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Little robots boost reliability of big power plants

March 22, 2016
The ink is fresh on a contract signed with Alinta Energy for GE to provide robotic inspections of 19 generators across all of the power company’s gas-fired plants in Australia and New Zealand. The robots didn’t sign, although their dexterity plays a major part in this agreement that underscores a new era of optimising machine productivity, and reducing the risk of damage to expensive assets—this may, eventually, have a cost benefit when it comes to insurance premiums paid to protect such assets.
GE’s “little” robots—which have been developed over several years by Alstom Inspection Robotics, part of GE’s 2015 acquisition of Alstom, a global leader in power generation—save unnecessary downtime in any power plant, by enabling inspection of generator components without removal of the rotor. Some 3,000 generators in the Asia-Pacific region alone could streamline maintenance procedures with GE’s robotic inspection technology.

The combined robotics package provided by GE to Alinta includes rotor-in-place inspection with the DIRIS ‘Small’ robot, a precision tool that sends probes through air gaps as small as 9 millimetres to conduct critical testing of the generator components, as well as the TurboRotoScan inspection, which tests the condition of the rotor retaining rings. Retaining rings are subject to extreme mechanical stress during operation of the generator; resulting stress corrosion cracks can lead to abrupt failure of the rings and irreversible damage to the generator.

Regular inspection of power generators has always been vital to ensuring their effective maintenance, but previously such inspections relied on the rotor being removed—a procedure that risks damage to the rotor and to other parts and was a major insurance-company concern. In the past, removing the rotor for inspections also cost Alinta three weeks in generator downtime. Inspection by purpose-built robots, on the other hand, can be conducted with the rotor in place, takes two to three days and dramatically reduces unnecessary risk of damage. If a robotic inspection indicates the need for repairs, the urgency of that need can be assessed, and downtime can be planned.
The biggest win for us was the ability to use this technology on non-GE units and apply it across our whole fleet.

As DIRIS slides its slender arm, accessorised with camera, tiny torch and compact mirror (for 360o vision), into the tight air gap between the rotor and stator, it records a video of the stator bore surface and the rotor body surface. During video inspection, the GE service engineer looks for signs of deterioration—such as delamination of the core packets—and checks for foreign objects and contamination. DIRIS also detects stator-core short circuits which, if undetected and not corrected, can lead to core burning and the need for costly, time-consuming repairs. Then there’s DIRIS’s wedge-tightness probe: it may have an uncomfortable ring to its name, but if the wedges in a generator come loose, vibration occurs which can lead to accelerated ageing of the winding insulation and support system. DIRIS flags the necessary fix.

“This generator-inspection system provides the current condition of the generator and indicates any issues or early warnings of failures,” says Gareth Williams, manager of engineering services, power generation, at Alinta Energy. The frequency of major inspections on power generators is typically every eight years. The speed and accuracy of a two-or-three-day robotic inspection will allow Alinta to increase the frequency of major inspections to every four years, doubling its opportunity to pick up component deterioration, while still vastly improving machine availability.

“Robotic inspection gives us a good indication of the condition of the machine and whether it’s going to run for the next four years, and whether we need to pull the rotor out then. Or we might decide, based on the condition, that we’ve got enough life to take us to the next four years. It provides us with greater flexibility.

“The biggest win for us,” he adds, “was the ability to use this technology on non-GE units and apply it across our whole fleet.” Williams describes the GE DIRIS as “probably a good 10 years ahead of most of the other competitors,” because of “its adaptability and the size which lets it fit into most machines—that’s one of the problems with some of the other robots, you can’t fit them into the smaller units”.

Alinta runs generators manufactured by GE, Alstom, Mitsubishi and Brush. That DIRIS and TurboRotoScan robots can be used to thoroughly inspect generators produced by almost any Original Equipment Manufacturer (OEM) allows GE to offer inspection services across the power-generation industry and provide a one-company solution to inspection needs.

GE TurboRotoscan

GE's TurboRotoscan detects stress corrosion cracks at the inner and outer surface of the retaining ring on wide range of generators.

Such offline (conducted when the generator is not in operation) inspections, as covered by the Alinta Energy contract, allow service engineers to provide detailed reports on the health of the machine asset. Over time, such reports contribute to an understanding of trends in machine-component wear and tear. Wynand Pienaar, senior product manager, GE Power Services, says, “When we do these robotic inspections on a regular basis, we develop a fingerprint of how situations develop. The robots provide consistent, comparable assessments, which allow us to evaluate and trend the condition of the generator. This allows us to determine the required work scope for the next scheduled outages, to mitigate risk of failure.”

Says Williams, “We worked closely with GE’s team and our insurance company, FM Global, to demonstrate the benefits of the DIRIS and TurboRotoScan technologies for ensuring effective generator inspections.” These processes,” he adds, “have been endorsed as best practice by our insurance company, which indicates that most generator claims arise through rotor removal and replacement activities”.

In Williams’s opinion, reliable, rotor-in-place testing could sufficiently reduce insurance company exposure to damage claims such that premiums can be negotiated downwards. “In time,” he says. “It’s obviously something that they look on favourably. We have a good relationship with our insurance company, and they actually have a lot of technical engineers within their business, so they support this procedure worldwide.”

As part of its expanding predictive-maintenance portfolio for worldwide power-plant operators, GE can combine offline robotic inspection with Generator Health Monitoring—an online monitoring platform that allows remote condition assessment of any generator during normal operation. Sensors fitted to the machines provide constant real-time data, to which GE applies analytical software that allows component failure to be predicted and addressed well before it occurs. “That’s another area we have been exploring with GE,” says Williams.