Track Corona Discharges with Ultrasonic Detector

By Stan Bullock, Manager
Infrared Imaging Div. Midwest Electrical Testing & Maintenance, Milwaukee, Wis.

Ultrasound detectors perfectly complement infrared instruments for the routine examination of electrical equipment. While infrared inspections allow operators to detect light that the eye cannot see, ultrasound allows them to detect sounds that the ear cannot hear. Corona, arcing, and tracking, which may not show up on infrared inspections, are revealed by ultrasound. Inspectors at Midwest Electrical Testing & Maintenance now pack an ultrasonic detector to scan primary power on all systems they inspect.

The instrument used is the Ultraprobe 2000, a hand-held ultrasonic detector manufactured by UE Systems, Elmsford, NY Devices such as this are battery-operated, portable, and have a convenient pistol grip. The Ultraprobe 2000 has an adjustable frequency dial that provides tuning capability from 20 to 100 kHz. Headphones allow the user to "hear" the ultrasound, complimenting the indications provided by the deflection of an analog meter.

A person with no experience can learn to use the instrument in about an hour. However, it takes more experience to be able to identify individual sounds. Usually the operator begins an inspection by fully opening the scale and broadly scanning the equipment. If a noise is detected, the operator uses the tuning and directional capabilities of the instrument to home in on the source.

Also available is a computer program that helps customers to perform annual trend analyses. When watching for trends, the dial is set at a specific level and data recorded. On returning the following year, the dial is reset to the same level to find if the problem has increased or decreased.

Detect tracking on arresters
In surveys performed for one electric utility, potheads, bushings, and arresters in substations were scanned (Fig 1). This utility was experiencing arrester failures that caused troublesome power interruptions. The ultrasonic detector indicated tracking on several of the arresters.

Further investigation isolated ultrasound coming from an area of intense corona and arcing. On inspection, the connection was found to be pitted and corroded. Fortunately the 138-kV equipment was only operating at 12% load at the time. Had it been operating at full load, the connections would probably have disintegrated, interrupting power to 80,000 to 100,000 customers.

Interlocked, metal-enclosed switchgear (Fig 2) serving a large shopping mall was examined with an ultrasonic detector. Ultrasound detected in a very localized area. Closer examination suggested arcing and tracing was occurring within the switch. Utility and mall maintenance personnel were called to de-energize the system and open the switch to perform a visual inspection. They found corona and tracking had destroyed 35% of the switchblade and that the contact surface was close to failure.

Transformer voltage traps that run hot cannot be detected with infrared equipment because the heat from the transformer barrel is greater than that from the loose connection. During inspection of a 1500-kVA dry transformer (Fig 3), serious tracking and arcing was heard in the secondary voltage tap. When the connections were dismantled, carbon buildup and arcing traces were found in the tap contacts.

At the powerhouse of a large manufacturer, 13.8-kV unshielded secondary power cables exit an old building in 4-in. fiber ducts, passing through 18-in. thick walls to supply power to five areas of the plant. Infrared scanning showed nothing wrong, whereas ultrasonic inspection (Fig 4) showed serious arcing in the fiber ducts. When the customer's maintenance personnel took the equipment apart they found that the cable was close to failure.

In each of these cases, problems were detected that could not have been found without de-energizing the equipment. In some cases, even a visual inspection would not have found the defects.