Transmission lines and related equipment carry the bulk supply of electricity in our electrical system. Like interstate highways and secondary roads, the electric power supply is carried along higher voltage transmission lines to the network of lower voltage lines and then to the roadside distribution lines that provide power to individual homes and businesses. Often referred to as “the grid”, the system includes the power lines and substations that house transformers, circuit breakers, and other equipment for transmitting, switching, and controlling electrical power.
The 345,000 volt (345 kV) Transmission System
The highest voltage transmission system in Maine operates at 345,000 volts (345 kV). This is the interstate highway system for electricity in Maine. It links CMP’s system to the bulk power grid in New Brunswick, Canada and southern New England and to the state’s largest generator at the W.F. Wyman power plant in Cumberland. Using 345 kV lines provides an efficient, reliable way to move large amounts of energy, because higher voltages result in lower line losses, i.e., energy lost as current travels through a conductor. The 345 kV system includes approximately 200 miles of transmission lines that feed the lower voltage transmission system through substations in Orrington, Windsor, Wiscasset, Pownal, Buxton, and Gorham.
The 115,000 volt (115 kV) Transmission System
The next highest level transmission lines operate at 115,000 volts (115 kV). This part of the grid can be compared to the network of larger state highways. It is the workhorse of the transmission system, carrying power from the 345 kV substations and medium-sized generators to smaller substations throughout the service territory. The 115 kV transmission system includes approximately one thousand miles of transmission lines with connections to more than 60 substations. Five 115 kV lines also connect CMP’s system to neighboring utilities to the north (Bangor Hydro Electric Company) and south (Public Service Company of New Hampshire). Many large industrial customers also have 115 kV service directly to their facilities.
The 34,500 volt Transmission System
The lowest tier of the transmission system operates at 34,500 volts (34.5 kV). These lines carry power to smaller distribution substations that may serve part of a city, an entire town, or several rural towns. Many medium-sized industrial and commercial customers also take their service directly from the 34.5 kV system. These lines also link supplies from smaller hydroelectric generators, bio-mass plants, and other small generators into the grid.
The Distribution System
The distribution system includes the familiar roadside lines and equipment that deliver power directly to customers. Distribution is connected to the transmission system through distribution substations. A substation may supply one or more distribution circuits, and the lines in a circuit may serve a small portion of a single town or they may extended along fifty or more miles of roadside through several towns. CMP’s system generally ends at the meter box on the outside of a customer’s home or business.
Distribution lines generally operate at 12,470 volts (12.47 kV). The energized (live), primary-voltage conductors are at the top of the pole on a horizontal crossarm or a pole-top insulator. A neutral wire is typically attached about four feet below the energized conductors. Additional equipment may include distribution transformers (round, grey canisters mounted just below the primaries), street lights, energized, secondary-voltage conductors, and service cables to individual customers. Below the electrical equipment, distribution poles also provide space for telecommunications attachments such as cable television, fiber optic, and telephone cables.