CN108876097B - Method for rapidly calculating frequency of alternating current micro-grid based on average amplitude difference compensation function - Google Patents

Method for rapidly calculating frequency of alternating current micro-grid based on average amplitude difference compensation function Download PDF

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CN108876097B
CN108876097B CN201810400358.1A CN201810400358A CN108876097B CN 108876097 B CN108876097 B CN 108876097B CN 201810400358 A CN201810400358 A CN 201810400358A CN 108876097 B CN108876097 B CN 108876097B
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frequency
function
average amplitude
amplitude difference
time
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李建科
王金全
徐晔
尹志勇
罗珊
黄克峰
张海涛
刘金宁
陈静静
庞玲玉
邢鸣
徐才华
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Jiangsu Zhenan Xinrun Power Technology Co ltd
Army Engineering University of PLA
Jiangsu Zhenan Power Equipment Co Ltd
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Army Engineering University of PLA
Jiangsu Zhenan Power Equipment Co Ltd
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Abstract

The invention provides a method for rapidly calculating the frequency of an alternating current micro-grid based on an average amplitude difference compensation function, which compensates the amplitude by constructing the average amplitude difference compensation function and simply adding, subtracting and absolute value operation, inhibits the attenuation of the amplitude of a peak value, ensures that the peak value of integral multiple of the period of a voltage signal is kept at a certain height, is convenient to find out a time coordinate corresponding to the peak value and improves the accuracy of frequency detection. The method adopts addition operation, obviously reduces the operation time and is beneficial to calculating the signal frequency in a long time; the amplitude is compensated under the condition that the time scale is not changed, and the time scale corresponding to the extreme value can be intuitively obtained. The method can calculate the alternating current frequency of each period, and is more suitable for alternating current frequency fluctuation caused by frequent fluctuation of the load.

