The thermal step method (TSM) is a nondestructive technique which measures the distribution of the electric field and space charge density across solid insulating materials. This work first reviews the principle and the basic equations of the TSM when used on a short circuited flat insulating structure (film or plate). An evolution of the method, which allows the determination of space charge distribution in an insulator while submitted to an external dc field, is then described. The fundamentals, the experimental setup and the validation of this technique on flat samples and on power cables are presented and discussed.

This paper proposes a technique for determining the distributions of the electric field and space charge in the insulation of power cables by using the data acquired with the thermal step method (TSM). The TSM consists of applying a low-temperature step to a short-circuited or dc-energized cable and of acquiring a transient capacitive current. The processing technique described in this paper is based on a series decomposition of the electric field, the coefficients of the series being identified via the measured current. The accuracy and the stability of the calculation method are evaluated by simulations performed using various distributions of the electric field and different noise levels. An application of the technique to space charge measurements in a dc conditioned power cable is then presented.

This paper describes the evolution of electric field and space charge in high voltage AC and DC cable polymeric insulation, followed using the thermal step method.

This paper is concerned with space charge accumulation and electric field distribution at XLPE/EPDM interfaces after thermal aging. XLPE, EPDM and multilayer XLPE/EPDM/XLPE flat samples have been manufactured and aged thermally during 240 and 480 h at 105 °C. Material degradation by oxidation during aging has been assessed by FTIR spectroscopy. The conductivities and permittivities of the samples have been measured before and during ageing. Using the values of conductivity and permittivity values obtained experimentally, the charge density values at the XLPE/EPDM interfaces and the electric field distribution across a multilayer XLPE/EPDM HVDC cable joint model has been calculated for different thermal aging times. The results show that the surface charge density values vary with the ageing time (decrease at aging beginning and then increase) and determine important enhancements of the electric field, in the vicinity of the inner but also of the outer semiconductor. The simultaneous presence of the volume and interfacial charges determines a less increase of the electric field for homocharges and more important for heterocharges, both for unaged and aged samples.

The presence of space charge changes locally the electric field distribution in power cable insulation and may play an important role in tree development, thus accelerating the dielectric breakdown. This paper is concerned with the computation of the electric field in polyethylene-insulated power cables affected by water trees which grow from the following: 1) the inner semiconducting layer; 2) the outer semiconducting layer; and 3) the inner and outer semiconducting layers, taking into account the space charge corresponding to the ions present in the treeing area. Space charge in plane samples where trees have been developed in an accelerated manner was estimated using the thermal step method. Average charge values given by space charge measurements were then used for the electric field computation in cable insulation with continuous or/and individual water trees. For the calculation of the electric field, an analytical and a numerical method have been used. This paper shows that the space charge changes the electric field distribution inside and outside the trees (the field increases in some areas and decreases in others) and that the field variations depend on the magnitude and on the polarity of the space charge, as well as on the dimensions of the water trees developed in the cable insulation. The obtained results show that, in the presence of water trees and space charge, the initiation of electric trees is more probable in the case of individual water trees than in the case of continuous water trees.

The thermal step method is a nondestructive technique for determining electric charge distribution across solid insulating structures. It consists in measuring and analyzing a transient capacitive current due to the redistribution of influence charges when the sample is crossed by a thermal wave. This work concerns the application of the technique to inhomogeneous insulating structures. A general equation of the thermal step current appearing in such a sample is established. It is shown that this expression is close to the one corresponding to a homogeneous sample and allows using similar techniques for calculating electric field and charge distribution.