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Constructing high voltage substation infrastructure  

Credit:  By Loren G. Flaugh, Correspondent | Cherokee Chronicle Times | Monday, December 29, 2014 | www.chronicletimes.com ~~

An earlier Chronicle Times newspaper story described how the 223-mile underground, 34,500 volt (kV) electrical collection system for MidAmerican Energy Company’s (MEC) Highland Wind Farm is designed and installed. Additionally, powering up utility scale wind farms requires a 345,000 volt (kV) transmission substation to connect power from wind farms onto the power grid.

After a great deal of consideration and consternation, the Highland location was finalized last spring when MEC secured a tract of land from Dale Township landowner Lowell Wilson. The site sets about 1,000′ east of MEC’s 345 kV power line that originates at the Raun Power Generating Station near Salix and goes to ITC Midwest’s Lakefield Junction switching substation NW of the Iowa Great Lakes.

Substation design engineers from MidAmerican Energy responded to numerous questions that were emailed last spring during the time when several rather intense substation discussions were held. Nearby landowners expressed concerns over the substation’s location and potential size.

Work on grading the Highland substation began in early May when Cleveringa Excavation from Alton staked out the boundaries and started stripping away topsoil from the approximately 500′ by 600′ site. Scrapers,bulldozers and other earth moving machines took weeks to remove the topsoil from the 7-acre site and stockpile it nearby.

With the Mill Creek flood plain so close, the ground is naturally low. Pertaining to the flood plain concern, MEC replied when asked, “Regarding the flood plain near the substation site, the Iowa Department of Natural Resources has examined the site and permitted MidAmerican to construct the substation and tap structures in the proposed locations.”

However, since the elevation needed to be raised at least to that of the gravel road past the facility, this meant that tens of thousands of cubic yards of clay, dirt, sand and crushed rock were hauled into the site with side dumps. This earthen material is capable of supporting the super heavy equipment that eventually comes later.

Perhaps upwards of 2,000 side dump trucks filled with as much as 20 to 25 cubic yards of material each was spread over the site and packed down layer by layer. Material for raising the elevation came from pits as far as Carnes some 40 miles away. Simply raising the elevation took many weeks.

American Site Builders from Amarillo, Texas moved their crews onto the site in early July. American Site Builders specializes in building high voltage substations and other complex facilities for the power generation and transmission industry.

Surveyors worked quickly to lay out the scores of small and large concrete pads that are required. The smaller diameter holes and cement foundations generally support tall pole structures, steel girders or H-beams that carry 345 kV conductors and other energized apparatus high overhead. Non-conductive insulators prevent the high voltages carried overhead from being too close to the grounded structures.

There’s very little margin for error. Tolerances are small and any deviation from engineering requirements can result in devices high overhead not properly connected. Consequently, foundations must be in precise locations for the overhead systems to fit.

ASB says on their website, “Due to liability and exposure risks, most construction companies are not eligible to build complex facilities like the larger substations seen in today’s electric transmission industry.”

Form builders and cement crews worked from July into September excavating, setting forms and pouring cement for the numerous pads with larger surface areas. These concrete pads support a modular control and communication building and two modular 34.5 kV switchgear assemblies. However, foundations containing the most concrete support the several hundred thousand pound step-up transformers typically found in facilities like this. One of the three huge single-phase transformers is now on site.

Between the existing power line and the west side of the substation, a tap structure must be built. What’s a tap structure MEC’s engineers were asked? Engineers explained that the tap structure is used to cut into an existing transmission line in order to tie substations to the high voltage grid. The structure itself is usually a self-supporting single-shafted pole with a large concrete foundation.

The tap structure for the Highland project will be constructed within the existing power line 145′ right-of-way. The overhead cables supported from two angle iron lattice structures are cut and then re-connected to the tap structure. Basically, the tap structure re-orients the cables for entering the substation.

MEC engineers said that contractors with specialized training and equipment for power line construction will install the tap structure and perform the necessary line work. The power line is de-energized while the tap structure is built. MidAmerican Energy was asked, but they couldn’t say, how long the outage would be while the tap structure is built and power lines are strung into the substation.

The power line is re-energized when the structure is completed and the overhead cables are re-connected. New overhead cables are strung to the substation dead-end structures located at the west side. Work on building that tap structure will be done in the spring when better working conditions arrive.

Meanwhile, high voltage linemen working inside the facility have been busy erecting the numerous vertical and horizontal steel structures. Working in a high voltage substation means linemen often find themselves working high off the ground. When 345 kV power lines cross over farmland and other terrain, the cables must maintain a minimum clearance of 28′ above the ground to meet electrical code regulations.

Driving by the substation site, linemen can often be seen using portable man lifts to reach iron structures and high voltage electrical devices 60 and 70 feet high off the ground. One truck crane lifts the components for the men working in the man lifts to hang in place and bolt together.

Recently, construction workers have started building the security fence around the north, west and south perimeter of the facility. With poles buried in cement spaced not far apart, the fencing will make the facility secure. Because the last of the large equipment is due in soon, some fencing on the east side may be done last.

Underground cables from the 23 wind turbine circuits enters the complex on the east side and goes into the two 345 kV switchgear assemblies. Trench Tech has a trenching machine parked nearby. But due to the frozen ground, burying these cables will wait until the warmer weather of spring and thawing ground.

Work on the substation facility was expected to be largely finished sometime in January. However, a wet June and July and the surprise onset of winter in November have resulted in some weather related delays. Comparing the larger work force of a month ago, to the shrinking crew seen now working at the substation site shows how weather can cause problems. Even so, the site is nearing completion.

Source:  By Loren G. Flaugh, Correspondent | Cherokee Chronicle Times | Monday, December 29, 2014 | www.chronicletimes.com

This article is the work of the source indicated. Any opinions expressed in it are not necessarily those of National Wind Watch.

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