CN111505375A - Frequency measurement method and device - Google Patents
Frequency measurement method and device Download PDFInfo
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- CN111505375A CN111505375A CN202010304298.0A CN202010304298A CN111505375A CN 111505375 A CN111505375 A CN 111505375A CN 202010304298 A CN202010304298 A CN 202010304298A CN 111505375 A CN111505375 A CN 111505375A
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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
- G01R23/02—Arrangements for measuring frequency, e.g. pulse repetition rate; Arrangements for measuring period of current or voltage
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R1/00—Details of instruments or arrangements of the types included in groups G01R5/00 - G01R13/00 and G01R31/00
- G01R1/30—Structural combination of electric measuring instruments with basic electronic circuits, e.g. with amplifier
Abstract
The invention discloses a frequency measurement method and a device, wherein a preposed low-pass filter and a postpositional low-pass filter are used for mutually matching and filtering harmonic components of different times to replace a high-order preposed low-pass filter, so that the occupation of a digital low-pass filter on the computing resources of an embedded device is greatly reduced, the frequency measurement precision and the tracking capability under the signal dynamic condition are improved, the accurate actual frequency is finally obtained, and the problems of the frequency measurement computing efficiency and the frequency measurement computing precision in the embedded device are effectively solved.
Description
Technical Field
The present invention relates to the field of power, and in particular, to a method and an apparatus for measuring frequency in a power system.
Background
In the field of electric power, frequency measurement is the basis of transformer substation embedded devices such as protection devices, measurement and control devices, synchronous phasor measurement devices and the like. The current frequency measurement methods can be divided into two types, namely hardware frequency measurement methods and software frequency measurement methods. Because the hardware frequency measurement method is realized through the filter shaping circuit and the phase-locked loop, certain cost is needed, the realization is complex, the method is not suitable for the development direction of secondary equipment of a power system, and the application is less and less. The software frequency measurement method comprises a zero-crossing point frequency measurement method, a least square method, a wavelet analysis method, a Fourier frequency measurement method and the like. The Fourier frequency measurement method has certain filtering capability and is better applied to an embedded device. The basic principle is to find the phase angle difference between 2 adjacent data windows, and when the frequency changes, the change of the frequency can be found from the phase angle difference. When the signal contains finite integral harmonic pollution, the frequency error is increased, and the larger the harmonic content is, the larger the frequency measurement result error is. In order to reduce errors, the pre-low-pass filter needs to be designed carefully in practical application.
Fig. 1 is a flow chart of a prior art fourier frequency measurement method, the method comprising:
s1: the alternating current conversion circuit carries out alternating current conversion on the input three-phase voltage analog signal to convert the three-phase voltage analog signal into a 5V weak current signal;
s2: the AD conversion circuit converts the three-phase voltage analog signal into a three-phase voltage discrete signal ua(k)，ub(k)，uc(k)；
S3: three-phase voltage discrete signal ua(k)，ub(k)，uc(k) Filtering harmonic waves through a pre-low-pass filter to obtain a three-phase voltage discrete signal u 'after filtering'a(k)，u′b(k)，u′c(k)；
S4: filtered three-phase voltage discrete signal u'a(k)，u′b(k)，u′c(k) And calculating to obtain a frequency value f by adopting Discrete Fourier Transform (DFT).
Common low-pass filters can effectively filter higher harmonics, but high-order filters are needed for filtering low-order harmonics such as 2 th order to 5 th order, and great pressure is brought to calculation of embedded devices. If the error is reduced by adopting the method of iterative averaging, the methods can achieve very high precision under the steady-state condition, but the frequency tracking performance is not good when the signal dynamically changes. And when the harmonic components needing to be suppressed are more, the order of the pre-filter is higher, more embedded device computing resources are occupied, the response time is longer, and the frequency tracking performance is influenced. Therefore, it is necessary to invent a frequency measurement method and device suitable for embedded devices.
Disclosure of Invention
Objects of the invention
The invention aims to provide a frequency measurement method and a frequency measurement device, wherein a digital post filter is added to enable the pre filter and the post filter to be matched with each other, so that the optimal filtering effect is achieved, the occupation of computing resources in an embedded device is reduced, and the frequency measurement precision and the tracking capability under the signal dynamic condition are improved.
