CN110907758A - Small current ground fault line selection method covering CT polarity self-correction - Google Patents

Small current ground fault line selection method covering CT polarity self-correction Download PDF

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CN110907758A
CN110907758A CN201911241132.2A CN201911241132A CN110907758A CN 110907758 A CN110907758 A CN 110907758A CN 201911241132 A CN201911241132 A CN 201911241132A CN 110907758 A CN110907758 A CN 110907758A
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zero
transient
branch
polarity
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CN110907758B (en
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郑玉平
潘书燕
夏磊
王小红
周华良
胡国
吴通华
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Nari Technology Co Ltd
NARI Nanjing Control System Co Ltd
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NARI Nanjing Control System Co Ltd
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    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
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Abstract

The invention discloses a small current ground fault line selection method covering CT polarity self-correction, which comprises the following steps: after the single-phase earth fault is started, extracting the transient state quantity in a characteristic frequency band for the bus zero-mode voltage and the zero-mode currents of all the branches through digital filtering, and screening out the branch with the maximum transient state quantity; checking, correcting and alarming the zero sequence CT polarities of all the branches, and performing fault line selection according to corrected data; and calculating the transient zero-mode current energy of the maximum branch circuit after filtering and the transient zero-mode current energy synthesized by all the branch circuits, and determining whether the fault point is positioned on the bus or the line according to the amplitude magnitude relation between the maximum branch circuit and the transient zero-mode current energy. The invention is suitable for a small current grounding system, and overcomes the problems of insufficient line selection accuracy and sensitivity caused by power frequency compensation quantity through the zero-mode current transient state quantity integral of digital filtering; and zero sequence CT polarity check, alarm and correction are carried out in real time in the fault line selection process, so that the problem of line selection error caused by CT polarity error is avoided.

Description

Small current ground fault line selection method covering CT polarity self-correction
Technical Field
The invention belongs to the power system fault monitoring technology, and particularly relates to a small current ground fault line selection method covering CT polarity self-correction.
Background
The grounding mode of the neutral point of the power system is mainly divided into two categories: the neutral point is directly grounded (high current grounding system) and the neutral point is not directly grounded (low current grounding system). The neutral point is not directly grounded, and includes a neutral point which is not grounded and a neutral point which is grounded via an arc suppression coil (resonance ground). In China, a small-current grounding system is mostly adopted in a medium-voltage power distribution network, but the problem of small-current grounding fault line selection is not completely and effectively solved so far because of unobtrusiveness (grounding current is very small and is generally only a few amperes), uncertainty (influence of arc coil compensating current in a resonance grounding system on power frequency current) and instability (high intermittent grounding and arc grounding occurrence rate) of fault amount.
At present, almost all small-current grounding line selection devices which are operated need to sample and analyze zero sequence current of a line to perform fault line selection, so that the correct polarity of zero sequence CT becomes a crucial factor which directly influences the accuracy of line selection. However, on one hand, in view of the characteristics of zero sequence current, when no ground fault occurs, the zero sequence current is very small and is basically 0 theoretically, so that the polarity of the CT cannot be accurately judged when the CT is installed; on the other hand, many situations in the engineering field do not allow zero sequence CT polarity and wiring uniformity check on the power failure of the switch cabinet, which often causes the situation of inaccurate line selection after the device is actually put into operation.
Aiming at the problem of fault line selection of a low-current grounding system, a great deal of research is carried out by many scholars, and line selection methods are divided into an active method and a passive method according to the source of a used signal. The active line selection method comprises an injection signal method, a medium resistance method, a residual current incremental method, a small disturbance method and the like; the passive line selection method can be divided into two types, namely a steady state characteristic quantity-based method and a transient state characteristic quantity-based method according to different used characteristic signals. Under the working condition of an arc suppression coil grounding system, the passive line selection method based on the steady-state information has the fault misjudgment of grounding faults and cannot meet the field requirements; the line selection method based on the transient characteristic quantity comprises a first half-wave method, a transient zero-mode current amplitude and polarity comparison method, a transient zero-mode current direction method, a transient energy method and the like. The zero-mode transient energy function is defined as the integral of the product of a zero-mode voltage derivative and zero-mode current in a time domain, the absolute value of the zero-mode transient energy function of a fault line is larger than that of a healthy line, the zero-mode transient energy function value of the fault line is a negative value, and the zero-mode transient energy function values of the healthy line are positive values, so that the judgment of the fault line is completed. When the grounding transition resistance is large, the zero-mode transient energy value is small, and because the zero-mode voltage is subjected to derivation operation, the zero-mode voltage and the zero-mode current at the fault occurrence moment are different from each other at the same moment, 1/4 cycles are formed, and the accumulation of transient energy is unfavorable, so that the accuracy of line selection during high-resistance grounding fault is further reduced, and the accuracy of line selection is reduced.