Description

Method for rapidly calculating frequency of alternating current micro-grid based on average amplitude difference compensation function
Technical Field
The invention belongs to the field of micro-grid power quality, and particularly relates to a method for rapidly calculating the frequency of an alternating current micro-grid based on an average amplitude difference compensation function, which is used for calculating the frequency change of the alternating current micro-grid caused by load fluctuation.
Background
When the pulse load consumes power, the stator current of the synchronous generator is increased in a short time to cause the electromagnetic torque to be increased, namely the total resisting moment is increased, because the accelerator actuating mechanism of the speed regulator has mechanical inertia, the response time is longer, when the pulse peak power comes, the output mechanical power of the diesel engine is smaller than the total resisting moment, the rotating speed is reduced according to the rotating shaft motion equation, and under the regulating action of the speed regulator, the rotating speed gradually rises back to the rated value; when the peak power of the pulse load disappears suddenly, the rotating speed is increased, and under the action of the speed regulator, the rotating speed is gradually reduced to a rated value, and the working characteristic of the pulse load is equivalent to that of the conventional load, wherein the power of the conventional load is increased suddenly and decreased suddenly and frequently. If the duty cycle of the pulse load is large and the duty ratio is small, the speed regulator and the voltage regulator of the diesel generator set can inhibit the fluctuation of frequency and voltage caused by sudden increase or sudden decrease of pulse peak power, but when the duty cycle of the pulse load is small and the duty ratio is large, the pulse is limited by the mechanical characteristics of the diesel generator set, when the influence of the current pulse on the diesel generator set is not removed, the next pulse comes, and the diesel generator set or the diesel generator set works in a new stable state or the frequency is continuously reduced until the diesel generator set is tripped in a protection mode. Therefore, the diesel generator set is frequently impacted by a high-power load, which is different from the single impact of the traditional large power grid, the analysis method of the circuit operation parameters is different, and a micro-grid operation analysis method and an evaluation index suitable for the pulse load must be provided.
The frequency fluctuation of the power grid caused by the conventional load finally tends to a stable value under the action of the speed regulator, namely, the period of the waveform is fixed at different moments, while the frequency of the microgrid is changed at different moments under the pulse load, and the working principle is shown in figure 1, namely, the frequency of the waveform is changed along with time under the same load and the same working mode, and is obviously different from that of the conventional load.
Scholars at home and abroad propose various power system frequency calculation methods, which can be generally divided into two types: hardware phase-locked loop technology and software frequency measurement algorithm. The hardware phase-locked loop technology is susceptible to noise and oscillation phenomenon can occur; the calculation method of the power system frequency generally comprises zero crossing point detection, recursive least square algorithm, DFT (fast Fourier transform) (FFT) algorithm, improved algorithm, Kalman filtering method, adaptive notch method and the like, and the algorithms have advantages and are widely applied to different fields and achieve good effects. The European legislation provides an electrical signal frequency tracking measurement algorithm based on a second-order infinite impulse response variable-step self-adaptive digital notch filter; the Marek, etc. converts the three-phase voltage signals into phasors, and utilizes a finite impulse response prefilter to inhibit noise influence; the method comprises the steps of bin expansion and the like, wherein estimated values of frequency and voltage amplitude are obtained through pre-estimation of the state of a power grid and are used for updating the coefficient of an infinite impulse response filter in real time to realize self-adaptive fundamental wave extraction of the power grid; karimi et al propose an orthogonal phase-locked loop structure, on this basis, Mojiri et al further simplify the algorithm steps by using an adaptive notch method. The above documents are directed to frequency calculation of an electric power system, and are directed to a utility grid with a small change in frequency. In a micro-grid, under the action of pulse load, the frequency is constantly changed, the amplitude is large, whether the methods are suitable for being worth researching or not is judged, in addition, some algorithms are easily influenced by noise, the algorithm flow is complex, the calculation amount is large, and the methods are not suitable for real-time calculation. Some documents mention stabilizing the frequency in the microgrid with certain control strategies, but do not indicate how to calculate the frequency. Shenzhen university Li Fepeng et al detects the pitch of the voice based on wavelet transformation and autocorrelation function, and the algorithm can be applied to a power system.
Disclosure of Invention
The purpose of the invention is as follows: in order to overcome the defects in the prior art, the invention provides a method for quickly calculating the frequency of an alternating current micro-grid based on an average amplitude difference compensation function.
The technical scheme is as follows:
the method for rapidly calculating the frequency of the alternating current micro-grid based on the average amplitude difference compensation function comprises the following steps:
step 1: intercepting a section of signal u of a microgrid alternating voltage signalw(t)；
Step 2: calculating the signal uwTotal number of sampling points N of (t):
N＝fN·tu
wherein f isNIs the sampling frequency; t is tuIs a signal uw(t) length of time;
and step 3: constructing an average amplitude difference compensation function, and calculating a function value R (tau);
the average amplitude difference compensation function is constructed as follows:
defining a short-time autocorrelation function:
Figure BDA0001645550170000021
wherein τ is a delay time, and τ is 0 to N; u. ofw(t) is a segment signal u of the AC side voltage signal u (t)w(t); n is the signal uw(t) the total number of sampling points is a positive integer; n represents each sample point;
an average amplitude difference function is thus constructed:
Figure BDA0001645550170000022
constructing an average amplitude difference compensation function based on the definition of the average amplitude difference function:
Figure BDA0001645550170000031
and 4, step 4: finding the time coordinate t corresponding to the peak value of the function value R (tau)jJ represents the number of the peak of the function value R (τ), j is 1,2,3, …;
and 5: calculating the frequency f corresponding to each periodj；
Figure BDA0001645550170000032
From this, the voltage signal u can be calculatedw(t) frequency of each cycle of (t).
Has the advantages that: the method adopts addition operation, obviously reduces the operation time and is beneficial to calculating the signal frequency in a long time; the amplitude is compensated under the condition that the time scale is not changed, and the time scale corresponding to the extreme value can be intuitively obtained. The method can calculate the alternating current frequency of each period, and is more suitable for alternating current frequency fluctuation caused by frequent fluctuation of the load.
Drawings
Fig. 1 is a schematic diagram of the operation of a pulsed load.
Fig. 2 is a waveform diagram of actually measured ac voltage and dc current.
Fig. 3 is a flow chart of the AMDCF algorithm.
Fig. 4 shows ACF calculation function values.
