Fundamental investigation on the thermal transfer coefficient due to arc faults
In order to determine the pressure rise due to arc faults in electrical installations, the portion of energy heating the surrounding gas of fault arcs has to be known. The ratio of the portion of energy to the electric energy, the thermal transfer coefficient, well known in literature as kp factor, is adopted here. This paper presents a theoretical approach for the determination of the pressure rise in electrical installations. It is based on the fundamental hydro- and thermodynamic conservation equations and the equation of gas state taking into account melting and evaporation of metals as well as chemical reactions with the surrounding gas. With respect to the dependence of the arc energy on gas density, the radiative effect of fault arcs on the energy balance is introduced. In consideration of the radiation, the more reasonable arc energy is applied for the estimation of the gas temperature and of the thermal transfer coefficient in the energy balance. In order to identify conveniently which factors essentially influence the development of pressure, the thermal transfer coefficient is studied and evaluated as an alternative variable of the gas pressure. The results for a test container show that factors such as the kind of insulating gases and of electrode materials, the size of test vessels, and the gas density considerably influence the thermal transfer coefficient and thus the pressure rise
A risk measurement tool for an underground electricity distribution system considering the consequences and uncertainties of manhole events
This paper explores a risk measure of underground vaults that considers the consequences of arc faults. The increasing use of underground systems, together with the aging of networks, the lack of maintenance and interference from other (third party) underground systems nearby have caused many accidents in urban areas, thus endangering human life. The involvement of a large number (hundreds or thousands) of underground vaults with different characteristics, the lack of historical data on modes of failure, the rarity of the occurrence of some faults, the magnitude of their consequences and the involvement of a complex environment surrounding the hazard zone make risk management even more complex and uncertain. Furthermore, given that the (monetary, time, staff, etc.) resources of an electrical power company are limited and scarce, it is necessary to use decision-making tools that aggregate the consequences and the uncertainties to assess the risks jointly with the preference structure of the company, thus solving the problem more realistically. Therefore, this paper puts forward the use of an additional risk analysis for manhole events in underground electrical distribution networks with a view to its being used as a decision aid tool in risk management. As an illustration of the use of the risk measurement tool proposed, a numerical application is presented. The result rather than showing a ranking of underground vaults, gives a measure of the risk used that can show the decision-maker (DM) how much better one group of alternatives (formed by alternatives with quite similar risk values) is than other groups, based on the DMâ€™s attitude to risk and grounded on the axiomatic structure of utility theory.
Transportfunktionen von Stickstoff bis 26000 °K
TheE(I)-characteristic and a large number ofT(r, I)-distributions measured in a 5 mm ØN2 cascade arc at normal pressure are used to evaluate the transport properties of nitrogen up to 26,000 °K. The electrical conductivityσ(T) and from this the cross-section for atom-electron collision and the Coulomb-cross-section are determined directly from theE(I)- and severalT(r, I)-curves. For the evaluation of the thermal conductivityκ(T) three temperature regions are discerned: Up to about 10,000 °Kκ(T) is derived directly from the energy equation since here the energy transport by radiation does not play an important role. Between 10,000 and 15,000 °K the radiative energy flux for different arc currents, the thermal conductivity and from this the charge exchange cross-section are determined in a good approximation utilizing the large number of measured temperature distributions. Above 15,000 °K the already evaluated collision cross-sections are used to computeκ(T). Withκ(T) known the radial distribution of the balance between emission and absorption of radiative power per unit volume is evaluated for different arc currents. It turns out, that at the highest measured arc current, i.e. 570 A, in the axial region of the arc about 95% of the supplied energy is carried off by radiation.