United Illuminating Co. (UI; Shelton, Connecticut, U.S.) was looking for ways to extract extra capacity from an existing pipe-type cable circuit. With additional EPRI funding, UI commissioned Power Delivery Consultants (PDC) to perform an ampacity audit of the circuit.
The resulting PDC review of utility records found insufficient as-built details to determine the burial depth. Burial depth is a critical parameter when determining the ability of the surrounding soil to transfer heat from the cable. This information would be critical to finding the as-built ampacity of the high-pressure gas-filled (HPGF) circuits, which have been in operation for more than 40 years. Fortunately, the limiting portion of the double circuit had a spare pipe running in parallel to the two energized pipes. Cable engineers ran a gyroscopic sensor (GyroData; Houston, Texas, U.S.) through the pipe. This “rate gyroscope,” used on spacecraft to determine orientation in the absence of gravity, measures the azimuth to 0.1-degree accuracy and inclination to 0.05-degree accuracy enabling the user to determine the X-Y-Z coordinates with better than 1-ft (0.3-m) accuracy. The gyroscope was pulled through the empty cable pipe at speeds up to 50 ft (15 m) per minute. The gyroscopic survey was coordinated to the same reference as a hydrographic survey, giving the burial depth for differences in the vertical coordinate along the route. The ampacity audit gave an accurate evaluation of installed capacity and provided UI with critical data to upgrade the ratings.
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