Fig. 5 shows the calculated function values of AMDF.
Fig. 6 shows the AMDCF calculated function values.
FIG. 7 is a graph of error versus time.
Detailed Description
The present invention will be further described with reference to the accompanying drawings.
The Auto Correlation Function (ACF) method is mainly used to study the periodicity of the signal waveform, and is a method for measuring the similarity between the signal and the self-translated waveform of the signal. Using a part of signal u (t) of AC side voltage signal u (t)w(t) for the study subject, defining a short-time autocorrelation function:
Figure BDA0001645550170000033
wherein τ is a delay time, and τ is 0 to N; n is the signal uw(t) the total number of sampling points is a positive integer; n denotes each sample point.
As shown in equation (1), the autocorrelation function correlates the original signal with the signal delayed by τ time. If the delay time is an integral multiple of the original signal period, the autocorrelation function value R will have an extremum, and the period of the extremum is the same as the original signal period. The frequency of the original signal can be obtained by detecting the frequency of the peak value of the function value R. However, there are a lot of multiplication operations inside the autocorrelation function if the truncated signal uwWhen (t) is longer, the calculation amount is large, and in order to overcome the defect of large calculation amount, relatively simple addition operation is adopted to define an Average amplitude Difference Function (AMDF):
Figure BDA0001645550170000041
equation (2) significantly reduces the complexity of the autocorrelation function algorithm, but as the delay time increases, uw(n) and uwThe overlap term of (n + tau) is gradually reduced, the value R of function2The peak value of (2) is also reduced correspondingly, and the extreme value of the function value is not easy to detect. For this purpose, based on the definition of the Average amplitude Difference Function, an Average amplitude Difference compensation Function (AMDCF) is constructed, defined as:
Figure BDA0001645550170000042
the detection method of the AMDCF comprises the following steps: voltage signal uwAnd (n) is a periodic signal, when the translation amount tau is an integral multiple of the period, the theoretical value of the average amplitude difference compensation function is zero, namely, maximum values appear at intervals in the AMDCF function value, the translation amount interval corresponding to the adjacent extreme value can be determined according to the translation amount corresponding to the maximum values, and then the time interval is determined. The function has only simple addition, subtraction and absolute value operation, compared with the ACF method,the operation amount is greatly reduced; compared with the AMDF method, the amplitude adjustment item 1/(N-tau) is added, the amplitude is compensated, the peak amplitude attenuation is restrained, the integral multiple of the voltage signal period is kept at a certain height, the time coordinate corresponding to the peak is convenient to find, and the accuracy of frequency detection is improved.
The method comprises the following specific steps:
step 1, intercepting a section of signal u of an alternating voltage signal of a micro-gridw(t)；
Step 2, calculating a signal uw(t) the total number of sampling points N;
N＝fN·tu
wherein f isNIs the sampling frequency; t is tuIs a signal uw(t) length of time;
step 3, calculating a function value R (tau) according to the constructed average amplitude difference compensation function;
step 4, finding out the time coordinate t corresponding to the peak value of the function value R (tau)j(j represents the number of the peak of the function value R (τ), and j is 1,2,3, …);
step 5, calculating the frequency f of the peak value of the function value R (tau)j；
Figure BDA0001645550170000051
The peak frequency is in one-to-one correspondence with the frequency of a single period of the signal, defined by the function value R (τ), from which the voltage signal u can be calculatedw(t) frequency of each cycle of (t).
To analyze the accuracy of the above algorithm, the ac voltage waveform under a pulsed load was measured, as shown in fig. 2.
As can be seen from fig. 2, the time length is 0.2s, and if the time length is calculated by power frequency, ten cycles are needed, it can be intuitively understood that the time length exceeds ten cycles shown in the graph, and the cycles are different, namely, the change of the rotating speed of the diesel engine caused by the change of the large current in the short time of the pulse load, and further, the frequency changes.
According to the formulas (1), (2) and (3), writing a program in MATLAB software by using an M file, importing voltage waveform data, and obtaining a calculated waveform. The AMDCF algorithm flow chart is shown in fig. 3.
According to the flow chart, a program (see appendix) is written, the same set of voltage data is calculated, the waveform of the function value R under the three functions is obtained, the autocorrelation function result is shown in fig. 4, the average amplitude difference function result is shown in fig. 5, and the average amplitude difference compensation function is shown in fig. 6.
The result shows that the amplitude of the ACF function is gradually reduced, the function value is approximately equal to zero when the sampling time is over, and the time consumption is long in the calculation process; the AMDF function adopts addition operation, so that the operation time is obviously reduced, the calculation of the signal frequency in a long time is facilitated, and the amplitude is gradually reduced; compared with the prior art, the AMDCF function operation inherits the advantage of short-time operation of the AMDCF function, the amplitude is compensated under the condition that the time scale is not changed, the time scale corresponding to the extreme value can be intuitively obtained, and the time scale obtained by the AMDCF function is the same as that obtained by the AMDCF function in view of the time scale, but the AMDCF function compensates the amplitude, so that the AMDCF function is more intuitive.
The difference of the time scales can be converted into corresponding frequency, and then the frequency of each period of the waveform is obtained, as shown in table 1, and the result obtained by the frequency measuring instrument is also given in the table.
TABLE 1 frequency calculation results
Figure BDA0001645550170000052
As can be seen from table 1, due to the effect of the pulse load, the frequency of the power grid fluctuates greatly, and even if the pulse load works in the same mode, the frequency of the power grid at different times is different, because frequent power sudden changes of the pulse load cause the diesel generator set to be in the dynamic regulation process all the time, which is closely related to the dynamic behaviors of the diesel engine and the synchronous generator. The electromechanical transient and electromagnetic transient processes of the diesel generator set are very complex, the diesel engine and the synchronous generator are strong nonlinear systems, and the set continuously adjusts the working state of the set to adapt to the change of the pulse load, so that the output frequency of the synchronous generator is fluctuated.
Example to verify the effectiveness of the algorithm presented herein, a comparison of the calculated errors was made, using the results of the frequency meter measurements as a reference, as shown in fig. 7.
As can be seen from FIG. 7, the AMDCF algorithm has relatively small error, can meet the measurement requirement, and is less in calculation amount, fast in calculation time and more suitable for fast calculation.
The above description is only of the preferred embodiments of the present invention, and it should be noted that: it will be apparent to those skilled in the art that various modifications and adaptations can be made without departing from the principles of the invention and these are intended to be within the scope of the invention.

