At annual meetings of the Generator Users Group (GUG), presentations by consultants and vendors, and participation by engineers from these companies in discussion sessions, are critical to the all-volunteer organization’s success. GUG activities are supported by the electric-power industry’s three major OEMs—GE/Alstom, Mitsubishi Hitachi Power Systems Americas (via MD&A), and Siemens Energy—and several third-party equipment and services providers. The latter group includes AGTServices, Doble Engineering, Environment One Corp, Ethos Energy, IRIS Power, National Electric Coil, Schweitzer Engineering, and Turbine Generator Maintenance (TGM).
Four consultant/vendor presentations at the 2016 meeting in San Antonio, August 22-25, are profiled below; the remainder will appear in the next issue of CCJ ONsite. The links provided enable quick access to the topics of greatest interest to you.
Electromagnetic interference (EMI) testing, Paul Spracklen, Doble Engineering.
Hydrogen safety, Steve Kilmartin, Environment One Corp.
Damaged steel: Mechanisms and symptoms, Neil Kilpatrick, GenMet LLC.
Field rewinds, stator-bar insulation diagnosis, and high-speed balancing, Keith Collins and Keith Campbell, Mechanical Dynamics & Analysis.
Users wanting to dig deeper into these areas can access the presentations on the Power Users website. You must be registered to participate in the forum, a relatively simple process if you’re not already signed up.
Condition-based maintenance (CBM) is an important goal in the power generation business. The focus is on preventing in-service failures by maintaining equipment only when needed and identifying where maintenance is not necessary. CBM conserves resources, reduces production costs, and minimizes the possibility of damage during maintenance (such as that caused by a rotor drop).
Electromagnetic interference (EMI) is a powerful tool for condition-based maintenance and is useful in diagnostics of both electrical and mechanical problems in the generator system. It has been used for 80 years to locate defects in power lines that cause radio and television interference. Application to powerplant equipment began in 1980.
EMI signals are collected with a split-core radio-frequency current transformer (RFCT) and radiated energy is measured with a simple hand-held instrument. These two techniques permit detailed condition and location identification. Maintenance recommendations can be given with the first test. Trending of numerous tests is not necessary to analyze data but may be helpful for long-term analysis.
There is no interference with plant operations while taking the EMI readings, which are passive and non-invasive. There is no applied signal and no risk whatsoever to equipment operation. The frequency spectrum is taken with an RFCT, typically applied to the generator neutral or a grounding cable, and the output signature can be analyzed on the screen of your personal computer.
The hand-held instrument measures radiated EMI and is simple to understand and use. In Fig 1, a transformer is scanned for radiated EMI. Switchgear typically can be scanned in a few moments (Fig 2). This technique can detect and identify the cubicle where there is deteriorated insulation or loose connections.
Using the RFCT approach, each system defect results in a distinctive radio-frequency spectrum unique to the physical location and type of defect present within that electrical insulation system. More than five-dozen conditions have been identified with this test. Comparison of data collected at two generator loads can determine if loose windings are developing. Substantial basic training is required to interpret the RFCT output curve, and backup interpretation can be obtained from Doble Engineering when interpretation results are uncertain.
Doble’s Paul Spracklen is a rotating-machinery systems expert
Hydrogen is very explosive, colorless, and odorless, as well as difficult to contain. Yet it has been used widely as a coolant for generators since 1938 (today there are over 10,000 hydrogen-cooled generators in service). Hydrogen is used for several reasons:
Windage/frictional losses are less than for air.
The relative density of hydrogen is four times less than that of air and its heat-transfer characteristics are better.
It is 14 times more efficient in removing heat than air.
Compounding the inherent dangers in handling hydrogen, the generator fleet is growing older, auxiliary hydrogen equipment is ageing, outage intervals are increasing, the workforce is getting younger and leaner, and training programs are not what they used to be. Thus the dangers of using hydrogen as a coolant may be increasing.
A recent newspaper headline stated: “Deadly explosion at ABS powerplant blamed on hydrogen gas.” Damage caused by recent explosions is shown in Figs 1 and 2. There was a death while unloading hydrogen at the plant in Fig 1. The explosion at the Fig 2 plant occurred because of inadequate hydrogen purging procedures. While there were no deaths at that facility, damage was significant.
It is essential for plant personnel to know and understand the hazards associated with hydrogen and that all equipment for handling and storing this gas be certified and maintained in first-class condition (Figs 3, 4). Finally, because purging is inherently complicated, and can be dangerous if performed improperly, all personnel involved must be well trained, non-sparking tools (bronze) must be used, carbon dioxide must be readily available in sufficient quantity, appropriate safety signage must be in evidence in critical areas, and keyed lock-outs must be provided for “air” and “hydrogen.”
E/One’s Steve Kilmartin has more than 30 years of generator experience—including time at an OEM and a major engineering company
Damaged steel: Mechanisms and symptoms
Neil Kilpatrick’s presentation was an hour-long lecture/discussion tutorial covering the following topics:
General machine construction.