(II) technical scheme
In order to solve the above problems, an aspect of the present invention provides a frequency measurement method, including detecting three-phase voltages or three-phase currents in an electric power system to obtain an analog detection signal; converting the analog detection signal into a discrete detection signal; carrying out primary filtering on the discrete detection signal through a pre-low-pass filter; performing discrete Fourier transform on the filtered discrete detection signal to calculate positive sequence voltage or positive sequence current, and calculating frequency according to the positive sequence voltage or the positive sequence current at equal time intervals to obtain a measurement frequency value sequence; and the measured frequency value sequence is subjected to secondary filtering through a post low-pass filter to obtain a frequency value.
According to one aspect of the invention, the pre-low pass filter and the post-low pass filter respectively filter out harmonic components of different orders.
According to an aspect of the invention, the stopband cut-off frequency of the pre-low-pass filter is higher than the stopband cut-off frequency of the post-low-pass filter.
According to one aspect of the invention, the pre-low pass filter or/and the post-low pass filter is/are a digital FIR low pass filter.
In another aspect of the present invention, a frequency measuring apparatus is provided, which includes a detecting unit for detecting three-phase voltages or three-phase currents in an electric power system to obtain an analog detection signal; the AD sampling loop is used for converting the analog detection signal into a discrete detection signal; the pre-low-pass filter is used for filtering the discrete detection signal for the first time; the calculating unit is used for performing discrete Fourier transform on the filtered discrete detection signal to calculate positive sequence voltage or positive sequence current, and calculating frequency according to the positive sequence voltage or the positive sequence current at equal time intervals to obtain a measurement frequency value sequence; and the post low-pass filter is used for carrying out secondary filtering on the measurement frequency value sequence to obtain a frequency value.
According to another aspect of the present invention, the pre-low pass filter and the post-low pass filter respectively filter out harmonic components of different orders.
According to another aspect of the invention, the stopband cut-off frequency of the pre-low-pass filter is higher than the stopband cut-off frequency of the post-low-pass filter.
According to another aspect of the invention, the pre-low pass filter or/and the post-low pass filter is/are a digital FIR low pass filter.
(III) advantageous effects
The invention utilizes the mutual matching of the preposed low-pass filter and the postposition low-pass filter to filter harmonic components with different times to replace a high-order preposed low-pass filter, thereby greatly reducing the occupation of the digital low-pass filter on the computing resources of the embedded device, improving the frequency measurement precision and the tracking capability under the signal dynamic condition, finally obtaining accurate actual frequency and effectively solving the problems of the frequency measurement computing efficiency and the computing precision in the embedded device.
Drawings
FIG. 1 is a flow chart of a prior art Fourier frequency measurement method;
FIG. 2 is a graph of the amplitude-frequency response of a pre-digital low pass filter provided by an embodiment of the present application;
FIG. 3 is a graph of the amplitude-frequency response of a post-digital low-pass filter provided by an embodiment of the present application;
fig. 4 is a flowchart of a frequency calculation method according to an embodiment of the present application.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention will be described in further detail with reference to the accompanying drawings in conjunction with the following detailed description. It should be understood that the description is intended to be exemplary only, and is not intended to limit the scope of the present invention. Moreover, in the following description, descriptions of well-known structures and techniques are omitted so as to not unnecessarily obscure the concepts of the present invention.
Take the typical 4000Hz sampling rate in an intelligent substation as an example. In order to suppress the influence of harmonics, a high-order digital low-pass filter is added to the front end when the frequency is calculated by a traditional Discrete Fourier Transform (DFT) algorithm. Taking the second harmonic component filtering as an example, the pre-FIR low-pass filter is often above 120 th order, which brings great computational pressure to the embedded device. One embodiment of the invention is a frequency calculation method, which uses a low-order pre-low-pass filter to filter out higher harmonics, and then adds a post-low-pass filter to the frequency calculation result to eliminate the influence of low harmonics. The method of matching the front low-pass filter with the rear low-pass filter is adopted, so that the calculated amount is effectively reduced.
The design method of the pre-digital low-pass filter and the post-low-pass filter is as follows. In an actual device, the sampling rate of the embedded device is 4000Hz, the rate of the device calculating frequency is 1000Hz, the calculation efficiency is considered according to the requirement of harmonic suppression, the pre-recorder and the post-filter need to be matched with each other, and a 20-order pre-digital low-pass filter and a 40-order post-digital low-pass filter can be adopted.
