CN113589106B - Single-phase earth fault line discrimination method for neutral point non-effective earthing medium-voltage micro-grid - Google Patents
Single-phase earth fault line discrimination method for neutral point non-effective earthing medium-voltage micro-grid Download PDFInfo
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Abstract
The single-phase earth fault circuit discrimination method of the neutral point non-effective grounding medium-voltage micro-grid comprises the steps of 1, collecting zero sequence voltage and current signals; step 2, extracting high-frequency zero-sequence current from the zero-sequence current signal; step 3, extracting fundamental frequency zero sequence voltage and fundamental frequency zero sequence current at two sides of the protected line from the zero sequence voltage signal and the zero sequence current signal; step 4, performing morphological mode coding on the high-frequency zero-sequence current, the fundamental frequency zero-sequence voltage and the fundamental frequency zero-sequence current to obtain initial variation trend characteristics of the high-frequency zero-sequence current and a peak value time difference value between the fundamental frequency zero-sequence voltage and the fundamental frequency zero-sequence current; and 5, constructing a main criterion according to the initial change trend characteristics of the high-frequency zero-sequence current, constructing an auxiliary criterion according to the fundamental frequency zero-sequence voltage and the peak time difference value between the fundamental frequency zero-sequence currents, and determining a fault line according to the main criterion and the auxiliary criterion. The method can accurately judge the fault line when the single-phase earth fault occurs to the neutral point non-effectively earthed microgrid.
Description
Technical Field
The invention relates to the technical field of power systems, in particular to a single-phase grounding fault line judgment method for a neutral point non-effectively grounded medium-voltage micro-grid.
Background
The micro-grid is a small power generation and distribution system organically integrating distributed micro-sources, loads, energy storage devices and protection monitoring devices, has island operation and high-reliability power supply capacity, can effectively improve the utilization efficiency of renewable energy sources such as solar energy, wind energy and the like, and is an important means for solving the power supply problem of islands and remote areas. The micro-grid has the characteristics of various distributed micro-source types, bidirectional power flow, flexible and changeable network topology and the like, can operate under two network topology structures of a ring type and a radiation type, wherein the ring type structure can reduce the power failure times and duration time of the micro-grid system caused by faults by providing a standby power supply path, improve the penetrating power of the distributed micro-source which can be accepted by the system, and can be converted into the radiation type network to continue to operate after the faults are eliminated. In addition, similar to the traditional medium voltage distribution network, the medium voltage microgrid also comprises two grounding modes of a neutral point effective grounding mode and a neutral point non-effective grounding mode, wherein the neutral point effective grounding mode is widely applied to distribution network systems below 600V and above 15kV in the North America region, and the neutral point non-effective grounding mode is mainly applied to the medium voltage distribution network systems in countries or regions such as China, Japan, Central Europe, and eastern Europe, and the system is allowed to continuously supply power for 1-2 hours after single-phase grounding faults occur, so that uninterrupted power supply of power users is guaranteed. According to statistics, the occurrence probability of the single-phase earth fault is far greater than that of other types of faults, accounts for about 80% of all faults, is mainly instantaneous fault, can be automatically extinguished, and can be eliminated without cutting off a fault line. Therefore, the power supply quality and the power supply reliability of the micro-grid can be further improved by adopting a neutral point non-effective grounding mode.
However, for the microgrid with the neutral point being inefficiently grounded, because no zero-sequence current channel exists between the micro source and the fault point, the difference between the single-phase grounding fault current and the normal operation current is very small, so that the previously proposed protection method for other fault types of the microgrid with or without the neutral point being grounded cannot be used for judging the single-phase grounding fault line of the microgrid with the neutral point being inefficiently grounded. Furthermore, the topological flexibility of the microgrid makes the single-phase earth fault response characteristic more complex, the fault line is difficult to distinguish, and the non-fault phase overvoltage caused by the single-phase earth fault can also deteriorate the equipment insulation, so that the interphase short-circuit fault is easily caused, and the personal safety and the equipment safety are threatened.
In recent years, experts and scholars at home and abroad mainly concentrate on the traditional radiation type power distribution network, and provide various single-phase earth fault line positioning methods according to the characteristic difference of zero-sequence current on a fault line and a non-fault line, wherein the methods mainly comprise two types, namely a steady-state component method based on information such as fundamental frequency zero-sequence current amplitude, phase and the like and a transient component method based on information such as zero-sequence current polarity, transient energy and the like. However, there are few relevant researches on the influence of the neutral point non-effective grounding mode on the response characteristic and the protection design of the medium-voltage microgrid single-phase grounding fault, and since the microgrid ring network is different from the single-bus radiation type network of the power distribution network and no upstream and downstream fault characteristic distinction is made, the method is not suitable for the neutral point non-effective grounding microgrid with a flexible and variable topological structure, and therefore, a method for judging the single-phase grounding fault line of the medium-voltage microgrid with the neutral point non-effective grounding is urgently needed to be researched in the industry.
Disclosure of Invention
The invention aims to solve the technical problem of providing a single-phase grounding fault line distinguishing method of a neutral point non-effective grounding micro-grid, which can realize that protection devices on two sides of a protected line accurately distinguish a fault line when the neutral point non-effective grounding micro-grid has a single-phase grounding fault.
