Autility's distribution system has the greatest impact on customer perception of utility services. This is because disturbances on the distribution system are responsible for about 90% of outages, power quality problems, and other drivers of customer satisfaction. Today’s distribution utility typically faces an ongoing problem of maintaining system reliability within a fixed or decreasing budget.
For the future, as states move at widely disparate rates toward their vision of tomorrow’s distribution utility, various scenarios could unfold. The distribution company of the future may have a strict “wires-only” commodity focus; or it may separate one product from another, targeting consumers with various added-value products and services. Survival in this uncertain and increasingly competitive environment requires combining both solid business strategy decisions with innovative engineering and operations options.
Issues
Issues and opportunities include, but are not limited to the following:
- Reducing Costs and extending the life of equipment.
Lower capital requirements for new facilities, while streamlining the budget for operation and maintenance of an aging overhead and underground infrastructure. - Getting Employees through industry change.
Identify internal changes needed to better respond to consumer demands. Institute training as appropriate. - Applying innovative new, efficient technologies for old problems.
Search for leading edge technologies that will streamline operations, improve reliability, power quality, and reduce costs.
Timely Actions
In order to reduce costs and extend the life of equipment, utilities will look for ways to identify expense categories for cost reduction and to apply new technologies. Forward-looking utilities will employ advanced techniques for assessing and predicting equipment age and wear. Utilities that are members of EPRI will repair substation and line equipment based on EPRI’s Reliability Centered Maintenance techniques. Integrating SCADA with other utility functions to smooth flow of information and coordinate employee efforts is an obvious option.
Not to be ignored is timely application of innovative new and efficient technologies that streamline operations, improve reliability, and reduce costs. Among the new techlologies are solid state breakers & static transfer switches, low-cost radios, wireless public networks, and fuel cells.
Overhead lines represent an area that is rich in opportunities. There are slow-release fungicides for wood poles, and operations software for work order management; the latter includes power management, materials management, and to programs to anticipate equipment failure and outages. EPRI's Maintenance Management Workstation, Distribution Engineering Workstation, and Transformer Maintenance Advisor provide valuable assists where needed. Finally, engineering software is available for predictive maintenance and equipment life extension, for system design and analysis, and to automate staking. Robotic tools offer safer, faster line repair.
To take advantage of automatic meter reading (AMR) enlightend utilities are assessing new meter and billing techniques to facilitate open competition. They must address the issue of meter ownership, develop programs to maximize the value of consumer data, use AMR to keep consumers informed and educated, and create a “New Millennium” home page that has a mix of engineering, marketing, accounting, and educational data. Internet sites are ideal for billing, collecting, crew dispatch, education, do-it-yourself energy audits, demand-side management etc.
Getting employees through industry change is another important challenge. Utilities may often institute training programs that integrate and coordinate ways of meeting customer needs. Employees must be informed of technical as well as customer conveniences important to their rate payers. The opportunity is there to instill an attitude of responsibility for consumer satisfaction in all personnel, regardless of their location/job function.
Best Practices
“We build main feeders to a higher safety factor to make sure of good safety margins on important feeders,” says Torey Bell , planning/design supervisor at Sulphur Springs Valley Electric Co-op in Willcox, Az. “That way, we can sleep at night, knowing that most outages on the distribution system will affect the fewest number of consumers.”
“Reduced conductor tensions reduce aeolian vibration and resulting conductor damage,” says Bell, “but just the same, all our new lines include spiral vibration dampers.” Adds Bell , dampers adjacent to line connections seem to be most critical, including the pole ground connector on the system neutral conductors. We include dampers at both line tap connections and where jumpers lead to equipment. Wherever feasible, we use line jumpers with “squeeze-on” type connectors, because “hot-line connectors” don't stand up well when subjected to repeated fault currents, which may occur on overhead lines.
