NERC Glossary

Glossary of Terms Used in
Reliability Standards

The newly approved terms
are included in the shaded
table rows below.

May 2, 2007
Term

Acronym

Definition

Adequacy

The ability of the electric system to supply the
aggregate electrical demand and energy requirements
of the end-use customers at all times, taking into
account scheduled and reasonably expected
unscheduled outages of system elements.

Adjacent Balancing Authority

A Balancing Authority Area that is interconnected
another Balancing Authority Area either directly or via a
multi-party agreement or transmission tariff.

Adverse Reliability Impact

The impact of an event that results in frequency-related
instability; unplanned tripping of load or generation; or
uncontrolled separation or cascading outages that
affects a widespread area of the Interconnection.

Agreement

A contract or arrangement, either written or verbal and
sometimes enforceable by law.

Altitude Correction Factor

A multiplier applied to specify distances, which adjusts
the distances to account for the change in relative air
density (RAD) due to altitude from the RAD used to
determine the specified distance. Altitude correction
factors apply to both minimum worker approach
distances and to minimum vegetation clearance
distances.

Ancillary Service

Those services that are necessary to support the
transmission of capacity and energy from resources to
loads while maintaining reliable operation of the
Transmission Service Provider's transmission system in
accordance with good utility practice. (From FERC order
888-A.)

Anti-Aliasing Filter

An analog filter installed at a metering point to remove
the high frequency components of the signal over the
AGC sample period.

Area Control Error

ACE

Arranged Interchange
Automatic Generation Control

The instantaneous difference between a Balancing
Authority’s net actual and scheduled interchange,
taking into account the effects of Frequency Bias and
correction for meter error.
The state where the Interchange Authority has received
the Interchange information (initial or revised).

AGC

Equipment that automatically adjusts generation in a
Balancing Authority Area from a central location to
maintain the Balancing Authority’s interchange schedule
plus Frequency Bias. AGC may also accommodate
automatic inadvertent payback and time error
correction.

Adopted by NERC Board of Trustees: May 2, 2007

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Glossary of Terms Used in Reliability Standards
Term

Acronym

Definition

Available Transfer Capability

ATC

A measure of the transfer capability remaining in the
physical transmission network for further commercial
activity over and above already committed uses. It is
defined as Total Transfer Capability less existing
transmission commitments (including retail customer
service), less a Capacity Benefit Margin, less a
Transmission Reliability Margin.

Balancing Authority

BA

The responsible entity that integrates resource plans
ahead of time, maintains load-interchange-generation
balance within a Balancing Authority Area, and supports
Interconnection frequency in real time.

Balancing Authority Area

The collection of generation, transmission, and loads
within the metered boundaries of the Balancing
Authority. The Balancing Authority maintains loadresource balance within this area.

Base Load

The minimum amount of electric power delivered or
required over a given period at a constant rate.

Blackstart Capability Plan

A documented procedure for a generating unit or
station to go from a shutdown condition to an operating
condition delivering electric power without assistance
from the electric system. This procedure is only a
portion of an overall system restoration plan.

Bulk Electric System

As defined by the Regional Reliability Organization, the
electrical generation resources, transmission lines,
interconnections with neighboring systems, and
associated equipment, generally operated at voltages of
100 kV or higher. Radial transmission facilities serving
only load with one transmission source are generally
not included in this definition.

Burden

Operation of the Bulk Electric System that violates or is
expected to violate a System Operating Limit or
Interconnection Reliability Operating Limit in the
Interconnection, or that violates any other NERC,
Regional Reliability Organization, or local operating
reliability standards or criteria.

Capacity Benefit Margin

CBM

The amount of firm transmission transfer capability
preserved by the transmission provider for LoadServing Entities (LSEs), whose loads are located on that
Transmission Service Provider’s system, to enable
access by the LSEs to generation from interconnected
systems to meet generation reliability requirements.
Preservation of CBM for an LSE allows that entity to
reduce its installed generating capacity below that
which may otherwise have been necessary without
interconnections to meet its generation reliability
requirements. The transmission transfer capability
preserved as CBM is intended to be used by the LSE
only in times of emergency generation deficiencies.

Adopted by NERC Board of Trustees: May 2, 2007

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Glossary of Terms Used in Reliability Standards
Term

Acronym

Definition

Capacity Emergency

A capacity emergency exists when a Balancing
Authority Area’s operating capacity, plus firm purchases
from other systems, to the extent available or limited
by transfer capability, is inadequate to meet its demand
plus its regulating requirements.

Cascading

The uncontrolled successive loss of system elements
triggered by an incident at any location. Cascading
results in widespread electric service interruption that
cannot be restrained from sequentially spreading
beyond an area predetermined by studies.

Cascading Outages

The uncontrolled successive loss of Bulk Electric System
Facilities triggered by an incident (or condition) at any
location resulting in the interruption of electric service
that cannot be restrained from spreading beyond a predetermined area.

