Cost-Effective Solution for Cable Reliability Issues
A case study on cable rejuvenation in rocky terrain.
Electrical Business Magazine, February, 2015 Issue
by Steve Hightower
The Empire District Electric Co. is a Missouri-based, investor-owned utility that services about 215,000 customers in portions of Missouri, Kansas, Arkansas and Oklahoma. The majority of Empire’s underground cable is 1/0 bare concentric neutral that was installed before 1980. Empire started experiencing increasing reliability issues, including power outages in residential and commercial areas, due to the age of the cable.
Empire initially had more than 17,000 feet of cable that needed attention. Much of the terrain within these areas is very rocky, and the quotes Empire received for replacing the failing cable were as much as $100/ft due to the need for replacing cable bored through rock. Cable injection quickly became an attractive solution.
The utility chose a hybrid injection process that uses a combination of Sustained Pressure Rejuvenation (SPR) and improved Unsustained
Pressure Rejuvenation (iUPR). Here are the four main reasons the utility chose the hybrid injection process:
1. The cost is less than half of replacing the failing cable.
2. Cable injection is much quicker than replacement. Most SPR-injected segments are completed in less than two hours.
3. The cable’s full dielectric strength is restored in seven days when the SPR process is utilized.
4. Reported injected cable segment failure rate is 0.4% with some suppliers.
In addition to the system’s reliability improvements and the savings in cost and time, the hybrid injection process also minimizes disruption to customers.
The initial injection project consisted of a two-man crew performing the injection while Empire’s own two-man crew completed all craftwork. Wes Robertson, line manager for Empire, said, “We particularly liked the hybrid approach to injection—treat each segment with the injection method that makes the most sense for that segment—since this approach allows for the maximum number of cables to be treated, and has the greatest impact on our system reliability”. (For this project, Empire chose to use a patented product called Cablecure 732 fluid, which uses both the iUPR and SPR injection processes. The Cablecure 732 fluid allowed Empire to take advantage of a 25-year warranty on segments injected using the iUPR process, and a 40-year warranty on segments injected using the SPR process.)
Circuit owners often choose to employ the hybrid injection approach, also known as Tailored Injection, to maximize the number of segments injected. The hybrid approach utilizes both the low-pressure iUPR and the moderate pressure SPR injection methods to maximize the number of segments injected, avoid digging difficult splices, and seal cable ends so fluid does not fill the elbows and splice bodies. The hybrid process delivers the best cable reliability, the maximum number of injected segments, and allows us to match the rejuvenation approach to the circuit owner’s budget and project needs.
The iUPR injection process is safer and more reliable than the legacy Unsustained Pressure Rejuvenation injection process, as it employs proprietary equipment and injection process improvements. One of these is reticular flash prevention (RFP) technology, which reduces the risk of injection port flashover. Another improvement is more robust and redundant seal designs that reduce fluid leaks and the possibility of transformer fires.
This process also eliminates the need for post-manufacture air-pressure testing to identify flashing from moulding defects, and reduces the oozing of treatment fluid from the elbow by 30% during and after injection. Fluid secretion can compromise elbow reliability under hot conditions, and lowers the postinjection reliability of the cable in all cases.
The iUPR process involves using low pressure (10-20 psi) to inject fluid through the cable strands. This low pressure allows the fluid
to flow through the existing splices without damaging them. Since low pressure is used, the injection process typically takes overnight for the rejuvenation fluid to reach the other end of the segment. After the injection equipment has been removed, the fluid permeates slowly into
the insulation, achieving full dielectric strength in about 18 months.
The SPR rejuvenation process uses moderate pressure to greatly accelerate the distribution of the rejuvenation fluid into the
insulation. The pressure used is well below the psi specified by cable manufacturers. Typical cable lengths only require about two hours to inject and the full dielectric strength of the cable segment is restored within seven days. The SPR injection process is used on clear runs and runs with blocked splices. When there is a blocked splice, the splice is excavated and the injection is performed in both directions from the splice toward the terminations before the splice is replaced and the pit is restored. The speed of the SPR method makes large injection
projects fast, effective and manageable for utilities like Empire.
Empire’s linemen were eager to assist in making the project successful. The rejuvenation went smoothly and very high
productivity rates were realized. What makes Empire a particularly interesting project, though, is that outstanding results were achieved in such challenging terrain. In one area near Branson, Mo. (where bedrock is a problem), nearly 100% of the cable segments were successfully injected, saving Empire the expensive replacement costs previously quoted for cable replacement. “We’ve been impressed with the results. Cable injection has saved us countless and countless failures,” said Robertson. “A side benefit was finding other issues we didn’t know about, such as leaking transformers.” These injection statistics have resulted in an increase in reliability for the utility, making Empire a believer in cable rejuvenation. “We have seen a significant increase in our system reliability since we began cable injecting. Right now we are seeing nine faults per year on uninjected cable versus 40 per year before we began injecting,” added Robertson.
By employing the hybrid injection approach, Empire was able to address reliability issues in challenging terrain for a fraction of the cost of cable replacement. After a complete cost analysis, Empire found it had saved more than 70% compared to the traditional method of addressing older cable—outright replacement. Customarily, most utilities realize a 50% or better cost savings when using cable rejuvenation versus replacement. The cost savings realized by Empire were higher because of the rocky terrain where the work was performed, not to mention the immeasurable benefit to the community by not having to disrupt transportation and daily life with dug up streets and sidewalks. Proof of Empire’s commitment in cable rejuvenation continued well beyond the initial work performed in 2011. In 2012, Empire continued to implement its cable rejuvenation process by treating over 39,000 feet of 1/0 and 4/0 bare cable. More than 25,000 feet of underground cable was being rejuvenated in 2014, and Empire plans to rejuvenate all of its aging underground cable over the next several years.
A regional sales manager for Novinium (novinium.com), Steve Hightower is a 15-year veteran of the utility industry, 10 of which have been spent in the cable rejuvenation business. Underground cable rejuvenation goes by several names, including chemical restoration, dielectric enhancement, cable treatment and silicone injection.