The authors present a review article of the properties of XLPE and an examination of the origins of space charge and how it is measured. The space charge in the insulation and the conduction current can affect the electric stress distribution in the insulation. The effect of morphology on the electrical properties of polyethylene is discussed in detail.

A general review of polymeric insulation for use in HVDC cables

This paper presents nanoscopic aspects of electronic aging in dielectrics, as the result of the interaction of excess non-thermalized carriers with the media. Hot electrons are hypothesized as the major source of non-thermal carriers and shown to be a decisive factor in electrical breakdown. The interaction of hot electrons with atomic and molecular solids forming dielectric films is investigated using high-resolution monochromatic electron beam and cryogenic techniques. In these experiments, electrons of a well-defined energy impinge on a dielectric nanometer-thick film grown at cryogenic temperatures in an ultra-high vacuum system. It is possible to observe directly the species created at specific energies by hot (i.e. low-energy) electrons within the range 0 to 20 eV; including highly reactive radicals, positive and negative ions and excited atoms and molecules, as well as the new compounds that result from non-thermal reactions. Energy losses to electronic, vibrational and phonon excitation also can be measured as a function of electron energy. The results of these experiments are reviewed. They serve to explain the creation of local defects, which then can act as inter- and intramolecular (or atomic) traps for electrons and holes in inhomogeneous media. This nanoscale information also allows one to identify the effects of the polaron and that of cumulative material damage and increases in leakage currents, observed on the macroscopic scale. More particularly, it is shown from the results of low-energy electron bombardment of slices of PE HV cables, that simpler molecular solids such as n-alkane films provide insight on the effect of 'hot' electrons in commercial insulating material. Recommendations are given for further research necessary to represent the evolution of the aging process in practical macroscopic materials of interest at the engineering level.

Nanoscopic aspects of electronic aging in dielectrics

Changes of crystallinity and unit cell parameters for a low-density polyethylene as a consequence of cross-linking by dicumyl peroxide and by electron beam irradiations are compared. Polyethylene samples cross-linked by irradiation in the solid state present variations essentially only of the a unit cell parameter while those cross-linked in the melt state present similar variations of both a and b unit cell parameters. A strict analogy with unit cell variations observed for ethene random copolymers with propene and with bulkier comonomers, respectively, is shown. This suggests inclusion of cross-links for solid-state cross-linked samples, which would lead to a gradual transformation from an orthorhombic to a pseudohexagonal phase, and exclusion of cross-links for melt state cross-linked samples, which would generate tensions at the orthorhombic crystallite interphases.

Polyethylene Unit Cell and Crystallinity Variations as a Consequence of Different Cross-Linking Processes

To study the degradation of low-density polyethylene (LDPE) and cross-linked polyethylene (XLPE) by partial discharge (PD), experiments were conducted in a nitrogen (N2) and (N2+O2) gas atmosphere. After being subjected to the PD, samples were examined using atomic force microscopy (AFM) and FTIR. The AFM images clearly revealed the creation of pits on the surface of degraded samples. A large difference in the pit pattern was observed between samples subjected to the PD in N2 and those subjected to the PD in (N2+O2). The FTIR spectra detected in samples subjected to the PD in (N2+O2) showed that carbonyl compounds were produced on the surface of degraded LDPE. Experiments were performed using an experimental system incorporating GC-MS to measure the gas components produced during degradation. Results of experiments showed that carbon dioxide (CO2) is produced when LDPE and XLPE are subjected to the PD in an oxygen-containing atmosphere. The amount of CO2 was found to increase concomitantly with the period of voltage application and oxygen concentration in the area where PD occurs. Based on experimental results, the processes of LDPE and XLPE degradation by PD were examined along with the antioxidant effect on degradation.

Degradation of low-density polyethylene and cross-linked polyethylene by partial discharge

A method and system for detecting defects in a wire (100) or wire bundle (200) includes a pair of sensors (150) wrapped around the wire or bundle, a data acquisition device (310) for monitoring pulses in the signal produced by the sensors around the wires, and a diagnostics engine (320). The diagnostics engine includes an analysis module (450) to determine features of the signals captured by the data acquisition device and a decision module (470) for determining whether the pulse represents a defect in the wire and the location of the defect. The system operates passively and can be used to monitor the wires while they are in use for carrying signals or current and does not require disconnecting the wires to be monitored.

