Key steps in inspecting, reconditioning generator rotors

Generators have a reputation for being so reliable that resource-challenged plant O&M staffs are inclined to focus their attention on gas turbines (GTs), heat-recovery steam generators, critical-service valves, and other equipment where problems are expected. But this strategy may not be in your best interest; it is important to give generators the minimal attention they typically require.

Some experts caution that the design conservatism built into generators 20 or more years ago is no longer in evidence given the competitive nature of today’s electric generation business and the “buy low cost” mentality of most powerplant owners. Evidence of eroding design margins can be found at many user-group meetings where generators have become a topic of growing importance. Open discussions among users also lead one to believe that O&M personnel having a deep understanding of, and experience with, generators and large transformers are in short supply.

Given the impact of the industry’s structural and philosophical changes over the last decade on equipment design, operation, and maintenance, owner/operators of gas-turbine-based peaking, cogeneration, and combinedcycle plants might consider carefully reviewing their generator inspection and overhaul procedures. Reason: To ensure that those procedures are consistent with the level of protection required by a given unit’s operating regime and contractual obligations.

Discussions with a cross section of generator experts reveal at least two schools of thought regarding inspection and overhaul. Those with decades of experience that obviously includes extensive hydro, nuclear, and conventional steam plant work generally suggest longer intervals between inspections than those with less experience on the conservatively designed equipment of the past and more on generators for GT-based plants.

Take rotors, for example. The “old school” suggests pulling the rotor at 100,000 hours (under normal circumstances) for a “caps off” in-shop inspection and reconditioning; the “new school” recommends the same at every GT major, or twice as often. Thinking on borescope inspection among the “new school” advocates is every 8000 operating hours, which typically would coincide with a GT combustor inspection on a base-load unit; “old school,” about 24,000 hours, or during a hot-gas-path inspection.

A thorough borescope inspection would include a look under the retaining rings. An experienced eye should be able to identify copper distortion or migration, block movement, contamination, loose or moving components, broken or cracked poleto- pole or coil-to-coil connections, cracking or pitting-corrosion attack on retaining rings, etc.

Preventive maintenance. National Electric Coil’s (Columbus, Ohio) Bill Moore, PE, well known by the user community, is a proponent of a proactive preventive maintenance (PM) program. At a recent meeting of the Combustion Turbine Operations Task Force (CTOTF), Moore stressed that generators should not be “taken for granted.”

He said that many generator problems can be traced to an ineffective PM program. Some reasons PM programs are not up to par, he continued:

  • Inspection and repair records lost, misplaced, and/or misfiled as a result of a change in plant ownership.
  • Experience gap caused by the loss of key personnel through workforce reductions.
  • Less money budgeted for equipment diagnostics, maintenance, and upgrades.

An effective PM program for generators, Moore stated, includes these three key elements: (1) inspection, (2) data collection and recordkeeping, and (3) testing. The information compiled is then used to initiate corrective action before failure. To accomplish these goals effectively, he said, requires the following:

  • Clearly defined responsibilities.
  • Sufficient time for inspections.
  • Well-defined schedules for testing and maintenance based on equipment type, age, service demands, operat ing condi t ions, safet y requirements, etc.
  • Established procedures.
  • On-going data trending and recordkeeping. They are particularly important. To illustrate: A winding short can be identified by a change in running vibration or an increase in excitation current to maintain a given load.

Regarding rotor electrical tests, Moore suggested the following, at a minimum:

  • Every outage, measure insulation resistance to determine the presence of contamination.
  • Every outage, measure winding resistance to verify the integrity of brazed connections.
  • Annually, and when the generator is operating, use a flux probe to identify shorted turns.
  • Every GT major, conduct a pole balance test to identify shorted turns if present.

Rotor inspection, refurbishment

Paul Heikkinen of Wood Group Generator Services Inc, Farmington, NM, views the flux probe as an important online condition monitoring instrument that should be included in every owner/operators arsenal. If you don’t have a flux probe, he suggests that you install one at the next outage. In Heikkinen’s experience, the turn-toturn shorts that the flux probe warns of account for about one-third of all rotor problems. The flux probe also alerts on coil-to-coil shorts.

The editors of the COMBINED CYCLE Journal visited Heikkinen at Wood Group’s generator repair facilities to learn how rotor inspections normally are conducted and to see first-hand the key steps in a rewind project. Since many supervisors and operators at GT-based plants have not seen a rotor being removed from the generator for inspection and/or repair, the accompanying photos offer valuable perspective. The information presented also suggests how you might go about conducting objective due diligence of alternative vendors when work on your generators is necessary.

