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Using accurate cable ampacity is critical to electrical power system design. An optimally sized cable results in minimum cost and high reliability. Wind and solar power plants, in particular, strive to optimize cable design by using ampacities that closely match maximum generation.
Cable ampacities have been estimated over the years based on engineering assumptions and site conditions. Various configurations require different parameters and assumptions. Cables placed underground require information about the ambient earth temperature, cable separation distance, soil thermal resistivity, etc. If these values are inaccurately estimated, the resulting cable size will be inaccurate. This may lead to cable overheating, if the cable is undersized, or increased cable cost, if the cable is oversized.
Underground cable ampacity is difficult to estimate because a primary factor determining ampacity, i.e., soil thermal resistivity, varies from moist to dry conditions, which in turn varies Manuscript received with cable loading. This paper compares the difference in results obtained when using various methods, including one recently proposed, to include the effects of soil thermal instability. These methods will include cable ampacities calculated using the Neher–McGrath method, IEEE Cable Ampacity tables, and a commercially available computer program.
Each of these methods requires some values that must be collected at the location where the cable will be installed. These include the soil thermal resistivity, also known as “rho” and measured in Kcm/W (◦C∗cm/W), and the maximum expected ambient temperature, at the depth of the hottest cable. The soil thermal resistivity, while critical, may not be as readily available as the ambient temperature. IEEE Std. 835 states: “In the past, when the thermalresistivity of the earth was not known a rho of 90 was recommended for rating the cable.
However, the ratings for buried cables are significantly affected by the earth’s portion of the thermal circuit and therefore correct knowledge of the effective soil thermal resistivity and soil thermal stability is paramount in establishing the correct rating for a buried cable system.” Measuring the in situ thermal resistivity is not a difficult process, as described in, but it is frequently not performed. It is likely that a cable size will be selected that is either smaller or larger than the optimal, if this step is skipped.
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