As part of the ERI initiative to help ensure the continued reliability of its energy supply, BGE initiated a project to construct a new transmission line in the southernmost portion of its service corridor. The project involved the installation of a 115-kV, 3-mile-long underground transmission cable by means of a concrete-encased duct bank system from BGE’s Raphael Road substation in Baltimore County to its Joppatowne substation in Harford County.
The installation of underground transmission lines is a demanding process, as there is no easy way for the heat generated by these expensive high-voltage cables to escape. If there is too much power running through a cable and its operating temperature becomes too hot, the result can be the derating of the cable ampacity, hot spots, reduction in cable lifespan and ultimately premature cable failure. To create a stable thermal environment underground, utilities must rely on custom-engineered thermal-grade concrete for the duct bank encasement and advanced thermal fill materials with optimal thermal resistivity properties for conducting heat away from the cable.
According to Zachary Lovett, value-added products manager at Aggregate Industries US, a significant cause of excessive heating and failure of underground transmission lines is the improper selection of thermal fill materials. “For the BGE project, the use of custom-designed thermal concrete and thermal fill solutions was of paramount importance to enhance heat dissipation, increase the allowable ampacity and ensure the new transmission line operated efficiently over its designed 40-year life expectancy,” he explained.
To meet the stringent performance criteria for the project, Aggregate Industries’ regional quality-control laboratory developed more than 15,350 cubic yards of Thermaflow™, an advanced thermal fill product that is a custom-engineered mix of local materials, pretested by regional laboratories and verified by third-party independent laboratories to ensure predictable and reliable material performance.
The strengths of the thermal concrete (duct bank encasement) and flowable thermal backfill materials for the BGE project were designed to achieve 4,000 psi and 300 psi at 28 days, respectively. The maximum thermal resistivity (rho values) of both materials was specified with a maximum thermal resistivity of 100° C-cm/W at 0% moisture. “Based on the specified performance characteristics of the material to be used in the project, we designed our mixes to those criteria, exceeded the performance and then consistently tested the material throughout the project cycle to confirm the delivery of the design parameters,” Lovett said.
The performance of the custom-engineered Thermaflow™ exceeded expectations, as the mixes delivered superior thermal resistivity values—from the mid-70s to the low-60s at 0% moisture—to ensure maximum heat dissipation from the cables. In addition, LafargeHolcim’s expertise in self-consolidating, nonsegregating mixes with precise spread and viscosity facilitated the development of thermal mix designs with the ideal flowability and small aggregate sizes to pack firmly around tight spaces between the runs of conduit.
“For the BGE project, the use of custom-designed thermal concrete and thermal fill solutions was of paramount importance to enhance heat dissipation, increase the allowable ampacity and ensure the new transmission line operated efficiently over its designed 40-year life expectancy.”