CN111766556A - Distribution network CT (computed tomography) calibration method - Google Patents
Distribution network CT (computed tomography) calibration method Download PDFInfo
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- CN111766556A CN111766556A CN202010387992.3A CN202010387992A CN111766556A CN 111766556 A CN111766556 A CN 111766556A CN 202010387992 A CN202010387992 A CN 202010387992A CN 111766556 A CN111766556 A CN 111766556A
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R35/00—Testing or calibrating of apparatus covered by the other groups of this subclass
- G01R35/02—Testing or calibrating of apparatus covered by the other groups of this subclass of auxiliary devices, e.g. of instrument transformers according to prescribed transformation ratio, phase angle, or wattage rating
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R23/00—Arrangements for measuring frequencies; Arrangements for analysing frequency spectra
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- G01R23/167—Spectrum analysis; Fourier analysis using filters with digital filters
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Abstract
The application provides a distribution network CT (computed tomography) calibration method which comprises the steps of obtaining an alternating current part in direct current voltage in high-voltage power transmission, and calling a digital low-pass filter to filter the alternating current part; carrying out autocorrelation operation on the filtered high-voltage power transmission signal based on the functional correlation to obtain analog outputs of the detected mutual inductor and the standard mutual inductor; and performing spectrum analysis on the analog output quantities of the detected transformer and the standard transformer by adopting discrete Fourier transform to determine the frequency response difference. By sequentially executing the direct current filtering algorithm, the signal time delay measuring algorithm and the frequency spectrum analysis algorithm, the defect that the fluctuation of the sampling value output by the detected direct current merging unit is large due to alternating current pulse signals in direct current voltage and current components in high-voltage power transmission can be reduced, and the acquisition precision of the sampling value is improved.
Description
Technical Field
The invention belongs to the field of error checking, and particularly relates to a distribution network CT (computed tomography) checking method.
Background
With the development of urbanization, in order to improve the power supply reliability of a distribution network and improve the service quality and meet the requirement of the beautiful life of people on high-quality power supply service, countries and companies all invest a large amount of manpower and material resources to strengthen the construction, upgrading and transformation of the distribution network. Especially, a large number of distribution network automation devices all contain mutual inductor equipment (such as PT or CT), and the equipment cannot meet the requirements of live operation due to the quality, process, structure and the like, so that accidents occur. Therefore, aiming at the research of the uninterrupted operation technology of the distribution network automation equipment containing the mutual inductor, the problem that the equipment is damaged or broken due to improper operation to affect the safety of the equipment and operators can be reduced or avoided, the research of the uninterrupted operation technology of the distribution network automation equipment is developed for the first time, the blank of the company in the aspect is filled, the power failure of engineering prearrangement is reduced, and the power supply reliability is improved.
The live-line disconnection drainage wire is the most basic conventional live-line work project of a 10kV overhead distribution line and accounts for about 60% of the live-line work times of each unit of a power company every year. In order to ensure the safety of live working, safety regulations require that when a no-load line is disconnected and connected in a live state, a circuit breaker (switch) and an isolating switch (knife switch) at the other end of the line are required to be confirmed to be disconnected, and a transformer and a voltage transformer which are connected to the line side can be carried out after the transformer and the voltage transformer are confirmed to be withdrawn from operation. However, with the overall progress of the automatic transformation work of the distribution lines of the urban power grid, various load switches are widely used, and a transformer (energy-taking PT for short) for providing power for a controller and an operating device of the load switch cannot quit operation during live-line work. Whether the energy taking PT can be installed in a charged state or not and the drainage wire with the PT is disconnected become the difficult problems which need to be solved urgently in the current charged work.
The mutual inductor is an important component of an electric energy metering device and is a legal metering appliance for performing fair and fair trade settlement between a power generation company and a power grid company, between the power grid company and a power supply company, between the power supply company and power users and accurately calculating and checking technical and economic indexes in a power system. According to the current national verification regulations, the transformers (including voltage transformers and current transformers) used for metering in the power grid must be regularly subjected to error characteristic detection. According to the conventional detection method, the error characteristic detection can be carried out only by taking an operating transformer off-line, which inevitably affects the power supply reliability. And the electric power system in China has a huge number of power transformers, the error performance workload of the power system in power failure test is huge, time and labor are wasted, and real data in a three-phase electrified state cannot be obtained.
Disclosure of Invention
In order to solve the defects in the prior art, the invention provides a distribution network CT (computed tomography) verification method, which comprises the following steps:
acquiring an alternating current part in direct current voltage in high-voltage power transmission, and calling a digital low-pass filter to filter the alternating current part;
carrying out autocorrelation operation on the filtered high-voltage power transmission signal based on the functional correlation to obtain analog outputs of the detected mutual inductor and the standard mutual inductor;
and performing spectrum analysis on the analog output quantities of the detected transformer and the standard transformer by adopting discrete Fourier transform to determine the frequency response difference.
