Subharmonic stator ground-fault protection
This is an important presentation for generator owner/operators because the failure to protect adequately against ground faults could lead to a hugely expensive outage in terms of equipment damage and prolonged loss of generation. Unfortunately, to fully understand the protection schemes available to prevent such damage it helps to be an electrical engineer with power experience—a rare species in an industry dominated by mechanical engineers.
Perhaps no one knows this better than Clyde V Maughan, who has degrees in both mechanical and electrical engineering, and was the muscle behind the founding of the Generator Users Group in 2015. He “blew the whistle” on the shortcomings of IEEE standards in adequately protecting against grounding issues in stator windings back in mid-2013 with his CCJ article, “Generator Protection: IEEE standards may not sufficiently address grounding issues in rotor, stator windings,” 2Q/2013, p 7.
The GUG2020 speaker reviewed some material covered in the Maughan article and provided details on a ground-fault experience that involved two unit trips several months apart. The second fault occurred because analysis of the first trip proved incomplete. This was a real “whodunit” in a sense because questions remain; however, the generating unit has been back in service for more than a year with no issues reported.
The presenter reminded attendees that large generators have high-impendence grounding (neutral) connections to limit ground-fault current (read “damage”). Other generator ground-fault protection methodologies require the unit to be online. The two methods typically used are the following:
- 59N—Generator Neutral Over-voltage Protection, which detects faults in the top 95% (approximately) of the generator up to the generator transformer.
- 27TN—Third-Harmonic Neutral Under-voltage Protection, which detects faults on the neutral side of the generator.
Together 59N and 27TN can provide full protection for the generator but many units still are protected only by the 59N relay—the 95% solution—and are at risk. Consider, too, that some generator designs may not produce sufficient third-harmonic voltages to allow reliable ground- fault protection schemes based on third-harmonic signals.
Given these concerns, voltage-injection relay systems (64S) have gained favor—especially where there are concerns regarding stator-winding condition and/or when the unit is deemed critical by the owner. A big advantage of the 64S is that it provides its own excitation source and assures the entire generator ground-fault protection when it is shutdown (at standstill or on turning gear), during startup, and at-speed offline or online.
Concerns regarding 64S are cost, more complicated analysis of issues, and more equipment to go wrong.
Remote technical support during generator maintenance
The second presentation at the conference, was insightful given the dearth of generator engineering talent in the electric power industry today (see previous article), and travel and other restrictions that have become the norm during the global pandemic. It was developed by Clyde V Maughan and would have been the 94-year-old’s first formal virtual presentation had a schedule conflict not interfered. Jim Timperley, well respected by generator users worldwide, made the presentation.
Maughan began by illustrating the need for, and drivers of, remote technical support, complete with examples. Next, he covered the advantages of working remotely—saves a lot of money and time. And the disadvantages: It’s difficult to see and hear about everything you might want to know without the close personal connections typically developed during a site visit. Regarding the last point, Maughan acknowledged this will be less of a concern in the future because of the rapid advancements in digital technologies and communications.
But wait, there’s more. With retirements, cutbacks in plant staffing, and the absence of traditional face-to-face conferences, Maughan thought it necessary to have a consultant register as a means for introducing generator-owner/operator personnel to candidate experts capable of fulfilling their needs.
So, he worked with General Manager Scott Schwieger to develop the Generator Consultant Skill Register, posted on CCJ’s website at www.ccj-online.com/generator-experts, where you can search for an expert with the specific skills required for your assignment. If you’re a generator expert who wants to be included in the register, present your qualifications for vetting at www.ccj-online.com/generator-expert-skill-register.
When a new stator requires a rewind: Vendor oversight lessons learned
This is a case- history that every user should read through. It tracks oversights/errors compounded over a period of six months and offers lessons for all plant personnel that don’t just apply to generators.
The “adventure” begins during initial startup testing: Generator vibration increased and the turbine tripped. Generator fans had been installed on incorrect ends of the machine and a blade liberated. There was no alert from the M&D center because it did not monitor units in commissioning. Damage to the machine was minimal, essentially limited to deposits of foreign material sprinkled throughout the generator. The unit was cleaned and the unit restarted relatively quickly after appropriate inspection and testing.
