CN114002559A - Flexible direct current transmission line traveling wave double-end distance measurement method and system - Google Patents
Flexible direct current transmission line traveling wave double-end distance measurement method and system Download PDFInfo
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Abstract
The invention relates to a traveling wave double-end distance measurement method and system for a flexible direct current transmission line, and belongs to the technical field of relay protection of power systems. Firstly, respectively acquiring capacitive current traveling wave signals from traveling wave coupling boxes at two ends of a flexible direct current transmission line; respectively extracting free oscillation components of the capacitive current traveling waves of the double-end traveling wave coupling box, and calculating frequency domain signals of the free oscillation components to obtain equal-interval frequency spectrums corresponding to the free oscillation components; and finally, calculating the equal interval frequency difference value of the frequency domain signal, calculating the ratio of the frequency difference values at two ends, and performing fault distance measurement by using the ratio. The invention realizes fault location by utilizing the equal-interval distribution frequency spectrum rule that fault traveling wave signals at two ends of the flexible direct current transmission line are reflected and refracted back and forth at the boundary of the flat wave reactors at the two ends of the line and the fault point, does not need to identify the property of a traveling wave head, calibrate the wave arrival time and synchronize clocks at two ends, and has the advantages of strong anti-jamming capability and anti-transition resistance capability, accurate and reliable ranging result, and good stability and robustness.
Description
Technical Field
The invention relates to a traveling wave double-end distance measurement method and system for a flexible direct current transmission line, and belongs to the technical field of relay protection of power systems.
Background
The flexible direct-current transmission technology has outstanding technical advantages in multiple aspects such as new energy grid connection, power grid asynchronous area interconnection, passive network power supply and the like. Along with the research and development of a novel power electronic device and a multi-level modular converter, the appearance of a sub-module topology with a fault self-clearing function and a novel direct-current breaker topology, the popularization of the application of the flexible direct-current transmission technology is further promoted. Compared with the traditional direct current transmission, the flexible direct current converter station has no problem of phase change failure, and is more reliable compared with the traditional direct current transmission. Compared with a direct-current cable, the overhead line is more suitable for long-distance power transmission, is convenient to erect and maintain, has lower cost, is easy to cause faults due to weather and environmental influences, and has the characteristics of high development speed and large fault current, so that the research and application of the fault location method for the flexible direct-current transmission line have important theoretical significance and engineering value.
The fault location method of the flexible direct current transmission line mainly refers to the fault location of the alternating current transmission line and the traditional direct current transmission line. The mainstream distance measurement method mainly comprises a fault analysis method, a traveling wave method and a natural frequency method according to different principles. The fault analysis method is characterized in that an equation is written according to the relation between the line parameters and the electrical quantity, fault distance measurement is realized through optimization solution, the method has low requirement on the sampling rate, an algorithm is realized by means of fixed line parameters, and the precision of the algorithm is greatly influenced by the variable characteristics of the line parameters. The traveling wave method distance measurement mainly utilizes the propagation characteristics of fault traveling waves, and is characterized in that a wave head reflecting a fault position is correctly found out. In comparison, the inherent frequency method has low requirement on sampling rate in distance measurement, and the distance measurement principle is that the inherent frequency dominant frequency of the fault traveling wave is extracted according to the periodic rule that the fault traveling wave is refracted and reflected back and forth at the fault point and the line boundary, and the fault distance is calculated according to the mathematical relationship between the wave speed, the inherent frequency dominant frequency and the fault distance. The inherent frequency method distance measurement has the advantages that the calculation result is stable, wave heads do not need to be calibrated, interference resistance and noise resistance are high, the distance measurement accuracy is affected by the magnitude of the dominant frequency value of the inherent frequency, the frequency is related to the boundary condition of the system, the length of a calculation time window and the wave speed, and the distance measurement result of the inherent frequency method deviates accordingly.
Disclosure of Invention
The invention aims to provide a method and a system for double-end ranging of a traveling wave of a flexible direct-current transmission line, which are used for solving the problem of fault ranging.
The technical scheme of the invention is as follows: a traveling wave double-end distance measurement method for a flexible direct current transmission line comprises the following specific steps:
step 1: and respectively acquiring capacitive current traveling wave signals from traveling wave coupling boxes at two ends of the flexible direct current transmission line to construct capacitive current ranging signals.
Step 2: and respectively extracting free oscillation components of the double-end capacitive current ranging signals, calculating frequency domain signals of the free oscillation components, and obtaining equidistant frequency spectrums corresponding to the free oscillation components.
Step 3: and calculating the equal interval frequency difference value of the frequency domain signal, calculating the ratio of the frequency difference values at two ends, and performing fault distance measurement by using the ratio.
The Step1 is specifically as follows:
step1.1: and respectively acquiring capacitive current traveling wave signals from traveling wave coupling boxes at two ends of the flexible direct current transmission line.
Step1.2: decoupling a direct current line through a Karenbauer transformation matrix, solving a polar space modulus capacitive current traveling wave signal, and solving the difference between values of adjacent sampling points of the signal as a new value of a previous sampling point so as to construct a capacitive current ranging signal.
The Step2 is specifically as follows:
step2.1: and respectively extracting free oscillation components of the double-end capacitive current ranging signals.
Step2.2: the signal after the higher order odd power of the resultant signal at Step2.1 is calculated.
