Determination of Hotspot Location and Conductor Temperatures from Spare Duct Temperatures in an Underground Installation

In this work a comparison has been made between the predictions from the models using both the present theory for the underground cable temperature prediction and the CYMCAP application and the field measurements to determine which, if any, models are capable of predicting the temperature and hotspot locations in an installation where the power cable is not embedded with the optical fibers and, therefore, where the cable temperatures must be inferred from the temperature measurements made in nearby spare ducts. The temperature measurements were collected from the underground 69 kV cable at the Brandow-Pickrell installation, which is a part of Salt River Project’s power sub-transmission system. The model development and the results are explained in detail. Results from the model developed have been compared and the factors affecting the cable temperature are highlighted.

Once the models were developed, it was observed that the earth surface temperature above the installation, solar radiation and other external factors such as underlying water lines, drain pipes, etc. play a key role in heating up or cooling down the power cables. It was also determined that the hotspot location in the power cable in the main duct was the same as the hotspot location in the spare duct inside the same installation.

It was also observed that the CYMCAP model had its limitations when the earth surface temperature variations were modeled in the software as the software only allows the earth’s ambient temperature to be modeled as a constant; further, results from the MATLAB model were more in line with the present theory of underground power cable temperature prediction. However, simulation results from both the MATLAB and CYMCAP model showed deviation from the measured data. It was also observed that the spare duct temperatures in this particular underground installation seemed to be affected by external factors such as solar radiation, underlying water lines, gas lines etc. which cannot be modeled in CYMCAP.