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    Glossary of terms

    Common electricity terms, explained

    Want to know the meaning of the terms we use? You’ll find explanations right here.

    For terms used in independent system operator rules, or Alberta Reliability Standards, read our Consolidated Authoritative Document Glossary.

    Start typing a term to filter, or browse terms alphabetically.

    • Acceptable operational reason

      Circumstances which reasonably prevent a generating asset from operating, or from operating safely.

    • Activation price

      The price paid to an operating reserve provider if the AESO System Controller dispatches the reserve.

    • Active operating reserve(s)

      Electricity reserves that meet operating requirements of the AIES under normal operating conditions.

    • Alberta Interconnected Electric System (AIES)

      The system of interconnected transmission power lines and generators in Alberta.

    • Alberta Internal Load (AIL)

      The total electricity consumption within the province of Alberta including behind-the-fence (BTF) load, the City of Medicine Hat and losses (transmission and distribution).

    • Alternating current (AC)

      A current that flows alternately in one direction and then in the reverse direction. In North America, the standard for alternating current is 60 complete cycles each second. Cycles per second is also referred to as Hertz (Hz).

    • Ancillary services

      Services necessary to support the transmission of energy from resources to loads based on consumption (for loads) and dispatch (for suppliers).

    • Baseload generation

      Generation capacity normally operated to serve load on an around-the-clock basis.

    • Behind-the-fence load (BTF)

      Industrial load served in whole, or in part, by onsite generation built on the host’s site.

    • Biomass

      Organic matter that is used to produce synthetic fuels or is burned in its natural state to produce energy. Biomass fuels include wood waste, peat, manure, grain by-products and food processing wastes.

    • Brownfield

      Land previously or currently used for industrial or certain commercial purposes.

    • Bulk transmission system

      The integrated system of transmission lines and substations that delivers electric power from major generating stations to load centres. The bulk system, which generally includes 500 kilovolt (kV) and 240 kV transmission lines and substations, also delivers/ receives power to and from adjacent control areas.

    • Bus (busbar)

      Electrically conductive structures in a substation to which elements such as transformers or transmission lines are connected.

    • Capability

      The maximum load that a generating unit, generating station or other electrical apparatus can carry under specified conditions for a given time period without exceeding limits of temperature and stress.

    • Capacitor/capacitor bank

      A device used to control voltages by eliminating a voltage drop in the system.

    • Capacity

      The amount of electric power delivered or required for a generator, turbine, transformer, transmission circuit, substation or system as rated by the manufacturer.

    • Capacity market model

      A market model where generators are paid for having generation available to supply, whether or not any energy is actually produced and supplied.

    • Carbon offset

      A financial instrument representing a reduction in greenhouse gas (GHG) emissions.

    • Circuit

      A conductor or a system of conductors through which electric current flows.

    • Cogeneration

      The simultaneous production of electricity and another form of useful thermal energy used for industrial, commercial, heating or cooling purposes.

    • Combined-cycle

      A system in which a gas turbine generates electricity and the waste heat is used to create steam to generate additional electricity using a steam turbine.

    • Competitive process

      A tendering process that enables all qualified bidders to compete in a fair, transparent and open environment for the right to develop, design, build, finance, own, operate and maintain major transmission infrastructure in Alberta. 

    • Conductor

      A metallic wire or combination of wires through which electric current is intended to flow.

    • Congestion

      The situation in which transactions that market participants wish to undertake are constrained by conditions on the transmission grid.

    • Constraint

      A restriction on a transmission system or segment of a transmission system that may limit the ability to transmit power between various locations.

    • Contingency

      An event occurring on the system resulting in the loss of a system element (e.g., outage of a generating unit or transmission line).

    • Contingency reserve(s)

      A type of reserve used to restore the supply/demand balance when a contingency occurs.

    • Control area

      A defined region of the electricity grid for which supply and demand are kept in balance by the control area’s system operator.

    • Converter station

      A location where electric energy is converted to direct current (DC) from alternating current (AC) or vice versa.

    • Cooling Degree Day (CDD)

      The number of degrees that a day’s average temperature is above a threshold temperature.

    • Cost-of-service model

      A traditional electric utility model where a utility is allowed to set rates based on the cost of providing service to customers and the right to earn a limited profit, all as set by a regulator such as a utilities commission.

    • Customer sectors

      Types of electric load classified according to type of use. Four sectors commonly used are residential, commercial, farm and industrial.

    • Cutplanes

      An imaginary line, often depicted on a map, that cuts across the transmission lines connecting two or more areas. The loading on these lines is summed together to measure the power flow across the cutplane.