Step2.3: and performing discrete Fourier transform on the high-order odd-power signals to respectively obtain the frequency spectrums of the high-order odd-power signals at two ends.
Step2.4: and judging whether the frequency spectrums obtained at the two ends are at equal intervals, if not, changing the length of a data time window, returning to execute step2.3, and if so, respectively calculating the frequency difference of the frequency domain signals at the two ends.
The length of the data time window is changed by taking the time tau of the traveling wave propagating in the full-length range of the line as a unit, and K tau is changed every time K is changed to 2n, and n is changed to 1,2 and 3 ….
The Step3 is specifically as follows:
step3.1: frequency values of spectral peak positions of the double-end equal interval frequency spectrums are respectively extracted, any two adjacent frequency values are subjected to difference, and frequency differences corresponding to the frequency spectrums at the two ends are obtained.
Step3.2: the ratio omega of the double-end frequency difference is obtained according to the formula (1)MNM denotes one end of the double-ended flexible direct current device, and N denotes the other end of the double-ended flexible direct current device.
In the formula (1), Δ ωM、ΔωNAre respectively provided withDenotes the angular frequency difference, Δ f, calculated at the M and N terminalsM、ΔfNRespectively representing the frequency difference obtained by the calculation of the M end and the N end, wherein the double-end frequency difference and the angular frequency difference respectively satisfy the relational expressions of the expressions (2) and (3).
Step3.3: using the ratio omega of the frequency differences of both endsMNThe constructed distance measurement formula (4) obtains a distance measurement estimation result k%:
in the formula (4), xfThe distance between the fault point and the end M is shown, k% represents the percentage of the distance between the fault point and the end M to the total length of the line, and l represents the total length of the direct current line.
Step3.4: calculating the check frequency difference value delta omega according to the whole length of the linel。
Step3.5: comparing the check frequency difference value delta omegalAnd Δ ωMAnd whether the difference between the ratio and the obtained distance measurement estimation result k% is smaller than a set check threshold value or not, if so, outputting the distance measurement result, otherwise, changing the length of the data time window, and returning to Step2.3.
The length of the data time window is changed by taking the time tau of the traveling wave propagating in the full-length range of the line as a unit, and K tau is changed every time K is changed to 2n, and n is changed to 1,2 and 3 ….
The Step3.2 can also be used for calculating the ratio omega of the double-end frequency difference according to the formula (5)MN。
The Step3.3 can also be realized by utilizing the ratio omega of double-end frequency differenceNMThe constructed distance measurement formula (6) obtains the distance measurement result (1-k)%.
The step3.5 may also be that the check frequency difference value Δ ω is comparedlAnd Δ ωNAnd whether the difference between the ratio and the obtained (1-k)% of the distance measurement estimation result is smaller than a set check threshold value or not is judged, if yes, the distance measurement result is output, and if not, the length of the data time window is changed, and Step2.3 is returned.
The length of the data time window is changed by taking the time tau of the traveling wave propagating in the full-length range of the line as a unit, and K tau is changed every time K is changed to 2n, and n is changed to 1,2 and 3 ….
A traveling wave double-end ranging system of a flexible direct current transmission line comprises:
and the acquisition module is used for respectively acquiring capacitive current traveling wave signals from traveling wave coupling boxes at two ends of the flexible direct current transmission line.
And the calculation module is used for constructing the capacitive current ranging signal and extracting the free oscillation component of the double-end capacitive current ranging signal.
And the signal analysis module is used for respectively calculating frequency domain signals of the ranging signals at the two ends and obtaining an equidistant frequency spectrum corresponding to the free oscillation component.
And the signal processing module is used for calculating the equal-interval frequency difference value of the frequency domain signal and calculating the ratio of the frequency difference values at the two ends.
And the distance measurement module is used for calculating and recording the fault distance measurement result.
The acquisition module comprises:
and the transmitting unit is used for converting the current signal of the secondary side of the mutual inductor into a signal acquired by the traveling wave device A/D.
And the acquisition unit is used for converting the current analog quantity signal into a digital signal.
And the storage unit is used for naming the recording data file according to the time mark and storing the recording data file in the local memory.
And the starting unit is used for judging whether the waveform mutation is larger than a set starting threshold value or not, and storing the current signal into a recording data file if the waveform mutation is larger than the set starting threshold value.
The calculation module comprises:
the calculation unit 1 decouples the direct current line through a Karenbauer transformation matrix, calculates a polar space modulus capacitive current traveling wave signal, calculates the difference between values of adjacent sampling points of the signal, and uses the difference as a new value of a previous sampling point to construct a capacitive current ranging signal.
And the calculating units 2 are used for respectively calculating the high-order odd power of the free oscillation component of the double-end capacitive current ranging signal.
The signal analysis module includes:
and the time-frequency transformation unit is used for performing discrete Fourier transformation on the high-order odd-power signals to respectively obtain the frequency spectrums of the high-order odd-power signals at two ends.
And the frequency spectrum judging unit is used for judging whether the frequency spectrums of the frequency domain signals at the two ends are at equal intervals.
The signal processing module includes:
and the frequency difference calculating unit is used for extracting the frequency values of the spectral peak positions of the double-end equally-spaced frequency spectrums, and performing difference on any two adjacent frequency values to obtain the frequency differences corresponding to the frequency spectrums at the two ends.
