High-voltage electrical emerging as a top interest area among gas-turbine users

Until relatively recently, the technical programs at user-group meetings focused almost exclusively on gas-turbine problems and solutions. But as issues were resolved and items crossed off punch lists, the reliability and availability of the basic engine improved to the point where other plant equipment was at least as likely to cause an outage—more so in some cases.

New topics appeared on meeting agendas. The CTOTF™ was the first user group to expand beyond the engine; its willingness to arrange sessions in parallel to cover the broad information needs of supervisory and management personnel made this possible. Under Former Chairman Bob Kirn’s leadership the program expanded to include environmental, NERC/FERC, and high-voltage-equipment roundtables. Others evidently believed the idea a good one.

The recent meeting of the 7F Users Group (Denver, May 11-15, 2015) is a case in point. It featured presentations on isolated-phase bus (IPB) duct, transformers, heat-recovery steam generator (HRSG) issues, catalysts for control of NOx and CO emissions—in addition to everything you wanted to know about the world’s most popular 60-Hz F-class engine.

There’s more: The ambitious 7F Users Group steering committee formed an umbrella organization, Power Users Inc, to coordinate programs for the 7F, Steam Turbine, and Combined Cycle Users Groups. STUG and CCUG met concurrently at the end of August with crossover allowed between meetings. There also was a robust HRSG workshop. Important: While 7F conferences focus solely on that GE frame, the STUG, CCUG, and HRSG programs cover equipment made by all manufacturers.

Next year, the recently formed Generator Users Group, which meets for the first time in early November, will join 7F, STUG, and CCUG under the umbrella. This will allow the 7F users to eliminate some non-engine topics from their program and provide more in-depth guidance on timely subjects—such as how to extend operating cycles, start/ramp faster, address ever-more-demanding grid requirements, etc. It’s no secret that operational flexibility and reliability are critical to profitability.

Isophase bus duct (IPBD) rarely has been a presentation or discussion topic at any user-group meeting—until recently. Evidently, its location on the far side of the generator made it easy for some personnel to ignore. But the failure to properly inspect and maintain IPBD regularly can lead to problems no plant manager wants. Gary Whitehead of Electrical Builders Inc. (EBI) brought this into sharp focus at the 7F meeting in Denver.

He began his presentation with the basics. IPB systems transfer the current from the generator to the generator step-up transformer (Fig 1). Also, auxiliary bus is used to tap off of the main bus to feed current to auxiliary equipment—including station service auxiliary transformers, exciter transformers, SCR auxiliary transformers, and metering equipment (surge and PT cubicles).

IPBD Fig 1

On large IPB systems, the speaker continued, chillers typically are used to blow cool, dry air through the bus duct to cool the conductors (Fig 2). This allows the bus to carry more current than one where the conductors are not convection cooled.

IPBD Fig 2

Whitehead told the group there are two basic types of IPB designs—continuous and non-continuous. Virtually all attendees had the former, in which the enclosures are electrically continuous throughout the bus system and bonded to each other at the bus ends. With this design, the enclosure for each phase absorbs nearly all the induced currents of that phase. The bonding at the ends of the bus allows the induced currents to cancel out each other. The system then is grounded at one end only. This design shields the majority of induced currents from the support steel.

IPB failures, Whitehead continued, generally are caused by (1) a failure in connected piece of equipment—the transformer, for example, (2) inadequate design or improper installation, and last, but certainly not least, (3) by improper inspection and maintenance. Regarding the first, the speaker said, the root cause may be difficult to determine; it might be in the connected equipment or in the IPB system itself.

The three most common failure causes attributed to inadequate design are limited maintenance access, enclosure expansion joints using cables for enclosure grounding, and single-insulator bus designs where the insulator supports the conductor in suspension. The time to deal with each of these is at the specification stage and to recognize that the cost of enhancements to a basic design is minimal in a lifecycle cost assessment. Example: If no access points (such as a removable IPB cover) are provided for proper maintenance of internals, how can you expect to operate problem-free?

