Interties

Interties are transmission facilities that connect two or more electric power systems or grids, enabling the transfer of electric energy between them. They can be overhead lines, underground or submarine cables, or high-voltage direct current (HVDC) links. Interties are designed to carry large powers at high voltages and are rated in megawatts (MW) or gigawatts (GW) to indicate their transfer capacity.

There are two main types of interties: alternating current (AC) interties and HVDC interties. AC interties synchronize the connected grids so power can flow while maintaining a common frequency and stable operation. HVDC interties connect grids that may operate out of step or at different frequencies, or where precise control of power flow is desirable. HVDC links provide directional control, reduced losses over long distances, and can improve stability by decoupling the dynamics of the two systems, though they require converter stations at each end.

Interties serve several core functions. They enable cross-border and regional energy exchange, increasing reliability by sharing reserves and providing alternative supply paths during outages. They help relieve transmission congestion, enable integration of remote or variable renewable resources, and support market operations by expanding trading opportunities. In some cases, interties also contribute to black-start capabilities and overall system resilience.

Operation and regulation involve coordination among transmission owners, independent system operators or regional entities, and market regulators. Interties must be designed, owned, and operated under interconnection agreements, reliability standards, and protective equipment schemes. Scheduling and dispatch are typically governed by market mechanisms and reliability criteria to maintain stability and prevent adverse flows.

Examples include HVDC interties linking distant regions, such as the Pacific Northwest and other areas, as well as numerous AC interties within large regional grids in North America and Europe.