A ratio calculation unit for calculating a ratio omega of the two-terminal frequency differencesMN、ΩNMM denotes one end of the double-ended dc device, and N denotes the other end of the double-ended dc device.
A check calculation unit for calculating a check frequency difference value [ delta ] omega ] according to the total length of the linel。
A check comparing unit for comparing the check frequency difference value delta omegalAnd Δ ωMAnd if the difference between the ratio and the obtained k% of the ranging estimation result is smaller than the set check threshold, outputting the ranging result, and if not, changing the length of the data time window.
The invention has the beneficial effects that:
the invention is not influenced by the boundary condition of the device, and avoids the errors caused by unreliable identification of the traveling wave head and inaccurate calibration of the wave arrival time.
The invention constructs the ranging equation by using the double-end frequency difference ratio, the ranging precision is not influenced by wave head distortion, waveform defect and head wave head loss, the influence of wave velocity attenuation is mathematically reduced, and the method does not depend on double-end synchronous time setting and has better robustness to weak fault modes such as high-resistance fault and the like.
The invention can utilize the traveling wave signal obtained by the traveling wave coupling box as the supplement and the assistance of the existing traveling wave distance measurement algorithm, the distance measurement result is expressed by the percentage of the fault position in the total line length, the line length does not participate in the distance measurement calculation, and the precision is not influenced by the actual change of the line length.
Drawings
FIG. 1 is a flowchart corresponding to example 1;
FIG. 2 is a flowchart illustrating the detailed steps of Step1 according to example 1;
FIG. 3 is a flowchart illustrating the detailed steps of Step2 according to example 1;
FIG. 4 is a flowchart illustrating the detailed steps of Step3 according to example 1;
fig. 5 is a functional block diagram of a system involved in embodiment 1;
figure 6 is a schematic diagram of a flexible dc power transmission system topology according to an embodiment;
FIG. 7 is a schematic diagram of current traveling waves obtained by the M-terminal and N-terminal traveling-wave coupling boxes in the embodiment;
fig. 8 is a diagram of an equally spaced spectrum of a signal after a higher order odd power corresponding to the M terminal and the N terminal in embodiment 2.
FIG. 9 is a diagram of capacitive current traveling-wave signals i respectively collected by traveling-wave coupling boxes at two ends of a flexible DC transmission line in embodiment 3Mc、iNcA diagram of;
FIG. 10 is a polar space modulus capacitive current traveling wave signal diagram in example 3;
FIG. 11 is a graph of free-running components of a two-terminal capacitive current ranging signal in example 3;
FIG. 12 is a spectrum diagram of an M-terminal higher order odd signal according to embodiment 3;
FIG. 13 is a spectrum diagram of an N-terminal higher order odd-power signal according to embodiment 3;
FIG. 14 shows the results obtained in example 4Capacitive current traveling wave signal i respectively collected by traveling wave coupling boxes at two ends of flexible direct current transmission lineMc、iNcA diagram of;
FIG. 15 is a polar space modulus capacitive current traveling wave signal diagram in example 4;
FIG. 16 is a graph of free-running components of a two-terminal capacitive current ranging signal in example 4;
FIG. 17 is a spectrum diagram of an M-terminal higher order odd signal in embodiment 4;
FIG. 18 is a spectrum diagram of an N-terminal higher order odd signal according to embodiment 4;
Detailed Description
The invention is further described with reference to the following drawings and detailed description.
Example 1: as shown in fig. 1, a method for measuring a distance between two ends of a traveling wave of a flexible direct current transmission line includes the following specific steps:
step 1: and respectively acquiring capacitive current traveling wave signals from traveling wave coupling boxes at two ends of the flexible direct current transmission line to construct capacitive current ranging signals.
Step 2: and respectively extracting free oscillation components of the double-end capacitive current ranging signals, calculating frequency domain signals of the free oscillation components, and obtaining equidistant frequency spectrums corresponding to the free oscillation components.
Step 3: and calculating the equal interval frequency difference value of the frequency domain signal, calculating the ratio of the frequency difference values at two ends, and performing fault distance measurement by using the ratio.
As shown in fig. 2, Step1 is specifically:
step1.1: and respectively acquiring capacitive current traveling wave signals from traveling wave coupling boxes at two ends of the flexible direct current transmission line.
Step1.2: decoupling a direct current line through a Karenbauer transformation matrix, solving a polar space modulus capacitive current traveling wave signal, and solving the difference between values of adjacent sampling points of the signal as a new value of a previous sampling point so as to construct a capacitive current ranging signal.
As shown in fig. 3, Step2 is specifically:
step2.1: and respectively extracting free oscillation components of the double-end capacitive current ranging signals.
Step2.2: the signal after the higher order odd power of the resultant signal at Step2.1 is calculated.
Step2.3: and performing discrete Fourier transform on the high-order odd-power signals to respectively obtain the frequency spectrums of the high-order odd-power signals at two ends.
Step2.4: and judging whether the frequency spectrums obtained at the two ends are at equal intervals, if not, changing the length of a data time window, returning to execute step2.3, and if so, respectively calculating the frequency difference of the frequency domain signals at the two ends.
The length of the data time window is changed by taking the time tau of the traveling wave propagating in the full-length range of the line as a unit, and K tau is changed every time K is changed to 2n, and n is changed to 1,2 and 3 ….
