Imagine standing in a swamp with knee-deep mud, rain pouring down and mosquitoes swarming on a hot muggy day in July. You are watching a 500-kV tower — destroyed by a tornado the night before — being hauled away so a temporary structure can be placed into the line and power can be restored. Now imagine it is three months later, the swamp still has knee-deep mud, mosquitoes are still swarming, and it is a bright and sunny humid day. The temporary structure installed in the 500-kV line is being dissembled and replaced with a permanent structure. Déjà vu?
In an effort to avoid this scenario, Dominion’s transmission lines group has adopted a strategy of restoration called permanent emergency. For all emergencies, whether minor or major, the restoration practice from 69-kV to 500-kV lines is to use materials, towers and poles in inventory to replace or restore the damaged parts of the line with permanent replacements. Simply put, the motto for emergencies is to “do it once and walk away.”
The Dominion transmission system consists of 6500 miles (10,461 km) of 69-kV, 115-kV, 138-kV, 230-kV and 500-kV lines covering most of Virginia and eastern North Carolina, U.S. Dominion placed the first 500-kV line into service in 1966 and then expanded the system from that line to cover most of its service territory. Because of their age and the demand for more power, most of the original lines have been rebuilt or are in the process of being rebuilt. As of June 2016, more than 5000 500-kV towers make up Dominion’s 500-kV system and 85% of these towers are tangent towers. In terms of operations, there are five major families of towers: 5, 5V, 5-2, F and 93 series.
Developing a functional strategy requires looking at both the design and historical events on the system. Prior to 1999, Dominion did not experience many structural failures on the 500-kV system. These failures were addressed with two temporary, in-house-designed, multidimensional aluminum H-frame structures. These structures were used once or twice in the early 1970s and then set aside. Spare structures consisted of extra towers and parts ordered for a project. They were not inventoried and were dumped on a back lot.
In 1999, an articulated mowing machine knocked a guyed V-tower off its foundation, resulting in a slight buckling of the two legs. The structural engineering group designed a temporary direct-buried guyed H-frame structure composed of three old single-shaft tubular-steel poles. This was later replaced by a lattice self-supporting tower identified from surplus.
In 2001, another guyed V-tower — located on a mountainside in West Virginia — failed as a result of a broken guy grip, most likely during a wind event. This was ironic because guy grips on towers in the adjacent valley, locally known as Hurricane Alley, had been inspected the previous month to determine whether a guy grip replacement program was needed. Early the next morning, engineers took a helicopter to the remote site, arriving before any construction personnel. The tower had fallen in the transverse direction into the tree line. It was readily apparent the foundation and anchors were intact. If — and that was a big if — a replacement tower was available, the new tower could be erected on the same site.
The only available option was the temporary H-frame previously described. After blasting holes in the rock to direct bury the legs, setting the longitudinal guys and erecting the structure, the line was energized five days later. A permanent self-supporting tower was ordered and installed months later, requiring another seven-day outage. The total cost of the restoration was US$1.1 million, and the restoration required two outages and 12 days.
A meeting was held for management, construction and transmission operations to discuss lessons learned and what could be improved. It was noted the two recent incidents involved the same tower type with no damage to the foundations and anchors. Considering the number and age of this tower type, as well as the time it took to install the temporary structure, Dominion decided to order two spare guyed V-towers.
PJM Expected Capabilities
In 2005, Dominion’s regional transmission organization, PJM, convened a committee of transmission engineers and operations representatives to determine the expected capabilities of service restoration for every member company. This applied to lines rated for 345 kV and 765 kV; 500 kV had been previously visited, but would be redeveloped.
Several meetings were held to discuss historical events, including specific territories, nationwide data and most typical failures; restoration strategies for how each utility responded to tower failures; and restoration capabilities, including temporary structures, material inventories, and engineering and construction capabilities.
From this discussion, a standard restoration scenario was developed for 345 kV and 500 kV: “For damage up to and including two completely destroyed structures and two minimally damaged structures, the transmission circuit will be restored to service within four days after damage assessment. Each member company shall individually maintain an adequate supply of materials/structures to effect this restoration scenario.”
This commitment excludes cases with exceptional conditions such as lines of multiple circuits, extensive area damage involving multiple corridors, a cascading event, and angle or dead-end structures.