The existing zero sequence CT polarity diagnosis method carries out CT polarity consistency comprehensive analysis based on multiple earth fault results, and has no CT polarity check in real time in the earth fault line selection process, and no measure for carrying out CT polarity self-correction according to the CT polarity check results, so that the problem of line selection error caused by CT polarity error cannot be avoided.
The existing small-current grounding line selection device is gradually configured with zero sequence CT polarity parameters, and although the polarity of the zero sequence CT of each branch circuit can be judged to be correct by manually checking and analyzing fault recording data after one or more single-phase grounding faults, the process needs manual participation, and the working efficiency is low. In addition, the existing line selection technologies do not perform CT polarity check in real time in the process of ground fault line selection, and do not perform CT polarity self-correction according to the CT polarity check result, and the correctness of the zero sequence CT polarity of each branch can be analyzed after a fault or n times of faults, so that the problem of one or more line selection errors caused by CT polarity errors cannot be avoided.
Meanwhile, the existing small-current ground fault line selection principle based on zero-mode transient energy adopts the integral of the product of a zero-mode voltage derivative (or zero-mode voltage phase shift) and a zero-mode current in a time domain, the zero-mode voltage derivative or the zero-mode voltage phase shift can cause that the change of the zero-mode voltage and the zero-mode current at the fault occurrence moment is not at the same moment, and the phase difference is 1/4 cycle waves, so that the accumulation of the transient energy is unfavorable, and particularly, when the high-resistance ground fault occurs, the transient energy is small, the time is only a few millisecond time window, the line selection sensitivity is poor, and the line selection accuracy is influenced; in addition, accumulation of energy in a characteristic frequency band is not considered before integration, and the line selection misjudgment rate is high when the power frequency component ratio of the system grounded through the arc suppression coil at the initial stage of the fault is high.
Disclosure of Invention
The purpose of the invention is as follows: aiming at the defects of the zero sequence CT polarity diagnosis prior art, the invention provides a small current ground fault line selection method covering CT polarity self-correction.
The technical scheme is as follows: a small current ground fault line selection method covering CT polarity self-correction is characterized in that fault identification is carried out on the basis of zero-mode voltage and zero-mode current, when any value of the zero-mode voltage and the zero-mode current exceeds a preset range value, a system is judged to have a ground fault, transient state quantity extraction in a characteristic frequency band is carried out on the bus zero-mode voltage and the zero-mode current of all branches, power frequency components of the bus zero-mode voltage and the zero-mode current of all branches are filtered, components in the characteristic frequency band are reserved, and branch selection with the maximum transient state quantity is carried out according to the transient state quantity; secondly, checking, alarming and correcting the zero sequence CT polarities of all branches with the maximum non-transient quantity; and then, continuing to perform the next judgment according to the corrected data:
firstly, carrying out comprehensive polarity analysis on the branch with the maximum transient quantity; if the judgment condition is met, determining the branch as a fault branch, and performing zero sequence CT polarity check, alarm and correction on the branch according to the transient zero-mode power direction; and if the transient zero-mode power of the branch circuit with the maximum transient quantity is in a positive direction, performing energy matching degree analysis on the transient zero-mode current square integrated in a time domain with the transient zero-mode current vector and the square of all branch circuits integrated in the time domain, otherwise, performing energy matching degree analysis on the transient zero-mode current square integrated in the time domain with all branch circuits with the polarity of the maximum branch circuit reversed to determine whether the fault point is located on the bus or the circuit and the zero-sequence CT polarity of the zero-sequence circuit is correct, and giving corresponding alarm and correction.
