Generator Users Group Week Two

Ryan LeClair, generator product-line manager, Gas Power Services, opened the GE Day program for the Generator Users Group (GUG) with an overview of the topics to be discussed, including: negative-sequence-current impacts and causes, fleet updates and new observations, 7FH2/B family and 2020 stator update, amortisseur spring axial migration cases and retention solutions, hi-pot testing, and third-harmonic stator ground. Highlights of the presentations made November 19 during the second week of the 2020 virtual meeting follow. To dig deeper, access the presentations on GE’s MyDashboard.website at https://mydashboard.gepower.com.

Negative-sequence-current impacts and causes

Janusz Bialik, principal engineer, and Ben Mancuso, generator design technical leader, Product Service, began by outlining their presentation: Current induction theory, overview of damper systems, case studies, and recommendations.

Negative sequence currents are a common and destructive phenomenon in generators— generally preventable by good design and operation, and the use of monitoring equipment. They can be caused by unbalanced three-phase currents, unbalanced loads, system faults, open phases, asynchronous operation, etc.

Recall that the phase currents and voltages in a three-phase power system can be represented by three single-phase components: Positive-, negative-, and zero-sequence. The first has the same rotation as the power system and represents balanced load. The second has a rotation opposite that of the power system; the zero-sequence component represents an unbalance that causes current flow in the neutral.

The negative-sequence current component circulating in the stator windings creates a magnetic flux in the air gap of the machine. This flux rotates at synchronous speed, but because its direction is opposite to the normal flux, eddy currents are induced in the rotor body. They tend to flow mainly in the outer regions of the rotor, because of the “skin effect,” which was explained by Bialik.

If large enough, the induced currents will spark and arc between wedges, wedges and forging, wedges and retaining rings, forging and retaining rings, and any component on the periphery of the rotor. Such sparking/arching can cause hardening of the metal in critical areas, followed by cracking.

Thus, negative-sequence currents have the potential to cause rapid heating and significant damage to the generator, dictating the need to protect against them. Loss of mechanical integrity or insulation failure can occur quickly.

Generator designers include many features to mitigate the effects of negative-sequence currents, Bialik told attendees. Examples include assuring good electrical contact between rotor structures to prevent arcing, a damper system (a/k/a amortisseur windings) in the rotor slots to form low-resistance paths across the rotor surface, aluminum slot wedges, etc. Use of a damper bar in the pole zone of the machine and a low-resistance endwinding design are illustrated in the presentation.

Generator fleet updates and new observations

Ross Sacharow, generator product service manager, began with an overview of what he considers one of the OEM’s most important—possibly the most important—maintenance document, GEK103566 Rev M. It includes a new section dedicated to brushless-exciter maintenance. This just-released guide is a complete update of Rev L, combining input from both the GE and Alstom fleets.

A particularly valuable slide for users, the fourth in the presentation, details the terminology changes important to owner/operators and defines first, borescope, and rotor-in robotic inspections. Access the presentation on GE’s MyDashboard website.

  • Sacharow told attendees that where inspections with robots are possible (air gap large enough to accommodate the robot), a rotor generally should be removed only for repairs and upgrades. He cited the following among the maintenance/repair activities that would require rotor removal:
  • Rotor tooth and wedge inspection and repair to remove hardened material.
  • Rings-off cleaning up to field rewind.
  • Partial or full stator re-wedge.
  • Core or wedge repair.
  • Partial or full stator rewind.
  • Retaining-rings-off inspection.

Another slide summarizes inspections and maintenance intervals as specified in Rev M alongside those published in Rev L with updates highlighted—a handy guide for maintenance managers.

  • GE polled the group to learn how users view the ability of robotic rotor-in versus rotor-out inspections to find a problem—assuming no known components to repair. Here’s what they said:
  • Very comparable, 13%.
  • Similar, 25%.
  • If it were up to me, I would always pull a rotor for a major inspection, 42%.
  • If it were up to me, I would always use a robot for a major inspection, 20%.

Recent findings regarding collector flashovers was Sacharow’s next topic. He said reducing selectivity—the unwillingness of brushes arranged in parallel to share current equally—was critical to reducing such incidents. Much has been published on the subject, yet flashovers still occur—typically a dozen incidents worldwide annually. For those requiring a refresher, the presentation has a slide dedicated to collector-system maintenance activities (daily, weekly, monthly, during an outage). This might be a good topic for a lunch-and-learn in the plant break room.

Next topic: Recent 7FH2 core-damage findings. At least six examples of turbine-end core damage at the step iron have been reported—five in the last year. All were found on generators made by a third-party. In each case, there was evidence of core ID looseness at the teeth. Risk is not clearly tied directly to operating hours or starts. Preliminary findings point to manufacturing practices as the cause of the looseness.

With the root-cause investigation ongoing, the following recommendation was made for units in this fleet not planning a generator outage in 2021: Perform a borescope inspection—as early as possible in the year—focusing on visual evidence of turbine-end step-iron core damage or greasing/red dusting.

