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.

Total projected costs are broken out as follows:

HVB inspection crew plus travel	$9500
Utility labor (1 man × 8 hr × 1 day)	$320

However, if a major maintenance inspection was scheduled for the 362-kV circuit breaker, it would have required at least a five-day outage. The purchase of a complete set of replacement parts would have been done prior to taking the breaker out of service. In most cases, many of these parts are unnecessary and the unused parts are either warehoused by the utility or sent back to the manufacturer for a large restocking fee. Generally, a complete set is ordered for one reason: The technicians have no way of knowing what will be needed until the breaker is opened. Even if parts are available, it takes time to order, ship and receive them. Any delay means keeping the tank open to the atmosphere, which must be kept to a minimum. Therefore, it's good practice to have everything on hand to keep the outage time to a minimum.

Normally, utilities also schedule a field engineer from the manufacturer to assist their crew with the inspection. Next, a four-person crew is assigned to the task. The crew brings vehicles, test equipment, a gas cart and replacement parts to the switching station. The crew removes the SF₆ gas, opens the tanks, inspects the interrupters, replaces the worn parts, checks for foreign parts, closes the tanks and refills the gas. Typical costs include:

Manufacturer's field engineer plus travel	$8000
Replacement parts	$30,000
Utility equipment	$10,000
Utility labor (4 men × 8 hr × 5 days)	$10,000
Total	$58,000

What Did It Prove?

The nondestructive x-ray testing performed at BA station would save a utility approximately $48,180 had the result been the same as PNM's breaker. The result of the open tank inspection confirms x-ray testing has the ability to show if a serious problem exists within the breaker. It also is able to show which parts need to be replaced. Because the utility would know exactly what the condition of the breaker is and what has to be replaced, it also would know exactly how long the work would take and would be able to predict if the maintenance could either be delayed or scheduled quickly. All of this would be accomplished on a circuit breaker with the tank closed; the gas still inside the breaker and the breaker would be quickly returned to service at the end of testing.

Intangible Benefits

Radiography equipment has been used for many years to verify welds and test the installation of equipment. Today, it's being used as a tool to reduce maintenance costs and time required to perform that maintenance on power circuit breakers. Its field testing was successful and is now being used by utilities in North America who are realizing additional intangible benefits inherent with the process.

Opening the tank of the circuit breaker exposes it to external moisture, introduces possible leaks to the gas system and increases the risk of contamination inside the breaker, not to mention the possibility of leaving foreign objects inside. Also, gas handling may result in contamination to the SF₆ gas, portions of SF₆ gas escaping into the atmosphere or personnel being exposed to the dangerous arching byproducts, which are both corrosive and toxic. By reducing the number of times a circuit breaker's tank is opened, the risks to personnel are minimized and the environment is protected.

Gene Wolf is the principal engineer for stations with Public Service Company of New Mexico (PNM). He is responsible for the design and construction of all EHV station facilities on the PNM system. In addition to his station responsibilities, Wolf is responsible for all HVDC and FACTS-related engineering support. A senior member of IEEE, he has been designing station facilities for more than 30 years.
GWOLF@pnm.com