Further, the method comprises a digital filtering step, wherein zero-mode voltage in a characteristic frequency band and zero-mode current transient quantities of all branches are extracted; correcting the polarity of CT, namely correcting the polarity of zero sequence CT of all branches in the system, including warning and correcting the branch with wrong CT polarity; fault line selection, wherein the fault line selection determines that a fault point is positioned on a bus or a line, and the method comprises the following steps:
(1) extracting transient state quantity in characteristic frequency band for bus zero-mode voltage and all branch zero-mode current, filtering power frequency component to obtain filtered transient state zero-mode voltage u0kfilter(t) and all branch transient zero-mode currents i0kfilter(t)；
(2) Calculating and sequencing the maximum values of transient zero-mode currents of all the branches after filtering, and screening out a branch H with the maximum transient quantity;
(3) checking the CT polarities of all branches outside the branch H with the maximum transient quantity screened in the step (2), judging whether the CT polarities of all branches outside the branch H are reversed according to the transient zero-mode power direction, correcting the polarities of the branches with the reversed CT polarities, and simultaneously negating a zero-mode current sampling value and a transient zero-mode current value of the branches;
(4) carrying out polarity comprehensive study and judgment analysis on the branch H with the maximum transient quantity, wherein the judgment condition comprises the fundamental wave amplitude I of the branch H after n cycles of fault occurrence0H_Non-maximum, transient process shiftDirect current component IDCoffsetGreater than a threshold value, and a transient direction IDir_TAnd the steady state direction IDir_SIf the two branches are inconsistent, if any condition is met, the branch H is a fault branch, and the step (5) is executed; otherwise, executing the step (6);
(5) judging transient direction I of branch HDir_TWhether the direction is positive or not, if so, the polarity of the branch H is correct; otherwise, the branch H is in reverse polarity connection, and the device corrects the polarity parameter and sends out warning information;
(6) judging the transient direction I of the branch circuit H according to the transient zero-mode power directionDir_TIf the direction is positive, executing the step (7) if the direction is positive, otherwise executing the step (10);
(7) solving the sum i of transient zero-mode current vectors of all the branches filtered in the step (1)0sum_filter(t)；
(8) Carrying out square integration on the sampling value in T2 time of the sum of the transient zero-mode current of the filtered branch circuit H in the step (1) and the transient zero-mode current vectors of all the branch circuits filtered in the step (7), thereby obtaining a transient zero-mode current energy set E of the branch circuit HH_filter(t) and all branch transient zero-mode current vector sum energy set Esum_filter(t)；
(9) In T2 time after fault starting, if all branch circuit transient zero-mode current vectors and energy sets are smaller than a branch circuit H transient zero-mode current energy set, judging that the polarity of a branch circuit H zero-sequence CT is correct and the branch circuit H zero-sequence CT is a fault branch circuit; otherwise, judging that the polarity of the branch circuit H zero sequence CT is reversely connected, correcting the polarity parameter by the device and sending alarm information, wherein the fault type is bus grounding;
(10) negating the transient zero-mode current value of the filtered branch circuit H, and obtaining a transient zero-mode current energy set of the branch circuit H and transient zero-mode current vectors and energy sets of all the branch circuits in T2 time after the fault starts by the method in the step (8);
(11) in the T2 time, if all branch transient zero-mode current vectors and energy sets are smaller than the branch H transient zero-mode current energy set, judging that the polarity of the branch H zero-sequence CT is reversed, correcting the polarity parameters by the device and sending out alarm information, wherein the branch is a fault branch; otherwise, judging that the polarity of the branch circuit H zero sequence CT is correct, and the fault type is bus grounding.
Furthermore, the expression for extracting and calculating the bus zero-mode voltage and the transient state quantity in the characteristic frequency band of the zero-mode currents of all the branches in the step (1) is as follows, and the designed filter is required to filter power frequency components and reserve the components in the frequency band range:
Figure BDA0002306255890000045
Figure BDA0002306255890000046
filter processing function
The calculation process for checking the polarity of the maximum non-transient branch CT in the step (3) is as follows:
Figure BDA0002306255890000041
wherein k ≠ H
DkCT with correct polarity > 0 >0k_filter＝i0k_filter；
DkLess than 0, CT polarity is connected reversely (alarm),
Figure BDA0002306255890000042
the transient zero-mode current vector sum calculation expression of all the branches after filtering in the step (7) is as follows:
Figure BDA0002306255890000043
m is the total number of branches actually put into operation.