On June 4, 2008, a tornado struck a valley, causing a 500-kV guyed V-tower to collapse. An adjacent 138-kV wood H-frame on the same right-of-way was unscathed. An on-site inspection revealed no damage to the foundation or three of the four anchors. The fourth anchor rod had a slight bend and was pull tested to verify capacity. A direct replacement tower was ordered from inventory and on-site the next day. Installation proceeded 24 hours a day, and the line was ready to energize in two-and-a-half days. The restoration cost $350,000 and required one outage. This effort enabled Dominion to win the Southeastern Electric Exchange Award of Excellence in the transmission lines category.
In 2010, management, engineering and operations personnel met again and decided to expand the permanent replacement strategy to include all self-supporting 500-kV towers. Dominion defines this strategy as “the ability to replace two 500-kV tangent towers with permanent replacement structures within four days, after assessment.” Additional tangent towers were ordered for the various tower families and placed into inventory.
The plan to support the strategy follows one of two paths, as determined during an initial assessment. Path one is for sites whose foundations are found to be intact; the damaged tower is removed and a direct replacement structure is installed. Path two is for sites with damaged foundations. For damaged foundations, there are three options:
1. If individual foundations can be repaired or replaced in a time frame to meet the defined strategy, direct permanent replacement is preferred.
2. If the foundations cannot be reused, the guyed V-tower will be installed as the permanent replacement of choice. The guys and foundation can be installed quickly and over a wide variety of terrain. The two legs provide an open window for the reinstallation of the center phase conductors without cutting and splicing.
3. The last option is to install a temporary structure. This decision may be a result of time constraints, material availability, terrain, weather or any combination of factors.
As part of the planning process, the development of failure scenarios is an expansion of the basic restoration strategy. They are real-life descriptions of restoration capabilities and include the following:
- A visual image for management approval, which is key to any strategy
- Defined material quantities, which eliminates individual opinions from inventory requirements
- Real cost versus level of restoration capability desired
- Documentation of restoration capabilities for future review
- Elimination of inventory purges without responsible management approval. Supply chain’s rationale is to reduce the inventories of low-turnover items. Emergency materials are like an insurance policy; utilities hope they are never used.
The failure scenarios developed include plans for single tower failures and catastrophic failures, which are defined as three or more structures down. Historically, there have only been single tower failures, but there is always the possibility of a larger event. Single tower failures of one to two structures will follow the path defined in the strategy.
For catastrophic failures, there are enough materials in inventory to restore several miles of 500-kV line. For all five families of towers on the system, the plan is to have tangent structures and double dead-end structures available in inventory. Although a time constraint is placed on the strategy, no constraint is placed on the failure scenarios as there are numerous factors that affect the time frame for restoration.
Success with a Sound Strategy
Dominion maintains a separate inventory for all emergency materials. Hardware and insulators are packaged in heavy-duty long-term storage containers, and warehoused for security and protection from weather. Tower components are stored in lay-down yards, packaged for long-term storage. Individual bundles are secured with heavy-duty stainless-steel banding and identified with embossed stainless-steel tagging. Pieces of steel within each bundle are separated from each other with plastic spacers to reduce white rust.
Dominion’s strategy on 500-kV circuit restorations has proven successful. Thanks to a sound strategy with supporting plans and guiding principles, service can be restored — with permanent replacement structures — within four days after a field assessment. Failure scenarios support the basic strategy and are used to define and quantify material inventory. Knowing response capabilities, material availability and the actual field assessment leads to the development of a plan for a successful and timely restoration.
Jill Hailey is an engineer I within Dominion Virginia Power’s transmission operations engineering group in Richmond, Virginia, U.S. She joined Dominion after graduating from Clemson University with a BSCSE degree. Her current role includes supporting field personnel and overseeing reliability projects to maintain integrity of the transmission system.
David E. Blaha, a consulting engineer, has worked for Dominion Virginia Power for nearly 40 years in various capacities, including line design, standards development and material specifications involving overhead transmission lines from 115 kV through 500 kV. His last 16 years have been in the transmission operations engineering group with a special interest in emergency restoration. He holds a BSCE degree from Virginia Tech and is a registered professional engineer in the state of Virginia.
Editor’s note: This topic was presented at the Southeastern Electric Exchange.