Utilities today are expected to reduce operating and maintenance costs while continuing to provide high-quality power and reliable service. This expectation is somewhat of an oxymoron. To achieve this expectation, utilities must resort to scheduling equipment outages in the spring or fall. Even removing a circuit breaker for inspection and maintenance must be scheduled far in advance. This task is made more difficult as utility systems are being operated at or near their rating limits; in addition, the time a device can be out of service is limited.

HVB AE Power Systems Inc. (HVB; Suwanee, Georgia, U.S.) was familiar with the x-ray technology used to inspect gas-insulated substations (GIS) after using it in its GIS equipment fabrication plant for years. In 2002, a customer asked why this technology couldn't be applied in the field. By performing x-ray analyses on circuit breakers in a utility's substation, the utility could get the results of a breaker's condition faster and less expensively than by taking the breaker apart. The idea certainly had merit. Reducing the cost of maintenance while increasing reliability would be ideal, yet several challenges remain when x-raying breakers.

For example, the interrupter tank is made from a thick metal because it's a pressure vessel designed to contain high-voltage and high-current interruptions. The breaker contact density is much greater than the density of any material found in a normal GIS facility. Moreover, the interrupter tank size is a problem, because the x-rays have to penetrate both the inner and outer walls of the vessel. This requires a powerful x-ray source. Furthermore, the x-ray device must be portable for fieldwork.

Devising a Test Process

By 2004, HVB had developed a testing process that worked in the factory. To see if it could be made portable, HVB turned to GE's Inspection Services (GEIS; Huntersville, North Carolina, U.S.), a company that had been using portable radiography equipment for performing nondestructive testing inspections (NDT) for more than 37 years in heavy industrial and power industries. Typically, testing included pressure piping, storage tanks, structural steel, rotating equipment, boiler tubing and aircraft components. GEIS saw this project as both a challenge and an opportunity to advance the technology of NDT. As a result, HVB and GEIS formed a partnership and began to develop procedures to enable field x-ray inspection of circuit breakers to become a reality.

Factory testing of the portable equipment took place in HVB's factory north of Atlanta, Georgia. After many months of simulated and staged tests, HVB was ready to try a field test on an actual breaker in an energized switching station. It had to be an HVB breaker because the x-ray's films had to be compared to the detailed production drawings of the circuit breaker parts. The engineers decided it would be best if the breaker had been in service for several years and nearing the end of a maintenance cycle.

Furthermore, it was important that the breaker be owned by a utility with whom HVB had a close working relationship. HVB had to trust the utility to keep the testing procedures and results confidential. What if the field test did not work, and HVB had to go back to the drawing board? The utility also had to have confidence in HVB that the testing would not damage the circuit breaker. A damaged breaker would negatively impact the utility's transmission system until it could be repaired. It also was necessary for the utility to have a competent and visible staff of engineers and technicians to add credibility to the validity of the test results.

Finding a Test Candidate

Public Service Company of New Mexico (PNM; Albuquerque, New Mexico, U.S.) met all of those requirements. PNM and HVB had maintained a close working relationship for the last five years. The majority of the PNM engineering staff has advanced engineering degrees and most are registered professional engineers. PNM had placed an HVB 362-kV circuit breaker in operation in the early 1980s, and the breaker was nearly due for a major maintenance inspection. After some discussion, PNM invited HVB and GEIS to come to its BA switching station north of Albuquerque for the investigation.

Field Testing Acid Test

Testing was scheduled for two days. The first day was dedicated to x-raying the “B” phase tank of the breaker. Filmstrips, much like those used in the health care industry, were taped on the opposite side of the breaker tank from the x-ray generator. Short-duration bursts of x-ray energy were passed through the tank at various locations. The filmstrips were developed and scanned into the computer. Then, technicians compared the images with the detailed drawings in the HVB database and measured dimensional points to detect wear, contact pitting and voids. They could verify if any hardware was loose by looking for gaps in the fastening devices. The process also can be used to detect any foreign objects in the breaker. All of this was completed with the breaker full of SF6 gas. Analysis of the PNM x-rays indicated some wear on the contacts, but nothing was shown that would require technicians to enter the breaker for maintenance.

On day two, the SF6 gas was removed from the breaker and the tank was opened. PNM technicians and HVB personnel disassembled the breaker's interrupter. Measurements were taken of all the contact parts and the x-rays were validated. All of the internal assembly was inspected, and the actual condition of the parts was exactly what the x-ray films had determined. Photos were taken of the parts, including the contacts to complete the historical record. The testing process had been proven in the factory with staged tests, verifying that loose and worn parts could be seen and analyzed from the x-rays. The field tests performed at PNM's BA switching station proved this technology could work in the field.

Does It Save Money?

Factory testing and field testing proved that both the concept and the process worked, but how did it translate into savings for the utility? In PNM's case, the testing proved the breaker did not need to have the scheduled maintenance. If this testing had been done as a commercial process rather than a test validation, it would have cost the utility around US$9800 to perform the inspection. As this investigation showed, there was no serious degradation to any of the components; therefore, no replacement parts or utility equipment were required.

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