A forced outage: emergency repair of an overheated generator

How do two companies, 12 hours apart, pull together to repair an overheated generator in the most effective and time-efficient way? This feature describes a particular emergency response to a forced outage, and describes step-by-step how the repairs were carried out.

Anuar Yusoff, Tenaga Nasional Berhad, Malaysia & Bill Moore, National Electric Coil, USA

What would you do if your generator had just suffered a forced outage due to an overheating incident so severe that molten solder from the stator winding connections flowed out of the machine?

This was the question and challenge facing Tenaga Nasional Berhad (TNB) of Malaysia, in regard to their Toshiba 124 MVA generator. This particular generator is part of the Sultan Ismail Power Station, located in Terengganu on the east cost of Malaysia. The unit was commissioned in 1984, and at that time, it was one of the largest combined-cycle power plants in the world.

In this instance, a loss of cooling water caused the generator to overheat so causing the melting of the stator winding series soldered connections. These electrical connections in the stator winding allow current to flow from one stator bar to another. Overheating caused solder to flow out of the joints, interrupting the current flow, melting away copper from the stator bar strands, damaging the stator bar insulation à‚— until the generator tripped offline and caused a forced outage (Figure 1).

Figure 1: The end of the damaged stator winding. Overheating from the loss of coolant resulted in a flashover that melted away stator coil copper. The generator was forced off-line and unable to produce power
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Damage assessment

Initial inspection of the winding showed that a flashover had occurred as a result of the overheating. Several coils were severely damaged resulting in as much as a 50 per cent loss of copper cross-sectional area (Figure 2).

Figure 2: The end of the damaged stator winding in one area. Overheating from the loss of coolant resulted in a flashover that severely melted away stator coil copper
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There were also many places where the solder from the series connection joints had melted and flowed out of the joints. The molten solder landed on other parts of the insulated winding, damaging the insulation. The lack of solder created a situation which led to further overheating of the joint à‚— due to an insufficient cross-section for heat transfer.

The joint lacked sufficient solder and the clip cap lacked sufficient resin fill, so both the solder at the winding connections and the resin flowed with the excessive heat. The leaking of the solder from numerous series connections was visible throughout the machine. Several insulating clip caps were found to be totally without resin and could be removed by hand pressure.

When melting occurs, large gaps can appear as the solder runs out à‚— this will inhibit heat transfer in the future, create additional hot spots, and will result in premature failure if not corrected (Figure 3).

Figure 3: In a melted series connection large openings and gaps occurred due to the flowing out of the molten solder. These gaps will inhibit heat transfer and the capacity of the joints to carry current
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Based on the initial winding inspection, it was thought that a total rewind was necessary. The machine had been in operation for 24 years, and the insulation life had been seriously degraded in the entire machine, even if localized repairs could be performed. In an ideal world, the winding would be successfully repaired and the unit rewound at some time in the future, on a planned basis rather than an emergency basis. This would result in significantly lower costs, to be performed at a time when power from the generator was needed least.

The options: repair or total rewind?

TNB researched the alternatives. One option was to consider the failure as a total loss of the generator winding and make plans to purchase and install a new replacement winding. Another option was to repair the generator rather than perform a rewind. The damage to the winding was very severe, and there was a high risk to performing a successful, reliable repair. The safest and most conservative approach would be to proceed with the rewind. However, the manufacturing and installation of new coils would leave the unit in a forced outage situation for several months. A reliable repair, on the other hand, could minimize the long delays and the higher costs associated with a full rewind.

These two options required a thorough evaluation by TNB. The first step was to identify vendors capable of reliably executing either of these two options. In addition to the original equipment manufacturer (OEM), National Electric Coil (NEC) à‚— a major rebuilder of large power generators but based in the US à‚— was identified as a possible candidate to carry out the repair work.

In addition to NEC’s own in-house expertise and capabilities, the company also provides additional resources through its Generator Service Network (GSN). NEC’s GSN is a worldwide support network of Generator Service Partner (GSP) and Network Service Partner (NSP) companies. These companies are ready to respond with NEC to on-site rewinds or repairs of large high-voltage generators anywhere in the world. For this project the NEC team would include its GSP, Australian Winders.

Both the OEM and NEC offered a repair solution, as well as a full rewind option. NEC had direct experience performing an almost identical repair on a similar generator of the same make à‚— Australian Winders had worked with NEC on other large generator projects and was experienced with NEC’s technical requirements for repairs and coil installations.