The design parameters of the 20 th-order pre-digital low-pass filter are as follows: the stop band cut-off frequency is 400Hz, and the stop band attenuation is 30 dB. The frequency response diagram is shown in fig. 2, and the values of the filter coefficients h (m) are obtained as follows:
h(m)＝[-0.02183968 -0.00729089 -0.00221872 0.00841156 0.024603790.04527615 0.06838147 0.09103711 0.11016914 0.12295228 0.12743347 0.122952280.11016914 0.09103711 0.06838147 0.04527615 0.02460379 0.00841156 -0.00221872-0.00729089 -0.02183968]。
the design parameters of the 40-order post-digital low-pass filter are as follows: the stop band cut-off frequency is 50Hz, and the stop band attenuation is 28 dB. The frequency response diagram is shown in fig. 3, and the values of the filter coefficients h (n) are obtained as follows:
h(n)＝[-0.02121753 -0.00448691 -0.004201721 -0.00333874 -0.001763970.00051964 0.00353596 0.00729981 0.01175050 0.01679081 0.02230810 0.028151890.03414425 0.04009611 0.04580241 0.05105413 0.05565791 0.05943935 0.062250800.06398373 0.06456965 0.06398373 0.06225080 0.05943935 0.05565791 0.051054130.04580241 0.04009611 0.03414425 0.02815189 0.02230810 0.01679081 0.011750500.00729981 0.00353596 0.00051964 -0.00176397 -0.00333874 -0.00420172 -0.00448691 -0.02121753]。
it should be noted that, in this embodiment, the pre-low pass filter and the post-low pass filter may be FIR low pass filters, or may be other types of low pass filters. The orders of the pre-low-pass filter and the post-low-pass filter are not limited to 20 th order and 40 th order, and those skilled in the art can design the pre-low-pass filter and the post-low-pass filter according to the requirement of harmonic suppression while considering calculation efficiency, and select other orders.
Referring to fig. 4, it can be seen that the frequency calculation method includes the following steps.
S1: the input three-phase voltage analog signal is converted into a 5V weak current signal through an alternating current conversion circuit;
s2: three-phase voltage analog signals are converted into three-phase voltage discrete signals u through an AD sampling loopa(k)，ub(k)，uc(k)；
S3: three-phase voltage discrete signal ua(k)，ub(k)，uc(k) Filtering out higher harmonics through a pre-low-pass filter to obtain a three-phase voltage discrete signal u 'after filtering'a(k)，u′b(k)，u′c(k)；
S4: filtered three-phase voltage discrete signal u'a(k)，u′b(k)，u′c(k) The Discrete Fourier Transform (DFT) is used to obtain the sequence of measurement frequency values f (0), f (1), f (2) … f (k).
The specific calculation method comprises the following steps: calculating three-phase voltage by adopting Discrete Fourier Transform (DFT) to obtain real part U of positive sequence voltage signalr(k) And imaginary part Ui(k) (ii) a The frequency is calculated once every 4 samples of data, i.e. at a rate of 1000 Hz. When calculating the frequency, calculating the phase difference by using positive sequence phasors with the interval of 20 milliseconds to obtain a measurement frequency value sequence f (0), f (1), f (2) … f (k);
s5: the frequency value f is obtained by filtering the low-order harmonic through a low-pass filter h (n) after the frequency value sequence f (0), f (1), f (2) … f (k).
If all harmonics are filtered out on the pre-filter, assuming that the filtering order is 120, the discrete fourier frequency measurement method in the prior art needs to operate a 120-order filter for 3 times because the frequency calculation generally adopts positive sequence voltage calculation. The invention only needs to operate the 20-order pre-filter and the 1-order 40-order post-filter for 3 times, and compared with the Fourier frequency measurement method in the prior art, the invention can greatly reduce the operation amount.
Simulation verification and actual device tests show that after the frequency is processed by the front filter and the rear filter, the calculation error of the frequency is less than 0.002Hz and is far lower than the traditional frequency measurement error of 0.01 Hz. Compared with the Fourier frequency measurement method in the prior art, the method effectively improves the calculation accuracy.
Another embodiment of the present invention is a frequency measurement apparatus including a detection unit, an AD sampling loop, a pre-low pass filter, a calculation unit, and a post-low pass filter. The detection unit is used for detecting three-phase voltage or current in the power system and converting the three-phase voltage or current into a weak current signal of 5V; the AD sampling circuit is used for discretizing the three-phase voltage analog signal and converting the three-phase voltage analog signal into a three-phase voltage discrete signal; the pre-low-pass filter is used for filtering the three-phase voltage discrete signals for the first time and filtering out higher harmonics; the calculating unit is used for performing discrete Fourier transform on the filtered discrete detection signal to calculate positive sequence voltage or positive sequence current, and calculating frequency according to the positive sequence voltage or the positive sequence current at equal time intervals to obtain a measurement frequency value sequence; the device is used for performing Discrete Fourier Transform (DFT) on the filtered three-phase voltage discrete signals, calculating positive sequence voltage, calculating phase difference according to the positive sequence voltage, and calculating to obtain a measurement frequency value sequence; and the post low-pass filter is used for carrying out secondary filtering on the measuring frequency value sequence, filtering low-order harmonic waves and obtaining a frequency value.