In order to solve the technical problems, the invention adopts the following technical method: the single-phase earth fault line distinguishing method of the neutral point non-effective earthing micro-grid comprises the following steps:
step 1, collecting zero sequence voltage and zero sequence current signals at two sides of a protected line in a neutral point non-effective grounding micro-grid;
step 2, extracting high-frequency zero-sequence currents on two sides of the protected line from the zero-sequence current signals obtained in the step 1;
step 3, respectively extracting fundamental frequency zero sequence voltage and fundamental frequency zero sequence current at two sides of the protected line from the zero sequence voltage signal and the zero sequence current signal obtained in the step 1;
step 4, performing morphological mode coding on the high-frequency zero-sequence current obtained in the step 2 and the fundamental frequency zero-sequence voltage and the fundamental frequency zero-sequence current obtained in the step 3 to obtain initial variation trend characteristics of the high-frequency zero-sequence current and a peak time difference value between the fundamental frequency zero-sequence voltage and the fundamental frequency zero-sequence current;
and 5, constructing a main criterion for identifying the fault line according to the initial change trend characteristic of the high-frequency zero-sequence current obtained in the step 4, constructing an auxiliary criterion for identifying the fault line according to the peak time difference value between the fundamental frequency zero-sequence voltage and the fundamental frequency zero-sequence current obtained in the step 4, and determining whether the protected line in the neutral point non-effective grounding microgrid has a fault or not according to the main criterion and the auxiliary criterion.
Further, step 6 is included, when one side of the protected line in the neutral point non-effectively grounded microgrid is determined to have a fault, the protection device of the side sends a trip signal to the circuit breaker of the side, and when one side of the protected line in the neutral point non-effectively grounded microgrid is determined to have no fault, the protection device of the side does not send a trip signal to the circuit breaker of the side.
Further, in step 1, two sides of a protected line in the neutral point non-effective grounding microgrid are respectively called as an m side and an n side, and the m side of the protected line acquires zero sequence voltage v on the m side in real time through a protection device on the m sidem0(k) And zero sequence current im0(k) The n side of the protected line acquires the zero sequence voltage v of the n side of the protected line in real time through a protection device of the n side of the protected linen0(k) And zero sequence current in0(k) Where k represents the kth sample value.
Further, in step 2, the zero sequence current i obtained from step 1 is transformed by wavelet methodm0(k) Zero sequence current in0(k) Respectively extracting high-frequency zero-sequence current im0h(k) And high frequency zero sequence current in0h(k)。
Further, in step 3, the zero sequence voltage v obtained from step 1 is extracted by the fundamental frequency component extraction methodm0(k) Zero sequence current im0(k) Zero sequence voltage vn0(k) And zero sequence current in0(k) Respectively extracting fundamental frequency zero sequence voltage vm0l(k) Fundamental frequency zero sequence current im0l(k) Fundamental frequency zero sequence voltage vn0l(k) And fundamental frequency zero sequence current in0l(k)。
The method for extracting the fundamental frequency component comprises the following steps: firstly, subtracting the average value of an original input signal x (t) to eliminate the direct current component in the signal x (t) to obtain a signal y (t); secondly, the signal y (t) is Hilbert transformed and multiplied by a fundamental frequency component offset factor
Taking the real part component to obtain a signal y' (t); then theSubtracting the average value of the signal y ' (t) to eliminate the direct current component in the signal y ' (t) to obtain a signal y ' (t); finally, the signal y "(t) is Hilbert transformed and multiplied by the fundamental frequency component offset factor
Taking a real part of the real component, and subtracting the real part from the signal y (t) to obtain a fundamental frequency component x in the original input signal x (t)l(t)。
y(t)＝x(t)-Mean[x(t)] (1)
y”(t)＝y'(t)-Mean[y'(t)] (3)
In the above formula, x (t) is the collected one-dimensional input signal, i.e. the zero-sequence voltage and zero-sequence current collected by the protection device, Mean (-) represents the average value of the input signal, H (-) represents hilbert transform, Re (-) represents the real component of the complex number,
and
as a factor of deviation of the fundamental frequency component, ω0Representing the angular velocity, x, of the fundamental frequencyl(t) is the fundamental frequency component of the original input signal x (t).
Further, the high-frequency zero-sequence current i obtained in the step 2m0h(k) High frequency zero sequence current in0h(k) And the fundamental frequency zero sequence voltage v obtained in the step 3m0l(k) Fundamental frequency zero sequence current im0l(k) Fundamental frequency zero sequence voltage vn0l(k) And fundamental frequency zero sequence current in0l(k) All of which are one-dimensional time series with obvious change trend characteristics, namely X ═ X1,x2,…,xn)。
Still further, in step 4, the high-frequency zero-sequence current i is appliedm0h(k) High-frequency zero-sequence current in0h(k) Fundamental frequency zero sequence voltage vm0l(k) Fundamental frequency zero sequence current im0l(k) Fundamental frequency zero sequence voltage vn0l(k) Fundamental frequency zero sequence current in0l(k) The ascending and descending change trend of the current is converted into a discrete sequence through morphological mode coding, and high-frequency zero-sequence current i is extracted from the discrete sequencem0h(k) Initial trend of change PmHigh-frequency zero-sequence current in0h(k) Initial trend of change PnAnd fundamental frequency zero sequence voltage vm0l(k) And fundamental frequency zero sequence current im0l(k) Is a peak time difference Δ TpmAnd fundamental frequency zero sequence voltage vn0l(k) And fundamental frequency zero sequence current in0l(k) Is a peak time difference Δ Tpn。
Furthermore, in step 4, the form mode coding adopts a form mode sequence including strings { -4, -3, -2, -1,0,1,2,3,4}, wherein the numerical values in the strings { -4, -3, -2, -1,0,1,2,3,4} sequentially represent nine variation trends of trough, steady after descending, steady after continuing, steady after ascending, and peak. The high-frequency zero-sequence current im0h(k) High frequency zero sequence current in0h(k) Fundamental frequency zero sequence voltage vm0l(k) Fundamental frequency zero sequence current im0l(k) Fundamental frequency zero sequence voltage vn0l(k) Fundamental frequency zero sequence current in0l(k) One-dimensional time series of (X ═ X)1,x2,…,xn) The correspondence relationship between the variation trend of (c) and the sequence of morphological patterns is shown in table 1 below.