Optionally, the obtaining an alternating current portion in a direct current voltage in high voltage power transmission, and calling a digital low-pass filter to perform filtering processing on the alternating current portion includes:
determining a transfer function expression and a difference equation of the digital filter;
adjusting an expression of the digital filter based on a preset performance parameter;
and obtaining an error calculation formula of the digital low-pass filter which meets the requirement after adjustment.
Optionally, the performing autocorrelation operation on the filtered high-voltage power transmission signal based on the functional correlation to obtain analog outputs of the detected transformer and the standard transformer includes:
obtaining an expression of an autocorrelation operation
In the above formula, x1(t) and x2(t) are respectively data received by the system at the same time, that is, analog outputs of the detected transformer and the standard transformer, D is delay, n1(t) and n2(t) are additive noises, which are assumed to be zero mean value and normal distribution random process with variance of 1, and are independent from the signal source s (t); the received correlation function of the data of the standard mutual inductor and the mutual inductor to be tested is as follows:
in the formula: rss represents the autocorrelation function of the source signal s (t), and E [. cndot. ] represents the mathematical expectation, where assuming that s (t), n1(t), and n2(t) are independent of each other, there are:
i.e. perfect orthogonality between the source signal and the noise and between the noise and the noise, then:
R12(τ)＝Rss(τ-D)
the autocorrelation function properties are:
|Rss(τ-D)|≤Rss(0)
in this case, when τ -D is 0, Rss(τ -D) takes the maximum value, i.e., the peak value.
The technical scheme provided by the invention has the beneficial effects that:
by sequentially executing the direct current filtering algorithm, the signal time delay measuring algorithm and the frequency spectrum analysis algorithm, the defect that the fluctuation of the sampling value output by the detected direct current merging unit is large due to alternating current pulse signals in direct current voltage and current components in high-voltage power transmission can be reduced, and the acquisition precision of the sampling value is improved.
Drawings
In order to more clearly illustrate the technical solution of the present invention, the drawings needed to be used in the description of the embodiments will be briefly introduced below, and it is obvious that the drawings in the following description are only some embodiments of the present invention, and it is obvious for those skilled in the art to obtain other drawings based on the drawings without creative efforts.
Fig. 1 is a schematic flow chart of a distribution network CT verification method proposed in the present application;
FIG. 2 is a FIR direct type structure proposed in an embodiment of the present application;
FIG. 3 is a schematic diagram of a phase relationship proposed in the embodiment of the present application;
fig. 4 is a schematic diagram of a butterfly form of an 8-point FFT proposed in the embodiment of the present application.
Detailed Description
To make the structure and advantages of the present invention clearer, the structure of the present invention will be further described with reference to the accompanying drawings.
Example one
The project requires that high-precision detection is carried out on high-voltage current direct-current components, high-precision detection is carried out on alternating-current pulsation in a range from 50Hz to 1200Hz of direct-current signals, and the high-precision detection can be realized only through related algorithms.
As shown in fig. 1, the distribution network CT verification method provided by the present application includes:
11. acquiring an alternating current part in direct current voltage in high-voltage power transmission, and calling a digital low-pass filter to filter the alternating current part;
12. carrying out autocorrelation operation on the filtered high-voltage power transmission signal based on the functional correlation to obtain analog outputs of the detected mutual inductor and the standard mutual inductor;
13. and performing spectrum analysis on the analog output quantities of the detected transformer and the standard transformer by adopting discrete Fourier transform to determine the frequency response difference.
The method for acquiring the alternating current part in the direct current voltage in the high-voltage power transmission and calling the digital low-pass filter to filter the alternating current part comprises the following steps:
determining a transfer function expression and a difference equation of the digital filter;
adjusting an expression of the digital filter based on a preset performance parameter;
and obtaining an error calculation formula of the digital low-pass filter which meets the requirement after adjustment. The method for carrying out autocorrelation operation on the filtered high-voltage power transmission signal based on the functional correlation to obtain the analog output of the detected mutual inductor and the standard mutual inductor comprises the following steps:
obtaining an expression of an autocorrelation operation
In the above formula, x1(t) and x2(t) are respectively data received by the system at the same time, that is, analog outputs of the detected transformer and the standard transformer, D is delay, n1(t) and n2(t) are additive noises, which are assumed to be zero mean value and normal distribution random process with variance of 1, and are independent from the signal source s (t); the received correlation function of the data of the standard mutual inductor and the mutual inductor to be tested is as follows:
in the formula: rss represents the autocorrelation function of the source signal s (t), and E [. cndot. ] represents the mathematical expectation, where assuming that s (t), n1(t), and n2(t) are independent of each other, there are:
i.e. perfect orthogonality between the source signal and the noise and between the noise and the noise, then:
R12(τ)＝Rss(τ-D)
the autocorrelation function properties are:
|Rss(τ-D)|≤Rss(0)
in this case, when τ -D is 0, Rss(τ -D) takes the maximum value, i.e., the peak value.
