Search

This article illustrates a method for the estimation of apparent trap-controlled mobility of charges (i.e., the mobility of charges trapped at different states due to chemical-physical features) in polymeric insulation, based on the results of space charge measurements. The information provided by the so-called depolarization characteristic, obtained measuring space charge with short-circuited electrodes after a given poling time, is used for this purpose. Two approximate expressions for apparent-trap controlled mobility estimation are compared, the first based on rough assumptions (essentially unipolar injection and uniform charge density within insulation thickness), the second obtained following an approach based on space charge profile conservation under depolarization. These expressions are applied to the results of space charge measurements performed on unaged and aged insulating polymers. The two approaches provide apparent mobility estimates versus depolarization time in substantial agreement, and of the same order of magnitude of apparent mobility data reported in the literature for similar materials. This seems to support the validity of the key idea of the work, i.e., deriving apparent trap-controlled mobility estimates from space charge observations, particularly when such estimates are used to compare different materials or to infer the degradation level of aged insulation for diagnostic purposes. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The aim of this paper is to infer partial discharge (PD) detection limits in extruded cables as a function of the distance between PD source and detection point. The investigation is carried out for different types of detectors, from IEC 60270-compliant to ultra-wide bandwidth systems. For this purpose, a medium voltage cable has been modeled as a transmission line with frequency-dependent parameters accounting for skin effect, dielectric and semicon characteristics. Pulse propagation through the cable has been predicted for very tong distances. Different filters have been used to simulate different detectors, emphasizing the behavior of these systems, particularly the features of sensitivity, noise rejection, PD source separation and PD source localization. [ABSTRACT FROM AUTHOR]

A new, physical, aging and life model for damage inception and growth in polymeric insulation starting at the level of microscopic cavities has been presented in Part 1, by illustrating extensively its theoretical background and derivation. In the present paper a parametric analysis, relevant to a polyethylene-based compound, is carried out that illustrates the sensitivity of the model to technological and material parameters, first of all applied electric field, temperature, conductivity and cavity size. In addition, fitting to experimental times-to-failure relevant to HVDC cable insulation is discussed in order to confirm soundness and validity of the proposed approach. [ABSTRACT FROM AUTHOR]

A comprehensive physical aging and life model is presented for damage inception and growth in polymeric insulation, starting at the level of microscopic cavities, that are known to be one of the most severe factors limiting design electric field and life of cable insulation. Degradation within a polyethylene-based material for HVDC cables is considered as a hot-electron induced bond-breaking process that cumulates with time into the polymer matrix at microvoid-polymer interface, fed by subsequent electron avalanches generated in microvoids. By this way, a conductive pit is formed, that may exceed eventually a critical size for the start-up of a rapid failure mechanism, thereby leading ultimately to electrical breakdown. The model, essentially based on the physical and microstructural characteristics of the insulation, is virtually free from disposable parameters. It is shown to reproduce qualitatively the dependence of insulation time to failure on applied field, temperature and cavity size and to fit quantitatively experimental times-to-failure relevant to PE-based materials in typical working conditions for HVDC cables. The first part of this paper presents the theoretical background and the derivation of model equations, as well as their discussion. The second part deals with parametric investigation and data fitting. [ABSTRACT FROM AUTHOR]

The article discusses the measurement and analysis of partial discharges (PD) as the main and the most effective tool in discovering local damage and flaws in cable systems. The methods of separation which is based on the deterioration of pulse frequency and identification resulted in improved reliability of PD assessment. Therefore, it offers information that can be used to develop maintenance procedure plans which is key in cutting operational costs in utility cable network.

Topics related to space charge phenomenology, particularly the origin of space charge and its interaction with the host insulating polymer, have been widely discussed in recent years. Due to the development of reliable techniques for direct observation of space charge it is now possible to obtain, besides obvious indications on space charge and electric field magnitude and location, quantities that provide additional information on space charge accumulation and depletion rate, apparent mobility and trap depth distribution. Therefore, commonly debated topics such as the nature of charge carriers, the conduction mechanism, the charge dynamics at interfaces and the relation with nanostructure can be re-examined on the basis of space charge observation and this is, indeed, the purpose of this paper. [ABSTRACT FROM AUTHOR]

