Air gap monitoring helps out at Grand Coulee

19 October 1998

Rated capacity at Grand Coulee Dam, the largest hydroelectric plant in North America, was increased from 6494 MWe to 6809 MWe after rewinding of three of the six generators in the plant's Third Powerhouse. With ratings of 805 MWe each, these three generators are the largest of their type in the world.

Three of the six units in the Third Powerhouse are of an air-cooled design supplied by Westinghouse while the other three – the ones that have been upgraded – are direct water cooled units supplied by Canadian General Electric.

Originally rated at 700 MWe each, these three units, G22, G23 and G24, were built and installed during the late 1970s. Almost from the start, they suffered from cooling system water leaks and operation problems caused by air gap irregularities. Canadian General Electric made extensive modifications after a stator core to rotor contact failure caused by an out-of-round rotor.

The original rotors were a floating rim design to allow the rotor to move on its spider and find its own centre of rotation. Mechanical instability of the rim and rotor pole assemblies resulted in a distorted rotor, causing structural failure of stator frame to core connections. The manufacturer corrected the mechanical problems by rounding out the rotor rims and shrink fitting them to the spiders. The water leakage problems were addressed, but continued to cause outages.

The US Bureau of Reclamation (USBR), the plant operator, concluded the only viable option was complete replacement of the windings. In 1992, Siemens won this contract, which included rebuilding the stators during an outage of 70 days per unit. The new stator for G22 was recommissioned in December 1995. The stator frame from that unit was rebuilt and installed in G24 the following year. The stator frame from G24 was rebuilt, installed in G23, and recommissioned in December 1997.

What made the project stand out was that the work was to be accomplished without removing the rotors or replacing any of the rotating elements' components. The 'build and exchange' philosophy was also unusual, helping Siemens achieve the 70-day turnaround time for each unit. Steve Kuehn of Siemens comments that the traditional build-in-place approach would have required an outage of around a year per unit.

Project engineers had to lift out the 740 t, 3.7 m high, 23 m diameter stator rings in one piece. For this, Siemens designed and built a special lifting frame that was fitted to the plant's 2000 t overhead rotor gantry crane. The structural elements of the frame were fabricated at Siemens Power Corporation's Milwaukee plant and welded together in the Grand Coulee erection bay.

Each lift took three to four hours, with the frame's lifting points and the loadings on the gantry crane's four hydraulic rams being constantly monitored. Once clear of the rotor, each stator was transported through the power house to the erection area. Work was carried out using a rotating, dual platform stacking machine, with two teams of technicians located 180o apart installing the core laminations. This enabled them to work non-stop, pausing only to reload the platforms with laminations and take intermediate measurements. Working a 60 hour week, crews of six to eight were able to complete an entire core in less than two months.

The upgraded stators give an additional 105 MWe per unit, making a significant contribution towards cost savings, estimated at $50 million net, after project costs of $27.5 million are taken into account.

The improvement was achieved using Siemens' proven double-dovetail core-to-frame attachment method, allowing the stator cores to expand uniformly within the frame, enabling them to operate at higher temperatures. Thermal expansion was improved by redesigning the sole plate keys to permit adequate radial expansion. The original design prevented proper frame expansion, forcing core to grow circumferentially and axially, causing core buckling.

The revised stator design led to the decision to use dynamic air gap testing during recommissioning. Stator-mounted capacitive air gap sensors were installed on each unit to evaluate the dynamic condition of the rotor and stator.

Sensors were installed on G23 and G24 prior to stator replacement, to evaluate the state of the floating rotor rim and analyse the existing air gap This provided data for subsequent rim re-shrinking and construction criteria for the new stator air gap.

The large size of the combination generator thrust and guide bearing results in very large cold clearances, to allow for thermal expansion as the bearings warm to operating temperature. According to Siemens, this results in a significant amount of movement of the centre of rotation of the rotor during warm-up. When the units were first commissioned, it was determined that this did not cause problems with the original stator and rotor combination. However, the design of the new stators differs from the old ones, so it was felt that air gap monitoring should be used to ensure that nothing was happening during warm-up that would be detrimental to generator operation. It would also enable in-depth analysis of the rotor and stator dynamics during recommissioning and early service life of the modified generators.

When the contract for replacing the windings on the three generators was being negotiated, suitable air-gap monitoring equipment was not readily available, so was not included within contract scope. However, between then and commencement of the first stator replacement, technology had advanced, making proven equipment available at a reasonable cost. USBR issued a change order, and VibroSystM Inc. contracted to install stator-mounted air gap sensors, with the first system installed on G22 during the summer of 1995. It comprises 12 capacitive sensors located at 30 degree intervals, 300 mm from the top of the core. Each sensor is connected to one of six data acquisition units mounted on the stator frame. Each of these units has dedicated signal processing capability for two channels and is linked to a PC-based controller via an RS485 cable.

Air gap monitoring proved its value during commissioning of G22, when it was proved that larger than anticipated currents in the winding eccentricity protective circuits were not caused by non-uniform air gaps. Without air gap data, extensive investigation of rotor and stator roundness would have been carried out, taking several days. In fact, the high current was the result of differences between the original lap winding and Siemens' new wave design.

The system was also useful to assess transitory dynamics. During an 800 MWe load rejection test, it was found that the air gap reduced equally at 90 degree intervals around the stator and the rotor expanded radially by 3 mm during acceleration to 155 per cent of the 86 r/min synchronous speed.

Subsequently, sensors were installed on the G23 and G24 stators. On G24, it was found that the rotor rim was loose and not concentric with its centre of rotation. In addition, the upper and lower rim layers were moving in different directions. These problems subjected the stator poles to high and unpredictable stresses. Similar, although more severe, conditions on G23 had resulted in a catastrophic rotor/stator strike in 1980 requiring a 20 per cent rewind and replacement of 15 per cent of the core laminations.

G24 was recommissioned in December 1996, and air gap data revealed significant improvement in rotor tightness. G23 was recommissioned a year later, in December 1997. Again, air gap information confirmed the rotor rim was significantly tighter during dynamic loading. Roundness, measured as the difference between maximum and minimum radius, improved from the original 2.31 mm to 1.40 mm after shrinking.

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