Nuclear safety: operational aspects. 4. Application of Wavelet Transform for Cable Aging Assessments

An aging assessment is an evaluation of appropriate information to determine the effects of aging on the current and future ability of systems, structures, and components to function within acceptance criteria. The aging of electrical insulating material has been recognized for many decades as a prime cause of the premature failure of a wide variety of plants; consequently, it is a topic that has received considerable attention. From the point of view of nuclear safety, the interest in these cables derives from their vital importance in providing the links between the transducers and the instrumentation and control systems that monitor and control the nuclear power plant. Most conventional cable aging assessment techniques mainly involve processing information based on the feature extraction of cable aging. Several researchers in recent years have presented results aimed at assessing cable more effectively. Hitherto, the algorithms developed have included the OIT/OITP method, the indenter method, the EAB method, and the TGA method. However, each of these techniques improves cable aging assessment to a certain extent, but each has its drawbacks as well. Most of the techniques developed to date have not been tested sufficiently to determine their effectiveness under different systems and conditions, which can be widespread. The wavelet transform is useful in analyzing the transient phenomena associated with cable cracks. It provides time information; it has the attribute of very effectively realizing non-stationary signals consisting of both low- and high-frequency components through the use of a variable window length of a signal. This paper describes a new cable aging assessment technique that involves capturing the current signals generated in a cable line under partial discharge; it is shown that the technique improves the performance of aging assessment by employing the absolute sum value based on the discrete wavelet transform (DWT). The detection process is performed through signal decomposition, thresholding of the wavelet transform coefficients, and duration time. Threshold value is determined by weighting the absolute sum value for one period in a moving window scheme. The performance is then examined for partial discharge signals under a variety of different systems and conditions encountered in practice. The wavelet transform normally consists of both the analysis and synthesis wavelet pair. Synthesis is used for waveform reconstruction. In the analysis process of the wavelet transform, the given signal is decomposed into its constituent wavelet sub-bands or levels. Each of these levels represents that part of the original signal occurring at that particular time and in that particular frequency band. These individual bands tend to be of uniform width, with respect to the log of their frequencies, as opposed to the uniform frequency widths of the Fourier spectral bands. The algorithm of multi-resolution decomposition is illustrated in Fig. 1, where three levels of decomposition are taken as an example for illustration. The details and approximations of the original signal S are obtained by passing it through a filter bank, which consists of low- and high-pass filters. A low-pass filter smoothes out the high-frequency components, while the high-pass filter picks out the high-frequency contents in the signal being analyzed. Under many circumstances, partial discharge (PD) in operating cable is intermittent; activity may depend on temperature, random vibrations, etc. Under these circumstances, electrical and acoustic tests are of limited value, as the PD source, even if suspected, may have a small duty cycle and is unlikely to be present during any scheduled field test. Such a problem may become known as a result of a PD-induced failure, which suggests that intermittent PD activity may be present in other similar locations. The study of partial discharge in cable is one of the most important subjects in the field of aging assessment in electrical cable. Much attention is being paid to PD detection to ensure sufficient reliability of cable when it is widely in service. To aid the development of the PD detection technique using the DWT, wavelet transform realization has been employed that determines a coefficient of d1 using different mother wavelets from an actual PD signal. The mother wavelet (Fig. 2) depicts the coefficient of d1 for different mother wavelets with the DWT realization. The behavior of the DWT for this actual PD signal is illustrated in Fig. 2; as expected, all the coefficients of d1 increase on PD inception, and there are small discernable differences in the DWT outputs for the five different mother wavelets considered. (authors)