The value of the direct current merging unit is from the sampling value of the standard power source and the detected merging unit, the signal quality of the standard power source is good, and the standard power source can meet the verification requirement, but the direct current voltage and current components in high-voltage power transmission contain abundant alternating current pulse signals, so that the fluctuation of the sampling value output by the detected direct current merging unit is large, and therefore an algorithm optimization scheme is required to be adopted to improve the acquisition precision.
The digital low-pass filter is a good choice, and can filter the digital signal collected by the front end through an FIR digital filter. According to the transfer function form of the FIR digital filter:
and the difference equation is as follows:
the system adopts a direct type digital filter design, and has the advantages of small calculation amount and simple structure, wherein the direct type structure is shown in figure 2.
To filter signals at 50Hz and above, the low pass filter has a cut-off frequency of 5 Hz. The design process of the FIR low-pass digital filter can be designed by using MATLAB design tool.
The FIR low pass filter in the scheme has the following characteristics: according to the Equiripple design method, the lower limit cut-off frequency of a pass band is 5Hz, the amplitude of the pass band is attenuated by 1dB, the cut-off frequency of a lower stop band is 10Hz, the amplitude of the lower stop band is attenuated by 80dB, and the sampling rate is 200 Hz. After the system is automatically designed, parameters can be automatically generated, and the scheme designs a direct FIR filter with 102 orders, namely N102, and simultaneously has the linear phase characteristic of type 2. The phase relationship is shown in fig. 3.
Therefore, the digital filter can attenuate the alternating current signals larger than 5Hz, and the direct current detection precision of the digital filter is greatly improved. Finally, through error calculation, the calculation mode is as follows:
in the formula: u shapesIs the secondary output value of the standard direct current power source; u shapecIs a measurement of the aligned stream combining unit. Therefore, the ratio error is obtained, and meanwhile, the data such as the ratio difference average value, the ratio difference maximum value and the ratio difference minimum value are obtained through methods such as data statistics.
Signal time delay measurement algorithm
The scheme adopts a correlation function segmentation delay algorithm, the relative delay range of two signals is smaller, so that one function time domain base number can be fixed in a certain time domain, the other function time domain base number moves, and the cross-correlation calculation of two discrete sequences is carried out once every moving unit. The application of the system can be better adapted by utilizing the idea. The autocorrelation operation process is as follows:
in the above formula, x1(t) and x2(t) are respectively data received by the system at the same time, that is, analog outputs of the detected transformer and the standard transformer, D is delay, n1(t) and n2(t) are additive noises, which are assumed to be zero mean value and normal distribution random process with variance of 1, and are independent from the signal source s (t); the received correlation function of the data of the standard mutual inductor and the mutual inductor to be tested is as follows:
in the formula: rss represents the autocorrelation function of the source signal s (t), and E [. cndot. ] represents the mathematical expectation, where assuming that s (t), n1(t), and n2(t) are independent of each other, there are:
i.e. perfect orthogonality between the source signal and the noise and between the noise and the noise, then:
R12(τ)＝Rss(τ-D)
the autocorrelation function properties are:
|Rss(τ-D)|≤Rss(0)
in this case, when τ -D is 0, Rss(τ -D) takes the maximum value, i.e., the peak value.
In the scheme, the delay D is obtained by adopting an active approximation method, and the specific error is derived from the sampling rate of the system, namely the resolution of a time domain. Meanwhile, in order to reduce the calculated amount, the active approximation method has a certain approximation process, namely, the moving direction of the active approximation method is detected to move towards the peak value in real time. The C language computation process of the autocorrelation function is generated directly by MATLAB. After determining its delay D, the delay time T may be determined by T ═ D × T, where T is the sampling period, i.e., T ═ 1/10240 s.
Spectral analysis algorithm
The time delay interval can be determined by performing spectrum analysis on the two paths of signals by DFT. For an analog quantity standard input channel, when the frequency is 50Hz, 256 sampling points per cycle are adopted, the sampling rate is 12.8kHz, and meanwhile, sampling points in the interval of DFT calculation are 1024 points, namely, the time of a time domain contained in each time of DFT calculation is 80ms, and sufficient energy spectrum distribution is provided for carrying out error analysis on signals. For the detected combination unit of digital quantity input, the sampling rate is 4kHz at 50Hz of fundamental wave, namely the number of points per cycle is 80 points.
The time decimation base 2DFT algorithm is adopted in the scheme, and the butterfly operation form has the advantages of simplicity, less memory, indexing and the like. In order to maximize the system precision, the twiddle factors are shaped by 32 bits, the twiddle factors required in the calculation process are generated by MATLAB and stored in DSP, and the DSP performs calculation in a table look-up mode, so that the calculation speed is greatly improved.
