Doyle Unit 1 is a legacy GE 7B gas turbine with a 64-MVA/14.4-kV, 30-psig hydrogen-cooled generator. A thorough investigation of the engine by Oglethorpe Power Corp engineers revealed degradation of its asphalt insulation and estimated remaining life at less than two years. Several equipment condition and operational performance gaps with the hydrogen cooling system also were identified during unit inspection and testing. Bringing the cooling system up to industry standards would have required an investment of $400,000 to $500,000 (est), according to Fleet Manager Michelle Crane.
The plant’s goal was to find a safe alternative to hydrogen coolant for the predicted remaining life of the generator. Staff researched the practicability of using helium as the cooling medium in a hydrogen-cooled generator and in April 2018 the idea was tested and implemented on Unit 1.
A hydrogen-to-helium conversion was considered primarily because of condition issues with both the hydrogen cooling system and generator stator. Regarding the former, the original OEM control panel was 50 years old, the hydrogen scavenge system was in poor condition, there was no haz-gas detection for hydrogen and no emergency auto-purge functionality, etc.
Regarding the stator, a rewind was recommended because of its deteriorating condition, polarization index was less than 2 megohms, debris from the degrading asphalt insulation was accumulating at both ends of the stator, glass tape was unraveling on series loops at the collector end, stator end basket was in a weakened condition, etc.
Doyle 1 was tested successfully using helium as the cooling medium on Apr 13, 2018. For verification purposes the machine was operated from a minimum load of 52 to 60 MW while varying reactive power from +30 to -10 MVAr. Helium and field- and stator-winding temperatures were monitored and captured throughout the trial run, along with gas pressure.
Generator temperatures were 20-25-deg-F warmer with helium as the cooling medium than with hydrogen. But the average gas temperature of 166F was well within the Class B insulation maximum temperature requirement of 266F. Helium purity, monitored with in-place calibrated instrumentation, was maintained without need for scavenging; no windage loss was observed. Testing proved the unit could operate with sufficient cooling to the generator, and without operating limitations, while using helium.
During the 2018 summer season, the machine was dispatched 12 times for peaking purposes, operating 57.2 hours. As predicted, a generator fault caused by age and insulation deterioration did occur as the summer was winding down; however, an inspection found not connection between helium cooling and the failure. The bottom line: Testing and operation of Doyle 1 verified the viability of helium as a possible alternative to hydrogen for generator cooling.
Wireless camera keeps extra set of eyes on Mulberry Cogen’s equipment
In any fast-paced O&M environment, there are times when certain pieces of equipment require additional, temporary, periodic observation—such as valves, exposed tank levels, pumps and motors, or even areas of the plant grounds.
Generally, this function is performed by roving operators who must break away from their rounds to travel to the area requiring observation. This can be a challenge, especially if the item requiring enhanced monitoring is located on top of the HRSG or in some other hard-to-access location. Plus, this approach permits critical observation for only as long as the operator can remain at the scene.
Simple solution. Most sites now have comprehensive security-camera systems and a wireless camera for equipment monitoring can be plugged into one of its unused channels, says Plant Manager Allen Czerkiewicz. Mulberry’s wireless camera is moved to wherever needed, allowing the CRO a live look at the area of concern while he or she remains in the control room. The display is viewed like any other camera on the system.
An added benefit is that the camera also records, enabling staff to look back at any event that occurred. After the temporary monitoring assignment is completed, the camera is picked up and brought inside or moved to another location.
The camera has been used extensively since it was purchased, and has been improved by affixing a magnet to the camera base (photo) and by purchasing attachments that allow the it to be installed practically anywhere—from round railings to flat metal surfaces. The camera and receiver are powered from 110-Vac outlets, although a battery-powered inverter can be used in remote locations.
Platform facilitates borescope access, improves safety at Lincoln
The gas turbines installed at Lincoln Generating Facility were not equipped with platforms for accessing the power-turbine sections of the units (photo left). For example, reaching the borescope ports on the lower portion of the turbine required staff to place two planks across the open space as a temporary work platform.
Plant Manager Brad Keaton said that during monthly VPP safety committee meetings this maintenance practice was discussed and participants agreed a change was required. Site personnel developed a work scope and solicited quotes from qualified contractors to install OSHA-approved platforms (photo right).
The safer work area created allows disassembly, inspection, and reassembly of the turbine during borescope inspections. Temporary scaffolding during outages is no longer required. The project has a three-year return on investment.
