D. E. Cooper

Thermal current transients in polyethylene

Abstract A thermal current transient technique, using a step input of radiations, has been employed in this work to study the nature of induced polarization in untreated and hexanetreated (commercial) low-density polyethylene film as a function of charging fields. The results show that for charging fields smaller than 4.0 × 10 7 V m −1 the current transients may arise from a presence of impurity ions, and at fields greater than 4.0 × 10 7 V m −1 injected space charge becomes the dominant contributor.

Role of Atmospheric Gases in the Space Charge Polarization of Low-Density Polyethylene in a Divergent AC Field

Charge injection into lowdensity polyethylene by a divergent ac field at room temperature has been investigated using a high-resolution ther mally stimulated discharge current technique. It was observed that TSDC spectra obtained after charging with ac fields were dissimilar to those obtained with dc fields of similar magnitude and were influenced by sam ple history and by the nature of the gaseous charging environment. Thoroughly outgassed samples charged in a high vacuum showed little change in charge release characteristics for ac and dc charging fields. Evidence of additional large polarizations was found in samples charged with ac fields only in a gaseous environment. It is suggested that this is due to the trapping of electrons by ionized gas molecules adsorbed by the polymer. The presence of oxygen gas initiated irreversible changes in the polarization behavior of LDPE by interaction with an ac field.

Non-uniform polyrization effects in D.C. stressed low-density polyethylene

Abstract Thermal current transients have been measured in untreated and hexane-treated low-density polyethylene as a function of electrical pre-stress. The results indicate a significant non-uniform polarization which is moderated by increasing fields in a complex manner.

Charge injection and storage in cross-linked polyethylene insulated cable

The authors present measurements of the trapped charge in short samples of commercial XLPE (cross-linked polyethylene) insulated power cable. Charge injection was investigated for both AC and DC electric stressing of the samples, for various conditions of temperature and gaseous environment. The injected space charge was examined using three techniques: thermally stimulated discharge current (TSDC), field-assisted TSDC, and the thermal transient current method. These results are compared and the viability of the latter, nondestructive technique to monitor the process of aging in commercial cables is discussed. The presence of a space-charge peak at 60 degrees C in prestressed XLPE insulation is clearly seen using the TSDC method. Field-assisted TSDC illustrates strong ionic conduction at higher temperatures.<<ETX>>

Anomalous polarization in polyethylene with A.C. charging

Abstract Thermal current transients in low density polyethylene have been measured as a function of a.c. electrical stress and its duration. The observed transients are similar in nature to those obtained with d.c. charging and indicate a presence of non-uniform polarization which is moderated by increasing fields and the charging time. The non-uniform polarizations are produced by a.c. charging fields, possibly as a result of the inhomogeneous nature of a virgin sample. Theremal treatment cannot completely erase the sample memory.

Thermally stimulated discharge current spectra from electrically stressed high voltage power cables

Cross-linked polyethylene is employed as the primary insulation in high voltage power transmission cables although its performance is known to deteriorate after a number of years in service. The origin of this ageing effect has been the subject of intense research interest and still remains largely unknown.

Dielectric behaviour of hydrated and electrically stressed cross-linked polyethylene in a power distribution cable

Cross-linked polythene (XLPE) cables are being used increasingly for high voltage power distribution because of their enhanced mechanical properties over traditional pressure assisted dielectric cables1,2. However, the expected service life of XLPE 15–36 kV distribution cables is yet to be achieved in practice. It has been suggested trial a premature failure of XLPE cables may be avoided by modifying the cable construction to prevent moisture ingress during operation. In general, water has very little effect on lyophobic organic hydrocarbons unless they contain impurities. However, there have been breakdowns of polythene and other hydrocarbon cables at low A.C. Stress under water due to the growth of ‘water trees’ for which there has been no definitive explanation as yet. The pressure of traces of salt, acids, alchohols or any substance which ionises in solution in water will be expected to worsen the effects of moisture. A reduction of electrical breakdown strength due to a pressure of moisture cannot be explained without a better-understanding of the dielectric polarisation process in such circumstances. With the above in view, we have undertaken in our laboratories a study of the electrical aging of XLPE distribution cables, immersed in water.

Field assisted thermally stimulated current in low-density polyethylene

The nature and quantity of release of injected charges in electrically pre-stressed low-density polyethylene (LDPE) have been studied by a technique in which the discharge current is monitored in the presence of a small drawing voltage while the temperature is slowly increased. Dielectric relaxation strength of (i) virgin, (ii) heat treated and (iii) heat-treated and electrically stressed LDPE have also been measured. The results indicate that at a charging field of 5.3 × 107 V m−1 and at a saturation drawing field of 2 × 106 V m−1 the injected charge density is significantly less than the initial defect and/or impurity density present in the virgin samples used in the present work.

An investigation of the electrical ageing in low density polyethylene

The electrical ageing effects in low density polyethylene (LDPE) may be related to an accumulation of electronic and ionic space charges in the bulk. A study of thermally stimulated discharge currents (TSDC), in which the sample is short circuited through an electrometer as its temperature is increased at a steady rate from sub-zero temperature to a few degrees below its melting point, have shown two unexpected results [1,2], The first is that the largest currents were obtained with the virgin samples (uncharged) and the second, that all samples produced a TSDC peak at the same temperature irrespective of the type and field of charging. Now the total charge released in a conventional TSDC experiment may not correspond to the actual trapped space charges due to a cancellation of a part of the charges flowing in opposite electrodes and recombination of ionic species. Furthermore, the observed current is the algebraic sum of the flow of charges of both polarities towards both electrodes in the existing internal field and, therefore, can lead to false conclusions regarding the polarity and the spatial location of the majority carriers.

High field polarisation behaviour of cross-linked polyethylene

The presence of persistent polarisation in an electrically stressed insulating polymer may originate from a significant alignment of the dipoles (structural and impurity) and injection and subsequent localization of charge carriers. A measure of the frozen-in polarisation may be obtained by several techniques including aborption current studies, thermally stimulated discharge current studies and an analysis of thermal current transients arising from a step input of thermal radiations incident on a polymer surface with electrodes shorted. Using the last technique1–3, mentioned above, it has been shown that in low density polythene (LDPE) films, the thermal current transients may arise from a presence of impurity dipoles at low electric field stressing. As the field is increased, the injected space charge becomes the dominant contributor to such current transients which are of pyroelectric nature. It has also been shown4 that the conductivity of hexane-treated LDPE (10−18 Ω−1 m−1) at room temperature is approximately two orders of magnitude lower than that of the untreated material. The thermal current transients with LDPE show that hexane treatment seems to provide a removal of impurity ions in the original polymer in such a manner that electrode injection becomes quite dominant even at moderate charging fields.