AEP’s bold new approach to overhead line design23 March 2016
The first installation of an overhead line using AEP’s new BOLD technology, with its distinctive crossbow-shaped transmission towers – the 22 mile Robison Park/Sorenson rebuild project near Fort Wayne, Indiana – is under construction (see photos, right) and due to be completed in June 2016.
AEP's BOLD (Breakthrough in Overhead Line Design) is an innovative line design featuring a compact conductor arrangement, based on a triangular (delta) configuration, suspended under a crescent-shaped cross arm. The power delivery capacity of a 345 kV BOLD line can exceed a conventional 345 kV line by up to 60% (in the same right of way), surpassing the capacity of a conventional 500 kV line, says AEP. For a given capacity, a BOLD design can be installed within a narrower right-of-way than a conventional design,while meeting all federal/state requirements.
A technology team at AEP developed BOLD to achieve higher capacity, less impedance and line loss, and improved reliability in a compact form factor.
Its arched cross-arm and lower overall height, compared to a traditional extra-high voltage line, help it to fit more aesthetically into the environment, its developers say.
BOLD was created to maximise the use of land for transmission line corridors (right-of-way) and avoid complex specialised equipment like series capacitors that can "create harmonic distortion that negatively impacts generator operations", AEP argues.
The benefits that BOLD provides can help utilities achieve the objectives of new resource integration and infrastructure renewal with the highest efficiency and least environmental and community impact, says AEP, the patented design providing an efficient and robust transmission solution at common domestic and international voltage classes representing, AEP believes, the best available technology in overhead line design.
According to AEP, BOLD has a cost advantage on a price/MW capacity basis relative to traditional overhead lines, and is significantly less expensive than underground lines.
The reduced energy losses arising from the lower impedance yield economic savings that can be significant according to AEP estimates. "The ability to replace and upgrade transmission lines with BOLD in existing corridors can save both time and money and there is also the ability to potentially reduce right-of-way width for new lines", AEP notes. In addition, the lower-profile can potentially reduce public resistance to new or upgraded lines and help expedite the siting and construction process.
With the retirement of fossil fuel generators and migration towards new renewable generation facilities, new transmission lines must be constructed AEP believes and suggests that BOLD is a particularly good fit for these applications because it can provide more capacity over longer distances, it can avoid series compensation (which can interfere with wind generators), it can contribute to increased voltage stability, and, as already noted, can reduce losses. Wind resources are generally located far away from load centres, requiring long transmission lines. Historically, series compensation has been used to increase transfer capability on these lines. But series capacitors can create harmonic interference known as sub-synchronous resonance (SSR) which can interfere with and even damage turbine generation facilities. Series capacitors are also complex and costly. BOLD's "compact line design, not requiring series compensation, is able to perform as well as or better than a traditional line with series compensation and avoid this potential complication", AEP suggests.
As noted, American Electric Power is currently constructing the first BOLD transmission line, in Indiana, USA. This initial deployment, for the 22 mile Robison Park/Sorenson rebuild project near Fort Wayne, uses a 345 kV/138 kV hybrid tubular steel design. In this project BOLD double-circuit towers will replace existing 138 kV towers in the same corridor. Construction began in April 2014, with completion expected in summer 2016.
The second BOLD project, utilising lattice tower structures, is scheduled to be constructed near Lafayette, also in Indiana, beginning in 2017. This is the AEP Meadow Lake/Reynolds rebuild. Spanning 20 miles, it will be the first double-circuit 345 kV application. The anticipated in- service date is June 2018.
A third installation is planned, for the 18 mile Magic Valley II wind interconnection, Raymondville, Texas. This will be a 345 kV lattice tower design (future double circuit). Planned in-service date August 2020.
ERCOT evaluation of a larger 345 kV project, 130 miles in total, is underway.
BOLD is currently designed for voltages ranging from 200 kV to 400 kV, with future voltages classes under consideration. Over 125 000 miles of 345 kV and 230 kV transmission lines are in operation today in North America, AEP estimates, and says "many of these lines will be reaching the end of their useful life in the coming years, creating an opportunity to replace and upgrade existing infrastructure with new technologies such as BOLD."
AEP has set up a subsidiary, BOLD Transmission, to market and licence the technology. BOLD Transmission holds the 14 patents granted or pending to date worldwide and hopes to sell the technology to other utilities, except those competing with AEP for transmission projects.
BOLD was developed in 2012 by AEP Transmission's Meihuan (Nancy) Fulk and Richard Gutman. Jeff Fleeman, AEP director of Advanced Transmission Studies and Technologies and VP at BOLD Transmission, came up with the name.
The capacity of a line is a function of surge impedance loading (SIL) and line length, and SIL = V2/surge impedance. So if higher voltage is not an option and fewer lines are required, lowering of surge impedance is needed to boost line capacity. Surge impedance of a line is approximately equal to the square root of L/C. So surge impedance can be reduced by decreasing L and increasing C, which can be achieved by: reducing space between phases; using more sub-conductors per bundle; employing larger bundle diameter; and having larger conductor diameters.
BOLD "leverages these principles", says AEP.
Design development has included collaboration with vendors on structure, insulators and hardware and construction. Full-scale prototype testing has included structural integrity/stress evaluation and electrical tests to examine corona and audible noise characteristics, switching behaviour and lightning performance, which has been found to be excellent.
For the 345 kV version o fBOLD, use of 3-954 ACSR conductor is proposed. This gives about 42% higher SIL compared with 2-954 ACSR conventional, with nearly 5000 A SE thermal capability (per circuit).
For the 230 kV version of BOLD, 2-795 ACSR conductor is proposed. This gives roughly 60% higher SIL compared with 1-1590 ACSR conventional, with about 3000 A SE thermal capability (per circuit).