    • De-rate

      A reduction in a generating unit’s or other piece of electric equipment’s net capacity.

    • Demand (electric)

      The volume of electric energy delivered to or by a system, part of a system, or piece of equipment at a given instant or averaged over any designated period of time.

    • Demand transmission service (DTS)

      The service provided to loads for interconnection access to the Alberta transmission system.

    • Demand-side management

      Activities that occur on the demand (customer) side of the meter and are implemented by the customer directly or by load-serving entities.

    • Deregulation

      An absence of a monopoly or central planning, where consumers can choose their supplier, similar to other common household service providers.

    • Direct assigned to TFOs

      In a regulated monopoly, projects are assigned to a transmission company based on their service territory. 

    • Direct current (DC)

      Current that flows continuously in the same direction (as opposed to alternating current). The current supplied from a battery is direct current.

    • Dispatch

      The process by which a system operator directs the real-time operation of a supplier or a purchaser to cause a specified amount of electric energy to be provided to, or taken off, the system.

    • Distribution facility owner

      Entities that own and operate distribution lines, the portion of the Alberta electrical system operating at 25 kilovolts (25,000 volts) or less. These distribution lines provide service to most consumers, except for some very large industries that are directly connected to the transmission grid.

    • Distribution-connected generation

      Small-scale power sources typically connected to a distribution system at customer locations.

    • Double circuit

      A line of supporting structures that carries two power circuits.

    • Dynamic stability

      The characteristic of a power system that, when disturbed from an original state through events such as short circuits, allows recovery to a normal state through damping of the oscillations generated by the disturbance events.

    • Emission intensity

      The ratio of a specific emission (such as carbon dioxide) to a measure of energy output. For the electricity sector, emission intensity is usually expressed as emissions per megawatt hour (MWh) of electricity generated.

    • Energy-only

      A market model where power plants are paid only for the energy they actually produce.

    • Equilibrium price

      A market price established through competition at the point where the quantity sought by buyers is equal to the amount produced by sellers. This price is also often called the competitive price or market clearing price.

    • Frequency excursion

      Any deviation, up or down, of the base frequency of a power system. In North America, the base power system frequency is 60 Hertz (60 cycles per second).

    • Gas turbine

      See simple-cycle.

    • Generating unit

      Any combination of an electrical generator physically connected to reactor(s), boiler(s), combustion or wind turbine(s) or other prime mover(s) and operated together to produce electric power.


    • Geothermal energy

      Where the prime mover is a turbine driven either by steam produced from hot water or by natural steam that derives its energy from heat found in rocks or fluids beneath the surface of the Earth.

    • Gigawatt (GW)

      One billion watts.

    • Gigawatt hour (GWh)

       One billion watt hours.

    • Greenfield

      Land being considered for development that has not previously been used for commercial or industrial purposes.

    • Greenhouse gas (GHG) emissions

      Gases that trap the heat of the sun in the Earth’s atmosphere, producing a greenhouse effect.

    • Grid

      A system of interconnected power lines and generators that is operated as a unified whole to supply customers at various locations. Also known as a transmission system.

    • Gross domestic product (GDP)

      One of the measures of national income and output for a given country’s economy. GDP is defined as the total market value of all final goods and services produced within the country in a given period of time (usually a calendar year). It is also considered as the sum of the value added at every stage of production (the intermediate stages) of all final goods and services produced within a country in a given period of time and is given a monetary value.

    • Heat rate

      A measure of generating plant thermal efficiency generally expressed as units of energy input per unit of energy output.

    • Heating Degree Day (HDD)

      The number of degrees that a day’s average temperature is below a threshold temperature.

    • High-voltage direct current (HVDC)

      The transmission of electricity using direct current.

    • Holding restrictions

      A legislated limit on the market share a company may hold.

    • Independent system operator (ISO)

      A system and market operator that is independent of other market interests. In Alberta the entity that fulfils this role is the Alberta Electric System Operator.

    • Index to the pool price

      A premium or a discount to the pool price.

    • Injection-withdrawal model

      A market model where access to the transmission system is allocated based on consumption (for loads) and dispatch (for suppliers).

    • Interconnection

      An arrangement of electrical lines and/or transformers that provides an interconnection to the transmission system for a generator or large commercial or industrial customer.

    • Intertie

      A transmission facility or facilities, usually transmission lines, which interconnect two adjacent electric systems and allow power to be imported and exported.

    • Kilowatt

      1,000 watts.

    • Kilowatt hour (KWh)

      1,000 watt hours. Imagine 10 lamps, lit with 100-watt bulbs for one hour. At the end of the hour, the lamps will have used one kilowatt hour of electricity. Electricity retailers typically bill households in kilowatt hours.

