Matthew S. Mashikian

Location and characterization of partial discharge sites in shielded power cables

An instrument capable of detecting and locating partial discharge sites and faults in shielded power cables was developed on behalf of a group of US electric utilities. The instrument uses state-of-the-art electronic hardware and advanced digital signal processing techniques. The partial discharge signals are reconstructed using the cable traveling wave characteristics (transfer function), and noise is reduced both through choice of hardware and by modern signal enhancement techniques. An adjunct instrument was also developed to correlate the location of a partial discharge site along the cable to the position along the surface of the cable burial trench. The principle and the implementation of the instrumentation package are described, and results obtained in the laboratory on a 175 m, 15 kV URD cable are discussed. >

Location of partial discharges in shielded cables in the presence of high noise

Updated information on a partial discharge location system intended to pinpoint the location of defects and/or faults on a shielded buried underground cable is presented. The system is based on the principle of reflectometry. Its salient characteristics include the following two features: (a) an ability to pinpoint the location of a partial discharge source not only along the cable length, but also along the surface of the trench where it is buried; and (b) the ability to operate in a high noise environment. Recent field and laboratory data obtained on noise and techniques for its mitigation are discussed in some detail. These techniques include analog/digital notch filters for the elimination of AM broadcast noise and an adaptive digital filtering scheme which incorporates the transfer function of the cable for added accuracy. >

Evaluation of field aged crosslinked polyethylene cables by partial discharge location

Partial discharge sites were located in several field aged crosslinked polyethylene insulated medium voltage power cables, using a newly developed instrument. Subsequent AC breakdown tests indicated that, with partial discharge inception voltages lower than 3 times operating voltage, the failures occurred at the partial discharge sites. In all cases, the failures originated at the sites of long water trees which affected the cables at discrete locations rather than uniformly over their entire lengths. Attributes of a partial discharge locator capable of performing in-situ nondestructive diagnostic tests on cables are discussed and field application procedures are described. >

Signal processing techniques for partial discharge site location in shielded cables

An instrumentation package capable of locating partial discharge sites in cables has been developed. The digitized partial discharge (PD) signals recorded from one cable end consist of a sequence of pulses whose separations contain information on the relative location of the PD site. The signals are often contaminated by noise and undergo substantial attenuation and phase change as they travel though the cable and the detection system. Moreover, overlap of two successive pulses is possible if the PD site is close to a cable end. The authors describe and illustrate two techniques-maximum likelihood (ML) estimation and deconvolution-for extracting pulse separation from such a time series of noisy and ambiguous signals. Both real and simulated measurements are used to demonstrate the potential of these methods. A procedure whereby knowledge of the combined cable-instrumentation transfer function can be incorporated into the maximum likelihood technique is also discussed. The ML method appears to be much more effective in the presence of cable noise. The main disadvantage of the ML method is that the approximate width of the wavelet or basic PD pulse should be known to give the best compromise between noise smoothing and peak resolution. This width can be determined by an impulse response test or by knowledge of cable length and parameters. >

Role of semiconducting compounds in water treeing of XLPE cable insulation

The connection between cleanliness of semiconducting compounds and water tree growth at the semiconductor-dielectric interface was demonstrated. Changes to semiconducting compounds have improved the reliability of solid dielectric cables.<<ETX>>

Partial discharge propagation model and location estimate

This paper describes a partial discharge propagation model and a location estimate which uses digital signal processing techniques. The model and the estimate are developed for an instrument which is capable of locating partial discharge sites in power cables non-destructively and in situ. Since the model utilizes actual cable response to partial discharge signals, it provides an excellent waveform match between simulations and measurements. The estimator can locate partial discharge sites in noisy environments with an error not exceeding 2 m. Applications of this model to field test data are also discussed.<<ETX>>

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

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.<<ETX>>

Role of semiconducting compounds in the premature aging of XLPE cable insulation

A clean, crosslinked polyethylene insulating material was sandwiched between two parallel layers of semiconducting shield compound in test cells which simulated extruded medium-voltage cables. Four commercial and four experimental semiconducting compounds were used. Cells were aged at 60 Hz, with a stress of 2.6 kV/mm (65 V/mil) or 3.4 kV/mm (85 V/mil), with one shielding layer exposed to air and the other exposed to distilled, deionized water. The effects of water soluble impurities in the semiconducting compounds on the number, size, and location of water trees developed in the insulation, at the semiconducting shield interfaces, are discussed. The movement of soluble impurities is also addressed.<<ETX>>

Field location of partial discharge in power cables using an adaptive noise mitigating system

The digitized partial discharge (PD) signal recorded at one power cable end consists of a sequence of pulses whose separation time contains information on the relative location of the PD sites. However, noisy PD signals can lead to a large PD site estimation error. The most annoying noise encountered during PD measurements consists primarily of amplitude modulated (AM) radio signals. The performance and the resolution of the wide band PD detection system are critically affected by this noise. This paper describes an adaptive noise mitigating system (ANMS) which is aimed at effectively reducing this noise.

Modeling of lightning surge protection in branched cable distribution network

A general-purpose computer simulation program was developed to model the behavior of voltage surges traveling on power cable networks with multiple branches. The program is capable of displaying the location and magnitude of the maximum surge voltage seen by the cable. It is also capable of reproducing the voltage variations at any point as a function of time, or the voltage profile along the cable at any given time. The results obtained have not been verified by actual field measurements but are in agreement with the results presented by previous authors. The computer simulation was used to study the effectiveness of different arrester placement schemes for branchless and single-branch systems. It was shown that, with prematurely aged cables whose BIL (basic impulse insulation level) has been considerably reduced, it is necessary to place arresters not only at the riser-pole, but also at all open points. This practice reduces the maximum voltage produced at any point along the cable and can therefore also be used with new cables to retard their aging process caused by exposure to repeated voltage surges. >