The Impact of a “Fuse Blow” Scheme on Overhead Distribution System Reliability and Power Quality

1. IEEE The Impact of a “Fuse Blow” Scheme on Overhead Distribution System Reliability and Power Quality Rural Electric Power Conference Craig A. O’Meally, P.E. Jim Burke, Fellow, IEEE April 27, 2009

2. Presentation Overview • Review of Fuse Blow • Review of Fuse Save • Impact of fault current • Case study of 19 actual utility feeders • Feeder Topology - Generalized Analysis • Conclusions

3. Review • Fuse Save • Minimizes customer interruption time by attempting to open the breaker or recloser faster than it takes to melt the fuse. • This saves the fuse and allows a simple momentary interruption • Fuse Blow • Eliminates the fast trip of the breaker or recloser by having the fuse blow for all permanent and temporary faults. • Minimizes momentary interruptions and increases SAIDI. Improves power quality but decreases reliability.

4. Fuse Blows Fuse Blow FUSE is BLOWN Lateral experiences sustained interruption

5. Fuse Blow • Fuse Blow • Used primarily to minimize momentary interruptions (reduces MAIFI) • Increases interruption duration (SAIDI) • Very successful in high short circuit areas • More suitable for industrial type customers having very sensitive loads

6. Breaker trips Fuse Save FUSE is SAVED Entire Feeder trips Momentary occurs Fuse does not Blow

7. Fuse Save • Fuse Save • Minimize customer interruption time • Reduce SAIDI • Increase MAIFI • May not work in high short circuit areas • Work well in most areas • Not suitable for certain industrial customers that cannot tolerate immediate reclosing • Works best for residential and small commercial customers

8. Both (Fuse Save & Fuse Blow) • Many utilities use both schemes for a variety of reasons • Fuse Blow for high short circuit current areas and Fuse Save where it will work. • Fuse Save on overhead and Fuse Blow on underground taps. • Fuse Save on rural and Fuse Blow on urban • Fuse Save on stormy days and Fuse Blow on nice days. • Fuse Save on some circuits and Fuse Blow on others depending on customer desires

9. Survey of 95 Utilities • Over 80% or 78 out 95 uses “Fuse Save” philosophy on some portion of their system

10. Short Circuit Courtesy of Milsoft’s LightTable • Many utilities with high short circuit levels use fairly fast fuses (e.g. 65K) on their laterals. • This fuse cannot be saved unless the short circuit current seen by the fuse is approximately 670 A or less • Fuse Save will not work on higher fault currents • Solution: go to a slower fuse by either changing the fuse type or increasing the fuse rating • For a 100KS, coordination is attained up to approximately 2600 amperes (about 2 miles from a typical substation)

11. Limits of Coordination • Using faster reclosers and breakers, as well as large fuse sizes, it is possible to achieve “fuse saving” beyond 4800 amperes. • Some utilities use 200 ampere fuse ratings and KS or T fuses to achieve these high levels of coordination.

12. High/Low Scheme • One technique used by a number of utilities is to use both a “fuse save” philosophy in lower short circuit areas and a “fuse blow” philosophy in high fault current areas (sometimes referred to as a High/Low Scheme)

13. High/Low Scheme • Reduces MAIFI as well as SAIDI. • SAIDI can be reduced since the midline device is normally faster (3 cycles) than a feeder breaker (6 cycles) and can be coordinated with the fuse in higher short circuit area • Another advantage is that it reduces the number of customers who are out of service for a permanent fault at the far end of the feeder

14. Fuse Save v. Fuse Blow Comparison • 19 actual utility feeders were investigated

15. Fuse Save Fuse Blow Fuse Save Effect SAIDI 1% - 42% SAIFI 2% - 47%

16. Fuse Save Fuse Blow Fuse Save Effect • Fuse save resulted in significant increase in the feeders’ MAIFI (20% - 1011%).

