Using Prony Method
Transcription
Using Prony Method
Spectral Estimation of Distorted Signals Using Prony Method Tadeusz Lobos, Jacek Rezmer Ah.rfracf -- Mudern Frequency power converters generate a widc spectrum of harmonic components. L a r p converter systems can also generale noncharacteristic harmonics and jnterharrnonics. Standard tools oC harmonic analysis based on the Fourier transform assume that o n l j harmonics are present and the periodicity intervals are fixed, while periodicity intervals in the presence of interharmonics are variable and very long. I n the case of frequency converters the periods of the main component are unknown. The Prony method as applied for signal analysis in power frequency converter was tested in the paper. The method does not show the disadvantages o f the traditional tools and algow cxact estimation the frequencies of all or dominant companents, cvcn when the periodicity intervals are unknown. To investigate the appmprjatencss of the method several experiments were performed. For comparison, similar experiments were repeated using the FlT. The comparison proved the superiority of the Prony method. The quality of voItage waveforms is nowadays an issue- of the ubnost importance for power utilities, electric energy consumers and also for the manufactures of electric and eiectronic equipment. The liberalization of European energy market will strengthen the competition and 1s expected to drive down the energy prices. This is reason for the requirements concerning the power quality. The voltage waveform is expected to be a pure sinusoldal with a given frequency and amplitude. Modem frequency power converters generate a wide spectmn of harmonic components witch deteriorate the quality of the delivered energy, increase the energy losses as well as decrease the reliab~lityof a power system. In some cases, large converters systems generate not only characteristic harmonics typical for the ideal converter operatjon, but also considerable amount of noncharactenstic harmonics and interharmonics which may strongly deteriorate the quality of the power supply voltage [I]. Interharmon~csare defined as noninteger harmonics of the main fundamental under consideratjon. The estlmation of the components is very important for control and protection tasks. The design of harmonics filters relies on the measurement of distorlions in both current and voltage waveforms. There are many different approaches for measuring harmonics, like FFT, application of adaptive filters, artificial neural networks, SVD, higher-order spectra, Authors arc wlth the Institute of Rlcctr~cal Engineering Fundamentals, Wrnclaw Un~verrityof Tcchnolog, 50-370 Wmclaw Poland, c-mall: [email protected] wrm.pI etc [2,3,4,5]. Most of them operate adequately only in the narrow range of frcquencres and a t moderate noise levels. The linear methods of system spectrum estlmation (Rlachan-Tukey), based on the Fourier transform, suffer from the major problem of resolution. Because of some invalid assumptions (zero d a ~or repetitive data outside the durat~on of observation) made in these methods, the est~rnated spectrum can be a smeared version of the true spectrum [6, 71. These methods usually assume that only harmonics are present and the periodicity intervals are fixed, while periodicity intervals in the presence of lnterhmonics are variable and very long [I]. It is very important to devclop better tools of parameter estimation of signal frequency camponen$. In the case of power frequency converter the periodicity intervals are unknown. Identification of some power converter faults can be a difficult task, especially in under-load- conditions. Different faults cause specific additional distortions of voltage and current waveforms. Detection o f add~tionalfrequency components can be used for fault identification. In this paper the frequencies of slgnal components are estimated usmg the Prony model. Prony method is a technique for modelling sampled data as a linear combmation of exponentials. Al~hough i t is not a spectral estimation technique, Prony method has a close relationship to the least squares linear prediction algorithms used for AR and ARMPL parameter estimation. Prony method seeks to fit a deterministic exponential model to the data in contrast to AR and ARMA methods that seek to fit a random modcl 20 the second-order data statistics. TI. PRONY METHOD Prony method is a technique for extracting sinusold or exponential signals from time serles data, by solving a set of linear equations. Assuming the N complex data samples w[ll ..., x [ ~ ] the investigated function can be approximated by M exponential functions: k=l n = 1,2, ..., )V Tp- sampling period, where AL - amplitude an - damping factor, ok- angular velocity vn - ~nitialphase. The discrete-time function expressed in the form may be concisely t=l m=O pp where hk = A,e''& , z, = e The estimation problem bases on the rn~nimizationof the squared error over the data values: -Pi M where E [n] = x[n] - v[n]= x 'Theright-hand sulnmation in (10) may be recognize as polynorn~aldefined by (81, evaluated a1 each of ils mots zk yielding the zero result- h ]- hi z;-' (4) k-l This turns out to be a difficult nonlinear problem. It can be solved using the Prony method that utilizes linear equation solutions. If as many data samples are used as there are exponential parameters, then an exact exponential fit to the data may be made. Consider the M-exponent discrete-time function: The