A Carbon Emission Accounting Method for Engineering Materials and Key Equipment in 110 kV Power Relocation Projects

Power transmission and transformation projects involve substantial quantities of engineering materials and key equipment with significant embodied carbon emissions, yet project-level carbon accounting methods remain insufficiently tailored to power engineering practices. This study takes a 110 kV transmission and transformation relocation project as a case study and develops a bill-of-quantities-based life-cycle carbon emission accounting method. The accounting scope covers civil engineering materials, cable systems, gas-insulated switchgear (GIS), and other key engineering components, and the carbon reduction potential of low-carbon material and equipment substitution is further evaluated. The proposed framework integrates engineering decomposition, hierarchical mapping from bill-of-quantities items to material and equipment inventories, component-level carbon factor modeling, carbon factor consistency calibration, and carbon-cost dual-objective scenario analysis. A cradle-to-grave boundary is adopted, covering material production (A1–A3), transportation (A4), construction (A5), operation and maintenance (B), and end-of-life treatment with recycling credits (C). Compared with conventional project-level carbon accounting that first aggregates engineering quantities into a flat material list, the proposed method preserves the bill-of-quantities hierarchy during inventory translation and assigns life-cycle modules at the item-to-component conversion step. This improves traceability from engineering packages to carbon hotspots and makes the accounting results more directly usable for procurement-oriented low-carbon decisions. The baseline case yields a net life-cycle carbon footprint of 455.5 t CO2e, with material production contributing 305.2 t CO2e (67.0%) and operation and maintenance contributing 147.2 t CO2e (32.3%). At the component level, 110 kV XLPE cables and SF6-related impacts dominate, accounting for 31.8% and 27.1% of the total, respectively. Individual substitution measures reduce emissions by 3.1–16.5%, while the combined strategy lowers the footprint to 355.8 t CO2e, achieving a 21.9% reduction with a 7.2% cost increase. The results demonstrate that the proposed method can effectively translate engineering quantity documents into traceable project-level life-cycle carbon accounting results, providing support for low-carbon material selection, equipment procurement, and early-stage decision making in 110 kV transmission and transformation relocation projects.