Engineering Design Tops List of NEC's Comprehensive Generator Services
NEC design engineers are ready to respond to any generator design issue that may arise during a generator repair or retrofit project. While the individual backgrounds of our engineers may vary, many began their careers at OEMs or worked for major utilities before coming to NEC, or both. This diversity of knowledge and experience gives our design team a wide-ranging base of experience with generating units of all ages, makes, sizes and types. Along with the winding design, our engineers address related issues of unit reliability, efficiency, maintainability and longevity. They also are involved in the design, development, evaluation and testing of semi-conducting coatings, varnishes and tapes for electrical field grading in the end-windings of high voltage stators.
Past projects have Involved generator rewinds, uprates or repairs to rotating electrical equipment manufactured by nearly every supplier in the world. A partial list includes General Electric, Westinghouse, Siemens, Allis Chalmers, Electric Machinery, NS Peebles, Brush, Elliott, Alstom, English Electric, Reliance, Toshiba, Hitachi, GEC, Mitsubishi and ABB. These projects have services turbogenerators, hydrogenerators, AC and DC motors, exciters, frequency changes and synchronous condensers. Sizes have ranged from motors of a couple thousand horsepower, to 1000 MVA generators.
In addition to complete uprates, including rewinds of both the rotors and stators, specific engineering tasks have included thermal analysis of complete generators, field winding thermal analysis, redesign of field windings to improve thermal performance, fan design, stator winding vibration analysis and testing, stator core vibration analysis and testing, electromagnetic force analysis, rotor component stress analysis, rotor and retaining rings specifications, and rewind specifications.
The following is a short explanation of the engineering tasks most common with the typical generator project:
- Uprate Studies done to determine what can be done to increase the output capability. These studies analyze the electrical, magnetic, mechanical and ventilation aspects of the generator in order to determine best combination of design parameters to meet output requirements.
- Life Extension Studies based on inspection, non-destructive testing and analyses that:
Estimate for the unit's remaining life
Identify what actions needed to extend the overall machine life or the time until a possible failure
Guide the development of a maintenance plan that maximizes unit reliability and availability
- Studies that address specific performance issues such as:
Noise Reduction – for generator designs that are susceptible to slot passing frequency noise or other harmonics
Reduction of Vibration due to intermodulation effect of magnetic circuits
Optimum Eddy Current Loss Reduction schemes
Effects of Changes – insulation and ventilation schemes, including calculation of steady state and transient temperatures of all components
Heating and Ventilation studies – evaluate existing schemes or to achieve improvements in friction and windage losses, reduction in hot spots, and general improvement in machine temperature.
Theoretical modeling and analysis of Generator Reliability
- Failure Analysis performed prior to any repairs – analyses include recommendations to mitigate the possibility of a similar failure on another machine, or a reoccurrence of the same failure.
- Re-Engineering Design performed, as necessary. Our advanced proprietary design software assures the compatibility of replacement components, so that replacement parts are equal or better in quality and performance to the originals. Features include but are not limited to:
- CAD/CAM design and element analysis
- Full electromagnetic calculations for all sizes and types of units
- Calculations of estimated machine characteristics from physical dimensions
- Calculation of performances, parameters, losses and the efficiency of the entire size range of fossil, hydro, and nuclear generators
- Specialized tooling design, such as tooling for large high voltage stator coils to fit into skewed stator cores
Typical Project Experience
NEC's team has been directly involved in hundreds of projects directly related to specific engineering problems with large generators and motors. A sampling of recent projects are listed below:
Turbogenerator Projects
- Failure analysis of water-cooled stator windings (TIL 1098)
- Analysis, redesign and manufacture of rotor fan blades to reduce blade failure (multiple projects)
- Retaining ring changeouts to upgrade retaining rings from non-magnetic18Mn5Cr to 18Mn18Cr rings. (TIL 1097-3 and OMM 089) (multiple projects)
- Forging and retaining ring modifications to repair rotor tooth top cracking (TIL 1292 and OMM 103) (multiple projects)
- Analysis, redesign and manufacture of winding and rewind of generator unit that had experienced two previous in-service failures prior to NEC involvement
- Clean, inspect, test and repaint winding during routine outage (multiple projects)
- Manufacture replacement stator winding and rewind unit on expedited schedule (multiple units)
- Complete emergency stator core inspection and stator rewind in 19 days
- Joint effort with partners to rewind both the stator and the rotor of an inner-water-cooled generator. NEC manufactured both stator and rotor windings. Stator rewind was complete in 20 days.
- Following a stator hydrogen cooler valve failure, a generator overheated and experienced thermal instability over the generator field at full load. This melted all soldered stator bar connections. Rotor was rewound and stator bar connections resoldered.
