Predictive maintenance for power plant operations is vital. Power plants must shut down at least once a year to check for damage to boilers and conduct other maintenance inspections – a time consuming, dangerous, and sometimes imprecise process. Robotic systems enable faster generator inspections without removing rotors from machines. To automate this, GE developed remote, robotic inspection tools, first deployed in April 2016, to inspect Alinta Energy’s gas-fired power plants in Australia and New Zealand. The project was developed through GE’s Alstom power generation business, which it acquired in November 2015. GE’s legacy Alstom business has been developing robots for generator inspection since the 1980s.
DIRIS and TurboRotoscan inspection systems
Developed by the Alstom power generation business, which GE purchased last year, GE’s Diagnostic Inspection Rotor In-Situ (DIRIS) and TurboRotoscan inspection systems will alert Alinta Energy engineers to potential generator issues and give them time to evaluate their options. Under the terms of the inspection agreement, GE will inspect 19 generators manufactured by GE, Alstom, Mitsubishi, and Brush, at seven of Alinta Energy’s gas-fired power plants in Australia and New Zealand. The DIRIS robot will provide Alinta Energy with modern robotic instrumentation and tooling to allow fast and reliable remote inspection of the turbogenerators. The DIRIS robot performs critical tests of the generator stator iron core laminations and stator radial wedging system by conducting a visual (video) inspection of the inside surfaces of the rotor and stator. These tests would normally be part of an overhaul regime after removing the rotor using manual semi-automated tooling. A low-flux test permits the identification of short circuits between the stator iron core laminations which could otherwise develop into critical hot spots severely damaging the generator. A tightness test of the radial wedging system permits identifying loose wedges that could otherwise promote movement of the stator bars and damage to the stator winding insulation system.
“These robotic inspection solutions have been endorsed by Alinta’s insurance company as a best practice for gas-fired power plants, reflecting the industry’s support for these technologies to support plant reliability,” says Philippe Machard, GE’s Power Services generator product line leader
GE’s robotic technology TurboRotoscan will inspect the generator retaining rings while the rotor remains in place and the retaining rings are mounted on the rotor. The scanner also contains probes to check the retaining ring’s outside surfaces.
“The development and deployment of robotic inspection technologies further enhances the capabilities of gas power plant operators to utilize predictive maintenance solutions to reduce costs and increase power plant reliability and efficiency,” Machard says.
In the past, baseload operators typically adopted schedule-based maintenance programs, performing service on key components at set intervals, Machard explains. However, this method is no longer suitable for the way power plants are operated in deregulated regions and areas where plants have to quickly respond to intermittent renewable generation. Many baseload plants are now being run in cycling or two-shifting operation and at varying loads, which puts key plant components under greater stress, potentially lowering equipment lifespan.
“Plant owners need a much deeper, real-time understanding of the condition of their equipment in order to ensure the availability and reliability of their power plants,” Machard explains.
Generator condition assessment
The generator is a key part of a plant, and early indication of component deterioration can facilitate cost-effective decision-making and planned maintenance, avoiding costly forced outages and emergency repairs.
The condition of a generator can be assessed either offline when the machine is at standstill, or online, when the machine is in operation. Offline, assessment can be performed with the rotor in-situ or with the rotor removed. Online, the generator can be monitored periodically, where a snapshot of the machine condition is taken and evaluated. Alternatively, the machine can be continuously monitored online. A remote connection to the GE Care Center provides the highest level of assurance. With GE’s Generator Health Monitoring plant operators can evaluate the information in real time, trend the condition over time, and identify any deterioration at an early stage to best plan outage activities.
“Most recently, there have been important developments in the area of robotics for offline inspection while the rotor is in place and continuous online monitoring,” Machard says. “The use of robotics to avoid the unnecessary removal of a rotor is a key area where advances are being made.”
DIRIS – GE’s legacy Alstom business has been developing robots for generator inspection since the 1980s. DIRIS allows fast and reliable inspection of the generator using robotic equipment while avoiding the time and costs associated with rotor removal. Many iterations of robots for DIRIS have been produced to inspect a large number of machine types.
The latest generation, DIRIS Small, is designed for use in small, air-cooled machines with cylindrical rotors. The robot has two drive units, one that sits on the rotor retaining ring to perform circumferential movements, and an axial drive unit that extends or retracts a thin, flexible, Teflon-coated band along the length of each slot. A magnetic probe carrier sits at the end of this flexible band.
One of the probes that can be attached to the DRIS Small robot includes a small video camera, a light source, and a mirror to enable visual inspections of areas that are inaccessible to humans while the rotor is in place. As the probe carrier moves up and down the length of each slot section, a video recording is made – both of the stator bore surface and of the rotor body surface.
“The visual inspection provides a 360° view of the inner stator core laminations, stator wedges, rotor surface and wedges, balancing weights, and the inboard ends of the retaining rings,” Machard says.
TurboRotoscan – Although reaching within the generator stator-rotor air gap to perform various assessments is important, it is also essential to check the integrity of the generator retaining rings. The rotor has two retaining rings located on the rotor shaft around the field-end winding. These are the generator components that are subject to the highest mechanical stress. Stress corrosion cracking (SCC) of rotor retaining rings, made from 18% manganese and 4% to 5% chromium alloy, emerged as a worldwide problem for the industry at the beginning of the 1980s. These cracks can lead to abrupt failure of the retaining ring in running conditions, causing irreversible damage to the generator. TurboRotoscan detects stress corrosion cracks at the inner and outer surface of the retaining ring on turbo-generator types.
Generator inspections will be performed through 2020. Machard says the technology is already available for large steam and nuclear plants, and GE is set to continue testing in Australia, Brazil, Dubai, France, Germany, India, Italy, Malaysia, Spain, Sweden, Switzerland, and the United States.
About the author: Arielle Campanalie is the associate editor of TES and she can be reached at firstname.lastname@example.org or 216.393.0240.