RELIABILITY OF DISTRIBUTION SYSTEM
#1

Submitted By
Yogesh Ranjan Mishra

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ABSTRACT
A distribution system is the part of an overall power system which links the bulk system to the individual customers. The primary goal of any distribution system is to supply reliable and quality power to the customers economically. The reliability indices like SAIFI, CAIDI, and ASAI and so on are very important performance indices which need to be satisfied from customer point of view. Calculation of these indices is essential to assess the reliability of a distribution system.
1. INTRODUCTION
Electric system reliability is a measure of the system’s ability to meet the electricity needs of customers [1, 2]. It is a term used by electric system planners and operators to measure aggregate system conditions, and as an aggregate measure, it generally applies to entire service territories or control regions. As such, the reliability of the electric system depends on the reliability of that system’s component parts, including, for example, power plants, transmission lines, substations, and distribution feeder lines. To help ensure a reliable system, planners and operators prefer having as much redundancy in these components as can be justified economically.
System reliability is also dependent on events that affect daily operations, including the decisions made by grid operators in real-time in response to changing system conditions. Operators like to have as much real time, and location-specific information as they can get about system conditions, as well as the ability to control power flows and dispatch power plants to enable effective response when problems occur. Weather is the primary reason for reliability problems, and includes problems caused by lightning strikes, high winds, snowfall, ice, and unexpectedly hot weather. The goal of both planners and operators is to have as resilient a system as possible that can adjust to problems without causing major consequences, and that when outages do occur, they are short-lived and affect the fewest number of customers as possible.
2. DISTRIBUTION SYSTEM
That part of power system which distributes electric power for local use is known as “distribution system”.
It mainly consists of three parts. Those are,
I. feeders
II. distributor
III. service mains
Feeders: - it is a conductor which connects the substation to the area where power is distributed.
Distributor: - it is a conductor from which tapings are taken for supply to the consumers.
Service mains: - it is generally a small cable which connects the distributor to consumer’s terminal.
Requirements of a good distribution system: -
I. Proper voltage – it ensures less variable voltage at the consumer’s terminal.
II. Availability of power on demand – it must be capable of supplying load demands of the consumers.
III. Reliability – as modern industries, office buildings and homes constantly require electric power the distribution system has to be reliable.
3. RELAIBILITY
In simple words, “Reliability is the ability of the power delivery system to provide uninterrupted service to the consumers”.
Reliability may be defined in many ways for any system. This is true for electric power systems, generally, and electric power distribution systems, in particular. The definitions of reliability in the literature address some common aspects of electric power systems. These include continuity of service, meeting customer demands, and the vulnerability of the power system.
Reliability concerns are often split into three categories: adequacy, or the capacity and energy to meet demand; security, or the ability to withstand disturbances; and quality, or acceptable frequency, voltage, and harmonic characteristics. Reliability cannot be discussed apart from the objectives of the system. The goals of the distribution system may be identified as (1) covering the territory (an aspect of adequacy), (2) having sufficient capacity for peak demand (another aspect of adequacy), (3) being able to operate under adverse conditions (security), and (4) providing a stable voltage (quality). Thus, the goals of the distribution system are congruent with reliability concerns [3].
The North American Reliability Council provides a Glossary of Terms. Reliability and the concepts of adequacy and security are defined there as follows (NERC Glossary of Terms):
Reliability: The degree of performance of the elements of the bulk electric system that results in electricity being delivered to customers within accepted standards and in the amount desired. Reliability may be measured by the frequency, duration, and magnitude of adverse effects on the electric supply. Electric system reliability can be addressed by considering two basic and functional aspects of the electric system: adequacy and security.
Adequacy: the ability of the electric system to supply the aggregate electrical demand and energy requirements of the customers at all times, taking into account scheduled and reasonably expected unscheduled outages of system elements.
Security: the ability of the electric system to withstand sudden disturbances such as electric short circuits or unanticipated loss of system elements. A more analytic definition, one possibly more often applied to generation, sets reliability to one minus the probability of system failure, or the inability to meet load. This probabilistic view uses the availability as an indicator of reliability, where availability is the steady-state probability that a component is in service. Another approach taken is to define reliability through indices
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#2
presented by:
Yogesh Ranjan Mishra

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TECHNICAL SEMINAR PRESENTATION-2011
INTRODUCTION

Electric system reliability is a measure of the system’s ability to meet the electricity needs of customers.
The reliability of the electric system depends on the reliability of that system’s component parts, including, for example, power plants, transmission lines, substations, and distribution feeder lines.
My objective is to present what is reliability in distribution systems.
DISTRBUTION SYSTEMS
• Definition: -
“Reliability is the ability of the power delivery system to provide uninterrupted service to the customers”.
• This definition is true for electric power systems, generally, and electric power distribution systems, in particular.
• Reliability concerns are often split into three categories:
- Adequacy
- Security
- Quality
HOW TO MEASURE RELIABILITY ?
• Reliability measures dealing with interruptions, address three factors: frequency, duration, and extent or severity.”
• The extent is the number of customers or load affected, which is determined by the layout of the distribution system.
• When assessing reliability, all three factors should be considered.
• In order to measure the reliability, there are many reliability indices.
• Each reliability index may be important for a different purpose.
RELIANILITY INDICES
• There are over forty reliability indices.
• The most commonly used ones are described below.
1. SAIFI
2. SAIDI
3. CAIFI
4. CAIDI
5. ASAI
• SAIFI and SAIDI are called system-oriented indices.
• CAIFI, CAIDI, ASAI are called customer-oriented indices.
SAIFI
• It is called “System average interruption frequency index”.
• Definition: - it is defined as total number of customer interruptions divided by total number of customer served.
‘λι’ is the failure rate, ‘Ni’ is the number of customers at load point ‘i’. R is the set of load points in the system.
SAIDI
• It is called “System average interruption duration index”.
• Definition: - it is defined as sum of customer interruption duration divided by total number of customer.
‘Ui’ is the annual outage time.
CAIFI
• It is called “Customer average interruption frequency index”.
• Definition: - it is defined as total number of customer interruptions divided by total number of customers interrupted.
‘CN’ is the number of customers interrupted.
CAIDI
• It is called “Customer average interruption duration index”.
• Definition: - it is defined as sum of customer interruption duration divided by total number of customer interruption.
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