CN113219300B - Power distribution network single-phase earth fault sensing method based on phase current transient state steady state - Google Patents

Power distribution network single-phase earth fault sensing method based on phase current transient state steady state Download PDF

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CN113219300B
CN113219300B CN202110438026.4A CN202110438026A CN113219300B CN 113219300 B CN113219300 B CN 113219300B CN 202110438026 A CN202110438026 A CN 202110438026A CN 113219300 B CN113219300 B CN 113219300B
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phase current
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CN113219300A (en
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齐郑
赵昕一
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North China Electric Power University
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R31/00—Arrangements for testing electric properties; Arrangements for locating electric faults; Arrangements for electrical testing characterised by what is being tested not provided for elsewhere
    • G01R31/08—Locating faults in cables, transmission lines, or networks
    • G01R31/081—Locating faults in cables, transmission lines, or networks according to type of conductors
    • G01R31/086—Locating faults in cables, transmission lines, or networks according to type of conductors in power transmission or distribution networks, i.e. with interconnected conductors
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R31/00—Arrangements for testing electric properties; Arrangements for locating electric faults; Arrangements for electrical testing characterised by what is being tested not provided for elsewhere
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Abstract

The application discloses a power distribution network single-phase earth fault sensing method based on phase current transient state steady state, which comprises the steps of collecting three-phase currents of a power distribution network, synthesizing zero-sequence currents, calculating fundamental wave amplitude, and acquiring fault signals when a single-phase earth fault occurs so as to acquire fault occurrence time and line fault component currents; solving a fundamental frequency component amplitude and a high frequency component amplitude of the fault component current, and judging whether the quantity relationship of the fundamental frequency component amplitude and the high frequency component amplitude meets a fault perception criterion; when the fault meets the criterion, judging that the system has a resistance grounding fault, and obtaining a fault path positioning result by using the steady-state characteristics of fault component current and the variance; otherwise, the system is judged to have the fault through the electric arc grounding, and the fault occurring time sampling point and the three-phase current mutation direction are obtained by utilizing the transient characteristics of the fault component current in combination with the improved S transformation to obtain the fault path positioning result. The method can be suitable for various single-phase earth fault types of the power distribution network, and effectively improves the operation reliability of the power distribution network.

Description

Power distribution network single-phase earth fault sensing method based on phase current transient state steady state
Technical Field
The invention belongs to the technical field of electric power automation, and relates to a power distribution network single-phase earth fault sensing method based on phase current transient state steady state.
Background
The power distribution network is used as a bridge for connecting a power system and a user, and the power utilization quality and safety of the power user are determined by the operational reliability of the power distribution network. With the continuous progress of the economic society, the power distribution network shows the development trend of scale enlargement and complicated topological structure, the single-phase earth fault problem of the power distribution network also has the characteristics of unobvious fault characteristics, various fault conditions and the like, the single-phase earth fault positioning problem is not well solved, and the economic and reliable operation requirements of the single-phase earth fault positioning problem cannot be met.
The existing power distribution network fault positioning technology can be roughly divided into two methods based on a line self signal and an external injection signal according to a judgment signal source. The two methods have higher action accuracy when meeting the requirements, but because the positioning technology based on external signal injection needs to additionally install signal injection equipment, the engineering economy is influenced, and the fault positioning technology based on the self signal of the line is usually adopted in the actual field.
Fault location technology based on the line signal is mostly used for completing the judgment by using the line zero sequence signal, but the location device cannot obtain the corresponding judgment signal due to the zero sequence current or voltage transformer loss in partial areas of the power distribution network, so that the application effect is poor. Meanwhile, the fault location technology based on the zero sequence signal has higher dependence on communication, and the algorithm can be failed when the communication fails.
In recent years researchers have attempted to use phase currents as decision signals to accomplish fault localization by analyzing the transient or steady state course of the phase currents. Some scholars consider that if the abrupt change directions of the three-phase currents are the same, the three-phase currents are a non-fault path, and if the abrupt change directions of the three-phase currents are different, the three-phase currents are a non-fault path. However, when the grounding resistance is large, the transient process of the three-phase current is very weak, and the accurate mutation direction cannot be obtained; and because the phase current is seriously interfered by load current and harmonic, the algorithm is easily misjudged by directly extracting the phase current as a judgment signal. In addition, some scholars think that after a single-phase earth fault occurs, the three-phase current change of a fault path is different from a non-fault path, so that whether a line has a fault or not can be judged. However, the load current in the power distribution network is far larger than the fault component current, detection errors are easily caused, and when the system is grounded through an electric arc, the fluctuation of the phase current in the steady state process is serious, so that the reliability of the algorithm is reduced.
Disclosure of Invention
In order to overcome the defects in the prior art, the application provides a power distribution network single-phase earth fault sensing method based on phase current transient state steady state, the power distribution network single-phase earth fault sensing is carried out based on transient state and steady state phase asymmetric signals, the problem that the power distribution network fault situation is complex can be solved, the fault locating requirements of multiple fault types and different neutral point earth modes can be met, and the requirements of high adaptability and weak dependence on communication can be met by utilizing a self-adaptive threshold value.
