The breakdown development of a covered rod/plane air-gap arrangement was investigated under lightning impulse stress. At positive polarity of the rod a flashover along the cover surface is followed by a breakdown of the air-gap. At negative polarity a breakdown of the air-gap is followed by a flashover along the cover surface

For an improved design of high voltage apparatus a material with a low dielectric constant and a relatively high breakdown strength is advantageous. Therefore the insulating properties of polymer foam with closed pores are investigated. It was found, that for a foam with closed gas filled spheres the breakdown voltage is close to the partial discharge inception voltage and it depends on the void size and the degree of foaming. For practical applications a void diameter lower than 50 /spl mu/m in combination with a high gas content is desirable. The tested foam had a breakdown strength of 6 kV/mm and a relative dielectric constant of 1.1. Using this material a decrease of 30% of the size of high voltage electrodes can be achieved.

In laboratory set-ups, polymer covering of the electrodes has resulted in the reduction of the insulating distances by a factor two. One of the problems with covered conductors is however that of creeping discharges on the surface of the covering. The aim of this work was to investigate how creeping discharges develop under lightning impulse (1.2/50 /spl mu/s) on two covered conductors at a 90-degree angle to each other. Furthermore, the aim was to investigate how to stop the propagation of a creeping discharge. This was achieved by inserting at least one cable termination, e.g. the type of termination used for high voltage cables, into the discharge path. The experiments showed that a set-up with two crossing cables can withstand very high voltages without any shed or termination of the creeping discharges. Further tests show a considerable increase of the breakdown voltage in other arrangements comprising covered electrodes. This was accomplished by using two cable terminations at each cable end to stop the propagation of creeping discharges.

The application of high DC voltage to a polymer/air insulation system can result in space charges on the interface. The space charges can cause significant distortions of the electric field distribution, which may trigger a surface flashover or breakdown in the insulation system. In the present investigation, an attempt has been made to measure the space charge on a polyester/air and PTFE/air interface by using a pressure wave propagation technique (PWP) under DC voltage

Dispositif concu pour proteger, dans une centrale electrique des objets (1, 7, 27) contre des surintensites provenant de defectuosites, lesdits objets etant relies a un reseau electrique (3) ou a un autre equipement de la centrale electrique. Un ensemble (5) servant a limiter les surintensites est relie a une ligne (2) reliant le reseau ou l'equipement (3) et l'objet (1, 7, 27) et est mis en service afin de limiter les surintensites avec l'aide d'un ensemble servant a detecter une condition de surintensite.

There has long been an intensive activity in the area of measurement technology for HV applications, both for testing, diagnostics and monitoring of HV devices and for investigation of material properties. This paper briefly surveys some methods developed and used: partial discharge (PD) detection in cable terminations using acoustics together with inductive techniques, inductive and capacitive on-line PD monitoring of power transformers, and measurement of charge distributions in solid dielectrics with the pressure-wave propagation and pulsed electro-acoustic techniques.< >

For partial discharges in oil, the relationship between, the apparent discharge magnitude, electrically measured in picocoulombs, and the acoustic signal amplitude from the same discharge, shows large variations. If the acoustic signal is detected with several detectors simultaneously, it is found that the acoustic amplitudes are correlated in a relatively narrow angular range. For larger angular separation, the acoustic amplitudes are uncorrelated and not anticorrelated. Some physical aspects of streamers in oil are illuminated by this observation and some theoretical considerations. The streamer velocity may, for example, be estimated.

An investigation on the barrier effects under DC voltage was carried out in a 50 mm rod/rod electrode system with a clean, particle-contaminated barrier and a barrier with small hole. The results show that a clean barrier with certain diameter is very effective to increase DC flashover voltage of the gap. The highest flashover voltage of the gap with a barrier depends on the barrier's diameter, position and dielectric strength. The flashover voltage can be slightly reduced by a conducting particle on the barrier surface. However, a small hole on the barrier results in strong decrease of the flashover voltage. In the paper, flashover characteristics of the rod-barrier-rod gap and the mechanism of barrier effects are discussed