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Solar Plant Ampacity cable study for a PV power plant is evaluated based on temperature measurements taken during normal operation for one year. Differences between the design and the installation are discussed regarding their impact on cable temperature. Soil thermal resistivity measurements are reviewed, highlighting the need for the design engineer to properly specify the soil conditions for the measurement. The measured temperature was significantly less than the temperature calculated using the Neher-McGrath method.
A solar photovoltaic (PV) power station located in Arizona was studied with the goal of comparing the pre-installation engineering analysis of cable temperature and solar plant ampacity with the values measured after the system was installed. The rated power output of the PV plant is approximately 10 MW using single axis tracking. In order to provide an optimal design for reliability and cost, the cables were specified using the NeherMcGrath calculation method and designed to operate near their maximum temperature rating. Several measurements of the soil thermal resistivity were made on soil samples from the installation site. Many of these measurements yielded soil resistivity values higher than 90 °C*cm/W used to calculate the tables in the National Electrical Code (NEC). These tables are commonly used by engineers to determine cable ampacity, sometimes without knowledge or consideration of the actual soil resistivity. Many of the measured resistivities also exceeded the highest value of RHO, 120°C*cm/W, used in the tables of IEEE Std. 835.
The highest thermal resistivity resulting from the testing onsite was 329°C*cm/W, which was given at a soil moisture of 0%. The engineers designing the cable system for the photovoltaic (PV) system used a soil resistivity value of 270°C*cm/W for their design. This was the average of all the tests taken at 0% soil moisture content. Furthermore, the design used a load factor of 53% that was based on measured data from similar projects and calculations from a commercially available software titled PVsyst. These parameters were used by the design engineer in the Neher-McGrath method to specify conductor size and such installation details as cable spacing and depth. The cable was selected based on a maximum operating temperature 75˚C, which is the limit of the cable terminations. Sensors were installed at the site to measure actual cable temperatures, and these measurements were compared with the temperatures anticipated from the design calculations.
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