Outage Management: Critical to Factor in the Bus Duct

By Cal Crader, PE

Properly planning and executing a maintenance plan during a planned outage is a critical practice in avoiding unnecessary downtime in a power plant. Although each individual plant has their own approach to scheduling outages, it is typical for plants to have a major outage occur once every two to four years. During these major outages, most plants will upgrade and/or replace major equipment. As the turbine generators and the boiler are usually top of mind when it comes to maintenance, outages tend to be scheduled around those particular pieces of equipment. However, understanding the amount of time and money that is required to shut down and entire plant for a given period of time, it is important to take full advantage of the outage.

In recent years, plants have leaned in the direction of having fewer, albeit longer outages to get critical repairs and upgrades completed. However, there are still smaller outages at these same plants generally in years where major outages do not occur. These smaller outages are just as essential to take advantage of. Why? Because the smaller outages provide an ideal opportunity to assess what equipment needs to be upgraded and/or repaired, and how much time will be needed to accomplish everything during the upcoming longer outage. During this shorter outage, managers need to make the decisions about what will and will not be part of their plans for the prolonged, major outage in the following year. At this time, it is critical to assess your bus duct system, whether that be the isolated phase bus or a non-segregated bus. Far too often, these crucial systems are not given the attention they require, leading to costly, untimely and most importantly, preventable power plant shutdowns.

Operators have the impression that because there are not as many moving parts as something such as a turbine generator, the bus duct system does not require the same amount of attention. This common misconception will lead decision-makers to overlook this system when planning upgrades/repairs during a major planned outage. Unfortunately, this misconception can ultimately turn out to be a very costly one. If a bus duct system fails at an unexpected time, the plant will need to shut down for emergency repairs. Due to time requirements, these emergency repairs can cost exponentially more than a preventative maintenance strategy would have. Couple this with all the lost revenue the plant experiences while being down for an extended period of time, and it becomes increasingly clear that it is in an operator’s best interest to give the bus duct system the attention it deserves.

ISOLATED PHASE BUS

Most plants either have an isolated phase bus or a non-segregated bus duct system. For the isolated phase bus, also known as the isophase bus, failure can be caused for a number of different reasons. Poor insulation, dirt, condensation and water intrusion can all be contributing factors as to why the isophase experiences complications. However, one of the biggest causes of isophase failure is localized overheating. Unfortunately, not only is this a common cause, but it also makes the repairs even more taxing, because crews must work in this extreme heat. Seeking assistance from a trained professional during a small, planned outage to assess the isophase’s condition and ensure that these potential pitfalls are documented and planned for is critical.

During the smaller, planned outage, a number of different areas should be checked in the isophase, starting with the bolted joints. The hardware should be correct, and the proper amount of torque should be present. Additionally, the plating condition, flexible connectors and the surface integrity needs to be documented.

Insulators are another area where small defects can lead to large problems. Any kind of damage needs to be observed (cracks, chips) and the amount of dirt build up should be assessed. Proper seals should be observed with the brushings and the hardware gaskets all need to be checked to see if they are functioning correctly. Another area that can lead to problems is the filter drain. Water build-up can be problematic.

All the bolted covers need to be removed so debris can be removed from the termination enclosures and all the insulators should be cleaned. If any equipment or hardware needs to be completely replaced, by having an assessment, operators and preventative maintenance experts can better plan the major outage. For everything to transpire effectively and efficiently, ordering required equipment and parts needs to be done ahead of the major outage so everything is on site when the plant is shut down and work needs to get done. Therefore, this is why it is so important to take advantage of the smaller outage and assess the challenge that lies ahead.

THE NON-SEGREGATED BUS

If a plant does not have an isophase bus duct, chances are there is a non-segregated (also known as non-seg) bus duct system present. Much like the isolated phase bus, the non-seg bus should have preventative maintenance done to it to keep it in working condition. Also, just like the isophase, assessing what damage and upgrades need to be taken care of should be done during the smaller outage.

Unique to the non-seg bus duct system, assessing the insulation on the bus bars is essential. As plants are continually uprating for maximum output, medium voltage bus bars also should be addressed to ensure they can take on this additional capacity. When plants fail to do this, failures are common. As bus bars experience overloads, the deterioration of their insulation is accelerated. Many maintenance personnel are observing that this added capacity requirement is leaving bus bars particularly vulnerable. Adding to the challenge is the type of material that most bus bars are insulated with. For a significant stretch of time between the years of 1960 and 1980 the most common material used was a polyphenylene oxide (PPO). It makes sense that this material was so popular; it was cost effective and did the job well. However, this material has a lifespan, and much of it has become very brittle over time. Replacing this PPO insulation on bus bars is essential.

There are new techniques recently developed to address this deteriorating insulation. A shrinkable tubing is now available made from a non-halogen based polymer that has been designed to withstand high voltage. Maintenance managers should have a qualified expert check the insulation on the bus bars to assess their condition. If the insulation has not been checked in years, there is a good chance that re-insulation is required and should be performed during a scheduled outage.

The bus duct system is a vital piece of the puzzle for a power plant. Preventative maintenance on the system is essential as emergency repairs can cost up to five times as much.