Case Studies: Brushless Exciters

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Project involved the total rewind and refurbishment of a hydrogen-cooled exciter, one of the largest of its type.

• Unit nameplate: Siemens MarkII, 4500 kW, 525 Volts, 8575 Amperes, and 3600 rpm. This unit provides excitation for a generator rated at 970 MVA, 22 kV, and 3600 rpm.
• The unit had been exhibiting an increasing demand for field current over a period of about a year.
• During the scheduled outage, inspection of the unit showed the failure of phase its lead retention band and the unit was scheduled for immediate refurbishment and rewind,
• Because the the failure of the retention banding there was considerable damage to phase leads and deformation of 34 of 36 phase lead connectors resulted.
• Lack of support also lead to extensive damage to inboard and outboard armature coil connections
• NEC redesigned phase lead connectors to add more stability. Profile was reshaped and cross section was increased.
• Diode wheel fuses and capacitors were tested and replaced.
• All parts of the exciter were thoroughly cleaned: two diode wheels, PMG, AC armature and its components.
• NEC rewound the unit's AC Exciter armature and replaced and the glass banding.
• After the start-up was completed and the unit running at full load, the exciter DC currents was measured
  at lower levels than the pre-outage level.