As shown in fig. 4, Step3 is specifically:
step3.1: frequency values of spectral peak positions of the double-end equal interval frequency spectrums are respectively extracted, any two adjacent frequency values are subjected to difference, and frequency differences corresponding to the frequency spectrums at the two ends are obtained.
Step3.2: the ratio omega of the double-end frequency difference is obtained according to the formula (1)MNM denotes one end of the double-ended flexible direct current device, and N denotes the other end of the double-ended flexible direct current device.
In the formula (1), Δ ωM、ΔωNRespectively representing the angular frequency difference, delta f calculated by the M terminal and the N terminalM、ΔfNRespectively representing the frequency difference obtained by the calculation of the M end and the N end, wherein the double-end frequency difference and the angular frequency difference respectively satisfy the relational expressions of the expressions (2) and (3).
Step3.3: using the ratio omega of the frequency differences of both endsMNAnd (4) obtaining a ranging estimation result k% by the constructed ranging formula (4).
In the formula (4), xfThe distance between the fault point and the end M is shown, k% represents the percentage of the distance between the fault point and the end M to the total length of the line, and l represents the total length of the direct current line.
Step3.4: calculating the check frequency difference value delta omega according to the whole length of the linel。
Step3.5: comparing the check frequency difference value delta omegalAnd Δ ωMAnd whether the difference between the ratio and the obtained distance measurement estimation result k% is smaller than a set check threshold value or not, if so, outputting the distance measurement result, otherwise, changing the length of the data time window, and returning to Step2.3.
The length of the data time window is changed by taking the time tau of the traveling wave propagating in the full-length range of the line as a unit, and K tau is changed every time K is changed to 2n, and n is changed to 1,2 and 3 ….
As shown in fig. 5, a traveling wave double-end ranging system for a flexible direct current transmission line includes:
and the acquisition module 101 is used for acquiring capacitive current traveling wave signals from traveling wave coupling boxes at two ends of the flexible direct current transmission line respectively.
And a calculating module 102, configured to construct a capacitive current ranging signal. Free oscillation components of the two-port capacitive current ranging signal are extracted.
And the signal analysis module 103 is used for respectively calculating frequency domain signals of the ranging signals at the two ends and obtaining an equidistant frequency spectrum corresponding to the free oscillation component.
And the signal processing module 104 is configured to calculate an equally spaced frequency difference value of the frequency domain signal, and calculate a ratio of the two end frequency difference values.
And the ranging module 105 is used for calculating and recording a fault ranging result.
The acquisition module 101 includes:
and the transmitting unit 1011 is used for converting the current signal on the secondary side of the transformer into a signal collected by the traveling wave device A/D.
And an acquisition unit 1012 for converting the current analog quantity signal into a digital signal.
And the storage unit 1013 is used for naming the wave recording data file according to the time stamp and storing the wave recording data file in the local storage.
And the starting unit 1014 is used for judging whether the waveform mutation is larger than a set starting threshold value, and if so, storing the current signal into a recording data file.
The calculation module 102 includes:
the calculation unit 1021A decouples the direct current line through the Karenbauer transformation matrix, obtains the polar space modulus capacitive current traveling wave signal, and obtains the difference between the values of the adjacent sampling points of the signal as the new value of the previous sampling point, so as to construct the capacitive current ranging signal.
The calculating unit 1021B calculates the high order odd power of the free oscillation component of the double-ended capacitive current ranging signal respectively.
The signal analysis module 103 includes:
and the time-frequency transformation unit 1031 performs discrete fourier transform on the high-order odd-power signal to obtain frequency spectrums of the two-end high-order odd-power signal.
The signal processing module 104 includes:
the frequency difference calculating unit 1041 is configured to extract frequency values at spectral peak positions of the double-end equally-spaced frequency spectrums, and perform a difference between any two adjacent frequency values to obtain equal frequency differences corresponding to the frequency spectrums at the two ends.
A ratio calculating unit 1042 for calculating a ratio Ω of the two-end frequency differenceMN、ΩNMM denotes one end of the double-ended dc device, and N denotes the other end of the double-ended dc device.
A calibration calculation unit 1043 for calculating a calibration frequency difference value Δ ω according to the total length of the linel。
A verification comparison unit 1044 for comparing the verification frequency difference value delta omegalAnd Δ ωMAnd if the difference between the ratio and the obtained k% of the ranging estimation result is smaller than the set check threshold, outputting the ranging result, and if not, changing the length of the data time window.
Example 2: as shown in fig. 6, the ± 500kV true bipolar double-ended flexible direct current transmission system (MMC-HVDC) is built under the PSCAD/EMTDC environment. The rectifying side is an M end, and the inverting side is an N end. A single bridge arm of the double-end MMC converter is provided with 200 half-bridge submodules respectively, and a bridge arm reactor Larm is 100 mH. The total length L of the line is 500km, the overhead line adopts a frequency-variable parameter model, the lightning conductor is reserved, the double-end current-limiting reactor L of the line is 150mH, and the sampling frequency is 100 kHz. Under the condition that a unipolar metallic grounding fault occurs at a position 100km away from an M end of a line, ranging is performed by using a traveling wave double-end ranging method of a flexible direct current transmission line, and the method comprises the following specific steps of:
step 1: and respectively collecting capacitive current traveling wave signals from traveling wave coupling boxes at two ends of the flexible direct current transmission line M, N to construct capacitive current ranging signals.