In order to achieve the above purpose, the invention adopts the following technical scheme:
a power distribution network single-phase earth fault sensing method based on phase current transient state steady state is characterized in that:
the method comprises the following steps:
step 1: collecting three-phase current of a power distribution network in real time, synthesizing zero-sequence current and calculating the fundamental wave amplitude of the zero-sequence current;
when the zero sequence current fundamental wave amplitude is larger than a set threshold value, judging that a single-phase earth fault occurs, and entering the step 2;
step 2: acquiring three-phase current, namely fault signals, and preprocessing the three-phase current;
and step 3: decomposing the fault signal preprocessed in the step 2 by using improved S transformation and acquiring the fault occurrence time by using a method of solving the maximum value of the first-order difference quotient;
and 4, step 4: subtracting three-phase currents which are different by a whole period before and after the fault occurrence time to obtain line fault component current;
and 5: method for solving fundamental frequency component amplitude A of fault component current by using improved S transformation0And a high frequency component amplitude A;
step 6: judging the fundamental frequency component amplitude A in the step 5 by using a fault perception criterion formed by the quantity relation of the fundamental frequency component amplitude and the high frequency component amplitude0Whether the quantitative relation with the high-frequency component amplitude A meets the fault perception criterion or not;
when the fault path positioning result meets the criterion, judging that the system has a resistance grounding fault, the steady state process of the fault component current is stable, and obtaining the fault path positioning result by using the steady state characteristics of the fault component current and the variance;
and otherwise, judging that the system has the fault through the arc grounding, and acquiring a fault occurrence time sampling point and a three-phase current mutation direction by combining the transient characteristics of the fault component current with the improved S transformation to obtain a fault path positioning result.
The invention further comprises the following preferred embodiments:
preferably, in step 2, after the three-phase current is obtained, zero-sequence current is synthesized by using an addition method and zero drift elimination pretreatment is performed by using an accumulation method.
Preferably, the formula of the improved S transform is as follows:
Figure BDA0003033832030000031
in the formula, tau is a position parameter of a control Gaussian window omega (tau-t, f) on a time axis t; f is the frequency; j is an imaginary unit, a is a Gaussian window amplitude adjustment coefficient, and b is a Gaussian window index adjustment coefficient.
Preferably, in step 4, the line fault component current is obtained by subtracting three-phase currents, which are different by an entire period before and after the fault occurrence time, specifically:
correspondingly subtracting signals of a period after the fault occurrence time and two periods before the fault occurrence to obtain fault component current, namely:
if(t)＝i(t0+T+t)-i(t0-2×T+t)(t＝1、2...T)；
if(t) is fault component current at time t, i (t) is three-phase current signal at time t, t0T is a sampling period for the time of occurrence of a fault.
Preferably, in step 6, the fault perception criterion is: kA·A0＞A，KAThe value range is as follows: k is more than or equal to 0.4A≤0.5。
Preferably, in step 6, if the variance is greater than the adaptive threshold or the abrupt change directions of the three-phase currents are not consistent, determining that the three-phase current is a fault path; and if the variance is less than or equal to the adaptive threshold or the abrupt change directions of the three-phase current are consistent, judging the path to be a non-fault path.
Preferably, the step 6 of obtaining the fault path positioning result by using the steady-state characteristics of the fault component current in combination with the variance specifically includes the following steps:
step 611: subtracting zero-sequence current from three-phase fault component current:
Figure BDA0003033832030000032
Figure BDA0003033832030000033
Figure BDA0003033832030000034
step 612: applying FFT to the subtraction result in the step 611 to obtain the fundamental wave amplitude of each current signal;
step 613: calculating the variance Var of the fundamental wave amplitude of the three-phase fault component currentK：
Figure BDA0003033832030000035
Step 614: setting self-adaptive threshold value Var related to zero sequence current fundamental wave amplitude0；
Step 615: comparison VarKAnd Var0When Var isK＞Var0The time is a fault path, and the reverse is a non-fault path.
Preferably, in step 614, an adaptive threshold is set
Figure BDA0003033832030000041
I0For the fundamental amplitude of zero-sequence current of this line, KrelAre adaptive coefficients.
Preferably, in step 614, KrelTake 0.85.
Preferably, in step 6, the transient characteristics of the fault component current are combined with the improved S transformation to obtain a fault occurrence time sampling point and a three-phase current abrupt change direction to obtain a fault path positioning result, specifically:
carrying out filtering processing on the three-phase current signals preprocessed in the step 2 by using improved S transformation to obtain fundamental frequency components, obtaining sampling points corresponding to the fault occurrence time by using a method of finding maximum values for the fundamental frequency components by using first-order difference quotient, and determining mutation directions by using the following method:
assuming that the current fundamental frequency component signal of a certain phase in the three-phase current is: and y ═ f (x), and acquiring a sampling point x corresponding to the fault occurrence time according to the improved S transformation0Defining a calculation interval in the vicinity of the sampling point
Figure BDA0003033832030000042
Wherein
Figure BDA0003033832030000043
The range of the interval can be calculated according to engineering experienceTaking 15;
calculating to obtain an absolute value of a difference value between a corresponding function value and a reference value in a calculation interval by using a function value corresponding to a sampling point at the fault occurrence moment as the reference value, wherein the sampling point corresponding to the maximum absolute value is used as a catastrophe point;
the maximum absolute value is:
Figure BDA0003033832030000044
the sampling point corresponding to the maximum absolute value is the catastrophe point.
