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Glossary of Transmission Grid Integration Terms

The following glossary defines common transmission grid integration terms and concepts.

Ancillary services

Services that help grid operators maintain balance on electric power systems. These include regulation and the contingency reserves: spinning, non-spinning, and, in some regions, supplemental operating.

Balancing authority area

A metered segment of the electric power system in which electrical balance is maintained. In a balancing authority area, the total of all generation must equal the total of all loads (as supplemented by electrical imports into and exports out of the area).

Balancing authority

The entity responsible for ensuring electrical balance within a balancing authority area.

Baseload generating plants

Typically coal or nuclear generat­ing units, these plants are usually committed and dispatched at constant or near-constant levels with minimum cycling. They are often the sources of lowest-cost energy when run at very high capacity factors.

Contingency reserves

Services sufficient to cover the unplanned trip (or disconnect) of a large generator or transmis­sion line to maintain system balance. Contingency reserves are generally split between spinning and non-spinning reserves and are often based on the largest single hazard (generator or trans­mission capacity). Contingency events are big (many megawatt) and fast (within a few cycles).

Demand response

Voluntary (and compen­sated) load reduction used as a bulk system reliability resource. Historically, utilities have controlled genera­tion to ensure reliability, but controlling load can also be effective.

Dispatch (economic dispatch)

A method by which system operators decide how much output should be scheduled from plants that have been committed or that can be started quickly.

Energy imbalance service

A market service that provides for the management of unscheduled deviations in individual generator output or load consumption.


The ability of the generation fleet to change its output (ramp) rapidly, start and stop with short notice, and achieve a low minimum turn-down level.

Frequency response

Generation (and responsive load) abil­ity to increase output (or reduce consumption) in response to a decline in system frequency and decrease output (or increase consumption) in response to an increase in system frequency. For generators, this requires governor response.

Generator trip

 A generator failure caused by electrical or mechanical malfunction that causes a contingency condition on a power system. Generator trips occur very quickly, usually within a few cycles. Contingency reserves provide sufficient online generation to replace tripped generators.

ISO (independent system operator)

An entity responsible for maintaining system balance, reliability, and electricity market operation.

Load forecast

A prediction of future demand. For normal operations, daily and weekly forecasts of hour-by-hour demand are used to develop generation schedules that ensure sufficient quantities and types of genera­tion are available when needed.

LMP (locational marginal price)

The price of a unit of energy at a particular location at a given time. LMPs are influenced by nearby generation, load level, and transmission constraints and losses.

Minimum run (turn-down) level

The minimum output that can be provided by a generator. Different generators have differ­ent minimum run levels based in part on fuel source, plant design, and common use.

Non-spinning reserve

Generation and responsive load that is offline but can be fully responsive within 30 minutes. Non-spinning reserve can be used to help manage generation variability and uncertainty for time frames that exceed 10 minutes (load following).

On the margin

The last generator to be dispatched at any point in time is the marginal generator or on the margin and typically sets the market price for that market period. Power system operators dispatch generators based on cost (sequentially from lowest to highest cost) and physical capabilities.

Operating reserves

A combination of contingency reserves, regula­tion reserves, and sometimes load-following reserves. This term is not standard; different definitions are relatively widespread.

Peaking plants

Plants (often combustion turbines with low capital cost and high or very high fuel costs) used sparingly (often only a handful of times a year during extreme peak periods of demand).


A change in generation output over some unit of time.

Ramp rate

The change in output of a generating unit, often measured in megawatts per minute.

Rated capacity

The maximum capacity of a generating unit.

Reactive supply and voltage control

The supply of dynamic reactive power, typically from generation, to control transmission system voltages.

Regulating reserves

Capacity devoted to providing a fast up-and-down balancing service. In the U.S., regulating capacity is controlled by computers (via automatic generation control). Regulation duty can sometimes be procured in a market.

RTO (regional transmission organization)

An entity responsible for maintaining system balance, reliability, and electricity market operation.


The practice of ensuring a generator is committed and available when needed. It also can refer to the scheduling of imports into and exports out of a balancing area.

Spinning reserve

Generation and responsive load that is online, can begin responding immediately, and is fully responsive within 10 minutes.

System control

The central control function performed by a system operator to manage generation, demand response, and transmission assets to reliably and economically serve load.


The collection of available generators arranged in eco­nomic order. This term is often applied in the context of a specific type of system operation. For example, dispatch stack refers to all generating units available for dispatch (that have been committed or are quick-start units). Commitment stack refers to all generation units that have been committed or are available for commitment.

Subhourly energy markets

Electricity markets that operate on time steps of five minutes. Approximately 60% of all electricity in the U.S. is currently traded in sub-hourly markets, running at 5-minute intervals so maximum flexibility can be obtained from the generation fleet.

Supplemental reserve

Generation and responsive load that is offline but can be fully responsive within 10 minutes to replace spinning reserve that has been activated in response to a con­tingency or other power system need.

Tie line

The transmission connection between two individual balancing areas.   

Turn-down capability

The minimum stable generation level that can be achieved by a generating unit.


A single generator that may be part of a multiple-generator power plant.

Unit commitment

The process of starting up a generator so that the plant is synchro­nized to the grid. This process can take many hours for a steam generator and depends on whether the plant is warm or hot from previous commitment.

Unloaded capacity

Generating capacity that is spinning and synchronized to the grid but is not providing energy.

Variable generation

Electricity generation technologies whose primary energy source varies over time and cannot easily be stored. Variable generation sources include solar, wind, ocean, and some hydro generation technologies.