△xmax＝max{|xi+1-xi|,i∈1,2,...,n-1} (5)
D(i)＝(xi+1-xi)/△xmax (6)
TABLE 1
Wherein, Δ xmaxFor inputting any two adjacent data x in time seriesiAnd xi+1A maximum value of a difference between (i ═ 1,2.. n-1), as shown in formula (5); d (i) is the difference between two adjacent data and the maximum difference Deltaxmaxζ is a division threshold value of the signal change tendency, and its value is set to Δ x as shown in equation (6)max0.1% of.
Preferably, in step 5, according to the high-frequency zero-sequence current i on both sides of the protected linem0h(k) Initial trend of change PmAnd a high frequency zero sequence current in0h(k) Initial trend of change PnThe main criterion is constructed by the difference, and is used for identifying a fault line in the neutral point non-effectively grounded microgrid and a non-fault line which does not contain a virtual tail end; according to the fundamental frequency zero sequence voltage v on two sides of the protected linem0l(k) Zero sequence current i with fundamental frequencym0l(k) Is a peak time difference Δ TpmAnd fundamental frequency zero sequence voltage vn0l(k) And fundamental frequency zero sequence current in0l(k) Is a peak time difference Δ TpnAnd constructing auxiliary criteria for identifying fault lines in the neutral point non-effectively grounded microgrid and non-fault lines comprising virtual terminals.
More preferably, in step 5, when the high-frequency zero-sequence current i is on both sides of the main feeder linem0h(k) Initial trend of change PmAnd high frequency zero sequence current in0h(k) Initial trend of change PnSame, said initial trend of change PmAnd initial trend of change PnWhen the product of the two is greater than 0, determining the main feeder line as a fault line; when high-frequency zero-sequence current i on two sides of a main feeder linem0h(k) Initial trend of change PmAnd high frequency zero sequence current in0h(k) Initial trend of change PnIn contrast, the initial variation tendency PmAnd initial trend of change PnIs less than 0, and the fundamental frequency zero sequence voltage v on both sides of the main feed linem0l(k) And fundamental frequency zero sequence current im0l(k) Is a peak time difference Δ Tpm<0 or fundamental frequency zero sequence voltage vn0l(k) And fundamental frequency zero sequence current in0l(k) Is a peak time difference Δ Tpn<When 0, determining the main feeder line as a fault line; when dividing intoFundamental frequency zero sequence voltage v of branch linem0l(k) And fundamental frequency zero sequence current im0l(k) Is a peak time difference Δ Tpm<When 0, determining the branch line as a fault line; the main feeder line is a line with a protection device arranged on both sides of a line in the neutral point non-effectively grounded microgrid; the branch line is a line with a protection device only arranged on one side of the line in the neutral point non-effective grounding micro-grid.
Compared with the traditional single-phase earth fault line positioning method, the single-phase earth fault line positioning method has the advantages that the design idea is different, the core of the method is that the single-phase earth fault line of the neutral point non-effective earth medium-voltage microgrid is determined by utilizing the difference of zero-sequence current characteristics of the fault line and the non-fault line, the method is high in accuracy, suitable for the microgrid with two topological structures of a ring type and a radiation type, low in requirements on communication bandwidth and speed, and capable of sharing a communication system with a microgrid protection method for non-single-phase earth faults such as an interphase short-circuit fault, a two-phase earth fault and a three-phase short-circuit fault. Specifically, the invention considers the influence of the neutral point non-effective grounding mode and the micro-grid topology flexibility change on the single-phase grounding fault response characteristics such as the direction of the high-frequency zero-sequence current, the fundamental frequency zero-sequence voltage, the fundamental frequency zero-sequence current phase difference and the like, converts the rising and falling change trends of the high-frequency zero-sequence current, the fundamental frequency zero-sequence voltage and the fundamental frequency zero-sequence current signals into discrete sequences through morphological mode coding, extracts the initial change trend characteristics of the high-frequency zero-sequence current at two sides of the protected line and the initial peak value time difference value of the zero-sequence fundamental frequency voltage and the current, constructs the main criterion for identifying the fault line according to the initial change trend difference of the high-frequency zero-sequence current at two sides of the fault line and the non-fault line without the virtual tail end point, distinguishes the fault line from the non-fault line without the virtual tail end point, and utilizes the initial peak value time difference value of the fundamental frequency zero-sequence voltage and the fundamental frequency zero-sequence current at two sides of the line, and constructing an auxiliary criterion, and distinguishing a fault line from a non-fault line comprising a virtual tail end point to form a single-phase ground fault line distinguishing method of the neutral point non-effectively grounded medium-voltage microgrid.