Using function value f (x) corresponding to the mutation pointmax) With said reference value f (x)0) Determining the sudden change direction of the phase current:
Figure BDA0003033832030000045
after respective sudden change directions of the three-phase current transient process are obtained, if the corresponding sudden change directions of the three-phase current are consistent, the three-phase current is judged to be a non-fault path; and if the three-phase current mutation directions are not consistent, judging the three-phase current mutation direction as a fault path.
The beneficial effect that this application reached:
1. the method can be suitable for various single-phase earth fault types of the power distribution network, has higher reliability in a system with a neutral point not grounded and an arc suppression coil earthed, can realize the positioning of various single-phase earth fault type sections of the power distribution network system with the neutral point passing the arc suppression coil and the non-grounded system, and can effectively improve the operation reliability of the power distribution network.
2. The invention only depends on three-phase current as a judgment signal, thereby avoiding the condition of poor algorithm adaptability caused by no zero-sequence current or voltage transformer on site.
3. The invention can realize the on-site judgment by setting the self-adaptive threshold, and can upload the judgment result to the master station for judgment, thereby improving the reliability of the algorithm and getting rid of the dependence on the communication system, and having higher adaptability.
4. The method has the advantages that the algorithm is simple in programming and easy to implement in practical application, a large number of secondary cable laying can be reduced in practical engineering, the economical efficiency of the algorithm is improved, the problem that the traditional positioning method is poor in effect due to the fact that various fault characteristics of the current power distribution network are complex is effectively solved, the positioning accuracy of the technology is high, and the method has high practical engineering application value.
Drawings
FIG. 1 is a flow diagram of a fault-sensing technique;
FIG. 2 is a zero sequence current equivalent diagram of a steady-state process of a neutral point ungrounded system;
FIG. 3 is a zero sequence current equivalent diagram in a steady state process of a neutral point arc suppression coil grounding system;
FIG. 4 is a fault path fault phase transient equivalent circuit for a neutral ungrounded system;
FIG. 5 is a normal path transient equivalent circuit of a neutral ungrounded system;
FIG. 6 is a structural simulation diagram of a single-phase earth fault system of a neutral point arc suppression coil grounding system of a physical test platform;
FIG. 7 is a high resistance ground fault path waveform diagram;
FIG. 8 is a high resistance ground non-fault path waveform diagram;
FIG. 9 is a diagram of a through-arc ground fault path waveform;
fig. 10 is a diagram of a through-arc ground non-fault path waveform.
Detailed Description
The present application is further described below with reference to the accompanying drawings. The following examples are only for illustrating the technical solutions of the present invention more clearly, and the protection scope of the present application is not limited thereby.
As shown in fig. 1, the method for sensing the single-phase earth fault of the power distribution network based on the transient steady state of the phase current, provided by the invention, comprises the following steps:
step 1: collecting three-phase current of a power distribution network in real time, synthesizing zero-sequence current and calculating the fundamental wave amplitude of the zero-sequence current;
when the zero sequence current fundamental wave amplitude is larger than a set threshold value, judging that a single-phase earth fault occurs, and entering the step 2;
step 2: acquiring three-phase current, namely a fault signal, and preprocessing, specifically:
after three-phase currents are obtained, zero-sequence currents are synthesized by using a phase addition method, and zero drift elimination pretreatment is performed by using a cumulative method.
And step 3: decomposing the fault signal preprocessed in the step 2 by using improved S transformation, and acquiring the fault occurrence time by using a method of solving the maximum value of the first-order difference quotient so as to distinguish the fault state of the fault signal;
since the conventional S-transform has a disadvantage of being too sensitive to signal dips and noise, it is not suitable for processing phase current information containing large noise. In order to avoid the error of the signal processing caused by the above situation, a Gaussian window amplitude adjusting coefficient a and a Gaussian window index adjusting coefficient b are added in the formula to improve the S transformation.
The formula for the improved S-transform is as follows:
Figure BDA0003033832030000061
in the formula, tau is a position parameter of a control Gaussian window omega (tau-t, f) on a time axis t; f is the frequency; j is an imaginary unit, a is a Gaussian window amplitude adjustment coefficient, and b is a Gaussian window index adjustment coefficient.
After the acquired discrete current signals i (t) are subjected to improved S transformation, a corresponding complex time-frequency matrix is obtained, and each element in the complex time-frequency matrix is subjected to modulus calculation to obtain the time-frequency matrix, wherein column vectors in the time-frequency matrix represent the amplitude-frequency characteristic of the signals at a certain moment, and row vectors represent the time-domain distribution of the signals at a certain frequency.
The first row n-0 corresponds to the signal dc component.
The difference in frequency between adjacent rows is:
Figure BDA0003033832030000062
wherein f issIs the sampling frequency; n is the number of sampling points；
The nth row corresponds to a frequency of:
Figure BDA0003033832030000063
and 4, step 4: three-phase currents which are different by a whole period before and after the fault occurrence moment are subtracted to obtain line fault component current, and the influence of load current on a judgment signal is reduced;
during specific implementation, the signals of the latter period and the first two periods of the fault are correspondingly subtracted at the moment of the fault occurrence to obtain the fault component current, namely:
if(t)＝i(t0+T+t)-i(t0-2×T+t)(t＝1、2...T)
if(t) is fault component current at time t, i (t) is three-phase current signal at time t, t0T is a sampling period for the time of occurrence of a fault.
