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.
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.
Flexibility
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).
Ramp
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.
Scheduling
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.
Stack
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.
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.
