CN113640616A - Time domain oscillation pulse conversion method based on frequency domain reflection - Google Patents
Time domain oscillation pulse conversion method based on frequency domain reflection Download PDFInfo
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Abstract
The invention relates to the technical field of power cables, and adopts the technical scheme that: the time domain oscillation pulse conversion method based on frequency domain reflection comprises the following main steps: s1, analyzing the frequency domain reflection test equipment to find the lowest test lower limit frequency, and the actual sampling data has data loss in the low frequency band; s2, obtaining a new time domain oscillation pulse through calculation; and S3, determining the polarity of the finally determined pulse by researching a reflection peak polarity judging method suitable for the Gaussian envelope narrow-band signal g (t). The polarity of the reflection wave peak of the insulation hot spot can be determined when the upper limit frequency is lower, and the depth analysis of the insulation hot spot is realized.
Description
Technical Field
The invention relates to the technical field of power cables, in particular to a time domain oscillation pulse conversion method based on frequency domain reflection.
Background
The traditional frequency domain reflection test in-process at long cable, cable length overlength leads to the high frequency composition of signal to attenuate great in the cable body, and the impedance mismatch of cable head end and test equipment anchor clamps junction can lead to signal high frequency composition further to attenuate simultaneously, therefore the high frequency composition attenuation of signal is very big in the frequency domain reflection test in-process of actual long cable, when test upper limit frequency sets up too high, not only can't gather the reflection information of high frequency part, but also can introduce great noise, reduce test effect.
Disclosure of Invention
The invention aims to solve the defects in the prior art, and provides a time domain oscillation pulse conversion method based on frequency domain reflection.
In order to achieve the above purposes, the technical scheme adopted by the invention is as follows: the time domain oscillation pulse conversion method based on frequency domain reflection comprises the following main steps:
s1, analyzing the frequency domain reflection test equipment to find the lowest test lower limit frequency, and the actual sampling data has data loss in the low frequency band;
s2, obtaining a new time domain oscillation pulse through calculation;
and S3, determining the polarity of the finally determined pulse by researching a reflection peak polarity judging method suitable for the Gaussian envelope narrow-band signal g (t).
Preferably, the S1 frequency domain reflection test is a single-point test of the cable parameters at each frequency point, and since the frequency of the frequency point is known, the narrow-band tuning receiver can be installed to reduce noise and unnecessary signals during the test, minimize the noise level of the instrument, and detect a small effective signal to obtain a high dynamic gain.
Preferably, the S2 is used to determine the frequency bandwidth of the gaussian envelope narrowband signal g (t), and the S2 is able to obtain the waveform of the gaussian envelope narrowband signal g (t).
Preferably, the S2 ensures that the gaussian envelope narrowband signal g (t) has only a main peak with the largest amplitude at the center of the time bandwidth, and the peak direction of the peak is upward, and the main peak can be subsequently used to determine the polarity of the reflection peak.
Preferably, in S2, the conjugate symmetry is performed on the positive frequency component to obtain reflection coefficient spectrum data of the negative frequency component, so that interpolated and extended reflection coefficient spectrum data Γ 4 is determined, and then a new time domain oscillation pulse y gaussian envelope narrowband signal g (t) is obtained through fourier spectrum calculation.
Preferably, in S3, it is required to explore a method for determining a polarity of a reflection peak applied to the narrow-band gaussian envelope signal g (t), simulating propagation of the narrow-band gaussian envelope signal g (t) in the cable by using a transfer function of the cable, where waveform parameters of the narrow-band gaussian envelope signal g (t) are set to fm 1.5MHz, ga is 1V, gb is 20 μ S and gc is 0.4 μ S, and in order to explore influences of attenuation and dispersion effects in the cable on a propagation waveform of the narrow-band gaussian envelope signal g (t), propagation waveforms of the narrow-band gaussian envelope signal g (t) in a case where only the attenuation effect exists and in a case where the attenuation and dispersion effects exist simultaneously are obtained by using a fourier spectrum calculation method, respectively.
Preferably, in the S3, the upper envelope waveform of the narrow-band gaussian envelope signal g (t) still has almost no change in the time domain, and the change in the time bandwidth is small, which indicates that the attenuation and dispersion phenomena have little influence on the upper envelope waveform of the narrow-band gaussian envelope signal g (t).