For units planning a generator outage in 2021, suggestion is to follow the standard inspection procedures presented in GEK103566 and use the outage to thoroughly inspect for evidence of core looseness or lamination damage. The recommended inspections (a list is included in the presentation) require removal of end shields, gas baffles, and at least one cooler. Questions? Contact your GE service rep.

7FH2/B family, 2020 stator update

Eric Buskirk, generator systems integration leader, responsible for guiding new unit and technology development, covered the rotor/stator configurations and manufacturing variables associated with this fleet, plus stator features, generator efficiency, fleet-wide issues and updates, and cyclic duty.

Presentation began with clarification of product nomenclature important to owner/operators. 7FH2 generators, made from about 1994 to 2015, dominate the fleet with about 850 units in service—the overwhelming majority coupled to 7F gas turbines. This product, renamed H33, has been phased out. Replacement is the 7FH2B, recently renamed H35, which has been manufactured since 2004. There are 75+ units in this fleet segment.

Users should be aware that during the gas-turbine bubble (1999 to 2005), GE expanded its generator manufacturing capability through partnerships with Doosan, Alstom, Melco, Toshiba, and Hitachi. While all insulation systems and generator processes were qualified by GE, there are minor manufacturing differences among the machines produced under license; some parts are interchangeable, some not—stator cores, for example. Valuable detail is provided in the presentation available on GE’s MyDashboard website.

Buskirk mentioned the availability of exchange fields for the H33 and H35, which could yield a small capacity uprate, noting that the stator may be limiting and auxiliaries may have to be upgraded in order to capture the full-capacity uprate entitlement. Generator details (leads up versus leads down, coolers and their arrangement, bushing design, connection-ring configuration, loop blocking, etc) are explained in a series of slides easily understandable by O&M personnel.

Generators usually are very efficient, the speaker reminded, but there are opportunities for small, but meaningful, performance improvements under certain conditions. Example: Cooling may be improved by raising hydrogen pressure, but at the expense of increased windage/fan losses. If a machine is running cold or not operating near nameplate, perhaps a reduction in gas pressure would be beneficial. For an H35, going from 45 to 30 psig saves about 75 kW. Raising hydrogen purity (by reducing seal-oil flow and making other adjustments, for example) can save up to another 100 kW or so. Several more possible improvements were explained as well.

Generator field amortisseur spring axial migration cases and retention solutions

Ben Mancuso, generator design engineering technical leader, opened his presentation reminding attendees that the purpose of the amortisseur winding is to divert the negative- sequence current (see first presentation summary above) from flowing in the rotor forging and causing arcing and/or overheating damage during normal operation. Be aware that some abnormal events can exceed amortisseur capability.

Mancuso provided valuable graphics and summary statements to explain the various slot amortisseur windings (Gen 1, 2, and 3, plus non-static start) found in the fleet and how they work. Plus, he addressed excessive axial misalignment—when a wedge vent hole exceeds 30% or more blockage—and its causes. Should wedge hole blockage exceed 50%—not typical—it’s possible to cause a hot spot in the winding, possibly leading to an electrical fault.

Finally, the speaker presented solutions to address component migration—including an amortisseur/spring axial retention modification package available from the OEM to lock the various components together.

 

Over-potential insulation dielectric testing (hi-pot)

Michael Villani, product and factory support, Generator Services, explained that periodic maintenance over-potential testing of generator stator windings is a generally accepted industry practice. He said that when this test is performed on healthy insulation and according to the OEM’s instructions, the insulation will not be damaged. Not everyone in the industry would agree with this. However, if the insulation’s integrity has been degraded prior to the test—such as from stator-bar abrasion, girth cracks, etc—the test will accentuate that damage.

Later, Villani noted that the final hi-pot test during generator manufacture is considered the most reliable factory test to determine the quality of the insulation system at the time of shipment. Thus, he concludes, continued periodic hi-pot testing in the field is the most reliable test to determine the suitability of an insulation system to perform its intended functions reliably. The customer ultimately decides what voltage it is most comfortable with for conducting the hi-pot.

Villani also reviewed the pros and cons of ac and dc hi-pot testing, noting that all new stator bars and stator windings are tested with ac hi-pot. With the ac test set large and requiring trucking and cranes to transport and move around onsite, and a significant power source, its cost is higher than for the dc test, which is selected in most instances.

Final topic in Villani’s presentation: Should hi-pot testing be closed or open? GE generally recommends hi-pot testing be performed with the unit degassed and opened up, so both ends of the generator are visible to the operator/second watch person and others observing the test. The benefit of this approach is that if a failure occurs it may be possible to see its location, thereby helping to quickly identify the failed winding.

 

Third-harmonic stator ground

Dhruv Bhatnagar’s (generator application engineering) presentation was the second on this topic at GUG2020, the first one delivered by an owner/operator during the Week One generator session (see previous section).

Content of both presentations is similar and those with interest in the subject matter should review both—the user’s on the Power Users website at www.powerusers.org and Bhatnagar’s, which provides GE’s recommendation for stator-ground protection, on MyDashboard. An important point: GE recommends simple and low-cost testing to ensure that the 27TN trip settings are properly configured. If the are not properly configured, significant generator damage can occur, as the examples provided in the presentation revealed.

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