Clock Hour

The 60-minute period ending at :00. All surveys,
measurements, and reports are based on Clock Hour
periods unless specifically noted.

Cogeneration

Production of electricity from steam, heat, or other
forms of energy produced as a by-product of another
process.

Compliance Monitor

The entity that monitors, reviews, and ensures
compliance of responsible entities with reliability
standards.

Confirmed Interchange

The state where the Interchange Authority has verified
the Arranged Interchange.

Congestion Management
Report

A report that the Interchange Distribution Calculator
issues when a Reliability Coordinator initiates the
Transmission Loading Relief procedure. This report
identifies the transactions and native and network load
curtailments that must be initiated to achieve the
loading relief requested by the initiating Reliability
Coordinator.

Constrained Facility

A transmission facility (line, transformer, breaker, etc.)
that is approaching, is at, or is beyond its System
Operating Limit or Interconnection Reliability Operating
Limit.

Contingency

The unexpected failure or outage of a system
component, such as a generator, transmission line,
circuit breaker, switch or other electrical element.

Contingency Reserve

The provision of capacity deployed by the Balancing
Authority to meet the Disturbance Control Standard
(DCS) and other NERC and Regional Reliability
Organization contingency requirements.

Contract Path

An agreed upon electrical path for the continuous flow
of electrical power between the parties of an
Interchange Transaction.

Adopted by NERC Board of Trustees: May 2, 2007

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Glossary of Terms Used in Reliability Standards
Term

Acronym

Definition

Control Performance
Standard

CPS

The reliability standard that sets the limits of a
Balancing Authority’s Area Control Error over a specified
time period.

Corrective Action Plan

A list of actions and an associated timetable for
implementation to remedy a specific problem.

Cranking Path

A portion of the electric system that can be isolated and
then energized to deliver electric power from a
generation source to enable the startup of one or more
other generating units.

Critical Assets

Facilities, systems, and equipment which, if destroyed,
degraded, or otherwise rendered unavailable, would
affect the reliability or operability of the Bulk Electric
System.

Critical Cyber Assets

Cyber Assets essential to the reliable operation of
Critical Assets.

Curtailment

A reduction in the scheduled capacity or energy delivery
of an Interchange Transaction.

Curtailment Threshold

The minimum Transfer Distribution Factor which, if
exceeded, will subject an Interchange Transaction to
curtailment to relieve a transmission facility constraint.

Cyber Assets

Programmable electronic devices and communication
networks including hardware, software, and data.

Cyber Security Incident

Any malicious act or suspicious event that:
•

Compromises, or was an attempt to
compromise, the Electronic Security Perimeter
or Physical Security Perimeter of a Critical
Cyber Asset, or,

•

Disrupts, or was an attempt to disrupt, the
operation of a Critical Cyber Asset.

Delayed Fault Clearing

Fault clearing consistent with correct operation of a
breaker failure protection system and its associated
breakers, or of a backup protection system with an
intentional time delay.

Demand

1. The rate at which electric energy is delivered to or
by a system or part of a system, generally
expressed in kilowatts or megawatts, at a given
instant or averaged over any designated interval of
time.
2. The rate at which energy is being used by the
customer.

Demand-Side Management

DSM

The term for all activities or programs undertaken by
Load-Serving Entity or its customers to influence the
amount or timing of electricity they use.

Adopted by NERC Board of Trustees: May 2, 2007

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Glossary of Terms Used in Reliability Standards
Term

Acronym

Definition

Direct Control Load
Management

DCLM

Demand-Side Management that is under the direct
control of the system operator. DCLM may control the
electric supply to individual appliances or equipment on
customer premises. DCLM as defined here does not
include Interruptible Demand.

Dispersed Load by
Substations
Distribution Factor

Substation load information configured to represent a
system for power flow or system dynamics modeling
purposes, or both.
DF

The portion of an Interchange Transaction, typically
expressed in per unit that flows across a transmission
facility (Flowgate).

Distribution Provider

Provides and operates the “wires” between the
transmission system and the end-use customer. For
those end-use customers who are served at
transmission voltages, the Transmission Owner also
serves as the Distribution Provider. Thus, the
Distribution Provider is not defined by a specific voltage,
but rather as performing the Distribution function at
any voltage.

Disturbance

1. An unplanned event that produces an abnormal
system condition.
2. Any perturbation to the electric system.
3. The unexpected change in ACE that is caused by the
sudden failure of generation or interruption of load.

Disturbance Control Standard

DCS

The reliability standard that sets the time limit following
a Disturbance within which a Balancing Authority must
return its Area Control Error to within a specified range.

Disturbance Monitoring
Equipment

DME

Devices capable of monitoring and recording system
data pertaining to a Disturbance. Such devices include
the following categories of recorders 1 :
• Sequence of event recorders which record
equipment response to the event

•

Fault recorders, which record actual waveform
data replicating the system primary voltages
and currents. This may include protective
relays.

