Something You Don’t See Every Day

Workers construct new gas-insulated bus assemblies (mainly the long, shiny metal structures along the ground) for the 345-kilovolt Harding-Chamberlin and Juniper-Harding transmission lines at the Juniper substation earlier this year.

October 16, 2019

Replacing transmission line conductors inside a substation is not an uncommon task. Replacing conductors sealed inside an enclosure filled with an inert gas, however, is a more unique undertaking. That’s exactly what was involved in a recent Energizing the Future project at the Juniper transmission substation in northern Ohio.

How common is gas-insulated equipment?

The Harding-Chamberlin and Juniper-Harding lines are the only two with gas-insulated segments within FirstEnergy’s transmission systems. Gas-insulated switchgear – which encases all the conductors, switches and breakers – is used at three substations with tight space limitations: Horizon in Cleveland, which powers Progressive Field, Rocket Mortgage Fieldhouse and other important downtown venues; and Morristown and Smithburg in northern and central New Jersey, respectively.

Back in 1976, the 345-kilovolt (kV) Harding-Chamberlin and Juniper-Harding transmission lines were connected to the Juniper substation – with a portion of these 345-kV conductors routed along the ground through gas-insulated transmission line (GIL) enclosures inside the substation to safely cross the path of overhead high-voltage conductors. Gas-insulated equipment is sealed inside metal enclosures filled with sulfur hexafluoride gas, which offers excellent electrical insulation and arc extinction properties. It is well suited to installations where space is tight, as in a few instances across our transmission systems (see sidebar).

While these GIL sections functioned well for more than 40 years, a project to replace them was initiated last year after a fault on the Harding-Chamberlin line. Jason Forristal, transmission project manager V, noted: “We’ve dealt with gas-insulated bus removal or maintenance, but this was the first new installation I’ve worked on. While the installation of the bus itself is a pretty standard operation, special environmental precautions and testing requirements are necessary because of the gas.”

After testing confirmed six updated GIL assemblies were leak-tight and new monitoring systems were functioning properly, the Harding-Chamberlin and Juniper-Harding lines were returned to service in late May.

Updated design features of this new GIL installation offer a number of improvements:

Segmenting each enclosure into three sections, instead of one long enclosure, will aid in detecting and locating problems. It also will minimize outage time since only the affected part of an enclosure will need to be isolated, drained and opened for maintenance or repairs.
Expanded, continuous monitoring capabilities enable the checking of equipment status and gas condition in real-time. These features supplement supervisory control and data acquisition (SCADA) gas pressure alarms duplicated from the previous setup. Status and condition data can be checked at the substation and is continuously monitored remotely by the equipment vendor.
Gas sampling capabilities now available on selected enclosure segments add reliability and protection by allowing gas samples to be drawn for laboratory analysis.

“This new installation significantly increases the resolution of gas monitoring and other local information,” said Ken Edwards, engineer V, Equipment & Standards. “This will help with moisture detection and watching how environmental changes affect the gas.” With these new capabilities, the net result is a clearer picture of the equipment’s function and condition at any time.