Performing square integral calculation expression of sampling values in T2 time on the vector sum of the filtered branch H transient zero-mode current and all branch transient zero-mode currents in the step (8) as follows:
Figure BDA0002306255890000044
Figure BDA0002306255890000051
EH_filter(t): a transient zero-mode current energy set after band-pass filtering of the corresponding branch H;
Figure BDA0002306255890000052
corresponding to the transient zero-mode current vector and the energy set after the band-pass filtering of all the branches;
T2the upper limit of the transient component integration time of the band-pass filtering from the fault starting moment.
In the step (9), the relevance of all branch transient zero-mode current vectors and energy sets and the branch H transient zero-mode current energy set in the T2 time is determined as follows:
at 0 to T2All satisfy
Figure BDA0002306255890000053
The zero sequence CT polarity of the branch H is correct and is a fault branch;
otherwise, the polarity of the zero sequence CT of the branch circuit H is reverse and the branch circuit H is grounded;
k is a reliability coefficient; eΔThrIs a reliable variation threshold.
The method comprises fault identification, and the fault identification comprises the following processes:
obtaining zero sequence voltage and zero sequence current values, when the amplitude of the zero sequence voltage exceeds a fixed value or the variation of the zero sequence voltage exceeds a fixed value, and the amplitude of the zero sequence current of any branch exceeds a fixed value or the variation of the zero sequence current exceeds a fixed value, judging that the system has a ground fault, and starting a digital filtering link, CT polarity correction and fault line selection logic.
Has the advantages that: compared with the prior art, the invention has the following remarkable effects:
(1) the invention can realize real-time checking, alarming and correcting of the zero sequence CT polarity of all the branches in the fault process, does not need to extract field fault recording data after the fault so as to comprehensively analyze the CT polarity correctness of the branches, does not need to power off the switch cabinet to check the mutual inductor and the secondary circuit, reduces the operation and maintenance cost, and avoids the problem of wrong line selection caused by wrong CT polarity.
(2) In a digital filtering link after fault starting, fault component extraction in a characteristic frequency band is carried out on zero-mode voltage and zero-mode currents of all branches by band-pass filtering or wavelet transformation and other methods, the influence of power frequency components is eliminated, main resonant frequency in a fault transient state reflecting process is reserved, and line selection accuracy is improved;
(3) the sum of the transient state quantities in the fault characteristic frequency bands of the zero-mode currents of all the branches is adopted, the sum is theoretically close to 0 when the circuit branches are in fault, the sum of the transient state quantities in the fault characteristic frequency bands of the zero-mode currents of all the branches is always larger than the transient state quantity in the fault characteristic frequency band of the zero-mode current of the largest branch when a bus is in fault, and the discrimination is obvious;
(4) a transient energy method in a zero-mode current fault characteristic frequency band is adopted, so that misjudgment caused by the influence of steady-state power frequency signals in a distributed arc coil grounding system on energy accumulation is eliminated;
(5) the fault identification starting link adopts the OR gate starting of zero-mode voltage and zero-mode current, and the line selection link adopts the integral of the zero-mode current in the time domain in the square direction, so that the zero-mode voltage is not needed, and the influence of PT line breakage is avoided;
(6) the zero-mode voltage is not needed in the line selection link, and the influence of distribution difference on the time dimension of zero-mode voltage and zero-mode current transient state quantity caused by zero-mode voltage derivation on the energy acquisition effect is overcome;
(7) the integration of the square of a zero-mode current sampling value in a characteristic frequency band on a time domain is adopted, so that the influence of insufficient sensitivity caused by high-resistance grounding is overcome;
(8) the method of the invention does not need to add additional primary equipment and does not need action coordination of other primary equipment;
(9) the invention is not influenced by the power frequency compensation quantity of the arc suppression coil, is self-adaptive to a neutral point ungrounded system, an arc suppression coil grounded system and a high-resistance grounded system, has wide application range and does not have a line selection blind area.
Drawings
FIG. 1 is a schematic flow diagram of the process of the present invention.
Detailed Description
For the purpose of explaining the technical solution disclosed in the present invention in detail, the following description is further made with reference to the accompanying drawings and specific embodiments.