•

Dynamic Disturbance Recorders (DDRs), which
record incidents that portray power system
behavior during dynamic events such as lowfrequency (0.1 Hz – 3 Hz) oscillations and
abnormal frequency or voltage excursions

1

Phasor Measurement Units and any other equipment that meets the functional requirements of DMEs may qualify
as DMEs.

Adopted by NERC Board of Trustees: May 2, 2007

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Glossary of Terms Used in Reliability Standards
Term
Dynamic Interchange
Schedule or
Dynamic Schedule

Acronym

Definition
A telemetered reading or value that is updated in real
time and used as a schedule in the AGC/ACE equation
and the integrated value of which is treated as a
schedule for interchange accounting purposes.
Commonly used for scheduling jointly owned generation
to or from another Balancing Authority Area.

Dynamic Transfer

The provision of the real-time monitoring, telemetering,
computer software, hardware, communications,
engineering, energy accounting (including inadvertent
interchange), and administration required to
electronically move all or a portion of the real energy
services associated with a generator or load out of one
Balancing Authority Area into another.

Economic Dispatch

The allocation of demand to individual generating units
on line to effect the most economical production of
electricity.

Electrical Energy

The generation or use of electric power by a device over
a period of time, expressed in kilowatthours (kWh),
megawatthours (MWh), or gigawatthours (GWh).

Electronic Security Perimeter

The logical border surrounding a network to which
Critical Cyber Assets are connected and for which
access is controlled.

Element

Any electrical device with terminals that may be
connected to other electrical devices such as a
generator, transformer, circuit breaker, bus section, or
transmission line. An element may be comprised of one
or more components.

Emergency or

Any abnormal system condition that requires automatic
or immediate manual action to prevent or limit the
failure of transmission facilities or generation supply
that could adversely affect the reliability of the Bulk
Electric System.

BES Emergency

Emergency Rating

The rating as defined by the equipment owner that
specifies the level of electrical loading or output, usually
expressed in megawatts (MW) or Mvar or other
appropriate units, that a system, facility, or element
can support, produce, or withstand for a finite period.
The rating assumes acceptable loss of equipment life or
other physical or safety limitations for the equipment
involved.

Energy Emergency

A condition when a Load-Serving Entity has exhausted
all other options and can no longer provide its
customers’ expected energy requirements.

Equipment Rating

The maximum and minimum voltage, current,
frequency, real and reactive power flows on individual
equipment under steady state, short-circuit and
transient conditions, as permitted or assigned by the
equipment owner.

Adopted by NERC Board of Trustees: May 2, 2007

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Glossary of Terms Used in Reliability Standards
Term

Acronym

Definition

Facility

A set of electrical equipment that operates as a single
Bulk Electric System Element (e.g., a line, a generator,
a shunt compensator, transformer, etc.)

Facility Rating

The maximum or minimum voltage, current, frequency,
or real or reactive power flow through a facility that
does not violate the applicable equipment rating of any
equipment comprising the facility.

Fault

An event occurring on an electric system such as a
short circuit, a broken wire, or an intermittent
connection.

Fire Risk

The likelihood that a fire will ignite or spread in a
particular geographic area.

Firm Demand

That portion of the Demand that a power supplier is
obligated to provide except when system reliability is
threatened or during emergency conditions.

Firm Transmission Service

The highest quality (priority) service offered to
customers under a filed rate schedule that anticipates
no planned interruption.

Flashover

An electrical discharge through air around or over the
surface of insulation, between objects of different
potential, caused by placing a voltage across the air
space that results in the ionization of the air space.

Flowgate

A designated point on the transmission system through
which the Interchange Distribution Calculator calculates
the power flow from Interchange Transactions.

Forced Outage

1. The removal from service availability of a generating
unit, transmission line, or other facility for
emergency reasons.
2. The condition in which the equipment is unavailable
due to unanticipated failure.

Frequency Bias

A value, usually expressed in megawatts per 0.1 Hertz
(MW/0.1 Hz), associated with a Balancing Authority
Area that approximates the Balancing Authority Area’s
response to Interconnection frequency error.

Frequency Bias Setting

A value, usually expressed in MW/0.1 Hz, set into a
Balancing Authority ACE algorithm that allows the
Balancing Authority to contribute its frequency response
to the Interconnection.

Frequency Deviation

A change in Interconnection frequency.

Frequency Error

The difference between the actual and scheduled
frequency. (FA – FS)

Adopted by NERC Board of Trustees: May 2, 2007

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Glossary of Terms Used in Reliability Standards
Term

Acronym

Definition

Frequency Regulation

The ability of a Balancing Authority to help the
Interconnection maintain Scheduled Frequency. This
assistance can include both turbine governor response
and Automatic Generation Control.

Frequency Response

(Equipment) The ability of a system or elements of the
system to react or respond to a change in system
frequency.
(System) The sum of the change in demand, plus the
change in generation, divided by the change in
frequency, expressed in megawatts per 0.1 Hertz
(MW/0.1 Hz).