The front low-pass filter and the rear low-pass filter respectively filter harmonic components of different times. The prepositive low-pass filter is used for filtering out higher harmonics, the postpositive low-pass filter is used for filtering out lower harmonics, and the stop band cut-off frequency of the prepositive low-pass filter is designed to be higher than that of the postpositive low-pass filter. The front low-pass filter and the rear low-pass filter can select a digital FIR low-pass filter.
In summary, the present invention provides a method and an apparatus for measuring frequency, the method converts the detected analog detection signal into a discrete detection signal, and performs a first filtering through a pre-low pass filter; calculating positive sequence voltage or positive sequence current by discrete Fourier transform, and calculating frequency according to the positive sequence voltage or the positive sequence current at equal time intervals to obtain a measurement frequency value sequence; and carrying out secondary filtering through a post low-pass filter to finally obtain a frequency value. The device comprises a detection unit, an AD sampling loop, a front low-pass filter, a calculation unit and a rear low-pass filter. According to the invention, the post low-pass filter is added, and the pre-low-pass filter and the post low-pass filter are matched with each other to filter harmonic components of different times, so that the occupation of a digital filter in the device on computing resources is reduced, the frequency measurement precision and the tracking capability under a signal dynamic condition are improved, accurate actual frequency is finally obtained, and the problems of the computing efficiency and the computing precision of frequency measurement of an embedded device are effectively solved.
It is to be understood that the above-described embodiments of the present invention are merely illustrative of or explaining the principles of the invention and are not to be construed as limiting the invention. Therefore, any modification, equivalent replacement, improvement and the like made without departing from the spirit and scope of the present invention should be included in the protection scope of the present invention. Further, it is intended that the appended claims cover all such variations and modifications as fall within the scope and boundaries of the appended claims or the equivalents of such scope and boundaries.
Claims (8)
1. A method of frequency measurement, comprising:
detecting three-phase voltage or three-phase current in the power system to obtain an analog detection signal;
converting the analog detection signal into a discrete detection signal;
carrying out primary filtering on the discrete detection signal through a pre-low-pass filter;
performing discrete Fourier transform on the filtered discrete detection signal to calculate positive sequence voltage or positive sequence current, and calculating frequency according to the positive sequence voltage or the positive sequence current at equal time intervals to obtain a measurement frequency value sequence;
and the measured frequency value sequence is subjected to secondary filtering through a post low-pass filter to obtain a frequency value.
2. The frequency measurement method according to claim 1, wherein the pre-low pass filter and the post-low pass filter out harmonic components of different orders, respectively.
3. The method according to claim 2, wherein the stopband cut-off frequency of the pre-low-pass filter is higher than the stopband cut-off frequency of the post-low-pass filter.
4. The frequency measurement method according to any one of claims 1-3, wherein the pre-low pass filter or/and the post-low pass filter employs a digital FIR low pass filter.
5. A frequency measuring device comprises
The detection unit is used for detecting three-phase voltage or three-phase current in the power system to obtain an analog detection signal;
the AD sampling loop is used for converting the analog detection signal into a discrete detection signal;
the pre-low-pass filter is used for filtering the discrete detection signal for the first time;
the calculating unit is used for performing discrete Fourier transform on the filtered discrete detection signal to calculate positive sequence voltage or positive sequence current, and calculating frequency according to the positive sequence voltage or the positive sequence current at equal time intervals to obtain a measurement frequency value sequence;
and the post low-pass filter is used for carrying out secondary filtering on the measurement frequency value sequence to obtain a frequency value.
6. The frequency measurement device according to claim 5, wherein the pre-low pass filter and the post-low pass filter respectively filter out harmonic components of different orders.
7. The frequency measurement device according to claim 6, wherein the stopband cut-off frequency of the pre-low pass filter is higher than the stopband cut-off frequency of the post-low pass filter.
8. The frequency measurement device according to any of claims 5-7, wherein the pre-low pass filter or/and the post-low pass filter employs a digital FIR low pass filter.
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