EPRI Terms & Definitions Exam Questions & Answers 2023/2024
OFF-PEAK - ANSWER-Those hours or other PERIODs defined by NAESB business practices, contract, AGREEMENTs, or guides as PERIODs of lower electrical DEMAND.
OHM'S LAW - ANSWER-A basic electrical law that relates the VOLTAGE (V), CURR...
OFF-PEAK - ANSWER-Those hours or other PERIODs defined by NAESB business practices, contract,
AGREEMENTs, or guides as PERIODs of lower electrical DEMAND.
OHM'S LAW - ANSWER-A basic electrical law that relates the VOLTAGE (V), CURRENT (I), and IMPEDANCE
(Z). Ohm's law is commonly stated as:V = I × Z
ON-PEAK - ANSWER-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) - ANSWER-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) - ANSWER-Electronic TRANSMISSION tariff accepted by the
U.S. FEDERAL ENERGY REGULATORYCOMMISSION requiring the TRANSMISSION SERVICE PROVIDER to
furnish to all shippers with non-discriminating service comparable to that provided by TRANSMISSION
OWNERS to themselves.
OPERATING AUTHORITY - ANSWER-An entity that: 1. Has ultimate accountability for a defined portion of
the BULK ELECTRIC SYSTEM to meet one or more of three RELIABILITY objectives -
GENERATION/DEMAND balance, TRANSMISSION security, and/or EMERGENCY preparedness; and 2. Is
accountable to NERC and its REGIONAL RELIABILITY ORGANIZATIONS for complying with NERC and
Regional Policies; and 3. Has the authority to control or direct the operation of generating
RESOURCES,TRANSMISSION facilities, or LOADS, to meet these Policies.
OPERATING PROCEDURE - ANSWER-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 RESERVE - ANSWER-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 - ANSWER-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.
OPERATING RESERVE - SUPPLEMENTAL - ANSWER-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 SECURITY LIMIT - ANSWER-The value of a SYSTEM operating parameter (for example, total
POWER transfer across an INTERFACE) that satisfies the most limiting of prescribed pre- and post-
CONTINGENCY OPERATINGCRITERIA as determined by EQUIPMENT loading capability and acceptable
STABILITY and VOLTAGE conditions.
OSCILLATIONS - ANSWER-Cyclic variations in VOLTAGE, CURRENT, FREQUENCY, or POWER flows. The
VOLTAGE and CURRENT of the POWER SYSTEM naturally oscillates at 60 HZ. The term oscillations is
typically used to refer to low FREQUENCY (a few HZ) oscillations that may occur.
OSCILLATORY STABILITY / INSTABILITY - ANSWER-An ANGLE STABILITY classification. The oscillatory
STABILITY LIMIT of a POWER SYSTEM is the maximum amount of ACTIVE POWER that can be transmitted
across the SYSTEM without excessive POWER OSCILLATIONS leading to a LOSS OF SYNCHRONISM. No
large DISTURBANCE need occur. The response of GENERATOR CONTROL SYSTEMS (governor and
EXCITER) are very important to oscillatory stability / instability.
OUTAGE - ANSWER-The PERIOD during which a generating unit, TRANSMISSION line, or other FACILITY is
out of service.
OUTAGE DISTRIBUTION FACTOR (ODF) - ANSWER-Expressed in per unit as the change in POWER flow on
a given TRANSMISSION path (or FLOWGATE)due to the OUTAGE of a specified TRANSMISSION
, ELEMENT(s) or GENERATOR(s). The affect of LOSSES is generally ignored:ODF = (Change in path flow) /
(Pre-CONTINGENCY flow on OUTAGED ELEMENTS)
OUTAGE TRANSFER DISTRIBUTION FACTOR (OTDF) - ANSWER-Expressed in per unit as the portion of
POWER flow on a path (or FLOWGATE) in response to a transfer, taking into consideration the PTDF of
the flow gate and the effects of an OUTAGED TRANSMISSION ELEMENT(s). The affect of LOSSES is
generally ignored:OTDF (FLOWGATE) = PTDF (FLOWGATE) + (ODF) * PTDF (OUTAGED ELEMENT)
OUT-OF-STEP (OOS) - ANSWER-To lose synchronism. Out-of-step is best viewed in terms of rotating
MAGNETIC FIELDs. When a GENERATOR is out-of-step with the POWER SYSTEM it connects to, the
GENERATOR's rotating MAGNETIC FIELD is no longer in-step or in synchronism with the rotating
MAGNETIC FIELD of the SYSTEM.
OUT-OF-STEP PROTECTIVE RELAY (78) - ANSWER-A PROTECTIVE RELAY that is designed to detect OUT-OF-
STEP conditions and initiate a user determined response. Out-of-step protective relays are often based
on the same principles as IMPEDANCE RELAYs. An OUT-OF-STEP RELAY differentiates between an OUT-
OF-STEP condition and a FAULT condition by measuring the speed at which the measured IMPEDANCE
changes.
OVERCURRENT RELAY - ANSWER-A PROTECTIVE RELAY that activates in response to a high CURRENT
magnitude. Overcurrent relays can be either timed or instantaneous and directional or non-directional.
OVEREXCITE - ANSWER-A GENERATOR is overexcited when the applied excitation is greater than that
needed to support the Generator's MAGNETIC FIELD. The excess REACTIVE POWER produced by the over
excitation condition flows to the SYSTEM. An overexcited GENERATOR supplies REACTIVE POWER to the
SYSTEM. The terms "LAGGING" and "boosting" are also used to refer to an overexcited GENERATOR. A
TRANSFORMER may also overexcite. Over excitation of a TRANSFORMER results from either applying to
high a VOLTAGE magnitude or operating at to low of a FREQUENCY.
OVERLAP ANGLE (µ) - ANSWER-When commutating between two VALVEs in an HVDC CONVERTER, a
natural overlap PERIOD occurs in which both VALVES are simultaneously conducting. The length of the
overlap PERIOD is measured in degrees and is called the overlap angle. The symbol for the overlap angle
is the Greek lower case letter mu (µ).
OVERLAP REGULATION SERVICE - ANSWER-A method of providing REGULATION SERVICE in which the
BALANCING AUTHORITY providing the REGULATION SERVICE incorporates another BALANCING
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