Method and system for passively detecting and locating wire harness defects

In order to detect, localize and interpret a partial discharge occurring in a partial discharge site along an electrical equipment, two measurement probes and a synchronization probe are installed along the electrical equipment. The measurement probes detect pulses travelling in the electrical equipment while the synchronization probe detects a phase angle in the electrical equipment and is usable for calibration purposes. A control unit receives the signals sensed by the probes and conditions them. Digital processings applied on the conditioned signals, involving their correlation, a time-frequency distribution and a form factor estimation, allows establishing a diagnosis indicating a detection of a partial discharge and its localization along the electrical equipment.

Detection, localization and interpretation of partial discharge

The long-term dielectric performance of underground power cable XLPE (cross-linked polyethylene) insulation suffers from poorly understood aging phenomena. A study of the morphological modifications of XLPE due to electrical aging may provide insight for a better understanding of aging mechanisms. The AFM (atomic force microscopy) technique has been used to study XLPE morphology of unaged, laboratory-aged and field-aged samples, whereby the soft XLPE surfaces were scanned with an oscillating AFM probe, to decrease the probability of surface modification. Images recorded on plasma-etched surfaces show many more structural details than an unetched surfaces. A XLPE cryo-microtomed surface from an unaged cable has been exposed to a low-energy electron beam, following which the exposed surface showed striking similarities with that of a field-aged cable.

Study on the morphology of XLPE power cable by means of atomic force microscopy

The long-term dielectric performance of underground power cable XLPE (cross-linked polyethylene) insulation suffers from poorly understood aging phenomena. A study of the morphological modifications of XLPE due to electrical aging may provide insight for a better understanding of aging mechanisms. The TEM technique has been used to study the XLPE morphology of unaged, laboratory-aged, and field-aged cable samples. A suitable image contrast enhancing and a stabilization of radiation damage were both successfully achieved by staining the sections with ruthenium tetroxide (RuO4), as individual XLPE lamellae were neatly and reproducibly resolved. Image analysis was used to help in the determination of any aging-induced morphological changes. XLPE samples from an unaged cable (D1) has been exposed to a low-energy electron beam, in an attempt to simulate certain conditions thought to occur in electric field-stressed dielectrics. © 1996 John Wiley & Sons, Inc.

Morphological study of aging phenomena in XLPE by TEM technique

Most of power failures occur at cable joints in Hydro-Quebec underground conduit network. Some are due to partial discharges inside these joints and the corresponding degradation of their electrical insulation. Hydro-Quebec Distribution conduit network consist mainly of XLPE cable with rubber premoulded accessories. In 2001, Hydro-Quebec started a research project for the diagnosis of underground cable and their accessories. This project was implemented for safety issues, to perform predictive maintenance and to withdraw defective accessories before failure.

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Detection, localization and interpretation of partial discharge in the underground distribution network at Hydro-Québec

Afin de detecter, localiser et interpreter une decharge partielle susceptibl e de survenir dans un site de decharge partielle le long d'un equipement electrique, deux sondes de mesure et une sonde de synchronisation sont installees le long de l'equipement electrique. Les sondes de mesure detectent des impulsions voyageant dans l'equipement electrique alors que la sonde de synchronisation detecte un ang le de phase dans l'equipement electrique et peut servir a des fins de calibration. Une unite de controle recoit les signaux captes par les sondes et les conditionne. Des traitements numeriques faits sur les signaux conditionnes, dont leur correlation, une distribution temps-frequence et une estimation de facteur de forme, permette nt d'etablir un diagnostic indiquant une detection d'une decharge partielle et sa localisation le long de l'equipement electrique.

Daniel Fournier

Papers

Detection, localization and interpretation of partial discharge

Study on the morphology of XLPE power cable by means of atomic force microscopy

Morphological study of aging phenomena in XLPE by TEM technique

Detection, localization and interpretation of partial discharge in the underground distribution network at Hydro-Québec

Detection, localisation et interpretation d'une decharge partielle