Edison Sault Electric (ESE, Sault Ste. Marie, Michigan, U.S.), a subsidiary of Wisconsin Energy Corp. (Milwaukee, Wisconsin, U.S.), serves the eastern half of the upper peninsula of Michigan.
This area is known as a sportman's paradise with many square miles of woods, miles of Great Lakes shoreline and inland lakes with many small islands.
ESE is the lowest cost supplier of electric power in Michigan. Hydro facilities at Sault Ste. Marie supply ESE with much of its energy, and the St. Mary's River rapids between Lake Superior and Lake Huron provide the water power. Within the ESE service territory is the Straits of Mackinac, which separate the upper and lower peninsulas of Michigan at the northern tip of Lake Michigan and Lake Huron.
Guarding the straits is Mackinac Island, a historic island inhabited at various times in history by Native Americans and French, British and American armies.
The 7 sq mi (18 sq km) island was used as a fort and commerce center during the fur trade of the 17th and 18th centuries and became a resort area during the late 19th century. The Victorian environment of the island resort and historic fort and buildings created one of the premier parks in the United States.
To maintain the history and charm of the area, the island officials allow no motor vehicles except for essential services and emergency vehicles. The Mackinac Island State Park Commission oversees the state-owned lands, which comprise 82% of the island. The city of Mackinac Island and the State Park Commission, through ordinances and policy decisions, strive to keep the island substantially as it was in the 1880s.
The State Park Commission staffs the fort and other historic buildings for tourists. The historic island, one of the major tourist destinations in the Midwest and the most prominent resort in Michigan, is separated from the mainland by 3 miles (5 km) of water, which is frozen between late December and early April. Ferries and barges transport almost all of the goods to the island. During the winter, horse-drawn sleighs and snowmobiles transport goods and services around the island; during the summer, horse-drawn drays and bicycles assume the responsibility.
Submarine cable from the mainland at St. Ignace, Michigan, supplies electric service to the island. Since the first cable installation in 1931, upgrades continue to serve the steady increase in demand. Two circuits of #4/0 AWG copper conductor — one installed in 1982 and the next in 1988 — replaced the three circuits of #1/0 AWG copper conductor put in service from 1931 to 1965. In 1980, the utility removed a backup generation station when it determined the station was inadequate to meet the rising demand. Another reason for removing the generator was the island's decision to convert power use from fossil generation to direct electrical service from the mainland.
The current substation is on the mainland at St. Ignace within 330 ft (100 m) of the landfall. The St. Ignace Substation, fed at 69 kV, provides service with 7.62/13.2-kV grounded wye distribution voltage. On the island, two feeders connect to the Pat Chambers switching station for re-regulation of the voltage and feeder protection with electronic oil circuit reclosers.
Two full-time linemen, who have a small bucket truck and digger for setting poles and for working energized primaries, serve the electric customers on the island. Except for emergencies, they travel to job sites on bicycles and work on projects in the main commercial area only in the off-season. ESE linemen from St. Ignace provide supplemental labor for large projects.
The fact that the island is a resort and state park requires barges to transport all heavy equipment, including concrete trucks and backhoes. During much of the year, ferries carry tourists and workers from the mainland to the island. Transportation is available from a year-round air service, which is the only reliable transportation during the winter due to the frozen straits. Snowmobiles cross the frozen straits along a path marked by Christmas trees placed in the ice. The commute and transportation of material and equipment, combined with the desire to maintain the Victorian atmosphere, create a challenge for any construction project.
The Need for Construction
In the summer of 2000, major load shifts from fossil fuels — mainly fuel oil and propane — severely loaded the two #4/0 AWG copper-cable circuits, which have a combined capacity of 12 MVA. The summer 2000 load reached 11 MVA, dramatically straining the 12 MVA rated cables. Hotels switching to electricity to fuel large water heaters increased the demand for electricity beyond the current peak. A load study determined that the island should expect a peak demand of 30 MVA within the next 25 years. To serve this load, the utility developed a comprehensive plan that considered some form of peak shaving or the installation of a submarine transmission circuit.
Devising a Solution
The island rejected the option of peak-shaving generation, because it would defeat the purpose of reducing the use of fossil fuels, and it would create unacceptable noise in the vicinity of the station. The island rejected another solution to use transmission-voltage submarine cable and the construction of a substation on the island because of space limitations, difficulties in installing underground transmission in rock and transformer noise. The island selected a plan to install sufficient 500-A submarine feeders from the St. Ignace Substation on the mainland to the island to carry the peak load with a single-contingency cable outage.
To meet the increased load and the requirement for increased reliability, the utility scheduled one new cable for installation in the fall of 2000 and a second cable installation in the spring of 2001. The two existing #4/0 cables would serve as a single feeder. The Pat Chambers Switching Station would provide space for an additional feeder and would double in size to distribute the power and provide improved sectionalizing protection. The mainland substation would upgrade to a 15/28 MVA transformer to support the cable capacity and near-term island load.