And 5: method for solving fundamental frequency component amplitude A of fault component current by using improved S transformation0And a high frequency component amplitude A;
step 6: when the grounding fault occurs through the resistor in the power distribution network system, the steady-state process of the fault component current is stable, the steady-state characteristic is obvious, and the fault path positioning can be completed accordingly.
However, when the system has a fault through arc grounding, the steady-state process of the fault component current fluctuates due to harmonic interference caused by the arc grounding itself, so that an accurate calculation result cannot be obtained, but the line state can be judged through transient state.
Therefore, the invention adopts the fundamental wave amplitude A of the zero sequence current in the line0Forming fault perception criterion K with high-frequency component amplitude AA·A0A is larger than A, so that the self-adaptive fault type distinguishing and fault characteristic selecting positioning method is as follows:
when the sensing criterion is met, the harmonic content in the line is low, the steady state process is stable, and the steady state criterion can complete a fault positioning task, namely when the criterion is met, the system is judged to have a fault grounded through a resistor, the steady state process of the fault component current is stable, and a fault path positioning result is obtained by combining the steady state characteristics of the fault component current and the variance;
when the sensing criterion is not satisfied, the harmonic content in the line is high, the arc grounding fault is likely to occur, the steady state process fluctuation is obvious, and the steady state criterion cannot be accurately judged, so the transient criterion is applied for judgment, namely when the sensing criterion is not satisfied, the arc grounding fault occurs in a judgment system, and the fault path positioning result is obtained by combining the transient characteristic of the fault component current with the improved S transformation to obtain the sampling point at the fault occurrence time and the three-phase current mutation direction.
If the current is larger than the self-adaptive threshold or the sudden change directions of the three-phase current are inconsistent, judging the current is a fault path; and if the variance is less than or equal to the adaptive threshold or the abrupt change directions of the three-phase current are consistent, judging the path to be a non-fault path.
Wherein, KACan be determined according to field actual measurement data and engineering experience, and is tested by a large amount of actual data, in order to clearly distinguish the grounding fault condition, KAThe value range is 0.4-0.5, which can meet most conditions.
The principle of obtaining the fault path positioning result by using the steady-state characteristics of the fault component current and the variance is as follows:
according to the symmetrical component method, the positive and negative zero sequence currents of the system can be obtained through the sequence network diagram.
Because the zero sequence impedance of the neutral point ungrounded system is different from that of the neutral point grounded system through the arc suppression coil by the arc suppression coil, the sequence currents are changed along with the zero sequence impedance.
However, zero sequence current flows through all lines of the system, so that the influence on the steady-state characteristics of the lines is obvious; and the positive and negative sequence currents only exist in a fault path, so that the influence on the steady-state characteristics of the line is small.
The invention analyzes zero sequence current distribution under two grounding modes, and takes a neutral point ungrounded system model and a neutral point arc suppression coil grounding system model with over compensation degree of 5 percent as an example to respectively analyze.
Because the zero sequence current forms a loop between the ground capacitance and the fault point on each line, the zero sequence current is shunted at the fault point and flows to all lines of the system. The zero sequence equivalent circuit of the neutral point ungrounded system is shown in the attached figure 2.
The obtained shunt coefficient of the zero sequence current on the ith line on the non-fault path is as follows:
Figure BDA0003033832030000081
Cicapacitance to ground on each line (i ═ 1, 2, 3); rfIs a ground resistor. The single-phase earth fault occurs in the A phase and C phase in the middle section of the line 22upThe capacitance to ground of the upstream line of the fault point; c2downIs the line-to-ground capacitance downstream of the fault point.
Because the zero sequence current on the fault path is the sum of the zero sequence currents except the downstream of the fault point, the shunt coefficient is as follows:
Figure BDA0003033832030000082
the zero sequence equivalent circuit of the neutral point through the arc suppression coil grounding system in the steady state process is shown in figure 3.
The obtained shunt coefficient of the zero sequence current on the ith line on the non-fault path is as follows:
Figure BDA0003033832030000083
Ci'is the capacitance to ground (i ═ 1, 2 and 3) and C'2upIs the capacitance to ground of the upstream line of the fault point, C'2downThe downstream line-to-ground capacitance of the fault point is obtained; l is0Is an arc suppression coil inductance.
Because the zero sequence current on the fault path is the sum of the zero sequence currents except the downstream of the fault point, the shunt coefficient is as follows:
Figure BDA0003033832030000084
for ease of description, of the failed pathThe flow dividing coefficient is unified as betaKThe shunt factor of the non-faulty path is betai。
According to the foregoing, the positive, negative and zero sequence three-sequence currents are contained in the fault component currents of the fault path, whereas only the zero sequence current is contained in the non-fault path.
For the sake of no loss of generality, the positive and negative zero three-sequence currents at the fault point, which are obtained by the system through the symmetrical component method, are respectively assumed to be
Figure BDA0003033832030000091
Then the three-sequence current on the fault path
Figure BDA0003033832030000092
As follows.
