برق. قدرت. کنترل. الکترونیک. مخابرات. تاسیسات.

دایره المعارف تاسیسات برق (اطلاعات عمومی برق)

American Electric Power, BOLD

The transmission grid has experienced ongoing and dramatic changes as a result of open access, regional markets and changes in the nature and location of generation sources. Widespread outages caused by natural events and the threat of physical attacks have driven the need for greater resiliency. At the same time, the public resists construction of new lines because of aesthetics and other reasons, especially higher-voltage lines that more efficiently deliver bulk power over long distances. Despite all of these drivers for change, transmission line designs have remained relatively unchanged over the last 40 years. Until now.

American Electric Power, BOLD
American Electric Power, BOLD
Crossarm and pole sections are gathered in the staging yard prior to assembly of the BOLD line.

Forces Driving Change

U.S. electric utilities today are engaged in transmission projects to enhance reliability, integrate new sources of generation and modernize the nation’s electric grid. When new energy sources are remote from load centers, transmission must deliver the power reliably and efficiently over longer distances. These trends are in response to mandatory reliability and renewable portfolio standards, environmental and efficiency demands, grid security and resiliency needs, and continued load growth.

In addition, the industry is adapting to the  (FERC) Order 1000, which eliminated an incumbent utility’s right of first refusal to build certain transmission facilities in its service territory, thus creating both challenges and opportunities. Concurrently, the U.S. transmission infrastructure is aging; significant renewal and rebuilding is underway and will continue. Most rebuilding will make use of existing rights-of-way (ROW) as a result of regulatory and public pressures to reduce the impact of line siting.

The Loadability Challenge

The power transport capability, or loadability, of an alternating-current transmission line is governed by thermal, voltage-drop and steady-state stability limitations. An outcome of the conductor or terminal equipment selection process, thermal rating is most limiting for lines shorter than 50 miles (80 km). Longer lines are limited primarily by voltage-drop or stability considerations, both of which are affected by the length-dependent line impedance.

The most effective way to reduce line impedance and improve loadability is to raise the transmission voltage class. However, this method faces public opposition, particularly when plans call for the highest available voltage transmission. Left with few options, utilities choose lower-voltage lines supported with series compensation to reduce transmission path impedance and attain required power-transfer objectives.

Traditionally, series compensation has served as a near-term remedy to stretch system capability until implementation of a longer-term solution or as a substitute for higher-voltage transmission. This is especially true when moving large blocks of power point-to-point over long distances. However, these applications may lead to operational issues such as sub-synchronous resonance (SSR) and sub-synchronous control interactions (SSCI), which pose a risk to electric machinery and can lead to instabilities. Other concerns include system protection complexities, maintenance or spare equipment requirements, limited life expectancy, electrical losses and future grid expansion issues such as tapping the compensated line.

American Electric Power, BOLD
One side of the crossarm is installed on the structure.

The BOLD Solution

The high-capacity, high-efficiency BOLD (breakthrough overhead line design) solution features a streamlined, low-profile structure with phase-conductor bundles arranged into compact delta configurations. The structure consists of an arched crossarm supporting both circuits set atop a tubular-steel pole, imparting a more favorable aesthetic appearance. BOLD can support either double-circuit or single-circuit (that can be expanded in the future to double-circuit) lines. Although the first BOLD development project is a 345-kV design, other voltages such as 230 kV are also included in the design series.

The average 100-ft (30-m) 345-kV structure is about one-third shorter than a traditional double-circuit design. Each phase contains multiple conductor bundles 18 inches to 32 inches (457 mm to 813 mm) in diameter. The separation distances among the three phases are as low as 14 ft (4.3 m) and are maintained using two interphase insulators per circuit; standard insulators attach each of these bundles to the crossarm and tubular structure bodies. The crossarm supports two shield wires positioned to provide zero-degree shield angle to the outmost phases.

American Electric Power, BOLD
Insulator installation is underway on the completed structure.

BOLD’s patented configuration packs roughly 50% more capacity into the ROW. It markedly improves line surge impedance loading (SIL), lowers series impedance and reduces ground-level electromagnetic field (EMF) effects. SIL is a convenient yardstick for measuring relative loadability among alternative line design solutions. BOLD uses three conductors per phase at 345 kV, offering significant gains in line loadability and energy efficiency for long-distance and local applications.

Although a typical 150-ft (46-m) 345-kV ROW may be used, the low-height, eye-catching profile can fit within a 105-ft (32-m) ROW, a potential reduction of nearly one-third of typical ROW width. These visual benefits are expected to improve public acceptance of new transmission projects.

American Electric Power, BOLD

BOLD vs. Traditional Design

The table summarizes the physical and electrical characteristics of a traditional 345-kV double-circuit line design and the BOLD solution (one of the three BOLD options). Note the new design’s significant improvements in SIL (43% increase), impedance (30% reduction) and energy loss (33% lower resistive loss), which most influence line loadability and efficiency. These are key advantages for a transmission lines built to carry power over long distances.

The table also compares thermal ratings, EMF and audible noise emissions. Especially notable is the 51% reduction in ground-level magnetic field. This is an area that continues to receive considerable attention in line-siting regulatory proceedings.

By using three-conductor phase bundles, significant gains are attained in thermal capacity and line efficiency, resulting in lower operating temperatures. BOLD improves overall system performance by unloading higher-impedance/lower-capacity lines. Alternative designs are possible, using between two conductors per phase and four conductors per phase.

When compared with conventional designs, the BOLD 230-kV solution will offer even greater relative electrical advantage in line impedance and loadability.

In projects that normally would require series compensation, an added bonus is the prevented need to install and maintain or replace the compensation equipment (including SSR/SSCI mitigation) over the long term, considering the long life expectancy of a transmission line. The associated financial impacts can be substantial over the full life of the line.

American Electric Power, BOLD
Conductor stringing dollies are in place on the completed structure.


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