UA North Substation

TEP is proposing to build a new 138-kilovolt (kV) substation in Central Tucson to strengthen electric reliability for residential and commercial customers in central Tucson and help satisfy growing energy needs in our community.

The new substation will interconnect with TEP’s planned Kino to DeMoss-Petrie 138 kV Transmission Line Project.

Project Details

Public Outreach Materials

Frequently Asked Questions

What are GIS substations and why build them?

TEP plans to design and construct UA North as a gas-insulated substation (GIS). TEP typically builds open-air substations on 5-7 acres near energy load demands where it can efficiently serve customers’ energy needs, considering proximity to existing infrastructure, geography, and other factors.

Electric utilities in the United States and other countries build GIS substations when challenged by space constraints. GIS substations use a nonflammable, nontoxic gas to insulate electrical equipment in sealed conduit, which prevents emissions. These substations require a smaller physical footprint and offer improved safety because circuit breakers and other equipment are enclosed.

What type of gas will be used as a dielectric insulator in the substation?

Sulfur hexafluoride (SF6) is a heavy, inert, nontoxic and incombustible gas. It’s among the most stable gaseous chemicals with an inertia similar to that of nitrogen gas. SF6 has excellent electric insulating and arc-quenching capacity. It has been used extensively by electric utilities in electrical transmission systems and electrical distributing devices.

Are there any potential health or safety concerns regarding SF6?

Sulfur hexafluoride is physiologically harmless to humans and animals. Although SF6 displaces oxygen, it generally does not present a concern in open areas. If an arc occurs inside the hermetically sealed interrupting chamber, the byproduct is considered harmful. However, this harmful byproduct would be contained within the sealed interrupting chamber inside the substation. TEP crews are specially trained to monitor for the byproduct, and to properly clean and dispose of it if an arc occurs.

Are there alternatives to using SF6?

At this time, no reliable, commercially viable alternatives are available for high-voltage operations. Oil-filled circuit breakers are no longer an option because they are no longer manufactured.  Studies suggest vacuum interrupters are not well suited to higher voltages.  Alternative gases are being studied. TEP currently is participating in a technical working group evaluating environmentally friendly alternatives to SF6.

What does TEP do to prevent SF6 gas from being released into the atmosphere?

Although SF6 is a potent greenhouse gas, it is used in an enclosed system that prevents the gas’ escape into the atmosphere.

TEP’s Tucson Substation uses closed-pressure equipment. Although documentation from the GIS manufacturer describes the SF6 leakage rate as less than half of one percent each per year, continual field testing indicates that Tucson Substation GIS equipment has experienced no leaks since being placed into service in 2010.

To ensure that SF6 emissions are minimized during operation of the Tucson Substation GIS equipment, TEP follows several safety and maintenance processes that include:

  • Conducting major substation inspections every 4 months, including checks for leaks.
  • Equipping breakers with alarms that are monitored 24/7 by TEP’s System Control office in the event of a change in gas levels.
  • Closely tracking gas levels. If gas needs to be added more than twice a year, field crews would schedule maintenance and perform leak detection with a special SF6 camera.
  • Enacting several safety measures if issues with purity, moisture, or arc byproduct are detected, including taking the breaker out of service and sending gas to a specialized contractor for destruction if it doesn’t pass testing.

These safety and maintenance processes align with the EPA’s Reducing SF6 Emissions in Electric Power Systems: Best Industry Practices.

Do any government agencies perform oversight of SF6 use?

SF6 emissions are reported to the Environmental Protection Agency’s Greenhouse Gas Reporting Program (GHGRP).