System Information

System Information

System Information Overview

The transmission system delivers electrical energy from generating stations to transmission customers and the distribution system. Originally, generating stations were located close to the load they served and the transmission system could be as simple as one line. Today a sophisticated networked system connects generating stations to substations which distribute electricity to customers. The transmission system in the strictest sense consists of:

  • wood poles and steel structures designed to safely support the wire conductors which run for many miles between substations; and
  • substations which contain protection, metering, and communications equipment, transformation to other voltages, and/or voltage control equipment.

This transmission system is designed and operated to transfer large quantities of electricity reliably, safely, and efficiently. But there is more. The items above are only the transmission facilities. The entire "system" (and it is a system in the literal sense, for they all work together) is composed of much more:

  • individual generators within a station, their associated excitation systems with fast voltage control, their associated turbines and governors to regulate frequency, their associated protective equipment, and the auxiliary loads to keep the generators operating;
  • the distribution system, with its voltage regulators, power factor compensation, protection systems, and transformers to provide customers their desired utilization voltages;
  • customer equipment, along with its protection and controls; and
  • control centers and dispatchers, which match generation to load, and control flows, voltages, and system frequency.

In fact, all of these facilities (not just transmission facilities) make up "the system" in the larger sense. Moreover, "the system" extends across our entire continent, operating in synchronism. That is why reliability criteria for transmission system designers and operators apply to generation facilities and consider the behavior of distribution systems and customer equipment. That is also why the models for large-scale studies of system behavior model all of the above facilities across the continent. The eastern interconnected systems make up the largest machine in the world.

The CMP Transmission System

The CMP transmission system is responsible for carrying bulk electricity between generators and ties to the rest of New England and Canada, and throughout the service territory to distribution substations and our customers. It consists of the highest capacity power lines, capacitors, reactors, transformers, circuit breakers, and other high voltage equipment used for transmitting, switching, and controlling electrical energy.

CMP's transmission system comprises approximately 2,300 miles of transmission lines and 300 substations. This system serves an 11,000-square-mile area, more than the size of Massachusetts and Rhode Island combined. Within this service territory, CMP serves more than 636,000 residential, commercial, industrial, and wholesale customers.

The CMP transmission system operates at three voltage levels: 345 kV, 115 kV, and 34.5 kV. The transmission and distribution lines can be compared to roads and the voltage may be compared to the speed limit. Substations act like the exit ramps or turnoffs. Voltage from one line to the next is changed at the substations. The following is a breakdown of CMP's three transmission operating voltages and their characteristics.

345 kV - The Interstate Highway

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.

115 kV - State Highways

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.

34.5 kV - Secondary Roads

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.


Since the early 1970s, CMP has developed and maintained strong interconnections with its neighbors in New Brunswick, New Hampshire and the rest of New England. CMP's interconnections allow the transfer of capacity and energy for both contractual arrangements and economic transactions. Actual interconnection capabilities with neighboring utilities are dependent on system conditions (such as load, status of generators, and transmission facility outages) both in Maine and in other areas at the time of the transfer. Some normal interconnection capabilities are shown below.

CMP Normal Interconnection Capabilities
Transmission Interface   Transfer Limit (MW)
New Brunswick to Maine   700
Maine to New Brunswick   150 to -350
Maine to New Hampshire   900 to 1,400
New Hampshire to Maine   1,100 to 1,500
NEPOOL Normal Interconnection Capabilities
Transmission Interface   Transfer Limit (MW)
Northern New England to Southern New England  1,700 to 3,000
Quebec to Sandy Pond   1,200 to 2,000
New England East-to-West   800 to 2,000
New England West-to-East   1,600 to 2,200
New York to New England   1,300 to 2,200
New England to New York   700 to 1,700
CMP Native Customer Load
Condition   Coincident Load (MW)
CMP System Peak Load   1,500
CMP minimum system load   600

This table illustrates that CMP and other New England transfer limits are large when compared to CMP's customer load. Moreover, these power transfer limits can vary significantly with conditions, and are not necessarily the same in both directions.
CMP is a small part of a much larger bulk power system. Therefore, the coordination of system design and operation extends well beyond individual system boundaries.


CMP is a participant in the New England Power Pool (NEPOOL), a voluntary organization formed in 1971 to assure reliability, and to attain maximum practicable economy with equitable sharing of benefits and costs. Therefore NEPOOL has a dual role: reliability and economy. Over 99% of the electrical energy supplied in New England is provided by participants in NEPOOL. New England operates as a single "Control Area". A series of standing Committees and Task Forces provides coordination and peer review for NEPOOL Participants. NEPOOL was originally formed by customer-serving electric utilities, but today includes Non-Utility Generators (NUGs) and Power Marketers and Brokers. NEPOOL relies on its "Reliability Standards", Criteria, Rules, and Standards (CRSs), Operating Guides (OGs), and Operating Procedures (OPs) to reliably design and operate the New England interconnected systems.

ISO New England, the Independent System Operator (ISO) for the New England Control Area, took over the operation of the NEPOOL system in 1997. ISO New England also hosts the OASIS site for all of the Transmission Providers in New England, and administers the NEPOOL Tariff.


CMP is a member of the Northeast Power Coordinating Council (NPCC), a voluntary organization formed after the 1965 Northeast Blackout to promote the reliability and efficiency of the electric bulk power systems by coordination of system design and operation. NPCC has a singular role; reliability. The NPCC region includes New England, New York, New Brunswick, Nova Scotia, Quebec, Ontario, Prince Edward Island, and Newfoundland. The Reliability Coordinating Committee and a series of standing Task Forces provides coordination and peer review for NPCC members. Working Groups also perform special reliability assignments. NPCC was originally formed by customer-serving electric utilities within the region, but has recently included other utilities, Non-Utility Generators (NUGs) and Power Marketers. NPCC relies on its "Reference Manual", composed of Criteria, Guides, and Procedures to reliably design and operate the NPCC interconnected systems.

Disturbances in Maine can affect the bulk power systems west and south of New York, to the Gulf Coast and the Rocky Mountains. Similarly, disturbances elsewhere on our continent can affect Maine and northeast North America. Therefore, the regional reliability councils, such as NPCC, have coordinated their efforts through the North American Electric Reliability Council (NERC).

About Reliability

Transmission system reliability incorporates dependability and security. Dependability relates to the continuity of electricity to customers. In the event of equipment failure, system security ensures that system failures are localized, and significant long-term damage is minimized.

In order to ensure an adequate level of reliability in New England and northeast North America, criteria have been established to guide the design, operation and maintenance of power systems. NEPOOL and NPCC members agree to abide by criteria and guidelines that have been established by experience over the years. This forms the basis for reliable operation of the bulk power system in Maine and the northeast. CMP also has reliability criteria in the form of our "Transmission Planning Criteria" and "Interconnection Requirements for Generation."

About Economy

The transmission system allows access to outside energy markets for both short-term and long-term contractual transactions. It also accommodates the flows necessary to allow the economic dispatch of generation and tie flows under the NEPOOL Agreement.




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