Drawings
FIG. 1 is a block diagram of a medium voltage microgrid system according to the present invention;
fig. 2 is a high-frequency zero-sequence current waveform on both sides of a line in an annular micro-grid related to the present invention, specifically, fig. 2(a) is a high-frequency zero-sequence current waveform on both sides of a fault line, and fig. 2(b) is a high-frequency zero-sequence current waveform on both sides of a non-fault line without a virtual end point;
fig. 3 is a waveform of high-frequency zero-sequence current at two sides of a line in a radiation type micro-grid according to the present invention, specifically, (a) in fig. 3 is a waveform of high-frequency zero-sequence current at two sides of a fault line; in fig. 3, (b) is a high-frequency zero-sequence current waveform on both sides of the non-fault line;
FIG. 4 shows waveforms of zero-sequence voltage, zero-sequence current, fundamental frequency zero-sequence voltage, and fundamental frequency zero-sequence current involved in the present invention; specifically, fig. 4(a) is a waveform diagram in which the phase difference between the fundamental frequency zero-sequence voltage and the fundamental frequency zero-sequence current is between 0 ° and 180 °, the solid line represents the original zero-sequence voltage and zero-sequence current waveforms, and the dotted line represents the extracted fundamental frequency zero-sequence voltage and fundamental frequency zero-sequence current waveforms; fig. 4(b) is a waveform diagram in which the phase difference between the fundamental frequency zero-sequence voltage and the fundamental frequency zero-sequence current is-180 ° to-0 °, the solid line represents the original zero-sequence voltage and zero-sequence current waveforms, and the dotted line represents the extracted fundamental frequency zero-sequence voltage and fundamental frequency zero-sequence current waveforms;
fig. 5 is a flowchart of a single-phase ground fault line discrimination method for a neutral point non-effectively grounded medium-voltage microgrid related in the present invention.
Detailed Description
In order to facilitate understanding of those skilled in the art, the present invention will be further described with reference to the following examples and drawings, which are not intended to limit the present invention.
Before explaining the present invention, it should be noted that a typical medium-voltage microgrid system structure related to the present invention is shown in fig. 1, and is composed of four 10kV buses, three distributed micro sources, two load and protection monitoring devices, and the like, wherein the distributed micro sources include a rotary type micro source diesel generator, and inverter type micro sources IBDG1 and IBDG2, which are connected to a 10kV network through interface transformers. For the neutral point non-effective grounding micro-grid, the low-voltage side of the interface transformer is in a Y-shaped winding connection mode, so that power is conveniently supplied to a single-phase load, the high-voltage side is in a delta-shaped winding connection mode, and the 10kV network is grounded through the grounding transformer. The non-effective grounding mode of the neutral point of the microgrid mainly comprises three modes of neutral point ungrounded mode, neutral point resonant grounding mode and neutral point high-resistance grounding mode.
When a single-phase earth fault occurs in the microgrid, no matter which neutral point non-effective grounding mode is adopted for the ring-type microgrid system with the closed microgrid switches in fig. 1, a virtual tail end point exists in a ring in a high-frequency range larger than 300 Hz. Based on the above, according to a double-end network equation between two ends of the fault line and the virtual tail end point, the directions of the high-frequency zero-sequence currents on two sides of the fault line are the same (the direction of the current flowing out of the bus is the positive direction of the current), and the fundamental frequency zero-sequence voltage on one side, far away from the grounding point, of the fault line is ahead of the fundamental frequency zero-sequence current, namely the fundamental frequency zero-sequence voltage on one side, far away from the grounding point, of the fault line and the current phase difference belong to 0-180 degrees; the directions of the high-frequency zero-sequence currents on the two sides of the non-fault line not including the virtual tail end point are opposite, the directions of the high-frequency zero-sequence currents on the two sides of the non-fault line including the virtual tail end point are the same, but the fundamental frequency zero-sequence voltages on the two ends of the line lag behind the fundamental frequency zero-sequence currents, namely the fundamental frequency zero-sequence voltages and the current phase difference are between-180 degrees and 0 degrees.
The invention provides a single-phase grounding fault line distinguishing method of a neutral point non-effective grounding medium-voltage micro-grid by utilizing the difference of the zero-sequence current characteristics of the fault line and the non-fault line, which comprises the following steps:
step 1, collecting zero sequence voltage and zero sequence current signals at two sides of a protected line in a neutral point non-effective grounding micro-grid;
step 2, extracting high-frequency components of zero sequence currents on two sides of the protected line from the zero sequence current signals obtained in the step 1;
step 3, extracting fundamental frequency components of zero sequence voltage and zero sequence current at two sides of the protected line from the zero sequence voltage signal and the zero sequence current signal obtained in the step 1;
step 4, performing morphological mode coding on the high-frequency component of the zero-sequence current obtained in the step 2 and the fundamental frequency components of the zero-sequence voltage and the current obtained in the step 3 to obtain the initial change trend characteristic of the high-frequency zero-sequence current and the peak time difference value between the fundamental frequency zero-sequence voltage and the fundamental frequency zero-sequence current;
and 6, when one side of the protected line in the neutral point non-effectively grounded microgrid is determined to have a fault, the protection device on the side sends a trip signal to the breaker on the side, and when one side of the protected line in the neutral point non-effectively grounded microgrid is determined to have no fault, the protection device on the side does not send a trip signal to the breaker on the side.
To facilitate understanding of those skilled in the art, the specific flow of the present embodiment will be described in more detail below.
Step 1, collecting zero sequence voltage and zero sequence current signals
The two sides of a protected line in a neutral point non-effective grounding micro-grid are respectively called as an m side and an n side, and the m side of the protected line acquires zero sequence voltage v on the m side in real time through a protection device on the m sidem0(k) And zero sequence current im0(k) The n side of the protected line acquires the zero sequence voltage v of the n side of the protected line in real time through a protection device of the n side of the protected linen0(k) And zero sequence current in0(k) Where k denotes the kth sampling value, in the present embodiment, the sampling frequency of the protection device is set to 10kHz, that is, 200 sampling points are collected in one fundamental wave period.