The invention provides a small current earth fault line selection method covering zero sequence CT polarity real-time check and self-correcting based on a transient zero-mode current energy set, namely, when a single-phase earth fault occurs, firstly, a bus zero-mode voltage and zero-mode currents of all branches are subjected to transient quantity extraction in a characteristic frequency band through digital filtering, and the maximum branch of the transient quantity is selected according to the transient quantity; then checking, alarming and correcting the zero sequence CT polarity of all the branches with the maximum non-transient quantity through the transient zero-mode power direction; and finally, carrying out comprehensive study and judgment on transient and steady amplitude and direction judgment, deviation direct current component judgment, transient zero-mode current energy set and transient zero-mode current vector and energy set matching degree judgment of all branches and the like on the branch with the maximum transient quantity according to the corrected data, determining whether the fault point is located on the bus or the line and the zero sequence CT polarity of the line is correct, and giving corresponding alarm and correction.
As shown in FIG. 1, the method comprises the following steps:
the method comprises the following steps: when the zero sequence voltage amplitude exceeds a fixed value or the zero sequence voltage variation exceeds a fixed value and the zero sequence current amplitude of any branch exceeds a fixed value or the zero sequence current variation exceeds a fixed value, judging that the system has a ground fault and entering a fault starting link to start fault route selection;
(U0＞U0setor
Figure BDA0002306255890000061
)||(I0k＞U0setOr
Figure BDA0002306255890000062
)
U0: a bus zero mode voltage amplitude;
ΔU0: bus zero-mode voltage miningThe amplitude calculated by the sample value variation;
U0seta bus zero-mode voltage starting threshold value;
Figure BDA0002306255890000071
starting a threshold value of the zero-mode voltage variation of the bus;
I0k: the amplitude of the zero-mode current of the kth branch circuit;
ΔI0k: the amplitude value calculated by the variation of the sampling value of the zero-mode current of the kth branch;
I0set: a zero mode current starting threshold value;
Figure BDA0002306255890000072
a zero mode current change threshold value.
Step two: extracting transient state quantity in a characteristic frequency band for bus zero-mode voltage and all branch zero-mode currents, wherein a designed filter is required to filter power frequency components, retain components in the characteristic frequency band and obtain filtered transient state zero-mode voltage u0kfilter(t) and all branch transient zero-mode currents i0kfilter(t)；
Figure BDA0002306255890000073
Figure BDA0002306255890000074
filter processing function
Step three: calculating and sequencing the maximum values of transient zero-mode currents of all the branches after filtering, and screening out a branch H with the maximum transient quantity;
step four: checking the polarities of all branches CT outside the branch H with the maximum transient quantity screened out in the third step, judging whether the polarities of all branches CT outside the branch H are reversed according to the transient zero-mode power direction, correcting the polarities of the branches with the reversed polarities of the branches CT, and simultaneously reversing the transient zero-mode current values of the branches;
Figure BDA0002306255890000075
wherein k ≠ H
DkCT with correct polarity > 0 >0k_filter＝i0k_filter；
DkLess than 0, CT polarity is connected reversely (alarm),
Figure BDA0002306255890000076
step five: carrying out polarity comprehensive study and judgment analysis on the branch H with the maximum transient quantity, wherein the judgment condition comprises the fundamental wave amplitude I of the branch H after n cycles of fault occurrence0H_HAmpNon-maximum, transient process offset DC component IDCoffsetGreater than a threshold value, and a transient direction IDir_TAnd the steady state direction IDir_SIf the two branches are inconsistent, if any condition is met, the branch H is a 'fault branch', and the sixth step is executed; otherwise, executing step seven;
I0H_HAmpnot max or IDCoffset＞IDCoffsetThrOr IDir_SAnd IDir_TDirection of non-uniformity
n, the number of cycles required for the current of each branch to transit to a steady state;
IDCoffsetThra DC offset component threshold value;
positive direction: the line flows to the bus; and (3) reverse direction: the bus bars flow to the lines.
Step six: judging transient direction I of branch HDir_TWhether the direction is positive or not, if so, the polarity of the branch H is correct; otherwise, the polarity of the branch H is reversed, the device corrects the polarity parameter and sends out warning information;
step seven: judging the transient direction I of the branch circuit H according to the transient zero-mode power direction methodDir_TIf the direction is positive, executing a step eight, and if not, executing a step eleven;
step eight: solving the sum i of transient zero-mode current vectors of all the operational branches filtered in the step two0sum_filter(t)；
Figure BDA0002306255890000081
m is the total number of branches actually put into operation.