Step 2: and respectively extracting free oscillation components of the double-end capacitive current ranging signals, calculating frequency domain signals of the free oscillation components, and obtaining equidistant frequency spectrums corresponding to the free oscillation components.
Step 3: and calculating the equal interval frequency difference value of the frequency domain signal, calculating the ratio of the frequency difference values at two ends, and performing fault distance measurement by using the ratio.
The Step1 specifically comprises the following steps:
step1.1: respectively collecting capacitive current traveling wave signals i from traveling wave coupling boxes at two ends of flexible direct current transmission lineMc、iNc。
Step1.2: obtaining polar space modulus capacitive current traveling wave signals through Karenbauer transformation matrix decoupling direct current line shown in formula (5)
In the formula (5), i1、i0Respectively representing the space modulus and the earth-mode component, I+、I_Respectively representing the anode current and the cathode current, and respectively obtaining the M-terminal and N-terminal polar space modulus capacitive current traveling wave signals i according to the formula (4)Mc1、iNc1For the M terminal and the N terminal, lower subscripts M and N are used, respectively, and the capacitive current is denoted by a lower subscript c.
The difference between the values of the adjacent sampling points of the signal is obtained according to the formula (6) and is used as the new value of the previous sampling point, so as to construct the capacitive current ranging signal.
Δic1(k)＝ic1(k+1)-ic1(k) (6)
Delta i for respectively obtaining capacitive current ranging signals of M end and N endMc1、△iNc1As shown in fig. 7.
The Step2 specifically comprises the following steps:
step2.1: and respectively extracting free oscillation components of the double-end capacitive current ranging signals. Taking time tau needed by traveling wave to propagate in the whole linear length range as a reference, respectively taking distance measurement signals with time windows tau, 2 tau and 4 tau before and after the first fault traveling wave arrives, wherein the distance measurement signals comprise the oscillation characteristics of multiple folding and reflection of the fault traveling wave from a fault point to a measurement end, and the oscillation characteristics are taken as free oscillation components.
Step2.2: calculating the Signal of the power alpha of the Signal obtained at Step2.1c1As shown in formula (7).
Signalc1(k)＝(Δic1(k))α,α＝2n+1,n＝1,2,3... (7)
Step2.3: discrete fourier transform is performed on the high-order odd-power signal to obtain the frequency spectrums of the two-end high-order odd-power signal, as shown in fig. 8.
Step2.4: and judging whether the frequency spectrums obtained at the two ends are at equal intervals, if so, respectively calculating the frequency difference of the frequency domain signals at the two ends.
The length of the data time window is changed by taking the time tau of the traveling wave propagating in the full-length range of the line as a unit, and K tau is changed every time K is changed to 2n, and n is changed to 1,2 and 3 ….
The Step3 specifically comprises the following steps:
step3.1: respectively extracting frequency values of spectral peak positions of the double-end equally-spaced frequency spectrums, and subtracting any two adjacent frequency values to obtain frequency differences delta f corresponding to the frequency spectrums at the two ends to be equalM＝372.64Hz,△fN＝248.38Hz。
Step3.2: the ratio omega of the double-end frequency difference is obtained according to the formula (1)MN1.5003, M representsThe rectification side of the double-ended flexible direct current device, and N denotes the inversion side of the double-ended flexible direct current device.
In the formula (1), Δ fM、ΔfNRespectively representing the frequency difference calculated by the M terminal and the N terminal,
step3.3 uses the ratio omega of the two-terminal frequency differencesMNWith the ranging equation (4) constructed, the ranging estimation result k% is 39.99%.
In the formula (4), xfThe distance between a fault point and the end M is shown, k% represents the percentage of the distance between the fault point and the end M in the total length of the line, and l is 500km, and the total length of the direct current line is shown.
Step3.4: calculating the check frequency difference value delta f according to the whole length of the linel。
Step3.5: comparing the frequency difference value delta flAnd Δ fMThe ratio of (A) to (B) was 39.98%.
The difference between 39.98% and the obtained estimated result k% of the distance measurement is-0.0005, which is smaller than the set check threshold ± 0.01, and the output result k% of the distance measurement is 39.99%.
Example 3: the +/-500 kV true bipolar double-end flexible direct current transmission system (MMC-HVDC) is built under the PSCAD/EMTDC environment. The rectifying side is an M end, and the inverting side is an N end. A single bridge arm of the double-end MMC converter is provided with 200 half-bridge submodules respectively, and a bridge arm reactor Larm is 100 mH. The total length L of the line is 500km, the overhead line adopts a frequency-variable parameter model, the lightning conductor is reserved, the double-end current-limiting reactor L of the line is 150mH, and the sampling frequency is 200 kHz. Under the condition that a single-pole grounding fault occurs at the midpoint of a line and the transition resistance is 500 ohms, the method for measuring the distance by using the traveling wave double-end distance measurement of the flexible direct-current transmission line comprises the following specific steps:
step 1: and respectively collecting capacitive current traveling wave signals from traveling wave coupling boxes at two ends of the flexible direct current transmission line M, N to construct capacitive current ranging signals.
Step1.1: respectively collecting capacitive current traveling wave signals i from traveling wave coupling boxes at two ends of flexible direct current transmission lineMc、iNcAs shown in fig. 9.
