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Glossary of terms
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Access — The contracted right to use an electrical system to transfer electrical energy.

Adequacy — The ability of the electric system to supply the aggregate electrical 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 electrical load-carrying ability, expressed in megawatts (MW) or megawatt-amperes (MVA) 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 – A group of lines and as­sociated equipment for the delivery of low-voltage electric 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 (fuel); 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 for electricity.


Import/export – Power flowing into or out of an area in order to serve load.

 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, distribution substations, etc.


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


LMP - The Locational Marginal Price is the cost to supply energy to the next increment of load at a specific location on the transmission grid.  Within the MISO market, the LMP accounts for the cost of transmission system congestion and losses.


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


Margin — The difference between capacity resources and internal demand.  Margin is usually expressed in megawatts (MW). 


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


MRO – Midwest Reliability Organization; one of the eight NERC Regional Entities in North America operating under authority from regulators in the United States and Canada through a delegation agreement with the North American Electric Reliability Corporation (NERC); (replaced the MAPP region on January 1, 2005).


MW – Megawatt; equal to 1 million watts.

NERC – North American Electric Reliability Corporation; Originally founded in 1968 by the electric utility industry as the North American Electric Reliability Council to develop and promote rules and protocols for the reliable operation of the bulk power electric transmission systems of North America.  Today, NERC is an international, independent, self-regulatory, not-for-profit organization, whose mission is to improve the reliability and security of the bulk power system in the United States, Canada and part of Mexico.


Network – A system of interconnected lines and other 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 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 unplanned.

Overloads – Occur when power flowing through wires or equipment is more than they were designed to 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 prefers to flow more heavily 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 or enhancements 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 electric system that results in electricity being delivered to customers within accepted standards and in the amount desired.    


RFC – ReliabilityFirst Corporation; as of January 1, 2006, one of the eight NERC Reliability Entities in North America operating under authority from regulators in the United States through a delegation agreement with the North American Electric Reliability Corporation (NERC).

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 conduct 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 (TLR) – A procedure used to limit power flows on lines or equipment when they could overload if an outage or 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 – Make the transmission system element able 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 during normal or under contingency conditions where the voltage dips low enough and either cannot recover quickly enough or not at all. Load may be unable to stay connected or equipment may be damaged under these conditions.


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


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