Compared with the prior art, the invention has the following beneficial effects:
the method comprises the steps of firstly carrying out Hilbert transformation on a time domain oscillation pulse obtained through conversion to obtain an upper envelope line and a lower envelope line in a waveform time domain, then determining the center of a time bandwidth of a reflected wave according to the positions of peak values of the upper envelope line and the lower envelope line, regarding the center as the peak value of the reflected wave, then determining the first peak shifted backwards in the center of the time bandwidth as a main peak, and judging the polarity of the reflected peak according to the polarity of the main peak so as to solve the problem of traveling wave distortion caused by dispersion and improve the accuracy of judging the polarity of the reflected peak of the time domain oscillation pulse.
Drawings
FIG. 1 is a waveform diagram of narrow band Gaussian envelope signals g (t) with different gd according to the method for transforming time domain oscillation pulses based on frequency domain reflection;
FIG. 2 is a graph showing the upper envelope curve and amplitude-frequency curve of a narrow-band Gaussian envelope signal g (t) with different gd according to the method for transforming a time-domain oscillation pulse based on frequency-domain reflection;
FIG. 3 is a waveform propagation result diagram of a Gaussian envelope narrowband signal g (t) with only attenuation according to the time domain oscillation pulse conversion method based on frequency domain reflection;
fig. 4 is a waveform propagation result diagram of a gaussian envelope narrowband signal g (t) with attenuation and dispersion according to the time domain oscillation pulse conversion method based on frequency domain reflection.
Detailed Description
The following description is presented to disclose the invention so as to enable any person skilled in the art to practice the invention. The preferred embodiments in the following description are given by way of example only, and other obvious variations will occur to those skilled in the art.
The time domain oscillation pulse conversion method based on frequency domain reflection is provided and comprises the following main steps:
s1, analyzing the frequency domain reflection test equipment to find the lowest test lower limit frequency, and the actual sampling data has data loss in the low frequency band;
s2, obtaining a new time domain oscillation pulse through calculation;
and S3, determining the polarity of the finally determined pulse by researching a reflection peak polarity judging method suitable for the Gaussian envelope narrow-band signal g (t).
The S1 frequency domain reflection test is to test the cable parameter of each frequency point separately, because the frequency of the frequency point is known, the noise and unnecessary signal can be reduced by installing narrow band tuning receiver, the noise level of the instrument is the lowest, the small effective signal can be detected, so as to obtain the higher dynamic gain.
Because the frequency domain reflection test equipment is a real instrument, the lowest test lower limit frequency exists, and data loss exists in the low frequency band of actual sampling data. When the upper test limit frequency is higher, the missing low-frequency-band data volume accounts for less in the total data volume, so that the baseline fluctuation in the converted time-domain pulse waveform is weaker, and the time-domain characteristics of the waveform are clear. In the frequency domain reflection test process of the short cable, the cable length is short, so that the attenuation of the high-frequency part of the signal in the cable body is weak, and the higher test upper limit frequency can be set in the frequency domain reflection test. In the frequency domain reflection test process of long cable, the cable length overlength leads to the high frequency composition of signal to attenuate great in the cable body, and the impedance mismatch of cable head end and test equipment anchor clamps junction can lead to the further decay of signal high frequency composition simultaneously, therefore the high frequency composition attenuation of signal is very big in the frequency domain reflection test process of actual long cable, when test upper limit frequency sets up too high, not only can't gather the reflection information of high frequency part, but also can introduce great noise, reduce test effect.
As shown in fig. 1-2, S2 can be used to determine the frequency bandwidth of the gaussian envelope narrowband signal g (t), and S2 can obtain the waveform of the gaussian envelope narrowband signal g (t), S2 ensures that the gaussian envelope narrowband signal g (t) has a main peak with the largest amplitude only at the central position of the time bandwidth, and the peak direction of the peak is upward, and then the main peak can be used to determine the polarity of the reflection peak, and S2 performs conjugate symmetry on the positive frequency component to obtain the reflection coefficient spectrum data of the negative frequency component, so as to determine the interpolated and extended reflection coefficient spectrum data r 4, and then obtains a new time-domain oscillation pulse y gaussian envelope narrowband signal g (t) through fourier spectrum calculation.
