Remember the slogan, "Never trust anyone over thirty?" Chances are you've joined the ranks of the over-thirty generation and are dealing with your own personal "maintenance" issues—not unlike the fleet of generators at larger U.S. power plants, whose average age is about 30. Given the continuing growth in U.S. electricity demand and the cost and difficulties of building a new coal-fired plant, it's safe to conclude that our existing coal-fired fleet will be with us for some time to come. An aging Mickey Mantle quipped, "If I knew I'd live so long, I would have taken better care of myself." Likewise, the utility industry needs to pay closer attention to generator maintenance to ensure the future viability of these "middle-aged" plants.
A significant proportion of the existing U.S. coal-fired fleet 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. Portland General Electric's (PGE) Boardman Power Plant has experienced these problems first-hand. But rather than yield to the temptation to make a quick fix, Boardman sought a permanent solution to the nagging problems.
The long road back
The single-unit, coal-fired Boardman plant, which celebrated its silver anniversary this year, looms over the high desert of eastern Oregon. Westinghouse Electric supplied the frame 2-105 x 245 generator, each of whose stator bars is directly cooled by hydrogen through a single tube stack (Figure 1). The generator has the following nameplate ratings at 60 psig H2:
- Output: 590 MVA
- Stator voltage: 24 kV
- Stator current: 14,193 A
- Rotor current: 4,337 A
- Rotor voltage: 475 V
- Power factor: 0.95
1. Hydrogen flows through the generator non-drive end stator bar cooling tubes. Courtesy: Alstom Power
During the first few years following its commissioning, the Boardman plant was operated on a very limited basis, and it essentially didn't operate at all between 1986 and 1988. The plant didn't reach a 40% capacity factor until 1990. But even during those early years, the following generator problems began to become apparent:
- Stress-corrosion cracking of the 18Mn-5Cr retaining rings. This led PGE to regularly inspect the rings to detect cracks before they could propagate to a critical size.
- Rotor top-tooth cracking. Westinghouse said this might be a problem, and PGE's experience confirmed that it was indeed.
- Loosening of the stator core laminations. This problem was first identified at the Boardman plant by a generator inspection during a 1993 outage. The laminations were not retightened at that time.
- High stator endwinding vibration levels. Throughout the 1980s and 1990s, the unit experienced relatively high endwinding vibrations, on the order of 10 mils. To mitigate the damage the vibration might cause, PGE instituted annual outages for endwinding inspections.
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