Colorado Springs Utilities linemen protect the underground system with fluid injection and surge arresters.
The Rocky Mountains tower over the city of Colorado Springs, and snow-capped Pikes Peak is visible nearly anywhere in town. So in 1970, the city council approved an ordinance requiring all new electric lines under 30 kV to be buried underground to allow for an unobstructed view.
Since then, Colorado Springs' population has tripled to nearly 400,000, and now about 75% of the municipally owned utility's electric system is underground. But providing citizens a clear view of the mountains comes with its challenges.
In the 1990s, 12.47-kV cable failures averaged about 40 per year with about 15% contribution to the System Average Interruption Duration Index (SAIDI). Beginning in 2002, those averages doubled with about 80 failures per year and contributed about 30% to the system SAIDI, which was 47.69 minutes in 2009.
While last year's SAIDI is better than the national average, Springs Utilities knows there is room for improvement.
In addition to the cost and complexity of dealing with underground cable, Springs Utilities also faces another challenge: the city has one of the highest incidences of lightning strikes in the nation.
The Pikes Peak region is considered the primary hot spot for lightning in Colorado with ground-flash densities of 3.0 to 5.5 flashes/sq km/year. Cable failures on the 12.47-kV system have been the largest, single contributor to total outage time for at least the past five years, and lightning strikes are one of the leading causes of cable failure.
Lightning is not the only force of nature working against buried electric cable. After some years, cable can break down because of water trees. Voids and contamination in the insulation, as well as other design and manufacturing deficiencies, lead to voltage stress concentrations within the cables. These elevated voltage stresses, combined with moisture ingress into the cable structure, create water trees. These dendritic growths of microscopic cavities degrade the insulation over time, ultimately causing the cables to fail. Treeing is the most common cause of dielectric cable failure.
Distribution cable failures affect more than 8 million U.S. electric customers each year. Most of the failures occur in cables installed before 1980. The failure rate for pre-1980 vintage cable is 20 times greater than for cable installed after 1985.
The first generations of medium-high-voltage polyethylene cables were particularly vulnerable to failure because of water trees, which come in many different types. Bow ties initiate inside the bulk of the dielectrics at voids or contaminations. Vented water trees start at protrusions on the interfaces between the dielectrics and conducting materials. Water buildup from load and thermal cycling causes halo water trees. Once these water trees are present, they can lead to electric trees. When this happens, it's often days or weeks until the cable fails.
Weighing the Options
To reduce the failure rate for older cables, utilities can replace the direct buried cable, but this can be cost prohibitive. In some circumstances, a less-expensive option is cable injection. Cable injection is typically one-third to one-half the cost of cable replacement and can extend the life of the cable by 20 years or more.
Utilities must consider several factors before they decide whether or not to replace the cable or rejuvenate it with injection fluid. Springs Utilities started a cable injection program in 2005 and discovered that the process has many distinct advantages. First of all, it is three to six times more productive than cable replacement. The injection process is clean, quiet and much less intrusive to electric customers. The utility also can proactively preempt interruptions due to cable faults in a cost-efficient manner, and the technology has a full cash-back warranty if a failure occurs within 20 years.
The Injection Process
Before the injection process can begin, linemen must de-energize and test the cable, inspect the enclosure/device and the cable, and then remove existing terminations. Next, they confirm the actual cable length using a time domain reflectometer. Field workers also use the device to confirm the condition of neutrals and identify the number of splices and locations.
Next, the linemen will perform flow and pressure tests by injecting nitrogen into the cable, typically at 15 psi to 20 psi, and then measuring the outflow and calculating the pneumatic resistance. The linemen then pressurize the cable and monitor it to ensure there are no leaks.
Based on the tests, length of cable and the breakeven costs compared to replacement, they will make the final decision on whether or not to inject the cable in the field. When they make the decision, the process begins. A feed tank injects fluid into the conductor strands at 15 psi to 20 psi, and the linemen place a vacuum tank on the collection end to hasten the fluid through and ensure a fill. The cable injection fluid then travels completely through the cable, and once the cable is flushed and filled, they remove the vacuum tank and install a permanent cap.
Once inside the insulation, the cable injection fluid reacts with the water in the tiny micro-voids and fills them with a dielectric oligomer. The fluid then repairs the damage caused by water trees and other defects. Since the molecules of the resulting oligomer are much larger than water molecules, they lock into place and retard the growth of future water trees. The process extends the life of even badly aged cable.
Adding Another Layer of Protection
For several years, the municipally owned utility debated whether or not the 12.47-kV underground system required additional surge arresters to better protect underground cable. Through a different approach to insulation coordination, Springs Utilities was able to show the need for an increase in margin of protection on the distribution underground system.
In 2009, the utility conducted a thorough review on how to better protect the underground cable system from lightning. Surge voltages due to lightning strikes typically enter the underground distribution system at the riser pole transition from the overhead to underground distribution. Surge arresters reduce the high-voltage surge caused by lightning to help prevent damage to cable and equipment. Surge arresters are especially important on underground systems where normal open points cause reflection of the traveling wave, also known as voltage doubling.
As a result of this study, the utility is installing metal-oxide surge arresters on normal open and endpoints on all 12.47-kV sub-feeder, laterals and branches 200 ft or greater from the tap. In addition to new construction, arresters will be placed in poor cable areas to help prolong the life of older cable.
Reliability of Arresters
Though the older metal-oxide technology had a higher failure rate, the modern metal-oxide varistor arresters have less than 1% failure rates annually. At Springs Utilities, the total surge arresters failures attributed 1.05% of the total number of interruptions and 1.52% of the total customer hours out from 2004 to 2008.
The useful life of surge arresters significantly depends on the number of surges seen by the device. MOV arresters have isolators designed to remove themselves from the circuit upon failure, so they should not cause an outage. However, the purpose of an arrester is to protect the equipment, not prevent an outage.
If the surge due to lightning exceeds the rating of the arrester, it may damage the arrester and will likely operate an upstream protective device. Unfortunately, without a thorough failure investigation, it is easy to quickly attribute these outage types to equipment failure even though the arrester performed its intended function. As cause investigations become more sophisticated, these outages should be distinguished from a defect in the arrester equipment.
Financial payback is nearly impossible to determine because of the variable nature of lightning over time. However, by showing the cost of cable failures versus the cost of the implementation of surge arresters, there seems to be little argument against such a critical insurance policy.
Springs Utilities spends about $6,000 each time it replaces cables, for a total of $500,000 annually for failed cable replacement. With the arresters costing between $60 and $200 each, the total cost of the surge arrester installation is estimated to be about $130,000 per year including labor. Based on the last three years, the company has spent $2 million to $3 million on cable injection or replacement projects.
Forty years after deciding to install all new electric lines underground, most citizens of Colorado Springs have an unhindered view of the mountains. That's what they've come to expect. They also expect reliable electric service and reasonable rates. The professionals at Springs Utilities are taking on the unique challenges of the underground system to deliver for their customers.
Dan Skokan (firstname.lastname@example.org), an operations supervisor for Colorado Springs Utilities, leads the cable rejuvenation program.
Rocky Bloesser (email@example.com) is the line crew supervisor for Colorado Springs Utilities and has been with the company for 24 years.
David Grossman (firstname.lastname@example.org) has been with Colorado Springs Utilities for the past eight years and is a senior public affairs specialist.
Colorado Springs Utilities' Susan Lovejoy, senior planning engineer, and Sarah LaBarre, operations superintendent, also have been significant contributors to these programs.
ELECTRIC UTILITY OPERATIONS
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