The narrow-band Gaussian envelope signal has excellent frequency band regulation capability, so the narrow-band Gaussian envelope signal is introduced as an incident signal, and is defined as the narrow-band Gaussian envelope signal g (t)
In the formula: t is a time factor, which indicates that the signal takes time as an independent variable. fm is the central frequency and can be used for determining the position of the amplitude-frequency curve of the Gaussian envelope narrowband signal g (t) on the frequency axis; ga is an amplitude factor that can be used to control the amplitude of the narrow-band gaussian envelope signal g (t); gb is a time factor, which can be used to determine the position of the gaussian envelope narrowband signal g (t) on the time axis, and at the same time, ensure that only one highest main peak exists in the gaussian envelope narrowband signal g (t), which is convenient for the judgment of the polarity of the subsequent reflection peak; gc is a pulse width factor and can be used for determining the time bandwidth and the frequency bandwidth of a Gaussian envelope narrow-band signal g (t), and the distance resolution of reflected waves in a time domain is controlled; fm is the central frequency and can be used for determining the position of the amplitude-frequency curve of the Gaussian envelope narrowband signal g (t) on the frequency axis; gd is the frequency modulation factor, which can be used to determine the frequency bandwidth of the narrow-band signal g (t) with gaussian envelope.
Since the final conversion result is subjected to the amplitude normalization process and the conversion process from the horizontal axis of time to the horizontal axis of distance from the head end, the parameter settings of ga and gb have no effect on the final result. First, we discuss gd, setting fm to 10MHz and gc to 0.1 μ s, and taking gd as 0rad/μ s2, 40rad/μ s2 and 80rad/μ s2 respectively, and obtaining the waveform of the narrow-band gaussian envelope signal g (t) as shown in fig. 1. In fig. 1, as gd increases, the frequency change rate of the gaussian envelope narrowband signal g (t) increases, and the frequency change phenomenon becomes more obvious. On the other hand, as can be seen from fig. 1, the presence of gb in equation (1-1) ensures that the gaussian envelope narrowband signal g (t) has only a main peak with the largest amplitude at the center of the time bandwidth, and the peak of the peak faces upwards, and the main peak can be subsequently used to perform the judgment of the polarity of the reflection peak.
For the convenience of observing the time bandwidth and the frequency bandwidth of the narrow-band gaussian envelope signal g (t) under different gd conditions in fig. 1, the time-domain upper envelope curve and the frequency-domain amplitude-frequency characteristic curve of the waveform in fig. 1 are respectively generated by using hilbert transform and fourier transform, as shown in fig. 2. In order to compare the envelope in the time domain, the narrow-band gaussian envelope signal g (t) is subjected to time shift under 3 groups of different gd conditions. As seen from fig. 2, when the remaining parameters in equation (1-1) are unchanged, the time bandwidth of the gaussian envelope narrowband signal g (t) is unchanged and the frequency bandwidth is increased as gd is increased. This means that as gd is larger, more frequency domain data is required to obtain the same time bandwidth, and therefore, for the frequency domain reflection technique, as gd is larger, a wider test band is required to obtain the same distance resolution of the reflected wave in the time domain, and a higher upper limit frequency of the test is required. In summary, the lower the set value of gd should be, the better, which is taken herein as 0rad/μ s2, when equation (1-1) is rewritten as
After the combination of the formula (1-2), the amplitude-frequency curve abs (g (f)) of the Gaussian envelope narrowband signal g (t) is obtained by Fourier transform
The discrete sampling frequency of the actual Gaussian envelope narrowband signal g (t) is ft, in order to ensure the waveform definition of the Gaussian envelope narrowband signal g (t), in this chapter, ft is taken as 30fmax, and the amplitude-frequency curve of the discrete signal Gaussian envelope narrowband signal g (t) is obtained as
As can be seen from the equations (1-4), taking f as the argument, abs (g (f)) is a Gaussian pulse function with a pulse amplitude factor of
The pulse time factor is fm and the pulse width factor is 1/(2 π gc). For the narrow-band gaussian envelope signal g (t), fm determines the center frequency of the signal and gc determines the frequency bandwidth of the signal. Because abs (g (f)) is a symmetric gaussian function, in order to ensure that the gaussian envelope narrowband signal g (t) is reasonably distributed in the frequency domain reflection test frequency band fmin-fmax, the center frequency fm of the gaussian envelope narrowband signal g (t) is set at the center of the frequency band fmin-fmax, that is, the center frequency fm is set at the center of the frequency band fmin-fmax
fm＝(fmax-fmin)/2+fmin (1-5)
As gc decreases, the time bandwidth of the gaussian envelope narrowband signal g (t) decreases, the resolution of the reflected wave on the range axis in the time domain increases, but the corresponding frequency bandwidth increases. Therefore, for the limited frequency band data obtained by the frequency domain reflection test, gc should be made as large as possible, so as to obtain a smaller time bandwidth and improve the distance resolution of the reflected wave in the time domain. This chapter therefore sets gc to
In the formula: k2 is a threshold value to ensure that abs (g (f)) is k2 times the peak of the function when f is fmax, which is taken to be 0.5% in this chapter.