Step 2, extracting high-frequency component of zero-sequence current
From the zero sequence current i obtained from step 1 by wavelet transform methodm0(k) Zero sequence current in0(k) Respectively extracting high-frequency zero-sequence current im0h(k) And high frequency zero sequence current in0h(k) In that respect Because the zero sequence current mainly comprises a steady-state fundamental frequency component, a direct current attenuation component andthe oscillation frequency of the oscillation attenuation component is usually within the range of 300-1500Hz, and therefore, the high-frequency zero-sequence current extracted in the present embodiment mainly refers to the oscillation attenuation component of the zero-sequence current, i.e. the characteristic frequency band component with the maximum energy except the fundamental frequency component.
Step 3, extracting fundamental frequency components of zero sequence voltage and zero sequence current
Zero sequence voltage v obtained from step 1 by fundamental frequency component extraction methodm0(k) Zero sequence current im0(k) Zero sequence voltage vn0(k) And zero sequence current in0(k) Respectively extracting fundamental frequency zero sequence voltage vm0l(k) Fundamental frequency zero sequence current im0l(k) Fundamental frequency zero sequence voltage vn0l(k) And fundamental frequency zero sequence current in0l(k)。
The method for extracting the fundamental frequency component comprises the following steps: firstly, subtracting the average value of an original input signal x (t) to eliminate the direct current component in the signal x (t) to obtain a signal y (t); secondly, the signal y (t) is Hilbert transformed and multiplied by a fundamental frequency component offset factor
Taking the real part component to obtain a signal y' (t); then, subtracting the average value of the signal y '(t) to eliminate the direct current component in the signal y' (t) to obtain a signal y "(t); finally, the signal y "(t) is Hilbert transformed and multiplied by the fundamental frequency component offset factor
Taking part of the real component, and subtracting the real component from the signal y (t) to obtain the fundamental component x in the original input signal x (t)l(t)。
y(t)＝x(t)-Mean[x(t)] (1)
y”(t)＝y'(t)-Mean[y'(t)] (3)
In the above formula, x (t) is the collected one-dimensional input signal, i.e. the zero-sequence voltage and zero-sequence current collected by the protection device, Mean (-) represents the average value of the input signal, H (-) represents hilbert transform, Re (-) represents the real component of the complex number,
and
as a factor of deviation of the fundamental frequency component, ω0Representing the angular velocity, x, of the fundamental frequencyl(t) is the fundamental frequency component of the original input signal x (t).
It should be noted here that, in the above step 2 and step 3, the high-frequency zero-sequence current im0h(k) High frequency zero sequence current in0h(k) Fundamental frequency zero sequence voltage vm0l(k) Fundamental frequency zero sequence current im0l(k) Fundamental frequency zero sequence voltage vn0l(k) And fundamental frequency zero sequence current in0l(k) All of which are one-dimensional time series with obvious change trend characteristics, namely X ═ X1,x2,…,xn)。
Step 4, obtaining the initial variation trend of the high-frequency zero-sequence current and the peak value time difference value between the fundamental frequency zero-sequence voltage and the fundamental frequency zero-sequence current
The high-frequency zero-sequence current i is converted intom0h(k) High frequency zero sequence current in0h(k) Fundamental frequency zero sequence voltage vm0l(k) Fundamental frequency zero sequence current im0l(k) Fundamental frequency zero sequence voltage vn0l(k) Fundamental frequency zero sequence current in0l(k) The ascending and descending change trend of the current is converted into a discrete sequence through form mode coding, and high-frequency zero-sequence current i is extracted from the discrete sequencem0h(k) Initial trend of change PmHigh frequency zero sequence current in0h(k) Initial variation tendency P ofnAnd fundamental frequency zero sequence voltage vm0l(k) And fundamental frequency zero sequence current im0l(k) Is a peak time difference Δ TpmAnd fundamental frequency zero sequence voltage vn0l(k) And fundamental frequency zero sequence current in0l(k) Is a peak time difference Δ Tpn。
Preferably, in step 4, the form mode coding adopts a form mode sequence composed of character strings { -4, -3, -2, -1,0,1,2,3,4}, wherein the numerical values in the character strings { -4, -3, -2, -1,0,1,2,3,4} sequentially represent nine variation trends of trough, stable and continuous decline after decline, stable and continuous and stable and ascending after smooth, stable and stable after ascending, and peak. About the aforementioned high frequency zero sequence current im0h(k) High frequency zero sequence current in0h(k) Fundamental frequency zero sequence voltage vm0l(k) Fundamental frequency zero sequence current im0l(k) Fundamental frequency zero sequence voltage vn0l(k) Fundamental frequency zero sequence current in0l(k) One-dimensional time series of (X ═ X)1,x2,…,xn) The correspondence relationship between the variation trend of (c) and the sequence of morphological patterns is shown in table 1 below.
△xmax＝max{|xi+1-xi|,i∈1,2,...,n-1} (5)
D(i)＝(xi+1-xi)/△xmax (6)
TABLE 1
Wherein, Δ xmaxFor inputting any two adjacent data x in time seriesiAnd xi+1A maximum value of a difference between (i ═ 1,2.. n-1), as shown in formula (5); d (i) is the difference between two adjacent data and the maximum difference Deltaxmaxζ is a division threshold value of the signal change tendency, and its value is set to Δ x as shown in equation (6)max0.1% of.
