Pumping Up Reliability

On a hot day in August near Farnham, a tractor and trailer hummed as 1,000 gallons of mineral oil were pumped out of a 10-megawatt transformer at NNEC’s Robley substation. High-powered pumps push the oil through several filters and heaters to clean the oil of any imperfections. The most extreme filter, .255 microns, filtered out particles smaller than a human blood cell.

To the everyday person, the process of using electricity to power the community might appear to be simple. A person contacts their electric utility, the utility hooks up a meter, and voila! That person has electricity! However, a closer look at that process proves that safely and effectively providing power to homes and businesses within a given service area is much more complex.

It includes many working parts, one of the most important being the capabilities and effectiveness of a utility’s substations. At NNEC, the main goal of the employees in our Engineering department is simple: Maintain the substations, protect the system, plan for the future, and, above all, keep the lights on.

NNEC’s power is generated by Old Dominion Electric Cooperative and transmitted over long distances to our service area. To make the trip with as little energy loss as possible, transmission lines operate at a very high voltage. The first step in making that electricity usable for our members is reducing the voltage. That’s where substations come in.

To do this, electricity passes through an array of transformers, breakers, regulators, and other equipment inside the substation. The substations are the most critical and expensive equipment owned by NNEC. Thousands of homes depend on the high operability of the transformers, and an item as small as a 1/4-inch washer can cause havoc within the transformer.

“If you don’t schedule maintenance, transformers will schedule it for you,” says Steve Minor, manager of engineering services and lead for NNEC’s substation maintenance program. “Several years ago, we began adding more tests and scheduled maintenance actions, which improve the effectiveness and efficiency of the transformers while also extending their useful life.”

For the work to proceed at the Robley substation in August, the Engineering team and contractors prepared and planned for months. Several tests and the oil cleaning would all occur within a 48-hour time period, calling for all hands on deck. The tests and work could not be completed while the transformer was operational, so NNEC brought in its mobile transformer. Thousands of members never experienced a disruption of power during the two days of work.

Southwest Electric Company and NNEC engineers performed a sweep frequency response analysis, power factor test, turns ratio test, Megger test, and resistance tests on the transformer before the maintenance was performed to obtain a baseline of data to evaluate the success of the maintenance. The same tests in future years to assess the rate of change of the various items.

After the tests, the mineral oil was pumped into a container, so engineers could do a visual inspection of the critical components of the transformer. Inside, a laminated core holds windings that either step up or step down the voltage. The windings are protected from one another by thousands of sheets of insulating paper coated in the mineral oil.

To perform the visual inspection, the entire area is cleared of loose nuts and bolts, and the inspector removes all jewelry, electric devices, loose clothing, and other items that could fall in. If anything were to fall in and were unknown or not found, it could ruin the transformer. Before the oil is pumped back into the transformer, it is pumped through the multimillion-dollar tractor trailer custom-built in Kentucky to perform substation maintenance. The heaters and filters remove all contaminants that could reduce the efficiency or degrade the life of the transformer. Heated to 1,300 degrees Fahrenheit and run through the filters, the oil is pumped back into the transformer and resealed.

After two days of tests and maintenance, the transformer hums back to life helping energize thousands of NNEC members, and is ready to work for another 30 years helping us deliver reliable and affordable electricity.

For the Armchair Scientist

Sweep Frequency Response Analysis (SFRA): Measures the unique frequency response, a “fingerprint” of the transformer. Comparing the measurements taken at various phases and comparing the measurements with the former results of the same transformer may provide indications of mechanical or electrical changes. The analysis of the frequencies can be used to see if windings are damaged.

Power Factor Test: Measures the power loss of the transformer’s insulation system by measuring the power angle between an applied alternating current voltage and the resultant current. The power factor is defined as the cosine of the phase angle between voltage and current. For ideal insulation, the phase angle is 90 degrees. The results can be compared over time to determine if the insulation is malfunctioning or decaying.

Turns Ratio Test: Using a specifically designed piece of equipment, readings of voltage are taken to the low-voltage and high-voltage windings after the voltage is applied to one winding. The ratio is the division between the high reading and the low reading. This test ensures that the transformer will provide the proper step-up or step-down in voltage.

Megger Test: Measures the quality of insulation within the transformer. Testing was completed with a megohmmeter, similar to a multimeter, but with a much higher amount of electricity. To pass, the insulation must demonstrate a higher resistance than the prescribed international standards set for that transformer. The insulation resistance of the device is measured while the phased and natural are short-circuited together. Resistances are measured between each winding and between the other windings and the ground.

Resistance Test: Measures the difference in resistance between windings and opens in the connections, ensuring that each circuit is properly wired and that all connections are tight. The testing can only be completed after the transformer has not been operational for several hours allowing the temperature inside to equalize. Performing this test also allows us to calculate and compensate for load losses.