Underground Power Cables
Underground cable installation is a “one-shot deal”, it may come back to haunt you if not constructed properly.
For Example: Underground cables installed at a wind-farm were tied directly to a main feeder cable. Unfortunately, the cables were simply placed in a trench using native soil as the backfill with minimal soil compaction. Ampacity calculations were performed using typical soil values, but thermal properties were not measured. Since wind turbines operate fairly continuously, the feeder cable often ran at maximum capacity. The heat generated from the feeder cables dried out the surrounding soil completely. Because the native soil was ‘poorly compacted fine silt’, it acted like an insulating blanket and the cable failed prematurely. A significant source of potential problems with underground circuits is the improper selection and installation of thermal backfill materials. To prevent premature failures, cable systems must be installed in a ‘thermally’ hospitable environment.
Few utilities have stringent specifications for conducting thermal surveys of the cable routes, and quality-assurance programs for the selection and installation of cable-trench backfill. This often leaves the decision up to the civil contractor. Unfortunately, the effects of incorrect thermal parameters of native soils and of poorly installed thermal backfills may not be evident for many years. When the cable loads increase and approach the design value, the temperatures rise beyond allowable levels, resulting in cable failures. The remedial cost of removing and replacing poor backfills is very high or impractical; especially under paved roads. The loss of revenues from de-rating a system may be even higher. Installing a new circuit may be the only, albeit expensive, option.
Importance of Soil and Backfill
Energized cables generate a lot of heat that must be dissipated through the backfill and native soil. The ability of the surrounding soil to transfer the heat determines whether an operating cable remains cool or overheats.
In order to determine the thermal properties of soils and backfills, one must conduct field and laboratory measurements using an industry standard test instrument such as Geotherm TPA2000; originally developed and designed under the sponsorship of Electrical Power Research Institute (EPRI EL-2128).
Fluidized Thermal Backfill (FTB)TM
In the 1970’s and 1980’s, utilities in Canada and the United States undertook the task of developing special thermal backfills for underground power cable installations. These materials were to provide very good thermal properties over a wide range of moisture contents, while simplifying the installation techniques. Much of this work was performed under the auspices of organizations such The Canadian Electrical Association (CEA), Electric Power Research Institute (EPRI) and the Civil Research Department of Ontario Hydro.
One of the best performing backfills was a Fluidized Thermal Backfill (FTB) containing sand, aggregate, cement and fly ash. The FTB is an engineered material that has very low thermal resistivity even when totally dry and has very long thermal dryout times (thermally very stable). By changing the mix proportions of the components, the strength and other mechanical parameters can be designed to suit specific field conditions and requirements. The component materials are carried by most concrete suppliers and FTB of a specific mix design is delivered in ready-mix concrete trucks. The overall installation and quality assurance tasks are very much simplified since the backfill can be poured into place and hardens to the desired density with no mechanical working of the material. Although granular type backfills (thermal sand and limestone screenings) are still being used by many utilities, the use of FTB over the past 25+ years has become very widespread.