When 500 megawatt (MW), 70,000-acre wind farms like the MidAmerican Energy Highland Wind Farm Project in O’Brien County are developed and built, the sum of specialized high-voltage, direct-burial cable used in the 34,500 volt (34.5 kV) underground electrical collection system is truly astonishing.
Drawings show the radial collection system is like a giant underground spider web with spindly tentacles that fan out perhaps 14 miles to distant turbine sites NW of Sutherland. Drawings filed with the O’Brien County Engineer’s Office show where 223 miles of trench must be opened to route the 3″ thick power cables back to the substation site 5 miles north of Paullina.
Because the power generated from a wind turbine is 3-phase, then take the trenched footage times 3 and that figure reveals the total amount of cable used in the radial collection system. Consequently, an astonishing 3,537,000 feet of cable, or 670 miles, must be manufactured and delivered to the site.
General Cable Company is a leading manufacturer and supplier of high voltage cable to the electric utility industry. Utility products director Joseph DeBolt replied to several emailed questions.
DeBolt says, “We supply a lot of cable to wind farms in Iowa. We look forward to more projects as they develop. We’ve also provided cable to Alliant Energy in the past for some of their projects in Iowa, Wisconsin and Minnesota. Our cables are all produced in the US for these projects and do not come from overseas as many components do for wind farms.”
How a wind turbine works
Comparing the tiny 650 kilowatt (KW) turbines used when the Enron Wind Farm near Alta was built in 1997 to the massive 2,300 KW machines used in the Highland project with their mammoth 374′ diameter rotors is like comparing David to Goliath.
Consisting of 176′ long blades bolted to an 8- to 10-ton ductile iron hub, the giant rotors turn a low-speed shaft at about 20 rpm’s. This low rpm drive train turns transmission gears in a complex gearbox that radically ups the speed to 1,200 to 1,800 rpm’s which is required for making electricity. The shaft then connects to an asynchronous electric generator.
This drive train is the heart of a wind turbine’s power generating capability. The generator transforms the energy from the wind turning the rotor into a stable three-phase, 700 volt 60 Hertz frequency. These 700 volt power cables go down to the base 260 feet below.
The power is then fed into a step-up transformer where it’s increased to 34.5 kV. Power from other nearby turbines is gathered together to make a circuit that may travel underground many miles to the wind farm substation. It’s connected onto massive step-up transformers that boost the voltage yet again to the 345,000 volt, 60 Hertz power grid.
Engineering wind farm circuits
At a February O’Brien County Supervisors meeting, landowner’s association president Daryl Haack wanted to better understand of how the tentacles of the electrical collection system might look. MidAmerican Energy Wind Energy Supervisor Adam Jablonski was present that day to answer questions.
Haack explained how Dale Township landowner Dennis Mars was concerned about too many individual trenches crossing his farm. Other landowners also wanted more information about the design of the 34.5 kV collection system that goes into the substation NW of the Mars farm.
Haack asked, “How much separation is there between the individual ditches? Does the County have any stipulations in the ordinance that limit the number of ditches that can cross someone’s property? Why can’t more than one circuit go into a trench?”
Jablonski replied, in part, “There is a separation distance between the collector lines, usually 10 to 12 feet. There are certain locations that will have more collector lines than others.” Jablonski noted that about 60% of all the 270 easement participants in the wind farm have collector lines only and no turbine sites. “Each circuit will have up to 10 turbine sites gathered together.”
Later on, Kevin Mars speculated about the potentially large number of trenches when he asked, “MidAmerican Energy hasn’t shown any plans of what the underground interconnecting cable circuits coming into the substation will look like. Can you imagine what my Dad’s farm will look like? They could tear the whole farm up.”
With 214 turbine sites, that means 23 circuits feed power into the substation. For example, distant wind turbine sites in Sections 8, 9, 17, 18 & 20, Center Township are combined in Section 9 southwest of Hartley. The combined output from the sites runs underground more than 12 miles SW, through the Mars farm and then into the nearby substation.
DeBolt answered a question about the maximum distance a wind turbine could be located from the substation. “There’s no real limit although underground cable costs and associated line losses eventually dictate that it is not cost effective. Some wind farms utilize overhead conductors for the final runs to the substation from areas of concentrated turbines. There can be some disadvantages to this like appearance, the potential to be taken down by ice or high winds and damage due to vandals,” DeBolt explained.
Highland’s 223 miles of trench
MidAmerican Energy Company said when the 500 MW wind farm building permit was approved last August that this would be Iowa’s largest wind farm. Within its boundaries are turbines in 4 townships.
MEC files underground utility applications that show where the expansive 34.5 kV radial collector system is located. These plans show where protective 6-inch diameter casings are bored under roads, wetlands and waterways. Plans also show where 3-inch diameter, 34.5 kV aluminum power cables pass through fiber glass junction boxes located at the outermost edges of fields.
Trench Tech has been laying mile after mile of underground high voltage cable since July 9th. Trench Tech specializes in underground utility installations, especially these large utility-scale wind farms like the Highland project. Semi trucks loaded with huge spools of 34.5 kV direct-burial power cable started arriving early in the spring at a storage area near Primghar.
DeBolt answered a question about the cable’s design life by saying, “These cables have a typical design life of 40 years, more than enough for the wind farm. There are many cases where these cables are lasting well beyond the design life in other utility applications.”
With corn stalks shoulder high and tassels emerging, crews moved trenching equipment onto the Mars farm in early July. A 50-acre patch of corn was knocked down and Trench Tech started burying the 34.5 kV high voltage cable. Trench Tech doesn’t plow in the direct burial aluminum cable.
An old Army semi truck carrying three reels of cable mounted to frames fastened down to a flat trailer moves slowing along in front of the trencher. Cable spools off onto the ground, as the trencher follows along and picks the cable up. Frames on the trenching machine guide the cable over the tracks and then around behind the auger chain teeth where the cable feeds down into the 4′ deep, 12″ wide trench.
Each trencher, and the crew and equipment that supports it, can excavate and backfill over a mile of trench each day. A second trencher was brought in to share the huge work load two weeks later.
Current progress as of Nov. 2nd shows Trench Tech has now opened about 75.1 miles of trench and buried about 225.3 miles of cable, with about 148 miles of trenching remaining.