CN106226590A - A kind of synchronous phase measuring in power system method - Google Patents

Abstract

The invention relates to the technical field of electric power automation, in particular to a method for measuring synchronized phasors in a power system. The measurement method of the present invention is based on the "phase invariance" of all-phase FFT (all-phase FFT, apFFT) to realize the accurate estimation of the phase angle, and uses the time-shift phase difference method to realize the correction estimation of the frequency, and then complete the system phasor Measurement. Practice has proved that the estimation accuracy of the algorithm is close to unbiased estimation in the case of no noise or low noise. Compared with the currently widely used algorithms, the proposed method has greatly improved the estimation accuracy and real-time performance without adding additional The amount of calculation and hardware cost is small, which is convenient for engineering realization.

Description

A method for measuring synchrophasors in power systems

technical field

The invention relates to the technical field of electric power automation, in particular to a method for measuring synchronized phasors in a power system.

Background technique

With the rapid development of my country's smart grid construction, the safe and stable operation of the power grid has become particularly important, and the construction of a dynamic and stable monitoring and control system for the entire network has become an urgent problem to be solved in the current system. The synchronized phasor measurement unit (PMU) based on the global positioning system (GPS) uses high-precision timing signals to realize the synchronous collection of data from each node of the wide-area power system, which promotes the rapid development of the wide-area measurement system (WAMS). A large number of PMUs have been used in power grid dynamic state estimation, fault location, wide-area protection, online parameter estimation and other fields, and their measurement accuracy will directly affect the performance of the above applications.

Synchronous phasor measurement is mainly aimed at estimating frequency, phase angle, and amplitude. The current practical PMU algorithms include zero-crossing detection method, discrete Fourier transform (DFT) method, Kalman filter method, wavelet transform method, etc., among which DFT algorithm is due to Its good harmonic suppression characteristics under static conditions are widely used in PMU devices of different voltage levels. However, when the operating frequency of the system deviates from 50Hz and the sampling data received by the PMU cannot meet the sampling conditions of the entire cycle, the spectrum leakage and frequency aliasing problems of the DFT algorithm itself will lead to large errors in the estimation of parameters such as frequency and phase, which will seriously affect the PMU. algorithm performance. Although the accuracy of parameter estimation such as frequency can be improved by adjusting the length of the data window, it also increases the computational burden. In addition, some PMUs use the improved recursive DFT to realize phasor measurement, which can reduce the frequency estimation error, but this algorithm still cannot completely solve the problems of spectrum leakage and frequency aliasing, and it needs to assume that the signal amplitude remains unchanged, which cannot be well applied to Dynamic grid conditions.

Contents of the invention

In view of the above technical problems, the present invention provides a method for measuring synchronized phasors in a power system.

Technical scheme of the present invention is:

A method for measuring synchrophasors in a power system, comprising the following steps:

Step 1. Perform two sets of fixed-length data sampling on the current and voltage observations received by the PMU to obtain two sets of PMU sequences:

The length of the PMU sequence in the two groups is 2N+1, and the delay is n ₀ .

Step 2: Carry out double-window full-phase spectrum analysis with a length of N on the two groups of PMU sequences, and according to the phase spectrum at the main spectral line k ^* , the phase difference expressions of the two groups of PMU sequences can be obtained:

Among them, 2n ₀ k ^* π/N is the phase difference compensation value after the digital angular frequency 2k ^* π/N at the main spectral line k ^* is delayed by n ₀ .

Step 3 derives the phase-compensated PMU signal frequency estimation and the frequency offset value expression on the main spectral line k ^* according to Equation 3:

in, For frequency estimation, dω is the frequency offset value on the main spectral line k ^* .

Step 4 Estimate the amplitude of the PMU sampling signal according to the frequency offset value dω on the main spectral line k ^* . For the dual-window apFFT spectrum analysis, there is

in, is the amplitude estimation, dω is the frequency offset value on the main spectral line k ^* , Y(k ^* ) is the full phase spectrum value of the observed signal at the main spectral line k ^* , F _g (dω) is the main spectral line frequency offset The value is substituted into the result of computing the full phase expression for the window function.