Figure BDA0003033832030000093
Figure BDA0003033832030000094
Direction factor a ═ ej120°On the faulty line, the fault component of each phase
Figure BDA0003033832030000095
The following were used:
Figure BDA0003033832030000096
Figure BDA0003033832030000097
Figure BDA0003033832030000098
positive and negative zero sequence three-sequence current on non-fault path
Figure BDA0003033832030000099
Comprises the following steps:
Figure BDA00030338320300000910
Figure BDA00030338320300000911
fault component of three-phase current on non-fault path
Figure BDA00030338320300000912
Respectively as follows:
Figure BDA00030338320300000913
Figure BDA00030338320300000914
Figure BDA00030338320300000915
the fault components of the three-phase current of the fault path and the non-fault path are analyzed to obtain:
on a fault path, the fault component of a fault phase is larger than that of a non-fault phase, and the fault components of the non-fault phases are equal; in the non-fault path, the fault components of the three-phase currents are all the same. After a single-phase earth fault occurs, a fault path comprises positive and negative zero-sequence currents, and a non-fault path only comprises the zero-sequence currents. In order to increase the difference between the fault path and the non-fault path and eliminate the influence of the shunt coefficient beta on the steady-state characteristic, the fault component current of the line is subtracted from the zero-sequence current, that is:
subtracting zero-sequence current from three-phase fault component current of the fault path:
Figure BDA00030338320300000916
Figure BDA00030338320300000917
Figure BDA00030338320300000918
subtracting zero-sequence current from three-phase fault component current of a non-fault path:
Figure BDA00030338320300000919
Figure BDA0003033832030000101
Figure BDA0003033832030000102
in order to further increase the distinction, FFT conversion is applied to the subtraction result to obtain the fundamental wave amplitude of each current signal, and then a variance formula is used for obtaining the variance calculation result of the three-phase fundamental wave amplitude of the circuit.
Variance Var of fault path fault component fundamental amplitudeKComprises the following steps:
Figure BDA0003033832030000103
variance Var of fundamental amplitude of non-failure path failure componentiComprises the following steps:
Vari＝0
setting adaptive thresholds
Figure BDA0003033832030000104
I0Zero sequence current of this lineThe amplitude of the fundamental wave;
comparison VarKAnd Var0When Var isK＞Var0The time is a fault path, and the reverse is a non-fault path.
Wherein KrelAs an adaptive coefficient, the sensitivity of the algorithm can be adjusted, and can be 0.85 according to engineering experience.
The method is characterized in that a fault path positioning result is obtained by combining transient characteristics of fault component current with improved S transformation to obtain a fault occurrence time sampling point and a three-phase current abrupt change direction, and the principle is as follows:
transient process after fault occurrence can be regarded as neutral point offset voltage u0The effect on the inductance and capacitance in the line. The invention firstly utilizes a neutral point ungrounded system to analyze, and transient current on each line flows back to the line through a fault point, a bus and a transformer. In order to simplify the calculation to obtain the current transient characteristics, a plurality of inductors and capacitors in an equivalent circuit can be combined into one inductor and capacitor, the approximation can cause the loss of high-frequency components, and the transient quantity process can be analyzed qualitatively.
Therefore, the fault path fault phase transient equivalent of the neutral point ungrounded system can be obtained, namely, a fault path fault phase fault current transient equivalent circuit diagram is shown in FIG. 4, and the transient fault current i can be obtainedf：
Figure BDA0003033832030000105
Wherein, omega is power frequency angular frequency;
Figure BDA0003033832030000106
is the initial phase of the zero sequence voltage; omegafIs the free-running current angular frequency. When R isAAt an increase, the system transient is diminished.
The transient equivalent circuit of the normal path of the system with the ungrounded neutral point, namely the transient equivalent circuit of the non-fault path and the transient equivalent circuit of the non-fault phase line of the fault path are shown in figure 5. From this, the non-fault line and the non-fault phase transient current i 'of the fault line can be obtained'fThe following were used:
Figure BDA0003033832030000111
in a system in which a neutral point is grounded via an arc suppression coil, since the compensation current of the arc suppression coil flows only through a fault phase in a fault path and the time constant of the arc suppression coil is large, the compensation current of the arc suppression coil is extremely small at the initial stage of the occurrence of a fault, and the arc suppression coil can be regarded as an open circuit. Therefore, when analyzing the transient process of the fault component current, the system of grounding the neutral point through the arc suppression coil is approximately the same as the system of ungrounded neutral point.
Through the analysis, in a system with a neutral point not grounded and a system with a neutral point grounded through an arc suppression coil, when the ground resistance is small or the system is grounded through an arc, the fault transient process is obvious. Because the transient current has a direct current attenuation component, the transient change process has a process of current mutation and then attenuation, and the mutation amount is maximum when a fault occurs. As fault transient current of each line flows to a fault point and flows back to the line through a fault phase line and a bus of a fault path, the fault component current transient process of the fault phase in the fault path and the transient processes in other lines are in an opposite relation, and the characteristic that three-phase current mutation directions of the fault phase and the non-fault phase in the fault path are different and three-phase current mutation directions in the non-fault path are the same is caused.
When a single-phase earth fault occurs in the power distribution network, the current mutation directions of a fault phase and a non-fault phase in a fault path are inconsistent; and in the non-fault path, the current abrupt change direction of the fault phase is the same as that of the non-fault phase. Because the phase current contains a large amount of harmonic waves and load current interference, a signal with high reliability cannot be obtained only by subtracting the current before and after the fault.
In order to avoid the influence of the high-frequency component on the direction of the sudden change of the signal, firstly, the three-phase current signal preprocessed in the step 2 is filtered by using improved S transformation to obtain a fundamental frequency component, and as the current signal generates the sudden change at the fault moment, a first-order difference quotient value at a sudden change point is larger, so that a method for searching the maximum value for the fundamental frequency component by using the first-order difference quotient obtains a sampling point corresponding to the fault occurrence moment, and the direction of the sudden change is determined by using the following method, specifically, the method comprises the following steps:
suppose that the fundamental frequency component signal of a phase current is: y ═ f (x).
