Achieving acceptable levels of sensitivity during online and/or onsite partial discharge (PD) measurements still continues to remain a very challenging task, primarily due to strong coupling of external (random, discrete spectral and stochastic pulsive) interferences. Many analog and digital approaches have been proposed for suppressing these interferences, and amongst these, rejection of the pulsive type of interferences is known to be very difficult, if not impossible. The time and frequency characteristics of the pulsive interference being very similar to that of the PD pulses is the main reason posing difficulty in their separation. In this paper, a novel, semi-automatic, and empirical wavelet-based method (using multi-resolution signal analysis) is proposed to recover PD pulses, buried in excessive noise/interference comprising of random, discrete spectral, pulsive, and any combination of these interferences occurring simultaneously and overlapping-in-time with the PD pulses. A critical assessment of the proposed method is carried out, by processing both simulated and practically acquired PD signals. The results obtained are also compared with those from the best digital filter (infinite impulse response, IIR and finite impulse response, FIR) method proposed in literature. From the results it emerges that, the wavelet approach is superior and further, has the unique capability of successfully rejecting all the three kinds of interferences, even when PD signals and one or all interferences occur simultaneously and overlap-in-time.

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Wavelet-based denoising of partial discharge signals buried in excessive noise and interference

A critical study of the performance of several digital filters for rejecting discrete spectral interference (DSI) in partial-discharge (PD) testing is reported. The filters are evaluated with particular reference to distortion introduced on successive PD pulses. A digital filtering method based on a cascaded second-order IIR lattice notch filter is proposed. The study shows that, for similar performance, methods based on linear prediction require longer filter orders when compared to cascaded IIR notch filters. The nonadaptive methods are found to be stable against impulsive disturbances, while adaptive methods are unstable. The method proposed gives the best performance, low distortion to a sequence of PD pulses, good stability, and fast filtering time. The performance at higher sampling rates, even with PD pulses of long widths, is also found to be superior. The method is implemented using fixed-point arithmetic leading to faster processing. Hence, the method is suitable for on-site PD measurements. >

/pdf/evaluation-of-digital-filters-for-rejecting-discrete-2c11t8q2yd.pdf

Evaluation of digital filters for rejecting discrete spectral interference in on-site PD measurements

Modern aircraft are using more electrical equipment and are therefore using higher voltages. These higher voltage systems will become increasingly common in the future. The forms of systems that are being used operate under frequencies/waveshapes different from those normally used on ground based electrical power systems, and the testing regimes are relatively undeveloped in comparison with the other standardized tests available for aerospace components. The reasons why higher voltage systems in aircraft may be susceptible to partial discharge are first introduced. Then two electrical test techniques that can be used to measure partial discharge activity are introduced. One of these, the balanced technique, is used to examine the partial discharge performance of four test objects that are stressed with high voltages at varying frequencies of both sine and square waveforms. The results that are presented show that, while the level of damage that can be attributed to partial discharges can be significantly increased as frequency rises, the inception voltage of such discharges does not significantly vary.

Partial Discharge Testing of Aerospace Electrical Systems

The Partial Discharge (PD) signals, which are characterized by high frequency current pulses, are strongly coupled with external noise. In on-site PD measurement de-noising this PD signal from noisy environment is one of the major challenges. This paper proposes a new nonlinear threshold function for de-noising PD signal using wavelet transform (WT) technique. This new threshold function is continuous as the soft threshold function and also uses the unused coefficients (i.e. which are less than threshold value) which are set to zero in different threshold function. This method first applied on two different type PD signals with White Gaussian Noise (WGN) and simulation results shows that new threshold function can give better performance by changing the variable parameter. Improved signal to noise ratio (SNR) and correlation coefficient indicate that the performance of this method is better than other de-noising technique. Finally, a field detected signal is tested and the de-noised signal indicates that method is effective and efficient for this application.

A new threshold function for de-noising partial discharge signal based on wavelet transform

The resolution and robustness of a cable PD locating instrument, based on time-domain reflectrometry (TDR), are critically affected by noise present in a field environment. The permanent background noise encountered during field tests has been shown to be mainly due to amplitude modulation (AM) radio signals. This paper describes a self-learning digital filter aimed at mitigating this noise. The filter, in conjunction with a signal averaging routine, has been successfully used to reduce noise and enhance the PD signal obtained from a cable in a field environment. Examples are provided to demonstrate the ability of the filter to significantly improve the signal-to-noise ratio (SNR) and effectively extract PD signals from field recordings. >

http://ieeexplore.ieee.org/iel3/3187/9053/00401520.pdf

Self-learning digital filter for the field location of partial discharge in cables

A PD (partial-discharger) measuring device to improve on-site measurements has been developed that is based on a digital filtering algorithm. The adaptive filter reduces periodic interference, e.g. from broadcasting stations. The filter principle is based on a fast Fourier transform that it suppresses external sinusoidal disturbances effectively. Even partial discharges with an apparent charge less than the basic interference level can easily be detected. >

J. Ott

Papers

An adaptive filter algorithm for on-site partial discharge measurements