With the 2023 Generator Users Group (GUG) approaching August 28-31 in Atlanta, part of the greater Power Users Combined Conference, reviewing some of the content from last year’s meeting should encourage you to attend or send someone from your organization to experience this valuable content in person. Presentation abstracts below are based on information available only to end users in the slide decks posted at www.powerusers.org. Those seeking deeper dives into specific topics should note the presentation titles in italics at the end of each summary and access the source material on the website.
Meaningful stats help guide improvement in plant performance
Equipment quality and personnel safety are top of mind at user-group meetings. The first presentation after lunch on Day One is valuable for the guidance it offers regarding generator components and processes to focus on at your plant to maintain top performance. The owner/operator’s database of regulated and unregulated assets is extensive: 227 non-hydro generators representing 48 different models from nine OEMs.
Stator windings are at the top of the “failing equipment” category with issues affecting 20% of the total fleet. Stator cooling water was next at 13%, followed by isophase bus/flex links and the rotor, each 10%. Brush rigging came in at 7%, seal-oil system at 6%, and AVR, exciter, and stator core each at 5%.
Process shortcomings were most evident in vendor execution, 24%; PM scope/inadequate frequency, 22%; and inadequate process/procedure, 14%. Installation error was next at 10%, with OEM design deficiency right behind at 8%.
Causes of poor quality identified included loss of vendor experience, substandard planning, poor procedures, complacency, lack of training, etc. Human performance tools that could have prevented many of the issues were better procedures and adherence to them, more positive attitude, when to stop work when problems arise, pre-job briefing that includes the consequences of incorrect actions.
The presentation can help you develop a better vendor qualification process, improve your procurement specifications, provide more effective vendor oversight, and improve vendor accountability.
Solutions for condensation in metal-enclosed bus ducts
“Metal-enclosed bus ducts should be part of your regular preventive maintenance program,” began Mohsen Tarassoly, EBI (Electrical Builders Inc), “but half of your issues will stem from condensation or water intrusion.” Designs for bus enclosures haven’t changed since the 1990s, but welded and bolted designs are more susceptible to moisture. None of the designs account for the frequent cycling these busses are exposed to, aggravating the situation.
The heart of the presentation focused on solutions, including drain plugs, isolated phase bus (IPB) insulator heaters, strip heaters, dry air systems, forced-air cooling systems, gel packs, and dessicants. Some of these impose their own maintenance concerns, he cautioned. During the Q&A, one audience member suggested that you turn on your forced-air system for several hours before starting the unit, especially in humid climates like Florida.
Hydrogen, the preferred cooling medium for large generators, demands respect
Hydrogen must be purged from generators prior to entry or exposure to air to prevent fire or explosion, which can occur with minimal ignition energy. Recall that the lower explosive limit for hydrogen in air is 4%. The upper explosive limit is 76%.
For maintenance, the inert gas of choice for purging hydrogen is CO₂, followed by an air purge. Once maintenance is complete another purge is needed to remove the air prior to recharging with H₂. In emergencies—such as fire, loss of shaft seal, low H₂ purity (less than 92%)—the generator is flooded with CO₂.
Andrew Slaugh, senior applications engineer for Airgas, told attendees that the traditional method of using CO₂ from cylinders for purging can be challenging. For example, it can take several people to monitor and maintain the purge flow: There can be interruptions in purge flow because of dry-ice formation, empty cylinders, change-out of cylinders, etc. Plus, it can take many hours to complete the purge.
An alternative is Airgas’ TurbinAL purge system which is said to provide uninterrupted gas flow at all times with no operator actions and with no power required for purging—as well as other benefits.
Presentation is valuable for its review of the fundamentals of safe hydrogen purging. It is recommended reading for anyone involved with hydrogen filling and removal.
Troubleshooting with EMSA
Cutsforth’s presentation offers some good observations on approaches to effective troubleshooting. The case study profiled, in progress, using Electromagnetic Signature Analysis, revealed increased activity in the 30 Hz to 100 MHz band, which typically indicates an issue with the isophase bus duct. However, the problem could be external to the bus duct—such as with the GSU or another non-generation-related source.
The continuously elevated EMSA level indicates that the source is not cyclical in nature—such as plant lighting. Next step likely would be to get the utility’s generator engineering group involved in the analysis. Stay tuned.