Whitehead stressed the need for access on both sides of seal-off bushings. Dirty insulators, or seal-off bushings cause tracking, which eventually leads to failure. Another thing he recommended “designing out” at the specification stage: Use of sliding covers as an expansion joint with multiple cables for enclosure grounding. Here’s the failure scenario: The cable with the best connection has the lowest resistance and will carry more of the ground current than others. This cable will overheat and fail, eventually. That current then is transferred to the cable with the next best connection. It, too, will overheat and eventually fail. Finally, as more cables fail, the overheating and failures accelerate, compromising the grounding system.

IPBD Fig 3, 4

IPB designs using insulators in tension should be avoided as well. The vibration inherent in all generation equipment will degrade ceramic inserts of bus insulators over time, Whitehead said (Fig 3). Failure of one insulator insert puts additional stress on the remaining insulators. If two adjacent insulators fail, the conductor will fall to the bottom of the enclosure.

As you might expect, the reasons for improper inspection and maintenance are independent of the equipment or system of concern. They include the absence of regularly scheduled inspection and maintenance, improper procedures, and the use of untrained electricians and/or inexperienced contractors. Things you should look for are cracking and failure of shunts (Fig 4), peeling or discolored paint from overheating (Fig 5) confirmed by infrared inspection (Fig 6). Whitehead explained that it was important to check for cracks in expansion joints because they allow moisture ingress and severe corrosion can occur when excessive ground currents are present (Fig 7).

IPBD Fig 5, 6

Safety was a recurring message during the presentation. Make a mistake around live bus duct and you might not get a second chance. What seemed to concern the speaker most were jobs involving non-electrical personnel unfamiliar with IPB. One example he gave was the possible need for HVAC technicians when the plant was running in cases where cool air must be forced through the bus duct to control temperature. He urged careful supervision by plant staff.

In closing, Whitehead suggested attendees adopt EBI’s full-spectrum five-point inspection and maintenance cycle to keep their IPB in good condition. It consists of the following: online EMI assessment of the bus duct to provide insight into impending issues, rigorous offline inspection, thorough cleaning (EBI recommends environmentally friendly cryogenic cleaning for the best results), repair/refurbish/replace components as necessary, and final verification and testing.

IPBD Fig 7

Some of the points made by Whitehead mirrored those of NAES Chief Engineer Bill Lovejoy at a recent CTOTF CT-Tech™ workshop. The latter recommended annual inspections of isophase duct and used an engineering drawing and photos to show attendees where to look for problems. Lovejoy said expansion joints are prone to cracking on cycling units because they typically are designed only for 1000 cycles. It doesn’t take long to get there on a daily-start unit. Also, he urged users to be sure drains are clear because water will accumulate over time and freeze in winter. Lovejoy noted that if you inadvertently ground isophase bus in a second spot you’ll burn off the paint. He suggested infrared thermography scans under load to identify second grounds.

Transformer condition assessment was the second high-voltage topic on the 2015 7F Users Group program. VP Paul Griffin of Doble Engineering Co presented on “Asset Health Review of Transformer Fleets Monitoring, Assessment, and Visualization.” A frequent speaker at user group meetings over the last several years, he has helped to raise awareness at the plant level regarding the need for proper monitoring and inspection of this critical piece of equipment. Keep in mind that loss of a plant’s main transformer could put the facility out of service for a year or more.

Griffin built his presentation around answers to the question: Why asset health review? The foundation for a health analysis is Doble’s data on a large population of transformers. It enables the company’s experts to gauge, with a high degree of accuracy, the condition of any transformer it inspects. Results of a health review include the following:

      • Provides a ranking from normal to “requires immediate attention.”

      • Finds problems in the early stages to manage them.

      • Identifies issues that require further study to better understand risks, plan solutions, and actions required.

      • Identifies patterns consistent with a specific population of transformers.

The asset health review may involve a thorough investigation by experts based on available data, deployment of testing, and review to identify problems and their seriousness. Griffin next discussed the “report card” Doble uses to present its overall condition assessment based on the health review.

Griffin’s presentation was comprehensive, encompassing more than 40 slides. He dug into the details of how to conduct an inspection to identify issues such as partial discharge, the importance of regular oil testing and what tests to run, dissolved gas analysis (DGA) and how to trend and convert data into information of value, etc. Registered users can access the presentation at www.powerusers.org in the forum section of the website. Registration is simple if you do not already have access.

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