of the neutral point small resistance and is the system zero-sequence voltage under fault.

[0063] Since the three-phase topology is symmetrical, if the fault is in phase B, then phase B can be renamed to phase A, and the original phase C can be renamed to phase B, and phase A can be renamed to phase C. The same is true for phase C faults. Therefore, from an analytical point of view, only phase A faults need to be considered, which is universal. Assume that the protection start setting is set to SU a0 , then the range of ground faults that can be started satisfies:

[0064]

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

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

[0067]

[0068] Asymmetry It is a phase quantity, and its direction is determined by the mutual relationship between the three-phase capacitance to ground. It is obviously different for different distribution systems. For the same distribution system, as the network structure, operation mode and meteorological conditions change, the three-phase capacitance to ground also changes. Moreover, when a fault occurs in the three phases ABC, the fault admittance is proportional to The phase relationships are also significantly different.

[0069] Therefore, for the above formula, we can define: Then, simplify it into the following form:

[0070]

[0071] Combined fault damping rate The mathematical relationship between the fault resistance and the zero-sequence voltage starting setting can be further obtained as follows:

[0072]

[0073] Where: I c is the system capacitance current.

[0074] S2. Establishing an optimization model based on the mathematical representation relationship;

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

[0076] The mathematical relationship between the zero-sequence voltage starting constant and the fault transition resistance described by S1 is converted into an optimization problem that can be solved with the help of optimization software, such as Gurobi and Cplex, to further obtain the upper and lower areas of the identifiable fault transition resistance under the zero-sequence voltage starting constant. In other words, r al The key to achieving the maximum and minimum values ​​lies in 2u i -2S 2 d r Whether the value is minimum or maximum.

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

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

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

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

[0081] The system grounding methods are divided into small current grounding and large current grounding, among which small current grounding mainly includes no grounding and grounding through a pin arc coil. The patent of this invention discusses the startup customization acquisition method under the large current grounding method.

[0082] Example 2 also discloses a phasor-based fault start-up detection method, which uses the parameters obtained by the method for obtaining the fault resistance interval corresponding to the starting constant of a small resistance grounding in Example 1 to set the detection criterion, and after meeting the detection criterion, starts the distribution network fault detection, wherein the detection criterion includes a start-up detection criterion based on the zero-sequence voltage amplitude and / or a start-up detection criterion based on the zero-sequence voltage amplitude mutation amount and / or a start-up detection criterion based on the zero-sequence voltage phasor mutation amount.

[0083] Embodiment 2 mainly proposes an analytical model for quantitatively analyzing three types of startup detection sensitivities, namely, zero-sequence voltage amplitude, zero-sequence voltage amplitude mutation, and zero-sequence voltage phasor mutation. It mainly combines the system zero-sequence voltage before and after the fault to derive the startup detection criteria under three modes, as follows:

[0084] Startup detection criteria based on zero-sequence voltage amplitude:

[0085]

[0086] Startup detection criterion based on the zero-sequence voltage amplitude mutation:

[0087]

[0088] Startup detection criterion based on zero-sequence voltage phase mutation:

[0089]

[0090] in, is the fault zero-sequence voltage after a ground fault occurs in the distribution network; Δ|U| represents the sudden change in amplitude of the zero-sequence voltage; is the zero-sequence voltage phasor mutation quantity; u 00 is the initial imbalance of the distribution network; d al is the fault damping rate of phase A; d r is the damping rate associated with the small resistance at the neutral point; is the A phase voltage.

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

[0092] Next, the present invention will be described using specific examples.

[0093] 1. Set the initial imbalance of the system The system current Ic = 60A, the neutral point small resistance R = 10Ω, the system initial zero-sequence voltage unit ratio after the small resistance is 0.001493, and the relationship curve between the upper and lower limits of fault detection and the starting setting can be drawn, such as Figure 2-Figure 4 Observation Figure 2-Figure 4 It can be learned that: 1) When using a small resistance grounding method, the upper limit curve of the fault resistance detection shows a steep downward trend with the increase of the starting setting, while the lower limit of the reliable detection resistance is relatively slow; 2) In practical applications, more attention is often paid to the reliability of protection startup. Here, the lower limit curve of reliable fault detection is analyzed, and it can be analyzed that the range of the detected fault resistance does not exceed 3000Ω. For example, the starting setting is set to 0.0016, which is slightly higher than the system initial zero-sequence voltage unit proportion of 0.001493 under the action of a small resistance, and the reliably detectable fault resistance is 1648Ω.