Likely, those involved breathed a sigh of relief at that point thinking they had dodged a bullet. Well, about a month after the first blade failure there was another, but this one was more severe, with over 500 damage locations on the endwinding. An engineering assessment based on bar voltage and damage location and depth revealed 14 bars at high risk, 10 medium risk, and three low risk.
The root-cause analysis of this failure revealed the blades that had been installed incorrectly on the turbine end of the generator before the first failure had been reused on the collector end. New blades were installed only on the turbine end after the first event. Engineers found that because the reused blades had operated backwards, they were prone to cracking from high-cycle fatigue. No NDE had been performed on the blades prior to their reuse.
The plant owner decided on bar repair to fix the damage done. Following lab testing of the proposed repair method, it was implemented on the affected generator. A follow-up ac hi-pot test failed on two phases, but neither failure occurred at a repair. Lab analysis determined the failures were caused by subsurface cracks attributed to debris impact. No visible surface damage was in evidence at these locations.
Next, a decision was made to proceed only with the replacement of the two failed top bars. But during this activity, six bottom bars were damaged by metal wedges. A full rewind was ordered.
Now four months into the project, during assembly of the stator bars, misalignment between the top and bottom bars was discovered. The bars originally were manufactured on the wrong bar form for the stator frame. When the top-bar replacements were planned, as-built dimensions were used instead of the design values.
You can only imagine what went on from that point until the generator was cleared for full- load operation about two months later. Get the details in the presentation, available to registered users only at www.powerusers.org.
Lessons learned were plentiful on this project, including these:
- Written communication between owner engineering and vendor field services is necessary for important recommendations and decisions. Too much can be lost or forgotten in verbal communication. Had this been done for the fan-blade replacement activity after the first blade failure, the second failure might not have happened.
- Owner oversight of critical activities is very important. It’s highly unlikely that an experienced owner’s eyes would have missed use of the destructive wedges during top-bar removal.
- Carefully vet the vendor’s human performance program regarding adherence to procedures, validation of assumptions, staff communication, and training. Don’t expect that the vendor’s human performance program is as rigorous as yours. Certain things were assumed during the manufacturing phase of this project that should have been verified before moving forward.
- Request shop quality exceptions. Recall that incorrectly manufactured bars were discovered during the rewind. There was no notification from the vendor during original manufacturing of the generator. The owner was told later such notifications had to be requested. Important to note here that the owner was not “allowed” to have a witness monitoring critical shop operations.
- Document and maintain vendor quality performance records. Don’t expect the vendor to do this for you.
- Be sure you have the “right” (read, most capable) people in the field to monitor contractor activities. It’s important that your team relentlessly follow up on QA/QC activities and collect and maintain meaningful data. Dotting eyes and crossing tees is vital to project success.
Effects of flexible operation on generators—stator winding.
Bill Moore, PE, EPRI’s technical executive for generators, began his presentation by reminding owner/operators that while the effects of stop/start operation on rotors are discussed relatively frequently, the impacts of flexible operation on stator windings have not been. He referred attendees to EPRI Report 1008351, publicly available at www.epri.com, which says the forced-outage rate of generators increased by 246% because of speed (stop/start) cycling.
In simple terms, Moore told the group that as a generator changes load its temperature changes. Given that endwinding materials (copper, blocking, insulation) have different coefficients of expansion, epoxy bonds break, blocks move and shift, dusting and greasing occurs and insulation can crack. Endwindings loosen as well.
The core of Moore’s presentation was an analysis by Siemens Energy which provides a calculated approach to accounting for stator-winding degradation in various types of generators for various load-cycling scenarios. Get the details in the presentation available to users at www.powerusers.org.
Moore shared the following conclusions/recommendations:
- Repeated major load swings cause a significantly higher degradation of the stator winding than smaller load swings.
- Areas of high degradation typically are inboard, towards the core, rather than outboard at the series connections.
- A typical indirect air-cooled generator is likely to suffer a higher rate of degradation from load cycling than a GVPI air-cooled generator.
Temperature control could be used to reduce temperature change during load swings and can provide the opportunity to reduce degradation.
