Distribution systems basic techniques and radial networks

Distribution systems analysis of the customer failure statistics of most utilities shows that the distribution system makes the greatest individual contribution to the unavailability of supply to a customer

Evaluation techniques A radial distribution system consists of a set of series components, including lines, cables, disconnects (or isolators), busbars, etc. A customer connected to any load point of such a system requires all components between himself and the supply point to be operating

The reliability indices that have been evaluated using classical concepts are the three primary ones of average failure rate, average outage duration and average annual unavailability or average annual outage time. These indices will be generally referred to in this book only as failure rate, outage duration and annual outage time. Although the three primary indices are fundamentally important, they do not always give a complete representation of the system behavior and response. Additional interruption indices

For instance, the same indices would be evaluated irrespective of whether one customer or 1 00 customers were connected to the load point or whether the average load at a load point was 10 kW or 100 MW. In order to reflect the severity or significance of a system outage, additional reliability indices can be and frequently are evaluated.

Additional interruption indices Customer-orientated indices –System average interruption frequency index, SAIFI

interruption indices Customer average interruption frequency index, CAIFl

interruption indices System average interruption duration index, SAIDI Customer average interruption duration index, CAIDI

Average service availability (unavailability) index, ASAI (ASU1)

Load- and energy-orientated indices Energy not supplied index, ENS Average energy not supplied, AENS or average system curtailment index, ASCI Average customer curtailment index, ACCI

The customer- and load-orientated indices described, are very useful for assessing the severity of system failures in future reliability prediction analysis. They can also be used, however, as a means of assessing the past performance of a system. In fact, at the present time, they are probably more widely used in this respect than as measures of future performance system performance

Assessment of system performance is a valuable procedure for three important reasons: –It establishes the chronological changes in system performance and therefore helps to identify weak areas and the need for reinforcement – It establishes existing indices which serve as a guide for acceptable values in future reliability assessments –It enables previous predictions to be compared with actual operating experience.

Example

System prediction

Application to radial systems

Effect of lateral distributor protection In this case a short circuit on a lateral distributor causes its appropriate fuse to blow; this causes disconnection of its load point until the failure is repaired but does not affect or cause the disconnection of any other load point.

Effect of lateral distributor protection

Effect of disconnects

Effect of transferring loads Unrestricted load transfer

Effect of transferring loads Transfer restrictions

Effect of transferring loads Transfer restrictions

Summary of indices