    • Levelized unit electricity cost (LUEC)

      The constant price required to cover all expenses incurred over the lifetime of a generating unit.

    • Load (electric)

      The amount of electric power used by devices connected to an electric system.

    • Load factor

      A measure of the average load, in kilowatts, supplied during a given period. It is generally used to determine the total amount of energy that would have been used if a given customer’s maximum load was sustained over an extended time period and, through comparison, show what percentage of potential load was actually used.

    • Long-term adequacy

      The ability of future electric system supply to meet expected electrical demand requirements over several years. 

    • Looped system

      A system of power lines in which circuits are connected between substations and then back to the same substation.

    • Megawatt (MW)

      One million watts.

    • Megawatt hour (MWh)

      One million watt hours. A megawatt hour measures the amount of electricity a generator produces in one hour.

    • Merchant transmission

      Transmission line(s), constructed by proponents that are not regulated utilities, for the purpose of selling transmission capacity to third parties such as generators or load customers who wish to make transactions over the line.

    • Merit order

      In the wholesale electricity market, the list used to dispatch electric generation to meet demand, based on offer price. The lowest cost generation is dispatched first. 

    • Meters

      Equipment that measures and registers the amount and direction of electrical quantities.

    • Needs Identification Document (NID)

      A document filed by the AESO with the Alberta Utilities Commission (AUC) to define the need to reinforce the transmission system to meet load growth and/or provide non-discriminatory access to interconnect new loads and generators to the system.

    • Non-wires solution

      Refers to a solution to address transmission reliability issues, such as congestion and constraint, that do not include the construction or upgrading of physical transmission infrastructure. One common example is “transmission must-run.”

    • Nuclear generation

      Where heat produced in a reactor by the fissioning of nuclear fuel is used to create steam in a boiler. This steam is then used to drive a turbine, which in turn drives the electric generator.

    • Offset

      See carbon offset.

    • Operating reserve

      Generating capacity that is held in reserve for system operations and can be brought online within a short period of time to respond to a contingency. Operating reserve may be provided by generation that is already online (synchronized) and loaded to less than its maximum output and is available to serve customer demand almost immediately. Operating reserve may also be provided by interruptible load.

    • Parallel path

      Electric power flows on all interconnected parallel paths in amounts inversely proportional to each path’s resistance. This also refers to the flow of electric power on one electric system’s transmission facilities resulting from scheduled electric power transfers between other electric systems.

    • Peak load/demand

      The maximum power demand (load) registered by a customer or a group of customers or a system in a stated time period. The value may be the maximum instantaneous load or, more usually, the average load over a designated interval of time such as one hour, and is normally stated in kilowatts or megawatts.

    • Peaking capacity

      The capacity of generating equipment normally reserved for operation during hours of the highest daily, weekly, or seasonal loads. Some generating equipment may be operated at certain times as peaking capacity, and at other times to serve loads on an around-the-clock basis.

    • Point-of-delivery (POD)

      Point(s) for interconnection on the transmission facility owner’s (TFO) system where capacity and/or energy is made available to the end-use customer.

    • Pool price

      The average of 60 one-minute system marginal prices accumulated over an hour.

    • Power Pool of Alberta

      The not-for-profit entity responsible for the operation of the wholesale electricity market from 1998 through 2002.

    • Premium price

      Price paid to an operating reserve provider that gives the AESO the option to call on the reserve if required.

    • Price fidelity

      A price that reflects economic fundamentals, free from distortion or intervention.

    • Price-quantity pairs

      Price-sensitive bid block information that consists of a quantity of megawatts and the available dollar price for the megawatts.

    • Reactive power

      The component of electric power that does not provide real work but is required to provide voltage.

    • Regulating reserve(s)

      A type of reserve responsive to automatic generation control that is sufficient to provide normal regulating margin.

    • Reliability

      The combined adequacy and security of an electric system. Adequacy is the ability of the electric system to supply the aggregate electrical demand and energy requirements of customers at all times, taking into account scheduled and unscheduled outages of system facilities. Security is the ability of the electric system to withstand sudden disturbances such as electric short circuits or unanticipated loss of system facilities. 

    • Reliability criteria

      A set of tests against which the operation of a power system is measured to ensure acceptable performance.

    • Reserve margin

      The percentage of installed capacity exceeding the expected peak demand during a specified period.

    • Service territories

      Geographic areas of the province served by a regulated electricity provider.

    • Short-term adequacy

      The ability of the electric system supply to meet expected electrical demand requirements in the immediate future (i.e., over the coming hours and days). 