Since the expansion of the existing right-of-way corridor on the island was prohibitly expensive because of the presence of solid limestone rock, overhead feeders would replace the existing two underground cable runs. The new overhead feeder designs would use four 636 mcm aluminum, 15-kV Hendrix spacer cable circuits. The construction would comprise three circuits with room for one future circuit.
ESE organized a team for the installation of the new submarine feeder involving several companies: Novak Engineering Inc. (Jackson, Michigan) for engineering, design, permitting and material specs; Durocher Dock & Dredge (Cheboygan, Michigan) for installation; Northwoods Surveying (Sault Ste. Marie) for the project's land-based surveys; and Fugro West Inc. (Ventura, California, U.S.) for the underwater surveys.
Installing time for the first cable was short, and required ordering, shipping and placing before the winter of 2000. Bad weather usually starts in late October with the season's worst storms hitting in November, requiring preparations before October 15. The various entities requiring permits included:
Army Corps of Engineers
State of Michigan Department of Environmental Quality
Department of Transportation for crossing Highway M-185
State of Michigan Department of Natural Resources (easements for the submarine cable)
Mackinac Island State Park Commission.
The governmental agencies cooperated by adjusting their schedules, holding special review meetings and hand-carrying documents to allow the project to move forward. With commitments in hand, ESE ordered the cable, which Nexans (Alcatel, Markham, Ontario, Canada) supplied. The cable plant in Oslo, Norway, was able to adjust its schedule to produce the cable before Thanksgiving. Nexans provided a cable basket and turntable to assist in the installation. To speed delivery of the cable, a dedicated ship transported the material across the Atlantic. The ship traveled through the St. Lawrence Seaway to the port in Erie, Pennsylvania, U.S., where a crane transferred the cable and basket to the Durocher barge, which transported the cable to the Durocher Dock & Dredge docks in Cheboygan.
Lake travel in November and December proved particularly treacherous with weather systems changing often and quickly, causing the barge captain to take refuge in safe harbors along the route. The typical 48-hour tug journey from Erie took six days. Upon arrival in Cheboygan, Durocher prepared the barge with the necessary equipment for laying the cable. As the cable was making the last leg of its journey, the trenches were excavated and prepared to receive the cable.
Durocher used a contract weather service to track weather patterns. A weather window appeared in early winter on December 18, and the crew began laying the island end of the cable at first light through a skim of ice near the shore. The cable crossing was guided by GPS linked to a computer preloaded with the cable route. The GPS survey technician communicated with tug captains, who pulled and steered the barge. One lead tug and two side tugs held the course in the shifting currents. The 3-mile (5-km) crossing has a limestone bottom and near shores that are 300 ft (91 m) long on each end before dropping off a shelf to a mid-crossing depth of 245 ft (75 m).
The contractor excavated the near shore with 3-ft (1-m)-deep trenches to a depth of 15 ft (5 m) of water to prevent damage from winter ice and small ship anchors. Beyond this excavation, the contractor laid the cable on the bottom of the lake in a corridor used as a safe-harbor shipping lane for ore carriers and by Coast Guard and Navy ships.
While the team expected the cable to suspend between lake bottom rock outcroppings, the inherent strength of the specified cable would prevent damage due to these suspended areas. If the installation crew could reach suspension points, it took remedial action to move the cable off of the protruding rock. In this connection, the cable construction had to withstand contact with small anchors, minor scraping from passing ships and the forces exerted by constantly shifting channel currents. Accordingly, the crew specified armored cable to provide protection from the harsh environment and to provide the necessary tensile strength if the cable needed splicing.
The Nexans cable was a 24-kV AKN three-conductor design. The individual phases consisted of 500 sq mm copper conductors (approximately 1000 kcmil), filled with semiconducting compound to prevent water ingress, an extruded layer of semiconducting XLPE for strand shielding, an insulation of 216 mils of XLPE and an insulation shield of semiconducting XLPE.
Polypropylene yarn with one 24-fiber single-mode fiber-optic cable filled the interstices. Semiconducting binder tapes were applied over the assembly followed by a 79-mil thick semiconducting polyethylene jacket. Fifty four #4 AWG galvanized wires provided the armoring for the cable with two final protective layers of polypropylene yarn and bitumen for additional protection. The overall diameter was 4.5 inches (11.4 cm) and the weight of the cable was 20.2 lbs per ft.
Near shore and onshore trenches required bedding and backfill materials with a thermal rho of 90 cm°c/w or less for dissipating heat from the cable to ensure maximum ampacity. Two fill materials available in the upper Great Lakes are sand and limestone. Samples of sand and various blends of crushed limestone revealed that two mixes of limestone grades were the preferred materials. Blend No. 1, used for the onshore trench, had 10% fines to fill the voids and maximize contact. Blend No. 2, used for the offshore trench, was similar to blend No.1 without the fines since these would be dispersed in the water. Sand was not considered because its thermal rho was too high at low moisture levels.
