Thierry Ditchi

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. >

Pressure wave propagation methods: a rich history and a bright future

The pressure wave propagation (PWP) method was developed 25 years ago to study space charges in solid dielectrics. This method has been a powerful tool in this field of research but has also proved its efficiency in adjacent fields for the characterization of materials or processes. In this paper the history of this method and its principle are recalled. The pulsed electro-acoustic (PEA) method, which can be physically described in a similar way and for which data can be analyzed in an equivalent fashion, is also presented. Examples of applications and developments are given which indicate the direction which these methods might take in the future

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.

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.

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.

A finite element method for the determination of space charge distributions in complex geometry

Electrical breakdown in insulators very often initiate near high field regions of the structure, as found near small-radius impurities or at electrode defects. This is attributed to the development of localized space charges. For this reason many efforts have been made to determine such charge. Various techniques are now available, but they are not directly applicable to complex geometries where it is difficult to determine analytically the field configuration and thus the relation between the measured variables and the space charge distribution. To solve this problem, we propose to use a numerical simulation using a finite element method (FEM). In this paper we describe how it can be implemented in the case of the pressure wave propagation (PWP) method. It is shown that measured signals in insulating samples with divergent electric field regions are well fitted by simulations. We show that this allows for the determination of space charge distribution in such samples.

A broad beam Doppler speed sensor for automotive applications

On‐board absolute speed sensors would be very useful in automotive applications in order to perform the measurement of the speed independently of the wheel rotation. Narrow beam Doppler sensors give a good accuracy but only operate if there is a reflecting obstacle in the antenna footprint on the road. Broad beam Doppler sensors have the advantage of offering a higher probability of getting a reflected wave, but they require multiple frequencies. A novel sensor is presented. It uses a single frequency emission, which leads to low cost devices. Simulations and measurements have been carried out which show that it is particularly well adapted to high‐risk situations, for instance icy or very wet road surfaces, when only a few reflecting obstacles are present. In such situations, the accuracy of the measurement is typically of the order of 0.5 per cent.

Can non-destructive space charge measurement techniques have fallout in other fields?

For more than 20 years, non-destructive space charge distribution measurement techniques have been successfully used to study electrical phenomena in insulators and dielectrics. Where other experimental techniques would require various hypotheses, they give directly spatial information. The implementation is carried out by producing a perturbation within the sample which may involve thermal diffusion, elastic wave propagation or electrical stress. A recent physical study of these methods gives a vision that offers new types of applications. In this paper the question of complex geometry sample and of free charge detection is considered.

Measurement of charge distributions in insulating materials for high voltage cables

Evolutions in time of charge distributions in thick dielectric insulators such as those used in high-voltage DC insulation are investigated. The pressure wave propagation (PWP) method has been applied to low-density, high-density, and cross-linked polyethylene under electric stress. It is shown that the impurities contained in carbon-loaded polyethylene electrodes play a very significant role in the development of a space charge distribution within the insulator. It is suggested that, by studying the evolution of the charge distributions, it is possible to determine whether a stabilization occurs and how much time is required to reach this stabilization, depending on the temperature and the applied voltage. Consequently, such a measurement made on samples having the same structure as the cables will help in defining tests and standard procedures for equipment evaluation.<<ETX>>

Direct observation of injected charges at insulator‐electrode interfaces in strongly diverging fields

Various theoretical models have been proposed to explain the behavior of insulator‐electrode interfaces in the case of strongly diverging electric fields. Until now, direct measurement of the charge transferred at these interfaces could not be performed. In this letter, we propose a novel structure which allows for such measurements. An application is presented in which charge transfer around 30 μm diameter wires embedded in an insulator is directly observed.