POWER DISTRIBUTION--Distribution Reliability

Reliability in engineering applications, as defined in The Authoritative Dictionary of IEEE Standard Terms (IEEE 100), is the probability that a de vice will function without failure over a specified time period or amount of usage. In the case of electric power distribution, reliability concerns have come from customers who want uninterrupted continuous power supplied to their facilities at minimum cost.

A typical goal for an electric utility is to have an overall average of one interruption of no more than two hours' duration per customer year. Given 8760 hours in a non-leap year, this goal corresponds to an Average Service Availability Index (ASAI) greater than or equal to 8758 service hours/8760 hours = 0.999772 = 99.9772%.

IEEE Standard 1366-2003 entitled, IEEE Guide for Electric Power Distribution Reliability Indices, defines the following distribution reliability indices:

System Average Interruption Frequency Index (SAIFI):

SAIFI = _ Total Number of Customers Interrupted/Total Number of Customers Served

System Average Interruption Duration Index (SAIDI):

SAIDI =Customer Interruption Duration Total Number of Customers Served

Customer Average Interruption Duration Index (CAIDI):

CAIDI = Customer Interruption Duration/Total Number of Customers Interrupted = SAIDI/SAIFI

Average Service Availability Index (ASAI): ASAI = Customer Hours Service Availability/Customer Hours Service Demands

In accordance with IEEE Std. 1366-2003, when calculating the above reliability indices, momentary interruption events are not included. A momentary interruption event has an interruption duration that's limited to the time required to restore service by an interrupting device (including multiple re closures of reclosers or circuit breakers). Switching operations must be completed within 5 minutes for a momentary interruption event. As such, customer interruption durations less than 5 minutes are excluded when calculating the reliability indices. IEEE Std. 1366-2003 also includes a method, when calculating reliability indices, for excluding major events, such as severe storms, for which the daily SAIDI exceeds a specified threshold.

The above formulas for reliability indices use customers out-of-service and customer-minutes out-of-service data. Electric utilities with outage management systems including geographical information systems (GIS) and customer information systems (CIS) are able to very accurately keep track of this data. Some utilities in the United States are required to report distribution reliability indices to state public service commissions, while other utilities may voluntarily report these indices to regional power associations. Typical values for these indices are given in Tbl. 6

The following example uses outage data given in IEEE Std. 1366-2003.

EX. 4 Distribution Reliability Indices

Tbl. 7 gives 2010 annual outage data (sustained interruptions) from a utility's CIS database for one feeder. This feeder (denoted circuit 7075) serves 2,000 customers with a total load of 4 MW. Excluding momentary interruption events (less than 5-minutes duration) and major events, which are omitted from Tbl. 7, calculate the SAIFI, SAIDI, CAIDI, and ASAI for this feeder.

Tbl. 8 lists basic outage reporting information recommended by an IEEE committee. Tbl. 9 lists generic and specific causes of outages, based on a U.S. Department of Energy study [16]. Many electric utilities routinely prepare distribution outage reports monthly, quarterly, and annually by town (municipality) or by district. The purposes of the reports are to monitor and evaluate distribution reliability, uncover weaknesses and potential problems, and make recommendations for improving reliability. These reports may include:

1. Frequency and duration reports, which provide data for the number of interruptions on distribution circuits together with power interrupted, average interruption duration, and causes.

2. Annual reports that sort outages according to cause of failure and ac cording to circuit classification (for example, sort for each primary volt age; or sort for each conductor type including overhead open wire, overhead spacer cable, underground direct-burial, and cable in conduit).

3. Five- or ten-year trends for reliability indices, and outage trends for specific causes such as tree-contact outages for overhead distribution or dig-in outages for underground distribution.

4. Lists of problem circuits such as the 20 ''worst'' (lowest ASAI) circuits in a district, or all circuits with repeated outages during the reporting period.

== TBL. 8 Basic Outage Reporting Information ==

1. Type, design, manufacturer, and other descriptions for classification purposes

2. Date of installation, location on system, length in case of a line

3. Mode of failure (short circuit, false operation, etc.)

4. Cause of failure (lightning, tree, etc.)

5. Times (both out of service and back in service, rather than outage duration alone), date, meteorological conditions when the failure occurred

6. Type of outage, forced or scheduled, transient or permanent (momentary or sustained)

TBL. 9 Generic and Specific Causes of Outages

Weather

Blizzard/snow

Cold

Flood

Heat

Hurricane

Ice

Lightning

Rain

Tornado

Wind

Other

Miscellaneous

Airplane/helicopter

Animal/bird/snake

Vehicle:

Automobile/truck

Crane Dig-in

Fire/explosion

Sabotage/vandalism

Tree

Unknown

Other

System Components

Electrical and mechanical:

Fuel supply

Generating unit failure

Transformer failure

Switchgear failure

Conductor failure

Tower, pole attachment

Insulation failure:

Transmission line, Substation, Surge arrester, Cable failure, Voltage control equipment:: Voltage regulator Automatic tap changer Capacitor Reactor Protection and control: Relay failure Communication signal error

Supervisory control error

System Operation

System conditions:

Stability

High/low voltage

High/low frequency Line overload Transformer overload

Unbalanced load Neighboring power system Public appeal:

Commercial and industrial

All customers

Voltage Reduction: 0-2% voltage reduction

Greater than 2-8% voltage reduction

Rotating Blackout

Utility personnel:

System operator error

Power plant operator error

Field operator error

Maintenance error

Other

Methods for improving distribution reliability include replacement of older distribution equipment, upgrades of problem circuits, crew staffing and training for fast responses to outages and rapid restoration of service, formal maintenance programs, and public awareness programs to reduce hazards in the vicinity of distribution equipment such as contractor dig-ins. Reliability evaluation has also become an important component of bid selections to pro cure new distribution equipment. Also, great strides in distribution reliability have come through distribution automation.