Installation of two pumped-storage turbines at Salt River Project’s (SRP) Mormon Flat and Horse Mesa pumped-storage powerhouses increased each facility’s generating capacity and efficiency but resulted in rough operation when the units were run too far off their best efficiency point (BEP). To correct this problem, SRP personnel applied depressing air to both units. Additional work in the form of a separate roughness air system was needed for the second unit. Since this fix, the units have operated acceptably.
Understanding the problem
The two 30-year-old Francis pumped-storage turbines were replaced in 2002 by GE Norway. One was at Horse Mesa Dam and the other at Mormon Flat Dam, both on the Salt River northeast of Phoenix. These are two of several dams operated by SRP and the only ones with pumped storage capability. The project involved replacing the turbines with new stainless steel runners that were more efficient and resulted in an uprate for both.
For Horse Mesa, the new turbine resulted in a capacity increase of 19 MW, to 115 MW from 96 MW. For the Mormon Flat unit, capacity increased to 54 MW from 40 MW. Of course, the project included replacement of the headcover, wickets gates, etc., and substantial modification of the discharge ring and draft tube.
However, one problem that operators of SRP noted almost immediately with the higher-efficiency turbines was that the units ran very rough when the wicket gates were operated more than just a few percent off the BEP. This situation caused additional mechanical problems in one of the units, particularly when the unit was transitioned from generate to condense mode.
Solving the problem
The existing control system was configured to apply depressing air when the unit “wickets at zero” switch was made. For one of the two new units, in an attempt to achieve acceptable operation without resultant mechanical issues, the vendor commissioning engineer asked SRP personnel if it would be possible to adjust the logic to apply depressing air earlier. At speed-no load, the wicket gates are about 20% to 22% open. The transition to condense logic drops the shutdown solenoids at that gate position, driving the gates closed at gate timing speed. Plant personnel adjusted the logic to open the main depressing air valve at 15% gate opening when transitioning to condense mode. Essentially, this used depressing air (normally used to depress the water out of the runner for condense mode operation and for pump starts) as roughness air.
|Installation of a blower with a variable frequency drive allows the Salt River Project to better control operation of a pumped-storage unit by shutting down and restarting as the unit unloads when the wicket gates approach best efficiency point.|
The difference during the operational transition from generate mode to condense mode in how the unit sounded and felt with this modification was so impressive, the same logic was applied to the other unit. With this change, there is very little difference in the amount of bubbles in the tailrace, there was a minimal additional drop in depressing air pressure for the transition, and the unit maximum megawatt consumption in the transition to condense was much lower. Note that this solution should work well on all pumped-storage units where the turbine is below tailrace level.
For the first unit, adding a manual bypass valve and air flow meter to the main and bypass depressing air valve header and throttling the manual valve to achieve about 250 cubic feet per minute (cfm) resulted in acceptable operation throughout the load range. On the second unit, however, testing indicated that one flow rate throughout the load range was unacceptable and that a separate roughness air system in addition to the existing depressing air system was needed. The testing included measuring decibel levels on the turbine deck with the unit at various wicket gate positions and determining how many cfm of air would be optimal for that gate position.
With this data in hand, we purchased and installed a blower with a variable frequency drive to control output. The blower for this unit is rated at 500 scfm at a discharge pressure of 10 pounds per square inch. As the wicket gates approach BEP, the blower shuts down and then restarts as the unit unloads, with additional attention paid to headcover vent position. This design minimizes wear and tear on the depressing air compressors and gives us a much more controllable system, using an analog out from the unit programmable logic controller to control blower discharge based on wicket gate position. Again, these modifications greatly improved how the units sound and feel as they load and unload.
Results and lessons learned
As hydroelectric plants age and rehabilitation work is undertaken, we all need to anticipate activities like this to maximize our return on investment. As part of the commissioning process, some need for improvement to the vendor design should be identified and considered.
— By John R. Hunter, now retired, was senior computer analyst and O&M supervisor in thermal and hydro plants and O&M supervisor for instrument, controls and electrical for hydro generation, Salt River Project
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