Step1.2: through a Karenbauer transformation matrix decoupling direct current line shown in formula (5), a polar space modulus capacitive current traveling wave signal is obtained, as shown in fig. 10:
in the formula (5), i1、i0Respectively representing the space modulus and the earth-mode component, I+、I_Respectively representing the anode current and the cathode current, and respectively obtaining the M-terminal and N-terminal polar space modulus capacitive current traveling wave signals i according to the formula (5)Mc1、iNc1For the M terminal and the N terminal, lower subscripts M and N are used, respectively, and the capacitive current is denoted by a lower subscript c.
The difference between the values of the adjacent sampling points of the signal is obtained according to the formula (6) and is used as the new value of the previous sampling point, so as to construct the capacitive current ranging signal.
Δic1(k)＝ic1(k+1)-ic1(k) (6)
Delta i for respectively obtaining capacitive current ranging signals of M end and N endMc1、△iNc1As shown in fig. 11.
Step 2: and respectively extracting free oscillation components of the double-end capacitive current ranging signals, calculating frequency domain signals of the free oscillation components, and obtaining equidistant frequency spectrums corresponding to the free oscillation components.
Step2.1: and respectively extracting free oscillation components of the double-end capacitive current ranging signals. Based on the time τ required for the traveling wave to propagate in the full line length range, the distance measurement signals with time windows τ,2 τ, and 4 τ before and after the arrival of the first fault traveling wave are respectively taken, and the distance measurement signals contain the oscillation characteristics of multiple folding and reflection of the fault traveling wave from the fault point to the measurement end as free oscillation components, as shown in fig. 11. Step2.2: calculation of the Step2.1 confidenceSignal of alpha power of numberc1As shown in formula (7).
Signalc1(k)＝(Δic1(k))α,α＝2n+1,n＝1,2,3... (7)
Step2.3: the discrete fourier transform is performed on the high-order odd-power signal to obtain the frequency spectrums of the two-end high-order odd-power signal, as shown in fig. 12 and 13.
Step2.4: and judging whether the frequency spectrums obtained at the two ends are at equal intervals, if so, respectively calculating the frequency difference of the frequency domain signals at the two ends.
Step 3: and calculating the equal interval frequency difference value of the frequency domain signal, calculating the ratio of the frequency difference values at two ends, and performing fault distance measurement by using the ratio.
Step3.1: respectively extracting frequency values of spectral peak positions of the double-end equally-spaced frequency spectrums, and subtracting any two adjacent frequency values to obtain frequency differences delta f corresponding to the frequency spectrums at the two ends to be equalM＝296.86Hz,△fN＝296.87Hz。
Step3.2: the ratio omega of the double-end frequency difference is obtained according to the formula (1)MNWhen 1, M denotes a rectification side of the double-ended flexible dc device, and N denotes an inversion side of the double-ended flexible dc device.
In the formula (1), Δ fM、ΔfNRespectively representing the frequency difference calculated by the M terminal and the N terminal.
Step3.3: using the ratio omega of the frequency differences of both endsMNThe constructed ranging equation (4) yields a ranging estimation result k% of 50%.
In the formula (4), xfThe distance between a fault point and the end M is shown, k% represents the percentage of the distance between the fault point and the end M in the total length of the line, and l is 500km, and the total length of the direct current line is shown.
Step3.4: calculating the check frequency difference value delta f according to the whole length of the linel。
Step3.5: comparing the frequency difference value delta flAnd Δ fMThe ratio of (A) to (B) is 50%.
The difference between 50% and the obtained estimated result k% is 0, which is smaller than the set check threshold ± 0.01, and the output result k% is 50%.
Example 4: the +/-500 kV true bipolar double-end flexible direct current transmission system (MMC-HVDC) is built under the PSCAD/EMTDC environment. The rectifying side is an M end, and the inverting side is an N end. A single bridge arm of the double-end MMC converter is provided with 200 half-bridge submodules respectively, and a bridge arm reactor Larm is 100 mH. The total length L of the line is 500km, the overhead line adopts a frequency-variable parameter model, the lightning conductor is reserved, the double-end current-limiting reactor L of the line is 150mH, and the sampling frequency is 1 MHz. Under the condition that a bipolar short-circuit fault occurs at the near end (50km) of a line distance M, the method for measuring the distance by using the traveling wave double-end distance of the flexible direct-current transmission line comprises the following specific steps:
step 1: and respectively collecting capacitive current traveling wave signals from traveling wave coupling boxes at two ends of the flexible direct current transmission line M, N to construct capacitive current ranging signals.
Step1.1: respectively collecting capacitive current traveling wave signals i from traveling wave coupling boxes at two ends of flexible direct current transmission lineMc、iNcAs shown in fig. 14.
Step1.2: polar space modulus capacitive current traveling wave signals are obtained by decoupling a direct current line through a Karenbauer transformation matrix shown in formula (5), as shown in FIG. 15.
In the formula (5), i1、i0Respectively representing the space modulus and the earth-mode component, I+、I_Respectively representing the anode current and the cathode current, and respectively obtaining the M-terminal and N-terminal polar space modulus capacitive current traveling wave signals i according to the formula (5)Mc1、iNc1For the M terminal and the N terminal, lower subscripts M and N are used, respectively, and the capacitive current is denoted by a lower subscript c.
The difference between the values of the adjacent sampling points of the signal is obtained according to the formula (6) and is used as the new value of the previous sampling point, so as to construct the capacitive current ranging signal.
