The reason the reliability of these cable systems is becoming a focused concern is due to the nature of the cable system failure profile over time. As shown in Figure 1, the expected failure of underground shielded power cable is likely to occur during the first five years due to either product defects or quality of installation. On the other hand, once the initial defects fail the system is likely to experience very few failures until the cable has been installed for at least 20 years. Around the 20-year point the age of the cable will begin to develop discreet defects at an increasing rate. The corresponding cable system failure curve vs. time is therefore shaped like a bathtub (Figure 1).
In most cases, the necessity to manage these assets proactively has been an omission in part due to the “out of sight, out of mind” nature of the asset. This is a concerning thought since many of the key assets being managed today are dependent on the connected cable to obtain the necessary electrical power to operate. The most sophisticated asset management process available will not perform effectively if key assets like power cable systems are omitted. There are two additional reasons why these critical assets have not been given the proper attention they deserve. The first reason is because of the “bathtub” curve in Figure 1. These assets, in most cases, have been very dependable through their years of operation. The second reason and subject of this article is that until now there has not been an available non-destructive means to effectively determine the actual condition of these cable systems.
The advancement of non-destructive power cable system diagnostic techniques, specifically, partial discharge defect specific diagnostics (DSD), empower asset managers and engineers to more effectively manage these key assets just as they are reaching a critical point in their life cycle. To date, there have been three options available:
1. Do nothing and react to failures as they occur.
2. Force failures by applying a destructive DC or AC withstand test.
3. Replace underground cable assets based on historical information (age).
Running the cable system to failure may be an acceptable approach while the cables are within the age range that places them at the bottom of the “bathtub” curve. However, as the assets age beyond that stage and outages began to occur more frequently this reactionary method of dealing with the asset becomes problematic on several fronts. Loss revenue, lower productivity, negative publicity and potential penalties in the utility segment are all potential negative results of this approach.
A common proactive approach has been to periodically apply a significant electrical stress to determine if the cable is capable of ‘withstanding” the event. The logic of this approach is to accelerate the growth of the worst potential defect to the point of failure under a controlled environment. Therefore, the failure happens during a scheduled outage vs. an unplanned event. This may not cause serious problems during the period when the age of the cable places it at the bottom of the “bathtub” curve and the number of cable defects is minimal. However, as the age of the cable increases and the cable system defects grow, this stressful “withstand” approach may begin to exasperate the development of multiple defects as depicted in the actual before and after results illustrated in Figure 2.
The other current practice has been to change out entire populations of aged cable with the belief if one 30-year-old cable is bad, then they must all be bad. This is a very expensive solution since evidence obtained utilizing non-destructive power frequency DSD indicates that this is entirely unnecessary. Data collected by applying the available DSD technique indicates that up to 85 percent of these aged cables are in excellent condition. Therefore the practice of blindly replacing cable based on age alone may end up costing three- to four-times more than necessary when you replace aged cable with 10-plus years of economic life remaining.
Partial discharge DSD enables asset managers and engineers to establish a solid baseline understanding of their valued cable system assets. This is encouraged at the time of commissioning of new installations in order to effectively enforce the installer warranty period or at the earliest possible opportunity to gain the necessary understanding of the current state of this important asset. Upon gaining the knowledge from performing this baseline condition assessment one is empowered to establish the necessary future asset plans.
This new DSD capability also provides a means of implementing a robust predictive diagnostic regimen to assure the future performance of your aging cable assets. This powerful asset management application empowers those responsible to proactively manage reliability at a significantly lower cost than arbitrarily replacing cable. The defect specific diagnostic approach provides actual defect location and severity information, so the option to perform inexpensive repairs vs. costly replacement can be considered. Remember, up to 85 percent of these cables are still in good operating condition and may not require any action to enhance their reliability. Therefore, the investment in having your cable assets diagnosed will actually provide you with the information to both improve your reliability and save a tremendous amount of money.
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