Understanding Transmission - How You Get Your Electricity »


AB 32 (The California Global Warming Solutions Act of 2006)

This law requires that the state’s greenhouse gas emissions be lowered to 2000 levels by 2010, and to 1990 levels by 2020, a reduction of approximately 50 million and 114 million metric tons, respectively.

Alternating Current (AC)

Electric current in which the direction of flow is reversed at frequent intervals. Most electric transmission systems in the U.S. are AC.

Ampere (Amp)

The basic unit for measuring the strength of an electric current. Also see Electricity Basics.

Balancing Authority (BA)

Regional agencies that control energy resources and transmission lines to balance energy supply and demand. Balancing authorities within California include the Balancing Authority of Northern California, California Independent System Operator, Imperial Irrigation District, Los Angeles Department of Water and Power, PacifiCorp-West, NV Energy, Turlock Irrigation District, and Western Area Lower Colorado.

Base Load

The unvarying (or slowly varying over many hours) portion of electric demand. Also see Load.

California Independent System Operator (CAISO)

A nonprofit public benefit corporation charged with operating the majority of California’s high-voltage wholesale power grid. The CAISO balances the demand for electricity with a supply of megawatts to serve more than 30 million consumers throughout California.

California Transmission Planning Group (CTPG)

A group of transmission owners and operators brought together to discuss how to address California’s current and future transmission needs.

California-Oregon Intertie (COI)

Consists of the California-Oregon Transmission Project (COTP) and the two Pacific AC Intertie (PACI) lines. The COI is an important regional system that transmits large amounts of electricity between the Northwest and California, benefitting both regions.

California-Oregon Transmission Project (COTP)

A 340 mile-long 500-kV AC transmission line that runs between Southern Oregon and Central California. It was constructed in 1993 by the Transmission Agency of Northern California, which continues to own and operate the line today.

Central Station Power

The generation of electricity in large power plants with distribution through a network of transmission lines (grid) for sale to a number of users throughout the state or region.

COI Path Operator

COTP operations are governed by the CAISO as California Path Operator, which together with the Northwest COI Path Operator, determines the COI operating limit for scheduling power transfers.


The wires of a transmission line that carry electric current.


Hollow tubes running from manhole to manhole in an underground transmission or distribution system.


Congestion occurs when available, low-cost energy cannot be delivered to all electric customers because insufficient transmission capacity is available.


A measurement of the amount of charge that passes a given point in a conductor every second. It is measured in units of amperes. Also see Electricity Basics.

Direct Current (DC)

Electric current in which electrons flow in one direction only; opposite of alternating current.

Electric Grid

An integrated system of electric transmission and distribution that connects electric generating sources to electricity consumers over a large area. Also see Electricity Basics.

Federal Energy Regulatory Commission (FERC)

The Federal agency that regulates the interstate transmission of electricity, natural gas, and oil. FERC also reviews proposals to build liquefied natural gas terminals and interstate natural gas pipelines and licenses hydroelectric power projects.


The source or entity from which electric power is first produced before it is transmitted to the electric grid. Examples of generators include natural gas, biomass, solar, wind and hydroelectric power plants.

Generation Interconnect (Gen-Tie)

These are facilities that connect the original source of electric power (generation) to the transmission system. They are typically less than five miles long.

Gigawatt (GW)

Watts (W) are the yardstick for measuring power. One gigawatt equals one billion watts. A gigwatt of electricity is enough to power about 750,000 average homes.


See Electric Grid or Transmission Grid.

Hertz (Hz)

In electric transmission, a term used interchangeably with cycles per second.

Independent Transmission Owners (TRANSCOS)

A TRANSCO’s duties would likely include construction and maintenance of energy corridors and transmission towers, coordination of transmission to wholesale customers (industries, utilities, co-ops, etc.), management of the energy grid in the regions in which they operate, and perhaps significant involvement in the wholesale energy market. At this time, many of a TRANSCO’s eventual functions are performed by an entity known as an independent system operator.

Kilovolts (kV)

Units of electrical voltage in transmission lines, one kilovolt equals 1,000 volts.

Kilowatt (kW)

Watts (W) are the yardstick for measuring power. One kilowatt equals 1,000 watts.

Kilowatt Hours (kWH)

Number of kilowatts used in an hour’s time.


Anything that draws electrical power, ranging in size from a light plugged into a wall outlet to a major industrial plant.

Load Center

A particular geographical area where energy is used; this more commonly refers to an area within a utility’s service territory where energy demand is the highest.

Load-Serving Entities (LSEs)

The utility company that provides the distribution, customer, and energy services for natural gas and electricity.

Megawatt (MW)

Watts (W) are the yardstick for measuring power. A megawatt (MW) is one million watts. One MW of electricity is enough to power about 750 average homes in North America. Also see Electricity Basics.

Megawatt Hour (MWh)

The measure of the total amount of energy consumed (in megawatts) over one hour.

North American Electricity Reliability Corporation (NERC)

A self-regulatory organization that monitors and ensures the reliability of North America’s bulk power system. TANC and transmission owners and operators throughout the U.S must comply with all applicable NERC standards in their operation and maintenance of transmission.


A measure of resistance or impedance of the flow of electric current. The standard unit for measuring resistance in a transmission conductor. Also see Electricity Basics.

Operating Reserve

The generating capacity available to the system operator within a short interval of time to meet demand in case a generator goes down or there is another disruption to the supply. Most power systems are designed so that, under normal conditions, the operating reserve is always at least the capacity of the largest generator plus a fraction of the peak load.

