When power plant operators consider implementing a preventative maintenance program in order to keep their equipment up and running reliably, far too often the non-segregated bus duct is overlooked. The system does not have any moving parts. For this reason, most do not provide the system with necessary maintenance during routine outages. Major equipment is always top of mind for operators (as it should be), but the non-segregated bus duct system is just as critical to the plant’s overall performance and needs just as much attention. There are documented incidents where this kind of neglect has led to very untimely and costly plant shutdowns. And as all plant operators know, it is far less costly to apply proactive preventative maintenance on a power plant during a scheduled outage than it is to conduct a reactive emergency repair.
When a non-segregated bus duct system fails, there is typically little or no redundancy. This means that there is no “quick fix.” The plant operations will essentially have to wait for the problem to be identified and corrected. Emergency repairs tend to be very costly. Historically, emergency repairs on a non-segregated bus duct system can cost plants up to five times as much as preventative maintenance on the system would. This figure takes into account the lost revenue experienced by the plant due to the unplanned outage and expedited materials.
With plants continuing to uprate for higher output, they will often fail to assess their medium-voltage bus bars for the additional capacity. This lack of attention to the bus bars has been leading to more and more instances of failure in the industry. With accelerated deterioration of the insulation on the bus bars from overloads, one could argue that the non-segregated bus duct system is in a more vulnerable position now than it has ever been.
The insulation condition is not something to be taken lightly. It is often one of the most common causes for non-segregated bus duct system failure in the industry. This is for a variety of reasons. The biggest is that this insulation will generally not be assessed on a regular basis. It is often one of those situations where OEM instructions are overlooked or ignored. This has been going on for the last three or four decades. Maintenance on the insulation only becomes top of mind when it is too late, and the entire system fails. Moreover, the material that has been most commonly applied to bus bars for the purpose of insulation is some kind of polyphenylene oxide (PPO). Between 1960 and 1980, PPO insulation was prevalent among the majority of nuclear power plants. It was an accepted practice because it was cost-effective and it performed well. However, what was not anticipated was the limited lifespan of this material. We now know that PPO insulation has been documented to break, crack, and become very brittle over time. We also know that if not addressed, this deterioration of insulation can lead to complete failure of the bus duct system. This, in turn, can cause an entire plant to be shut down to make the necessary repairs. There have been many documented incidents where this lack of insulation integrity has led to dangerous situations and costly plant shutdowns.
The question becomes, “What is the process to ensure that the insulation remains effective on a particular plant’s bus bars?”
First and foremost, awareness is the most important piece of the puzzle. This is an issue that many plant operators have simply not paid particularly close attention to until relatively recently. Under most circumstances, there is no preventative maintenance or inspections made on bus systems during planned outages. Furthermore, insulation inspection on certain bus bars has not been performed in years. Step one is making sure that people understand that this is an issue that needs to be addressed before it leads to a complete system failure. Once an operator knows that they may have a problem on their hands, what is the next step? What kind of processes are available to upgrade the insulation during a scheduled outage while other preventative maintenance tasks are being performed? Luckily, there is a state-of-the-art process available to power plants that solves this problem effectively and efficiently.
Above all else, it is critical to have a qualified expert assess the current condition of the non-segregated bus system. This should be performed during a scheduled outage to take full advantage of the planned downtime. Some key visual indicators of a problem are arc tracking, discoloration, cracked or noticeably damaged insulation, and moisture/debris buildup within the enclosure. This is also an opportunity to check and see if anything else needs to be upgraded or replaced in the non-segregated bus duct system. This includes assessing the cover gaskets and hardware, boots, and support insulators and determining whether those should be replaced. After all the inspections have taken place, a plan of action should be put together so the work can be completed in the allotted time provided by the planned outage.
To properly upgrade the insulation, the bus bars should be removed and the old insulation needs to be stripped and replaced with a more modern alternative. There is shrinkable tubing now available made from a modern, non-halogen based polymer designed specifically for high voltage environments. The material’s strength, durability, and ability to age well have caught the eye of the preventative maintenance community. It is widely regarded as a definitive upgrade to the old PPO style insulation. It is critical to consult with a preventative maintenance company that provides this kind of insulation upgrade, as not all do.
Once the re-insulation is complete, the next step is to reinstall the bus bars with new connecting hardware. The insulating boots should be hand cleaned and testing should be performed on the connection points. Then, as necessary, it is important to replace the cover gaskets and support blocks to avoid unforeseen complications.
This kind of routine preventative maintenance performed per OEM instructions can increase the reliability of the bus duct system. Additionally, failure to insulate properly can result in fires, plant transients, reactor trips, and other safety concerns. Taking this into consideration and addressing the potential issues during planned outages can be the difference between a plant facing an unexpected shutdown, costing a plant significant revenue.