The trenches were in solid to fractured limestone, requiring drilling and blasting prior to excavation, with particular care required to protect the existing cables. Excavation was completed and filled with the spoils. Open trenches, which could not withstand the constant wave action, were reopened just before the cable arrived. Divers directing the cable up the onshore trench were engulfed in waves freezing into a slush as the lay completed.
Using a temporary movable shanty, the cable and termination materials were kept warm while building the terminations. The contractor completed the cable terminations on the ground and pulled up the riser poles. Typically, terminations are completed on the pole with the cable strapped in place. However, this would require 35 ft (11 m) of enclosed scaffolding to protect from the low temperatures experienced during this phase of the project.
The cable installation, scheduled for the spring of 2001, provided its own set of challenges. Since the tourist season begins in early May, the installation team had to coordinate its efforts to minimize the impact of the construction on the island visitors. The onshore trench on the island crossed a two-lane paved state highway heavily traveled by bike riders, horse-drawn carriages, skaters and walkers. Since the only practical way to cross the highway with the cable was by open cutting, the contractor made a special effort to coordinate the required road closures. The contractor delivered the heavy equipment to the site before dawn, diverted traffic during the scheduled road closures and constructed temporary roads to accommodate the traffic. Since heavy equipment was in use continuously during the installation, the contractor had to turn off diesel engines and escort horses through the construction zone to prevent them from bolting. Families on bike tours were assisted across the gravel temporary roads to prevent injuries.
The island overhead line construction used a self-supporting steel pole at the shore, allowing the Hendrix cables to deadend without the use of environmentally intrusive anchors and guy wires. At the request of the State Park Commission, the pole installed near the scenic highway was painted brown to blend in with the surrounding forest. The installation of spacer cable allowed the use of a very narrow corridor with the least disruption to trees and property. The line design used 45-ft (14-m) Class 1 and 2 poles with relatively short spans averaging 120 ft (36 m). The line design had to withstand NESC heavy loading conditions and any added strain imposed by fallen trees.
The cable for the second circuit used a three-conductor, 15-kV armored Kerite cable, 4.22 inches (10.7 cm) in diameter, with 500 kcmil copper conductors insulated with 220 mils of 133% EPR. A 5-mil copper tape shield followed by an 80-mil polyethylene jacket covered the extruded semicon-ductive layer. The interstices comprised of fillers, except for a 12-fiber single-mode fiber-optic cable placed in one interstice and a #2/0 AWG copper neutral placed in another. Core tape binds the entire cable and is covered with polypropylene bedding. The Kerite cable used an innovative design for the armor, consisting of #6 BWG hardened aluminum alloy wires individually wrapped with 45 mils of HDPE. The use of aluminum resulted in a 20% weight saving over standard steel armor. The cable weighed 11.2 lbs per ft allowing the cable to be shipped by rail on a single spool.
At the completion of the installation, ESE brought DTE Energy Technologies (Farmington Hills, Michigan) to test the cables using the Cable Wise system to determine the integrity of their insulation. DTE established a set of criteria for judging cable quality based on partial discharge (PD) tests.
PD testing offers a technique for testing cables in place, in a non-destructive manner, while the cable is in normal on-line operation. The PD levels detected from the cable classify in one of five levels, depending on magnitude and repetition of the measured PD pulses. The levels are as follows:
Level 1 is a system free of discharges, indicating that no action is required.
Level 2 is a system with trivial discharge, which usually indicates activity in joints and terminations with no action recommended. If the discharge is located within the cable, retesting is recommended in two years.
Level 3 is a system with a moderate amount of discharge. Based on its experience, DTE claims that this level of activity indicates a low probability of failure but recommends retesting in one year.
Level 4 is a system with active discharge activity, indicating probable failure within two years. Under these circumstances, consider replacement or some other remedial action.
Level 5 is a system with high discharge activity, indicating almost certain failure within two years. Replacement is recommended.
For cables now in place, the testing revealed that the new cables tested in the Level 1 category as expected. The 1982 cable tested in the Level 1 category, with some sections testing in the Level 2 category. The 1988 cable contained sections tested in the Level 1, 2 and 3 categories. Upon further inspection, the Level 3 category was found to be in the cable section on the island shore. This section of cable was replaced. Subsequent testing of this section revealed that the section was now in the Level 1 category.
The expectation is that ESE will schedule annual testing of all of the cables to ensure continued reliability of the circuits.
Ernest H. Maas is the vice president of engineering and operations for Edison Sault Electric Co. He joined the company in 1984 as an engineering trainee, having received the BS degree in electronic engineering technology from Lake Superior State College.
Craig R. Davidson is the St. Ignace Division Manager for Edison Sault Electric Co. He joined the company in 1976 and was assigned to the meter shop. He has completed course work in Construction Technology at Lake Superior State College.