As I drove toward the office one morning, it occurred to me that we were due for an emergency call. I was about to text a colleague (hands-free voice recognition) when the phone rang. Soon I rerouted the navigation system and I was off to the races! Along the way, I notified the office and called a technician to assist and ensure his van was equipped with transformer test equipment since the customer suggested they might have lost a transformer.
The technician and I arrived simultaneously at the guard shack of one of the nation’s most iconic thoroughbred horse racetracks. Focusing on the task at hand was not initially easy with the perfectly groomed track nestled against a backdrop of the snow-capped San Gabriel Mountains and horses walking by, but we quickly got down to business.
The facility takes 4,160 volts from the local utility at several points and distributes scores of dry-type and fluid-filled transformers via underground cable and a few sections of overhead lines. Our client escorted us to the remote end of the park, where the secondary voltage at the carpenter’s shop was improper.
We observed the overhead line terminating to a medium-voltage cable riser into an oil fuse cutout (OFC) with a small transformer adjacent. The transformer was humming and a quick look at the well-worn nameplate indicated a 4,160 V to 480 V delta-delta configuration. We made our way into the carpenter’s shop, which was black, and measured voltage at the 200A low-voltage panel. It was quickly apparent that balanced three-phase 480 V indeed was not present, and we began to also suspect a faulty transformer.
Plan of Attack
We wanted to shut down the transformer for testing, but did not want to risk operating the oil-filled cutout (OFC). These types of apparatus have provided a means of safe, economical, medium-voltage protection and switching for many years, but can be very unsafe due to oil contamination or loss of oil over time from faulty gaskets. The G&W Electric-type switch present also carries a manufacturer’s warning letter recommending replacement with more modern oil-free switch technology.
Shutting down the circuit was not immediately possible as some facilities, including the equine hospital, could not afford power loss until another hour. As we waited for the outage window, we decided to take a look at the OFC cables after determining a cover could be removed safely.
We expected to see an obvious termination or jumper failure but were disappointed to find the cables intact. Additionally, voltage appeared to be present on all three phases using a tic-tracer, but we cautioned ourselves that the transformer primary delta winding would allow voltage to be present on all three phases.
The outage availability was soon confirmed, and the 4,160 V main breaker was opened. This breaker is very old and is also a candidate for replacement.
After confirming no voltage present at the OFC input, we focused our efforts on opening the OFC so the possible faulty transformer could be isolated and the remainder of the circuit could be energized to other facilities.
The OFC handle bolt on one of the phases was galled and required quite a bit of coaxing to free up so the switch could be opened. We discharged the cables and opened the transformer, but again, no obvious signs of damage were noted. After opening the low-voltage main, the transformer was ready for testing, and we chose to proceed with a transformer turns ratio (TTR) test.
We initially obtained unexpected results, but without a vector diagram to go by, we thought we should see how the results sorted out. The transformer did state “center tapped,” and as we went through the TTR tests, we observed the secondary winding conductors coming from the coils, which helped us determine how the center tap was derived from the center coil.
Looking at the TTR test results, no problem was apparent. We proceeded to perform insulation resistance tests, which also passed. Quick inspection of the OFC fuse links and fluid also did not reveal any problems.
We decided to leave the OFC in the open position and close the main 4,160 V breaker to restore power to the other buildings on the circuit. After doing so, our technician stationed at the OFC measured voltage, and B-phase voltage was found to be absent. The facility manager and I started back to the OFC but first went to a building on the circuit and measured balanced 480 V three-phase power.
So we have power at the origination, but we are missing a phase at the end of the circuit. We inspected the overhead line and soon found an open B-phase jumper, which was subsequently repaired using a bucket truck.
It appears that the aluminum jumper developed a hot spot that eventually burned open. Complete power was successfully restored.
Reflecting on this successful and efficient troubleshooting adventure, lessons can always be learned:
- Gather as many details from the facility as possible.
- If something just failed, be ultra-concerned with operating equipment safely or avoid energized operation as much as possible.
- Come up with a logical plan of attack, based upon likely failed equipment.
- Perform visual inspection and tests to rule out components.
Don A. Genutis presently serves as President of Halco Testing Services, Inc., a NETA Accredited Company with offices in Los Angeles and Houston. He has held various principal positions during his 35-plus year career in the electrical testing field, primarily focused on advancing no-outage testing techniques for the last 20 years, with emphasis on cable and switchgear on-line partial discharge testing techniques. Early in his career, Don acquired and operated the former Westinghouse East Pittsburgh Insulation Research Laboratory, where he gained valuable experience in the understanding of insulation material performance. Don holds a BS in electrical engineering from Carnegie Mellon University and is a NETA Certified Technician. Don has authored 50 technical articles for NETA World and has been featured in EC&M and Uptime magazines. Don’s work is summarized in his book, Partial Discharge & Other No-Outage Testing Methods, published in 2019.