The high-voltage transmission interties that connect today's electric grid were engineering achievements and were carefully developed over decades to improve reliability, lower costs and make better use of generation resources, according to a new report from Grid Strategies.

The report, Making Connections: A History of the Regional Electrical Lines That Shaped the U.S. Grid, traces the evolution of transmission interconnections from the earliest utility tie lines in the late 1800s through the development of major regional systems like the Pennsylvania-New Jersey-Maryland (PJM) Interconnection and the Pacific Intertie.

Rather than focusing solely on technological milestones, the report examines how regulators and utilities consistently viewed transmission interties through three primary objectives: improving efficiency, strengthening reliability and taking advantage of complementary generation resources across regions.

"Interties, also known as interconnections, ties, or tie-ins, are high voltage transmission links that enable the bulk transfer of power between independently operated and geographically dispersed electric utilities," the report states. Over time, these connections transformed isolated utility systems into integrated regional networks that improved operations and allowed utilities to coordinate hydroelectric and thermal generation across broad geographic areas.

The report highlights how early engineers recognized the potential of interconnected systems. At the 1904 International Electrical Congress, Utah Light & Power engineer R.F. Hayward predicted that interconnected utilities could one day make it "possible to-day to operate a string of steam and water plants from the Atlantic to the Pacific Coast," foreshadowing the modern interconnected grid.

Three historical projects illustrate the evolution of regional transmission planning.

The first, the 1931 Greenbush-Dunwoodie transmission line in New York, connected hydroelectric resources in upstate New York with steam-powered generation serving New York City. Regulators concluded the line would defer costly new generation investments while improving service reliability by allowing the two systems to support one another during seasonal fluctuations and peak demand.

The report next examines the Roseland-Bushkill transmission line, completed in 1927 between New Jersey and Pennsylvania. The project created what became the foundation of today's PJM Interconnection while demonstrating that utilities could coordinate operations without merging into a single company. Regulators emphasized that interconnected utilities remained operationally independent while sharing resources to reduce redundant infrastructure and improve reliability.

The final case study focuses on the Pacific Intertie, constructed between 1965 and 1970 to link hydroelectric resources in the Pacific Northwest with California's growing electricity demand. Federal agencies promoted the project as both an economic and conservation measure, enabling seasonal power exchanges between regions with complementary generation profiles.

The report notes that by the postwar period, federal agencies increasingly viewed transmission integration as essential for meeting rapidly growing electricity demand while maintaining system reliability. Following the Northeast Blackout of 1965, the Federal Power Commission described interties as "vital factors in assuring reliable bulk power supply" that had proven their value over decades of operation.

Throughout the report, the authors argue that the same principles that drove historical transmission development remain relevant today as utilities confront rising electricity demand and increasing pressure to expand the grid.

"The history traced in this report shows that interconnection was consistently pursued according to a recognizable set of technical, economic, and regulatory logics shared by utilities and government authorities," the report concludes.

Across more than a century of transmission development, the authors write, interconnections consistently enabled utilities to exchange surplus energy, reduce the need for redundant infrastructure, improve system resilience and make more efficient use of available energy resources.