Connections to lines and equipment jumpers with limited clearances are always trouble spots. Sulphur Springs Valley Electric Co-op uses covered jumpers wherever birds could cause either a phase-to-phase or line to ground fault, such as at line taps, transformers, and reclosers. “We install bird guards at every equipment bushing along with covered jumpers,” adds Bell . “The covered conductors and bushings provide extra barriers between energized conductors and between them and conductors at other voltage levels. Whatever it takes to eliminate an unneeded outage,” says Bell .
Good sectionalizing practices, such as locating reclosers or fuses past load centers and important customers always helps. With about 40,000 meters and 3,100 miles of distribution lines, a typical utility has well over 200 line reclosers and many more sectionalizers. Cooper’s electronic sequence coordination function on reclosers is truly valuable; it reduces the number of blinks substantially, plus we use fault indicators at reclosers.
Joint use of poles by CATV companies creates problems for utilities, especially when older poles are involved. Jim Byrne, engineering manager of Poudre Valley REA in Fort Collins, Co. reports that the RUS has issued a guide for joint use contracts with CATV companies. “Poudre Valley’s contracts require quality inspections to limit liability, specify strength requirements, and address the reliability issues,” says Byrne. “For example, we’re wary of CATV companies pulling their cables up too tight during installation and changing the sag on our distribution lines. We also require a 5-ft separation whenever a third attachment is requested below the telephone and CATV; otherwise, linemen can't safely climb through the maze.”
Cuming County Public Power District, in West Point, Ne. took a drastic approach to reducing maintenance, using steel distribution poles, which also shunt lightning directly to ground. Serving 3000 consumers 70 miles northwest of Omaha, Ne., the co-op built 60 miles of distribution line that have absolutely no wood. The steel poles were manufactured by Valmont Industries, Valley, Ne. and the fiberglass crossarms were manufactured at LEWTEX by Shakespeare, Newberry, SC.
“This application exploited the naturally high strength-to-weight ratio of both materials,” notes Earl Boston, director of purchasing at Cuming CPPD. “We’ve had no outages or even blinks in the three years that the steel poles have been up, operating at 7.2/12.47 kV with an overhead neutral. We don’t understand the reason, but somehow the use of a tubular steel pole with a broad grounding foot at the base provides a free flowing path for lightning energy, without stressing any electrical equipment along the way.”
Fiberglass crossarms weigh less than wood and have excellent dielectric characteristics, about 155 kV BIL per foot. “Overhead neutral construction with steel poles and fiberglass crossarms gave us the optimum configuration for distribution lines that are quick and easy to build, provided the highest degree of reliability, and are the easiest to maintain,” says Boston.
“We can build a steel/fiberglass line in 40 percent less time than it takes with wood. Poles can be framed out using four, nine-inch machine bolts—two bolts for the braceless fiberglass crossarm, and two bolts for the pole top pin. If the steel pole has been ordered with the self-grounding attachment, the pole will be automatically grounded when set and tamped. After the conductor is sagged, we just connect the neutral to the pole, and the entire structure is grounded. No longer will squirrels or raccoons be able to climb our poles or woodpeckers ravage our crossarms,” boasts Boston.
Continues Boston, “Holes & climbing slots were predrilled at the factory, which reduced drilling time in the field.” Pole diameters were always the same, which allowed for less inventory and reduced guess-work as to bolt length, diameter, thread length, etc. Steel poles are automatically equipotentially grounded, so when a line technician applies protective grounds, it makes all maintenance work safer. Steel is also impervious to ditch fires or grass fires at the base of the pole.
Maximized line capacity is the primary objective of good distribution line design, which means using shunt capacitors to operate as close to unity power factor as possible. But how much fixed capacitance is it safe to install? “No more than one-half the peak kVA on the system,” suggests one utility. For example, take a system that has an inherent 85 percent power factor, and the current lags the voltage by about 30 electrical degrees. Using a 30/60 triangle for illustration, the side opposite the 30 deg. angle (KVAR) is half the length of the hypotenuse (kVA), which is in phase with the current. The remaining side is in phase with the voltage and is the true power (kW). Adding more than half the peak kVA in fixed capacitance may overcompensate and result in a leading power factor.