Δic1(k)＝ic1(k+1)-ic1(k) (6)
Delta i for respectively obtaining capacitive current ranging signals of M end and N endMc1、△iNc1As shown in fig. 16.
Step 2: and respectively extracting free oscillation components of the double-end capacitive current ranging signals, calculating frequency domain signals of the free oscillation components, and obtaining equidistant frequency spectrums corresponding to the free oscillation components.
Step2.1: and respectively extracting free oscillation components of the double-end capacitive current ranging signals. Based on the time τ required for the traveling wave to propagate in the full line length range, the distance measurement signals with time windows τ,2 τ, and 4 τ before and after the arrival of the first fault traveling wave are respectively taken, and the distance measurement signals contain the oscillation characteristics of multiple folding and reflection of the fault traveling wave from the fault point to the measurement end as free oscillation components, as shown in fig. 16.
Step2.2: calculating the Signal of the power alpha of the Signal obtained at Step2.1c1. As shown in equation (7).
Signalc1(k)＝(Δic1(k))α,α＝2n+1,n＝1,2,3... (7)
Step2.3: the discrete fourier transform is performed on the high-order odd-power signal to obtain the frequency spectrums of the two-end high-order odd-power signal, as shown in fig. 17 and fig. 18.
Step2.4: and judging whether the frequency spectrums obtained at the two ends are at equal intervals, if so, respectively calculating the frequency difference of the frequency domain signals at the two ends.
Step 3: and calculating the equal interval frequency difference value of the frequency domain signal, calculating the ratio of the frequency difference values at two ends, and performing fault distance measurement by using the ratio.
Step3.1: respectively extracting frequency values of spectral peak positions of the double-end equally-spaced frequency spectrums, and subtracting any two adjacent frequency values to obtain frequency differences delta f corresponding to the frequency spectrums at the two ends to be equalM＝1489.6Hz，△fN＝165.87Hz。
Step3.2: determining the frequency difference between the two ends according to equation (1)Ratio omegaMNWhen the voltage is equal to 8.98, M denotes a rectification side of the double-ended flexible dc device, and N denotes an inversion side of the double-ended flexible dc device.
In the formula (1), Δ fM、ΔfNRespectively representing the frequency difference calculated by the M terminal and the N terminal.
Step3.3: using the ratio omega of the frequency differences of both endsMNThe constructed ranging equation (4) yields a ranging estimation result k% of 10.02%.
In the formula (4), xfThe distance between a fault point and the end M is shown, k% represents the percentage of the distance between the fault point and the end M in the total length of the line, and l is 500km, and the total length of the direct current line is shown.
Step3.4: calculating the check frequency difference value delta f according to the whole length of the linel。
Step3.5: comparing the frequency difference value delta flAnd Δ fMThe ratio of (A) to (B) is 10%.
The difference between 10% and the obtained estimated result k% of the distance measurement is-0.00017, which is smaller than the set check threshold ± 0.01, and the output result k% of the distance measurement is 10.02%.
While the present invention has been described in detail with reference to the embodiments shown in the drawings, the present invention is not limited to the embodiments, and various changes can be made without departing from the spirit and scope of the present invention.
Claims (12)
1. A traveling wave double-end distance measurement method for a flexible direct current transmission line is characterized by comprising the following steps:
step 1: respectively collecting capacitive current traveling wave signals from traveling wave coupling boxes at two ends of a flexible direct current transmission line to construct capacitive current ranging signals;
step 2: respectively extracting free oscillation components of the double-end capacitive current ranging signals, calculating frequency domain signals of the free oscillation components, and obtaining equal-interval frequency spectrums corresponding to the free oscillation components;
step 3: and calculating the equal interval frequency difference value of the frequency domain signal, calculating the ratio of the frequency difference values at two ends, and performing fault distance measurement by using the ratio.
2. The traveling wave double-end ranging method for the flexible direct-current transmission line according to claim 1, wherein Step1 specifically comprises:
step1.1: respectively collecting capacitive current traveling wave signals from traveling wave coupling boxes at two ends of a flexible direct current transmission line;
step1.2: decoupling a direct current line through a Karenbauer transformation matrix, solving a polar space modulus capacitive current traveling wave signal, and solving the difference between values of adjacent sampling points of the signal as a new value of a previous sampling point so as to construct a capacitive current ranging signal.
3. The traveling wave double-end ranging method for the flexible direct-current transmission line according to claim 1, wherein Step2 specifically comprises:
step2.1: respectively extracting free oscillation components of the double-end capacitive current ranging signals;
step2.2: calculating a signal after the high-order odd power of the signal obtained by Step2.1;
step2.3: performing discrete Fourier transform on the high-order odd-power signals to respectively obtain frequency spectrums of the high-order odd-power signals at two ends;
step2.4: and judging whether the frequency spectrums obtained at the two ends are at equal intervals, if not, changing the length of a data time window, returning to execute step2.3, and if so, respectively calculating the frequency difference of the frequency domain signals at the two ends.
