Hearing about the catastrophic failure of a similar plant in Austria prompted Pacific Gas and Electric Company to proactively repair all three units at its 1,212-MW Helms Pumped Storage facility. This work, completed in 2012, will avoid a potentially serious accident that posed significant employee safety concerns and would have cost millions of dollars.
by Pacific Gas and Electric Company
Pacific Gas and Electric Company’s 1,212-MW Helms Pumped Storage Facility, located high in the Sierra Nevada Mountains in California about 50 miles east of Fresno, produces enough energy for 900,000 homes. This plant is an essential part of California’s electric grid, and a long-term outage at the facility could be costly and lead to significant operational challenges. In fact, Helms is the largest energy storage facility on the California electric system.
Construction of the facility began in 1977. The Helms powerhouse, situated between the Wishon (lower) and Courtright (upper) reservoirs at about 7,000 feet of elevation, began operating in June 1984. The powerhouse is buried deep inside a granite mountain and contains three vertically mounted pump turbine-generator units manufactured by Westinghouse. The units are fed by a headrace tunnel 10,511 feet long, connected to a 2,248-foot-long steel penstock that trifurcates into three individual penstocks. Water from the three units is discharged into the lower reservoir via a 3,797-foot-long tailrace tunnel. The effective hydraulic head is 1,625 feet.
Upon hearing about the failure mechanism of an Austrian facility in 2010, PG&E began to perform proactive inspection and repairs on the three units at Helms to prevent such a catastrophic failure. All inspection and repair work was completed on the final unit in August 2012. All units were successfully returned to service and have operated reliably since that time.
“As a result of this effort, a potential serious accident and a multi-year outage was avoided, millions of dollars will be saved and Helms will once again provide safe, reliable, affordable, clean power to PG&E customers,” said Mike Jones, a director in power generation with PG&E.
How it happened is a tale that combines technology, teamwork and a deep understanding of how the Helms facility works to move water and generate electricity.
|The rotors of all three units at the 1,212-MW Helms pumped-storage plant were determined to have cracks after plant personnel took them off line to perform proactive condition inspections.|
Problems at an Austrian hydro plant
The story begins in the summer of 2009, more than 5,000 miles away in Austria, when a major mechanical failure forced a pumped-storage facility out of service. A generator in the 276-MW Rodund II facility, owned by Vorarlberger Illwerke, had been damaged as a result of a field pole attachment failure.1 The unit was operating when the field pole attachment failed, seriously damaging equipment and creating a dangerous situation for workers. At the time of the failure, the unit had been operating since 1976. Fixing the damage at this plant took more than two years and cost many millions of dollars.
PG&E personnel became aware of the Austrian incident in 2010. While the manufacturers of the turbine-generator equipment for the two facilities are different, most pumped-storage facilities (including Helms) operate under similarly challenging conditions – multiple starts and stops daily. This operating flexibility is even more important as grid operators seek to integrate intermittent renewable resources, such as wind and solar. In fact, the Helms plant can go from dead stop to full generation in just 8 minutes.
With Helms Unit 2 scheduled to undergo a planned maintenance outage in fall 2011, PG&E didn’t take any chances and assembled a team of experts to inspect and evaluate the field pole assemblies and their attachments on this unit. The team included personnel from Voith Hydro, the Materials Testing Institute at the University of Stuttgart, and Exponent, as well as several independent and internal consultants.
Cracking was discovered on the Unit 2 rotor, so inspections were required for the remaining two units. These inspections also revealed cracks.
The repair job had to be carefully orchestrated because of the size of the parts involved. Each rotor is 20 feet in diameter and 10 feet tall, weighs 1 million pounds and spins at 360 rpm (revolutions per minute). The rotors have 20 electro-magnetic field poles attached around the circumference, and each pole weighs more than 13,000 pounds. As the rotor spins, each pole passes by the stator at 250 mph and exerts a force equal to 5.5 million pounds as it tries to separate from the rotor. The rotor assembly consists of more than 10,000 punched steel laminations stacked and held together by 600 bolts. The field poles are attached to the rotor in a double dovetail-shaped fit.
While there was cracking in various locations in the rotor field pole fit areas, the most severe cracking was located near the top and bottom of the rotor assemblies. The overhanging weight of the field pole end turns exerts a higher loading distribution in this area of the rotor. Coupled with through-rotor cooling air ducts in these regions and multiple daily starts and stops, these cyclic loads create an opportunity for fatigue cracking in the pole attachment dovetail on the rotor.
In each case, the cracking mechanism was evaluated using two- and three-dimensional finite element-based crack initiation and growth fracture mechanics calculations. Repairs consisted of removing the cracks by grinding and polishing the surfaces. That left the challenge of determining if the remaining rotor rim material was sufficiently strong to hold the pole assemblies in place with a sufficient factor of safety for continued operation.
PG&E workers used a high-tech, three-dimensional technology scanner called EXAscan to accurately assess the effect of the grinding performed on the rotor. The scanner, which was manufactured by Creaform of Levis, Quebec, Canada, is a handheld device that produces a color-coded view of the piece of equipment being scanned on a monitor, with accuracy within 40 microns.
PG&E’s Applied Technology Services (ATS) lab in San Ramon, Calif., led the innovative use of these hand-held scanners. The device also allowed crews to assess the effectiveness of the repairs by helping to create 2D and 3D models for evaluating the integrity of the newly repaired rotors. These models were used to finalize the serviceability calculations of the rotors, which were determined to be able to be returned to service with limits on start/stop cycles. All this work has established a confidence that the original rotors could remain in service until replacement rotors could be designed, built and installed on site.
Bottom line: The quick response by PG&E and Helms workers identified the problems early, preventing the failure that brought production at the Austrian hydro plant to a halt. Although Helms was temporarily off line for the repairs, Unit 2 returned to service ahead of schedule in February 2012. Unit 3, where the cracking was more severe, came back online just before Memorial Day weekend in 2012, just in time to the meet summer’s high-peak demands. Unit 1 returned to service Aug. 20, 2012.
|Repair work on the three Helms units consisted of removing the rotor cracks by grinding and polishing the surfaces, then determining if the remaining rotor rim material was sufficiently strong to hold the pole assemblies in place with a sufficient factor of safety for continued operation.|
“All in a day’s work”
Asked how he sums up the situation, looking back at the race to make repairs, Jones replied, “All in a day’s work.” He explained that he really doesn’t like to overplay what could have happened: “We work very hard to understand what’s going on in the industry and the condition of our equipment. When we find something like this, the whole team engages and works tirelessly to meet our customers’ needs for reliable energy.”
Overall, Jones said the equipment at Helms is pretty robust and has endured a very challenging operating history. “Helms is such an important flexible resource in both pumping and generating modes that it is started and stopped far more frequently than ever imagined,” said Jones. “Even with substantial cracking, the generators were well-built and durable and gave us some real confidence in their ability to continue to operate safely and reliably after the repairs.”
1Ingram, Elizabeth A., “Worldwide Pumped Storage Activity,” HRW-Hydro Review Worldwide, Volume 18, No. 4, September 2010, pages 12-20.