Alcoa doesn’t like surprises, specifically unplanned downtime that interrupts power from its Warrick Power Plant, Newburgh, Ind. But in a generating station where the newest unit is dated 1970, it's hard to eliminate unplanned downtime without thermographic switchgear surveys.
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The days of opening panel doors for those surveys ended with the plant's adoption of Alcoa's Electrical High Voltage Safety Standard 32.60, which is even more stringent than NFPA 70E. “Our two options for open-door inspection of medium-voltage switchgear under the Alcoa safety standard are to wear 100-calorie protective gear or operate the infrared camera remotely from a safe distance,” says Brent Welz, senior power reliability engineer at Warrick. “The suit imposes physical limits on what a thermographer can do and see while using the IR camera, not to mention that it can be intolerably hot inside that suit. Remote operation of the camera eliminates the suit, but adds its own hurdles to camera setup and aiming.”
IR surveys have proven their worth for Alcoa, so Welz explored IR-transparent windows that allow closed-door inspections. The windows used on electrical cabinets typically transmit IR energy only in the wavelength of 0.13 μm to 10 μm. However, the best range for an infrared camera is 8 μm to 14 μm. If the window cuts off at 10 μm, the camera misses more than 70% of the energy emitted by the target. This reduction results in lower image quality and compromises temperature values computed with camera algorithms established for the 8 μm to 14 μm range.
“The reduction in transmitted IR energy is critical when you're looking at switchgear,” Welz explains. “Switchgear is usually oversized for the load, so it runs cool. This means the IR camera must detect subtle temperature differences in an environment with very low levels of emitted infrared.”
IR windows have other practical issues as well. The hygroscopic crystal materials absorb moisture over time, reducing image quality. The material is also easily scratched or cracked, so a camera lens barrel cannot be pushed against it. Windows can break if the panel flexes or absorbs an impact, and will accumulate dirt and oil over time, so doors must be opened to clean the inside surfaces.
“The greater concern for us was the cost of the windows - $300 to $600 each,” Welz says. “Using a camera's standard lens, you'd need more than one window for each cabinet, or with a wide-angle lens, you'd probably need a larger, more expensive window because the wide angle usually has a larger-diameter front optic.”
Warrick engineers planned to try IR windows despite cost concerns, but then learned about the Viewport from Mikron Infrared (www.mikroninfrared.com). “At $50 for a non-locking port and $85 for the locking version, it was an easy decision to change direction,” says Welz. “The Viewport is part of a system that requires a special wide-angle lens, but our thermography service has the lens.”
The port uses no window and allows a wide-angle Mikron SpyGlass lens to scan the interior of a cabinet through an opening just 1/2-in. diameter. The patented port is UL approved for NEMA Type 1, 2, 3, 3R, 4, 5, 12, 12K and 13 enclosures. The lens/port system enables closed-door thermal inspections of connected electrical switchgear, while maintaining the original safety rating on cabinets energized to 15,000 volts.
The Viewport is designed to dock with the cone-shaped tip of the SpyGlass barrel. The tip works with the port like a ball-and-socket joint, allowing the camera to be rocked at different angles to look up, down and to the sides, as well as straight ahead. When not in use, the port is covered by a sealed, screw-on or lockable cap.
The SpyGlass lens has a wide field of view (53° horizontal by 40° vertical, or 66° diagonal) and provides temperature measurement accuracy within 3°C. Focusing as close as 2 in. and providing great depth of field lets it work where components are crowded or near the door.
“We currently have about 60 ports installed, and we add more during scheduled outages,” Welz says. “With the port and the lens, there's a defined area you can view at a certain distance, and we position the port to adequately cover the cabinet interior. We've been using one port per cabinet.”
Coverage and image quality are as good today as when they were allowed to remove the cabinet doors, he adds. “And an entire unit's switchgear can be scanned in about 45 minutes with the port, compared to 4 to 5 hours if we removed the doors and worked the camera remotely or with the required PPE.”
The only PPE needed is Alcoa’s Level 2: hardhat, safety glasses, ear plugs and no synthetic fabrics. “It's a big step down from the 100-calorie suit, and makes the job fast and easy,” Welz adds. “The ports may be even more valuable as we apply them on our 480-V equipment where the cramped nature of the cabinet makes a close-focusing, wide-angle lens valuable. We think the ports will also facilitate our use of ultrasound detection, which we're currently implementing. Eventually, we may have enough of them installed to justify buying a SpyGlass lens for our own Mikron 7515 camera, which we use for day to day work and verification of our repairs, etc.”