Generator Operator

The entity that operates generating unit(s) and
performs the functions of supplying energy and
Interconnected Operations Services.

Generator Owner

Entity that owns and maintains generating units.

Generator Shift Factor

GSF

A factor to be applied to a generator’s expected change
in output to determine the amount of flow contribution
that change in output will impose on an identified
transmission facility or Flowgate.

Generator-to-Load
Distribution Factor

GLDF

The algebraic sum of a Generator Shift Factor and a
Load Shift Factor to determine the total impact of an
Interchange Transaction on an identified transmission
facility or Flowgate.

Host Balancing Authority

1. A Balancing Authority that confirms and implements
Interchange Transactions for a Purchasing Selling
Entity that operates generation or serves customers
directly within the Balancing Authority’s metered
boundaries.
2. The Balancing Authority within whose metered
boundaries a jointly owned unit is physically located.

Hourly Value

Data measured on a Clock Hour basis.

Implemented Interchange

The state where the Balancing Authority enters the
Confirmed Interchange into its Area Control Error
equation.

Inadvertent Interchange

The difference between the Balancing Authority’s Net
Actual Interchange and Net Scheduled Interchange.
(IA – IS)

Independent Power Producer

IPP

Institute of Electrical and
Electronics Engineers, Inc.

IEEE

Any entity that owns or operates an electricity
generating facility that is not included in an electric
utility’s rate base. This term includes, but is not limited
to, cogenerators and small power producers and all
other nonutility electricity producers, such as exempt
wholesale generators, who sell electricity.

Adopted by NERC Board of Trustees: May 2, 2007

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Glossary of Terms Used in Reliability Standards
Term

Acronym

Definition

Interchange Distribution
Calculator

IDC

The mechanism used by Reliability Coordinators in the
Eastern Interconnection to calculate the distribution of
Interchange Transactions over specific Flowgates. It
includes a database of all Interchange Transactions and
a matrix of the Distribution Factors for the Eastern
Interconnection.

Interchange

Energy transfers that cross Balancing Authority
boundaries.

Interchange Authority

The responsible entity that authorizes implementation
of valid and balanced Interchange Schedules between
Balancing Authority Areas, and ensures communication
of Interchange information for reliability assessment
purposes.

Interchange Schedule

An agreed-upon Interchange Transaction size
(megawatts), start and end time, beginning and ending
ramp times and rate, and type required for delivery and
receipt of power and energy between the Source and
Sink Balancing Authorities involved in the transaction.

Interchange Transaction

An agreement to transfer energy from a seller to a
buyer that crosses one or more Balancing Authority
Area boundaries.

Interchange Transaction Tag

The details of an Interchange Transaction required for
its physical implementation.

or
Tag
Interconnected Operations
Service

A service (exclusive of basic energy and transmission
services) that is required to support the reliable
operation of interconnected Bulk Electric Systems.

Interconnection

When capitalized, any one of the three major electric
system networks in North America: Eastern, Western,
and ERCOT.

Interconnection Reliability
Operating Limit

IROL

A System Operating Limit that, if violated, could lead to
instability, uncontrolled separation, or Cascading
Outages that adversely impact the reliability of the Bulk
Electric System.

Interconnection Reliability
Operating Limit Tv

IROL Tv

The maximum time that an Interconnection Reliability
Operating Limit can be violated before the risk to the
interconnection or other Reliability Coordinator Area(s)
becomes greater than acceptable. Each Interconnection
Reliability Operating Limit’s Tv shall be less than or
equal to 30 minutes.

Intermediate Balancing
Authority

A Balancing Authority Area that has connecting facilities
in the Scheduling Path between the Sending Balancing
Authority Area and Receiving Balancing Authority Area
and operating agreements that establish the conditions
for the use of such facilities

Adopted by NERC Board of Trustees: May 2, 2007

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Glossary of Terms Used in Reliability Standards
Term

Acronym

Interruptible Load

Definition
Demand that the end-use customer makes available to
its Load-Serving Entity via contract or agreement for
curtailment.

or
Interruptible Demand
Joint Control

Automatic Generation Control of jointly owned units by
two or more Balancing Authorities.

Limiting Element

The element that is 1. )Either operating at its
appropriate rating, or 2,) Would be following the
limiting contingency. Thus, the Limiting Element
establishes a system limit.

Load

An end-use device or customer that receives power
from the electric system.

Load Shift Factor

LSF

A factor to be applied to a load’s expected change in
demand to determine the amount of flow contribution
that change in demand will impose on an identified
transmission facility or monitored Flowgate.

Load-Serving Entity

Secures energy and transmission service (and related
Interconnected Operations Services) to serve the
electrical demand and energy requirements of its enduse customers.

Misoperation

ƒ

Any failure of a Protection System element to
operate within the specified time when a fault or
abnormal condition occurs within a zone of
protection.

ƒ

Any operation for a fault not within a zone of
protection (other than operation as backup
protection for a fault in an adjacent zone that is
not cleared within a specified time for the
protection for that zone).

