Underground cable ampacity: a fresh look at addressing the future electric grid

Abstract

Underground power cables serve a critical purpose in electric power applications and the electric grid. Many have experienced the frustration of a power outage resulting from a failed cable. This dissertation addresses underground electric power cable ampacity and provides analytical, experimental, and operational test results for underground cables. The motivation for this work stems from challenges facing the industry in determining cable ampacity due to the uncertainty in soil thermal resistivity and soil thermal stability. Analytical results compare multiple software models. Experimental results consist of radial temperature measurements of a buried cable at 3 heat rates for 5 to 21 days. Operational results include measurements of 1 kV DC combiner circuits installed at a 10 MW photovoltaic (PV) power plant. There are numerous methodologies for calculating ampacity that can result in substantial differences. Some of these differences stem from the concern of soil dry-out described as soil thermal stability. This dissertation proposes a method to address using a soil parameter called the Non-Drying Heat Rate. Experimental results indicate that soil around a cable dries based on the magnitude of heat flux and length of time but not directly proportional to the cable diameter as proposed in the Law of Times. A set of experiments was performed on a direct buried cable to compare with the Neher-McGrath method and commercially available software programs. The results show the Neher-McGrath calculations and CYMCAP software outputs overestimated the measured temperature with a mean error of 4% ± 10% for the 6 experiments performed. Soil drying was not predicted to occur based on the non-drying heat rate measurements, and the experimental results confirmed this. A PV power plant design was used as a case study concluding that the measurements for the DC combiner cables were significantly lower than the calculated temperatures. It illustrates the need for an industry accepted standard that provides a clear methodology for addressing soil thermal resistivity and soil thermal stability. This dissertation makes the following contributions: 1. Illustrates the need for an industry standard that addresses soil thermal stability 2. Proposes the non-drying heat rate method to address soil thermal stability 3. Indicates that the Neher-McGrath method is conservative by experimentation 4. Indicates no soil drying occurred during experimentation, as predicted by the non-drying heat rate method 5. Provides cable temperature measurements of an operational PV power plant