The coefficient K_{0} 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 N_{2}/SF_{6} gas mixture with a majority of 80%N_{2} and a minority of 20%SF_{6}. 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 N_{2} and SF_{6} gas.
SF_{6} 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, SF_{6} 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 SF_{6} altogether. According to today’s consideration the most suitable insulating alternatives apart from N_{2} are carbon-dioxide (CO_{2}), a mixtures of N_{2}/CO_{2}, or dry air. N_{2}/CO_{2} and dry air show similar results to the N_{2}/SF_{6} gas mixture, but with a lower level of absolute insulation capability and in consequence a larger dimensioning.
$K_{0 gas1} \left(- V_{\mathrm{gas}} + 1\right)^{0.75} + K_{0 gas2} V_{\mathrm{gas}}^{0.75}$ | mixture of two gases, CIGRE Guide 218 (2003) |
$K_{0 gas1} \left(- V_{\mathrm{gas}} + 1\right) + K_{0 gas2} V_{\mathrm{gas}}$ | mixture of two gases, linear approximation |
$K_{\mathrm{c}} c_{\mathrm{gas}}$ | Vermeer1983, basic formula |
Id | Gas | Cableizer | Vermeer1983 | Itaka1978 |
---|---|---|---|---|
Air | dry Air | 5.864 | ||
N2 | N_{2} | 5.819 | 5.83 | 14.9 |
SF6 | SF_{6} | 11.341 | 11.3 | 24.4 |
CO2 | CO_{2} | 4.5 |