Purpose of Report: To explain current SEU policy on line construction

Background: SEU recently undertook to construct a section of overhead line on Sangre Road north of 19^th Street. This line has caused questions from some concerning the reasoning for overhead construction instead of underground type construction in certain areas. This report will attempt to explain the present departmental plan concerning the policy.

Discussion:

Section line roads provide the main travel ways throughout Stillwater. They are usually straight for mile lengths, have dedicated rights of way which are usually clear, and are the roads from which other side streets originate. These side roads provide access to residential subdivisions, apartment complexes, businesses, parks, and schools. Due to their purpose, the section line roads are also the logical paths for main electrical feeders that carry power circuits to these other areas.

SEU uses section line roads for substation feeder class overhead distribution lines. These circuits are rated for 600 amps at 12,470 volts and are designed ultimately to be supplied from either direction from two different substations. This configuration allows SEU to perform switching to do maintenance work on parts of the line without requiring outages to customers.

From these section line circuits, fused tap circuits are constructed that are rated for 200 amps to supply subdivisions, businesses, apartment complexes, etc. If a problem originates on the tap circuit, the fuse blows shutting off the power to the problem area and protecting the section line circuit from tripping off. This design keeps the other customers in service and only disconnects the problem line from the main feeder.

These 200 amp tap circuits can be either overhead or underground, but the section line circuits are almost always overhead construction. This design condition is due to several factors, the main one of which is cost.

For a specific example, a 1/2 mile long section of overhead construction rated for 600 amps was estimated to cost $17,366 including materials and labor. That line can be tapped conveniently at any of the poles for overhead service extensions. If a pole is not readily available where it is needed, another can be installed for a tap point, or one could be moved to the correct location without excessive expense or trouble. Street lights can be suspended from the poles to light the roadway. If underground service is required, an underground "riser" can be installed on a pole and then underground conductor can be run from that location to the padmounted transformer needed to serve the given customer. These are all rather simple attachments to the overhead line and are used throughout the city. The average cost of this type line construction is about $6.68 per foot assuming a 2,600 foot line.

The underground system which is equivalent to the above overhead one is much more complicated. The 600 amp rated conductor is 1-3/4" in diameter and costs approximately $10.27 per foot based upon an October 1998 estimate. The wire weighs a bit more than 4 pounds per foot and because it is rated for 15,000 volts, it requires special terminations and splices at all end points.

When a tap is to be made on this type wire, a piece of padmounted switchgear rated for 600 amps is required. The box this switchgear is mounted in is 5'-7" wide, 3'-8" tall, and 5' deep. It contains two, three-phase switches for the main line (in and out) and two, three-phase fused disconnects for the tap lines. This switch cubicle costs $10,000 and the concrete foundation upon which is rests ranges from $500 to $750 depending upon its depth. At each of these cubicles, high voltage terminations are required on each conductor. Taps cannot be made "mid span" on this type wire and if another tap is needed, the conduit system and wire must be cut, another manhole and switchgear installed, and new wire pulled and terminated on that switchgear and the switchgear units on either side of it. The point to note here is that it is costly to make taps on high voltage cable, so it is important to design the system with switch cabinets at known lot corners for future taps onto the circuit because it becomes very expensive to relocate or add to these tap points and cabinets once the circuit is constructed.

For a 1/2 mile section of 600 amp underground feeder along a section line road, manholes were planned at 300 foot intervals for pulling purposes. It was estimated that four switchgear would be required for tap lines extending from this main circuit. All the conductors would be installed in conduit with one conductor per conduit. One spare conduit was included in the installation costs. The total cost of materials and trench for this 1/2 mile length of underground circuitry was estimated to be $170,173 for an average cost of about $65.45 per foot. It is noted that of this cost, $86,637 is for the underground conductors alone.

This estimate did not include the provision of street lighting for either of the options. However, for the overhead system, it is easy to install a fixture and arm on the given poles. Needed transformers can be hung on the poles and the lighting system is functional. These type lights cost about $100 each and the arms are $30.

For the underground option, poles with fixtures are required and cost about $500 each. For that construction, pad mounted transformers would need to be located in the road right of way at intervals. They would supply power to an additional duct system which would tie the light standards together. In this instance, poles would still be installed on the right of way, but they would be aluminum ones.

Both the overhead and underground systems have expected lives of about 30 years. The argument is sometimes made that since underground is not harmed by the normal "above ground" elements, it is more cost effective in the long run because it does not need repair as often. It is true that during the 30 year life cycle, the overhead line may require repairs due to wind damage and vehicles may hit the poles. If trees grow up near the lines, then they will also require trimming. In most cases, though, the need for corrective actions can be readily seen and the repairs made fairly simply and quickly.

Failures in the underground conductor are usually due to dig ins or insulation breakdown. If the problem is a dig in, then finding the location to repair is evident. If the insulation breaks down, then the location of the problem may be harder. Fault indicators may help localize the problem between two of the locator devices, but the repair requires determining which section of line is bad. In either case, the repair involves replacing the bad segment between two adjacent manholes.

In the present design, that means the removal and replacement of 300 feet of cable. The expense of the cable is $3,081 at the estimated price plus the cost of the terminators. That cost would cover a substantial amount of the usual maintenance expected with an overhead line. If a vehicle hits and damages one of the switchgear boxes, then the replacement may cost $10,000.

From these estimated cost calculations, it is noted that in this feeder construction class used on section line roads, the cost between an overhead and underground circuit of the same general configuration is the difference between $6.68 per foot for overhead and $65.45 per foot for underground. This is a substantial difference which cannot be justified by staff given the functionality of the overhead line and the lack of the same with an underground circuit. The much higher expense of the underground circuitry also goes against staff's mission to "responsibly use the resources which have been entrusted to us."

It appears that the main reason for using underground circuitry for this class of circuit is one of aesthetics. While that desire may have an overriding influence in specific locations, such as within industrial or commercial parks that also require large blocks of power to given locations, the aesthetic reason cannot be used to justify the general use of 600 amp underground feeders throughout the city's service area due to the substantial financial repercussions on the utility. Somebody has to pay for these higher cost installations and it has been the understanding of staff that SEU customers have not wanted to invest that much money just for aesthetic purposes. In a period when deregulation may soon put more financial pressures on the utility, changing the current policy to require the higher cost underground option to achieve the same functional result as the overhead system does not seem prudent.

Recommendation:

It is the recommendation of staff that the Trustees allow SEU to continue the policy of installing overhead electrical circuitry on section line roads.

Recommended by:

___________________________

Carl Weinaug

City Manager