Heikkinen began the tour by noting that a “caps-off” inspection and reconditioning of a rotor with no apparent problems takes about two weeks of shop time. He acknowledged that this work can be done in the plant if the customer prefers, but suggested that the benefits associated with dynamically rebalancing the rotor after retaining rings are reinstalled favor a shop environment. By contrast, a total rewind generally takes four or five weeks of shop time.

After the rotor is removed from the generator, be sure it is handled by experienced riggers and rotor-transport veterans (Fig 1). A safe, smooth ride to the shop (and return) is important. Heikkinen said the trucks Wood Group uses are equipped with seismic recorders to verify impactfree transportation.

Your evaluation of shop capability begins with crane capability and operator certification (Fig 2). First stop for the rotor is an incoming mechanical baseline inspection (Fig 3). Electrical tests are next, including these:

Static turn insulation integrity test (STIIT™) is used to measure and compare readings for individual coils within a two-pole rotor. Readings for like coils of opposite poles are compared to each other as well as to industrial standards. Differences in readings are indicative of the severity of turn-to-turn shorts in a particular coil (Fig 4).

A pole drop test is performed by inducing a voltage into a rotor winding, measuring the voltage drop across each individual pole, and then comparing the two against each other. If readings differ by more than 5%, turn-to-turn shorts are suspected in the pole generating the lower reading (Fig 5).

An ac impedance test is another way to test for turn-to turn shorts. It is performed with the rotor at rest or at any speed by inducing into the rotor winding a series of voltages that increase incrementally. Voltage and amperage are measured and recorded. Resistance is calculated and graphed against the applicable voltage levels.

Nondestructive examination determines the condition of retaining rings, rotor body wedges, blower hubs, couplings, etc. Even the forging itself should be examined by the most suitable method among ultrasonic, dye-penetrant, Zyglo®, eddy-current, and magnetic-particle techniques (Fig 6).

After the incoming inspection, component parts are match-marked and the rotor is carefully dismantled. A water-cooled induction heating system is used to facilitate removal of the retaining rings, not an open flame (Fig 7). With the retaining rings off the rotor, end windings can be inspected thoroughly. They often are found dirty and heavily contaminated (Fig 8); in some cases, distorted and misshapen because of thermal and centrifugal forces (Fig 9).

Next, individual coils are carefully removed from the rotor body, such that the end-strap shape is maintained (Fig 10). The bare rotor then is stripped of all existing balance weights and prepared for cleaning with blasting media designed to remove foreign material but not metal (Fig 11).

After cleaning, the rotor body forging is inspected for cracking and fatigue (Fig 12). If new coils are required in your machine, be sure the industry standard silver-bearing copper is specified (Fig 13). For planning and budgeting purposes, note that the metals markets are now controlled by the sellers. Copper prices have doubled in the last two years, Heikkinen cautioned, and the time for short-term premium deliveries has gone from one week to three or four.

Completed coils, inspected and measured to assure proper form and fit, must be handled carefully to prevent damage (Fig 14). While coils are being made from reels of copper strip, craftsmen install slot insulation (Fig 15). Next comes rewinding in a clean-room environment to prevent the introduction of contaminants into the windings (Fig 16). Finishing touches ensure winding alignment (Figs 17, 18) and the condition of slot insulation (Fig 19) are perfect.

The completed windings are compressed using fixtures made to fit your rotor. While under compression, the windings are induction-baked to cure all epoxies in the turn insulation system (Fig 20). After the rotor cools to ambient, the compression fixtures are removed and the permanent blocking system is installed. Top filler material and rotor body wedges also are installed at this time (Fig 21).

End-winding compression fixtures are reinstalled for a second curing cycle (Fig 22). That step complete, the rotor cools to ambient and the fixtures are removed (Fig 23). Then the retaining rings are thermally expanded and moved into position (Fig 24). Mechanical inspection and electrical testing is next for the completely assembled rotor (Fig 25). After passing those tests, the rotor is lowered into the bunker for an at-speed balance, overspeed confirmation, and running electrical tests (Fig 26). After passing one more round of mechanical inspection, visual examination, and electrical testing (Fig 27), the rotor is ready for shipment back to the plant. ccj oh