As shown in fig. 2 and 3, for the ring-type and radiation-type micro-grid systems, the variation trends of the high-frequency zero-sequence currents at two sides of the fault line, which have the same direction, are firstly reduced, then increased, then reduced, and so on. In the present embodiment, as can be obtained from table 1, the pattern mode sequences of the high-frequency zero-sequence current sequences (including 20 sampling points after the fault) on the m side and the n side of the fault line shown in fig. 2(a) are all-2, -2, -2, -2, -2, -2, -2, -2, -2, -2), and the initial trend P of the high-frequency zero-sequence current on two sides of the fault line can be knownmAnd PnAre all-1, PmAnd PnThe product of (d) is greater than 0. The phase difference between the fundamental frequency zero sequence voltage and the current of the fault line far away from the grounding point is between 0 and 180 degrees, and as shown in figure 4(a), the initial peak value time T of the fundamental frequency zero sequence voltagepvAnd fundamental frequency zero sequence current initial peak value time TpiDifference value Δ T ofpBetween-10 ms and 0.
For a non-fault line without a virtual end point, the directions of the high-frequency zero-sequence currents on the two sides of the line are opposite, and the initial trend P of the high-frequency zero-sequence currents on the two sides of the non-fault line shown in fig. 2(b)mAnd PnAre respectively 1 and-1, PmAnd PnThe product of (d) is less than 0. For a non-fault line comprising a virtual tail end point, the directions of high-frequency zero-sequence currents on two sides of the line are the same, and P ismAnd PnThe product of (c) is less than 0, the phase difference of the fundamental frequency zero sequence voltage and the current at the two sides of the line is-180 degrees to-0 degrees, as shown in fig. 4(b), the initial peak value time T of the fundamental frequency zero sequence voltage at the two sides of the line ispvAnd fundamental frequency zero sequence current initial peak value time TpiDifference value Δ T ofpmAnd Δ TpnBoth between 0 and 10 ms.
According to high-frequency zero-sequence current i at two sides of a protected linem0h(k) Initial variation tendency P ofmAnd a high frequency zero sequence current in0h(k) Initial trend of change PnThe difference of the neutral point and the virtual tail point is used for constructing a main criterion, and the main criterion is used for identifying a fault line of the neutral point non-effectively grounded microgrid and a non-fault line which does not contain the virtual tail point; according to the fundamental frequency zero sequence voltage v on two sides of the protected linem0l(k) Zero sequence current i with fundamental frequencym0l(k) Is a peak time difference Δ TpmAnd fundamental frequency zero sequence voltage vn0l(k) And fundamental frequency zero sequence current in0l(k) Is a peak time difference Δ TpnConstructing auxiliary criterion for identifying fault line of neutral point non-effective grounding microgrid and virtual end pointIs not a faulty line.
The single-phase earth fault line of the neutral point non-effective earthing micro-grid can be judged by utilizing the main criterion and the auxiliary criterion. Specifically, in this embodiment, when the high-frequency zero-sequence current i is on both sides of the main feeder linem0h(k) Initial trend of change PmAnd high frequency zero sequence current in0h(k) Initial trend of change PnSame, i.e. initial trend of change PmAnd initial trend of change PnWhen the product of (a) and (b) is greater than 0, the main feeder may be determined to be a faulty line (i.e., an in-zone fault). When high-frequency zero-sequence current i at two sides of the main feeder linem0h(k) Initial trend of change PmAnd high frequency zero sequence current in0h(k) Initial trend of change PnIn contrast, i.e. initial trend of change PmAnd initial trend of change PnIs less than 0, and the fundamental frequency zero sequence voltage v on both sides of the main feed linem0l(k) Zero-sequence current i of sum fundamental frequencym0l(k) Is a peak time difference Δ Tpm<0 or fundamental frequency zero sequence voltage vn0l(k) And fundamental frequency zero sequence current in0l(k) Is a peak time difference Δ Tpn<When 0, the main feeder line may also be determined as a faulty line (i.e., an intra-area fault), otherwise, the main feeder line is determined as a non-faulty line (i.e., an extra-area fault). When the fundamental frequency of the branch line is zero sequence voltage vm0l(k) And fundamental frequency zero sequence current im0l(k) Is a peak time difference Δ Tpm<When 0, the branch line may be determined to be a faulty line (i.e., an intra-area fault), otherwise, the branch line is determined to be a non-faulty line (i.e., an extra-area fault). The main feeder line is a line with a protection device arranged on both sides of a line in the neutral point non-effective grounding microgrid, and the branch line is a line with a protection device arranged on only one side of the line in the neutral point non-effective grounding microgrid.
Step 6, fault response
If the protective device on the m side of the protected line in the neutral point non-effectively grounded microgrid judges that the line is a fault line, the protective device sends a tripping signal to a breaker on the m side; if the line is determined to be a non-faulty line, the protection device does not transmit a trip signal to the m-side breaker. If the protective device on the n side of the protected line determines that the line is a fault line, the protective device sends a trip signal to the n side breaker, and if the line is determined to be a non-fault line, the protective device does not send a trip signal to the n side breaker.
In the present embodiment, a 10kV microgrid system model as shown in fig. 1 is established in the PSCAD/EMTDC, wherein the diesel generator has a capacity of 1MVA, the IBDG1 and the IBDG2 have capacities of 500kW, the inverters thereof have a constant power control strategy, and the loads 1 and 2 have capacities of (1+ j0.03) MVA. The line lengths of the lines AB, BC, CD and AD are all 2km, the line length of the line B1 is 1km, the positive sequence impedance and the zero sequence impedance of the unit length of the line are respectively 0.265+ j0.078 omega/km and 2.7+ j0.318 omega/km, and the positive sequence admittance and the zero sequence admittance of the unit length are respectively j106.5 mu S/km and j87.96 mu S/km.
