Aikaterini D. Polykrati

Calculation of the activation energy of oil-paper insulation in a distribution transformer

This article presents measurements of insulating resistance in distribution transformers, at various temperatures, from which the activation energy of the oil-paper system is calculated.

A model for calculating the temperature of aluminium particles ejected from overhead low-voltage lines owing to a short-circuit

Wildfires, which are uncontrolled fires spreading readily over vast areas, are usually the result of human negligence, arson or lightning. There are cases of fires close to electrical distribution lines for which the network has been blamed. In the present paper, the risk of a wildfire breaking out owing to the temperature of molten metal particles that are possibly created on bare conductors of low-voltage networks in short-circuit faults (unless they are interrupted by the protection systems) is examined. Thus, a mathematical model is proposed for the estimated temperature rise of those molten metal particles ejected from bare conductors of low-voltage overhead lines. Moreover, this model can be applied to medium- or high-voltage networks. The model takes into account the weather conditions and particles’ height above the ground. Further, an arithmetic example for an incandescent particle ejected from aluminium conductors of a low-voltage network is given. According to this example, there is no risk of dead leaves or wood catching fire owing to this particle.

High-Voltage Lines in Fire Environment

This letter concerns the change in bending which can manifest in a high-voltage line, when it lies in a fire environment. It is shown that in the case where aluminum conductors steel-reinforced conductors attain a temperature of more than 500°C, a drastic fall of the steel core mechanical strength occurs, resulting in permanent elongation of the line which drastically decreases the clearance from the underlying vegetation.

THE AVERAGE VALUE OF ELECTRONIC ENERGY DURING DIELECTRIC AGEING OF POLYMERIC SOLID INSULATORS UNDER HVAC STRESS

This work examines the average value of electronic kinetic energy during the dielectric ageing of solid insulators. For this purpose, samples of various insulators with a relative permittivity e r =2.5-4.5 (e.g. industrial bakelite) were electrically stressed by the imposition of a high AC voltage. The electrode set-up used was metal tip-samplemetal plane in insulating transformer oil environment. Thus, the imposed field was inhomogenous, such as those found in practical applications. Measurements were taken by increasing the imposed voltage in steps of 1 kV until the ultimate breakdown of the insulators and the charge which flows through the sample, as a result of partial discharges, was measured. The partial discharges appeared in the oscillographs in the form of potential steps of the measuring capacitor voltage, which were placed in series with the electrode set-up. From these measurements the increase of the average value of the electronic kinetic energy in relation in relation to the value of the imposed electric field was noted. The results for this material samples are in agreement with other results regarding DC or impulse voltage stress.

PROTECTIVE MEASURES AGAINST ELECTRICAL HAZARDS OF CONSUMER INSTALLATIONS

The protection system of a home electrical installation concerns exclusively the private consumer himself and all necessary measures insuring its proper and safe operation is his own responsibility. Electrical installations with an inadequate protection system in conjunction with unforeseeable events like the abruption of an overhead power line have often caused fatal accidents and property damage. The aim of this paper is to focus on such events and to propose measures to prevent similar accidents in the future.

Abruption Cross-Section and Bending Change of ACSR Energy Lines in Fire Environment

An ACSR (Aluminum Conductors Steel Reinforced) power line, when it lies in a fire environment, can manifest a permanent elongation and change in bending, as when the conductors attain a temperature of over 500°C, a drastic fall of the mechanical strength of the steel core is caused. This work is a study about the conductor cross-section at the position of the abruption, in case it occurs. This cross section appears to be approximately 62.5% of the original value. Subsequently, two approximative methods of calculating the bending change of overhead lines are shown.