4. The traveling wave double-end ranging method for the flexible direct-current transmission line according to claim 1, wherein Step3 specifically comprises:
step3.1: respectively extracting frequency values of spectral peak positions of double-end equal-interval frequency spectrums, and carrying out difference on any two adjacent frequency values to obtain corresponding equal frequency differences of the frequency spectrums at the two ends;
step3.2: the ratio omega of the double-end frequency difference is obtained according to the formula (1)MNM represents one end of the double-end flexible direct current device, and N represents the other end of the double-end flexible direct current device;
in the formula (1), Δ ωM、ΔωNRespectively representing the angular frequency difference, delta f calculated by the M terminal and the N terminalM、ΔfNRespectively representing the frequency difference obtained by the calculation of the M end and the N end, wherein the double-end frequency difference and the angular frequency difference respectively satisfy the relational expressions of the expressions (2) and (3);
step3.3: using the ratio omega of the frequency differences of both endsMNThe constructed distance measurement formula (4) obtains a distance measurement estimation result k%:
in the formula (4), xfThe distance between a fault point and the end M is represented, k% represents the percentage of the distance between the fault point and the end M to the total length of the line, and l represents the total length of the direct-current line;
step3.4: calculating the check frequency difference value delta omega according to the whole length of the linel；
Step3.5: comparing the check frequency difference value delta omegalAnd Δ ωMIf the difference between the ratio and the k% of the obtained distance measurement estimation result is less than the set check threshold, if yes, outputting the distance measurement result, if no, changing the length of the data time window,return to Step2.3.
5. The traveling wave double-end ranging method for the flexible direct-current transmission line according to claim 4, characterized in that: the Step3.2 can also be used for calculating the ratio omega of the double-end frequency difference according to the formula (5)NM；
The Step3.3 can also be realized by utilizing the ratio omega of double-end frequency differenceNMThe constructed distance measurement formula (6) obtains the distance measurement result (1-k)%.
6. The traveling wave double-end ranging method for the flexible direct-current transmission line according to claim 5, characterized in that: the step3.5 may also be that the check frequency difference value Δ ω is comparedlAnd Δ ωNAnd whether the difference between the ratio and the obtained (1-k)% of the distance measurement estimation result is smaller than a set check threshold value or not is judged, if yes, the distance measurement result is output, and if not, the length of the data time window is changed, and Step2.3 is returned.
7. The method of claim 3, 4 or 6, wherein the method comprises the following steps: the length of the data time window is in the unit of the time tau of the traveling wave propagating in the whole length range of the line, and K tau is changed every time K is changed, wherein K is 2n, and n is 1,2 and 3 ….
8. The utility model provides a flexible direct current transmission line travelling wave bi-polar range finding system which characterized in that includes:
the acquisition module is used for respectively acquiring capacitive current traveling wave signals from traveling wave coupling boxes at two ends of the flexible direct current transmission line;
the calculation module is used for constructing a capacitive current ranging signal and extracting a free oscillation component of the double-end capacitive current ranging signal;
the signal analysis module is used for respectively calculating frequency domain signals of the ranging signals at the two ends to obtain equal-interval frequency spectrums corresponding to the free oscillation components;
the signal processing module is used for calculating the equal-interval frequency difference value of the frequency domain signal and calculating the ratio of the frequency difference values at the two ends;
and the distance measurement module is used for calculating and recording the fault distance measurement result.
9. The traveling wave double-end ranging method for the flexible direct-current transmission line according to claim 8, wherein the acquisition module comprises:
the transmitting unit is used for converting the current signal of the secondary side of the mutual inductor into a signal acquired by the traveling wave device A/D;
the acquisition unit is used for converting the current analog quantity signal into a digital signal;
the storage unit is used for naming the recording data files according to the time stamps and storing the recording data files in a local memory;
and the starting unit is used for judging whether the waveform mutation is larger than a set starting threshold value or not, and storing the current signal into a recording data file if the waveform mutation is larger than the set starting threshold value.
10. The traveling wave double-ended ranging method for the flexible direct-current transmission line according to claim 8, wherein the calculation module comprises:
the calculation unit 1 is used for decoupling a direct current line through a Karenbauer transformation matrix, solving a polar space modulus capacitive current traveling wave signal, solving the difference between values of adjacent sampling points of the signal, and taking the difference as a new value of a previous sampling point to construct a capacitive current ranging signal;
and the calculating units 2 are used for respectively calculating the high-order odd power of the free oscillation component of the double-end capacitive current ranging signal.
11. The traveling wave double-end ranging method for the flexible direct current transmission line according to claim 8, wherein the signal analysis module comprises:
the time-frequency transformation unit is used for performing discrete Fourier transformation on the high-order odd-power signals to respectively obtain frequency spectrums of the high-order odd-power signals at two ends;
and the frequency spectrum judging unit is used for judging whether the frequency spectrums of the frequency domain signals at the two ends are at equal intervals.
12. The traveling wave double-ended ranging method for the flexible direct current transmission line according to claim 8, wherein the signal processing module comprises:
the frequency difference calculating unit is used for extracting frequency values of spectral peak positions of the double-end equal-interval frequency spectrums, and performing difference on any two adjacent frequency values to obtain the corresponding equal frequency differences of the frequency spectrums at the two ends;
a ratio calculation unit for calculating a ratio omega of the two-terminal frequency differencesMN、ΩNMM denotes one end of the double-ended dc device, and N denotes the other end of the double-ended dc device.
A check calculation unit for calculating a check frequency difference value [ delta ] omega ] according to the total length of the linel；
A check comparing unit for comparing the check frequency difference value delta omegalAnd Δ ωMAnd if the difference between the ratio and the obtained k% of the ranging estimation result is smaller than the set check threshold, outputting the ranging result, and if not, changing the length of the data time window.
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