Utilities are always looking to optimize their power-delivery networks to ensure customers receive appropriate levels of power quality and reliability. Although there is no magic bullet in managing power-quality issues, utilities can maximize network performance and better serve customers by diligently addressing trouble-prone areas.
Significant reliability and power-quality concerns for customers include sustained interruptions, momentary interruptions and voltage sags. These problems are all caused by faults on the utility power system, with most occurring on the distribution system. Two key strategies to improve reliability and power quality to customers are:
- Minimize the effect of faults on customers
This can be achieved by sectionalizing and restoring circuits more quickly (automation), using more protective devices (more fuses, more reclosers), reclosing more quickly and improving coordination.
- Eliminate faults
Better tree maintenance, animal protection, equipment replacement programs and arrester application, as well as thorough construction work audits to ensure quality and line inspections, all help to eliminate faults.
Recently, EPRI has focused on practical approaches to improving reliability and quality of electric power to customers. Much of EPRI's work has been based on Duke Power's experience successfully applying specific measures to enhance its delivery system.
Protective Device Strategies
Fuses, sectionalizing switches, re-closers, sectionalizers — the more components there are, the more faults are isolated to smaller chunks of circuitry, and the fewer customers are interrupted. Taps are almost universally fused, primarily for reliability. Fuses also make cheap fault finders. Ideally, a high percentage of a circuit's exposure should be on fused taps, so that when permanent faults occur on those sections, only a small number of customers are interrupted.
Short fused taps are often overlooked. Many utilities have more unfused taps than they realize. If the unfused taps use a smaller conductor size, faults are more likely to burn down such conductors, because a circuit breaker will take longer to clear a fault than a fuse. Because such taps may be on side streets, crews can easily forget to inspect them for damage during patrols, thus increasing the interruption time. Therefore, one way to improve reliability is to make sure that taps are fused.
Optimizing reclosing strategies is another way to minimize the impact of faults on customers. Choosing the number of reclosing attempts is a balancing act. Each reclosing attempt has some chance that the circuit will hold, but the probability diminishes quickly with each successive attempt. Excessive reclosing increases stress on the system and results in additional damage at the fault location (faults in equipment do more damage, wire burndowns are more likely), voltage sags, through-fault stress to transformers and other equipment (especially connectors) and ratcheting of overcurrent relays. IEEE surveys find that most utilities attempt two or three reclose operations before locking out. Anything more than that is unwarranted.
Using an “immediate” reclose attempt on the first shot is one way to reduce the impact on customers. An immediate reclose means having no intentional time delay (or a very short time delay) on the first reclose attempt on circuit breakers and reclosers. Many residential devices — such as digital clocks, VCRs and microwaves — can ride through a half-second interruption but not a five-second interruption. Thus, a fast reclose helps reduce residential complaints.
Reviewing circuit breaker and fuse coordination is another approach to improving reliability. Circuits are usually designed for either fuse saving or fuse blowing. With fuse saving, the instantaneous relay element on a breaker or the fast curve on a recloser clears faults before downstream lateral fuses operate. Because most faults on distribution circuits are temporary, fuse saving improves reliability by reducing tap fuse operations. With fuse blowing, the fuse is set to blow before the upstream breaker operates, greatly reducing the number of momentary interruptions on the circuit. About one-third of utilities use fuse saving, one-third use fuse blowing and one-third use a combination of strategies, with the trend predominately toward fuse blowing.
Neither a fuse-saving nor a fuse-blowing protection scheme is the best choice for all applications. One scheme is better in some applications than others. The best choice depends on many factors, including fusing practices, wire type used, the mix and location of customers on a circuit, and the utility philosophy. It is helpful to review the choice made (even on a circuit-by-circuit basis) because many situations would be better served by a different choice.
Other protection-related strategies also are valuable. Automated switches or recloser loops can automatically sectionalize a circuit after a permanent fault, greatly decreasing the restoration time. Reviewing and improving protective device coordination can reduce the number of customers affected by a fault. Sequence coordination is a way to coordinate device fast curves to prevent excessive momentary interruptions. Placing single-phase protective devices on three-phase circuits can help on residential circuits by interrupting only one-third of the customers for single-phase faults.
Faults are not evenly distributed along lines. Not all faults are inevitable “acts of nature.” Most of them are from specific deficiencies at specific structures. On overhead circuits, most faults result from inadequate clearances, inadequate insulation, old equipment, or from trees or branches falling onto a line.
A first step in eliminating faults is to identify what is causing them. Keeping in mind that most faults result from specific structural deficiencies, field identification of fault sources is a key part of construction-improvement programs. Field personnel can be trained to spot pole structures where faults have occurred or might likely occur. Common structural deficiencies include poor jumper clearances; old equipment (such as expulsion arresters); bushings or other equipment unprotected against animals, ground leads or grounded guys near phase conductors; poor clearances with polymer arresters; damaged insulators; damaged covered wire; and dangerous trees or branches present.
The photo on page 51 is an obvious example of a location where a fault has occurred — probably repeated faults. The arrester and pole are severely blackened from arc burn products, and the transformer bushing has cracked and is missing a piece. This structure has several severe deficiencies:
The externally gapped arrester (which is possibly failed internally) provides a very short gap, easily flashed by birds or squirrels. Also, it would take only a small lightning surge to flashover this structure.
Neither the arrester nor the transformer bushing has animal protection.
The guy wire attached near a phase conductor is uninsulated.
The armless type of construction using post insulators provides little insulation on the pole.
The equipment has no local fuse.