Operational Transfer Capability (OTC)

The amount of power that can be reliably transmitted through a transmission path given current or forecasted system conditions.

Pacific AC Intertie (PACI)

Two 500-kV AC lines that form a link that that allows large amounts of power to be transmitted between the Pacific Northwest and California. The Intertie and the COTP make up the California Oregon Intertie.

Pacific DC Intertie (PDCI)

The 500-kV direct current transmission line between the Pacific Northwest and the Los Angeles area.

Peak Load / Peak Demand

The maximum power requirement of a system at a given time, or the amount of power needed to supply customers at a time when the need is greatest. In California, peak demand usually occurs in the late afternoon during the hottest days of the year.

Reliability Standards

The reliability requirements for planning and operating the North American bulk power system. In short, these are the standards that must be met to ensure the lights stay on even if there is a catastrophic loss (such as a tower collapse) somewhere within the transmission system. The reliability standards are developed through NERC and regulated and enforced by FERC.

Renewable Energy

Any form of energy that is replaced by nature, with or without human assistance. Common forms of renewable energy include wind, solar, geothermal and tidal energy.

Renewable Energy Transmission Initiative (RETI)

An initiative to help identify the transmission projects needed to accommodate California’s renewable energy goals, support future energy policy, and facilitate transmission corridor designation and transmission and generation siting and permitting.


An area of land within which a transmission line is constructed, operated, maintained and repaired.

California Renewables Portfolio Standard (RPS)

One of the most ambitious renewable energy standards in the U.S. By the end of 2010 (later extended until 2013), 20 percent of the electricity sold by California investor-owned utilities is required to come from qualifying renewable resources (not including large hydroelectric). In 2008, Governor Schwarzenegger signed an executive order that requires that by 2020, 33 percent of all electricity sold in California will be generated by qualifying renewable resources. In 2011, Governor Jerry Brown signed a law codifying this requirement that all California utilities get 33% of the electricity they sell from renewable sources by the year 2020.

Stage 1 Emergency

A Stage 1 Emergency is called when the minimum required Operating Reserve level falls below approximately seven percent. This amount can vary depending on the type of generation that is running. Under a Stage 1 Emergency, voluntary conservation is called for to avoid severe conditions.

Stage 2 Emergency

A Stage 2 Emergency is called when the minimum Operating Reserve level falls below five percent. Under a Stage 2 Emergency, “interruptible” programs operated by the utilities can be called upon. Typically, participants in these programs are commercial and industrial customers who receive a lower electricity rate and in exchange, are required to reduce their energy usage by a predetermined amount when called upon during a Stage 2 Emergency.

Stage 3 Emergency

A Stage 3 Emergency is called when Operating Reserves fall below the minimum requirements, which can result in rotating power outages. This is a last resort, used only when a climbing demand for energy is close to surpassing the available supply.


A high-voltage electric system facility used to switch generators, equipment and circuits or transmission lines in and out of a system. It also is used to change AC voltages from one level to another, and/or change AC to DC current or reverse in order to help power flow most efficiently through the system.


Transmission lines that carry energy at reduced voltages from the major transmission line system, typically, 34.5 kV to 69 kV. This power is sent to regional distribution substations. Sometimes the subtransmission voltage is tapped along the way for use in industrial or large commercial operations. Some utilities categorize these as transmission lines.


A component of a transmission grid used to step up voltage from generators to high voltage transmission lines and then step down voltage to the local distribution system.

Transmission Expansion Policy Planning Committee

A WECC sponsored Committee with a role in the western transmission planning process is to provide region-wide services in three areas: 1) overseeing development and management of a common database for economic analysis of transmission needs, 2) providing policy and management of the regional planning process across the region, and 3) guiding analyses and modeling for Western Interconnection economic transmission expansion planning.

Transmission Grid

A system of interconnected generating facilities, substations and transmission lines that provide energy from its sources to its end users.

Transmission Lines

Electricity infrastructure that transmits high-voltage electricity (typically above 69-kV) from the generation source to electric customers.

Transmission Losses

The amount of power lost in the course of transmission due to resistance of the conductors. Transmitting energy at higher voltages reduces the amount of energy lost. In high-voltage transmission, these losses typically amount to approximately 5-7 percent of the total power put into the system. Also see Electricity Basics.


In transmission, voltage refers to the amount of “push” used to move electrons through a transmission conductor. One volt is the force required to send one ampere of electrical current through a resistance of one ohm. Also see Electricity Basics.

Watt (W)

The yardstick for measuring power. 1,000 watts equal one kilowatt (kW). 1,000 kWs equal one megawatt (MW). 1,000 MWs equal one gigawatt (GW). Also see Electricity Basics.


A consortium of utility companies that provide transmission of electricity throughout the Central and Western U.S. WestConnect’s members work collaboratively to assess stakeholder and market needs and develop cost-effective enhancements to the western wholesale electricity market.

Western Area Power Administration

An agency within the U.S. Department of Energy that markets Federal hydroelectric power to preference customers. These customers include municipalities, cooperatives, irrigation districts, Federal and State agencies, and Native American tribes. Western’s service territory covers 15 western states, including California. Western owns and operates more than 17,000 miles of high-voltage transmission lines.

Western Electricity Coordinating Council (WECC)

The Regional Entity responsible for coordinating and promoting bulk electric system reliability in the Western Interconnection. In addition, WECC assures open and non-discriminatory transmission access among members, provides a forum for resolving transmission access disputes, and provides an environment for coordinating the operating and planning activities of its members.