The butterfly form of 8-point FFT is shown in figure 4, the system respectively carries out fast DFT operation on 1024-point sequences of two signals, and the frequency response difference between a detected combination unit and a check meter can be compared through the operation result, so that the frequency response performance of the system within the range of 50-1200 Hz can be determined.
The sequence numbers in the above embodiments are merely for description, and do not represent the sequence of the assembly or the use of the components.
The above description is only exemplary of the present invention and should not be taken as limiting the invention, as any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.
Claims (3)
1. The distribution network CT verification method is characterized by comprising the following steps:
acquiring an alternating current part in direct current voltage in high-voltage power transmission, and calling a digital low-pass filter to filter the alternating current part;
carrying out autocorrelation operation on the filtered high-voltage power transmission signal based on the functional correlation to obtain analog outputs of the detected mutual inductor and the standard mutual inductor;
and performing spectrum analysis on the analog output quantities of the detected transformer and the standard transformer by adopting discrete Fourier transform to determine the frequency response difference.
2. The distribution network CT verification method according to claim 1, wherein the obtaining of the alternating current part of the direct current voltage in the high-voltage transmission and the filtering of the alternating current part by using a digital low-pass filter comprise:
determining a transfer function expression and a difference equation of the digital filter;
adjusting an expression of the digital filter based on a preset performance parameter;
and obtaining an error calculation formula of the digital low-pass filter which meets the requirement after adjustment.
3. The distribution network CT verification method of claim 1, wherein the autocorrelation operation is performed on the filtered high-voltage transmission signal based on the functional correlation to obtain analog outputs of the detected transformer and the standard transformer, and the method comprises the following steps:
obtaining an expression of an autocorrelation operation
In the above formula, x1(t) and x2(t) are respectively data received by the system at the same time, that is, analog outputs of the detected transformer and the standard transformer, D is delay, n1(t) and n2(t) are additive noises, which are assumed to be zero mean value and normal distribution random process with variance of 1, and are independent from the signal source s (t); the received correlation function of the data of the standard mutual inductor and the mutual inductor to be tested is as follows:
in the formula: rss represents the autocorrelation function of the source signal s (t), and E [. cndot. ] represents the mathematical expectation, where assuming that s (t), n1(t), and n2(t) are independent of each other, there are:
i.e. perfect orthogonality between the source signal and the noise and between the noise and the noise, then:
R12(τ)＝Rss(τ-D)
the autocorrelation function properties are:
|Rss(τ-D)|≤Rss(0)
in this case, when τ -D is 0, Rss(τ -D) takes the maximum value, i.e., the peak value.
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|---|---|---|---|---|
| CN101604009A (en) * | 2009-07-22 | 2009-12-16 | 天津市电力公司 | Method for verifying universal electronic type mutual inductor |
| CN104833941A (en) * | 2015-05-26 | 2015-08-12 | 广东电网有限责任公司电力科学研究院 | DC mutual inductor check meter combining analog quantity and digital quantity verification function |
| CN106772199A (en) * | 2017-01-05 | 2017-05-31 | 云南电网有限责任公司电力科学研究院 | A kind of DC current transformer frequency response characteristic check system and method |
| CN108872921A (en) * | 2018-07-31 | 2018-11-23 | 中国电力科学研究院有限公司 | Device and method for verifying broadband characteristics of direct current transformer |
| CN110210081A (en) * | 2019-05-17 | 2019-09-06 | 武汉理工大学 | A kind of SS-OCT system k-clock delay correcting algorithm |
| CN110488216A (en) * | 2019-09-20 | 2019-11-22 | 云南电网有限责任公司电力科学研究院 | A kind of digital output DCVT field calibration system |
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2020
- 2020-05-09 CN CN202010387992.3A patent/CN111766556A/en active Pending
Patent Citations (6)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN101604009A (en) * | 2009-07-22 | 2009-12-16 | 天津市电力公司 | Method for verifying universal electronic type mutual inductor |
| CN104833941A (en) * | 2015-05-26 | 2015-08-12 | 广东电网有限责任公司电力科学研究院 | DC mutual inductor check meter combining analog quantity and digital quantity verification function |
| CN106772199A (en) * | 2017-01-05 | 2017-05-31 | 云南电网有限责任公司电力科学研究院 | A kind of DC current transformer frequency response characteristic check system and method |
| CN108872921A (en) * | 2018-07-31 | 2018-11-23 | 中国电力科学研究院有限公司 | Device and method for verifying broadband characteristics of direct current transformer |
| CN110210081A (en) * | 2019-05-17 | 2019-09-06 | 武汉理工大学 | A kind of SS-OCT system k-clock delay correcting algorithm |
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Application publication date: 20201013 |