Hawaiian Electric Co. has nearly 1300 structures on its 28 138-kV transmission circuits that span the island of Oahu. The system is isolated with no interconnections to other utilities. A single structure failure should not cause customer outages, but the system is at an increased risk for overload and outages if additional failures occur. The 138-kV transmission structures generally are one of the following types: wood poles, steel poles, fiberglass poles, aluminum frames, and steel or aluminum lattice towers. The wide variety of structures is needed for the topography of the island, ranging from mountain ridges, valleys, side slopes and urban areas.
Steel and wood poles comprise most of the structures, typically located in the mountainous areas of Oahu. Currently, more than 35% of the transmission structures have been in service more than 40 years and nearly 20% have been in service more than 50 years. Because of its aging transmission infrastructure, Hawaiian Electric has begun replacing the existing transmission structures.