Air conditioners get more efficient at high temperatures when they are loaded to the hilt, hence drawing less reactive power. You still need capacitors on peak, but less than required just before peak. If you over compensate, you are asking for serious over-voltage problems, leading power factors, and the associated high losses. One shouldn’t use the minimum peak monthly power bill to size fixed capacitors, either. That’s too many, Your minimum peak is probably half the peak of the lowest month, and that's the load level you should use to size the fixed capacitors. All the rest should be switched; otherwise, your system losses are likely to be high in off-peak months. Extra capacitors on line cause a leading power factor, which generates higher losses. Nobody needs that.
Underground Installation & Replacement
Although underground installation and maintenance is a mature technology, new ideas make it an area of constant change. “There’s always a new way to splice a cable, terminate a joint, or protect a cable in underground residential distribution (URD). The key is to stay flexible, alert and receptive to change,” says Berl Davis, VP of engineering and operations at Palmetto Electric Co-operative in Hilton Head, NC and chairman of NRECA’s Underground Distribution Subcommittee. “You never know when you’ll see an improvement that might benefit your system operation.”
Utilities already design their URD systems to achieve reliable, long lasting and economical electric service. But no two utilities do it in the same way. Variations exist in work practices, depending on soil, weather, and system operating conditions. Take trenching versus direct-buried cable, for example. One company uses trenching in all new subdivisions, and installs jacketed EPR cable in PVC stick conduit throughout for ease of future cable replacement. “But when we have to replace a cable under an established lawn, driveway, or parking lot, we use guided underground boring to avoid disrupting a lawn or garden. The extra cost is usually justified, compared with the consumer relations problem we would create otherwise,” notes one operations manager. He replaces old bare-concentric neutral cable with jacketed EPR (ethylene propylene rubber) because EPR is more flexible and because the quality of splices and connections on EPR is more uniform. “We also use flexible PE (polyethylene) conduit when replacing cable,” he adds.
Nationwide, utilities use guided boring and flexible PE for 40 percent of all cable replacement, with an average run reaching 250 ft, according to the Insulated Conductors Committee of IEEE. “But 60 percent of all new underground cable is still direct buried,” reports the committee. “One utility says it still direct buries all new primary because it feels the cable should last forty or fifty years without imposing the extra cost of conduit. However, they do use flexible PE and guided boring on cable replacements.
One utility in mid-Carolina also uses a weak link on all flexible PE pulls to avoid “neck-down.” The weak link and extra lubrication keeps pulling tension down to 2000 lbs., thus preventing the tensioning rigs from stretching the PE. The utility also uses thicker wall PE supplied by Arnco Corp., of Elyria, Ohio and others to prevent “ovalling out” on bends. Otherwise, it’s impossible to pull cable through the bends afterwards. “We don’t use schedule 40 PVC for bores anymore, but specify SDR 13.5 or SDR 11 flexible PE, and vary the weak link according to wall thickness. (The SDR [standard dimensional ratio] indicates the wall thickness, a lower SDR number indicating a thicker wall.)”
The IEEE Guide for Installation of Cable Using the Guided Boring Method, IEEE Std 1333-1994 recommends methods and equipment for proper and economical installation of insulated conductors and conduits. The 18-page guide also addresses installation of cable pre-installed in conduit (CIC), and is well worth the investment of time and effort to read. Dantzler references the guide when soliciting quotes for underground services.