The NEC team proposed a repair solution similar to a successful repair. The other failed unit also had soldered end winding connections that had melted and flowed out as a result of overheating from a loss of cooling incident. It required the rebuilding of all stator bar series connections with new, improved soldered connections.

NEC’s proposed TNB repair would include all new clips and caps (NEC uses the word ‘clip’ to refer to the copper series connection plates, connecting the top and bottom coils; the word ‘cap’ refers to the insulating molded caps that are placed over the copper series connections).

Figure 4 illustrates typical bars and clips prior to the soldering of the clips to the top and bottom bars. The photograph (from a similar generator) is used to illustrate clearly the series connection arrangement.

Figure 4: The series connection assembly, which includes the top and bottom bar strand package joined together by copper plates called ‘clips’
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In this machine the copper clips are brazed to the stator bar strands using a high temperature braze alloy. In addition, copper blocks and tapered wedges are inserted between top and bottom strand packages to further promote current transfer between the two coils. In the TNB generator, solder (which melts at a significantly lower temperature) was originally used to join the series connections together.

The quickest and most reliable repair would involve reuse of solder for the new connections. In addition, NEC utilized new copper blocks and tapered copper wedges in this machine to promote better current transfer through the cross-section of the clip.

The NEC team was able to demonstrate in its proposal, a reliable, proven repair scope with the necessary ‘higher value’ to TNB that would bring the machine back online in the shortest period of time. The decision to execute the repair work and award it to NEC paid off in good dividends, but due to the complex nature of the repair the project was not without its challenges.

The repair works

NEC, along with its Generator Service Partner, Australian Winders, proceeded to repair the existing stator winding. Initial electrical tests were done by TNB to determine the baseline condition of the stator winding. In particular, the copper winding resistance measurement was carefully performed in order to compare it to factory values and assure consistency of measurement values with the final values.

The extensive repairs included the removal of all (138 total) existing insulated clip caps and copper clips over the series joints. The soldered clips were removed by flame torch. Extreme care was needed in this operation so as not to damage the adjacent coil insulation.

Special heat sink cloth was used to extract heat away from the area and protect the coil insulation during the heating process. The coil insulation was protected during this operation, and also was trimmed back to allow for a longer insulating clip assembly to be installed.

In addition to the task of removing all the existing insulating clip caps and copper clips, special repairs were needed on the coils that had been severely damaged by the flashover; these coils had large sections of copper strands melted and blown away.

To effectively repair, each melted and solidified strand in the strand package had to be opened, separated and cleaned. All existing solder material on the copper strands had to be removed so the proper bonding of new solder could occur. These coil strands would be reassembled into a strand package to accommodate restoration of the copper cross-sectional area.

After clip removal, careful cleaning of the copper strands was performed to remove all resin that might prevent good bonding of solder on the new clips. Induction heat and a special hydraulic clamp were used.

Coils with severe damage on the ends had new clips soldered in place (Figure 5). Tapered copper wedges were installed to ensure full surface electrical contact. All clips were re-inspected after soldering to be sure that there were no voids and the surface area showed adequate electrical contact.

Figure 5: New soldered copper tapered wedges inserted into the series connection clip assembly
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A second pass of solder was applied as necessary to fill in any voids that would cause a potential loss of electrical contact. The tapered wedges were then cut off, even with the end of the coils, and the whole connections were cleaned as shown in Figure 6.

Figure 6: The final appearance of end winding soldered connections before installation of the insulating clip caps
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The resistance of each clip was measured after this operation and each showed a consistent value. A resistance measurement was done on each phase of the winding with consistent results obtained.

All phase leads had been stripped and inspected for overheating damage, which would be evident by any discolouration. Since all the phase leads were insulated, possible discoloration could not be observed without the stripping.

No discolouration was found, and the phase leads were reinsulated with 12 layers of mica tape, painting between each layer with an ambient temperature curing epoxy resin compound.

Next, all new insulating clip caps were installed over the refurbished series connections. New clip caps have several advantages over the method of taping the series connections: often the caps can be installed much more efficiently, so reducing the overall repair time, and in addition, their uniform size and shape assures consistent spacing of the series connections and coils.

Finally, their rigid construction, along with the internal resin compound (which cures rock hard and completely encapsulates the series connection) provides a secure, void-free insulating barrier which will protect the connection for a very long time.