Step nine: and performing square integration on the sampling value within T2 time on the filtered transient zero-mode current of the branch circuit H in the step two and the sum of the vectors of the transient zero-mode currents of all the branch circuits filtered in the step eight, so as to obtain an energy set E of the transient zero-mode current of the branch circuit HH_filter(t) and all branch transient zero-mode current vector sum energy set Esum_filter(t)；
Figure BDA0002306255890000082
Figure BDA0002306255890000083
EH_filter(t): a transient zero-mode current energy set after band-pass filtering of the corresponding branch H;
Figure BDA0002306255890000084
corresponding to the transient zero-mode current vector and the energy set after the band-pass filtering of all the branches;
T2the upper limit of the transient component integration time of the band-pass filtering from the fault starting moment.
Step ten: in T2 time after fault starting, if all branch circuit transient zero-mode current vectors and energy sets are smaller than a branch circuit H transient zero-mode current energy set, judging that the polarity of a branch circuit H zero-sequence CT is correct and the branch circuit H zero-sequence CT is a fault branch circuit; otherwise, judging that the polarity of the branch circuit H zero sequence CT is reversely connected, correcting the polarity parameter by the device and sending alarm information, wherein the fault type is bus grounding;
at 0 to T2All satisfy
Figure BDA0002306255890000085
The zero sequence CT polarity of the branch H is correct and is a fault branch;
otherwise, the polarity of the zero sequence CT of the branch circuit H is reverse and the branch circuit H is grounded;
k is a reliability coefficient; eΔThrIs a reliable variation threshold.
Step eleven: inverting the transient zero-mode current value of the filtered branch circuit H, and acquiring the transient zero-mode current energy set E of the branch circuit H in T2 time after the fault starts by the synchronous step nine methodH_filter(t) and all branch transient zero-mode current vector sum energy set Esum_filter(t)；
i0H_filter(t)＝-i0H_filter(t)
Step twelve: in the T2 time, if all branch transient zero-mode current vectors and energy sets are smaller than the branch H transient zero-mode current energy set, the polarity of the branch H zero-sequence CT is judged to be reversed, the device corrects the polarity parameter and sends out alarm information, and the branch is a fault branch; otherwise, judging that the polarity of the branch circuit H zero sequence CT is correct, and judging that the fault type is bus grounding;
at 0 to T2All satisfy
Figure BDA0002306255890000091
The zero sequence CT polarity of the branch H is correct and is a fault branch;
otherwise, the polarity of the zero sequence CT of the branch circuit H is reverse and the branch circuit H is grounded;
k is a reliability coefficient; eΔThrIs a reliable variation threshold.
The method adopts an OR gate starting based on zero-mode voltage quantity and zero-mode current quantity, and when any variable exceeds a fixed value, the system is judged to have a ground fault; after fault starting, extracting transient state quantity in a characteristic frequency band for bus zero-mode voltage and all branch zero-mode currents, filtering power frequency components of the transient state quantity, calculating the maximum value of the transient state zero-mode currents of all the branches after filtering, and screening out the branch with the maximum transient state quantity; in the fault judging process, the polarities of the zero sequence CTs of all the branches are checked in real time, and the branch with wrong CT polarity is alarmed and corrected on software; fault line selection is carried out based on data after zero sequence CT correction, the integral of the square of the transient zero-mode current of the maximum branch circuit after filtering in the time domain, the integral of the vector and the square of the transient zero-mode current of all the branch circuits in the time domain are calculated, whether the fault point is located on a bus or a circuit is determined according to the amplitude magnitude relation between the two, and when the fault point belongs to the circuit fault, the method can simultaneously carry out fault branch circuit judgment. The line selection link extracts the transient state quantity in the characteristic frequency band, and the influence of power frequency compensation quantity on the line selection accuracy is solved; zero-mode voltage is not needed, the problem that distribution difference in time dimension of zero-mode voltage and zero-mode current transient state quantity caused by PT disconnection influence and zero-mode voltage derivation influences energy acquisition effect is solved, and the problem that line selection accuracy is influenced by power frequency compensation quantity of an arc suppression coil grounding system is solved; meanwhile, the method adopts an algorithm of integrating transient zero-mode current square in a time domain, so that the problem of insufficient sensitivity caused by high-resistance grounding fault is solved; meanwhile, CT polarity check, alarm and correction are carried out in real time in the fault line selection process, so that the problem of line selection error caused by CT polarity error is avoided, the trouble caused by power failure check can be avoided, and the operation and maintenance cost is reduced.