Firstly, according to sampling points corresponding to the accurate occurrence moments of the faults obtained by improving S conversion, finding out the phase abrupt change sampling points of each phase current according to the first-order difference quotient, and defining a calculation interval near the sampling points
Figure BDA0003033832030000112
Wherein
Figure BDA0003033832030000113
For calculating the interval range, 15 can be taken according to the fault transient duration; the reasonable design interval can avoid errors caused by inaccurate sampling points at the moment of fault occurrence;
then, a function value corresponding to a sampling point at the moment of the fault occurrence can be used as a reference value, an absolute value of a difference value between the corresponding function value and the reference value in a calculation interval is calculated, and a sampling point corresponding to the maximum absolute value is used as a catastrophe point;
the maximum absolute value is:
ymax＝max{f(x0+1)-f(x0),f(x0+2)-f(x0)...f(x0+15)-f(x0)}
the sampling point corresponding to the maximum absolute value is the catastrophe point.
Finally, the function value f (x) corresponding to the mutation point is utilizedmax) With said reference value f (x)0) Determining the sudden change direction of the phase current:
Figure BDA0003033832030000121
according to the method, the respective mutation directions of the three-phase current transient processes can be obtained.
If the corresponding sudden change directions of the three-phase currents are consistent, judging that the three-phase currents are a non-fault path; and if the three-phase current mutation directions are not consistent, judging the three-phase current mutation direction as a fault path.
Example 1
As shown in fig. 6, fig. 6 is a simulation model structure diagram of a system in which a neutral point of a physical simulation platform is grounded through an arc suppression coil.
In fig. 6, lines 1 and 3 are normal lines, line 2 is a fault line, and detection devices are respectively installed at the head ends of lines 1 and 3, line 2 is divided into a front line and a rear line, and detection devices are respectively installed at the front section and the rear section of a fault point.
According to the invention, a physical simulation platform is utilized to simulate high-resistance grounding and arc grounding to carry out fault perception technology simulation verification, wherein the high-resistance grounding is a special case of grounding through a resistor and is a case of difficult line selection.
For the single-phase earth fault of a neutral point arc suppression coil grounding system or a neutral point ungrounded system, the single-phase earth fault sensing method of the power distribution network based on the phase current transient stable state can be adopted, and after a fault sensing technical device is installed at the head end of each section of line in a circuit such as in a graph 6, the single-phase earth fault path positioning can be completed. The specific implementation steps are shown in fig. 1.
Step 1: each detection device measures phase current change in real time and synthesizes zero-sequence current;
once the zero sequence current amplitude of the line is judged to be larger than the threshold value, the fault can be judged to occur, and the wave recording device transmits fault waveforms, namely fault signals, to the positioning device;
in order to avoid the fluctuation of the zero sequence current to trigger the device to act, an operating threshold value is set in the device. When the zero sequence current amplitude value monitored by the monitoring device is larger than a working threshold value, judging that a fault occurs in the system; in practical application, the working threshold value can be set according to the actual situation of the field zero-sequence current.
Step 2: when a fault occurs in the system, the device is triggered and the wave recording device transmits a fault waveform to the positioning device;
and step 3: decomposing the fault signal preprocessed in the step 2 by using improved S transformation and acquiring the fault occurrence time by using a method of solving the maximum value of the first-order difference quotient;
and 4, step 4: subtracting three-phase currents which are different by a whole period before and after the fault occurrence time to obtain line fault component current;
and 5: calculating a fundamental wave amplitude and a high-frequency component amplitude after the zero sequence current fault occurs;
step 6: and judging whether the fundamental wave amplitude and the high-frequency component amplitude meet the fault perception criterion.
Because the signal fault characteristics of the line are changed due to different fault types, the fault type is judged by utilizing a fault perception algorithm, the adaptive signal characteristics are selected to complete fault positioning, and the coefficient K in the fault perception criterionACan be adjusted according to the actual situation on site, and in the embodiment, 0.49 is taken.
If the fault perception criterion is met, the device judges by utilizing the steady-state process of the fault component current and obtains a calculation result by utilizing a variance method.
If the fault perception criterion is not met, the device judges by using a fault component current transient process, and obtains a three-phase current mutation direction after determining a mutation point by using improved S transformation.
In a high-resistance grounding experiment, the ratio of the amplitude of the high-frequency component to the amplitude of the fundamental wave at the detection points of the front section of the system line 1, the front section of the line 2 and the rear section of the line 2 is 0.0548, 0.0452, 0.0168 and 0.0264, and the fault perception criterion is met, so the device selects a steady-state process adopting fault component current to complete judgment.
And after the zero sequence current of the line is subtracted from the three-phase fault component current, the three-phase fundamental wave amplitude is obtained by FFT conversion, and the corresponding variance of the line is calculated by using a variance formula.
