The power supply outage experienced by customers on Mackinac Island has reminded everyone of the need for a reliable source of electric power.

At one time, the power requirements for Mackinac Island were supplied by diesel generator units located on the Island. As the electric load continued to grow, a power supply system from the mainland at St. Ignace was installed using special submarine cables to help supply a portion of the Island’s requirements from the Edison Sault diesel generator plant at St. Ignace. Eventually, the diesel power plants could no longer efficiently meet the Island’s growing load, and the cable supply system became the sole source of electric power to the Island utilizing a transformer at the St. Ignace Substation.

The use of special submarine cable will continue to be the case as this portion of the system has been very reliable. The cables have been replaced over the years as the need to deliver more power to meet Island customer requirements required larger capacity cables. The most recent cable replacements were in 1988 for one of the two circuits with the other one having been replaced in 1983. The industry norm for the expected life of these cables is about 30 years.

The use of underground cable has grown in popularity with customers because it eliminates much of the visible portion of the electric distribution system. This has been the preference on the Island for many years to help maintain its unique character.

Residential customers are now commonly and reliably connected to the utility distribution system using 120/240-volt underground wires direct buried to the residence (also known as "secondary distribution"). However, the use of underground cables for high-voltage "primary" distribution facilities (such as the 13,200-volt distribution lines on the Island) is a more demanding situation. Installation of such cables in the ground exposes them to a potentially harsh environment, particularly in the rock- and gravel-bearing soils found on the Island. For example, over time, moisture that can work its way into the cable’s insulation system, migration of rocks in soil that can physically damage the cable, and damage to cables through inadvertent "dig-ins" can result in widespread customer outages because primary distribution supplies many customers on one circuit.

The recent power outage on Mackinac Island was due to failure of the land-based primary distribution cables used in the power supply system. Edison Sault will be examining with Island officials the advantages and disadvantages of underground versus overhead distribution lines for that portion of the power supply system between the shoreline connection to the submarine cables and the switching station located at Stonecliff.

Because the submarine cables supplying Mackinac Island are known to be in a stressed condition, Edison Sault intends to replace all of the electric power supply system cable facilities from St. Ignace to the Pat Chambers Switching Station on the Island as soon as possible. Immediately after the power supply outage was resolved, Company officials began work with its consulting engineer (Novak Engineering, Inc.) to design a new power supply system both in the water and on land.

Development of a comprehensive specification for new replacement submarine cables capable of delivering 17 MW of power (70% more than the summer 2000 peak) to Island customers was recently completed. On August 22, 2000, the specification was sent to four suppliers of special submarine cable as part of a request for proposal to furnish and install the new cable supply system.

It is Edison Sault’s plan to install two new submarine cable circuits by the end of 2001. The Company is working diligently with suppliers to determine if one of the replacement circuits can be installed by the end of 2000. During the next few weeks, Edison Sault representatives will be visiting three of the cable suppliers in North America to review their engineering and design capabilities and ability to expedite delivery and installation of the cables before winter arrives.

Edison Sault is also examining a number of issues to improve the distribution system to Mackinac Island and on the Island. These issues include:

• The type of termination facilities (cabinets, support structures, etc.) that will be required at the shoreline for the new submarine cables.

• The type of distribution facilities (type of cable or wire, direct-buried or overhead, etc.) that should be installed from the shoreline to the Pat Chambers Switching Station at Stonecliff.

• Options for installing additional distribution circuits on the Island to divide the customers into smaller groups as this would reduce the impact of outages on these circuits as well as facilitate the investigations performed to determine the location of failed equipment.

Edison Sault has applied standard utility practices (annual visual inspections of equipment and continuous monitoring of operating parameters such as power flow in cables) in its inspection program. The Company also employs the services of Bisbee Company to perform a special infrared scan of equipment to locate overheating and possible bad connections that could lead to a failure and equipment outage. In addition, annual testing the power supply transformer at St. Ignace Substation is performed including dissolved gas in oil analysis, voltage breakdown of transformer oil, etc.

Edison Sault’s inspection and testing program has generally been successful in maintaining the reliability of its power supply system to the Island. Based on the recent experience with the submarine cable portion of the supply system, the Company will be employing special testing, such as the newly-developed partial discharge testing provided by DTE Energy Technologies, for these cables on an annual basis.

As a result of the recent power supply outage to Mackinac Island, Edison Sault plans to develop further studies to determine the viability of having permanent generation (gas, diesel) on the Island as an alternate backup plan.