# Coefficient K for a gas under GIL conditions

The coefficient K0 is used in the formula from Elektra 100 and is equal to the coefficient $C$ which was used by J. Vermeer in the paper: 'A simple formula for the calculation of the convective heat transfer between conductor and sheath in compressed gas insulated (CGI) cables', 1983 as published in Elektra 87.

The cost, and the need for insulation alone and not switching, led to the use of an N2/SF6 gas mixture with a majority of 80%N2 and a minority of 20%SF6. The mixture has different dielectric and thermal properties and the CIGRE Guide 218 (2003) provides an expression to calculate the this coefficient to be calculated for different mixtures of N2 and SF6 gas.

SF6 is a potent greenhouse gas with a high global warming potential, and its concentration in the earth atmosphere has been rapidly increasing over the last few decades. And during its working cycle, SF6 decomposes under electrical stress, forming toxic byproducts that are a health threat for working personnel in the event of exposure. For these reasons, other insulating gases and mixtures are tested to replace SF6 altogether. According to today’s consideration the most suitable insulating alternatives apart from N2 are carbon-dioxide (CO2), a mixtures of N2/CO2, or dry air. N2/CO2 and dry air show similar results to the N2/SF6 gas mixture, but with a lower level of absolute insulation capability and in consequence a larger dimensioning.

The parameters for N2 and SF6 are taken from Vermeer' paper or from the paper from Itaka (1978) in case this method is selected. The parameters for N2 and SF6 were recalculated using the method described in the paper and the researched gas parameters.

The parameters for Air, CO2 and O2 were calculated using the method described in the paper and the researched gas parameters.

Symbol
$K_{0}$
Formulae
 $K_{0 gas1} \left(1 - V_{gas}\right)^{0.75} + K_{0 gas2} V_{gas}^{0.75}$ mixture of two gases, CIGRE Guide 218 (2003) $K_{0 gas1} \left(1 - V_{gas}\right) + K_{0 gas2} V_{gas}$ mixture of two gases, linear approximation $K_{vermeer} c_{gas}$ Vermeer1983, basic formula
$V_{gas}$
Used in
$W_{conv_{ce}}$
Choices
IdGasCableizerVermeer1983Itaka1978
Airdry Air5.864
N2N25.8195.8314.9
SF6SF611.34111.324.4
CO2CO26.275
O2O25.623