This paper presents a discussion of the concept of threshold for trapped space charge accumulation in solid dielectrics submitted to a dc field. The starting point is the fact that it is often possible to define a critical field separating an ohmic type of conduction from a non-linear regime in the current-voltage characteristic of solid dielectrics. In the space charge limited conduction theory, this critical field corresponds to the onset of space charge accumulation. However, other conduction processes, such as hopping conduction for example, can also explain non-linearity in the current-voltage characteristic, which does not involve space charge. It is proposed to check for the existence of a critical field for space charge accumulation using complementary techniques, i.e., space charge detection and electroluminescence techniques. Polyethylene, polyester and polycarbonate were investigated as being representative of three different families of polymers. It is shown that similar values of thresholds are found for a given material using the three above-mentioned techniques, lending support to the physical explanation of a threshold for trapped space charge accumulation. [ABSTRACT FROM AUTHOR]

Luminescence experiments have been used to probe the nature of trapping sites and the dynamic behavior of charge carriers in low density polyethylene (LDPE), antioxidant containing LDPE (LDPE + AO) and cross-linked polyethylene (XLPE). It is shown that deep traps of different natures are present in these materials. For example carbonyl functions provide deep traps in LDPE. New traps are seen in LDPE + AO and XLPE, associated with the anti oxidant itself or with its reaction products, and to the by-products of the cross-linking reaction. Among those, acetophenone, cumyl alcohol and α-methylstyrene cannot account for the observed spectra. It is shown, in addition, that two XLPE compounds based on two different base resins exhibit the same behaviour demonstrating that the chemical and/or structural modifications introduced by the cross-linking process itself control the charge storage in XLPE. The electrical properties of the same materials are discussed in the light of the work reported in Part 1 (in this series of two papers). [ABSTRACT FROM AUTHOR]

Applications of a new methodology, aimed at the identification of defects occurring in insulation systems of HV apparatus and based on partial discharge (PD) measurements, are presented in this paper. This methodology relies upon the digital acquisition of a large amount of PD pulses and separates the acquired pulses into homogeneous sub-classes. Signal processing tools recognize the presence of noise among the different classes. Identification of basic PD source typologies (i.e., internal, corona and surface discharges) is then achieved, resorting to fuzzy algorithms. The proposed procedure is applied to measurements performed on different HV apparatus, such as cables, transformers and rotating machines. The purpose of this paper is to show that the identification process is robust, regarding the measuring circuit, and flexible, so that it can constitute an advanced tool for condition based maintenance, guiding maintenance experts in making decisions on the condition of the insulation system under test. [ABSTRACT FROM AUTHOR]

An experimental investigation of the behavior of partial discharges occurring in insulation cavities as a function of light irradiation conditions is presented in this paper. Partial discharge inception voltage (PDIV) measurements were performed for different irradiation conditions of the test objects (visible light, of variable intensity, ultra violet (UV) light or absence of irradiation), defect geometry and conductivity of insulating material. It is shown that changes of such factors involve con siderable alteration of partial discharge inception features, in terms of both inception voltage and delay time. Depending on applied field, defect geometry, insulating material characteristics and irradiation condition, in fact, partial discharge inception voltage may be influenced prevailingly by background radiation, as generally stated in literature, or by field-assisted electron detrapping from material. cavity interface. [ABSTRACT FROM AUTHOR]

Polymer nanocomposites are defined as polymers in which small amounts of nanometer size fillers are homogeneously dispersed by only several weight percent- ages. Addition of just a few weight percent of the nanofillers has profound impact on the physical, chemical, mechanical and electrical properties of polymers. Such change is often favorable for engineering purpose. This nanocomposite technology has emerged from the field of engineering plastics, and potentially expanded its application to structural materials, coatings, and packaging to medical/biomedical products, and electronic and photonic devices. Recently these `hi-tech' materials with excellent properties have begun to attract research people in the field of dielectrics and electrical insulation. Since new properties are brought about from the interactions of nanofillers with polymer matrices, mesoscopic properties are expected to come out, which would be interesting to both scientists and engineers. Improved characteristics are expected as dielectrics and electrical insulation. Several interesting results to indicate foreseeable future have been revealed, some of which are described on materials and processing in the paper together with basic concepts and future direction. [ABSTRACT FROM AUTHOR]