Similar to s (t) in chapter 5, the gaussian envelope narrowband signal g (t) is inconsistent with the frequency range of the reflection coefficient spectrum obtained by testing, and the frequency sampling points of the gaussian envelope narrowband signal g (t) and the reflection coefficient spectrum are inconsistent, so that interpolation and expansion processing of the reflection coefficient spectrum data are also required. In the aspect of interpolation, reflection coefficient spectrum data of each frequency point corresponding to the gaussian envelope narrowband signal g (t) in the frequency band fmin-fmax is solved by adopting segmented cubic spline interpolation, and in the frequency bands of 0-fmin and fmax-30 fmax, because no frequency component of the gaussian envelope narrowband signal g (t) is distributed in the region, the reflection coefficient spectrum data of each frequency point corresponding to the gaussian envelope narrowband signal g (t) in the frequency bands of 0-fmin and fmax-30 fmax can be set to zero. In the aspect of expansion, the conjugate symmetry is carried out on the positive frequency component to obtain the reflection coefficient spectrum data of the negative frequency component, and the interpolated and expanded reflection coefficient spectrum data F4 is determined.
Then, a new time domain oscillation pulse y Gaussian envelope narrowband signal g (t) is obtained by Fourier spectrum calculation
yg(t)＝IFT(FT(g(t))Γ4) (1-7)。
As shown in fig. 3-4, in S3, it is necessary to search a method for determining the polarity of a reflection peak applied to a gaussian-envelope narrowband signal g (t), and simulate propagation of the gaussian-envelope narrowband signal g (t) in a cable by using a transfer function of the cable, where waveform parameters of the gaussian-envelope narrowband signal g (t) are set to fm 1.5MHz, ga 1V, gb 20 μ S and gc 0.4 μ S, and in order to search for the influence of attenuation and dispersion effects on the propagation waveform of the gaussian-envelope narrowband signal g (t) in the cable, propagation waveforms of the gaussian-envelope narrowband signal g (t) in a case where only attenuation effects exist and in a case where attenuation and dispersion effects exist simultaneously are obtained by using a fourier spectrum calculation method, respectively, in S3, an upper waveform of the gaussian-envelope narrowband signal g (t) still has almost no change, and a time bandwidth change is small, which indicates that the influence of the attenuation and dispersion effects on an upper envelope waveform of the gaussian-envelope narrowband envelope signal g (t) is not large .
Because the gaussian envelope narrowband signal g (t) is a time domain oscillation pulse, unlike the unipolar pulse of s (t), the polarity of the pulse cannot be directly determined, and therefore, a method for determining the polarity of the reflection peak applicable to the gaussian envelope narrowband signal g (t) needs to be explored. Simulating a Gaussian envelope narrow-band signal g (t) to propagate in the cable by using a transfer function of the cable, wherein the waveform parameters of the Gaussian envelope narrow-band signal g (t) are set to fm 1.5MHz, ga 1V, gb 20 mus and gc 0.4 mus. In order to investigate the influence of attenuation and dispersion effects in the cable on the propagation waveform of the gaussian envelope narrowband signal g (t), the propagation waveforms of the gaussian envelope narrowband signal g (t) obtained by using a fourier spectrum calculation method under the condition that only the attenuation effect exists and under the condition that the attenuation and dispersion effects exist simultaneously are shown in fig. 3 and 4, and amplitude-frequency characteristic curves in the upper envelope and frequency domain in the time domain under the two different conditions are shown in fig. 3 and 4. It should be noted that, in order to visually analyze the distortion phenomenon of the gaussian envelope narrowband signal g (t) propagation waveform, the waveforms of fig. 3 and 4 are both subjected to amplitude normalization processing, and time-axis translation processing is performed on the time-domain waveform and the upper envelope in the time domain.
As can be seen from fig. 3, in the frequency domain, the attenuation effect causes the center frequency of the gaussian envelope narrowband signal g (t) to decrease, and the oscillation period of the waveform becomes large. In the time domain, the envelope waveform on the signal is almost unchanged, the time bandwidth is slightly changed, and meanwhile, the main peak is located in the middle of the time bandwidth and is not shifted.