[0094] 2. Set the initial imbalance of the system When the system current Ic = 60A and the neutral point small resistance R = 10Ω, the initial zero-sequence voltage unit ratio of the system under the action of the small resistance is 0.001493. The relationship curve between the system zero-sequence voltage amplitude, zero-sequence voltage amplitude mutation and zero-sequence voltage phasor mutation with the fault damping rate can be drawn, as shown in Figure 5 , Figure 6 Observation Figure 5-Figure 6 , we can know that: 1) after using small resistance, the three starting modes constructed by zero-sequence voltage all show linear detection characteristics, which is conducive to the setting of starting constants without being subject to the comprehensive constraints of system working conditions and grounding methods; 2) compared with the other two, the starting detection criterion constructed by zero-sequence voltage phasor has a slight detection advantage, which can be verified by the fact that the change of zero-sequence voltage phasor after the fault is higher than its amplitude and amplitude change; 3) when the constants of the three starting modes 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 slightly higher than the initial zero-sequence voltage unit proportion of 0.001493 under the action of 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 zero-sequence voltage phasor has more advantages.

[0095] 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 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; 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 mathematical relationship between the starting value of the zero-sequence voltage and the corresponding fault resistance interval is: in, It indicates the asymmetry degree of the distribution network during normal operation, that is, the initial imbalance 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 resistance of the neutral point; S is the set zero-sequence voltage starting fixed value threshold; is the voltage of phase A, d r is the damping rate associated with the small resistance at the neutral point; r al is the fault resistance; u i Represents the imaginary part of the asymmetry.

3. The method for obtaining the fault resistance interval corresponding to the starting setting value of a low resistance grounding according to claim 2 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; Y a , Y b , Y c and Y0 are the conductance and susceptance of phase A, phase B, phase C to ground and the reciprocal of the small resistance of the neutral point; e is an operator, is the voltage of phase A; d r is the damping ratio 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 neutral point resistance, C a , C b and C c 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, d r is the damping rate associated with the small resistance at the neutral point; d al 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: Among them, S is the set zero-sequence voltage starting constant threshold; Combination The following formula is derived: Among them, u r represents the real part of the asymmetry, u i 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: Among them, g al Represents the inverse of the fault resistance, r al represents the fault resistance, X c Represents capacitive reactance, U a0 Indicates the A-phase voltage of the three-phase voltage in the distribution network, I c Indicates the maximum capacitive current when a ground fault occurs in the distribution network.

4. The method for obtaining the fault resistance interval corresponding to the starting setting value of a small resistance grounding according to claim 2 or 3, characterized in that: The specific expression of the asymmetry is:

5. The method for obtaining the fault resistance interval corresponding to the starting setting value of a small resistance grounding according to claim 2 or 3, characterized in that: The optimization model is specifically: obj: 2u i -2S 2 d r subject to u i 2 +u r 2 =u 00 2 Among them, obj represents the objective function and subject to represents the constraint conditions.

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

7. 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 1-6, 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.

8. The method for detecting a fault initiation based on a phasor method according to claim 7, characterized in that: The specific expression of the startup detection criterion based on the zero-sequence voltage amplitude is: in, u is the fault zero-sequence voltage after a ground fault occurs in the distribution network; 00 is the initial imbalance of the distribution network; d al is the fault damping rate of phase A; d r 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 7, characterized in that: The specific expression of the startup detection criterion based on the zero-sequence voltage amplitude mutation amount is: Among them, Δ|U| represents 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; u 00 is the initial imbalance of the distribution network; d al is the fault damping rate of phase A; d r is the damping rate associated with the small resistance at the neutral point; is the A phase voltage.

10. The method for detecting a fault initiation based on a phasor method according to claim 7, 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; u 00 is the initial imbalance of the distribution network; d al is the fault damping rate of phase A; d r is the damping rate associated with the small resistance at the neutral point; is the A phase voltage.

Images