    • Simple-cycle

      Where a gas turbine is the prime mover in a plant. A gas turbine consisting typically of one or more combustion chambers where liquid or gaseous fuel is burned. The hot gases are passed to the turbine where they expand, driving the turbine that in turn drives the generator.

    • Single circuit

      A transmission line where one circuit is carried on a set of structures (poles or lattice towers).

    • Solar (power)

      A process that produces electricity by converting solar radiation into electricity or to thermal energy to produce steam to drive a turbine.

    • Spinning reserve(s)

      The amount of unloaded generation that is synchronized to the Alberta Interconnected Electric System and ready to serve additional demand. 

    • Standby operating reserve(s)

      A type of reserve used when all active operating reserves have been dispatched.

    • Substation/switching station

      A facility where equipment is used to tie together two or more electric circuits through switches (circuit breakers). The switches are selectively arranged to permit a circuit to be disconnected or to change the electric connection between the circuits.

    • Supplemental reserve(s)

      Generation that is capable of being connected to the AIES and loaded within 10 minutes, or load that can be reduced in 10 minutes. 

    • Supply transmission service (STS)

      The service provided to generators for interconnection access to the Alberta transmission system.

    • System marginal price

      The price in dollars per megawatt hour determined for each minute of a specific settlement interval.

    • T-2

      Two hours prior to the delivery hour, when the ability to change offers in the market is closed.

    • Tap

      A point of connection along a transmission line between substations.

    • Tariff (Transmission)

      The terms and conditions under which transmission services are provided, including the rates or charges that users must pay.

    • Thermal overload

      A condition where the thermal limit of a piece of electrical equipment such as a conductor or transformer is exceeded.

    • 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 (or paths) between those areas under specified system conditions. The units of transfer capability can be expressed in megawatts. 

    • Transformer

      An electrical device for changing the voltage of alternating current.

    • Transmission

      The transfer of electricity over a group of interconnected lines and associated equipment, between points of supply and points at which it is transformed for delivery to consumers, or is delivered to other electric systems.

    • Transmission Administrator

      The for-profit entity responsible for the management of the transmission system from 1998 through 2002.

    • Transmission facility owner (TFO)

      The owner of the system of high-voltage power lines and equipment that links generating units to large customer loads and to distribution systems. 

    • Transmission losses

      Energy that is lost through the process of transmitting electricity. 

    • Transmission must-run (TMR)

      Where a generator is required to operate at a minimum specified output level to maintain system reliability in the event of an outage to certain transmission system elements.

    • Transmission path

      One or more transmission lines that form the transmission connection between two points on the system.

    • Transmission rights

      A tradable, contractual right to flow energy over a specific transmission path.

    • Transmission system (electric)

      An interconnected group of electric transmission lines and associated equipment for moving or transferring electricity in bulk between points of supply and points at which it is transformed for delivery over the distribution system lines to consumers, or is delivered to other electric systems.

    • Transmission tag(s)

      An electronic identifier which contains specific transactional information on transmission, distribution and retail operations.

    • Unit force majeure

      Any occurrence that is beyond the reasonable control of a market participant, which could not have been avoided by the exercise of reasonable diligence, and which prevents a market participant from performing its obligations under the ISO Rules.

    • Vertically integrated

      In the case of electricity, a vertically integrated company owns generation, transmission, distribution and retail operations.

    • Voltage

      The difference of electrical potential between two points of an electrical circuit expressed in volts. It is the measurement of the potential for an electric field to cause an electric current in an electrical conductor. Depending on the amount of difference of electrical potential, it is referred to as extra low voltage, low voltage, high voltage or extra high voltage.

    • Voltage stability

      Operation within acceptable voltage ranges. Normal voltage limits are defined as the operating voltage range on the interconnected system that is acceptable on a sustained basis. Emergency voltage limits are defined as the operating voltage range on the interconnected system that is acceptable for the time sufficient for system adjustments to be made following a facility outage or system disturbance.

    • Voltage violation

      A measured or calculated condition where the voltage at a point on the transmission system is outside the acceptable limits as described in the criteria.

    • Watt

      The unit of power equal to one joule of energy per second. It measures a rate of energy conversion. A typical household incandescent light bulb uses electrical energy at a rate of 25 to 100 watts.

    • Watt hour (Wh)

      An electrical energy unit of measure equal to one watt of power supplied to or taken from an electric circuit steadily for one hour.

    • Western Electricity Coordinating Council (WECC)

      An organization formed to coordinate and promote electric system reliability for the system that interconnects Alberta, B.C., 14 western U.S. states and part of one Mexican state.

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