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Non-segregated bus maintenance

Energy Tech Magazine

Routine maintenance of a non-segregated bus duct system is critical to the overall health and functionality of a power generating station. Without a consistent preventative maintenance program in place, power generating stations are subject to an increased risk of failure, leading to costly, unplanned outages. Unfortunately, far too often, operators do not give this element of their system the proper attention. It does not have visible moving parts as a turbine generator does, leading some operators to incorrectly put it out of their mind. Typically, absolutely no preventative or routine maintenance or inspections are performed on bus systems. However, maintenance and proper care of a station’s non-segregated bus system is just as important as any other element, and its failure can become an overwhelming problem. This is mostly due to the fact that there is no redundant backup. Additionally, plants will often uprate for a higher output, but ultimately fail to uprate their medium-voltage bus bars. This makes the bus system an even more critical component to care for. Emergency repairs on the bus duct can cost more than five times as much as a preventative maintenance program. This should be more than enough incentive for a proper program to be in place. All power generating stations have planned outages on a schedule, giving operators an opportunity to upgrade their non-segregated bus duct; they just need to take advantage of the opportunity.

One of the most common issues with maintaining a non-segregated bus duct system is the degrading and eroding of its insulation. More often than not, this insulation does not get upgraded or replaced in a timely fashion per OEM recommendations, leading to a variety of problems. For the past 30 to 40 years, it has been commonplace in the non-segregated bus duct to use polyphenylene oxide (PPO) insulation. It was an effective material and became the norm for decades. However, due to its characteristics, this PPO type of insulation has a documented history of becoming brittle. It cracks and ultimately degrades over time. There have been a disproportionately high number of bus duct failures document as a result of this insulation degradation.

So what can be done?

There are processes available that can adequately essentially re-insulate the bus bars during a scheduled outage. The first step is to have a qualified expert assess the current situation a non-segregated bus system is in. This can be done during a scheduled outage with a visual inspection. The expert should look for arc tracking and other indications that the insulation needs to be addressed. Additionally, he or she will take note of cracked or damaged insulation and the moisture and/or debris buildup inside the enclosure. These are all signs for insulation that needs to be replaced in the non-segregated bus system. Finally, the cover gaskets and cover hardware should be inspected to determine if they need to be replaced as well.

After the visual inspection has concluded, a proper plan of action must be developed. In this plan, areas should be outlined that are determined to need insulation replacement. The old bus bar insulation will need to be removed and replaced with tubing made from a modern, non-halogen based polymer designed for high voltage environments. This kind of material has become more and more popular as opposed to classic PPO insulation due to its strength, durability and aging characteristics.

After the process is implemented to remove and replace the insulation on the bus bars, the bolting hardware and the boots should also be replaced. Finally, the cover gaskets and hardware need to be addressed, if it is determined that these elements need to be replaced during the inspection phase.

As previously stated, all of these repairs and upgrades should be scheduled during a planned outage. While maintenance on other parts of the power generating station is taking place, care and attention should be given to the non-segregated bus duct system. Otherwise, very costly emergency repairs may need to take place. Failure to insulate properly can result in bus bar faults, fires, plant transients, reactor trips and numerous other safety concerns. There are many documented incidents from all across the United States where insulation maintenance like that discussed in this article was ignored. Many were caused by cracked and damaged PPO insulation. This led to water and debris accumulated on the bus bar which led to tracking and ultimately, failure. This could have all been avoided had the plant operators paid proper attention to the non-segregated bus duct system during a routine inspection during the plant’s most recent planned outage.

The most important takeaway is that preventative maintenance performed per OEM instruction can prevent nearly all faults in bus systems, and the most common culprit for this failure is ignored, faulty insulation.

About the author

Randy Jansen is the president of SE Energy, LLC, a nationwide specialty electrical construction, engineering and consulting firm serving clients in the utility and power generation, transmission and distribution, and heavy industrial markets. SE Energy specializes in the engineering, consulting, installation, repair, and maintenance of distributed generation, modular substations, battery/energy storage, protective relay systems, medium voltage switchgear and bus, isolated phase bus duct, non-segregated bus duct, control system upgrades, excitation Systems, 24/7 emergency response and disaster recovery. For more information, visit: www.se-energy.com.