17. Feeder Topology • Is it possible to obtain improvements in all three indices, or at a minimum, improve SAIDI and SAIFI, while keeping deterioration of MAIFI to a minimum with the implementation of the fuse saving scheme? • What topologies stand to gain the most by implementing fuse save? The topology of the feeder that lends itself to such improvements is of great interest and was a major goal of our investigation

18. Generalized Analysis • Several completely overhead feeder topologies were analyzed, on a generalized basis, with the goal of simultaneously seeing the impact of a fuse blow philosophy on SAIFI, SAIDI, and MAIFI.

19. Generalized Analysis SAIDI • Shorter feeders consisting of longer laterals are the greatest SAIFI and SAIDI beneficiaries SAIFI A 7 mile feeder having 2 mile laterals may reduce the feeders SAIFI and SAIDI by 27% and 11% A 7 mile feeder with 0.5 mile lateral taps may have 10% and 3% reduction in SAIFI and SAIDI

20. Generalized Analysis • Longer feeders with short laterals (number of laterals remaining constant) are likely to have a smaller increase in MAIFI, as a result of a fuse save philosophy

21. Conclusions • Most utilities only report SAIDI, CAIDI, and SAIFI, to their Commissions, so from this point of view a “fuse save” scheme is superior. • Although fuse saving will improve system SAIFI and SAIDI at the expense of MAIFI, the negative impact on power quality may be reduced by carefully selecting only those feeders where sensitive customers (especially industrial) are absent, or choosing longer feeders consisting of short laterals. • The use of a mid circuit recloser and downstream fuse saving may also be used to increase distribution reliability with minor impact, if any, on power quality

22. Conclusions • The nature of the customers, and the overall benefits to reliability must first be evaluated before implementation. • If the existing MAIFI is high, then alternate means of improving the feeder’s reliability is recommended. • Rural electric customers are usually more forgiving and tolerant of momentaries than they are of sustained outages hence the rewards of fuse saving is likely to outweigh those of fuse blow.

23. Conclusion • Longer feeders with short laterals (number of laterals remaining constant) are likely to have a smaller increase in MAIFI, as a result of a fuse save philosophy than • Similar size feeders with longer laterals • Shorter feeders with laterals of similar length • Shorter feeders with longer laterals • Longer feeders with longer laterals

24. Questions?

25. Thank You Craig O’Meally Jim Burke comeally@quanta-technology.com James.Burke@quanta-technology.com

26. The End

27. Supplementary Info

28. Analysis of reliability on laterals and feeders • The devices used on the electrical distribution system (like most equipment) have a relatively constant failure rate (λ). • The failure rate of electrical equipment may be assumed to follow an exponential distribution. • The reliability, or probability that the equipment does not fail over a given time is

29. Analysis of reliability on laterals and feeders • Since the components operate independently, then the system life distribution of a section with n components is also exponential. The failure rate parameter of the system or section (λs) is: • In a period of one year, a typical overhead lateral that is 1 mile long, will have an average probability of failing of approximately 15.5% (ranging between 2.37% to a high of 28.1%) Probability of failing = Where Probability of failing =

30. Analysis of reliability on laterals and feeders • Typical permanent failure rates of key distribution components are illustrated Source: Electric Power Distribution Reliability by Richard Brown

31. Analysis of reliability on laterals and feeders • Since a feeder is comprised of multiple lateral taps, the probability of failure is increased as the number and length of laterals increase. • An increase in the length of the feeder also results in a decrease in the reliability of the feeder. • The physical characteristics of the feeder and its laterals influence the overall reliability of the feeder.

32. Analysis of reliability on laterals and feeders • The history and the characteristics of the laterals, along with their probability of developing a fault are worth investigating prior to using fuse save or fuse blow. • Other parameters that are worth closer analysis include • Typical length mains and laterals • Short mains and long laterals • Long mains and short laterals • Impact of load density • Actual system results (from previous studies)