The United Illuminating Transmission Business


UI’s transmission business unit, established in 2006, achieves growth by identifying, pursuing and capturing transmission opportunities to enhance reliability and market efficiency and deploying infrastructure solutions that provide stakeholder value.

The 345,000 volt (345 kV) Transmission System

The highest level of transmission voltage currently within the UI system is 345,000 volts (345 kV).  This level of the transmission system consists of approximately 11 miles each of overhead high voltage lines and underground cable systems, totaling 22 miles of 345kV transmission.  The system is interconnected to the New England 345 kV electric grid and our neighboring utilities.

The 115,000 volt (115 kV) Transmission System

The next highest transmission level in the UI system is 115,000 volts (115 kV) consisting of approximately 93 miles of overhead high voltage lines and approximately 17 miles of underground cable systems.  A majority of the 115 kV system overhead high voltage lines are within the Connecticut Department of Transportation railroad right of way between New Haven and Fairfield, while the underground system is typically within metropolitan areas of New Haven and Bridgeport.  The 115 kV transmission system provides power to several transmission (115 kV) to distribution (13.8 kV) level substations.  The 13.8 kV distribution infrastructure at these substations provides power to localized commercial, industrial and residential customers.

Recent achievements:

  • Middletown Norwalk 345 kV Project – Completed in 2008, 345kV XLPE cables, 115kV XLPE cables, Gas Insulated Switchgear (GIS) equipment technology, 345 KV variable shunt reactors, 600 MVA 345/115 kV auto-transformers
  • Grand Avenue – New 115 kV GIS Switching Station in New Haven.
  • Several 115 kV reliability upgrades within Southwest Connecticut
  • Pootatuck 115/13.8 kV Substation in Shelton