Access – The contracted right to use an electrical system to transfer electrical energy.

Adequacy – The ability of the electric system to supply the total electric demand and energy requirements of the customers at all times, taking into account scheduled and reasonably expected unscheduled outages of system elements.

Capacitor banks – System elements that support the voltages necessary to provide reliable service to customers.

Capacity – The load-carrying ability, expressed in megawatts (MW) of generation, transmission or other electrical equipment.

Contingency – Outage of a transmission line, generator or other piece of equipment, which affects the flow of power on the transmission network and impacts other network elements.

Demand – The rate at which electric energy is delivered to or by a system or part of a system, generally expressed in kilowatts (kW) or megawatts (MW), at a given instant or averaged over any designated interval of time.

Distribution – An interconnected group of lines and associated equipment for the delivery of low-voltage energy between the transmission network and end users.

FERC – Federal Energy Regulatory Commission; an independent agency that regulates the interstate transmission of natural gas, oil and electricity.

Generation – The process of producing electrical energy from other forms of energy; also, the amount of electric energy produced, usually expressed in kilowatt hours (kWh) or megawatt hours (MWh).

Heavy loads – High volume of electricity flowing on a line, transformer or other equipment to meet a high demand.

Import/export – Ability of the transmission system to bring power into or out of an area in order to serve electricity needs.

kV – Kilovolt; equal to 1,000 volts.

Line rebuild – Removing an existing line and replacing it with a new, higher capacity line.

Line reconductor – Removing the conductors (wires) from an existing transmission line and replacing them with higher capacity conductors.

Load – All the devices that consume electricity and make up the total demand for power at any given moment, like factories, businesses, schools, homes, etc.

Loading relief – A system reinforcement of operating action that results in lower power flows on equipment that is heavily loaded or overloaded.

Low voltages – A situation that can occur in parts of the system that are heavily loaded or which have high motor loads. Think of a clothesline pulled taut with nothing handing on it, but which then tends to sag when more and more clothes (i.e. loads or motors) are attached. Low voltages negatively impact the ability to serve loads reliably.

MAIN – Mid America Interconnected Network; one of the nine NERC Regional Reliability Councils.

Margin – The difference between (1) generation resources and electric demand or (2) capacity of a transmission line and the power flowing on it. Margin is usually expressed in megawatts (MW).

MISO – Midwest Independent Transmission System Operator; a not-for-profit transmission system operator that serves the electric transmission needs of much of the Midwest.

MW – Megawatt; equal to 1 million watts.

NERC – North American Reliability Council; a not-for-profit company formed by the electric utility industry in 1968 to promote the reliability of the electricity supply in North America. NERC consists of nine Regional Reliability Councils and one Affiliate whose members account for virtually all the electricity supplied in the United States, Canada and a portion of Mexico.

Network – A system of interconnected lines and electrical equipment.

OASIS – Open Access Same Time Information System; an electronic posting system for transmission access data that allows all transmission customers to view the data and request transmission service simultaneously.

Off-peak - Those hours or other periods defined by contract or other agreements or guides as periods of lower electrical demand; generally nights and weekends.

On-peak – Those hours or other periods defined by contract or other agreements or guides as periods of higher electrical demand; generally weekdays.

Operating guides – Procedures carried out by transmission operators when certain events occur on the system that may compromise system reliability if no action is taken.

Outage – The unavailability of electrical equipment; could be planned or forced.

Overloads – Occur when power flowing through wires or equipment is more than they can carry without incurring damage.

Parallel path flows – When electricity flows from a power plant over the transmission system, it obeys the laws of physics and flows over the paths of least resistance. Though there may be direct connection between a power plant and a particular load area, some of the power will instead flow over other network lines parallel to the direct connection.

Planning – The process by which the performance of the electric system is evaluated and future changes and additions to the bulk electric systems are determined.

Power flows – Electricity moving through lines or other equipment.

Reliable – Meets standard industry and specific ATC system performance criteria.

Reliability – The degree of performance of the elements of the bulk electric system that results in electricity being delivered to customers within accepted standards and in the amount desired.

Reserve – The difference between an electric system's capability and the expected peak demand for electricity.

Security – The ability of the electric system to withstand sudden disturbances such as unanticipated loss of system elements.

Serve load – Reliably deliver the amounts of electricity needed to match what consumer would like to use at any given time.

Shed load – Reduce the level of power flowing by disconnecting load from the network in order to prevent major equipment damage or widespread outages. This is usually a last resort emergency action.

Single contingency – The sudden, unexpected failure or outage of a system facility(s) or element(s) (generating unit, transmission line, transformer, etc.). Elements removed from service as part of the operation of a remedial action scheme are considered part of a single contingency.

Stability – The ability of an electric system to maintain a state of equilibrium during normal and abnormal system conditions or disturbances.

Substation – Place where transmission lines connect to each other and where protective equipment like circuit breakers are located. Also where transformers are located to step the voltage up or down in order to put power into or take power out of the transmission network.

T-D interconnection – Transmission to distribution interconnection; place where distribution substations connect to transmission system.

Thermal rating – The maximum amount of electrical current that a transmission line or electrical facility can carry over a specified time period before it sustains permanent damage by overheating or before it violates public safety requirements.

Thermal overloads – Power flows on lines or equipment that exceed their capacity limits.

Transfer capability – The measure of the ability of interconnected electric systems to move or transfer power in a reliable manner from one area to another over all transmission lines between those areas under specified system conditions.

Transformers – Devices that change voltage levels.

Transmission – An interconnected group of lines and associated equipment for the movement or transfer of electric energy between points of supply and points at which it is transformed for delivery to customers or is delivered to other electric systems.

Transmission loading relief – A procedure used to limit power flows on lines or equipment when they could overload if an outage on another system element occurred. The result is an interruption of specific power transactions that contribute to the power flow on the affected line or equipment.

Uprates – Allows the transmission system element to carry more electricity than it currently can. This can include increasing line clearances or replacing limiting pieces of equipment to enable the safe carrying of more power.

Voltage collapse – Can occur after a contingency where the voltage dips low enough and cannot recover quickly enough. In this situation, protective equipment automatically will disconnect lines and/or transformers, causing load to be shed.

Voltage stability – System is able to maintain the proper voltages needed to serve load.