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Transmission Line Transformers (TLT) based on the use of ferrite beads are presented. This work aims at optimising the performance of the voltage gain and the compactness of the TLT according to the position, the type and the number of ferrites used. The ferrites have been selected taking into account two main parameters: the maximum currents flowing through the outer conductor of the lines of a TLT without ferrite beads and the current frequencies. The function of these ferrites is to decrease the effect of parasitic short-circuit transmission lines between the outer conductors of the coaxial cables of the TLT. Voltages and secondary currents are measured on each cable. The voltage gain achieved reaches the optimum theoretical value thanks to the use of ferrites for a 2, 4 and 10-stage transmission line transformer. Each cable is shorter than 1.5 meter in length which provides an unquestionable advantage of compactness.

Transmission Line Transformers (TLT) based on the use of ferrite beads are presented. This work aims at optimising the performance of the voltage gain and the compactness of the TLT according to the position, the type and the number of ferrites used. The ferrites have been selected taking into account two main parameters: the maximum currents flowing through the outer conductor of the lines of a TLT without ferrite beads and the current frequencies. The function of these ferrites is to decrease the effect of parasitic short-circuit transmission lines between the outer conductors of the coaxial cables of the TLT. Voltages and secondary currents are measured on each cable. The voltage gain achieved reaches the optimum theoretical value thanks to the use of ferrites for a 2, 4 and 10-stage transmission line transformer. Each cable is shorter than 1.5 meter in length which provides an unquestionable advantage of compactness.

This paper presents a numerical simulation of the first negative corona in SF₆ at atmospheric pressure, in a point-to-plane electrode system. A few experiments are first carried out in order to acquire a basic knowledge of the physical processes involved in the first negative corona. Since it does not seem convincing that photoemission would be the predominant secondary process at the cathode surface, field effect emission will be introduced into the numerical modelling of the ignition and development of the first negative corona. The axial and radial development of the discharge is also optically measured. Continuity equations of electron, positive- and negative-ion densities are numerically solved on a non-uniform mesh with a monotonic upwind scheme for conservation law (MUSCL) method. These equations are coupled with Poisson's equation by using the classical method of disks introduced by Davies. The novelty of this model bears upon the choice of the boundary conditions which take field effect emission into account. According to the numerical simulation, this secondary effect could indeed explain the fast rise time of the typical negative corona current pulse, and enables numerical simulation results to be in agreement with the experimental ones.

The aim of the present work is to provide data to understand better and quantify the dielectric strength of pure compressed N2(1 < P< 8 bar). The lack of agreement of results found in the literature leads us to study the electrical and physical characteristics of this gas as a function of a wide set of parameters. Experiments are mainly performed in point to plane nitrogen gaps up to 40 mm length using three different rod radii. Gas conditioning phenomenon in positive polarity and corresponding change in characteristics of discharge mechanism are demonstrated here. In our experimental conditions, no leader is involved in the discharge development whatever the polarity is. The values of the breakdown reduced field $E_{\rm b}/P$are determined as a function of the pressure. Two different experiments are performed in order to take into account the mechanisms of initiatory electron production: the influence of the γ-ray irradiated N2and that of the surface roughness of the HV electrode. In negative polarity, the free electrons are provided from the cathode whereas they are provided from the gas under positive polarity. Voltage waveform influences on the U50breakdown voltage are dealt with in a large pressure range (1 < P< 12 bar). The role of the non-uniformity of the applied electric field on the U50value is pointed out in both polarities. Finally, the reduced guiding field of negative streamer is experimentally measured.

The aim of the present work is to provide data to understand better and quantify the dielectric strength of pure compressed N$_2$ ($1< P< 8$ bar). The lack of agreement of results found in the literature leads us to study the electrical and physical characteristics of this gas as a function of a wide set of parameters. Experiments are mainly performed in point to plane nitrogen gaps up to 40 mm length using three different rod radii. Gas conditioning phenomenon in positive polarity and corresponding change in characteristics of discharge mechanism are demonstrated here. In our experimental conditions, no leader is involved in the discharge development whatever the polarity is. The values of the breakdown reduced field $E_{\rm b}/P$ are determined as a function of the pressure. Two different experiments are performed in order to take into account the mechanisms of initiatory electron production: the influence of the $\gamma$-ray irradiated N$_2$ and that of the surface roughness of the HV electrode. In negative polarity, the free electrons are provided from the cathode whereas they are provided from the gas under positive polarity. Voltage waveform influences on the $U_{50}$ breakdown voltage are dealt with in a large pressure range ($1< P< 12$ bar). The role of the non-uniformity of the applied electric field on the $U_{50}$ value is pointed out in both polarities. Finally, the reduced guiding field of negative streamer is experimentally measured.