- Strip global VPI stator, clean and test stator core, rewind stator (multiple units)
- Stator and rotor winding manufacture for OEM for use in its own rewind projects (multiple)
- Stator and rotor winding manufacture for NEC partner for use its own projects or in joint projects (multiple projects for multiple partners)
- Manufacture and install new winding in 42 days; including redesign of "original" winding design to eliminate partial discharge problem caused by absence of side filler packing material
- Strip WESTAC generator stator and clean and test the stator core. remove, reinsulate and reinstall existing bus rings, surge rings, and support brackets. Remove, repair and restack stator core. Furnish and install stator winding with RTDs
- Design, manufacture and install new stator winding that eliminates lead group transposition to provide maximum upratabe potential. Also provided new enhanced bus rings
- Rewedge stator (multiple projects)
- Rewind and restack four-pole generator stator on-site and concurrently repair and reinsulate rotor poles and remount them to rotor off-site in NEC's Service facility
- Rewind and partial restack, including replacement of all through bolts with new nitronic type bolts
- Refurbish damaged 20-year-old spare winding
- Refurbishment and reinstallation of aluminum rotor windings (multiple projects)
- Refurbishment and reinstallation with improved adhesives of rotor windings experiencing turn insulation migration (TIL 1308-2R2) (multiple projects)
- Repairs and rewind of rotor experiencing turn to turn shorts and/or thermal imbalance (TIL 1143-3 and TIL 1158-3) (multiple projects)
- Repairs and refurbishment of collector rings and journals
- Redesign and repair of rotor leads and pole crossover connectors to eliminate cracking (AIB 8004 and AIB 8306) (multiple projects)
- Refurbishment of large, inner-hydrogen-cooled brushless exciter
- Refurbishment of various brushless exciters (AIB 8205) (multiple projects)
Hydrogenerator Projects
- Bulb generator stator core failure analysis
- Spider thrust bearing failure analysis
- Thrust bearing load capacity study
- Spider stress analysis
- Analysis of rotor spring key migration
- Field winding analysis
- Finite element analysis of failed damper bars
- Guide bearing failure analysis
- Ventilation analysis
- Thermal analyses
- Armature winding corona analysis
- 50-year and other generator life extension analyses
- Inspect 1920 vintage generator, advise of suitability for continued use
- Excessive slip ring brush wear
- Noise reduction analysis
- Determination of cause of high temperature and vibration at core splits for pumped storage unit
- Determination of cause of excessive stator core vibration in generator unit
- Pole density calculations
- Oil lift panel redesign
- Redesign of field pole connectors
- Redesign rotor rim
- Re-design thrust bearing cooling oil system
- Blower blade re-design for ventilation upgrade
- Redesign hydrogenerator to change the number of coolers
- Design of new field poles (convert from cast iron to laminated)
- Design new sound proof housing
- Design new housings, coolers, fans, etc., to convert open ventilation system to closed system
- Where a stator failed to ground, partial rewind and rewedge of stator, and cleaning and testing of rotor.
- Uprate of generator, providing and installing stator coils, new bus rings, and new stator core and clamping structure (multiple projects)
- Manufacture, installation and testing for new armature winding, new bus rings, spare coils or winding (multiple projects)
- Manufacture and installation of windings for 11 units at two plants; nine units made at same rating by by three different OEMs, Mitsubishi, AEG and BBC and two units made by Rade Koncar a different rating
- Manufacture and installation of new stator winding and new stator core, plus re-rounding of the core (multiple projects)
- Frequency change of 12 units: Installation of uprated windings and redesign and configuration of rotors for additional poles and manufacture of all new pole pieces
- Stator rewind and uprate plus extensive work on core, frame and fingerplates to repair damage
- Re-plumbing of generator/turbine shaft, redrilling of stator frame and fabrication of new dowels for stator frame and upper bridge to round out the stator frame
- Replacement of the generator cooling piping
- Installation of a PDA coupler systems
Exciter Projects
- Refurbishment, including armature rewind, of a 4500 kW, 525 Volts, 8575 Amperes, 3600 rpm hydrogen-cooled brushless exciter. This exciter had a total of 144 diodes, with 24 diodes in parallel
per phase per polarity, and 72 capacitors and 72 fuses. It is only one of a handful
of hydrogen-cooled exciters
- Repairs to a hydrogenertor stator's main and pilot exciters
Motor Projects
- Multiple projects involving the full analysis and redesign of squirrel cage induction motors, changing of bar profiles to change speed-torque characteristics and speed, and to increase fatigue life
Capabilities for Exciters
- Inspect, test and clean
- Fuse and diode replacement
- Repair or replace glass banding
- Skid refurbishment
- TC replacement
- Rewind
READ MORE about NEC's exciter services.
Capabilities for High-Voltage Motors
Squirrel Cage Induction Motors
READ MORE about NEC's services for squirrel cage rotors.
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