ƒ

Any unintentional Protection System operation
when no fault or other abnormal condition has
occurred unrelated to on-site maintenance and
testing activity.

Native Load

The end-use customers that the Load-Serving Entity is
obligated to serve.

Net Actual Interchange

The algebraic sum of all metered interchange over all
interconnections between two physically Adjacent
Balancing Authority Areas.

Net Energy for Load

Net Balancing Authority Area generation, plus energy
received from other Balancing Authority Areas, less
energy delivered to Balancing Authority Areas through
interchange. It includes Balancing Authority Area
losses but excludes energy required for storage at
energy storage facilities.

Net Interchange Schedule

The algebraic sum of all Interchange Schedules with
each Adjacent Balancing Authority.

Adopted by NERC Board of Trustees: May 2, 2007

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Glossary of Terms Used in Reliability Standards
Term

Acronym

Definition

Net Scheduled Interchange

The algebraic sum of all Interchange Schedules across a
given path or between Balancing Authorities for a given
period or instant in time.

Network Integration
Transmission Service

Service that allows an electric transmission customer to
integrate, plan, economically dispatch and regulate its
network reserves in a manner comparable to that in
which the Transmission Owner serves Native Load
customers.

Non-Firm Transmission
Service

Transmission service that is reserved on an as-available
basis and is subject to curtailment or interruption.

Non-Spinning Reserve

1. That generating reserve not connected to the
system but capable of serving demand within a
specified time.
2. Interruptible load that can be removed from the
system in a specified time.

Normal Clearing

A protection system operates as designed and the fault
is cleared in the time normally expected with proper
functioning of the installed protection systems.

Normal Rating

The rating as defined by the equipment owner that
specifies the level of electrical loading, usually
expressed in megawatts (MW) or other appropriate
units that a system, facility, or element can support or
withstand through the daily demand cycles without loss
of equipment life.

Nuclear Plant Generator
Operator

Any Generator Operator or Generator Owner that is a
Nuclear Plant Licensee responsible for operation of a
nuclear facility licensed to produce commercial power.

Nuclear Plant Off-site Power
Supply (Off-site Power)

The electric power supply provided from the electric
system to the nuclear power plant distribution system
as required per the nuclear power plant license.
Requirements included in the design basis of the
nuclear plant and statutorily mandated for the operation
of the plant, including nuclear power plant licensing
requirements for:
1) Off-site power supply to enable safe shutdown of
the plant during an electric system or plant
event; and

Nuclear Plant Licensing
Requirements (NPLRs)

2) Avoiding preventable challenges to nuclear safety
as a result of an electric system disturbance,
transient, or condition.
Nuclear Plant Interface
Requirements (NPIRs)

The requirements based on NPLRs and Bulk Electric
System requirements that have been mutually agreed
to by the Nuclear Plant Generator Operator and the
applicable Transmission Entities.

Off-Peak

Those hours or other periods defined by NAESB
business practices, contract, agreements, or guides as
periods of lower electrical demand.

Adopted by NERC Board of Trustees: May 2, 2007

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Glossary of Terms Used in Reliability Standards
Term

Acronym

On-Peak

Definition
Those hours or other periods defined by NAESB
business practices, contract, agreements, or guides as
periods of higher electrical demand.

Open Access Same Time
Information Service

OASIS

An electronic posting system that the Transmission
Service Provider maintains for transmission access data
and that allows all transmission customers to view the
data simultaneously.

Open Access Transmission
Tariff

OATT

Electronic transmission tariff accepted by the U.S.
Federal Energy Regulatory Commission requiring the
Transmission Service Provider to furnish to all shippers
with non-discriminating service comparable to that
provided by Transmission Owners to themselves.

Operating Plan

A document that identifies a group of activities that
may be used to achieve some goal. An Operating Plan
may contain Operating Procedures and Operating
Processes. A company-specific system restoration plan
that includes an Operating Procedure for black-starting
units, Operating Processes for communicating
restoration progress with other entities, etc., is an
example of an Operating Plan.

Operating Procedure

A document that identifies specific steps or tasks that
should be taken by one or more specific operating
positions to achieve specific operating goal(s). The
steps in an Operating Procedure should be followed in
the order in which they are presented, and should be
performed by the position(s) identified. A document
that lists the specific steps for a system operator to
take in removing a specific transmission line from
service is an example of an Operating Procedure.

Operating Process

A document that identifies general steps for achieving a
generic operating goal. An Operating Process includes
steps with options that may be selected depending
upon Real-time conditions. A guideline for controlling
high voltage is an example of an Operating Process.

Operating Reserve

That capability above firm system demand required to
provide for regulation, load forecasting error,
equipment forced and scheduled outages and local area
protection. It consists of spinning and non-spinning
reserve.

Operating Reserve – Spinning

The portion of Operating Reserve consisting of:
• Generation synchronized to the system and fully
available to serve load within the Disturbance
Recovery Period following the contingency event;
or
• Load fully removable from the system within the
Disturbance Recovery Period following the
contingency event.

