Tillamook 115kV UG Feasibility Assessment Page 1 of 14

115-kV UNDERGROUND CONSTRUCTION ALONG FRONT STREET

LOCATION AND ALIGNMENT

Tillamook PUD plans to construct a 115-kV overhead transmission line between the Bonneville Power Administration Tillamook Substation east of the City of Tillamook and a proposed PUD substation between the towns of Netarts and Oceanside. The City has asked Tillamook PUD to investigate the concept of placing a 0.5-mile-long segment of the alignment underground along Front Street in Tillamook. The underground portion would parallel the south side of Hoquarten Slough. The underground facility would cross under Oregon Highway 101 and would need to accommodate the future planned rearrangement of Highway 101 at the Hoquarten Bridge. The following map indicates the location of the alignment and the end points of the underground segment.

Project location

SYSTEM CONFIGURATION

The system configuration is determined by the following criteria:

1. Reliability Requires a 4-Cable System. The Oceanside 115-kV Transmission Line will be the only 115-kV circuit serving the Netarts-Oceanside Area. This circuit is composed of three energized wires. If the underground segment along Front Street were constructed with three cables, a failure of any component of the underground segment would cause an area-wide outage in the area served by the new Netarts/Oceanside substation. Repair of an overhead transmission line is usually accomplished in a matter of hours with the PUDs linemen, equipment, spare conductor, and hardware. However, failure of an underground cable, splice, or riser terminator will require a minimum of several weeks or months to repair the failed components. This

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is because high-voltage underground systems and the individual components are specialized devices, carefully matched with each other, and requiring technicians of carefully maintained training and certification. The PUD would store spare components, but with no underground transmission technicians on staff, the PUD would need to schedule a specialist to mobilize to Tillamook with the proper equipment to make the repairs.

Although cable rated 35 kV and below is common, cable for transmission voltages is rarely justified, and is therefore rarely installed. Because transmission line outages can affect thousands of consumers, transmission cables are manufactured to higher dimensional and purity standards than the common underground distribution cable. To reduce deterioration due to the high voltage stress, 115-kV transmission cable incorporates a welded copper or lead moisture barrier to preclude any water molecules from the insulation. As a result, the terminations and splices must be installed in clean room laboratory conditions. Consequently, field installations require building a climate-controlled room around termination sites or inside the splice vaults. The cable ends must be prepared to exacting tolerances and cleanliness. It is typical to expect that the installation of each high-voltage terminal or splice will require one 8-hour shift by a skilled technician.

Even with the best of transmission cable installations, there remains a probability of an equipment failure due to manufacturer or installation defect. An underground transmission system has an expected life of about 50 years, and the failure probability tends to increase with age. In addition, there is always the risk of damage due to severe storm, vehicle accident, fire, or vandalism. Because repairs typically require weeks, owners of transmission systems must mitigate the risk of a failure. Usually, the risk is covered by constructing a redundant circuit on a separate alignment. These looped systems are typical in metropolitan transmission networks.

The PUDs plan for the proposed Oceanside 115-kV overhead transmission line does not include a second 115-kV transmission source to the proposed Oceanside Substation. Instead, the PUD plans to improve its existing 25-kV distribution line to the Netarts-Oceanside area so that it can serve as an emergency power supply in the case of a brief transmission outage.

However, a failure in the 115-kV underground segment would take weeks or months to correct. The planned emergency distribution tie line to the Netarts/Oceanside area will be designed to serve the area for a few hours or days at a time, but it will not be practical to design this tie line with enough efficiency and reliability for long-term (weeks or months) transmission line outages. Therefore, in my opinion, because a single-component 115-kV underground failure is the is most likely mode of failure, the PUD should install four, terminated, and tested cables; this way, the failure of any one of the three normally-energized cables can be by-passed with the fourth (spare) cable to restore service. With the spare 115-kV cable operating, The PUD can order parts and arrange for qualified repair personnel and equipment. When finally ready for the permanent repairs to the failed component, the 115-kV transmission would be de-energized and the emergency distribution tie line would be used to carry the load. With the fourth cable in place, the emergency tie will only operate during the several days that the 115-kV line is actually shut down for repairs.

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Three of the 115-kV cables would be normally energized. The fourth (emergency) cable will be terminated on the side of the riser pole where the overhead conductors are deadended. The spare cable termination would be positioned between the overhead conductors so that it can be jumpered to take the place of the failed cable or terminator. This process can be done with a man-lift truck and with the system de-energized and grounded.

The design and planning of the permanent repair must begin as soon as possible. The nature and cost of the repair will depend on the extent of damage. Replacement cable, terminators, and additional underground splice vaults may be required.

115-kV CABLES

The cables specified for this system will have a stranded copper conductor at its core and insulated with cross-linked polyethylene. The cable construction will include a conductor shield material, an insulation shield material, an impervious copper or lead inner jacket and a tough plastic outer jacket. The underground cables will be designed to match the thermal capacity of the overhead conductors of this transmission line. Given the thermal properties of the duct bank and concrete encasement, the size of the stranded copper conductor of the underground cable is calculated to keep the cable temperature under the manufacturers maximum specified operating temperature. Depending on the required size of the copper conductor, the finished diameter is expected to be approximately 3.5 inches in diameter and weigh about 9.5 pounds per foot.

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DUCT BANK

A typical transmission line duct bank design has four large PVC pipes (conduits or ducts) placed in a square arrangement using plastic spacer devices. Each of the transmission line cables must be placed in its own conduit to facilitate the pulling-in, or replacement, of the cables, and to provide for heat dissipation. Two smaller conduits would also be installed above the transmission conduits: usually one for a transmission line protective relay communication cable, and another conduit used as a spare or for substation data acquisition and control. The entire duct bank is anchored to the floor of the trench with an initial placement of 3-4 of concrete, which is allowed to set. Then the entire duct bank assembly is encased with a high-strength, thermally-conductive concrete mix, which is designed and tested for this purpose. Concrete encasement under city streets and highways include steel reinforcing bars on the corners of the duct bank to improve the strength of the encasement and control cracking. Because steel reinforcement rods must have at least 3 of concrete around them, construction of duct banks under roads is more complex, and the trench width must be increased.

For protection against dig-ins, the top of the transmission line conduit is typically placed 36-40 from grade; the trench will be about 6 feet deep. However, power cables generate heat and soil acts as insulation that causes the cable temperature to rise. Engineers test the soil thermal characteristic and use this value in the design of the optimum duct bank depth and encasement. The design objective is to economically limit the maximum cable temperature to the manufacturers specification.

Under pavement, the roadway occupancy permit may allow the duct bank to be placed in a shallow trench. Shallow placement under roadways may be allowed due to the intrinsic protection against dig-ins. If a narrow trench is required, the top of the encasement could be just under the pavement material and the trench will be approximately 3 feet deep. If a 5-foot-wide trench is allowed, the conduits may be laid flat and a trench depth of 2 feet may be practical.

A shallower trench along Front Street would interfere with fewer of the existing underground utilities. However, the wide-trench option to lay the conduits flat, would increase surfacing demolition and repair work.

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Typical Duct Bank under soil (under roadway, excavation will be shallower) Note: This illustration does not include the fiber optic conduit.

Typical construction in an asphalt roadway: Pave