Series blocking was installed between the newly fitted and cured insulating clip caps. The blocks were tied in place with heat shrinking tie cord and then coated with an epoxy resin. This blocking, plus additional phase lead blocking, provided secure bracing to resist end turn vibration, which could lead to cracked conductors and shorted strands in the future.

During the outage to repair the end winding, TNB performed a full stator re-wedge on the unit and assisted with electrical testing and bump testing of the end windings. Initial bump tests done by TNB were performed on each individual coil without a complete modal analysis. NEC and Electromechanical Engineering Associates (EME), another NEC Generator Service Network member, assisted with these tests.

Both companies have experience with performing modal analysis on other repaired and rewound generators. This process included a ‘roving hammer impact test.’ Modal analysis uses different techniques than the bump testing of the individual coils. The test results were converted on an expedited basis into modal format by EME. Bump test data files were sent back to NEC’s engineering team in the US for review and analysis.

Final bump tests on the end winding showed that the N=2 oval mode was sufficiently ‘low tuned’ away from the double operating frequency of 100 Hz. Final insulation resistance tests showed acceptable values.

After the completion of this generator repair work, the steam turbine was smoothly and successfully re-commissioned on 14th November 2008 (Figure 7 & 8).

Figure 7: The final painting in progress after the repair
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Figure 8: A view of the repaired generator after painting
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As part of the overall project, NEC provided a final condition assessment of the stator winding; this assessment was based in part on EMI (Electro-Magnetic Interference) testing and evaluation. EMI is a new generator testing method that can be done with the unit online, with testing techniques and applications being developed by Doble Engineering, Boston, Massachusetts.

The repaired generator actually showed lower EMI readings than two other generators in operation at the same location. However, even with those low values, considering the degree of overheating that occurred coupled with the previous service life of 24 years, this generator is still a candidate for a rewind in the near future.

NEC recommended that a new winding should be purchased and held in storage, so that if an unexpected failure does occur forced outage time would be kept to a minimum.

Issues and challenges

Communication on this project was especially critical, due not only to the urgency of the repair but the significant time difference of 12 hours between Malaysia and the US, making effective communications challenging. Where possible, designated times for meetings were established which minimized disruption.

Primary communication between members of each company during the planning stages was also key à‚— communication was made on a regular basis, daily if necessary. Communication was kept simple and relevant, by establishing action items and follow-up responsibilities.

New participants were introduced as they joined the project, and as a whole, the communication on this project between TNB Malaysia, NEC and Australian Winders was excellent. At the start of the project, transportation of equipment and materials quickly became another issue.

Due to the urgency of the repair (with the generator on forced outage) special tooling including NEC’s induction heating equipment, needed to be at site on an expedited basis.

NEC found that with the use of standard commercial shipments, even expedited deliveries were subject to delays, inspections, re-distributions, with unmarked component parts being removed from their original shipping containers and misdirected.

NEC switched to dedicated carriers with proven track records and clearly marked all component pieces within each container with the correct identification and contact details à‚— should the container be unpacked during customs or transport. Tracking began immediately on subsequent shipments of equipment and materials, in order to detect any breakdown in the execution of the shipper’s obligations.

Summary and conclusions

This feature describes the emergency response procedure in repairing a failed generator in Malaysia. The generator had overheated due to an incident with a loss of cooling.

Severe damage to the stator winding forced the unit out of service and several repair alternatives were considered, including a full stator rewind à‚— this option would involve extensive delays due to the time necessary to manufacture new stator bars and perform the rewind.

TNB, the owners, chose to authorize an emergency repair by NEC. Although NEC was a new supplier to TNB, a thorough evaluation showed NEC had proven experience in rebuilds and repairs of large power generators, along with engineering staff capable of handling unforeseen challenges.

The total repair took 24 days once NEC’s equipment had arrived onsite. TNB exhibited the confidence and courage to work with a new supplier, with headquarters in different time zones.

Because of the NEC Generator Service Network team’s capability to perform a large-scale emergency repair, TNB was generously rewarded by a newly reliable, rebuilt generator, now more robust and upgraded and ready to withstand similar fault conditions in the future.

TNB and NEC’s ability to effectively co-ordinate and perform the repair under such an urgent basis, with tooling and manpower halfway around the world, combined with effective project management from both companies, good communication, as well as highly skilled service and engineering staffs, resulted in a very successful repair.

Ahmad Shahrizan, Tenaga Nasional Berhad, Malaysia (www.tnb.com.my) and Ron Roman, National Electric Coil, USA (www.highvoltagecoils.com), also contributed to this article.

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