Adopted by NERC Board of Trustees: May 2, 2007

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Glossary of Terms Used in Reliability Standards
Term

Acronym

Operating Reserve –
Supplemental

Definition
The portion of Operating Reserve consisting of:
• Generation (synchronized or capable of being
synchronized to the system) that is fully available
to serve load within the Disturbance Recovery
Period following the contingency event; or
• Load fully removable from the system within the
Disturbance Recovery Period following the
contingency event.

Operating Voltage

The voltage level by which an electrical system is
designated and to which certain operating
characteristics of the system are related; also, the
effective (root-mean-square) potential difference
between any two conductors or between a conductor
and the ground. The actual voltage of the circuit may
vary somewhat above or below this value.

Overlap Regulation Service

A method of providing regulation service in which the
Balancing Authority providing the regulation service
incorporates another Balancing Authority’s actual
interchange, frequency response, and schedules into
providing Balancing Authority’s AGC/ACE equation.

Peak Demand

1. The highest hourly integrated Net Energy For Load
within a Balancing Authority Area occurring within a
given period (e.g., day, month, season, or year).
2. The highest instantaneous demand within the
Balancing Authority Area.

Performance-Reset Period

The time period that the entity being assessed must
operate without any violations to reset the level of non
compliance to zero.

Physical Security Perimeter

The physical, completely enclosed (“six-wall”) border
surrounding computer rooms, telecommunications
rooms, operations centers, and other locations in which
Critical Cyber Assets are housed and for which access is
controlled.

Planning Authority

The responsible entity that coordinates and integrates
transmission facility and service plans, resource plans,
and protection systems.

Point of Delivery

POD

A location that the Transmission Service Provider
specifies on its transmission system where an
Interchange Transaction leaves or a Load-Serving Entity
receives its energy.

Point of Receipt

POR

A location that the Transmission Service Provider
specifies on its transmission system where an
Interchange Transaction enters or a Generator delivers
its output.

Point to Point Transmission
Service

PTP

The reservation and transmission of capacity and
energy on either a firm or non-firm basis from the
Point(s) of Receipt to the Point(s) of Delivery.

Adopted by NERC Board of Trustees: May 2, 2007

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Glossary of Terms Used in Reliability Standards
Term

Acronym

Definition

Pro Forma Tariff

Usually refers to the standard OATT and/or associated
transmission rights mandated by the U.S. Federal
Energy Regulatory Commission Order No. 888.

Protection System

Protective relays, associated communication systems,
voltage and current sensing devices, station batteries
and DC control circuitry.

Pseudo-Tie

A telemetered reading or value that is updated in real
time and used as a “virtual” tie line flow in the AGC/ACE
equation but for which no physical tie or energy
metering actually exists. The integrated value is used
as a metered MWh value for interchange accounting
purposes.

Purchasing-Selling Entity

The entity that purchases or sells, and takes title to,
energy, capacity, and Interconnected Operations
Services. Purchasing-Selling Entities may be affiliated or
unaffiliated merchants and may or may not own
generating facilities.

Ramp Rate

(Schedule) The rate, expressed in megawatts per
minute, at which the interchange schedule is attained
during the ramp period.

or
Ramp

(Generator) The rate, expressed in megawatts per
minute, that a generator changes its output.

Rated Electrical Operating
Conditions

The specified or reasonably anticipated conditions under
which the electrical system or an individual electrical
circuit is intend/designed to operate

Rating

The operational limits of a transmission system element
under a set of specified conditions.

Reactive Power

The portion of electricity that establishes and sustains
the electric and magnetic fields of alternating-current
equipment. Reactive power must be supplied to most
types of magnetic equipment, such as motors and
transformers. It also must supply the reactive losses
on transmission facilities. Reactive power is provided
by generators, synchronous condensers, or electrostatic
equipment such as capacitors and directly influences
electric system voltage. It is usually expressed in
kilovars (kvar) or megavars (Mvar).

Real Power

The portion of electricity that supplies energy to the
load.

Reallocation

The total or partial curtailment of Transactions during
TLR Level 3a or 5a to allow Transactions using higher
priority to be implemented.

Real-time

Present time as opposed to future time. (From
Interconnection Reliability Operating Limits standard.)

Receiving Balancing Authority

The Balancing Authority importing the Interchange.

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Glossary of Terms Used in Reliability Standards
Term

Acronym

Regional Reliability
Organization

Definition
1. An entity that ensures that a defined area of the
Bulk Electric System is reliable, adequate and
secure.
2. A member of the North American Electric Reliability
Council. The Regional Reliability Organization can
serve as the Compliance Monitor.

Regional Reliability Plan

The plan that specifies the Reliability Coordinators and
Balancing Authorities within the Regional Reliability
Organization, and explains how reliability coordination
will be accomplished.

Regulating Reserve

An amount of reserve responsive to Automatic
Generation Control, which is sufficient to provide
normal regulating margin.