Damage mechanisms in steel, for non-metallurgists.
Where different damage mechanisms sometimes are identified.
Observable symptoms that might be present, with comments on symptom severity.
This tutorial focused on the generator rotor. Topics included, among others: damper current damage, electrical joint failure, fretting, deposition of decomposition products on visible rotor surfaces, overheating of retaining rings and other forgings, and stationary/rotating rub damage. Material presented on the last topic is summarized below to offer perspective on the depth of coverage and the value of participation in GUG meetings.
Stationary rotating rub failure sequence (refer to the diagram):
1. Contact is established and maintained. Frictional heating occurs over the contact surface and heat flows into both contact elements.
2. A heat-source zone is established. The heat-input plane is the contact area at the interface.
3. Heat flows inward and along the surface.
4. The temperature rise depends on the amount of energy input and the time rate of input.
5. As temperature builds in the hot zone, the metal tries to expand, but the cold surrounding metal is much stronger and more stable and compressional yielding occurs. Increasing hot-zone peak temperature means more compressional yielding; as temperature increases, expansion increases, and strength drops.
6. This kind of rub can result in local metal temperatures in excess of 1300F, with metallurgical transformation to austenite.
7. Some hot metal will be “smeared” by adhesive interaction.
8. When rubbing stops, the hot zone effectively is quenched to the surrounding metal temperature. In typical magnetic rotor steel components, this means that a hardening transformation occurs. But, at the same time, a significant contraction of the former hot zone occurs, and the stress state of transformed metal zone will change to what can be a very high tensile stress.
9. The result is a zone of metal with high tensile stress, additive to normal operating stress, and with a ductility and toughness which tends to be very poor.
10. Intensity of damage tends to correlate with the local volume of damaged metal; high volume relates to more severe damage with increased cracking tendencies.
11. Crack initiation and propagation cannot be predicted, but, clearly, the probability of cracking must be significant.
12. This condition means that the part (rotor forging, blower hub, blower blade, etc) is now capable of erratic and unpredictable behavior.
Unless this is a superficial condition, repair/replacement likely will be required.
Metallurgical problems are widespread on generators and additional topics include these: braze-joint failure processes, torsional fatigue symptoms and analysis, general rotor overheating symptoms and analysis, coupling-bolt failure modes and analysis, and rub-induced bending analysis and repair.
Neil Kilpatrick recently opened his own shop—GenMet LLC—after accumulating more than 45 years of generator metallurgy experience at Westinghouse Electric Corp and Siemens Energy
Field rewinds, stator-bar insulation diagnosis, high-speed balance
This presentation was divided into three segments, with Keith Collins covering field rewinds and high-speed balancing, and Keith Campbell stator insulation. Collins opened the session with a general summary of cooling methods for field windings. But the focus of his presentation was on the merits of reusing copper versus rewinding with new copper (Fig 1).
The steps for a rewind with existing copper are the following:
Visual inspection of the copper.
Nondestructive removal and cleaning of copper fit for reuse.
Repairs, if any.
Re-annealing and final inspection.
For a new-copper rewind, the steps discussed were these:
Procure new copper and verify it meets specs, including shape.
Remove the old winding—destructively if necessary.
Install the new winding.
The pros and cons of using new copper versus old copper were discussed in detail, with excellent photography illustrating best practices, lessons learned, etc. Much can be learned by accessing the presentation on the user group’s website.
Key takeaways based on MD&A’s experience:
New copper isn’t required for a field rewind; nearly all damage to existing copper can be repaired by splicing and/or brazing.
If damage is so bad that the use of new copper is suggested, there probably is a bigger problem at hand—such as forging damage.
Reverse-engineer coils during rewinds to gather data for future use.
If new copper is the path taken, be sure it is procured long before the scheduled outage.
Stator insulation. Campbell took over the speaking duties from Collins and listed these five factors as contributors to insulation degradation: time, thermal, mechanical, electrical, and the introduction of contaminants. Various aspects of stator-bar groundwall insulation degradation were considered and illustrated—including mechanical vibration (Fig 2) and electrical phenomena such as partial discharge and vibration sparking.
High-speed balance. Collins returned to the front of the room and began his second presentation with a backgrounder on the evolution of balance equipment. He recommended high-speed balancing of generator rotors following a rewind with new or existing copper, after the replacement of a major component, and after any machining. Remainder of the presentation offered details on MD&A’s balance facility in St. Louis (Fig 3), which can handle rotors up to about 90 tons, 13 ft in diameter, and 49 ft long. Plus, is has full high-speed thermal test capability to accommodate electrical testing of the rotor at speed.
Information disseminated at the meeting showed only seven high-speed balance facilities in the country in addition to MD&A’s, with most in the East—Schenectady, NY; Richmond, Va; Pooler, Ga; Columbus, Ohio; Charlotte, NC. The other locations: West Allis, Wisc, and Farmington, NM.
MD&A’s Keith Collins is operations manager of the high-speed balance facility;
Keith Campbell is a generator specialist