When a single-phase ground fault occurs at F1 in the microgrid of fig. 1 and the fault resistance is 0, the discrimination results of the single-phase ground fault line of the ring-type microgrid and the radiating-type microgrid which adopt different neutral point non-effective grounding modes are respectively shown in tables 2 and 3, and "Mode 1", "Mode 2" and "Mode 3" respectively represent a neutral point non-grounding Mode, a neutral point resonance grounding Mode and a neutral point high-resistance grounding Mode. From table 1-2, under different neutral point non-effective grounding modes and micro-grid topological structures, according to the characteristics of the initial variation trend of the high-frequency zero-sequence current on two sides of the line and the time difference value of the initial peak value of the fundamental frequency zero-sequence voltage and the initial peak value of the current, the method can accurately judge that the line AB is a fault line and other lines are non-fault lines.
TABLE 2
TABLE 3
Therefore, when a single-phase earth fault occurs in the neutral point non-effective-earthing medium-voltage microgrid in the embodiment, the protection devices on two sides of the protected line only need to exchange the initial variation trend characteristic of the high-frequency zero-sequence current and the information of the time difference value of the initial peak value of the fundamental frequency zero-sequence voltage and the initial peak value of the current, so that the fault line can be determined. Generally, the identification method for the single-phase ground fault of the medium-voltage microgrid with the neutral point not effectively grounded in the embodiment has the advantages of small communication data volume and low requirements on communication bandwidth and speed, and can share a communication system with a microgrid protection method for the non-single-phase ground faults such as an interphase short-circuit fault, a two-phase ground fault and a three-phase short-circuit fault.
The above embodiments are preferred implementations of the present invention, and the present invention can be implemented in other ways, and any obvious substitutions without departing from the spirit of the present invention are within the scope of the present invention.
Some of the drawings and descriptions of the present invention have been simplified to facilitate the understanding of the improvements over the prior art by those skilled in the art, and other elements have been omitted from this document for the sake of clarity, and it should be appreciated by those skilled in the art that such omitted elements may also constitute the subject matter of the present invention.
Claims (9)
1. The single-phase earth fault line distinguishing method of the neutral point non-effective grounding medium-voltage micro-grid is characterized by comprising the following steps:
step 1, collecting zero sequence voltage and zero sequence current signals at two sides of a protected line in a neutral point non-effective grounding micro-grid;
step 2, extracting high-frequency zero-sequence currents on two sides of the protected line from the zero-sequence current signals obtained in the step 1;
step 3, respectively extracting fundamental frequency zero sequence voltage and fundamental frequency zero sequence current at two sides of the protected line from the zero sequence voltage signal and the zero sequence current signal obtained in the step 1;
step 4, performing morphological mode coding on the high-frequency zero-sequence current obtained in the step 2 and the fundamental frequency zero-sequence voltage and the fundamental frequency zero-sequence current obtained in the step 3 to obtain initial variation trend characteristics of the high-frequency zero-sequence current and a peak time difference value between the fundamental frequency zero-sequence voltage and the fundamental frequency zero-sequence current; the form mode sequence adopted by the form mode coding comprises character strings { -4, -3, -2, -1,0,1,2,3,4}, and numerical values in the character strings { -4, -3, -2, -1,0,1,2,3,4} sequentially represent nine change trends of wave trough, stable descending, continuous descending, stable descending after stable descending, continuous stable ascending, stable ascending after stable ascending, and wave crest after ascending;
and 5, constructing a main criterion for identifying the fault line according to the initial change trend characteristic of the high-frequency zero-sequence current obtained in the step 4, constructing an auxiliary criterion for identifying the fault line according to the peak time difference value between the fundamental frequency zero-sequence voltage and the fundamental frequency zero-sequence current obtained in the step 4, and determining whether the protected line in the neutral point non-effective grounding microgrid has a fault or not according to the main criterion and the auxiliary criterion.
2. The method for identifying a single-phase earth fault line of a neutral point non-actively grounded medium voltage microgrid according to claim 1, characterized in that: and 6, when one side of the protected line in the neutral point non-effectively grounded microgrid is determined to have a fault, the protection device of the side sends a trip signal to the breaker of the side, and when one side of the protected line in the neutral point non-effectively grounded microgrid is determined to have no fault, the protection device of the side does not send a trip signal to the breaker of the side.
3. The method for identifying a single-phase earth fault line of a neutral point non-actively grounded medium voltage microgrid according to claim 2, characterized in that: in step 1, two sides of a protected line in the neutral point non-effective grounding microgrid are respectively called as an m side and an n side, and the m side of the protected line acquires zero sequence voltage v on the m side in real time through a protection device on the m sidem0(k) And zero sequence current im0(k) The n side of the protected line acquires the zero sequence voltage v of the n side of the protected line in real time through a protection device of the n side of the protected linen0(k) And zero sequence current in0(k) Where k represents the kth sample value.
