Today, more of the grid is being placed underground, minimizing issues with certain types of weather and accidents. Undergrounding is up to five times as expensive per mile as overhead transmission; even so, some communities are mandating any new or rebuilt circuits be placed underground.
Direct burial cable installed between 1965 and 1980 was mostly composed of an insulated cable that had a flaw in the insulation material and now requires either total replacement or the injection of gel to prevent further electrical treeing. Both methods are being done with success, but this work adds to the cost of maintaining existing underground systems.
Buried cable location was approximated on many drawings, being routed around trees and other objects, or laid in the approximate location of the drawings for expediency. This means that finding issues with underground cables in order to repair or replace them takes extra time. Groundpenetrating radar can be helpful in updating actual locations of underground cable.
Many of the conduits that were installed before 1990 were concrete or clay pipes, installed under streets that now see much heavier vehicles and heavier traffic. As such, these conduits have, in many places, collapsed. The use of ground-penetrating radar can find the locations of such collapsed conduit.
With the electrification of transportation and other areas where fossil fuel is used, larger or higher numbers of conductors are going to be needed. In an underground system, large cables are hard to pull, especially around bends. Changing standards to provide larger conduits and larger radius corners will make both of those upgrade options easier. Oversizing conduit for today’s needs, will mean that if strong enough conduit is used that larger or more conductor can be pulled at a future date. Another alternative is to install an extra circuit’s worth of conduit when trenches are opened to install new duct banks.
An advantage of installing larger underground conduit is that longer lengths of conductor can be installed in a single pull. Georgia Power, for example, has pulled underground cables over 1,500 feet, with better conduit design and specialized equipment. Since splices are the cause of 70% of outages in the underground system, the longer distances between splices can mean fewer outages and less operations and maintenance cost.
Underground conductors should have high-quality jackets to avoid future electrical treeing incidents. Ideally, underground conductors will last for 50 or more years when properly installed and spliced. The extra capital cost for quality cables typically can be justified with reduced costs for operations and maintenance.
Like overhead systems, underground cable systems should work to pad-mount as much of the accessory equipment as possible. This allows for quicker, safer inspections and battery changes and makes wireless communications more reliable. While excellent, highly durable antennas exist to be installed in the center of manhole covers, pad mounting is in the long run easier to deal with from an operations and maintenance standpoint. The other advantage of pad mounting is that less training and fewer stock keeping units are required when the underground system equipment matches overhead equipment.
Paper-wrapped lead splices are commonly disliked among utility workers. They require work to be performed in extremely tight quarters with little room to move and at chest height to the worker. When stacked brick was used to provide support for manholes, this kind of tight quarters was required. Today, structural design has advanced so significantly that manholes and vaults can be made large enough to support all maintenance activities.
With arc flash, air quality and other safety concerns, these larger underground structures are justified. Installing permanent cable racks and monitoring equipment in these spaces becomes both possible and desirable. High-pressure air lines can be permanently installed in manholes, ready for a hookup to a compressor or tank on the truck. This means faster degassing and a quicker turnaround for inspections and maintenance, just as airline hose masks or air packs provide for utility workers. Permanent insertion points for safety rails and hoist rigs also lessen setup time and make the equipment safer and sturdier. These measures add up to make workers safer, reducing the chance of injury while entering and leaving an underground structure.
In spot networks, the standard is to avoid having multiple feeders that follow the same route. Larger vaults provide the means to put distance between feeders when required. They give space to create fire-resistant structures in the manhole to shield one or more feeders from the rest of the conductors in the vault. New materials that provide better fire resistance are on the market, and standards engineers should consider adopting them.