Another problem that is under revision by ICC’s Subcommittee 1050 on elbows and bushings is the occurrence of flashovers on 200-amp rated connectors during switching operations. Most flashovers tend to occur at low temperatures (in the range of 20-30 deg. F). Tom Champion, a research engineer at NEETRAC (National Electric Energy Testing, Research, and Applications Center) chairs the subcommittee and says the mechanism for flashover is fairly well understood. “When the load-break elbow is withdrawn from the bushing, a partial vacuum can be formed across the mating interface. The formation of a vacuum depends on the condition of the silicone grease lubricant, the time since the components were last operated, and the ambient temperature. When a partial vacuum forms, the dielectric strength across the mating interface is greatly reduced. The breakdown strength of air is much less at a partial vacuum than at either normal air pressure or under the high vacuum conditions used in vacuum-insulated switchgear. At low temperatures, the combination of conditions appears to be right for flashover,” says Champion.
The range of temperatures over which most flashovers occur has caused some utilities to try heating the elbow before switching, which has been somewhat successful. In addition, one manufacturer, Elastimold has experimented with a solution that attempts to vent the interface, preventing the formation of a vacuum. Vent holes placed around the cuff of the elbow allow air to rush into the interface, reducing the likelihood of flashover. “However, these are only temporary solutions. Further research is needed to eliminate the partial vacuum problem without compromising other aspects of the interface dielectric strength,” notes Champion.
For fault detection, many utilities use acoustic, resettable fault indicators in place of optical devices because they are easier to locate in case of overgrown shrubbery, and they don't require that a hole be drilled in the padmount transformer enclosure.
Cables are particularly sensitive to voltage surges and have come to be regarded as the tender spot in many utility systems. Lightning poses a problem anytime it strikes at or near a cable riser pole (where a cable rises to connect with an overhead primary). Arresters are installed at riser poles to protect cables from lightning, but discharge voltages from arresters travel through a cable at half the speed of light, and when it reaches an open point on a loop, it reflects back on itself at double the voltage at the open point. Design guides recommend that the leads to the arrester and cable ground (neutral) be as short as possible to limit induced voltages. Arrester lead length is the combined line and ground lead length in series with the arrester and in parallel with the cable’s termination. As illustrated in the guides, however, arrester lead length can be virtually eliminated by carrying the line and ground connections to the arrester terminals first, and then to the conductor and ground terminals of the cable termination. This also takes full advantage of the protective margin provided by the riser-class arresters, which are installed to limit voltage surges.
Fault locating uses an electric discharge technique, combining a high energy discharge (called a thumper) with TDR to locate a highresistance fault. At first, the TDR gives a rough measure of where the fault is located on the cable, then as the equipment is moved closer, the exact location can be pinpointed. Firms that offer this equipment include HDW Electronics, Bethlehem, PA., Von Corp., Birmingham, AL., Hipotronics, Brewster, NY, and AVO, International, Dallas, TX. One line superintendent at Beltrami Elecric Co-op in Bimidji, MN. has used HDW’s device for several years, and claims a 75 percent savings in time to locate faults.
Refinements in technology not only bring better solutions for underground systems, but suggest changes and improvements in a utility’s operating practices. Engineers would be well served to constantly monitor progress and change, which is reported in the industry trade press, at trade shows, by vendor representatives, and when networking with each other. This cannot but help them to understand the hostile conditions under which underground equipment operates, and this will ultimately enable them to anticipate and prevent problems before they arise.
John Marks graduated from the University of Nebraska in 1954, earning a BSc in electrical engineering. After serving four years in the U.S. Navy as a pilot, he returned to industry, first with Allis-Chalmers in Milwaukee and then General Electric X-Ray. At both firms he worked as a technical advertising specialist.
Next, he obtained a position as editor of Electric Light & Power magazine in Boston MA, a trade magazine serving the electric utility industry. After ten years with the magazine, Mr. Marks took a position as Manager of Technical Information in the Communication Division of the Electric Power Research Institute in Palo Alto, CA. While at EPRI, he gave his course on EffectiveWriting to over 100 employees, and has presented his course to numerous private individuals.
Now semi-retired, Mr. Marks spends his time writing for the electric utility trade press and presenting writing courses to interested industrial firms.
