What’s known, needs to be learned about stator magnetic core failures

Consultant Clyde Maughan, one of the industry’s leading authorities on the causes of generator failures, called the CCJ office a few weeks back to talk about stator magnetic core failures. Say what? The Clyde, as the near-nonagenarian has come to be known, seems to relish the idea of throwing curve balls to turbine guys who might believe they know something about generators.

He eased into the subject to keep the “students’” interest, beginning the lesson this way: Stator cores historically have required minor, but common, maintenance to repair impact damage from small foreign objects and cuts and bruises related to wedging operations. In addition, minor local or general looseness occasionally has been experienced and typically repaired successfully by tightening of the clamping flange and/or insertion of tapered wedges in the teeth.

But major failures, Maughan continued, have occurred for several identifiable reasons—including widespread damage from a large magnetic object in the air gap, fracture of a stator bar, over-fluxing, severe impact by the field during installation or removal, and gross clamping-force looseness. There was good news, too: Catastrophic, spontaneous meltdowns have been rare and seem confined to a few models.

Maughan’s objective for the call was to review basic core design features and components, and to provide owner/operators a summary of core incidents worth remembering during generator inspections and review of operating data. He said, of all generator components, the core may be the least well understood. It is deceptively simple in construction, but complicated in the extreme in its magnetic, mechanical, and electrical duties—and in the diverse ways it can fail.

The core provides three basic functions, the consultant continued. It does the following:

      • Serves as the stationary path for the magnetic flux in conjunction with the rotating field.

      • Contains and supports the stator bars.

      • Shields the stator-bar conductors from the potent air-gap flux.

Editorial timeout: The editors suggested that the details of stator core design and construction (Figs 1-3) would be much too complex for them to grasp over the phone. Maughan agreed and offered to develop an article for the next issue of CCJ based on a presentation he was working on for the first annual meeting of the Generator Users Group next month. In the meantime, he suggested users review of Chapter 2, Design, in his text, Maintenance of Turbine-Driven Generators.

Stator Core Figs 1-3

Maughan set aside the planned primer on core design and construction to address inspection challenges, testing, and core failure modes—all of considerable interest to owner/operator personnel with generator responsibilities in engineering offices and at powerplants.

Thorough inspection of the core on a large generator can be a tedious, time-consuming, and frustrating experience, the consultant acknowledged, but it is highly important There is a great amount of surface to be inspected, he continued, with a significant portion of that awkwardly overhead, making it difficult to see clearly what you must.

Initial inspection of the surface can be done with the naked eye, Maughan said, as virtually all defects and problems can be identified visually. With the field removed, the core ID is fully exposed. There is much less access to the core OD, access generally available only through manhole covers, cooler openings, and high-voltage bushing manholes. But all accessible surfaces should be inspected, the editors were told. However, since most operational problems related to core OD surfaces tend to concentrate at the ends of the core, inspection of the core OD can focus on about the last 2 ft of each end of the core.

Any conditions of concern can be inspected in more detail with better lighting and by using a magnifying glass. On generators with radial ventilation ducts, some suspect locations may be seen using a borescope—for example, core iron at the bottom of the slot.

There are two common tests to determine core condition: low flux and high flux. Here are some details on both that Maughan believes users should be aware of:

      • El Cid (Electromagnetic Core Imperfection Detection) is the most widely used low-flux test. It came into common use in around 1980 and is simple, convenient, and completely safe to both the core and personnel (Figs 4, 5). However, like all generator tests, it has weaknesses. Numerous incidents of questionable results have occurred. Nevertheless, low-flux testing is an important core evaluation tool and should be used (with caution and judgment) in conjunction with inspection for routine assessment of core condition. It should also be used before and after any work is done involving possible damage to core iron—such as stator-slot rewedging and partial or complete stator rewind.

      • High-flux testing has been used since the infancy of the power-generation industry. However, it requires a high-voltage power source, a suitable breaker, and long length of cable (Fig 6). The high current and voltage involved are hazards to both personnel and the core. Thus the test is not simple, convenient, or safe. With the advent of low-flux testing, the high-flux test is now largely used only when low-flux test results are called into question.

Stator Core Figs 4-6

Major failure modes. Root causes of many common core failures are well known. By contrast, a few major core failures are not well understood. One example: On a particular line of large generators, there have been several total core meltdowns for which the root cause only has been speculated (Fig 7).

Typical of other unexplained core failures is the incident shown in Fig 8. During routine stator rewind, the core was inspected and the hot spot in evidence was detected. But since the El Cid test did not indicate a problem, a high flux test was not conducted. A new winding was installed and the core immediately failed at that location.

Stator Core Figs 7, 8

In Maughan’s mind it seems certain that stator cores will continue to be an infrequent but major repair component on generators. The expectation is that in time (and broader dialogue among core experts with participation by owner/operators in industry forums such as the Generator Users Group and the International Generator Technical Community) core deterioration and failure mechanisms will become better understood.

The foregoing is only the tip of the iceberg, so to speak, of what Maughan and others will be contributing to the industry’s collective knowledge on stator magnetic core failures, and many other issues as well, at the upcoming Generator User Group meeting. Don’t miss the opportunity to learn from the industry’s top experts. Register today. 

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