Specifically, the data sampling in step 1 is non-period sampling.

Specifically, in step 4, the hanning window function is used for dual-window apFFT spectrum analysis, and the corresponding current and voltage amplitude correction estimation expressions are:

in, For amplitude estimation, dω is the frequency offset value on the main spectral line k ^* , and Y(k ^* ) is the full phase spectrum value of the observed signal at the main spectral line k ^* .

A PMU phasor measurement system includes a front-end measured analog quantity access circuit, a signal conditioning circuit, a synchronous A/D converter, a microprocessor and an external system. The front-end analog input circuit is composed of a voltage transformer and a current transformer, and the signal conditioning circuit is composed of an integrated operational amplifier and a digital potentiometer, and the digital potentiometer is the feedback resistor of the integrated operational amplifier. The output terminals of the voltage transformer and the current transformer are connected to the input terminal of the integrated operational amplifier, the output terminal of the integrated operational amplifier is connected to the input terminal of the synchronous A/D converter, and the output terminal of the synchronous A/D converter is connected to the output of the microprocessor One end of the microprocessor is connected to the control end of the digital potentiometer, and the external system is connected to the input/output end of the microprocessor.

Specifically, the microprocessor in the PMU phasor measurement system is an ARM9LPC3250 microcontroller.

Specifically, the synchronous A/D converter in the PMU phasor measurement system is an ADS8568 8-channel synchronous sampling A/D converter.

Beneficial effects of the present invention: the measurement method of the present invention is based on the "phase invariance" of all-phase FFT (all-phase FFT, apFFT) to realize the accurate estimation of the phase angle, and utilize the time-shift phase difference method to realize the correction estimation of the frequency, Then complete the system phasor measurement. Practice has proved that the estimation accuracy of the algorithm is close to unbiased estimation in the case of no noise or low noise. Compared with the currently widely used algorithms, the proposed method has greatly improved the estimation accuracy and real-time performance without adding additional The amount of calculation and hardware cost is small, which is convenient for engineering realization.

Description of drawings

Fig. 1 is a flowchart of a method for measuring synchrophasors in a power system in the present invention.

Fig. 2 is a schematic circuit diagram of the PMU phasor measurement system in the present invention.

Fig. 3 is the signal processing flowchart of dual-window apFFT.

Figure 4 shows the traditional windowed FFT amplitude and phase spectrum.

Figure 5 shows the dual-window apFFT amplitude and phase spectra.

Fig. 6 is the test signal and its apFFT spectrum in the embodiment.

detailed description

Refer to Figure 2 to build a PMU phasor measurement system, and refer to Figure 1 to perform apFFT-based phasor measurement. The PMU phasor measurement system is composed of front-end measured analog quantity access circuit, signal conditioning circuit, synchronous A/D converter, ARM9LPC3250 microprocessor and external system. Among them, the front-end analog quantity access circuit is mainly composed of voltage and current transformers, which are responsible for isolating the measured AC signal before it is connected to the system, so as to improve the safety of the entire measurement system. The analog signal conditioning circuit is composed of an integrated operational amplifier and a digital potentiometer, which amplifies the voltage and current signals, and the digital potentiometer adjusts the feedback resistance under the control of the ARM processor to realize automatic variable range measurement. Synchronous A/D can choose TI's ADS8568 8-channel synchronous sampling A/D converter. LPC3250 is an industrial-grade microcontroller with an ARM9 core, with a working frequency of 200MHz. It is the control and computing core of the PMU measurement system. The external SPI-type FLASH memory is used to store measurement information and system parameters. The Ethernet port is used for Realize communication with the remote machine to realize data upload, and SDRAM is used to store and execute the specific test main program code. The equipment is equipped with an LCD color LCD screen and a matrix keyboard to realize local functions such as parameter setting, result query, function selection, and waveform display. The SD card is used to store the measured parameters or waveforms selected by the user for a long time.