As can be seen from fig. 4, in the frequency domain, the center frequency of the gaussian envelope narrowband signal g (t) decreases, and the oscillation period of the waveform becomes large, which is mainly caused by the attenuation effect as can be seen from comparing fig. 3. In the time domain, the upper envelope waveform of the gaussian envelope narrowband signal g (t) still has almost no change, and the change of the time bandwidth is small, which shows that the attenuation and dispersion phenomena have little influence on the upper envelope waveform of the gaussian envelope narrowband signal g (t). The main peak is no longer located in the middle of the time bandwidth, but a hysteresis occurs, and the amplitude of the main peak starts to decrease and the amplitude of the side peak preceding the main peak starts to increase, as can be seen from a comparison of fig. 3, which is mainly caused by the dispersion effect. The phenomenon that the amplitude of the main peak is reduced and the amplitude of the side peak in front of the main peak is increased can cause the cross-correlation algorithm and other traditional methods to easily cause misjudgment on the main peak, cause mispositioning on an insulation hot spot, and simultaneously cause misjudgment on the polarity of the reflection peak of the insulation hot spot, and cause misanalysis on the insulation hot spot.
Comprehensively, the Hilbert transform [64-65] is carried out on the time domain oscillation pulse obtained through conversion to obtain an upper envelope line and a lower envelope line [66-67] in a waveform time domain, then the center of the time bandwidth of the reflected wave is determined according to the positions of the peak values of the upper envelope line and the lower envelope line, the center is regarded as the peak value of the reflected wave, then the first peak shifted backwards in the center of the time bandwidth is determined as a main peak, the polarity of the reflected peak is determined according to the polarity of the main peak, the problem of traveling wave distortion caused by chromatic dispersion is solved, and the accuracy of judging the polarity of the reflected peak of the time domain oscillation pulse is improved.
The foregoing shows and describes the general principles, essential features, and advantages of the invention. It will be understood by those skilled in the art that the present invention is not limited to the embodiments described above, which are merely illustrative of the principles of the invention, but that various changes and modifications may be made without departing from the spirit and scope of the invention, which fall within the scope of the invention as claimed. The scope of the invention is defined by the appended claims and equivalents thereof.
Claims (7)
1. The time domain oscillation pulse conversion method based on frequency domain reflection is characterized in that: the method comprises the following main steps:
s1, analyzing the frequency domain reflection test equipment to find the lowest test lower limit frequency, and the actual sampling data has data loss in the low frequency band;
s2, obtaining a new time domain oscillation pulse through calculation;
and S3, determining the polarity of the finally determined pulse by researching a reflection peak polarity judging method suitable for the Gaussian envelope narrow-band signal g (t).
2. The method of claim 1, wherein the method comprises: the S1 can determine the polarity of the reflection peak of the insulation hot spot when the upper limit frequency is lower, and realize the depth analysis of the insulation hot spot.
3. The method of claim 1, wherein the method comprises: the S2 can be used to determine the frequency bandwidth of the gaussian envelope narrowband signal g (t), and the S2 can obtain the waveform of the gaussian envelope narrowband signal g (t).
4. The method of claim 1, wherein the method comprises: the S2 ensures that only one main peak with the largest amplitude appears at the center of the time bandwidth, and the peak direction of the peak is upward, and the main peak can be subsequently used to determine the polarity of the reflection peak.
5. The method of claim 1, wherein the method comprises: in the step S2, the conjugate symmetry is performed on the positive frequency component to obtain reflection coefficient spectrum data of the negative frequency component, so that interpolated and expanded reflection coefficient spectrum data Γ 4 is determined, and then a new time domain oscillation pulse y gaussian envelope narrowband signal g (t) is obtained through fourier spectrum calculation.
6. The method of claim 1, wherein the method comprises: in S3, a method for determining the polarity of a reflection peak applied to the narrow-band gaussian envelope signal g (t) needs to be explored, the narrow-band gaussian envelope signal g (t) is simulated to propagate through the cable by using a transfer function of the cable, where waveform parameters of the narrow-band gaussian envelope signal g (t) are set to fm 1.5MHz, ga 1V, gb 20 μ S and gc 0.4 μ S, and in order to explore the influence of attenuation and dispersion effects in the cable on the propagation waveform of the narrow-band gaussian envelope signal g (t), the propagation waveforms of the narrow-band gaussian envelope signal g (t) are obtained by using a fourier spectrum calculation method respectively when only the attenuation effect exists and when the attenuation and dispersion effects exist at the same time.
7. The method of claim 1, wherein the method comprises: in the S3, in the time domain, the upper envelope waveform of the gaussian envelope narrowband signal g (t) still has almost no change, and the change of the time bandwidth is small, which indicates that the attenuation and dispersion phenomena have little influence on the upper envelope waveform of the gaussian envelope narrowband signal g (t).
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