Experimental Study on the Effect of Soil Moisture Content on Critical Temperature Rise for Typical Cable Backfill Materials

Abstract

The increased utilization of power grid requires operating power cables close to their thermal limit. Buried power cables in such condition experience thermal instability where thermal resistance increases as the moisture migrates away from the proximity of the cable forming a dry-out zone. While it is common to use a two-zone model to account for the dry-out zone in ampacity calculations, there is a limited number of studies on characterizing backfill materials for their critical temperature rise ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\Delta \theta_{x}$</tex-math></inline-formula> ), especially for crushed rock sands. In addition, dependency of the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\Delta \theta_{x}$</tex-math></inline-formula> on moisture content is sometimes not acknowledged. This study measures the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\Delta \theta_{x}$</tex-math></inline-formula> for three typical backfill materials and investigates the relationship between <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\Delta \theta_{x}$</tex-math></inline-formula> and moisture content. The results show that crushed rock sand has higher <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\Delta \theta_{x}$</tex-math></inline-formula> compared to natural sand. Overall, large range of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\Delta \theta_{x}$</tex-math></inline-formula> was measured depending on moisture content and type of soil. Ampacity calculations with the established <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\Delta \theta_{x}$</tex-math></inline-formula> show that at low moisture content, the thermal resistivity of the soil has a higher influence on ampacity than <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\Delta \theta_{x}$</tex-math></inline-formula> . At relatively high moisture content, the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\Delta \theta_{x}$</tex-math></inline-formula> becomes the predominant factor governing the overall ampacity.