Steven Boggs

Partial discharge: overview and signal generation

It is shown how a partial discharge (PD) within a test sample generates a signal that can be measured outside the apparatus under test. Because the transfer function between the phenomenon within the apparatus under test and the signal that can be measured can vary widely, measured partial-discharge magnitudes rarely have absolute meaning and are often referred to as effective partial-discharge magnitude to acknowledge this uncertainty. The most common sources of PD are floating components, corona, and voids. Each of these is considered in turn, and the fundamentals of how the phenomenon generates a current in the external circuit are explained.<<ETX>>

Fundamentals of partial discharge in the context of field cable testing

An article providing background on fundamentals of partial discharge, especially as related to field PD testing of cable systems.

Partial discharge. XXII. High frequency attenuation in shielded solid dielectric power cable and implications thereof for PD location

PD testing and location is an essential part of factory testing of shielded solid dielectric power cable. The accuracy with which a PD source can be located is limited by high frequency attenuation of PD pulses as they propagate through the cable.

The case for frequency domain PD testing in the context of distribution cable

Frequency domain PD testing offers a rich set of PD characterization tools, the most basic of which are described in the article. PD detection sensitivity under field conditions is one to two orders of magnitude greater than for time domain testing as a result of (i) no need to trigger on the first PD pulse above the broadband noise, and (ii) the filtering effect of the cable between the PD detection site and the terminations. As a result of this greatly increased sensitivity and the rich set of characterization tools, frequency domain PD testing has been developed into a highly sensitive and reliable tool for characterizing the condition of distribution cable during normal operation, the sensitivity and accuracy of which have been confirmed through independent testing.

Diffusion of low molecular weight siloxane from bulk to surface

Silicone-based materials for outdoor insulators have the advantage that low molecular weight (LMW) components migrate through the material and coat the surface, thereby restoring hydrophobicity over a period of hours. By measuring the IR absorption of siloxane migrating to the silicone surface through a thin carbon coating, the time constant for migration was calculated. According to the time dependence of IR-absorbance, the time constant for migration ranged from 1.6 to 5.3 h depending on alumina trihydrate (ATH) filler concentration.

Effect of Alteration of Antioxidant by UV Treatment on the Dielectric Strength of BOPP Capacitor Film

The effect of altered products of the antioxidant by UV irradiation on dielectric breakdown strength of metallized biaxially-oriented polypropylene (BOPP) capacitor film was studied. Results from Weibull statistical analysis show that an optimal UV exposure level of ~3.4 J/cm2 (200-400 nm) increases the breakdown strength at 5% probability by ~20%. As indicated by UV spectroscopic analysis, the antioxidant in BOPP was consumed almost entirely to form 2,6-di-tert-butyl-p-benzoquinone (2,6-DTB-PBQ). Density functional theory computations indicate that this compound is electronegative, which probably accounts for the increased dielectric strength.

Partial Discharge Pulse Propagation in Shielded Power Cable and Implications for Detection Sensitivity

Boggs and Stone (1982) defined the fundamental limits to the electrical detection of corona and partial discharge (PD), i.e., wideband detection of a PD-induced pulse in the presence of thermal noise. This paper treated the effect of frequency-dependent attenuation in shielded power cable in that context. However, most of the plots in that paper were the result of numerical computations. In the same year, Stone and Boggs set out a theory for high-frequency attenuation of shielded power cable. They showed good agreement between attenuation predicted from measured material properties and measured, high-frequency attenuation of shielded power cable. Since 1982, measurements of high-frequency cable attenuation have been reported by a number of authors for a variety of cables. In addition, software tools have become available that facilitate an analytic solution for the parameters of interest. This article summarizes the theory for PD propagation in shielded power cable for both symmetric (Gaussian) and asymmetric PD-pulse waveforms, based on the assumption that the attenuation constant (dB/m or Nepers/m) of the cable is proportional to frequency. This appears to be the most complete possible analytic exposition of PD attenuation in shielded-power cable, which has obvious applications to field PD measurements of such cable.

A rational consideration of space charge

Space charge formation and its increase with field under DC conditions is not a mystical phenomenon. The increase in space charge with applied field is an obvious and inevitable result of the interaction of field-dependent current density with spatially inhomogeneous resistivity. For common polymeric dielectrics, the current density makes a transition from a linear function of field to an exponential function of field at around 10 kV/mm. This causes a similar transition in the sample space charge in the same field region. However, this transition has no obvious connection with aging, and if it does prove to have a connection, the driving force is the rapidly increasing current density and not the space charge, which is a result thereof. As to the issue of the accuracy of space charge measurements, it may be more important to focus on providing a physical mechanism for 1-eV deep traps at an average separation of 3 nm within a polymeric dielectric. This would bring about the ability to engineer important dielectric properties as well as improved understanding of the physical basis of aging and other important phenomena in dielectrics.

Effect of Insulation Properties on the Field Grading of Solid Dielectric DC Cable

The development of solid dielectric dc transmission class cable is a priority throughout much of the world, to avoid risks associated with placing hydrocarbon fluids in underwater environments. The conductivity of polymeric solid dielectrics tends to be a strong function of temperature and electric field, however. Based on measured material properties, we demonstrate the effect of such dependencies on the field grading of dc cable for the range of measured material properties and provide an analytical approximation for computing the field of resistively graded dielectrics, including the effect of temperature and field-dependent conductivity.

Partial discharge. II. Detection sensitivity

For pt.I see ibid., vol.6, no.4, p.33-9 (1990). Options for detecting and characterizing the signal generated by a partial discharge (PD) within a test piece or test configuration are examined. Important parameters considered include minimum detectable signal (PD sensitivity), circuit time constants that limit PD waveform characterization (where this may be desirable), and general applicability of the detection scheme. In general, it is found that detection schemes that are most sensitive tend to be somewhat application specific, whereas detection schemes that are of general applicability tend to sacrifice some sensitivity.<<ETX>>