Claims (1)

1. The method for rapidly calculating the frequency of the alternating current micro-grid based on the average amplitude difference compensation function is characterized by comprising the following steps of: the method comprises the following steps:
step 1: intercepting a section of signal u of a microgrid alternating voltage signalw(t)；
Step 2: calculating the signal uwTotal number of sampling points N of (t):
N＝fNgtu
wherein f isNIs the sampling frequency; t is tuIs a signal uw(t) length of time;
and step 3: constructing an average amplitude difference compensation function, and calculating a function value R (tau);
the average amplitude difference compensation function is constructed as follows:
defining a short-time autocorrelation function:
Figure FDA0003157454160000011
wherein τ is a delay time, and τ is 0 to N; u. ofw(t) is a signal u of the alternating voltage signal u (t)w(t); n is the signal uw(t) the total number of sampling points is a positive integer; n represents each sample point;
an average amplitude difference function is thus constructed:
Figure FDA0003157454160000012
constructing an average amplitude difference compensation function based on the definition of the average amplitude difference function:
Figure FDA0003157454160000013
and 4, step 4: finding the time coordinate t corresponding to the peak value of the function value R (tau)jJ represents the number of the peak of the function value R (τ), j is 1,2,3, …;
and 5: calculating the frequency f corresponding to each periodj；
Figure FDA0003157454160000014
From this, the voltage signal u can be calculatedw(t) frequency of each cycle of (t).
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Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN103426122A (en) * 2013-05-17 2013-12-04 中国能源建设集团广东省电力设计研究院 Comprehensive evaluation method of micro-grid
CN107480837A (en) * 2017-09-29 2017-12-15 山东理工大学 A kind of islet operation micro-grid coordination control method that frequency is synchronously determined based on GPS

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* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US10333308B2 (en) * 2016-08-15 2019-06-25 Nec Corporation Two-level predictive based reactive power coordination and voltage restoration for microgrids

Patent Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN103426122A (en) * 2013-05-17 2013-12-04 中国能源建设集团广东省电力设计研究院 Comprehensive evaluation method of micro-grid
CN107480837A (en) * 2017-09-29 2017-12-15 山东理工大学 A kind of islet operation micro-grid coordination control method that frequency is synchronously determined based on GPS

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