Regulation Service

The process whereby one Balancing Authority contracts
to provide corrective response to all or a portion of the
ACE of another Balancing Authority. The Balancing
Authority providing the response assumes the obligation
of meeting all applicable control criteria as specified by
NERC for itself and the Balancing Authority for which it
is providing the Regulation Service.

Reliability Coordinator

The entity that is the highest level of authority who is
responsible for the reliable operation of the Bulk Electric
System, has the Wide Area view of the Bulk Electric
System, and has the operating tools, processes and
procedures, including the authority to prevent or
mitigate emergency operating situations in both nextday analysis and real-time operations. The Reliability
Coordinator has the purview that is broad enough to
enable the calculation of Interconnection Reliability
Operating Limits, which may be based on the operating
parameters of transmission systems beyond any
Transmission Operator’s vision.

Reliability Coordinator Area

The collection of generation, transmission, and loads
within the boundaries of the Reliability Coordinator. Its
boundary coincides with one or more Balancing
Authority Areas.

Reliability Coordinator
Information System

RCIS

The system that Reliability Coordinators use to post
messages and share operating information in real time.

Remedial Action Scheme

RAS

See “Special Protection System”

Reportable Disturbance

Any event that causes an ACE change greater than or
equal to 80% of a Balancing Authority’s or reserve
sharing group’s most severe contingency. The
definition of a reportable disturbance is specified by
each Regional Reliability Organization. This definition
may not be retroactively adjusted in response to
observed performance.

Adopted by NERC Board of Trustees: May 2, 2007

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Glossary of Terms Used in Reliability Standards
Term

Acronym

Definition

Reserve Sharing Group

A group whose members consist of two or more
Balancing Authorities that collectively maintain,
allocate, and supply operating reserves required for
each Balancing Authority’s use in recovering from
contingencies within the group. Scheduling energy
from an Adjacent Balancing Authority to aid recovery
need not constitute reserve sharing provided the
transaction is ramped in over a period the supplying
party could reasonably be expected to load generation
in (e.g., ten minutes). If the transaction is ramped in
quicker (e.g., between zero and ten minutes) then, for
the purposes of Disturbance Control Performance, the
Areas become a Reserve Sharing Group.

Resource Planner

The entity that develops a long-term (generally one
year and beyond) plan for the resource adequacy of
specific loads (customer demand and energy
requirements) within a Planning Authority Area.

Response Rate

The Ramp Rate that a generating unit can achieve
under normal operating conditions expressed in
megawatts per minute (MW/Min).

Request for Interchange

RFI

A collection of data as defined in the NAESB RFI
Datasheet, to be submitted to the Interchange
Authority for the purpose of implementing bilateral
Interchange between a Source and Sink Balancing
Authority.

Right-of-Way (ROW)

A corridor of land on which electric lines may be
located. The Transmission Owner may own the land in
fee, own an easement, or have certain franchise,
prescription, or license rights to construct and maintain
lines.

Scenario

Possible event.

Schedule

(Verb) To set up a plan or arrangement for an
Interchange Transaction.
(Noun) An Interchange Schedule.

Scheduled Frequency

60.0 Hertz, except during a time correction.

Scheduling Entity

An entity responsible for approving and implementing
Interchange Schedules.

Scheduling Path

The Transmission Service arrangements reserved by
the Purchasing-Selling Entity for a Transaction.

Sending Balancing Authority

The Balancing Authority exporting the Interchange.

Sink Balancing Authority

The Balancing Authority in which the load (sink) is
located for an Interchange Transaction. (This will also
be a Receiving Balancing Authority for the resulting
Interchange Schedule.)

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Glossary of Terms Used in Reliability Standards
Term

Acronym

Definition

Source Balancing Authority

The Balancing Authority in which the generation
(source) is located for an Interchange Transaction.
(This will also be a Sending Balancing Authority for the
resulting Interchange Schedule.)

Special Protection System

An automatic protection system designed to detect
abnormal or predetermined system conditions, and take
corrective actions other than and/or in addition to the
isolation of faulted components to maintain system
reliability. Such action may include changes in demand,
generation (MW and Mvar), or system configuration to
maintain system stability, acceptable voltage, or power
flows. An SPS does not include (a) underfrequency or
undervoltage load shedding or (b) fault conditions that
must be isolated or (c) out-of-step relaying (not
designed as an integral part of an SPS). Also called
Remedial Action Scheme.

(Remedial Action Scheme)

Spinning Reserve

Unloaded generation that is synchronized and ready to
serve additional demand.

Stability

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

Stability Limit

The maximum power flow possible through some
particular point in the system while maintaining stability
in the entire system or the part of the system to which
the stability limit refers.

Supervisory Control and Data
Acquisition

SCADA

A system of remote control and telemetry used to
monitor and control the transmission system.

Supplemental Regulation
Service

A method of providing regulation service in which the
Balancing Authority providing the regulation service
receives a signal representing all or a portion of the
other Balancing Authority’s ACE.

