Uncertainty is the enemy of a utility. Accordingly, processes and procedures have been set up to manage or mitigate risk for many of the electromechanical devices that make up and control the power grid. But what can a utility do when something unexpected happens, resulting in a forced outage? How can a utility determine if a critical asset is fit to be returned to service? Sweep frequency response analysis (SFRA) can serve as a key indicator of whether a transformer can be re-energized or if there is a need to work out an alternative.

SFRA interpretation looks complicated at first, but the technique is simpler than it appears. A trace represents the fingerprint of a given winding; changes to that fingerprint mean something has changed from one measurement to the next. Sometimes this can be a dangerous mechanical change, like a winding being damaged, and sometimes it represents a natural process, like a byproduct of fluid movement inside the tank or a change in the grounding circuit. When SFRA traces are compared, one wants to consider both the traces themselves, how the traces might have changed over time and, most importantly, the context surrounding the measurement to determine what the next steps might be.

SFRA at Lower Colorado River Authority

Relocation case: Open-circuit measurements before and after the move. Low-frequency differences like this are commonly attributed to magnetic viscosity effects. A lack of any high-frequency differences reinforces this analysis.Lower Colorado River Authority (LCRA) was one of Doble's first partners to apply SFRA as a testing technique. LCRA began using SFRA in 2002 to assess the condition of its transformer fleet. Technicians performing the tests were able to quickly develop skills in field analysis because of the graphical nature of results, allowing for quick adoption of the tool as a part of routine field testing. In particular, after a significant fault event, LCRA viewed SFRA as a way to provide a more detailed analysis of physical changes to the core and coil assembly of a transformer than traditional methods of testing. When a transformer is off-line, many are counting on the utility to return it to service as soon as possible. The ability to quickly analyze results and compare them to baseline values provided LCRA with the means to make timely determinations about re-energizing transformers.

LCRA has integrated SFRA into three of its tests:

  • Validation of factory tests during on-site acceptance testing

  • Production of baseline data sets, after the assembly and processing of a new transformer

  • Routine maintenance testing and comparison to baseline data established during acceptance testing.

SFRA also is used after abnormal operations and occurrences. After a tripping operation, it's important to exercise caution to limit the voltage levels applied as a result of the presence of combustible gases within a transformer tank. Because SFRA uses low voltages, LCRA uses it to perform initial troubleshooting after an event.

Any significant deviations are taken to indicate physical changes that may require further investigation. SFRA provides quick analysis, therefore, it has become essential to LCRA's ability to make timely determinations about re-energizing transformers.