equatron can be solved for the polynvm~al coeficients. In the second step the roots of the polynomial defined by (8) can be calculated. The damping factors and sinusoidal frequenc~esmay hc determined from the roots z k For pract~calsituations, the number of data points N usually exceeds the minimum number needed to fit a model of exponentials, i.e. N > 2M . In the overdetermined data case, the linear equatlon ( 1 I ) must be modified to: m=O The estimation problem bases on the minimization of the total squared error: n=M+l The M equations of ( 5 ) may be expressed in matrix from as: The matrix equation represents a set of linear equations that can be solved for the unknown vector of amplitudes. Prony proposed to define the polynomial that has the zt exponents as its roots: When estimating the parameters of the srgnaI components, in the case of heavy distorted waveforms, we identify apart horn some sinusoidal components, also some components with relativ great damping factors. In reality they do not exist, but are caused by noise and computation errors. To eliminate the components we divide the estimated amplltudcs by damping factors and choose the components 1~1ththe biggest results of the division. The investigations have been carried out using also the modified Prony algorithm [&I, which maximises the likelihood of the fitting problem. For different numbers of the estimated exponential components M (5...AJZ, changed by step of 5) the square error of the approximation was cdculated, As optrmal was chosen the number whrch assures the minimal error. From the calculated components, the components with the snlallest relation AJa has been neglected. The polynomial may be represented as the sum: III. INDUSTRIAL FREQUENCY CONVERTER (8) Shifiing the index on ( 5 ) from n to n-m a"d muEtiplying by the parameter ~ [ myield: ] (9) The (9) can be modified into: (ID) The investigated dnve represents a typical configuration of industrial drives, consisting o f a three-phase asynchronous motor and a power converter, composed of a single-phase halfcontrolled bridge rectifier and a voltage source converter [Fig. 1) ( 1 01. Thecurrents waveforms at the converter input (I-input), at the converter ot~tput(I-output)as well as in the intermediate circuit (1-internled), during normal conditions and under fault conditions (capacitor or switch failure) were investigated usmg the Prony and FFT methods. The main frequency of the wavrform at thc converter input was 50 Hz, and at the converter oulput 40 Hz. Figs 2, 3 and 4 show the current waveforms In the in~ermediatecircuit (I-interned). Under fault free nor- Fig. 1 Industrial convener drive 0 -1 0.015 Time is] 0.01 0.005 0.02 0.025 L - 0 b - 0.01 i - _ - L - - 0.02 0.03 Time Is3 0.04 0.05 lr .... 1.5 : - - - - - - - . . i N - - 0 1000 2000 3000 4000 Frequency [Hz] C t . 0 500 1000 Frequency [Hz] 1500 1.5 I Frequency [Hz] Frequency [Hz] Fig. 2. Cumnt In thc ~ntcrmed~atc clrcu~t(I-~nterrned),undcr normal conditions (a), investiption results: Prony N=250, M=12O @); FFT N=250 (c), f,=10 kHz Fig. 3. Current ~nthc intcrmcdiatc circuit (I-~ntcrmcd),after capacltar failure (a). investigation results Frony N=250, M=105 (b), FFT N=250 (c), fp=5kHz ma1 conditions (Fig. 2), apart from dc component, the following frequencies have been detected, when applying the sampling window equal to 25 rns.,: ca. 240, 720, 970, 1460, 1700, 2400, 3100 Hz. Capacitor or switch failure changes significantly the current wavefornl in the intermediate circuit (Figs. 3, 4, 5). Figs. 3 and 5 show the investigation results for [he sampling window equaI to 50 ms. For comparison, Fig. 4 shows the results for the sampling window equal to 30 ms. In the first case the results arc more exact. Additional frequency compunent havc been detccled. In thc case of capacitor failure (Fig. 3) 80 and 640 Hz and in the case of switch failure (Fig. 5) 40 Hz. Detection of the component indicates a fault. 1Y.CONCLUSIONS It has been shown that a high-resolution spectrum est~mationmethod, such as Prony method could be effectively used for estimation of the frequencies of signal components. The accuracy of the estimation depends on the slgnal distortion, the sampling window and on number of samples taken into the esti~nationprocess. ol0 b - I 0.02 0.01 7-: 0.03 Time [s] . --. -! r-.-.-..---- I Frequency [Hz] Frequency [Hz] 21- --- 500 Time [s] -..-,----- 1000 1500 Frequency [Hz] ---- 2000 Frequency [Hz] F i g 4. C w e n t in the lntcrmediate circuit (I-intcrmed), aftcr capacitor fa~lurc(a), ~nvcstigationrcsults: Prony N=150, M=?5 @); FFT N= 150 (c), f,=5 kHz Fig 5 . Current In thc intermediate circuit [I-intcmcd), aftcr switch failure [a)$ investigation rcsuIts: Prony N=5OO, M=250 (b); F R N=500.(c), f,=10 kHz The proposed method was investigated under different conditions and found to be a variable and efficient b o l for detection of all higher harmonlc existing in a slgnal. It also makes it possible the estimation of frequencies of interhmonics. Detection of some non-characteristic signal components can be used for identification of frequency converter f a d t s. [4] f.Loboq, T. Korina and H.J. Koglin, "Powcr System Harrnonlcs Estrmation Using Linear Least Squares Mclhod and SVD", IEE Pmc. on Gem Transrn, and Drstrib., vol. V. REFERENCES [ I ] R Carbone, D. Menniti, N. Sorrcntino, and A. Tcsta, "Itcmtrvc H m o n l c s and Intcrhanwn~c Analysts in Mult~convcncrIndustrial Systems", qhIn!. 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