The experimental results are as follows:
line Phase A calculation result/A Phase B calculation result/A C phase calculation result/A Variance results
Front section of line 2 3.3542 1.4719 1.3781 1.242804
Rear section of line 2 0.0569 0.2409 0.1641 0.008541
Line 1 0.1608 0.1374 0.1714 0.000303
Line 3 0.1026 0.0968 0.1002 8.49E-06
In an arc grounding experiment, the ratio of the amplitude of the high-frequency component to the amplitude of the fundamental wave at the detection points of the front section of the system line 1, the front section of the line 2 and the rear section of the line 2 is 0.6725, 0.5862, 0.5698 and 0.5126, and the device selects a transient process of fault component current to finish judgment because the fault perception criterion is not met.
And determining a mutation point by using the improved S transformation and obtaining the three-phase current mutation direction. The experimental results are as follows:
line Direction of phase A mutation Direction of phase B mutation Direction of phase C mutation The judgment result
Front section of line 2 -1 -1 1 Direction of abrupt change is different
Rear section of line 2 -1 -1 -1 The direction of mutation is the same
Line 1 -1 -1 -1 The direction of mutation is the same
Line 3 -1 -1 -1 The direction of mutation is the same
If the variance of the calculated result of the device is larger than the threshold value or the three-phase mutation directions are inconsistent, judging the path as a fault path; and if the variance calculated by the device is smaller than the threshold value or the three-phase mutation direction is always, judging the path as a non-fault path.
The result obtained by the high-resistance grounding test is compared with an adaptive threshold 0.89383 set in the device, the variance result corresponding to the front-stage line of the line 2 is greater than the threshold, and the line is a fault path; the variance calculation results of the lines 1 and 3 corresponding to the rear sections of the line 2 are smaller than a threshold value, and the line is a non-fault line. Therefore, the fault path positioning can be completed, the fault is positioned at the front section of the line 2, and the fault phase can be obtained to be the A phase.
Fig. 7-8 illustrate the fault path and non-fault path current waveforms with high impedance grounding.
The results obtained by the arc grounding test can obtain that the abrupt change directions of the front section of the line 2 are different, and the line is a fault path; the corresponding abrupt change directions of the line 1 and the line 3 and the rear section of the line 2 are the same, and the lines are non-fault lines. Therefore, the fault path positioning can be completed, the fault is positioned in the front section of the line 2, and the fault phase can be obtained to be the A phase.
9-10, fault path and non-fault path current waveforms for arcing to ground.
In conclusion, the three-phase current of the power distribution network is measured in real time, and when a single-phase earth fault occurs in the system, the fault signal is decomposed by using improved S transformation, and the fault occurrence time is obtained to distinguish the fault state of the wave recording signal; then, the phase currents which are different by a whole period before and after the fault occurrence time are subtracted to obtain line fault component current, so that the influence of load current on a judgment signal is reduced; and then, solving the high-frequency component and the fundamental frequency component amplitude of the fault component current by using improved S transformation, and judging the fault type by using a fault perception criterion formed by the quantity relation of the high-frequency component and the fundamental frequency component: when the criterion is met, judging that the steady-state process of the system is stable, and obtaining a fault section positioning result by combining the steady-state characteristics of fault component current with the variance; and otherwise, judging that the system has the grounding fault through the electric arc, and acquiring a sampling point at the fault occurrence moment and the three-phase current mutation direction by using the transient characteristic of the fault component current in combination with the improved S transformation to obtain a section positioning result.
Further, the amplitude A of the fundamental frequency component of the fault component is utilized0Setting fault perception criterion K with high-frequency component amplitude AA·A0Is > A. The criterion can effectively distinguish the grounding fault through the electric arc from the grounding fault through the resistance, realizes the purpose of positioning different faults by adopting a fault positioning method with higher adaptability, and improves the reliability of fault positioning. Meanwhile, the invention also provides different positioning methods by utilizing the transient and steady processes of the three-phase current respectively: the method comprises the steps of obtaining respective mutation points of three-phase currents by utilizing three-phase current transient process signals and improved S transformation, determining mutation directions, judging as a fault path when the mutation directions are inconsistent, and judging as a non-fault path when the mutation directions are consistent; the technology distinguishes a fault path from a non-fault path by using the magnitude of the fundamental wave amplitude variance in the three-phase current steady-state process, and sets an adaptive threshold value Var by using the zero-sequence current fundamental wave amplitude of the circuit0And when the variance value is larger than the threshold value, judging the path as a fault path, otherwise, judging the path as a non-fault path. The positioning method can not only realize the fault positioning after uploading the fault information, but also can finish the judgment on site, thereby realizing the weak dependence on the communication system.
The present applicant has described and illustrated embodiments of the present invention in detail with reference to the accompanying drawings, but it should be understood by those skilled in the art that the above embodiments are merely preferred embodiments of the present invention, and the detailed description is only for the purpose of helping the reader to better understand the spirit of the present invention, and not for limiting the scope of the present invention, and on the contrary, any improvement or modification made based on the spirit of the present invention should fall within the scope of the present invention.

