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Midlife Crises Now Under Way for 30-plus-year-old Generators with Hydrogen-Inner-Cooled Stators

Authored by professional engineers from Alstom Power and Portland General Electric, an article with the optimistically promising title "A permanent solution to generator vibration problems" appeared in the April 2006 issue of Power Magazine, the regularly published online journal of the Electric Power Conference and Exhibition.

It concerned upgrading the high-pressure turbine at PGE Boardman, a large coal-fired power plant producing 15 percent of the electricity in Oregon, in advance of a major refit to boost its output and run Boardman as a baseload unit.  Like many other coal-fired plants of the last 40 years, Boardman "has generators whose stators are directly inner-cooled by hydrogen.  With three decades of experience operating these generators now under its belt, the industry has identified several fleetwide chronic maintenance problems that can result in extended, unplanned outages and significant lost-generation and repair costs."

In 2004, PGE realized their planned upgrade also gave them the opportunity of "permanently solving...four known generator problems.  The solution would be costly, bu the payback would be significant--long-term, reliable operation of the plant at a much higher output."  Those problems included cracking of turbine retaining rings and rotor top teeth, loosening of stator core laminations, and high stator endwinding vibration levels that required significant annual downtime for inspections.  Even at 40 percent output, "the unit experienced relatively high endwinding vibrations, on the order of 10 mils [hundredths of an inch]."

During the overhaul, Alstom Power Inc. rectified each of these.  To address the vibration issue, "Alstom designed a free-floating support system...[with] multiple pressing plates equally spaced around the outside of the winding head," which, "creates a very cohesive endwinding basket structure that is resistant to vibration."  As a result, tests after the turbine overhaul had less than 25 percent of the vibration of pre-overhaul levels.  Significantly, this was verified by an "endwinding vibration monitoring system," enabling technicians to keep an eye on that aspect of turbine wear in the furture, too.

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