C. Alquie

Evidence of strong correlation between space-charge buildup and breakdown in cable insulation

Many processes have been considered over the years to explain the origin of breakdown in cable insulation. Such effects as space charge build-up, tree growth, charge injection, etc. have all been discussed. Various techniques are now available to measure, in a nondestructive way, space charge distributions in insulators. These techniques, for instance the pressure wave propagation (PWP) method, can be used under applied electric stress and thus make it possible to follow the development of space charge in selected regions of the insulators. In this paper we present new evidence linking space charge buildup, tree growth and breakdown in XLPE. We have used the PWP method to monitor the charge distribution as a function of time under dc stress in high insulating thickness cable. We show that for certain insulation systems the space charge buildup can increases the local field to a value which is more than 8/spl times/ the applied electric field, leading to breakdown. Post-mortem analysis followed by optical microscopy shows the presence of electrical trees, the breakdown channel being centered on one of them. The study of space charge evolution in practical insulations permits an understanding of the role of space charge in dc breakdowns. This understanding enables the development of technologies to suppress this effect and hence realize practical dc XLPE transmission cables.

Electrical properties of electrode/polyethylene/electrode structures

The pressure-wave propagation (PWP) method allows for the nondestructive measurement of charge distributions in dielectric materials. This method has been used to study the electrical properties of electrode/polyethylene/electrode structures such as those involved in high-voltage cables. The influence of the composition of the insulating resin itself and that of the electrodes are analyzed in polyethylene samples. According to the chosen combination, charge transfer at the interfaces, migration of ionizable impurities, or a strong decrease of both is observed. This application of the PWP method is of particular interest, since it allows for a suitable choice of the materials and structures involved in insulator/conductor interfaces. >

The Pressure wave Propagation Metod for the Analysis of Insulating Materials: Application to LDPE Used in HV Cables

In the field of electrical insulation, polymers are progressively replacing formerly used materials. When such an insulator is subjected to a voltage gradient, an internal space charge and a polarization develop, eventually reducing the apparent electrical strength of the material. We have applied the pressure wave propagation (PWP) method to the study of HV insulators, such as low density polyethylene (LDPE) used for HV cables. The spatial charge distribution is successively measured in samples subjected to various processes. The influence of additives in the development of a space charge under electric stress is directly shown. We have also observed that the nature of the electrode-iinsulator interface strongly affects the charge distribution throughout the insulator, and that charge injection from these interfaces occurs at much lower applied voltage gradients than previously assumed. The evolution in time of the charge distribution in samples submitted to voltage gradients, at 20 and 700C, is studied. These results show that an equilibrium is reached, from which the life expectancy of materials and cables can be evaluated.

Broadband determination of ultrasonic attenuation and phase velocity in insulating materials

Ultrasonic attenuation and phase velocity in a solid insulating material are determined in a broad frequency band by studying the propagation of a pressure pulse in a sample of this material submitted to an electric field of known value. During the propagation of the pulse in the sample, which is placed between short‐circuited electrodes, a current is generated in the external circuit. This signal gives directly the time dependence of the pressure pulses entering and exiting the sample, from which the frequency‐dependent attenuation and phase velocity are deduced by Fourier analysis. The pressure pulse is generated by the impact of a laser pulse on an absorbing surface adjacent to one face of the sample. New results obtained in polyethylene and silicone samples are presented. The proposed method presents the following advantages: It avoids the use of a transducer, reduces the impedance matching requirements, and provides information in a broad frequency band by a single measurement.

Application of the pressure wave propagation method to the study of interfacial effects in e-irradiated polymer films

The pressure wave propagation method is applied here to the study of charging and discharging processes of polymer films of spatial interest. The irradiation of the materials is performed by a monoenergetic electron beam in the range 5–50 keV. Since the irradiated surface of the sample is free, we extended the model of the induced signals, previously developed, to take into account the total reflection of the pressure at the polymer‐vacuum interface. All the experiments, irradiations and measurements, are operated in a vacuum environment. After the description of the experimental setup, typical signals showing the interfacial effects are discussed. They concern the behavior of the sample‐vacuum interface and charge injection at the target‐sample interface. These experiments, which were carried out both on metallized and nonmetallized samples bonded to the target electrode, allow a direct observation of charge injection at interfaces.

Determination of the polarization‐depth distribution in poled ferroelectric ceramics using thermal and pressure pulse techniques

This paper is the first of a series with the common theme of comparing thermal and acoustic pulse methods of measuring charge or polarization profiles across the thickness of slab‐shaped samples that are representative of different types of materials. In this paper, thermal and pressure pulse measurements are reported of the polarization distribution in poled, ferroelectric ceramic samples. The results obtained from both methods are complementary so that there is a benefit to using both. The results also demonstrate that large deviations from uniform polarization can be induced by processing differences.

Comparison of polarization distribution measurement by the LIMM and PWP methods

Two different methods of measurement of polarization distributions in piezoelectric materials, the laser intensity modulation method (LIMM) and the pressure wave propagation (PWP) method, are compared. The two techniques are described, and measurements made on different PVDF-based samples, one with a symmetric and the other one with an antisymmetric structure, are reported. Although the LIMM responds to diffusion of heat and the PWP method to the propagation of a pressure wave, the direct observation of the raw data gives similar information in both cases as far as the symmetry of the polarization distribution is concerned. The specific characteristics of each method are analyzed. >

Measurement of space-charge distributions in insulators under very rapidly varying voltage

In this paper we describe a new instrument for real-time measurements of space charge distributions in insulators submitted to rapidly varying voltages. It is illustrated with the pressure wave propagation method but can also be adapted for use with the pulsed electro-acoustic method. A new acoustic generator has been designed to produce high amplitude pressure pulses, at a high rate, and with good reproducibility. This generator, made of a thin transducer coupled with a waveguide and a backing medium, responds with a pressure pulse when a step voltage is applied. A special sample holder with a signal decoupling circuit, and a storage unit have been developed. The described instrument allows for measurements at a rate of 3200 Hz. Its potentialities are illustrated by studying the evolution of space charge in polyethylene samples stressed by both pulsed and 50 Hz ac voltages.

The pressure-pulse method for measuring space-charge distribution in irradiated insulators

The authors report on the determination of the spatial charge distribution in electron-irradiated Teflon films by the pressure-wave-propagation method. A pressure pulse generated in the sample by a Nd-YAG pulsed laser is used as a virtual probe of the charge distribution. The authors show that positive charges are injected at the metal/dielectric interface. The charges remain trapped after a surface discharge, but buried incident electrons disappear. The method could also be applied to insulators used in a vacuum. >

Study of charge injection in insulators submitted to diverging fields

Various theoretical models have been proposed to explain the behavior of insulator-electrode interfaces in the case of strongly divergent electric fields. However until recently a direct measurement of the charge transferred at these interfaces could not be performed and very indirect ways had to be used. In this paper we propose a structure which allows for direct measurements of this charge using the PWP (pressure wave propagation) method. We present evidence of charge transferred around small diameter wires, embedded in an insulator, submitted to an applied voltage.