For some time now, electric utilities have been engaged in a delicate balancing act with respect to optimizing plant investment for maintaining a high degree of reliability in their operations. For example, with the present constraints on existing transmission facilities and the absence of available rights-of-way (R/W) for new lines, the question persists about how to increase line capacity to handle the new generation being built. Reconductoring, or add-bundling, is an option, provided the structures are strong enough to handle the added conductor weight. Another option is to re-rate existing conductors to operate at higher temperatures to carry the increased electrical load. This condition would be acceptable if there were sufficient clearance to accommodate the increased sag that would result from the high-temperature operation. Lacking the necessary clearance, while lattice-type structures could be raised by employing leg extensions, wood-pole structures would require a completely different treatment.

In today's electric-utility operations, it is clear the transmission environment is impacted, not only by the new generation independent power producers (IPPs) are building, but also by telecom companies seeking ways to install fiber-optic cables on existing structures and by natural gas companies that use transmission corridors to lay their pipes. The effect on clearances has been significant for a plant originally designed for the sole purpose of transmitting electric power over long distances.

Solving the Clearance Problem

The idea of adding material to lengthen a structural member is unremarkable to anyone who has ever nailed two boards together, end to end, in effect, performing a splicing operation. A similar approach can be used to add length to a pole to raise the height of conductors that are supported on crossarms at the top of the pole. To achieve the required pole height, the pole is cut about 5 ft (1.5 m) from the ground line, raised hydraulically to provide a vertical gap of anywhere between 5 ft and 20 ft (1.5 m to 6 m) between the cut ends and stabilized in place by steel members that are bolted to the ends of the separated pole.

Central Maine Power Co. (CMP, Augusta, Maine, U.S.) used this lifting system to accommodate an added load of 1600 MW from five new merchant plants that were built in its service territory. Its 345-kV line, designed on the basis of a 120-degree sag curve, did not have adequate ground clearance to handle the added load, which would require a design based on a 212-degree sag. The existing conductors, 850.8 kcmil ACSR 45/7, were rated at 447.8 MVA for the original sag design. For the proposed sag design, the rating would be increased to 1427.9 MVA. All structures were H-frame, built in the late 1960s and mid 1970s.

Initially, during the summer of 1999 when it was first apprised of the necessity of taking on the added load, CMP estimated that 67 structures would need to be rebuilt to achieve the added ground clearance necessary to carry the load from the new generating plants. By the time the project was complete, 270 structures were involved. Contractual agreements required the work be completed by May 2000.

A conventional structural change-out project would require taking the line out of service during the peak winter months. Even if materials could be secured and a labor force enlisted, removing the line from service was not an option. The situation called for a new solution, which was available with the PhaseRaiser technology developed by Laminated Wood Systems (LWS, Seward, Nebraska, U.S.).

The technology was based on a lifting procedure, designed to raise existing structures without taking the line out of service. Tests on poles that had been subjected to the PhaseRaiser splicing technology showed that the modified poles not only met but exceeded National Electric Safety Code (NESC) load requirements.

The work was contracted to On Target Utility Services (Portland, Maine) with a single six-man crew. LWS trained this crew on a de-energized line, with the expectation that this core crew would train subsequent crews.

At the outset of the project, the 345-kV line was taken out of service for the first three lifts. The ease with which the lifts were accomplished permitted the remainder of the work to be done while the line was in service. The total number of structures that required lifting grew to more than four times the original projected number, which made the deadline for completion even more restrictive.

To meet this new demand on productivity, new PhaseRaiser trailers were constructed and additional crews were trained in the lifting process. Ultimately, there were four six-man crews, which used personnel from the Pole-Setting Division, Underground Division and Overhead Line Division. None of the workers on the crews were rated higher than a 3^rd Class Line-worker, illustrating that the system could be implemented with minimum training and a non-rated line force.

The process required only basic hand tools, consisting of impact guns, hammers, electric drills and chain saws with a 20-inch (51-cm) minimum bar to cut through the poles. For environmental reasons, CMP prohibited the use of rubber-tired vehicles on its R/Ws, thus requiring the use of a flex-track-mounted derrick and the modification of LWS's trailers with tracks to tow behind the flex track.

The biggest problem LWS encountered was the girth of the cedar poles. The poles were larger than LWS typically encountered, so its drill guides were not large enough to reach across the poles. LWS obtained a larger tool that enabled guides to be used for the cuts. The work was completed on time with the PhaseRaiser structure-lifting system proving to be a valuable tool for solving the problem CMP faced.

John Joyce is senior project manager for On Target Utility Services, and is responsible for managing all transmission and substation projects. He holds the BS degree in industrial technology from the University of Southern Maine.

How the System Works

Major bottlenecks that exist on certain transmission lines inhibit their ability to accommodate the ever-increasing demand for power. Utilities searching for solutions to this problem can replace the lines with larger conductors that would significantly increase the current-carrying capacity of the line, but at a cost in terms of having to reinforce — or replace — existing structures. Utilities usually consider this option as a last resort because of the high cost of such an undertaking. A preferred approach is to operate the conductors at higher currents, which would result in corresponding increases in conductor temperature and conductor sag. The increase in sag, however, would impair ground clearances and would be in violation of existing codes.

The issue of an increase in conductor sag can be addressed by physically lifting the height of the conductors. This is done by increasing conductor tension using special V-string insulator assemblies to modify the conductor attachment points, or by physically lifting the structure to gain added height. To raise the height of the conductors on its wood-pole line, Central Maine Power selected Lamintaed Wood Systems' PhaseRaiser technology to increase the height of the poles.

At the outset, because wind can affect a lifted structure, the wind speed is checked to ensure it is less than 25 mph. Once it has been established that work can proceed, the ground wire on the structure is cut and connected to a floating jumper to provide sufficient length during the lifting process. Poles are then marked to establish cut lines and vang locations where the hydraulic cylinders will be mounted. After the vangs have been installed, the poles are notched to permit later cut-throughs without any interference from the steel-reinforcing members. The steel is then erected and bolted onto the bottom pole, which remains embedded in the ground. Using working ladders on the steel, the upper vang is installed, nylon straps secure the entire assembly and the pole is cut all the way through, separating the pole in the ground from the upper portion. With vangs in place, the hydraulic cylinders are positioned and attached to both the upper and lower vangs.

Once the required height is reached, x-bolts are installed to secure the steel to the pole segments and the hydraulic cylinders are removed. As a precaution, exposed wood surfaces are treated with a copper naphthenate spray or grease. The final step is to install side shields to provide a uniform steel surface from the groundline to the lower portion of the top pole section.