4. The method for identifying a single-phase earth fault line of a neutral point non-actively grounded medium voltage microgrid according to claim 3, characterized in that: in step 2, the zero sequence current i obtained from step 1 is transformed by waveletm0(k) Zero sequence current in0(k) Respectively extracting high-frequency zero-sequence current im0h(k) And high frequency zero sequence current in0h(k)。
5. The method for identifying a single-phase earth fault line of a neutral point non-actively grounded medium voltage microgrid according to claim 4, characterized in that: in step 3, the zero sequence voltage v obtained from step 1 is extracted by a fundamental frequency component extraction methodm0(k) Zero sequence current im0(k) Zero sequence voltage vn0(k) And zero sequence current in0(k) Respectively extracting fundamental frequency zero sequence voltage vm0l(k) Fundamental frequency zero sequence current im0l(k) Fundamental frequency zero sequence voltage vn0l(k) And fundamental frequency zero sequence current in0l(k)；
The method for extracting the fundamental frequency component comprises the following steps: firstly, subtracting the average value of an original input signal x (t) to eliminate the direct current component in the signal x (t) to obtain a signal y (t); secondly, the signal y (t) is Hilbert transformed and multiplied by a fundamental frequency component offset factor
Taking the real part component to obtain a signal y' (t); then, subtracting the average value of the signal y '(t) to eliminate the direct current component in the signal y' (t) to obtain a signal y "(t); finally, the signal y "(t) is Hilbert transformed and multiplied by the fundamental frequency component offset factor
Taking a real part of the real component, and subtracting the real part from the signal y (t) to obtain a fundamental frequency component x in the original input signal x (t)l(t)；
y(t)＝x(t)-Mean[x(t)] (1)
y”(t)＝y'(t)-Mean[y'(t)] (3)
In the above formula, x (t) is the collected one-dimensional input signal, i.e. the zero-sequence voltage and zero-sequence current collected by the protection device, Mean (-) represents the average value of the input signal, H (-) represents hilbert transform, Re (-) represents the real component of the complex number,
and
as a factor of deviation of the fundamental frequency component, ω0Representing the angular velocity, x, of the fundamental frequencyl(t) is the fundamental frequency component of the original input signal x (t).
6. The method for identifying a single-phase earth fault line of a neutral point non-actively grounded medium voltage microgrid according to claim 5, characterized in that: the high-frequency zero-sequence current i obtained in the step 2m0h(k) High frequency zero sequence current in0h(k) And the fundamental frequency zero sequence voltage v obtained by the step 3m0l(k) Fundamental frequency zero sequence current im0l(k) Fundamental frequency zero sequence voltage vn0l(k) And fundamental frequency zero sequence current in0l(k) All of the one-dimensional time sequences have obvious change trend characteristics, namely X ═ X1,x2,…,xn)。
7. The method for identifying a single-phase earth fault line of a neutral point non-actively grounded medium voltage microgrid according to claim 6, characterized in that: in step 4, the high-frequency zero-sequence current i is processedm0h(k) High frequency zero sequence current in0h(k) Fundamental frequency zero sequence voltage vm0l(k) Fundamental frequency zero sequence current im0l(k) Fundamental frequency zero sequence voltage vn0l(k) Fundamental frequency zero sequence current in0l(k) The ascending and descending change trend of the current is converted into a discrete sequence through form mode coding, and high-frequency zero-sequence current i is extracted from the discrete sequencem0h(k) Initial trend of change PmHigh frequency zero sequence current in0h(k) Initial variation tendency P ofnAnd fundamental frequency zero sequence voltage vm0l(k) And fundamental frequency zero sequence current im0l(k) Is a peak time difference Δ TpmAnd fundamental frequency zero sequence voltage vn0l(k) And fundamental frequency zero sequence current in0l(k) Is a peak time difference Δ Tpn。
8. The method for identifying a single-phase earth fault line of a neutral point non-operatively grounded medium voltage microgrid according to claim 7, characterized in that: in step 5, according to the high-frequency zero-sequence current i at two sides of the protected linem0h(k) Initial trend of change PmAnd high frequency zero sequence current in0h(k) Initial trend of change PnThe difference of the neutral point and the virtual tail point is used for constructing a main criterion, and the main criterion is used for identifying a fault line of the neutral point non-effectively grounded microgrid and a non-fault line which does not contain the virtual tail point; according to the fundamental frequency zero sequence voltage v on two sides of the protected linem0l(k) Zero-sequence current of fundamental frequency im0l(k) Is a peak time difference Δ TpmAnd fundamental frequency zero sequence voltage vn0l(k) And fundamental frequency zero sequence current in0l(k) Is a peak time difference Δ TpnAnd constructing auxiliary criteria for identifying fault lines in the neutral point non-effectively grounded microgrid and non-fault lines comprising the virtual end points.
9. The method for identifying a single-phase earth fault line of a neutral point non-actively grounded medium voltage microgrid according to claim 8, characterized in that: in step 5, when the high-frequency zero-sequence current i on both sides of the main feeder linem0h(k) Initial trend of change PmAnd high frequency zero sequence current in0h(k) Initial trend of change PnSame, said initial trend of changePmAnd initial trend of change PnWhen the product of the two is greater than 0, determining the main feeder line as a fault line; when high-frequency zero-sequence current i at two sides of the main feeder linem0h(k) Initial trend of change PmAnd high frequency zero sequence current in0h(k) Initial trend of change PnIn contrast, the initial variation tendency PmAnd initial trend of change PnIs less than 0, and the fundamental frequency zero sequence voltage v on both sides of the main feeder linem0l(k) And fundamental frequency zero sequence current im0l(k) Is a peak time difference Δ Tpm<0 or fundamental frequency zero sequence voltage vn0l(k) And fundamental frequency zero sequence current in0l(k) Is a peak time difference Δ Tpn<When 0, determining the main feeder line as a fault line; when fundamental frequency zero sequence voltage v of branch linem0l(k) And fundamental frequency zero sequence current im0l(k) Peak time difference value Δ T ofpm<When 0, determining the branch line as a fault line; the main feeder line is a line with a protection device arranged on both sides of a line in the neutral point non-effectively grounded microgrid; the branch line is a line with a protection device only arranged on one side of the line in the neutral point non-effective grounding micro-grid.
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