A ventilation study was performed on a Hydro Generator as part of an uprate study. The objective of the study was to determine if the existing ventilation would be sufficient to maintain satisfactory operating temperatures when the unit for a proposed uprate to 134 MVA from 82.5 MVA (62% increase).
Combining test measurements of stator vent velocities with EME equivalent resistance network modeling, a major stator vent air flow unbalance was discovered and corrected. Through the modeling, it was determined that the cause for the severe stator vent flow imbalance was instability of operating the rim mounted blowers in parallel and that the issue could be resolved by making which of the modifications identified below?
What would you do?
A.) Re-design rim fans
B.) Re-design rim fan shrouds
C.) Install baffles between the poles
D.) Modify end-turn vent restrictors
The answer is C.) Install baffles (inter-pole baffles)
The selected solution was to install the inter-pole baffles as this addressed the root cause of the axial flow imbalance. The model indicated that the bottom fan was supplying all of the air, while the top fan was only maintaining the pressure (operation at shut-off pressure). As a result, nearly 75% of the total stator vent air flow was through the top half of the stator. The root cause of the flow imbalance was the low resistance to axial air flow through the inter-polar spaces. Installation of the inter-pole baffles increased the resistance to axial air flow and balanced the flow through the stator vents to within a few percent as predicted by the model, and then confirmed by measurement.
Options B.) and D.) were also presented to the customer as additional modifications that could be implemented with the pole baffle installation to increase the overall air flow through the stator vents. The customer elected to reduce the amount of the up-rate, and as a result the additional corrective actions were not required. Additionally, the performance was significantly improved through installation of the inter-pole baffles, further reducing the need for additional corrective action. One of the units has been operating for about 20 years with the new inter-polar baffles.
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