Surge

A transient variation of current, voltage, or power flow
in an electric circuit or across an electric system.

Sustained Outage

The deenergized condition of a transmission line
resulting from a fault or disturbance following an
unsuccessful automatic reclosing sequence and/or
unsuccessful manual reclosing procedure.

System

A combination of generation, transmission, and
distribution components.

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Glossary of Terms Used in Reliability Standards
Term
System Operating Limit

Acronym

Definition
The value (such as MW, MVar, Amperes, Frequency or
Volts) that satisfies the most limiting of the prescribed
operating criteria for a specified system configuration to
ensure operation within acceptable reliability criteria.
System Operating Limits are based upon certain
operating criteria. These include, but are not limited
to:
• Facility Ratings (Applicable pre- and postContingency equipment or facility ratings)
• Transient Stability Ratings (Applicable pre- and
post-Contingency Stability Limits)
• Voltage Stability Ratings (Applicable pre- and postContingency Voltage Stability)
• System Voltage Limits (Applicable pre- and postContingency Voltage Limits)

System Operator

An individual at a control center (Balancing Authority,
Transmission Operator, Generator Operator, Reliability
Coordinator) whose responsibility it is to monitor and
control that electric system in real time.

Telemetering

The process by which measurable electrical quantities
from substations and generating stations are
instantaneously transmitted to the control center, and
by which operating commands from the control center
are transmitted to the substations and generating
stations.

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 sags to the point
that it violates public safety requirements.

Tie Line

A circuit connecting two Balancing Authority Areas.

Tie Line Bias

A mode of Automatic Generation Control that allows the
Balancing Authority to 1.) maintain its Interchange
Schedule and 2.) respond to Interconnection frequency
error.

Time Error

The difference between the Interconnection time
measured at the Balancing Authority(ies) and the time
specified by the National Institute of Standards and
Technology. Time error is caused by the accumulation
of Frequency Error over a given period.

Time Error Correction

An offset to the Interconnection’s scheduled frequency
to return the Interconnection’s Time Error to a
predetermined value.

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Glossary of Terms Used in Reliability Standards
Term

Acronym

TLR Log

Total Transfer Capability

Definition
Report required to be filed after every TLR Level 2 or
higher in a specified format. The NERC IDC prepares
the report for review by the issuing Reliability
Coordinator. After approval by the issuing Reliability
Coordinator, the report is electronically filed in a public
area of the NERC Web site.

TTC

The amount of electric power that can be moved or
transferred reliably from one area to another area of
the interconnected transmission systems by way of all
transmission lines (or paths) between those areas
under specified system conditions.

Transaction

See Interchange Transaction.

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 are in terms
of electric power, generally expressed in megawatts
(MW). The transfer capability from “Area A” to “Area B”
is not generally equal to the transfer capability from
“Area B” to “Area A.”

Transfer Distribution Factor

See Distribution Factor.

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 Constraint

A limitation on one or more transmission elements that
may be reached during normal or contingency system
operations.

Transmission Customer

1. Any eligible customer (or its designated agent) that
can or does execute a transmission service
agreement or can or does receive transmission
service.
2. Any of the following responsible entities: Generator
Owner, Load-Serving Entity, or Purchasing-Selling
Entity.

Transmission Line

A system of structures, wires, insulators and associated
hardware that carry electric energy from one point to
another in an electric power system. Lines are
operated at relatively high voltages varying from 69 kV
up to 765 kV, and are capable of transmitting large
quantities of electricity over long distances.

Transmission Operator

The entity responsible for the reliability of its “local”
transmission system, and that operates or directs the
operations of the transmission facilities.

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Glossary of Terms Used in Reliability Standards
Term

Acronym

Definition

Transmission Owner

The entity that owns and maintains transmission
facilities.

Transmission Planner

The entity that develops a long-term (generally one
year and beyond) plan for the reliability (adequacy) of
the interconnected bulk electric transmission systems
within its portion of the Planning Authority Area.

Transmission Reliability
Margin

TRM

The amount of transmission transfer capability
necessary to provide reasonable assurance that the
interconnected transmission network will be secure.
TRM accounts for the inherent uncertainty in system
conditions and the need for operating flexibility to
ensure reliable system operation as system conditions
change.

Transmission Service

Services provided to the Transmission Customer by the
Transmission Service Provider to move energy from a
Point of Receipt to a Point of Delivery.

Transmission Service Provider

The entity that administers the transmission tariff and
provides Transmission Service to Transmission
Customers under applicable transmission service
agreements.

Vegetation

All plant material, growing or not, living or dead.

Vegetation Inspection

The systematic examination of a transmission corridor
to document vegetation conditions.

Wide Area

The entire Reliability Coordinator Area as well as the
critical flow and status information from adjacent
Reliability Coordinator Areas as determined by detailed
system studies to allow the calculation of
Interconnected Reliability Operating Limits.

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