Abstruct Previous researchers have shown, using FFT spectrum analyzers, that arcing causes a significant increase in the third harmonic content of the leakage current of polluted insulators. This paper investigates the feasibility of using modern high resolution digital spectral analysis methods to obtain better spectral estimates. Techniques discussed include the classical periodogram, correlogram, autoregressive (AR) and autoregressive-moving average (ARMA) methods. The leakage current was continuously acquired and stored (for about 30 mins) during flashover tests. The analog waveform was then digitized off-line and the digitized samples used to compute the power spectral density (PSD). It was observed that the relative magnitudes of the odd harmonics in the leakage current increase with increase in arcing intensity. In particular the third harmonic PSD increased from almost zero at the start of the experiment to about 20% of the fundamental frequency just prior to flashover.K m r d s : insulators, pollution, leakage current, signature analysis, power spectral density
L INTRODUCTIONNear industrial and coastal areas contamination is a frequent cause of insulator flashover, most cases of which result in lengthy outages. The insulators have to be washed or cleaned before the restoration of the service. These problems occur both in transmission lines and substations. The measure of pollution severity and the prediction of flashover was investigated by several researchers [2]. Arizona State University performed laboratory studies where the leakage current of artificially polluted insulators was monitored and the surface discharge photographed [ 11. The leakage current was analyzed using FFT. It was found that the surface discharge has distinctive stages of development and that the upcoming flashover modifies the harmonic content. These findings suggested that the frequency signature of the leakage current may indicate the imminence of flashover.Reliable spectral estimates of the harmonics were required in order to quanm the amount of harmonic content that might indicate the certainty of flashover. Although the FFT is computationally efficient, one pertinent limitation is the implicit windowing of the data. Windowing causes 'leakage' in the frequency domain, i.e. energy in the main lobe of a spectral response leaks into sidelobes, distorting other spectral responses. Also the resolution at the spectral peaks is very much degraded.The objective of this paper is to introduce some of the modern digital spectral estimation methods available, more specifically time-series (parametric) modeling which alleviates the limitations of the FFT and the frequency signature obtained through those methods. The generaI spectral analysis problem reduces to that of computing a set of AR parameters and subsequently the power spectral density (PSD) or equivalently, computing the autocorrelation sequence (ACS) of a random process and Fourier transforming to obtain the PSD.
IL MEASUREMENT SYSTEM FOR LEAKAGE
CURRENTThe experimental setup co...
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