Claims (6)

1. A power distribution network single-phase earth fault sensing method based on phase current transient state steady state is characterized in that:
the method comprises the following steps:
step 1: collecting three-phase current of a power distribution network in real time, synthesizing zero-sequence current and calculating the fundamental wave amplitude of the zero-sequence current;
when the zero sequence current fundamental wave amplitude is larger than a set threshold value, judging that a single-phase earth fault occurs, and entering the step 2;
step 2: acquiring three-phase current, namely fault signals, and preprocessing the three-phase current;
and step 3: decomposing the fault signal preprocessed in the step 2 by using improved S transformation and acquiring the fault occurrence time by using a method of solving the maximum value of the first-order difference quotient;
and 4, step 4: subtracting three-phase currents which are different by a whole period before and after the fault occurrence time to obtain line fault component current;
and 5: method for solving fundamental frequency component amplitude A of fault component current by using improved S transformation0And a high frequency component amplitude A;
step 6: judging the fundamental frequency component amplitude A in the step 5 by using a fault perception criterion formed by the quantity relation of the fundamental frequency component amplitude and the high frequency component amplitude0Whether the quantitative relation with the high-frequency component amplitude A meets the fault perception criterion or not;
the fault perception criterion is as follows: kA·A0＞A，KAThe value range is as follows: k is more than or equal to 0.4A≤0.5；
When the fault path positioning result meets the criterion, judging that the system has a resistance grounding fault, the steady state process of the fault component current is stable, and obtaining the fault path positioning result by using the steady state characteristics of the fault component current and the variance;
otherwise, judging that the system has a fault through electric arc grounding, and acquiring a fault occurrence moment sampling point and a three-phase current mutation direction by utilizing the transient characteristic of the fault component current in combination with the improved S transformation to obtain a fault path positioning result;
the method for obtaining the fault path positioning result by using the steady-state characteristics of the fault component current and the variance specifically comprises the following steps:
step 611: subtracting zero-sequence current from three-phase fault component current:
Figure FDA0003374355860000011
Figure FDA0003374355860000012
Figure FDA0003374355860000013
step 612: applying FFT to the subtraction result in the step 611 to obtain the fundamental wave amplitude of each current signal;
step 613: calculating the variance Var of the fundamental wave amplitude of the three-phase fault component currentK：
Figure FDA0003374355860000021
Step 614: setting self-adaptive threshold value Var related to zero sequence current fundamental wave amplitude0；
Step 615: comparison VarKAnd Var0When Var isK＞Var0The time is a fault path, otherwise, the time is a non-fault path;
the method for obtaining the fault path positioning result by using the transient characteristics of the fault component current in combination with the improved S transformation to obtain the fault occurrence time sampling point and the three-phase current mutation direction specifically comprises the following steps:
carrying out filtering processing on the three-phase current signals preprocessed in the step 2 by using improved S transformation to obtain fundamental frequency components, obtaining sampling points corresponding to the fault occurrence time by using a method of finding maximum values for the fundamental frequency components by using first-order difference quotient, and determining mutation directions by using the following method:
assuming that the current fundamental frequency component signal of a certain phase in the three-phase current is: and y ═ f (x), and acquiring a sampling point x corresponding to the fault occurrence time according to the improved S transformation0Defining a calculation interval in the vicinity of the sampling point
Figure FDA0003374355860000022
Wherein
Figure FDA0003374355860000023
Calculating the range of the interval;
calculating to obtain an absolute value of a difference value between a corresponding function value and a reference value in a calculation interval by using a function value corresponding to a sampling point at the fault occurrence moment as the reference value, wherein the sampling point corresponding to the maximum absolute value is used as a catastrophe point;
the maximum absolute value is:
Figure FDA0003374355860000024
sampling points corresponding to the maximum absolute value are catastrophe points; using function value f (x) corresponding to the mutation pointmax) With said reference value f (x)0) Determining the sudden change direction of the phase current:
Figure FDA0003374355860000025
after respective sudden change directions of the three-phase current transient process are obtained, if the corresponding sudden change directions of the three-phase current are consistent, the three-phase current is judged to be a non-fault path; and if the three-phase current mutation directions are not consistent, judging the three-phase current mutation direction as a fault path.
2. The method for sensing the single-phase earth fault of the power distribution network based on the phase current transient steady state is characterized in that:
and 2, after three-phase currents are obtained, synthesizing zero-sequence currents by using a phase addition method and performing zero drift elimination pretreatment by using an accumulation method.
3. The method for sensing the single-phase earth fault of the power distribution network based on the phase current transient steady state is characterized in that:
the formula for the improved S-transform is as follows:
Figure FDA0003374355860000031
in the formula, tau is a position parameter of a control Gaussian window omega (tau-t, f) on a time axis t; f is the frequency; j is an imaginary unit, a is a Gaussian window amplitude adjustment coefficient, and b is a Gaussian window index adjustment coefficient.
4. The method for sensing the single-phase earth fault of the power distribution network based on the phase current transient steady state is characterized in that:
and 4, subtracting three-phase currents which are different by a whole period before and after the fault occurrence time to obtain line fault component current, wherein the method specifically comprises the following steps:
correspondingly subtracting signals of a period after the fault occurrence time and two periods before the fault occurrence to obtain fault component current, namely:
if(t)＝i(t0+T+t)-i(t0-2×T+t)(t＝1、2...T)；
if(t) is fault component current at time t, i (t) is three-phase current signal at time t, t0T is a sampling period for the time of occurrence of a fault.
5. The method for sensing the single-phase earth fault of the power distribution network based on the phase current transient steady state is characterized in that:
in step 614, an adaptive threshold is set
Figure FDA0003374355860000032
I0For the fundamental amplitude of zero-sequence current of this line, KrelAre adaptive coefficients.
6. The method for sensing the single-phase earth fault of the power distribution network based on the phase current transient steady state is characterized in that:
in step 614, KrelTake 0.85.
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