Claims (8)

1. A small current ground fault line selection method covering CT polarity self-correction is characterized in that: after the single-phase earth fault is started, firstly, extracting transient state quantity in a characteristic frequency band for bus zero-mode voltage and zero-mode currents of all branches through digital filtering, filtering power frequency components of the transient state quantity, reserving components in the characteristic frequency band, and selecting the branch with the maximum transient state quantity; secondly, checking, alarming and correcting the zero sequence CT polarities of all branches with the maximum non-transient quantity; and then continuing to perform the next judgment according to the corrected data, wherein the judgment is as follows:
comprehensively studying, judging and analyzing the branch with the maximum transient quantity, wherein the judging conditions comprise that the fundamental wave amplitude of the branch is not maximum after n cycles of fault occurrence, the offset direct current component in the transient process is larger than a threshold value, and the transient and steady directions are inconsistent, if any judging condition is met, the branch is determined to be a fault branch, and zero sequence CT polarity check, alarm and correction are carried out on the branch according to the transient zero-mode power direction; and if the transient zero-mode power of the branch circuit with the maximum transient quantity is in the positive direction, performing energy matching degree analysis on the transient zero-mode current square integrated in the time domain with the transient zero-mode current vectors and squares of all the branch circuits, otherwise, performing energy matching degree analysis on the transient zero-mode current square integrated in the time domain with all the branch circuits with the reversed polarity of the maximum branch circuit, determining whether the fault point is located on the bus or the circuit and the zero-sequence CT polarity of the circuit is correct, and giving corresponding alarm and correction.
2. The method of claim 1, wherein the method comprises the steps of: the method comprises the processes of digital filtering, CT polarity correction and fault line selection, wherein the digital filtering is to extract the transient state quantity in a characteristic frequency band for zero-mode voltage and zero-mode current of all branches; the CT polarity correction is used for correcting the polarities of zero sequence CTs of all branches in the system, and comprises the steps of warning and correcting branches with wrong CT polarities; the fault line selection method for determining whether the fault point is located in a bus or a line comprises the following steps:
(1) extracting transient state quantity in a characteristic frequency band from the bus zero-mode voltage and all branch zero-mode currents, filtering power frequency components of the transient state quantity, and obtaining filtered transient state zero-mode voltage u0kfilter(t) and all branch transient zero-mode currents i0kfilter(t)；
(2) Calculating and sequencing the maximum values of transient zero-mode currents of all the branches after filtering, and screening out a branch H with the maximum transient quantity;
(3) checking the CT polarities of all branches outside the branch H with the maximum transient quantity screened in the step (2), judging whether the CT polarities of all branches outside the branch H are reversed according to the transient zero-mode power direction, correcting the polarities of the branches with the reversed CT polarities, and simultaneously negating a zero-mode current sampling value and a transient zero-mode current value of the branches;
(4) carrying out polarity comprehensive study and judgment analysis on the branch H with the maximum transient quantity, wherein the judgment condition comprises the fundamental wave amplitude I of the branch H after n cycles of fault occurrence0H_HAmpWhether it is not maximum, the transient process shifts the DC component IDCoffsetWhether it is greater than a threshold value, and a transient direction IDir_TAnd the steady state direction IDir_SIf the two branches are not consistent, if any condition is met, the branch H is a fault branch, and the step (5) is executed; otherwise, executing the step (6);
(5) judging transient direction I of branch HDir_TWhether the direction is positive or not, if so, judging the polarity of the branch H to be correct; otherwise, the branch circuit H judges that the polarity is reverse, and the device corrects the polarity parameter and sends out warning information;
(6) judging the transient direction I of the branch circuit H according to the transient zero-mode power directionDir_TIf the direction is positive, executing the step (7) if the direction is positive, otherwise executing the step (10);
(7) calculating the sum i of transient zero-mode current vectors of all the branches filtered in the step (1)0sum_filter(t)；