The ARM9 core computing unit of the PMU executes the apFFT-based phasor measurement algorithm proposed by the present invention, and the flow process is shown in FIG. 1 . In Fig. 1, the signal processing flow diagram of the dual-window apFFT calculation unit is shown in Fig. 3 . Use a convolution window w _c with a length of 2N-1 to window the input samples, and translate and superimpose two pieces of data with an interval of N to generate N new data samples y(n), (n=0,1,...,N- 1), and then perform FFT on y(n) to obtain the spectral analysis result. In FIG. 3, N=4.

In Figure 3, the convolution window w _c =[w _c (-N+1),...,w _c (-1), w _c (0), w _c (1),...,w _c (N-1)] ^T is convolved by the front window f and the flipped back window b, namely:

The corresponding Fourier transform satisfies

W _c (e ^jω )＝F(e ^jω )B(e ^-jω )＝F(e ^jω )B ^* (e ^jω ) Formula 9

Let the front window f and the rear window b be both symmetrical windows, then w _c is also a symmetrical window, that is: w _c (n)=w _c (-n). According to different convolution window forms, apFFT can be divided into three cases: no window, single window and double window.

It can be seen that the full-phase spectrum analysis fully considers all N kinds of segmented FFT spectrum analysis situations with a length of N including the sample point x(0). The above-mentioned organic synthesis results have derived some unique properties of apFFT, such as amplitude spectrum suppression leakage performance, and phase flat distribution characteristics near spectral peaks. It should be pointed out that although apFFT considers N kinds of sample segmented FFT spectrum analysis, it only needs to perform one FFT to complete it. Compared with the traditional PMU algorithm, it does not increase the amount of additional calculations, and the engineering application effect is good.

The following analyzes the technical effect of the phasor measurement method in the embodiment from two aspects: signal phase estimation performance and algorithm performance under harmonic conditions.

1. Signal phase estimation performance

Utilize the measuring device of the present invention to estimate the phase parameters of the collected current waveform, and the collection sequence is a composite cosine current sampling containing 3 different frequency components and initial phases

where, ω ₁ =20.0, ω ₂ =60.2, ω ₃ =100.4, N=256. The corresponding windowed FFT and double-window apFFT spectra are shown in Figure 4 and Figure 5, respectively. Here the window function uses the hanning window (raised cosine window), the expression is as follows:

As shown in Figures 4 to 5, in terms of amplitude spectrum, all-phase FFT has a better ability to suppress leakage than traditional FFT, and the energy leakage caused by non-full-period sampling and crosstalk between multiple frequency components is controlled by two within the spectral line. In terms of phase spectrum, the traditional FFT phase spectrum is relatively messy. Only at k=20 (full cycle sampling), the spectral value is close to the theoretical value of 10°. At k=60 and 100, the measured phase deviates greatly from the real phase. The apFFT phase spectrum presents a regular flat distribution near the frequency of the measured component, that is, it has "phase invariance", and can directly reflect the initial phase of each component without any error correction, which has certain advantages in the estimation of stable signal phase. Table 1 shows the phase measurement results of the two spectral analysis methods.

Table 1 Comparison of phase spectrum between traditional FFT and all-phase FFT (N=256)

2. Algorithm performance under harmonic conditions

The signal source adopts a high-precision three-phase synchrophasor test device calibrator, which meets the "GB/T26862-2011 Power System Synchronized Phasor Measurement Device Testing Specification", "Q/GDW416-2010 Power System Synchronized Phasor Measurement Device (PMU) Standard requirements such as "Technical Specifications for Testing". The fundamental frequency of the measured current signal is 50.5Hz, the phase angle is 40°, and it contains two current harmonic components with frequencies of 200Hz and 300Hz. The expression of the signal is:

x(t)＝5cos(50.5×2πt/f _s +40π/180)+0.25cos(200×2πt/f _s )+1.75cos(300×2πt/f _s ) Formula 12

The dual-window apFFT spectrum analysis of the current signal is shown in Figure 6. The frequency, phase, and amplitude parameters of the signal are estimated by using the measurement device of the present invention and the conventional algorithm PMU device respectively, and the comparison results are shown in Table 2.

