C.G. Karagiannopoulos

A theoretic and experimental investigation in stationary electric contacts

Stationary contacts of practical applications under nominal current operation may exhibit nonlinear behaviour. This may be attributed to the microstructures metal-isolator-metal, metal-oxide-metal and metal-gas-metal which are formed on the apparent contacting area, as oxides and other chemical compounds create interfacial layers. Work function changes of the contacting metals can modulate the effective barrier height and width, resulting in a significant change of the crossing resistance in contacts, even during a time interval of one period. Nonlinear behaviour is developed beyond a threshold field value determinated by the voltage drop at the contact. According to the experimental results it becomes evident that at the stage of moderate degradation, tunnelling occurs. An equivalent circuit is proposed which is valid for stationary contacts of practical applications. The theoretic investigation is based upon commonly accepted relations concerning quantum mechanical tunnelling. Between the experimental results and the theoretical ones a satisfactory agreement was found.

Theoretical transmission-line study of symmetrical indoor triple-pole cables for single-phase HF signalling

Theoretical study of symmetrical single-phase indoor tricels is performed with respect to high-frequency (HF) transmission-line modeling, aimed at potential communication applications over the low-voltage (LV) in-house grid. The analysis is based on the fundamental transverse-electro-magnetic (TEM) wave propagation concept, commencing from the three-conductor transmission-line model. In the three-wire cable, any transmission circuit consisting of two conductors-out of all three-is by virtue of symmetry reduced to a two-conductor formulation. The distributed parameters of the resulting model are moreover extracted via proper observation of cables field structure, where complete conformance with the uniform isolated two-conductor line is obtained. Propagation of high frequency signalling transmitted between phase and neutral may thence in practice be treated and formulated independently, as if no ground wire existed. In addition, verification of the TEM mode assumption is demonstrated, and hints on the applicability of the work presented are suggested.

Electrical contact overheating under short-circuit currents

Abstract In this paper, a method is proposed, concerning the temperature rise estimation at stationary electrical contacts, operating in the power supply network, during short-circuit. The method consists in the formation of a mathematical model, which simulates the operation of the contact/conductors system under heavy fault currents. The model comprises analytical mathematical expressions that enable temperature estimations lengthwise the aforementioned system. From the analysis results, it can be concluded that the conductor (bar) cross-section is of primary importance as far as overheating is concerned. It is convincingly shown that the equipment deterioration can be delayed by lowering the maximum contact temperature during short-circuit transient phenomena. An approximate equation is also proposed as an extra calculating test for the contact and bar overheating under short-circuit conditions. The proposed model could be a criterion for the selection of conductors and switchgear components.

Temperature distribution of fuse elements during the pre-arcing period

Abstract This work models the function of medium and low voltage fuses so as to estimate the increase in temperature across the fuse elements during nominal current operation for different types, diameters, element lengths and currents. The power balance in a thin metallic conductor, which simulates the fuse element, was derived and the resulting differential equation solved analytically. The theoretical results were close to the experimental measurements that we made, and the experimental work of other authors. The mathematical model that was developed will be useful in the design of fuses, as the computation of the axial temperature distribution across the fuse elements for the different parameters mentioned above is relatively simple.

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.

A Wildfire Model for the Estimation of the Temperature Rise of an Overhead Line Conductor

Fires in open land covered with grass, brush, or timber are often called wildfires. During a wildfire, the overhead conductors of a passing through distribution line suffer a thermal stress that deteriorates their mechanical properties. This paper presents a fire model that allows the estimation of the temperature rise of the conductors if we know some characteristics of the fire. The knowledge of the temperature rise is necessary for the study of the effects on the overhead line conductors.

Statistical treatment of insulation failure data with the application of simplified regression method in the high-voltage laboratory

Abstract In this article, we propose a new and simplified regression method for estimating the parameters of the Weibull distribution for high-voltage insulation failure data. This method is simpler to apply than the widely used maximum likelihood method and has some interesting properties suitable for small sample size data and short lifetime test data. This fact makes the method appealing for practical purposes. A simulation is applied for comparison of the introduced simplified regression method with the maximum likelihood method. Comparison is also realized by using experimental data obtained from lifetime recordings during stressing of pressboard and XLPE specimens with 50Hz a.c. voltages of constant value in the high-voltage laboratory.

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.

Model for temperature estimation of dc-contactors with double-break main contacts

Abstract The double-break arrangement of dc-contactors is advantageous for electrical arc quenching, so the most important disadvantage of the double-break, the higher temperature rise during current carrying operation, is acceptable. However, overheating can be a serious problem. In this paper, a mathematical model is proposed for the steady-state temperature difference distribution of double-break dc-contactors and an experimental verification of the theoretical results attempted. The influence of different dc-contactors’ parameters on the temperature distribution is investigated, and it is convincingly shown that the heat generated is dissipated mainly by the conductors, and that this dominates the optimisation of the dc-contactor temperature rise.

Measurement of fusible elements during current interruption and interpretation of related phenomena

Abstract The physicochemical process that occurs during the operation of fuse elements under excessive or heavy fault currents is complex and not well understood. In this paper, measurements obtained using analog/digital (A/D) converters are presented and the observed phenomena are investigated. Multiple arcing and a short-duration lowering of the voltage drop are explained, and interpretations of related phenomena during short-circuit are presented.