Constantinos D. Halevidis

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.

Thermal Effect of the Recloser Operation Cycle on Bare Overhead Conductors

A new calculation method of the temperature rise generated from multiple (consecutive) short circuits, to which bare overhead conductors are subjected, is proposed in this paper. Multiple short circuits are common in medium-voltage lines protected by reclosers due to their operation cycle. The proposed method treats each short circuit as a separate event. In additiony, the cooling between each short circuit is taken into account. The results of the proposed method are compared to the results given by the method of the IEC 60865-1 standard. The application of the proposed method results in greater temperature rises when the equivalent thermal current density is greater than a threshold. Furthermore, alterations to the recloser operation cycle are performed in order to investigate its effect on the generated temperature rise, namely, alterations to the reclosing interval between successive tripping operations and the ratio of fast to delayed trips. Finally, the proposed method was compared to an exact numerical solution of the heating process.

Power flow in PME distribution systems during an open neutral condition

The open neutral condition, in distribution networks, can cause significant damage to customer installations. Due to this, the continuity of the neutral conductor is important to the power utilities. In this paper, a power flow algorithm for three-phase four-wire distribution networks with protective multiple earthing (PME), during an open neutral condition, is presented. This algorithm is based on a modified backwards-forwards technique. Additionally, the return current flow is explicitly modeled. Results from three different distribution feeders with different load unbalance and topology are presented and discussed. Furthermore, the effects of the grounding resistances, the load model (constant admittance or constant power), and the neutral breakage point (abruption point) on the voltage levels, are investigated.

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.

Magnetic-Field Measurements Near Two-Pole-Type Distribution Substations

This paper presents the mean values of the resulting magnetic field in the low-frequency range, which were recorded during spring and summer near two-pole-type distribution substations in urban areas. The mitigation of the magnetic field that was achieved after covering the low-voltage (LV) fuse boxes with thin aluminum plates is also presented. The mean attenuation achieved is approximately 23% during spring and 18% during the summer at the region of up to 30 cm from the substations and at a height of 1.0 m from the ground, at the measuring points near the LV fuse boxes. All of the measured magnetic-field values that were observed were lower in comparison with those specified as limits and introduced by ICNIRP.

Partial Discharges Measurements on Coated and Non-Coated Solid Dielectric in Air

In this work, high voltage pulses of the form 1.2/50μs were applied on a metal tip solid dielectric metal plate combination in the air using coated and non coated specimens in order to investigate the reduction of partial discharges. The measurements indicated the effect of the coating, as it reduces the partial discharges on the dielectric.

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.