Table 2 Test signal parameter estimation results

The above results show that, compared with the traditional estimation algorithm based on DFT, the dual-window apFFT phasor measurement algorithm proposed by the present invention has high estimation accuracy and strong ability to suppress noise.

The phasor measurement method proposed by the invention has no limitation on the value range of the measured current and voltage parameters, and the collector does not need to sample the signal in the whole cycle, which is convenient for engineering implementation. In addition, although apFFT considers N kinds of sample segmented FFT spectrum analysis, it only needs to perform one FFT to complete, which does not increase the amount of additional calculation compared with the traditional PMU algorithm, and the engineering application effect is good.

The implementation manners described above are only preferred embodiments of the present invention, rather than an exhaustive list of feasible implementations of the present invention. For those skilled in the art, any obvious changes made without departing from the principle and spirit of the present invention should be considered to be included in the protection scope of the claims of the present invention.

Claims (6)

1. A power system synchronous phasor measurement method is characterized by comprising the following steps:

step 1, carrying out two groups of data sampling with fixed length on current and voltage observed quantities received by PMUs to obtain two groups of PMU sequences:

two groups of PMU sequences have the length of 2N +1 and the time delay of N₀；

Step 2, respectively carrying out double-window full-phase spectrum analysis with the length of N on the two groups of PMU sequences, and according to a main spectral line k^*Obtaining a phase difference expression of two groups of PMU sequences:

wherein, 2n₀k^*pi/N as main spectral line k^*At a digital angular frequency of 2k^*pi/N warp size N₀The delayed phase difference compensation value;

step 3, the PMU signal frequency estimation after phase compensation and the main spectral line k are derived according to the formula 3^*The upper frequency offset value expression:

wherein,for frequency estimation, d ω is the dominant line k^*An upper frequency offset value;

step 4 according to the main spectral line k^*The upper frequency offset value d omega is used for estimating the amplitude of the PMU sampling signal, and for the double-window apFFT spectrum analysis, the upper frequency offset value d omega is used for estimating the amplitude of the PMU sampling signal

Wherein,for amplitude estimation, d ω is the dominant line k^*Upper frequency offset value, Y (k)^*) For observing signals at main line k^*Value of the full phase spectrum of (F)_gAnd (d omega) is a calculation result of substituting the main spectral line frequency offset value into the window function full-phase expression.

2. The method according to claim 1, wherein the data sampling in step 1 is non-integer period sampling.

3. The method for measuring the synchronous phasor of the power system according to claim 1, wherein a hang window function is adopted in step 4 for double-window apFFT spectrum analysis, and the corresponding current and voltage amplitude correction estimation expressions are as follows:

wherein,for amplitude estimation, d ω is the dominant line k^*Upper frequency offset value, Y (k)^*) For observing signals at main line k^*The value of the full phase spectrum of (b).

4. A PMU phasor measurement system is characterized in that it comprises a front-end analog access circuit to be measured, a signal conditioning circuit, a synchronous A/D converter, a microprocessor and an external system, the front-end analog quantity access circuit consists of a voltage transformer and a current transformer, the signal conditioning circuit consists of an integrated operational amplifier and a digital potentiometer, the digital potentiometer is a feedback resistor of the integrated operational amplifier, the output ends of the voltage transformer and the current transformer are connected with the input end of the integrated operational amplifier, the output end of the integrated operational amplifier is connected with the input end of the synchronous A/D converter, the output end of the synchronous A/D converter is connected with the output end of the microprocessor, one output end of the microprocessor is connected with the control end of the digital potentiometer, and the external system is connected with the input/output end of the microprocessor.

5. The PMU phasor measurement system according to claim 4, characterized in that the microprocessor is an ARM9LPC3250 microcontroller.

6. A PMU phasor measurement system according to claim 4, characterized in that the synchronous A/D converter is an ADS8568 type 8-channel synchronous sampling A/D converter.