(8) Carrying out square integration on sampling values in T2 time on the filtered transient zero-mode current of the branch circuit H in the step (1) and the sum of the vectors of the transient zero-mode currents of all the branch circuits filtered in the step (7) to obtain a transient zero-mode current energy set E of the branch circuit HH_filter(t) and all branch transient zero-mode current vector sum energy set Esum_filter(t)；
(9) In T2 time after fault starting, if all branch circuit transient zero-mode current vectors and energy sets are smaller than a branch circuit H transient zero-mode current energy set, judging that the branch circuit H zero-sequence CT is correct in polarity and is a fault branch circuit; otherwise, judging that the polarity of the branch circuit H zero sequence CT is reversely connected, correcting the polarity parameter by the device and sending alarm information, wherein the fault type is bus grounding;
(10) negating the transient zero-mode current value of the filtered branch circuit H, and obtaining a transient zero-mode current energy set of the branch circuit H and transient zero-mode current vectors and energy sets of all the branch circuits in T2 time after the fault starts in the same step (8);
(11) in the T2 time, if all branch transient zero-mode current vectors and energy sets are smaller than the branch H transient zero-mode current energy set, judging that the polarity of the branch H zero-sequence CT is reversed, correcting the polarity parameters by the device and sending out alarm information, wherein the branch is a fault branch; otherwise, judging that the polarity of the branch circuit H zero sequence CT is correct, and the fault type is bus grounding.
3. The method of claim 2, wherein the method comprises the steps of: in the step (1), the expressions for extracting and calculating the bus zero-mode voltage and the transient state quantity in the characteristic frequency band of all branch zero-mode currents are as follows, the designed filter is required to filter power frequency components, the components in the characteristic frequency band are reserved, and the filtering expressions are as follows:
Figure FDA0002306255880000021
Figure FDA0002306255880000022
in the formula, the filter represents a filter processing function.
4. The method of claim 2, wherein the method comprises the steps of: the calculation process for checking the polarity of the non-transient maximum branch CT in the step (3) is as follows:
Figure FDA0002306255880000031
wherein k ≠ H
DkCT with correct polarity > 0 >0k_filter＝i0k_filter；
DkCT polarity reversal < 0 >0k_filter＝-i0k_filter。
5. The method of claim 2, wherein the method comprises the steps of: the transient zero-mode current vector sum calculation expression of all the branches after filtering in the step (7) is as follows:
Figure FDA0002306255880000032
m is the total number of branches actually put into operation.
6. The method of claim 2, wherein the method comprises the steps of: performing square integral calculation expression of sampling values in T2 time on the vector sum of the filtered branch H transient zero-mode current and all branch transient zero-mode currents in the step (8) as follows:
Figure FDA0002306255880000033
Figure FDA0002306255880000034
EH_filter(t): a transient zero-mode current energy set after band-pass filtering of the corresponding branch H;
Figure FDA0002306255880000035
corresponding to the transient zero-mode current vector and the energy set after the band-pass filtering of all the branches;
T2the upper limit of the transient component integration time of the band-pass filtering from the fault starting moment.
7. The method of claim 2, wherein the method comprises the steps of: in the step (9), the relevance of all branch transient zero-mode current vectors and energy sets and the branch H transient zero-mode current energy set in the T2 time is determined as follows:
at 0 to T2All satisfy
Figure FDA0002306255880000036
The zero sequence CT polarity of the branch H is correct and is a fault branch; otherwise, the polarity of the zero sequence CT of the branch circuit H is reverse and the branch circuit H is grounded; k is a reliability coefficient; eΔThrIs a reliable variation threshold.
8. The low-current ground fault line selection method covering CT polarity self-correction according to claim 1 or 2, characterized in that: the method comprises fault identification, and the fault identification comprises the following processes:
obtaining zero sequence voltage and zero sequence current values, when the amplitude of the zero sequence voltage exceeds a fixed value or the variation of the zero sequence voltage exceeds a fixed value, and the amplitude of the zero sequence current of any branch exceeds a fixed value or the variation of the zero sequence current exceeds a fixed value, judging that the system has a ground fault, and starting a digital filtering link, CT polarity correction and fault line selection logic.
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