Overall, this structure is a disaster. It is highly susceptible to lightning, animals, small tree branches and other debris. Fixing this pole requires a complete overhaul: replace the arrester, replace the transformer bushing, add animal guards, add a guy insulator, add a local fuse, and reframe with crossarms or other materials to increase insulation and electrical clearances. While this example contains many blatant problems, any one of the problems by itself can be a source of faults.
Inspections of structures to identify fault sources should be done from the ground. Implement training out in the field for best results; show examples of fault sources. Walk the line and use binoculars — it is more effective than “riding the line.” Some fault sources are not obvious and require looking at a structure from different angles.
When evaluating structures and possible fault causes, note the distinction between the cause of the fault and the deficiency. The cause of the fault may have been a squirrel, but the underlying source of the problem may have been poor electrical clearances (unprotected bushings, tight spacings and so forth). Although the squirrels cannot be eliminated, structures can be made more resistant to squirrel contacts.
Programs to Reduce Fault Rates
The best way to reduce faults over time is to “institutionalize” fault-reduction practices. After identifying the most common fault sources, implement programs to address these so that performance improves continually. Options for such programs include:
- Design review
The first step in implementing fault-resistant designs is to start with good designs.
- Outage follow-ups
On-site outage reviews can help identify weak points and reduce fault rates. Faults tend to repeat at the same locations and follow patterns. By identifying the location of the fault and any structural deficiencies that contributed to the fault, the deficiencies can be corrected to prevent repeated future faults.
- Construction audits
Construction audits reinforce practices to ensure that crews build to the specifications. Audits also educate crews on why things are done and how the designs minimize faults.
- Problem-circuit audits
Just as there are pole locations that can have repeated faults, faults can cluster on some circuits. These faults may be from consistently poor construction on a circuit, heavy tree exposure or a few poor structures with repeated faults. The goals of a problem-circuit audit are (1) to identify the deficiencies that are the most probable sources of faults and (2) to correct deficiencies to avoid repeat fault events.
- Upgrade and maintenance projects
Utilities can shore up weak areas by any one of several construction upgrade programs: animal-guard implementations, arrester replacements or new applications, and cable-replacement programs. Maintenance projects include tree clearance, danger tree programs, and pole inspection and replacements.
There are no quick fixes. Utilities must maintain consistency. For the most part, these are not one- or two-year programs. Maintaining fault resistance is an ongoing process that becomes a core part of utility operations.
Duke Power pioneered the strategy of identifying and removing sources of faults on its system. By doing this, Duke Power is able to maintain respectable reliability numbers, despite the fact that its service territory has regular severe weather, and Duke Power is mainly an overhead utility with predominantly all-radial systems. Since first implementing fault reduction programs, Duke has reduced its SAIFI (system average interruption frequency index) by more than 15% during a 10-year period.
Because fault sources are most often at equipment poles, consider programs targeted specifically at these poles. Duke Power has many single-phase CSP transformers with old arresters, minimal animal protection and no local fusing. These locations are the second-largest contributor of outage minutes on Duke's system. To address this problem area, Duke has a “transformer retrofit” program where crews install local cutouts with surge-resistant fuses, new lightning arresters, insulated leads and animal guards. While crews set up at a pole, they insulate uninsulated primary guys and remove pole grounds above the transformer tank. Duke applies this program on a circuit-by-circuit basis, with each region developing a prioritization plan based on outage database records.
It costs less to upgrade construction when a crew is already set up at a pole. Consider implementing procedures and checklists for crews to fix problems on structures whenever they are set up to work on a pole.
On a final note, these strategies and programs for improving reliability can be targeted for maximum benefit. Outage databases help identify problems leading to faults. They also help identify which circuits to target, which areas have the most problems with trees, which areas have the most problems with animals, and so on. In addition, outage databases judge the effectiveness of improvement programs.
Tom Short is a senior engineer with EPRI PEAC in Schenectady, New York. Prior to joining EPRI PEAC, he worked for Power Technologies Inc. for 10 years. For several utilities, Short has performed lightning protection, voltage sag, flicker, capacitor application and load flow analysis studies, and has been involved in several monitoring projects on distribution systems. He received an MSEE degree from Montana State University in 1990. As chair of the IEEE Working Group on the Lightning Performance of Distribution Lines, he led the development of IEEE Std. 1410-1997; for this effort, he was awarded the 2002 Technical Committee Distinguished Service Award. Recently, Short completed the Electric Power Distribution Handbook, published by CRC Press (2004).
Lee Taylor has worked as a distribution engineer at Duke Power for 33 years, and has been the lead distribution reliability engineer there for the past 15 years. Taylor holds the title of consulting engineer, the highest technical position in the company. His specialties include distribution reliability data analysis, field investigations of fault sources, tactical and strategic fault prevention, reliability training and assessment of field personnel, and construction quality aspects of reliability and power quality. Lee received his bachelor's degree in physics from the University of North Carolina in 1971. He is a licensed professional engineer, IEEE member, and currently serves as an officer on the SEE Power Quality and Reliability Subcommittee. Lee has served on several IEEE committees and working groups in the area of distribution reliability, and has co-authored a number of papers on distribution system outage data analysis.
Electric Power Distribution Handbook, Short, T.A., CRC Press (Boca Raton, Florida, U.S.) 2004.
Power Quality Improvement Methodology for Wires Companies, EPRI (Palo Alto, California, U.S.) 2003. 1001665.
Power Quality Implications of Distribution Construction, EPRI (Palo Alto, California, U.S.) 2004. 1002188.