Definitions are given here to aid the user in understanding the factors that affect index calculation. Many of these definitions were taken directly from IEEE Std 100-1996. If there is a conflict between the definitions in this guide and IEEE Std 100-1996, the definitions in this guide take precedence. Others are given because they have a new interpretation within this guide or have not been defined before.

connected load:
The connected transformer kVA, peak load, or metered demand (to be clearly specified when reporting) on the circuit or portion of circuit that is interrupted. When reporting, the report should state whether it is based on an annual peak or on a reporting period peak.

customer count:
The number of customers or number of meters. The number of customers is the preferred item to count if the counting system is not already in place.

distribution system:
That portion of an electric system that delivers electric energy from transformation points on the transmission system to the customer. Note: The distribution system is generally considered to be anything from the distribution substation fence to the customer meter. Often the initial overcurrent protection and voltage regulator are within the substation fence.

duration interruption:
The period (measured in seconds, or minutes, or hours, or days) from the initiation of an interruption to a customer or other facility until service has been restored to that customer or facility. An interruption may require step-restoration tracking to provide reliable index calculation. It may be desirable to record the duration of each interruption.

forced interruption:
An interruption caused by a forced outage.

interrupting device:
A device capable of being reclosed whose purpose is to interrupt faults and restore service or disconnect loads. These devices can be manual, automatic, or motor-operated. Examples may include transmission breakers, feeder breakers, line reclosers, and motor-operated switches.

interrupting device event:
The operation associated with the interrupting device for cases where a reclosing device operates but does not lockout and where a switch is opened only temporarily.

interrupting device operation:
The operation associated with a reclosing device for cases where the switch opens and closes once but does not lockout.

The loss of service to one or more customers. Note: It is the result of one or more component outages, depending on system configuration. See: outage.
interruptions caused by events outside of distribution: For most utilities, this type of interruption is a small percentage of the total interruptions. It will be defined here to account for the cases where outside influences are a major occurrence. Three categories that may be helpful to monitor are: transmission, generation, and substations.

The final operation of a recloser or circuit breaker in an attempt to clear a persistent fault. The overcurrent protective device locks open their contacts under these conditions.

loss of service:
The loss of electrical power, a complete loss of voltage, to one or more customers or meters. This does not include any of the power quality issues (sags, swells, impulses, or harmonics).

major event:
A catastrophic event that exceeds design limits of the electric power system and that is characterized by the following (as defined by the utility):
a) Extensive damage to the electric power system;
b) More than a specified percentage of customers simultaneously out of service;
c) Service restoration times longer than specified.
Some examples are extreme weather, such as a one in five year event, or earthquakes.

momentary event interruption:
An interruption of duration limited to the period required to restore service by an interrupting device. Note: Such switching operations must be completed in a specified time not to exceed 5 min. This definition includes all reclosing operations that occur within 5 min of the first interruption. For example, if a recloser or breaker operates two, three, or four times and then holds, the event shall be considered one momentary interruption event.

momentary interruption:
Single operation of an interrupting device that results in a voltage zero. For example, two breaker or recloser operations equals two momentary interruptions.

outage (electric power systems):
The state of a component when it is not available to perform its intended function due to some event directly associated with that component. Notes:
1. An outage may or may not cause an interruption of service to customers, depending on system configuration.
2. This definition derives from transmission and distribution applications and does not apply to generation outages.

reporting period:
A period assumed to be one year unless otherwise stated.

scheduled interruption (electric power systems):
A loss of electric power that results when a component is deliberately taken out of service at a selected time, usually for the purposes of construction, preventative maintenance, or repair.
1. This derives from transmission and distribution applications and does not apply to generation interruptions.
2. The key test to determine if an interruption should be classified as a forced or scheduled interruption is as follows. If it is possible to defer the interruption when such deferment is desirable, the interruption is a scheduled interruption; otherwise, the interruption is a forced interruption. Deferring an interruption may be desirable, for example, to prevent overload of facilities or interruption of service to customers.

step restoration:
The restoration of service to blocks of customers in an area until the entire area or feeder is restored.

sustained interruption:
Any interruption not classified as a momentary event. Any interruption longer
than 5 min.

total number of customers served:
The total number of customers served on the last day of the reporting period. If a different customer total is used, it must be clearly defined within the report.

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